Abstract
The specificity of JHM virus (JHMV) tropism for rat oligodendrocytes, as one of the primary host cells in the central nervous system, is maintained after explanation (S. Beushausen and S. Dales, Virology 141:89-101, 1985). The temporal correlation between onset of resistance to JHMV infection in vivo, completion of myelination, and maturation of the central nervous system can be simulated in vitro by inducers of oligodendrocyte differentiation (Beushausen and Dales, Virology, 1985). Stimulation of differentiation through the elevation of intracellular cyclic AMP (cAMP) levels suggests a possible connection between activation of the adenylate cyclase system and coronavirus expression. Chromatographic analysis of cAMP-dependent protein kinase activity in cytosol extracts prepared from astrocytes or oligodendrocytes revealed that both glial cell types were deficient in protein kinase I, indicating that expression of coronavirus in differentiated cells was not contingent upon the presence of protein kinase I. However, treatment with N6,2'-O-dibutyryladenosine-3',5'-cyclic monophosphate (dbcAMP) resulted in a severalfold enhancement of the free regulatory subunit (RI) in oligodendrocytes but not in astrocytes. The RII subunit in both neural cell types was relatively unaffected. Rapid increase in RI due to dbcAMP treatment was correlated with inhibition of JHMV expression. Other differentiation inducers, including 8-Br cAMP and forskolin which, by contrast, caused a decrease in detectable RI, also blocked JHMV expression. This apparent anomaly can be attributed to an increased turnover of RI due to destabilization of the molecule which occurs upon site-specific binding of the cyclic nucleotides. On the basis of these observations, we conclude that the state of oligodendrocyte differentiation manifested with the modulation of RI regulates JHMV expression. The differentiation process did not affect either virus adsorption or sequestration but appeared to inhibit the expression of viral RNA and proteins, implying that replication was inhibited at some step between penetration and initiation of genomic functions, perhaps at the stage of uncoating. We therefore examined the possibility that protein kinases and phosphatases, which influence cellular regulation during cAMP-induced differentiation, may be responsible for the phenomenon of coronavirus suppression in oligodendrocytes. Evidence was obtained indicating that normal processing of the phosphorylated nucleocapsid protein is inhibited in differentiated oligodendrocytes, consistent with the notion that JHMV replication might be arrested during uncoating.