The distribution of actin immunoreactivity in rhabdomeres of tipulid flies in relation to extracellular membrane shedding

Identification and immunolocalization of actin cytoskeletal components in light- and dark-adapted octopus retinas

Photoreceptors in the octopus retina are of the rhabdomeric type, with rhabdomeres arising from the plasma membrane on opposite sides of the cylindrical outer segment. Each rhabdomere microvillus has an actin filament core, but other actin-binding proteins have not been identified. We used immunoblotting techniques to identify actin-binding proteins in octopus retinal extracts and immunofluorescence microscopy to localize the same proteins in fixed tissue. Antibodies directed against alpha-actinin and vinculin recognized single protein bands on immunoblots of octopus retinal extract with molecular weights comparable to the same proteins in other tissues. Anti-filamin identified two closely spaced bands similar in molecular weight to filamin in other species. Antibodies to the larger of the Drosophila ninaC gene products, p174, identified two bands lower in molecular weight than p174. Anti-villin localized a band that was significantly less in molecular weight than villin found in other cells. Epifluorescence and confocal microscopy were used to map the location of the same actin-binding proteins in dark- and light-adapted octopus photoreceptors and other retinal cells. Antibodies to most of the actin-binding proteins showed heavy staining of the photoreceptor proximal/supportive cell region accompanied by rhabdom membrane and rhabdom tip staining, although subtle differences were detected with individual antibodies. In dark-adapted retinas anti-alpha-actinin stained the photoreceptor proximal/supportive cell region where an extensive junctional complex joins these two cell types, but in the light, immunoreactivity extended above the junctional complex into the rhabdom bases. Most antibodies densely stained the rhabdom tips but anti-villin exhibited a striated pattern of localization at the tips. We believe that the actin-binding proteins identified in the octopus retina may play a significant role in the formation of new rhabdomere microvilli in the dark. We speculate that these proteins and actin remain associated with an avillar membrane that connects opposing sets of rhabdomeres in light-adapted retinas. Association of these cytoskeletal proteins with the avillar membrane would constitute a pool of proteins that could be recruited for rapid microvillus formation from the previously avillar region.

Actin-dependent light-induced translocation of mitochondria and ER cisternae in the photoreceptor cells of the locust Schistocerca gregaria

Light-dependent changes in the positioning of organelles in photoreceptor cells of arthropods are a well-known phenomenon. In this study, we examine the role of the cytoskeleton in these light-dependent antagonistic movements. In dark-adapted photoreceptor cells of the locust Schistocerca gregaria, prominent sacs of smooth endoplasmic reticulum (ER) oppose the bases of the photoreceptive microvilli. Light stimulation causes a translocation of the ER elements towards the main cell body, and an aggregation of mitochondria adjacent to the microvilli. Immunofluorescence studies and electron-microscopic examination of chemically fixed or high-pressure-frozen, freeze-substituted specimens demonstrate a lack of microtubules in the submicrovillar region. However, numerous filament bundles are aligned in close association with mitochondria and ER elements, along the track of their movement. Fluorescent phallotoxins and monoclonal anti-actin antibodies label filament bundles in the submicrovillar region, indicating that they are composed of F-actin. Finally, depolymerization of the submicrovillar actin filaments by incubation with cytochalasin B results in a blockade of the movement of mitochondria and ER cisternae towards the rhabdom. These results suggest that the light-dependent translocation of both ER cisternae and mitochondria occurs along actin filaments.