NEMO regulates a cell death switch in TNF signaling by inhibiting recruitment of RIPK3 to the cell death-inducing complex II.

Incontinentia Pigmenti (IP) is a rare X-linked disease characterized by early male lethality and multiple abnormalities in heterozygous females. IP is caused by NF-κB essential modulator (NEMO) mutations. The current mechanistic model suggests that NEMO functions as a crucial component mediating the recruitment of the IκB-kinase (IKK) complex to tumor necrosis factor receptor 1 (TNF-R1), thus allowing activation of the pro-survival NF-κB response. However, recent studies have suggested that gene activation and cell death inhibition are two independent activities of NEMO. Here we describe that cells expressing the IP-associated NEMO-A323P mutant had completely abrogated TNF-induced NF-κB activation, but retained partial antiapoptotic activity and exhibited high sensitivity to death by necroptosis. We found that robust caspase activation in NEMO-deficient cells is concomitant with RIPK3 recruitment to the apoptosis-mediating complex. In contrast, cells expressing the ubiquitin-binding mutant NEMO-A323P did not recruit RIPK3 to complex II, an event that prevented caspase activation. Hence NEMO, independently from NF-κB activation, represents per se a key component in the structural and functional dynamics of the different TNF-R1-induced complexes. Alteration of this process may result in differing cellular outcomes and, consequently, also pathological effects in IP patients with different NEMO mutations.