Chaitin M H, Burnside B
Bascom Palmer Eye Institute, Department of Cell Biology and Anatomy, University of Miami School of Medicine, Florida 33101.
Invest Ophthalmol Vis Sci. 1989 Dec;30(12):2461-9.
The rod outer segment (ROS) is attached to the visual cell body by a connecting cilium. Axonemal components of this cilium extend into the ROS, an organelle which undergoes continuous renewal throughout life. New membranous disks are added at the ROS base, while older ones are shed from the tip. The formation of new disks is believed to result from plasma membrane evaginations at the distal end of the connecting cilium, but the mechanism responsible for disk morphogenesis is not yet understood. Within the ciliary axoneme, at the base of the ROS, an actin-rich domain has been localized with immunoelectron microscopy, and filamentous actin has been detected with fluorescent phallotoxin. However, actin filaments have not previously been observed in electron micrographs of this region. We now report that a meshwork of decorated actin filaments was observed within the center of the ciliary axoneme, at the base of the ROS, after visual cells were permeabilized with saponin and incubated with myosin subfragment-1 (S-1). Furthermore, individual filaments were seen to extend from the center of the axoneme into the base of the ROS disk stack by passing between pairs of ciliary microtubule doublets. Arrowheads on these filaments uniformly pointed toward the cilium, while the barbed (or fast-growing) ends were oriented in the direction of disk expansion and were often associated with the ROS plasma membrane. In control retinas, undecorated filaments were observed. Thus, S-1 binding did not induce filament formation. These results suggest that an actin filament network may provide cytoskeletal support and guidance for the growing ROS disks.
视杆细胞外段（ROS）通过连接纤毛与视觉细胞体相连。该纤毛的轴丝成分延伸至ROS，ROS是一种在整个生命过程中持续更新的细胞器。新的膜盘在ROS基部添加，而较旧的膜盘则从顶端脱落。新膜盘的形成被认为是由于连接纤毛远端的质膜内陷，但负责膜盘形态发生的机制尚不清楚。在纤毛轴丝内，在ROS基部，通过免疫电子显微镜定位了一个富含肌动蛋白的区域，并且用荧光鬼笔环肽检测到了丝状肌动蛋白。然而，以前在该区域的电子显微照片中未观察到肌动蛋白丝。我们现在报告，在用皂素使视觉细胞通透并与肌球蛋白亚片段-1（S-1）孵育后，在ROS基部的纤毛轴丝中心观察到了装饰有肌动蛋白丝的网络。此外，单个丝被观察到从轴丝中心延伸到ROS膜盘堆栈基部，穿过成对的纤毛微管双联体之间。这些丝上的箭头均匀地指向纤毛，而带刺（或快速生长）的末端则朝向膜盘扩展的方向，并且经常与ROS质膜相关联。在对照视网膜中，观察到未装饰的丝。因此，S-1结合不会诱导丝的形成。这些结果表明，肌动蛋白丝网络可能为生长中的ROS膜盘提供细胞骨架支持和引导。
