Caspase-7 activates ASM to repair gasdermin and perforin pores

Igniting hope: Harnessing NLRP3 inflammasome-GSDMD-mediated pyroptosis for cancer immunotherapy

In the contemporary landscape of oncology, immunotherapy, represented by immune checkpoint blockade (ICB) therapy, stands out as a beacon of innovation in cancer treatment. Despite its promise, the therapy's progression is hindered by suboptimal clinical response rates. Addressing this challenge, the modulation of the NLRP3 inflammasome-GSDMD-mediated pyroptosis pathway holds promise as a means to augment the efficacy of immunotherapy. In the pathway, the NLRP3 inflammasome serves as a pivotal molecular sensor that responds to inflammatory stimuli within the organism. Its activation leads to the release of cytokines interleukin 1β and interleukin 18 through the cleavage of GSDMD, thereby forming membrane pores and potentially resulting in pyroptosis. This cascade of processes exerts a profound impact on tumor development and progression, with its function and expression exhibiting variability across different tumor types and developmental stages. Consequently, understanding the specific roles of the NLRP3 inflammasome and GSDMD-mediated pyroptosis in diverse tumors is imperative for comprehending tumorigenesis and crafting precise therapeutic strategies. This review aims to elucidate the structure and activation mechanisms of the NLRP3 inflammasome, as well as the induction mechanisms of GSDMD-mediated pyroptosis. Additionally, we provide a comprehensive overview of the involvement of this pathway in various cancer types and its applications in tumor immunotherapy, nanotherapy, and other fields. Emphasis is placed on the feasibility of leveraging this approach to enhance ICB therapy within the field of immunotherapy. Furthermore, we discuss the potential applications of this pathway in other immunotherapy methods, such as chimeric antigen receptor T-cell (CAR-T) therapy and tumor vaccines.

Exploring caspase functions in mouse models

Caspases are enzymes with protease activity. Despite being known for more than three decades, caspase investigation still yields surprising and fascinating information. Initially associated with cell death and inflammation, their functions have gradually been revealed to extend beyond, targeting pathways such as cell proliferation, migration, and differentiation. These processes are also associated with disease mechanisms, positioning caspases as potential targets for numerous pathologies including inflammatory, neurological, metabolic, or oncological conditions. While in vitro studies play a crucial role in elucidating molecular pathways, they lack the context of the body's complexity. Therefore, laboratory animals are an indispensable part of successfully understanding and applying caspase networks. This paper aims to summarize and discuss recent knowledge, understanding, and challenges in caspase knock-out mice.

Palmitoylation at a conserved cysteine residue facilitates gasdermin D-mediated pyroptosis and cytokine release

Gasdermin D (GSDMD)-mediated pyroptotic cell death drives inflammatory cytokine release and downstream immune responses upon inflammasome activation, which play important roles in host defense and inflammatory disorders. Upon activation by proteases, the GSDMD N-terminal domain (NTD) undergoes oligomerization and membrane translocation in the presence of lipids to assemble pores. Despite intensive studies, the molecular events underlying the transition of GSDMD from an autoinhibited soluble form to an oligomeric pore form inserted into the membrane remain incompletely understood. Previous work characterized S-palmitoylation for gasdermins from bacteria, fungi, invertebrates, as well as mammalian gasdermin E (GSDME). Here, we report that a conserved residue Cys191 in human GSDMD was S-palmitoylated, which promoted GSDMD-mediated pyroptosis and cytokine release. Mutation of Cys191 or treatment with palmitoyltransferase inhibitors cyano-myracrylamide (CMA) or 2-bromopalmitate (2BP) suppressed GSDMD palmitoylation, its localization to the membrane and dampened pyroptosis or IL-1β secretion. Furthermore, Gsdmd-dependent inflammatory responses were alleviated by inhibition of palmitoylation in vivo. By contrast, coexpression of GSDMD with palmitoyltransferases enhanced pyroptotic cell death, while introduction of exogenous palmitoylation sequences fully restored pyroptotic activities to the C191A mutant, suggesting that palmitoylation-mediated membrane localization may be distinct from other molecular events such as GSDMD conformational change during pore assembly. Collectively, our study suggests that S-palmitoylation may be a shared regulatory mechanism for GSDMD and other gasdermins, which points to potential avenues for therapeutically targeting S-palmitoylation of gasdermins in inflammatory disorders.

NLRP3, NLRP6, and NLRP12 are inflammasomes with distinct expression patterns

Inflammasomes are sensors that detect cytosolic microbial molecules or cellular damage, and in response they initiate a form of lytic regulated cell death called pyroptosis. Inflammasomes signal via homotypic protein-protein interactions where CARD or PYD domains are crucial for recruiting downstream partners. Here, we screened these domains from NLR family proteins, and found that the PYD domain of NLRP6 and NLRP12 could activate caspase-1 to induce cleavage of IL-1β and GSDMD. Inflammasome reconstitution verified that full length NLRP6 and NLRP12 formed inflammasomes in vitro, and NLRP6 was more prone to auto-activation. NLRP6 was highly expressed in intestinal epithelial cells (IEC), but not in immune cells. Molecular phylogeny analysis found that NLRP12 was closely related to NLRP3, but the activation mechanisms are different. NLRP3 was highly expressed in monocytes and macrophages, and was modestly but appreciably expressed in neutrophils. In contrast, NLRP12 was specifically expressed in neutrophils and eosinophils, but was not detectable in macrophages. NLRP12 mutations cause a periodic fever syndrome called NLRP12 autoinflammatory disease. We found that several of these patient mutations caused spontaneous activation of caspase-1 in vitro, which likely causes their autoinflammatory disease. Different cell types have unique cellular physiology and structures which could be perturbed by a pathogen, necessitating expression of distinct inflammasome sensors to monitor for signs of infection.

Dihydroartemisinin Modulates Enteric Glial Cell Heterogeneity to Alleviate Colitis

The precise mechanism underlying the therapeutic effects of dihydroartemisinin (DHA) in alleviating colitis remains incompletely understood. A strong correlation existed between the elevation of glial fibrillary acidic protein (GFAP)+/S100 calcium binding protein B (S100β)+ enteric glial cells (EGCs) in inflamed colonic tissues and the disruption of the intestinal epithelial barrier (IEB) and gut vascular barrier (GVB) observed in chronic colitis. DHA demonstrated efficacy in restoring the functionality of the dual gut barrier while concurrently attenuating intestinal inflammation. Mechanistically, DHA inhibited the transformation of GFAP+ EGCs into GFAP+/S100β+ EGCs while promoting the differentiation of GFAP+/S100β+ EGCs back into GFAP+ EGCs. Furthermore, DHA induced apoptosis in GFAP+/S100β+ EGCs by inducing cell cycle arrest at the G0/G1 phase. The initial mechanism is further validated that DHA regulates EGC heterogeneity by improving dysbiosis in colitis. These findings underscore the multifaceted therapeutic potential of DHA in ameliorating colitis by improving dysbiosis, modulating EGC heterogeneity, and preserving gut barrier integrity, thus offering promising avenues for novel therapeutic strategies for inflammatory bowel diseases.

AIM2 enhances Candida albicans infection through promoting macrophage apoptosis via AKT signaling

Candida albicans is among the most prevalent invasive fungal pathogens for immunocompromised individuals and novel therapeutic approaches that involve immune response modulation are imperative. Absent in melanoma 2 (AIM2), a pattern recognition receptor for DNA sensing, is well recognized for its involvement in inflammasome formation and its crucial role in safeguarding the host against various pathogenic infections. However, the role of AIM2 in host defense against C. albicans infection remains uncertain. This study reveals that the gene expression of AIM2 is induced in human and mouse innate immune cells or tissues after C. albicans infection. Furthermore, compared to their wild-type (WT) counterparts, Aim2-/- mice surprisingly exhibit resistance to C. albicans infection, along with reduced inflammation in the kidneys post-infection. The resistance of Aim2-/- mice to C. albicans infection is not reliant on inflammasome or type I interferon production. Instead, Aim2-/- mice display lower levels of apoptosis in kidney tissues following infection than WT mice. The deficiency of AIM2 in macrophages, but not in dendritic cells, results in a phenocopy of the resistance observed in Aim2-/- mice against C. albican infection. The treatment of Clodronate Liposome, a reagent that depletes macrophages, also shows the critical role of macrophages in host defense against C. albican infection in Aim2-/- mice. Furthermore, the reduction in apoptosis is observed in Aim2-/- mouse macrophages following infection or treatment of DNA from C. albicans in comparison with controls. Additionally, higher levels of AKT activation are observed in Aim2-/- mice, and treatment with an AKT inhibitor reverses the host resistance to C. albicans infection. The findings collectively demonstrate that AIM2 exerts a negative regulatory effect on AKT activation and enhances macrophage apoptosis, ultimately compromising host defense against C. albicans infection. This suggests that AIM2 and AKT may represent promising therapeutic targets for the management of fungal infections.

Atorvastatin causes developmental and behavioral toxicity in yellowstripe goby (Mugilogobius chulae) embryos/larvae via disrupting lipid metabolism and autophagy processes

Atorvastatin (ATV) is one of the most commonly prescribed lipid-lowering drugs detected frequently in the environment due to its high use and low degradation rate. However, the toxic effects of residual ATV in the aquatic environment on non-target organisms and its toxic mechanisms are still largely unknown. In the present study, embryos of a native estuarine benthic fish, Mugilogobius chulae, were employed to investigate the developmental and behavioral toxic effects of ATV including environmentally relevant concentrations. The aim of this study was to provide a scientific basis for ecological risk assessment of ATV in the aquatic environment by investigating the changes of biological endpoints at multiple levels in M. chulae embryos/larvae. The results showed that ATV had significantly lethal and teratogenic effects on M. chulae embryos/larvae and caused abnormal changes in developmental parameters including hatch rate, body length, heart rate, and spontaneous movement. ATV exposure caused oxidative stress in M. chulae embryos/larvae subsequently inhibited autophagy and activated apoptosis, leading to abnormal developmental processes and behavioral changes in M. chulae embryos/larvae. The disruptions of lipid metabolism, autophagy, and apoptosis in M. chulae embryos/larvae caused by ATV exposure may pose a potential ecological risk at the population level.

Inflammasomes in epithelial innate immunity: front line warriors

Our epithelium represents a battle ground against a variety of insults including pathogens and danger signals. It encodes multiple sensors that detect and respond to such insults, playing an essential role in maintaining and defending tissue homeostasis. One key set of defense mechanisms is our inflammasomes which drive innate immune responses including, sensing and responding to pathogen attack, through the secretion of pro-inflammatory cytokines and cell death. Identification of physiologically relevant triggers for inflammasomes has greatly influenced our ability to decipher the mechanisms behind inflammasome activation. Furthermore, identification of patient mutations within inflammasome components implicates their involvement in a range of epithelial diseases. This review will focus on exploring the roles of inflammasomes in epithelial immunity and cover: the diversity and differential expression of inflammasome sensors amongst our epithelial barriers, their ability to sense local infection and damage and the contribution of the inflammasomes to epithelial homeostasis and disease.

Synthetic protein circuits for programmable control of mammalian cell death

Natural cell death pathways such as apoptosis and pyroptosis play dual roles: they eliminate harmful cells and modulate the immune system by dampening or stimulating inflammation. Synthetic protein circuits capable of triggering specific death programs in target cells could similarly remove harmful cells while appropriately modulating immune responses. However, cells actively influence their death modes in response to natural signals, making it challenging to control death modes. Here, we introduce naturally inspired "synpoptosis" circuits that proteolytically regulate engineered executioner proteins and mammalian cell death. These circuits direct cell death modes, respond to combinations of protease inputs, and selectively eliminate target cells. Furthermore, synpoptosis circuits can be transmitted intercellularly, offering a foundation for engineering synthetic killer cells that induce desired death programs in target cells without self-destruction. Together, these results lay the groundwork for programmable control of mammalian cell death.

ASM variants in the spotlight: A structure-based atlas for unraveling pathogenic mechanisms in lysosomal acid sphingomyelinase

Lysosomal acid sphingomyelinase (ASM), a critical enzyme in lipid metabolism encoded by the SMPD1 gene, plays a crucial role in sphingomyelin hydrolysis in lysosomes. ASM deficiency leads to acid sphingomyelinase deficiency, a rare genetic disorder with diverse clinical manifestations, and the protein can be found mutated in other diseases. We employed a structure-based framework to comprehensively understand the functional implications of ASM variants, integrating pathogenicity predictions with molecular insights derived from a molecular dynamics simulation in a lysosomal membrane environment. Our analysis, encompassing over 400 variants, establishes a structural atlas of missense variants of lysosomal ASM, associating mechanistic indicators with pathogenic potential. Our study highlights variants that influence structural stability or exert local and long-range effects at functional sites. To validate our predictions, we compared them to available experimental data on residual catalytic activity in 135 ASM variants. Notably, our findings also suggest applications of the resulting data for identifying cases suited for enzyme replacement therapy. This comprehensive approach enhances the understanding of ASM variants and provides valuable insights for potential therapeutic interventions.

Strategies of bacterial detection by inflammasomes

Mammalian innate immunity is regulated by pattern-recognition receptors (PRRs) and guard proteins, which use distinct strategies to detect infections. PRRs detect bacterial molecules directly, whereas guards detect host cell manipulations by microbial virulence factors. Despite sensing infection through different mechanisms, both classes of innate immune sensors can activate the inflammasome, an immune complex that can mediate cell death and inflammation. Inflammasome-mediated immune responses are crucial for host defense against many bacterial pathogens and prevent invasion by non-pathogenic organisms. In this review, we discuss the mechanisms by which inflammasomes are stimulated by PRRs and guards during bacterial infection, and the strategies used by virulent bacteria to evade inflammasome-mediated immunity.

New insights into Gasdermin D pore formation

Gasdermin D (GSDMD) is a pore-forming protein that perforates the plasma membrane (PM) during pyroptosis, a pro-inflammatory form of cell death, to induce the unconventional secretion of inflammatory cytokines and, ultimately, cell lysis. GSDMD is activated by protease-mediated cleavage of its active N-terminal domain from the autoinhibitory C-terminal domain. Inflammatory caspase-1, -4/5 are the main activators of GSDMD via either the canonical or non-canonical pathways of inflammasome activation, but under certain stimuli, caspase-8 and other proteases can also activate GSDMD. Activated GSDMD can oligomerize and assemble into various nanostructures of different sizes and shapes that perforate cellular membranes, suggesting plasticity in pore formation. Although the exact mechanism of pore formation has not yet been deciphered, cysteine residues are emerging as crucial modulators of the oligomerization process. GSDMD pores and thus the outcome of pyroptosis can be modulated by various regulatory mechanisms. These include availability of activated GSDMD at the PM, control of the number of GSDMD pores by PM repair mechanisms, modulation of the lipid environment and post-translational modifications. Here, we review the latest findings on the mechanisms that induce GSDMD to form membrane pores and how they can be tightly regulated for cell content release and cell fate modulation.

DHX9 maintains epithelial homeostasis by restraining R-loop-mediated genomic instability in intestinal stem cells

Epithelial barrier dysfunction and crypt destruction are hallmarks of inflammatory bowel disease (IBD). Intestinal stem cells (ISCs) residing in the crypts play a crucial role in the continuous self-renewal and rapid recovery of intestinal epithelial cells (IECs). However, how ISCs are dysregulated in IBD remains poorly understood. Here, we observe reduced DHX9 protein levels in IBD patients, and mice with conditional DHX9 depletion in the intestinal epithelium (Dhx9ΔIEC) exhibit an increased susceptibility to experimental colitis. Notably, Dhx9ΔIEC mice display a significant reduction in the numbers of ISCs and Paneth cells. Further investigation using ISC-specific or Paneth cell-specific Dhx9-deficient mice demonstrates the involvement of ISC-expressed DHX9 in maintaining epithelial homeostasis. Mechanistically, DHX9 deficiency leads to abnormal R-loop accumulation, resulting in genomic instability and the cGAS-STING-mediated inflammatory response, which together impair ISC function and contribute to the pathogenesis of IBD. Collectively, our findings highlight R-loop-mediated genomic instability in ISCs as a risk factor in IBD.

The gasdermin family: emerging therapeutic targets in diseases

The gasdermin (GSDM) family has garnered significant attention for its pivotal role in immunity and disease as a key player in pyroptosis. This recently characterized class of pore-forming effector proteins is pivotal in orchestrating processes such as membrane permeabilization, pyroptosis, and the follow-up inflammatory response, which are crucial self-defense mechanisms against irritants and infections. GSDMs have been implicated in a range of diseases including, but not limited to, sepsis, viral infections, and cancer, either through involvement in pyroptosis or independently of this process. The regulation of GSDM-mediated pyroptosis is gaining recognition as a promising therapeutic strategy for the treatment of various diseases. Current strategies for inhibiting GSDMD primarily involve binding to GSDMD, blocking GSDMD cleavage or inhibiting GSDMD-N-terminal (NT) oligomerization, albeit with some off-target effects. In this review, we delve into the cutting-edge understanding of the interplay between GSDMs and pyroptosis, elucidate the activation mechanisms of GSDMs, explore their associations with a range of diseases, and discuss recent advancements and potential strategies for developing GSDMD inhibitors.

Potential mechanism of ginseng in the treatment of periodontitis based on network pharmacology and molecular docking

OBJECTIVES

To explore the mechanism of ginseng in the treatment of periodontitis based on network pharmacology and molecular docking technology.

METHODS

Potential targets of ginseng and periodontitis were obtained through various databases. The intersection targets of ginseng and periodontitis were obtained by using VENNY, the protein-protein interaction network relationship diagram was formed on the STRING platform, the core target diagram was formed by Cytoscape software, and the ginseng-active ingredient-target network diagram was constructed. The selected targets were screened for gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analysis. The core targets of ginseng's active ingredients in treating periodontitis were analyzed by molecular docking technique.

RESULTS

The 22 ginseng's active ingredients, 591 potential targets of ginseng's active ingredients, 2 249 periodontitis gene targets, and 145 ginseng-periodontitis intersection targets were analyzed. Ginseng had strong binding activity on core targets such as vascular endothelial growth factor A and epidermal growth factor receptor, as well as hypoxia induced-factor 1 (HIF-1) signaling pathway and phosphatidylinositol 3-kinase-protein kinase B (PI3K-Akt) signaling pathway.

CONCLUSIONS

Ginseng and its active components can regulate several signaling pathways such as HIF-1 and PI3K-Akt, thereby indicating that ginseng may play a role in treating periodontitis through multiple pathways.

Caspase-1 activates gasdermin A in non-mammals

Gasdermins oligomerize to form pores in the cell membrane, causing regulated lytic cell death called pyroptosis. Mammals encode five gasdermins that can trigger pyroptosis: GSDMA, B, C, D, and E. Caspase and granzyme proteases cleave the linker regions of and activate GSDMB, C, D, and E, but no endogenous activation pathways are yet known for GSDMA. Here, we perform a comprehensive evolutionary analysis of the gasdermin family. A gene duplication of GSDMA in the common ancestor of caecilian amphibians, reptiles, and birds gave rise to GSDMA-D in mammals. Uniquely in our tree, amphibian, reptile, and bird GSDMA group in a separate clade than mammal GSDMA. Remarkably, GSDMA in numerous bird species contain caspase-1 cleavage sites like YVAD or FASD in the linker. We show that GSDMA from birds, amphibians, and reptiles are all cleaved by caspase-1. Thus, GSDMA was originally cleaved by the host-encoded protease caspase-1. In mammals the caspase-1 cleavage site in GSDMA is disrupted; instead, a new protein, GSDMD, is the target of caspase-1. Mammal caspase-1 uses exosite interactions with the GSDMD C-terminal domain to confer the specificity of this interaction, whereas we show that bird caspase-1 uses a stereotypical tetrapeptide sequence to confer specificity for bird GSDMA. Our results reveal an evolutionarily stable association between caspase-1 and the gasdermin family, albeit a shifting one. Caspase-1 repeatedly changes its target gasdermin over evolutionary time at speciation junctures, initially cleaving GSDME in fish, then GSDMA in amphibians/reptiles/birds, and finally GSDMD in mammals.

Plasma membrane repair empowers the necrotic survivors as innate immune modulators

The plasma membrane is crucial to the survival of animal cells, and damage to it can be lethal, often resulting in necrosis. However, cells possess multiple mechanisms for repairing the membrane, which allows them to maintain their integrity to some extent, and sometimes even survive. Interestingly, cells that survive a near-necrosis experience can recognize sub-lethal membrane damage and use it as a signal to secrete chemokines and cytokines, which activate the immune response. This review will present evidence of necrotic cell survival in both in vitro and in vivo systems, including in C. elegans, mouse models, and humans. We will also summarize the various membrane repair mechanisms cells use to maintain membrane integrity. Finally, we will propose a mathematical model to illustrate how near-death experiences can transform dying cells into innate immune modulators for their microenvironment. By utilizing their membrane repair activity, the biological effects of cell death can extend beyond the mere elimination of the cells.

Death and survival from executioner caspase activation

Executioner caspases are evolutionarily conserved regulators of cell death under apoptotic stress. Activated executioner caspases drive apoptotic cell death through cleavage of diverse protein substrates or pyroptotic cell death in the presence of gasdermin E. On the other hand, activation of executioner caspases can also trigger pro-survival and pro-proliferation signals. In recent years, a growing body of studies have demonstrated that cells can survive from executioner caspase activation in response to stress and that the survivors undergo molecular and phenotypic alterations. This review focuses on death and survival from executioner caspase activation, summarizing the role of executioner caspases in apoptotic and pyroptotic cell death and discussing the potential mechanism and consequences of survival from stress-induced executioner caspase activation.

Pyroptosis in health and disease

The field of cell death has witnessed significant advancements since the initial discovery of apoptosis in the 1970s. This review delves into the intricacies of pyroptosis, a more recently identified form of regulated, lytic cell death, and explores the roles of pyroptotic effector molecules, with a strong emphasis on their mechanisms and relevance in various diseases. Pyroptosis, characterized by its proinflammatory nature, is driven by the accumulation of large plasma membrane pores comprised of gasdermin family protein subunits. In different contexts of cellular homeostatic perturbations, infections, and tissue damage, proteases, such as caspase-1 and caspase-4/5, play pivotal roles in pyroptosis by cleaving gasdermins. Gasdermin-D (GSDMD), the most extensively studied member of the gasdermin protein family, is expressed in various immune cells and certain epithelial cells. Upon cleavage by caspases, GSDMD oligomerizes and forms transmembrane pores in the cell membrane, leading to the release of proinflammatory cytokines. GSDMD-N, the NH2-terminal fragment, displays an affinity for specific lipids, contributing to its role in pore formation in pyroptosis. While GSDMD is the primary focus, other gasdermin family members are also discussed in detail. These proteins exhibit distinct tissue-specific functions and contribute to different facets of cell death regulation. Additionally, genetic variations in some gasdermins have been linked to diseases, underscoring their clinical relevance. Furthermore, the interplay between GSDM pores and the activation of other effectors, such as ninjurin-1, is elucidated, providing insights into the complexity of pyroptosis regulation. The findings underscore the molecular mechanisms that govern pyroptosis and its implications for various physiological and pathological processes.

To die or not to die: Gasdermins in intestinal health and disease

Intestinal homeostasis is achieved by the balance among intestinal epithelium, immune cells, and gut microbiota. Gasdermins (GSDMs), a family of membrane pore forming proteins, can trigger rapid inflammatory cell death in the gut, mainly pyroptosis and NETosis. Importantly, there is increasing literature on the non-cell lytic roles of GSDMs in intestinal homeostasis and disease. While GSDMA is low and PJVK is not expressed in the gut, high GSDMB and GSDMC expression is found almost restrictively in intestinal epithelial cells. Conversely, GSDMD and GSDME show more ubiquitous expression among various cell types in the gut. The N-terminal region of GSDMs can be liberated for pore formation by an array of proteases in response to pathogen- and danger-associated signals, but it is not fully understood what cell type-specific mechanisms activate intestinal GSDMs. The host relies on GSDMs for pathogen defense, tissue tolerance, and cancerous cell death; however, pro-inflammatory milieu caused by pyroptosis and excessive cytokine release may favor the development and progression of inflammatory bowel disease and cancer. Therefore, a thorough understanding of spatiotemporal mechanisms that control gasdermin expression, activation, and function is essential for the development of future therapeutics for intestinal disorders.

Control of Cell Death in Health and Disease

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

Response of lipid metabolism, energy supply, and cell fate in yellowstripe goby (Mugilogobius chulae) exposed to environmentally relevant concentrations atorvastatin

The usage of typical pharmaceuticals and personal care products (PPCPs) such as cardiovascular and lipid-modulating drugs in clinical care accounts for the largest share of pharmaceutical consumption in most countries. Atorvastatin (ATV), one of the most commonly used lipid-lowering drugs, is frequently detected with lower concentrations in aquatic environments owing to its wide application, low removal, and degradation rates. However, the adverse effects of ATV on non-target aquatic organisms, especially the molecular mechanisms behind the toxic effects, still remain unclear. Therefore, this study investigated the potentially toxic effects of ATV exposure (including environmental concentrations) on yellowstripe goby (Mugilogobius chulae) and addressed the multi-dimensional responses. The results showed that ATV caused typical hepatotoxicity to M. chulae. ATV interfered with lipid metabolism by blocking fatty acid β-oxidation and led to the over-consumption of lipids. Thus, the exposed organism was obliged to alter the energy supply patterns and substrates utilization pathways to keep the normal energy supply. In addition, the higher concentration of ATV exposure caused oxidative stress to the organism. Subsequently, M. chulae triggered the autophagy and apoptosis processes with the help of key stress-related transcriptional regulators FOXOs and Sestrins to degrade the damaged organelles and proteins to maintain intracellular homeostasis.

The Septin Cytoskeleton is Required for Plasma Membrane Repair

Mammalian cells are frequently exposed to mechanical and biochemical stressors resulting in plasma membrane injuries. Repair mechanisms reseal the plasma membrane to restore homeostasis and prevent cell death. In the present work, a silencing RNA screen was performed to uncover plasma membrane repair mechanisms of cells exposed to a pore-forming toxin (listeriolysin O). This screen identified molecules previously known to repair the injured plasma membrane such as annexin A2 (ANXA2) as well as novel plasma membrane repair candidate proteins. Of the novel candidates, we focused on septin 7 (SEPT7) because the septins are an important family of conserved eukaryotic cytoskeletal proteins. Using diverse experimental approaches, we established for the first time that SEPT7 plays a general role in plasma membrane repair of cells perforated by pore-forming toxins and mechanical wounding. Remarkably, upon cell injury, the septin cytoskeleton is extensively redistributed in a Ca 2+ -dependent fashion, a hallmark of plasma membrane repair machineries. The septins reorganize into subplasmalemmal domains arranged as knob and loop (or ring) structures containing F-actin, myosin II, and annexin A2 (ANXA2) and protrude from the cell surface. Importantly, the formation of these domains correlates with the plasma membrane repair efficiency. Super-resolution microscopy shows that septins and actin are arranged in intertwined filaments associated with ANXA2. Silencing SEPT7 expression prevented the formation of the F-actin/myosin II/ANXA2 domains, however, silencing expression of ANXA2 had no observable effect on their formation. These results highlight the key structural role of the septins in remodeling the plasma membrane and in the recruitment of the repair molecule ANXA2. Collectively, our data support a novel model in which the septin cytoskeleton acts as a scaffold to promote the formation of plasma membrane repair domains containing contractile F-actin and annexin A2.

Caspase-1 activates gasdermin A in non-mammals

Gasdermins oligomerize to form pores in the cell membrane, causing regulated lytic cell death called pyroptosis. Mammals encode five gasdermins that can trigger pyroptosis: GSDMA, B, C, D, and E. Caspase and granzyme proteases cleave the linker regions of and activate GSDMB, C, D, and E, but no endogenous activation pathways are yet known for GSDMA. Here, we perform a comprehensive evolutionary analysis of the gasdermin family. A gene duplication of GSDMA in the common ancestor of caecilian amphibians, reptiles and birds gave rise to GSDMA-D in mammals. Uniquely in our tree, amphibian, reptile and bird GSDMA group in a separate clade than mammal GSDMA. Remarkably, GSDMA in numerous bird species contain caspase-1 cleavage sites like YVAD or FASD in the linker. We show that GSDMA from birds, amphibians, and reptiles are all cleaved by caspase-1. Thus, GSDMA was originally cleaved by the host-encoded protease caspase-1. In mammals the caspase-1 cleavage site in GSDMA is disrupted; instead, a new protein, GSDMD, is the target of caspase-1. Mammal caspase-1 uses exosite interactions with the GSDMD C-terminal domain to confer the specificity of this interaction, whereas we show that bird caspase-1 uses a stereotypical tetrapeptide sequence to confer specificity for bird GSDMA. Our results reveal an evolutionarily stable association between caspase-1 and the gasdermin family, albeit a shifting one. Caspase-1 repeatedly changes its target gasdermin over evolutionary time at speciation junctures, initially cleaving GSDME in fish, then GSDMA in amphibians/reptiles/birds, and finally GSDMD in mammals.

miR-223-3p Prevents Necroptotic Macrophage Death by Targeting Ripk3 in a Negative Feedback Loop and Consequently Ameliorates Advanced Atherosclerosis

BACKGROUND

The formation of large necrotic cores results in vulnerable atherosclerotic plaques, which can lead to severe cardiovascular diseases. However, the specific regulatory mechanisms underlying the development of necrotic cores remain unclear.

METHODS

To evaluate how the modes of lesional cell death are reprogrammed during the development of atherosclerosis, the expression levels of key proteins that are involved in the necroptotic, apoptotic, and pyroptotic pathways were compared between different stages of plaques in humans and mice. Luciferase assays and loss-of-function studies were performed to identify the microRNA-mediated regulatory mechanism that protects foamy macrophages from necroptotic cell death. The role of this mechanism in atherosclerosis was determined by using a knockout mouse model with perivascular drug administration and tail vein injection of microRNA inhibitors in Apoe-/- mice.

RESULTS

Here, we demonstrate that the necroptotic, rather than the apoptotic or pyroptotic, pathway is more activated in advanced unstable plaques compared with stable plaques in both humans and mice, which closely correlates with necrotic core formation. The upregulated expression of Ripk3 (receptor-interacting protein kinase 3) promotes the C/EBPβ (CCAAT/enhancer binding protein beta)-dependent transcription of the microRNA miR-223-3p, which conversely inhibits Ripk3 expression and forms a negative feedback loop to regulate the necroptosis of foamy macrophages. The knockout of the Mir223 gene in bone marrow cells accelerates atherosclerosis in Apoe-/- mice, but this effect can be rescued by Ripk3 deficiency or treatment with the necroptosis inhibitors necrostatin-1 and GSK-872. Like the Mir223 knockout, treating Apoe-/- mice with miR-223-3p inhibitors increases atherosclerosis.

CONCLUSIONS

Our study suggests that miR-223-3p expression in macrophages protects against atherosclerotic plaque rupture by limiting the formation of necrotic cores, thus providing a potential microRNA therapeutic candidate for atherosclerosis.

Apoptotic signaling clears engineered Salmonella in an organ-specific manner

Pyroptosis and apoptosis are two forms of regulated cell death that can defend against intracellular infection. When a cell fails to complete pyroptosis, backup pathways will initiate apoptosis. Here, we investigated the utility of apoptosis compared to pyroptosis in defense against an intracellular bacterial infection. We previously engineered Salmonella enterica serovar Typhimurium to persistently express flagellin, and thereby activate NLRC4 during systemic infection in mice. The resulting pyroptosis clears this flagellin-engineered strain. We now show that infection of caspase-1 or gasdermin D deficient macrophages by this flagellin-engineered S. Typhimurium induces apoptosis in vitro. Additionally, we engineered S. Typhimurium to translocate the pro-apoptotic BH3 domain of BID, which also triggers apoptosis in macrophages in vitro. During mouse infection, the apoptotic pathway successfully cleared these engineered S. Typhimurium from the intestinal niche but failed to clear the bacteria from the myeloid niche in the spleen or lymph nodes. In contrast, the pyroptotic pathway was beneficial in defense of both niches. To clear an infection, cells may have specific tasks that they must complete before they die; different modes of cell death could initiate these 'bucket lists' in either convergent or divergent ways.

Gasdermin D-Mediated Pyroptosis in Diabetic Cardiomyopathy: Molecular Mechanisms and Pharmacological Implications

Diabetic cardiomyopathy (DCM) is a pathophysiological condition triggered by diabetes mellitus (DM), which can lead to heart failure (HF). One of the most important cellular processes associated with DCM is the death of cardiomyocytes. Gasdermin D (GSDMD) plays a key role in mediating pyroptosis, a type of programmed cell death closely associated with inflammasome activation. Recent studies have revealed that pyroptosis is induced during hyperglycemia, which is crucial to the development of DCM. Although the effects of pyroptosis on DCM have been discussed, the relationship between DCM and GSDMD is not fully clarified. Recent studies gave us the impetus for clarifying the meaning of GSDMD in DCM. The purpose of this review is to summarize new and emerging insights, mainly discussing the structures of GSDMD and the mechanism of pore formation, activation pathways, molecular mechanisms of GSDMD-mediated pyroptosis, and the therapeutic potential of GSDMD in DCM. The implications of this review will pave the way for a new therapeutic target in DCM.

Gasdermin D is the only Gasdermin that provides protection against acute Salmonella gut infection in mice

Gasdermins (GSDMs) share a common functional domain structure and are best known for their capacity to form membrane pores. These pores are hallmarks of a specific form of cell death called pyroptosis and mediate the secretion of pro-inflammatory cytokines such as interleukin 1β (IL1β) and interleukin 18 (IL18). Thereby, Gasdermins have been implicated in various immune responses against cancer and infectious diseases such as acute Salmonella Typhimurium (S.Tm) gut infection. However, to date, we lack a comprehensive functional assessment of the different Gasdermins (GSDMA-E) during S.Tm infection in vivo. Here, we used epithelium-specific ablation, bone marrow chimeras, and mouse lines lacking individual Gasdermins, combinations of Gasdermins or even all Gasdermins (GSDMA1-3C1-4DE) at once and performed littermate-controlled oral S.Tm infections in streptomycin-pretreated mice to investigate the impact of all murine Gasdermins. While GSDMA, C, and E appear dispensable, we show that GSDMD i) restricts S.Tm loads in the gut tissue and systemic organs, ii) controls gut inflammation kinetics, and iii) prevents epithelium disruption by 72 h of the infection. Full protection requires GSDMD expression by both bone-marrow-derived lamina propria cells and intestinal epithelial cells (IECs). In vivo experiments as well as 3D-, 2D-, and chimeric enteroid infections further show that infected IEC extrusion proceeds also without GSDMD, but that GSDMD controls the permeabilization and morphology of the extruding IECs, affects extrusion kinetics, and promotes overall mucosal barrier capacity. As such, this work identifies a unique multipronged role of GSDMD among the Gasdermins for mucosal tissue defense against a common enteric pathogen.

Programmed Cell Death in Unicellular Versus Multicellular Organisms

Programmed cell death (self-induced) is intrinsic to all cellular life forms, including unicellular organisms. However, cell death research has focused on animal models to understand cancer, degenerative disorders, and developmental processes. Recently delineated suicidal death mechanisms in bacteria and fungi have revealed ancient origins of animal cell death that are intertwined with immune mechanisms, allaying earlier doubts that self-inflicted cell death pathways exist in microorganisms. Approximately 20 mammalian death pathways have been partially characterized over the last 35 years. By contrast, more than 100 death mechanisms have been identified in bacteria and a few fungi in recent years. However, cell death is nearly unstudied in most human pathogenic microbes that cause major public health burdens. Here, we consider how the current understanding of programmed cell death arose through animal studies and how recently uncovered microbial cell death mechanisms in fungi and bacteria resemble and differ from mechanisms of mammalian cell death.

Mechanisms of Endothelial Cell Membrane Repair: Progress and Perspectives

Endothelial cells are the crucial inner lining of blood vessels, which are pivotal in vascular homeostasis and integrity. However, these cells are perpetually subjected to a myriad of mechanical, chemical, and biological stresses that can compromise their plasma membranes. A sophisticated repair system involving key molecules, such as calcium, annexins, dysferlin, and MG53, is essential for maintaining endothelial viability. These components orchestrate complex mechanisms, including exocytosis and endocytosis, to repair membrane disruptions. Dysfunctions in this repair machinery, often exacerbated by aging, are linked to endothelial cell death, subsequently contributing to the onset of atherosclerosis and the progression of cardiovascular diseases (CVD) and stroke, major causes of mortality in the United States. Thus, identifying the core machinery for endothelial cell membrane repair is critically important for understanding the pathogenesis of CVD and stroke and developing novel therapeutic strategies for combating CVD and stroke. This review summarizes the recent advances in understanding the mechanisms of endothelial cell membrane repair. The future directions of this research area are also highlighted.

Non-Canonical Inflammasome Pathway: The Role of Cell Death and Inflammation in Ehrlichiosis

Activating inflammatory caspases and releasing pro-inflammatory mediators are two essential functions of inflammasomes which are triggered in response to pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs). The canonical inflammasome pathway involves the activation of inflammasome and its downstream pathway via the adaptor ASC protein, which causes caspase 1 activation and, eventually, the cleavage of pro-IL-1b and pro-IL-18. The non-canonical inflammasome pathway is induced upon detecting cytosolic lipopolysaccharide (LPS) by NLRP3 inflammasome in Gram-negative bacteria. The activation of NLRP3 triggers the cleavage of murine caspase 11 (human caspase 4 or caspase 5), which results in the formation of pores (via gasdermin) to cause pyroptosis. Ehrlichia is an obligately intracellular bacterium which is responsible for causing human monocytic ehrlichiosis (HME), a potentially lethal disease similar to toxic shock syndrome and septic shock syndrome. Several studies have indicated that canonical and non-canonical inflammasome activation is a crucial pathogenic mechanism that induces dysregulated inflammation and host cellular death in the pathophysiology of HME. Mechanistically, the activation of canonical and non-canonical inflammasome pathways affected by virulent Ehrlichia infection is due to a block in autophagy. This review aims to explore the significance of non-canonical inflammasomes in ehrlichiosis, and how the pathways involving caspases (with the exception of caspase 1) contribute to the pathophysiology of severe and fatal ehrlichiosis. Improving our understanding of the non-canonical inflammatory pathway that cause cell death and inflammation in ehrlichiosis will help the advancement of innovative therapeutic, preventative, and diagnostic approaches to the treatment of ehrlichiosis.

Early Endosomes Undergo Calcium-Triggered Exocytosis and Enable Repair of Diffuse and Focal Plasma Membrane Injury

Cells are routinely exposed to agents that cause plasma membrane (PM) injury. While pore-forming toxins (PFTs), and chemicals cause nanoscale holes dispersed throughout the PM, mechanical trauma causes focal lesions in the PM. To examine if these distinct injuries share common repair mechanism, membrane trafficking is monitored as the PM repairs from such injuries. During the course of repair, dispersed PM injury by the PFT Streptolysin O activates endocytosis, while focal mechanical injury to the PM inhibits endocytosis. Consequently, acute block of endocytosis prevents repair of diffuse, but not of focal injury. In contrast, a chronic block in endocytosis depletes cells of early endosomes and inhibits repair of focal injury. This study finds that both focal and diffuse PM injury activate Ca2+ -triggered exocytosis of early endosomes. The use of markers including endocytosed cargo, Rab5, Rab11, and VAMP3, all reveal injury-triggered exocytosis of early endosomes. Inhibiting Rab5 prevents injury-triggered early endosome exocytosis and phenocopies the failed PM repair of cells chronically depleted of early endosomes. These results identify early endosomes as a Ca2+ -regulated exocytic compartment, and uncover the requirement of their dual functions - endocytosis and regulated exocytosis, to differentially support PM repair based on the nature of the injury.

Epithelial inflammasomes, gasdermins, and mucosal inflammation - Lessons from Salmonella and Shigella infected mice

Besides its crucial function in nutrient absorbance and as barrier against the microbiota, the gut epithelium is essential for sensing pathogenic insults and mounting of an appropriate early immune response. In mice, the activation of the canonical NAIP/NLRC4 inflammasome is critical for the defense against enterobacterial infections. Activation of the NAIP/NLRC4 inflammasome triggers the extrusion of infected intestinal epithelial cells (IEC) into the gut lumen, concomitant with inflammasome-mediated lytic cell death. The membrane permeabilization, a hallmark of pyroptosis, is caused by the pore-forming proteins called gasdermins (GSDMs). Recent work has revealed that NAIP/NLRC4-dependent extrusion of infected IECs can, however, also be executed in the absence of GSDMD. In fact, several reports highlighted that various cell death pathways (e.g., pyroptosis or apoptosis) and unique mechanisms specific to particular infection models and stages of gut infection are in action during epithelial inflammasome defense against intestinal pathogens. Here, we summarize the current knowledge regarding the underlying mechanisms and speculate on the putative functions of the epithelial inflammasome activation and cell death, with a particular emphasis on mouse infection models for two prominent enterobacterial pathogens, Salmonella Typhimurium and Shigella flexneri.

An innate granuloma eradicates an environmental pathogen using Gsdmd and Nos2

Granulomas often form around pathogens that cause chronic infections. Here, we discover an innate granuloma model in mice with an environmental bacterium called Chromobacterium violaceum. Granuloma formation not only successfully walls off, but also clears, the infection. The infected lesion can arise from a single bacterium that replicates despite the presence of a neutrophil swarm. Bacterial replication ceases when macrophages organize around the infection and form a granuloma. This granuloma response is accomplished independently of adaptive immunity that is typically required to organize granulomas. The C. violaceum-induced granuloma requires at least two separate defense pathways, gasdermin D and iNOS, to maintain the integrity of the granuloma architecture. This innate granuloma successfully eradicates C. violaceum infection. Therefore, this C. violaceum-induced granuloma model demonstrates that innate immune cells successfully organize a granuloma and thereby resolve infection by an environmental pathogen.

Inflammasome activation occurs in CD4+ and CD8+ T cells during graft-versus-host disease

A severe complication of hematopoietic stem cell transplantation is graft-versus-host disease (GvHD), a reaction that occurs following the transfer of donor immune cells (the graft) into an allogeneic host. Transplanted cells recognize host alloantigens as foreign, resulting in the activation of donor T cells and migration of these pathological cells into host tissues. In this study, we found that caspase-1 is activated in alloreactive murine and human CD4+ and CD8+ T cells early during acute GvHD (aGvHD). The presence of inflammasome-bound active caspase-1 (p33) and ASC-speck formation confirmed inflammasome activation in these cells. We further measured gasdermin D (GSDMD) cleavage and IL-18 secretion from alloreactive T cells ex vivo. Isolated T cells with high levels of active caspase-1 had a strong inflammatory transcriptional signature and a metabolic phenotype similar to inflammatory myeloid cells, including the upregulation of proinflammatory cytokines and metabolic switch from oxidative phosphorylation to aerobic glycolysis. We also observed oxidative stress, mitochondrial dysfunction, and cell death phenotypes consistent with inflammatory cell death in alloreactive T cells. For the first time, this study characterizes caspase-1 activation in transplanted T cells during aGvHD, using mouse and human models, adding to a body of literature supporting inflammasome function in cells of the adaptive immune system.

Pyroptosis-induced inflammation and tissue damage

Pyroptosis is a programmed necrotic cell death executed by gasdermins, a family of pore-forming proteins. The cleavage of gasdermins by specific proteases enables their pore-forming activity. The activation of the prototype member of the gasdermin family, gasdermin D (GSDMD), is linked to innate immune monitoring by inflammasomes. Additional gasdermins such as GSDMA, GSDMB, GSDMC, and GSDME are activated by inflammasome-independent mechanisms. Pyroptosis is emerging as a key host defense strategy against pathogens. However, excessive pyroptosis causes cytokine storm and detrimental inflammation leading to tissue damage and organ dysfunction. Consequently, dysregulated pyroptotic responses contribute to the pathogenesis of various diseases, including sepsis, atherosclerosis, acute respiratory distress syndrome, and neurodegenerative disorders. This review will discuss the inflammatory consequences of pyroptosis and the mechanisms of pyroptosis-induced tissue damage and disease pathogenesis.

Unconventional protein secretion by gasdermin pores

Unconventional protein secretion (UPS) allows the release of specific leaderless proteins independently of the classical endoplasmic reticulum (ER)-Golgi secretory pathway. While it remains one of the least understood mechanisms in cell biology, UPS plays an essential role in immunity as it controls the release of the IL-1 family of cytokines, which coordinate host defense and inflammatory responses. The unconventional secretion of IL-1β and IL-18, the two most prominent members of the IL-1 family, is initiated by inflammasome complexes - cytosolic signaling platforms that are assembled in response to infectious or noxious stimuli. Inflammasomes activate inflammatory caspases that proteolytically mature IL-1β/- 18, but also induce pyroptosis, a lytic form of cell death. Pyroptosis is caused by gasdermin-D (GSDMD), a member of the gasdermin protein family, which is activated by caspase cleavage and forms large β-barrel plasma membrane pores. This pore-forming activity is shared with other family members that are activated during infection or upon treatment with chemotherapy drugs. While the induction of cell death was assumed to be the main function of gasdermin pores, accumulating evidence suggests that they have also non-lytic functions, such as in the release of cytokines and alarmins, or in regulating ion fluxes. This has raised the possibility that gasdermin pores are one of the main mediators of UPS. Here, I summarize and discuss new insights into gasdermin activation and pore formation, how gasdermin pores achieve selective cargo release, and how gasdermin pore formation and ninjurin-1-driven plasma membrane rupture are executed and regulated.

Apoptotic caspase-7 activation inhibits non-canonical pyroptosis by GSDMB cleavage

GSDMB is associated with several inflammatory diseases, such as asthma, sepsis and colitis. GZMA is released by cytotoxic lymphocytes and cleaves GSDMB at the K244 site and to induce GSDMB N-terminus dependent pyroptosis. This cleavage of GSDMB is noncell autonomous. In this study, we demonstrated that the GSDMB-N domain (1-91 aa) was important for a novel cell-autonomous function and that GSDMB could bind caspase-4 and promote noncanonical pyroptosis. Furthermore, activated caspase-7 cleaved GSDMB at the D91 site to block GSDMB-mediated promotion of noncanonical pyroptosis during apoptosis. Mechanistically, the cleaved GSDMB-C-terminus (92-417 aa) binds to the GSDMB-N-terminus (1-91 aa) to block the function of GSDMB. During E. coli and S. Typhimurium infection, inhibition of the caspase-7/GSDMB axis resulted in more pyroptotic cells. Furthermore, in a septic mouse model, caspase-7 inhibition or deficiency in GSDMB-transgenic mice led to more severe disease phenotypes. Overall, we demonstrate that apoptotic caspase-7 activation inhibits non-canonical pyroptosis by cleaving GSDMB and provide new targets for sepsis therapy.

Pyroptosis in defense against intracellular bacteria

Pathogenic microbes invade the human body and trigger a host immune response to defend against the infection. In response, host-adapted pathogens employ numerous virulence strategies to overcome host defense mechanisms. As a result, the interaction between the host and pathogen is a dynamic process that shapes the evolution of the host's immune response. Among the immune responses against intracellular bacteria, pyroptosis, a lytic form of cell death, is a crucial mechanism that eliminates replicative niches for intracellular pathogens and modulates the immune system by releasing danger signals. This review focuses on the role of pyroptosis in combating intracellular bacterial infection. We examine the cell type specific roles of pyroptosis in neutrophils and intestinal epithelial cells. We discuss the regulatory mechanisms of pyroptosis, including its modulation by autophagy and interferon-inducible GTPases. Furthermore, we highlight that while host-adapted pathogens can often subvert pyroptosis, environmental microbes are effectively eliminated by pyroptosis.

Bucket lists must be completed during cell death

Regulated cell death occurs in many forms, including apoptosis, pyroptosis, necroptosis, and NETosis. Most obviously, the purpose of these pathways is to kill the cell. However, many cells need to complete a set of effector programs before they die, which we define as a cellular 'bucket list'. These effector programs are specific to the cell type, and mode and circumstances of death. For example, intestinal epithelial cells need to complete the process of extrusion before they die. Cells use regulatory mechanisms to temporarily prolong their life, including endosomal sorting complex required for transport (ESCRT)- and acid sphingomyelinase (ASM)-driven membrane repair. These allow cells to complete their bucket lists before they die.

Pyroptosis modulation by bacterial effector proteins

Pyroptosis is a proinflammatory form of programmed cell death featured with membrane pore formation that causes cellular swelling and allows the release of intracellular inflammatory mediators. This cell death process is elicited by the activation of the pore-forming proteins named gasdermins, and is intricately orchestrated by diverse regulatory factors in mammalian hosts to exert a prompt immune response against infections. However, growing evidence suggests that bacterial pathogens have evolved to regulate host pyroptosis for evading immune clearance and establishing progressive infection. In this review, we highlight current understandings of the functional role and regulatory network of pyroptosis in host antibacterial immunity. Thereafter, we further discuss the latest advances elucidating the mechanisms by which bacterial pathogens modulate pyroptosis through adopting their effector proteins to drive infections. A better understanding of regulatory mechanisms underlying pyroptosis at the interface of host-bacterial interactions will shed new light on the pathogenesis of infectious diseases and contribute to the development of promising therapeutic strategies against bacterial pathogens.

Pyroptosis and the cellular consequences of gasdermin pores

The family of gasdermin proteins plays a key role in the host response against external and internal pathogenic signals by mediating the form of inflammatory regulated cell death known as pyroptosis. One of the most well-studied gasdermins within innate immunity is gasdermin D, which is cleaved, oligomerizes, and forms plasma membrane pores. Gasdermin D pores lead to a number of downstream cellular consequences including plasma membrane rupture, or cell lysis. In this review, we describe mechanisms of activation for each of the gasdermins, their cell type specificity and some disease associations. We then discuss downstream consequences of gasdermin pore formation, including cellular mechanisms of membrane repair. Finally, we present some important next steps to better understand pyroptosis and the cellular consequences of gasdermin pore formation.

Molecular mechanisms of gasdermin D pore-forming activity

The regulated disruption of the plasma membrane, which can promote cell death, cytokine secretion or both is central to organismal health. The protein gasdermin D (GSDMD) is a key player in this process. GSDMD forms membrane pores that can promote cytolysis and the release of interleukin-1 family cytokines into the extracellular space. Recent discoveries have revealed biochemical and cell biological mechanisms that control GSDMD pore-forming activity and its diverse downstream immunological effects. Here, we review these multifaceted regulatory activities, including mechanisms of GSDMD activation by proteolytic cleavage, dynamics of pore assembly, regulation of GSDMD activities by posttranslational modifications, membrane repair and the interplay of GSDMD and mitochondria. We also address recent insights into the evolution of the gasdermin family and their activities in species across the kingdoms of life. In doing so, we hope to condense recent progress and inform future studies in this rapidly moving field in immunology.

A 360° view of the inflammasome: Mechanisms of activation, cell death, and diseases

Inflammasomes are critical sentinels of the innate immune system that respond to threats to the host through recognition of distinct molecules, known as pathogen- or damage-associated molecular patterns (PAMPs/DAMPs), or disruptions of cellular homeostasis, referred to as homeostasis-altering molecular processes (HAMPs) or effector-triggered immunity (ETI). Several distinct proteins nucleate inflammasomes, including NLRP1, CARD8, NLRP3, NLRP6, NLRC4/NAIP, AIM2, pyrin, and caspases-4/-5/-11. This diverse array of sensors strengthens the inflammasome response through redundancy and plasticity. Here, we present an overview of these pathways, outlining the mechanisms of inflammasome formation, subcellular regulation, and pyroptosis, and discuss the wide-reaching effects of inflammasomes in human disease.

Gasdermins assemble; recent developments in bacteriology and pharmacology

The discovery of gasdermin D (GSDMD) as the terminal executioner of pyroptosis provided a large piece of the cell death puzzle, whilst simultaneously and firmly putting the gasdermin family into the limelight. In its purest form, GSDMD provides a connection between the innate alarm systems to an explosive, inflammatory form of cell death to jolt the local environment into immunological action. However, the gasdermin field has moved rapidly and significantly since the original seminal work and novel functions and mechanisms have been recently uncovered, particularly in response to infection. Gasdermins regulate and are regulated by mechanisms such as autophagy, metabolism and NETosis in fighting pathogen and protecting host. Importantly, activators and interactors of the other gasdermins, not just GSDMD, have been recently elucidated and have opened new avenues for gasdermin-based discovery. Key to this is the development of potent and specific tool molecules, so far a challenge for the field. Here we will cover some of these recently discovered areas in relation to bacterial infection before providing an overview of the pharmacological landscape and the challenges associated with targeting gasdermins.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Regulation of gasdermins in pyroptosis and cytokine release

Gasdermins are effectors of pyroptosis downstream of diverse signaling pathways. Emerging evidence suggests that a number of post-translational modifications regulate the function of gasdermins in pyroptosis, a highly inflammatory form of cell death, and lytic or non-lytic secretion of intracellular contents. These include processing by different caspases and other proteases that may activate or suppress pyroptosis, ubiquitination by a bacterial E3 ligase that suppresses pyroptosis as an immune evasion mechanism, modifications at Cys residues in mammalian or microbial gasdermins that promote or inhibit pyroptosis, and potential phosphorylation that represses pyroptosis. Such diverse regulatory mechanisms by host and microbial proteases, ubiquitin ligases, acyltransferases, kinases and phosphatases may underlie the divergent physiological and pathological functions of gasdermins, and furnish opportunities for therapeutic targeting of gasdermins in infectious diseases and inflammatory disorders.

An innate granuloma eradicates an environmental pathogen using Gsdmd and Nos2

Granulomas often form around pathogens that cause chronic infections. Here, we discover a novel granuloma model in mice. Chromobacterium violaceum is an environmental bacterium that stimulates granuloma formation that not only successfully walls off but also clears the infection. The infected lesion can arise from a single bacterium that replicates in the presence of a neutrophil swarm. Bacterial replication ceases when macrophages organize around the infection and form a granuloma. This granuloma response is accomplished independently of adaptive immunity that is typically required to organize granulomas. The C. violaceum -induced granuloma requires at least two separate defense pathways, gasdermin D and iNOS, to maintain the integrity of the granuloma architecture. These innate granulomas successfully eradicate C. violaceum infection. Therefore, this new C. violaceum -induced granuloma model demonstrates that innate immune cells successfully organize a granuloma and thereby eradicate infection by an environmental pathogen.

The Niemann-Pick type diseases – A synopsis of inborn errors in sphingolipid and cholesterol metabolism

Disturbances of lipid homeostasis in cells provoke human diseases. The elucidation of the underlying mechanisms and the development of efficient therapies represent formidable challenges for biomedical research. Exemplary cases are two rare, autosomal recessive, and ultimately fatal lysosomal diseases historically named "Niemann-Pick" honoring the physicians, whose pioneering observations led to their discovery. Acid sphingomyelinase deficiency (ASMD) and Niemann-Pick type C disease (NPCD) are caused by specific variants of the sphingomyelin phosphodiesterase 1 (SMPD1) and NPC intracellular cholesterol transporter 1 (NPC1) or NPC intracellular cholesterol transporter 2 (NPC2) genes that perturb homeostasis of two key membrane components, sphingomyelin and cholesterol, respectively. Patients with severe forms of these diseases present visceral and neurologic symptoms and succumb to premature death. This synopsis traces the tortuous discovery of the Niemann-Pick diseases, highlights important advances with respect to genetic culprits and cellular mechanisms, and exposes efforts to improve diagnosis and to explore new therapeutic approaches.

Resisting T cell attack: tumor-cell-intrinsic defense and reparation mechanisms

Cytotoxic T lymphocytes (CTLs) are antigen-specific killer cells equipped to identify and eliminate host cells that have been altered through infection or transformation. Both chimeric antigen-receptor (CAR) T cell therapies and immune checkpoint blockade (ICB) therapies are based on successful elimination of tumor cells by cytotoxic effectors. In this opinion article, we outline cell-intrinsic mechanisms by which tumor cells defend against CTLs, highlighting pathways that confer resistance and proposing opportunities for combination therapies. We discuss how exogenous killing entities [e.g., supramolecular attack particles (SMAPs)] offer a novel strategy to circumvent cellular resistance mechanisms. Our opinion article highlights the importance of identifying, quantifying, and targeting tumor defense mechanisms at the interface between tumor cells and CTLs as a critical consideration in the development of immunotherapy approaches.