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Guiberson NGL, Black LS, Haller JE, Brukner A, Abramov D, Ahmad S, Xie YX, Sharma M, Burré J. Disease-linked mutations in Munc18-1 deplete synaptic Doc2. Brain 2024; 147:2185-2202. [PMID: 38242640 PMCID: PMC11146428 DOI: 10.1093/brain/awae019] [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: 05/31/2022] [Revised: 11/20/2023] [Accepted: 01/09/2024] [Indexed: 01/21/2024] Open
Abstract
Heterozygous de novo mutations in the neuronal protein Munc18-1/STXBP1 cause syndromic neurological symptoms, including severe epilepsy, intellectual disability, developmental delay, ataxia and tremor, summarized as STXBP1 encephalopathies. Although haploinsufficiency is the prevailing disease mechanism, it remains unclear how the reduction in Munc18-1 levels causes synaptic dysfunction in disease as well as how haploinsufficiency alone can account for the significant heterogeneity among patients in terms of the presence, onset and severity of different symptoms. Using biochemical and cell biological readouts on mouse brains, cultured mouse neurons and heterologous cells, we found that the synaptic Munc18-1 interactors Doc2A and Doc2B are unstable in the absence of Munc18-1 and aggregate in the presence of disease-causing Munc18-1 mutants. In haploinsufficiency-mimicking heterozygous knockout neurons, we found a reduction in Doc2A/B levels that is further aggravated by the presence of the disease-causing Munc18-1 mutation G544D as well as an impairment in Doc2A/B synaptic targeting in both genotypes. We also demonstrated that overexpression of Doc2A/B partially rescues synaptic dysfunction in heterozygous knockout neurons but not heterozygous knockout neurons expressing G544D Munc18-1. Our data demonstrate that STXBP1 encephalopathies are not only characterized by the dysfunction of Munc18-1 but also by the dysfunction of the Munc18-1 binding partners Doc2A and Doc2B, and that this dysfunction is exacerbated by the presence of a Munc18-1 missense mutant. These findings may offer a novel explanation for the significant heterogeneity in symptoms observed among STXBP1 encephalopathy patients.
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Affiliation(s)
- Noah Guy Lewis Guiberson
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
| | - Luca S Black
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
| | - Jillian E Haller
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
| | - Aniv Brukner
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
| | - Debra Abramov
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
| | - Saad Ahmad
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
| | - Yan Xin Xie
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
| | - Manu Sharma
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
| | - Jacqueline Burré
- Helen and Robert Appel Alzheimer’s Disease Research Institute, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY 10021, USA
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2
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Zhang H, Lei M, Zhang Y, Li H, He Z, Xie S, Zhu L, Wang S, Liu J, Li Y, Lu Y, Ma C. Phosphorylation of Doc2 by EphB2 modulates Munc13-mediated SNARE complex assembly and neurotransmitter release. SCIENCE ADVANCES 2024; 10:eadi7024. [PMID: 38758791 PMCID: PMC11100570 DOI: 10.1126/sciadv.adi7024] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/12/2023] [Accepted: 04/12/2024] [Indexed: 05/19/2024]
Abstract
At the synapse, presynaptic neurotransmitter release is tightly controlled by release machinery, involving the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and Munc13. The Ca2+ sensor Doc2 cooperates with Munc13 to regulate neurotransmitter release, but the underlying mechanisms remain unclear. In our study, we have characterized the binding mode between Doc2 and Munc13 and found that Doc2 originally occludes Munc13 to inhibit SNARE complex assembly. Moreover, our investigation unveiled that EphB2, a presynaptic adhesion molecule (SAM) with inherent tyrosine kinase functionality, exhibits the capacity to phosphorylate Doc2. This phosphorylation attenuates Doc2 block on Munc13 to promote SNARE complex assembly, which functionally induces spontaneous release and synaptic augmentation. Consistently, application of a Doc2 peptide that interrupts Doc2-Munc13 interplay impairs excitatory synaptic transmission and leads to dysfunction in spatial learning and memory. These data provide evidence that SAMs modulate neurotransmitter release by controlling SNARE complex assembly.
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Affiliation(s)
- Hong Zhang
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, 430074 Wuhan, China
| | - Mengshi Lei
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, 430074 Wuhan, China
| | - Yu Zhang
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, 430074 Wuhan, China
| | - Hao Li
- Institute for Brain Research, Wuhan Center of Brain Science, Huazhong University of Science and Technology, Wuhan 430030, China
- Department of Pathophysiology, School of Basic Medicine and Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, China
| | - Zhen He
- Department of Pathogen Biology, School of Basic Medicine, Tongji Medical College, Huazhong University of Science and Technology, 430030 Wuhan, China
| | - Sheng Xie
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, 430074 Wuhan, China
| | - Le Zhu
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, 430074 Wuhan, China
| | - Shen Wang
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, 430074 Wuhan, China
| | - Jianfeng Liu
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, 430074 Wuhan, China
| | - Yan Li
- Department of Pathogen Biology, School of Basic Medicine, Tongji Medical College, Huazhong University of Science and Technology, 430030 Wuhan, China
| | - Youming Lu
- Institute for Brain Research, Wuhan Center of Brain Science, Huazhong University of Science and Technology, Wuhan 430030, China
- Department of Pathophysiology, School of Basic Medicine and Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, China
| | - Cong Ma
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, 430074 Wuhan, China
- Institute for Brain Research, Wuhan Center of Brain Science, Huazhong University of Science and Technology, Wuhan 430030, China
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3
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Deurloo MHS, Eide S, Turlova E, Li Q, Spijker S, Sun HS, Groffen AJA, Feng ZP. Rasal1 regulates calcium dependent neuronal maturation by modifying microtubule dynamics. Cell Biosci 2024; 14:13. [PMID: 38246997 PMCID: PMC10800070 DOI: 10.1186/s13578-024-01193-w] [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: 10/29/2023] [Accepted: 01/03/2024] [Indexed: 01/23/2024] Open
Abstract
BACKGROUND Rasal1 is a Ras GTPase-activating protein which contains C2 domains necessary for dynamic membrane association following intracellular calcium elevation. Membrane-bound Rasal1 inactivates Ras signaling through its RasGAP activity, and through such mechanisms has been implicated in regulating various cellular functions in the context of tumors. Although highly expressed in the brain, the contribution of Rasal1 to neuronal development and function has yet to be explored. RESULTS We examined the contributions of Rasal1 to neuronal development in primary culture of hippocampal neurons through modulation of Rasal1 expression using molecular tools. Fixed and live cell imaging demonstrate diffuse expression of Rasal1 throughout the cell soma, dendrites and axon which localizes to the neuronal plasma membrane in response to intracellular calcium fluctuation. Pull-down and co-immunoprecipitation demonstrate direct interaction of Rasal1 with PKC, tubulin, and CaMKII. Consequently, Rasal1 is found to stabilize microtubules, through post-translational modification of tubulin, and accordingly inhibit dendritic outgrowth and branching. Through imaging, molecular, and electrophysiological techniques Rasal1 is shown to promote NMDA-mediated synaptic activity and CaMKII phosphorylation. CONCLUSIONS Rasal1 functions in two separate roles in neuronal development; calcium regulated neurite outgrowth and the promotion of NMDA receptor-mediated postsynaptic events which may be mediated both by interaction with direct binding partners or calcium-dependent regulation of down-stream pathways. Importantly, the outlined molecular mechanisms of Rasal1 may contribute notably to normal neuronal development and synapse formation.
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Affiliation(s)
- M H S Deurloo
- Department of Physiology, University of Toronto, Toronto, Canada
| | - S Eide
- Department of Physiology, University of Toronto, Toronto, Canada
| | - E Turlova
- Department of Physiology, University of Toronto, Toronto, Canada
| | - Q Li
- Department of Physiology, University of Toronto, Toronto, Canada
| | - S Spijker
- Department Molecular and Cellular Neurobiology, Neurogenomics and Cognition Research, VU University of Amsterdam, Amsterdam, The Netherlands
| | - H-S Sun
- Department of Physiology, University of Toronto, Toronto, Canada
- Department of Surgery, University of Toronto, Toronto, Canada
| | - A J A Groffen
- Department of Functional Genomics, Center for Neurogenomics and Cognition Research, VU University Amsterdam, Amsterdam, The Netherlands
| | - Z-P Feng
- Department of Physiology, University of Toronto, Toronto, Canada.
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4
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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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5
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Abramov D, Guiberson NGL, Burré J. STXBP1 encephalopathies: Clinical spectrum, disease mechanisms, and therapeutic strategies. J Neurochem 2021; 157:165-178. [PMID: 32643187 PMCID: PMC7812771 DOI: 10.1111/jnc.15120] [Citation(s) in RCA: 48] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2020] [Revised: 06/24/2020] [Accepted: 06/30/2020] [Indexed: 12/13/2022]
Abstract
Mutations in Munc18-1/STXBP1 (syntaxin-binding protein 1) are linked to various severe early epileptic encephalopathies and neurodevelopmental disorders. Heterozygous mutations in the STXBP1 gene include missense, nonsense, frameshift, and splice site mutations, as well as intragenic deletions and duplications and whole-gene deletions. No genotype-phenotype correlation has been identified so far, and patients are treated by anti-epileptic drugs because of the lack of a specific disease-modifying therapy. The molecular disease mechanisms underlying STXBP1-linked disorders are yet to be fully understood, but both haploinsufficiency and dominant-negative mechanisms have been proposed. This review focuses on the current understanding of the phenotypic spectrum of STXBP1-linked disorders, as well as discusses disease mechanisms in the context of the numerous pathways in which STXBP1 functions in the brain. We additionally evaluate the available animal models to study these disorders and highlight potential therapeutic approaches for treating these devastating diseases.
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Affiliation(s)
- Debra Abramov
- Appel Institute for Alzheimer's Disease Research, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY, USA
| | - Noah Guy Lewis Guiberson
- Appel Institute for Alzheimer's Disease Research, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY, USA
| | - Jacqueline Burré
- Appel Institute for Alzheimer's Disease Research, Brain and Mind Research Institute, Weill Cornell Medicine, New York, NY, USA
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6
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Carvalhais LG, Martinho VC, Ferreiro E, Pinheiro PS. Unraveling the Nanoscopic Organization and Function of Central Mammalian Presynapses With Super-Resolution Microscopy. Front Neurosci 2021; 14:578409. [PMID: 33584169 PMCID: PMC7874199 DOI: 10.3389/fnins.2020.578409] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2020] [Accepted: 12/03/2020] [Indexed: 12/22/2022] Open
Abstract
The complex, nanoscopic scale of neuronal function, taking place at dendritic spines, axon terminals, and other minuscule structures, cannot be adequately resolved using standard, diffraction-limited imaging techniques. The last couple of decades saw a rapid evolution of imaging methods that overcome the diffraction limit imposed by Abbe's principle. These techniques, including structured illumination microscopy (SIM), stimulated emission depletion (STED), photo-activated localization microscopy (PALM), and stochastic optical reconstruction microscopy (STORM), among others, have revolutionized our understanding of synapse biology. By exploiting the stochastic nature of fluorophore light/dark states or non-linearities in the interaction of fluorophores with light, by using modified illumination strategies that limit the excitation area, these methods can achieve spatial resolutions down to just a few tens of nm or less. Here, we review how these advanced imaging techniques have contributed to unprecedented insight into the nanoscopic organization and function of mammalian neuronal presynapses, revealing new organizational principles or lending support to existing views, while raising many important new questions. We further discuss recent technical refinements and newly developed tools that will continue to expand our ability to delve deeper into how synaptic function is orchestrated at the nanoscopic level.
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Affiliation(s)
- Lia G Carvalhais
- Center for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Portugal.,Center for Innovative Biomedicine and Biotechnology, University of Coimbra, Coimbra, Portugal.,Institute for Interdisciplinary Research, University of Coimbra, Coimbra, Portugal
| | - Vera C Martinho
- Center for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Portugal.,Center for Innovative Biomedicine and Biotechnology, University of Coimbra, Coimbra, Portugal.,Institute for Interdisciplinary Research, University of Coimbra, Coimbra, Portugal
| | - Elisabete Ferreiro
- Center for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Portugal.,Center for Innovative Biomedicine and Biotechnology, University of Coimbra, Coimbra, Portugal.,Institute for Interdisciplinary Research, University of Coimbra, Coimbra, Portugal
| | - Paulo S Pinheiro
- Center for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Portugal.,Center for Innovative Biomedicine and Biotechnology, University of Coimbra, Coimbra, Portugal.,Institute for Interdisciplinary Research, University of Coimbra, Coimbra, Portugal
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7
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Kreutzberger AJB, Kiessling V, Doyle CA, Schenk N, Upchurch CM, Elmer-Dixon M, Ward AE, Preobraschenski J, Hussein SS, Tomaka W, Seelheim P, Kattan I, Harris M, Liang B, Kenworthy AK, Desai BN, Leitinger N, Anantharam A, Castle JD, Tamm LK. Distinct insulin granule subpopulations implicated in the secretory pathology of diabetes types 1 and 2. eLife 2020; 9:e62506. [PMID: 33164744 PMCID: PMC7738183 DOI: 10.7554/elife.62506] [Citation(s) in RCA: 21] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2020] [Accepted: 11/06/2020] [Indexed: 12/12/2022] Open
Abstract
Insulin secretion from β-cells is reduced at the onset of type-1 and during type-2 diabetes. Although inflammation and metabolic dysfunction of β-cells elicit secretory defects associated with type-1 or type-2 diabetes, accompanying changes to insulin granules have not been established. To address this, we performed detailed functional analyses of insulin granules purified from cells subjected to model treatments that mimic type-1 and type-2 diabetic conditions and discovered striking shifts in calcium affinities and fusion characteristics. We show that this behavior is correlated with two subpopulations of insulin granules whose relative abundance is differentially shifted depending on diabetic model condition. The two types of granules have different release characteristics, distinct lipid and protein compositions, and package different secretory contents alongside insulin. This complexity of β-cell secretory physiology establishes a direct link between granule subpopulation and type of diabetes and leads to a revised model of secretory changes in the diabetogenic process.
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Affiliation(s)
- Alex J B Kreutzberger
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
| | - Volker Kiessling
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
| | - Catherine A Doyle
- Department of Pharmacology, University of VirginiaCharlottesvilleUnited States
| | - Noah Schenk
- Department of Pharmacology, University of MichiganAnn ArborUnited States
| | - Clint M Upchurch
- Department of Pharmacology, University of VirginiaCharlottesvilleUnited States
| | - Margaret Elmer-Dixon
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
| | - Amanda E Ward
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
| | - Julia Preobraschenski
- Department of Neurobiology, Max Planck Institute for Biophysical ChemistryGöttingenGermany
- Cluster of Excellence in Multiscale Bioimaging: from Molecular Machines to Networks of Excitable Cells and Institute for Auditory Neuroscience, University of GöttingenGöttingenGermany
| | - Syed S Hussein
- Department of Microbiology, University of VirginiaCharlottesvilleUnited States
| | - Weronika Tomaka
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
| | - Patrick Seelheim
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
| | - Iman Kattan
- Department of Neurobiology, Max Planck Institute for Biophysical ChemistryGöttingenGermany
| | - Megan Harris
- Department of Cell Biology, University of VirginiaCharlottesvilleUnited States
| | - Binyong Liang
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
| | - Anne K Kenworthy
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
| | - Bimal N Desai
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department of Pharmacology, University of VirginiaCharlottesvilleUnited States
| | - Norbert Leitinger
- Department of Pharmacology, University of VirginiaCharlottesvilleUnited States
| | - Arun Anantharam
- Department of Pharmacology, University of MichiganAnn ArborUnited States
| | - J David Castle
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department of Cell Biology, University of VirginiaCharlottesvilleUnited States
| | - Lukas K Tamm
- Center for Membrane and Cell Physiology, University of VirginiaCharlottesvilleUnited States
- Department for Molecular Physiology and Biological Physics, University of VirginiaCharlottesvilleUnited States
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8
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Doc2 Proteins Are Not Required for the Increased Spontaneous Release Rate in Synaptotagmin-1-Deficient Neurons. J Neurosci 2020; 40:2606-2617. [PMID: 32098902 DOI: 10.1523/jneurosci.0309-19.2020] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2019] [Revised: 01/10/2020] [Accepted: 01/23/2020] [Indexed: 11/21/2022] Open
Abstract
Regulated secretion is controlled by Ca2+ sensors with different affinities and subcellular distributions. Inactivation of Syt1 (synaptotagmin-1), the main Ca2+ sensor for synchronous neurotransmission in many neurons, enhances asynchronous and spontaneous release rates, suggesting that Syt1 inhibits other sensors with higher Ca2+ affinities and/or lower cooperativities. Such sensors could include Doc2a and Doc2b, which have been implicated in spontaneous and asynchronous neurotransmitter release and compete with Syt1 for binding SNARE complexes. Here, we tested this hypothesis using triple-knock-out mice. Inactivation of Doc2a and Doc2b in Syt1-deficient neurons did not reduce the high spontaneous release rate. Overexpression of Doc2b variants in triple-knock-out neurons reduced spontaneous release but did not rescue synchronous release. A chimeric construct in which the C2AB domain of Syt1 was substituted by that of Doc2b did not support synchronous release either. Conversely, the soluble C2AB domain of Syt1 did not affect spontaneous release. We conclude that the high spontaneous release rate in synaptotagmin-deficient neurons does not involve the binding of Doc2 proteins to Syt1 binding sites in the SNARE complex. Instead, our results suggest that the C2AB domains of Syt1 and Doc2b specifically support synchronous and spontaneous release by separate mechanisms. (Both male and female neurons were studied without sex determination.)SIGNIFICANCE STATEMENT Neurotransmission in the brain is regulated by presynaptic Ca2+ concentrations. Multiple Ca2+ sensor proteins contribute to synchronous (Syt1, Syt2), asynchronous (Syt7), and spontaneous (Doc2a/Doc2b) phases of neurotransmitter release. Genetic ablation of synchronous release was previously shown to affect other release phases, suggesting that multiple sensors may compete for similar release sites, together encoding stimulus-secretion coupling over a large range of synaptic Ca2+ concentrations. Here, we investigated the extent of functional overlap between Syt1, Doc2a, and Doc2b by reintroducing wild-type and mutant proteins in triple-knock-out neurons, and conclude that the sensors are highly specialized for different phases of release.
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9
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Synaptotagmin-1 and Doc2b Exhibit Distinct Membrane-Remodeling Mechanisms. Biophys J 2019; 118:643-656. [PMID: 31952804 DOI: 10.1016/j.bpj.2019.12.021] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2019] [Revised: 12/13/2019] [Accepted: 12/16/2019] [Indexed: 11/24/2022] Open
Abstract
Synaptotagmin-1 (Syt1) is a calcium sensor protein that is critical for neurotransmission and is therefore extensively studied. Here, we use pairs of optically trapped beads coated with SNARE-free synthetic membranes to investigate Syt1-induced membrane remodeling. This activity is compared with that of Doc2b, which contains a conserved C2AB domain and induces membrane tethering and hemifusion in this cell-free model. We find that the soluble C2AB domain of Syt1 strongly affects the probability and strength of membrane-membrane interactions in a strictly Ca2+- and protein-dependent manner. Single-membrane loading of Syt1 yielded the highest probability and force of membrane interactions, whereas in contrast, Doc2b was more effective after loading both membranes. A lipid-mixing assay with confocal imaging reveals that both Syt1 and Doc2b are able to induce hemifusion; however, significantly higher Syt1 concentrations are required. Consistently, both C2AB fragments cause a reduction in the membrane-bending modulus, as measured by a method based on atomic force microscopy. This lowering of the energy required for membrane deformation may contribute to Ca2+-induced fusion.
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10
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Doc2b Ca 2+ binding site mutants enhance synaptic release at rest at the expense of sustained synaptic strength. Sci Rep 2019; 9:14408. [PMID: 31594980 PMCID: PMC6783474 DOI: 10.1038/s41598-019-50684-1] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2019] [Accepted: 09/11/2019] [Indexed: 12/01/2022] Open
Abstract
Communication between neurons involves presynaptic neurotransmitter release which can be evoked by action potentials or occur spontaneously as a result of stochastic vesicle fusion. The Ca2+-binding double C2 proteins Doc2a and –b were implicated in spontaneous and asynchronous evoked release, but the mechanism remains unclear. Here, we compared wildtype Doc2b with two Ca2+ binding site mutants named DN and 6A, previously classified as gain- and loss-of-function mutants. They carry the substitutions D218,220N or D163,218,220,303,357,359A respectively. We found that both mutants bound phospholipids at low Ca2+ concentrations and were membrane-associated in resting neurons, thus mimicking a Ca2+-activated state. Their overexpression in hippocampal primary cultured neurons had similar effects on spontaneous and evoked release, inducing high mEPSC frequencies and increased short-term depression. Together, these data suggest that the DN and 6A mutants both act as gain-of-function mutants at resting conditions.
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11
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Nomiyama R, Emoto M, Fukuda N, Matsui K, Kondo M, Sakane A, Sasaki T, Tanizawa Y. Protein kinase C iota facilitates insulin-induced glucose transport by phosphorylation of soluble nSF attachment protein receptor regulator (SNARE) double C2 domain protein b. J Diabetes Investig 2019; 10:591-601. [PMID: 30369065 PMCID: PMC6497606 DOI: 10.1111/jdi.12965] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/25/2018] [Revised: 09/25/2018] [Accepted: 10/11/2018] [Indexed: 12/24/2022] Open
Abstract
AIMS/INTRODUCTION Double C2 domain protein b (DOC2b), one of the synaptotagmins, has been shown to translocate to the plasma membrane, and to initiate membrane-fusion processes of vesicles containing glucose transporter 4 proteins on insulin stimulation. However, the mechanism by which DOC2b is regulated remains unclear. Herein, we identified the upstream regulatory factors of DOC2b in insulin signal transduction. We also examined the role of DOC2b on systemic homeostasis using DOC2b knockout (KO) mice. MATERIALS AND METHODS We first identified DOC2b binding proteins by immunoprecipitation and mutagenesis experiments. Then, DOC2b KO mice were generated by disrupting the first exon of the DOC2b gene. In addition to the histological examination, glucose metabolism was assessed by measuring parameters on glucose/insulin tolerance tests. Insulin-stimulated glucose uptake was also measured using isolated soleus muscle and epididymal adipose tissue. RESULTS We identified an isoform of atypical protein kinase C (protein kinase C iota) that can bind to DOC2b and phosphorylates one of the serine residues of DOC2b (S34). This phosphorylation is essential for DOC2b translocation. DOC2b KO mice showed insulin resistance and impaired oral glucose tolerance on insulin and glucose tolerance tests, respectively. Insulin-stimulated glucose uptake was impaired in isolated soleus muscle and epididymal adipose tissues from DOC2b KO mice. CONCLUSIONS We propose a novel insulin signaling mechanism by which protein kinase C iota phosphorylates DOC2b, leading to glucose transporter 4 vesicle translocation, fusion and facilitation of glucose uptake in response to insulin. The present results also showed DOC2b to play important roles in systemic glucose homeostasis.
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Affiliation(s)
- Ryuta Nomiyama
- Division of Endocrinology, Metabolism, Hematological Sciences and TherapeuticsYamaguchi University Graduate School of MedicineUbeJapan
| | - Masahiro Emoto
- Division of Endocrinology, Metabolism, Hematological Sciences and TherapeuticsYamaguchi University Graduate School of MedicineUbeJapan
- Emoto ClinicUbeJapan
| | - Naofumi Fukuda
- Division of Endocrinology, Metabolism, Hematological Sciences and TherapeuticsYamaguchi University Graduate School of MedicineUbeJapan
| | - Kumiko Matsui
- Division of Endocrinology, Metabolism, Hematological Sciences and TherapeuticsYamaguchi University Graduate School of MedicineUbeJapan
| | - Manabu Kondo
- Division of Endocrinology, Metabolism, Hematological Sciences and TherapeuticsYamaguchi University Graduate School of MedicineUbeJapan
| | - Ayuko Sakane
- Department of BiochemistryTokushima University Graduate School of Medical SciencesTokushimaJapan
| | - Takuya Sasaki
- Department of BiochemistryTokushima University Graduate School of Medical SciencesTokushimaJapan
| | - Yukio Tanizawa
- Division of Endocrinology, Metabolism, Hematological Sciences and TherapeuticsYamaguchi University Graduate School of MedicineUbeJapan
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12
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Abstract
Various forms of synaptic plasticity underlie aspects of learning and memory. Synaptic augmentation is a form of short-term plasticity characterized by synaptic enhancement that persists for seconds following specific patterns of stimulation. The mechanisms underlying this form of plasticity are unclear but are thought to involve residual presynaptic Ca2+ Here, we report that augmentation was reduced in cultured mouse hippocampal neurons lacking the Ca2+ sensor, Doc2; other forms of short-term enhancement were unaffected. Doc2 binds Ca2+ and munc13 and translocates to the plasma membrane to drive augmentation. The underlying mechanism was not associated with changes in readily releasable pool size or Ca2+ dynamics, but rather resulted from superpriming a subset of synaptic vesicles. Hence, Doc2 forms part of the Ca2+-sensing apparatus for synaptic augmentation via a mechanism that is molecularly distinct from other forms of short-term plasticity.
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13
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Courtney NA, Briguglio JS, Bradberry MM, Greer C, Chapman ER. Excitatory and Inhibitory Neurons Utilize Different Ca 2+ Sensors and Sources to Regulate Spontaneous Release. Neuron 2018; 98:977-991.e5. [PMID: 29754754 DOI: 10.1016/j.neuron.2018.04.022] [Citation(s) in RCA: 44] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2017] [Revised: 03/19/2018] [Accepted: 04/17/2018] [Indexed: 01/16/2023]
Abstract
Spontaneous neurotransmitter release (mini) is an important form of Ca2+-dependent synaptic transmission that occurs in the absence of action potentials. A molecular understanding of this process requires an identification of the underlying Ca2+ sensors. Here, we address the roles of the relatively low- and high-affinity Ca2+ sensors, synapotagmin-1 (syt1) and Doc2α/β, respectively. We found that both syt1 and Doc2 regulate minis, but, surprisingly, their relative contributions depend on whether release was from excitatory or inhibitory neurons. Doc2α promoted glutamatergic minis, while Doc2β and syt1 both regulated GABAergic minis. We identified Ca2+ ligand mutations in Doc2 that either disrupted or constitutively activated the regulation of minis. Finally, Ca2+ entry via voltage-gated Ca2+ channels triggered miniature GABA release by activating syt1, but had no effect on Doc2-driven minis. This work reveals an unexpected divergence in the regulation of spontaneous excitatory and inhibitory transmission in terms of both Ca2+ sensors and sources.
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Affiliation(s)
- Nicholas A Courtney
- Department of Neuroscience and Howard Hughes Medical Institute, University of Wisconsin-Madison, 1111 Highland Avenue, Madison, WI 53705, USA
| | - Joseph S Briguglio
- Department of Neuroscience and Howard Hughes Medical Institute, University of Wisconsin-Madison, 1111 Highland Avenue, Madison, WI 53705, USA
| | - Mazdak M Bradberry
- Department of Neuroscience and Howard Hughes Medical Institute, University of Wisconsin-Madison, 1111 Highland Avenue, Madison, WI 53705, USA
| | - Christina Greer
- Department of Neuroscience and Howard Hughes Medical Institute, University of Wisconsin-Madison, 1111 Highland Avenue, Madison, WI 53705, USA
| | - Edwin R Chapman
- Department of Neuroscience and Howard Hughes Medical Institute, University of Wisconsin-Madison, 1111 Highland Avenue, Madison, WI 53705, USA.
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14
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Houy S, Groffen AJ, Ziomkiewicz I, Verhage M, Pinheiro PS, Sørensen JB. Doc2B acts as a calcium sensor for vesicle priming requiring synaptotagmin-1, Munc13-2 and SNAREs. eLife 2017; 6:27000. [PMID: 29274147 PMCID: PMC5758110 DOI: 10.7554/elife.27000] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2017] [Accepted: 12/21/2017] [Indexed: 01/08/2023] Open
Abstract
Doc2B is a cytosolic protein with binding sites for Munc13 and Tctex-1 (dynein light chain), and two C2-domains that bind to phospholipids, Ca2+ and SNAREs. Whether Doc2B functions as a calcium sensor akin to synaptotagmins, or in other calcium-independent or calcium-dependent capacities is debated. We here show by mutation and overexpression that Doc2B plays distinct roles in two sequential priming steps in mouse adrenal chromaffin cells. Mutating Ca2+-coordinating aspartates in the C2A-domain localizes Doc2B permanently at the plasma membrane, and renders an upstream priming step Ca2+-independent, whereas a separate function in downstream priming depends on SNARE-binding, Ca2+-binding to the C2B-domain of Doc2B, interaction with ubMunc13-2 and the presence of synaptotagmin-1. Another function of Doc2B – inhibition of release during sustained calcium elevations – depends on an overlapping protein domain (the MID-domain), but is separate from its Ca2+-dependent priming function. We conclude that Doc2B acts as a vesicle priming protein.
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Affiliation(s)
- Sébastien Houy
- Neuronal Secretion Group, Department of Neuroscience, University of Copenhagen, København, Denmark
| | - Alexander J Groffen
- Department of Clinical Genetics, Center for Neurogenomics and Cognitive Research, VU Medical Center, Amsterdam, Netherlands
| | - Iwona Ziomkiewicz
- Neuronal Secretion Group, Department of Neuroscience, University of Copenhagen, København, Denmark.,Discovery Sciences, Innovative Medicines and Early Development, AstraZeneca R&D, Cambridge, United Kingdom
| | - Matthijs Verhage
- Department of Clinical Genetics, Center for Neurogenomics and Cognitive Research, VU Medical Center, Amsterdam, Netherlands.,Department of Functional Genomics, Faculty of Science, Center for Neurogenomics and Cognitive Research, VrijeUniversiteit, Amsterdam, Netherlands
| | - Paulo S Pinheiro
- Neuronal Secretion Group, Department of Neuroscience, University of Copenhagen, København, Denmark
| | - Jakob Balslev Sørensen
- Neuronal Secretion Group, Department of Neuroscience, University of Copenhagen, København, Denmark
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15
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Michaeli L, Gottfried I, Bykhovskaia M, Ashery U. Phosphatidylinositol (4, 5)-bisphosphate targets double C2 domain protein B to the plasma membrane. Traffic 2017; 18:825-839. [PMID: 28941037 DOI: 10.1111/tra.12528] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2017] [Revised: 09/18/2017] [Accepted: 09/18/2017] [Indexed: 01/05/2023]
Abstract
Double C2 domain protein B (DOC2B) is a high-affinity Ca2+ sensor that translocates from the cytosol to the plasma membrane (PM) and promotes vesicle priming and fusion. However, the molecular mechanism underlying its translocation and targeting to the PM in living cells is not completely understood. DOC2B interacts in vitro with the PM components phosphatidylserine, phosphatidylinositol (4, 5)-bisphosphate [PI(4, 5)P2 ] and target SNAREs (t-SNAREs). Here, we show that PI(4, 5)P2 hydrolysis at the PM of living cells abolishes DOC2B translocation, whereas manipulations of t-SNAREs and other phosphoinositides have no effect. Moreover, we were able to redirect DOC2B to intracellular membranes by synthesizing PI(4, 5)P2 in those membranes. Molecular dynamics simulations and mutagenesis in the calcium and PI(4, 5)P2 -binding sites strengthened our findings, demonstrating that both calcium and PI(4, 5)P2 are required for the DOC2B-PM association and revealing multiple PI(4, 5)P2 -C2B interactions. In addition, we show that DOC2B translocation to the PM is ATP-independent and occurs in a diffusion-like manner. Our data suggest that the Ca2+ -triggered translocation of DOC2B is diffusion-driven and aimed at PI(4, 5)P2 -containing membranes.
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Affiliation(s)
- Lirin Michaeli
- Department of Neurobiology, Faculty of Life Sciences, Tel-Aviv University, Tel Aviv, Israel
| | - Irit Gottfried
- Department of Neurobiology, Faculty of Life Sciences, Tel-Aviv University, Tel Aviv, Israel
| | | | - Uri Ashery
- Department of Neurobiology, Faculty of Life Sciences, Tel-Aviv University, Tel Aviv, Israel.,Sagol School of Neuroscience, Tel-Aviv University, Tel Aviv, Israel
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16
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Direct quantitative detection of Doc2b-induced hemifusion in optically trapped membranes. Nat Commun 2015; 6:8387. [PMID: 26395669 PMCID: PMC4598560 DOI: 10.1038/ncomms9387] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2015] [Accepted: 08/17/2015] [Indexed: 01/08/2023] Open
Abstract
Ca2+-sensor proteins control the secretion of many neuroendocrine substances. Calcium-secretion coupling may involve several mechanisms. First, Ca2+-dependent association of their tandem C2 domains with phosphatidylserine may induce membrane curvature and thereby enhance fusion. Second, their association with SNARE complexes may inhibit membrane fusion in the absence of a Ca2+ trigger. Here we present a method using two optically trapped beads coated with SNARE-free synthetic membranes to elucidate the direct role of the C2AB domain of the soluble Ca2+-sensor Doc2b. Contacting membranes are often coupled by a Doc2b-coated membrane stalk that resists forces up to 600 pN upon bead separation. Stalk formation depends strictly on Ca2+ and phosphatidylserine. Real-time fluorescence imaging shows phospholipid but not content mixing, indicating membrane hemifusion. Thus, Doc2b acts directly on membranes and stabilizes the hemifusion intermediate in this cell-free system. In living cells, this mechanism may co-occur with progressive SNARE complex assembly, together defining Ca2+-secretion coupling. Membrane fusion in cells is triggered by an increase in Ca2+ and involves SNARE complexes and calcium-sensing proteins, but the mechanism underlying the Ca2+-sensors' role in fusion remains unclear. Here the authors show in vitro that the Ca2+-sensor Doc2b acts directly on membranes and induces a hemifusion intermediate in the presence of calcium.
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17
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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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18
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Doc2b serves as a scaffolding platform for concurrent binding of multiple Munc18 isoforms in pancreatic islet β-cells. Biochem J 2015; 464:251-8. [PMID: 25190515 DOI: 10.1042/bj20140845] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Biphasic glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells involves soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor (SNARE) protein-regulated exocytosis. SNARE complex assembly further requires the regulatory proteins Munc18c, Munc18-1 and Doc2b. Munc18-1 and Munc18c are required for first- and second-phase GSIS respectively. These distinct Munc18-1 and Munc18c roles are related to their transient high-affinity binding with their cognate target (t-)SNAREs, Syntaxin 1A and Syntaxin 4 respectively. Doc2b is essential for both phases of GSIS, yet the molecular basis for this remains unresolved. Because Doc2b binds to Munc18-1 and Munc18c via its distinct C2A and C2B domains respectively, we hypothesized that Doc2b may provide a plasma membrane-localized scaffold/platform for transient docking of these Munc18 isoforms during GSIS. Towards this, macromolecular complexes composed of Munc18c, Doc2b and Munc18-1 were detected in β-cells. In vitro interaction assays indicated that Doc2b is required to bridge the interaction between Munc18c and Munc18-1 in the macromolecular complex; Munc18c and Munc18-1 failed to associate in the absence of Doc2b. Competition-based GST-Doc2b interaction assays revealed that Doc2b could simultaneously bind both Munc18-1 and Munc18c. Hence these data support a working model wherein Doc2b functions as a docking platform/scaffold for transient interactions with the multiple Munc18 isoforms operative in insulin release, promoting SNARE assembly.
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19
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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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20
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Doc2b synchronizes secretion from chromaffin cells by stimulating fast and inhibiting sustained release. J Neurosci 2013; 33:16459-70. [PMID: 24133251 DOI: 10.1523/jneurosci.2656-13.2013] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Synaptotagmin-1 and -7 constitute the main calcium sensors mediating SNARE-dependent exocytosis in mouse chromaffin cells, but the role of a closely related calcium-binding protein, Doc2b, remains enigmatic. We investigated its role in chromaffin cells using Doc2b knock-out mice and high temporal resolution measurements of exocytosis. We found that the calcium dependence of vesicle priming and release triggering remained unchanged, ruling out an obligatory role for Doc2b in those processes. However, in the absence of Doc2b, release was shifted from the readily releasable pool to the subsequent sustained component. Conversely, upon overexpression of Doc2b, the sustained component was largely inhibited whereas the readily releasable pool was augmented. Electron microscopy revealed an increase in the total number of vesicles upon Doc2b overexpression, ruling out vesicle depletion as the cause for the reduced sustained component. Further experiments showed that, in the absence of Doc2b, the refilling of the readily releasable vesicle pools is faster, but incomplete. Faster refilling leads to an increase in the sustained component as newly primed vesicles fuse while the [Ca(2+)]i following stimulation is still high. We conclude that Doc2b acts to inhibit vesicle priming during prolonged calcium elevations, thus protecting unprimed vesicles from fusing prematurely, and redirecting them to refill the readily releasable pool after relaxation of the calcium signal. In sum, Doc2b favors fast, synchronized release, and limits out-of-phase secretion.
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21
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Giladi M, Michaeli L, Almagor L, Bar-On D, Buki T, Ashery U, Khananshvili D, Hirsch JA. The C2B domain is the primary Ca2+ sensor in DOC2B: a structural and functional analysis. J Mol Biol 2013; 425:4629-41. [PMID: 23994332 DOI: 10.1016/j.jmb.2013.08.017] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2013] [Revised: 08/12/2013] [Accepted: 08/21/2013] [Indexed: 12/21/2022]
Abstract
DOC2B (double-C2 domain) protein is thought to be a high-affinity Ca(2+) sensor for spontaneous and asynchronous neurotransmitter release. To elucidate the molecular features underlying its physiological role, we determined the crystal structures of its isolated C2A and C2B domains and examined their Ca(2+)-binding properties. We further characterized the solution structure of the tandem domains (C2AB) using small-angle X-ray scattering. In parallel, we tested structure-function correlates with live cell imaging tools. We found that, despite striking structural similarity, C2B binds Ca(2+) with considerably higher affinity than C2A. The C2AB solution structure is best modeled as two domains with a highly flexible orientation and no difference in the presence or absence of Ca(2+). In addition, kinetic studies of C2AB demonstrate that, in the presence of unilamellar vesicles, Ca(2+) binding is stabilized, as reflected by the ~10-fold slower rate of Ca(2+) dissociation than in the absence of vesicles. In cells, isolated C2B translocates to the plasma membrane (PM) with an EC50 of 400 nM while the C2A does not translocate at submicromolar Ca(2+) concentrations, supporting the biochemical observations. Nevertheless, C2AB translocates to the PM with an ~2-fold lower EC50 and to a greater extent than C2B. Our results, together with previous studies, reveal that the C2B is the primary Ca(2+) sensing unit in DOC2B, whereas C2A enhances the interaction of C2AB with the PM.
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Affiliation(s)
- Moshe Giladi
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel
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22
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Ramalingam L, Oh E, Yoder SM, Brozinick JT, Kalwat MA, Groffen AJ, Verhage M, Thurmond DC. Doc2b is a key effector of insulin secretion and skeletal muscle insulin sensitivity. Diabetes 2012; 61:2424-32. [PMID: 22698913 PMCID: PMC3447898 DOI: 10.2337/db11-1525] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/31/2011] [Accepted: 04/02/2012] [Indexed: 11/13/2022]
Abstract
Exocytosis of intracellular vesicles, such as insulin granules, is carried out by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) and Sec1/Munc18 (SM) proteins. An additional regulatory protein, Doc2b (double C2 domain), has recently been implicated in exocytosis from clonal β-cells and 3T3-L1 adipocytes. Here, we investigated the role of Doc2b in insulin secretion, insulin sensitivity, and the maintenance of whole-body glucose homeostasis. Doc2b heterozygous (Doc2b(+/-)) and homozygous (Doc2b(-/-)) knockout mice exhibited significant whole-body glucose intolerance and peripheral insulin resistance, compared with wild-type littermates. Correspondingly, Doc2b(+/-) and Doc2b(-/-) mice exhibited decreased responsiveness of pancreatic islets to glucose in vivo, with significant attenuation of both phases of insulin secretion ex vivo. Peripheral insulin resistance correlated with ablated insulin-stimulated glucose uptake and GLUT4 vesicle translocation in skeletal muscle from Doc2b-deficient mice, which was coupled to impairments in Munc18c-syntaxin 4 dissociation and in SNARE complex assembly. Hence, Doc2b is a key positive regulator of Munc18c-syntaxin 4-mediated insulin secretion as well as of insulin responsiveness in skeletal muscle, and thus a key effector for glucose homeostasis in vivo. Doc2b's actions in glucose homeostasis may be related to its ability to bind Munc18c and/or directly promote fusion of insulin granules and GLUT4 vesicles in a stimulus-dependent manner.
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Affiliation(s)
- Latha Ramalingam
- Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana
| | - Eunjin Oh
- Department of Pediatrics, Herman B Wells Center, Indiana University School of Medicine, Indianapolis, Indiana
| | - Stephanie M. Yoder
- Department of Pediatrics, Herman B Wells Center, Indiana University School of Medicine, Indianapolis, Indiana
| | | | - Michael A. Kalwat
- Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana
| | - Alexander J. Groffen
- Department of Functional Genomics and Department of Clinical Genetics, Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, the Netherlands
| | - Matthijs Verhage
- Department of Functional Genomics and Department of Clinical Genetics, Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, the Netherlands
| | - Debbie C. Thurmond
- Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana
- Department of Pediatrics, Herman B Wells Center, Indiana University School of Medicine, Indianapolis, Indiana
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23
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Kasai H, Takahashi N, Tokumaru H. Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis. Physiol Rev 2012; 92:1915-64. [DOI: 10.1152/physrev.00007.2012] [Citation(s) in RCA: 120] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.
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Affiliation(s)
- Haruo Kasai
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
| | - Noriko Takahashi
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
| | - Hiroshi Tokumaru
- Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; and Faculty of Pharmaceutical Sciences at Kagawa, Tokushima Bunri University, Kagawa, Japan
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24
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Oh E, Kalwat MA, Kim MJ, Verhage M, Thurmond DC. Munc18-1 regulates first-phase insulin release by promoting granule docking to multiple syntaxin isoforms. J Biol Chem 2012; 287:25821-33. [PMID: 22685295 DOI: 10.1074/jbc.m112.361501] [Citation(s) in RCA: 57] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Attenuated levels of the Sec1/Munc18 (SM) protein Munc18-1 in human islet β-cells is coincident with type 2 diabetes, although how Munc18-1 facilitates insulin secretion remains enigmatic. Herein, using conventional Munc18-1(+/-) and β-cell specific Munc18-1(-/-) knock-out mice, we establish that Munc18-1 is required for the first phase of insulin secretion. Conversely, human islets expressing elevated levels of Munc18-1 elicited significant potentiation of only first-phase insulin release. Insulin secretory changes positively correlated with insulin granule number at the plasma membrane: Munc18-1-deficient cells lacked 35% of the normal component of pre-docked insulin secretory granules, whereas cells with elevated levels of Munc18-1 exhibited a ∼20% increase in pre-docked granule number. Pre-docked syntaxin 1-based SNARE complexes bound by Munc18-1 were detected in β-cell lysates but, surprisingly, were reduced by elevation of Munc18-1 levels. Paradoxically, elevated Munc18-1 levels coincided with increased binding of syntaxin 4 to VAMP2 at the plasma membrane. Accordingly, syntaxin 4 was a requisite for Munc18-1 potentiation of insulin release. Munc18c, the cognate SM isoform for syntaxin 4, failed to bind SNARE complexes. Given that Munc18-1 does not pair with syntaxin 4, these data suggest a novel indirect role for Munc18-1 in facilitating syntaxin 4-mediated granule pre-docking to support first-phase insulin exocytosis.
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Affiliation(s)
- Eunjin Oh
- Department of Pediatrics, Herman B. Wells Center, Indiana University School of Medicine, Indianapolis, Indiana 46202, USA
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Yao J, Gaffaney JD, Kwon SE, Chapman ER. Doc2 is a Ca2+ sensor required for asynchronous neurotransmitter release. Cell 2011; 147:666-77. [PMID: 22036572 DOI: 10.1016/j.cell.2011.09.046] [Citation(s) in RCA: 163] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2010] [Revised: 07/19/2011] [Accepted: 09/29/2011] [Indexed: 11/26/2022]
Abstract
Synaptic transmission involves a fast synchronous phase and a slower asynchronous phase of neurotransmitter release that are regulated by distinct Ca(2+) sensors. Though the Ca(2+) sensor for rapid exocytosis, synaptotagmin I, has been studied in depth, the sensor for asynchronous release remains unknown. In a screen for neuronal Ca(2+) sensors that respond to changes in [Ca(2+)] with markedly slower kinetics than synaptotagmin I, we observed that Doc2--another Ca(2+), SNARE, and lipid-binding protein--operates on timescales consistent with asynchronous release. Moreover, up- and downregulation of Doc2 expression levels in hippocampal neurons increased or decreased, respectively, the slow phase of synaptic transmission. Synchronous release, when triggered by single action potentials, was unaffected by manipulation of Doc2 but was enhanced during repetitive stimulation in Doc2 knockdown neurons, potentially due to greater vesicle availability. In summary, we propose that Doc2 is a Ca(2+) sensor that is kinetically tuned to regulate asynchronous neurotransmitter release.
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Affiliation(s)
- Jun Yao
- Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin, Madison, WI 53706, USA
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26
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Affiliation(s)
- Sascha Martens
- Max F. Perutz Laboratories, University of Vienna, Vienna, Austria
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Sato M, Mori Y, Matsui T, Aoki R, Oya M, Yanagihara Y, Fukuda M, Tsuboi T. Role of the polybasic sequence in the Doc2alpha C2B domain in dense-core vesicle exocytosis in PC12 cells. J Neurochem 2010; 114:171-81. [PMID: 20403080 DOI: 10.1111/j.1471-4159.2010.06739.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The double C2 (Doc2) family is characterized by an N-terminal Munc13-1-interacting domain and C-terminal tandem C2 domains, and it comprises three isoforms, Doc2alpha, Doc2beta, and Doc2gamma, in humans and mice. Doc2alpha, the best-characterized, brain-specific isoform, exhibits Ca(2+)-dependent phospholipid-binding activity through its C2A domain, and the Ca(2+)-binding activity is thought to be important for the regulation of Ca(2+)-dependent exocytosis. In contrast to the C2A domain, however, nothing is known about the physiological functions of the C2B domain in regulated exocytosis. In this study, we demonstrated by a mutation analysis that the polybasic sequence in the C2B domain of Doc2alpha (306 KKSKHKTCVKKK 317) is required for binding of syntaxin-1a/synaptosome-associated protein of 25 kDa (SNAP-25) heterodimer. We also investigated the effect of Lys-to-Gln (named KQ) mutations in the polybasic sequence of the C2B domain on vesicle dynamics by total internal reflection fluorescence microscopy in PC12 cells. A Doc2alpha(KQ) mutant, which lacks binding activity toward syntaxin-1a/SNAP-25 heterodimer, significantly decreased the number of plasma membrane-docked vesicles before stimulation and strongly inhibited high-KCl-induced exocytosis from the plasma membrane-docked vesicles. These results indicate that the polybasic sequence in the C2B domain functions as a binding site for syntaxin-1a/SNAP-25 heterodimer and controls the number of 'readily releasable' vesicles in neuroendocrine cells.
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Affiliation(s)
- Mai Sato
- Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Meguro, Tokyo, Japan
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Groffen AJ, Martens S, Arazola RD, Cornelisse LN, Lozovaya N, de Jong APH, Goriounova NA, Habets RLP, Takai Y, Borst JG, Brose N, McMahon HT, Verhage M. Doc2b is a high-affinity Ca2+ sensor for spontaneous neurotransmitter release. Science 2010; 327:1614-8. [PMID: 20150444 PMCID: PMC2846320 DOI: 10.1126/science.1183765] [Citation(s) in RCA: 238] [Impact Index Per Article: 17.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Synaptic vesicle fusion in brain synapses occurs in phases that are either tightly coupled to action potentials (synchronous), immediately following action potentials (asynchronous), or as stochastic events in the absence of action potentials (spontaneous). Synaptotagmin-1, -2, and -9 are vesicle-associated Ca2+ sensors for synchronous release. Here we found that double C2 domain (Doc2) proteins act as Ca2+ sensors to trigger spontaneous release. Although Doc2 proteins are cytosolic, they function analogously to synaptotagmin-1 but with a higher Ca2+ sensitivity. Doc2 proteins bound to N-ethylmaleimide-sensitive factor attachment receptor (SNARE) complexes in competition with synaptotagmin-1. Thus, different classes of multiple C2 domain-containing molecules trigger synchronous versus spontaneous fusion, which suggests a general mechanism for synaptic vesicle fusion triggered by the combined actions of SNAREs and multiple C2 domain-containing proteins.
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Affiliation(s)
- Alexander J. Groffen
- Department of Functional Genomics, CNCR, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, The Netherlands
| | - Sascha Martens
- MRC Laboratory of Molecular Biology, Hills Road, Cambridge, UK
| | - Rocío Díez Arazola
- Department of Functional Genomics, CNCR, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, The Netherlands
| | - L. Niels Cornelisse
- Department of Functional Genomics, CNCR, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, The Netherlands
| | - Natalia Lozovaya
- Department of Functional Genomics, CNCR, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, The Netherlands
- Department of Integrative Neurophysiology, CNCR, VU University, Amsterdam, The Netherlands
| | - Arthur P. H. de Jong
- Department of Functional Genomics, CNCR, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, The Netherlands
| | - Natalia A. Goriounova
- Department of Functional Genomics, CNCR, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, The Netherlands
- Department of Integrative Neurophysiology, CNCR, VU University, Amsterdam, The Netherlands
| | - Ron L. P. Habets
- Department of Neuroscience, Erasmus MC, University Medical Center Rotterdam, The Netherlands
| | - Yoshimi Takai
- Department of Biochemistry and Molecular Biology, Kobe University Graduate School of Medicine, Japan
| | - J. Gerard Borst
- Department of Neuroscience, Erasmus MC, University Medical Center Rotterdam, The Netherlands
| | - Nils Brose
- Max-Planck-Institut für Experimentelle Medizin, Abteilung Molekulare Neurobiologie, Göttingen, Germany
| | | | - Matthijs Verhage
- Department of Functional Genomics, CNCR, Neuroscience Campus Amsterdam, VU University and VU Medical Center, Amsterdam, The Netherlands
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Friedrich R, Yeheskel A, Ashery U. DOC2B, C2 domains, and calcium: A tale of intricate interactions. Mol Neurobiol 2010; 41:42-51. [PMID: 20052564 DOI: 10.1007/s12035-009-8094-8] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2009] [Accepted: 12/09/2009] [Indexed: 11/28/2022]
Abstract
Ca(+2)-dependent exocytosis involves vesicle docking, priming, fusion, and recycling. This process is performed and regulated by a vast number of synaptic proteins and depends on proper protein-protein and protein-lipid interactions. Double C2 domain (DOC2) is a protein family of three isoforms found while screening DNA libraries with a C2 probe. DOC2 has three domains: the Munc13-interacting domain and tandem C2s (designated C2A and C2B) connected by a short polar linker. The C2 domain binds phospholipids in a Ca(2+)-dependent manner. This review focuses on the ubiquitously expressed isoform DOC2B. Sequence alignment of the tandem C2 protein family in mouse revealed high homology (81%) between rabphilin-3A and DOC2B proteins. We created a structural model of DOC2B's C2A based on the crystal structure of rabphilin-3A with and without calcium and found that the calcium-binding loops of DOC2B move upon calcium binding, enabling efficient plasma membrane penetration of its C2A. Here, we discuss the potential relation between the DOC2B bioinformatical model and its function and suggest a possible working model for its interaction with other proteins of the exocytotic machinery, including Munc13, Munc18, and syntaxin.
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Affiliation(s)
- Reut Friedrich
- Department of Neurobiology, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel
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Phospholipid-binding protein EhC2A mediates calcium-dependent translocation of transcription factor URE3-BP to the plasma membrane of Entamoeba histolytica. EUKARYOTIC CELL 2009; 9:695-704. [PMID: 20023071 DOI: 10.1128/ec.00346-09] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The Entamoeba histolytica upstream regulatory element 3-binding protein (URE3-BP) is a transcription factor that binds DNA in a Ca(2+)-inhibitable manner. The protein is located in both the nucleus and the cytoplasm but has also been found to be enriched in the plasma membrane of amebic trophozoites. We investigated the reason for the unusual localization of URE3-BP at the amebic plasma membrane. Here we identify and characterize a 22-kDa Ca(2+)-dependent binding partner of URE3-BP, EhC2A, a novel member of the C2-domain superfamily. Immunoprecipitations of URE3-BP and EhC2A showed that the proteins interact and that such interaction was enhanced in the presence of Ca(2+). Recombinant and native EhC2A bound phospholipid liposomes in a Ca(2+)-dependent manner, with half-maximal binding occurring at 3.4 muM free Ca(2+). A direct interaction between EhC2A and URE3-BP was demonstrated by the ability of recombinant EhC2A to recruit recombinant URE3-BP to phospholipid liposomes in a Ca(2+)-dependent manner. URE3-BP and EhC2A were observed to translocate to the amebic plasma membrane upon an increase in the intracellular Ca(2+) concentration of trophozoites, as revealed by subcellular fractionation and immunofluorescent staining. Short hairpin RNA-mediated knockdown of EhC2A protein expression significantly modulated the mRNA levels of URE3-BP-regulated transcripts. Based on these results, we propose a model for EhC2A-mediated regulation of the transcriptional activities of URE3-BP via Ca(2+)-dependent anchoring of the transcription factor to the amebic plasma membrane.
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Miyazaki M, Emoto M, Fukuda N, Hatanaka M, Taguchi A, Miyamoto S, Tanizawa Y. DOC2b is a SNARE regulator of glucose-stimulated delayed insulin secretion. Biochem Biophys Res Commun 2009; 384:461-5. [PMID: 19410553 DOI: 10.1016/j.bbrc.2009.04.133] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2009] [Accepted: 04/26/2009] [Indexed: 11/24/2022]
Abstract
Insulin secretion is precisely regulated by blood glucose with unique biphasic pattern. The regulatory mechanism of the second-phase insulin release is unclear. In this study, we report that DOC2b (double C2 domain protein isoform b), a SNARE related protein, was associated with insulin vesicles and translocated to plasma membrane within several minutes upon high-glucose stimulation followed by an interaction with syntaxin4, but not syntaxin1. This binding specificity and the time course of DOC2b translocation were suitable for the regulation of second-phase insulin release. Increased DOC2b expression enhanced glucose-stimulated insulin secretion. In contrast, silencing DOC2b inhibited delayed release of insulin, without affecting rapid (approximately 7min) phase secretion. Interestingly, DOC2b had no effects on KCl-triggered insulin release. These data suggest that DOC2b may be a regulator for delayed (second-phase) insulin secretion in MIN6 cells.
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Affiliation(s)
- Mutsuko Miyazaki
- Division of Endocrinology, Metabolism, Hematological Sciences and Therapeutics, Department of Bio-Signal Analysis, Yamaguchi University Graduate School of Medicine, 1-1-1 Minami-Kogushi, Ube 755-8505, Japan
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32
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Abstract
Calcium-dependent exocytosis is regulated by a vast number of proteins. DOC2B is a synaptic protein that translocates to the plasma membrane (PM) after small elevations in intracellular calcium concentration. The aim of this study was to investigate the role of DOC2B in calcium-triggered exocytosis. Using biochemical and biophysical measurements, we demonstrate that the C2A domain of DOC2B interacts directly with the PM in a calcium-dependent manner. Using a combination of electrophysiological, morphological, and total internal reflection fluorescent measurements, we found that DOC2B acts as a priming factor and increases the number of fusion-competent vesicles. Comparing secretion during repeated stimulation between wild-type DOC2B and a mutated DOC2B that is constantly at the PM showed that DOC2B enhances catecholamine secretion also during repeated stimulation and that DOC2B has to translocate to the PM to exert its facilitating effect, suggesting that its activity is dependent on calcium. The hypothesis that DOC2B exerts its effect at the PM was supported by the finding that DOC2B affects the fusion kinetics of single vesicles and interacts with the PM SNAREs (soluble NSF attachment receptors). We conclude that DOC2B is a calcium-dependent priming factor and its activity at the PM enables efficient expansion of the fusion pore, leading to increased catecholamine release.
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33
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Martens S, McMahon HT. Mechanisms of membrane fusion: disparate players and common principles. Nat Rev Mol Cell Biol 2008; 9:543-56. [DOI: 10.1038/nrm2417] [Citation(s) in RCA: 524] [Impact Index Per Article: 32.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
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Higashio H, Nishimura N, Ishizaki H, Miyoshi J, Orita S, Sakane A, Sasaki T. Doc2α and Munc13-4 Regulate Ca2+-Dependent Secretory Lysosome Exocytosis in Mast Cells. THE JOURNAL OF IMMUNOLOGY 2008; 180:4774-84. [DOI: 10.4049/jimmunol.180.7.4774] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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Ke B, Oh E, Thurmond DC. Doc2beta is a novel Munc18c-interacting partner and positive effector of syntaxin 4-mediated exocytosis. J Biol Chem 2007; 282:21786-97. [PMID: 17548353 DOI: 10.1074/jbc.m701661200] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The widely expressed Sec/Munc18 (SM) protein Munc18c is required for SNARE-mediated insulin granule exocytosis from islet beta cells and GLUT4 vesicle exocytosis in skeletal muscle and adipocytes. Although Munc18c function is known to involve binding to the t-SNARE Syntaxin 4, a paucity of Munc18c-binding proteins has restricted elucidation of the mechanism by which it facilitates these exocytosis events. Toward this end, we have identified the double C2 domain protein Doc2beta as a new binding partner for Munc18c. Unlike its granule/vesicle localization in neuronal cells, Doc2beta was found principally in the plasma membrane compartment in islet beta cells and adipocytes. Moreover, co-immunoprecipitation and GST interaction assays showed Doc2beta-Munc18c binding to be direct and complexes to be devoid of Syntaxin 4. Supporting the notion of Munc18c binding with Syntaxin 4 and Doc2beta in mutually exclusive complexes, in vitro competition with Syntaxin 4 effectively displaced Munc18c from binding to Doc2beta. The second C2 domain (C2B) of Doc2beta and an N-terminal region of Munc18c were sufficient to confer complex formation. Disruption of endogenous Munc18c-Doc2beta complexes by addition of the Doc2beta binding domain of Munc18c (residues 173-255) was found to selectively inhibit glucose-stimulated insulin release. Moreover, increased expression of Doc2beta enhanced glucose-stimulated insulin secretion by approximately 40%, whereas siRNA-mediated depletion of Doc2beta attenuated insulin release. All changes in secretion correlated with parallel alterations in VAMP2 granule docking with Syntaxin 4. Taken together, these data support a model wherein Munc18c transiently switches from association with Syntaxin 4 to association with Doc2beta at the plasma membrane to facilitate exocytosis.
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Affiliation(s)
- Ban Ke
- Department of Biochemistry and Molecular Biology, Center for Diabetes Research, Indiana University School of Medicine, Indianapolis, IN 46202, USA
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36
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Nili U, de Wit H, Gulyas-Kovacs A, Toonen RF, Sørensen JB, Verhage M, Ashery U. Munc18-1 phosphorylation by protein kinase C potentiates vesicle pool replenishment in bovine chromaffin cells. Neuroscience 2006; 143:487-500. [PMID: 16997485 DOI: 10.1016/j.neuroscience.2006.08.014] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2006] [Revised: 08/07/2006] [Accepted: 08/08/2006] [Indexed: 10/24/2022]
Abstract
Activation of protein kinase C (PKC) after robust stimulation is necessary for vesicle pool replenishment in secretory cells. Here we studied the contribution of a prominent downstream PKC target, Munc18-1, to this process in bovine chromaffin cells. In these cells, both activation of endogenous PKC and overexpressing of Munc18-1 promote vesicle pool replenishment after an extensive stimulation. In order to study the physiological relevance of PKC-dependent Munc18-1 phosphorylation, we generated two Munc18-1 phospho-mutants; one that mimics a constitutively PKC-phosphorylated Munc18-1 (i.e. a phosphomimetic mutant; Munc18-1(S313D)) and a second that cannot be PKC-phosphorylated (Munc18-1(3A)). Overexpression of Munc18-1(3A) caused a significant decrease in vesicle pool replenishment following a depleting stimulation, while Munc18-1(S313D) caused a significant increase in vesicle pool replenishment. These findings suggested that the phosphorylation of Munc18-1 by PKC potentiates vesicle pool replenishment. This hypothesis was further strengthened by the finding that overexpression of wild type Munc18-1 in the presence of a PKC inhibitor caused a significant reduction in vesicle pool replenishment, similar to that observed with Munc18-1(3A). Moreover, overexpression of Munc18-1(S313D) in the presence of the PKC inhibitor partly alleviated this attenuation, elucidating Munc18-1's unique contribution to vesicle pool replenishment. Finally, we demonstrate that Munc18-1 promotes vesicle docking in a phosphorylation-independent manner. This is deduced from the findings that both the wild type and the two Munc18-1 phospho-mutants enhanced docking to the same extent in bovine chromaffin cells. We conclude that Munc18-1 facilitates docking in a PKC phosphorylation-independent manner, and that its phosphorylation by PKC potentiates vesicle pool replenishment following a depleting stimulation, at a post-docking stage.
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Affiliation(s)
- U Nili
- Department of Neurobiochemistry, Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel
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Groffen AJA, Friedrich R, Brian EC, Ashery U, Verhage M. DOC2A and DOC2B are sensors for neuronal activity with unique calcium-dependent and kinetic properties. J Neurochem 2006; 97:818-33. [PMID: 16515538 DOI: 10.1111/j.1471-4159.2006.03755.x] [Citation(s) in RCA: 78] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Elevation of the intracellular calcium concentration ([Ca2+]i) to levels below 1 microm alters synaptic transmission and induces short-term plasticity. To identify calcium sensors involved in this signalling, we investigated soluble C2 domain-containing proteins and found that both DOC2A and DOC2B are modulated by submicromolar calcium levels. Fluorescent-tagged DOC2A and DOC2B translocated to plasma membranes after [Ca2+]i elevation. DOC2B translocation preceded DOC2A translocation in cells co-expressing both isoforms. Half-maximal translocation occurred at 450 and 175 nm[Ca2+]i for DOC2A and DOC2B, respectively. This large difference in calcium sensitivity was accompanied by a modest kinetic difference (halftimes, respectively, 2.6 and 2.0 s). The calcium sensitivity of DOC2 isoforms can be explained by predicted topologies of their C2A domains. Consistently, neutralization of aspartates D218 and D220 in DOC2B changed its calcium affinity. In neurones, both DOC2 isoforms were reversibly recruited to the plasma membrane during trains of action potentials. Consistent with its higher calcium sensitivity, DOC2B translocated at lower depolarization frequencies. Styryl dye uptake experiments in hippocampal neurones suggest that the overexpression of mutated DOC2B alters the synaptic activity. We conclude that both DOC2A and DOC2B are regulated by neuronal activity, and hypothesize that their calcium-dependent translocation may regulate synaptic activity.
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Affiliation(s)
- Alexander J A Groffen
- Department of Functional Genomics, Center for Neurogenomics and Cognition Research, Vrije Universiteit (VU) and VU Medical Centre, Amsterdam, the Netherlands.
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Lambrecht NWG, Yakubov I, Zer C, Sachs G. Transcriptomes of purified gastric ECL and parietal cells: identification of a novel pathway regulating acid secretion. Physiol Genomics 2006; 25:153-65. [PMID: 16403840 DOI: 10.1152/physiolgenomics.00271.2005] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The gastric entero-chromaffin-like (ECL) cell plays a key regulatory role in peripheral regulation of acid secretion due to the release of histamine that stimulates acid secretion by the parietal cell. Studies in intact animals, gastric glands, and isolated cells after short-term culture have shown expression of stimulatory CCK2 and PAC1 and inhibitory SST2 and Gal1 receptors as well as histidine decarboxylase. However, the pattern of its gene expression as a neuroendocrine cell has not been explored. Comparison of gene expression by 95% pure ECL cells obtained by density gradient, elutriation, and fluorescence-assisted cell sorting with isolates of the intact fundic gastric epithelium (i.e., "subtractive hybridization") identified a variety of additional expressed gene families characteristic of this neuroendocrine cell. These include genes 1) involved in neuropeptide synthesis and secretory vesicle exocytosis, 2) involved in control of inflammation, 3) implicated in healing of the epithelium, 4) encoding inhibitory Gi protein-coupled receptors, 5) playing a role in neuroendocrine regulation of food intake, and 6) encoding proteins likely involved in maintenance of circadian rhythm, in addition to the ECL cell-specific genes histidine decarboxylase and monoamine transporter. Particularly, the inhibitory apelin receptor gene, APJ, was highly expressed in the ECL cell preparation. Because parietal cells express apelin, immunohistochemical and functional studies showed that there is an inhibitory feed back loop between the parietal and ECL cell during gastrin stimulation, providing evidence for a novel pathway of downregulation of acid secretion due to interaction between these two cell types.
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Affiliation(s)
- Nils W G Lambrecht
- Department of Pathology, David Geffen School of Medicine, University of California Los Angeles, California, USA.
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39
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Malkinson G, Spira ME. Calcium concentration threshold and translocation kinetics of EGFP-DOC2B expressed in cultured Aplysia neurons. Cell Calcium 2006; 39:85-93. [PMID: 16305808 DOI: 10.1016/j.ceca.2005.10.007] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2005] [Revised: 10/10/2005] [Accepted: 10/17/2005] [Indexed: 11/20/2022]
Abstract
The double C2 domain protein family (DOC2) is characterized by two calcium-binding domains (C2). Upon binding to calcium, the affinity of the protein to phospholipids is significantly increased, leading to translocation of the protein from the cytosol to the plasma membrane. These properties, and the binding domain of DOC2B to Munc13, suggested that DOC2B could play a role in augmentation and potentiation of synaptic release. Nevertheless, the level of the free intracellular calcium concentration ([Ca(2+)](i)) which triggers its translocation under in vivo conditions, is not known. Using cultured Aplysia neurons that express rat EGFP-DOC2B, we found that the [Ca(2+)](i) increment necessary to induce EGFP-DOC2B translocation is approximately 200 nM in the bulk of the cytoplasm. The rate of EGFP-DOC2B recruitment to the plasma membrane is slower than the [Ca(2+)](i) elevation rate, while the detachment of EGFP-DOC2B from it is faster than the calcium removal. The extent of EGFP-DOC2B translocation to the plasma membrane reflects local submembrane [Ca(2+)](i). Our observations are consistent with the view that DOC2B can participate in the regulation of neurotransmitter release. It should be noted that EGFP-DOC2B could be used as a tool to map sub-membrane calcium dynamics under physiological conditions.
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Affiliation(s)
- Guy Malkinson
- Department of Neurobiology, The Life Sciences Institute, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
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