Multiple Na⁺-P_i Cotransporters in Rat Odontoblastic and Osteoblastic Cells

Transcellular ion transport systems play a cardinal role during osteogenesis and dentinogenesis, as a considerable bulk transport of the ions constituting the mineral occurs. While Ca²⁺ transporters have been comparatively extensively studied, less is understood about phosphate handling in calcified tissue-forming cells, and little is known about which P_i transporters are active. Objectives: The aim of this study was to investigate the expression of known sodium-dependent phosphate transporter genes in rat incisor odontoblasts, MRPC-1 odontoblast-like cells and UMR-106 osteoblast-like cells. Methods: Expression of the type II Na⁺-P_i cotransporter genes Npt-2a and Npt-2b was analyzed by gene specific RT-PCR, Northern blot hybridization and immunological detection of NaPi-2a and NaPi-2b protein in these cells as well as on histological sections from the mineralizing tooth. Expression of type I (Npt-1) and type III (Glvr-1 and Ram-1) Na⁺-P_i cotransporters was studied at the mRNA level. The sodium-dependent P_i uptake, its regulation by phosphate starvation, and the effect of PTH alone and in combination with P_i deprivation were determined. Results: Both MRPC-1 and UMR-106 cell lines exhibited a high Na⁺-dependent phosphate uptake, significantly increased by P_i depletion. Both type II high-capacity Na⁺-P_i cotransporters, Npt-2a and Npt-2b, were highly expressed in the three cell types investigated. All cells studied expressed low levels of Glvr-1, UMR-106 expressed Npt-1, while both odontoblasts and MRPC-1 expressed Ram-1. Immunohistochemistry demonstrated both Npt-2a and Npt-2b in murine odontoblasts, ameloblasts and osteoblasts in vivo. Conclusion: Npt-2a and Npt-2b have hitherto not been demonstrated in calcified tissue-forming cells, and such a simultaneous expression has not been found in any cell type. The findings suggest an instrumental role for Na⁺-P_i cotransporters during biomineralization, where the type II cotransporters would be prime candidates for the large transcellular P_i flow taking place. This study was supported by the Swedish Medical Research Council grants 2789 and 14100.