Synaptotagmin-7 Is Essential for Ca2+-Triggered Delayed Asynchronous Release But Not for Ca2+-Dependent Vesicle Priming in Retinal Ribbon Synapses
Synaptotagmin-7 Is Essential for Ca2+-Triggered Delayed Asynchronous Release But Not for Ca2+-Dependent Vesicle Priming in Retinal Ribbon Synapses
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DOI：
10.1523/jneurosci.0759-15.2015
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发表时间：
2015-08-05
影响因子：
5.3
通讯作者：
Suedhof, Thomas C.
中科院分区：
文献类型：
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作者：
Luo, Fujun;Bacaj, Taulant;Suedhof, Thomas C.
大多数突触在两个阶段释放神经递质：（1）持续几毫秒的快速同步阶段;（2）持续数百毫秒的延迟“异步”阶段。Ca 2+通过与synaptotagmin-1、synaptotagmin-2或synaptotagmin-9结合触发快速同步神经递质释放，但Ca 2+如何触发延迟异步释放长期以来一直是个谜。最近的研究结果表明，与突触结合蛋白-7在神经内分泌胞吐中的Ca 2+传感器功能一致，突触结合蛋白-7也可以作为突触囊泡胞吐的Ca 2+传感器，但在延迟异步释放过程中起作用。令人困惑的是，随后的一项研究假设，突触结合蛋白-7不是释放的Ca 2+传感器，但介导强烈刺激后的Ca 2+依赖性囊泡再引发。为了解决这些问题，我们在这里分析了突触传递在杆双极神经元AII无长突细胞突触急性小鼠视网膜切片作为模型系统。使用配对录音，我们表明，敲除synaptotagmin-7选择性损害延迟异步释放，但不快速同步释放。延迟异步释放被阻断在野生型突触细胞内添加高浓度的缓慢的钙离子螯合剂EGTA，但EGTA没有影响synaptotagmin-7敲除神经元，因为延迟异步释放已经受损。此外，直接测量囊泡再引发未能揭示突触结合蛋白-7敲除对囊泡再引发的影响。我们的数据表明，synaptotagmin-7是选择性的视网膜杆双极细胞突触的钙依赖性延迟异步释放必不可少的，它的功能可以通过简单地引入一个缓慢的钙缓冲到细胞中被阻止，synaptotagmin-7是不需要正常的囊泡重启动。
Most synapses release neurotransmitters in two phases: (1) a fast synchronous phase lasting a few milliseconds; and (2) a delayed "asynchronous" phase lasting hundreds of milliseconds. Ca2+ triggers fast synchronous neurotransmitter release by binding to synaptotagmin-1, synaptotagmin-2, or synaptotagmin-9, but how Ca2+ triggers delayed asynchronous release has long remained enigmatic. Recent results suggested that consistent with the Ca2+-sensor function of synaptotagmin-7 in neuroendocrine exocytosis, synaptotagmin-7 also functions as a Ca2+ sensor for synaptic vesicle exocytosis but operates during delayed asynchronous release. Puzzlingly, a subsequent study postulated that synaptotagmin-7 is not a Ca2+ sensor for release but mediates Ca2+-dependent vesicle repriming after intense stimulation. To address these issues, we here analyzed synaptic transmission at rod bipolar neuron-AII amacrine cell synapses in acute mouse retina slices as a model system. Using paired recordings, we show that knock-out of synaptotagmin-7 selectively impairs delayed asynchronous release but not fast synchronous release. Delayed asynchronous release was blocked in wildtype synapses by intracellular addition of high concentrations of the slow Ca2+-chelator EGTA, but EGTA had no effect in synaptotagmin-7 knock-out neurons because delayed asynchronous release was already impaired. Moreover, direct measurements of vesicle repriming failed to uncover an effect of the synaptotagmin-7 knock-out on vesicle repriming. Our data demonstrate that synaptotagmin-7 is selectively essential for Ca2+-dependent delayed asynchronous release in retinal rod bipolar cell synapses, that its function can be blocked by simply introducing a slow Ca2+ buffer into the cells, and that synaptotagmin-7 is not required for normal vesicle repriming.