Quantifying the synaptic Ca2+-binding kinetics of Synaptotagmin-1, the Ca2+ sensor for transmitter release in the forebrain

中枢神经系统神经元通过Ca2+触发释放神经递质从突触前终末进行通信。在终端电压门控Ca2+通道的去极化打开和流入的Ca2+建立一个陡峭的，短暂的浓度梯度周围的开口通道的入口，迅速减少与通道的距离增加。Ca2+结合到专门的Ca2+传感器蛋白（Synaptotagmins, Syt），从而触发神经递质填充囊泡与突触前活性区质膜的融合，并随后释放递质。由于Ca2+梯度的陡峭和短暂的持续时间，在这个过程中永远不会建立化学平衡。这使得释放传感器的细胞内Ca2+结合动力学（相对于其在平衡状态下的亲和力）在控制释放的速度、可靠性和调节能力方面处于中心地位。因此，为了定量了解突触信息传递及其通过突触可塑性进行的调节，需要了解释放传感器的细胞内Ca2+结合动力学。两种Syt亚型，Syt1和Syt2，是触发大脑快速递质释放的主要Ca2+传感器；然而，只有Ca2+与Syt2的结合，后脑的显性同工异构体在突触中被详细研究。前脑中的主要传感器Syt1被认为表现不同，但其突触Ca2+结合动力学的详细定量数据缺失。解决这一主要的不确定性，以理解突触信息流及其调控在前脑区域，如新皮层是本提案的主题。

Central nervous system neurons communicate via Ca2+-triggered release of neurotransmitters from presynaptic terminals. Upon depolarization of a terminal voltage-gated Ca2+ channels open and the inflowing Ca2+ builds a steep, short lasting concentration gradient around the mouth of the open channel that rapidly diminishes with increasing distance from the channel. Ca2+ binds to specialized Ca2+ sensor proteins (Synaptotagmins, Syt), thereby, triggering the fusion of neurotransmitter filled vesicles with the plasma membrane of the presynaptic active zone and the subsequent release of the transmitter. Due to the steepness and short duration of the Ca2+ gradient a chemical equilibrium is never established in this process. This makes the intracellular Ca2+-binding kinetics of the release sensor (as opposed by its affinity at equilibrium) central in the control of speed, reliability and modulation capability of release. Thus, in order to derive a quantitative understanding of synaptic information transfer but also of its modulation via synaptic plasticity it is required to know the intracellular Ca2+-binding kinetics of the release sensor. Two Syt isoforms, Syt1 and Syt2, are the main Ca2+ sensors triggering fast transmitter release in the brain; however, only Ca2+-binding to Syt2, the dominant isoform in the hindbrain has been studied in detail in synapses. Syt1, the dominating sensor in the forebrain, is thought to behave differently but detailed quantitative data of its synaptic Ca2+-binding kinetics are missing. Resolving this major uncertainty for understanding synaptic information flow and its regulation in forebrain regions like the neocortex is topic of the present proposal.