Shiga Toxin/Lipopolysaccharide Activates Caspase-4 and Gasdermin D to Trigger Mitochondrial Reactive Oxygen Species Upstream of the NLRP3 Inflammasome

DGAT2 Inhibition Potentiates Lipid Droplet Formation To Reduce Cytotoxicity in APOL1 Kidney Risk Variants

BACKGROUND

Two variants in the gene encoding apolipoprotein L1 (APOL1) that are highly associated with African ancestry are major contributors to the large racial disparity in rates of human kidney disease. We previously demonstrated that recruitment of APOL1 risk variants G1 and G2 from the endoplasmic reticulum to lipid droplets leads to reduced APOL1-mediated cytotoxicity in human podocytes.

METHODS

We used CRISPR-Cas9 gene editing of induced pluripotent stem cells to develop human-derived APOL1G0/G0 and APOL1G2/G2 kidney organoids on an isogenic background, and performed bulk RNA sequencing of organoids before and after treatment with IFN-γ. We examined the number and distribution of lipid droplets in response to treatment with inhibitors of diacylglycerol O-acyltransferases 1 and 2 (DGAT1 and DGAT2) in kidney cells and organoids.

RESULTS

APOL1 was highly upregulated in response to IFN-γ in human kidney organoids, with greater increases in organoids of high-risk G1 and G2 genotypes compared with wild-type (G0) organoids. RNA sequencing of organoids revealed that high-risk APOL1G2/G2 organoids exhibited downregulation of a number of genes involved in lipogenesis and lipid droplet biogenesis, as well as upregulation of genes involved in fatty acid oxidation. There were fewer lipid droplets in unstimulated high-risk APOL1G2/G2 kidney organoids than in wild-type APOL1G0/G0 organoids. Whereas DGAT1 inhibition reduced kidney organoid lipid droplet number, DGAT2 inhibition unexpectedly increased organoid lipid droplet number. DGAT2 inhibition promoted the recruitment of APOL1 to lipid droplets, with associated reduction in cytotoxicity.

CONCLUSIONS

Lipogenesis and lipid droplet formation are important modulators of APOL1-associated cytotoxicity. Inhibition of DGAT2 may offer a potential therapeutic strategy to attenuate cytotoxic effects of APOL1 risk variants.

It’s All in the PAN: Crosstalk, Plasticity, Redundancies, Switches, and Interconnectedness Encompassed by PANoptosis Underlying the Totality of Cell Death-Associated Biological Effects

The innate immune system provides the first line of defense against cellular perturbations. Innate immune activation elicits inflammatory programmed cell death in response to microbial infections or alterations in cellular homeostasis. Among the most well-characterized programmed cell death pathways are pyroptosis, apoptosis, and necroptosis. While these pathways have historically been defined as segregated and independent processes, mounting evidence shows significant crosstalk among them. These molecular interactions have been described as ‘crosstalk’, ‘plasticity’, ‘redundancies’, ‘molecular switches’, and more. Here, we discuss the key components of cell death pathways and note several examples of crosstalk. We then explain how the diverse descriptions of crosstalk throughout the literature can be interpreted through the lens of an integrated inflammatory cell death concept, PANoptosis. The totality of biological effects in PANoptosis cannot be individually accounted for by pyroptosis, apoptosis, or necroptosis alone. We also discuss PANoptosomes, which are multifaceted macromolecular complexes that regulate PANoptosis. We consider the evidence for PANoptosis, which has been mechanistically characterized during influenza A virus, herpes simplex virus 1, Francisella novicida, and Yersinia infections, as well as in response to altered cellular homeostasis, in inflammatory diseases, and in cancers. We further discuss the role of IRF1 as an upstream regulator of PANoptosis and conclude by reexamining historical studies which lend credence to the PANoptosis concept. Cell death has been shown to play a critical role in infections, inflammatory diseases, neurodegenerative diseases, cancers, and more; therefore, having a holistic understanding of cell death is important for identifying new therapeutic strategies.

Gene expression profile and injury sites in mice treated with Shiga toxin 2 and lipopolysaccharide as a Shiga toxin-associated hemolytic uremic syndrome model

Shiga toxin 2 (Stx2) and lipopolysaccharide (LPS) contribute to the development of hemolytic uremic syndrome (HUS). Mouse models of HUS induced by LPS/Stx2 have been used for elucidating HUS pathophysiology and for therapeutic development. However, the underlying molecular mechanisms and detailed injury sites in this model remain unknown. We analyzed mouse kidneys after LPS/Stx2 administration using microarrays. Decreased urinary osmolality and urinary potassium were observed after LPS/Stx2 administration, suggestive of distal nephron disorders. A total of 1212 and 1016 differentially expressed genes were identified in microarrays at 6 and 72 h after LPS/Stx2 administration, respectively, compared with those in controls. Ingenuity pathway analysis revealed activation of TNFR1/2, iNOS, and IL-6 signaling at both time points, and inhibition of pathways associated with lipid metabolism at 72 h only. The strongly downregulated genes in the 72-h group were expressed in the distal nephrons. In particular, genes associated with distal convoluted tubule (DCT) 2 /connecting tubule (CNT) and principal cells of the cortical collection duct (CCD) were downregulated to a greater extent than those associated with DCT1 and intercalated cells. Stx receptor globotriaosylceramide 3 (Gb3) revealed no colocalization with DCT1-specific Pvalb and intercalated cell-specific Slc26a4 but did present colocalization with Slc12a3 (present in both DCT1 and DCT2), and Aqp2 in principal cells. Gb3 localization tended to coincide with the segment in which the downregulated genes were present. Thus, the LPS/Stx2-induced kidney injury model represents damage to DCT2/CNT and principal cells in the CCD, based on molecular, biological, and physiological findings.

Subtilase cytotoxin from Shiga-toxigenic Escherichia coli impairs the inflammasome and exacerbates enteropathogenic bacterial infection

Subtilase cytotoxin (SubAB) is an AB₅ toxin mainly produced by the locus of enterocyte effacement-negative Shiga-toxigenic Escherichia coli (STEC) strain such as O113:H21, yet the contribution of SubAB to STEC infectious disease is unclear. We found that SubAB reduced activation of the STEC O113:H21 infection-induced non-canonical NLRP3 inflammasome and interleukin (IL)-1β and IL-18 production in murine macrophages. Downstream of lipopolysaccharide signaling, SubAB suppressed caspase-11 expression by inhibiting interferon-β/STAT1 signaling, followed by disrupting formation of the NLRP3/caspase-1 assembly. These inhibitions were regulated by PERK/IRE1α-dependent endoplasmic reticulum (ER) stress signaling initiated by cleavage of the host ER chaperone BiP by SubAB. Our murine model of SubAB-producing Citrobacter rodentium demonstrated that SubAB promoted C. rodentium proliferation and worsened symptoms such as intestinal hyperplasia and diarrhea. These findings highlight the inhibitory effect of SubAB on the NLRP3 inflammasome via ER stress, which may be associated with STEC survival and infectious disease pathogenicity in hosts.

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SubAB from STEC inhibits inflammasome activation and IL-1β/IL-18 production

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SubAB prevents caspase-11 expression via IRE1α/PERK-dependent inhibition of STAT1

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SubAB reduces LPS-induced pro-IL-1β production via IRE1α/PERK-dependent pathway

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SubAB promotes C. rodentium survival in mouse colon and facilitates the infection

TSPO Deficiency Exacerbates GSDMD-Mediated Macrophage Pyroptosis in Inflammatory Bowel Disease

Background: the 18-kDa translocator protein (TSPO) is a mitochondrial outer membrane protein, and its expression tends to increase in response to inflammatory stimulation, rapidly. However, the role of TSPO in inflammation and pyroptosis is not yet clear. Here, we identified TSPO as a novel key regulator of pyroptosis. (2) Methods: TSPO knockout and DSS induced mouse inflammatory bowel disease (IBD) models were employed to assess the roles of TSPO in the pathogenesis of IBD. Primary peritoneal macrophages from TSPO knockout mice were applied to evaluate the mechanism of TSPO in cell pyroptosis. Conclusions: in response to inflammatory injury, TSPO expression is rapidly upregulated and provides a protective function against GSDMD-mediated pyroptosis, which helps us better understand the biological role of TSPO and a novel regulatory mechanism of the pyroptosis process.

Mechanisms and Consequences of Noncanonical Inflammasome-Mediated Pyroptosis

The noncanonical inflammasome, comprising inflammatory caspases 4, 5, or 11, monitors the cytosol for bacterial lipopolysaccharide (LPS). Intracellular LPS-elicited autoproteolysis of these inflammatory caspases leads to the cleavage of the pore-forming protein gasdermin D (GSDMD). GSDMD pore formation induces a lytic form of cell death known as pyroptosis and the release of inflammatory cytokines and DAMPs, thereby promoting inflammation. The noncanonical inflammasome-dependent innate sensing of cytosolic LPS plays important roles in bacterial infections and sepsis pathogenesis. Exciting studies in the recent past have significantly furthered our understanding of the biochemical and structural basis of the caspase-4/11 activation of GSDMD, caspase-4/11's substrate specificity, and the biological consequences of noncanonical inflammasome activation of GSDMD. This review will discuss these recent advances and highlight the remaining gaps in our understanding of the noncanonical inflammasome and pyroptosis.

Gasdermin D in Different Subcellular Locations Predicts Diverse Progression, Immune Microenvironment and Prognosis in Colorectal Cancer

Background

Pyroptosis is a type of cell death that causes an immune reaction. Gasdermin D (GSDMD), as an executor of pyroptosis, has become an attractive target in cancer research. However, the clinical significance of GSDMD expression in different subcellular locations remains unclear.

Methods

GSDMD was detected by immunohistochemistry in 178 cases of colorectal cancer with follow-up information. General data and information on systemic inflammatory indicators were collected from case records, and the clinicopathological parameters were reviewed by microscopy. CD3⁺, CD4⁺, and CD8⁺ T lymphocytes, CD20⁺ B lymphocytes, and CD68⁺ macrophages were detected by immunohistochemistry. Univariate survival analysis (Kaplan–Meier method, Log rank test) and a multivariate Cox proportional hazard model were used to analyze the impact of GSDMD on overall survival.

Results

Survival analysis showed that high expression of cytoplasmic GSDMD was an independent favorable indicator for prognosis (P=0.027) and improved the efficacy of chemotherapy (P=0.012). Positive cytoplasmic GSDMD expression indicated lower probability of distant metastasis (P=0.024), yet nuclear GSDMD expression predicted deeper infiltration depth (P=0.007). Membranous GSDMD expression positively correlated with CD68⁺ macrophages in tumor center (P=0.002) and CD8⁺ lymphocytes in tumor invasive front (P=0.007). However, nuclear GSDMD was negatively related to CD68⁺ macrophages in tumor invasive front (P<0.001) and CD8⁺ lymphocytes in tumor center (P=0.069). Cytoplasmic GSDMD was associated with more CD3⁺ lymphocytes both in tumor center (P=0.066) and tumor invasive front (P=0.008). Moreover, positive membranous GSDMD indicated a lower neutrophil-to-lymphocyte ratio (P=0.013).

Conclusion

GSDMD subcellular localization patterns are related to CRC progression and immune reaction, and should be investigated in future studies.

The Versatile Gasdermin Family: Their Function and Roles in Diseases

The gasdermin (GSDM) family, a novel group of structure-related proteins, consists of GSDMA, GSDMB, GSDMC, GSDMD, GSDME/DNFA5, and PVJK/GSDMF. GSDMs possess a C-terminal repressor domain, cytotoxic N-terminal domain, and flexible linker domain (except for GSDMF). The GSDM-NT domain can be cleaved and released to form large oligomeric pores in the membrane that facilitate pyroptosis. The emerging roles of GSDMs include the regulation of various physiological and pathological processes, such as cell differentiation, coagulation, inflammation, and tumorigenesis. Here, we introduce the basic structure, activation, and expression patterns of GSDMs, summarize their biological and pathological functions, and explore their regulatory mechanisms in health and disease. This review provides a reference for the development of GSDM-targeted drugs to treat various inflammatory and tissue damage-related conditions.

GSDMD Mediates LPS-Induced Septic Myocardial Dysfunction by Regulating ROS-dependent NLRP3 Inflammasome Activation

Myocardial dysfunction is a serious consequence of sepsis and contributes to high mortality. Currently, the molecular mechanism of myocardial dysfunction induced by sepsis remains unclear. In the present study, we investigated the role of gasdermin D (GSDMD) in cardiac dysfunction in septic mice and the underlying mechanism. C57BL/6 wild-type (WT) mice and age-matched Gsdmd-knockout (Gsdmd ^-/-) mice were intraperitoneally injected with lipopolysaccharide (LPS) (10 mg/kg) to mimic sepsis. The results showed that GSDMD-NT, the functional fragment of GSDMD, was upregulated in the heart tissue of septic WT mice induced by LPS, which was accompanied by decreased cardiac function and myocardial injury, as shown by decreased ejection fraction (EF) and fractional shortening (FS) and increased cardiac troponin I (cTnI), creatine kinase isoenzymes MB (CK-MB), and lactate dehydrogenase (LDH). Gsdmd ^-/- mice exhibited protection against LPS-induced myocardial dysfunction and had a higher survival rate. Gsdmd deficiency attenuated LPS-induced myocardial injury and cell death. Gsdmd deficiency prevented LPS-induced the increase of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in serum, as well as IL-1β and TNF-α mRNA levels in myocardium. In addition, LPS-mediated inflammatory cell infiltration into the myocardium was ameliorated and activation of NF-κB signaling pathway and the NOD-like receptor protein 3 (NLPR3) inflammasome were suppressed in Gsdmd ^-/- mice. Further research showed that in the myocardium of LPS-induced septic mice, GSDMD-NT enrichment in mitochondria led to mitochondrial dysfunction and reactive oxygen species (ROS) overproduction, which further regulated the activation of the NLRP3 inflammasome. In summary, our data suggest that GSDMD plays a vital role in the pathophysiology of LPS-induced myocardial dysfunction and may be a crucial target for the prevention and treatment of sepsis-induced myocardial dysfunction.

Aeromonas sobria Induces Proinflammatory Cytokines Production in Mouse Macrophages via Activating NLRP3 Inflammasome Signaling Pathways

Aeromonas sobria, a common conditional pathogenic bacteria, is widely distributed in the environment and causes gastroenteritis in humans or septicemia in fish. Of all Aeromonas species, A. sobria is the most frequently isolated from human infections especially in immunocompromised subjects. Innate immunity is the first protection system of organism to resist non-specific pathogens invasion; however, the immune response process of hosts against A. sobria infection re\mains unexplored. The present study established an A. sobria infection model using primary mouse peritoneal macrophages (PMφs). The adherence and cytotoxicity of A. sobria on PMφs were determined by May-Grünwald Giemsa staining and LDH release measurement. Pro-inflammatory cytokine expression levels were measured using qPCR, western blotting, and ELISA methods. We also investigated the levels of ASC oligomerization and determined the roles of active caspase-1 in IL-1β secretion through inhibition assays and explored the activated pattern recognition receptors through immunofluorescence. We further elucidated the roles of activated inflammasome in regulating the host's inflammatory response through inhibition combined with ELISA assays. Our results showed that A. sobria induced lytic cell death and LDH release, whereas it had no adhesive properties on PMφs. A. sobria triggered various proinflammatory cytokine transcription level upregulation, and IL-1β occupied the highest levels. The pro-IL-1β protein expression levels increased in a dose-dependent manner with MOI ranging from 1 to 100. This process was regulated by ASC-dependent inflammasome, which cleavage pro-IL-1β into active IL-1β p17 with activated caspase-1 p20. Meanwhile, the expression levels of NLRP3 receptor significantly increased, location analysis revealed puncta-like surrounding nuclear, and inhibition of NLRP3 inflammasome downregulated caspase-1 activation and IL-1β secretion. Blocking of NLRP3 inflammasome activation through K⁺ efflux and cathepsin B or caspase approaches downregulated A. sobria-induced proinflammatory cytokine production. Overall, these data indicated that A. sobria induced proinflammatory cytokine production in PMφs through activating NLRP3 inflammasome signaling pathways.

Unraveling enterohemorrhagic Escherichia coli infection: the promising role of dietary compounds and probiotics in bacterial elimination and host innate immunity boosting

The innate immune system has developed sophisticated strategies to defense against infections. Host cells utilize the recognition machineries such as toll-like receptors and nucleotide binding and oligomerization domain-like receptors to identify the pathogens and alert immune system. However, some pathogens have developed tactics to evade host defenses, including manipulation of host inflammatory response, interference with cell death pathway, and highjack of phagocytosis signaling for a better survival and colonization in host. Enterohemorrhagic Escherichia coli (EHEC) is a notorious foodborne pathogen that causes severe tissue damages and gastrointestinal diseases, which has been reported to disturb host immune responses. Diverse bioactive compounds such as flavonoids, phenolic acids, alkaloids, saccharides, and terpenoids derived from food varieties and probiotics have been discovered and investigated for their capability of combating bacterial infections. Some of them serve as novel antimicrobial agents and act as immune boosters that harness host immune system. In this review, we will discuss how EHEC, specifically E. coli O157:H7, hijacks the host immune system and interferes with host signaling pathway; and highlight the promising role of food-derived bioactive compounds and probiotics in harnessing host innate immunity and eliminating E. coli O157:H7 infection with multiple strategies.

A genome-wide screen uncovers multiple roles for mitochondrial nucleoside diphosphate kinase D in inflammasome activation

Noncanonical inflammasome activation by cytosolic lipopolysaccharide (LPS) is a critical component of the host response to Gram-negative bacteria. Cytosolic LPS recognition in macrophages is preceded by a Toll-like receptor (TLR) priming signal required to induce transcription of inflammasome components and facilitate the metabolic reprograming that fuels the inflammatory response. Using a genome-scale arrayed siRNA screen to find inflammasome regulators in mouse macrophages, we identified the mitochondrial enzyme nucleoside diphosphate kinase D (NDPK-D) as a regulator of both noncanonical and canonical inflammasomes. NDPK-D was required for both mitochondrial DNA synthesis and cardiolipin exposure on the mitochondrial surface in response to inflammasome priming signals mediated by TLRs, and macrophages deficient in NDPK-D had multiple defects in LPS-induced inflammasome activation. In addition, NDPK-D was required for the recruitment of TNF receptor-associated factor 6 (TRAF6) to mitochondria, which was critical for reactive oxygen species (ROS) production and the metabolic reprogramming that supported the TLR-induced gene program. NDPK-D knockout mice were protected from LPS-induced shock, consistent with decreased ROS production and attenuated glycolytic commitment during priming. Our findings suggest that, in response to microbial challenge, NDPK-D-dependent TRAF6 mitochondrial recruitment triggers an energetic fitness checkpoint required to engage and maintain the transcriptional program necessary for inflammasome activation.

Luteolin prevents THP-1 macrophage pyroptosis by suppressing ROS production via Nrf2 activation

Pyroptosis plays an important role in the pathogenesis of numerous infectious, autoimmune, and inflammatory diseases, which makes it a promising target for intervention. In this study, the effect of luteolin on pyroptosis and the underlying mechanism were investigated using the canonical NLRP3 inflammasome in THP-1 macrophages induced by LPS/ATP. The results showed that luteolin exhibited a potent preventive effect on THP-1 macrophage pyroptosis, as evidenced by the increase in cell viability and the decrease in LDH release. Moreover, luteolin was found to significantly reduce the expression of NLRP3, pro-CASP-1 and CASP-1, which are the key components of NLRP3 inflammasome, as well as the expression of N-GSDMD and IL-1β, and we proved that the inhibition of luteolin on NLRP3 inflammasome activation is ROS-dependent. Furthermore, it was demonstrated that luteolin promoted Nrf2 nuclear translocation, thereby increasing the expression of HO-1 that reduces ROS production, while the anti-pyroptotic effect of luteolin was reversed by a specific Nrf2 inhibitor. Additionally, luteolin inhibited NF-κB p65 phosphorylation and nuclear translocation. In summary, we conclude that luteolin prevents THP-1 macrophage pyroptosis by suppressing ROS production via Nrf2 activation as well as NF-κB inactivation. These results support luteolin as a potential bioactive chemical against pyroptosis-related inflammatory diseases.

Autophagy: A Novel Horizon for Hair Cell Protection

As a general sensory disorder, hearing loss was a major concern worldwide. Autophagy is a common cellular reaction to stress that degrades cytoplasmic waste through the lysosome pathway. Autophagy not only plays major roles in maintaining intracellular homeostasis but is also involved in the development and pathogenesis of many diseases. In the auditory system, several studies revealed the link between autophagy and hearing protection. In this review, we aimed to establish the correlation between autophagy and hair cells (HCs) from the aspects of ototoxic drugs, aging, and acoustic trauma and discussed whether autophagy could serve as a potential measure in the protection of HCs.

Gasdermin family proteins as a permeabilization factor of cell membrane in pyroptosis process

The type of cell death, i.e. apoptosis, autophagy, necrosis or pyroptosis, depends on the inducing factor and the phase of the cell cycle. The main role in immunological response to microorganisms is played by a process called pyroptosis. Pyroptosis induces various types of inflammatory factors in response to molecular patterns associated with pathogens, e.g., bacterial lipopolysaccharide in the canonical or non-canonical pathway depending on the type of caspases involved. In pyroptosis, the gasdermin D protein belonging to the gasdermin protein family (A, B, C, D, E and DFNB59) plays an important role, which is characterized by specific tissue gene expression mainly in epithelial cells, skin and the digestive system and is responsible for regulating the proliferation and differentiation of cells and is responsible for inhibiting or developing cancers in various organs. The GSDM family is responsible for the formation of pores in the cell membrane, enabling the secretion of proinflammatory cytokines (IL-1β and IL-18) involved in initiating inflammatory response pathways by recruiting and activating immune cells at the site of infection. The gasdermin D protein plays an essential role in the non-canonical pyroptosis process, whose N-terminal forming pores in the cell membrane leads to edema, osmotic lysis and, consequently, to the death of the infected cell.

Mitochondrial quality control in kidney injury and repair

Mitochondria are essential for the activity, function and viability of eukaryotic cells and mitochondrial dysfunction is involved in the pathogenesis of acute kidney injury (AKI) and chronic kidney disease, as well as in abnormal kidney repair after AKI. Multiple quality control mechanisms, including antioxidant defence, protein quality control, mitochondrial DNA repair, mitochondrial dynamics, mitophagy and mitochondrial biogenesis, have evolved to preserve mitochondrial homeostasis under physiological and pathological conditions. Loss of these mechanisms may induce mitochondrial damage and dysfunction, leading to cell death, tissue injury and, potentially, organ failure. Accumulating evidence suggests a role of disturbances in mitochondrial quality control in the pathogenesis of AKI, incomplete or maladaptive kidney repair and chronic kidney disease. Moreover, specific interventions that target mitochondrial quality control mechanisms to preserve and restore mitochondrial function have emerged as promising therapeutic strategies to prevent and treat kidney injury and accelerate kidney repair. However, clinical translation of these findings is challenging owing to potential adverse effects, unclear mechanisms of action and a lack of knowledge of the specific roles and regulation of mitochondrial quality control mechanisms in kidney resident and circulating cell types during injury and repair of the kidney.

Combined Therapy With Polyethylene Glycol-20k and MCC950 Preserves Post-Resuscitated Myocardial Function in a Rat Model of Cardiac Arrest and Cardiopulmonary Resuscitation

Background To investigate the therapeutic potential of combined therapy with polyethylene glycol-20k (PEG-20k) and MCC950 on post-resuscitation myocardial function in a rat model of cardiac arrest. Methods and Results Thirty rats were randomized into 5 groups: Sham, Control, PEG-20k, MCC950, PEG-20k+ MCC950. Except for sham, animals were subjected to 6 minutes of ventricular fibrillation followed by 8 minutes cardiopulmonary resuscitation. Two milliliters PEG-20k was administered by intravenous injection coincident with the start of cardiopulmonary resuscitation; MCC950 (10 mg/kg), a highly selective NLRP3 inflammasome inhibitor, was delivered immediately after restoration of spontaneous circulation. Myocardial function, sublingual microcirculation, mitochondrial function, plasma cardiac troponin I, and interleukin-1β, expression of proteins in SIRT1 (sirtuin 1)/PGC-1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha) and NLRP3 (the NOD-like receptor family protein 3) inflammasome pathways were evaluated. Following cardiopulmonary resuscitation, myocardial function was compromised with a significantly decreased cardiac output, ejection fraction, and increased myocardial performance index, cardiac troponin I. Sublingual microcirculation was disturbed with impaired perfused vessel density and microvascular flow index. Cardiac arrest reduced mitochondrial routine respiration, Complex I-linked respiration, respiratory control rates and oxidative phosphorylation coupling efficiency. PEG-20k or MCC950 alone restored mitochondrial respiratory function, restituted sublingual microcirculation, and preserved myocardial function, whereas a combination of PEG-20k and MCC950 further improved these aspects. PEG-20k restored the expression of SIRT1 and PGC-1α, and blunted activation of NLRP3 inflammasomes. MCC950 suppressed expression of cleaved-caspase-1/pro-caspase-1, ASC (apoptosis-associated speck-like protein), GSDMD [gasdermin d], and interleukin-1β. Conclusions Combined therapy with PEG-20k and MCC950 is superior to either therapy alone for preserving post-resuscitated myocardial function, restituting sublingual microcirculation at restoration of spontaneous circulation at 6 hours. The responsible mechanisms involve upregulated expression of SIRT1/PGC1-α in tandem with inhibition of NLRP3 inflammasomes.

Punching Holes in Cellular Membranes: Biology and Evolution of Gasdermins

The gasdermin (GSDM) family has evolved as six gene clusters (GSDMA-E and Pejvakin, PJVK), and GSDM proteins are characterized by a unique N-terminal domain (N-GSDM). With the exception of PJVK, the N-GSDM domain is capable of executing plasma membrane permeabilization. Depending on the cell death modality, several protease- and kinase-dependent mechanisms directly regulate the activity of GSDME and GSDMD, the two most widely expressed and best-studied GSDMs. We provide an overview of all GSDMs in terms of biological function, tissue expression, activation, regulation, and structure. In-depth phylogenetic analysis reveals that GSDM genes show many gene duplications and deletions, suggesting that strong evolutionary forces and a unique position of the PJVK gene are associated with the occurrence of complex inner-ear development in vertebrates.

Gasdermin D Protects Mouse Podocytes Against High-Glucose-Induced Inflammation and Apoptosis via the C-Jun N-Terminal Kinase (JNK) Pathway

Background

The inflammation and apoptosis of podocytes contribute to the pathological progression of diabetic nephropathy. Gasdermin D (GSDMD) plays an executive role in pyroptosis, but its effect on high-glucose (HG)-induced inflammation and apoptosis remains unclear. The aim of this study was to investigate the effect of GSDMD on high-glucose-induced inflammation and apoptosis in podocytes.

Material/Methods

Mouse podocytes were cultivated by high- or normal-glucose medium. We used western blot analysis, reverse transcription-quantitative polymerase chain reaction (RT-qPCR), and immunofluorescence to detect the expression and localization of GSDMD in high-glucose-induced podocytes, and the expression of apoptosis-related proteins Bax and Bcl-2, inflammatory factors IL-1β, IL-6, and TNF-α, and JNK pathways in high-glucose-induced podocytes. Western blot and immunofluorescence were used to detect the expression and localization of synaptopodin under GSDMD knockdown and JNK-specific blocker SP600125. MitoSOX Red was used to detect the production of ROS in mitochondria under siGSDMD. The intracellular ROS generation was detected using a reactive oxygen species assay kit.

Results

We found that GSDMD knockdown and JNK inhibition reduced the expression of Bax, Bcl-2, cleaved caspase-3, IL-1β, IL-6, and TNF-α. Our results showed that GSDMD knockdown can inhibit HG-induced mitochondrial ROS production and JNK phosphorylation.

Conclusions

This study indicates that GSDMD knockdown can attenuate HG-induced inflammation and apoptosis by inhibiting the phosphorylation of JNK via mitochondrial ROS.

Hypoxia and Porphyromonas gingivalis-lipopolysaccharide synergistically induce NLRP3 inflammasome activation in human gingival fibroblasts

OBJECTIVE

To investigate the effects of hypoxia and Porphyromonas gingivalis- lipopolysaccharide (P. gingivalis-LPS) on activation of the NACHT leucine-rich repeat protein 3 (NLRP3) inflammasome in human gingival fibroblasts (HGFs).

DESIGN

Periodontitis was optimally simulated using a hypoxic concentration of 1%. HGFs were stimulated using P. gingivalis-LPS (1.0 μg/ml) in normoxia and hypoxia for 3 h and 6 h, respectively. The expression levels of genes and proteins of hypoxia-inducible factor-1α (HIF-1α), interleukin-1β, gasdermin D (GSDMD) and the NLRP3 inflammasome, including NLRP3, apoptosis-associated speck-like protein containing CARD (ASC), caspase-1 and its activated forms, were measured using quantitative real-time polymerase chain reaction and western blot. ELISA was used to detect and determine levels of the inflammatory factor interleukin-1β in cell supernatants. Lactate dehydrogenase (LDH) release assay, caspase-1 activity assay and Hoechst 33342/Propidium Iodide (PI) staining were performed to further verify the presence of pyroptosis.

RESULTS

The NLRP3 inflammasome (i.e., NLRP3, ASC, caspase-1) was not affected by individual stimulation using P. gingivalis-LPS or hypoxia. However, the combination of both hypoxia and P. gingivalis-LPS stimulation significantly enhanced inflammasome activation and promoted the expression of interleukin-1β, gasdermin D and HIF-1α at gene and protein levels; PI positive cells and the release of LDH were also elevated.

CONCLUSION

Hypoxia and P. gingivalis-LPS synergistically induced NLRP3 inflammasome activation in HGFs, and subsequently high levels of interleukin-1β and GSDMD-mediated pyroptosis can cause an HGF inflammatory response, which plays an important role in the pathogenesis of periodontitis.

Activation of the Nlrp3 Inflammasome Contributes to Shiga Toxin-Induced Hemolytic Uremic Syndrome in a Mouse Model

Objective

To explore the role of the Nlrp3 inflammasome activation in the development of hemolytic uremic syndrome (HUS) induced by Stx2 and evaluate the efficacy of small molecule Nlrp3 inhibitors in preventing the HUS.

Methods

Peritoneal macrophages (PMs) isolated from wild-type (WT) C57BL/6J mice and gene knockout mice (Nlrc4 ^-/-, Aim2 ^-/-, and Nlrp3 ^-/-) were treated with Stx2 in vitro and their IL-1β releases were measured. WT mice and Nlrp3 ^-/- mice were also treated with Stx2 in vivo by injection, and the biochemical indices (serum IL-1β, creatinine [CRE] and blood urea nitrogen [BUN]), renal injury, and animal survival were compared. To evaluate the effect of the Nlrp3 inhibitors in preventing HUS, WT mice were pretreated with different Nlrp3 inhibitors (MCC950, CY-09, Oridonin) before Stx2 treatment, and their biochemical indices and survival were compared with the WT mice without inhibitor pretreatment.

Results

When PMs were stimulated by Stx2 in vitro, IL-1β release in Nlrp3 ^-/- PMs was significantly lower compared to the other PMs. The Nlrp3 ^-/- mice treated by Stx2 in vivo, showed lower levels of the biochemical indices, alleviated renal injuries, and increased survival rate. When the WT mice were pretreated with the Nlrp3 inhibitors, both the biochemical indices and survival were significantly improved compared to those without inhibitor pretreatment, with Oridonin being most potent.

Conclusion

Nlrp3 inflammasome activation plays a vital role in the HUS development when mice are challenged by Stx2, and Oridonin is effective in preventing HUS.

Activation mechanisms of inflammasomes by bacterial toxins

Inflammasomes are cytosolic innate immune complexes, which assemble in mammalian cells in response to microbial components and endogenous danger signals. A major family of inflammasome activators is bacterial toxins. Inflammasome sensor proteins, such as the nucleotide-binding oligomerisation domain-like receptor (NLR) family members NLRP1b and NLRP3, and the tripartite motif family member Pyrin⁺ efflux triggered by pore-forming toxins or by other toxin-induced homeostasis-altering events such as lysosomal rupture. Pyrin senses perturbation of host cell functions induced by certain enzymatic toxins resulting in impairment of RhoA GTPase activity. Assembly of the inflammasome complex activates the cysteine protease caspase-1, leading to the proteolytic cleavage of the proinflammatory cytokines IL-1β and IL-18, and the pore-forming protein gasdermin D causing pyroptosis. In this review, we discuss the latest progress in our understanding on the activation mechanisms of inflammasome complexes by bacterial toxins and effector proteins and explore avenues for future research into the relationships between inflammasomes and bacterial toxins.

Ac-FLTD-CMK inhibits pyroptosis and exerts neuroprotective effect in a mice model of traumatic brain injury

Pyroptosis has been reported to contribute to the traumatic brain injury (TBI) process. Ac-FLTD-CMK is a newly synthesized pyroptosis inhibitor. However, whether Ac-FLTD-CMK inhibits pyroptosis and plays a neuroprotective role after TBI is unknown. The present study aimed to determine the effects of Ac-FLTD-CMK on TBI in a mouse model. Male C57BL/6 mice were randomly divided into sham, TBI + vehicle, and TBI + Ac-FLTD-CMK groups. TBI was induced using a weight-drop apparatus. Intraventricular injection of Ac-FLTD-CMK was performed 30 min after TBI. Caspase-1, caspase-11, gasdermin-D (GSDMD), and caspase-3 expression in the peri-contusional cortex were assessed by western blotting. Interleukin-1β (IL-1β) and interleukin-18 (IL-18) expression in the peri-contusional cortex were measured using ELISA. Behavioral experiments, brain water content, Evans blue extravasation, lactate dehydrogenase (LDH) release, and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling staining were also performed. The results showed that Ac-FLTD-CMK administration significantly downregulated caspase-1 p20, caspase-11 p20, GSDMD N-terminal, IL-1β, and IL-18 expression; reduced LDH release; alleviated neuronal death; attenuated brain edema and blood-brain barrier damage; and improved neurobehavioral function. These findings indicate that Ac-FLTD-CMK treatment suppresses pyroptosis and protects mice against TBI.

Gasdermin D restricts Burkholderia cenocepacia infection in vitro and in vivo

Burkholderia cenocepacia (B. cenocepacia) is an opportunistic bacterium; causing severe life threatening systemic infections in immunocompromised individuals including cystic fibrosis patients. The lack of gasdermin D (GSDMD) protects mice against endotoxin lipopolysaccharide (LPS) shock. On the other hand, GSDMD promotes mice survival in response to certain bacterial infections. However, the role of GSDMD during B. cenocepacia infection is not yet determined. Our in vitro study shows that GSDMD restricts B. cenocepacia replication within macrophages independent of its role in cell death through promoting mitochondrial reactive oxygen species (mROS) production. mROS is known to stimulate autophagy, hence, the inhibition of mROS or the absence of GSDMD during B. cenocepacia infections reduces autophagy which plays a critical role in the restriction of the pathogen. GSDMD promotes inflammation in response to B. cenocepacia through mediating the release of inflammasome dependent cytokine (IL-1β) and an independent one (CXCL1) (KC). Additionally, different B. cenocepacia secretory systems (T3SS, T4SS, and T6SS) contribute to inflammasome activation together with bacterial survival within macrophages. In vivo study confirmed the in vitro findings and showed that GSDMD restricts B. cenocepacia infection and dissemination and stimulates autophagy in response to B. cenocepacia. Nevertheless, GSDMD promotes lung inflammation and necrosis in response to B. cenocepacia without altering mice survival. This study describes the double-edged functions of GSDMD in response to B. cenocepacia infection and shows the importance of GSDMD-mediated mROS in restriction of B. cenocepacia.

An overview of the non-canonical inflammasome

Inflammasomes are large cytosolic multiprotein complexes assembled in response to infection and cellular stress, and are crucial for the activation of inflammatory caspases and the subsequent processing and release of pro-inflammatory mediators. While caspase-1 is activated within the canonical inflammasome, the related caspase-4 (also known as caspase-11 in mice) and caspase-5 are activated within the non-canonical inflammasome upon sensing of cytosolic lipopolysaccharide (LPS) from Gram-negative bacteria. However, the consequences of canonical and non-canonical inflammasome activation are similar. Caspase-1 promotes the processing and release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and the release of danger signals, as well as a lytic form of cell death called pyroptosis, whereas caspase-4, caspase-5 and caspase-11 directly promote pyroptosis through cleavage of the pore-forming protein gasdermin D (GSDMD), and trigger a secondary activation of the canonical NLRP3 inflammasome for cytokine release. Since the presence of the non-canonical inflammasome activator LPS leads to endotoxemia and sepsis, non-canonical inflammasome activation and regulation has important clinical ramifications. Here we discuss the mechanism of non-canonical inflammasome activation, mechanisms regulating its activity and its contribution to health and disease.

Shiga toxin suppresses noncanonical inflammasome responses to cytosolic LPS

Inflammatory caspase-dependent cytosolic lipopolysaccharide (LPS) sensing is a critical arm of host defense against bacteria. How pathogens overcome this pathway to establish infections is largely unknown. Enterohemorrhagic Escherichia coli (EHEC) is a clinically important human pathogen causing hemorrhagic colitis and hemolytic uremic syndrome. We found that a bacteriophage-encoded virulence factor of EHEC, Shiga toxin (Stx), suppresses caspase-11-mediated activation of the cytosolic LPS sensing pathway. Stx was essential and sufficient to inhibit pyroptosis and interleukin-1 (IL-1) responses elicited specifically by cytosolic LPS. The catalytic activity of Stx was necessary for suppression of inflammasome responses. Stx impairment of inflammasome responses to cytosolic LPS occurs at the level of gasdermin D activation. Stx also suppresses inflammasome responses in vivo after LPS challenge and bacterial infection. Overall, this study assigns a previously undescribed inflammasome-subversive function to a well-known bacterial toxin, Stx, and reveals a new phage protein-based pathogen blockade of cytosolic immune surveillance.

Targeting pyroptosis to regulate ischemic stroke injury: Molecular mechanisms and preclinical evidences

Stroke is one of the leading causes of death worldwide with limited therapies. After ischemic stroke occurs, a robust sterile inflammatory response happens and lasts for days and determines neurological prognosis. Pyroptosis is an inflammatory programmed cell death characterized by cleavage of pore-forming proteins gasdermins as a result of activating caspases and inflammasomes. It has morphological characteristics of rapid plasma-membrane rupture and release of proinflammatory intracellular contents as well as cytokines. Recent researches implicate pyroptosis involvement in the pathogenesis of ischemic stroke and inhibition of pyroptosis attenuates ischemic brain injury. In this review, we discussed molecular mechanisms of pyroptosis, evidences for pyroptosis involvement in different kinds of the central nervous system cells, as well as potential inhibitors for intervention of pyroptosis. Based on the review, we hypothesize the feasibility of therapeutic strategies targeting pyroptosis in the context of ischemic stroke.

Priming Is Dispensable for NLRP3 Inflammasome Activation in Human Monocytes In Vitro

Interleukin (IL)-18 and IL-1β are potent pro-inflammatory cytokines that contribute to inflammatory conditions such as rheumatoid arthritis and Alzheimer's disease. They are produced as inactive precursors that are activated by large macromolecular complexes called inflammasomes upon sensing damage or pathogenic signals. NLRP3 inflammasome activation is regarded to require a priming step that causes NLRP3 and IL-1β gene upregulation, and also NLRP3 post-translational licencing. A subsequent activation step leads to the assembly of the complex and the cleavage of pro-IL-18 and pro-IL-1β by caspase-1 into their mature forms, allowing their release. Here we show that human monocytes, but not monocyte derived macrophages, are able to form canonical NLRP3 inflammasomes in the absence of priming. NLRP3 activator nigericin caused the processing and release of constitutively expressed IL-18 in an unprimed setting. This was mediated by the canonical NLRP3 inflammasome that was dependent on K+ and Cl- efflux and led to ASC oligomerization, caspase-1 and Gasdermin-D (GSDMD) cleavage. IL-18 release was impaired by the NLRP3 inhibitor MCC950 and by the absence of NLRP3, but also by deficiency of GSDMD, suggesting that pyroptosis is the mechanism of release. This work highlights the readiness of the NLRP3 inflammasome to assemble in the absence of priming in human monocytes and hence contribute to the very early stages of the inflammatory response when IL-1β has not yet been produced. It is important to consider the unprimed setting when researching the mechanisms of NLRP3 activation, as to not overshadow the pathways that occur in the absence of priming stimuli, which might only enhance this response.

An Update on CARD Only Proteins (COPs) and PYD Only Proteins (POPs) as Inflammasome Regulators

Inflammasomes are protein scaffolds required for the activation of caspase-1 and the subsequent release of interleukin (IL)-1β, IL-18, and danger signals, as well as the induction of pyroptotic cell death to restore homeostasis following infection and sterile tissue damage. However, excessive inflammasome activation also causes detrimental inflammatory disease. Therefore, extensive control mechanisms are necessary to prevent improper inflammasome responses and inflammatory disease. Inflammasomes are assembled by sequential nucleated polymerization of Pyrin domain (PYD) and caspase recruitment domain (CARD)-containing inflammasome components. Once polymerization is nucleated, this process proceeds in a self-perpetuating manner and represents a point of no return. Therefore, regulation of this key step is crucial for a controlled inflammasome response. Here, we provide an update on two single domain protein families containing either a PYD or a CARD, the PYD-only proteins (POPs) and CARD-only proteins (COPs), respectively. Their structure allows them to occupy and block access to key protein-protein interaction domains necessary for inflammasome assembly, thereby regulating the threshold of these nucleated polymerization events, and consequently, the inflammatory host response.

ROS systems are a new integrated network for sensing homeostasis and alarming stresses in organelle metabolic processes

Reactive oxygen species (ROS) are critical for the progression of cardiovascular diseases, inflammations and tumors. However, the mechanisms of how ROS sense metabolic stress, regulate metabolic pathways and initiate proliferation, inflammation and cell death responses remain poorly characterized. In this analytic review, we concluded that: 1) Based on different features and functions, eleven types of ROS can be classified into seven functional groups: metabolic stress-sensing, chemical connecting, organelle communication, stress branch-out, inflammasome-activating, dual functions and triple functions ROS. 2) Among the ROS generation systems, mitochondria consume the most amount of oxygen; and nine types of ROS are generated; thus, mitochondrial ROS systems serve as the central hub for connecting ROS with inflammasome activation, trained immunity and immunometabolic pathways. 3) Increased nuclear ROS production significantly promotes cell death in comparison to that in other organelles. Nuclear ROS systems serve as a convergent hub and decision-makers to connect unbearable and alarming metabolic stresses to inflammation and cell death. 4) Balanced ROS levels indicate physiological homeostasis of various metabolic processes in subcellular organelles and cytosol, while imbalanced ROS levels present alarms for pathological organelle stresses in metabolic processes. Based on these analyses, we propose a working model that ROS systems are a new integrated network for sensing homeostasis and alarming stress in metabolic processes in various subcellular organelles. Our model provides novel insights on the roles of the ROS systems in bridging metabolic stress to inflammation, cell death and tumorigenesis; and provide novel therapeutic targets for treating those diseases. (Word count: 246).

Targeting of BCL-2 Family Members during Anticancer Treatment: A Necessary Compromise between Individual Cell and Ecosystemic Responses?

The imbalance between BCL-2 homologues and pro-death counterparts frequently noted in cancer cells endows them with a cell autonomous survival advantage. To eradicate ectopic cells, inhibitors of these homologues (BH3 mimetics) were developed to trigger, during anticancer treatment, full activation of the canonical mitochondrial apoptotic pathway and related caspases. Despite efficiency in some clinical settings, these compounds do not completely fulfill their initial promise. We herein put forth that a growing body of evidence indicates that mitochondrial integrity, controlled by BCL-2 family proteins, and downstream caspases regulate other cell death modes and influence extracellular signaling by committed cells. Moreover, intercellular communications play a key role in spreading therapeutic response across cancer cell populations and in engaging an immune response. We thus advocate that BH3 mimetics administration would be more efficient in the long term if it did not induce apoptosis in all sensitive cells at the same time, but if it could instead allow (or trigger) death signal production by non-terminally committed dying cell populations. The development of such a trade-off strategy requires to unravel the effects of BH3 mimetics not only on each individual cancer cell but also on homotypic and heterotypic cell interactions in dynamic tumor ecosystems.

Mitochondrial GSDMD Pores DAMPen Pyroptosis

Noncanoncial inflammasome activation by cytosolic lipopolysaccharide (LPS) causes pyroptotic cell death facilitated by gasdermin D (GSDMD) pore formation. In this issue of Immunity, Huang et al. describe how cytosolic LPS in endothelial cells does not cause cell death but restrains endothelial cell proliferation.

Transcriptional network inference and master regulator analysis of the response to ribosome-inactivating proteins in leukemia cells

Gene-regulatory networks reconstruction has become a very popular approach in applied biology to infer and dissect functional interactions of Transcription Factors (TFs) driving a defined phenotypic state, termed as Master Regulators (MRs). In the present work, cutting-edge bioinformatic methods were applied to re-analyze experimental data on leukemia cells (human myelogenous leukemia cell line THP-1 and acute myeloid leukemia MOLM-13 cells) treated for 6 h with two different Ribosome-Inactivating Proteins (RIPs), namely Shiga toxin type 1 (400 ng/mL) produced by Escherichia coli strains and the plant toxin stenodactylin (60 ng/mL), purified from the caudex of Adenia stenodactyla Harms. This analysis allowed us to identify the common early transcriptional response to 28S rRNA damage based on gene-regulatory network inference and Master Regulator Analysis (MRA). Both toxins induce a common response at 6 h which involves inflammatory mediators triggered by AP-1 family transcriptional factors and ATF3 in leukemia cells. We describe for the first time the involvement of MAFF, KLF2 and KLF6 in regulating RIP-induced apoptotic cell death, while receptor-mediated downstream signaling through ANXA1 and TLR4 is suggested for both toxins.

Gasdermin D activity in inflammation and host defense

The mechanisms underlying the release of interleukin-1 (IL-1) family cytokines from phagocytes have been the subject of intense investigations for more than 30 years. The absence of an amino-terminal secretion signal from members of this family suggests a previously unknown mechanism of protein secretion that transfers cytosolic IL-1 directly across the plasma membrane into the extracellular space. The pore-forming protein gasdermin D (GSDMD) has emerged as the conduit for IL-1 secretion from the cytosol, serving to induce the release of IL-1 from living (hyperactive) or dead (pyroptotic) cells. In this Review, we discuss the mechanism by which GSDMD pore formation is regulated by the activity of inflammatory caspases, which are commonly associated with inflammasomes. We discuss how GSDMD promotes IL-1 release from hyperactive or pyroptotic cells, with a specific focus on defining how these distinct cell fates associated with GSDMD activity can be regulated. Last, the physiological consequences of GSDMD activity and therapeutic potential of targeting this pore-forming protein are discussed, which highlight the abundance of questions that remain to be answered by the community.

Molecular Biology of Escherichia Coli Shiga Toxins' Effects on Mammalian Cells

Shiga toxins (Stxs), syn. Vero(cyto)toxins, are potent bacterial exotoxins and the principal virulence factor of enterohemorrhagic Escherichia coli (EHEC), a subset of Shiga toxin-producing E. coli (STEC). EHEC strains, e.g., strains of serovars O157:H7 and O104:H4, may cause individual cases as well as large outbreaks of life-threatening diseases in humans. Stxs primarily exert a ribotoxic activity in the eukaryotic target cells of the mammalian host resulting in rapid protein synthesis inhibition and cell death. Damage of endothelial cells in the kidneys and the central nervous system by Stxs is central in the pathogenesis of hemolytic uremic syndrome (HUS) in humans and edema disease in pigs. Probably even more important, the toxins also are capable of modulating a plethora of essential cellular functions, which eventually disturb intercellular communication. The review aims at providing a comprehensive overview of the current knowledge of the time course and the consecutive steps of Stx/cell interactions at the molecular level. Intervention measures deduced from an in-depth understanding of this molecular interplay may foster our basic understanding of cellular biology and microbial pathogenesis and pave the way to the creation of host-directed active compounds to mitigate the pathological conditions of STEC infections in the mammalian body.

NLRP12 collaborates with NLRP3 and NLRC4 to promote pyroptosis inducing ganglion cell death of acute glaucoma

BACKGROUND

Acute glaucoma, characterized by a sudden elevation in intraocular pressure (IOP) and retinal ganglion cells (RGCs) death, is a major cause of irreversible blindness worldwide that lacks approved effective therapies, validated treatment targets and clear molecular mechanisms. We sought to explore the potential molecular mechanisms underlying the causal link between high IOP and glaucomatous RGCs death.

METHODS

A murine retinal ischemia/ reperfusion (RIR) model and an in vitro oxygen and glucose deprivation/reoxygenation (OGDR) model were used to investigate the pathogenic mechanisms of acute glaucoma.

RESULTS

Our findings reveal a novel mechanism of microglia-induced pyroptosis-mediated RGCs death associated with glaucomatous vision loss. Genetic deletion of gasdermin D (GSDMD), the effector of pyroptosis, markedly ameliorated the RGCs death and retinal tissue damage in acute glaucoma. Moreover, GSDMD cleavage of microglial cells was dependent on caspase-8 (CASP8)-hypoxia-inducible factor-1α (HIF-1α) signaling. Mechanistically, the newly identified nucleotide-binding leucine-rich repeat-containing receptor (NLR) family pyrin domain-containing 12 (NLRP12) collaborated with NLR family pyrin domain-containing 3 (NLRP3) and NLR family CARD domain-containing protein 4 (NLRC4) downstream of the CASP8-HIF-1α axis, to elicit pyroptotic processes and interleukin-1β (IL-1β) maturation through caspase-1 activation, facilitating pyroptosis and neuroinflammation in acute glaucoma. Interestingly, processing of IL-1β in turn magnified the CASP8-HIF-1α-NLRP12/NLRP3/NLRC4-pyroptosis circuit to accelerate inflammatory cascades.

CONCLUSIONS

These data not only indicate that the collaborative effects of NLRP12, NLRP3 and NLRC4 on pyroptosis are responsible for RGCs death, but also shed novel mechanistic insights into microglial pyroptosis, paving novel therapeutic avenues for the treatment of glaucoma-induced irreversible vision loss through simultaneously targeting of pyroptosis.

Gasdermin A3-Mediated Cell Death Causes Niche Collapse and Precocious Activation of Hair Follicle Stem Cells

Hair follicles undergo recurrent growth, regression, and resting phases throughout postnatal life, which is supported by hair follicle stem cells. The niche components of hair follicle stem cells are important to maintain their quiescence and stemness. Gsdma3 gain-of-function mutations in mice cause chronic skin inflammation, aberrant hair cycle, and progressive hair loss, reminiscent of scarring alopecia in humans. However, the mechanism underlying these defects remains elusive. Here, we used a combined Cre/loxP and rtTA/TRE system to study the spatiotemporal effect of Gsdma3 overexpression on distinct hair cycle stages. We found that Gsdma3-mediated cell death affects anagen initiation, anagen progression, and catagen-telogen transition. Induced Gsdma3 expression causes bulge inner layer collapse and precocious hair follicle stem cell activation, leading to subsequent hair follicle degeneration. Although macrophages and dendritic cells are recruited to the bulge region, in vivo depletion of these cells using a neutralizing antibody does not alleviate cell death in the bulge or hair germ, indicating that macrophages are less likely to cause immediate hair follicle deletion. Our data suggest that dysregulated Gsdma3 causes bulge inner layer necrosis to induce club hair shedding and immediate anagen reentry without going through telogen morphology, which implicates a role for Gsdma3 in hair follicle stem cell niche maintenance.

Vying for the control of inflammasomes: The cytosolic frontier of enteric bacterial pathogen-host interactions

Enteric pathogen-host interactions occur at multiple interfaces, including the intestinal epithelium and deeper organs of the immune system. Microbial ligands and activities are detected by host sensors that elicit a range of immune responses. Membrane-bound toll-like receptors and cytosolic inflammasome pathways are key signal transducers that trigger the production of pro-inflammatory molecules, such as cytokines and chemokines, and regulate cell death in response to infection. In recent years, the inflammasomes have emerged as a key frontier in the tussle between bacterial pathogens and the host. Inflammasomes are complexes that activate caspase-1 and are regulated by related caspases, such as caspase-11, -4, -5 and -8. Importantly, enteric bacterial pathogens can actively engage or evade inflammasome signalling systems. Extracellular, vacuolar and cytosolic bacteria have developed divergent strategies to subvert inflammasomes. While some pathogens take advantage of inflammasome activation (e.g. Listeria monocytogenes, Helicobacter pylori), others (e.g. E. coli, Salmonella, Shigella, Yersinia sp.) deploy a range of virulence factors, mainly type 3 secretion system effectors, that subvert or inhibit inflammasomes. In this review we focus on inflammasome pathways and their immune functions, and discuss how enteric bacterial pathogens interact with them. These studies have not only shed light on inflammasome-mediated immunity, but also the exciting area of mammalian cytosolic immune surveillance.

Minocycline alleviates NLRP3 inflammasome-dependent pyroptosis in monosodium glutamate-induced depressive rats

BACKGROUND

Inflammasome activation and followed by the release of proinflammatory cytokines play a pivotal role in the development and progression of depression. However, the involvement of gasdermin D (GSDMD)-mediated pyroptosis in inflammasome-associated depression has not been studied. The present study aimed to determine the involvement of pyroptosis in the development of depression.

METHODS

The rat depressive model was established by the administration of monosodium glutamate (MSG) in postnatal rats. Minocycline (an anti-inflammatory agent) and VX-765 (a specific inhibitor of caspase-1) were given as intervention treatments when rats were two-month-old. Rat depressive behaviors were evaluated by behavioral tests, including open field test, sucrose preference test, and forced swim test. Rat hippocampi were collected for western blotting and immunofluorescence examination.

RESULTS

MSG administration induced depressive-like behavior in rats. MSG upregulated protein presences of caspase-1, GSDMD, interleukin-1β (IL-1β), interleukin-18 (IL-18), NLR pyrin domain-containing 3 (NLRP3), apoptosis-associated speck-like protein (ASC), high mobility group box 1 protein (HMGB1), and the receptor for advanced glycation end products (RAGE) in the hippocampus. Protein presences of HMGB1, NLRP3 and GSDMD were upregulated in Olig2+ oligodendrocytes in the hippocampus. The data suggest that both HMGB1/RAGE/NLRP3 signalings and GSDMD-dependent pyroptosis were activated. Both minocycline and VX-765 treatments improved depressive-like behaviors. Minocycline treatment significantly reduced both HMGB1/RAGE/NLRP3 inflammasome signalings and GSDMD-dependent pyroptosis. VX-765 downregulated GSDMD-dependent pyroptosis, but not HMGB1/RAGE signalings, indicating that GSDMD-dependent pyroptosis is a key player in the progress of depression.

CONCLUSION

In rats hippocampus, NLRP3 inflammasome activates GSDMD mediated-pyroptosis in the hippocampus of MSG-induced depressive rats.

mtDNA Activates cGAS Signaling and Suppresses the YAP-Mediated Endothelial Cell Proliferation Program to Promote Inflammatory Injury

Cytosolic DNA acts as a universal danger-associated molecular pattern (DAMP) signal; however, the mechanisms of self-DNA release into the cytosol and its role in inflammatory tissue injury are not well understood. We found that the internalized bacterial endotoxin lipopolysaccharide (LPS) activated the pore-forming protein Gasdermin D, which formed mitochondrial pores and induced mitochondrial DNA (mtDNA) release into the cytosol of endothelial cells. mtDNA was recognized by the DNA sensor cGAS and generated the second messenger cGAMP, which suppressed endothelial cell proliferation by downregulating YAP1 signaling. This indicated that the surviving endothelial cells in the penumbrium of the inflammatory injury were compromised in their regenerative capacity. In an experimental model of inflammatory lung injury, deletion of cGas in mice restored endothelial regeneration. The results suggest that targeting the endothelial Gasdermin D activated cGAS-YAP signaling pathway could serve as a potential strategy for restoring endothelial function after inflammatory injury.

Role of Parkin-mediated mitophagy in the protective effect of polydatin in sepsis-induced acute kidney injury

BACKGROUND

We have reported that polydatin (PD) alleviates mitochondrial dysfunction in rat models of sepsis-induced acute kidney injury (SI-AKI), but the mechanism is not well understood. Here, we investigated the role of Parkin-mediated mitophagy in the protective effects of PD in SI-AKI in mice.

METHODS

Sepsis was induced in the mice by caecal ligation and puncture. Mitophagy was determined by mitochondrial mass. NLRP3 inflammasome activation was determined by NLRP3, ASC and caspase-1. Mitophagy was blocked by treatment with mitochondrial division inhibitor-1 and Parkin knockout.

KEY RESULTS

PD treatment increased the sepsis-induced loss of mitochondrial mass, indicating the upregulation of mitophagy. Furthermore, PD treatment mediated Parkin translocation from the cytoplasm to the mitochondria. This suggests that Parkin-mediated mitophagy is an underlying mechanism. This was confirmed by the suppression of PD-induced mitophagy in Parkin-/- mice and in mice that were treated with a mitophagy inhibitor. PD-induced Parkin translocation and mitophagy were blocked by inhibiting SIRT1; thus, activation of SIRT1 might be an important molecular mechanism that is triggered by PD. Additionally, PD treatment protected against sepsis-induced kidney injury. These effects were blocked by inhibition of Parkin-dependent mitophagy. Furthermore, PD also protected against mitochondrial dysfunction and mitochondria-dependent apoptosis, and the effect was blocked when Parkin-dependent mitophagy was inhibited. Finally, PD suppressed NLRP3 inflammasome activation that was also dependent on Parkin-mediated mitophagy.

CONCLUSIONS

These findings indicate that Parkin-mediated mitophagy is important for the protective effect of PD in SI-AKI, and the underlying mechanisms include the inhibition of mitochondrial dysfunction and NLRP3 inflammasome activation.

Mechanism and Regulation of Gasdermin-Mediated Cell Death

The innate immune system senses and responds to pathogens and endogenous damage through supramolecular protein complexes known as inflammasomes. Cytosolic inflammasome sensor proteins trigger inflammasome assembly on detection of infection and danger. Assembled inflammasomes activate a cascade of inflammatory caspases, which process procytokines and gasdermin D (GSDMD). Cleaved GSDMD forms membrane pores that lead to cytokine release and/or programmed lytic cell death, called pyroptosis. In this review, we provide a primer on pyroptosis and focus on its executioner, the GSDM protein family. In addition to inflammasome-mediated GSDMD pore formation, we describe recently discovered GSDMD activation by caspase-8 and elastase in Yersinia-infected macrophages and aging neutrophils, respectively, and GSDME activation by apoptotic caspases. Finally, we discuss strategies that host cells and pathogens use to restrict GSDMD pore formation, in addition to therapeutics targeting the GSDM family.

The role of inflammasomes in kidney disease

Inflammasomes are multiprotein innate immune complexes that regulate caspase-dependent inflammation and cell death. Pattern recognition receptors, such as nucleotide-binding oligomerization domain (NOD)-like receptors and absent in melanoma 2 (AIM2)-like receptors, sense danger signals or cellular events to activate canonical inflammasomes, resulting in caspase 1 activation, pyroptosis and the secretion of IL-1β and IL-18. Non-canonical inflammasomes can be activated by intracellular lipopolysaccharides, toxins and some cell signalling pathways. These inflammasomes regulate the activation of alternative caspases (caspase 4, caspase 5, caspase 11 and caspase 8) that lead to pyroptosis, apoptosis and the regulation of other cellular pathways. Many inflammasome-related genes and proteins have been implicated in animal models of kidney disease. In particular, the NLRP3 (NOD-, LRR- and pyrin domain-containing 3) inflammasome has been shown to contribute to a wide range of acute and chronic microbial and non-microbial kidney diseases via canonical and non-canonical mechanisms that regulate inflammation, pyroptosis, apoptosis and fibrosis. In patients with chronic kidney disease, immunomodulation therapies targeting IL-1β such as canakinumab have been shown to prevent cardiovascular events. Moreover, findings in experimental models of kidney disease suggest that small-molecule inhibitors targeting NLRP3 and other inflammasome components are promising therapeutic agents.

Research Progress on the Role of Inflammasomes in Kidney Disease

Inflammasomes are multimeric complexes composed of cytoplasmic sensors, apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC or PYCARD), and procaspase-1 and play roles in regulating caspase-dependent inflammation and cell death. Inflammasomes are assembled by sensing pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) and initiate inflammatory responses by activating caspase-1. Activated caspase-1 promotes the release of the inflammatory cytokines interleukin-1β (IL-1β) and IL-18 and eventually induces pyroptosis. Inflammasomes are closely related to kidney diseases. In particular, the NLRP3 (NACHT, LRR, and PYD domain-containing protein 3) inflammasome has been shown to cause acute and chronic kidney diseases by regulating canonical and noncanonical mechanisms of inflammation. Small-molecule inhibitors that target NLRP3 and other components of the inflammasome are potential options for the treatment of kidney-related diseases such as diabetic nephropathy. This article will focus on the research progress on inflammasomes and the key pathogenic roles of inflammasomes in the development and progression of kidney diseases and explore the potential of this intracellular inflammation to further prevent or block the development of the kidney disease.

The involvement of regulated cell death forms in modulating the bacterial and viral pathogenesis

Apoptosis, necroptosis and pyroptosis represent three distinct types of regulated cell death forms, which play significant roles in response to viral and bacterial infections. Whereas apoptosis is characterized by cell shrinkage, nuclear condensation, bleb formation and retained membrane integrity, necroptosis and pyroptosis exhibit osmotic imbalance driven cytoplasmic swelling and early membrane damage. These three cell death forms exert distinct immune stimulatory potential. The caspase driven apoptotic cell demise is considered in many circumstances as anti-inflammatory, whereas the two lytic cell death modalities can efficiently trigger immune response by releasing damage associated molecular patterns to the extracellular space. The relevance of these cell death modalities in infections can be best demonstrated by the presence of viral proteins that directly interfere with cell death pathways. Conversely, some pathogens hijack the cell death signaling routes to initiate a targeted attack against the immune cells of the host, and extracellular bacteria can benefit from the destruction of intact extracellular barriers upon cell death induction. The complexity and the crosstalk between these cell death modalities reflect a continuous evolutionary race between pathogens and host. This chapter discusses the current advances in the research of cell death signaling with regard to viral and bacterial infections and describes the network of the cell death initiating molecular mechanisms that selectively recognize pathogen associated molecular patterns.

Role of Shiga Toxins in Cytotoxicity and Immunomodulatory Effects of Escherichia coli O157:H7 during Host-Bacterial Interactions in vitro

Enterohemorrhagic Escherichia coli (EHEC) strains are food-borne pathogens that can cause different clinical conditions. Shiga toxin 2a and/or 2c (Stx2)-producing E. coli O157:H7 is the serotype most frequently associated with severe human disease. In this work we analyzed the hypothesis that host cells participate in Stx2 production, cell damage, and inflammation during EHEC infection. With this aim, macrophage-differentiated THP-1 cells and the intestinal epithelial cell line HCT-8 were incubated with E. coli O157:H7. A time course analysis of cellular and bacterial survival, Stx2 production, stx₂ transcription, and cytokine secretion were analyzed in both human cell lines. We demonstrated that macrophages are able to internalize and kill EHEC. Simultaneously, Stx2 produced by internalized bacteria played a major role in macrophage death. In contrast, HCT-8 cells were completely resistant to EHEC infection. Besides, macrophages and HCT-8 infected cells produce IL-1β and IL-8 inflammatory cytokines, respectively. At the same time, bacterial stx₂-specific transcripts were detected only in macrophages after EHEC infection. The interplay between bacteria and host cells led to Stx production, triggering of inflammatory response and cell damage, all of which could contribute to a severe outcome after EHEC infections.

Fiery Cell Death: Pyroptosis in the Central Nervous System

Pyroptosis ('fiery death') is an inflammatory type of regulated cell death (RCD), which occurs downstream of inflammasome activation. Pyroptosis is mediated directly by the recently identified family of pore-forming proteins known as gasdermins, the best characterized of which is gasdermin D (GSDMD). Recent investigations implicate pyroptosis in the pathogenesis of multiple neurological diseases. In this review, we discuss molecular mechanisms that drive pyroptosis, evidence for pyroptosis within the CNS, and emerging therapeutic strategies for its inhibition in the context of neurological disease.

The gasdermins, a protein family executing cell death and inflammation

The gasdermins are a family of recently identified pore-forming effector proteins that cause membrane permeabilization and pyroptosis, a lytic pro-inflammatory type of cell death. Gasdermins contain a cytotoxic N-terminal domain and a C-terminal repressor domain connected by a flexible linker. Proteolytic cleavage between these two domains releases the intramolecular inhibition on the cytotoxic domain, allowing it to insert into cell membranes and form large oligomeric pores, which disrupts ion homeostasis and induces cell death. Gasdermin-induced pyroptosis plays a prominent role in many hereditary diseases and (auto)inflammatory disorders as well as in cancer. In this Review, we discuss recent developments in gasdermin research with a focus on mechanisms that control gasdermin activation, pore formation and functional consequences of gasdermin-induced membrane permeabilization.