The CARD plays a critical role in ASC foci formation and inflammasome signalling

NLRP3/1-mediated pyroptosis: beneficial clues for the development of novel therapies for Alzheimer's disease

The inflammasome is a multiprotein complex involved in innate immunity that mediates the inflammatory response leading to pyroptosis, which is a lytic, inflammatory form of cell death. There is accumulating evidence that nucleotide-binding domain and leucine-rich repeat pyrin domain containing 3 (NLRP3) inflammasome-mediated microglial pyroptosis and NLRP1 inflammasome-mediated neuronal pyroptosis in the brain are closely associated with the pathogenesis of Alzheimer's disease. In this review, we summarize the possible pathogenic mechanisms of Alzheimer's disease, focusing on neuroinflammation. We also describe the structures of NLRP3 and NLRP1 and the role their activation plays in Alzheimer's disease. Finally, we examine the neuroprotective activity of small-molecule inhibitors, endogenous inhibitor proteins, microRNAs, and natural bioactive molecules that target NLRP3 and NLRP1, based on the rationale that inhibiting NLRP3 and NLRP1 inflammasome-mediated pyroptosis can be an effective therapeutic strategy for Alzheimer's disease.

Innate immune responses and monocyte-derived phagocyte recruitment in protective immunity to pathogenic bacteria: insights from Legionella pneumophila

Legionella species are Gram-negative intracellular bacteria that evolved in soil and freshwater environments, where they infect and replicate within various unicellular protozoa. The primary virulence factor of Legionella is the expression of a type IV secretion system (T4SS), which contributes to the translocation of effector proteins that subvert biological processes of the host cells. Because of its evolution in unicellular organisms, T4SS effector proteins are not adapted to subvert specific mammalian signaling pathways and immunity. Consequently, Legionella pneumophila has emerged as an interesting infection model for investigating immune responses against pathogenic bacteria in multicellular organisms. This review highlights recent advances in our understanding of mammalian innate immunity derived from studies involving L. pneumophila. This includes recent insights into inflammasome-mediated mechanisms restricting bacterial replication in macrophages, mechanisms inducing cell death in response to infection, induction of effector-triggered immunity, activation of specific pulmonary cell types in mammalian lungs, and the protective role of recruiting monocyte-derived cells to infected lungs.

Andrographolide Attenuates NLRP3 Inflammasome Activation and Airway Inflammation in Exacerbation of Chronic Obstructive Pulmonary Disease

Purpose

The aim of this study is to uncover the anti-inflammatory propertity of andrographolide (AGP) in acute exacerbation of chronic obstructive pulmonary disease (AECOPD) and the underlying mechanisms related to the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome pathway.

Methods

An in vivo experiment was conducted on murine model of AECOPD through endotracheal atomization of elastase and lipopolysaccharide (LPS). Intraperitoneal AGP was administered four times. NLRP3 inflammasome pathway molecules were examined using real-time quantitative polymerase chain reaction (RT-qPCR) and Western blot analysis. By using enzyme-linked immunosorbent assay (ELISA), we tested interleukin (IL)-1β levels in bronchoalveolar lavage fluid. An in vitro study was conducted to determine how AGP impacts the NLRP3 inflammasome in THP-1 derived macrophages. The levels of molecules involved in the pathway were measured. Furthermore, molecular docking analyses were carried out to investigate the interactions between AGP and pathway targets.

Results

In the in vivo study, NLRP3 inflammasome activation was observed in mice experiencing AECOPD. The administration of high-dose AGP demonstrated a mitigating effect on inflammatory cells infiltration in the lungs. Moreover, AGP administration effectively suppressed the expression of NLRP3, apoptosis associated speck-like protein that contains a CARD (PYCARD), cysteinyl aspartate-specific protease-1 (Caspase-1), IL-1β, and IL-18 at both the genetic and protein levels. In the in vitro experiment, IL-1β levels were significantly elevated in THP-1 derived macrophages with activated inflammasome compared to the control group. Furthermore, the downregulation of NLRP3, CASP1, and IL1B genes was observed upon the inhibition of NLRP3 expression through small interfering RNA (siRNA). AGP demonstrated inhibitory effects on the gene expression and protein levels of NLRP3, Caspase-1, and IL-1β. Additionally, molecular docking analysis confirmed that AGP exhibited a favorable binding affinity with all five targets of the pathway.

Conclusion

AGP effectively inhibited NLRP3 inflammasome activation and mitigated the inflammatory reaction of AECOPD both in animal models and in vitro experiments, highlighting the potential of AGP as a treatment for AECOPD with anti-inflammatory properties.

Vitamin D regulates COVID-19 associated severity by suppressing the NLRP3 inflammasome pathway

BACKGROUND

The role of vitamin D3 (VitD3) in modulating innate and adaptive immunity has been reported in different disease contexts. Since the start of the coronavirus disease-2019 (COVID-19) pandemic, the role of VitD3 has been highlighted in many correlational and observational studies. However, the exact mechanisms of action are not well identified. One of the mechanisms via which VitD3 modulates innate immunity is by regulating the NLRP3-inflammasome pathway, being a main underlying cause of SARS-CoV-2-induced hyperinflammation.

AIMS AND MAIN METHODS

Blood specimens of severe COVID-19 patients with or without VitD3 treatment were collected during their stay in the intensive care unit and patients were followed up for 29 days. qPCR, western blot, and ELISA were done to investigate the mechanism of action of VitD3 on the NLRP3 inflammasome activation.

KEY FINDINGS

We here report the ability of VitD3 to downregulate the NLRP3-inflammsome pathway in severe COVID-19 patients. Lower inflammasome pathway activation was observed with significantly lower gene and protein expression of NLRP3, cleaved caspase-1, ASC and IL-1β among severe COVID-19 patients treated with VitD3. The reduction of the inflammasome pathway was associated with a reduction in disease severity markers and enhancement of type I IFN pathway.

SIGNIFICANCE

Our data reveals an important anti-inflammatory effect of VitD3 during SARS-CoV-2 infection. Further investigations are warranted to better characterize the ability of VitD3 to control disease pathogenesis and prevent progression to severe states. This will allow for a more efficient use of a low cost and accessible treatment like VitD3.

Identification of a short sequence motif in the influenza A virus pathogenicity factor PB1-F2 required for inhibition of human NLRP3

Influenza A virus infection activates the NLRP3 inflammasome, a multiprotein signaling complex responsible for the proteolytic activation and release of the proinflammatory cytokine IL-1β from monocytes and macrophages. Some influenza A virus (IAV) strains encode a short 90-amino acid peptide (PB1-F2) on an alternative open reading frame of segment 2, with immunomodulatory activity. We recently demonstrated that contemporary IAV PB1-F2 inhibits the activation of NLRP3, potentially by NEK7-dependent activation. PB1-F2 binds to NLRP3 with its C-terminal 50 amino acids, but the exact binding motif was unknown. On the NLRP3 side, the interface is formed through the leucine-rich-repeat (LRR) domain, potentially in conjunction with the pyrin domain. Here, we took advantage of PB1-F2 sequences from IAV strains with either weak or strong NLRP3 interaction. Sequence comparison and structure prediction using Alphafold2 identified a short four amino acid sequence motif (TQGS) in PB1-F2 that defines NLRP3-LRR binding. Conversion of this motif to that of the non-binding PB1-F2 suffices to lose inhibition of NLRP3 dependent IL-1β release. The TQGS motif further alters the subcellular localization of PB1-F2 and its colocalization with NLRP3 LRR and pyrin domain. Structural predictions suggest the establishment of additional hydrogen bonds between the C-terminus of PB1-F2 and the LRR domain of NLRP3, with two hydrogen bonds connecting to threonine and glutamine of the TQGS motif. Phylogenetic data show that the identified NLRP3 interaction motif in PB1-F2 is widely conserved among recent IAV-infecting humans. Our data explain at a molecular level the specificity of NLRP3 inhibition by influenza A virus.

IMPORTANCE

Influenza A virus infection is accompanied by a strong inflammatory response and high fever. The human immune system facilitates the swift clearance of the virus with this response. An essential signal protein in the proinflammatory host response is IL-1b. It is released from inflammatory macrophages, and its production and secretion depend on the function of NLRP3. We had previously shown that influenza A virus blocks NLRP3 activation by the expression of a viral inhibitor, PB1-F2. Here, we demonstrate how this short peptide binds to NLRP3 and provide evidence that a four amino acid stretch in PB1-F2 is necessary and sufficient to mediate this binding. Our data identify a new virus-host interface required to block one signaling path of the innate host response against influenza A virus.

Anti-inflammatory effect of Trichospira verticillata via suppression of the NLRP3 inflammasome in neutrophilic asthma

Trichospira verticillata is an annual herb that belongs to the family Asteraceae. Trichospira verticillata extract (TVE) elicits anti-plasmodial activity; however, there has been no detailed report about its anti-inflammatory effects and molecular mechanisms. In addition, herbal plants exhibit anti-inflammatory effects by suppressing the NLRP3 inflammasome. Therefore, the primary goal of this study was to examine the effects of TVE on NLRP3 inflammasome activation by measuring interleukin-1β (IL-1β) secretion. We treated lipopolysaccharides (LPS)-primed J774A.1 and THP-1 cells with TVE, which attenuated NLRP3 inflammasome activation. Notably, TVE did not affect nuclear factor-kappa B (NF-κB) signalling or intracellular reactive oxygen species (ROS) production and potassium efflux, suggesting that it inactivates the NLRP3 inflammasome via other mechanisms. Moreover, TVE suppressed the formation of apoptosis-associated speck-like protein (ASC) speck and oligomerization. Immunoprecipitation data revealed that TVE reduced the binding of NLRP3 to NIMA-related kinase 7 (NEK7), resulting in reduced ASC oligomerization and speck formation. Moreover, TVE alleviated neutrophilic asthma (NA) symptoms in mice. This study demonstrates that TVE modulates the binding of NLPR3 to NEK7, thereby reporting novel insights into the mechanism by which TVE inhibits NLRP3 inflammasome. These findings suggest TVE as a potential therapeutic of NLRP3 inflammasome-mediated diseases, particularly NA.

The Role of the NLRP3 Inflammasome in the Molecular and Biochemical Mechanisms of Cervical Ripening: A Comprehensive Review

The uterine cervix is one of the key factors involved in ensuring a proper track of gestation and labor. At the end of the gestational period, the cervix undergoes extensive changes, which can be summarized as a transformation from a non-favorable cervix to one that is soft and prone to dilation. During a process called cervical ripening, fundamental remodeling of the cervical extracellular matrix (ECM) occurs. The cervical ripening process is a derivative of many interlocking and mutually driving biochemical and molecular pathways under the strict control of mediators such as inflammatory cytokines, nitric oxide, prostaglandins, and reactive oxygen species. A thorough understanding of all these pathways and learning about possible triggering factors will allow us to develop new, better treatment algorithms and therapeutic goals that could protect women from both dysfunctional childbirth and premature birth. This review aims to present the possible role of the NLRP3 inflammasome in the cervical ripening process, emphasizing possible mechanisms of action and regulatory factors.

Cryo-electron tomography of NLRP3-activated ASC complexes reveals organelle co-localization

NLRP3 induces caspase-1-dependent pyroptotic cell death to drive inflammation. Aberrant activity of NLRP3 occurs in many human diseases. NLRP3 activation induces ASC polymerization into a single, micron-scale perinuclear punctum. Higher resolution imaging of this signaling platform is needed to understand how it induces pyroptosis. Here, we apply correlative cryo-light microscopy and cryo-electron tomography to visualize ASC/caspase-1 in NLRP3-activated cells. The puncta are composed of branched ASC filaments, with a tubular core formed by the pyrin domain. Ribosomes and Golgi-like or endosomal vesicles permeate the filament network, consistent with roles for these organelles in NLRP3 activation. Mitochondria are not associated with ASC but have outer-membrane discontinuities the same size as gasdermin D pores, consistent with our data showing gasdermin D associates with mitochondria and contributes to mitochondrial depolarization.

Corilagin alleviates LPS-induced sepsis through inhibiting pyroptosis via targeting TIR domain of MyD88 and binding CARD of ASC in macrophages

Sepsis is a dysregulated systemic inflammatory response caused by infection that leads to multiple organ injury and high mortality without effective treatment. Corilagin, a natural polyphenol extracted from traditional Chinese herbs, exhibits strong anti-inflammatory properties. However, the role for Corilagin in lipopolysaccharide (LPS)-induced sepsis and the molecular mechanisms underlying this process have not been completely explored. Here we determine the effect of Corilagin on LPS-treated mice and use a screening approach integrating surface plasmon resonance with liquid chromatography-tandem mass spectrometry (SPR-LC-MS/MS) to further explore the therapeutic target of Corilagin. We discovered that Corilagin significantly prolonged the survival time of septic mice, attenuated the multi-organ injury and the expression of pyroptosis-related proteins in tissues of LPS-treated mice. In vitro studies revealed that Corilagin inhibited pyroptosis and NLRP3 inflammasome activation in LPS-treated macrophages followed with ATP stimulation, as reflected by decreased levels of GSDMD-NT and activated caspase-1, and reduced ASC specks formation. Mechanistically, Corilagin alleviated the formation of ASC specks and blocked the interaction of ASC and pro-caspase1 by competitively binding with the caspase recruitment domain (CARD) of ASC. Additionally, Corilagin interrupted the TLR4-MyD88 interaction through targeting TIR domain of MyD88, leading to the inhibition of NF-κB activation and NLRP3 production. In addition, Corilagin downregulated genes associated with several inflammatory responses and inflammasome-related signaling pathways in LPS-stimulated macrophages. Overall, our results indicate that the inhibitory effect of Corilagin on pyroptosis through targeting TIR domain of MyD88 and binding the CARD domain of ASC in macrophages plays an essential role in protection against LPS-induced sepsis.

Inflammasome: structure, biological functions, and therapeutic targets

Inflammasomes are a group of protein complex located in cytoplasm and assemble in response to a wide variety of pathogen-associated molecule patterns, damage-associated molecule patterns, and cellular stress. Generally, the activation of inflammasomes will lead to maturation of proinflammatory cytokines and pyroptotic cell death, both associated with inflammatory cascade amplification. A sensor protein, an adaptor, and a procaspase protein interact through their functional domains and compose one subunit of inflammasome complex. Under physiological conditions, inflammasome functions against pathogen infection and endogenous dangers including mtROS, mtDNA, and so on, while dysregulation of its activation can lead to unwanted results. In recent years, advances have been made to clarify the mechanisms of inflammasome activation, the structural details of them and their functions (negative/positive) in multiple disease models in both animal models and human. The wide range of the stimuli makes the function of inflammasome diverse and complex. Here, we review the structure, biological functions, and therapeutic targets of inflammasomes, while highlight NLRP3, NLRC4, and AIM2 inflammasomes, which are the most well studied. In conclusion, this review focuses on the activation process, biological functions, and structure of the most well-studied inflammasomes, summarizing and predicting approaches for disease treatment and prevention with inflammasome as a target. We aim to provide fresh insight into new solutions to the challenges in this field.

The NLRP3 inflammasome in viral infection (Review)

The interplay between pathogen and host determines the immune response during viral infection. The Nod‑like receptor (NLR) protein 3 inflammasome is a multiprotein complex that induces the activation of inflammatory caspases and the release of IL‑1β, which play an important role in the innate immune responses. In the present review, the mechanisms of the NLR family pyrin domain containing 3 inflammasome activation and its dysregulation in viral infection were addressed.

ASC oligomer favors caspase-1CARD domain recruitment after intracellular potassium efflux

Signaling through the inflammasome is important for the inflammatory response. Low concentrations of intracellular K+ are associated with the specific oligomerization and activation of the NLRP3 inflammasome, a type of inflammasome involved in sterile inflammation. After NLRP3 oligomerization, ASC protein binds and forms oligomeric filaments that culminate in large protein complexes named ASC specks. ASC specks are also initiated from different inflammasome scaffolds, such as AIM2, NLRC4, or Pyrin. ASC oligomers recruit caspase-1 and then induce its activation through interactions between their respective caspase activation and recruitment domains (CARD). So far, ASC oligomerization and caspase-1 activation are K+-independent processes. Here, we found that when there is low intracellular K+, ASC oligomers change their structure independently of NLRP3 and make the ASCCARD domain more accessible for the recruitment of the pro-caspase-1CARD domain. Therefore, conditions that decrease intracellular K+ not only drive NLRP3 responses but also enhance the recruitment of the pro-caspase-1 CARD domain into the ASC specks.

Insights Regarding the Role of Inflammasomes in Leukemia: What Do We Know?

Inflammation is a physiological mechanism of the immune response and has an important role in maintaining the hematopoietic cell niche in the bone marrow. During this process, the participation of molecules produced by innate immunity cells in response to a variety of pathogen-associated molecular patterns and damage-associated molecular patterns is observed. However, chronic inflammation is intrinsically associated with leukemogenesis, as it induces DNA damage in hematopoietic stem cells and contributes to the creation of the preleukemic clone. Several factors influence the malignant transformation within the hematopoietic microenvironment, with inflammasomes having a crucial role in this process, in addition to acting in the regulation of hematopoiesis and its homeostasis. Inflammasomes are intracellular multimeric complexes responsible for the maturation and secretion of the proinflammatory cytokines interleukin-1β and interleukin-18 and the cell death process via pyroptosis. Therefore, dysregulation of the activation of these complexes may be a factor in triggering several diseases, including leukemias, and this has been the subject of several studies in the area. In this review, we summarized the current knowledge on the relationship between inflammation and leukemogenesis, in particular, the role of inflammasomes in different types of leukemias, and we describe the potential therapeutic targets directed at inflammasomes in the leukemic context.

Role of inflammasome in severe, steroid-resistant asthma

Purpose of review

Asthma is a common heterogeneous group of chronic inflammatory diseases with different pathological phenotypes classified based on the various clinical, physiological and immunobiological profiles of patients. Despite similar clinical symptoms, asthmatic patients may respond differently to treatment. Hence, asthma research is becoming more focused on deciphering the molecular and cellular pathways driving the different asthma endotypes. This review focuses on the role of inflammasome activation as one important mechanism reported in the pathogenesis of severe steroid resistant asthma (SSRA), a Th2-low asthma endotype. Although SSRA represents around 5-10% of asthmatic patients, it is responsible for the majority of asthma morbidity and more than 50% of asthma associated healthcare costs with clear unmet need. Therefore, deciphering the role of the inflammasome in SSRA pathogenesis, particularly in relation to neutrophil chemotaxis to the lungs, provides a novel target for therapy.

Recent findings

The literature highlighted several activators of inflammasomes that are elevated during SSRA and result in the release of proinflammatory mediators, mainly IL-1β and IL-18, through different signaling pathways. Consequently, the expression of NLRP3 and IL-1β is shown to be positively correlated with neutrophil recruitment and negatively correlated with airflow obstruction. Furthermore, exaggerated NLRP3 inflammasome/IL-1β activation is reported to be associated with glucocorticoid resistance.

Summary

In this review, we summarized the reported literature on the activators of the inflammasome during SSRA, the role of IL-1β and IL-18 in SSRA pathogenesis, and the pathways by which inflammasome activation contributes to steroid resistance. Finally, our review shed light on the different levels to target inflammasome involvement in an attempt to ameliorate the serious outcomes of SSRA.

Oligomeric states of ASC specks regulate inflammatory responses by inflammasome in the extracellular space

Inflammasomes are multi-protein complexes and play a crucial role in host defense against pathogens. Downstream inflammatory responses through inflammasomes are known to be related to the oligomerization degree of ASC specks, but the detailed mechanism still remains unexplored. Here, we demonstrate that oligomerization degrees of ASC specks regulate the caspase-1 activation in the extracellular space. A protein binder specific for a pyrin domain (PYD) of ASC (ASC^PYD) was developed, and structural analysis revealed that the protein binder effectively inhibits the interaction between PYDs, disassembling ASC specks into low oligomeric states. ASC specks with a low oligomerization degree were shown to enhance the activation of caspase-1 by recruiting and processing more premature caspase-1 through interactions between CARD of caspase-1 (caspase-1^CARD) and CARD of ASC (ASC^CARD). These findings can provide insight into controlling the inflammasome-mediated inflammatory process as well as the development of inflammasome-targeting drugs.

Targeting the NLRP3 inflammasome in diabetic retinopathy: From Pathogenesis to Therapeutic Strategies

Diabetic retinopathy (DR) is a common diabetic microvascular complication and the main cause of vision loss in working-aged people. The NLRP3 inflammasome is a cytosolic multimeric complex that plays a significant role in innate immunity. After sensing injury, the NLRP3 inflammasome induces inflammatory mediator secretion and triggers a form of inflammatory cell death known as pyroptosis. Studies over the past five years have shown increased expression of NLRP3 and related inflammatory mediators in vitreous samples from DR patients at different clinical stages. Many NLRP3-targeted inhibitors have shown great antiangiogenic and anti-inflammatory effects in diabetes mellitus models, suggesting that the NLRP3 inflammasome is involved in the progression of DR. This review covers the molecular mechanisms of NLRP3 inflammasome activation. Furthermore, we discuss the implications of the NLRP3 inflammasome in DR, including the induction of pyroptosis and inflammation and the promotion of microangiopathy and retinal neurodegeneration. We also summarize the research progress on targeting the NLRP3 inflammasome in DR therapeutics with the expectation of providing new insights into DR progression and treatment.

Involvement of inflammasomes in tumor microenvironment and tumor therapies

Inflammasomes are macromolecular platforms formed in response to damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns, whose formation would cause maturation of interleukin-1 (IL-1) family members and gasdermin D (GSDMD), leading to IL-1 secretion and pyroptosis respectively. Several kinds of inflammasomes detecting different types of dangers have been found. The activation of inflammasomes is regulated at both transcription and posttranscription levels, which is crucial in protecting the host from infections and sterile insults. Present findings have illustrated that inflammasomes are involved in not only infection but also the pathology of tumors implying an important link between inflammation and tumor development. Generally, inflammasomes participate in tumorigenesis, cell death, metastasis, immune evasion, chemotherapy, target therapy, and radiotherapy. Inflammasome components are upregulated in some tumors, and inflammasomes can be activated in cancer cells and other stromal cells by DAMPs, chemotherapy agents, and radiation. In some cases, inflammasomes inhibit tumor progression by initiating GSDMD-mediated pyroptosis in cancer cells and stimulating IL-1 signal-mediated anti-tumor immunity. However, IL-1 signal recruits immunosuppressive cell subsets in other cases. We discuss the conflicting results and propose some possible explanations. Additionally, we also summarize interventions targeting inflammasome pathways in both preclinical and clinical stages. Interventions targeting inflammasomes are promising for immunotherapy and combination therapy.

Intracellular DAMPs in Neurodegeneration and Their Role in Clinical Therapeutics

Neuroinflammation is the major implication of neurodegeneration. This is a complex process which initiates from the cellular injury triggering the innate immune system which gives rise to damage-associated molecular patterns (DAMPs) which are also recognized as endogenous danger indicators. These originate from various compartments of the cell under pathological stimulus. These are very popular candidates having their origin in the intracellular compartments and organelles of the cell and may have their site of action itself in the intracellular or at the extracellular spaces. Under the influence of the pathological stimuli, they interact with the pattern-recognition receptor to initiate their pro-inflammatory cascade followed by the cytokine release. This provides a good opportunity for diagnostic and therapeutic interventions creating better conditions for repair and reversal. Since the major contributors arise from the intracellular compartment, in this review, we have attempted to focus on the DAMP molecules arising from the intracellular compartments and their specific roles in the neurodegenerative events explaining their downstream mediators and signaling. Moreover, we have tried to cover the latest interventions in terms of DAMPs as clinical biomarkers which can assist in detecting the disease and also target it to reduce the innate-immune activation response which can help in reducing the sterile neuroinflammation having an integral role in the neurodegenerative processes.

Molecular characteristics and the roles of CaASC and its restriction to Aeromonas hydrophila in Carassius auratus

Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), as a critical adaptor molecule in inflammasome complexes, plays an important role in mediating inflammation reaction. In this study, the complete cDNA of ASC gene with 891 bp was cloned in Qihe crucian carp Carassius auratus (named as CaASC), which was composed of a 5'-UTR of 36 bp, a 3'-UTR of 252 bp, and an ORF of 603 bp encoded 200 amino acids with a predicted isoelectric point of 5.61 and a molecular mass of 22.0 kDa. Multiple sequence alignment and motif analysis revealed that CaASC contained a conserved N-terminal Pyrin domain (PYD) and a C-terminal Caspase recruitment domain (CARD). CaASC mRNA and protein expressions were detected in selected tissues, with the highest mRNA level in the spleen. Meanwhile, CaASC gene expressions were clearly altered in intestine, gill, skin, spleen, liver and head kidney of fish challenged by Aeromonas hydrophila, LPS, and polyI:C, respectively. The recombined proteins of CaASC with fluorescent tag were over-expressed in transfected 293T cells, and the green specks were observed obviously and located in the cytoplasm. Furthermore, knockdown of CaASC reduced the expression of IL-1β and promoted the bacterial colonization in fish tissues, while overexpression of CaASC increased the expression of IL-1β and hampered the bacterial colonization in these tissues. Taken together, these results identified the molecular characteristics of CaASC in C. auratus, and revealed its role in regulating IL-1β expression and restricting bacterial infection in vivo.

Enhanced activity of NLRP3 inflammasome and its proinflammatory effect in influenza A viral pneumonia

Aim: The aim of this study was to investigate the activity of NLRP3 inflammasome and its effect on inducing severe pneumonia 1 week after influenza A virus (IAV) infection. Materials & methods: The expression levels of NLRP3, caspase-1 and IL-1β were assessed in murine macrophages stimulated with IAV. And the severity of viral pneumonia in mice was explored. Results & conclusion: The data showed that although the expression of NLRP3 diverged, activity of NLRP3 inflammasome was enhanced 1 week after IAV infection, and more severe viral pneumonia was associated with IL-1β in serum. It infers that enhanced activity of NLRP3 inflammasome induces augmented expression of IL-1β and severe pneumonia in a NLRP3-independent way, 1 week after IAV infection.

The Significance of NLRP Inflammasome in Neuropsychiatric Disorders

The NLRP inflammasome is a multi-protein complex which mainly consists of the nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain. Its activation is linked to microglial-mediated neuroinflammation and partial neuronal degeneration. Many neuropsychiatric illnesses have increased inflammatory responses as both a primary cause and a defining feature. The NLRP inflammasome inhibition delays the progression and alleviates the deteriorating effects of neuroinflammation on several neuropsychiatric disorders. Evidence on the central effects of the NLRP inflammasome potentially provides the scientific base of a promising drug target for the treatment of neuropsychiatric disorders. This review elucidates the classification, composition, and functions of the NLRP inflammasomes. It also explores the underlying mechanisms of NLRP inflammasome activation and its divergent role in neuropsychiatric disorders, including Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, depression, drug use disorders, and anxiety. Furthermore, we explore the treatment potential of the NLRP inflammasome inhibitors against these disorders.

The central inflammasome adaptor protein ASC activates the inflammasome after transition from a soluble to an insoluble state

Apoptosis-associated speck-like protein containing a caspase recruitment domain (CARD) (ASC) is a 22 kDa protein that functions as the central adaptor for inflammasome assembly. ASC forms insoluble specks in monocytes undergoing pyroptosis, and the polymerization of ASC provides a template of CARDs that leads to proximity-mediated autoactivation of caspase-1 in canonical inflammasomes. However, specks are insoluble protein complexes, and solubility is typically important for protein function. Therefore, we sought to define whether ASC specks comprise active inflammasome complexes or are simply the end stage of exhausted ASC polymers. Using a THP-1 cell–lysing model of caspase-1 activation that is ASC dependent, we compared caspase-1 activation induced by preassembled insoluble ASC specks and soluble monomeric forms of ASC. Unexpectedly, after controlling for the concentration dependence of ASC oligomerization, we found that only insoluble forms of ASC promoted caspase-1 autocatalysis. This link to insolubility was recapitulated with recombinant ASC. We show that purified recombinant ASC spontaneously precipitated and was functional, whereas the maltose-binding protein–ASC fusion to ASC (promoting enhanced solubility) was inactive until induced to insolubility by binding to amylose beads. This functional link to insolubility also held true for the Y146A mutation of the CARD of ASC, which avoids insolubility and caspase-1 activation. Thus, we conclude that the role of ASC insolubility in inflammasome function is inextricably linked to its pyrin domain–mediated and CARD-mediated polymerizations. These findings will support future studies into the molecular mechanisms controlling ASC solubility.

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.

The role of inflammasomes in vascular cognitive impairment

There is an increasing prevalence of Vascular Cognitive Impairment (VCI) worldwide, and several studies have suggested that Chronic Cerebral Hypoperfusion (CCH) plays a critical role in disease onset and progression. However, there is a limited understanding of the underlying pathophysiology of VCI, especially in relation to CCH. Neuroinflammation is a significant contributor in the progression of VCI as increased systemic levels of the proinflammatory cytokine interleukin-1β (IL-1β) has been extensively reported in VCI patients. Recently it has been established that CCH can activate the inflammasome signaling pathways, involving NLRP3 and AIM2 inflammasomes that critically regulate IL-1β production. Given that neuroinflammation is an early event in VCI, it is important that we understand its molecular and cellular mechanisms to enable development of disease-modifying treatments to reduce the structural brain damage and cognitive deficits that are observed clinically in the elderly. Hence, this review aims to provide a comprehensive insight into the molecular and cellular mechanisms involved in the pathogenesis of CCH-induced inflammasome signaling in VCI.

Identification of an ASC oligomerization inhibitor for the treatment of inflammatory diseases

The ASC (apoptosis-associated speck-like protein containing a caspase recruitment domain (CARD)) protein is an scaffold component of different inflammasomes, intracellular multiprotein platforms of the innate immune system that are activated in response to pathogens or intracellular damage. The formation of ASC specks, initiated by different inflammasome receptors, promotes the recruitment and activation of procaspase-1, thereby triggering pyroptotic inflammatory cell death and pro-inflammatory cytokine release. Here we describe MM01 as the first-in-class small-molecule inhibitor of ASC that interferes with ASC speck formation. MM01 inhibition of ASC oligomerization prevents activation of procaspase-1 in vitro and inhibits the activation of different ASC-dependent inflammasomes in cell lines and primary cultures. Furthermore, MM01 inhibits inflammation in vivo in a mouse model of inflammasome-induced peritonitis. Overall, we highlight MM01 as a novel broad-spectrum inflammasome inhibitor for the potential treatment of multifactorial diseases involving the dysregulation of multiple inflammasomes.

The role of NLRP3 inflammasome in the pathogenesis of rheumatic disease

Inflammasome is a molecular platform that is formed in the cytosolic compartment to mediate host immune responses to infection and cellular damage. Inflammasome can activate caspase-1, leading to the maturation of two inflammatory cytokines interleukin 1β (IL-1β) and IL-18 and initiation of a proinflammatory form of cell death called pyroptosis. Among various inflammasome complexes, the NLRP3 inflammasome is by far the most studied inflammasome. NLRP3 inflammasome is a key factor in regulating host immune defense against infectious microbes and cellular damage. However, the dysregulated NLRP3 inflammasome activation also participates in the pathogenesis of many human disorders. NLRP3 inflammasome plays an important role in the pathogenesis of rheumatic disease such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), ankylosing spondylitis (AS), Sjögren's syndrome (SS), dermatomyositis/polymyositis (DM/PM), gout, and systemic sclerosis (SSc). For example, NLRP3 inflammasome has been found highly activated in synovial tissues and peripheral blood mononuclear cells from RA patients. In this paper, we will discuss the role of NLRP3 inflammasome in the pathogenesis of rheumatic disease.

Actions of the NLRP3 and NLRC4 inflammasomes overlap in bone resorption

Overwhelming evidence indicates that excessive stimulation of innate immune receptors of the NOD-like receptor (NLR) family causes significant damage to multiple tissues, yet the role of these proteins in bone metabolism is not well known. Here, we studied the interaction between the NLRP3 and NLRC4 inflammasomes in bone homeostasis and disease. We found that loss of NLRP3 or NLRC4 inflammasome attenuated osteoclast differentiation in vitro. At the tissue level, lack of NLRP3, or NLRC4 to a lesser extent, resulted in higher baseline bone mass compared to wild-type (WT) mice, and conferred protection against LPS-induced inflammatory osteolysis. Bone mass accrual in mutant mice correlated with lower serum IL-1β levels in vivo. Unexpectedly, the phenotype of Nlrp3-deficient mice was reversed upon loss of NLRC4 as bone mass was comparable between WT mice and Nlrp3;Nlrc4 knockout mice. Thus, although bone homeostasis is perturbed to various degrees by the lack of NLRP3 or NLRC4, this tissue appears to function normally upon compound loss of the inflammasomes assembled by these receptors.

The APC10 subunit of the anaphase-promoting complex/cyclosome orchestrates NLRP3 inflammasome activation during the cell cycle

The activation of the NLRP3 inflammasome plays a crucial role in the innate immune response. During cell division, NLRP3 inflammasome activation must be strictly controlled. In this study, we discover that the anaphase-promoting complex subunit 10 (APC10), a substrate recognition protein of the anaphase-promoting complex/cyclosome (APC/C), is a critical mediator of NLRP3 inflammasome activation. During interphase, APC10 interacts with NLRP3 to promote NLRP3 inflammasome activation, whereas during mitosis, APC10 disassociates from the NLRP3 inflammasome to repress inflammatory responses. This study reveals a distinct mechanism by which APC10 serves as a switch for NLRP3 inflammasome activation during the cell cycle.

NLRP3 inflammasome in cancer and metabolic diseases

The NLRP3 inflammasome is a multimeric cytosolic protein complex that assembles in response to cellular perturbations. This assembly leads to the activation of caspase-1, which promotes maturation and release of the inflammatory cytokines interleukin-1β (IL-1β) and IL-18, as well as inflammatory cell death (pyroptosis). The inflammatory cytokines contribute to the development of systemic low-grade inflammation, and aberrant NLRP3 activation can drive a chronic inflammatory state in the body to modulate the pathogenesis of inflammation-associated diseases. Therefore, targeting NLRP3 or other signaling molecules downstream, such as caspase-1, IL-1β or IL-18, has the potential for great therapeutic benefit. However, NLRP3 inflammasome-mediated inflammatory cytokines play dual roles in mediating human disease. While they are detrimental in the pathogenesis of inflammatory and metabolic diseases, they have a beneficial role in numerous infectious diseases and some cancers. Therefore, fine tuning of NLRP3 inflammasome activity is essential for maintaining proper cellular homeostasis and health. In this Review, we will cover the mechanisms of NLRP3 inflammasome activation and its divergent roles in the pathogenesis of inflammation-associated diseases such as cancer, atherosclerosis, diabetes and obesity, highlighting the therapeutic potential of targeting this pathway.

NETosis occurs independently of neutrophil serine proteases

Neutrophils are primary host innate immune cells defending against pathogens. One proposed mechanism by which neutrophils prevent the spread of pathogens is NETosis, the extrusion of cellular DNA resulting in neutrophil extracellular traps (NETs). The protease neutrophil elastase (NE) has been implicated in the formation of NETs through proteolysis of nuclear proteins leading to chromatin decondensation. In addition to NE, neutrophils contain three other serine proteases that could compensate if the activity of NE was neutralized. However, whether they do play such a role is unknown. Thus, we deployed recently described specific inhibitors against all four of the neutrophil serine proteases (NSPs). Using specific antibodies to the NSPs along with our labeled inhibitors, we show that catalytic activity of these enzymes is not required for the formation of NETs. Moreover, the NSPs that decorate NETs are in an inactive conformation and thus cannot participate in further catalytic events. These results indicate that NSPs play no role in either NETosis or arming NETs with proteolytic activity.

Noncanonical Roles of Caspase-4 and Caspase-5 in Heme-Driven IL-1β Release and Cell Death

Excessive release of heme from RBCs is a key pathophysiological feature of several disease states, including bacterial sepsis, malaria, and sickle cell disease. This hemolysis results in an increased level of free heme that has been implicated in the inflammatory activation of monocytes, macrophages, and the endothelium. In this study, we show that extracellular heme engages the human inflammatory caspases, caspase-1, caspase-4, and caspase-5, resulting in the release of IL-1β. Heme-induced IL-1β release was further increased in macrophages from patients with sickle cell disease. In human primary macrophages, heme activated caspase-1 in an inflammasome-dependent manner, but heme-induced activation of caspase-4 and caspase-5 was independent of canonical inflammasomes. Furthermore, we show that both caspase-4 and caspase-5 are essential for heme-induced IL-1β release, whereas caspase-4 is the primary contributor to heme-induced cell death. Together, we have identified that extracellular heme is a damage-associated molecular pattern that can engage canonical and noncanonical inflammasome activation as a key mediator of inflammation in macrophages.

Neutrophils: Many Ways to Die

Neutrophils or polymorphonuclear leukocytes (PMN) are key participants in the innate immune response for their ability to execute different effector functions. These cells express a vast array of membrane receptors that allow them to recognize and eliminate infectious agents effectively and respond appropriately to microenvironmental stimuli that regulate neutrophil functions, such as activation, migration, generation of reactive oxygen species, formation of neutrophil extracellular traps, and mediator secretion, among others. Currently, it has been realized that activated neutrophils can accomplish their effector functions and simultaneously activate mechanisms of cell death in response to different intracellular or extracellular factors. Although several studies have revealed similarities between the mechanisms of cell death of neutrophils and other cell types, neutrophils have distinctive properties, such as a high production of reactive oxygen species (ROS) and nitrogen species (RNS), that are important for their effector function in infections and pathologies such as cancer, autoimmune diseases, and immunodeficiencies, influencing their cell death mechanisms. The present work offers a synthesis of the conditions and molecules implicated in the regulation and activation of the processes of neutrophil death: apoptosis, autophagy, pyroptosis, necroptosis, NETosis, and necrosis. This information allows to understand the duality encountered by PMNs upon activation. The effector functions are carried out to eliminate invading pathogens, but in several instances, these functions involve activation of signaling cascades that culminate in the death of the neutrophil. This process guarantees the correct elimination of pathogenic agents, damaged or senescent cells, and the timely resolution of the inflammation that is essential for the maintenance of homeostasis in the organism. In addition, they alert the organism when the immunological system is being deregulated, promoting the activation of other cells of the immune system, such as B and T lymphocytes, which produce cytokines that potentiate the microbicide functions.

The involvement of NLRP3 inflammasome in the treatment of neurodegenerative diseases

In an ageing society, neurodegenerative diseases have attracted attention because of their high incidence worldwide. Despite extensive research, there is a lack of conclusive insights into the pathogenesis of neurodegenerative diseases, which limit the strategies for symptomatic treatment. Therefore, better elucidation of the molecular mechanisms involved in neurodegenerative diseases can provide an important theoretical basis for the discovery of new and effective prevention and treatment methods. The innate immune system is activated during the ageing process and in response to neurodegenerative diseases. Inflammasomes are multiprotein complexes that play an important role in the activation of the innate immune system. They mediate inflammatory reactions and pyroptosis, which are closely involved in neurodegeneration. There are different types of inflammasomes, although the nucleotide-binding oligomerization domain-like receptor pyrin domain-containing 3 (NLRP3) inflammasome is the most common inflammasome; NLRP3 plays an important role in the pathogenesis of neurodegenerative diseases. In this review, we will discuss the mechanisms that are involved in the activation of the NLRP3 inflammasome and its crucial role in the pathology of neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis. We will also review various treatments that target the NLRP3 inflammasome pathway and alleviate neuroinflammation. Finally, we will summarize the novel treatment strategies for neurodegenerative disorders.

Inhibition of NLRP3 Inflammasome Activation and Pyroptosis in Macrophages by Taraxasterol Is Associated With Its Regulation on mTOR Signaling

Taraxasterol (TAS) is an active ingredient of Dandelion (Taraxacum mongolicum Hand. -Mazz.), a medicinal plant that has long been used in China for treatment of inflammatory disorders. But the underlying mechanism for its therapeutic effects on inflammatory disorders is not completely clear. Inflammasome activation is a critical step of innate immune response to infection and aseptic inflammation. Among the various types of inflammasome sensors that has been reported, NLR family pyrin domain containing 3 (NLRP3) is implicated in various inflammatory diseases and therefore has been most extensively studied. In this study, we aimed to explore whether TAS could influence NLPR3 inflammasome activation in macrophages. The results showed that TAS dose-dependently suppressed the activation of caspase-1 in lipopolysaccharide (LPS)-primed murine primary macrophages upon nigericin treatment, resulting in reduced mature interleukin-1β (IL-1β) release and gasdermin D (GSDMD) cleavage. TAS greatly reduced ASC speck formation upon the stimulation of nigericin or extracellular ATP. Consistent with reduced cleavage of GSDMD, nigericin-induced pyroptosis was alleviated by TAS. Interestingly, TAS time-dependently suppressed the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) and mTORC2 signaling induced by LPS priming. Like TAS, both INK-128 (inhibiting both mTORC1 and mTORC2) and rapamycin (inhibiting mTORC1 only) also inhibited NLRP3 inflammasome activation, though their effects on mTOR signaling were different. Moreover, TAS treatment alleviated mitochondrial damage by nigericin and improved mouse survival from bacterial infection, accompanied by reduced IL-1β levels in vivo. Collectively, by inhibiting the NLRP3 inflammasome activation, TAS displayed anti-inflammatory effects likely through regulation of the mTOR signaling in macrophages, highlighting a potential action mechanism for the anti-inflammatory activity of Dandelion in treating inflammation-related disorders, which warrants further clinical investigation.

Biological Functions of Gasdermins in Cancer: From Molecular Mechanisms to Therapeutic Potential

Pyroptosis is a type of lytic programmed cell death triggered by various inflammasomes that sense danger signals. Pyroptosis has recently attracted great attention owing to its contributory role in cancer. Pyroptosis plays an important role in cancer progression by inducing cancer cell death or eliciting anticancer immunity. The participation of gasdermins (GSDMs) in pyroptosis is a noteworthy recent discovery. GSDMs have emerged as a group of pore-forming proteins that serve important roles in innate immunity and are composed of GSDMA-E and Pejvakin (PJVK) in human. The N-terminal domains of GSDMs, expect PJVK, can form pores on the cell membrane and function as effector proteins of pyroptosis. Remarkably, it has been found that GSDMs are abnormally expressed in several forms of cancers. Moreover, GSDMs are involved in cancer cell growth, invasion, metastasis and chemoresistance. Additionally, increasing evidence has indicated an association between GSDMs and clinicopathological features in cancer patients. These findings suggest the feasibility of using GSDMs as prospective biomarkers for cancer diagnosis, therapeutic intervention and prognosis. Here, we review the progress in unveiling the characteristics and biological functions of GSDMs. We also focus on the implication and molecular mechanisms of GSDMs in cancer pathogenesis. Investigating the relationship between GSDMs and cancer biology could assist us to explore new therapeutic avenues for cancer prevention and treatment.

Structure, Activation and Regulation of NLRP3 and AIM2 Inflammasomes

The inflammasome is a three-component (sensor, adaptor, and effector) filamentous signaling platform that shields from multiple pathogenic infections by stimulating the proteolytical maturation of proinflammatory cytokines and pyroptotic cell death. The signaling process initiates with the detection of endogenous and/or external danger signals by specific sensors, followed by the nucleation and polymerization from sensor to downstream adaptor and then to the effector, caspase-1. Aberrant activation of inflammasomes promotes autoinflammatory diseases, cancer, neurodegeneration, and cardiometabolic disorders. Therefore, an equitable level of regulation is required to maintain the equilibrium between inflammasome activation and inhibition. Recent advancement in the structural and mechanistic understanding of inflammasome assembly potentiates the emergence of novel therapeutics against inflammasome-regulated diseases. In this review, we have comprehensively discussed the recent and updated insights into the structure of inflammasome components, their activation, interaction, mechanism of regulation, and finally, the formation of densely packed filamentous inflammasome complex that exists as micron-sized punctum in the cells and mediates the immune responses.

Mechanism of filament formation in UPA-promoted CARD8 and NLRP1 inflammasomes

NLRP1 and CARD8 are related cytosolic sensors that upon activation form supramolecular signalling complexes known as canonical inflammasomes, resulting in caspase-1 activation, cytokine maturation and/or pyroptotic cell death. NLRP1 and CARD8 use their C-terminal (CT) fragments containing a caspase recruitment domain (CARD) and the UPA (conserved in UNC5, PIDD, and ankyrins) subdomain for self-oligomerization, which in turn form the platform to recruit the inflammasome adaptor ASC (apoptosis-associated speck-like protein containing a CARD) or caspase-1, respectively. Here, we report cryo-EM structures of NLRP1-CT and CARD8-CT assemblies, in which the respective CARDs form central helical filaments that are promoted by oligomerized, but flexibly linked, UPAs surrounding the filaments. Through biochemical and cellular approaches, we demonstrate that the UPA itself reduces the threshold needed for NLRP1-CT and CARD8-CT filament formation and signalling. Structural analyses provide insights on the mode of ASC recruitment by NLRP1-CT and the contrasting direct recruitment of caspase-1 by CARD8-CT. We also discover that subunits in the central NLRP1CARD filament dimerize with additional exterior CARDs, which roughly doubles its thickness and is unique among all known CARD filaments. Finally, we engineer and determine the structure of an ASCCARD-caspase-1CARD octamer, which suggests that ASC uses opposing surfaces for NLRP1, versus caspase-1, recruitment. Together these structures capture the architecture and specificity of the active NLRP1 and CARD8 inflammasomes in addition to key heteromeric CARD-CARD interactions governing inflammasome signalling.

Insights Into Mucosal Innate Immune Responses in House Dust Mite-Mediated Allergic Asthma

The prevalence of asthma has been rising steadily for several decades, and continues to be a major public health and global economic burden due to both direct and indirect costs. Asthma is defined as chronic heterogeneous inflammatory diseases characterized by airway obstruction, mucus production and bronchospasm. Different endotypes of asthma are being recognized based on the distinct pathophysiology, genetic predisposition, age, prognosis, and response to remedies. Mucosal innate response to environmental triggers such as pollen, cigarette smoke, fragrances, viral infection, and house dust mite (HDM) are now recognized to play an important role in allergic asthma. HDM are the most pervasive allergens that co-habitat with us, as they are ubiquitous in-house dusts, mattress and bedsheets, and feed on a diet of exfoliated human skin flakes. Dermatophagoides pteronyssinus, is one among several HDM identified up to date. During the last decade, extensive studies have been fundamental in elucidating the interactions between HDM allergens, the host immune systems and airways. Moreover, the paradigm in the field of HDM-mediated allergy has been shifted away from being solely a Th2-geared to a complex response orchestrated via extensive crosstalk between the epithelium, professional antigen presenting cells (APCs) and components of the adaptive immunity. In fact, HDM have several lessons to teach us about their allergenicity, the complex interactions that stimulate innate immunity in initiating and perpetuating the lung inflammation. Herein, we review main allergens of Dermatophagoides pteronyssinus and their interactions with immunological sentinels that promote allergic sensitization and activation of innate immunity, which is critical for the development of the Th2 biased adaptive immunity to HDM allergens and development of allergic asthma.

Aggravated MRSA pneumonia secondary to influenza A virus infection is derived from decreased expression of IL-1β

Secondary methicillin-resistant Staphylococcus aureus (MRSA) infection is a cause of severe pneumonia with high mortality during influenza A virus (IAV) pandemics. Alveolar macrophages (AMs) mount cellular defenses against IAV and MRSA infection, which occurs via the nucleotide-binding domain-like receptor protein 3 (NLRP3) inflammasome. However, the activity and function of the NLRP3 inflammasome in MRSA pneumonia secondary to IAV infection remain unclear. To clarify this, we studied MRSA infection secondary to IAV both in vitro and in mouse model. The expression of the NLRP3 inflammasome was evaluated by quantitative reverse transcription polymerase chain reaction, immunofluorescence, Western blot, and enzyme-linked immunosorbent assay. The lung pathology and the rate of weight change were observed. We found that IAV infection for 1 week activated NLRP3 inflammasome. The enhanced expression of NLRP3, caspase-1, and cleaved caspase-1 was associated with MRSA infection secondary to IAV, but the expression of interleukin (IL)-1β decreased in superinfection with MRSA both in vitro and in vivo. The aggravated inflammatory pathology in MRSA pneumonia secondary to IAV infection was associated with decreased expression of IL-1β. And increased weight loss in MRSA pneumonia secondary to IAV infection was related to decreased concentration of IL-1β in serum. It infers that superinfection with MRSA reduces expression of IL-1β someway, and decreased expression of IL-1β impairs the host immunity and leads to aggravated pneumonia. These results contributed to our understanding of the detailed activity of the NLRP3 inflammasome, IL-1β, and their relationship with aggravation of MRSA pneumonia secondary to IAV infection. Immunotherapy targeting the IL-1β signaling pathway could be possible therapeutic strategy for secondary MRSA pneumonia.

HDAC6 mediates an aggresome-like mechanism for NLRP3 and pyrin inflammasome activation

Inflammasomes are supramolecular complexes that play key roles in immune surveillance. This is accomplished by the activation of inflammatory caspases, which leads to the proteolytic maturation of interleukin 1β (IL-1β) and pyroptosis. Here, we show that nucleotide-binding domain, leucine-rich repeat, and pyrin domain-containing protein 3 (NLRP3)- and pyrin-mediated inflammasome assembly, caspase activation, and IL-1β conversion occur at the microtubule-organizing center (MTOC). Furthermore, the dynein adapter histone deacetylase 6 (HDAC6) is indispensable for the microtubule transport and assembly of these inflammasomes both in vitro and in mice. Because HDAC6 can transport ubiquitinated pathological aggregates to the MTOC for aggresome formation and autophagosomal degradation, its role in NLRP3 and pyrin inflammasome activation also provides an inherent mechanism for the down-regulation of these inflammasomes by autophagy. This work suggests an unexpected parallel between the formation of physiological and pathological aggregates.

Neuroinflammation Mediated by NLRP3 Inflammasome After Intracerebral Hemorrhage and Potential Therapeutic Targets

Intracerebral hemorrhage (ICH) is the most fatal subtype of stroke; there is still a lack of effective treatment. Microglia are a major component of the innate immune system, and they respond to acute brain injury by activating and forming classic M1-like (pro-inflammatory) or alternative M2-like (anti-inflammatory) phenotype. The existence of the polarization indicates that the role of microglia in disease's progression and recovery after ICH is still unclear, perhaps involving microglial secretion of anti-inflammatory or pro-inflammatory cytokines and chemokines. The NOD-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome is considered to be the main participant in neuroinflammation. Recent evidence has shown that NLRP3 inflammasome can be activated after ICH, resulting in inflammatory cascade reactions and aggravating brain injury. Furthermore, previous studies have reported that NLRP3 inflammasome is mainly present in microglia, so we speculate that its activation may be strongly associated with microglial polarization. Many scholars have investigated the role of brain injury caused by NLRP3 inflammasome after ICH, but the precise operating mechanisms remain uncertain. This review summarized the activation mechanism of NLRP3 inflammasome after ICH and the possible mechanism of NLRP3 inflammasome promoting neuroinflammation and aggravating nerve injury and discussed the relevant potential therapeutic targets.

Pyroptotic and non-pyroptotic effector functions of caspase-11

Innate immune cells, epithelial cells, and many other cell types are capable of detecting infection or tissue injury, thus mounting regulated immune response. Inflammasomes are highly sophisticated and effective orchestrators of innate immunity. These oligomerized multiprotein complexes are at the center of various innate immune pathways, including modulation of the cytoskeleton, production and maturation of cytokines, and control of bacterial growth and cell death. Inflammasome assembly often results in caspase‐1 activation, which is an inflammatory caspase that is involved in pyroptotic cell death and release of inflammatory cytokines in response to pathogen patterns and endogenous danger stimuli. However, the nature of stimuli and inflammasome components are diverse. Caspase‐1 activation mediated release of mature IL‐1β and IL‐18 in response to canonical stimuli initiated by NOD‐like receptor (NLR), and apoptosis‐associated speck‐like protein containing a caspase recruitment domain (ASC). On the other hand, caspase‐11 delineates a non‐canonical inflammasome that promotes pyroptotic cell death and non‐pyroptotic functions in response to non‐canonical stimuli. Caspase‐11 in mice and its homologues in humans (caspase‐4/5) belong to caspase‐1 family of cysteine proteases, and play a role in inflammation. Knockout mice provided new genetic tools to study inflammatory caspases and revealed the role of caspase‐11 in mediating septic shock in response to lethal doses of lipopolysaccharide (LPS). Recognition of LPS mediates caspase‐11 activation, which promotes a myriad of downstream effects that include pyroptotic and non‐pyroptotic effector functions. Therefore, the physiological functions of caspase‐11 are much broader than its previously established roles in apoptosis and cytokine maturation. Inflammation induced by exogenous or endogenous agents can be detrimental and, if excessive, can result in organ and tissue damage. Consequently, the existence of sophisticated mechanisms that tightly regulate the specificity and sensitivity of inflammasome pathways provides a fine‐tuning balance between adequate immune response and minimal tissue damage. In this review, we summarize effector functions of caspase‐11.

Selective inhibition of NLRP3 inflammasome by designed peptide originating from ASC

NACHT, LRR, and PYD domains-containing protein 3 (NLRP3) inflammasome is a multiprotein complex which forms within cells in response to various microbial and self-derived triggers. Mutations in the gene encoding NLRP3 cause rare cryopyrin-associated periodic syndromes (CAPS) and growing evidence links NLRP3 inflammasome to common diseases such as Alzheimer´s disease. In order to modulate different stages of NLRP3 inflammasome assembly nine peptides whose sequences correspond to segments of inflammasome components NLRP3 and apoptosis-associated speck-like protein containing a CARD (ASC) were selected. Five peptides inhibited IL-1β release, caspase-1 activation and ASC oligomerization in response to soluble and particulate NLRP3 triggers. Modulatory peptides also attenuated IL-1β maturation induced by constitutive CAPS-associated NLRP3 mutants. Peptide corresponding to H2-H3 segment of ASC pyrin domain selectively inhibited NLRP3 inflammasome by binding to NLRP3 pyrin domain in the micromolar range. The peptide had no effect on AIM2 and NLRC4 inflammasomes as well as NF-κB pathway. The peptide effectively dampened neutrophil infiltration in the silica-induced peritonitis and when equipped with Antennapedia or Angiopep-2 motifs crossed the blood-brain barrier in a mouse model. Our study demonstrates that peptides represent an important tool for targeting multiprotein inflammatory complexes and can serve as the basis for the development of novel anti-inflammatory strategies for neurodegeneration.

Effects of PYRIN-containing Apaf1-like protein 1 on isoflurane-induced postoperative cognitive dysfunction in aged rats

Postoperative cognitive dysfunction (POCD) is a prevalent neurocognitive disorder following surgery and anesthesia, particularly in elderly patients. Isoflurane is a widely used anesthetic agent, which is associated with the development of POCD; however, the precise mechanisms remain unclear. In the present study, aged rats were exposed to 2% isoflurane to establish a POCD model. The expression of PYRIN-containing Apaf1-like protein 1 (PYPAF1) was knocked down using a lentivirus containing specific short hairpin RNA. Subsequently, the spatial learning ability of rats was assessed using the Morris water maze. In addition, mRNA and protein expression levels were detected using reverse transcription-quantitative PCR and western blot analysis, respectively. Immunofluorescence double staining was also used to determine the expression of PYPAF1 and Iba-1 in the hippocampus. Neural apoptosis was observed using TUNEL-NeuN double staining. The results revealed that isoflurane exposure impaired the spatial learning ability of rats, while PYPAF1 knockdown alleviated cognitive impairment. In addition, isoflurane exposure induced activation of the PYPAF1 inflammasome, as evidenced by elevated expression of PYPAF1 and apoptosis-associated speck-like protein containing a caspase recruitment domain, while silencing of PYPAF1 partially reversed this effect. Furthermore, isoflurane exposure promoted the activation of microglia and caspase-1, and the secretion of interleukin (IL)-1β and IL-18, all of which were alleviated following PYPAF1 silencing. Moreover, isoflurane exposure induced neuronal apoptosis, elevated the levels of Bax and cleaved caspase-3, and inhibited the expression of Bcl-2; all of these effects were partially abrogated following PYPAF1 silencing. In conclusion, the results of the present study indicated that PYPAF1 silencing partially abolished isoflurane-induced cognitive impairment, neuroinflammation and neuronal apoptosis. Therefore, PYPAF1 may be a potential therapeutic target for treatment of POCD.

Multiple roles of caspase-8 in cell death, inflammation, and innate immunity

Caspase-8 is an apical caspase involved in the programmed form of cell death called apoptosis that is critically important for mammalian development and immunity. Apoptosis was historically described as immunologically silent in contrast to other types of programmed cell death such as necroptosis or pyroptosis. Recent reports suggest considerable crosstalk between these different forms of cell death. It is becoming increasingly clear that caspase-8 has many non-apoptotic roles, participating in multiple processes including regulation of necroptosis (mediated by receptor-interacting serine/threonine kinases, RIPK1-RIPK3), inflammatory cytokine expression, inflammasome activation, and cleavage of IL-1β and gasdermin D, and protection against shock and microbial infection. In this review, we discuss the involvement of caspase-8 in cell death and inflammation and highlight its role in innate immune responses and in the relationship between different forms of cell death. Caspase-8 is one of the central components in this type of crosstalk.

Inhibition of NLRP3 Inflammasome: A Prospective Target for the Treatment of Ischemic Stroke

Stroke is one of the major devastating diseases with no effective medical therapeutics. Because of the high rate of disability and mortality among stroke patients, new treatments are urgently required to decrease brain damage following a stroke. In recent years, the inflammasome is a novel breakthrough point that plays an important role in the stroke, and the inhibition of inflammasome may be an effective method for stroke treatment. Briefly, inflammasome is a multi-protein complex that causes activation of caspase-1 and subsequent production of pro-inflammatory factors including interleukin (IL)-18 and IL-1β. Among them, the NLRP3 inflammasome is the most typical inflammasome, which can detect cell damage and mediate inflammatory response to tissue damage in ischemic stroke. The NLRP3 inflammasome has become a key mediator of post-ischemic inflammation, leading to a cascade of inflammatory reactions and cell death eventually. Thus, NLRP3 inflammasome is an ideal therapeutic target due to its important role in the inflammatory response after ischemic stroke. In this mini review article, we will summarize the structure, assembly, and regulation of NLRP3 inflammasome, the role of NLRP3 inflammasome in ischemic stroke, and several treatments targeting NLRP3 inflammasome in ischemic stroke. The further understanding of the mechanism of NLRP3 inflammasome in patients with ischemic stroke will provide novel targets for the treatment of cerebral ischemic stroke patients.

Characterization of the inflammasome component SmASC in turbot (Scophthalmus maximus)

Apoptosis-associated speck-like protein containing a C-terminal caspase recruit domain (ASC) is an important adapter protein in the inflammasome complex that mediates inflammatory caspase activation and host innate immunity in mammals. However, the function of inflammasome components in lower vertebrate remains poorly understood. In this study, full length of SmASC was cloned from turbot (Scophthalmus maximus). Through bioinformatic analysis, we found that SmASC shares relatively high identity with ASC in bony fish. Furthermore, we found that the intact SmASC can form an oligomeric speck-like structure, while the PYD segment of SmASC can form the filamentous structure. Moreover, expression of SmASC was induced after intraperitoneal injection of Edwardsiella piscicida (E. piscicida) in vivo. To further explore the role of SmASC during infection, we constructed SmASC knockdown and overexpression models by administration of siRNA and overexpression plasmids in vivo, respectively. Expression of SmASC decreased the propagation of E. piscicida in different immune organs. In summary, our results characterize the function of SmASC in S. maximus, suggesting that the SmASC plays a critical role in turbot immune responses.

Comprehensive review of ASC structure and function in immune homeostasis and disease

Apoptosis associated speck like protein containing CARD (ASC) is widely researched and recognized as an adaptor protein participating in inflammasome assembly and pyroptosis. It contains a bipartite structure comprising of a pyrin and a caspase recruitment domain (CARD) domain. These two domains help ASC function as an adaptor molecule. ASC is encoded by the gene PYCARD. ASC plays pivotal role in various diseases as well as different homeostatic processes. ASC plays a regulatory role in different cancers showing differential regulation with respect to tissue and stage of disease. Besides cancer, ASC also plays a central role in sensing, regulation, and/or disease progression in bacterial infections, viral infections and in varied inflammatory diseases. ASC is expressed in different types of immune and non-immune cells. Its localization pattern also varies with different kinds of stimuli encountered by cell. This review will summarize the literature on the structure cellular and tissue expression, localization and disease association of ASC.

DROSHA-Dependent miRNA and AIM2 Inflammasome Activation in Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive interstitial lung disease. Chronic lung inflammation is linked to the pathogenesis of IPF. DROSHA, a class 2 ribonuclease III enzyme, has an important role in the biogenesis of microRNA (miRNA). The function of miRNAs has been identified in the regulation of the target gene or protein related to inflammatory responses via degradation of mRNA or inhibition of translation. The absent-in-melanoma-2 (AIM2) inflammasome is critical for inflammatory responses against cytosolic double stranded DNA (dsDNA) from pathogen-associated molecular patterns (PAMPs) and self-DNA from danger-associated molecular patterns (DAMPs). The AIM2 inflammasome senses double strand DNA (dsDNA) and interacts with the adaptor apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), which recruits pro-caspase-1 and regulates the maturation and secretion of interleukin (IL)-1β and IL-18. A recent study showed that inflammasome activation contributes to lung inflammation and fibrogenesis during IPF. In the current review, we discuss recent advances in our understanding of the DROSHA-miRNA-AIM2 inflammasome axis in the pathogenesis of IPF.