Soluble Klotho Protects against Uremic Cardiomyopathy Independently of Fibroblast Growth Factor 23 and Phosphate

Genetic Ablation of Fgf23 or Klotho Does not Modulate Experimental Heart Hypertrophy Induced by Pressure Overload

Left ventricular hypertrophy (LVH) ultimately leads to heart failure in conditions of increased cardiac pre- or afterload. The bone-derived phosphaturic and sodium-conserving hormone fibroblast growth factor-23 (FGF23) and its co-receptor Klotho have been implicated in the development of uremic LVH. Using transverse aortic constriction (TAC) in gene-targeted mouse models, we examine the role of Fgf23 and Klotho in cardiac hypertrophy and dysfunction induced by pressure overload. TAC profoundly increases serum intact Fgf23 due to increased cardiac and bony Fgf23 transcription and downregulation of Fgf23 cleavage. Aldosterone receptor blocker spironolactone normalizes serum intact Fgf23 levels after TAC by reducing bony Fgf23 transcription. Notably, genetic Fgf23 or Klotho deficiency does not influence TAC-induced hypertrophic remodelling, LV functional impairment, or LV fibrosis. Despite the profound, aldosterone-mediated increase in circulating intact Fgf23 after TAC, our data do not support an essential role of Fgf23 or Klotho in the pathophysiology of pressure overload-induced cardiac hypertrophy.

Circulating Klotho Associates With Cardiovascular Morbidity and Mortality During Hemodialysis

BACKGROUND

Klotho gene was identified as an aging suppressor. In animals, klotho overexpression extends life span, and defective klotho results in rapid aging and early death. The kidney is the main contributor to circulating klotho levels, and, during chronic kidney disease, renal klotho gene expression is drastically reduced in animals and humans as well.

OBJECTIVE

We aimed to determine the consequences of a serum klotho (seKL) defect on cardiovascular morbidity and mortality during chronic dialysis.

DESIGN

The ARNOGENE study was designed to prospectively follow a cohort of hemodialysis patients for 2 years without specific intervention. A total of 769 patients was recruited and followed from the end of 2008 until January 2011. A total of 238 patients was analyzed due to a technical sample conservation issue with other samples.

RESULTS

The median seKL was markedly reduced, 360.4 ng/L (interquartile range 176.5) as compared with nondialysis chronic kidney disease patients or healthy volunteers. Patients with a seKL above the first quartile (≥280 ng/L) had a significantly reduced occurrence of outcome combining cardiovascular events and cardiovascular death [odds ratio (OR) = 0.39; 0.19 to 0.78, P = 0.008] compared with patient with klotho <280 ng/L. This effect persisted (OR = 0.86; 0.76 to 0.99, P = 0.03) after adjustment on age, sex, diabetes, cardiac insufficiency, dialysis vintage, and serum hemoglobin, albumin, fibroblast growth factor-23, phosphate, and calcium.

CONCLUSIONS

These results suggest that, during chronic hemodialysis, conservation of seKL >280 ng/L is associated with a better 2-year cardiovascular protection. Thus, a preserved klotho function supports cardiovascular protection and may represent a prognostic tool and therapeutic target for cardiovascular disease.

Modeled structural basis for the recognition of α2-3-sialyllactose by soluble Klotho

Soluble Klotho (sKlotho) is the shed ectodomain of antiaging membrane Klotho that contains 2 extracellular domains KL1 and KL2, each of which shares sequence homology to glycosyl hydrolases. sKlotho elicits pleiotropic cellular responses with a poorly understood mechanism of action. Notably, in injury settings, sKlotho confers cardiac and renal protection by down-regulating calcium-permeable transient receptor potential canonical type isoform 6 (TRPC6) channels in cardiomyocytes and glomerular podocytes. Inhibition of PI3K-dependent exocytosis of TRPC6 is thought to be the underlying mechanism, and recent studies showed that sKlotho interacts with α2-3-sialyllactose-containing gangliosides enriched in lipid rafts to inhibit raft-dependent PI3K signaling. However, the structural basis for binding and recognition of α2-3-sialyllactose by sKlotho is unknown. Using homology modeling followed by docking, we identified key protein residues in the KL1 domain that are likely involved in binding sialyllactose. Functional experiments based on the ability of Klotho to down-regulate TRPC6 channel activity confirm the importance of these residues. Furthermore, KL1 domain binds α2-3-sialyllactose, down-regulates TRPC6 channels, and exerts protection against stress-induced cardiac hypertrophy in mice. Our results support the notion that sialogangliosides and lipid rafts are membrane receptors for sKlotho and that the KL1 domain is sufficient for the tested biologic activities. These findings can help guide the design of a simpler Klotho mimetic.-Wright, J. D., An, S.-W., Xie, J., Yoon, J., Nischan, N., Kohler, J. J., Oliver, N., Lim, C., Huang, C.-L. Modeled structural basis for the recognition of α2-3-sialyllactose by soluble Klotho.

MicroRNA-21 regulates peroxisome proliferator-activated receptor alpha, a molecular mechanism of cardiac pathology in Cardiorenal Syndrome Type 4

Cardiovascular events are the leading cause of death in patients with chronic kidney disease (CKD), although the pathological mechanisms are poorly understood. Here we longitudinally characterized left ventricle pathology in a 5/6 nephrectomy rat model of CKD and identify novel molecular mediators. Next-generation sequencing of left ventricle mRNA and microRNA (miRNA) was performed at physiologically distinct points in disease progression, identifying alterations in genes in numerous immune, lipid metabolism, and inflammatory pathways, as well as several miRNAs. MiRNA miR-21-5p was increased in our dataset and has been reported to regulate many identified pathways. Suppression of miR-21-5p protected rats with 5/6 nephrectomy from developing left ventricle hypertrophy and improved left ventricle function. Next-generation mRNA sequencing revealed that miR-21-5p suppression altered gene expression in peroxisome proliferator-activated receptor alpha (PPARα) regulated pathways in the left ventricle. PPARα, a miR-21-5p target, is the primary PPAR isoform in the heart, importantly involved in regulating fatty acid metabolism. Therapeutic delivery of low-dose PPARα agonist (clofibrate) to rats with 5/6 nephrectomy improved cardiac function and prevented left ventricle dilation. Thus, comprehensive characterization of left ventricle molecular changes highlights the involvement of numerous signaling pathways not previously explored in CKD models and identified PPARα as a potential therapeutic target for CKD-related cardiac dysfunction.

Tissue expression and source of circulating αKlotho

αKlotho (Klotho), a type I transmembrane protein and a coreceptor for Fibroblast Growth Factor-23, was initially thought to be expressed only in a limited number of tissues, most importantly the kidney, parathyroid gland and choroid plexus. Emerging data may suggest a more ubiquitous Klotho expression pattern which has prompted reevaluation of the restricted Klotho paradigm. Herein we systematically review the evidence for Klotho expression in various tissues and cell types in humans and other mammals, and discuss potential reasons behind existing conflicting data. Based on current literature and tissue expression atlases, we propose a classification of tissues into high, intermediate and low/absent Klotho expression. The functional relevance of Klotho in organs with low expression levels remain uncertain and there is currently limited data on a role for membrane-bound Klotho outside the kidney. Finally, we review the evidence for the tissue source of soluble Klotho, and conclude that the kidney is likely to be the principal source of circulating Klotho in physiology.

Potential application of klotho in human chronic kidney disease

The extracellular domain of transmembrane alpha-Klotho (αKlotho, hereinafter simply called Klotho) is cleaved by secretases and released into the circulation as soluble Klotho. Soluble Klotho in the circulation starts to decline early in chronic kidney disease (CKD) stage 2 and urinary Klotho possibly even earlier in CKD stage 1. Therefore soluble Klotho could serve as an early and sensitive marker of kidney function decline. Moreover, preclinical animal data support Klotho deficiency is not just merely a biomarker, but a pathogenic factor for CKD progression and extrarenal CKD complications including cardiovascular disease and disturbed mineral metabolism. Prevention of Klotho decline, re-activation of endogenous Klotho production or supplementation of exogenous Klotho are all associated with attenuation of renal fibrosis, retardation of CKD progression, improvement of mineral metabolism, amelioration of cardiomyopathy, and alleviation of vascular calcification in CKD. Therefore Klotho is not only a diagnostic and/or prognostic marker for CKD, but the treatment of Klotho deficiency may be a promising strategy to prevent, retard, and decrease the burden of comorbidity in CKD.

[Myocardial fibrosis: Current aspects of the problem]

Fibrosis is one of the main components in the progression of most cardiovascular diseases, including coronary heart disease, by causing structural changes in the myocardium and vascular wall. The quantitative and qualitative characteristics of fibrosis of the myocardium are responsible for decreasing its elastic properties, developing diastolic dysfunction, impairing myocardial contractility, developing systolic dysfunction and cardiac arrhythmias, and worsening coronary blood flow in patients with heart failure of different etiologies. The important aspect of studying fibrosis is not only its interpretation as a model of the typical pathological process, but also its consideration as a systemic lesion of various organs and tissues. At the same time, the identification of myocardial fibrosis biomarkers that are available for their determination in circulating blood is of particular interest. Since there was evidence for the role of fibrosis in developing dysfunction of various organs and ensuring the systematicity of most diseases, especially at their development stages, the process of fibrosis came to be regarded as a promising therapeutic target. It is relevant to further investigate myocardial fibrosis, which is aimed at increasing the efficiency of its diagnosis and predicting its course and pathogenetically sound therapy.

The Association Between Fibroblast Growth Factor 23 and Renal Transplantation Outcome Is Modified by Follow-up Duration and Glomerular Filtration Rate Assessment Method

Introduction

Fibroblast growth factor 23 (FGF23) could contribute to cardiovascular morbidity in chronic kidney disease. In studies of kidney transplant recipients, a high circulating level of FGF23 has been associated with death and graft loss independently of estimated glomerular filtration rate (GFR). Whether FGF23 is associated with adverse outcomes in the early posttransplantation period is unknown.

Methods

We analyzed a cohort of 845 kidney transplant recipients in stable condition who had GFR measured in the first years after transplantation with a median follow-up of 71 months.

Results

A high FGF23 concentration was associated with death or graft loss in univariate analysis, but this association was lost after adjustment for measured GFR. In contrast, FGF23 remained significantly associated with the composite outcome when estimated GFR was substituted for measured GFR. We also observed that follow-up duration modified the association between FGF23 and outcome. Although FGF23 was not associated with any endpoint in the full duration of the study, we found an independent association between FGF23 and the incidence of graft loss within the 4 years after FGF23 measurement. We did not find an association between FGF23 levels and left ventricular mass in a subgroup of 227 patients who had echocardiography performed within 3 months of FGF23 measurement.

Discussion

This study demonstrates that FGF23 measured during the first year after transplantation is not an independent predictor of death and graft loss and is not associated with left ventricular hypertrophy in the posttransplantation period. It further unveils important factors modifying the association between FGF23 and outcome in this population.

Effect of Paricalcitol on FGF-23 and Klotho in Kidney Transplant Recipients

BACKGROUND

Paricalcitol decreases intact parathyroid hormone and the frequency of secondary hyperparathyroidism after kidney transplantation. This proof-of-concept study aimed to assess the effect of paricalcitol on fibroblast growth factor-23/KLOTHO axis in renal transplants.

METHODS

Twenty-nine subjects with secondary hyperparathyroidism received oral paricalcitol 1 μg/d for 3 months, and 8 patients matched by age, sex, and creatinine clearance, but with intact parathyroid hormone less than 100 pg/mL, were included as controls.

RESULTS

Intact parathyroid hormone decreased in paricalcitol-treated patients (P < 0.0001). Serum fibroblast growth factor-23 enhanced (P < 0.01), whereas KLOTHO concentrations showed a trend to increase (P = 0.067). KLOTHO gene expression in peripheral blood mononuclear cells increased by 45.7% in paricalcitol-treated patients (P < 0.01), without change in controls. Paricalcitol administration resulted in a median percent decrease of 56% in methylated DNA levels of KLOTHO promoter (P < 0.001). The ratio of the unmethylated/methylated KLOTHO promoter DNA did not change in controls, but it increased by 177% in paricalcitol-treated subjects (P < 0.0001). The increase in this ratio was independently associated with the change in serum KLOTHO (r = 0.55, P < 0.01) and messenger RNA expression levels (r = 0.40, P < 0.05).

CONCLUSIONS

Paricalcitol administration to renal transplant patients significantly reduced intact parathyroid hormone and increased fibroblast growth factor-23, with a trend to increase in serum KLOTHO. Paricalcitol-treated patients showed a decrease in the methylation of the KLOTHO promoter with an increment in the ratio of un-methyated/methylated DNA, which was associated with an increase of KLOTHO gene expression levels and serum KLOTHO concentrations. Long-term studies are needed to assess whether paricalcitol-induced increase in KLOTHO gene expression and serum concentrations may translate into beneficial clinical effects.

Extrarenal effects of FGF23

Chronic kidney disease (CKD) is associated with an increased risk of cardiovascular mortality, infections, and impaired cognitive function. It is characterized by excessively increased levels of the phosphaturic hormone fibroblast growth factor 23 (FGF23) and a deficiency of its co-receptor Klotho. Despite the important physiological effect of FGF23 in maintaining phosphate homeostasis, there is increasing evidence that higher FGF23 levels are a risk factor for mortality and cardiovascular disease. FGF23 directly induces left ventricular hypertrophy via activation of the FGF receptor 4/calcineurin/nuclear factor of activated T cells signaling pathway. By contrast, the impact of FGF23 on endothelial function and the development of atherosclerosis are poorly understood. The results of recent experimental studies indicate that FGF23 directly impacts on hippocampal neurons and may thereby impair learning and memory function in CKD patients. Finally, it has been shown that FGF23 interferes with the immune system by directly acting on polymorphonuclear leukocytes and macrophages. In this review, we discuss recent data from clinical and experimental studies on the extrarenal effects of FGF23 with respect to the cardiovascular, central nervous, and immune systems.

The role of the anti-ageing protein Klotho in vascular physiology and pathophysiology

Klotho is an anti-ageing protein that functions in many pathways that govern ageing, like regulation of phosphate homeostasis, insulin signaling, and Wnt signaling. Klotho expression levels and levels in blood decline during ageing. The vascular phenotype of Klotho deficiency features medial calcification, intima hyperplasia, endothelial dysfunction, arterial stiffening, hypertension, and impaired angiogenesis and vasculogenesis, with characteristics similar to aged human arteries. Klotho-deficient phenotypes can be prevented and rescued by Klotho gene expression or protein supplementation. High phosphate levels are likely to be directly pathogenic and are a prerequisite for medial calcification, but more important determinants are pathways that regulate cellular senescence, suggesting that deficiency of Klotho renders cells susceptible to phosphate toxicity. Overexpression of Klotho is shown to ameliorate medial calcification, endothelial dysfunction, and hypertension. Endogenous vascular Klotho expression is a controversial subject and, currently, no compelling evidence exists that supports the existence of vascular membrane-bound Klotho expression, as expressed in kidney. In vitro, Klotho has been shown to decrease oxidative stress and apoptosis in both SMCs and ECs, to reduce SMC calcification, to maintain the contractile SMC phenotype, and to prevent μ-calpain overactivation in ECs. Klotho has many protective effects with regard to the vasculature and constitutes a very promising therapeutic target. The purpose of this review is to explore the etiology of the vascular phenotype of Klotho deficiency and the therapeutic potential of Klotho in vascular disease.

The systemic nature of CKD

The accurate definition and staging of chronic kidney disease (CKD) is one of the major achievements of modern nephrology. Intensive research is now being undertaken to unravel the risk factors and pathophysiologic underpinnings of this disease. In particular, the relationships between the kidney and other organs have been comprehensively investigated in experimental and clinical studies in the last two decades. Owing to technological and analytical limitations, these links have been studied with a reductionist approach focusing on two organs at a time, such as the heart and the kidney or the bone and the kidney. Here, we discuss studies that highlight the complex and systemic nature of CKD. Energy balance, innate immunity and neuroendocrine signalling are highly integrated biological phenomena. The diseased kidney disrupts such integration and generates a high-risk phenotype with a clinical profile encompassing inflammation, protein-energy wasting, altered function of the autonomic and central nervous systems and cardiopulmonary, vascular and bone diseases. A systems biology approach to CKD using omics techniques will hopefully enable in-depth study of the pathophysiology of this systemic disease, and has the potential to unravel critical pathways that can be targeted for CKD prevention and therapy.

Novel Faces of Fibroblast Growth Factor 23 (FGF23): Iron Deficiency, Inflammation, Insulin Resistance, Left Ventricular Hypertrophy, Proteinuria and Acute Kidney Injury

FGF23 is a hormone that appears as the core regulator of phosphate metabolism. Great deal of data has accumulated to demonstrate increased FGF23 secretion from the bone to compensate for even subtle increases in serum phosphorus long before intact PTH. However, recent evidence points to the fact that actions and interactions of FGF23 are not limited solely to phosphate metabolism. FGF23 may be implicated in iron metabolism and erythropoiesis, inflammation, insulin resistance, proteinuria, acute kidney injury and left ventricular hypertrophy. In this review, we will summarize latest experimental and clinical data examining impact of FGF23 on aforementioned pathophysiologic pathways/disorders.

Recombinant α-Klotho may be prophylactic and therapeutic for acute to chronic kidney disease progression and uremic cardiomyopathy

α-Klotho is highly expressed in the kidney, and its extracellular domain is cleaved and released into the circulation. Chronic kidney disease (CKD) is a state of α-Klotho deficiency, which exerts multiple negative systemic effects on numerous organs including the cardiovascular system. Since acute kidney injury (AKI) greatly escalates the risk of CKD development, we explored the effect of α-Klotho on prevention and treatment on post-AKI to CKD progression and cardiovascular disease. Therein, ischemia reperfusion injury-induced AKI was followed by early administration of recombinant α-Klotho or vehicle starting one day and continued for four days after kidney injury (CKD prevention protocol). A CKD model was generated by unilateral nephrectomy plus contralateral ischemia reperfusion injury. Late administration of α-Klotho in this model was started four weeks after injury and sustained for 12 weeks (CKD treatment protocol). The prevention protocol precluded AKI to CKD progression and protected the heart from cardiac remodeling in the post-AKI model. One important effect of exogenous α-Klotho therapy was the restoration of endogenous α-Klotho levels long after the cessation of exogenous α-Klotho therapy. The treatment protocol still effectively improved renal function and attenuated cardiac remodeling in CKD, although these parameters did not completely return to normal. In addition, α-Klotho administration also attenuated high phosphate diet-induced renal and cardiac fibrosis, and improved renal and cardiac function in the absence of pre-existing renal disease. Thus, recombinant α-Klotho protein is safe and efficacious, and might be a promising prophylactic or therapeutic option for prevention or retardation of AKI-to-CKD progression and uremic cardiomyopathy.

Mineral metabolism and cardiovascular disease in CKD

The mineral bone disorder of CKD, called Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD), has a major role in the etiology and progression of cardiovascular disease in CKD patients. Since the main emphasis in CKD-MBD is on three categories (bone abnormalities, laboratory abnormalities, and vascular calcifications), we have routinely accepted ectopic cardiovascular calcifications as a central risk factor in the pathophysiology of CKD-MBD for cardiac events. However, recent compelling evidence suggests that some CKD-MBD-specific factors other than vascular calcification might contribute to the onset of cardiovascular disease. Most notable is fibroblast growth factor-23 (FGF23), which is thought to be independently associated with cardiac remodeling. Slow progression of cardiac disorders, such as vascular calcification and cardiac remodeling, characterizes cardiac disease due to CKD-MBD. In contrast, fatal arrhythmia may be induced when QT prolongation occurs with CKD-MBD treatment, such as with lower Ca dialysate or the use of calcimimetics. Sudden onset of fatal cardiac events, such as heart failure and sudden cardiac death, due to fatal arrhythmia would be another distinctive phenomenon of CKD-MBD. This may be defined as CKD-MBD-specific cardiac complex syndrome.

Klotho May Ameliorate Proteinuria by Targeting TRPC6 Channels in Podocytes

Klotho is a type-1 membrane protein predominantly produced in the kidney, the extracellular domain of which is secreted into the systemic circulation. Membranous and secreted Klotho protect organs, including the kidney, but whether and how Klotho directly protects the glomerular filter is unknown. Here, we report that secreted Klotho suppressed transient receptor potential channel 6 (TRPC6)-mediated Ca2+ influx in cultured mouse podocytes by inhibiting phosphoinositide 3-kinase-dependent exocytosis of the channel. Furthermore, soluble Klotho reduced ATP-stimulated actin cytoskeletal remodeling and transepithelial albumin leakage in these cells. Overexpression of TRPC6 by gene delivery in mice induced albuminuria, and exogenous administration of Klotho ameliorated the albuminuria. Notably, immunofluorescence and in situ hybridization revealed Klotho expression in podocytes of mouse and human kidney. Heterozygous Klotho-deficient CKD mice had aggravated albuminuria compared with that in wild-type CKD mice with a similar degree of hypertension and reduced clearance function. Finally, disrupting the integrity of glomerular filter by saline infusion-mediated extracellular fluid volume expansion increased urinary Klotho excretion. These results reveal a potential novel function of Klotho in protecting the glomerular filter, and may offer a new therapeutic strategy for treatment of proteinuria.

New Insights into the Mechanism of Action of Soluble Klotho

The klotho gene encodes a type I single-pass transmembrane protein that contains a large extracellular domain, a membrane spanning segment, and a short intracellular domain. Klotho protein exists in several forms including the full-length membrane form (mKl) and a soluble circulating form [soluble klotho (sKl)]. mKl complexes with fibroblast growth factor receptors to form coreceptors for FGF23, which allows it to participate in FGF23-mediated signal transduction and regulation of phosphate and calcium homeostasis. sKl is present in the blood, urine, and cerebrospinal fluid where it performs a multitude of functions including regulation of ion channels/transporters and growth factor signaling. How sKl exerts these pleiotropic functions is poorly understood. One hurdle in understanding sKl's mechanism of action as a "hormone" has been the inability to identify a receptor that mediates its effects. In the body, the kidneys are a major source of sKl and sKl levels decline during renal disease. sKl deficiency in chronic kidney disease makes the heart susceptible to stress-induced injury. Here, we summarize the current knowledge of mKl's mechanism of action, the mechanistic basis of sKl's protective, FGF23-independent effects on the heart, and provide new insights into the mechanism of action of sKl focusing on recent findings that sKl binds sialogangliosides in membrane lipid rafts to regulate growth factor signaling.

Inhibition of TRPC6 channels ameliorates renal fibrosis and contributes to renal protection by soluble klotho

Fibrosis is an exaggerated form of tissue repair that occurs with serious damage or repetitive injury and ultimately leads to organ failure due to the excessive scarring. Increased calcium ion entry through the TRPC6 channel has been associated with the pathogenesis of heart and glomerular diseases, but its role in renal interstitial fibrosis is unknown. We studied this by deletion of Trpc6 in mice and found it decreased unilateral ureteral obstruction-induced interstitial fibrosis and blunted increased mRNA expression of fibrosis-related genes in the ureteral obstructed kidney relative to that in the kidney of wild-type mice. Administration of BTP2, a pyrazol derivative known to inhibit function of several TRPC channels, also ameliorated obstruction-induced renal fibrosis and gene expression in wild-type mice. BTP2 inhibited carbachol-activated TRPC3 and TRPC6 channel activities in HEK293 cells. Ureteral obstruction caused over a 10-fold increase in mRNA expression for TRPC3 as well as TRPC6 in the kidneys of obstructed relative to the sham-operated mice. The magnitude of protection against obstruction-induced fibrosis in Trpc3 and Trpc6 double knockout mice was not different from that in Trpc6 knockout mice. Klotho, a membrane and soluble protein predominantly produced in the kidney, is known to confer protection against renal fibrosis. Administration of soluble klotho significantly reduced obstruction-induced renal fibrosis in wild-type mice, but not in Trpc6 knockout mice, indicating that klotho and TRPC6 inhibition act in the same pathway to protect against obstruction-induced renal fibrosis. Thus klotho and TRPC6 may be pharmacologic targets for treating renal fibrosis.

Klotho/FGF23 Axis in Chronic Kidney Disease and Cardiovascular Disease

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.

Response of fibroblast growth factor 23 to volume interventions in arterial hypertension and diabetic nephropathy

Fibroblast growth factor 23 (FGF-23) rises progressively in chronic kidney disease and is associated with adverse cardiovascular outcomes. FGF-23 putatively induces volume retention by upregulating the sodium-chloride cotransporter (NCC). We studied whether, conversely, interventions in volume status affect FGF-23 concentrations.We performed a post hoc analysis of 1) a prospective saline infusion study with 12 patients with arterial hypertension who received 2 L of isotonic saline over 4 hours, and 2) a randomized controlled trial with 45 diabetic nephropathy (DN) patients on background angiotensin-converting enzyme -inhibition (ACEi), who underwent 4 6-week treatment periods with add-on hydrochlorothiazide (HCT) or placebo, combined with regular sodium (RS) or low sodium (LS) diet in a cross-over design. Plasma C-terminal FGF-23 was measured by ELISA (Immutopics) after each treatment period in DN and before and after saline infusion in hypertensives.The patients with arterial hypertension were 45 ± 13 (mean ± SD) years old with an estimated glomerular filtration rate (eGFR) of 101 ± 18 mL/min/1.73 m. Isotonic saline infusion did not affect FGF-23 (before infusion: 68 median [first to third quartile: 58-97] relative unit (RU)/mL, after infusion: 67 [57-77] RU/mL, P = 0.37). DN patients were 65 ± 9 years old. During ACEi + RS treatment, eGFR was 65 ± 25 mL/min/1.73 m and albuminuria 649 mg/d (230-2008 mg/d). FGF23 level was 94 (73-141) RU/mL during ACEi therapy. FGF-23 did not change significantly by add-on HCT (99 [74-148] RU/mL), LS diet (99 [75-135] RU/mL), or their combination (111 [81-160] RU/mL, P = 0.15).Acute and chronic changes in volume status did not materially change FGF-23 in hypertensive patients and DN, respectively. Our data do not support a direct feedback loop between volume status and FGF-23 in hypertension or DN.

Association between FGF23, α-Klotho, and Cardiac Abnormalities among Patients with Various Chronic Kidney Disease Stages

Background

Several experimental studies have demonstrated that fibroblast growth factor 23 (FGF23) may induce myocardial hypertrophy via pathways independent of α-Klotho, its co-factor in the induction of phosphaturia. On the other hand, few studies have clearly demonstrated the relationship between FGF23 level and left ventricular hypertrophy among subjects without chronic kidney disease (CKD; i.e., CKD stage G1 or G2).

Purpose

To investigate the data from 903 patients admitted to the cardiology department with various degrees of renal function, including 234 patients with CKD stage G1/G2.

Methods and Results

Serum levels of full-length FGF23 and α-Klotho were determined by enzyme immunoassay. After adjustment for sex, age, and estimated glomerular filtration rate (eGFR), the highest FGF23 tertile was significantly associated with left ventricular hypertrophy among patients with CKD stage G1/G2 and those with CKD stage G3a/G3b/G4 as compared with the lowest FGF23 tertile, and the association retained significance after further adjustment for serum levels of corrected calcium, inorganic phosphate, and C-reactive protein, as well as diuretic use, history of hypertension, and systolic blood pressure. FGF23 was also associated with low left ventricular ejection fraction among patients with CKD stage G1/G2 and those with CKD stage G3a/G3b/G4 after adjusting for age, sex, eGFR, corrected calcium, and inorganic phosphate. On the other hand, compared with the highest α-Klotho tertile, the lowest α-Klotho tertile was associated with left ventricular hypertrophy and systolic dysfunction only among patients with CKD stage G3b and stage G3a, respectively.

Conclusions

An association between FGF23 and cardiac hypertrophy and systolic dysfunction was observed among patients without CKD as well as those with CKD after multivariate adjustment. However, the association between α-Klotho and cardiac hypertrophy and systolic dysfunction was significant only among patients with CKD G3b and G3a, respectively.

Phosphate Toxicity in CKD: The Killer among Us

Maintenance of a normal serum phosphate level depends on absorption in the gut, reabsorption and excretion by the kidney, and the flux between the extracellular and skeletal pools. Phosphate homeostasis is a coordinated, complex system of crosstalk between the bone, intestine, kidney, and parathyroid gland. Dysfunction of this system has serious clinical consequences in healthy individuals and those with conditions, such as CKD, in which hyperphosphatemia is associated with increased risks of cardiovascular morbidity and mortality. The last half-century of renal research has helped define the contribution of the parathyroid hormone, calcitriol, fibroblast growth factor 23, and Klotho in the regulation of phosphate. However, despite new discoveries and insights gained during this time, what remains unchanged is the recognition that phosphate retention is the initiating factor for the development of many of the complications observed in CKD, namely secondary hyperparathyroidism and bone and cardiovascular diseases. Controlling phosphate load remains the primary goal in the treatment of CKD. This review discusses the clinical effects of dysregulated phosphate metabolism, particularly in CKD, and its association with cardiovascular disease. The importance of early control of phosphate load in the treatment of CKD is emphasized, and the latest research in the treatment of phosphate retention is discussed.

Klotho, the Holy Grail of the kidney: from salt sensitivity to chronic kidney disease

The Klotho gene displays an extremely shortened life span with loss of function missense mutations leading to premature multiple organ failure, thus resembling human premature aging syndromes. The transmembrane form of Klotho protein functions as an obligatory co-receptor for FGF23. Klotho and FGF23 are crucial components for the regulation of vitamin D metabolism and subsequently blood phosphate levels. The secreted Klotho protein has multiple regulatory functions, including effects on electrolyte homeostasis, on growth factor pathways as well as on oxidative stress, which are currently the object of extensive research. Klotho protein deficiency is observed in many experimental and clinical disease models. Genetic polymorphisms such as the G-395A polymorphism in the promoter region of the Klotho gene have been associated with the development of essential hypertension. The kidneys are the primary site of Klotho production, and renal Klotho is decreased in CKD, followed by a reduction in plasma Klotho. Klotho deficiency has been both associated with progression of CKD as well as with its cardinal systemic manifestations, including cardiovascular disease. Thus, Klotho has been suggested both as a risk biomarker for early detection of CKD and additionally as a potential therapeutic tool in the future.

Bone mineral disorder in chronic kidney disease: Klotho and FGF23; cardiovascular implications

Cardiovascular factors are one of the main causes of morbidity and mortality in patients with chronic kidney disease. Bone mineral metabolism disorders and inflammation are pathological conditions that involve increased cardiovascular risk in chronic kidney disease. The cardiovascular risk involvement of bone mineral metabolism classical biochemical parameters such as phosphorus, calcium, vitamin D and PTH is well known. The newest markers, FGF23 and klotho, could also be implicated in cardiovascular disease.

αKlotho and Chronic Kidney Disease

Alpha-Klotho (αKlotho) protein is encoded by the gene, Klotho, and functions as a coreceptor for endocrine fibroblast growth factor-23. The extracellular domain of αKlotho is cleaved by secretases and released into the circulation where it is called soluble αKlotho. Soluble αKlotho in the circulation starts to decline in chronic kidney disease (CKD) stage 2 and urinary αKlotho in even earlier CKD stage 1. Therefore soluble αKlotho is an early and sensitive marker of decline in kidney function. Preclinical data from numerous animal experiments support αKlotho deficiency as a pathogenic factor for CKD progression and extrarenal CKD complications including cardiac and vascular disease, hyperparathyroidism, and disturbed mineral metabolism. αKlotho deficiency induces cell senescence and renders cells susceptible to apoptosis induced by a variety of cellular insults including oxidative stress. αKlotho deficiency also leads to defective autophagy and angiogenesis and promotes fibrosis in the kidney and heart. Most importantly, prevention of αKlotho decline, upregulation of endogenous αKlotho production, or direct supplementation of soluble αKlotho are all associated with attenuation of renal fibrosis, retardation of CKD progression, improvement of mineral metabolism, amelioration of cardiac function and morphometry, and alleviation of vascular calcification in CKD. Therefore in rodents, αKlotho is not only a diagnostic and prognostic marker for CKD but the enhancement of endogenous or supplement of exogenous αKlotho are promising therapeutic strategies to prevent, retard, and decrease the comorbidity burden of CKD.

Deficiency of Soluble α-Klotho as an Independent Cause of Uremic Cardiomyopathy

Cardiovascular disease (CVD) is the major cause of mortality for patients with chronic kidney disease (CKD). Cardiac hypertrophy, occurring in up to 95% patients with CKD (also known as uremic cardiomyopathy), increases their risk for cardiovascular death. Many CKD-specific risk factors of uremic cardiomyopathy have been recognized, such as secondary hyperparathyroidism, indoxyl sulfate (IS)/p-cresyl, and vitamin D deficiency. However, several randomized controlled trials have recently shown that these risk factors have little impact on the mortality of CVD. Klotho is a type 1 membrane protein predominantly produced in the kidney, and CKD is known to be a Klotho-deficient state. Because of its important role in FGF23 and phosphate metabolism, Klotho is believed to affect cardiac growth and function indirectly through FGF23 and phosphate. Recent studies showed that soluble Klotho protects the heart against stress-induced cardiac hypertrophy by inhibiting TRPC6 channel-mediated abnormal Ca(2+) signaling in the heart, and the decreased level of circulating soluble Klotho in CKD is an important cause of uremic cardiomyopathy independent of FGF23 and phosphate. These new evidence suggested that Klotho is an independent contributing factor for uremic cardiomyopathy and a possible new target for treatment of this disease.

Klotho inhibits angiotensin II-induced cardiomyocyte hypertrophy through suppression of the AT1R/beta catenin pathway

Myocardial hypertrophy is an independent risk factor for cardiac morbidity and mortality. The antiaging protein klotho reportedly possesses a protective role in cardiac diseases. However, the precise mechanisms underlying the cardioprotective effects of klotho remain unknown. This study was aimed to determine the effects of klotho on angiotensin II (Ang II)-induced hypertrophy in neonatal rat cardiomyocytes and the possible mechanism of actions. We found that klotho significantly inhibited Ang II-induced hypertrophic growth of neonatal cardiomyocytes, as evidenced by decreased [(3)H]-Leucine incorporation, cardiomyocyte surface area and β-myosin heavy chain (β-MHC) mRNA expression. Meanwhile, klotho inhibited Ang II-stimulated activation of the Wnt/β-catenin pathway in cardiomyocytes, as evidenced by decreased protein expression of active β-catenin, downregulated protein and mRNA expression of the β-catenin target genes c-myc and cyclin D1, and increased β-catenin phosphorylation. Inhibition of the Wnt/β-catenin pathway by the specific inhibitor XAV939 markedly attenuated Ang II-induced cardiomyocyte hypertrophy. The further study revealed that klotho treatment significantly downregulated protein expression of Ang II receptor type I (AT1R) but not type II (AT2R). The AT1R antagonist losartan inhibited Ang II-stimulated activation of the Wnt/β-catenin pathway and cardiomyocyte hypertrophy. Our findings suggest that klotho inhibits Ang II-induced cardiomyocyte hypertrophy through suppression of the AT1R/β-catenin signaling pathway, which may provide new insights into the mechanism underlying the protective effects of klotho in heart diseases, and raise the possibility that klotho may act as an endogenous antihypertrophic factor by inhibiting the Ang II signaling pathway.

Intermedin1-53 attenuates vascular calcification in rats with chronic kidney disease by upregulation of α-Klotho

Deficiency in α-Klotho is involved in the pathogenesis of vascular calcification. Since intermedin (IMD)1-53 (a calcitonin/calcitonin gene-related peptide) protects against vascular calcification, we studied whether IMD1-53 inhibits vascular calcification by upregulating α-Klotho. A rat model of chronic kidney disease (CKD) with vascular calcification induced by the 5/6 nephrectomy plus vitamin D3 was used for study. The aortas of rats with CKD showed reduced IMD content but an increase of its receptor, calcitonin receptor-like receptor, and its receptor modifier, receptor activity-modifying protein 3. IMD1-53 treatment reduced vascular calcification. The expression of α-Klotho was greatly decreased in the aortas of rats with CKD but increased in the aortas of IMD1-53-treated rats with CKD. In vitro, IMD1-53 increased α-Klotho protein level in calcified vascular smooth muscle cells. α-Klotho knockdown blocked the inhibitory effect of IMD1-53 on vascular smooth muscle cell calcification and their transformation into osteoblast-like cells. The effect of IMD1-53 to upregulate α-Klotho and inhibit vascular smooth muscle cell calcification was abolished by knockdown of its receptor or its modifier protein, or treatment with the protein kinase A inhibitor H89. Thus, IMD1-53 may attenuate vascular calcification by upregulating α-Klotho via the calcitonin receptor/modifying protein complex and protein kinase A signaling.

Induction of cardiac FGF23/FGFR4 expression is associated with left ventricular hypertrophy in patients with chronic kidney disease

BACKGROUND

In chronic kidney disease (CKD), serum concentrations of fibroblast growth factor 23 (FGF23) increase progressively as glomerular filtration rate declines, while renal expression of the FGF23 coreceptor Klotho decreases. Elevated circulating FGF23 levels are strongly associated with mortality and with left ventricular hypertrophy (LVH), which is a major cause of cardiovascular death in CKD patients. The cardiac FGF23/FGF receptor (FGFR) system and its role in the development of LVH in humans have not been addressed previously.

METHODS

We conducted a retrospective case-control study in 24 deceased patients with childhood-onset end-stage renal disease (dialysis: n = 17; transplanted: n = 7), and 24 age- and sex-matched control subjects. Myocardial autopsy samples of the left ventricle were evaluated for expression of endogenous FGF23, FGFR isoforms, Klotho, calcineurin and nuclear factor of activated T-cells (NFAT) by immunohistochemistry, immunofluorescence microscopy, qRT-PCR and western blotting.

RESULTS

The majority of patients presented with LVH (67%). Human cardiomyocytes express full-length FGF23, and cardiac FGF23 is excessively high in patients with CKD. Enhanced myocardial expression of FGF23 in concert with Klotho deficiency strongly correlates with the presence of LVH. Cardiac FGF23 levels associate with time-averaged serum phosphate levels, up-regulation of FGFR4 and activation of the calcineurin-NFAT signaling pathway, an established mediator of cardiac remodelling and LVH. These changes are detected in patients on dialysis but not in those with a functioning kidney transplant.

CONCLUSIONS

Our results indicate a strong association between LVH and enhanced expression levels of FGF23, FGFR4 and calcineurin, activation of NFAT and reduced levels of soluble Klotho in the myocardium of patients with CKD. These alterations are not observed in kidney transplant patients.

Cardiorenal Syndrome in End-Stage Kidney Disease

Background: Cardiorenal syndrome (CRS) in patients with end-stage kidney disease (ESKD) represents mainly cardiovascular disease (CVD) due to various complications associated with renal dysfunction—defined as type 4 CRS by Ronco et al.—because the effect of cardiac dysfunction on the kidneys does not need to be taken into consideration, unlike in non-dialysis dependent chronic kidney disease (CKD). Summary: Patients with ESKD are often in a state of chronic inflammation due to the upregulation of proinflammatory cytokines. Chronic inflammation leads to malnutrition and consequently to vascular endothelial dysfunction and vascular calcification, which is referred to as malnutrition-inflammation-atherosclerosis (MIA) syndrome and acts as a major risk factor for CVD. Anemia also plays a crucial role in CVD, and individuals with erythropoietin-resistant anemia have a particularly high risk of CVD. However, caution is emphasized because not only anemia itself, but also the overtreatment of anemia with erythropoiesis-stimulating agents aimed at elevating hemoglobin to ≥13 g/dl can also increase the risk of CVD. In CKD-mineral and bone disorder (CKD-MBD), phosphate load triggers the interactions between various factors such as calcium, parathyroid hormone, vitamin D, and fibroblast growth factor 23, promoting vascular calcification and thus becoming a risk factor for CVD. Key Messages: In addition to traditional atherosclerosis risk factors such as hypertension, diabetes, and dyslipidemia, the involvement of MIA syndrome, anemia, and CKD-MBD accompanying CKD have also become a focus for investigation as major players in CRS in patients with ESKD.

The impact of fibroblast growth factor-23 on the cardiovascular system in chronic kidney disease

Chronic kidney disease is associated with an accelerated risk of cardiovascular (CV) mortality. Seminal work over the last decade has identified abnormal bone metabolism as an important modulator of the increased CV burden in this cohort. In particular, FGF23, a phosphaturic hormone with serum levels found to be markedly elevated in chronic kidney disease, is independently associated with increased risks of all-cause mortality and CV events. This editorial will discuss the proposed mechanisms linking FGF23 to CV disease in chronic kidney disease, namely, direct cardiac myocyte toxicity, endothelial dysfunction and vascular calcification.

Cardio-Pulmonary-Renal Interactions: A Multidisciplinary Approach

Over the past decade, science has greatly advanced our understanding of interdependent feedback mechanisms involving the heart, lung, and kidney. Organ injury is the consequence of maladaptive neurohormonal activation, oxidative stress, abnormal immune cell signaling, and a host of other mechanisms that precipitate adverse functional and structural changes. The presentation of interorgan crosstalk may include an acute, chronic, or acute on chronic timeframe. We review the current, state-of-the-art understanding of cardio-pulmonary-renal interactions and their related pathophysiology, perpetuating nature, and cycles of increased susceptibility and reciprocal progression. To this end, we present a multidisciplinary approach to frame the diverse spectrum of published observations on the topic. Assessment of organ functional reserve and use of biomarkers are valuable clinical strategies to screen and detect disease, assist in diagnosis, assess prognosis, and predict recovery or progression to chronic disease.

Soluble klotho and mortality: the Ludwigshafen Risk and Cardiovascular Health Study

BACKGROUND

Experimental evidence suggests that soluble klotho (s-klotho), a co-receptor for fibroblast growth factor 23 (FGF23), may modulate cardiovascular risk through multiple mechanisms. However, the predictive value of s-klotho in patients remains unclear. Therefore, the present study examined in a large cohort of patients referred for coronary angiography whether s-klotho is associated with cardiovascular and total mortality.

METHODS

The longitudinal associations between baseline s-klotho and FGF23 concentrations and mortality were evaluated in 2948 participants of the Ludwigshafen Risk and Cardiovascular Health Study (LURIC), referred for coronary angiography.

RESULTS

Mean age of participants was: 63 ± 10 years. Patients with diabetes mellitus (n = 1136) had elevated s-klotho: [440 (430-449) versus 414 (406-421) pg/mL, p < 0.001]. S-klotho decreased in parallel to glomerular filtration rate (GFR) and increased in parallel to FGF23. During a median follow-up of 9.9 years, 874 deaths (30%) occurred, 539 (18%) of which were cardiovascular. After adjustment for cardiovascular risk factors, the hazard ratios in the fourth quartile compared to the first quartile of s-klotho were 1.14 (95%CI, 0.94-1.38; p = 0.187) for all-cause mortality and 1.03 (95%CI, 0.80-1.31; p = 0.845) for cardiovascular mortality. Excess mortality prediction by high levels of baseline FGF23 was not modified by adjustment for baseline s-klotho levels.

CONCLUSIONS

Klotho does not add predictive power to cardiovascular and mortality risk assessment in patients with normal renal function.

Klotho, stem cells, and aging

Aging is an inevitable and progressive biological process involving dysfunction and eventually destruction of every tissue and organ. This process is driven by a tightly regulated and complex interplay between genetic and acquired factors. Klotho is an antiaging gene encoding a single-pass transmembrane protein, klotho, which serves as an aging suppressor through a wide variety of mechanisms, such as antioxidation, antisenescence, antiautophagy, and modulation of many signaling pathways, including insulin-like growth factor and Wnt. Klotho deficiency activates Wnt expression and activity contributing to senescence and depletion of stem cells, which consequently triggers tissue atrophy and fibrosis. In contrast, the klotho protein was shown to suppress Wnt-signaling transduction, and inhibit cell senescence and preserve stem cells. A better understanding of the potential effects of klotho on stem cells could offer novel insights into the cellular and molecular mechanisms of klotho deficiency-related aging and disease. The klotho protein may be a promising therapeutic agent for aging and aging-related disorders.

Association between Serum Soluble Klotho Levels and Mortality in Chronic Hemodialysis Patients

Klotho is a single-pass transmembrane protein predominantly expressed in the kidney. The extracellular domain of Klotho is subject to ectodomain shedding and is released into the circulation as a soluble form. Soluble Klotho is also generated from alternative splicing of the Klotho gene. In mice, defects in Klotho expression lead to complex phenotypes resembling those observed in dialysis patients. However, the relationship between the level of serum soluble Klotho and overall survival in hemodialysis patients, who exhibit a state of Klotho deficiency, remains to be delineated. Here we prospectively followed a cohort of 63 patients with a mean duration of chronic hemodialysis of 6.7 ± 5.4 years for a median of 65 months. Serum soluble Klotho was detectable in all patients (median 371 pg/mL, interquartile range 309-449). Patients with serum soluble Klotho levels below the lower quartile (<309 pg/mL) had significantly higher cardiovascular and all-cause mortality rates. Furthermore, the higher all-cause mortality persisted even after adjustment for confounders (hazard ratio 4.14, confidence interval 1.29-13.48). We conclude that there may be a threshold for the serum soluble Klotho level associated with a higher risk of mortality.