Animal Physiology, Department of Biology, Friedrich-Alexander-University Erlangen-Nuremberg Erlangen, Germany.
Front Cell Neurosci. 2014 Feb 25;8:60. doi: 10.3389/fncel.2014.00060. eCollection 2014.
Synaptic vesicle exocytosis at chemical synapses is followed by compensatory endocytosis. Multiple pathways including Clathrin-mediated retrieval of single vesicles, bulk retrieval of large cisternae, and kiss-and-run retrieval have been reported to contribute to vesicle recycling. Particularly at the continuously active ribbon synapses of retinal photoreceptor and bipolar cells, compensatory endocytosis plays an essential role to provide ongoing vesicle supply. Yet, little is known about the mechanisms that contribute to endocytosis at these highly complex synapses. To identify possible specializations in ribbon synaptic endocytosis during different states of activity, we exposed mice to controlled lighting conditions and compared the distribution of endocytotic proteins at rod and cone photoreceptor, and ON bipolar cell ribbon synapses with light and electron microscopy. In mouse ON bipolar cell terminals, Clathrin-mediated endocytosis seemed to be the dominant mode of endocytosis at all adaptation states analyzed. In contrast, in mouse photoreceptor terminals in addition to Clathrin-coated pits, clusters of membranously connected electron-dense vesicles appeared during prolonged darkness. These clusters labeled for Dynamin3, Endophilin1, and Synaptojanin1, but not for AP180, Clathrin LC, and hsc70. We hypothesize that rod and cone photoreceptors possess an additional Clathrin-independent mode of vesicle retrieval supporting the continuous synaptic vesicle supply during prolonged high activity.
在化学突触中，突触小泡的胞吐作用之后会伴随着补偿性内吞作用。已经报道了多种途径，包括网格蛋白介导的单个囊泡回收、大潴泡的批量回收以及吻-跑回收，这些途径都有助于囊泡的再循环。特别是在视网膜光感受器和双极细胞的持续活跃的带状突触中，补偿性内吞作用对于提供持续的囊泡供应起着至关重要的作用。然而，对于这些高度复杂的突触中内吞作用的机制，我们知之甚少。为了确定在不同活动状态下带状突触内吞作用的可能特殊化，我们让小鼠暴露在受控的光照条件下，并通过光镜和电镜比较了在 rod 和 cone 光感受器以及 ON 双极细胞带状突触中内吞蛋白的分布。在小鼠的 ON 双极细胞末端，网格蛋白介导的内吞作用似乎是在所有分析的适应状态下的主要内吞作用模式。相比之下，在小鼠光感受器末端，除了网格蛋白包被的凹陷外，在长时间的黑暗中还出现了膜连接的电子致密囊泡簇。这些簇标记了 Dynamin3、Endophilin1 和 Synaptojanin1，但不标记 AP180、网格蛋白 LC 和 hsc70。我们假设 rod 和 cone 光感受器具有额外的网格蛋白非依赖性囊泡回收方式，这种方式支持在长时间的高活性期间持续的突触囊泡供应。
