CULTURED SCHWANN-CELLS CONSTITUTIVELY EXPRESS THE MYELIN PROTEIN P-0

It is widely thought that mammalian Schwann cells do not express P-0, the major glycoprotein in peripheral myelin, unless they are induced t o do so by axonal signals that can be mimicked by agents that trigger cAMP signaling pathways. In contrast, we find that cultured Schwann ce lls make large amounts of P-0 without the addition of any axonal-like signal, provided they have not been exposed to serum during the cultur e process. We also report that glial growth factor/neuregulin inhibits this constitutive P-0 expression. Myelin basic protein is regulated i n a similar way. We suggest that expression of P-0 by Schwann cells be fore the onset of myelination may be prevented by inhibitory signals w ithin the nerve, rather than by the absence of a positive signal from axons.