The Pseudokinase MLKL and the Kinase RIPK3 Have Distinct Roles in Autoimmune Disease Caused by Loss of Death-Receptor-Induced Apoptosis

Abstract

SUMMARY The kinases RIPK1 and RIPK3 and the pseudo-kinase MLKL have been identified as key regulators of the necroptotic cell death pathway, although a role for MLKL within the whole animal has not yet been established. Here, we have shown that MLKL deficiency rescued the embryonic lethality caused by loss of Caspase-8 or FADD. Casp8−/−Mlkl−/− and Fadd−/−Mlkl−/− mice were viable and fertile but rapidly developed severe lymphadenopathy, systemic autoimmune disease and thrombocytopenia. These morbidities occurred more rapidly and with increased severity in Casp8−/−Mlkl−/− and Fadd−/−Mlkl−/− mice compared to Casp8−/−Ripk3−/− or Fadd−/−Ripk3−/− mice, respectively. These results demonstrate that MLKL is an essential effector of necroptosis in embryos caused by loss of Caspase-8 or FADD. Furthermore, they suggest that RIPK3 and/or MLKL may exert functions independently of necroptosis. It appears that non-necroptotic functions of RIPK3 contribute to the lymphadenopathy, autoimmunity and excess cytokine production that occur when FADD or caspase-8 mediated apoptosis is abrogated.

Complex Pathologic Roles of RIPK1 and RIPK3: Moving Beyond Necroptosis

“…Along these lines, even though the loss of both RIPK3 and caspase-8 alleviates many acute injuries, it leads to autoimmune disease, which is further exacerbated in Mlkl -/- ; caspase-8 -/- animals [112, 113]. Overall, we interpret these and other published data to indicate that effector mechanisms activated by RIPK1 kinase are carefully balanced and their modulation should be considered with caution as it may fundamentally change the wiring of the regulation, leading to unexpected detrimental responses.…”

“…Both groups observed that while Mlkl -/- rescued embryonic lethality due to activation of necroptosis in Caspase-8 -/- and Fadd -/- mice, these mice rapidly succumbed to extremely severe systemic autoimmune disease, lymphadenopathy, and thrombocytopenia [112, 113]. Interestingly, while the development of autoimmunity may be hypothesized to result from a deficiency of both apoptosis and necroptosis in these mice; however, cell death-deficient Ripk3 -/- ;Caspase-8 -/- and Ripk3 -/- ; Fadd -/- mice developed much milder autoimmune disease.…”

“…Interestingly, while the development of autoimmunity may be hypothesized to result from a deficiency of both apoptosis and necroptosis in these mice; however, cell death-deficient Ripk3 -/- ;Caspase-8 -/- and Ripk3 -/- ; Fadd -/- mice developed much milder autoimmune disease. One potential clue to understanding this discrepancy comes from the observation that deficiency in MLKL, which has been previously shown to mediate necroptosis exclusively, led to the increased systemic inflammation, and this was not seen in RIPK3-deficient mice [113]. This may reflect our recent observation that caspase-8 inhibition in macrophages promoted inflammatory gene expression, which was blocked in Ripk3 -/- , but not Mlkl -/- cells [37].…”

Interrogating the relevance of mitochondrial apoptosis for vertebrate development and postnatal tissue homeostasis

“…This excess cell death that destroys the vasculature of the embryo, leading to developmental arrest around E10.5 (Varfolomeev et al 1998;Yeh et al 1998Yeh et al , 2000, can be fully rescued by codeletion of RIPK3 or MLKL, the key driver and effector of necroptosis, respectively. FADD/MLKL-deficient and Caspase 8/MLKL-deficient mice, which lack literally all DR-mediated apoptosis as well as programmed necroptosis are born at a normal Mendelian frequency and show no developmental abnormalities, as do mice lacking FADD plus RIPK3 or Caspase 8 plus RIPK3 (Kaiser et al 2011;Oberst et al 2011;Alvarez-Diaz et al 2016;Zhang et al 2016). Tissue homeostasis also appears unaffected in these mice, with one notable exception, the hematopoietic compartment, which gradually expands and provokes fatal autoimmunity in aged mice (Alvarez-Diaz et al 2016;Zhang et al 2016).…”

“…FADD/MLKL-deficient and Caspase 8/MLKL-deficient mice, which lack literally all DR-mediated apoptosis as well as programmed necroptosis are born at a normal Mendelian frequency and show no developmental abnormalities, as do mice lacking FADD plus RIPK3 or Caspase 8 plus RIPK3 (Kaiser et al 2011;Oberst et al 2011;Alvarez-Diaz et al 2016;Zhang et al 2016). Tissue homeostasis also appears unaffected in these mice, with one notable exception, the hematopoietic compartment, which gradually expands and provokes fatal autoimmunity in aged mice (Alvarez-Diaz et al 2016;Zhang et al 2016). As such, all of these mutants mimic the pathology noted in lpr mutant mice that lack functional FAS/CD95 but develop normally otherwise (Watanabe-Fukunaga et al 1992).…”