Sequential Engagement of Distinct MLKL Phosphatidylinositol-Binding Sites Executes Necroptosis

Polymorphisms in Necroptosis Pathway Genes: Novel Prognostic Markers for Multiple Myeloma Treatment Outcomes

Multiple myeloma is a neoplastic disease characterised by the proliferation of clonal, atypical plasma cells. In cancer cells, the balance between two paths of cell death, necroptosis and apoptosis, is disrupted. The aim of this study was to analyse the occurrence of polymorphisms in genes encoding key proteins for the necroptosis process, i.e., RIPK-1, RIPK-3 and MAPKAPK2. We investigated the potential relations between the occurrence of genetic variability and the clinical course of the disease. We analysed six single-nucleotide polymorphisms in a population of patients with multiple myeloma (n = 205) and healthy volunteers (n = 100): RIPK1 rs2272990, RIPK1 rs9391981, RIPK3 rs724165, RIPK3rs3212243, MAPKAPK2, rs45514798 and MAPKAPK2 rs4073250. We found that genotypes rs9391981 CG, rs724165 CG, rs3212243 GG, and rs4073250 AA were independent predictors of overall survival, while genotype MAPKAPK2 rs4073250 AA was an independent predictor of progression-free survival. MAPKAPK2 rs45514798 AA was associated with polyneuropathy after thalidomide therapy. In conclusion, some of the SNPs tested have potential prognostic value and could be used as marker of survival in patients with multiple myeloma.

Advances in MLKL-targeted inhibitors and PROTACs for necroptosis therapeutics

Necroptosis is a highly regulated form of cell death. Mixed lineage kinase domain-like protein (MLKL) serves as its central effector and plays a critical role in various physiological and pathological processes. Given its close association with multiple diseases, MLKL has emerged as a promising therapeutic target. This review highlights recent advances in the development of necroptosis inhibitors and degraders targeting MLKL. The optimization of active compounds, structural modifications, and the applications of proteolysis-targeting chimeras (PROTACs) are emphasized. Furthermore, this study systematically evaluates the structural characteristics and biological activities of these compounds, thereby providing critical insights to inform future investigations and pharmaceutical development within this field.

RIPK3 in necroptosis and cancer

Receptor-interacting protein kinase-3 is essential for the cell death pathway called necroptosis. Necroptosis is activated by the death receptor ligands and pattern recognition receptors of the innate immune system, leading to significant consequences in inflammation and in diseases, particularly cancer. Necroptosis is highly proinflammatory compared with other modes of cell death because cell membrane integrity is lost, resulting in releases of cytokines and damage-associated molecular patterns that potentiate inflammation and activate the immune system. We discuss various ways that necroptosis is triggered along with its potential role in cancer and therapy.

Targeting Programmed Cell Death in Acquired Sensorineural Hearing Loss: Ferroptosis, Necroptosis, and Pyroptosis

Sensorineural hearing loss (SNHL), the most commonly-occurring form of hearing loss, is caused mainly by injury to or the loss of hair cells and spiral ganglion neurons in the cochlea. Numerous environmental and physiological factors have been shown to cause acquired SNHL, such as ototoxic drugs, noise exposure, aging, infections, and diseases. Several programmed cell death (PCD) pathways have been reported to be involved in SNHL, especially some novel PCD pathways that have only recently been reported, such as ferroptosis, necroptosis, and pyroptosis. Here we summarize these PCD pathways and their roles and mechanisms in SNHL, aiming to provide new insights and potential therapeutic strategies for SNHL by targeting these PCD pathways.

The function of necroptosis in liver cancer

Liver cancer is one of the most lethal cancers, and apoptosis resistance is a major obstacle contributing to chemotherapy failure in liver cancer treatment. Inducing cancer cell death by bypassing the apoptotic pathway is considered a promising approach to overcome this problem. Necroptosis is a non-caspase-dependent regulated mode of cell death mainly mediated by receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like (MLKL) protein, and the utilization of necroptosis for treating hepatocellular carcinoma (HCC) also offers a new hope for addressing liver cancer in the clinic. In this paper, the role of necroptosis in HCC as well as the effect on differentiation of liver cancer are reviewed. We also comparatively analyze the relationship among necroptosis, apoptosis, and necrosis, as well as summarize the characteristics and functions of key proteins involved in this pathway. The bidirectional regulation of necroptosis and the mitochondrial machinery within this pathway deserve attention.

Teleost GSDMEc regulates GSDMEa-mediated pyroptosis

INTRODUCTION

Gasdermin (GSDM) is a family of proteins that execute pyroptosis after being activated by caspase (CASP) cleavage. Mammals possess five GSDM members (A - E) with pyroptotic ability. Teleosts possess only one pyroptotic GSDM, GSDME, that exists in three orthologs, GSDMEa, b, and c. GSDMEa and GSDMEb are known to induce pyroptosis, but the function of GSDMEc is unknown.

OBJECTIVES

The present study aimed to elucidate the function of teleost GSDMEc and examine the interplay among teleost GSDME orthologs by using snakehead Channa argus as a representative species.

METHODS

Pyroptosis was assessed via microscopy and biochemical assays. GSDME cleavage, oligomerization, and membrane translocation were examined via immunoblotting. The interactions of GSDME products were examined using confocal microscopy and co-immunoprecipitation. GSDME knockdown in fish and in vivo bacterial infection were performed.

RESULTS

C. argus possessed three GSDME variants (CaGSDMEa, CaGSDMEc1, and CaGSDMEc2). CaGSDMEa was cleaved by C. argus CASP (CaCASP) 1/8 to produce an N-terminal fragment (NT), NT261, that induced pyroptosis. CaGSDMEc1 and CaGSDMEc2 were also cleaved by CaCASP1/8, but the resulting NTs, NT123 and NT108, respectively, were unable to induce pyroptosis. However, both NT123 and NT108 could bind and promote the pyroptotic activity of NT261 by facilitating NT261 oligomerization and membrane translocation. The interaction between NT261 and NT123/NT108 depended on a positively charged motif that is conserved in the metazoan GSDME and is essential to the membrane localization of NT123 and the pyroptotic activity of NT261. Bacterial infection induced CaGSDMEa/CaCASP8 activation and CaGSDMEc1/c2 cleavage in snakehead cells, resulting in pyroptosis, IL-1β/18 maturation cleavage, and extracellular DNA-net formation. CaGSDMEa/c1 knockdown significantly increased bacterial dissemination in fish tissues and reduced fish survival.

CONCLUSIONS

Our results revealed the functions and interactive mechanism of teleost GSDME orthologs, and provided new insights into the regulation of pyroptosis in lower vertebrates.

Optogenetically Activatable MLKL as a Standalone Functional Module for Necroptosis and Therapeutic Applications in Antitumoral Immunity

Necroptosis plays a crucial role in the progression of various diseases and has gained substantial attention for its potential to activate antitumor immunity. However, the complex signaling networks that regulate necroptosis have made it challenging to fully understand its mechanisms and translate this knowledge into therapeutic applications. To address these challenges, an optogenetically activatable necroptosis system is developed that allows for precise spatiotemporal control of key necroptosis regulators, bypassing complex upstream signaling processes. The system, specifically featuring optoMLKL, demonstrates that it can rapidly assemble into functional higher-order "hotspots" within cellular membrane compartments, independent of RIPK3-mediated phosphorylation. Moreover, the functional module of optoMLKL significantly enhances innate immune responses by promoting the release of iDAMPs and cDAMPs, which are critical for initiating antitumor immunity. Furthermore, optoMLKL exhibits antitumor effects when activated in patient-derived pancreatic cancer organoids, highlighting its potential for clinical application. These findings will pave the way for innovative cancer therapies by leveraging optogenetic approaches to precisely control and enhance necroptosis.

BBB breakdown caused by plasma membrane pore formation

The blood-brain barrier, recently reintroduced as the blood-brain border (BBB), is a dynamic interface between the central nervous system (CNS) and the bloodstream. Disruption of the BBB exposes the CNS to peripheral pathogens and harmful substances, causing or worsening various CNS diseases. While traditional views attribute BBB failure to tight junction disruption or increased transcytosis, recent studies highlight the critical role of gasdermin D (GSDMD) pore formation in brain endothelial cells (bECs) during BBB disruption by lipopolysaccharide (LPS) or bacterial infections. This mechanism may also be involved in neurological complications like the 'brain fog' seen in long COVID. Pore formation in bECs may represent a prevalent mechanism causing BBB leakage. Investigating membrane-permeabilizing pores or channels and their effects on BBB integrity is a growing area of research. Further exploration of molecular processes that maintain, disrupt, and restore bEC membrane integrity will advance our understanding of brain vasculature and aid in developing new therapies for BBB-related diseases.

MicroRNA-21 plays a role in exacerbating chronic obstructive pulmonary disease by regulating necroptosis and apoptosis in bronchial epithelial cells

INTRODUCTION

Bronchial epithelial cell damage is an important determinant of the severity of chronic obstructive pulmonary (COPD). However, the exact molecular mechanisms underlying this cell death in COPD development are not well understood. This study investigates the involvement of microRNA-21 (miR-21/miRNA-21) in COPD and its underlying molecular mechanism.

METHODS

A mouse model of COPD was created by exposing the mice to cigarette smoke (CS) and injecting them with cigarette smoke extract (CSE). Both wild-type mice and miR-21 knockout (miR-21-/-) mice were used to investigate the role of microRNA-21 (miR-21) in exacerbating COPD. Various assays and analyses were performed, including HE staining, tunel staining, enzyme-linked immunosorbent assay (ELISA), flow cytometry, quantitative real-time polymerase chain reaction (RT-qPCR), and western blotting (WB) to measure outcomes such as the pathological morphological changes, necroptosis, apoptosis, and levels of inflammatory factors.

RESULTS

Our results revealed an upregulation of miR-21 in the lung tissue of COPD model mice. Additionally, knockout of miR-21 resulted in decreased levels of bronchial epithelial cell necroptosis and apoptosis, as evidenced by the downregulation of tumor necrosis factor receptor 1 (TNFR1), phosphoryl-mixed lineage kinase domain-like protein (p-MLKL) and caspase-3. This downregulation of necroptosis and apoptosis ultimately led to a reduction of inflammatory factors and damage-associated molecular patterns (DAMPs), such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL- 1β), and interleukin-6 (IL-6) and high mobility group protein B1(HMGB1) in the lungs, thereby ameliorating COPD.

CONCLUSIONS

Our findings suggest that miR-21 contributes to the worsening of chronic obstructive pulmonary disease by modulating necroptosis and apoptosis in bronchial epithelial cells, providing a new theoretical basis for the pathogenesis of chronic obstructive pulmonary disease.

MLKL as an emerging machinery for modulating organelle dynamics: regulatory mechanisms, pathophysiological significance, and targeted therapeutics

Mixed lineage kinase domain-like protein (MLKL) is a pseudokinase featured by a protein kinase-like domain without catalytic activity. MLKL was originally discovered to be phosphorylated by receptor-interacting protein kinase 1/3, typically increase plasma membrane permeabilization, and disrupt the membrane integrity, ultimately executing necroptosis. Recent evidence uncovers the association of MLKL with diverse cellular organelles, including the mitochondrion, lysosome, endosome, endoplasmic reticulum, and nucleus. Thus, this review mainly focuses on the regulatory functions, mechanisms, and targets of MLKL in organelles rather than necroptosis and summarize the medical significance in multiple diseases. On this basis, we conclude and analyze the current progress and prospect for the development of MLKL-related drugs, from natural products, small-molecule chemical compounds, to proteolysis-targeting chimera. This review is aimed to propel the development of MLKL as a valid drug target and the discovery of novel MLKL-related drugs, and promote their further applications.

Necroptosis in obesity: a complex cell death event

Obesity is an exceedingly prevalent and frequent health issue in today's society. Fat deposition is accompanied by low-grade inflammation in fat tissue and throughout the body, leading to metabolic disorders that ultimately promote the onset of obesity-related diseases. The development of obesity is accompanied by cell death events such as apoptosis as well as pyroptosis, however, the role of necroptosis in obesity has been widely reported in recent years. Necroptosis, a mode of cell death distinct from apoptosis and necrosis, is associated with developing many inflammatory conditions and their associated diseases. It also exhibits modulation of apoptosis and pyroptosis. It is morphologically similar to necroptosis, characterized by the inhibition of caspase-8, the formation of membrane pores, and the subsequent rupture of the plasma membrane. This paper focuses on the key pathways and molecules of necroptosis, exploring its connections with apoptosis and pyroptosis, and its implications in obesity. This paper posits that the modulation of necroptosis-related targets may represent a novel potential therapeutic avenue for the prevention and treatment of obesity-induced systemic inflammatory responses, and provides a synopsis of potential molecular targets that may prove beneficial in obesity-associated inflammatory diseases.

MLKL-USP7-UBA52 signaling is indispensable for autophagy in brain through maintaining ubiquitin homeostasis

Individuals with genetic elimination of MLKL (mixed lineage kinase domain like pseudokinase) exhibit an increased susceptibility to neurodegenerative diseases like Alzheimer disease (AD). However, the mechanism is not yet fully understood. Here, we observed significant compromise in macroautophagy/autophagy in the brains of mlkl knockout (KO) mice, as evidenced by the downregulation of BECN1/Beclin1 and ULK1 (unc-51 like autophagy activating kinase 1). We identified UBA52 (ubiquitin A-52 residue ribosomal protein fusion product 1) as the binding partner of MLKL under physiological conditions. Loss of Mlkl induced a decrease in ubiquitin levels by preventing UBA52 cleavage. Furthermore, we demonstrated that the deubiquitinase (DUB) USP7 (ubiquitin specific peptidase 7) mediates the processing of UBA52, which is regulated by MLKL. Moreover, our results indicated that the reduction of BECN1 and ULK1 upon Mlkl loss is attributed to a decrease in their lysine 63 (K63)-linked polyubiquitination. Additionally, single-nucleus RNA sequencing revealed that the loss of Mlkl resulted in the disruption of multiple neurodegenerative disease-related pathways, including those associated with AD. These results were consistent with the observation of cognitive impairment in mlkl KO mice and exacerbation of AD pathologies in an AD mouse model with mlkl deletion. Taken together, our findings demonstrate that MLKL-USP7-UBA52 signaling is required for autophagy in brain through maintaining ubiquitin homeostasis, and highlight the contribution of Mlkl loss-induced ubiquitin deficits to the development of neurodegeneration. Thus, the maintenance of adequate levels of ubiquitin may provide a novel perspective to protect individuals from multiple neurodegenerative diseases through regulating autophagy.Abbreviations: 4HB: four-helix bundle; AAV: adeno-associated virus; AD: Alzheimer disease; AIF1: allograft inflammatory factor 1; APOE: apolipoprotein E; APP: amyloid beta precursor protein; Aβ: amyloid β; BECN1: beclin 1; co-IP: co-immunoprecipitation; DEGs: differentially expressed genes; DLG4: discs large MAGUK scaffold protein 4; DUB: deubiquitinase; EBSS: Earle's balanced salt solution; GFAP: glial fibrillary acidic protein; HRP: horseradish peroxidase; IL1B: interleukin 1 beta; IL6: interleukin 6; IPed: immunoprecipitated; KEGG: Kyoto Encyclopedia of Genes and Genomes; KO: knockout; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MLKL: mixed lineage kinase domain like pseudokinase; NSA: necrosulfonamide; OPCs: oligodendrocyte precursor cells; PFA: paraformaldehyde; PsKD: pseudo-kinase domain; SYP: synaptophysin; UB: ubiquitin; UBA52: ubiquitin A-52 residue ribosomal protein fusion product 1; UCHL3: ubiquitin C-terminal hydrolase L3; ULK1: unc-51 like autophagy activating kinase 1; UMAP: uniform manifold approximation and projection; UPS: ubiquitin-proteasome system; USP7: ubiquitin specific peptidase 7; USP9X: ubiquitin specific peptidase 9 X-linked.

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.

How NINJ1 mediates plasma membrane rupture and why NINJ2 cannot

Ninjurin-1 (NINJ1) is an active executioner of plasma membrane rupture (PMR), a process previously thought to be a passive osmotic lysis event in lytic cell death. Ninjurin-2 (NINJ2) is a close paralog of NINJ1 but cannot mediate PMR. Using cryogenic electron microscopy (cryo-EM), we show that NINJ1 and NINJ2 both assemble into linear filaments that are hydrophobic on one side but hydrophilic on the other. This structural feature and other evidence point to a PMR mechanism by which NINJ1 filaments wrap around and solubilize membrane fragments and, less frequently, form pores in the plasma membrane. In contrast to the straight NINJ1 filament, the NINJ2 filament is curved toward the intracellular space, preventing its circularization or even assembly on a relatively flat membrane to mediate PMR. Mutagenesis studies further demonstrate that the NINJ2 filament curvature is induced by strong association with lipids, particularly a cholesterol molecule, at the cytoplasmic leaflet of the lipid bilayer.

Unveiling cellular changes in leukaemia cell lines after cannabidiol treatment through lipidomics

The present study was aimed at revealing the metabolic changes that occurred in the cellular lipid pattern of acute and chronic myeloid leukaemia cells following treatment with cannabidiol (CBD). CBD is a non-psychoactive compound present in Cannabis sativa L., which has shown an antiproliferative action in these type of cancer cells. CBD treatment reduced cell viability and initiated apoptotic and necrotic processes in both cancer cell lines in a time and dose-dependent manner, showing acute myeloid leukaemia (HL-60) cells greater sensitivity than chronic myeloid leukaemia ones (K-562), without differences in the activation of caspases 3/7. Then, control and treated cells of HL-60 and K-562 cell lines were studied through an untargeted lipidomic approach. The treatment was carried out with CBD at a concentration of 10 μM for HL-60 cells and 23 µM CBD for K-562 cells for 48 h. After the extraction of the lipid content from cell lysates, the samples were analysed by UHPLC-QTOF-MS/MS both in the positive and the negative ionization modes. The comprehensive characterization of cellular lipids unveiled several classes significantly affected by CBD treatment. Most of the differences correspond to phospholipids, including cardiolipins (CL), phosphatidylcholines (PC) and phosphosphingolipids (SM), and also triacylglycerols (TG), being many TG species increased after CBD treatment in the acute and chronic models, whereas phospholipids were found to be decreased. The results highlight some important lipid alterations related to CBD treatment, plausibly connected with different metabolic mechanisms involved in the process of cell death by apoptosis in cancer cell lines.

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.

Regulation of ubiquitination in sepsis: from PAMP versus DAMP to peripheral inflammation and cell death

Sepsis (sepsis) is a systemic inflammatory response triggered by infection, and its pathologic features include overproduction of peripheral inflammatory factors (e.g., IL-1β, IL-6, TNF-α), which ultimately leads to cytokine storm and multiple organ dysfunction syndrome (MODS). Pathogen-associated molecular patterns (PAMP) and damage-associated molecular patterns (DAMP) induce strong immune responses and exacerbate inflammation by activating pattern recognition receptors (PRRs) in the host. Ubiquitination, as a key protein post-translational modification, dynamically regulates the activity of several inflammation-associated proteins (e.g., RIPK1, NLRP3) through the coordinated action of the E1, E2, and E3 enzymes, affects cell death pathways such as necroptosis and pyroptosis, and ultimately regulates the release of peripheral inflammatory factors. Deubiquitinating enzymes (DUBs), on the other hand, influence the intensity of the inflammatory response in sepsis by counter-regulating the ubiquitination process and balancing pro- and anti-inflammatory signals. This review focuses on how PAMP and DAMP activate inflammatory pathways via PRRs, and the central role of ubiquitination and deubiquitination in the development of sepsis, especially the mechanisms in regulating the secretion of peripheral inflammatory factors and cell death. By deeply dissecting the impact of the balance of ubiquitination and deubiquitination on inflammatory regulation, we further envision its potential as a therapeutic target in sepsis.

A new perspective on targeting pulmonary arterial hypertension: Programmed cell death pathways (Autophagy, Pyroptosis, Ferroptosis)

Pulmonary arterial hypertension (PAH) is a severe cardiovascular disease characterized by elevated pulmonary vascular resistance, progressive increases in pulmonary artery pressures, ultimately leading to right-sided heart failure, and potentially mortality. Pulmonary vascular remodeling is pivotal in PAH onset and progression. While targeted drug therapies have notably ameliorated PAH prognosis, current medications primarily focus on vascular vasodilation, with limited ability to reverse pulmonary vascular remodeling fundamentally, resulting in suboptimal patient prognoses. Cellular death in pulmonary vasculature, once thought to be confined to apoptosis and necrosis, has evolved with the identification of pyroptosis, autophagy, and ferroptosis, revealing their association with vascular injury in PAH. These novel forms of regulated cellular death impact reactive oxygen species (ROS) generation, calcium stress, and inflammatory cascades, leading to pulmonary vascular cell loss, exacerbating vascular injury, and mediating adverse remodeling, inflammation, immune anomalies, and current emerging mechanisms (such as endothelial-mesenchymal transition, abnormal energy metabolism, and epigenetic regulation) in the pathogenesis of PAH. This review comprehensively delineates the roles of autophagy, pyroptosis, and ferroptosis in PAH, elucidating recent advances in their involvement and regulation of vascular injury. It juxtaposes their distinct functions in PAH and discusses the interplay of these programmed cell deaths in pulmonary vascular injury, highlighting the benefits of combined targeted therapies in mitigating pulmonary arterial hypertension-induced vascular injury, providing novel insights into targeted treatments for pulmonary arterial hypertension.

Cell death: Revisiting the roads to ruin

A paradigm shift in the study of cell death is currently occurring: whereas previously we had always considered that there were "points of no return" in any cell death pathway, we now realize that in many types of active, regulated cell death, this is not the case. We are also learning that cells that "almost die," but nevertheless survive, can transiently take on an altered state, with potential implications for understanding cancer therapies and relapse. In this perspective, we parse the many forms of cell death by analogy to suicide, sabotage, and murder, and consider how cells that might be "instructed" to engage a cell death pathway might nevertheless survive.

The Evolution and Biological Activity of Metazoan Mixed Lineage Kinase Domain-Like Protein (MLKL)

In mammals, mixed lineage kinase domain-like protein (MLKL) is the executor of necroptosis. MLKL comprises an N-terminal domain (NTD), which alone suffices to trigger necroptosis by forming pores in the plasma membrane, and a C-terminal domain that inhibits the NTD activity. Evolutionarily, MLKL is poorly conserved in animals and not found in Protostomia. Although MLKL orthologs exist in invertebrate Deuterostomia, the biological activity of invertebrate MLKL is unknown. Herein, we examined 34 metazoan phyla and detected MLKL not only in Deuterostomia but also in Protostomia (Rotifera). The Rotifera MLKL exhibited low identities with non-Rotifera MLKL but shared relatively high identities with non-metazoan MLKL. In invertebrates, MLKL formed two phylogenetic clades, one of which was represented by Rotifera. In vertebrates, MLKL expression was tissue-specific and generally rich in immune organs. When expressed in human cells, the MLKL-NTD of Rotifera, Echinodermata, Urochordata, and Cephalochordata induced strong necroptosis. The necroptotic activity of Rotifera MLKL depended on a number of conserved residues. Together these findings provided new insights into the evolution of MLKL in Metazoa and revealed the biological activity of invertebrate MLKL.

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.

Modulatory effects of necroptosis: A potential preventive approach to control diseases in fish

Necroptosis is a caspase-independent programmed cell death process characterized by morphological similarities to necrosis and the potential to cause significant inflammatory reactions. The initiation, execution, and inhibition of necroptosis involve a complex interplay of various signaling proteins. When death receptors bind to ligands, necroptosis is triggered through the receptor-interacting serine/threonine-protein kinase 1 (RIPK1)/RIPK3/Mixed Lineage Kinase Domain-Like (MLKL) axis, leading to inflammatory reactions in the surrounding tissues. This process encompasses numerous physiological regulatory mechanisms and contributes to the development and progression of certain diseases. The mechanisms of necroptosis were not well conserved across terrestrial and aquatic organisms, with differences in some components and functions. Given the significant challenges that aquatic animal diseases pose to aquaculture, research interest in necroptosis has surged recently, particularly in studies focusing on fish. Understanding necroptosis in fish can lead to interventions that offer potential breakthroughs in disease inhibition and fish health improvement.

Intricate interplay between cell metabolism and necroptosis regulation in metabolic dysfunction-associated steatotic liver disease: A narrative review

Metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD), encompasses a progressive spectrum of liver conditions, ranging from steatosis to metabolic dysfunction-associated steatohepatitis, characterised by hepatocellular death and inflammation, potentially progressing to cirrhosis and/or liver cancer. In both experimental and human MASLD, necroptosis-a regulated immunogenic necrotic cell death pathway-is triggered, yet its exact role in disease pathogenesis remains unclear. Noteworthy, necroptosis-related signalling pathways are emerging as key players in metabolic reprogramming, including lipid and mitochondrial metabolism. Additionally, metabolic dysregulation is a well-established contributor to MASLD development and progression. This review explores the intricate interplay between cell metabolism and necroptosis regulation and its impact on MASLD pathogenesis. Understanding these cellular events may offer new insights into the complexity of MASLD pathophysiology, potentially uncovering therapeutic opportunities and unforeseen metabolic consequences of targeting necroptosis.

A kinase fusion protein from Aegilops longissima confers resistance to wheat powdery mildew

Many disease resistance genes have been introgressed into wheat from its wild relatives. However, reduced recombination within the introgressed segments hinders the cloning of the introgressed genes. Here, we have cloned the powdery mildew resistance gene Pm13, which is introgressed into wheat from Aegilops longissima, using a method that combines physical mapping with radiation-induced chromosomal aberrations and transcriptome sequencing analysis of ethyl methanesulfonate (EMS)-induced loss-of-function mutants. Pm13 encodes a kinase fusion protein, designated MLKL-K, with an N-terminal domain of mixed lineage kinase domain-like protein (MLKL_NTD domain) and a C-terminal serine/threonine kinase domain bridged by a brace. The resistance function of Pm13 is validated through transient and stable transgenic complementation assays. Transient over-expression analyses in Nicotiana benthamiana leaves and wheat protoplasts reveal that the fragment Brace-Kinase122-476 of MLKL-K is capable of inducing cell death, which is dependent on a functional kinase domain and the three α-helices in the brace region close to the N-terminus of the kinase domain.

Emerging connectivity of programmed cell death pathways and pulmonary vascular remodelling during pulmonary hypertension

Pulmonary hypertension (PH) is a chronic progressive vascular disease characterized by abnormal pulmonary vascular resistance and pulmonary artery pressure. The major structural alteration during PH is pulmonary vascular remodelling, which is mainly caused by the imbalance between proliferation and apoptosis of pulmonary vascular cells. Previously, it was thought that apoptosis was the only type of programmed cell death (PCD). Soon afterward, other types of PCD have been identified, including autophagy, pyroptosis, ferroptosis and necroptosis. In this review, we summarize the role of the above five forms of PCD in mediating pulmonary vascular remodelling, and discuss their guiding significance for PH treatment. The current review could provide a better understanding of the correlation between PCD and pulmonary vascular remodelling, contributing to identify new PCD-associated drug targets for PH.

Cell Death in Liver Disease and Liver Surgery

Cell death is crucial for maintaining tissue balance and responding to diseases. However, under pathological conditions, the surge in dying cells results in an overwhelming presence of cell debris and the release of danger signals. In the liver, this gives rise to hepatic inflammation and hepatocellular cell death, which are key factors in various liver diseases caused by viruses, toxins, metabolic issues, or autoimmune factors. Both clinical and in vivo studies strongly affirm that hepatocyte death serves as a catalyst in the progression of liver disease. This advancement is characterized by successive stages of inflammation, fibrosis, and cirrhosis, culminating in a higher risk of tumor development. In this review, we explore pivotal forms of cell death, including apoptosis, pyroptosis, and necroptosis, examining their roles in both acute and chronic liver conditions, including liver cancer. Furthermore, we discuss the significance of cell death in liver surgery and ischemia-reperfusion injury. Our objective is to illuminate the molecular mechanisms governing cell death in liver diseases, as this understanding is crucial for identifying therapeutic opportunities aimed at modulating cell death pathways.

The lipid basis of cell death and autophagy

ABBREVIATIONS

ACSL: acyl-CoA synthetase long chain family; DISC: death-inducing signaling complex; DAMPs: danger/damage-associated molecular patterns; Dtgn: dispersed trans-Golgi network; FAR1: fatty acyl-CoA reductase 1; GPX4: glutathione peroxidase 4; LPCAT3: lysophosphatidylcholine acyltransferase 3; LPS: lipopolysaccharide; MUFAs: monounsaturated fatty acids; MOMP: mitochondrial outer membrane permeabilization; MLKL, mixed lineage kinase domain like pseudokinase; oxPAPC: oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine; OxPCs: oxidized phosphatidylcholines; PUFAs: polyunsaturated fatty acids; POR: cytochrome p450 oxidoreductase; PUFAs: polyunsaturated fatty acids; RCD: regulated cell death; RIPK1: receptor interacting serine/threonine kinase 1; SPHK1: sphingosine kinase 1; SOAT1: sterol O-acyltransferase 1; SCP2: sterol carrier protein 2; SFAs: saturated fatty acids; SLC47A1: solute carrier family 47 member 1; SCD: stearoyl-CoA desaturase; VLCFA: very long chain fatty acids.

Dopaminergic neuronal death via necroptosis in Parkinson's disease: A review of the literature

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive dysfunction and loss of dopaminergic neurons of the substantia nigra pars compacta (SNc). Several pathways of programmed cell death are likely to play a role in dopaminergic neuron death, such as apoptosis, necrosis, pyroptosis and ferroptosis, as well as cell death associated with proteasomal and mitochondrial dysfunction. A better understanding of the molecular mechanisms underlying dopaminergic neuron death could inform the design of drugs that promote neuron survival. Necroptosis is a recently characterized regulated cell death mechanism that exhibits morphological features common to both apoptosis and necrosis. It requires activation of an intracellular pathway involving receptor-interacting protein 1 kinase (RIP1 kinase, RIPK1), receptor-interacting protein 3 kinase (RIP3 kinase, RIPK3) and mixed lineage kinase domain-like pseudokinase (MLKL). The potential involvement of this programmed cell death pathway in the pathogenesis of PD has been studied by analysing biomarkers for necroptosis, such as the levels and oligomerization of phosphorylated RIPK3 (pRIPK3) and phosphorylated MLKL (pMLKL), in several PD preclinical models and in PD human tissue. Although there is evidence that other types of cell death also have a role in DA neuron death, most studies support the hypothesis that this cell death mechanism is activated in PD tissues. Drugs that prevent or reduce necroptosis may provide neuroprotection for PD. In this review, we summarize the findings from these studies. We also discuss how manipulating necroptosis might open a novel therapeutic approach to reduce neuronal degeneration in PD.

Acylation of MLKL Impacts Its Function in Necroptosis

Mixed lineage kinase domain-like (MLKL) is a key signaling protein of necroptosis. Upon activation by phosphorylation, MLKL translocates to the plasma membrane and induces membrane permeabilization, which contributes to the necroptosis-associated inflammation. Membrane binding of MLKL is initially initiated by electrostatic interactions between the protein and membrane phospholipids. We previously showed that MLKL and its phosphorylated form (pMLKL) are S-acylated during necroptosis. Here, we characterize the acylation sites of MLKL and identify multiple cysteines that can undergo acylation with an interesting promiscuity at play. Our results show that MLKL and pMLKL undergo acylation at a single cysteine, with C184, C269, and C286 as possible acylation sites. Using all-atom molecular dynamic simulations, we identify differences that the acylation of MLKL causes at the protein and membrane levels. Through investigations of the S-palmitoyltransferases that might acylate pMLKL in necroptosis, we showed that zDHHC21 activity has the strongest effect on pMLKL acylation, inactivation of which profoundly reduced the pMLKL levels in cells and improved membrane integrity. These results suggest that blocking the acylation of pMLKL destabilizes the protein at the membrane interface and causes its degradation, ameliorating the necroptotic activity. At a broader level, our findings shed light on the effect of S-acylation on MLKL functioning in necroptosis and MLKL-membrane interactions mediated by its acylation.

IE1 of Human Cytomegalovirus Inhibits Necroptotic Cell Death via Direct and Indirect Modulation of the Necrosome Complex

Programmed necrosis is an integral part of intrinsic immunity, serving to combat invading pathogens and restricting viral dissemination. The orchestration of necroptosis relies on a precise interplay within the necrosome complex, which consists of RIPK1, RIPK3 and MLKL. Human cytomegalovirus (HCMV) has been found to counteract the execution of necroptosis during infection. In this study, we identify the immediate-early 1 (IE1) protein as a key antagonist of necroptosis during HCMV infection. Infection data obtained in a necroptosis-sensitive cell culture system revealed a robust regulation of post-translational modifications (PTMs) of the necrosome complex as well as the importance of IE1 expression for an effective counteraction of necroptosis. Interaction analyses unveiled an association of IE1 and RIPK3, which occurs in an RHIM-domain independent manner. We propose that this interaction manipulates the PTMs of RIPK3 by promoting its ubiquitination. Furthermore, IE1 was found to exert an indirect activity by modulating the levels of MLKL via antagonizing its interferon-mediated upregulation. Overall, we claim that IE1 performs a broad modulation of innate immune signaling to impede the execution of necroptotic cell death, thereby generating a favorable environment for efficient viral replication.

Anti-Necroptotic Effects of Itaconate and its Derivatives

Itaconate is an unsaturated dicarboxylic acid that is derived from the decarboxylation of the Krebs cycle intermediate cis-aconitate and has been shown to exhibit anti-inflammatory and anti-bacterial/viral properties. But the mechanisms underlying itaconate's anti-inflammatory activities are not fully understood. Necroptosis, a lytic form of regulated cell death (RCD), is mediated by receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL) signaling. It has been involved in the pathogenesis of organ injury in many inflammatory diseases. In this study, we aimed to explore whether itaconate and its derivatives can inhibit necroptosis in murine macrophages, a mouse MPC-5 cell line and a human HT-29 cell line in response to different necroptotic activators. Our results showed that itaconate and its derivatives dose-dependently inhibited necroptosis, among which dimethyl itaconate (DMI) was the most effective one. Mechanistically, itaconate and its derivatives inhibited necroptosis by suppressing the RIPK1/RIPK3/MLKL signaling and the oligomerization of MLKL. Furthermore, DMI promoted the nuclear translocation of Nrf2 that is a critical regulator of intracellular redox homeostasis, and reduced the levels of intracellular reactive oxygen species (ROS) and mitochondrial superoxide (mtROS) that were induced by necroptotic activators. Consistently, DMI prevented the loss of mitochondrial membrane potential induced by the necroptotic activators. In addition, DMI mitigated caerulein-induced acute pancreatitis in mice accompanied by reduced activation of the necroptotic signaling in vivo. Collectively, our study demonstrates that itaconate and its derivatives can inhibit necroptosis by suppressing the RIPK1/RIPK3/MLKL signaling, highlighting their potential applications for treating necroptosis-associated diseases.

MLKL polymerization-induced lysosomal membrane permeabilization promotes necroptosis

Mixed lineage kinase-like protein (MLKL) forms amyloid-like polymers to promote necroptosis; however, the mechanism through which these polymers trigger cell death is not clear. We have determined that activated MLKL translocates to the lysosomal membrane during necroptosis induction. The subsequent polymerization of MLKL induces lysosome clustering and fusion and eventual lysosomal membrane permeabilization (LMP). This LMP leads to the rapid release of lysosomal contents into the cytosol, resulting in a massive surge in cathepsin levels, with Cathepsin B (CTSB) as a significant contributor to the ensuing cell death as it cleaves many proteins essential for cell survival. Importantly, chemical inhibition or knockdown of CTSB protects cells from necroptosis. Furthermore, induced polymerization of the MLKL N-terminal domain (NTD) also triggers LMP, leading to CTSB release and subsequent cell death. These findings clearly establish the critical role of MLKL polymerization induced lysosomal membrane permeabilization (MPI-LMP) in the process of necroptosis.

Cell death classification: A new insight based on molecular mechanisms

Cells tend to disintegrate themselves or are forced to undergo such destructive processes in critical circumstances. This complex cellular function necessitates various mechanisms and molecular pathways in order to be executed. The very nature of cell death is essentially important and vital for maintaining homeostasis, thus any type of disturbing occurrence might lead to different sorts of diseases and dysfunctions. Cell death has various modalities and yet, every now and then, a new type of this elegant procedure gets to be discovered. The diversity of cell death compels the need for a universal organizing system in order to facilitate further studies, therapeutic strategies and the invention of new methods of research. Considering all that, we attempted to review most of the known cell death mechanisms and sort them all into one arranging system that operates under a simple but subtle decision-making (If \ Else) order as a sorting algorithm, in which it decides to place and sort an input data (a type of cell death) into its proper set, then a subset and finally a group of cell death. By proposing this algorithm, the authors hope it may solve the problems regarding newer and/or undiscovered types of cell death and facilitate research and therapeutic applications of cell death.

Acylation of MLKL impacts its function in necroptosis

Mixed lineage kinase domain-like (MLKL) is a key signaling protein of necroptosis. Upon activation by phosphorylation, MLKL translocates to the plasma membrane and induces membrane permeabilization which contributes to the necroptosis-associated inflammation. Membrane binding of MLKL is initially initiated by the electrostatic interactions between the protein and membrane phospholipids. We previously showed that MLKL and its phosphorylated form (pMLKL) are S-acylated during necroptosis. Here, we characterize acylation sites of MLKL and identify multiple cysteines that can undergo acylation with an interesting promiscuity at play. Our results show that MLKL and pMLKL undergo acylation at a single cysteine, C184, C269 and C286 are the possible acylation sites. Using all atom molecular dynamic simulations, we identify differences that the acylation of MLKL causes at the protein and membrane level. Through systematic investigations of the S-palmitoyltransferases that might acylate MLKL in necroptosis, we showed that zDHHC21 activity has the strongest effect on pMLKL acylation, inactivation of which profoundly reduced the pMLKL levels in cells and improved membrane integrity. These results suggest that blocking the acylation of pMLKL destabilizes the protein at the membrane interface and causes its degradation, ameliorating necroptotic activity. At a broader level, our findings shed light on the effect of S-acylation on MLKL functioning in necroptosis and MLKL-membrane interactions mediated by its acylation.

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.

Higher Mixed lineage Kinase Domain-like protein (MLKL) is associated with worst overall survival in adult-type diffuse glioma patients

INTRODUCTION

Recently, the search for novel molecular markers in adult-type diffuse gliomas has grown substantially, yet with few novel breakthroughs. As the presence of a necrotic center is a differential diagnosis for more aggressive entities, we hypothesized that genes involved in necroptosis may play a role in tumor progression.

AIM

Given that MLKL is the executioner of the necroptotic pathway, we evaluated whether this gene would help to predict prognosis of adult gliomas patients.

METHODS

We analyzed a publicly available retrospective cohort (n = 530) with Kaplan Meier survival analysis (p<0.0001) and both uni- and multivariate Cox regression models.

RESULTS

We determined that MLKL is an independent predictive prognostic marker for overall survival in these patients (HR: 2.56, p<0.001), even when controlled by the CNS5 gold-standard markers, namely IDH mutation and 1p/19q Codeletion (HR: 1.68, p = 0.013). These findings were confirmed in a validation cohort (n = 325), using the same cutoff value. Interestingly, higher expression of MLKL is associated with worse clinical outcome for adult-type diffuse glioma patients, which is opposite to what was found in other cell cancer types, suggesting that necroptosis undertakes an atypical detrimental role in glioma progression.

Identity, structure, and function of the mitochondrial permeability transition pore: controversies, consensus, recent advances, and future directions

The mitochondrial permeability transition (mPT) describes a Ca2+-dependent and cyclophilin D (CypD)-facilitated increase of inner mitochondrial membrane permeability that allows diffusion of molecules up to 1.5 kDa in size. It is mediated by a non-selective channel, the mitochondrial permeability transition pore (mPTP). Sustained mPTP opening causes mitochondrial swelling, which ruptures the outer mitochondrial membrane leading to subsequent apoptotic and necrotic cell death, and is implicated in a range of pathologies. However, transient mPTP opening at various sub-conductance states may contribute several physiological roles such as alterations in mitochondrial bioenergetics and rapid Ca2+ efflux. Since its discovery decades ago, intensive efforts have been made to identify the exact pore-forming structure of the mPT. Both the adenine nucleotide translocase (ANT) and, more recently, the mitochondrial F1FO (F)-ATP synthase dimers, monomers or c-subunit ring alone have been implicated. Here we share the insights of several key investigators with different perspectives who have pioneered mPT research. We critically assess proposed models for the molecular identity of the mPTP and the mechanisms underlying its opposing roles in the life and death of cells. We provide in-depth insights into current controversies, seeking to achieve a degree of consensus that will stimulate future innovative research into the nature and role of the mPTP.

Mass Spectrometry Detects Sphingolipid Metabolites for Discovery of New Strategy for Cancer Therapy from the Aspect of Programmed Cell Death

Sphingolipids, a type of bioactive lipid, play crucial roles within cells, serving as integral components of membranes and exhibiting strong signaling properties that have potential therapeutic implications in anti-cancer treatments. However, due to the diverse group of lipids and intricate mechanisms, sphingolipids still face challenges in enhancing the efficacy of different therapy approaches. In recent decades, mass spectrometry has made significant advancements in uncovering sphingolipid biomarkers and elucidating their impact on cancer development, progression, and resistance. Primary sphingolipids, such as ceramide and sphingosine-1-phosphate, exhibit contrasting roles in regulating cancer cell death and survival. The evasion of cell death is a characteristic hallmark of cancer cells, leading to treatment failure and a poor prognosis. The escape initiates with long-established apoptosis and extends to other programmed cell death (PCD) forms when patients experience chemotherapy, radiotherapy, and/or immunotherapy. Gradually, supportive evidence has uncovered the fundamental molecular mechanisms underlying various forms of PCD leading to the development of innovative molecular, genetic, and pharmacological tools that specifically target sphingolipid signaling nodes. In this study, we provide a comprehensive overview of the sphingolipid biomarkers revealed through mass spectrometry in recent decades, as well as an in-depth analysis of the six main forms of PCD (apoptosis, autophagy, pyroptosis, necroptosis, ferroptosis, and cuproptosis) in aspects of tumorigenesis, metastasis, and tumor response to treatments. We review the corresponding small-molecule compounds associated with these processes and their potential implications in cancer therapy.

Necroptosis in the pathophysiology of preeclampsia

Necroptosis is a newly-identified form of gene-regulated cell necrosis that is increasingly considered to be a pathway associated with human pathophysiological conditions. Cells undergoing necroptosis exhibit necrotic phenotypes, including disruption of the plasma membrane integrity, organelle swelling, and cytolysis. Accumulating evidence suggests that trophoblast necroptosis plays a complex role in preeclampsia (PE). However, the exact pathogenesis remains unclear. Its unique mechanisms of action in various diseases are expected to provide prospects for the treatment of PE. Therefore, it is necessary to further explore its molecular mechanism in PE in order to identify potential therapeutic options. This review examines the current knowledge regarding the role and mechanisms of necroptosis in PE and provides a theoretical basis for new therapeutic targets for PE.

SARS-CoV-2 Infection Induces HMGB1 Secretion Through Post-Translational Modification and PANoptosis

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection induces excessive pro-inflammatory cytokine release and cell death, leading to organ damage and mortality. High-mobility group box 1 (HMGB1) is one of the damage-associated molecular patterns that can be secreted by pro-inflammatory stimuli, including viral infections, and its excessive secretion levels are related to a variety of inflammatory diseases. Here, the aim of the study was to show that SARS-CoV-2 infection induced HMGB1 secretion via active and passive release. Active HMGB1 secretion was mediated by post-translational modifications, such as acetylation, phosphorylation, and oxidation in HEK293E/ACE2-C-GFP and Calu-3 cells during SARS-CoV-2 infection. Passive release of HMGB1 has been linked to various types of cell death; however, we demonstrated for the first time that PANoptosis, which integrates other cell death pathways, including pyroptosis, apoptosis, and necroptosis, is related to passive HMGB1 release during SARS-CoV-2 infection. In addition, cytoplasmic translocation and extracellular secretion or release of HMGB1 were confirmed via immunohistochemistry and immunofluorescence in the lung tissues of humans and angiotensin-converting enzyme 2-overexpressing mice infected with SARS-CoV-2.

Single-Molecule Monitoring of Membrane Association of the Necroptosis Executioner MLKL with Discernible Anchoring and Insertion Dynamics

The dynamics of membrane proteins that are well-folded in water and become functional after self-insertion into cell membranes is not well understood. Herein we report on single-molecule monitoring of membrane association dynamics of the necroptosis executioner MLKL. We observed that, upon landing, the N-terminal region (NTR) of MLKL anchors onto the surface with an oblique angle and then is immersed in the membrane. The anchoring end does not insert into the membrane, but the opposite end does. The protein is not static, switching slowly between water-exposed and membrane-embedded conformations. The results suggest a mechanism for the activation and function of MLKL in which exposure of H4 is critical for MLKL to adsorb on the membrane, and the brace helix H6 regulates MLKL rather than inhibits it. Our findings provide deeper insights into membrane association and function regulation of MLKL and would have impacts on biotechnological applications.

Human gasdermin D and MLKL disrupt mitochondria, endocytic traffic and TORC1 signalling in budding yeast

Gasdermin D (GSDMD) and mixed lineage kinase domain-like protein (MLKL) are the pore-forming effectors of pyroptosis and necroptosis, respectively, with the capacity to disturb plasma membrane selective permeability and induce regulated cell death. The budding yeast Saccharomyces cerevisiae has long been used as a simple eukaryotic model for the study of proteins associated with human diseases by heterologous expression. In this work, we expressed in yeast both GSDMD and its N-terminal domain (GSDMD(NT)) to characterize their cellular effects and compare them to those of MLKL. GSDMD(NT) and MLKL inhibited yeast growth, formed cytoplasmic aggregates and fragmented mitochondria. Loss-of-function point mutants of GSDMD(NT) showed affinity for this organelle. Besides, GSDMD(NT) and MLKL caused an irreversible cell cycle arrest through TORC1 inhibition and disrupted endosomal and autophagic vesicular traffic. Our results provide a basis for a humanized yeast platform to study GSDMD and MLKL, a useful tool for structure-function assays and drug discovery.

Activation of immune signals during organ transplantation

The activation of host's innate and adaptive immune systems can lead to acute and chronic graft rejection, which seriously impacts graft survival. Thus, it is particularly significant to clarify the immune signals, which are critical to the initiation and maintenance of rejection generated after transplantation. The initiation of response to graft is dependent on sensing of danger and stranger molecules. The ischemia and reperfusion of grafts lead to cell stress or death, followed by releasing a variety of damage-associated molecular patterns (DAMPs), which are recognized by pattern recognition receptors (PRRs) of host immune cells to activate intracellular immune signals and induce sterile inflammation. In addition to DAMPs, the graft exposed to 'non-self' antigens (stranger molecules) are recognized by the host immune system, stimulating a more intense immune response and further aggravating the graft damage. The polymorphism of MHC genes between different individuals is the key for host or donor immune cells to identify heterologous 'non-self' components in allogeneic and xenogeneic organ transplantation. The recognition of 'non-self' antigen by immune cells mediates the activation of immune signals between donor and host, resulting in adaptive memory immunity and innate trained immunity to the graft, which poses a challenge to the long-term survival of the graft. This review focuses on innate and adaptive immune cells receptor recognition of damage-associated molecular patterns, alloantigens and xenoantigens, which is described as danger model and stranger model. In this review, we also discuss the innate trained immunity in organ transplantation.

Butyrate induces development-dependent necrotizing enterocolitis-like intestinal epithelial injury via necroptosis

BACKGROUND

The accumulation of short-chain fatty acids (SCFAs) from bacterial fermentation may adversely affect the under-developed gut as observed in premature newborns at risk for necrotizing enterocolitis (NEC). This study explores the mechanism by which specific SCFA fermentation products may injure the premature newborn intestine mucosa leading to NEC-like intestinal cell injury.

METHODS

Intraluminal injections of sodium butyrate were administered to 14- and 28-day-old mice, whose small intestine and stool were harvested for analysis. Human intestinal epithelial stem cells (hIESCs) and differentiated enterocytes from preterm and term infants were treated with sodium butyrate at varying concentrations. Necrosulfonamide (NSA) and necrostatin-1 (Nec-1) were used to determine the protective effects of necroptosis inhibitors on butyrate-induced cell injury.

RESULTS

The more severe intestinal epithelial injury was observed in younger mice upon exposure to butyrate (p = 0.02). Enterocytes from preterm newborns demonstrated a significant increase in sensitivity to butyrate-induced cell injury compared to term newborn enterocytes (p = 0.068, hIESCs; p = 0.038, differentiated cells). NSA and Nec-1 significantly inhibited the cell death induced by butyrate.

CONCLUSIONS

Butyrate induces developmental stage-dependent intestinal injury that resembles NEC. A primary mechanism of cell injury in NEC is necroptosis. Necroptosis inhibition may represent a potential preventive or therapeutic strategy for NEC.

IMPACT

Butyrate induces developmental stage-dependent intestinal injury that resembles NEC. A primary mechanism of cell injury caused by butyrate in NEC is necroptosis. Necroptosis inhibitors proved effective at significantly ameliorating the enteral toxicity of butyrate and thereby suggest a novel mechanism and approach to the prevention and treatment of NEC in premature newborns.

Dietary nitrate improves jaw bone remodelling in zoledronate-treated mice

Bisphosphonate-related osteonecrosis of the jaw (BRONJ) is a serious complication that occurs in patients with osteoporosis or metastatic bone cancer treated with bisphosphonate. There is still no effective treatment and prevention strategy for BRONJ. Inorganic nitrate, which is abundant in green vegetables, has been reported to be protective in multiple diseases. To investigate the effects of dietary nitrate on BRONJ-like lesions in mice, we utilized a well-established mouse BRONJ model, in which tooth extraction was performed. Specifically, 4 mM sodium nitrate was administered in advance through drinking water to assess the short- and long-term effects on BRONJ. Zoledronate injection could induce severe healing inhibition of the tooth extraction socket, while addition of pretreating dietary nitrate could alleviate the inhibition by reducing monocyte necrosis and inflammatory cytokines production. Mechanistically, nitrate intake increased plasma nitric oxide levels, which attenuated necroptosis of monocytes by downregulating lipid and lipid-like molecule metabolism via a RIPK3 dependent pathway. Our findings revealed that dietary nitrate could inhibit monocyte necroptosis in BRONJ, regulate the bone immune microenvironment and promote bone remodelling after injury. This study contributes to the understanding of the immunopathogenesis of zoledronate and supports the feasibility of dietary nitrate for the clinical prevention of BRONJ.

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.

Necroptosis in CNS diseases: Focus on astrocytes

In the last few years, necroptosis, a recently described type of cell death, has been reported to play an important role in the development of various brain pathologies. Necroptosis is a cell death mechanism that has morphological characteristics similar to necrosis but is mediated by fundamentally different molecular pathways. Necroptosis is initiated by signaling through the interaction of RIP1/RIP3/MLKL proteins (receptor-interacting protein kinase 1/receptor-interacting protein kinase 3/mixed lineage kinase domain-like protein). RIPK1 kinase is usually inactive under physiological conditions. It is activated by stimulation of death receptors (TNFR1, TNFR2, TLR3, and 4, Fas-ligand) by external signals. Phosphorylation of RIPK1 results in the formation of its complex with death receptors. Further, complexes with the second member of the RIP3 and MLKL cascade appear, and the necroptosome is formed. There is enough evidence that necroptosis plays an important role in the pathogenesis of brain ischemia and neurodegenerative diseases. In recent years, a point of view that both neurons and glial cells can play a key role in the development of the central nervous system (CNS) pathologies finds more and more confirmation. Astrocytes play complex roles during neurodegeneration and ischemic brain damage initiating both impair and protective processes. However, the cellular and molecular mechanisms that induce pathogenic activity of astrocytes remain veiled. In this review, we consider these processes in terms of the initiation of necroptosis. On the other hand, it is important to remember that like other types of programmed cell death, necroptosis plays an important role for the organism, as it induces a strong immune response and is involved in the control of cancerogenesis. In this review, we provide an overview of the complex role of necroptosis as an important pathogenetic component of neuronal and astrocyte death in neurodegenerative diseases, epileptogenesis, and ischemic brain damage.

Omics approaches to better understand the molecular mechanism of necroptosis and their translational implications

Necroptosis is a type of programed cell death characterized by an inflammatory phenotype due to extensive membrane permeabilization and rupture. Initiation of necroptosis involves activation of tumor necrosis factor receptors by tumor necrosis factor alpha (TNFα) followed by coordinated activities of receptor-interacting protein kinases and mixed lineage kinase-like protein (MLKL). Subsequently, MLKL undergoes phosphorylation and translocates to the plasma membrane, leading to permeabilization. Such permeabilization results in the release of various cytokines and causes extensive inflammatory activity at the organismal level. This inflammatory activity is one of the major differences between apoptosis and necroptosis and links necroptosis to several human pathologies that exhibit inflammation, in addition to the ultimate cell death phenotype. Given the crosstalk between the activation of cell death pathway and inflammatory activity, approaches that provide insights on the regulation of transcripts, proteins and their processing at the global level have substantially improved our understanding of necroptosis and its involvement in different disease states. In this review, we highlight recent omic studies probing the transcriptome, proteome and lipidome which elucidate potential new mechanisms and signaling pathways during necroptosis and the necroptosis-associated inflammatory activity observed in various diseases. We specifically focus on studies investigating the transcriptome and intracellular and released proteome that contribute to inflammatory nature of necroptotic cells. We also highlight different lipids that have been implicated in necroptosis and lipidomic studies identifying lipid players in necroptosis. Finally, we review studies which suggest certain necroptosis-related genes as potential prognosis markers for different cancers and discuss their translational implications.

Modeling the molecular fingerprint of protein-lipid interactions of MLKL on complex bilayers

Lipids, the structural part of membranes, play important roles in biological functions. However, our understanding of their implication in key cellular processes such as cell division and protein-lipid interaction is just emerging. This is the case for molecular interactions in mechanisms of cell death, where the role of lipids for protein localization and subsequent membrane permeabilization is key. For example, during the last stage of necroptosis, the mixed lineage kinase domain-like (MLKL) protein translocates and, eventually, permeabilizes the plasma membrane (PM). This process results in the leakage of cellular content, inducing an inflammatory response in the microenvironment that is conducive to oncogenesis and metastasis, among other pathologies that exhibit inflammatory activity. This work presents insights from long all-atom molecular dynamics (MD) simulations of complex membrane models for the PM of mammalian cells with an MLKL protein monomer. Our results show that the binding of the protein is initially driven by the electrostatic interactions of positively charged residues. The protein bound conformation modulates lipid recruitment to the binding site, which changes the local lipid environment recruiting PIP lipids and cholesterol, generating a unique fingerprint. These results increase our knowledge of protein-lipid interactions at the membrane interface in the context of molecular mechanisms of the necroptotic pathway, currently under investigation as a potential treatment target in cancer and inflamatory diseases.

Immune infiltration and a necroptosis-related gene signature for predicting the prognosis of patients with cervical cancer

Background: Cervical cancer (CC), the fourth most common cancer among women worldwide, has high morbidity and mortality. Necroptosis is a newly discovered form of cell death that plays an important role in cancer development, progression, and metastasis. However, the expression of necroptosis-related genes (NRGs) in CC and their relationship with CC prognosis remain unclear. Therefore, we screened the signature NRGs in CC and constructed a risk prognostic model. Methods: We downloaded gene data and clinical information of patients with cervical squamous cell carcinoma and endocervical adenocarcinoma (CESC) from The Cancer Genome Atlas (TCGA) database. We performed functional enrichment analysis on the differentially expressed NRGs (DENRGs). We constructed prognostic models and evaluated them by Cox and LASSO regressions for DENRGs, and validated them using the International Cancer Genome Consortium (ICGC) dataset. We used the obtained risk score to classify patients into high- and low-risk groups. We employed the ESTIMATE and single sample gene set enrichment analysis (ssGSEA) algorithms to explore the relationship between the risk score and the clinical phenotype and the tumor immune microenvironment. Results: With LASSO regression, we established a prognostic model of CC including 16 signature DENRGs (TMP3, CHMP4C, EEF1A1, FASN, TNF, S100A10, IL1A, H1.2, SLC25A5, GLTP, IFNG, H2AC13, TUBB4B, AKNA, TYK2, and H1.5). The risk score was associated with poor prognosis in CC. Survival was lower in the high-risk group than the low-risk group. The nomogram based on the risk score, T stage, and N stage showed good prognostic predictive power. We found significant differences in immune scores, immune infiltration analysis, and immune checkpoints between the high- and low-risk groups (p < 0.05). Conclusion: We screened for DENRGs based on the TCGA database by using bioinformatics methods, and constructed prognostic models based on the signature DENRGs, which we confirmed as possibly having important biological functions in CC. Our study provides a new perspective on CC prognosis and immunity, and offers a series of new targets for future treatment.