Extracellular Matrix with Defective Collagen Cross-linking Affects the Differentiation of Bone Cells

Fibrillar type I collagen, the predominant organic component in bone, is stabilized by lysyl oxidase (LOX)-initiated covalent intermolecular cross-linking, an important determinant of bone quality. However, the impact of collagen cross-linking on the activity of bone cells and subsequent tissue remodeling is not well understood. In this study, we investigated the effect of collagen cross-linking on bone cellular activities employing a loss-of-function approach, using a potent LOX inhibitor, β-aminopropionitrile (BAPN). Osteoblastic cells (MC3T3-E1) were cultured for 2 weeks in the presence of 0-2 mM BAPN to obtain low cross-linked collagen matrices. The addition of BAPN to the cultures diminished collagen cross-links in a dose-dependent manner and, at 1 mM level, none of the major cross-links were detected without affecting collagen production. After the removal of cellular components from these cultures, MC3T3-E1, osteoclasts (RAW264.7), or mouse primary bone marrow-derived stromal cells (BMSCs) were seeded. MC3T3-E1 cells grown on low cross-link matrices showed increased alkaline phosphatase (ALP) activity. The number of multinucleate tartrate-resistant acid phosphatase (TRAP)-positive cells increased in RAW264.7 cells. Initial adhesion, proliferation, and ALP activity of BMSCs also increased. In the animal experiments, 4-week-old C57BL/6 mice were fed with BAPN-containing diet for 8 weeks. At this point, biochemical analysis of bone demonstrated that collagen cross-links decreased without affecting collagen content. Then, the diet was changed to a control diet to minimize the direct effect of BAPN. At 2 and 4 weeks after the change, histological samples were prepared. Histological examination of femur samples at 4 weeks showed a significant increase in the number of bone surface osteoblasts, while the bone volume and surface osteoclast numbers were not significantly affected. These results clearly demonstrated that the extent of collagen cross-linking of bone matrix affected the differentiation of bone cells, underscoring the importance of collagen cross-linking in the regulation of cell behaviors and tissue remodeling in bone. Characterization of collagen cross-linking in bone may be beneficial to obtain insight into not only bone mechanical property, but also bone cellular activities.