Human and murine osteocalcin gene expression: conserved tissue restricted expression and divergent responses to 1,25-dihydroxyvitamin D3 in vivo

Targeting osteocytes vs osteoblasts

Although osteoblasts and osteocytes are descended from the same lineage, they each have unique and essential roles in bone. Targeting gene deletion to osteoblasts and osteocytes using the Cre/loxP system has greatly increased our current understanding of how these cells function. Additionally, the use of the Cre/loxP system in conjunction with cell-specific reporters has enabled lineage tracing of these bone cells both in vivo and ex vivo. However, concerns have been raised regarding the specificity of the promoters used and the resulting off-target effects on cells within and outside of the bone. In this review, we have summarized the main mouse models that have been used to determine the functions of specific genes in osteoblasts and osteocytes. We discuss the expression patterns and specificity of the different promoter fragments during osteoblast to osteocyte differentiation in vivo. We also highlight how their expression in non-skeletal tissues may complicate the interpretation of study results. A thorough understanding of when and where these promoters are activated will enable improved study design and greater confidence in data interpretation.

G-CSF Receptor Deletion Amplifies Cortical Bone Dysfunction in Mice With STAT3 Hyperactivation in Osteocytes

Bone strength is determined by the structure and composition of its thickened outer shell (cortical bone), yet the mechanisms controlling cortical consolidation are poorly understood. Cortical bone maturation depends on SOCS3-mediated suppression of IL-6 cytokine-induced STAT3 phosphorylation in osteocytes, the cellular network embedded in bone matrix. Because SOCS3 also suppresses granulocyte-colony-stimulating factor receptor (G-CSFR) signaling, we here tested whether global G-CSFR (Csf3r) ablation altereed bone structure in male and female mice lacking SOCS3 in osteocytes, (Dmp1^Cre :Socs3^f/f mice). Dmp1^Cre :Socs3^f/f :Csf3r^-/- mice were generated by crossing Dmp1^Cre :Socs3^f/f mice with Csf3r^-/- mice. Although G-CSFR is not expressed in osteocytes, Csf3r deletion further delayed cortical consolidation in Dmp1^Cre :Socs3^f/f mice. Micro-CT images revealed extensive, highly porous low-density bone, with little true cortex in the diaphysis, even at 26 weeks of age; including more low-density bone and less high-density bone in Dmp1^Cre :Socs3^f/f :Csf3r^-/- mice than controls. By histology, the area where cortical bone would normally be found contained immature compressed trabecular bone in Dmp1^Cre :Socs3^f/f :Csf3r^-/- mice and greater than normal levels of intracortical osteoclasts, extensive new woven bone formation, and the presence of more intracortical blood vessels than the already high levels observed in Dmp1^Cre :Socs3^f/f controls. qRT-PCR of cortical bone from Dmp1^Cre :Socs3^f/f :Csf3r^-/- mice also showed more than a doubling of mRNA levels for osteoclasts, osteoblasts, RANKL, and angiogenesis markers. The further delay in cortical bone maturation was associated with significantly more phospho-STAT1 and phospho-STAT3-positive osteocytes, and a threefold increase in STAT1 and STAT3 target gene mRNA levels, suggesting G-CSFR deletion further increases STAT signaling beyond that of Dmp1^Cre :Socs3^f/f bone. G-CSFR deficiency therefore promotes STAT1/3 signaling in osteocytes, and when SOCS3 negative feedback is absent, elevated local angiogenesis, bone resorption, and bone formation delays cortical bone consolidation. This points to a critical role of G-CSF in replacing condensed trabecular bone with lamellar bone during cortical bone formation. © 2022 American Society for Bone and Mineral Research (ASBMR).

Osteocytes in bone aging: Advances, challenges, and future perspectives

Osteocytes play a critical role in maintaining bone homeostasis and in regulating skeletal response to hormones and mechanical loading. Substantial evidence have demonstrated that osteocytes and their lacunae exhibit morphological changes in aged bone, indicating the underlying involvement of osteocytes in bone aging. Notably, recent studies have deciphered aged osteocytes to have characteristics such as impaired mechanosensitivity, accumulated cellular senescence, dysfunctional perilacunar/canalicular remodeling, and degenerated lacuna-canalicular network. However, detailed molecular mechanisms of osteocytes remain unclear. Nonetheless, osteocyte transcriptomes analyzed via advanced RNA sequencing (RNA-seq) techniques have identified several bone aging-related genes and signaling pathways, such as Wnt, Bmp/TGF, and Jak-STAT. Moreover, inflammation, immune dysfunction, energy shortage, and impaired hormone responses possibly affect osteocytes in age-related bone deterioration. In this review, we summarize the hallmarks of aging bone and osteocytes and discuss osteocytic mechanisms in age-related bone loss and impaired bone quality. Furthermore, we provide insights into the challenges faced and their possible solutions when investigating osteocyte transcriptomes. We also highlight that single-cell RNA-seq can decode transcriptomic messages in aged osteocytes; therefore, this technique can promote novel single cell-based investigations in osteocytes once a well-established standardized protocol specific for osteocytes is developed. Interestingly, improved understanding of osteocytic mechanisms have helped identify promising targets and effective therapies for aging-related osteoporosis and fragile fractures.

The Osteocyte Transcriptome: Discovering Messages Buried Within Bone

PURPOSE OF THE REVIEW

Osteocytes are cells embedded within the bone matrix, but their function and specific patterns of gene expression remain only partially defined; this is beginning to change with recent studies using transcriptomics. This unbiased approach can generate large amounts of data and is now being used to identify novel genes and signalling pathways within osteocytes both at baseline conditions and in response to stimuli. This review outlines the methods used to isolate cell populations containing osteocytes, and key recent transcriptomic studies that used osteocyte-containing preparations from bone tissue.

RECENT FINDINGS

Three common methods are used to prepare samples to examine osteocyte gene expression: digestion followed by sorting, laser capture microscopy, and the isolation of cortical bone shafts. All these methods present challenges in interpreting the data generated. Genes previously not known to be expressed by osteocytes have been identified and variations in osteocyte gene expression have been reported with age, sex, anatomical location, mechanical loading, and defects in bone strength. A substantial proportion of newly identified transcripts in osteocytes remain functionally undefined but several have been cross-referenced with functional data. Future work and improved methods (e.g. scRNAseq) likely provide useful resources for the study of osteocytes and important new information on the identity and functions of this unique cell type within the skeleton.

Endothelium-specific depletion of LRP1 improves glucose homeostasis through inducing osteocalcin

The vascular endothelium is present within metabolic organs and actively regulates energy metabolism. Here we show osteocalcin, recognized as a bone-secreted metabolic hormone, is expressed in mouse primary endothelial cells isolated from heart, lung and liver. In human osteocalcin promoter-driven green fluorescent protein transgenic mice, green fluorescent protein signals are enriched in endothelial cells lining aorta, small vessels and capillaries and abundant in aorta, skeletal muscle and eye of adult mice. The depletion of lipoprotein receptor-related protein 1 induces osteocalcin through a Forkhead box O -dependent pathway in endothelial cells. Whereas depletion of osteocalcin abolishes the glucose-lowering effect of low-density lipoprotein receptor-related protein 1 depletion, osteocalcin treatment normalizes hyperglycemia in multiple mouse models. Mechanistically, osteocalcin receptor-G protein-coupled receptor family C group 6 member A and insulin-like-growth-factor-1 receptor are in the same complex with osteocalcin and required for osteocalcin-promoted insulin signaling pathway. Therefore, our results reveal an endocrine/paracrine role of endothelial cells in regulating insulin sensitivity, which may have therapeutic implications in treating diabetes and insulin resistance through manipulating vascular endothelium.

Associations of Osteocalcin Forms With Metabolic Syndrome and Its Individual Components in Older Men: The Health In Men Study

CONTEXT

The osteoblast-derived polypeptide, osteocalcin (OC), has been associated with lower risk of type 2 diabetes and metabolic syndrome (MetS) in several epidemiological studies. Animal studies have indicated the undercarboxylated form of OC (ucOC) drives its association with metabolic outcomes.

OBJECTIVE

We compared associations of ucOC and carboxylated OC (cOC) with MetS and its components in older men.

METHODS

A cross-sectional analysis of 2575 men aged ≥70 years and older resident in Perth, Western Australia. ucOC was assayed using a hydroxyapatite-binding method, and cOC calculated by subtracting ucOC from total OC. Main outcome measures were MetS and its components.

RESULTS

Both lower serum ucOC and cOC levels, and the proportion of cOC (%cOC) were associated with less favorable metabolic parameters (higher waist circumference, triglyceride, glucose, blood pressure, and lower high-density lipoprotein cholesterol), whereas inverse associations were found with %ucOC. Men in the lowest quintile of ucOC had higher risk of MetS compared to men in the highest quintile (Q1 ≤ 7.7 vs Q5 > 13.8 ng/mL; OR = 2.4; 95% CI, 1.8-3.2). Men in the lowest quintile of cOC had higher risk of MetS compared to those in the highest quintile (≤ 5.8 vs > 13.0 ng/mL; OR = 2.4; 95% CI, 1.8-3.2).

CONCLUSION

Lower concentrations of serum ucOC or cOC were associated with less favorable metabolic parameters and a higher risk of MetS. In contrast, a lower proportion of ucOC was associated with better metabolic parameters and lower MetS risk. Further research is warranted to determine whether ucOC and cOC are suitable biomarkers for cardiometabolic risk in men.

Myogenic, genomic and non-genomic influences of the vitamin D axis in skeletal muscle

Despite vitamin D-deficiency clinically presenting with myopathy, muscle weakness and atrophy, the mechanisms by which vitamin D exerts its homeostatic effects upon skeletal muscle remain to be fully established. Recent studies have shown that the receptor by which 1α,25-dihydroxyvitamin D₃ (1,25[OH]₂ D₃ ) exerts its biological actions (ie, the vitamin D receptor, VDR) elicits both genomic and non-genomic effects upon skeletal muscle. The controversy surrounding skeletal muscle VDR mRNA/protein expression in post-natal muscle has been allayed by myriad recent studies, while dynamic expression of VDR throughout myogenesis, and association of higher VDR levels during muscle regeneration/immature muscle cells, suggests a role in myogenesis and perhaps an enrichment of VDR in satellite cells. Accordingly, in vitro studies have demonstrated 1,25(OH)₂ D₃ is anti-proliferative in myoblasts, yet pro-differentiation in latter stages of myogenesis. These effects involve modulation of gene expression (VDR as a transcriptional co-activator controls ~3% of the genome) and post-genomic intracellular signalling for example, via c-Src and alterations to intramuscular calcium homeostasis and proteostasis. The aim of this review is to consider the biomolecular role for the vitamin D/VDR axis in myogenesis, while also exploring global evidence for genomic and non-genomic mechanisms of action for 1,25(OH)₂ D₃ /VDR.

Osteocalcin and measures of adiposity: a systematic review and meta-analysis of observational studies

Osteocalcin, the osteoblast-derived protein, has been shown to be modulated in patients with problematic glucose metabolism. Our systematic review and meta-analysis found that in humans, higher blood osteocalcin level is associated with lower body indices of fat.

PURPOSE/INTRODUCTION

Osteocalcin (OC) was found to be inversely correlated with measures of glucose and energy metabolism, with some groups suggesting the undercarboxylated form (ucOC) to be metabolically active, although the link is not clear, especially in humans. Given obesity is a major risk factor for metabolic disorders, we aimed to assess the correlation between OC and two measures of body weight: body mass index (BMI) and percentage body fat (%BF).

METHODS

MEDLINE and EMBASE were searched to identify observational studies in adult populations that reported the crude correlation coefficients (r) between OC and BMI and %BF. Pool r were obtained using random-effects models.

RESULTS

Fifty-one publications were included in this analysis. Both total OC (TOC) (pooled r = - 0.151, 95% CI - 0.17, - 0.130; I² = 52%) and ucOC (pooled r = - 0.060, 95% CI - 0.103, - 0.016; I² = 54%) were inversely correlated with BMI. The pooled r between TOC and BMI in Caucasian-and-other-regions (r = - 0.187) were stronger than those in Asian populations (r = - 0.126; intra-group p = 0.002; R² = 0.21). The mean/median BMI of the reported cohort was the major contributor to between-study heterogeneity in correlation between TOC/ucOC and BMI (R² = 0.28 and 0.77, respectively). Both TOC and ucOC were also inversely correlated with %BF (TOC: pooled r = - 0.185, 95% CI - 0.257 to - 0.112; ucOC: pooled r = - 0.181, 95% CI - 0.258 to - 0.101).

CONCLUSION

Higher OC and ucOC were correlated with lower BMI and %BF. The inverse correlations between TOC/ucOC and BMI appear to be affected by ethnicity and obesity status.

Cortical bone maturation in mice requires SOCS3 suppression of gp130/STAT3 signalling in osteocytes

Bone strength is determined by its dense cortical shell, generated by unknown mechanisms. Here we use the Dmp1Cre:Socs3f/f mouse, with delayed cortical bone consolidation, to characterise cortical maturation and identify control signals. We show that cortical maturation requires a reduction in cortical porosity, and a transition from low to high density bone, which continues even after cortical shape is established. Both processes were delayed in Dmp1Cre:Socs3f/f mice. SOCS3 (suppressor of cytokine signalling 3) inhibits signalling by leptin, G-CSF, and IL-6 family cytokines (gp130). In Dmp1Cre:Socs3f/f bone, STAT3 phosphorylation was prolonged in response to gp130-signalling cytokines, but not G-CSF or leptin. Deletion of gp130 in Dmp1Cre:Socs3f/f mice suppressed STAT3 phosphorylation in osteocytes and osteoclastic resorption within cortical bone, leading to rescue of the corticalisation defect, and restoration of compromised bone strength. We conclude that cortical bone development includes both pore closure and accumulation of high density bone, and that these processes require suppression of gp130-STAT3 signalling in osteocytes.

Osteoblast-specific expression of Panx3 is dispensable for postnatal bone remodeling

Since cost-effective osteoanabolic treatment options remain to be established, it is relevant to identify specific molecules physiologically regulating osteoblast differentiation and/or activity that are principally accessible as drug targets. Specific or predominant gene expression in a given cell type often predicts a relevant function in the respective tissue. Thus, we aimed to identify genes encoding membrane-associated proteins with selective expression in differentiated osteoblasts. We therefore applied an unbiased approach, i.e. Affymetrix Gene Chip hybridization, to compare global gene expression in primary murine osteoblasts at two stages of differentiation. For the most strongly induced genes we analyzed their expression pattern in different tissues, which led us to identify known and unknown osteoblast differentiation markers with predominant expression in bone. One of these genes was Panx3, encoding a transmembrane hemichannel with ill-defined function in skeletal remodeling. To decipher the role of Panx3 in osteoblasts we first generated Panx3-fl/fl mice carrying a Runx2-Cre transgene. Using undecalcified histology followed by bone-specific histomorphometry we did not observe any significant difference between 24 weeks old Cre-negative and Cre-positive littermates. We additionally generated and analyzed mice with ubiquitous Panx3 deletion, where a delay of endochondral ossification did not translate into a detectable skeletal phenotype after weaning, possibly explained by compensatory induction of Panx1. Of note, newborn Panx3-deficient mice displayed significantly reduced serum glucose levels, which was not the case in older animals. Our findings demonstrate that Panx3 expression in osteoblasts is not required for postnatal bone remodeling, which essentially rules out its suitability as a target protein for osteoanabolic medication.

Histological Analysis of Bone Repair in Mandibular Body Osteotomy Using Internal Fixation System in Three Different Gaps without Bone Graft in an Animal Model

The aim was to analyze histologically the bone repair in a mandibular osteotomy model with different gaps between the segments. Nine male rabbits who underwent osteotomies on the mandibular body were fixed with a 1.5 system plate and no bone graft; group 1 (2 mm gap between segments), group 2 (5 mm gap between segments), and group 3 (8 mm gap between segments) were included. After 8 weeks they were euthanized and the sample was processed for histological analysis. Group 1 showed advanced bone repair with cartilaginous tissue and cancellous bone, showing osteoblasts and type III collagenous fibers. In group 2, a more delayed ossification was observed, with an extensive area of peripheral ossifying cartilage and chondrocytes in greater number at the center of the defect; group 3 showed no evidence of ossification with fibrous tissue, a very low level of chondrocytes, and some bone sequestrate. We can conclude that, in this animal model, 2 or 5 mm gap in the osteotomy could be repaired as bone when fixation is used. The size of the gap is an important factor for the use of bone grafts considering endochondral ossification. This model can be used for graft analysis and related technologies.

The Cells of Bone and Their Interactions

Bone tissue is comprised of a collagen-rich matrix containing non-collagenous organic compounds, strengthened by mineral crystals. Bone strength reflects the amount and structure of bone, as well as its quality. These qualities are determined and maintained by osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells) on the surface of the bone and osteocytes embedded within the bone matrix. Bone development and growth also involves cartilage cells (chondrocytes). These cells do not act in isolation, but function in a coordinated manner, including co-ordination within each lineage, between the cells of bone, and between these cells and other cell types within the bone microenvironment. This chapter will briefly outline the cells of bone, their major functions, and some communication pathways responsible for controlling bone development and remodeling.

Insulin and osteocalcin: further evidence for a mutual cross-talk

PURPOSE

In the last few years, bone has been recognized as an endocrine organ that modulates glucose metabolism by secretion of osteocalcin, an osteoblast-specific hormone, that influences fat deposition and blood sugar levels. To date, however, very few in vitro models have been developed to investigate, at the molecular levels, the relationship between glucose, insulin and osteocalcin. This study aims at covering this gap.

METHODS

We studied osteogenic differentiation, osteocalcin gene expression, and osteblast-mediated insulin secretion, using cultured MG-63 human osteoblast-like cells that underwent glucotoxicity and insulin resistance. In addition, we investigated whether a correlation existed between hyperglycemia and/or insulin resistance and total osteocalcin serum concentrations in patients.

RESULTS

While insulin and low glucose increased osteocalcin gene expression, disruption of insulin signaling in MG-63 osteoblasts and high glucose concentration in cell culture medium decreased osteocalcin gene transcription and reduced osteogenic differentiation. Concomitantly, insulin secretion was significantly impaired in rat INS-1 β-cells treated with conditioned medium from insulin resistant MG-63 cells or cells exposed to high glucose concentrations. Also, chronic hyperglycemia, but not insulin resistance, inversely correlated with circulating osteocalcin levels in patients.

CONCLUSION

Our results further support the existence of an endocrine axis between bone, where osteocalcin is produced, and pancreatic β-cells, and add new insights into the molecular details of this relationship. These findings may contribute to the understanding of osteocalcin regulation and its role in metabolism.

Ectodermal-Neural Cortex 1 Isoforms Have Contrasting Effects on MC3T3-E1 Osteoblast Mineralization and Gene Expression

The importance of Wnt pathway signaling in development of bone has been well established. Here we investigated the role of a known Wnt target, ENC1 (ectodermal-neural cortex 1; NRP/B), in osteoblast differentiation. Enc1 expression was detected in mouse osteoblasts, chondrocytes, and osteocytes by in situ hybridization, and osteoblastic expression was verified in differentiating primary cultures and MC3T3-E1 pre-osteoblast cells, with 57 kDa and 67 kDa ENC1 protein isoforms detected throughout differentiation. Induced knockdown of both ENC1 isoforms reduced alkaline phosphatase staining and virtually abolished MC3T3-E1 mineralization. At culture confluence, Alpl (alkaline phosphatase liver/bone/kidney) expression was markedly reduced compared with control cells, and there was significant and coordinated alteration of other genes involved in cellular phosphate biochemistry. In contrast, with 67 kDa-selective knockdown mineralized nodule formation was enhanced and there was a two-fold increase in Alpl expression at confluence. There was enhanced expression of Wnt/β-catenin target genes with knockdown of both isoforms at this time-point and a five-fold increase in Frzb (Frizzled related protein) with 67 kDa-selective knockdown at mineralization, indicating possible ENC1 interactions with Wnt signaling in osteoblasts. These results are the first to demonstrate a role for ENC1 in the control of osteoblast differentiation. Additionally, the contrasting mineralization phenotypes and transcriptional patterns seen with coordinate knockdown of both ENC1 isoforms vs selective knockdown of 67 kDa ENC1 suggest opposing roles for the isoforms in regulation of osteoblastic differentiation, through effects on Alpl expression and phosphate cellular biochemistry. This study is the first to report differential roles for the ENC1 isoforms in any cell lineage. J. Cell. Biochem. 118: 2141-2150, 2017. © 2016 Wiley Periodicals, Inc.

Visual reporters for study of the osteoblast lineage

Advancing our understanding of osteoblast biology and differentiation is critical to elucidate the pathological mechanisms responsible for skeletal diseases such as osteoporosis. Histology and histomorphometry, the classical methods to study osteoblast biology, identify osteoblasts based on their location and morphology and ability to mineralize matrix, but do not clearly define their stage of differentiation. Introduction of visual transgenes into the cells of osteoblast lineage has revolutionized the field and resulted in a paradigm shift that allowed for specific identification and isolation of subpopulations within the osteoblast lineage. Knowledge acquired from the studies based on GFP transgenes has allowed for more precise interpretation of studies analyzing targeted overexpression or deletion of genes in the osteoblast lineage. Here, we provide a condensed overview of the currently available promoter-fluorescent reporter transgenic mice that have been generated and evaluated to varying extents. We cover different stages of the lineage as transgenes have been utilized to identify osteoprogenitors, pre-osteoblasts, osteoblasts, or osteocytes. We show that each of these promoters present with advantages and disadvantages. The studies based on the use of these reporter mice have improved our understanding of bone biology. They constitute attractive models to target osteoblasts and help to understand their cell biology.

Skeletal characterization of an osteoblast-specific vitamin D receptor transgenic (ObVDR-B6) mouse model

BACKGROUND

Overexpression of the human vitamin D receptor (hVDR) transgene under control of the human osteocalcin promoter in FVB/N mice (OSVDR) was previously demonstrated to exhibit increased cortical and trabecular bone volume and strength due to decreased bone resorption and increased bone formation. An important question to address is whether the OSVDR bone phenotype persists on an alternative genetic background such as C57Bl6/J.

METHODS

OSVDR mice (OSV3 line) were backcrossed onto the C57Bl6/J genetic background for at least 6 generations to produce OSVDR mice with 98.4% C57Bl6/J congenicity (ObVDR-B6 mice). Hemizygous male and female ObVDR-B6 and littermate wild-type (WT) mice were fed a standard laboratory chow diet and killed at 3, 9 and 20 weeks of age for analyses of biochemical and structural variables and dynamic indices of bone histomorphometry.

RESULTS

At 9 weeks of age, both cortical and trabecular femoral bone volumes were increased in both male and female ObVDR-B6 mice, when compared to WT levels (P<0.05), without systemic changes to calciotropic parameters. The increase in femoral trabecular bone volume was associated with increase in MAR (P<0.01) and reduced osteoclast size (P<0.05). However, in female mice trabecular bone volume was unchanged in femoral metaphysis of 20 weeks mice and in vertebra both at 9 and 20 weeks of age. Increased cortical bone in both male and female ObVDR-B6 mice was due largely to increased periosteal expansion and was associated with increased cortical strength at 20 weeks of age.

CONCLUSION

Overexpression of the human VDR gene in mature osteoblasts of C57Bl6/J mice increases cortical and trabecular bone volumes and confirms the previous reports of increased bone in OSVDR mice on the FVB/N background. However, site-specific and gender-related differences in bone volume suggest that the effects of osteoblast-specific VDR overexpression are more complex than hitherto recognised.

Chondrocytic ephrin B2 promotes cartilage destruction by osteoclasts in endochondral ossification

The majority of the skeleton arises by endochondral ossification, whereby cartilaginous templates expand and are resorbed by osteoclasts then replaced by osteoblastic bone formation. Ephrin B2 is a receptor tyrosine kinase expressed by osteoblasts and growth plate chondrocytes that promotes osteoblast differentiation and inhibits osteoclast formation. We investigated the role of ephrin B2 in endochondral ossification using Osx1Cre-targeted gene deletion. Neonatal Osx1Cre.Efnb2(Δ/Δ) mice exhibited a transient osteopetrosis demonstrated by increased trabecular bone volume with a high content of growth plate cartilage remnants and increased cortical thickness, but normal osteoclast numbers within the primary spongiosa. Osteoclasts at the growth plate had an abnormal morphology and expressed low levels of tartrate-resistant acid phosphatase; this was not observed in more mature bone. Electron microscopy revealed a lack of sealing zones and poor attachment of Osx1Cre.Efnb2(Δ/Δ) osteoclasts to growth plate cartilage. Osteoblasts at the growth plate were also poorly attached and impaired in their ability to deposit osteoid. By 6 months of age, trabecular bone mass, osteoclast morphology and osteoid deposition by Osx1Cre.Efnb2(Δ/Δ) osteoblasts were normal. Cultured chondrocytes from Osx1Cre.Efnb2(Δ/Δ) neonates showed impaired support of osteoclastogenesis but no significant change in Rankl (Tnfsf11) levels, whereas Adamts4 levels were significantly reduced. A population of ADAMTS4(+) early hypertrophic chondrocytes seen in controls was absent from Osx1Cre.Efnb2(Δ/Δ) neonates. This suggests that Osx1Cre-expressing cells, including hypertrophic chondrocytes, are dependent on ephrin B2 for their production of cartilage-degrading enzymes, including ADAMTS4, and this might be required for attachment of osteoclasts and osteoblasts to the cartilage surface during endochondral ossification.

PKCδ is required for Jagged-1 induction of human mesenchymal stem cell osteogenic differentiation

JAG1, the gene for the Jagged-1 ligand (Jag1) in the Notch signaling pathway, is variably mutated in Alagille Syndrome (ALGS). ALGS patients have skeletal defects, and additionally JAG1 has been shown to be associated with low bone mass through genome-wide association studies. Plating human osteoblast precursors (human mesenchymal stem cells-hMSCs) on Jag1 is sufficient to induce osteoblast differentiation; however, exposure of mouse MSC (mMSC) to Jag1 actually inhibits osteoblastogenesis. Overexpression of the notch-2 intracellular domain (NICD2) is sufficient to mimic the effect of Jag1 on hMSC osteoblastogenesis, while blocking Notch signaling with a γ-secretase inhibitor or with dominant-negative mastermind inhibits Jag1-induced hMSC osteoblastogenesis. In pursuit of interacting signaling pathways, we discovered that treatment with a protein kinase C δ (PKCδ) inhibitor abrogates Jag1-induced hMSC osteoblastogenesis. Jag1 results in rapid PKCδ nuclear translocation and kinase activation. Furthermore, Jag1 stimulates the physical interaction of PKCδ with NICD. Collectively, these results suggest that Jag1 induces hMSC osteoblast differentiation through canonical Notch signaling and requires concomitant PKCδ signaling. This research also demonstrates potential deficiencies in using mouse models to study ALGS bone abnormalities.

Vitamin D: direct effects of vitamin D metabolites on bone: lessons from genetically modified mice

The vitamin D endocrine system has clear beneficial effects on bone as demonstrated by prevention of rickets in children and by reducing the risk of osteomalacia or osteoporosis in adults or elderly subjects. Depending on the design of the study of genetically modified animals, however, 1,25(OH)2D and the vitamin D receptor (VDR) may have no effect, beneficial or even deleterious direct effects on bone. We present here a comprehensive model of the direct effects of vitamin D on bone. In case of sufficient calcium supply, vitamin D and its metabolites can improve the calcium balance and facilitate mineral deposition in bone matrix largely without direct effects on bone cells, although some beneficial effects may occur via mature osteoblasts, as demonstrated in mice with osteoblast-specific overexpression of VDR or 1α-hydroxylase. In case of calcium deficiency, however, 1,25(OH)2D enhances bone resorption, whereas simultaneously inhibiting bone mineralization, so as to defend serum calcium homeostasis at the expense of bone mass. This dual role probably provides a survival benefit for land vertebrates living in a calcium-poor environment.

Talking among ourselves: paracrine control of bone formation within the osteoblast lineage

While much research focuses on the range of signals detected by the osteoblast lineage that originate from endocrine influences, or from other cells within the body, there are also multiple interactions that occur within this family of cells. Osteoblasts exist as teams and form extensive communication networks both on, and within, the bone matrix. We provide four snapshots of communication pathways that exist within the osteoblast lineage between different stages of their differentiation, as follows: (1) PTHrP, a factor produced by early osteoblasts that stimulates the activity of more mature bone-forming cells and the most mature osteoblast embedded within the bone matrix, the osteocyte; (2) sclerostin, a secreted factor, released by osteocytes into their extensive communication network to restrict the activity of younger osteoblasts on the bone surface; (3) oncostatin M, a member of the IL-6/gp130 family of cytokines, expressed throughout osteoblast differentiation and acting to stimulate osteoblast activity that works on a different receptor in the mature osteocyte compared to the preosteoblast; and (4) Eph/ephrins, cell-contact-dependent kinases, and the osteoblast-lineage-specific interaction of EphB4 and ephrinB2, which provides a checkpoint for entry to the late stages of osteoblast differentiation and restricts RANKL expression.

In vivo determination of vitamin d function using transgenic mice carrying a human osteocalcin luciferase reporter gene

Vitamin D is an essential factor for ossification, and its deficiency causes rickets. Osteocalcin, which is a noncollagenous protein found in bone matrix and involved in mineralization and calcium ion homeostasis, is one of the major bone morphogenetic markers and is used in the evaluation of osteoblast maturation and osteogenic activation. We established transgenic mouse line expressing luciferase under the control of a 10-kb osteocalcin enhancer/promoter sequence. Using these transgenic mice, we evaluated the active forms of vitamins D2 and D3 for their bone morphogenetic function by in vivo bioluminescence. As the result, strong activity for ossification was observed with 1α,25-hydroxyvitamin D3. Our mouse system can offer a feasible detection method for assessment of osteogenic activity in the development of functional foods and medicines by noninvasive screening.

Bioluminescence imaging of bone formation using hairless osteocalcin-luciferase transgenic mice

Osteocalcin is a major noncollagenous protein component of bone extracellular matrix, synthesized and secreted exclusively by osteoblastic cells during the late stage of maturation. We introduced a 10kb human osteocalcin enhancer/promoter (OC)-luciferase (Luc) construct into a hairless mouse line. Examination of tissue RNAs from these transgenic mice showed a predominant restriction of Luc mRNA expression to bone-associated tissues. Immunohistochemical staining of calvaria tissue sections revealed the localization of Luc protein to osteoblasts. Utilizing in vivo bioluminescence imaging, supplementation of 1α,25-dihydroxyvitamin D(3) increased Luc activity throughout the skeleton, consistent with in vitro transient transfection studies in osteoblast-like cells. Moreover, we observed an abrupt decrease in bioluminescence activity as the mice reached puberty, and a further decrease gradually thereafter. Using a radius skeletal repair model, we observed enhanced bioluminescence at the fracture site in both young (14-22 weeks old) and aged (50-66 weeks old) mice. However, peak bioluminescence was delayed in aged mice compared with young mice, suggesting retarded osteocalcin expression with aging. Our in vivo imaging system may contribute to the therapy and prevention of various bone metabolic disorders through its effective monitoring of the bone formation process.

Molecular differentiation between osteophytic and articular cartilage--clues for a transient and permanent chondrocyte phenotype

OBJECTIVE

To identify the molecular differences between the transient and permanent chondrocyte phenotype in osteophytic and articular cartilage.

METHODS

Total RNA was isolated from the cartilaginous layer of osteophytes and from intact articular cartilage from knee joints of 15 adult human donors and subjected to cDNA microarray analysis. The differential expression of relevant genes between these two cartilaginous tissues was additionally validated by quantitative reverse transcriptase polymerase chain reaction (RT-PCR) and by immunohistochemistry.

RESULTS

Among 47,000 screened transcripts, 600 transcripts were differentially expressed between osteophytic and articular chondrocytes. Osteophytic chondrocytes were characterized by increased expression of genes involved in the endochondral ossification process [bone gamma-carboxyglutamate protein/osteocalcin (BGLAP), bone morphogenetic protein-8B (BMP8B), collagen type I, alpha 2 (COL1A2), sclerostin (SOST), growth arrest and DNA damage-induced gene 45ß (GADD45ß), runt-related transcription factor 2 (RUNX2)], and genes encoding tissue remodeling enzymes [matrix metallopeptidase (MMP)9, 13, hyaluronan synthase 1 (HAS1)]. Articular chondrocytes expressed increased transcript levels of antagonists and inhibitors of the BMP- and Wnt-signaling pathways [Gremlin-1 (GREM1), frizzled-related protein (FRZB), WNT1 inducible signaling pathway protein-3 (WISP3)], as well as factors that inhibit terminal chondrocyte differentiation and endochondral bone formation [parathyroid hormone-like hormone (PTHLH), sex-determining region Y-box 9 (SOX9), stanniocalcin-2 (STC2), S100 calcium binding protein A1 (S100A1), S100 calcium binding protein B (S100B)]. Immunohistochemistry of tissue sections for GREM1 and BGLAP, the two most prominent differentially expressed genes, confirmed selective detection of GREM1 in articular chondrocytes and that of BGLAP in osteophytic chondrocytes and bone.

CONCLUSIONS

Osteophytic and articular chondrocytes significantly differ in their gene expression pattern. In articular cartilage, a prominent expression of antagonists inhibiting the BMP- and Wnt-pathway may serve to lock and stabilize the permanent chondrocyte phenotype and thus prevent their terminal differentiation. In contrast, osteophytic chondrocytes express genes with roles in the endochondral ossification process, which may account for their transient phenotype.

BMP2 protein regulates osteocalcin expression via Runx2-mediated Atf6 gene transcription

Bone morphogenetic protein 2 (BMP2) activates unfolded protein response (UPR) transducers, such as PERK and OASIS, in osteoblast cells. ATF6, a bZIP transcription factor, is also a UPR transducer. However, the involvement of ATF6 in BMP2-induced osteoblast differentiation has not yet been elucidated. In the present study, BMP2 treatment was shown to markedly induce the expression and activation of ATF6 with an increase in alkaline phosphatase (ALP) and OC expression in MC3T3E1 cells. In contrast, ATF6 activation by BMP2 was not observed in the Runx2(-/-) primary calvarial osteoblasts, and Runx2 overexpression recovered BMP2 action. BMP2 stimulated ATF6 transcription by enhancing the direct binding of Runx2 to the osteoblast-specific cis-acting element 2 (OSE2, ACCACA, -205 to -200 bp) motif of the Atf6 promoter region. In addition, the overexpression of ATF6 increased the Oc promoter activity by enhancing the direct binding to a putative ATF6 binding motif (TGACGT, -1126 to -1121 bp). The inhibition of ATF6 function with the dominant negative form of ATF6 (DN-ATF6) blocked BMP2- or Runx2-induced OC expression. Interestingly, OASIS, which is structurally similar to ATF6, did not induce Oc expression. ALP and Alizarin red staining results confirmed that BMP2-induced matrix mineralization was also dependent on ATF6 in vitro. Overall, these results suggest that BMP2 induces osteoblast differentiation through Runx2-dependent ATF6 expression, which directly regulates Oc transcription.

Genetic evidence points to an osteocalcin-independent influence of osteoblasts on energy metabolism

The skeleton has been shown recently to regulate glucose metabolism through an osteoblast-specific hormone, osteocalcin, which favors β-cell proliferation, insulin secretion, insulin sensitivity, and energy expenditure. An implication of this finding is that a decrease in osteoblast numbers would compromise glucose metabolism in an osteocalcin-dependent manner. To test this hypothesis, osteoblasts were inducibly ablated by cross-breeding transgenic mice expressing a tamoxifen-regulated Cre under the control of the osteocalcin promoter with mice in which an inactive form of the diphtheria toxin A chain was introduced into a ubiquitously expressed locus. Ablation of osteoblasts in adult mice profoundly affected glucose metabolism. In a manner similar to what is seen in the case of osteocalcin deficiency, a partial ablation of this cell population resulted in hypoinsulinemia, hyperglycemia, glucose intolerance, and decreased insulin sensitivity. However, and unlike what is seen in osteocalcin-deficient mice, osteoblast ablation also decreased gonadal fat and increased energy expenditure and the expression of resistin, an adipokine proposed to mediate insulin resistance. While administration of osteocalcin reversed (fully) the glucose intolerance and reinstated normal blood glucose and insulin levels, it only partially restored insulin sensitivity and did not affect the improved gonadal fat weight and energy expenditure in osteoblast-depleted mice. These observations not only strengthen the notion that osteoblasts are necessary for glucose homeostasis and energy expenditure but also suggest that in addition to osteocalcin, other osteoblast-derived hormones may contribute to the emerging function of the skeleton as a regulator of energy metabolism.

Enhanced reporter gene assay for the detection of osteogenic differentiation

Detection of osteogenic differentiation is crucial for bone tissue engineering. Despite established standard end point assays, there is increasing demand for methods allowing noninvasive kinetic differentiation monitoring. Reporter gene assays employing tissue-specific promoters and suitable reporter genes fulfill these requirements. Many promoters, however, exhibit only weak cis-activating potential, thus limiting their application to generate sensitive reporter gene assays. Therefore, the aim of this study was to design a reporter gene assay employing elements of the murine osteocalcin promoter coupled to a viral enhancer for signal amplification. Additionally, the system's practicability was enhanced by introducing a secreted luciferase as a quantifiable reporter gene. The constructs were tested in C2C12 cells stimulated with recombinant human bone morphogenetic protein 2 for osteogenic differentiation in two-dimensional and three-dimensional culture. Osteogenic differentiation was confirmed by standard assays for osteogenesis. The reporter gene signal was detected through a secreted luciferase or fluorescence microscopy for enhanced yellow fluorescent protein. The constructs exhibited strong activation upon treatment with recombinant human bone morphogenetic protein 2. Weak background expression was observable in negative controls, attributed to the pan-active viral enhancer. In conclusion, a novel enhancer/tissue-specific promoter combination allows specific signal-amplified, kinetic monitoring of osteogenic differentiation in a nonsample-destructive manner.

Misexpression of CCAAT/enhancer binding protein beta causes osteopenia

CCAAT/enhancer binding proteins (C/EBPs) are expressed by osteoblasts and adipocytes during differentiation. C/EBP beta is critical for adipogenesis; however, its role in osteoblastogenesis is unclear, and its function in the postnatal skeleton is not known. To study C/EBP beta in osteoblasts in vivo, we created transgenic mice expressing full length C/EBP beta under the control of a 3.8 kb fragment of the human osteocalcin promoter. Two transgenic lines were established in a friend leukemia virus strain B genetic background, and compared with wild type littermate controls. Both C/EBP beta transgenic lines exhibited osteopenia, with a 30% decrease in bone volume, due to a decrease in trabecular number. The number of osteoblasts and osteoclasts per bone perimeter was not changed. Bone marrow stromal cells from C/EBP beta transgenics showed reduced mineralization, and reduced alkaline phosphatase mRNA levels. Calvarial osteoblasts from C/EBP beta transgenics displayed reduced alkaline phosphatase activity. To determine the consequences of the Cebpb deletion in vivo, the phenotype of Cebpb null mice was compared with that of wild type controls of identical genetic composition. Cebpb null mice exhibited reduced weight, body fat, and bone mineral density, and decreased bone volume, due to a decrease in trabecular number. The number of osteoblasts and osteoclasts per bone perimeter was not changed. C/EBP beta downregulation by RNA interference in calvarial osteoblasts had no effect on osteoblast differentiation/function. The phenotype of the Cebpb inactivation may be secondary to systemic indirect effects, and to direct effects of C/EBP beta in osteoblasts. In conclusion, C/EBP beta plays a role in mesenchymal cell differentiation and its misexpression in vivo causes osteopenia.

Gene expression in normotopic and heterotopic human bone: increased level of SP7 mRNA in pathological tissue

Head injury-induced heterotopic ossification (HO) develops at vicinity of joints and in severe cases requires surgical intervention. Our previous study demonstrated high mRNA levels of osteocalcin (OC), type 1 collagen (COL1), osteonectin and RUNX2/CBFA1 in osteocytes and lining osteoblasts from non-evolutive HO compared to equivalent healthy cells from the proximal femoral shaft of patients receiving prosthesis. This allowed a first molecular characterisation of this pathological bone. The aims of this study is to extend the analysis to 10 more genes and determine those involved in the high OC mRNA level observed in pathological bone samples. RNAs were prepared from normotopic and heterotopic human bone samples digested by collagenase. After cDNA synthesis, mRNA levels were determined by real-time PCR and normalised using beta actin and glyceraldehyde-3-phosphate dehydrogenase. OSTERIX/SP7 expression was observed for the first time in human adult bone biopsies. In HO samples higher levels of SP7 (four- to sevenfold increase) and 1alpha,25-dihydroxy vitamin D(3) receptor (VDR) (two- to threefold increase) were observed compared to control samples. Moreover, SP7 level was correlated to OC and RUNX2 levels. In control samples, OC and SP7 levels were correlated. Our study further confirms that the involvement of SP7 in bone physiology is not only limited to the developmental step. Moreover, our results support the hypothesis that in HO the high level of OC expression could be due not only to an increase in RUNX2, but also in SP7 or VDR or to an imbalance in their respective activities.

Skeletal overexpression of connective tissue growth factor impairs bone formation and causes osteopenia

Connective tissue growth factor (CTGF), a member of the CCN family of proteins, is expressed in skeletal cells, and the ctgf null mutation leads to neonatal lethality due to defects in skeletal development. To define the function of CTGF in the postnatal skeleton, we created transgenic mice overexpressing CTGF under the control of the human osteocalcin promoter. CTGF transgenic female and male mice exhibited a significant decrease in bone mineral density, compared with wild-type littermate controls. Bone histomorphometry revealed that CTGF overexpression caused decreased trabecular bone volume due to impaired osteoblastic activity because mineral apposition and bone formation rates were decreased. Osteoblast and osteoclast number and bone resorption were not altered. Calvarial osteoblasts and stromal cells from CTGF transgenics displayed decreased alkaline phosphatase and osteocalcin mRNA levels and reduced bone morphogenetic protein (BMP) signaling mothers against decapentaplegic, Wnt/beta-catenin, and IGF-I/Akt signaling. In conclusion, CTGF overexpression in vivo causes osteopenia, secondary to decreased bone formation, possibly by antagonizing BMP, Wnt, and IGF-I signaling and activity.

EphrinB2 regulation by PTH and PTHrP revealed by molecular profiling in differentiating osteoblasts

With the aim of identifying new pathways and genes regulated by PTH(1-34) and PTH-related protein 1-141 [PTHrP(1-141)] in osteoblasts, this study was carried out using a mouse marrow stromal cell line, Kusa 4b10, that acquires features of the osteoblastic phenotype in long-term culture conditions. After the appearance of functional PTH receptor 1 (PTHR1) in Kusa 4b10 cells, they were treated with either PTH(1-34) or PTHrP(1-141), and RNA was subjected to Affymetrix whole mouse genome array. The microarray data were validated using quantitative real-time RT-PCR on independently prepared RNA samples from differentiated Kusa 4b10, UMR106 osteosarcoma cells, and primary mouse calvarial osteoblasts, as well as in vivo using RNA from metaphyseal bone after a single PTH injection to 3-wk-old and 6-mo-old ovariectomized rats. Of the 45,101 probes used on the microarray, 4675 were differentially expressed by >or=1.5 fold, with a false discovery rate <0.1. Among the regulated genes, ephrinB2 mRNA was upregulated in response to both PTH and PTHrP. This was confirmed by quantitative real-time PCR in vitro and in vivo. Increased ephrinB2 protein was also shown in vitro by Western blotting, and immunostaining of femur sections showed ephrinB2 in both osteoclasts and osteoblasts. Production of ephrinB2, as well as other ephrins or Eph family members, did not change during differentiation of Kusa 4b10 cells. Blockade of ephrinB2/EphB4 interaction resulted in inhibition of mineralization of Kusa 4b10 cells. Together with the shown effect of ephrinB2 promoting osteoblast differentiation and bone formation through action on EphB4, the data raise the possibility that PTH or PTHrP might regulate ephrinB2 to act in a paracrine or autocrine manner on EphB4 or EphB2 in the osteoblast, contributing as a local event to the anabolic action of PTH or PTHrP.

Constitutive activity of the osteoblast Ca2+-sensing receptor promotes loss of cancellous bone

Changes in extracellular [Ca2+] modulate the function of bone cells in vitro via the extracellular Ca2+-sensing receptor (CaR). Within bone microenvironments, resorption increases extracellular [Ca2+] locally. To determine whether enhanced CaR signaling could modulate remodeling and thereby bone mass in vivo, we generated transgenic mice with a constitutively active mutant CaR (Act-CaR) targeted to their mature osteoblasts by the 3.5 kb osteocalcin promoter. Longitudinal microcomputed tomography of cancellous bone revealed reduced bone volume and density, accompanied by a diminished trabecular network, in the Act-CaR mice. The bone loss was secondary to an increased number and activity of osteoclasts, demonstrated by histomorphometry of secondary spongiosa. Histomorphometry, conversely, indicates that bone formation rates were unchanged in the transgenic mice. Constitutive signaling of the CaR in mature osteoblasts resulted in increased expression of RANK-L (receptor activator of nuclear factor-kappaB ligand), the major stimulator of osteoclast differentiation and activation, which is the likely underlying mechanism for the bone loss. The phenotype of Act-CaR mice is not attributable to systemic changes in serum [Ca2+] or PTH levels. We provide the first in vivo evidence that increased signaling by the CaR in mature osteoblasts can enhance bone resorption and further propose that fluctuations in the [Ca2+] within the bone microenvironment may modulate remodeling via the CaR.

Nephroblastoma overexpressed (Nov) inhibits osteoblastogenesis and causes osteopenia

Nephroblastoma overexpressed (Nov), a member of the Cyr 61, connective tissue growth factor, Nov (CCN) family of proteins, is expressed by osteoblasts, but its function in cells of the osteoblastic lineage is not known. We investigated the effects of Nov overexpression by transducing murine ST-2 stromal and MC3T3 osteoblastic cells with a retroviral vector where Nov is under the control of the cytomegalovirus promoter. We also examined the skeletal phenotype of transgenic mice expressing Nov under the control of the human osteocalcin promoter. Overexpression of Nov in ST-2 cells inhibited the appearance of mineralized nodules and decreased alkaline phosphatase activity and osteocalcin mRNA levels. Nov overexpression inhibited the effect of bone morphogenetic protein (BMP)-2 on the phosphorylation of Smad 1/5/8; on the transactivation of 12xSBE-Oc-pGL3, a BMP/Smad signaling reporter construct, and of Wnt 3 on cytoplasmic beta-catenin levels; and on the transactivation of the Wnt/beta-catenin signaling reporter construct 16xTCF-Luc. Nov overexpression did not activate Notch or transforming growth factor beta signaling. Glutathione S-transferase pulldown assays demonstrated direct Nov-BMP interactions. Nov transgenic mice exhibited osteopenia. In conclusion, Nov binds BMP-2 and antagonizes BMP-2 and Wnt activity, and its overexpression inhibits osteoblastogenesis and causes osteopenia.

CCAAT/Enhancer-binding protein homologous protein (CHOP) decreases bone formation and causes osteopenia

CCAAT enhancer-binding protein (C/EBP) homologous protein (CHOP), is a member of the C/EBP family of nuclear proteins and plays a role in osteoblastic and adipocytic cell differentiation. CHOP is necessary for normal bone formation, but the consequences of its overexpression in vivo are not known. To investigate the direct actions of CHOP on bone remodeling in vivo, we generated transgenic mice overexpressing CHOP under the control of the human osteocalcin promoter. CHOP transgenics exhibited normal weight and reduced bone mineral density. Static and dynamic femoral bone histomorphometry revealed that CHOP overexpression caused reduced trabecular bone volume, secondary to decreased bone formation rates. One of 2 lines displayed a decrease in the number of osteoblasts, but in vivo bromodeoxyuridine labeling demonstrated that CHOP overexpression did not have an effect on osteoblastic cell replication. The decreased osteoblast cell number was accounted by an increase in apoptosis, as determined by DNA fragmentation measured by transferase-mediated digoxigenin-deoxyuridine triphosphate (dUTP) in situ nick-end labeling (TUNEL) reaction. In conclusion, transgenic mice overexpressing CHOP in the bone microenvironment have impaired osteoblastic function leading to osteopenia.

Vitamin D action and regulation of bone remodeling: suppression of osteoclastogenesis by the mature osteoblast

Vitamin D acts through the immature osteoblast to stimulate osteoclastogenesis. Transgenic elevation of VDR in mature osteoblasts was found to inhibit osteoclastogenesis associated with an altered OPG response. This inhibition was confined to cancellous bone. This study indicates that vitamin D-mediated osteoclastogenesis is regulated locally by OPG production in the mature osteoblast.

INTRODUCTION

Vitamin D stimulates osteoclastogenesis acting through its nuclear receptor (VDR) in immature osteoblast/stromal cells. This mobilization of calcium stores does not occur in a random manner, with bone preferentially removed from cancellous bone. The process whereby the systemic, humoral regulator is targeted to a particular region of the skeleton is unclear.

MATERIALS AND METHODS

Bone resorption was assessed in mice with vitamin D receptor transgenically elevated in mature osteoblasts (OSVDR). Vitamin D-mediated osteoclastogenesis was examined in vitro using OSVDR osteoblasts and osteoblastic RANKL: osteoprotegerin (OPG) examined in vivo and in vitro after vitamin D treatment.

RESULTS

Vitamin D-mediated osteoclastogenesis was reduced in OSVDR mice on chow and calcium-restricted diets, with effects confined to cancellous bone. OSVDR osteoblasts had a reduced capacity to support osteoclastogenesis in culture. The vitamin D-mediated reduction in OPG expression was reduced in OSVDR osteoblasts in vivo and in vitro, resulting in a reduced RANKL/OPG ratio in OSVDR compared with wildtype, after exposure to vitamin D.

CONCLUSIONS

Mature osteoblasts play an inhibitory role in bone resorption, with active vitamin D metabolites acting through the VDR to increase OPG. This inhibition is less active in cancellous bone, effectively targeting this region for resorption after the systemic release of activated vitamin D metabolites.

Both direct and collagen-mediated signals are required for active vitamin D3-elicited differentiation of human osteoblastic cells: roles of osterix, an osteoblast-related transcription factor

In order to investigate the mechanisms by which 1alpha,25(OH)2 vitamin D3 (VD3) stimulates the differentiation of human osteoblasts, we cultured MG-63, which is a human osteoblastic cell line, in the presence or absence of VD3 and/or L-ascorbic acid 2-phosphate (Asc 2-P), a long-acting vitamin C derivative. The cell growth rate was decreased by the presence of VD3 in the culture medium. Type I collagen synthesis and alkaline phosphatase (ALP) activity, which are markers of early stage osteoblast differentiation, were stimulated by the presence of VD3 as well as by that of Asc 2-P. The co-presence of Asc 2-P and VD3 had a synergistic effect on the collagen synthesis and ALP activity of the cells. Inhibition of collagen synthesis by the addition of inhibitors of collagen synthesis to the medium attenuated the stimulative effect of VD3 and Asc 2-P on the ALP activity. Transfection of the cells with siRNA-expressing vectors for COL1A1 decreased the expression level of ALP mRNA in addition to that of COL1A1. On the other hand, ALP activity was significantly increased, and the growth rate was decreased, when the cells were cultured on type I collagen-coated dishes. These effects were not seen when the cells were cultured on dishes coated with heat-denatured collagen. VD3 also increased the mRNA levels for Runx2 and osterix, which are transcription factors critical for osteoblast differentiation, as well as those of differentiation markers such as bone/liver/kidney type ALP, COL1A1, (the gene for the alpha1 chain of type I collagen), and osteocalcin, in the cells. Normal human osteoblasts and human bone marrow-derived mesenchymal stem cells (hBMSC) showed quite similar responses to VD3. These results indicate that VD3-stimulated gene expression of type I collagen and that mature type I collagen produced in the presence of Asc 2-P mediates at least a part of the stimulative effects of Asc 2-P and VD3 on the differentiation of these human osteoblastic cells. Levels of mRNAs for ALP and COL1A1 were increased, but the level of Runx2 was decreased, by the expression of osterix in MG-63 cells. These results also suggest that VD3 controls the growth and differentiation of human osteoblastic cells by regulating the gene expression of osteoblast-related transcription factors as well as that of type I collagen, and that the co-presence of both signals is essential for VD3 to express full activity toward the differentiation of human osteoblasts.

Visualizing levels of osteoblast differentiation by a two-color promoter-GFP strategy: Type I collagen-GFPcyan and osteocalcin-GFPtpz

A 3.9 kb DNA fragment of human osteocalcin promoter and 3.6 kb DNA fragment of the rat collagen type1a1 promoter linked with visually distinguishable GFP isomers, topaz and cyan, were used for multiplex analysis of osteoblast lineage progression. Three patterns of dual transgene expression can be appreciated in primary bone cell cultures derived from the transgenic mice and by histology of their corresponding bones. Our data support the interpretation that strong pOBCol3.6GFPcyan alone is found in newly formed osteoblasts, while strong pOBCol3.6GFPcyan and hOC-GFPtpz are present in osteoblasts actively making a new matrix. Osteoblasts expressing strong hOC-GFPtpz and weak pOBCol3.6GFPcyan are also present and may or may not be producing mineralized matrix. This multiplex approach reveals the heterogeneity within the mature osteoblast population that cannot be appreciated by current histological methods. It should be useful to identify and isolate populations of cells within an osteoblast lineage as they progress through stages of differentiation.

Large-scale in silico and microarray-based identification of direct 1,25-dihydroxyvitamin D3 target genes

1alpha,25-Dihydroxyvitamin D3 [1,25(OH)2D3] regulates calcium homeostasis and controls cellular differentiation and proliferation. The vitamin D receptor (VDR) is a ligand-regulated transcription factor that recognizes cognate vitamin D response elements (VDREs) formed by direct or everted repeats of PuG(G/T)TCA motifs separated by 3 or 6 bp (DR3 or ER6). Here, we have identified direct 1,25(OH)2D3 target genes by combining 35,000+ gene microarrays and genome-wide screens for consensus DR3 and ER6 elements, and DR3 elements containing single nucleotide substitutions. We find that the effect of a nucleotide substitution on VDR binding in vitro does not predict VDRE function in vivo, because substitutions that disrupted binding in vitro were found in several functional elements. Hu133A microarray analyses, performed with RNA from human SCC25 cells treated with 1,25(OH)2D3 and protein synthesis inhibitor cycloheximide, identified more than 900 regulated genes. VDREs lying within -10 to +5 kb of 5'-ends were assigned to 65% of these genes, and VDR binding was confirmed to several elements in vivo. A screen of the mouse genome identified more than 3000 conserved VDREs, and 158 human genes containing conserved elements were 1,25(OH2)D3-regulated on Hu133A microarrays. These experiments also revealed 16 VDREs in 11 of 12 genes induced more than 10-fold in our previous microarray study, five elements in the human gene encoding the epithelial calcium channel TRPV6, as well as novel 1,25(OH2)D3 target genes implicated in regulation of cell cycle progression. The combined approaches used here thus provide numerous insights into the direct target genes underlying the broad physiological actions of 1,25(OH)2D3.

Osteoblast-specific targeting of soluble colony-stimulating factor-1 increases cortical bone thickness in mice

The soluble and membrane-bound forms of CSF-1 are synthesized by osteoblasts and stromal cells in the bone microenvironment. Transgenic mice, generated to selectively express sCSF-1 in bone, showed increased cortical thickness in the femoral diaphysis caused by new bone formation along the endosteal surface. The ability of sCSF-1 to enhance bone cell activity in vivo is potentially relevant for increasing cortical bone in a variety of disorders.

INTRODUCTION

The soluble form of colony-stimulating factor-1 (sCSF-1) and the membrane-bound form of CSF-1 (mCSF-1) have been shown to support osteoclastogenesis in vitro; however, the effect of each peptide on bone remodeling in vivo is unclear. To determine the effect of sCSF-1, selectively expressed in bone, the skeletal phenotype of transgenic mice harboring the human sCSF-1 cDNA under the control of the osteocalcin promoter was assessed.

METHODS

At 5 and 14 weeks, mice were analyzed for CSF-1 protein levels, weighed, and X-rayed, and femurs were removed for peripheral quantitative computed tomography, histology, and histomorphometry.

RESULTS

High levels of human sCSF-1 were detected in bone extracts and, to a lesser extent, in plasma. Adult transgenic mice showed normal body weight and increased circulating monocytic cells. At 5 weeks, the femoral diaphysis was similar in CSF-1T and wt/wt littermates. However, by 14 weeks, the femoral diaphysis in CSF-1T mice showed increased cortical thickness and bone mineral density. In contrast to the diaphysis, the femoral metaphysis of CSF-1T mice showed normal cancellous bone comparable with wt/wt littermates at each time point. Histological sections demonstrated increased woven bone along the endosteal surface of the diaphysis and intracortical remodeling. Fluorochrome-labeling analysis confirmed endocortical bone formation in CSF-1T, with a 3.1-fold increase in the percentage of double-labeled surfaces and a 3.6-fold increase in the bone formation rate compared with wt/wt mice. Although remodeling resulted in a slightly porous cortex, sCSF-1 preferentially stimulated endocortical bone formation, leading to increased cortical thickness.

CONCLUSIONS

These findings indicate that sCSF-1 is a key determinant of bone cell activity in the corticoendosteal envelope.

Molecular imaging of the skeleton: quantitative real-time bioluminescence monitoring gene expression in bone repair and development

Monitoring gene expression in vivo, noninvasively, is a critical issue in effective gene therapy systems. To date, there are no adequate molecular imaging techniques, which quantitatively monitor gene expression in vivo in skeletal development and repair. The aim of this study was to monitor gene expression in skeletal development and repair, using a real-time molecular imaging system, which quantitatively and noninvasively detects bioluminescence in vivo. Our experimental model consisted of transgenic mice harboring the luciferase marker gene under the regulation of the human osteocalcin (hOC) promoter. A new light detection cooled charge coupled device (CCCD) camera was applied to monitor luciferase expression. In vitro, mesenchymal stem cells (MSCs) isolated from bone marrow of transgenic mice exhibited hOC promoter regulation, detected by luciferase expression that correlated with their osteogenic differentiation. During development from 1 week to 1.5 years, transgenic mice exhibited transgene expression in a wide spectrum of skeletal organs, including calvaria, vertebra, tail, and limbs, reaching a peak at 1 week in most of the skeletal organs. In two skeletal repair models, bone fracture and marrow ablation, the noninvasive CCCD system revealed a peak of luciferase expression at 6 days postsurgery. All quantitative, noninvasive, real-time CCCD measurements correlated with a luciferase biochemical assay and luciferase immunohistochemistry, which demonstrated luciferase expression in hypertrophic chondrocytes and trabecular osteoblasts. Our studies show for the first time (1) the CCCD detection system is a reliable quantitative gene detection tool for the skeleton in vivo, (2) expression of luciferase regulated by the hOC promoter is significantly decreased with age in most skeletal sites, and (3) the dynamics of hOC regulation during mice skeletal development and repair in real time, quantitatively and noninvasively.

In vitro differentiation of embryonic stem cells into mineralized osteoblasts

Embryonic stem cells are pluripotent cells derived from the inner cell mass of mouse blastocysts that have been shown to differentiate spontaneously into cell types representing all three germ layers. This study shows that ES cells were induced to differentiate in vitro into mineralized osteoblasts under the influence of ascorbic acid, beta-glycerophosphate and 1alpha,25-OH vitamin D3. The activity of alkaline phosphatase, an early osteoblast marker, was found to be increased around day 12 of culture. Mineralized cells were clearly identified by histochemical staining, which detects mineralized calcium. The major noncollagenous component of bone matrix, osteocalcin, was localized to the mineralized cells by immunofluorescence. The expression of bone-specific genes was analyzed by real-time quantitative PCR. Osteocalcin and bone sialoprotein (BSP) were identified as early as in the fourth week of embryonic stem cell culture, both being characteristic for late stages of osteoblastic differentiation, indicating that at this time of culture the identified cells represent "mature" osteoblasts. The osteoblast-specific transcription factor Cbfa1 was induced a few days earlier. The expression of osteopontin and osteonectin, both being involved in binding calcium ions and hydroxyapatite during mineralization processes, as well as of collagen type I, representing by far the most predominant collagen in vertebrate organisms, is enhanced at the beginning of the second culture week upon addition of supplements. In the third week of culture, treated cells showed a second peak of osteopontin, osteonectin and collagen type I expression, osteopontin and osteonectin being stimulated 3-4-fold and collagen type I being induced 6-fold over control values. Alkaline phosphatase (ALP) expression was enhanced at the beginning of the third week of culture and was found to be increased again at later stages of culture at days 27-34. The in vitro differentiation of mouse embryonic stem cells into osteoblasts may provide a suitable model for studying the molecular processes of osteoblastic development in vivo.

Osteoprotegerin reduces osteoclast numbers and prevents bone erosion in collagen-induced arthritis

Rheumatoid arthritis is characterized by progressive synovial inflammation and joint destruction. While matrix metalloproteinases (MMPs) are implicated in the erosion of unmineralized cartilage, bone destruction involves osteoclasts, the specialized cells that resorb calcified bone matrix. RANK ligand (RANKL) expressed by stromal cells and T cells, and its cognate receptor, RANK, were identified as a critical ligand-receptor pair for osteoclast differentiation and survival. A decoy receptor for RANKL, osteoprotegerin, (OPG) impinges on this system and regulates osteoclast numbers and activity. RANKL is also expressed in collagen-induced arthritis (CIA) in which focal collections of osteoclasts are prominent at sites of bone destruction. To determine the role of RANK signaling events in the effector phase of CIA, we investigated effects of Fc-osteoprotegerin fusion protein (Fc-OPG) in CIA. After induction of CIA in Dark Agouti rats, test animals were treated with or without Fc-OPG (3 mg/kg/day) subcutaneously for 5 days, beginning at the onset of disease. Paraffin-embedded joints were then analyzed histologically and the adjacent bone assessed by histomorphometry. Osteoclasts were identified using TRAP staining and expression of the mRNA for OPG and RANKL was identified by in situ hybridization. The results indicated that short-term Fc-OPG effectively prevented joint destruction, even though it had no impact on the inflammatory aspects of CIA. In arthritic joints, Fc-OPG depleted osteoclast numbers by over 75% and diminished bone erosion scores by over 60%. Although cartilage loss was also reduced by Fc-OPG, the effects on cartilage were less striking than those on bone. In arthritic joints OPG mRNA was highly expressed and co-localized with RANK ligand, and treatment with Fc-OPG did not affect the expression of endogenous RANKL or OPG mRNA. These data demonstrate that short term Fc-OPG treatment has powerful anti-erosive effects, principally on bone, even though synovitis is not affected. These findings indicate the potential utility of disrupting RANK signaling to preserve skeletal integrity in inflammatory arthritis.

Regulation of human osteocalcin promoter in hormone-independent human prostate cancer cells

Osteocalcin (OC) is a small (6 kDa) polypeptide whose expression was thought to be limited to mature osteoblasts. The discovery of OC expression in prostate cancer specimens led us to study the regulation of OC gene in androgen-independent metastatic human prostate PC3 cells. An 800-bp human OC (hOC) promoter-luciferase construct exhibited strong basal and vitamin D-induced activity in OC-positive human prostate and osteosarcoma cell lines. Through deletion analysis of the hOC promoter, the functional hierarchy of the cis-acting elements, OSE1, OSE2, and AP-1/VDRE, was established in PC3 cells (OSE1 > AP-1/VDRE > OSE2). By juxtaposing dimers of these 3 cis-elements, we produced a minimal hOC promoter capable of displaying high tissue specific activity in prostate cancer cells. Our study demonstrated three groups of transcription factors, Runx2, JunD/Fra-2, and Sp1, responsible for the high hOC promoter activity in PC3 cells by binding to the OSE2, AP-1/VDRE, and OSE1 elements, respectively. Among the three groups of transcription factors, the expression levels of Runx2 and Fra-2 are higher in the OC-positive PC3 cells and osteoblasts, compared with the OC-negative LNCaP cells. Interestingly, unlike the mouse OC promoter, the OSE1 site in hOC promoter is regulated by members of Sp1 family instead of the osteoblast-specific factor Osf1. The molecular basis for androgen-independent prostate cancer cells behaving like mature osteoblasts may be explained by the interplay and coordination of these transcription factors under the tight regulation of autocrine and paracrine mediators.

Establishment of an osteoid preosteocyte-like cell MLO-A5 that spontaneously mineralizes in culture

The mechanisms controlling the initiation of mineralization of bone matrix are not clear. To examine this process, we established a cell line called MLO-A5 that mineralizes in sheets, not nodules, within 3 days of culture in the presence of beta-glycerophosphate (beta-GP) and ascorbic acid and within 7 days in the absence of beta-GP and ascorbic acid. The mineral formed in both cases was shown to be bonelike apatite by Fourier transformed infrared (FTIR) spectroscopy. Mineral-to-matrix ratios (min/matrix) calculated from the FTIR data, which are related directly to ash weight, were approximately 0.4 in the absence of beta-GP and ascorbic acid and approximately 1.2 in the presence of beta-GP and ascorbic acid. By comparison, these ratios in fetal rat calvarial cells without beta-GP equal 0 and with beta-GP 1.9. This cell line and three others (MLO-A2, -D1, and -D6) were isolated from the long bones of transgenic mice expressing the large T-antigen driven by the osteocalcin promoter, the same mice from which the osteocyte-like cell line MLO-Y4 was isolated.(1) The cell lines were selected based on a dendritic or stellate morphology. MLO-A5 cells express high alkaline phosphatase, collagen type 1, parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor, bone sialoprotein (BSP), and osteocalcin (767 ng/10(6) cells compared with <1-2.2 ng/10(6) cell for primary mouse osteoblasts and five osteoblast cell lines). The single unique feature of the MLO-A5 cells compared with the other three nonmineralizing cell lines is the high expression of messenger RNA (mRNA) for BSP. These cell lines may represent stages of osteocyte differentiation and the MLO-A5 cells represent the postosteoblast, preosteocyte responsible for triggering mineralization of osteoid.

Cross-talk between 1,25-dihydroxyvitamin D3 and transforming growth factor-beta signaling requires binding of VDR and Smad3 proteins to their cognate DNA recognition elements

1,25-Dihydroxyvitamin D(3) (vitamin D) and transforming growth factor-beta (TGF-beta) regulate diverse biological processes including cell proliferation and differentiation through modulation of the expression of target genes. Members of the Smad family of proteins function as effectors of TGF-beta signaling pathways whereas the vitamin D receptor (VDR) confers vitamin D signaling. We investigated the molecular mechanisms by which TGF-beta and vitamin D signaling pathways interact in the regulation of the human osteocalcin promoter. Synergistic activation of the osteocalcin gene promoter by TGF-beta and vitamin D was observed in transient transfection experiments. However, in contrast to a previous report by Yanagisawa, J., Yanagi, Y., Masuhiro, Y., Suzawa, M., Watanabe, M., Kashiwagi, K., Toriyabe, T., Kawabata, M., Miyazono, K., and Kato, S. (1999) Science, 283, 1317-1321, synergistic activation was not detectable when the osteocalcin vitamin D response element (VDRE) alone was linked to a heterologous promoter. Inclusion of the Smad binding elements (SBEs) with the VDRE in the heterologous promoter restored synergistic activation. Furthermore, this synergy was dependent on the spacing between VDRE and SBEs. The Smad3-Smad4 heterodimer was found to bind in gel shift assay to two distinct DNA segments of the osteocalcin promoter: -1030 to -989 (SBE3) and -418 to -349 (SBE1). Deletion of SBE1, which is proximal to the VDRE, but not the distal SBE3 in this promoter reporter abolished TGF-beta responsiveness and eliminated synergistic co-activation with vitamin D. Thus the molecular mechanism, whereby Smad3 and VDR mediate cross-talk between the TGF-beta and vitamin D signaling pathways, requires both a VDRE and a SBE located in close proximity to the target promoter.

Increased formation and decreased resorption of bone in mice with elevated vitamin D receptor in mature cells of the osteoblastic lineage

The microarchitecture of bone is regulated by complex interactions between the bone-forming and resorbing cells, and several compounds regulate both actions. For example, vitamin D, which is required for bone mineralization, also stimulates bone resorption. Transgenic mice overexpressing the vitamin D receptor solely in mature cells of the osteoblastic bone-forming lineage were generated to test the potential therapeutic value of shifting the balance of vitamin D activity in favor of bone formation. Cortical bone was 5% wider and 15% stronger in these mice due to a doubling of periosteal mineral apposition rate without altered body weight or calcium homeostatic hormone levels. A 20% increase in trabecular bone volume in transgenic vertebrae was also observed, unexpectedly associated with a 30% reduction in resorption surface rather than greater bone formation. These findings indicate anabolic vitamin D activity in bone and identify a previously unknown pathway from mature osteoblastic cells to inhibit osteoclastic bone resorption, counterbalancing the known stimulatory action through immature osteoblastic cells. A therapeutic approach that both stimulates cortical anabolic and inhibits trabecular resorptive pathways would be ideal for treatment of osteoporosis and other osteopenic disorders.

Species-divergent regulation of human and mouse osteocalcin genes by calciotropic hormones

Although osteocalcin is the most abundant noncollagenous protein in bone, its role remains undefined. Recent studies have reported diametrically opposing responses in the vitamin D regulation of the mouse vs the human and rat osteocalcin genes. The aim of this study was to increase the understanding of these differences and further elucidate the physiological function and regulation of osteocalcin. Direct comparison of the regulation of both the endogenous mouse osteocalcin gene (mOC) and a human osteocalcin promoter-chloramphenicol acetyl transferase (hOC-CAT) reporter as integrated templates was undertaken in primary osteoblastic cultures from OSCAT transgenic mice. Expression of both genes was up-regulated with the onset of mineralization. Long-term chronic 1, 25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) treatment and acute (2 day) PTH treatment inhibited both mOC and hOC-CAT expression. At all stages of osteoblastic development studied, hOC-CAT was up-regulated by acute 1,25-(OH)(2)D(3), whereas mOC was unaffected or inhibited. Mouse osteopontin was strongly up-regulated by acute 1, 25-(OH)(2)D(3) treatment. Thus, the divergence of the osteocalcin responses to 1,25-(OH)(2)D(3) is specific for the osteocalcin gene and for an acute 1,25-(OH)(2)D(3) treatment regime. Elucidation of this unique aspect of bone physiology will provide valuable insights into the still incompletely understood roles of osteocalcin and 1, 25-(OH)(2)D(3) in bone.

Bone tissue specific transcriptional control: options for targeting gene therapy to the skeleton

BACKGROUND

The transplantation of multipotential bone marrow cells containing bone tissue specific promoter-controlled transgenes provides an efficacious approach to deliver therapeutic gene expression to osteoblasts for the treatment of patients with bone disorders or tumor metastasis to the skeleton. The specificity of tissue-restricted gene therapy can be refined by utilization of a 31-amino-acid segment of the hematopoietic and osteogenic AML/CBF transcription factors that direct the regulatory proteins to subnuclear sites that support gene expression.

METHODS

Unfractionated adherent bone marrow cells from transgenic mice constructed with the proximal 1.7 kb of the osteocalcin gene promoter fused to a CAT reporter were transplanted by intravenous infusion. Engraftment and expression at the single-cell level within the context of tissue organization was established by immunohistochemistry using an anti-CAT antibody. Sequences that support the intranuclear trafficking of AML/CBF transcription factors to subnuclear sites that support transcription were determined by the expression and visualization of mutated and epitope tagged AML/CBF proteins.

RESULTS

Immunohistochemical staining of an extensive series of tissue sections from mice posttransplantation using an anti-CAT antibody indicated that CAT-positive osteoblasts and osteocytes were present in bone sections. These findings indicate that donor bone marrow-derived cells engraft in bone tissue in an environment that supports maturation to the developmental stage at which a bone specific osteocalcin promoter is transcriptionally active. Characterization of functional domains in AML/CBF transcription factors has established that there are at least two regulated events that are required for targeting the factors to transcriptionally active nuclear domains: A nuclear localization signal in the amino terminal region controls nuclear import and retention, and a nuclear matrix targeting signal in the carboxyl region controls association with nuclear matrix-linked sites where transcription occurs.

CONCLUSIONS

The specificity of hematopoietic and bone phenotypic promoters, together with the additional level of specificity inherent in the AML/CBF family of hematopoietic and osteogenic intranuclear targeting signals, offers viable options for constructing gene therapy regimens that are targeted to the skeleton for the control of metastatic disease. It is realistic to anticipate that, as additional parameters of gene regulatory mechanisms are defined, particularly components of transcriptional control that are operative within a three-dimensional context of nuclear architecture, opportunities for enhancing the effectiveness of treating patients with tumors that metastasize to bone will be extended.

Osteoblast-specific gene expression after transplantation of marrow cells: implications for skeletal gene therapy

Somatic gene therapies require targeted transfer of the therapeutic gene(s) into stem cells that proliferate and then differentiate and express the gene in a tissue-restricted manner. We have developed an approach for gene therapy using marrow cells that takes advantage of the osteoblast specificity of the osteocalcin promoter to confine expression of chimeric genes to bone. Adherent marrow cells, carrying a reporter gene [chloramphenicol acetyltransferase (CAT)] under the control of a 1.7-kilobase rat osteocalcin gene promoter, were expanded ex vivo. After transplantation by intravenous infusion, engrafted donor cells in recipient mice were detected by the presence of the transgene in a broad spectrum of tissues. However, expression of the transgene was restricted to osteoblasts and osteocytes, as established by biochemical analysis of CAT activity and immunohistochemical analysis of CAT expression at the single cell level. Our data indicate that donor cells achieved long-term engraftment in various tissues of the recipients and that the CAT gene under control of the osteocalcin promoter is expressed specifically in bone. Thus, transplantation of multipotential marrow cells containing the osteocalcin promoter-controlled transgene provides an efficacious approach to deliver therapeutic gene expression to osteoblasts for treatment of bone disorders or tumor metastasis to the skeleton.

Expression of rat homeobox gene, rHOX, in developing and adult tissues in mice and regulation of its mRNA expression in osteoblasts by bone morphogenetic protein 2 and parathyroid hormone-related protein

The rat homeobox gene, rHox, was cloned from a rat osteosarcoma cDNA library. Southwestern and gel mobility shift analyses showed that rHox binds to the promoter regions of collagen (alpha1)I and osteocalcin genes while transient transfection with rHox resulted in repression of their respective promoter activities. In situ hybridization studies showed that rHox mRNA was widely expressed in osteoblasts, chondrocytes, skeletal muscle, skin epidermis, and bronchial and intestinal epithelial cells, as well as cardiac muscle in embryonic and newborn mice. However in 3-month-old mice, rHox mRNA expression was restricted to osteoblasts, megakaryocytes, and myocardium. Bone morphogenetic protein 2, a growth factor that commits mesenchymal progenitor cells to differentiate into osteoblasts, down-regulated rHox mRNA expression by 40-50% in UMR 201, a rat preosteoblast cell line, in a time- and dose-dependent manner. In contrast, PTH-related protein (PTHrP), recently shown to be a negative regulator of chondrocyte differentiation, significantly enhanced rHox mRNA expression in UMR 106-06 osteoblastic cells by 3-fold at 24 h while at the same time down-regulating expression of pro-alpha1(I) collagen mRNA by 60%. Expression of rHox mRNA in calvarial osteoblasts derived from PTHrP -/- mice was approximately 15% of that observed in similar cells obtained from normal mice. In conclusion, current evidence suggests that rHox acts as a negative regulator of osteoblast differentiation. Furthermore, down-regulation of rHox mRNA by bone morphogenetic protein 2 and its up-regulation by PTHrP support a role of the homeodomain protein, rHox, in osteoblast differentiation.