Novel Conformation-Specific Antibodies Against Matrix γ-Carboxyglutamic Acid (Gla) Protein

Abstract

Objective-Matrix ␥-carboxyglutamic acid (Gla) protein (MGP), a vitamin K-dependent protein, is a potent in vivo inhibitor of arterial calcification. We hypothesized that low endogenous production of MGP and impaired carboxylation of MGP may contribute to the development or the progression of vascular disease. Methods and Results-Novel conformation-specific antibodies against MGP were used for immunohistochemistry of healthy and sclerotic arteries. In healthy arteries, MGP was mainly displayed around the elastin fibers in the tunica media. The staining colocalized with that for carboxylated MGP, whereas undercarboxylated MGP (ucMGP) was not detected. In atherosclerotic arteries, ucMGP was found in the intima, where it was associated with vesicular structures. In Mönckeberg's sclerosis of the media, ucMGP was localized around all areas of calcification. The results indicate that ucMGP is strongly associated with vascular calcification of different etiologies. In a separate study, serum MGP concentrations in a cohort of 172 subjects who had undergone percutaneous coronary intervention were significantly reduced compared with an apparently healthy population. Key Words: matrix Gla protein (MGP) Ⅲ vitamin K Ⅲ calcification Ⅲ atherosclerosis T he extracellular fluids in the human body contain calcium and phosphate in high concentrations, even exceeding the solubility product for spontaneous precipitation. 1 However, physiological calcification is restricted to bone and teeth, whereas soft tissue calcification is regarded as pathological. Vascular calcification can occur at 3 anatomic sites: the intima where it is associated with atherosclerosis, the tunica media, and the heart valves. Huang et al showed that coronary artery calcification does not significantly affect stability of atheroma, 2 and the involvement of calcium salt accretion on cardiovascular disease is not known yet. However, overall vascular calcification is regarded as one of the major complications of cardiovascular disease and is an independent risk factor for myocardial infarction (MI) and cardiac death. [3][4][5][6] Therefore, prevention of vascular calcification is a prerequisite for human health. Inhibition of calcification is regarded presently as an active process in which a variety of proteins are involved throughout the body. 7 In the vasculature, a major calcification inhibitory factor is matrix ␥-carboxyglutamic acid (Gla) protein (MGP), a vitamin K-dependent protein synthesized by vascular smooth muscle cells (VSMCs). 8,9 Its 5 Gla residues are formed in a post-translational carboxylation reaction in which vitamin K functions as an essential cofactor. 10,11 MGP in its carboxylated form will be designated here as GlaMGP. The presence of the Gla residues is critical for MGP function, and undercarboxylated, inactive species of MGP (designated as GluMGP) are formed during inadequate vitamin K status or as a result of vitamin K antagonists. In animal models, it was demonstrated that impaired MGP synthesis 12,13 as well as treatment with vitamin ...

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

“…For each of the three forms of MGP, the amino acid sequences recognized by the respective monoclonal antibodies corresponded exactly to those of a recent study performed in human arteries [11]. In the vascular wall, the inhibition of γ-carboxylation of Glu residues yielded the undercarboxylation of MGP and the subsequent calcification of the tunica media [10].…”

“…This indicates that the MGP protein present in bone, where calcification is a physiological event, is predominantly in its active form, and demonstrates that, at least locally, there is not a significant vitamin K deficiency. Interestingly, in the intima of atherosclerotic arteries and in Mönckeberg's sclerosis of the media, where pathological calcifications of different etiologies occur, undercarboxylated MGP was almost exclusively localized in sites of calcification, while total and carboxylated MGP were mostly found in the noncalcified areas of the tunica media [11].…”

“…MGP is a protein mainly found in bone, dentine, and cartilage, which contains five vitamin-K-dependent γ-carboxyglutamic acid (Gla) residues that confer it a high affinity for calcium and phosphate ions, and for hydroxyapatite crystals. The presence of Gla residues, therefore, is regarded as critical for the function of MGP, while undercarboxylated MGP, which may result from poor vitamin K status, is considered inactive [10,11]. Therefore, fetuin-A and MGP can inhibit extraosseous and specifically arterial calcifications, as well as bone mineralization [2,12].…”

Intake of Vitamin K Antagonists and Worsening of Cardiac and Vascular Disease: Results From the Population‐Based Gutenberg Health Study

“…Unlike coagulation factors, which are synthesized and carboxylated within the liver, MGP and Gas‐6 are carboxylated within the vasculature 4. Increased levels of undercarboxylated MGP have been associated with vascular calcification 5. The deficiency of active MGP and Gas‐6 provokes cell death, decreased contractility of vascular smooth muscle cells, and accelerated vascular calcification 4, 6.…”

Calcium scores and matrix Gla protein levels: association with vitamin K status

“…In other study populations, with higher cardiovascular risk and more calcification or with renal insufficiency, lower serum ucMGP levels have been found [22]. A possible explanation is that, with progressing calcification most of the ucMGP is bound to the calcium in the vascular wall and is than trapped, ultimately leading to a fall in serum levels [22,31,33,34]. In addition, in the presence of sufficient vitamin K, ucMGP is carboxylated to cMGP giving rise to lower ucMGP levels as is supported by the positive beta in our regression analysis.…”