The regulation of sphingolipid metabolism controls the survival of retina photoreceptors

ABRAHAN CE; ROTSTEIN NP

Buenos Aires, Argentina

Congreso; International Congress on Eye Research (ICER); 2006

International SOciety for Eye Research (ISER)

DOCOSAHEXAENOIC ACID CONTROLS THE SURVIVAL OF RETINA PHOTORECEPTORS THROUGH THE REGULATION OF SPHINGOLIPID METABOLISM. Abrahan C. and Rotstein N. Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur-CONICET. 8000Bahia Blanca, Buenos Aires, Argentina   Photoreceptors die through apoptosis during several retinal neurodegenerative diseases. Though oxidative stress has been proposed to participate in triggering this death, the pathways and mediators concerned are still ill-defined. We have recently shown that the sphingolipid ceramide is involved in triggering the apoptosis of photoreceptors after oxidative stress. This apoptotic death can be prevented by docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina; to achieve this protection, DHA decreases ceramide intracellular pool by glucosylating this sphingolipid. In this study, we investigated whether other pathways of the metabolism of sphingolipids might be involved in the regulation of photoreceptor apoptosis. The intracellular content of ceramide can decrease through its breakdown by ceramidases, leading to the formation of sphingosine, which may also trigger apoptosis. In turn, sphingosine can be phosphorylated by sphingosine kinase (SphK) to sphingosine-1-phophate (S1P), which has anti-apoptotic functions. To establish whether sphingosine induced apoptosis, rat retina neurons in culture were treated with this sphingolipid (??µM); after 24 hs., sphingosine increased the amount of photoreceptors having apoptotic nuclei and decreased the amount of those preserving their mitochondrial potential. DHA supplementation of the cultures prevented sphingosine-induced apoptosis. To investigate if ceramide was metabolized to sphingosine in order to trigger apoptosis, an inhibitor of alkaline ceramidase was added to the cultures before ceramide treatment. Addition of this inhibitor prevented ceramide-induced apoptosis pf photoreceptors, suggesting that ceramide is transformed to sphingosine, which in turn might be a mediator of photoreceptor apoptosis. We then investigated if DHA might protect photoreceptors from apoptosis induced by oxidative stress through increasing metabolization of ceramide and/or sphingosine to S1P. DHA-supplemented cultures were treated with or without an inhibitor of SphK, before adding either ceramide or the oxidant paraquat. Addition of ceramide induced photoreceptor apoptosis and DHA completely prevented it; however, inhibition of SphK blocked the protective effect of DHA; similarly, the SphK inhibitor blocked DHA protection against paraquat-induced apoptosis. This suggests that the activation of this enzyme and the subsequent increase in S1P synthesis might promote photoreceptor survival, through the combination of S1P antiapoptotic effect and the depletion of the proapoptotic mediators, ceramide and sphingosine. These results lead us to propose that oxidative stress might induce an increase in ceramide and sphingosine levels, which trigger photoreceptor apoptosis, while DHA activates intracellular pathways simultaneously decreasing the levels of these sphingolipids and increasing those of S1P, thus preventing photoreceptor apoptosis.