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Peroxynitrite generated by inducible nitric oxide synthase and NADPH oxidase mediates microglial toxicity to oligodendrocytes.

ABSTRACT: Reactive microglia in the CNS have been implicated in the pathogenesis of white matter disorders, such as periventricular leukomalacia and multiple sclerosis. However, the mechanism by which activated microglia kill oligodendrocytes (OLs) remains elusive. Here we show that lipopolysaccharide (LPS)-induced death of developing OLs is caused by microglia-derived peroxynitrite, the reaction product of nitric oxide (NO) and superoxide anion. Blocking peroxynitrite formation with nitric oxide synthase inhibitors, superoxide dismutase mimics, or a decomposition catalyst abrogated the cytotoxicity. Only microglia, but not OLs, expressed inducible NO synthase (iNOS) after LPS challenge; microglia from iNOS knockout mice were not cytotoxic upon activation. The molecular source for superoxide was identified as the superoxide-generating enzyme NADPH oxidase. The oxidase was activated upon LPS exposure, and its inhibition prevented microglial toxicity toward OLs. Furthermore, microglia isolated from mice deficient in the catalytic component of the oxidase, gp91(phox), failed to induce cell death. Our results reveal a role for NADPH oxidase in LPS-induced OL death and suggest that peroxynitrite produced by iNOS and NADPH oxidase in activated microglia may play an important role in the pathogenesis of white matter disorders.

SUBMITTER: Li J

PROVIDER: S-EPMC1174990 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The phagocytic NADPH oxidase generates superoxide by transferring electrons from cytosolic NADPH to extracellular O2. The activity of the oxidase at the plasma membrane can be measured as electron current (I(e)), and the voltage dependence of I(e) was recently reported to exhibit a strong rectification in human eosinophils, with the currents being nearly voltage independent at negative potentials. To investigate the underlying mechanism, we performed voltage-clamp experiments on inside-out patches from human eosinophils activated with PMA. Electron current was evoked by bath application of different concentrations of NADPH, whereas slow voltage ramps (0.8 mV/ms), ranging from -120 to 200 mV, were applied to obtain 'steady-state' current-voltage relationships (I-V). The amplitude of I(e) recorded at -40 mV was minimal at 8 microM NADPH and saturated above 1 mM, with half-maximal activity (K(m)) observed at approx. 110 microM NADPH. Comparison of I-V values obtained at different NADPH concentrations revealed that the voltage-dependence of I(e) is strongly influenced by the substrate concentration. Above 0.1 mM NADPH, I(e) was markedly voltage-dependent and steeply decreased with depolarization within the physiological membrane potential range (-60 to 60 mV), the I-V curve strongly rectifying only below -100 mV. At lower NADPH concentrations the I-V curve was progressively shifted to more positive potentials and I(e) became voltage-independent also within the physiological range. Consequently, the K(m) of the oxidase decreased by approx. 40% (from 100 to 60 microM) when the membrane potential increased from -60 to 60 mV. We concluded that the oxidase activity depends on both membrane potential and [NADPH], and that the shape of the I(e)-V curve is influenced by the concentration of NADPH in the submillimolar range. The surprising voltage-independence of I(e) reported in whole-cell perforated patch recordings was most likely due to substrate limitation and is not an intrinsic property of the oxidase.

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Peroxynitrite generated by inducible nitric oxide synthase and NADPH oxidase mediates microglial toxicity to oligodendrocytes.

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