We have identified a novel chordin-like protein, CHL2, which is structurally most homologous to CHL/neuralin/ventroptin. When injected into Xenopus embryos, CHL2 RNA induced a secondary axis. Recombinant CHL2 protein interacted directly with BMPs in a competitive manner to prevent binding to the type I BMP receptor ectodomain, and inhibited BMP-dependent induction of alkaline phosphatase in C2C12 cells. Thus, CHL2 behaves as a secreted BMP-binding inhibitor. In situ hybridization revealed that CHL2 expression is restricted to chondrocytes of various developing joint cartilage surfaces and connective tissues in reproductive organs. Adult mesenchymal progenitor cells expressed CHL2, and its levels decreased during chondrogenic differentiation. Addition of CHL2 protein to a chondrogenic culture system reduced cartilage matrix deposition. Consistently,CHL2 transcripts were weakly detected in normal adult joint cartilage. However, CHL2 expression was upregulated in middle zone chondrocytes in osteoarthritic joint cartilage (where hypertrophic markers are induced). CHL2 depressed chondrocyte mineralization when added during the hypertrophic differentiation of cultured hyaline cartilage particles. Thus, CHL2 may play negative roles in the (re)generation and maturation of articular chondrocytes in the hyaline cartilage of both developing and degenerated joints.

RANKL is an essential mediator of bone erosions, but the role of RANKL in systemic bone loss had not been studied in arthritis. RANKL protein was increased in rat joint extracts and serum at the earliest stages of arthritis. Osteoprotegerin (OPG) treatment reversed local and systemic bone loss, suggesting that RANKL is both a marker and mediator of bone loss in arthritis.Introduction: RANKL is well established as an essential mediator of bone erosions in inflammatory arthritis, but the role of RANKL in systemic bone loss in arthritis had not been studied. We hypothesized that serum RANKL could serve as both a mediator and as a novel biomarker for local and systemic bone loss in arthritis. We challenged this hypothesis in two established rat models of inflammatory arthritis. We sought to determine whether serum RANKL was elevated early in disease progression and whether RANKL suppression could prevent both local and systemic bone loss in these models. Materials and Methods: Detailed time-course studies were conducted in animals with collagen-induced (CIA) or adjuvant-induced (AIA) arthritis to evaluate the onset and progression of inflammation (paw swelling), bone erosions, osteoclast numbers, and RANKL protein levels in arthritic joints and in serum. Additional CIA and AIA rats (n ‫ס‬ 8/group) received placebo (PBS) or recombinant OPG (3 mg/kg three times weekly) for 10 days beginning 4 days after disease onset (first macroscopic evidence of hind paw erythema and edema) to assess the role of RANKL in local and systemic bone loss. Results: RANKL protein was significantly elevated in the joints and serum of CIA and AIA rats within 1-2 days of disease onset. Increased RANKL levels were associated with local (hind paw) and systemic (vertebral) osteopenia in both models. The RANKL inhibitor OPG prevented local and systemic osteopenia in both models of established disease. Conclusions: RANKL protein is significantly increased both locally and systemically during the earliest stages of inflammatory arthritis in rats, suggesting that serum RANKL might have prognostic value for bone erosions and systemic osteopenia in this condition. RANKL inhibition through OPG prevented local and systemic bone loss in these arthritis models, suggesting that RANKL inhibition is a promising new approach for treating bone loss in arthritis.

Objective. To investigate whether the bonepreserving effects of a RANKL antagonist or a tumor necrosis factor (TNF) antagonist could be further improved by the addition of a bone anabolic agent in inflammatory arthritis.Methods. Lewis rats with either adjuvant-induced arthritis (AIA) or collagen-induced arthritis (CIA) were treated for 10 days with PEGylated soluble tumor necrosis factor receptor type I (PEG sTNFRI), interleukin-1 receptor antagonist (IL-1Ra), osteoprotegerin (OPG), parathyroid hormone (PTH), or combinations of these agents starting on day 4 after disease onset. Treatment effects were assessed clinically, radiologically, and histologically, and by morphometry for the extent of paw swelling, bone erosive changes, and synovial inflammation.Results. Paw swelling and synovial inflammation were significantly inhibited by PEG sTNFRI in AIA and CIA, and by IL-1Ra in CIA. OPG and PTH had no significant effect on these parameters. Analysis of bone erosion revealed a significant bone-sparing effect of monotherapy with PEG sTNFRI or OPG in both models, whereas IL-1Ra was only effective in CIA. PTH treatment alone did not show a bone-protective effect in either model. With the combination of PEG sTNFRI and PTH, erosion scores (؊74% in AIA and ؊61% in CIA versus controls) were significantly lower than those elicited by PEG sTNFRI alone (؊41% and ؊29%, respectively, versus controls). Similar results were also obtained with the combination of OPG and PTH (؊88% in AIA and ؊73% in CIA, compared with ؊70% and ؊55%, respectively, with OPG monotherapy). Coadministration of IL-1Ra and PTH had no synergistic bonesparing effect. Morphometric analysis revealed that the addition of PTH to PEG sTNFRI or OPG resulted in higher bone volume and higher osteoblast numbers in both AIA and CIA.Conclusion. The bone-protective effects resulting from RANKL or TNF antagonism can be further improved by the addition of a bone anabolic agent.

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SummaryWe determined the expression of ORAI1 protein in rodent and non-rodent tissues using a monoclonal antibody directed against an extracellular loop of the protein. Previous reports using antibodies directed at the C-terminus of ORAI1 have not detected central nervous system (CNS) expression. Our results demonstrate broad tissue expression that includes the CNS using a unique monoclonal antibody specific to an extracellular loop of ORAI1. In addition, we present in situ hybridization (ISH) results using a probe within the middle of the mouse coding region showing CNS expression of Orai1 RNA. We contrast the patterns of rodent and human tissue expression and conclude that rodents have similar expression of ORAI1 in most tissue types when compared to primates, with an important exception being the male reproductive system, where human-specific expression is observed. (J Histochem Cytochem 62:864-878, 2014)