Hypoxic injury to oligodendrocytes: reversible inhibition of ATP-dependent transport of ceramide from the endoplasmic reticulum to the Golgi.

Previous studies have shown that gradual progressive hypoxia specifically inhibits the synthesis of the major myelin lipid galactosylceramide (GalCer) in cultured neonatal rat oligodendrocytes (OLG) (Kendler and Dawson, J Biol Chem 265:12259-12266, 1990). The inhibition of de novo synthesized GalCer (measured by [3H]palmitate incorporation) was accompanied by an increase in the [3H]labeled pool of nonhydroxy fatty acid ceramide, the precursor of GalCer. The decreased galactosylation of NFACer was not due to an inhibition of UDP-Gal:ceramide:galactosyltransferase activity or to a depletion in available UDP-Gal. Analysis of subcellular fractionations of OLG membranes on Percoll gradients indicated that NFA ceramide was accumulating in the endoplasmic reticulum (ER) during hypoxia, suggesting that the transport of NFACer from its site of synthesis (ER) to its site of galactosylation, presumably the Golgi, was blocked by hypoxia. This accumulation of ceramide was replicated by lowering ATP levels to 80-90% of control by treating OLG with 12 nM oligomycin, and was reversed by reoxygenation of the cells. Conversion of [3H]palmitate-labeled NFACer to GalCer in semi-intact OLG required both exogenous UDP-Gal and ATP, further suggesting that the transport of NFACer from the ER to its site of synthesis (cis-Golgi) is an energy-dependent step that is highly susceptible to relatively minor ATP depletion associated with early hypoxic injury. Our results further suggest that ceramide appears to be a good marker for ER and GalCer is a good marker for the cis-Golgi.