Rip Proteins in Innate Immune Signaling

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The death domain containing serine/threonine kinase Ripk1 is the core component of TNF-induced signaling complexes mediating NF¿B and MAP kinase activation, apoptosis and an alternative form of caspase independent cell death called necroptosis. In unbiased genome wide siRNA screens for regulators of necroptosis, Ripk1, its deubiquitinase Cyld, the related Ripk3 as well as other proteins belonging to the interferon and Toll-like receptor signaling systems were identified. These studies and those using an allosteric Ripk1 kinase inhibitor or Ripk3-deficient mice establish necroptosis as a prominent host defense against viral infection. Necroptosis requires the kinase activities of Ripk1 and Ripk3 but precisely how Rip kinases initiate and execute necroptotic cell death is unknown. We have shown that Ripk1 has additional signaling functions beyond TNF, in nucleic acid sensing pathways mediated by TLR3 and Rig-I. Our preliminary studies additionally implicate Ripk1 in a cytosolic DNA sensing pathway involving Sting, Tbk1 and Irf3. Collectively, these studies predict a central role for Ripk1 in innate anti-viral immunity, however the perinatal lethality associated with a Ripk1-deficiency has precluded in vivo analyses. Goals of the current proposal are to test a requirement for Ripk1 in the anti-viral innate immune response using the conditional Ripk1 mice we have generated. We have also introduced a mutation into the Ripk1 locus that impairs the kinase activity of Ripk1. Our preliminary studies in Ripk1 kinase inactive MEFs and macrophages find these cells protected from TNF- and TLR3-induced necroptosis, respectively. An additional objective of this proposal is to examine the contribution of viral-initiated necroptosis to host defense in these newly engineered Ripk1 kinase inactive mice. In addition to virus-induced injury and inflammation, Rip kinases respond to non-microbial signals called danger-associated molecular patterns (DAMPs) released upon tissue injury. The long-term goal of these studies is to selectively inhibit Rip kinases in sterile inflammation without impairing innate immunity.