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(1), and suppresses necroptosis mediated by the kinase RIPK3 and the pseudokinase MLKL2-4. Mice that lack caspase-8 display MLKL-dependent embryonic lethality(4), as do mice that express catalytically inactive CASP8(C362A)(5). Casp8(C362A/C362A)Mlkl(-/-) mice die during the perinatal period(5), whereas Casp8(-/-)Mlkl(-/-) mice are viable(4), 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 Casp8(C362A/C362A)Mlkl(-/-)Casp1(-/-) and Casp8(C362A/C362A)Mlkl(-/-)Asc(-/-) 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 Casp8(C362A/C362A)Mlkl(-/-) and Casp8(C362A/C362A)Mlkl(-/-)Casp1(-/-) mice also contributed to lethality because Casp8(C362A/C362A)Mlkl(-/-)Casp1(-/-)Casp11(-/-) (Casp11 is also known as Casp4) neonates survived more often than Casp8(C362A/C362A)Mlkl(-/-)Casp1(-/-) neonates. Finally, Casp8(C362A/C362A)Ripk3(-/-)Casp1(-/-)Casp11(-/-) mice survived longer than Casp8(C362A/C362A)Mlkl(-/-)Casp1(-/-)Casp11(-/-) 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.