Mechanisms of Gasdermin Family Members in Inflammasome Signaling and Cell Death

The Gasdermin (GSDM) family consists of Gasdermin A (GSDMA), Gasdermin B (GSDMB), Gasdermin C (GSDMC), Gasdermin D (GSDMD), Gasdermin E (GSDME) and Pejvakin (PJVK). GSDMD is activated by inflammasome-associated inflammatory caspases. Cleavage of GSDMD by human or mouse caspase-1, human caspase-4, human caspase-5, and mouse caspase-11 liberates the N-terminal effector domain from the C-terminal inhibitory domain. The N-terminal domain oligomerizes in the cell membrane and forms a pore of 10–16?nm in diameter, through which substrates of a smaller diameter, such as interleukin-1β and interleukin-18, are secreted. The increasing abundance of membrane pores ultimately leads to membrane rupture and pyroptosis, releasing the entire cellular content. Other than GSDMD, the N-terminal domain of all GSDMs, with the exception of PJVK, have the ability to form pores. There is evidence to suggest that GSDMB and GSDME are cleaved by apoptotic caspases. Here, we review the mechanistic functions of GSDM proteins with respect to their expression and signaling profile in the cell, with more focused discussions on inflammasome activation and cell death.     

Emerging insights into molecular mechanisms underlying pyroptosis and functions of inflammasomes in diseases

Pyroptosis is a form of necrotic and inflammatory programmed cell death, which could be characterized by cell swelling, pore formation on plasma membranes, and release of proinflammatory cytokines (IL-1β and IL-18). The process of pyroptosis presents as dual effects: protecting multicellular organisms from microbial infection and endogenous dangers; leading to pathological inflammation if overactivated. Two pathways have been found to trigger pyroptosis: caspase-1 mediated inflammasome pathway with the involvement of NLRP1-, NLRP3-, NLRC4-, AIM2-, pyrin-inflammasome (canonical inflammasome pathway) and caspase-4/5/11-mediated inflammasome pathway (noncanonical inflammasome pathway). Gasdermin D (GSDMD) has been proved to be a substrate of inflammatory caspases (caspase-1/4/5/11), and the cleaved N-terminal domain of GSDMD oligomerizes to form cytotoxic pores on the plasma membrane. Here, we mainly reviewed the up to date mechanisms of pyroptosis, and began with the inflammasomes as the activator of caspase-1/caspase-11, 4, and 5. We further discussed these inflammasomes functions in diseases, including infectious diseases, sepsis, inflammatory autoimmune diseases, and neuroinflammatory diseases.     

Caspase-1-driven neutrophil pyroptosis and its role in host susceptibility to Pseudomonas aeruginosa

Multiple regulated neutrophil cell death programs contribute to host defense against infections. However, despite expressing all necessary inflammasome components, neutrophils are thought to be generally defective in Caspase-1-dependent pyroptosis. By screening different bacterial species, we found that several Pseudomonas aeruginosa (P. aeruginosa) strains trigger Caspase-1-dependent pyroptosis in human and murine neutrophils. Notably, deletion of Exotoxins U or S in P. aeruginosa enhanced neutrophil death to Caspase-1-dependent pyroptosis, suggesting that these exotoxins interfere with this pathway. Mechanistically, P. aeruginosa Flagellin activates the NLRC4 inflammasome, which supports Caspase-1-driven interleukin (IL)-1β secretion and Gasdermin D (GSDMD)-dependent neutrophil pyroptosis. Furthermore, P. aeruginosa-induced GSDMD activation triggers Calcium-dependent and Peptidyl Arginine Deaminase-4-driven histone citrullination and translocation of neutrophil DNA into the cell cytosol without inducing extracellular Neutrophil Extracellular Traps. Finally, we show that neutrophil Caspase-1 contributes to IL-1β production and susceptibility to pyroptosis-inducing P. aeruginosa strains in vivo. Overall, we demonstrate that neutrophils are not universally resistant for Caspase-1-dependent pyroptosis.     

Pore formation by GSDMD is the effector mechanism of pyroptosis

Pyroptosis is a unique, pro‐inflammatory form of lytic cell death that is initiated by the activation of inflammatory caspases. The caspase substrate gasdermin D (GSDMD) plays a critical function in pyroptosis, yet the precise mode of action of this molecule in cell death execution remained unclear. Several recent reports, including a The EMBO Journal article, show that the caspase‐matured N‐terminal fragment of GSDMD is recruited to lipid membranes to form pore‐like structures, which constitutes the key effector mechanism of pyroptotic cell death.     

Gasdermin D is an executor of pyroptosis and required for interleukin-1β secretion

Inflammasome is an intracellular signaling complex of the innate immune system. Activation of inflammasomes promotes the secretion of interleukin 1β (IL-1β) and IL-18 and triggers pyroptosis. Caspase-1 and -11 (or -4/5 in human) in the canonical and non-canonical inflammasome pathways, respectively, are crucial for inflammasome-mediated inflammatory responses. Here we report that gasdermin D (GSDMD) is another crucial component of inflammasomes. We discovered the presence of GSDMD protein in nigericin-induced NLRP3 inflammasomes by a quantitative mass spectrometry-based analysis. Gene deletion of GSDMD demonstrated that GSDMD is required for pyroptosis and for the secretion but not proteolytic maturation of IL-1β in both canonical and non-canonical inflammasome responses. It was known that GSDMD is a substrate of caspase-1 and we showed its cleavage at the predicted site during inflammasome activation and that this cleavage was required for pyroptosis and IL-1β secretion. Expression of the N-terminal proteolytic fragment of GSDMD can trigger cell death and N-terminal modification such as tagging with Flag sequence disrupted the function of GSDMD. We also found that pro-caspase-1 is capable of processing GSDMD and ASC is not essential for GSDMD to function. Further analyses of LPS plus nigericin- or Salmonella typhimurium-treated macrophage cell lines and primary cells showed that apoptosis became apparent in Gsdmd^−/− cells, indicating a suppression of apoptosis by pyroptosis. The induction of apoptosis required NLRP3 or other inflammasome receptors and ASC, and caspase-1 may partially contribute to the activation of apoptotic caspases in Gsdmd^−/− cells. These data provide new insights into the molecular mechanisms of pyroptosis and reveal an unexpected interplay between apoptosis and pyroptosis.     

SARS‐CoV‐2 nucleocapsid suppresses host pyroptosis by blocking Gasdermin D cleavage

SARS‐CoV‐2 is an emerging coronavirus that causes dysfunctions in multiple human cells and tissues. Studies have looked at the entry of SARS‐CoV‐2 into host cells mediated by the viral spike protein and human receptor ACE2. However, less is known about the cellular immune responses triggered by SARS‐CoV‐2 viral proteins. Here, we show that the nucleocapsid of SARS‐CoV‐2 inhibits host pyroptosis by blocking Gasdermin D (GSDMD) cleavage. SARS‐CoV‐2‐infected monocytes show enhanced cellular interleukin‐1β (IL‐1β) expression, but reduced IL‐1β secretion. While SARS‐CoV‐2 infection promotes activation of the NLRP3 inflammasome and caspase‐1, GSDMD cleavage and pyroptosis are inhibited in infected human monocytes. SARS‐CoV‐2 nucleocapsid protein associates with GSDMD in cells and inhibits GSDMD cleavage in vitro and in vivo. The nucleocapsid binds the GSDMD linker region and hinders GSDMD processing by caspase‐1. These insights into how SARS‐CoV‐2 antagonizes cellular inflammatory responses may open new avenues for treating COVID‐19 in the future.     

Regulation of Lytic and Non-Lytic Functions of Gasdermin Pores

Pyroptosis is a necrotic form of cell death that was initially found to be induced upon activation of inflammatory caspases by inflammasome complexes. Mechanistically, pyroptosis induction requires cleavage of the caspase substrate gasdermin D (GSDMD), and the release of the GSDMD N-terminal fragment, which targets the plasma membrane to form large β-barrel pores. GSDMD shares this pore-forming ability with other gasdermin family members, which induce pyroptosis during infection or upon treatment with chemotherapy drugs. While induction of cell death has been assumed to be the main function of the gasdermin pores, increasing evidence suggests that these pores have non-lytic functions, such as in releasing cytokines or alarmins and in regulating intracellular signaling via ionic fluxes. Here we discuss how gasdermin pore formation is regulated to induce membrane permeabilization or lysis, how gasdermin pores achieve specificity for cargo-release and how cells repair gasdermin-induced damage to the plasma membrane.     

GSDMD membrane pore formation constitutes the mechanism of pyroptotic cell death

Pyroptosis is a lytic type of cell death that is initiated by inflammatory caspases. These caspases are activated within multi‐protein inflammasome complexes that assemble in response to pathogens and endogenous danger signals. Pyroptotic cell death has been proposed to proceed via the formation of a plasma membrane pore, but the underlying molecular mechanism has remained unclear. Recently, gasdermin D (GSDMD), a member of the ill‐characterized gasdermin protein family, was identified as a caspase substrate and an essential mediator of pyroptosis. GSDMD is thus a candidate for pyroptotic pore formation. Here, we characterize GSDMD function in live cells and in vitro. We show that the N‐terminal fragment of caspase‐1‐cleaved GSDMD rapidly targets the membrane fraction of macrophages and that it induces the formation of a plasma membrane pore. In vitro, the N‐terminal fragment of caspase‐1‐cleaved recombinant GSDMD tightly binds liposomes and forms large permeability pores. Visualization of liposome‐inserted GSDMD at nanometer resolution by cryo‐electron and atomic force microscopy shows circular pores with variable ring diameters around 20 nm. Overall, these data demonstrate that GSDMD is the direct and final executor of pyroptotic cell death.     

Saikosaponin-D induces the pyroptosis of lung cancer by increasing ROS and activating the NF-κB/NLRP3/caspase-1/GSDMD pathway

Saikosaponin-D (SSD), an active ingredient in Bupleurum chinense, exerts anticancer effects in various cancers by inhibiting cancer proliferation and inducing apoptosis. However, whether SSD can induce other forms of cell death is unknown. The current study aims to demonstrate that SSD can induce pyroptosis in non-small-cell lung cancer. In this study, HCC827 and A549 non-small-cell lung cancer cells were treated with different concentrations of SSD for 1.5 h. HE and TUNEL staining were used to verify cell damage caused by SSD. Immunofluorescence and western blotting were performed to verify the effect of SSD on the NF-κB/NLRP3/caspase-1/gasdermin D (GSDMD) pathway. Changes in inflammatory factors were detected by ELISAs. Finally, the reactive oxygen species (ROS) scavenger N-acetylcysteine (NAC) was introduced to verify that SSD induces pyroptosis through the ROS/NF-κB pathway. The results of the HE and TUNEL staining showed that SSD resulted in balloon-like swelling of NSCLC cells accompanied by increased DNA damage. Immunofluorescence and western blot assays confirmed that SSD treatment activated the NLRP3/caspase-1/GSDMD pathway, stimulated an increase in ROS levels and activated NF-κB in lung cancer cells. The ROS scavenger N-acetylcysteine significantly attenuated SSD-induced NF-κB/NLRP3/caspase-1/GSDMD pathway activation and inhibited the release of the inflammatory cytokines IL-1β and IL-18. In conclusion, SSD induced lung cancer cell pyroptosis by inducing ROS accumulation and activating the NF-κB/NLRP3/caspase-1/GSDMD pathway. These experiments lay the foundation for the application of SSD in the treatment of non-small-cell lung cancer and regulation of the lung cancer immune microenvironment.     

Genetic targeting of Card19 is linked to disrupted NINJ1 expression,impaired cell lysis,and increased susceptibility to Yersinia infection

Cell death plays a critical role in inflammatory responses. During pyroptosis, inflammatory caspases cleave Gasdermin D (GSDMD) to release an N-terminal fragment that generates plasma membrane pores that mediate cell lysis and IL-1 cytokine release. Terminal cell lysis and IL-1β release following caspase activation can be uncoupled in certain cell types or in response to particular stimuli, a state termed hyperactivation. However, the factors and mechanisms that regulate terminal cell lysis downstream of GSDMD cleavage remain poorly understood. In the course of studies to define regulation of pyroptosis during Yersinia infection, we identified a line of Card19-deficient mice (Card19^lxcn) whose macrophages were protected from cell lysis and showed reduced apoptosis and pyroptosis, yet had wild-type levels of caspase activation, IL-1 secretion, and GSDMD cleavage. Unexpectedly, CARD19, a mitochondrial CARD-containing protein, was not directly responsible for this, as an independently-generated CRISPR/Cas9 Card19 knockout mouse line (Card19^Null) showed no defect in macrophage cell lysis. Notably, Card19 is located on chromosome 13, immediately adjacent to Ninj1, which was recently found to regulate cell lysis downstream of GSDMD activation. RNA-seq and western blotting revealed that Card19^lxcn BMDMs have significantly reduced NINJ1 expression, and reconstitution of Ninj1 in Card19^lxcn immortalized BMDMs restored their ability to undergo cell lysis in response to caspase-dependent cell death stimuli. Card19^lxcn mice exhibited increased susceptibility to Yersinia infection, whereas independently-generated Card19^Null mice did not, demonstrating that cell lysis itself plays a key role in protection against bacterial infection, and that the increased infection susceptibility of Card19^lxcn mice is attributable to loss of NINJ1. Our findings identify genetic targeting of Card19 being responsible for off-target effects on the adjacent gene Ninj1, disrupting the ability of macrophages to undergo plasma membrane rupture downstream of gasdermin cleavage and impacting host survival and bacterial control during Yersinia infection.     

Mafenide derivatives inhibit neuroinflammation in Alzheimer's disease by regulating pyroptosis

The main mechanism of pyroptosis is Caspase-1–mediated GSDMD cleavage, and GSDMD is also the executive protein of pyroptosis. Our previous study has shown that mafenide can inhibit pyroptosis by inhibiting the GSDMD-Asp275 site to suppress cleavage. In this study, sulfonamide was used as the parent nucleus structure to synthesize sulfa-4 and sulfa-20. Screening of drug activity in the pyroptosis model of BV2 and iBMDM cell lines revealed the efficacy of five compounds were superior to mafenide, which exerted a better inhibitory effect on the occurrence of pyroptosis. For in vivo assay, Sulfa-4 and Sulfa-22 were intervened in the neuroinflammation APP/PS1 mice. As a result, the administration of Sulfa-4 and Sulfa-22 could significantly inhibit the activation of microglia, decrease the expression of inflammatory factors in the central nervous system and simultaneously suppress the production of p30-GSDMD as well as the expression of upstream NLRP3 inflammasome and Caspase-1 protein. Immunoprecipitation and Biotin-labelled assay confirmed the targeted binding relationship of Sulfa-4 and Sulfa-22 with GSDMD protein in the iBMDM model in vitro. In this study, we investigated a new type inhibitor of GSDMD cleavage, which exerted a good inhibitory effect on pyroptosis and provided new references for the development of inflammatory drugs in the future.     

New mechanism of nerve injury in Alzheimer’s disease: β‐amyloid‐induced neuronal pyroptosis

The present study was designed to investigate the role of β‐amyloid (Aβ_1‐42) in inducing neuronal pyroptosis and its mechanism. Mice cortical neurons (MCNs) were used in this study, LPS + Nigericin was used to induce pyroptosis in MCNs (positive control group), and Aβ_1‐42 was used to interfere with MCNs. In addition, propidium iodide (PI) staining was used to examine cell permeability, lactate dehydrogenase (LDH) release assay was employed to detect cytotoxicity, immunofluorescence (IF) staining was used to investigate the expression level of the key protein GSDMD, Western blot was performed to detect the expression levels of key proteins, and enzyme‐linked immunosorbent assay (ELISA) was utilized to determine the expression levels of inflammatory factors in culture medium, including IL‐1β, IL‐18 and TNF‐α. Small interfering RNA (siRNA) was used to silence the mRNA expression of caspase‐1 and GSDMD, and Aβ_1‐42 was used to induce pyroptosis, followed by investigation of the role of caspase‐1‐mediated GSDMD cleavage in pyroptosis. In addition, necrosulfonamide (NSA), an inhibitor of GSDMD oligomerization, was used for pre‐treatment, and Aβ_1‐42 was subsequently used to observe the pyroptosis in MCNs. Finally, AAV9‐siRNA‐caspase‐1 was injected into the tail vein of APP/PS1 double transgenic mice (Alzheimer's disease mice) for caspase‐1 mRNA inhibition, followed by observation of behavioural changes in mice and measurement of the expression of inflammatory factors and pyroptosis‐related protein. As results, Aβ_1‐42 could induce pyroptosis in MCNs, increase cell permeability and enhance LDH release, which were similar to the LPS + Nigericin‐induced pyroptosis. Meanwhile, the expression levels of cellular GSDMD and p30‐GSDMD were up‐regulated, the levels of NLRP3 inflammasome and GSDMD‐cleaved protein caspase‐1 were up‐regulated, and the levels of inflammatory factors in the medium were also up‐regulated. siRNA intervention in caspase‐1 or GSDMD inhibited Aβ_1‐42‐induced pyroptosis, and NSA pre‐treatment also caused the similar inhibitory effects. The behavioural ability of Alzheimer's disease (AD) mice was relieved after the injection of AAV9‐siRNA‐caspase‐1, and the expression of pyroptosis‐related protein in the cortex and hippocampus was down‐regulated. In conclusion, Aβ_1‐42 could induce pyroptosis by GSDMD protein, and NLRP3‐caspase‐1 signalling was an important signal to mediate GSDMD cleavage, which plays an important role in Aβ_1‐42‐induced pyroptosis in neurons. Therefore, GSDMD is expected to be a novel therapeutic target for AD.     

African swine fever virus cysteine protease pS273R inhibits pyroptosis by noncanonically cleaving gasdermin D

African swine fever (ASF) is a viral hemorrhagic disease that affects domestic pigs and wild boar and is caused by the African swine fever virus (ASFV). The ASFV virion contains a long double-stranded DNA genome, which encodes more than 150 proteins. However, the immune escape mechanism and pathogenesis of ASFV remain poorly understood. Here, we report that the pyroptosis execution protein gasdermin D (GSDMD) is a new binding partner of ASFV-encoded protein S273R (pS273R), which belongs to the SUMO-1 cysteine protease family. Further experiments demonstrated that ASFV pS273R-cleaved swine GSDMD in a manner dependent on its protease activity. ASFV pS273R specifically cleaved GSDMD at G107-A108 to produce a shorter N-terminal fragment of GSDMD consisting of residues 1 to 107 (GSDMD-N_1–107). Interestingly, unlike the effect of GSDMD-N_1–279 fragment produced by caspase-1-mediated cleavage, the assay of LDH release, cell viability, and virus replication showed that GSDMD-N_1–107 did not trigger pyroptosis or inhibit ASFV replication. Our findings reveal a previously unrecognized mechanism involved in the inhibition of ASFV infection-induced pyroptosis, which highlights an important function of pS273R in inflammatory responses and ASFV replication.     

miR-135b-5p抑制脓毒症引起的小鼠急性肺损伤(ALI)的作用及机制*

目的: 探究miR-135b-5p在小鼠脓毒症(sepsis)引起的急性肺损伤(ALI)模型中的表达水平及其对小鼠肺部炎症反应和细胞焦亡的影响。方法: 将C57BL/6小鼠随机分为6组,每组8只,通过盲肠结扎穿刺法(CLP)手术构建CLP诱导的脓毒症小鼠模型:腹腔注射0.1 mg/kg的巴比妥麻醉,腹部纵向切开暴露盲肠,结扎盲肠并用注射器针头进行穿孔,挤出部分肠道内容物后缝合伤口。假手术组(Sham组)开腹后不做任何处理缝合伤口,无CLP手术处理。治疗组分为CLP+NC mimic组,CLP+miR-135b-5p mimic组,CLP+NC mimic+empty vector组,CLP+消皮素D (GSDMD)组,CLP+miR-135b-5p mimic+GSDMD组。治疗组小鼠在CLP手术前一周皮下注射200 μl溶解于生理盐水的NC mimic(200 nmol/L),miR-135b-5p mimic(200 nmol/L),empty vector(100 nmol/L),GSDMD vector(100 nmol/L),每天注射1次,连续一周。术后24 h采用二氧化碳窒息法实施安乐死。采用qRT-PCR检测小鼠肺组织样本中miR-135b-5p和GSDMD mRNA的表达水平;苏木精-伊红(HE)染色检测小鼠肺组织形态和损伤状态;采用5 ml生理盐水冲洗小鼠右肺3次,每次持续约3~5 min,收集肺泡灌洗液(BALF),酶联免疫吸附实验(ELISA)检测小鼠肺泡灌洗液(BALF)中GSDMD、白介素1β(IL-1β)和白介素18(IL-18)的表达水平;蛋白免疫印迹法检测小鼠肺组织内含NLR家族PYRIN域蛋白3(NLRP3),半胱氨酸天冬氨酸蛋白水解酶1(caspase 1)以及切割后的N-端GSDMD端蛋白结构域(cleaved-GSDMD-N)的表达水平。双荧光素酶报告基因检测系统验证miR-135b-5p与GSDMD的靶向结合关系。结果: 与对照组相比,CLP组小鼠肺组织中有大量的炎症细胞浸润,肺泡损伤,细胞间质水肿及肺泡塌陷等病理特征,小鼠肺组织内细胞焦亡相关蛋白(NLRP3,caspase-1和GSDMD)的表达水平明显增加(P<0.01),但miR-135b-5p的表达水平明显下调(P<0.01);与CLP组相比,超表达miR-135b-5p能够明显抑制CLP诱导的小鼠肺组织内细胞焦亡(P<0.01),靶向抑制GSDMD的表达水平(P<0.01);超表达GSDMD能够逆转超表达miR-135b-5p对肺组织细胞焦亡的抑制作用(P<0.01),超表达miR-135b-5p能够通过靶向GSDMD抑制小鼠BALF中IL-1β及IL-18的表达水平(P<0.01)。结论: miR-135b-5p靶向下调GSDMD抑制细胞焦亡,改善脓毒症引起的ALI,为脓毒症诱导的ALI治疗提供了潜在的治疗靶点和理论依据。     

Synoviolin promotes IRE1 ubiquitination and degradation in synovial fibroblasts from mice with collagen-induced arthritis

The E3 ubiquitin ligase synoviolin (SYVN1) functions as an anti-apoptotic factor that is responsible for the outgrowth of synovial cells during the development of rheumatoid arthritis. The molecular mechanisms underlying SYVN1 regulation of cell death are largely unknown. Here, we report that elevated SYVN1 expression correlates with decreased levels of the protein inositol-requiring enzyme 1 (IRE1)-a pro-apoptotic factor in the endoplasmic reticulum (ER)-stress-induced apoptosis pathway-in synovial fibroblasts from mice with collagen-induced arthritis (CIA). SYVN1 interacts with and catalyses IRE1 ubiquitination and consequently promotes IRE1 degradation. Suppression of SYVN1 expression in synovial fibroblasts from CIA mice restores IRE1 protein expression and reverses the resistance of ER-stress-induced apoptosis of CIA synovial fibroblasts. These results show that SYVN1 causes the overgrowth of synovial cells by degrading IRE1, and therefore antagonizes ER-stress-induced cell death.     

细胞焦亡参与肿瘤发生分子机制的研究进展

细胞焦亡是一种依赖gasdermin家族蛋白的成孔活性,由半胱氨酸-天冬氨酸特异性蛋白激酶1、4、5、11等介导的以细胞肿胀、质膜穿孔及炎性小分子的释放为特征的新的裂解性、促炎性程序性细胞死亡.目前发现,细胞焦亡参与了多种疾病的发生与发展过程,特别是对肿瘤及肿瘤微环境的形成具有促进及抑制的双重功能,使焦亡成为抗肿瘤药物...     

Dehydroepiandrosterone exacerbates nigericin-induced abnormal autophagy and pyroptosis via GPER activation in LPS-primed macrophages

As a widely acknowledged FDA-approved dietary supplement or over-the-counter medicines, dehydroepiandrosterone (DHEA) exerts anti-inflammatory and immunomodulatory function. Pyroptosis is an important form of programmed cell death (PCD), and which acts a key role in the body’s anti-infection and inflammatory responses. But the effects and mechanisms of DHEA on pyroptosis remain unclear. Here, we found that DHEA inhibited the NLRP3 inflammasome components expression by blocking inflammatory signals in lipopolysaccharide (LPS)-primed macrophages, and prevented the bacterial toxin nigericin (Nig)-induced NLRP3 inflammasome assembly. However, DHEA exacerbated NLRP3-independent cell death in Nig-treated inflammatory macrophages. During this process, DHEA induced the abnormal autophagy, which reflected as the blocking of autophagic flux and the accumulation of autophagy receptor p62 (SQSTM1) protein. In addition, DHEA caused a burst of reactive oxygen species (ROS) and activated extracellular signal-regulated kinase (ERK) phosphorylation in LPS plus Nig-stimulated macrophages but not in LPS-treated macrophages. Mechanistically, the present study certified that the activation of G protein-coupled estrogen receptor (GPER) signal mediated the cell death induced by DHEA in Nig-stimulated inflammatory macrophages, as GPER specific inhibitor G15 alleviated the abnormal autophagy and ultimately prevented the gasdermin D (GSDMD)-mediated pyroptosis induced by DHEA. Collectively, DHEA can exacerbate Nig-induced abnormal autophagy and pyroptosis via activation of GPER in LPS-primed macrophages, which prompts us the potential application value of DHEA in anti-infection or anti-tumor immunity.Subject terms: Cell death and immune response, Immune cell death     

Infiltration of inflammatory macrophages and neutrophils and widespread pyroptosis in lung drive influenza lethality in nonhuman primates

Severe influenza kills tens of thousands of individuals each year, yet the mechanisms driving lethality in humans are poorly understood. Here we used a unique translational model of lethal H5N1 influenza in cynomolgus macaques that utilizes inhalation of small-particle virus aerosols to define mechanisms driving lethal disease. RNA sequencing of lung tissue revealed an intense interferon response within two days of infection that resulted in widespread expression of interferon-stimulated genes, including inflammatory cytokines and chemokines. Macaques with lethal disease had rapid and profound loss of alveolar macrophages (AMs) and infiltration of activated CCR2⁺ CX3CR1⁺ interstitial macrophages (IMs) and neutrophils into lungs. Parallel changes of AMs and neutrophils in bronchoalveolar lavage (BAL) correlated with virus load when compared to macaques with mild influenza. Both AMs and IMs in lethal influenza were M1-type inflammatory macrophages which expressed neutrophil chemotactic factors, while neutrophils expressed genes associated with activation and generation of neutrophil extracellular traps (NETs). NETs were prominent in lung and were found in alveolar spaces as well as lung parenchyma. Genes associated with pyroptosis but not apoptosis were increased in lung, and activated inflammatory caspases, IL-1β and cleaved gasdermin D (GSDMD) were present in bronchoalveolar lavage fluid and lung homogenates. Cleaved GSDMD was expressed by lung macrophages and alveolar epithelial cells which were present in large numbers in alveolar spaces, consistent with loss of epithelial integrity. Cleaved GSDMD colocalized with viral NP-expressing cells in alveoli, reflecting pyroptosis of infected cells. These novel findings reveal that a potent interferon and inflammatory cascade in lung associated with infiltration of inflammatory macrophages and neutrophils, elaboration of NETs and cell death by pyroptosis mediates lethal H5N1 influenza in nonhuman primates, and by extension humans. These innate pathways represent promising therapeutic targets to prevent severe influenza and potentially other primary viral pneumonias in humans.     

Caspase-7 mediates caspase-1-induced apoptosis independently of Bid

Inflammasomes are innate immune mechanisms that activate caspase-1 in response to a variety of stimuli, including Salmonella infection. Active caspase-1 has a potential to induce two different types of cell death, depending on the expression of the pyroptosis mediator gasdermin D (GSDMD); following caspase-1 activation, GSDMD-sufficient and GSDMD-null/low cells undergo pyroptosis and apoptosis, respectively. Although Bid, a caspase-1 substrate, plays a critical role in caspase-1 induction of apoptosis in GSDMD-null/low cells, an additional mechanism that mediates this cell death independently of Bid has also been suggested. This study investigated the Bid-independent pathway of caspase-1-induced apoptosis. Caspase-1 has been reported to process caspase-6 and caspase-7. Silencing of caspase-7, but not caspase-6, significantly reduced the activation of caspase-3 induced by caspase-1, which was activated by chemical dimerization, in GSDMD/Bid-deficient cells. CRISPR/Cas9-mediated depletion of caspase-7 had the same effect on the caspase-3 activation. Moreover, in the absence of GSDMD and Bid, caspase-7 depletion reduced apoptosis induced by caspase-1 activation. Caspase-7 was activated following caspase-1 activation independently of caspase-3, suggesting that caspase-7 acts downstream of caspase-1 and upstream of caspase-3. Salmonella induced the activation of caspase-3 in GSDMD-deficient macrophages, which relied partly on Bid and largely on caspase-1. The caspase-3 activation and apoptotic morphological changes seen in Salmonella-infected GSDMD/Bid-deficient macrophages were attenuated by caspase-7 knockdown. These results suggest that in addition to Bid, caspase-7 can also mediate caspase-1-induced apoptosis and provide mechanistic insights into inflammasome-associated cell death that is one major effector mechanism of inflammasomes.