骨骼发育中的转录因子Cbfa1

骨骼由骨和软骨共同构成.最近的研究表明,转录因子Cbfa1不仅控制骨的形成和生长,还影响软骨组织成熟,并且可能与破骨细胞分化和软骨血管侵入有关.     

Role of Runx genes in chondrocyte differentiation

Runx2/Cbfa1 plays a central role in skeletal development as demonstrated by the absence of osteoblasts/bone in mice with inactivated Runx2/Cbfa1 alleles. To further investigate the role of Runx2 in cartilage differentiation and to assess the potential of Runx2 to induce bone formation, we cloned chicken Runx2 and overexpressed it in chick embryos using a retroviral system. Infected chick wings showed multiple phenotypes consisting of (1) joint fusions, (2) expansion of carpal elements, and (3) shortening of skeletal elements. In contrast, bone formation was not affected. To investigate the function of Runx2/Cbfa1 during cartilage development, we have generated transgenic mice that express a dominant negative form of Runx2 in cartilage. The selective inactivation of Runx2 in chondrocytes results in a severe shortening of the limbs due to a disturbance in chondrocyte differentiation, vascular invasion, osteoclast differentiation, and periosteal bone formation. Analysis of the growth plates in transgenic mice and in chick limbs shows that Runx2 is a positive regulator of chondrocyte differentiation and vascular invasion. The results further indicate that Runx2 promotes chondrogenesis either by maintaining or by initiating early chondrocyte differentiation. Furthermore, Runx2 is essential but not sufficient to induce osteoblast differentiation. To analyze the role of runx genes in skeletal development, we performed in situ hybridization with Runx2- and Runx3-specific probes. Both genes were coexpressed in cartilaginous condensations, indicating a cooperative role in the regulation of early chondrocyte differentiation and thus explaining the expansion/maintenance of cartilage in the carpus and joints of infected chick limbs.     

Smad6/Smurf1 overexpression in cartilage delays chondrocyte hypertrophy and causes dwarfism with osteopenia

Biochemical experiments have shown that Smad6 and Smad ubiquitin regulatory factor 1 (Smurf1) block the signal transduction of bone morphogenetic proteins (BMPs). However, their in vivo functions are largely unknown. Here, we generated transgenic mice overexpressing Smad6 in chondrocytes. Smad6 transgenic mice showed postnatal dwarfism with osteopenia and inhibition of Smad1/5/8 phosphorylation in chondrocytes. Endochondral ossification during development in these mice was associated with almost normal chondrocyte proliferation, significantly delayed chondrocyte hypertrophy, and thin trabecular bone. The reduced population of hypertrophic chondrocytes after birth seemed to be related to impaired bone growth and formation. Organ culture of cartilage rudiments showed that chondrocyte hypertrophy induced by BMP2 was inhibited in cartilage prepared from Smad6 transgenic mice. We then generated transgenic mice overexpressing Smurf1 in chondrocytes. Abnormalities were undetectable in Smurf1 transgenic mice. Mating Smad6 and Smurf1 transgenic mice produced double-transgenic pups with more delayed endochondral ossification than Smad6 transgenic mice. These results provided evidence that Smurf1 supports Smad6 function in vivo.     

Adenovirus-mediated transfer of siRNA against Runx2/Cbfa1 inhibits the formation of heterotopic ossification in animal model

The heterotopic ossification of muscles, tendons, and ligaments is a common problem faced by orthopaedic surgeons. Runx2/Cbfa1 plays an essential role during the osteoblast differentiation and is considered as a molecular switch in osteoblast biology. RNA interference technology is a powerful tool for silencing endogenous or exogenous genes in mammalian cells. In this study, we investigated the effect of Runx2/Cbfa1-specific siRNA on osteoblast differentiation and mineralization in osteoblastic cells, and then constructed adenovirus containing siRNA against Runx2/Cbfa1 (Ad-Runx2-siRNA) to inhibit the formation of heterotopic ossification induced by BMP4, demineralized bone matrix, and trauma in animal model. Our results showed that the Runx2/Cbfa1-specific siRNA could inhibit the expression of Runx2/Cbfa1 at the level of mRNA and protein. Analysis of the expression of osteoblast maturation genes including type I collagen, osteopontin, bone sialoprotein, and osteocalcin, alkaline phosphatase activity, and matrix mineralization (von kossa) revealed that osteoblast differentiation was inhibited in cultured primary mouse osteoblasts transduced with Ad-Runx2-siRNA. Furthermore, adenovirus-mediated transfer of siRNA against Runx2/Cbfa1 could inhibit the formation of heterotopic ossification induced by BMP4, demineralized bone matrix, and trauma in animal model. It is likely that the inhibition of Runx2/Cbfa1 by RNAi could be developed as a powerful approach to prevent or treat heterotopic ossification.     

Regulation of MMP-3 expression and secretion by the chemokine eotaxin-1 in human chondrocytes

Background  

Osteoarthritis (OA) is characterized by the degradation of articular cartilage, marked by the breakdown of matrix proteins. Studies demonstrated the involvement of chemokines in this process, and some may potentially serve as diagnostic markers and therapeutic targets; however, the underlying signal transductions are not well understood.     

Role of CDMP-1 in Skeletal Morphogenesis: Promotion of Mesenchymal Cell Recruitment and Chondrocyte Differentiation

Cartilage provides the template for endochondral ossification and is crucial for determining the length and width of the skeleton. Transgenic mice with targeted expression of recombinant cartilage-derived morphogenetic protein-1 (CDMP-1), a member of the bone morphogenetic protein family, were created to investigate the role of CDMP-1 in skeletal formation. The mice exhibited chondrodysplasia with expanded cartilage, which consists of the enlarged hypertrophic zone and the reduced proliferating chondrocyte zone. Histologically, CDMP-1 increased the number of chondroprogenitor cells and accelerated chondrocyte differentiation to hypertrophy. Expression of CDMP-1 in the notochord inhibited vertebral body formation by blocking migration of sclerotome cells to the notochord. These results indicate that CDMP-1 antagonizes the ventralization signals from the notochord. Our study suggests a molecular mechanism by which CDMP-1 regulates the formation, growth, and differentiation of the skeletal elements.     

Paraoxonase 1 overexpression in mice and its effect on high-density lipoproteins.

Paraoxonase (PON1) is a high-density lipoprotein (HDL)-associated enzyme believed to protect against the early events of atherogenesis by its ability to hydrolyze oxidized phospholipids. A transgenic mouse overexpressing PON1 (mPON1) was developed to address the question of whether overexpression of PON1 is important in protecting HDL function during oxidative stress. Transgenic mice were obtained that have up to a 5-fold increase in mPON1 activity measured as arylesterase activity [52.7 +/- 17.3 U/ml versus 251.7 +/- 25.1 U/ml for wild-type (WT) and mPON1 high expressers, respectively]; this increase in mPON1 activity was reflected by a 5.3-fold increase in relative mass of the enzyme. Excess mPON1 was associated solely with HDL but did not alter HDL composition, size, or charge. Lecithin:cholesterol acyltransferase (LCAT) on HDL is a sensitive indicator of oxidative stress; exposure of plasmas from both WT and mPON1 overexpresser mice to 0.4 mM copper ions for 2 h showed a 30-40% protection of LCAT activity in mPON1 overexpressers compared to WT. Excess mPON1 also inhibited lipid hydroperoxide formation on HDL. These data strongly suggest that overexpression of mPON1 protects HDL integrity and function.     

Spatial distribution of osteoblast-specific transcription factor Cbfa1 and bone formation in atherosclerotic arteries

The mechanisms of ectopic bone formation in arteries are poorly understood. Osteoblasts might originate either from stem cells that penetrate atherosclerotic plaques from the blood stream or from pluripotent mesenchymal cells that have remained in the arterial wall from embryonic stages of the development. We have examined the frequency of the expression and spatial distribution of osteoblast-specific factor-2/core binding factor-1 (Osf2/Cbfa1) in carotid and coronary arteries. Cbfa1-expressing cells were rarely observed but were found in all tissue specimens in the deep portions of atherosclerotic plaques under the necrotic cores. The deep portions of atherosclerotic plaques under the necrotic cores were characterized by the lack of capillaries of neovascularization. In contrast, plaque shoulders, which were enriched by plexuses of neovascularization, lacked Cbfa1-expressing cells. No bone formation was found in any of the 21 carotid plaques examined and ectopic bone was observed in only two of 12 coronary plaques. We speculate that the sparse invasion of sprouts of neovascularization into areas underlying the necrotic cores, where Cbfa1-expressing cells reside, might explain the rarity of events of ectopic bone formation in the arterial wall. This study has also revealed that Cbfa1-expressing cells contain alpha-smooth muscle actin and myofilaments, indicating their relationship with arterial smooth muscle cells.     

Four distinct chondrocyte populations in the fetal bovine growth plate: highest expression levels of PTH/PTHrP receptor,Indian hedgehog,and MMP-13 in hypertrophic chondrocytes and their suppression by PTH (1-34) and PTHrP (1-40)

Differentiation and growth of chondrocytes in fetal growth plates of vertebrate long bones and ribs appear to occur in a gradual, continuous manner between the resting zone through the proliferation zone, maturation zone, and upper and lower hypertrophic zones, with a continuous increase in cell size up to 10-fold of the volume of a resting chondrocyte. Here we provide evidence, however, that after centrifugation through a continuous Percoll gradient growth plate chondrocytes separate into four distinct cell populations (B1 to B4) which differ markedly in density, size, and gene expression. These populations collect in the absence of any phase borders in the gradient which might serve as concentration barriers. Fractions B1 and B2 contained the largest cells with the lowest buoyant density and showed the highest expression levels for type X collagen (Col X), but only the B1 population expressed high levels of matrix metalloproteinase-13 (collagenase 3). Cells in fraction B3 were significantly smaller and expressed little Col X, while cells in fraction B4 were of similar size to cells in the resting zone without significant Col X expression. The highest levels of parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor (PTHR-1), and Indian hedgehog (Ihh) expression were also found in the hypertrophic fractions B1 and B2 and not in the prehypertrophic fraction B3, as expected from in situ hybridization data on PTHR-1 expression in fetal rodent or chicken growth plates. Incubation of fractions B1 to B3 with the amino-terminal fragments PTH (1-34) or PTHrP (1-40) suppressed the expression of Col X and PTHR-1 by more than 50% and the expression of Ihh nearly completely. In contrast, the mid-regional PTH fragment PTH (28-48) and PTH (52-84) consistently stimulated the expression of PTHR-1 by 10-20% in fractions B1 to B3. These findings confirm the existence of distinct differentiation stages within chondrocytes of the growth plate and support the hypothesis proposed by Vortkamp et al. (Science 273(1996)613) of a regulatory feedback loop of Ihh and PTH/PTHrP fragments controlling the differentiation of proliferating to prehypertrophic chondrocytes, but extend the ability to respond to PTH/PTHrP hypertrophic chondrocytes.     

Cartilage proteoglycan aggregate and fibronectin can modulate the expression of type X collagen by embryonic chick chondrocytes cultured in collagen gels

Chick embryo sternal chondrocytes from the caudal and cephalic regions were cultured within type I collagen gels and type I collagen/proteoglycan aggregate composite gels in normal serum. Caudal region chondrocytes were also cultured within type I collagen gels in the presence of fibronectindepleted serum. There was a marked stimulation of type X collagen synthesis by the caudal region chondrocytes after 9 days in the presence of fibronectin-depleted serum and after 14 days in the presence of proteoglycan aggregate. These results provide evidence for the ability of chondrocytes from a zone of permanent cartilage to synthesise type X collagen and for the involvement of extracellular matrix components in the control of type X collagen gene expression.     

A dominant negative form of Rac1 affects myogenesis of adult thoracic muscles in Drosophila

Blocking Rac1 function in precursors of the indirect flight muscle of Drosophila severely disrupts muscle formation. The DLM fibers that develop using larval scaffolds are reduced in number and fiber size, while the DVMs, which develop using founder cells, are mostly absent. These adult muscle phenotypes are in part due to a reduced myoblast pool present at the third larval instar. BrDU labeling studies indicated that this is primarily due to a reduction in proliferation. In addition, DVM myoblasts display altered morphology and are unable to segregate into primordia. This defect precedes the evident block in fusion. We also show that the recently described DVM founder cells can be labeled with 22C10 and beta-3 tubulin, and that they are present under conditions of dominant negative Rac1(N17) expression. Despite the presence of founder cells, DVM fiber formation is rarely observed. Although DLM myoblasts are able to segregate around their larval scaffolds, the pace of fusion is reduced and consequently there is a delay in DLM fiber formation. Thus, in addition to its well-established role in fusion, Rac1 is also involved in the regulation of myoblast proliferation and segregation during adult myogenesis. These are two new roles for Rac1 in Drosophila.     

Estrogen-induced regulation of the ATP-binding cassette transporter A1 (ABCA1) in mice: A possible mechanism of atheroprotection by estrogen

Estrogens are suggested to be antiatherogenic by affecting the vessel wall components. Since ABCA1 was recently shown to be atheroprotective, it was examined if estrogen-induced atheroprotection occurs partly via the regulation of the ABCA1. Since hepatic ABCA1 expression was also suggested to contribute to the bulk HDL levels, regulation of the ABCA1 under conditions of high or low levels of HDL were investigated in mice expressing normal or elevated levels of apoAI. To delineate whether estrogen's effect occurs via estrogen receptor--mediated pathway, the estrogen receptor--deficient (ER-)^–/– mice were also administered either placebo or -estradiol for 5 consecutive days. Estrogen treatments decreased circulating HDL levels by 30%, but increased hepatic and intestinal ABCA1 mRNA by 2- and 1.5-fold, respectively. Hepatic ABCA1 mRNA also increased in the ER-^–/– mice by 3-fold. These results suggest that estrogen, despite lowering the levels of HDL, it up-regulated the hepatic ABCA1 mRNA, and in the absence of ER-, ER- could compensate for ER-. To study whether HDL levels correlate with the ABCA1 expression, wild-type (WT) and the apoAI transgenic (A1-Tg) mice were fed high fat (HF) diet with or without cholic acid (CA) for 3 weeks. One group of mice was treated with fenofibrate, known to elevate HDL levels. CA without HF decreased HDL levels, while fenofibrate increased HDL levels. However, neither CA nor fenofibrate altered hepatic ABCA1 mRNA levels. HF diet increased the hepatic ABCA1 mRNA 1.8-fold in WT, but lowered ABCA1 mRNA by 2-fold in A1-Tg mice, suggesting that ABCA1 levels did not correlate with circulating HDL levels, while basal levels of HDL influenced ABCA1 expression. These data show for the first time that estrogen's antiatherogenic effects may occur via ABCA1-mediated pathway, and circulating HDL levels may influence expression of ABCA1.     

用鼻咽相对特异性调控区建立N-LMP1转基因小鼠

为了研究EBV LMP1在鼻咽癌发生发展中的作用 ,构建了EDL 2、PLUNC p双启动子调控鼻咽癌来源的LMP1(latentmembraneprotein 1,潜伏膜蛋白 1)的表达载体 ,采用受精卵前核显微注射法构建转基因小鼠。结果表明 ,在所获得的 5 8只转基因首建鼠中 ,4只整合阳性 ,其中的一只转基因小鼠在鼻咽、前胃、舌根等部位检测到了外源基因的表达。