Lehigh University, Department of Biological Sciences, Bethlehem, PA 18015, USA.
Neuroscience. 2010 Oct 13;170(2):441-52. doi: 10.1016/j.neuroscience.2010.07.021. Epub 2010 Jul 16.
Synapsin is a phosphoprotein reversibly associated with synaptic vesicles. We investigated synapsin function in mediating synaptic activity during intense stimulation at Drosophila motor boutons. Electron microscopy analysis of synapsin(-) boutons demonstrated that synapsin maintains vesicle clustering over the periphery of the bouton. Cyclosporin A pretreatment disrupted peripheral vesicle clustering, presumably due to increasing synapsin phosphorylated state. Labeling recycling vesicles with a fluorescent dye FM1-43 followed by photoconversion of the dye into electron dense product demonstrated that synapsin deficiency does not affect mixing of the reserve and recycling vesicle pools but selectively reduces the size of the reserve pool. Intense stimulation produced a significant increase in vesicle abundance and vesicle redistribution toward the central core of synapsin (+) boutons, while in synapsin (-) boutons the area occupied by vesicles did not change and the increase in vesicle numbers was not as prominent. However, intense stimulation produced an increase in basal release at synapsin(-) but not in synapsin(+) boutons, suggesting that synapsin may direct vesicles to the reserve pool. Finally, synapsin deficiency inhibited an increase in quantal size and formation of endosome-like cisternae, which was activated either by intense electrical stimulation or by high K(+) application. Taken together, these results elucidate a novel synapsin function, specifically, promoting vesicle reuptake and reserve pool formation upon intense stimulation.
突触结合蛋白是一种与突触小泡可逆结合的磷酸化蛋白。我们研究了突触结合蛋白在介导果蝇运动终板强刺激过程中的突触活动中的功能。突触结合蛋白（-）终板的电子显微镜分析表明，突触结合蛋白维持着囊泡在终板周边的聚集。环孢素 A 预处理破坏了周边囊泡的聚集，推测是由于突触结合蛋白磷酸化状态的增加。用荧光染料 FM1-43 标记再循环囊泡，然后将染料光转化为电子致密产物，表明突触结合蛋白缺乏不会影响储备和再循环囊泡池的混合，但选择性地减小储备池的大小。强烈刺激会导致囊泡数量显著增加，并向突触结合蛋白（+）终板的中央核心重新分布，而在突触结合蛋白（-）终板中，囊泡所占面积没有变化，囊泡数量的增加也不明显。然而，强烈刺激会导致突触结合蛋白（-）终板而不是突触结合蛋白（+）终板基础释放增加，这表明突触结合蛋白可能将囊泡引导到储备池。最后，突触结合蛋白缺乏抑制了量子大小的增加和内体样小凹的形成，这是由强烈的电刺激或高钾应用激活的。综上所述，这些结果阐明了突触结合蛋白的一个新功能，即在强烈刺激下促进囊泡再摄取和储备池的形成。
