Cone opsin determines the time course of cone photoreceptor degeneration in Leber congenital amaurosis

Mutations in RPE65 or lecithin-retinol acyltransferase (LRAT) disrupt 11-cis-retinal recycling and cause Leber congenital amaurosis (LCA), the most severe retinal dystrophy in early childhood. We used Lrat^-/-, a murine model for LCA, to investigate the mechanism of rapid cone degeneration. Although both M and S cone opsins mistrafficked as reported previously, mislocalized M-opsin was degraded whereas mislocalized S-opsin accumulated in Lrat^-/- cones before the onset of massive ventral/central cone degeneration. As the ventral and central retina express higher levels of Sopsin than the dorsal retina in mice, our results may explain why ventral and central cones degenerate more rapidly than dorsal cones in Rpe65^-/- and Lrat^-/- LCA models. In addition, human blue opsin and mouse S-opsin, but not mouse M-opsin or human red/green opsins, aggregated to form cytoplasmic inclusions in transfected cells, which may explain why blue cone function is lost earlier than red/green-cone function in patients with LCA. The aggregation of short-wavelength opsins likely caused rapid cone degenerations through an endoplasmic reticulum stress pathway, as demonstrated in both the Lrat ^-/- retina and transfected cells. Replacing rhodopsin with S-opsin in Lrat^-/- rods resulted in mislocalization and aggregation of S-opsin in the inner segment and the synaptic region of rods, ER stress, and dramatically accelerated rod degeneration. Our results demonstrate that cone opsins play a major role in determining the degeneration rate of photoreceptors in LCA.