Tumor-Intrinsic SIRPA Drives Pyroptosis Evasion in Head and Neck Cancer

Pyroptosis, a gasdermin-mediated immunogenic cell death, has been shown to elicit adaptive antitumor immune responses, thereby augmenting the response to cancer immunotherapy when pyroptosis is therapeutically activated. However, despite increased gasdermin E (GSDME) expression, significant pyroptosis remains elusive in certain tumor types, and the underlying regulatory mechanisms are poorly understood. In this study, we observed high signal regulatory protein α1 (SIRPA) expression in head and neck squamous cell carcinoma (HNSCC) cells, a target in cancer immunotherapy. Intriguingly, SIRPA inhibition markedly augmented pyroptosis activity in tumor tissues and modulated tumor growth in a HNSCC mouse model. Subsequent investigations revealed that SIRPA knockout upregulated GSDME expression and potentiated cisplatin-induced pyroptosis in cancer cells. Integrative transcriptomics and metabolomics analysis suggested that the SIRPA knockout profoundly altered protein ubiquitination and augmented argininosuccinic acid levels in cancer cells. Specifically, we demonstrated that ubiquitin-specific peptidase 18 (USP18), a deubiquitinating enzyme, targets GSDME for deubiquitination and that USP18 knockdown suppressed cisplatin-induced pyroptosis. Notably, we found that succinylation of GSDME, which is mediated by succinyl-CoA, promotes GSDME cleavage without affecting caspase-3 activation. Further experiments indicated that SIRPA expression in tumor cells can decrease the antitumor efficacy of chemotherapy and immunotherapy in HNSCC mouse models. In summary, our findings reveal a novel mechanism of pyroptosis evasion in HNSCC, whereby tumor-intrinsic SIRPA enhances GSDME ubiquitylation and inhibits its succinylation. These insights suggest that inhibiting SIRPA expression may improve the efficacy of immunotherapy for HNSCC by inducing pyroptosis.

Abalone Haliotis discus caspase 8 is an apoptosis effector and a pyroptosis activator

In mammals, caspase 8 (CASP8) is a well-known initiator caspase of apoptosis. In invertebrates, the function of CASP8 is poorly understood. Herein, we examined the function of abalone Haliotis discus CASP8 (HdCASP8). Compared to mammalian CASP8, HdCASP8 possesses the conserved DED and CASc domains but also has an extra death domain (DD). HdCASP8 induced marked apoptosis of HEK293T cells without activating CASP3/6/7. Consistently, HdCASP8 did not cleave H. discus CASP3 (HdCASP3). HdCASP8 exhibited CASP3/6-like cleavage specificity and cleaved the apoptotic substrate DFF45. HdCASP3 is known to activate abalone pyroptosis by cleaving H. discus gasdermin E (HdGSDME) at two sites, DQVD and DEID. In the present work, HdCASP8 was found to interact with HdGSDME at its C-terminal region and induce pyroptosis by cleaving HdGSDME at DQVD but not at DEID. During bacterial infection, the expressions of HdCASP8 and HdGSDME were significantly upregulated in multiple tissues of abalone in a time-dependent manner. Together these results indicate that, most likely owing to its unique structural feature, HdCASP8 differs from the classical CASP8 by acting as an apoptosis/pyroptosis-regulating CASP3 and from the classical CASP3 in certain aspects of substrate specificity. These findings provide new insights into CASP8-mediated programmed cell death in invertebrates.

Potential Impact of Climate Change-Induced Alterations on Pyroptotic Cell Death in Animal Cells: A Review

Climate change-induced alterations in temperature variation, ozone exposure, water salinity and acidification, and hypoxia might influence immunity and thus survival in diverse groups of animals from fish to mammals. Pyroptosis is a type of lytic pro-inflammatory programmed cell death, which participates in the innate immune response, and is involved in multiple diseases characterized by inflammation and cell death, mostly studied in human cells. Diverse extrinsic factors can induce pyroptosis, leading to the extracellular release of pro-inflammatory molecules such as IL-18. Climate change-related factors, either directly or indirectly, can also promote animal cell death via different regulated mechanisms, impacting organismal fitness. However, pyroptosis has been relatively less studied in this context compared to another cell death process, apoptosis. This review covers previous research pointing to the potential impact of climate change, through various abiotic stressors, on pyroptotic cell death in different animal cells in various contexts. It was proposed that temperature, ozone exposure, water salinity, water acidification and hypoxia have the potential to induce pyroptotic cell death in animal cells and promote inflammation, and that these pyroptotic events should be better understood to be able to mitigate the adverse effects of climate change on animal physiology and health. This is of high importance considering the increasing frequency, intensity and duration of climate-based changes in these environmental parameters, and the critical function of pyroptosis in immune responses of animals and in their predisposition to multiple diseases including cancer. Furthermore, the need for further mechanistic studies showing the more direct impact of climate change-induced environmental alterations on pyroptotic cell death in animals at the organismal level was highlighted. A complete picture of the association between climate change and pyroptosis in animals will be also highly valuable in terms of ecological and clinical applications, and it requires an interdisciplinary approach. SIGNIFICANCE: Climate change-induced alterations might influence animal physiology. Pyroptosis is a form of cell death with pro-inflammatory characteristics. Previous research suggests that temperature variation, ozone exposure, water salinity and acidification, and hypoxia might have the potential to contribute to pyroptotic cell death in certain cell types and contexts. Climate change-induced pyroptotic cell death should be better understood to be able to mitigate the adverse effects of climate change on animal health.

Cnidaria XIAP activates caspase-mediated cell death

In vertebrates, X-linked inhibitor of apoptosis (XIAP) is a potent inhibitor of apoptosis. XIAP inhibits apoptosis by interacting with proapoptotic caspases via its baculovirus IAP repeat (BIR) domains and mediating caspase ubiquitination via its really interesting new gene (RING) domain and ubiquitin-associated (UBA) domain. In invertebrates, and mainly in arthropods, XIAP is also known as an apoptosis inhibitor. However, no studies on basal metazoan XIAP have been documented to date. In the present work, we examined the biological activity of XIAP from the jellyfish Aurelia coerulea (AcXIAP) and other non-bilaterians. AcXIAP has three BIRs and one RING domain but lacks a UBA domain. AcXIAP enhanced the apoptosis-inducing activity of all four A. coerulea caspases identified in this study, including both initiator and effector clades. AcXIAP activated caspase via one of the BIRs, which bound and stabilized the caspase, and the RING domain, which mediated ubiquitination of the caspase p20 subunit in a lysine-independent manner. Similar caspase-activating properties were also observed in the XIAP of hydra, coral, and sponge. In hydra, XIAP knockdown markedly decreased cell death induced by an apoptosis inducer. Together these results revealed the unconventional function and working mechanism of XIAP in Cnidaria, and shed new light on the functional and structural evolution of XIAP.

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.

Structural basis for the pore-forming activity of a complement-like toxin

Pore-forming proteins comprise a highly diverse group of proteins exemplified by the membrane attack complex/perforin (MACPF), cholesterol-dependent cytolysin (CDC), and gasdermin superfamilies, which all form gigantic pores (>150 angstroms). A recently found family of pore-forming toxins, called CDC-like proteins (CDCLs), are wide-spread in gut microbes and are a prevalent means of antibacterial antagonism. However, the structural aspects of how CDCLs assemble a pore remain a mystery. Here, we report the crystal structure of a proteolytically activated CDCL and cryo-electron microscopy structures of a prepore-like intermediate and a transmembrane pore providing detailed snapshots across the entire pore-forming pathway. These studies reveal a sophisticated array of regulatory features to ensure productive pore formation, and, thus, CDCLs straddle the MACPF, CDC, and gasdermin lineages of the giant pore superfamilies.

Mechanistic insights into gasdermin-mediated pyroptosis

Pyroptosis, a novel mode of inflammatory cell death, is executed by membrane pore-forming gasdermin (GSDM) family members in response to extracellular or intracellular injury cues and is characterized by a ballooning cell morphology, plasma membrane rupture and the release of inflammatory mediators such as interleukin-1β (IL-1β), IL-18 and high mobility group protein B1 (HMGB1). It is a key effector mechanism for host immune defence and surveillance against invading pathogens and aberrant cancerous cells, and contributes to the onset and pathogenesis of inflammatory and autoimmune diseases. Manipulating the pore-forming activity of GSDMs and pyroptosis could lead to novel therapeutic strategies. In this Review, we discuss the current knowledge regarding how GSDM-mediated pyroptosis is initiated, executed and regulated, its roles in physiological and pathological processes, and the crosstalk between different modes of programmed cell death. We also highlight the development of drugs that target pyroptotic pathways for disease treatment.

GSDMD-mediated pyroptosis: molecular mechanisms, diseases and therapeutic targets

Pyroptosis is a regulated form of inflammatory cell death in which Gasdermin D (GSDMD) plays a central role as the key effector molecule. GSDMD-mediated pyroptosis is characterized by complex biological features and considerable heterogeneity in its expression, mechanisms, and functional outcomes across various tissues, cell types, and pathological microenvironments. This heterogeneity is particularly pronounced in inflammation-related diseases and tumors. In the context of inflammatory diseases, GSDMD expression is typically upregulated, and its activation in macrophages, neutrophils, T cells, epithelial cells, and mitochondria triggers both pyroptotic and non-pyroptotic pathways, leading to the release of pro-inflammatory cytokines and exacerbation of tissue damage. However, under certain conditions, GSDMD-mediated pyroptosis may also serve a protective immune function. The expression of GSDMD in tumors is regulated in a more complex manner, where it can either promote immune evasion or, in some instances, induce tumor cell death. As our understanding of GSDMD's role continues to progress, there have been advancements in the development of inhibitors targeting GSDMD-mediated pyroptosis; however, these therapeutic interventions remain in the preclinical phase. This review systematically examines the cellular and molecular complexities of GSDMD-mediated pyroptosis, with a particular emphasis on its roles in inflammation-related diseases and cancer. Furthermore, it underscores the substantial therapeutic potential of GSDMD as a target for precision medicine, highlighting its promising clinical applications.

Vastly different energy landscapes of the membrane insertions of monomeric gasdermin D and A3

Gasdermin D and gasdermin A3 belong to the same family of pore-forming proteins and executors of pyroptosis, a form of programmed cell death. To unveil the process of their pore formation, we examine the energy landscapes upon insertion of the gasdermin D and A3 monomers into a lipid bilayer by extensive atomistic molecular dynamics simulations. We reveal a lower free energy barrier of membrane insertion for gasdermin D than for gasdermin A3 and a preference of gasdermin D for the membrane-inserted and of gasdermin A3 for the membrane-adsorbed state, suggesting that gasdermin D first inserts and then oligomerizes while gasdermin A3 oligomerizes and then inserts. Gasdermin D stabilizes itself in the membrane by forming more salt bridges and pulling phosphatidylethanolamine lipids and more water into the membrane. Gasdermin-lipid interactions support the pore formation. Our findings suggest that both the gasdermin species and the lipid composition modulate gasdermin pore formation.

Mechanisms of RCD-1 pore formation and membrane bending

Regulator of cell death-1 (RCD-1) governs the heteroallelic expression of RCD-1-1 and RCD-1-2, a pair of fungal gasdermin (GSDM)-like proteins, which prevent cytoplasmic mixing during allorecognition and safeguard against mycoparasitism, genome exploitation, and deleterious cytoplasmic elements (e.g., senescence plasmids) by effecting a form of cytolytic cell death. However, the underlying mechanisms by which RCD-1 acts on the cell membrane remain elusive. Here, we demonstrate that RCD-1 binds acidic lipid membranes, forms pores, and induces membrane bending. Using atomic force microscopy (AFM) and AlphaFold, we show that RCD-1-1 and RCD-1-2 form heterodimers that further self-assemble into ~14.5 nm-wide transmembrane pores (~10 heterodimers). Moreover, through AFM force spectroscopy and micropipette aspiration, we reveal that RCD-1 proteins bend membranes with low bending moduli. This combined action of pore formation and membrane deformation may constitute a conserved mechanism within the broader GSDM family.

Advances in research on the impact and mechanisms of pathogenic microorganism infections on pyroptosis

Pyroptosis, also known as inflammatory necrosis, is a form of programmed cell death characterized by the activation of gasdermin proteins, leading to the formation of pores in the cell membrane, continuous cell swelling, and eventual membrane rupture. This process results in the release of intracellular contents, including pro-inflammatory cytokines like IL-1β and IL-18, which subsequently trigger a robust inflammatory response. This process is a crucial component of the body's innate immune response and plays a significant role in combating infections. There are four main pathways through which pathogenic microorganisms induce pyroptosis: the canonical inflammasome pathway, the non-canonical inflammasome pathway, the apoptosis-associated caspase-mediated pathway, and the granzyme-mediated pathway. This article provides a brief overview of the effects and mechanisms of pathogen infections on pyroptosis.

Targeting GSDME-mediated macrophage polarization for enhanced antitumor immunity in hepatocellular carcinoma

Despite the notable efficacy of anti-PD1 therapy in the management of hepatocellular carcinoma (HCC) patients, resistance in most individuals necessitates additional investigation. For this study, we collected tumor tissues from nine HCC patients receiving anti-PD1 monotherapy and conducted RNA sequencing. These findings revealed significant upregulation of GSDME, which is predominantly expressed by tumor-associated macrophages (TAMs), in anti-PD1-resistant patients. Furthermore, patients with elevated levels of GSDME+ macrophages in HCC tissues presented a poorer prognosis. The analysis of single-cell sequencing data and flow cytometry revealed that the suppression of GSDME expression in nontumor cells resulted in a decrease in the proportion of M2-like macrophages within the tumor microenvironment (TIME) of HCC while concurrently augmenting the cytotoxicity of CD8 + T cells. The non-N-terminal fragment of GSDME within macrophages combines with PDPK1, thereby activating the PI3K-AKT pathway and facilitating M2-like polarization. The small-molecule Eliprodil inhibited the increase in PDPK1 phosphorylation mediated by GSDME site 1. The combination of Eliprodil and anti-PD1 was effective in the treatment of both spontaneous HCC in c-Myc + /+;Alb-Cre + /+ mice and in a hydrodynamic tail vein injection model, which provides a promising strategy for novel combined immunotherapy.

Paralichthys olivaceus GSDME-mediated pyroptosis is regulated by multiple caspases in different manners

Pyroptosis is a type of programmed cell death mediated by gasdermin (GSDM). GSDM is activated by caspase (CASP), which cleaves GSDM to release the N-terminal (NT) fragment that forms channels in the plasma membrane and leads to cell death. To date, research on pyroptosis in teleost is limited. In this study, we examined the activation and regulation mechanism of pyroptosis in flounder Paralichthys olivaceus. P. olivaceus gasdermin E (PoGSDME) was found to be cleaved by six P. olivaceus caspases (PoCASP1/3a/3b/7/8a/8b). PoCASP1/3a/3b/7 cleaved primarily at 245FEAD248, which generated an NT fragment (NT248) that induced robust pyroptosis. PoCASP8a/8b cleaved both the full length PoGSDME and NT248 at 202IEKD205, thus destroying the biological activity of PoGSDME and NT248. Nine residues crucial for PoGSDME function were identified, of which, F2, L19, and G85 were essential to plasma membrane translocation. During bacterial infection, PoGSDME and PoCASP1 expressions were significantly upregulated in flounder tissues, and PoGSDME, as well as PoCASP1, activation occurred in flounder cells accompanied with the processing cleavage of IL-1β and IL-18. Together these results revealed both the activation and the inhibition mechanisms of GSDME-mediated pyroptosis in flounder, and added new insights into the regulation of pyroptosis in fish.

CRADD and cIAP1 antagonistically regulate caspase-9-mediated apoptosis in teleost

Caspase 9 (CASP9) is a well-known initiator caspase of intrinsic apoptosis. In humans, cIAP1 binds and induces degradation of the activated form of CASP9, but not pro-CASP9. In fish, the activity and regulation of CASP9 remain unknown. In this work, using flounder Paralichthys olivaceus as a representative species, we examined the regulatory mechanism of CASP9 in teleost. P. olivaceus CASP9 (PoCASP9) induced robust apoptosis, which was inhibited by P. olivaceus cIAP1 (PocIAP1). Unlike human cIAP1, PocIAP1 bound both pro- and active PoCASP9 and induced their degradation via the RING domain-involved proteasome pathway. In humans, the adaptor molecule CRADD cannot interact with CASP9. In contrast, P. olivaceus CRADD (PoCRADD) bound both pro- and active PoCASP9 via CARD-CARD interaction and enhanced apoptosis by promoting the cellular levels of pro- and active PoCASP9. Furthermore, PoCRADD abrogated the inhibition of PoCASP9 by PocIAP1 by preventing PocIAP1-PoCASP9 interaction. Together these results reveal a CASP9 regulation mechanism in teleost that differs from that in humans and demonstrate that teleost CASP9 is tightly and directly controlled by both negative and positive regulators that exert a regulation effect both before and after CASP9 activation. These findings advance our understanding of the regulation of CASP9-mediated apoptosis in vertebrates.

The Evolution of NLR Inflammasome and Its Mediated Pyroptosis in Metazoa

Nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) inflammasomes are multiprotein signaling platforms that control the inflammatory response and coordinate antimicrobial defense. In the present study, the distribution of NLR, Caspase-1, and gasdermin (GSDM) homologues and their structural characteristics and evolutionary relationships were systematically analyzed in metazoa according to the genomes of species. In invertebrates, there were only NLRC and/or NLRD presented from sponge to amphioxus, and according to the evolutionary tree, NLR from sponge located in the most primitive position. Caspase-1 existed in some metazoan phyla (Brachiopoda, Ectoprocta, Arthropoda, Mollusca, Annelia, Nematoda, Platyelminthes, Coelenterate, and Porifera) and its activation sites were relatively conserved. The amino acid sequences and three-dimensional structures of N-terminal CARD/Death domain of NLR and Caspase-1 were similar in species from sponge to human. NLR and Caspase-1 co-existed in species of Brachiopoda, Mollusca, Annelia, Coelenterate, and Porifera. There was only GSDME or PJVK found in some phyla of invertebrates and their cleavage sites were conserved (DxxD). And it was predicted that the NLR inflammasome in inducing pyroptosis could occur in species of Brachiopoda, Mollusca, Annelia, and Coelenterate. These studies indicated that NLR inflammasome emerged early in sponges of metazoa, and NLR inflammasome in inducing pyroptosis first appeared in Coelenterate, suggesting that inflammasome and its mediated pyroptosis had existed in the early stage of metazoa, but they had been lost in many species during evolution.

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.

Gasdermins as evolutionarily conserved executors of inflammation and cell death

The gasdermins are a family of pore-forming proteins that have recently emerged as executors of pyroptosis, a lytic form of cell death that is induced by the innate immune system to eradicate infected or malignant cells. Mammalian gasdermins comprise a cytotoxic N-terminal domain, a flexible linker and a C-terminal repressor domain. Proteolytic cleavage in the linker releases the cytotoxic domain, thereby allowing it to form β-barrel membrane pores. Formation of gasdermin pores in the plasma membrane eventually leads to a loss of the electrochemical gradient, cell death and membrane rupture. Here we review recent work that has expanded our understanding of gasdermin biology and function in mammals by revealing their activation mechanism, their regulation and their roles in autoimmunity, host defence and cancer. We further highlight fungal and bacterial gasdermin pore formation pointing to a conserved mechanism of cell death induction.

Pufferfish gasdermin Ea is a significant player in the defense against bacterial pathogens

Gasdermins (GSDMs) are proteins cleaved by caspase (CASP) to trigger pyroptosis. In teleosts, pyroptosis is mediated by gasdermin E (GSDME). The Pufferfish, Takifugu rubripes, possesses two GSDME orthologs: named TrGSDMEa and TrGSDMEb. TrGSDMEa is cleaved by CASP3/7 to liberate the N-terminal (NT) domain that can trigger pyroptosis in mammalian cells. However, the biological function of TrGSDMEa in pufferfish is unknown, and TrGSDMEb is poorly studied. We found that TrGSDMEb was cleaved by CASP1/3/6/7/8, but the resulting NT domain, despite its similarity to TrGSDMEa-NT domain in sequence and structure, failed to induce pyroptosis. TrGSDMEa and TrGSDMEb exhibited similar expression patterns in pufferfish under normal physiological conditions but were up- and downregulated, respectively, in expression during Vibrio harveyi and Edwardsiella tarda infection. Bacterial infection induced the activation of TrGSDMEa and CASP3/7 in pufferfish cells, resulting in pyroptosis accompanied with IL-1β production and maturation. Inhibition of TrGSDMEa-mediated pyroptosis via TrCASP3/7 reduced the death of pufferfish cells and augmented bacterial dissemination in fish tissues. Structure-oriented mutagenesis identified 16 conserved residues in teleost GSDMEa that were required for the pore formation or auto-inhibition of GSDMEa. This study illustrates the role of GSDMEa-mediated pyroptosis in teleost defense against bacterial pathogens and provides new insights into the structure-based function of vertebrate GSDME.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-024-00237-x.

Unanticipated Loss of Inflammasomes in Birds

Inflammasomes are multiprotein complexes that form in response to ligands originating from pathogens as well as alterations of normal cell physiology caused by infection or tissue damage. These structures engage a robust inflammatory immune response that eradicates environmental microbes before they cause disease, and slow the growth of bona fide pathogens. Despite their undeniable utility in immunity, inflammasomes are radically reduced in birds. Perhaps most surprising is that, within all birds, NLRP3 is retained, while its signaling adapter ASC is lost, suggesting that NLRP3 signals via a novel unknown adapter. Crocodilian reptiles and turtles, which share a more recent common ancestor with birds, retain many of the lost inflammasome components, indicating that the deletion of inflammasomes occurred after birds diverged from crocodiles. Some bird lineages have even more extensive inflammasome loss, with songbirds continuing to pare down their inflammasomes until only NLRP3 and CARD8 remain. Remarkably, songbirds have lost caspase-1 but retain the downstream targets of caspase-1: IL-1β, IL-18, and the YVAD-linker encoding gasdermin A. This suggests that inflammasomes can signal through alternative proteases to activate cytokine maturation and pyroptosis in songbirds. These observations may reveal new contexts of activation that may be relevant to mammalian inflammasomes and may suggest new avenues of research to uncover the enigmatic nature of the poorly understood NLRP3 inflammasome.

Induction of apoptosis by double-stranded RNA was present in the last common ancestor of cnidarian and bilaterian animals

Apoptosis, a major form of programmed cell death, is an essential component of host defense against invading intracellular pathogens. Viruses encode inhibitors of apoptosis to evade host responses during infection, and to support their own replication and survival. Therefore, hosts and their viruses are entangled in a constant evolutionary arms race to control apoptosis. Until now, apoptosis in the context of the antiviral immune system has been almost exclusively studied in vertebrates. This limited phyletic sampling makes it impossible to determine whether a similar mechanism existed in the last common ancestor of animals. Here, we established assays to probe apoptosis in the sea anemone Nematostella vectensis, a model species of Cnidaria, a phylum that diverged approximately 600 million years ago from the rest of animals. We show that polyinosinic:polycytidylic acid (poly I:C), a synthetic long double-stranded RNA mimicking viral RNA and a primary ligand for the vertebrate RLR melanoma differentiation-associated protein 5 (MDA5), is sufficient to induce apoptosis in N. vectensis. Furthermore, at the transcriptomic level, apoptosis related genes are significantly enriched upon poly(I:C) exposure in N. vectensis as well as bilaterian invertebrates. Our phylogenetic analysis of caspase family genes in N. vectensis reveals conservation of all four caspase genes involved in apoptosis in mammals and revealed a cnidarian-specific caspase gene which was strongly upregulated. Altogether, our findings suggest that apoptosis in response to a viral challenge is a functionally conserved mechanism that can be traced back to the last common ancestor of Bilateria and Cnidaria.

Cleavage of gasdermin by apoptotic caspases triggers pyroptosis restricting bacterial colonization in Hydra

Gasdermin (GSDM) proteins, as the direct executors of pyroptosis, are structurally and functionally conserved among vertebrates and play crucial roles in host defense against infection, inflammation, and cancer. However, the origin of functional GSDMs remains elusive in the animal kingdom. Here, we found that functional GSDME homologs first appeared in the cnidarian. Moreover, these animal GSDME homologs share evolutionarily conserved apoptotic caspase cleavage sites. Thus, we verified the functional conservation of apoptotic caspase-GSDME cascade in Hydra, a representative species of cnidarian. Unlike vertebrate GSDME homologs, HyGSDME could be cleaved by four Hydra caspase homologs with caspase-3 activity at two sites. Furthermore, in vivo activation of Hydra caspases resulted in HyGSDME cleavage to induce pyroptosis, exacerbating injury and restricting bacterial burden, which protects Hydra from pathogen invasion. In conclusion, these results suggest that GSDME-dependent pyroptosis may be an ancient and conserved host defense mechanism, which may contribute to better understanding on the origin and evolution of GSDMs.

Inflammasome-independent pyroptosis

Gasdermins are membrane pore-forming proteins that cause pyroptosis, an inflammatory cell death in which cells burst and release cytokines, chemokines, and other host alarm signals, such as ATP and HMGB1, which recruit and activate immune cells at sites of infection and danger. There are five gasdermins in humans - gasdermins A to E. Pyroptosis was first described in myeloid cells and mucosal epithelia, which express gasdermin D and activate it when cytosolic sensors of invasive infection or tissue damage assemble into large macromolecular structures, called inflammasomes. Inflammasomes recruit and activate inflammatory caspases (caspase 1, 4, 5, and 11), which cut gasdermin D to remove an inhibitory C-terminal domain, allowing the N-terminal domain to bind to membrane acidic lipids and oligomerize into pores. Recent studies have identified inflammasome-independent proteolytic pathways that activate gasdermin D and the other gasdermins. Here, we review inflammasome-independent pyroptosis pathways and what is known about their role in normal physiology and disease.

Chicken gasdermins mediate pyroptosis after the cleavage by caspases

Gasdermin (GSDM) proteins are executioners of pyroptosis in many species. Gasdermin proteins can be cleaved at their linker region between the amino domain (NT) and carboxyl domain (CT) by enzymes. The released GSDM-NTs bind cell membrane and form pores, thereby leading to the release of cellular components and lytic cell death. GSDM-mediated pyroptosis is considered to play important role in immune responses. However, little is known about the GSDM proteins and GSDM-mediated pyroptosis in birds. In the current study, genes encoding chicken gasdermin A (chGSDMA) and chGSDME were cloned. The cleavage of chGSDMA and chGSDME by chicken caspase-1 (chCASP1), chCASP3 and chCASP7 and the cleavage sites were determined. The chGSDMA-NT obtained form chCASP1-mediated cleavage and chGSDME-NT obtained from chCASP3/chCASP7-mediated cleavage could bind and damage cell membrane and lead to cell death of HEK293 cells. chGSDMA-NT also strongly localized to and formed puncta in nucleus. Besides, both chGSDMA-NT and chGSDME-NT showed growth inhibition and bactericidal activity to bacteria. In chickens challenged with Pasteurella multocida and Salmonella typhimurium, the expression of chGSDMA and chGSDME was upregulated and the activation of chCASP3 and the cleavage of chGSDME were observed. The work provides essential information for expanding our knowledge on pyroptosis in birds.

Cleavage-independent activation of ancient eukaryotic gasdermins and structural mechanisms

Gasdermins (GSDMs) are pore-forming proteins that execute pyroptosis for immune defense. GSDMs are two-domain proteins activated by proteolytic removal of the inhibitory domain. In this work, we report two types of cleavage-independent GSDM activation. First, TrichoGSDM, a pore-forming domain-only protein from the basal metazoan Trichoplax adhaerens, is a disulfides-linked autoinhibited dimer activated by reduction of the disulfides. The cryo-electron microscopy (cryo-EM) structure illustrates the assembly mechanism for the 44-mer TrichoGSDM pore. Second, RCD-1-1 and RCD-1-2, encoded by the polymorphic regulator of cell death-1 (rcd-1) gene in filamentous fungus Neurospora crassa, are also pore-forming domain-only GSDMs. RCD-1-1 and RCD-1-2, when encountering each other, form pores and cause pyroptosis, underlying allorecognition in Neurospora. The cryo-EM structure reveals a pore of 11 RCD-1-1/RCD-1-2 heterodimers and a heterodimerization-triggered pore assembly mechanism. This study shows mechanistic diversities in GSDM activation and indicates versatile functions of GSDMs.

Evolution of Caspases and the Invention of Pyroptosis

The protein scaffold that includes the caspases is ancient and found in all domains of life. However, the stringent specificity that defines the caspase biologic function is relatively recent and found only in multicellular animals. During the radiation of the Chordata, members of the caspase family adopted roles in immunity, events coinciding with the development of substrates that define the modern innate immune response. This review focuses on the switch from the non-inflammatory cellular demise of apoptosis to the highly inflammatory innate response driven by distinct members of the caspase family, and the interplay between these two regulated cell death pathways.

Characterization of the Apoptotic and Antimicrobial Activities of Two Initiator Caspases of Sea Cucumber Apostichopus japonicus

Caspase (CASP) is a protease family that plays a vital role in apoptosis, development, and immune response. Herein, we reported the identification and characterization of two CASPs, AjCASPX1 and AjCASPX2, from the sea cucumber Apostichopus japonicus, an important aquaculture species. AjCASPX1/2 share similar domain organizations with the vertebrate initiator caspases CASP2/9, including the CARD domain and the p20/p10 subunits with conserved functional motifs. However, compared with human CASP2/9, AjCASPX1/2 possess unique structural features in the linker region between p20 and p10. AjCASPX1, but not AjCASPX2, induced marked apoptosis of human cells by activating CASP3/7. The recombinant proteins of AjCASPX2 and the CARD domain of AjCASPX2 were able to bind to a wide range of bacteria, as well as bacterial cell wall components, and inhibit bacterial growth. AjCASPX1, when expressed in Escherichia coli, was able to kill the host bacteria. Under normal conditions, AjCASPX1 and AjCASPX2 expressions were most abundant in sea cucumber muscle and coelomocytes, respectively. After bacterial infection, both AjCASPX1 and AjCASPX2 expressions were significantly upregulated in sea cucumber tissues and cells. Together, these results indicated that AjCASPX1 and AjCASPX2 were initiator caspases with antimicrobial activity and likely functioned in apoptosis and immune defense against pathogen infection.

Structure and assembly of a bacterial gasdermin pore

In response to pathogen infection, gasdermin (GSDM) proteins form membrane pores that induce a host cell death process called pyroptosis1-3. Studies of human and mouse GSDM pores have revealed the functions and architectures of assemblies comprising 24 to 33 protomers4-9, but the mechanism and evolutionary origin of membrane targeting and GSDM pore formation remain unknown. Here we determine a structure of a bacterial GSDM (bGSDM) pore and define a conserved mechanism of pore assembly. Engineering a panel of bGSDMs for site-specific proteolytic activation, we demonstrate that diverse bGSDMs form distinct pore sizes that range from smaller mammalian-like assemblies to exceptionally large pores containing more than 50 protomers. We determine a cryo-electron microscopy structure of a Vitiosangium bGSDM in an active 'slinky'-like oligomeric conformation and analyse bGSDM pores in a native lipid environment to create an atomic-level model of a full 52-mer bGSDM pore. Combining our structural analysis with molecular dynamics simulations and cellular assays, our results support a stepwise model of GSDM pore assembly and suggest that a covalently bound palmitoyl can leave a hydrophobic sheath and insert into the membrane before formation of the membrane-spanning β-strand regions. These results reveal the diversity of GSDM pores found in nature and explain the function of an ancient post-translational modification in enabling programmed host cell death.

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.

The molecular mechanism and evolutionary divergence of caspase 3/7-regulated gasdermin E activation

Caspase (CASP) is a family of proteases involved in cleavage and activation of gasdermin, the executor of pyroptosis. In humans, CASP3 and CASP7 recognize the same consensus motif DxxD, which is present in gasdermin E (GSDME). However, human GSDME is cleaved by CASP3 but not by CASP7. The underlying mechanism of this observation is unclear. In this study, we identified a pyroptotic pufferfish GSDME that was cleaved by both pufferfish CASP3/7 and human CASP3/7. Domain swapping between pufferfish and human CASP and GSDME showed that the GSDME C-terminus and the CASP7 p10 subunit determined the cleavability of GSDME by CASP7. p10 contains a key residue that governs CASP7 substrate discrimination. This key residue is highly conserved in vertebrate CASP3 and in most vertebrate (except mammalian) CASP7. In mammals, the key residue is conserved in non-primates (e.g., mouse) but not in primates. However, mouse CASP7 cleaved human GSDME but not mouse GSDME. These findings revealed the molecular mechanism of CASP7 substrate discrimination and the divergence of CASP3/7-mediated GSDME activation in vertebrate. These results also suggested that mutation-mediated functional alteration of CASP probably enabled the divergence and specialization of different CASP members in the regulation of complex cellular activities in mammals.

Gasdermin and MLKL necrotic cell death effectors: Signaling and diseases

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

Paralichthys olivaceus MLKL-mediated necroptosis is activated by RIPK1/3 and involved in anti-microbial immunity

Necroptosis is a type of proinflammatory programmed necrosis essential for innate immunity. The receptor interacting protein kinases 1/3 (RIPK1/3) and the substrate mixed lineage kinase domain-like protein (MLKL) are core components of the necroptotic axis. The activation and immunological function of necroptosis in fish remain elusive. Herein, we studied the function and activation of RIPK1/3 (PoRIPK1/3) and MLKL (PoMLKL) in teleost Paralichthys olivaceus. Bacterial infection increased the expression of RIPK1/3 and MLKL. The N-terminal four-helix bundle (4HB) domain of PoMLKL exhibited necroptosis-inducing activity, and the C-terminal pseudokinase domain exerted auto-inhibitory effect on the 4HB domain. PoRIPK3 was capable of phosphorylating the T360/S361 residues in the PoMLKL C-terminal domain and initiated necroptosis, and this necroptosis-inducing activity was enhanced by PoRIPK1. PoRIPK1/3 interacted with PoMLKL in a manner that depended on the RIP homotypic interaction motif (RHIM), and deletion of RHIM from PoRIPK1/3 led to the dissociation of PoRIPK1/3 with PoMLKL. Inhibition of PoMLKL-mediated necroptosis increased Edwardsiella tarda infection in fish cells and tissues, and led to significantly enhanced lethality of the host. Taken together, these results revealed the activation mechanism of PoRIPK1/3-PoMLKL signaling pathway and the immunological function of necroptosis in the immune defense of teleost.

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.

Stress/cell death pathways, neuroinflammation, and neuropathic pain

Neuropathic pain is a common and debilitating modality of chronic pain induced by a lesion or disease of the somatosensory nervous system. Albeit the elucidation of numerous pathophysiological mechanisms and the development of potential treatment compounds, safe and reliable therapies of neuropathic pain remain poor. Multiple stress/cell death pathways have been shown to be implicated in neuroinflammation during neuropathic pain. Here, we summarize the current knowledge of stress/cell death pathways and present an overview of the roles and molecular mechanisms of stress/cell death pathways in neuroinflammation during neuropathic pain, covering intrinsic and extrinsic apoptosis, autophagy, mitophagy, ferroptosis, pyroptosis, necroptosis, and phagoptosis. Small molecule compounds that modulate stress/cell death pathways in alleviating neuropathic pain are discussed mainly based on preclinical neuropathic pain models. These findings will contribute to in-depth understanding of the pathological processes during neuropathic pain as well as bridge the gap between basic and translational research to uncover new neuroprotective interventions.

Hydra gasdermin-gated pyroptosis signalling regulates tissue regeneration

Pyroptosis, an inflammatory form of programmed cell death, is directly executed by gasdermin (GSDM) depending on its N-terminal pore-forming fragment-mediated membrane-disrupting, triggering intracellular contents release, which plays important roles in mammalian anti-infection and anti-tumor immune responses. However, whether pyroptosis engages in the regulation of tissue regeneration remains largely unknown. Here, utilizing Hydra vulgaris as the research model, we found that an HyCARD2-HyGSDME-mediated pyroptosis signalling is activated in both head and foot regenerated tips after amputation. Impeding pyroptosis by knocking down the expression of either HyGSDME or HyCARD2 significantly hampered both head and foot regeneration in Hydra. Mechanistically, the activation of HyCARD2-HyGSDME axis at wound sites is dependent of intracellular mitochondrial reactive oxygen species (mtROS), the removing of which hindered Hydra head regeneration. Moreover, the HyCARD2-HyGSDME axis-gated pyroptosis was found to enhance the initial secretion and upregulated expression of Wnt3. Collectively, these findings indicate that gasdermin-gated pyroptosis is critical for the evoking of Wnt signalling to facilitate Hydra tissue regeneration, which provides insights into functional diversification within the gasdermin family in the animal kingdom.

Evolution of the gasdermin family and pyroptosis

Gasdermins have been identified as playing a prominent role in the innate immune response as the executors of a specific type of cell death called pyroptosis. Specific proteolytic cleavage of gasdermins generates an N-terminal that oligomerizes and forms pores in the cell membrane. Although pyroptosis has been widely described in mammals, the importance of gasdermins and gasdermin-like proteins in inducing cell death in other vertebrates, in invertebrates and in other taxa including fungi and bacteria is still being determined. Mammalian, fungal and bacterial gasdermins have in common the fact that they go through the same stages (such as proteolytic activation) when inducing membrane rupture, which suggests that pyroptosis is as an ancient mechanism. In this review, we summarize the evolution and function of the gasdermin and gasdermin-like proteins in animals, fungi and bacteria.

Functional roles of CcGSDMEa-like in common carp (Cyprinus carpio) after Aeromonas hydrophila infection

GSDMs could punch holes in cell membrane and participate in the immune response to bacterial infections. In current study, the molecular and structural characteristics of CcGSDMEa-like were analyzed, and the role of CcGSDMEa-like in the inflammatory response against Aeromonas hydrophila was studied. The results showed that the CcGSDMEa-like shared the conserved structural characteristics with GSDMEs of other teleosts. The CcGSDMEa-like mRNA and protein expression levels were significantly affected by A. hydrophila challenge. When the CcGSDMEa-like was overexpressed, the expression of CcIL-1β were significantly increased in fish and EPC cells, and bacterial contents were significantly decreased in fish tissues. While, when the CcGSDMEa-like was knocked down, the expression and secretion of CcIL-1β were significantly decreased in vivo and in vitro, and the bacterial contents were increased in vivo after A. hydrophila infection 12 h and 24 h. In brief, CcGSDMEa-like could regulate the content of bacteria in fish through mediating the expression and secretion of CcIL-1β. Bactericidal assay and cytotoxicity assay showed that CcGSDMEa-like had no bactericidal activity to Escherichia coli, and did not disrupt cytomembrane integrity of HEK293T cells. This study suggested that CcGSDMEa-like could play roles in the antibacterial and inflammatory processes in fish.

Structure and assembly of a bacterial gasdermin pore

In response to pathogen infection, gasdermin (GSDM) proteins form membrane pores that induce a host cell death process called pyroptosis1-33. Studies of human and mouse GSDM pores reveal the functions and architectures of 24-33 protomers assemblies4-9, but the mechanism and evolutionary origin of membrane targeting and GSDM pore formation remain unknown. Here we determine a structure of a bacterial GSDM (bGSDM) pore and define a conserved mechanism of pore assembly. Engineering a panel of bGSDMs for site-specific proteolytic activation, we demonstrate that diverse bGSDMs form distinct pore sizes that range from smaller mammalian-like assemblies to exceptionally large pores containing >50 protomers. We determine a 3.3 Å cryo-EM structure of a Vitiosangium bGSDM in an active slinky-like oligomeric conformation and analyze bGSDM pores in a native lipid environment to create an atomic-level model of a full 52-mer bGSDM pore. Combining our structural analysis with molecular dynamics simulations and cellular assays, our results support a stepwise model of GSDM pore assembly and suggest that a covalently bound palmitoyl can leave a hydrophobic sheath and insert into the membrane before formation of the membrane-spanning β-strand regions. These results reveal the diversity of GSDM pores found in nature and explain the function of an ancient post-translational modification in enabling programmed host cell death.

Research Progress on Pyroptosis in Hematological Malignancies

OPINION STATEMENT

Pyroptosis is a kind of programmed cell death dependent on the caspase pathway that is different from apoptosis and necrosis. Recent studies have shown that pyroptosis can be involved in the pathological processes of many diseases, such as cancers, atherosclerosis, diabetic nephropathy, and blood diseases. However, the specific mechanisms by which pyroptosis participates in the occurrence and development of hematological malignant tumors still need further exploration. This article reviews the characteristics of pyroptosis and the regulatory mechanisms promoting or inhibiting pyroptosis and discusses the role of pyroptosis in hematological malignant tumors, which could provide ideas for the clinical treatment of such tumors in the future.

Gasdermins gone wild: new roles for GSDMs in regulating cellular homeostasis

Since their discovery, members of the gasdermin (GSDM) family of proteins have been firmly established as executors of pyroptosis, with the N-terminal fragment of most GSDMs capable of forming pores in the plasma membrane. More recent findings suggest that some GSDMs can drive additional cell death pathways, such as apoptosis and necroptosis, through mechanisms independent of plasma membrane perforation. There is also emerging evidence that by associating with cellular compartments such as mitochondria, peroxisomes, endosomes, and the nucleus, GSDMs regulate cell death-independent aspects of cellular homeostasis. Here, we review the diversity of GSDM function across several cell types and explore how various cellular stresses can promote relocalization - and thus refunctionalization - of GSDMs.

An introduction to lesions and histology of scleractinian corals

Stony corals (Scleractinia) are in the Phylum Cnidaria (cnidae referring to various types of stinging cells). They may be solitary or colonial, but all secrete an external, supporting aragonite skeleton. Large, colonial members of this phylum are responsible for the accretion of coral reefs in tropical and subtropical waters that form the foundations of the most biodiverse marine ecosystems. Coral reefs worldwide, but particularly in the Caribbean, are experiencing unprecedented levels of disease, resulting in reef degradation. Most coral diseases remain poorly described and lack clear case definitions, while the etiologies and pathogenesis are even more elusive. This introductory guide is focused on reef-building corals and describes basic gross and microscopic lesions in these corals in order to serve as an invitation to other veterinary pathologists to play a critical role in defining and advancing the field of coral pathology.

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.

Different types of cell death and their shift in shaping disease

Cell death is the irreversible stop of life. It is also the basic physiological process of all organisms which involved in the embryonic development, organ maintenance and autoimmunity of the body. In recent years, we have gained more comprehension of the mechanism in cell death and have basically clarified the different types of "programmed cell death", such as apoptosis, necroptosis, autophagy, and pyroptosis, and identified some key genes in these processes. However, in these previous studies, the conversion between different cell death modes and their application in diseases are rarely explored. To sum up, although many valued discoveries have been discovered in the field of cell death in recent years, there are still many unknown problems to be solved in this field. Facts have proved that cell death is a very complex game, and a series of core players have the ability to destroy the delicate balance of the cell environment, from survival to death, from anti-inflammatory to pro-inflammatory. With the thorough research of the complex regulatory mechanism of cell death, there will certainly be exciting new research in this field in the next few years. The sake of this paper is to emphasize the complex mechanism of overturning the balance between different cell fates and provide relevant theoretical basis for the connection between cell death transformation and disease treatment in the future.

Caspase-mediated LPS sensing and pyroptosis signaling in Hydra

Inflammatory caspases sensing lipopolysaccharide (LPS) to drive gasdermin (GSDM)-mediated pyroptosis is an important immune response mechanism for anti-infection defense in mammals. In this work, we resolved an LPS-induced and GSDM-gated pyroptosis signaling cascade in Cnidarians. Initially, we identified a functional GSDM protein, HyGSDME, in Hydra, executing cytosolic LPS-induced pyroptosis in a caspase-dependent manner. Further, we identified a proinflammatory caspase, HyCaspA, capable of sensing cytosolic LPS by an uncharacterized N-terminal domain relying on its unique hydrophobic property, thereby triggering its oligomerization and self-activation. Subsequently, the LPS-activated HyCaspA cleaved an apoptotic caspase, HyCARD2, to trigger HyGSDME-gated pyroptosis. Last, HyGSDME exhibited an enriched distribution on the ectodermal layer of Hydra polyps, exerting a canonical immune defense function against surface-invading bacteria. Collectively, our work resolved an ancient pyroptosis signaling cascade in Hydra, suggesting that inflammatory caspases sensing cytosolic LPS to initiate GSDM-gated pyroptosis are a conserved immune defense mechanism from Cnidarians to mammals.

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.

Pyroptosis in fish research: A promising target for disease management

Pyroptosis is a newly discovered programmed cell death pathway that plays an essential role in the host's defense against pathogenic infections. This process is orchestrated by inflammasomes, which are intricate multiprotein complexes that orchestrate the activation of caspase and instigate the liberation of proinflammatory cytokines. Additionally, gasdermin family proteins execute their role by forming pores in the cell membrane, ultimately leading to cell lysis. In recent years, pyroptosis has emerged as a promising target for disease management in fish, particularly in the context of infectious diseases. In this review, we provide an overview of the current understanding regarding the role of pyroptosis in fish, focusing on its involvement in host-pathogen interactions and its potential as a therapeutic target. We also highlighted the latest advancements in the field development of pyroptosis inhibitors and their potential applications in fish disease management. Subsequently, we deliberate on the obstacles and future prospects for pyroptosis research in fish, emphasizing the necessity of conducting more comprehensive investigations to unravel the intricate regulatory mechanisms governing this process across diverse fish species and environmental contexts. Finally, this review will also highlight the current limitations and future perspectives of pyroptosis research in aquaculture.

The gasdermin protein family: emerging roles in gastrointestinal health and disease

Since the identification and characterization of gasdermin (GSDM) D as the main effector of inflammatory regulated cell death (or pyroptosis), literature on the GSDM family of pore-forming proteins is rapidly expanding, revealing novel mechanisms regulating their expression and functions that go beyond pyroptosis. Indeed, a growing body of evidence corroborates the importance of GSDMs within the gastrointestinal system, underscoring their critical contributions to the pathophysiology of gastrointestinal cancers, enteric infections and gut mucosal inflammation, such as inflammatory bowel disease. However, with this increase in knowledge, several important and controversial issues have arisen regarding basic GSDM biology and its role(s) during health and disease states. These include critical questions centred around GSDM-dependent lytic versus non-lytic functions, the biological activities of cleaved versus full-length proteins, the differential roles of GSDM-expressing mucosal immune versus epithelial cells, and whether GSDMs promote pathogenic or protective effects during specific disease settings. This Review provides a comprehensive summary and interpretation of the current literature on GSDM biology, specifically focusing on the gastrointestinal tract, highlighting the main controversial issues and their clinical implications, and addressing future areas of research to unravel the specific role(s) of this intriguing, yet enigmatic, family of proteins.

Pyroptotic gasdermin exists in Mollusca and is vital to eliminating bacterial infection

Gasdermin (GSDM) is a family of proteins that execute pyroptosis in vertebrate. In invertebrate, pyroptotic GSDM was documented only in coral. Recent studies identified abundant GSDM structural homologs in Mollusca, but their functions are unclear. Herein, we report a functional GSDM from Pacific abalone Haliotis discus (HdGSDME). HdGSDME is specifically activated by abalone caspase 3 (HdCASP3) cleavage at two distinct sites, generating two active isoforms with pyroptotic and cytotoxic activities. HdGSDME possesses evolutionarily conserved residues that proved to be essential to the N-terminal pore-formation and C-terminal auto-inhibition capacities. Bacterial challenge activates the HdCASP3-HdGSDME pathway and induces pyroptosis and extracellular traps in abalone. Blockage of the HdCASP3-HdGSDME axis promotes bacterial invasion and host mortality. Collectively, this study reveals the existence of functionally conserved and yet distinct-featured GSDM in Mollusca and provides insights into the function and evolution of invertebrate GSDM.

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.

Characterization of GSDME in amphioxus provides insights into the functional evolution of GSDM-mediated pyroptosis

Members of the gasdermin (GSDM) family are pore-forming effectors that cause membrane permeabilization and pyroptosis, a lytic proinflammatory type of cell death. To reveal the functional evolution of GSDM-mediated pyroptosis at the transition from invertebrates to vertebrates, we conducted functional characterization of amphioxus GSDME (BbGSDME) and found that it can be cleaved by distinct caspase homologs, yielding the N253 and N304 termini with distinct functions. The N253 fragment binds to cell membrane, triggers pyroptosis, and inhibits bacterial growth, while the N304 performs negative regulation of N253-mediated cell death. Moreover, BbGSDME is associated with bacteria-induced tissue necrosis and transcriptionally regulated by BbIRF1/8 in amphioxus. Interestingly, several amino acids that are evolutionarily conserved were found to be important for the function of both BbGSDME and HsGSDME, shedding new lights on the functional regulation of GSDM-mediated inflammation.

Duck gasdermin E is a substrate of caspase-3/-7 and an executioner of pyroptosis

Gasdermin (GSDM)-mediated cell death is an ancient immune defensive mechanism that plays an essential role in bacteria, fungi, coral, teleost, and mammals. After being cleaved by proteases of hosts or pathogens, amino-terminal (NT) fragment of GSDMs (GSDM-NTs) form pores in the membrane structure of cells, thereby leading to pyroptotic cell death. However, the expression profile, activation mechanism and function of avian GSDMs have not been studied in depth yet. In the current study, genes encoding duck gasdermin E (duGSDME), caspase-3 (ducaspase-3) and ducaspase-7 were cloned from mRNA of a virus-challenged duck embryo. The cleavage of duGSDME by ducaspase-3/-7 was verified in the cell-free system and/or in human embryonic kidney cells (HEK293). Ducaspase-3/-7 could recognize and cleave duGSDME at 270DAVD273. Overexpression of duGSDME-NT (1-273aa) fragment led to pyroptosis-like morphological change, increased lactic dehydrogenase (LDH) release and propidium iodide uptake of HEK293 cells, which indicated that duGSDME-NTs could cause cell membrane damage. In addition, recombinantly expressed duGSDME-NT showed bactericidal activity to an enterotoxic Escherichia coli (F5+) strain. The expression level of duGSDME was low in duckling tissues. DHAV-3 challenge upregulated the expression of duGSDME and ducaspase-3 in different tissues and led to the activation of ducaspase-3 and cleavage of duGSDME. The results indicated that duGSDME is a substrate of ducapsase-3/-7, and duGSDME-NT can cause pyroptosis. In addition, duGSDME may play a role in the immune defense of ducks against infectious diseases after being cleaved by ducaspase-3. The current study provides essential information for further investigation of the mechanisms of avian innate immunity and avian diseases.