NLRP7: From inflammasome regulation to human disease

Nucleotide‐binding oligomerization domain (NOD) and leucine‐rich repeat (LRR)‐containing receptors or NOD‐like receptors (NLRs) are cytosolic pattern recognition receptors, which sense conserved microbial patterns and host‐derived danger signals to elicit innate immune responses. The activation of several prototypic NLRs, including NLR and pyrin domain (PYD) containing (NLRP) 1, NLRP3 and NLR and caspase recruitment domain (CARD) containing (NLRC) 4, results in the assembly of inflammasomes, which are large, cytoplasmic multiprotein signalling platforms responsible for the maturation and release of the pro‐inflammatory cytokines IL‐1β and IL‐18, and for the induction of a specialized form of inflammatory cell death called pyroptosis. However, the function of other members of the NLR family, including NLRP7, are less well understood. NLRP7 has been linked to innate immune signalling, but its precise role is still controversial as it has been shown to positively and negatively affect inflammasome responses. Inflammasomes are essential for homeostasis and host defence, but inappropriate inflammasome responses due to hereditary mutations and somatic mosaicism in inflammasome components and defective regulation have been linked to a broad spectrum of human diseases. A compelling connection between NLRP7 mutations and reproductive diseases, and in particular molar pregnancy, has been established. However, the molecular mechanisms by which NLRP7 mutations contribute to reproductive diseases are largely unknown. In this review, we focus on NLRP7 and discuss the current evidence of its role in inflammasome regulation and its implication in human reproductive diseases.     

NLRX1 protein attenuates inflammatory responses to infection by interfering with the RIG-I-MAVS and TRAF6-NF-κB signaling pathways

The nucleotide-binding domain and leucine-rich-repeat-containing (NLR) proteins regulate innate immunity. Although the positive regulatory impact of NLRs is clear, their inhibitory roles are not well defined. We showed that Nlrx1(-/-) mice exhibited increased expression of antiviral signaling molecules IFN-β, STAT2, OAS1, and IL-6 after influenza virus infection. Consistent with increased inflammation, Nlrx1(-/-) mice exhibited marked morbidity and histopathology. Infection of these mice with an influenza strain that carries a mutated NS-1 protein, which normally prevents IFN induction by interaction with?RNA and the intracellular RNA sensor RIG-I, further exacerbated IL-6 and type I IFN signaling. NLRX1 also weakened cytokine responses to the 2009 H1N1 pandemic influenza virus in human cells.?Mechanistically, Nlrx1 deletion led to constitutive interaction of MAVS and RIG-I. Additionally, an inhibitory function is identified for NLRX1 during LPS activation of macrophages where the MAVS-RIG-I pathway was not involved. NLRX1 interacts with TRAF6 and inhibits NF-κB activation. Thus, NLRX1 functions as a checkpoint of overzealous inflammation.     

连续传代培养对hUC-MSCs上NLR家族mRNA表达的影响*

 目的：探讨连续传代培养对人脐带间充质干细胞（hUC-MSCs）表达NOD样受体（NLR）家族所有23个成员mRNA的影响,为改进hUC-MSCs传代培养质量与增加hUC-MSCs在实验和临床应用中的数量及安全性寻找切入途径。方法：取剖宫产无菌新生儿脐带,经胶原酶II消化结合贴壁选择分离纯化hUC-MSCs,并连续传代培养;采用流式细胞术、诱导分化和RT-qPCR对连续培养传代前后的hUC-MSCs（第3和28代）进行鉴定并对23个NLR家族成员mRNA的表达进行定量分析。结果：连续传代培养前后hUC-MSCs的细胞表型均为CD29+/CD44+/CD105+/CD31-/CD34-/CD40-/CD45-/CD106-/HLA-DR-,能被诱导为成骨和脂肪细胞,符合国际细胞治疗协会建议的MSCs基本特征。全部NLR家族成员的mRNA在培养第3代hUC-MSCs上均有表达,且NOD1、NLRC4、NLRC5、NLRP1、NLRP3、NLRP10、NAIP、NLRX1和APAF1高表达,其余成员低表达。培养传代至第28代除NLRP10 的mRNA表达上升、NLRC5和NLRX1 mRNA表达基本不变外,其余成员的mRNA表达均有所下降,其中NLRP1 mRNA表达差异有统计学意义（P<0.05）。结论：体外连续传代培养对hUC-MSCs表达NLR家族成员的影响是多向性的。这些影响与MSCs的增殖、分化及其免疫调节功能的联系有待进一步实验探讨。     

NLRC5 Mediates Cytokine Secretion in RAW264.7 Macrophages and Modulated by the JAK2/STAT3 Pathway

The nucleotide-binding domain leucine-rich repeat proteins (NLRs), a class of innate immune receptors that respond to pathogen attack or cellular stress, have gained increasing attention. NLRC5 is the largest member of NLR family, which has recently been identified as a critical regulator of immune responses. In this study, we explore the role of NLRC5 in cytokine secretion and the role of the JAK2/STAT3 signaling pathway in lipopolysaccharide-induced NLRC5 expression in RAW264.7 cells. We demonstrated that overexpression of NLRC5 results in a downregulation of interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) secretion; on the other hand, knockdown of NLRC5 by transfecting siRNA enhanced IL-6 and TNF-α secretion in RAW264.7 cells. These results indicated that NLRC5 plays a negative role in the regulation of IL-6 and TNF-α. Meanwhile, AG490 (a specific inhibitor of the JAK2/STAT3 signaling pathway) and JAK2 siRNA were used to manipulate JAK2/STAT3 activity. Finally, the results showed that AG490 and JAK2 siRNA inhibited NLRC5 expression and the expression levels of p-JAK2 and p-STAT3. We, for the first time, demonstrate that the inhibition of the JAK2/STAT3 signaling pathway results in decrease of NLRC5 expression.     

NLRC5 interacts with RIG‐I to induce a robust antiviral response against influenza virus infection

The NLR protein, NLRC5 is an important regulator of MHC class I gene expression, however, the role of NLRC5 in other innate immune responses is less well defined. In the present study, we report that NLRC5 binds RIG‐I and that this interaction is critical for robust antiviral responses against influenza virus. Overexpression of NLRC5 in the human lung epithelial cell line, A549, and normal human bronchial epithelial cells resulted in impaired replication of influenza virus A/Puerto Rico/8/34 virus (PR8) and enhanced IFN‐β expression. Influenza virus leads to induction of IFN‐β that drives RIG‐I and NLRC5 expression in host cells. Our results suggest that NLRC5 extends and stabilizes influenza virus induced RIG‐I expression and delays expression of the viral inhibitor protein NS1. We show that NS1 binds to NLRC5 to suppress its function. Interaction domain mapping revealed that NLRC5 interacts with RIG‐I via its N‐terminal death domain and that NLRC5 enhanced antiviral activity in an leucine‐rich repeat domain independent manner. Taken together, our findings identify a novel role for NLRC5 in RIG‐I‐mediated antiviral host responses against influenza virus infection, distinguished from the role of NLRC5 in MHC class I gene regulation.     

NOD-like subfamily of the nucleotide-binding domain and leucine-rich repeat containing family receptors and their expression in channel catfish

The NLRs (nucleotide-binding domain and leucine-rich repeat containing family receptors) are a recently identified family of pattern recognition receptors in vertebrates. Several subfamilies of NLRs have been characterized in human, mouse, and zebrafish, but studies of NLRs in other species, especially teleost species, have been lacking. Here we report characterization of five NLRs from channel catfish: NOD1, NOD2, NLRC3, NLRC5, and NLRX1. Structural analysis indicated that the genes were organized in a similar fashion as in the mammals and in zebrafish. Phylogenetic analysis suggested that they were orthologous to the NOD-like subfamily of NLRs. All five NOD-like genes exist as a single copy gene in the catfish genome. Hybridization of gene-specific probes allowed mapping of three NLR genes to the catfish physical map, laying a foundation for genome characterization and for establishing orthologies with NLR genes from other species. These genes are widely expressed in various tissues and leukocyte cell lines. While the majority of the NLR genes appeared to be constitutively expressed, NOD1 was induced after infection with a bacterial pathogen, Edwardsiella ictaluri, the causative agent of enteric septicemia of catfish (ESC), suggesting its involvement in immunity against the intracellular pathogen.     

Autophagy as an innate immunity paradigm: expanding the scope and repertoire of pattern recognition receptors

Autophagy is rapidly developing into a new immunological paradigm. The latest links now include overlaps between autophagy and innate immune signaling via TBK-1 and IKKα/β, and the role of autophagy in inflammation directed by the inflammasome. Autophagy's innate immunity connections include responses to pathogen and damage-associated molecular patterns including alarmins such as HMGB1 and IL-1β, Toll-like receptors, Nod-like receptors including NLRC4, NLRP3 and NLRP4, and RIG-I-like receptors. Autophagic adaptors referred to as SLRs (sequestosome 1/p62-like receptors) are themselves a category of pattern recognition receptors. SLRs empower autophagy to eliminate intracellular microbes by direct capture and by facilitating generation and delivery of antimicrobial peptides, and also serve as inflammatory signaling platforms. SLRs contribute to autophagic control of intracellular microbes, including Mycobacterium tuberculosis, Salmonella, Listeria, Shigella, HIV-1 and Sindbis virus, but act as double-edged sword and contribute to inflammation and cell death. Autophagy roles in innate immunity continue to expand vertically and laterally, and now include antimicrobial function downstream of vitamin D3 action in tuberculosis and AIDS. Recent data expand the connections between immunity-related GTPases and autophagy to include not only IRGM but also several members of the Gbp (guanlyate-binding proteins) family. The efficacy with which autophagy handles microbes, microbial products and sterile endogenous irritants governs whether the outcome will be with suppression of or with excess inflammation, the latter reflected in human diseases that have strong inflammatory components including tuberculosis and Crohn's disease.     

Toll-like receptors (TLRs) and Nod-like receptors (NLRs) in inflammatory disorders

Toll-like receptors (TLRs) and Nod-like receptors (NLRs) are two major forms of innate immune sensors, which provide immediate responses against pathogenic invasion or tissue injury. Activation of these sensors induces the recruitment of innate immune cells such as macrophages and neutrophils, initiates tissue repair processes, and results in adaptive immune activation. Abnormalities in any of these innate sensor-mediated processes may cause excessive inflammation due to either hyper responsive innate immune signaling or sustained compensatory adaptive immune activation. Recent gene association studies appear to reveal strong associations of NLR gene mutations and development of several idiopathic inflammatory disorders. In contrast, TLR polymorphisms are less often associated with inflammatory disorders. Nevertheless, TLRs are up-regulated in the affected tissue of most inflammatory disorders, suggesting TLR signaling is involved in the pathogenesis of chronic and/or idiopathic inflammatory disorders. NLR signaling results in the formation of a molecular scaffold complex (termed an inflammasome) and orchestrates with TLRs to induce IL-1β and IL-18, both of which are important mediators in the majority of inflammatory disorders. Therefore, understanding the roles of TLRs and NLRs in the pathogenesis of chronic and idiopathic inflammatory disorders may provide novel targets for the prevention and/or treatment of many common and uncommon diseases involving inflammation.     

What is new with Nods?

Over the last few years, much research has focused on determining the function of members of the cytosolic Nod-like receptor (NLR) family in terms of their triggers and the signaling pathways that they control. Members of this family include the NLRP proteins, which play a role in sensing both microbial and danger signals and triggering the caspase-1 dependent inflammasome, and the Nod subfamily characterized by proteins with a caspase-activating and recruitment domain (CARD) or a so-called 'X' domain. Nod1, Nod2, NLRX1 and NLRC5 are all members of this subfamily and in this review, we will focus on recent work that has shown the importance of these molecules in both pathogen sensing and regulation of innate and adaptive immunity.     

NLRC4/IPAF: a CARD carrying member of the NLR family

The NOD-like receptor (NLR) family of proteins is involved in the regulation of innate immune responses and cell death pathways. Recent findings show that the NLR family member NLRC4 (also known as IPAF) has important roles in innate immune responses to Gram-negative bacteria. Macrophages infected with Legionella pneumophila, Salmonella typhimurium, Shigella flexneri, or Pseudomonas aeruginosa activate caspase-1 in an NLRC4-dependent manner leading to macrophage cell death and the release of proinflammatory cytokines. This review will discuss these findings as well as the role of bacterial type III and type IV secretion systems and flagellin in NLRC4-mediated caspase-1 activation.     

Polymorphisms in inflammasome' genes and susceptibility to HIV-1 infection

The involvement of inflammasome genes in the susceptibility to HIV-1 infection was investigated. Twelve single nucleotide polymorphisms within NLRP1, NLRP3, NLRC4, CARD8, CASP1, and IL1B genes were analyzed in 150 HIV-1-infected Brazilian subjects and 158 healthy controls. The 2 polymorphisms rs10754558 in NLRP3 and rs1143634 in IL1B were significantly associated to the HIV-1 infection. These findings supported the previously hypothesized involvement of NALP3-inflammasome in HIV-1 pathogenesis, underlining once more the key role of inflammation and innate immunity in the susceptibility to HIV-1 infection.     

Expression pattern of NLRC5 in the postnatal mouse brain

Nucleotide oligomerization domain-like receptors (NLRs), belonging to a large family of pattern recognition receptors, participate in the host’s first line of defense against invading pathogens. Caspase recruitment domain containing 5 (NLRC5), the largest member in the NLR family, is demonstrated to be involved in the innate immune response and inflammatory diseases far and wide. Recent studies report that NLRC5 is associated with some central nervous system (CNS) diseases. Besides, NLRC5 is a mastery regulator for the expression of MHC class I both in the immune system and the CNS, while MHC class I is expressed and exerts its function in the brain. Therefore, it is necessary to investigate the expression pattern of NLRC5 in the developing and adult CNS. In our study, postnatal brain sections of C57BL/6 J mice are analyzed for the expression of NLRC5 protein by immunofluorescence. In the postnatal stages of developing telencephalon, NLRC5 exhibits a spatial and temporal expression pattern. NLRC5 is time-specifically expressed in subfields of hippocampus and different layers of prefrontal cortex. Moreover, it is shown that NLRC5 is highly cell type specific. It can be expressed in large quantities by neurons and microglia, but rarely expressed by astrocytes. Taken together, our research is important for further understanding the biological characteristics of NLRC5 and its function in the CNS.     

The protein kinase PKR is critical for LPS‐induced iNOS production but dispensable for inflammasome activation in macrophages

Inflammasomes are multi‐protein platforms that drive the activation of caspase‐1 leading to the processing and secretion of biologically active IL‐1β and IL‐18. Different inflammasomes including NOD‐like receptor (NLR) family pyrin domain‐containing 3 (NLRP3), NLR caspase‐recruitment domain‐containing 4 (NLRC4) and absent in melanoma 2 (AIM2) are activated and assembled in response to distinct microbial or endogenous stimuli. However, the mechanisms by which upstream stimuli trigger inflammasome activation remain poorly understood. Double‐stranded RNA‐activated protein kinase (PKR), a protein kinase activated by viral infection, has been recently shown to be required for the activation of the inflammasomes. Using macrophages from two different mouse strains deficient in PKR, we found that PKR is important for the induction of the inducible nitric oxide synthase (iNOS). However, PKR was dispensable for caspase‐1 activation, processing of pro‐IL‐1β/IL‐18 and secretion of IL‐1β induced by stimuli that trigger the activation of NLRP3, NLRC4 and AIM2. These results indicate that PKR is not required for inflammasome activation in macrophages.     

A new eye on NLR proteins: focused on clarity or diffused by complexity?

The nucleotide-binding domain leucine-rich repeat proteins (NLRs) represent the major class of intracellular innate immune receptors in plants and animals. Understanding their functions is a major challenge in immunology. This review highlights recent efforts toward elucidating NLR functions in human and plants. We compare unconventional aspects of NLR proteins across the two kingdoms. We review recent advances describing P-loop independent activation, nuclear-cytoplasmic trafficking, oligomerization and multimerization requirements for signaling, and for expanded functions beyond pathogen recognition by several NLR proteins.     

Molecular Mechanism of NLRP3 Inflammasome Activation

The inflammasome is an intracellular multimolecular complex that controls caspase-1 activity in the innate immune system. NLRP3, a member of the NLR family of cytosolic pattern recognition receptors, along with the adaptor protein ASC, mediates caspase-1 activation via assembly of the inflammasome in response to various pathogen-derived factors as well as danger-associated molecules. The active NLRP3 inflammasome drives innate immune response towards invading pathogens and cellular damage, and regulates adaptive immune response. Here, we review identified agonists of the NLRP3 inflammasome and the molecular mechanism by which they induce NLRP3 inflammasome activation. Three signaling pathways involving potassium efflux, generation of reactive oxygen species, and cathepsin B release are discussed.