Inhibition of neutral sphingomyelinase-2 facilitates remyelination

Abstract

For reasons that are not completely understood, remyelination is often incomplete, producing thin myelin sheaths with disorganized structure. We investigated the cellular basis for this altered myelin structure, and found that the response of oligodendrocyte progenitor cells (OPCs), and mature oligodendrocytes to TNFα and IL-1β is modified by the expression of the sphingomyelin hydrolase nSMase2. OPCs do not express nSMase2, and exhibit a protective response to these cytokines manifest by decreased ceramide, increased sphingosine 1-phosphate, and increased cell motility. Mature oligodendrocytes express nSMase2, and respond to TNFα and IL-1β with a stress phenotype, evidenced by increased ceramide, decreased sphingosine, and active caspase 3. Pharmacological inhibition or a targeted genetic deletion of nSMase2 in vivo increased myelin thickness, and enhanced myelin compaction. These results suggest that inhibition of nSMase2 improves the quality of new myelin by protecting maturing/myelinating oligodendrocytes. Pharmacological inhibition of nSMase2 following a demyelinating event could stabilize the structure of these newly formed myelin sheaths and protect them from secondary demyelination.Electrostatic interactions between negatively charged phosphatidyl serine (PS) head groups, and positively charged surface groups on MBP, combined with attractive van der Walls force creates a highly stable interface (Min, Kristiansen et al., 2009). Adhesion at the intraperiod line is less stable, and relies heavily on the interactions of proteolipid protein (PLP) with membrane lipids through van der Walls force to overcome repulsive energies originating from thermal undulations. Small changes in the lipid components can modify these adhesive forces and result in the decompaction and/or degeneration of myelin. We previously reported modifications in the lipid composition of normal appearing white matter (NAWM) at sights adjacent to lesions in multiple sclerosis (MS) brains, and calculated that this change in lipid composition would produce an order of magnitude increase in the repulsive energy between bilayers, that would destabilize myelin structure (Bandaru, Mielke et al., 2013). Although these results were compelling, using autopsy material we were unable to determine the mechanism(s) that contributed to these perturbations in the lipid content of regenerated myelin. In this study we show that pharmacological inhibition or genetic deletion of the sphingomyelin hydrolase neutral sphingomyelinase 2 (nSMase2) protects oligodendrocytes during re-myelination, resulting in a partial normalization of myelin lipid content and restoration of myelin compaction. Results Modifications in the lipid composition of myelin following remyelinationCompared with control mice fed a standard diet, we observed an 89 ± 9.5% reduction of black gold staining in the posterior corpus callosum (pCC) of mice fed a cuprizone containing diet for 4 weeks, consistent with a robust demyelination in this brain region. Four weeks after return to a standard die...