Cleavage of RIPK1 by caspase-8 is crucial for limiting apoptosis and necroptosis

The aspartate-specific cysteine protease caspase-8 suppresses necroptotic cell death mediated by RIPK3 and MLKL. Indeed, mice that lack caspase-8 die in a RIPK3- and MLKL-dependent manner during embryogenesis1-3. In humans, caspase-8 deficiency is associated with immunodeficiency4 or very early onset inflammatory bowel disease5. The substrates that are cleaved by caspase-8 to prevent necroptosis in vivo have not been defined. Here we show that knock-in mice that express catalytically inactive caspase-8(C362A) die as embryos owing to MLKL-dependent necroptosis, similar to caspase-8-deficient mice. Thus, caspase-8 must cleave itself, other proteins or both to inhibit necroptosis. Mice that express caspase-8(D212A/D218A/D225A/D387A), which cannot cleave itself, were viable, as were mice that express c-FLIP or CYLD proteins that had been mutated to prevent cleavage by caspase-8. By contrast, mice that express RIPK1(D325A), in which the caspase-8 cleavage site Asp325 had been mutated, died mid-gestation. Embryonic lethality was prevented by inactivation of RIPK1, loss of TNFR1, or loss of both MLKL and the caspase-8 adaptor FADD, but not by loss of MLKL alone. Thus, RIPK1(D325A) appears to trigger cell death mediated by TNF, the kin...Continue Reading