The protease Omi cleaves the mitogen-activated protein kinase kinase MEK1 to inhibit microglial activation.

Inflammation in Parkinson's disease is closely associated with disease pathogenesis. Mutations in Omi, which encodes the protease Omi, are linked to neurodegeneration and Parkinson's disease in humans and in mouse models. The severe neurodegeneration and neuroinflammation that occur in mnd2 (motor neuron degeneration 2) mice result from loss of the protease activity of Omi by the point mutation S276C; however, the substrates of Omi that induce neurodegeneration are unknown. We showed that Omi was required for the production of inflammatory molecules by microglia, which are the resident macrophages in the central nervous system. Omi suppressed the activation of the mitogen-activated protein kinases (MAPKs) extracellular signal-regulated kinase 1 and 2 (ERK1/2) by cleaving the upstream kinase MEK1 (mitogen-activated or extracellular signal-regulated protein kinase kinase 1). Knockdown of Omi in microglial cell lines led to activation of ERK1/2 and resulted in degradation of IκBα [α inhibitor of nuclear factor κB (NF-κB)], resulting in NF-κB activation and the expression of genes encoding inflammatory molecules, such as tumor necrosis factor-α and inducible nitric oxide synthase. The production of inflammatory molecules induced by the knockdown of Omi was blocked by the MEK1-specific inhibitor U0126. Furthermore, expression of the protease-deficient S276C Omi mutant in a microglial cell line had no effect on MEK1 cleavage or ERK1/2 activation. In the brains of mnd2 mice, we observed increased transcription of several genes encoding inflammatory molecules, as well as activation of astrocytes and microglia. Therefore, our study demonstrates that Omi is an intrinsic cellular factor that inhibits neuroinflammation.