RIPK1 is shown to have a crucial role—independent of its known kinase function—in suppressing epithelial cell apoptosis and necroptosis in mice, thereby regulating homeostasis and preventing inflammation in barrier tissues. Receptor-interacting protein 1 kinase (RIPK1) is involved in the activation of various cell death pathways and in the control of inflammatory signalling. Two separate groups reporting in this issue use contrasting techniques to show that as well as promoting cell death, RIPK1 has a paradoxical function in supporting the survival of mouse epithelial cells that is independent of its kinase function. RIPK1 suppresses epithelial cell apoptosis and necroptosis by preventing FADD/caspase-8-mediated apoptosis and RIPK3-dependent necroptosis. These findings, together with genetic data, suggest that RIPK1 is a master regulator of epithelial cell survival, homeostasis and inflammation in the intestine and the skin. Necroptosis has emerged as an important pathway of programmed cell death in embryonic development, tissue homeostasis, immunity and inflammation1,2,3,4,5,6,7,8. RIPK1 is implicated in inflammatory and cell death signalling9,10,11,12,13 and its kinase activity is believed to drive RIPK3-mediated necroptosis14,15. Here we show that kinase-independent scaffolding RIPK1 functions regulate homeostasis and prevent inflammation in barrier tissues by inhibiting epithelial cell apoptosis and necroptosis. Intestinal epithelial cell (IEC)-specific RIPK1 knockout caused IEC apoptosis, villus atrophy, loss of goblet and Paneth cells and premature death in mice. This pathology developed independently of the microbiota and of MyD88 signalling but was partly rescued by TNFR1 (also known as TNFRSF1A) deficiency. Epithelial FADD ablation inhibited IEC apoptosis and prevented the premature death of mice with IEC-specific RIPK1 knockout. However, mice lacking both RIPK1 and FADD in IECs displayed RIPK3-dependent IEC necroptosis, Paneth cell loss and focal erosive inflammatory lesions in the colon. Moreover, a RIPK1 kinase inactive knock-in delayed but did not prevent inflammation caused by FADD deficiency in IECs or keratinocytes, showing that RIPK3-dependent necroptosis of FADD-deficient epithelial cells only partly requires RIPK1 kinase activity. Epidermis-specific RIPK1 knockout triggered keratinocyte apoptosis and necroptosis and caused severe skin inflammation that was prevented by RIPK3 but not FADD deficiency. These findings revealed that RIPK1 inhibits RIPK3-mediated necroptosis in keratinocytes in vivo and identified necroptosis as a more potent trigger of inflammation compared with apoptosis. Therefore, RIPK1 is a master regulator of epithelial cell survival, homeostasis and inflammation in the intestine and the skin.

RIPK1 maintains epithelial homeostasis by inhibiting apoptosis and necroptosis