Lack of effect of microfilament or microtubule cytoskeleton-disrupting agents on restriction of externalized phosphatidylserine to rod photoreceptor outer segment tips.

In the mammalian retina, life-long renewal of rod photoreceptor outer segments involves circadian shedding of distal outer segment tips and their prompt phagocytosis by the adjacent retinal pigment epithelium (RPE) every morning after light onset. Failure of this process causes retinal dystrophy in animal models and its decline likely contributes to retinal aging and some forms of degeneration of the human retina. We previously found that surface exposure of the membrane phospholipid phosphatidylserine (PS) is restricted to outer segment tips with discrete boundaries in mouse retina and that both frequency and length of tips exposing PS peak after light onset. Here, we sought to test mechanisms photoreceptors use to restrict PS specifically to their outer segment tips. To this end, we tested whether nocodazole or cytochalasin D, perturbing microtubule or F-actin microfilament cytoskeleton, respectively, affect localization of externalized PS at outer segment tips. Fluorescence imaging of PS exposed by rods in freshly dissected, live mouse retina showed normal PS demarcation of outer segment tips regardless of drug treatment. These results suggest that the mechanism that restricts externalized PS to rod tips is independent of F-actin and microtubule cytoskeletal systems.