Stimulation of TNFR1 by TNFalpha can promote three distinct alternative mechanisms of cell death: necroptosis, RIPK1-independent and -dependent apoptosis. How cells decide which way to die is unclear. Here, we report that TNFalpha-induced phosphorylation of RIPK1 in the intermediate domain by TAK1 plays a key role in regulating this critical decision. Using phospho-Ser321 as a marker, we show that the transient phosphorylation of RIPK1 intermediate domain induced by TNFalpha leads to RIPK1-independent apoptosis when NF-kappaB activation is inhibited by cycloheximide. On the other hand, blocking Ser321 phosphorylation promotes RIPK1 activation and its interaction with FADD to mediate RIPK1-dependent apoptosis (RDA). Finally, sustained phosphorylation of RIPK1 intermediate domain at multiple sites by TAK1 promotes its interaction with RIPK3 and necroptosis. Thus, absent, transient and sustained levels of TAK1-mediated RIPK1 phosphorylation may represent distinct states in TNF-RSC to dictate the activation of three alternative cell death mechanisms, RDA, RIPK1-independent apoptosis and necroptosis.TNFalpha can promote three distinct mechanisms of cell death: necroptosis, RIPK1-independent and dependent apoptosis. Here the authors show that TNFalpha-induced phosphorylation of RIPK1 in the intermediate domain by TAK1 plays a key role in regulating this decision.

Mutations in the optineurin (OPTN) gene have been implicated in both familial and sporadic amyotrophic lateral sclerosis (ALS). However, the role of this protein in the central nervous system (CNS) and how it may contribute to ALS pathology are unclear. Here, we found that optineurin actively suppressed receptor-interacting kinase 1 (RIPK1)-dependent signaling by regulating its turnover. Loss of OPTN led to progressive dysmyelination and axonal degeneration through engagement of necroptotic machinery in the CNS, including RIPK1, RIPK3, and mixed lineage kinase domain-like protein (MLKL). Furthermore, RIPK1- and RIPK3-mediated axonal pathology was commonly observed in SOD1(G93A) transgenic mice and pathological samples from human ALS patients. Thus, RIPK1 and RIPK3 play a critical role in mediating progressive axonal degeneration. Furthermore, inhibiting RIPK1 kinase may provide an axonal protective strategy for the treatment of ALS and other human degenerative diseases characterized by axonal degeneration.