A novel role for bone-derived cells in ankylosing spondylitis: Focus on IL-23

RNA Sequencing Reveals the Expression Profiles of circRNAs and Indicates Hsa_circ_0070562 as a Pro-osteogenic Factor in Bone Marrow-Derived Mesenchymal Stem Cells of Patients With Ankylosing Spondylitis

Recent studies have reported that circular RNAs (circRNAs) play a crucial regulatory role in a variety of human diseases. However, the roles of circRNAs in pathological osteogenesis in ankylosing spondylitis (AS) remain unclear. We conducted circRNA and miRNA expression profiling of osteogenically differentiated bone marrow-derived mesenchymal stem cells (BMSCs) of patients with AS compared with those of healthy donors (HDs) by RNA sequencing (RNA-seq). Results showed that a total of 31806 circRNAs were detected in the BMSC samples, of which 418 circRNAs were significantly differentially expressed (DE) with a fold change ≥2 and p value <0.05. Among these, 204 circRNAs were upregulated, and 214 were downregulated. GO and KEGG analyses demonstrated that the DE circRNAs were mainly involved in the regulation of biological processes of the cell matrix adhesion and the TGF-beta signaling pathway, which are closely related to AS. circRNA-miRNA interaction networks related to the TGF-beta signaling pathway were established. The results of qRT-PCR showed that has_circ_0070562 was significantly up-regulated in AS-MSCs. In vitro experiments showed that silencing of has_circ_0070562 weakened osteogenesis of AS-BMSCs. In conclusion, we identified numerous circRNAs that were dysregulated in AS-BMSCs compared with HD-BMSCs. Bioinformatic analyses suggested that these dysregulated circRNAs might play important functional roles in AS-BMSCs osteogenesis. Circ_0070562 functioned as a pro-ostegenic factor and might serve as a potential biomarker and a therapeutic target for AS.

Osteoimmunological insights into the pathogenesis of ankylosing spondylitis

Ankylosing spondylitis (AS) is inflammatory arthritis predominantly affecting the spine, which is involved in the disorders of both immune and skeletal systems. The exact pathogenesis of AS is not fully understood. Osteoimmunology is a new subject of study in inflammatory arthritis, in particular the pathogenic events involved in the cross-regulation of both skeletal and immune systems. In this review, we discuss osteoimmunological and pathological changes of AS in the spine that are characterized by altered osteogenesis and osteolytic bone destruction, accompanied by the changes of the immune system. It was revealed that bone cells like mesenchymal stem cells, osteoblast, and osteoclast in crossing talking with immune cells such as T cells, B cells coregulate to the pathogenesis of AS. Further, an array of cytokines and molecules expressed by both skeletal and immune systems contribute to these complex interplays. Understanding the cellular and molecular mechanisms underlying the pathogenesis of AS will lay a foundation for the exploration of the potential new treatment to AS.

HLA-B27-mediated activation of TNAP phosphatase promotes pathogenic syndesmophyte formation in ankylosing spondylitis

Ankylosing spondylitis (AS) is a type of axial inflammation. Over time, some patients develop spinal ankylosis and permanent disability; however, current treatment strategies cannot arrest syndesmophyte formation completely. Here, we used mesenchymal stem cells (MSCs) from AS patients (AS MSCs) within the enthesis involved in spinal ankylosis to delineate that the HLA-B27-mediated spliced X-box-binding protein 1 (sXBP1)/retinoic acid receptor-β (RARB)/tissue-nonspecific alkaline phosphatase (TNAP) axis accelerated the mineralization of AS MSCs, which was independent of Runt-related transcription factor 2 (Runx2). An animal model mimicking AS pathological bony appositions was established by implantation of AS MSCs into the lumbar spine of NOD-SCID mice. We found that TNAP inhibitors, including levamisole and pamidronate, inhibited AS MSC mineralization in vitro and blocked bony appositions in vivo. Furthermore, we demonstrated that the serum bone-specific TNAP (BAP) level was a potential prognostic biomarker to predict AS patients with a high risk for radiographic progression. Our study highlights the importance of the HLA-B27-mediated activation of the sXBP1/RARB/TNAP axis in AS syndesmophyte pathogenesis and provides a new strategy for the diagnosis and prevention of radiographic progression of AS.

CCAAT/enhancer-binding protein beta (C/EBPβ) is an important mediator of 1,25 dihydroxyvitamin D3 (1,25D3)-induced receptor activator of nuclear factor kappa-B ligand (RANKL) expression in osteoblasts

Receptor activator of nuclear factor kappa B ligand (RANKL) expression in osteoblasts is regulated by 1,25-dihydroxyvitamin D3 (1,25D3). CCAAT/enhancer-binding protein beta (C/EBPβ) has been proposed to function as a transcription factor and upregulate RANKL expression, but it is still uncertain how C/EBPβ is involved in 1,25D3-induced RANKL expression of osteoblasts. 1,25D3 stimulation increased the expression of RANKL and C/EPBβ genes in osteoblasts and enhanced phosphorylation and stability of these proteins. Moreover, induction of RANKL expression by 1,25D3 in osteoblasts was downregulated upon knockdown of C/EBPβ. In contrast, C/EBPβ overexpression directly upregulated RANKL promoter activity and exhibited a synergistic effect on 1,25D3-induced RANKL expression. In particular, 1,25D3 treatment of osteoblasts increased C/EBPβ protein binding to the RANKL promoter. In conclusion, C/EBPβ is required for induction of RANKL by 1,25D3. [BMB Reports 2019; 52(6): 391-396].

Cytokines and Bone: Osteoimmunology

Cytokines and hematopoietic growth factors have traditionally been thought of as regulators of the development and function of immune and blood cells. However, an ever-expanding number of these factors have been discovered to have major effects on bone cells and the development of the skeleton in health and disease (Table 1). In addition, several cytokines have been directly linked to the development of osteoporosis in both animal models and in patients. In order to understand the mechanisms regulating bone cells and how this may be dysregulated in disease states, it is necessary to appreciate the diverse effects that cytokines and inflammation have on osteoblasts, osteoclasts, and bone mass. This chapter provides a broad overview of this topic with extensive references so that, if desired, readers can access specific references to delve into individual topics in greater detail.

Targeting inflammatory pathways in axial spondyloarthritis

The triggers and pathogenesis of axial spondyloarthritis (axSpA) are not yet completely understood. However, therapeutic agents targeting tumor necrosis factor-α and interleukin-17 inflammatory pathways have proven successful in suppressing many of the clinical symptoms and signs of axSpA, giving us an indication of which pathways are responsible for initiating and maintaining the inflammation. The mechanisms that eventuate in syndesmophytes and ankyloses are less clear. This review addresses these two critical pathways of inflammation, discussing their nature and these factors that may activate or enhance the pathways in patients with axSpA. In addition, genetic and other markers important to the inflammatory pathways implicated in axSpA are explored, and prognostic biomarkers are discussed. Treatment options available for the management of axSpA and their associated targets are highlighted.

Regulation of osteoblasts by alkaline phosphatase in ankylosing spondylitis

AIM

Ankylosing spondylitis (AS) is characterized by excessive spinal ankylosis and bone formation. Alkaline phosphatase (ALP) activity is reported to be high in AS, but little is known about the molecular relationship between ALP and AS. The aims of this study were to investigate the relevance of ALP to AS and the role of ALP in the regulation of osteoblast differentiation in AS.

METHODS

High-throughput data with accession numbers GSE73754 and GSE41038 were downloaded from the Gene Expression Omnibus. We retrospectively collected and compared the ALP levels of male patients with AS to those of sex- and age-matched healthy controls (HC) and rheumatoid arthritis (RA) patients. Total serum ALP and ALP activity were measured in the AS and RA groups. ALP gene expression and intracellular ALP activity were analyzed in microarray data from primary diseases control (Ct) and AS-bone-derived cells (BdCs) and in vitro experiments. Furthermore, the effect of ALP inhibitor was examined in both primary Ct- and AS-BdCs under osteoblast differentiation. Regulation of runt-related transcription factor 2 (RUNX2) by ALP was also analyzed.

RESULTS

Alkaline phosphatase level was higher in AS compared with RA and HC and was associated with radiograph progression. ALP expression was also enriched in the bone tissue of AS patients. Furthermore, AS-BdCs exhibited increasing ALP activity, leading to accelerated osteoblastic activity and differentiation. Intriguingly, inhibition of ALP reduced RUNX2 expression, a master transcriptional factor in osteoblasts, and differentiation status of both primary Ct- and AS-BdCs. Treatment of ALP activator or inhibitor modulated RUNX2 protein level and RUNX2 regulated ALP promoter activity, indicating a reciprocal ALP-RUNX2 positive feedback to regulate osteoblast differentiation.

CONCLUSION

Alkaline phosphatase was highly expressed in AS patients, may be involved in the ankylosis of AS, and represents a possible therapeutic target for ankylosis.

IL-17A induces osteoblast differentiation by activating JAK2/STAT3 in ankylosing spondylitis

BACKGROUND

IL-17A has recently emerged as a potential target that regulates the extensive inflammation and abnormal bone formation observed in ankylosing spondylitis (AS). Blocking IL-17A is expected to inhibit bony ankylosis. Here, we investigated the effects of anti IL-17A agents in AS.

METHODS

TNFα, IL-17A, and IL-12/23 p40 levels in serum and synovial fluid from patients with ankylosing spondylitis (AS), rheumatoid arthritis (RA), osteoarthritis (OA), or healthy controls (HC) were measured by ELISA. Bone tissue samples were obtained at surgery from the facet joints of ten patients with AS and ten control (Ct) patients with noninflammatory spinal disease. The functional relevance of IL-17A, biological blockades, Janus kinase 2 (JAK2), and non-receptor tyrosine kinase was assessed in vitro with primary bone-derived cells (BdCs) and serum from patients with AS.

RESULTS

Basal levels of IL-17A and IL-12/23 p40 in body fluids were elevated in patients with AS. JAK2 was also highly expressed in bone tissue and primary BdCs from patients with AS. Furthermore, addition of exogenous IL-17A to primary Ct-BdCs promoted the osteogenic stimulus-induced increase in ALP activity and mineralization. Intriguingly, blocking IL-17A with serum from patients with AS attenuated ALP activity and mineralization in both Ct and AS-BdCs by inhibiting JAK2 phosphorylation and downregulating osteoblast-involved genes. Moreover, JAK2 inhibitors effectively reduced JAK2-driven ALP activity and JAK2-mediated events.

CONCLUSIONS

Our findings indicate that IL-17A regulates osteoblast activity and differentiation via JAK2/STAT3 signaling. They shed light on AS pathogenesis and suggest new rational therapies for clinical AS ankylosis.