Purification, characterization, and studies on biosynthesis of a 59-kDa bone sialic acid-containing protein (BSP) from rat mandible using a monoclonal antibody. Evidence that 59-kDa BSP may be the rat counterpart of human alpha 2-HS glycoprotein and is synthesized by both hepatocytes and osteoblasts

Osteoclasts secrete osteopontin into resorption lacunae during bone resorption

Osteopontin (OPN) is a non-collagenous extracellular sialylated glycoprotein located in bone. It is believed to be one of the key components in osteoclast attachment to bone during resorption. In this study, we characterized OPN and other glycoproteins found in the resorption lacunae to confirm the role of osteoclasts in OPN secretion using electron microscopy and mass spectrometry. Additionally, we examined the glycan epitopes of resorption pits and the effects of different glycan epitopes on the differentiation and function of osteoclasts. Osteoarthritic femoral heads were examined by immunohistochemistry to reveal the presence of OPN in areas of increased bone metabolism in vivo. Our results demonstrate that human osteoclasts secrete OPN into resorption lacunae on native human bone and on carbonated hydroxyapatite devoid of natural OPN. OPN is associated with an elevated bone turnover in osteoarthritic bone under experimental conditions. Our data further confirm that osteoclasts secrete OPN into the resorption pit where it may function as a chemokine for subsequent bone formation. We show that α2,3- and α2,6-linked sialic acids have a role in the process of osteoclast differentiation. OPN is one of the proteins that has both of the above sialic residues, hence we propose that de-sialylation can effect osteoclast differentiation in bone.

FGF23-regulated production of Fetuin-A (AHSG) in osteocytes

INTRODUCTION

AHSG, a serum glycoprotein with recognized anti-calcification activity, has also been suggested to modulate both bone formation and resorption. Though the bulk of AHSG is mostly synthesized in the liver, it has been claimed that also bone cells might produce it. However, the extent of the bone AHSG production and the potential controlling factors remain to be definitively proven. A relevant number of studies support the notion that FGF23, a bone-derived hormone, not only regulates the most important mineral metabolism (MM) related factors (phosphate, parathyroid hormone, vitamin D, etc.), but might be also involved in cardiovascular (CV) outcome, both in chronic kidney disease (CKD) patients and in the general population. Furthermore, in addition to some direct autocrine and paracrine effects in bone, FGF23 has been suggested to interact with AHSG. In this study we investigated if AHSG is really produced by bone cells, and if its bone production is related and/or controlled by FGF23, using cultured bone cells, according to a new method recently published by our group.

RESULTS

Our data show that AHSG is consistently produced in osteocytes and to a far lesser extent in osteoblasts. Both FGF23 addition to the culture medium and its over-expression in osteocytes were associated with a consistent increase of both AHSG mRNA and protein, while FGF23 silencing was followed by opposite effects. Though most of these results were largely affected by the blockage of FGF23 receptors, the role of these receptors in the different experimental sets is still not completely clarified. In addition, we found that FGF23 and AHSG proteins co-localized both in cytoplasm and nucleus, which suggests a possible reciprocal interactivity.

CONCLUSIONS

Our data not only confirm that AHSG is produced in bone, mainly in osteocytes, but show for the first time that its production is modulated by FGF23. Since both proteins play important roles in the bone and cardiovascular pathology, these results add new pieces to the puzzling relationship between bone and vascular pathology, in particular in CKD patients, prompting future investigations in this field.

Retention of fetuin-A in renal tubular lumen protects the kidney from nephrocalcinosis in rats

The serum glycoprotein fetuin-A is an important inhibitor of extraosseous calcification. The importance of fetuin-A has been confirmed in fetuin-A null mice, which develop widespread extraosseous calcification including the kidney. However, the mechanism how fetuin-A protects kidneys from nephrocalcinosis remains uncertain. Here, we demonstrate that intratubular fetuin-A plays a role in the prevention of nephrocalcinosis in the proximal tubules. Although normal rat kidney did not express mRNA for fetuin-A, we found punctate immunohistochemical staining of fetuin-A mainly in the S1 segment of the proximal tubules. The staining pattern suggested that fetuin-A passed through the slit diaphragm, traveled in the proximal tubular lumen, and was introduced into proximal tubular cells by megalin-mediated endocytosis. To test this hypothesis, we inhibited the function of megalin by intravenous injection of histidine-tagged soluble receptor-associated protein (His-sRAP), a megalin inhibitor. His-sRAP injection diminished fetuin-A staining in the proximal tubules and led to urinary excretion of fetuin-A. We further analyzed the role of fetuin-A in nephrocalcinosis. Continuous injection of parathyroid hormone (PTH) 1-34 induced nephrocalcinosis mainly in the proximal tubules in rats. His-sRAP retained fetuin-A in renal tubular lumen and thereby protected the kidneys of PTH-treated rats from calcification. Our findings suggest that tubular luminal fetuin-A works as a natural inhibitor against calcification in the proximal tubules under PTH-loaded condition.

Reduction of orthodontic tooth movement by experimentally induced periodontal inflammation in mice

Orthodontic therapy is known to have an aggravating effect on the progression of destructive periodontitis if oral hygiene is not maintained. However, it is largely unknown how active periodontitis affects the velocity of orthodontic tooth movement. In this study, we examined the effect of periodontal inflammation on orthodontic tooth movement using a mouse model. Orthodontic force was applied on the maxillary first molar of mice, with or without ligature wire to induce experimental periodontitis. The distance moved by the first molar was significantly reduced by the ligature-induced experimental periodontitis. Tartrate-resistant acid phosphatase staining revealed that the number of osteoclasts present during orthodontic treatment was lower in the pressure zone of alveolar bone in the presence of periodontal inflammation. Consistently, the expression level of receptor activator of nuclear factor-kappaB ligand (RANKL) in the pressure zone was decreased in the ligature group. By contrast, experimental periodontitis increased the expression of cyclooxygenase-2 mRNA in the periodontal tissues, while in vitro treatment with prostaglandin E(2) decreased extracellular signal-regulated kinase phosphorylation and RANKL expression induced by mechanical stress in osteoblasts. Taken together, these results suggest that the orthodontic force-induced osteoclastogenesis in alveolar bone was inhibited by the accompanying periodontal inflammation, at least partly through prostaglandin E(2), resulting in reduced orthodontic tooth movement.

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.

Characterization of granulations of calcium and apatite in serum as pleomorphic mineralo-protein complexes and as precursors of putative nanobacteria

Calcium and apatite granulations are demonstrated here to form in both human and fetal bovine serum in response to the simple addition of either calcium or phosphate, or a combination of both. These granulations are shown to represent precipitating complexes of protein and hydroxyapatite (HAP) that display marked pleomorphism, appearing as round, laminated particles, spindles, and films. These same complexes can be found in normal untreated serum, albeit at much lower amounts, and appear to result from the progressive binding of serum proteins with apatite until reaching saturation, upon which the mineralo-protein complexes precipitate. Chemically and morphologically, these complexes are virtually identical to the so-called nanobacteria (NB) implicated in numerous diseases and considered unusual for their small size, pleomorphism, and the presence of HAP. Like NB, serum granulations can seed particles upon transfer to serum-free medium, and their main protein constituents include albumin, complement components 3 and 4A, fetuin-A, and apolipoproteins A1 and B100, as well as other calcium and apatite binding proteins found in the serum. However, these serum mineralo-protein complexes are formed from the direct chemical binding of inorganic and organic phases, bypassing the need for any biological processes, including the long cultivation in cell culture conditions deemed necessary for the demonstration of NB. Thus, these serum granulations may result from physiologically inherent processes that become amplified with calcium phosphate loading or when subjected to culturing in medium. They may be viewed as simple mineralo-protein complexes formed from the deployment of calcification-inhibitory pathways used by the body to cope with excess calcium phosphate so as to prevent unwarranted calcification. Rather than representing novel pathophysiological mechanisms or exotic lifeforms, these results indicate that the entities described earlier as NB most likely originate from calcium and apatite binding factors in the serum, presumably calcification inhibitors, that upon saturation, form seeds for HAP deposition and growth. These calcium granulations are similar to those found in organisms throughout nature and may represent the products of more general calcium regulation pathways involved in the control of calcium storage, retrieval, tissue deposition, and disposal.

Immunohistochemical localization and mRNA expression of matrix Gla protein and fetuin-A in bone biopsies of hemodialysis patients

Matrix Gla protein (MGP) and fetuin-A are inhibitors of arterial calcifications. In blood of rats, calcium-phosphate-fetuin-MGP complexes, produced in bone, have been identified. Indeed, an association between bone resorption, release of such complexes, and arterial calcifications has been reported. We have investigated the synthesis and localization of fetuin-A and MGP in bone of hemodialysis patients and the possible contribution of bone cells in arterial calcifications. Bone biopsies from 11 hemodialysis patients were used for histology, in situ hybridization of fetuin-A and MGP messenger RNA (mRNA), immunohistochemistry of fetuin-A, and total, carboxylated, and non-carboxylated MGP proteins. Patients showed various types of renal osteodystrophy, or normal bone. MGP was synthesized and expressed (total and carboxylated) by osteoblasts, osteocytes, and most osteoclasts, while fetuin-A by osteoblasts and osteocytes. Fetuin-A and carboxylated MGP proteins were positive in the calcified matrix, while total MGP was negative. Osteoid seams were negative to fetuin-A, lightly positive to carboxylated MGP, and occasionally positive to total MGP. Undercarboxylated MGP was mostly undetectable. In adult humans, fetuin-A is produced also by osteoblasts, and not only by hepatocytes, as previously believed. MGP, essentially carboxylated, is synthesized by osteoblasts and most osteoclasts. Increased bone turnover can be an important contributor to arterial calcifications.

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.

The comparative analysis of serum proteomes for the discovery of biomarkers for acute myeloid leukemia

OBJECTIVE

Acute myeloid leukemia (AML) develops as the consequence of a series of genetic changes in a hematopoietic precursor cell. However, the definitive diagnostic protein biomarkers for AML are still unclear. In our study to identify the biomarkers for an initial diagnosis, detection of relapse, and monitoring the minimal residual disease in AML by a less invasive method, serum proteins reflecting alterations in their proteomes were analyzed.

MATERIALS AND METHODS

We compared the two-dimensional electrophoresis patterns of human sera of 12 patients with AML with those of 12 normal subjects. The differentially expressed spots were identified by matrix-assisted laser desorption/ionization time-of-flight and electrospray ionization quadupole time-of-flight mass spectrometries.

RESULTS

Eight proteins that expressed differentially in the AML group were found. The expression levels of alpha-2-HS-glycoprotein, complement-associated protein SP-40, 40, RBP4 gene product, lipoprotein C-III, and an unknown protein were downregulated in serum of AML patients, whereas the other three proteins, including immunoglobulin heavy-chain variant, proteosome 26S ATPase subunit 1, and haptoglobin-1 were upregulated.

CONCLUSION

These results suggest that these proteins can be used as less invasive diagnostic and monitoring biomarkers of AML if further studies are done.

Processing and transport of matrix gamma-carboxyglutamic acid protein and bone morphogenetic protein-2 in cultured human vascular smooth muscle cells: evidence for an uptake mechanism for serum fetuin

Matrix gamma-carboxyglutamic acid protein (MGP) is a member of the vitamin K-dependent protein family with unique structural and physical properties. MGP has been shown to be an inhibitor of arterial wall and cartilage calcification. One inhibitory mechanism is thought to be binding of bone morphogenetic protein-2. Binding has been shown to be dependent upon the vitamin K-dependent gamma-carboxylation modification of MGP. Since MGP is an insoluble matrix protein, this work has focused on intracellular processing and transport of MGP to become an extracellular binding protein for bone morphogenetic protein-2. Human vascular smooth muscle cells (VSMCs) were infected with an adenovirus carrying the MGP construct, which produced non-gamma-carboxylated MGP and fully gamma-carboxylated MGP. Both forms of MGP were found in the cytosolic and microsomal fractions obtained from the cells by differential centrifugation. The crude microsomal fraction was shown to contain an additional, more acidic Ser-phosphorylated form of MGP believed to be the product of Golgi casein kinase. The data suggest that phosphorylation of MGP dictates different transport routes for MGP in VSMCs. A proteomic approach failed to identify a larger soluble precursor of MGP or an intracellular carrier protein for MGP. Evidence is presented for a receptor-mediated uptake mechanism for fetuin by cultured human VSMCs. Fetuin, shown by mass spectrometry not to contain MGP, was found to be recognized by anti-MGP antibodies. Fetuin uptake and secretion by proliferating and differentiating cells at sites of calcification in the arterial wall may represent an additional protective mechanism against arterial calcification.

Mineralization of decalcified bone occurs under cell culture conditions and requires bovine serum but not cells

The purpose of this study was to develop an in vitro model system for bone matrix mineralization in the absence of cells. For this model, we utilized EDTA-decalcified new-born rat tibias with the cartilaginous ends intact, allowing us to visually determine the specificity of mineralization within the bone. Our results show that supplementation of DMEM culture medium with 10mM beta-glycerophosphate and 15% fetal bovine serum (FBS) results in non-physiological mineral percipitation in the tibia because of the generation of supraphysiological (5mM) levels of inorganic phosphate in the medium. The same medium supplemented only with inorganic phosphate to a final concentration of 2mM failed to mineralize a decalcified tibia matrix. However, additional supplementation of this medium with as little as 5% FBS resulted in mineralization of those regions of the type I collagen where mineral was found prior to decalcification, with no evidence for mineralization in the cartilage at the bone ends or in the periosteum. Analysis of the mineral by Fourier-transform infrared spectroscopy and powder X-ray diffraction shows that tibias that have been decalcified and then remineralized contain an apatitic mineral that is strikingly similar to the mineral in normal bone. Tendon, a type I collagen matrix not normally mineralized in vivo, also mineralizes when incubated in DMEM containing 2mM Pi and as little as 1.5% FBS, but not when incubated in DMEM without serum. These data indicate that serum contains a nucleator of type I collagen matrix mineralization, and that mineralization of type I collagen under cell culture conditions requires serum but not living cells.

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.

Tornado extraction: a method to enrich and purify RNA from the nephrogenic zone of the neonatal rat kidney

BACKGROUND

Development of the kidney is a complicated and tightly regulated process. Although several genes responsible for the renal development have been identified to date, the precise mechanisms of spatial and temporal regulation remain to be elucidated. Therefore, expanding our knowledge of molecules that are associated with nephrogenesis will be helpful to understand the whole process.

METHODS

To extract RNA selectively from the nephrogenic zone of the developing kidney, we developed a simple and reliable method.

RESULTS

This method, named "tornado extraction," enriched RNA of the nephrogenic zone by about 30-fold. In combination with the suppression subtractive hybridization, a considerable number of genes that were differentially expressed in the nephrogenic zone were obtained. These genes included a series of endodermal markers such as albumin and alpha-fetoprotein as well as GDNF (glia-derived neurotrophic factor), osteoblast-specific factor-2 (OSF-2)/periostin and fetuin (one of the major serum proteins in the fetus).

CONCLUSION

Tornado extraction has great value in studying genes in the nephrogenic zone of the developing kidney. Since the quality of RNA obtained by this method is excellent, tornado extraction is suitable in combination with other techniques including the subtractive hybridization method and DNA microarray analysis.

Alpha2-HS glycoprotein: a protein in search of a function

In this issue (pp. 305-310) Jun Ren and Amy J. Davidoff author an article entitled 'Alpha2-HS glycoprotein, a putative inhibitor of tyrosine kinase, prevents glucose toxicity associated with cardiomyocyte dysfunction'. The protein responsible for this biological activity has recently come to the forefront of research on the biological activity of plasma proteins.

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.

Fetuin and fetuin messenger RNA in granulosa cells of the rat ovary

The hardening reaction that occurs in the zona pellucida to block polyspermy can be overcome in oocyte cultures in the presence of fetal serum or the serum component fetuin. Fetuin may also prevent precocious zona hardening by inhibiting a ZP2 proteinase released spontaneously by cortical granules during maturation of the oocyte. We demonstrated fetuin mRNA in the rat ovary by reverse transcriptase-polymerase chain reaction and localized it by in situ hybridization. Fetuin mRNA was present in all granulosa cells of growing and large follicles. Immunohistochemical analysis revealed that the fetuin protein was only present in some of the small, growing follicles. In large, healthy follicles, fetuin protein was confined to cumulus cells and granulosa cells bordering the antrum. Fetuin was present in atretic follicles, but the staining pattern differed from that of healthy follicles. The follicular antrum contained a substantial amount of fetuin, but whether granulosa cells secreted it or it originated in the ovarian blood supply could not be confirmed. We concluded that at least a portion of the fetuin is produced by granulosa cells of growing and large follicles, suggesting that fetuin may function in a paracrine manner to maintain the zona pellucida in a penetrable state for fertilization.

Fetuin in the developing neocortex of the rat: distribution and origin

Immunocytochemical distribution of the fetal protein fetuin in the neocortex of developing rat brain and the presence of its mRNA, as detected by using reverse transcriptase-polymerase chain reaction analysis, was studied in fetuses at embryonic day 15 (E15) through E22, in neonates at postnatal day 0 (P0) through P20, and in adults. Quantitative estimates of fetuin in cerebrospinal fluid (CSF) and plasma were obtained over the same period. Exogenous (bovine) fetuin injected intraperitoneally into fetal and postnatal rats was used to study the uptake of fetuin into CSF and brain and its distribution compared with endogenous fetuin; bovine albumin was used as a control. Fetuin was identified immunocytochemically in the cortical plate and subplate cells of the developing neocortex. In the rat fetus, fetuin first was apparent at E17, mainly in cell processes, but a few subplate cells also were positive. By E18, there was strong staining in subplate neurons and in inner cells of the cortical plate. At E21, these inner cells of the cortical plate were beginning to differentiate into layer VI neurons, many of which were positive for fetuin. By P0-P1, more layer VI neurons and some layer V neurons had become positive for fetuin. Fetuin immunoreactivity generally was weaker at P1, and, by P2-P3, it had disappeared from all of the layers of the developing neocortex. Bovine fetuin (but not albumin), probably taken up through CSF over the neocortical dorsal surface, had a cytoplasmic distribution; endogenous rat fetuin was both cytoplasmic and membrane bound. Thus, much of this fetuin can be accounted for by uptake, although the presence of fetuin mRNA indicates that in situ synthesis may also contribute.

Activation of integrin alpha(V)beta(3) regulates cell adhesion and migration to bone sialoprotein

alpha(V)beta(3), a broadly distributed member of the integrin family of adhesion receptors, has been implicated in a variety of physiological and pathophysiological events, including control of bone density, angiogenesis, apoptosis, tumor growth, and metastasis. Recently, it has been shown that activation of alpha(V)beta(3), its transition from a low- to a high-affinity/avidity state, influences its recognition of certain ligands. Bone sialoprotein (BSP) is recognized as an important ligand for alpha(V)beta(3) in processes ranging from bone formation to the homing of metastatic tumor cells. Here, the influence of alpha(V)beta(3) activation on the adhesion and migration of relevant cells to BSP has been examined. Stimulation of lymphoblastoid, osteoblastoid, and human umbilical vein endothelial cells (HUVEC) with PMA or Mn(2+) markedly enhanced alpha(V)beta(3)-dependent adhesion to BSP. alpha(V)beta(3)-mediated migration of HUVEC or osteoblastic cells to BSP was substantially enhanced by stimulation, demonstrating that alpha(V)beta(3) activation enhances both adhesive and migratory responses. However, adhesion and/or migration of certain tumor cell lines, including M21 melanoma and MDA MB435 and SKBR3 breast carcinoma cell lines, to BSP was constitutively high and was not augmented by alpha(V)beta(3)-activating stimuli. Inhibitors of the intracellular signaling molecules, phosphatidylinositol 3-kinase with wortmannin, hsp90-dependent kinases with geldanamycin, and calpain with calpeptin, but not MAPKK with PD98059, reduced the high spontaneous adhesion and migration of the M21 cells to BSP, consistent with the constitutive activation of the receptor on these tumor cells. These results indicate that the activation state of alpha(V)beta(3) can regulate cell migration and adhesion to BSP and, by extension, to other ligands of this receptor. The constitutive activation of alpha(V)beta(3) on neoplastic cells may contribute to tumor growth and metastatic potential.

Effects of calcitonin and parathyroid hormone on calcification of primary cultures of chicken growth plate chondrocytes

Few studies have been directed toward elucidating the action of calcitonin (CT) and parathyroid hormone (PTH) on growth plate chondrocytes, cells directly involved in longitudinal bone growth and provisional calcification. In this study, primary cultures of avian growth plate chondrocytes that calcify without the supplement of beta-glycerophosphate were used to investigate the effects of synthetic human CT and 1-34 bovine PTH on (1) cell division and growth; (2) the deposition of Ca2+ and inorganic phosphate (Pi); (3) the activity of alkaline phosphatase (AP), an enzyme long associated with the mineralization process; (4) the levels of proteoglycans; and (5) the synthesis of collagens. Added continually to preconfluent cultures from day 6 until harvest, CT (1-30 nM) and PTH (0.1-1.0 nM) increased mineral deposition; the maximal increase was seen between days 18-21 at 10 nM CT (175-260%) and 0.5 nM PTH (approximately 170-280%), both p < 0.001. CT had no significant effect on cellular protein, or AP-specific activity, whereas PTH increased cellular protein, DNA, proteoglycan, and collagen content of the cultures in a dosage-dependent manner. AP activity and levels of Type II and X collagens and fibronectin in the culture medium showed a biphasic response to PTH; maximal increases were seen at 0.5 nM between days 15-18. Longer exposure (days 21-27) to PTH at higher levels (5-10 nM) caused a marked decreased in AP activity but a lesser decrease in the collagens. These results indicate that CT and PTH can act directly on chondrocytes to stimulate mineralization, but that PTH specifically stimulated cell division and synthesis of cellular and extracellular proteins by growth plate chondrocytes. The implications of these findings with regard to Ca2+ homeostasis and bone formation are discussed.

Effect of phosphorylated rat fetuin on the growth of hepatocytes in primary culture in the presence of human hepatocyte-growth factor. Evidence that phosphorylated fetuin is a natural modulator of hepatocyte-growth factor

Rat fetuin, a counterpart of human alpha 2-HS glycoprotein and bovine fetuin, that is synthesized and secreted by hepatocytes is mostly phosphorylated, though rat fetuin isolated from bone matrix does not contain phosphorus. A rat 63-kDa phosphorylated N-glycoprotein (pp63) is the phosphorylated form of rat fetuin and pp63 has been shown to inhibit insulin-receptor tyrosine kinase activity. Therefore, we examined the effect of phosphorylated rat fetuin (phosphofetuin) on DNA synthesis in rat hepatocytes in culture in the presence of human hepatocyte-growth factor (HGF), since the human receptor of HGF, c-Met, is known to contain a tyrosine-kinase domain in its intracellular domain. We found that phosphofetuin from conditioned medium of rat-hepatocyte cultures dose-dependently decreased HGF-stimulated DNA synthesis in hepatocytes, whereas addition of non-phosphorylated rat fetuin had no effect. Addition of anti-(rat fetuin) Ig to the culture medium increased HGF-stimulated DNA synthesis by hepatocytes. Immunoprecipitation and cross-linking experiments showed that phosphofetuin bound to human HGF. We found that phosphofetuin interfered with binding of HGF to its specific receptor(s). These observations suggest that phosphofetuin synthesized by hepatocytes may be a natural modulator of HGF as a chalone, and that regulation of expression of phosphofetuin by growth factors and cytokines may be involved in liver regeneration under inflammatory conditions, such as in hepatitis.

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.

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.

Structure and molecular regulation of bone matrix proteins

The organic matrix of bone contains several protein families, including collagens, proteoglycans, and glycoproteins, all of which may be extensively modified by posttranslational events, such as phosphorylation and sulfation. Many of the glycoproteins contain Arg-Gly-Asp (RGD), the integrin-binding sequence, within their structure, whereas other constituent proteins contain gamma-carboxyglutamic acid. The deposition of bone matrix by cells in the osteoblastic lineage is regulated by extrinsic factors, such as systemic and local growth factors and physical forces, and factors that are intrinsic to the cell, such as position in the cell cycle, maturational stage, and developmental age of the donor. Recent studies of several bone matrix gene promoters have identified cis- and trans-acting elements that are responsible for gene activity, although the precise sequence of regulatory events is not known. Development of in vitro assays, coupled with studies of the appearance of these proteins during development in vivo, provides insight into the functions of these proteins during the various stages of bone metabolism. Potential roles for these proteins include proliferation and maturation of stem cells, formation of matrix scaffolding elaborated by bone-forming cells, modeling, and remodeling. Changes in the functional properties of the extracellular matrix may be involved in a variety of disease processes, including osteoporosis and oral bone loss.

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.

Immunohistochemical detection of an enamel protein-related epitope in rat bone at an early stage of osteogenesis

Monoclonal antibody MI315 was produced against hamster tooth germ homogenate by in vitro immunization. It was found that MI315 reacted with enamel matrix, ameloblasts, and bone matrix at an early stage of osteogenesis. Decalcified tissues of rat femurs and mandibles were examined with MI315 using indirect immunofluorescence. In endochondral ossification of femurs, immunoreactivity was found in bone extracellular matrix (ECM) deposited on the surface of the cartilage core of primary spongiosa, but not in the cartilage core itself. In intramembranous ossification of 0-day-old rat mandibles, intense immunofluorescence was detected in bone ECM and a few young osteocytes, but not in osteoblasts. Immunoreactivity in bone ECM of 2-day-old rats decreased and almost disappeared from bone ECM of 4-day-old rats. Although in nondecalcified sections of 0-day-old rats, negligible immunofluorescence was detected in bone ECM which showed positive staining in decalcified tissues, the immunostaining appeared after decalcification using ethylenediaminetetraacetic acid (EDTA). These results indicate that a substance(s), which had a common epitope with an enamel-derived protein(s), existed in immature bone ECM of both endochondral and intramembranous ossification, and that it might be masked by bone mineral. Monoclonal antibody MI315 is a useful tool to investigate the time- and position-specific changes in osteogenesis and amelogenesis.

Haemonectin, a granulocytic-cell-binding protein, is related to the plasma glycoprotein fetuin

Haemonectin, a protein present in rabbit bone marrow extracellular matrix extracts, has been reported to bind granulocytes in a developmentally regulated manner. We have purified haemonectin from such extracts and determined the partial amino-acid sequence. The sequence obtained shows 60-70% similarity with the sequence of the plasma glycoprotein fetuin from other mammal species. This difference is consistent with the difference between fetuins from different species. We conclude that the rabbit haemonectin molecule is related to fetuin. The similarity between haemonectin and fetuin is reinforced by analysis with Western blots of one- and two-dimensional gels. These show that haemonectin, like fetuin, is present in serum and that migration of haemonectin from serum and extracellular matrix extracts, on two-dimensional gels, co-incides with that of human fetuin (alpha 2HS-glycoprotein) from serum, extracellular matrix extracts and in purified form. Also, antihaemonectin antibodies cross react with human fetuin. These data imply that the rabbit haemonectin molecule is closely related to fetuin, but do not rule out the possibility that these molecules are functionally distinct.

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.