In vivo expression of transcripts encoding the Glvr-1 phosphate transporter retrovirus receptor during bone development

In vitro observations suggest that inorganic phosphate (:Pi) transport play s an important functional role in osteogenic tells and in their matrix vesi cles for the initiation of matrix calcification. Recent studies have shown that the type III sodium-dependent Pi transporters, Glvr-1 and Glvr-2, are expressed in human osteoblast-like cells and have suggested I! potential ro le for type III transporters in regulated Pi handling in osteogenic cells, To address the relevance of thee findings in the contest of bone formation iv vi ro and, in particular, in relation to matrix calcification, we invest igated expression of the Glvr-1 transporter by in situ hybridization in dev eloping embryonic murine metatarsals, using human Glvr-1 cDNA as a probe, I n this model of endochondral ossification, expression of transcripts encodi ng Glvr-1 could be detected from day 17 of embryonic development. A hybridi zation signal for Glvr-1 was specifically observed in a subset of hypertrop hic chondrocytes and could not be detected in osteoblasts. The expression o f Glvr-1 mRNA was compared with that of transcripts encoding extracellular matrix proteins. Glvr-1 mRNA expression was confined to a population of ear ly hypertrophic chondrocytes expressing type X collagen and to slightly mor e mature cells that express transcripts encoding osteopontin but lack type X collagen mRNA. No Glvr-1 transcripts were detected in fully differentiate d hypertrophic chondrocytes. This pattern of Glvr-1 mRNA expression was mai ntained throughout embryonic development until after birth. In conclusion, the Glvr-1 phosphate transporter is selectively expressed in a subset of hy pertrophic chondrocytes during endochondral bone formation, in a region whe re matrix mineralization proceeds. This observation represents the first in vivo evidence consistent with a potential role for this phosphate transpor ter ill matrix calcification. (C) 1999 by Elsevier Science Inc. All rights reserved.