Osteoblasts express NLRP3, a nucleotide-binding domain and leucine-rich repeat region containing receptor implicated in bacterially induced cell death

Inhibition of NLRP3 alleviates calcium oxalate crystal-induced renal fibrosis and crystal adhesion

Kidney stones are gaining attention as one of the most common urological diseases. In this study, we first constructed a mouse model of calcium oxalate (CaOx) crystal deposition by intraperitoneal injection of glyoxalate (Gly) and found that the levels of NLRP3, CASP1 and ASC, which constitute the NLRP3 inflammasome, as well as the level of its downstream product, IL-1β, were elevated in the kidneys of the model group of mice, as determined by RNA-seq. We then examined NLRP3 expression via immunohistochemistry, immunofluorescence, qPCR, and Western blotting in human samples, calcium oxalate monohydrate (COM)-stimulated HK2 cells, and a model of calcium oxalate crystal deposition via intraperitoneal injection. We then constructed systemic NLRP3 knockout mice and found via RNA-seq that CaOx crystal-induced renal fibrosis and crystal adhesion may be attenuated after the knockout of NLRP3. We further substantiated these findings by knocking down NLRP3 both in vitro and in NLRP3-knockout mice. Consistently, we observed more pronounced calcium oxalate crystal-induced renal fibrosis and enhanced crystal adhesion upon overexpression of NLRP3 in vitro and in vivo. Ultimately, we utilized the NLRP3 inhibitor MCC950 to support the potential of NLRP3 as a therapeutic target. Our research revealed that NLRP3 plays a pivotal role in kidney stone formation by mitigating renal fibrosis and reducing crystal adhesion induced by CaOx crystals.

Periodontitis increases the risk of gastrointestinal dysfunction: an update on the plausible pathogenic molecular mechanisms

Periodontitis is an immuno-inflammatory disease of the soft tissues surrounding the teeth. Periodontitis is linked to many communicable and non-communicable diseases such as diabetes, cardiovascular disease, rheumatoid arthritis, and cancers. The oral-systemic link between periodontal disease and systemic diseases is attributed to the spread of inflammation, microbial products and microbes to distant organ systems. Oral bacteria reach the gut via swallowed saliva, whereby they induce gut dysbiosis and gastrointestinal dysfunctions. Some periodontal pathogens like Porphyromonas. gingivalis, Klebsiella, Helicobacter. Pylori, Streptococcus, Veillonella, Parvimonas micra, Fusobacterium nucleatum, Peptostreptococcus, Haemophilus, Aggregatibacter actinomycetomcommitans and Streptococcus mutans can withstand the unfavorable acidic, survive in the gut and result in gut dysbiosis. Gut dysbiosis increases gut inflammation, and induce dysplastic changes that lead to gut dysfunction. Various studies have linked oral bacteria, and oral-gut axis to various GIT disorders like inflammatory bowel disease, liver diseases, hepatocellular and pancreatic ductal carcinoma, ulcerative colitis, and Crohn's disease. Although the correlation between periodontitis and GIT disorders is well established, the intricate molecular mechanisms by which oral microflora induce these changes have not been discussed extensively. This review comprehensively discusses the intricate and unique molecular and immunological mechanisms by which periodontal pathogens can induce gut dysbiosis and dysfunction.

DAMPs, PAMPs, NLRs, RIGs, CLRs and TLRs - Understanding the Alphabet Soup in the Context of Bone Biology

PURPOSE OF REVIEW

The purpose of this review is to summarize the current understanding of cell-autonomous innate immune pathways that contribute to bone homeostasis and disease.

RECENT FINDINGS

Germ-line encoded pattern recognition receptors (PRRs) are the first line of defense against danger and infections. In the bone microenvironment, PRRs and downstream signaling pathways, that mount immune defense, interface intimately with the core cellular processes in bone cells to alter bone formation and resorption. The role of PRR engagement on bone remodeling has been best described as a result of activated macrophages secreting effector molecules that reshape the characteristics of bone-resident cells. However, it is being increasingly recognized that local bone resident-cells like osteoclasts and osteoblasts possess an arsenal of PRRs. The engagement of these PRRs by stimuli in the bone niche can drive cell-autonomous (aka cell-intrinsic) responses that, in turn, impact bone-remodeling dramatically, irrespective of immune cell effectors. Indeed, this vital role for cell-autonomous innate immune responses is evident in how reduced PRR activity within osteoclast progenitors correlates with their reduced differentiation and abnormal bone remodeling. Further, cell-intrinsic PRR activity has now been shown to influence the behavior of osteoblasts, osteocytes and other local immune/non-immune cell populations. However, distinct PRR families have varying impact on bone homeostasis and inflammation, emphasizing the importance of investigating these different nodes of innate immune signaling in bone cells to better identify how they synergistically and/or antagonistically regulate bone remodeling in the course of an immune response. Innate immune sensing within bone resident cells is a critical determinant for bone remodeling in health and disease.

Pyroptosis in Skeleton Diseases: A Potential Therapeutic Target Based on Inflammatory Cell Death

Skeletal disorders, including fractures, osteoporosis, osteoarthritis, rheumatoid arthritis, and spinal degenerative conditions, along with associated spinal cord injuries, significantly impair daily life and impose a substantial burden. Many of these conditions are notably linked to inflammation, with some classified as inflammatory diseases. Pyroptosis, a newly recognized form of inflammatory cell death, is primarily triggered by inflammasomes and executed by caspases, leading to inflammation and cell death through gasdermin proteins. Emerging research underscores the pivotal role of pyroptosis in skeletal disorders. This review explores the pyroptosis signaling pathways and their involvement in skeletal diseases, the modulation of pyroptosis by other signals in these conditions, and the current evidence supporting the therapeutic potential of targeting pyroptosis in treating skeletal disorders, aiming to offer novel insights for their management.

Immunostimulatory nucleic acid nanoparticles (NANPs) augment protective osteoblast and osteoclast type I interferon responses to Staphylococcus aureus

Recalcitrant staphylococcal osteomyelitis may be due, in part, to the ability of Staphylococcus aureus to invade bone cells. However, osteoclasts and osteoblasts are now recognized to shape host responses to bacterial infection and we have recently described their ability to produce IFN-β following S. aureus infection and limit intracellular bacterial survival/propagation. Here, we have investigated the ability of novel, rationally designed, nucleic acid nanoparticles (NANPs) to induce the production of immune mediators, including IFN-β, following introduction into bone cells. We demonstrate the successful delivery of representative NANPs into osteoblasts and osteoclasts via endosomal trafficking when complexed with lipid-based carriers. Their delivery was found to differentially induce immune responses according to their composition and architecture via discrete cytosolic pattern recognition receptors. Finally, the utility of this nanoparticle technology was supported by the demonstration that immunostimulatory NANPs augment IFN-β production by S. aureus infected bone cells and reduce intracellular bacterial burden.

Osteoblast-specific down-regulation of NLRP3 inflammasome by aptamer-functionalized liposome nanoparticles improves bone quality in postmenopausal osteoporosis rats

Rationale: NLRP3 inflammasome is critical in the development and progression of many metabolic diseases driven by chronic inflammation, but its effect on the pathology of postmenopausal osteoporosis (PMOP) remains poorly understood. Methods: We here firstly examined the levels of NLRP3 inflammasome in PMOP patients by ELISA. Then we investigated the possible mechanisms underlying the effect of NLRP3 inflammasome on PMOP by RNA sequencing of osteoblasts treated with NLRP3 siRNA and qPCR. Lastly, we accessed the effect of decreased NLRP3 levels on ovariectomized (OVX) rats. To specifically deliver NLRP3 siRNA to osteoblasts, we constructed NLRP3 siRNA wrapping osteoblast-specific aptamer (CH6)-functionalized lipid nanoparticles (termed as CH6-LNPs-siNLRP3). Results: We found that the levels of NLRP3 inflammasome were significantly increased in patients with PMOP, and were negatively correlated with estradiol levels. NLRP3 knock-down influenced signal pathways including immune system process, interferon signal pathway. Notably, of the top ten up-regulated genes in NLRP3-reduced osteoblasts, nine genes (except Mx2) were enriched in immune system process, and five genes were related to interferon signal pathway. The in vitro results showed that CH6-LNPs-siNLRP3 was relatively uniform with a dimeter of 96.64 ± 16.83 nm and zeta potential of 38.37 ± 1.86 mV. CH6-LNPs-siNLRP3 did not show obvious cytotoxicity and selectively delivered siRNA to bone tissue. Moreover, CH6-LNPs-siNLRP3 stimulated osteoblast differentiation by activating ALP and enhancing osteoblast matrix mineralization. When administrated to OVX rats, CH6-LNPs-siNLRP3 promoted bone formation and bone mass, improved bone microarchitecture and mechanical properties by decreasing the levels of NLRP3, IL-1β and IL-18 and increasing the levels of OCN and Runx2. Conclusion: NLRP3 inflammasome may be a new biomarker for PMOP diagnosis and plays a key role in the pathology of PMOP. CH6-LNPs-siNLRP3 has potential application for the treatment of PMOP.

The Impact of NLRP3 Inflammasome on Osteoblasts and Osteogenic Differentiation: A Literature Review

Osteoblasts (OBs), which are a crucial type of bone cells, derive from bone marrow mesenchymal stem cells (MSCs). Accumulating evidence suggests inflammatory cytokines can inhibit the differentiation and proliferation of OBs, as well as interfere with their ability to synthesize bone matrix, under inflammatory conditions. NLRP3 inflammasome is closely associated with cellular pyroptosis, which can lead to excessive release of pro-inflammatory cytokines, causing tissue damage and inflammatory responses, however, the comprehensive roles of NLRP3 inflammasome in OBs and their differentiation have not been fully elucidated, making targeting NLRP3 inflammasome approaches to treat diseases related to OBs uncertain. In this review, we provide a summary of NLRP3 inflammasome activation and its impact on OBs. We highlight the significant roles of NLRP3 inflammasome in regulating OBs differentiation and function. Furthermore, current available strategies to affect OBs function and osteogenic differentiation targeting NLRP3 inflammasome are listed and analyzed. Finally, through the prospective discussion, we seek to provide novel insights into the crucial role of NLRP3 inflammasome in diseases related to OBs and offer valuable information for devising treatment strategies.

NLRP3 Inhibition Leads to Impaired Mucosal Fibroblast Function in Patients with Inflammatory Bowel Diseases

BACKGROUND AND AIMS

Inflammatory bowel diseases (IBD) are characterized by mucosal inflammation and sequential fibrosis formation, but the exact role of the hyperactive NLRP3 inflammasome in these processes is unclear. Thus, we studied the expression and function of the NLRP3 inflammasome in the context of inflammation and fibrosis in IBD.

METHODS

We analysed intestinal NLRP3 expression in mucosal immune cells and fibroblasts from IBD patients and NLRP3-associated gene expression via single-cell RNA sequencing and microarray analyses. Furthermore, cytokine secretion of NLRP3 inhibitor treated blood and mucosal cells, as well as proliferation, collagen production, and cell death of NLRP3 inhibitor treated intestinal fibroblasts from IBD patients were studied.

RESULTS

We found increased NLRP3 expression in the inflamed mucosa of IBD patients and NLRP3 inhibition led to reduced IL-1β and IL-18 production in blood cells and diminished the bioactive form of mucosal IL-1β. Single cell analysis identified overlapping expression patterns of NLRP3 and IL-1β in classically activated intestinal macrophages and we also detected NLRP3 expression in CD163+ macrophages. In addition, NLRP3 expression was also found in intestinal fibroblasts from IBD patients. Inhibition of NLRP3 led to reduced proliferation of intestinal fibroblasts, which was associated with a marked decrease in production of collagen type I and type VI in IBD patients. Moreover, NLRP3 inhibition in intestinal fibroblasts induced autophagy, a cellular process involved in collagen degradation.

CONCLUSIONS

In the presented study, we demonstrate that inhibiting NLRP3 might pave the way for novel therapeutic approaches in IBD, especially to prevent the severe complication of intestinal fibrosis formation.

Staphylococcus aureus enhances osteoclast differentiation and bone resorption by stimulating the NLRP3 inflammasome pathway

BACKGROUND

Osteomyelitis is one of the most challenging infectious diseases and is mainly caused by Staphylococcus aureus (S. aureus). In this study, we analyzed the effect of S. aureus on osteoclast differentiation and its possible molecular mechanism.

METHODS

We cultured RAW 264.7 cells with live S. aureus for 5 days. We assessed cell viability and the formation of resorption pits. We tested the NLRP3 inflammasome signaling pathways and measured the mRNA expression levels of osteoclastspecific genes, including TRAP, MMP9, cathepsin K, calcitonin receptor and ATP6V0d2. Furthermore, we analyzed the protein expression levels of the protein in the NF-κB and p38 MAPK signaling pathways to clarify the signaling pathways by which S. aureus promotes osteoclast differentiation.

RESULTS

Staphylococcus aureus induced NLRP3 inflammasome activation. S. aureus promoted bone resorption and enhanced the expression of osteoclastspecific genes, such as TRAP, MMP9, cathepsin K, calcitonin receptor and ATP6V0d2. MCC950 was used to inhibit NLRP3 inflammasome activity. Osteoclast differentiation and the expression of osteoclastspecific genes induced by S. aureus were inhibited by MCC950 pretreatment. The degradation of IκBα and phosphorylation of P65 were increased under the induction of S. aureus, but proteins in the p38 MAPK signaling pathway did not change significantly.

CONCLUSION

Staphylococcus aureus induces osteoclast differentiation and promotes bone resorption in vitro, and the NLRP3 inflammasome signaling pathway plays a significant role in this process. S. aureus-induced NLRP3 inflammasome activation was mainly dependent on the NF-κB signaling pathway during osteoclastogenesis.

Selenomethionine against titanium particle-induced osteolysis by regulating the ROS-dependent NLRP3 inflammasome activation via the β-catenin signaling pathway

Wear debris-induced osteolysis, especially titanium (Ti) particles-induced osteolysis, is the most common cause of arthroplasty failure with no effective therapy. Previous studies have suggested that inflammation and impaired osteogenesis are associated with Ti particles -induced osteolysis. Selenium (Se) is an essential trace element in the human body, which forms selenomethionine (Se-Met) in nature, and selenoproteins has strong anti-inflammatory and antioxidant stress effects. In this study, the effects of Se-Met on Ti particles-induced osteolysis were observed and the potential mechanism was explored. We found that exogenous Se-Met relieved osteolysis induced by Ti particles in two animal models and MC3T3-E1 cells. We found that the addition of Se-Met effectively inhibited Ti particle-induced inflammation by regulating reactive oxygen species-dependent (ROS-dependent) NOD-like receptor protein 3 (NLRP3) inflammasome activation. These therapeutic effects were abrogated in MC3T3-E1 cells that had received a β-catenin antagonist, suggesting that Se-Met alleviates inflammatory osteolysis via the β-catenin signaling pathway. Collectively, these findings indicated that Se-Met may serve as a potential therapeutic agent for treating Ti particle-induced osteolysis.

A Review on Pathophysiology, and Molecular Mechanisms of Bacterial Chondronecrosis and Osteomyelitis in Commercial Broilers

Modern day broilers have a great genetic potential to gain heavy bodyweights with a huge metabolic demand prior to their fully mature ages. Moreover, this made the broilers prone to opportunistic pathogens which may enter the locomotory organs under stress causing bacterial chondronecrosis and osteomyelitis (BCO). Such pathogenic colonization is further accelerated by microfractures and clefts that are formed in the bones due to rapid growth rate of the broilers along with ischemia of blood vessels. Furthermore, there are several pathways which alter bone homeostasis like acute phase response, and intrinsic and extrinsic cell death pathways. In contrast, all the affected birds may not exhibit clinical lameness even with the presence of lameness associated factors causing infection. Although Staphylococcus, E. coli, and Enterococcus are considered as common bacterial pathogens involved in BCO, but there exist several other non-culturable bacteria. Any deviation from maintaining a homeostatic environment in the gut might lead to bacterial translocation through blood followed by proliferation of pathogenic bacteria in respective organs including bones. It is important to alleviate dysbiosis of the blood which is analogous to dysbiosis in the gut. This can be achieved by supplementing pro, pre, and synbiotics which helps in providing a eubiotic environment abating the bacterial translocation that was studied to the incidence of BCO. This review focused on potential and novel biomarkers, pathophysiological mechanism, the economic significance of BCO, immune mechanisms, and miscellaneous factors causing BCO. In addition, the role of gut microbiomes along with their diversity and cell culture models from compact bones of chicken in better understanding of BCO were explored.

Interplay of gene expression and regulators under salinity stress in gill of Labeo rohita

BACKGROUND

Labeo rohita is the most preferred freshwater carp species in India. The concern of increasing salinity concentration in freshwater bodies due to climate change may greatly impact the aquatic environment. Gills are one of the important osmoregulatory organs and have direct contact with external environment. Hence, the current study is conducted to understand the gill transcriptomic response of L. rohita under hypersalinity environment.

RESULTS

Comprehensive analysis of differentially expressed long non-coding RNAs (lncRNAs), microRNAs (miRNAs) and mRNAs was performed in gills of L. rohita treated with 2, 4, 6 and 8ppt salinity concentrations. Networks of lncRNA-miRNA-mRNA revealed involvement of 20, 33, 52 and 61 differentially expressed lncRNAs, 11, 13, 26 and 21 differentially expressed miRNAs in 2, 4, 6 and 8ppt groups between control and treatment respectively. These lncRNA-miRNA pairs were regulating 87, 214, 499 and 435 differentially expressed mRNAs (DE mRNAs) in 2, 4, 6 and 8ppt treatments respectively. Functional analysis of these genes showed enrichment in pathways related to ion transportation and osmolyte production to cope with induced osmotic pressure due to high salt concentration. Pathways related to signal transduction (MAPK, FOXO and phosphatidylinositol signaling), and environmental information processing were also upregulated under hypersalinity. Energy metabolism and innate immune response pathways also appear to be regulated. Protein turnover was high at 8ppt as evidenced by enrichment of the proteasome and aminoacyl tRNA synthesis pathways, along with other enriched KEGG terms such as apoptosis, cellular senescence and cell cycle.

CONCLUSION

Altogether, the RNA-seq analysis provided valuable insights into competitive endogenous (lncRNA-miRNA-mRNA) regulatory network of L. rohita under salinity stress. L. rohita is adapting to the salinity stress by means of upregulating protein turnover, osmolyte production and removing the damaged cells using apoptotic pathway and regulating the cell growth and hence diverting the essential energy for coping with salinity stress.

Vitamin D deficiency triggers intrinsic apoptosis by impairing SPP1-dependent antiapoptotic signaling in chronic hematogenous osteomyelitis

Chronic hematogenous osteomyelitis (CHOM) is a common bone disease characterized by the development of sequestra after bacterial infection. Emerging evidence has shown that vitamin D (VD) deficiency raises the risk of osteomyelitis, but the underlying mechanisms remain obscure. Here, we establish a CHOM model in VD diet-deficient mice by intravenous inoculation of Staphylococcus aureus. Whole-genome microarray analyses using osteoblast cells isolated from sequestra reveal significant downregulation of SPP1 (secreted phosphoprotein 1). Molecular basis investigations show that VD sufficiency activates the VDR/RXR (VD receptor/retinoid X receptor) heterodimer to recruit NCOA1 (nuclear receptor coactivator 1) and transactivate SPP1 in healthy osteoblast cells. Secreted SPP1 binds to the cell surface molecule CD40 to activate serine/threonine-protein kinase Akt1, which then phosphorylates forkhead box O3a (FOXO3a), blocking FOXO3a-mediated transcription. By contrast, VD deficiency impairs the NCOA1-VDR/RXR-mediated overexpression of SPP1, leading to the inactivation of Akt1 and the accumulation of FOXO3a. FOXO3a then upregulates the expression of the apoptotic genes BAX (Bcl2-associated X-protein), BID (BH3 interacting death domain), and BIM (Bcl2-interacting mediator of cell death), to induce apoptosis. Administration of the NCOA1 inhibitor gossypol to the CHOM mice also promotes the occurrence of sequestra. VD supplementation can reactivate the SPP1-dependent antiapoptotic signaling and improve the outcomes of CHOM. Collectively, our data reveal that VD deficiency promotes bone destruction in CHOM by the removal of SPP1-dependent antiapoptotic signaling.

Posttraumatic Osteomyelitis Risks Associated with NLRP3 Gene Polymorphisms in the Chinese Population

The purpose of this case-control study was to examine possible links between NLRP3 gene polymorphisms and the risk of developing posttraumatic osteomyelitis (PTOM) in the Chinese population. A total of 306 patients with PTOM and 368 normal controls were genotyped for NLRP3 (rs35829419, rs10754558, rs7525979, rs4612666), ELP2 (rs1785929, rs1789547, rs1785928, rs12185396, rs681757, rs8299, rs2032206, rs559289), STAT3 (rs4796793, rs744166, rs1026916, rs2293152, rs1053004), CASP1 (rs501192, rs580253, rs556205, rs530537), NFKBIA (rs696), NFKB1 (rs4648068), CARD8 (rs204321), and CD14 (rs2569190) using the genotyping technique SNaPshot. The genotype distributions of NLRP3 gene rs10754558 (p = 0.047) and rs7525979 (p = 0.048) significantly differed between the patients and the healthy controls. Additionally, heterozygous models indicated a significant association between NLRP3 rs10754558 and the likelihood of developing PTOM (OR = 1.600, p = 0.039), as did recessive and homozygous models of NLRP3 rs7525979 (OR = 0.248, p = 0.019 and 0.239, p = 0.016, respectively). Collectively, our findings suggest that, in the Chinese population, the risk of developing PTOM was increased by the association between NLRP3 rs10754558 and rs7525979. Therefore, our findings may provide novel insights and guidance in the prevention and development of PTOM.

Computational Evaluation of Azadirachta indica-Derived Bioactive Compounds as Potential Inhibitors of NLRP3 in the Treatment of Alzheimer's Disease

BACKGROUND

The development of therapeutic agents against Alzheimer's disease (AD) has stalled recently. Drug candidates targeting amyloid-β (Aβ) deposition have often failed clinical trials at different stages, prompting the search for novel targets for AD therapy. The NLRP3 inflammasome is an integral part of innate immunity, contributing to neuroinflammation and AD pathophysiology. Thus, it has become a promising new target for AD therapy.

OBJECTIVE

The study sought to investigate the potential of bioactive compounds derived from Azadirachta-indica to inhibit the NLRP3 protein implicated in the pathophysiology of AD.

METHODS

Structural bioinformatics via molecular docking and density functional theory (DFT) analysis was utilized for the identification of novel NLRP3 inhibitors from A. indica bioactive compounds. The compounds were further subjected to pharmacokinetic and drug-likeness analysis. Results obtained from the compounds were compared against that of oridonin, a known NLRP3 inhibitor.

RESULTS

The studied compounds optimally saturated the binding site of the NLRP3 NACHT domain, forming principal interactions with the different amino acids at its binding site. The studied compounds also demonstrated better bioactivity and chemical reactivity as ascertained by DFT analysis and all the compounds except 7-desacetyl-7-benzoylazadiradione, which had two violations, conformed to Lipinski's rule of five.

CONCLUSION

In silico studies show that A. indica derived compounds have better inhibitory potential against NLRP3 and better pharmacokinetic profiles when compared with the reference ligand (oridonin). These compounds are thus proposed as novel NLRP3 inhibitors for the treatment of AD. Further wet-lab studies are needed to confirm the potency of the studied compounds.

A patent review of NLRP3 inhibitors to treat autoimmune diseases

INTRODUCTION

NOD-like receptor family pyrin domain containing 3 (NLRP3) can sense a plethora of exogenous and endogenous dangers. Upon activation, a multimeric protein complex, the NLRP3 inflammasome, is formed to initiate the innate immune responses. Emerging studies have implicated the pathophysiological roles of this protein complex in human disorders, highlighting that it represents a druggable target for therapeutics development.

AREAS COVERED

The current review summarizes the functional facets of the NLRP3 inflammasome, its association with autoimmune diseases, and recent patents on the development of NLRP3 inhibitors. Literature search was conducted using SciFinder and Google Patents with the key word NLRP3 and NLRP3 inhibitors.

EXPERT OPINION

Although significant advances have been made in understanding the NLRP3 inflammasome, more studies are still needed to elucidate the molecular mechanisms underlying its roles in autoimmune diseases. A number of NLRP3 inhibitors have been patented, however, none of them have been approved for clinical use. Due to the complex nature of the NLRP3 inflammasome, novel screening assays along with target engagement methods could benefit the drug discovery and clinical translation. In addition, clinical trials on NLRP3 inhibitors are still in their early stages, and continuous investigations are needed to fully assess their safety and effectiveness.

Inflammasomes and the IL-1 Family in Bone Homeostasis and Disease

PURPOSE OF REVIEW

Inflammasomes are multimeric protein structures with crucial roles in host responses against infections and injuries. The importance of inflammasome activation goes beyond host defense as a dysregulated inflammasome and subsequent secretion of IL-1 family members is believed to be involved in the pathogenesis of various diseases, some of which also produce skeletal manifestations. The purpose of this review is to summarize recent developments in the understanding of inflammasome regulation and IL-1 family members in bone physiology and pathology and current therapeutics will be discussed.

RECENT FINDINGS

Small animal models have been vital to help understand how the inflammasome regulates bone dynamics. Animal models with gain or loss of function in various inflammasome components or IL-1 family signaling have illustrated how these systems can impact numerous bone pathologies and have been utilized to test new inflammasome therapeutics. It is increasingly clear that a tightly regulated inflammasome is required not only for host defense but for skeletal homeostasis, as a dysregulated inflammasome is linked to diseases of pathological bone accrual and loss. Given the complexities of inflammasome activation and redundancies in IL-1 activation and secretion, targeting these pathways is at times challenging. Ongoing research into inflammasome-mediated mechanisms will allow the development of new therapeutics for inflammasome/IL-1 diseases.

The Role of Inflammasome NLPR3 in the Development and Therapy of Periodontitis

Periodontitis is a chronic inflammatory disease that affects tooth-supporting tissues and even leads to tooth loss. NLRP3 inflammasomes play a critical role in periodontitis pathogenesis. Aberrant activation or overexpression of NLRP3 inflammasomes in cellular players, including osteoclasts, osteoblasts, periodontal ligament fibroblasts, and leukocytes often contributes to cellular dysfunction and environment abnormality, thus resulting in the disorganization of ligament and alveolar bone. In this review, we mainly focus on the negative regulation of NLRP3 inflammasome in periodontitis and highlight the importance of NLRP3 inflammasome as a candidate therapeutic target in periodontitis treatment. Then we elucidate the development status of NLRP3 inflammasome inhibitors and show their application potential for treating periodontitis. In summary, this review reveals the recent progress and perspectives of NLRP3 inflammasome and the therapeutic potential of NLRP3 inflammasome inhibitors in periodontitis.

Chlorogenic acid suppresses mitochondrial apoptotic effectors Bax/Bak to counteract Nod-like receptor pyrin domain 3 (NLRP3) inflammasome in thiram exposed chondrocytes

BACKGROUND

Tibial dyschondroplasia (TD) is a common disease characterized by proliferation and the deterioration of growth plate's chondrocytes due to widespread utilization of thiram in the agriculture and industrial sector.

PURPOSE

In recent years, Nod-like receptor pyrin domain 3 (NLRP3) inflammasome has become a dilemma in the occurrence of many diseases. According to many research investigations, NLRP3 inflammasome has been linked to various diseases caused by pesticides and environmental toxins. Its involvement in such conditions opens up new treatment approaches. However, the role of the NLRP3 inflammasome in the development of TD is not fully understood under the impact of chlorogenic acid (CGA).

METHODS

Chondrocytes were cultured with our previously developed methodology from growth plates. After morphological and molecular identification, chondrocytes were split into different groups to investigate the efficacy of chlorogenic acid. Cell apoptosis was determined through flow cytometry and Tunnel assay. Furthermore, RT-qPCR, immunofluorescence, and western blotting techniques were used to check marker genes and proteins expression.

RESULTS

In thiram-induced TD, Bax/Bak activation persuade a parallel pathway, mediated by the NLRP3 base inflammasome. It is worth mentioning that the apoptotic executioners (caspase-3 and caspase-7) act upstream for inflammasome. Furthermore, chondrocytes' ability to undergo mitochondrial apoptosis was governed by anti-apoptotic members, e.g., Bcl-2 and Bcl-xl. Equilibrium of these anti-apoptotic proteins ensured appropriate regulation of apoptosis during the development and survival of chondrocytes.

CONCLUSION

Chondrocytes have ability to undergo Bax/Bak-mediated apoptosis and generate pro-inflammatory signals, e.g., NLRP3 in thiram-induced TD. So, the Nod-like receptor pyrin domain 3 is the potential target to eliminate TD at all stages of pathology, while drugs, e.g., CGA, can significantly improve chondrocytes' survival by targeting these pro-inflammatory signals.

RelA/MicroRNA-30a/NLRP3 signal axis is involved in rheumatoid arthritis via regulating NLRP3 inflammasome in macrophages

NLRP3 inflammasome plays an important role in the pathogenesis of rheumatoid arthritis (RA). However, the post-transcriptional regulation of NLRP3 expression by miRNA in synovial macrophages is still not well understood. The aim of the study is to elucidate the mechanisms of RA with the focus on miRNAs mediated post-transcriptional regulation of the NLRP3 inflammasome. Here, we used NLRP3-deficient mice (NLRP3^KO) to cross with TNFα-transgenic mice (TNF^TG) to generate NLRP3^KO/TNF^TG mice, and compared their joint phenotypes with those of their TNF^TG and wild-type (WT) littermates at 5 months of age. In comparison to WT mice, articular bone volume and cartilage area are decreased, whereas inflammed area, eroded surface, ALP+ osteoblast number, TRAP+ osteoclast number, and the areas of RelA+F4/80+, Caspase-1+F4/80+, IL-1β+F4/80+ synoviocytes are increased in the TNF^TG mice. Knockout of NLRP3 ameliorates joint inflammation and bone damage in TNF^TG mice. Further, in TNFα-primed BMDMs, RelA positively regulates NLRP3 expression, but negatively regulates miR-30a. Additionally, miR-30a negatively mediates NLRP3 expression by directly binding to its 3' UTR, suggesting a miR-30a-mediated feedforward loop acting on NLRP3. Finally, intra-articular injection of AAV-miR-30a inhibits NLRP3 inflammasome activation, reduces joint inflammation, and attenuates bone damage in TNF^TG mice. Thus, RelA/miR-30a/NLRP3 signal axis is involved in RA through regulating NLRP3 Inflammasome in macrophages.

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.

Protective effects of sirtuin 3 on titanium particle-induced osteogenic inhibition by regulating the NLRP3 inflammasome via the GSK-3β/β-catenin signalling pathway

Periprosthetic osteolysis (PPO) remains the key factor in implant failure and subsequent revision surgery and is mainly triggered by wear particles. Previous studies have shown that inhibition of osteoblastic differentiation is the most widespread incident affecting the interface of trabecular and loosening prostheses. Additionally, the NLRP3 inflammasome is activated by prosthetic particles. Sirtuin3, an NAD⁺-dependent deacetylase of mitochondria, regulates the function of mitochondria in diverse activities. However, whether SIRT3 can mitigate wear debris-induced osteolysis by inhibiting the NLRP3 inflammasome and enhancing osteogenesis has not been previously reported. Therefore, we investigated the role of SIRT3 during the process of titanium (Ti) particle-induced osteolysis. We revealed that upregulated SIRT3 dramatically attenuated Ti particle-induced osteogenic inhibition through suppression of the NLRP3 inflammasome and improvement of osteogenesis in vivo and in vitro. Moreover, we found that SIRT3 interference in the process of Ti particle-induced osteolysis relied on the GSK-3β/β-catenin signalling pathway. Collectively, these findings indicated that SIRT3 may serve as a rational new treatment against debris-induced PPO by deacetylase-dependent inflammasome attenuation.

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Effect of SIRT3 on rescued wear particles-induced osteogenic inhibition.

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Effect of SIRT3 on inhibited the activation of pyroptosis-related NLRP3 inflammasome.

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Effect of SIRT3 on mitigated periprosthetic osteolysis via GSK-3β/β-catenin signaling.

NLRP3 regulates alveolar bone loss in ligature-induced periodontitis by promoting osteoclastic differentiation

Objectives

NLRP3 inflammasome is a critical part of the innate immune system and plays an important role in a variety of inflammatory diseases. However, the effects of NLRP3 inflammasome on periodontitis have not been fully studied.

Materials and methods

We used ligature‐induced periodontitis models of NLRP3 knockout mice (NLRP3^KO) and their wildtype (WT) littermates to compare their alveolar bone phenotypes. We further used Lysm‐Cre/Rosa^nTnG mouse to trace the changes of Lysm‐Cre⁺ osteoclast precursors in ligature‐induced periodontitis with or without MCC950 treatment. At last, we explored MCC950 as a potential drug for the treatment of periodontitis in vivo and in vitro.

Results

Here, we showed that the number of osteoclast precursors, osteoclast differentiation and alveolar bone loss were reduced in NLRP3^KO mice compared with WT littermates, by using ligature‐induced periodontitis model. Next, MCC950, a specific inhibitor of the NLRP3 inflammasome, was used to inhibit osteoclast precursors differentiation into osteoclast. Further, we used Lysm‐Cre/Rosa^nTnG mice to demonstrate that MCC950 decreases the number of Lysm‐Cre⁺ osteoclast precursors in ligature‐induced periodontitis. At last, treatment with MCC950 significantly suppressed alveolar bone loss with reduced IL‐1β activation and osteoclast differentiation in ligature‐induced periodontitis.

Conclusion

Our findings reveal that NLRP3 regulates alveolar bone loss in ligature‐induced periodontitis by promoting osteoclastic differentiation.

The Role of NLRP3 Inflammasome Activities in Bone Diseases and Vascular Calcification

Continuous stimulation of inflammation is harmful to tissues of an organism. Inflammatory mediators not only have an effect on metabolic and inflammatory bone diseases but also have an adverse effect on certain genetic and periodontal diseases associated with bone destruction. Inflammatory factors promote vascular calcification in various diseases. Vascular calcification is a pathological process similar to bone development, and vascular diseases play an important role in the loss of bone homeostasis. The NLRP3 inflammasome is an essential component of the natural immune system. It can recognize pathogen-related molecular patterns or host-derived dangerous signaling molecules, recruit, and activate the pro-inflammatory protease caspase-1. Activated caspase-1 cleaves the precursors of IL-1β and IL-18 to produce corresponding mature cytokines or recognizes and cleaves GSDMD to mediate cell pyroptosis. In this review, we discuss the role of NLRP3 inflammasome in bone diseases and vascular calcification caused by sterile or non-sterile inflammation and explore potential treatments to prevent bone loss.

Multitasking by the OC Lineage during Bone Infection: Bone Resorption, Immune Modulation, and Microbial Niche

Bone infections, also known as infectious osteomyelitis, are accompanied by significant inflammation, osteolysis, and necrosis. Osteoclasts (OCs) are the bone-resorbing cells that work in concert with osteoblasts and osteocytes to properly maintain skeletal health and are well known to respond to inflammation by increasing their resorptive activity. OCs have typically been viewed merely as effectors of pathologic bone resorption, but recent evidence suggests they may play an active role in the progression of infections through direct effects on pathogens and via the immune system. This review discusses the host- and pathogen-derived factors involved in the in generation of OCs during infection, the crosstalk between OCs and immune cells, and the role of OC lineage cells in the growth and survival of pathogens, and highlights unanswered questions in the field.

Relevance of Caspase-1 and Nlrp3 Inflammasome on Inflammatory Bone Resorption in A Murine Model of Periodontitis

This study investigates the role of NLRP3 inflammasome and its main effector Caspase-1 in inflammation and alveolar bone resorption associated with periodontitis. Heat-killed Aggregatibacter actinomycetemcomitans (Aa) was injected 3x/week (4 weeks) into gingival tissues of wild-type (WT), Nlrp3-KO and Caspase1-KO mice. Bone resorption was measured by µCT and osteoclast number was determined by tartrate-resistant acid phosphatase (TRAP) staining. Inflammation was assessed histologically (H/E staining and immunofluorescence of CD45 and Ly6G). In vitro studies determined the influence of Nlrp3 and Caspase-1 in Rankl-induced osteoclast differentiation and activity and on LPS-induced expression of inflammation-associated genes. Bone resorption was significantly reduced in Casp1-KO but not in Nlrp3-KO mice. Casp1-KO mice had increased in osteoclast numbers, whereas the inflammatory infiltrate or on gene expression were similar to those of WT and Nlrp3-KO mice. Strikingly, osteoclasts differentiated from Nlrp3-deficient macrophages had increased resorbing activity in vitro. LPS-induced expression of Il-10, Il-12 and Tnf-α was significantly reduced in Nlrp3- and Casp1-deficient macrophages. As an inceptive study, these results suggest that Nlrp3 inflammasome does not play a significant role in inflammation and bone resorption in vivo and that Caspase-1 has a pro-resorptive role in experimental periodontal disease.

NLRC4 inflammasome has a protective role on inflammatory bone resorption in a murine model of periodontal disease

There is virtually no information on the role of NLRC4 inflammasome on bone resorption and inflammation associated with periodontitis. Bacterial-associated experimental periodontitis was induced in wild-type (WT) and Nlrc4-KO C57BL/6 mice. 3 μL of a 1 × 10⁹ UFC/mL PBS suspension of heat-killed Gram-negative bacteria were injected (3x/week for 4 weeks) directly into the gingival tissues of WT and Nlrc4-KO mice (n = 6/genotype). Control animals were injected bilaterally (3x/week for 4 weeks) in the same sites with the same volume of the PBS vehicle. Alveolar bone resorption was quantified by μCT. Inflammatory infiltrate in the gingival tissues was assessed qualitatively in H&E-stained slides and by the detection of a pan-leukocyte marker (CD45) and a neutrophil marker (Ly6G) using immunofluorescence. Modulation of Rankl, Mmp-13, Tnf-a, Il-6 and Il-10 expression in the gingival tissues was determined by RT-qPCR. Osteoclastogenesis was assessed in vivo by biochemical staining for TRAP. The relevance of NLRC4 for RANKL-induced osteoclastic differentiation and activity was investigated in vitro using bone marrow-derived macrophages from WT and Nlrc4-KO mice. Bone resorption was significantly greater in Nlrc4-KO mice; however there were no differences between WT and Nlrc4-KO mice on osteoclast numbers and on the inflammatory infiltrate. In vitro, osteoclast activity was significantly enhanced in Nlrc4-deficient macrophages; whereas RANKL-induced differentiation was not affected. Expression of the selected candidate genes was also similarly increased by the induction of experimental periodontal disease, except for the expression of Tnf-alpha and Il-10, which was already significantly higher in the gingival tissues of Nlrc4-KO mice. We conclude that NLRC4 inflammasome has a protective role on inflammatory bone resorption in this experimental model. Furthermore, the bone-sparing effect may be related with the modulation of osteoclast activity.

Omnipresence of inflammasome activities in inflammatory bone diseases

The inflammasomes are intracellular protein complexes that are assembled in response to a variety of perturbations including infections and injuries. Failure of the inflammasomes to rapidly clear the insults or restore tissue homeostasis can result in chronic inflammation. Recurring inflammation is also provoked by mutations that cause the constitutive assembly of the components of these protein platforms. Evidence suggests that chronic inflammation is a shared mechanism in bone loss associated with aging, dysregulated metabolism, autoinflammatory, and autoimmune diseases. Mechanistically, inflammatory mediators promote bone resorption while suppressing bone formation, an imbalance which over time leads to bone loss and increased fracture risk. Thus, while acute inflammation is important for the maintenance of bone integrity, its chronic state damages this tissue. In this review, we discuss the role of the inflammasomes in inflammation-induced osteolysis.

Double-Stranded RNA Is a Novel Molecular Target in Osteomyelitis Pathogenesis: A Translational Avian Model for Human Bacterial Chondronecrosis with Osteomyelitis

Osteomyelitis remains a serious inflammatory bone disease that affects millions of individuals worldwide and for which there is no effective treatment. Despite scientific evidence that Staphylococcus bacteria are the most common causative species for human bacterial chondronecrosis with osteomyelitis (BCO), much remains to be understood about the underlying virulence mechanisms. Herein, we show increased levels of double-stranded RNA (dsRNA) in infected bone in a Staphylococcus-induced chicken BCO model and in human osteomyelitis samples. Administration of synthetic [poly(I:C)] or genetic (Alu) dsRNA induces human osteoblast cell death. Similarly, infection with Staphylococcus isolated from chicken BCO induces dsRNA accumulation and cell death in human osteoblast cell cultures. Both dsRNA administration and Staphylococcus infection activate NACHT, LRR and PYD domains-containing protein (NLRP)3 inflammasome and increase IL18 and IL1B gene expression in human osteoblasts. Pharmacologic inhibition with Ac-YVAD-cmk of caspase 1, a critical component of the NLRP3 inflammasome, prevents DICER1 dysregulation- and dsRNA-induced osteoblast cell death. NLRP3 inflammasome and its components are also activated in bone from BCO chickens and humans with osteomyelitis, compared with their healthy counterparts. These findings provide a rationale for the use of chicken BCO as a human-relevant spontaneous animal model for osteomyelitis and identify dsRNA as a new treatment target for this debilitating bone pathogenesis.

NLRP3: A promising therapeutic target for autoimmune diseases

NLRP3, a member of nucleotide-binding domain-(NOD) like receptor family, can be found in large varieties of immune and non-immune cells. Upon activation, the NLRP3, apoptosis-associated speck-like protein (ASC) and pro-caspase-1 would assemble into a multimeric protein, called the NLRP3 inflammasome. Then the inflammasome promotes inflammation (through specific cleavage and production of bioactive IL-1β and IL-18) and pyroptotic cell death. Previous studies have indicated the importance of NLRP3 in regulating innate immunity. Recently, numerous studies have revealed their significance in autoimmune diseases, such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc) and inflammatory bowel disease (IBD). In this review, we will briefly discuss the biological features of NLRP3 and summarize the recent progression of the involvement of NLRP3 in the development and pathogenesis of autoimmune diseases, as well as its clinical implications and therapeutic potential.

Updating osteoimmunology: regulation of bone cells by innate and adaptive immunity

Osteoimmunology encompasses all aspects of the cross-regulation of bone and the immune system, including various cell types, signalling pathways, cytokines and chemokines, under both homeostatic and pathogenic conditions. A number of key areas are of increasing interest and relevance to osteoimmunology researchers. Although rheumatoid arthritis has long been recognized as one of the most common autoimmune diseases to affect bone integrity, researchers have focused increased attention on understanding how molecular triggers and innate signalling pathways (such as Toll-like receptors and purinergic signalling pathways) related to pathogenic and/or commensal microbiota are relevant to bone biology and rheumatic diseases. Additionally, although most discussions relating to osteoimmune regulation of homeostasis and disease have focused on the effects of adaptive immune responses on bone, evidence exists of the regulation of immune cells by bone cells, a concept that is consistent with the established role of the bone marrow in the development and homeostasis of the immune system. The active regulation of immune cells by bone cells is an interesting emerging component of investigations that seek to understand how to control immune-associated diseases of the bone and joints.

Enterococcus faecalis Lipoteichoic Acid-induced NLRP3 Inflammasome via the Activation of the Nuclear Factor Kappa B Pathway

INTRODUCTION

We wished to examine the effects of the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome on periapical periodontitis induced by Enterococcus faecalis and to investigate the molecular mechanisms of lipoteichoic acid (LTA) derived from E. faecalis on the expression of the NLRP3 inflammasome.

METHODS

A model of periapical periodontitis by sealing E. faecalis into the pulp chambers of rats was established. We then examined the relationship between the expression, location, distribution, and concentration of NLRP3, caspase-1, and interleukin 1β with the inflammatory progression by immunohistochemistry and undertook correlation analyses. RAW264.7 cells were cultured in the absence or presence of LTA together with or without nuclear factor kappa B (NF-κB) inhibitor BAY 11-7082; NLRP3 inflammasome expression was measured by Western blotting, the enzyme-linked immunosorbent assay, and real-time quantitative polymerase chain reaction. An immunofluorescence study was conducted to further detect whether NF-κB can be completely inhibited by BAY 11-7082 or activated by LTA.

RESULTS

An animal model of periapical periodontitis was established successfully. Expression of NLRP3, caspase-1, and interleukin 1β protein was observed in the inflamed area. The expression of these 3 proteins had a significant positive correlation (P < .05). Overall, our results showed that, compared with the negative control group, LTA could directly activate expression of messenger RNA and protein of the NLRP3 inflammasome (P < .05), whereas BAY 11-7082 inhibited it (P < .05).

CONCLUSIONS

Our results suggested that LTA can act as a directly stimulating factor associated with expression of the NLRP3 inflammasome during periapical periodontitis, which is mainly linked with the NF-κB signaling activation pathway.

Chronic inflammation triggered by the NLRP3 inflammasome in myeloid cells promotes growth plate dysplasia by mesenchymal cells

Skeletal complications are common features of neonatal-onset multisystem inflammatory disease (NOMID), a disorder caused by NLRP3-activating mutations. NOMID mice in which NLRP3 is activated globally exhibit several characteristics of the human disease, including systemic inflammation and cartilage dysplasia, but the mechanisms of skeletal manifestations remain unknown. In this study, we find that activation of NLRP3 in myeloid cells, but not mesenchymal cells triggers chronic inflammation, which ultimately, causes growth plate and epiphyseal dysplasia in mice. These responses are IL-1 signaling-dependent, but independent of PARP1, which also functions downstream of NLRP3 and regulates skeletal homeostasis. Mechanistically, inflammation causes severe anemia and hypoxia in the bone environment, yet down-regulates the HIF-1α pathway in chondrocytes, thereby promoting the demise of these cells. Thus, activation of NLRP3 in hematopoietic cells initiates IL-1β-driven paracrine cascades, which promote abnormal growth plate development in NOMID mice.

Inflammatory osteolysis: a conspiracy against bone

There are many causes of inflammatory osteolysis, but regardless of etiology and cellular contexts, the osteoclast is the bone-degrading cell. Thus, the impact of inflammatory cytokines on osteoclast formation and function was among the most important discoveries advancing the treatment of focal osteolysis, leading to development of therapeutic agents that either directly block the bone-resorptive cell or do so indirectly via cytokine arrest. Despite these advances, a substantial number of patients with inflammatory arthritis remain resistant to current therapies, and even effective anti-inflammatory drugs frequently do not repair damaged bone. Thus, insights into events such as those impacted by inflammasomes, which signal through cytokine-dependent and -independent mechanisms, are needed to optimize treatment of inflammatory osteolysis.

NOD-Like Receptors: Guardians of Intestinal Mucosal Barriers

The NOD-like receptors (NLRs) are cytosolic pattern-recognition receptors, which are critically involved in mucosal immune defense. The association of the NLR, NOD2, with inflammatory bowel disease first pointed to the NLRs potential function as guardians of the intestinal barrier. Since then, several studies have emphasized the importance of NLRs in maintaining gut homeostasis and intestinal infections, and in shaping the microbiota. In this review, we will highlight the function of NLRs in intestinal inflammation.

NLRP3 Inflammasome Plays an Important Role in the Pathogenesis of Collagen-Induced Arthritis

Objective. To investigate the relationship between NLRP3 and the pathogenesis of collagen-induced arthritis. Methods. We used the collagen-induced arthritis (CIA) mouse model. The mice were divided into two groups: the model group (CIA, n = 16) and the control group (Normal, n = 8). The mice were sacrificed seven weeks after immunization. The arthritis score and imaging evaluation (X-rays, Micro-CT, and MRI) were performed. Synovial tissue NLRP3 expression and peripheral blood cytokine levels were analyzed. Results. The arthritis score (6.00 ± 2.52), imaging score (4.63 ± 0.92), and synovial tissue NLRP3 expression (4.00 ± 2.03) significantly increased in the CIA mice. The expression of synovial NLRP3 was positively correlated with arthritis clinical and radiographic scores (r = 0.792 and r = 0.669, resp.). Conclusions. The synovial NLRP3 expression increased at the early onset of RA. Synovial NLRP3 expression level was correlated with the clinical arthritis severity and extent of radiological destruction, suggesting that NLRP3 is involved in the pathogenesis of RA.

[Calcitonin gene-related peptide inhibits the expression of Nod-like receptor protein 3 to Dromote osteoblast differentiation in mouse osteoblasts in vitro]

OBJECTIVE

This study aims to investigate the regulatory effects of calcitonin gene-related peptide (CGRP) on Nod-like receptor protein 3 (NLRP3) and interleukin-1β (IL-1β) to promote osteoblast differentiation.

METHODS

Different concentrations of CGRP (0, 10, 30, 100 ng · mL⁻¹) were added to mouse osteoblasts in vitro. The mRNA and protein expression levels of both NLRP3 and IL-1β were examined using Real-time polymerase chain reaction and Western blot, respectively. Moreover, the concentrations of IL-1β protein and intracellular reactive oxygen species (ROS) were detected using enzyme-linked immunosorbent assay and flow cytometry, respectively. The osteogenic differentiation of mouse osteoblasts was identified through alizarin red staining.

RESULTS

The protein and mRNA expression levels of both NLRP3 and IL-1β significantly decreased (P < 0.05) with increasing CGRP concentration. Moreover, the contents of intracellular ROS gradually decreased (P<0.05). The osteogenic differentiation of the osteoblasts was more enhanced in the group treated with 100 ng · mL⁻¹ CGRP than in the empty group (0 ng · mL⁻¹ CGRP).

CONCLUSION

CGRP promotes osteoblast differentiation by inhibiting the expression of inflammatory factors.

Fine mapping of bone structure and strength QTLs in heterogeneous stock rat

We previously demonstrated that skeletal structure and strength phenotypes vary considerably in heterogeneous stock (HS) rats. These phenotypes were found to be strongly heritable, suggesting that the HS rat model represents a unique genetic resource for dissecting the complex genetic etiology underlying bone fragility. The purpose of this study was to identify and localize genes associated with bone structure and strength phenotypes using 1524 adult male and female HS rats between 17 to 20 weeks of age. Structure measures included femur length, neck width, head width; femur and lumbar spine (L3-5) areas obtained by DXA; and cross-sectional areas (CSA) at the midshaft, distal femur and femoral neck, and the 5th lumbar vertebra measured by CT. In addition, measures of strength of the whole femur and femoral neck were obtained. Approximately 70,000 polymorphic SNPs distributed throughout the rat genome were selected for genotyping, with a mean linkage disequilibrium coefficient between neighboring SNPs of 0.95. Haplotypes were estimated across the entire genome for each rat using a multipoint haplotype reconstruction method, which calculates the probability of descent at each locus from each of the 8 HS founder strains. The haplotypes were then tested for association with each structure and strength phenotype via a mixed model with covariate adjustment. We identified quantitative trait loci (QTLs) for structure phenotypes on chromosomes 3, 8, 10, 12, 17 and 20, and QTLs for strength phenotypes on chromosomes 5, 10 and 11 that met a conservative genome-wide empiric significance threshold (FDR=5%; P<3×10(-6)). Importantly, most QTLs were localized to very narrow genomic regions (as small as 0.3 Mb and up to 3 Mb), each harboring a small set of candidate genes, both novel and previously shown to have roles in skeletal development and homeostasis.

Staphylococcus aureus vs. Osteoblast: Relationship and Consequences in Osteomyelitis

Bone cells, namely osteoblasts and osteoclasts work in concert and are responsible for bone extracellular matrix formation and resorption. This homeostasis is, in part, altered during infections by Staphylococcus aureus through the induction of various responses from the osteoblasts. This includes the over-production of chemokines, cytokines and growth factors, thus suggesting a role for these cells in both innate and adaptive immunity. S. aureus decreases the activity and viability of osteoblasts, by induction of apoptosis-dependent and independent mechanisms. The tight relationship between osteoclasts and osteoblasts is also modulated by S. aureus infection. The present review provides a survey of the relevant literature discussing the important aspects of S. aureus and osteoblast interaction as well as the ability for antimicrobial peptides to kill intra-osteoblastic S. aureus, hence emphasizing the necessity for new anti-infectious therapeutics.

NLRP3 gene is associated with ulcerative colitis (UC), but not Crohn's disease (CD), in Chinese Han population

OBJECTIVE

This study aimed to investigate whether NLRP3 is associated with IBD in Chinese Han population.

METHODS

Three SNPs were genotyped using polymerase chain reaction with sequence-specific primers in 288 patients [232 Crohn's disease (CD) patients, 56 ulcerative colitis (UC) patients] and 274 controls.

RESULTS

In IBD group, the results showed no significant association. When subdivided to CD and UC, it showed in CD subgroup, there was no significant association. However, in UC subgroup, rs10754558 (P allele=0.015272, P genotype=0.029776, OR [95% CI]=0.604190[0.401200-0.909886]) and rs10925019 (P allele=0.013042, P genotype=0.037045, OR [95% CI]=2.022613[1.149854-3.557812]) have significant associations with UC. The G and T alleles were risk factors of the susceptibility of UC, the GG and TT genotypes may increase risk of this disease. Rs4925648 has no association with UC. The haplotypes analysis results showed as follow: for rs4925648-rs10925019, CC and TT are risk factors for UC (for CC, χ2=3.605, P=0.057613, OR [95% CI]=1.645 [0.980-2.761], for TT, χ2=5.522, P=0.018804, OR [95% CI]=0.426[0.205-0.884]), and for rs10754558-rs10925019, CT and GC haplotypes are risk factors for UC (for CT, χ2=3.545, P=0.059739, OR [95% CI]=0.571[0.317-1.029], for GC, χ2=9.359, P=0.002228, OR [95% CI]=1.904 [1.255-2.887]).

CONCLUSIONS

We first demonstrated that rs10754558 and rs10925019 are significantly associated with the susceptibility of UC, but not CD in Chinese Han population, suggesting that NLRP3 may play an important role in the pathogenesis of UC.

The NLRP3 inflammasome: role in airway inflammation

Asthma is characterized by airway inflammation, airway hyperresponsiveness and airway remodelling. Uncontrolled airway inflammation or repeated asthma exacerbations can lead to airway remodelling, which cannot be reversed by current pharmacological treatment, and consequently lead to decline in lung function. Thus, it is critical to understand airway inflammation in asthma and infectious exacerbation. The inflammasome has emerged as playing a key role in innate immunity and inflammation. Upon ligand sensing, inflammasome components assemble and self-oligomerize, leading to caspase-1 activation and maturation of pro-IL-1β and pro-IL-18 into bioactive cytokines. These bioactive cytokines then play a pivotal role in the initiation and amplification of inflammatory processes. In addition to facilitating the proteolytic activation of IL-1β and IL-18, inflammasomes also participate in cell death through caspase-1-mediated pyroptosis. In this review, we describe the structure and function of the inflammasome and provide an overview of our current understanding of role of the inflammasome in airway inflammation. We focus on nucleotide-binding domain and leucine-rich repeat protein 3 (NLRP3) inflammasome as it is the best-characterized subtype shown expressed in airway and considered to play a key role in chronic airway diseases such as asthma.

NLRP3 inflammasome is required for apoptosis of Aggregatibacter actinomycetemcomitans-infected human osteoblastic MG63 cells

Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans) is a Gram-negative bacterium which is implicated in the pathogenesis of human periodontal disease and in particular aggressive periodontitis. Virulence factors from A. actinomycetemcomitans have been shown to induce apoptosis of osteoblasts, however, the underlying mechanisms of the induction of apoptosis are poorly understood. In the present study, the infection of A. actinomycetemcomitans in human osteoblastic MG63 cells was established. Accordingly, A. actinomycetemcomitans infection enhanced significant apoptosis of MG63 cells. We found that both expression levels of NLRP3 and ASC were increased dramatically after MG63 cell cultures exposed to A. actinomycetemcomitans. Moreover, the secretion of mature interleukin-1β (IL-1β) and IL-18 were extensively induced in A. actinomycetemcomitans-infected cells as compared with non-invasion group of MG63 cell cultures, indicating the activation of the NLRP3 inflammasome during infection. Finally, we found that the knockdown expression of NLRP3 by specific small interfering RNA (siRNA) attenuated apoptosis of A. actinomycetemcomitans-infected MG63 cells. Our data suggest that A. actinomycetemcomitans promotes apoptosis of human osteoblasts at least partially through the NLRP3 inflammasome.

Lipopolysaccharide/adenosine triphosphate induces IL‑1β and IL-18 secretion through the NLRP3 inflammasome in RAW264.7 murine macrophage cells

The NOD-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome plays pivotal roles in inflammation and autoimmunity. The NLRP3 inflammasome is activated in response to various signals, including pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs). However, its role in inflammation remains unclear. In this study, we used lipopolysaccharide (LPS) and adenosine triphosphate (ATP) to simulate an inflammatory environment as the testing model. We found that the exposure of RAW264.7 cells to LPS/ATP triggered the activation of caspase-1 (P<0.01) and the cleavage of interleukin (IL)-1β (P<0.01), as well as the release of other cytokines, such as IL-18 (P<0.01) and IL-33 (P<0.01). Extracellular potassium chloride at a high concentration (150 mM) abrogated the secretion of IL-1β and IL-18 (P<0.01), but did not reduce the processing of IL-33 (P>0.05). In addition, the silencing of NLRP3 with small interfering RNA (siRNA) suppressed the generation of proinflammatory cytokines, such as IL-1β (P<0.01), IL-18 (P<0.01), but not IL-33 (P>0.05), along with the decreased mRNA and protein expression of NLRP3 and caspase-1 (P<0.05). However, extracellular potassium at a high concentration and NLRP3 siRNA did not affect the level of apoptosis-associated speck-like protein containing a caspase recruitment domain (CARD) (ASC; P>0.05). Our results suggest that the NLRP3/ASC/caspase-1 axis participates in the regulation of pro-imflammatory cytokine secretion in RAW264.7 cells, particularly the generation of IL-1β and IL-18.

Apoptosis-associated uncoupling of bone formation and resorption in osteomyelitis

The mechanisms underlying the destruction of bone tissue in osteomyelitis are only now being elucidated. While some of the tissue damage associated with osteomyelitis likely results from the direct actions of bacteria and infiltrating leukocytes, perhaps exacerbated by bacterial manipulation of leukocyte survival pathways, infection-induced bone loss predominantly results from an uncoupling of the activities of osteoblasts and osteoclasts. Bacteria or their products can directly increase osteoclast formation and activity, and the inflammatory milieu at sites of infection can further promote bone resorption. In addition, osteoclast activity is critically regulated by osteoblasts that can respond to bacterial pathogens and foster both inflammation and osteoclastogenesis. Importantly, bone loss during osteomyelitis is also brought about by a decline in new bone deposition due to decreased bone matrix synthesis and by increased rates of osteoblast apoptosis. Extracellular bacterial components may be sufficient to reduce osteoblast viability, but the causative agents of osteomyelitis are also capable of inducing continuous apoptosis of these cells by activating intrinsic and extrinsic cell death pathways to further uncouple bone formation and resorption. Interestingly, bacterial internalization appears to be required for maximal osteoblast apoptosis, and cytosolic inflammasome activation may act in concert with autocrine/paracrine death receptor-ligand signaling to induce cell death. The manipulation of apoptotic pathways in infected bone cells could be an attractive new means to limit inflammatory damage in osteomyelitis. However, the mechanism that is the most important in bacterium-induced bone loss has not yet been identified. Furthermore, it remains to be determined whether the host would be best served by preventing osteoblast cell death or by promoting apoptosis in infected cells.

NLRP3 promotes autophagy of urate crystals phagocytized by human osteoblasts

Introduction

Monosodium urate (MSU) microcrystals present in bone tissues of chronic gout can be ingested by nonprofessional phagocytes like osteoblasts (OBs) that express NLRP3 (nucleotide-binding domain and leucine-rich repeat region containing family of receptor protein 3). MSU is known to activate NLRP3 inflammasomes in professional phagocytes. We have identified a new role for NLRP3 coupled to autophagy in MSU-stimulated human OBs.

Methods

Normal human OBs cultured in vitro were investigated for their capacity for phagocytosis of MSU microcrystals by using confocal microscopy. Subsequent mineralization and matrix metalloproteinase activity were evaluated, whereas regulatory events of phagocytosis were deciphered by using signaling inhibitors, phosphokinase arrays, and small interfering RNAs. Statistics were carried out by using paired or unpaired t tests, and the one-way ANOVA, followed by multiple comparison test.

Results

Most of the OBs internalized MSU in vacuoles. This process depends on signaling via PI3K, protein kinase C (PKC), and spleen tyrosine kinase (Syk), but is independent of Src kinases. Simultaneously, MSU decreases phosphorylation of the protein kinases TOR (target of rapamycin) and p70S6K. MSU activates the cleavage of microtubule-associated protein light chain 3 (LC3)-I into LC3-II, and MSU microcrystals are coated with GFP-tagged LC3. However, MSU-stimulated autophagy in OBs absolutely requires the phagocytosis process. We find that MSU upregulates NLRP3, which positively controls the formation of MSU-autophagosomes in OBs. MSU does not increase death and late apoptosis of OBs, but reduces their proliferation in parallel to decreasing their competence for mineralization and to increasing their matrix metalloproteinase activity.

Conclusions

MSU microcrystals, found locally encrusted in the bone matrix of chronic gout, activate phagocytosis and NLRP3-dependent autophagy in OBs, but remain intact in permanent autophagosomes while deregulating OB functions.

Staphylococcus aureus biofilms decrease osteoblast viability, inhibits osteogenic differentiation, and increases bone resorption in vitro

Background

Osteomyelitis is a severe and often debilitating disease characterized by inflammatory destruction of bone. Despite treatment, chronic infection often develops which is associated with increased rates of treatment failure, delayed osseous-union, and extremity amputation. Within affected bone, bacteria exist as biofilms, however the impact of biofilms on osteoblasts during disease are unknown. Herein, we evaluated the effect of S. aureus biofilms on osteoblast viability, osteogenic potential, and the expression of the pro-osteoclast factor, receptor activator of NF-kB ligand (RANK-L).

Methods

Osteoblasts were exposed to biofilm conditioned media (BCM) from clinical wound isolates of Staphylococcus aureus under normal growth and osteogenic conditions to assess cellular viability and osteoblast differentiation, respectively. Cell viability was evaluated using a live/dead assay and by quantifying total cellular DNA at days 0, 1, 3, 5, and 7. Apoptosis following treatment with BCM was measured by flow-cytometry using the annexin V-FITC/PI apoptosis kit. Osteogenic differentiation was assessed by measuring alkaline phosphatase activity and intracellular accumulation of calcium and osteocalcin for up to 21 days following exposure to BCM. Expression of genes involved in osteogenic differentiation and osteoclast regulation, were also evaluated by quantitative real-time PCR.

Results

BCM from clinical strains of S. aureus reduced osteoblast viability which was accompanied by an increase in apoptosis. Osteogenic differentiation was significantly inhibited following treatment with BCM as indicated by decreased alkaline phosphatase activity, decreased intracellular accumulation of calcium and inorganic phosphate, as well as reduced expression of transcription factors and genes involved in bone mineralization in viable cells. Importantly, exposure of osteoblasts to BCM resulted in up-regulated expression of RANK-L and increase in the RANK-L/OPG ratio compared to the untreated controls.

Conclusions

Together these studies suggest that soluble factors produced by S. aureus biofilms may contribute to bone loss during chronic osteomyelitis simultaneously by: (1) reducing osteoblast viability and osteogenic potential thereby limiting new bone growth and (2) promoting bone resorption through increased expression of RANK-L by osteoblasts. To our knowledge these are the first studies to demonstrate the impact of staphylococcal biofilms on osteoblast function, and provide an enhanced understanding of the pathogenic role of staphylococcal biofilms during osteomyelitis.