Open brain gene product Rab23: expression pattern in the adult mouse brain and functional characterization.

The gene mutated in the mouse open brain (opb) phenotype antagonizes sonic hedgehog-mediated signaling and encodes a small GTPase of the Rab family, Rab23. To date, the brain expression profile and exact mechanism of function of the Rab23 protein has remained unknown. Specific antibodies generated against Rab23 showed that the protein is highly enriched in the adult rodent brain and present in low levels in multiple tissues of the adult rodent. Rab23 is found in the cytosol as well as being associated with the plasma and endosomal membranes. In the adult mouse brain, Rab23 is found in betaIII tubulin (TuJ) positive neuronal cell bodies and are most prominent in the cortex, hypothalamus and the cerebellum. It is, however, absent from glial fibrillary acidic protein (GFAP) positive astrocytes or CNPase positive oligodendrocytes. Despite the plasma membrane/endosomal membrane localization of Rab23, neither overexpression of the GTP-restricted nor the GDP-bound mutant forms affect internalization of transferrin or epidermal growth factor. Exogenous overexpression of Rab23 or its mutants also did not affect the morphological differentiation of thalamic neurons in culture. Expression of Rab23 in the adult brain is suggestive, however, of having a postnatal function beyond its role in embryonic development.