Bacaj Taulant, Wu Dick, Burré Jacqueline, Malenka Robert C, Liu Xinran, Südhof Thomas C
Department of Molecular and Cellular Physiology and Howard Hughes Medical Institute, Stanford University, Stanford, California, United States of America.
Department of Molecular and Cellular Physiology and Howard Hughes Medical Institute, Stanford University, Stanford, California, United States of America; Nancy Pritzker Laboratory, Department of Psychiatry and Behavioral Sciences, Stanford University Medical School, Stanford, California, United States of America.
PLoS Biol. 2015 Oct 5;13(10):e1002267. doi: 10.1371/journal.pbio.1002267. eCollection 2015 Oct.
In forebrain neurons, Ca(2+) triggers exocytosis of readily releasable vesicles by binding to synaptotagmin-1 and -7, thereby inducing fast and slow vesicle exocytosis, respectively. Loss-of-function of synaptotagmin-1 or -7 selectively impairs the fast and slow phase of release, respectively, but does not change the size of the readily-releasable pool (RRP) of vesicles as measured by stimulation of release with hypertonic sucrose, or alter the rate of vesicle priming into the RRP. Here we show, however, that simultaneous loss-of-function of both synaptotagmin-1 and -7 dramatically decreased the capacity of the RRP, again without altering the rate of vesicle priming into the RRP. Either synaptotagmin-1 or -7 was sufficient to rescue the RRP size in neurons lacking both synaptotagmin-1 and -7. Although maintenance of RRP size was Ca(2+)-independent, mutations in Ca(2+)-binding sequences of synaptotagmin-1 or synaptotagmin-7--which are contained in flexible top-loop sequences of their C2 domains--blocked the ability of these synaptotagmins to maintain the RRP size. Both synaptotagmins bound to SNARE complexes; SNARE complex binding was reduced by the top-loop mutations that impaired RRP maintenance. Thus, synaptotagmin-1 and -7 perform redundant functions in maintaining the capacity of the RRP in addition to nonredundant functions in the Ca(2+) triggering of different phases of release.
在前脑神经元中，Ca(2+) 通过与突触结合蛋白 -1 和 -7 结合来触发易释放囊泡的胞吐作用，从而分别诱导快速和慢速囊泡胞吐。突触结合蛋白 -1 或 -7 的功能丧失分别选择性地损害释放的快速和慢速阶段，但不会改变通过用高渗蔗糖刺激释放所测量的易释放囊泡池（RRP）的大小，也不会改变囊泡引发进入RRP的速率。然而，我们在此表明，突触结合蛋白 -1 和 -7 的同时功能丧失会显著降低RRP的容量，同样不会改变囊泡引发进入RRP的速率。突触结合蛋白 -1 或 -7 中的任何一种都足以挽救缺乏突触结合蛋白 -1 和 -7 的神经元中的RRP大小。尽管RRP大小的维持不依赖于Ca(2+)，但突触结合蛋白 -1 或突触结合蛋白 -7 的Ca(2+) 结合序列中的突变（这些序列包含在其C2结构域的柔性顶部环序列中）会阻断这些突触结合蛋白维持RRP大小的能力。两种突触结合蛋白都与SNARE复合体结合；SNARE复合体结合因损害RRP维持的顶部环突变而减少。因此，除了在Ca(2+) 触发不同释放阶段中的非冗余功能外，突触结合蛋白 -1 和 -7 在维持RRP容量方面具有冗余功能。
