[Phosphate sensing and osteocyte]

Osteocyte expresses molecules responsible for hereditary hypophosphatemic rickets such as FGF23, DMP1 and PHEX. FGF23 exerts its effects in kidney by decreasing both renal phosphate (Pi) reabsorption and vitamin D activation. It is suggested that DMP1 and PHEX negatively regulate the expression of FGF23, although the underlying mechanisms remain to be elucidated. Circulating level of FGF23 appears to be regulated by systemic factors as well, including vitamin D and PTH. Although it is unknown how mammals sense the Pi availability, it has been recently reported that extracellular Pi itself triggers signal transduction in some cell types such as osteoblasts and chondrocytes, suggesting that osteocytes also respond to the alteration in extracellular Pi level.