Sherry David M, Heidelberger Ruth
University of Houston College of Optometry, Houston, Texas 77204-2020, USA.
J Comp Neurol. 2005 Apr 18;484(4):440-57. doi: 10.1002/cne.20504.
Current models of synaptic transmission require retrieval of membrane from the presynaptic terminal following neurotransmitter exocytosis. Dynamin, a GTPase, is thought to be critical for this retrieval process. At ribbon synapses of retinal bipolar neurons, however, compensatory endocytosis does not require GTP hydrolysis, suggesting that endocytosis mechanisms may differ among synapses. To understand better the synaptic vesicle recycling at conventional and ribbon synapses, the distributions of dynamin and two associated proteins, amphiphysin and clathrin, were examined in the retinas of goldfish and mouse by using immunocytochemical methods. Labeling for dynamin, clathrin, and amphiphysin was distributed differentially among conventional and ribbon synapses in retinas of both species. Ribbon synapses of photoreceptors and most bipolar cells labeled only weakly for dynamin relative to conventional synapses. Amphyiphysin labeling was strong at many ribbon synapses, and labeling in rod terminals was stronger than in cone terminals in the mouse retina. Clathrin labeling was heterogeneous among ribbon synapses. Similarly to the case with amphiphysin, mouse rod terminals showed stronger clathrin labeling than cone terminals. Among conventional synapses, there was heterogeneous labeling for all three endocytic proteins. Some labeling for each protein might have been associated with postsynaptic terminals. The differential distribution of labeling for these proteins among identified synapses in the retina suggests considerable heterogeneity in the molecular mechanisms underlying synaptic membrane retrieval, even among synapses with similar active zone ultrastructure. Thus, as with exocytosis, mechanisms of synaptic membrane retrieval may be tuned by the precise complement of proteins expressed within the synaptic terminal.
当前的突触传递模型认为，神经递质胞吐作用后，突触前终末需要回收膜。动力蛋白是一种GTP酶，被认为对这一回收过程至关重要。然而，在视网膜双极神经元的带状突触中，代偿性内吞作用并不需要GTP水解，这表明不同突触的内吞机制可能存在差异。为了更好地理解传统突触和带状突触处的突触小泡循环，我们利用免疫细胞化学方法检测了金鱼和小鼠视网膜中动力蛋白以及两种相关蛋白——发动蛋白和网格蛋白的分布情况。在这两个物种的视网膜中，动力蛋白、网格蛋白和发动蛋白的标记在传统突触和带状突触中的分布存在差异。相对于传统突触，光感受器和大多数双极细胞的带状突触对动力蛋白的标记较弱。在许多带状突触处，发动蛋白的标记很强，并且在小鼠视网膜中，视杆终末的标记比视锥终末更强。网格蛋白的标记在带状突触中是异质性的。与发动蛋白的情况类似，小鼠视杆终末的网格蛋白标记比视锥终末更强。在传统突触中，这三种内吞蛋白的标记也是异质性的。每种蛋白的一些标记可能与突触后终末有关。这些蛋白在视网膜中已确定的突触间标记的差异分布表明，即使在具有相似活性区超微结构的突触中，突触膜回收的分子机制也存在相当大的异质性。因此，与胞吐作用一样，突触膜回收机制可能由突触终末内表达的精确蛋白组合来调节。
