Klotho prevents renal calcium loss

Abstract

Disturbed calcium (Ca2+) homeostasis, which is implicit to the aging phenotype of klotho-deficient mice, has been attributed to altered vitamin D metabolism, but alternative possibilities exist. We hypothesized that failed tubular Ca2+ absorption is primary, which causes increased urinary Ca2+ excretion, leading to elevated 1,25-dihydroxyvitamin D 3 [1,25(OH)2D3] and its sequelae. Here, we assessed intestinal Ca2+ absorption, bone densitometry, renal Ca 2+ excretion, and renal morphology via energy-dispersive x-ray microanalysis in wild-type and klotho-/- mice. We observed elevated serum Ca2+ and fractional excretion of Ca2+ (FE Ca) in klotho-/- mice. Klotho-/- mice also showed intestinal Ca2+ hyperabsorption, osteopenia, and renal precipitation of calcium-phosphate. Duodenal mRNA levels of transient receptor potential vanilloid 6 (TRPV6) and calbindin-D9K increased. In the kidney, klotho-/- mice exhibited increased expression of TRPV5 and decreased expression of the sodium/calcium exchanger (NCX1) and calbindin-D 28K, implying a failure to absorb Ca2+ through the distal convoluted tubule/connecting tubule (DCT/CNT) via TRPV5. Gene and protein expression of the vitamin D receptor (VDR), 25-hydroxyvitamin D-1-α-hydroxylase (1αOHase), and calbindin-D9K excluded renal vitamin D resistance. By modulating the diet, we showed that the renal Ca2+ wasting was not secondary to hypercalcemia and/or hypervitaminosis D. In summary, these findings illustrate a primary defect in tubular Ca2+ handling that contributes to the precipitation of calcium-phosphate in DCT/CNT. This highlights the importance of klotho to the prevention of renal Ca2+ loss, secondary hypervitaminosis D, osteopenia, and nephrocalcinosis. Copyright © 2009 by the American Society of Nephrology.