The primary site of the acrocephalic feature in Apert Syndrome is a dwarf cranial base with accelerated chondrocytic differentiation due to aberrant activation of the FGFR2 signaling

Activation of osteoblastic bone anabolism in the calvarial sutures is considered to be the essential pathologic condition underlying mutant FGFR2-related craniofacial dysostosis. However, early clinical investigations indicated that abnormal cartilage development in the cranial base was rather a primary site of abnormal feature in Apert Syndrome (AS). To examine the significance of cartilaginous growth of the cranial base in AS, we generated a transgenic mouse bearing AS-type mutant Fgfr2IIIc under the control of the Col2a1 promoter-enhancer (Fgfr2IIIc(P253R) mouse). Despite the lacking expression of Fgfr2IIIc(P253R) in osteoblasts, exclusive disruption of chondrocytic differentiation and growth reproduced AS-like acrocephaly accompanied by short anterior cranial base with fusion of the cranial base synchondroses, maxillary hypoplasia and synostosis of the calvarial sutures with no significant abnormalities in the trunk and extremities. Gene expression analyses demonstrated upregulation of p21, Ihh and Mmp-13 accompanied by modest increase in expression of Sox9 and Runx2, indicating acceleration of chondrocytic maturation and hypertrophy in the cranial base of the Fgfr2IIIc(P253R) mice. Furthermore, an acquired affinity and specificity of mutant FGFR2IIIc(P253R) receptor with FGF2 and FGF10 is suggested as a mechanism of activation of FGFR2 signaling selectively in the cranial base. In this report, we strongly suggest that the acrocephalic feature of AS is not alone a result of the coronal suture synostosis, but is a result of the primary disturbance in growth of the cranial base with precocious endochondral ossification.     

Smad6 is essential to limit BMP signaling during cartilage development

Bone morphogenetic protein (BMP) signaling pathways regulate multiple aspects of endochondral bone formation. The importance of extracellular antagonists as regulators of BMP signaling has been defined. In vitro studies reveal that the intracellular regulators, inhibitory Smads 6 and 7, can regulate BMP‐mediated effects on chondrocytes. Although in vivo studies in which inhibitory Smads were overexpressed in cartilage have shown that inhibitory Smads have the potential to limit BMP signaling in vivo, the physiological relevance of inhibitory Smad activity in skeletal tissues is unknown. In this study, we have determined the role of Smad6 in endochondral bone formation. Loss of Smad6 in mice leads to defects in both axial and appendicular skeletal development. Specifically, Smad6?/? mice exhibit a posterior transformation of the seventh cervical vertebra, bilateral ossification centers in lumbar vertebrae, and bifid sternebrae due to incomplete sternal band fusion. Histological analysis of appendicular bones revealed delayed onset of hypertrophic differentiation and mineralization at midgestation in Smad6?/? mice. By late gestation, however, an expanded hypertrophic zone, associated with an increased pool of proliferating cells undergoing hypertrophy, was evident in Smad6 mutant growth plates. The mutant phenotype is attributed, at least in part, to increased BMP responsiveness in Smad6‐deficient chondrocytes. Overall, our results show that Smad6 is required to limit BMP signaling during endochondral bone formation. © 2011 American Society for Bone and Mineral Research     

Interrelationship of Cranial Suture Fusion, Basicranial Development, and Resynostosis Following Suturectomy in Twist1+/? Mice, a Murine Model of Saethre-Chotzen Syndrome

The interrelationships among suture fusion, basicranial development, and subsequent resynostosis in syndromic craniosynostosis have yet to be examined. The objectives of this study were to determine the potential relationship between suture fusion and cranial base development in a model of syndromic craniosynostosis and to assess the effects of the syndrome on resynostosis following suturectomy. To do this, posterior frontal and coronal suture fusion, postnatal development of sphenooccipital synchondrosis, and resynostosis in Twist1(+/+) (WT) and Twist1(+/-) litter-matched mice (a model for Saethre-Chotzen syndrome) were quantified by evaluating μCT images with advanced image-processing algorithms. The coronal suture in Twist(+/-) mice developed, fused, and mineralized at a faster rate than that in normal littermates at postnatal days 6-30. Moreover, premature fusion of the coronal suture in Twist1(+/-) mice preceded alterations in cranial base development. Analysis of synchondrosis showed faster mineralization in Twist(+/-) mice at postnatal days 25-30. In a rapid resynostosis model, there was an inability to fuse both the midline posterior frontal suture and craniotomy defects in 21-day-old Twist(+/-) mice, despite having accelerated mineralization in the posterior frontal suture and defects. This study showed that dissimilarities between Twist1(+/+) and Twist1(+/-) mice are not limited to a fused coronal suture but include differences in fusion of other sutures, the regenerative capacity of the cranial vault, and the development of the cranial base.     

Dullard/Ctdnep1 Regulates Endochondral Ossification via Suppression of TGF‐β Signaling

Transforming growth factor (TGF)‐β signaling plays critical roles during skeletal development and its excessive signaling causes genetic diseases of connective tissues including Marfan syndrome and acromelic dysplasia. However, the mechanisms underlying prevention of excessive TGF‐β signaling in skeletogenesis remain unclear. We previously reported that Dullard/Ctdnep1 encoding a small phosphatase is required for nephron maintenance after birth through suppression of bone morphogenetic protein (BMP) signaling. Unexpectedly, we found that Dullard is involved in suppression of TGF‐β signaling during endochondral ossification. Conditional Dullard‐deficient mice in the limb and sternum mesenchyme by Prx1‐Cre displayed the impaired growth and ossification of skeletal elements leading to postnatal lethality. Dullard was expressed in early cartilage condensations and later in growth plate chondrocytes. The tibia growth plate of newborn Dullard mutant mice showed reduction of the proliferative and hypertrophic chondrocyte layers. The sternum showed deformity of cartilage primordia and delayed hypertrophy. Micromass culture experiments revealed that Dullard deficiency enhanced early cartilage condensation and differentiation, but suppressed mineralized hypertrophic chondrocyte differentiation, which was reversed by treatment with TGF‐β type I receptor kinase blocker LY‐364947. Dullard deficiency induced upregulation of protein levels of both phospho‐Smad2/3 and total Smad2/3 in micromass cultures without increase of Smad2/3 mRNA levels, suggesting that Dullard may affect Smad2/3 protein stability. The phospho‐Smad2/3 level was also upregulated in perichondrium and hypertrophic chondrocytes in Dullard‐deficient embryos. Response to TGF‐β signaling was enhanced in Dullard‐deficient primary chondrocyte cultures at late, but not early, time point. Moreover, perinatal administration of LY‐364947 ameliorated the sternum deformity in vivo. Thus, we identified Dullard as a new negative regulator of TGF‐β signaling in endochondral ossification. © 2014 American Society for Bone and Mineral Research.     

Deficiency of Smad3 results in enhanced inducible nitric oxide synthase–mediated hypotension in lipopolysaccharide-induced endotoxemia

Background

Smad3 is a principal intracellular mediator of signaling for transforming growth factor β, a cytokine involved in pleiotropic pathophysiological processes including inflammation and immunity. The function of Smad3 in regulating inducible nitric oxide synthase (iNOS) expression and septic shock has not been characterized.

Methods

Smad3^−/−(referred hereafter as KO) and wild-type (WT) mice were injected intraperitoneally with lipopolysaccharide (LPS) to induce the septic hypotension. Mortality, blood pressure, and plasma levels of nitrite were measured. The iNOS messenger RNA and protein levels in lung, kidney, and spleen were also analyzed.

Results

Mice lacking functional Smad3 respond to LPS with greater mortality than their WT littermates. The high mortality of KO mice is accompanied by enhanced hypotension after intraperitoneal injection of LPS. Both KO and WT mice displayed an increase in plasma nitrite during the experimental period; however, LPS administration caused more dramatic changes in KO mice than WT mice. Likewise, the iNOS messenger RNA and protein levels in lung, kidney, and spleen were more strongly increased in KO mice than in WT mice after LPS administration.

Conclusions

Defects in the Smad3 gene may increase susceptibility to the development of septic hypotension because of enhanced iNOS production.     

Increased mandibular condylar growth in mice with estrogen receptor beta deficiency

Temporomandibular joint (TMJ) disorders predominantly afflict women of childbearing age, suggesting a role for female hormones in the disease process. In long bones, estrogen acting via estrogen receptor beta (ERβ) inhibits axial skeletal growth in female mice. However, the role of ERβ in the mandibular condyle is largely unknown. We hypothesize that female ERβ‐deficient mice will have increased mandibular condylar growth compared to wild‐type (WT) female mice. This study examined female 7‐day‐old, 49‐day‐old, and 120‐day‐old WT and ERβ knockout (KO) mice. There was a significant increase in mandibular condylar cartilage thickness as a result of an increased number of cells, in the 49‐day‐old and 120‐day‐old female ERβ KO compared with WT controls. Analysis in 49‐day‐old female ERβ KO mice revealed a significant increase in collagen type X, parathyroid hormone–related protein (Pthrp), and osteoprotegerin gene expression and a significant decrease in receptor activator for nuclear factor κ B ligand (Rankl) and Indian hedgehog (Ihh) gene expression, compared with WT controls. Subchondral bone analysis revealed a significant increase in total condylar volume and a decrease in the number of osteoclasts in the 49‐day‐old ERβ KO compared with WT female mice. There was no difference in cell proliferation in condylar cartilage between the genotypes. However, there were differences in the expression of proteins that regulate the cell cycle; we found a decrease in the expression of Tieg1 and p57 in the mandibular condylar cartilage from ERβ KO mice compared with WT mice. Taken together, our results suggest that ERβ deficiency increases condylar growth in female mice by inhibiting the turnover of fibrocartilage. © 2013 American Society for Bone and Mineral Research.     

Knocking out Smad3 favors allogeneic mouse fetal skin development in adult wounds

Fetal skin development represents a process of the interaction between skin progenitor cells and their unique extracellular matrix niche, which is also important for the mechanism study of skin progenitor cell differentiation and fetal scarless wound healing. Thus, a change in the niche environment, such as altered expression levels of growth factors or cytokines, may also change the outcome of fetal skin development. This study tested the hypothesis that deletion of mouse Smad3 creates a favorable environment for fetal skin development in adult wounds. Fetal skin of green fluorescent protein mouse (C57BL/B6) of gestational day 16.5 was respectively transplanted to the wound beds of wild‐type (WT), heterozygous (HT), and homologous (KO) Smad3 deletion mice (C57BL/B6 × 129SV). The results showed that green fluorescent protein fetal mouse skin after its transplantation developed much better into hair follicle containing skin in KO or HT wound beds than in WT wound beds with significant differences in the number of follicles per mm² among the three groups at 1, 2, and 3 weeks posttransplantation (p < 0.05). In addition, less fibrosis was observed in KO wounds than in HT and WT wounds with significant difference in the wound bed thickness among the three groups at 3 weeks posttransplantation (p < 0.05). Interestingly, there was a delayed graft rejection in the KO group when compared with the HT and WT groups. In conclusion, deletion of Smad3 in a wound bed creates a better environment for skin progenitor cell differentiation and fetal skin development. Translation of such a concept to the creation of a wound environment that is favorable for adult stem cell differentiation and skin appendage formation may become an important strategy for the regeneration of wounded skin.     

The immunosuppressant FK506 promotes development of the chondrogenic phenotype in human synovial stromal cells via modulation of the Smad signaling pathway

OBJECTIVE: To assess the effect of the immunosuppressant FK506 on chondrogenic differentiation of human synovial stromal cells (hSSCs). METHODS: hSSCs were isolated from synovium of the knee joint and 2x10(5) cells were subjected to pellet culture in chondrogenic culture medium for 3 weeks with or without growth factors [bone morphogenetic protein 2 (BMP2) or transforming growth factor beta(1) (TGFbeta(1))] and +/- addition of FK506 in chondrogenic culture media was evaluated. Chondrogenesis was assessed by the size of the pellet, the production of proteoglycans, and messenger RNA (mRNA) levels for chondrogenic markers. Furthermore, levels and intracellular location of phosphorylated Smad proteins related to BMP signaling and TGFbeta signaling were evaluated following exposure to FK506. RESULTS: FK506 enhanced the differentiation of hSSCs toward a chondrogenic phenotype in a dose-dependent manner associated with increases in glycosaminoglycan synthesis and increased mRNA levels for chondrogenic marker genes. Additionally, FK506 further enhanced chondrogenesis of synovial stromal cells (SSCs) induced by BMP2 and TGFbeta(1), also in a dose-dependent manner. Notably, phosphorylation of Smad1/5/8 and Smad3 was significantly increased by FK506. Also, the ratio of nuclear translocation to cytoplasmic levels of phosphorylated Smad1/5/8 and Smad3 were increased following exposure of SSCs to FK506. Moreover, inhibition of Smad signaling significantly abrogated FK506-induced chondrogenic differentiation of SSCs. CONCLUSION: This study demonstrated that FK506 promotes chondrogenic differentiation of hSSCs likely via impact on Smad signaling pathways. With further optimization, FK506 could potentially be a unique therapeutic tool to promote cartilage repair in clinical situations, as well as enhance development of tissue engineered cartilage in vitro.     

Augmentation of smad‐dependent BMP signaling in neural crest cells causes craniosynostosis in mice

Craniosynostosis describes conditions in which one or more sutures of the infant skull are prematurely fused, resulting in facial deformity and delayed brain development. Approximately 20% of human craniosynostoses are thought to result from gene mutations altering growth factor signaling; however, the molecular mechanisms by which these mutations cause craniosynostosis are incompletely characterized, and the causative genes for diverse types of syndromic craniosynostosis have yet to be identified. Here, we show that enhanced bone morphogenetic protein (BMP) signaling through the BMP type IA receptor (BMPR1A) in cranial neural crest cells, but not in osteoblasts, causes premature suture fusion in mice. In support of a requirement for precisely regulated BMP signaling, this defect was rescued on a Bmpr1a haploinsufficient background, with corresponding normalization of Smad phosphorylation. Moreover, in vivo treatment with LDN‐193189, a selective chemical inhibitor of BMP type I receptor kinases, resulted in partial rescue of craniosynostosis. Enhanced signaling of the fibroblast growth factor (FGF) pathway, which has been implicated in craniosynostosis, was observed in both mutant and rescued mice, suggesting that augmentation of FGF signaling is not the sole cause of premature fusion found in this model. The finding that relatively modest augmentation of Smad‐dependent BMP signaling leads to premature cranial suture fusion suggests an important contribution of dysregulated BMP signaling to syndromic craniosynostoses and potential strategies for early intervention.     

COX-2 from the injury milieu is critical for the initiation of periosteal progenitor cell mediated bone healing

Although a critical role of COX-2 in bone repair has been established, the mechanism involved remains unclear. During early inflammatory phase of bone healing, COX-2 is produced by the surrounding inflammatory cells as well as bone/cartilage progenitors. Based on the temporal and spatial expression of COX-2 during the early phase of fracture healing, we hypothesize that COX-2 from both sources is critical for progenitor cell activation, proliferation and differentiation. To directly test this we utilized a murine femoral grafting model, in which live segmental grafts from the same strains were transplanted and donor versus host cell involvement in healing was assessed. Specifically, fresh femur cortical bone grafts of 4 mm in length from COX-2(-/-) (KO) mice were transplanted into wild type (WT) mice with the same sized segmental defect in femurs. Similarly, grafts from WT were transplanted into the defects in KO mice. As controls, transplantations between wild types, and transplantations between KO were also performed. Histologic analyses showed that WT-to-WT transplantation resulted in normal endochondral bone healing as evidenced by markedly induction of neovascularization and periosteal bone formation on donor graft. In contrast, transplantation of KO graft into KO host led to 96% reduction of bone formation and near elimination of donor cell-initiated periosteal bone formation. Similarly, transplantation of WT graft into a KO host resulted in 87% reduction of bone formation (n=8, p>0.05), indicating that KO host impaired WT donor progenitor cell expansion and differentiation. When a KO graft was transplanted into WT host, KO donor periosteal cell-initiated endochondral bone formation was restored. Histomorphometric analyses demonstrated 10-fold increase in bone formation and 3-fold increase in cartilage formation compared to KO-to-KO transplantation (n=8, p<0.05), suggesting that COX-2 deficient donor cells were capable to differentiate and form bone when placed in a WT host. Taken together, our data strongly suggest that COX-2 is critical for initiation of periosteal cortical bone healing. The early induction of COX-2 constitutes a crucial host-healing environment for activation and differentiation of donor periosteal progenitors. Elimination of COX-2 at the early stage of healing could lead to detrimental effects on periosteal progenitor cell-initiated cortical bone repair.     

Cyclooxygenase-2 gene disruption promotes proliferation of murine calvarial osteoblasts in vitro

Cyclooxygenase-2 (COX-2) is highly expressed in osteoblasts, and COX-2 produced prostaglandins (PGs) can increase osteoblastic differentiation in vitro. The goal of this study was to examine effects of COX-2 expression on calvarial osteoblastic proliferation and apoptosis. Primary osteoblasts (POBs) were cultured from calvariae of COX-2 wild-type (WT) and knockout (KO) mice. POB proliferation was evaluated by (3)H-thymidine incorporation and analysis of cell replication and cell cycle distribution by flow cytometry. POB apoptosis was evaluated by annexin and PI staining on flow cytometry. As expected, PGE(2) production and alkaline phosphatase (ALP) activity were increased in WT cultures compared to KO cultures. In contrast, cell numbers were decreased in WT compared to KO cells by day 4 of culture. Proliferation, measured on days 3-7 of culture, was 2-fold greater in KO than in WT POBs and associated with decreased Go/G1 and increased S cell cycle distribution. There was no significant effect of COX-2 genotype on apoptosis under basal culture conditions on day 5 of culture. Cell growth was decreased in KO POBs by the addition of PGE(2) or a protein kinase A agonist and increased in WT POBs by the addition of NS398, a selective COX-2 inhibitor. In contrast, differentiation and cell growth in marrow stromal cell (MSC) cultures, evaluated by ALP and crystal violet staining respectively, were increased in MSCs from WT mice compared to MSCs from KO mice, and exogenous PGE(2) increased cell growth in KO MSC cultures. We conclude that PGs secondary to COX-2 expression decrease osteoblastic proliferation in cultured calvarial cells but increase growth of osteoblastic precursors in MSC cultures.     

Partial loss of Smad7 function impairs bone remodeling,osteogenesis and enhances osteoclastogenesis in mice

Smad7 is well demonstrated as a negative regulator of TGF-β signaling. Its alteration in expression often results in diseases such as cancer and fibrosis. However, the exact role of Smad7 in regulating bone remodeling during mammalian development has not been properly delineated. In this study we performed experiments to clarify the involvement of Smad7 in regulating osteogenesis and osteoclastogenesis both invivo and invitro. Genetically engineered Smad7^ΔE1 (KO) mice were used, whereby partial functional of Smad7 is lost by deleting exon I of the Smad7 gene and the truncated proteins cause a hypomorphic allele. Analysis with μCT imagery and bone histomorphometry showed that the KO mice had lower TbN, TbTh, higher TbSp in the metaphysic region of the femurs at 6, 12, 24 weeks from birth, as well as decreased MAR and increased osteoclast surface compared with the WT mice. In vitro BM-MSC multi-lineage differentiation evaluation showed that the KO group had reduced osteogenic potential, fewer mineralized nodules, lower ALP activity, and reduced gene expression of Col1A1, Runx2 and OCN. The adipogenic potential was elevated in the KO group with more formation of lipid droplets, and increased gene expression of Adipsin and C/EBPα. The osteoclastogenic potential of KO mice BMMs was elevate, with emergence of more osteoclasts, larger resorptive areas, and increased gene expression of TRAP and CTR. Our results indicate that partial loss of Smad7 function in mice leads to compromised bone formation and enhanced bone resorption. Thus, Smad7 is acknowledged as a novel key regulator between osteogenesis and osteoclastogenesis.     

Sirt1-deficient mice exhibit an altered cartilage phenotype

ObjectiveWe previously demonstrated that Sirt1 regulates apoptosis in cartilage in vitro. Here we attempt to examine in vivo cartilage homeostasis, using Sirt1 total body knockout (KO) mice.MethodArticular cartilage was harvested from hind paws of 1-week and 3-week-old mice carrying wild type (WT) or null Sirt1 gene. Knees of Sirt1 haploinsufficient mice also were examined, at 6 months. Joint cartilage was processed for histologic examination or biochemical analyses of chondrocyte cultures.ResultsWe found that articular cartilage tissue sections from Sirt1 KO mice up to 3 weeks of age exhibited low levels of type 2 collagen, aggrecan, and glycosaminoglycan content. In contrast, protein levels of MMP-13 were elevated in the Sirt1 KO mice, leading to a potential increase of cartilage breakdown, already shown in the heterozygous mice. Additional results showed elevated chondrocyte apoptosis in Sirt1 KO mice, as compared to WT controls. In addition to these observations, PTP1b (protein tyrosine phosphatase b) was elevated in the Sirt1 KO mice, in line with previous reports.ConclusionThe findings from this animal model demonstrated that Sirt1 KO mice presented an altered cartilage phenotype, with an elevated apoptotic process and a potential degradative cartilage process.     

CCN4/WISP-1 positively regulates chondrogenesis by controlling TGF-β3 function

The CCN family of proteins plays important roles in development and homeostasis of bone and cartilage. To understand the role of CCN4 in chondrogenesis, human bone marrow stromal cells (hBMSCs) were transduced with CCN4 adenovirus (adCCN4) or siRNA to CCN4 (siCCN4) in the presence or absence of transforming growth factor-β3 (TGF-β3). Overexpression of CCN4 enhanced TGF-β3-induced SMAD2/3 phosphorylation and chondrogenesis of hBMSCs in an in vitro assay using a micromass culture model. On the other hand, knockdown of CCN4 inhibited the TGF-β3-induced SMAD2/3 phosphorylation and synthesis of cartilage matrix in micromass cultures of hBMSCs. Immunoprecipitation–western blot analysis revealed that CCN4 bound to TGF-β3 and regulated the ability of TGF-β3 to bind to hBMSCs. In vivo analysis confirmed there was a significant decrease in the gene expression levels of chondrocyte markers in cartilage samples from Ccn4-knock out (KO) mice, compared to those from wild type (WT) control. In order to investigate the regenerative properties of the articular cartilage in Ccn4-KO mice, articular cartilage defects were surgically performed in the knee joints of young mice, and the results showed that the cartilage was partially repaired in WT mice, but not in Ccn4-KO mice. In conclusion, these results show, for the first time, that CCN4 has a positive influence on chondrogenic differentiation by modulating the effects of TGF-β3.     

Ameloblastin,an Extracellular Matrix Protein,Affects Long Bone Growth and Mineralization

Matrix molecules such as the enamel‐related calcium‐binding phosphoprotein ameloblastin (AMBN) are expressed in multiple tissues, including teeth, bones, and cartilage. Here we have asked whether AMBN is of functional importance for timely long bone development and, if so, how it exerts its function related to osteogenesis. Adolescent AMBN‐deficient mice (AMBN^Δ5–6) suffered from a 33% to 38% reduction in femur length and an 8.4% shorter trunk spinal column when compared with WT controls, whereas there was no difference between adult animals. On a cellular level, AMBN truncation resulted in a shortened growth plate and a 41% to 49% reduction in the number of proliferating tibia chondrocytes and osteoblasts. Bone marrow stromal cells (BMSCs) isolated from AMBN mutant mice displayed defects in proliferation and differentiation potential as well as cytoskeleton organization. Osteogenesis‐related growth factors, such as insulin‐like growth factor 1 (IGF1) and BMP7, were also significantly (46% to 73%) reduced in AMBN‐deficient BMSCs. Addition of exogenous AMBN restored cytoskeleton structures in AMBN mutant BMSCs and resulted in a dramatic 400% to 600% increase in BMP2, BMP7, and Col1A expression. Block of RhoA diminished the effect of AMBN on osteogenic growth factor and matrix protein gene expression. Addition of exogenous BMP7 and IGF1 rescued the proliferation and differentiation potential of AMBN‐deficient BMSCs. Confirming the effects of AMBN on long bone growth, back‐crossing of mutant mice with full‐length AMBN overexpressors resulted in a complete rescue of AMBN^Δ5–6 bone defects. Together, these data indicate that AMBN affects extracellular matrix production and cell adhesion properties in the long bone growth plate, resulting in altered cytoskeletal dynamics, increased osteogenesis‐related gene expression, as well as osteoblast and chondrocyte proliferation. We propose that AMBN facilitates rapid long bone growth and an important growth spurt during the skeletogenesis of adolescent tooth‐bearing vertebrates. © 2016 American Society for Bone and Mineral Research.     

Enhanced Osteoclastogenesis Causes Osteopenia in Twisted Gastrulation‐Deficient Mice Through Increased BMP Signaling

The uncoupling of osteoblastic and osteoclastic activity is central to disorders such as osteoporosis, osteolytic malignancies, and periodontitis. Numerous studies have shown explicit functions for bone morphogenetic proteins (BMPs) in skeletogenesis. Their signaling activity has been shown in various contexts to be regulated by extracellular proteins, including Twisted gastrulation (TWSG1). However, experimental paradigms determining the effects of BMP regulators on bone remodeling are limited. In this study, we assessed the role of TWSG1 in postnatal bone homeostasis. Twsg1‐deficient (Twsg1^?/?) mice developed osteopenia that could not be explained by defective osteoblast function, because mineral apposition rate and differentiation markers were not significantly different compared with wildtype (WT) mice. Instead, we discovered a striking enhancement of osteoclastogenesis in Twsg1^?/? mice, leading to increased bone resorption with resultant osteopenia. Enhanced osteoclastogenesis in Twsg1^?/? mice was caused by increased cell fusion, differentiation, and function of osteoclasts. Furthermore, RANKL‐mediated osteoclastogenesis and phosphorylated Smad1/5/8 levels were enhanced when WT osteoclasts were treated with recombinant BMP2, suggesting direct regulation of osteoclast differentiation by BMPs. Increase in detectable levels of phosphorylated Smad 1/5/8 was noted in osteoclasts from Twsg1^?/? mice compared with WT mice. Furthermore, the enhanced osteoclastogenesis in Twsg1^?/? mice was reversed in vitro in a dose‐dependent manner with exposure to Noggin, a BMP antagonist, strongly suggesting that the enhanced osteoclastogenesis in Twsg1 mutants is attributable to increased BMP signaling. Thus, we present a novel and previously uncharacterized role for TWSG1 in inhibiting osteoclastogenesis through regulation of BMP activity.