Prevention of Arterial Elastocalcinosis: Differential Roles of the Conserved Glutamic Acid and Serine Residues of Matrix Gla Protein

Abstract

Background: Mutations in matrix Gla protein gene (MGP ) lead to Keutel syndrome, a rare disease hallmarked by ectopic calcification of cartilage and vascular tissues. Although MGP (matrix Gla protein) acts as a strong inhibitor of arterial elastic lamina calcification (elastocalcinosis), its mode of action is unknown. Two sets of conserved residues undergoing posttranslational modifications—4 glutamic acid residues, which are γ-carboxylated by gamma-glutamyl carboxylase; and 3 serine residues, which are phosphorylated by yet unknown kinase(s)—are thought to be essential for MGP’s function. Methods: We pursued a genetic approach to study the roles of MGP’s conserved residues. First, a transgenic line ( SM22a-GlamutMgp ) expressing a mutant form of MGP, in which the conserved glutamic acid residues were mutated to alanine, was generated. The transgene was introduced to Mgp − /− mice to generate a compound mutant, which produced the mutated MGP only in the vascular tissues. We generated a second mouse model ( Mgp S3mut/S3mut ) to mutate MGP’s conserved serine residues to alanine. The initiation and progression of vascular calcification in these models were analyzed by alizarin red staining, histology, and micro-computed tomography imaging. Results: On a regular diet, the arterial walls in the Mgp −/− ; SM22α-GlamutMgp mice were not calcified. However, on a high phosphorus diet, these mice showed wide-spread arterial calcification. In contrast, Mgp S3mut/S3mut mice on a regular diet recapitulated arterial calcification traits of Mgp −/− mice, although with lesser severity. Conclusions: For the first time, we show here that MGP’s conserved serine residues are indispensable for its antimineralization function in the arterial tissues. Although the conserved glutamic acid residues are not essential for this function, they are needed to prevent phosphate-induced arterial elastocalcinosis.

Should vitamin K antagonists ever be used in ESRD?

“…Thus, in this population that is already at risk for vascular calcification, the effect of warfarin is quantitatively very large and likely to lead to substantial morbidity. The greater effect of warfarin in ESRD is consistent with recent studies in which genetic inactivation of MGP did not result in vascular calcification unless the mice were fed a high phosphate diet, 18 presumably mimicking the hyperphosphatemia that is common in ESRD.…”

Cardiovascular Calcification Heterogeneity in Chronic Kidney Disease

“…173 Both, the conserved glutamic acid residues and N-terminal conserved serine residues contribute to its antimineralization function. 174,175 Vitamin K is essential for the activation and mineral inhibitory effects of MGP, and individuals with CKD often exhibit vitamin K deficiency, 176 likely contributing to the decreased active MGP in these patients and subsequent vascular calcification. The elevated thromboembolic risk in patients with CKD requires anticoagulation.…”

“… 173 Both, the conserved glutamic acid residues and N-terminal conserved serine residues contribute to its antimineralization function. 174 , 175 …”

Roles of vitamin K‑dependent protein in biomineralization (Review)

“…In addition, the expression of MGP in chondrocytes is regulated by extracellular inorganic phosphorus, which may represent a feedback regulatory mechanism to inhibit inappropriate calcification in cartilage ( 84 ). Of note, ectopic expression of MGP in human osteosarcoma (OS) cells significantly increased cancer cell metastasis to the lung in patients with OS, which may lead to poor prognosis ( 88 ). In general, MGP maintains bone health by affecting cartilage mineralization, osteoblast function and osteoclast activity.…”

“…However, a recent study suggested something different. Parashar et al ( 88 ) constructed MGP mutant mice and found that the serine residues at the MGP N-terminus and glutamate at the C-terminus had a synergistic effect to inhibit vascular tissue calcification; however, the serine residues had a more critical role. In addition, a study used immunohistochemistry to confirm that cMGP and pMGP have anti-calcification effects in veins ( 89 ).…”