Opsin, G-protein and 48-kDa protein in normal and rd mouse retinas: Developmental expression of mRNAS and proteins and light/dark cycling of mRNAs

Abstract

Retinal degeneration in rd mice is manifested during the most rapid period of postnatal photoreceptor differentiation and is hypothesized to be caused by a lesion in cGMP metabolism. We have studied the sequence of developmental expression of three proteins involved in the cGMP cascade and the mRNAs from which they are translated, in rd and control mouse retinas. Slot blot analysis of retinal RNAs indicates that the mRNAs coding for opsin, the α, β and γ subunits of G-protein and 48-kDa protein each has the same time for onset of expression in normal and diseased retinas. Gβ and 48-kDa protein mRNAs are already detectable at birth, opsin mRNA appears by postnatal day 5 (P5), Gγ mRNA at P6 and Gα mRNA by P8. The levels of all these mRNAs decrease in the diseased retinas after P11–P12, correlating with the reduction in photoreceptor cell number that characterizes the rd disease. Immunocytochemistry indicates that the 48-kDa protein is present at birth, Gγ and opsin are detectable at P4 and Gα at P7. After P7, opsin and G-protein immunoreactivity are localized throughout the photoreceptor cell in the rd retinas but they are found only in the outer segment in control retinas. The 48-kDa protein immunoreactivity, which is observed in the whole photoreceptor layer both in rd and control retinas throughout development, is the only one of all immunoreactivities analysed that remains at 2 months of age in the rd retina and is probably localized in cones. However, at 6 months of age, 48-kDa protein immunoreactive cells are no longer present in the rd retina. We have also investigated whether there is a daily rhythm for the levels of mRNA present at different times during the light/dark periods in developing rd rd and rd/+ retinas and in adult normal (+/+) retinas. We find that the levels of each mRNA analysed appear to cycle in the +/+ adult retina, with the greatest amount of opsin and the three subunits of G-protein mRNAs occurring just before light onset and the greatest amount of 48-kDa protein mRNA occurring just before lights off. Cycling in the developing diseased or control retinas (P0–P12) is not observed and may be masked by the pronounced cell growth that occurs during this period.