Klotho Prevents Renal Calcium Loss

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 D₃ [1,25(OH)₂D₃] and its sequelae. Here, we assessed intestinal Ca²⁺ absorption, bone densitometry, renal Ca²⁺ 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-D_9K 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-D_9K 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.Characterization of a mouse that showed a phenotype comparable to human aging led to the identification of the hormone klotho.¹ Klotho^−/− mice have atherosclerosis, osteopenia, soft tissue calcifications, pulmonary emphysema, and altered glucose metabolism.¹ It has been suggested that the etiology of many of these findings is a primary defect in phosphorous [P(i)] and calcium (Ca²⁺) homeostasis.^2,3 Klotho^−/− mice have elevated serum levels of Ca²⁺.^1,4,5 The mechanism mediating hypercalcemia is poorly understood. A possible explanation invokes the role of klotho in vitamin D homeostasis. Klotho has been proposed to participate in a negative feedback circuit to inhibit 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] synthesis.^6,7 Specifically, klotho is necessary to transduce the signal of fibroblast growth factor 23 (FGF23) through the FGF receptor, thereby suppressing CYP1b expression, the enzyme that mediates the conversion of 25-hydroxyvitamin D into 1,25(OH)₂D₃. Thus, the absence of klotho results in increased serum levels of 1,25(OH)₂D₃ and reduced serum concentrations of the calciotropic hormone parathyroid hormone.^4,7,8 This would drive increased resorption of Ca²⁺ from bone, hyperabsorption from the intestine, increased serum levels of Ca²⁺, and consequently increase renal Ca²⁺ excretion. Definitive proof of this is lacking because the molecular control of Ca²⁺ homeostasis in klotho^−/− mice has yet to be delineated.Consistent with the above hypothesis is the observation that klotho^−/− mice display hypercalciuria^4,5,9 and that normalization of serum 1,25(OH)₂D₃ levels reverts many, but not all, of their abnormalities.⁶ The published literature supports an alternative, complementary hypothesis.^9–11 A primary defect in tubular Ca²⁺ handling might cause hypervitaminosis D and renal Ca²⁺ wasting observed in klotho^−/− mice. Consistent with this idea, in vitro, klotho mediates an increase in cell surface expression of transient receptor potential vanilloid 5 (TRPV5)^10,11 the distal convoluted tubule/connecting tubule (DCT/CNT) channel responsible for the transcellular absorption of Ca²⁺.¹² This process is itself implicit to Ca²⁺ homeostasis as TRPV5 is the predominant regulator of urinary Ca²⁺ excretion.¹³ Therefore, we set out to test the hypothesis that klotho^−/− mice have a primary renal Ca²⁺ leak that contributes to a secondary increase in 1,25(OH)₂D₃ synthesis and its consequences.