Characteristics and culture of osteoblasts derived from avian long bone

Osteopoietic engraftment after bone marrow transplantation: effect of inbred strain of mice

OBJECTIVE

Transplantable osteoprogenitors, as well as hematopoietic progenitors, reside in bone marrow. We previously reported the first clinical trial of bone marrow transplantation (BMT) for a genetic disorder of bone, osteogenesis imperfecta. Although the patients demonstrated striking clinical benefits after transplantation, measured osteopoietic engraftment was low and did not seem to be durable. Therefore, we sought an animal model, which closely reflects the clinical experience, to facilitate development of strategies to improve the efficiency of osteoprogenitor engraftment after BMT.

MATERIALS AND METHODS

We transplanted unfractionated bone marrow cells from green fluorescent protein-transgenic mice into lethally irradiated recipients in four combinations of inbred mouse strains: from C57BL/6 into C57BL/6 (C-C), from C57BL/6 into FVB/N (C-F), from FVB/N into C57BL/6 (F-C), and from FVB/N into FVB/N (F-F). At 2 weeks after transplantation, we assessed donor hematopoietic and osteopoietic engraftment by flow cytometry, using a novel mean fluorescence assay, and by immunohistochemical staining for green fluorescent protein.

RESULTS

Hematopoietic reconstitution by donor cells was complete in all four combinations. Although osteopoietic engraftment of the transplanted cells was also documented in all the four groups, the magnitude of osteopoietic engraftment differed markedly among the strains where F-F > C-F > F-C > C-C.

CONCLUSION

Our findings indicate that the genetic background of inbred mouse strains affects efficiency of osteopoietic engraftment after BMT. Thus, the murine strain must be considered when comparing experimental outcomes. Moreover, comparing the genetic variation among murine strains may lend insight into the factors governing osteopoietic differentiation of transplanted marrow cells.

Characterization of the non-collagenous proteins in avian cortical and medullary bone

Northern blotting, RT-PCR, and Western blotting techniques were used to characterize the matrix constituents of avian cortical and medullary bone. Extracts of bone tissue were found to contain multiple isoforms of bone sialoprotein (BSP), osteopontin (OPN), osteonectin (ON), osteocalcin (OC), and dentin matrix protein-1 (DMP-1). Only single transcripts were observed with Northern blotting; therefore it was concluded that the isoforms were due to differences in post-translational modifications. Since medullary bone is rich in keratan sulfate (KS), RT-PCR was used to investigate the expression of known keratan sulfate-containing proteoglycans (KSPGs). Although this tissue was found to express lumican and osteoglycin/mimecan, there was little evidence to suggest that these proteoglycans were a major source of the keratan sulfate glycosaminoglycans. Treatment of medullary bone extracts with keratanase resulted in the appearance of a BSP immunoactive band of approximately 59 kDa. However, it was not possible to isolate and identify the intact keratan sulfate proteoglycan.

The effect of donor age on the sensitivity of osteoblasts to the proliferative effects of TGF(beta) and 1,25(OH(2)) vitamin D(3)

The loss of osteoblast function in aging bone is one of the major causes of osteopenia, or loss of bone mass. In this study, this loss of function was investigated by examining the proliferative response of rat long bone periosteal osteoblasts to TGF(beta1) and 1,25-dihydroxy vitamin D(3) (1,25-D(3)) as a function of donor age. Using a DNA binding fluorescent dye, DNA levels were measured in osteoblast cultures derived from either young adult (3-4 months) or old (14-15 months) rats following treatment with two concentrations (10(-9) M or 10(-12) M) of either 1,25-D(3) or TGF(beta1) or with vehicle. Cells from young rat bone, when treated with 1, 25-D(3), showed a dose-dependent increase in proliferation when treated with the higher dose and a decrease in proliferation when treated with the lower dose. Osteoblasts isolated from old rats did not respond to 1, 25-D(3) treatment. A similar pattern of response to TGF(beta1) was found. When treated with 10(-9) M TGF(beta1), the rate of proliferation increased for young rat osteoblasts, but the old rat derived cells were unresponsive. The 10(-12) M dose of TGF(beta1) was ineffective for both young and old cells. This study has shown that osteoblasts derived from old donors are impaired in their ability to respond to vitamin D and TGF(beta), two of the major controlling factors of skeletal development and maintenance.

Inhibition of Na+/Ca2+ exchange with KB-R7943 or bepridil diminished mineral deposition by osteoblasts

Osteoblasts form new bone by secreting a complex extracellular matrix that has the capacity to mineralize when adequate amounts of calcium and phosphate are supplied. The studies reported here show that long-term treatment of cultured, primary osteoblasts with Na+/Ca2+ exchanger (NCX) inhibitors, bepridil and KB-R7943, impacts in a dose-dependent manner the ability of the cells to form a calcified matrix. Treatment of confluent osteoblast cultures for 14 days with low levels of bepridil (3.0 microM) or KB-R7943 (1.0 microM and 0.1 microM) resulted in a significantly diminished capacity of these cells to mineralize bone matrix, without significantly altering cell morphology, viability, or cell differentiation. The data indicate that inhibition of NCX reduces mineral accumulation in the bone matrix by blocking the efflux of Ca2+ from the osteoblast into the bone fluid. In addition, immunocytochemistry of type I collagen (COLI) and bone sialoprotein (BSP) suggests that inhibition of NCX by 1.0 microM KB-R7943 also may impair the secretion of bone matrix proteins by the osteoblasts. This study is the first to show that NCX is an important regulator of the bone fluid microenvironment and that NCX appears critical to the mineralization process.

Expression of translation initiation factor IF2 is regulated during osteoblast differentiation

We isolated and characterized a cDNA for the N-terminal half of the eukaryotic initiation of translation factor 2 (cIF2) during a screen of chicken osteoblast cDNAs. The apparent size of the message for this protein, approximately 5.6 kb, is slightly larger in size than that for human IF2 (hIF2). There is a high degree of sequence similarity between the human and chicken N-terminal portions of the protein that extends to the encoding nucleotide sequence. The tissue specific expression pattern for cIF2 and hIF2 are similar, being moderately abundant in brain, liver, and skeletal muscle, and detectable in kidney, chondrocytes, and freshly isolated osteoblasts. The ratio of message for cIF2 to that of beta-actin was 0.10 and 0.18 for liver and brain. Message levels peak in osteoblasts between 8 and 12 days of culture, coinciding with high levels of matrix protein synthesis. At peak expression, the ratio of cIF2:beta-actin for 8 day osteoblasts was 0.76. Treatment of osteoblast cultures with cycloheximide markedly reduces the level of cIF2 message indicating that novel protein synthesis is required for its expression. Hybridization of RNA samples from either chicken osteoblasts or a human osteoblast cell line with a probe for a subunit of human eukaryotic initiation of translation factor 2 (eIF2alpha), the housekeeping initiation factor, indicates that levels of eIF2 remain low. With hIF2, cIF2 represents the only other vertebrate homolog of IF2 for which a major portion of the coding sequence has been identified. This is the first report of regulated expression for a eukaryotic IF2 and is the first demonstration of its abundance in osteoblasts.

Perspectives on osteoblast and osteoclast function

We present evidence that the polar, matrix-forming osteoblasts are connected laterally to form an impervious layer of cells. Next, the possible mechanisms by which calcium ions are translocated across the layer of cells into sites of mineralization are analyzed. Finally, mechanisms of attachment of bone-resorbing osteoclasts are considered. Osteoclasts adhere to matrix, in part, though an arginine-glycine-aspartic acid (RGD)-dependent mechanism. Adherence is under control of parathyroid hormone and 17beta-estradiol.

Confocal imaging and timing of secretion of matrix proteins by osteoblasts derived from avian long bone

Primary osteoblasts derived from avian long bone have been evaluated in terms of spatial and temporal expression of known osteoblastic marker proteins during the early phases of cell culture. Confocal imaging of matrix proteins revealed that osteocalcin, bone sialoprotein, osteopontin, and osteonectin were restricted to the cell interior at day 4 of culture; secretion and deposition into the extra-cellular matrix of bone sialoprotein and osteopontin was evident at 8 and 12 days of culture. Osteocalcin and osteonectin were not deposited in the matrix within the timeframe of the study. Total collagen levels produced and alkaline phosphatase activity were substantial by day 4 of culture, and increased from that point 4.0- and 5.5-fold, respectively, by culture day 12. The expression of type I collagen, PTHrP receptor, osteopontin, bone sialoprotein and osteocalcin was followed by Northern blot analysis. Type I collagen and osteopontin mRNA were expressed at constant levels throughout the culture period. Over the 12 days of culture both PTH/PTHrP receptor and bone sialoprotein mRNA expression were found to increase by 2.3- and 2.5-fold, respectively. In contrast, the expression of osteocalcin message decreased by 2.5-fold by day 8 of culture.

Asymmetric distribution of functional sodium-calcium exchanger in primary osteoblasts

To understand calcium translocation in osteoblasts, we have determined the location of sodium-calcium (Na-Ca) exchanger (NCX) in relation to actin and alpha-tubulin in primary cultures of avian osteoblasts. Osteoblasts derived from the periosteal surface of tibias from growing chickens were cultured for 8 days in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum. Lysates immunoblotted with antibodies raised against the canine cardiac Na-Ca antibodies revealed a 70 kDa exchanger protein. Cross-reactivity of the anti-NCX antibody was confirmed by enriching for NCX in protein samples derived from plasma membrane vesicles by affinity chromatography using the exchanger inhibitory peptide. Fractions enriched for the exchanger were eluted from the column and subjected to immunoblotting with the anti-NCX antibody, revealing an intense single band at 70 kDa. Examination of live cells loaded with Calcium Green-1 AM ester by confocal microscopy demonstrated sodium-dependent calcium uptake, confirming the presence of functional NCX in intact cells. Immunolocalization studies of osteoblasts stained with anti-NCX antibodies revealed asymmetric localization of the exchanger in cultured osteoblasts, residing almost entirely within two 0.5-microm optical sections along the substrate adherent side of the cells. Since NCX is known to be a low-affinity, high-capacity calcium translocating molecule and also appears to be asymmetrically positioned, it is likely to play a key role in bone formation.

Mitochondrial membrane potential changes in osteoblasts treated with parathyroid hormone and estradiol

This study assessed mitochondrial membrane potential changes in cultured osteoblasts treated with hormones known to regulate osteoblasts. A fluorescent carbocyanine dye, 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine++ + iodide, also called JC-1, was used as a probe. JC-1 emits photons at 585 nm (orange-red) when the membrane potential in mitochondria is highly negative, but when the potential becomes reduced emission occurs at 527 nm (green). Osteoblasts were rinsed in serum-free medium for 5 min, then loaded with 1 x 10(-6) M JC-1 for 10 min. The distribution and intensity of JC-1 fluorescence were evaluated with a laser-scanning confocal microscope system. Hormone treatments included parathyroid hormone (PTH; 10(-8) M), 17beta-estradiol (10(-8) M), and thyroxine (T4; 10(-8) M). The potassium ionophore valinomycin (10(-6) M) was used as a control since it is known to disrupt the electrochemical gradient of mitochondria without interfering with the pH gradient. Valinomycin caused a profound, rapid increase (22.5% above untreated values) in the green/red ratio, which indicated a lowering of the mitochondrial membrane potential in all samples evaluated. PTH caused a less pronounced, but significant (7-14%), reduction in membrane potential in all cells examined. PTH is known to affect osteoblasts in a number of ways and is inhibitory to mitochondrial respiration; the results confirm this effect. For estradiol, half of the cells responded at a significant level, with a membrane potential reduction of 6 to 13% being recorded; the other half did not respond. Thyroxine did not alter mitochondrial membrane potential. Responses were detectable within 20 s for valinomycin, but occurred at a slower rate, over 200 to 300 s, following PTH and estradiol treatment. Responses to PTH and estradiol could be due to mitochondrial uptake of cytosolic Ca2+.

Maltol complexes of vanadium (IV) and (V) regulate in vitro alkaline phosphatase activity and osteoblast-like cell growth

Vanadium compounds have been found to possess insulin- and growth factor-mimetic effects. In consequence, these derivatives are potentially useful as effective oral therapeutic agents in diabetic patients. However, their use has been limited by various toxic side-effects and by the low solubility of different derivatives. Recently, vanadium complex with maltol, a sugar used as a common food additive, have been synthesised and investigated in animals, showing possible insulin-mimetic effects with low toxic side-effects. In the present study we have investigated the effect of bis(maltolato)oxovanadium (IV) (BMOV) and bis(maltolato)dioxovanadium (V) (BMV) on bone cells in culture as well as their direct effect on alkaline phosphatase in vitro. A comparison was also made with the action of vanadate and vanadyl cation. Vanadium compounds regulated cell proliferation in a biphasic manner with similar potencies. Osteoblast differentiation, assessed by alkaline phosphatase activity, was found to be dose-dependent, with the inhibitory effect being stronger for vanadate and BMOV than for vanadyl and BMV. All vanadium compounds directly inhibited bovine intestinal ALP with a similar potency. Thus, maltol vanadium derivatives behave in a similar way to vanadate and vanadyl in osteoblast-like UMR 106 cells in culture.

Development of a new method for obtaining osteoclasts from endosteal surfaces

Techniques for the isolation of a highly pure population of viable osteoclasts are limited. For this reason, we developed an isolation procedure that results in a high yield of osteoclast-like cells, up to 92% pure, from 3-wk-old chicken tibias. The unique feature of the method is the migration of cells from marrow-free endosteal surfaces to vitronectin-coated plates. The cells retain the osteoclast phenotype and remain viable in culture for a minimum of 1 wk. The cells were characterized and compared to two populations of authentic avian osteoclasts, which were isolated on the basis of association with fibronectin-coated plates. The cells contained substantial amounts of tartrate-resistant acid phosphatase. Alkaline phosphatase levels were negligible, suggesting little contamination by osteoblasts. Response to parathyroid hormone, dibutyryl cyclic adenosine monophosphate, calcitonin, acetazolamide, 17 beta-estradiol, and prostaglandin E2 was evident, as detected by measuring acid production. The vitronectin-associating cells contained numerous mitochondria, had the ability to resorb bone in an in vitro bone slice assay, and specifically bound biotinylated vitronectin. At 5 d of culture, the cells demonstrated marginal multinuclearity, having two to three nuclei. A large number (approximately 1 x 10(6) cells/tibia) of viable cells that exhibit characteristics of authentic osteoclasts can be obtained by the method described. Potentially, this method could be applied to other species.

Characterization of calcium translocation across the plasma membrane of primary osteoblasts using a lipophilic calcium-sensitive fluorescent dye, calcium green C18

The synthesis of Calcium Green C18, a lipophilic fluorescent calcium-sensitive dye, and its use as a monitor of Ca2+ efflux from cells is described. This indicator consists of a Calcium Green-1 molecule conjugated to a lipophilic 18-carbon alkyl chain which will intercalate into cell membranes. The Kd of the indicator for Ca2+ in aqueous solution (pH 7.2, 22 degrees C, ionic strength 0.1 M) is 0.23 +/- 0.04 microM and in the presence of liposomes is 0.062 +/- 0.007 microM. Due to its high negativity, the calcium chelating fluorophore faces the cell exterior, when loaded under a defined set of conditions. The dye was found largely on the surface of the cells when loaded at a concentration of 5 microM for 10 min at 37 degrees C. Five minutes after introduction of EGTA, 83-95% fluorescence disappeared, indicating that most of the fluorophore was on the cell surface. Photobleaching was minimal (3-13%). A confocal laser scanning microscope was used to detect and quantify fluorescence. Internalized dye was apparent in cells loaded for longer times (30-60 min) and in membrane-impaired cells, as shown by uptake of propidium iodide. Under defined confocal laser scanning microscope settings, a transient fluorescence at the periphery of approximately 30% of the cells was observed following 10(-8) M parathyroid hormone treatment, indicating the presence of outwardly directed calcium transport across the plasma membrane. Calcium efflux usually lasted 7-10 min, peaking at around 2-3 min. Changes in cell shape were also observed. Calcium efflux was shown to be sensitive to (a) 10 microM quercetin and 10 microM vanadate, partially specific inhibitors of plasma membrane Ca(2+)-ATPase, to (b) 0.1 mM trifluoperazine, an agent which renders calmodulin ineffective, and to (c) 10 mM neomycin sulfate, which blocks release of Ca2+ from intracellular stores. Thapsigargin (5 microM), an inhibitor of Ca(2+)-ATPase of the endoplasmic reticulum, prolonged fluorescence. These observations indicate that cell surface fluorescence was due to the capture of Ca2+ by Calcium Green C18 after Ca2+ had been translocated across osteoblast plasma membranes. Involvement of the plasma membrane Ca(2+)-ATPase, known to be present in osteoblasts in substantial amounts, is implicated.

Characterization of calcium efflux by osteoblasts derived from long bone periosteum

Inside-out vesicles were prepared from the plasma membrane of osteoblasts which had been isolated from the periosteum of 2-3-week-old chicken tibia and cultured for 6-8 days. Calcium uptake was initiated by adding 0.4 mM ATP and detected as a reduction in fluorescence from the reaction medium using the Ca(2+)-specific fluoroprobe, fluo-3. The reaction medium contained ouabain (1 mM) to block Na+, K(+)-ATPase activity and oligomycin (20 micrograms/ml) to block mitochondrial activity. Thapsigargin (5 microM) had little effect, indicating that contributions to Ca2+ uptake by endoplasmic reticulum derived microsomes were minimal. The Ca2+ uptake rate was 9.9 +/- 2.3 nmol/mg protein/min. Trifluoperazine (0.1 mM), which impairs the capacity of calmodulin to activate Ca(2+)-ATPase, substantially inhibited transport, as did quercetin (10 mM) and vanadate (10 microM), inhibitors of Ca(2+)-ATPases. This study has shown the presence of an outwardly directed, calmodulin-sensitive calcium transport system in the primary osteoblast plasma membrane. The pumping rate is substantially less than rates found in the intestine, a tissue which is involved in massive transport of Ca2+, but is similar to rates found in many other tissues. It is concluded that the enzyme does not support calcium translocation to sites of mineralization.