Analysis of Internal Deletions of a Rat Col1a1 Promoter Fragment in Transfected ROS17/2.8 Cells

The aim of this paper is identification of regulatory sequences downstream of –1683 base pairs (bp) in the rat Col1a1 promoter important for expression in osteoblasts. Previous findings suggest that a rat Col1a1 gene fragment extending from –1719 to +115 bp linked to the chloramphenicol acetyl transferase (CAT) reporter gene (ColCAT1719) is highly and selectively expressed in osteoblasts. Three internal deletions within the ColCAT1719 construct were generated and stably transfected into ROS 17/2.8 cells. CAT activity was measured in cell extracts. An internal deletion of ColCAT1719 from –1637 to –504 bp caused an almost complete loss of CAT activity, whereas deletions of –1284 to –905 bp and –1284 to –451 bp had little effect on CAT activity. We hypothesized that removal of a Runx2/Cbfa1 consensus site at –1376 bp may have caused the loss of activity produced by the –1637 to –504 bp deletion. To test this hypothesis, we produced a more restricted internal deletion of ColCAT1719 from –1418 to –1284 bp, which removes this site. This deletion did not affect promoter activity. Our results suggest that the Runx2 site at –1376 bp by itself does not influence Col1719 promoter activity. Future studies will focus on the region between –1637 to 1418 bp, which contains several potentially interesting transcription factor binding sites

Comparison of Proliferation and Differentiation of Calvarial Osteoblast Cultures Derived from Msx2 Deficient and Wild Type Mice

We analyzed proliferation and differentiation of calvarial osteoblasts derived from Msx2 deficient in comparison with wild type mice. Calvarial osteoblast cultures from five to eight days old Msx2 deficient, heterozygous and wild type mice were studied for difference in proliferation and differentiation. Proliferation rate was assessed by counting cell number, BrdU and Calcein AM labeling. Differentiation was assessed by Von Kossa and alkaline phosphatase staining, northern blot hybridization with bone differentiation markers, infection of cell cultures with retrovirus expressing GFP under the control of type I collagen promoter fragment. At day six, cell number in cell culture derived from Msx2 deficient mice was 20% lower then in culture from wild type mice. There were 16.8% BrdU labeled cells in cell culture from Msx2 deficient mice, 20.9% in culture from heterozygous mice and 21.6% in culture from wild type mice. Cell cultures from Msx2 deficient mice showed lower intensity of fluorescence when marked with Calcein AM then cultures from wild type mice. Von Kossa staining showed increased mineralization and northern blot analysis showed increased levels of bone differentiation markers in cell cultures derived from Msx2 deficient mice. GFP came on earlier in Msx2 deficient cultures after infection with Col2.3 GFP retrovirus. We conclude that calvarial osteoblasts derived from Msx2 deficient mice have a lower rate of proliferation and demonstrate increased osteoblastic differentiation when compared to osteoblasts derived from wild type mice

Expression and function of Dlx genes in the osteoblast lineage

AbstractOur laboratory and others have shown that overexpression of Dlx5 stimulates osteoblast differentiation. Dlx5−/−/Dlx6−/− mice have more severe craniofacial and limb defects than Dlx5−/−, some of which are potentially due to defects in osteoblast maturation. We wished to investigate the degree to which other Dlx genes compensate for the lack of Dlx5, thus allowing normal development of the majority of skeletal elements in Dlx5−/− mice. Dlx gene expression in cells from different stages of the osteoblast lineage isolated by FACS sorting showed that Dlx2, Dlx5 and Dlx6 are expressed most strongly in less mature osteoblasts, whereas Dlx3 is very highly expressed in differentiated osteoblasts and osteocytes. In situ hybridization and Northern blot analysis demonstrated the presence of endogenous Dlx3 mRNA within osteoblasts and osteocytes. Dlx3 strongly upregulates osteoblastic markers with a potency comparable to Dlx5. Cloned chick or mouse Dlx6 showed stimulatory effects on osteoblast differentiation. Our results suggest that Dlx2 and Dlx6 have the potential to stimulate osteoblastic differentiation and may compensate for the absence of Dlx5 to produce relatively normal osteoblastic differentiation in Dlx5 knockout mice, while Dlx3 may play a distinct role in late stage osteoblast differentiation and osteocyte function

Direct cell–cell contact between mature osteoblasts and osteoclasts dynamically controls their functions in vivo

Bone homeostasis is regulated by communication between bone-forming mature osteoblasts (mOBs) and bone-resorptive mature osteoclasts (mOCs). However, the spatial–temporal relationship and mode of interaction in vivo remain elusive. Here we show, by using an intravital imaging technique, that mOB and mOC functions are regulated via direct cell–cell contact between these cell types. The mOBs and mOCs mainly occupy discrete territories in the steady state, although direct cell–cell contact is detected in spatiotemporally limited areas. In addition, a pH-sensing fluorescence probe reveals that mOCs secrete protons for bone resorption when they are not in contact with mOBs, whereas mOCs contacting mOBs are non-resorptive, suggesting that mOBs can inhibit bone resorption by direct contact. Intermittent administration of parathyroid hormone causes bone anabolic effects, which lead to a mixed distribution of mOBs and mOCs, and increase cell–cell contact. This study reveals spatiotemporal intercellular interactions between mOBs and mOCs affecting bone homeostasis in vivo

Utilization of Transgenic Models in Evaluation of Osteogenic Differentiation of Embryonic Stem Cells

Previous studies reported that embryonic stem cells (ESCs) can be induced to differentiate into cells showing a mature osteoblastic phenotype by culturing them under osteo-inductive conditions. It is probable that osteogenic differentiation requires that ESCs undergo differentiation through an intermediary step involving a mesenchymal lineage precursor. Based on our previous studies indicating that adult mesenchymal progenitor cells express αSMA, we have generated ESCs from transgenic mice in which an αSMA promoter directs the expression of red fluorescent protein (RFP) to mesenchymal progenitor cells. To track the transition of ESC-derived MSCs into mature osteoblasts, we have utilized a bone-specific fragment of rat type I collagen promoter driving green fluorescent protein (Col2.3GFP). Following osteogenic induction in ESCs, we have observed expression of alkaline phosphatase and subsequent mineralization as detected by von Kossa staining. After one week of osteogenic induction, ESCs begin to express αSMARFP. This expression was localized to the peripheral area encircling a typical ESC colony. Nevertheless, these αSMARFP positive cells did not show activation of the Col2.3GFP promoter, even after 7 weeks of osteogenic differentiation in vitro. In contrast, Col2.3GFP expression was detected in vivo, in mineralized areas following teratoma formation. Our results indicate that detection of alkaline phosphatase activity and mineralization of ESCs cultured under osteogenic conditions is not sufficient to demonstrate osteogenic maturation. Our study indicates the utility of the promoter-visual transgene approach to assess the commitment and differentiation of ESCs into the osteoblast lineage

Stepwise Differentiation of Pluripotent Stem Cells into Osteoblasts Using Four Small Molecules under Serum-free and Feeder-free Conditions

Pluripotent stem cells are a promising tool for mechanistic studies of tissue development, drug screening, and cell-based therapies. Here, we report an effective and mass-producing strategy for the stepwise differentiation of mouse embryonic stem cells (mESCs) and mouse and human induced pluripotent stem cells (miPSCs and hiPSCs, respectively) into osteoblasts using four small molecules (CHIR99021 [CHIR], cyclopamine [Cyc], smoothened agonist [SAG], and a helioxanthin-derivative 4-(4-methoxyphenyl)pyrido[4′,3′:4,5]thieno[2,3-b]pyridine-2-carboxamide [TH]) under serum-free and feeder-free conditions. The strategy, which consists of mesoderm induction, osteoblast induction, and osteoblast maturation phases, significantly induced expressions of osteoblast-related genes and proteins in mESCs, miPSCs, and hiPSCs. In addition, when mESCs defective in runt-related transcription factor 2 (Runx2), a master regulator of osteogenesis, were cultured by the strategy, they molecularly recapitulated osteoblast phenotypes of Runx2 null mice. The present strategy will be a platform for biological and pathological studies of osteoblast development, screening of bone-augmentation drugs, and skeletal regeneration