Retinol-dependent visual sensitivity and interphoto-receptor retinoid-binding protein (irbp) content in mouse

Purpose : IRBP, a retinoid-and fatty acid-binding glycoprotein secreted by the photoreceptor cells, is believed to be involved in the rhodopsin cycle. In vitamin A deficient rats, rhodopsin, outer segment size, visual sensitivity and IRBP were all reduced. These reductions were recovered upon vitamin A repletion, but IRBP recovered most rapidly. To understand the mechanism of IRBP recovery, we determined whether transcription or translation of the IRBP gene is affected by the status of vitamin A. Methods: Wild-type and transgenic mice with the IRBP promoter-CAT reporter gene were maintained on a retinol deficient diet supplemented with retinoic acid (-A) or on a control diet (+A) for 30-60 weeks postweaning. Some -A mice were repleted for vitamin A for 7 days (-A+A). Visual sensitivities were examined by ERG. The retinal distribution of IRBP was examined by EM immunocytochemistry. IRBP and its mRNA levels were measured by western/slot blot and northern blot, respectively. Results : ERG revealed alterations in waveform and a decline in the b-wave sensitivity in the -A mice; retinol repletion effected a full recovery of sensitivity. EM showed significant levels of IRBP between the microvilli of the retinal pigment epithelium and outer segments in both the +A and -A+A mice. Western/slot blots revealed a 24% reduction of IRBP in the -A mice, fully recovered in the -A+A mice. Northern blot showed no difference in the mRNA of IRBP or CAT between the three treatment groups. Conclusion : These results suggest that the retinol regulation of IRBP is not at the level of transcription.