Coordinate expression of novel genes during osteoblast differentiation

Isolation and Culture of Neonatal Mouse Calvarial Osteoblasts

This chapter describes the isolation and culture of neonatal mouse calvarial osteoblasts. This primary cell population is obtained by sequential enzymatic digestion of the calvarial bone matrix and is capable of differentiating in vitro into mature osteoblasts that deposit a collagen extracellular matrix and form mineralized bone nodules. Maturation of the cultures can be monitored by gene expression analyses and staining for the presence of alkaline phosphatase or matrix mineralization. This culture system, therefore, provides a powerful model in which to test how various experimental conditions, such as the manipulation of gene expression, may affect osteoblast maturation and/or function.

Deletion of calponin 2 attenuates the development of calcific aortic valve disease in ApoE-/- mice

Calcific aortic valve disease (CAVD) is a leading cause of cardiovascular mortality and lacks non-surgical treatment. The pathogenesis of CAVD involves perturbation of valvular cells by mechanical stimuli, including shear stress, pressure load and leaflet stretch, of which the molecular mechanism requires further elucidation. We recently demonstrated that knockout (KO) of Cnn2 gene that encodes calponin isoform 2, a mechanoregulated cytoskeleton protein, attenuates atherosclerosis in ApoE KO mice. Here we report that Cnn2 KO also decreased calcification of the aortic valve in ApoE KO mice, an established model of CAVD. Although myeloid cell-specific Cnn2 KO highly effectively attenuated vascular atherosclerosis that shares many pathogenic processes with CAVD, it did not reduce aortic valve calcification in ApoE KO mice. Indicating a function in the pathogenesis of CAVD, calponin 2 participates in myofibroblast differentiation that is a leading step in the development of CAVD. The aortic valves of ApoE KO mice exhibited increased expression of calponin 2 and smooth muscle actin (SMA), a hallmark of myofibroblasts. The expression of calponin 2 increased during myofibroblast-like differentiation of primary sheep aortic valve interstitial cells and during the osteogenic differentiation of mouse myofibroblasts. Cnn2 KO attenuated TGFβ1-induced differentiation of myofibroblasts in culture as shown by the lower expression of SMA and less calcification than that of wild type (WT) cells. These findings present calponin 2 as a novel molecular target for the treatment and prevention of CAVD.

17-β-estradiol up-regulates apolipoprotein genes expression during osteoblast differentiation in vitro

Apolipoproteins are of great physiological importance and are associated with different diseases. Many independent studies of patterns of gene expression during osteoblast differentiation have been described, and some apolipoproteins have been induced during this process. 17-β-estradiol (E2) may enhance osteoblast physiological function. However, no studies have indicated whether E2 can modulate the expression of apolipoproteins during osteoblast differentiation in vitro. The aim of the current study was to observe the regulation of apolipoprotein mRNA expression by E2 during this process. Primary osteoblasts were collected from the calvaria of newborn mice and were subjected to osteoblast differentiation in vitro with serial concentrations of E2. RNA was isolated on days 0, 5, and 25 of differentiation. Real-time PCR was performed to analyze the levels of apolipoprotein mRNA. Results showed that during osteoblast differentiation all of the apolipoprotein genes were up-regulated by E2 in a dose-dependent manner. Moreover, only ApoE was strongly induced during the mineralization of cultured osteoblasts. This result suggests that ApoE might be involved in osteoblast differentiation. The hypothesis is that E2 promotes osteoblast differentiation by up-regulating ApoE gene expression, though further study is needed to confirm this hypothesis.

Isolation and culture of neonatal mouse calvarial osteoblasts

This chapter describes the isolation and culture of neonatal mouse calvarial osteoblasts. This primary cell population is obtained by sequential enzymatic digestion of the calvarial bone matrix and is capable of differentiating in vitro into mature osteoblasts that deposit a collagen extracellular matrix and form mineralized bone nodules. Maturation of the cultures can be monitored by gene expression analyses and staining for the presence of alkaline phosphatase or matrix mineralization. This culture system, therefore, provides a powerful model to test how various experimental conditions, such as the manipulation of gene expression, may affect osteoblast maturation and/or function.

Comparative study of human aortic and mitral valve interstitial cell gene expression and cellular function

Valve interstitial cells (VICs) are essential for valvular pathogenesis. However, the transcriptional profiles and cellular functions of human aortic VICs (hAVICs) and mitral VICs (hMVICs) have not been directly compared. We performed NimbleGen gene expression profiling analyses of hAVICs and hMVICs. Seventy-eight known genes were differentially expressed between hAVICs and hMVICs. Higher expression of NKX2-5, TBX15, OGN, OMD, and CDKN1C and lower expression of TBX5, MMP1, and PCDH10 were found in hAVICs compared to hMVICs. The differences in these genes, excepting OGN and OMD, remained in rheumatic VICs. We also compared cell proliferation, migration, and response to mineralization medium. hMVICs proliferated more quickly but showed more calcium deposition and alkaline phosphatase activity than hAVICs after culture in mineralization medium, indicating that hMVICs were more susceptible to in vitro calcification. Our findings reveal differences in the transcription profiles and cellular functions of hAVICs and hMVICs.

Rab24 is required for normal cell division

Rab24 is an atypical member of the Rab GTPase family whose distribution in interphase cells has been characterized; however, its function remains largely unknown. In this study, we have analyzed the distribution of Rab24 throughout cell division. We have observed that Rab24 was located at the mitotic spindle in metaphase, at the midbody during telophase and in the furrow during cytokinesis. We have also observed partial co-localization of Rab24 and tubulin and demonstrated its association to microtubules. Interestingly, more than 90% of transiently transfected HeLa cells with Rab24 presented abnormal nuclear connections (i.e., chromatin bridges). Furthermore, in CHO cells stably transfected with GFP-Rab24wt, we observed a large percentage of binucleated and multinucleated cells. In addition, these cells presented an extremely large size and multiple failures in mitosis, as aberrant spindle formation (metaphase), delayed chromosomes (telophase) and multiple cytokinesis. A marked increase in binucleated, multinucleated and multilobulated nucleus formation was observed in HeLa cells depleted of Rab24. We also present evidence that a fraction of Rab24 associates with microtubules. In addition, Rab24 knock down resulted in misalignment of chromosomes and abnormal spindle formation in metaphase leading to the appearance of delayed chromosomes during late telophase and failures in cytokinesis. Our findings suggest that an adequate level of Rab24 is necessary for normal cell division. In summary, Rab24 modulates several mitotic events, including chromosome segregation and cytokinesis, perhaps through the interaction with microtubules.

Comparative analysis of the transcriptome in tissues secreting purple and white nacre in the pearl mussel Hyriopsis cumingii

The triangle sail mussel Hyriopsis cumingii (Lea) is the most important mussel species used for commercial freshwater pearl production in China. Mussel color is an important indicator of pearl quality. To identify genes involved in the nacre coloring, we conducted RNA-seq and obtained 541,268 sequences (298 bp average size) and 440,034 sequences (293 bp average size) in secreting purple and white nacre libraries (P- and W-libraries), respectively. The 981,302 Expressed Sequence Tags (ESTs) were assembled into 47,812 contigs and 289,386 singletons. In BLASTP searches of the deduced protein, 22,495 were proteins with functional annotations. Thirty-three genes involved in pearl or shell formation were identified. Digital expression analysis identified a total of 358 differentially expressed genes, and 137 genes in the P-library and 221 genes in the W-library showed significantly higher expression. Furthermore, a set of SSR motifs and SNPs between the two samples was identified from the ESTs, which provided the markers for genetic linkage, QTL analysis and future breeding. These EST sequences provided valuable information to further understand the molecular mechanisms involved in the formation, color determination and evolution of the pearl or shell.

Osteoblast-specific expression of MEF induces osteopenia through downregulation of osteoblastogenesis and upregulation of osteoclastogenesis

In bone remodeling, various transcriptional factors are involved, and the deficiency or overexpression of some of these factors results in bone defects. Myeloid elf-1-like factor (MEF) is one of the Ets transcription factors that control the expression of genes that are critical for biologic processes such as cell proliferation, differentiation, and death. Previously, we reported that MEF promotes cell proliferation and functions as a negative regulator of osteogenic differentiation by interacting directly with Runx2 and suppressing its transcriptional activity. To investigate the in vivo function of MEF in bone formation and bone remodeling in vivo, we generated transgenic mice that overexpress MEF in osteoblasts under the control of the 2.3-kb Col1α1 promoter, named Col1α1-MEF. Femoral bone in Col1α1-MEF transgenic mice exhibited low bone mass with fewer trabecular bones and thinner and less developed cortical bones. The mineralized volume fraction (BV/TV) and bone-forming rate (BFR) were remarkably decreased to about 63% and 40%, respectively, in 6-week-old MEF transgenic mice compared with wild-type mice. In addition, reduced bone mineral density was observed in lumbar vertebrae of Col1α1-MEF transgenic mice. The number of TRACP(+) osteoclasts was increased in Col1α1-MEF transgenic mice and MEF-overexpressing MC3T3-E1 cells. All these in vivo results suggest that MEF suppresses bone formation by osteoblasts and facilitates bone resorption by activating osteoclasts indirectly.

Latent TGF-beta binding proteins (LTBPs)-1 and -3 coordinate proliferation and osteogenic differentiation of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) possess the capability to differentiate into bone forming cells, osteoblasts, and thus represent a new therapeutic tool in regenerative medicine. Transforming growth factor (TGF)-beta is abundantly present in bone tissue where it regulates osteoblast and osteoclast functions in a complex manner. Latent TGF-beta binding protein (LTBP)-1 mediates the extracellular matrix (ECM) targeting and accumulation of most TGF-beta in the bone. We describe here an important regulatory role for LTBP-3 in TGF-beta activation and autocrine growth control in MSCs. LTBP-3 knockdown via siRNA mediated silencing resulted in reduced cell proliferation and reduced osteogenic differentiation. When MSCs were induced to undergo differentiation, LTBP-3 levels became downregulated in parallel with reduced TGF-beta activation. These changes coincided with the matrix maturation phase of osteogenic differentiation. The mechanism of LTBP-3 is most likely via TGF-beta activation in the early proliferative phase of the differentiation process. Later, when TGF-beta activity would inhibit further maturation and mineralization, LTBP-3 expression becomes downregulated and LTBP-1 containing large latent TGF-beta1 complexes accumulate into the ECM. These complexes represent readily available targets for osteoclast mediated release and activation of TGF-beta in bone tissue. Our results provide evidence that LTBP isoforms can differentially regulate TGF-beta activation and ECM accumulation during osteogenic differentiation.

Gene expression of types I, II, and VI collagen, aggrecan, and chondroitin-6-sulfotransferase in the human annulus: in situ hybridization findings

BACKGROUND CONTEXT

Within each lamellar bundle in the annulus, disc cells produce a complex and sophisticated architectural organization which acts to meet the unique biomechanical needs of the disc. How cells coordinate expression of genes throughout the disc is an important but as yet poorly understood process. For the annulus, such coordination probably involves cell-cell communication as well as growth factor and mechanoreceptor signaling to appropriately maintain the disc extracellular matrix (ECM) for the prevention of annular tears.

PURPOSE

To determine the percentage and patterns of gene expression for types I, II, and VI collagen, aggrecan, and chondroitin-6-sulfotransferase in the human annulus.

STUDY DESIGN/SETTING

Human annulus specimens were obtained from surgical subjects and a control donor in a study approved by the authors' Human Subjects Institutional Review Board.

PATIENT SAMPLE

Four Thompson grade II, three grade III, and four grade IV annulus specimens were evaluated with in situ hybridization to determine gene expression.

OUTCOME MEASURES

The percentages of cells in the human annulus expressing type I, II, and VI collagen, aggrecan, and chondroitin-6-sulfotransferase.

METHODS

In situ hybridization, a technique with high temporal and spatial resolution, was used to detect gene expression of types I, II, and VI collagen, aggrecan, and chondroitin-6 sulfotransferase in cells in adjacent sections of annulus from discs with Thompson grades of II, III, and IV.

RESULTS

Overall, 30.8% of cells expressed aggrecan, 38.4% type I collagen, 45.6% type II collagen, 48.1% type VI collagen, and 57.7% chondroitin-6-sulfotransferase. An important finding was that adjacent cells could be expressing, or not expressing, the gene of interest. These data could not have been gained from other global molecular techniques such as microarray analysis or reverse transcription polymerase chain reaction (RT-PCR). Information on gene expression by individual disc cells is important to better understand disc matrix homeostasis, the pathogenesis of disc degeneration, and to formulate potential biologic therapies for disc degeneration.

CONCLUSIONS

This in situ hybridization study revealed the important finding that adjacent cells differ in their gene expression patterns for specific genes. Factors that could contribute to this difference in adjacent cell gene expression include cellular heterogeneity within the annulus, the presence of senescent cells with altered gene expression, and/or loss of coordinated disc cell function as a result of disruption of cell-cell communication.

Expression patterns of Ets2 protein correlate with bone-specific proteins in cell-seeded three-dimensional bone constructs

The transcription factor Ets2 and its transcriptional targets osteopontin (OPN) and osteocalcin (OC) are expressed in tissue-engineered bone constructs in vitro. Up to now little is known about the role of Ets2 in tissue-engineering applications. This study was intended to investigate the hypothesis that protein expression of Ets2 is correlated with the expression of bone-specific proteinsin tissue-engineeredbone constructs. Cell-seeded three-dimensional bone constructs manufactured with osteoblastic cells and poly(lactic-co-glycolic acid) polymer fleeces over a period of 21 days were analyzed by SDS-PAGE and Western blotting. The protein expression of OPN, OC, osteonectin and collagen type I was analyzed. Cellularity, alkaline phosphatase-specific activity and histology confirmed the osteoblastic phenotype of the constructs. Correlations between Ets2 expression and OPN and Ets2 and collagen type I expression could be detected during the phase of late osteoblastic differentiation between days 9 and 21. The correlation between OC and collagen type I was significant in this late stage of osteoblastic differentiation. These results suggest that there is a strong interplay of Ets2 with bone-specific proteins in cell-seeded three-dimensional bone constructs. This study is a crucial step to elucidate the complex interplay of bone-related proteins in the application of bone tissue engineering.

Ki-energy (life-energy) stimulates osteoblastic cells and inhibits the formation of osteoclast-like cells in bone cell culture models

Some practitioners of the Nishino Breathing Method (NBM) were found to have a higher bone density than the average values of age- and gender-matched non-practitioners. Using bone cell culture models, we investigated a possible mechanism behind this observation. For the study of bone mineralization, we performed the following two experiments using cultured osteoblastic MC3T3-E1 cells: (i) Kozo Nishino, a Japanese Ki expert, sent Ki-energy to the cells once for 5 or 10 min after they were seeded in culture dishes in the presence of 10% fetal bovine serum (FBS). They were incubated for 72 h and the cells were counted. The number in the dish with 10-min Ki-exposure was significantly greater than that in the control (P < 0.01 with n = 8). We performed a reverse transcription-polymerase chain reaction (RT-PCR) study using these cells, but the mRNA expressions did not change significantly. (ii) After cells were incubated for 72 h without Ki-exposure (in the presence of FBS), they were further cultured for 48 h (in the absence of FBS) to promote differentiation. At the beginning of the second culture stage, Ki was applied once for 10 min. After 48 h, RT-PCR was performed. The mRNA expressions which are related to bone mineralization, such as Runx2, alpha1(I) collagen, alkaline phosphatase and osteocalcin, increased significantly (P < 0.05 and n = 4 for all). For the bone resorption study, we used mouse marrow cultures, which can form osteoclast-like cells in the presence of (1-34) parathyroid hormone (PTH), and stimulate resorption. We exposed these cells to Ki-energy twice for the duration of 5 or 10 min on day 0 and day 4. On day 7, the cells were counted. The number of osteoclast-like cells in dishes with Ki exposure was significantly smaller than those in control dishes (P < 0.05 with n = 5). The difference between 5-min exposure and 10-min exposure was not statistically significant. All of our data suggest that the Ki-effect on osteoporosis should be further explored.

Transcriptome analysis reveals an osteoblast-like phenotype for human osteotropic breast cancer cells

Metastatic breast cancer cells exhibit the selective ability to seed and grow in the skeleton. We and others have previously reported that human breast tumors which metastasize to the skeleton overexpress bone matrix extracellular proteins. In an attempt to reveal the osteoblast-like phenotype of osteotropic breast cancer cells, we performed a microarray study on a model of breast cancer bone metastasis consisting of the MDA-MB-231 human cell line and its variant B02 selected for its high capacity to form bone metastases in vivo. Analysis of B02 cells transcriptional profile revealed that 11 and 9 out of the 50 most up- and down-regulated mRNAs, respectively, corresponded to genes which expression has been previously associated with osteoblastic differentiation process. Thus, osteoblast specific cadherin 11 which mediates the differentiation of mesenchymal cells into osteoblastic cells is up-regulated in B02. While S100A4, recently described as a key negative regulator of osteoblast differentiation, is the most down-regulated gene in B02 cells. RT-PCR and western blotting experiments allowed the validation of the modulation of several genes of interest. Using immunohistochemistry, performed on human breast primary tumors and their matched liver and bone metastases, we were able to confirm that the osteoblast-like pattern of gene expression observed in our model holds true in vivo. This is the first report demonstrating a gene-expression pattern corresponding to the acquisition of an osteomimetic phenotype by bone metastatic breast cancer cells.

Proteomic analysis of cartilage- and bone-associated samples

The skeleton of the human body is built of cartilage and bone, which are tissues that contain extensive amounts of extracellular matrix (ECM). In bone, inorganic mineral hydroxyapatite forms 50-70% of the whole weight of the tissue. Although the organic matrix of bone consists of numerous proteins, 90% of it is composed of type I collagen. In cartilage, ECM forms a major fraction of the tissue, type II collagen and aggrecans being the most abundant macromolecules. It is obvious that the high content of ECM components causes analytical problems in the proteomic analysis of cartilage and bone, analogous to those in the analysis of low-abundance proteins present in serum. The massive contents of carbohydrates present in cartilage proteoglycans, and hydroxyapatite in bone, further complicate the situation. However, the development of proteomic tools makes them more and more tempting also for research of musculoskeletal tissues. Application of proteomic techniques to the research of chondrocytes, osteoblasts, osteocytes, and osteoclasts in cell cultures can immediately benefit from the present knowledge. Here we make an overview to previous proteomic research of cartilage- and bone-associated samples and evaluate the future prospects of applying proteomic techniques to investigate key events, such as cellular signal transduction, in cartilage- and bone-derived cells.

beta-cryptoxanthin stimulates cell differentiation and mineralization in osteoblastic MC3T3-E1 cells

The effect of beta-cryptoxanthin, a kind of carotenoid, on cell differentiation and mineralization in osteoblastic MC3T3-E1 cells was investigated. Cells were cultured for 72 h in a minimum essential medium containing 10% fetal bovine serum (FBS), and the cells with subconfluency were changed to a medium containing either vehicle or beta-cryptoxanthin (10(-8) to 10(-6) M) without FBS. Cells were cultured for 3 to 21 days. Gene expression in osteoblastic cells was determined using reverse transcription-polymerase chain reaction (RT-PCR). Culture with beta-cryptoxanthin (10(-7) or 10(-6) M) for 3 days caused a significant increase in Runx2 type 1, Runx2 type 2, alpha1 (I) collagen, and alkaline phosphatase mRNA levels in osteoblastic cells. These increases were completely blocked in the presence of cycloheximide, an inhibitor of protein synthesis, or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), an inhibitor of transcriptional activity. Meanwhile, vitamin A (10(-6) M) did not have a significant effect on Runx2 type 1 mRNA expression in the cells. The effect of beta-cryptoxanthin (10(-6) M) in stimulating Runx2 type 1 and alpha1 (I) collagen mRNA levels, protein content, and alkaline phosphatase activity in the cells was also seen in the presence of vitamin A (10(-6) M), suggesting that the mode of beta-cryptoxanthin action differs from that of vitamin A. Prolonged culture with beta-cryptoxanthin (10(-6) M) for 3 to 21 days caused a significant increase in cell number, deoxyribonucleic acid (DNA) content, protein content, and alkaline phosphatase activity in osteoblastic cells, suggesting that beta-cryptoxanthin stimulates cell proliferation and differentiation. Moreover, culture with beta-cryptoxanthin (10(-7) or 10(-6) M) for 5 to 21 days caused a remarkable increase in mineralization. This study demonstrates that beta-cryptoxanthin has a stimulatory effect on cell differentiation and mineralization due to enhancing gene expression of proteins, which involve in bone formation in osteoblastic MC3T3-E1 cells.

Collagen, type V, alpha1 (COL5A1) is regulated by TGF-beta in osteoblasts

Bone matrix contains high concentrations of growth factors that are known to play important regulatory roles during osteogenesis, particularly transforming growth factor-beta (TGF-beta). Divergent effects of TGF-beta on bone formation have been reported both in vitro and in vivo depending upon experimental conditions, cells employed and their stage of maturation. In this study, we have used a clonal osteoblastic cell line MC3T3-E1, derived from newborn mouse calvaria, as an in vitro model of bone development. These cells undergo an ordered, time-dependent developmental sequence characterized by three stages (proliferation, differentiation and mineralization), over a 30-35-day period. In this study, cDNA microarray technology was used to study the expression profile of 8470 genes, in the presence of TGF-beta1 during osteoblast development. Microarray analysis revealed 120 cDNAs to be differentially expressed in MC3T3-E1 osteoblasts that had been treated with TGF-beta1. From the 120 differentially expressed genes, we selected Collagen, type V, alpha1 (COL5A1) {differential expression=+4.9} for further studies since it represents a previously uncharacterized component of the bone matrix. Using Northern blotting, we found that, when MC3T3-E1 cells were treated with TGF-beta1, COL5A1 was up-regulated during the proliferation and differentiation phases of osteogenesis. Furthermore, by a combination of RNA in situ hybridization and Northern blotting, we found COL5A1 mRNA to be expressed in the calvaria and developing bone of the E17.5 mouse embryos. Lastly, significant COL5A1 protein expression was observed by immunohistochemistry in the developing bone of the E17.5 mouse embryos. In conclusion, by the use of in vitro and in vivo approaches, we have discovered that the COL5A1 gene is a target of TGF-beta during osteogenesis.

Suppressive effect of regucalcin on cell differentiation and mineralization in osteoblastic MC3T3-E1 cells

The role of regucalcin in the regulation of osteoblastic cell function was investigated. Osteoblastic MC3T3-E1 cells with subconfluent monolayers were cultured in a medium containing regucalcin (10(-10)-10(-8) M) without fetal bovine serum (FBS). The proliferation of osteoblastic cells was not significantly altered in the presence of regucalcin. The results of reverse transcription-polymerase chain reaction (RT-PCR) analysis with specific primers showed that the expression of Runx2 (Cbfa1) and insulin-like growth factor-I (IGF-I) mRNAs in osteoblastic cells was significantly suppressed in the presence of regucalcin (10(-10) or 10(-9) M). Transforming growth factor-beta1 mRNA levels were significantly enhanced in the 24 h-culture with regucalcin (10(-10) or 10(-9) M). Alpha1(I) collagen and glyceroaldehyde-3-phosphate dehydrogenase (G3PDH) mRNA levels were not significantly changed by culture with regucalcin (10(-10) or 10(-9) M). Alkaline phosphatase activity was significantly decreased in the lysate of cells cultured for 24 or 48 h with regucalcin (10(-10)-10(-8) M). Moreover, the expression of regucalcin in osteoblastic cells was demonstrated by RT-PCR and Western blot analysis. When regucalcin (10(-7) M) was added into the enzyme reaction mixture containing the lysate of osteoblastic cells cultured in the absence of regucalcin, alkaline phosphatase activity was significantly decreased. Also, Ca2+/calmodulin-dependent nitric oxide (NO) synthase activity in the cell lysate was significantly decreased by addition of regucalcin (10(-10)-10(-8) M) into the reaction mixture. The presence of anti-regucalcin monoclonal antibody (5 or 10 ng/ml) in the enzyme reaction mixture caused a significant increase in NO synthase activity in the cell lysate in the presence or absence of Ca2+/calmodulin, suggesting a role of endogenous regucalcin. When osteoblastic cells with subconfluency were cultured in the presence of regucalcin (10(-10) or 10(-9) M) for 3, 9, or 18 days, the results with Alizarin red staining showed that the mineralization was markedly suppressed by culture with regucalcin for 3, 9, or 18 days. This study demonstrates that regucalcin regulates the function of osteoblastic cells, and that the protein suppresses cell differentiation and mineralization.

Calcyclin, a Ca2+ ion-binding protein, contributes to the anabolic effects of simvastatin on bone

In vitro treatment with a pharmacological dose of simvastatin, a potent pro-drug of a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, stimulates bone formation. In our study, simvastatin stimulated differentiation of osteoblasts remarkably in a dose-dependent manner, with minimal effect on proliferation. To identify the mediators of the anabolic effects of simvastatin on osteoblasts, we tried to identify and characterize simvastatin-induced proteins by using proteomic analysis. Calcyclin was significantly up-regulated by more than 10 times, and annexin I was also up-regulated by simvastatin. However, annexin III, vimentin, and tropomyosin were down-regulated. Up-regulated calcyclin mRNA by simvastatin was validated by reverse transcription in mouse calvarial cells. In confocal microscope analysis, green fluorescence protein-calcyclin fusion protein was ubiquitously observed in the of MC3T3-E1 cells transfected with green fluorescence protein-calcyclin cDNA containing plasmid and was quickly concentrated in the nucleus 20 min after simvastatin treatment. Overexpression of calcyclin cDNA stimulated both the proliferation and expression of alkaline phosphatase mRNA significantly, without exposure to simvastatin in MC3T3-E1 cells. However, both the rate of proliferation of the osteoblasts and the expression of alkaline phosphatase mRNA were suppressed significantly 1 day after treatment with the calcyclin-specific small interference RNA, and furthermore, simvastatin did not overcome this suppression in the small interference RNA-pretreated MC3T3-E1 cells. In conclusion, calcyclin is one of the candidate proteins that plays a role in osteoblastogenesis in response to simvastatin, although the precise functions of calcyclin in osteoblast remain to be verified.

Non-functional Fas ligand increases the formation of cartilage early in the endochondral bone induction by rhBMP-2

It has previously been shown that mice with a defect in Fas ligand-mediated apoptosis have an enhancement of ectopic bone formation. We investigated the expression of bone-related markers--alkaline phosphatase, collagen, bone sialoprotein, osteocalcin, osteopontin, and bone morphogenetic proteins (BMP) -2, -4, and -7; and cytokines interleukin-1alpha (IL-1), IL-1beta, and tumor necrosis factor-alpha (TNF-alpha) in ectopic new bone induced by recombinant human (rh) BMP-2 in mice without functional Fas-ligand (gld mice). At day 6 after rhBMP-2 implantation, gld mice formed more cartilage and mesenchyme compared with their wild type littermates. At later stages, gld mice did not differ from the control mice in the volume of newly formed tissue, expressing higher level of BMP genes and lower levels of genes involved in osteoblast maturation--bone sialoprotein and osteopontin. Differences in the levels of expression of IL-1alpha and TNF-alpha were observed only at day 12 after rhBMP-2 implantation. These results suggest that gld mice have an increased recruitment of cells of mesenchymal origin and an abnormal pattern of differentiation and maturation of the newly formed mesenchymal tissues.

Calcyclin mediates serum response element (SRE) activation by an osteoblastic extracellular cation-sensing mechanism

The molecular mechanism of sensing extracellular cations in osteoblasts is controversial. Using an expression-cloning strategy, the calcium-binding protein calcyclin was found to mediate the response of MC3T3-E1 osteoblasts to extracellular cations, but not the calcimimetic NPS-568, indicating the presence of another cation-sensing mechanism. Further understanding of calcyclin function in osteoblasts may identify novel targets for regulating bone formation.

INTRODUCTION

Extracellular calcium and other cations seem to regulate the function of osteoblasts through a distinct calcium-sensing mechanism that is coupled to activation of c-fos gene transcription. The identity of this calcium-sensing mechanism is unknown.

METHODS

To identify molecules that participate in this extracellular cation-sensing pathway, we developed an expression cloning strategy in COS-7 cells using cation stimulation of a serum response element (SRE) luciferase reporter derived from the c-fos promoter to screen a mouse MC3T3-E1 osteoblast cDNA library.

RESULTS AND CONCLUSIONS

We identified calcyclin (S100A6), a calcium-binding protein of the EF-hand type belonging to the S100 family, as being responsible for transferring a cation-sensing response from osteoblasts to COS-7 cells. Transfection of the calcyclin cDNA into COS-7 and HEK-293 cells confirmed that the overexpression of calcylin caused these cells to gain the ability to sense extracellular cations, including aluminum, gadolinium, calcium, and magnesium. Conversely, we found that an antisense calcyclin construct reduced calcyclin expression and partially inhibited the cation-sensing response in MC3T3-E1 osteoblasts. These results implicate calcyclin in the activation of SRE and establish a role for calcyclin as an accessory protein involved in the cation-sensing pathway in osteoblasts.

Antisense oligonucleotide against 47-kDa heat shock protein (Hsp47) inhibits wound-induced enhancement of collagen production

It is well known that excessive collagen synthesis during the wound-healing process causes scar formation. Our recent in-vivo study indicates that antisense treatment against 47-kDa heat shock protein (Hsp47), a collagen-specific molecular chaperone, relieves scar formation following skin wounds in rats [Wang et al., Plast. Reconstr. Surg., in press]. In order to understand the mechanism of this phenomenon, we examined the effects of antisense treatment on the expression of mRNAs and proteins of Hsp47 and collagens in fibroblasts derived from wounded rat tongues. Hsp47 and procollagen alpha1(I) and alpha1(III) mRNAs were consistently increased after wounding and were maximal at day 5 post-injury. Treatment with antisense oligonucleotide against Hsp47 efficiently blocked the production of procollagen alpha2(I) and alpha1(III) proteins, but had little effect on their mRNA levels. Therefore, we conclude that antisense oligonucleotide against Hsp47 inhibits the production of procollagen type I and III proteins in fibroblasts derived from wounded tongues, overcoming the increase in their mRNAs.

Stromal cell-derived factor-1 (SDF-1) recruits osteoclast precursors by inducing chemotaxis, matrix metalloproteinase-9 (MMP-9) activity, and collagen transmigration

Signals targeting OCs to bone and resorption sites are not well characterized. A chemoattractant receptor (CXCR4), highly expressed in murine OC precursors, mediated their chemokine (SDF-1)-induced chemoattraction, collagen transmigration, and MMP-9 expression. Thus, bone vascular and stromal SDF-1 may direct OC precursors into bone and marrow sites for development and bone resorption.

INTRODUCTION

Although chemokines are essential for trafficking and homing of circulating hematopoietic cells under normal and pathological conditions, their potential roles in osteoclast (OC) recruitment or function are generally unknown. CXCR4 and its unique ligand, stromal cell-derived factor-1 (SDF-1), critically control the matrix metalloproteinase (MMP)-dependent targeting of hematopoietic cells into bone and within the marrow microenvironment. Therefore, SDF-1/CXCR4 may regulate OC precursor recruitment to sites for development and activation.

METHODS

Chemokine receptor mRNA expression was analyzed during OC formation induced by RANKL in murine RAW 264.7 cells. SDF-1 versus RANKL effects on chemotaxis, transcollagen migration, MMP-9 expression and activity, OC development, and bone resorption were evaluated in RAW cells or RAW-OCs.

RESULTS

CXCR4 was highly expressed in RAW cells and downregulated during their RANKL development into bone-resorptive RAW-OCs. SDF-1, but not RANKL, elicited RAW cell chemotaxis. Conversely, RANKL, but not SDF-1, promoted RAW-OC development, TRAP activity, cathepsin K expression, and bone pit resorption, and SDF-1 did not modify these RANKL responses. Both SDF-1 and RANKL increased MMP-9, a matrix-degrading enzyme essential for OC precursor migration into developing bone marrow cavities, and increased transcollagen migration of RAW cells in a MMP-dependent manner. SDF-1 also upregulated MMP-9 in various primary murine OC precursor cells. Because RANKL induced a higher, more sustained expression of MMP-9 in RAW cells than did SDF-1, MMP-9 may have an additional role in mature OCs. Consistent with this, MMP-9 upregulation during RANKL-induced RAW-OC development was necessary for initiation of bone pit resorption.

CONCLUSIONS

SDF-1, a chemokine highly expressed by bone vascular endothelial and marrow stromal cells, may be a key signal for the selective attraction of circulating OC precursors into bone and their migration within marrow to appropriate perivascular stromal sites for RANKL differentiation into resorptive OCs. Thus, SDF-1 and RANKL likely serve complementary physiological functions, partly mediated through increases in MMP-9, to coordinate stages of OC precursor recruitment, development, and function.

A gene expression profile for endochondral bone formation: oligonucleotide microarrays establish novel connections between known genes and BMP-2-induced bone formation in mouse quadriceps

Endochondral bone formation has been fairly well characterized from a morphological perspective and yet this process remains largely undefined at molecular and biochemical levels. In vitro and in vivo studies have shown that human bone morphogenetic protein-2 (hBMP-2) is an important developmental growth and differentiation factor, capable of inducing ectopic bone formation in vivo. This study evaluated several aspects of the osteogenic effect of hBMP-2 protein injected into quadriceps of female C57B1/6J SCID mice. Mice were euthanized 1, 2, 3, 4, 7, and 14 days postinjection and muscles were collected for several methods of analysis. Hematoxylin and eosin-stained sections of muscles injected with formulation buffer showed no evidence of osteogenesis. In contrast, sections of muscles injected with hBMP-2 showed evidence of endochondral bone formation that progressed to mineralized bone by day 14. In addition, radiographs of mice injected with hBMP-2 showed that much of the quadriceps muscle had undergone mineralization by day 14. Labeled mRNA solutions were prepared and hybridized to oligonucleotide arrays designed to monitor approximately 1300 murine, full-length genes. Changes in gene expression associated with hBMP-2 were determined from time-matched comparisons between buffer and hBMP-2 samples. A gene expression profile was created for 215 genes that showed greater than 4-fold changes at one or more of the indicated time points. One hundred twenty-two of these genes have previously been associated with bone or cartilage metabolism and showed significant increases in expression, e.g., aggrecan (Agc1), runt related transcription factor 2 (Runx2), bone Gla protein 1 (Bglap1), and procollagens type II (Col2a1) and X (Col10a1). In addition, there were 93 genes that have not been explicitly associated with bone or cartilage metabolism. Two of these genes, cytokine receptor-like factor-1 (Crlf1) and matrix metalloproteinase 23 (Mmp23), showed peak changes in gene expression of 15- and 40-fold on days 4 and 7, respectively. In situ hybridizations of muscle sections showed that Mmp23 and Crlf1 mRNAs were expressed in chondrocytes and osteoblasts, suggesting a role for both proteins in some aspect of cartilage or bone formation. In conclusion, oligonucleotide arrays enabled a broader view of endochondral bone formation than has been reported to date. An increased understanding of the roles played by these gene products will improve our understanding of skeletogenesis, fracture repair, and pathological conditions such as osteoporosis.

Transcriptional profiling of human osteoblast differentiation

Osteoblast differentiation is a key aspect of bone formation and remodeling. To further our understanding of the differentiation process, we have developed a collection of conditionally immortalized adult human osteoblast cell lines representing discrete stages of differentiation. To evaluate changes in gene expression associated with differentiation, polyA((+)) RNA from pre-osteoblasts, early and late osteoblasts, and pre-osteocytes was subjected to gene chip analysis using the Affymetrix Hu6800 chip in conjunction with an Affymetrix custom chip enriched in bone and cartilage cDNAs. Overall, the expression of 47 genes was found to change threefold or more on both chips between the pre-osteoblastic and pre-osteocytic stages of differentiation. Many of the observed differences, including down-regulation of collagen type I and collagen-processing enzymes, reflect expected patterns and support the relevance of our results. Other changes have not been reported and offer new insight into the osteoblast differentiation process. Thus, we observed regulation of factors controlling cell cycle and proliferation, reflecting decreased proliferation, and increased apoptosis in pre-osteocytic cells. Elements maintaining the cytoskeleton, extracellular matrix, and cell-cell adhesion also changed with differentiation reflecting profound alterations in cell architecture associated with the differentiation process. We also saw dramatic down-regulation of several components of complement and other immune response factors that may be involved in recruitment and differentiation of osteoclasts. The decrease in this group of genes may provide a mechanism for controlling bone remodeling of newly formed bone. Our screen also identified several signaling proteins that may control osteoblast differentiation. These include an orphan nuclear receptor DAX1 and a small ras-related GTPase associated with diabetes, both of which increased with increasing differentiation, as well as a high mobility group-box transcription factor, SOX4, that was down-regulated during differentiation. In summary, our study provides a comprehensive transcriptional profile of human osteoblast differentiation and identifies several genes of potential importance in controlling differentiation of osteoblasts.

Identification of genes regulated during osteoblastic differentiation by genome-wide expression analysis of mouse calvaria primary osteoblasts in vitro

Although several independent studies of gene expression patterns during osteoblast differentiation in cultures from calvaria and other in vitro models have been reported, only a small portion of the mRNAs expressed in osteoblasts have been characterized. We have previously analyzed the behavior of several known markers in osteoblasts, using Affymetrix GeneChip murine probe arrays (27,000 genes). In the present study we report larger groups of transcripts displaying significant expression modulation during the culture of osteoblasts isolated from mice calvaria. The expression profiles of 601 such regulated genes, classified in distinct functional families, are presented and analyzed here. Although some of these genes have previously been shown to play important roles in bone biology, the large majority of them have never been demonstrated to be regulated during osteoblast differentiation. Despite the fact that the precise involvement of these genes in osteoblast differentiation and function needs to be evaluated, the data presented herein will aid in the identification of genes that play a significant role in osteoblasts. This will provide a better understanding of the regulation of osteoblast differentiation and maturation.

Simulated microgravity suppresses osteoblast phenotype, Runx2 levels and AP-1 transactivation

Conditions of disuse such as bed rest, space flight, and immobilization result in decreased mechanical loading of bone, which is associated with reduced bone mineral density and increased fracture risk. Mechanisms involved in this process are not well understood but involve the suppression of osteoblast function. To elucidate the influence of mechanical unloading on osteoblasts, a rotating wall vessel (RWV) was employed as a ground based model of simulated microgravity. Mouse MC3T3-E1 osteoblasts were grown on microcarrier beads for 14 days and then placed in the RWV for 24 h. Consistent with decreased bone formation during actual spaceflight conditions, alkaline phosphatase and osteocalcin expression were decreased by 80 and 50%, respectively. In addition, runx2 expression and AP-1 transactivation, key regulators of osteoblast differentiation and bone formation, were reduced by more than 60%. This finding suggests that simulated microgravity could promote dedifferentiation and/or transdifferentiation to alternative cell types; however, markers of adipocyte, chondrocyte, and myocyte lineages were not induced by RWV exposure. Taken together, our results indicate that simulated microgravity may suppress osteoblast differentiation through decreased runx2 and AP-1 activities.

Genomic approaches to identifying transcriptional regulators of osteoblast differentiation

Recent microarray studies of mouse and human osteoblast differentiation in vitro have identified novel transcription factors that may be important in the establishment and maintenance of differentiation. These findings help unravel the pattern of gene-expression changes that underly the complex process of bone formation.

Comprehensive microarray analysis of bone morphogenetic protein 2-induced osteoblast differentiation resulting in the identification of novel markers for bone development

Osteoblasts are cells responsible for matrix deposition during bone development and although temporal expression of many genes has been related to osteoblast differentiation, a complete description of osteoblast-specific gene regulation will lead to a better understanding of osteoblast function. In this study, microarray technology was used to analyze gene expression on a broad scale during osteoblast differentiation. Expression analysis of 9596 sequences revealed 342 genes and expressed sequence tags (ESTs) to be modulated differentially during a time course experiment in which murine C2C12 mesenchymal progenitor cells were induced to differentiate into mature osteoblasts by treatment with bone morphogenetic protein 2 (BMP-2). By means of hierarchical clustering, these genes were grouped by similarities in their expression profiles, resulting in subsets of early, intermediate, and late response genes, which are representative of the distinct stages of osteoblast differentiation. To identify new bone markers, the bone specificity of the late response genes was determined by comparing BMP-induced expression in C2C12 and MC3T3 osteoblasts with that in NIH3T3 fibroblasts. This resulted in the identification of nine novel genes and ESTs that were induced specifically in osteoblasts, in addition to the well-known markers ALP and osteocalcin. For at least one of these novel genes, Wnt inhibitory factor 1, and two of the ESTs, expression in developing bone was verified in vivo by in situ hybridization of E16.5 mouse embryos. In conclusion, by a combination of in vitro and in vivo screening approaches, a set of new genes related to osteoblast differentiation and skeletal development has been identified.

The anti-proliferative gene TIS21 is involved in osteoclast differentiation

The remodeling process of bone is accompanied by complex changes in the expression levels of various genes. Several approaches have been employed to detect differentially-expressed genes in regard to osteoclast differentiation. In order to identify the genes that are involved in osteoclast differentiation, we used a cDNAarray-nylon membrane. Among 1,200 genes that showed a measurable signal, 19 genes were chosen for further study. Eleven genes were up-regulated; eight genes were downregulated. TIS21 was one of the up-regulated genes which were highly expressed in mature osteoclasts. To verify the cDNA microarray results, we carried out RT-PCR and real-time RT-PCR for the TIS21 gene. The TIS21 mRNA level was higher in differentiated-osteoclasts when compared to undifferentiated bone-marrow macrophages. Furthermore, the treatment with 1 mM of a TIS21 antisense oligonucleotide reduced the formation of osteoclasts from the bone-marrow-precursor cells by approximately 30%. These results provide evidence for the potential role of TIS21 in the differentiation of osteoclasts

Behavior of osteoblast, adipocyte, and myoblast markers in genome-wide expression analysis of mouse calvaria primary osteoblasts in vitro

Several genes, such as alkaline phosphatase, osteocalcin, and Cbfa1/Osf2, are known to be regulated during osteoblastic differentiation and are commonly used as "osteoblast markers" for in vitro or in vivo studies. The number of these genes is very limited, however, and it is of major interest to identify new genes that are activated or repressed during the process of osteoblast differentiation and bone formation as well as to extend the available information on gene families relevant to this particular differentiation pathway. To identify such genes, we have implemented a genome-wide analysis by determining changes in expression levels of 27,000 genes during in vitro differentiation of primary osteoblasts isolated from mouse calvaria. This study focuses on the description of the analytical and filtering process applied; on the transcriptional analysis of well-established "bone," "adipocyte," and "muscle" pathway markers; and on a description of the regulation profiles for genes recently described in the Skeletal Gene Database. We also demonstrate that new array technologies constitute reliable and powerful tools to monitor the transcription of genes involved in osteoblastic differentiation, allowing a more integrated vision of the biological pathways regulated during osteoblast commitment, differentiation, and function.

Lumican is a major proteoglycan component of the bone matrix

MC3T3-E1 mouse calvaria cells are a clonal population of committed osteoprogenitors that in the presence of appropriate supplements form a mineralized bone matrix. The development of the MC3T3-E1 cells can be divided into three major stages, namely, proliferation, differentiation, and mineralization. Recently, using the cDNA microarray technology we found lumican to be abundantly expressed during the mineralization and differentiation stages of the MC3T3-E1 development and not during the proliferation stage. Lumican has been shown to play essential roles in regulating collagen fibril formation in different extracellular matrices but its expression in the developing bone matrix remains elusive. By examining the expression profile of this gene during the different stages of MC3T3-E1 development, utilizing the 'real-time' PCR technology, we observed that the expression of lumican increases as the osteoblast culture differentiates and matures, suggesting that lumican may be involved in regulating collagen fibrillogenesis in bone matrices. Using immunostaining, we observed that during the early embryonic development of mouse (E11 to E13), lumican is mainly expressed in the cartilaginous matrices. However, in the older embryos (E14 to E16), the expression of lumican is more prominent in the developing bone matrices. Our data suggest that lumican is a significant proteoglycan component of bone matrix, which is secreted by differentiating and mature osteoblasts only and therefore it can be used as a marker to distinguish proliferating pre-osteoblasts from the differentiating osteoblasts.

Correct identification of genes from serial analysis of gene expression tag sequences

SAGE (serial analysis of gene expression) is a remarkable technique for genome-wide analysis of gene expression. It is crucial to understand the extent to which SAGE can accurately indicate a gene or expressed sequence tag (EST) with a single tag. We analyzed the effect of the size of SAGE tag on gene identification. Our observation indicates that SAGE tags are in general not long enough to achieve the degree of uniqueness of identification originally envisaged. Our observations also indicate that the limitation of using SAGE tag to identify a gene can be overcome by converting SAGE tags into longer 3' EST sequences with the generation of longer cDNA fragments from SAGE tages for gene identification (GLGI) method.

Cloning and characterization of osteoactivin, a novel cDNA expressed in osteoblasts

Osteoblast development is a complex process involving the expression of specific growth factors and regulatory proteins that control cell proliferation, differentiation, and maturation. In this study, we used the rat mutation, osteopetrosis (op), to examine differences in skeletal gene expression between mutant op and normal littermates. Total RNA isolated from long bone and calvaria was used as a template for mRNA differential display. One of many cDNAs that were selectively expressed in either normal or mutant bone was cloned and sequenced and found to share some homology to the human nmb and Pmel 17 genes. This novel cDNA was named osteoactivin. Osteoactivin has an open reading frame of 1716 bp that encodes a protein of 572 amino acids with a predicted molecular weight of 63.8 kD. Protein sequence analysis revealed the presence of a signal peptide and a cleavage site at position 23. The protein also has thirteen predicted N-linked glycosylation sites and a potential RGD integrin recognition site at position 556. Northern blot analysis confirmed that osteoactivin was 3- to 4-fold overexpressed in op versus normal bone. RT-PCR analysis showed that osteoactivin is most highly expressed in bone compared with any of the other non-osseous tissues examined. In situ hybridization analysis of osteoactivin in normal bone revealed that it is primarily expressed in osteoblasts actively engaged in bone matrix production and mineralization. In primary rat osteoblast cultures, osteoactivin showed a temporal pattern of expression being expressed at highest levels during the later stages of matrix maturation and mineralization and correlated with the expression of alkaline phosphatase and osteocalcin. Our findings show that osteoactivin expression in bone is osteoblast-specific and suggest that it may play an important role in osteoblast differentiation and matrix mineralization. Furthermore, osteoactivin overexpression in op mutant bone may be secondary to the uncoupling of bone resorption and formation resulting in abnormalities in osteoblast gene expression and function.

Discovery of osteoblast-associated genes using cDNA microarrays

Osteoblast maturation is a complex process and involves distinct genotypic changes that are accompanied by specific phenotypic alterations. To identify new bone-related genes in osteoblasts we utilized the high-density mouse GEM1 microarray gene chip from IncyteGenomics, Inc. (St. Louis, Mo). We examined the expression profiles of over 8700 genes during the proliferation (day 3) and the mineralization (day 34) phases of MC3T3-E1 development. More than 8600 genes provided measurable signals. Of these genes, 252 were found to be differentially expressed on days 3 and 34. A large number of these genes have never been previously recognized in the context of osteoblast development. Approximately, 60% of the genes with expressions that were dominant in proliferating osteoblasts consisted of growth-related genes such as TACC3 and Pr22. The expressions of TIS21/BTG2, and a novel gene EST350, were found to peak during the differentiation phase (day 12), suggesting that they may play important roles in osteoblast differentiation. The majority of the genes with expressions that were dominant during the mineralization phase consisted of signal transduction genes and extracellular matrix (ECM) proteins such as lumican and cystatin-C. It is significant that lumican expression could not be detected on day 3, which indicates that this gene may serve as an important marker of postmitotic osteoblasts. The establishment of the expression profiles of these and other genes with various phases of MC3T3-E1 osteoblast development will allow us to distinguish the molecular events at different phases of osteoblast biology.

Genome-wide screening by cDNA microarray of genes associated with matrix mineralization by human mesenchymal stem cells in vitro

Using a culture system that facilitates osteogenic differentiation of bone marrow-derived human mesenchymal stem cells, we analyzed gene-expression profiles during the mineralization process by means of a cDNA microarray system consisting of 23,040 genes. We compared expression profiles of the cells at days 3, 15, and 27 of incubation in media containing either a combination of 0.1 microM dexamethasone, 0.05 mM ascorbic acid-2-phosphate, and 10 mM beta-glycerophosphate, dexamethasone only, ascorbic acid-2-phosphate plus beta-glycerophosphate, or medium without any of these osteogenic supplements. Histochemical analysis revealed osteogenic differentiation of cells incubated in the presence of all three agents, but not in the other cultures. Comparison of the expression profiles disclosed transcriptional stimulation of 55 genes and repression of 82 genes among more than 20,000 examined. A set of differentially expressed genes we report here should contribute to a better understanding of the process of mineralization in the matrix surrounding human mesenchymal stem cells.

Ets transcription factors and targets in osteogenesis

Bone formation in vivo is a complex phenomenon whereby recruitment and replication of mesenchymal precursors of osteoblasts, differentiation into preosteoblasts, osteoblasts, and mature osteoblasts ultimately result in the accumulation and mineralization of the extracellular matrix. MC3T3-E1, a clonal osteoblastic cell line, was derived from mouse calvaria and undergoes an ordered and time dependent developmental sequence leading to formation of multilayered bone nodules over a 30 - 35 day period. This developmental pattern is characterized by the replication of preosteoblasts followed by growth arrest and expression of mature osteoblastic characteristics such as matrix maturation and eventual formation of multilayered nodules with a mineralized extracellular matrix. We have found that Ets1 is expressed in proliferating preosteoblastic cells whereas Ets2 is expressed by differentiating and mature osteoblasts. In addition, the expression of Ets1 can be induced in MC3T3-E1 and fetal rat calvaria cells by retinoic acid (RA) which is known to exert profound effects on skeletal growth and development, bone turnover, and induce specific cellular responses in bone cells. Thus the multiple functions of RA in bone cells are likely to be mediated in part by Ets1. Also, Ets2 transgenic mice develop multiple neurocranial, viserocranial, and cervical skeletal abnormalities. Significantly, these abnormalities are similar to the skeletal anomalies found in trisomy-16 mice and in humans with Down's syndrome, wherein the dosage of Ets2 is known to be increased. These results indicate that Ets2 has an important role in skeletal development and that Ets2 overexpression in transgenics is responsible for the genesis of the same type of skeletal abnormalities that are seen in Down's syndrome. Thus the genetic programs regulated by Ets1 and Ets2 may significantly affect the development and differentiation of osteoblasts, and in fact, Ets1 has been shown to interact with the 'quintessential' osteoblast transcription factor CbfA1. This review will examine in detail the role and possible targets of Ets1 and Ets2 in osteoblast differentiation and bone formation.