Osteoblasts sense extracellular levels of phosphate to control the local expression of phosphatases for matrix mineralisation

The provision of inorganic phosphate (P_i) for biomineralisation is under systemic and local control. Locally, osteoblast production of phosphatases such as tissue-nonspecific alkaline phosphatase (TNAP) and PHOSPHO1 is required for normal skeletal mineralisation and osteoblasts may sense extracellular P_i concentrations to control local phosphatase activity and thereby “fine-tune” P_i production and delivery for biomineralisation. This has been poorly explored and this study examined the ability of osteoblasts to sense and respond to extracellular P_i to control the local expression of TNAP and PHOSPHO1.

Extracellular P_i downregulated the expression of PHOSPHO1 and TNAP by human primary osteoblasts at both mRNA and protein levels. Increasing P_i concentrations also reduced the mRNA expression of the type III Na- P_i co-transporters, PiT-1 and PiT-2 and selectively enhanced ERK1/2 phosphorylation. Inhibition of PiT-1 and PiT-2 by Foscarnet or MEK1/2 by UO126 abolished the downregulation of PHOSPHO1 and ALPL expression by extracellular Pi. Moreover, extracellular P_i phosphorylated fibroblast growth factor receptor (FGFR) substrate 2α (FRS2α) and this activation was abolished by Foscarnet. Also, blocking FGFR signalling inhibited the phosphorylation of ERK1/2 and prevented the decrease in ALPL and PHOSPHO1 expression by extracellular P_i. Similar results were observed in cultured murine calvaria. Osteoblast matrix mineralisation by extracellular P_i was dependent upon type III Na- P_i co-transporters and FGFR signalling.

In conclusion, these results suggest an interplay between FGFR and P_i transporters is required for osteoblasts to sense and respond to extracellular P_i. This understanding advances our knowledge of the molecular control of physiological bone mineralisation by osteoblasts.