Renal Na+ -Phosphate Cotransport in Murine X-linked Hypophosphatemic Rickets: Molecular Characterization

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The X-linked Hyp mouse is characterized by a specific defect in proximal tubular phosphate (Pi) reabsorption that is associated with a decrease in Vmax of the high affinity Na+ -Pi cotransport system in the renal brush border membrane. To understand the mechanism for Vmax reduction, we examined the effect of the Hyp mutation on renal expression of Na+ -Pi cotransporter mRNA and protein. Northern hybridization of renal RNA with a rat, renal-specific Na+ -Pi cotransporter cDNA probe (NaPi-2) (Magagnin et al 1993. Proc. Natl. Acad. Sci. USA. 90:5979-5983.) demonstrated a reduction in a 2.6-kb transcript in kidneys of Hyp mice relative to normal littermates (NaPi-2/beta-actin mRNA = 57+/-6% of normal in Hyp mice, n = 6, P < 0.01). Na+ -Pi cotransport, but not Na+ -sulfate cotransport, was (approximately) 50% lower in Xenopus oocytes injected with renal mRNA extracted from Hyp mice when compared with that from normal mice. Hybrid depletion experiments documented that the mRNA-dependent expression of Na+ -Pi cotransport in oocytes was related to NaPi-2. Western analysis demonstrated that NaPi-2 protein is also significantly reduced in brush border membranes of Hyp mice when compared to normals. The present data demonstrate that the specific reduction in renal Na+ -Pi cotransport in brush border membranes of Hyp mice can be ascribed to a proportionate decrease in the abundance of Na+ -Pi cotransporter mRNA and protein. (J. Clin. Invest. 1994. 93:671-676.) Key words: X-linked hypophosphatemia. vitamin D resistant rickets. familial hypophosphatemic rickets. brush border membrane. proximal tubule. NaPi-1. NaPi-2. NaPi-3

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