Sphingosine-1-Phosphate Is a Key Regulator of Proliferation and Differentiation in Retina Photoreceptors

Abstract

PURPOSE. Identifying the cues required for survival and development of photoreceptors is essential for treating retina neurodegenerations. We previously established that glial derived neurotrophic factor (GDNF) stimulates proliferation and that docosahexaenoic acid (DHA) promotes photoreceptor survival and differentiation. Our later finding that ceramide triggers photoreceptor apoptosis suggested sphingolipids might also control photoreceptor development. We now investigated whether sphingosine-1-phophate (S1P), which promotes survival and differentiation in several cell types, regulates photoreceptor proliferation and differentiation and whether it is a mediator in GDNF and DHA effects. METHODS. Rat retina neuronal cultures were supplemented at day 0 or 1 with S1P, GDNF or DHA and treated with DL-threo-dihydrosphingosine (DHS) to inhibit S1P synthesis or with Brefeldin A (BFA) to block intracellular trafficking. Proliferation was quantified determining bromodeoxyuridine uptake and number of mitotic figures. Opsin, peripherin and sphingosine kinase (SphK), the enzyme required for S1P synthesis, were quantified by immunocytochemistry and Western blot. RESULTS. S1P increased proliferation of photoreceptor progenitors. It also stimulated formation of apical processes, enhanced opsin and peripherin expression and promoted their localization in these processes; DHA had similar effects. BFA prevented S1P and DHA enhancement of apical process formation, without affecting opsin expression. While GDNF and DHA enhanced SphK expression in photoreceptors, inhibiting S1P synthesis blocked GDNF mitogenic effect and DHA effects on differentiation. CONCLUSIONS. We propose S1P as a key regulator in photoreceptor development. GDNF and DHA might upregulate SphK levels to promote S1P synthesis, which would initially promote proliferation and then advance photoreceptor differentiation.