Exposure of rat optic nerves to nitric oxide causes protein S-nitrosation and myelin decompaction

This study investigates the effect of nitric oxide ( NO) on both the chemical modi. cations of CNS proteins and the architecture of the myelinated internode. Incubation of rat optic nerves for 2 h with 1 mM concentration of the NO-donors S-nitroso-N-acetyl-penicillamine (SNAP), ethyl- 2-[hydroxyimino]-5-nitro-3-hexeneamide (NOR-3), and 4-phenyl-3-furoxan carbonitrile (PFC) led to decompaction of myelin at the level of the intraperiod line (IPL). In contrast, incubation with 1 mM sodium nitroprusside, which slowly releases NO, sodium nitrite, and N-nitrosopyrrolidine failed to cause myelin disassembly. This suggests that free NO and/or some of its direct oxidation products (e.g., N2O3) are the active molecular species. NO-induced alterations in myelin architecture could not be assigned to protein or lipid degradation, lipid peroxidation, ATP depletion, calcium uptake, protein nitration, protein carbonylation, and nerve depolarization. NO-treatment, however, resulted in the S-nitrosation of a number of proteins. In myelin, one of the major S-nitrosated substrates was identified as proteolipid protein (PLP), an abundant cysteine-rich protein that is responsible for IPL stabilization. Peripheral nervous system myelin, whose stability depends on proteins other than PLP, was not decompacted upon incubation of sciatic nerves with SNAP. It is proposed that NO-mediated nitrosation of sulfhydryl groups is likely to interfere with the normal function of PLP and other important CNS myelin proteins leading to the structural demise of this membrane. These findings are relevant to multiple sclerosis and other inflammatory demyelinating disorders where both excessive NO production and myelin instability are known to occur.