(M)Unc13s in Active Zone Diversity: A Drosophila Perspective

The so-called active zones at pre-synaptic terminals are the ultimate filtering devices, which couple between action potential frequency and shape, and the information transferred to the post-synaptic neurons, finally tuning behaviors. Within active zones, the release of the synaptic vesicle operates from specialized “release sites.” The (M)Unc13 class of proteins is meant to define release sites topologically and biochemically, and diversity between Unc13-type release factor isoforms is suspected to steer diversity at active zones. The two major Unc13-type isoforms, namely, Unc13A and Unc13B, have recently been described from the molecular to the behavioral level, exploiting Drosophila being uniquely suited to causally link between these levels. The exact nanoscale distribution of voltage-gated Ca2+ channels relative to release sites (“coupling”) at pre-synaptic active zones fundamentally steers the release of the synaptic vesicle. Unc13A and B were found to be either tightly or loosely coupled across Drosophila synapses. In this review, we reported recent findings on diverse aspects of Drosophila Unc13A and B, importantly, their nano-topological distribution at active zones and their roles in release site generation, active zone assembly, and pre-synaptic homeostatic plasticity. We compared their stoichiometric composition at different synapse types, reviewing the correlation between nanoscale distribution of these two isoforms and release physiology and, finally, discuss how isoform-specific release components might drive the functional heterogeneity of synapses and encode discrete behavior.

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A stochastic model of active zone material mediated synaptic vesicle docking and priming at resting active zones

Scientific Reports ◽

10.1038/s41598-017-00360-z ◽

2017 ◽

Vol 7 (1) ◽

Cited By ~ 2

Author(s):

Jae Hoon Jung ◽

Sebatian Doniach

Keyword(s):

Stochastic Model ◽

Synaptic Vesicle ◽

Active Zone ◽

Vesicle Docking ◽

Active Zones

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Hypothesis Relating the Structure, Biochemistry and Function of Active Zone Material Macromolecules at a Neuromuscular Junction

Frontiers in Synaptic Neuroscience ◽

10.3389/fnsyn.2021.798225 ◽

2022 ◽

Vol 13 ◽

Author(s):

Joseph A. Szule

Keyword(s):

Membrane Fusion ◽

Synaptic Vesicle ◽

Active Zone ◽

Structural Integrity ◽

Neuromuscular Junctions ◽

Vesicle Trafficking ◽

Material Structure ◽

Neurotransmitter Secretion ◽

Active Zones ◽

And Function

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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Reconstructing essential active zone functions within a synapse

10.1101/2021.11.21.469466 ◽

2021 ◽

Author(s):

Chao Tan ◽

Shan Shan H Wang ◽

Giovanni de Nola ◽

Pascal S Kaeser

Keyword(s):

Fusion Protein ◽

Synaptic Vesicle ◽

Active Zone ◽

Neurotransmitter Release ◽

Molecular Machines ◽

Vesicle Docking ◽

Active Zones ◽

Ca2 Channels

Active zones are molecular machines that control neurotransmitter release through synaptic vesicle docking and priming, and through coupling of these vesicles to Ca2+ entry. The complexity of active zone machinery has made it challenging to determine which mechanisms drive these roles in release. Here, we induce RIM+ELKS knockout to eliminate active zone scaffolding networks, and then reconstruct each active zone function. Re-expression of RIM1-Zn fingers positioned Munc13 on undocked vesicles and rendered them release-competent. Reconstitution of release-triggering required docking of these vesicles to Ca2+ channels. Fusing RIM1-Zn to CaVbeta4-subunits sufficed to restore docking, priming and release-triggering without reinstating active zone scaffolds. Hence, exocytotic activities of the 80 kDa CaVbeta4-Zn fusion protein bypassed the need for megadalton-sized secretory machines. These data define key mechanisms of active zone function, establish that fusion competence and docking are mechanistically separable, and reveal that active zone scaffolding networks are not required for release.

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Rapid active zone remodeling consolidates presynaptic potentiation

10.1101/493452 ◽

2018 ◽

Author(s):

Mathias A. Böhme ◽

Anthony W. McCarthy ◽

Andreas T. Grasskamp ◽

Christine B. Beuschel ◽

Pragya Goel ◽

...

Keyword(s):

Neurotransmitter Release ◽

Release Site ◽

Scaffold Proteins ◽

Homeostatic Plasticity ◽

Discrete Subset ◽

Molecular Sequence ◽

Central Neurons ◽

Presynaptic Plasticity ◽

Active Zones ◽

Plastic Changes

AbstractSynaptic transmission is mediated by neurotransmitter release at presynaptic active zones (AZs) followed by postsynaptic neurotransmitter detection. Plastic changes in transmission maintain functionality during perturbations and enable memory formation. Postsynaptic plasticity targets neurotransmitter receptors, but presynaptic plasticity mechanisms directly regulating the neurotransmitter release apparatus remain largely enigmatic. Here we describe that AZs consist of nano-modular release site units and identify a molecular sequence adding more modules within minutes of plasticity induction. This requires cognate transport machinery and a discrete subset of AZ scaffold proteins. Structural remodeling is not required for the immediate potentiation of neurotransmitter release, but rather necessary to sustain this potentiation over longer timescales. Finally, mutations in Unc13 that disrupt homeostatic plasticity at the neuromuscular junction also impair shot-term memory when central neurons are targeted, suggesting that both forms of plasticity operate via Unc13. Together, while immediate synaptic potentiation capitalizes on available material, it triggers the coincident incorporation of modular release sites to consolidate stable synapse function.

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RIM proteins and their role in synapse function

Biological Chemistry ◽

10.1515/bc.2010.064 ◽

2010 ◽

Vol 391 (6) ◽

Cited By ~ 35

Author(s):

Tobias Mittelstaedt ◽

Elena Alvaréz-Baron ◽

Susanne Schoch

Keyword(s):

Synaptic Vesicle ◽

Active Zone ◽

Neurotransmitter Release ◽

Multidomain Proteins ◽

Short Term ◽

Presynaptic Nerve Terminal ◽

Presynaptic Plasticity ◽

Active Zones ◽

Synaptic Vesicle Release ◽

The Brain

Abstract Active zones are specialized areas of the plasma membrane in the presynaptic nerve terminal that mediate neurotransmitter release and synaptic plasticity. The multidomain proteins RIM1 and RIM2 are integral components of the cytomatrix at the active zone, interacting with most other active zone-enriched proteins as well as synaptic vesicle proteins. In the brain, RIMs are present in multiple isoforms (α, β, γ) diverging in their structural composition, which mediate overlapping and distinct functions. Here, we summarize recent findings about the specific roles of the various RIM isoforms in basic synaptic vesicle release as well as long- and short-term presynaptic plasticity.

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Resolving the molecular architecture of the photoreceptor active zone by MINFLUX nanoscopy

10.1101/2021.05.28.446138 ◽

2021 ◽

Author(s):

Chad Grabner ◽

Isabella Jansen ◽

Jakob Neef ◽

Tobias Weiss ◽

Roman Schmidt ◽

...

Keyword(s):

Synaptic Vesicle ◽

Molecular Complex ◽

Active Zone ◽

Single Layer ◽

Release Site ◽

Molecular Architecture ◽

Synaptic Ribbon ◽

Rod Photoreceptors ◽

Macromolecular Complexes ◽

Axis Of Symmetry

Cells assemble macromolecular complexes into scaffoldings that serve as substrates for catalytic processes. Years of molecular neurobiology indicate that neurotransmission depends on such optimization strategies, yet 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. To facilitate MINFLUX nanoscopy of the AZ, we developed and verified an immobilization technique, we name Heat Assisted Rapid Dehydration (HARD). Here fresh retinal slices are directly stamped onto glass coverslips yielding a single layer of rod AZs. These AZs exhibited excellent labeling efficiency and minimal signal redundancy in the Z-direction. Our data indicate that the SV release site is a molecular complex of bassoon-Rab3-binding molecule 2 (RIM2)-ubMunc13-2-CAST. The complexes are serially duplicated longitudinally, and reflected in register along the axis of symmetry of the synaptic ribbon.

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