Oligodendrocyte substratum adhesion modulates expression of adenylate cyclase-linked receptors

Abstract

The molecular mechanisms of myelin formation/reformation in the central nervous system are unknown. In previous work we have demonstrated that mature oligodendrocytes (OLG) respond to a signal(s), elicited by their adhesion to a substratum, by turning on a myelinogenic metabolism. Events occurring within 24 hr of adhesion include generation of diacylglycerol, activation of protein kinase C, phosphorylation of myelin basic protein, and enhanced synthesis of myelin lipids and proteins. To elucidate the mechanism(s) of signal transduction, we have investigated whether OLG-substratum interaction influences the level of basal cAMP and the expression of receptors coupled to adenylate cyclase. By using ovine brain OLG we have found that adhesion to a polylysine-coated surface for 24 hr increased the basal level of cAMP 2-fold and altered the expression (assessed by cAMP production) of receptors coupled to adenylate cyclase. Isoproterenol (beta-adrenergic agonist) augmented cAMP from 4 to 26 pmol/mg of protein in adhering OLG but had no such effect in nonattached OLG. Adhesion of OLG was accompanied by rapid synthesis of ethanolamine plasmalogen, a class of lipids believed to be associated with beta-adrenergic receptors. Nonattached OLG responded to prostaglandin E1 with only a 3-fold stimulation in their cAMP content; in attached OLG, 6-fold stimulation was observed. In contrast, vasoactive intestinal polypeptide elicited a 3-fold increase in cAMP in nonattached OLG but, following 24 hr of attachment, OLG did not respond to vasoactive intestinal polypeptide. The increase of cellular cAMP levels was accompanied by a 2.5-fold gain in protein kinase A. OLG-substratum adhesion resulted also in phosphorylation of the OLG/myelin protein, 2',3'-cyclic nucleotide 2'-phosphodiesterase, which proved to be a substrate for cAMP and phospholipid-, Ca2+-dependent protein kinases. These findings, in conjunction with our earlier work, implicate cAMP and diacylglycerol in signaling myelinogenesis; they suggest that phosphorylation/dephosphorylation of myelin basic protein and 2',3'-cyclic nucleotide 2'-phosphodiesterase may be key processes in the cascade of events that are initiated by adhesion of OLG to a polylysine surface (possibly acting as a surrogate for axons) and culminate in the reformation of myelin.