The interaction between RIPK1 and FADD controls perinatal lethality and inflammation.

Embryonic development is a complex process where cellular division, differentiation and death are harmonically regulated. The regulated cell death (RCD) pathways of apoptosis and necroptosis critically influence embryogenesis. RIPK1 interacts with a protein complex consisting of FADD, caspase-8 and cFLIPL, suppressing both extrinsic apoptosis and necroptosis. Here, we generated ripk1 mutant animals (ripk1R588E) in which the interaction between FADD and RIPK1 is disrupted. These animals were expected to die mid gestation (E10.5) but survive to late gestation (~E16.5). Further removing the kinase activity of ripk1 (ripk1REKA) did not prevent embryonic lethality. The developing ripk1RE and ripk1REKA embryos die in a RIPK3-dependent manner since removal of ripk3 allowed animals to survive to adulthood. While embryonic lethality in ripk1RE mice was prevented by ablation of the necroptosis effector MLKL, animals succumbed to lethal inflammation shortly after birth, and this was not prevented by further ablation of FADD. In contrast, embryonic lethality in ripk1REKA mice was not prevented by ablation of MLKL, but animals survived to adulthood when both MLKL and caspase-8 were co-ablated. Thus, RIPK1 prevents RIPK3-caspase-8-mediated cell death before birth. The signals that induce apoptosis and necroptosis are mediated, in part, by ZBP1, since ablation of zbp1 largely prevented death of both ripk1RE and ripk1REKA embryos. Together, we identified that ZBP1 induces cell death during embryogenesis with death outcome being regulated by the death domain and kinase activity of RIPK1. Thus, RCD can be induced by various signals and is differentially regulated at distinctive stages of development.