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Saha Roy D, Gupta A, Vishvakarma V, Krupa P, Li MS, Maiti S. Serotonin Promotes Vesicular Association and Fusion by Modifying Lipid Bilayers. J Phys Chem B 2024; 128:4975-4985. [PMID: 38743687 DOI: 10.1021/acs.jpcb.4c00115] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/16/2024]
Abstract
The primary event in chemical neurotransmission involves the fusion of a membrane-limited vesicle at the plasma membrane and the subsequent release of its chemical neurotransmitter cargo. The cargo itself is not known to have any effect on the fusion event. However, amphiphilic monoamine neurotransmitters (e.g., serotonin and dopamine) are known to strongly interact with lipid bilayers and to affect their mechanical properties, which can in principle impact membrane-mediated processes. Here, we probe whether serotonin can enhance the association and fusion of artificial lipid vesicles in vitro. We employ fluorescence correlation spectroscopy and total internal reflection fluorescence microscopy to measure the attachment and fusion of vesicles whose lipid compositions mimic the major lipid components of synaptic vesicles. We find that the association between vesicles and supported lipid bilayers is strongly enhanced in a serotonin dose-dependent manner, and this drives an increase in the rate of spontaneous fusion. Molecular dynamics simulations and fluorescence spectroscopy data show that serotonin insertion increases the water content of the hydrophobic part of the bilayer. This suggests that the enhanced membrane association is likely driven by an energetically favorable drying transition. Other monoamines, such as dopamine and norepinephrine, but not other related species, such as tryptophan, show similar effects on membrane association. Our results reveal a lipid bilayer-mediated mechanism by which monoamines can themselves modulate vesicle fusion, potentially adding to the control toolbox for the tightly regulated process of neurotransmission in vivo.
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Affiliation(s)
- Debsankar Saha Roy
- Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
| | - Ankur Gupta
- Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
| | - Vicky Vishvakarma
- Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
| | - Pawel Krupa
- Institute of Physics, Polish Academy of Sciences, Warsaw 02-668, Poland
| | - Mai Suan Li
- Institute of Physics, Polish Academy of Sciences, Warsaw 02-668, Poland
- Institute for Computational Science and Technology, Quang Trung Software City, Tan Chanh Hiep Ward, District 12, 729110 Ho Chi Minh City, Vietnam
| | - Sudipta Maiti
- Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
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2
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Jung JH, Chen X, Reese TS. Cryo-EM tomography and automatic segmentation delineate modular structures in the postsynaptic density. Front Synaptic Neurosci 2023; 15:1123564. [PMID: 37091879 PMCID: PMC10117989 DOI: 10.3389/fnsyn.2023.1123564] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2022] [Accepted: 03/02/2023] [Indexed: 04/08/2023] Open
Abstract
Postsynaptic densities (PSDs) are large protein complexes associated with the postsynaptic membrane of excitatory synapses important for synaptic function including plasticity. Conventional electron microscopy (EM) typically depicts PSDs as compact disk-like structures of hundreds of nanometers in size. Biochemically isolated PSDs were also similar in dimension revealing a predominance of proteins with the ability to polymerize into an extensive scaffold; several EM studies noted their irregular contours with often small granular structures (<30 nm) and holes. Super-resolution light microscopy studies observed clusters of PSD elements and their activity-induced lateral movement. Furthermore, our recent EM study on PSD fractions after sonication observed PSD fragments (40–90 nm in size) separate from intact PSDs; however, such structures within PSDs remained unidentified. Here we examined isolated PSDs by cryo-EM tomography with our new approach of automatic segmentation that enables delineation of substructures and their quantitative analysis. The delineated substructures broadly varied in size, falling behind 30 nm or exceeding 100 nm and showed that a considerable portion of the substructures (>38%) in isolated PSDs was in the same size range as those fragments. Furthermore, substructures spanning the entire thickness of the PSD were found, large enough to contain both membrane-associated and cytoplasmic proteins of the PSD; interestingly, they were similar to nanodomains in frequency. The structures detected here appear to constitute the isolated PSD as modules of various compositions, and this modular nature may facilitate remodeling of the PSD for proper synaptic function and plasticity.
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Warner JM, An D, Stratton BS, O'Shaughnessy B. A hemifused complex is the hub in a network of pathways to membrane fusion. Biophys J 2023; 122:374-385. [PMID: 36463406 PMCID: PMC9892611 DOI: 10.1016/j.bpj.2022.12.003] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2022] [Revised: 06/25/2022] [Accepted: 11/30/2022] [Indexed: 12/12/2022] Open
Abstract
Membrane fusion is a critical step for many essential processes, from neurotransmission to fertilization. For over 40 years, protein-free fusion driven by calcium or other cationic species has provided a simplified model of biological fusion, but the mechanisms remain poorly understood. Cation-mediated membrane fusion and permeation are essential in their own right to drug delivery strategies based on cell-penetrating peptides or cation-bearing lipid nanoparticles. Experimental studies suggest calcium drives anionic membranes to a hemifused intermediate that constitutes a hub in a network of pathways, but the pathway selection mechanism is unknown. Here we develop a mathematical model that identifies the network hub as a highly dynamic hemifusion complex. Multivalent cations drive expansion of this high-tension hemifusion interface between interacting vesicles during a brief transient. The fate of this interface determines the outcome, either fusion, dead-end hemifusion, or vesicle lysis. The model reproduces the unexplained finding that calcium-driven fusion of vesicles with planar membranes typically stalls at hemifusion, and we show the equilibrated hemifused state is a novel lens-shaped complex. Thus, membrane fusion kinetics follow a stochastic trajectory within a network of pathways, with outcome weightings set by a hemifused complex intermediate.
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Affiliation(s)
- Jason M Warner
- Department of Chemical Engineering, Columbia University, New York, New York
| | - Dong An
- Department of Chemical Engineering, Columbia University, New York, New York
| | | | - Ben O'Shaughnessy
- Department of Chemical Engineering, Columbia University, New York, New York.
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4
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The premetazoan ancestry of the synaptic toolkit and appearance of first neurons. Essays Biochem 2022; 66:781-795. [PMID: 36205407 DOI: 10.1042/ebc20220042] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2022] [Revised: 08/31/2022] [Accepted: 09/13/2022] [Indexed: 12/13/2022]
Abstract
Neurons, especially when coupled with muscles, allow animals to interact with and navigate through their environment in ways unique to life on earth. Found in all major animal lineages except sponges and placozoans, nervous systems range widely in organization and complexity, with neurons possibly representing the most diverse cell-type. This diversity has led to much debate over the evolutionary origin of neurons as well as synapses, which allow for the directed transmission of information. The broad phylogenetic distribution of neurons and presence of many of the defining components outside of animals suggests an early origin of this cell type, potentially in the time between the first animal and the last common ancestor of extant animals. Here, we highlight the occurrence and function of key aspects of neurons outside of animals as well as recent findings from non-bilaterian animals in order to make predictions about when and how the first neuron(s) arose during animal evolution and their relationship to those found in extant lineages. With advancing technologies in single cell transcriptomics and proteomics as well as expanding functional techniques in non-bilaterian animals and the close relatives of animals, it is an exciting time to begin unraveling the complex evolutionary history of this fascinating animal cell type.
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Grabner CP, Jansen I, Neef J, Weihs T, Schmidt R, Riedel D, Wurm CA, Moser T. Resolving the molecular architecture of the photoreceptor active zone with 3D-MINFLUX. SCIENCE ADVANCES 2022; 8:eabl7560. [PMID: 35857490 PMCID: PMC9286502 DOI: 10.1126/sciadv.abl7560] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/11/2023]
Abstract
Cells assemble macromolecular complexes into scaffoldings that serve as substrates for catalytic processes. Years of molecular neurobiology research indicate that neurotransmission depends on such optimization strategies. However, the molecular topography of the presynaptic active zone (AZ), where transmitter is released upon synaptic vesicle (SV) fusion, remains to be visualized. Therefore, we implemented MINFLUX optical nanoscopy to resolve the AZ of rod photoreceptors. This was facilitated by a novel sample immobilization technique that we name heat-assisted rapid dehydration (HARD), wherein a thin layer of rod synaptic terminals (spherules) was transferred onto glass coverslips from fresh retinal slices. Rod ribbon AZs were readily immunolabeled and imaged in 3D with a precision of a few nanometers. Our 3D-MINFLUX results indicate that the SV release site in rods is a molecular complex of bassoon-RIM2-ubMunc13-2-Cav1.4, which repeats longitudinally on both sides of the ribbon.
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Affiliation(s)
- Chad P. Grabner
- Institute for Auditory Neuroscience, University Medical Center Göttingen, 37075 Göttingen, Germany
- Auditory Neuroscience and Synaptic Nanophysiology Group, Max Planck Institute for Multidisciplinary Sciences, 37077 Göttingen, Germany
- Collaborative Research Center 1286, University of Göttingen, Göttingen, Germany
- Cluster of Excellence “Multiscale Bioimaging: from Molecular Machines to Networks of Excitable Cells”, University of Göttingen, 37075 Göttingen, Germany
- Corresponding author. (C.P.G.); (C.A.W.); (T.M.)
| | - Isabelle Jansen
- Abberior Instruments, Hans-Adolf-Krebs-Weg 1, 37077 Göttingen, Germany
| | - Jakob Neef
- Institute for Auditory Neuroscience, University Medical Center Göttingen, 37075 Göttingen, Germany
- Auditory Neuroscience and Synaptic Nanophysiology Group, Max Planck Institute for Multidisciplinary Sciences, 37077 Göttingen, Germany
- Collaborative Research Center 1286, University of Göttingen, Göttingen, Germany
- Cluster of Excellence “Multiscale Bioimaging: from Molecular Machines to Networks of Excitable Cells”, University of Göttingen, 37075 Göttingen, Germany
| | - Tobias Weihs
- Abberior Instruments, Hans-Adolf-Krebs-Weg 1, 37077 Göttingen, Germany
| | - Roman Schmidt
- Abberior Instruments, Hans-Adolf-Krebs-Weg 1, 37077 Göttingen, Germany
| | - Dietmar Riedel
- Laboratory of Electron Microscopy, Max Planck Institute for Multidisciplinary Sciences, 37077 Göttingen, Germany
| | - Christian A. Wurm
- Abberior Instruments, Hans-Adolf-Krebs-Weg 1, 37077 Göttingen, Germany
- Corresponding author. (C.P.G.); (C.A.W.); (T.M.)
| | - Tobias Moser
- Institute for Auditory Neuroscience, University Medical Center Göttingen, 37075 Göttingen, Germany
- Auditory Neuroscience and Synaptic Nanophysiology Group, Max Planck Institute for Multidisciplinary Sciences, 37077 Göttingen, Germany
- Collaborative Research Center 1286, University of Göttingen, Göttingen, Germany
- Cluster of Excellence “Multiscale Bioimaging: from Molecular Machines to Networks of Excitable Cells”, University of Göttingen, 37075 Göttingen, Germany
- Corresponding author. (C.P.G.); (C.A.W.); (T.M.)
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Laghaei R, Meriney SD. Microphysiological Modeling of the Structure and Function of Neuromuscular Transmitter Release Sites. Front Synaptic Neurosci 2022; 14:917285. [PMID: 35769072 PMCID: PMC9236679 DOI: 10.3389/fnsyn.2022.917285] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2022] [Accepted: 05/12/2022] [Indexed: 11/13/2022] Open
Abstract
The general mechanism of calcium-triggered chemical transmitter release from neuronal synapses has been intensely studied, is well-known, and highly conserved between species and synapses across the nervous system. However, the structural and functional details within each transmitter release site (or active zone) are difficult to study in living tissue using current experimental approaches owing to the small spatial compartment within the synapse where exocytosis occurs with a very rapid time course. Therefore, computer simulations offer the opportunity to explore these microphysiological environments of the synapse at nanometer spatial scales and on a sub-microsecond timescale. Because biological reactions and physiological processes at synapses occur under conditions where stochastic behavior is dominant, simulation approaches must be driven by such stochastic processes. MCell provides a powerful simulation approach that employs particle-based stochastic simulation tools to study presynaptic processes in realistic and complex (3D) geometries using optimized Monte Carlo algorithms to track finite numbers of molecules as they diffuse and interact in a complex cellular space with other molecules in solution and on surfaces (representing membranes, channels and binding sites). In this review we discuss MCell-based spatially realistic models of the mammalian and frog neuromuscular active zones that were developed to study presynaptic mechanisms that control transmitter release. In particular, these models focus on the role of presynaptic voltage-gated calcium channels, calcium sensors that control the probability of synaptic vesicle fusion, and the effects of action potential waveform shape on presynaptic calcium entry. With the development of these models, they can now be used in the future to predict disease-induced changes to the active zone, and the effects of candidate therapeutic approaches.
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Affiliation(s)
- Rozita Laghaei
- Biomedical Applications Group, Pittsburgh Supercomputing Center, Carnegie Mellon University, Pittsburgh, PA, United States
| | - Stephen D. Meriney
- Department of Neuroscience, Center for Neuroscience, University of Pittsburgh, Pittsburgh, PA, United States
- *Correspondence: Stephen D. Meriney
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Qin N, Chen Z, Xue R. A two-subpopulation model that reflects heterogeneity of large dense core vesicles in exocytosis. Cell Cycle 2022; 21:531-546. [PMID: 35067177 PMCID: PMC8942488 DOI: 10.1080/15384101.2022.2026576] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
Exocytosis of large dense core vesicles is responsible for hormone secretion in neuroendocrine cells. The population of primed vesicles ready to release upon cell excitation demonstrates large heterogeneity. However, there are currently no models that clearly reflect such heterogeneity. Here, we develop a novel model based on single vesicle release events from amperometry recordings of PC12 cells using carbon fiber microelectrode. In this model, releasable vesicles can be grouped into two subpopulations, namely, SP1 and SP2. SP1 vesicles replenish quickly, with kinetics of ~0.0368 s-1, but likely undergo slow fusion pore expansion (amperometric signals rise at ~2.5 pA/ms), while SP2 vesicles demonstrate slow replenishment (kinetics of ~0.0048 s-1) but prefer fast dilation of fusion pore, with an amperometric signal rising rate of ~9.1 pA/ms. Phorbol ester enlarges the size of SP2 partially via activation of protein kinase C and conveys SP1 vesicles into SP2. Inhibition of Rho GTPase-dependent actin rearrangement almost completely depletes SP2. We also propose that the phorbol ester-sensitive vesicle subpopulation (SP2) is analogous to the subset of superprimed synaptic vesicles in neurons. This model provides a meticulous description of the architecture of the readily releasable vesicle pool and elucidates the heterogeneity of the vesicle priming mechanism.
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Affiliation(s)
- Nan Qin
- Shanghai Key Laboratory of Maternal Fetal Medicine, Clinical and Translational Research Center of Shanghai First Maternity & Infant Hospital, Frontier Science Center for Stem Cell Research, School of Life Sciences and Technology, Tongji University, Shanghai, China
| | - Zhixi Chen
- Shanghai Key Laboratory of Maternal Fetal Medicine, Clinical and Translational Research Center of Shanghai First Maternity & Infant Hospital, Frontier Science Center for Stem Cell Research, School of Life Sciences and Technology, Tongji University, Shanghai, China
| | - Renhao Xue
- Shanghai Key Laboratory of Maternal Fetal Medicine, Clinical and Translational Research Center of Shanghai First Maternity & Infant Hospital, Frontier Science Center for Stem Cell Research, School of Life Sciences and Technology, Tongji University, Shanghai, China,CONTACT Renhao Xue Shanghai Key Laboratory of Maternal Fetal Medicine, Clinical and Translational Research Center of Shanghai First Maternity & Infant Hospital, Frontier Science Center for Stem Cell Research, School of Life Sciences and Technology, Tongji University, Shanghai, China
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Martínez-Valencia A, Ramírez-Santiago G, De-Miguel FF. Dynamics of Neuromuscular Transmission Reproduced by Calcium-Dependent and Reversible Serial Transitions in the Vesicle Fusion Complex. Front Synaptic Neurosci 2022; 13:785361. [PMID: 35242023 PMCID: PMC8885725 DOI: 10.3389/fnsyn.2021.785361] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2021] [Accepted: 12/30/2021] [Indexed: 11/28/2022] Open
Abstract
Neuromuscular transmission, from spontaneous release to facilitation and depression, was accurately reproduced by a mechanistic kinetic model of sequential maturation transitions in the molecular fusion complex. The model incorporates three predictions. First, calcium-dependent forward transitions take vesicles from docked to preprimed to primed states, followed by fusion. Second, prepriming and priming are reversible. Third, fusion and recycling are unidirectional. The model was fed with experimental data from previous studies, whereas the backward (β) and recycling (ρ) rate constant values were fitted. Classical experiments were successfully reproduced with four transition states in the model when every forward (α) rate constant had the same value, and both backward rate constants were 50–100 times larger. Such disproportion originated an abruptly decreasing gradient of resting vesicles from docked to primed states. By contrast, a three-state version of the model failed to reproduce the dynamics of transmission by using the same set of parameters. Simulations predict the following: (1) Spontaneous release reflects primed to fusion spontaneous transitions. (2) Calcium elevations synchronize the series of forward transitions that lead to fusion. (3) Facilitation reflects a transient increase of priming following the calcium-dependent maturation transitions. (4) The calcium sensors that produce facilitation are those that evoke the transitions form docked to primed states. (5) Backward transitions and recycling restore the resting state. (6) Depression reflects backward transitions and slow recycling after intense release. Altogether, our results predict that fusion is produced by one calcium sensor, whereas the modulation of the number of vesicles that fuse depends on the calcium sensors that promote the early transition states. Such finely tuned kinetics offers a mechanism for collective non-linear transitional adaptations of a homogeneous vesicle pool to the ever-changing pattern of electrical activity in the neuromuscular junction.
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Affiliation(s)
- Alejandro Martínez-Valencia
- Posgrado en Ciencias Físicas, Universidad Nacional Autónoma de México, Ciudad de México, Mexico
- Instituto de Fisiología Celular-Neurociencias, Universidad Nacional Autónoma de México, Ciudad de México, Mexico
| | | | - Francisco F. De-Miguel
- Instituto de Fisiología Celular-Neurociencias, Universidad Nacional Autónoma de México, Ciudad de México, Mexico
- *Correspondence: Francisco F. De-Miguel,
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Szule JA. Hypothesis Relating the Structure, Biochemistry and Function of Active Zone Material Macromolecules at a Neuromuscular Junction. Front Synaptic Neurosci 2022; 13:798225. [PMID: 35069169 PMCID: PMC8766674 DOI: 10.3389/fnsyn.2021.798225] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2021] [Accepted: 12/14/2021] [Indexed: 12/22/2022] Open
Abstract
This report integrates knowledge of in situ macromolecular structures and synaptic protein biochemistry to propose a unified hypothesis for the regulation of certain vesicle trafficking events (i.e., docking, priming, Ca2+-triggering, and membrane fusion) that lead to neurotransmitter secretion from specialized “active zones” of presynaptic axon terminals. Advancements in electron tomography, to image tissue sections in 3D at nanometer scale resolution, have led to structural characterizations of a network of different classes of macromolecules at the active zone, called “Active Zone Material’. At frog neuromuscular junctions, the classes of Active Zone Material macromolecules “top-masts”, “booms”, “spars”, “ribs” and “pins” direct synaptic vesicle docking while “pins”, “ribs” and “pegs” regulate priming to influence Ca2+-triggering and membrane fusion. Other classes, “beams”, “steps”, “masts”, and “synaptic vesicle luminal filaments’ likely help organize and maintain the structural integrity of active zones. Extensive studies on the biochemistry that regulates secretion have led to comprehensive characterizations of the many conserved proteins universally involved in these trafficking events. Here, a hypothesis including a partial proteomic atlas of Active Zone Material is presented which considers the common roles, binding partners, physical features/structure, and relative positioning in the axon terminal of both the proteins and classes of macromolecules involved in the vesicle trafficking events. The hypothesis designates voltage-gated Ca2+ channels and Ca2+-gated K+ channels to ribs and pegs that are connected to macromolecules that span the presynaptic membrane at the active zone. SNARE proteins (Syntaxin, SNAP25, and Synaptobrevin), SNARE-interacting proteins Synaptotagmin, Munc13, Munc18, Complexin, and NSF are designated to ribs and/or pins. Rab3A and Rabphillin-3A are designated to top-masts and/or booms and/or spars. RIM, Bassoon, and Piccolo are designated to beams, steps, masts, ribs, spars, booms, and top-masts. Spectrin is designated to beams. Lastly, the luminal portions of SV2 are thought to form the bulk of the observed synaptic vesicle luminal filaments. The goal here is to help direct future studies that aim to bridge Active Zone Material structure, biochemistry, and function to ultimately determine how it regulates the trafficking events in vivo that lead to neurotransmitter secretion.
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Shortened tethering filaments stabilize presynaptic vesicles in support of elevated release probability during LTP in rat hippocampus. Proc Natl Acad Sci U S A 2021; 118:2018653118. [PMID: 33875591 DOI: 10.1073/pnas.2018653118] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Long-term potentiation (LTP) is a cellular mechanism of learning and memory that results in a sustained increase in the probability of vesicular release of neurotransmitter. However, previous work in hippocampal area CA1 of the adult rat revealed that the total number of vesicles per synapse decreases following LTP, seemingly inconsistent with the elevated release probability. Here, electron-microscopic tomography (EMT) was used to assess whether changes in vesicle density or structure of vesicle tethering filaments at the active zone might explain the enhanced release probability following LTP. The spatial relationship of vesicles to the active zone varies with functional status. Tightly docked vesicles contact the presynaptic membrane, have partially formed SNARE complexes, and are primed for release of neurotransmitter upon the next action potential. Loosely docked vesicles are located within 8 nm of the presynaptic membrane where SNARE complexes begin to form. Nondocked vesicles comprise recycling and reserve pools. Vesicles are tethered to the active zone via filaments composed of molecules engaged in docking and release processes. The density of tightly docked vesicles was increased 2 h following LTP compared to control stimulation, whereas the densities of loosely docked or nondocked vesicles congregating within 45 nm above the active zones were unchanged. The tethering filaments on all vesicles were shorter and their attachment sites shifted closer to the active zone. These findings suggest that tethering filaments stabilize more vesicles in the primed state. Such changes would facilitate the long-lasting increase in release probability following LTP.
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Goult BT. The Mechanical Basis of Memory - the MeshCODE Theory. Front Mol Neurosci 2021; 14:592951. [PMID: 33716664 PMCID: PMC7947202 DOI: 10.3389/fnmol.2021.592951] [Citation(s) in RCA: 20] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/08/2020] [Accepted: 02/05/2021] [Indexed: 12/11/2022] Open
Abstract
One of the major unsolved mysteries of biological science concerns the question of where and in what form information is stored in the brain. I propose that memory is stored in the brain in a mechanically encoded binary format written into the conformations of proteins found in the cell-extracellular matrix (ECM) adhesions that organise each and every synapse. The MeshCODE framework outlined here represents a unifying theory of data storage in animals, providing read-write storage of both dynamic and persistent information in a binary format. Mechanosensitive proteins that contain force-dependent switches can store information persistently, which can be written or updated using small changes in mechanical force. These mechanosensitive proteins, such as talin, scaffold each synapse, creating a meshwork of switches that together form a code, the so-called MeshCODE. Large signalling complexes assemble on these scaffolds as a function of the switch patterns and these complexes would both stabilise the patterns and coordinate synaptic regulators to dynamically tune synaptic activity. Synaptic transmission and action potential spike trains would operate the cytoskeletal machinery to write and update the synaptic MeshCODEs, thereby propagating this coding throughout the organism. Based on established biophysical principles, such a mechanical basis for memory would provide a physical location for data storage in the brain, with the binary patterns, encoded in the information-storing mechanosensitive molecules in the synaptic scaffolds, and the complexes that form on them, representing the physical location of engrams. Furthermore, the conversion and storage of sensory and temporal inputs into a binary format would constitute an addressable read-write memory system, supporting the view of the mind as an organic supercomputer.
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Affiliation(s)
- Benjamin T. Goult
- School of Biosciences, University of Kent, Canterbury, United Kingdom
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12
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Akimov SA, Molotkovsky RJ, Kuzmin PI, Galimzyanov TR, Batishchev OV. Continuum Models of Membrane Fusion: Evolution of the Theory. Int J Mol Sci 2020; 21:E3875. [PMID: 32485905 PMCID: PMC7312925 DOI: 10.3390/ijms21113875] [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: 04/29/2020] [Revised: 05/24/2020] [Accepted: 05/27/2020] [Indexed: 12/18/2022] Open
Abstract
Starting from fertilization, through tissue growth, hormone secretion, synaptic transmission, and sometimes morbid events of carcinogenesis and viral infections, membrane fusion regulates the whole life of high organisms. Despite that, a lot of fusion processes still lack well-established models and even a list of main actors. A merger of membranes requires their topological rearrangements controlled by elastic properties of a lipid bilayer. That is why continuum models based on theories of membrane elasticity are actively applied for the construction of physical models of membrane fusion. Started from the view on the membrane as a structureless film with postulated geometry of fusion intermediates, they developed along with experimental and computational techniques to a powerful tool for prediction of the whole process with molecular accuracy. In the present review, focusing on fusion processes occurring in eukaryotic cells, we scrutinize the history of these models, their evolution and complication, as well as open questions and remaining theoretical problems. We show that modern approaches in this field allow continuum models of membrane fusion to stand shoulder to shoulder with molecular dynamics simulations, and provide the deepest understanding of this process in multiple biological systems.
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Affiliation(s)
- Sergey A. Akimov
- Laboratory of Bioelectrochemistry, A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31/4 Leninskiy Prospekt, 119071 Moscow, Russia; (R.J.M.); (P.I.K.); (T.R.G.); (O.V.B.)
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13
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Bowers MR, Reist NE. Synaptotagmin: Mechanisms of an electrostatic switch. Neurosci Lett 2020; 722:134834. [DOI: 10.1016/j.neulet.2020.134834] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2020] [Revised: 02/06/2020] [Accepted: 02/09/2020] [Indexed: 02/09/2023]
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14
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Ge D, Noakes PG, Lavidis NA. What are Neurotransmitter Release Sites and Do They Interact? Neuroscience 2020; 425:157-168. [DOI: 10.1016/j.neuroscience.2019.11.017] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2019] [Revised: 11/10/2019] [Accepted: 11/11/2019] [Indexed: 12/22/2022]
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15
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Jung JH. Synaptic Vesicles Having Large Contact Areas with the Presynaptic Membrane are Preferentially Hemifused at Active Zones of Frog Neuromuscular Junctions Fixed during Synaptic Activity. Int J Mol Sci 2019; 20:ijms20112692. [PMID: 31159267 PMCID: PMC6600287 DOI: 10.3390/ijms20112692] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2019] [Revised: 05/23/2019] [Accepted: 05/29/2019] [Indexed: 12/29/2022] Open
Abstract
Synaptic vesicles dock on the presynaptic plasma membrane of axon terminals and become ready to fuse with the presynaptic membrane or primed. Fusion of the vesicle membrane and presynaptic membrane results in the formation of a pore between the membranes, through which the vesicle’s neurotransmitter is released into the synaptic cleft. A recent electron tomography study on frog neuromuscular junctions fixed at rest showed that there is no discernible gap between or merging of the membrane of docked synaptic vesicles with the presynaptic membrane, however, the extent of the contact area between the membrane of docked synaptic vesicles and the presynaptic membrane varies 10-fold with a normal distribution. The study also showed that when the neuromuscular junctions are fixed during repetitive electrical nerve stimulation, the portion of large contact areas in the distribution is reduced compared to the portion of small contact areas, suggesting that docked synaptic vesicles with the largest contact areas are greatly primed to fuse with the membrane. Furthermore, the finding of several hemifused synaptic vesicles among the docked vesicles was briefly reported. Here, the spatial relationship of 81 synaptic vesicles with the presynaptic membrane at active zones of the neuromuscular junctions fixed during stimulation is described in detail. For the most of the vesicles, the combined thickness of each of their contact sites was not different from the sum of the membrane thicknesses of the vesicle membrane and presynaptic membrane, similar to the docked vesicles at active zones of the resting neuromuscular junctions. However, the combined membrane thickness of a small portion of the vesicles was considerably less than the sum of the membrane thicknesses, indicating that the membranes at their contact sites were fixed in a state of hemifusion. Moreover, the hemifused vesicles were found to have large contact areas with the presynaptic membrane. These findings support the recently proposed hypothesis that, at frog neuromuscular junctions, docked synaptic vesicles with the largest contact areas are most primed for fusion with the presynaptic membrane, and that hemifusion is a fusion intermediate step of the vesicle membrane with the presynaptic membrane for synaptic transmission.
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Affiliation(s)
- Jae Hoon Jung
- Department of Biology, Texas A&M University, College Station, TX 77845, USA.
- Department of Physics, Stanford University School of Humanities and Sciences, Stanford, CA 94309, USA.
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16
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Chakrabarti R, Wichmann C. Nanomachinery Organizing Release at Neuronal and Ribbon Synapses. Int J Mol Sci 2019; 20:E2147. [PMID: 31052288 PMCID: PMC6539712 DOI: 10.3390/ijms20092147] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2019] [Revised: 04/26/2019] [Accepted: 04/26/2019] [Indexed: 11/17/2022] Open
Abstract
A critical aim in neuroscience is to obtain a comprehensive view of how regulated neurotransmission is achieved. Our current understanding of synapses relies mainly on data from electrophysiological recordings, imaging, and molecular biology. Based on these methodologies, proteins involved in a synaptic vesicle (SV) formation, mobility, and fusion at the active zone (AZ) membrane have been identified. In the last decade, electron tomography (ET) combined with a rapid freezing immobilization of neuronal samples opened a window for understanding the structural machinery with the highest spatial resolution in situ. ET provides significant insights into the molecular architecture of the AZ and the organelles within the presynaptic nerve terminal. The specialized sensory ribbon synapses exhibit a distinct architecture from neuronal synapses due to the presence of the electron-dense synaptic ribbon. However, both synapse types share the filamentous structures, also commonly termed as tethers that are proposed to contribute to different steps of SV recruitment and exocytosis. In this review, we discuss the emerging views on the role of filamentous structures in SV exocytosis gained from ultrastructural studies of excitatory, mainly central neuronal compared to ribbon-type synapses with a focus on inner hair cell (IHC) ribbon synapses. Moreover, we will speculate on the molecular entities that may be involved in filament formation and hence play a crucial role in the SV cycle.
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Affiliation(s)
- Rituparna Chakrabarti
- Molecular Architecture of Synapses Group, Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, 37075 Göttingen, Germany.
- Center for Biostructural Imaging of Neurodegeneration, University Medical Center Göttingen, 37075 Göttingen, Germany.
- Collaborative Research Center 889 "Cellular Mechanisms of Sensory Processing", 37099 Göttingen, Germany.
| | - Carolin Wichmann
- Molecular Architecture of Synapses Group, Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, 37075 Göttingen, Germany.
- Center for Biostructural Imaging of Neurodegeneration, University Medical Center Göttingen, 37075 Göttingen, Germany.
- Collaborative Research Center 889 "Cellular Mechanisms of Sensory Processing", 37099 Göttingen, Germany.
- Collaborative Research Center 1286 "Quantitative Synaptology", 37099 Göttingen, Germany.
- Auditory Neuroscience Group, Max Planck Institute for Experimental Medicine, 37075 Göttingen, Germany.
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17
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Gundersen CB. Fast, synchronous neurotransmitter release: Past, present and future. Neuroscience 2019; 439:22-27. [PMID: 31047980 DOI: 10.1016/j.neuroscience.2019.04.030] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2019] [Revised: 04/11/2019] [Accepted: 04/12/2019] [Indexed: 01/23/2023]
Abstract
This mini-review starts with a summary of the crucial contributions Ricardo Miledi made to our understanding of how the action potential triggers fast, synchronous transmitter release. It then transitions to the discovery of synaptotagmin and its role as the exocytotic Ca2+ sensor at nerve terminals. The final section confronts the array of unique models that have been proposed to explain the membrane fusion step of exocytosis. More than a dozen different hypotheses seek to explain the terminal steps of the exocytotic cascade. It will be an interesting challenge for the field to distinguish among these possibilities. Nevertheless, with ongoing technological advances, perhaps we will have a better picture of this process by the end of the coming decade. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.
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Affiliation(s)
- Cameron B Gundersen
- Department of Molecular and Medical Pharmacology, David Geffen UCLA School of Medicine, Los Angeles, CA 90095.
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18
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Zuber B, Lučić V. Molecular architecture of the presynaptic terminal. Curr Opin Struct Biol 2019; 54:129-138. [PMID: 30925443 DOI: 10.1016/j.sbi.2019.01.008] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2018] [Revised: 01/11/2019] [Accepted: 01/26/2019] [Indexed: 10/27/2022]
Abstract
Neurotransmitter release at the presynaptic terminal is one of the fundamental processes in neuronal communication. It is a complex process comprising signaling pathways that exert a precise spatio-temporal coordination to prepare and bring synaptic vesicles to exocytosis. While many molecular components involved have been identified, their direct observation at different stages of the neurotransmitter release is lacking. Three-dimensional imaging by electron tomography provided remarkable views of the synaptic vesicles and the cytomatrix. Imaging fully hydrated, vitrified samples allowed a direct visualization, precise localization and a quantitative characterization of pleomorphic synaptic vesicle-bound complexes in situ, as well as the elucidation of their function in the neurotransmitter release.
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Affiliation(s)
- Benoît Zuber
- Institute of Anatomy, University of Bern, Bern 3012, Switzerland
| | - Vladan Lučić
- Max-Planck-Institute of Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany.
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19
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Jung JH, Szule JA, Stouder K, Marshall RM, McMahan UJ. Active Zone Material-Directed Orientation, Docking, and Fusion of Dense Core Vesicles Alongside Synaptic Vesicles at Neuromuscular Junctions. Front Neuroanat 2018; 12:72. [PMID: 30271328 PMCID: PMC6146030 DOI: 10.3389/fnana.2018.00072] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2018] [Accepted: 08/17/2018] [Indexed: 12/13/2022] Open
Abstract
Active zone material is an organelle that is common to active zones along the presynaptic membrane of chemical synapses. Electron tomography on active zones at frog neuromuscular junctions has provided evidence that active zone material directs the docking of synaptic vesicles (SVs) on the presynaptic membrane at this synapse. Certain active zone material macromolecules connect to stereotypically arranged macromolecules in the membrane of undocked SVs, stably orienting a predetermined fusion domain of the vesicle membrane toward the presynaptic membrane while bringing and holding the two membranes together. Docking of the vesicles is required for the impulse-triggered vesicle membrane-presynaptic membrane fusion that releases the vesicles’ neurotransmitter into the synaptic cleft. As at other synapses, axon terminals at frog neuromuscular junctions contain, in addition to SVs, vesicles that are larger, are much less frequent and, when viewed by electron microscopy, have a distinctive electron dense core. Dense core vesicles at neuromuscular junctions are likely to contain peptides that are released into the synaptic cleft to regulate formation, maintenance and behavior of cellular apparatus essential for synaptic impulse transmission. We show by electron tomography on axon terminals of frog neuromuscular junctions fixed at rest and during repetitive impulse transmission that dense core vesicles selectively dock on and fuse with the presynaptic membrane alongside SVs at active zones, and that active zone material connects to the dense core vesicles undergoing these processes in the same way it connects to SVs. We conclude that undocked dense core vesicles have a predetermined fusion domain, as do undocked SVs, and that active zone material directs oriented docking and fusion of these different vesicle types at active zones of the presynaptic membrane by similar macromolecular interactions.
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Affiliation(s)
- Jae H Jung
- Department of Biology, Texas A&M University, College Station, TX, United States.,Department of Neurobiology, Stanford University School of Medicine, Stanford University, Stanford, CA, United States
| | - Joseph A Szule
- Department of Biology, Texas A&M University, College Station, TX, United States.,Department of Neurobiology, Stanford University School of Medicine, Stanford University, Stanford, CA, United States
| | - Kylee Stouder
- Department of Biology, Texas A&M University, College Station, TX, United States
| | - Robert M Marshall
- Department of Biology, Texas A&M University, College Station, TX, United States.,Department of Neurobiology, Stanford University School of Medicine, Stanford University, Stanford, CA, United States
| | - Uel J McMahan
- Department of Biology, Texas A&M University, College Station, TX, United States.,Department of Neurobiology, Stanford University School of Medicine, Stanford University, Stanford, CA, United States
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20
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Dobson KL, Smith ZH, Bellamy TC. Distribution of vesicle pools in cerebellar parallel fibre terminals after depression of ectopic transmission. PLoS One 2018; 13:e0200937. [PMID: 30024947 PMCID: PMC6053221 DOI: 10.1371/journal.pone.0200937] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2017] [Accepted: 07/05/2018] [Indexed: 11/18/2022] Open
Abstract
At parallel fibre terminals in the cerebellar cortex, glutamate released outside of the active zone can activate AMPA receptors on juxtaposed Bergmann glial cell processes. This process is termed “ectopic” release, and allows for directed transmission to astroglial cells that is functionally independent of synaptic transmission to postsynaptic Purkinje neurons. The location of ectopic sites in presynaptic terminals is uncertain. Functional evidence suggests that stimulation of parallel fibres at 1 Hz exhausts ectopic transmission due to a failure to rapidly recycle vesicles to the ectopic pool, and so would predict a loss of vesicles in the near vicinity of extrasynaptic glial processes. In this study we used this stimulation protocol to investigate whether the distribution of vesicles within the presynaptic terminal is altered after suppression of ectopic release. Stimulation at 1 Hz had only a minor impact on the distribution of vesicles in presynaptic terminals when analysed with electron microscopy. Vesicle number and terminal size were unaffected by 1 Hz stimulation, but the relative abundance of vesicles in close proximity to the active zone was marginally reduced. In contrast, the fraction of vesicles facing glial membranes was unchanged after suppression of ectopic transmission. 1 Hz stimulation also resulted in a small but statistically-significant increase in the distance between glial membrane and presynaptic terminal, suggesting withdrawal of glial membranes from synapses is detectable in ultrastructural anatomy within minutes. These results raise doubts about the location of ectopic release sites, but indicate that neuron-glial association varies on a dynamic time scale.
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Affiliation(s)
- Katharine L. Dobson
- School of Life Sciences, University of Nottingham Medical School, Nottingham, United Kingdom
- * E-mail:
| | - Zoe H. Smith
- School of Life Sciences, University of Nottingham Medical School, Nottingham, United Kingdom
| | - Tomas C. Bellamy
- School of Life Sciences, University of Nottingham Medical School, Nottingham, United Kingdom
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21
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Tao CL, Liu YT, Sun R, Zhang B, Qi L, Shivakoti S, Tian CL, Zhang P, Lau PM, Zhou ZH, Bi GQ. Differentiation and Characterization of Excitatory and Inhibitory Synapses by Cryo-electron Tomography and Correlative Microscopy. J Neurosci 2018; 38:1493-1510. [PMID: 29311144 PMCID: PMC5815350 DOI: 10.1523/jneurosci.1548-17.2017] [Citation(s) in RCA: 113] [Impact Index Per Article: 18.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2017] [Revised: 12/17/2017] [Accepted: 12/24/2017] [Indexed: 11/21/2022] Open
Abstract
As key functional units in neural circuits, different types of neuronal synapses play distinct roles in brain information processing, learning, and memory. Synaptic abnormalities are believed to underlie various neurological and psychiatric disorders. Here, by combining cryo-electron tomography and cryo-correlative light and electron microscopy, we distinguished intact excitatory and inhibitory synapses of cultured hippocampal neurons, and visualized the in situ 3D organization of synaptic organelles and macromolecules in their native state. Quantitative analyses of >100 synaptic tomograms reveal that excitatory synapses contain a mesh-like postsynaptic density (PSD) with thickness ranging from 20 to 50 nm. In contrast, the PSD in inhibitory synapses assumes a thin sheet-like structure ∼12 nm from the postsynaptic membrane. On the presynaptic side, spherical synaptic vesicles (SVs) of 25-60 nm diameter and discus-shaped ellipsoidal SVs of various sizes coexist in both synaptic types, with more ellipsoidal ones in inhibitory synapses. High-resolution tomograms obtained using a Volta phase plate and electron filtering and counting reveal glutamate receptor-like and GABAA receptor-like structures that interact with putative scaffolding and adhesion molecules, reflecting details of receptor anchoring and PSD organization. These results provide an updated view of the ultrastructure of excitatory and inhibitory synapses, and demonstrate the potential of our approach to gain insight into the organizational principles of cellular architecture underlying distinct synaptic functions.SIGNIFICANCE STATEMENT To understand functional properties of neuronal synapses, it is desirable to analyze their structure at molecular resolution. We have developed an integrative approach combining cryo-electron tomography and correlative fluorescence microscopy to visualize 3D ultrastructural features of intact excitatory and inhibitory synapses in their native state. Our approach shows that inhibitory synapses contain uniform thin sheet-like postsynaptic densities (PSDs), while excitatory synapses contain previously known mesh-like PSDs. We discovered "discus-shaped" ellipsoidal synaptic vesicles, and their distributions along with regular spherical vesicles in synaptic types are characterized. High-resolution tomograms further allowed identification of putative neurotransmitter receptors and their heterogeneous interaction with synaptic scaffolding proteins. The specificity and resolution of our approach enables precise in situ analysis of ultrastructural organization underlying distinct synaptic functions.
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Affiliation(s)
- Chang-Lu Tao
- National Laboratory for Physical Sciences at the Microscale
- School of Life Sciences
| | - Yun-Tao Liu
- National Laboratory for Physical Sciences at the Microscale
- School of Life Sciences
| | - Rong Sun
- National Laboratory for Physical Sciences at the Microscale
| | - Bin Zhang
- Chinese Academy of Sciences Key Laboratory of Brain Function and Disease
- School of Life Sciences
| | - Lei Qi
- Chinese Academy of Sciences Key Laboratory of Brain Function and Disease
- School of Life Sciences
| | - Sakar Shivakoti
- National Laboratory for Physical Sciences at the Microscale
- School of Life Sciences
| | - Chong-Li Tian
- Chinese Academy of Sciences Key Laboratory of Brain Function and Disease
- School of Life Sciences
| | - Peijun Zhang
- Division of Structural Biology, Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford OX37BN, United Kingdom
| | - Pak-Ming Lau
- Chinese Academy of Sciences Key Laboratory of Brain Function and Disease
- School of Life Sciences
| | - Z Hong Zhou
- National Laboratory for Physical Sciences at the Microscale,
- School of Life Sciences
- The California NanoSystems Institute, and
- Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, Los Angeles, California 90095
| | - Guo-Qiang Bi
- National Laboratory for Physical Sciences at the Microscale,
- School of Life Sciences
- Chinese Academy of Sciences Center for Excellence in Brain Science and Intelligence Technology, Innovation Center for Cell Signaling Network, University of Science and Technology of China, Hefei, Anhui 230026, China
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22
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Homan AE, Laghaei R, Dittrich M, Meriney SD. Impact of spatiotemporal calcium dynamics within presynaptic active zones on synaptic delay at the frog neuromuscular junction. J Neurophysiol 2017; 119:688-699. [PMID: 29167324 DOI: 10.1152/jn.00510.2017] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The spatiotemporal calcium dynamics within presynaptic neurotransmitter release sites (active zones, AZs) at the time of synaptic vesicle fusion is critical for shaping the dynamics of neurotransmitter release. Specifically, the relative arrangement and density of voltage-gated calcium channels (VGCCs) as well as the concentration of calcium buffering proteins can play a large role in the timing, magnitude, and plasticity of release by shaping the AZ calcium profile. However, a high-resolution understanding of the role of AZ structure in spatiotemporal calcium dynamics and how it may contribute to functional heterogeneity at an adult synapse is currently lacking. We demonstrate that synaptic delay varies considerably across, but not within, individual synapses at the frog neuromuscular junction (NMJ). To determine how elements of the AZ could contribute to this variability, we performed a parameter search using a spatially realistic diffusion reaction-based computational model of a frog NMJ AZ (Dittrich M, Pattillo JM, King JD, Cho S, Stiles JR, Meriney SD. Biophys J 104: 2751-2763, 2013; Ma J, Kelly L, Ingram J, Price TJ, Meriney SD, Dittrich M. J Neurophysiol 113: 71-87, 2015). We demonstrate with our model that synaptic delay is sensitive to significant alterations in the spatiotemporal calcium dynamics within an AZ at the time of release caused by manipulations of the density and organization of VGCCs or by the concentration of calcium buffering proteins. Furthermore, our data provide a framework for understanding how AZ organization and structure are important for understanding presynaptic function and plasticity. NEW & NOTEWORTHY The structure of presynaptic active zones (AZs) can play a large role in determining the dynamics of neurotransmitter release across many model preparations by influencing the spatiotemporal calcium dynamics within the AZ at the time of vesicle fusion. However, less is known about how different AZ structural schemes may influence the timing of neurotransmitter release. We demonstrate that variations in AZ structure create different spatiotemporal calcium profiles that, in turn, lead to differences in synaptic delay.
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Affiliation(s)
- Anne E Homan
- Department of Neuroscience, Center for Neuroscience, University of Pittsburgh , Pittsburgh, Pennsylvania
| | - Rozita Laghaei
- Biomedical Applications Group, Pittsburgh Supercomputing Center, Carnegie Mellon University , Pittsburgh, Pennsylvania
| | | | - Stephen D Meriney
- Department of Neuroscience, Center for Neuroscience, University of Pittsburgh , Pittsburgh, Pennsylvania
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23
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A Membrane-Fusion Model That Exploits a β-to-α Transition in the Hydrophobic Domains of Syntaxin 1A and Synaptobrevin 2. Int J Mol Sci 2017; 18:ijms18071582. [PMID: 28753981 PMCID: PMC5536069 DOI: 10.3390/ijms18071582] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2017] [Revised: 07/14/2017] [Accepted: 07/18/2017] [Indexed: 11/16/2022] Open
Abstract
Parallel zippering of the SNARE domains of syntaxin 1A/B, SNAP-25, and VAMP/synaptobrevin 2 is widely regarded as supplying the driving force for exocytotic events at nerve terminals and elsewhere. However, in spite of intensive research, no consensus has been reached concerning the molecular mechanism by which these SNARE proteins catalyze membrane fusion. As an alternative to SNARE-based models, a scenario was developed in which synaptotagmin 1 (or, 2) can serve as a template to guide lipid movements that underlie fast, synchronous exocytosis at nerve terminals. This “dyad model” advanced a novel proposal concerning the membrane disposition of the palmitoylated, cysteine-rich region of these synaptotagmins. Unexpectedly, it now emerges that a similar principle can be exploited to reveal how the hydrophobic, carboxyl-terminal domains of syntaxin 1A and synaptobrevin 2 can perturb membrane structure at the interface between a docked synaptic vesicle and the plasma membrane. These “β-to-α transition” models will be compared and contrasted with other proposals for how macromolecules are thought to intervene to drive membrane fusion.
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24
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Takamori M. Synaptic Homeostasis and Its Immunological Disturbance in Neuromuscular Junction Disorders. Int J Mol Sci 2017; 18:ijms18040896. [PMID: 28441759 PMCID: PMC5412475 DOI: 10.3390/ijms18040896] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2017] [Revised: 04/04/2017] [Accepted: 04/19/2017] [Indexed: 12/14/2022] Open
Abstract
In the neuromuscular junction, postsynaptic nicotinic acetylcholine receptor (nAChR) clustering, trans-synaptic communication and synaptic stabilization are modulated by the molecular mechanisms underlying synaptic plasticity. The synaptic functions are based presynaptically on the active zone architecture, synaptic vesicle proteins, Ca2+ channels and synaptic vesicle recycling. Postsynaptically, they are based on rapsyn-anchored nAChR clusters, localized sensitivity to ACh, and synaptic stabilization via linkage to the extracellular matrix so as to be precisely opposed to the nerve terminal. Focusing on neural agrin, Wnts, muscle-specific tyrosine kinase (a mediator of agrin and Wnts signalings and regulator of trans-synaptic communication), low-density lipoprotein receptor-related protein 4 (the receptor of agrin and Wnts and participant in retrograde signaling), laminin-network (including muscle-derived agrin), extracellular matrix proteins (participating in the synaptic stabilization) and presynaptic receptors (including muscarinic and adenosine receptors), we review the functional structures of the synapse by making reference to immunological pathogenecities in postsynaptic disease, myasthenia gravis. The synapse-related proteins including cortactin, coronin-6, caveolin-3, doublecortin, R-spondin 2, amyloid precursor family proteins, glia cell-derived neurotrophic factor and neurexins are also discussed in terms of their possible contribution to efficient synaptic transmission at the neuromuscular junction.
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Affiliation(s)
- Masaharu Takamori
- Neurological Center, Kanazawa-Nishi Hospital, Kanazawa, Ishikawa 920-0025, Japan.
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25
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A stochastic model of active zone material mediated synaptic vesicle docking and priming at resting active zones. Sci Rep 2017; 7:278. [PMID: 28325932 PMCID: PMC5428245 DOI: 10.1038/s41598-017-00360-z] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2016] [Accepted: 02/21/2017] [Indexed: 11/09/2022] Open
Abstract
Synaptic vesicles (SVs) fuse with the presynaptic membrane (PM) at specialized regions called active zones for synaptic transmission. SVs are associated with dense aggregates of macromolecules called active zone material (AZM) that has been thought to be involved in SV release. However, its role has recently begun to be elucidated. Several morphological studies proposed distinctively different AZM mediated SV docking and priming models: sequential and concurrent SV docking/priming. To explore ways to reconcile the contradictory models we develop a stochastic AZM mediated SV docking and priming model. We assume that the position of each connection site of the AZM macromolecules on their SV, directly linking the SV with the PM, varies by random shortening and lengthening of the macromolecules at resting active zones. We also perform computer simulations of SVs near the PM at resting active zones, and the results show that the distribution of the AZM connection sites can significantly affect the SV's docking efficiency and distribution of its contact area with the PM, thus priming and that the area correlates with the shape of the SVs providing a way to account for seemingly irreconcilable observations reported about the spatial relationship of SVs with the PM at active zones.
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26
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Gundersen CB. The Structure of the Synaptic Vesicle-Plasma Membrane Interface Constrains SNARE Models of Rapid, Synchronous Exocytosis at Nerve Terminals. Front Mol Neurosci 2017; 10:48. [PMID: 28280457 PMCID: PMC5321675 DOI: 10.3389/fnmol.2017.00048] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2016] [Accepted: 02/13/2017] [Indexed: 12/27/2022] Open
Abstract
Contemporary models of neurotransmitter release invoke direct or indirect interactions between the Ca2+ sensor, synaptotagmin and the incompletely zippered soluble, N-ethyl-maleimide-sensitive factor attachment protein receptor (SNARE) complex. However, recent electron microscopic (EM) investigations have raised pragmatic issues concerning the mechanism by which SNAREs trigger membrane fusion at nerve terminals. The first issue is related to the finding that the area of contact between a “fully primed” synaptic vesicle and the plasma membrane can exceed 600 nm2. Approximately four-thousands lipid molecules can inhabit this contact zone. Thus, renewed efforts will be needed to explain how the zippering of as few as two SNARE complexes mobilizes these lipids to achieve membrane fusion. The second issue emerges from the finding that “docking filaments” are sandwiched within the area of vesicle-plasma membrane contact. It is challenging to reconcile the location of these filaments with SNARE models of exocytosis. Instead, this commentary outlines how these data are more compatible with a model in which a cluster of synaptotagmins catalyzes exocytotic membrane fusion.
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Affiliation(s)
- Cameron B Gundersen
- Department of Molecular and Medical Pharmacology, David Geffen UCLA School of Medicine Los Angeles, CA, USA
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27
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Zhao WD, Hamid E, Shin W, Wen PJ, Krystofiak ES, Villarreal SA, Chiang HC, Kachar B, Wu LG. Hemi-fused structure mediates and controls fusion and fission in live cells. Nature 2016; 534:548-52. [PMID: 27309816 PMCID: PMC4930626 DOI: 10.1038/nature18598] [Citation(s) in RCA: 99] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2015] [Accepted: 05/24/2016] [Indexed: 11/09/2022]
Abstract
Membrane fusion and fission are vital for eukaryotic life. For three decades, it has been proposed that fusion is mediated by fusion between the proximal leaflets of two bilayers (hemi-fusion) to produce a hemi-fused structure, followed by fusion between the distal leaflets, whereas fission is via hemi-fission, which also produces a hemi-fused structure, followed by full fission. This hypothesis remained unsupported owing to the lack of observation of hemi-fusion or hemi-fission in live cells. A competing fusion hypothesis involving protein-lined pore formation has also been proposed. Here we report the observation of a hemi-fused Ω-shaped structure in live neuroendocrine chromaffin cells and pancreatic β-cells, visualized using confocal and super-resolution stimulated emission depletion microscopy. This structure is generated from fusion pore opening or closure (fission) at the plasma membrane. Unexpectedly, the transition to full fusion or fission is determined by competition between fusion and calcium/dynamin-dependent fission mechanisms, and is notably slow (seconds to tens of seconds) in a substantial fraction of the events. These results provide key missing evidence in support of the hemi-fusion and hemi-fission hypothesis in live cells, and reveal the hemi-fused intermediate as a key structure controlling fusion and fission, as fusion and fission mechanisms compete to determine the transition to fusion or fission.
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Affiliation(s)
- Wei-Dong Zhao
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Edaeni Hamid
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Wonchul Shin
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Peter J Wen
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Evan S Krystofiak
- National Institute on Deafness and other Communication Disorders, 35A Convent Drive, Room 3D-824, Bethesda, Maryland 20892, USA
| | - Seth A Villarreal
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Hsueh-Cheng Chiang
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
| | - Bechara Kachar
- National Institute on Deafness and other Communication Disorders, 35A Convent Drive, Room 3D-824, Bethesda, Maryland 20892, USA
| | - Ling-Gang Wu
- National Institute of Neurological Disorders and Stroke, 35 Convent Drive, Room 2B-1012, Bethesda, Maryland 20892, USA
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Szule JA, Jung JH, McMahan UJ. The structure and function of 'active zone material' at synapses. Philos Trans R Soc Lond B Biol Sci 2016; 370:rstb.2014.0189. [PMID: 26009768 DOI: 10.1098/rstb.2014.0189] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
The docking of synaptic vesicles on the presynaptic membrane and their priming for fusion with it to mediate synaptic transmission of nerve impulses typically occur at structurally specialized regions on the membrane called active zones. Stable components of active zones include aggregates of macromolecules, 'active zone material' (AZM), attached to the presynaptic membrane, and aggregates of Ca(2+)-channels in the membrane, through which Ca(2+) enters the cytosol to trigger impulse-evoked vesicle fusion with the presynaptic membrane by interacting with Ca(2+)-sensors on the vesicles. This laboratory has used electron tomography to study, at macromolecular spatial resolution, the structure and function of AZM at the simply arranged active zones of axon terminals at frog neuromuscular junctions. The results support the conclusion that AZM directs the docking and priming of synaptic vesicles and essential positioning of Ca(2+)-channels relative to the vesicles' Ca(2+)-sensors. Here we review the findings and comment on their applicability to understanding mechanisms of docking, priming and Ca(2+)-triggering at other synapses, where the arrangement of active zone components differs.
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Affiliation(s)
- Joseph A Szule
- Department of Biology, Texas A&M University, College Station, TX 77845, USA
| | - Jae Hoon Jung
- Department of Biology, Texas A&M University, College Station, TX 77845, USA
| | - Uel J McMahan
- Department of Biology, Texas A&M University, College Station, TX 77845, USA
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