In search of the fountain of youth.

In Greek mythology, Klotho spins the thread of life. In human biology, Klotho protein was so named when researchers discovered that mutations that interrupt its normal function result in a shorter thread of life that is attributable to accelerated aging.1 From its initial discovery, the kidney and its role in phosphate and calcium handling have been at the epicenter of the Klotho story. a, b, and g Klotho are single-pass transmembrane proteins that serve as coreceptors for the fibroblast growth factor 19 (FGF-19) subgroup of endocrine FGFs.2 Heterodimeric complexes of Klotho and FGF receptors at the cell surface increase the binding affinity of ubiquitously expressed FGF receptors for specific FGF ligands and thereby confer the end-organ specificity of the main actions of the endocrine FGFs. bKlotho serves as the coreceptor for FGF-19, which regulates biliary tract function, and for FGF-21, which regulates hepatic and adipocyte energy metabolism.2 The functions of gKlotho are poorly understood. aKlotho (shortened to Klotho below) is the coreceptor for FGF-23 and is the Klotho protein that is most relevant to normal renal function and to diseases of the kidney. Klotho is most highly expressed in the kidney, parathyroid glands, and choroid plexus.3 Activation of FGF receptor-Klotho complexes by FGF-23 regulates phosphate homeostasis through effects on proximal tubular expression of the sodium-phosphate cotransporters, NaPi2a and NaPi2c. FGF-23 also helps regulate calcium homeostasis by influencing parathyroid hormone (PTH) secretion and by regulating circulating levels of 1,25dihydroxyvitamin D via Klotho-dependent inhibition of CYP27B1 and stimulation of CYP24A1.4 The actions of Klotho in the brain are unknown. In addition to its 130-kD transmembrane form, a slightly shorter, 120to 130-kD soluble form of Klotho that lacks the short transmembrane and intracellular domains circulates freely.2 Soluble Klotho can be derived from alternative splicing of the Klotho gene or from proteolytic cleavage of the extracellular domains of membrane-bound Klotho from the cell surface. Soluble Klotho appears to exert its own FGF23–independent effects on mineral metabolism through enzymatically cleaving sugar residues from phosphate and calcium transporters that result in increased urinary phosphate excretion and decreased urinary calcium excretion.5 In addition to these effects, the literature abounds with reports of protean actions of soluble Klotho too vast to comprehensively detail in this space, but broadly including antifibrotic, anticalcification, anti-inflammatory, antimetastasis, and antioxidant effects, among others.5 Despite our growing understanding of Klotho biology, numerous fundamental questions remain unanswered. High on the list is resolving the relative contributions of the kidney versus other organs to the overall systemic effects of Klotho and to the pool of circulating solubleKlotho. In this issue of JASN, Lindberg et al. report on the role of the kidney using Cre-LoxP recombination technology to create a kidney-specific deletion of Klotho (Six2-KL) that they contrasted with a global Klotho knockout.6 In the kidney-specific Six2-KL mice, Klotho mRNA expression was reduced by approximately 80%, but Klotho protein was completely undetectable by immunofluorescence staining of kidney sections and by Western blotting of kidney extracts. In addition, unlike explanted wild-type kidneys, explanted Six2-KL kidneys shed no soluble Klotho into culture medium. Consistent with the absence of soluble Klotho shedding from Six2-KL kidneys ex vivo, Six2-KL mice demonstrated 80% reductions in circulating soluble Klotho levels in vivo. The authors confirmed that they successfully deleted Klotho only in the kidney by demonstrating unperturbed Klotho expression in the parathyroid glands and choroid plexus. Metabolically, the kidney-specific deletion of Klotho caused hyperphosphatemia due to defective FGF-23–mediated renal phosphate excretion, which caused FGF-23 levels to rise dramatically while NaPi2a expression remained abundant in the proximal tubular brush border. Renal Klotho deletion also caused hypercalcemia due to increased levels of 1,25dihydroxyvitamin D, which could not be adequately suppressed by FGF-23 in the absence of Klotho. Hypercalcemia suppressed PTH levels despite severe hyperphosphatemia, consistent with the known effect of PTH to primarily respond to calcium rather than phosphate when they exert opposing demands on PTH secretion. In summary, kidney-specific deletion of Klotho reproduced the biochemical phenotype of mutations that globally interrupt Klotho function.1 Systemically, kidney-specific deletion of Klotho caused severe growth retardation, inanition, kyphosis, bone demineralization, soft tissue and arterial calcifications, skin atrophy, Published online ahead of print. Publication date available at www.jasn.org.