Empirical evaluation of bone extraction protocols

The application of high-resolution analytical techniques to characterize ancient bone proteins requires clean, efficient extraction to obtain high quality data. Here, we evaluated many different protocols from the literature on ostrich cortical bone and moa cortical bone to evaluate their yield and relative purity using the identification of antibody-antigen complexes on enzyme-linked immunosorbent assay and gel electrophoresis. Moa bone provided an ancient comparison for the effectiveness of bone extraction protocols tested on ostrich bone. For the immunological part of this study, we focused on collagen I, osteocalcin, and hemoglobin because collagen and osteocalcin are the most abundant proteins in the mineralized extracellular matrix and hemoglobin is common in the vasculature. Most of these procedures demineralize the bone first, and then the remaining organics are chemically extracted. We found that the use of hydrochloric acid, rather than ethylenediaminetetraacetic acid, for demineralization resulted in the cleanest extractions because the acid was easily removed. In contrast, the use of ethylenediaminetetraacetic acid resulted in smearing upon electrophoretic separation, possibly indicating these samples were not as pure. The denaturing agents sodium dodecyl sulfate, urea, and guanidine HCl have been used extensively for the solubilization of proteins in non-biomineralized tissue, but only the latter has been used on bone. We show that all three denaturing agents are effective for extracting bone proteins. One additional method tested uses ammonium bicarbonate as a solubilizing buffer that is more appropriate for post-extraction analyses (e.g., proteomics) by removing the need for desalting. We found that both guanidine HCl and ammonium bicarbonate were effective for extracting many bone proteins, resulting in similar electrophoretic patterns. With the increasing use of proteomics, a new generation of scientists are now interested in the study of proteins from not only extant bone but also from ancient bone.

Recombinant expression of mouse osteocalcin protein in Escherichia coli

Osteocalcin is the most abundant non-collagenous protein of bone. Recombinant mouse osteocalcin protein (mOC) that includes the highly conserved central domain for binding to hydroxyapatite (HA), a mineral component of bone, was expressed in Escherichia coli. Purified mOC protein exhibited a significant increase in HA adhesion and differentiation in osteoblast cells as well as binding to HA with high affinity.

Extraction, purification, and development of an enzyme-linked immunosorbent assay for osteocalcin

The present study describes the isolation and purification of osteocalcin (OC) from bovine bones and the development of an enzyme-linked immunosorbent assay (ELISA) for OC as a marker of bone formation, for assessing bone health. Bone proteins were extracted from about 90 g of bovine bone powder using 20% formic acid. The protein extract was fractionated by gel permeation chromatography on Sephadex G-50 column followed by fast protein liquid chromatography (FPLC) on a MONO-Q column. The immunoreactive active fraction was then purified by chromatofocusing, using FPLC on a MONO P column and a single homogeneous band of molecular size of about 5.8kDa, as judged by Tricine SDS-PAGE following silver staining of the gel, was obtained. It reacted specifically with its antibodies in an ELISA. About 678 microg of purified OC was yielded from about 90 g of bovine bones. The purified OC was subsequently used for the raising antisera, which was used in the development of an indirect ELISA. The developed ELISA has a sensitivity of 2.5-4.0 ng/mL and was used in estimating levels of OC in women of various age groups.

Osteocalcin protein sequences of Neanderthals and modern primates

We report here protein sequences of fossil hominids, from two Neanderthals dating to approximately 75,000 years old from Shanidar Cave in Iraq. These sequences, the oldest reported fossil primate protein sequences, are of bone osteocalcin, which was extracted and sequenced by using MALDI-TOF/TOF mass spectrometry. Through a combination of direct sequencing and peptide mass mapping, we determined that Neanderthals have an osteocalcin amino acid sequence that is identical to that of modern humans. We also report complete osteocalcin sequences for chimpanzee (Pan troglodytes) and gorilla (Gorilla gorilla gorilla) and a partial sequence for orangutan (Pongo pygmaeus), all of which are previously unreported. We found that the osteocalcin sequences of Neanderthals, modern human, chimpanzee, and orangutan are unusual among mammals in that the ninth amino acid is proline (Pro-9), whereas most species have hydroxyproline (Hyp-9). Posttranslational hydroxylation of Pro-9 in osteocalcin by prolyl-4-hydroxylase requires adequate concentrations of vitamin C (l-ascorbic acid), molecular O(2), Fe(2+), and 2-oxoglutarate, and also depends on enzyme recognition of the target proline substrate consensus sequence Leu-Gly-Ala-Pro-9-Ala-Pro-Tyr occurring in most mammals. In five species with Pro-9-Val-10, hydroxylation is blocked, whereas in gorilla there is a mixture of Pro-9 and Hyp-9. We suggest that the absence of hydroxylation of Pro-9 in Pan, Pongo, and Homo may reflect response to a selective pressure related to a decline in vitamin C in the diet during omnivorous dietary adaptation, either independently or through the common ancestor of these species.

Osteocalcin fragment in bone matrix enhances osteoclast maturation at a late stage of osteoclast differentiation

Although 14-day-old mouse embryonic calvarial cells cultured in plastic culture dishes in the presence of 1alpha,25-dihydroxyvitamin D3[1alpha,25-(OH)2D3] for 7 days could barely resorb bone slices, the same calvarial cells cultured with an ethylenediaminetetraacetic acid (EDTA) extract from bovine bone powder under the same conditions stimulated pit formation on bone slices in a dose-dependent manner. Therefore, the present study was conducted to purify and characterize this osteoclast maturation-inducing factor(s) from the bone matrix. The protein having osteoclast maturation-inducing activity in the EDTA extract was purified by gel filtration over Superdex 75 preparation grade and chromatography on hydroxyapatite, Mono Q, and C8 reversed-phase HPLC by monitoring the ability of the eluted fractions to elicit pit formation on bone slices. The molecular weight of the purified protein estimated by high-resolution polyacrylamide gel electrophoresis was 5.7 kDa and 6.8 kDa in the respective absence and presence of 2-mercaptoethanol. The sequence of the 30-amino-acid purified protein corresponded to the 7th to 36th residues of bovine osteocalcin. The osteocalcin fragment, missing the initial 6 residues at the N-terminal region, exhibited higher osteoclast maturation-inducing ability than bovine intact osteocalcin on a per weight basis. The osteocalcin fragment had no effect on the expression of receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) and osteoprotegerin (OPG) genes in calvarial cells, nor did it enhance the bone-resorbing activity of mature osteoclasts. When the osteocalcin fragment was added to late-stage (days 4-7) or to early-stage (days 0-3) cultures of calvarial cells pretreated with 1alpha,25-(OH)2D3, its stimulatory effect was observed in the late-stage cultures rather than in the early-stage ones. In addition, the osteocalcin fragment directly enhanced the formation of osteoclasts with bone-resorbing ability from Mac-1+ c-Fms+ cells in the presence of macrophage colony-stimulating factor (MCSF) and RANKL. These results suggest that the osteocalcin fragment in bone matrix is involved in osteoclast maturation, especially at a late stage of osteoclast differentiation.

Immunoreactive osteocalcin forms in conditioned media from human osteoblast culture and in sera from healthy adult control subjects and patients with bone pathologies

BACKGROUND

The aim of this work was to study the immunoreactive forms of bone Gla protein (BGP) present in conditioned media of human osteoblast cultures (BGP released from osteoblast) and in the sera of healthy adult control subjects and patients with bone pathologies (chronic renal failure on haemodialysis, Paget's disease of bone and post-menopausal osteoporosis).

METHODS

The technical procedure used was a combination of high-performance liquid chromatography (HPLC) and different BGP assays with several specificities to analyse BGP levels in the different HPLC fractions. Aliquots of conditioned media or sera were purified through a Sephadex G-50m column and by HPLC (C4 reverse-phase column) in a 25-40% acetonitrile gradient. Two-minute fractions were collected and divided into three aliquots in order to determine osteocalcin content using three different assays: (a) ELSA-OST-NAT IRMA, which only detects intact osteocalcin; (b) ELSA-OSTEO IRMA, which detects intact osteocalcin and N-terminal fragments; and (c) OSCA Test RIA, which detects intact osteocalcin, C-terminal and other fragments.

RESULTS

We found different immunoreactive forms of osteocalcin in the culture medium of human osteoblasts and in sera from control subjects and patients for the bone pathologies studied. We did not find great qualitative differences between the immunoreactive osteocalcin profile found in the culture medium from human osteoblasts and the sera from healthy control subjects. However, the different bone pathologies show different characteristic patterns of immunoreactive forms of osteocalcin.

CONCLUSIONS

An interesting finding has been the detection, both in sera and in osteoblast culture media, of several immunoreactive forms of intact osteocalcin that eluted from HPLC at different acetonitrile percentages, and therefore correspond to different molecular forms.

[Separation and purefied of calf osteocalcin, set up radioimmunoassay for osteocalcin]

OBJECTIVE AND METHODS

We extracted and purified osteocalcin (BGP) from calf femur by gel filtration, FPLC and HPLC subsequently. The rabbits were immunized by this antigen and produced antibody. With this antibody (final 1: 35,000 dilution) we established RIA for BGP. This assay is stable and sensitive, the kd was 2.29 x 10(11). The intra assay variation was 2.1% and the inter assay variaton was 5.6%. It has no crossreaction with insulin, parathyroid hormone or calcitonin.

RESULTS

The serum BGP values were detected in normal subjects and six kinds of bone metabolic diseases. In 81 normal subjects, the average serum BGP level was (5.3 +/- 1.5) ng/ml and in postmenopausal osteoporosis the serum BGP level [(6.2 +/- 1.9) ng/ml, n = 42] was higher than the normal (P < 0.05). The serum BGP levels were increased much in primary hyperparathyroidism [(12.8 +/- 7.0) ng/ml, n = 21, P < 0.001]. Increased values of BGP were also found in patients with chronic renal failure [(8.5 +/- 2.4) ng/ml, n = 18, P < 0.01] and Paget's disease [(6.7 +/- 2.2) ng/ml, n = 12, P < 0.05]. Decreased serum BGP values were found in Cushing syndrome [(2.9 +/- 0.9) ng/ml, n = 16, P < 0.001] and hypoparathyroidism [(3.4 +/- 1.1) ng/ml, n = 42, P < 0.001]. This result showed that in diseases with high bone turnover, the serum BGP levels were increased and in diseases with low bone turnover, the serum BGP levels were decreased.

CONCLUSION

The data suggest that serum BGP is a sensitive marker for bone turnover. It is very improtant in studying bone metabolic diseases.

The mature osteoblast phenotype is characterized by extensive plasticity

While both morphological and biochemical-molecular attributes demarcate differentiation stages in specific cell and tissue types, what constitutes necessary and sufficient expression to define particular cell types is not always known. For example, mature osteoblasts (OBs) are defined morphologically as the cuboidal, biosynthetically active, basophilic cells residing on bone surfaces and responsible for the deposition of osteoid matrix. However, several recent observations suggest that not all mature OBs are identical. To explore further the validity of the hypothesis that heterogeneity of phenotype exists among mature OBs, we grew fetal rat calvaria cells in vitro at low density under conditions in which bone nodules form and mineralize in isolation of other contaminating cell and colony types. Cells resident in mature OB colonies, i.e., those comprising mainly cuboidal cells associated with an osteoid matrix that had begun to mineralize, were analyzed in situ for protein expression by immunocytochemistry with antibodies against collagen type I, alkaline phosphatase, osteopontin, bone sialoprotein, and osteocalcin. Consistent with the expected phenotype of mature OBs, many OBs expressed high levels of all of these markers, but strikingly even adjacent morphologically indistinguishable cuboidal OBs had differences in protein expression, especially in relation to osteopontin, bone sialoprotein, and osteocalcin expression. Double-labeling with Hoechst 33258 and osteocalcin indicated that the variation in antibody labeling intensity/protein expression appeared independent of a variation in cell cycle. To further ascertain the extent of this heterogeneity, 20 single cells were micromanipulated from colonies and subjected to poly(A)-PCR to analyze the simultaneous coexpression profiles of the same five markers analyzed by immunocytochemistry and two other markers, the OB-osteocyte transition marker E11 and the parathyroid hormone/parathyroid hormone-related protein receptor. Notably, the repertoire of genes expressed and their levels of expression varied markedly in individual OBs. The observed heterogeneity suggests that the mature OB phenotype is not a single unique phenotype but rather encompasses a flexible pattern of expression from the repertoire of OB-associated markers.

Purification and characterization of recombinant osteocalcin fusion protein expressed in Escherichia coli

Human osteocalcin (hOC) is a 49-amino-acid peptide produced mainly by bone osteoblasts. The amount of hOC in the circulation reflects the status of bone metabolism and it is used to monitor various bone-related diseases. The aim of this study was to produce recombinant human osteocalcin (rhOC) in Escherichia coli and use it for designing new osteocalcin fluorescence immunoassays. Recombinant DNA technology was used to fuse synthetic hOC coding sequences to an affinity handle system based on glutathione S-transferase (GST) gene. GST-rhOC fusion protein was produced in a bacterial intracellular expression system mainly in a soluble form. The affinity-purified fusion protein was cleaved with activated protease factor X releasing the rhOC portion. The structure of rhOC was confirmed by mass spectrometry and amino acid sequencing. The fusion protein and its proteolytic cleavage product proved to be immunoreactive as shown by Western blotting analysis and by a new osteocalcin immunoassay based on time-resolved fluorescence. When osteocalcin was tested for its ability to bind to hydroxyapatite, there were no differences between the recombinant forms and native human osteocalcin purified from bone, suggesting that the Gla residues might be important only in oriented high-affinity binding.

Amino-acid sequence of bone Gla protein from the African clawed toad Xenopus laevis and the fish Sparus aurata

As an initial step in the analysis of bone Gla protein (BGP; osteocalcin) function in lower vertebrates, we have developed a simple and rapid method for the isolation of BGP from bone and have applied this to the isolation of BGP from the African clawed toad Xenopus laevis and the fish Sparus aurata. We have also determined the complete amino-acid sequence of Sparus and Xenopus BGP, including the identification of the sites of y-carboxylation. Since the addition of Xenopus and Sparus BGP sequences significantly extends the range of species whose BGP structures are known, we have compared the 18 presently known BGP sequences. Twelve amino acids are invariant in these 18 BGP sequences and are therefore presumably critical to BGP conformation or function. Eight of these 12 invariant amino acids are also invariant in all presently known matrix Gla protein sequences (shark, mouse, rat, cow, human), an observation which strongly supports the evolutionary relationship between these two vitamin K-dependent bone proteins and suggests that the proteins may adapt similar tertiary structures.

Multi-site-specificity of the vitamin K-dependent carboxylase: in vitro carboxylation of des-gamma-carboxylated bone Gla protein and Des-gamma-carboxylated pro bone Gla protein

The vitamin K-dependent carboxylase processes multiple glutamic acid residues to gamma-carboxyglutamic acid (Gla) residues in a limited number of proteins. The targeted proteins are synthesized with an amino-terminal propeptide which has been shown to play an important role in gamma-carboxylation. The specificity of the enzyme for each potential Gla site, the direction of carboxylation, and the influence of a bound propeptide on these events are not understood. Des-gamma-carboxy forms of bone Gla protein (BGP), which contain potential Gla residues at positions 17, 21, and 24, were employed as model substrates to determine the multi-site-specificity of the enzyme. Recombinant bovine des-gamma-carboxylated proBGP (rdproBGP) and heat-decarboxylated BGP (dBGP), lacking a propeptide, were used as substrates for a bovine liver carboxylase, and the in vitro reaction products were analyzed for the formation of 14CO2 Gla. The di-Gla species was found to be the predominant product of in vitro carboxylation of both rdproBGP and dBGP at less than saturating concentrations of each substrate. Carboxylation of both substrates occurred preferentially at the more C-terminal potential Gla sites, residues 21 and 24. A similar pattern of carboxylation was observed with a rat bone cell carboxylase, suggesting no species or tissue variation in the enzyme specificity. Some tricarboxylated product accumulated during carboxylation of rdproBGP but not dBGP, suggesting that the covalently bound propeptide directs more complete carboxylation of the Gla domain. In addition, monocarboxylated rdproBGP was found to accumulate in the absence but not in the presence of a free noncovalently attached propeptide, indicating that free propeptide affects more efficient carboxylation of rdproBGP.

Direct demonstration that the vitamin K-dependent bone Gla protein is incompletely gamma-carboxylated in humans

Incomplete vitamin K-dependent gamma-carboxylation has been found in bone Gla protein (BGP) isolated from each of 20 different human bone samples. Using N-terminal protein sequencing of the methyl-esterified protein (Anal Biochem 1991;199:93-97), a method that directly measures the percentage of gamma-carboxylation at each target glutamate residue, the extent of incomplete BGP gamma-carboxylation was found to depend strongly on sequence position, with (chi +/- SD) 67 +/- 14% gamma-carboxylation at residue 17.88 +/- 9% gamma-carboxylation at residue 21, and 93 +/- 4% gamma-carboxylation at residue 24. There is a strong correlation between the incomplete gamma-carboxylation at glutamate residues 17 and 21 for BGP purified from the 20 bone samples (p < 0.001), which suggests that individual differences in the efficiency of BGP gamma-carboxylation during synthesis probably cause the observed differences in percentage BGP gamma-carboxylation between bone samples. These results have been interpreted using a kinetic treatment of gamma-carboxylation. This treatment predicts the existence of differences in the extent of gamma-carboxylation between glutamate residues in BGP, as well as the correlation between percentage carboxylation at Glu17 and Glu21. Although the molecular basis of incomplete BGP gamma-carboxylation is at present unknown, if incomplete BGP gamma-carboxylation were caused only by differences in the availability of vitamin K in bone cells, this kinetic treatment predicts that the range in BGP gamma-carboxylation observed in the 20 human bone samples studied here could be explained by a relatively modest fivefold range in the vitamin K levels of these individuals.

The displacement of calcium from osteocalcin at submicromolar concentrations of free lead

Lead, an environmental toxin, is known to impair some of the functional properties of osteocalcin, a small protein (MW, 5700) active in bone mineralization and resorption. To investigate a possible mechanism of lead toxicity at the molecular level, we have studied the interaction of lead with osteocalcin using 43Ca and 1H NMR. The measured 43Ca NMR linewidth as well as longitudinal relaxation rate (1/T1) of 43CaCl2 progressively increased with increasing amounts of added osteocalcin. A titration measuring 43Ca linewidth as a function of [Ca2+]/[Osteocalcin] ratio could be fitted to a single metal binding site with a dissociation constant of 7 microM. The 43Ca 1/T1 of Ca-osteocalcin decreased in the presence of Pb2+ due to competitive displacement of Ca2+ by Pb2+. The magnitude of decrease in the effect of osteocalcin on 43Ca 1/T1 in the presence of Pb2+ was consistent with the existence of only one tight divalent cation binding site. An analysis of the NMR T1 data in osteocalcin solutions containing both Pb2+ and Ca2+ yielded a Pb-osteocalcin dissociation constant of about 2 nM. The 1H NMR spectra showed Pb-induced changes in the same aliphatic and aromatic resonances of osteocalcin that are also affected by Ca(2+)-binding, supporting interaction of Pb2+ at the Ca2+ site. However, the existence of significant differences between the Pb-osteocalcin and Ca-osteocalcin NMR spectra indicates some differences in the structures of the two complexes. Since Pb2+ inhibits the binding of osteocalcin to hydroxyapatite, the high affinity of Pb2+ for osteocalcin would indicate significant inactivation of osteocalcin even at submicromolar free lead levels. Pb(2+)-induced inactivation of osteocalcin could affect bone mineral dynamics and may be related to the observed inverse correlation between blood Pb(2+)-levels and stature and chest circumference observed in growing children.

TGF-beta 1 and 25-hydroxycholesterol stimulate osteoblast-like vascular cells to calcify

Previous studies in our laboratory demonstrated messenger RNA for bone morphogenetic protein-2a in human calcified plaque, suggesting that arterial calcification is a regulated process, similar to osteogenesis. To further test this hypothesis, we have isolated and cloned a subpopulation of cells from bovine aortic media that show osteoblastic potential. These novel cells are primarily distinguished from smooth muscle cells by expression of a surface marker preliminarily identified as a modified form of the ganglioside sialyl-lactosylceramide (GM3). Osteoblastic potential was indicated by high levels of alkaline phosphatase and collagen I, expression of osteopontin and osteonectin (SPARC), and production of bone-specific osteocalcin and hydroxyapatite. Cultures of these cells were stimulated to form increased numbers of calcium-mineral-producing nodules by the oxysterol 25-hydroxycholesterol as well as by transforming growth factor-beta 1, both known to be present in atherosclerotic lesions. The stimulation of calcifying vascular cells in the artery wall by these two factors suggests a possible mechanism for the colocalization of calcification with atherosclerosis in vivo.

The putative RGD-dependent cell adhesion activity of matrix Gla protein is due to higher molecular weight contaminants

This study was carried out to further characterize the RGD-dependent cell adhesion activity which was previously observed in the vitamin K-dependent matrix Gla protein (MGP) (Loeser, R. F., and Wallin, R. (1992) J. Biol. Chem. 267, 9459-9462). We have found that this cell adhesion activity can be completely removed from the 10-kDa MGP by gel filtration over Sephacryl S-200-HR. The higher molecular weight contaminants removed by the gel filtration step display potent cell adhesion activity. The additional evidence previously adduced in support of the putative cell adhesion activity of MGP is that heat decarboxylation of the vitamin K-dependent gamma-carboxyglutamate residues in MGP abolished the adhesion activity. The heat decarboxylation conditions used, however, appear to cause other chemical changes in proteins in addition to decarboxylation, as evidenced by the fact that the cell adhesion activity of fibronectin, which is not a vitamin K-dependent protein, is also destroyed by this procedure. The present evidence that the putative cell adhesion activity of MGP is caused by contaminating higher molecular weight cell adhesion proteins accounts for two apparent anomalies in the previously reported cell adhesion activity of MGP, the failure of antibodies raised against a synthetic peptide corresponding to the C-terminal 19 residues of bovine MGP to inhibit the cell adhesion activity of the intact, 79-residue bovine protein, and the potent inhibition of the cell adhesion activity of MGP by a synthetic peptide containing an RGD sequence, even though MGP does not contain this sequence.

Isolation and sequence of the vitamin K-dependent matrix Gla protein from the calcified cartilage of the soupfin shark

High levels of the vitamin K-dependent matrix Gla protein (MGP) have been found in the calcified costal cartilage of the cow and the calcified vertebral cartilage of the soupfin shark (Galeorhinus galeus). In both species, MGP accounts for 35-40% of the total protein in the acid demineralization extract of calcified cartilage, and the mineral content of calcified cartilage is comparable to that of bovine cortical bone. Shark and bovine MGP are both nearly insoluble in neutral buffers, a conserved property that indicates that self-aggregation could be important to the as yet unknown function of MGP. The complete amino acid sequence of shark MGP was determined to compare the structure of the elasmobranch protein to the several currently known mammalian MGP sequences. Shark MGP contains 4 residues of the vitamin K-dependent amino acid gamma-carboxyglutamic acid in its 102 residue sequence and has a calculated molecular weight = 12,770 daltons. The first 76 residues of shark MGP are homologous in sequence to mammalian MGPs, with 37% sequence identity, but the C-terminal 23 residues of the shark protein have no counterpart in the mammalian MGPs. This C-terminal segment of shark MGP contains 8 basic residues and no acidic residues. Among the features conserved in shark MGP, in all mammalian MGPs, and in all other currently known vitamin K-dependent mammalian proteins are a 15-residue region of sequence homology that has been shown to function as the gamma-carboxylase recognition sequence and an invariant sequence of unknown function, Gla-Xaa-Xaa-Xaa-Gla-Xaa-Cys.

Metal ion binding to dog osteocalcin studied by 1H NMR spectroscopy

One-dimensional 1H NMR was employed to study the effects of Ca2+ and Lu3+ binding on the apo and calcium-saturated forms of dog bone Gla protein (BGP, osteocalcin). Titration of apo dog BGP with Ca2+ in 20 mM NaCl showed spectral perturbations consistent with the binding of 5 mol equiv of calcium in the NMR slow-exchange limit. The first 2 Ca2+ equiv induced significant conformational changes in the apoprotein, binding cooperatively with a Kd1 approximately 5.0 x 10(-4) M and a Hill coefficient H = 2.3 in 20 mM NaCl. The last 3 equiv bound with a slightly weaker affinity and did not induce significant structural changes. Neither the affinity nor the stoichiometry of calcium binding was significantly altered at 150 mM NaCl. The addition of only 1 Lu3+ equiv to apo dog osteocalcin was sufficient to induce the same spectral perturbations as 2 Ca2+ ions. The addition of 2 Lu3+ equiv to calcium-saturated osteocalcin had little effect on its 1H NMR spectrum, and BGP aggregated at [Lu3+]o/[BGP]o ratios greater than 2 in either the presence or absence of calcium. The spectrum of calcium-saturated osteocalcin was invariant at < or = 55 degrees C (< or = 50 degrees C in 150 mM NaCl), after which the proton resonances shifted to frequencies more characteristic of apo BGP. Saturation with calcium somewhat stabilized the apo dog osteocalcin protein against conformational changes induced at pH extremes; apo BGP was stable at 6.0 < or = pH < or = 10, and calcium-saturated BGP was stable at 5.8 < or = pH < or = 10. Both our NMR and gel filtration data indicate that calcium-saturated osteocalcin exists as a dimer at both high and low protein concentrations. A conformational change in dog osteocalcin was thus induced by the cooperative association of Ca2+ to two high-affinity sites on the protein and stabilized by the association of 3 additional Ca2+ equiv. The results of our temperature and calcium binding studies were consistent with an estimated Kd1 approximately 5.0 x 10(-4) M for the two high-affinity sites. Lutetium induced the same structural changes in osteocalcin as calcium, but the two high-affinity Ca2+ binding sites did not have equal affinities for Lu3+. The BGP:Ca2+ complex was unstable at the low pH conditions induced by osteoclasts during bone resorption, yet the osteocalcin protein retained a BGP:Ca(2+)-like conformation at low pH. However, unlike the calcium-saturated form of the protein, osteocalcin was monomeric at low pH.

Clonal analysis for developmental potential of chick periosteum-derived cells: agar gel culture system

The developmental potential of periosteum-derived cells was clonally assessed with an agar gel culture system. Morphologically, two types of colonies were predominantly observed. By immunocytochemical observation with antibodies against aggrecan or bone Gla protein, one type of colony was judged to be chondrogenic, and the other osteogenic. By chronological observation, each type of colony did not convert to the other. Supplementation with transforming growth factor (TGF)-beta 1 shortened the time course of chondrogenesis and also increased colony forming efficiency of chondrogenic colonies. On the other hand, colony forming efficiency of osteogenic colonies decreased with TGF-beta 1 treatment, whereas the time course of osteogenesis remained unaffected. These observations suggest that there are both committed osteoprogenitor and chondroprogenitor cells present in the periosteal cell population, and TGF-beta 1 stimulates proliferation and differentiation of chondrogenic cell population by its targeted action.

Isolation and complete amino acid sequence of osteocalcin from canine bone

Osteocalcin was purified in high yield and to homogeneity from the diaphysis of dog femora by the following steps: (1) acid demineralization of bone powder, (2) solid-phase extraction of acid-soluble proteins on Sep-Pak C18 cartridges, (3) gel filtration on Sephadex G-50, and (4) fast protein liquid chromatography on an Accell-QMA anion-exchange column. Starting from 30 g washed bone powder, approximately 7-10 mg pure protein was obtained in 2 days. The key step is the initial solid-phase extraction of osteocalcin from a large volume of a demineralized bone solution. The primary structure was established by automated sequence analyses of two tryptic peptides, of two endoproteinase Glu-C carboxy-terminal peptides, and of the first 30 amino acid residues of the intact protein. Dog osteocalcin contains 49 amino acids, has a molecular mass of 5654 daltons, contains no Thr, Met, Hyp, or Trp, has a disulfide bond between Cys 23 and 29, and is fully gamma-carboxylated at residues 17, 21, and 24. Dog osteocalcin does not contain a pair of basic amino acids found at positions 43-44 in most other osteocalcins from mammals and birds. A computer search for homology indicated 88, 90, 84, 88, 66, and 57% sequence identity of dog osteocalcin with human, bovine, cat, monkey, chicken, and swordfish osteocalcin, respectively, and weaker homologies with the gamma-carboxylated domains of blood-clotting proteins and the Pro-rich N-terminal extensions of myosin light-chain A1 and beta-crystalline B1. The possible relevance of these homologies to the structure and potential functions of osteocalcin is discussed.

Immunohistochemical study of bone GLA protein in primary bone tumors

METHODS

The immunoreactivity of bone GLA protein (BGP) in primary bone tumors, including osteosarcoma, chondrosarcoma, malignant fibrous histiocytoma of bone (MFH), and giant cell tumor of bone (GCT), was investigated with anti-BGP rabbit serum and peroxidase-antiperoxidase complex.

RESULTS

As to intracellular localization, BGP antigenicity was detected in 33 of 35 cases of osteosarcoma and 12 of 25 cases of chondrosarcoma. However, there were no positive findings in all 15 cases of MFH or 20 cases of GCT. In chondrosarcoma, the frequency of positively stained cases increased according to pathologic grading (i.e., 3 of 14 cases of Grade 1, 7 of 9 cases of Grade 2, and 2 of 2 cases of Grade 3). Although the multinucleated cells in MFH or GCT were not immunostained, BGP antigenicity was observed in the multinucleated cells of osteosarcoma (12 of 15 cases). In the matrix of osteosarcoma, BGP immunoreactivity of the tumorous osteoid was observed in 28 of 32 cases. However, in the matrices of chondrosarcoma, MFH, and GCT, BGP immunoreactivity was not observed.

CONCLUSION

These results suggest that the immunohistochemical study of BGP is useful for the differential diagnosis of bone tumors.

Characterization of a new human osteosarcoma cell line OHS-4

We present a new human osteosarcoma cell line designated OHS-4. These cells showed a high alkaline phosphatase activity that is not regulated by 1,25 dihydroxyvitamin D3. They exhibited a sensitive adenylate cyclase response to parathyroid hormone but not to prostaglandin E2 or human calcitonin. By Northern blot analysis we could detect type I collagen mRNA but none for type III collagen. The cells were able to produce human osteocalcin at a maximum level of 35 ng per million cells when exposed to 2.4 nM 1,25-dihydroxyvitamin D3 for 96 h. We purified this protein from conditioned media using successive chromatography and assessed its identity by partial amino acid sequencing. When injected into nude mice, the cells retained their osteogenic activity and developed calcified tumors. After Von Kossa staining, we observed nonmineralized osteoid deposits and mineralized deposits with a structure similar to that of trabecular bone by light microscopy. On the basis of its osteoblastic characteristics, this new osteosarcoma cell line may represent the human counterpart of the ROS 17/2 cell line. This cell line represents a valuable model for the isolation and characterization of human bone specific proteins.

A colorimetric assay specific for gamma-carboxyglutamic acid-containing proteins: its utility in protein purification procedures

A colorimetric method for the detection of gamma-carboxyglutamic acid (Gla)-containing proteins after reaction with 4-diazobenzenesulfonic acid is presented. Proteins can be visualized after electroblotting from polyacrylamide gels onto membrane supports, after dot-blotting onto membranes, or in solution as a red colored product with an absorbance maximum at 530 nm. The method is specific since other proteins without gamma-carboxyglutamic acid do not form a red color. The presence of other proteins does not inhibit or affect color production by gamma-carboxyglutamic acid-containing proteins. Application of the method for staining a Western blot of a crude extract of bone resulted in staining of only the gamma-carboxyglutamic acid-containing proteins. The usefulness of the method was verified when a second gamma-carboxyglutamic acid-containing protein, prothrombin, also resulted in red color production. A linear color response is seen up to 17 microM for the gamma-carboxyglutamic acid-containing protein bone Gla protein and up to 27 microM for the amino acid. The detection limit is down to 1 microgram of bone Gla protein or 0.17 nmol of the protein on electroblots or dot blots. The simplicity of the method allows rapid screening for gamma-carboxyglutamic acid-containing proteins or allows monitoring of purifications of these proteins in chromatographic or electrophoretic separations.