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Renal calcification in mice homozygous for the disrupted type IIa Na/Pi cotransporter gene Npt2
Hien Chau1, Sherif El-Maadawy, Marc D McKee
1Department of Biology, McGill University, Montreal, Quebec, Canada.
Abstract:
Mice homozygous for the disrupted renal type IIa sodium/phosphate (Na/Pi) cotransporter gene (Npt2-/-) exhibit renal Pi wasting, hypophosphatemia, and an adaptive increase in the serum concentration of 1,25-dihydroxyvitamin D with associated hypercalcemia and hypercalciuria. Because hypercalciuria is a risk factor for nephrocalcinosis, we determined whether Npt2-/- mice form renal stones. Analysis of renal sections by von Kossa staining and intact kidneys by microcomputed tomography revealed renal calcification in adult Npt2-/- mice but not in Npt2+/+ littermates. Energy-dispersive spectroscopy and selected-area electron diffraction indicated that the calcifications are comprised of calcium and Pi with an apatitic mineral phase. To determine the age of onset of nephrocalcinosis, we examined renal sections of newborn and weanling mice. At both ages, mutant but not wild-type mice display renal calcification, which is associated with renal Pi wasting and hypercalciuria. Immunohistochemistry revealed that osteopontin co-localizes with the calcifications. Furthermore, renal osteopontin messenger RNA abundance is significantly elevated in Npt2-/- mice compared with Npt2+/+ mice. The onset of renal stones correlated developmentally with the absence of Npt2 expression and the expression of the genes responsible for the renal production (1alpha-hydroxylase) and catabolism (24-hydroxylase) of 1,25-dihydroxyvitamin D. In summary, we show that Npt2 gene ablation is associated with renal calcification and suggest that mutations in the NPT2 gene may contribute to nephrocalcinosis in a subset of patients with familial hypercalciuria.
Insights
Mice lacking the sodium/phosphate cotransporter gene (Npt2-/-) develop kidney stones due to impaired phosphate reabsorption. This study links Npt2 gene mutations to nephrocalcinosis, a condition causing kidney calcification.
Area of Science:
- Nephrology
- Molecular Biology
- Mineral Metabolism
Background:
- Renal type IIa sodium/phosphate (Na/Pi) cotransporter (Npt2) is crucial for phosphate reabsorption.
- Npt2 knockout mice (Npt2-/-) exhibit phosphate wasting, hypophosphatemia, and altered vitamin D metabolism.
- Hypercalciuria, a consequence of Npt2 deficiency, is a known risk factor for nephrocalcinosis.
Purpose of the Study:
- To investigate whether Npt2-/- mice develop renal stones (nephrocalcinosis).
- To determine the age of onset and mineral composition of renal calcifications in Npt2-/- mice.
- To explore the association between Npt2 gene ablation, vitamin D metabolism, and nephrocalcinosis.
Main Methods:
- Von Kossa staining and microcomputed tomography for detecting renal calcification.
- Energy-dispersive spectroscopy and selected-area electron diffraction for mineral analysis.
- Immunohistochemistry and quantitative PCR to assess osteopontin expression and localization.
Main Results:
- Npt2-/- mice exhibited significant renal calcification, confirmed by von Kossa staining and micro-CT.
- Calcifications consisted of calcium and phosphate with an apatitic mineral phase.
- Renal calcification was present from birth, associated with phosphate wasting, hypercalciuria, and elevated osteopontin.
Conclusions:
- Npt2 gene ablation in mice leads to nephrocalcinosis.
- The findings suggest a potential role for NPT2 gene mutations in familial hypercalciuria-associated nephrocalcinosis.
- Developmental correlation observed between Npt2 absence, vitamin D regulatory gene expression, and renal stone formation.