The death domain of FADD is essential for embryogenesis, lymphocyte development, and proliferation.

The Fas-associated death domain-containing protein (FADD) is an adaptor for relaying apoptotic signals initiated by death receptors such as Fas. Whereas a lack of death receptors has no effect on mouse development, FADD deficiency results in early embryonic lethality, indicating that FADD has additional functions independent of death receptors. We have previously shown that conditional deletion of FADD not only impairs apoptosis but also leads to defective lymphocyte proliferation. The non-apoptotic signaling mediated by FADD remains poorly understood. Earlier studies have suggested that FADD carboxyl terminal serine phosphorylation likely plays a role in FADD-mediated proliferation signaling in T cells. The FADD death domain is presumably only required for apoptotic signaling, as it interacts with death receptors which are dispensable during embryonic development and lymphocyte proliferation. To test this hypothesis, we have performed mutational analyses of the FADD death domain and identified a mutant, R117Q, which lacks binding to Fas and, thus, is incapable of apoptotic signaling in cell lines. Unexpectedly, this death domain point mutation disrupted mouse embryonic development as shown by in vivo functional reconstitution analyses. Interestingly, a second FADD death domain mutant, V121N, retained normal Fas binding and apoptotic signaling ability but also failed to support mouse development. Furthermore, lymphocyte proliferation responses were impaired by V121N. This reverse genetic study has revealed a previously unappreciated role of the FADD death domain, which likely functions as a molecular switch regulating two distinct signals leading to apoptosis and cell proliferation and is critical for embryogenesis, lymphocyte development, and proliferation.