Soft-tissue calcification is a prominent feature in both chronic kidney disease (CKD) and experimental Klotho deficiency, but whether Klotho deficiency is responsible for the calcification in CKD is unknown. Here, wild-type mice with CKD had very low renal, plasma, and urinary levels of Klotho. In humans, we observed a graded reduction in urinary Klotho starting at an early stage of CKD and progressing with loss of renal function. Despite induction of CKD, transgenic mice that overexpressed Klotho had preserved levels of Klotho, enhanced phosphaturia, better renal function, and much less calcification compared with wild-type mice with CKD. Conversely, Klotho-haploinsufficient mice with CKD had undetectable levels of Klotho, worse renal function, and severe calcification. The beneficial effect of Klotho on vascular calcification was a result of more than its effect on renal function and phosphatemia, suggesting a direct effect of Klotho on the vasculature. In vitro, Klotho suppressed Na⁺-dependent uptake of phosphate and mineralization induced by high phosphate and preserved differentiation in vascular smooth muscle cells. In summary, Klotho is an early biomarker for CKD, and Klotho deficiency contributes to soft-tissue calcification in CKD. Klotho ameliorates vascular calcification by enhancing phosphaturia, preserving glomerular filtration, and directly inhibiting phosphate uptake by vascular smooth muscle. Replacement of Klotho may have therapeutic potential for CKD.

Cardiac dysfunction in CKD is characterized by aberrant cardiac remodeling with hypertrophy and fibrosis. CKD is a state of severe systemic Klotho deficiency, and restoration of Klotho attenuates vascular calcification associated with CKD. We examined the role of Klotho in cardiac remodeling in models of Klotho deficiency—genetic Klotho hypomorphism, high dietary phosphate intake, aging, and CKD. Klotho-deficient mice exhibited cardiac dysfunction and hypertrophy before 12 weeks of age followed by fibrosis. In wild-type mice, the induction of CKD led to severe cardiovascular changes not observed in control mice. Notably, non-CKD mice fed a high-phosphate diet had lower Klotho levels and greatly accelerated cardiac remodeling associated with normal aging compared with those on a normal diet. Chronic elevation of circulating Klotho because of global overexpression alleviated the cardiac remodeling induced by either high-phosphate diet or CKD. Regardless of the cause of Klotho deficiency, the extent of cardiac hypertrophy and fibrosis correlated tightly with plasma phosphate concentration and inversely with plasma Klotho concentration, even when adjusted for all other covariables. High-fibroblast growth factor–23 concentration positively correlated with cardiac remodeling in a Klotho-deficient state but not a Klotho-replete state. In vitro, Klotho inhibited TGF-β1–, angiotensin II–, or high phosphate–induced fibrosis and abolished TGF-β1– or angiotensin II–induced hypertrophy of cardiomyocytes. In conclusion, Klotho deficiency is a novel intermediate mediator of pathologic cardiac remodeling, and fibroblast growth factor–23 may contribute to cardiac remodeling in concert with Klotho deficiency in CKD, phosphotoxicity, and aging.

αKlotho is a multifunctional protein highly expressed in the kidney. Soluble αKlotho is released through cleavage of the extracellular domain from membrane αKlotho by secretases to function as an endocrine/paracrine substance. The role of the kidney in circulating αKlotho production and handling is incompletely understood, however. Here, we found higher αKlotho concentration in suprarenal compared with infrarenal inferior vena cava in both rats and humans. In rats, serum αKlotho concentration dropped precipitously after bilateral nephrectomy or upon treatment with inhibitors of αKlotho extracellular domain shedding. Furthermore, the serum half-life of exogenous αKlotho in anephric rats was four- to five-fold longer than that in normal rats, and exogenously injected labeled recombinant αKlotho was detected in the kidney and in urine of rats. Both in vivo (micropuncture) and in vitro (proximal tubule cell line) studies showed that αKlotho traffics from the basal to the apical side of the proximal tubule via transcytosis. Thus, we conclude that the kidney has dual roles in αKlotho homeostasis, producing and releasing αKlotho into the circulation and clearing αKlotho from the blood into the urinary lumen.

Background: Membrane αKlotho (hereinafter called Klotho) is highly expressed in the kidney and functions as a coreceptor of FGF receptors (FGFRs) to activate specific fibroblast growth factor 23 (FGF23) signal pathway. FGF23 is produced in bones and participates in the maintenance of mineral homeostasis. The extracellular domain of transmembrane Klotho can be cleaved by secretases and released into the circulation as soluble Klotho. Soluble Klotho does not only weakly activate FGFRs to transduce the FGF23 signaling pathway, but also functions as an enzyme and hormonal substance to play a variety of biological functions. FGF23 exerts its biological effects through activation of FGFRs in a Klotho-dependent manner. However, extremely high FGF23 can exert its pathological action in a Klotho-independent manner. Summary: The decline in serum and urinary Klotho followed by a rise in serum FGF23 at an early stage of chronic kidney disease (CKD) functions as an early biomarker for kidney dysfunction and can also serve as a predictor for risk of cardiovascular disease (CVD) and mortality in both CKD patients and the general population. Moreover, Klotho deficiency is a pathogenic factor for CKD progression and CVD. FGF23 may also contribute to CVD. Prevention of Klotho decline, reactivation of endogenous Klotho production, or supplementation of exogenous Klotho attenuate renal fibrosis, retard CKD progression, improve mineral metabolism, ameliorate cardiomyopathy, and alleviate vascular calcification in CKD. However, the poor CVD outcome after depletion of FGF23 with FGF23 antibody stimulates the generation of a more specific inhibitor of FGF23 for CKD treatment. Key Message: Klotho/FGF23 may not only be diagnostic and/or prognostic biomarkers for CKD and CVD, but are also pathogenic contributors to CKD progression and CVD development. The Klotho/FGF23 axis should be a novel target for renal clinics.