Osteogenic protein-1 up-regulation of the collagen X promoter activity is mediated by a MEF-2-like sequence and requires an adjacent AP-1 sequence

Thrombospondin-1 prevents excessive ossification in cartilage repair tissue induced by osteogenic protein-1

This study investigated the effect of thrombospondin-1 (TSP-1) on the formation of cartilage repair tissue in combination with stimulation by osteogenic protein-1 (OP-1). In miniature pigs, articular cartilage lesions in the femoral trochlea were treated by the microfracture technique and either received no further treatment (MFX), or were treated by additional application of recombinant osteogenic protein-1 (MFX+OP-1), recombinant TSP-1 (MFX+TSP-1), or a combination of both proteins (MFX+TSP-1+OP-1). Six and 26 weeks after surgery, the repair tissue and the degree of endochondral ossification were assessed by histochemical and immunohistochemical methods detecting collagen types I, II, X, TSP-1, and CD31. Microfracture treatment merely induced the formation of inferior fibrocartilaginous repair tissue. OP-1 stimulated chondrogenesis, but also induced chondrocyte hypertrophy, characterized by synthesis of collagen type X, and excessive bone formation. Application of TSP-1 inhibited inadvertant endochondral ossification, but failed to induce chondrogenesis. In contrast, the simultaneous application of both TSP-1 and OP-1 induced and maintained a permanent, nonhypertrophic chondrocyte-like phenotype within cartilage repair tissue. The data of this study demonstrate that OP-1 and TSP-1 complement each other in a functional manner. While OP-1 induces chondrogenesis of the ingrowing cells, TSP-1 prevents their further hypertrophic differentiation and prevents excessive endochondral ossification within the lesions.

Generation and characterization of Col10a1-mcherry reporter mice

We report here on the generation of a new fluorescent protein reporter transgenic mouse line, Col10a1-mCherry, which can be used as a tool to study chondrocyte biology and pathology. Collagen, Type X, alpha 1 (Col10a1) is highly expressed in hypertrophic chondrocytes and commonly used as a gene marker for this cell population. The Col10a1-mCherry reporter line was generated using a bacterial recombination strategy with the mouse BAC clone RP23-192A7. To aid in the characterization of this animal model, we intercrossed Col10a1-mCherry mice with Collagen, Type II, alpha 1 (Col2a1) enhanced cyan fluorescent protein (ECFP) reporter mice and characterized the expression of both chondrocyte reporters during embryonic skeletal development from days E10.5 to E17.5. Additionally, at postnatal day 0, Col10a1-mCherry reporter expression was compared to endogenous Col10a1 mRNA expression in long bones and revealed that mCherry fluorescence extended past the Col10a1 expression domain. However, in situ hybridization for mCherry was consistent with the zone of Col10a1 mRNA expression, indicating that the persistent detection of mCherry fluorescence was a result of the long protein half life of mCherry in conjunction with a very rapid phase of skeletal growth and not due to aberrant transcriptional regulation. Taking advantage of the continued fluorescence of hypertrophic chondrocytes at the chondro-osseus junction, we intercrossed Col10a1-mCherry mice with two different Collagen, Type 1, alpha 1, (Col1a1) osteoblast reporter mice, pOBCol3.6-Topaz and pOBCol2.3-Emerald to investigate the possibility that hypertrophic chondrocytes transdifferentiate into osteoblasts. Evaluation of long bones at birth suggests that residual hypertrophic chondrocytes and osteoblasts in the trabecular zone exist as two completely distinct cell populations. genesis 49:410-418, 2011.

Prostaglandin E2 inhibits BMP signaling and delays chondrocyte maturation

While cyclooxygenases are important in endochondral bone formation during fracture healing, mechanisms involved in prostaglandin E2 (PGE2) regulation of chondrocyte maturation are incompletely understood. The present study was undertaken to determine if PGE2 effects on chondrocyte differentiation are related to modulation of the bone morphogenetic protein (BMP) signaling pathway. In primary murine sternal chondrocytes, PGE2 differentially regulated genes involved in differentiation. PGE2 induced type II collagen and MMP-13, had minimal effects on alkaline phosphatase, and inhibited the expression of the maturational marker, type X collagen. In BMP-2-treated cultures, PGE2 blocked the induction of type X collagen. All four EP receptors were expressed in chondrocytes and tended to be inhibited by BMP-2 treatment. RCJ3.1C5.18 chondrocytes transfected with the protein kinase A (PKA) responsive reporter, CRE-luciferase, showed luciferase induction following exposure to PGE2, consistent with activation of PKA signaling and the presence of the EP2 and EP4 receptors. Both PGE2 and the PKA agonist, dibutyryl cAMP, blocked the induction of the BMP-responsive reporter, 12XSBE, by BMP-2 in RCJ3.1C5.18 chondrocytes. In contrast, PGE2 increased the ability of TGF-beta to activate the TGF-beta-responsive reporter, 4XSBE. Finally, PGE2 down-regulated BMP-mediated phosphorylation of Smads 1, 5, and 8 in RCJ3.1C5.18 cells and in primary murine sternal chondrocytes. Altogether, the findings show that PGE2 regulates chondrocyte maturation in part by targeting BMP/Smad signaling and suggest an important role for PGE2 in endochondral bone formation.

Inhibin and the regulation of bone mass

Inhibins A and B are gonadal peptide members of the transforming growth factor-beta superfamily that serve as negative feedback regulators of pituitary follicle-stimulating hormone (FSH). Accumulating evidence suggests that bone turnover and bone loss increase in women before menopause and the decrease in serum estradiol levels. Increased FSH levels have been correlated with some of these perimenopausal changes, whereas decreased inhibins strongly correlate with increases in bone formation and resorption across the menopause transition, and predict lumbar bone mass in perimenopausal women, likely resulting from the direct inhibin suppression of osteoblast and osteoclast development. Interestingly, continuous exposure of mice to inhibin A in vivo is anabolic and protective against gonadectomy-induced bone loss. Together, these data suggest inhibins contribute to the endocrine regulation of bone metabolism via a bimodal mechanism of action such that cycling inhibin exposure suppresses bone turnover, and continuous exposure to inhibins is anabolic.

Axin1 and Axin2 are regulated by TGF- and mediate cross-talk between TGF- and Wnt signaling pathways

Chondrocyte maturation during endochondral bone formation is regulated by a number of signals that either promote or inhibit maturation. Among these, two well-studied signaling pathways play crucial roles in modulating chondrocyte maturation: transforming growth factor-beta (TGF-beta)/Smad3 signaling slows the rate of chondrocyte maturation, while Wingless/INT-1-related (Wnt)/beta-catenin signaling enhances the rate of chondrocyte maturation. Axin1 and Axin2 are functionally equivalent and have been shown to inhibit Wnt/beta-catenin signaling and stimulate TGF-beta signaling. Here we show that while Wnt3a stimulates Axin2 in a negative feedback loop, TGF-beta suppresses the expression of both Axin1 and Axin2 and stimulates beta-catenin signaling. In Axin2 -/- chondrocytes, TGF-beta treatment results in a sustained increase in beta-catenin levels compared to wild-type chondrocytes. In contrast, overexpression of Axin enhanced TGF-beta signaling while overexpression of beta-catenin inhibited the ability of TGF-beta to induce Smad3-sensitive reporters. Finally, the suppression of the Axins is Smad3-dependent since the effect is absent in Smad3 -/- chondrocytes. Altogether these findings show that the Axins act to integrate signals between the Wnt/beta-catenin and TGF-beta/Smad pathways. Since the suppression Axin1 and Axin2 expression by TGF-beta reduces TGF-beta signaling and enhances Wnt/beta-catenin signaling, the overall effect is a shift from TGF-beta toward Wnt/beta-catenin signaling and an acceleration of chondrocyte maturation.

[Use of bone marrow mesenchymal stem cells for ex vivo cartilage regeneration]

Articular cartilage disorders and injuries often result in lifelong chronic pain and compromised quality of life. When it comes to local articular cartilage defects, modern medicine is limited to short-term pain relief and inflammation control. In extreme cases the affected tissue is surgically removed and replaced by a synthetic prosthesis of limited durability. Cell-based therapies to regenerate articular cartilage have been in use since 1994. Such therapies provide a healthy population of cells to the injured site and require differentiated chondrocytes from the uninjured site as base material. Their usage often leads to donor site morbidity and they generate rigid fibrous cartilage where more flexible hyaline cartilage is required. The major restrictive factors for such methods are inadequate number and limited proliferation capacity of chondrocytes in vitro. Tissue engineering of adult marrow stromal cells/mesenchymal stem cells (MSCs) with their almost unlimited proliferation potential and proven capability to differentiate into chondrocytes for ex vivo generation of cartilage tissue still remains a vision. For optimal harnessing of MSCs as chondroprogenitor cells, basic background information regarding commitment to the lineage, cartilage differentiation and the regulatory factors and molecules involved is essential.

Leptin regulates chondrogenic differentiation in ATDC5 cell-line through JAK/STAT and MAPK pathways

Leptin, the satiety hormone, has been found to affect growth-plate cartilage development. In the present study, some of the signal transduction pathways that mediate leptin signaling in the ATDC5 chondrogenic cell-line, a model for endochondral ossification, were analyzed. For this purpose, real-time PCR, Western blots and immunofluorescence techniques were used. It was found that leptin increased phosphorylation of ERK1/2, p38, and STAT3 in a time- and dose-dependent manner. Specific inhibition of STAT3 or ERK1/2, but not of P38, blocked the stimulatory effect of leptin on type X collagen mRNA levels. Moreover, leptin induced the translocation of ERK1/2 into the nucleus, as well as c-fos expression, indicating full activation of this cascade. Leptin-induced JNK phosphorylation was not observed, although leptin significantly and rapidly increased JNK protein levels and c-jun mRNA levels. In addition, ERK5 was identified in these cells, but there was no apparent effect of leptin on either its phosphorylation or protein level. The study indicates that the effects of leptin on growth-plate chondrocytes are specifically mediated through ERK1/2 and STAT3, while P38 is not essential for leptin-induced type X collagen expression. This is the first demonstration that these pathways are involved in leptin-induced growth.

Induction of apoptosis of human primary osteoclasts treated with a transcription factor decoy mimicking a promoter region of estrogen receptor alpha

In this paper we investigated how the increase of human estrogen receptor alfa (ERalpha) gene expression may affect breast, osteoblast and osteoclast cells. Increase of ERalpha expression was obtained by interfering with the activity of a negative transcription factor and by removing it with a short and powerful decoy oligonucleotide (RA4-3') mimicking a region of distal promoter C of ERalpha gene. We provide evidence that this decoy was able to induce apoptosis in osteoclasts, but not in osteoblasts and in breast cancer cells, in an estrogen dependent manner. This effect was associated with increase of the levels of Caspase 3 and Fas receptor. Since ERalpha is important in the transcription of different genes and is involved in several pathological processes, including neoplastic and osteopenic diseases, our findings may be of relevance for a possible new therapeutical approach of such diseases.

Transcription factor decoy against promoter C of estrogen receptor α gene induces a functional ERα protein in breast cancer cells

This study addresses the hypothesis that transfection of oligonucleotide mimicking a negative regulatory sequence of promoter C of estrogen receptor alpha (ER alpha) gene is sufficient for its re-expression in ER-negative human cancer cell lines. Even if the negative transcription regulator subtracted by the transcription factor decoy is not yet been identified, we demonstrated that after this decoy treatment, the cells produced a functional ER alpha protein able to respond to 17-beta-estradiol and to transactivate a transfected estrogen response element (ERE)-regulated reporter gene. The effects of reactivated ER alpha protein and its estrogen dependence on endogenous target gene expression level, such as ER beta, have been also assessed. The proliferation of the cells transfected with low levels of decoy was significantly increased by estrogen and not by tamoxifen, suggesting that the levels of reactivated ER alpha in these decoy conditions confers a certain hormone sensitivity. On the contrary, high-level expression of ER alpha obtained at high doses of transfected decoy molecule produced a progressive decrease of cell proliferation. Since ER alpha is important in the transcription of different genes and its loss is involved in several pathological processes including neoplastic and chronic diseases, our findings may be of relevance for a possible new therapeutical approach of such diseases.

SP3/SP1 transcription activity regulates specific expression of collagen type X in hypertrophic chondrocytes

Previously, we have shown that two non-canonical specificity protein (SP)-binding sites within the proximal promoter (nucleotide (nt) -139 to +5) of the chicken Col10a1 gene are involved in conferring tissue-specific expression of type X collagen to hypertrophic chondrocytes. In the present study, we examined the role of SP3/SP1 transcription factors in the regulation of the Col10a1 promoter. The SP3/SP1 ratio is higher in hypertrophic versus non-hypertrophic chondrocytes, due to the significant decrease in SP1 in hypertrophic cells detected by real-time PCR and Western blot analyses. Functional analyses by transfection-mediated overexpression of SP1 and SP3 suggest that SP1 inhibits the Col10a1 promoter. This effect is negated by an interaction with SP3 in hypertrophic chondrocytes. Additionally, mutation analysis showed that the 40-bp intervening sequence (nt -115 to -75) is required for expression of the Col10a1 gene. In this sequence, a binding site for Dlx5/6 transcription factors (nt -99 to -87) retards a protein specific for hypertrophic chondrocytes in electrophoretic mobility shift assay. Endogenous levels of Dlx5 are 3-fold higher in hypertrophic versus non-hypertrophic cells by real-time PCR analysis, and overexpression of Dlx5 in non-hypertrophic chondrocytes activates the proximal Col10a1 promoter 3-fold. These results indicate that the SP3/SP1 ratio and Dlx5 are important regulators of the proximal Col10a1 promoter in hypertrophic cartilage and suggest that interactions between SP3 and SP1 regulate expression of different types of collagen during chondrocyte differentiation.

Identification of alternative promoter usage for the matrix Gla protein gene. Evidence for differential expression during early development in Xenopus laevis

Recent cloning of the Xenopus laevis (Xl) matrix Gla protein (MGP) gene indicated the presence of a conserved overall structure for this gene between mammals and amphibians but identified an additional 5'-exon, not detected in mammals, flanked by a functional, calcium-sensitive promoter, 3042 bp distant from the ATG initiation codon. DNA sequence analysis identified a second TATA-like DNA motif located at the 3' end of intron 1 and adjacent to the ATG-containing second exon. This putative proximal promoter was found to direct transcription of the luciferase reporter gene in the X. laevis A6 cell line, a result confirmed by subsequent deletion mutant analysis. RT-PCR analysis of XlMGP gene expression during early development identified a different temporal expression of the two transcripts, strongly suggesting differential promoter activation under the control of either maternally inherited or developmentally induced regulatory factors. Our results provide further evidence of the usefulness of nonmammalian model systems to elucidate the complex regulation of MGP gene transcription and raise the possibility that a similar mechanism of regulation may also exist in mammals.

The AP1 transcription factor Fra2 is required for efficient cartilage development

The Fos-related AP1 transcription factor Fra2 (encoded by Fosl2)is expressed in various epithelial cells as well as in cartilaginous structures. We studied the role of Fra2 in cartilage development. The absence of Fra2 in embryos and newborns leads to reduced zones of hypertrophic chondrocytes and impaired matrix deposition in femoral and tibial growth plates, probably owing to impaired differentiation into hypertrophic chondrocytes. In addition, hypertrophic differentiation and ossification of primordial arches of the developing vertebrae are delayed in Fra2-deficient embryos. Primary Fosl2–/– chondrocytes exhibit decreased hypertrophic differentiation and remain in a proliferative state longer than wild-type cells. As pups lacking Fra2 die shortly after birth, we generated mice carrying `floxed' Fosl2 alleles and crossed them to coll2a1-Cre mice, allowing investigation of postnatal cartilage development. The coll2a1-Cre, Fosl2f/f mice die between 10 and 25 days after birth, are growth retarded and display smaller growth plates similar to Fosl2–/– embryos. In addition, these mice suffer from a kyphosis-like phenotype, an abnormal bending of the spine. Hence, Fra2 is a novel transcription factor important for skeletogenesis by affecting chondrocyte differentiation.

Identification of an osteogenic protein-1 responsive element in the aggrecan promoter

Previous studies have demonstrated that osteogenic protein-1 (OP-1), a member of the bone morphogenetic protein (BMP) subfamily of the TGF-beta superfamily, stimulates aggrecan synthesis. To investigate transcriptional regulation of the aggrecan gene by OP-1, we constructed a clone containing a 1 kb region of the 5'-upstream sequence of the mouse aggrecan gene fused to the promoter-less luciferase reporter gene in pGL2-Basic vector. A series of promoter deletion constructs were also generated. Studies of the promoter activity of these DNA constructs in transient-transfected cells revealed that deletion of a 33 bp region rendered the promoter unresponsive to OP-1, BMP-6, and CDMP-1 without affecting BMP-2 and TGF-beta1 responsiveness. Thus, the expression of the mouse aggrecan gene in response to BMPs appears to be the result of a unique combination of different cis-acting elements.

p38 MAP kinase signalling is required for hypertrophic chondrocyte differentiation

Longitudinal growth of endochondral bones is accomplished through the co-ordinated proliferation and hypertrophic differentiation of growth plate chondrocytes. The molecular mechanisms and signalling cascades controlling these processes are not well understood. To analyse the expression and roles of p38 mitogen-activated protein kinases in this process, we have established a micromass system for the reproducible hypertrophic differentiation of mouse mesenchymal limb bud cells. Our results show that all four mammalian p38 kinase genes are expressed during the chondrogenic programme, as well as their upstream regulators MKK3 (mitogen-activated protein kinase kinase 3) and MKK6. Treatment of micromass cultures with pharmacological inhibitors of p38 results in a marked delay in hypertrophic differentiation in micromass cultures, indicating a requirement for p38 signalling in chondrocyte differentiation. Inhibition of p38 kinase activity leads to reduced and delayed induction of alkaline phosphatase activity and matrix mineralization. In addition, p38 inhibition causes reduced expression of hypertrophic marker genes such as collagen X, matrix metalloproteinase 13 and bone sialoprotein. The function of p38 in hypertrophic differentiation appears to be mediated, at least in part, by the transcription factor myocyte enhancer factor 2C. In summary, we have demonstrated a novel requirement for p38 signalling in hypertrophic differentiation of chondrocytes and identified myocyte enhancer factor 2C as an important regulator of chondrocyte gene expression.

CREB Cooperates with BMP-stimulated Smad signaling to enhance transcription of the Smad6 promoter

Growth plate chondrocytes integrate a multitude of growth factor signals during maturation. PTHrP inhibits maturation through stimulation of PKA/CREB signaling while the bone morphogenetic proteins (BMPs) stimulate maturation through Smad mediated signaling. In this manuscript, we show that interactions between CREB and the BMP associated Smads are promoter specific, and demonstrate for the first time the requirement of CREB signaling for Smad mediated activation of a BMP responsive region of the Smad6 promoter. The 28 base pairs (bp) BMP responsive element of the Smad6 promoter contains an 11 bp Smad binding region and an adjacent 17 bp region in which we characterize a putative CRE site. PKA/CREB gain of function enhanced BMP stimulation of this reporter, while loss of CREB function diminished transcriptional activity. In contrast, ATF-2 and AP-1 transcription factors had minimal effects. Electrophoretic mobility shift assay (EMSA) confirmed CREB binding to the Smad6 promoter element. Mutations eliminating binding resulted in loss of transcriptional activity, while mutations that maintained CREB binding had continued reporter activation by CREB and BMP-2. The Smad6 gene was similarly regulated by CREB. Dominant negative CREB reduced BMP-2 stimulated Smad6 gene transcription by 50%, but markedly increased BMP-2 mediated stimulation of colX and Ihh expression. In contrast, PTHrP which activates CREB signaling, blocked the stimulatory effect of BMP-2 on colX and Ihh, but minimally inhibited the stimulatory effect of BMP on Smad6. These findings are the first to demonstrate a cooperative association between CREB and BMP regulated Smads in cells from vertebrates and demonstrate that promoter-specific rather than generalized interactions between PKA/CREB and BMP signaling regulate gene expression in chondrocytes.

Transcriptional regulation of BMP4 synexpression in transgenic Xenopus

Synexpression groups are genetic modules composed of genes that share both a complex expression pattern and the biological process in which they function. Here we investigate the regulation of BMP4 synexpression by studying the enhancers of bambi, smad7 and vent2 in Xenopus. We find that a BMP4 synexpression promoter module is compact and (i) requires direct BMP responsiveness through Smad and Smad-cofactor binding motifs, (ii) may contain an evolutionary conserved BMP-responsive element, bre7 (TGGCGCC), that is crucial for expression of bambi and smad7 and is highly prognostic for novel BMP-responsive enhancers (BREs); and (iii) requires a narrow window of BMP inducibility, because minor enhancement or reduction of BMP responsiveness abolishes synexpression. Furthermore, we used a bioinformatic model to predict in silico 13 novel BREs, and tested five of them that were found in the id1-4 genes. The results highlight that in vivo analysis is required to reveal the physiological, spatio-temporal regulation of BMP-responsive genes.

MAP kinases in chondrocyte differentiation

The majority of the vertebrate skeleton develops through the process of endochondral ossification and involves successive steps of chondrogenesis, chondrocyte proliferation, and hypertrophic chondrocyte differentiation. Interruption of this program through gene mutations and hormonal or environmental factors contributes to numerous diseases, including growth disorders and chondrodysplasias. While a large number of growth factors and hormones have been implicated in the regulation of chondrocyte biology, relatively little is known about the intracellular signaling pathways involved. Recent data provide novel insights into the mechanisms governing acquisition of new phenotypes within the chondrogenic program and suggest multiple pivotal roles for members of the mitogen-activated protein kinase family and their downstream targets in cartilage development. These data are summarized and discussed here.

Role of c-fos in the regulation of type X collagen gene expression by PTH and PTHrP: localization of a PTH/PTHrP-responsive region in the human COL10A1 enhancer

PTH and PTHrP have been shown to inhibit maturation of growth plate chondrocytes and the expression of type X collagen. In order to examine the regulatory mechanisms involved, fetal bovine growth plate chondrocytes were incubated for 24-48 h under serum-free conditions with PTH and PTHrP and various aminoterminal, midregional, and carboxyterminal fragments of these hormones. Analysis of type X collagen mRNA levels by Northern hybridization showed a significant suppression by PTH (1-84), PTH (1-34), and PTHrP (1-40), but not by PTH (28-48) or PTH (53-84). PTH fragment (3-34) did not reduce alpha1(X) mRNA levels, while bis-indolylmaleimide, an inhibitor of the protein-kinase C pathway, did not affect alpha1(X) mRNA suppression by PTH, supporting the notion that the inhibition of type X collagen expression by PTH involves predominantly the adenylate cyclase pathway of the PTH/PTHrP-receptor. Since PTH and PTHrP have been shown to induce c-fos in osteoblasts and chondrocytes, the possibility was tested that c-fos mediated the suppressive effect of PTH/PTHrP on collagen X expression. In fetal bovine hypertrophic chondrocytes PTH (1-34), but not PTH (3-34) nor the midregional or C-terminal PTH fragments induced c-fos expression. In order to identify cis- and trans-acting elements in the COL10A1 gene involved in c-fos-mediated inhibition of collagen X expression by PTH/PTHrP, reporter gene constructs carrying various fragments of the human COL10A1 promoter coupled to the luciferase gene were transfected into hypertrophic chondrocytes. A tissue-specific, strong enhancer region, which we had previously located in the promoter of the human type X collagen gene COL10A1, was further narrowed down to a 530-bp sequence, located between - 1,870- and - 2,407 bp upstream of the transcription start site. The transcriptional activity of this enhancer element in transfected hypertrophic chondrocytes was significantly reduced after incubation with PTH (1-34) or PTHrP (1-40). Transcription of these reporter genes was also inhibited when chondrocytes were cotransfected with a c-fos expression vector. These results indicate the presence of a PTH/PTHrP responsive element in the human COL10A1 enhancer, which may be represented by multiple putative AP-1 sites located in this region.

An enhancer complex confers both high-level and cell-specific expression of the human type X collagen gene

Type X collagen expression is restricted to hypertrophic chondrocytes in the endochondral growth plate. Transient transfection of reporter constructs containing the human collagen X promoter into primary growth plate chondrocytes identified a cis-acting positive regulatory DNA element(s) that has cell-specific enhancer properties and binds a nuclear protein expressed specifically in growth plate chondrocytes. Functional disruption of this region results in a significant reduction in the activation of reporter gene transcription. The identified enhancer is a major element controlling both high-level and cell-specific expression of type X collagen gene.

Predicting transcription factor synergism

Transcriptional regulation is mediated by a battery of transcription factor (TF) proteins, that form complexes involving protein-protein and protein-DNA interactions. Individual TFs bind to their cognate cis-elements or transcription factor-binding sites (TFBS). TFBS are organized on the DNA proximal to the gene in groups confined to a few hundred base pair regions. These groups are referred to as modules. Various modules work together to provide the combinatorial regulation of gene transcription in response to various developmental and environmental conditions. The sets of modules constitute a promoter model. Determining the TFs that preferentially work in concert as part of a module is an essential component of understanding transcriptional regulation. The TFs that act synergistically in such a fashion are likely to have their cis-elements co-localized on the genome at specific distances apart. We exploit this notion to predict TF pairs that are likely to be part of a transcriptional module on the human genome sequence. The computational method is validated statistically, using known interacting pairs extracted from the literature. There are 251 TFBS pairs up to 50 bp apart and 70 TFBS pairs up to 200 bp apart that score higher than any of the known synergistic pairs. Further investigation of 50 pairs randomly selected from each of these two sets using PubMed queries provided additional supporting evidence from the existing biological literature suggesting TF synergism for these novel pairs.

Chondrogenic differentiation of mesenchymal stem cells from bone marrow: differentiation-dependent gene expression of matrix components

Transforming growth factor (TGF)-beta-induced chondrogenesis of mesenchymal stem cells derived from bone marrow involves the rapid deposition of a cartilage-specific extracellular matrix. The sequential events in this pathway leading from the undifferentiated stem cell to a mature chondrocyte were investigated by analysis of key matrix elements. Differentiation was rapidly induced in cells cultured in the presence of TGF-beta 3 or -beta 2 and was accompanied by the early expression of fibromodulin and cartilage oligomeric matrix protein. An increase in aggrecan and versican core protein synthesis defined an intermediate stage, which also involved the small leucine-rich proteoglycans decorin and biglycan. This was followed by the appearance of type II collagen and chondroadherin. The pathway was also characterized by the appearance of type X collagen, usually associated with hypertrophic cartilage. There was also a change in the pattern of sulfation of chondroitin sulfate, with a progressive increase in the proportion of 6-sulfated species. The major proportion of newly synthesized glycosaminoglycan was part of an aggregating proteoglycan network. These data allow us to define the phenotype of the differentiated cell and to understand in greater detail the sequential process of matrix assembly.

A novel cell culture model of chondrocyte differentiation during mammalian endochondral ossification

Endochondral ossification (EO) occurs in the growth plate where chondrocytes pass through discrete stages of proliferation, maturation, hypertrophy, and calcification. We have developed and characterized a novel bovine cell culture model of EO that mirrors these events and will facilitate in vitro studies on factors controlling chondrocyte differentiation. Chondrocytes derived from the epiphyses of long bones of fetal calves were treated with 5-azacytidine (aza-C) for 48 h. Cultures were maintained subsequently without aza-C and harvested at selected time points for analyses of growth and differentiation status. A chondrocytic phenotype associated with an extensive extracellular matrix rich in proteoglycans and collagen types II and VI was observed in aza-C-treated and -untreated cultures. aza-C-treated cultures were characterized by studying the expression of several markers of chondrocyte differentiation. Parathyroid hormone-related protein (PTHrP) and its receptor, both markers of maturation, were expressed at days 5-9. Type X collagen, which is restricted to the stage of hypertrophy, was expressed from day 11 onward. Hypertrophy was confirmed by a 14-fold increase in cell size by day 15 and an increased synthesis of alkaline phosphatase during the hypertrophic period (days 14-28). The addition of PTHrP to aza-C-treated cultures at day 14 led to the down-regulation of type X collagen by 6-fold, showing type X collagen expression is under the control of PTHrP as in vivo. These findings show that aza-C can induce fetal bovine epiphyseal chondrocytes to differentiate in culture in a manner consistent with that which occurs during the EO process in vivo.

Identification of an osteogenic protein-1 (bone morphogenetic protein-7)-responsive element in the promoter of the rat insulin-like growth factor-binding protein-5 gene

Osteogenic protein-1 (OP-1), a member of the bone morphogenetic protein subfamily of the transforming growth factor-beta superfamily, induces new bone formation in vivo and regulates the expression of numerous growth factors. We previously showed that OP-1 down-regulates the transcription of the insulin-like growth factor-binding protein-5 (IGFBP-5) in primary cultures of fetal rat calvaria (FRC) cells. In the present study we identified, within the IGFBP-5 promoter, a 21-bp region that confers OP-1 responsiveness in FRC cells. Within this region lie three putative cis-acting regulatory elements, viz. a CAAT-like sequence, a CCAAT/enhancer-binding protein (C/EBPalpha)-like element, and a c-Myb or E-box-like motif. Mutations in the CAAT-like sequence reduced the promoter activity in both control and OP-1-treated cells, but did not abrogate the OP-1-induced down-regulation. Mutations in the C/EBPalpha-like element reduced the promoter activity in both control and OP-1-treated cells without significantly affecting the extent of down-regulation. Mutations in the putative c-Myb or E-box-like motif reduced the promoter activity in both the OP-1-treated and control cells and completely abolished the inhibitory effect of OP-1 on the IGFBP-5 promoter activity. Gel mobility shift analyses further showed specific interaction between nuclear protein(s) in FRC cells and the 21-bp region. OP-1 down-regulates the nuclear regulatory protein interaction with the 21-bp region by reducing either the cellular concentration of the regulatory protein(s) or the affinity of the regulatory protein(s) for the OP-1 responsive element. In conclusion, we identified an OP-1 response region in the rat IGFBP-5 promoter and further showed that OP-1 down-regulates the nuclear protein interaction with the response element(s).

Estrogen opposes the apoptotic effects of bone morphogenetic protein 7 on tissue remodeling

Interactions between estrogen and growth factor signaling pathways at the level of gene expression play important roles in the function of reproductive tissues. For example, estrogen regulates transforming growth factor beta (TGFbeta) in the uterus during the proliferative phase of the mammalian reproductive cycle. Bone morphogenetic protein 7 (BMP-7), a member of the TGFbeta superfamily, is also involved in the development and function of reproductive tissues. However, relatively few studies have addressed the expression of BMP-7 in reproductive tissues, and the role of BMP-7 remains unclear. As part of an ongoing effort to understand how estrogen represses gene expression and to study its interactions with other signaling pathways, chick BMP-7 (cBMP-7) was cloned. cBMP-7 mRNA levels are repressed threefold within 8 h following estrogen treatment in the chick oviduct, an extremely estrogen-responsive reproductive tissue. This regulation occurs at the transcriptional level. Estrogen has a protective role in many tissues, and withdrawal from estrogen often leads to tissue regression; however, the mechanisms mediating regression of the oviduct remain unknown. Terminal transferase-mediated end-labeling and DNA laddering assays demonstrated that regression of the oviduct during estrogen withdrawal involves apoptosis, which is a novel observation. cBMP-7 mRNA levels during estrogen withdrawal increase concurrently with the apoptotic index of the oviduct. Furthermore, addition of purified BMP-7 induces apoptosis in primary oviduct cells. This report demonstrates that the function of BMP-7 in the oviduct involves the induction of apoptosis and that estrogen plays an important role in opposing this function.

Smad6 is a Smad1/5-induced smad inhibitor. Characterization of bone morphogenetic protein-responsive element in the mouse Smad6 promoter

Smad6 is an inhibitory Smad that is induced by bone morphogenetic proteins (BMPs) and interferes with BMP signaling. We have isolated the mouse Smad6 promoter and identified the regions responsible for transcriptional activation by BMPs. The proximal BMP-responsive element (PBE) in the Smad6 promoter is important for the transcriptional activation by BMPs and contains a 28-base pair GC-rich sequence including four overlapping copies of the GCCGnCGC-like motif, which is a binding site for Drosophila Mad and Medea. We generated a luciferase reporter construct (3GC2-Lux) containing three repeats of the GC-rich sequence derived from the PBE. BMPs and BMP receptors induced transcriptional activation of 3GC2-Lux in various cell types, and this activation was enhanced by cotransfection of BMP-responsive Smads, i.e. Smad1 or Smad5. Moreover, direct DNA binding of BMP-responsive Smads and common-partner Smad4 to the GC-rich sequence of PBE was observed. These results indicate that the expression of Smad6 is regulated by the effects of BMP-activated Smad1/5 on the Smad6 promoter.

Characterization of a bone morphogenetic protein-responsive Smad-binding element

Bone morphogenetic proteins (BMPs) are pleiotropic growth and differentiation factors belonging to the transforming growth factor-beta (TGF-beta) superfamily. Signals of the TGF-beta-like ligands are propagated to the nucleus through specific interaction of transmembrane serine/threonine kinase receptors and Smad proteins. GCCGnCGC has been suggested as a consensus binding sequence for Drosophila Mad regulated by a BMP-like ligand, Decapentaplegic. Smad1 is one of the mammalian Smads activated by BMPs. Here we show that Smad1 binds to this motif upon BMP stimulation in the presence of the common Smad, Smad4. The binding affinity is likely to be relatively low, because Smad1 binds to three copies of the motif weakly, but more repeats of the motif significantly enhance the binding. Heterologous reporter genes (GCCG-Lux) with multiple repeats of the motif respond to BMP stimulation but not to TGF-beta or activin. Mutational analyses reveal several bases critical for the responsiveness. A natural BMP-responsive reporter, pTlx-Lux, is activated by BMP receptors in P19 cells but not in mink lung cells. In contrast, GCCG-Lux responds to BMP stimulation in both cells, suggesting that it is a universal reporter that directly detects Smad phosphorylation by BMP receptors.

Serum induction of the collagen X promoter requires the Raf/MEK/ERK and p38 pathways

The collagen X gene is expressed exclusively by differentiated, hypertrophic chondrocytes. The mechanisms controlling collagen X expression remain largely unknown. Here we show that collagen X promoter activity can be induced by serum stimulation of chondrogenic MCT cells. The serum response is conferred by a 462 nucleotide promoter fragment. Both the c-Raf/MEK/ERK and p38 MAP kinase pathways are required for this effect, whereas phosphatidylinositol-3-kinase and protein kinase A repress promoter activation. These data are the first to demonstrate serum inducibility of the collagen X promoter and to identify signal transduction pathways involved.

Osteogenic protein-1 protects against cerebral infarction induced by MCA ligation in adult rats

BACKGROUND AND PURPOSE

Osteogenic protein-1 (OP1) not only possesses trophic activity on bone tissue but also influences neuronal survival and differentiation in vitro. Specific receptors for OP1 are present in brain and spinal cord and can be upregulated during cerebral contusion. OP1 is a member of the transforming growth factor-beta superfamily, several of whose members possess neuroprotective activity. In this study, the neuroprotective effect of OP1 in cerebral ischemia was evaluated in adult animals.

METHODS

Adult male Sprague-Dawley rats were anesthetized with chloral hydrate. OP1 or vehicle was administered intracortically or intracerebroventricularly to the rats. Thirty minutes, 24 hours, or 72 hours after OP1 injection, the right middle cerebral artery (MCA) was ligated for 90 minutes. Twenty-four hours after reperfusion, animals were tested for motor behavior. The animals were subsequently anesthetized with urethane and perfused intracardially with saline. Brain tissue was removed, sliced, and incubated with 2% triphenyltetrazolium chloride to localize the area of infarction.

RESULTS

Only animals pretreated with OP1 24 hours before MCA ligation showed a reduction in motor impairment. OP1, given 30 minutes or 72 hours before MCA ligation, did not reduce cortical infarction. In contrast, pretreatment with OP1 24 hours before MCA ligation significantly attenuated the volume of infarction in the cortex, in agreement with the behavioral findings.

CONCLUSIONS

Intracerebral administration of OP1 24 hours before MCA ligation reduces ischemia-induced injury in the cerebral cortex.

A BMP responsive transcriptional region in the chicken type X collagen gene

Bone morphogenetic proteins (BMPs) were originally identified by their ability to induce ectopic bone formation and have been shown to promote both chondrogenesis and chondrocyte hypertrophy. BMPs have recently been found to activate a membrane serine/threonine kinase signaling mechanism in a variety of cell types, but the downstream effectors of BMP signaling in chondrocyte differentiation remain unidentified. We have previously reported that BMP-2 markedly stimulates type X collagen expression in prehypertrophic chick sternal chondrocytes, and that type X collagen mRNA levels in chondrocytes cultured under serum-free (SF) conditions are elevated 3- to 5-fold within 24 h. To better define the molecular mechanisms of induction of chondrocyte hypertrophy by BMPs, we examined the effect of BMPs on type X collagen production by 15-day chick embryo sternal chondrocytes cultured under SF conditions in the presence or absence of 30 ng/ml BMP-2, BMP-4, or BMP-7. Two populations of chondrocytes were used: one representing resting cartilage isolated from the caudal third of the sterna and the second representing prehypertrophic cartilage from the cephalic third of the sterna. BMP-2, BMP-4, and BMP-7 all effectively promoted chondrocyte maturation of cephalic sternal chondrocytes as measured by high levels of alkaline phosphatase, diminished levels of type II collagen, and induction of the hypertrophic chondrocyte-specific marker, type X collagen. To test whether BMP control of type X collagen expression occurs at the transcriptional level, we utilized plasmid constructs containing the chicken collagen X promoter and 5' flanking regions fused to a reporter gene. Constructs were transiently transfected into sternal chondrocytes cultured under SF conditions in the presence or absence of 30 ng/ml BMP-2, BMP-4, or BMP-7. A 533 bp region located 2.4-2.9 kb upstream from the type X collagen transcriptional start site was both necessary and sufficient for strong BMP responsiveness in cells destined for hypertrophy, but not in chondrocytes derived from the lower sterna.

Signal transduction by bone morphogenetic proteins

Bone morphogenetic proteins (BMPs) are multifunctional cytokines, which are members of the transforming growth factor-beta (TGF-beta) superfamily. Activities of BMPs are extracellularly regulated by BMP-binding proteins, Noggin and Chordin. BMPs bind to two different types of serine-threonine kinase receptors, type I and type II. Two BMP type I receptors and a BMP type II receptor have been identified in mammals. Intracellular signals are transduced by Smad proteins. Smad1, Smad5 and probably MADH6, are activated by BMP receptors, form heteromeric complexes with Smad4, and translocate into the nucleus where they may activate transcription of various genes. Smad6 and Smad7 are inhibitory Smads, and may act as autocrine switch-off signals. In Drosophila melanogaster, Decapentaplegic (Dpp) is a homologue of mammalian BMPs. In this review, mechanism of action of Dpp will be discussed in comparison with that of BMPs.