Virus inhibition of RIP3-dependent necrosis

Down the rabbit hole: Is necroptosis truly an innate response to infection?

Pathogenic microbes have evolved countless sophisticated mechanisms to subvert host immune responses and cause disease. Understanding evasion strategies employed by pathogens has led to numerous discoveries on specific host cell processes that are critical for controlling infection. Programmed cell death (PCD) is a key host defence to microbial infection, as well as being critical for organ development and cellular homeostasis in multicellular organisms. Much of our current understanding of PCD as a host response to infection has stemmed from the discovery and study of viral inhibitors of apoptosis, and more recently viral inhibition of the newly characterised from of PCD termed necroptosis, the mechanisms of which are still under intense investigation. Many bacterial pathogens also encode inhibitors of PCD, yet these discoveries are relatively more recent and thus the biological significance of such mechanisms is still under debate. In this viewpoint article, we will argue the concept that necroptosis is merely a "back-up" mechanism in the event that apoptosis is inhibited, or whether it is a true host innate response to infection that has evolved in response to a growing arsenal of microbial evasion strategies.

Murine cytomegalovirus IE3-dependent transcription is required for DAI/ZBP1-mediated necroptosis

DNA-dependent activator of interferon regulatory factors/Z-DNA binding protein 1 (DAI/ZBP1) is a crucial sensor of necroptotic cell death induced by murine cytomegalovirus (MCMV) in its natural host. Here, we show that viral capsid transport to the nucleus and subsequent viral IE3-dependent early transcription are required for necroptosis. Necroptosis induction does not depend on input virion DNA or newly synthesized viral DNA A putative RNA-binding domain of DAI/ZBP1, Zα2, is required to sense virus and trigger necroptosis. Thus, MCMV IE3-dependent transcription from the viral genome plays a crucial role in activating DAI/ZBP1-dependent necroptosis. This implicates RNA transcripts generated by a large double-stranded DNA virus as a biologically relevant ligand for DAI/ZBP1 during natural viral infection.

Inhibition of Necroptosis Attenuates Kidney Inflammation and Interstitial Fibrosis Induced By Unilateral Ureteral Obstruction

BACKGROUND

Inflammation plays a crucial role in renal interstitial fibrosis, the pathway of chronic kidney diseases. Necroptosis is a novel form of regulated cell death, which plays a potential role in inflammation and renal diseases. The small molecule necrostatin-1 (Nec-1) is a specific inhibitor of necroptosis. This study was aimed at determining the role of necroptosis, RIP1/RIP3/mixed lineage kinase domain-like (MLKL) signaling pathway, in renal inflammation and interstitial fibrosis related to primitive tubulointerstitial injury. It was also aimed at evaluating the effect of Nec-1 in renal fibrosis induced by unilateral ureteral obstruction (UUO).

METHODS

Renal histology, immunohistochemistry, western blot, and real-time polymerase chain reaction were performed using UUO C57BL/6J mice model. Moreover, we tested whether Nec-1 was renal-protective in the interstitial fibrosis kidney. Mice were exposed to UUO and injected intraperitoneal with Nec-1 or vehicle.

RESULTS

The levels of RIP1/RIP3/MLKL protein and mRNA were increased in the obstructed kidneys 7 days after UUO; this was accompanied by changes in renal pathological lesions. Renal histological examination showed lesser renal damage in Nec-1-treated UUO mice. Renal inflammation, assessed by tumor necrosis factor-α, interleukin-1β, and monocyte chemotactic protein-1 was markedly attenuated by Nec-1. Furthermore, Nec-1 treatment also significantly reduced TGF-β and α-smooth muscle actin, indicating lesser renal interstitial fibrosis.

CONCLUSION

These findings suggest that the participation of necroptosis in UUO is partly demonstrated. And necroptosis inhibition may have a potential role in the treatment of diseases with increased inflammatory response and interstitial fibrosis in renal.

Sensing of viral and endogenous RNA by ZBP1/DAI induces necroptosis

Nucleic acids are potent triggers for innate immunity. Double‐stranded DNA and RNA adopt different helical conformations, including the unusual Z‐conformation. Z‐DNA/RNA is recognised by Z‐binding domains (ZBDs), which are present in proteins implicated in antiviral immunity. These include ZBP1 (also known as DAI or DLM‐1), which induces necroptosis, an inflammatory form of cell death. Using reconstitution and knock‐in models, we report that mutation of key amino acids involved in Z‐DNA/RNA binding in ZBP1's ZBDs prevented necroptosis upon infection with mouse cytomegalovirus. Induction of cell death was cell autonomous and required RNA synthesis but not viral DNA replication. Accordingly, ZBP1 directly bound to RNA via its ZBDs. Intact ZBP1‐ZBDs were also required for necroptosis triggered by ectopic expression of ZBP1 and caspase blockade, and ZBP1 cross‐linked to endogenous RNA. These observations show that Z‐RNA may constitute a molecular pattern that induces inflammatory cell death upon sensing by ZBP1.

Culling of APCs by inflammatory cell death pathways restricts TIM3 and PD-1 expression and promotes the survival of primed CD8 T cells

We evaluated the impact of premature cell death of antigen-presenting cells (APCs) by Caspase-1- and RipK3-signaling pathways on CD8⁺ T-cell priming during infection of mice with Salmonella typhimurium (ST). Our results indicate that Caspase1 and RipK3 synergize to rapidly eliminate infected APCs, which does not influence the initial activation of CD8⁺ T cells. However, the maintenance of primed CD8⁺ T cells was greatly compromised when both these pathways were disabled. Caspase-1- and RipK3-signaling did not influence NF-κB signaling in APCs, but synergized to promote processing of IL-1 and IL-18. Combined deficiency of Caspase1 and RipK3 resulted in compromised innate immunity and accelerated host fatality due to poor processing of IL-18. In contrast, synergism in cell death by Caspase-1- and RipK3 resulted in restriction of PD-1 and TIM3 expression on primed CD8⁺ T cells, which promoted the survival of activated CD8⁺ T cells.

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.

Initiation and execution mechanisms of necroptosis: an overview

Necroptosis is a form of regulated cell death, which is induced by ligand binding to TNF family death domain receptors, pattern recognizing receptors and virus sensors. The common feature of these receptor systems is the implication of proteins, which contain a receptor interaction protein kinase (RIPK) homology interaction motif (RHIM) mediating recruitment and activation of receptor-interacting protein kinase 3 (RIPK3), which ultimately activates the necroptosis executioner mixed lineage kinase domain-like (MLKL). In case of the TNF family members, the initiator is the survival- and cell death-regulating RIPK1 kinase, in the case of Toll-like receptor 3/4 (TLR3/4), a RHIM-containing adaptor, called TRIF, while in the case of Z-DNA-binding protein ZBP1/DAI, the cytosolic viral sensor itself contains a RHIM domain. In this review, we discuss the different protein complexes that serve as nucleation platforms for necroptosis and the mechanism of execution of necroptosis. Transgenic models (knockout, kinase-dead knock-in) and pharmacologic inhibition indicate that RIPK1, RIPK3 or MLKL are implicated in many inflammatory, degenerative and infectious diseases. However, the conclusion of necroptosis being solely involved in the etiology of diseases is blurred by the pleiotropic roles of RIPK1 and RIPK3 in other cellular processes such as apoptosis and inflammasome activation.

A RIPtide Protects Neurons from Infection

RIPK3 and RIPK1 limit virus spread by executing either apoptotic or necroptotic cell death in response to infection. In a recent issue of Cell, Daniels et al. (2017) unveil an unexpected cell death-independent requirement of RIP kinase activity in coordinating neuroinflammation, restricting West Nile virus pathogenesis in neurons.

RIP1 is a central signaling protein in regulation of TNF-α/TRAIL mediated apoptosis and necroptosis during Newcastle disease virus infection

Newcastle disease virus (NDV) is an oncolytic virus which selectively replicates in tumor cells and exerts anti-tumor cytotoxic activity by promoting cell death. In this study, we focus on characterization of the underlying mechanisms of NDV-induced cell death in HeLa cells. We find that NDV Herts/33 strain triggers both extrinsic and intrinsic apoptosis at late infection times. The activation of NF-кB pathway and subsequent up-regulation of TNF-α/TRAIL initiates extrinsic apoptosis, leading to activation of caspase 8 and cleavage of Bid into tBid. tBid transmits the extrinsic apoptotic signals to mitochondria and mediates intrinsic apoptosis, which is hallmarked by cleavage of caspase 9. Moreover, RIP1 is cleaved into RIP1-N and RIP1-C at D324 by caspase 8, and this cleavage promotes apoptosis. Surprisingly, over expression of RIP1 reduces apoptosis and depletion of RIP1 promotes apoptosis, suggesting full length RIP1 is anti-apoptotic. Moreover, necroptosis hallmark protein MLKL is activated by phosphorylation at 12-24 h.p.i., and RIP1 regulates the level of phosphor-MLKL. Immunostaining shows that RIP1 aggregates to stress granules (SGs) at 8-24 h.p.i., and phosphor-MLKL is also recruited to SGs, instead of migrating to plasma membrane to exert its necrotic function. Immunoprecipitation study demonstrates that RIP1 bind to phosphor-MLKL, and depletion of RIP1 reduces the aggregation of MLKL to SGs, suggesting that RIP1 recruits MLKL to SGs. Altogether, NDV infection initiates extrinsic apoptosis via activation of NF-кB and secretion of TNF-α/TRAIL. Activation of caspase 8 by TNF-α/TRAIL and subsequent cleavage of Bid and RIP1 transmit the death signals to mitochondria. Meanwhile, virus subverts the host defensive necroptosis via recruiting phosphor-MLKL by RIP1 to SGs. Thus, RIP1 is a central signaling protein in regulation of apoptosis and necroptosis during NDV infection.

Apoptosis and Necroptosis as Host Defense Strategies to Prevent Viral Infection

Antiviral transcriptional responses and regulated cell death are crucial components of the host response to virus infection. However, in contrast to the signaling pathways that promote antiviral transcription, those that initiate cell death following virus infection are less understood. Several recent studies have identified pattern recognition receptors (PRRs) of the mammalian innate immune system that activate cell death pathways. These same receptors also have established roles in the induction of antiviral gene expression. In this review we discuss the mechanisms by which PRRs can serve dual roles as initiators of inflammatory gene expression and as inducers of apoptosis and necroptosis following virus infection.

Noncanonical cell death program independent of caspase activation cascade and necroptotic modules is elicited by loss of TGFβ-activated kinase 1

Programmed cell death (PCD) occurs in several forms including apoptosis and necroptosis. Apoptosis is executed by the activation of caspases, while necroptosis is dependent on the receptor interacting protein kinase 3 (RIPK3). Precise control of cell death is crucial for tissue homeostasis. Indeed, necroptosis is triggered by caspase inhibition to ensure cell death. Here we identified a previously uncharacterized cell death pathway regulated by TAK1, which is unexpectedly provoked by inhibition of caspase activity and necroptosis cascades. Ablation of TAK1 triggers spontaneous death in macrophages. Simultaneous inhibition of caspases and RIPK3 did not completely restore cell viability. Previous studies demonstrated that loss of TAK1 in fibroblasts causes TNF-induced apoptosis and that additional inhibition of caspase leads to necroptotic cell death. However, we surprisingly found that caspase and RIPK3 inhibitions do not completely suppress cell death in Tak1-deficient cells. Mechanistically, the execution of the third cell death pathway in Tak1-deficient macrophages and fibroblasts were mediated by RIPK1-dependent rapid accumulation of reactive oxygen species (ROS). Conversely, activation of RIPK1 was sufficient to induce cell death. Therefore, loss of TAK1 elicits noncanonical cell death which is mediated by RIPK1-induced oxidative stress upon caspase and necroptosis inhibition to further ensure induction of cell death.

Cooperative inhibition of RIP1-mediated NF-κB signaling by cytomegalovirus-encoded deubiquitinase and inactive homolog of cellular ribonucleotide reductase large subunit

Several viruses have been found to encode a deubiquitinating protease (DUB). These viral DUBs are proposed to play a role in regulating innate immune or inflammatory signaling. In human cytomegalovirus (HCMV), the largest tegument protein encoded by UL48 contains DUB activity, but its cellular targets are not known. Here, we show that UL48 and UL45, an HCMV-encoded inactive homolog of cellular ribonucleotide reductase (RNR) large subunit (R1), target receptor-interacting protein kinase 1 (RIP1) to inhibit NF-κB signaling. Transfection assays showed that UL48 and UL45, which binds to UL48, interact with RIP1 and that UL48 DUB activity and UL45 cooperatively suppress RIP1-mediated NF-κB activation. The growth of UL45-null mutant virus was slightly impaired with showing reduced accumulation of viral late proteins. Analysis of a recombinant virus expressing HA-UL45 showed that UL45 interacts with both UL48 and RIP1 during virus infection. Infection with the mutant viruses also revealed that UL48 DUB activity and UL45 inhibit TNFα-induced NF-κB activation at late times of infection. UL48 cleaved both K48- and K63-linked polyubiquitin chains of RIP1. Although UL45 alone did not affect RIP1 ubiquitination, it could enhance the UL48 activity to cleave RIP1 polyubiquitin chains. Consistently, UL45-null virus infection showed higher ubiquitination level of endogenous RIP1 than HA-UL45 virus infection at late times. Moreover, UL45 promoted the UL48-RIP1 interaction and re-localization of RIP1 to the UL48-containing virion assembly complex. The mouse cytomegalovirus (MCMV)-encoded DUB, M48, interacted with mouse RIP1 and M45, an MCMV homolog of UL45. Collectively, our data demonstrate that cytomegalovirus-encoded DUB and inactive R1 homolog target RIP1 and cooperatively inhibit RIP1-mediated NF-κB signaling at the late stages of HCMV infection.

Activation of NF-κB signaling leads to expression of proinflammatory cytokines and chemokines and plays a key role in regulating innate immune response and inflammation to virus infection. HCMV upregulates and downregulates NF-κB signaling during the course of infection. Upregulation of NF-κB signaling may promote viral gene expression or viral dissemination, but its downregulation may be necessary for suppression of excessive immune responses. Recently, it was demonstrated that viral late functions downregulate TNFα- and IL-1β-induced NF-κB activation. However, the viral proteins involved and the underlying mechanisms are not understood. In the present study, we demonstrate that two HCMV proteins, the largest tegument protein harboring deubiquitinase activity and the inactive homolog of cellular ribonucleotide reductase large subunit, cooperatively inhibit RIP1-mediated NF-κB signaling at the late stages of infection. This study for the first time identified RIP1 as a substrate of viral deubiquitinase and highlights the importance of the negative regulation of NF-κB during virus infection.

Exploitation of necroptosis for treatment of caspase-compromised cancers

Programmed necrosis, or necroptosis, is a type of specialized cell death with necrotic characteristics, including the loss of membrane integrity and swollen organelles in dying cells. However, unlike simple necrosis, it may be induced as an alternative form of cell death when apoptosis is blocked and it is mediated in an orchestrated manner, similar to apoptosis, by a series of signaling molecules. Necroptosis-associated proteins and their specific small molecules have been extensively identified in order to illuminate the underlying mechanisms by which necroptosis is activated through a novel signaling pathway. However, the biological significance of necroptosis, which is known as a secondary route of apoptosis, remains under debate. Concurrent with these concerns, the clinical application of necroptosis has been cautiously proposed to treat necroptosis-associated diseases, and to overcome resistance to anticancer drugs. Accordingly, the present review will highlight the harnessing of necroptosis for anticancer therapy. To this end, the state-of-the art technique of necroptosis as a cancer therapy will be briefly described, and then its potential for clinical purposes will be delineated. For a further understanding of necroptosis, the present review begins with a basic introduction to necroptosis and its multifaceted physiological consequences.

Lysis of human neutrophils by community-associated methicillin-resistant Staphylococcus aureus

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) causes infections associated with extensive tissue damage and necrosis. In vitro, human neutrophils fed CA-MRSA lyse by an unknown mechanism that is inhibited by necrostatin-1, an allosteric inhibitor of receptor-interacting serine/threonine kinase 1 (RIPK-1). RIPK-1 figures prominently in necroptosis, a specific form of programmed cell death dependent on RIPK-1, RIPK-3, and the mixed-lineage kinase-like protein (MLKL). We previously reported that necrostatin-1 inhibits lysis of human neutrophils fed CA-MRSA and attributed the process to necroptosis. We now extend our studies to examine additional components in the programmed cell death pathway to test the hypothesis that neutrophils fed CA-MRSA undergo necroptosis. Lysis of neutrophils fed CA-MRSA was independent of tumor necrosis factor α, active RIPK-1, and MLKL, but dependent on active RIPK-3. Human neutrophils fed CA-MRSA lacked phosphorylated RIPK-1, as well as phosphorylated or oligomerized MLKL. Neutrophils fed CA-MRSA possessed cytoplasmic complexes that included inactive caspase 8, RIPK-1, and RIPK-3, and the composition of the complex remained stable over time. Together, these data suggest that neutrophils fed CA-MRSA underwent a novel form of lytic programmed cell death via a mechanism that required RIPK-3 activity, but not active RIPK-1 or MLKL, and therefore was distinct from necroptosis. Targeting the molecular pathways that culminate in lysis of neutrophils during CA-MRSA infection may serve as a novel therapeutic intervention to limit the associated tissue damage.

RIPK3 interacts with MAVS to regulate type I IFN-mediated immunity to Influenza A virus infection

The type I interferon pathway plays a critical role in both host defense and tolerance against viral infection and thus requires refined regulatory mechanisms. RIPK3-mediated necroptosis has been shown to be involved in anti-viral immunity. However, the exact role of RIPK3 in immunity to Influenza A Virus (IAV) is poorly understood. In line with others, we, herein, show that Ripk3-/- mice are highly susceptible to IAV infection, exhibiting elevated pulmonary viral load and heightened morbidity and mortality. Unexpectedly, this susceptibility was linked to an inability of RIKP3-deficient macrophages (Mφ) to produce type I IFN in the lungs of infected mice. In Mφ infected with IAV in vitro, we found that RIPK3 regulates type I IFN both transcriptionally, by interacting with MAVS and limiting RIPK1 interaction with MAVS, and post-transcriptionally, by activating protein kinase R (PKR)-a critical regulator of IFN-β mRNA stability. Collectively, our findings indicate a novel role for RIPK3 in regulating Mφ-mediated type I IFN anti-viral immunity, independent of its conventional role in necroptosis.

RIPK3 Restricts Viral Pathogenesis via Cell Death-Independent Neuroinflammation

Receptor-interacting protein kinase-3 (RIPK3) is an activator of necroptotic cell death, but recent work has implicated additional roles for RIPK3 in inflammatory signaling independent of cell death. However, while necroptosis has been shown to contribute to antiviral immunity, death-independent roles for RIPK3 in host defense have not been demonstrated. Using a mouse model of West Nile virus (WNV) encephalitis, we show that RIPK3 restricts WNV pathogenesis independently of cell death. Ripk3^-/- mice exhibited enhanced mortality compared to wild-type (WT) controls, while mice lacking the necroptotic effector MLKL, or both MLKL and caspase-8, were unaffected. The enhanced susceptibility of Ripk3^-/- mice arose from suppressed neuronal chemokine expression and decreased central nervous system (CNS) recruitment of T lymphocytes and inflammatory myeloid cells, while peripheral immunity remained intact. These data identify pleiotropic functions for RIPK3 in the restriction of viral pathogenesis and implicate RIPK3 as a key coordinator of immune responses within the CNS.

Mouse cytomegalovirus M36 and M45 death suppressors cooperate to prevent inflammation resulting from antiviral programmed cell death pathways

The complex interplay between caspase-8 and receptor-interacting protein (RIP) kinase RIP 3 (RIPK3) driving extrinsic apoptosis and necroptosis is not fully understood. Murine cytomegalovirus triggers both apoptosis and necroptosis in infected cells; however, encoded inhibitors of caspase-8 activity (M36) and RIP3 signaling (M45) suppress these antiviral responses. Here, we report that this virus activates caspase-8 in macrophages to trigger apoptosis that gives rise to secondary necroptosis. Infection with double-mutant ΔM36/M45mutRHIM virus reveals a signaling pattern in which caspase-8 activates caspase-3 to drive apoptosis with subsequent RIP3-dependent activation of mixed lineage kinase domain-like (MLKL) leading to necroptosis. This combined cell death signaling is highly inflammatory, greater than either apoptosis induced by ΔM36 or necroptosis induced by M45mutRHIM virus. IL-6 production by macrophages is dramatically increased during double-mutant virus infection and correlates with faster antiviral responses in the host. Collaboratively, M36 and M45 target caspase-8 and RIP3 pathways together to suppress this proinflammatory cell death. This study reveals the effect of antiviral programmed cell death pathways on inflammation, shows that caspase-8 activation may go hand-in-hand with necroptosis in macrophages, and revises current understanding of independent and collaborative functions of M36 and M45 in blocking apoptotic and necroptotic cell death responses.

Viral RNA at Two Stages of Reovirus Infection Is Required for the Induction of Necroptosis

Necroptosis, a regulated form of necrotic cell death, requires the activation of the RIP3 kinase. Here, we identify that infection of host cells with reovirus can result in necroptosis. We find that necroptosis requires sensing of the genomic RNA within incoming virus particles via cytoplasmic RNA sensors to produce type I interferon (IFN). While these events that occur prior to the de novo synthesis of viral RNA are required for the induction of necroptosis, they are not sufficient. The induction of necroptosis also requires late stages of reovirus infection. Specifically, efficient synthesis of double-stranded RNA (dsRNA) within infected cells is required for necroptosis. These data indicate that viral RNA interfaces with host components at two different stages of infection to induce necroptosis. This work provides new molecular details about events in the viral replication cycle that contribute to the induction of necroptosis following infection with an RNA virus.IMPORTANCE An appreciation of how cell death pathways are regulated following viral infection may reveal strategies to limit tissue destruction and prevent the onset of disease. Cell death following virus infection can occur by apoptosis or a regulated form of necrosis known as necroptosis. Apoptotic cells are typically disposed of without activating the immune system. In contrast, necroptotic cells alert the immune system, resulting in inflammation and tissue damage. While apoptosis following virus infection has been extensively investigated, how necroptosis is unleashed following virus infection is understood for only a small group of viruses. Here, using mammalian reovirus, we highlight the molecular mechanism by which infection with a dsRNA virus results in necroptosis.

Increased hepatic receptor interacting protein kinase 3 expression due to impaired proteasomal functions contributes to alcohol-induced steatosis and liver injury

Chronic alcohol exposure increased hepatic receptor-interacting protein kinase (RIP) 3 expression and necroptosis in the liver but its mechanisms are unclear. In the present study, we demonstrated that chronic alcohol feeding plus binge (Gao-binge) increased RIP3 but not RIP1 protein levels in mouse livers. RIP3 knockout mice had decreased serum alanine amino transferase activity and hepatic steatosis but had no effect on hepatic neutrophil infiltration compared with wild type mice after Gao-binge alcohol treatment. The hepatic mRNA levels of RIP3 did not change between Gao-binge and control mice, suggesting that alcohol-induced hepatic RIP3 proteins are regulated at the posttranslational level. We found that Gao-binge treatment decreased the levels of proteasome subunit alpha type-2 (PSMA2) and proteasome 26S subunit, ATPase 1 (PSMC1) and impaired hepatic proteasome function. Pharmacological or genetic inhibition of proteasome resulted in the accumulation of RIP3 in mouse livers. More importantly, human alcoholics had decreased expression of PSMA2 and PSMC1 but increased protein levels of RIP3 compared with healthy human livers. Moreover, pharmacological inhibition of RIP1 decreased Gao-binge-induced hepatic inflammation, neutrophil infiltration and NF-κB subunit (p65) nuclear translocation but failed to protect against steatosis and liver injury induced by Gao-binge alcohol. In conclusion, results from this study suggest that impaired hepatic proteasome function by alcohol exposure may contribute to hepatic accumulation of RIP3 resulting in necroptosis and steatosis while RIP1 kinase activity is important for alcohol-induced inflammation.

Complex Pathologic Roles of RIPK1 and RIPK3: Moving Beyond Necroptosis

A process of regulated necrosis, termed necroptosis, has been recognized as a major contributor to cell death and inflammation occurring under a wide range of pathologic settings. The core event in necroptosis is the formation of the detergent-insoluble 'necrosome' complex of homologous Ser/Thr kinases, receptor protein interacting kinase 1 (RIPK1) and receptor interacting protein kinase 3 (RIPK3), which promotes phosphorylation of a key prodeath effector, mixed lineage kinase domain-like (MLKL), by RIPK3. Core necroptosis mediators are under multiple controls, which have been a subject of intense investigation. Additional, non-necroptotic functions of these factors, primarily in controlling apoptosis and inflammatory responses, have also begun to emerge. This review will provide an overview of the current understanding of the human disease relevance of this pathway, and potential therapeutic strategies, targeting necroptosis mediators in various pathologies.

Murine Cytomegalovirus Deubiquitinase Regulates Viral Chemokine Levels To Control Inflammation and Pathogenesis

Maintaining control over inflammatory processes represents a paradox for viral pathogens. Although many viruses induce host inflammatory responses to facilitate infection, control is necessary to avoid overactivation. One way is through the manipulation of proinflammatory chemokine levels, both host and viral. Murine cytomegalovirus (MCMV), a model betaherpesvirus, encodes a viral C-C chemokine, MCK2, which promotes host inflammatory responses and incorporates into virions to facilitate viral dissemination. Here, we show that the activity of M48, the conserved MCMV deubiquitinating enzyme (DUB), regulates MCK2 levels during infection. Inactivation of M48 DUB activity results in viral attenuation and exacerbates virally induced, MCK2-dependent inflammatory responses. M48 DUB activity also influences MCK2 incorporation into virions. Importantly, attenuation of DUB-mutant virus acute replication in vitro and in vivo is largely ameliorated by targeted deletion of MCK2. Thus, uncontrolled MCK2 levels appear to mediate DUB-mutant virus attenuation in specific tissues or cell types. This demonstrates that MCMV M48 DUB activity plays a previously unappreciated role in controlling MCK2 levels, thereby managing MCK2-dependent processes. These findings reveal a novel intrinsic control mechanism of virally induced inflammation and support the identification of betaherpesvirus DUBs as possible new targets for antiviral therapies.

Human cytomegalovirus infections represent a tremendous burden not only to those afflicted but also to health care systems worldwide. As cytomegalovirus infections are a leading cause of nongenetic sensory loss and neurodevelopmental delay, it is imperative that valuable model systems exist in order that we might understand what viral factors contribute to replication and pathogenesis. Currently, the only approved drug treatments against CMV infection are nucleoside analogues, to which some strains have become resistant. Understanding unique viral enzymatic contributions to infections will allow the development of novel pharmacological therapies. Here, we show that M48, the conserved MCMV deubiquitinase, is critical for MCMV replication in mice and demonstrate that attenuation is due to deregulated production of a viral proinflammatory chemokine. The deubiquitinases of both human and murine CMV represent structurally unique DUBs and are therefore attractive targets for pharmacological intervention. Continued research into the substrates of these DUBs will lend additional insight into their potential as targets.

Controlled detonation: evolution of necroptosis in pathogen defense

Necroptosis is a lytic form of programmed cell death that involves the swelling and rupture of dying cells. Although several necroptosis-inducing stimuli have been defined, in most cells this pathway is kept in check by the action of the pro-apoptotic protease caspase-8 and the IAP ubiquitin ligases. How and when necroptosis is triggered under physiological conditions therefore remains a persistent question. Because necroptosis likely arose as a defensive mechanism against viral infection, exploration of this question requires a consideration of host-pathogen interactions, and how the sensing of infection could sensitize cells to necroptosis. Here, we will discuss the role of necroptosis in the response to viral infection, consider why the necroptotic pathway has been favored during evolution, and describe emerging evidence for death-independent functions of key necroptotic signaling components.

Necroptotic cell death in failing heart: relevance and proposed mechanisms

As cardiomyocytes have a limited capability for proliferation, renewal, and repair, the loss of heart cells followed by replacement with fibrous tissue is considered to result in the development of ventricular dysfunction and progression to heart failure (HF). The loss of cardiac myocytes in HF has been traditionally believed to occur mainly due to programmed apoptosis or unregulated necrosis. While extensive research work is being carried out to define the exact significance and contribution of both these cell death modalities in the development of HF, recent knowledge has indicated the existence and importance of a different form of cell death called necroptosis in the failing heart. This new cell damaging process, resembling some of the morphological features of passive necrosis as well as maladaptive autophagy, is a programmed process and is orchestrated by a complex set of proteins involving receptor-interacting protein kinase 1 and 3 (RIP1, RIP3) and mixed lineage kinase domain-like protein (MLKL). Activation of the RIP1-RIP3-MLKL signaling pathway leads to disruption of cation homeostasis, plasma membrane rupture, and finally cell death. It seems likely that inhibition of any site in this pathway may prove as an effective pharmacological intervention for preventing the necroptotic cell death in the failing heart. This review is intended to describe general aspects of the signaling pathway associated with necroptosis, to describe its relationship with cardiac dysfunction in some models of cardiac injury and discuss its potential relevance in various types of HF with respect to the underlying pathologic mechanisms.

Necroptosis: Mechanisms and Relevance to Disease

Necroptosis is a form of regulated cell death that critically depends on receptor-interacting serine-threonine kinase 3 (RIPK3) and mixed lineage kinase domain-like (MLKL) and generally manifests with morphological features of necrosis. The molecular mechanisms that underlie distinct instances of necroptosis have just begun to emerge. Nonetheless, it has already been shown that necroptosis contributes to cellular demise in various pathophysiological conditions, including viral infection, acute kidney injury, and cardiac ischemia/reperfusion. Moreover, human tumors appear to obtain an advantage from the downregulation of key components of the molecular machinery for necroptosis. Although such an advantage may stem from an increased resistance to adverse microenvironmental conditions, accumulating evidence indicates that necroptosis-deficient cancer cells are poorly immunogenic and hence escape natural and therapy-elicited immunosurveillance. Here, we discuss the molecular mechanisms and relevance to disease of necroptosis.

Human Papillomavirus Downregulates the Expression of IFITM1 and RIPK3 to Escape from IFNγ- and TNFα-Mediated Antiproliferative Effects and Necroptosis

The clearance of a high-risk human papillomavirus (hrHPV) infection takes time and requires the local presence of a strong type 1 cytokine T cell response, suggesting that hrHPV has evolved mechanisms to resist this immune attack. Using an unique system for non, newly, and persistent hrHPV infection, we show that hrHPV infection renders keratinocytes (KCs) resistant to the antiproliferative- and necroptosis-inducing effects of IFNγ and TNFα. HrHPV-impaired necroptosis was associated with the upregulation of several methyltransferases, including EZH2, and the downregulation of RIPK3 expression. Restoration of RIPK3 expression using the global histone methyltransferase inhibitor 3-deazaneplanocin increased necroptosis in hrHPV-positive KCs. Simultaneously, hrHPV effectively inhibited IFNγ/TNFα-mediated arrest of cell growth at the S-phase by downregulating IFITM1 already at 48 h after hrHPV infection, followed by an impaired increase in the expression of the antiproliferative gene RARRES1 and a decrease of the proliferative gene PCNA. Knockdown of IFITM1 in uninfected KCs confirmed its role on RARRES1 and its antiproliferative effects. Thus, our study reveals how hrHPV deregulates two pathways involved in cell death and growth regulation to withstand immune-mediated control of hrHPV-infected cells.

A motive for killing: effector functions of regulated lytic cell death

Immunological responses activated by pathogen recognition come in many guises. The proliferation, differentiation and recruitment of immune cells, and the production of inflammatory cytokines and chemokines are central to lifelong immunity. Cell death serves as a key function in the resolution of innate and adaptive immune responses. It also coordinates cell-intrinsic effector functions to restrict infection. Necrosis was formally considered a passive form of cell death or a consequence of pathogen virulence factor expression, and necrotic tissue is frequently associated with infection. However, there is now emerging evidence that points to a role for regulated forms of necrosis, such as pyroptosis and necroptosis, driving inflammation and shaping the immune response.

DAI Senses Influenza A Virus Genomic RNA and Activates RIPK3-Dependent Cell Death

Influenza A virus (IAV) is an RNA virus that is cytotoxic to most cell types in which it replicates. IAV activates the host kinase RIPK3, which induces cell death via parallel pathways of necroptosis, driven by the pseudokinase MLKL, and apoptosis, dependent on the adaptor proteins RIPK1 and FADD. How IAV activates RIPK3 remains unknown. We report that DAI (ZBP1/DLM-1), previously implicated as a cytoplasmic DNA sensor, is essential for RIPK3 activation by IAV. Upon infection, DAI recognizes IAV genomic RNA, associates with RIPK3, and is required for recruitment of MLKL and RIPK1 to RIPK3. Cells lacking DAI or containing DAI mutants deficient in nucleic acid binding are resistant to IAV-triggered necroptosis and apoptosis. DAI-deficient mice fail to control IAV replication and succumb to lethal respiratory infection. These results identify DAI as a link between IAV replication and RIPK3 activation and implicate DAI as a sensor of RNA viruses.

Casp8p41: The Protean Mediator of Death in CD4 T-cells that Replicate HIV

HIV cure is now the focus of intense research after Timothy Ray Brown (the Berlin patient) set the precedent of being the first and only person cured. A major barrier to achieving this goal on a meaningful scale is an elimination of the latent reservoir, which is thought to comprise CD4-positive cells that harbor integrated, replication-competent HIV provirus. These cells do not express viral proteins, are indistinguishable from uninfected CD4 cells, and are thought to be responsible for HIV viral rebound—that occurs within weeks of combination anti retroviral therapy (cART) interruption. Modalities to engineer transcriptional stimulation (reactivation) of this dormant integrated HIV provirus, leading to expression of cytotoxic viral proteins, are thought to be a specific way to eradicate the latently infected CD4 pool and are becoming increasingly relevant in the era of HIV cure. HIV protease is one such protein produced after HIV reactivation that cleaves procaspase-8 to generate a novel protein Casp8p41. Casp8p41 then binds to the BH3 domain of BAK, leading to BAK oligomerization, mitochondrial depolarization, and apoptosis. In central memory T cells (TCMs) from HIV-infected patients, an elevated Bcl-2/procaspase-8 ratio was observed, and Casp8p41 binding to Bcl-2 was associated with a lack of reactivation-induced cell death. This was reversed by priming cells with a specific Bcl-2 antagonist prior to reactivation, resulting in increased cell death and decreased HIV DNA in a Casp8p41-dependent pathway. This review describes the biology, clinical relevance, and implications of Casp8p41 for a potential cure.

The Inflammatory Signal Adaptor RIPK3: Functions Beyond Necroptosis

Receptor interacting protein kinase 3 (RIPK3) is an essential serine/threonine kinase for necroptosis, a type of regulated necrosis. A variety of stimuli can cause RIPK3 activation through phosphorylation. Activated RIPK3 in turn phosphorylates and activates the downstream necroptosis executioner mixed lineage kinase domain-like (MLKL). Necroptosis is a highly inflammatory type of cell death because of the release of intracellular immunogenic contents from disrupted plasma membrane. Indeed, RIPK3-deficient mice exhibited reduced inflammation in many inflammatory disease models. These results have been interpreted as evidence that necroptosis is a key driver for RIPK3-induced inflammation. Interestingly, recent studies show that RIPK3 also regulates NF-κB, inflammasome activation, and kinase-independent apoptosis. These studies also reveal that these nonnecroptotic functions contribute significantly to disease pathogenesis. In this review, we summarize our current understanding of necroptotic and nonnecroptotic functions of RIPK3 and discuss how these effects contribute to RIPK3-mediated inflammation.

Diverse mechanisms evolved by DNA viruses to inhibit early host defenses

In mammalian cells, early defenses against infection by pathogens are mounted through a complex network of signaling pathways shepherded by immune-modulatory pattern-recognition receptors. As obligate parasites, the survival of viruses is dependent on the evolutionary acquisition of mechanisms that tactfully dismantle and subvert the cellular intrinsic and innate immune responses. Here, we review the diverse mechanisms by which viruses that accommodate DNA genomes are able to circumvent activation of cellular immunity. We start by discussing viral manipulation of host defense protein levels by either transcriptional regulation or protein degradation. We next review viral strategies used to repurpose or inhibit these cellular immune factors by molecular hijacking or by regulating their post-translational modification status. Additionally, we explore the infection-induced temporal modulation of apoptosis to facilitate viral replication and spread. Lastly, the co-evolution of viruses with their hosts is highlighted by the acquisition of elegant mechanisms for suppressing host defenses via viral mimicry of host factors. In closing, we present a perspective on how characterizing these viral evasion tactics both broadens the understanding of virus-host interactions and reveals essential functions of the immune system at the molecular level. This knowledge is critical in understanding the sources of viral pathogenesis, as well as for the design of antiviral therapeutics and autoimmunity treatments.

Molecular Pathogenesis of NASH

Nonalcoholic steatohepatitis (NASH) is the main cause of chronic liver disease in the Western world and a major health problem, owing to its close association with obesity, diabetes, and the metabolic syndrome. NASH progression results from numerous events originating within the liver, as well as from signals derived from the adipose tissue and the gastrointestinal tract. In a fraction of NASH patients, disease may progress, eventually leading to advanced fibrosis, cirrhosis and hepatocellular carcinoma. Understanding the mechanisms leading to NASH and its evolution to cirrhosis is critical to identifying effective approaches for the treatment of this condition. In this review, we focus on some of the most recent data reported on the pathogenesis of NASH and its fibrogenic progression, highlighting potential targets for treatment or identification of biomarkers of disease progression.

Necroptosis inhibitors as therapeutic targets in inflammation mediated disorders - a review of the current literature and patents

INTRODUCTION

Recent studies have shown substantial interplay between the apoptosis and necroptosis pathways. Necroptosis, a form of programmed cell death, has been found to stimulate the immune system contributing to the pathophysiology of several inflammation-mediated disorders. Determining the contribution of necroptotic signaling pathways to inflammation may lead to the development of selective and specific molecular target implicated necroptosis inhibitors. Areas covered: This review summarizes the recently published and patented necroptosis inhibitors as therapeutic targets in inflammation-mediated disorders. The role of several necroptosis inhibitors, focusing on specific signaling molecules, was discussed with particular attention to inflammation-mediated disorders. Data was obtained from Espacenet®, WIPO®, USPTO® patent websites, and other relevant sources (2006-2016). Expert opinion: Necroptosis inhibitors hold promise for treatment of inflammation-mediated clinical conditions in which necroptotic cell death plays a major role. Although necroptosis inhibitors reviewed in this survey showed inhibitory effects against several inflammation-mediated disorders, only a few have passed to the stage of clinical testing and need extensive research for therapeutic practice. Revisiting the existing drugs and developing novel necroptosis inhibiting agents as well as understanding their mechanism are essential. A detailed study of necroptosis function in animal models of inflammation may provide us an alternative strategy for the development of drug-like necroptosis inhibitors.

Viruses and the Diversity of Cell Death

Cell death is a common outcome of virus infection. In some cases, cell death curbs virus replication. In others, cell death enhances virus dissemination and contributes to tissue injury, exacerbating viral disease. Three forms of cell death are observed following virus infection-apoptosis, necroptosis, and pyroptosis. In this review, I describe the core machinery needed for each of these forms of cell death. Using representative viruses, I highlight how distinct stages of virus replication initiate signaling pathways that elicit these forms of cell death. I also discuss viral strategies to overcome the deleterious effects of cell death on virus propagation and the consequences of cell death for host physiology.

RIPK1 and RIPK3 Kinases Promote Cell-Death-Independent Inflammation by Toll-like Receptor 4

Macrophages are a crucial component of the innate immune system in sensing pathogens and promoting local and systemic inflammation. RIPK1 and RIPK3 are homologous kinases, previously linked to activation of necroptotic death. In this study, we have described roles for these kinases as master regulators of pro-inflammatory gene expression induced by lipopolysaccharide, independent of their well-documented cell death functions. In primary macrophages, this regulation was elicited in the absence of caspase-8 activity, required the adaptor molecule TRIF, and proceeded in a cell autonomous manner. RIPK1 and RIPK3 kinases promoted sustained activation of Erk, cFos, and NF-κB, which were required for inflammatory changes. Utilizing genetic and pharmacologic tools, we showed that RIPK1 and RIPK3 account for acute inflammatory responses induced by lipopolysaccharide in vivo; notably, this regulation did not require exogenous manipulation of caspases. These findings identified a new pharmacologically accessible pathway that may be relevant to inflammatory pathologies.

An evolutionary perspective on the necroptotic pathway

Throughout the animal kingdom, innate immune receptors protect the organism from microbial intruders by activating pathways that mediate inflammation and pathogen clearance. Necroptosis contributes to the innate immune response by killing pathogen-infected cells and by alerting the immune system through the release of danger signals. Components of the necroptotic signaling axis - TIR-domain-containing adapter-inducing interferon-β (TRIF), Z-DNA sensor DAI, receptor-interacting kinase (RIPK)1, RIPK3 and mixed-lineage kinase domain-like protein (MLKL) - are therefore expected to be found in all animals. However, a phylogenetic analysis reveals that the necroptotic axis, except for RIPK1, is poorly conserved in the animal kingdom, suggesting that alternative mechanisms regulate necroptosis in these species or that necroptosis would apparently be absent. These findings question the universal role of necroptosis during innate immunity in the animal kingdom.

Ferroptosis, but Not Necroptosis, Is Important in Nephrotoxic Folic Acid-Induced AKI

AKI is histologically characterized by necrotic cell death and inflammation. Diverse pathways of regulated necrosis have been reported to contribute to AKI, but the molecular regulators involved remain unclear. We explored the relative contributions of ferroptosis and necroptosis to folic acid (FA)-induced AKI in mice. FA-AKI in mice associates with lipid peroxidation and downregulation of glutathione metabolism proteins, features that are typical of ferroptotic cell death. We show that ferrostatin-1 (Fer-1), an inhibitor of ferroptosis, preserved renal function and decreased histologic injury, oxidative stress, and tubular cell death in this model. With respect to the immunogenicity of ferroptosis, Fer-1 prevented the upregulation of IL-33, an alarmin linked to necroptosis, and other chemokines and cytokines and prevented macrophage infiltration and Klotho downregulation. In contrast, the pancaspase inhibitor zVAD-fmk did not protect against FA-AKI. Additionally, although FA-AKI resulted in increased protein expression of the necroptosis mediators receptor-interacting protein kinase 3 (RIPK3) and mixed lineage domain-like protein (MLKL), targeting necroptosis with the RIPK1 inhibitor necrostatin-1 or genetic deficiency of RIPK3 or MLKL did not preserve renal function. Indeed, compared with wild-type mice, MLKL knockout mice displayed more severe AKI. However, RIPK3 knockout mice with AKI had less inflammation than their wild-type counterparts, and this effect associated with higher IL-10 concentration and regulatory T cell-to-leukocyte ratio in RIPK3 knockout mice. These data suggest that ferroptosis is the primary cause of FA-AKI and that immunogenicity secondary to ferroptosis may further worsen the damage, although necroptosis-related proteins may have additional roles in AKI.

Vibrio cholerae GbpA elicits necrotic cell death in intestinal cells

Vibrio choleraeN-acetylglucosamine-binding protein GbpA is a secretory protein that facilitates the initial adherence of bacteria in the human intestine. Until now, considerable progress in the characterization of GbpA has been done, yet little is known about its role in host response. Our present studies demonstrated that GbpA at the amount secreted in the intestine resulted in decreased cell viability, altered cell morphology, disruption of cell membrane integrity and damage of cellular DNA indicating necrotic cell death. We observed that GbpA exposure leads to mitochondrial dysfunction, characterized by accumulation of reactive oxygen species (ROS), depolarization of mitochondrial membrane potential and depletion of ATP pool in host cells. Additionally, the intra-cellular ROS, accumulated in response to GbpA, were found to induce the migration of NF-κB from cytoplasm into nucleus in host cells. Taken together, these results prompted us to conclude that GbpA orchestrates a necrotic response in host cells which may have implications in immune response.

Cloaked in ubiquitin, a killer hides in plain sight: the molecular regulation of RIPK1

In the past decade, studies have shown how instrumental programmed cell death (PCD) can be in innate and adaptive immune responses. PCD can be a means to maintain homeostasis, prevent or promote microbial pathogenesis, and drive autoimmune disease and inflammation. The molecular machinery regulating these cell death programs has been examined in detail, although there is still much to be explored. A master regulator of programmed cell death and innate immunity is receptor-interacting protein kinase 1 (RIPK1), which has been implicated in orchestrating various pathologies via the induction of apoptosis, necroptosis, and nuclear factor-κB-driven inflammation. These and other roles for RIPK1 have been reviewed elsewhere. In a reflection of the ability of tumor necrosis factor (TNF) to induce either survival or death response, this molecule in the TNF pathway can transduce either a survival or a death signal. The intrinsic killing capacity of RIPK1 is usually kept in check by the chains of ubiquitin, enabling it to serve in a prosurvival capacity. In this review, the intricate regulatory mechanisms responsible for restraining RIPK1 from killing are discussed primarily in the context of the TNF signaling pathway and how, when these mechanisms are disrupted, RIPK1 is free to unveil its program of cellular demise.

The role of necroptosis in pulmonary diseases

By regulating the cell number and eliminating harmful cells, programmed cell death plays a critical role in development, homeostasis, and disease. While apoptosis is a recognized form of programmed cell death, necrosis was considered a type of uncontrolled cell death induced by extreme physical or chemical stress. However, recent studies have revealed the existence of a genetically programmed and regulated form of necrosis, termed necroptosis. Necroptosis is defined as necrotic cell death that is dependent on receptor-interacting protein kinase 3 (RIPK3). RIPK3, receptor-interacting protein kinase 1 (RIPK1), and a mixed-lineage kinase domain-like protein (MLKL) form a multiprotein complex called a necrosome. Although necroptosis generally provides a cell-autonomous host defense, on the other hand, cell rupture caused by necroptosis induces inflammation through the release of damage-associated molecular patterns, such as mitochondrial DNA, HMGB1, and IL-1. Previously, necroptosis was considered an alternative to apoptosis, but it is becoming increasingly clear that necroptosis itself is relevant to clinical disease, independent of apoptosis. According to some recent studies, autophagy, a cellular process for organelle and protein turnover, regulates necroptosis. This review outlines the principal components of necroptosis and provides an overview of the emerging importance of necroptosis in the pathogenesis of pulmonary disease, including chronic obstructive pulmonary disease, lung cancer, infection, and sepsis. We also discuss the molecular relationship between necroptosis and autophagy. Strategies targeting necroptosis may yield novel therapies for pulmonary diseases.

K45A mutation of RIPK1 results in poor necroptosis and cytokine signaling in macrophages, which impacts inflammatory responses in vivo

Receptor interacting protein kinase 1 (RIPK1) participates in several cell signaling complexes that promote cell activation and cell death. Stimulation of RIPK1 in the absence of caspase signaling induces regulated necrosis (necroptosis), which promotes an inflammatory response. Understanding of the mechanisms through which RIPK1 promotes inflammation has been unclear. Herein we have evaluated the impact of a K45A mutation of RIPK1 on necroptosis of macrophages and the activation of inflammatory response. We show that K45A mutation of RIPK1 results in attenuated necroptosis of macrophages in response to stimulation with LPS, TNFα and IFNβ in the absence of caspase signaling. Impairment in necroptosis correlated with poor phosphorylation of RIPK1, RIPK3 and reduced trimerization of MLKL. Furthermore, K45A mutation of RIPK1 resulted in poor STAT1 phosphorylation (at S727) and expression of RANTES and MIP-1α following TNF-R engagement in the absence of caspase activation. Our results further indicate that in the absence of stimulation by pathogen-associated molecular patterns (PAMPs), cellular inhibitors of apoptotic proteins (cIAPs) prevent the K45-dependent auto-phosphorylation of RIPK1, leading to resistance against necroptosis. Finally, RIPK1(K45A) mice displayed attenuated inflammatory response in vivo as they were significantly resistant against endotoxin shock, but highly susceptible against a challenge with Salmonella typhimurium. This correlated with reduced expression of IL-1β and ROS, and poor processing of caspase 8 by RIPK1(K45A) macrophages. Overall, these results indicate that K45 mediated kinase activity of RIPK1 is not only important for necroptosis but it also has a key role in promoting cytokine signaling and host response to inflammatory stimuli.

CYLD Proteolysis Protects Macrophages from TNF-Mediated Auto-necroptosis Induced by LPS and Licensed by Type I IFN

Tumor necrosis factor (TNF) induces necroptosis, a RIPK3/MLKL-dependent form of inflammatory cell death. In response to infection by Gram-negative bacteria, multiple receptors on macrophages, including TLR4, TNF, and type I IFN receptors, are concurrently activated, but it is unclear how they crosstalk to regulate necroptosis. We report that TLR4 activates CASPASE-8 to cleave and remove the deubiquitinase cylindromatosis (CYLD) in a TRIF- and RIPK1-dependent manner to disable necroptosis in macrophages. Inhibiting CASPASE-8 leads to CYLD-dependent necroptosis caused by the TNF produced in response to TLR4 ligation. While lipopolysaccharides (LPS)-induced necroptosis was abrogated in Tnf(-/-) macrophages, a soluble TNF antagonist was not able to do so in Tnf(+/+) macrophages, indicating that necroptosis occurs in a cell-autonomous manner. Surprisingly, TNF-mediated auto-necroptosis of macrophages requires type I IFN, which primes the expression of key necroptosis-signaling molecules, including TNFR2 and MLKL. Thus, the TNF necroptosis pathway is regulated by both negative and positive crosstalk.

Biomarkers for the detection of necroptosis

Necroptosis has been extensively studied recently, and the receptor-interacting kinase 3 (RIP3 or RIPK3) and its substrate, the pseudokinase mixed lineage kinase domain-like protein, have been discovered to be core components of this process. Classical necroptosis requires RIP1 (or RIPK1) for the activation of RIP3 through the induction of RIP1/RIP3 necrosomes. Increasing evidence from genetic and pharmacological studies has been expanding the view that necroptosis plays important roles in the etiology and/or progression of many human diseases, such as pancreatitis, ischemic injury, and neurodegenerative diseases, among others. Ongoing progress in translational research about necroptosis has highlighted the increasingly important need for the identification of biomarkers for use in disease diagnosis, monitoring, and drug development. This review presents a discussion of the current status of biomarkers that can be used to detect necroptosis both in vitro and in vivo.

Necroptosis-independent signaling by the RIP kinases in inflammation

Recent advances have identified a signaling cascade involving receptor interacting protein kinase 1 (RIPK1), RIPK3 and the pseudokinase mixed lineage kinase domain-like (MLKL) that is crucial for induction of necroptosis, a non-apoptotic form of cell death. RIPK1-RIPK3-MLKL-mediated necroptosis has been attributed to cause many inflammatory diseases through the release of cellular damage-associated molecular patterns (DAMPs). In addition to necroptosis, emerging evidence suggests that these necroptosis signal adaptors can also facilitate inflammation independent of cell death. In particular, the RIP kinases can drive NF-κB and inflammasome activation independent of cell death. In this review, we will discuss recent discoveries that led to this realization and present arguments why cell death-independent signaling by the RIP kinases may have a more important role in inflammation than necroptosis.

Identification of a Transcription Factor That Regulates Host Cell Exit and Virulence of Mycobacterium tuberculosis

The interaction of Mycobacterium tuberculosis (Mtb) with host cell death signaling pathways is characterized by an initial anti-apoptotic phase followed by a pro-necrotic phase to allow for host cell exit of the bacteria. The bacterial modulators regulating necrosis induction are poorly understood. Here we describe the identification of a transcriptional repressor, Rv3167c responsible for regulating the escape of Mtb from the phagosome. Increased cytosolic localization of MtbΔRv3167c was accompanied by elevated levels of mitochondrial reactive oxygen species and reduced activation of the protein kinase Akt, and these events were critical for the induction of host cell necrosis and macroautophagy. The increase in necrosis led to an increase in bacterial virulence as reflected in higher bacterial burden and reduced survival of mice infected with MtbΔRv3167c. The regulon of Rv3167c thus contains the bacterial mediators involved in escape from the phagosome and host cell necrosis induction, both of which are crucial steps in the intracellular lifecycle and virulence of Mtb.

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, is a highly successful human pathogen. Following entry into host phagocytic cells, Mtb resides within a modified phagosomal compartment and inhibits apoptotic host cell death. Recent studies have demonstrated that Mtb eventually translocates from the phagosomal compartment to the cytosol. This event is followed by the induction of necrotic host cell death allowing the bacteria to exit the host cell and infect naive cell populations. Our study adds to this relatively unexplored aspect of Mtb pathogenesis by revealing that the transcriptional repressor Rv3167c of Mtb negatively regulates phagosomal escape and host cell necrosis. We furthermore demonstrate that the increased necrosis induction by the Mtb mutant strain deficient in Rv3167c required elevated reactive oxygen species levels within host cell mitochondria and reduced activation of the protein kinase Akt. In addition, the increased virulence of the Mtb mutant strain observed after aerosol infection of mice strengthens the link between the ability of the bacteria to induce host cell necrosis and virulence. The Mtb genes negatively regulated by Rv3167c are thus potential virulence factors that can be targeted for drug and vaccine development.

The interplay between human herpes simplex virus infection and the apoptosis and necroptosis cell death pathways

Human herpes simplex virus (HSV) is a ubiquitous human pathogen that establishes a lifelong latent infection and is associated with mucocutaneous lesions. In multicellular organisms, cell death is a crucial host defense mechanism that eliminates pathogen-infected cells. Apoptosis is a well-defined form of programmed cell death executed by a group of cysteine proteases, called caspases. Studies have shown that HSV has evolved strategies to counteract caspase activation and apoptosis by encoding anti-apoptotic viral proteins such as gD, gJ, Us3, LAT, and the ribonucleotide reductase large subunit (R1). Recently, necroptosis has been identified as a regulated form of necrosis that can be invoked in the absence of caspase activity. Receptor-interacting kinase 3 (RIP3 or RIPK3) has emerged as a central signaling molecule in necroptosis; it is activated via interaction with other RIP homotypic interaction motif (RHIM)-containing proteins such as RIP1 (or RIPK1). There is increasing evidence that HSV R1 manipulates necroptosis via the RHIM-dependent inactivation or activation ofRIP3 in a species-specific manner. This review summarizes the current understanding of the interplay between HSV infection and cell death pathways, with an emphasis on apoptosis and necroptosis.

Regulated necrosis: disease relevance and therapeutic opportunities

The discovery of regulated cell death presents tantalizing possibilities for gaining control over the life-death decisions made by cells in disease. Although apoptosis has been the focus of drug discovery for many years, recent research has identified regulatory mechanisms and signalling pathways for previously unrecognized, regulated necrotic cell death routines. Distinct critical nodes have been characterized for some of these alternative cell death routines, whereas other cell death routines are just beginning to be unravelled. In this Review, we describe forms of regulated necrotic cell death, including necroptosis, the emerging cell death modality of ferroptosis (and the related oxytosis) and the less well comprehended parthanatos and cyclophilin D-mediated necrosis. We focus on small molecules, proteins and pathways that can induce and inhibit these non-apoptotic forms of cell death, and discuss strategies for translating this understanding into new therapeutics for certain disease contexts.

Poly-ubiquitination in TNFR1-mediated necroptosis

Tumor necrosis factor (TNF) is a master pro-inflammatory cytokine, and inappropriate TNF signaling is implicated in the pathology of many inflammatory diseases. Ligation of TNF to its receptor TNFR1 induces the transient formation of a primary membrane-bound signaling complex, known as complex I, that drives expression of pro-survival genes. Defective complex I activation results in induction of cell death, in the form of apoptosis or necroptosis. This switch occurs via internalization of complex I components and assembly and activation of secondary cytoplasmic death complexes, respectively known as complex II and necrosome. In this review, we discuss the crucial regulatory functions of ubiquitination—a post-translational protein modification consisting of the covalent attachment of ubiquitin, and multiples thereof, to target proteins—to the various steps of TNFR1 signaling leading to necroptosis.

Expression Profile and Tissue-Specific Distribution of the Receptor-Interacting Protein 3 in BALB/c Mice

RIP3, a member of receptor-interacting protein family, is serine/threonine kinase that contributes to necrosis and promotes systematic inflammation. However, detailed information of the expression pattern and tissue distribution in BALB/c mice, a commonly used laboratory animal model, is still unavailable. Here, we provided the basic data of expression profile and histologic distribution of RIP3 in tissues of BALB/c mice. Rip3 mRNA expression levels and tissue distribution were detected by real-time quantitative PCR and immunohistochemical detection, respectively. Rip3 mRNA expression showed the highest level in the spleen and duodenum, while with the lowest level in brain. Immunohistochemical detection revealed this protein located in different type cells in different tissues. What's more, the obvious positive staining in nuclear was detected in liver cells and neurons in cerebral cortex of the brain, while cells in other organs, including heart, spleen, lung, kidney, stomach, duodenum and trachea, showed strong positive mainly in cytoplasm. The results will help us to further understand the site-specific functions of RIP3 in necrosis and inflammatory responses.

Necroptosis and Inflammation

Necroptosis is a regulated form of necrosis, with the dying cell rupturing and releasing intracellular components that can trigger an innate immune response. Toll-like receptor 3 and 4 agonists, tumor necrosis factor, certain viral infections, or the T cell receptor can trigger necroptosis if the activity of the protease caspase-8 is compromised. Necroptosis signaling is modulated by the kinase RIPK1 and requires the kinase RIPK3 and the pseudokinase MLKL. Either RIPK3 deficiency or RIPK1 inhibition confers resistance in various animal disease models, suggesting that inflammation caused by necroptosis contributes to tissue damage and that inhibitors of these kinases could have therapeutic potential. Recent studies have revealed unexpected complexity in the regulation of cell death programs by RIPK1 and RIPK3 with the possibility that necroptosis is but one mechanism by which these kinases promote inflammation.