Activation of the p38 MAP/kinase pathway is required for retinoic acid effects on retina photoreceptors

POLITI, L.E.; DE GENARO, P.; SIMÓN, M.V.; ROTSTEIN, N.P.

Fort Lauderdale

Congreso; ARVO Annual Meeting; 2012

Purpose: Retinoic acid (RA) has a pivotal role in promoting cell differentiation in several tissues. This vitamin A metabolite is critical during the development of the nervous system, including the retina. In addition, RA has been shown to induce apoptosis in diverse cell types. However, the mechanisms by which RA induces these seemingly contradictory effects remain elusive. We here investigated these effects of RA in rat retina neurons.Methods: Pure neuronal cultures prepared from rat retinas were grown in chemically defined media and supplemented at day 0 with RA, with or without docosahexaenoic acid (DHA), a survival lipid molecule for photoreceptors (PHRs). To evaluate p38 involvement in RA effects, phosphorylated-p38 levels were determined; cultures were also treated with or without SB203580, a p38 specific inhibitor, before RA addition. Several parameters of differentiation and apoptosis were evaluated at different times in vitro. The effect of the pan caspase inhibitor Z-VAD-FMK on apoptosis was also determined.Results: RA significantly increased opsin and peripherin expression in PHRs and stimulated axon outgrowth in PHRs and amacrine cells. RA selectively increased apoptosis of PHRs by day 3 in vitro, which was prevented by pre-treatment with the caspase pan-inhibitor Z-VAD-FMK. RA effects on opsin expression and apoptosis were mediated by activation of the p38-MAPK pathway; RA promoted p38 phosphorylation, whereas SB203580 blocked these effects, without affecting axon outgrowth. Noteworthy, RA early induction of apoptosis was prevented by supplementing the cultures with DHA.Conclusions: This work shows that RA promoted differentiation in PHR while simultaneously inducing an early onset of apoptosis. Both effects were mediated by activation of the p38-MAPK pathway, whereas RA stimulation of axon outgrowth was not, implying that RA simultaneously activated different signaling pathways in retinal neurons. DHA prevention of RA-induced apoptosis suggests that RA might activate apoptosis in PHRs with a deficient supply of trophic factors, underscoring the need of an exquisite coordination in the supply of these factors. Thus, the dual effects of RA might help to establish the final number of photoreceptors during retina development.