Dissection of the new pro-inflammatory components of the necrosome complex

Tufts University Boston, Boston MA

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Abstract

RIPK1 and RIPK3, two homologous Ser/Thr kinases, originally attracted major interest as mediators of regulated necrotic cell death, termed necroptosis, under a wide range of pathologic settings, including ischemia- reperfusion injuries, autoimmune conditions and CNS pathologies. Necroptosis can be induced in vitro in macrophage cells following activation of TLR4 receptor by its ligand lipopolysaccharide (LPS), present in the cell membranes of gram negative bacteria. Importantly, sensing of LPS by TLR4 is also well established to lead to the induction of inflammatory responses, which are an important component of innate immunity. Our recently published data identifies a new necroptosis-independent function of RIPK1 and RIPK3 in mediating TLR4- induced inflammatory gene expression in vitro and in vivo. Our studies further delineate downstream signaling cascades controlling RIPK-dependent production of inflammatory molecules, including TNF? and IFNß. In particular, we show Erk1/2 and TBK1/IKK? represent key effectors serving to promote inflammatory gene expression downstream from RIPK1/RIPK3. In addition, our data reveal a putative role for another TLR-induced inflammatory kinase TAK1 in the control of RIPK1 signaling in macrophages. Overall, these data reveal new roles of RIPK1/RIPK3 kinases in controlling TLR4-induced responses. However, the molecular mechanisms connecting RIPK1/RIPK3 to other players in this regulation remain largely unknown. Our new preliminary data reveal that detergent-insoluble RIPK1/RIPK3 necrosome complexes, which form in response to TLR4 activation, may act to recruit and directly activate Erk1/2 and TBK1/IKK?. We also report new phosphorylation sites on RIPK1, which may be targeted by TAK1. Extending on these data, our research plan encompasses further in-depth characterization of the necrosome composition aimed at revealing its pro-inflammatory components, and evaluation of the phosphorylation changes in the known RIPK1/RIPK3 pathway components to define connections between necrosome-associated kinases. In sum, our work will elucidate mechanisms of RIPK1/RIPK3-dependent inflammation, which are just beginning to emerge and are highly relevant for both the physiologic and pathologic functions of these kinases.