Cellular ceramide acts as 2nd messenger mediating photoreceptor cell death in mammalian retinal degeneration

Abstract

Purpose: Ceramide is the key metabolite of cellular sphingolipid biosynthesis, and its level in a cell is tightly regulated by the balanced activity of catabolic and anabolic enzymes. Several environmental factors and cell death signals can activate ceramide synthesis by modulating its metabolic enzymes, and the elevated free ceramide in a cell signals for apoptosis. We hypothesized that ceramide plays a role of cellular 2nd messenger in photoreceptor apoptosis in various forms of mammalian retinal degenerations (RDs).

Methods: In this study, we used well-characterized rat models of RD such as light-induced RD (LIRD) in albino SD rats, mutant rhodopsin rats (P23H-1), and RCS rats. We measured cellular ceramide levels at different time points before and during the process of degeneration by mass spectrometric analysis. We assayed the expression of the genes involved by RT-PCR, and measured the activity of major enzymes, in order to determine the association of ceramide production with retinal degeneration and to identify the mechanism by which ceramide levels increase in the retina during degeneration. We used ceramide synthesis inhibitors in these retinal degeneration models to validate the role of ceramide in the process of photoreceptor degeneration.

Results: We observed significant increases in ceramide level in the retina in the LIRD model of SD rats before apoptosis and in P23H-1 and RCS rats during active photoreceptor cell death. In these models, we found differences in the mechanism of ceramide production: the de novo pathway of ceramide production was involved in the LIRD model, and sphingomyelinase activation occurred along with the activation of the de novo pathway in P23H1 and RCS retinas. We also found inhibition of de novo ceramide biosynthesis prevents photoreceptor cell death in light-induced retinal degeneration and in P23H-1 rats.

Conclusions: We conclude that ceramide signaling is associated with some forms of RD, which can be targeted for therapeutic development.

Keywords: 695 retinal degenerations: cell biology • 648 photoreceptors • 426 apoptosis/cell death