Human Remyelination Promoting Antibody Stimulates Astrocytes Proliferation Through Modulation of the Sphingolipid Rheostat in Primary Rat Mixed Glial Cultures

Remyelination promoting human IgMs effectively increase the number of myelinated axons in animal models of multiple sclerosis. Hence, they ultimately stimulate myelin production by oligodendrocytes (OLs); however, their exact mechanism of action remains to be elucidated, and in particular, it remains unclear whether they are directly targeting OLs, or their action is mediated by effects on other cell types. We assessed the effect of remyelination promoting antibody rHIgM22 on the proliferative response and on the ceramide/sphingosine 1-phosphate rheostat in mixed glial cell cultures (MGCs). rHIgM22 treatment caused a time-dependent increase in PDGFαR protein in MGCs. Forty-eight hours of treatment with rHIgM22 induced a dose-dependent proliferative response (evaluated as total cell number and as EdU(+) cell number) in MGCs. When the proliferation response of MGCs to rHIgM22 was analyzed as a function of the cell types, the most significant proliferative response was associated with GLAST(+) cells, i.e., astrocytes. In many cell types, the balance between different sphingolipid mediators (the "sphingolipid rheostat"), in particular ceramide and sphingosine 1-phosphate, is critical in determining the cell fate. rHIgM22 treatment in MGCs induced a moderate but significant inhibition of total acidic sphingomyelinase activity (measured in vitro on cell lysates), the main enzyme responsible for the stimulus-mediated production of ceramide, when treatment was performed in serum containing medium, but no significant differences were observed when antibody treatment was performed in the absence of serum. Moreover, rHIgM22 treatment, either in the presence or in absence of serum, had no effects on ceramide levels. On the other hand, rHIgM22 treatment for 24 h induced increased production and release of sphingosine 1-phosphate in the extracellular milieu of MGC. Release of sphingosine 1-phosphate upon rHIgM22 treatment was strongly reduced by a selective inhibitor of PDGFαR. Increased sphingosine 1-phosphate production does not seem to be mediated by regulation of the biosynthetic enzymes, sphingosine kinase 1 and 2, since protein levels of these enzymes and phosphorylation of sphingosine kinase 1 were unchanged upon rHIgM22 treatment. Instead, we observed a significant reduction in the levels of sphingosine 1-phosphate lyase 1, one of the key catabolic enzymes. Remarkably, rHIgM22 treatment under the same experimental conditions did not induce changes in the production and/or release of sphingosine 1-phosphate in pure astrocyte cultures. Taken together, these data suggest that rHIgM22 indirectly influences the proliferation of astrocytes in MGCs, by affecting the ceramide/sphingosine 1-phosphate balance. The specific cell population directly targeted by rHIgM22 remains to be identified, however our study unveils another aspect of the complexity of rHIgM22-induced remyelinating effect.