Two animal models of the human disorder hypophosphatemic vitamin-D resistant rickets exist, the Hyp and Gy mice. Both affected mice and humans manifest an X-linked phenotype. Affected mice and humans show decreased Na+/Pi transport activity in the renal proximal tubules, which is characterized by a decreased maximal velocity (Vmax). The defect in the Hyp mice is most likely due to a decreased transcription rate of the renal Na+/Pi transporter gene (Collins, J.F., Ghishan, F.K., Am. J. Physiol. 269(38):439-448). The current studies were designed to define the molecular defect in the Gy mice. Sodium-dependent uptake of phosphate (Pi) in renal BBMV showed uptake levels of 170 ± 25 and 66 ± 11 pmol/mg protein/6s in normal and Gy mice respectively (n=3, p=0.0102). Glucose uptake levels in the BBMV were 1.94 ± 0.87 and 1.91 ± 0.35 pmol/mg protein/ 6s in normal and Gy mice respectively (n=3). Northern blot analysis of kidney cortex in both mice revealed nearly equivalent message levels (normal/Gy = 1.01 ± 0.12, n=3). In-situ hybridization localized the mRNA to the renal cortex in both mice, and confirmed equal message levels. Western blot analysis of renal BBM proteins, utilizing a polyclonal antiserum, showed one predominant band at 87 kDa in both mouse samples with intensities being decreased in the Gy mice (normal/Gy = 4.129 ± 0.70, n=4, p<0.04). Immunohistochemical analysis localized the protein to the apical membrane of proximal tubules in both mice. Furthermore, the Gy mouse Na+/Pi transporter cDNA was cloned by PCR and the 5′ end was sequenced. No sequence differences were present when compared to the normal mouse cDNA sequence. These results suggest that the molecular defect in the Gy mice is distinct from that in the Hyp mice, and likely involves abnormal translation of the message, or defective protein processing or translocation to the BBM. These results further suggest that two X-Iinked factors regulate different steps in the expression pathway of the Na+/Pi transporter gene.
|Original language||English (US)|
|Journal||Journal of Investigative Medicine|
|State||Published - 1996|
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)