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

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.

scholarly journals The structure and function of ‘active zone material’ at synapses

2015 ◽  
Vol 370 (1672) ◽  
pp. 20140189 ◽  
Author(s):  
Joseph A. Szule ◽  
Jae Hoon Jung ◽  
Uel J. McMahan
Keyword(s):  
Spatial Resolution ◽  
Active Zone ◽  
Synaptic Vesicles ◽  
Neuromuscular Junctions ◽  
Electron Tomography ◽  
Structure And Function ◽  
Presynaptic Membrane ◽  
Axon Terminals ◽  
Active Zones ◽  
And Function

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.

scholarly journals Bruchpilot, a Protein with Homology to ELKS/CAST, Is Required for Structural Integrity and Function of Synaptic Active Zones in Drosophila

Neuron ◽  
2006 ◽  
Vol 51 (2) ◽  
pp. 275 ◽  
Author(s):  
Dhananjay A. Wagh ◽  
Tobias M. Rasse ◽  
Esther Asan ◽  
Alois Hofbauer ◽  
Isabell Schwenkert ◽  
...  
Keyword(s):  
Structural Integrity ◽  
Active Zones ◽  
And Function

Release and Recycling of the Readily Releasable Vesicle Population in a Synapse Possessing No Reserve Population

2007 ◽  
Vol 97 (6) ◽  
pp. 4048-4057 ◽  
Author(s):  
J. H. Koenig ◽  
Kazuo Ikeda
Keyword(s):  
High Frequency ◽  
Active Zone ◽  
Neuromuscular Junctions ◽  
Recycling Rate ◽  
Spontaneous Release ◽  
Electron Microscopic ◽  
Single Action ◽  
Active Zones ◽  
Evoked Activity ◽  
Microscopic Techniques

We previously demonstrated that the tergotrochanteral muscle (TTM) of Drosophila is innervated by unique synapses that possess a small readily releasable/recycling vesicle population (active zone population), but not the larger reserve vesicle population observed at other neuromuscular junctions in this animal. Using light and electron microscopic techniques and intracellular recording from the G1 muscle fiber of the TTM, the release and recycling characteristics of the readily releasable/recycling population were observed without any possible contribution from a reserve population. Our results indicate 1) the total number of vesicles in synapses presynaptic to the G1 fiber correlates with the total number of quanta that can be released onto this fiber; 2) the number of quanta released by a single action potential onto the G1 fiber is about one half the number of morphologically “docked” vesicles in active zones onto the G1, and this ratio decreases in a partially depleted state; 3) the recycling rate at 1-Hz stimulation, a frequency that does not cause any depression, is 0.24 recycled vesicle/active zone/s; and 4) normal-appearing spontaneous release occurs from the active zone vesicle population and, unlike synapses that possess a reserve population, the frequency of this release is reduced after high-frequency evoked activity.

scholarly journals Bruchpilot, a Protein with Homology to ELKS/CAST, Is Required for Structural Integrity and Function of Synaptic Active Zones in Drosophila

Neuron ◽  
2006 ◽  
Vol 49 (6) ◽  
pp. 833-844 ◽  
Author(s):  
Dhananjay A. Wagh ◽  
Tobias M. Rasse ◽  
Esther Asan ◽  
Alois Hofbauer ◽  
Isabell Schwenkert ◽  
...  
Keyword(s):  
Structural Integrity ◽  
Active Zones ◽  
And Function

scholarly journals Synaptic vesicle cycling is not impaired in a glutamatergic and a cholinergic synapse that exhibit deficits in acidification and filling

2012 ◽  
Vol 48 (1) ◽  
pp. 155-161
Author(s):  
Bento João Abreu ◽  
Luciana Ferreira Leite ◽  
Débora Lopes Oliveira ◽  
Ernani Amaral
Keyword(s):  
Synaptic Vesicle ◽  
Neuromuscular Junctions ◽  
Vesicle Trafficking ◽  
Experimental Models ◽  
Frog Neuromuscular Junction ◽  
Experimental Conditions ◽  
Pharmacological Agents ◽  
Vesicle Cycling ◽  
Functional Differences ◽  
Styryl Dye

The purpose of the present work was to investigate synaptic vesicle trafficking when vesicles exhibit alterations in filling and acidification in two different synapses: a cholinergic frog neuromuscular junction and a glutamatergic ribbon-type nerve terminal in the retina. These synapses display remarkable structural and functional differences, and the mechanisms regulating synaptic vesicle cycling might also differ between them. The lipophilic styryl dye FM1-43 was used to monitor vesicle trafficking. Both preparations were exposed to pharmacological agents that collapse ΔpH (NH4Cl and methylamine) or the whole ΔµH+ (bafilomycin), a necessary situation to provide the driving force for neurotransmitter accumulation into synaptic vesicles. The results showed that FM1-43 loading and unloading in neuromuscular junctions did not differ statistically between control and experimental conditions (P > 0.05). Also, FM1-43 labeling in bipolar cell terminals proved highly similar under all conditions tested. Despite remarkable differences in both experimental models, the present findings show that acidification and filling are not required for normal vesicle trafficking in either synapse.

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

2022 ◽  
Vol 13 ◽  
Author(s):  
Chengji Piao ◽  
Stephan J. Sigrist
Keyword(s):  
Synaptic Vesicle ◽  
Active Zone ◽  
Stoichiometric Composition ◽  
Release Site ◽  
Homeostatic Plasticity ◽  
Release Factor ◽  
Loosely Coupled ◽  
Action Potential Frequency ◽  
Active Zones ◽  
Potential Frequency

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.

scholarly journals Reconstructing essential active zone functions within a synapse

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.

scholarly journals Regeneration of the active zone at the frog neuromuscular junction.

1984 ◽  
Vol 98 (5) ◽  
pp. 1685-1695 ◽  
Author(s):  
C P Ko
Keyword(s):  
Active Zone ◽  
Transmitter Release ◽  
Neuromuscular Junctions ◽  
Freeze Fracture ◽  
Frog Neuromuscular Junction ◽  
Normal Pattern ◽  
Intramembrane Particles ◽  
Total Length ◽  
Secondary Stage ◽  
Active Zones

The active zone is a unique specialization of the presynaptic membrane and is believed to be the site of transmitter release. The formation of the active zone and the relationship of this process to transmitter release were studied at reinnervated neuromuscular junctions in the frog. At different times after a nerve crush, the cutaneous pectoris muscles were examined with intracellular recording recording and freeze-fracture electron microscopy. The P face of a normal active zone typically consists of two double rows of particles lined up in a continuous segment located opposite a junctional fold. In the initial stage of reinnervation, clusters of large intramembrane particles surrounding membrane elevations appeared on the P face of nerve terminals. Like normal active zones, these clusters were aligned with junctional folds. Vesicle openings, which indicate transmitter release, were seen at these primitive active zones, even though intramembrane particles were not yet organized into the normal pattern of two double rows. The length of active zones at this stage was only approximately 15% of normal. During the secondary stage, every junction was reinnervated and most active zones had begun to organize into the normal pattern with normal orientation. Unlike normal, there were often two or more discontinuous short segments of active zone aligned with the same junctional fold. The total length of active zone per junctional fold increased to one-third of normal, mainly because of the greater number of segments. In the third stage, the number of active zone segments per junctional fold showed almost no change when compared with the secondary stage. However, individual segments elongated and increased the total length of all active zone segments per junctional fold to about two-thirds of the normal length. The dynamic process culminated in the final stage, during which elongating active zones appeared to join together and the number of active zone segments per junctional fold decreased to normal. Thus, in most regions, regeneration of the active zones was complete. These results suggest that the normal organization of two double rows is not necessary for the active zone to be functional. Furthermore, localization of regenerating active zones is related to junctional folds and/or their associated structures.

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