cDNA and deduced amino acid sequence of mouse matrix gla protein: one of five glutamic acid residues potentially modified to gla is not conserved in the mouse sequence

Characteristics and performance of an immunosorbent assay for human matrix Gla-protein

BACKGROUND

Matrix gammacarboxyglutamate (Gla)-protein (MGP) is a strong inhibitor of soft tissue calcification and is mainly produced by chondrocytes and vascular smooth muscle cells (VSMCs). MGP-deficient mice have extensive calcifications of cartilage and arteries leading to osteopenia, fractures and blood vessel ruptures. Promotor polymorphisms resulting in decreased expression levels were found to be associated with an increased risk for cardiovascular disease in humans.

METHODS

Recently, an ELISA-based assay has become available with which MGP may be detected in the circulation. The principle of the test kit is that of a competitive immunoassay using a monoclonal antibody against MGP bound to the microtiter plate.

RESULTS

Here, we report on a critical evaluation of this assay and its potential diagnostic utility in diseases associated with the degeneration of the arterial vessel wall and cartilage. The biochemical performance of the kit is satisfactory, and significant differences were found between a number of patient cohorts and the reference population. Serum MGP concentrations were significantly decreased in patients with angina pectoris and in various cartilage diseases.

CONCLUSIONS

The assay allows comparison of groups and may become a suitable marker for risk assessment or diagnosis in cardiovascular disease and osteoarthritis.

Enhanced expression of Runx2/PEBP2alphaA/CBFA1/AML3 during fracture healing

The cause of the dramatic increase in expression of the osteopontin gene during fracture healing was studied in a mouse experimental model. Semiquantitative reverse transcription-polymerase chain reaction, Northern blotting, and in situ hybridization analysis showed that the enhanced expression took place prior to callus formation. The change in the expression pattern of collagenous and noncollagenous bone matrix proteins in addition to Ets-1 and Runx2, major transcription factors of osteopontin, were examined and compared to that of osteopontin. Although Ets-1 expression showed no significant change during fracture healing, enhanced expression of Runx2 corresponding to that of osteopontin was observed. Furthermore, in situ hybridization demonstrated that osteopontin-expressing cells also express the Runx2 gene. The results indicated the possibility that Runx2 is a major regulator of osteopontin during fracture healing.

Matrix Gla protein in Xenopus laevis: molecular cloning, tissue distribution, and evolutionary considerations

Matrix Gla protein (MGP) belongs to the family of vitamin K-dependent, Gla-containing proteins and in higher vertebrates, is found in the extracellular matrix of mineralized tissues and soft tissues. MGP synthesis is highly regulated at the transcription and posttranscription levels and is now known to be involved in the regulation of extracellular matrix calcification and maintenance of cartilage and soft tissue integrity during growth and development. However, its mode of action at the molecular level remains unknown. Because there is a large degree of conservation between amino acid sequences of shark and human MGP, the function of MGP probably has been conserved throughout evolution. Given the complexity of the mammalian system, the study of MGP in a lower vertebrate might be advantageous to relate the onset of MGP expression with specific events during development. Toward this goal, MGP was purified from Xenopus long bones and its N-terminal amino acid sequence was determined and used to clone the Xenopus MGP complementary DNA (cDNA) by a mixture of reverse-transcription (RT)- and 5'- rapid amplification of cDNA ends (RACE)-polymerase chain reaction (PCR). MGP messenger RNA (mRNA) was present in all tissues analyzed although predominantly expressed in Xenopus bone and heart and its presence was detected early in development at the onset of chondrocranium development and long before the appearance of the first calcified structures and metamorphosis. These results show that in this system, as in mammals, MGP may be required to delay or prevent mineralization of cartilage and soft tissues during the early stages of development and indicate that Xenopus is an adequate model organism to further study MGP function during growth and development.

Coordinated expression of matrix Gla protein is required during endochondral ossification for chondrocyte survival

Matrix Gla protein (MGP) is a 14-kD extracellular matrix protein of the mineral-binding Gla protein family. Studies of MGP-deficient mice suggest that MGP is an inhibitor of extracellular matrix calcification in arteries and the epiphyseal growth plate. In the mammalian growth plate, MGP is expressed by proliferative and late hypertrophic chondrocytes, but not by the intervening chondrocytes. To investigate the functional significance of this biphasic expression pattern, we used the ATDC5 mouse chondrogenic cell line. We found that after induction of the cell line with insulin, the differentiating chondrocytes express MGP in a stage-specific biphasic manner as in vivo. Treatment of the ATDC5 cultures with MGP antiserum during the proliferative phase leads to their apoptosis before maturation, whereas treatment during the hypertrophic phase has no effect on chondrocyte viability or mineralization. After stable transfection of ATDC5 cells with inducible sense or antisense MGP cDNA constructs, we found that overexpression of MGP in maturing chondrocytes and underexpression of MGP in proliferative and hypertrophic chondrocytes induced apoptosis. However, overexpression of MGP during the hypertrophic phase has no effect on chondrocyte viability, but it does reduce mineralization. This work suggests that coordinated levels of MGP are required for chondrocyte differentiation and matrix mineralization.

Total chemical synthesis of human matrix Gla protein

Human matrix Gla protein (MGP) is a vitamin K-dependent extracellular matrix protein that binds Ca2+ ions and that is involved in the prevention of vascular calcification. MGP is a 10.6-kD protein (84 amino acids) containing five gamma-carboxyglutamic acid (Gla) residues and one disulfide bond. Studies of the mechanism by which MGP prevents calcification of the arterial media are hampered by the low solubility of the protein (<10 microg/mL). Because of solubility problems, processing of a recombinantly expressed MGP-fusion protein chimera to obtain MGP was unsuccessful. Here we describe the total chemical synthesis of MGP by tBoc solid-phase peptide synthesis (SPPS) and native chemical ligation. Peptide Tyr1-Ala53 was synthesized on a derivatized resin yielding a C-terminal thioester group. Peptide Cys54-Lys84 was synthesized on Lys-PAM resin yielding a C-terminal carboxylic acid. Subsequent native chemical ligation of the two peptides resulted in the formation of a native peptide bond between Ala53 and Cys54. Folding of the 1-84-polypeptide chain in 3 M guanidine (pH 8) resulted in a decrease of molecular mass from 10,605 to 10,603 (ESI-MS), representing the loss of two protons because of the formation of the Cys54-Cys60 internal disulfide bond. Like native MGP, synthetic MGP had the same low solubility when brought into aqueous buffer solutions with physiological salt concentrations, confirming its native like structure. However, the solubility of MGP markedly increased in borate buffer at pH 7.4 in the absence of sodium chloride. Ca2+-binding to MGP was confirmed by analytical HPLC, on which the retention time of MGP was reduced in the presence of CaCl2. Circular dichroism studies revealed a sharp increase in alpha-helicity at 0.2 mM CaCl2 that may explain the Ca2+-dependent shift in high-pressure liquid chromatography (HPLC)-retention time of MGP. In conclusion, facile and efficient chemical synthesis in combination with native chemical ligation yielded MGP preparations that can aid in unraveling the mechanism by which MGP prevents vascular calcification.

In vitro metabolic and respiratory acidosis selectively inhibit osteoblastic matrix gene expression

Clinically, a decrease in blood pH may be due to either a reduction in bicarbonate concentration ([HCO(-)(3)], metabolic acidosis) or an increase in PCO(2) (respiratory acidosis). In mammals, metabolic acidosis induces a far greater increase in urine calcium excretion than respiratory acidosis. In cultured bone, metabolic acidosis induces a marked increase in calcium efflux and a decrease in osteoblastic collagen synthesis, whereas isohydric respiratory acidosis has little effect on either parameter. We have shown that metabolic acidosis prevents the normal developmental increase in the expression of RNA for matrix Gla protein and osteopontin in chronic cultures of primary murine calvarial bone cells (predominantly osteoblasts) but does not alter expression of osteonectin. To compare the effects of isohydric metabolic and respiratory acidosis on expression of these genes, bone cell cultures were incubated in medium at pH approximately 7.2 to model metabolic ([HCO(-)(3)], approximately 13 mM) or respiratory (PCO(2), approximately 80 mmHg) acidosis or at pH approximately 7.4 as a control. Cells were sampled at weeks 4, 5, and 6 to assess specific RNA content. At all time periods studied, both metabolic and respiratory acidosis inhibited the expression of RNA for matrix Gla protein and osteopontin to a similar extent, whereas there was no change in osteonectin expression. In contrast to the significant difference in the effects of metabolic and respiratory acidosis on bone calcium efflux and osteoblastic collagen synthesis, these two forms of acidosis have a similar effect on osteoblastic RNA expression of both matrix Gla protein and osteopontin. Thus, although several aspects of bone cell function are dependent on the type of acidosis, expression of these two matrix genes appears to be regulated by extracellular pH, independently of the type of acidosis.

Chronic metabolic acidosis reversibly inhibits extracellular matrix gene expression in mouse osteoblasts

Chronic metabolic acidosis induces net calcium efflux from bone mineral through an increase in osteoclastic resorption and a decrease in osteoblastic matrix deposition and mineralization. To determine the effects of chronic metabolic acidosis on the expression of genes necessary for mineralization, we grew primary bone cells, which are principally osteoblasts, to confluence in neutral pH (7.5) medium and then switched the cells either to a neutral pH or to an acidic pH (7.1) differentiation medium. Cells were harvested for RNA at 4- to 7-day intervals for up to 44 days. By 36 days, there was extensive bone nodule formation and mineralization in cells cultured in neutral medium; however, there was a substantial decrease in nodule formation and mineralization in cells cultured in acidic medium. There was a marked increase in matrix Gla protein RNA and an increase in osteopontin RNA in neutral cultures; however, acidic medium almost completely prevented any increase. In contrast, RNA levels for osteonectin and transforming growth factor-beta1 were not altered by chronic acidosis. Additional cells were incubated in acid differentiation medium for 1, 2, or 3 wk and then transferred to neutral medium; in each case, there was recovery of matrix Gla protein RNA and osteopontin RNA expression. Still other cells were incubated in neutral differentiation medium for 1, 2, or 3 wk and then transferred to acid medium; in each case there was inhibition of matrix Gla protein RNA and osteopontin RNA expression. Thus metabolic acidosis appears to specifically inhibit RNA accumulation of certain genes whose products may be essential for formation of mature bone matrix.

Molecular cloning of avian matrix Gla protein

Matrix Gla protein plays an essential role in preventing the calcification of blood vessel walls, cartilage and other tissues. We report here the primary structure of chicken matrix Gla protein as deduced from the cDNA sequence. The avian protein exhibited the characteristic motifs previously identified in the mammalian proteins, but its amino acid sequence shared only 51-56% identity with the latter proteins. Moreover, a region proposed to function as binding site for gamma-carboxylase in the mammalian proteins was poorly conserved in the chicken protein. Our sequence data should be helpful in the design of mutational analyses which are intended to characterize functional interactions of matrix Gla proteins with other proteins.

Isolation and characterization of a novel secretory protein, stromal cell-derived factor-2 (SDF-2) using the signal sequence trap method

With use of the signal sequence trap method, we isolated a cDNA encoding a novel secretory protein, SDF-2, from the mouse stromal cell line, ST2. The human homologue of SDF-2 was also isolated. The amino acid (aa) sequences deduced from both the clones were conserved more than 92%. The chromosomal localization of the human SDF-2 gene was mapped to 17q11.2. The aa sequence of SDF-2 shows similarity to those of yeast dolichyl phosphate-D-mannose:protein mannosyltransferases, Pmt1p [Strahl-Bolsinger et al. (1993) Proc. Natl. Acad. Sci. USA 90, 8164-8168] and Pmt2p [Lussier et al. (1995) J. Biol. Chem. 270, 2770-2775], whose activities have not been detected in higher eukaryotes.

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

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

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.

Conserved phosphorylation of serines in the Ser-X-Glu/Ser(P) sequences of the vitamin K-dependent matrix Gla protein from shark, lamb, rat, cow, and human

The present studies demonstrate that matrix Gla protein (MGP), a 10-kDa vitamin K-dependent protein, is phosphorylated at 3 serine residues near its N-terminus. Phosphoserine was identified at residues 3, 6, and 9 of bovine, human, rat, and lamb MGP by N-terminal protein sequencing. All 3 modified serines are in tandemly repeated Ser-X-Glu sequences. Two of the serines phosphorylated in shark MGP, residues 2 and 5, also have glutamate residues in the n + 2 position in tandemly repeated Ser-X-Glu sequences, whereas the third, shark residue 3, would acquire an acidic phosphoserine in the n + 2 position upon phosphorylation of serine 5. The recognition motif found for MGP phosphorylation, Ser-X-Glu/Ser(P), has been seen previously in milk caseins, salivary proteins, and a number of regulatory peptides. A review of the literature has revealed an intriguing dichotomy in the extent of serine phosphorylation among secreted proteins that are phosphorylated at Ser-X-Glu/Ser(P) sequences. Those phosphoproteins secreted into milk or saliva are fully phosphorylated at each target serine, whereas phosphoproteins secreted into the extracellular environment of cells are partially phosphorylated at target serine residues, as we show here for MGP and others have shown for regulatory peptides and the insulin-like growth factor binding protein 1. We propose that the extent of serine phosphorylation regulates the activity of proteins secreted into the extracellular environment of cells, and that partial phosphorylation can therefore be explained by the need to ensure that the phosphoprotein be poised to gain or lose activity with regulated changes in phosphorylation status.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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