Caspase-4 mediates non-canonical activation of the NLRP3 inflammasome in human myeloid cells

Pyroptosis in bone loss

Pyroptosis could be responsible for the bone loss from bone metabolic diseases, leading to the negative impact on people's health and life. It has been shown that osteoclasts, osteoblasts, macrophages, chondrocytes, periodontal and gingival cells may be involved in bone loss linked with pyroptosis. So far, the involved mechanisms have not been fully elucidated. In this review, we introduced the related cells involved in the pyroptosis associated with bone loss and summarized the role of these cells in the bone metabolism during the process of pyroptosis. We also discuss the clinical potential of targeting mechanisms in the osteoclasts, osteoblasts, macrophages, chondrocytes, periodontal and gingival cells touched upon pyroptosis to treat bone loss from bone metabolic diseases as well as the challenges of avoiding potential side effects and producing efficient treatment methods.

Pseudomonas aeruginosa Cytotoxins: Mechanisms of Cytotoxicity and Impact on Inflammatory Responses

Pseudomonas aeruginosa is one of the most virulent opportunistic Gram-negative bacterial pathogens in humans. It causes many acute and chronic infections with morbidity and mortality rates as high as 40%. P. aeruginosa owes its pathogenic versatility to a large arsenal of cell-associated and secreted virulence factors which enable this pathogen to colonize various niches within hosts and protect it from host innate immune defenses. Induction of cytotoxicity in target host cells is a major virulence strategy for P. aeruginosa during the course of infection. P. aeruginosa has invested heavily in this strategy, as manifested by a plethora of cytotoxins that can induce various forms of cell death in target host cells. In this review, we provide an in-depth review of P. aeruginosa cytotoxins based on their mechanisms of cytotoxicity and the possible consequences of their cytotoxicity on host immune responses.

Uncoupled pyroptosis and IL-1β secretion downstream of inflammasome signaling

Inflammasomes are supramolecular platforms that organize in response to various damage-associated molecular patterns and pathogen-associated molecular patterns. Upon activation, inflammasome sensors (with or without the help of ASC) activate caspase-1 and other inflammatory caspases that cleave gasdermin D and pro-IL-1β/pro-IL-18, leading to pyroptosis and mature cytokine secretion. Pyroptosis enables intracellular pathogen niche disruption and intracellular content release at the cost of cell death, inducing pro-inflammatory responses in the neighboring cells. IL-1β is a potent pro-inflammatory regulator for neutrophil recruitment, macrophage activation, and T-cell expansion. Thus, pyroptosis and cytokine secretion are the two main mechanisms that occur downstream of inflammasome signaling; they maintain homeostasis, drive the innate immune response, and shape adaptive immunity. This review aims to discuss the possible mechanisms, timing, consequences, and significance of the two uncoupling preferences downstream of inflammasome signaling. While pyroptosis and cytokine secretion may be usually coupled, pyroptosis-predominant and cytokine-predominant uncoupling are also observed in a stimulus-, cell type-, or context-dependent manner, contributing to the pathogenesis and development of numerous pathological conditions such as cryopyrin-associated periodic syndromes, LPS-induced sepsis, and Salmonella enterica serovar Typhimurium infection. Hyperactive cells consistently release IL-1β without LDH leakage and pyroptotic death, thereby leading to prolonged inflammation, expanding the lifespans of pyroptosis-resistant neutrophils, and hyperactivating stimuli-challenged macrophages, dendritic cells, monocytes, and specific nonimmune cells. Death inflammasome activation also induces GSDMD-mediated pyroptosis with no IL-1β secretion, which may increase lethality in vivo. The sublytic GSDMD pore formation associated with lower expressions of pyroptotic components, GSDMD-mediated extracellular vesicles, or other GSDMD-independent pathways that involve unconventional secretion could contribute to the cytokine-predominant uncoupling; the regulation of caspase-1 dynamics, which may generate various active species with different activities in terms of GSDMD or pro-IL-1β, could lead to pyroptosis-predominant uncoupling. These uncoupling preferences enable precise reactions to different stimuli of different intensities under specific conditions at the single-cell level, promoting cooperative cell and host fate decisions and participating in the pathogen "game". Appropriate decisions in terms of coupling and uncoupling are required to heal tissues and eliminate threats, and further studies exploring the inflammasome tilt toward pyroptosis or cytokine secretion may be helpful.

Deciphering the Role of Pyroptosis Impact on Cardiovascular Diseases

Pyroptosis has become a noteworthy area of focus in recent years due to its association with inflammatory diseases. Pyroptosis is a type of programmed cell death accompanied by an inflammatory response, and the discovery of the gasdermin family has expanded the study of pyroptosis. The primary characteristics of pyroptosis include cell expansion, membrane penetration, and the ejection of cell contents. In healthy physiology, pyroptosis is an essential part of the host's defence against pathogen infection. Excessive Pyroptosis, however, can lead to unchecked and persistent inflammatory responses, including the emergence of inflammatory diseases. More precisely, gasdermin family members have a role in the creation of membrane holes during pyroptosis, which leads to cell lysis. It is also related to how pro-inflammatory intracellular substances, including IL-1, IL-18, and High mobility group box 1 (HMGB1), are used. Two different signalling pathways, one of which is regulated by caspase-1 and the other by caspase-4/5/11, are the primary causes of pyroptosis. Cardiovascular diseases are often associated with cell death and acute or chronic inflammation, making this area of research particularly relevant. In this review, we first systematically summarize recent findings related to Pyroptosis, exploring its characteristics and the signalling pathway mechanisms, as well as various treatment strategies based on its modulation that has emerged from the studies. Some of these strategies are currently undergoing clinical trials. Additionally, the article elaborates on the scientific evidence indicating the role of Pyroptosis in various cardiovascular diseases. As a whole, this should shed insight into future paths and present innovative ideas for employing Pyroptosis as a strong disease-fighting weapon.

Cancer-associated pyroptosis: A new license to kill tumor

Pyroptosis is a programmed necrotic cell death mediated by pore-forming Gasdermin (GSDM) proteins. After being unleashed from the C-terminal auto-inhibitory domains by proteolytic cleavage, the N-terminal domains of GSDMs oligomerize and perforate on the plasma membrane to induce cytolytic pyroptosis, releasing immune mediators and alarming the immune system. Upon infection or danger signal perception, GSDMD that functions downstream of the inflammasome, a supramolecular complex for inflammatory caspase activation, is cleaved and activated by inflammasome-activated caspase-1/4/5/11 in immune cells and epithelial cells to trigger pyroptosis and exert anti-infection protection. Unlike this inflammasome-activated pyroptosis (IAP), recent studies also suggest an emerging role of cancer-associated pyroptosis (CAP), mediated by other GSDMs in cancer cells, in provoking anti-tumor immunity. IAP and CAP share common features like cell membrane rupture but also differ in occurrence sites, activating mechanisms, secreting cytokines and biological outcomes. Here we review the most recent knowledge of cancer-associated pyroptosis and present a promising avenue for developing therapeutic interventions to enhance anti-tumor immunity for cancer treatment.

Noncanonical NLRP3 Inflammasome Activation: Standard Protocols

The canonical activation of multimeric inflammasomes usually occurs through caspase-1 activation, and it is characterized by the presence of extracellular IL-1β and IL-18 or measuring danger signal proteins, such as HMGB1 using enzyme-linked immunosorbent assay (ELISA) or Western blots; these assays differentiate non-cleaved and cleaved forms of these two cytokines (the cleaved form is the mature and active form). Similar techniques can be used to assess noncanonical inflammasome activation. Real-time PCR can measure the relative mRNA expression for a specific gene, whereas Western blots or immunocytochemistry can detect the presence of proteins by binding of specific antibodies to their antigens in biological samples. Moreover, noncanonical inflammasome activation can be evaluated through the cleavage of the amino and the carboxy terminals of one important component, gasdermin D (GSDMD), whose cleavage induces its pyroptotic activity. Thus, the analysis of cleaved GSDMD is an ideal pathway to study the noncanonical inflammasome. ELISA and immunoblot can be performed on cell culture supernatants or cell extracts.

What type of cell death occurs in chronic cerebral hypoperfusion? A review focusing on pyroptosis and its potential therapeutic implications

Chronic cerebral hypoperfusion (CCH) is a major global disease with chronic cerebral blood flow reduction. It is also the main cause of cognitive impairment and neurodegenerative diseases. Pyroptosis, a novel form of cell death, is characterized by the rupture of the cell membrane and the release of pro-inflammatory mediators. In recent years, an increasing number of studies have identified the involvement of pyroptosis and its mediated inflammatory response in the pathological process of CCH. Therefore, preventing the activation of pyroptosis following CCH is beneficial to inhibit the inflammatory cascade and reduce brain injury. In this review, we discuss the research progress on the relationship between pyroptosis and CCH, in order to provide a reference for research in related fields.

Protocol to purify recombinant inflammatory caspases and assess their catalytic activity in vitro

The inflammatory caspases, such as caspase-1, caspase-4, or caspase-11, are key enzymes in mammalian innate immunity as they control inflammasome-dependent inflammation. Assessing the specific proteolytic activities of these caspases in the context of a cell remains challenging, which is why in vitro studies of their catalytic activity have proven useful. Herein, we describe a detailed protocol for the purification of recombinant inflammatory caspases after heterologous expression in bacteria and how to assess and quantify cleavage of full-length protein substrates. For complete details on the use and execution of this protocol, please refer to Devant et al. (2021).1.

Sidestream Smoke Extracts from Harm-Reduction and Conventional Camel Cigarettes Inhibit Osteogenic Differentiation via Oxidative Stress and Differential Activation of intrinsic Apoptotic Pathways

Epidemiological studies suggest cigarette smoking as a probable environmental factor for a variety of congenital anomalies, including low bone mass, increased fracture risk and poor skeletal health. Human and animal in vitro models have confirmed hypomineralization of differentiating cell lines with sidestream smoke being more harmful to developing cells than mainstream smoke. Furthermore, first reports are emerging to suggest a differential impact of conventional versus harm-reduction tobacco products on bone tissue as it develops in the embryo or in vitro. To gather first insight into the molecular mechanism of such differences, we assessed the effect of sidestream smoke solutions from Camel (conventional) and Camel Blue (harm-reduction) cigarettes using a human embryonic stem cell osteogenic differentiation model. Sidestream smoke from the conventional Camel cigarettes concentration-dependently inhibited in vitro calcification triggered by high levels of mitochondrially generated oxidative stress, loss of mitochondrial membrane potential, and reduced ATP production. Camel sidestream smoke also induced DNA damage and caspase 9-dependent apoptosis. Camel Blue-exposed cells, in contrast, invoked only intermediate levels of reactive oxygen species insufficient to activate caspase 3/7. Despite the absence of apoptotic gene activation, damage to the mitochondrial phenotype was still noted concomitant with activation of an anti-inflammatory gene signature and inhibited mineralization. Collectively, the presented findings in differentiating pluripotent stem cells imply that embryos may exhibit low bone mineral density if exposed to environmental smoke during development.

The human inflammasomes

Two decades of inflammasome research has led to a vast body of knowledge on the complex regulatory mechanisms and pathological roles of canonical and non-canonical inflammasome activation in a plethora of research models of primarily rodent origin. More recently, the field has made notable progress in characterizing human-specific inflammasomes and their regulation mechanisms, including an expansion of inflammasome biology to adaptive immune cells. These exciting developments in basic research have been accompanied by potentially transformative results from large clinical trials and translational efforts to develop inflammasome-targeted small molecule inhibitors for therapeutic use. Here, we will discuss recent findings in the field with a specific emphasis on activation mechanisms of human inflammasomes and their potential role in auto-inflammatory, metabolic and neoplastic diseases.

Pyroptosis in inflammatory bone diseases: Molecular insights and targeting strategies

Inflammatory bone diseases include osteoarthritis (OA) and rheumatoid arthritis (RA), which can cause severe bone damage in a chronic inflammation state, putting tremendous pressure on the patients' families and government agencies regarding medical costs. In addition, the complexity of osteoimmunology makes research on these diseases difficult. Hence, it is urgent to determine the potential mechanisms and find effective drugs to target inflammatory bone diseases to reduce the negative effects of these diseases. Recently, pyroptosis, a gasdermin-induced necrotic cell death featuring secretion of pro-inflammatory cytokines and lysis, has become widely known. Based on the effect of pyroptosis on immunity, this process has gradually emerged as a vital component in the etiopathogenesis of inflammatory bone diseases. Herein, we review the characteristics and mechanisms of pyroptosis and then focus on its clinical significance in inflammatory bone diseases. In addition, we summarize the current research progress of drugs targeting pyroptosis to enhance the therapeutic efficacy of inflammatory bone diseases and provide new insights for future directions.

Pyroptosis: A Newly Discovered Therapeutic Target for Ischemia-Reperfusion Injury

Ischemia-reperfusion (I/R) injury, uncommon among patients suffering from myocardial infarction, stroke, or acute kidney injury, can result in cell death and organ dysfunction. Previous studies have shown that different types of cell death, including apoptosis, necrosis, and autophagy, can occur during I/R injury. Pyroptosis, which is characterized by cell membrane pore formation, pro-inflammatory cytokine release, and cell burst, and which differentiates itself from apoptosis and necroptosis, has been found to be closely related to I/R injury. Therefore, targeting the signaling pathways and key regulators of pyroptosis may be favorable for the treatment of I/R injury, which is far from adequate at present. This review summarizes the current status of pyroptosis and its connection to I/R in different organs, as well as potential treatment strategies targeting it to combat I/R injury.

Molecular mechanisms and roles of pyroptosis in acute lung injury

ABSTRACT

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), which are characterized by excessive inflammation and accompanied by diffuse injury of alveoli, can result in severe respiratory failures. The morbidity and mortality of patients remain high because the major treatments for ALI/ARDS are mainly supportive due to the lack of effective therapies. Numerous studies have demonstrated that the aggravation of coronavirus disease 2019 (COVID-19) leads to severe pneumonia and even ARDS. Pyroptosis, a biological process identified as a type of programed cell death, is mainly triggered by inflammatory caspase activation and is directly meditated by the gasdermin protein family, as well as being associated with the secretion and release of pro-inflammatory cytokines. Clinical and experimental evidence corroborates that pyroptosis of various cells in the lung, such as immune cells and structural cells, may play an important role in the pathogenesis of "cytokine storms" in ALI/ARDS, including those induced by COVID-19. Here, with a focus on ALI/ARDS and COVID-19, we summarized the recent advances in this field and proposed the theory of an inflammatory cascade in pyroptosis to identify new targets and pave the way for new approaches to treat these diseases.

The emerging role of pyroptosis in pediatric cancers: from mechanism to therapy

Pediatric cancers are the driving cause of death for children and adolescents. Due to safety requirements and considerations, treatment strategies and drugs for pediatric cancers have been so far scarcely studied. It is well known that tumor cells tend to progressively evade cell death pathways, which is known as apoptosis resistance, one of the hallmarks of cancer, dominating tumor drug resistance. Recently, treatments targeting nonapoptotic cell death have drawn great attention. Pyroptosis, a newly specialized form of cell death, acts as a critical physiological regulator in inflammatory reaction, cell development, tissue homeostasis and stress response. The action in different forms of pyroptosis is of great significance in the therapy of pediatric cancers. Pyroptosis could be induced and consequently modulate tumorigenesis, progression, and metastasis if treated with local or systemic therapies. However, excessive or uncontrolled cell death might lead to tissue damage, acute inflammation, or even cytokine release syndrome, which facilitates tumor progression or recurrence. Herein, we aimed to describe the molecular mechanisms of pyroptosis, to highlight and discuss the challenges and opportunities for activating pyroptosis pathways through various oncologic therapies in multiple pediatric neoplasms, including osteosarcoma, neuroblastoma, leukemia, lymphoma, and brain tumors.

NLRP3 inflammasome: The rising star in cardiovascular diseases

Cardiovascular diseases (CVDs) are the prevalent cause of mortality around the world. Activation of inflammasome contributes to the pathological progression of cardiovascular diseases, including atherosclerosis, abdominal aortic aneurysm, myocardial infarction, dilated cardiomyopathy, diabetic cardiomyopathy, heart failure, and calcific aortic valve disease. The nucleotide oligomerization domain-, leucine-rich repeat-, and pyrin domain-containing protein 3 (NLRP3) inflammasome plays a critical role in the innate immune response, requiring priming and activation signals to provoke the inflammation. Evidence shows that NLRP3 inflammasome not only boosts the cleavage and release of IL-1 family cytokines, but also leads to a distinct cell programmed death: pyroptosis. The significance of NLRP3 inflammasome in the CVDs-related inflammation has been extensively explored. In this review, we summarized current understandings of the function of NLRP3 inflammasome in CVDs and discussed possible therapeutic options targeting the NLRP3 inflammasome.

Pyroptosis in development, inflammation and disease

In the early 2000s, caspase-1, an important molecule that has been shown to be involved in the regulation of inflammation, cell survival and diseases, was given a new function: regulating a new mode of cell death that was later defined as pyroptosis. Since then, the inflammasome, the inflammatory caspases (caspase-4/5/11) and their substrate gasdermins (gasdermin A, B, C, D, E and DFNB59) has also been reported to be involved in the pyroptotic pathway, and this pathway is closely related to the development of various diseases. In addition, important apoptotic effectors caspase-3/8 and granzymes have also been reported to b involved in the induction of pyroptosis. In our article, we summarize findings that help define the roles of inflammasomes, inflammatory caspases, gasdermins, and other mediators of pyroptosis, and how they determine cell fate and regulate disease progression.

Bibliometric analysis of global research trends on pyroptosis in lung disease

Background

Pyroptosis is a lytic pro-inflammatory programmed cell death mode that depends on caspase, inflammasome, and Gasdermin D (GSDMD). A growing number of studies have shown that pyroptosis is closely related to the pathophysiological mechanism of lung. The purpose of this study is to analyze the literature from Science Citation Index Expanded (SCI-expanded) of Web of Science Core Collection (WoSCC) and visualize the current trends and hotspots in the research of pyroptosis in lung disease.

Methods

On February 20, 2022, we retrieved all articles on pyroptosis in lung disease from SCI-expanded of WoSCC. Original articles and reviews published in English from 2007 to 2021 were included in the analysis. VOSviewer 1.6.17 and CiteSpace 5.8.R2 were used to analyze the retrieved data and visualize the results.

Result

1798 qualified original articles and reviews on pyroptosis in lung disease were included in the bibliometric analysis. So far, the research in this field is still in a period of growth, and the number of global publications has increased yearly. Among the 66 countries that have published relevant articles, China ranked first in the number of publications, and the USA ranked first in the number of cited articles. Holian,A. was the author with the largest number of articles, including 21 published. The University of California System in the USA was the organization with the largest number of articles, totaling 55. Frontiers in Immunology was the journal with the most publications in pyroptosis. After bibliometric analysis, the frequently used keywords are: “NOD-like receptor3 (NLRP3) inflammasome”, “inflammation”, “oxidative stress”, and “acute lung injury (ALI)”.

Conclusion

The research on pyroptosis in lung disease is in its growth stage. The information released in this article may help researchers better understand the hotspots and developmental trends in this field, the cooperation network information of authors, countries, and institutions, and the citation correlation between articles. With the in-depth study of the mechanism of pyroptosis, the focus has shifted to increasing research on the connections and influences of different diseases. So far, increasing attention has been paid to the research field of the relationship between ALI and pyroptosis related to COVID-19.

Cigarette smoke promotes inflammasome-independent activation of caspase-1 and -4 leading to gasdermin D cleavage in human macrophages

Mechanisms and consequences of gasdermin D (GSDMD) activation in cigarette smoke (CS)-associated inflammation and lung disease are unknown. GSDMD is a downstream effector of caspase-1, -8, and -4. Upon cleavage, GSDMD generates pores into cell membranes. Different degrees of GSDMD activation are associated with a range of physiological outputs ranging from cell hyperactivation to pyroptosis. We have previously reported that in human monocyte-derived macrophages CS extract (CSE) inhibits the NLRP3 inflammasome and shifts the response to lipopolysaccharide (LPS) towards the TLR4-TRIF axis leading to activation of caspase-8, which, in turn, activates caspase-1. In the present work, we investigated whether other ASC-dependent inflammasomes could be involved in caspase activation by CSE and whether caspase activation led to GSDMD cleavage and other downstream effects. Presented results demonstrate that CSE promoted ASC-independent activation of caspase-1 leading to GSDMD cleavage and increased cell permeability, in the absence of cell death. GSDMD cleavage was strongly enhanced upon stimulation with LPS+CSE, suggesting a synergistic effect between the two stimuli. Noteworthy, CSE promoted LPS internalization leading to caspase-4 activation, thus contributing to increased GSDMD cleavage. Caspase-dependent GSDMD cleavage was associated with mitochondrial superoxide generation. Increased cleaved GSDMD was found in lung macrophages of smokers compared to ex-smokers and non-smoking controls. Our findings revealed that ASC-independent activation of caspase-1, -4, and -8 and GSDMD cleavage upon exposure to CS may contribute to macrophage dysfunction and feed the chronic inflammation observed in the smokers' lung.

The role of pyroptosis and its crosstalk with immune therapy in breast cancer

Pyroptosis is a brand-new category of programmed cell death (PCD) that is brought on by multitudinous inflammasomes, which can recognize several stimuli to pilot the cleavage of and activate inflammatory cytokines like IL-18 and IL-1β is believed to have dual effects on the development of multiple cancers including breast cancer. However, pyroptosis has different effects on cancers depending on the type of tissues and their distinct heredity. Recently, the association between pyroptosis and breast cancer has received more and more attention, and it is thought that inducing pyroptosis could be used as a cancer treatment option. In addition, a great deal of evidence accumulating over the past decades has evinced the crosstalk between pyroptosis and tumor immunological therapy. Thus, a comprehensive summary combining the function of pyroptosis in breast cancer and antitumor immunity is imperative. We portray the prevalent knowledge of the multidimensional roles of pyroptosis in cancer and summarize the pyroptosis in breast cancer principally. Moreover, we elucidate the influence of inflammasomes and pyroptosis-produced cytokines on the tumor microenvironment (TME) of breast cancer. Taken together, we aim to provide a clue to harness pyroptosis rationally and apply it to augment immunotherapy efficiency for breast cancer.

Lipid A Variants Activate Human TLR4 and the Noncanonical Inflammasome Differently and Require the Core Oligosaccharide for Inflammasome Activation

Detection of Gram-negative bacterial lipid A by the extracellular sensor, myeloid differentiation 2 (MD2)/Toll-like receptor 4 (TLR4), or the intracellular inflammasome sensors, CASP4 and CASP5, induces robust inflammatory responses. The chemical structure of lipid A, specifically its phosphorylation and acylation state, varies across and within bacterial species, potentially allowing pathogens to evade or suppress host immunity. Currently, it is not clear how distinct alterations in the phosphorylation or acylation state of lipid A affect both human TLR4 and CASP4/5 activation. Using a panel of engineered lipooligosaccharides (LOS) derived from Yersinia pestis with defined lipid A structures that vary in their acylation or phosphorylation state, we identified that differences in phosphorylation state did not affect TLR4 or CASP4/5 activation. However, the acylation state differentially impacted TLR4 and CASP4/5 activation. Specifically, all tetra-, penta-, and hexa-acylated LOS variants examined activated CASP4/5-dependent responses, whereas TLR4 responded to penta- and hexa-acylated LOS but did not respond to tetra-acylated LOS or penta-acylated LOS lacking the secondary acyl chain at the 3' position. As expected, lipid A alone was sufficient for TLR4 activation. In contrast, both core oligosaccharide and lipid A were required for robust CASP4/5 inflammasome activation in human macrophages, whereas core oligosaccharide was not required to activate mouse macrophages expressing CASP4. Our findings show that human TLR4 and CASP4/5 detect both shared and nonoverlapping LOS/lipid A structures, which enables the innate immune system to recognize a wider range of bacterial LOS/lipid A and would thereby be expected to constrain the ability of pathogens to evade innate immune detection.

Immunohistochemical analysis of caspase expression in the brains of individuals with obesity or overweight

Mechanisms underlying the negative effects of obesity on the brain are still unknown. Obesity is associated with oxidative stress in the brain and neuroinflammation that promotes neurodegenerative diseases. Chronic low-grade neuroinflammation in obesity could be associated with lower volumes of gray matter and lower neuronal density. If neuroinflammation mediated by the expression of cytokines and chemokines leads to apoptosis, this can be assessed by examining caspase expression. The aim of this study was to compare the expression of caspases in the 16 brains of donors with obesity/overweight (n = 8; Body Mass Index [BMI] = 31.6 ± 4.35 kg/m²; 2 females; Age = 52.9 ± 4.76 years) and normal weight (n = 8; BMI = 21.8 ± 1.5 kg/m²; 3 females; Age = 37.8 ± 19.2 years). Sixteen human brain samples were processed. Serial paraffin sections were examined by anti-caspase immunochemistry (caspase-3, caspase-4, caspase-6, caspase-1, caspase-8, and caspase-9 antibodies). Postmortem samples of cerebral cortex tissue were captured as photomicrographs and the images obtained were analyzed using ImageJ software to obtain the percentage of positive caspase expression. Nonparametric Mann-Whitney U tests were performed to compare caspase expression between samples from donors with obesity/overweight and normal weight. Taking into consideration the immunohistochemistry results, the Search Tool for the Retrieval of Interacting Genes was used to model molecular interactions. Results showed that brain samples from individuals with obesity/overweight exhibited significantly greater values of positive expression for Caspase-1 (U = 16.5, p = 0.05, Cohen d = 0.89) and -8 (U = 15, p = 0.03, Cohen d = 0.99) than those from donors with normal weight. This study contributes to the knowledge about the inflammatory effects of obesity/overweight on brain, suggesting the activation of the alternative inflammasome pathway in which interact caspase-1 and -8.

Bacterial subversion of NLR-mediated immune responses

Members of the mammalian Nod-like receptor (NLR) protein family are important intracellular sensors for bacteria. Bacteria have evolved under the pressure of detection by host immune sensing systems, leading to adaptive subversion strategies to dampen immune responses for their benefits. These include modification of microbe-associated molecular patterns (MAMPs), interception of innate immune pathways by secreted effector proteins and sophisticated instruction of anti-inflammatory adaptive immune responses. Here, we summarise our current understanding of subversion strategies used by bacterial pathogens to manipulate NLR-mediated responses, focusing on the well-studied members NOD1/2, and the inflammasome forming NLRs NLRC4, and NLRP3. We discuss how bacterial pathogens and their products activate these NLRs to promote inflammation and disease and the range of mechanisms used by bacterial pathogens to evade detection by NLRs and to block or dampen NLR activation to ultimately interfere with the generation of host immunity. Moreover, we discuss how bacteria utilise NLRs to facilitate immunotolerance and persistence in the host and outline how various mechanisms used to attenuate innate immune responses towards bacterial pathogens can also aid the host by reducing immunopathologies. Finally, we describe the therapeutic potential of harnessing immune subversion strategies used by bacteria to treat chronic inflammatory conditions.

Emerging mechanisms of pyroptosis and its therapeutic strategy in cancer

Pyroptosis, a type of inflammatory programmed cell death, is triggered by caspase cleavage of gasdermin family proteins. Based on accumulating evidence, pyroptosis is closely associated with tumour development, but the molecular mechanism underlying pyroptosis activation and the signalling pathways regulated by pyroptosis remain unclear. In this review, we first briefly introduce the definition, morphological characteristics, and activation pathways of pyroptosis and the effect of pyroptosis on anticancer immunity. Then we review recent progress concerning the complex role of pyroptosis in various tumours. Importantly, we summarise various FDA-approved chemotherapy drugs or natural compounds that exerted antitumor properties by inducing pyroptosis of cancer cells. Moreover, we also focus on the current application of nanotechnology-induced pyroptosis in tumour therapy. In addition, some unsolved problems and potential future research directions are also raised.

Lipid-protein interactions regulating the canonical and the non-canonical NLRP3 inflammasome

The inflammatory response is a complex regulated effector mechanism of the innate immune system that is initiated after tissue injury or infection. The NLRP3 inflammasome is an important initiator of inflammation by regulating the activation of caspase-1, the maturation of pro-inflammatory cytokines and the induction of pyroptotic cell death. Numerous studies demonstrate that the NLRP3 inflammasome could be modulated by lipids, existing a relation between lipids and the activation of different inflammatory processes. In this review we will summarize how the mechanism of NLRP3 inflammasome activation is regulated by different lipids and how these lipids control specific cellular localization of NLRP3 during activation. Although being a cytosolic protein, NLRP3 interacts with lipids accessible in neighbor membranes. Also, the modulation of NLRP3 by endogenous lipids has been found causative of different metabolic diseases and bacterial-pathogenic lipids lead to NLRP3 activation during infection. The understanding of the modulation of the NLRP3 inflammasome by lipids has resulted not only in a better knowledge about the mechanism of NLRP3 activation and its implication in disease, but also opens a new avenue for the development of novel therapeutics and vaccines, as NLRP3 could be modulated by synthetic lipids used as adjuvants.

Salmonella enterica Serovar Typhimurium Induces NAIP/NLRC4- and NLRP3/ASC-Independent, Caspase-4-Dependent Inflammasome Activation in Human Intestinal Epithelial Cells

Salmonella enterica serovar Typhimurium is a Gram-negative pathogen that causes diseases ranging from gastroenteritis to systemic infection and sepsis. Salmonella uses type III secretion systems (T3SS) to inject effectors into host cells. While these effectors are necessary for bacterial invasion and intracellular survival, intracellular delivery of T3SS products also enables detection of translocated Salmonella ligands by cytosolic immune sensors. Some of these sensors form multimeric complexes called inflammasomes, which activate caspases that lead to interleukin-1 (IL-1) family cytokine release and pyroptosis. In particular, the Salmonella T3SS needle, inner rod, and flagellin proteins activate the NAIP/NLRC4 inflammasome in murine intestinal epithelial cells (IECs), which leads to restriction of bacterial replication and extrusion of infected IECs into the intestinal lumen, thereby preventing systemic dissemination of Salmonella. While these processes are quite well studied in mice, the role of the NAIP/NLRC4 inflammasome in human IECs remains unknown. Unexpectedly, we found the NAIP/NLRC4 inflammasome is dispensable for early inflammasome responses to Salmonella in both human IEC lines and enteroids. Additionally, NLRP3 and the adaptor protein ASC are not required for inflammasome activation in Caco-2 cells. Instead, we observed a necessity for caspase-4 and gasdermin D pore-forming activity in mediating inflammasome responses to Salmonella in Caco-2 cells. These findings suggest that unlike murine IECs, human IECs do not rely on NAIP/NLRC4 or NLRP3/ASC inflammasomes and instead primarily use caspase-4 to mediate inflammasome responses to Salmonella pathogenicity island 1 (SPI-1)-expressing Salmonella.

Activation and Pharmacological Regulation of Inflammasomes

Inflammasomes are intracellular signaling complexes of the innate immune system, which is part of the response to exogenous pathogens or physiological aberration. The multiprotein complexes mainly consist of sensor proteins, adaptors, and pro-caspase-1. The assembly of the inflammasome upon extracellular and intracellular cues drives the activation of caspase-1, which processes pro-inflammatory cytokines IL-1β and IL-18 to maturation and gasdermin-D for pore formation, leading to pyroptosis and cytokine release. Inflammasome signaling functions in numerous infectious or sterile inflammatory diseases, including inherited autoinflammatory diseases, metabolic disorders, cardiovascular diseases, cancers, neurodegenerative disorders, and COVID-19. In this review, we summarized current ideas on the organization and activation of inflammasomes, with details on the molecular mechanisms, regulations, and interventions. The recent developments of pharmacological strategies targeting inflammasomes as disease therapeutics were also covered.

Targeting Novel Regulated Cell Death: Pyroptosis, Necroptosis, and Ferroptosis in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the primary causes of visual impairment in the working-age population. Retinal cell death is recognized as a prominent feature in the pathological changes of DR. Several types of cell death occurrence have been confirmed in DR, which might be the underlying mechanisms of retinal cell loss. Regulated cell death (RCD) originates from too intense or prolonged perturbations of the intracellular or extracellular microenvironment for adaptative responses to cope with stress and restore cellular homeostasis. Pyroptosis, necroptosis, and ferroptosis represent the novel discovered RCD forms, which contribute to retinal cell death in the pathogenesis of DR. This evidence provides new therapeutic targets for DR. In this review, we summarize the mechanisms of three types of RCD and analyse recent advances on the association between novel RCD and DR, aiming to provide new insights into the underlying pathogenic mechanisms and propose a potential new strategy for DR therapy.

Gasdermins in Innate Host Defense Against Entamoeba histolytica and Other Protozoan Parasites

Gasdermins (GSDMs) are a group of proteins that are cleaved by inflammatory caspases to induce pore formation in the plasma membrane to cause membrane permeabilization and lytic cell death or pyroptosis. All GSDMs share a conserved structure, containing a cytotoxic N-terminal (NT) pore-forming domain and a C-terminal (CT) repressor domain. Entamoeba histolytica (Eh) in contact with macrophages, triggers outside-in signaling to activate inflammatory caspase-4/1 via the noncanonical and canonical pathway to promote cleavage of gasdermin D (GSDMD). Cleavage of GSDMD removes the auto-inhibition that masks the active pore-forming NT domain in the full-length protein by interactions with GSDM-CT. The cleaved NT-GSDMD monomers then oligomerize to form pores in the plasma membrane to facilitate the release of IL-1β and IL-18 with a measured amount of pyroptosis. Pyroptosis is an effective way to counteract intracellular parasites, which exploit replicative niche to avoid killing. To date, most GSDMs have been verified to perform pore-forming activity and GSDMD-induced pyroptosis is rapidly emerging as a mechanism of anti-microbial host defence. Here, we review our comprehensive and current knowledge on the expression, activation, biological functions, and regulation of GSDMD cleavage with emphases on physiological scenario and related dysfunctions of each GSDM member as executioner of cell death, cytokine secretion and inflammation against Eh and other protozoan parasitic infections.

The Relevance of Physico-Chemical Properties and Protein Corona for Evaluation of Nanoparticles Immunotoxicity-In Vitro Correlation Analysis on THP-1 Macrophages

Alongside physiochemical properties (PCP), it has been suggested that the protein corona of nanoparticles (NPs) plays a crucial role in the response of immune cells to NPs. However, due to the great variety of NPs, target cells, and exposure protocols, there is still no clear relationship between PCP, protein corona composition, and the immunotoxicity of NPs. In this study, we correlated PCP and the protein corona composition of NPs to the THP-1 macrophage response, focusing on selected toxicological endpoints: cell viability, reactive oxygen species (ROS), and cytokine secretion. We analyzed seven commonly used engineered NPs (SiO2, silver, and TiO2) and magnetic NPs. We show that with the exception of silver NPs, all of the tested TiO2 types and SiO2 exhibited moderate toxicities and a transient inflammatory response that was observed as an increase in ROS, IL-8, and/or IL-1β cytokine secretion. We observed a strong correlation between the size of the NPs in media and IL-1β secretion. The induction of IL-1β secretion was completely blunted in NLR family pyrin domain containing 3 (NLRP3) knockout THP-1 cells, indicating activation of the inflammasome. The correlations analysis also implicated the association of specific NP corona proteins with the induction of cytokine secretion. This study provides new insights toward a better understanding of the relationships between PCP, protein corona, and the inflammatory response of macrophages for different engineered NPs, to which we are exposed on a daily basis.

Caspase-4 and -5 Biology in the Pathogenesis of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic relapsing inflammatory disease of the gastrointestinal tract, associated with high levels of inflammatory cytokine production. Human caspases-4 and -5, and their murine ortholog caspase-11, are essential components of the innate immune pathway, capable of sensing and responding to intracellular lipopolysaccharide (LPS), a component of Gram-negative bacteria. Following their activation by LPS, these caspases initiate potent inflammation by causing pyroptosis, a lytic form of cell death. While this pathway is essential for host defence against bacterial infection, it is also negatively associated with inflammatory pathologies. Caspases-4/-5/-11 display increased intestinal expression during IBD and have been implicated in chronic IBD inflammation. This review discusses the current literature in this area, identifying links between inflammatory caspase activity and IBD in both human and murine models. Differences in the expression and functions of caspases-4, -5 and -11 are discussed, in addition to mechanisms of their activation, function and regulation, and how these mechanisms may contribute to the pathogenesis of IBD.

A TLR4-independent critical role for CD14 in intracellular LPS sensing

Intracellular lipopolysaccharide (LPS) sensing by the noncanonical inflammasome comprising caspase-4 or −11 governs antibacterial host defense. How LPS gains intracellular access in vivo is largely unknown. Here, we show that CD14—an LPS-binding protein with a well-documented role in TLR4 activation—plays a vital role in intracellular LPS sensing in vivo. By generating Cd14^−/− and Casp11^−/− mice strains on a Tlr4^−/− background, we dissociate CD14’s known role in TLR4 signaling from its role in caspase-11 activation and show a TLR4-independent role for CD14 in GSDMD activation, pyroptosis, alarmin release, and the lethality driven by cytosolic LPS. Mechanistically, CD14 enables caspase-11 activation by mediating cytosolic localization of LPS in a TLR4-independent manner. Overall, our findings attribute a critical role for CD14 in noncanonical inflammasome sensing of LPS in vivo and establish—together with previous literature—CD14 as an essential proximal component of both TLR4-based extracellular and caspase-11-based intracellular LPS surveillance.

How LPS attains cytosolic access in vivo is unclear. Vasudevan et al. define a TLR4-independent role for CD14 in the cytosolic localization of LPS, triggering noncanonical inflammasome activation and pyroptosis in vivo. This finding positions CD14 as an integral component of both extracellular and intracellular LPS surveillance pathways.

Ars moriendi: Proteases as sculptors of cellular suicide

The Ars moriendi, which translates to "The Art of Dying," encompasses two Latin texts that gave advice on how to die well and without fear according to the Christian precepts of the late Middle Ages. Given that ten to hundred billion cells die in our bodies every day, it is obvious that the concept of a well and orderly ("regulated") death is also paramount at the cellular level. In apoptosis, as the most well-studied form of regulated cell death, proteases of the caspase family are the central mediators. However, caspases are not the only proteases that act as sculptors of cellular suicide, and therefore, we here provide an overview of the impact of proteases in apoptosis and other forms of regulated cell death.

The TLR-chaperone CNPY3 is a critical regulator of NLRP3-Inflammasome activation

Toll like receptors (TLRs) mediate the recognition of microbial and endogenous insults to orchestrate the inflammatory response. TLRs localize to the plasma membrane or endomembranes, depending on the member, and rely critically on endoplasmic reticulum-resident chaperones to mature and reach their subcellular destinations. The chaperone canopy FGF signaling regulator 3 (CNPY3) is necessary for the proper trafficking of multiple TLRs including TLR1/2/4/5/9 but not TLR3. However, the exact role of CNPY3 in inflammatory signalling downstream of TLRs has not been studied in detail. Consistent with the reported client specificity, we report here that functional loss of CNPY3 in engineered macrophages impairs downstream signalling by TLR2 but not TLR3. Unexpectedly, CNPY3-deficient macrophages show reduced interleukin-1β (IL-1ß) and IL-18 processing and production independent of the challenged upstream TLR species, demonstrating a separate, specific role for CNPY3 in inflammasome activation. Mechanistically, we document that CNPY3 regulates caspase-1 localization to the apoptosis speck and auto-activation of caspase-1. Importantly, we were able to recapitulate these findings in macrophages from an early infantile epileptic encephalopathy (EIEE) patient with a novel CNPY3 loss-of-function variant. Summarizing, our findings reveal a hitherto unknown, TLR-independent role of CNPY3 in inflammasome activation, highlighting a more complex and dedicated role of CNPY3 to the inflammatory response than anticipated. This article is protected by copyright. All rights reserved.

The potential convergence of NLRP3 inflammasome, potassium, and dopamine mechanisms in Parkinson's disease

The pathology of Parkinson's disease (PD) is characterized by α-synuclein aggregation, microglia-mediated neuroinflammation, and dopaminergic neurodegeneration in the substantia nigra with collateral striatal dopamine signaling deficiency. Microglial NLRP3 inflammasome activation has been linked independently to each of these facets of PD pathology. The voltage-gated potassium channel Kv1.3, upregulated in microglia by α-synuclein and facilitating potassium efflux, has also been identified as a modulator of neuroinflammation and neurodegeneration in models of PD. Evidence increasingly suggests that microglial Kv1.3 is mechanistically coupled with NLRP3 inflammasome activation, which is contingent on potassium efflux. Potassium conductance also influences dopamine release from midbrain dopaminergic neurons. Dopamine, in turn, has been shown to inhibit NLRP3 inflammasome activation in microglia. In this review, we provide a literature framework for a hypothesis in which Kv1.3 activity-induced NLRP3 inflammasome activation, evoked by stimuli such as α-synuclein, could lead to microglia utilizing dopamine from adjacent dopaminergic neurons to counteract this process and fend off an activated state. If this is the case, a sufficient dopamine supply would ensure that microglia remain under control, but as dopamine is gradually siphoned from the neurons by microglial demand, NLRP3 inflammasome activation and Kv1.3 activity would progressively intensify to promote each of the three major facets of PD pathology: α-synuclein aggregation, microglia-mediated neuroinflammation, and dopaminergic neurodegeneration. Risk factors overlapping to varying degrees to render brain regions susceptible to such a mechanism would include a high density of microglia, an initially sufficient supply of dopamine, and poor insulation of the dopaminergic neurons by myelin.

Probiotics as Potential Biological Immunomodulators in the Management of Oral Lichen Planus: What's New?

Oral lichen planus (OLP) is a T cell-mediated chronic inflammatory disorder with multifactorial aetiology and malignant transformation potential. Despite the treatments so far identified, new tailored and safe specific measures are needed. Recently, human microbiota imbalance has been linked to several immune-mediated diseases, opening new therapeutic perspectives for probiotics; besides their ability to directly interact with the host microbiota, they also display a strain-specific immune-modulatory effect. Thus, this non-systematic review aims to elucidate the molecular pathways underlying probiotic activity, mainly those of Lactobacilli and Bifidobacteria and their metabolites in OLP pathogenesis and malignant transformation, focusing on the most recent in vitro and in vivo research evidence. Findings related to their activity in other immune-mediated diseases are here included, suggesting a probiotic translational use in OLP. Probiotics show immune-modulatory and microbiota-balancing activities; they protect the host from pathogens, hamper an excessive effector T cell response, reduce nuclear factor-kappa B (NF-kB) signalling and basal keratinocytes abnormal apoptosis, shifting the mucosal response towards the production of anti-inflammatory cytokines, thus preventing uncontrolled damage. Therefore, probiotics could be a highly encouraging prevention and immunotherapeutic approach for a safer and more sustainable OLP management.

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.

Dopamine signaling modulates microglial NLRP3 inflammasome activation: implications for Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is characterized by the loss of nigral dopaminergic neurons leading to impaired striatal dopamine signaling, α-synuclein- (α-syn-) rich inclusions, and neuroinflammation. Degenerating neurons are surrounded by activated microglia with increased secretion of interleukin-1β (IL-1β), driven largely by the NLRP3 inflammasome. A critical role for microglial NLRP3 inflammasome activation in the progression of both dopaminergic neurodegeneration and α-syn pathology has been demonstrated in parkinsonism mouse models. Fibrillar α-syn activates this inflammasome in mouse and human macrophages, and we have shown previously that the same holds true for primary human microglia. Dopamine blocks microglial NLRP3 inflammasome activation in the MPTP model, but its effects in this framework, highly relevant to PD, remain unexplored in primary human microglia and in other in vivo parkinsonism models.

METHODS

Biochemical techniques including quantification of IL-1β secretion and confocal microscopy were employed to gain insight into dopamine signaling-mediated inhibition of the NLRP3 inflammasome mechanism in primary human microglia and the SYN120 transgenic mouse model. Dopamine and related metabolites were applied to human microglia together with various inflammasome activating stimuli. The involvement of the receptors through which these catecholamines were predicted to act were assessed with agonists in both species.

RESULTS

We show in primary human microglia that dopamine, L-DOPA, and high extracellular K⁺, but not norepinephrine and epinephrine, block canonical, non-canonical, and α-syn-mediated NLRP3 inflammasome-driven IL-1β secretion. This suggests that dopamine acts as an inflammasome inhibitor in human microglia. Accordingly, we provide evidence that dopamine exerts its inhibitory effect through dopamine receptor D1 and D2 (DRD1 and DRD2) signaling. We also show that aged mice transgenic for human C-terminally truncated (1-120) α-syn (SYN120 tg mice) display increased NLRP3 inflammasome activation in comparison to WT mice that is diminished upon DRD1 agonism.

CONCLUSIONS

Dopamine inhibits canonical, non-canonical, and α-syn-mediated activation of the NLRP3 inflammasome in primary human microglia, as does high extracellular K⁺. We suggest that dopamine serves as an endogenous repressor of the K⁺ efflux-dependent microglial NLRP3 inflammasome activation that contributes to dopaminergic neurodegeneration in PD, and that this reciprocation may account for the specific vulnerability of these neurons to disease pathology.

Inflammatory Caspases: Toward a Unified Model for Caspase Activation by Inflammasomes

Inflammasomes are inflammatory signaling complexes that provide molecular platforms to activate the protease function of inflammatory caspases. Caspases-1, -4, -5, and -11 are inflammatory caspases activated by inflammasomes to drive lytic cell death and inflammatory mediator production, thereby activating host-protective and pathological immune responses. Here, we comprehensively review the mechanisms that govern the activity of inflammatory caspases. We discuss inflammatory caspase activation and deactivation mechanisms, alongside the physiological importance of caspase activity kinetics. We also examine mechanisms of caspase substrate selection and how inflammasome and cell identities influence caspase activity and resultant inflammatory and pyroptotic cellular programs. Understanding how inflammatory caspases are regulated may offer new strategies for treating infection and inflammasome-driven disease. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Gene Expression-Based Predication of RNA Pseudouridine Modification in Tumor Microenvironment and Prognosis of Glioma Patients

Aberrant expression of methyltransferases and demethylases may augment tumor initiation, proliferation and metastasis through RNA modification, such as m⁶A and m⁵C. However, activity of pseudouridine (Ψ) modification of RNA remains unknown in glioma, the most common malignant intracranial tumor. In this study, we explored the expression profiles of the Ψ synthase genes in glioma and constructed an efficient prediction model for glioma prognosis based on the CGGA and TCGA datasets. In addition, the risk-score signature was positively associated with malignancy of gliomas and the abundance of tumor-infiltrating immune cells such as macrophages M0 and regulatory T cells (Tregs), but negatively associated with the abundance of monocytes, NK cell activation and T cell CD4⁺ naive. In terms of mechanism, the risk-score signature was positively associated with the expression of inflammatory molecules such as S100A11 and CASP4 in glioma. Overall, this study provided evidence for the activity of RNA Ψ modification in glioma malignancy and local immunity.

To die or not to die: Programmed cell death responses and their interactions with Coxiella burnetii infection

Coxiella burnetii is a Gram-negative, obligate intracellular, macrophage-tropic bacterium and the causative agent of the zoonotic disease Q fever. The epidemiology of Q fever is associated with the presence of infected animals; sheep, goats, cattle, and humans primarily become infected by inhalation of contaminated aerosols. In humans, the acute phase of the disease is characterized primarily by influenza-like symptoms, and approximately 3-5% of the infected individuals develop chronic infection. C. burnetii infection activates many types of immune responses, and the bacteria's genome encodes for numerous effector proteins that interact with host immune signaling mechanisms. Here, we will discuss two forms of programmed cell death, apoptosis and pyroptosis. Apoptosis is a form of non-inflammatory cell death that leads to phagocytosis of small membrane-bound bodies. Conversely, pyroptosis results in lytic cell death accompanied by the release of proinflammatory cytokines. Both apoptosis and pyroptosis have been implicated in the clearance of intracellular bacterial pathogens, including C. burnetii. Finally, we will discuss the role of autophagy, the degradation of unwanted cellular components, during C. burnetii infection. Together, the review of these forms of programmed cell death will open new research questions aimed at combating this highly infectious pathogen for which treatment options are limited.

The Role of Inflammasomes in the Pathogenesis of Neurodegenerative Diseases

Abstract—Protein misfolding and accumulation of protein aggregates is a distinctive feature of most neurodegenerative diseases. They lead to disruption of cellular homeostasis, loss of synaptic connections, and therefore cellular apoptosis. It has been demonstrated that some innate immune responses play an important role in the emergence and progression of neurodegenerative diseases. Inflammasomes are components of innate immunity that play a major role in the maintenance of chronic inflammation. Inflammasomes function as intracellular sensors, detecting both exogenous and endogenous stimuli. They also take part in caspase-1 activation and the synthesis of pro-inflammatory cytokines. In the central nervous system (CNS), inflammasomes are predominantly expressed by microglia, the key cells of innate immunity responsible for activation and maintenance of inflammation. In addition to microglia, inflammasomes can be expressed and activated by astrocytes and neurons, as well as infiltrating myeloid cells. Understanding the mechanisms of activation and functioning of inflammasomes is crucial for the development of novel drugs targeted at modulation of the immune response associated with their excessive activation. This review provides up-to-date information on the inflammasome structure and mechanisms of action, the role of protein misfolding, aggregation and the influence of these factors on inflammasome activation, as well as potential therapeutic targets in neurodegenerative diseases.

Human NAIP/NLRC4 and NLRP3 inflammasomes detect Salmonella type III secretion system activities to restrict intracellular bacterial replication

Salmonella enterica serovar Typhimurium is a Gram-negative pathogen that uses two distinct type III secretion systems (T3SSs), termed Salmonella pathogenicity island (SPI)-1 and SPI-2, to deliver virulence factors into the host cell. The SPI-1 T3SS enables Salmonella to invade host cells, while the SPI-2 T3SS facilitates Salmonella's intracellular survival. In mice, a family of cytosolic immune sensors, including NAIP1, NAIP2, and NAIP5/6, recognizes the SPI-1 T3SS needle, inner rod, and flagellin proteins, respectively. Ligand recognition triggers assembly of the NAIP/NLRC4 inflammasome, which mediates caspase-1 activation, IL-1 family cytokine secretion, and pyroptosis of infected cells. In contrast to mice, humans encode a single NAIP that broadly recognizes all three ligands. The role of NAIP/NLRC4 or other inflammasomes during Salmonella infection of human macrophages is unclear. We find that although the NAIP/NLRC4 inflammasome is essential for detecting T3SS ligands in human macrophages, it is partially required for responses to infection, as Salmonella also activated the NLRP3 and CASP4/5 inflammasomes. Importantly, we demonstrate that combinatorial NAIP/NLRC4 and NLRP3 inflammasome activation restricts Salmonella replication in human macrophages. In contrast to SPI-1, the SPI-2 T3SS inner rod is not sensed by human or murine NAIPs, which is thought to allow Salmonella to evade host recognition and replicate intracellularly. Intriguingly, we find that human NAIP detects the SPI-2 T3SS needle protein. Critically, in the absence of both flagellin and the SPI-1 T3SS, the NAIP/NLRC4 inflammasome still controlled intracellular Salmonella burden. These findings reveal that recognition of Salmonella SPI-1 and SPI-2 T3SSs and engagement of both the NAIP/NLRC4 and NLRP3 inflammasomes control Salmonella infection in human macrophages.

NLRP3 Inflammasome: Checkpoint Connecting Innate and Adaptive Immunity in Autoimmune Diseases

Autoimmune diseases are a broad spectrum of human diseases that are characterized by the breakdown of immune tolerance and the production of autoantibodies. Recently, dysfunction of innate and adaptive immunity is considered to be a key step in the initiation and maintenance of autoimmune diseases. NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is a multimeric protein complex, which can detect exogenous pathogen irritants and endogenous danger signals. The main function of NLRP3 inflammasome is to promote secretion of interleukin (IL)-1β and IL-18, and pyroptosis mediated by caspase-1. Served as a checkpoint in innate and adaptive immunity, aberrant activation and regulation of NLRP3 inflammasome plays an important role in the pathogenesis of autoimmune diseases. This paper reviewed the roles of NLRP3 inflammasome in autoimmune diseases, which shows NLRP3 inflammasome may be a potential target for autoimmune diseases deserved further study.

Internalization of the Membrane Attack Complex Triggers NLRP3 Inflammasome Activation and IL-1β Secretion in Human Macrophages

Interleukin 1β (IL-1β) plays a major role in inflammation and is secreted by immune cells, such as macrophages, upon recognition of danger signals. Its secretion is regulated by the inflammasome, the assembly of which results in caspase 1 activation leading to gasdermin D (GSDMD) pore formation and IL-1β release. During inflammation, danger signals also activate the complement cascade, resulting in the formation of the membrane attack complex (MAC). Here, we report that stimulation of LPS-primed human macrophages with sub-lytic levels of MAC results in activation of the NOD-like receptor 3 (NLRP3) inflammasome and GSDMD-mediated IL-1β release. The MAC is first internalized into endosomes and then colocalizes with inflammasome components; adapter protein apoptosis associated speck-like protein containing a CARD (ASC) and NLRP3. Pharmacological inhibitors established that MAC-triggered activation of the NLRP3 inflammasome was dependent on MAC endocytosis. Internalization of the MAC also caused dispersion of the trans-Golgi network. Thus, these data uncover a role for the MAC in activating the inflammasome and triggering IL-1β release in human macrophages.

Chronic low-grade inflammation in heart failure with preserved ejection fraction

Heart failure (HF) with preserved ejection fraction (HFpEF) is currently the predominant form of HF with a dramatic increase in risk with age. Low-grade inflammation, as occurs with aging (termed "inflammaging"), is a common feature of HFpEF pathology. Suppression of proinflammatory pathways has been associated with attenuated HFpEF disease severity and better outcomes. From this perspective, inflammasome signaling plays a central role in mediating chronic inflammation and cardiovascular disease progression. However, the causal link between the inflammasome-immune signaling axis on the age-dependent progression of HFpEF remains conjectural. In this review, we summarize the current understanding of the role of inflammatory pathways in age-dependent cardiac function decline. We will also evaluate recent advances and evidence regarding the inflammatory pathway in the pathophysiology of HFpEF, with special attention to inflammasome signaling.

Evolution-inspired redesign of the LPS receptor caspase-4 into an interleukin-1β converting enzyme

Innate immune signaling pathways comprise multiple proteins that promote inflammation. This multistep means of information transfer suggests that complexity is a prerequisite for pathway design. Herein, we test this hypothesis by studying caspases that regulate inflammasome-dependent inflammation. Several caspases differ in their ability to recognize bacterial LPS and cleave interleukin-1β (IL-1β). No caspase is known to contain both activities, yet distinct caspases with complementary activities bookend an LPS-induced pathway to IL-1β cleavage. Using caspase-1/4 hybrid proteins present in canines as a guide, we identified molecular determinants of IL-1β cleavage specificity within caspase-1. This knowledge enabled the redesign of human caspase-4 to operate as a one-protein signaling pathway, which intrinsically links LPS detection to IL-1β cleavage and release, independent of inflammasomes. We identified caspase-4 homologues in multiple carnivorans which display the activities of redesigned human caspase-4. These findings illustrate natural signaling pathway diversity and highlight how multistep innate immune pathways can be condensed into a single protein.

Pyroptosis, metabolism, and tumor immune microenvironment

In response to a wide range of stimulations, host cells activate pyroptosis, a kind of inflammatory cell death which is provoked by the cytosolic sensing of danger signals and pathogen infection. In manipulating the cleavage of gasdermins (GSDMs), researchers have found that GSDM proteins serve as the real executors and the deterministic players in fate decisions of pyroptotic cells. Whether inflammatory characteristics induced by pyroptosis could cause damage the host or improve immune activity is largely dependent on the context, timing, and response degree. Here, we systematically review current points involved in regulatory mechanisms and the multidimensional roles of pyroptosis in several metabolic diseases and the tumor microenvironment. Targeting pyroptosis may reveal potential therapeutic avenues.

Interruption of Helicobacter pylori-Induced NLRP3 Inflammasome Activation by Chalcone Derivatives

Helicobacter pylori causes chronic gastritis through cag pathogenicity island (cagPAI), vacuolating cytotoxin A (VacA), lipopolysaccharides (LPS), and flagellin as pathogen-related molecular patterns (PAMPs), which, in combination with the pattern recognition receptors (PRRs) of host cells promotes the expression and secretion of inflammation-causing cytokines and activates innate immune responses such as inflammasomes. To identify useful compounds against H. pylori-associated gastric disorders, the effect of chalcone derivatives to activate the nucleotide-binding oligomerization domain (NOD)-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome was examined in an H. pylori-infected human monocytic THP-1 cell line in this study. Among the five synthetic structurally-related chalcone derivatives examined, 2'-hydroxy-4',6'-dimethoxychalcone (8) and 2'-hydroxy-3,4,5- trimethoxychalcone (12) strongly blocked the NLRP3 inflammasome in H. pylori-infected THP-1 cells. At 10 μM, these compounds inhibited the production of active IL-1β, IL-18, and caspase-1, and apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) oligomerization, but did not affect the expression levels of NLRP3, ASC, and pro-caspase-1. The interruption of NLRP3 inflammasome activation by these compounds was found to be mediated via the inhibition of the interleukin-1 receptor-associated kinase 4 (IRAK4)/IκBα/NF-κB signaling pathway. These compounds also inhibited caspase-4 production associated with non-canonical NLRP3 inflammasome activation. These results show for the first time that certain chalcones could interrupt the activation of the NLRP3 inflammasome in H. pylori-infected THP-1 cells. Therefore, these chalcones may be helpful in alleviating H. pylori-related inflammatory disorders including chronic gastritis.

New Insights on the Early Interaction Between Typhoid and Non-typhoid Salmonella Serovars and the Host Cells

Salmonella enterica is a common source of food and water-borne infections, causing a wide range of clinical ailments in both human and animal hosts. Immunity to Salmonella involves an interplay between different immune responses, which are rapidly initiated to control bacterial burden. However, Salmonella has developed several strategies to evade and modulate the host immune responses. In this sense, the main knowledge about the pathogenicity of this bacterium has been obtained by the study of mouse models with non-typhoidal serovars. However, this knowledge is not representative of all the pathologies caused by non-typhoidal serovars in the human. Here we review the most important features of typhoidal and non-typhoidal serovars and the diseases they cause in the human host, describing the virulence mechanisms used by these pathogens that have been identified in different models of infection.

Inflammasomes in Alveolar Bone Loss

Bone remodeling is tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Fine tuning of the osteoclast-osteoblast balance results in strict synchronization of bone resorption and formation, which maintains structural integrity and bone tissue homeostasis; in contrast, dysregulated bone remodeling may cause pathological osteolysis, in which inflammation plays a vital role in promoting bone destruction. The alveolar bone presents high turnover rate, complex associations with the tooth and periodontium, and susceptibility to oral pathogenic insults and mechanical stress, which enhance its complexity in host defense and bone remodeling. Alveolar bone loss is also involved in systemic bone destruction and is affected by medication or systemic pathological factors. Therefore, it is essential to investigate the osteoimmunological mechanisms involved in the dysregulation of alveolar bone remodeling. The inflammasome is a supramolecular protein complex assembled in response to pattern recognition receptors and damage-associated molecular patterns, leading to the maturation and secretion of pro-inflammatory cytokines and activation of inflammatory responses. Pyroptosis downstream of inflammasome activation also facilitates the clearance of intracellular pathogens and irritants. However, inadequate or excessive activity of the inflammasome may allow for persistent infection and infection spreading or uncontrolled destruction of the alveolar bone, as commonly observed in periodontitis, periapical periodontitis, peri-implantitis, orthodontic tooth movement, medication-related osteonecrosis of the jaw, nonsterile or sterile osteomyelitis of the jaw, and osteoporosis. In this review, we present a framework for understanding the role and mechanism of canonical and noncanonical inflammasomes in the pathogenesis and development of etiologically diverse diseases associated with alveolar bone loss. Inappropriate inflammasome activation may drive alveolar osteolysis by regulating cellular players, including osteoclasts, osteoblasts, osteocytes, periodontal ligament cells, macrophages, monocytes, neutrophils, and adaptive immune cells, such as T helper 17 cells, causing increased osteoclast activity, decreased osteoblast activity, and enhanced periodontium inflammation by creating a pro-inflammatory milieu in a context- and cell type-dependent manner. We also discuss promising therapeutic strategies targeting inappropriate inflammasome activity in the treatment of alveolar bone loss. Novel strategies for inhibiting inflammasome signaling may facilitate the development of versatile drugs that carefully balance the beneficial contributions of inflammasomes to host defense.