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McDonald and colleagues reported osteoclast-related dynamic mechanisms that lead, by fission, to osteomorphs; motile, fusion-competent cells capable of forming bone-resorbing osteoclasts. scRNA-seq analyses revealed that osteomorphs are transcriptionally distinct from osteoclasts and macrophages and might be implicated in rare and common bone diseases in humans., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

The transcription factor Runx2 and the Wnt/β-catenin pathway are major regulators of bone formation. Our aim was to assess the interactions between the Wnt/β-catenin pathway and Runx2 that contribute to bone resorption. Our results indicate that the activity of the canonical Wnt/β-catenin pathway depends on Runx2. Runx2 overexpression inhibited β-catenin levels and activity in vitro and in vivo. Inhibition of Gsk3b using lithium chloride in Runx2-overexpressing osteoporotic female mice rescued the Wnt/β-catenin signaling in vivo and completely restored trabecular bone volume by increasing bone formation and decreasing bone resorption. The activation of Wnt/β-catenin signaling by lithium chloride treatment reduced the number and activity of bone marrow-derived osteoclast-like cells in vitro, suggesting that the restoration of trabecular bone in vivo was due to decreased bone resorption, consistent with the reduced receptor activator of NF-κB ligand/osteoprotegerin ratio in Runx2-overexpressing osteoblasts. Lithium chloride also increased osteoblast differentiation and activity in vitro in agreement with the increase in mineral apposition rate and osteocalcin expression detected in vivo. Our results indicate that the activity of the canonical Wnt/β-catenin pathway in osteoblast is modulated by Runx2. To conclude, our in vivo and in vitro results highlight the role of Runx2 as a negative regulator of Wnt/β-catenin pathway activity in osteoblasts and indicate that the abnormal Wnt/β-catenin activity seen in Runx2 transgenic mice affects both osteoblast and osteoclast differentiation and activity., (Copyright © 2016 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

Background: Osteoarthritis (OA) is characterised by cartilage degradation and bone lesions. Subchondral bone may be involved in the pathogenesis of cartilage matrix breakdown., Objective: To assess the role of bone remodelling in OA by studying the effect of bisphosphonate on OA development in mice with high bone remodelling., Methods: Mice overexpressing Runx2 (Runx2-Tg) under the control of collagen type I that displayed high bone remodelling were used. Joint instability was performed by partial medial meniscectomy to induce OA., Results: Six weeks after surgery, tibial cartilage of Runx2-Tg mice displayed an increased number of ADAMTS-4- and ADAMTS-5-expressing chondrocytes compared with controls (p<0.05). This increase was higher in Runx2-Tg mice than in wild-type mice, although their OA score did not differ (2.5+/-0.6 vs 2.4+/-0.2, P=NS). Pamidronate reduced the OA score in Runx2-Tg mice but not in wild-type littermates (1.2+/-0.5 vs 2.7+/-0.4; p<0.05) despite the reduction of bone resorption and of the expression of cartilage proteases in both genotypes., Conclusions: These findings support the hypothesis that the level of bone resorption influences cartilage metabolism and that inhibition might prevent the progression of OA. Targeting bone resorption might therefore provide an approach to the treatment of high bone resorbing forms of OA.

The Runx2 gene is essential for osteoblast differentiation and function. In vivo over-expression of Runx2 in osteoblasts increases bone resorption, and blocks terminal osteoblast differentiation. Several lines of evidence suggest that osteoblastic matrix metalloproteinases (MMPs) could contribute to the increased bone resorption observed in mice over-expressing Runx2 (Runx2 mice). The goal of our study was to use a transgenic approach to find out whether the inhibition of osteoblastic MMPs can reduce the bone loss induced by the over-expression of Runx2. We analyzed the effect of the in vivo over-expression of the TIMP-1 in osteoblasts on the severe osteopenic phenotype in Runx2 mice. Females with the different genotypes (WT, Runx2, TIMP-1 and TIMP-1/Runx2) were analyzed for bone density, architecture, osteoblastic and osteoclastic activity and gene expression using qPCR. TIMP-1 over-expression reduces the bone loss in adult Runx2 mice. The prevention of the bone loss in TIMP-1/Runx2 mice was due to a combination of reduced bone resorption and sustained bone formation. We present evidence that the ability of osteoblastic cells to induce osteoclastic differentiation is lower in TIMP-1/Runx2 mice than in Runx2 mice, probably due to a reduction in the expression of RANK-L and of the Runx2 transgene. Osteoblast primary cells from TIMP-1/Runx2 mice, but not from Runx2 mice, were able to differentiate into fully mature osteoblasts producing high osteocalcin levels. In conclusion, our findings suggest that osteoblastic MMPs can affect osteoblast differentiation. Our work also indicates that osteoblastic MMPs are partly responsible for the bone loss observed in Runx2 transgenic mice.

Distal-less (Dlx) homeobox transcription factors play a central role in the control of osteogenesis. In particular, Dlx5 regulates osteoblasts/osteoclasts coupling during perinatal bone formation. We analyze here the effect of Dlx5 allelic reduction in the control of bone remodeling. We first show that Dlx5 expression persists during postnatal bone development. We then compare the skeletal phenotype of 10- and 20-week-old Dlx5(+/-) mice to that of wild-type (WT) littermates. Dlx5(+/-) male mice exhibit lower bone mineral density (BMD) at both ages while only 20-week-old females are affected. microCT analyses reveal a reduction in cortical thickness of femoral midshafts in Dlx5(+/-) mice. Histomorphometry on distal femora shows no changes in trabecular structure and confirms a reduction in Dlx5(+/-) cortical thickness. The cortical decrease of 10-week-old mice does not derive from a reduction in periosteal bone apposition, but results from increased bone resorption with a significantly higher number of endosteal osteoclasts per bone surface and a larger marrow diameter. Urinary level of deoxypyridinoline is also higher in heterozygous mice confirming an increase in bone resorption activity. Our findings might be relevant for understanding complex, multifactorial diseases such as osteoporosis in which quantitative deregulation of gene expression leads to disruption of bone homeostasis., ((c) 2009 Wiley-Liss, Inc.)

Matrix metalloproteinases (MMPs) are key mediators in extra-cellular matrix remodelling and implicated primarily in bone growth, and particularly in osteoclastic bone resorption. We hypothesise that MMPs have a role in the increased bone remodelling resulting from oestrogen deficiency. Transgenic (TG) mice overexpressing TIMP-1 in their osteoblastic cells and their wild-type (WT) littermates were ovariectomised. One month after surgery, bone mineral density (BMD) and bone microarchitecture were assessed. Primary cells from WT and TG mice were used to determine how TIMP-1 affects osteoclast and osteoblastic cells. The reduction of BMD induced by ovariectomy in WT mice was not observed in the transgenic mice. The transgene overexpression also dampened the post-ovariectomy increase in bone resorption in contrast to the WT mice. In vivo, osteoclastic surfaces and D-pyridinoline were not increased in TG mice, and ex vivo, the differentiation of osteoclasts from TG bone marrow precursor cells were unaffected by in vivo oestrogen deficiency or treatment. We showed also that TIMP-1 overexpression reduces and delays the osteoblastic proliferation and differentiation respectively, and reduced the generation of the active form of TGFbeta1 in the supernatant of TG osteoblasts. Our findings support the hypothesis that in vivo inhibition of osteoblastic MMPs prevented the bone loss induced by oestrogen deficiency, with a significant decrease in bone resorption. This effect was presumably resulting from (1) a direct inhibition of osteoclastic resorption activity by the TIMP-1 and (2) the modification in the local activation of extra-cellular signalling factors such as TGFbeta1 and the OPG/RANKL ratio.

The runt family transcription factor core-binding factor alpha1 (Cbfa1) is essential for bone formation during development. Surprisingly, transgenic mice overexpressing Cbfa1 under the control of the 2.3-kb collagen type I promoter developed severe osteopenia that increased progressively with age and presented multiple fractures. Analysis of skeletally mature transgenic mice showed that osteoblast maturation was affected and that specifically in cortical bone, bone resorption as well as bone formation was increased, inducing high bone turnover rates and a decreased degree of mineralization. To understand the origin of the increased bone resorption, we developed bone marrow stromal cell cultures and reciprocal coculture of primary osteoblasts and spleen cells from wild-type or transgenic mice. We showed that transgenic cells of the osteoblastic lineage induced an increased number of tartrate-resistant acid phosphatase-positive multinucleated cells, suggesting that primary osteoblasts as well as bone marrow stromal cells from transgenic mice have stronger osteoclastogenic properties than cells derived from wild-type animals. We investigated the candidate genes whose altered expression could trigger this increase in bone resorption, and we found that the expression of receptor activator of NF-kappaB ligand (RANKL) and collagenase 3, two factors involved in bone formation-resorption coupling, was markedly increased in transgenic cells. Our data thus suggest that overexpression of Cbfa1 in cells of the osteoblastic lineage does not necessarily induce a substantial increase in bone formation in the adult skeleton but has a positive effect on osteoclast differentiation in vitro and can also dramatically enhance bone resorption in vivo, possibly through increased RANKL expression.