Leptin increases osteoblast-specific osteocalcin release through a hypothalamic relay

Enhanced long-term expression of leptin by gene therapy selectively in the hypothalamus, without leakage to the systemic circulation, abrogated skeletal abnormalities and reinstated weight and insulin-glucose homeostasis in leptin-deficient ob/ob mice. Whether increases in osteocalcin, a hormone produced by osteoblasts and known to play a role in bone growth and recently in glucose-insulin homeostasis, may link these benefits of central leptin was assessed. The effects of a single intraventricular injection of non-immunogenic, non-pathogenic recombinant adeno-associated virus vector encoding leptin gene (rAAV-lep) or green fluorescent protein gene (rAAV-GFP, control) were studied in three genotypes, wild type (wt), obese diabetic, hyperinsulinemic ob/ob and non-obese, diabetic insulinopenic Akita mice. Selective hypothalamic leptin expression with central rAAV-lep treatment decreased weight, fat mass, food intake, suppressed insulin levels in ob/ob and wt mice, and conferred euglycemia by suppressing blood glucose in all three genotypes. Contemporaneously, rAAV-lep treatment also augmented blood osteocalcin levels. In wt mice, osteocalcin rose by 51% and, whereas, basal osteocalcin levels in ob/ob and Akita mice were significantly lower as compared to those in wt mice (26% and 55%, respectively), gene therapy reinstated levels to the control range in ob/ob mice, and raised 40% above the wt range even in the absence of insulin in Akita mice. These findings demonstrate that the central beneficial effects of leptin on bone growth involve increased hypothalamic relay of signals that augment osteocalcin efflux from osteoblasts into the general circulation, a response that, in turn, may also modulate glucose-insulin and weight homeostasis.

Zfp521 antagonizes Runx2, delays osteoblast differentiation in vitro, and promotes bone formation in vivo

Zfp521, a 30 C2H2 Kruppel-like zinc finger protein, is expressed at high levels at the periphery of early mesenchymal condensations prefiguring skeletal elements and in all developing bones in the perichondrium and periosteum, in osteoblast precursors and osteocytes, and in chondroblast precursors and growth plate prehypertrophic chondrocytes. Zfp521 expression in cultured mesenchymal cells is decreased by BMP-2 and increased by PTHrP, which promote and antagonize osteoblast differentiation, respectively. In vitro, Zfp521 overexpression reduces the expression of several downstream osteoblast marker genes and antagonizes osteoblast differentiation. Zfp521 binds Runx2 and represses its transcriptional activity, and Runx2 dose-dependently rescues Zfp521's inhibition of osteoblast differentiation. In contrast, osteocalcin promoter-targeted overexpression of Zfp521 in osteoblasts in vivo results in increased bone formation and bone mass. We propose that Zfp521 regulates the rate of osteoblast differentiation and bone formation during development and in the mature skeleton, in part by antagonizing Runx2.

Pref-1 regulates mesenchymal cell commitment and differentiation through Sox9

Pref-1 is an EGF repeat-containing transmembrane protein that produces a biologically active soluble form by TACE-mediated cleavage. Although Pref-1 inhibition of adipogenesis has been well established, the specific target(s) of Pref-1 or the Pref-1 function in mesenchymal cell commitment/differentiation are not known. Here, we show that Sox9 downregulation is required for adipocyte differentiation and that Pref-1 inhibits adipocyte differentiation through upregulating Sox9 expression. Sox9 directly binds to the promoter regions of C/EBPbeta and C/EBPdelta to suppress their promoter activity, preventing adipocyte differentiation. Furthermore, we also show that, by inducing Sox9, Pref-1 promotes chondrogenic induction of mesenchymal cells but prevents chondrocyte maturation as well as osteoblast differentiation, with supporting in vivo evidence in Pref-1 null and Pref-1 transgenic mice. Thus, Sox9 is a Pref-1 target, and Pref-1 directs multipotent mesenchymal cells to the chondrogenic lineage but inhibits differentiation into adipocytes as well as osteoblasts and chondrocytes.

Metabolic acidosis increases intracellular calcium in bone cells through activation of the proton receptor OGR1

Metabolic acidosis increases urine Ca without increasing intestinal absorption, leading to bone Ca loss. It is unclear how bone cells detect the increase in proton concentration. To determine which G protein-coupled proton sensing receptors are expressed in bone, PCR was performed, and products were detected for OGR1, TDAG8, G2A, and GPR4. We tested the hypothesis that the G protein-coupled proton sensor, OGR1, is an H(+)-sensing receptor in bone. To determine whether acid-induced bone resorption involves OGR1, we incubated mouse calvariae in neutral pH (NTL) or acidic (MET) medium +/- the OGR1 inhibitor CuCl(2). CuCl(2) decreased MET-induced Ca efflux. We used fluorescent imaging of perfused bone cells to determine whether MET increases Ca(i). Perfusion with MET induced a rapid, flow-independent, increase in Ca(i) in individual bone cells. To determine whether transfection of OGR1 into a heterologous cell type would increase Ca(i) in response to H(+), we perfused Chinese hamster ovary (CHO) cells transfected with mouse OGR1 cDNA. Perfusion with MET induced a rapid increase in Ca(i) in OGR1-transfected CHO cells. These data indicate that OGR1 induces an increase in Ca(i) in response to MET and is a prime candidate for an osteoblast proton sensor.

Endogenous bone morphogenetic proteins mediate 1alpha, 25-dihydroxyvitamin D(3)-induced expression of osteoblast differentiation markers in human dermal fibroblasts

Human dermal fibroblasts are generally considered to be restricted to a fibroblastic lineage. Although dermal fibroblasts do not typically express markers of osteoblastic differentiation, they have previously been shown to undergo osteoinduction when stimulated with bone morphogenetic proteins (BMPs) or vitamin D(3). However, involvement of BMP signaling in vitamin D(3)-mediated osteoinduction has not been reported. In this study, human dermal fibroblasts were cultured in chemically defined medium containing vitamin D(3), in the presence of the BMP antagonist noggin or neutralizing antibodies specific for BMP-4 or BMP-6, and characterized for markers of osteoblastic differentiation. Treatment of dermal fibroblasts with vitamin D(3) induced expression of BMP-4 (1.2 +/- 0.2, 1.7 +/- 0.2, and 1.8 +/- 0.2 relative fold increase) and BMP-6 (9.1 +/- 0.3, 23.3 +/- 2.1, and 30.4 +/- 3.0 relative fold increase) at 3, 14, and 21 days, respectively. Vitamin D(3) was also shown to induce the expression of the osteoblast-specific markers, alkaline phosphatase and osteocalcin, in a dose-dependent manner in human dermal fibroblasts. Addition of noggin, BMP-4 antibodies, and BMP-6 antibodies resulted in a downregulation of alkaline phosphatase activity (by 42%, 22%, and 20%, respectively) and secreted osteocalcin (by 20%, 31%, and 49%, respectively) after 21 days in culture. However, blocking BMP signaling did not result in complete recovery of a fibroblastic phenotype. Taken together, these results suggest that BMP signaling plays a role in the induction of an osteoblastic phenotype in human dermal fibroblasts in response to vitamin D(3) stimulation.

Ethanol impairs estrogen receptor signaling resulting in accelerated activation of senescence pathways, whereas estradiol attenuates the effects of ethanol in osteoblasts

Epidemiological and animal studies have suggested that chronic alcohol consumption is a major risk factor for osteoporosis. Using bone from cycling female rats infused chronically with ethanol (EtOH) in vivo and osteoblastic cells in vitro, we found that EtOH significantly increased estrogen receptor alpha (ERalpha) and beta (ERbeta) mRNA and ERalpha protein levels. Treatment with 17beta-estradiol (E2) in vivo and in vitro interfered with these effects of EtOH on bone and osteoblastic cells. ERalpha agonist propylpyrazoletriol (PPT) and ERbeta agonist diarylpropionitrile (DPN) attenuated EtOH-induced ERalpha and ERbeta gene overexpression, respectively. Similar to the ER antagonist ICI 182780, EtOH blocked nuclear translocation of ERalpha-ECFP in the presence of E2 in UMR-106 osteoblastic cells. EtOH also downregulated ERE-luc reporter activity. On the other hand, EtOH by itself upregulated some common ERalpha- and ERbeta-mediated genes apparently by an ER-independent pathway. EtOH also transactivated the luciferase activity of the p21 promoter region independent of additional exogenous ERalpha, activated p21 and p53, and stimulated senescence-associated beta-galactosidase activity in rat stromal osteoblasts. E2 treatment attenuated these EtOH actions. We conclude that inhibitory cross-talk between EtOH and E2 in osteoblasts on ERs, p53/p21, and cell senescence provides a pathophysiologic mechanism underlying bone loss and the protective effects of estrogens in alcohol-exposed females.

Osteoclasts have multiple roles in bone in addition to bone resorption

Osteoclasts are the cells that degrade bone to initiate normal bone remodeling and mediate bone loss in pathologic conditions by increasing their resorptive activity. They are derived from precursors in the myeloid/ monocyte lineage that circulate in the blood after their formation in the bone marrow. These osteoclast precursors (OCPs) are attracted to sites on bone surfaces destined for resorption and fuse with one another to form the multinucleated cells that resorb calcified matrixes under the influence of osteoblastic cells in bone marrow. Recent studies have identified functions for OCPs and osteoclasts in and around bone other than bone resorption. For example, they regulate the differentiation of osteoblast precursors and the movement of hematopoietic stem cells from the bone marrow to the bloodstream; they participate in immune responses, and secrete cytokines that can affect their own functions and those of other cells in inflammatory and neoplastic processes affecting bone. Here, we review these findings, which define new roles for osteoclasts and OCPs in the growing field of osteoimmunology and in common pathologic conditions in which bone resorption is increased.

Adipocyte differentiation of bone marrow-derived mesenchymal stem cells: cross talk with the osteoblastogenic program

Bone marrow mesenchymal stem cells (MSCs) are multipotent cells, which among other cell lineages, give rise to adipocytes and osteoblasts. Within the bone marrow, the differentiation of MSCs into adipocytes or osteoblasts is competitively balanced; mechanisms that promote one cell fate actively suppress mechanisms that induce the alternative lineage. This occurs through the cross talk between complex signaling pathways including those derived from bone morphogenic proteins (BMPs), winglesstype MMTV integration site (Wnt) proteins, hedgehogs, delta/jagged proteins, fibroblastic growth factors (FGF), insulin, insulin-like growth factors (IGF), and transcriptional regulators of adipocyte and osteoblast differentiation including peroxisome proliferator-activated receptor-gamma (PPAR gamma) and runt-related transcription factor 2 (Runx2). Here, we discuss the molecular regulation of bone marrow adipogenesis with emphasis on signals that interact with osteoblastogenic pathways and highlight the possible therapeutic implications of these interactions.

Age-dependent Wnt gene expression in bone and during the course of osteoblast differentiation

Wnt signaling is vital for osteoblast differentiation and recently has been associated with aging. Because impaired osteoblastogenesis is a cellular characteristic of age-induced bone loss, we investigated whether this process is associated with an altered expression of Wnt signaling-related proteins in bone and osteoblasts. Bone marrow cells were isolated from male C57BL/6 mice, aged 6 weeks, 6 months, and 18 months, respectively. Osteogenic differentiation was induced for 3 weeks and assessed using alizarin red staining. Gene expression of Wnt1, 3a, 4, 5a, 5b, 7b, 9b, 10b, lipoprotein receptor-related protein (LRP)-5/6, as well as dickkopf-1 (Dkk-1), sclerostin, and secreted frizzled related protein-1 (sFRP-1) was determined in bone tissue and osteoblasts on days 7, 14, and 21 by real-time RT-PCR. Osteoblast differentiation was significantly reduced in aged mice compared with young and adult mice. In bone tissue, expression levels of all genes assessed were decreased in adult and old mice, respectively, compared with young mice. Mature osteoblasts of aged compared with those of young mice showed enhanced expression of Wnt9b, LRP-6, and Dkk-1, and decreased expression of Wnt5a and 7b. In early osteoblasts, mRNA levels of Wnt1, 5a, 5b, and 7b were increased significantly in aged mice. The expression of Wnt3a, 4, LRP-5, and sclerostin was not altered in aged osteoblasts. In conclusion, osteoblastic expression of each Wnt-related protein is regulated individually by aging. The overall decreased expression of Wnt-related proteins in bone tissue of aged mice underlines the newly discovered association of Wnt signaling with aging.

Progressive recruitment of Runx2 to genomic targets despite decreasing expression during osteoblast differentiation

The mRNAs encoding Runx2, a master osteoblast transcription factor, and its target gene Osteocalcin (OC), are commonly used as markers of osteoblast differentiation. We found that while OC mRNA levels do indeed increase during development of the osteoblast phenotype in MC3T3-E1 cultures, Runx2 mRNA levels surprisingly decrease. Neither translational control of Runx2 (based on Western analysis) nor regulation of its DNA-binding ability (assessed by electrophoretic mobility shift assay) could explain the unexpected opposite patterns of Runx2 and OC expression. Instead, a series of chromatin immunoprecipitation (ChIP) assays during osteoblast differentiation revealed that early on, when Runx2 protein amount and DNA-binding activity are maximal, it is practically absent from the OC promoter. At later stages, Runx2 is recruited to the OC promoter while Runx2 mRNA, protein, and in vitro DNA binding progressively decrease. We also followed Runx2 occupancy at a novel genomic target discovered by ChIP-Chip analysis of cells in which the OC promoter is maximally occupied. The results revealed that Runx2 is recruited to this locus and to the OC promoter with a remarkably similar temporal pattern. These observations highlight a mechanism that restrains Runx2-mediated transcriptional control by confining its access to genomic targets to a narrow window of time. The need for such stringent control is consistent with the severe consequences of Runx2 over-expression in vivo.

Overexpression of bone sialoprotein leads to an uncoupling of bone formation and bone resorption in mice

The purpose of this study was to determine the effects of bone sialoprotein (BSP) overexpression in bone metabolism in vivo by using a homozygous transgenic mouse line that constitutively overexpresses mouse BSP cDNA driven by the cytomegalovirus (CMV) promoter. CMV-BSP transgenic (TG) mice and wildtype mice were weighed, and their length, BMD, and trabecular bone volume were measured. Serum levels of RANKL, osteocalcin, osteoprotegerin (OPG), TRACP5b, and PTH were determined. Bone histomorphometry, von Kossa staining, RT-PCR analysis, Western blot, MTS assay, in vitro mineralization assay, and TRACP staining were also performed to delineate phenotypes of this transgenic mouse line. Compared with wildtype mice, adult TG mice exhibit mild dwarfism, lower values of BMD, and lower trabecular bone volume. TG mice serum contained increased calcium levels and decreased PTH levels, whereas the levels of phosphorus and magnesium were within normal limits. TG mice serum also exhibited lower levels of osteoblast differentiation markers and higher levels of markers, indicating osteoclastic activity and bone resorption. H&E staining, TRACP staining, and bone histomorphometry showed that adult TG bones were thinner and the number of giant osteoclasts in TG mice was higher, whereas there were no significant alterations in osteoblast numbers between TG mice and WT mice. Furthermore, the vertical length of the hypertrophic zone in TG mice was slightly enlarged. Moreover, ex vivo experiments indicated that overexpression of BSP decreased osteoblast population and increased osteoclastic activity. Partly because of its effects in enhancing osteoclastic activity and decreasing osteoblast population, BSP overexpression leads to an uncoupling of bone formation and resorption, which in turn results in osteopenia and mild dwarfism in mice. These findings are expected to help the development of therapies to metabolic bone diseases characterized by high serum level of BSP.

Synergistic induction of CX3CL1 by TNF alpha and IFN gamma in osteoblasts from rheumatoid arthritis: involvement of NF-kappa B and STAT-1 signaling pathways

To explore the regulation of CX3CL1 in inflammatory bone diseases, CX3CL1 expression by osteoblasts (OB) was examined. Human OB isolated from rheumatoid arthritis (RA) patients, osteoarthritis patients, and normal individuals were incubated in the presence of cytokines. Soluble CX3CL1 levels were determined with an enzyme-linked immunosorbent assay. Expression of CX3CL1 mRNA was examined using quantitative real-time polymerase chain reaction. Although tumor necrosis factor (TNF)-α or interferon (IFN)-γ alone RA OB induced negligible CX3CL1 secretion, the combination of TNF-α and IFN-γ induced dramatic increases in both soluble CX3CL1 protein and mRNA transcripts. This synergistic effect was more pronounced in OB from RA than in OB from either osteoarthritis or normal individuals. The expression of CX3CL1 was markedly reduced by specific inhibitors of the nuclear factor-κB (NF-κB) or STAT-1 transcription factor. These findings suggest that osteoblasts are an important cellular source of CX3CL1 and may play roles in inflammatory bone/joint diseases.

Eph receptors and ephrin signaling pathways: a role in bone homeostasis

The maintenance of bone homeostasis is tightly controlled, and largely dependent upon cellular communication between osteoclasts and osteoblasts, and the coupling of bone resorption to bone formation. This tight coupling is essential for the correct function and maintenance of the skeletal system, repairing microscopic skeletal damage and replacing aged bone. A range of pathologic diseases, including osteoporosis and cancer-induced bone disease, disrupt this coupling and cause subsequent alterations in bone homeostasis. Eph receptors and their associated ligands, ephrins, play critical roles in a number of cellular processes including immune regulation, neuronal development and cancer metastasis. Eph receptors are also expressed by cells found within the bone marrow microenvironment, including osteoclasts and osteoblasts, and there is increasing evidence to implicate this family of receptors in the control of normal and pathological bone remodeling.

Use of an alpha-smooth muscle actin GFP reporter to identify an osteoprogenitor population

Identification of a reliable marker of skeletal precursor cells within calcified and soft tissues remains a major challenge for the field. To address this, we used a transgenic model in which osteoblasts can be eliminated by pharmacological treatment. Following osteoblast ablation a dramatic increase in a population of alpha-smooth muscle actin (alpha-SMA) positive cells was observed. During early recovery phase from ablation we have detected cells with the simultaneous expression of alpha-SMA and a preosteoblastic 3.6GFP marker, indicating the potential for transition of alpha-SMA+ cells towards osteoprogenitor lineage. Utilizing alpha-SMAGFP transgene, alpha-SMAGFP+ positive cells were detected in the microvasculature and in the osteoprogenitor population within bone marrow stromal cells. Osteogenic and adipogenic induction stimulated expression of bone and fat markers in the alpha-SMAGFP+ population derived from bone marrow or adipose tissue. In adipose tissue, alpha-SMA+ cells were localized within the smooth muscle cell layer and in pericytes. After in vitro expansion, alpha-SMA+/CD45-/Sca1+ progenitors were highly enriched. Following cell sorting and transplantation of expanded pericyte/myofibroblast populations, donor-derived differentiated osteoblasts and new bone formation was detected. Our results show that cells with a pericyte/myofibroblast phenotype have the potential to differentiate into functional osteoblasts.

Cadherin-11 promotes the metastasis of prostate cancer cells to bone

Bone is the most common site of metastases from prostate cancer. The mechanism by which prostate cancer cells metastasize to bone is not fully understood, but interactions between prostate cancer cells and bone cells are thought to initiate the colonization of metastatic cells at that site. Here, we show that cadherin-11 (also known as osteoblast-cadherin) was highly expressed in prostate cancer cell line derived from bone metastases and had strong homophilic binding to recombinant cadherin-11 in vitro. Down-regulation of cadherin-11 in bone metastasis-derived PC3 cells with cadherin-11-specific short hairpin RNA (PC3-shCad-11) significantly decreased the adhesion of those cells to cadherin-11 in vitro. In a mouse model of metastasis, intracardiac injection of PC3 cells led to metastasis of those cells to bone. However, the incidence of PC3 metastasis to bone in this model was reduced greatly when the expression of cadherin-11 by those cells was silenced. The clinical relevance of cadherin-11 in prostate cancer metastases was further studied by examining the expression of cadherin-11 in human prostate cancer specimens. Cadherin-11 was not expressed by normal prostate epithelial cells but was detected in prostate cancer, with its expression increasing from primary to metastatic disease in lymph nodes and especially bone. Cadherin-11 expression was not detected in metastatic lesions that occur in other organs. Collectively, these findings suggest that cadherin-11 is involved in the metastasis of prostate cancer cells to bone.

Vitamin D receptor-dependent 1 alpha,25(OH)2 vitamin D3-induced anti-apoptotic PI3K/AKT signaling in osteoblasts

Osteoblast apoptosis plays a crucial role in bone remodeling. Physiological doses of 1 alpha,25(OH)(2)-vitamin D(3) (1,25D) protect osteoblasts against apoptosis by means of mechanisms only partially understood. We studied activation of an Akt survival cascade downstream of 1,25D nongenomic stimulation of phosphatidylinositide-3'-kinase (PI3K) in osteoblastic cells. We measured a dose- and time-dependent 1,25D induction of Akt phosphorylation (p-Akt) in cultured osteoblastic cells. Maximal response was achieved with 10 nM 1,25D after 5 min. We found that staurosporine (STSP)-induced apoptosis was significantly reduced in 1,25D-pretreated osteoblasts. 1,25D prosurvival effects were abolished when cells were preincubated with inhibitors of PI3K activation. By means of siRNA silencing, we proved that 1,25D induction of p-Akt requires a classic vitamin D receptor (VDR) in osteoblasts. Furthermore, non-osteoblastic CV-1 cells transfected with an enhanced green fluorescent protein (EGFP)-VDR construct responded to 1,25D treatment with a rapid p-Akt response associated with increased cell survival not detected in native, nontransfected cells. We measured increased levels of p-Akt substrates p-Bad and p-FKHR and significantly reduced activity of caspases 8 and 3/7 after 1,25D treatment. In addition, 1,25D-induced protection against apoptosis was abolished when osteoblasts were preincubated with pertussis toxin. We conclude that anti-apoptotic effects of 1,25D in osteoblasts occur through nongenomic activation of a VDR/PI3K/Akt survival pathway that includes phosphorylation of multiple p-Akt substrates and reduction of caspase activities.

BMP-2 vs. BMP-4 expression and activity in glucocorticoid-arrested MC3T3-E1 osteoblasts: Smad signaling, not alkaline phosphatase activity, predicts rescue of mineralization

Pharmacological glucocorticoids (GCs) inhibit bone formation, leading to osteoporosis. GCs inhibit bone morphogenetic protein-2 (Bmp2) expression, and rhBMP-2 restores mineralization in GC-arrested osteoblast cultures. To better understand how GCs regulate BMPs, we investigated Bmp transcription, as well as rhBMP-induced Smad and alkaline phosphatase (ALP) activity. Bmp2 cis-regulatory regions were analyzed by reporter plasmids and LacZ-containing bacterial artificial chromosomes. We found that GCs inhibited Bmp2 via a domain > 50 kb downstream of the coding sequence. Bmp expression was evaluated by RT-PCR; whereas GCs strongly inhibited Bmp2, Bmp4 was abundantly expressed and resistant to GCs. Both rhBMP-2 and rhBMP-4 restored mineralization in GC-arrested cultures; rhBMP-2 was 5-fold more effective when dosing was based on ALP activation, however, the rhBMPs were equipotent when dosing was based on Smad transactivation. In conclusion, GCs regulate Bmp2 via a far-downstream domain, and activation of Smad, not ALP, best predicts the pro-mineralization potential of rhBMPs.

Osteoimmunology: cytokines and the skeletal system

It has become clear that complex interactions underlie the relationship between the skeletal and immune systems. This is particularly true for the development of immune cells in the bone marrow as well as the functions of bone cells in skeletal homeostasis and pathologies. Because these two disciplines developed independently, investigators with an interest in either often do not fully appreciate the influence of the other system on the functions of the tissue that they are studying. With these issues in mind, this review will focus on several key areas that are mediated by crosstalk between the bone and immune systems. A more complete appreciation of the interactions between immune and bone cells should lead to better therapeutic strategies for diseases that affect either or both systems.

Oxygen tension regulates preosteocyte maturation and mineralization

Oxygen availability is a critical signal for proper development of many tissues, however there is limited knowledge of its role in the maturation of bone cells. To test the hypothesis that low pO2 regulates bone cell mineralization, MLO-A5 and MLO-Y4 cells were cultured in monolayer and three-dimensional alginate scaffolds in hypoxia (2% O2) or normoxia (20% O2). Hypoxia reduced mineralization and decreased alkaline phosphatase activity of preosteocyte-like MLO-A5 cells in both monolayer and alginate cultures. Similar changes in osteogenic activity were seen when the were subjected to chemical hypoxia. Likewise, Osteocyte-like MLO-Y4 cells also exhibited reduced osteogenic activity in hypoxia relative to normoxic controls. Based on these observations, it is concluded that a low pO2 decreased the mineralization potential of bone cells at both early and late stages of maturation. Since the oxemic state is transduced by the transcription factor, HIF-1alpha, experiments were performed to determine if this protein was responsible for the observed changes in mineral formation. It was noted that when HIF-1alpha was silenced, mineralization activities were not restored. Indeed, in hypoxia, in relationship to wild type controls, the mineralization potential of the knockdown cells was further reduced. Based on these findings, it is concluded that the osteogenic activity of preosteocyte-like cells is dependent on both the O2 tension and the expression of HIF-1alpha.

The study of abnormal bone development in the Apert syndrome Fgfr2+/S252W mouse using a 3D hydrogel culture model

Apert syndrome is caused by mutations in fibroblast growth factor receptor 2 (Fgfr2) and is characterized by craniosynostosis and other skeletal abnormalities. The Apert syndrome Fgfr2+/S252W mouse model exhibits perinatal lethality. A 3D hydrogel culture model, derived from tissue engineering strategies, was used to extend the study of the effect of the Fgfr2+/S252W mutation in differentiating osteoblasts postnatally. We isolated cells from the long bones of Apert Fgfr2+/S252W mice (n=6) and cells from the wild-type sibling mice (n=6) to be used as controls. During monolayer expansion, Fgfr2+/S252W cells demonstrated increased proliferation and ALP activity, as well as altered responses of these cellular functions in the presence of FGF ligands with different binding specificity (FGF2 or FGF10). To better mimic the in vivo disease development scenario, cells were also encapsulated in 3D hydrogels and their phenotype in 3D in vitro culture was compared to that of in vivo tissue specimens. After 4 weeks in 3D culture in osteogenic medium, Fgfr2+/S252W cells expressed 2.8-fold more collagen type I and 3.3-fold more osteocalcin than did wild-type controls (p<0.01). Meanwhile, Fgfr2+/S252W cells showed decreased bone matrix remodeling and expressed 87% less Metalloprotease-13 and 71% less Noggin (p<0.01). The S252W mutation also led to significantly higher production of collagen type I and II in 3D as shown by immunofluorescence staining. In situ hybridization and alizarin red S staining of postnatal day 0 (P0) mouse limb sections demonstrated significantly higher levels of osteopontin expression and mineralization in Fgfr2+/S252W mice. Complementary to in vivo findings, this 3D hydrogel culture system provides an effective in vitro venue to study the pathogenesis of Apert syndrome caused by the analogous mutation in humans.

Iodothyronine deiodinase enzyme activities in bone

Euthyroid status is essential for normal skeletal development and maintenance of the adult skeleton, but the mechanisms which control supply of thyroid hormone to bone cells are poorly understood. Thyroid hormones enter target cells via monocarboxylate transporter-8 (MCT8), which provides a functional link between thyroid hormone uptake and metabolism in the regulation of T3-action but has not been investigated in bone. Most circulating active thyroid hormone (T3) is derived from outer ring deiodination of thyroxine (T4) mediated by the type 1 deiodinase enzyme (D1). The D2 isozyme regulates intra-cellular T3 supply and determines saturation of the nuclear T3-receptor (TR), whereas a third enzyme (D3) inactivates T4 and T3 to prevent hormone availability and reduce TR-saturation. The aim of this study was to determine whether MCT8 is expressed in the skeleton and whether chondrocytes, osteoblasts and osteoclasts express functional deiodinases. Gene expression was analyzed by RT-PCR and D1, D2 and D3 function by sensitive and highly specific determination of enzyme activities. MCT8 mRNA was expressed in chondrocytes, osteoblasts and osteoclasts at all stages of cell differentiation. D1 activity was undetectable in all cell types, D2 activity was only present in mature osteoblasts whereas D3 activity was evident throughout chondrocyte, osteoblast and osteoclast differentiation in primary cell cultures. These data suggest that T3 availability especially during skeletal development may be limited by D3-mediated catabolism rather than by MCT8 mediated cellular uptake or D2-dependent T3 production.

The use of N-terminal immobilization of PTH(1-34) on PLGA to enhance bioactivity

The objective of this work was to control the orientation of bioactive molecules immobilized on a biodegradable substrate to improve their accessibility for binding to cell surface receptors and, therefore, to increase bioactivity. The osteotropic peptide, parathyroid hormone (1-34) (PTH(1-34)), was used to demonstrate the approach. To this end, the intrinsic N-terminal serine residue was oxidized to create an aldehyde group that specifically bound to hydrazide-derivatized poly(lactide-co-glycolide) under neutral conditions to form a hydrazone bond. Use of dihydrazide spacers significantly increased the amount of peptide immobilized compared to simple adsorption or direct, random attachment. In probing accessibility of immobilized PTH(1-34), attachment using longer dihydrazide spacers enhanced binding of an antibody against an epitope in the N-terminal region of the peptide. The longest spacer also increased binding of a C-terminal antibody. Furthermore, substrates with peptide tethered via spacers stimulated intracellular synthesis of cAMP, with activity increasing with dihydrazide length. PTH(1-34) immobilized using the longest spacer was significantly more effective than both random binding and adsorption. Site-directed binding of bioactive peptides to surfaces presents biomolecules for binding with cells so as to enhance interaction with receptors, and therefore the approach may be useful for obtaining preferred localized tissue responses.

Osteopenia in transgenic mice with osteoblast-targeted expression of the inducible cAMP early repressor

ICER is a member of the CREM family of basic leucine zipper transcription factors that acts as a dominant negative regulator of gene transcription. Four different isoforms of ICER (I, Igamma, II and IIgamma) are transcribed from the P2 promoter of the Crem gene. We previously found that each of the ICER isoforms is induced by parathyroid hormone in osteoblasts. The goal of the present study was to assess the function of ICER in bone by overexpressing ICER in osteoblasts of transgenic mice. ICER I and ICER II cDNAs, each containing an N-terminal FLAG epitope tag, were cloned downstream of a fragment containing 3.6 kb of the rat Col1a1 promoter and most of the rat Col1a1 first intron to produce pOBCol3.6-ICER I and pOBCol3.6-ICER II transgenes, respectively. Multiple lines of mice were generated bearing the ICER I and ICER II transgenes. At 8 weeks of age, ICER I and ICER II transgenic mice had lower body weights and decreased bone mineral density of femurs and vertebrae. Further studies were done with ICER I transgenic mice, which had greatly reduced trabecular bone volume and a markedly decreased bone formation rate in femurs. Osteoblast differentiation and osteocalcin expression were reduced in ex vivo bone marrow cultures from ICER I transgenic mice. ICER I antagonized the activity of ATF4 at its consensus DNA binding site in the osteocalcin promoter in vitro. Thus, transgenic mice with osteoblast-targeted overexpression of ICER exhibited osteopenia caused primarily by reduced bone formation. We speculate that ICER regulates the activity and/or expression of ATF/CREB factors required for normal bone formation.

TIEG-null mice display an osteopenic gender-specific phenotype

TGFbeta inducible early gene-1 (TIEG) was originally cloned from human osteoblasts (OB) and has been shown to play an important role in TGFbeta/Smad signaling, regulation of gene expression and OB growth and differentiation. To better understand the biological role of TIEG in the skeleton, we have generated congenic TIEG-null (TIEG(-/-)) mice in a pure C57BL/6 background. Through the use of DXA and pQCT analysis, we have demonstrated that the femurs and tibias of two-month-old female TIEG(-/-) mice display significant decreases in total bone mineral content, density, and area relative to wild-type (WT) littermates. However, no differences were observed for any of these bone parameters in male mice. Further characterization of the bone phenotype of female TIEG(-/-) mice involved mechanical 3-point bending tests, micro-CT, and histomorphometric analyses of bone. The 3-point bending tests revealed that the femurs of female TIEG(-/-) mice have reduced strength with increased flexibility compared to WT littermates. Micro-CT analysis of femurs of two-month-old female TIEG(-/-) mice revealed significant decreases in cortical bone parameters compared to WT littermates. Histomorphometric evaluation of the distal femur revealed that female TIEG(-/-) mice also display a 31% decrease in cancellous bone area, which is primarily due to a decrease in trabecular number. At the cellular level, female TIEG(-/-) mice exhibit a 42% reduction in bone formation rate which is almost entirely due to a reduction in double labeled perimeter. Differences in mineral apposition rate were not detected between WT and TIEG(-/-) mice. Taken together, these findings suggest that female TIEG(-/-) mice are osteopenic mainly due to a decrease in the total number of functional/mature OBs.

Cell-cell communication in the osteoblast/osteocyte lineage

Skeletal development (bone modeling) and its maintenance in post-natal life in response to local and systemic stimuli (bone remodeling) require coordinated activity among osteoblasts (bone forming cells), osteocytes (cells embedded in bone) and osteoclasts (bone resorbing cells), in order to meet the needs of structural integrity, mechanical competence and maintenance of mineral homeostasis. One mechanism of cell-cell interaction is via direct cell-cell communication via gap junctions. These are transmembrane channels that allow continuity of cytoplasms between communicating cells. The biologic importance of connexin43 (Cx43), the most abundant gap junction protein in the skeleton is demonstrated by the skeletal malformations present in oculodentodigital dysplasia (ODDD), a disease linked to Cx43 gene (GJA1) mutations, and by the low bone mass and osteoblast dysfunction in Gja1 ablated mice. The presence of Cx43 is required for osteoblast differentiation and function, and by forming either gap junctions or "hemichannels" Cx43 allows participation of cell networks to responses to extracellular stimuli, via propagation of specific signals converging upon connexin sensitive transcriptional units. Hence, Cx43 is involved in skeletal responsiveness to anabolic signals, as those provided by parathyroid hormone and physical load, the latter function probably involving osteocyte-osteoblast communication.