Stimulation of calcification of growth plate cartilage matrix vesicles by binding to type II and X collagens

Matrix vesicles (MV), microstructures which rapidly accumulate Ca²⁺ and induce mineral formation in vitro, are linked to type II and X collagens and proteoglycans in the hypertrophic cartilage. However, the roles of these matrix proteins on MV function are not known. This led us to investigate the influence of type II and X collagen binding on Ca²⁺ uptake by MV. MV isolated from chicken growth plate cartilage were treated with pure bacterial collagenase and 1 M NaCl in synthetic cartilage lymph to selectively and completely remove associated type II and X collagens. Uptake of ⁴⁵Ca²⁺ by these collagen-depleted vesicles was markedly reduced. Further treatment with detergent, which disrupted the membrane, restored Ca²⁺ uptake, indicating that the vesicle membrane structure and the nucleational core inside the vesicle lumen were still intact after the collagenase and 1 M NaCl treatments. Readdition of either native type II or X collagen to the collagenase, 1 M NaCl-treated MV stimulated their Ca²⁺ uptake to levels similar to those of untreated vesicles. Pepsin-treated type II and X collagens were less effective in stimulating Ca²⁺ uptake, indicating that non-triple helical domains of these collagens were involved. The pepsin treatment of these collagens also decreased their binding to annexin V (anchorin CII), one of three annexins found in MV, suggesting that annexin V is involved in mediating the binding of type II and X collagens to the MV surface. Furthermore, treatment of collagenase, 1 M NaCl-treated MV with chymotrypsin, which damaged annexin V as well as many other MV proteins, prevented the stimulation of Ca²⁺ uptake by these collagens. Thus, the interaction between type II and X collagens with MV activates the influx of Ca²⁺ into MV and may play an important role in calcification of the vesicles.