Sharp Ca²⁺ nanodomains beneath the ribbon promote highly synchronous multivesicular release at hair cell synapses.
Sharp Ca²⁺ nanodomains beneath the ribbon promote highly synchronous multivesicular release at hair cell synapses.
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DOI：
10.1523/jneurosci.1866-11.2011
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发表时间：
2011-11-16
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影响因子：
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通讯作者：
von Gersdorff H
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作者：
Graydon CW;Cho S;Li GL;Kachar B;von Gersdorff H
毛细胞带状突触表现出几个显著特征。在结构上，致密体或带状物锚定在突触前膜上，并束缚突触囊泡;在功能上，神经递质释放由看似同步的多泡释放产生的大EPSC事件主导。然而，突触带在促进这种形式的释放中的具体作用仍然是难以捉摸的。使用完整的超微结构重建和电容测量的牛蛙两栖类乳头毛细胞透析与高浓度的缓慢的Ca 2+缓冲液（10 mM EGTA），我们发现，突触囊泡的数量在该带的基础上密切相关的那些囊泡，最迅速和有效地释放，而其余的带状拴系囊泡相关的第二个，较慢的池的囊泡。结合持久性的多泡释放在极端的Ca 2+缓冲条件下（10 mM BAPTA），我们的数据反对Ca 2+依赖的化合物融合的带状拴系囊泡在毛细胞突触。此外，在毛细胞去极化，我们的研究结果表明，升高的钙离子水平提高囊泡池补充率。最后，使用Ca 2+扩散模拟，我们建议，丝带和它的囊泡定义一个小的细胞质体积，其中Ca 2+缓冲液是饱和的，尽管10 mM BAPTA条件。这种局部缓冲饱和允许突触带下方的释放位点附近的快速和大的Ca 2+上升，其可以触发多量子EPSC。我们的结论是，通过限制可用的突触前体积，丝带可能创造条件同步释放一小群停靠囊泡。
Hair cell ribbon synapses exhibit several distinguishing features. Structurally, a dense body, or ribbon, is anchored to the presynaptic membrane and tethers synaptic vesicles; functionally, neurotransmitter release is dominated by large EPSC events produced by seemingly synchronous multivesicular release. However, the specific role of the synaptic ribbon in promoting this form of release remains elusive. Using complete ultrastructural reconstructions and capacitance measurements of bullfrog amphibian papilla hair cells dialyzed with high concentrations of a slow Ca2+ buffer (10 mM EGTA), we found that the number of synaptic vesicles at the base of the ribbon correlated closely to those vesicles that released most rapidly and efficiently, while the rest of the ribbon-tethered vesicles correlated to a second, slower pool of vesicles. Combined with the persistence of multivesicular release in extreme Ca2+ buffering conditions (10 mM BAPTA), our data argues against the Ca2+-dependent compound fusion of ribbon-tethered vesicles at hair cell synapses. Moreover, during hair cell depolarization, our results suggest that elevated Ca2+ levels enhance vesicle pool replenishment rates. Finally, using Ca2+ diffusion simulations, we propose that the ribbon and its vesicles define a small cytoplasmic volume where Ca2+ buffer is saturated, despite 10 mM BAPTA conditions. This local buffer saturation permits fast and large Ca2+ rises near release sites beneath the synaptic ribbon that can trigger multiquantal EPSCs. We conclude that, by restricting the available presynaptic volume, the ribbon may be creating conditions for the synchronous release of a small cohort of docked vesicles.