Identification of the rat bone 60K acidic glycoprotein as alpha 2HS-glycoprotein

Tissue chaperoning—the expanded functions of fetuin-A beyond inhibition of systemic calcification

Traditionally, fetuin-A embodies the prototype anti-calcification protein in the blood, preventing cardiovascular calcification. Low serum fetuin-A is generally associated with mineralization dysbalance and enhanced mortality in end stage renal disease. Recent evidence indicates that fetuin-A is a crucial factor moderating tissue inflammation and fibrosis, as well as a systemic indicator of acute inflammatory disease. Here, the expanded function of fetuin-A is discussed in the context of mineralization and inflammation biology. Unbalanced depletion of fetuin-A in this context may be the critical event, triggering a vicious cycle of progressive calcification, inflammation, and tissue injury. Hence, we designate fetuin-A as tissue chaperone and propose the potential use of exogenous fetuin-A as prophylactic agent or emergency treatment in conditions that are associated with acute depletion of endogenous protein.

Live Imaging of Calciprotein Particle Clearance and Receptor Mediated Uptake: Role of Calciprotein Monomers

Background

The liver-derived plasma protein fetuin A is a systemic inhibitor of ectopic calcification. Fetuin-A stabilizes calcium phosphate mineral initially as ion clusters to form calciprotein monomers (CPM), and then as larger multimeric consolidations containing amorphous calcium phosphate (primary CPP, CPP 1) or more crystalline phases (secondary CPP, CPP 2). CPM and CPP mediate excess mineral stabilization, transport and clearance from circulation.

Methods

We injected i.v. synthetic fluorescent CPM and studied their clearance by live two-photon microscopy. We analyzed organ sections by fluorescence microscopy to assess CPM distribution. We studied cellular clearance and cytotoxicity by flow cytometry and live/dead staining, respectively, in cultured macrophages, liver sinusoidal endothelial cells (LSEC), and human proximal tubule epithelial HK-2 cells. Inflammasome activation was scored in macrophages. Fetuin A monomer and CPM charge were analyzed by ion exchange chromatography.

Results

Live mice cleared CPP in the liver as published previously. In contrast, CPM were filtered by kidney glomeruli into the Bowman space and the proximal tubules, suggesting tubular excretion of CPM-bound calcium phosphate and reabsorption of fetuin A. Fetuin-A monomer clearance was negligible in liver and low in kidney. Anion exchange chromatography revealed that fetuin A monomer was negatively charged, whereas CPM appeared neutral, suggesting electrochemical selectivity of CPM versus fetuin A. CPM were non-toxic in any of the investigated cell types, whereas CPP 1 were cytotoxic. Unlike CPP, CPM also did not activate the inflammasome.

Conclusions

Fetuin-A prevents calcium phosphate precipitation by forming CPM, which transform into CPP. Unlike CPP, CPM do not trigger inflammation. CPM are readily cleared in the kidneys, suggesting CPM as a physiological transporter of excess calcium and phosphate. Upon prolonged circulation, e.g., in chronic kidney disease, CPM will coalesce and form CPP, which cannot be cleared by the kidney, but will be endocytosed by liver sinusoidal endothelial cells and macrophages. Large amounts of CPP trigger inflammation. Chronic CPM and CPP clearance deficiency thus cause calcification by CPP deposition in blood vessels and soft tissues, as well as inflammation.

Mineralization by inhibitor exclusion: the calcification of collagen with fetuin

One of our goals is to understand the mechanisms that deposit mineral within collagen fibrils, and as a first step we recently determined the size exclusion characteristics of the fibril. This study revealed that apatite crystals up to 12 unit cells in size can access the water within the fibril, whereas molecules larger than a 40-kDa protein are excluded. Based on these observations, we proposed a novel mechanism for fibril mineralization: that macromolecular inhibitors of apatite growth favor fibril mineralization by selectively inhibiting crystal growth in the solution outside of the fibril. To test this mechanism, we developed a system in which crystal formation is driven by homogeneous nucleation at high calcium phosphate concentration and the only macromolecule in solution is fetuin, a 48-kDa inhibitor of apatite growth. Our experiments with this system demonstrated that fetuin determines the location of mineral growth; in the presence of fetuin mineral grows exclusively within the fibril, whereas in its absence mineral grows in solution outside the fibril. Additional experiments showed that fetuin is also able to localize calcification to the interior of synthetic matrices that have size exclusion characteristics similar to those of collagen and that it does so by selectively inhibiting mineral growth outside of these matrices. We termed this new calcification mechanism "mineralization by inhibitor exclusion," the selective mineralization of a matrix using a macromolecular inhibitor of mineral growth that is excluded from that matrix. Future studies will be needed to evaluate the possible role of this mechanism in bone mineralization.

Fetuin-A and BMD in older persons: the Health Aging and Body Composition (Health ABC) study

Fetuin-A is a hepatic secretory protein that promotes bone mineralization in vitro. Whether fetuin-A levels are associated with BMD in humans is unknown. The Health Aging and Body Composition study enrolled 3075 well-functioning black and white persons 70-79 yr of age and measured BMD. This cross-sectional study measured serum fetuin-A using ELISA among a random sample of 508 participants within sex and race strata. Multivariate linear regression analysis evaluated the associations of fetuin-A with BMD. Among women (n = 257), higher fetuin-A levels were significantly associated with higher total hip (p = 0.02), lumbar spine (p = 0.03), and whole body BMD (p = 0.01) in models adjusted for age, race, diabetes, alcohol and tobacco use, physical activity, body mass index, C-reactive protein levels, calcium supplement, and estrogen use. For example, each SD (0.38 g/liter) higher level of fetuin-A was associated with 0.016 g/cm(2) higher total hip areal BMD. The association was of similar magnitude and direction for femoral neck BMD but did not reach statistical significance (p = 0.11). In contrast, among men (n = 251), fetuin-A had no significant associations with total hip (p = 0.79), lumbar spine (p = 0.35), whole body (p = 0.46), or femoral neck BMD (p = 0.54) in multivariable models. We conclude that higher fetuin-A levels are independently associated with higher BMD among well-functioning community-dwelling older women but not older men. Future studies should evaluate whether fetuin-A may refine fracture risk assessment in older women.

The essential role of fetuin in the serum-induced calcification of collagen

The mineral in bone is located primarily within the collagen fibril, and during mineralization the fibril is formed first and then water within the fibril is replaced with mineral. Our goal is to understand the mechanism of fibril mineralization, and as a first step we recently determined the size exclusion characteristics of the fibril. This study indicates that apatite crystals up to 12 unit cells in size can access the water within the fibril while molecules larger than a 40-kDa protein are excluded. We proposed a novel mechanism for fibril mineralization based on these observations, one that relies exclusively on agents excluded from the fibril. One agent generates crystals outside the fibril, some of which diffuse into the fibril and grow, and the other selectively inhibits crystal growth outside of the fibril. We have tested this mechanism by examining the impact of removing the major serum inhibitor of apatite growth, fetuin, on the serum-induced calcification of collagen. The results of this test show that fetuin determines the location of serum-driven mineralization: in fetuin's presence, mineral forms only within collagen fibrils; in fetuin's absence, mineral forms only in solution outside the fibrils. The X-ray diffraction spectrum of serum-induced mineral is comparable to the spectrum of bone crystals. These observations show that serum calcification activity consists of an as yet unidentified agent that generates crystal nuclei, some of which diffuse into the fibril, and fetuin, which favors fibril mineralization by selectively inhibiting the growth of crystals outside the fibril.

Vascular calcification and fetuin-A deficiency in chronic kidney disease

In past decades, extraosseous (including cardiovascular) calcification was predominantly regarded as a passive process of limited pathophysiological significance. It was thought that a calcium (Ca) x phosphate (P) product in excess of Ca and P solubility was the key trigger of progressive hydroxyapatite precipitation and deposition in vessels and soft tissues. Recently, however, it became apparent that calcification is a complex and highly regulated process involving inhibitors, inducers, and cell differentiation processes. It further became evident that cardiovascular manifestations of calcification predict patient outcomes in general populations, but in particular, in patients with chronic kidney disease. This review discusses the role of fetuin-A in the regulation of extraosseous and especially cardiovascular calcification processes. Fetuin-A is an inflammation-related Ca-regulatory glycoprotein and the prototype of a systemically acting calcification inhibitor. The emerging role of fetuin-A deficiency as a risk factor in dialysis patients was documented in cross-sectional studies demonstrating a significant correlation with all-cause and cardiovascular mortality.

Serum levels of the fetuin-mineral complex correlate with artery calcification in the rat

The present studies were carried out to evaluate the possible association between the presence of the fetuin-mineral complex in serum and vitamin D-induced artery calcification. The first experiment shows that there is a fetuin-mineral complex in the blood of rats in which extensive calcification of the artery media has been induced by treatment with vitamin D for 96 h, and that there is no detectable fetuin-mineral complex in the blood of rats in which artery calcification has been inhibited by concurrent treatment with ibandronate or osteoprotegerin. The second experiment shows that the timing of vitamin D-induced artery calcification correlates with the timing of the maximal increase in serum fetuin-mineral complex levels. Whereas both results indicate that serum levels of the fetuin-mineral complex are indeed associated with vitamin D-induced artery calcification, the biochemical basis for this association is presently unclear. One possibility is that high levels of the fetuin-mineral complex cause defects in the ability of fetuin to prevent the growth of the mineral component, which then seeds artery calcification. Another possibility is that the fetuin-mineral complex is the downstream product of a pathway that begins with the true causative agent, and that the serum level of the fetuin-mineral complex is a marker for the activity of this agent in blood. An unexpected finding of the present studies is that vitamin D-induced artery calcification is also correlated with a 65 to 75% reduction in serum fetuin, a reduction that appears to be caused by the clearance of the fetuin-mineral complex from serum.

Biochemical characterization of the serum fetuin-mineral complex

The present study was carried out to characterize the fetuin-mineral complex (FMC), a high molecular mass complex of calcium phosphate mineral and the proteins fetuin and matrix Gla protein (MGP) that was initially discovered in serum of rats treated with etidronate and appears to play a critical role in inhibiting calcification in vivo. Fetuin purified from the FMC contains 3.3 mol of protein-bound phosphate. There is 1.3 mg of FMC/ml of serum 6 h after etidronate injection, and the FMC is 46% fetuin and 53% mineral by mass. Formation of the FMC in the first 6 h after etidronate injection does not increase serum fetuin despite the fact that 50% of serum fetuin is associated with the FMC, and clearance of the FMC in the 9-24-h interval lowers total serum fetuin by 50%. These observations suggest that the fetuin component of the FMC is derived from fetuin initially in serum and that clearance of the FMC removes the associated fetuin from circulation. One additional protein was consistently present in all preparations of the FMC, spp24 (secreted phosphoprotein 24). This 24-kDa protein is similar in domain structure to fetuin and, like fetuin and MGP, contains several residues of phosphoserine and accumulates in bone. Exogenous spp24 associated strongly with the FMC when added to serum containing it. These observations suggest that spp24 may, like fetuin and MGP, play a role in inhibiting calcification.

The inhibition of calcium phosphate precipitation by fetuin is accompanied by the formation of a fetuin-mineral complex

The present studies show that the previously reported ability of fetuin to inhibit the precipitation of hydroxyapatite from supersaturated solutions of calcium and phosphate in vitro is accompanied by the formation of the fetuin-mineral complex, a high molecular mass complex of calcium phosphate mineral and the proteins fetuin and matrix Gla protein that was initially discovered in the serum of rats treated with etidronate and that appears to play a critical role in inhibiting calcification in vivo. Rat serum potently inhibited the precipitation of calcium phosphate mineral when the concentration of calcium and phosphate were increased by 10 mm each, and the modified serum was incubated at 37 degrees C for 9 days; in the absence of serum, precipitation occurred in seconds. Large amounts of the fetuin-mineral complex were generated in the first 3 h of this incubation and remained throughout the 9-day incubation. Purified bovine fetuin inhibited the precipitation of mineral for over 14 days in a solution containing 5 mM calcium and phosphate at pH 7.4 at 22 degrees C, whereas precipitation occurred in minutes without fetuin. There was a biphasic drop in ionic calcium in the fetuin solution, however, from 5 to 3 mM in the first hour and from 3 to 0.9 mM between 20 and 24 h; these changes in ionic calcium are due to the formation of complexes of calcium, phosphate, and fetuin. The complex found at 24 h to 14 days is identical to the fetuin-mineral complex found in the serum of etidronate-treated rats, whereas the complex found between 1 and 20 h is less stable.

The interaction of recombinant decorin with alpha2HS-glycoprotein-implications for structural and functional investigations

Isolated protein preparations of the small leucine-rich proteoglycans (SLRPs) associated with mineralized tissues have provided important information in understanding their structural and functional interactions within extracellular matrices and their potential roles in mineralization. Two important SLRPs, decorin and biglycan, copurify following extraction and purification from mineralized tissues using standard procedures, and to overcome this problem decorin was synthesized within a mammalian expression system to obtain pure preparations. The expressed protein was purified from the culture medium using anion-exchange chromatography, and characterization confirmed the presence of a decorin-rich fraction. However, N-terminal sequencing revealed the additional presence of alpha2HS-glycoprotein (alpha2HSG), representing approximately 35% of the total purified fraction. The decorin-rich fraction was subjected to selected further purification techniques to separate decorin from alpha2HSG. Application of the sample at a low concentration (1 mg/ml) to a second anion-exchange procedure and elution over an expanded sodium chloride gradient resulted in a high degree of purity (98%), with a single protein isolate demonstrable by SDS-PAGE. Electroelution achieved partial purification ( approximately 89%), but immunoprecipitation with antibodies against the glycosaminoglycan chain and the polyhistidine tag failed to separate the two proteins. This study suggests there is a strong interaction between recombinantly produced decorin and alpha2 HSG and highlights the importance of the purification technique to the application of recombinantly produced proteins or those that have been extracted from mineralized tissues for use in structural and functional interactions.

Discovery of a high molecular weight complex of calcium, phosphate, fetuin, and matrix gamma-carboxyglutamic acid protein in the serum of etidronate-treated rats

In the present study we report the discovery of a novel protein-mineral complex in the serum of rats treated with doses of the bone-active bisphosphonate etidronate that inhibit normal bone mineralization. The composition of this high molecular mass protein-mineral complex consists of about 18% mineral, 80% fetuin, and 2% matrix Gla protein (MGP) by weight, and the presence of the complex in serum after an injection of 8 mg etidronate/100 g of body weight elevates calcium by 1.8-fold (to 4.3 mm), phosphate by 1.6-fold (to 5.6 mm), and MGP by 25-fold (to 12 microg/ml). The serum mineral complex reaches maximal levels at 6 h after subcutaneous injection of etidronate and is subsequently cleared from serum by 24 h. This highly specific complex of fetuin, MGP, and mineral prevents the growth, aggregation, and precipitation of the mineral component, which indicates that the previously reported calcification inhibitory activities of fetuin and MGP may be related to their ability to form stable complexes with nascent mineral nuclei. Treatment with the vitamin K-antagonist warfarin prevents the increase in serum MGP after etidronate injection, which shows that the increase in serum MGP is due to new synthesis and that the gamma-carboxylation of MGP is necessary for its binding to the serum mineral complex.

A comparative study of sialic acid-rich proteins in rat bone and dentin

Four sialic acid-rich (SA-rich) proteins found in bone and dentin, osteopontin (OPN), bone sialoprotein (BSP), bone acidic glycoprotein-75 (BAG-75), and dentin matrix protein 1 (DMP1), share some common features. We used SDS-PAGE and Western immunoblots to analyze and compare SA-rich proteins in bone and dentin extracts from rats with a single chromatographic procedure. OPN was detected in dentin extracts, with a relative level less than one-seventieth of that in bone. Both bone and dentin BSP demonstrated an extremely broad distribution pattern, probably due to a high degree of heterogeneity in post-translational modifications. BAG-75 in both bone and dentin was detected as an 83 kDa band, dramatically distinct from that of DMPI. Using a polyclonal antibody raised against a purified bone 57 kDa protein (a portion of DMPI), we detected 150 kDa protein bands in bone fraction; the same bands were recognized by antirecombinant rat DMPI antibody. Bands from dentin migrating at about 150 kDa in earlier fractions and progressing to 200 kDa in later fractions showed a clear immunoreactivity to the anti-57 kDa antibody. We conclude that the majority of DMPI in rat bone is processed into fragments, whereas that in dentin remains intact.

Calcium signals in prostate cancer cells: specific activation by bone-matrix proteins

Cancer of the prostate commonly metastasizes to bony sites where cells acquire an aggressive, rapidly proliferating, androgen-independent phenotype. The interaction between bone and prostate, thus, becomes a key factor in disease progression. Fluctuations in intracellular ionized Ca2+ [Ca2+]i are rapid, regulated signal transduction events often associated with cell proliferation. Hence, Ca2+ signals provide a convenient measure of early events in cancer cell growth. This study developed single cell fluorescent imaging techniques to visualize Ca2+ signals in Fura-2 loaded prostatic cancer cell lines of various metastatic phenotypes. Solubilized bone fractions containing extracellular matrix and associated proteins were tested for the ability to trigger Ca2+ signals in prostate cancer cell lines. Fractions representing the complete repertoire of non-collagenous proteins present in mineralized bone were tested. Results demonstrated that two bone fractions termed D3b- and D4a-triggered Ca2+ signals in prostate cancer cells derived from bone (PC-3), but not brain (DU-145) metastases of prostate cancer. Lymph-node derived LNCaP cells also did not produce a Ca2+ signal in response to addition of soluble bone matrix. No other bone fractions produced a Ca2+ signal in PC-3 cells. It is of interest that bone fractions D3b and D4a contain a number of non-collagenous matrix proteins including osteonectin (SPARC) and osteopontin (OPN), as well as prothrombin. Moreover, antibody LM609 that recognizes the alpha v beta 3 integrin, blocks the ability of OPN to trigger a Ca2+ transient in PC-3 cells. These studies support a conclusion that bone-matrix proteins play a role in the growth and progression of metastatic prostate cancer, and that prior growth in bone may be associated with development of a bone-matrix-responsive phenotype.

Blood circulation as source for osteopontin in acellular extrinsic fiber cementum and other mineralizing tissues

Osteopontin (OPN) is one of the major non-collagenous proteins in root cementum and other mineralized tissues. Although most of this mineral-seeking protein is thought to be produced by local tissue cells, some of it might enter the mineralizing matrix from the blood. To test this hypothesis, we followed the distribution of a single dose of purified porcine or rat 125I-labeled OPN injected i.v. in rats, in mineralizing and non-mineralizing tissues and in subcutaneously implanted collagenous implants. The animals were killed 30 or 48 hrs after injection. Tissues (calvaria, tibia, lower and upper jaws) were harvested and processed for radioautography and biochemical analysis. Tissues as well as calcifying collagenous implants proved to have taken up radiolabel. In EDTA extracts of long bones, the majority of the radiolabel was demonstrated to be associated with intact OPN. The iodinated protein was also found in the acellular extrinsic fiber cementum (acellular cementum) layer investing the continuously growing incisors, in laminae limitantes, cement lines, and in forming bone near the mineralization front. Further, the label was present in the circumpulpal dentin of the incisors, and some of it appeared to have been incorporated into developing enamel. It is concluded that OPN in acellular cementum and other mineralizing tissues may-at least partially-originate from sources outside the direct environment following its transportation via serum.

Friends and relations of the cystatin superfamily--new members and their evolution

The cystatin "superfamily" encompasses proteins that contain multiple cystatin-like sequences. Some of the members are active cysteine protease inhibitors, while others have lost or perhaps never acquired this inhibitory activity. In recent years, several new members of the superfamily have characterized, including proteins from insects and plants. Based on partial amino acid homology, new members, such as the invariant chain (Ii), and the transforming growth factor-beta receptor type II (TGF-beta receptor II) may, in fact, represent members of an emerging family within the superfamily that may have used some common building blocks to form functionally diverse proteins. Cystatin super-family members have been found throughout evolution and members of each family of the superfamily are present in mammals today. In this review, the new and older, established members of the family are arranged into a possible evolutionary order, based on sequence homology and functional similarities.

Limited proteolysis of human alpha2-HS glycoprotein/fetuin. Evidence that a chymotryptic activity can release the connecting peptide

alpha2-HS glycoprotein is a major protein of human plasma whose function is still obscure. A proteolytically processed form of alpha2-HS glycoprotein lacking a segment of 40 amino acid residues bridging its heavy and light chain portions ("connecting peptide") has been described suggesting that this peptide is released by post-translational processing to fulfill biological role(s) of alpha2-HS glycoprotein. To test this hypothesis we investigated how the connecting peptide is released from the parental molecule by limited proteolysis. We developed monoclonal antibodies to various portions of the connecting peptide and its NH2-terminal flanking region which cross-react with the native alpha2-HS glycoprotein. Purified alpha2-HS glycoprotein from human plasma was subjected to limited proteolysis by proteinases including trypsin, chymotrypsin, elastase plasmin, kallikrein, thrombin, and renin. Immunoprint analysis of the proteolytic digests indicated that alpha2-HS glycoprotein is readily cleaved in its connecting peptide region. NH2-terminal amino sequence analysis of the generated fragments demonstrated that a single proteinase, chymotrypsin, cleaves the critical Leu-Leu bond flanking the NH2-terminal portion of the connecting peptide region. Most but not all of the other proteinase cleavage sites map to a short stretch of 9 residues located in the center portion of the connecting peptide region. Immunoprint analysis of plasma samples from patients with sepsis demonstrate that the connecting peptide region is cleaved under pathological conditions. Our results indicate that the connecting peptide and/or fragments thereof are readily releasable from alpha2-HS glycoprotein in vitro and in vivo.

The serum protein alpha2-HS glycoprotein/fetuin inhibits apatite formation in vitro and in mineralizing calvaria cells. A possible role in mineralization and calcium homeostasis

We present data suggesting a function of alpha2-HS glycoproteins/fetuins in serum and in mineralization, namely interference with calcium salt precipitation. Fetuins occur in high serum concentration during fetal life. They accumulate in bones and teeth as a major fraction of noncollagenous bone proteins. The expression pattern in fetal mice confirms that fetuin is predominantly made in the liver and is accumulated in the mineralized matrix of bones. We arrived at a hypothesis on the molecular basis of fetuin function in bones using primary rat calvaria osteoblast cultures and salt precipitation assays. Our results indicate that fetuins inhibit apatite formation both in cell culture and in the test tube. This inhibitory effect is mediated by acidic amino acids clustering in cystatin-like domain D1. Fetuins account for roughly half of the capacity of serum to inhibit salt precipitation. We propose that fetuins inhibit phase separation in serum and modulate apatite formation during mineralization.

Secretion of Osteopontin by macrophages and its accumulation at tissue surfaces during wound healing in mineralized tissues: a potential requirement for macrophage adhesion and phagocytosis

Osteopontin (OPN), a noncollagenous, extracellular matrix sialoprotein found at relatively high levels in both normal and pathological mineralized tissues, is expressed by tissue-specific cells in bone, calcified cartilage, and teeth. On the other hand, a hallmark of OPN expression in pathologically mineralizing tissue, and in other soft tissues experiencing a more generalized type of necrotic injury, is the production of OPN by macrophages at the lesion site. In the present study, we have localized OPN and other noncollagenous proteins by ultrastructural colloidal-gold immunocytochemistry using a rat model in which mineralized tissue defects are surgically created in mandibular bone and teeth. The healing response was examined by immunocytochemistry and transmission electron microscopy at 10 min, 3 days and 7 days post-surgery using antibodies against OPN, bone sialoprotein, osteocalcin, bone acidic glycoprotein-75, fibronectin, and amelogenin. Whereas most of these proteins were characteristically distributed within their respective extracellular matrices as described previously, OPN was additionally observed to accumulate as a lamina limitans at surgically exposed bone and tooth surfaces, as well as at the surface of particulate, mineralized tissue debris. Intracellular labeling of the Golgi apparatus and secretory granules of macrophages at the lesion site demonstrated that OPN production by macrophages was a prominent secretory event of the inflammatory response during wound healing in mineralized tissues. Pseudopodal and lamellipodal cytoplasmic extensions of macrophages were observed in direct contact with the OPN-containing lamina limitans at these surfaces. Particulate, calcified debris internalized by macrophages also displayed a prominent surface "coating" of OPN. In conclusion, our interpretation of the present data is that OPN secreted by macrophages may serve as a macrophage adhesion protein, and where concentrated at the surface of small particulate, mineralized tissue debris, may act as an opsonin, thereby facilitating cell adhesion and phagocytosis by macrophages, a process likely mediated by integrin-binding, signal transduction, and cytoskeletal restructuring.

Electrophoretic gels of dentin matrix proteins as diffusion media for in vitro mineralization

Non-collagenous proteins of dentin and bone have important effects on mineralization which have been studied by various in vitro systems. We developed an in vitro mineralization system using electrophoretic gels as diffusion media of calcium and phosphate ions. Calcium and phosphate ions were diffused naturally or propelled by electric potential. Calcium phosphate was precipitated in the gel, and the precipitation was affected by proteins in the gel which had been separated by electrophoresis. We applied this system to analysis of non-collagenous proteins of dentin. Among the proteins, phosphophoryns promoted calcium phosphate precipitation in the natural-diffusion system. A non-collagenous protein having a molecular mass of 60 kDa inhibited precipitation. The results were different, however, in the electric-diffusion system, in which phosphophoryns had a negative effect. The present system enabled us to compare the effects of plural proteins rapidly, even using unpurified material.

Isolation, characterization, and primary structure of a calcium-binding 63-kDa bone protein

A novel noncollagenous protein of the mineralized matrix of bovine bone was isolated by ion exchange and gel permeation chromatography. The apparent M(r) of the protein is 63,000 as determined by SDS-polyacrylamide gel electrophoresis. The protein is a rather minor constituent in bone and could not be detected in other connective tissues by enzyme-linked immunosorbent assay of guanidine HCl extracts. The 63-kDa protein was detected in the osteoid and around the osteocytes upon immuno-histochemical staining of bovine compact bone. The sequence of the 63-kDa protein was deduced from cDNA clones isolated from a rat calvaria lambda gt11 expression library. The protein contains two centrally located EF-hand Ca(2+)-binding domains. Seven heptad repeats are present indicating the ability of the protein for coiled-coil interactions. Ability to bind calcium was confirmed by 45Ca2+ binding to protein blotted onto nitrocellulose membrane. The protein was synthesized in calvaria explants as detected by immunoprecipitation of radiolabeled protein from the culture medium. Although the protein can be detected in biochemical amounts in bone only, varying amounts of mRNA for this protein were detected in several rat tissues by RNase protection assay with highest levels in rat calvaria. This extracellular protein corresponds to a mouse protein called nucleobindin.

Bone matrix proteins in osteogenesis and remodelling in the neonatal rat mandible as studied by immunolocalization of osteopontin, bone sialoprotein, alpha 2HS-glycoprotein and alkaline phosphatase

The neonatal rat mandible was used as a model to study bone formation, mineralization, quiescence, and resorption, using immunolocalization and a variety of tissue-processing techniques. Monospecific antibodies for osteopontin (OPN), bone sialoprotein (BSP), alkaline phosphatase (AP) and alpha 2HS-glycoprotein (alpha 2HS-GP) were used on fixed paraffin-embedded tissue, fixed frozen tissue and unfixed frozen tissue. Immunostaining was correlated with mineral content by two procedures, the von Kossa and the morin techniques. Morin fluorescence was used with secondary immunostaining to provide a way of closely correlating bone matrix proteins and matrix mineralization. Co-immunolocalization procedures were used to compare the sites of bone proteins in the matrix. AP was found earliest during osteogenic cell differentiation, appearing in the preosteoblasts, followed by OPN and BSP, which first appeared in osteoblasts. alpha 2HS-GP expression was not observed in cells. The results provide clear evidence for the presence of OPN in osteoid, while BSP and alpha 2HS-GP were confined to the mineralized matrix. Immunostaining of bone proteins is highly technique-dependent: immunolocalization investigations required several methods of approach to ensure adequate demonstration of these proteins in cells and matrix. The results support the contention that osteopontin is multifunctional in bone metabolism, and that alpha 2HS-GP, though produced in the liver, is abundant in bone matrix and may also have a function in bone metabolism.

The matrix Gla protein gene is a marker of the chondrogenesis cell lineage during mouse development

Matrix Gla protein (MGP) is, along with osteocalcin, a skeletal member of the family of extracellular mineral-binding Gla proteins. Although the precise function of these proteins remains obscure, circumstantial evidence suggests that they play a role in endochondral ossification. As a first step toward understanding MGP function we have performed a preliminary characterization of its promoter element and studied the developmental pattern of expression of this gene. DNA transfection experiments indicate that the mouse MGP promoter functions better in cells expressing the MGP gene than in cells that do not express the gene. During mouse development, MGP gene expression is detectable as early as day 10.5 of embryonic development (E10.5), before any skeletal structures are identifiable. In situ hybridization analysis shows that MGP mRNA is initially present at the mesenchymal epithelial interphase in lung and limb buds. As development proceeds, MGP gene is predominantly expressed in cells of the chondrocytic lineage in areas that will undergo endochondral ossification as well as in areas that will remain cartilaginous, such as the trachea and bronchi. In growth plate cartilage, MGP mRNA is present in resting, proliferative, and late hypertrophic chondrocytes. Surprisingly, MGP mRNA is absent from the early hypertrophic chondrocytes and from the osteoblasts. Finally, the MGP gene is expressed at a lower level in kidney medulla and uterus smooth muscle but not in brain, spleen, or heart during development. This study demonstrates that during development MGP gene expression occurs early and is predominant at the epithelial mesenchymal interfaces, principally of lung and limb buds, and in cells of the chondrocytic lineage. This finding raises the intriguing possibility that MGP may play distinct roles during embryogenesis and in the adult organism.

Expression of Mac-2 antigen in the preosteoclast and osteoclast identified in the op/op mouse injected with macrophage colony-stimulating factor

Osteoclast deficiency in op/op mice is cured by a single injection of 5 micrograms recombinant human macrophage colony-stimulating factor (rhM-CSF). In this study, we found that mouse osteoclasts are positive for Mac-2 antigen, but not for F4/80, MOMA-2, Mac-1, or BM8 antigen. By using F4/80 and MOMA-2 monoclonal antibodies, we confirmed the absence of mature macrophages in the femora of op/op mice and found that multiple injections of rhM-CSF are required for the recruitment of macrophages in the bones. After a single rhM-CSF injection, we found Mac-2 positive mononuclear cells in the femora of op/op mice. The time course of the appearance of Mac-2-positive cells was very similar to that of tartrate-resistant acid phosphatase (TRAP)-positive cells. In bone sections prepared from the mutant mice that received rhM-CSF 3 days earlier, 91% of the TRAP-positive mononuclear cells were also positive for Mac-2 antigen. These results demonstrate the expression of Mac-2 antigen in preosteoclasts. The antigen was detected on the plasma membrane of preosteoclasts, as well as in their cytoplasm and nucleus, and in the extracellular matrix in the space between the cells and bone. Since Mac-2 is a galactose-specific lectin, a potential role of the lectin in cell-cell and cell-matrix adhesion during osteoclast differentiation is suggested.

Immunolocalization of osteopontin, osteocalcin, and dentin sialoprotein during dental root formation and early cementogenesis in the rat

Using immunohistochemical methods we studied the tissue localization of the extracellular matrix proteins osteopontin (OPN), osteocalcin (OC), and dentin sialoprotein (DSP) during the formation of acellular and cellular cementum in newly born rats. In the layer of acellular cementum of developing incisor and molar teeth we found a very strong staining for OPN but not for DSP or OC. Many cells immediately adjacent to acellular cementum and PDL cells were also positive for OPN but not for DSP or for OC. In contrast, cellular cementum in molar teeth stained strongly for OPN and OC but not for DSP. Consistent with these observations, the cells engaged in the formation of cellular cementum (cementoblasts and cementocytes) reacted strongly for OPN and OC but not for DSP. In advanced stages of dentinogenesis, both crown and root odontoblasts and dentin stained for OPN, OC, and DSP. Cells and matrices of surrounding alveolar bone stained for OPN and OC but not for DSP. We conclude that cementoblasts and cementocytes of cellular cementum produce OPN and OC but not DSP and thus express an osteoblast-like, not an odontoblast-like, phenotype. The cells responsible for the production of acellular cementum are likely cells of the PDL in close contact with the dental root surface. These fibroblast-like cells express OPN but not OC or DSP and accordingly express only a partial osteoblastic phenotype.

Dissimilar expression patterns for the extracellular matrix proteins osteopontin (OPN) and collagen type I in dental tissues and alveolar bone of the neonatal rat

Osteopontin (OPN) is a phosphorylated, sialic acid containing glycoprotein that can be extracted from the mineralized extracellular matrix of bone. In the present study we determined the expression patterns of OPN in dental tissues and alveolar bone of 1-3 day old (neonatal) rats by means of 1) immunohistochemistry, 2) Northern blotting and 3) in situ hybridization. We compared these patterns with those of type I collagen. We localized collagen type I expression in osteoblasts adjacent to alveolar bone and in odontoblasts lining predentin/dentin, but not in the epithelial ameloblasts. For OPN, we observed a weak antigenicity in predentin. Although generally no cellular immunostaining was found, very occasionally a minor immunoreactivity was detected in a small number of pre-mineralizing incisor odontoblasts. On the mRNA level, however, no OPN transcripts could be detected in odontoblasts, either by in situ or by Northern hybridization analyses. Also the odontoblasts of the bone-like dentin (osteodentin) region in the tip of incisors were negative for OPN. In contrast, however, osteoblasts of alveolar bone showed strong positive signals with all three techniques, confirming the sensitivity and specificity of the detection methods. From the data obtained in this study, it can be concluded that during early stages of dentinogenesis OPN presumably is not expressed in developing rat tooth germs. The weak immunostaining observed sporadically in some young odontoblasts is probably due to resorption of OPN of non-dental origin entrapped in the predentin.

A major non-collagenous 62 kDa protein from rat bone mineralized matrix is identical to pp63 a phosphorylated glycoprotein from liver

A protein present as a M(r) 62 k monomer and as several differently sized disulfide-bonded oligomers has been isolated from rat bone mineralized matrix. Its overall tissue distribution determined by ELISA immunoassays showed the protein present only in bone, tooth and in serum while aorta, cartilage, intestine, kidney, liver, muscle, skin, spleen and tendon were all negative. Despite that the 62 kDa protein was abundant and selectively found in bone, no positive cDNA clone could be identified in several rat bone libraries. Positive clones were, however, identified in a rat liver expression library. A cDNA clone of 1.3 kb hybridized in a Northern blotting assay to a 1.8 kb mRNA in rat liver. No hybridization signal was detected with RNA from bone, brain, lung, muscle, spleen and kidney. Sequence analysis of the isolated cDNA clone revealed a 50-bp untranslated region followed by an open reading frame of 357 amino acids. The open reading frame can be divided into a 17-amino acid signal peptide followed by the mature protein of 340 amino acids with alanine as its N-terminal amino acid. A short N-terminal amino acid sequence from the isolated 62-kDa bone protein verified the molecular identity of the cDNA clone. The primary structure of the 62-kDa liver protein was identical to a that of a 63-kDa phosphorylated glycoprotein (pp63) from liver.

Dentin sialoprotein: biosynthesis and developmental appearance in rat tooth germs in comparison with amelogenins, osteocalcin and collagen type-I

A non-collagenous protein, extracted from rat incisor dentin, is a dentin sialoprotein (DSP). We examined immunohistochemically the developmental appearance and tissue distribution of DSP in 1 to 3-day-old rat molar and incisor tooth germs. The earliest staining for DSP was observed in newly differentiated odontoblasts. In more advanced stages, immunostaining for DSP gradually increased in pre-dentin, odontoblasts and dentin, and appeared in many cells of the dental papilla. In early stages of development before the breakdown of the dental basement membrane, pre-ameloblasts were also positive for DSP. This staining disappeared from the ameloblast cell body soon after deposition of the first layer of mineralized dentin. Radiolabelling of tooth matrix proteins with 14C-serine in vitro followed by immunoprecipitation and fluorography confirmed that DSP was synthesized by tooth-forming cells. The immunolocalization for DSP was different from that of either collagen type-I, osteocalcin or the amelogenins. Whereas collagen type-I and osteocalcin were restricted to the mesenchymal dental tissues, the amelogenins were detectable in both epithelial and mesenchymal dental cells and tissues at the epithelio-mesenchymal interface at early stages of development, prior to the onset of dentin mineralization. We conclude that DSP is expressed in and secreted by odontoblasts and some dental papilla cells from early stages of dentinogenesis onwards, i.e. later than type-I collagen, but before deposition of the first layer of mineralized dentin. In pre-mineralizing stages, some of the matrix proteins may be endocytosed from the pre-dentin by both cell types involved in the epithelio-mesenchymal interaction.

Ultrastructural immunolocalization of noncollagenous (osteopontin and osteocalcin) and plasma (albumin and alpha 2HS-glycoprotein) proteins in rat bone

The high-resolution, postembedding protein A-gold immunocytochemical technique was used to visualize the distribution of two noncollagenous bone proteins, osteopontin (OPN) and osteocalcin (OC), and two plasma proteins, alpha 2HS-glycoprotein (alpha 2HS-GP) and albumin (ALB), in sections of Lowicryl K4M-embedded rat tibial and alveolar bone. In the primary spongiosa of the metaphysis, a seam of organic material (lamina limitans) that labeled intensely with OPN and OC antibodies was observed at the bone/calcified cartilage interface just below the zone of vascular invasion of the growth plate. With deposition of bone matrix proper by osteoblasts in this region and its subsequent mineralization, extensive areas of bone were heavily labeled with anti-OPN, anti-OC, and anti-alpha 2HS-GP antibodies, where the majority of gold particles were associated with amorphous, electron-dense patches of organic material throughout the mineralized bone. In the unmineralized osteoid, substantially less labeling was observed, and where occasional mineralization loci were dispersed throughout the osteoid layer, these sometimes showed a concentration of gold particles. ALB labeling, on the other hand, was moderate and generally diffuse throughout the mineralized bone matrix and the osteoid. In alveolar bone, labeling patterns were generally similar to those found in tibial bone. Particularly striking in alveolar bone, however, was an intense anti-OPN labeling of (1) the lamina limitans at cell-lined bone surfaces, including that surrounding cell processes and osteocytes, (2) cement (reversal, resting) lines, and (3) the perilacumar matrix of some osteocytes. In summary, these data suggest that certain plasma proteins, such as alpha 2HS-GP, interact with bone matrix proteins, such as OPN and OC, at sites of tissue mineralization and that the presence of OPN in mineralized bone and at bone surfaces (lamina limitans) and cement lines has a multifunctional role, including regulation of mineralization and mediation of cell dynamics during endochondral and intramembranous bone modeling and remodeling.

Molecular cloning and sequence analysis of cDNA for a 59 kD bone sialoprotein of the rat: demonstration that it is a counterpart of human alpha 2-HS glycoprotein and bovine fetuin

A complementary DNA (cDNA) for the 59 kD bone sialoprotein, which is supposed to be the rat counterpart of human alpha 2-HS glycoprotein (alpha 2-HSG) and is synthesized by both hepatocytes and osteoblasts, has been cloned from a rat liver cDNA library. Polyclonal rabbit antibodies to rat 59 kD bone sialoprotein were used to identify and isolate the cDNA. The amino acid sequence of 59 kD bone sialoprotein deduced from the cDNA revealed that the entire protein consisted of 352 amino acid residues, including a signal peptide of 18 amino acid residues, and contained three possible N-glycosylation sites. On Northern blot analysis of rat liver, an mRNA of about 1.5 kilobases was detected. An mRNA of 59 kD bone sialoprotein was also detectable in rat bone but not in other tissues, such as kidney, brain, and lung. A computer search of protein and nucleic acid data bases revealed that 68.2, 63.2, and 97.4% amino acid residues of 59 kD bone sialoprotein were identical with those of human alpha 2-HSG, bovine fetuin, and rat phosphorylated N-glycoprotein (pp63), respectively. The positions of cysteine residues in 59 kD bone sialoprotein also completely matched those in human alpha 2-HSG and bovine fetuin, indicating that the sialoprotein is the rat counterpart of human alpha 2-HSG and bovine fetuin. In addition, comparison of the nucleotide sequence of cDNA for rat fetuin/alpha 2-HSG with that for pp63 recently corrected showed only two differences in nucleotides in the entire protein coding regions of the two proteins, and immunoreactive rat fetuin/alpha 2-HSG in the conditioned medium of adult rat hepatocytes in primary culture was found to be phosphorylated. Thus, because rat fetuin/alpha 2-HSG isolated from bone and synthesized by osteoblasts in culture does not contain phosphorus, it seems to be pp63 dephosphorylated during circulation or in the bone matrix.

High-resolution immunolocalization of osteopontin and osteocalcin in bone and cartilage during endochondral ossification in the chicken tibia

The ultrastructural distribution of two noncollagenous proteins, osteopontin (OPN) and osteocalcin (OC), originally extracted from bone matrix and proposed to play an important role in bone formation, was examined in the matrices of bone and cartilage from embryonic and postnatal chicken tibial growth plates by high-resolution immunocytochemistry using the colloidal gold technique. In bone, immunolabeling patterns using polyclonal antibodies against chicken OPN and OC were generally similar in that both showed an intense, but regionally variable, labeling of mineralized bone matrix and small mineralization loci dispersed throughout the osteoid and containing prominent condensed organic material. Unmineralized osteoid showed weak-to-moderate labeling. In the mineralized bone matrix proper, labeling was predominantly associated with amorphous, electron-dense patches of organic material among the collagen fibrils. In growth plate cartilage, both proteins first appeared related to calcified cartilage in the hypertrophic zone, although the labeling patterns were somewhat different. For OPN, gold particles were mostly associated with an organic lamina limitans-like density containing condensed, filamentous organic matrix at the periphery of small nodules and large masses of calcified cartilage, with additional moderate labeling throughout the interior of the calcified cartilage. For OC, labeling was observed over filamentous structures throughout the calcified cartilage matrix, with some, but less, labeling at the periphery. In the lowermost zones of the growth plate, the major reaction using both antibodies was found over a layer of dense, amorphous organic material at the periphery of the calcified cartilage at the future bone/calcified cartilage interface, a labeling pattern that persisted following bone deposition at these sites. OPN and to a lesser extent OC were also concentrated in cement (resting, reversal) lines. Throughout the bone and cartilage of the tibia, cells of both the osteoblastic and the osteoclastic lineages were found directly apposed to labeled surfaces and lamina limitans of organic matrix containing OPN and OC. In summary, it is concluded from the immunocytochemical data presented here that the association of OPN and OC with mineralized regions of the extracellular matrices of bone and cartilage and the accumulation of these proteins at tissue surfaces and interfaces are consistent with the hypotheses that they play a role in the extracellular mineralization process per se and/or that they may mediate cell adhesion and dynamics.

Evidence for the formation of a complex between osteopontin and osteocalcin

We hypothesize that the mechanisms governing bone formation and remodeling involve the assembly of some of the components of the extracellular matrix into supramolecular complexes. We have examined the associations of osteopontin (OPN) with other proteins isolated from demineralized rat long bones. Three ligand binding techniques were used to demonstrate the formation of complexes between osteopontin and osteocalcin (OCN). Using gel overlay assays, the binding between soluble 125I-OPN and OCN immobilized in acrylamide gels was visualized. Competition for 125I-OPN-OCN complexes was demonstrated when unlabeled OCN-enriched bone extract was included in gel overlay solutions. Also, gel overlay assays showed 125I-OCN binding to OPN. Saturable binding was shown in solid-phase filter binding assays, which yielded an equilibrium binding constant of moderately high affinity (approximately 10(-8) M). Specificity of OPN-OCN complex formation was confirmed by measuring binding in the presence of unlabeled OPN and OCN versus a bone-localized serum protein, alpha 2HS-glycoprotein. Finally, the formation of soluble complexes were demonstrated in a modified Hummel-Dreyer gel filtration assay. These results indicate that OPN and OCN form complexes in vitro. The possible functions of OPN-OCN complexes in osteoclast recruitment and attachment are discussed.

The nucleotide and partial amino acid sequences of rat fetuin. Identity with the natural tyrosine kinase inhibitor of the rat insulin receptor

Fetuins are among the major plasma proteins, yet their biological role has remained elusive. Here we report the molecular cloning of rat fetuin and the sequence analysis of a full-length clone, RF619 of 1456 bp with an open reading frame of 1056 bp encoding 352 amino acid residues. The coding part of RF619 was identical with the cDNA sequence of the natural inhibitor of the insulin receptor tyrosine kinase from rat (pp63) except for four substitutions and a single base insertion causing divergence of the predicted protein sequences. Partial amino acid sequences of rat plasma fetuin were in agreement with the predictions based on the RF619 cDNA. Purified rat fetuin inhibited the insulin receptor tyrosine kinase in vitro. Therefore, we conclude that RF619 and pp63 cDNA encode the same protein, i.e. authentic rat fetuin which is a functional tyrosine kinase inhibitor.