Matrix Gla protein gene expression and protein accumulation colocalize with cartilage distribution during development of the teleost fish Sparus aurata.

Matrix Gla protein (MGP) is a member of the family of extracellular mineral-binding Gla proteins, expressed in several tissues with high accumulation in bone and cartilage. Although the precise molecular mechanism of action of this protein remains unknown, all available evidence indicates that MGP plays a role as an inhibitor of mineralization. We investigated the sites of gene expression and protein accumulation of MGP throughout development of the bony fish Sparus aurata, by in situ hybridization, Northern and RT-PCR Southern hybridization, and immunohistochemistry. The results obtained were compared with the patterns of developmental appearance of cartilaginous and mineralized structures in this species, identified by histological techniques and by detection of mRNA presence and protein accumulation of osteocalcin (Bone Gla protein), a marker for osteoblasts known to accumulate in bone mineralized extracellular matrix. The expression of MGP mRNA was first detected at 2 days posthatching (dph) by Northern analysis, RT-PCR amplification, and in situ hybridization, and thereafter continuously detected at various levels of intensity, until 130 dph. In situ hybridization analysis performed in parallel with immunohistochemistry indicated that until ca. 45 dph, the MGP gene was highly expressed in a number of different tissues including skull, jaw, neural and hemal arches, and heart and the protein accumulated in cartilaginous tissues. At 85 dph, a stage when most skeletal structures are mineralized, MGP gene expression and protein accumulation were restricted to the remaining cartilaginous structures, whereas osteocalcin gene expression and protein accumulation were localized in most mineralized structures. MGP gene expression was also detected in heart and kidney, although in situ hybridization only detected MGP mRNA in heart, located in the arterial bulbus and not in the cardiac muscle. Our results are in agreement with those recently described for MGP localization in adult tissues of another teleost fish, as well as available data from higher vertebrates, strengthening the hypothesis of a conserved function for MGP from teleost fish to human, a period of more than 200 million years of evolution. In addition, Sparus aurata, a marine teleost fish routinely grown in captivity, appears to be a good model to further analyze MGP gene expression and regulation.