The plasma membrane and matrix vesicles of mouse growth plate chondrocytes during differentiation as revealed in freeze-fracture replicas

The epiphyseal cartilage in mouse tibia and fibula was investigated with the freeze-fracture method. Cytodifferentiation of growth plate chondrocytes was found to be marked by changes in both cell membrane and extracellular matrix vesicle membranes. Exocytosis and endocytosis were observed in all zones of differentiation, with endocytosis being predominant in the reserve and proliferative zones and exocytosis occurring with greatest frequency during hypertrophy. Intramembraneous particles (IMPs) on the plasma membrane were distributed evenly on the reserve and proliferative cell membranes, whereas in the hypertrophic zone IMPs tended to be distributed asymmetrically. Several types of matrix vesicles were identifiable on the basis of IMP distribution: IMP-free, IMP-aggregated, and IMP-random. The distribution pattern of IMPs on vesicles varied with differentiation of the chondrocytes. For proliferative and prehypertrophic cells, most matrix vesicles belonged to the IMP-random category. IMP-aggregated and IMP-free matrix vesicles became increasingly frequent in the later stages of differentiation, particularly in the late hypertrophic stage. IMPs were observed more frequently on the convex protoplasmic fracture face of matrix vesicles than on the concave exoplasmic fracture face, as was also observed for the plasma membrane. Matrix vesicles formation appears to occur by budding from chondrocyte projections and bulges at the smooth surfaces of the cells and from cell disintegration. Crystals of mineral were apparent in cross-fractured matrix vesicles of the calcifying zone, but not in the other zones.

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.

Regulation of differentiated osteoclasts

Osteoclasts respond to many factors, including endocrines, cytokines, cell-cell interactions, and cell-matrix contacts. For mature osteoclasts, the first level of control occurs through signaling that follows binding to an appropriate substrate. Mononuclear and multinucleate osteoclasts are activated when cell surface integrins, notably but not exclusively alphavbeta3 integrins, bind to calcified matrices. The binding process results in actin ring formation and deployment of adhesive proteins into a ring shape such that a seal is formed. As this ring forms, components of the ruffled border assemble from diffuse distribution to form the resorption apparatus, which includes the vacuolar-ATPase, carbonic anhydrase, and other key molecules. This review focuses on the control of osteoclast activity, beginning with attachment and ruffled border assembly. Direct and indirect regulation by PTH/PTHrP, genomic and nongenomic effects of estrogen, and gene expression of ruffled border components, carbonic anhydrase, and vacuolar ATPase are reviewed. Finally, the need to understand complex signaling pathway interaction is discussed.

Simultaneous detection and functional response of testosterone and estradiol receptors in osteoblast plasma membranes

Osteoblasts derived from the periosteal surfaces of two-three-week-old male broiler chicken tibias were cultured for eight days. The cells were then loaded with fura-2/AM ester to detect surges in intracellular Ca(2+). Treatment with 10(-7) M testosterone (T) or 17beta-estradiol (E) elicited a rapid (within seconds) response that was substantially reduced by introducing the calcium chelating agent EGTA or the calcium-channel blocker verapamil. The hormones were equally effective when covalently linked to bovine serum albumin (BSA), a procedure that ensures the hormone does not enter the cells. The rapid response to surface-bound steroids indicates that the responses were invoked through plasma-membrane receptors. The source of Ca(2+) was shown to be through entry from external sources, as well as from intracellular stores. Flow cytometry of fluorescein-tagged T-BSA and E-BSA revealed that osteoblasts derived from male chickens had similar and substantial levels of both receptors.

Gene expression and tibial dyschondroplasia

Tibial dyschondroplasia (TD) is a skeletal deformity associated with rapid growth in a number of avian species. The disease is the result of a disruption in the cascade of events that occur in the epiphyseal growth plate. Whereas the incidence of TD is susceptible to genetic selection, no specific genetic defect has been identified. Although there are extensive data describing the morphological and biochemical characteristics of the lesion, the mechanism of lesion formation is unknown. However, naturally occurring or induced genetic mutations in other species can provide important clues to possible mechanisms responsible for lesion development. Disruption of normal chondrocyte differentiation by constitutive activation of the parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor, inactivation of the fibroblast growth factor receptor-3 (FGFR-3) receptor, and blocking vascular endothelial growth factor (VEGF) signaling all result in lesions that resemble TD. Impairment of vascular penetration due to the ablation of matrix metalloproteinase-9 (MMP-9) or tartrate-resistant acid phosphatase (TRAP) activity also results in similar cartilage abnormalities. We have integrated these observations with our current knowledge of TD to describe a hypothesis for the sequence of events responsible for the development of tibial dyschondroplastic lesions.

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.

Estrogen stimulates protein tyrosine phosphorylation and Src kinase activity in avian osteoclasts

The estrogen, 17beta-estradiol, stimulated a profound increase in phosphotyrosine immunostaining of proteins that localized along the site of attachment in avian osteoclasts within 1 min of treatment. By 10 min, this rapidly occurring event had returned to basal levels. Pretreatment with 1 microM herbimycin A, a tyrosine kinase inhibitor, prevented the response. Immunoblotting revealed that Src kinase was one of the phosphorylated intermediates. Src kinase also appeared to translocate to the periphery of the cells during the 1 min 17beta-estradiol treatment and became dispersed by 10 min. Src kinase activity measurements indicated an increase in phosphotransferase activity after the 1 min estradiol treatment; this effect diminished with longer exposures to estrogen. Pretreatment of osteoclasts with 1 microg/ml cytochalasin B, an inhibitor of actin polymerization, delayed the appearance of increased phosphotyrosine immunostaining at attachment sites, possibly through inhibition of Src kinase translocation. These findings demonstrate that estrogen stimulates rapid tyrosine phosphorylation in osteoclasts, a process that involves activation and translocation of Src kinase to the plasma membrane.

Depolarization of osteoclast plasma membrane potential by 17beta-estradiol

The effect of estrogen on plasma membrane potential of isolated avian osteoclasts was examined through the use of a fluorescent potential-sensitive dye, bis-(1,3-dibutylbarbiturate) trimethine oxonol, also known as bis-oxonol. A decrease in potential was observed within seconds of addition of 17beta-estradiol. Ouabain, a specific Na+K+-ATPase inhibitor, and BaCl2, an inhibitor of the inwardly rectifying K+ channel, blocked the estrogen response. Verapamil and lanthanum chloride (LaCl3), inhibitors of inward Ca2+ channels, and 4'4-diisothiocyanatostilbene-2'2-disulfonic acid (DIDS), an inhibitor of Cl- channels, did not affect the depolarization. Herbimycin A, a tyrosine kinase inhibitor, also had no effect on the decreased membrane potential. These data provide evidence which suggests that estrogen regulates osteoclasts through ion channel activities. The change in K+ channel activity was observed within seconds of addition of 17beta-estradiol, indicating an action at the level of the plasma membrane.

Localization of carbonic anhydrase in living osteoclasts with bodipy 558/568-modified acetazolamide, a thiadiazole carbonic anhydrase inhibitor

We describe the synthesis of Bodipy 558/568-modified acetazolamide, a fluorescent inhibitor of carbonic anhydrase and its use to localize the enzyme in living cells. The modified acetazolamide, with its specific sulfonamide group intact, labeled cells at concentrations as low as 10(-9) M, with a minimal loading time of 5 min. The staining was decreased by 57.4% by preincubating cells with unaltered acetazolamide (1:100) or with trifluoromethane sulfonamide, 6-ethoxyzolamide, and 5-(3-hydroxybenzoyl)-thiophene-2-sulfonamide. The efficacy of the inhibitor was unchanged by the fluorescent label, as determined by an acridine orange assay that detects acidification of osteoclasts, the cell model used in this study. This compound should prove to be useful for studying carbonic anhydrase in many organisms because of the high degree of conservation of the active site of this enzyme. (J Histochem Cytochem 47:545-550, 1999)

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.

Spectrin localization in osteoclasts: immunocytochemistry, cloning, and partial sequencing

The presence of spectrin was demonstrated in chick osteoclasts by Western blotting and light and electron microscopic immunolocalization. Additionally, screening of a chick osteoclast cDNA library revealed the presence of alpha-spectrin. Light microscope level immunocytochemical staining of osteoclasts in situ revealed spectrin staining throughout the cytoplasm with heavier staining found at the marrow-facing cell margin and around the nuclei. Confocal microscopy of isolated osteoclasts plated onto a glass substrate showed that spectrin encircled the organelle-rich cell center. Nuclei and cytoplasmic inclusions were also stained and the plasma membrane was stained in a nonuniform, patchy distribution corresponding to regions of apparent membrane ruffling. Ultracytochemical localization showed spectrin to be found at the plasma membrane and distributed throughout the cytoplasm with especially intense staining of the nuclear membrane and filaments within the nuclear compartment.

Detection of endogenous biotin-containing proteins in bone and cartilage cells with streptavidin systems

When utilizing streptavidin systems with Western blots of chondrocyte, osteoblast and osteoclast lysates, proteins of the molecular weights 116 kDa, 75 kDa and 67 kDa were observed to be bound by streptavidin alone. Streptavidin binding could not be blocked by pre-incubation with an RGD containing peptide. The same proteins were bound by ExtrAvidin which lacks the RGD sequence present in streptavidin. Pre-incubation with free biotin completely abolished the binding of both streptavidin and ExtrAvidin. The three proteins observed are believed to be the biotin containing carboxylases: pyruvate carboxylase, 3-methylcrotonyl carboxylase, and propionyl carboxylase. The findings of this study underscore the need to apply vigorous controls to distinguish between endogenous biotinylated proteins and biotin used as a means to amplify avidin detection systems since a wide variety of proteins with relevance to bone and cartilage biology have molecular weights similar to the biotin carboxylases.

Type X collagen biosynthesis and expression in avian tibial dyschondroplasia

OBJECTIVE

Tibial dyschondroplasia (TD) is an abnormality of growth plate cartilage characterized by the presence of non-vascularized, non-mineralized tissue. The objective of this study was to examine structural and functional alterations of the growth plate-specific type X collagen in TD cartilage.

DESIGN

Collagen biosynthesis was examined in organ cultures and in cultured chondrocytes from normal growth plate and TD cartilage. Thermal stability of type X collagen extracted from normal and TD cartilage organ cultures to protease digestions by trypsin plus chymotrypsin or bacterial collagenase was determined. The expression of collagen genes was examined in cultured normal and TD chondrocytes.

RESULTS

Synthesis of total collagen and of type X collagen was greater than threefold higher in organ cultures from the TD lesion compared with normal growth plate. The increase in type X collagen synthesis in the lesion was compensated by a reduction in the relative proportions of types II and XI collagens. The thermal denaturation and collagenase cleavage properties of purified types II and X collagens from TD cartilage were normal. The expression of type X collagen gene was threefold higher in cultured TD chondrocytes compared to chondrocytes from normal growth plate. Normal growth plate chondrocytes in primary cultures synthesized predominantly type X collagen (80% of total collagen). In contrast, TD chondrocytes synthesized mainly types I and II collagens and type X collagen represented only 22% of total collagen. TD cells initiated the synthesis of type I collagen within 5 days of primary culture, whereas normal chondrocytes did not synthesize this collagen during the same culture period. Although type X collagen synthesis was reduced in TD chondrocytes, the mRNA levels for type X collagen were substantially higher than in normal chondrocytes.

CONCLUSION

Accumulation of type X collagen in TD cartilage results from its increased biosynthesis which is due largely to increased expression of the gene for this collagen, although, the chondrocyte culture studies suggest the possibility of postranscriptional defect in type X collagen synthesis or processing in TD lesion. Moreover, the TD chondrocytes in contrast with normal chondrocytes display evidence of prompt loss of their specific phenotype during short-term primary cultures.

Chondrocytes of the tibial dyschondroplastic lesion are apoptotic

Tibialdyschondroplasia (TD) is a disease characterized by the formation of an avascular, non-mineralized lesion along the mature face of the epiphyseal growth plate in rapidly growing chickens. In the normal growth plate, cells progress from a proliferative phase to hypertrophy where the tissue is vascularized and replaced by trabecular bone. In TD, cells apparently cease their development early in the transition to hypertrophy. These diseased cells are not removed by vascularization nor does mineralization occur. The resulting lesion increases in size as proliferative cells continue to divide in the absence of removal and replacement of cartilage by bone. This laboratory has previously reported that cells of the TD lesion have the morphological appearance of necrotic cells or in some cases apoptotic cells. In this study we examine in more detail the status of cells comprising the TD lesion using molecular techniques. Genomic DNA isolated from cells of severe TD lesions show the nucleosomal laddering indicative of apoptosis, while DNA isolated from proliferative and hypertrophic cells does not. This result was confirmed by the use of the Cell Death Detection ELISA which shows quantitatively that cells from severe TD lesions contain nearly twice as many nucleosomal fragments as cells from the hypertrophic zone while proliferative chondrocytes do not have significant fragmentation. In situ examination of the epiphyseal growth plate with terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) clearly shows that the cells of the severe TD lesion are apoptotic. Cells from smaller lesions are stained to a lesser extent or not at all by TUNEL. We believe that the apoptosis seen in TD is a secondary effect of the disease and not its primary cause.

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+.

Parathyroid hormone uses both adenylate cyclase and protein kinase C to regulate acid production in osteoclasts

Osteoclasts, isolated from the endosteum of 2.5- to 3-week-old chickens, were treated with acridine orange, a hydrogen ion concentration-sensitive fluorescent dye, in order to monitor changes in acid production. The adenylate cyclase inhibitor, alloxan, blocked parathyroid hormone (PTH)-stimulated acid production. Dibutyryl cyclic adenosine monophosphate, a membrane-permeant form of cyclic adenosine monophosphate, mimicked the PTH effect. Bisindolylmaleimide, a specific inhibitor of protein kinase C (PKC), blocked the initial stimulation (15, 30, and 60 min) of acid production by PTH but had no effect on long-term stimulation (120 min). Confocal microscopy of osteoclasts stained with fluorescein-conjugated bisindolylmateimide revealed a shift in location of PKC from the cytoplasm to the plasma membrane region after treatment with parathyroid hormone. The results of these studies support the hypothesis that PTH regulation of acid production in osteoclasts involves both adenylate cyclase and PKC as effectors.

Multiple G-protein involvement in parathyroid hormone regulation of acid production by osteoclasts

The involvement of multiple G-proteins in parathyroid hormone regulation of acid production was demonstrated in a highly enriched osteoclast population. Osteoclasts were isolated from the endosteum of 2.5 to 3-week-old chicken tibia using sequential enzymatic digestion. Single cell analysis of acid production was accomplished using microscope photometry and vital staining with acridine orange, a hydrogen ion concentration sensitive fluorescent dye. Lithium chloride, an uncoupler of G-proteins from their respective receptors, blocked parathyroid hormone stimulated production of acid. Cholera toxin, which permanently activates Gs-proteins, mimicked PTH stimulation. Pertussis toxin, which prevents receptor interaction with Gi- and Go-proteins, blocked both 10(-8) M and 10(-11) M PTH stimulated acid production, suggesting that the pertussis toxin-sensitive G-protein is utilized at both PTH concentrations. Immunoblots of osteoclast plasma membrane proteins, using a panel of antibodies generated against specific G-protein alpha subunits, revealed a 48 kDa Gs alpha, a 41 kDa Go alpha, a 34 kDa Gi alpha-3, and a unique 68 kDa G alpha subunit, with the 41 kDa and 34 kDa bands being the most intense. Immunoblots of osteoblast plasma membrane proteins had a substantially different profile with the most intense bands being a Gs alpha (48 kDa) and a Go alpha (36 and 38 kDa). The studies suggest the utilization of at least two different G-proteins in the parathyroid hormone regulation of acid formation by osteoclasts, a Gs and a pertussis toxin-sensitive G-protein (Go and/or Gi alpha-3).

Immunolocalization of osteonectin in avian tibial dyschondroplastic cartilage

Osteonectin is an acidic calcium-binding protein found in cartilage, bone matrix, vascular endothelium, and areas of tissue repair. Using immunocytochemistry, osteonectin has been localized in all zones of the normal avian epiphyseal growth plate with notably high amounts in the hypertrophic zone. In the proximal portion of this zone the staining was intracellular, while in the distal calcifying portion of the hypertrophic zone staining was both intracellular and extracellular. Osteonectin was also detected in the growth plate associated with lesions of chickens with tibial dyschondroplasia (TD). Intense intracellular staining was observed in hypertrophic chondrocytes proximal to the lesion; staining was markedly diminished in the TD lesion; extracellular matrix was devoid of staining. Staining intensity was high along the peripheral edges of the lesion that were undergoing vascularization and resorption. This was the only area in the dysplastic cartilage where staining was observed in the extracellular matrix as well as intracellularly. Similar patterns were viewed in all TD lesions examined, whether they were spontaneous or induced by dietary treatments or genetic selection.

Basic fibroblast growth factor: an autocrine growth factor for epiphyseal growth plate chondrocytes

Basic fibroblast growth factor (bFGF) is a permissive mitogen for cultured chondrocytes and has been localized in the specific zones of the epiphyseal growth plate. In this study, we demonstrate that bFGF present in cartilage originates from within the cellular constituents of this tissue. Utilizing reverse transcription coupled to the polymerase chain reaction (PCR), bFGF mRNA was found in extracts of cartilage tissue. Immunocytochemical studies revealed that bFGF was present intracellularly in freshly isolated proliferative chondrocytes and in the extracellular matrix (ECM) after 24 h of culture. Western blot analysis of protein extracts from isolated proliferative chondrocytes identified a bFGF immunoreactive species with a molecular weight of approximately 18 kDa. In situ hybridization confirmed the presence of bFGF mRNA in freshly isolated proliferative chondrocytes. The bFGF in the ECM seemed to be sequestered and not available for biological activity, since these cells still required exogenous bFGF for cell proliferation. This sequestered bFGF could be released to stimulate cell proliferation when cultures were treated with plasmin, a proteolytic enzyme. These data support the hypothesis that bFGF is synthesized by chondrocytes and functions as an autocrine/paracrine mitogen via its deposition into the ECM with subsequent release from the ECM of cartilage being a critical step in biological activity. In addition, the study provides further evidence that locally produced bFGF plays an important role in normal growth and development of cartilage tissue.

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.

Role of microscopy in elucidating the mechanism and regulation of the osteoclast resorptive apparatus

Microscopic studies have assisted in revealing some of the components of the resorptive apparatus of osteoclasts, specifically carbonic anhydrase and the proton-translocating ATPase. Further, microscopy has helped substantiate the types of proteolytic enzymes secreted into the resorption lacuna. Regulatory agents affecting the resorptive process in vitro include parathyroid hormone, 17 beta-estradiol, calcitonin, and 1,25-dihydroxyvitamin D3. Studies showing the specific binding of parathyroid hormone, estradiol, and calcitonin to osteoclast plasma membrane are discussed. While specific binding suggests that direct effects may occur, further investigation is needed to substantiate this possibility.

Immunolocalization of basic fibroblastic growth factor in avian tibial dyschondroplastic cartilage

Recent research in our laboratory has demonstrated that basic fibroblast growth factor (bFGF) is a permissive mitogen for epiphyseal growth plate chondrocytes. Immunocytochemistry demonstrated the presence of bFGF in the proliferative and hypertrophic zones of normal epiphyseal growth plates of 4-wk-old broiler chickens. The purpose of this investigation was to extend this research to include examination of the status of bFGF in the cartilage lesion associated with tibial dyschondroplasia (TD). Immunocytochemistry revealed that the distribution of bFGF in the growth plate proximal to the TD lesion was similar to that observed with normal growth plate. However, the intensity of immunofluorescence was greatly diminished in the TD lesion. The number of chondrocytes staining positive for bFGF was also reduced. In the peripheral edges of the lesion where cartilage was being actively resorbed, the staining intensity was greatly increased when compared to the rest of the TD lesion. Similar patterns were observed in all TD tissues examined whether the lesions were spontaneous or induced by dietary treatments or genetic selection. It is hypothesized that the decrease in bFGF, a potent angiogenic factor, may be responsible for the poor vascularization of the TD lesion.

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.

Isolation and localization of basic fibroblast growth factor-immunoreactive substance in the epiphyseal growth plate

Previous research in our laboratory has shown basic fibroblast growth factor (bFGF) to be a permissive mitogen for isolated avian growth plate chondrocytes. The present study was conducted to determine whether bFGF is present in avian growth plate and, if present, to determine its localization within the tissue. Immunohistochemical studies revealed that bFGF is present in the resting proliferative and hypertrophic calcifying zones of the growth plate but is absent from the prehypertrophic zone. Basic FGF appears to be associated with the extracellular matrix of the proliferative zone, but it is predominantly intracellular in the hypertrophic and mineralizing zone chondrocytes. Partial purification of cartilage-derived bFGF was performed on crude extracts of cartilage using heparin-Sepharose affinity chromatography. The presence of bFGF in the heparin-Sepharose column fractions was confirmed by immunoblotting and radioimmunoassay. Furthermore, western blot analysis of the extracts showed multiple protein bands having bFGF immunoreactivity, in the molecular weight range 14.4-18 kD. The data support the hypothesis that bFGF has a dual role in the growth plate. In the proliferative zone it acts as a chondrocyte mitogen, whereas when released from terminal hypertrophic chondrocytes, bFGF may serve as a chemotactic signal for metaphyseal blood vessel proliferation.

Characteristics and culture of osteoblasts derived from avian long bone

A method is presented for isolating primary osteoblasts from the periosteal surface of chick tibia. The culture system identified supports both cell proliferation and phenotype retention. Cell numbers increased 8-fold in Week 1 and 20-fold over a total of 12 days. Well-established osteoblast markers, alkaline phosphatase staining, gamma-carboxyglutamic acid, osteocalcin, type I collagen, and parathyroid hormone binding were detected. Osteocalcin, gamma-carboxyglutamic acid, and type I collagen were present on culture Day 4, and were increased in amount by Day 8, but were similar to the earlier level on Day 12, suggesting that the phenotype may revert to a less differentiated state by 12 days in culture. Alkaline phosphatase staining was intense at all three assay times, however. During the last 4 days of the 12-day culture period, proliferation rates were higher than in the previous 8 days.

Surface binding and clearance of calcitonin by avian osteoclasts

Osteoclasts, isolated from chick tibias and maintained in culture for 4-6 days, were treated with biotinylated calcitonin followed by fluorescent avidin. In 2 min the entire cell surface fluoresced, by 7.5 min fluorescence was centrally clustered and by 10 min fluorescence had diminished to background levels. Specific binding was blocked by excess unlabeled calcitonin and not influenced by unrelated peptide. The results show that avian osteoclasts bind calcitonin specifically and that the bound calcitonin is rapidly cleared from the cell surface, which indicates involvement of receptor-mediated endocytosis.

Specific binding of estrogen to osteoclast surfaces

Specific plasma membrane binding sites for 17 beta-estradiol were shown to exist on avian osteoclasts by the use of a fluorescent conjugated estrogen, 17 beta-estradiol-6-carboxymethyloxime- bovine serum albumin-fluorescein isothiocyanate. Binding was dose and time dependent, as well as specific, since 17 beta-estradiol blocked the binding of the complex. Tamoxifen was also an effective blocking agent. Rapid responses to 17 beta-estradiol included decreased acid production (41.5%) and distinct cell shape changes.

Vitamin K deficiency does not functionally impair skeletal metabolism of laying hens and their progeny

The purpose of this research was to determine the effect of vitamin K deficiency on indices of skeletal metabolism in laying hens, developing embryos and young growing chickens. Laying hens were fed a vitamin K-deficient diet for 28 wk, which resulted in impaired blood clotting and reduced bone gamma-carboxyglutamic acid (Gla) concentration compared with vitamin K-sufficient hens. However, this treatment did not influence egg production, eggshell deposition or other reproductive performance criteria. Vitamin K-deficient embryos were able to mobilize sufficient quantities of calcium for normal skeletal development, although they exhibited severe reduction in blood clotting and bone Gla concentration. Similar results were obtained from progeny of both vitamin K-sufficient and -deficient hens fed deficient diets for 4 wk after hatching. These results indicate that a severe reduction in skeletal protein Gla concentration does not interfere with normal development of this tissue.

Effect of estrogen on acidification in osteoclasts

Isolated chicken osteoclasts were neutralized with NH4Cl, then recovery of acid formation was monitored by measuring acridine orange fluorescence over a period of 4 hrs. Estradiol-17 beta and diethylstilbestrol (DES), in nanomolar amounts, blocked acidification 20% to 60%. Effects were detectable in 30 min and maximal by one hr. Estradiol-17 alpha was ineffective. This action of estradiol is believed to be mediated at the plasma membrane since the response was rapid and was not affected by cycloheximide inhibition of protein synthesis. Stimulation of acidification by PTH and cyclic AMP was blocked by estradiol. Estradiol was inhibitory to the same extent as calcitonin; these effects were not additive. Estradiol-17 beta and DES, in micromolar but not in nanomolar amounts, blocked proton pumping in isolated plasma membrane vesicles.

Evidence that estrogen binding sites are present in bone cells and mediate medullary bone formation in Japanese quail

Studies were carried out in Japanese quail to characterize the binding of estrogen to bone cells and to determine whether induction of medullary bone by estrogen is mediated by estrogen receptors. First, attempts were made to identify specific, high affinity nuclear binding sites for estrogen with a nuclear exchange assay in oviduct and femora from laying female quail and in liver, kidney, and femora from adult male quail treated with estradiol valerate (2 mg per bird). High-affinity nuclear estrogen binding sites were detected in each of the tissues except femora. Second, estrogen binding sites in femora from male quail were localized by radioautography after treatment with 100 microCi of [3H]-17 beta-estradiol ([3H]-E). Estrogen binding sites were present at 1 and 3 h after administration of [3H]-E, and binding of the radiolabeled hormone was prevented by the simultaneous administration of an excess of radioinert estrogen. Third, estradiol valerate (.4 mg) was given as a bolus to adult male Japanese quail. Five days later, the quail had hypercalcemia due to accumulation of phosvitin in serum, had an extensive network of medullary bone at the femur midshaft, and had reduced cortical bone area. The nonsteroidal, anti-estrogen tamoxifen (2 mg/day) prevented estrogen-induced hypercalcemia, medullary bone formation, and reduction of cortical bone area. Fourth, the estrogen target cells in femora from adult male quail that were induced to differentiate to osteoblasts by estrogen treatment were located by [3H]-proline radioautography as early as 12 h following administration of the hormone. These results are interpreted as evidence that induction of medullary bone is a process mediated by estrogen receptor.

Demonstration of calmodulin-sensitive calcium translocation by isolated osteoclast plasma membrane vesicles

Plasma membrane vesicles were prepared from chicken osteoclasts, and active calcium transport was demonstrated in a spectrofluorimetric assay using the fluorescent calcium concentration indicator, fura-2. Transport activity was inhibited by quercetin (10 microM), sodium vanadate (10 microM), and the anticalmodulin agents, compound 48/80 (20 and 200 micrograms/ml) and calmidazolium (10 and 20 microM). The transport rate (Vmax, 1.3 nmol/mg protein/min) was not altered in the presence of the protonophore, nigericin (1 microM), indicating that proton transport was not driving calcium transport. Release of accumulated calcium in the vesicles occurred with the addition of bromo-A23187 (5 microM) or ionomycin (5 microM). Increasing calcium transport occurred with increasing calcium concentration. Finally, the calmodulin content of the vesicles was demonstrated to be 54-134 U/mg protein. These results demonstrate that a calmodulin-sensitive, ATP-dependent calcium transporter is present in the osteoclast plasma membrane.

Specific binding of parathyroid hormone to living osteoclasts

We show that osteoclasts bind parathyroid hormone (PTH) in a manner that displays the properties of receptor-dependent hormone binding, that is, saturability, time dependence, temperature dependence, and hormone specificity. Osteoclasts were isolated from the endosteum of 2 to 3 week chick tibiae and maintained in culture for 4-6 days. Bovine PTH-(1-84) was biotinylated with N-hydroxysuccinimidobiotin. Biotinyl-PTH (btPTH, 10(-5)-10(-11) M) was added to the cultured osteoclasts for 2-20 minutes. After rinsing away unbound btPTH, fluorescein isothiocyanate-labeled avidin (FITC-avidin) at a concentration of 66 micrograms/ml was applied. Receptor binding characteristics were assessed: (1) saturation occurred at around 10(-6) M btPTH; (2) competition of excess unlabeled PTH was found, namely, a 10-fold excess abolished fluorescence; (3) specificity was shown by adding other polypeptide hormones (insulin, glucagon, and calcitonin) in 10- to 100-fold excess--no effect on PTH binding was observed; and (4) affinity of btPTH for its binding site was indicated by half-maximal binding approximately equal to 10(-7) M for both osteoclasts and osteoblasts. Biotin (10(-5) M) or FITC-avidin (66 micrograms/ml) alone did not cause fluorescence. The time course of btPTH on the cell exterior was short: at 2 and 5 minutes dots of fluorescence were randomly dispersed over the cell surface, by 10 minutes most of the fluorescence was clustered in one region of the membrane, and by 20 minutes most of the hormone was no longer present on the surface of the cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the functional stages of osteoclasts by enzyme histochemistry and electron microscopy

To investigate the functional stages of osteoclasts, the ultrastructural histochemical distribution of the lysosomal enzymes [acid phosphatase (tartrate-sensitive) and neutral phosphatase], the plasma membrane enzymes [alkaline phosphatase, Ca(++)-ATPase, and alkaline ouabain-insensitive p-nitrophenylphosphatase (alkaline p-NPPase)], and the mitochondrial enzyme (cytochrome C oxidase) was evaluated in the chicken tibial metaphysis. Both active-appearing and detached (resting) osteoclasts were studied. Serial sectioning was used to identify detached osteoclasts which were present in the perivascular space. The ultrastructure of detached osteoclasts was similar to that of active osteoclasts, except for the lack of a ruffled border and clear zone, and an altered distribution pattern of small vesicles. Small vesicles were uniformly distributed in the cytoplasm of resting osteoclasts, whereas they were concentrated beneath the ruffled border of active osteoclasts. Alkaline p-NPPase, a marker enzyme for the basal ruffled border, was also apparent on the membrane of small vesicles. However, the vesicles did not possess Ca(++)-ATPase, a marker enzyme for the apical plasma membrane. These findings support the concept that small vesicles serve as a membrane reservoir for the ruffled border membrane. Pre-osteoclasts contained abundant mitochondria and lysosomes, prominent Golgi complexes, moderately developed endoplasmic reticulum, and lacked small vesicles. Pre-osteoclasts appear to fuse with osteoclasts which are attached to the bone surface, but not with detached osteoclasts. The small vesicles, from which the ruffled border arises, are absent from pre-osteoclasts, suggesting that they develop after fusion with pre-existing osteoclasts or after attachment to the bone surface. Alkaline p-NPPase appears to be a marker for differentiation of pre-osteoclasts to mature osteoclasts.

Ultrastructural localization of adenylate cyclase activity in chicken osteoclasts

Using lead citrate as a capture reagent and adenylate-(beta, gamma-methylene) diphosphate (AMP-PCP) as a substrate, we localized adenylate cyclase activity on the non-ruffled border plasma membrane of approximately half of the osteoclasts on trabecular bone surfaces in the tibial metaphyses of chickens fed a low (0.3%)-calcium diet. The enzyme was not detectable in osteoclasts when chickens were fed a normal calcium diet. Activity was observed on the entire plasma membrane of detached osteoclasts that were situated between osteoblasts on the bone surface and blood vessels in the marrow cavity. Detection of activity on detached osteoclasts required the presence of an activator, implying lower levels in these cells than in those with ruffled borders. Staining was greater on the lateral sides of osteoblasts and osteoclasts when they were in contact with each other. Reaction specificity was indicated by the demonstration of stimulation by forskolin, guanylate-(beta, gamma-methylene) diphosphate (GMP-PCP), dimethylsulfoxide, and NaF, inhibition by alloxan and 2',5'-dideoxyadenosine, and absence of activity when sections were incubated in substrate-free medium or when GMP-PCP replaced AMP-PCP as a substrate. The finding of adenylate cyclase in osteoclast plasma membrane provides structural evidence that the adenylate cyclase-cyclic AMP system has a role in regulation of osteoclast cell function. The low-calcium diet appears to have resulted in increased amounts of adenylate cyclase in osteoclasts.

Ultrastructural localization of tartrate-resistant acid phosphatase (purple acid phosphatase) activity in chicken cartilage and bone

Tartrate-resistant acid adenosine triphosphatase activity at pH 6.5, using a lead-salt method, was localized at light and electron microscopic levels in cartilage and bone matrices, osteoclasts, and chondroclasts. Cartilage matrix staining occurred after vascular invasion of the growth plate. In osteoclasts, activity was present in lysosomes, extracellular ruffled border channels, and the underlying cartilage and bone matrices. Staining artifacts occurred at lower pH levels (pH 5.4, 5.0). Adenosine diphosphate, p-nitrophenylphosphate, thiamine pyrophosphate, and alpha-naphthylphosphate also acted as substrates; but no activity was observed when adenosine monophosphate, adenylate-(beta, gamma-methylene) diphosphate, and beta-glycerophosphate were used. The activity was inhibited by NaF, dithionite, and a high concentration of p-chloromercuribenzoic acid, and activated by simultaneous addition of FeCl2 and ascorbic acid, as has been shown in biochemical studies. These histochemical results support the view that the adenosine triphosphate hydrolyzing activity at pH 6.5 is due to tartrate-resistant acid phosphatase (TRAP). There were some differences in ultrastructural localization between TRAP and tartrate-sensitive acid phosphatase (TSAP) activities in osteoclasts: TSAP activity was more intense in lysosomes and Golgi complexes and TRAP was stronger in the cartilage and bone matrices. It is suggested, therefore, that most of TRAP is in an inactive form in cells and is activated when secreted.

Ultrastructural localization of guanylate cyclase in bone cells

Guanylyl imidodiphosphate (GMP-PNP) hydrolyzing enzyme activity as a means of detecting plasma membrane guanylate cyclase was demonstrated in osteoblasts of chicken tibial metaphysis using a lead citrate histochemical method at the electron microscopic level. Activity was not discerned in osteoclasts or osteocytes. The reaction product development was completely abolished when the sections were incubated with substrate-free or MnCl2-free medium. Guanylate-(beta, gamma-methylene) diphosphate (GMP-PCP) was a less effective substrate than GMP-PNP, and Mn++ was a stronger stimulator than Mg++. No reaction product was observed on the plasma membrane of osteoblasts when beta-glycerophosphate or p-nitrophenylphosphate was used as substrate instead of GMP-PNP. The results implicate guanylate cyclase as a significant effector of osteoblast regulation at the site of the plasma membrane.

In vitro resorptive activity of isolated chick osteoclasts: effects of carbonic anhydrase inhibition

A potent inhibitor of carbonic anhydrase, 5-[3-hydroxybenzoyl]thiophene-2-sulfonamide (HTS), was shown to cause a 37% reduction in the area of resorption pits formed by isolated chick osteoclasts when used at a dose of 10(-7) M. HTS at doses of 10(-9) and 10(-7) M was also effective in reducing acid formation by the osteoclasts (14 and 36%, respectively). Additionally, the effect of HTS was found to be readily reversed by removing the agent, showing that it does not exert a toxic effect on the cells. This study indicates that the inhibitory effect of HTS on bone resorption is at the level of the acid-forming mechanism in osteoclasts and supports the view that carbonic anhydrase has a central role in the process.

Effects of parathyroid hormone, calcitonin, and dibutyryl-cyclic AMP on osteoclast area in cultured chick tibia

Slices of osteoclast-enriched endosteal surfaces of 3 week chick tibia were cultured for 1-3 days. Osteoclasts on the bone surface were made visible by acridine orange fluorescence. Osteoclast area was measured by image analysis. Parathyroid hormone (PTH) caused osteoclasts to increase in area about 40%, and calcitonin (CT) caused a decrease in area also of about 40%. Subsequent addition of dibutyryl cyclic AMP to PTH- or to CT-treated cells resulted in a further change of 40 and 30%, respectively. Application of the cyclic AMP analog alone had no effect. All responses were rapid, occurring in 2-4 minutes.

Biochemical characterization of an electrogenic vacuolar proton pump in purified chicken osteoclast plasma membrane vesicles

A well-characterized chicken osteoclast plasma membrane vesicle preparation manifested Mg2(+)-dependent ATP hydrolyzing activity of 0.213 mumol inorganic phosphate released per mg protein per minute (n = 7). The Mg2+ dependence showed a high-affinity component with a KMg of 1.293 microM and Vmax of 0.063 mumol Pi per mg protein per minute, and a low-affinity component with a KMg of 297.6 microM and a Vmax of 0.232 mumol Pi per mg protein per minute. The Mg2(+)-ATPase activity was inhibited by N,N'-dicyclohexylcarbodiimide (DCCD, 0.2 mM, 50.7%), N-ethylmaleimide (0.5 mM, 34.6%), nolinium bromide (1 mM, 29.9%), 4,4'-diisothiocyano-2,2'-stilbene sulfonic acid (DIDS, 1 mM, 45.1%), and p-chloromercuribenzoic acid (PCMB, 0.1 mM, 33.8%). Sodium orthovanadate (Na3 VO4) at 1 microM had no effect but caused 29.5% inhibition at 1 mM. Na+ could substitute for K+ without loss of activity, NO3- caused 19.5% inhibition when substituted for Cl-, and acetate replacement of Cl- resulted in 36.4% stimulation of Mg2(+)-ATPase. ATP, GTP, ITP, CTP, and ADP were all hydrolyzed effectively. DCCD (0.2 mM), NEM (0.5 mM), nolinium bromide (1 mM), and DIDS (50 microM) almost completely abolished proton transport as measured spectrofluorometrically by acridine orange quenching. Na3 VO4 (1 mM) had no effect, and duramycin (80 micrograms/ml) inhibited transport 52.7%. K+ replacement of Na+ caused a 79.2% increase in initial proton transport rate. NO3- and acetate substitution of Cl- resulted in a 46.1 and 55.7% decrease in transport, respectively. ATP supports transport far more effectively than the other nucleotides tested. ADP was ineffective. Experiments using the potassium ionophore, valinomycin, indicated that the proton pump functions electrogenically, with Cl- most likely cotransported by an anion transporter. The proton pump also seems to have at least one anion-sensitive site, elucidated by experiments in the presence of NO3- and Cl-.

Characterization of a Ca2(+)-ATPase in osteoclast plasma membrane

The plasma membrane fraction of chicken osteoclasts was purified utilizing 20% continuous Percoll gradients. Biochemical marker enzyme analysis (ouabain-sensitive Na+,K(+)-ATPase and 5'-nucleotidase) indicated that plasma membrane enrichment was 11.87-fold and 7.25-fold, respectively, and contamination with mitochondria, endoplasmic reticulum, and lysosomes was low as determined by succinic dehydrogenase, NADH dehydrogenase, and N-acetylglucosaminidase activities, respectively. SDS latency of Na+,K(+)-ATPase and 5'-nucleotidase activities of the isolated plasma membranes revealed that 43-50% of vesicles were sealed, with 10-16% in the inside-out orientation, depending on the membrane fraction used. Electron microscopy confirmed the vesicular nature of the plasma membrane fraction. The plasma membrane Ca2(+)-ATPase had a high-affinity (KCa = 0.22 microM; Vmax = 0.16 mumol/mg per min) and a low-affinity (KCa = 148 microM; Vmax = 0.37 mumol/mg per min) component. Calmodulin (0.12 microM) had no effect on Ca2(+)-ATPase activity. However, trifluoperazine (0.1 mM), a calmodulin antagonist, strongly inhibited especially the high-affinity component of the enzyme. Vanadate and lanthanum also caused inhibition. In the presence of CDTA, a potent Ca2+ and Mg2+ chelating agent, high-affinity Ca2(+)-ATPase activity was abolished, indicating that trace Mg2+ was essential for activity. The Ca2(+)-ATPase substrate curve using ATP showed a high-affinity (Km = 12.3 microM; Vmax = 0.022 mumol/mg per min) and a low-affinity (Km = 43.8 microM; Vmax = 0.278 mumol/mg per min) component. These results demonstrate that osteoclasts have a plasma membrane Ca2(+)-ATPase with characteristics similar to the enzyme responsible for active calcium extrusion in other cells.

Fluid shear stress as a mediator of osteoblast cyclic adenosine monophosphate production

Effects of interstitial fluid flow on osteoblasts were investigated. Intracellular cyclic adenosine monophosphate (cAMP) levels were monitored in cultured osteoblasts subjected to shear rates ranging from 10 to 3,500 sec-1. Cyclic AMP levels were significantly increased at all shear rates from 1 pmole/mg protein to 10-16 pmole/mg protein. Osteoblasts subjected to a shear rate of 430 sec-1 for 0.5-15 minutes exhibited elevated levels (12-fold) of intracellular cAMP, which were sustained throughout the perfusion period. Osteoblasts were three times more sensitive to flow stimulation than human umbilical vein endothelial cells and baby hamster kidney fibroblasts, which also displayed higher cAMP levels (4-fold) after exposure to flow. To distinguish streaming potential effects from shear stress effects, viscosity was increased 5-fold by addition of neutral dextran to the perfusing medium. Shear stress is a function of viscosity, and streaming potentials are not for a given shear rate. The mechanism of this cellular response to flow was shown to be shear stress dependent. Inhibition of cyclooxygenase by 20 microM ibuprofen completely inhibited the flow-dependent cAMP response, indicating the cAMP response is mediated by prostaglandins. Our results suggest that fluid flow induced by mechanical stress may be an important mediator of bone remodeling.

Ultrastructural localization of adenylate cyclase in chicken bone cells

We investigated adenylate cyclase distribution in 6-day-and 3-week-old calvariae and in 6-day-old long bone metaphyses from chickens. Reaction product distribution was on the plasma membrane of osteoblasts, pre-osteoblasts, and forming osteocytes which contacted one another. Osteoclasts and mature osteocytes lacked reaction product. Six-day calvariae reacted less intensely than the other two tissues. In controls, reaction product was markedly diminished or eliminated by removal of forskolin or substrates, or by addition of the inhibitor 2',5'-dideoxyadenosine. The results indicate the importance of cyclic AMP in osteoblast regulation. Osteocytes and osteoclasts may involve alternate mechanisms as major regulatory systems.