The role of cyclic AMP in oligodendrocyte differentiation

Adenosine 3′,5′-cyclic monophosphate (cAMP) accelerates the rate of oligodendrocyte differentiation in primary glial cell cultures, while inhibiting proliferation among precursors. It has been unclear whether cAMP regulates differentiation of oligodendrocytes in the absence of experimental manipulation, nor have biological ligands been identified that regulate oligodendroglia via cAMP. Finally, little is known about the expression and regulation of the enzymes that synthesize and degrade cAMP in oligodendroglia. To address these issues, we developed a novel immunohistochemical technique to visualize cAMP within single cells. As oligodendrocytes underwent endogenous differentiation in cell culture, basal cAMP levels remained uniformly low. No consistent pattern was detected between basal cAMP levels and oligodendroglial developmental stages, suggesting that differentiation is not initiated by substantial changes in basal cAMP. However, oligodendroglia did require basal cAMP signaling for normal morphological differentiation to occur: in the absence of cAMP-dependent protein kinase activity, cells expressed antigenic markers of differentiation, but did not extend elaborate processes, a hallmark of oligodendrocyte differentiation. The biological agonists calcitonin, corticotropin releasing hormone, and vasoactive intestinal peptide elicited cAMP production within oligodendroglia. Prostanoids and β-adrenergic agonists, which have previously been reported to elevate cAMP in oligodendroglia, did not increase cAMP immunofluorescence in oligodendroglia, though they stimulated non-oligodendroglial cells robustly. Cyclic AMP phosphodiesterase (PDE) activity was shown to play an important role in regulating cAMP in oligodendrocytes. Isobutylmethylxanthine, a broad-spectrum inhibitor of most PDE types, vinpocetine, a specific inhibitor of the PDE1 family, and rolipram, a specific inhibitor of the PDE4 family, inhibited PDE activity in oligodendroglia. Western blot analysis and immunofluorescence staining with subtype-specific antibodies confirmed the presence of PDE4 isoforms in oligodendroglia. cAMP regulates diverse developmental processes in oligodendroglia, and is itself precisely controlled. These studies are the first to directly demonstrate that cAMP levels are regulated in oligodendroglia by biological agonists. Additionally, they reveal a novel regulatory role for cAMP in oligodendrocyte process extension. The significant contribution PDE to the regulation of intracellular cAMP levels, which has received little study in oligodendroglia, is emphasized by our findings