TAB2 controls a TAK1-independent cell death checkpoint at the level of TNFR1 complex II in the TNF pathway

Tumor necrosis factor (TNF) signaling determines the cell's fate by promoting either survival or cell death via apoptosis, necroptosis or pyroptosis. Excessive or chronic cell death by TNF was shown to drive inflammatory pathologies, highlighting the importance of the mechanisms that normally block TNF cytotoxicity. This study investigates the role of TAB2, an adaptor protein traditionally linked to TAK1 activation in the TNF pathway. Contrary to expectations, TAB2 deficiency did not impair TAK1-dependent NF-κB or MAPK signaling, nor did it affect TAK1- and IKK-dependent inhibitory phosphorylation of RIPK1 in TNFR1 complex I, indicating that TAK1 remains functional in absence of TAB2. Still, TAB2 deficiency switches the TNF response from survival to apoptosis, demonstrating a crucial TAK1-independent pro-survival function of TAB2 in the pathway. This switch was absent in TAB3-deficient conditions, highlighting a non-redundant function of TAB2. We show that TAB2 is an integral part of TNFR1 complex II, limiting the abundance of the cytotoxic complex through direct association. The ubiquitin-binding NZF domain of TAB2 is critical for this function as cells expressing a NZF-deficient mutant switched the TNF response towards apoptosis, while not affecting TAK1-mediated signaling. Moreover, we found that the NZF domain of TAB2 also represses TNF-mediated necroptosis in conditions of Caspase-8 inhibition, thereby showing that TAB2 represses two different cell death modalities in the TNF pathway by limiting the abundance of two variations of the cytotoxic complex II. Together, our results reveal the existence of a new cell death checkpoint in the TNF pathway that is controlled by TAB2, through a mechanism independent of its established function in TAK1 recruitment and activation.