1
|
Bose D, Bera M, Norman CA, Timofeeva Y, Volynski KE, Krishnakumar SS. A minimal presynaptic protein machinery mediating synchronous and asynchronous exocytosis and short-term plasticity. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.04.15.589559. [PMID: 38659918 PMCID: PMC11042279 DOI: 10.1101/2024.04.15.589559] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/26/2024]
Abstract
Neurotransmitters are released from synaptic vesicles with remarkable precision in response to presynaptic Ca2+ influx but exhibit significant heterogeneity in exocytosis timing and efficacy based on the recent history of activity. This heterogeneity is critical for information transfer in the brain, yet its molecular basis remains poorly understood. Here, we employ a biochemically-defined fusion assay under physiologically-relevant conditions to delineate the minimal protein machinery sufficient to account for different modes of Ca2+-triggered vesicle fusion and short-term facilitation. We find that Synaptotagmin-1, Synaptotagmin-7, and Complexin, synergistically restrain SNARE complex assembly, thus preserving vesicles in a stably docked state at rest. Upon Ca2+ activation, Synaptotagmin-1 induces rapid vesicle fusion, while Synaptotagmin-7 mediates delayed fusion. Competitive binding of Synaptotagmin-1 and Synaptotagmin-7 to the same SNAREs, coupled with differential rates of Ca2+-triggered fusion clamp reversal, govern the kinetics of vesicular fusion. Under conditions mimicking sustained neuronal activity, the Synaptotagmin-7 fusion clamp is destabilized by the elevated basal Ca2+ concentration, thereby enhancing the synchronous component of fusion. These findings provide a direct demonstration that a small set of proteins is sufficient to account for how nerve terminals adapt and regulate the Ca2+-evoked neurotransmitter exocytosis process to support their specialized functions in the nervous system.
Collapse
Affiliation(s)
- Dipayan Bose
- Yale Nanobiology Institute, Yale University School of Medicine, New Haven, USA
- Department of Neurology, Yale University School of Medicine, New Haven, USA
| | - Manindra Bera
- Yale Nanobiology Institute, Yale University School of Medicine, New Haven, USA
- Cell Biology, Yale University School of Medicine, New Haven, USA
| | - Chris A Norman
- Department of Clinical and Experimental Epilepsy, UCL Queen Square Institute of Neurology, University College London, UK
- Department of Computer Science, University of Warwick, Coventry, UK
| | - Yulia Timofeeva
- Department of Computer Science, University of Warwick, Coventry, UK
| | - Kirill E Volynski
- Cell Biology, Yale University School of Medicine, New Haven, USA
- Department of Clinical and Experimental Epilepsy, UCL Queen Square Institute of Neurology, University College London, UK
| | - Shyam S Krishnakumar
- Yale Nanobiology Institute, Yale University School of Medicine, New Haven, USA
- Department of Neurology, Yale University School of Medicine, New Haven, USA
- Department of Clinical and Experimental Epilepsy, UCL Queen Square Institute of Neurology, University College London, UK
| |
Collapse
|
2
|
Wu Z, Kusick GF, Berns MMM, Raychaudhuri S, Itoh K, Walter AM, Chapman ER, Watanabe S. Synaptotagmin 7 docks synaptic vesicles to support facilitation and Doc2α-triggered asynchronous release. eLife 2024; 12:RP90632. [PMID: 38536730 PMCID: PMC10972563 DOI: 10.7554/elife.90632] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/11/2024] Open
Abstract
Despite decades of intense study, the molecular basis of asynchronous neurotransmitter release remains enigmatic. Synaptotagmin (syt) 7 and Doc2 have both been proposed as Ca2+ sensors that trigger this mode of exocytosis, but conflicting findings have led to controversy. Here, we demonstrate that at excitatory mouse hippocampal synapses, Doc2α is the major Ca2+ sensor for asynchronous release, while syt7 supports this process through activity-dependent docking of synaptic vesicles. In synapses lacking Doc2α, asynchronous release after single action potentials is strongly reduced, while deleting syt7 has no effect. However, in the absence of syt7, docked vesicles cannot be replenished on millisecond timescales. Consequently, both synchronous and asynchronous release depress from the second pulse onward during repetitive activity. By contrast, synapses lacking Doc2α have normal activity-dependent docking, but continue to exhibit decreased asynchronous release after multiple stimuli. Moreover, disruption of both Ca2+ sensors is non-additive. These findings result in a new model whereby syt7 drives activity-dependent docking, thus providing synaptic vesicles for synchronous (syt1) and asynchronous (Doc2 and other unidentified sensors) release during ongoing transmission.
Collapse
Affiliation(s)
- Zhenyong Wu
- Department of Neuroscience, University of Wisconsin-MadisonMadisonUnited States
- Howard Hughes Medical InstituteMadisonUnited States
| | - Grant F Kusick
- Department of Cell Biology, Johns Hopkins University, School of MedicineBaltimoreUnited States
- Biochemistry, Cellular and Molecular Biology Graduate Program, Johns Hopkins University School of MedicineBaltimoreUnited States
| | - Manon MM Berns
- Department of Neuroscience, University of CopenhagenCopenhagenDenmark
| | - Sumana Raychaudhuri
- Department of Cell Biology, Johns Hopkins University, School of MedicineBaltimoreUnited States
| | - Kie Itoh
- Department of Cell Biology, Johns Hopkins University, School of MedicineBaltimoreUnited States
| | - Alexander M Walter
- Department of Neuroscience, University of CopenhagenCopenhagenDenmark
- Molecular and Theoretical Neuroscience, Leibniz-Institut für Molekulare Pharmakologie, FMP im CharitéCrossOverBerlinGermany
| | - Edwin R Chapman
- Department of Neuroscience, University of Wisconsin-MadisonMadisonUnited States
- Howard Hughes Medical InstituteMadisonUnited States
| | - Shigeki Watanabe
- Department of Cell Biology, Johns Hopkins University, School of MedicineBaltimoreUnited States
- Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of MedicineBaltimoreUnited States
| |
Collapse
|
3
|
Beaven AH, Bikkumalla V, Chon NL, Matthews AE, Lin H, Knight JD, Sodt AJ. Synaptotagmin 7 C2 domains induce membrane curvature stress via electrostatic interactions and the wedge mechanism. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.01.10.575084. [PMID: 38313280 PMCID: PMC10837831 DOI: 10.1101/2024.01.10.575084] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 02/06/2024]
Abstract
Synaptotagmin 7 (Syt-7) is part of the synaptotagmin protein family that regulates exocytotic lipid membrane fusion. Among the family, Syt-7 stands out by its membrane binding strength and stabilization of long-lived membrane fusion pores. Given that Syt-7 vesicles form long-lived fusion pores, we hypothesize that its interactions with the membrane stabilize the specific curvatures, thicknesses, and lipid compositions that support a metastable fusion pore. Using all-atom molecular dynamics simulations and FRET-based assays of Syt-7's membrane-binding C2 domains (C2A and C2B), we found that Syt-7 C2 domains sequester anionic lipids, are sensitive to cholesterol, thin membranes, and generate lipid membrane curvature by two competing, but related mechanisms. First, Syt-7 forms strong electrostatic contacts with the membrane, generating negative curvature stress. Second, Syt-7's calcium binding loops embed in the membrane surface, acting as a wedge to thin the membrane and induce positive curvature stress. These curvature mechanisms are linked by the protein insertion depth as well as the resulting protein tilt. Simplified quantitative models of the curvature-generating mechanisms link simulation observables to their membrane-reshaping effectiveness.
Collapse
Affiliation(s)
- Andrew H. Beaven
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD
- Postdoctoral Research Associate Program, National Institute of General Medical Sciences, National Institutes of Health, Bethesda, MD
| | | | - Nara L. Chon
- Department of Chemistry, University of Colorado Denver, Denver, CO
| | | | - Hai Lin
- Department of Chemistry, University of Colorado Denver, Denver, CO
| | | | - Alexander J. Sodt
- Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD
| |
Collapse
|
4
|
Courtney KC, Mandal T, Mehta N, Wu L, Li Y, Das D, Cui Q, Chapman ER. Synaptotagmin-7 outperforms synaptotagmin-1 to promote the formation of large, stable fusion pores via robust membrane penetration. Nat Commun 2023; 14:7761. [PMID: 38012142 PMCID: PMC10681989 DOI: 10.1038/s41467-023-42497-8] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2023] [Accepted: 10/11/2023] [Indexed: 11/29/2023] Open
Abstract
Synaptotagmin-1 and synaptotagmin-7 are two prominent calcium sensors that regulate exocytosis in neuronal and neuroendocrine cells. Upon binding calcium, both proteins partially penetrate lipid bilayers that bear anionic phospholipids, but the specific underlying mechanisms that enable them to trigger exocytosis remain controversial. Here, we examine the biophysical properties of these two synaptotagmin isoforms and compare their interactions with phospholipid membranes. We discover that synaptotagmin-1-membrane interactions are greatly influenced by membrane order; tight packing of phosphatidylserine inhibits binding due to impaired membrane penetration. In contrast, synaptotagmin-7 exhibits robust membrane binding and penetration activity regardless of phospholipid acyl chain structure. Thus, synaptotagmin-7 is a super-penetrator. We exploit these observations to specifically isolate and examine the role of membrane penetration in synaptotagmin function. Using nanodisc-black lipid membrane electrophysiology, we demonstrate that membrane penetration is a critical component that underlies how synaptotagmin proteins regulate reconstituted, exocytic fusion pores in response to calcium.
Collapse
Affiliation(s)
- Kevin C Courtney
- Howard Hughes Medical Institute and the Department of Neuroscience, University of Wisconsin, 1111 Highland Avenue, Madison, WI, 53705, USA
- Department of Biochemistry and Molecular Medicine, West Virginia University, Morgantown, WV, 26506, USA
| | - Taraknath Mandal
- Department of Chemistry, Boston University, Boston, MA, 02215, USA
- Department of Physics, Indian Institute of Technology - Kanpur, Kanpur, 208016, India
| | - Nikunj Mehta
- Howard Hughes Medical Institute and the Department of Neuroscience, University of Wisconsin, 1111 Highland Avenue, Madison, WI, 53705, USA
| | - Lanxi Wu
- Howard Hughes Medical Institute and the Department of Neuroscience, University of Wisconsin, 1111 Highland Avenue, Madison, WI, 53705, USA
| | - Yueqi Li
- Howard Hughes Medical Institute and the Department of Neuroscience, University of Wisconsin, 1111 Highland Avenue, Madison, WI, 53705, USA
- Center for Bioanalytical Chemistry, University of Science and Technology of China, Hefei, 230026, China
| | - Debasis Das
- Howard Hughes Medical Institute and the Department of Neuroscience, University of Wisconsin, 1111 Highland Avenue, Madison, WI, 53705, USA
- Department of Biological Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Navy Nagar, Colaba, Mumbai, 400005, India
| | - Qiang Cui
- Department of Chemistry, Boston University, Boston, MA, 02215, USA
| | - Edwin R Chapman
- Howard Hughes Medical Institute and the Department of Neuroscience, University of Wisconsin, 1111 Highland Avenue, Madison, WI, 53705, USA.
| |
Collapse
|
5
|
Norman CA, Krishnakumar SS, Timofeeva Y, Volynski KE. The release of inhibition model reproduces kinetics and plasticity of neurotransmitter release in central synapses. Commun Biol 2023; 6:1091. [PMID: 37891212 PMCID: PMC10611806 DOI: 10.1038/s42003-023-05445-2] [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: 04/12/2023] [Accepted: 10/11/2023] [Indexed: 10/29/2023] Open
Abstract
Calcium-evoked release of neurotransmitters from synaptic vesicles (SVs) is catalysed by SNARE proteins. The predominant view is that, at rest, complete assembly of SNARE complexes is inhibited ('clamped') by synaptotagmin and complexin molecules. Calcium binding by synaptotagmins releases this fusion clamp and triggers fast SV exocytosis. However, this model has not been quantitatively tested over physiological timescales. Here we describe an experimentally constrained computational modelling framework to quantitatively assess how the molecular architecture of the fusion clamp affects SV exocytosis. Our results argue that the 'release-of-inhibition' model can indeed account for fast calcium-activated SV fusion, and that dual binding of synaptotagmin-1 and synaptotagmin-7 to the same SNARE complex enables synergistic regulation of the kinetics and plasticity of neurotransmitter release. The developed framework provides a powerful and adaptable tool to link the molecular biochemistry of presynaptic proteins to physiological data and efficiently test the plausibility of calcium-activated neurotransmitter release models.
Collapse
Affiliation(s)
- Christopher A Norman
- University College London Institute of Neurology, University College London, London, WC1N 3BG, UK
- Department of Computer Science, University of Warwick, Coventry, CV4 7AL, UK
- Mathematics for Real-World Systems Centre for Doctoral Training, University of Warwick, Coventry, CV4 7AL, UK
| | - Shyam S Krishnakumar
- University College London Institute of Neurology, University College London, London, WC1N 3BG, UK.
- Department of Neurology, Yale Nanobiology Institute, Yale University School of Medicine, New Haven, CT, 06510, USA.
| | - Yulia Timofeeva
- University College London Institute of Neurology, University College London, London, WC1N 3BG, UK.
- Department of Computer Science, University of Warwick, Coventry, CV4 7AL, UK.
| | - Kirill E Volynski
- University College London Institute of Neurology, University College London, London, WC1N 3BG, UK.
- Department of Cell Biology, Yale University School of Medicine, New Haven, CT, 06510, USA.
| |
Collapse
|
6
|
Hikima T, Witkovsky P, Khatri L, Chao MV, Rice ME. Synaptotagmins 1 and 7 Play Complementary Roles in Somatodendritic Dopamine Release. J Neurosci 2022; 42:3919-3930. [PMID: 35361702 PMCID: PMC9097777 DOI: 10.1523/jneurosci.2416-21.2022] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2021] [Revised: 02/18/2022] [Accepted: 03/18/2022] [Indexed: 11/21/2022] Open
Abstract
The molecular mechanisms underlying somatodendritic dopamine (DA) release remain unresolved, despite the passing of decades since its discovery. Our previous work showed robust release of somatodendritic DA in submillimolar extracellular Ca2+ concentration ([Ca2+]o). Here we tested the hypothesis that the high-affinity Ca2+ sensor synaptotagmin 7 (Syt7), is a key determinant of somatodendritic DA release and its Ca2+ dependence. Somatodendritic DA release from SNc DA neurons was assessed using whole-cell recording in midbrain slices from male and female mice to monitor evoked DA-dependent D2 receptor-mediated inhibitory currents (D2ICs). Single-cell application of an antibody to Syt7 (Syt7 Ab) decreased pulse train-evoked D2ICs, revealing a functional role for Syt7. The assessment of the Ca2+ dependence of pulse train-evoked D2ICs confirmed robust DA release in submillimolar [Ca2+]o in wild-type (WT) neurons, but loss of this sensitivity with intracellular Syt7 Ab or in Syt7 knock-out (KO) mice. In millimolar [Ca2+]o, pulse train-evoked D2ICs in Syt7 KOs showed a greater reduction in decreased [Ca2+]o than seen in WT mice; the effect on single pulse-evoked DA release, however, did not differ between genotypes. Single-cell application of a Syt1 Ab had no effect on train-evoked D2ICs in WT SNc DA neurons, but did cause a decrease in D2IC amplitude in Syt7 KOs, indicating a functional substitution of Syt1 for Syt7. In addition, Syt1 Ab decreased single pulse-evoked D2ICs in WT cells, indicating the involvement of Syt1 in tonic DA release. Thus, Syt7 and Syt1 play complementary roles in somatodendritic DA release from SNc DA neurons.SIGNIFICANCE STATEMENT The respective Ca2+ dependence of somatodendritic and axonal dopamine (DA) release differs, resulting in the persistence of somatodendritic DA release in submillimolar Ca2+ concentrations too low to support axonal release. We demonstrate that synaptotagmin7 (Syt7), a high-affinity Ca2+ sensor, underlies phasic somatodendritic DA release and its Ca2+ sensitivity in the substantia nigra pars compacta. In contrast, we found that synaptotagmin 1 (Syt1), the Ca2+ sensor underlying axonal DA release, plays a role in tonic, but not phasic, somatodendritic DA release in wild-type mice. However, Syt1 can facilitate phasic DA release after Syt7 deletion. Thus, we show that both Syt1 and Syt7 act as Ca2+ sensors subserving different aspects of somatodendritic DA release processes.
Collapse
Affiliation(s)
- Takuya Hikima
- Department of Neurosurgery, New York University Grossman School of Medicine, New York, New York 10016
| | - Paul Witkovsky
- Department of Neurosurgery, New York University Grossman School of Medicine, New York, New York 10016
| | - Latika Khatri
- Department of Cell Biology, New York University Grossman School of Medicine, New York, New York 10016
| | - Moses V Chao
- Department of Cell Biology, New York University Grossman School of Medicine, New York, New York 10016
- Department of Psychiatry, New York University Grossman School of Medicine, New York, New York 10016
- Department of Neuroscience & Physiology, New York University Grossman School of Medicine, New York, New York 10016
| | - Margaret E Rice
- Department of Neurosurgery, New York University Grossman School of Medicine, New York, New York 10016
- Department of Neuroscience & Physiology, New York University Grossman School of Medicine, New York, New York 10016
| |
Collapse
|
7
|
Alnaas AA, Watson-Siriboe A, Tran S, Negussie M, Henderson JA, Osterberg JR, Chon NL, Harrott BM, Oviedo J, Lyakhova T, Michel C, Reisdorph N, Reisdorph R, Shearn CT, Lin H, Knight JD. Multivalent lipid targeting by the calcium-independent C2A domain of synaptotagmin-like protein 4/granuphilin. J Biol Chem 2020; 296:100159. [PMID: 33277360 PMCID: PMC7857503 DOI: 10.1074/jbc.ra120.014618] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2020] [Revised: 11/03/2020] [Accepted: 12/04/2020] [Indexed: 12/15/2022] Open
Abstract
Synaptotagmin-like protein 4 (Slp-4), also known as granuphilin, is a Rab effector responsible for docking secretory vesicles to the plasma membrane before exocytosis. Slp-4 binds vesicular Rab proteins via an N-terminal Slp homology domain, interacts with plasma membrane SNARE complex proteins via a central linker region, and contains tandem C-terminal C2 domains (C2A and C2B) with affinity for phosphatidylinositol-(4,5)-bisphosphate (PIP2). The Slp-4 C2A domain binds with low nanomolar apparent affinity to PIP2 in lipid vesicles that also contain background anionic lipids such as phosphatidylserine but much weaker when either the background anionic lipids or PIP2 is removed. Through computational and experimental approaches, we show that this high-affinity membrane binding arises from concerted interaction at multiple sites on the C2A domain. In addition to a conserved PIP2-selective lysine cluster, a larger cationic surface surrounding the cluster contributes substantially to the affinity for physiologically relevant lipid compositions. Although the K398A mutation in the lysine cluster blocks PIP2 binding, this mutated protein domain retains the ability to bind physiological membranes in both a liposome-binding assay and MIN6 cells. Molecular dynamics simulations indicate several conformationally flexible loops that contribute to the nonspecific cationic surface. We also identify and characterize a covalently modified variant that arises through reactivity of the PIP2-binding lysine cluster with endogenous bacterial compounds and binds weakly to membranes. Overall, multivalent lipid binding by the Slp-4 C2A domain provides selective recognition and high-affinity docking of large dense core secretory vesicles to the plasma membrane.
Collapse
Affiliation(s)
- Aml A Alnaas
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | | | - Sherleen Tran
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | - Mikias Negussie
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | - Jack A Henderson
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | - J Ryan Osterberg
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | - Nara L Chon
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | - Beckston M Harrott
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | - Julianna Oviedo
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | - Tatyana Lyakhova
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA
| | - Cole Michel
- Department of Pharmaceutical Sciences, School of Medicine, University of Colorado Anschutz Medical Campus, Aurora, Colorado, USA
| | - Nichole Reisdorph
- Department of Pharmaceutical Sciences, School of Medicine, University of Colorado Anschutz Medical Campus, Aurora, Colorado, USA
| | - Richard Reisdorph
- Department of Pharmaceutical Sciences, School of Medicine, University of Colorado Anschutz Medical Campus, Aurora, Colorado, USA
| | - Colin T Shearn
- Department of Pediatrics, Division of Pediatric Gastroenterology, Hepatology and Nutrition, School of Medicine, University of Colorado Anschutz Medical Campus, Aurora, Colorado, USA
| | - Hai Lin
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA.
| | - Jefferson D Knight
- Department of Chemistry, University of Colorado Denver, Denver, Colorado, USA.
| |
Collapse
|
8
|
Kobbersmed JR, Grasskamp AT, Jusyte M, Böhme MA, Ditlevsen S, Sørensen JB, Walter AM. Rapid regulation of vesicle priming explains synaptic facilitation despite heterogeneous vesicle:Ca 2+ channel distances. eLife 2020; 9:51032. [PMID: 32077852 PMCID: PMC7145420 DOI: 10.7554/elife.51032] [Citation(s) in RCA: 23] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2019] [Accepted: 02/14/2020] [Indexed: 12/27/2022] Open
Abstract
Chemical synaptic transmission relies on the Ca2+-induced fusion of transmitter-laden vesicles whose coupling distance to Ca2+ channels determines synaptic release probability and short-term plasticity, the facilitation or depression of repetitive responses. Here, using electron- and super-resolution microscopy at the Drosophila neuromuscular junction we quantitatively map vesicle:Ca2+ channel coupling distances. These are very heterogeneous, resulting in a broad spectrum of vesicular release probabilities within synapses. Stochastic simulations of transmitter release from vesicles placed according to this distribution revealed strong constraints on short-term plasticity; particularly facilitation was difficult to achieve. We show that postulated facilitation mechanisms operating via activity-dependent changes of vesicular release probability (e.g. by a facilitation fusion sensor) generate too little facilitation and too much variance. In contrast, Ca2+-dependent mechanisms rapidly increasing the number of releasable vesicles reliably reproduce short-term plasticity and variance of synaptic responses. We propose activity-dependent inhibition of vesicle un-priming or release site activation as novel facilitation mechanisms. Cells in the nervous system of all animals communicate by releasing and sensing chemicals at contact points named synapses. The ‘talking’ (or pre-synaptic) cell stores the chemicals close to the synapse, in small spheres called vesicles. When the cell is activated, calcium ions flow in and interact with the release-ready vesicles, which then spill the chemicals into the synapse. In turn, the ‘listening’ (or post-synaptic) cell can detect the chemicals and react accordingly. When the pre-synaptic cell is activated many times in a short period, it can release a greater quantity of chemicals, allowing a bigger reaction in the post-synaptic cell. This phenomenon is known as facilitation, but it is still unclear how exactly it can take place. This is especially the case when many of the vesicles are not ready to respond, for example when they are too far from where calcium flows into the cell. Computer simulations have been created to model facilitation but they have assumed that all vesicles are placed at the same distance to the calcium entry point: Kobbersmed et al. now provide evidence that this assumption is incorrect. Two high-resolution imaging techniques were used to measure the actual distances between the vesicles and the calcium source in the pre-synaptic cells of fruit flies: this showed that these distances are quite variable – some vesicles sit much closer to the source than others. This information was then used to create a new computer model to simulate facilitation. The results from this computing work led Kobbersmed et al. to suggest that facilitation may take place because a calcium-based mechanism in the cell increases the number of vesicles ready to release their chemicals. This new model may help researchers to better understand how the cells in the nervous system work. Ultimately, this can guide experiments to investigate what happens when information processing at synapses breaks down, for example in diseases such as epilepsy.
Collapse
Affiliation(s)
- Janus Rl Kobbersmed
- Department of Mathematical Sciences, University of Copenhagen, København, Denmark.,Department of Neuroscience, University of Copenhagen, København, Denmark
| | - Andreas T Grasskamp
- Molecular and Theoretical Neuroscience, Leibniz-Forschungsinstitut für Molekulare Pharmakologie, FMP im CharitéCrossOver, Berlin, Germany
| | - Meida Jusyte
- Molecular and Theoretical Neuroscience, Leibniz-Forschungsinstitut für Molekulare Pharmakologie, FMP im CharitéCrossOver, Berlin, Germany.,Einstein Center for Neuroscience, Berlin, Germany
| | - Mathias A Böhme
- Molecular and Theoretical Neuroscience, Leibniz-Forschungsinstitut für Molekulare Pharmakologie, FMP im CharitéCrossOver, Berlin, Germany
| | - Susanne Ditlevsen
- Department of Mathematical Sciences, University of Copenhagen, København, Denmark
| | | | - Alexander M Walter
- Molecular and Theoretical Neuroscience, Leibniz-Forschungsinstitut für Molekulare Pharmakologie, FMP im CharitéCrossOver, Berlin, Germany.,Einstein Center for Neuroscience, Berlin, Germany
| |
Collapse
|
9
|
Synaptotagmin 7 Mediates Both Facilitation and Asynchronous Release at Granule Cell Synapses. J Neurosci 2019; 38:3240-3251. [PMID: 29593071 DOI: 10.1523/jneurosci.3207-17.2018] [Citation(s) in RCA: 54] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2017] [Revised: 02/08/2018] [Accepted: 02/13/2018] [Indexed: 01/17/2023] Open
Abstract
When an action potential invades a presynaptic terminal it evokes large, brief Ca2+ signals that trigger vesicle fusion within milliseconds that is followed by a small residual Ca2+ (Cares) signal. At many synapses Cares produces synaptic facilitation that lasts up to hundreds of milliseconds and, although less common, Cares can also evoke asynchronous release (AR) that persists for tens of milliseconds. The properties of facilitation and AR are very different, which suggests that they are mediated by distinct mechanisms. However, recently it has been shown that the slow calcium sensor synaptotagmin 7 (Syt7) mediates facilitation at many synapses where AR does not occur, and conversely Syt7 can mediate AR without mediating facilitation. Here we study cerebellar granule cell synapses onto stellate cells and Purkinje cells in mice of both sexes to assess the role of Syt7 in these phenomena at the same synapse. This is of particular interest at granule cell synapses where AR is much more calcium dependent and shorter-lived than facilitation. We find that Syt7 can mediate these two processes despite their divergent properties. In Syt7 knock-out animals, facilitation and AR are smaller and shorter lived than in wild-type animals, even though the initial probability of release and Cares signals are unchanged. Although there are short-lived Syt7-independent mechanisms that mediate facilitation and AR in Syt7 KO animals, we find that at granule cell synapses AR and facilitation are both mediated primarily by Syt7.SIGNIFICANCE STATEMENT At synapses made by cerebellar granule cells, presynaptic activity elevates calcium for tens of milliseconds, which in turn evokes both asynchronous release (AR) and synaptic facilitation. AR is more calcium sensitive and shorter-lived than facilitation at these synapses, suggesting that they are mediated by different mechanisms. However, we find that the slow calcium sensor synaptotagmin 7 mediates both of these phenomena. Small, rapidly decaying components of AR and facilitation are present in Syt7 KO animals, indicating that additional mechanisms can contribute to both AR and facilitation at these synapses.
Collapse
|
10
|
Harsini FM, Bui AA, Rice AM, Chebrolu S, Fuson KL, Turtoi A, Bradberry M, Chapman ER, Sutton RB. Structural Basis for the Distinct Membrane Binding Activity of the Homologous C2A Domains of Myoferlin and Dysferlin. J Mol Biol 2019; 431:2112-2126. [PMID: 31004665 DOI: 10.1016/j.jmb.2019.04.006] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/01/2019] [Revised: 04/04/2019] [Accepted: 04/04/2019] [Indexed: 02/03/2023]
Abstract
Dysferlin has been implicated in acute membrane repair processes, whereas myoferlin's activity is maximal during the myoblast fusion stage of early skeletal muscle cell development. Both proteins are similar in size and domain structure; however, despite the overall similarity, myoferlin's known physiological functions do not overlap with those of dysferlin. Here we present for the first time the X-ray crystal structure of human myoferlin C2A to 1.9 Å resolution bound to two divalent cations, and compare its three-dimensional structure and membrane binding activities to that of dysferlin C2A. We find that while dysferlin C2A binds membranes in a Ca2+-dependent manner, Ca2+ binding was the rate-limiting kinetic step for this interaction. Myoferlin C2A, on the other hand, binds two calcium ions with an affinity 3-fold lower than that of dysferlin C2A; and, surprisingly, myoferlin C2A binds only marginally to phospholipid mixtures with a high fraction of phosphatidylserine.
Collapse
Affiliation(s)
- Faraz M Harsini
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA; Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA
| | - Anthony A Bui
- Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, 79409, USA
| | - Anne M Rice
- Department of Biophysics, Johns Hopkins University, Baltimore, MD, 21205, USA
| | - Sukanya Chebrolu
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA
| | - Kerry L Fuson
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA
| | - Andrei Turtoi
- Tumor Microenvironment and Resistance to Treatment Lab, Institut de Recherche en Cancrologie de Montpellier, 34090 Montpellier, France; Institut du Cancer Montpellier, 34090 Montpellier, France; Universit Montpellier, 34298 Montpellier, France
| | - Mazdak Bradberry
- Department of Neuroscience, University of Wisconsin-Madison, Madison, WI, 53705, USA
| | - Edwin R Chapman
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI, 53705, USA; Department of Neuroscience, University of Wisconsin-Madison, Madison, WI, 53705, USA
| | - R Bryan Sutton
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA; Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA.
| |
Collapse
|
11
|
Tran HT, Anderson LH, Knight JD. Membrane-Binding Cooperativity and Coinsertion by C2AB Tandem Domains of Synaptotagmins 1 and 7. Biophys J 2019; 116:1025-1036. [PMID: 30795874 DOI: 10.1016/j.bpj.2019.01.035] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2018] [Revised: 12/21/2018] [Accepted: 01/30/2019] [Indexed: 02/04/2023] Open
Abstract
Synaptotagmin-1 (Syt-1) and synaptotagmin-7 (Syt-7) contain analogous tandem C2 domains, C2A and C2B, which together sense Ca2+ to bind membranes and promote the stabilization of exocytotic fusion pores. Syt-1 triggers fast release of neurotransmitters, whereas Syt-7 functions in processes that involve lower Ca2+ concentrations such as hormone secretion. Syt-1 C2 domains are reported to bind membranes cooperatively, based on the observation that they penetrate farther into membranes as the C2AB tandem than as individual C2 domains. In contrast, we previously suggested that the two C2 domains of Syt-7 bind membranes independently, based in part on measurements of their liposome dissociation kinetics. Here, we investigated C2A-C2B interdomain cooperativity with Syt-1 and Syt-7 using directly comparable measurements. Equilibrium Ca2+ titrations demonstrate that the Syt-7 C2AB tandem binds liposomes lacking phosphatidylinositol-4,5-bisphosphate (PIP2) with greater Ca2+ sensitivity than either of its individual domains and binds to membranes containing PIP2 even in the absence of Ca2+. Stopped-flow kinetic measurements show differences in cooperativity between Syt-1 and Syt-7: Syt-1 C2AB dissociates from PIP2-free liposomes much more slowly than either of its individual C2 domains, indicating cooperativity, whereas the major population of Syt-7 C2AB has a dissociation rate comparable to its C2A domain, suggesting a lack of cooperativity. A minor subpopulation of Syt-7 C2AB dissociates at a slower rate, which could be due to a small cooperative component and/or liposome clustering. Measurements using an environment-sensitive fluorescent probe indicate that the Syt-7 C2B domain inserts deeply into membranes as part of the C2AB tandem, similar to the coinsertion previously reported for Syt-1. Overall, coinsertion of C2A and C2B domains is coupled to cooperative energetic effects in Syt-1 to a much greater extent than in Syt-7. The difference can be understood in terms of the relative contributions of C2A and C2B domains toward membrane binding in the two proteins.
Collapse
Affiliation(s)
- Hai T Tran
- Department of Chemistry, University of Colorado Denver, Denver, Colorado
| | - Lauren H Anderson
- Department of Chemistry, University of Colorado Denver, Denver, Colorado
| | - Jefferson D Knight
- Department of Chemistry, University of Colorado Denver, Denver, Colorado.
| |
Collapse
|
12
|
Burke KJ, Keeshen CM, Bender KJ. Two Forms of Synaptic Depression Produced by Differential Neuromodulation of Presynaptic Calcium Channels. Neuron 2018; 99:969-984.e7. [PMID: 30122380 PMCID: PMC7874512 DOI: 10.1016/j.neuron.2018.07.030] [Citation(s) in RCA: 34] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2018] [Revised: 06/03/2018] [Accepted: 07/18/2018] [Indexed: 01/09/2023]
Abstract
Neuromodulators are important regulators of synaptic transmission throughout the brain. At the presynaptic terminal, neuromodulation of calcium channels (CaVs) can affect transmission not only by changing neurotransmitter release probability, but also by shaping short-term plasticity (STP). Indeed, changes in STP are often considered a requirement for defining a presynaptic site of action. Nevertheless, some synapses exhibit non-canonical forms of neuromodulation, where release probability is altered without a corresponding change in STP. Here, we identify biophysical mechanisms whereby both canonical and non-canonical presynaptic neuromodulation can occur at the same synapse. At a subset of glutamatergic terminals in prefrontal cortex, GABAB and D1/D5 dopamine receptors suppress release probability with and without canonical increases in short-term facilitation by modulating different aspects of presynaptic CaV function. These findings establish a framework whereby signaling from multiple neuromodulators can converge on presynaptic CaVs to differentially tune release dynamics at the same synapse.
Collapse
Affiliation(s)
- Kenneth J Burke
- Neuroscience Graduate Program, Department of Neurology, University of California, San Francisco, San Francisco, CA, USA
| | - Caroline M Keeshen
- Weill Institute for Neurosciences, Department of Neurology, University of California, San Francisco, San Francisco, CA, USA
| | - Kevin J Bender
- Neuroscience Graduate Program, Department of Neurology, University of California, San Francisco, San Francisco, CA, USA; Weill Institute for Neurosciences, Department of Neurology, University of California, San Francisco, San Francisco, CA, USA.
| |
Collapse
|
13
|
Synergistic control of neurotransmitter release by different members of the synaptotagmin family. Curr Opin Neurobiol 2018; 51:154-162. [DOI: 10.1016/j.conb.2018.05.006] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2018] [Revised: 05/19/2018] [Accepted: 05/21/2018] [Indexed: 01/06/2023]
|
14
|
MacDougall DD, Lin Z, Chon NL, Jackman SL, Lin H, Knight JD, Anantharam A. The high-affinity calcium sensor synaptotagmin-7 serves multiple roles in regulated exocytosis. J Gen Physiol 2018; 150:783-807. [PMID: 29794152 PMCID: PMC5987875 DOI: 10.1085/jgp.201711944] [Citation(s) in RCA: 34] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2017] [Accepted: 05/07/2018] [Indexed: 12/19/2022] Open
Abstract
MacDougall et al. review the structure and function of the calcium sensor synaptotagmin-7 in exocytosis. Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types. Here, we review the known and emerging roles of Syt-7 in various contexts and stress the importance of its actions. Unique functions of Syt-7 are discussed in light of recent imaging, electrophysiological, and computational studies. Particular emphasis is placed on Syt-7–dependent regulation of synaptic transmission and neuroendocrine cell secretion. Finally, based on biochemical and structural data, we propose a mechanism to link Syt-7’s role in membrane fusion with its role in subsequent fusion pore expansion via strong calcium-dependent phospholipid binding.
Collapse
Affiliation(s)
| | - Zesen Lin
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
| | - Nara L Chon
- Department of Chemistry, University of Colorado, Denver, CO
| | - Skyler L Jackman
- Vollum Institute, Oregon Health & Science University, Portland, OR
| | - Hai Lin
- Department of Chemistry, University of Colorado, Denver, CO
| | | | - Arun Anantharam
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
| |
Collapse
|
15
|
Bendahmane M, Bohannon KP, Bradberry MM, Rao TC, Schmidtke MW, Abbineni PS, Chon NL, Tran S, Lin H, Chapman ER, Knight JD, Anantharam A. The synaptotagmin C2B domain calcium-binding loops modulate the rate of fusion pore expansion. Mol Biol Cell 2018; 29:834-845. [PMID: 29444959 PMCID: PMC5905296 DOI: 10.1091/mbc.e17-11-0623] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2017] [Revised: 01/22/2018] [Accepted: 02/07/2018] [Indexed: 01/19/2023] Open
Abstract
In chromaffin cells, the kinetics of fusion pore expansion vary depending on which synaptotagmin isoform (Syt-1 or Syt-7) drives release. Our recent studies have shown that fusion pores of granules harboring Syt-1 expand more rapidly than those harboring Syt-7. Here we sought to define the structural specificity of synaptotagmin action at the fusion pore by manipulating the Ca2+-binding C2B module. We generated a chimeric Syt-1 in which its C2B Ca2+-binding loops had been exchanged for those of Syt-7. Fusion pores of granules harboring a Syt-1 C2B chimera with all three Ca2+-binding loops of Syt-7 (Syt-1:7C2B123) exhibited slower rates of fusion pore expansion and neuropeptide cargo release relative to WT Syt-1. After fusion, this chimera also dispersed more slowly from fusion sites than WT protein. We speculate that the Syt-1:7 C2B123 and WT Syt-1 are likely to differ in their interactions with Ca2+ and membranes. Subsequent in vitro and in silico data demonstrated that the chimera exhibits a higher affinity for phospholipids than WT Syt-1. We conclude that the affinity of synaptotagmin for the plasma membrane, and the rate at which it releases the membrane, contribute in important ways to the rate of fusion pore expansion.
Collapse
Affiliation(s)
- Mounir Bendahmane
- Department of Pharmacology, University of Michigan, Ann Arbor, MI 48109
| | - Kevin P. Bohannon
- Department of Pharmacology, University of Michigan, Ann Arbor, MI 48109
| | - Mazdak M. Bradberry
- Department of Neuroscience, Howard Hughes Medical Institute. University of Wisconsin–Madison, Madison, WI 53705
| | - Tejeshwar C. Rao
- Department of Pharmacology, University of Michigan, Ann Arbor, MI 48109
| | | | | | - Nara L. Chon
- Department of Chemistry, University of Colorado Denver, Denver, CO 80217
| | - Sherleen Tran
- Department of Chemistry, University of Colorado Denver, Denver, CO 80217
| | - Hai Lin
- Department of Chemistry, University of Colorado Denver, Denver, CO 80217
| | - Edwin R. Chapman
- Department of Neuroscience, Howard Hughes Medical Institute. University of Wisconsin–Madison, Madison, WI 53705
| | | | - Arun Anantharam
- Department of Pharmacology, University of Michigan, Ann Arbor, MI 48109
| |
Collapse
|
16
|
Sun L, Gao Y, Xu Y, Chao J, Liu H, Wang L, Li D, Fan C. Real-Time Imaging of Single-Molecule Enzyme Cascade Using a DNA Origami Raft. J Am Chem Soc 2017; 139:17525-17532. [DOI: 10.1021/jacs.7b09323] [Citation(s) in RCA: 77] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Affiliation(s)
- Lele Sun
- Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
- University of Chinese Academy of Sciences, Beijing 100049, China
| | - Yanjing Gao
- Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
- University of Chinese Academy of Sciences, Beijing 100049, China
| | - Yan Xu
- Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
- University of Chinese Academy of Sciences, Beijing 100049, China
| | - Jie Chao
- Key
Laboratory for Organic Electronics and Information Displays (KLOEID),
Institute of Advanced Materials (IAM), School of Materials Science
and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210046, China
| | - Huajie Liu
- Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
| | - Lianhui Wang
- Key
Laboratory for Organic Electronics and Information Displays (KLOEID),
Institute of Advanced Materials (IAM), School of Materials Science
and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210046, China
| | - Di Li
- Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
| | - Chunhai Fan
- Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
| |
Collapse
|
17
|
Synaptotagmin 7 confers frequency invariance onto specialized depressing synapses. Nature 2017; 551:503-506. [PMID: 29088700 PMCID: PMC5892411 DOI: 10.1038/nature24474] [Citation(s) in RCA: 52] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2017] [Accepted: 10/09/2017] [Indexed: 12/22/2022]
Abstract
At most synapses in the brain, short-term plasticity dynamically modulates synaptic strength. Rapid frequency-dependent changes in synaptic strength play critical roles in sensory adaptation, gain control and many other neural computations1,2. However, some auditory, vestibular and cerebellar synapses maintain constant strength over a wide range of firing frequencies3–5, and as a result efficiently encode firing rates. Despite its apparent simplicity, frequency-invariant transmission is difficult to achieve because of inherent synaptic nonlinearities6. Here we study frequency-invariant transmission at Purkinje cell to deep cerebellar nuclear (PC to DCN) synapses and vestibular synapses. Prolonged activation of these synapses leads to initial depression, which is followed by steady-state responses that are frequency invariant for their physiological activity range. We find that Synaptotagmin 7 (Syt7), a recently identified calcium sensor for short-term facilitation7, is present at both synapses. It was unclear why a sensor for facilitation would be present at these and other depressing synapses. We find that at PC and vestibular synapses, Syt7 supports a hidden component of facilitation that can be unmasked in wildtype animals but is absent in Syt7 knockout animals. In wildtype mice, facilitation increases with firing frequency and counteracts depression to produce frequency-invariant transmission. In Syt7 knockout mice, PC and vestibular synapses exhibit conventional use-dependent depression, weakening to a greater extent as the firing frequency is increased. Presynaptic rescue of Syt7 expression restores both facilitation and frequency-invariant transmission. Our results identify a function for Syt7 at synapses that exhibit overall depression, and demonstrate that facilitation plays an unexpected and important role in producing frequency-invariant transmission.
Collapse
|
18
|
Doussau F, Schmidt H, Dorgans K, Valera AM, Poulain B, Isope P. Frequency-dependent mobilization of heterogeneous pools of synaptic vesicles shapes presynaptic plasticity. eLife 2017; 6:28935. [PMID: 28990927 PMCID: PMC5648531 DOI: 10.7554/elife.28935] [Citation(s) in RCA: 30] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2017] [Accepted: 10/06/2017] [Indexed: 12/03/2022] Open
Abstract
The segregation of the readily releasable pool of synaptic vesicles (RRP) in sub-pools that are differentially poised for exocytosis shapes short-term plasticity. However, the frequency-dependent mobilization of these sub-pools is poorly understood. Using slice recordings and modeling of synaptic activity at cerebellar granule cell to Purkinje cell synapses of mice, we describe two sub-pools in the RRP that can be differentially recruited upon ultrafast changes in the stimulation frequency. We show that at low-frequency stimulations, a first sub-pool is gradually silenced, leading to full blockage of synaptic transmission. Conversely, a second pool of synaptic vesicles that cannot be released by a single stimulus is recruited within milliseconds by high-frequency stimulation and support an ultrafast recovery of neurotransmitter release after low-frequency depression. This frequency-dependent mobilization or silencing of sub-pools in the RRP in terminals of granule cells may play a role in the filtering of sensorimotor information in the cerebellum.
Collapse
Affiliation(s)
- Frédéric Doussau
- Institut des Neurosciences Cellulaires et Intégratives, CNRS, Université de Strasbourg, Strasbourg, France
| | - Hartmut Schmidt
- Carl-Ludwig Institute for Physiology, University of Leipzig, Leipzig, Germany
| | - Kevin Dorgans
- Institut des Neurosciences Cellulaires et Intégratives, CNRS, Université de Strasbourg, Strasbourg, France
| | - Antoine M Valera
- Institut des Neurosciences Cellulaires et Intégratives, CNRS, Université de Strasbourg, Strasbourg, France
| | - Bernard Poulain
- Institut des Neurosciences Cellulaires et Intégratives, CNRS, Université de Strasbourg, Strasbourg, France
| | - Philippe Isope
- Institut des Neurosciences Cellulaires et Intégratives, CNRS, Université de Strasbourg, Strasbourg, France
| |
Collapse
|
19
|
Exceptionally tight membrane-binding may explain the key role of the synaptotagmin-7 C 2A domain in asynchronous neurotransmitter release. Proc Natl Acad Sci U S A 2017; 114:E8518-E8527. [PMID: 28923929 DOI: 10.1073/pnas.1710708114] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Synaptotagmins (Syts) act as Ca2+ sensors in neurotransmitter release by virtue of Ca2+-binding to their two C2 domains, but their mechanisms of action remain unclear. Puzzlingly, Ca2+-binding to the C2B domain appears to dominate Syt1 function in synchronous release, whereas Ca2+-binding to the C2A domain mediates Syt7 function in asynchronous release. Here we show that crystal structures of the Syt7 C2A domain and C2AB region, and analyses of intrinsic Ca2+-binding to the Syt7 C2 domains using isothermal titration calorimetry, did not reveal major differences that could explain functional differentiation between Syt7 and Syt1. However, using liposome titrations under Ca2+ saturating conditions, we show that the Syt7 C2A domain has a very high membrane affinity and dominates phospholipid binding to Syt7 in the presence or absence of l-α-phosphatidylinositol 4,5-diphosphate (PIP2). For Syt1, the two Ca2+-saturated C2 domains have similar affinities for membranes lacking PIP2, but the C2B domain dominates binding to PIP2-containing membranes. Mutagenesis revealed that the dramatic differences in membrane affinity between the Syt1 and Syt7 C2A domains arise in part from apparently conservative residue substitutions, showing how striking biochemical and functional differences can result from the cumulative effects of subtle residue substitutions. Viewed together, our results suggest that membrane affinity may be a key determinant of the functions of Syt C2 domains in neurotransmitter release.
Collapse
|
20
|
Hamilton DJ, Coffman MD, Knight JD, Reed SM. Lipid-Coated Gold Nanoparticles and FRET Allow Sensitive Monitoring of Liposome Clustering Mediated by the Synaptotagmin-7 C2A Domain. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2017; 33:9222-9230. [PMID: 28850236 DOI: 10.1021/acs.langmuir.7b01397] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Synaptotagmin (Syt) family proteins contain tandem C2 domains, C2A and C2B, which insert into anionic membranes in response to increased cytosolic Ca2+ concentration and facilitate exocytosis in neuronal and endocrine cells. The C2A domain from Syt7 binds lipid membranes much more tightly than the corresponding domain from Syt1, but the implications of this difference for protein function are not yet clear. In particular, the ability of the isolated Syt7 C2A domain to initiate membrane apposition and/or aggregation has been previously unexplored. Here, we demonstrate that Syt7 C2A induces apposition and aggregation of liposomes using Förster resonance energy transfer (FRET) assays, dynamic light scattering, and spectroscopic techniques involving lipid-coated gold nanoparticles (LCAuNPs). Protein-membrane binding, membrane apposition, and macroscopic aggregation are three separate phenomena with distinct Ca2+ requirements: the threshold Ca2+ concentration for membrane binding is lowest, followed by apposition and aggregation. However, aggregation is highly sensitive to protein concentration and can occur even at submicromolar Syt7 C2A; thus, highly sensitive assays are needed for measuring apposition without complications arising from aggregation. Notably, the localized surface plasmon resonance of the LCAuNP is sensitive to ≤10 nM Syt7 C2A concentrations. Furthermore, when the LCAuNPs were added into a FRET-based liposome apposition assay, the resultant energy transfer increased; possible explanations are discussed. Overall, LCAuNP-based methods allow for highly sensitive detection of protein-induced membrane apposition under conditions that miminize large-scale aggregation.
Collapse
Affiliation(s)
- Desmond J Hamilton
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Matthew D Coffman
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Jefferson D Knight
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Scott M Reed
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| |
Collapse
|
21
|
Jackman SL, Regehr WG. The Mechanisms and Functions of Synaptic Facilitation. Neuron 2017; 94:447-464. [PMID: 28472650 DOI: 10.1016/j.neuron.2017.02.047] [Citation(s) in RCA: 223] [Impact Index Per Article: 31.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2016] [Revised: 02/23/2017] [Accepted: 02/28/2017] [Indexed: 12/22/2022]
Abstract
The ability of the brain to store and process information relies on changing the strength of connections between neurons. Synaptic facilitation is a form of short-term plasticity that enhances synaptic transmission for less than a second. Facilitation is a ubiquitous phenomenon thought to play critical roles in information transfer and neural processing. Yet our understanding of the function of facilitation remains largely theoretical. Here we review proposed roles for facilitation and discuss how recent progress in uncovering the underlying molecular mechanisms could enable experiments that elucidate how facilitation, and short-term plasticity in general, contributes to circuit function and animal behavior.
Collapse
Affiliation(s)
- Skyler L Jackman
- Department of Neurobiology, Harvard Medical School, Boston, MA 02118, USA
| | - Wade G Regehr
- Department of Neurobiology, Harvard Medical School, Boston, MA 02118, USA.
| |
Collapse
|
22
|
Vermaas JV, Tajkhorshid E. Differential Membrane Binding Mechanics of Synaptotagmin Isoforms Observed in Atomic Detail. Biochemistry 2016; 56:281-293. [PMID: 27997124 DOI: 10.1021/acs.biochem.6b00468] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Synaptotagmin (Syt) is a membrane-associated protein involved in vesicle fusion through the SNARE complex that is found throughout the human body in 17 different isoforms. These isoforms have two membrane-binding C2 domains, which sense Ca2+ and thereby promote anionic membrane binding and lead to vesicle fusion. Through molecular dynamics simulations using the highly mobile membrane mimetic acclerated bilayer model, we have investigated how small protein sequence changes in the Ca2+-binding loops of the C2 domains may give rise to the experimentally determined difference in binding kinetics between Syt-1 and Syt-7 isoforms. Syt-7 C2 domains are found to form more close contacts with anionic phospholipid headgroups, particularly in loop 1, where an additional positive charge in Syt-7 draws the loop closer to the membrane and causes the anchoring residue F167 to insert deeper into the bilayer than the corresponding methionine in Syt-1 (M173). By performing additional replica exchange umbrella sampling calculations, we demonstrate that these additional contacts increase the energetic cost of unbinding the Syt-7 C2 domains from the bilayer, causing them to unbind more slowly than their counterparts in Syt-1.
Collapse
Affiliation(s)
- Josh V Vermaas
- Center for Biophysics and Quantitative Biology, Department of Biochemistry, and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Emad Tajkhorshid
- Center for Biophysics and Quantitative Biology, Department of Biochemistry, and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| |
Collapse
|
23
|
Crawford DC, Kavalali ET. Molecular underpinnings of synaptic vesicle pool heterogeneity. Traffic 2015; 16:338-64. [PMID: 25620674 DOI: 10.1111/tra.12262] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2014] [Revised: 01/06/2015] [Indexed: 12/31/2022]
Abstract
Neuronal communication relies on chemical synaptic transmission for information transfer and processing. Chemical neurotransmission is initiated by synaptic vesicle fusion with the presynaptic active zone resulting in release of neurotransmitters. Classical models have assumed that all synaptic vesicles within a synapse have the same potential to fuse under different functional contexts. In this model, functional differences among synaptic vesicle populations are ascribed to their spatial distribution in the synapse with respect to the active zone. Emerging evidence suggests, however, that synaptic vesicles are not a homogenous population of organelles, and they possess intrinsic molecular differences and differential interaction partners. Recent studies have reported a diverse array of synaptic molecules that selectively regulate synaptic vesicles' ability to fuse synchronously and asynchronously in response to action potentials or spontaneously irrespective of action potentials. Here we discuss these molecular mediators of vesicle pool heterogeneity that are found on the synaptic vesicle membrane, on the presynaptic plasma membrane, or within the cytosol and consider some of the functional consequences of this diversity. This emerging molecular framework presents novel avenues to probe synaptic function and uncover how synaptic vesicle pools impact neuronal signaling.
Collapse
Affiliation(s)
- Devon C Crawford
- Department of Neuroscience, UT Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9111, USA
| | | |
Collapse
|
24
|
Chon NL, Osterberg JR, Henderson J, Khan HM, Reuter N, Knight JD, Lin H. Membrane Docking of the Synaptotagmin 7 C2A Domain: Computation Reveals Interplay between Electrostatic and Hydrophobic Contributions. Biochemistry 2015; 54:5696-711. [DOI: 10.1021/acs.biochem.5b00422] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Affiliation(s)
- Nara Lee Chon
- Department
of Chemistry, University of Colorado Denver, Denver, Colorado 80217-3364, United States
| | - J. Ryan Osterberg
- Department
of Chemistry, University of Colorado Denver, Denver, Colorado 80217-3364, United States
| | - Jack Henderson
- Department
of Chemistry, University of Colorado Denver, Denver, Colorado 80217-3364, United States
| | - Hanif M. Khan
- Department
of Molecular Biology, University of Bergen, 5008 Bergen, Norway
- Computational
Biology Unit, Department of Informatics, University of Bergen, 5008 Bergen, Norway
| | - Nathalie Reuter
- Department
of Molecular Biology, University of Bergen, 5008 Bergen, Norway
- Computational
Biology Unit, Department of Informatics, University of Bergen, 5008 Bergen, Norway
| | - Jefferson D. Knight
- Department
of Chemistry, University of Colorado Denver, Denver, Colorado 80217-3364, United States
| | - Hai Lin
- Department
of Chemistry, University of Colorado Denver, Denver, Colorado 80217-3364, United States
| |
Collapse
|
25
|
Osterberg JR, Chon NL, Boo A, Maynard FA, Lin H, Knight JD. Membrane Docking of the Synaptotagmin 7 C2A Domain: Electron Paramagnetic Resonance Measurements Show Contributions from Two Membrane Binding Loops. Biochemistry 2015; 54:5684-95. [PMID: 26322740 DOI: 10.1021/acs.biochem.5b00421] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
The synaptotagmin (Syt) family of proteins plays an important role in vesicle docking and fusion during Ca(2+)-induced exocytosis in a wide variety of cell types. Its role as a Ca(2+) sensor derives primarily from its two C2 domains, C2A and C2B, which insert into anionic lipid membranes upon binding Ca(2+). Syt isoforms 1 and 7 differ significantly in their Ca(2+) sensitivity; the C2A domain from Syt7 binds Ca(2+) and membranes much more tightly than the C2A domain from Syt1, at least in part because of greater contributions from the hydrophobic effect. While the structure and membrane activity of Syt1 have been extensively studied, the structural origins of differences between Syt1 and Syt7 are unknown. This study used site-directed spin labeling and electron paramagnetic resonance spectroscopy to determine depth parameters for the Syt7 C2A domain, for comparison to analogous previous measurements with the Syt1 C2A domain. In a novel approach, the membrane docking geometry of both Syt1 and Syt7 C2A was modeled by mapping depth parameters onto multiple molecular dynamics-simulated structures of the Ca(2+)-bound protein. The models reveal membrane penetration of Ca(2+) binding loops 1 (CBL1) and 3 (CBL3), and membrane binding is more sensitive to mutations in CBL3. On average, Syt7 C2A inserts more deeply into the membrane than Syt1 C2A, although depths vary among the different structural models. This observation provides a partial structural explanation for the hydrophobically driven membrane docking of Syt7 C2A.
Collapse
Affiliation(s)
- J Ryan Osterberg
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Nara Lee Chon
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Arthur Boo
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Favinn A Maynard
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Hai Lin
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | - Jefferson D Knight
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| |
Collapse
|
26
|
Vasquez JK, Chantranuvatana K, Giardina DT, Coffman MD, Knight JD. Lateral diffusion of proteins on supported lipid bilayers: additive friction of synaptotagmin 7 C2A-C2B tandem domains. Biochemistry 2014; 53:7904-13. [PMID: 25437758 PMCID: PMC4278679 DOI: 10.1021/bi5012223] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
![]()
The
synaptotagmin (Syt) family of proteins contains tandem C2 domains,
C2A and C2B, which bind membranes in the presence of Ca2+ to trigger vesicle fusion during exocytosis. Despite recent progress,
the role and extent of interdomain interactions between C2A and C2B
in membrane binding remain unclear. To test whether the two domains
interact on a planar lipid bilayer (i.e., experience thermodynamic
interdomain contacts), diffusion of fluorescent-tagged C2A, C2B, and
C2AB domains from human Syt7 was measured using total internal reflection
fluorescence microscopy with single-particle tracking. The C2AB tandem
exhibits a lateral diffusion constant approximately half the value
of the isolated single domains and does not change when additional
residues are engineered into the C2A–C2B linker. This is the
expected result if C2A and C2B are separated when membrane-bound;
theory predicts that C2AB diffusion would be faster if the two domains
were close enough together to have interdomain contact. Stopped-flow
measurements of membrane dissociation kinetics further support an
absence of interdomain interactions, as dissociation kinetics of the
C2AB tandem remain unchanged when rigid or flexible linker extensions
are included. Together, the results suggest that the two C2 domains
of Syt7 bind independently to planar membranes, in contrast to reported
interdomain cooperativity in Syt1.
Collapse
Affiliation(s)
- Joseph K Vasquez
- Department of Chemistry, University of Colorado Denver , Denver, Colorado 80217, United States
| | | | | | | | | |
Collapse
|
27
|
Lyakhova TA, Knight JD. The C2 domains of granuphilin are high-affinity sensors for plasma membrane lipids. Chem Phys Lipids 2013; 182:29-37. [PMID: 24184645 DOI: 10.1016/j.chemphyslip.2013.10.009] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2013] [Revised: 09/30/2013] [Accepted: 10/19/2013] [Indexed: 11/30/2022]
Abstract
Membrane-targeting proteins are crucial components of many cell signaling pathways, including the secretion of insulin. Granuphilin, also known as synaptotagmin-like protein 4, functions in tethering secretory vesicles to the plasma membrane prior to exocytosis. Granuphilin docks to insulin secretory vesicles through interaction of its N-terminal domain with vesicular Rab proteins; however, the mechanisms of granuphilin plasma membrane targeting and release are less clear. Granuphilin contains two C2 domains, C2A and C2B, that interact with the plasma membrane lipid phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P2]. The goal of this study was to determine membrane-binding mechanisms, affinities, and kinetics of both granuphilin C2 domains using fluorescence spectroscopic techniques. Results indicate that both C2A and C2B bind anionic lipids in a Ca(2+)-independent manner. The C2A domain binds liposomes containing a physiological mixture of lipids including 2% PI(4,5)P2 or PI(3,4,5)P3 with high affinity (apparent K(d, PIPx) of 2-5 nM), and binds nonspecifically with moderate affinity to anionic liposomes lacking phosphatidylinositol phosphate (PIPx) lipids. The C2B domain binds with sub-micromolar affinity to liposomes containing PI(4,5)P2 but does not have a measurable affinity for background anionic lipids. Both domains can be competed away from their target lipids by the soluble PIPx analog inositol-(1,2,3,4,5,6)-hexakisphosphate (IP6), which is a positive regulator of insulin secretion. Potential roles of these interactions in the docking and release of granuphilin from the plasma membrane are discussed.
Collapse
Affiliation(s)
- Tatyana A Lyakhova
- Department of Chemistry, University of Colorado Denver, Campus Box 194, P.O. Box 173364, Denver, CO 80217, USA
| | - Jefferson D Knight
- Department of Chemistry, University of Colorado Denver, Campus Box 194, P.O. Box 173364, Denver, CO 80217, USA.
| |
Collapse
|
28
|
Marty NJ, Holman CL, Abdullah N, Johnson CP. The C2 domains of otoferlin, dysferlin, and myoferlin alter the packing of lipid bilayers. Biochemistry 2013; 52:5585-92. [PMID: 23859474 DOI: 10.1021/bi400432f] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Ferlins are large multi-C2 domain membrane proteins involved in membrane fusion and fission events. In this study, we investigate the effects of binding of the C2 domains of otoferlin, dysferlin, and myoferlin on the structure of lipid bilayers. Fluorescence measurements indicate that multi-C2 domain constructs of myoferlin, dysferlin, and otoferlin change the lipid packing of both small unilamellar vesicles and giant plasma membrane vesicles. The activities of these proteins were enhanced in the presence of calcium and required negatively charged lipids like phosphatidylserine or phosphatidylglycerol for activity. Experiments with individual domains uncovered functional differences between the C2A domain of otoferlin and those of dysferlin and myoferlin, and truncation studies suggest that the effects of each subsequent C2 domain on lipid ordering appear to be additive. Finally, we demonstrate that the activities of these proteins on membranes are insensitive to high salt concentrations, suggesting a nonelectrostatic component to the interaction between ferlin C2 domains and lipid bilayers. Together, the data indicate that dysferlin, otoferlin, and myoferlin do not merely passively adsorb to membranes but actively sculpt lipid bilayers, which would result in highly curved or distorted membrane regions that could facilitate membrane fusion, membrane fission, or recruitment of other membrane-trafficking proteins.
Collapse
Affiliation(s)
- Naomi J Marty
- Department of Biochemistry and Biophysics, Oregon State University, Corvallis, OR 97331, USA
| | | | | | | |
Collapse
|