Collagen/annexin V interactions regulate chondrocyte mineralization

Physiological mineralization in growth plate cartilage is highly regulated and restricted to terminally differentiated chondrocytes. Because mineralization occurs in the extracellular matrix, we asked whether major extracellular matrix components (collagens) of growth plate cartilage are directly involved in regulating the mineralization process. Our findings show that types II and X collagen interacted with cell surfaceexpressed annexin V. These interactions led to a stimulation of annexin Vmediated Ca2+ influx resulting in an increased intracellular Ca2+ concentration, [Ca2+](i), and ultimately increased alkaline phosphatase activity and mineralization of growth plate chondrocytes. Consequently, stimulation of these interactions (ascorbate to stimulate collagen synthesis, culturing cells on type II collagencoated dishes, or overexpression of fulllength annexin V) resulted in increase of [Ca2+](i), alkaline phosphatase activity, and mineralization of growth plate chondrocytes, whereas inhibition of these interactions (3,4dehydroLproline to inhibit collagen secretion, K201, a specific annexin channel blocker, overexpression of N terminusdeleted mutant annexin V that does not bind to type II collagen and shows reduced Ca2+ channel activities) decreased [Ca2+](i), alkaline phosphatase activity, and mineralization. In conclusion, the interactions between collagen and annexin V regulate mineralization of growth plate cartilage. Because annexin V is upregulated during pathological mineralization events of articular cartilage, it is possible that these interactions also regulate pathological mineralization.