Active Zone Material-Directed Orientation, Docking, and Fusion of Dense Core Vesicles Alongside Synaptic Vesicles at Neuromuscular Junctions.

Jae H Jung,Uel J Mcmahan,Robert M Marshall,Joseph A Szule,Kylee Stouder

doi:10.3389/fnana.2018.00072

Jae H Jung, Uel J Mcmahan + Show 3 more

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https://doi.org/10.3389/fnana.2018.00072

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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.

Highlights

Active zones along the presynaptic plasma membrane of chemical synapses are characterized by one or more dense aggregates of proteinaceous macromolecules called active zone material (AZM), which are attached to the presynaptic membrane and extend into the cytosol
Alongside and connected to the AZM macromolecules was an -shaped profile containing a dense core, which we interpret as a former dense core vesicles (DCVs) that had fused with the presynaptic membrane (Figure 1C)
In addition to AZM macromolecules, the 10 docked or docking DCVs at active zones fixed at rest and during impulse activity, as well as the former docked DCV that had fused with the presynaptic membrane, were connected to elongate nonAZM macromolecules (e.g., Figures 2, 3)

Summary

Introduction

Active zones along the presynaptic plasma membrane of chemical synapses are characterized by one or more dense aggregates of proteinaceous macromolecules (variously shaped filaments) called active zone material (AZM), which are attached to the presynaptic membrane and extend into the cytosol. The size, shape, and distribution of AZM can vary from one synaptic type to another within an animal species and for the same synaptic type between species (Palade, 1954; Palay, 1956; Zhai and Bellen, 2004). Despite such variation, synaptic vesicles (SVs), which contain small molecule neurotransmitters, move toward and dock on, i.e., are held in contact with, the presynaptic membrane alongside AZM. Several lines of evidence make it seem likely that the AZM macromolecules include proteins shown by biochemistry to regulate both the docking and fusion of SVs (Harlow et al, 2001; Szule et al, 2012; Jung et al, 2016)

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