Differential sensitivity of oligodendrocytes and motor neurons to reactive nitrogen species: implications for multiple sclerosis

Depending on its concentration, nitric oxide (NO) has beneficial or toxic effects. In pathological conditions, NO reacts with superoxide to form peroxynitrite, which nitrates proteins forming nitrotyrosine residues (3NY), leading to loss of protein function, perturbation of signal transduction, and cell death. 3NY immunoreactivity is present in many CNS diseases, particularly multiple sclerosis. Here, using the high flux NO donor, spermine-NONOate, we report that oligodendrocytes are resistant to NO, while motor neurons are NO sensitive. Motor neuron sensitivity correlates with the NO-dependent formation of 3NY, which is significantly more pronounced in motor neurons when compared with oligodendrocytes, suggesting peroxynitrite as the toxic molecule. The heme-metabolizing enzyme, heme-oxygenase-1 (HO1), is necessary for oligodendrocyte NO resistance, as demonstrated by loss of resistance after HO1 inhibition. Resistance is reinstated by peroxynitrite scavenging with uric acid further implicating peroxynitrite as responsible for NO sensitivity. Most importantly, differential sensitivity to NO is also present in cultures of primary oligodendrocytes and motor neurons. Finally, motor neurons cocultured with oligodendrocytes, or oligodendrocyte-conditioned media, become resistant to NO toxicity. Preliminary studies suggest oligodendrocytes release a soluble factor that protects motor neurons. Our findings challenge the current paradigm that oligodendrocytes are the exclusive target of multiple sclerosis pathology.