Misexpression of Dickkopf-1 in endothelial cells, but not in chondrocytes or hypertrophic chondrocytes, causes defects in endochondral ossification

Developing cartilage serves as a template for long-bone development during endochondral ossification. Although the coupling of cartilage and bone development with angiogenesis is an important regulatory step for endochondral ossification, the molecular mechanisms are poorly understood. One possible mechanism involves the action of Dickkopf (DKK), which is a family of soluble canonical Wnt antagonists with four members (DKK1-4). We initially observed opposite expression patterns of Dkk1 and Dkk2 during angiogenesis and chondrocyte differentiation: downregulation of Dkk1 and upregulation of Dkk2. We examined the in vivo role of Dkk1 and Dkk2 in linking cartilage/bone development and angiogenesis by generating transgenic (TG) mice that specifically express Dkk1 or Dkk2 in chondrocytes, hypertrophic chondrocytes, or endothelial cells. Despite specific expression pattern during cartilage development, chondrocyte- and hypertrophic chondrocyte-specific Dkk1 and Dkk2 TG mice showed normal developmental phenotypes. However, Dkk1 misexpression in endothelial cells resulted in defects of endochondral ossification and reduced skeletal size. The defects are caused by the inhibition of angiogenesis in developing bone and subsequent inhibition of apoptosis of hypertrophic chondrocytes and cartilage resorption.     

Physiological function of the angiotensin AT1a receptor in bone remodeling

In order to determine whether the renin-angiotensin system (RAS) has any physiologic function in bone metabolism, mice lacking the gene encoding the major angiotensin II receptor isoform, AT1a, were studied using micro CT scanning, histomorphometric, and biochemical techniques. Three-dimensional (3D) micro CT analysis of the tibial metaphysis revealed that both male and female AT1a knockout mice exhibited an increased trabecular bone volume along with increased trabecular number and connectivity. Histomorphometric analysis of the tibial metaphysis indicated that the parameters of bone formation as well as resorption were increased, which was also supported by elevated serum osteocalcin and carboxy-terminal collagen crosslink (CTX) concentrations in the AT1a-deficient mice. Osteoclastogenesis and osteoblastogenesis assays in ex vivo cultures, however, did not reveal any intrinsic alterations in the differentiation potential of AT1a-deficient cells. Quantitative RT-PCR using RNA isolated from the tibia and femur revealed that the receptor activator of NF-κB ligand (RANKL)/osteoprotegerin (OPG) ratio and the expression of stromal cell-derived factor (SDF)1α were increased, whereas that of SOST was decreased in AT1a-deficient bone, which may account for the increased bone resorption and formation, respectively. AT1a-deficient mice also displayed a lean phenotype with reduced serum leptin levels. They maintained high bone mass with advancing age, and were protected from bone loss induced by ovariectomy. Collectively, the data suggest that RAS has a physiologic function in bone remodeling, and that signaling through AT1a negatively regulates bone turnover and bone mass.     

Depletion of annexin A5, annexin A6, and collagen X causes no gross changes in matrix vesicle–mediated mineralization,but lack of collagen X affects hematopoiesis and the Th1/Th2 response

Numerous biochemical studies have pointed to an essential role of annexin A5 (AnxA5), annexin A6 (AnxA6), and collagen X in matrix vesicle–mediated biomineralization during endochondral ossification and in osteoarthritis. By binding to the extracellular matrix protein collagen X and matrix vesicles, annexins were proposed to anchor matrix vesicles in the extracellular space of hypertrophic chondrocytes to initiate the calcification of cartilage. However, mineralization appears to be normal in mice lacking AnxA5 and AnxA6, whereas collagen X–deficient mice show only subtle alterations in the growth plate organization. We hypothesized that the simultaneous lack of AnxA5, AnxA6, and collagen X in vivo induces more pronounced changes in the growth plate development and the initiation of mineralization. In this study, we generated and analyzed mice deficient for AnxA5, AnxA6, and collagen X. Surprisingly, mice were viable, fertile, and showed no obvious abnormalities. Assessment of growth plate development indicated that the hypertrophic zone was expanded in Col10a1^?/? and AnxA5^?/?AnxA6^?/?Col10a1^?/? newborns, whereas endochondral ossification and mineralization were not affected in 13‐day‐ and 1‐month‐old mutants. In peripheral quantitative computed tomography, no changes in the degree of biomineralization were found in femora of 1‐month‐ and 1‐year‐old mutants even though the diaphyseal circumference was reduced in Col10a1^?/? and AnxA5^?/?AnxA6^?/?Col10a1^?/? mice. The percentage of naive immature IgM⁺/IgM⁺ B cells and peripheral T‐helper cells were increased in Col10a1^?/? and AnxA5^?/?AnxA6^?/?Col10a1^?/? mutants, and activated splenic T cells isolated from Col10a1^?/? mice secreted elevated levels of IL‐4 and GM‐CSF. Hence, collagen X is needed for hematopoiesis during endochondral ossification and for the immune response, but the interaction of annexin A5, annexin A6, and collagen X is not essential for physiological calcification of growth plate cartilage. Therefore, annexins and collagen X may rather fulfill functions in growth plate cartilage not directly linked to the mineralization process. © 2012 American Society for Bone and Mineral Research.     

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.     

Expression of p21CIP1/WAF1 in Chondrocytes

During endochondral ossification, proliferative activity of chondrocytes is arrested and the cells undergo terminal hypertrophic differentiation. We examined the expression of the cyclin-dependent kinase inhibitor, p21^CIP1/WAF1 in permanent cartilage (xyphoid and articular cartilage) and in cartilage undergoing endochondral ossification (growth plate, epiphyseal ossification centers, and costochondral junctions) to determine if p21 is up-regulated in chondrocytes during hypertrophic differentiation. Northern blot analyses demonstrated expression of p21 in chondrocytes undergoing endochondral ossification and from sites of permanent cartilage. Quantitative analyses of Northern data showed an association between expression of the hypertrophic-specific marker, collagen type X, and the level of 21 expression. In situ hybridization of rodent femoropatellar joints and costochondral junctions localized p21 mRNA to chondrocytes within both the proliferative and hypertrophic zones of the growth plates, in chondrocytes involved in formation of the epiphyseal ossification centers, and in articular chondrocytes. Immunohistochemical analyses of p21 expression in the same tissues demonstrated comparatively higher levels of p21 protein in postmitotic chondrocytes. These data suggest that p21 is active in cell cycle regulation in chondrocytes, and that increased p21 expression is associated with hypertrophic differentiation. Received: 11 October 1996 / Accepted: 23 April 1997     

Biomarkers in WNT1 and PLS3 Osteoporosis: Altered Concentrations of DKK1 and FGF23

Recent advancements in genetic research have uncovered new forms of monogenic osteoporosis, expanding our understanding of the molecular pathways regulating bone health. Despite active research, knowledge on the pathomechanisms, disease-specific biomarkers, and optimal treatment in these disorders is still limited. Mutations in WNT1, encoding a WNT/β-catenin pathway ligand WNT1, and PLS3, encoding X chromosomally inherited plastin 3 (PLS3), both result in early-onset osteoporosis with prevalent fractures and disrupted bone metabolism. However, despite marked skeletal pathology, conventional bone markers are usually normal in both diseases. Our study aimed to identify novel bone markers in PLS3 and WNT1 osteoporosis that could offer diagnostic potential and shed light on the mechanisms behind these skeletal pathologies. We measured several parameters of bone metabolism, including serum dickkopf-1 (DKK1), sclerostin, and intact and C-terminal fibroblast growth factor 23 (FGF23) concentrations in 17 WNT1 and 14 PLS3 mutation-positive subjects. Findings were compared with 34 healthy mutation-negative subjects from the same families. Results confirmed normal concentrations of conventional metabolic bone markers in both groups. DKK1 concentrations were significantly elevated in PLS3 mutation-positive subjects compared with WNT1 mutation-positive subjects (p < .001) or the mutation-negative subjects (p = .002). Similar differences were not seen in WNT1 subjects. Sclerostin concentrations did not differ between any groups. Both intact and C-terminal FGF23 were significantly elevated in WNT1 mutation-positive subjects (p = .039 and p = .027, respectively) and normal in PLS3 subjects. Our results indicate a link between PLS3 and DKK1 and WNT1 and FGF23 in bone metabolism. The normal sclerostin and DKK1 levels in patients with impaired WNT signaling suggest another parallel regulatory mechanism. These findings provide novel information on the molecular networks in bone. Extended studies are needed to investigate whether these biomarkers offer diagnostic value or potential as treatment targets in osteoporosis. © 2020 American Society for Bone and Mineral Research.     

Mutations in Profilin 1 Cause Early-Onset Paget's Disease of Bone With Giant Cell Tumors

Paget's disease of bone (PDB) is a late-onset chronic progressive bone disease characterized by abnormal activation of osteoclasts that results in bone pain, deformities, and fractures. PDB is very rare in Asia. A subset of PDB patients have early onset and can develop malignant giant cell tumors (GCTs) of the bone (PDB/GCTs), which arise within Paget bone lesions; the result is a significantly higher mortality rate. SQSTM1, TNFRSF11A, OPG, VCP, and HNRNPA2B1 have been identified as pathogenic genes of PDB, and ZNF687 is the only confirmed gene to date known to cause PDB/GCT. However, the molecular mechanism underlying PDB/GCT has not been fully elucidated. Here, we investigate an extended Chinese pedigree with eight individuals affected by early-onset and polyostotic PDB, two of whom developed GCTs. We identified a heterozygous 4-bp deletion in the Profilin 1 (PFN1) gene (c.318_321delTGAC) by genetic linkage analysis and exome sequencing for the family. Sanger sequencing revealed another heterozygous 1-bp deletion in PFN1 (c.324_324delG) in a sporadic early-onset PDB/GCT patient, further proving its causative role. Interestingly, a heterozygous missense mutation of PFN1 (c.335 T > C) was identified in another PDB/GCT family, revealing that not only deletion but also missense mutations in PFN1 can cause PDB/GCT. Furthermore, we established a Pfn1-mutated mouse model (C57BL/6J mice) and successfully obtained Pagetic phenotypes in heterozygous mice, verifying loss of function of PFN1 as the cause of PDB/GCT development. In conclusion, our findings reveal mutations in PFN1 as the pathological mechanism in PDB/GCT, and we successfully established Pfn1-mutated mice as a suitable animal model for studying PDB-associated pathological mechanisms. The identification of PFN1 mutations has great diagnostic value for identifying PDB individuals predisposed toward developing GCTs. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).     

Ovarian Cancer G protein‐Coupled Receptor 1 Is Involved in Acid‐Induced Apoptosis of Endplate Chondrocytes in Intervertebral Discs

Ovarian cancer G protein‐coupled receptor 1 (OGR1) has been shown to be a receptor for protons. We investigated the role of proton‐sensing G protein‐coupled receptors in the apoptosis of endplate chondrocytes induced by extracellular acid. The expression of proton‐sensing G protein‐coupled receptors was examined in rat lumbar endplate chondrocytes. Knockdown of OGR1 was achieved by transfecting chondrocytes with specific short hairpin RNA (shRNA) for OGR1. Apoptotic changes were evaluated by DNA fragmentation ELISA, electron microscopy, and flow cytometry. Intracellular calcium ([Ca²⁺]i) was analyzed with laser scanning confocal microscopy. The mechanism of OGR1 in acid‐induced apoptosis of endplate chondrocytes was also investigated. We found that OGR1 was predominantly expressed in rat endplate chondrocytes, and its expression was highly upregulated in response to acidosis. Knocking down OGR1 with shRNAs effectively attenuated acid‐induced apoptosis of endplate chondrocytes and increased [Ca²⁺]i. Blocking OGR1‐mediated [Ca²⁺]i elevation inhibited acid‐induced calcium‐sensitive proteases such as calpain and calcineurin, and also inhibited the activation of Bid, Bad, and Caspase 3 and cleavage of poly (ADP‐ribose) polymerase (PARP). OGR1‐mediated [Ca²⁺]i elevation has a crucial role in apoptosis of endplate chondrocytes by regulating activation of calcium‐sensitive proteases and their downstream signaling. © 2014 American Society for Bone and Mineral Research.     

Absence of Calcitriol Causes Increased Lactational Bone Loss and Lower Milk Calcium but Does Not Impair Post‐lactation Bone Recovery in Cyp27b1 Null Mice

We hypothesized that adaptation to calcium supply demands of pregnancy and lactation do not require calcitriol. Adult Cyp27b1 null mice lack calcitriol and have hypocalcemia, hypophosphatemia, and rickets. We studied wild‐type (WT) and null sister pairs raised on a calcium‐, phosphorus‐, and lactose‐enriched “rescue” diet that prevents hypocalcemia and rickets. Bone mineral content (BMC) increased >30% in pregnant nulls, declined 30% during lactation, and increased 30% by 4 weeks post‐weaning. WT showed less marked changes. Micro‐CT revealed loss of trabecular bone and recovery in both genotypes. In lactating nulls, femoral cortical thickness declined >30%, whereas endocortical perimeter increased; both recovered to baseline after weaning; there were no such changes in WT. Histomorphometry revealed a profound increase in osteoid surface and thickness in lactating nulls, which recovered after weaning. By three‐point bend test, nulls had a >50% decline in ultimate load to failure that recovered after weaning. Although nulls showed bone loss during lactation, their milk calcium content was 30% lower compared with WT. Serum parathyroid hormone (PTH) was markedly elevated in nulls at baseline, reduced substantially in pregnancy, but increased again during lactation and remained high post‐weaning. In summary, pregnant Cyp27b1 nulls gained BMC with reduced secondary hyperparathyroidism, implying increased intestinal calcium delivery. Lactating nulls lost more bone mass and strength than WT, accompanied by increased osteoid, reduced milk calcium, and worsened secondary hyperparathyroidism. This implies suboptimal intestinal calcium absorption. Post‐weaning, bone mass and strength recovered to baseline, whereas BMC exceeded baseline by 40%. In conclusion, calcitriol‐independent mechanisms regulate intestinal calcium absorption and trabecular bone metabolism during pregnancy and post‐weaning but not during lactation; calcitriol may protect cortical bone during lactation. © 2017 American Society for Bone and Mineral Research.     

Osteoblast‐Specific Expression of the Fibrous Dysplasia (FD)–Causing Mutation GsαR201C Produces a High Bone Mass Phenotype but Does Not Reproduce FD in the Mouse

We recently reported the generation and initial characterization of the first direct model of human fibrous dysplasia (FD; OMIM #174800), obtained through the constitutive systemic expression of one of the disease‐causing mutations, Gsα^R201C, in the mouse. To define the specific pathogenetic role(s) of individual cell types within the stromal/osteogenic system in FD, we generated mice expressing Gsα^R201C selectively in mature osteoblasts using the 2.3kb Col1a1 promoter. We show here that this results in a striking high bone mass phenotype but not in a mimicry of human FD. The high bone mass phenotype involves specifically a deforming excess of cortical bone and prolonged and ectopic cortical bone remodeling. Expression of genes characteristic of late stages of bone cell differentiation/maturation is profoundly altered as a result of expression of Gsα^R201C in osteoblasts, and expression of the Wnt inhibitor Sost is reduced. Although high bone mass is, in fact, a feature of some types/stages of FD lesions in humans, it is marrow fibrosis, localized loss of adipocytes and hematopoietic tissue, osteomalacia, and osteolytic changes that together represent the characteristic pathological profile of FD, as well as the sources of specific morbidity. None of these features are reproduced in mice with osteoblast‐specific expression of Gsα^R201C. We further show that hematopoietic progenitor/stem cells, as well as more mature cell compartments, and adipocyte development are normal in these mice. These data demonstrate that effects of Gsα mutations underpinning FD‐defining tissue changes and morbidity do not reflect the effects of the mutations on osteoblasts proper. © 2015 American Society for Bone and Mineral Research. © 2014 American Society for Bone and Mineral Research     

Abnormal Response to Physical Activity in Femurs after Heterozygous Inactivation of One Allele of the Col2a1 Gene for Type II Collagen in Mice

The objective of this study was to evaluate the influence of heterozygous inactivation of one allele of the type II collagen gene (Col2a1) on biomechanical properties and mineral density of bone under physical loading conditions. C57BL/6−TGN mice with heterozygous knockout (HZK) inactivation of Col2a1 gene and their nontransgenic littermate controls were housed in individual cages with running wheels for 9 and 15 months. The running activity of each mouse was monitored continuously throughout the experiment. Bone mineral density (BMD) of mice femora was measured using dual-energy X-ray absorptiometry (DXA) and peripheral quantitative computerized tomography (pQCT). Biomechanical properties were determined using three-point bending tests. Vertebral bone samples were prepared for quantitative polarized light microscopy and digital densitometry of proteoglycans. The concentration of total collagen and collagen cross-links were analyzed using high-performance liquid chromatograpy (HPLC). The average daily running distance was shorter for the HZK mice between the age of 4 and 15 months as compared with normal runners (P < 0.05). The ultimate breaking force was 14.8% and 23.6% (9 vs. 15 months) lower in HZK-runners than in wild-type runners. BMD of the femur was 6.1% lower in HZK-runners at the age of 9 months (P < 0.05). Physical activity increased cortical BMD in wild-type runners but not in the HZK runners at the age of 9 months. The collagen network of the HZK mice was less organized. There were only minor changes in BMD and mechanical and structural properties between sedentary HZK mice and their wild-type controls. Increased physical activity induced significantly lower bone density, mechanical properties, and organization of collagen fibers in male HZK mice. However, there were no major differences in biomechanical parameters between sedentary HZK and wild-type male mice. This suggests an important guiding role of collagen type II in bone remodelling and maturation.