RIPK1 Regulates RIPK3-MLKL-Driven Systemic Inflammation and Emergency Hematopoiesis

Abstract

Upon ligand binding, RIPK1 is recruited to tumor necrosis factor receptor superfamily (TNFRSF) and Toll-like receptor (TLR) complexes promoting prosurvival and inflammatory signaling. RIPK1 also directly regulates caspase-8-mediated apoptosis or, if caspase-8 activity is blocked, RIPK3-MLKL-dependent necroptosis. We show that C57BL/6 Ripk1(-/-) mice die at birth of systemic inflammation that was not transferable by the hematopoietic compartment. However, Ripk1(-/-) progenitors failed to engraft lethally irradiated hosts properly. Blocking TNF reversed this defect in emergency hematopoiesis but, surprisingly, Tnfr1 deficiency did not prevent inflammation in Ripk1(-/-) neonates. Deletion of Ripk3 or Mlkl, but not Casp8, prevented extracellular release of the necroptotic DAMP, IL-33, and reduced Myd88-dependent inflammation. Reduced inflammation in the Ripk1(-/-)Ripk3(-/-), Ripk1(-/-)Mlkl(-/-), and Ripk1(-/-)Myd88(-/-) mice prevented neonatal lethality, but only Ripk1(-/-)Ripk3(-/-)Casp8(-/-) mice survived past weaning. These results reveal a key function for RIPK1 in inhibiting necroptosis and, thereby, a role in limiting, not only promoting, inflammation.

TAK1 control of cell death

“…Interestingly, even though RIPK1 was believed to be an essential mediator of RIPK3 activation, necroptosis can still be engaged with Ripk1 deletion. 80,81 Thus, necroptosis may be mediated through not only TAK1-RIPK1-RIPK3 complex but also through alternative complexes lacking either TAK1 or RIPK1. In summary, despite the fact that Tak1 deletion leads to hypersensitivity to TNFa-induced cell death (apoptosis), activation of TAK1 is also associated with TNFa-induced cell death (necroptosis).…”

“…Two lines of evidence support the idea that necroptosis inhibits apoptosis: (1) inhibition of RIPK3 by expressing a kinase-dead version of RIPK3 is reported to cause apoptotic cell death in vivo, which results in blood vessel abnormalities similar to endothelial-specific deletion of Tak1; 37,97 (2) deletion of Ripk1 primarily induces apoptotic cell death and tissue injury in vivo, although necroptosis is also induced. 80,81 Thus, inhibition and/or deletion of any part of the necroptotic protein kinase cascade (TAK1, RIPK1 or RIPK3) activates apoptotic cell death in vivo. Based on these, we propose that apoptosis and necroptosis are reciprocally regulated ( Figure 4).…”

RIP kinases: key decision makers in cell death and innate immunity

“…75 This may, at least partly, underlie the perinatal lethality associated with RIP1 deficiency but would require that any such protective effects of RIP1 are independent of kinase activity as RIP1 kinase dead knockin mice survive to adulthood. 63,76,77 In addition, during development the physiological role of RIP1 in regulating RIP3-driven necroptosis appears to be highly dependent on the stage of development with RIP1 being required for TNF-induced necroptosis at E10.5 78 but inhibiting necroptosis and associated inflammation at later stages of development. 78,79 Although RIP3-driven necroptosis contributes to the perinatal defects associated with RIP1 deficiency, it is not the sole underlying mechanism.…”

“…63,76,77 In addition, during development the physiological role of RIP1 in regulating RIP3-driven necroptosis appears to be highly dependent on the stage of development with RIP1 being required for TNF-induced necroptosis at E10.5 78 but inhibiting necroptosis and associated inflammation at later stages of development. 78,79 Although RIP3-driven necroptosis contributes to the perinatal defects associated with RIP1 deficiency, it is not the sole underlying mechanism. 63 This is supported by recent studies demonstrating an important role for RIP1 in protecting against TNF-and caspase 8-driven apoptosis.…”

“…RIG-1 also triggers TBK1/IKKi-mediated activation of IRF3 and induction of type I IFNs. RIP1 recruits caspase 8 to the RIG-1 complex resulting in RIP1 cleavage and termination of RIG-I signalling RIP kinases and innate immunity F Humphries et al necroptosis, 78 a recent report has indicated that RIP1 blocks TLR3-, TRIF-and IFN-driven necroptosis before birth. 79 TLRs that do not employ TRIF can also induce necroptosis in an indirect manner by inducing TNF to trigger necrosis via TNFR-1 as described above.…”