Background: Inactivating mutations in matrix Gla protein (MGP) lead to Keutel syndrome, a rare disease hallmarked by ectopic calcification of cartilage and vascular tissues. Although MGP 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 (MgpS3mut/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, MgpS3mut/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 on a regular diet, they are needed to prevent phosphate-induced arterial elastocalcinosis.
Keywords: CRISPR-Cas systems; Keutel syndrome; matrix Gla protein; phosphorylation; posttranslational modification; vascular calcification.