Antiapoptotic Strategies in Retinal Degeneration: a Biochemical and Functional Approach

The sphingolipid ceramide exerts a pro-apoptotic role in variety of cellular and organ systems and increased of de-novo synthesis of ceramide are associated with initiation of cell death. In Retinitis Pigmentosa (RP) photoreceptor death occurs by apoptosis but the individual pathways of this process are unknown. We employed an animal model of RP, the rd10 mutant mouse, to assess the role of ceramide in inherited photoreceptor degeneration. We used Myriocin, a known inhibitor of serine palmitoyltransferase (SPT, the rate-limiting enzyme of ceramide biosynthesis) which was either injected intravitreally in a single dose or administered daily to rd10 mice as eye drops of Solid Lipid Nanoparti- cles (SLNs). Control mice were given intravitreal injections of vehicle alone or unloaded lipid particles, respectively. We found that retinal ceramide levels in rd10 mice double from P14 to P30, the time interval of maximum photoreceptor death in this strain. Intraocular treatment with Myriocin decreases the number of pycnotic photoreceptors in rd10 mice by approximately 50%. Electroretino- gram (ERG) recordings were obtained from animals of various ages chronically treated with Myriocin-SLNs. ERG a-waves persist after P30 in treated mice while these responses are virtually extinct in control littermates. Retinal sec- tions from ERG recorded animals were examined at a confocal microscope to estimate photoreceptor survival. Morphometric analysis of retinas from rd10 mice aged P24 (peak of rod apoptosis) up to P30 showed prolonged survival of photoreceptors in treated animals. This study demonstrates in a mammalian model of RP that it is possible to decrease the rate of apoptotic death of pho- toreceptors in vivo by lowering retinal ceramide levels through inhibition of the de-novo biosynthesis of this molecule. Non-invasive, chronic administrations of nanoparticles loaded with SPT inhibitors are effective in prolonging survival and light responsiveness of photoreceptors.