Cloning of matrix Gla protein in a marine cartilaginous fish, Prionace glauca: preferential protein accumulation in skeletal and vascular systems.

Matrix Gla protein (MGP) belongs to the family of vitamin K dependent, Gla containing proteins and, in mammals, birds and Xenopus, its mRNA has been previously detected in bone, cartilage and soft tissue extracts, while the accumulation of the protein was found mainly in calcified tissues. More recently, the MGP gene expression was also studied in marine teleost fish where it was found to be associated with chondrocytes, smooth muscle and endothelial cells. To date no information is available on the sites of MGP expression or accumulation in cartilaginous fishes that diverged from osteichthyans, a group that includes mammals, over 400 million years ago. The main objectives of this work were to study the sites of MGP gene expression and protein accumulation by means of in situ hybridization and immunohistochemistry. MGP mRNA and protein were localized as expected not only in cartilage from branchial arches and vertebra but also in the endothelia of the vascular system as well as in the tubular renal endothelium. The accumulation of MGP in non mineralized soft tissues was unexpected and suggests differences in localization or regulation of this protein in shark soft tissues compared to tetrapods and teleosts. Our results also corroborate the hypothesis that in Prionace glauca, as previously shown in mammals, the MGP protein probably also acts as a calcification inhibitor, protecting soft tissues from abnormal and ectopic calcification.