Mechanisms of pathologic new bone formation

The 'Tree trunk and root' model: key imaging findings may anatomically differentiate axial psoriatic arthritis and DISH from axial spondyloarthropathy

Variable axial skeleton inflammation and axial skeleton tissue remodelling with aberrant ligamentous soft-tissue ossification occurs across the axial spondyloarthritis (ax-SpA) axial psoriatic arthritis (ax-PsA) and the diffuse idiopathic skeletal hyperostosis (DISH) spectrum. In this article, we show how imaging has resulted in an enthesis-centric model for different disease pathology compartmentalisation or a 'root and trunk' model for pathological process development. Whilst ankylosing spondylitis is predominantly characterised by early entheseal bony anchorage-related osteitis (root inflammation) and DISH is characterised by ligamentous soft-tissue ossification, ax-PsA is more heterogenous. Whilst ax-PsA may share an identical osteitis pattern to ax-SpA, a substantial proportion of ax-PsA cases have a soft tissue or tree trunk pathology that manifests as back pain with lack of osteitis but prominent ligamentous trunk ossification at later stages. We illustrate this using different imaging modalities to create a base for imaging research to elucidate this pattern of pathology.

Exploration of molecular biomarkers in ankylosing spondylitis transcriptomics

Background

Inflammation of the spine and sacroiliac joints is a hallmark of the chronic, progressive inflammatory illness known as ankylosing spondylitis (AS). The insidious onset and non-specific early symptoms of AS often lead to delays in diagnosis and treatment, which may result in the onset of disability. It is therefore imperative to identify new biomarkers.

Methods

In this study, datasets GSE73754 and GSE25101 were derived from the Gene Expression Omnibus (GEO). Key genes were identified through differential expression analysis and weighted gene co-expression network analysis (WGCNA). A model was then established using LASSO regression, and then it was subjected to the receiver operating characteristic (ROC) curve analysis for evaluation of the diagnostic accuracy of the genes. Subsequently, immune infiltration analysis was conducted to demonstrate the immune infiltration status of the samples and the correlation between key genes and immune infiltration. Finally, the expression levels of key genes in peripheral blood mononuclear cells (PBMCs) and their correlation with clinical indicators were validated via RT-qPCR assay.

Results

Through WGCNA and differential expression analysis, 6 genes were identified. Ultimately, five key genes (ACSL1, SLC40A1, GZMM, TRIB1, XBP1) were determined using LASSO regression. The area under the ROC curve (AUC) for these genes was greater than 0.7, indicating favorable diagnostic performance. Immune infiltration analysis showed that AS was associated with infiltration levels of various immune cells. RT-qPCR validated that the expression of ACSL1, SLC40A1, GZMM, and XBP1 was consistent with the predictive model, whereas TRIB1 expression was contrary to the predictive model. Clinical correlation analysis of key genes revealed that ACSL1 was positively linked to hsCRP levels, GZMM was negatively linked to, hsCRP levels, and neutrophil absolute values, SLC40A1 was positively linked to ESR, hsCRP levels and neutrophil absolute values, and XBP1 was negatively linked to ESR, hsCRP levels, and neutrophil absolute values.

Conclusion

This study identified key genes that may reveal a potential association between AS and ferroptosis, demonstrating high diagnostic value. Furthermore, the expression levels of these genes in peripheral blood mononuclear cells (PBMCs) are strongly correlated with disease activity. These findings not only suggest potential biomarkers for the diagnosis of AS but also offer important references for exploring new therapeutic targets, highlighting their substantial clinical applicability.

Disease-specific definitions of new bone formation on spine radiographs: a systematic literature review

Objectives

We aimed to explore the radiographic definitions of types of New Bone formation (NBF) by focusing on the terminology, description and location of the findings.

Methods

Three systematic literature reviews were conducted in parallel to identify the radiographic spinal NBF definitions for spondyloarthritis (SpA), Diffuse Idiopathic Skeletal Hyperostosis (DISH) and Osteorathritis (OA). Study characteristics and definitions were extracted independently by two reviewers. Definitions were analysed and collated based on whether they were unique, modified or established from previous research.

Results

We identified 33 studies that indicated a definition for the NBF in SpA, 10 for DISH and 7 for spinal OA. In SpA, the variations in syndesmophytes included the description as well as the subtypes and locations. The differentiation of syndesmophytes from osteophytes were included in 12 articles, based on the origin and the angle of the NBF and associated findings. The definitions of DISH varied in the number of vertebrae, level and laterality. For OA, five articles indicated that osteophytes arose from the anterior or lateral aspects of the vertebral bodies, and two studies required a size cut-off.

Discussion

Our ultimate aim is to create formal NBF definitions for SpA, DISH and OA guided by an atlas, through a Delphi exercise with international experts. The improved ability to differentiate these conditions radiographically will not only allow the clinicians to accurately approach patients but also will help the researchers to better classify patient phenotypes and focus on accurate radiographic outcomes.

Revealing mechanism of Methazolamide for treatment of ankylosing spondylitis based on network pharmacology and GSEA

Methazolamide is a carbonic anhydrase (CA) inhibitor with satisfactory safety. Our previous studies have demonstrated the elevation of CA1 expression and the therapeutic effect of Methazolamide in Ankylosing spondylitis (AS). In this study, we explored the pathogenic role of CA1 and the pharmacological mechanism of Methazolamide in AS through Gene Set Enrichment Analysis (GSEA) and network pharmacology. Seven out of twelve CA1 related gene sets were enriched in AS group. CA1 was core enriched in above seven gene sets involving zinc ion binding, arylesterase activity and one carbon metabolic process. Functional analysis of the candidate target genes obtained from the intersection of AS associated genes and Methazolamide target genes indicated that Methazolamide exerts therapeutic effects on AS mainly through inflammatory pathways which regulate the production of tumor necrosis factor, IL-6 and nitric oxide. PTGS2, ESR1, GSK3β, JAK2, NOS2 and CA1 were selected as therapeutic targets of Methazolamide in AS. Molecular docking and molecular dynamics simulations were performed successfully. In addition, we innovatively obtained the intersection of Gene Ontology (GO)/Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses and GSEA results, and found that 18 GO terms and 5 KEGG terms were indicated in the pharmacological mechanism of Methazolamide in AS, involving bone mineralization, angiogenesis, inflammation, and chemokine signaling pathways. Nevertheless, validation for these mechanisms is needed in vivo/vitro experiments.

Rotating Magnetic Field Mitigates Ankylosing Spondylitis Targeting Osteocytes and Chondrocytes via Ameliorating Immune Dysfunctions

Ankylosing spondylitis (AS) is clinically characterized by bone fusion that is induced by the pathological formation of extra bone. Unfortunately, the fundamental mechanism and related therapies remain unclear. The loss of SHP-2 (encoded by Ptpn11) in CD4-Cre;Ptpn11^f/f mice resulted in the induction of AS-like pathological characteristics, including spontaneous cartilage and bone lesions, kyphosis, and arthritis. Hence, this mouse was utilized as an AS model in this study. As one of the basic physical fields, the magnetic field (MF) has been proven to be an effective treatment method for articular cartilage degeneration. In this study, the effects of a rotating magnetic field (RMF; 0.2 T, 4 Hz) on an AS-like mouse model were investigated. The RMF treatment (2 h/d, 0.2 T, 4 Hz) was performed on AS mice from two months after birth until the day before sampling. The murine specimens were subjected to transcriptomics, immunomics, and metabolomics analyses, combined with molecular and pathological experiments. The results demonstrated that the mitigation of inflammatory deterioration resulted in an increase in functional osteogenesis and a decrease in dysfunctional osteolysis due to the maintenance of bone homeostasis via the RANKL/RANK/OPG signaling pathway. Additionally, by regulating the ratio of CD4+ and CD8+ T-cells, RMF treatment rebalanced the immune microenvironment in skeletal tissue. It has been observed that RMF interventions have the potential to alleviate AS, including by decreasing pathogenicity and preventing disease initiation. Consequently, RMF, as a moderately physical therapeutic strategy, could be considered to alleviate the degradation of cartilage and bone tissue in AS and as a potential option to halt the progression of AS.

Purine metabolites promote ectopic new bone formation in ankylosing spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease that mainly affects the axial skeleton, whose typical features are inflammatory back pain, bone structural damage and pathological new bone formation. The pathology of ectopic new bone formation is still little known. In this study, we found increased purine metabolites in plasma of patients with AS. Similarly, metabolome analysis indicated increased purine metabolites in both serum of CD4-Cre; Ptpn11^fl/fl and SHP2-deficient chondrocytes. SHP2-deficient chondrocytes promoted the growth of wild type chondrocytes and differentiation of osteoblasts in CD4-Cre; Ptpn11^fl/fl mice, which spontaneously developed AS-like bone disease. Purine metabolites, along with PTHrP derived from SHP2-deficient chondrocytes, accelerated the growth of chondrocytes and ectopic new bone formation through PKA/CREB signaling. Moreover, Suramin, a purinergic receptor antagonist, suppressed pathological new bone formation in AS-like bone disease. Overall, these results highlight the potential role of targeting purinergic signaling in retarding ectopic new bone formation in AS.

Targeting chondrocytes for arresting bony fusion in ankylosing spondylitis

Bony fusion caused by pathological new bone formation manifests the clinical feature of ankylosing spondylitis (AS). However, the underlying mechanism remains elusive. Here we discovered spontaneous kyphosis, arthritis and bony fusion in mature CD4-Cre;Ptpn11f/f mice, which present the pathophysiological features of AS. A population of CD4-Cre-expressing proliferating chondrocytes was SHP2 deficient, which could differentiate into pre-hypertrophic and hypertrophic chondrocytes. Functionally, SHP2 deficiency in chondrocytes impeded the fusion of epiphyseal plate and promoted chondrogenesis in joint cavity and enthesis. Mechanistically, aberrant chondrocytes promoted ectopic new bone formation through BMP6/pSmad1/5 signaling. It is worth emphasizing that such pathological thickness of growth plates was evident in adolescent humans with enthesitis-related arthritis, which could progress to AS in adulthood. Targeting dysfunctional chondrogenesis with Smo inhibitor sonidegib significantly alleviated the AS-like bone disease in mice. These findings suggest that blockade of chondrogenesis by sonidegib would be a drug repurposing strategy for AS treatment.

Contributions of muscle-resident progenitor cells to homeostasis and disease

Adult skeletal muscle maintains a homeostatic state with modest levels of cellular turnover, unlike the skin or blood. However, the muscle is highly sensitive to tissue injury, which unleashes a cascade of regenerative and inflammatory processes. Muscle regeneration involves cross-talk between numerous cytokine signaling axes, and the coordinated activity of multiple muscle-resident and circulating progenitor populations. Satellite cells, closely associated with myofibers, are established as the canonical muscle stem cell, with self-renewal and myofiber-regenerating capacity. However, a heterogeneous group of mesenchymal progenitor cells residing within the muscle interstitium are also highly responsive to muscle injury and exhibit varying degrees of regenerative potential. These cells interact with satellite cells via direct and indirect mechanisms to regulate regeneration or repair. We describe the known phylogenetic and functional relationships of the multiple progenitor populations residing within skeletal muscle, their putative roles in the coordination of injury repair, and their possible contributions to health and disease.

Anti-TNFα treatment decreases the previously increased serum Indian Hedgehog levels in patients with ankylosing spondylitis and affects the expression of functional Hedgehog pathway target genes

OBJECTIVE

Indian Hedgehog (Ihh) is the ligand that activates the Hedgehog pathway (HH) in the skeleton-the main controller of endochondral ossification. We aimed at assessing serum levels of Ihh in patients with ankylosing spondylitis (AS) and the effect of serum from patients with AS on HH pathway activation.

METHODS

Serum Ihh levels were measured in 59 patients with AS, 70 patients with rheumatoid arthritis (RA), and 53 healthy subjects. The effect of serum from patients with AS on HH pathway activation was evaluated using an osteoblast-like cell line model.

RESULTS

Patients with AS not on anti-TNFα treatment had significantly higher Ihh levels compared to patients with RA not on anti-TNFα treatment (mean ± SEM of OD: 0.370 ± 0.025 vs. 0.279 ± 0.026 for patients with AS and RA, respectively, p = 0.027) and healthy subjects (p = 0.031). Patients with AS on anti-TNFα treatment had significantly lower Ihh levels compared to patients with AS not on such treatment (p = 0.028). Patients with RA on anti-TNF treatment had higher levels of Ihh compared to patients not on such treatment (p = 0.013). PTHrP levels were similar in patients with RA, AS, and healthy subjects and were not affected by anti-TNFα treatment. We next assessed HH pathway activation in Saos2 cells following incubation with serum from AS patients prior to and following anti-TNF treatment. The HH pathway was downregulated following treatment.

CONCLUSIONS

Ihh levels are increased in patients with AS and decrease following anti-TNFα treatment; this finding may have pathogenic and clinical implications.

Glast-expressing progenitor cells contribute to heterotopic ossification

Heterotopic ossification (HO), acquired or hereditary, is the formation of true bone outside the normal skeleton. Although the lineages of cells contributing to bone formation during normal development are well defined, the precise lineages of cells that contribute to HO are not clear. This study utilized Cre-lox based genetic lineage tracing to examine the contribution to HO of cells that expressed either FoxD1 or Glast. Both lineages contributed broadly to different normal tissues, and FoxD1-cre labeled cells contributed to normal bone formation. Despite the similarity in labeling patterns of normal tissues, and the significant contribution of FoxD1-cre labeled cells to normal bone, only Glast-creERT labeled progenitors contributed significantly to HO at all stages, suggesting that the cell populations that normally contribute to physiological bone formation, such as the Foxd1-cre labeled cells, may not participate in pathological HO. Further, identification of Glast-expressing cells as precursors that give rise to HO should help with the molecular targeting of this population both for the prevention and for the treatment of HO.

Positive regulators of osteoclastogenesis and bone resorption in rheumatoid arthritis

Bone destruction is a frequent and clinically serious event in patients with rheumatoid arthritis (RA). Local joint destruction can cause joint instability and often necessitates reconstructive or replacement surgery. Moreover, inflammation-induced systemic bone loss is associated with an increased fracture risk. Bone resorption is a well-controlled process that is dependent on the differentiation of monocytes to bone-resorbing osteoclasts. Infiltrating as well as resident synovial cells, such as T cells, monocytes and synovial fibroblasts, have been identified as sources of osteoclast differentiation signals in RA patients. Pro-inflammatory cytokines are amongst the most important mechanisms driving this process. In particular, macrophage colony-stimulating factor, RANKL, TNF, IL-1 and IL-17 may play dominant roles in the pathogenesis of arthritis-associated bone loss. These cytokines activate different intracellular pathways to initiate osteoclast differentiation. Thus, over the past years several promising targets for the treatment of arthritic bone destruction have been defined.

Are current available therapies disease-modifying in spondyloarthritis?

Disease modification in spondyloarthritis should target the improvement of symptoms and preservation of function. Therefore, inhibition of structural damage caused by the disease processes appears essential. In spondyloarthritis, structural damage results mainly in progressive ankylosis of the spine and peripheral joint destruction. Currently available therapies for the treatment of spondyloarthritis appear effective at inhibiting tissue destruction but, with the exception of celecoxib, do not appear to affect new tissue formation leading to ankylosis. In this article, we discuss clinical and pathophysiological concepts of disease modification in spondyloarthritis, challenges in its evaluation, recent clinical data and new concepts that may help explain structural damage as well as the onset and progression of disease.

Increased expression of carbonic anhydrase I in the synovium of patients with ankylosing spondylitis

Background

One of the most distinctive features of ankylosing spondylitis (AS) is new bone formation and bone resorption at sites of chronic inflammation. Previous studies have indicated that the hyperplasia and inflammation of synovial tissues are significantly related to the pathogenic process of AS. The present study used a proteomic approach to identify novel AS-specific proteins by simultaneously comparing the expression profiles of synovial membranes from patients with AS, rheumatoid arthritis (RA) and osteoarthritis (OA).

Methods

Synovial tissues were collected from the hip joints of patients with AS and knee joints of patients with RA or OA (n = 10 for each disease) during joint replacement surgery. Proteins extracted from the synovial tissues were separated by 2-D electrophoresis (2-DE), and the proteins with significantly increased expression in the AS samples were subjected to MALDI-TOF/TOF-MS analysis. The results were verified using western blotting and immunohistochemistry. Levels of the candidate proteins in synovial fluids from knee joints (n = 40 for each disease) were measured using ELISA.

Results

The proteomic approach revealed significantly increased expression of carbonic anhydrase I (CA1) in the synovial membrane of patients with AS as compared with the RA and OA tissue samples. Immunohistochemistry and western blotting analysis confirmed the findings described above. The ELISA detected a higher level of CA1 in synovial fluids from patients with AS than those with OA. The mean value of the CA1 level was also higher in AS patients as compared with RA patients. This study also detected increased expression of alpha-1-antitrypsin in the synovial tissues from AS patients, which is in agreement with other reports.

Conclusion

In vitro experiments by other groups indicated that CA1 catalyzes the generation of HCO₃^- through the hydration of CO₂, which then combines with Ca²⁺ to form a CaCO3 precipitate. Calcification is an essential step of bone formation. Substantial evidence indicates that carbonic anhydrase also stimulates bone resorption. Hence, overexpression of CA1 in the synovial tissues of AS patients may promote improper calcification and bone resorption in AS.

Effects of time of initial exposure to MSV sarcoma on bone induction by dentine matrix implants and on orthotopic femora

HCl-demineralized murine lower incisors were implanted intramuscularly into syngeneic BALB/c mice to induce heterotopic osteogenesis. Implants were exposed at the early, preosteogenic stage (4), or at the later, osteogenic stage (12) to the Moloney sarcoma virus (MSV), which within 3–4 days results in a sarcoma. The yield of bone induction was determined by weight of dry bone mass following NaOH hydrolysis of soft tissues. To verify the effect of this sarcoma on orthotopic local femoral bone, the dry mass of the tumor-exposed femora was measured and compared with the weight of MSV-unexposed contralateral controls. MSV-sarcoma or cells involved with their spontaneous rejection have a stimulatory effect on the periosteal membrane of the tumor-adjacent femoral bones, increasing their dry mass on average by 18%. No stimulatory effect on heterotopic bone induction was observed when the MSV sarcoma grew during the early, preosteogenic stage (4 onward), but when the tooth matrix had been exposed to such tumor at the already bone-forming stage, (12 onward), the yield of bone induction was enhanced. Thus, it is postulated that lesions induced by MSV during the early, preosteogenic stage inhibit recruitment of osteoprogenitor cells or degrade Bone Morphogenetic Proteins (BMPs) released by matrix resorbing inflammatory cells, whereas when acting on already existing bone they have a stimulatory effect.

Baseline and 1-year magnetic resonance imaging of the sacroiliac joint and lumbar spine in very early inflammatory back pain. Relationship between symptoms, HLA-B27 and disease extent and persistence

BACKGROUND

The precise anatomical location of pathology associated with inflammatory back pain (IBP) in early spondyloarthropathy (SpA) remains unclear.

OBJECTIVE

To use MRI to study the sacroiliac joint (SIJ) and lumbar spine (LS) and explore the relationship between sites and extent of inflammation and HLA-B27 status over 12 months.

METHODS

54 patients with IBP; median duration 24 weeks (54% HLA-B27 positive; median Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) 5.65) and 22 control subjects (11 with mechanical back pain; 11 volunteers) were recruited and 63% (n = 34) were reassessed at 1 year. Fat saturation and T1-weighted MRI was performed with images being scored for active bone marrow oedema (BMO) lesions representative of inflammation.

RESULTS

At baseline 46/54 (85%) patients had BMO (SIJs and LS) compared with 40% in the control group. The majority of affected patients had inflammation at the SIJ level (96% (n = 44); 23.5% (n = 12) LS) and 28.3% (n = 13) at both sites simultaneously. The SIJ activity score confirmed more severe inflammation (BMO grade 2 or 3: 52.2%) in the IBP group (controls = BMO grade 1: 100%; p<0.001). HLA-B27 was associated with both the severity (p = 0.009) and number of baseline SIJ lesions (p = 0.045) and with persistence (SIJ or LS) at 1 year (p = 0.02). 90% of reattenders fulfilled European Spondyloarthropathy Study Group criteria; 73.5% showed MRI inflammation despite clinical improvement (median BASDAI 5.65 to 3.05; p<0.009).

CONCLUSION

LS and SIJ involvement may occur simultaneously in very early SpA and may be differentiated from non-inflammatory back pain by the severity of MRI lesions. HLA-B27 is associated with both the severity of osteitis and its persistence.

Progress in spondylarthritis. Mechanisms of new bone formation in spondyloarthritis

Targeted therapies that neutralize tumour necrosis factor are often able to control the signs and symptoms of spondyloarthritis. However, recent animal model data and clinical observations indicate that control of inflammation may not be sufficient to impede disease progression toward ankylosis in these patients. Bone morphogenetic proteins and WNTs (wingless-type like) are likely to play an important role in ankylosis and could be therapeutic targets. The relationship between inflammation and new bone formation is still unclear. This review summarizes progress made in our understanding of ankylosis and offers an alternative view of the relationship between inflammation and ankylosis.

Heterotopic osteogenesis by murine demineralized incisors at lesions sites induced by concanavalin a in mice

The aim of this study was to examine the effects of Concanavalin A (Con A) administration on the early (preosteogenic) and late stages of osteogenesis induced by implantation of demineralized murine incisors into syngeneic mice. Local administration of Con A resulted in an increased yield of demineralized incisor-induced bone when injected during the preosteogenic stage of induction. This effect was not observed when Con A was injected after heterotopic osteogenesis had been established. This suggests that Con A recruits osteoprogenitor cells, but does not stimulate differentiated chondroblasts and osteoblasts.

Joint homeostasis, restoration, and remodeling in osteoarthritis

Osteoarthritis is the major cause of joint failure. The outcome of the disease process is determined by complex interactions between cells and molecules steering homeostasis, destruction, restoration, and remodeling. The articular cartilage has a limited restoration and repair capacity. Genetic studies in humans and the development of mouse models have identified the role of signaling pathways that are important for skeletal development in the postnatal biology and pathology of articular cartilage. These include bone morphogenetic protein, transforming growth factor beta, fibroblast growth factor, wingless-type signaling, and their respective antagonists such as noggin and frizzled related protein. The synovium is prone to inflammation and emerging evidence suggests that innate and adaptive immune responses are important. Bone and cartilage form a biomechanical unit; stiffer bones might impair cartilage homeostasis. The biology of frizzled related protein provides a basis for the hypothesized inverse relationship between osteoarthritis and osteoporosis.

Bone morphogenetic proteins in destructive and remodeling arthritis

Joint destruction and tissue responses determine the outcome of chronic arthritis. Joint inflammation and damage are often the dominant clinical presentation. However, in some arthritic diseases, in particular the spondyloarthritides, joint remodeling is a prominent feature, with new cartilage and bone formation leading to ankylosis and contributing to loss of function. A role for bone morphogenetic proteins in joint remodeling has been demonstrated in the formation of both enthesophytes and osteophytes. Data from genetic models support a role for bone morphogenetic protein signaling in cartilage homeostasis. Finally, this signaling pathway is likely to play a steering role in the synovium.

Osteoporosis and vertebral fractures in ankylosing spondylitis

PURPOSE OF REVIEW

To review recent data, in the context of what is already known, about an increased risk of vertebral fractures in ankylosing spondylitis patients.

RECENT FINDINGS

Osteoporosis and fractures of the vertebral body and its dorsal arch are now well recognized features in patients with ankylosing spondylitis, but their diagnosis is still often a challenge. The risk factors and clinical consequences for fractures in ankylosing spondylitis are increasingly understood in the context of osteoimmunology and of spinal biomechanical changes in material and structural components in the spine that result in bone failure.

SUMMARY

Diagnosing fractures of the vertebral body and its dorsal arch remains a challenge in studies and in clinical practice. Prospective studies are needed to evaluate to what degree such fractures can be prevented in ankylosing spondylitis.