Role of the progressive ankylosis gene (ank) in cartilage mineralization

Mineralization of growth plate cartilage is a critical event during endochondral bone formation, which allows replacement of cartilage by bone. Ankylosis protein (Ank), which transports intracellular inorganic pyrophosphate (PP(i)) to the extracellular milieu, is expressed by hypertrophic and, especially highly, by terminally differentiated mineralizing growth plate chondrocytes. Blocking Ank transport activity or ank expression in terminally differentiated mineralizing growth plate chondrocytes led to increases of intra- and extracellular PP(i) concentrations, decreases of alkaline phosphatase (APase) expression and activity, and inhibition of mineralization, whereas treatment of these cells with the APase inhibitor levamisole led to an increase of extracellular PP(i) concentration and inhibition of mineralization. Ank-overexpressing hypertrophic nonmineralizing growth plate chondrocytes showed decreased intra- and extracellular PP(i) levels; increased mineralization-related gene expression of APase, type I collagen, and osteocalcin; increased APase activity; and mineralization. Treatment of Ank-expressing growth plate chondrocytes with a phosphate transport blocker (phosphonoformic acid [PFA]) inhibited uptake of inorganic phosphate (P(i)) and gene expression of the type III Na(+)/P(i) cotransporters Pit-1 and Pit-2. Furthermore, PFA or levamisole treatment of Ank-overexpressing hypertrophic chondrocytes inhibited APase expression and activity and subsequent mineralization. In conclusion, increased Ank activity results in elevated intracellular PP(i) transport to the extracellular milieu, initial hydrolysis of PP(i) to P(i), P(i)-mediated upregulation of APase gene expression and activity, further hydrolysis and removal of the mineralization inhibitor PP(i), and subsequent mineralization.