Search

The cloning of the renal NaPi-2a (SLC34A1) and NaPi-2c (SLC34A3) phosphate transporters has made it possible to characterize the molecular and biophysical regulation of renal proximal tubular reabsorption of inorganic phosphate (Pi). Dietary factors, such as Pi and K, and several hormones and phosphatonins, including parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and glucocorticoids, regulate the transporters through various transcriptional, translational, and post-translational mechanisms that involve acute trafficking via endocytosis or exocytosis, interactions with PDZ domain proteins, lipid microdomains, and diffusion and clustering in the apical brush border membrane. The visualization of these trafficking events by means of novel microscopy techniques that includes fluorescence lifetime imaging microscopy (FLIM), Förster resonance energy transfer (FRET), fluctuation correlation spectroscopy (FCS), and modulation tracking (MT), is the primary focus of this review. [ABSTRACT FROM AUTHOR]

We analyzed vitamin D receptor (VDR) (−/−) mice fed either a normal diet or a rescue diet. Weanling VDR (−/−) mice had hypophosphatemia and hyperphosphaturia. Renal Na-dependent inorganic phosphate (Pi) cotransport activity was significantly decreased in weanling VDR (−/−) mice. In VDR (+/+) mice, renal Npt2a/Npt2c/PiT-2 protein levels were significantly increased at 21 and 28 days of age compared with that at 1 day of age. Npt2c and PiT-2 protein levels were maximally expressed at 28 days of age. Npt2a protein levels were significantly decreased in mice at 28 days of age compared with 21 and 60 days of age. In VDR (−/−) mice, Npt2a/Npt2c/PiT-2 protein levels were considerably lower than those in age-matched VDR (+/+) mice at 21 and 28 days of age. The reduced Npt2a/Npt2c/PiT-2 protein recovered completely in VDR-null mice fed the rescue diet. Although Pi transport activity and Npt2b were reduced in the proximal intestine in VDR (−/−) mice, Npt2b protein levels were not reduced in the distal intestine in VDR (−/−) mice. The rescue diet did not affect intestinal Npt2b protein levels in VDR (−/−) mice. Thus, reduced intestinal Pi absorption in VDR (−/−) mice does not seem to be the only factor that causes hypophosphatemia; reduced Npt2a, Npt2c, or PiT-2 protein levels during development might also cause hypophosphatemia and rickets in VDR (−/−) mice. Furthermore, dietary intervention completely normalized the expression of the renal phosphate transporters (Npt2a/Npt2c/PiT-2) in VDR (−/−) mice, suggesting that the lack of VDR activity is not the cause of impaired renal phosphate reabsorption. [ABSTRACT FROM AUTHOR]

Pi transport by vascular smooth muscle cells (VSMC) has been proposed to play an important role in the pathogenesis of vascular calcification. In this study, we have determined the correlation between calcification induced by Pi, platelet-derived growth factor (PDGF)-BB, and tumor necrosis factor-α and Pi transport activity in primary cultures of rat aortic VSMC. These agents induced calcification and increased the expression of Cbfa1, Msx2, and Bmp2 osteogene messenger RNA in rat aortic VSMC, while Pi transport rate was not modified per milligram of protein. Only PDGF increased Pi transport when it was expressed per unit of DNA, as PDGF also increased total cell protein by 100%, while DNA content and number of cells were not modified. PDGF increased the expression of the Pi transporter, Pit-1, but membrane protein biotinylation showed that Pit-1 abundance was not modified in the cell surface. Immunofluorescence revealed that, under basal conditions, Pit-1 is only slightly expressed at the cell membrane, but strongly expressed inside the cell. The intracellular signal colocalizes with endoplasmic reticulum (ER) markers, and PDGF increases Pit-1 expression in the ER but not the cell membrane. In conclusion, Pi transport across the plasma membrane does not correlate directly with calcification, but the expression of Pit-1 in the ER opens new possibilities for the study of the pathogenesis of vascular calcification. [ABSTRACT FROM AUTHOR]