ABSTRACT

Thy1 (CD90), a glycosylated, glycophosphatidylinositol-anchored membrane protein highly expressed by subsets of mesenchymal stem cells and fibroblasts, inhibits adipogenesis. The role of Thy1 on bone structure and function has been poorly studied and represents a major knowledge gap. Therefore, we analyzed the long bones of wild-type (WT) and Thy1 knockout (KO) mice with micro-computed tomography (micro-CT) and histomorphometry to compare changes in bone architecture and overall bone structure. micro-CT analysis of long bones revealed Thy1 KO and WT mice fed a high-fat diet demonstrated bone structural parameters at 4 mo that differed significantly between WT and KO mice. A significant reduction in trabecular bone volume was noted in Thy1 KO mice. The most prominent differences were observed in trabecular bone volume ratio and trabecular bone connectivity density. Consistent with micro-CT measurements, histomorphometric analysis also showed decreased bone volume in the obese Thy1 KO mice compared to obese WT mice. In vitro assays revealed that osteogenic conditions increased Thy1 expression during OB differentiation and absence of Thy1 attenuated osteoblastogenesis. Together, these findings support the concept that Thy1 serves as a major mechanistic link to regulate bone formation and negatively regulate adipogenesis.-Paine, A., Woeller, C. F., Zhang, H., Garcia-Hernandez, M. L., Huertas, N., Xing, L., Phipps, R. P., Ritchlin, C. T. Thy1 is a positive regulator of osteoblast differentiation and modulates bone homeostasis in obese mice.

Psoriatic arthritis (PsA) is an inflammatory rheumatic disorder that occurs in patients with psoriasis and predominantly affects musculoskeletal structures, skin and nails. The etiology of PsA is not well understood but evidence supports interplay of genetic, immunologic and environmental factors which promote pathological bone remodeling and joint damage in PsA. Localized and systemic bone loss due to increased activity of osteoclasts is well established in PsA based on animal models and translational studies. In contrast, the mechanisms responsible for pathological bone remodeling in PsA remain enigmatic although new candidate molecules and pathways have been identified. Recent reports have revealed novel findings related to bone erosion and pathologic bone formation in PsA. Many associated risk factors and contributing molecular mechanisms have also been identified. In this review, we discuss new developments in the field, point out unresolved questions regarding the pathogenetic origins of the wide array of bone phenotypes in PsA and discuss new directions for investigation.

Purpose of review To highlight and emphasize how new knowledge of mechanisms linked to the IL-23/IL-17 pathway are relevant to the pathophysiology of axial spondyloarthritis (axSpA) and to demonstrate how molecules in IL-23/IL-17 pathway provide novel therapeutic targets for axSpA patients. Recent findings Similar to AS, the increased frequency of Th17 cells in nr-axSpA patients underscores the concept that these disorders can be viewed on a spectrum. Recent findings suggest that the contribution of IL-23/IL-17 signaling pathways possibly differ in male and female AS patients. The finding that IL-17 and IL-22 secreting type 3 innate lymphoid cells (ILC3), are increased in AS patients point to their potential role in the pathogenesis of axSpA. Reports of dysbiosis in the gut microbiome of AS patients supports previous work indicating a possible causal relationship between altered gut flora, ileocolonic inflammation and axSpA. Of important clinical relevance, are results from clinical trials supporting the efficacy and safety of agents that block IL-12/23 (ustekinumab) and IL-17 (secukinumab and ixekizumab) in AS patients. Summary Recent studies further establish the central position of the IL-23/IL-17 pathway in the pathogenesis of axSpA. Targeting IL-23/IL-17 pathway appears to be a safe and effective strategy for treatment of axSpA patients.

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Imaging studies have provided new insights into new bone formation phenotypes in PsA. The IL-23/IL-17 pathway is of central importance in psoriatic bone remodeling where, apart from CD4 T helper cells, other IL-17 and IL-22-secreting innate and adaptive cells may also be involved. Insights from study of the microbiome and from omics technologies will set the stage for new advances in our understanding of bone disorders in psoriatic diseases.

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