RIPK3-mediated necroptosis regulates cardiac allograft rejection

An acidic pH environment converts necroptosis to apoptosis

Cardiac ischemia results in anaerobic metabolism and lactic acid accumulation and with time, intracellular and extracellular acidosis. Ischemia and subsequent reperfusion injury (IRI) lead to various forms of programmed cell death. Necroptosis is a major form of programmed necrosis that worsens cardiac function directly and also promotes inflammation by the release of cellular contents. Potential effects of increasing acidosis on programmed cell death and their specific components have not been well studied. While apoptosis is caspase-dependent, in contrast, necroptosis is mediated by the receptor-interacting protein kinases 1 and 3 (RIPK1/3). In our study, we observed that at physiological pH = 7.4, caspase-8 inhibition did not prevent TNFα-induced cell death in mouse cardiac vascular endothelial cells (MVECs) but promoted necroptotic cell death. As expected, necroptosis was blocked by RIPK1 inhibition. However, at pH = 6.5, TNFα induced an apoptosis-like pattern which was inhibited by caspase-8 inhibition. Interestingly phosphorylation of necroptotic molecules RIPK1, RIPK3, and mixed lineage kinase domain-like protein (MLKL) was enhanced in an acidic pH environment. However, RIPK3 and MLKL phosphorylation was self-limited which may have limited their participation in necroptosis. In addition, an acidic pH promoted apoptosis-inducing factor (AIF) cleavage and nuclear translocation. AIF RNA silencing inhibited cell death, supporting the role of AIF in this cell death. In summary, our study demonstrated that the pH of the micro-environment during inflammation can bias cell death pathways by altering the function of necroptosis-related molecules and promoting AIF-mediated cell death. Further insights into the mechanisms by which an acidic cellular micro-environment influences these and perhaps other forms of regulated cell death, may lead to therapeutic strategies to attenuate IRI.

The CaMK Family Differentially Promotes Necroptosis and Mouse Cardiac Graft Injury and Rejection

Organ transplantation is associated with various forms of programmed cell death which can accelerate transplant injury and rejection. Targeting cell death in donor organs may represent a novel strategy for preventing allograft injury. We have previously demonstrated that necroptosis plays a key role in promoting transplant injury. Recently, we have found that mitochondria function is linked to necroptosis. However, it remains unknown how necroptosis signaling pathways regulate mitochondrial function during necroptosis. In this study, we investigated the receptor-interacting protein kinase 3 (RIPK3) mediated mitochondrial dysfunction and necroptosis. We demonstrate that the calmodulin-dependent protein kinase (CaMK) family members CaMK1, 2, and 4 form a complex with RIPK3 in mouse cardiac endothelial cells, to promote trans-phosphorylation during necroptosis. CaMK1 and 4 directly activated the dynamin-related protein-1 (Drp1), while CaMK2 indirectly activated Drp1 via the phosphoglycerate mutase 5 (PGAM5). The inhibition of CaMKs restored mitochondrial function and effectively prevented endothelial cell death. CaMKs inhibition inhibited activation of CaMKs and Drp1, and cell death and heart tissue injury (n = 6/group, p < 0.01) in a murine model of cardiac transplantation. Importantly, the inhibition of CaMKs greatly prolonged heart graft survival (n = 8/group, p < 0.01). In conclusion, CaMK family members orchestrate cell death in two different pathways and may be potential therapeutic targets in preventing cell death and transplant injury.

Cellular and molecular mechanisms of cell damage and cell death in ischemia-reperfusion injury in organ transplantation

Ischemia-reperfusion injury (IRI) is a critical pathological condition in which cell death plays a major contributory role, and negatively impacts post-transplant outcomes. At the cellular level, hypoxia due to ischemia disturbs cellular metabolism and decreases cellular bioenergetics through dysfunction of mitochondrial electron transport chain, causing a switch from cellular respiration to anaerobic metabolism, and subsequent cascades of events that lead to increased intracellular concentrations of Na+, H+ and Ca2+ and consequently cellular edema. Restoration of blood supply after ischemia provides oxygen to the ischemic tissue in excess of its requirement, resulting in over-production of reactive oxygen species (ROS), which overwhelms the cells' antioxidant defence system, and thereby causing oxidative damage in addition to activating pro-inflammatory pathways to cause cell death. Moderate ischemia and reperfusion may result in cell dysfunction, which may not lead to cell death due to activation of recovery systems to control ROS production and to ensure cell survival. However, prolonged and severe ischemia and reperfusion induce cell death by apoptosis, mitoptosis, necrosis, necroptosis, autophagy, mitophagy, mitochondrial permeability transition (MPT)-driven necrosis, ferroptosis, pyroptosis, cuproptosis and parthanoptosis. This review discusses cellular and molecular mechanisms of these various forms of cell death in the context of organ transplantation, and their inhibition, which holds clinical promise in the quest to prevent IRI and improve allograft quality and function for a long-term success of organ transplantation.

Advances in the regulatory mechanisms of mTOR in necroptosis

The mammalian target of rapamycin (mTOR), an evolutionarily highly conserved serine/threonine protein kinase, plays a prominent role in controlling gene expression, metabolism, and cell death. Programmed cell death (PCD) is indispensable for maintaining homeostasis by removing senescent, defective, or malignant cells. Necroptosis, a type of PCD, relies on the interplay between receptor-interacting serine-threonine kinases (RIPKs) and the membrane perforation by mixed lineage kinase domain-like protein (MLKL), which is distinguished from apoptosis. With the development of necroptosis-regulating mechanisms, the importance of mTOR in the complex network of intersecting signaling pathways that govern the process has become more evident. mTOR is directly responsible for the regulation of RIPKs. Autophagy is an indirect mechanism by which mTOR regulates the removal and interaction of RIPKs. Another necroptosis trigger is reactive oxygen species (ROS) produced by oxidative stress; mTOR regulates necroptosis by exploiting ROS. Considering the intricacy of the signal network, it is reasonable to assume that mTOR exerts a bifacial effect on necroptosis. However, additional research is necessary to elucidate the underlying mechanisms. In this review, we summarized the mechanisms underlying mTOR activation and necroptosis and highlighted the signaling pathway through which mTOR regulates necroptosis. The development of therapeutic targets for various diseases has been greatly advanced by the expanding knowledge of how mTOR regulates necroptosis.

Regulated cell death and inflammasome activation in gut injury following traumatic surgery in vitro and in vivo: implication for postoperative death due to multiorgan dysfunction

Postoperative multi-organ dysfunction (MOD) is associated with significant mortality and morbidity. Necroptosis has been implicated in different types of solid organ injury; however, the mechanisms linking necroptosis to inflammation require further elucidation. The present study examines the involvement of necroptosis and NLR family pyrin domain containing 3 (NLRP3) inflammasome in small intestine injury following traumatic surgery. Kidney transplantation in rats and renal ischaemia-reperfusion (I/R) in mice were used as traumatic and laparotomic surgery models to study necroptosis and inflammasome activation in the small intestinal post-surgery; additional groups also received receptor-interacting protein kinase 1 (RIPK1) inhibitor necrostatin-1s (Nec-1s). To investigate whether necroptosis regulates inflammasome activity in vitro, necroptosis was induced in human colonic epithelial cancer cells (Caco-2) by a combination of tumour necrosis factor-alpha (TNFα), SMAC mimetic LCL-161 and pan-caspase inhibitor Q-VD-Oph (together, TLQ), and necroptosis was blocked by Nec-1s or mixed lineage kinase-domain like (MLKL) inhibitor necrosulfonamide (NSA). Renal transplantation and renal ischaemia-reperfusion (I/R) upregulated the expression of necroptosis mediators (RIPK1; RIPK3; phosphorylated-MLKL) and inflammasome components (P2X purinoceptor subfamily 7, P2X7R; NLRP3; caspase-1) in the small intestines at 24 h, and Nec-1s suppressed the expression of inflammasome components. TLQ treatment induced NLRP3 inflammasome, promoted cleavage of caspase-1 and interleukin-1 beta (IL-1β), and stimulated extracellular ATP release from Caco-2 cells, and MLKL inhibitor NSA prevented TLQ-induced inflammasome activity and ATP release from Caco-2 cells. Our work suggested that necroptosis and inflammasome interactively promote remote postoperative small intestinal injury, at least in part, through ATP purinergic signalling. Necroptosis-inflammasome axis may be considered as novel therapeutic target for tackling postoperative MOD in the critical care settings.

Ferroptosis in the post-transplantation inflammatory response

Transplantation is a life-saving therapy for patients with end-stage organ disease. Successful outcomes after transplantation require mitigation of the post-transplant inflammatory response, limiting alloreactivity, and prevention of organ rejection. Traditional immunosuppressive regimens aim to dampen the adaptive immune response; however, recent studies have shown the feasibility and efficacy of targeting the innate immune response. Necroinflammation initiated by donor organ cell death is implicated as a critical mediator of primary graft dysfunction, acute rejection, and chronic rejection. Ferroptosis is a form of regulated cell death that triggers post-transplantation inflammation and drives the activation of both innate and adaptive immune cells. There is a growing acceptance of the clinical relevance of ferroptosis to solid organ transplantation. Modulating ferroptosis may be a potentially promising strategy to reduce complications after organ transplantation.

Necroptosis in Organ Transplantation: Mechanisms and Potential Therapeutic Targets

Organ transplantation remains the only treatment option for patients with end-stage organ dysfunction. However, there are numerous limitations that challenge its clinical application, including the shortage of organ donations, the quality of donated organs, injury during organ preservation and reperfusion, primary and chronic graft dysfunction, acute and chronic rejection, infection, and carcinogenesis in post-transplantation patients. Acute and chronic inflammation and cell death are two major underlying mechanisms for graft injury. Necroptosis is a type of programmed cell death involved in many diseases and has been studied in the setting of all major solid organ transplants, including the kidney, heart, liver, and lung. It is determined by the underlying donor organ conditions (e.g., age, alcohol consumption, fatty liver, hemorrhage shock, donation after circulatory death, etc.), preservation conditions and reperfusion, and allograft rejection. The specific molecular mechanisms of necroptosis have been uncovered in the organ transplantation setting, and potential targeting drugs have been identified. We hope this review article will promote more clinical research to determine the role of necroptosis and other types of programmed cell death in solid organ transplantation to alleviate the clinical burden of ischemia-reperfusion injury and graft rejection.

The pathophysiological role of receptor-interacting protein kinase 3 in cardiovascular disease

Recent studies have found that receptor interacting protein kinase 3 (RIPK3) can mediate CaMK Ⅱ phosphorylation and oxidation, open mitochondrial permeability transition pore (mPTP), and induce myocardial necroptosis. The increased expression or phosphorylation of RIPK3 is one of the important markers of necroptosis; Inhibition of CaMK Ⅱ phosphorylation or oxidation significantly reduces RIPK3 mediated myocardial necroptosis; Studies have shown that necroptosis plays an important role in the occurrence and development of cardiovascular diseases; Using the selective inhibitor GSK '872 of RIPK3 can effectively inhibit the occurrence and development of cardiovascular diseases, and can reverse cardiovascular and cardiac dysfunction caused by overexpression of RIPK3. In this review, we provide a brief overview of the current knowledge on RIPK3 in mediating necroptosis, inflammatory response, and oxidative stress, and discussed the role of RIPK3 in cardiovascular diseases such as atherosclerosis, myocardial ischaemia, myocardial infarction, and heart failure.

Consensus Gene Network Analysis Identifies the Key Similarities and Differences in Endothelial and Epithelial Cell Dynamics after Candida albicans Infection

Endothelial and epithelial cells are morphologically different and play a critical role in host defense during Candida albicans infection. Both cells respond to C. albicans infection by activating various signaling pathways and gene expression patterns. Their interactions with these pathogens can have beneficial and detrimental effects, and a better understanding of these interactions can help guide the development of new therapies for C. albicans infection. To identify the differences and similarities between human endothelial and oral epithelial cell transcriptomics during C. albicans infection, we performed consensus WGCNA on 32 RNA-seq samples by relating the consensus modules to endothelial-specific modules and analyzing the genes connected. This analysis resulted in the identification of 14 distinct modules. We demonstrated that the magenta module correlates significantly with C. albicans infection in each dataset. In addition, we found that the blue and cyan modules in the two datasets had opposite correlation coefficients with a C. albicans infection. However, the correlation coefficients and p-values between the two datasets were slightly different. Functional analyses of the hub of genes from endothelial cells elucidated the enrichment in TNF, AGE-RAGE, MAPK, and NF-κB signaling. On the other hand, glycolysis, pyruvate metabolism, amino acid, fructose, mannose, and vitamin B6 metabolism were enriched in epithelial cells. However, mitophagy, necroptosis, apoptotic processes, and hypoxia were enriched in both endothelial and epithelial cells. Protein-protein interaction analysis using STRING and CytoHubba revealed STAT3, SNRPE, BIRC2, and NFKB2 as endothelial hub genes, while RRS1, SURF6, HK2, and LDHA genes were identified in epithelial cells. Understanding these similarities and differences may provide new insights into the pathogenesis of C. albicans infections and the development of new therapeutic targets and interventional strategies.

RIPK necrotic cell death pathway in both donor photoreceptor and host immune cells synergize to affect photoreceptor graft survival

Photoreceptor transplant has been put forward as a repair strategy to tackle degenerated retinas. Nonetheless, cell death and immune rejection seriously limit the success of this strategy, with only a small fraction of transplanted cells surviving. Improving the survival of transplanted cells is of critical importance. Recent evidence has identified receptor-interacting protein kinase 3 (RIPK3) as a molecular trigger controlling necroptotic cell death and inflammation. However, its role in photoreceptor transplantation and regenerative medicine has not been studied. We hypothesized that modulation of RIPK3 to address both cell death and immunity could have advantageous effects on photoreceptor survival. In a model of inherited retinal degeneration, deletion of RIPK3 in donor photoreceptor precursors significantly increases the survival of transplanted cells. Simultaneous RIPK3 deletion in donor photoreceptors and recipients maximizes graft survival. Lastly, to discern the role of RIPK3 in the host immune response, bone marrow transplant experiments demonstrated that peripheral immune cell RIPK3 deficiency is protective for both donor and host photoreceptor survival. Interestingly, this finding is independent of photoreceptor transplantation, as the peripheral protective effect is also observed in an additional retinal detachment photoreceptor degeneration model. Altogether, these results indicate that immunomodulatory and neuroprotective strategies targeting the RIPK3 pathway can aid regenerative therapies of photoreceptor transplantation.

Role of necroptosis in kidney health and disease

Cell death, particularly that of tubule epithelial cells, contributes critically to the pathophysiology of kidney disease. A body of evidence accumulated over the past 15 years has ascribed a central pathophysiological role to a particular form of regulated necrosis, termed necroptosis, to acute tubular necrosis, nephron loss and maladaptive renal fibrogenesis. Unlike apoptosis, which is a non-immunogenic process, necroptosis results in the release of cellular contents and cytokines, which triggers an inflammatory response in neighbouring tissue. This necroinflammatory environment can lead to severe organ dysfunction and cause lasting tissue injury in the kidney. Despite evidence of a link between necroptosis and various kidney diseases, there are no available therapeutic options to target this process. Greater understanding of the molecular mechanisms, triggers and regulators of necroptosis in acute and chronic kidney diseases may identify shortcomings in current approaches to therapeutically target necroptosis regulators and lead to the development of innovative therapeutic approaches.

Xenon attenuated neonatal lipopolysaccharide exposure induced neuronal necroptosis and subsequently improved cognition in juvenile rats

Background: Neonatal sepsis is known to cause neurodevelopment impairment and has been reported to increase risks for neurological/psychiatric disorders. In this study, we investigated the effect of xenon, a well-known potent neuroprotective gas, on neonatal sepsis-induced neurodevelopment impairment in rats together with underlying mechanism by focusing on receptor-interacting protein kinase (RIP) mediated neuronal necroptosis. Methods: 3-day-old Sprague-Dawley rat pups were exposed to either 70% xenon or N2 balanced with O2 for 6 h, during which lipopolysaccharide (LPS) was injected intraperitoneally for 3 times (500 μg/kg for the 1st and 250 μg/kg for the second and third dose; n = 6-10/group). In another cohort of 3-day-old rat pups, intracerebroventricular injection of necrostatin-1 (4 µg in 4 µl saline, a RIP-1-targeted inhibitor of necroptosis) was performed 20 min after the third dose of LPS. The learning ability and memory were assessed 25 days after LPS injection. Then, their hippocampus was collected for neuronal necroptosis with RIP and MIKL assessments using western blot and in situ immunostaining. Systemic and neuro-inflammation was also assessed. Results: LPS insult resulted in elevation of pro-inflammatory cytokine TNF-𝝰 and IL-6, caused neuronal necroptosis and damaged synaptic integrity at the brain developing stage, which finally led to the long-term cognitive impairment. Xenon inhibited necroptosis associated mediator RIP-1, RIP-3, and MLKL activation, protected neurons and attenuated cognitive dysfunction induced by LPS. Like xenon, the similar pattern changes induced by a RIP-1 inhibitor Necrostatin-1 were also found. Conclusion: This study indicates that necroptosis is involved in neonatal sepsis-induced neurofunctional impairments and xenon may be a novel therapeutic strategy to prevent/treat cognitive impairment in neonatal septic patients.

Updated Views on Neutrophil Responses in Ischemia-Reperfusion Injury

Ischemia-reperfusion injury is an inevitable event during organ transplantation and represents a primary risk factor for the development of early graft dysfunction in lung, heart, liver, and kidney transplant recipients. Recent studies have implicated recipient neutrophils as key mediators of this process and also have found that early innate immune responses after transplantation can ultimately augment adaptive alloimmunity and affect late graft outcomes. Here, we discuss signaling pathways involved in neutrophil recruitment and activation after ischemia-mediated graft injury in solid organ transplantation with an emphasis on lung allografts, which have been the focus of recent studies. These findings suggest novel therapeutic interventions that target ischemia-reperfusion injury-mediated graft dysfunction in transplant recipients.

Necroptosis: A Pathogenic Negotiator in Human Diseases

Over the past few decades, mechanisms of programmed cell death have attracted the scientific community because they are involved in diverse human diseases. Initially, apoptosis was considered as a crucial mechanistic pathway for programmed cell death; recently, an alternative regulated mode of cell death was identified, mimicking the features of both apoptosis and necrosis. Several lines of evidence have revealed that dysregulation of necroptosis leads to pathological diseases such as cancer, cardiovascular, lung, renal, hepatic, neurodegenerative, and inflammatory diseases. Regulated forms of necrosis are executed by death receptor ligands through the activation of receptor-interacting protein kinase (RIPK)-1/3 and mixed-lineage kinase domain-like (MLKL), resulting in the formation of a necrosome complex. Many papers based on genetic and pharmacological studies have shown that RIPKs and MLKL are the key regulatory effectors during the progression of multiple pathological diseases. This review focused on illuminating the mechanisms underlying necroptosis, the functions of necroptosis-associated proteins, and their influences on disease progression. We also discuss numerous natural and chemical compounds and novel targeted therapies that elicit beneficial roles of necroptotic cell death in malignant cells to bypass apoptosis and drug resistance and to provide suggestions for further research in this field.

T cell-derived tumor necrosis factor induces cytotoxicity by activating RIPK1-dependent target cell death

TNF ligation of TNF receptor 1 (TNFR1) promotes either inflammation and cell survival by (a) inhibiting RIPK1's death-signaling function and activating NF-κB or (b) causing RIPK1 to associate with the death-inducing signaling complex to initiate apoptosis or necroptosis. The cellular source of TNF that results in RIPK1-dependent cell death remains unclear. To address this, we employed in vitro systems and murine models of T cell-dependent transplant or tumor rejection in which target cell susceptibility to RIPK1-dependent cell death could be genetically altered. We show that TNF released by T cells is necessary and sufficient to activate RIPK1-dependent cell death in target cells and thereby mediate target cell cytolysis independently of T cell frequency. Activation of the RIPK1-dependent cell death program in target cells by T cell-derived TNF accelerates murine cardiac allograft rejection and synergizes with anti-PD1 administration to destroy checkpoint blockade-resistant murine melanoma. Together, the findings uncover a distinct immunological role for TNF released by cytotoxic effector T cells following cognate interactions with their antigenic targets. Manipulating T cell TNF and/or target cell susceptibility to RIPK1-dependent cell death can be exploited to either mitigate or augment T cell-dependent destruction of allografts and malignancies to improve outcomes.

Receptor Interacting Protein Kinases 1/3: The Potential Therapeutic Target for Cardiovascular Inflammatory Diseases

The receptor interacting protein kinases 1/3 (RIPK1/3) have emerged as the key mediators in cell death pathways and inflammatory signaling, whose ubiquitination, phosphorylation, and inhibition could regulate the necroptosis and apoptosis effectually. Recently, more and more studies show great interest in the mechanisms and the regulator of RIPK1/3-mediated inflammatory response and in the physiopathogenesis of cardiovascular diseases. The crosstalk of autophagy and necroptosis in cardiomyocyte death is a nonnegligible conversation of cell death. We elaborated on RIPK1/3-mediated necroptosis, pathways involved, the latest regulatory molecules and therapeutic targets in terms of ischemia reperfusion, myocardial remodeling, myocarditis, atherosclerosis, abdominal aortic aneurysm, and cardiovascular transplantation, etc.

Cyclophilin D Regulates the Nuclear Translocation of AIF, Cardiac Endothelial Cell Necroptosis and Murine Cardiac Transplant Injury

Ischemia-reperfusion injury (IRI) is an inevitable consequence of organ transplant procedure and associated with acute and chronic organ rejection in transplantation. IRI leads to various forms of programmed cell death, which worsens tissue damage and accelerates transplant rejection. We recently demonstrated that necroptosis participates in murine cardiac microvascular endothelial cell (MVEC) death and murine cardiac transplant rejection. However, MVEC death under a more complex IRI model has not been studied. In this study, we found that simulating IRI conditions in vitro by hypoxia, reoxygenation and treatment with inflammatory cytokines induced necroptosis in MVECs. Interestingly, the apoptosis-inducing factor (AIF) translocated to the nucleus during MVEC necroptosis, which is regulated by the mitochondrial permeability molecule cyclophilin D (CypD). Furthermore, CypD deficiency in donor cardiac grafts inhibited AIF translocation and mitigated graft IRI and rejection (n = 7; p = 0.002). Our studies indicate that CypD and AIF play significant roles in MVEC necroptosis and cardiac transplant rejection following IRI. Targeting CypD and its downstream AIF may be a plausible approach to inhibit IRI-caused cardiac damage and improve transplant survival.

Toll-like receptor 3 is an endogenous sensor of cell death and a potential target for induction of long-term cardiac transplant survival

Inflammation posttransplant is directly linked to cell death programs including apoptosis and necrosis. Cell death leads to the release of cellular contents which can promote inflammation. Targeting of these pathways should be an effective strategy to prevent transplant rejection. Toll-like receptor 3 (TLR3) is emerging as a major endogenous sensor of inflammation. In this study, we assessed the role of TLR3 on cell death and transplant rejection. We showed that TLR3 is highly expressed on mouse microvascular endothelial cell (ECs) and the endothelium of cardiac grafts. We demonstrated that TLR3 interacting with dsRNA or self-RNA triggered apoptosis and necroptosis in ECs. Interestingly, TLR3-induced necroptosis led mitochondrial damage. Inhibition of the mitochondrial membrane permeability molecule Cyclophilin D prevented necroptosis in ECs. In vivo, endothelium damage and activities of caspase-3 and mixed lineage kinase domain-like protein were inhibited in TLR3^-/- cardiac grafts compared with C57BL/6 grafts posttransplant (n = 5, p < .001). Importantly, TLR3^-/- cardiac grafts had prolonged survival in allogeneic BALB/c mice (mean survival = 121 ± 67 vs. 31 ± 6 days of C57BL/6 grafts, n = 7, p = .002). In summary, our study suggests that TLR3 is an important cell death inducer in ECs and cardiac grafts and thus a potential therapeutic target in preventing cardiac transplant rejection.

Ferroptosis and Necroptosis in the Kidney

In the last decade, the role of apoptosis in the pathophysiology of acute kidney injury (AKI) and AKI to chronic kidney disease (CKD) progression has been revisited as our understanding of ferroptosis and necroptosis has emerged. A growing body of evidence, reviewed here, ascribes a central pathophysiological role for ferroptosis and necroptosis to AKI, nephron loss, and acute tubular necrosis. We will introduce concepts to the non-cell-autonomous manner of kidney tubular injury during ferroptosis, a phenomenon that we refer to as a "wave of death." We hypothesize that necroptosis might initiate cell death propagation through ferroptosis. The remaining necrotic debris requires effective removal processes to prevent a secondary inflammatory response, referred to as necroinflammation. Open questions include the differences in the immunogenicity of ferroptosis and necroptosis, and the specificity of necrostatins and ferrostatins to therapeutically target these processes to prevent AKI-to-CKD progression and end-stage renal disease.

Targeting necroptosis as therapeutic potential in chronic myocardial infarction

Cardiovascular diseases (CVDs) are considered the predominant cause of morbidity and mortality globally. Of these, myocardial infarction (MI) is the most common cause of CVD mortality. MI is a life-threatening condition which occurs when coronary perfusion is interrupted leading to cardiomyocyte death. Subsequent to MI, consequences include adverse cardiac remodeling and cardiac dysfunction mainly contribute to the development of heart failure (HF). It has been shown that loss of functional cardiomyocytes in MI-induced HF are associated with several cell death pathways, in particular necroptosis. Although the entire mechanism underlying necroptosis in MI progression is still not widely recognized, some recent studies have reported beneficial effects of necroptosis inhibitors on cell viability and cardiac function in chronic MI models. Therefore, extensive investigation into the necroptosis signaling pathway is indicated for further study. This article comprehensively reviews the context of the underlying mechanisms of necroptosis in chronic MI-induced HF in in vitro, in vivo and clinical studies. These findings could inform ways of developing novel therapeutic strategies to improve the clinical outcomes in MI patients from this point forward.

Identification of cell-free DNA methylation patterns unique to the human left ventricle as a potential indicator of acute cellular rejection

Increased levels of donor-derived cell-free DNA (dd-cfDNA) in recipient plasma have been associated with rejection after transplantation. DNA sequence differences have been used to distinguish between donor and recipient, but epigenetic differences could also potentially identify dd-cfDNA. This pilot study aimed to identify ventricle-specific differentially methylated regions of DNA (DMRs) that could be detected in cfDNA. We identified 24 ventricle-specific DMRs and chose two for further study, one on chromosome 9 and one on chromosome 12. The specificity of both DMRs for the left ventricle was confirmed using genomic DNA from multiple human tissues. Serial matched samples of myocardium (n = 33) and plasma (n = 24) were collected from stable adult heart transplant recipients undergoing routine endomyocardial biopsy for rejection surveillance. Plasma DMR levels increased with biopsy-proven rejection grade for individual patients. Mean cellular apoptosis in biopsy samples increased significantly with rejection severity (2.4%, 4.4% and 10.0% for ACR 0R, 1R, and 2R, respectively) but did not show a consistent relationship with DMR levels. We identified multiple DNA methylation patterns unique to the human ventricle and conclude that epigenetic differences in cfDNA populations represent a promising alternative strategy for the non-invasive detection of rejection.

Extracellular HMGB1 Contributes to the Chronic Cardiac Allograft Vasculopathy/Fibrosis by Modulating TGF-β1 Signaling

Cardiac allograft vasculopathy (CAV) charactered with aberrant remodeling and fibrosis usually leads to the loss of graft after heart transplantation. Our previous work has reported that extracellular high-mobility group box 1 (HMGB1) participated in the CAV progression via promoting inflammatory cells infiltration and immune damage. The aim of this study was to investigate the involvement of HMGB1 in the pathogenesis of CAV/fibrosis and potential mechanisms using a chronic cardiac rejection model in mice. We found high levels of transforming growth factor (TGF)-β1 in cardiac allografts after transplantation. Treatment with HMGB1 neutralizing antibody markedly prolonged the allograft survival accompanied by attenuated fibrosis of cardiac allograft, decreased fibroblasts-to-myofibroblasts conversion, and reduced synthesis and release of TGF-β1. In addition, recombinant HMGB1 stimulation promoted release of active TGF-β1 from cardiac fibroblasts and macrophages in vitro, and subsequent phosphorylation of Smad2 and Smad3 which were downstream of TGF-β1 signaling. These data indicate that HMGB1 contributes to the CAV/fibrosis via promoting the activation of TGF-β1/Smad signaling. Targeting HMGB1 might become a new therapeutic strategy for inhibiting cardiac allograft fibrosis and dysfunction.

The role of RIPK3-regulated cell death pathways and necroptosis in the pathogenesis of cardiac ischaemia-reperfusion injury

Despite advancements in management of acute myocardial infarction, this disease remains one of the leading causes of death. Timely reestablishment of epicardial coronary blood flow is the cornerstone of therapy; however, substantial amount of damage can occur as a consequence of cardiac ischaemia/reperfusion (I/R) injury. It has been previously proposed that the pathway leading to major cell death, apoptosis, is responsible for cardiac I/R injury. Nevertheless, there is compelling evidence to suggest that necroptosis, a programmed necrosis, contributes remarkably to both myocardial injury and microcirculatory dysfunction following cardiac I/R injury. Receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed-lineage kinase domain-like pseudokinase (MLKL) are shown as the major mediators of necroptosis. In addition to the traditional perception that RIPK1/RIPK3/MLKL-dependent plasma membrane rupture is fundamental to this process, several RIPK3-related pathways such as endoplasmic reticulum stress and mitochondrial fragmentation have also been implicated in cardiac I/R injury. In this review, reports from both in vitro and in vivo studies regarding the roles of necroptosis and RIPK3-regulated necrosis in cardiac I/R injury have been collectively summarized and discussed. Furthermore, reports on potential interventions targeting these processes to attenuate cardiac I/R insults to the heart have been presented in this review. Future investigations adding to the knowledge obtained from these previous studies are needed in the pursuit of discovering the most effective pharmacological agent to improve cardiac I/R outcomes.

Sterile inflammation in thoracic transplantation

The life-saving benefits of organ transplantation can be thwarted by allograft dysfunction due to both infectious and sterile inflammation post-surgery. Sterile inflammation can occur after necrotic cell death due to the release of endogenous ligands [such as damage-associated molecular patterns (DAMPs) and alarmins], which perpetuate inflammation and ongoing cellular injury via various signaling cascades. Ischemia-reperfusion injury (IRI) is a significant contributor to sterile inflammation after organ transplantation and is associated with detrimental short- and long-term outcomes. While the vicious cycle of sterile inflammation and cellular injury is remarkably consistent amongst different organs and even species, we have begun understanding its mechanistic basis only over the last few decades. This understanding has resulted in the developments of novel, yet non-specific therapies for mitigating IRI-induced graft damage, albeit with moderate results. Thus, further understanding of the mechanisms underlying sterile inflammation after transplantation is critical for identifying personalized therapies to prevent or interrupt this vicious cycle and mitigating allograft dysfunction. In this review, we identify common and distinct pathways of post-transplant sterile inflammation across both heart and lung transplantation that can potentially be targeted.

Receptor-interacting protein kinase 1 (RIPK1) as a therapeutic target

Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) is a key mediator of cell death and inflammation. The unique hydrophobic pocket in the allosteric regulatory domain of RIPK1 has enabled the development of highly selective small-molecule inhibitors of its kinase activity, which have demonstrated safety in preclinical models and clinical trials. Potential applications of these RIPK1 inhibitors for the treatment of monogenic and polygenic autoimmune, inflammatory, neurodegenerative, ischaemic and acute conditions, such as sepsis, are emerging. This article reviews RIPK1 biology and disease-associated mutations in RIPK1 signalling pathways, highlighting clinical trials of RIPK1 inhibitors and potential strategies to mitigate development challenges.

Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) — a key mediator of cell death and inflammation — is activated in human diseases. Here, Yuan and colleagues discuss current understanding of RIPK1 biology and its association with diseases including inflammatory and autoimmune disorders, neurodegenerative diseases and sepsis. The clinical development of small-molecule RIPK1 inhibitors and associated challenges are discussed.

Prolonged Cold Ischemia Induces Necroptotic Cell Death in Ischemia-Reperfusion Injury and Contributes to Primary Graft Dysfunction after Lung Transplantation

Primary graft dysfunction (PGD) is a major cause of morbidity and mortality after lung transplantation. Ischemia-reperfusion injury (IRI) is a key event that contributes to PGD, though complex interactions affect donor lungs status, such as preceding brain death (BD), hemorrhagic shock (HS), and pre-engraftment lung management, the latter recognized as important risk factors for PGD. We hypothesized that a multi-hit isogenic mouse model of lung transplantation is more closely linked to PGD than IRI alone. Left lung transplants were performed between inbred C57BL/6 mice. A one-hit model of IRI was established by inducing cold ischemia (CI) of the donor lungs at 0°C for 1, 72, or 96 hours before engraftment. Multi-hit models were established by inducing 24 hours of HS and/or 3 hours of BD before 24 hours of CI. The recipients were killed at 24 hours after transplant and lung graft samples were analyzed. In the one-hit model of IRI, up to 72-hour CI time resulted in minimal cellular infiltration near small arteries after 24-hour reperfusion. Extension of CI time to 96 hours led to increased cellular infiltration and necroptotic pathway activation, without evidence of apoptosis, after 24-hour reperfusion. In a multi-hit model of PGD, "HS + BD + IRI" demonstrated increased lung injury, cellular infiltration, and activation of necroptotic and apoptotic pathways compared with IRI alone. Treatment with an inhibitor of receptor-interacting protein kinase 1 kinase, necrostatin-1, resulted in a significant decrease of downstream necroptotic pathway activation in both single- and multi-hit models of IRI. Thus, activation of necroptosis is a central event in IRI after prolonged CI, though it may not be sufficient to cause PGD alone. Pathological evaluation of donor lungs after CI-induced IRI, in conjunction with pre-engraftment donor lung factors in our multi-hit model, demonstrated early evidence of lung injury consistent with PGD. Our findings support the premise that pre-existing donor lung status is more important than CI time alone for inflammatory pathway activation in PGD, which may have important clinical implications for donor lung retrieval.

Necrostatin‑1 protects mice from acute lung injury by suppressing necroptosis and reactive oxygen species

Acute lung injury (ALI) is characterized by tissue damage and inflammatory cytokine secretion; however, the therapeutic options available to treat ALI remain limited. Necrostatin-1 (Nec-1) has the ability to attenuate cell necroptosis in various inflammatory diseases. The present study evaluated the protective effects of Nec-1 on a mouse model of lipopolysaccharide-induced ALI. Histological alterations in the lungs were evaluated through hematoxylin and eosin staining, and the expression levels of cytokines in the bronchoalveolar lavage fluid and lung tissues were determined by ELISA. In addition, accumulated production of reactive oxygen species was determined by staining with DCFH-DA probes, western blotting and immunofluorescence. The results revealed that treatment with the necroptosis inhibitor, Nec-1, exerted significant protective effects on ALI-induced inflammation and necroptosis. The key proteins involved in necroptosis were markedly reduced, including receptor-interacting serine/threonine-protein kinase (RIP)1 and RIP3. Notably, antioxidant proteins were upregulated by Nec-1, which may attenuate oxidative stress. Furthermore, treatment with Nec-1 markedly suppressed necroptosis in the pulmonary alveoli RLE-6TN cell line. Taken together, these data revealed a novel association between ALI and necroptosis, and suggested that necroptosis inhibitors may be used as effective anti-inflammatory drugs to treat ALI.

HMGB1-associated necroptosis and Kupffer cells M1 polarization underlies remote liver injury induced by intestinal ischemia/reperfusion in rats

Reperfusion of the ischemic intestine often leads to drive distant organ injury, especially injuries associated with hepatocellular dysfunction. The precise molecular mechanisms and effective multiple organ protection strategies remain to be developed. In the current study, significant remote liver dysfunction was found after 6 hours of reperfusion according to increased histopathological scores, serum lactate dehydrogenase (LDH), alanine aminotransferase (ALT)/aspartate aminotransferase (AST) levels, as well as enhanced bacterial translocation in a rat intestinal ischemia/reperfusion (I/R) injury model. Moreover, receptor-interacting protein kinase 1/3 (RIP1/3) and phosphorylated-MLKL expressions in tissue were greatly elevated, indicating that necroptosis occurred and resulted in acute remote liver function impairment. Inhibiting the necroptotic pathway attenuated HMGB1 cytoplasm translocation and tissue damage. Meanwhile, macrophage-depletion study demonstrated that Kupffer cells (KCs) are responsible for liver damage. Blocking HMGB1 partially restored the liver function via suppressed hepatocyte necroptosis, tissue inflammation, hepatic KCs, and circulating macrophages M1 polarization. What's more, HMGB1 neutralization further protects against intestinal I/R-associated liver damage in microbiota-depleted rats. Therefore, intestinal I/R is likely associated with acute liver damage due to hepatocyte necroptosis, and which could be ameliorated by Nec-1 administration and HMGB1 inhibition with the neutralizing antibody and inhibitor. Necroptosis inhibition and HMGB1 neutralization/inhibition, may emerge as effective pharmacological therapies to minimize intestinal I/R-induced acute remote organ dysfunction.

Tumor necrosis factor-driven cell death in donor organ as a barrier to immunological tolerance

PURPOSE OF REVIEW

Regulated cell death (RCD) is likely to play a role in organ rejection but it is unclear how it may be invoked. A well-known trigger of regulated cell death is tumor necrosis factor-alpha (TNF), which activates both caspase-dependent apoptosis and caspase-independent necroptosis. TNF is best known as a pro-inflammatory cytokine because it activates NFκB and MAPK signaling to induce expression of pro-inflammatory genes.

RECENT FINDINGS

Emerging data from animal models now suggest that TNF-induced cell death can also be inflammatory. Therefore, the role of cellular demise in regulating immunity should be considered. In transplantation, TNF could have a role in cellular injury or death from ischemia reperfusion (IR) injury and this may dictate organ survival. The default response to TNF in most cells is survival, rather than death, because of the presence of cell death checkpoints. However, cells succumb to TNF-driven death when these checkpoints are disrupted, and sensitivity to death likely reflects a reduction in molecules that fortify these checkpoints. We propose that a cell's propensity to die in response to TNF may underlie allograft rejection.

SUMMARY

Genetic, epigenetic, and posttranslational control of death checkpoint regulators in donor tissues may determine graft survival. Therapeutically, drugs that prevent donor cell demise could be useful in preventing organ rejection.

Collateral damage: necroptosis in the development of lung injury

Cell death is increasingly recognized as a driving factor in the development of acute lung injury. Necroptosis, an immunogenic regulated cell death program important in innate immunity, has been implicated in the development of lung injury in a diverse range of conditions. Characterized by lytic cell death and consequent extracellular release of endogenous inflammatory mediators, necroptosis can be both beneficial and deleterious to the host, depending on the context. Here, we review recent investigations linking necroptosis and the development of experimental lung injury. We assess the consequences of necroptosis during bacterial pneumonia, viral infection, sepsis, and sterile injury, highlighting increasing evidence from in vitro studies, animal models, and clinical studies that implicates necroptosis in the pathogenesis of ARDS. Lastly, we highlight current challenges in translating laboratory findings to the bedside.

Plin5/p-Plin5 Guards Diabetic CMECs by Regulating FFAs Metabolism Bidirectionally

Background

Hyper-free fatty acidemia (HFFA) impairs cardiac capillaries, as well as type 2 diabetes mellitus (T2DM). Perilipin 5 (Plin5) maintains metabolic balance of free fatty acids (FFAs) in high oxidative tissues via the states of nonphosphorylation and phosphorylation. However, when facing to T2DM-HFFA, Plin5's role in cardiac microvascular endothelial cells (CMECs) is not defined.

Methods

In mice of WT or Plin5^−/−, T2DM models were rendered by high-fat diet combined with intraperitoneal injection of streptozocin. CMECs isolated from left ventricles were incubated with high glucose (HG) and high FFAs (HFFAs). Plin5 phosphorylation was stimulated by isoproterenol. Plin5 expression was knocked down by small interfering RNA (siRNA). We determined cardiac function by small animal ultrasound, apoptotic rate by flow cytometry, microvessel quantity by immunohistochemistry, microvascular integrity by scanning electron microscopy, intracellular FFAs by spectrophotometry, lipid droplets (LDs) by Nile red staining, mRNAs by quantitative real-time polymerase chain reaction, proteins by western blots, nitric oxide (NO) and reactive oxygen species (ROS) by fluorescent dye staining and enzyme-linked immunosorbent assay kits.

Results

In CMECs, HFFAs aggravated cell injury induced by HG and activated Plin5 expression. In mice with T2DM-HFFA, Plin5 deficiency reduced number of cardiac capillaries, worsened structural incompleteness, and enhanced diastolic dysfunction. Moreover, in CMECs treated with HG-HFFAs, both ablation and phosphorylation of Plin5 reduced LDs content, increased intracellular FFAs, stimulated mitochondrial β-oxidation, added ROS generation, and reduced the expression and activity of endothelial nitric oxide synthase (eNOS), eventually leading to increased apoptotic rate and decreased NO content, all of which were reversed by N-acetyl-L-cysteine.

Conclusion

Plin5 preserves lipid balance and cell survival in diabetic CMECs by regulating FFAs metabolism bidirectionally via the states of nonphosphorylation and phosphorylation.

RIPK1 inhibitor Cpd-71 attenuates renal dysfunction in cisplatin-treated mice via attenuating necroptosis, inflammation and oxidative stress

Acute kidney injury (AKI) is a destructive clinical condition induced by multiple insults including ischemic reperfusion, nephrotoxic drugs and sepsis. It is characterized by a sudden decline in renal function, in addition to excessive inflammation, oxidative stress and programmed cell death of renal tubular epithelial cells. RIPK1-mediated necroptosis plays an important role in AKI. In the present study, we evaluated the treatment effects of Compound-71 (Cpd-71), a novel RIPK1 inhibitor, by comparing with Necrostatin-1 (Nec-1), a classic RIPK1 inhibitor, which has several drawbacks like the narrow structure–activity relationship (SAR) profile, moderate potency and non-ideal pharmacokinetic properties, in vivo and in vitro. Our results showed that pretreatment of Cpd-71 attenuated cisplatin-induced renal injury, restored renal function and suppressed renal inflammation, oxidative stress and cell necroptosis. In addition, Cpd-71 inhibited renal damage while reducing the up-regulated serum creatinine (Cr) and blood urea nitrogen (BUN) levels in established AKI mice model. Consistently, we confirmed that Cpd-71 exhibited more effectively suppressive effect on cisplatin-induced renal tubular cell necroptosis than Nec-1, by physically binding to the allosteric type III ligand binding site of RIPK1, thereby reduced RIPK1 kinase activity, RIPK1/RIPK3 complex formation and phosphor-MLKL membrane translocation by molecular docking, Western blot, co-immunoprecipitation and cellular thermal shift assay (CETSA). Taken together, we currently showed that targeting RIPK1 with Cpd-71 may serve as a promising clinical candidate for AKI treatment.

Loss of receptor interacting protein kinases 3 and caspase-8 augments intrinsic apoptosis in tubular epithelial cell and promote kidney ischaemia-reperfusion injury

BACKGROUND

Ischaemia-reperfusion injury (IRI) is associated with programmed cell death that promotes inflammation and organ dysfunction. Necroptosis is mediated by members of receptor interacting protein kinases (RIPK1/3). Inhibition of RIPK1/3 provides a pro-survival benefit in kidney IRI. Caspase-8 initiates apoptosis and contributes to IRI. We studied whether inhibiting both RIPK3 and caspase-8 would provide an additional benefit in kidney IRI.

METHODS

A clamp was applied to the left kidney pedicle for 45 min followed by right kidney nephrectomy. Kidney and serum from wild type, RIPK3^-/- , and RIPK3^-/- caspase-8^-/- double knockout (DKO) mice were collected post-IRI for assessment of injury. Tubular epithelial cells (TEC) isolated from wild type, RIPK3^-/- , and DKO mice were treated with interferons-γ and interleukin-1β to induce apoptotic death.

RESULTS

Kidney IRI of DKO mice did not show improvement over RIPK3^-/- mice. We have found that DKO triggered 'intrinsic' apoptosis in TEC in response to interleukin-1β and interferons-γ. Up-regulation of the B-cell lymphoma 2 (Bcl-2)-associated death promoter, the Bcl-2-homologous antagonist killer and Bcl-2-associated X protein and enhanced activation of caspase-3 and 9 were found in DKO TEC. TEC infected with Murine cytomegalovirus that encodes multiple cell death inhibitors resist to death.

CONCLUSION

We show that the deletion of both RIPK3 and caspase-8 does not provide additive benefit in IRI or TEC death and may enhance injury by up-regulation of intrinsic apoptosis. This suggests blocking multiple death pathways may be required for the prevention of kidney IRI clinically.

Mitochondrial permeability regulates cardiac endothelial cell necroptosis and cardiac allograft rejection

Transplantation is invariably associated with programmed cell death including apoptosis and necrosis, resulting in delayed graft function and organ rejection. We have demonstrated the contribution of necroptosis to mouse microvascular endothelial cell (MVEC) death and transplant rejection. Organ injury results in the opening of mitochondrial permeability transition pores (mPTPs), which can trigger apoptotic molecules release that ultimately results in cell death. The effect of mPTPs in the necroptotic pathway remains controversial; importantly, their role in transplant rejection is not clear. In this study, tumor necrosis factor-α triggered MVECs to undergo receptor-interacting protein kinase family (RIPK1/3)-dependent necroptosis. Interestingly, inhibition of mPTP opening could also inhibit necroptotic cell death. Cyclophilin-D (Cyp-D) is a key regulator of the mPTPs. Both inhibition and deficiency of Cyp-D protected MVECs from necroptosis (n = 3, P < .00001). Additionally, inhibition of Cyp-D attenuated RIPK3-downstream mixed-lineage kinase domain-like protein phosphorylation. In vivo, Cyp-D-deficient cardiac grafts showed prolonged survival in allogeneic BALB/c mice posttransplant compared with wild-type grafts (n = 7, P < .0001). Our study results suggest that the mPTPs may be important mechanistic mediators of necroptosis in cardiac grafts. There is therapeutic potential in targeting cell death via inhibition of the mPTP-regulating molecule Cyp-D to prevent cardiac graft rejection.

Ferroptotic cell death and TLR4/Trif signaling initiate neutrophil recruitment after heart transplantation

Non-apoptotic forms of cell death can trigger sterile inflammation through the release of danger-associated molecular patterns, which are recognized by innate immune receptors. However, despite years of investigation the mechanisms which initiate inflammatory responses after heart transplantation remain elusive. Here, we demonstrate that ferrostatin-1 (Fer-1), a specific inhibitor of ferroptosis, decreases the level of pro-ferroptotic hydroperoxy-arachidonoyl-phosphatidylethanolamine, reduces cardiomyocyte cell death and blocks neutrophil recruitment following heart transplantation. Inhibition of necroptosis had no effect on neutrophil trafficking in cardiac grafts. We extend these observations to a model of coronary artery ligation-induced myocardial ischemia reperfusion injury where inhibition of ferroptosis resulted in reduced infarct size, improved left ventricular systolic function, and reduced left ventricular remodeling. Using intravital imaging of cardiac transplants, we uncover that ferroptosis orchestrates neutrophil recruitment to injured myocardium by promoting adhesion of neutrophils to coronary vascular endothelial cells through a TLR4/TRIF/type I IFN signaling pathway. Thus, we have discovered that inflammatory responses after cardiac transplantation are initiated through ferroptotic cell death and TLR4/Trif-dependent signaling in graft endothelial cells. These findings provide a platform for the development of therapeutic strategies for heart transplant recipients and patients, who are vulnerable to ischemia reperfusion injury following restoration of coronary blood flow.

The pathological features of regulated necrosis

Necrosis of a cell is defined by the loss of its plasma membrane integrity. Morphologically, necrosis occurs in several forms such as coagulative necrosis, colliquative necrosis, caseating necrosis, fibrinoid necrosis, and others. Biochemically, necrosis was demonstrated to represent a number of genetically determined signalling pathways. These include (i) kinase-mediated necroptosis, which depends on receptor interacting protein kinase 3 (RIPK3)-mediated phosphorylation of the pseudokinase mixed lineage kinase domain like (MLKL); (ii) gasdermin-mediated necrosis downstream of inflammasomes, also referred to as pyroptosis; and (iii) an iron-catalysed mechanism of highly specific lipid peroxidation named ferroptosis. Given the molecular understanding of the nature of these pathways, specific antibodies may allow direct detection of regulated necrosis and correlation with morphological features. Necroptosis can be specifically detected by immunohistochemistry and immunofluorescence employing antibodies to phosphorylated MLKL. Likewise, it is possible to generate cleavage-specific antibodies against epitopes in gasdermin protein family members. In ferroptosis, however, specific detection requires quantification of oxidative lipids by mass spectrometry (oxylipidomics). Together with classical cell death markers, such as TUNEL staining and detection of cleaved caspase-3 in apoptotic cells, the extension of the arsenal of necrosis markers will allow pathological detection of specific molecular pathways rather than isolated morphological descriptions. These novel pieces of information will be extraordinarily helpful for clinicians as inhibitors of necroptosis (necrostatins), ferroptosis (ferrostatins), and inflammasomes have emerged in clinical trials. Anatomical pathologists should embrace these novel ancillary tests and the concepts behind them and test their impact on diagnostic precision, prognostication, and the prediction of response to the upcoming anti-necrotic therapies. Copyright © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

ARC regulates programmed necrosis and myocardial ischemia/reperfusion injury through the inhibition of mPTP opening

Necrosis is a key factor in myocardial injury during cardiac pathological processes, such as myocardial infarction (MI), ischemia/reperfusion (I/R) injury and heart failure. Increasing evidence suggests that several aspects of necrosis are programmed and tightly regulated, so targeting the necrosis process has become a new trend for myocardial protection. Multiple cellular signaling pathways have been implicated in necrotic cell death, such as the death receptor-mediated extrinsic and mitochondrial intrinsic pathways. However, the precise mechanisms underlying myocardial necrosis remain unclear. In this study, we showed that apoptosis repressor with caspase recruitment domain (ARC) participated in the mitochondrial intrinsic pathway and inhibited myocardial necrosis by preventing the opening of the mitochondrial permeability transition pore (mPTP). ARC attenuated necrotic cell death triggered by exposure to 500 μM hydrogen peroxide (H₂O₂) in the cardiomyocyte cell line H9c2. In mice, ARC ameliorated myocardial necrosis, reduced the myocardial infarct size and improved long-term heart function during I/R injury. Mechanistically, it has been shown that the inhibition of necrosis by ARC was dependent on its mitochondrial localization and that ARC prevented the opening of mPTP by targeting CypD, the main regulator of mPTP. In addition, ARC expression was negatively regulated by the transcription factor p53 at the transcriptional level during the necrosis process. These findings identified the novel role of ARC in myocardial necrosis and delineated the p53-ARC-CypD/mPTP necrosis pathway during ischemia- and oxidative stress-induced myocardial damage, which can provide a new strategy for cardiac protection.

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ARC attenuates necrosis both in cardiomyocytes exposed to H₂O₂ and in mouse hearts with ischemia/reperfusion injury.

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The attenuation of necrosis by ARC depends on its mitochondrial localization and its inhibition of mPTP opening.

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ARC targets CypD and prevents mPTP opening.

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p53 regulates necrosis by suppression of ARC expression at the transcriptional level.

Necrostatin-1 Protects Against D-Galactosamine and Lipopolysaccharide-Induced Hepatic Injury by Preventing TLR4 and RAGE Signaling

Fulminant hepatic failure (FHF) is a life-threatening clinical syndrome results in massive inflammation and hepatocyte death. Necroptosis is a regulated form of necrotic cell death that is emerging as a crucial control point for inflammatory diseases. The kinases receptor interacting protein (RIP) 1 and RIP3 are known as key modulators of necroptosis. In this study, we investigated the impact of necroptosis in the pathogenesis of FHF and molecular mechanisms, particularly its linkage to damage-associated molecular pattern (DAMP)-mediated pattern recognition receptor (PRR) signaling pathways. Male C57BL/6 mice were given an intraperitoneal injection of necrostatin-1 (Nec-1, RIP1 inhibitor; 1.8 mg/kg; dissolved in 2% dimethyl sulfoxide in phosphate-buffered saline) 1 h before receiving D-galactosamine (GalN; 800 mg/kg)/lipopolysaccharide (LPS; 40 μg/kg). Hepatic RIP1, RIP3 protein expression, their phosphorylation, and RIP1/RIP3 complex formation upregulated in the GalN/LPS group were attenuated by Nec-1. Nec-1 markedly reduced the increases in mortality and serum alanine aminotransferase activity induced by GalN/LPS. Increased serum high mobility group box 1 (HMGB1) and interleukin (IL)-33 release, HMGB1-toll-like receptor 4 and HMGB1-receptor for advanced glycation end products (RAGE) interaction, and nuclear protein expressions of NF-κB and early growth response protein-1 (egr-1) were attenuated by Nec-1. Our finding suggests that necroptosis is responsible for GalN/LPS-induced liver injury through DAMP-activated PRR signaling.

Regulated Cell Death Seen through the Lens of Islet Transplantation

Clinical islet transplantation effectively restores euglycemia and corrects glycosylated hemoglobin in labile type 1 diabetes mellitus (T1DM). Despite marked improvements in islet transplantation outcomes, acute islet cell death remains a substantial obstacle that compromises long-term engraftment outcomes. Multiple organ donors are routinely required to achieve insulin independence. Therapeutic agents that ameliorate cell death and/or control injury-related inflammatory cascades offer potential to improve islet transplant success. Apoptotic cell death has been identified as a major contributor to cellular demise and therapeutic strategies that subvert initiation and consequences of apoptotic cell death have shown promise in pre-clinical models. Indeed, in numerous pathologies and diseases apoptosis has been the most extensively described form of regulated cell death. However, recent identification of novel, alternative regulated cell death pathways in other disease states and solid organ transplantation suggest that these additional pathways may also have substantial relevance in islet transplantation. These regulated, non-apoptotic cell death pathways exhibit distinct biochemical characteristics but have yet to be fully characterized within islet transplantation. We review herein the various regulated cell death pathways and highlight their relative potential contributions to islet viability, engraftment failure and islet dysfunction.

Endothelial Dysfunction in Kidney Transplantation

Kidney transplantation entails a high likelihood of endothelial injury. The endothelium is a target of choice for injury by ischemia-reperfusion, alloantibodies, and autoantibodies. A certain degree of ischemia-reperfusion injury inevitably occurs in the immediate posttransplant setting and can manifest as delayed graft function. Acute rejection episodes, whether T-cell or antibody-mediated, can involve the graft micro- and macrovasculature, leading to endothelial injury and adverse long-term consequences on graft function and survival. In turn, caspase-3 activation in injured and dying endothelial cells favors the release of extracellular vesicles (apoptotic bodies and apoptotic exosome-like vesicles) that further enhance autoantibody production, complement deposition, and microvascular rarefaction. In this review, we present the evidence for endothelial injury, its causes and long-term consequences on graft outcomes in the field of kidney transplantation.

Inhibition of regulated necrosis attenuates receptor-interacting protein kinase 1-mediated ischemia-reperfusion injury after lung transplantation

BACKGROUND

Increasing evidence indicates that regulated necrosis plays a critical role during cell death caused by ischemia-reperfusion (IR) injury. Necroptosis is one form of regulated necrosis. Necrostatin-1 (Nec-1), an inhibitor of receptor-interacting protein kinase 1 (RIPK1), is known to reduce necroptosis. We investigated the effect of Nec-1 treatment on IR-induced lung injury in a rat lung transplant model.

METHODS

Lewis rats were divided into 4 groups (n = 6 each): (1) Control (no treatment), (2) Donor treatment (D), (3) Recipient treatment (R), and (4) Donor plus Recipient treatment (D+R) groups. Donor lungs were flushed and preserved for 18 hours at 4ºC before transplantation. Recipient animals underwent a left single lung transplant. After 2 hours of reperfusion, we assessed the physiologic function, cytokine expression, pathway activation, and the extent of necrosis.

RESULTS

Pulmonary gas exchange in D+R group was significantly better than in the other 3 groups (p = 0.003). Lung edema was significantly lower in the D+R group compared with the Control group (p = 0.006). The expression of interleukin-6 in lung tissue and plasma was significantly reduced in the D+R group compared with the Control group (p = 0.036). The percentage of necrotic cells in D+R group was significantly lower than in the Control and D groups (p = 0.01), indicating Nec-1inhibited regulated necrosis.

CONCLUSIONS

The administration of Nec-1 to both donor and recipient improved graft function after lung transplantation through the reduction of necroptosis. The inhibition of regulated necrosis appears to be a promising strategy to attenuate IR lung injury after lung transplantation.

Necrostatin-1 Attenuates Cisplatin-Induced Nephrotoxicity Through Suppression of Apoptosis and Oxidative Stress and Retains Klotho Expression

Aim: Cisplatin is an effective chemotherapeutic drug, but the application in clinical is greatly limited by its nephrotoxicity. Necrostatin-1 (Nec-1), an inhibitor of RIP1 kinase, has been reported to inhibit RIP-mediated necroptosis. The aim of this study is to detect the protective effects of Nec-1 on the nephrotoxicity of cisplatin and to investigate its renoprotection mechanism. Methods: 8-week-old male C57BL/6 mice were randomly assigned into four groups: Control, Nec-1, Cisplatin, and Cisplatin+Nec-1. Mice were treated with cisplatin with or without Nec-1 pre-treatment. Renal function, histological changes, necroptosis, and apoptotic markers were investigated. NFκB pathway related proteins, proinflammatory cytokines, oxidative stress markers, renal Klotho, and autophagy-related proteins levels were also examined. Results: Renal function and histological data displayed that the treatment with Nec-1 significantly attenuates cisplatin-induced renal damage. The expression of RIPK1/RIPK3/MLKL were significantly enhanced in cisplatin group as compared to the control group (p < 0.05) and was significantly reduced by pre-treatment of Nec-1 (p < 0.05). The level of stress and apoptosis-related protein, including p-JNK, p-c-Jun, p-p38, Bax/Bcl-2 ratio, and caspase-3 showed the similar trend. Pre-treatment with Nec-1 inhibit NFκB signaling, reduced proinflammatory cytokines and oxidative stress, up-regulated renal Klotho, and autophagy-related proteins levels. Conclusion: Our results suggest that Nec-1 could be a potential therapeutic drug against the cisplatin-induced nephrotoxicity through its anti-necroptosis, anti-apoptotic, anti-inflammatory anti-oxidant and retain Klotho expression and activate autophagy effects in the kidney.

The in vivo evidence for regulated necrosis

Necrosis is a hallmark of several widespread diseases or their direct complications. In the past decade, we learned that necrosis can be a regulated process that is potentially druggable. RIPK3- and MLKL-mediated necroptosis represents by far the best studied pathway of regulated necrosis. During necroptosis, the release of damage-associated molecular patterns (DAMPs) drives a phenomenon referred to as necroinflammation, a common consequence of necrosis. However, most studies of regulated necrosis investigated cell lines in vitro in a cell autonomous manner, which represents a non-physiological situation. Conclusions based on such work might not necessarily be transferrable to disease states in which synchronized, non-cell autonomous effects occur. Here, we summarize the current knowledge of the pathophysiological relevance of necroptosis in vivo, and in light of this understanding, we reassess the morphological classification of necrosis that is generally used by pathologists. Along these lines, we discuss the paucity of data implicating necroptosis in human disease. Finally, the in vivo relevance of non-necroptotic forms of necrosis, such as ferroptosis, is addressed.

Necroptosis in microglia contributes to neuroinflammation and retinal degeneration through TLR4 activation

Inflammation has emerged to be a critical mechanism responsible for neural damage and neurodegenerative diseases. Microglia, the resident innate immune cells in retina, are implicated as principal components of the immunological insult to retinal neural cells. The involvement of microglia in retinal inflammation is complex and here we propose for the first time that necroptosis in microglia triggers neuroinflammation and exacerbates retinal neural damage and degeneration. We found microglia experienced receptor-interacting protein kinase 1 (RIP1)- and RIP3-dependent necroptosis not only in the retinal degenerative rd1 mice, but also in the acute retinal neural injury mice. The necroptotic microglia released various pro-inflammatory cytokines and chemokines, such as tumor necrosis factor-α and chemokine (C-C motif) ligand 2, which orchestrated the retinal inflammation. Importantly, necroptosis blockade using necrostatin-1 could suppress microglia-mediated inflammation, rescue retinal degeneration or prevent neural injury in vivo. Meanwhile, cultured microglia underwent RIP1/3-mediated necroptosis and the necroptotic microglia produced large amounts of pro-inflammatory cytokines in response to lipopolysaccharide or oxidative stress in vitro. Mechanically, TLR4 deficiency ameliorated microglia necroptosis with decreased expression levels of machinery molecules RIP1 and RIP3, and suppressed retinal inflammation, suggesting that TLR4 signaling was required in microglia necroptosis-mediated inflammation. Thus, we proposed that microglia experienced necroptosis through TLR4 activation, promoting an inflammatory response that serves to exacerbate considerable neural damage and degeneration. Necroptosis blockade therefore emerged as a novel therapeutic strategy for tempering microglia-mediated neuroinflammation and ameliorating neural injury and neurodegenerative diseases.

Intracellular pH Regulates TRAIL-Induced Apoptosis and Necroptosis in Endothelial Cells

During ischemia or inflammation of organs, intracellular pH can decrease if acid production exceeds buffering capacity. Thus, the microenvironment can expose parenchymal cells to a reduced extracellular pH which can alter pH-dependent intracellular functions. We have previously shown that while silencing caspase-8 in an in vivo ischemia reperfusion injury (IRI) model results in improved organ function and survival, removal of caspase-8 function in a donor organ can paradoxically result in enhanced receptor-interacting protein kinase 1/3- (RIPK1/3-) regulated necroptosis and accelerated graft loss following transplantation. In our current study, TRAIL- (TNF-related apoptosis-inducing ligand-) induced cell death in vitro at neutral pH and caspase-8 inhibition-enhanced RIPK1-dependent necroptotic death were confirmed. In contrast, both caspase-8 inhibition and RIPK1 inhibition attenuated cell death at a cell pH of 6.7. Cell death was attenuated with mixed lineage kinase domain-like (MLKL) silencing, indicating that MLKL membrane rupture, a distinctive feature of necroptosis, occurs regardless of pH. In summary, there is a distinct regulatory control of apoptosis and necroptosis in endothelial cells at different intracellular pH. These results highlight the complexity of modulating cell death and therapeutic strategies that may need to consider different consequences on cell death dependent on the model.

Necroptosis: A Novel Cell Death Modality and Its Potential Relevance for Critical Care Medicine

Cell death is intertwined with life in development, homeostasis, pathology, and aging. Until recently, apoptosis was the best known form of programmed cell death, whereas necrosis was for a long time considered accidental owing to physicochemical injury. However, identification of crucial signaling and execution molecules, which are highly regulated, revealed that necrosis encompasses several cell death modalities that can be therapeutically targeted. The best understood form of regulated necrosis is necroptosis, which is transduced by the kinase activities of receptor interacting protein kinase-1 and receptor interacting protein kinase-3, eventually leading to the activation of mixed lineage kinase domain-like and plasma membrane permeabilization. We are only beginning to appreciate the role of necroptosis in different pathological conditions, including critical illnesses. In this review, we discuss the molecular mechanisms of necroptosis and analyze the effect of inhibiting necroptosis in experimental models of critical illnesses. In view of the identification of an increasing number of cell death modalities, we also briefly discuss the simultaneous targeting of multiple cell death modalities because, depending on the cell type and cellular conditions, various types of cell death may contribute to the pathology.

Pharmacological modulation of cell death in organ transplantation

New options to pharmacologically modulate fundamental mechanisms of regulated cell death are rapidly evolving and found first clinical applications in cancer therapy. Here, we present an overview on how the recent advances in the understanding of the biology and pharmacology of cell death might influence research and clinical practice in solid organ transplantation. Of particular interest are the novel opportunities related to organ preservation and immunomodulation, which might contribute to promote organ repair and to develop more selective ways to modulate allogeneic immune responses to prevent rejection and induce immunological tolerance.

Necroptosis: Modules and molecular switches with therapeutic implications

Among the various programmed cell death (PCD) pathways, "Necroptosis" has gained much importance as a novel paradigm of cell death. This pathway has emerged as a backup mechanism when physiologically conserved PCD (apoptosis) is non-functional either genetically or pathogenically. The expanding spectrum of necroptosis from physiological development to diverse patho-physiological disorders, including xenobiotics-mediated toxicity has now grabbed the attention worldwide. The efficient role of necroptosis regulators in disease development and management are under constant examination. In fact, few regulators (e.g. MLKL) have already paved their way towards clinical trials and others are in queue. In this review, emphasis has been paid to the various contributing factors and molecular switches that can regulate necroptosis. Here we linked the overview of current knowledge of this enigmatic signaling with magnitude of therapeutics that may underpin the opportunities for novel therapeutic approaches to suppress the pathogenesis of necroptosis-driven disorders.