Role of Klotho in aging, phosphate metabolism, and CKD

Abstract

The klotho gene (KL) was identified first as a putative aging-suppressor gene that extended life span when overexpressed and accelerated aging-like phenotypes when disrupted in mice. It encodes a single-pass transmembrane protein and is expressed predominantly in kidney, where it functions as an obligate coreceptor for fibroblast growth factor 23 (FGF-23). FGF-23 is a bone-derived hormone that suppresses phosphate reabsorption and 1,25 dihydroxyvitamin D(3) (vitamin D) synthesis in the kidney. Klotho also is expressed in the parathyroid gland, where FGF-23 decreases parathyroid hormone expression and secretion, further suppressing vitamin D synthesis in kidney. Thus, FGF-23 functions as a phosphaturic hormone and a counter-regulatory hormone for vitamin D, thereby inducing negative phosphate balance. Mice lacking either FGF-23 or Klotho show hyperphosphatemia in addition to developing multiple aging-like phenotypes, which can be rescued by resolving phosphate retention. These findings have unveiled an unexpected link between aging and phosphate. In patients with chronic kidney disease (CKD), phosphate retention is seen universally and has been associated with increased mortality risk. Patients with CKD have high serum FGF-23 levels with decreased klotho expression in the kidney and parathyroid, rendering FGF-23 and Klotho as potential biomarkers and therapeutic targets for CKD. The Klotho protein not only serves as a coreceptor for FGF-23, but also functions as a humoral factor. Klotho's extracellular domain is released into blood and urine by ectodomain shedding and exerts various functions independently of FGF-23, including regulation of multiple ion channels and transporters. Decreased urinary Klotho protein level has been identified as one of the earliest biomarkers of CKD progression. This review focuses on the current understanding of Klotho protein function, with emphasis on its potential involvement in the pathophysiologic process of CKD.

Figure 1

Endocrine regulation of phosphate homeostasis. Ten years ago, two principal calcium-regulating hormones, 1,25 dihydroxyvitamin D₃ (vitamin D) and parathyroid hormone (PTH), were thought to regulate phosphate metabolism. PTH increases vitamin D synthesis in kidney (①). Vitamin D in turn decreases PTH (②), thereby closing a negative feedback loop. Now the fibroblast growth factor 23 (FGF-23)-klotho system has emerged as the principal phosphate-regulating endocrine axes. FGF23 is secreted from bone and acts on kidney to reduce vitamin D synthesis (③). Because vitamin D increases FGF23 expression in bone (④), a negative feedback loop exists between FGF-23 and vitamin D. FGF23 also acts on parathryroid to reduce PTH (⑤). Because PTH increases FGF-23 expression (⑥), there exists another negative feedback loop between PTH and FGF-23.

Figure 2

Changes in phosphate-regulating factors during chronic kidney disease (CKD) progression. (A) Decrease in urinary klotho levels is probably the earliest change detectable in early stage CKD.^, Increase in fibroblast growth factor 23 (FGF-23), decrease in vitamin D, and increase in parathyroid hormone (PTH) precede hyperphosphatemia during CKD progression. (B) A model that explains panel A. Decrease in klotho induces end-organ resistance to FGF-23, resulting in compensatory increase in FGF-23 to maintain normal serum phosphate levels. However, this is at the expense of vitamin D synthesis. Because vitamin D increases klotho gene expression, low vitamin D further reduces klotho expression. In addition, low vitamin D increases PTH and high PTH further stimulates FGF-23. Phosphate retention, which occurs later, further amplifies the abnormality in all four factors. This constellation thus forms a deterioration spiral of decreased klotho and increased FGF-23.

Figure 3

Multiple functions of membrane klotho and secreted klotho. Membrane klotho functions as an obligate co-receptor to fibroblast growth factor 23 (FGF-23) in the kidney and parathyroid gland. This activity ensures a negative phosphate balance by 1) induction of phosphaturia and suppression of vitamin D synthesis in kidney and by 2) suppression of parathyroid hormone (PTH) expression and secretion in the parathyroid gland. Secreted klotho exists in luminal fluid of renal tubules as well as in urine and functions as an enzyme that modifies N-linked glycans of TRPV5 and ROMK1 (renal outer medullary K⁺ channel) on the apical membrane of renal distal tubules, which increases calcium reabsorption and potassium excretion. Secreted klotho also modifies N-linked glycans of type-2a Na-dependent phosphate co-transporters (NaPi-2a) on the apical brush border membrane of renal proximal tubules, which reduces phosphate reabsorption independently of FGF-23.