Receptor-interacting protein kinases 1 and 3 (RIPK1/3) have best been described for their role in mediating a regulated form of necrosis, referred to as necroptosis. During this process, RIPK3 phosphorylates mixed lineage kinase domain-like (MLKL) to cause plasma membrane rupture. RIPK3-deficient mice have recently been demonstrated to be protected in a series of disease models, but direct evidence for activation of necroptosis in vivo is still limited. Here, we sought to further examine the activation of necroptosis in kidney ischemia-reperfusion injury (IRI) and from TNFα-induced severe inflammatory response syndrome (SIRS), two models of RIPK3-dependent injury. In both models, MLKL-ko mice were significantly protected from injury to a degree that was slightly, but statistically significantly exceeding that of RIPK3-deficient mice. We also demonstrated, for the first time, accumulation of pMLKL in the necrotic tubules of human patients with acute kidney injury. However, our data also uncovered unexpected elevation of blood flow in MLKL-ko animals, which may be relevant to IRI and should be considered in the future. To further understand the mode of regulation of cell death by MLKL, we screened a panel of clinical plasma membrane channel blockers and we found phenytoin to inhibit necroptosis. However, we further found that phenytoin attenuated RIPK1 kinase activity in vitro, likely due to the hydantoin scaffold also present in necrostatin-1, and blocked upstream necrosome formation steps in the cells undergoing necroptosis. We further report that this clinically used anti-convulsant drug displayed protection from kidney IRI and TNFα-induces SIRS in vivo. Overall, our data reveal the relevance of RIPK3-pMLKL regulation for acute kidney injury and identifies an FDA-approved drug that may be useful for immediate clinical evaluation of inhibition of pro-death RIPK1/RIPK3 activities in human diseases.

Endothelial progenitor cells (EPCs) may be relevant contributors to endothelial cell (EC) repair in various organ systems. In this study, we investigated the potential role of EPCs in renal EC repair. We analyzed the major EPC subtypes in murine kidneys, blood, and spleens after induction of selective EC injury using the concanavalin A/anti-concanavalin A model and after ischemia/reperfusion (I/R) injury as well as the potential of extrarenal cells to substitute for injured local EC. Bone marrow transplantation (BMTx), kidney transplantation, or a combination of both were performed before EC injury to allow distinction of extrarenal or BM-derived cells from intrinsic renal cells. During endothelial regeneration, cells expressing markers of endothelial colony-forming cells (ECFCs) were the most abundant EPC subtype in kidneys, but were not detected in blood or spleen. Few cells expressing markers of EC colony-forming units (EC-CFUs) were detected. In BM chimeric mice (C57BL/6 with tandem dimer Tomato-positive [tdT+] BM cells), circulating and splenic EC-CFUs were BM-derived (tdT+), whereas cells positive for ECFC markers in kidneys were not. Indeed, most BM-derived tdT+ cells in injured kidneys were inflammatory cells. Kidneys from C57BL/6 donors transplanted into tdT+ recipients with or without prior BMTx from C57BL/6 mice were negative for BM-derived or extrarenal ECFCs. Overall, extrarenal cells did not substitute for any intrinsic ECs. These results demonstrate that endothelial repair in mouse kidneys with acute endothelial lesions depends exclusively on local mechanisms.

Necroinflammation is defined as the inflammatory response to necrotic cell death. Different necrotic cell death pathways exhibit different immune reponses, despite a comparable level of intracellular content release (referred to as damage associated molecular patterns or DAMPs). In addition to DAMP release, which is inevitably associated with necrotic cell death, the active production of pro/anti-inflammatory cytokines characterizes certain necrotic pathways. Necroptosis, ferroptosis and pyroptosis, therefore, are immunogenic to a different extent. In this review, we discuss the clinical relevance of necroinflammation highlighting potential human serum markers. We focus on the role of the adrenal glands and the lungs as central organs affected by systemic and/or local DAMP release and underline their role in intensive care medicine. In addition, data from models of acute kidney injury (AKI) and kidney transplantation have significantly shaped the field of necroinflammation and may be helpful for the understanding of the potential role of dialysis and plasma exchange to treat ongoing necroinflammation upon intensive care unit (ICU) conditions. In conclusion, we are only beginning to understand the importance of necroinflammation in diseases and transplantation, including xenotransplantation. However, given the existing efforts to develop inhibitors of necrotic cell death (ferrostatins, necrostatins, etc), we consider it likely that interference with necroinflammation reaches clinical routine in the near future.