Activity of caspase-8 determines plasticity between cell death pathways.

Caspase-8 is a protease with both pro-death and pro-survival functions: it mediates apoptosis induced by death receptors such as TNFR1, and suppresses necroptosis mediated by the kinase RIPK3 and the pseudokinase MLKL. Mice that lack caspase-8 display MLKL-dependent embryonic lethality, as do mice that express catalytically inactive CASP8(C362A). Casp8Mlkl mice die during the perinatal period, whereas Casp8Mlkl mice are viable, which indicates that inactive caspase-8 also has a pro-death scaffolding function. Here we show that mutant CASP8(C362A) induces the formation of ASC (also known as PYCARD) specks, and caspase-1-dependent cleavage of GSDMD and caspases 3 and 7 in MLKL-deficient mouse intestines around embryonic day 18. Caspase-1 and its adaptor ASC contributed to the perinatal lethal phenotype because a number of Casp8MlklCasp1 and Casp8MlklAsc mice survived beyond weaning. Transfection studies suggest that inactive caspase-8 adopts a distinct conformation to active caspase-8, enabling its prodomain to engage ASC. Upregulation of the lipopolysaccharide sensor caspase-11 in the intestines of both Casp8Mlkl and Casp8MlklCasp1 mice also contributed to lethality because Casp8MlklCasp1Casp11 (Casp11 is also known as Casp4) neonates survived more often than Casp8MlklCasp1 neonates. Finally, Casp8Ripk3Casp1Casp11 mice survived longer than Casp8MlklCasp1Casp11 mice, indicating that a necroptosis-independent function of RIPK3 also contributes to lethality. Thus, unanticipated plasticity in death pathways is revealed when caspase-8-dependent apoptosis and MLKL-dependent necroptosis are inhibited.