Keratan sulphate proteoglycans associated with the bone formation of embryonic chick femurs

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The type and sistribution of sulphated proteoglycans (PGs) in the midshaft subperiosteal bone of 15-18-day embryonic chick femurs were studied immunocytochemically and biochemically, using four monoclonal antibodies (MAb 2B6, 3B3, 1B5, and 5D4). These MAb specifically recognize epitopes in chondroitin 4-sulphate (C4-S) and dermatan sulphate (DS) ; chondroitin 6-sulphate (C6-S) and unsulphated chondroitin (C0-S) ; C0-S ; and keratan sulphate (KS) respectively. Immunohistochemistry showed that staining of C4-S, DS, and KS, but not of C6-S and C0-S, was limited to osteoid, the cell surface of osteocytes, and to the walls of osteocytic lacunae and bone canaliculi in 15-18-day embryonic specimens. However, no significant difference in the distribution and intensity of immunostaining was observed in these specimens. Bone proteins were extracted from fresh 18-day embryonic specimens with a three extraction procedure, 4 M guanidine HCl (GdnCl, G-1 extract), 0.4M EDTA (E-ectract), followed by GdnCl (G-2extract), to characterize mineral binding and collagenous matrix associated PGs in E-and G2-extracts respectively. Western blot analysis of E-and G2-extracts demonstrated that chondroitinase ABC-digested PGs with a molecular weight (M_r) approximately of 45,000 containing GAGs predominantly corresponding to C4-S and/or DS, with no detectable C6-S or C0-S present in the mineral and matrix phase, whereas KSPGs having an M_r of approximately 72,000 are only present in the mineral phase. These results indicate that embryonic chick bone contains small PGs having C4-S, DS, and KS chains with preferential localization to osteoid, the cell surface of osteocytes, and to the walls of osteocytic lacunac and bone canaliculi.