The renal sodium-phosphate transporter and X-linked hypophosphatemic vitamin D-resistant rickets

The sodium-phosphate (Na⁺/P(i)) transporter is a brush-border membrane specific protein located in the epithelia of the kidney and intestine of mammals. This transporter is involved in the pathogenesis of X-linked hypophosphatemic rickets. X-linked hypophosphatemic vitamin D-resistant rickets is the most common form of rickets in man, and is characterized by low serum phosphate levels. The (Hyp) mouse is thought to be an animal homologue of this disease in humans. Decreased renal proximal tubular reabsorption of phosphate has been observed in the (Hyp) mouse, resulting in hyperphosphaturia. The Na⁺/P(i) transporter was cloned from the mouse, and cRNA generated from this cDNA showed increased sodium-dependent phosphate uptake in Xenopus oocytes. Northern blot analysis showed recognition of three transcripts in normal and (Hyp) mice at 2.6, 4.6, and 10 kilobases, with all three bands being decreased in the (Hyp) mouse. In-situ hybridization localized the message to the renal proximal tubules in both mice. Polyclonal antiserum, raised against a C-terminal synthetic peptide specific for this transporter, recognized several bands at 152, 87, 82, 77, and 37 kDa on Western blots of renal brush-border membrane proteins from both mice. The intensity of all bands was decreased in the (Hyp) mouse samples by 5-10 fold. Immunohistochemistry localized the immunoreactive protein to the apical membrane of the proximal tubules in both mice. The transcription rate of this gene was decreased by 2-fold in the (Hyp) mouse. Dietary phosphate deprivation resulted in increased transporter activity, message levels, and immunoreactive protein levels in both (Hyp) and control mice. Cloning of the PEX gene (from the Hyp) locus on chromosome X of patients with hypophosphatemic rickets would suggest that a product of the PEX gene alters the function of the Na⁺/P(i) transporter.