RIP1 kinase activity-dependent roles in embryonic development of Fadd-deficient mice

RIP1 is an essential regulator of TNF-induced signaling complexes mediating NF-κB activation, apoptosis and necroptosis. Loss of Rip1 rescues the embryonic lethality of Fadd or Caspase-8-deficient mice, even though the double knockout mice die shortly after birth like Rip1-deficient mice. Recent studies demonstrated that mice expressing RIP1 kinase-dead mutants developed normally and resisted necroptotic stimuli in vitro and in vivo. However, the impact of RIP1 kinase activity on Fadd^-/- embryonic development remains unknown. Here, we engineered two RIP1 kinase inactive mutant mouse lines, a Rip1^K45A/K45A mouse line as previously reported and a novel Rip1^Δ/Δ mouse line with an altered P-loop in the kinase domain. While RIP1^K45A could not rescue the embryonic lethality of Fadd-deficient mice at E11.5, RIP1^Δ rescued lethality of Fadd^-/- mice at E11.5 and Fadd^-/-Rip1^Δ/Δ mice eventually died at E16.5 due to excessive death of fetal liver cells and unregulated inflammation. Under necropotosis-inducing conditions, comparing to Rip1^K45A/K45A cells, Rip1^Δ/Δcells displayed reduced phosphorylation and oligomerization of RIP3 and MLKL, which lead to increased cell viability. Thus, our study provides genetic evidence that different kinase inactive mutations have distinct impacts on the embryogenesis of Fadd-deficient mice, which might attribute to their extents of protection on necroptosis signaling.