Andrews L D, Cohen A I
J Cell Biol. 1983 Sep;97(3):749-55. doi: 10.1083/jcb.97.3.749.
We performed electron microscopy of replicas from freeze-fractured retinas exposed during or after fixation to the cholesterol-binding antibiotic, filipin. We observed characteristic filipin-induced perturbations throughout the disk and plasma membranes of retinal rod outer segments of various species. It is evident that a prolonged exposure to filipin in fixative enhances rather than reduces presumptive cholesterol detection in the vertebrate photoreceptor cell. In agreement with the pattern seen in our previous study (Andrews, L.D., and A. I. Cohen, 1979, J. Cell Biol., 81:215-228), filipin-binding in membranes exhibiting particle-free patches seemed largely confined to these patches. Favorably fractured photoreceptors exhibited marked filipin-binding in apical inner segment plasma membrane topologically confluent with and proximate to the outer segment plasma membrane, which was comparatively free of filipin binding. A possible boundary between these differing membrane domains was suggested in a number of replicas exhibiting lower filipin binding to the apical plasma membrane of the inner segment in the area surrounding the cilium. This area contains a structure (Andrews, L. D., 1982, Freeze-fracture studies of vertebrate photoreceptors, In Structure of the Eye, J. G. Hollyfield and E. Acosta Vidrio, editors, Elsevier/North-Holland, New York, 11-23) that resembles the active zones of the nerve terminals for the frog neuromuscular junction. These observations lead us to hypothesize that these structures may function to direct vesicle fusion to occur near them, in a domain of membrane more closely resembling outer than inner segment plasma membrane. The above evidence supports the views that (a) all disk membranes contain cholesterol, but the particle-free patches present in some disks trap cholesterol from contiguous particulate membrane regions; (b) contiguous inner and outer segment membranes may greatly differ in cholesterol content; and (c) the suggested higher cholesterol in the inner segment than in the outer segment plasma membrane may help direct newly inserted photopigment molecules to the outer segment.
我们对在固定过程中或固定后暴露于胆固醇结合抗生素制霉菌素的冷冻断裂视网膜复制品进行了电子显微镜观察。我们在各种物种的视网膜视杆外段的盘膜和质膜中观察到了制霉菌素诱导的特征性扰动。显然，在固定剂中长时间暴露于制霉菌素会增强而非降低脊椎动物光感受器细胞中假定胆固醇的检测。与我们之前的研究（Andrews, L.D., 和 A.I. Cohen, 1979, J. Cell Biol., 81:215 - 228）中看到的模式一致，在呈现无颗粒斑块的膜中，制霉菌素结合似乎主要局限于这些斑块。有利断裂的光感受器在与外段质膜拓扑连续且紧邻的顶端内段质膜中表现出明显的制霉菌素结合，而外段质膜相对较少制霉菌素结合。在一些复制品中，在围绕纤毛的区域内段顶端质膜的制霉菌素结合较低，这表明了这些不同膜结构域之间可能存在边界。该区域包含一种结构（Andrews, L.D., 1982, Freeze - fracture studies of vertebrate photoreceptors, In Structure of the Eye, J.G. Hollyfield 和 E. Acosta Vidrio, editors, Elsevier/North - Holland, New York, 11 - 23），类似于青蛙神经肌肉接头神经末梢的活性区。这些观察结果使我们推测，这些结构可能起到引导囊泡融合在它们附近发生的作用，发生在一个更类似于外段而非内段质膜的膜结构域中。上述证据支持以下观点：(a) 所有盘膜都含有胆固醇，但一些盘中存在的无颗粒斑块会从相邻的颗粒膜区域捕获胆固醇；(b) 相邻的内段和外段膜在胆固醇含量上可能有很大差异；(c) 内段中推测的胆固醇含量高于外段质膜，这可能有助于将新插入的光色素分子引导至外段。
