1
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Small C, Harper C, Jiang A, Kontaxi C, Pronot M, Yak N, Malapaka A, Davenport EC, Wallis TP, Gormal RS, Joensuu M, Martínez-Mármol R, Cousin MA, Meunier FA. SV2A controls the surface nanoclustering and endocytic recruitment of Syt1 during synaptic vesicle recycling. J Neurochem 2024. [PMID: 39091022 DOI: 10.1111/jnc.16186] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2024] [Revised: 07/04/2024] [Accepted: 07/06/2024] [Indexed: 08/04/2024]
Abstract
Following exocytosis, the recapture of plasma membrane-stranded vesicular proteins into recycling synaptic vesicles (SVs) is essential for sustaining neurotransmission. Surface clustering of vesicular proteins has been proposed to act as a 'pre-assembly' mechanism for endocytosis that ensures high-fidelity retrieval of SV cargo. Here, we used single-molecule imaging to examine the nanoclustering of synaptotagmin-1 (Syt1) and synaptic vesicle protein 2A (SV2A) in hippocampal neurons. Syt1 forms surface nanoclusters through the interaction of its C2B domain with SV2A, which are sensitive to mutations in this domain (Syt1K326A/K328A) and SV2A knockdown. SV2A co-clustering with Syt1 is reduced by blocking SV2A's cognate interaction with Syt1 (SV2AT84A). Surprisingly, impairing SV2A-Syt1 nanoclustering enhanced the plasma membrane recruitment of key endocytic protein dynamin-1, causing accelerated Syt1 endocytosis, altered intracellular sorting and decreased trafficking of Syt1 to Rab5-positive endocytic compartments. Therefore, SV2A and Syt1 are segregated from the endocytic machinery in surface nanoclusters, limiting dynamin recruitment and negatively regulating Syt1 entry into recycling SVs.
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Affiliation(s)
- Christopher Small
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
| | - Callista Harper
- Centre for Discovery Brain Sciences, University of Edinburgh, Edinburgh, UK
- Muir Maxwell Epilepsy Centre, University of Edinburgh, Edinburgh, UK
- Simons Initiative for the Developing Brain, University of Edinburgh, Edinburgh, UK
| | - Anmin Jiang
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
| | - Christiana Kontaxi
- Centre for Discovery Brain Sciences, University of Edinburgh, Edinburgh, UK
- Muir Maxwell Epilepsy Centre, University of Edinburgh, Edinburgh, UK
- Simons Initiative for the Developing Brain, University of Edinburgh, Edinburgh, UK
| | - Marie Pronot
- Centre for Discovery Brain Sciences, University of Edinburgh, Edinburgh, UK
- Muir Maxwell Epilepsy Centre, University of Edinburgh, Edinburgh, UK
- Simons Initiative for the Developing Brain, University of Edinburgh, Edinburgh, UK
| | - Nyakuoy Yak
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
| | - Anusha Malapaka
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
| | - Elizabeth C Davenport
- Centre for Discovery Brain Sciences, University of Edinburgh, Edinburgh, UK
- Muir Maxwell Epilepsy Centre, University of Edinburgh, Edinburgh, UK
- Simons Initiative for the Developing Brain, University of Edinburgh, Edinburgh, UK
| | - Tristan P Wallis
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
| | - Rachel S Gormal
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
| | - Merja Joensuu
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
| | - Ramón Martínez-Mármol
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
| | - Michael A Cousin
- Centre for Discovery Brain Sciences, University of Edinburgh, Edinburgh, UK
- Muir Maxwell Epilepsy Centre, University of Edinburgh, Edinburgh, UK
- Simons Initiative for the Developing Brain, University of Edinburgh, Edinburgh, UK
| | - Frédéric A Meunier
- Clem Jones Centre for Ageing Dementia Research, Queensland Brain Institute, the University of Queensland, Brisbane, Queensland, Australia
- School of Biomedical Sciences, the University of Queensland, Brisbane, Queensland, Australia
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2
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Beaven AH, Bikkumalla V, Chon NL, Matthews AE, Lin H, Knight JD, Sodt AJ. Synaptotagmin 7 C2 domains induce membrane curvature stress via electrostatic interactions and the wedge mechanism. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.01.10.575084. [PMID: 38313280 PMCID: PMC10837831 DOI: 10.1101/2024.01.10.575084] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 02/06/2024]
Abstract
Synaptotagmin 7 (Syt-7) is part of the synaptotagmin protein family that regulates exocytotic lipid membrane fusion. Among the family, Syt-7 stands out by its membrane binding strength and stabilization of long-lived membrane fusion pores. Given that Syt-7 vesicles form long-lived fusion pores, we hypothesize that its interactions with the membrane stabilize the specific curvatures, thicknesses, and lipid compositions that support a metastable fusion pore. Using all-atom molecular dynamics simulations and FRET-based assays of Syt-7's membrane-binding C2 domains (C2A and C2B), we found that Syt-7 C2 domains sequester anionic lipids, are sensitive to cholesterol, thin membranes, and generate lipid membrane curvature by two competing, but related mechanisms. First, Syt-7 forms strong electrostatic contacts with the membrane, generating negative curvature stress. Second, Syt-7's calcium binding loops embed in the membrane surface, acting as a wedge to thin the membrane and induce positive curvature stress. These curvature mechanisms are linked by the protein insertion depth as well as the resulting protein tilt. Simplified quantitative models of the curvature-generating mechanisms link simulation observables to their membrane-reshaping effectiveness.
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Affiliation(s)
- Andrew H. Beaven
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD
- Postdoctoral Research Associate Program, National Institute of General Medical Sciences, National Institutes of Health, Bethesda, MD
| | | | - Nara L. Chon
- Department of Chemistry, University of Colorado Denver, Denver, CO
| | | | - Hai Lin
- Department of Chemistry, University of Colorado Denver, Denver, CO
| | | | - Alexander J. Sodt
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD
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3
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Huang W, Yang Y, Che F, Wu H, Ma Y, Zhao Y. Lethal variant in the C2A domain may cause severe SYT1-associated neurodevelopmental disorder in the newborns. Neurogenetics 2024; 25:27-31. [PMID: 37930470 DOI: 10.1007/s10048-023-00738-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2023] [Accepted: 10/22/2023] [Indexed: 11/07/2023]
Abstract
Synaptotagmin-1 (SYT1) plays a pivotal role in regulating presynaptic processes, including neurotransmitter release. SYT1 variants perturb synaptic vesicle endocytosis and exocytosis, resulting in a series of neurodevelopmental disorders defined as Baker-Gordon syndrome. Herein, we report the case of a newborn with dysmorphic facial appearance, severe hypotonia, poor feeding, gastroesophageal reflux, and an inability to eat and breathe, diagnosed with Baker-Gordon syndrome. A retrospective search was performed on a newborn with Baker-Gordon syndrome. Medical charts were reviewed, with focus on the clinical presentation, diagnostic process, and treatment outcomes. Whole-genome high-throughput DNA sequencing was performed to identify genetic variants. Whole-exome sequencing identified the likely pathogenic variant as SYT1 C.551 T > C(p.V184A). Sanger sequencing results indicated that this variant was a de novo mutation in a conservative site located in the C2A domain of the protein. The patient died at 57 days old because of severe feeding and breathing problems. Our findings of a novel lethal variant in the C2A domain of SYT1 in the youngest patient diagnosed infantile Baker-Gordon syndrome who presented with the most severe hypotonia reported to date expands the spectrum of SYT1- associated neurodevelopmental disorders.
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Affiliation(s)
- Wendi Huang
- Department of Neonatology, Xi'an Children's Hospital, No. 69 Xijuyuan Lane, Lianhu District, Xi'an City, 710003, Shaanxi, China
| | - Ying Yang
- Shaanxi Institute for Pediatric Diseases, Xi'an Children's Hospital, Xi'an, China
| | - Fengyu Che
- Shaanxi Institute for Pediatric Diseases, Xi'an Children's Hospital, Xi'an, China
| | - Haibin Wu
- Shaanxi Institute for Pediatric Diseases, Xi'an Children's Hospital, Xi'an, China
| | - Ying Ma
- Department of Neonatology, Xi'an Children's Hospital, No. 69 Xijuyuan Lane, Lianhu District, Xi'an City, 710003, Shaanxi, China
| | - Yujuan Zhao
- Department of Neonatology, Xi'an Children's Hospital, No. 69 Xijuyuan Lane, Lianhu District, Xi'an City, 710003, Shaanxi, China.
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4
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Palfreyman MT, West SE, Jorgensen EM. SNARE Proteins in Synaptic Vesicle Fusion. ADVANCES IN NEUROBIOLOGY 2023; 33:63-118. [PMID: 37615864 DOI: 10.1007/978-3-031-34229-5_4] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/25/2023]
Abstract
Neurotransmitters are stored in small membrane-bound vesicles at synapses; a subset of synaptic vesicles is docked at release sites. Fusion of docked vesicles with the plasma membrane releases neurotransmitters. Membrane fusion at synapses, as well as all trafficking steps of the secretory pathway, is mediated by SNARE proteins. The SNAREs are the minimal fusion machinery. They zipper from N-termini to membrane-anchored C-termini to form a 4-helix bundle that forces the apposed membranes to fuse. At synapses, the SNAREs comprise a single helix from syntaxin and synaptobrevin; SNAP-25 contributes the other two helices to complete the bundle. Unc13 mediates synaptic vesicle docking and converts syntaxin into the permissive "open" configuration. The SM protein, Unc18, is required to initiate and proofread SNARE assembly. The SNAREs are then held in a half-zippered state by synaptotagmin and complexin. Calcium removes the synaptotagmin and complexin block, and the SNAREs drive vesicle fusion. After fusion, NSF and alpha-SNAP unwind the SNAREs and thereby recharge the system for further rounds of fusion. In this chapter, we will describe the discovery of the SNAREs, their relevant structural features, models for their function, and the central role of Unc18. In addition, we will touch upon the regulation of SNARE complex formation by Unc13, complexin, and synaptotagmin.
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Affiliation(s)
- Mark T Palfreyman
- School of Biological Sciences, and Howard Hughes Medical Institute, University of Utah, Salt Lake City, UT, USA
| | - Sam E West
- School of Biological Sciences, and Howard Hughes Medical Institute, University of Utah, Salt Lake City, UT, USA
| | - Erik M Jorgensen
- School of Biological Sciences, and Howard Hughes Medical Institute, University of Utah, Salt Lake City, UT, USA.
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5
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Zhou Q. Calcium Sensors of Neurotransmitter Release. ADVANCES IN NEUROBIOLOGY 2023; 33:119-138. [PMID: 37615865 DOI: 10.1007/978-3-031-34229-5_5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/25/2023]
Abstract
Calcium (Ca2+) plays a critical role in triggering all three primary modes of neurotransmitter release (synchronous, asynchronous, and spontaneous). Synaptotagmin1, a protein with two C2 domains, is the first isoform of the synaptotagmin family that was identified and demonstrated as the primary Ca2+ sensor for synchronous neurotransmitter release. Other isoforms of the synaptotagmin family as well as other C2 proteins such as the double C2 domain protein family were found to act as Ca2+ sensors for different modes of neurotransmitter release. Major recent advances and previous data suggest a new model, release-of-inhibition, for the initiation of Ca2+-triggered synchronous neurotransmitter release. Synaptotagmin1 binds Ca2+ via its two C2 domains and relieves a primed pre-fusion machinery. Before Ca2+ triggering, synaptotagmin1 interacts Ca2+ independently with partially zippered SNARE complexes, the plasma membrane, phospholipids, and other components to form a primed pre-fusion state that is ready for fast release. However, membrane fusion is inhibited until the arrival of Ca2+ reorients the Ca2+-binding loops of the C2 domain to perturb the lipid bilayers, help bridge the membranes, and/or induce membrane curvatures, which serves as a power stroke to activate fusion. This chapter reviews the evidence supporting these models and discusses the molecular interactions that may underlie these abilities.
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Affiliation(s)
- Qiangjun Zhou
- Department of Cell and Developmental Biology, Vanderbilt Brain Institute, Vanderbilt University, Nashville, TN, USA.
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6
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Wang X, Yu D, Wang H, Lei Z, Zhai Y, Sun M, Chen S, Yin P. Rab3 and synaptotagmin proteins in the regulation of vesicle fusion and neurotransmitter release. Life Sci 2022; 309:120995. [PMID: 36167148 DOI: 10.1016/j.lfs.2022.120995] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2022] [Revised: 09/13/2022] [Accepted: 09/21/2022] [Indexed: 10/31/2022]
Abstract
Ca2+-triggered neurotransmitter release involves complex regulatory mechanisms, including a series of protein-protein interactions. Three proteins, synaptobrevin (VAMP), synaptosomal-associated protein of 25kDa (SNAP-25) and syntaxin, constitute the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) core complex that plays key roles in controlling vesicle fusion and exocytosis. Many other proteins participate in the regulation of the processes via direct and/or indirect interaction with the SNARE complex. Although much effort has been made, the regulatory mechanism for exocytosis is still not completely clear. Accumulated evidence indicates that the small GTPase Rab3 and synaptotagmin proteins play important regulatory roles during vesicle fusion and neurotransmitter release. This review outlines our present understanding of the two regulatory proteins, with the focus on the interaction of Rab3 with synaptotagmin in the regulatory process.
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Affiliation(s)
- Xianchun Wang
- State Key Laboratory of Developmental Biology of Freshwater Fish, Protein Chemistry Laboratory, College of Life Sciences, Hunan Normal University, Changsha 410081, China.
| | - Dianmei Yu
- State Key Laboratory of Developmental Biology of Freshwater Fish, Protein Chemistry Laboratory, College of Life Sciences, Hunan Normal University, Changsha 410081, China
| | - Haiyan Wang
- State Key Laboratory of Developmental Biology of Freshwater Fish, Protein Chemistry Laboratory, College of Life Sciences, Hunan Normal University, Changsha 410081, China
| | - Zhixiang Lei
- State Key Laboratory of Developmental Biology of Freshwater Fish, Protein Chemistry Laboratory, College of Life Sciences, Hunan Normal University, Changsha 410081, China
| | - Yiwen Zhai
- State Key Laboratory of Developmental Biology of Freshwater Fish, Protein Chemistry Laboratory, College of Life Sciences, Hunan Normal University, Changsha 410081, China
| | - Minlu Sun
- State Key Laboratory of Developmental Biology of Freshwater Fish, Protein Chemistry Laboratory, College of Life Sciences, Hunan Normal University, Changsha 410081, China
| | - Si Chen
- State Key Laboratory of Developmental Biology of Freshwater Fish, Protein Chemistry Laboratory, College of Life Sciences, Hunan Normal University, Changsha 410081, China
| | - Panfeng Yin
- State Key Laboratory of Developmental Biology of Freshwater Fish, Protein Chemistry Laboratory, College of Life Sciences, Hunan Normal University, Changsha 410081, China
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7
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Téllez-Arreola JL, Martínez-Torres A, Flores-Moran AE, Lazaro-Guevara JM, Estrada-Mondragón A. Analysis of the MCTP Amino Acid Sequence Reveals the Conservation of Putative Calcium- and Lipid-Binding Pockets Within the C2 Domains In Silico. J Mol Evol 2022; 90:271-282. [PMID: 35604448 DOI: 10.1007/s00239-022-10057-1] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2021] [Accepted: 05/10/2022] [Indexed: 11/24/2022]
Abstract
MCTPs (Multiple C2 Domains and Transmembrane region Proteins) are evolutionarily and structurally related to other C2 proteins, which are central to exocytosis and membrane trafficking; however, their specific function has been little studied. MCTPs are associated with endosomes and the endoplasmic reticulum and possess three C2 domains (C2A-C2C) and two transmembrane regions (TMRs) well conserved in different species. Here, we generated structural models of the MCTP C2 domains of C. elegans and analyzed their putative function by docking, which revealed that these domains possess Ca2+- and lipid-binding pockets, suggesting that MCTPs play a significant, calcium-dependent role in membrane physiology.
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Affiliation(s)
- José Luis Téllez-Arreola
- Departamento de Neurobiología Celular Y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, 76215, Juriquilla, Querétaro, México.
| | - Ataúlfo Martínez-Torres
- Departamento de Neurobiología Celular Y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, 76215, Juriquilla, Querétaro, México
| | - Adriana E Flores-Moran
- Unit for Basic and Applied Microbiology, School of Natural Sciences, Autonomous University of Queretaro, Queretaro, Mexico
| | - José M Lazaro-Guevara
- Department of Human Genetics, University of Utah, Salt Lake City, UT, USA.,Department of Botany, University of British Columbia, Vancouver, BC, Canada.,Biodiversity Research Centre, University of British Columbia, Vancouver, BC, Canada
| | - Argel Estrada-Mondragón
- Department of Biomedical and Clinical Sciences (BKV), Linköping University, 581 83, Linköping, Sweden.
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8
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Bourgeois-Jaarsma Q, Miaja Hernandez P, Groffen AJ. Ca 2+ sensor proteins in spontaneous release and synaptic plasticity: Limited contribution of Doc2c, rabphilin-3a and synaptotagmin 7 in hippocampal glutamatergic neurons. Mol Cell Neurosci 2021; 112:103613. [PMID: 33753311 DOI: 10.1016/j.mcn.2021.103613] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2020] [Revised: 03/09/2021] [Accepted: 03/13/2021] [Indexed: 11/28/2022] Open
Abstract
Presynaptic neurotransmitter release is strictly regulated by SNARE proteins, Ca2+ and a number of Ca2+ sensors including synaptotagmins (Syts) and Double C2 domain proteins (Doc2s). More than seventy years after the original description of spontaneous release, the mechanism that regulates this process is still poorly understood. Syt-1, Syt7 and Doc2 proteins contribute predominantly, but not exclusively, to synchronous, asynchronous and spontaneous phases of release. The proteins share a conserved tandem C2 domain architecture, but are functionally diverse in their subcellular location, Ca2+-binding properties and protein interactions. In absence of Syt-1, Doc2a and -b, neurons still exhibit spontaneous vesicle fusion which remains Ca2+-sensitive, suggesting the existence of additional sensors. Here, we selected Doc2c, rabphilin-3a and Syt-7 as three potential Ca2+ sensors for their sequence homology with Syt-1 and Doc2b. We genetically ablated each candidate gene in absence of Doc2a and -b and investigated spontaneous and evoked release in glutamatergic hippocampal neurons, cultured either in networks or on microglial islands (autapses). The removal of Doc2c had no effect on spontaneous or evoked release. Syt-7 removal also did not affect spontaneous release, although it altered short-term plasticity by accentuating short-term depression. The removal of rabphilin caused an increased spontaneous release frequency in network cultures, an effect that was not observed in autapses. Taken together, we conclude that Doc2c and Syt-7 do not affect spontaneous release of glutamate in hippocampal neurons, while our results suggest a possible regulatory role of rabphilin-3a in neuronal networks. These findings importantly narrow down the repertoire of synaptic Ca2+ sensors that may be implicated in the spontaneous release of glutamate.
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Affiliation(s)
- Quentin Bourgeois-Jaarsma
- Department of Functional Genomics, Faculty of Science, Center for Neurogenomics and Cognitive Research, VU University, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands
| | - Pablo Miaja Hernandez
- Department of Functional Genomics, Faculty of Science, Center for Neurogenomics and Cognitive Research, VU University, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands
| | - Alexander J Groffen
- Department of Functional Genomics, Faculty of Science, Center for Neurogenomics and Cognitive Research, VU University, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands; Department of Clinical Genetics, VU Medical Center, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands.
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9
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Characterization of PROPPIN-Phosphoinositide Binding by Stopped-Flow Fluorescence Spectroscopy. Methods Mol Biol 2021. [PMID: 33481242 DOI: 10.1007/978-1-0716-1142-5_15] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
Abstract
PROPPINs (β-propellers that bind polyphosphoinositides) are a protein family that binds preferentially phosphatidylinositol 3-phosphate (PtdIns(3)P) and phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2) via its FRRG motif. PROPPINs are involved in autophagic functions, but their molecular mechanism is still elusive. To unravel the molecular mechanism of PROPPINs, it is essential to understand the PROPPIN-phosphoinositide binding. Here, we describe a protocol to study the kinetics of the PROPPIN-phosphoinositide binding using a fluorescence resonance energy transfer (FRET) stopped-flow approach. We use FRET between fluorophore-labeled protein and fluorophore-labeled liposomes, monitoring the increase of the acceptor emission in labeled liposomes after the protein-membrane binding. Through this approach, we studied the kinetics of the PROPPIN Atg18 (Autophagy-related protein 18) from Pichia angusta (PaAtg18) and a mutant of its FRRG motif, called FTTG mutant. Stopped-flow experiments demonstrated that the main function of the FRRG motif is to retain, instead of to drive, Atg18 to the membrane, decreasing the Atg18 dissociation rate. Furthermore, this method is suitable for the study of other PI-binding proteins.
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10
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Wang Q, Jiang M, Isupov MN, Chen Y, Littlechild JA, Sun L, Wu X, Wang Q, Yang W, Chen L, Li Q, Wu Y. The crystal structure of Arabidopsis BON1 provides insights into the copine protein family. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2020; 103:1215-1232. [PMID: 32369638 DOI: 10.1111/tpj.14797] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/11/2019] [Revised: 03/17/2020] [Accepted: 04/20/2020] [Indexed: 06/11/2023]
Abstract
The Arabidopsis thaliana BON1 gene product is a member of the evolutionary conserved eukaryotic calcium-dependent membrane-binding protein family. The copine protein is composed of two C2 domains (C2A and C2B) followed by a vWA domain. The BON1 protein is localized on the plasma membrane, and is known to suppress the expression of immune receptor genes and to positively regulate stomatal closure. The first structure of this protein family has been determined to 2.5-Å resolution and shows the structural features of the three conserved domains C2A, C2B and vWA. The structure reveals the third Ca2+ -binding region in C2A domain is longer than classical C2 domains and a novel Ca2+ binding site in the vWA domain. The structure of BON1 bound to Mn2+ is also presented. The binding of the C2 domains to phospholipid (PSF) has been modeled and provides an insight into the lipid-binding mechanism of the copine proteins. Furthermore, the selectivity of the separate C2A and C2B domains and intact BON1 to bind to different phospholipids has been investigated, and we demonstrated that BON1 could mediate aggregation of liposomes in response to Ca2+ . These studies have formed the basis of further investigations into the important role that the copine proteins play in vivo.
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Affiliation(s)
- Qianchao Wang
- State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, 350002, P. R. China
- University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
| | - Meiqin Jiang
- State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, 350002, P. R. China
- University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
| | - Michail N Isupov
- Henry Wellcome Center for Biocatalysis, Biosciences, University of Exeter, Stocker Road, Exeter, EX4 4QD, UK
| | - Yayu Chen
- Provincial University Key Laboratory of Cellular Stress Response and Metabolic Regulation, College of Life Science, Fujian Normal University, Fuzhou, 350117, P. R. China
| | - Jennifer A Littlechild
- Henry Wellcome Center for Biocatalysis, Biosciences, University of Exeter, Stocker Road, Exeter, EX4 4QD, UK
| | - Lifang Sun
- Provincial University Key Laboratory of Cellular Stress Response and Metabolic Regulation, College of Life Science, Fujian Normal University, Fuzhou, 350117, P. R. China
| | - Xiuling Wu
- State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, 350002, P. R. China
- University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
| | - Qin Wang
- State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, 350002, P. R. China
- University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
| | - Wendi Yang
- Provincial University Key Laboratory of Cellular Stress Response and Metabolic Regulation, College of Life Science, Fujian Normal University, Fuzhou, 350117, P. R. China
| | - Lifei Chen
- Provincial University Key Laboratory of Cellular Stress Response and Metabolic Regulation, College of Life Science, Fujian Normal University, Fuzhou, 350117, P. R. China
| | - Qi Li
- Department of Environmental Science & Engineering, Fudan University, Shanghai, 200433, P. R. China
| | - Yunkun Wu
- Provincial University Key Laboratory of Cellular Stress Response and Metabolic Regulation, College of Life Science, Fujian Normal University, Fuzhou, 350117, P. R. China
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11
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Bradberry MM, Courtney NA, Dominguez MJ, Lofquist SM, Knox AT, Sutton RB, Chapman ER. Molecular Basis for Synaptotagmin-1-Associated Neurodevelopmental Disorder. Neuron 2020; 107:52-64.e7. [PMID: 32362337 DOI: 10.1016/j.neuron.2020.04.003] [Citation(s) in RCA: 34] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2019] [Revised: 03/09/2020] [Accepted: 04/01/2020] [Indexed: 12/13/2022]
Abstract
At neuronal synapses, synaptotagmin-1 (syt1) acts as a Ca2+ sensor that synchronizes neurotransmitter release with Ca2+ influx during action potential firing. Heterozygous missense mutations in syt1 have recently been associated with a severe but heterogeneous developmental syndrome, termed syt1-associated neurodevelopmental disorder. Well-defined pathogenic mechanisms, and the basis for phenotypic heterogeneity in this disorder, remain unknown. Here, we report the clinical, physiological, and biophysical characterization of three syt1 mutations from human patients. Synaptic transmission was impaired in neurons expressing mutant variants, which demonstrated potent, graded dominant-negative effects. Biophysical interrogation of the mutant variants revealed novel mechanistic features concerning the cooperative action, and functional specialization, of the tandem Ca2+-sensing domains of syt1. These mechanistic studies led to the discovery that a clinically approved K+ channel antagonist is able to rescue the dominant-negative heterozygous phenotype. Our results establish a molecular cause, basis for phenotypic heterogeneity, and potential treatment approach for syt1-associated neurodevelopmental disorder.
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Affiliation(s)
- Mazdak M Bradberry
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA; Medical Scientist Training Program, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA
| | - Nicholas A Courtney
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA
| | - Matthew J Dominguez
- Department of Cell Physiology and Molecular Biophysics and Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX 79430, USA
| | - Sydney M Lofquist
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA
| | - Andrew T Knox
- Department of Neurology, Section of Pediatric Neurology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA
| | - R Bryan Sutton
- Department of Cell Physiology and Molecular Biophysics and Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX 79430, USA
| | - Edwin R Chapman
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA.
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12
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Bowers MR, Reist NE. The C2A domain of synaptotagmin is an essential component of the calcium sensor for synaptic transmission. PLoS One 2020; 15:e0228348. [PMID: 32032373 PMCID: PMC7006929 DOI: 10.1371/journal.pone.0228348] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2019] [Accepted: 01/13/2020] [Indexed: 01/27/2023] Open
Abstract
The synaptic vesicle protein, synaptotagmin, is the principle Ca2+ sensor for synaptic transmission. Ca2+ influx into active nerve terminals is translated into neurotransmitter release by Ca2+ binding to synaptotagmin’s tandem C2 domains, triggering the fast, synchronous fusion of multiple synaptic vesicles. Two hydrophobic residues, shown to mediate Ca2+-dependent membrane insertion of these C2 domains, are required for this process. Previous research suggested that one of its tandem C2 domains (C2B) is critical for fusion, while the other domain (C2A) plays only a facilitatory role. However, the function of the two hydrophobic residues in C2A have not been adequately tested in vivo. Here we show that these two hydrophobic residues are absolutely required for synaptotagmin to trigger vesicle fusion. Using in vivo electrophysiological recording at the Drosophila larval neuromuscular junction, we found that mutation of these two key C2A hydrophobic residues almost completely abolished neurotransmitter release. Significantly, mutation of both hydrophobic residues resulted in more severe deficits than those seen in synaptotagmin null mutants. Thus, we report the most severe phenotype of a C2A mutation to date, demonstrating that the C2A domain is absolutely essential for synaptotagmin’s function as the electrostatic switch. The postulated role of synaptotagmin’s C2A domain in triggering neurotransmitter release has fluctuated wildly over the years. Early biochemical experiments suggested that the C2A domain was essential, while the C2B domain was superfluous. Then, functional experiments measuring neurotransmitter release in vivo following disruptions in Ca2+ binding suggested that C2B was essential, while C2A was superfluous. Subsequently, the use of more refined mutations to disrupt Ca2+ binding indicated that C2A played a facilitatory role. Here we show two hydrophobic residues of the C2A domain are absolutely required for synaptotagmin-triggered neurotransmitter release. Thus, after over twenty years of research, we now demonstrate that the C2A domain of synaptotagmin is an essential component of the Ca2+ sensor for triggering synaptic transmission in vivo.
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Affiliation(s)
- Matthew R. Bowers
- Department of Biomedical Sciences, Molecular, Cellular, Integrative Neurosciences Program, Colorado State University, Fort Collins, CO, United States of America
| | - Noreen E. Reist
- Department of Biomedical Sciences, Molecular, Cellular, Integrative Neurosciences Program, Colorado State University, Fort Collins, CO, United States of America
- * E-mail:
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13
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Rafi SK, Fernández-Jaén A, Álvarez S, Nadeau OW, Butler MG. High Functioning Autism with Missense Mutations in Synaptotagmin-Like Protein 4 (SYTL4) and Transmembrane Protein 187 (TMEM187) Genes: SYTL4- Protein Modeling, Protein-Protein Interaction, Expression Profiling and MicroRNA Studies. Int J Mol Sci 2019; 20:E3358. [PMID: 31323913 PMCID: PMC6651166 DOI: 10.3390/ijms20133358] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2019] [Revised: 06/10/2019] [Accepted: 06/17/2019] [Indexed: 01/31/2023] Open
Abstract
We describe a 7-year-old male with high functioning autism spectrum disorder (ASD) and maternally-inherited rare missense variant of Synaptotagmin-like protein 4 (SYTL4) gene (Xq22.1; c.835C>T; p.Arg279Cys) and an unknown missense variant of Transmembrane protein 187 (TMEM187) gene (Xq28; c.708G>T; p. Gln236His). Multiple in-silico predictions described in our study indicate a potentially damaging status for both X-linked genes. Analysis of predicted atomic threading models of the mutant and the native SYTL4 proteins suggest a potential structural change induced by the R279C variant which eliminates the stabilizing Arg279-Asp60 salt bridge in the N-terminal half of the SYTL4, affecting the functionality of the protein's critical RAB-Binding Domain. In the European (Non-Finnish) population, the allele frequency for this variant is 0.00042. The SYTL4 gene is known to directly interact with several members of the RAB family of genes, such as, RAB27A, RAB27B, RAB8A, and RAB3A which are known autism spectrum disorder genes. The SYTL4 gene also directly interacts with three known autism genes: STX1A, SNAP25 and STXBP1. Through a literature-based analytical approach, we identified three of five (60%) autism-associated serum microRNAs (miRs) with high predictive power among the total of 298 mouse Sytl4 associated/predicted microRNA interactions. Five of 13 (38%) miRs were differentially expressed in serum from ASD individuals which were predicted to interact with the mouse equivalent Sytl4 gene. TMEM187 gene, like SYTL4, is a protein-coding gene that belongs to a group of genes which host microRNA genes in their introns or exons. The novel Q236H amino acid variant in the TMEM187 in our patient is near the terminal end region of the protein which is represented by multiple sequence alignments and hidden Markov models, preventing comparative structural analysis of the variant harboring region. Like SYTL4, the TMEM187 gene is expressed in the brain and interacts with four known ASD genes, namely, HCFC1; TMLHE; MECP2; and GPHN. TMM187 is in linkage with MECP2, which is a well-known determinant of brain structure and size and is a well-known autism gene. Other members of the TMEM gene family, TMEM132E and TMEM132D genes are associated with bipolar and panic disorders, respectively, while TMEM231 is a known syndromic autism gene. Together, TMEM187 and SYTL4 genes directly interact with recognized important ASD genes, and their mRNAs are found in extracellular vesicles in the nervous system and stimulate target cells to translate into active protein. Our evidence shows that both these genes should be considered as candidate genes for autism. Additional biological testing is warranted to further determine the pathogenicity of these gene variants in the causation of autism.
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Affiliation(s)
- Syed K Rafi
- Departments of Psychiatry & Behavioral Sciences and Pediatrics, University of Kansas Medical Center, Kansas City, KS 66160, USA.
| | | | - Sara Álvarez
- Genomics and Medicine, NIM Genetics, 28108 Madrid, Spain
| | - Owen W Nadeau
- Department of Biochemistry and Molecular Biology, University of Kansas Medical Center, Kansas City, KS 66160, USA
| | - Merlin G Butler
- Departments of Psychiatry & Behavioral Sciences and Pediatrics, University of Kansas Medical Center, Kansas City, KS 66160, USA.
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14
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Ciruelas K, Marcotulli D, Bajjalieh SM. Synaptic vesicle protein 2: A multi-faceted regulator of secretion. Semin Cell Dev Biol 2019; 95:130-141. [PMID: 30826548 DOI: 10.1016/j.semcdb.2019.02.003] [Citation(s) in RCA: 32] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2018] [Revised: 01/11/2019] [Accepted: 02/21/2019] [Indexed: 01/01/2023]
Abstract
Synaptic Vesicle Protein 2 (SV2) comprises a recently evolved family of proteins unique to secretory vesicles that undergo calcium-regulated exocytosis. In this review we consider SV2s' structural features, evolution, and function and discuss its therapeutic potential as the receptors for an expanding class of drugs used to treat epilepsy and cognitive decline.
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Affiliation(s)
- Kristine Ciruelas
- Department of Pharmacology, University of Washington, Seattle, WA, United States
| | - Daniele Marcotulli
- Department of Pharmacology, University of Washington, Seattle, WA, United States
| | - Sandra M Bajjalieh
- Department of Pharmacology, University of Washington, Seattle, WA, United States.
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15
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Tran HT, Anderson LH, Knight JD. Membrane-Binding Cooperativity and Coinsertion by C2AB Tandem Domains of Synaptotagmins 1 and 7. Biophys J 2019; 116:1025-1036. [PMID: 30795874 DOI: 10.1016/j.bpj.2019.01.035] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2018] [Revised: 12/21/2018] [Accepted: 01/30/2019] [Indexed: 02/04/2023] Open
Abstract
Synaptotagmin-1 (Syt-1) and synaptotagmin-7 (Syt-7) contain analogous tandem C2 domains, C2A and C2B, which together sense Ca2+ to bind membranes and promote the stabilization of exocytotic fusion pores. Syt-1 triggers fast release of neurotransmitters, whereas Syt-7 functions in processes that involve lower Ca2+ concentrations such as hormone secretion. Syt-1 C2 domains are reported to bind membranes cooperatively, based on the observation that they penetrate farther into membranes as the C2AB tandem than as individual C2 domains. In contrast, we previously suggested that the two C2 domains of Syt-7 bind membranes independently, based in part on measurements of their liposome dissociation kinetics. Here, we investigated C2A-C2B interdomain cooperativity with Syt-1 and Syt-7 using directly comparable measurements. Equilibrium Ca2+ titrations demonstrate that the Syt-7 C2AB tandem binds liposomes lacking phosphatidylinositol-4,5-bisphosphate (PIP2) with greater Ca2+ sensitivity than either of its individual domains and binds to membranes containing PIP2 even in the absence of Ca2+. Stopped-flow kinetic measurements show differences in cooperativity between Syt-1 and Syt-7: Syt-1 C2AB dissociates from PIP2-free liposomes much more slowly than either of its individual C2 domains, indicating cooperativity, whereas the major population of Syt-7 C2AB has a dissociation rate comparable to its C2A domain, suggesting a lack of cooperativity. A minor subpopulation of Syt-7 C2AB dissociates at a slower rate, which could be due to a small cooperative component and/or liposome clustering. Measurements using an environment-sensitive fluorescent probe indicate that the Syt-7 C2B domain inserts deeply into membranes as part of the C2AB tandem, similar to the coinsertion previously reported for Syt-1. Overall, coinsertion of C2A and C2B domains is coupled to cooperative energetic effects in Syt-1 to a much greater extent than in Syt-7. The difference can be understood in terms of the relative contributions of C2A and C2B domains toward membrane binding in the two proteins.
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Affiliation(s)
- Hai T Tran
- Department of Chemistry, University of Colorado Denver, Denver, Colorado
| | - Lauren H Anderson
- Department of Chemistry, University of Colorado Denver, Denver, Colorado
| | - Jefferson D Knight
- Department of Chemistry, University of Colorado Denver, Denver, Colorado.
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16
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Katti S, Her B, Srivastava AK, Taylor AB, Lockless SW, Igumenova TI. High affinity interactions of Pb 2+ with synaptotagmin I. Metallomics 2018; 10:1211-1222. [PMID: 30063057 DOI: 10.1039/c8mt00135a] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Lead (Pb) is a potent neurotoxin that disrupts synaptic neurotransmission. We report that Synaptotagmin I (SytI), a key regulator of Ca2+-evoked neurotransmitter release, has two high-affinity Pb2+ binding sites that belong to its cytosolic C2A and C2B domains. The crystal structures of Pb2+-complexed C2 domains revealed that protein-bound Pb2+ ions have holodirected coordination geometries and all-oxygen coordination spheres. The on-rate constants of Pb2+ binding to the C2 domains of SytI are comparable to those of Ca2+ and are diffusion-limited. In contrast, the off-rate constants are at least two orders of magnitude smaller, indicating that Pb2+ can serve as both a thermodynamic and kinetic trap for the C2 domains. We demonstrate, using NMR spectroscopy, that population of these sites by Pb2+ ions inhibits further Ca2+ binding despite the existing coordination vacancies. Our work offers a unique insight into the bioinorganic chemistry of Pb(ii) and suggests a mechanism by which low concentrations of Pb2+ ions can interfere with the Ca2+-dependent function of SytI in the cell.
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Affiliation(s)
- Sachin Katti
- Department of Biochemistry and Biophysics, Texas A&M University, 300 Olsen Boulevard, College Station, TX 77843, USA.
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17
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Synergistic control of neurotransmitter release by different members of the synaptotagmin family. Curr Opin Neurobiol 2018; 51:154-162. [DOI: 10.1016/j.conb.2018.05.006] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2018] [Revised: 05/19/2018] [Accepted: 05/21/2018] [Indexed: 01/06/2023]
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18
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Synaptotagmin oligomerization is essential for calcium control of regulated exocytosis. Proc Natl Acad Sci U S A 2018; 115:E7624-E7631. [PMID: 30038018 DOI: 10.1073/pnas.1808792115] [Citation(s) in RCA: 37] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023] Open
Abstract
Regulated exocytosis, which underlies many intercellular signaling events, is a tightly controlled process often triggered by calcium ion(s) (Ca2+). Despite considerable insight into the central components involved, namely, the core fusion machinery [soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)] and the principal Ca2+ sensor [C2-domain proteins like synaptotagmin (Syt)], the molecular mechanism of Ca2+-dependent release has been unclear. Here, we report that the Ca2+-sensitive oligomers of Syt1, a conserved structural feature among several C2-domain proteins, play a critical role in orchestrating Ca2+-coupled vesicular release. This follows from pHluorin-based imaging of single-vesicle exocytosis in pheochromocytoma (PC12) cells showing that selective disruption of Syt1 oligomerization using a structure-directed mutation (F349A) dramatically increases the normally low levels of constitutive exocytosis to effectively occlude Ca2+-stimulated release. We propose a parsimonious model whereby Ca2+-sensitive oligomers of Syt (or a similar C2-domain protein) assembled at the site of docking physically block spontaneous fusion until disrupted by Ca2+ Our data further suggest Ca2+-coupled vesicular release is triggered by removal of the inhibition, rather than by direct activation of the fusion machinery.
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19
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Luttrell SM, Su YH, Swalla BJ. Getting a Head with Ptychodera flava Larval Regeneration. THE BIOLOGICAL BULLETIN 2018; 234:152-164. [PMID: 29949438 DOI: 10.1086/698510] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Severe injury to the central nervous system of chordates often results in permanent and irreversible mental and physical challenges. While some chordates are able to repair and/or regenerate portions of their nervous system, no chordate has been shown to be able to regenerate all regions of its central nervous system after catastrophic injury or amputation. Some hemichordates, on the other hand, are able to efficiently regenerate all neural structures, including their dorsal, hollow neural tube after complete ablation. Solitary hemichordates are marine acorn worms and a sister group to the echinoderms. The hemichordate Ptychodera flava progresses from a pelagic, feeding tornaria larva to a tripartite benthic worm with an anterior proboscis, a middle collar region, and a long posterior trunk. The adult worm regenerates all body parts when bisected in the trunk, but it was unknown whether the regeneration process was present in tornaria larvae. Now, we show that P. flava larvae are capable of robust regeneration after bisection through the sagittal, coronal, and axial planes. We also use antibody staining to show that the apical sensory organ regenerates a rich, serotonin-positive complex of cells within two weeks after amputation. Cells labeled with 5-ethynyl-2'-deoxyuridine confirm that regeneration is occurring through epimorphic processes as new cells are added at the cut site and throughout the regenerating tissue. This study verifies that P. flava larvae can be used for future functional studies aimed at identifying the genetic and morphological mechanisms controlling central nervous system regeneration in a stem deuterostome.
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20
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MacDougall DD, Lin Z, Chon NL, Jackman SL, Lin H, Knight JD, Anantharam A. The high-affinity calcium sensor synaptotagmin-7 serves multiple roles in regulated exocytosis. J Gen Physiol 2018; 150:783-807. [PMID: 29794152 PMCID: PMC5987875 DOI: 10.1085/jgp.201711944] [Citation(s) in RCA: 34] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2017] [Accepted: 05/07/2018] [Indexed: 12/19/2022] Open
Abstract
MacDougall et al. review the structure and function of the calcium sensor synaptotagmin-7 in exocytosis. Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types. Here, we review the known and emerging roles of Syt-7 in various contexts and stress the importance of its actions. Unique functions of Syt-7 are discussed in light of recent imaging, electrophysiological, and computational studies. Particular emphasis is placed on Syt-7–dependent regulation of synaptic transmission and neuroendocrine cell secretion. Finally, based on biochemical and structural data, we propose a mechanism to link Syt-7’s role in membrane fusion with its role in subsequent fusion pore expansion via strong calcium-dependent phospholipid binding.
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Affiliation(s)
| | - Zesen Lin
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
| | - Nara L Chon
- Department of Chemistry, University of Colorado, Denver, CO
| | - Skyler L Jackman
- Vollum Institute, Oregon Health & Science University, Portland, OR
| | - Hai Lin
- Department of Chemistry, University of Colorado, Denver, CO
| | | | - Arun Anantharam
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
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21
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Kaempf N, Maritzen T. Safeguards of Neurotransmission: Endocytic Adaptors as Regulators of Synaptic Vesicle Composition and Function. Front Cell Neurosci 2017; 11:320. [PMID: 29085282 PMCID: PMC5649181 DOI: 10.3389/fncel.2017.00320] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2017] [Accepted: 09/26/2017] [Indexed: 11/13/2022] Open
Abstract
Communication between neurons relies on neurotransmitters which are released from synaptic vesicles (SVs) upon Ca2+ stimuli. To efficiently load neurotransmitters, sense the rise in intracellular Ca2+ and fuse with the presynaptic membrane, SVs need to be equipped with a stringently controlled set of transmembrane proteins. In fact, changes in SV protein composition quickly compromise neurotransmission and most prominently give rise to epileptic seizures. During exocytosis SVs fully collapse into the presynaptic membrane and consequently have to be replenished to sustain neurotransmission. Therefore, surface-stranded SV proteins have to be efficiently retrieved post-fusion to be used for the generation of a new set of fully functional SVs, a process in which dedicated endocytic sorting adaptors play a crucial role. The question of how the precise reformation of SVs is achieved is intimately linked to how SV membranes are retrieved. For a long time both processes were believed to be two sides of the same coin since Clathrin-mediated endocytosis (CME), the proposed predominant SV recycling mode, will jointly retrieve SV membranes and proteins. However, with the recent proposal of Clathrin-independent SV recycling pathways SV membrane retrieval and SV reformation turn into separable events. This review highlights the progress made in unraveling the molecular mechanisms mediating the high-fidelity retrieval of SV proteins and discusses how the gathered knowledge about SV protein recycling fits in with the new notions of SV membrane endocytosis.
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Affiliation(s)
- Natalie Kaempf
- Molecular Physiology and Cell Biology Section, Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP), Berlin, Germany
| | - Tanja Maritzen
- Molecular Physiology and Cell Biology Section, Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP), Berlin, Germany
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22
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Shields MC, Bowers MR, Fulcer MM, Bollig MK, Rock PJ, Sutton BR, Vrailas-Mortimer AD, Lochmüller H, Whittaker RG, Horvath R, Reist NE. Drosophila studies support a role for a presynaptic synaptotagmin mutation in a human congenital myasthenic syndrome. PLoS One 2017; 12:e0184817. [PMID: 28953919 PMCID: PMC5617158 DOI: 10.1371/journal.pone.0184817] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2016] [Accepted: 08/31/2017] [Indexed: 12/04/2022] Open
Abstract
During chemical transmission, the function of synaptic proteins must be coordinated to efficiently release neurotransmitter. Synaptotagmin 2, the Ca2+ sensor for fast, synchronized neurotransmitter release at the human neuromuscular junction, has recently been implicated in a dominantly inherited congenital myasthenic syndrome associated with a non-progressive motor neuropathy. In one family, a proline residue within the C2B Ca2+-binding pocket of synaptotagmin is replaced by a leucine. The functional significance of this residue has not been investigated previously. Here we show that in silico modeling predicts disruption of the C2B Ca2+-binding pocket, and we examine the in vivo effects of the homologous mutation in Drosophila. When expressed in the absence of native synaptotagmin, this mutation is lethal, demonstrating for the first time that this residue plays a critical role in synaptotagmin function. To achieve expression similar to human patients, the mutation is expressed in flies carrying one copy of the wild type synaptotagmin gene. We now show that Drosophila carrying this mutation developed neurological and behavioral manifestations similar to those of human patients and provide insight into the mechanisms underlying these deficits. Our Drosophila studies support a role for this synaptotagmin point mutation in disease etiology.
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Affiliation(s)
- Mallory C. Shields
- Department of Biomedical Sciences, Colorado State University, Fort Collins, CO, United States of America
- Molecular, Cellular, and Integrative Neuroscience Program, Colorado State University, Fort Collins, CO, United States of America
| | - Matthew R. Bowers
- Department of Biomedical Sciences, Colorado State University, Fort Collins, CO, United States of America
- Molecular, Cellular, and Integrative Neuroscience Program, Colorado State University, Fort Collins, CO, United States of America
| | - McKenzie M. Fulcer
- Department of Biomedical Sciences, Colorado State University, Fort Collins, CO, United States of America
| | - Madelyn K. Bollig
- Department of Biomedical Sciences, Colorado State University, Fort Collins, CO, United States of America
| | - Patrick J. Rock
- School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas, United States of America
| | - Bryan R. Sutton
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas, United States of America
| | - Alysia D. Vrailas-Mortimer
- Department of Biological Sciences, University of Denver, Denver, CO, United States of America
- School of Biological Sciences, Illinois State University, Normal, IL, United States of America
| | - Hanns Lochmüller
- John Walton Muscular Dystrophy Research Centre, Institute of Genetic Medicine, Newcastle University, Newcastle upon Tyne, NE1 3BZ, United Kingdom
| | - Roger G. Whittaker
- Institute of Neuroscience, Newcastle University, Newcastle upon Tyne, NE2 4HH, United Kingdom
| | - Rita Horvath
- John Walton Muscular Dystrophy Research Centre, Institute of Genetic Medicine, Newcastle University, Newcastle upon Tyne, NE1 3BZ, United Kingdom
| | - Noreen E. Reist
- Department of Biomedical Sciences, Colorado State University, Fort Collins, CO, United States of America
- Molecular, Cellular, and Integrative Neuroscience Program, Colorado State University, Fort Collins, CO, United States of America
- * E-mail:
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23
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Jackman SL, Regehr WG. The Mechanisms and Functions of Synaptic Facilitation. Neuron 2017; 94:447-464. [PMID: 28472650 DOI: 10.1016/j.neuron.2017.02.047] [Citation(s) in RCA: 229] [Impact Index Per Article: 32.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2016] [Revised: 02/23/2017] [Accepted: 02/28/2017] [Indexed: 12/22/2022]
Abstract
The ability of the brain to store and process information relies on changing the strength of connections between neurons. Synaptic facilitation is a form of short-term plasticity that enhances synaptic transmission for less than a second. Facilitation is a ubiquitous phenomenon thought to play critical roles in information transfer and neural processing. Yet our understanding of the function of facilitation remains largely theoretical. Here we review proposed roles for facilitation and discuss how recent progress in uncovering the underlying molecular mechanisms could enable experiments that elucidate how facilitation, and short-term plasticity in general, contributes to circuit function and animal behavior.
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Affiliation(s)
- Skyler L Jackman
- Department of Neurobiology, Harvard Medical School, Boston, MA 02118, USA
| | - Wade G Regehr
- Department of Neurobiology, Harvard Medical School, Boston, MA 02118, USA.
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24
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Bartholome O, Van den Ackerveken P, Sánchez Gil J, de la Brassinne Bonardeaux O, Leprince P, Franzen R, Rogister B. Puzzling Out Synaptic Vesicle 2 Family Members Functions. Front Mol Neurosci 2017; 10:148. [PMID: 28588450 PMCID: PMC5438990 DOI: 10.3389/fnmol.2017.00148] [Citation(s) in RCA: 73] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2017] [Accepted: 05/02/2017] [Indexed: 01/18/2023] Open
Abstract
Synaptic vesicle proteins 2 (SV2) were discovered in the early 80s, but the clear demonstration that SV2A is the target of efficacious anti-epileptic drugs from the racetam family stimulated efforts to improve understanding of its role in the brain. Many functions have been suggested for SV2 proteins including ions or neurotransmitters transport or priming of SVs. Moreover, several recent studies highlighted the link between SV2 and different neuronal disorders such as epilepsy, Schizophrenia (SCZ), Alzheimer's or Parkinson's disease. In this review article, we will summarize our present knowledge on SV2A function(s) and its potential role(s) in the pathophysiology of various brain disorders.
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Affiliation(s)
- Odile Bartholome
- Laboratory of Nervous System Disorders and Therapy, GIGA-Neurosciences, University of LiègeLiège, Belgium
| | | | - Judit Sánchez Gil
- Laboratory of Nervous System Disorders and Therapy, GIGA-Neurosciences, University of LiègeLiège, Belgium
| | | | - Pierre Leprince
- Laboratory of Nervous System Disorders and Therapy, GIGA-Neurosciences, University of LiègeLiège, Belgium
| | - Rachelle Franzen
- Laboratory of Nervous System Disorders and Therapy, GIGA-Neurosciences, University of LiègeLiège, Belgium
| | - Bernard Rogister
- Laboratory of Nervous System Disorders and Therapy, GIGA-Neurosciences, University of LiègeLiège, Belgium.,Department of Neurology, Centre Hospitalier Universitaire de Liège (CHU), University of LiègeLiège, Belgium
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25
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Tang X, Xie C, Wang Y, Wang X. Localization of Rab3A-binding site on C2A domain of synaptotagmin I to reveal its regulatory mechanism. Int J Biol Macromol 2017; 96:736-742. [DOI: 10.1016/j.ijbiomac.2016.12.074] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2016] [Revised: 12/22/2016] [Accepted: 12/29/2016] [Indexed: 10/20/2022]
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26
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Guo T, Duan Z, Chen J, Xie C, Wang Y, Chen P, Wang X. Pull-down combined with proteomic strategy reveals functional diversity of synaptotagmin I. PeerJ 2017; 5:e2973. [PMID: 28194317 PMCID: PMC5301975 DOI: 10.7717/peerj.2973] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2016] [Accepted: 01/10/2017] [Indexed: 12/05/2022] Open
Abstract
Synaptotagmin I (Syt I) is most abundant in the brain and is involved in multiple cellular processes. Its two C2 domains, C2A and C2B, are the main functional regions. Our present study employed a pull-down combined with proteomic strategy to identify the C2 domain-interacting proteins to comprehensively understand the biological roles of the C2 domains and thus the functional diversity of Syt I. A total of 135 non-redundant proteins interacting with the C2 domains of Syt I were identified. Out of them, 32 and 64 proteins only bound to C2A or C2B domains, respectively, and 39 proteins bound to both of them. Compared with C2A, C2B could bind to many more proteins particularly those involved in synaptic transmission and metabolic regulation. Functional analysis indicated that Syt I may exert impacts by interacting with other proteins on multiple cellular processes, including vesicular membrane trafficking, synaptic transmission, metabolic regulation, catalysis, transmembrane transport and structure formation, etc. These results demonstrate that the functional diversity of Syt I is higher than previously expected, that its two domains may mediate the same and different cellular processes cooperatively or independently, and that C2B domain may play even more important roles than C2A in the functioning of Syt I. This work not only further deepened our understanding of the functional diversity of Syt I and the functional differences between its two C2 domains, but also provided important clues for the further related researches.
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Affiliation(s)
- Tianyao Guo
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Zhigui Duan
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Jia Chen
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Chunliang Xie
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Ying Wang
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Ping Chen
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Xianchun Wang
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
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27
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Pinheiro PS, Houy S, Sørensen JB. C2-domain containing calcium sensors in neuroendocrine secretion. J Neurochem 2016; 139:943-958. [DOI: 10.1111/jnc.13865] [Citation(s) in RCA: 44] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2016] [Revised: 09/17/2016] [Accepted: 10/05/2016] [Indexed: 12/11/2022]
Affiliation(s)
- Paulo S. Pinheiro
- Center for Neuroscience and Cell Biology; University of Coimbra; Coimbra Portugal
| | - Sébastien Houy
- Department of Neuroscience and Pharmacology; Faculty of Health and Medical Sciences; University of Copenhagen; Copenhagen Denmark
| | - Jakob B. Sørensen
- Department of Neuroscience and Pharmacology; Faculty of Health and Medical Sciences; University of Copenhagen; Copenhagen Denmark
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28
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Pérez-Lara Á, Thapa A, Nyenhuis SB, Nyenhuis DA, Halder P, Tietzel M, Tittmann K, Cafiso DS, Jahn R. PtdInsP 2 and PtdSer cooperate to trap synaptotagmin-1 to the plasma membrane in the presence of calcium. eLife 2016; 5. [PMID: 27791979 PMCID: PMC5123861 DOI: 10.7554/elife.15886] [Citation(s) in RCA: 71] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2016] [Accepted: 10/25/2016] [Indexed: 12/16/2022] Open
Abstract
The Ca2+-sensor synaptotagmin-1 that triggers neuronal exocytosis binds to negatively charged membrane lipids (mainly phosphatidylserine (PtdSer) and phosphoinositides (PtdIns)) but the molecular details of this process are not fully understood. Using quantitative thermodynamic, kinetic and structural methods, we show that synaptotagmin-1 (from Rattus norvegicus and expressed in Escherichia coli) binds to PtdIns(4,5)P2 via a polybasic lysine patch in the C2B domain, which may promote the priming or docking of synaptic vesicles. Ca2+ neutralizes the negative charges of the Ca2+-binding sites, resulting in the penetration of synaptotagmin-1 into the membrane, via binding of PtdSer, and an increase in the affinity of the polybasic lysine patch to phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2). These Ca2+-induced events decrease the dissociation rate of synaptotagmin-1 membrane binding while the association rate remains unchanged. We conclude that both membrane penetration and the increased residence time of synaptotagmin-1 at the plasma membrane are crucial for triggering exocytotic membrane fusion. DOI:http://dx.doi.org/10.7554/eLife.15886.001 The human nervous system contains billions of neurons that communicate with each other across junctions called synapses. When a neuron is activated, the levels of calcium ions inside the cell rise. This causes molecules called neurotransmitters to be released from the neuron at a synapse to make contact with the second neuron. The neurotransmitters are stored inside cells within compartments known as synaptic vesicles and are released when these vesicles fuse with the membrane surrounding the cell. Proteins called SNAREs regulate the membrane fusion process. These proteins assemble into bundles that help to drive vesicle and cell membranes together. Another protein called synaptotagmin-1 sticks out from the vesicle membrane and senses the levels of calcium ions in the cell to trigger membrane fusion at the right time. Synaptotagmin-1 has two regions that can bind to calcium ions, known as the C2 domains. When calcium ion levels rise, these domains insert into the cell membrane by binding to two fat molecules in the membrane called phosphatidylserine (PtdSer) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). Synaptotagmin-1 also interacts with the SNARE proteins, but it is not known whether synaptotagmin-1 triggers fusion by binding directly to SNAREs, or by the way it inserts into the cell membrane. Pérez-Lara et al. used several biophysical methods to investigate how synaptotagmin-1 binds to PtdSer and PtdInsP2. The experiments show that these molecules bind to different regions of synaptotagmin-1 and work together to attach the protein to the cell membrane and insert the C2 domains. Calcium ions increase the affinity of synaptotagmin-1 binding to the cell membrane by making it harder for synaptotagmin-1 to separate from the membrane, rather than by increasing its ability to bind to it. Further experiments show that synaptotagmin-1 prefers to bind to membranes that contain PtdInsP2 over binding to the SNARE proteins. Together, the findings of Pérez-Lara et al. suggest that calcium ions may trigger the release of neurotransmitters by trapping synaptotagmin-1 at the cell membrane rather than by directly affecting how it interacts with SNARE proteins. Further work will be needed to establish exactly how the SNARE proteins, PtdInsP2 and synaptotagmin-1 interact. DOI:http://dx.doi.org/10.7554/eLife.15886.002
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Affiliation(s)
- Ángel Pérez-Lara
- Department of Neurobiology, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Anusa Thapa
- Department of Chemistry and Center for Membrane Biology, University of Virginia, Charlottesville, United States
| | - Sarah B Nyenhuis
- Department of Chemistry and Center for Membrane Biology, University of Virginia, Charlottesville, United States
| | - David A Nyenhuis
- Department of Chemistry and Center for Membrane Biology, University of Virginia, Charlottesville, United States
| | - Partho Halder
- Department of Neurobiology, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Michael Tietzel
- Department of Molecular Enzymology, Göttingen Center for Molecular Biosciences, Georg-August University Göttingen, Göttingen, Germany
| | - Kai Tittmann
- Department of Molecular Enzymology, Göttingen Center for Molecular Biosciences, Georg-August University Göttingen, Göttingen, Germany
| | - David S Cafiso
- Department of Chemistry and Center for Membrane Biology, University of Virginia, Charlottesville, United States
| | - Reinhard Jahn
- Department of Neurobiology, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
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Abstract
The synaptotagmin family of vesicle proteins is believed to mediate calcium-dependent regulation of membrane trafficking. Detailed biochemical and in vivo studies of the most characterized isoform, synaptotagmin 1 (syt 1), have provided compelling evidence that it functions as a calcium sensor for fast neurotransmitter release at synapses. However, the function of the remaining isoforms is unclear, and multiple roles have been hypothesized for several of these. Recent evidence in Drosophila has given insight into the function of some of the remaining synaptotagmin family members. Of the five evolutionarily conserved isoforms in Drosophila, only two, syt 1 and syt 4, localize to most, if not all, synapses. The former is localized to presynaptic terminals, whereas the latter is predominantly postsynaptic. This suggests an intriguing possibility that syt 4 may mediate a postsynaptic vesicle trafficking pathway, providing a molecular basis for an evolutionarily conserved bidirectional vesicular trafficking communication system at synapses.
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Affiliation(s)
- Motojiro Yoshihara
- The Picower Center for Learning and Memory, Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
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30
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Calcium binding promotes conformational flexibility of the neuronal Ca(2+) sensor synaptotagmin. Biophys J 2016; 108:2507-2520. [PMID: 25992729 DOI: 10.1016/j.bpj.2015.04.007] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2015] [Revised: 04/07/2015] [Accepted: 04/09/2015] [Indexed: 01/12/2023] Open
Abstract
Synaptotagmin 1 (Syt1) is a synaptic vesicle protein that serves as a calcium sensor of neuronal secretion. It is established that calcium binding to Syt1 triggers vesicle fusion and release of neuronal transmitters, however, the dynamics of this process is not fully understood. To investigate how Ca(2+) binding affects Syt1 conformational dynamics, we performed prolonged molecular dynamics (MD) simulations of Ca(2+)-unbound and Ca(2+)-bound forms of Syt1. MD simulations were performed at a microsecond scale and combined with Monte Carlo sampling. We found that in the absence of Ca(2+) Syt1 structure in the solution is represented by an ensemble of conformational states with tightly coupled domains. To investigate the effect of Ca(2+) binding, we used two different strategies to generate a molecular model of a Ca(2+)-bound form of Syt1. First, we employed subsequent replacements of monovalent cations transiently captured within Syt1 Ca(2+)-binding pockets by Ca(2+) ions. Second, we performed MD simulations of Syt1 at elevated Ca(2+) levels. All the simulations produced Syt1 structures bound to four Ca(2+) ions, two ions chelated at the binding pocket of each domain. MD simulations of the Ca(2+)-bound form of Syt1 revealed that Syt1 conformational flexibility drastically increased upon Ca(2+) binding. In the presence of Ca(2+), the separation between domains increased, and interdomain rotations became more frequent. These findings suggest that Ca(2+) binding to Syt1 may induce major changes in the Syt1 conformational state, which in turn may initiate the fusion process.
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31
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Gordon SL, Cousin MA. The iTRAPs: Guardians of Synaptic Vesicle Cargo Retrieval During Endocytosis. Front Synaptic Neurosci 2016; 8:1. [PMID: 26903854 PMCID: PMC4746236 DOI: 10.3389/fnsyn.2016.00001] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2015] [Accepted: 01/25/2016] [Indexed: 11/30/2022] Open
Abstract
The reformation of synaptic vesicles (SVs) during endocytosis is essential for the maintenance of neurotransmission in central nerve terminals. Newly formed SVs must be generated with the correct protein cargo in the correct stoichiometry to be functional for exocytosis. Classical clathrin adaptor protein complexes play a key role in sorting and clustering synaptic vesicle cargo in this regard. However it is becoming increasingly apparent that additional “fail-safe” mechanisms exist to ensure the accurate retrieval of essential cargo molecules. For example, the monomeric adaptor proteins AP180/CALM and stonin-2 are required for the efficient retrieval of synaptobrevin II (sybII) and synaptotagmin-1 respectively. Furthermore, recent studies have revealed that sybII and synaptotagmin-1 interact with other SV cargoes to ensure a high fidelity of retrieval. These cargoes are synaptophysin (for sybII) and SV2A (for synaptotagmin-1). In this review, we summarize current knowledge regarding the retrieval mechanisms for both sybII and synaptotagmin-1 during endocytosis. We also define and set criteria for a new functional group of SV molecules that facilitate the retrieval of their interaction partners. We have termed these molecules intrinsic trafficking partners (iTRAPs) and we discuss how the function of this group impacts on presynaptic performance in both health and disease.
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Affiliation(s)
- Sarah L Gordon
- Florey Institute of Neuroscience and Mental Health, The University of Melbourne Parkville, VIC, Australia
| | - Michael A Cousin
- Centre for Integrative Physiology, University of Edinburgh Edinburgh, UK
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32
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Evans CS, He Z, Bai H, Lou X, Jeggle P, Sutton RB, Edwardson JM, Chapman ER. Functional analysis of the interface between the tandem C2 domains of synaptotagmin-1. Mol Biol Cell 2016; 27:979-89. [PMID: 26792839 PMCID: PMC4791141 DOI: 10.1091/mbc.e15-07-0503] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2015] [Accepted: 01/12/2016] [Indexed: 11/20/2022] Open
Abstract
Synaptotagmin (syt)-1 is a Ca2+ sensor that triggers rapid synaptic vesicle exocytosis. Mutations that disrupt physical interactions between the tandem Ca2+-sensing modules (C2 domains) of syt-1 disrupt regulated membrane fusion in reconstituted fusion reactions and in neurons. Hence contacts between these domains are important for function. C2 domains are widespread motifs that often serve as Ca2+-binding modules; some proteins have more than one copy. An open issue is whether these domains, when duplicated within the same parent protein, interact with one another to regulate function. In the present study, we address the functional significance of interfacial residues between the tandem C2 domains of synaptotagmin (syt)-1, a Ca2+ sensor for neuronal exocytosis. Substitution of four residues, YHRD, at the domain interface, disrupted the interaction between the tandem C2 domains, altered the intrinsic affinity of syt-1 for Ca2+, and shifted the Ca2+ dependency for binding to membranes and driving membrane fusion in vitro. When expressed in syt-1 knockout neurons, the YHRD mutant yielded reductions in synaptic transmission, as compared with the wild-type protein. These results indicate that physical interactions between the tandem C2 domains of syt-1 contribute to excitation–secretion coupling.
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Affiliation(s)
- Chantell S Evans
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275 Molecular and Cellular Pharmacology Program, University of Wisconsin-Madison, Madison, WI 53705-2275
| | - Zixuan He
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275
| | - Hua Bai
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275
| | - Xiaochu Lou
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275
| | - Pia Jeggle
- Department of Pharmacology, University of Cambridge, Cambridge CB2 1PD, UK
| | - R Bryan Sutton
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX 79430
| | | | - Edwin R Chapman
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275 Molecular and Cellular Pharmacology Program, University of Wisconsin-Madison, Madison, WI 53705-2275
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33
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Abstract
UNLABELLED The Ca(2+) sensor synaptotagmin-1 (syt-1) regulates neurotransmitter release by interacting with anionic phospholipids. Here we test the idea that the intrinsic kinetics of syt-membrane interactions determine, in part, the time course of synaptic transmission. To tune the kinetics of this interaction, we grafted structural elements from the slowest isoform, syt-7, onto the fastest isoform, syt-1, resulting in a chimera with intermediate kinetic properties. Moreover, the chimera coupled a physiologically irrelevant metal, Sr(2+), to membrane fusion in vitro. When substituted for syt-1 in mouse hippocampal neurons, the chimera slowed the kinetics of synaptic transmission. Neurons expressing the chimera also evinced rapid and efficient Sr(2+) triggered release, in contrast to the weak response of neurons expressing syt-1. These findings reveal presynaptic sensor-membrane interactions as a major factor regulating the speed of the release machinery. Finally, the chimera failed to clamp the elevated spontaneous fusion rate exhibited by syt-1 KO neurons, indicating that the metal binding loops of syt-1 regulate the two modes of release by distinct mechanisms. SIGNIFICANCE STATEMENT In calcium, synaptotagmin-1 triggers neurotransmitter release by interacting with membranes. Here, we demonstrate that intrinsic properties of this interaction control the time course of synaptic transmission. We engineered a "chimera" using synaptotagmin-1 and elements of a slower isoform, synaptotagmin-7. When expressed in neurons, the chimera slowed the rate of neurotransmitter release. Furthermore, unlike native synaptotagmin-1, the chimera was able to function robustly in the presence of strontium-a metal not present in cells. We exploited this ability to show that a key function of synaptotagmin-1 is to penetrate cell membranes. This work sheds light on fundamental mechanisms of neurotransmitter release.
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34
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Osterberg JR, Chon NL, Boo A, Maynard FA, Lin H, Knight JD. Membrane Docking of the Synaptotagmin 7 C2A Domain: Electron Paramagnetic Resonance Measurements Show Contributions from Two Membrane Binding Loops. Biochemistry 2015; 54:5684-95. [PMID: 26322740 DOI: 10.1021/acs.biochem.5b00421] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
The synaptotagmin (Syt) family of proteins plays an important role in vesicle docking and fusion during Ca(2+)-induced exocytosis in a wide variety of cell types. Its role as a Ca(2+) sensor derives primarily from its two C2 domains, C2A and C2B, which insert into anionic lipid membranes upon binding Ca(2+). Syt isoforms 1 and 7 differ significantly in their Ca(2+) sensitivity; the C2A domain from Syt7 binds Ca(2+) and membranes much more tightly than the C2A domain from Syt1, at least in part because of greater contributions from the hydrophobic effect. While the structure and membrane activity of Syt1 have been extensively studied, the structural origins of differences between Syt1 and Syt7 are unknown. This study used site-directed spin labeling and electron paramagnetic resonance spectroscopy to determine depth parameters for the Syt7 C2A domain, for comparison to analogous previous measurements with the Syt1 C2A domain. In a novel approach, the membrane docking geometry of both Syt1 and Syt7 C2A was modeled by mapping depth parameters onto multiple molecular dynamics-simulated structures of the Ca(2+)-bound protein. The models reveal membrane penetration of Ca(2+) binding loops 1 (CBL1) and 3 (CBL3), and membrane binding is more sensitive to mutations in CBL3. On average, Syt7 C2A inserts more deeply into the membrane than Syt1 C2A, although depths vary among the different structural models. This observation provides a partial structural explanation for the hydrophobically driven membrane docking of Syt7 C2A.
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Affiliation(s)
- J Ryan Osterberg
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Nara Lee Chon
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Arthur Boo
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Favinn A Maynard
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Hai Lin
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Jefferson D Knight
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
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35
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Structural elements that underlie Doc2β function during asynchronous synaptic transmission. Proc Natl Acad Sci U S A 2015. [PMID: 26195798 DOI: 10.1073/pnas.1502288112] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
Double C2-like domain-containing proteins alpha and beta (Doc2α and Doc2β) are tandem C2-domain proteins proposed to function as Ca(2+) sensors for asynchronous neurotransmitter release. Here, we systematically analyze each of the negatively charged residues that mediate binding of Ca(2+) to the β isoform. The Ca(2+) ligands in the C2A domain were dispensable for Ca(2+)-dependent translocation to the plasma membrane, with one exception: neutralization of D220 resulted in constitutive translocation. In contrast, three of the five Ca(2+) ligands in the C2B domain are required for translocation. Importantly, translocation was correlated with the ability of the mutants to enhance asynchronous release when overexpressed in neurons. Finally, replacement of specific Ca(2+)/lipid-binding loops of synaptotagmin 1, a Ca(2+) sensor for synchronous release, with corresponding loops from Doc2β, resulted in chimeras that yielded slower kinetics in vitro and slower excitatory postsynaptic current decays in neurons. Together, these data reveal the key determinants of Doc2β that underlie its function during the slow phase of synaptic transmission.
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36
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Baker K, Gordon SL, Grozeva D, van Kogelenberg M, Roberts NY, Pike M, Blair E, Hurles ME, Chong WK, Baldeweg T, Kurian MA, Boyd SG, Cousin MA, Raymond FL. Identification of a human synaptotagmin-1 mutation that perturbs synaptic vesicle cycling. J Clin Invest 2015; 125:1670-8. [PMID: 25705886 DOI: 10.1172/jci79765] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2014] [Accepted: 01/08/2015] [Indexed: 12/14/2022] Open
Abstract
Synaptotagmin-1 (SYT1) is a calcium-binding synaptic vesicle protein that is required for both exocytosis and endocytosis. Here, we describe a human condition associated with a rare variant in SYT1. The individual harboring this variant presented with an early onset dyskinetic movement disorder, severe motor delay, and profound cognitive impairment. Structural MRI was normal, but EEG showed extensive neurophysiological disturbances that included the unusual features of low-frequency oscillatory bursts and enhanced paired-pulse depression of visual evoked potentials. Trio analysis of whole-exome sequence identified a de novo SYT1 missense variant (I368T). Expression of rat SYT1 containing the equivalent human variant in WT mouse primary hippocampal cultures revealed that the mutant form of SYT1 correctly localizes to nerve terminals and is expressed at levels that are approximately equal to levels of endogenous WT protein. The presence of the mutant SYT1 slowed synaptic vesicle fusion kinetics, a finding that agrees with the previously demonstrated role for I368 in calcium-dependent membrane penetration. Expression of the I368T variant also altered the kinetics of synaptic vesicle endocytosis. Together, the clinical features, electrophysiological phenotype, and in vitro neuronal phenotype associated with this dominant negative SYT1 mutation highlight presynaptic mechanisms that mediate human motor control and cognitive development.
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37
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Vasquez JK, Chantranuvatana K, Giardina DT, Coffman MD, Knight JD. Lateral diffusion of proteins on supported lipid bilayers: additive friction of synaptotagmin 7 C2A-C2B tandem domains. Biochemistry 2014; 53:7904-13. [PMID: 25437758 PMCID: PMC4278679 DOI: 10.1021/bi5012223] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
![]()
The
synaptotagmin (Syt) family of proteins contains tandem C2 domains,
C2A and C2B, which bind membranes in the presence of Ca2+ to trigger vesicle fusion during exocytosis. Despite recent progress,
the role and extent of interdomain interactions between C2A and C2B
in membrane binding remain unclear. To test whether the two domains
interact on a planar lipid bilayer (i.e., experience thermodynamic
interdomain contacts), diffusion of fluorescent-tagged C2A, C2B, and
C2AB domains from human Syt7 was measured using total internal reflection
fluorescence microscopy with single-particle tracking. The C2AB tandem
exhibits a lateral diffusion constant approximately half the value
of the isolated single domains and does not change when additional
residues are engineered into the C2A–C2B linker. This is the
expected result if C2A and C2B are separated when membrane-bound;
theory predicts that C2AB diffusion would be faster if the two domains
were close enough together to have interdomain contact. Stopped-flow
measurements of membrane dissociation kinetics further support an
absence of interdomain interactions, as dissociation kinetics of the
C2AB tandem remain unchanged when rigid or flexible linker extensions
are included. Together, the results suggest that the two C2 domains
of Syt7 bind independently to planar membranes, in contrast to reported
interdomain cooperativity in Syt1.
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Affiliation(s)
- Joseph K Vasquez
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
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38
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Ma J, Kelly L, Ingram J, Price TJ, Meriney SD, Dittrich M. New insights into short-term synaptic facilitation at the frog neuromuscular junction. J Neurophysiol 2014; 113:71-87. [PMID: 25210157 DOI: 10.1152/jn.00198.2014] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Short-term synaptic facilitation occurs during high-frequency stimulation, is known to be dependent on presynaptic calcium ions, and persists for tens of milliseconds after a presynaptic action potential. We have used the frog neuromuscular junction as a model synapse for both experimental and computer simulation studies aimed at testing various mechanistic hypotheses proposed to underlie short-term synaptic facilitation. Building off our recently reported excess-calcium-binding-site model of synaptic vesicle release at the frog neuromuscular junction (Dittrich M, Pattillo JM, King JD, Cho S, Stiles JR, Meriney SD. Biophys J 104: 2751-2763, 2013), we have investigated several mechanisms of short-term facilitation at the frog neuromuscular junction. Our studies place constraints on previously proposed facilitation mechanisms and conclude that the presence of a second class of calcium sensor proteins distinct from synaptotagmin can explain known properties of facilitation observed at the frog neuromuscular junction. We were further able to identify a novel facilitation mechanism, which relied on the persistent binding of calcium-bound synaptotagmin molecules to lipids of the presynaptic membrane. In a real physiological context, both mechanisms identified in our study (and perhaps others) may act simultaneously to cause the experimentally observed facilitation. In summary, using a combination of computer simulations and physiological recordings, we have developed a stochastic computer model of synaptic transmission at the frog neuromuscular junction, which sheds light on the facilitation mechanisms in this model synapse.
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Affiliation(s)
- Jun Ma
- Biomedical Applications Group, Pittsburgh Supercomputing Center, Carnegie Mellon University, Pittsburgh, Pennsylvania; Joint Carnegie Mellon-University of Pittsburgh PhD Program in Computational Biology, School of Computer Science, Carnegie Mellon University, Pittsburgh, Pennsylvania
| | - Lauren Kelly
- Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania; and
| | - Justin Ingram
- Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania; and
| | - Thomas J Price
- Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania; and
| | - Stephen D Meriney
- Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania; and
| | - Markus Dittrich
- Biomedical Applications Group, Pittsburgh Supercomputing Center, Carnegie Mellon University, Pittsburgh, Pennsylvania; Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania; and Department of Computational and Systems Biology, University of Pittsburgh, Pittsburgh, Pennsylvania
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Padmanarayana M, Hams N, Speight LC, Petersson EJ, Mehl RA, Johnson CP. Characterization of the lipid binding properties of Otoferlin reveals specific interactions between PI(4,5)P2 and the C2C and C2F domains. Biochemistry 2014; 53:5023-33. [PMID: 24999532 PMCID: PMC4144714 DOI: 10.1021/bi5004469] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
![]()
Otoferlin
is a transmembrane protein consisting of six C2 domains,
proposed to act as a calcium sensor for exocytosis. Although otoferlin
is believed to bind calcium and lipids, the lipid specificity and
identity of the calcium binding domains are controversial. Further,
it is currently unclear whether the calcium binding affinity of otoferlin
quantitatively matches the maximal intracellular presynaptic calcium
concentrations of ∼30–50 μM known to elicit exocytosis.
To characterize the calcium and lipid binding properties of otoferlin,
we used isothermal titration calorimetry (ITC), liposome sedimentation
assays, and fluorescence spectroscopy. Analysis of ITC data indicates
that with the exception of the C2A domain, the C2 domains of otoferlin
bind multiple calcium ions with moderate (Kd = 25–95 μM) and low affinities (Kd = 400–700 μM) in solution. However, in the presence
of liposomes, the calcium sensitivity of the domains increased by
up to 10-fold. It was also determined that calcium enhanced liposome
binding for domains C2B–C2E, whereas the C2F domain bound liposomes
in a calcium-independent manner. Mutations that abrogate calcium binding
in C2F do not disrupt liposome binding, supporting the conclusion
that the interaction of the C2F domain with phosphatidylserine is
calcium-independent. Further, domains C2C and C2F, not domains C2A,
C2B, C2D, and C2E, bound phosphatidylinositol 4,5-bisphosphate 1,2-dioleoyl-sn-glycero-3-phospho(1′-myoinositol-4′,5′-bisphosphate) [PI(4,5)P2], which preferentially
steered them toward liposomes harboring PI(4,5)P2. Remarkably, lysine
mutations L478A and L480A in C2C selectively weaken the PI(4,5)P2
interaction while leaving phosphatidylserine binding unaffected. Finally,
shifts in the emission spectra of an environmentally sensitive fluorescent
unnatural amino acid indicate that the calcium binding loops of the
C2F domain directly interact with the lipid bilayer of negatively
charged liposomes in a calcium-independent manner. On the basis of
these results, we propose that the C2F and C2C domains of otoferlin
preferentially bind PI(4,5)P2 and that PI(4,5)P2 may serve to target
otoferlin to the presynapse in a calcium-independent manner. This
positioning would facilitate fast calcium-dependent exocytosis at
the hair cell synapse.
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Affiliation(s)
- Murugesh Padmanarayana
- Department of Biochemistry and Biophysics, Oregon State University , Corvallis, Oregon 973331, United States
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40
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Liu H, Bai H, Xue R, Takahashi H, Edwardson JM, Chapman ER. Linker mutations reveal the complexity of synaptotagmin 1 action during synaptic transmission. Nat Neurosci 2014; 17:670-7. [PMID: 24657966 PMCID: PMC4139111 DOI: 10.1038/nn.3681] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2013] [Accepted: 02/21/2014] [Indexed: 12/13/2022]
Abstract
The Ca(2+) sensor for rapid synaptic vesicle exocytosis, synaptotagmin 1 (syt), is largely composed of two Ca(2+)-sensing C2 domains, C2A and C2B. We investigated the apparent synergy between the tandem C2 domains by altering the length and rigidity of the linker that connects them. The behavior of the linker mutants revealed a correlation between the ability of the C2 domains to penetrate membranes in response to Ca(2+) and to drive evoked neurotransmitter release in cultured mouse neurons, uncovering a step in excitation-secretion coupling. Using atomic force microscopy, we found that the synergy between these C2 domains involved intra-molecular interactions between them. Thus, syt function is markedly affected by changes in the physical nature of the linker that connects its tandem C2 domains. Moreover, the linker mutations uncoupled syt-mediated regulation of evoked and spontaneous release, revealing that syt also acts as a fusion clamp before the Ca(2+) trigger.
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Affiliation(s)
- Huisheng Liu
- 1] Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, USA. [2] [3]
| | - Hua Bai
- 1] Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, USA. [2]
| | - Renhao Xue
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, USA
| | | | | | - Edwin R Chapman
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, USA
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41
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Gaffaney JD, Xue R, Chapman ER. Mutations that disrupt Ca²⁺-binding activity endow Doc2β with novel functional properties during synaptic transmission. Mol Biol Cell 2013; 25:481-94. [PMID: 24356452 PMCID: PMC3923640 DOI: 10.1091/mbc.e13-10-0571] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Double C2-domain protein (Doc2) is a Ca(2+)-binding protein implicated in asynchronous and spontaneous neurotransmitter release. Here we demonstrate that each of its C2 domains senses Ca(2+); moreover, the tethered tandem C2 domains display properties distinct from the isolated domains. We confirm that overexpression of a mutant form of Doc2β, in which two acidic Ca(2+) ligands in the C2A domain and two in the C2B domain have been neutralized, results in markedly enhanced asynchronous release in synaptotagmin 1-knockout neurons. Unlike wild-type (wt) Doc2β, which translocates to the plasma membrane in response to increases in [Ca(2+)](i), the quadruple Ca(2+)-ligand mutant does not bind Ca(2+) but is constitutively associated with the plasma membrane; this effect is due to substitution of Ca(2+) ligands in the C2A domain. When overexpressed in wt neurons, Doc2β affects only asynchronous release; in contrast, Doc2β Ca(2+)-ligand mutants that constitutively localize to the plasma membrane enhance both the fast and slow components of synaptic transmission by increasing the readily releasable vesicle pool size; these mutants also increase the frequency of spontaneous release events. Thus, mutations in the C2A domain of Doc2β that were intended to disrupt Ca(2+) binding result in an anomalous enhancement of constitutive membrane-binding activity and endow Doc2β with novel functional properties.
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Affiliation(s)
- Jon D Gaffaney
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin, Madison, WI 53706
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42
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Phosphatidylserine-mediated cellular signaling. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2013; 991:177-93. [PMID: 23775696 DOI: 10.1007/978-94-007-6331-9_10] [Citation(s) in RCA: 256] [Impact Index Per Article: 23.3] [Reference Citation Analysis] [Abstract] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Phosphatidylserine (PS), a phospholipid with a negatively charged head group, is an important constituent of eukaryotic membranes. Rather than being a passive component of cellular membranes, PS plays an important role in a number of signaling pathways. Signaling is mediated by proteins that are recruited and/or activated by PS in one of two ways: via domains that stereospecifically recognize the head group, or by electrostatic interactions with membranes that are rich in PS and therefore display negative surface charge. Such interactions are key to both intracellular and extracellular signaling cascades. PS, exposed extracellularly, is instrumental in triggering blood clotting and also serves as an "eat me" signal for the clearance of apoptotic cells. Inside the cell, a number of pathways depend of PS; these include kinases, small GTPases and fusogenic proteins. This review will discuss the generation and distribution of PS, current methods of phospholipid visualization within live cells, as well as the current understanding of the role of PS in both extracellular and intracellular signaling events.
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43
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Synaptotagmins 1 and 2 as mediators of rapid exocytosis at nerve terminals: The dyad hypothesis. J Theor Biol 2013; 332:149-60. [DOI: 10.1016/j.jtbi.2013.04.029] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2013] [Accepted: 04/24/2013] [Indexed: 11/16/2022]
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Genetic analysis of synaptotagmin C2 domain specificity in regulating spontaneous and evoked neurotransmitter release. J Neurosci 2013; 33:187-200. [PMID: 23283333 DOI: 10.1523/jneurosci.3214-12.2013] [Citation(s) in RCA: 59] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
Synaptic vesicle fusion mediates communication between neurons and is triggered by rapid influx of Ca(2+). The Ca(2+)-triggering step for fusion is regulated by the synaptic vesicle transmembrane protein Synaptotagmin 1 (Syt1). Syt1 contains two cytoplasmic C2 domains, termed C2A and C2B, which coordinate Ca(2+) binding. Although C2A and C2B share similar topology, binding of Ca(2+) ions to the C2B domain has been suggested as the only critical trigger for evoked vesicle release. If and how C2A domain function is coordinated with C2B remain unclear. In this study, we generated a panel of Syt1 chimeric constructs in Drosophila to delineate the unique and shared functions of each C2 domain in regulation of synaptic vesicle fusion. Expression of Syt 1 transgenes containing only individual C2 domains, or dual C2A-C2A or C2B-C2B chimeras, failed to restore Syt1 function in a syt1(-/-) null mutant background, indicating both C2A and C2B are specifically required to support fast synchronous release. Mutations that disrupted Ca(2+) binding to both C2 domains failed to rescue evoked release, but supported synaptic vesicle docking and endocytosis, indicating that these functions of Syt1 are Ca(2+)-independent. The dual C2 domain Ca(2+)-binding mutant also enhanced spontaneous fusion while dramatically increasing evoked release when coexpressed with native Syt1. Together, these data indicate that synaptic transmission can be regulated by Syt1 multimerization and that both C2 domains of Syt1 are uniquely required for modulating Ca(2+)-independent spontaneous fusion and Ca(2+)-dependent synchronous release.
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45
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Saludes JP, Morton LA, Ghosh N, Beninson LA, Chapman ER, Fleshner M, Yin H. Detection of highly curved membrane surfaces using a cyclic peptide derived from synaptotagmin-I. ACS Chem Biol 2012; 7:1629-35. [PMID: 22769435 DOI: 10.1021/cb3002705] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The generation of highly curved membranes is essential to cell growth, division, and movement. Recent research in the field is focused to answer questions related to the consequences of changes in the topology of the membrane once it is created, broadly termed as membrane curvature sensing. Most probes that are used to study curvature sensing are intact membrane active proteins such as DP1/Yop1p, ArfGAP1, BAR domains, and Synaptotagmin-I (Syt1). Taking a cue from nature, we created the cyclic peptide C2BL3C based on the membrane penetration C2B loop 3 of Syt1 via "Click" chemistry. Using a combination of spectroscopic techniques, we investigated the peptide-lipid interactions of this peptide with synthetic phospholipid vesicles and exosomes from rat blood plasma. We found that the macrocycle peptide probe was selective for lipid vesicles with highly curved surfaces (d < 100 nm). These results suggested that C2BL3C functions as a selective detector of highly curved phospholipid bilayers.
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Affiliation(s)
| | | | | | | | - Edwin R. Chapman
- Howard Hughes Medical Institute
and Department of Neuroscience, University of Wisconsin, Madison, Wisconsin 53706, United States
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46
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Calcium binding by synaptotagmin's C2A domain is an essential element of the electrostatic switch that triggers synchronous synaptic transmission. J Neurosci 2012; 32:1253-60. [PMID: 22279210 DOI: 10.1523/jneurosci.4652-11.2012] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Synaptotagmin is the major calcium sensor for fast synaptic transmission that requires the synchronous fusion of synaptic vesicles. Synaptotagmin contains two calcium-binding domains: C2A and C2B. Mutation of a positively charged residue (R233Q in rat) showed that Ca2+-dependent interactions between the C2A domain and membranes play a role in the electrostatic switch that initiates fusion. Surprisingly, aspartate-to-asparagine mutations in C2A that inhibit Ca2+ binding support efficient synaptic transmission, suggesting that Ca2+ binding by C2A is not required for triggering synchronous fusion. Based on a structural analysis, we generated a novel mutation of a single Ca2+-binding residue in C2A (D229E in Drosophila) that inhibited Ca2+ binding but maintained the negative charge of the pocket. This C2A aspartate-to-glutamate mutation resulted in ∼80% decrease in synchronous transmitter release and a decrease in the apparent Ca2+ affinity of release. Previous aspartate-to-asparagine mutations in C2A partially mimicked Ca2+ binding by decreasing the negative charge of the pocket. We now show that the major function of Ca2+ binding to C2A is to neutralize the negative charge of the pocket, thereby unleashing the fusion-stimulating activity of synaptotagmin. Our results demonstrate that Ca2+ binding by C2A is a critical component of the electrostatic switch that triggers synchronous fusion. Thus, Ca2+ binding by C2B is necessary and sufficient to regulate the precise timing required for coupling vesicle fusion to Ca2+ influx, but Ca2+ binding by both C2 domains is required to flip the electrostatic switch that triggers efficient synchronous synaptic transmission.
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47
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Koch M, Holt M. Coupling exo- and endocytosis: an essential role for PIP₂ at the synapse. Biochim Biophys Acta Mol Cell Biol Lipids 2012; 1821:1114-32. [PMID: 22387937 DOI: 10.1016/j.bbalip.2012.02.008] [Citation(s) in RCA: 57] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2011] [Revised: 02/12/2012] [Accepted: 02/13/2012] [Indexed: 12/24/2022]
Abstract
Chemical synapses are specialist points of contact between two neurons, where information transfer takes place. Communication occurs through the release of neurotransmitter substances from small synaptic vesicles in the presynaptic terminal, which fuse with the presynaptic plasma membrane in response to neuronal stimulation. However, as neurons in the central nervous system typically only possess ~200 vesicles, high levels of release would quickly lead to a depletion in the number of vesicles, as well as leading to an increase in the area of the presynaptic plasma membrane (and possible misalignment with postsynaptic structures). Hence, synaptic vesicle fusion is tightly coupled to a local recycling of synaptic vesicles. For a long time, however, the exact molecular mechanisms coupling fusion and subsequent recycling remained unclear. Recent work now indicates a unique role for the plasma membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP(2)), acting together with the vesicular protein synaptotagmin, in coupling these two processes. In this work, we review the evidence for such a mechanism and discuss both the possible advantages and disadvantages for vesicle recycling (and hence signal transduction) in the nervous system. This article is part of a Special Issue entitled Lipids and Vesicular Transport.
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Affiliation(s)
- Marta Koch
- Laboratory of Neurogenetics, VIB Center for the Biology of Disease and K.U. Leuven Center for Human Genetics, O&N4 Herestraat 49, 3000 Leuven, Belgium
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48
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Hui E, Gaffaney JD, Wang Z, Johnson CP, Evans CS, Chapman ER. Mechanism and function of synaptotagmin-mediated membrane apposition. Nat Struct Mol Biol 2011; 18:813-21. [PMID: 21642967 PMCID: PMC3130839 DOI: 10.1038/nsmb.2075] [Citation(s) in RCA: 62] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2010] [Accepted: 04/28/2011] [Indexed: 11/09/2022]
Abstract
Synaptotagmin-I (syt) is a Ca2+ sensor that triggers synchronous neurotransmitter release. The first documented biochemical property of syt was its ability to aggregate membranes in response to Ca2+. However, the mechanism and function of syt-mediated membrane aggregation are poorly understood. Here, we demonstrate that syt-mediated vesicle aggregation is driven by trans interactions between syt molecules bound to different membranes. We observed a strong correlation between the ability of Ca2+-syt to aggregate vesicles and to stimulate SNARE-mediated membrane fusion. Moreover, artificial aggregation of membranes - using non-syt proteins - also efficiently promoted fusion of SNARE-bearing liposomes. Finally, using a modified fusion assay, we observed that syt drives the assembly of otherwise non-fusogenic individual t-SNARE proteins into fusion competent heterodimers, in an aggregation-independent manner. Thus, membrane aggregation and t-SNARE assembly appear to be two key aspects of Ca2+-syt-regulated, SNARE-catalyzed fusion reactions.
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Affiliation(s)
- Enfu Hui
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, USA
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49
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Membrane penetration by synaptotagmin is required for coupling calcium binding to vesicle fusion in vivo. J Neurosci 2011; 31:2248-57. [PMID: 21307261 DOI: 10.1523/jneurosci.3153-09.2011] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The vesicle protein synaptotagmin I is the Ca(2+) sensor that triggers fast, synchronous release of neurotransmitter. Specifically, Ca(2+) binding by the C(2)B domain of synaptotagmin is required at intact synapses, yet the mechanism whereby Ca(2+) binding results in vesicle fusion remains controversial. Ca(2+)-dependent interactions between synaptotagmin and SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment receptor) complexes and/or anionic membranes are possible effector interactions. However, no effector-interaction mutations to date impact synaptic transmission as severely as mutation of the C(2)B Ca(2+)-binding motif, suggesting that these interactions are facilitatory rather than essential. Here we use Drosophila to show the functional role of a highly conserved, hydrophobic residue located at the tip of each of the two Ca(2+)-binding pockets of synaptotagmin. Mutation of this residue in the C(2)A domain (F286) resulted in a ∼50% decrease in evoked transmitter release at an intact synapse, again indicative of a facilitatory role. Mutation of this hydrophobic residue in the C(2)B domain (I420), on the other hand, blocked all locomotion, was embryonic lethal even in syt I heterozygotes, and resulted in less evoked transmitter release than that in syt(null) mutants, which is more severe than the phenotype of C(2)B Ca(2+)-binding mutants. Thus, mutation of a single, C(2)B hydrophobic residue required for Ca(2+)-dependent penetration of anionic membranes results in the most severe disruption of synaptotagmin function in vivo to date. Our results provide direct support for the hypothesis that plasma membrane penetration, specifically by the C(2)B domain of synaptotagmin, is the critical effector interaction for coupling Ca(2+) binding with vesicle fusion.
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50
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Wan C, Kiessling V, Cafiso DS, Tamm LK. Partitioning of synaptotagmin I C2 domains between liquid-ordered and liquid-disordered inner leaflet lipid phases. Biochemistry 2011; 50:2478-85. [PMID: 21322640 DOI: 10.1021/bi101864k] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Synaptotagmin I is the calcium sensor in synchronous neurotransmitter release caused by fusion of synaptic vesicles with the presynaptic membrane in neurons. Synaptotagmin I interacts with acidic phospholipids, but also with soluble N-ethylmaleimide-sensitive factor attachment receptors (SNAREs), at various stages in presynaptic membrane fusion. Because SNAREs can be organized into small cholesterol-dependent clusters in membranes, it is important to determine whether the C2 domains of synaptotagmin target membrane domains with different cholesterol contents. To address this question, we used a previously developed asymmetric two-phase lipid bilayer system to investigate the membrane binding and lipid phase targeting of soluble C2A and C2AB domains of synaptotagmin. We found that both domains target more disordered cholesterol-poor domains better than highly ordered cholesterol-rich domains. The selectivity is greatest (∼3-fold) for C2A binding to disordered domains that are formed in the presence of 5 mol % PIP(2) and 15 mol % PS. It is smallest (∼1.4-fold) for C2AB binding to disordered domains that are formed in the presence of 40 mol % PS. In the course of these experiments, we also found that C2A domains in the presence of Ca(2+) and C2AB domains in the absence of Ca(2+) are quite reliable reporters of the acidic lipid distribution between ordered and disordered lipid phases. Accordingly, PS prefers the liquid-disordered phase over the liquid-ordered phase by ∼2-fold, but PIP(2) has an up to 3-fold preference for the liquid-disordered phase.
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Affiliation(s)
- Chen Wan
- Center for Membrane Biology, Department of Molecular Physiology and Biological Physics, University of Virginia, Snyder Building, P.O. Box 800886, Charlottesville, Virginia 22908, United States
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