RGMb protects against acute kidney injury by inhibiting tubular cell necroptosis via an MLKL-dependent mechanism

RIPK3 mediates renal tubular epithelial cell apoptosis in endotoxin‑induced acute kidney injury

Renal tubular epithelial cell apoptosis is an important pathological mechanism of septic acute kidney injury (AKI). Endotoxin, also known as lipopolysaccharide (LPS), has a key role in septic AKI and can directly induce tubular epithelial cell apoptosis. The upregulation of receptor-interacting protein kinase 3 (RIPK3) in tubular epithelial cells has been reported in septic AKI, with RIPK3 mediating apoptosis in several cell types. In the present study, the effect of RIPK3 on endotoxin-induced AKI was investigated in mouse tubular epithelial cell apoptosis in vitro and in vivo. It was found that the expression of RIPK3 was markedly increased in endotoxin-induced AKI. Endotoxin-induced AKI and tubular epithelial cell apoptosis could be attenuated by GSK′872, a RIPK3 inhibitor. LPS stimulation also upregulated RIPK3 expression in tubular epithelial cells in a time-dependent manner. Both RIPK3 inhibitor and small interfering RNA (siRNA) targeting RIPK3 reduced LPS-induced tubular epithelial cell apoptosis in vitro. The expression of the proapoptotic protein Bax was induced by LPS and reversed by GSK′872 or RIPK3-siRNA. The present study revealed that RIPK3 mediated renal tubular cell apoptosis in endotoxin-induced AKI. RIPK3 may be a potential target for the prevention of renal tubular cell apoptosis in endotoxin-induced AKI.

Regulated necrosis in kidney ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) is the outcome of an inflammatory process that is triggered when an organ undergoes a transient reduction or cessation of blood flow, followed by re-establishment of perfusion. In the clinical setting, IRI contributes to significant acute kidney injury, patient morbidity and mortality, and adverse outcomes in transplantation. Tubular cell death by necrosis and apoptosis is a central feature of renal IRI. Recent research has challenged traditional views of cell death by identifying new pathways in which cells die in a regulated manner but with the morphologic features of necrosis. This regulated necrosis (RN) takes several forms, with necroptosis and ferroptosis being the best described. The precise mechanisms and relationships between the RN pathways in renal IRI are currently the subject of active research. The common endpoint of RN is cell membrane rupture, resulting in the release of cytosolic components with subsequent inflammation and activation of the immune system. We review the evidence and mechanisms of RN in the kidney following renal IRI, and discuss the use of small molecule inhibitors and genetically modified mice to better understand this process and guide potentially novel therapeutic interventions.

Blockade of ALK4/5 signaling suppresses cadmium- and erastin-induced cell death in renal proximal tubular epithelial cells via distinct signaling mechanisms

Various types of cell death, including apoptosis, necrosis, necroptosis, and ferroptosis, are induced in renal tubular epithelial cells following exposure to environmental stresses and toxicants such as osmotic stress, ischemia/reperfusion injury, cisplatin, and cadmium. This is known to cause renal dysfunction, but the cellular events preceding stress-induced cell death in renal tubules are not fully elucidated. The activin receptor-like kinase (ALK) 4/5, also known as activin-transforming growth factor (TGF) β receptor, is involved in stress-induced renal injury. We, therefore, studied the role of ALK4/5 signaling in HK-2 human proximal tubular epithelial cell death induced by cisplatin, cadmium, hyperosmotic stress inducer, sorbitol, and the ferroptosis activator, erastin. We found that ALK4/5 signaling is involved in cadmium- and erastin-induced cell death, but not sorbitol- or cisplatin-induced apoptotic cell death. Cadmium exposure elevated the level of phosphorylated Smad3, and treatment with the ALK4/5 kinase inhibitors, SB431542 or SB505124, suppressed cadmium-induced HK-2 cell death. Cadmium-induced cell death was attenuated by siRNA-mediated ALK4 or Smad3 silencing, or by treatment with SIS3, a selective inhibitor of TGFβ1-dependent Smad3 phosphorylation. Furthermore, ALK4/5 signaling activated Akt signaling to promote cadmium-induced HK-2 cell death. In contrast, siRNA-mediated Inhibin-bA silencing or treatment with TGFβ1 or activin A had little effect on cadmium-induced HK-2 cell death. On the other hand, treatment with SB431542 or SB505124 attenuated erastin-induced ferroptosis by hyperactivating Nrf2 signaling in HK-2 cells. These results suggest that blockade of ALK4/5 signaling protects against cadmium- and erastin-induced HK-2 cell death via Akt and Nrf2 signaling pathways, respectively.

The Structural Basis of Necroptotic Cell Death Signaling

The recent implication of the cell death pathway, necroptosis, in innate immunity and a range of human pathologies has led to intense interest in the underlying molecular mechanism. Unlike the better-understood apoptosis pathway, necroptosis is a caspase-independent pathway that leads to cell lysis and release of immunogens downstream of death receptor and Toll-like receptor (TLR) ligation. Here we review the role of recent structural studies of the core machinery of the pathway, the protein kinases receptor-interacting protein kinase (RIPK)1 and RIPK3, and the terminal effector, the pseudokinase mixed lineage kinase domain-like protein (MLKL), in shaping our mechanistic understanding of necroptotic signaling. Structural studies have played a key role in establishing models that describe MLKL's transition from a dormant monomer to a killer oligomer and revealing important interspecies differences.

hsa-miR-500a-3P alleviates kidney injury by targeting MLKL-mediated necroptosis in renal epithelial cells

MLKL is a central mediator for necroptosis. Its knockout significantly relieves acute kidney injury (AKI). However, its upstream regulatory mechanism in AKI has not been fully elucidated. We recently reviewed how microRNAs (miRNAs), a type of well-studied epigenetic regulator, play critical roles in AKI. Here, we evaluated miRNAs that potentially target MLKL and evaluated their function in human tubular epithelial cells in response to toxic and ischemic insults. TargetScan analysis showed that miR-194-5P, miR-338-3P, miR-500a-3P, and miR-577 had MLKL binding sites. Although all 4 miRNAs are reduced in AKI, our data show that only hsa-miR-500a-3P was significantly suppressed in cisplatin-treated human tubular epithelial (HK2) cells. We further found that hsa-miR-500a-3P alleviated cisplatin-induced HK2 cell death, which was confirmed by transmission electron microscopy and flow cytometry. In addition, overexpression of hsa-miR-500a-3P decreased kidney injury molecule-1 mRNA and protein levels. Real-time PCR, ELISA, and immunofluorescence data show that hsa-miR-500a-3P protected against inflammatory response, evidenced by decreased monocyte chemotactic protein-1 and proinflammatory cytokines TNF-α and IL-8. Further, hsa-miR-500a-3P attenuated P65 NF-κB phosphorylation and promoter activity. Mechanistically, luciferase reporter assay showed that hsa-miR-500a-3P bound the 3'UTR of MLKL, thereby suppressing phosphorylation and membrane translocation of MLKL. In agreement with these findings, we identified that overexpression of hsa-miR-500a-3P attenuated cell injury and the inflammatory response in response to sodium azide treatment in an in vitro model. Results show that circulating exosomes from patients with AKI down-regulated miR-500a-3P, which suppressed cell injury and inflammation in HK2 cells. hsa-miR-500a-3P alleviated toxic and ischemic insults that were triggered by cell necroptosis and the inflammatory response in human HK2 cells by targeting MLKL. This may serve as a novel therapeutic target for treatment of AKI.-Jiang, L., Liu, X.-Q., Ma, Q., Yang, Q., Gao, L., Li, H.-D., Wang, J.-N., Wei, B., Wen, J., Li, J., Wu, Y.-G., Meng, X.-M. hsa-miR-500a-3P alleviates kidney injury by targeting MLKL-mediated necroptosis in renal epithelial cells.

Programmed Necrosis and Disease:We interrupt your regular programming to bring you necroinflammation

Compared to the tidy and immunologically silent death during apoptosis, necrosis seems like a chaotic and unorganized demise. However, we now recognize that there is a method to its madness, as many forms of necrotic cell death are indeed programmed and function beyond lytic cell death to support homeostasis and immunity. Inherently more immunogenic than their apoptotic counterpart, programmed necrosis, such as necroptosis, pyroptosis, ferroptosis, and NETosis, releases inflammatory cytokines and danger-associated molecular patterns (DAMPs), skewing the milieu to a pro-inflammatory state. Moreover, impaired clearance of dead cells often leads to inflammation. Importantly, these pathways have all been implicated in inflammatory and autoimmune diseases, therefore careful understanding of their molecular mechanisms can have long lasting effects on how we interpret their role in disease and how we translate these mechanisms into therapy.

Progress in studies of necroptosis and its relationship to disease processes

This review briefly introduces the mechanism and detection methods of necroptosis in recent years. The most significant points of this review focus on the involvement of necroptotic proteins in disease progression. The following aspects are summarized: 1) RIPs, MLKL, and the upstream and downstream molecules that mediate necroptosis; 2) The development of detection methods for necroptosis; 3) The involvement of related necroptotic proteins in diverse diseases etiology; and 4) The application of necroptotic proteins in disease diagnosis.

RIPK3-MLKL-mediated necroinflammation contributes to AKI progression to CKD

Necroptosis predominates functionally over apoptosis in the pathophysiology of renal ischemia-reperfusion injury (IRI). Inhibition of the core components of the necroptotic pathway—receptor-interacting protein kinase 1 (RIPK1), RIPK3 or mixed lineage kinase domain-like protein (MLKL) reduced renal injury after ischemia/reperfusion (IR). Necrosis can initiate inflammation, which enhances necrosis in a positive feedback loop, subsequently leading to triggering more inflammation, termed as necroinflammation. However, the mechanisms underlying necroinflammation driven by renal tubular cell necroptosis in progression of AKI to CKD are still largely unknown. Here we showed that the upregulated expression and interactions between RIPK3 and MLKL induced necroptosis of renal proximal tubular cells and contributed to NLRP3 inflammasome activation under the conditions of IRI. Gene deletion of Ripk3 or Mlkl ameliorated renal tubular cell necroptosis, macrophage infiltration and NLRP3 inflammasome activation with a reduction in caspase-1 activation and maturation of IL-1β, and then finally reduced interstitial fibrogenesis in the long term after IRI. Bone marrow chimeras confirmed that RIPK3-MLKL-dependent necroptosis is responsible for the initiation of the early renal injury after IRI, and then necroptosis triggered NLRP3 inflammasome activation, which subsequently accelerates necroptosis and triggers more inflammation in an auto-amplification loop. These data indicate that necroinflammation driven by RIPK3-MLKL-dependent necroptosis plays a crucial role in the progression of IRI to CKD.

Necroptosis in Acute Kidney Injury

BACKGROUND/AIMS

Regulated necrosis is an expanding research field with important implications for acute kidney injury (AKI). A focused review of the evolving evidence for necroptosis in AKI, one of several forms of regulated necrosis defines the known and unknown.

METHODS

A literature search was performed in PUBMED and ScienceDirect between January 1957 and April 2018 using the following keywords: "acute kidney injury," "necrosis," "necroptosis," "necroinflammation."

RESULTS

The necroptosis signaling cascade involves a number of proteins including receptor-interacting protein-1 (RIPK1), RIPK3, and mixed lineage kinase domain-like pseudokinase (MLKL) as well as the MLKL regulator RGMb. The existing experimental evidence in AKI based on mice with genetic deletions of these proteins, more or less specific inhibitory compounds, and diverse experimental AKI models is reviewed.

CONCLUSION

There is broad consistency suggesting a role for necroptosis in AKI, but some studies report divergent evidence potentially relating to the specific model used and the time point of analysis. Mlkl-deficient mice are currently the most specific and reliable experimental tool to study necroptosis in vivo (in kidney disease). The clinical potential of necroptosis inhibition in AKI is to be evaluated, but conceptual problems in AKI definitions and in complex clinical scenarios remain a concern.

Novel Insights into Crystal-Induced Kidney Injury

Background

The entity of crystal nephropathies encompasses a spectrum of different kidney injuries induced by crystal-formed intrinsic minerals, metabolites, and proteins or extrinsic dietary components and drug metabolites. Depending on the localization and dynamics of crystal deposition, the clinical presentation can be acute kidney injury, progressive chronic kidney disease, or renal colic.

Summary

The molecular mechanisms involving crystal-induced injury are diverse and remain poorly understood. Type 1 crystal nephropathies arise from crystals in the vascular lumen (cholesterol embolism) or the vascular wall (atherosclerosis) and involve kidney infarcts or chronic ischemia, respectively. Type 2 crystal nephropathies arise from intratubular crystal deposition causing obstruction, interstitial inflammation, and tubular cell injury. NLRP3 inflammasome and necroptosis drive renal necroinflammation in acute settings. Type 3 is represented by crystal and stone formation in the draining urinary tract, i.e., urolithiasis, causing renal colic and chronic obstruction.

Key Messages

Dissecting the types of injury is the first step towards a better understanding of the pathophysiology of crystal nephropathies. Crystal-induced acti-vation of the inflammasome and necroptosis, crystal adhesion, crystallization inhibitors, extratubulation, and granulo-ma formation are only a few of certainly many involved pathomechanisms that deserve further studies to eventually form the basis for innovative cures for these diseases.