Regulated Production of Mineralization-Competent Matrix Vesicles by Terminally Differentiated Chondrocytes

摘要

Biomineralization is a highly regulated process which is under strict cellular control. In this study, we show that treatment of hypertrophic chondrocytes with retinoic acid (RA) led to terminal differentiation of these cells and the release of matrix vesicles (MV), which initiate the mineralization process. These vesicles contain high amounts of annexins II, V and VI, and alkaline phosphatase activity. The annexins form Ca~(2+) channels in these vesicles, enabling the influx of Ca~(2+) into the vesicles and the formation of the first crystal phase inside the vesicle lumen. RA-treatment of chondrocyte cultures led to a 3-fold increase in the cytosolic calcium concentration, followed by a relocation of annexins II, V, and VI, which require Ca~(2+) to bind to phospholipids, from the cytoplasm to the plasma membrane, and the release of annexin-containing MV. Chelation of cytosolic calcium with BAPTA-2AM led to significant decrease of mineralization in RA-treated cultures, and to a reduction of the amount of annexins and alkaline phosphatase activity in MV. In addition, these vesicles were not able to take up Ca~(2+). In conclusion, changes in the concentration of cytosolic calcium regulate the release of mineralization-competent MV from the plasma membrane of terminally differentiated chondrocytes and subsequent mineralization.