Delayed rhodopsin regeneration and altered distribution of interphotoreceptor retinoid binding protein (IRBP) in the mi(vit)/mi(vit) (vitiligo) mouse

Rhodopsin regeneration requires attachment between the retinal pigment epithelium (RPE) and rod outer segments; however, in experimentally induced retinal detachment, rhodopsin regeneration can be restored partially upon addition of IRBP (interphotoreceptor retinoid binding protein). The mi(vit)/mi(vit) (vitiligo) mutant mouse, a model of slowly progressing photoreceptor cell degeneration, has a marked elevation of IRBP at 4 weeks as well as progressive detachment of the retina. The purpose of this study was to determine whether this mutant is capable of regenerating rhodopsin within a few hours following an intense light bleach. Rhodopsin regeneration was determined spectrophotometrically in mice after an intense one hour light bleach followed by 0,1, 2, 4 or 24 h of dark recovery. IRBP was localized immunohistochemically in fixed frozen tissue at the light microscopic level and in LR Gold embedded tissue at the ultrastructural level. Rhodopsin regeneration experiments indicated that rhodopsin levels following 0, 1, 2 and 4 h dark-recovery were significantly less in mi(vit)/mi(vit) mutants compared with controls. Immunohistochemical detection of IRBP indicated an altered distribution of the protein in the mutant mice compared with controls. There was accumulation in the region of the inner segments in mutant retinas rather than distribution only to the RPE/OS apical regions as in controls. The data suggest that regeneration of rhodopsin is reduced by 4 weeks postnatally in the mi(vit)/mi(vit) mouse. There is partial detachment of the retina at this age; and IRBP, thought to be essential for proper functioning of the visual cycle, is aberrantly distributed in this mutant.