Relationships between bone protein and mineral in developing porcine long bone and calvaria

Several proteins in the bone matrix have been implicated in the regulation of mineral crystal formation and growth. To investigate the relationships between these proteins and the mineral phase at various stages of mineral maturation, fetal porcine calvariae and long bones were fragmented and the particles (20 microm) separated by density gradient sedimentation into fractions of increasing density (1.8 to >2.2 g/cm3). Samples from each fraction were analyzed by X-ray diffraction to obtain the average crystal size/strain and chemical composition. Other samples were sequentially extracted, first with 4.0 mol/L guanidium hydrochloride (GuHCl) (G1), then with 0.5 mol/L ethylene-diamine tetraacetic acid (EDTA) (E), and again with 4.0 mol/L Gu-HCI (G2), for analysis of proteins in different tissue compartments. Based on the mineral density distribution and crystal size, fetal porcine bone protein content was determined for tissue residue and each extract and the protein composition analyzed by sodium dodecyl-polyacrylamide gel electrophoresis (SDS-PAGE). Although the insoluble organic matrix decreased with mineral density the collagen and protein content remained fairly constant, representing approximately 10% of the tissue weight, except in the highest density fraction. Whereas the total extractable protein, representing predominantly noncollagenous proteins, did not show density-related differences, differences were observed for individual proteins on SDS-PAGE. Consistent with their presence in osteoid, the content of bone sialoprotein (BSP), tyrosine-rich acidic matrix protein (TRAMP), and a series of small proteins with cell attachment properties in the G1 extract decreased with mineral density, whereas TRAMP and BSP were increased in G2 extracts. Mineral-associated proteins, including alpha2HS-glycoprotein, BSP, osteopontin (OPN), and osteocalcin, increased with mineral density, whereas secreted protein acidic and rich in cysteine (SPARC)/osteonectin, and some minor proteins, appeared to decrease. Differences of individual proteins within and between the calvarial and long bones could be related to the role of these proteins in the formation and maturation of hydroxyapatite crystals. Collectively, these studies demonstrate mineral density-associated changes in protein composition that reflect a rapid maturation of mineral crystals in embryonic porcine bones.