Schonn Jean-Sébastien, Maximov Anton, Lao Ye, Südhof Thomas C, Sørensen Jakob B
Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen, Germany.
Proc Natl Acad Sci U S A. 2008 Mar 11;105(10):3998-4003. doi: 10.1073/pnas.0712373105. Epub 2008 Feb 28.
Synaptotagmin-1, the canonical isoform of the synaptotagmin family, is a Ca(2+) sensor for fast synchronous neurotransmitter release in forebrain neurons and chromaffin cells. Even though deletion of synaptotagmin-1 abolishes fast exocytosis in chromaffin cells, it reduces overall secretion by only 20% because of the persistence of slow exocytosis. Therefore, another Ca(2+) sensor dominates release in these cells. Synaptotagmin-7 has a higher Ca(2+) affinity and slower binding kinetics than synaptotagmin-1, matching the proposed properties for the second, slower Ca(2+) sensor. Here, we examined Ca(2+)-triggered exocytosis in chromaffin cells from KO mice lacking synaptotagmin-7, and from knockin mice containing normal levels of a mutant synaptotagmin-7 whose C(2)B domain does not bind Ca(2+). In both types of mutant chromaffin cells, Ca(2+)-triggered exocytosis was decreased dramatically. Moreover, in chromaffin cells lacking both synaptotagmin-1 and -7, only a very slow release component, accounting for approximately 30% of WT exocytosis, persisted. These data establish synaptotagmin-7 as a major Ca(2+) sensor for exocytosis in chromaffin cells, which, together with synaptotagmin-1, mediates almost all of the Ca(2+) triggering of exocytosis in these cells, a surprising result, considering the lack of a role of synaptotagmin-7 in synaptic vesicle exocytosis.
突触结合蛋白-1是突触结合蛋白家族的典型亚型，是前脑神经元和嗜铬细胞中快速同步神经递质释放的Ca(2+)传感器。尽管删除突触结合蛋白-1会消除嗜铬细胞中的快速胞吐作用，但由于慢速胞吐作用的持续存在，它只会使总体分泌减少20%。因此，另一种Ca(2+)传感器主导了这些细胞中的释放。与突触结合蛋白-1相比，突触结合蛋白-7具有更高的Ca(2+)亲和力和更慢的结合动力学，与所提出的第二种较慢的Ca(2+)传感器的特性相匹配。在这里，我们研究了来自缺乏突触结合蛋白-7的基因敲除小鼠以及含有正常水平的C(2)B结构域不结合Ca(2+)的突变型突触结合蛋白-7的基因敲入小鼠的嗜铬细胞中Ca(2+)触发的胞吐作用。在这两种类型的突变嗜铬细胞中，Ca(2+)触发的胞吐作用都显著降低。此外，在同时缺乏突触结合蛋白-1和-7的嗜铬细胞中，仅持续存在一种非常缓慢的释放成分，约占野生型胞吐作用的30%。这些数据表明突触结合蛋白-7是嗜铬细胞中胞吐作用的主要Ca(2+)传感器，它与突触结合蛋白-1一起介导了这些细胞中几乎所有的Ca(2+)触发的胞吐作用，考虑到突触结合蛋白-7在突触小泡胞吐作用中缺乏作用，这是一个令人惊讶的结果。
