A rat G protein-coupled receptor selectively expressed in myelin-forming cells

By screening an olfactory bulb cDNA library using dopamine receptor probes, we isolated the cDNA coding for the rat counterpart of an orphan receptor known as Edg-2, homologous to G protein-coupled receptors. In situ hybridization analysis showed that Edg-2 mRNA expression is restricted to myelinated structures, e.g. corpus callosum or peripheral nerves. A weaker expression in various peripheral organs was also detected in newborns. A 3.8-kb transcript was found at high levels in highly myelinated brain structures and sciatic nerve, and, at lower levels, in poorly myelinated peripheral organs, consistent with its occurrence in Schwann cells in the peripheral nervous system. One hundred percent of Edg-2 mRNA-containing cells in the brain also expressed mRNA encoding myelin-basic-protein, a marker of oligodendrocytes. This restricted olygodendrocytes localization was confirmed by the absence of cellular colocalization of Edg-2 and glial fibrillary acidic protein, an astrocytic marker. During prenatal development, Edg-2 mRNA expression was high in the cortical neuroepithelium and meningeal layer at E16, extended later to other neuroepithelia, and disappeared shortly after birth. During brain postnatal development, Edg-2 mRNA expression in myelinated structures followed a caudo-rostral gradient, similar to that of myelination. Thus, Edg-2 is the first G protein-coupled receptor found to be selectively expressed in myelin-forming cells in the nervous system and its temporal expression pattern is consistent with a dual role (i) in neurogenesis, during embryonic development, and (ii) in myelination and myelin maintenance, during postnatal life.