Actin filaments in the photoreceptor cilium of the rds mutant mouse

Abstract

An actin filament meshwork was recently demonstrated within the ciliary axoneme at the base of the photoreceptor outer segment (OS) in rat retina. Individual filaments of a uniform polarity extended from the cilium and entered into the bottom of the OS disc stack where they associated with the plasma membrane in the region of new disc assembly. This and other studies have indicated that an actin-mediated mechanism may regulate OS disc morphogenesis. The homozygous rds mouse exhibits an absence of OS formation, although cilia do develop and opsin is contained within the ciliary plasma membrane. The rds abnormality is believed to result from a defect in OS disc assembly. Immunogold labeling has shown that actin is situated within the distal end of rds photoreceptor cilia, as well as in the distal cilium of normal mice prior to the onset of OS differentiation. However, anti-actin antibodies do not distinguish between monomer and filamentous actin. In the current study, neural retinas from rds and control mice were permeabilized with saponin, incubated with myosin subfragment-1 (S-1), and prepared for electron microscopy. Following this treatment, a meshwork of myosin S-1 decorated actin filaments could be observed within the axoneme in the distal end of each rds photoreceptor cilium. As in normal visual cells, actin filaments exited the axoneme by passing between pairs of microtubule doublets. These filaments had the correct polarity, with all arrowheads pointing toward the axoneme, and they associated with the ciliary plasma membrane in the region where OS disc morphogenesis would normally occur. Additionally, filament-containing membranous structures were observed in close proximity to the distal cilium. Some of these may correspond to the opsin-rich vesicles previously identified in the subretinal space of the rds mouse.