Endogenous Klotho Research Articles

Klotho as a potential biomarker and therapy for acute kidney injury

Klotho is a single-pass transmembrane protein that is highly expressed in the kidney and is known to act as a coreceptor for fibroblast growth factor 23. The extracellular domain can be produced independently or shed from membrane-bound Klotho and functions as an endocrine substance with multiple functions including antioxidation, modulation of ion transport, suppression of fibrosis, and preservation of stem cells. Emerging evidence has revealed that Klotho deficiency is an early event in acute kidney injury (AKI), and a pathogenic factor that exacerbates acute kidney damage and contributes to long-term consequences. Restoration by exogenous supplementation or stimulation of endogenous Klotho might prevent and ameliorate injury, promote recovery, and suppress fibrosis to mitigate development of chronic kidney disease. Although data are still emerging, in this Perspectives article we discuss why this renal-derived protein is a highly promising candidate as both an early biomarker and therapeutic agent for AKI.

Vascular Klotho Deficiency Potentiates the Development of Human Artery Calcification and Mediates Resistance to Fibroblast Growth Factor 23

Klotho is known to function as a cofactor for the phosphatonin, fibroblast growth factor (FGF)-23 at the kidney. FGF-23 levels rise in chronic kidney disease (CKD) despite progression of accelerated vascular calcification. There are currently conflicting data on whether FGF-23 may exhibit direct vasculoprotective effects in CKD. In this study, we describe for the first time endogenous Klotho expression in human arteries and human aortic smooth muscle cells. We show that CKD is a state of vascular Klotho deficiency promoted by chronic circulating stress factors, including proinflammatory, uremic, and disordered metabolic conditions. Mechanistic studies demonstrated that Klotho knockdown potentiated the development of accelerated calcification through a Runx2 and myocardin-serum response factor-dependent pathway. Klotho knockdown studies further revealed that vascular cells are a Klotho-dependent target tissue for FGF-23. FGF-23 mediated cellular activation of p-ERK, p-AKT, and cellular proliferative effects, which were abrogated following Klotho knockdown. We next showed that vascular Klotho deficiency driven by procalcific stressors could be restored by vitamin D receptor activators, in vitro and further confirmed using human arterial organ cultures from CKD patients, in vivo. Furthermore, restoration of suppressed Klotho expression by vitamin D receptor activators conferred human aortic smooth muscle cells responsive to FGF-23 signaling and unmasked potential anticalcific effects. Chronic metabolic stress factors found in CKD promote vascular Klotho deficiency. Mechanistic studies revealed a bifunctional role for local vascular Klotho, first, as an endogenous inhibitor of vascular calcification and, second, as a cofactor required for vascular FGF-23 signaling. Furthermore, vitamin D receptor activators can restore Klotho expression and unmask FGF-23 anticalcific effects.

Klotho deficiency is an early biomarker of renal ischemia–reperfusion injury and its replacement is protective

Klotho is an antiaging substance with pleiotropic actions including regulation of mineral metabolism. It is highly expressed in the kidney and is present in the circulation and urine but its role in acute kidney injury (AKI) is unknown. We found that ischemia-reperfusion injury (IRI) in rodents reduced Klotho in the kidneys, urine, and blood, all of which were restored upon recovery. Reduction in kidney and plasma Klotho levels were earlier than that of neutrophil gelatinase-associated lipocalin (NGAL), a known biomarker of kidney injury. Patients with AKI were found to have drastic reductions in urinary Klotho. To examine whether Klotho has a pathogenic role, we induced IRI in mice with different endogenous Klotho levels ranging from heterozygous Klotho haploinsufficient, to wild-type (WT), to transgenic mice overexpressing Klotho. Klotho levels in AKI were lower in haploinsufficient and higher in transgenic compared with WT mice. The haploinsufficient mice had more extensive functional and histological alterations compared with WT mice, whereas these changes were milder in overexpressing transgenic mice, implying that Klotho is renoprotective. Rats with AKI given recombinant Klotho had higher Klotho protein, less kidney damage, and lower NGAL than rats with AKI given vehicle. Hence, AKI is a state of acute reversible Klotho deficiency, low Klotho exacerbates kidney injury and its restoration attenuates renal damage and promotes recovery from AKI. Thus, endogenous Klotho not only serves as an early biomarker for AKI but also functions as a renoprotective factor with therapeutic potential.

Regulation of Fibroblast Growth Factor-23 Signaling by Klotho

The aging suppressor gene Klotho encodes a single-pass transmembrane protein. Klotho-deficient mice exhibit a variety of aging-like phenotypes, many of which are similar to those observed in fibroblast growth factor-23 (FGF23)-deficient mice. To test the possibility that Klotho and FGF23 may function in a common signal transduction pathway(s), we investigated whether Klotho is involved in FGF signaling. Here we show that Klotho protein directly binds to multiple FGF receptors (FGFRs). The Klotho-FGFR complex binds to FGF23 with higher affinity than FGFR or Klotho alone. In addition, Klotho significantly enhanced the ability of FGF23 to induce phosphorylation of FGF receptor substrate and ERK in various types of cells. Thus, Klotho functions as a cofactor essential for activation of FGF signaling by FGF23.