Molecular mechanism for NLRP6 inflammasome assembly and activation

Unraveling the NLRP family: Structure, function, activation, critical influence on tumor progression, and potential as targets for cancer therapy

The innate immune system serves as the body's initial defense, swiftly detecting danger via pattern recognition receptors (PRRs). Among these, nucleotide-binding oligomerization domain-like receptor family pyrin domain-containing proteins (NLRPs) are pivotal in recognizing pathogen-associated and damage-associated molecular patterns, thereby triggering immune responses. NLRPs, the most extensively studied subset within the NLR family, form inflammasomes that regulate inflammation, essential for innate immunity activation. Recent research highlights NLRPs' significant impact on various human diseases, including cancer. With differential expression across organs, NLRPs influence cancer progression by modulating immune reactions, cell fate, and proliferation. Their clinical significance in cancer makes them promising therapeutic targets. This review provides a comprehensive overview of the structure, function, activation mechanism of the NLRPs family and its potential role in cancer progression. In addition, we particularly focused on the concept of NLRP as a therapeutic target and its potential value in combination with immune checkpoint inhibitors.

Exploring the correlation between innate immune activation of inflammasome and regulation of pyroptosis after intracerebral hemorrhage: From mechanism to treatment

Stroke has emerged as the primary cause of disability and death globally in recent years. Intracerebral hemorrhage (ICH), a particularly severe kind of stroke, is occurring in an increasing number of people. The two main clinical treatments for ICH now in use are conservative pharmaceutical therapy and surgical intervention, both of which have risks and drawbacks. Consequently, it is crucial to look into the pathophysiology of ICH and consider cutting-edge therapeutic approaches. Recent research has revealed that pyroptosis is a newly identified type of cell death distinguished by the break of the cell membrane and the discharge of pro-inflammatory substances through different routes. Following ICH, glial cells experience pyroptosis, which worsens neuroinflammation. Hence, the onset and progression of ICH are strongly linked to pyroptosis, which is facilitated by different inflammasomes. It is essential to conduct a comprehensive investigation of ICH damage processes and uncover new targets for treatment. The impact and function of pyroptosis in ICH, as well as the activation and regulation of inflammasomes and their mediated pyroptosis pathways will be fully discussed in this review.

Nucleic Acid Materials-Mediated Innate Immune Activation for Cancer Immunotherapy

Abnormally localized nucleic acids (NAs) are considered as pathogen associated molecular patterns (PAMPs) in innate immunity. They are recognized by NAs-specific pattern recognition receptors (PRRs), leading to the activation of associated signaling pathways and subsequent production of type I interferons (IFNs) and pro-inflammatory cytokines, which further trigger the adaptive immunity. Notably, NAs-mediated innate immune activation is highly dependent on the conformation changes, especially the aggregation of PRRs. Evidence indicates that the characteristics of NAs including their length, concentration and even spatial structure play essential roles in inducing the aggregation of PRRs. Therefore, nucleic acid materials (NAMs) with high valency of NAs and high-order structures hold great potential for activating innate and adaptive immunity, making them promising candidates for cancer immunotherapy. In recent years, a variety of NAMs have been developed and have demonstrated significant efficacy in achieving satisfactory anti-tumor immunity in multiple mouse models, exhibiting huge potential for clinical application in cancer treatment. This review aims to discuss the mechanisms of NAMs-mediated innate immune response, and summarize their applications in cancer immunotherapy.

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

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

Mechanistic insights from inflammasome structures

Inflammasomes are supramolecular complexes that form in the cytosol in response to pathogen-associated and damage-associated stimuli, as well as other danger signals that perturb cellular homoeostasis, resulting in host defence responses in the form of cytokine release and programmed cell death (pyroptosis). Inflammasome activity is closely associated with numerous human disorders, including rare genetic syndromes of autoinflammation, cardiovascular diseases, neurodegeneration and cancer. In recent years, a range of inflammasome components and their functions have been discovered, contributing to our knowledge of the overall machinery. Here, we review the latest advances in inflammasome biology from the perspective of structural and mechanistic studies. We focus on the most well-studied components of the canonical inflammasome - NAIP-NLRC4, NLRP3, NLRP1, CARD8 and caspase-1 - as well as caspase-4, caspase-5 and caspase-11 of the noncanonical inflammasome, and the inflammasome effectors GSDMD and NINJ1. These structural studies reveal important insights into how inflammasomes are assembled and regulated, and how they elicit the release of IL-1 family cytokines and induce membrane rupture in pyroptosis.

Nlrp6 protects from corticosterone-induced NSPC ferroptosis by modulating RIG-1/MAVS-mediated mitophagy

Hippocampal neural stem/progenitor cells (NSPCs) are highly vulnerable to different stress stimuli, resulting in adult neurogenesis decline and eventual cognitive defects. Our previous study demonstrated that NOD-like receptor family pyrin domain-containing 6 (Nlrp6) highly expressed in NSPCs played a critical role in sustaining hippocampal neurogenesis to resist stress-induced depression, but the underlying mechnistms are still unclear. Here, we found that Nlrp6 depletion led to cognitive defects and hippocampal NSPC loss in mice. RNA-sequencing analysis of the primary NSPCs revealed that Nlrp6 deficiency altered gene expression profiles of mitochondrial energy generation and ferroptotic process. Upon siNlrp6 transfection, as well as corticosterone (CORT) exposure, downregulation of Nlrp6 suppressed retinoic acid-inducible gene I (RIG-1)/mitochondrial antiviral signaling proteins (MAVS)-mediated autophagy, but drove NSPC ferroptotic death. More interesting, short chain fatty acids (SCFAs) upregulated Nlrp6 expression and promoted RIG-1/MAVS-mediated mitophagy, preventing CORT-induced NSPC ferroptosis. Our study further demonstrates that Nlrp6 should be a sensor for RIG-1/MAVS-mediated mitophagy and play a critical role in maintain mitochondrial homeostasis of hippocampal NSPCs. These results suggests that Nlrp6 should be a potential drug target to combat neurodegenerative diseases relative with chronic stress.

Liquid-liquid phase separation in innate immunity

Intrinsic and innate immune responses are essential lines of defense in the body's constant surveillance of pathogens. The discovery of liquid-liquid phase separation (LLPS) as a key regulator of this primal response to infection brings an updated perspective to our understanding of cellular defense mechanisms. Here, we review the emerging multifaceted role of LLPS in diverse aspects of mammalian innate immunity, including DNA and RNA sensing and inflammasome activity. We discuss the intricate regulation of LLPS by post-translational modifications (PTMs), and the subversive tactics used by viruses to antagonize LLPS. This Review, therefore, underscores the significance of LLPS as a regulatory node that offers rapid and plastic control over host immune signaling, representing a promising target for future therapeutic strategies.

Role of pattern recognition receptors in the development of MASLD and potential therapeutic applications

Metabolic dysfunction-associated steatotic liver disease (MASLD) has become one of the most prevalent liver diseases worldwide, and its occurrence is strongly associated with obesity, insulin resistance (IR), genetics, and metabolic stress. Ranging from simple fatty liver to metabolic dysfunction-associated steatohepatitis (MASH), even to severe complications such as liver fibrosis and advanced cirrhosis or hepatocellular carcinoma, the underlying mechanisms of MASLD progression are complex and involve multiple cellular mediators and related signaling pathways. Pattern recognition receptors (PRRs) from the innate immune system, including Toll-like receptors (TLRs), C-type lectin receptors (CLRs), NOD-like receptors (NLRs), RIG-like receptors (RLRs), and DNA receptors, have been demonstrated to potentially contribute to the pathogenesis for MASLD. Their signaling pathways can induce inflammation, mediate oxidative stress, and affect the gut microbiota balance, ultimately resulting in hepatic steatosis, inflammatory injury and fibrosis. Here we review the available literature regarding the involvement of PRR-associated signals in the pathogenic and clinical features of MASLD, in vitro and in animal models of MASLD. We also discuss the emerging targets from PRRs for drug developments that involved agent therapies intended to arrest or reverse disease progression, thus enabling the refinement of therapeutic targets that can accelerate drug development.

A novel pyroptosis-related gene signature exhibits distinct immune cells infiltration landscape in Wilms' tumor

BACKGROUND

Wilms' tumor (WT) is the most common renal tumor in childhood. Pyroptosis, a type of inflammation-characterized and immune-related programmed cell death, has been extensively studied in multiple tumors. In the current study, we aim to construct a pyroptosis-related gene signature for predicting the prognosis of Wilms' tumor.

METHODS

We acquired RNA-seq data from TARGET kidney tumor projects for constructing a gene signature, and snRNA-seq data from GEO database for validating signature-constructing genes. Pyroptosis-related genes (PRGs) were collected from three online databases. We constructed the gene signature by Lasso Cox regression and then established a nomogram. Underlying mechanisms by which gene signature is related to overall survival states of patients were explored by immune cell infiltration analysis, differential expression analysis, and functional enrichment analysis.

RESULTS

A pyroptosis-related gene signature was constructed with 14 PRGs, which has a moderate to high predicting capacity with 1-, 3-, and 5-year area under the curve (AUC) values of 0.78, 0.80, and 0.83, respectively. A prognosis-predicting nomogram was established by gender, stage, and risk score. Tumor-infiltrating immune cells were quantified by seven algorithms, and the expression of CD8( +) T cells, B cells, Th2 cells, dendritic cells, and type 2 macrophages are positively or negatively correlated with risk score. Two single nuclear RNA-seq samples of different histology were harnessed for validation. The distribution of signature genes was identified in various cell types.

CONCLUSIONS

We have established a pyroptosis-related 14-gene signature in WT. Moreover, the inherent roles of immune cells (CD8( +) T cells, B cells, Th2 cells, dendritic cells, and type 2 macrophages), functions of differentially expressed genes (tissue/organ development and intercellular communication), and status of signaling pathways (proteoglycans in cancer, signaling pathways regulating pluripotent of stem cells, and Wnt signaling pathway) have been elucidated, which might be employed as therapeutic targets in the future.

NLRP inflammasomes in health and disease

NLRP inflammasomes are a group of cytosolic multiprotein oligomer pattern recognition receptors (PRRs) involved in the recognition of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) produced by infected cells. They regulate innate immunity by triggering a protective inflammatory response. However, despite their protective role, aberrant NLPR inflammasome activation and gain-of-function mutations in NLRP sensor proteins are involved in occurrence and enhancement of non-communicating autoimmune, auto-inflammatory, and neurodegenerative diseases. In the last few years, significant advances have been achieved in the understanding of the NLRP inflammasome physiological functions and their molecular mechanisms of activation, as well as therapeutics that target NLRP inflammasome activity in inflammatory diseases. Here, we provide the latest research progress on NLRP inflammasomes, including NLRP1, CARD8, NLRP3, NLRP6, NLRP7, NLRP2, NLRP9, NLRP10, and NLRP12 regarding their structural and assembling features, signaling transduction and molecular activation mechanisms. Importantly, we highlight the mechanisms associated with NLRP inflammasome dysregulation involved in numerous human auto-inflammatory, autoimmune, and neurodegenerative diseases. Overall, we summarize the latest discoveries in NLRP biology, their forming inflammasomes, and their role in health and diseases, and provide therapeutic strategies and perspectives for future studies about NLRP inflammasomes.

MNDA, a PYHIN factor involved in transcriptional regulation and apoptosis control in leukocytes

Inflammation control is critical during the innate immune response. Such response is triggered by the detection of molecules originating from pathogens or damaged host cells by pattern-recognition receptors (PRRs). PRRs subsequently initiate intra-cellular signalling through different pathways, resulting in i) the production of inflammatory cytokines, including type I interferon (IFN), and ii) the initiation of a cascade of events that promote both immediate host responses as well as adaptive immune responses. All human PYRIN and HIN-200 domains (PYHIN) protein family members were initially proposed to be PRRs, although this view has been challenged by reports that revealed their impact on other cellular mechanisms. Of relevance here, the human PYHIN factor myeloid nuclear differentiation antigen (MNDA) has recently been shown to directly control the transcription of genes encoding factors that regulate programmed cell death and inflammation. While MNDA is mainly found in the nucleus of leukocytes of both myeloid (neutrophils and monocytes) and lymphoid (B-cell) origin, its subcellular localization has been shown to be modulated in response to genotoxic agents that induce apoptosis and by bacterial constituents, mediators of inflammation. Prior studies have noted the importance of MNDA as a marker for certain forms of lymphoma, and as a clinical prognostic factor for hematopoietic diseases characterized by defective regulation of apoptosis. Abnormal expression of MNDA has also been associated with altered levels of cytokines and other inflammatory mediators. Refining our comprehension of the regulatory mechanisms governing the expression of MNDA and other PYHIN proteins, as well as enhancing our definition of their molecular functions, could significantly influence the management and treatment strategies of numerous human diseases. Here, we review the current state of knowledge regarding PYHIN proteins and their role in innate and adaptive immune responses. Emphasis will be placed on the regulation, function, and relevance of MNDA expression in the control of gene transcription and RNA stability during cell death and inflammation.

NLRP6 deficiency suppresses colorectal cancer liver metastasis growth by modulating M-MDSC-induced immunosuppressive microenvironment

Colorectal cancer liver metastasis (CRLM) a profound influence on the prognosis of patients with colorectal cancer (CRC), prompting a comprehensive inquiry into its underlying mechanisms. Amidst the multifaceted tumor microenvironment, myeloid-derived suppressor cells (MDSCs) have emerged as pivotal orchestrators of immune modulation. However, their specific contributions to the CRLM have not been explored. The role of NLRP6, a member of the NOD-like receptor family, is of interest. Employing a liver metastasis model, our investigation revealed a heightened accumulation of monocytic MDSCs (M-MDSCs) within metastatic sites, culminating in an immunosuppressive milieu characterized by depleted CD8+ T cell populations. Remarkably, the absence of NLRP6 disrupts this intricate immunosuppressive network, highlighting its nuanced role in sculpting the trajectory of CRLM. This study elucidates the interplay between NLRP6 and MDSCs, potentially guiding novel therapeutic strategies to recalibrate the immune microenvironment in CRLM and enhance patient outcomes.

The role of NLRP6 in the development and progression of neurological diseases

The nervous system possesses the remarkable ability to undergo changes in order to store information; however, it is also susceptible to damage caused by invading pathogens or neurodegenerative processes. As a member of nucleotide-binding oligomerization domain-like receptor (NLR) family, the NLRP6 inflammasome serves as a cytoplasmic innate immune sensor responsible for detecting microbe-associated molecular patterns. Upon activation, NLRP6 can recruit the adapter protein apoptosis-associated speck-like protein (ASC) and the inflammatory factors caspase-1 or caspase-11. Consequently, inflammasomes are formed, facilitating the maturation and secretion of pro-inflammatory cytokines such as inflammatory factors-18 (IL-18) and inflammatory factors-1β (IL-1β). Precise regulation of NLRP6 is crucial for maintaining tissue homeostasis, as dysregulated inflammasome activation can contribute to the development of various diseases. Furthermore, NLRP6 may also play a role in the regulation of extraintestinal diseases. In cells of the brain, such as astrocytes and neurons, NLRP6 inflammasome are also present. Here, the assembly and subsequent activation of caspase-1 mediated by NLRP6 contribute to disease progression. This review aims to discuss the structure and function of NLRP6, explain clearly the mechanisms that induce and activate NLRP6, and explore its role within the central and peripheral nervous system.

A mouse model to distinguish NLRP6-mediated inflammasome-dependent and -independent functions

The NOD-like receptor (NLR) family pyrin domain containing 6 (NLRP6) serves as a sensor for microbial dsRNA or lipoteichoic acid (LTA) in intestinal epithelial cells (IECs), and initiating multiple pathways including inflammasome pathway and type I interferon (IFN) pathway, or regulating nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) pathways. NLRP6 can exert its function in both inflammasome-dependent and inflammasome-independent manners. However, there is no tool to distinguish the contribution of individual NLRP6-mediated pathway to the physiology and pathology in vivo. Here, we validated that Arg39 and Trp50 residues in the pyrin domain (PYD) of murine NLRP6 are required for ASC recruitment and inflammasome activation, but are not important for the RNA binding and PYD-independent NLRP6 oligomerization. We further generated the Nlrp6R39E&W50E mutant mice, which showed reduced inflammasome activation in either steady state intestine or during viral infection. However, the type I IFN production in cells or intestine tissue from Nlrp6R39E&W50E mutant mice remain normal. Interestingly, NLRP6-mediated inflammasome activation or the IFN-I production seems to play distinct roles in the defense responses against different types of RNA viruses. Our work generated a useful tool to study the inflammasome-dependent role of NLRP6 in vivo, which might help to understand the complexity of multiple pathways mediated by NLRP6 in response to the complicated and dynamic environmental cues in the intestine.

NLRP12 interacts with NLRP3 to block the activation of the human NLRP3 inflammasome

Inflammasomes are multiprotein complexes that drive inflammation and contribute to protective immunity against pathogens and immune pathology in autoinflammatory diseases. Inflammasomes assemble when an inflammasome scaffold protein senses an activating signal and forms a signaling platform with the inflammasome adaptor protein ASC. The NLRP subfamily of NOD-like receptors (NLRs) includes inflammasome nucleators (such as NLRP3) and also NLRP12, which is genetically linked to familial autoinflammatory disorders that resemble diseases caused by gain-of-function NLRP3 mutants that generate a hyperactive NLRP3 inflammasome. We performed a screen to identify ASC inflammasome-nucleating proteins among NLRs that have the canonical pyrin-NACHT-LRR domain structure. Only NLRP3 and NLRP6 could initiate ASC polymerization to form "specks," and NLRP12 failed to nucleate ASC polymerization. However, wild-type NLRP12 inhibited ASC inflammasome assembly induced by wild-type and gain-of-function mutant NLRP3, an effect not seen with disease-associated NLRP12 mutants. The capacity of NLRP12 to suppress NLRP3 inflammasome assembly was limited to human NLRP3 and was not observed for wild-type murine NLRP3. Furthermore, peripheral blood mononuclear cells from patients with an NLRP12 mutant-associated inflammatory disorder produced increased amounts of the inflammatory cytokine IL-1β in response to NLRP3 stimulation. Thus, our findings provide insights into NLRP12 biology and suggest that NLRP3 inhibitors in clinical trials for NLRP3-driven diseases may also be effective in treating NLRP12-associated autoinflammatory diseases.

Design principles for inflammasome inhibition by pyrin-only-proteins

Inflammasomes are filamentous signaling platforms essential for host defense against various intracellular calamities such as pathogen invasion and genotoxic stresses. However, dysregulated inflammasomes cause an array of human diseases including autoinflammatory disorders and cancer. It was recently identified that endogenous pyrin-only-proteins (POPs) regulate inflammasomes by directly inhibiting their filament assembly. Here, by combining Rosetta in silico, in vitro, and in cellulo methods, we investigate the target specificity and inhibition mechanisms of POPs. We find here that POP1 is ineffective in directly inhibiting the central inflammasome adaptor ASC. Instead, POP1 acts as a decoy and targets the assembly of upstream receptor pyrin-domain (PYD) filaments such as those of AIM2, IFI16, NLRP3, and NLRP6. Moreover, not only does POP2 directly suppress the nucleation of ASC, but it can also inhibit the elongation of receptor filaments. In addition to inhibiting the elongation of AIM2 and NLRP6 filaments, POP3 potently suppresses the nucleation of ASC. Our Rosetta analyses and biochemical experiments consistently suggest that a combination of favorable and unfavorable interactions between POPs and PYDs is necessary for effective recognition and inhibition. Together, we reveal the intrinsic target redundancy of POPs and their inhibitory mechanisms.

The role of inflammasomes in human diseases and their potential as therapeutic targets

Inflammasomes are large protein complexes that play a major role in sensing inflammatory signals and triggering the innate immune response. Each inflammasome complex has three major components: an upstream sensor molecule that is connected to a downstream effector protein such as caspase-1 through the adapter protein ASC. Inflammasome formation typically occurs in response to infectious agents or cellular damage. The active inflammasome then triggers caspase-1 activation, followed by the secretion of pro-inflammatory cytokines and pyroptotic cell death. Aberrant inflammasome activation and activity contribute to the development of diabetes, cancer, and several cardiovascular and neurodegenerative disorders. As a result, recent research has increasingly focused on investigating the mechanisms that regulate inflammasome assembly and activation, as well as the potential of targeting inflammasomes to treat various diseases. Multiple clinical trials are currently underway to evaluate the therapeutic potential of several distinct inflammasome-targeting therapies. Therefore, understanding how different inflammasomes contribute to disease pathology may have significant implications for developing novel therapeutic strategies. In this article, we provide a summary of the biological and pathological roles of inflammasomes in health and disease. We also highlight key evidence that suggests targeting inflammasomes could be a novel strategy for developing new disease-modifying therapies that may be effective in several conditions.

NLRP6 potentiates PI3K/AKT signalling by promoting autophagic degradation of p85α to drive tumorigenesis

The PI3K/AKT pathway plays an essential role in tumour development. NOD-like receptors (NLRs) regulate innate immunity and are implicated in cancer, but whether they are involved in PI3K/AKT pathway regulation is poorly understood. Here, we report that NLRP6 potentiates the PI3K/AKT pathway by binding and destabilizing p85α, the regulatory subunit of PI3K. Mechanistically, NLRP6 recruits the E3 ligase RBX1 to p85α and ubiquitinates lysine 256 on p85α, which is recognized by the autophagy cargo receptor OPTN, causing selective autophagic degradation of p85α and subsequent activation of the PI3K/AKT pathway by reducing PTEN stability. We further show that loss of NLRP6 suppresses cell proliferation, colony formation, cell migration, and tumour growth in glioblastoma cells in vitro and in vivo. Disruption of the NLRP6/p85α interaction using the Pep9 peptide inhibits the PI3K/AKT pathway and generates potent antitumour effects. Collectively, our results suggest that NLRP6 promotes p85α degradation via selective autophagy to drive tumorigenesis, and the interaction between NLRP6 and p85α can be a promising therapeutic target for tumour treatment.

Dysregulation of inflammasome activation in glioma

Gliomas are the most common brain tumors characterized by complicated heterogeneity. The genetic, molecular, and histological pathology of gliomas is characterized by high neuro-inflammation. The inflammatory microenvironment in the central nervous system (CNS) has been closely linked with inflammasomes that control the inflammatory response and coordinate innate host defenses. Dysregulation of the inflammasome causes an abnormal inflammatory response, leading to carcinogenesis in glioma. Because of the clinical importance of the various physiological properties of the inflammasome in glioma, the inflammasome has been suggested as a promising treatment target for glioma management. Here, we summarize the current knowledge on the contribution of the inflammasomes in glioma and therapeutic insights. Video Abstract.

Structural basis for negative regulation of the Escherichia coli maltose system

Proteins from the signal transduction ATPases with numerous domains (STAND) family are known to play an important role in innate immunity. However, it remains less well understood how they function in transcriptional regulation. MalT is a bacterial STAND that controls the Escherichia coli maltose system. Inactive MalT is sequestered by different inhibitory proteins such as MalY. Here, we show that MalY interacts with one oligomerization interface of MalT to form a 2:2 complex. MalY represses MalT activity by blocking its oligomerization and strengthening ADP-mediated MalT autoinhibition. A loop region N-terminal to the nucleotide-binding domain (NBD) of MalT has a dual role in mediating MalT autoinhibition and activation. Structural comparison shows that ligand-binding induced oligomerization is required for stabilizing the C-terminal domains and conferring DNA-binding activity. Together, our study reveals the mechanism whereby a prokaryotic STAND is inhibited by a repressor protein and offers insights into signaling by STAND transcription activators.

Spotlight on NLRP6 and Tumor Research Situation: A Potential Cancer Participant

NOD-like receptor family pyrin domain containing 6 (NLRP6) is a new pattern recognition receptor in the mammalian innate immune system. Both the liver and the gut exhibit substantial levels of cytoplasmic expression. It can speed up cell response to endogenous danger signals or exogenous pathogen infection. NLRP6 can function in various ways as an inflammasome or a noninflammasome. The understanding of NLRP6 is steadily increasing thanks to ongoing investigations, but due to discrepancies in how those studies have described their link with tumors, the significance of NLRP6 in the emergence of cancer is still debatable as of this writing. This article will use the structure and function of NLRP6 as the pivotal point and thoroughly explain the present interactions between NLRP6 and tumors and any possible clinical benefits.

Intestinal mucus components and secretion mechanisms: what we do and do not know

Damage to the colon mucus barrier, the first line of defense against microorganisms, is an important determinant of intestinal diseases such as inflammatory bowel disease and colorectal cancer, and disorder in extraintestinal organs. The mucus layer has attracted the attention of the scientific community in recent years, and with the discovery of new mucosal components, it has become increasingly clear that the mucosal barrier is a complex system composed of many components. Moreover, certain components are jointly involved in regulating the structure and function of the mucus barrier. Therefore, a comprehensive and systematic understanding of the functional components of the mucus layer is clearly warranted. In this review, we summarize the various functional components of the mucus layer identified thus far and describe their unique roles in shaping mucosal structure and function. Furthermore, we detail the mechanisms underlying mucus secretion, including baseline and stimulated secretion. In our opinion, baseline secretion can be categorized into spontaneous Ca²⁺ oscillation-mediated slow and continuous secretion and stimulated secretion, which is mediated by massive Ca²⁺ influx induced by exogenous stimuli. This review extends the current understanding of the intestinal mucus barrier, with an emphasis on host defense strategies based on fortification of the mucus layer.

Role of the inflammasome in insulin resistance and type 2 diabetes mellitus

The inflammasome is a protein complex composed of a variety of proteins in cells and which participates in the innate immune response of the body. It can be activated by upstream signal regulation and plays an important role in pyroptosis, apoptosis, inflammation, tumor regulation, etc. In recent years, the number of metabolic syndrome patients with insulin resistance (IR) has increased year by year, and the inflammasome is closely related to the occurrence and development of metabolic diseases. The inflammasome can directly or indirectly affect conduction of the insulin signaling pathway, involvement the occurrence of IR and type 2 diabetes mellitus (T2DM). Moreover, various therapeutic agents also work through the inflammasome to treat with diabetes. This review focuses on the role of inflammasome on IR and T2DM, pointing out the association and utility value. Briefly, we have discussed the main inflammasomes, including NLRP1, NLRP3, NLRC4, NLRP6 and AIM2, as well as their structure, activation and regulation in IR were described in detail. Finally, we discussed the current therapeutic options-associated with inflammasome for the treatment of T2DM. Specially, the NLRP3-related therapeutic agents and options are widely developed. In summary, this article reviews the role of and research progress on the inflammasome in IR and T2DM.

Pyroptosis and degenerative diseases of the elderly

Pyroptosis is a recently described mechanism of programmed cell death mediated by proteins of the gasdermin family. Widely recognized signaling cascades include the classical, non-classical, caspase-3-dependent gasdermin E and caspase-8-dependent gasdermin D pathways. Additional pyroptotic pathways have been subsequently reported. With the rising prevalence of advanced age, the role of pyroptosis in the degenerative diseases of the elderly has attracted increased research attention. This article reviews the primary mechanisms of pyroptosis and summarizes progress in the research of degenerative diseases of the elderly such as presbycusis, age-related macular degeneration, Alzheimer's disease, intervertebral disc degeneration, and osteoarthritis.

The NLRP6 protein is very faintly expressed in several normal and cancerous epithelial cells and may be confused with an unrelated protein

Nod-Like Receptor Pyrin domain-containing protein 6 (NLRP6), a member of the Nucleotide-oligomerization domain-Like Receptor (NLR) family of proteins, assembles together with the ASC protein to form an inflammasome upon stimulation by bacterial lipoteichoic acid and double-stranded DNA. Besides its expression in myeloid cells, NLRP6 is also expressed in intestinal epithelial cells where it may contribute to the maintenance of gut homeostasis and negatively controls colorectal tumorigenesis. Here, we report that NLRP6 is very faintly expressed in several colon cancer cell lines, detected only in cytoplasmic small dots were it colocalizes with ASC. Consequently, it is very hardly detected by standard western-blotting techniques by several presently available commercial antibodies which, in contrast, highly cross-react with a protein of 90kDa that we demonstrate to be unrelated to NLRP6. We report here these results to caution the community not to confuse the 90kDa protein with the endogenous human NLRP6.

Cryo-EM structures of the active NLRP3 inflammasome disc

Inflammasomes are cytosolic innate immune complexes that activate caspase-1 following detection of pathogenic and endogenous dangers1-5, and NACHT-, leucine-rich repeat (LRR)- and pyrin domain (PYD)-containing protein  3 (NLRP3) is an inflammasome sensor of membrane damage highly important in regard to the induction of inflammation2,6,7. Here we report cryogenic electron microscopy structures of disc-shaped active NLRP3 oligomers in complex with adenosine 5'-O-(3-thio)triphosphate, the centrosomal NIMA-related kinase 7 (NEK7) and the adaptor protein ASC, which recruits caspase-1. In these NLRP3-NEK7-ASC complexes, the central NACHT domain of NLRP3 assumes an ATP-bound conformation in which two of its subdomains rotate by about 85° relative to the ADP-bound inactive conformation8-12. The fish-specific NACHT-associated domain conserved in NLRP3 but absent in most NLRPs13 becomes ordered in its key regions to stabilize the active NACHT conformation and mediate most interactions in the disc. Mutations on these interactions compromise NLRP3-mediated caspase-1 activation. The N-terminal PYDs from all NLRP3 subunits combine to form a PYD filament that recruits ASC PYD to elicit downstream signalling. Surprisingly, the C-terminal LRR domain and the LRR-bound NEK7 do not participate in disc interfaces. Together with previous structures of an inactive NLRP3 cage in which LRR-LRR interactions play an important role8-11, we propose that the role of NEK7 is to break the inactive cage to transform NLRP3 into the active NLRP3 inflammasome disc.

Qing-Chang-Hua-Shi granule ameliorates DSS-induced colitis by activating NLRP6 signaling and regulating Th17/Treg balance

BACKGROUND

Chinese herbal medicine Qing-Chang-Hua-Shi granule (QCHS) is widely used to treat ulcerative colitis in China. However, the molecular mechanisms of QCHS remains largely unknown.

PURPOSE

To assess the therapeutic effects of QCHS on colitis and to reveal its mechanisms of action.

METHODS

The main components of QCHS were identified using a UHPLC-QTOF-MS method and the efficacy of QCHS was evaluated using an DSS-induced mice model. The inflammatory responses and mucosal integrity in colon were comprehensively assessed. Flow cytometry was used to analysis the proportion of Th17 and Treg cells. Detect the signal transduction of the NOD-like receptor family pyrin domain containing 6 (NLRP6) both in vitro and in vivo. Furthermore, siNLRP6 transfection was used to validate the functional targets of QCHS.

RESULTS

QCHS treatment significantly alleviated colitis in mice by improving symptoms and pathological damage. Moreover, QCHS treatment suppressed the inflammatory response and preserved the integrity of colon tissue. Most importantly, QCHS balanced the Th17/Treg response of UC mice. Mechanistically, by activating NLRP6 inflammasome pathway, QCHS regulated the maturation of interleukin (IL)-1β and IL-18 to affect inflammation and drive Th17 cell differentiation.

CONCLUSIONS

The effect of QCHS on UC mice is dose-dependent, with high-dose QCHS being superior to 5-Aminosalicylic acid (200 mg/kg/day). QCHS acts through the NLRP6 signaling pathway to modulate Th17/Treg balance, resulting in the protective effects against colitis. This study investigated the relevant pharmacological mechanisms of QCHS, providing further evidence for the application of QCHS in UC treatment.

NLRP6 negatively regulates type 2 immune responses in mice

BACKGROUND

Inflammasomes are large protein complexes that assemble in the cytosol in response to danger such as tissue damage or infection. Following activation, inflammasomes trigger cell death and the release of biologically active forms of pro-inflammatory cytokines interleukin (IL)-1β and IL-18. NOD-like receptor family pyrin domain containing 6 (NLRP6) inflammasome is required for IL-18 secretion by intestinal epithelial cells, macrophages, and T cells, contributing to homeostasis and self-defense against pathogenic microbes. However, the involvement of NLRP6 in type 2 lung inflammation remains elusive.

METHODS

Wild-type (WT) and Nlrp6^-/- mice were used. Birch pollen extract (BPE)-induced allergic lung inflammation, eosinophil recruitment, Th2-related cytokine and chemokine production, airway hyperresponsiveness, and lung histopathology, Th2 cell differentiation, GATA3, and Th2 cytokines expression, were determined. Nippostrongylus brasiliensis (Nb) infection, worm count in intestine, type 2 innate lymphoid cell (ILC2), and Th2 cells in lungs were evaluated.

RESULTS

We demonstrate in Nlrp6^-/- mice that a mixed Th2/Th17 immune responses prevailed following birch pollen challenge with increased eosinophils, ILC2, Th2, and Th17 cell induction and reduced IL-18 production. Nippostrongylus brasiliensis infected Nlrp6^-/- mice featured enhanced early expulsion of the parasite due to enhanced type 2 immune responses compared to WT hosts. In vitro, NLRP6 repressed Th2 polarization, as shown by increased Th2 cytokines and higher expression of the transcription factor GATA3 in the absence of NLRP6. Exogenous IL-18 administration partially reduced the enhanced airways inflammation in Nlrp6^-/- mice.

CONCLUSIONS

In summary, our data identify NLRP6 as a negative regulator of type 2 immune responses.

Activation and regulation mechanisms of NOD-like receptors based on structural biology

Innate immunity is a primary defense system against microbial infections. Innate immune pattern recognition receptors (PRRs) play pivotal roles in detection of invading pathogens. When pathogens, such as bacteria and viruses, invade our bodies, their components are recognized by PRRs as pathogen-associated molecular patterns (PAMPs), activating the innate immune system. Cellular components such as DNA and RNA, acting as damage-associated molecular patterns (DAMPs), also activate innate immunity through PRRs under certain conditions. Activation of PRRs triggers inflammatory responses, interferon-mediated antiviral responses, and the activation of acquired immunity. Research on innate immune receptors is progressing rapidly. A variety of these receptors has been identified, and their regulatory mechanisms have been elucidated. Nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) constitute a major family of intracellular PRRs and are involved in not only combating pathogen invasion but also maintaining normal homeostasis. Some NLRs are known to form multi-protein complexes called inflammasomes, a process that ultimately leads to the production of inflammatory cytokines and induces pyroptosis through the proteolytic cascade. The aberrant activation of NLRs has been found to be associated with autoimmune diseases. Therefore, NLRs are considered targets for drug discovery, such as for antiviral drugs, immunostimulants, antiallergic drugs, and autoimmune disease drugs. This review summarizes our recent understanding of the activation and regulation mechanisms of NLRs, with a particular focus on their structural biology. These include NOD2, neuronal apoptosis inhibitory protein (NAIP)/NLRC4, NLR family pyrin domain containing 1 (NLRP1), NLRP3, NLRP6, and NLRP9. NLRs are involved in a variety of diseases, and their detailed activation mechanisms based on structural biology can aid in developing therapeutic agents in the future.

Physiological and pathophysiological functions of NLRP6: pro- and anti-inflammatory roles

The nucleotide-binding oligomerization and leucine-rich repeat receptor (NLR) protein family consists of important immune sensors that form inflammasomes, a cytosolic multi-protein platform that induces caspase-1 activation and is involved in different inflammatory pathologies. The NLR family pyrin domain containing 6 (NLRP6) is a receptor that can signal by forming inflammasomes, but which can also play an important role without forming inflammasomes. NLRP6 regulates intestinal homeostasis and inflammation, but also is involved in cancer, the nervous system or liver diseases, with both protective and deleterious consequences. In the present article, we review the different roles of NLRP6 in these processes and offer new insights into NLRP6 activation.

This review discusses emerging roles for the NLR family pyrin domain containing 6 receptor (NLRP6) in intestinal homeostasis, inflammation, cancer, the nervous system and liver disease.

Insights into inflammasome regulation: cellular, molecular, and pathogenic control of inflammasome activation

Maintenance of immune homeostasis is an intricate process wherein inflammasomes play a pivotal role by contributing to innate and adaptive immune responses. Inflammasomes are ensembles of adaptor proteins that can trigger a signal following innate sensing of pathogens or non-pathogens eventuating in the inductions of IL-1β and IL-18. These inflammatory cytokines substantially influence the antigen-presenting cell's costimulatory functions and T helper cell differentiation, contributing to adaptive immunity. As acute and chronic disease conditions may accompany parallel tissue damage, we analyze the critical role of extracellular factors such as cytokines, amyloids, cholesterol crystals, etc., intracellular metabolites, and signaling molecules regulating inflammasome activation/inhibition. We develop an operative framework for inflammasome function and regulation by host cell factors and pathogens. While inflammasomes influence the innate and adaptive immune components' interplay modulating the anti-pathogen adaptive immune response, pathogens may target inflammasome inhibition as a survival strategy. As trapped between health and diseases, inflammasomes serve as promising therapeutic targets and their modus operandi serves as a scientific rationale for devising better therapeutic strategies.

Directionality of PYD filament growth determined by the transition of NLRP3 nucleation seeds to ASC elongation

Inflammasomes sense intrinsic and extrinsic danger signals to trigger inflammatory responses and pyroptotic cell death. Homotypic pyrin domain (PYD) interactions of inflammasome forming nucleotide-binding oligomerization domain (NOD)-like receptors with the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD) mediate oligomerization into filamentous assemblies. We describe the cryo-electron microscopy (cryo-EM) structure of the human NLRP3PYD filament and identify a pattern of highly polar interface residues that form the homomeric interactions leading to characteristic filament ends designated as A- and B-ends. Coupling a titration polymerization assay to cryo-EM, we demonstrate that ASC adaptor protein elongation on NLRP3PYD nucleation seeds is unidirectional, associating exclusively to the B-end of the filament. Notably, NLRP3 and ASC PYD filaments exhibit the same symmetry in rotation and axial rise per subunit, allowing a continuous transition between NLRP3 and ASC. Integrating the directionality of filament growth, we present a molecular model of the ASC speck consisting of active NLRP3, ASC, and Caspase-1 proteins.

Cardiac Remodeling in Heart Failure: Role of Pyroptosis and Its Therapeutic Implications

Pyroptosis is a kind of programmed cell death closely related to inflammation. The pathways that mediate pyroptosis can be divided into the Caspase-1-dependent canonical pathway and the Caspase4/5/11-dependent non-canonical pathway. The most significant difference from other cell death is that pyroptosis rapidly causes rupture of the plasma membrane, cell expansion, dissolution and rupture of the cell membrane, the release of cell contents and a large number of inflammatory factors, and send pro-inflammatory signals to adjacent cells, recruit inflammatory cells and induce inflammatory responses. Cardiac remodeling is the basic mechanism of heart failure (HF) and the core of pathophysiological research on the underlying mechanism. A large number of studies have shown that pyroptosis can cause cardiac fibrosis, cardiac hypertrophy, cardiomyocytes death, myocardial dysfunction, excessive inflammation, and cardiac remodeling. Therefore, targeting pyroptosis has a good prospect in improving cardiac remodeling in HF. In this review, the basic molecular mechanism of pyroptosis is summarized, the relationship between pyroptosis and cardiac remodeling in HF is analyzed in-depth, and the potential therapy of targeting pyroptosis to improve adverse cardiac remodeling in HF is discussed, providing some ideas for improving the study of adverse cardiac remodeling in HF.

SPARC: Structural properties associated with residue constraints

SPARC facilitates the generation of plausible hypotheses regarding underlying biochemical mechanisms by structurally characterizing protein sequence constraints. Such constraints appear as residues co-conserved in functionally related subgroups, as subtle pairwise correlations (i.e., direct couplings), and as correlations among these sequence features or with structural features. SPARC performs three types of analyses. First, based on pairwise sequence correlations, it estimates the biological relevance of alternative conformations and of homomeric contacts, as illustrated here for death domains. Second, it estimates the statistical significance of the correspondence between directly coupled residue pairs and interactions at heterodimeric interfaces. Third, given molecular dynamics simulated structures, it characterizes interactions among constrained residues or between such residues and ligands that: (a) are stably maintained during the simulation; (b) undergo correlated formation and/or disruption of interactions with other constrained residues; or (c) switch between alternative interactions. We illustrate this for two homohexameric complexes: the bacterial enhancer binding protein (bEBP) NtrC1, which activates transcription by remodeling RNA polymerase (RNAP) containing σ⁵⁴, and for DnaB helicase, which opens DNA at the bacterial replication fork. Based on the NtrC1 analysis, we hypothesize possible mechanisms for inhibiting ATP hydrolysis until ADP is released from an adjacent subunit and for coupling ATP hydrolysis to restructuring of σ⁵⁴ binding loops. Based on the DnaB analysis, we hypothesize that DnaB 'grabs' ssDNA by flipping every fourth base and inserting it into cavities between subunits and that flipping of a DnaB-specific glutamine residue triggers ATP hydrolysis.

The structure of NLRP9 reveals a unique C-terminal region with putative regulatory function

Nucleotide-binding and oligomerization domain-like receptors (NLRs) can form inflammasomes that activate caspase-1 and pro-interleukin-1β and induce pyroptosis. NLR family pyrin domain-containing 9 (NLRP9) forms an inflammasome and activates innate immune responses during virus infection, but little is known about this process. Here, we report the crystal and cryo-electron microscopy structures of NLRP9 in an ADP-bound state, revealing inactive and closed conformations of NLRP9 and its similarities to other structurally characterised NLRs. Moreover, we found a C-terminal region interacting with the concave surface of the leucine-rich repeat domain of NLRP9. This region is unique among NLRs and might be involved in the specific function of NLRP9. These data provide the structural basis for understanding the mechanism of NLRP9 regulation and activation.

Involvement of Inflammasome Components in Kidney Disease

Inflammasomes are multiprotein complexes with an important role in the innate immune response. Canonical activation of inflammasomes results in caspase-1 activation and maturation of cytokines interleukin-1β and -18. These cytokines can elicit their effects through receptor activation, both locally within a certain tissue and systemically. Animal models of kidney diseases have shown inflammasome involvement in inflammation, pyroptosis and fibrosis. In particular, the inflammasome component nucleotide-binding domain-like receptor family pyrin domain containing 3 (NLRP3) and related canonical mechanisms have been investigated. However, it has become increasingly clear that other inflammasome components are also of importance in kidney disease. Moreover, it is becoming obvious that the range of molecular interaction partners of inflammasome components in kidney diseases is wide. This review provides insights into these current areas of research, with special emphasis on the interaction of inflammasome components and redox signalling, endoplasmic reticulum stress, and mitochondrial function. We present our findings separately for acute kidney injury and chronic kidney disease. As we strictly divided the results into preclinical and clinical data, this review enables comparison of results from those complementary research specialities. However, it also reveals that knowledge gaps exist, especially in clinical acute kidney injury inflammasome research. Furthermore, patient comorbidities and treatments seem important drivers of inflammasome component alterations in human kidney disease.

NLRP3 cages revealed by full-length mouse NLRP3 structure control pathway activation

The NACHT-, leucine-rich-repeat- (LRR), and pyrin domain-containing protein 3 (NLRP3) is emerging to be a critical intracellular inflammasome sensor of membrane integrity and a highly important clinical target against chronic inflammation. Here, we report that an endogenous, stimulus-responsive form of full-length mouse NLRP3 is a 12- to 16-mer double-ring cage held together by LRR-LRR interactions with the pyrin domains shielded within the assembly to avoid premature activation. Surprisingly, this NLRP3 form is predominantly membrane localized, which is consistent with previously noted localization of NLRP3 at various membrane organelles. Structure-guided mutagenesis reveals that trans-Golgi network dispersion into vesicles, an early event observed for many NLRP3-activating stimuli, requires the double-ring cages of NLRP3. Double-ring-defective NLRP3 mutants abolish inflammasome punctum formation, caspase-1 processing, and cell death. Thus, our data uncover a physiological NLRP3 oligomer on the membrane that is poised to sense diverse signals to induce inflammasome activation.

Phase separation drives RNA virus-induced activation of the NLRP6 inflammasome

NLRP6 is important in host defense by inducing functional outcomes including inflammasome activation and interferon production. Here, we show that NLRP6 undergoes liquid-liquid phase separation (LLPS) upon interaction with double-stranded RNA (dsRNA) in vitro and in cells, and an intrinsically disordered poly-lysine sequence (K350-354) of NLRP6 is important for multivalent interactions, phase separation, and inflammasome activation. Nlrp6-deficient or Nlrp6^K350-354A mutant mice show reduced inflammasome activation upon mouse hepatitis virus or rotavirus infection, and in steady state stimulated by intestinal microbiota, implicating NLRP6 LLPS in anti-microbial immunity. Recruitment of ASC via helical assembly solidifies NLRP6 condensates, and ASC further recruits and activates caspase-1. Lipoteichoic acid, a known NLRP6 ligand, also promotes NLRP6 LLPS, and DHX15, a helicase in NLRP6-induced interferon signaling, co-forms condensates with NLRP6 and dsRNA. Thus, LLPS of NLRP6 is a common response to ligand stimulation, which serves to direct NLRP6 to distinct functional outcomes depending on the cellular context.

Gasdermin D in pyroptosis

Pyroptosis is the process of inflammatory cell death. The primary function of pyroptosis is to induce strong inflammatory responses that defend the host against microbe infection. Excessive pyroptosis, however, leads to several inflammatory diseases, including sepsis and autoimmune disorders. Pyroptosis can be canonical or noncanonical. Upon microbe infection, the canonical pathway responds to pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), while the noncanonical pathway responds to intracellular lipopolysaccharides (LPS) of Gram-negative bacteria. The last step of pyroptosis requires the cleavage of gasdermin D (GsdmD) at D275 (numbering after human GSDMD) into N- and C-termini by caspase 1 in the canonical pathway and caspase 4/5/11 (caspase 4/5 in humans, caspase 11 in mice) in the noncanonical pathway. Upon cleavage, the N-terminus of GsdmD (GsdmD-N) forms a transmembrane pore that releases cytokines such as IL-1β and IL-18 and disturbs the regulation of ions and water, eventually resulting in strong inflammation and cell death. Since GsdmD is the effector of pyroptosis, promising inhibitors of GsdmD have been developed for inflammatory diseases. This review will focus on the roles of GsdmD during pyroptosis and in diseases.

Pyroptosis and Redox Balance in Kidney Diseases

Significance: Kidney diseases remain a worldwide public health problem resulting in millions of deaths each year; they are characterized by progressive destruction of renal function by sustained inflammation. Pyroptosis is a lytic type of programmed cell death involved in inflammation, as well as a key fibrotic mechanism that is critical in the development of kidney pathology. Pyroptosis is induced by the cleavage of Gasdermins by various caspases and is executed by the insertion of the N-terminal fragment of cleaved Gasdermins into the plasma membrane, creating oligomeric pores and allowing the release of diverse proinflammatory products into the extracellular space. Inflammasomes are multiprotein complexes leading to the activation of caspase-1, which will cleave Gasdermin D, releasing several proinflammatory cytokines; this results in the initiation and amplification of the inflammatory response. Recent Advances: The efficacy of Gasdermin D cleavage is reduced by a change in the redox balance. Recently, several studies have shown that the attenuation of reactive oxygen species (ROS) production induced by antioxidant pathways results in a reduction of renal pyroptosis. In this review, we discuss the role of pyroptosis in the pathogenesis of chronic kidney disease (CKD) and acute kidney disease; summarize the clinical outcomes and different molecular mechanisms leading to Gasdermin activation; and examine studies about the capacity of antioxidants, particularly Nrf2 activators, to ameliorate Gasdermin activity. Future Directions: We illustrate the potential influence of the deregulation of redox balance on inflammasome activity and pyroptosis as a novel therapeutic approach for the treatment of kidney diseases. Antioxid. Redox Signal. 35, 40-60.

Hepatocyte-specific deletion of Nlrp6 in mice exacerbates the development of non-alcoholic steatohepatitis

OBJECTIVE

Previous studies have established that deficiency in Nucleotide-binding and oligomerization domain (NOD)-like receptor family pyrin domain containing 6 (Nlrp6) changes the configuration of the gut microbiota, which leads to hepatic steatosis. Here, we aimed to determine the hepatic function of Nlrp6 in lipid metabolism and inflammation and its role in the development of non-alcoholic steatohepatitis (NASH).

METHODS

Nlrp6^Loxp/Loxp and hepatocyte-specific Nlrp6-knockout mice were fed a high-fat diet (HFD) or methionine-choline deficient (MCD) diet to induce fatty liver or steatohepatitis, respectively. Primary hepatocytes were isolated to further explore the underlying mechanisms in vitro. In addition, we used adenovirus to overexpress Nlrp6 in ob/ob mice to demonstrate its role in NASH.

RESULTS

Hepatic Nlrp6 expression was downregulated in NASH patients and in obese mice. Hepatocyte-specific Nlrp6 deficiency promoted HFD- or MCD diet-induced lipid accumulation and inflammation, whereas Nlrp6 overexpression in ob/ob mice had beneficial effects. In vitro studies demonstrated that knockdown of Nlrp6 aggravated hepatic steatosis and inflammation in hepatocytes, but its overexpression markedly attenuated these abnormalities. Moreover, both in vitro and in vivo study demonstrated that Nlrp6 inhibited Cd36-mediated lipid uptake. Nlrp6 deficiency-enhanced fatty acid uptake was blocked by a Cd36 inhibitor in hepatocytes. Nlrp6 ablation increased the expression of proinflammatory cytokines, likely as a result of increased NF-κB phosphorylation and activation. Mechanistically, Nlrp6 promoted the degradation of transforming growth factor-β-activated kinase 1 (TAK1)-binding protein 2/3 (TAB2/3) via a lysosomal-dependent pathway, which suppressed NF-κB activation.

CONCLUSIONS

Nlrp6 may play a key role in the pathological process of NASH by inhibiting Cd36 and NF-κB pathways. It may be a potential therapeutic target for NASH.

Distinct axial and lateral interactions within homologous filaments dictate the signaling specificity and order of the AIM2-ASC inflammasome

Inflammasomes are filamentous signaling platforms integral to innate immunity. Currently, little is known about how these structurally similar filaments recognize and distinguish one another. A cryo-EM structure of the AIM2PYD filament reveals that the architecture of the upstream filament is essentially identical to that of the adaptor ASCPYD filament. In silico simulations using Rosetta and molecular dynamics followed by biochemical and cellular experiments consistently demonstrate that individual filaments assemble bidirectionally. By contrast, the recognition between AIM2 and ASC requires at least one to be oligomeric and occurs in a head-to-tail manner. Using in silico mutagenesis as a guide, we also identify specific axial and lateral interfaces that dictate the recognition and distinction between AIM2 and ASC filaments. Together, the results here provide a robust framework for delineating the signaling specificity and order of inflammasomes.

The intestinal parasite Cryptosporidium is controlled by an enterocyte intrinsic inflammasome that depends on NLRP6

The intestinal immune system is able to control pathogens while tolerating and interpreting microbial cues from an abundant microbiome. The mechanisms of innate recognition are crucial to differentiating between pathogen and commensal in this tissue and to mounting an appropriate inflammatory response. Persistent inflammation can alter the cellular architecture and physiology of the gut and have lasting impact on the nutritional state of children who face frequent infection with certain enteric pathogens. We demonstrate that the widespread parasite Cryptosporidium acts as a potent trigger for an enterocyte-intrinsic inflammasome that depends on the NOD-like receptor pyrin domain-6 and results in the local release of the proinflammatory cytokine IL-18.

The apicomplexan parasite Cryptosporidium infects the intestinal epithelium. While infection is widespread around the world, children in resource-poor settings suffer a disproportionate disease burden. Cryptosporidiosis is a leading cause of diarrheal disease, responsible for mortality and stunted growth in children. CD4 T cells are required to resolve this infection, but powerful innate mechanisms control the parasite prior to the onset of adaptive immunity. Here, we use the natural mouse pathogen Cryptosporidium tyzzeri to demonstrate that the inflammasome plays a critical role in initiating this early response. Mice lacking core inflammasome components, including caspase-1 and apoptosis-associated speck-like protein, show increased parasite burden and caspase 1 deletion solely in enterocytes phenocopies whole-body knockout (KO). This response was fully functional in germfree mice and sufficient to control Cryptosporidium infection. Inflammasome activation leads to the release of IL-18, and mice that lack IL-18 are more susceptible to infection. Treatment of infected caspase 1 KO mice with recombinant IL-18 is remarkably efficient in rescuing parasite control. Notably, NOD-like receptor family pyrin domain containing 6 (NLRP6) was the only NLR required for innate parasite control. Taken together, these data support a model of innate recognition of Cryptosporidium infection through an NLRP6-dependent and enterocyte-intrinsic inflammasome that leads to the release of IL-18 required for parasite control.

Therapeutic implications of inflammasome in inflammatory bowel disease

Inflammatory bowel disease (IBD) remains a persistent health problem with a global burden surging over 6.8 million cases currently. Clinical pathology of IBD is complicated; however, hyperactive inflammatory and immune responses in the gut is shown to be one of the persistent causes of the disease. Human gut inflammasome, the activator of innate immune system is believed to be a primary underlying cause for the pathology and is largely associated with the progression of IBD. To manage IBD, there is a need to fully understand the role of inflammasome activation in IBD. Since inflammasome potentially play a significant role in IBD, systemic modulation of inflammasome may provide an effective therapeutic and clinical approach to control IBD symptoms. In this review, we have focused on this association between IBD and gut inflammasome, and recent advances in the research and therapeutic strategies for IBD. We have discussed inflammasomes and their components, outcomes from the experimental animals and human studies, inflammasome inhibitors, and developments in the inflammasome-targeted therapies for IBD.

NLRP6 in host defense and intestinal inflammation

NLRP6 is a member of the NLR (nucleotide-oligomerization domain-like receptor) family of proteins that recognize pathogen-derived factors and damage-associated molecular patterns in the cytosol. The function of NLRP6 has been attributed to the maintenance of epithelial integrity and host defense against microbial infections. Under some physiological conditions, NLRP6 forms a complex with ASC and caspase-1 or caspase-11 to form an inflammasome complex cleaving pro-interleukin-1β (IL-1β) and IL-18 into their biologically active forms. Here, we summarize recent advances in the understanding of the mechanisms of activation of the NLRP6 inflammasome and discuss its relevance to human disease.

Techniques to Study Inflammasome Activation and Inhibition by Small Molecules

Inflammasomes are immune cytosolic oligomers involved in the initiation and progression of multiple pathologies and diseases. The tight regulation of these immune sensors is necessary to control an optimal inflammatory response and recover organism homeostasis. Prolonged activation of inflammasomes result in the development of chronic inflammatory diseases, and the use of small drug-like inhibitory molecules are emerging as promising anti-inflammatory therapies. Different aspects have to be taken in consideration when designing inflammasome inhibitors. This review summarizes the different techniques that can be used to study the mechanism of action of potential inflammasome inhibitory molecules.

NLRC5 promotes cell migration and invasion by activating the PI3K/AKT signaling pathway in endometrial cancer

Objective

NOD-like receptor family caspase recruitment domain family domain-containing 5 (NLRC5) is involved in the development of cancer. Our objective was to explore the role of NLRC5 in the progression of endometrial cancer (EC).

Methods

The roles of NLRC5 in migration and invasion of AN3CA EC cells were examined by cell wound-healing assay, Transwell migration, and invasion analysis. Overexpression of NLRC5 was achieved with NLRC5 plasmid, and knockdown of NLRC5 was achieved using small interfering (si)RNA-NLRC5 in AN3CA cells. The expression of NLRC5 was detected by immunohistochemical, western blot, and quantitative real-time PCR. LY294002 was used to inhibit the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway.

Results

NLRC5 was downregulated in EC tissue compared with normal endometrium. Overexpression of NLRC5 led to upregulation of cell migration and invasion in AN3CA cells and expression of matrix metallopeptidase (MMP)-9. Inhibition of NLRC5 restricted migration and invasion of AN3CA cells and expression of MMP9. Overexpression of NLRC5 promoted the activation of PI3K/AKT signaling pathway. Inhibiting PI3K/AKT signaling pathway by using LY294002 blocked the positive role of NLRC5 in migration and invasion of AN3CA cells and expression of MMP9.

Conclusions

These results demonstrate that NLRC5 promotes EC progression by activating the PI3K/AKT signaling pathway.

Structural basis for distinct inflammasome complex assembly by human NLRP1 and CARD8

Nod-like receptor (NLR) proteins activate pyroptotic cell death and IL-1 driven inflammation by assembling and activating the inflammasome complex. Closely related sensor proteins NLRP1 and CARD8 undergo unique auto-proteolysis-dependent activation and are implicated in auto-inflammatory diseases; however, their mechanisms of activation are not understood. Here we report the structural basis of how the activating domains (FIINDUPA-CARD) of NLRP1 and CARD8 self-oligomerize to assemble distinct inflammasome complexes. Recombinant FIINDUPA-CARD of NLRP1 forms a two-layered filament, with an inner core of oligomerized CARD surrounded by an outer ring of FIINDUPA. Biochemically, self-assembled NLRP1-CARD filaments are sufficient to drive ASC speck formation in cultured human cells-a process that is greatly enhanced by NLRP1-FIINDUPA which forms oligomers in vitro. The cryo-EM structures of NLRP1-CARD and CARD8-CARD filaments, solved here at 3.7 Å, uncover unique structural features that enable NLRP1 and CARD8 to discriminate between ASC and pro-caspase-1. In summary, our findings provide structural insight into the mechanisms of activation for human NLRP1 and CARD8 and reveal how highly specific signaling can be achieved by heterotypic CARD interactions within the inflammasome complexes.