Orientation of actin filaments in teleost retinal pigment epithelial cells, and the effect of the lectin, Concanavalin A, on melanosome motility.

Retinal pigment epithelial cells of teleosts contain numerous melanosomes (pigment granules) that exhibit light-dependent motility. In light, melanosomes disperse out of the retinal pigment epithelium (RPE) cell body (CB) into long apical projections that interdigitate with rod photoreceptors, thus shielding the photoreceptors from bleaching. In darkness, melanosomes aggregate through the apical projections back into the CB. Previous research has demonstrated that melanosome motility in the RPE CB requires microtubules, but in the RPE apical projections, actin filaments are necessary and sufficient for motility. We used myosin S1 labeling and platinum replica shadowing of dissociated RPE cells to determine actin filament polarity in apical projections. Actin filament bundles within RPE apical projections are uniformly oriented with barbed ends toward the distal tips. Treatment of RPE cells with the tetravalent lectin, Concanavalin A, which has been shown to suppress cortical actin flow by crosslinking of cell-surface proteins, inhibited melanosome aggregation and stimulated ectopic filopodia formation but did not block melanosome dispersion. The polarity orientation of F-actin in apical projections suggests that a barbed-end directed myosin motor could effect dispersion of melanosomes from the CB into apical projections. Inhibition of aggregation, but not dispersion, by ConA confirms that different actin-dependent mechanisms control these two processes and suggests that melanosome aggregation is sensitive to treatments previously shown to disrupt actin cortical flow.