Klotho Deficiency Causes Vascular Calcification in Chronic Kidney Disease

Abstract

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 ϩ -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.

Renal Production, Uptake, and Handling of Circulating αKlotho

“…aKlotho is found in urine and serum of rodents and humans 2,20,42,56 ; and the kidney has the highest abundance of aKlotho compared with other organs. Clinical and experimental animal studies showed low renal tubular aKlotho expression in both acute kidney injury and CKD 10,40,42,43,[56][57][58][59][60][61][62] and low circulating aKlotho levels in kidney diseases of a variety of etiologies in animals 42,43,56,62 and humans. 32,44,61,63 These data suggest that the kidney contributes to circulating aKlotho.…”

“…71 Furthermore, we did not see decreased negative charge in the GBM when kidney sections were directly incubated with aKlotho (Supplemental Figure 5) and in vivo studies did not show increased urinary protein excretion in normal rats acutely perfused with aKlotho and Tg-Kl mice with chronic higher levels of aKlotho. 21,43 Free-flow micropuncture from Bowman's space showed low radioactivity after injection with 125 I-aKlotho ( Figure 5C). Thus, the exogenous aKlotho that ends up in the urine is tubular in origin probably through transcytosis across proximal renal tubules.…”

“…Understanding the source and clearance of Klotho is critical because restoration of aKlotho in deficient states reverses the phenotype, 2,26,43 which has great therapeutic potential. 2,26,43 We show that the kidney is the principal contributor to circulating aKlotho and simultaneously, is also the major organ where circulating aKlotho is taken up from the circulation. Thus kidney disease is expected to affect both production and clearance of aKlotho in a complex fashion.…”

Fractional Excretion of Phosphorus Modifies the Association between Fibroblast Growth Factor-23 and Outcomes

“…16 Kidney biopsy studies show that persons with CKD and ESRD have lower renal tubular klotho expression, 17 low serum soluble klotho, and low urine klotho concentrations. 18,19 Prior studies in mice show that klotho haplo insufficiency (approximately 50% reduction in klotho) is accompanied by an approximately threefold higher FGF23 concentration, suggesting that FGF23 may be increased in a compensatory fashion to induce its biologic effects when klotho is decreased. 14,15 In this context, our findings could suggest that concurrent high FGF23 and low FePi may be serving as a noninvasive marker of low kidney klotho expression or activity.…”

Soluble Klotho and Autosomal Dominant Polycystic Kidney Disease

“…The extracellular domain of Klotho is shed on the cell surface after being cleaved by membrane-anchored proteases (ADAM 10 and 17) and is detected in the blood and cerebrospinal fluid in mice and humans (35,36). Serum (and urine) Klotho levels correlated with Klotho expression in the kidney of Klotho knock-in and knock-out mice, indicating that circulating Klotho may represent renal Klotho expression (37). This shedded soluble Klotho molecule acts as a hormone on the renal proximal tubule, where it has recently been postulated to pass through the basolateral membrane and to migrate through the cytosol; it ultimately reaches the brush-border membrane, where it causes endocytosis of the Na-Pi cotransporters (32).…”