Proteoglycans of developing bone.

Abstract

We purified and characterized the bone proteoglycans from fetal calves, growing rats, and human fetuses. The major proteoglycan is part of the mineralized tissue matrix and only 10-20% can be extracted prior to demineralization. This bone proteoglycan is a small glycoconjugate (Mr = 80,000-120,000) containing approximately 20-30% protein and either one or two chondroitin sulfate chains (Mr = 40,000) attached to a relatively monodisperse protein core (Mr = 38,000). "O"-linked and "N"-linked oligosaccharide units are also present. Antibodies directed against the protein core of calf bone proteoglycan do not cross-react with cartilage, skin, corneal, or basement membrane proteoglycans in immunoassays and have minimal cross-reactivity with scleral proteoglycans. Quantitative immunoassays and indirect immunofluorescence were used to show that the molecule is localized to forming bone trabeculae and dentin, but not to any other tissue. Osteoblasts and osteoprogenitor cells adjacent to areas undergoing rapid osteogenesis also contain this small proteoglycan. A second proteoglycan (Mr approximately equal to 1,000,000) was extracted from newly forming bone prior to demineralization. This large proteoglycan, which was isolated from the cartilage-free areas of developing intramembranous bone, has a protein core similar to that of the cartilage aggregating proteoglycan and cross-reacts with antisera raised against these cartilage proteoglycans but not with the small mineral-entrapped proteoglycan. It contains larger (Mr = 40,000) and fewer chondroitin sulfate chains than its cartilage-derived analogue, and is localized to the soft connective tissue mesenchyme lying between growing bone trabeculae. More fully formed compact bone did not contain detectable quantities of this proteoglycan.