Rhodopsin, 11- cis vitamin A, and interstitial retinol-binding protein (IRBP) during retinal development in normal and rd mutant mice

Biochemical and immunological techniques were used to determine the emergence of interstitial retinol binding protein (IRBP), rhodopsin, and stored retinyl esters (all- trans and 11- cis) during retinal development in normal and rd mice. IRBP could be demonstrated at embryonic Day 17 (E17), corresponding to an early stage of inner segment development. Although all- trans retinyl esters were present earlier, 11- cis retinyl esters did not appear until postnatal Days 6–7 (P6–P7), corresponding to rod outer segment (ROS) disc formation. Rhodopsin was detected at the same developmental stage. The proportion of 11- cis retinyl esters reached a maximum of 40–50% at P15–P20. Thereafter, the proportion dropped, due to more rapid accumulation of the all- trans isomer. Rhodopsin and IRBP increased in parallel with ROS elongation up to P25, when the ROS had reached their mature lengths. The increases then continued up to P40–P50. In rd (retinal degeneration) mice, IRBP and rhodopsin were identical with the controls until P12, but then dropped as the photoreceptors degenerated. Synthesis and secretion of IRBP in vitro was less than 10% of the controls in rd retinas at P26, when only 4–5% of the photoreceptors survived. The quantities of retinyl esters (mainly stearate and palmitate in the ratio of 6:1, respectively) stored in dark-adapted mouse eyes progressively increased as the animals aged, representing 0.5 mole eq. of the rhodopsin at 8 months. Although retinyl esters (11- cis and all- trans) also accumulated in rd mouse eyes up to P12, little further increase occurred. At P93, the retinyl esters (0.01 nmole · eye −1) were only 4% of the controls at P91. A peak in the proportion of 11- cis isomer occurred at P10–P20, but it averaged only 15% of the total ester and declined to 5% at P93. These findings support the hypothesis that IRBP is synthesized by the rods and cones, and suggest that its synthesis and secretion are initiated when the photoreceptor inner segments start to differentiate. 11- cis Retinoids and rhodopsin do not appear until the outer segments start to form. It is suggested that in the rd mouse the absence of photoreceptors, perhaps coupled with lack of normal interphotoreceptor matrix, leads to a loss in the ability of the pigment epithelium to store retinyl esters.