Variable hormone responsiveness of osteoblast populations isolated at different stages of embryogenesis and its relationship to the osteogenic lineage

Decoding the Regulatory Landscape of Ageing in Musculoskeletal Engineered Tissues Using Genome-Wide DNA Methylation and RNASeq

Mesenchymal stem cells (MSC) are capable of multipotent differentiation into connective tissues and as such are an attractive source for autologous cell-based regenerative medicine and tissue engineering. Epigenetic mechanisms, like DNA methylation, contribute to the changes in gene expression in ageing. However there was a lack of sufficient knowledge of the role that differential methylation plays during chondrogenic, osteogenic and tenogenic differentiation from ageing MSCs. This study undertook genome level determination of the effects of DNA methylation on expression in engineered tissues from chronologically aged MSCs. We compiled unique DNA methylation signatures from chondrogenic, osteogenic, and tenogenic engineered tissues derived from young; n = 4 (21.8 years ± 2.4 SD) and old; n = 4 (65.5 years±8.3SD) human MSCs donors using the Illumina HumanMethylation 450 Beadchip arrays and compared these to gene expression by RNA sequencing. Unique and common signatures of global DNA methylation were identified. There were 201, 67 and 32 chondrogenic, osteogenic and tenogenic age-related DE protein-coding genes respectively. Findings inferred the nature of the transcript networks was predominantly for 'cell death and survival', 'cell morphology', and 'cell growth and proliferation'. Further studies are required to validate if this gene expression effect translates to cell events. Alternative splicing (AS) was dysregulated in ageing with 119, 21 and 9 differential splicing events identified in chondrogenic, osteogenic and tenogenic respectively, and enrichment in genes associated principally with metabolic processes. Gene ontology analysis of differentially methylated loci indicated age-related enrichment for all engineered tissue types in 'skeletal system morphogenesis', 'regulation of cell proliferation' and 'regulation of transcription' suggesting that dynamic epigenetic modifications may occur in genes associated with shared and distinct pathways dependent upon engineered tissue type. An altered phenotype in engineered tissues was observed with ageing at numerous levels. These changes represent novel insights into the ageing process, with implications for stem cell therapies in older patients. In addition we have identified a number of tissue-dependant pathways, which warrant further studies.

Vitamin D deficiency and its role in muscle-bone interactions in the elderly

In this commentary, we focus on common 'downstream' links of vitamin D between muscle and bone health. Both direct and indirect effects of 1,25 dihydroxyvitamin D (1,25(OH)D) link the mutual age-related decline in muscle function and bone density, independent of physical activity. Changes in calcium absorption associated with vitamin D deficiency affect both muscle and bone mass. The age-related decline in vitamin D receptor expression and 1,25(OH)D activity impact on proinflammatory cytokines such as tumor necrosis factor -α and interleukin-6 in skeletal muscle and vitamin D deficiency appears to enhance both bone marrow adipogenesis and intramuscular adipose tissue impacting as reduced functionality in both skeletal tissues. Controversial findings on the role of 1,25(OH)D on skeletal muscle may relate to differences in vitamin D receptor expression throughout different stages of muscle cell differentiation. Prolonged vitamin D insufficiency in the elderly is associated with reductions in both bone mineral density and type 2 muscle fibers with the outcomes of skeletal fragility in combination with reduced muscle power, leading to increased risk of falls and fracture.

Vitamin D endocrine system and osteoblasts

The interaction between vitamin D and osteoblasts is complex. In the current review we will give an overview of the current knowledge of the vitamin D endocrine system in osteoblasts. The presence of the vitamin D receptor in osteoblasts enables direct effects of 1α,25dihydroxyvitamin D3 (1α,25D3) on osteoblasts, but the magnitude of the effects is subject to the presence of many other factors. Vitamin D affects osteoblast proliferation, as well as differentiation and mineralization, but these effects vary with the timing of treatment, dosage and origin of the osteoblasts. Vitamin D effects on differentiation and mineralization are mostly stimulatory in human and rat osteoblasts, and inhibitory in murine osteoblasts. Several genes and mechanisms are studied to explain the effects of 1α,25D3 on osteoblast differentiation and bone formation. Besides the classical VDR, osteoblasts also express a membrane-localized receptor, and in vitro studies have shown that osteoblasts are capable of the synthesis of 1α,25D3.

Evolution of the interaction between Runx2 and VDR, two transcription factors involved in osteoblastogenesis

Background

The mineralized skeleton is a major evolutionary novelty that has contributed to the impressive morphological diversifications of the vertebrates. Essential to bone biology is the solidified extracellular matrix secreted by highly specialized cells, the osteoblasts. We now have a rather complete view of the events underlying osteogenesis, from a cellular, molecular, genetic, and epigenetic perspective. Because this knowledge is still largely restricted to mammals, it is difficult, if not impossible, to deduce the evolutionary history of the regulatory network involved in osteoblasts specification and differentiation. In this study, we focused on the transcriptional regulators Runx2 and VDR (the Vitamin D Receptor) that, in mammals, directly interact together and stabilize complexes of co-activators and chromatin remodellers, thereby allowing the transcriptional activation of target genes involved in extracellular matrix mineralization. Using a combination of functional, biochemical, and histological approaches, we have asked if the interaction observed between Runx2 and VDR represents a recent mammalian innovation, or if it results from more ancient changes that have occurred deep in the vertebrate lineage.

Results

Using immunohistochemistry and in situ hybridization in developing embryos of chick, frog and teleost fishes, we have revealed that the co-expression of Runx2 and VDR in skeletal elements has been particularly strengthened in the lineage leading to amniotes. We show that the teleost Runx2 orthologue as well as the three mammalian Runx1, Runx2 and Runx3 paralogues are able to co-immunoprecipitate with the VDR protein present in nuclear extracts of rat osteoblasts stimulated with 1α,25-dihydroxyvitamin D₃. In addition, the teleost Runx2 can activate the transcription of the mammalian osteocalcin promoter in transfection experiments, and this response can be further enhanced by 1α,25-dihydroxyvitamin D₃. Finally, using pull-down experiments between recombinant proteins, we show that the VDR homologue from teleosts, but not from ascidians, is able to directly interact with the mammalian Runx2 homologue.

Conclusions

We propose an evolutionary scenario for the assembly of the molecular machinery involving Runx2 and VDR in vertebrates. In the last common ancestor of actinopterygians and sacropterygians, the three Runx paralogues possessed the potential to physically and functionally interact with the VDR protein. Therefore, 1α,25-dihydroxyvitamin D₃ might have been able to modulate the transcriptional activity of Runx1, Runx2 or Runx3 in the tissues expressing VDR. After the split from amphibians, in the lineage leading to amniotes, Runx2 and VDR became robustly co-expressed in developing skeletal elements, and their regulatory interaction was incorporated in the genetic program involved in the specification and differentiation of osteoblasts.

Bone mineral density before and after OLT: long-term follow-up and predictive factors

Fracturing after liver transplantation (OLT) occurs due to the combination of preexisting low bone mineral density (BMD) and early posttransplant bone loss, the risk factors for which are poorly defined. The prevalence and predictive factors for hepatic osteopenia and osteoporosis, posttransplant bone loss, and subsequent bone gain were studied by the long-term posttransplant follow-up of 360 consecutive adult patients with end-stage primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC). Only 20% of patients with advanced PBC or PSC have normal bone mass. Risk factors for low spinal BMD are low body mass index, older age, postmenopausal status, muscle wasting, high alkaline phosphatase and low serum albumin. A high rate of spinal bone loss occurred in the first 4 posttransplant months (annual rate of 16%) especially in those with younger age, PSC, higher pretransplant bone density, no inflammatory bowel disease, shorter duration of liver disease, current smoking, and ongoing cholestasis at 4 months. Factors favoring spinal bone gain from 4 to 24 months after transplantation were lower baseline and/or 4-month bone density, premenopausal status, lower cumulative glucocorticoids, no ongoing cholestasis, and higher levels of vitamin D and parathyroid hormone. Bone mass therefore improves most in patients with lowest pretransplant BMD who undergo successful transplantation with normal hepatic function and improved gonadal and nutritional status. Patients transplanted most recently have improved bone mass before OLT, and although bone loss still occurs early after OLT, these patients also have a greater recovery in BMD over the years following OLT.

Mouse embryo-derived NIH3T3 fibroblasts adopt an osteoblast-like phenotype when treated with 1alpha,25-dihydroxyvitamin D(3) and dexamethasone in vitro

This study examines the capability of NIH3T3 fibroblasts to express osteoblastic markers following stimulation with a number of hormones and growth factors in vitro. Of the agents tested, 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) dose-dependently induced alkaline phosphatase (ALP) activity in NIH3T3 cells, and this effect was enhanced by the addition of dexamethasone (Dex), which when administered alone caused no detectable ALP expression. The combined use of 1,25(OH)(2)D(3) and Dex also stimulated the synthesis of osteocalcin, and osteopontin. Furthermore, cells treated with the both hormones, in the presence of beta-glycerophosphate and l-ascorbic acid, formed mineralized plaques, indicating an osteoblast (OB) phenotype. By contrast, the differentiation induced by 1,25(OH)(2)D(3) or 1,25(OH)(2)D(3) plus Dex was significantly antagonized by transforming growth factor-beta1 and all trans-retinoic acid. These data indicate that NIH3T3 cells have the potential to adopt an OB-like phenotype and may prove to be a convenient model for studying the early events of osteogenic differentiation and the specific interactions of 1,25(OH)(2)D(3) with glucocorticoids in controlling this process in vitro.

Bone metabolism in advanced cholestatic liver disease: analysis by bone histomorphometry

Despite the clinical importance of cholestatic osteopenia, little is known about its pathophysiologic mechanism. By tetracycline-labeled histomorphometric analysis of bone biopsies taken at the time of liver transplantation, we prospectively evaluated bone resorption and formation in 50 consecutive patients with advanced primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC). Histomorphometric analysis confirmed low bone volume parameters, consistent with the mean T-score of the lumbar spine of -1.9 by dual energy x-ray absorptiometry. Dynamic (bone formation rates, adjusted apposition rates) and static (osteoid markers, osteoblast number) parameters of bone formation were decreased in cholestatic patients with no abnormalities in mineralization. Increased osteoclast numbers and increased eroded surface areas suggested increased bone resorption, and this was supported in female patients by increased trabecular separation and decreased trabecular number. Male cholestatic patients, however, did not have significant increases in resorption parameters, although they were as osteopenic as female patients and had low bone formation markers. Bone histomorphometric changes were similar in PBC and PSC, suggesting an etiologic effect of chronic cholestasis rather than the individual diseases. Cancellous bone volume and osteoid markers correlated with bone mineral density measurements but no correlations were found between histomorphometric parameters and biochemical markers of bone metabolism. In conclusion, cholestatic osteopenia appears to result from a combination of decreased bone formation and increased resorption, especially in female patients, but the relative importance of these two abnormalities and their actual etiology remain to be elucidated.

PTH-dependent adenylyl cyclase activation in SaOS-2 cells: passage dependent effects on G protein interactions

Parathyroid hormone (PTH) sensitive adenylyl cyclase activity (ACA) in SaOS-2 cells varies as a function of cell passage. In early passage (EP) cells (< 6), ACA in response to PTH and forskolin (FOR) was relatively low and equivalent, whereas in late passage (LP) cells (> 22), PTH exceeded FOR dependent ACA. Potential biochemical mechanisms for this passage dependent change in ACA were considered. In EP, prolonged exposure to pertussis toxin (PT) markedly enhanced ACA activity in response to PTH, Isoproterenol and Gpp(NH)p, whereas ACA in response to FOR was decreased. In contrast, the identical treatment of LP with PT diminished all ACA in response to PTH, Gpp(NH)p, and FOR. The dose dependent effects of PT on subsequent [(32)P]ADP-ribosylation of its substrates, GTPase activity, as well as FOR-dependent ACA, were equivalent in EP and LP. The relative amounts of G(alpha)i and G(alpha)s proteins, as determined both by Western blot, PT and cholera toxin (CT) dependent [(32)P]ADP-ribosylation, were quantitatively similar in EP and LP. Western blot levels of G(alpha)s and G(alpha)i proteins were not influenced by prior exposure to PT. Both PT and CT dependent [(32)P]ADP-ribosylation were dose-dependently decreased following exposure to PT. However, the PT-dependent decline in CT-dependent [(32)P]ADP-ribosylation occurred with enhanced sensitivity in LP. The protein synthesis inhibitor cycloheximide partially reversed the PT associated decrease in FOR dependent ACA in EP. In contrast, cycloheximide completely reversed the PT associated decrease in FOR and as well as PTH dependent ACA in LP. G(alpha)s activity, revealed by cyc(-) reconstitution, was not altered either by cell passage or exposure to PT. The results suggest that the coupling between the components of the complex may be pivotally important in the differential responsiveness of early and late passage SaOS-2 cells to PTH.

Expression and regulation of Runx2/Cbfa1 and osteoblast phenotypic markers during the growth and differentiation of human osteoblasts

The runt family transcription factor (AML-3/PEBP2alphaA1/Cbfa1/RUNX2) plays a crucial role in formation of the mineralized skeleton during embryogenesis and regulates maturation of the osteoblast phenotype. Because steroid hormones and growth factors significantly influence growth and differentiation properties of osteoblasts, we addressed Cbfa1 as a target gene for regulation by dexamethasone (Dex), 1,25(OH)D(3) (vitamin D(3)), 17beta-estradiol, and transforming growth factor-beta1 (TGF-beta1). The representation of functional protein levels by Western blot analyses and gel mobility shift assays was examined during the growth and mineralization of several conditionally immortalized human osteoblast cell lines HOB 04-T8, 03-CE6, and 03-CE10, each representing different stages of maturation. In situ immunofluorescence demonstrates Cbfa1 is associated with nuclear matrix in punctate domains, some of which are transcriptionally active, colocalizing with phosphorylated RNA polymerase II. Although each of the cell lines exhibited different responses to the steroid hormones and to TGF-beta1, all cell lines showed a similar increase in Cbfa1 protein and DNA binding activity induced only by Dex. On the other hand, Cbfa1 mRNA levels were not altered by Dex treatment. This regulation of Cbfa1 by steroid hormones in human osteoblasts contrasts to modifications in Cbfa1 expression in primary rat calvarial osteoblasts and the mouse MC3T3-E1 osteoblast cell line. Thus, these results reveal multiple levels of regulation of Cbfa1 expression and activity in osteoblasts. Moreover, the data suggest that in committed human osteoblasts, constitutive expression of Cbfa1 may be required to sustain the osteoblast phenotype.

Neuropilin-1 expression in osteogenic cells: down-regulation during differentiation of osteoblasts into osteocytes

The expression of neuropilin-1 (NRP1), a recently described VEGF and semaphorin receptor expressed by endothelial cells (EC) but some non-EC types as well, was analyzed in osteoblasts in vitro and in vivo. Cultured MC3T3-E1 osteoblasts expressed NRP1 mRNA and bound VEGF(165) but not VEGF(121), characteristic of the VEGF isoform-specific binding of NRP1. These cells did not express VEGFR-1 or VEGFR-2 so that VEGF binding to osteoblasts was strictly NRP1-dependent. In a chick osteocyte differentiation system, NRP1 was expressed by osteoblasts but its expression was absent as the cells matured into osteocytes. Immunohistochemical localization of NRP1 within the developing bones of 36-day-old mice and embryonic Day 17 chicks demonstrated that NRP1 was expressed by osteoblasts migrating alongside invading blood vessels within the metaphysis of the growth plate, as well as by osteoblasts at the developing edge of trabeculae within the marrow cavity. On the other hand, NRP1 was not expressed by osteocytes in either species, consistent with the in vitro results. In addition to osteogenic cells, NRP1 expression by EC was observed throughout the bone. Together these results suggest that NRP1 might have a dual function in bone by mediating osteoblast function directly as well as angiogenesis.

Maturation state determines the response of osteogenic cells to surface roughness and 1,25-dihydroxyvitamin D3

In this study we assessed whether osteogenic cells respond in a differential manner to changes in surface roughness depending on their maturation state. Previous studies using MG63 osteoblast-like cells, hypothesized to be at a relatively immature maturation state, showed that proliferation was inhibited and differentiation (osteocalcin production) was stimulated by culture on titanium (Ti) surfaces of increasing roughness. This effect was further enhanced by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. In the present study, we examined the response of three additional cell lines at three different maturation states: fetal rat calvarial (FRC) cells (a mixture of multipotent mesenchymal cells, osteoprogenitor cells, and early committed osteoblasts), OCT-1 cells (well-differentiated secretory osteoblast-like cells isolated from calvaria), and MLO-Y4 cells (osteocyte-like cells). Both OCT-1 and MLO-Y4 cells were derived from transgenic mice transformed with the SV40 large T-antigen driven by the osteocalcin promoter. Cells were cultured on Ti disks with three different average surface roughnesses (Ra): PT, 0.5 microm; SLA, 4.1 microm; and TPS, 4.9 microm. When cultures reached confluence on plastic, vehicle or 10(-7) M or 10(-8) M 1,25(OH)2D3 was added for 24 h to all of the cultures. At harvest, cell number, alkaline phosphatase-specific activity, and production of osteocalcin, transforming growth factor beta1 (TGF-beta1) and prostaglandin E2 (PGE2) were measured. Cell behavior was sensitive to surface roughness and depended on the maturation state of the cell line. Fetal rat calvarial (FRC) cell number and alkaline phosphatase-specific activity were decreased, whereas production of osteocalcin, TGF-beta1, and PGE2 were increased with increasing surface roughness. Addition of 1,25(OH)2D3 to the cultures further augmented the effect of roughness for all parameters in a dose-dependent manner; only TGF-beta1 production on plastic and PT was unaffected by 1,25(OH)2D3. OCT-1 cell number and alkaline phosphatase (SLA > TPS) were decreased and production of PGE2, osteocalcin, and TGF-beta1 were increased on SLA and TPS. Response to 1,25(OH)2D3 varied with the parameter being measured. Addition of the hormone to the cultures had no effect on cell number or TGF-beta1 production on any surface, while alkaline phosphatase was stimulated on SLA and TPS; osteocalcin production was increased on all Ti surfaces but not on plastic; and PGE2 was decreased on plastic and PT, but unaffected on SLA and TPS. In MLO-Y4 cultures, cell number was decreased on SLA and TPS; alkaline phosphatase was unaffected by increasing surface roughness; and production of osteocalcin, TGF-beta1, and PGE2 were increased on SLA and TPS. Although 1,25(OH)2D3 had no effect on cell number, alkaline phosphatase, or production of TGF-beta1 or PGE2 on any surface, the production of osteocalcin was stimulated by 1,25(OH)2D3 on SLA and TPS. These results indicate that surface roughness promotes osteogenic differentiation of less mature cells, enhancing their responsiveness to 1,25(OH)2D3. As cells become more mature, they exhibit a reduced sensitivity to their substrate but even the terminally differentiated osteocyte is affected by changes in surface roughness.

Spaceflight effects on cultured embryonic chick bone cells

A model calcifying system of primary osteoblast cell cultures derived from normal embryonic chicken calvaria has been flown aboard the shuttle, Endeavour, during the National Aeronautics and Space Administration (NASA) mission STS-59 (April 9-20, 1994) to characterize unloading and other spaceflight effects on the bone cells. Aliquots of cells (approximately 7 x 10(6)) grown in Dulbecco's modified Eagle's medium (DMEM) + 10% fetal bovine serum (FBS) were mixed with microcarrier beads, inoculated into cartridge culture units of artificial hollow fiber capillaries, and carried on the shuttle. To promote cell differentiation, cartridge media were supplemented with 12.5 microg/ml ascorbate and 10 mM beta-glycerophosphate for varying time periods before and during flight. Four cartridges contained cells from 17-day-old embryos grown for 5 days in the presence of ascorbate prior to launch (defined as flight cells committed to the osteoblastic lineage) and four cartridges supported cells from 14-day-old embryos grown for 10 days with ascorbate before launch (uncommitted flight cells). Eight cartridges prepared in the same manner were maintained under normal gravity throughout the flight (control cells) and four additional identical cartridges under normal gravity were terminated on the day of launch (basal cells). From shuttle launch to landing, all cartridges were contained in closed hardware units maintaining 5% CO2, 37 degrees C, and media delivery at a rate of approximately 1.5 ml/6 h. During day 3 and day 5 of flight, duplicate aliquots of conditioned media and accumulated cell products were collected in both the flight and the control hardware units. At the mission end, comparisons among flight, basal, and control samples were made in cell metabolism, gene expression for type I collagen and osteocalcin, and ultrastructure. Both committed and uncommitted flight cells were metabolically active, as measured by glucose uptake and lactate production, at approximately the same statistical levels as control counterparts. Flight cells elaborated a less extensive extracellular matrix, evidenced by a reduced collagen gene expression and collagen protein appearance compared with controls. Osteocalcin was expressed by all cells, a result indicating progressive differentiation of both flight and control osteoblasts, but its message levels also were reduced in flight cells compared with ground samples. This finding suggested that osteoblasts subjected to flight followed a slower progression toward a differentiated function. The summary of data indicates that spaceflight, including microgravity exposure, demonstrably affects bone cells by down-regulating type I collagen and osteocalcin gene expression and thereby inhibiting expression of the osteogenic phenotype notably by committed osteoblasts. The information is important for insight into the response of bone cells to changes of gravity and of force in general.

The developmental control of osteoblast-specific gene expression: role of specific transcription factors and the extracellular matrix environment

Bone formation is a carefully controlled developmental process involving morphogen-mediated patterning signals that define areas of initial mesenchyme condensation followed by induction of cell-specific differentiation programs to produce chondrocytes and osteoblasts. Positional information is conveyed via gradients of molecules, such as Sonic Hedgehog that are released from cells within a particular morphogenic field together with region-specific patterns of hox gene expression. These, in turn, regulate the localized production of bone morphogenetic proteins and related molecules which initiate chondrocyte- and osteoblast-specific differentiation programs. Differentiation requires the initial commitment of mesenchymal stem cells to a given lineage, followed by induction of tissue-specific patterns of gene expression. Considerable information about the control of osteoblast-specific gene expression has come from analysis of the promoter regions of genes encoding proteins like osteocalcin that are selectively expressed in bone. Both general and tissue-specific transcription factors control this promoter. Osf2/Cbfa1, the first osteoblast-specific transcription factor to be identified, is expressed early in the osteoblast lineage and interacts with specific DNA sequences in the osteocalcin promoter essential for its selective expression in osteoblasts. The OSF2/CBFA1 gene is necessary for the development of mineralized tissues, and its mutation causes the human disease, cleidocranial dysplasia. Committed osteoprogenitor cells already expressing Osf2/Cbfa1 must synthesize a collagenous ECM before they will differentiate. A cell:ECM interaction mediated by integrin-type cell-surface receptors is essential for the induction of osteocalcin and other osteoblast-related proteins. This interaction stimulates the binding of Osf2/Cbfa1 to the osteocalcin promoter through an as-yet-undefined mechanism.

IL-1 up-regulates osteopontin expression in experimental crescentic glomerulonephritis in the rat

Osteopontin (OPN) is a macrophage chemotactic and adhesion molecule that acts to promote macrophage infiltration in rat anti-glomerular basement membrane (GBM) glomerulonephritis. The present study investigated the role of interleukin-1 (IL-1) in the up-regulation of renal OPN expression in this disease model. Accelerated anti-GBM glomerulonephritis was induced in groups of six rats. Animals were treated by a constant infusion of the IL-1 receptor antagonist or saline (control) over days -1 to 14 (induction phase) or days 7 to 21 (established disease). In normal rat kidney, OPN was expressed in a few tubules (<5%) and absent from glomeruli. During the development of rat anti-GBM disease (days 7 to 21), there was substantial up-regulation of OPN mRNA and protein expression in glomeruli (>5 cells per glomerular cross-section) and tubular epithelial cells (50-75% OPN-positive). Up-regulation of OPN expression was associated with macrophage accumulation within the kidney, severe proteinuria, loss of renal function, and severe histological damage including glomerular crescentic formation and tubulointerstitial fibrosis. In contrast, IL-1 receptor antagonist treatment of either the induction phase of disease or established disease significantly reduced OPN mRNA and protein expression in glomeruli (/75-85%, P < 0.001) and tubules (/45-60%, P < 0.001). The reduction in OPN expression was associated with significant inhibition of macrophage accumulation and progressive renal injury. In vitro, the addition of IL-1 to the normal rat tubular epithelial cell line NRK52E up-regulated OPN mRNA and protein levels, an effect that was dose-dependent and inhibited by the addition of IL-1 receptor antagonist, thus demonstrating that IL-1 can act directly to up-regulate renal OPN expression. In conclusion, this study provides in vivo and in vitro evidence that IL-1 up-regulates OPN expression in experimental kidney disease and support for the argument that inhibition of OPN expression is one mechanism by which IL-1 receptor antagonist treatment suppresses macrophage-mediated renal injury.

Morphological and immunocytochemical characterization of primary osteogenic cell cultures derived from fetal rat cranial tissue

Enzymatic digestion of bone tissue potentially releases a mixture of precursor, differentiating, and mature cells. Conceptually, early fetal osteogenic tissue should provide a more uniform population of cells than late embryonic or newborn bone in which cells have already differentiated. In this context, we have applied sequential enzymatic digestion to obtain and culture cells from 15-16-day fetal rat cranial tissue, a developmental age where deposition of bone matrix has not yet started at this site. These cultures were compared with those of osteogenic cells isolated from newborn rat calvariae and grown under similar conditions. Matrix production and composition were examined by colloidal gold immunocytochemistry using antibodies to bone sialoprotein (BSP), osteocalcin (OC), and osteopontin (OPN). The plated cells formed mineralized nodules by day 14. The presence of mineral was determined by von Kossa staining and backscattered electron imaging (BEI), and the accumulation of calcium and phosphorus within the nodules was demonstrated by X-ray microanalysis and elemental mapping. At early time intervals, cells were generally cuboidal in shape and showed a well-developed Golgi apparatus, which occasionally was immunoreactive for OPN. Labeling for BSP and OPN was found over mineralization foci and electron-dense material within, and at the periphery, of larger mineralized masses and over accumulations of afibrillar matrix at the dish surface. Osteocalcin immunoreactivity was also associated with electron-dense portions of the bone-like matrix. These data demonstrate the potential of presumptive fetal rat calvarial cells to form a bone-like matrix in vitro and suggest that the assembly and mineralization pattern show similarities to the process of intramembranous ossification. Such a culture system is of interest not only for studying cellular and matrix events of bone formation, but also factors which influence mesenchymal cells in committing themselves to the osteogenic pathway.

Chondrogenic potential of skeletal cell populations: selective growth of chondrocytes and their morphogenesis and development in vitro

Most vertebrate embryonic and post-embryonic skeletal tissue formation occurs through the endochondral process in which cartilage serves a transitory role as the anlage for the bone structure. The differentiation of chondrocytes during this process in vivo is characterized by progressive morphological changes associated with the hypertrophy of these cells and is defined by biochemical changes that result in the mineralization of the extracellular matrix. The mechanisms, which, like those in vivo, promote both chondrogenesis in presumptive skeletal cell populations and endochondral progression of chondrogenic cells, may be examined in vitro. The work presented here describes mechanisms by which cells within presumptive skeletal cell populations become restricted to a chondrogenic lineage as studied within cell populations derived from 12-day-old chicken embryo calvarial tissue. It is found that a major factor associated with selection of chondrogenic cells is the elimination of growth within serum-containing medium. Chondrogenesis within these cell populations appears to be the result of permissive conditions which select for chondrogenic proliferation over osteogenic cell proliferation. Data suggest that chondrocyte cultures produce autocrine factors that promote their own survival or proliferation. The conditions for promoting cell growth, hypertrophy, and extracellular matrix mineralization of embryonic chicken chondrocytes in vitro include ascorbic acid supplementation and the presence of an organic phosphate source. The differentiation of hypertrophic chondrocytes in vitro is associated with a 10-15-fold increase in alkaline phosphatase enzyme activity and deposition of mineral within the extracellular matrix. Temporal studies of the biochemical changes coincident with development of hypertrophy in vitro demonstrate that proteoglycan synthesis decreases 4-fold whereas type X collagen synthesis increases 10-fold within the same period. Ultrastructural examination reveals cellular and extracellular morphology similar to that of hypertrophic cells in vivo with chondrocytes embedded in a well formed extracellular matrix of randomly distributed collagen fibrils and proteoglycan. Mineral deposition is seen in the interterritorial regions of the matrix between the cells and is apatitic in nature. These characteristics of chondrogenic growth and development are very similar in vivo and in vitro and they suggest that studies of chondrogenesis in vitro may provide a valuable model for the process in vivo.

Glucocorticoid stimulation of Na+-dependent ascorbic acid transport in osteoblast-like cells

Ascorbic acid (AA) is an essential cofactor for osteoblast differentiation both in vivo and in vitro. Before it can function, this vitamin must be transported into cells via a specific Na+-dependent AA transporter. In this study, we examine the regulation of this transport activity by glucocorticoids, a class of steroid hormones known to stimulate in vitro osteoblast differentiation. Dexamethasone stimulated Na+-dependent AA transport activity approximately twofold in primary rat calvarial osteoblasts. Effects of hormone on ascorbic acid transport were rapid (detected within 24 h) and were maximally stimulated by 25-50 nM dexamethasone. Similar effects of dexamethasone on transport activity were also observed in murine MC3T3-E1 cells. This preosteoblast cell line was used for a more detailed characterization of the glucocorticoid response. Transport activity was stimulated selectively by glucocorticoids (dexamethasone > corticosterone) relative to other steroid hormones (progesterone and 17-beta-estradiol) and was blocked when cells were cultured in the presence of cycloheximide, a protein synthesis inhibitor. Kinetic analysis of AA transporter activity in control and dexamethasone-treated cells indicated a Km of approximately 17 microM for both groups. In contrast, dexamethasone increased Vmax by approximately 2.5-fold. Cells also contained an Na+-independent glucose transport activity that has been reported in other systems to transport vitamin C as oxidized dehydroascorbic acid. In marked contrast to Na+-dependent AA transport, this activity was inhibited by dexamethasone. Thus, glucocorticoids increase Na+-dependent AA transport in osteoblasts, possibly via up-regulation of transporter synthesis, and this response can be resolved from actions of glucocorticoids on glucose transport.

Osteoblast cytoskeletal modulation in response to mechanical strain in vitro

The structural integrity of microfilaments has been shown to be necessary for the signal transduction of mechanical stimuli within osteoblasts. Qualitative and quantitative changes within the cytoskeleton of osteoblasts may therefore be crucial components of the signal transduction processes of these cells in response to mechanical stimulation. Avian osteoblasts were strained with a device that deforms a flexible, cell-laden membrane at a defined frequency and intensity in a uniform biaxial manner. We examined the effects of mechanical strain on the accumulation of protein and the expression of the major cytoskeletal elements and specific integrin-binding (arginine-glycine-aspartic acid) proteins of these cells. Mechanical strain increased the level of total extracellular matrix-accumulated fibronectin by approximately 150% and decreased that of osteopontin by approximately 60% but had no quantifiable effect on the accumulation of beta1 integrin subunit or collagen type I. An examination of the major elements of the cytoskeleton demonstrated that neither the level of actin nor that of the intermediate filament protein vimentin changed; however, the amount of tubulin decreased by approximately 75% and the amount of vinculin, a major protein of focal adhesion complexes, increased by approximately 250%. An analysis of protein synthesis by two-dimensional gel electrophoresis of [35S]methionine-labeled cytoskeletal proteins demonstrated that the changes in the accumulation of vinculin and tubulin resulted from their altered synthesis. Messenger RNA analysis confirmed that the changes in accumulation and protein synthesis observed for vinculin, fibronectin, and osteopontin were controlled at a pretranslational level. Immunofluorescent microscopy demonstrated that mechanical strain led to increased formation and thickening of actin stress fibers, with a commensurate dissociation in microtubules and a clear increase in levels of vinculin at the peripheral edges of the cells. In conclusion, the elevated rate of synthesis and the increased accumulation of vinculin and fibronectin, as well as the increase in the number and size of stress fibers and focal adhesion complexes, suggest that mechanical strain leads to a coordinated change both in the cytoskeleton and in extracellular matrix proteins that will facilitate tighter adhesion of an osteoblast to its extracellular matrix.

Osteocalcin production in primary osteoblast cultures derived from normal and Hyp mice

Rickets and osteomalacia are characteristic features of the Hyp mouse model of human X-linked hypophosphatemia. Hyp mice demonstrate elevated circulating osteocalcin levels, as well as altered regulation of osteocalcin by 1,25(OH)2D3. Whether this osteocalcin abnormality is intrinsic to the osteoblast, or mediated by the in vivo milieu, has not been established. We therefore characterized osteocalcin production and its regulation by 1,25(OH)2D3 in primary cultures of murine osteoblasts and examined osteocalcin and its messenger RNA in response to 1,25(OH)2D3 in cultures of Hyp mouse-derived osteoblasts. Cell viability and osteocalcin production are optimal when murine cells are harvested within 36 h of age. Murine primary osteoblast cultures mineralize and produce osteocalcin in a maturation-dependent fashion (as demonstrated in other species), and continuous exposure to 1,25(OH)2D3, beginning at day 9 of culture, inhibits osteoblast differentiation and osteocalcin production and prevents mineralization of the culture. However, in contrast to other species, exposure to 1,25(OH)2D3, added later (days 17-25) in culture, does not stimulate osteocalcin but arrests osteocalcin production at current levels. Ambient media levels of osteocalcin were no different in cultures from Hyp mice and their normal litter mates, and the down-regulatory response to 1,25(OH)2D3 was comparable in cultures from normal and Hyp mice. Furthermore, expression of osteocalcin messenger RNA in murine cultures is reduced with exposure to 1,25(OH)2D3, and there is no difference between normal and Hyp cultures in this response. Thus, primary murine osteoblasts manifest a species-specific effect of 1,25(OH)2D3 on osteocalcin production. Furthermore, the increased serum osteocalcin production seen in intact Hyp mice, and the altered response to 1,25(OH)2D3 in Hyp mice, are not observed in osteoblast cultures derived from the mutant strain. These data indicate that abnormalities of osteocalcin described in intact Hyp mice require factors other than those present in cultured cells.

Developmental restriction of embryonic calvarial cell populations as characterized by their in vitro potential for chondrogenic differentiation

The mechanism(s) by which the cells within the calvaria tissue are restricted into the osteogenic versus the chondrogenic lineage during intramembranous bone formation were examined. Cells were obtained from 12-day chicken embryo calvariae after tissue condensation, but before extensive osteogenic differentiation, and from 17-day embryo calvariae when osteogenesis is well progressed. Only cell populations from the younger embryos showed chondrogenic differentiation as characterized by the expression of collagen type II. The chondrocytes underwent a temporal progression of maturation and endochondral development, demonstrated by the expression of collagen type II B transcript and expression of collagen type X mRNA. Cell populations from both ages of embryos showed progressive osteogenic differentiation, based on the expression of osteopontin, bone sialoprotein, and osteocalcin mRNAs. Analysis using lineage markers for either chondrocytes or osteoblasts demonstrated that when the younger embryonic cultures were grown in conditions that were permissive for chondrogenesis, the number of chondrogenic cells increased from approximately 15 to approximately 50% of the population, while the number of osteogenic cells remained almost constant at approximately 35-40%. Pulse labeling of the cultures with BrdU showed selective labeling of the chondrogenic cells in comparison with the osteogenic cells. These data indicate that the developmental restriction of skeletal cells of the calvaria is not a result of positive selection for osteogenic differentiation but a negative selection against the progressive growth of chondrogenic cells in the absence of a permissive or inductive environment. These results further demonstrate that while extrinsic environmental factors can modulate the lineage progression of skeletal cells within the calvariae, there is a progressive restriction during embryogenesis in the number of cells within the calvaria with a chondrogenic potential. Finally, these data suggest that the loss of cells with chondrogenic potential from the calvaria may be related to the progressive limitation of the reparative capacity of the cranial bones.

Species-specific glucocorticoid and 1,25-dihydroxyvitamin D responsiveness in mouse MC3T3-E1 osteoblasts: dexamethasone inhibits osteoblast differentiation and vitamin D down-regulates osteocalcin gene expression

The mouse MC3T3-E1 cell line is nontumorigenic and undergoes a typical program of osteoblast differentiation in vitro, producing a bone-like mineralized extracellular matrix. We report responses of these cells to dexamethasone (Dex) and 1,25-(OH)2D3 that are in contrast to findings from other osteoblast culture systems. First, chronic exposure of both early- and late-passaged MC3T3-E1 cells to 10(-7) M Dex, initiated during the proliferation period, blocked osteoblast differentiation, in contrast to the enhanced differentiation observed in cultures of fetal rat calvarial-derived cells. Secondly, 1,25-(OH)2D3 did not up-regulate expression (messenger RNA or protein synthesis) of the endogenous mouse osteocalcin (OC) gene. Several lines of evidence are presented that suggest this response is caused by sequence specific properties of the mouse OC vitamin D response element. We also observed both qualitative and quantitative differences in expression of cell growth (histone H2B) and phenotype-related genes (collagen, OC, osteopontin, glucocorticoid receptor, and 1, 25-(OH)2D3 receptor), between pre- and postmineralization stage osteoblasts, in response to 24 h steroid hormone treatment. Our findings in MC3T3-E1 cells are consistent with current concepts of selective influences of 1,25-(OH)2D3 and glucocorticoids as a function of osteoblast maturation. However, the inhibition of osteoblast differentiation by chronic Dex at 10(-7) M and the down-regulation of OC by 1,25-(OH)2D3 are novel observations relevant to species-specific responsiveness of mouse bone-expressed genes to steroid hormones during osteoblast differentiation.

Signal transduction pathways mediating parathyroid hormone stimulation of bone sialoprotein gene expression in osteoblasts

Bone sialoprotein is a major noncollagenous protein of bone. Parathyroid hormone (PTH) was shown to cause a 2-4-fold increase in the steady-state levels of bone sialoprotein mRNAs within primary cultures of embryonic osteoblasts. The induction could be mimicked by both forskolin and phorbol 12-myristate 13-acetate and was not inhibited by cycloheximide. Transient expression of a approximately 1200-base pair avian 5' bsp promoter/reporter construct demonstrated similar inductions as mRNA levels. Co-transfection of an expression plasmid encoding heat-stable inhibitor of cAMP-dependent protein kinase, a peptide inhibitor of PKA, decreased both the basal and PTH-induced bsp transcription, while co-expression of the catalytic subunit of PKA-induced bsp expression 3-fold. Protein kinase C activation, on the other hand, did not appear to work through its activation of c-fos, since co-transfection of an expression clone for c-fos had no effect. Interestingly, heat-stable inhibitor of cAMP-dependent protein kinase also inhibited the phorbol 12-myristate 13-acetate induction, suggesting that the protein kinase C acts through some form of interaction with the cAMP/PKA pathway. A half-cAMP response element site in the bsp promoter was identified as the cis-acting element that mediated the PTH response by the transient transfections with reporter constructs containing nested deletions of the promoter or a heterologous promoter containing the cAMP response element. In conclusion, these data indicate that PTH stimulation of bsp gene expression is specific to osteoblasts and mediated by changing cellular cAMP/PKA levels. They further suggest that although protein kinase C is capable of stimulating the gene by itself, it plays a minimal role in mediating the PTH induction of bone sialoprotein.