Retinoic acid promotes apoptosis and differentiation in photoreceptors by activating the p38 MAP kinase pathway

Abstract

Purpose Retinoic acid (RA) has a critical role during development of the retina, controlling cell fate and differentiation. We here investigated RA effects on photoreceptor differentiation and apoptosis and the intracellular pathways involved in these effects. Methods Rat retinal neuronal cultures were supplemented with RA and with or without docosahexaenoic acid (DHA), a photoreceptor trophic factor. Photoreceptor apoptosis and differentiation were evaluated at different times of development. The effect of the pan caspase inhibitor Z-VAD-FMK on apoptosis and differentiation was determined. RA activation of p38 MAP kinase was determined by evaluating phosphorylated (P)-p38 levels in cultures with or without RA and the effect of pre-treatment with SB203580, a p38 specific inhibitor. Results RA addition at day 0, when cells were still proliferating, selectively increased apoptosis in photoreceptors, whereas addition at day 2 no longer caused cell death. DHA and Z-VAD-FMK prevented this early apoptosis. RA stimulated opsin and peripherin expression and neurite outgrowth regardless of time of development. RA addition rapidly increased P-p38 levels, whereas p38 inhibition decreased RA-induced differentiation and apoptosis, without affecting neurite outgrowth. Conclusions Our results show that RA stimulated differentiation and induced an early apoptosis in photoreceptors, which were rescued by DHA addition. Noteworthy, activation of the p38 intracellular pathway was required for RA effects on both differentiation and apoptosis. This suggests that RA induction of differentiation during early development must be counterbalanced by survival factors, such as DHA, to prevent photoreceptor death and this interplay might help to establish the final number of photoreceptors.