Connections of Synaptic Vesicles to Active Zone Material Before and after Docking on the Presynaptic

Abstract

The active zones of axon terminals, which are where the initial events in synaptic transmission occur, are characterized in part by dense aggregates of macromolecules, called active zone material (AZM), attached to the cytoplasmic surface of the presynaptic plasma membrane and by synaptic vesicles docked on the presynaptic membrane, which contain neurotransmitter and fuse with the presynaptic membrane to release their neurotransmitter during synaptic transmission. Previous electron tomography (ET) studies on the 15nm of AZM next to the presynaptic membrane of axon terminals at resting frog and mouse neuromuscular junctions (NMJs) showed that it is composed of an organized network of elongate macromolecules that fall into several classes, one of which, ribs, connects to each docked vesicle.We used ET to study in axon terminals at resting frog NMJs the remaining 45nm of the AZM network deep to the presynaptic membrane. Like that in the initial 15nm, it is a highly organized network containing several classes of elongate macromolecules. Some of these, spars and booms, also connect to docked vesicles. Thus, each docked vesicle is connected to 3-4 ribs, 2-3 spars and 4-8 booms. For NMJs that had been fixed during electrical stimulation, we found that at those active zones where the docked vesicles had fused with and flattened into the presynaptic membrane, the vesicles moving from the reserve pool to replace them at the docking sites on the presynaptic membrane are connected to the ribs, spars and booms when 20-30 nm from the membrane. Altogether the findings support the hypothesis (Harlow et al., Nature 409: 479-484, 2001; Nagwaney et al., J. Comp. Neurol. 513: 457-468, 2009) that AZM helps regulate the docking of synaptic vesicles on the presynaptic membrane.