PARP-1 promotes microglial activation,proliferation,and MMP-9-mediated neuron death

Activated microglia contribute to cell death in ischemic and neurodegenerative disorders of the central nervous system. Microglial activation is regulated in part by NF-κB, and the nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1) enhances NF-κB binding to DNA. Here, the role of PARP-1 in microglia-mediated neurotoxicity was assessed using microglia from wild type (wt) and PARP-1−/− mice. Cultured microglia were incubated with tumor necrosis factor á (TNFα), a cytokine that is upregulated in many neurological disorders. When stimulated with TNFá, wt microglia proliferated, underwent morphological changes characteristic of activation, and killed neurons placed in co-culture. The effects of TNFα were markedly attenuated both in PARP-1−/− microglia and in wt microglia treated with the PARP enzymatic inhibitor 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2 h)-isoquinolinone (DPQ). These effects were also blocked by (E)-3-(4-methylphenylsulfonyl)-2-propenenenitrile (BAY 11-7082), which inhibits translocation of NF-κB to the nucleus. TNFα also upregulated microglial release of matrix metalloproteinase-9 (MMP-9), an enzyme with potential neurotoxic properties that is transcriptionally regulated by NF-κB. This upregulation was blocked in PARP-1−/− microglia and in wt microglia by the PARP inhibitor DPQ. Microglia from MMP-9−/− mice were used to evaluate the contribution of MMP-9 to microglial neurotoxicity. MMP-9−/− microglia treated with TNFα-showed substantially reduced neurotoxicity relative to the wt microglia. TNFα-stimulated wt microglia treated with the MMP inhibitor ilomastat also showed reduced neurotoxicity. These findings suggest that PARP-1 activation is required for both TNFα - induced microglial activation and the neurotoxicity resulting from TNFα - induced MMP-9 release.