Differential expression of interphotoreceptor retinoid-binding protein, opsin, cellular retinaldehyde-binding protein, and basic fibroblastic growth factor.
The mRNA for rat interphotoreceptor retinoid-binding protein (IRBP) consists of 5.2 kb and 6.4 kb transcripts which are expressed early during retinal development. Here, we characterized rat IRBP cDNA and genomic clones, determined the mechanism generating the two transcripts and compared their expression to mRNAs for opsin, cellular retinaldehyde-binding protein (CRAlBP) and basic fibroblast growth factor (bFGF). We found that human and rat IRBPs have a high degree of sequence homology (86% amino acid identity over the fourth repeat). RNA-PCR studies and Northern blot analysis, showed that the 6.4 kb mRNA has a longer 3'-untranslated region (UTR) than the 5.2 kb message. Both IRBP transcripts, but not the mRNA for opsin are present in the neonatal and adult pineal. During development, the mRNAs for IRBP and opsin reach one-half of their maximal levels by P5 and P11, respectively. The ratio of the two IRBP mRNAs remains constant throughout life. In contrast, the ratio of the longest to the shortest opsin mRNA decreases between E19 and 1 year of age. Immunohistochemistry demonstrated a marked increase in the amount of IRBP in the interphotoreceptor matrix between P1 and P9 corresponding to the enhanced expression of this mRNA. Up-regulation of opsin mRNA expression during the second postnatal week corresponds to the emergence of the outer segments. The temporal expression of CRAlBP is similar to IRBP while bFGF is not expressed until after photoreceptor differentiation is complete. In summary, two 3'-transcription termination sites explain the difference in IRBP mRNA sizes. The different temporal and tissue specific patterns of IRBP and opsin expression suggest that separate mechanisms control the expression of these two genes. The expression patterns of IRBP and CRAlBP are consistent with the role of vitamin A as a morphogen and bFGF in neuronal maintenance.