Cellular distribution of estrogen receptor beta in neonatal rat bone

Estrogens affect bone metabolism, and ovariectomy of rats results in marked bone loss caused by stimulation of osteoclastic bone resorption. Estrogen receptors (ER) have been demonstrated in osteoblasts and bone marrow stromal cells, but their presence in osteoclasts is controversial. Until recently, only one type of ER (now renamed ERalpha) had been identified. After the discovery of a novel ER subtype (ERbeta), it became necessary to re-investigate the ER expression in human and rodent bone. In the present study we examined the expression of ER mRNA in neonatal rat bone. Expression of ER alpha and beta mRNA (RT-PCR) was evident in femurs of 3-week-old male and female rats. In situ hybridization histochemistry of femural bones with digoxigenin labelled riboprobes, as well as radioactively labeled riboprobes, revealed that ERbeta mRNA was predominantly expressed in osteoblasts covering the metaphyseal bone trabecular surface. The presence of ERbeta mRNA in osteoblasts of rat bone suggests that ERbeta is involved in the mechanism of action of estrogens in bone.     

<Emphasis Type="Italic">In Vivo</Emphasis> Mechanical Loading Modulates Insulin-Like Growth Factor Binding Protein-2 Gene Expression in Rat Osteocytes

Mechanical stimulation is essential for maintaining skeletal integrity. Mechanosensitive osteocytes are important during the osteogenic response. The growth hormone-insulin-like growth factor (GH-IGF) axis plays a key role during regulation of bone formation and remodeling. Insulin-like growth factor binding proteins (IGFBPs) are able to modulate IGF activity. The aim of this study was to characterize the role of IGFBP-2 in the translation of mechanical stimuli into bone formation locally in rat tibiae. Female Wistar rats were assigned to three groups (n = 5): load, sham, and control. The four-point bending model was used to induce a single period of mechanical loading on the tibial shaft. The effect on IGFBP-2 mRNA expression 6 hours after stimulation was determined with nonradioactive in situ hybridization on decalcified tibial sections. Endogenous IGFBP-2 mRNA was expressed in trabecular and cortical osteoblasts, some trabecular and subendocortical osteocytes, intracortical endothelial cells of blood vessels, and periosteum. Megakaryocytes, macrophages, and myeloid cells also expressed IGFBP-2 mRNA. Loading and sham loading did not affect IGFBP-2 mRNA expression in osteoblasts, bone marrow cells, and chondrocytes. An increase of IGFBP-2 mRNA-positive osteocytes was shown in loaded (1.68-fold) and sham-loaded (1.35-fold) endocortical tibial shaft. In conclusion, 6 hours after a single loading session, the number of IGFBP-2 mRNA-expressing osteocytes at the endosteal side of the shaft and inner lamellae was increased in squeezed and bended tibiae. Mechanical stimulation modulates IGFBP-2 mRNA expression in endocortical osteocytes. We suggest that IGFBP-2 plays a role in the lamellar bone formation process.     

低氧诱导因子-1α在绝经后骨质疏松中调控作用的实验研究

目的探讨在绝经后骨质疏松的发生、发展过程中,低氧诱导因子-1α（HIF-1α）对于成骨细胞功能的调控作用。方法2004年10月至2006年5月,应用Cre—Loxp重组酶技术,建立成骨细胞条件性敲除HIF-1α小鼠,取3个月龄雌性野生型和敲除型小鼠各24只行卵巢切除术,术后0、4、8周取材行HE染色、四环素荧光双标记、Micro-CT、RT—PCR、Western-blotting检测。结果与野生型小鼠相比,敲除型小鼠骨小梁的数目、体积、厚度,骨密度,骨矿沉积速度,血管内皮生长因子（VEGF）、RunX2、ALP、OC基因在mRNA水平的表达,VEGF、RunX2在蛋白水平的表达均明显降低,尤其以术后8周最为明显。结论在绝经后骨质疏松的发生、发展过程中,成骨细胞条件性敲除HIF-1α后成骨功能降低,HIF-1α能够调控成骨细胞的成骨功能。     

Expression of fibrillins and other microfibril-associated proteins in human bone and osteoblast-like cells

Fibrillin-containing microfibrils are structural components of extracellular matrices of a diverse range of tissues, including bone. Their importance in bone biology is illustrated by the skeletal abnormalities manifest in the congenital disorder, Marfan syndrome, which results from mutations in the fibrillin-1 gene. We investigated the expression of fibrillins and other microfibril-associated proteins in human bone and bone-derived osteoblasts. Analysis of RNA extracted from cancellous bone showed expression of mRNAs encoding fibrillin-1 and -2, MAGP-1 and -2, LTBP-2, and MP78/70 (Big-h3). In demineralized normal mature bone, fibrillin-1 was immunolocalized to fibrils within the bone matrix and pericellularly to cells lining the endosteal surfaces of trabecular bone, some osteocytes, and cells associated with blood vessels. LTBP-2 was also identified at the endosteal surface and within the bone matrix in a lamellar fashion. In addition, primary osteoblast-like cells cultured from human trabecular bone (obtained from patients at joint replacement surgery) were found to express abundant mRNA for fibrillins and associated glycoproteins. Moreover, using western blot analysis, fibrillin-1 protein was shown to be secreted into the medium and to be deposited into the cell layer. Immunofluorescence staining of the cell layer visualized fibrillin-1 in the matrix as a three-dimensional network of fine filaments. Expression of fibrillin-1 by osteoblast-like cells was constitutive, and a number of skeletally active agents had little effect on mRNA or protein levels. These results show that human osteoblasts from mature bone express fibrillins and other microfibril-associated proteins, and suggest a role for these molecules in adult human bone.     

Androgen Receptor Action in Osteoblasts in Male Mice Is Dependent on Their Stage of Maturation

Androgen action via the androgen receptor (AR) is essential for normal skeletal growth and bone maintenance post‐puberty in males; however, the molecular and cellular mechanisms by which androgens exert their actions in osteoblasts remains relatively unexplored in vivo. To identify autonomous AR actions in osteoblasts independent of AR signaling in other tissues, we compared the extent to which the bone phenotype of the Global‐ARKO mouse was restored by replacing the AR in osteoblasts commencing at either the 1) proliferative or 2) mineralization stage of their maturation. In trabecular bone, androgens stimulated trabecular bone accrual during growth via the AR in proliferating osteoblasts and maintained trabecular bone post‐puberty via the AR in mineralizing osteoblasts, with its predominant action being to inhibit bone resorption by decreasing the ratio of receptor activator of NF‐κB ligand (RANKL) to osteoprotegerin (OPG) gene expression. During growth, replacement of the AR in proliferating but not mineralizing osteoblasts of Global‐ARKOs was able to partially restore periosteal circumference, supporting the concept that androgen action in cortical bone to increase bone size during growth is mediated via the AR in proliferating osteoblasts. This study provides further significant insight into the mechanism of androgen action via the AR in osteoblasts, demonstrating that it is dependent on the stage of osteoblast maturation. © 2014 American Society for Bone and Mineral Research.     

Twisted gastrulation, a bone morphogenetic protein agonist/antagonist, is not required for post-natal skeletal function

Twisted gastrulation (Tsg) is a secreted glycoprotein that binds bone morphogenetic proteins (BMP)-2 and -4 and can display both BMP agonist and antagonist functions. Tsg promotes BMP-mediated endochondral ossification, but its activity in adult bone is not known. We created tsg null mice and examined the consequences of the tsg deletion on the skeleton in vivo and on osteoblast function in vitro. Analysis of the skeletal phenotype of 4-week-old tsg null mice revealed a 40% decrease in trabecular bone volume, but osteoblast and osteoclast number, and bone formation and resorption were not affected. The phenotype was transient, and at 7 weeks of age tsg null mice were not different from control wild-type mice. The decreased trabecular bone is congruent with a defect in endochondral bone formation. In osteoblasts isolated from tsg null mice, tsg gene inactivation decreased the BMP-2 stimulatory effects on osteocalcin expression and alkaline phosphatase activity, indicating that in the bone microenvironment endogenous Tsg enhances BMP activity. Accordingly, tsg null cells displayed impaired BMP signaling. These results were confirmed by Tsg down-regulation in primary osteoblasts from wild-type mice using RNA interference. In conclusion, endogenous Tsg is required for normal BMP activity in osteoblastic cells in vitro, but it plays a minor role in the regulation of adult bone homeostasis in vivo.     

转录因子Runx2在青少年特发性脊柱侧凸患者成骨细胞中的表达及其意义

目的 从成骨细胞(OB)水平探讨转录因子Runx2与青少年特发性脊柱侧凸(AIS)患者骨量降低的关系.方法 2008年3月至12月行后路手术的AIS患者28例为试验组,男性2例,女性26例;年龄12～18岁,平均14.9岁;Cobb角40～94°,平均57.3°.试验组根据骨密度(BMD)情况又分为:A组(骨量正常组)15例,B组(骨量减低组)13例.正常对照组(C组)为住院治疗的非脊柱畸形的8例患者,男性6例,女性2例;年龄12～18岁,平均15.3岁.各组均采用双能X线吸收测量仪(DEXA)测量骨密度(BMD),测量部位包括非优势侧股骨近端及腰椎.所有受试者术中取适量髂前上嵴的松质骨,运用植块法培养OB.培养至P2代后行表型鉴定,用RT-PCR和Western blot法检测各组Runx2 mRNA及蛋白的表达水平并进行统计学分析.结果 Runx2的mRNA及蛋白水平的表达,B组较A组和c组均降低,差异均有统计学意义(P<0.05),A组与C组差异无统计学意义(P>0.05).结论 Runx2在OB水平mRNA及蛋白表达水平的异常可能与MS骨量降低的分子机制相关.     

Iodothyronine deiodinase enzyme activities in bone

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

TGFβ-1 mRNA Expression and Proliferation of Human Osteoblastic Cells in Nonosteoporotic and Osteoporotic Women under Influence of TGFβ-1 and IGF-I

Currently, primary osteoporosis is the most frequent metabolic disease in women after menopause [1]. The resulting loss of bone mass is accompanied by an increased risk of skeletal fragility. One reason for the development of osteoporosis might be an impaired function of mature osteoblasts. To evaluate the involvement of specific growth factors in bone remodeling, cell cultures of osteoblastic cells derived from nonosteoporotic and osteoporotic postmenopausal women were established. The influences of TGFβ-1 and IGF-I on proliferation and mRNA expression of TGFβ-1 were investigated by [³H]-thymidine incorporation and competitive RT-PCR. We found IGF-I to have no significant effect on proliferation in cells of osteoporotic and nonosteoporotic patients. In contrast, differences were found in TGFβ-1 mRNA expression after application of IGF-I. Application of TGFβ-1 enhanced its own mRNA expression in both groups in a similar manner. Whereas the proliferation of cells of nonosteoporotic patients was inhibited by (10⁻¹⁰ M) TGFβ-1, this treatment led to an increased proliferation of cells of osteoporotic patients. Received: 10 June 1996 / Accepted: 13 January 1997     

Trabecular bone turnover, bone marrow cell development, and gene expression of bone matrix proteins after low calcium feeding in rats

Low-calcium-fed animals have been accepted as one of the experimental models showing a reduction in bone mass. However, the effects of short-term low-calcium feeding on bone turnover, the development of osteoprogenitor cells, and gene expression of bone matrix proteins have not been reported. In this study, we examined the effect of a low-calcium diet on rat tibia and analyzed the changes in the bone by histomorphometry, bone marrow cell culture, and in situ and Northern hybridization of the bone matrix proteins. Rats were fed either a low-calcium diet (0.05% Ca) or a normal calcium diet (0.5% Ca) using the pair feeding technique. They were killed at day 0, 12 h, and days 1, 2, and 3. In the low-calcium group, the serum parathyroid hormone (PTH) level was temporarily increased in 12 h after feeding the low-calcium diet. Bone mineral density in the trabecular bone was significantly decreased from 1 day after the low-calcium feeding, but cortical bone did not show any changes during the experimental period. The bone volume per tissue volume in the proximal tibia also decreased from day 1 in the low-calcium group. The number of osteoclasts and osteoblasts on the trabecular bone surface was increased in the low-calcium group compared with the normal-calcium group. An ex vivo study showed that the number of progenitors of osteoclasts and osteoblasts in bone marrow was also increased in the low-calcium group of rats. The localization of type I collagen mRNA was observed in osteoblasts in the low-calcium group. The Northern hybridization study showed that the gene expression of type I collagen, osteopontin, and osteocalcin was increased at day 3 in the low-calcium group. These results indicated that the trabecular bone surface quickly responded to the low-calcium feeding and that bone remodeling activity was activated probably by PTH. The changes in bone marrow cell populations and the gene expression of bone matrix proteins are closely associated with increased bone turnover induced by the low-calcium diet, resulting in rapid bone loss of the trabecular bone.     

The protein tyrosine phosphatase Rptpzeta is expressed in differentiated osteoblasts and affects bone formation in mice

Tyrosine phosphorylation of intracellular substrates is one mechanism to regulate cellular proliferation and differentiation. Protein tyrosine phosphatases (PTPs) act by dephosphorylation of substrates and thereby counteract the activity of tyrosine kinases. Few PTPs have been suggested to play a role in bone remodeling, one of them being Rptpzeta, since it has been shown to be suppressed by pleiotrophin, a heparin-binding molecule affecting bone formation, when over-expressed in transgenic mice. In a genome-wide expression analysis approach we found that Ptprz1, the gene encoding Rptpzeta, is strongly induced upon terminal differentiation of murine primary calvarial osteoblasts. Using RT-PCR and Western Blotting we further demonstrated that differentiated osteoblasts, in contrast to neuronal cells, specifically express the short transmembrane isoform of Rptpzeta. To uncover a potential role of Rptpzeta in bone remodeling we next analyzed the skeletal phenotype of a Rptpzeta-deficient mouse model using non-decalcified histology and histomorphometry. Compared to wildtype littermates, the Rptpzeta-deficient mice display a decreased trabecular bone volume at the age of 50 weeks, caused by a reduced bone formation rate. Likewise, Rptpzeta-deficient calvarial osteoblasts analyzed ex vivo display decreased expression of osteoblast markers, indicating a cell-autonomous defect. This was confirmed by the finding that Rptpzeta-deficient osteoblasts had a diminished potential to form osteocyte-like cellular extensions on Matrigel-coated surfaces. Taken together, these data provide the first evidence for a physiological role of Rptpzeta in bone remodeling, and thus identify Rptpzeta as the first PTP regulating bone formation in vivo.     

Dysregulated TGF‐β signaling alters bone microstructure in a mouse model of Loeys‐Dietz syndrome

Loeys‐Dietz syndrome (LDS) is a connective tissue disorder characterized by vascular and skeletal abnormalities resembling Marfan syndrome, including a predisposition for pathologic fracture. LDS is caused by heterozygous mutations in the genes encoding transforming growth factor‐β (TGF‐β) type 1 and type 2 receptors. In this study, we characterized the skeletal phenotype of mice carrying a mutation in the TGF‐β type 2 receptor associated with severe LDS in humans. Cortical bone in LDS mice showed significantly reduced tissue area, bone area, and cortical thickness with increased eccentricity. However, no significant differences in trabecular bone volume were observed. Dynamic histomorphometry performed in calcein‐labeled mice showed decreased mineral apposition rates in cortical and trabecular bone with normal numbers of osteoblasts and osteoclasts. Mechanical testing of femurs by three‐point bending revealed reduced femoral strength and fracture resistance. In vitro, osteoblasts from LDS mice demonstrated increased mineralization with enhanced expression of osteoblast differentiation markers compared with control cells. These changes were associated with impaired TGF‐β1–induced Smad2 and Erk1/2 phosphorylation and upregulated TGF‐β1 ligand mRNA expression, compatible with G357W as a loss‐of‐function mutation in the TGF‐β type 2 receptor. Paradoxically, phosphorylated Smad2/3 in cortical osteocytes measured by immunohistochemistry was increased relative to controls, possibly suggesting the cross‐activation of TGF‐β–related receptors. The skeletal phenotype observed in the LDS mouse closely resembles the principal structural features of bone in humans with LDS and establishes this mouse as a valid in vivo model for further investigation of TGF‐β receptor signaling in bone. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1447–1454, 2015.