Active zone material at the nervous system's synapses is situated next to synaptic vesicles that are docked at the presynaptic plasma membrane, and calcium channels that are anchored in the membrane. Here we use electron microscope tomography to show the arrangement and associations of structural components of this compact organelle at a model synapse, the frog's neuromuscular junction. Our findings indicate that the active zone material helps to dock the vesicles and anchor the channels, and that its architecture provides both a particular spatial relationship and a structural linkage between them. The structural linkage may include proteins that mediate the calcium-triggered exocytosis of neurotransmitter by the synaptic vesicles during synaptic transmission.

The docking of synaptic vesicles at active zones on the presynaptic plasma membrane of axon terminals is essential for their fusion with the membrane and exocytosis of their neurotransmitter to mediate synaptic impulse transmission. Dense networks of macromolecules, called active zone material, (AZM) are attached to the presynaptic membrane next to docked vesicles. Electron tomography has shown that some AZM macromolecules are connected to docked vesicles, leading to the suggestion that AZM is somehow involved in the docking process. We used electron tomography on the simply arranged active zones at frog neuromuscular junctions to characterize the connections of AZM to docked synaptic vesicles and to search for the establishment of such connections during vesicle docking. We show that each docked vesicle is connected to 10–15 AZM macromolecules, which fall into four classes based on several criteria including their position relative to the presynaptic membrane. In activated axon terminals fixed during replacement of docked vesicles by previously undocked vesicles, undocked vesicles near vacated docking sites on the presynaptic membrane have connections to the same classes of AZM macromolecules that are connected to docked vesicles in resting terminals. The number of classes and the total number of macromolecules to which the undocked vesicles are connected are inversely proportional to the vesicles’ distance from the presynaptic membrane. We conclude that vesicle movement toward and maintenance at docking sites on the presynaptic membrane are directed by an orderly succession of stable interactions between the vesicles and distinct classes of AZM macromolecules positioned at different distances from the membrane. Establishing the number, arrangement and sequence of association of AZM macromolecules involved in vesicle docking provides an anatomical basis for testing and extending concepts of docking mechanisms provided by biochemistry.

Electron tomography was used to view macromolecules composing active zone material (AZM) in axon terminals at mouse neuromuscular junctions. Connections of the macromolecules to each other, to calcium channels in the presynaptic membrane and to synaptic vesicles docked on the membrane prior to fusing with it during synaptic transmission were similar to those of AZM macromolecules at frog neuromuscular junctions previously examined by electron tomography and support the hypothesis that AZM regulates vesicle docking and fusion. A species difference in the arrangement of AZM relative to docked vesicles may help account for a greater vesicle-presynaptic membrane contact area during docking and a greater probability of fusion during synaptic transmission in mouse. Certain AZM macromolecules in mouse were connected to synaptic vesicles contacting the presynaptic membrane at sites where fusion does not occur. These secondary docked vesicles had a different relationship to the membrane and AZM macromolecules than primary docked vesicles consistent with their having a different AZM-regulated behavior.

The priming of a docked synaptic vesicle determines the probability of its membrane (VM) fusing with the presynaptic membrane (PM) when a nerve impulse arrives. To gain insight into the nature of priming, we searched by electron tomography for structural relationships correlated with fusion probability at active zones of axon terminals at frog neuromuscular junctions. For terminals fixed at rest, the contact area between the VM of docked vesicles and PM varied >10-fold with a normal distribution. There was no merging of the membranes. For terminals fixed during repetitive evoked synaptic transmission, the normal distribution of contact areas was shifted to the left, due in part to a decreased number of large contact areas, and there was a subpopulation of large contact areas where the membranes were hemifused, an intermediate preceding complete fusion. Thus, fusion probability of a docked vesicle is related to the extent of its VM-PM contact area. For terminals fixed 1 h after activity, the distribution of contact areas recovered to that at rest, indicating the extent of a VM-PM contact area is dynamic and in equilibrium. The extent of VM-PM contact areas in resting terminals correlated with eccentricity in vesicle shape caused by force toward the PM and with shortness of active zone material macromolecules linking vesicles to PM components, some thought to include Ca 2+ channels. We propose that priming is a variable continuum of events imposing variable fusion probability on each vesicle and is regulated by force-generating shortening of active zone material macromolecules in dynamic equilibrium.synapse | hemifusion | priming | active zone material | electron tomography S ynaptic vesicles (SVs) move toward and dock on (are held in contact with) the presynaptic plasma membrane (PM) of a neuron's axon terminal before fusing with the PM and releasing their neurotransmitter into the synaptic cleft to mediate synaptic transmission (1, 2). Docking is achieved by force-generating interactions of the vesicle membrane (VM) protein synaptobrevin with the PM proteins syntaxin and SNAP25 (3, 4). These interactions, which produce force by forming a coiled coil called the SNARE core complex, are regulated by auxiliary proteins (1, 5-7). Such force on the VM-PM contact site may also play a role in their fusion (8). At typical synapses, docking and fusion take place at structurally specialized regions along the PM called active zones (9, 10). Several lines of evidence suggest that the formation of the SNARE core complex occurs in the macromolecules composing the common active zone organelle, active zone material (AZM) (2,(11)(12)(13)(14), which is positioned near Ca 2+ channels concentrated in the PM at active zones (15-17). Influx of Ca 2+ through the channels after the arrival of a nerve impulse triggers fusion of the VM of docked SVs with the PM.Priming is a step in synaptic transmission between the docking of an SV on, and fusion with, the PM and accounts for the observation that relatively few docked SVs fuse with the PM...