Viral MLKL Homologs Subvert Necroptotic Cell Death by Sequestering Cellular RIPK3

Genetic analysis of mixed lineage kinase domain-like pseudokinase (MLKL) in oral squamous cell carcinoma: A comparative evaluation between young and old patients

BACKGROUND

Oral squamous cell carcinoma (OSCC) is characterized by dysregulation of multiple cell signaling pathways, including the necroptotic pathway. Recently, the incidence of OSCC is increasing among the young population (below the age of 40 years). These patients exhibit differences in the pathobiological characteristics and treatment response compared to the older cohorts. There is a notable lack of research exploring the role of necroptotic proteins in younger OSCC patients.

AIM

To investigate the expression of Mixed Lineage Kinase domain Like Pseudokinase (MLKL), a key necroptotic protein, in young and old patients with OSCC.

METHODOLOGY

The study included sixty histopathologically confirmed cases of OSCC, categorized into two groups; Group I - 30 patients aged > 40 years and Group II - 30 patients aged ≤ 40 years. Each of these groups consisted of 10 cases each of well differentiated, moderately differentiated and poorly differentiated OSCC. The samples were evaluated for the MLKL gene expression using Real time PCR and the results were analyzed using the 2-ΔΔCT method.

RESULTS

The real-time PCR analysis showed a 31 % decrease in MLKL gene expression in the younger age group (Group II) compared to the older group. A decrease of 40 % in WDSCC, 67 % in MDSCC, and 38 % in PDSCC was observed in the younger group compared to the older age group.

CONCLUSION

Our findings suggest age-related differences in necroptotic cell death regulation through MLKL, with decreased MLKL expression observed in younger patients compared to older patients. Modulating necroptotic cell death pathways in OSCC can promote switching between different cell death pathways and provide effective therapeutic strategies.

Evolutionary and functional analyses reveal a role for the RHIM in tuning RIPK3 activity across vertebrates

Receptor interacting protein kinases (RIPK) RIPK1 and RIPK3 play important roles in diverse innate immune pathways. Despite this, some RIPK1/3-associated proteins are absent in specific vertebrate lineages, suggesting that some RIPK1/3 functions are conserved, while others are more evolutionarily labile. Here, we perform comparative evolutionary analyses of RIPK1-5 and associated proteins in vertebrates to identify lineage-specific rapid evolution of RIPK3 and RIPK1 and recurrent loss of RIPK3-associated proteins. Despite this, diverse vertebrate RIPK3 proteins are able to activate NF-κB and cell death in human cells. Additional analyses revealed a striking conservation of the RIP homotypic interaction motif (RHIM) in RIPK3, as well as other human RHIM-containing proteins. Interestingly, diversity in the RIPK3 RHIM can tune activation of NF-κB while retaining the ability to activate cell death. Altogether, these data suggest that NF-κB activation is a core, conserved function of RIPK3, and the RHIM can tailor RIPK3 function to specific needs within and between species.

Oligomerised RIPK1 is the main core component of the CD95 necrosome

The necrosome is the key macromolecular signaling platform initiating necroptosis, i.e., a RIPK1/RIPK3-dependent program of cell death with an important role in the control of inflammation in multicellular organisms. However, the composition and structure of the necrosome remain incompletely understood. Here we use biochemical assays, quantitative mass spectrometry, and AlphaFold modeling to decipher the composition and derive a structural model of the CD95L/BV6-induced necrosome. We identify RIPK1 as the central component of the necrosome, forming the core of this complex. In addition, AlphaFold modeling provides insights into the structural mechanisms underlying RIPK1 oligomerization, highlighting the critical role of type-II interactions between the Death Domains (DDs) of FADD and RIPK1 in the assembly of RIPK1-mediated complexes. The role of type-II DD interactions in necroptosis induction is further validated through structure-guided site-directed mutagenesis. Our findings could be useful for the pharmacological targeting of the necroptosis network to treat diseases associated with dysregulated cell death and inflammation.

Bat adaptations in inflammation and cell death regulation contribute to viral tolerance

Bats are reservoirs for multiple viruses, some of which are known to cause global disease outbreaks. Virus spillovers from bats have been implicated in zoonotic transmission. Some bat species can tolerate viral infections, such as infections with coronaviruses and paramyxoviruses, better than humans and with less clinical consequences. Bat species are speculated to have evolved alongside these viral pathogens, and adaptations within the bat immune system are considered to be associated with viral tolerance. Inflammation and cell death in response to zoonotic virus infections prime human immunopathology. Unlike humans, bats have evolved adaptations to mitigate virus infection-induced inflammation. Inflammatory cell death pathways such as necroptosis and pyroptosis are associated with immunopathology during virus infections, but their regulation in bats remains understudied. This review focuses on the regulation of inflammation and cell death pathways in bats. We also provide a perspective on the possible contribution of cell death-regulating proteins, such as caspases and gasdermins, in modulating tissue damage and inflammation in bats. Understanding the role of these adaptations in bat immune responses can provide valuable insights for managing future disease outbreaks, addressing human disease severity, and improving pandemic preparedness.

Lytic coelomocyte death is tuned by cleavage but not phosphorylation of MLKL in echinoderms

Lytic cell death including necroptosis and pyroptosis is induced by mixed lineage kinase domain-like protein (MLKL) phosphorylation and inflammatory caspase specific cleavage Gasdermins in higher mammals, respectively. In this study, we identified a novel MLKL homolog containing a tetrapeptide recognition motif (14-LVAD-17) of inflammatory caspase from Apostichopus japonicus,which was absent of Gasdermins member by genome screening. Functional analysis revealed that AjMLKL was involved in the regulation of Vibrio splendidus AJ01 infection induced lytic coelomocyte death in a cleavage-dependent manner, but not through RIPK3-dependent phosphorylation as mammals. Mechanistically, the activated form of cysteine-aspartic specific proteases-1 (AjCASP-1) bound to the tetrapeptide site of AjMLKL and cleaved it at Asp17. Cleaved AjMLKL18-491 displayed higher binding affinities towards phosphatidylinositol phosphate and cardiolipin compared to those of un-cleaved form. In addition, cleaved AjMLKL18-491 exerted stronger ability in disrupting the membrane integrity of liposome. More importantly, AjMLKL18-491 caused a large non-selective ionic coelomocyte pore and could directly kill the invasive AJ01. Moreover, activation of inflammatory AjCASP-1 was further found to be dependent on forming an inflammasome-like complex via CASc domain of AjCASP-1 and the N-terminal Ig domains of internalized AjNLRC4. All our results proved first evidence that lytic cell death was activated through MLKL cleavage, not MLKL phosphorylation in echinoderm, which offered insights into the functional, evolutionary mechanisms of lytic cell death in invertebrates.

Protein shapeshifting in necroptotic cell death signaling

Necroptosis is a mode of programmed cell death executed by the mixed lineage kinase domain-like (MLKL) pseudokinase following its activation by the upstream receptor-interacting protein kinase-3 (RIPK3), subsequent to activation of death, Toll-like, and pathogen receptors. The pathway originates in innate immunity, although interest has surged in therapeutically targeting necroptosis owing to its dysregulation in inflammatory diseases. Here, we explore how protein conformation and higher order assembly of the pathway effectors - Z-DNA-binding protein-1 (ZBP1), RIPK1, RIPK3, and MLKL - can be modulated by post-translational modifications, such as phosphorylation, ubiquitylation, and lipidation, and intermolecular interactions to tune activities and modulate necroptotic signaling flux. As molecular level knowledge of cell death signaling grows, we anticipate targeting the conformations of key necrosomal effector proteins will emerge as new avenues for drug development.

Evolutionary and functional analyses reveal a role for the RHIM in tuning RIPK3 activity across vertebrates

Receptor interacting protein kinases (RIPK) RIPK1 and RIPK3 play important roles in diverse innate immune pathways. Despite this, some RIPK1/3-associated proteins are absent in specific vertebrate lineages, suggesting that some RIPK1/3 functions are conserved while others are more evolutionarily labile. Here, we perform comparative evolutionary analyses of RIPK1-5 and associated proteins in vertebrates to identify lineage-specific rapid evolution of RIPK3 and RIPK1 and recurrent loss of RIPK3-associated proteins. Despite this, diverse vertebrate RIPK3 proteins are able to activate NF-κB and cell death in human cells. Additional analyses revealed a striking conservation of the RIP homotypic interaction motif (RHIM) in RIPK3, as well as other human RHIM-containing proteins. Interestingly, diversity in the RIPK3 RHIM can tune activation of NF-κB while retaining the ability to activate cell death. Altogether, these data suggest that NF-κB activation is a core, conserved function of RIPK3, and the RHIM can tailor RIPK3 function to specific needs within and between species.

Targeting necroptosis in MCF-7 breast cancer cells: In Silico insights into 8,12-dimethoxysanguinarine from Eomecon Chionantha through molecular docking, dynamics, DFT, and MEP studies

Breast cancer remains a significant challenge in oncology, highlighting the need for alternative therapeutic strategies that target necroptosis to overcome resistance to conventional therapies. Recent investigations into natural compounds have identified 8,12-dimethoxysanguinarine (SG-A) from Eomecon chionantha as a potential necroptosis inducer. This study presents the first computational exploration of SG-A interactions with key necroptotic proteins-RIPK1, RIPK3, and MLKL-through molecular docking, molecular dynamics (MD), density functional theory (DFT), and molecular electrostatic potential (MEP) analyses. Molecular docking revealed that SG-A exhibited a stronger affinity for MLKL (-9.40 kcal/mol) compared to the co-crystallized ligand (-6.29 kcal/mol), while its affinity for RIPK1 (-6.37 kcal/mol) and RIPK3 (-7.01 kcal/mol) was lower. MD simulations further demonstrated the stability of SG-A within the MLKL site, with RMSD values stabilizing between 1.4 and 3.3 Å over 300 ns, indicating a consistent interaction pattern. RMSF analysis indicated the preservation of protein backbone flexibility, with average fluctuations under 1.7 Å. The radius of gyration (Rg) results indicated a consistent value of ~15.3 Å across systems, confirming the role of SG-A in maintaining protein integrity. Notably, SG-A maintains two critical H-bonds within the active site of MLKL, reinforcing the stability of the interaction. Principal component analysis (PCA) indicated a significant reduction in MLKL's conformational space upon SG-A binding, implying enhanced stabilization. Dynamic cross-correlation map (DCCM) analysis further revealed that SG-A induced highly correlated motions, reducing internal fluctuations within MLKL compared to the co-crystallized ligand. MM-PBSA revealed the enhanced binding efficacy of SG-A, with a significant binding free energy of -31.03 ± 0.16 kcal/mol against MLKL, surpassing that of the control (23.96 ± 0.11 kcal/mol). In addition, the individual residue contribution analysis highlighted key interactions, with ARG149 showing a significant contribution (-176.24 kcal/mol) in the MLKL-SG-A complex. DFT and MEP studies corroborated these findings, revealing that the electronic structure of SG-A is conducive to stable binding interactions, characterized by a narrow band gap (~0.16 units) and distinct electrostatic potential favourable for necroptosis induction. In conclusion, SG-A has emerged as a compelling inducer of necroptosis for breast cancer therapy, warranting further experimental validation to fully realize its therapeutic potential.

An Evolutionary Framework Exploiting Virologs and Their Host Origins to Inform Poxvirus Protein Functions

Poxviruses represent evolutionary successful infectious agents. As a family, poxviruses can infect a wide variety of species including humans, fish, and insects. While many other viruses are species-specific, an individual poxvirus species is often capable of infecting diverse hosts and cell types. For example, the prototypical poxvirus, vaccinia, is well known to infect numerous human cell types but can also infect cells from divergent hosts like frog neurons. Notably, poxvirus infections result in both detrimental human and animal diseases. The most infamous disease linked to a poxvirus is smallpox caused by variola virus. Poxviruses are large double-stranded DNA viruses, which uniquely replicate in the cytoplasm of cells. The model poxvirus genome encodes ~200 nonoverlapping protein-coding open reading frames (ORFs). Poxvirus gene products impact various biological processes like the production of virus particles, the host range of infectivity, and disease pathogenesis. In addition, poxviruses and their gene products have biomedical application with several species commonly engineered for use as vaccines and oncolytic virotherapy. Nevertheless, we still have an incomplete understanding of the functions associated with many poxvirus genes. In this chapter, we outline evolutionary insights that can complement ongoing studies of poxvirus gene functions and biology, which may serve to elucidate new molecular activities linked to this biomedically relevant class of viruses.

Divergent roles of RIPK3 and MLKL in high-fat diet-induced obesity and MAFLD in mice

Cell death frequently occurs in the pathogenesis of obesity and metabolic dysfunction-associated fatty liver disease (MAFLD). However, the exact contribution of core cell death machinery to disease manifestations remains ill-defined. Here, we show via the direct comparison of mice genetically deficient in the essential necroptotic regulators, receptor-interacting protein kinase-3 (RIPK3) and mixed lineage kinase domain-like (MLKL), as well as mice lacking apoptotic caspase-8 in myeloid cells combined with RIPK3 loss, that RIPK3/caspase-8 signaling regulates macrophage inflammatory responses and drives adipose tissue inflammation and MAFLD upon high-fat diet feeding. In contrast, MLKL, divergent to RIPK3, contributes to both obesity and MAFLD in a manner largely independent of inflammation. We also uncover that MLKL regulates the expression of molecules involved in lipid uptake, transport, and metabolism, and congruent with this, we discover a shift in the hepatic lipidome upon MLKL deletion. Collectively, these findings highlight MLKL as an attractive therapeutic target to combat the growing obesity pandemic and metabolic disease.

MLKL deficiency elevates testosterone production in male mice independently of necroptotic functions

Mixed lineage kinase domain-like (MLKL) is a pseudokinase, best known for its role as the terminal effector of the necroptotic cell death pathway. MLKL-mediated necroptosis has long been linked to various age-related pathologies including neurodegeneration, atherosclerosis and male reproductive decline, however many of these attributions remain controversial. Here, we investigated the role of MLKL and necroptosis in the adult mouse testis: an organ divided into sperm-producing seminiferous tubules and the surrounding testosterone-producing interstitium. We find that sperm-producing cells within seminiferous tubules lack expression of key necroptotic mediators and thus are resistant to a pro-necroptotic challenge. By comparison, coordinated expression of the necroptotic pathway occurs in the testicular interstitium, rendering cells within this compartment, especially the lysozyme-positive macrophages, vulnerable to necroptotic cell death. We also uncover a non-necroptotic role for MLKL in regulating testosterone levels. Thus, MLKL serves two roles in the mouse testes - one involving the canonical response of macrophages to necroptotic insult, and the other a non-canonical function in male reproductive hormone control.

Death at a funeral: Activation of the dead enzyme, MLKL, to kill cells by necroptosis

Necroptosis is a lytic form of programmed cell death implicated in inflammatory pathologies, leading to intense interest in the underlying mechanisms and therapeutic prospects. Here, we review our current structural understanding of how the terminal executioner of the pathway, the dead kinase, mixed lineage kinase domain-like (MLKL), is converted from a dormant to killer form by the upstream regulatory kinase, RIPK3. RIPK3-mediated phosphorylation of MLKL's pseudokinase domain toggles a molecular switch that induces dissociation from a cytoplasmic platform, assembly of MLKL oligomers, and trafficking to the plasma membrane, where activated MLKL accumulates and permeabilises the lipid bilayer to induce cell death. We highlight gaps in mechanistic knowledge of MLKL's activation, how mechanisms diverge between species, and the power of modelling in advancing structural insights.

Inhibitors identify an auxiliary role for mTOR signalling in necroptosis execution downstream of MLKL activation

Necroptosis is a lytic and pro-inflammatory form of programmed cell death executed by the terminal effector, the MLKL (mixed lineage kinase domain-like) pseudokinase. Downstream of death and Toll-like receptor stimulation, MLKL is trafficked to the plasma membrane via the Golgi-, actin- and microtubule-machinery, where activated MLKL accumulates until a critical lytic threshold is exceeded and cell death ensues. Mechanistically, MLKL's lytic function relies on disengagement of the N-terminal membrane-permeabilising four-helix bundle domain from the central autoinhibitory brace helix: a process that can be experimentally mimicked by introducing the R30E MLKL mutation to induce stimulus-independent cell death. Here, we screened a library of 429 kinase inhibitors for their capacity to block R30E MLKL-mediated cell death, to identify co-effectors in the terminal steps of necroptotic signalling. We identified 13 compounds - ABT-578, AR-A014418, AZD1480, AZD5363, Idelalisib, Ipatasertib, LJI308, PHA-793887, Rapamycin, Ridaforolimus, SMI-4a, Temsirolimus and Tideglusib - each of which inhibits mammalian target of rapamycin (mTOR) signalling or regulators thereof, and blocked constitutive cell death executed by R30E MLKL. Our study implicates mTOR signalling as an auxiliary factor in promoting the transport of activated MLKL oligomers to the plasma membrane, where they accumulate into hotspots that permeabilise the lipid bilayer to cause cell death.

Necroptosis in immunity, tissue homeostasis, and cancer

Immune and tissue homeostasis is achieved through balancing signals that regulate cell survival, proliferation, and cell death. Recent studies indicate that certain cell death programs can stimulate inflammation and are often referred as 'immunogenic cell death' (ICD). ICD is a double-edged sword that can confer protection against pathogen infection but also cause tissue damage. Necroptosis is a key ICD module that has been shown to participate in host defense against pathogen infection, tissue homeostasis, and cancer response to immunotherapy. Here, we will review recent findings on the regulation of necroptosis signaling and its role in pathogen infection, tissue homeostasis, and cancer.

An immunohistochemical atlas of necroptotic pathway expression

Necroptosis is a lytic form of regulated cell death reported to contribute to inflammatory diseases of the gut, skin and lung, as well as ischemic-reperfusion injuries of the kidney, heart and brain. However, precise identification of the cells and tissues that undergo necroptotic cell death in vivo has proven challenging in the absence of robust protocols for immunohistochemical detection. Here, we provide automated immunohistochemistry protocols to detect core necroptosis regulators - Caspase-8, RIPK1, RIPK3 and MLKL - in formalin-fixed mouse and human tissues. We observed surprising heterogeneity in protein expression within tissues, whereby short-lived immune barrier cells were replete with necroptotic effectors, whereas long-lived cells lacked RIPK3 or MLKL expression. Local changes in the expression of necroptotic effectors occurred in response to insults such as inflammation, dysbiosis or immune challenge, consistent with necroptosis being dysregulated in disease contexts. These methods will facilitate the precise localisation and evaluation of necroptotic signaling in vivo.

Rotavirus non-structural protein 4 usurps host cellular RIPK1-RIPK3 complex to induce MLKL-dependent necroptotic cell death

The dynamic interface between invading viral pathogens and programmed cell death (PCD) of the host is a finely regulated process. Host cellular demise at the end of the viral life cycle ensures the release of progeny virions to initiate new infection cycles. Rotavirus (RV), a diarrheagenic virus with double-stranded RNA genome, has been reported to trigger different types of PCD such as apoptosis and pyroptosis in a highly regulated way to successfully disseminate progeny virions. Recently our lab also showed that induction of MLKL-driven programmed necroptosis by RV. However, the host cellular machinery involved in RV-induced necroptosis and the upstream viral trigger responsible for it remained unaddressed. In the present study, the signalling upstream of MLKL-driven necroptosis has been delineated where the involvement of Receptor interacting serine/threonine kinase 3 (RIPK3) and 1 (RIPK1) from the host side and RV non-structural protein 4 (NSP4) as the viral trigger for necroptosis has been shown. Interestingly, RV-NSP4 was found to be an integral component of the necrosome complex by interacting with RIPK1, thereby bypassing the requirement of RIPK1 kinase activity. Subsequently, NSP4-driven elevated cytosolic Ca2+ concentration and Ca2+-binding to NSP4 lead further to RHIM domain-dependent RIPK1-RIPK3 interaction, RIPK3-dependent MLKL phosphorylation, and eventual necroptosis. Overall, this study presents the interplay between RV-NSP4 and the host cellular necrosome complex to induce necroptotic death of host cells.

Gasdermin and MLKL necrotic cell death effectors: Signaling and diseases

Diverse inflammatory conditions, from infections to autoimmune disease, are often associated with cellular damage and death. Apoptotic cell death has evolved to minimize its inflammatory potential. By contrast, necrotic cell death via necroptosis and pyroptosis-driven by membrane-damaging MLKL and gasdermins, respectively-can both initiate and propagate inflammatory responses. In this review, we provide insights into the function and regulation of MLKL and gasdermin necrotic effector proteins and drivers of plasma membrane rupture. We evaluate genetic evidence that MLKL- and gasdermin-driven necrosis may either provide protection against, or contribute to, disease states in a context-dependent manner. These cumulative insights using gene-targeted mice underscore the necessity for future research examining pyroptotic and necroptotic cell death in human tissue, as a basis for developing specific necrotic inhibitors with the potential to benefit a spectrum of pathological conditions.

Necroptosis does not drive disease pathogenesis in a mouse infective model of SARS-CoV-2 in vivo

Necroptosis, a type of lytic cell death executed by the pseudokinase Mixed Lineage Kinase Domain-Like (MLKL) has been implicated in the detrimental inflammation caused by SARS-CoV-2 infection. We minimally and extensively passaged a single clinical SARS-CoV-2 isolate to create models of mild and severe disease in mice allowing us to dissect the role of necroptosis in SARS-CoV-2 disease pathogenesis. We infected wild-type and MLKL-deficient mice and found no significant differences in viral loads or lung pathology. In our model of severe COVID-19, MLKL-deficiency did not alter the host response, ameliorate weight loss, diminish systemic pro-inflammatory cytokines levels, or prevent lethality in aged animals. Our in vivo models indicate that necroptosis is dispensable in the pathogenesis of mild and severe COVID-19.

Control of Cell Death in Health and Disease

Apoptosis, necroptosis, and pyroptosis are genetically programmed cell death mechanisms that eliminate obsolete, damaged, infected, and self-reactive cells. Apoptosis fragments cells in a manner that limits immune cell activation, whereas the lytic death programs of necroptosis and pyroptosis release proinflammatory intracellular contents. Apoptosis fine-tunes tissue architecture during mammalian development, promotes tissue homeostasis, and is crucial for averting cancer and autoimmunity. All three cell death mechanisms are deployed to thwart the spread of pathogens. Disabling regulators of cell death signaling in mice has revealed how excessive cell death can fuel acute or chronic inflammation. Here we review strategies for modulating cell death in the context of disease. For example, BCL-2 inhibitor venetoclax, an inducer of apoptosis, is approved for the treatment of certain hematologic malignancies. By contrast, inhibition of RIPK1, NLRP3, GSDMD, or NINJ1 to limit proinflammatory cell death and/or the release of large proinflammatory molecules from dying cells may benefit patients with inflammatory diseases.

Cell death

Cell death supports morphogenesis during development and homeostasis after birth by removing damaged or obsolete cells. It also curtails the spread of pathogens by eliminating infected cells. Cell death can be induced by the genetically programmed suicide mechanisms of apoptosis, necroptosis, and pyroptosis, or it can be a consequence of dysregulated metabolism, as in ferroptosis. Here, we review the signaling mechanisms underlying each cell-death pathway, discuss how impaired or excessive activation of the distinct cell-death processes can promote disease, and highlight existing and potential therapies for redressing imbalances in cell death in cancer and other diseases.

Type I Interferon: Monkeypox/Mpox Viruses Achilles Heel?

Poxviruses are notorious for having acquired/evolved numerous genes to counteract host innate immunity. Chordopoxviruses have acquired/evolved at least three different inhibitors of host necroptotic death: E3, which blocks ZBP1-dependent necroptotic cell death, and vIRD and vMLKL that inhibit necroptosis downstream of initial cell death signaling. While this suggests the importance of the necroptotic cell death pathway in inhibiting chordopoxvirus replication, several chordopoxviruses have lost one or more of these inhibitory functions. Monkeypox/mpox virus (MPXV) has lost a portion of the N-terminus of its E3 homologue. The N-terminus of the vaccinia virus E3 homologue serves to inhibit activation of the interferon-inducible antiviral protein, ZBP1. This likely makes MPXV unique among the orthopoxviruses in being sensitive to interferon (IFN) treatment in many mammals, including humans, which encode a complete necroptotic cell death pathway. Thus, IFN sensitivity may be the Achille's Heel for viruses like MPXV that cannot fully inhibit IFN-inducible, ZBP1-dependent antiviral pathways.

β1 integrin signaling governs necroptosis via the chromatin-remodeling factor CHD4

Fibrosis, characterized by sustained activation of myofibroblasts and excessive extracellular matrix (ECM) deposition, is known to be associated with chronic inflammation. Receptor-interacting protein kinase 3 (RIPK3), the central kinase of necroptosis signaling, is upregulated in fibrosis and contributes to tumor necrosis factor (TNF)-mediated inflammation. In bile-duct-ligation-induced liver fibrosis, we found that myofibroblasts are the major cell type expressing RIPK3. Genetic ablation of β1 integrin, the major profibrotic ECM receptor in fibroblasts, not only abolished ECM fibrillogenesis but also blunted RIPK3 expression via a mechanism mediated by the chromatin-remodeling factor chromodomain helicase DNA-binding protein 4 (CHD4). While the function of CHD4 has been conventionally linked to the nucleosome-remodeling deacetylase (NuRD) and CHD4-ADNP-HP1(ChAHP) complexes, we found that CHD4 potently repressed a set of genes, including Ripk3, with high locus specificity but independent of either the NuRD or the ChAHP complex. Thus, our data uncover that β1 integrin intrinsically links fibrotic signaling to RIPK3-driven inflammation via a novel mode of action of CHD4.

Phosphorylation-dependent pseudokinase domain dimerization drives full-length MLKL oligomerization

The necroptosis pathway is a lytic, pro-inflammatory mode of cell death that is widely implicated in human disease, including renal, pulmonary, gut and skin inflammatory pathologies. The precise mechanism of the terminal steps in the pathway, where the RIPK3 kinase phosphorylates and triggers a conformation change and oligomerization of the terminal pathway effector, MLKL, are only emerging. Here, we structurally identify RIPK3-mediated phosphorylation of the human MLKL activation loop as a cue for MLKL pseudokinase domain dimerization. MLKL pseudokinase domain dimerization subsequently drives formation of elongated homotetramers. Negative stain electron microscopy and modelling support nucleation of the MLKL tetramer assembly by a central coiled coil formed by the extended, ~80 Å brace helix that connects the pseudokinase and executioner four-helix bundle domains. Mutational data assert MLKL tetramerization as an essential prerequisite step to enable the release and reorganization of four-helix bundle domains for membrane permeabilization and cell death.

A common human MLKL polymorphism confers resistance to negative regulation by phosphorylation

Across the globe, 2-3% of humans carry the p.Ser132Pro single nucleotide polymorphism in MLKL, the terminal effector protein of the inflammatory form of programmed cell death, necroptosis. Here we show that this substitution confers a gain in necroptotic function in human cells, with more rapid accumulation of activated MLKLS132P in biological membranes and MLKLS132P overriding pharmacological and endogenous inhibition of MLKL. In mouse cells, the equivalent Mlkl S131P mutation confers a gene dosage dependent reduction in sensitivity to TNF-induced necroptosis in both hematopoietic and non-hematopoietic cells, but enhanced sensitivity to IFN-β induced death in non-hematopoietic cells. In vivo, MlklS131P homozygosity reduces the capacity to clear Salmonella from major organs and retards recovery of hematopoietic stem cells. Thus, by dysregulating necroptosis, the S131P substitution impairs the return to homeostasis after systemic challenge. Present day carriers of the MLKL S132P polymorphism may be the key to understanding how MLKL and necroptosis modulate the progression of complex polygenic human disease.

Virologs, viral mimicry, and virocell metabolism: the expanding scale of cellular functions encoded in the complex genomes of giant viruses

The phylum Nucleocytoviricota includes the largest and most complex viruses known. These "giant viruses" have a long evolutionary history that dates back to the early diversification of eukaryotes, and over time they have evolved elaborate strategies for manipulating the physiology of their hosts during infection. One of the most captivating of these mechanisms involves the use of genes acquired from the host-referred to here as viral homologs or "virologs"-as a means of promoting viral propagation. The best-known examples of these are involved in mimicry, in which viral machinery "imitates" immunomodulatory elements in the vertebrate defense system. But recent findings have highlighted a vast and rapidly expanding array of other virologs that include many genes not typically found in viruses, such as those involved in translation, central carbon metabolism, cytoskeletal structure, nutrient transport, vesicular trafficking, and light harvesting. Unraveling the roles of virologs during infection as well as the evolutionary pathways through which complex functional repertoires are acquired by viruses are important frontiers at the forefront of giant virus research.

Oncolytic virotherapy: basic principles, recent advances and future directions

Oncolytic viruses (OVs) have attracted growing awareness in the twenty-first century, as they are generally considered to have direct oncolysis and cancer immune effects. With the progress in genetic engineering technology, OVs have been adopted as versatile platforms for developing novel antitumor strategies, used alone or in combination with other therapies. Recent studies have yielded eye-catching results that delineate the promising clinical outcomes that OVs would bring about in the future. In this review, we summarized the basic principles of OVs in terms of their classifications, as well as the recent advances in OV-modification strategies based on their characteristics, biofunctions, and cancer hallmarks. Candidate OVs are expected to be designed as "qualified soldiers" first by improving target fidelity and safety, and then equipped with "cold weapons" for a proper cytocidal effect, "hot weapons" capable of activating cancer immunotherapy, or "auxiliary weapons" by harnessing tactics such as anti-angiogenesis, reversed metabolic reprogramming and decomposing extracellular matrix around tumors. Combinations with other cancer therapeutic agents have also been elaborated to show encouraging antitumor effects. Robust results from clinical trials using OV as a treatment congruously suggested its significance in future application directions and challenges in developing OVs as novel weapons for tactical decisions in cancer treatment.

Norovirus MLKL-like protein initiates cell death to induce viral egress

Non-enveloped viruses require cell lysis to release new virions from infected cells, suggesting that these viruses require mechanisms to induce cell death. Noroviruses are one such group of viruses, but there is no known mechanism that causes norovirus infection-triggered cell death and lysis1-3. Here we identify a molecular mechanism of norovirus-induced cell death. We found that the norovirus-encoded NTPase NS3 contains an N-terminal four-helix bundle domain homologous to the membrane-disruption domain of the pseudokinase mixed lineage kinase domain-like (MLKL). NS3 has a mitochondrial localization signal and thus induces cell death by targeting mitochondria. Full-length NS3 and an N-terminal fragment of the protein bound the mitochondrial membrane lipid cardiolipin, permeabilized the mitochondrial membrane and induced mitochondrial dysfunction. Both the N-terminal region and the mitochondrial localization motif of NS3 were essential for cell death, viral egress from cells and viral replication in mice. These findings suggest that noroviruses have acquired a host MLKL-like pore-forming domain to facilitate viral egress by inducing mitochondrial dysfunction.

Norovirus MLKL-like pore forming protein initiates programed cell death for viral egress

Non-enveloped viruses require cell lysis to release new virions from infected cells, suggesting that these viruses require mechanisms to induce cell death. Noroviruses are one such group of viruses, but a mechanism of norovirus-infection triggered cell death and lysis are unknown. Here we have identified a molecular mechanism of norovirus-induced cell death. We found that the norovirus-encoded NTPase contains a N-terminal four helix bundle domain homologous to the pore forming domain of the pseudokinase Mixed Lineage Kinase Domain-Like (MLKL). Norovirus NTPase acquired a mitochondrial localization signal, thereby inducing cell death by targeting mitochondria. NTPase full length (NTPase-FL) and N-terminal fragment (NTPase-NT) bound mitochondrial membrane lipid cardiolipin, permeabilized mitochondrial membrane and induced mitochondrial dysfunction. Both the N-terminal region and the mitochondrial localization motif of NTPase were essential for cell death, virus egress from cells and virus replication in mice. These findings suggest that noroviruses stole a MLKL-like pore forming domain and co-opted it to facilitate viral egress by inducing mitochondrial dysfunction.

From (Tool)Bench to Bedside: The Potential of Necroptosis Inhibitors

Necroptosis is a regulated caspase-independent form of necrotic cell death that results in an inflammatory phenotype. This process contributes profoundly to the pathophysiology of numerous neurodegenerative, cardiovascular, infectious, malignant, and inflammatory diseases. Receptor-interacting protein kinase 1 (RIPK1), RIPK3, and the mixed lineage kinase domain-like protein (MLKL) pseudokinase have been identified as the key components of necroptosis signaling and are the most promising targets for therapeutic intervention. Here, we review recent developments in the field of small-molecule inhibitors of necroptosis signaling, provide guidelines for their use as chemical probes to study necroptosis, and assess the therapeutic challenges and opportunities of such inhibitors in the treatment of a range of clinical indications.

MLKL post-translational modifications: road signs to infection, inflammation and unknown destinations

Necroptosis is a caspase-independent modality of cell death that requires the activation of the executioner MLKL. In the last ten years the field gained a substantial amount of evidence regarding its involvement in host response to pathogens, TNF-induced inflammatory diseases as well as pathogen recognition receptors (PRR)-induced inflammation. However, there are still a lot of questions that remain unanswered. While it is clear that there are specific events needed to drive MLKL activation, substantial differences between human and mouse MLKL not only highlight different evolutionary pressure, but also provide potential insights on alternative modalities of activation. While in TNF-induced necroptosis it is clear the involvement of the RIPK3 mediated phosphorylation, it still remains to be understood how certain inflammatory in vivo phenotypes are not equally rescued by either RIPK3 or MLKL loss. Moreover, the plethora of different reported phosphorylation events on MLKL, even in cells that do not express RIPK3, suggest indeed that there is more to MLKL than RIPK3-mediated activation, not only in the execution of necroptosis but perhaps in other inflammatory conditions that include IFN response. The recent discovery of MLKL ubiquitination has highlighted a new checkpoint in the regulation of MLKL activation and the somewhat conflicting evidence reported certainly require some untangling. In this review we will highlight the recent findings on MLKL activation and involvement to pathogen response with a specific focus on MLKL post-translational modifications, in particular ubiquitination. This review will highlight the outstanding main questions that have risen from the last ten years of research, trying at the same time to propose potential avenues of research.

Dying in self-defence: a comparative overview of immunogenic cell death signalling in animals and plants

Host organisms utilise a range of genetically encoded cell death programmes in response to pathogen challenge. Host cell death can restrict pathogen proliferation by depleting their replicative niche and at the same time dying cells can alert neighbouring cells to prepare environmental conditions favouring future pathogen attacks. As expected, many pathogenic microbes have strategies to subvert host cell death to promote their virulence. The structural and lifestyle differences between animals and plants have been anticipated to shape very different host defence mechanisms. However, an emerging body of evidence indicates that several components of the host-pathogen interaction machinery are shared between the two major branches of eukaryotic life. Many proteins involved in cell death execution or cell death-associated immunity in plants and animals exert direct effects on endomembrane and loss of membrane integrity has been proposed to explain the potential immunogenicity of dying cells. In this review we aim to provide a comparative view on how cell death processes are linked to anti-microbial defence mechanisms in plants and animals and how pathogens interfere with these cell death programmes. In comparison to the several well-defined cell death programmes in animals, immunogenic cell death in plant defence is broadly defined as the hypersensitive response. Our comparative overview may help discerning whether specific types of immunogenic cell death exist in plants, and correspondingly, it may provide new hints for previously undiscovered cell death mechanism in animals.

The web of death: the expanding complexity of necroptotic signaling

The past decade has seen the emergence of the necroptosis programmed cell death pathway as an important contributor to the pathophysiology of myriad diseases. The receptor interacting protein kinase (RIPK)1 and RIPK3, and the pseudokinase executioner protein, mixed lineage kinase domain-like (MLKL), have grown to prominence as the core pathway components. Depending on cellular context, these proteins also serve as integrators of signals, such as post-translational modifications and protein or metabolite interactions, adding layers of complexity to pathway regulation. Here, we describe the emerging picture of the web of proteins that tune necroptotic signal transduction and how these events have diverged across species, presumably owing to selective pressures of pathogens upon the RIPK3-MLKL protein pair.

Reovirus Type 3 Dearing Variants Do Not Induce Necroptosis in RIPK3-Expressing Human Tumor Cell Lines

Reoviruses are used as oncolytic viruses to destroy tumor cells. The concomitant induction of anti-tumor immune responses enhances the efficacy of therapy in tumors with low amounts of immune infiltrates before treatment. The reoviruses should provoke immunogenic cell death (ICD) to stimulate a tumor cell-directed immune response. Necroptosis is considered a major form of ICD, and involves receptor-interacting protein kinase 1 (RIPK1), RIPK3 and phosphorylation of mixed-lineage kinase domain-like protein (MLKL). This leads to cell membrane disintegration and the release of damage-associated molecular patterns that can activate immune responses. Reovirus Type 3 Dearing (T3D) can induce necroptosis in mouse L929 fibroblast cells and mouse embryonic fibroblasts. Most human tumor cell lines have a defect in RIPK3 expression and consequently fail to induce necroptosis as measured by MLKL phosphorylation. We used the human colorectal adenocarcinoma HT29 cell line as a model to study necroptosis in human cells since this cell line has frequently been described in necroptosis-related studies. To stimulate MLKL phosphorylation and induce necroptosis, HT29 cells were treated with a cocktail consisting of TNFα, the SMAC mimetic BV6, and the caspase inhibitor Z-VAD-FMK. While this treatment induced necroptosis, three different reovirus T3D variants, i.e., the plasmid-based reverse genetics generated virus (T3D^K), the wild-type reovirus T3D isolate R124, and the junction adhesion molecule-A-independent reovirus mutant (jin-1) failed to induce necroptosis in HT29 cells. In contrast, these viruses induced MLKL phosphorylation in murine L929 cells, albeit with varying efficiencies. Our study shows that while reoviruses efficiently induce necroptosis in L929 cells, this is not a common phenotype in human cell lines. This study emphasizes the difficulties of translating the results of ICD studies from murine cells to human cells.

Role of necroptosis in kidney health and disease

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

Subversion of Programed Cell Death by Poxviruses

Poxviruses have been long regarded as potent inhibitors of apoptotic cell death. More recently, they have been shown to inhibit necroptotic cell death through two distinct strategies. These strategies involve either blocking virus sensing by the host pattern recognition receptor, ZBP1 (also called DAI) or by influencing receptor interacting protein kinase (RIPK)3 signal transduction by inhibition of activation of the executioner of necroptosis, mixed lineage kinase-like protein (MLKL). Vaccinia virus E3 specifically blocks ZBP1 → RIPK3 → MLKL necroptosis, leaving virus-infected cells susceptible to the TNF death-receptor signaling (e.g., TNFR1 → FADD → RIPK1 → RIPK3 → MLKL), and, potentially, TLR3 → TRIF → RIPK3 → MLKL necroptosis. While E3 restriction of necroptosis appears to be common to many poxviruses that infect vertebrate hosts, another modulatory strategy not observed in vaccinia or variola virus manifests through subversion of MLKL activation. Recently described viral mimics of MLKL in other chordopoxviruses inhibit all three modes of necroptotic cell death. As with inhibition of apoptosis, the evolution of potentially redundant viral mechanisms to inhibit programmed necroptotic cell death emphasizes the importance of this pathway in the arms race between pathogens and their hosts.

Endothelial Caspase-8 prevents fatal necroptotic hemorrhage caused by commensal bacteria

Caspase-8 transduces signals from death receptor ligands, such as tumor necrosis factor, to drive potent responses including inflammation, cell proliferation or cell death. This is a developmentally essential function because in utero deletion of endothelial Caspase-8 causes systemic circulatory collapse during embryogenesis. Whether endothelial Caspase-8 is also required for cardiovascular patency during adulthood was unknown. To address this question, we used an inducible Cre recombinase system to delete endothelial Casp8 in 6-week-old conditionally gene-targeted mice. Extensive whole body vascular gene targeting was confirmed, yet the dominant phenotype was fatal hemorrhagic lesions exclusively within the small intestine. The emergence of these intestinal lesions was not a maladaptive immune response to endothelial Caspase-8-deficiency, but instead relied upon aberrant Toll-like receptor sensing of microbial commensals and tumor necrosis factor receptor signaling. This lethal phenotype was prevented in compound mutant mice that lacked the necroptotic cell death effector, MLKL. Thus, distinct from its systemic role during embryogenesis, our data show that dysregulated microbial- and death receptor-signaling uniquely culminate in the adult mouse small intestine to unleash MLKL-dependent necroptotic hemorrhage after loss of endothelial Caspase-8. These data support a critical role for Caspase-8 in preserving gut vascular integrity in the face of microbial commensals.

Programmed Necrosis in Host Defense

Host control over infectious disease relies on the ability of cells in multicellular organisms to detect and defend against pathogens to prevent disease. Evolution affords mammals with a wide variety of independent immune mechanisms to control or eliminate invading infectious agents. Many pathogens acquire functions to deflect these immune mechanisms and promote infection. Following successful invasion of a host, cell autonomous signaling pathways drive the production of inflammatory cytokines, deployment of restriction factors and induction of cell death. Combined, these innate immune mechanisms attract dendritic cells, neutrophils and macrophages as well as innate lymphoid cells such as natural killer cells that all help control infection. Eventually, the development of adaptive pathogen-specific immunity clears infection and provides immune memory of the encounter. For obligate intracellular pathogens such as viruses, diverse cell death pathways make a pivotal contribution to early control by eliminating host cells before progeny are produced. Pro-apoptotic caspase-8 activity (along with caspase-10 in humans) executes extrinsic apoptosis, a nonlytic form of cell death triggered by TNF family death receptors (DRs). Over the past two decades, alternate extrinsic apoptosis and necroptosis outcomes have been described. Programmed necrosis, or necroptosis, occurs when receptor interacting protein kinase 3 (RIPK3) activates mixed lineage kinase-like (MLKL), causing cell leakage. Thus, activation of DRs, toll-like receptors (TLRs) or pathogen sensor Z-nucleic acid binding protein 1 (ZBP1) initiates apoptosis as well as necroptosis if not blocked by virus-encoded inhibitors. Mammalian cell death pathways are blocked by herpesvirus- and poxvirus-encoded cell death suppressors. Growing evidence has revealed the importance of Z-nucleic acid sensor, ZBP1, in the cell autonomous recognition of both DNA and RNA virus infection. This volume will explore the detente between viruses and cells to manage death machinery and avoid elimination to support dissemination within the host animal.

Approaches to Evaluating Necroptosis in Virus-Infected Cells

The immune system functions to protect the host from pathogens. To counter host defense mechanisms, pathogens have developed unique strategies to evade detection or restrict host immune responses. Programmed cell death is a major contributor to the multiple host responses that help to eliminate infected cells for obligate intracellular pathogens like viruses. Initiation of programmed cell death pathways during the early stages of viral infections is critical for organismal survival as it restricts the virus from replicating and serves to drive antiviral inflammation immune recruitment through the release of damage-associated molecular patterns (DAMPs) from the dying cell. Necroptosis has been implicated as a critical programmed cell death pathway in a diverse set of diseases and pathological conditions including acute viral infections. This cell death pathway occurs when certain host sensors are triggered leading to the downstream induction of mixed-lineage kinase domain-like protein (MLKL). MLKL induction leads to cytoplasmic membrane disruption and subsequent cellular destruction with the release of DAMPs. As the role of this cell death pathway in human disease becomes apparent, methods identifying necroptosis patterns and outcomes will need to be further developed. Here, we discuss advances in our understanding of how viruses counteract necroptosis, methods to quantify the pathway, its effects on viral pathogenesis, and its impact on cellular signaling.

ZBP1-Mediated Necroptosis: Mechanisms and Therapeutic Implications

Cell death is a fundamental pathophysiological process in human disease. The discovery of necroptosis, a form of regulated necrosis that is induced by the activation of death receptors and formation of necrosome, represents a major breakthrough in the field of cell death in the past decade. Z-DNA-binding protein (ZBP1) is an interferon (IFN)-inducing protein, initially reported as a double-stranded DNA (dsDNA) sensor, which induces an innate inflammatory response. Recently, ZBP1 was identified as an important sensor of necroptosis during virus infection. It connects viral nucleic acid and receptor-interacting protein kinase 3 (RIPK3) via two domains and induces the formation of a necrosome. Recent studies have also reported that ZBP1 induces necroptosis in non-viral infections and mediates necrotic signal transduction by a unique mechanism. This review highlights the discovery of ZBP1 and its novel findings in necroptosis and provides an insight into its critical role in the crosstalk between different types of cell death, which may represent a new therapeutic option.

Tumor necrosis factor is a necroptosis-associated alarmin

Necroptosis is a form of regulated cell death that can occur downstream of several immune pathways. While previous studies have shown that dysregulated necroptosis can lead to strong inflammatory responses, little is known about the identity of the endogenous molecules that trigger these responses. Using a reductionist in vitro model, we found that soluble TNF is strongly released in the context of necroptosis. On the one hand, necroptosis promotes TNF translation by inhibiting negative regulatory mechanisms acting at the post-transcriptional level. On the other hand, necroptosis markedly enhances TNF release by activating ADAM proteases. In studying TNF release at single-cell resolution, we found that TNF release triggered by necroptosis is activated in a switch-like manner that exceeds steady-state TNF processing in magnitude and speed. Although this shedding response precedes massive membrane damage, it is closely associated with lytic cell death. Further, we found that lytic cell death induction using a pore-forming toxin also triggers TNF shedding, indicating that the activation of ADAM proteases is not strictly related to the necroptotic pathway but likely associated with biophysical changes of the cell membrane upon lytic cell death. These results demonstrate that lytic cell death, particularly necroptosis, is a critical trigger for TNF release and thus qualify TNF as a necroptosis-associated alarmin.

RIPK1 and RIPK3 in antibacterial defence

Upon sensing pathogenic bacterial infection, host cells activate a multitude of inflammatory and immunogenic responses to promote bacterial clearance and restore tissue homeostasis. RIPK1 and RIPK3 are two key players in antimicrobial defence, by either driving inflammatory signalling or inducing programmed cell death activation, ranging from apoptosis, pyroptosis to necroptosis. In this review, we first discuss the mechanisms by which RIPK1 and RIPK3 promote the assembly of death-inducing complexes and how these cell death pathways are activated as host responses to counteract pathogenic bacteria. We further outline the immunological importance of cell death in antibacterial defence and highlight outstanding questions in the field.

Necroptosis at a glance

Necroptosis, or programmed necrosis, is an inflammatory form of cell death with important functions in host defense against pathogens and tissue homeostasis. The four cytosolic receptor-interacting protein kinase homotypic interaction motif (RHIM)-containing adaptor proteins RIPK1, RIPK3, TRIF (also known as TICAM1) and ZBP1 mediate necroptosis induction in response to infection and cytokine or innate immune receptor activation. Activation of the RHIM adaptors leads to phosphorylation, oligomerization and membrane targeting of the necroptosis effector protein mixed lineage kinase domain-like (MLKL). Active MLKL induces lesions on the plasma membrane, leading to the release of pro-inflammatory damage-associated molecular patterns (DAMPs). Thus, activities of the RHIM adaptors and MLKL are tightly regulated by posttranslational modifications to prevent inadvertent release of immunogenic contents. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of the regulatory mechanisms of necroptosis and its biological functions in tissue homeostasis, pathogen infection and other inflammatory diseases.

Membrane permeabilization is mediated by distinct epitopes in mouse and human orthologs of the necroptosis effector, MLKL

Necroptosis is a lytic programmed cell death pathway with origins in innate immunity that is frequently dysregulated in inflammatory diseases. The terminal effector of the pathway, MLKL, is licensed to kill following phosphorylation of its pseudokinase domain by the upstream regulator, RIPK3 kinase. Phosphorylation provokes the unleashing of MLKL's N-terminal four-helix bundle (4HB or HeLo) domain, which binds and permeabilizes the plasma membrane to cause cell death. The precise mechanism by which the 4HB domain permeabilizes membranes, and how the mechanism differs between species, remains unclear. Here, we identify the membrane binding epitope of mouse MLKL using NMR spectroscopy. Using liposome permeabilization and cell death assays, we validate K69 in the α3 helix, W108 in the α4 helix, and R137/Q138 in the first brace helix as crucial residues for necroptotic signaling. This epitope differs from the phospholipid binding site reported for human MLKL, which comprises basic residues primarily located in the α1 and α2 helices. In further contrast to human and plant MLKL orthologs, in which the α3-α4 loop forms a helix, this loop is unstructured in mouse MLKL in solution. Together, these findings illustrate the versatility of the 4HB domain fold, whose lytic function can be mediated by distinct epitopes in different orthologs.

Viral-mediated activation and inhibition of programmed cell death

Viruses are ubiquitous intracellular genetic parasites that heavily rely on the infected cell to complete their replication life cycle. This dependency on the host machinery forces viruses to modulate a variety of cellular processes including cell survival and cell death. Viruses are known to activate and block almost all types of programmed cell death (PCD) known so far. Modulating PCD in infected hosts has a variety of direct and indirect effects on viral pathogenesis and antiviral immunity. The mechanisms leading to apoptosis following virus infection is widely studied, but several modalities of PCD, including necroptosis, pyroptosis, ferroptosis, and paraptosis, are relatively understudied. In this review, we cover the mechanisms by which viruses activate and inhibit PCDs and suggest perspectives on how these affect viral pathogenesis and immunity.

Ubiquitylation of RIPK3 beyond-the-RHIM can limit RIPK3 activity and cell death

Pathogen recognition and TNF receptors signal via receptor interacting serine/threonine kinase-3 (RIPK3) to cause cell death, including MLKL-mediated necroptosis and caspase-8-dependent apoptosis. However, the post-translational control of RIPK3 is not fully understood. Using mass-spectrometry, we identified that RIPK3 is ubiquitylated on K469. The expression of mutant RIPK3 K469R demonstrated that RIPK3 ubiquitylation can limit both RIPK3-mediated apoptosis and necroptosis. The enhanced cell death of overexpressed RIPK3 K469R and activated endogenous RIPK3 correlated with an overall increase in RIPK3 ubiquitylation. Ripk3^K469R/K469R mice challenged with Salmonella displayed enhanced bacterial loads and reduced serum IFNγ. However, Ripk3^K469R/K469R macrophages and dermal fibroblasts were not sensitized to RIPK3-mediated apoptotic or necroptotic signaling suggesting that, in these cells, there is functional redundancy with alternate RIPK3 ubiquitin-modified sites. Consistent with this idea, the mutation of other ubiquitylated RIPK3 residues also increased RIPK3 hyper-ubiquitylation and cell death. Therefore, the targeted ubiquitylation of RIPK3 may act as either a brake or accelerator of RIPK3-dependent killing.

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RIPK3 can be ubiquitylated on K469 to limit RIPK3-induced necroptosis and apoptosis

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Ripk3^K469R/K469R mice are more susceptible to Salmonella infection

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Several ubiquitylated or surface exposed lysines can limit RIPK3-induced cell death

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Hyper-ubiquitylated RIPK3 correlates with RIPK3 signaling and cell death

Revisiting Regulated Cell Death Responses in Viral Infections

The fate of a viral infection in the host begins with various types of cellular responses, such as abortive, productive, latent, and destructive infections. Apoptosis, necroptosis, and pyroptosis are the three major types of regulated cell death mechanisms that play critical roles in viral infection response. Cell shrinkage, nuclear condensation, bleb formation, and retained membrane integrity are all signs of osmotic imbalance-driven cytoplasmic swelling and early membrane damage in necroptosis and pyroptosis. Caspase-driven apoptotic cell demise is considered in many circumstances as an anti-inflammatory, and some pathogens hijack the cell death signaling routes to initiate a targeted attack against the host. In this review, the selected mechanisms by which viruses interfere with cell death were discussed in-depth and were illustrated by compiling the general principles and cellular signaling mechanisms of virus–host-specific molecule interactions.

Human RIPK3 C-lobe phosphorylation is essential for necroptotic signaling

Necroptosis is a caspase-independent, pro-inflammatory mode of programmed cell death which relies on the activation of the terminal effector, MLKL, by the upstream protein kinase RIPK3. To mediate necroptosis, RIPK3 must stably interact with, and phosphorylate the pseudokinase domain of MLKL, although the precise molecular cues that provoke RIPK3 necroptotic signaling are incompletely understood. The recent finding that RIPK3 S227 phosphorylation and the occurrence of a stable RIPK3:MLKL complex in human cells prior to exposure to a necroptosis stimulus raises the possibility that additional, as-yet-unidentified phosphorylation events activate RIPK3 upon initiation of necroptosis signaling. Here, we sought to identify phosphorylation sites of RIPK3 and dissect their regulatory functions. Phosphoproteomics identified 21 phosphorylation sites in HT29 cells overexpressing human RIPK3. By comparing cells expressing wild-type and kinase-inactive D142N RIPK3, autophosphorylation sites and substrates of other cellular kinases were distinguished. Of these 21 phosphosites, mutational analyses identified only pT224 and pS227 as crucial, synergistic sites for stable interaction with MLKL to promote necroptosis, while the recently reported activation loop phosphorylation at S164/T165 negatively regulate the kinase activity of RIPK3. Despite being able to phosphorylate MLKL to a similar or higher extent than wild-type RIPK3, mutation of T224, S227, or the RHIM in RIPK3 attenuated necroptosis. This finding highlights the stable recruitment of human MLKL by RIPK3 to the necrosome as an essential checkpoint in necroptosis signaling, which is independent from and precedes the phosphorylation of MLKL.

CRISPR deletions in cell lines for reconstitution studies of pseudokinase function

The non-catalytic cousins of protein kinases, the pseudokinases, have grown to prominence as indispensable signaling entities over the past decade, despite their lack of catalytic activity. Because their importance has only been fully embraced recently, many of the 10% of the human kinome categorized as pseudokinases are yet to be attributed biological functions. The advent of CRISPR-Cas9 editing to genetically delete pseudokinases in a cell line of interest has proven invaluable to dissecting many functions and remains the method of choice for gene knockout. Here, using the terminal effector pseudokinase in the necroptosis cell death pathway, MLKL, as an exemplar, we describe a method for genetic knockout of pseudokinases in cultured cells. This method does not retain the CRISPR guide sequence in the edited cells, which eliminates possible interference in subsequent reconstitution studies where mutant forms of the pseudokinase can be reintroduced into cells exogenously for detailed mechanistic characterization.

Co-expression of recombinant RIPK3:MLKL complexes using the baculovirus-insect cell system

Pseudokinase domains are found throughout the kingdoms of life and serve myriad roles in cell signaling. These domains, which resemble protein kinases but are catalytically-deficient, have been described principally as protein interaction domains. Broadly, pseudokinases have been reported to function as: allosteric regulators of conventional enzymes; scaffolds to nucleate assembly and/or localization of signaling complexes; molecular switches; or competitors of signaling complex assembly. From detailed structural and biochemical studies of individual pseudokinases, a picture of how they mediate protein interactions is beginning to emerge. Many such studies have relied on recombinant protein production in insect cells, where endogenous chaperones and modifying enzymes favor bona fide folding of pseudokinases. Here, we describe methods for co-expression of pseudokinases and their interactors in insect cells, as exemplified by the MLKL pseudokinase, which is the terminal effector in the necroptosis cell death pathway, and its upstream regulator kinase RIPK3. These methods are broadly applicable to co-expression of other pseudokinases with their interaction partners from bacmids using the baculovirus-insect cell expression system.

Ars moriendi: Proteases as sculptors of cellular suicide

The Ars moriendi, which translates to "The Art of Dying," encompasses two Latin texts that gave advice on how to die well and without fear according to the Christian precepts of the late Middle Ages. Given that ten to hundred billion cells die in our bodies every day, it is obvious that the concept of a well and orderly ("regulated") death is also paramount at the cellular level. In apoptosis, as the most well-studied form of regulated cell death, proteases of the caspase family are the central mediators. However, caspases are not the only proteases that act as sculptors of cellular suicide, and therefore, we here provide an overview of the impact of proteases in apoptosis and other forms of regulated cell death.