Differences in matrix composition between calvaria and long bone in mice suggest differences in biomechanical properties and resorption

The mammalian skeleton consists of bones that are formed in two different ways: long bones via endochondral ossification and flat bones via intramembranous ossification. These different formation modes may result in differences in the composition of the two bone types. Using the 2D-difference in gel electrophoresis technique and mass spectrometry, we analyzed the composition of murine mineral-associated proteins of calvaria and long bone. Considerable differences in protein composition were observed. Flat bones (calvariae) contained more soluble collagen (8×), pigment epithelium derived factor (3×) and osteoglycin (4×); whereas long bones expressed more chondrocalcin (3×), thrombospondin- 1 (4×), fetuin (4×), secreted phosphoprotein 24 (3×), and thrombin (7×). Although cystatin motifs containing proteins, such as secreted phosphoprotein 24 and fetuin are highly expressed in long bone, they did not inhibit the activity of the cysteine proteinases cathepsin B and K. The solubility of collagen differed which coincided with differences in collagen crosslinking, long bone containing 3× more (hydroxylysine)-pyridinoline. The degradation of long bone collagen by MMP2 (but not by cathepsin K) was impaired. These differences in collagen crosslinking may explain the differences in the proteolytic pathways osteoclasts use to degrade bone. Our data demonstrate considerable differences in protein composition of flat and long bones and strongly suggest functional differences in formation, resorption, and mechanical properties of these bone types.