Sequential Engagement of Distinct MLKL Phosphatidylinositol-Binding Sites Executes Necroptosis

Locking mixed-lineage kinase domain-like protein in its auto-inhibited state prevents necroptosis

As an alternative pathway of controlled cell death, necroptosis can be triggered by tumor necrosis factor via the kinases RIPK1/RIPK3 and the effector protein mixed-lineage kinase domain-like protein (MLKL). Upon activation, MLKL oligomerizes and integrates into the plasma membrane via its executioner domain. Here, we present the X-ray and NMR costructures of the human MLKL executioner domain covalently bound via Cys86 to a xanthine class inhibitor. The structures reveal that the compound stabilizes the interaction between the auto-inhibitory brace helix α6 and the four-helix bundle by stacking to Phe148. An NMR-based functional assay observing the conformation of this helix showed that the F148A mutant is unresponsive to the compound, providing further evidence for the importance of this interaction. Real-time and diffusion NMR studies demonstrate that xanthine derivatives inhibit MLKL oligomerization. Finally, we show that the other well-known MLKL inhibitor Necrosulfonamide, which also covalently modifies Cys86, must employ a different mode of action.

Distinct photo-oxidation-induced cell death pathways lead to selective killing of human breast cancer cells

Lack of effective treatments for aggressive breast cancer is still a major global health problem. We have previously reported that photodynamic therapy using methylene blue as photosensitizer (MB-PDT) massively kills metastatic human breast cancer, marginally affecting healthy cells. In this study, we aimed to unveil the molecular mechanisms behind MB-PDT effectiveness and specificity towards tumor cells. Through lipidomics and biochemical approaches, we demonstrated that MB-PDT efficiency and specificity rely on polyunsaturated fatty acid-enriched membranes and on the better capacity to deal with photo-oxidative damage displayed by non-tumorigenic cells. We found out that, in tumorigenic cells, lysosome membrane permeabilization is accompanied by ferroptosis and/or necroptosis. Our results also pointed at a cross-talk between lysosome-dependent cell death (LDCD) and necroptosis induction after photo-oxidation, and contributed to broaden the understanding of MB-PDT-induced mechanisms and specificity in breast cancer cells. Therefore, we demonstrated that efficient approaches could be designed on the basis of lipid composition and metabolic features for hard-to-treat cancers. The results further reinforce MB-PDT as a therapeutic strategy for highly aggressive human breast cancer cells.

Pore formation in regulated cell death

The discovery of alternative signaling pathways that regulate cell death has revealed multiple strategies for promoting cell death with diverse consequences at the tissue and organism level. Despite the divergence in the molecular components involved, membrane permeabilization is a common theme in the execution of regulated cell death. In apoptosis, the permeabilization of the outer mitochondrial membrane by BAX and BAK releases apoptotic factors that initiate the caspase cascade and is considered the point of no return in cell death commitment. Pyroptosis and necroptosis also require the perforation of the plasma membrane at the execution step, which involves Gasdermins in pyroptosis, and MLKL in the case of necroptosis. Although BAX/BAK, Gasdermins and MLKL share certain molecular features like oligomerization, they form pores in different cellular membranes via distinct mechanisms. Here, we compare and contrast how BAX/BAK, Gasdermins, and MLKL alter membrane permeability from a structural and biophysical perspective and discuss the general principles of membrane permeabilization in the execution of regulated cell death.

Necroptotic movers and shakers: cell types, inflammatory drivers and diseases

The necroptotic cell death pathway has received significant attention for its ability to trigger inflammatory responses and its potential involvement in related conditions. Recent insights into the essential membrane damaging necroptotic pseudokinase effector, Mixed lineage kinase domain like (MLKL), have revealed a number of diverse MLKL functions that contribute to the inflammatory nature of necroptosis. Here we review distinct MLKL signalling roles and document the immunogenic molecules released by necroptosis. We discuss specific in vivo MLKL-driven responses, the activation of inflammasome complexes and innate lymphoid cells, which have been documented to drive disease. Finally, we list necroptotic competent cell types and their involvement in MLKL-driven cell death-associated and inflammatory-associated conditions.

Influenza Virus Z-RNAs Induce ZBP1-Mediated Necroptosis

Influenza A virus (IAV) is a lytic RNA virus that triggers receptor-interacting serine/threonine-protein kinase 3 (RIPK3)-mediated pathways of apoptosis and mixed lineage kinase domain-like pseudokinase (MLKL)-dependent necroptosis in infected cells. ZBP1 initiates RIPK3-driven cell death by sensing IAV RNA and activating RIPK3. Here, we show that replicating IAV generates Z-RNAs, which activate ZBP1 in the nucleus of infected cells. ZBP1 then initiates RIPK3-mediated MLKL activation in the nucleus, resulting in nuclear envelope disruption, leakage of DNA into the cytosol, and eventual necroptosis. Cell death induced by nuclear MLKL was a potent activator of neutrophils, a cell type known to drive inflammatory pathology in virulent IAV disease. Consequently, MLKL-deficient mice manifest reduced nuclear disruption of lung epithelia, decreased neutrophil recruitment into infected lungs, and increased survival following a lethal dose of IAV. These results implicate Z-RNA as a new pathogen-associated molecular pattern and describe a ZBP1-initiated nucleus-to-plasma membrane "inside-out" death pathway with potentially pathogenic consequences in severe cases of influenza.

Hepatic inflammation after burn injury is associated with necroptotic cell death signaling

BACKGROUND

Burn injury still has a high attributable mortality. The elevated mortality rate of severe burns is still concerning. Hepatic inflammation and injury are common after burns and are associated with poor outcomes. Necroptosis is a programmed cell death linked with inflammation. Thus, assessing necroptotic pathways in the liver can lead to new therapeutic modalities to improve mortality after severe burns.

METHODS

Mice underwent 15% total body surface area burn or sham injury. Three hours after burn, the mice were euthanized to collect blood and livers. Histology, injury markers, genes expression, and tissue protein levels were compared between groups.

RESULTS

Compared with sham, burned mice had heightened liver inflammatory cell infiltration and edema. Serum aspartate aminotransferase and alanine aminotransferase were increased by 4.9- and 3.4-fold, respectively, in burned mice relative to sham (p < 0.05). Expression of tumor necrosis factor α, interleukin-6, interleukin-1β, and CXCL1 (KC) genes were elevated in livers of burned mice by 10-, 86-, 10-, and 828-fold, respectively, compared with sham (p < 0.05). Expression of necroptotic genes, namely, receptor-interacting protein kinases 1 and 3, and mixed lineage kinase domain-like in livers of burned mice were increased by 10-, 13-, and 4.5-fold, respectively, relative to sham (p < 0.05). Receptor-interacting protein kinase 1 and phosphorylated mixed lineage kinase domain-like protein levels measured by Western-blot in livers after burn injury were elevated by 22- and 17-fold, respectively, compared with sham (p < 0.05).

CONCLUSION

Liver damage occurs early after burns in mice and is associated with elevation of proinflammatory cytokines, chemokine, and proteins involved in the necroptotic pathway. This study suggests that necroptosis plays a role in the pathogenesis of liver failure secondary to burn injury.

Up-Regulation of RIP3 Alleviates Prostate Cancer Progression by Activation of RIP3/MLKL Signaling Pathway and Induction of Necroptosis

Background

The receptor-interacting protein kinase 3 (RIP3/RIPK3) was recently found to be a critical regulator of programmed necrosis/necroptosis. However, the biological role and clinical significance of RIP3 in prostate cancer remain obscure.

Methods

Western blotting and QRT-PCR were performed to detect the level of RIP3 in prostate cancer cells. Fixed cancer tissue and normal tissue specimens were subjected to immunohistochemical analysis of RIP3. Cell migration and invasion abilities were evaluated by transwell assays. In vitro proliferative ability was examed by MTS. And in vivo nude mice model were used to evaluate the effect of RIP3 ectopic expression on proliferative capability. Cell cycle of prostate cancer cells were analyzed by flow cytometry. Changes in some related proteins caused by RIP3 overexpression were explored using Western blotting.

Results

RIP3 was significantly down-regulated in prostate cancer cell lines and clinical prostate tumor samples. And over-expressing RIP3 suppressed the migration and invasion of prostate cancer cells. Two important matrix metalloproteinases MMP2, MMP9 which enables the destruction of the histological barrier of tumor cell invasion and three mesenchymal markers Vimentin, fibronectin, and N-cadherin were under-expressed due to the overexpression of RIP3, but the E-cadherin level which is the epithelial marker was increased. Furthermore, our results also showed that RIP3 can inhibit the proliferation and tumorigenicity of prostate cancer cells both in vitro and in vivo by phosphorylating MLKL, which were reversed by MLKL inhibitor treatment, indicating that necroptosis was involved in cell death.

Conclusion

Taken together, these findings indicated that RIP3 is responsible for the progression of prostate cancer, suggesting that RIP3 might have the potential to be a prognostic marker or a therapeutic target against prostate cancer.

RIPK3 Activation Leads to Cytokine Synthesis that Continues after Loss of Cell Membrane Integrity

Necroptosis is a form of programmed cell death that is defined by activation of the kinase RIPK3 and subsequent cell membrane permeabilization by the effector MLKL. RIPK3 activation can also promote immune responses via production of cytokines and chemokines. How active cytokine production is coordinated with the terminal process of necroptosis is unclear. Here, we report that cytokine production continues within necroptotic cells even after they have lost cell membrane integrity and irreversibly committed to death. This continued cytokine production is dependent on mRNA translation and requires maintenance of endoplasmic reticulum integrity that remains after plasma membrane integrity is lost. The continued translation of cytokines by cellular corpses contributes to necroptotic cell uptake by innate immune cells and priming of adaptive immune responses to antigens associated with necroptotic corpses. These findings imply that cell death and production of inflammatory mediators are coordinated to optimize the immunogenicity of necroptotic cells.

Identification of MYC as an antinecroptotic protein that stifles RIPK1-RIPK3 complex formation

The underlying mechanism of necroptosis in relation to cancer is still unclear. Here, MYC, a potent oncogene, is an antinecroptotic factor that directly suppresses the formation of the RIPK1-RIPK3 complex. Gene set enrichment analyses reveal that the MYC pathway is the most prominently down-regulated signaling pathway during necroptosis. Depletion or deletion of MYC promotes the RIPK1-RIPK3 interaction, thereby stabilizing the RIPK1 and RIPK3 proteins and facilitating necroptosis. Interestingly, MYC binds to RIPK3 in the cytoplasm and inhibits the interaction between RIPK1 and RIPK3 in vitro. Furthermore, MYC-nick, a truncated form that is mainly localized in the cytoplasm, prevented TNF-induced necroptosis. Finally, down-regulation of MYC enhances necroptosis in leukemia cells and suppresses tumor growth in a xenograft model upon treatment with birinapant and emricasan. MYC-mediated suppression of necroptosis is a mechanism of necroptosis resistance in cancer, and approaches targeting MYC to induce necroptosis represent an attractive therapeutic strategy for cancer.

The Killer Pseudokinase Mixed Lineage Kinase Domain-Like Protein (MLKL)

Whereas the apoptosis cell death pathway typically enables cells to undergo death in an immunologically silent manner, cell death by necroptosis induces cell lysis and release of cellular constituents known to elicit an immune response. Consequently, the origins of necroptosis likely originated in host defense against pathogens, although recently it has emerged that dysregulation of the pathway underlies many human pathologies. The past decade has seen a rapid advance in our understanding of the molecular mechanisms underlying necroptotic cell death, including the implication of the pseudokinase, mixed lineage kinase domain-like protein (MLKL), as the terminal effector in the pathway. Here, I review our current understanding of how MLKL is activated by the upstream receptor interacting protein kinase (RIPK)3, the proposed mechanism(s) by which MLKL kills cells, and recently described layers of regulation that tune MLKL's killing activity.

Metabolic Flexibility and Innate Immunity in Renal Ischemia Reperfusion Injury: The Fine Balance Between Adaptive Repair and Tissue Degeneration

Renal ischemia reperfusion injury (IRI), a common event after renal transplantation, causes acute kidney injury (AKI), increases the risk of delayed graft function (DGF), primes the donor kidney for rejection, and contributes to the long-term risk of graft loss. In the last decade, epidemiological studies have linked even mild episodes of AKI to chronic kidney disease (CKD) progression, and innate immunity seems to play a crucial role. The ischemic insult triggers an acute inflammatory reaction that is elicited by Pattern Recognition Receptors (PRRs), expressed on both infiltrating immune cells as well as tubular epithelial cells (TECs). Among the PRRs, Toll-like receptors (TLRs), their synergistic receptors, Nod-like receptors (NLRs), and the inflammasomes, play a pivotal role in shaping inflammation and TEC repair, in response to renal IRI. These receptors represent promising targets to modulate the extent of inflammation, but also function as gatekeepers of tissue repair, protecting against AKI-to-CKD progression. Despite the important considerations on timely use of therapeutics, in the context of IRI, treatment options are limited by a lack of understanding of the intra- and intercellular mechanisms associated with the activation of innate immune receptors and their impact on adaptive tubular repair. Accumulating evidence suggests that TEC-associated innate immunity shapes the tubular response to stress through the regulation of immunometabolism. Engagement of innate immune receptors provides TECs with the metabolic flexibility necessary for their plasticity during injury and repair. This could significantly affect pathogenic processes within TECs, such as cell death, mitochondrial damage, senescence, and pro-fibrotic cytokine secretion, well-known to exacerbate inflammation and fibrosis. This article provides an overview of the past 5 years of research on the role of innate immunity in experimental and human IRI, with a focus on the cascade of events activated by hypoxic damage in TECs: from programmed cell death (PCD) and mitochondrial dysfunction-mediated metabolic rewiring of TECs to maladaptive repair and progression to fibrosis. Finally, we will discuss the important crosstalk between metabolism and innate immunity observed in TECs and their therapeutic potential in both experimental and clinical research.

A missense mutation in the MLKL brace region promotes lethal neonatal inflammation and hematopoietic dysfunction

MLKL is the essential effector of necroptosis, a form of programmed lytic cell death. We have isolated a mouse strain with a single missense mutation, Mlkl^D139V, that alters the two-helix 'brace' that connects the killer four-helix bundle and regulatory pseudokinase domains. This confers constitutive, RIPK3 independent killing activity to MLKL. Homozygous mutant mice develop lethal postnatal inflammation of the salivary glands and mediastinum. The normal embryonic development of Mlkl^D139V homozygotes until birth, and the absence of any overt phenotype in heterozygotes provides important in vivo precedent for the capacity of cells to clear activated MLKL. These observations offer an important insight into the potential disease-modulating roles of three common human MLKL polymorphisms that encode amino acid substitutions within or adjacent to the brace region. Compound heterozygosity of these variants is found at up to 12-fold the expected frequency in patients that suffer from a pediatric autoinflammatory disease, chronic recurrent multifocal osteomyelitis (CRMO).

Microfluidic Immunoassays for Time-Resolved Measurement of Protein Secretion from Single Cells

Measurement of humoral factors secreted from cells has served as an indispensable method to monitor the states of a cell ensemble because humoral factors play crucial roles in cell-cell interaction and aptly reflect the states of individual cells. Although a cell ensemble consisting of a large number of cells has conventionally been the object of such measurements, recent advances in microfluidic technology together with highly sensitive immunoassays have enabled us to quantify secreted humoral factors even from individual cells in either a population or a temporal context. Many groups have reported various miniaturized platforms for immunoassays of proteins secreted from single cells. This review focuses on the current status of time-resolved assay platforms for protein secretion with single-cell resolution. We also discuss future perspectives of time-resolved immunoassays from the viewpoint of systems biology.

Immunological Significance of HMGB1 Post-Translational Modification and Redox Biology

Most extracellular proteins are secreted via the classical endoplasmic reticulum (ER)/Golgi-dependent secretion pathway; however, some proteins, including a few danger-associated molecular patterns (DAMPs), are secreted via non-classical ER/Golgi-independent secretion pathways. The evolutionarily conserved high mobility group box1 (HMGB1) is a ubiquitous nuclear protein that can be released by almost all cell types. HMGB1 lacks signal peptide and utilizes diverse non-canonical secretion mechanisms for its extracellular export. Although the post-translational modifications of HMGB1 were demonstrated, the oxidation of HMGB1 and secretion mechanisms are not highlighted yet. We currently investigated that peroxiredoxins I and II (PrxI/II) induce the intramolecular disulfide bond formation of HMGB1 in the nucleus. Disulfide HMGB1 is preferentially transported out of the nucleus by binding to the nuclear exportin chromosome-region maintenance 1 (CRM1). We determined the kinetics of HMGB1 oxidation in bone marrow-derived macrophage as early as a few minutes after lipopolysaccharide treatment, peaking at 4 h while disulfide HMGB1 accumulation was observed within the cells, starting to secrete in the late time point. We have shown that HMGB1 oxidation status, which is known to determine the biological activity in extracellular HMGB1, is crucial for the secretion of HMGB1 from the nucleus. This review summarizes selected aspects of HMGB1 redox biology relevant to the induction and propagation of inflammatory diseases. We implicate the immunological significance and the need for novel HMGB1 inhibitors through mechanism-based studies.

Beclin 1 functions as a negative modulator of MLKL oligomerisation by integrating into the necrosome complex

Necroptosis is a form of regulated cell death caused by formation of the necrosome complex. However, the factors modulating this process and the systemic pathophysiological effects of necroptosis are yet to be understood. Here, we identified that Beclin 1 functions as an anti-necroptosis factor by being recruited into the necrosome complex upon treatment with TNFα, Smac mimetic, and pan-caspase inhibitor and by repressing MLKL oligomerisation, thus preventing the disruption of the plasma membrane. Cells ablated or knocked-out for Beclin 1 become sensitised to necroptosis in an autophagy-independent manner without affecting the necrosome formation itself. Interestingly, the recruitment of Beclin 1 into the necrosome complex is dependent on the activation and phosphorylation of MLKL. Biochemically, the coiled-coil domain (CCD) of Beclin 1 binds to the CCD of MLKL, which restrains the oligomerisation of phosphorylated MLKL. Finally, Beclin 1 depletion was found to promote necroptosis in leukaemia cells and enhance regression of xenografted-tumour upon treatment with Smac mimetics and caspase inhibitors. These results suggest that Beclin 1 functions as a negative regulator in the execution of necroptosis by suppressing MLKL oligomerisation.

The necroptosis pathway and its role in age-related neurodegenerative diseases: will it open up new therapeutic avenues in the next decade?

INTRODUCTION

Necroptosis is a programmed form of necrotic cell death. Growing evidence demonstrates that necroptosis contributes to cell demise in different pathological conditions including age-dependent neurodegenerative diseases (NDs). These findings open new avenues for understanding the mechanisms of neuronal loss in NDs, which might eventually translate into novel therapeutic interventions.

AREAS COVERED

We reviewed key aspects of necroptosis, in health and disease, focusing on evidence demonstrating its involvement in the pathogenesis of age-related NDs. We then highlight the activation of this pathway in the mechanism of axonal degeneration. We searched on PubMed the literature regarding necroptosis published between 2008 and 2020 and reviewed all publications were necroptosis was studied in the context of age-related NDs.

EXPERT OPINION

Axonal loss and neuronal death are the ultimate consequences of NDs that translate into disease phenotypes. Targeting degenerative mechanisms of the neuron appears as a strategy that might cover a wide range of diseases. Thus, the participation of necroptosis as a common mediator of neuronal demise emerges as a promising target for therapeutic intervention. Considering evidence demonstrating that necroptosis mediates axonal degeneration, we propose and discuss the potential of targeting necroptosis-mediated axonal destruction as a strategy to tackle NDs before neuronal loss occurs.

A novel neurodegenerative spectrum disorder in patients with MLKL deficiency

Mixed lineage kinase domain-like (MLKL) is the main executor of necroptosis, an inflammatory form of programmed cell death. Necroptosis is implicated in combating infections, but also in contributing to numerous other clinical conditions, including cardiovascular diseases and neurodegenerative disorders. Inhibition of necroptosis is therefore of therapeutic interest. Here we report two siblings both of whom over the course of 35 years developed a similar progressive, neurodegenerative spectrum disorder characterized by paresis, ataxia and dysarthria. Magnetic resonance imaging of their central nervous system (CNS) revealed severe global cerebral volume loss and atrophy of the cerebellum and brainstem. These brothers are homozygous for a rare haplotype identified by whole genome sequencing carrying a frameshift variant in MLKL, as well as an in-frame deletion of one amino acid in the adjacent fatty acid 2-hydroxylase (FA2H) gene. Functional studies of patient-derived primary cells demonstrated that the variant in MLKL leads to a deficiency of MLKL protein resulting in impairment of necroptosis. Conversely, shotgun lipidomic analysis of the variant in FA2H shows no impact on either the abundance or the enzymatic activity of the encoded hydroxylase. To our knowledge, this is the first report of complete necroptosis deficiency in humans. The findings may suggest that impaired necroptosis is a novel mechanism of neurodegeneration, promoting a disorder that shares some clinical features with primary progressive multiple sclerosis (PPMS) and other neurodegenerative diseases. Importantly, the necroptotic deficiency does not cause symptoms outside the nervous system, nor does it confer susceptibility to infections. Given the current interest in pharmacological inhibition of necroptosis by targeting MLKL and its associated pathways, this strategy should be developed with caution, with careful consideration of the possible development of adverse neurological effects.

Identification of MLKL membrane translocation as a checkpoint in necroptotic cell death using Monobodies

The necroptosis cell death pathway has been implicated in host defense and in the pathology of inflammatory diseases. While phosphorylation of the necroptotic effector pseudokinase Mixed Lineage Kinase Domain-Like (MLKL) by the upstream protein kinase RIPK3 is a hallmark of pathway activation, the precise checkpoints in necroptosis signaling are still unclear. Here we have developed monobodies, synthetic binding proteins, that bind the N-terminal four-helix bundle (4HB) "killer" domain and neighboring first brace helix of human MLKL with nanomolar affinity. When expressed as genetically encoded reagents in cells, these monobodies potently block necroptotic cell death. However, they did not prevent MLKL recruitment to the "necrosome" and phosphorylation by RIPK3, nor the assembly of MLKL into oligomers, but did block MLKL translocation to membranes where activated MLKL normally disrupts membranes to kill cells. An X-ray crystal structure revealed a monobody-binding site centered on the α4 helix of the MLKL 4HB domain, which mutational analyses showed was crucial for reconstitution of necroptosis signaling. These data implicate the α4 helix of its 4HB domain as a crucial site for recruitment of adaptor proteins that mediate membrane translocation, distinct from known phospholipid binding sites.

Partners in Crime: The Interplay of Proteins and Membranes in Regulated Necrosis

Pyroptosis, necroptosis, and ferroptosis are well-characterized forms of regulated necrosis that have been associated with human diseases. During regulated necrosis, plasma membrane damage facilitates the movement of ions and molecules across the bilayer, which finally leads to cell lysis and release of intracellular content. Therefore, these types of cell death have an inflammatory phenotype. Each type of regulated necrosis is mediated by a defined machinery comprising protein and lipid molecules. Here, we discuss how the interaction and reshaping of these cellular components are essential and distinctive processes during pyroptosis, necroptosis, and ferroptosis. We point out that although the plasma membrane is the common target in regulated necrosis, different mechanisms of permeabilization have emerged depending on the cell death form. Pore formation by gasdermins (GSDMs) is a hallmark of pyroptosis, while mixed lineage kinase domain-like (MLKL) protein facilitates membrane permeabilization in necroptosis, and phospholipid peroxidation leads to membrane damage in ferroptosis. This diverse repertoire of mechanisms leading to membrane permeabilization contributes to define the specific inflammatory and immunological outcome of each type of regulated necrosis. Current efforts are focused on new therapies that target critical protein and lipid molecules on these pathways to fight human pathologies associated with inflammation.

Life, death, and autophagy in cancer: NF-κB turns up everywhere

Escaping programmed cell death is a hallmark of cancer. NF-κB transcription factors are key regulator of cell survival and aberrant NF-κB signaling has been involved in the pathogenesis of most human malignancies. Although NF-κB is best known for its antiapoptotic role, other processes regulating the life/death balance, such as autophagy and necroptosis, seem to network with NF-κB. This review discusses how the reciprocal regulation of NF-κB, autophagy and programmed cell death affect cancer development and progression.

Regulation of Necroptosis by Phospholipids and Sphingolipids

Several non-apoptotic regulated cell death pathways have been recently reported. Necroptosis, a form of necrotic-regulated cell death, is characterized by the involvement of receptor-interacting protein kinases and/or the pore-forming mixed lineage kinase domain-like protein. Recent evidence suggests a key role for lipidic molecules in the regulation of necroptosis. The purpose of this mini-review is to outline the regulation of necroptosis by sphingolipids and phospholipids.

Molecular mechanisms of necroptosis and relevance for neurodegenerative diseases

Necroptosis is a regulated cell death pathway morphologically similar to necrosis that depends on the kinase activity of receptor interacting protein 3 (RIP3) and the subsequent activation of the pseudokinase mixed lineage kinase domain-like protein (MLKL), being also generally dependent on RIP1 kinase activity. Necroptosis can be recruited during pathological conditions, usually following the activation of death receptors under specific cellular contexts. In this regard, necroptosis has been implicated in the pathogenesis of multiple disorders, including acute and chronic neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases, and multiple sclerosis. Here, we summarize the molecular mechanisms regulating the induction of necroptosis and downstream effectors of this form of cell death, besides exploring non-necroptotic roles for necroptosis-related proteins that may impact on alternative cell death pathways and inflammatory mechanisms in disease. Finally, we outline the recent evidence implicating necroptosis in neurodegenerative conditions and the emerging therapeutic perspectives targeting necroptosis in these diseases.

Lipids that directly regulate innate immune signal transduction

Pattern Recognition Receptors (PRRs) detect evidence of infection and tissue damage. The activation of these receptors and their downstream signal transduction pathways initiate a protective immune response. These signaling pathways are influenced by their spatial context, and precise subcellular positioning of proteins and protein complexes in these pathways is essential for effective immune responses in vivo . This organization is not limited to transmembrane proteins that reside in specific organelles, but also to proteins that engage membrane lipid head groups for proper positioning. In this review, we focus on the role of cell membranes and protein–lipid interactions in innate immune signal transduction and how their mechanisms of localization regulate the immune response. We will discuss how lipids spatially regulate the sensing of damage or infection, mediate effector activity, and serve as messengers of cell death and tissue damage.

Plant immune signaling: Advancing on two frontiers

Plants have evolved multiple defense strategies to cope with pathogens, among which plant immune signaling that relies on cell-surface localized and intracellular receptors takes fundamental roles. Exciting breakthroughs were made recently on the signaling mechanisms of pattern recognition receptors (PRRs) and intracellular nucleotide-binding site (NBS) and leucine-rich repeat (LRR) domain receptors (NLRs). This review summarizes the current view of PRRs activation, emphasizing the most recent discoveries about PRRs' dynamic regulation and signaling mechanisms directly leading to downstream molecular events including mitogen-activated protein kinase (MAPK) activation and calcium (Ca²⁺ ) burst. Plants also have evolved intracellular NLRs to perceive the presence of specific pathogen effectors and trigger more robust immune responses. We also discuss the current understanding of the mechanisms of NLR activation, which has been greatly advanced by recent breakthroughs including structures of the first full-length plant NLR complex, findings of NLR sensor-helper pairs and novel biochemical activity of Toll/interleukin-1 receptor (TIR) domain.

A lipid perspective on regulated cell death

Lipids are fundamental to life as structural components of cellular membranes and for signaling. They are also key regulators of different cellular processes such as cell division, proliferation, and death. Regulated cell death (RCD) requires the engagement of lipids and lipid metabolism for the initiation and execution of its killing machinery. The permeabilization of lipid membranes is a hallmark of RCD that involves, for each kind of cell death, a unique lipid profile. While the permeabilization of the mitochondrial outer membrane allows the release of apoptotic factors to the cytosol during apoptosis, permeabilization of the plasma membrane facilitates the release of intracellular content in other nonapoptotic types of RCD like necroptosis and ferroptosis. Lipids and lipid membranes are important accessory molecules required for the activation of protein executors of cell death such as BAX in apoptosis and MLKL in necroptosis. Peroxidation of membrane phospholipids and the subsequent membrane destabilization is a prerequisite to ferroptosis. Here, we discuss how lipids are essential players in apoptosis, the most common form of RCD, and also their role in necroptosis and ferroptosis. Altogether, we aim to highlight the contribution of lipids and membrane dynamics in cell death regulation.

Molecular Insights into the Mechanism of Necroptosis: The Necrosome As a Potential Therapeutic Target

Necroptosis, or regulated necrosis, is an important type of programmed cell death in addition to apoptosis. Necroptosis induction leads to cell membrane disruption, inflammation and vascularization. It plays important roles in various pathological processes, including neurodegeneration, inflammatory diseases, multiple cancers, and kidney injury. The molecular regulation of necroptotic pathway has been intensively studied in recent years. Necroptosis can be triggered by multiple stimuli and this pathway is regulated through activation of receptor-interacting protein kinase 1 (RIPK1), RIPK3 and pseudokinase mixed lineage kinase domain-like (MLKL). A better understanding of the mechanism of regulation of necroptosis will further aid to the development of novel drugs for necroptosis-associated human diseases. In this review, we focus on new insights in the regulatory machinery of necroptosis. We further discuss the role of necroptosis in different pathologies, its potential as a therapeutic target and the current status of clinical development of drugs interfering in the necroptotic pathway.

Understanding allosteric interactions in hMLKL protein that modulate necroptosis and its inhibition

Mixed Lineage Kinase domain-Like (MLKL), a key player in necroptosis, is a multi-domain protein with an N-terminal 4 helical bundle (4HB) and a pseudokinase domain (PsK) connected by brace helices. Phosphorylation of PsK domain of MLKL is a key step towards oligomerization of 4HB domain that causes cell death. Necrosulfonamide (NSA) binds to the 4HB domain of MLKL to inhibit necroptosis. To understand the molecular details of MLKL function and it's inhibition, we have performed a molecular dynamic study on hMLKL protein in apo, phosphorylated and NSA-bound states for a total 3 μs simulation time. Our simulations show increased inter-domain flexibility, increased rigidification of the activation loop and increased alpha helical content in the brace helix region revealing a form of monomeric hMLKL necessary for oligomerization upon phosphorylation as compared to apo state. NSA binding disrupts this activated form and causes two main effects on hMLKL conformation: (1) locking of the relative orientation of 4HB and PsK domains by the formation of several new interactions and (2) prevention of key 4HB residues to participate in cross-linking for oligomer formation. This new understanding of the effect of hMLKL conformations on phosphorylation and NSA binding suggest new avenues for designing effective allosteric inhibitors of hMLKL.

Current translational potential and underlying molecular mechanisms of necroptosis

Cell death has a fundamental impact on the evolution of degenerative disorders, autoimmune processes, inflammatory diseases, tumor formation and immune surveillance. Over the past couple of decades extensive studies have uncovered novel cell death pathways, which are independent of apoptosis. Among these is necroptosis, a tightly regulated, inflammatory form of cell death. Necroptosis contribute to the pathogenesis of many diseases and in this review, we will focus exclusively on necroptosis in humans. Necroptosis is considered a backup mechanism of apoptosis, but the in vivo appearance of necroptosis indicates that both caspase-mediated and caspase-independent mechanisms control necroptosis. Necroptosis is regulated on multiple levels, from the transcription, to the stability and posttranslational modifications of the necrosome components, to the availability of molecular interaction partners and the localization of receptor-interacting serine/threonine-protein kinase 1 (RIPK1), receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and mixed lineage kinase domain-like protein (MLKL). Accordingly, we classified the role of more than seventy molecules in necroptotic signaling based on consistent in vitro or in vivo evidence to understand the molecular background of necroptosis and to find opportunities where regulating the intensity and the modality of cell death could be exploited in clinical interventions. Necroptosis specific inhibitors are under development, but >20 drugs, already used in the treatment of various diseases, have the potential to regulate necroptosis. By listing necroptosis-modulated human diseases and cataloging the currently available drug-repertoire to modify necroptosis intensity, we hope to kick-start approaches with immediate translational potential. We also indicate where necroptosis regulating capacity should be considered in the current applications of these drugs.

Uncovering human mixed lineage kinase domain-like activation in necroptosis

MLKL and its obligate upstream receptor interacting protein kinase 3 are essential components of necroptosis. It is well established that MLKL is the executioner of plasma membrane rupture in necroptosis. In healthy cells MLKL is dormant. Several dormant configurations have emerged from high-resolution structural studies revealing distinct mechanisms of MLKL autoinhibition in mammals. MLKL is activated through the concerted actions of receptor interacting protein kinase 3, which phosphorylates MLKL, and, in the case of the human pathway, inositol phosphate (IP) metabolites synthesized by the IP kinases of the IP metabolic pathway. Here, we highlight recent progress toward understanding the mechanisms of regulation of human MLKL, and survey the latest opportunities for targeting MLKL in pathophysiology.

Membrane Disruption by Very Long Chain Fatty Acids during Necroptosis

Necroptosis is a form of regulated cell death which results in loss of plasma membrane integrity, release of intracellular contents, and an associated inflammatory response. We previously found that saturated very long chain fatty acids (VLCFAs), which contain ≥20 carbons, accumulate during necroptosis. Here, we show that genetic knockdown of Fatty Acid (FA) Elongase 7 (ELOVL7) reduces accumulation of specific very long chain FAs during necroptosis, resulting in reduced necroptotic cell death and membrane permeabilization. Conversely, increasing the expression of ELOVL7 increases very long chain fatty acids and membrane permeabilization. In vitro, introduction of the VLCFA C24 FA disrupts bilayer integrity in liposomes to a greater extent than a conventional C16 FA. To investigate the microscopic origin of these observations, atomistic Molecular Dynamics (MD) simulations were performed. MD simulations suggest that fatty acids cause clear differences in bilayers based on length and that it is the interdigitation of C24 FA between the individual leaflets that results in disorder in the region and, consequently, membrane disruption. We synthesized clickable VLCFA analogs and observed that many proteins were acylated by VLCFAs during necroptosis. Taken together, these results confirm the active role of VLCFAs during necroptosis and point to multiple potential mechanisms of membrane disruption including direct permeabilization via bilayer disruption and permeabilization by targeting of proteins to cellular membranes by fatty acylation.

MLKL mediates apoptosis via a mutual regulation with PERK/eIF2α pathway in response to reactive oxygen species generation

The pseudokinase mixed lineage kinase domain-like protein (MLKL) is a core effector of necroptosis, and its function in necroptosis is widely studied. However, the function of MLKL in apoptosis remains unclear. In the present study, the role of MLKL in chelerythrine (CHE)-promoted apoptosis was studied. A special band of MLKL (i.e., *MLKL) was observed after treatment with CHE. MLKL and *MLKL were accumulated in the nucleus upon treatment with CHE and MLKL silencing reversed the CHE-induced apoptosis. Blockade of CHE-triggered reactive oxygen species (ROS) generation or inhibition of CHE-activated protein kinase-like endoplasmic reticulum kinase (PERK)-eukaryotic initiation factor 2 α subunit (eIF2α) pathway reversed the apoptosis. A decreased ROS level inhibited CHE-mediated nuclear translocation of MLKL and *MLKL and the activation of eIF2α, whereas MLKL or eIF2α silencing did not affect the CHE-triggered ROS generation. Furthermore, MLKL silencing prevented the CHE-activated eIF2α signal, and eIF2α silencing blocked the CHE-induced nuclear translocation of MLKL and *MLKL. Our studies suggested that CHE possibly induces apoptosis through the nuclear translocation of MLKL and *MLKL, which is promoted by a mutual regulation between MLKL and PERK-eIF2α pathway in response to ROS formation. The present study clarified the new function of MLKL in apoptosis.

Guidelines for evaluating myocardial cell death

Cell death is a fundamental process in cardiac pathologies. Recent studies have revealed multiple forms of cell death, and several of them have been demonstrated to underlie adverse cardiac remodeling and heart failure. With the expansion in the area of myocardial cell death and increasing concerns over rigor and reproducibility, it is important and timely to set a guideline for the best practices of evaluating myocardial cell death. There are six major forms of regulated cell death observed in cardiac pathologies, namely apoptosis, necroptosis, mitochondrial-mediated necrosis, pyroptosis, ferroptosis, and autophagic cell death. In this article, we describe the best methods to identify, measure, and evaluate these modes of myocardial cell death. In addition, we discuss the limitations of currently practiced myocardial cell death mechanisms.

Help wanted: helper NLRs and plant immune responses

Plant nucleotide-binding domain and leucine-rich repeat-containing (NLR) proteins function as intracellular receptors in response to pathogens and activate effector-triggered immune responses (ETI). The activation of some sensor NLRs (sNLR) by their corresponding pathogen effector is well studied. However, the mechanisms by which the recently defined helper NLRs (hNLR) function to transduce sNLR activation into ETI-associated cell death and disease resistance remains poorly understood. We briefly summarize recent examples of sNLR activation and we then focus on hNLR requirements in sNLR-initiated immune responses. We further discuss how shared sequence homology with fungal self-incompatibility proteins and the mammalian mixed lineage kinase domain like pseudokinase (MLKL) proteins informs a plausible model for the structure and function of an ancient clade of plant hNLRs, called RNLs.

Redox (phospho)lipidomics of signaling in inflammation and programmed cell death

In addition to the known prominent role of polyunsaturated (phospho)lipids as structural blocks of biomembranes, there is an emerging understanding of another important function of these molecules as a highly diversified signaling language utilized for intra- and extracellular communications. Technological developments in high-resolution mass spectrometry facilitated the development of a new branch of metabolomics, redox lipidomics. Analysis of lipid peroxidation reactions has already identified specific enzymatic mechanisms responsible for the biosynthesis of several unique signals in response to inflammation and regulated cell death programs. Obtaining comprehensive information about millions of signals encoded by oxidized phospholipids, represented by thousands of interactive reactions and pleiotropic (patho)physiological effects, is a daunting task. However, there is still reasonable hope that significant discoveries, of at least some of the important contributors to the overall overwhelmingly complex network of interactions triggered by inflammation, will lead to the discovery of new small molecule regulators and therapeutic modalities. For example, suppression of the production of AA-derived pro-inflammatory mediators, HXA3 and LTB4, by an iPLA2 γ inhibitor, R-BEL, mitigated injury associated with the activation of pro-inflammatory processes in animals exposed to whole-body irradiation. Further, technological developments promise to make redox lipidomics a powerful approach in the arsenal of diagnostic and therapeutic instruments for personalized medicine of inflammatory diseases and conditions.

Necroptosis mediators RIPK3 and MLKL suppress intracellular Listeria replication independently of host cell killing

RIPK3, a key mediator of necroptosis, has been implicated in the host defense against viral infection primary in immune cells. However, gene expression analysis revealed that RIPK3 is abundantly expressed not only in immune organs but also in the gastrointestinal tract, particularly in the small intestine. We found that orally inoculated Listeria monocytogenes, a bacterial foodborne pathogen, efficiently spread and caused systemic infection in Ripk3-deficient mice while almost no dissemination was observed in wild-type mice. Listeria infection activated the RIPK3-MLKL pathway in cultured cells, which resulted in suppression of intracellular replication of Listeria Surprisingly, Listeria infection-induced phosphorylation of MLKL did not result in host cell killing. We found that MLKL directly binds to Listeria and inhibits their replication in the cytosol. Our findings have revealed a novel functional role of the RIPK3-MLKL pathway in nonimmune cell-derived host defense against Listeria invasion, which is mediated through cell death-independent mechanisms.

Direct Activation of Human MLKL by a Select Repertoire of Inositol Phosphate Metabolites

Necroptosis is an inflammatory form of programmed cell death executed through plasma membrane rupture by the pseudokinase mixed lineage kinase domain-like (MLKL). We previously showed that MLKL activation requires metabolites of the inositol phosphate (IP) pathway. Here we reveal that I(1,3,4,6)P₄, I(1,3,4,5,6)P₅, and IP₆ promote membrane permeabilization by MLKL through directly binding the N-terminal executioner domain (NED) and dissociating its auto-inhibitory region. We show that IP₆ and inositol pentakisphosphate 2-kinase (IPPK) are required for necroptosis as IPPK deletion ablated IP₆ production and inhibited necroptosis. The NED auto-inhibitory region is more extensive than originally described and single amino acid substitutions along this region induce spontaneous necroptosis by MLKL. Activating IPs bind three sites with affinity of 100-600 μM to destabilize contacts between the auto-inhibitory region and NED, thereby promoting MLKL activation. We therefore uncover MLKL's activating switch in NED triggered by a select repertoire of IP metabolites.

Regulated necrosis in kidney ischemia-reperfusion injury

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

The pseudokinase MLKL regulates hepatic insulin sensitivity independently of inflammation

Objective

The mixed lineage kinase domain like (MLKL) protein, receptor interacting protein (RIPK) 1, and RIPK3 are key regulators of necroptosis, a highly pro-inflammatory mode of cell death that has been implicated in various pathological processes and human diseases. However, the role of these necroptotic regulators in diabetes remains unknown. Here we sought to delineate the role of MLKL in insulin resistance and type 2 diabetes (T2D).

Methods

We first analyzed the expression of key necroptotic regulators in obese/diabetic mouse models. We then utilized MLKL knockout (MLKL^−/−) mice to evaluate the effects of MLKL on obesity-induced metabolic complications. We further determined the consequences of MLKL inhibition on hepatic insulin signaling and explored the underlying mechanism. Finally, we assessed the potential therapeutic effects of necroptotic inhibitor, necrostatin-1 (Nec-1), in ob/ob mice.

Results

In wild-type or obese mice (ob/ob, db/db, or diet-induced obesity), MLKL was increased in certain obesity-associated tissues, particularly in the liver. Whole-body deficiency of MLKL prevented obesity-induced insulin resistance and glucose intolerance. Inhibition of MLKL or other key necroptotic regulators enhanced hepatic insulin sensitivity. MLKL modulated insulin-stimulated PI(3,4,5)P3 production in liver cells but did not affect the expression of inflammatory genes in vitro and in vivo. Nec-1 administration ameliorated insulin resistance and glucose intolerance in ob/ob mice.

Conclusions

These findings reveal MLKL as a regulator of insulin sensitivity and suggest necroptotic regulators might be potential therapeutic targets for insulin resistance and T2D.

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Hepatic MLKL is increased in diabetic mouse models.

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MLKL deficiency prevents obesity-induced metabolic complications.

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Inhibition of MLKL enhances insulin sensitivity independent of inflammation.

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Pharmacological inhibition of RIPK1 in ob/ob mice ameliorates insulin resistance.

Phosphoinositides: multipurpose cellular lipids with emerging roles in cell death

Phosphorylated phosphatidylinositol lipids, or phosphoinositides, critically regulate diverse cellular processes, including signalling transduction, cytoskeletal reorganisation, membrane dynamics and cellular trafficking. However, phosphoinositides have been inadequately investigated in the context of cell death, where they are mainly regarded as signalling secondary messengers. However, recent studies have begun to highlight the importance of phosphoinositides in facilitating cell death execution. Here, we cover the latest phosphoinositide research with a particular focus on phosphoinositides in the mechanisms of cell death. This progress article also raises key questions regarding the poorly defined role of phosphoinositides, particularly during membrane-associated events in cell death such as apoptosis and secondary necrosis. The review then further discusses important future directions for the phosphoinositide field, including therapeutically targeting phosphoinositides to modulate cell death.

Pyroptosis versus necroptosis: similarities, differences, and crosstalk

Pyroptosis and necroptosis represent two pathways of genetically encoded necrotic cell death. Although these cell death programmes can protect the host against microbial pathogens, their dysregulation has been implicated in a variety of autoimmune and auto-inflammatory conditions. The disease-promoting potential of necroptosis and pyroptosis is likely a consequence of their ability to induce a lytic cell death. This cell suicide mechanism, distinct from apoptosis, allows the release of immunogenic cellular content, including damage-associated molecular patterns (DAMPs), and inflammatory cytokines such as interleukin-1β (IL-1β), to trigger inflammation. In this Review, we discuss recent discoveries that have advanced our understanding on the primary functions of pyroptosis and necroptosis, including evidence for the specific cytokines and DAMPs responsible for driving inflammation. We compare the similar and unique aspects of pyroptotic- and necroptotic-induced membrane damage, and explore how these may functionally impact distinct intracellular organelles and signalling pathways. We also examine studies highlighting the crosstalk that can occur between necroptosis and pyroptosis signalling, and evidence supporting the physiological significance of this convergence. Ultimately, a better understanding of the similarities, unique aspects and crosstalk of pyroptosis and necroptosis will inform as to how these cell death pathways might be manipulated for therapeutic benefit.

Interferons Transcriptionally Up-Regulate MLKL Expression in Cancer Cells

Interferons (IFNs) are key players in the tumor immune response and act by inducing the expression of IFN-stimulated genes (ISGs). Here, we identify the mixed-lineage kinase domain-like pseudokinase (MLKL) as an ISG in various cancer cell lines. Both type I and type II IFNs increase the expression of MLKL indicating that MLKL up-regulation is a general feature of IFN signaling. IFNγ up-regulates mRNA as well as protein levels of MLKL demonstrating that IFNγ transcriptionally regulates MLKL. This notion is further supported by Actinomycin D chase experiments showing that IFNγ-stimulated up-regulation of MLKL is prevented in the presence of the transcriptional inhibitor Actinomycin D. Also, knockdown of the transcription factor IFN-regulatory factor 1 (IRF1) and signal transducer and activator of transcription (STAT) 1 as well as knockout of IRF1 significantly attenuate IFNγ-mediated induction of MLKL mRNA levels. Up-regulation of MLKL by IFNγ provides a valuable tool to sensitize cells towards necroptotic cell death and to overcome apoptosis resistance of cancer cells.

The Structural Basis of Necroptotic Cell Death Signaling

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

Receptor-interacting Ser/Thr kinase 1 (RIPK1) and myosin IIA-dependent ceramidosomes form membrane pores that mediate blebbing and necroptosis

Formation of membrane pores/channels regulates various cellular processes, such as necroptosis or stem cell niche signaling. However, the roles of membrane lipids in the formation of pores and their biological functions are largely unknown. Here, using the cellular stress model evoked by the sphingolipid analog drug FTY720, we show that formation of ceramide-enriched membrane pores, referred to here as ceramidosomes, is initiated by a receptor-interacting Ser/Thr kinase 1 (RIPK1)-ceramide complex transported to the plasma membrane by nonmuscle myosin IIA-dependent trafficking in human lung cancer cells. Molecular modeling/simulation coupled with site-directed mutagenesis revealed that Asp¹⁴⁷ or Asn¹⁶⁹ of RIPK1 are key for ceramide binding and that Arg²⁵⁸ or Leu²⁹³ residues are involved in the myosin IIA interaction, leading to ceramidosome formation and necroptosis. Moreover, generation of ceramidosomes independently of any external drug/stress stimuli was also detected in the plasma membrane of germ line stem cells in ovaries during the early stages of oogenesis in Drosophila melanogaster Inhibition of ceramidosome formation via myosin IIA silencing limited germ line stem cell signaling and abrogated oogenesis. In conclusion, our findings indicate that the RIPK1-ceramide complex forms large membrane pores we named ceramidosomes. They further suggest that, in addition to their roles in stress-mediated necroptosis, these ceramide-enriched pores also regulate membrane integrity and signaling and might also play a role in D. melanogaster ovary development.

MLKL-PITPα signaling-mediated necroptosis contributes to cisplatin-triggered cell death in lung cancer A549 cells

Necroptosis has been reported to be involved in cisplatin-induced cell death, but the mechanisms underlying the occurrence of necroptosis are not fully elucidated. In this study, we show that apart from apoptosis, cisplatin induces necroptosis in A549 cells. The alleviation of cell death by two necroptosis inhibitors-necrostatin-1 (Nec-1) and necrosulfonamide (NSA), and the phosphorylation of mixed lineage kinase domain-like protein (MLKL) at serine 358, suggest the involvement of receptor-interacting protein kinase 1 (RIPK1)-RIPK3-MLKL signaling in cisplatin-treated A549 cells. Additionally, the initiation of cisplatin-induced necroptosis relies on autocrine tumor necrosis factor alpha (TNF-α). Furthermore, we present the first evidence that phosphatidylinositol transfer protein alpha (PITPα) is involved in MLKL-mediated necroptosis by interacting with the N terminal MLKL on its sixth helix and the preceding loop, which facilitates MLKL oligomerization and plasma membrane translocation in necroptosis. Silencing of PITPα expression interferes with MLKL function and reduces cell death. Our data elucidate that cisplatin-treated lung cancer cells undergo a new type of programmed cell death called necroptosis and shed new light on how MLKL translocates to the plasma membrane.

A FRET biosensor for necroptosis uncovers two different modes of the release of DAMPs

Necroptosis is a regulated form of necrosis that depends on receptor-interacting protein kinase (RIPK)3 and mixed lineage kinase domain-like (MLKL). While danger-associated molecular pattern (DAMP)s are involved in various pathological conditions and released from dead cells, the underlying mechanisms are not fully understood. Here we develop a fluorescence resonance energy transfer (FRET) biosensor, termed SMART (a sensor for MLKL activation by RIPK3 based on FRET). SMART is composed of a fragment of MLKL and monitors necroptosis, but not apoptosis or necrosis. Mechanistically, SMART monitors plasma membrane translocation of oligomerized MLKL, which is induced by RIPK3 or mutational activation. SMART in combination with imaging of the release of nuclear DAMPs and Live-Cell Imaging for Secretion activity (LCI-S) reveals two different modes of the release of High Mobility Group Box 1 from necroptotic cells. Thus, SMART and LCI-S uncover novel regulation of the release of DAMPs during necroptosis.

Necroptotic cells activate MLKL and release inflammatory DAMPs, although the underlying regulatory mechanisms of this process are poorly understood. Here, Murai et al. develop a necroptosis-specific FRET sensor (SMART) that monitors MLKL membrane translocation to identify two modes of DAMP release.

Characterization of MLKL-mediated Plasma Membrane Rupture in Necroptosis

Necroptosis is a programmed cell death pathway triggered by activation of receptor interacting protein kinase 3 (RIPK3), which phosphorylates and activates the mixed lineage kinase-like domain pseudokinase, MLKL, to rupture or permeabilize the plasma membrane. Necroptosis is an inflammatory pathway associated with multiple pathologies including autoimmunity, infectious and cardiovascular diseases, stroke, neurodegeneration, and cancer. Here, we describe protocols that can be used to characterize MLKL as the executioner of plasma membrane rupture in necroptosis. We visualize the process of necroptosis in cells using live-cell imaging with conventional and confocal fluorescence microscopy, and in fixed cells using electron microscopy, which together revealed the redistribution of MLKL from the cytosol to the plasma membrane prior to induction of large holes in the plasma membrane. We present in vitro nuclear magnetic resonance (NMR) analysis using lipids to identify putative modulators of MLKL-mediated necroptosis. Based on this method, we identified quantitative lipid-binding preferences and phosphatidyl-inositol phosphates (PIPs) as critical binders of MLKL that are required for plasma membrane targeting and permeabilization in necroptosis.