Regulation of cell death is vital for embryonic development and homeostasis in somatic cells, and the Fas-associated death domain (FADD) is a critical signalling adaptor for extrinsic apoptotic pathways. In a study of potential interactions between FADD and receptor interacting protein kinase-1 (RIP1/RIPK1), double deficiency of FADD and RIPK1 is shown to rescue the defects in embryonic development and lymphocyte proliferation seen in mice with single gene deficiencies. This suggests that FADD (presumably in conjunction with caspase-8 and c-FLIP) keeps necrosis in check by causing cleavage of RIPK1. Double deficiency of FADD and RIPK1 is shown to rescue the defects in mouse embryonic development and lymphocyte proliferation that are characteristic for mice with single gene deficiencies. This work suggests that the activity of FADD (presumably in conjunction with caspase-8 and c-FLIP) is to keep necrosis in check by causing the cleavage of RIPK1. FADD is a common adaptor shared by several death receptors for signalling apoptosis through recruitment and activation of caspase 8 (refs 1–3). Death receptors are essential for immune homeostasis, but dispensable during embryogenesis. Surprisingly, Fadd−/− mice die in utero4,5 and conditional deletion of FADD leads to impaired lymphocyte proliferation6,7. How FADD regulates embryogenesis and lymphocyte responses has been a long-standing enigma. FADD could directly bind to RIP1 (also known as RIPK1), a serine/threonine kinase that mediates both necrosis and NF-κB activation. Here we show that Fadd−/− embryos contain raised levels of RIP1 and exhibit massive necrosis. To investigate a potential in vivo functional interaction between RIP1 and FADD, null alleles of RIP1 were crossed into Fadd−/− mice. Notably, RIP1 deficiency allowed normal embryogenesis of Fadd−/− mice. Conversely, the developmental defect of Rip1−/− lymphocytes was partially corrected by FADD deletion. Furthermore, RIP1 deficiency fully restored normal proliferation in Fadd−/− T cells but not in Fadd−/− B cells. Fadd−/−Rip1−/− double-knockout T cells are resistant to death induced by Fas or TNF-α and show reduced NF-κB activity. Therefore, our data demonstrate an unexpected cell-type-specific interplay between FADD and RIP1, which is critical for the regulation of apoptosis and necrosis during embryogenesis and lymphocyte function.

Functional complementation between FADD and RIP1 in embryos and lymphocytes