The detection of estrogen receptor-α (ERα) in osteoblasts and osteoclasts over 20 years ago suggested that direct effects of estrogens on both of these cell types are responsible for their beneficial effects on the skeleton, but the role of ERα in osteoblast lineage cells has remained elusive. In addition, estrogen activation of ERα in osteoclasts can only account for the protective effect of estrogens on the cancellous, but not the cortical, bone compartment that represents 80% of the entire skeleton. Here, we deleted ERα at different stages of differentiation in murine osteoblast lineage cells. We found that ERα in osteoblast progenitors expressing Osterix1 (Osx1) potentiates Wnt/β-catenin signaling, thereby increasing proliferation and differentiation of periosteal cells. Further, this signaling pathway was required for optimal cortical bone accrual at the periosteum in mice. Notably, this function did not require estrogens. The osteoblast progenitor ERα mediated a protective effect of estrogens against endocortical, but not cancellous, bone resorption. ERα in mature osteoblasts or osteocytes did not influence cancellous or cortical bone mass. Hence, the ERα in both osteoblast progenitors and osteoclasts functions to optimize bone mass but at distinct bone compartments and in response to different cues.
Maria Almeida, Srividhya Iyer, Marta Martin-Millan, Shoshana M. Bartell, Li Han, Elena Ambrogini, Melda Onal, Jinhu Xiong, Robert S. Weinstein, Robert L. Jilka, Charles A. O’Brien, Stavros C. Manolagas
The osteoporosis associated with human hyperthyroidism has traditionally been attributed to elevated thyroid hormone levels. There is evidence, however, that thyroid-stimulating hormone (TSH), which is low in most hyperthyroid states, directly affects the skeleton. Importantly, Tshr-knockout mice are osteopenic. In order to determine whether low TSH levels contribute to bone loss in hyperthyroidism, we compared the skeletal phenotypes of wild-type and Tshr-knockout mice that were rendered hyperthyroid. We found that hyperthyroid mice lacking TSHR had greater bone loss and resorption than hyperthyroid wild-type mice, thereby demonstrating that the absence of TSH signaling contributes to bone loss. Further, we identified a TSH-like factor that may confer osteoprotection. These studies suggest that therapeutic suppression of TSH to very low levels may contribute to bone loss in people.
Ramkumarie Baliram, Li Sun, Jay Cao, Jianhua Li, Rauf Latif, Amanda K. Huber, Tony Yuen, Harry C. Blair, Mone Zaidi, Terry F. Davies
The adenosine diphosphate (ADP) receptor P2RY12 (purinergic receptor P2Y, G protein coupled, 12) plays a critical role in platelet aggregation, and P2RY12 inhibitors are used clinically to prevent cardiac and cerebral thrombotic events. Extracellular ADP has also been shown to increase osteoclast (OC) activity, but the role of P2RY12 in OC biology is unknown. Here, we examined the role of mouse P2RY12 in OC function. Mice lacking P2ry12 had decreased OC activity and were partially protected from age-associated bone loss. P2ry12–/– OCs exhibited intact differentiation markers, but diminished resorptive function. Extracellular ADP enhanced OC adhesion and resorptive activity of WT, but not P2ry12–/–, OCs. In platelets, ADP stimulation of P2RY12 resulted in GTPase Ras-related protein (RAP1) activation and subsequent αIIbβ3 integrin activation. Likewise, we found that ADP stimulation induced RAP1 activation in WT and integrin β3 gene knockout (Itgb3–/–) OCs, but its effects were substantially blunted in P2ry12–/– OCs. In vivo, P2ry12–/– mice were partially protected from pathologic bone loss associated with serum transfer arthritis, tumor growth in bone, and ovariectomy-induced osteoporosis: all conditions associated with increased extracellular ADP. Finally, mice treated with the clinical inhibitor of P2RY12, clopidogrel, were protected from pathologic osteolysis. These results demonstrate that P2RY12 is the primary ADP receptor in OCs and suggest that P2RY12 inhibition is a potential therapeutic target for pathologic bone loss.
Xinming Su, Desiree H. Floyd, Alun Hughes, Jingyu Xiang, Jochen G. Schneider, Ozge Uluckan, Emanuela Heller, Hongju Deng, Wei Zou, Clarissa S. Craft, Kaiming Wu, Angela C. Hirbe, Dorota Grabowska, Mark C. Eagleton, Sarah Townsley, Lynne Collins, David Piwnica-Worms, Thomas H. Steinberg, Deborah V. Novack, Pamela B. Conley, Michelle A. Hurchla, Michael Rogers, Katherine N. Weilbaecher
Serotonin is a critical regulator of bone mass, fulfilling different functions depending on its site of synthesis. Brain-derived serotonin promotes osteoblast proliferation, whereas duodenal-derived serotonin suppresses it. To understand the molecular mechanisms of duodenal-derived serotonin action on osteoblasts, we explored its transcriptional mediation in mice. We found that the transcription factor FOXO1 is a crucial determinant of the effects of duodenum-derived serotonin on bone formation We identified two key FOXO1 complexes in osteoblasts, one with the transcription factor cAMP-responsive element–binding protein 1 (CREB) and another with activating transcription factor 4 (ATF4). Under normal levels of circulating serotonin, the proliferative activity of FOXO1 was promoted by a balance between its interaction with CREB and ATF4. However, high circulating serotonin levels prevented the association of FOXO1 with CREB, resulting in suppressed osteoblast proliferation. These observations identify FOXO1 as the molecular node of an intricate transcriptional machinery that confers the signal of duodenal-derived serotonin to inhibit bone formation.
Aruna Kode, Ioanna Mosialou, Barbara C. Silva, Marie-Therese Rached, Bin Zhou, Ji Wang, Tim M. Townes, Rene Hen, Ronald A. DePinho, X. Edward Guo, Stavroula Kousteni
Osteoporotic bones have reduced spongy bone mass, altered bone architecture, and increased marrow fat. Bone marrow stem cells from osteoporotic patients are more likely to differentiate into adipocytes than control cells, suggesting that adipocyte differentiation may play a role in osteoporosis. VEGF is highly expressed in osteoblastic precursor cells and is known to stimulate bone formation. Here we tested the hypothesis that VEGF is also an important regulator of cell fate, determining whether differentiation gives rise to osteoblasts or adipocytes. Mice with conditional VEGF deficiency in osteoblastic precursor cells exhibited an osteoporosis-like phenotype characterized by reduced bone mass and increased bone marrow fat. In addition, reduced VEGF expression in mesenchymal stem cells resulted in reduced osteoblast and increased adipocyte differentiation. Osteoblast differentiation was reduced when VEGF receptor 1 or 2 was knocked down but was unaffected by treatment with recombinant VEGF or neutralizing antibodies against VEGF. Our results suggested that VEGF controls differentiation in mesenchymal stem cells by regulating the transcription factors RUNX2 and PPARγ2 as well as through a reciprocal interaction with nuclear envelope proteins lamin A/C. Importantly, our data support a model whereby VEGF regulates differentiation through an intracrine mechanism that is distinct from the role of secreted VEGF and its receptors.
Yanqiu Liu, Agnes D. Berendsen, Shidong Jia, Sutada Lotinun, Roland Baron, Napoleone Ferrara, Bjorn R. Olsen
Mutations in the coactivator CREB-binding protein (CBP) are a major cause of the human skeletal dysplasia Rubinstein-Taybi syndrome (RTS); however, the mechanism by which these mutations affect skeletal mineralization and patterning is unknown. Here, we report the identification of 3-phosphoinositide-dependent kinase 1 (PDK1) as a key regulator of CBP activity and demonstrate that its functions map to both osteoprogenitor cells and mature osteoblasts. In osteoblasts, PDK1 activated the CREB/CBP complex, which in turn controlled runt-related transcription factor 2 (RUNX2) activation and expression of bone morphogenetic protein 2 (BMP2). These pathways also operated in vivo, as evidenced by recapitulation of RTS spectrum phenotypes with osteoblast-specific Pdk1 deletion in mice (Pdk1osx mice) and by the genetic interactions observed in mice heterozygous for both osteoblast-specific Pdk1 deletion and either Runx2 or Creb deletion. Finally, treatment of Pdk1osx and Cbp+/– embryos with BMPs in utero partially reversed their skeletal anomalies at birth. These findings illustrate the in vivo function of the PDK1-AKT-CREB/CBP pathway in bone formation and provide proof of principle for in utero growth factor supplementation as a potential therapy for skeletal dysplasias.
Jae-Hyuck Shim, Matthew B. Greenblatt, Anju Singh, Nicholas Brady, Dorothy Hu, Rebecca Drapp, Wataru Ogawa, Masato Kasuga, Tetsuo Noda, Sang-Hwa Yang, Sang-Kyou Lee, Vivienne I. Rebel, Laurie H. Glimcher
Mutations in human FYVE, RhoGEF, and PH domain–containing 1 (FGD1) cause faciogenital dysplasia (FGDY; also known as Aarskog syndrome), an X-linked disorder that affects multiple skeletal structures. FGD1 encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase CDC42. However, the mechanisms by which mutations in FGD1 affect skeletal development are unknown. Here, we describe what we believe to be a novel signaling pathway in osteoblasts initiated by FGD1 that involves the MAP3K mixed-lineage kinase 3 (MLK3). We observed that MLK3 functions downstream of FGD1 to regulate ERK and p38 MAPK, which in turn phosphorylate and activate the master regulator of osteoblast differentiation, Runx2. Mutations in FGD1 found in individuals with FGDY ablated its ability to activate MLK3. Consistent with our description of this pathway and the phenotype of patients with FGD1 mutations, mice with a targeted deletion of Mlk3 displayed multiple skeletal defects, including dental abnormalities, deficient calvarial mineralization, and reduced bone mass. Furthermore, mice with knockin of a mutant Mlk3 allele that is resistant to activation by FGD1/CDC42 displayed similar skeletal defects, demonstrating that activation of MLK3 specifically by FGD1/CDC42 is important for skeletal mineralization. Thus, our results provide a putative biochemical mechanism for the skeletal defects in human FGDY and suggest that modulating MAPK signaling may benefit these patients.
Weiguo Zou, Matthew B. Greenblatt, Jae-Hyuck Shim, Shashi Kant, Bo Zhai, Sutada Lotinun, Nicholas Brady, Dorothy Zhang Hu, Steven P. Gygi, Roland Baron, Roger J. Davis, Dallas Jones, Laurie H. Glimcher
Osteoclasts are terminally differentiated leukocytes that erode the mineralized bone matrix. Osteoclastogenesis requires costimulatory receptor signaling through adaptors containing immunoreceptor tyrosine-based activation motifs (ITAMs), such as Fc receptor common γ (FcRγ) and DNAX-activating protein of 12 kDa. Identification of these ITAM-containing receptors and their ligands remains a high research priority, since the stimuli for osteoclastogenesis are only partly defined. Osteoclast-associated receptor (OSCAR) was proposed to be a potent FcRγ-associated costimulatory receptor expressed by preosteoclasts in vitro, but OSCAR lacks a cognate ligand and its role in vivo has been unclear. Using samples from mice and patients deficient in various ITAM signaling pathways, we show here that OSCAR costimulates one of the major FcRγ-associated pathways required for osteoclastogenesis in vivo. Furthermore, we found that OSCAR binds to specific motifs within fibrillar collagens in the ECM that become revealed on nonquiescent bone surfaces in which osteoclasts undergo maturation and terminal differentiation in vivo. OSCAR promoted osteoclastogenesis in vivo, and OSCAR binding to its collagen motif led to signaling that increased numbers of osteoclasts in culture. Thus, our results suggest that ITAM-containing receptors can respond to exposed ligands in collagen, leading to the functional differentiation of leukocytes, which provides what we believe to be a new concept for ITAM regulation of cytokine receptors in different tissue microenvironments.
Alexander David Barrow, Nicolas Raynal, Thomas Levin Andersen, David A. Slatter, Dominique Bihan, Nicholas Pugh, Marina Cella, Taesoo Kim, Jaerang Rho, Takako Negishi-Koga, Jean-Marie Delaisse, Hiroshi Takayanagi, Joseph Lorenzo, Marco Colonna, Richard W. Farndale, Yongwon Choi, John Trowsdale
The heterotrimeric G protein subunit Gsα stimulates cAMP-dependent signaling downstream of G protein–coupled receptors. In this study, we set out to determine the role of Gsα signaling in cells of the early osteoblast lineage in vivo by conditionally deleting Gsα from osterix-expressing cells. This led to severe osteoporosis with fractures at birth, a phenotype that was found to be the consequence of impaired bone formation rather than increased resorption. Osteoblast number was markedly decreased and osteogenic differentiation was accelerated, resulting in the formation of woven bone. Rapid differentiation of mature osteoblasts into matrix-embedded osteocytes likely contributed to depletion of the osteoblast pool. In addition, the number of committed osteoblast progenitors was diminished in both bone marrow stromal cells (BMSCs) and calvarial cells of mutant mice. In the absence of Gsα, expression of sclerostin and dickkopf1 (Dkk1), inhibitors of canonical Wnt signaling, was markedly increased; this was accompanied by reduced Wnt signaling in the osteoblast lineage. In summary, we have shown that Gsα regulates bone formation by at least two distinct mechanisms: facilitating the commitment of mesenchymal progenitors to the osteoblast lineage in association with enhanced Wnt signaling; and restraining the differentiation of committed osteoblasts to enable production of bone of optimal mass, quality, and strength.
Joy Y. Wu, Piia Aarnisalo, Murat Bastepe, Partha Sinha, Keertik Fulzele, Martin K. Selig, Min Chen, Ingrid J. Poulton, Louise E. Purton, Natalie A. Sims, Lee S. Weinstein, Henry M. Kronenberg
A fine balance between bone resorption by osteoclasts and bone formation by osteoblasts maintains bone homeostasis. In patients with cherubism, gain-of-function mutations in 3BP2, which is encoded by SH3-domain binding protein 2 (SH3BP2), cause cystic lesions with activated osteoclasts that lead to craniofacial abnormalities. However, little is known about the function of wild-type 3BP2 in regulating bone homeostasis. Here we have shown that 3BP2 is required for the normal function of both osteoblasts and osteoclasts. Initial analysis showed that Sh3bp2–/–mice developed osteoporosis as a result of reduced bone formation despite the fact that bone resorption was impaired. We demonstrated using reciprocal bone marrow chimeras, a cell-intrinsic defect of the osteoblast and osteoclast compartments in vivo. Further, Sh3bp2–/– osteoblasts failed to mature and form mineralized nodules in vitro, while Sh3bp2–/– osteoclasts spread poorly and were unable to effectively degrade dentine matrix in vitro. Finally, we showed that 3BP2 was required for Abl activation in osteoblasts and Src activation in osteoclasts, and demonstrated that the in vitro defect of each cell type was restored by the respective expression of activated forms of these kinases. These findings reveal an unanticipated role for the 3BP2 adapter protein in osteoblast function and in coordinating bone homeostatic signals in both osteoclast and osteoblast lineages.
Noam Levaot, Paul D. Simoncic, Ioannis D. Dimitriou, Andrew Scotter, Jose La Rose, Adeline H.M. Ng, Thomas L. Willett, Chiachien J. Wang, Salima Janmohamed, Marc Grynpas, Ernst Reichenberger, Robert Rottapel