Department of Neurobiology, Stanford University School of Medicine, Stanford, California, USA.
PLoS One. 2013 Jul 22;8(7):e69410. doi: 10.1371/journal.pone.0069410. Print 2013.
Synaptic vesicles dock at active zones on the presynaptic plasma membrane of a neuron's axon terminals as a precondition for fusing with the membrane and releasing their neurotransmitter to mediate synaptic impulse transmission. Typically, docked vesicles are next to aggregates of plasma membrane-bound macromolecules called active zone material (AZM). Electron tomography on tissue sections from fixed and stained axon terminals of active and resting frog neuromuscular junctions has led to the conclusion that undocked vesicles are directed to and held at the docking sites by the successive formation of stable connections between vesicle membrane proteins and proteins in different classes of AZM macromolecules. Using the same nanometer scale 3D imaging technology on appropriately stained frog neuromuscular junctions, we found that ∼10% of a vesicle's luminal volume is occupied by a radial assembly of elongate macromolecules attached by narrow projections, nubs, to the vesicle membrane at ∼25 sites. The assembly's chiral, bilateral shape is nearly the same vesicle to vesicle, and nubs, at their sites of connection to the vesicle membrane, are linked to macromolecules that span the membrane. For docked vesicles, the orientation of the assembly's shape relative to the AZM and the presynaptic membrane is the same vesicle to vesicle, whereas for undocked vesicles it is not. The connection sites of most nubs on the membrane of docked vesicles are paired with the connection sites of the different classes of AZM macromolecules that regulate docking, and the membrane spanning macromolecules linked to these nubs are also attached to the AZM macromolecules. We conclude that the luminal assembly of macromolecules anchors in a particular arrangement vesicle membrane macromolecules, which contain the proteins that connect the vesicles to AZM macromolecules during docking. Undocked vesicles must move in a way that aligns this arrangement with the AZM macromolecules for docking to proceed.
突触小泡在神经元轴突末梢的突触前质膜的活性区停靠，作为与膜融合并释放其神经递质以介导突触冲动传递的前提条件。通常，停靠的小泡位于称为活性区物质（AZM）的质膜结合大分子聚集物旁边。对固定和染色的活性和静止蛙运动神经元突触的轴突末梢的组织切片进行电子断层扫描，得出的结论是，未停靠的小泡通过囊泡膜蛋白与不同类别的 AZM 大分子中的蛋白之间稳定连接的连续形成被引导到停靠位点并保持在该位点。在适当染色的蛙运动神经元突触上使用相同的纳米级 3D 成像技术，我们发现小泡内腔体积的约 10%被一组拉长的大分子占据，这些大分子通过狭窄的突起、小结与囊泡膜相连，在约 25 个位点处。该组装体的手性、双侧形状在囊泡之间几乎相同，而在与囊泡膜连接的位点处的小结与跨膜的大分子相连。对于停靠的小泡，组装体形状相对于 AZM 和突触前质膜的取向在囊泡之间是相同的，而对于未停靠的小泡则不是。大多数小结在停靠小泡膜上的连接位点与调节停靠的不同类别的 AZM 大分子的连接位点配对，与这些小结相连的跨膜大分子也与 AZM 大分子相连。我们得出结论，内腔大分子的组装以特定的方式锚定囊泡膜大分子，这些大分子包含将囊泡与 AZM 大分子连接的蛋白质，在停靠过程中。未停靠的小泡必须以一种方式移动，使这种排列与 AZM 大分子对齐，以便进行停靠。
