Cutting Edge: RIP1 Kinase Activity Is Dispensable for Normal Development but Is a Key Regulator of Inflammation in SHARPIN-Deficient Mice

Abstract

RIP1 (RIPK1) kinase is a key regulator of TNF-induced NF-κB activation, apoptosis, and necroptosis through its kinase and scaffolding activities. Dissecting the balance of RIP1 kinase activity and scaffolding function in vivo during development and TNF-dependent inflammation has been hampered by the perinatal lethality of RIP1-deficient mice. In this study, we generated RIP1 kinase–dead (Ripk1K45A) mice and showed they are viable and healthy, indicating that the kinase activity of RIP1, but not its scaffolding function, is dispensable for viability and homeostasis. After validating that the Ripk1K45A mice were specifically protected against necroptotic stimuli in vitro and in vivo, we crossed them with SHARPIN-deficient cpdm mice, which develop severe skin and multiorgan inflammation that has been hypothesized to be mediated by TNF-dependent apoptosis and/or necroptosis. Remarkably, crossing Ripk1K45A mice with the cpdm strain protected against all cpdm-related pathology. Together, these data suggest that RIP1 kinase represents an attractive therapeutic target for TNF-driven inflammatory diseases.

The Inflammatory Caspases-1 and -11 Mediate the Pathogenesis of Dermatitis in Sharpin-Deficient Mice

“…It was reported recently that dermatitis of cpdm mice is initiated through TNFR1-dependent cell death, because genetic ablation of Tnfr1 from cpdm mice prevented skin disease manifestation up to 35 wk of age (45). However, although it is agreed that TNFinduced cell death is pathogenic in this context, there is a discrepancy about the primary cell death modality that drives cpdm skin pathology (39,44). Because both apoptosis and necroptosis are induced by TNF in cpdm keratinocytes ex vivo (31), it is conceivable that they are both detrimental.…”

“…1G shows increased expression and processing of inflammatory caspase-1 and -11 and enhanced processing of IL-18 in the skin, indicative of inflammasome activation. Because LUBAC regulates cell death downstream of TNF signaling, which was implicated in cpdm skin disease pathogenesis (39,44,45), we examined, in parallel, the levels of cell death effectors in the skin of young cpdm mice. The kinases Ripk1 and Ripk3, which are involved in both apoptosis and necroptosis, also were induced in cpdm skin (Fig.…”

“…This is unlike AD, in which neutralizing the IL-4R common a subunit proved to be of high therapeutic value in human patients (38). TNF-mediated cell death pathways play a critical role in cpdm disease pathogenesis, because genetic loss of Tnf (31) or transgenic expression of a kinasedead Ripk1 (39), which is capable of inducing apoptosis but not necroptosis, rescued cpdm skin disease. Loss of IL-1RAP, a common adaptor of IL-1R and IL-33R, also was shown to attenuate skin inflammation, implicating these inflammatory pathways in disease pathogenesis (40).…”

SPATA2 regulates the activation of RIPK1 by modulating linear ubiquitination

“…TRADD is also involved in the recruitment of TRAF2/TRAF5, which function as redundant adaptors to recruit cIAP1/2, two important E3 ubiquitin ligases that mediate the ubiquitination of RIPK1 by K63-and K11-linked chains (Dynek et al 2010;de Almagro et al 2015). The ubiquitination of RIPK1 in the TNF-RSC regulates the activation of its kinase activity, which in turn activates RIPK3 and MLKL to promote necroptosis, a regulated necrotic cell death pathway (Berger et al 2014;de Almagro et al 2017).…”

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.…”