A novel in vitro culture system for analysis of functional role of phosphate transport in endochondral ossification

In vivo expression of the type III sodium-dependent phosphate transporter ( NaPiT) Glvr-1 during endochondral ossification, suggests a functional role for inorganic phosphate (Pi) transport in cartilage calcification. For furt her analysis of this relationship, an in vitro model of endochondral ossifi cation is required. In this context, we investigated the characteristics of Pi transport in the new chondrogenic cell line ATDC5 in relation to extrac ellular matrix (ECM) formation and mineralization, Pi uptake in ATDC-5 cell s and in isolated matrix vesicles (MVs) is mediated by an Na-dependent Pi t ransporter with a pH dependency characteristic of a type III Pi carrier (lo wer activity at alkaline pH). Northern blot analysis indicated that ATDC-5 cells express Glvr-1 transcripts during the various stages of their maturat ion with a maximal level during the proliferating stage. In isolated MVs, P i transport activity was maximal at day 21, concomitant with the beginning of type X collagen messenger RNA expression. These events preceded the init iation of matrix mineralization, which was apparent at day 25, and then gra dually increased until day 47, This temporal relationship between maximal P i transport activity in MVs and the expression of a marker of mineralizing chondrocytes is compatible with the possible involvement of Pi transport in the ECM calcification observed in ATDC-5 cell cultures. In conclusion, the se observations suggest that ATCD-5 cells in culture represent a promising model for the analysis of a functional role of Pi transport in the initial events of endochondral ossification. (Bone 27:69-74; 2000) (C) 2000 by Else vier Science Inc. All rights reserved.