Summary Receptor-interacting protein kinase (RIPK) 1 functions as a key mediator of tissue homeostasis via formation of Caspase-8 activating ripoptosome complexes, positively and negatively regulating apoptosis, necroptosis, and inflammation. Here, we report an unanticipated cell-death- and inflammation-independent function of RIPK1 and Caspase-8, promoting faithful chromosome alignment in mitosis and thereby ensuring genome stability. We find that ripoptosome complexes progressively form as cells enter mitosis, peaking at metaphase and disassembling as cells exit mitosis. Genetic deletion and mitosis-specific inhibition of Ripk1 or Caspase-8 results in chromosome alignment defects independently of MLKL. We found that Polo-like kinase 1 (PLK1) is recruited into mitotic ripoptosomes, where PLK1’s activity is controlled via RIPK1-dependent recruitment and Caspase-8-mediated cleavage. A fine balance of ripoptosome assembly is required as deregulated ripoptosome activity modulates PLK1-dependent phosphorylation of downstream effectors, such as BUBR1. Our data suggest that ripoptosome-mediated regulation of PLK1 contributes to faithful chromosome segregation during mitosis.
The receptor-interacting serine/threonine protein kinase 1 (RIPK1) is a key mediator of regulated cell death and inflammation. Recent studies suggest that RIPK1 inhibition would fundamentally improve the therapy of RIPK1-dependent organ damage in stroke, myocardial infarction, kidney failure, and systemic inflammatory response syndrome. Additionally, it could ameliorate or prevent multi-organ failure induced by cytokine release in the context of hyperinflammation, as seen in COVID-19 patients. Therefore, we searched for a RIPK1 inhibitor and present the aromatic antiepileptic and FDA-approved drug primidone (Liskantin®) as a potent inhibitor of RIPK1 activation in vitro and in a murine model of TNFα-induced shock, which mimics the hyperinflammatory state of cytokine release syndrome. Furthermore, we detected for the first time RIPK1 activation in the respiratory tract epithelium of hospitalized patients who tested positive for SARS-CoV-2 infection. Our data provide a strong rationale for evaluating the drug primidone in conditions of hyperinflammation in humans.
Necroptosis is a lytic, inflammatory form of cell death that not only contributes to pathogen clearance but can also lead to disease pathogenesis. Necroptosis is triggered by RIPK3-mediated phosphorylation of MLKL, which is thought to initiate MLKL oligomerisation, membrane translocation and membrane rupture, although the precise mechanism is incompletely understood. Here, we show that K63-linked ubiquitin chains are attached to MLKL during necroptosis and that ubiquitylation of MLKL at K219 significantly contributes to the cytotoxic potential of phosphorylated MLKL. The K219R MLKL mutation protects animals from necroptosis-induced skin damage and renders cells resistant to pathogen-induced necroptosis. Mechanistically, we show that ubiquitylation of MLKL at K219 is required for higher-order assembly of MLKL at membranes, facilitating its rupture and necroptosis. We demonstrate that K219 ubiquitylation licenses MLKL activity to induce lytic cell death, suggesting that necroptotic clearance of pathogens as well as MLKL-dependent pathologies are influenced by the ubiquitin-signalling system.
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