Virus inhibition of RIP3-dependent necrosis

Post-translational control of RIPK3 and MLKL mediated necroptotic cell death

Several programmed lytic and necrotic-like cell death mechanisms have now been uncovered, including the recently described receptor interacting protein kinase-3 (RIPK3)-mixed lineage kinase domain-like (MLKL)-dependent necroptosis pathway. Genetic experiments have shown that programmed necrosis, including necroptosis, can play a pivotal role in regulating host-resistance against microbial infections. Alternatively, excess or unwarranted necroptosis may be pathological in autoimmune and autoinflammatory diseases. This review highlights the recent advances in our understanding of the post-translational control of RIPK3-MLKL necroptotic signaling. We discuss the critical function of phosphorylation in the execution of necroptosis, and highlight the emerging regulatory roles for several ubiquitin ligases and deubiquitinating enzymes. Finally, based on current evidence, we discuss the potential mechanisms by which the essential, and possibly terminal, necroptotic effector, MLKL, triggers the disruption of cellular membranes to cause cell lysis.

Herpes Simplex Virus 1 (HSV-1) and HSV-2 Mediate Species-Specific Modulations of Programmed Necrosis through the Viral Ribonucleotide Reductase Large Subunit R1

Receptor-interacting protein kinase 3 (RIP3) and its substrate mixed-lineage kinase domain-like protein (MLKL) are core regulators of programmed necrosis. The elimination of pathogen-infected cells by programmed necrosis acts as an important host defense mechanism. Here, we report that human herpes simplex virus 1 (HSV-1) and HSV-2 had opposite impacts on programmed necrosis in human cells versus their impacts in mouse cells. Similar to HSV-1, HSV-2 infection triggered programmed necrosis in mouse cells. However, neither HSV-1 nor HSV-2 infection was able to induce programmed necrosis in human cells. Moreover, HSV-1 or HSV-2 infection in human cells blocked tumor necrosis factor (TNF)-induced necrosis by preventing the induction of an RIP1/RIP3 necrosome. The HSV ribonucleotide reductase large subunit R1 was sufficient to suppress TNF-induced necrosis, and its RIP homotypic interaction motif (RHIM) domain was required to disrupt the RIP1/RIP3 complex in human cells. Therefore, this study provides evidence that HSV has likely evolved strategies to evade the host defense mechanism of programmed necrosis in human cells.

IMPORTANCE

This study demonstrated that infection with HSV-1 and HSV-2 blocked TNF-induced necrosis in human cells while these viruses directly activated programmed necrosis in mouse cells. Expression of HSV R1 suppressed TNF-induced necrosis of human cells. The RHIM domain of R1 was essential for its association with human RIP3 and RIP1, leading to disruption of the RIP1/RIP3 complex. This study provides new insights into the species-specific modulation of programmed necrosis by HSV.

Death in the fast lane: what's next for necroptosis?

Necroptosis is a form of programmed cell death that is both mechanistically and morphologically distinct from apoptosis, the canonical mechanism of cell suicide. Although early descriptions of necroptosis date back decades, the last 5 years have seen a proliferation of studies of this process. This surge in interest has included the recent publication of several excellent, in-depth reviews of the literature [Chan FK-M et al. (2014) Annu Rev Immunol 33, 141210135520002; Weinlich R & Green DR (2014) Mol Cell 56, 469-480; Silke J et al. (2015) Nat Immunol 16, 689-697; Linkermann A & Green DR (2014) N Engl J Med 370, 455-465]. Rather than contribute another summary to this well-summarized field, in this Minireview I will briefly discuss key recent findings, then touch on some of the major outstanding questions - the known unknowns - that remain.

Upregulation of the receptor-interacting protein 3 expression and involvement in neural tissue damage after spinal cord injury in mice

Background

Necroptosis is a newly identified type of programmed cell death that differs from apoptosis. Recent studies have demonstrated that necroptosis is involved in multiple pathologies of various human diseases. Receptor-interacting protein 3 (RIP3) is known to be a critical regulator of necroptosis. This study investigated alterations in the RIP3 expression and the involvement in neural tissue damage after spinal cord injury (SCI) in mice.

Results

Immunohistochemical analysis demonstrated that the RIP3 expression was significantly increased in the lesion site after spinal cord hemisection. The increased expression of RIP3 started at 24 h, peaked at 3 days and lasted for at least 21 days after hemisection. The RIP3 expression was observed in neurons, astrocytes and oligodendrocytes. Western blot analysis also demonstrated the RIP3 protein expression significantly upregulated in the injured spinal cord. RIP3 staining using propidium iodide (PI)-labeled sections showed most of the PI-labeled cells were observed as RIP3-positive. Double staining of TUNEL and RIP3 demonstrated that TUNEL-positive cells exhibiting shrunken or fragmented nuclei, as generally observed in apoptotic cells, rarely expressed RIP3.

Conclusions

The present study first demonstrated that the expression of RIP3 is dramatically upregulated in various neural cells in the injured spinal cord and peaked at 3 days after injury. Additionally, most of the PI-labeled cells expressed RIP3 in response to neural tissue damage after SCI. The present study suggested that the upregulation of the RIP3 expression may play a role as a novel molecular mechanism in secondary neural tissue damage following SCI. However, further study is needed to clarify the specific molecular mechanism underlying the relationship between the RIP3 expression and cell death in the injured spinal cord.

A tale of two domains - a structural perspective of the pseudokinase, MLKL

Recently, the programmed necrosis or 'necroptosis' cell death pathway has attracted much interest because of its implication in multiple pathologies, including inflammatory diseases and the cell death arising from ischaemia reperfusion injuries. Pharmacologically, necroptosis is an attractive target because, unlike the counterpart pathway, apoptosis, it is dispensable for mammalian development. In particular, the most terminal-known obligate effector in the necroptosis pathway, the pseudokinase MLKL (mixed lineage kinase domain-like), holds particular appeal because, thus far, its only known function is as a mediator of necroptotic cell death. We review the current understanding and gaps in knowledge relating to how MLKL can be activated by receptor interacting protein kinase (RIPK)3 downstream of tumour necrosis factor receptor 1:RIPK1, Toll like receptor-3:TRIF and viral DNA: DAI (DNA-dependent activator of interferon regulatory factors)/ZBF1. We also discuss the potential mechanism(s) by which MLKL induces necroptotic cell death, with particular emphasis on insights arising from structural studies of mouse and human MLKL.

Deficiencies in Cellular Processes Modulated by the Retinoblastoma Protein Do Not Account for Reduced Human Cytomegalovirus Replication in Its Absence

Despite encoding multiple viral proteins that modulate the retinoblastoma (Rb) protein in a manner classically defined as inactivation, human cytomegalovirus (HCMV) requires the presence of the Rb protein to replicate efficiently. In uninfected cells, Rb controls numerous pathways that the virus also commandeers during infection. These include cell cycle progression, senescence, mitochondrial biogenesis, apoptosis, and glutaminolysis. We investigated whether a potential inability of HCMV to regulate these Rb-controlled pathways in the absence of the Rb protein was the reason for reduced viral productive replication in Rb knockdown cells. We found that HCMV was equally able to modulate these pathways in the parental Rb-expressing and Rb-depleted cells. Our results suggest that Rb may be required to enhance a specific viral process during HCMV productive replication.

IMPORTANCE

The retinoblastoma (Rb) tumor suppressor is well established as a repressor of E2F-dependent transcription. Rb hyperphosphorylation, degradation, and binding by viral oncoproteins are also codified. Recent reports indicate Rb can be monophosphorylated, repress the transcription of antiviral genes in association with adenovirus E1A, modulate cellular responses to polycomb-mediated epigenetic methylations in human papillomavirus type 16 E7 expressing cells, and increase the efficiency of human cytomegalovirus (HCMV) productive replication. Since Rb function also now extends to regulation of mitochondrial function (apoptosis, metabolism), it is clear that our current understanding of this protein is insufficient to explain its roles in virus-infected cells and tumors. Work here reinforces this concept, showing the known roles of Rb are insufficient to explain its positive impact on HCMV replication. Therefore, HCMV, along with other viral systems, provide valuable tools to probe functions of Rb that might be modulated with therapeutics for cancers with viral or nonviral etiologies.

Structural basis of cell apoptosis and necrosis in TNFR signaling

The tumor necrosis factor receptors (TNFRs) play essential roles in innate and adaptive immunity. Depending on conditions, TNFR induces multiple cell fates including cell survival, cell apoptosis, and cell programmed necrosis. Here, we review recent progress in structural studies of the TNFR signaling pathway. The structural basis for the high order signal complexes, including the DISC, ripoptosome, necrosome, and RIP3/MLKL complex, may provide novel insights for understanding the biophysical principles of cell signaling cascades.

Innate immunity in tuberculosis: how the sensing of mycobacteria and tissue damage modulates macrophage death

The success of Mycobacterium tuberculosis as a human pathogen has been attributed to the ability of the bacillus to proliferate inside macrophages and to induce cell death. This review describes how the sensors of the innate immune system modulate the cell death pathways in infected macrophages and, consequently, the pathogenesis of tuberculosis.

A new kind of cell suicide: mechanisms and functions of programmed necrosis

Classically, there are two major forms of cell death: necrosis, an unregulated digestion of cellular components; and apoptosis, a programmed mechanism that is promoted by caspases. However, another form of cell death has recently been identified that is inhibited by caspases, and yet occurs through a regulated mechanism, termed programmed necrosis or necroptosis. The biochemical basis of this program has begun to emerge, with the discovery of the receptor-interacting kinase RIP3 and its substrate, the pseudokinase mixed lineage kinase domain-like protein (MLKL), as core components. Furthermore, animal models have revealed significant functions for RIP3/MLKL-mediated necrotic cell death in immune responses against microbial infection and in the etiology of diseases involving tissue damage. This review discusses recent advances in our understanding of the mechanistic details and physiological functions of programmed necrosis.

TNF-α-induced programmed cell death in the pathogenesis of acquired aplastic anemia

The mechanism of acquired aplastic anemia (AA), a bone marrow hematopoiesis failure disease, has not been fully understood. TNF-α is a pleiotropic cytokine involved in cell proliferation, differentiation and death, and inflammation through binding to specific receptors on cell membranes. Aberrant secretion of TNF-α contributes to a number of human diseases, including tumor development and inflammation. TNF-α is also an important negative regulator of hematopoiesis. Over-expression of TNF-α not only directly inhibits the proliferation and differentiation of hematopoietic cells, but also initiates the intracellular death pathway to induce hematopoietic cell death, leading to bone marrow hematopoiesis failure. In this review, we summarize the mechanisms underlying extrinsic apoptosis and necroptosis of hematopoietic cells induced by TNF-α, and discuss the role of TNF-α-induced programmed cell death in the pathogenesis of acquired AA.

Programmed cell death with a necrotic-like phenotype

Programmed cell death is the process by which an individual cell in a multicellular organism commits cellular 'suicide' to provide a long-term benefit to the organism. Thus, programmed cell death is important for physiological processes such as development, cellular homeostasis, and immunity. Importantly, in this process, the cell is not eliminated in response to random events but in response to an intricate and genetically defined set of internal cellular molecular events or 'program'. Although the apoptotic process is generally very well understood, programmed cell death that occurs with a necrotic-like phenotype has been much less studied, and it is only within the past few years that the necrotic program has begun to be elucidated. Originally, programmed necrosis was somewhat dismissed as a nonphysiological phenomenon that occurs in vitro. Recent in vivo studies, however, suggest that regulated necrosis is an authentic classification of cell death that is important in mammalian development and other physiological processes, and programmed necrosis is now considered a significant therapeutic target in major pathological processes as well. Although the RIP1-RIP3-dependent necrosome complex is recognized as being essential for the execution of many instances of programmed necrosis, other downstream and related necrotic molecules and pathways are now being characterized. One of the current challenges is understanding how and under what conditions these pathways are linked together.

New Insights into the Role of Ubiquitin Networks in the Regulation of Antiapoptosis Pathways

Ubiquitin is a small modifier protein that conjugates on lysine (Lys) residues of substrates, and it can be targeted by another ubiquitin molecule to form chains through conjugation on the intrinsic Lys residues and methionine (Met) 1 residue. Ubiquitination of substrates by such chains determines the fate of substrates, thereby influencing various biological processes. In this chapter, we focus on apoptosis with an emphasis on the regulation by ubiquitination. The signal transduction of apoptosis is governed not only by the classical function of ubiquitin, which is proteasome-dependent degradation of substrates, but also by the apoptosis signaling complex formation guided by different types of ubiquitin chains. Ubiquitinations of pro- and antiapoptotic proteins are tightly regulated by particular sets of enzymes, such as ubiquitin E3 ligases and deubiquitinases (DUBs). We further discuss ubiquitination in the tumor necrosis factor (TNF) signaling pathway as an example for the ubiquitin-dependent regulation of apoptosis and cell survival.

Necroptosis is a key pathogenic event in human and experimental murine models of non-alcoholic steatohepatitis

Hepatocyte cell death, inflammation and oxidative stress constitute key pathogenic mechanisms underlying non-alcoholic fatty liver disease (NAFLD). We aimed to investigate the role of necroptosis in human and experimental NAFLD and its association with tumour necrosis factor α (TNF-α) and oxidative stress. Serum markers of necrosis, liver receptor-interacting protein 3 (RIP3) and phosphorylated mixed lineage kinase domain-like (MLKL) were evaluated in control individuals and patients with NAFLD. C57BL/6 wild-type (WT) or RIP3-deficient (RIP3(-/-)) mice were fed a high-fat choline-deficient (HFCD) or methionine and choline-deficient (MCD) diet, with subsequent histological and biochemical analysis of hepatic damage. In primary murine hepatocytes, necroptosis and oxidative stress were also assessed after necrostatin-1 (Nec-1) treatment or RIP3 silencing. We show that circulating markers of necrosis and TNF-α, as well as liver RIP3 and MLKL phosphorylation were increased in NAFLD. Likewise, RIP3 and MLKL protein levels and TNF-α expression were increased in the liver of HFCD and MCD diet-fed mice. Moreover, RIP3 and MLKL sequestration in the insoluble protein fraction of NASH (non-alcoholic steatohepatitis) mice liver lysates represented an early event during stetatohepatitis progression. Functional studies in primary murine hepatocytes established the association between TNF-α-induced RIP3 expression, activation of necroptosis and oxidative stress. Strikingly, RIP3 deficiency attenuated MCD diet-induced liver injury, steatosis, inflammation, fibrosis and oxidative stress. In conclusion, necroptosis is increased in the liver of NAFLD patients and in experimental models of NASH. Further, TNF-α triggers RIP3-dependent oxidative stress during hepatocyte necroptosis. As such, targeting necroptosis appears to arrest or at least impair NAFLD progression.

A cellular screen identifies ponatinib and pazopanib as inhibitors of necroptosis

Necroptosis is a form of regulated necrotic cell death mediated by receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and RIPK3. Necroptotic cell death contributes to the pathophysiology of several disorders involving tissue damage, including myocardial infarction, stroke and ischemia-reperfusion injury. However, no inhibitors of necroptosis are currently in clinical use. Here we performed a phenotypic screen for small-molecule inhibitors of tumor necrosis factor-alpha (TNF-α)-induced necroptosis in Fas-associated protein with death domain (FADD)-deficient Jurkat cells using a representative panel of Food and Drug Administration (FDA)-approved drugs. We identified two anti-cancer agents, ponatinib and pazopanib, as submicromolar inhibitors of necroptosis. Both compounds inhibited necroptotic cell death induced by various cell death receptor ligands in human cells, while not protecting from apoptosis. Ponatinib and pazopanib abrogated phosphorylation of mixed lineage kinase domain-like protein (MLKL) upon TNF-α-induced necroptosis, indicating that both agents target a component upstream of MLKL. An unbiased chemical proteomic approach determined the cellular target spectrum of ponatinib, revealing key members of the necroptosis signaling pathway. We validated RIPK1, RIPK3 and transforming growth factor-β-activated kinase 1 (TAK1) as novel, direct targets of ponatinib by using competitive binding, cellular thermal shift and recombinant kinase assays. Ponatinib inhibited both RIPK1 and RIPK3, while pazopanib preferentially targeted RIPK1. The identification of the FDA-approved drugs ponatinib and pazopanib as cellular inhibitors of necroptosis highlights them as potentially interesting for the treatment of pathologies caused or aggravated by necroptotic cell death.

Live to die another way: modes of programmed cell death and the signals emanating from dying cells

All life ends in death, but perhaps one of life's grander ironies is that it also depends on death. Cell-intrinsic suicide pathways, termed programmed cell death (PCD), are crucial for animal development, tissue homeostasis and pathogenesis. Originally, PCD was almost synonymous with apoptosis; recently, however, alternative mechanisms of PCD have been reported. Here, we provide an overview of several distinct PCD mechanisms, namely apoptosis, autophagy and necroptosis. In addition, we discuss the complex signals that emanate from dying cells, which can either trigger regeneration or instruct additional killing. Further advances in understanding the physiological roles of the various mechanisms of cell death and their associated signals will be important to selectively manipulate PCD for therapeutic purposes.

HSV cheats the executioner

In this issue of Cell Host & Microbe, Huang et al. (2015) and Guo et al. (2015), along with recent work by Wang et al. (2014), reveal that HSV ribonucleotide reductase has opposing activities in either inducing or preventing necroptosis, depending on the host species. This evolutionary twist underscores the importance of selective pressure in virus-host relationships.

Loss of caspase-3 sensitizes colon cancer cells to genotoxic stress via RIP1-dependent necrosis

Caspase-3 is the best known executioner caspase in apoptosis. We generated caspase-3 knockout (C3KO) and knockdown human colorectal cancer cells, and found that they are unexpectedly sensitized to DNA-damaging agents including 5-fluorouracil (5-FU), etoposide, and camptothecin. C3KO xenograft tumors also displayed enhanced therapeutic response and cell death to 5-FU. C3KO cells showed intact apoptosis and activation of caspase-7 and -9, impaired processing of caspase-8, and induction of necrosis in response to DNA-damaging agents. This form of necrosis is associated with HMGB1 release and ROS production, and suppressed by genetic or pharmacological inhibition of RIP1, MLKL1, or caspase-8, but not inhibitors of pan-caspases or RIP3. 5-FU treatment led to the formation of a z-VAD-resistant pro-caspase-8/RIP1/FADD complex, which was strongly stabilized by caspase-3 KO. These data demonstrate a key role of caspase-3 in caspase-8 processing and suppression of DNA damage-induced necrosis, and provide a potentially novel way to chemosensitize cancer cells.

Macrophage- and RIP3-dependent inflammasome activation exacerbates retinal detachment-induced photoreceptor cell death

Detachment of photoreceptors from the retinal pigment epithelium is seen in various retinal disorders, resulting in photoreceptor death and subsequent vision loss. Cell death results in the release of endogenous molecules that activate molecular platforms containing caspase-1, termed inflammasomes. Inflammasome activation in retinal diseases has been reported in some cases to be protective and in others to be detrimental, causing neuronal cell death. Moreover, the cellular source of inflammasomes in retinal disorders is not clear. Here, we demonstrate that patients with photoreceptor injury by retinal detachment (RD) have increased levels of cleaved IL-1β, an end product of inflammasome activation. In an animal model of RD, photoreceptor cell death led to activation of endogenous inflammasomes, and this activation was diminished by Rip3 deletion. The major source of Il1b expression was found to be infiltrating macrophages in the subretinal space, rather than dying photoreceptors. Inflammasome inhibition attenuated photoreceptor death after RD. Our data implicate the infiltrating macrophages as a source of damaging inflammasomes after photoreceptor detachment in a RIP3-dependent manner and suggest a novel therapeutic target for treatment of retinal diseases.

Die another way--non-apoptotic mechanisms of cell death

Regulated, programmed cell death is crucial for all multicellular organisms. Cell death is essential in many processes, including tissue sculpting during embryogenesis, development of the immune system and destruction of damaged cells. The best-studied form of programmed cell death is apoptosis, a process that requires activation of caspase proteases. Recently it has been appreciated that various non-apoptotic forms of cell death also exist, such as necroptosis and pyroptosis. These non-apoptotic cell death modalities can be either triggered independently of apoptosis or are engaged should apoptosis fail to execute. In this Commentary, we discuss several regulated non-apoptotic forms of cell death including necroptosis, autophagic cell death, pyroptosis and caspase-independent cell death. We outline what we know about their mechanism, potential roles in vivo and define outstanding questions. Finally, we review data arguing that the means by which a cell dies actually matters, focusing our discussion on inflammatory aspects of cell death.

Molecular crosstalk between apoptosis, necroptosis, and survival signaling

Our current knowledge of the molecular mechanisms regulating the signaling pathways leading to cell survival, cell death, and inflammation has shed light on the tight mutual interplays between these processes. Moreover, the fact that both apoptosis and necrosis can be molecularly controlled has greatly increased our interest in the roles that these types of cell death play in the control of general processes such as development, homeostasis, and inflammation. In this review, we provide a brief update on the different cell death modalities and describe in more detail the intracellular crosstalk between survival, apoptotic, necroptotic, and inflammatory pathways that are activated downstream of death receptors. An important concept is that the different cell death processes modulate each other by mutual inhibitory mechanisms, serve as alternative back-up death routes in the case of a defect in the first-line cell death response, and are controlled by multiple feedback loops. We conclude by discussing future perspectives and challenges in the field of cell death and inflammation research.

Chronic Treatment with a Water-Soluble Extract from the Culture Medium of Ganoderma lucidum Mycelia Prevents Apoptosis and Necroptosis in Hypoxia/Ischemia-Induced Injury of Type 2 Diabetic Mouse Brain

Type 2 diabetes mellitus has been known to increase systemic oxidative stress by chronic hyperglycemia and visceral obesity and aggravate cerebral ischemic injury. On the basis of our previous study regarding a water-soluble extract from the culture medium of Ganoderma lucidum mycelia (designed as MAK), which exerts antioxidative and neuroprotective effects, the present study was conducted to evaluate the preventive effects of MAK on apoptosis and necroptosis (a programmed necrosis) induced by hypoxia/ischemia (H/I) in type 2 diabetic KKAy mice. H/I was induced by a combination of unilateral common carotid artery ligation with hypoxia (8% O₂ for 20 min) and subsequent reoxygenation. Pretreatment with MAK (1 g/kg, p.o.) for a week significantly reduced H/I-induced neurological deficits and brain infarction volume assessed at 24 h of reoxygenation. Histochemical analysis showed that MAK significantly suppressed superoxide production, neuronal cell death, and vacuolation in the ischemic penumbra, which was accompanied by a decrease in the numbers of TUNEL- or cleaved caspase-3-positive cells. Furthermore, MAK decreased the expression of receptor-interacting protein kinase 3 mRNA and protein, a key molecule for necroptosis. These results suggest that MAK confers resistance to apoptotic and necroptotic cell death and relieves H/I-induced cerebral ischemic injury in type 2 diabetic mice.

Manipulation of apoptosis and necroptosis signaling by herpesviruses

Like apoptosis, necroptosis is an innate immune mechanism that eliminates pathogen-infected cells. Receptor-interacting protein kinase (RIP)3 (also called RIPK3) mediates necrotic death by phosphorylating an executioner protein, MLKL, leading to plasma membrane leakage. The pathway is triggered against viruses that block caspase 8. In murine CMV, the viral inhibitor of caspase 8 activation prevents extrinsic apoptosis but also has the potential to unleash necroptosis. This virus encodes the viral inhibitor of RIP activation to prevent RIP homotypic interaction motif (RHIM)-dependent signal transduction and necroptosis. Recent investigations reveal a similar mechanism at play in the human alpha-herpesviruses, herpes simplex virus (HSV)1 and HSV2, where RHIM competitor function and caspase 8 suppression are carried out by the virus-encoded large subunit of ribonucleotide reductase (R1). In human cells, R1 inhibition of caspase 8 prevents TNF-induced apoptosis, but sensitizes to TNF-induced necroptosis. The RHIM and caspase 8 interaction domains of R1 collaborate to prevent RIP3-dependent steps and enable both herpesviruses to deflect host cell death machinery that would cut short infection. In mouse cells, HSV1 infection by itself triggers necroptosis by driving RIP3 protein kinase activity. HSV1 R1 contributes to the activation of RIP3 adaptor function in mice, a popular host animal for experimental infection. Based on these studies, infection of RIP3-kinase inactive mice should be explored in models of pathogenesis and latency. The necrotic death pathway that is suppressed during infection in the natural host becomes a cross-species barrier to infection in a non-natural host.

Ppm1b negatively regulates necroptosis through dephosphorylating Rip3

The auto-phosphorylation of murine receptor-interacting protein 3 (Rip3) on Thr 231 and Ser 232 in the necrosome is required to trigger necroptosis. However, how Rip3 phosphorylation is regulated is still largely unknown. Here we identified protein phosphatase 1B (Ppm1b) as a Rip3 phosphatase and found that Ppm1b restricts necroptosis in two settings: spontaneous necroptosis caused by Rip3 auto-phosphorylation in resting cells, and tumour necrosis factor-α (TNF)-induced necroptosis in cultured cells. We revealed that Ppm1b selectively suppresses necroptosis through the dephosphorylation of Rip3, which then prevents the recruitment of mixed lineage kinase domain-like protein (Mlkl) to the necrosome. We further showed that Ppm1b deficiency (Ppm1b(d/d)) in mice enhanced TNF-induced death in a Rip3-dependent manner, and the role of Ppm1b in inhibiting necroptosis was evidenced by elevated Rip3 phosphorylation and tissue damage in the caecum of TNF-treated Ppm1b(d/d) mice. These data indicate that Ppm1b negatively regulates necroptosis through dephosphorylating Rip3 in vitro and in vivo.

Innate immune recognition of DNA: A recent history

Innate immune DNA sensing underpins many physiological and pathological responses to DNA, including anti-viral immunity to DNA viruses. Although it has been appreciated for many years that cytosolic DNA can evoke a type I interferon response, it is only within the past decade that the cellular mechanisms responsible for such a response have been defined. Here we review the discoveries that led to an appreciation of the existence of cytosolic DNA sensor proteins, and discuss two key such sensors, cGAS and IFI16, in detail. DNA sensors operate via STING, a protein shown to have a central role in controlling altered gene induction in response to DNA in vivo, and as such to be central to a rapidly expanding list of both protective and harmful responses to DNA. We also discuss recent insights into how and when DNA stimulates innate immunity, and highlight current outstanding questions in the DNA sensing field.

Necroptosis: The Trojan horse in cell autonomous antiviral host defense

Herpesviruses suppress cell death to assure sustained infection in their natural hosts. Murine cytomegalovirus (MCMV) encodes suppressors of apoptosis as well as M45-encoded viral inhibitor of RIP activation (vIRA) to block RIP homotypic interaction motif (RHIM)-signaling and recruitment of RIP3 (also called RIPK3), to prevent necroptosis. MCMV and human cytomegalovirus encode a viral inhibitor of caspase (Casp)8 activation to block apoptosis, an activity that unleashes necroptosis. Herpes simplex virus (HSV)1 and HSV2 incorporate both RHIM and Casp8 suppression strategies within UL39-encoded ICP6 and ICP10, respectively, which are herpesvirus-conserved homologs of MCMV M45. Both HSV proteins sensitize human cells to necroptosis by blocking Casp8 activity while preventing RHIM-dependent RIP3 activation and death. In mouse cells, HSV1 ICP6 interacts with RIP3 and, surprisingly, drives necroptosis. Thus, herpesviruses have illuminated the contribution of necoptosis to host defense in the natural host as well as its potential to restrict cross-species infections in nonnatural hosts.

Suppression of RIP3-dependent necroptosis by human cytomegalovirus

Necroptosis is an alternate programmed cell death pathway that is unleashed by caspase-8 compromise and mediated by receptor-interacting protein kinase 3 (RIP3). Murine cytomegalovirus (CMV) and herpes simplex virus (HSV) encode caspase-8 inhibitors that prevent apoptosis together with competitors of RIP homotypic interaction motif (RHIM)-dependent signal transduction to interrupt the necroptosis. Here, we show that pro-necrotic murine CMV M45 mutant virus drives virus-induced necroptosis during nonproductive infection of RIP3-expressing human fibroblasts, whereas WT virus does not. Thus, M45-encoded RHIM competitor, viral inhibitor of RIP activation, sustains viability of human cells like it is known to function in infected mouse cells. Importantly, human CMV is shown to block necroptosis induced by either TNF or M45 mutant murine CMV in RIP3-expressing human cells. Human CMV blocks TNF-induced necroptosis after RIP3 activation and phosphorylation of the mixed lineage kinase domain-like (MLKL) pseudokinase. An early, IE1-regulated viral gene product acts on a necroptosis step that follows MLKL phosphorylation prior to membrane leakage. This suppression strategy is distinct from RHIM signaling competition by murine CMV or HSV and interrupts an execution process that has not yet been fully elaborated.

The molecular relationships between apoptosis, autophagy and necroptosis

Cells are constantly subjected to a vast range of potentially lethal insults, which may activate specific molecular pathways that have evolved to kill the cell. Cell death pathways are defined partly by their morphology, and more specifically by the molecules that regulate and enact them. As these pathways become more thoroughly characterized, interesting molecular links between them have emerged, some still controversial and others hinting at the physiological and pathophysiological roles these death pathways play. We describe specific molecular programs controlling cell death, with a focus on some of the distinct features of the pathways and the molecular links between them.

A novel function of RIP1 in postnatal development and immune homeostasis by protecting against RIP3-dependent necroptosis and FADD-mediated apoptosis

RIP1 is an adaptor kinase originally identified as being able to associate with TNFR1 and Fas, and is later shown to be involved in signaling induced by TLRs. Major signaling pathways regulated by RIP1 include necroptosis, apoptosis, and pro-survival/inflammation NF-κB activation. Previous studies show that RIP1 deficiency has no effect on mouse embryogenesis, but blocks postnatal development. This phenotype could not readily be explained, since mice lacking TNFR1, Fas, or TLRs show no apparent developmental defect. Certain types of RIP1-deficient cells are hypersensitive to TNF-induced apoptosis. However, in our previous study, deletion of the apoptotic adaptor protein, FADD, provides marginal improvement of postnatal development of rip1^−/− mice. Remarkably, the current data shows that haploid insufficiency of RIP3, a known mediator of necroptosis, allowed survival of rip1^−/−fadd^−/− mice beyond weaning age, although the resulting rip1^−/−fadd^−/− rip3^+/− mice were significant smaller in size and weight. Moreover, complete absence of RIP3 further improved postnatal development of the resulting rip1^−/−fadd^−/−rip3^−/− mice, which display normal size and weight. In such triple knockout (TKO) mice, lymphocytes underwent normal development, but progressively accumulated as mice age. This lymphoproliferative (lpr) disease in TKO mice is, however, less severe than that of fadd^−/−rip3^−/− double knockout mice. In total, the data show that the postnatal developmental defect in rip1^−/− mice is due in part to FADD-mediated apoptosis as well as RIP3-dependent necroptosis. Moreover, the function of RIP1 contributes to development of lpr diseases.

The role of necroptosis in neurosurgical diseases

Programmed necrosis or necroptosis is an alternative form of cell death that is executed through a caspase-independent pathway. Necroptosis has been implicated in many pathological conditions. Genetic or pharmacological inhibition of necroptotic signaling has been shown to confer neuroprotection after traumatic and ischemic brain injury. Therefore, the necroptotic pathway represents a potential target for neurological diseases that are managed by neurosurgeons. In this review, we summarize recent advances in the understanding of necroptotic signaling pathways and explore the role of necroptotic cell death in craniocerebral trauma, brain tumors, and cerebrovascular diseases.

Necroptosis: Pathway diversity and characteristics

Regulated cell death is a physiological process that controls organismal homeostasis. Deregulation of cell death can lead to the development of a number of human diseases and tissue damage. Apoptosis is a best-known model of caspase-dependent regulated cell death, but recently necroptosis has garnered a lot of attention as a form of regulated cell death not mediated by caspases. Different stimuli can trigger necroptosis, and all of them converge at the activation of the protein kinase RIP3 (receptor-interacting protein 3) and the pseudokinase MLKL (mixed lineage kinase domain-like). Necroptosis activation relies on the unique protein-interaction motif RHIM (RIP homology interaction motif). Different RHIM-containing proteins (RIP1, DAI and TRIF) transduce necroptotic signals from the cell death trigger to the cell death mediators RIP3-MLKL. RIP1 has a particularly important and complex role in necroptotic cell death regulation ranging from cell death activation to inhibition, often in a cell type and context dependent fashion.

RIPK1 and RIPK3: critical regulators of inflammation and cell death

RIPK1 and RIPK3 (receptor-interacting serine/threonine protein kinases 1/3) interact by virtue of their RIP homotypic interaction motifs to mediate a form of cell death called necroptosis, although mice lacking these kinases have very different phenotypes. RIPK1-deficient mice die soon after birth, whereas RIPK3-deficient mice are healthy. Necroptosis involves cell rupture and is triggered by tumor necrosis factor (TNF), Toll-like receptors (TLRs), or the T cell receptor (TCR) when pro-apoptotic caspase-8 is inhibited. Various mouse models of disease are ameliorated by RIPK3 deficiency, suggesting that necroptosis contributes to pathology. Genetic rescue experiments now reveal why RIPK3-deficient are viable but RIPK1-deficient mice are not. These and other experiments indicate unexpected complexity in the regulation of both apoptosis and necroptosis by RIPK1 and RIPK3.

Necroptosis suppresses inflammation via termination of TNF- or LPS-induced cytokine and chemokine production

TNF promotes a regulated form of necrosis, called necroptosis, upon inhibition of caspase activity in cells expressing RIPK3. Because necrosis is generally more pro-inflammatory than apoptosis, it is widely presumed that TNF-induced necroptosis may be detrimental in vivo due to excessive inflammation. However, because TNF is intrinsically highly pro-inflammatory, due to its ability to trigger the production of multiple cytokines and chemokines, rapid cell death via necroptosis may blunt rather than enhance TNF-induced inflammation. Here we show that TNF-induced necroptosis potently suppressed the production of multiple TNF-induced pro-inflammatory factors due to RIPK3-dependent cell death. Similarly, necroptosis also suppressed LPS-induced pro-inflammatory cytokine production. Consistent with these observations, supernatants from TNF-stimulated cells were more pro-inflammatory than those from TNF-induced necroptotic cells in vivo. Thus necroptosis attenuates TNF- and LPS-driven inflammation, which may benefit intracellular pathogens that evoke this mode of cell death by suppressing host immune responses.

Evasion of innate cytosolic DNA sensing by a gammaherpesvirus facilitates establishment of latent infection

Herpesviruses are DNA viruses harboring the capacity to establish lifelong latent-recurrent infections. There is limited knowledge about viruses targeting the innate DNA-sensing pathway, as well as how the innate system impacts on the latent reservoir of herpesvirus infections. In this article, we report that murine gammaherpesvirus 68 (MHV68), in contrast to α- and β-herpesviruses, induces very limited innate immune responses through DNA-stimulated pathways, which correspondingly played only a minor role in the control of MHV68 infections in vivo. Similarly, Kaposi's sarcoma-associated herpesvirus also did not stimulate immune signaling through the DNA-sensing pathways. Interestingly, an MHV68 mutant lacking deubiquitinase (DUB) activity, embedded within the large tegument protein open reading frame (ORF)64, gained the capacity to stimulate the DNA-activated stimulator of IFN genes (STING) pathway. We found that ORF64 targeted a step in the DNA-activated pathways upstream of the bifurcation into the STING and absent in melanoma 2 pathways, and lack of the ORF64 DUB was associated with impaired delivery of viral DNA to the nucleus, which, instead, localized to the cytoplasm. Correspondingly, the ORF64 DUB active site mutant virus exhibited impaired ability to establish latent infection in wild-type, but not STING-deficient, mice. Thus, gammaherpesviruses evade immune activation by the cytosolic DNA-sensing pathway, which, in the MHV68 model, facilitates establishment of infections.

Connections between various trigger factors and the RIP1/ RIP3 signaling pathway involved in necroptosis

Programmed cell death is a basic cellular process that is critical to maintaining tissue homeostasis. In contrast to apoptosis, necrosis was previously regarded as an unregulated and uncontrollable process. However, as research has progressed, necrosis, also known as necroptosis or programmed necrosis, is drawing increasing attention, not least becasu of its possible impications for cancer research. Necroptosis exhibits a unique signaling pathway that requires the involvement of receptor interaction protein kinases 1 and 3 (RIP1 and RIP3), mixed lineage kinase domain-like (MLKL), and phosphoglycerate mutase 5 (PGAM5) and can be specifically inhibited by necrostatins. Not only does necroptosis serve as a backup cell death program when apoptosis is inhibited, but it is now recognized to play a pivotal role in regulating various physiological processes and the pathogenesis of a variety of human diseases such as ischemic brain injury, immune system disorders and cancer. The control of necroptosis by various defined trigger factors and signaling pathways now offers the opportunity to target this cellular process for therapeutic purposes. The purpose of this paper is to review current findings concerning the connections between various trigger factors and the RIP1/RIP3 signaling pathway as it relates to necroptosis.

Programmed cell death and its role in inflammation

Cell death plays an important role in the regulation of inflammation and may be the result of inflammation. The maintenance of tissue homeostasis necessitates both the recognition and removal of invading microbial pathogens as well as the clearance of dying cells. In the past few decades, emerging knowledge on cell death and inflammation has enriched our molecular understanding of the signaling pathways that mediate various programs of cell death and multiple types of inflammatory responses. This review provides an overview of the major types of cell death related to inflammation. Modification of cell death pathways is likely to be a logical therapeutic target for inflammatory diseases.

Emerging Roles for RIPK1 and RIPK3 in Pathogen-Induced Cell Death and Host Immunity

Receptor-interacting protein kinases 1 and 3 (RIPK1 and RIPK3 ) are homologous serine-threonine kinases that were recognized for their roles in directing programmed necrotic cell death or necroptosis under a broad range of pathologic settings. Emerging evidence suggests new physiologic roles for RIPK1 and RIPK3 in mediating cell death of innate immune responses. Our review discusses current evidence on the mechanisms and the impact of RIPK1- and/or RIPK3-dependent cell death in responses to a variety of viral and bacterial pathogens. Furthermore, the discussion also summarizes emerging roles for RIPK1 and RIPK3 in other facets of host immunity, including the maintenance of epithelial barrier function and pro-inflammatory processes that may, in some cases, manifest independent of cell death. Finally, we briefly consider the therapeutic opportunities in targeting RIPK1- and RIPK3-dependent processes in infection and immunity.

Evolutionary link between metazoan RHIM motif and prion-forming domain of fungal heterokaryon incompatibility factor HET-s/HET-s

The Rip homotypic interaction motif (RHIM) is a short, non-globular sequence stretch that mediates a key interaction of mammalian necroptosis signaling. In order to understand its unusual oligomerization properties, we set out to trace the evolutionary origins of the RHIM motif by identifying distantly related protein motifs that might employ the same binding mode. The RHIM motif was found to be related to the prion-forming domain of the HET-s protein, which oligomerizes by forming structurally well-characterized fibrils and is involved in fungal heterokaryon incompatibility. This evolutionary relationship explains the recently reported propensity of mammalian RHIM motifs to form amyloid fibrils, but suggests that these fibrils have a different structural architecture than currently assumed. These findings, together with numerous observations of RHIM-like motifs in immunity proteins from a wide range of species, provide insight to the modern innate immunity pathways in animals, plants and fungi.

Programmed necrosis in the cross talk of cell death and inflammation

Cell proliferation and cell death are integral elements in maintaining homeostatic balance in metazoans. Disease pathologies ensue when these processes are disturbed. A plethora of evidence indicates that malfunction of cell death can lead to inflammation, autoimmunity, or immunodeficiency. Programmed necrosis or necroptosis is a form of nonapoptotic cell death driven by the receptor interacting protein kinase 3 (RIPK3) and its substrate, mixed lineage kinase domain-like (MLKL). RIPK3 partners with its upstream adaptors RIPK1, TRIF, or DAI to signal for necroptosis in response to death receptor or Toll-like receptor stimulation, pathogen infection, or sterile cell injury. Necroptosis promotes inflammation through leakage of cellular contents from damaged plasma membranes. Intriguingly, many of the signal adaptors of necroptosis have dual functions in innate immune signaling. This unique signature illustrates the cooperative nature of necroptosis and innate inflammatory signaling pathways in managing cell and organismal stresses from pathogen infection and sterile tissue injury.

Regulated cell death in AKI

AKI is pathologically characterized by sublethal and lethal damage of renal tubules. Under these conditions, renal tubular cell death may occur by regulated necrosis (RN) or apoptosis. In the last two decades, tubular apoptosis has been shown in preclinical models and some clinical samples from patients with AKI. Mechanistically, apoptotic cell death in AKI may result from well described extrinsic and intrinsic pathways as well as ER stress. Central converging nodes of these pathways are mitochondria, which become fragmented and sensitized to membrane permeabilization in response to cellular stress, resulting in the release of cell death-inducing factors. Whereas apoptosis is known to be regulated, tubular necrosis was thought to occur by accident until recent work unveiled several RN subroutines, most prominently receptor-interacting protein kinase-dependent necroptosis and RN induced by mitochondrial permeability transition. Additionally, other cell death pathways, like pyroptosis and ferroptosis, may also be of pathophysiologic relevance in AKI. Combination therapy targeting multiple cell-death pathways may, therefore, provide maximal therapeutic benefits.

The role of RIP1 and RIP3 in the development of aplastic anemia induced by cyclophosphamide and busulphan in mice

This study aimed to investigate the role of RIP1 and RIP3 in the pathogenesis of aplastic anemia (AA) induced by cyclophosphamide and busulphan in mice. Animals were randomly divided into three groups: the control group, the AA group, and the Nec-1 group. Mouse AA model was established by intraperitoneal injection of cyclophosphamide (40 mg/kg/d) and busulfan (20 mg/kg/d) for 12 days. The Nec-1 group mice received intraperitoneal injection of Nec-1 (2 mg/kg/d) for 12 days prior to intraperitoneal injection of cyclophosphamide (40 mg/kg/d) and busulfan (20 mg/kg/d) for 12 days. The control mice received intraperitoneal injection of equal volume of saline. At 12 h after the last intraperitoneal injection, blood and bone marrow tissues were collected from mice. Peripheral blood cells were analyzed using hematology analyzer and the histological changes of bone marrow tissues were examined using scanning electron microscopy (SEM). The levels of RIP3 and RIP3 in bone marrow were measured using Western blot analysis and the interaction of RIP1 and RIP3 proteins was investigated on the basis of immunoprecipitation analysis. ELISA was used to measure the levels of IL-6, TNF-α, and FLT-3L in bone marrow tissue supernatant. Apoptosis and necrosis of bone marrow cells were analyzed using flow cytometry. Western blot showed that the expression of RIP1 and RIP3 was significantly increases in AA mice compared to the normal controls. Immunoprecipitation detected the pro-necrotic RIP1-RIP3 complex, suggesting that RIP1 and RIP3 mediated necroptosis may involved in the damage of bone marrow cells. Compared to the AA mice, Nec-1 group mice exhibited significantly increase of peripheral blood cells and mononuclear cells in bone marrow tissues and decrease of the apoptosis/necrosis of bone marrow cells. In addition, we observed significant decrease of IL-6, TNF-α, and FLT-3L in bone marrow tissue supernatant in the Nec-1 group mice compared to AA mice. Our results suggest that Nec-1 can prevent the development of AA by inhibiting bone marrow cells necrosis and the production of inflammatory mediators. RIP1 and RIP3-mediated necroptosis may involve in the pathogenesis of AA induced by cyclophosphamide and busulfan in mice.

A long-awaited merger of the pathways mediating host defence and programmed cell death

Historically, cell death and inflammation have been closely linked, but the necessary divergence of the fields in the past few decades has enriched our molecular understanding of the signalling pathways that mediate various programmes of cell death and multiple types of inflammatory responses. The fields have now come together again demonstrating a surprising level of integration. Intimate interconnections at multiple levels are revealed between the cell death and inflammatory signal transduction pathways that are mobilized in response to the engagement of pattern recognition receptors during microbial infection. Molecules such as receptor-interacting protein kinase 1 (RIPK1), RIPK3, FAS-associated death domain protein (FADD), FLICE-like inhibitory protein (FLIP) and caspase 8 - which are associated with different forms of cell death - are incorporated into compatible and exceedingly dynamic Toll-like receptor, NOD-like receptor and RIG-I-like receptor signalling modules. These signalling modules have a high capacity to switch from inflammation to cell death, or a programmed execution of both, all in an orchestrated battle for host defence and survival.

The two faces of receptor interacting protein kinase-1

Receptor Interacting Protein Kinase-1 (RIPK1), a key player in inflammation and cell death, assumes opposite functions depending on the cellular context and its posttranslational modifications. Genetic evidence supported by biochemical and cellular biology approaches sheds light on the circumstances in which RIPK1 promotes or inhibits these processes.

RIP3 induces apoptosis independent of pronecrotic kinase activity

Receptor-interacting protein kinase 3 (RIP3 or RIPK3) has emerged as a central player in necroptosis and a potential target to control inflammatory disease. Here, three selective small-molecule compounds are shown to inhibit RIP3 kinase-dependent necroptosis, although their therapeutic value is undermined by a surprising, concentration-dependent induction of apoptosis. These compounds interact with RIP3 to activate caspase 8 (Casp8) via RHIM-driven recruitment of RIP1 (RIPK1) to assemble a Casp8-FADD-cFLIP complex completely independent of pronecrotic kinase activities and MLKL. RIP3 kinase-dead D161N mutant induces spontaneous apoptosis independent of compound, whereas D161G, D143N, and K51A mutants, like wild-type, only trigger apoptosis when compound is present. Accordingly, RIP3-K51A mutant mice (Rip3(K51A/K51A)) are viable and fertile, in stark contrast to the perinatal lethality of Rip3(D161N/D161N) mice. RIP3 therefore holds both necroptosis and apoptosis in balance through a Ripoptosome-like platform. This work highlights a common mechanism unveiling RHIM-driven apoptosis by therapeutic or genetic perturbation of RIP3.

Programmed necrosis and necroptosis signalling

In recent years, the paradigm of cell death regulation has changed. Nowadays, not only apoptosis but also several forms of necrosis (e.g. necroptosis) are considered to be regulated. The central roles of receptor-interacting serine/threonine protein kinase1 (RIPK1), RIPK3, and mixed-lineage kinase domain-like protein, and the molecular signalling platforms in which these molecules participate, are being intensively studied. In particular, the role of RIPK1, being both a kinase and a scaffold molecule, in different cell death regulatory complexes is of great relevance for the field. This minireview aims to introduce the emerging and dynamic field of necroptosis to the reader, with a specific focus on intracellular signalling pathways involved in this process.

Inhibition of Theiler's virus-induced apoptosis in infected murine macrophages results in necroptosis

In mice Theiler's murine encephalomyelitis virus (TMEV) persists in macrophages that eventually undergo apoptosis. TMEV infection of macrophages in culture induces apoptosis through the intrinsic pathway, restricting virus yields. We show that inhibition of TMEV-induced apoptosis leads to phosphorylation of receptor interacting protein 1 (RIP1), localization of RIP1 and RIP3 to mitochondria, ROS production independent of MAPK activation and programmed necrosis (necroptosis). Blocking both apoptosis and necroptosis restored virus yields.

Direct activation of RIP3/MLKL-dependent necrosis by herpes simplex virus 1 (HSV-1) protein ICP6 triggers host antiviral defense

The receptor-interacting kinase-3 (RIP3) and its downstream substrate mixed lineage kinase domain-like protein (MLKL) have emerged as the key cellular components in programmed necrotic cell death. Receptors for the cytokines of tumor necrosis factor (TNF) family and Toll-like receptors (TLR) 3 and 4 are able to activate RIP3 through receptor-interacting kinase-1 and Toll/IL-1 receptor domain-containing adapter inducing IFN-β, respectively. This form of cell death has been implicated in the host-defense system. However, the molecular mechanisms that drive the activation of RIP3 by a variety of pathogens, other than the above-mentioned receptors, are largely unknown. Here, we report that human herpes simplex virus 1 (HSV-1) infection triggers RIP3-dependent necrosis. This process requires MLKL but is independent of TNF receptor, TLR3, cylindromatosis, and host RIP homotypic interaction motif-containing protein DNA-dependent activator of IFN regulatory factor. After HSV-1 infection, the viral ribonucleotide reductase large subunit (ICP6) interacts with RIP3. The formation of the ICP6-RIP3 complex requires the RHIM domains of both proteins. An HSV-1 ICP6 deletion mutant failed to cause effective necrosis of HSV-1-infected cells. Furthermore, ectopic expression of ICP6, but not RHIM mutant ICP6, directly activated RIP3/MLKL-mediated necrosis. Mice lacking RIP3 exhibited severely impaired control of HSV-1 replication and pathogenesis. Therefore, this study reveals a previously uncharacterized host antipathogen mechanism.

Role of Bax in death of uninfected retinal cells during murine cytomegalovirus retinitis

PURPOSE

Extensive death of uninfected bystander neuronal cells is an important component of the pathogenesis of cytomegalovirus retinitis. Our previous results have shown that caspase 3-dependent and -independent pathways are involved in death of uninfected bystander cells during murine cytomegalovirus (MCMV) retinitis and also that Bcl-2, an important inhibitor of apoptosis via the Bax-mediated mitochondrial pathway, is downregulated during this process. The purpose of this study was to determine whether Bax-mediated mitochondrial damage has a significant role in the death of uninfected retinal cells.

METHODS

BALB/c mice, Bax(-/-) mice, or Bax(+/+) mice were immunosuppressed with methylprednisolone and infected with 5 × 10(3) plaque-forming units (PFU) of the K181 strain of MCMV via the supraciliary route. Injected eyes were analyzed by plaque assay, electron microscopy, hematoxylin and eosin (H&E) staining, TUNEL assay, Western blot (for caspase 3, caspase 12, Bax, receptor interacting protein-1 [RIP1] and receptor interacting protein-3 [RIP3]), as well as immunohistochemical staining for MCMV early antigen and cleaved caspase 3.

RESULTS

Significantly more Bax was detected in mitochondrial fractions of MCMV-infected eyes than in mitochondrial fractions of mock-infected control eyes. Furthermore, the level of cleaved caspase 3 was significantly lower in MCMV-infected Bax(-/-) eyes than in MCMV-infected Bax(+/+) eyes. However, more caspase 3-independent cell death of uninfected bystander retinal cells and more cleaved RIP1 were observed in Bax(-/-) than in Bax(+/+) eyes.

CONCLUSIONS

During MCMV retinitis, Bax is activated and has an important role in death of uninfected bystander retinal cells by caspase 3-dependent apoptosis. Although the exact mechanism remains to be deciphered, active Bax might also prevent death of some types of uninfected retinal cells by a caspase 3-independent pathway.

RIPK1 can function as an inhibitor rather than an initiator of RIPK3-dependent necroptosis

Tumour necrosis factor and lipopolysaccharide can promote a regulated form of necrosis, called necroptosis, upon inhibition of caspase activity in cells expressing receptor-interacting serine/threonine kinase (RIPK)3. Because inhibitors of RIPK1 kinase activity such as necrostatin-1 block necroptosis in many settings, RIPK1 is thought to be required for activation of RIPK3, leading to necroptosis. However, here we show that, although necrostatin potently inhibited tumour necrosis factor-induced, lipopolysaccharide-induced and polyIC-induced necroptosis, RIPK1 knockdown unexpectedly potentiated this process. In contrast, RIPK3 knockdown potently suppressed necroptosis under the same conditions. Significantly, necrostatin failed to block necroptosis in the absence of RIPK1, indicating that its ability to suppress necroptosis was indeed RIPK1-dependent. These data argue that RIPK1 is dispensable for necroptosis and can act as an inhibitor of this process. Our observations also suggest that necrostatin enhances the inhibitory effects of RIPK1 on necroptosis, as opposed to blocking its participation in this process.