The roles of annexins and types II and X collagen in matrix vesicle-mediated mineralization of growth plate cartilage

Annexins II, V, and VI are major components of matrix vesicles (MV), i.e. particles that have the critical role of initiating the mineralization process in skeletal tissues. Furthermore, types II and X collagen are associated with MV, and these interactions mediated by annexin V stimulate Ca²⁺ uptake and mineralization of MV. However, the exact roles of annexin II, V, and VI and the interaction between annexin V and types II and X collagen in MV function and initiation of mineralization are not well understood. In this study, we demonstrate that annexin II, V, or VI mediate Ca²⁺ influx into phosphatidylserine (PS)-enriched liposomes, liposomes containing lipids extracted from authentic MV, and intact authentic MV. The annexin Ca²⁺ channel blocker, K-201, not only inhibited Ca²⁺ influx into fura-2-loaded PSenriched liposomes mediated by annex-in II, V, or VI, but also inhibited Ca²⁺ uptake by authentic MV. Types II and X collagen only bound to liposomes in the presence of annexin V but not in the presence of annexin II or VI. Binding of these collagens to annexin V stimulated its Ca²⁺ channel activities, leading to an increased Ca²⁺ influx into the liposomes. These findings indicate that the formation of annexin II, V, and VI Ca²⁺ channels in MV together with stimulation of annexin V channel activity by collagen (types II and X) binding can explain how MV are able to rapidly take up Ca²⁺ and initiate the formation of the first crystal phase.