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 TNFR1¹, and suppresses necroptosis mediated by the kinase RIPK3 and the pseudokinase MLKL^2-4. Mice that lack caspase-8 display MLKL-dependent embryonic lethality⁴, as do mice that express catalytically inactive CASP8(C362A)⁵. Casp8^C362A/C362AMlkl^-/- mice die during the perinatal period⁵, whereas Casp8^-/-Mlkl^-/- mice are viable⁴, 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/C362AMlkl^-/-Casp1^-/- and Casp8^C362A/C362AMlkl^-/-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/C362AMlkl^-/- and Casp8^C362A/C362AMlkl^-/-Casp1^-/- mice also contributed to lethality because Casp8^C362A/C362AMlkl^-/-Casp1^-/-Casp11^-/- (Casp11 is also known as Casp4) neonates survived more often than Casp8^C362A/C362AMlkl^-/-Casp1^-/- neonates. Finally, Casp8^C362A/C362ARipk3^-/-Casp1^-/-Casp11^-/- mice survived longer than Casp8^C362A/C362AMlkl^-/-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.

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