Retinoid‐binding proteins in the cerebellum and choroid plexus and their relationship to regionalized retinoic acid synthesis and degradation

The expression of cellular retinoic‐acid‐binding protein (CRABP) and cellular retinol‐binding protein (CRBP), as well as their relationship to retinoic acid (RA) synthesis and degradation were examined in the developing mouse cerebellum and choroid plexus of the fourth ventricle. The choroid plexus, which expresses the RA‐synthesizing retinaldehyde dehydrogenase RALDH‐2, is likely to represent a diffusion source of RA for the closely apposed cerebellum, regulating its development. We found CRBP to be expressed in the choroid plexus and, in an in‐vitro assay, addition of recombinant CRBP to RALDH‐2 increased RA synthesis from retinaldehyde, with the amount of increase depending on the CRBP/retinaldehyde ratio. A technique that characterizes RA‐binding proteins according to their isoelectric point showed both CRABP I and CRABP II to be present in the cerebellum and P19 cells, and only CRABP II to be present in the choroid plexus. With this technique, CRABP I could also be detected in the HL60 cell line. In addition to the two known acidic RA‐binding proteins CRABP I and II, the cerebellum expressed a third RA‐binding protein distinguishable by its neutral isoelectric point; the same binding protein was also detected in the olfactory bulb, kidney and testes. We used the RA‐binding technique to follow the rate of elimination of bound RA from the cerebellum. A systemic injection of 0.3 μmols RA into postnatal day‐1 mice was almost completely removed after 8 hours. These results suggest mechanisms by which the retinoid‐binding protein may regulate the equilibrium of RA synthesis and catabolism in the cerebellum and choroid plexus.