Actin filament polarity at the site of rod outer segment disk morphogenesis

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.