Soluble klotho and autosomal dominant polycystic kidney disease

Association of hyperphosphatemia with renal prognosis in patients with autosomal dominant polycystic kidney disease

BACKGROUND

Serum phosphate (P) levels are generally lower in autosomal dominant polycystic kidney disease (ADPKD) than in other kidney disorders, potentially masking the clinical significance of hyperphosphatemia. This study aimed to determine if serum P levels can predict renal outcomes in ADPKD patients.

METHODS

We included 235 patients with ADPKD who were not taking drugs to treat hyperphosphatemia. Survival analysis was performed for the renal outcome of a 50% reduction in estimated glomerular filtration rate or initiation of renal replacement therapy.

RESULTS

Multivariable Cox regression analyses revealed that serum P (1 mg/dL increase, HR = 2.03, P < 0.0001) was a significant risk factor for kidney disease progression. Similarly, hyperphosphatemia (P > 3.5 mg/dL, HR = 2.05; P > 4.0 mg/dL, HR = 1.90; P > 4.5 mg/dL, HR = 2.78; P > 5.0 mg/dL, HR = 27.22) was significantly associated with renal prognosis. Kaplan-Meier analysis showed significantly lower kidney survival rates in patients with P > 3.5 mg/dL than in those without hyperphosphatemia (log-rank test, P < 0.0001), and similar Kaplan-Meier analysis results were found for P > 4.0 mg/dL, P > 4.5 mg/dL, and P > 5.0 mg/dL. The 2 year kidney survival rate for ADPKD patients with P > 3.5 mg/dL was 66.7% overall and 41.4% in those with stage 4-5 CKD. For patients with P > 4.0 mg/dL, the survival rate dropped to 46.8% overall and 28.2% in those with stage 4-5 CKD, indicating a very poor prognosis.

CONCLUSION

Hyperphosphatemia was associated with renal prognosis in patients with ADPKD. In these patients, attention should be paid to even mild serum P elevation of > 3.5 or > 4.0 mg/dL.

Kidney phosphate wasting predicts poor outcome in polycystic kidney disease

BACKGROUND

Patients with autosomal dominant polycystic kidney disease (ADPKD) have disproportionately high levels of fibroblast growth factor 23 (FGF-23) for their chronic kidney disease stage, however only a subgroup develops kidney phosphate wasting. We assessed factors associated with phosphate wasting and hypothesize that it identifies patients with more severe disease and predicts disease progression.

METHODS

We included 604 patients with ADPKD from a multicenter prospective observational cohort (DIPAK; Developing Intervention Strategies to Halt Progression of Autosomal Dominant Polycystic Kidney Disease) in four university medical centers in the Netherlands. We measured parathyroid hormone (PTH) and total plasma FGF-23 levels, and calculated the ratio of tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmP/GFR) with <0.8 mmol/L defined as kidney phosphate wasting. We analysed the association of TmP/GFR with estimated GFR (eGFR) decline over time and the risk for a composite kidney outcome (≥30% eGFR decline, kidney failure or kidney replacement therapy).

RESULTS

In our cohort (age 48 ± 12 years, 39% male, eGFR 63 ± 28 mL/min/1.73 m2), 59% of patients had phosphate wasting. Male sex [coefficient -0.2, 95% confidence interval (CI) -0.2; -0.1], eGFR (0.002, 95% CI 0.001; 0.004), FGF-23 (0.1, 95% CI 0.03; 0.2), PTH (-0.2, 95% CI -0.3; -0.06) and copeptin (-0.08, 95% CI -0.1; -0.08) were associated with TmP/GFR. Corrected for PTH, FGF-23 and eGFR, every 0.1 mmol/L decrease in TmP/GFR was associated with a greater eGFR decline of 0.2 mL/min/1.73 m2/year (95% CI 0.01; 0.3) and an increased hazard ratio of 1.09 (95% CI 1.01; 1.18) of the composite kidney outcome.

CONCLUSION

Our study shows that in patients with ADPKD, phosphate wasting is prevalent and associated with more rapid disease progression. Phosphate wasting may be a consequence of early proximal tubular dysfunction and insufficient suppression of PTH.

Autosomal Dominant Polycystic Kidney Disease: Extrarenal Involvement

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disorder, but kidneys are not the only organs involved in this systemic disorder. Individuals with the condition may display additional manifestations beyond the renal system, involving the liver, pancreas, and brain in the context of cystic manifestations, while involving the vascular system, gastrointestinal tract, bones, and cardiac valves in the context of non-cystic manifestations. Despite kidney involvement remaining the main feature of the disease, thanks to longer survival, early diagnosis, and better management of kidney-related problems, a new wave of complications must be faced by clinicians who treated patients with ADPKD. Involvement of the liver represents the most prevalent extrarenal manifestation and has growing importance in the symptom burden and quality of life. Vascular abnormalities are a key factor for patients' life expectancy and there is still debate whether to screen or not to screen all patients. Arterial hypertension is often the earliest onset symptom among ADPKD patients, leading to frequent cardiovascular complications. Although cardiac valvular abnormalities are a frequent complication, they rarely lead to relevant problems in the clinical history of polycystic patients. One of the newest relevant aspects concerns bone disorders that can exert a considerable influence on the clinical course of these patients. This review aims to provide the "state of the art" among the extrarenal manifestation of ADPKD.

Fibroblast Growth Factor 23 Bone Regulation and Downstream Hormonal Activity

Mineral homeostasis of calcium and phosphate levels is one critical component to the maintenance of bone mineral density (BMD) and strength. Diseases that disrupt calcium and phosphate balanced have highlighted not only the role these minerals play in overall bone homeostasis, but also the factors, hormones and downstream transporters, responsible for mineral metabolism. The key phosphaturic hormone elucidated from studying rare heritable disorders of hypophosphatemia is Fibroblast Growth Factor 23 (FGF23). FGF23 is predominantly secreted from bone cells in an effort to maintain phosphate balance by directly controlling renal reabsorption and indirectly affecting intestinal uptake of this mineral. Multiple factors have been shown to enhance bone mRNA expression; however, FGF23 can also undergo proteolytic cleavage to control secretion of the biologically active form of the hormone. The review focuses specifically on the regulation of FGF23 and its secretion from bone as well as its hormonal actions under physiological and disease conditions.

Reduced Levels of the Antiaging Hormone Klotho are Associated With Increased Aortic Stiffness in Diabetic Kidney Disease

Introduction

Aortic pulse wave velocity (Ao-PWV) predicts cardiovascular and kidney disease in type 2 diabetes (T2D). Klotho is a circulating antiaging hormone (sKlotho) with putative cardiorenal protective effects. The relationship between sKlotho and Ao-PWV in diabetic kidney disease (DKD) is unknown.

Methods

In a cross-sectional cohort study, the correlation of sKlotho measured by a validated immunoassay, and Ao-PWV measured by applanation tonometry, was investigated in 172 participants with T2D and early stage DKD (all had estimated glomerular filtration rate [eGFR] >45 ml/min) on stable renin angiotensin system (RAS) inhibition. In cultured human aortic smooth muscle cells (HASMCs) stimulated with angiotensin II (AngII), the effects of recombinant human sKlotho pretreatment were assessed on intracellular calcium ([Ca²⁺]_i) responses and expression of proteins associated with proosteogenic HASMC phenotypes.

Results

Mean (range) age of the cohort was 61.3 years (40-82) and 65% were male. Mean (±SD) Ao-PWV was 11.4 (±2.3) m/s, eGFR 78.8 (±23.5) and median (interquartile range) sKlotho of 358.5 (194.2-706.3) pg/ml. In multivariable linear regression analyses, we observed a statistically significant inverse relationship between sKlotho and Ao-PWV, which was independent of clinical risk factors for cardiorenal disease. Pretreatment of cultured HASMC with sKlotho significantly attenuated AngII-stimulated [Ca²⁺]_i transients and reduced osteogenic collagen (Col1a2) expression.

Conclusions

In individuals with T2D and early DKD, lower levels of sKlotho are associated with increased Ao-PWV. Taken together with the direct effect of sKlotho on mediators of aortic wall stiffness in vitro, these findings may explain the enhanced risk of cardiorenal disease in DKD.

Does Dapagliflozin influence arterial stiffness and levels of circulating anti-aging hormone soluble Klotho in people with type 2 diabetes and kidney disease? Results of a randomized parallel group clinical trial

Objective

The mechanisms that explain the cardio-renal benefits of sodium glucose co-transporter 2 (SGLT-2) inhibitors are unknown. The effect of SGLT-2 inhibitors on arterial aging, measured by Aortic Pulse Wave Velocity (Ao-PWV) and Soluble Klotho (s-Klotho), a circulating anti-aging biomarker of arterial health are also unclear.

Design/Setting

A 24-week single center randomized controlled trial (registry number/ EudraCT Number: 2013-004042-42) comparing Dapagliflozin and Ramipril (D+R) versus Ramipril (R) on the primary endpoint of urine albumin excretion rate (AER) and pre-specified secondary endpoints of Ao-PWV and biomarkers of arterial aging [s-Klotho and Fibroblast Growth Factor 23 (FGF-23)]. People with type 2 diabetes who had estimated glomerular filtration rate (eGFR) > 60 ml/min and residual microalbuminuria on maximum tolerated renin angiotensin system (RAS) inhibition were included in this study.

Results

In total, 33 participants (male 73%) were randomized to either D+R (n = 17) or R (n = 16) arms. After 24 weeks of treatment, Ao-PWV (mean ± SD) did not change significantly from baseline D +R [9.06 ± 1.91 m/s to 9.13 ± 2.03 m/s], and R [9.88 ± 2.12 m/s to 10.0 ± 1.84 m/s]. AER fell significantly by 43.5% (95% CI: −57.36%, −29.56%; p < 0.01) in people in the D+ R arm only. We do not observe any significant changes in FGF-23 or s-Klotho. HbA1c and Angiotensin 1–7 fell significantly only in D + R arm.

Conclusions

The combination of Dapagliflozin and Ramipril had no effects on Ao-PWV and s-Klotho which are biomarkers of arterial aging and cardio-renal risk. Our data suggest that the early cardio-renal benefits observed with SGLT-2 inhibitors are unlikely to be related to an improvement in arterial aging.

A Decreased Level of Soluble Klotho Can Predict Cardiovascular Death in No or Mild Abdominal Aortic Calcification Hemodialysis Patients

Background: Soluble Klotho plays an important role in cardiovascular disease and death in chronic kidney disease (CKD). We assessed the relationship between serum soluble Klotho (sKL) level and outcome in MHD patients.

Methods: Soluble Klotho was detected by ELISA. Cox regression analysis and Kaplan-Meier analysis showed the relationship between sKL and cardiovascular disease (CVD) mortality in maintenance hemodialysis (MHD) patients.

Results: There were 45 cases (35.2%) of all-cause death and 36 cases (28.1%) of CVD mortality. Multivariate linear regression analysis showed that Log[iPTH] (γ = −0.224, P = 0.015) was an independent predictor of sKL level. Cox regression showed that lower sKL was associated with higher CVD mortality rate [OR = 0.401, 95% CI (0.183–0.867), P = 0.022]. Kaplan-Meier analysis showed that the CVD mortality rate increased significantly in patients with low sKL (P = 0.006). Compared with high sKL patients, low sKL patients with no or mild vascular calcification [aortic calcification score (AACs) ≤ 4] had no significant difference in all-cause mortality rate. The CVD mortality rate was significantly lower in high sKL patients (P = 0.004) than in those with low sKL. In the severe calcification group (AACs ≥ 5), all-cause and CVD mortality rates were similar between different sKL groups (P = 0.706 and 0.488, respectively). The area under the receiver-operating characteristic curve (AUC) of soluble Klotho for predicting the CVD in MHD patients with AACs ≤ 4 was 0.796 (0.647–0.946, P = 0.017), sensitivity was 0.921, and specificity was 0.50 for a cutoff value of 307.69 pg/ml.

Conclusions: Lower sKL was associated with higher CVD mortality rate. Lower sKL concentration in MHD patients with no or mild calcification can predict CVD mortality.

Prognostic Value of Fibroblast Growth Factor 23 in Autosomal Dominant Polycystic Kidney Disease

INTRODUCTION

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by progressive cyst growth and a loss of functioning renal mass, but a decline in glomerular filtration rate (GFR) and onset of end-stage renal disease (ESRD) occur late in the disease course. There is therefore a great need for early prognostic biomarkers in this disorder.

METHODS

We measured baseline serum fibroblast growth factor 23 (FGF23) levels in 192 patients with ADPKD from the Consortium for Radiologic Imaging Studies of PKD (CRISP) cohort that were followed for a median of 13 years and tested the association between FGF23 levels and change over time in height-adjusted total kidney volume (htTKV), GFR, and time to the composite endpoints of ESRD, death, and doubling of serum creatinine.

RESULTS

Patients in the highest quartile for baseline FGF23 level had a higher rate of increase in htTKV (0.95% per year, P = 0.0016), and faster rate of decline in GFR (difference of -1.03 ml/min/1.73 m2 per year, P = 0.005) compared with the lowest quartile, after adjusting for other covariates, including htTKV and genotype. The highest quartile of FGF23 was also associated with a substantial increase in risk for the composite endpoint of ESRD, death, or doubling of serum creatinine (hazard ratio [HR] of 2.45 in the fully adjusted model, P = 0.03).

CONCLUSION

FGF23 is a prognostic biomarker for disease progression and clinically important outcomes in ADPKD, and has additive value to established imaging and genetic biomarkers.

Fibroblast growth factor 23 decreases PDE4 expression in heart increasing the risk of cardiac arrhythmia; Klotho opposes these effects

The concentration of fibroblast growth factor 23 (FGF23) rises progressively in renal failure (RF). High FGF23 concentrations have been consistently associated with adverse cardiovascular outcomes or death, in chronic kidney disease (CKD), heart failure or liver cirrhosis. We identified the mechanisms whereby high concentrations of FGF23 can increase the risk of death of cardiovascular origin. We studied the effects of FGF23 and Klotho in adult rat ventricular cardiomyocytes (ARVMs) and on the heart of mice with CKD. We show that FGF23 increases the frequency of spontaneous calcium waves (SCWs), a marker of cardiomyocyte arrhythmogenicity, in ARVMs. FGF23 increased sarcoplasmic reticulum Ca²⁺ leakage, basal phosphorylation of Ca²⁺-cycling proteins including phospholamban and ryanodine receptor type 2. These effects are secondary to a decrease in phosphodiesterase 4B (PDE4B) in ARVMs and in heart of mice with RF. Soluble Klotho, a circulating form of the FGF23 receptor, prevents FGF23 effects on ARVMs by increasing PDE3A and PDE3B expression. Our results suggest that the combination of high FGF23 and low sKlotho concentrations decreases PDE activity in ARVMs, which favors the occurrence of ventricular arrhythmias and may participate in the high death rate observed in patients with CKD.

Interactions between FGF23 and Genotype in Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

Higher serum intact fibroblast growth factor 23 (iFGF23) was associated with disease progression in participants with autosomal dominant polycystic kidney disease (ADPKD) in the HALT-PKD Studies. PKD mutation is also an important determinant of progression. We hypothesized that serum levels of iFGF23 and vitamin D metabolites (1,25-dihydroxyvitamin D [1,25(OH)₂D] and 25-hydroxyvitamin D [25[OH]D]) differ according to ADPKD mutation and differentially predict clinical end points according to genotype (significant interaction between genotype and mineral metabolites).

METHODS

A total of 864 individuals with ADPKD who participated in the HALT-PKD Study A or B and had measurements of mineral metabolites (1,25[OH]₂D, 25[OH]D, iFGF23) were categorized by PKD mutation (PKD1 truncating, PKD1 nontruncating, PKD2, or no mutation detected [NMD]). The association of the interactions of genotype × iFGF23, genotype × 1,25(OH)2D, and genotype × 25(OH)D with (1) annualized change in eGFR; (2) mean annualized percentage change in height-corrected total kidney volume (Study A only); and (3) time to a composite of 50% reduction in eGFR, ESKD, or death were evaluated using linear regression and Cox proportional hazards regression.

RESULTS

Median (interquartile range) iFGF23 differed (PKD1 truncating, 55.8 [40.7-76.8]; PKD1 nontruncating, 49.9 [37.7-71.0]; PKD2, 49.0 [33.8-70.5]; NMD, 50.3 [39.7-67.4] pg/ml; P=0.03) and mean±SD 1,25(OH)₂D differed (PKD1 truncating, 32.8±12.8; PKD1 nontruncating, 33.4±12.5; PKD2, 34.1±13.1; NMD, 38.0±14.6 pg/ml; P=0.02) according to PKD genotype. There was a significant interaction between iFGF23 and genotype (P=0.02) for the composite end point in fully adjusted models, but no significant interaction between 1,25(OH)₂D or 25(OH)D and genotype for clinical end points.

CONCLUSIONS

ADPKD genotype interacts significantly with FGF23 to influence clinical end points. Whereas the worst outcomes were in individuals with a PKD1-truncating or -nontruncating mutation and the highest iFGF23 tertile, risk of the composite end point differed according to iFGF23 the most in the PKD1-nontruncating and PKD2 groups.

Biomarkers of vascular calcification in serum

Over the last decades, the association between vascular calcification (VC) and all-cause/cardiovascular mortality, especially in patients with high atherogenic status, such as those with diabetes and/or chronic kidney disease, has been repeatedly highlighted. For over a century, VC has been noted as a passive, degenerative, aging process without any treatment options. However, during the past decades, studies confirmed that mineralization of the arteries is an active, complex process, similar to bone genesis and formation. The main purpose of this review is to provide an update of the existing biomarkers of VC in serum and develop the various pathogenetic mechanisms underlying the calcification process, including the pivotal roles of matrix Gla protein, osteoprotegerin, bone morphogenetic proteins, fetuin-a, fibroblast growth-factor-23, osteocalcin, osteopontin, osteonectin, sclerostin, pyrophosphate, Smads, fibrillin-1 and carbonic anhydrase II.

Klotho/FGF23 and Wnt Signaling as Important Players in the Comorbidities Associated with Chronic Kidney Disease

Fibroblast Growth Factor 23 (FGF23) and Klotho play an essential role in the regulation of mineral metabolism, and both are altered as a consequence of renal failure. FGF23 increases to augment phosphaturia, which prevents phosphate accumulation at the early stages of chronic kidney disease (CKD). This effect of FGF23 requires the presence of Klotho in the renal tubules. However, Klotho expression is reduced as soon as renal function is starting to fail to generate a state of FGF23 resistance. Changes in these proteins directly affect to other mineral metabolism parameters; they may affect renal function and can produce damage in other organs such as bone, heart, or vessels. Some of the mechanisms responsible for the changes in FGF23 and Klotho levels are related to modifications in the Wnt signaling. This review examines the link between FGF23/Klotho and Wnt/β-catenin in different organs: kidney, heart, and bone. Activation of the canonical Wnt signaling produces changes in FGF23 and Klotho and vice versa; therefore, this pathway emerges as a potential therapeutic target that may help to prevent CKD-associated complications.

High Serum Phosphate Level as a Risk Factor to Determine Renal Prognosis in Autosomal Dominant Polycystic Kidney Disease: A Retrospective Study

Background: Serum phosphate levels, which are associated with the progression of renal dysfunction in chronic kidney disease, in patients with autosomal dominant polycystic kidney disease (ADPKD) are lower than those in patients with other kidney diseases. However, their role in ADPKD remains unclear. This study aimed to determine whether serum phosphate levels could have an association with renal prognoses among patients with ADPKD. Methods: In total, 55 patients with PKD1 or PKD2 mutations but not undergoing dialysis were evaluated. Data regarding serum phosphate levels were collected, and Cox regression analyses were used to calculate hazard ratios (HRs) with renal replacement therapy as the endpoint. Results: The median (quartile 1; quartile 3) serum phosphate concentration was 3.4 (3.1; 3.9) mg/dL, and the estimated glomerular filtration rate (eGFR) was 39.5 (17.6; 65.7) mL/min/1.73 m². The multivariate analysis that included age, PKD1 mutation, eGFR, urinary protein excretion, hyperuricemia, and serum phosphate determined that eGFR (HR, 0.82; 95% confidence interval (CI), 0.74-0.90; p < 0.0001) and serum phosphate (HR, 6.78; 95% CI, 1.94-34.02; p = 0.0021) were independently associated with renal replacement therapy. Conclusions: We found that serum phosphate levels were significantly associated with poor renal prognoses in patients with ADPKD.

Klotho supplementation ameliorates blood pressure and renal function in DBA/2-pcy mice, a model of polycystic kidney disease

Klotho interacts with various membrane proteins such as receptors for transforming growth factor-β (TGF-β) and insulin-like growth factor (IGF). Renal expression of klotho is diminished in polycystic kidney disease (PKD). In the present study, the effects of klotho supplementation on PKD were assessed. Recombinant human klotho protein (10 μg·kg^-1·day^-1) or a vehicle was administered daily by subcutaneous injection to 6-wk-old mice with PKD (DBA/2-pcy). Blood pressure was measured using tail-cuff methods. After 2 mo, mice were killed, and the kidneys were harvested for analysis. Exogenous klotho protein supplementation reduced kidney weight, cystic area, systolic blood pressure, renal angiotensin II levels, and 8-epi-PGF_2α excretion (P < 0.05). Klotho protein supplementation enhanced glomerular filtration rate, renal expression of superoxide dismutase, and klotho itself (P < 0.05). Klotho supplementation attenuated renal expressions of TGF-β and collagen type I and diminished renal abundance of Twist, phosphorylated Akt, and mammalian target of rapamycin (P < 0.05). Pathological examination revealed that klotho decreased the fibrosis index and nuclear staining of Smad in PKD kidneys (P < 0.05). Our data indicate that klotho protein supplementation ameliorates the renin-angiotensin system, reducing blood pressure in PKD mice. Furthermore, the present results implicate klotho supplementation in the suppression of Akt/mammalian target of rapamycin signaling, slowing cystic expansion. Finally, our findings suggest that klotho protein supplementation attenuated fibrosis at least partly by inhibiting epithelial mesenchymal transition in PKD.

A distinct bone phenotype in ADPKD patients with end-stage renal disease

Autosomal dominant polycystic kidney disease (ADPKD) is among the most common hereditary nephropathies. Low bone turnover osteopenia has been reported in mice with conditional deletion of the PKD1 and PKD2 genes in osteoblasts, and preliminary clinical data also suggest suppressed bone turnover in patients with ADPKD. The present study compared the bone phenotype between patients with end stage renal disease (ESRD) due to ADPKD and controls with ESRD due to other causes. Laboratory parameters of bone mineral metabolism (fibroblast growth factor 23 and sclerostin), bone turnover markers (bone alkaline phosphatase, tartrate-resistant acid phosphatase 5b) and bone mineral density (BMD, by dual energy x-ray absorptiometry, DXA) were assessed in 518 patients with ESRD, including 99 with ADPKD. Bone histomorphometry data were available in 71 patients, including 10 with ADPKD. Circulating levels of bone alkaline phosphatase were significantly lower in patients with ADPKD (17.4 vs 22.6 ng/mL), as were histomorphometric parameters of bone formation. Associations between ADPKD and parameters of bone formation persisted after adjustment for classical determinants including parathyroid hormone, age, and sex. BMD was higher in skeletal sites rich in cortical bone in patients with ADPKD compared to non-ADPKD patients (Z-score midshaft radius -0.04 vs -0.14; femoral neck -0.72 vs -1.02). Circulating sclerostin levels were significantly higher in ADPKD patients (2.20 vs 1.84 ng/L). In conclusion, patients with ESRD due to ADPKD present a distinct bone and mineral phenotype, characterized by suppressed bone turnover, better preserved cortical BMD, and high sclerostin levels.

Hemodialysis Product and Hip Fracture in Hemodialysis Patients: A Nationwide Cohort Study in Japan

Some have raised concerns that longer and more frequent hemodialysis (HD) would be associated with bone fractures due to excess phosphate removal. We examined the effects of hemodialysis product (HDP) on hip fracture incidence among Japanese HD patients using registry data of the Japanese Society for Dialysis Therapy. During a 1-year study period, 1411 hip fractures occurred among 135 984 patients. After adjusting for demographic and clinical factors, patients with a high HDP did not show a significant risk of hip fracture. Interestingly, patients with polycystic kidney disease had a lower risk of hip fracture. Our findings did not support the hypothesis that patients undergoing longer and more frequent HD would face a higher risk of hip fracture than those undergoing shorter and less frequent HD. Polycystic kidney disease was identified as a new significant factor for hip fracture; relative to glomerulonephritis, this condition was associated with a lower risk of hip fracture.

Recent advances in the clinical management of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic systemic disorder causing the development of renal and hepatic cysts and decline in renal function. It affects around 1 in 1,000 live births. Early hypertension and progressive renal failure due to massive enlargement of cysts and fibrosis are hallmarks of the disease. This article reviews recent advances in ADPKD and focuses mainly on diagnosis, management, and prediction of the course of the disease.

The role of klotho in chronic kidney disease

Chronic kidney disease (CKD) is an inherently systemic disease that refers to a long-term loss of kidney function. The progression of CKD has repercussions for other organs, leading to many kinds of extrarenal complications. Intensive studies are now being undertaken to reveal the risk factors and pathophysiological mechanism of this disease. During the past 20 years, increasing evidence from clinical and basic studies has indicated that klotho, which was initially known as an anti-aging gene and is mainly expressed in the kidney, is significantly correlated with the development and progression of CKD and its complications. Here, we discuss in detail the role and pathophysiological implications of klotho in ion disorders, the inflammation response, vascular calcification, mineral bone disorders, and renal fibrosis in CKD. Based on the pathogenic mechanism of klotho deficiency and klotho decline in urine early in CKD stage 2 and even earlier in CKD stage 1, it is not difficult to understand that soluble klotho can serve as an early and sensitive marker of CKD. Moreover, the prevention of klotho decline by several mechanisms can attenuate renal injuries, retard CKD progression, ameliorate extrarenal complications, and improve renal function. In this review, we focus on the functions and pathophysiological implications of klotho in CKD and its extrarenal complications as well as its potential applications as a diagnostic and/or prognostic biomarker for CKD and as a novel treatment strategy to improve and decrease the burden of comorbidity in CKD.

Hepatic Production of Fibroblast Growth Factor 23 in Autosomal Dominant Polycystic Kidney Disease

CONTEXT

The bone-derived hormone fibroblast growth factor (FGF) 23 controls phosphate homeostasis and urinary phosphate excretion. FGF23 plasma levels increase in the early stage of renal insufficiency to prevent hyperphosphatemia. Recent evidence suggests that this increase has effects on cardiac and immune cells that compromise patients' health. Patients with autosomal dominant polycystic kidney disease (ADPKD) have been reported to have higher FGF23 concentrations than other patients with similar renal function. The significance of this finding has remained unknown.

METHODS AND RESULTS

Analyzing the FGF23 plasma levels in 434 patients with ADPKD and 355 control subjects with a measured glomerular filtration rate (mGFR) between 60 and 120 mL/min per 1.73 m2, we confirmed that patients with ADPKD had higher FGF23 plasma concentrations than controls. Remarkably, this difference did not translate into renal phosphate leakage. Using different assays for FGF23, we found that this discrepancy was explained by a predominant increase in the cleaved C-terminal fragment of FGF23, which lacks phosphaturic activity. We found that FGF23 plasma concentration independently correlated with the severity of cystic liver disease in ADPKD. We observed that, in contrast to control liver tissues, the cystic liver from patients with ADPKD markedly expressed FGF23 messenger RNA and protein. In line with this finding, the surgical reduction of polycystic liver mass was associated with a decrease in FGF23 plasma levels independently of any modification in mGFR, phosphate, or iron status.

CONCLUSION

Our findings demonstrate that severely polycystic livers produce FGF23 and increase levels of circulating FGF23 in patients with ADPKD.

A decreased soluble Klotho level with normal eGFR, FGF23, serum phosphate, and FEP in an ADPKD patient with enlarged kidneys due to multiple cysts

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary renal disorder. ADPKD is characterized clinically by the presence of multiple bilateral renal cysts that lead to chronic renal failure. The cysts evolve from renal tubular epithelial cells that express the Klotho gene. Notably, Klotho acts as a co-receptor for fibroblast growth factor 23 (FGF23); in this context, it induces phosphaturia and maintains serum phosphate at a normal level. Many reports have shown that decreases in the soluble Klotho level and increases in the FGF23 level are associated with glomerular filtration rate (GFR) decline, but a recent study observed these changes in patient with normal eGFR. It remains unclear whether the decrease in the Klotho level precedes the increase in FGF23. Here, we present an ADPKD patient with enlarged kidneys due to multiple cysts who had a decreased soluble Klotho level but a normal eGFR and a normal FGF23 level. The patient's serum phosphate level was normal, as was the fractional excretion of phosphate (FEP). This appears to be the first reported case to show a decreased soluble Klotho level plus normal eGFR, FGF23, and FEP. These results suggest that Klotho decreases before FGF23 increases and further suggest that Klotho is not required to maintain normal serum phosphate levels in ADPKD if the FEP and serum phosphate levels are normal.

Reduced Levels of Anti-Ageing Hormone Klotho Predict Renal Function Decline in Type 2 Diabetes

CONTEXT AND OBJECTIVE

Soluble Klotho (sKlotho) is a circulating hormone with cardiovascular-renal protective effects. Whether sKlotho predicts estimated glomerular filtration rate (eGFR) decline in patients with type 2 diabetes mellitus (T2DM) with relatively preserved renal function is unknown.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

Single-center observational follow-up study of 101 patients with T2DM and eGFR >45 mL/min [91% on renin angiotensin system (RAS) blockade] followed for a median of 9 years (range, 2 to 13 years).

MAIN OUTCOME

Primary outcome was a >50% decline in eGFR. sKlotho, serum phosphorus, serum calcium, and fibroblast growth factor-23 levels were measured from stored samples collected at baseline. Patients were followed up with standardized clinical and biochemical measurements.

RESULTS

Patients with residual microalbuminuria (MA) despite RAS blockade (n = 53) had significantly lower levels of sKlotho [median, 184.7 pg/mL; interquartile range (IQR), 130.5 to 271.8 pg/mL) compared with patients without MA (n = 39; median, 235.2 pg/mL; IQR, 172.0 to 289.4 pg/mL; P = 0.03). Of the cohort, 21% reached the primary outcome. In a competing risk analysis, a 10% higher sKlotho level reduced the incidence of the primary outcome by 12% (hazard ratio, 0.27; 95% confidence interval, 0.15 to 0.52; P < 0.001] independent of traditional risk factors. Patients with sKlotho below the median of 204.4 pg/mL had nearly a fourfold higher cumulative incidence of the primary outcome compared with those above the median (24% vs 6.2%; P = 0.01).

CONCLUSIONS

In patients with T2DM with relatively preserved eGFR, reduced levels of sKlotho predict renal function decline independent of traditional risk markers. sKlotho is a biomarker of renal dysfunction and a potential treatment target for renoprotection in T2DM.

Evidence for Bone and Mineral Metabolism Alterations in Children With Autosomal Dominant Polycystic Kidney Disease

CONTEXT

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disease. Hypophosphatemia was demonstrated in adult patients with preserved renal function, together with high fibroblast growth factor 23 (FGF23) and low soluble Klotho levels. The latter explained the relative FGF23 hyporesponsiveness in this cohort.

OBJECTIVE

Evaluating phosphate and bone mineral metabolism in children with ADPKD compared with what is known in adult ADPKD patients.

DESIGN

Observational cross-sectional study.

SETTING

Multicenter study via ambulatory care in tertiary centers.

PARTICIPANTS

Ninety-two children with ADPKD (52 males; mean ± standard deviation age, 10.2 ± 5.0 years) and 22 healthy controls (HCs, 10 males; mean ± standard deviation age, 10.3 ± 4.1 years).

MAIN OUTCOME MEASURES

The predictor was early ADPKD stage. Bone mineral metabolism and renal phosphate handling were the main outcome measures. Performed measurements were serum phosphate, tubular maximum phosphorus reabsorption per glomerular filtration rate, FGF23, soluble Klotho, sclerostin, and bone alkaline phosphatase.

RESULTS

ADPKD children had significantly lower serum phosphate levels compared with HC. Low tubular maximum phosphorus reabsorption per glomerular filtration rate was observed in 24% of patients, although not significantly different from HC. Serum FGF23 and soluble Klotho levels were comparable between patients and HC. In addition, we showed decreased bone alkaline phosphatase levels in ADPKD children, suggesting suppressed bone formation.

CONCLUSIONS

This report demonstrates hypophosphatemia and suppressed bone formation in a pediatric ADPKD cohort, with preserved renal function, compared with HC. Although FGF23 levels were not different from controls, they should be considered inappropriate, given the concomitant hypophosphatemia. Further studies are required to elucidate underlying pathophysiology and potential clinical consequences.

Serum calcitriol levels in a patient with X-linked hypophosphatemia complicated by autosomal dominant polycystic kidney disease

Serum calcitriol [1,25(OH)2D] levels are low normal in the presence of hypophosphatemia in X-linked dominant hypophosphatemic rickets (XLH) due to elevated serum fibroblast growth factor 23 (FGF23) levels. We report a peculiar finding of markedly elevated serum 1,25(OH)2D levels in a patient with XLH complicated by autosomal dominant polycystic kidney disease (ADPKD) and retinitis pigmentosa (RP). She was diagnosed with XLH, ADPKD and RP at the age of 5, 13 and 15 years, respectively. After the diagnosis of ADPKD, the 1,25(OH)2D levels increased up to 282 pg/mL treated with a pharmacological dose of 1αOHD3 (1αOHD3). At the age of 17 years, under a physiological dose, serum 25-hydroxyvitamin D3 (25OHD3) and 1,25(OH)2D levels were 14.0 ng/mL and 93 pg/mL, respectively. With off-therapy for 3 days, the latter decreased to 9 pg/mL with an increased ratio of 24,25-dihydroxyvitamin D3 (24,25(OH)2D3)/25OHD3. Serum FGF23 and circulating soluble α-klotho (sKlotho) levels were 552 and 873 pg/mL, respectively, with the latter being almost half of the normal control. Since the coexistence of elevated serum FGF23 and 1,25(OH)2D levels contradicted the conventional FGF23 actions, we speculated on the possible association of decreased sKlotho with the peculiar vitamin D metabolism, although mechanisms in detail remained unknown.

Perturbations of the anti-ageing hormone Klotho in patients with type 1 diabetes and microalbuminuria

AIMS/HYPOTHESIS

Patients with type 1 diabetes and microalbuminuria are at high risk of cardiovascular disease (CVD) and end-stage renal disease. Soluble Klotho is an anti-ageing circulating hormone involved in phosphate metabolism and vascular homeostasis through protective effects on the endothelium and antioxidant actions. The role of soluble Klotho in patients with type 1 diabetes and microalbuminuria is unknown.

METHODS

In a cross-sectional single-centre study we evaluated the levels of circulating serum soluble Klotho in 33 participants with type 1 diabetes and a history of microalbuminuria (receiving renin-angiotensin system [RAS] inhibitors) and 45 participants with type 1 diabetes without a history of microalbuminuria (not receiving RAS or other antihypertensive drugs). All participants had an eGFR >45 ml/min, duration of diabetes >20 years and no history of CVD. Serum soluble Klotho levels were measured by a validated immunoassay.

RESULTS

Participants with microalbuminuria had significantly lower levels of serum Klotho compared with those without microalbuminuria (median [interquartile range], 659.3 [525.3, 827.6] vs 787.7 [629.5, 1007]; p = 0.023). This difference persisted after adjustment for variables including age and eGFR. In a subgroup of 30 individuals with and without microalbuminuria, other markers of phosphate balance were not significantly different.

CONCLUSIONS/INTERPRETATION

In individuals with type 1 diabetes, microalbuminuria is associated with soluble Klotho deficiency. Further studies are required to determine whether soluble Klotho is causally related to the development of cardio-renal disease in type 1 diabetes.

Is Autosomal Dominant Polycystic Kidney Disease Becoming a Pediatric Disorder?

Autosomal dominant polycystic kidney disease (ADPKD) affects 1 in 400 to 1,000 live births, making it the most common monogenic cause of renal failure. Although no definite cure is available yet, it is important to affect disease progression by influencing modifiable factors such as hypertension and proteinuria. Besides this symptomatic management, the only drug currently recommended in Europe for selected adult patients with rapid disease progression, is the vasopressin receptor antagonist tolvaptan. However, the question remains whether these preventive interventions should be initiated before extensive renal damage has occurred. As renal cyst formation and expansion begins early in life, frequently in utero, ADPKD should no longer be considered an adult-onset disease. Moreover, the presence of hypertension and proteinuria in affected children has been reported to correlate well with disease severity. Until now, it is controversial whether children at-risk for ADPKD should be tested for the presence of the disease, and if so, how this should be done. Herein, we review the spectrum of pediatric ADPKD and discuss the pro and contra of testing at-risk children and the challenges and unmet needs in pediatric ADPKD care.

The importance of klotho in phosphate metabolism and kidney disease

The discovery of fibroblast growth factor-23 (FGF23) and its co-receptor α-klotho has broadened our understanding of mineral metabolism and led to a renewed research focus on phosphate homeostatic pathways in kidney disease. Expanding knowledge of these mechanisms, both in normal physiology and in pathology, identifies targets for potential interventions designed to reduce the complications of renal disease, particularly the cardiovascular sequelae. FGF23 has emerged as a major α-klotho-dependent endocrine regulator of mineral metabolism, functioning to activate vitamin D and as a phosphatonin. However, increasingly there is an appreciation that klotho may act independently as a phosphate regulator, as well as having significant activity in other key biological processes. This review outlines our current understanding of klotho, and its potential contribution to kidney disease and cardiovascular health.

New Insights into the Molecular Mechanisms Targeting Tubular Channels/Transporters in PKD Development

BACKGROUND

Autosomal dominant polycystic kidney disease (PKD) or autosomal recessive PKD is caused by a mutation in the PKD1, PKD2 or PKHD1 gene, which encodes polycystin-1, polycystin-2 or fibrocystin, respectively. Embryonic and postnatal mutation studies show that transport or channel function is dysregulated before the initiation of cystogenesis, suggesting that the abnormality of transport or channel function plays a critical role in the pathology of PKD.

SUMMARY

Polycystin-2 by itself is a calcium-permeable cation channel, and its channel function can be regulated by polycystin-1 or fibrocystin. In this paper, we reviewed the current knowledge about calcium transports and cyclic adenosine monophosphate (cAMP)-driven chloride transports in PKD. In addition, the function and the underlining mechanism of glucose transporters, phosphate transporters and water channels in PKD are also discussed.

KEY MESSAGES

Abnormalities in calcium handling and exuberant cAMP-dependent cystic fibrosis transmembrane conductance regulator-mediated fluid secretion in the collecting duct are the most important issues in the pathogenesis of PKD.

Renal Production, Uptake, and Handling of Circulating αKlotho

αKlotho is a multifunctional protein highly expressed in the kidney. Soluble αKlotho is released through cleavage of the extracellular domain from membrane αKlotho by secretases to function as an endocrine/paracrine substance. The role of the kidney in circulating αKlotho production and handling is incompletely understood, however. Here, we found higher αKlotho concentration in suprarenal compared with infrarenal inferior vena cava in both rats and humans. In rats, serum αKlotho concentration dropped precipitously after bilateral nephrectomy or upon treatment with inhibitors of αKlotho extracellular domain shedding. Furthermore, the serum half-life of exogenous αKlotho in anephric rats was four- to five-fold longer than that in normal rats, and exogenously injected labeled recombinant αKlotho was detected in the kidney and in urine of rats. Both in vivo (micropuncture) and in vitro (proximal tubule cell line) studies showed that αKlotho traffics from the basal to the apical side of the proximal tubule via transcytosis. Thus, we conclude that the kidney has dual roles in αKlotho homeostasis, producing and releasing αKlotho into the circulation and clearing αKlotho from the blood into the urinary lumen.

Fibroblast growth factor 23 and acute kidney injury

Fibroblast growth factor 23 (FGF23), which is produced in bone, participates in the maintenance of phosphate metabolism and can serve as a biomarker for adverse cardiovascular outcomes in patients with chronic kidney disease and end-stage renal disease. Circulating FGF23 rapidly increases after acute kidney injury (AKI), preceding other known markers such as neutrophil gelatinase-associated lipocalin and serum creatinine. The increase in FGF23 in AKI appears to be independent of parathyroid hormone, vitamin D signaling pathways, and dietary phosphate. The potential mechanisms include: (1) increased production of FGF23 in the bone by yet-to-be-identified factors; (2) ectopic production of FGF23 by injured renal tubules; and (3) decreased renal clearance of circulating FGF23. Circulating FGF23 determined by intact FGF23 enzyme-linked immunosorbent assay (ELISA) is a more reliable biomarker of AKI than FGF23 C-terminal ELISA (a mixed readout of C-terminal fragment and intact FGF23). Given that FGF23 can be ectopically expressed in differentiated renal tubules and iron modulates FGF23 metabolism, an effect of iron on FGF23 expression in renal tubules is conceivable but remains to be confirmed. More clinical and experimental studies are required to validate the use of circulating FGF23 as a biomarker for the early identification of AKI and prediction of short- and long-term adverse outcomes post-AKI. More importantly, the biologic effect of increased FGF23 in AKI needs to be defined.

αKlotho and vascular calcification: an evolving paradigm

PURPOSE OF REVIEW

Cardiovascular disease remains the single most serious contributor to mortality in chronic kidney disease (CKD). Although conventional risk factors are prevalent in CKD, both cardiomyopathy and vasculopathy can be caused by pathophysiologic mechanisms specific to the uremic state. CKD is a state of systemic αKlotho deficiency. Although the molecular mechanism of action of αKlotho is not well understood, the downstream targets and biologic functions of αKlotho are astonishingly pleiotropic. An emerging body of literature links αKlotho to uremic vasculopathy.

RECENT FINDINGS

The expression of αKlotho in the vasculature is controversial because of conflicting data. Regardless of whether αKlotho acts as a circulating or resident protein, there are good data associating changes in αKlotho levels with vascular pathology including vascular calcification and in-vitro data of the direct action of αKlotho on both the endothelium and vascular smooth muscle cells in terms of cytoprotection and prevention of mineralization.

SUMMARY

It is critical to understand the pathogenic role of αKlotho on the integral endothelium-vascular smooth muscle network rather than each cell type in isolation in uremic vasculopathy, as αKlotho can serve as a potential prognostic biomarker and a biological therapeutic agent.

Osteoblast-specific deletion of Pkd2 leads to low-turnover osteopenia and reduced bone marrow adiposity

Polycystin-1 (Pkd1) interacts with polycystin-2 (Pkd2) to form an interdependent signaling complex. Selective deletion of Pkd1 in the osteoblast lineage reciprocally regulates osteoblastogenesis and adipogenesis. The role of Pkd2 in skeletal development has not been defined. To this end, we conditionally inactivated Pkd2 in mature osteoblasts by crossing Osteocalcin (Oc)-Cre;Pkd2^+/null mice with floxed Pkd2 (Pkd2^flox/flox) mice. Oc-Cre;Pkd2^flox/null (Pkd2^Oc-cKO) mice exhibited decreased bone mineral density, trabecular bone volume, cortical thickness, mineral apposition rate and impaired biomechanical properties of bone. Pkd2 deficiency resulted in diminished Runt-related transcription factor 2 (Runx2) expressions in bone and impaired osteoblastic differentiation ex vivo. Expression of osteoblast-related genes, including, Osteocalcin, Osteopontin, Bone sialoprotein (Bsp), Phosphate-regulating gene with homologies to endopeptidases on the X chromosome (Phex), Dentin matrix protein 1 (Dmp1), Sclerostin (Sost), and Fibroblast growth factor 23 (FGF23) were reduced proportionate to the reduction of Pkd2 gene dose in bone of Oc-Cre;Pkd2^flox/+ and Oc-Cre;Pkd2^flox/null mice. Loss of Pkd2 also resulted in diminished peroxisome proliferator-activated receptor γ (PPARγ) expression and reduced bone marrow fat in vivo and reduced adipogenesis in osteoblast culture ex vivo. Transcriptional co-activator with PDZ-binding motif (TAZ) and Yes-associated protein (YAP), reciprocally acting as co-activators and co-repressors of Runx2 and PPARγ, were decreased in bone of Oc-Cre;Pkd2^flox/null mice. Thus, Pkd1 and Pkd2 have coordinate effects on osteoblast differentiation and opposite effects on adipogenesis, suggesting that Pkd1 and Pkd2 signaling pathways can have independent effects on mesenchymal lineage commitment in bone.

The demonstration of αKlotho deficiency in human chronic kidney disease with a novel synthetic antibody

BACKGROUND

αKlotho is the prototypic member of the Klotho family and is most highly expressed in the kidney. αKlotho has pleiotropic biologic effects, and in the kidney, its actions include regulation of ion transport, cytoprotection, anti-oxidation and anti-fibrosis. In rodent models of chronic kidney disease (CKD), αKlotho deficiency has been shown to be an early biomarker as well as a pathogenic factor. The database for αKlotho in human CKD remains controversial even after years of study.

METHODS

We used a synthetic antibody library to identify a high-affinity human antigen-binding fragment that recognizes human, rat and mouse αKlotho primarily in its native, rather than denatured, form.

RESULTS

Using an immunoprecipitation-immunoblot (IP-IB) assay, we measured both serum and urinary levels of full-length soluble αKlotho in humans and established that human CKD is associated with αKlotho deficiency in serum and urine. αKlotho levels were detectably lower in early CKD preceding disturbances in other parameters of mineral metabolism and progressively declined with CKD stages. We also found that exogenously added αKlotho is inherently unstable in the CKD milieu suggesting that decreased production may not be the sole reason for αKlotho deficiency.

CONCLUSION

Synthetic antibody libraries harbor tremendous potential for a variety of biomedical and clinical applications. Using such a reagent, we furnish data in support of αKlotho deficiency in human CKD, and we set the foundation for the development of diagnostic and therapeutic applications of anti-αKlotho antibodies.

Association of serum levels of FGF23 and α-Klotho with glomerular filtration rate and proteinuria among cardiac patients

Background

Expression and/or excretion of fibroblast growth factor-23 (FGF23) and its co-receptor Klotho are altered in patients with end-stage renal disease. The possibility that the FGF23/α-Klotho system mediates the aggravated cardiovascular outcome among patients with chronic kidney disease (CKD) has been suggested. We determined whether FGF23 and α-Klotho concentrations are altered among patients with reduced renal function and proteinuria.

Methods

Serum FGF23 and α-Klotho were measured in cardiology patients who were not undergoing chronic hemodialysis. Estimated glomerular filtration rate (eGFR) was correlated negatively with FGF23 and positively with α-Klotho.

Results

The correlation between FGF23 and the renal tubular maximum reabsorption rate of phosphate to the GFR (TmP/GFR) was not significant, but that between FGF23 and serum calcium or inorganic phosphate was significant among patients with an estimated GFR of less than 60 mL/min/m². By stepwise multivariate regression analysis, eGFR was selected as significant predictor for FGF23 or α-Klotho among patients with an estimated GFR of less than 60 mL/min/m²; however, urine albumin/creatinine ratio was not selected as a predictor for FGF23 or α-Klotho irrespective of the eGFR levels. In patients with eGFR of <60 mL/min/1.73 m², UACR was significantly associated with log(FGF23); but, this association did not remain statistically significant in a multivariate model.

Conclusions

Among cardiology patients with various stages of CKD, serum concentrations of FGF23 and α-Klotho were associated with renal function, but not with the extent of proteinuria.

Left ventricular mass progression despite stable blood pressure and kidney function in stage 3 chronic kidney disease

BACKGROUND/AIMS

Progressive chronic kidney disease (CKD) is associated with worsening cardiovascular (CV) risk not explained by traditional risk factors. Left ventricular (LV) hypertrophy (LVH) is an important CV risk factor, but its progression has not been documented in early CKD. We explored whether progression of LVH in early CKD would occur despite stable kidney function.

METHODS

We conducted a post hoc analysis of a 12-month study of lanthanum carbonate in stage 3 CKD, which included longitudinal assessments of CV biomarkers. Primary outcome for the analysis was the change in LV mass (LVM) indexed to height in meters(2.7) (LVM/Ht(2.7)). Secondary outcomes were changes in blood pressure (BP), pulse-wave velocity, LV systolic/diastolic function, fibroblast growth factor 23 (FGF23), klotho, and estimated glomerular filtration rate (eGFR).

RESULTS

Thirty-one of 38 original subjects had sufficient data for analysis. LVM/Ht(2.7) increased (47 ± 13 vs. 53 ± 13 g/m(2.7), p = 0.006) over 12 months despite stable BP, stable eGFR and normal LV systolic function. Vascular stiffness and LV diastolic dysfunction persisted throughout the study. Klotho levels decreased (748 ± 289 to 536 ± 410 pg/ml, p = 0.03) but were unrelated to changes in LVM/Ht(2.7). The change in FGF23/klotho ratio was strongly correlated with changes in LVM/Ht(2.7) (r2 = 0.582, p = 0.03).

CONCLUSION

Subjects with stage 3 CKD exhibited increasing LVM, persistent LV diastolic dysfunction and vascular stiffness despite stable kidney function, BP and LV systolic function. Abnormal FGF23 signaling due to reduced klotho expression may be associated with increasing LVM.

Soluble Klotho levels in adult renal transplant recipients are modulated by recombinant human erythropoietin

BACKGROUND

Data on serum soluble Klotho levels in chronic kidney disease are contradictory and even less is known after renal transplantation. Experimental studies demonstrated that recombinant human erythropoietin (rhEPO) treatment mitigates Klotho reduction caused by renal damage. Therefore, this study aimed to determine serum Klotho levels in a cohort of kidney transplant recipients (KTR) and to evaluate whether rhEPO treatment can modulate, in vivo and in vitro, soluble Klotho.

METHODS

117 KTR and 22 healthy subjects (HS) were enrolled. In 17 KTR, rhEPO was discontinued for 5 weeks and Klotho levels were compared to 34 propensity score-matched controls. Moreover, we evaluated Klotho mRNA expression and protein secretion in HK-2 tubular cells treated with cyclosporin A (CyA) and rhEPO, alone or in combination.

RESULTS

Serum Klotho levels in KTR were significantly higher than in HS (0.68 vs. 0.37, p = 0.002) and significantly associated with estimated glomerular filtration rate (r = -0.378, p = 0.003) and fibroblast growth factor 23 (r = -0.307, p < 0.0001). After 5 weeks of rhEPO discontinuation, treated KTR showed a sharper reduction of Klotho levels than controls (-0.56 vs. -0.11 ng/ml, p < 0.0001). In HK-2 cells CyA treatment induced a Klotho down-regulation that was mitigated by rhEPO pre-treatment. In the same experimental conditions, our results revealed that cells treated with CyA + rhEPO secreted higher soluble Klotho levels than those exposed to CyA or rhEPO alone.

CONCLUSIONS

Our results demonstrate that KTR have higher serum Klotho levels than HS and that rhEPO treatment modulates these concentrations, suggesting a link between rhEPO and soluble Klotho in KTR.

Urine proteomes of healthy aging humans reveal extracellular matrix (ECM) alterations and immune system dysfunction

Aging is a complex physiological process that poses considerable conundrums to rapidly aging societies. For example, the risk of dying from cardiovascular diseases and/or cancer steadily declines for people after their 60s, and other causes of death predominate for seniors older than 80 years of age. Thus, physiological aging presents numerous unanswered questions, particularly with regard to changing metabolic patterns. Urine proteomics analysis is becoming a non-invasive and reproducible diagnostic method. We investigated the urine proteomes in healthy elderly people to determine which metabolic processes were weakened or strengthened in aging humans. Urine samples from 37 healthy volunteers aged 19-90 years (19 men, 18 women) were analyzed for protein expression by liquid chromatography-tandem mass spectrometry. This generated a list of 19 proteins that were differentially expressed in different age groups (young, intermediate, and old age). In particular, the oldest group showed protein changes reflective of altered extracellular matrix turnover and declining immune function, in which changes corresponded to reported changes in cardiovascular tissue remodeling and immune disorders in the elderly. Thus, urinary proteome changes in the elderly appear to reflect the physiological processes of aging and are particularly clearly represented in the circulatory and immune systems. Detailed identification of "protein trails" creates a more global picture of metabolic changes that occur in the elderly.

FGF23 and Klotho in chronic kidney disease

PURPOSE OF REVIEW

The wealth of data regarding fibroblast growth factor-23 (FGF23) and Klotho in chronic kidney disease (CKD) has risen exponentially over the past decade. This review is an attempt to summarize pivotal aspects of previous research, provide an update of recent findings and define important areas for future investigation.

RECENT FINDINGS

The phosphaturic hormone FGF23 increases dramatically as renal function declines. Identification of contributing stimuli to the rise in FGF23 is fundamental and recent evidence suggest a multifactorial cause which entails perturbed osteocyte function and renal mechanisms such as Klotho deficiency and, somewhat paradoxically, systemic Klotho excess. Circulating FGF23 predicts adverse outcomes, particularly cardiovascular disease, in CKD as well as in the general population. The concept of FGF23 merely as a biomarker and regulator of mineral metabolism is currently challenged by data linking FGF23 to pathological processes such as cardiac hypertrophy. Conversely, tissue level of the FGF23 coreceptor Klotho declines in early CKD and this deficiency is linked to accelerated ageing, cellular senescence, vascular calcification, oxidative stress and renal fibrosis. At present, methodological difficulties limit the utility of soluble Klotho measurements. Animal proof-of-concept studies have demonstrated beneficial effects of Klotho delivery in CKD, whereas anti-FGF23 therapy using neutralizing antibodies improved biochemical and bone parameters at the expense of increased vascular calcification and mortality.

SUMMARY

Pathological alterations of FGF23-Klotho in CKD are implicated as clinical biomarkers and may provide novel therapeutic strategies to alleviate the cardiovascular risk and slow CKD progression.

Renal expression of FGF23 and peripheral resistance to elevated FGF23 in rodent models of polycystic kidney disease

Fibroblast growth factor 23 (FGF23) regulates phosphate homeostasis and is linked to cardiovascular disease and all-cause mortality in chronic kidney disease. FGF23 rises in patients with CKD stages 2-3, but in patients with autosomal dominant polycystic kidney disease, the increase of FGF23 precedes the first measurable decline in renal function. The mechanisms governing FGF23 production and effects in kidney disease are largely unknown. Here we studied the relation between FGF23 and mineral homeostasis in two animal models of PKD. Plasma FGF23 levels were increased 10-fold in 4-week-old cy/+ Han:SPRD rats, whereas plasma urea and creatinine concentrations were similar to controls. Plasma calcium and phosphate levels as well as TmP/GFR were similar in PKD and control rats at all time points examined. Expression and activity of renal phosphate transporters, the vitamin D3-metabolizing enzymes, and the FGF23 co-ligand Klotho in the kidney were similar in PKD and control rats through 8 weeks of age, indicating resistance to FGF23, although phosphorylation of the FGF receptor substrate 2α protein was enhanced. In the kidneys of rats with PKD, FGF23 mRNA was highly expressed and FGF23 protein was detected in cells lining renal cysts. FGF23 expression in bone and spleen was similar in control rats and rats with PKD. Similarly, in an inducible Pkd1 knockout mouse model, plasma FGF23 levels were elevated, FGF23 was expressed in kidneys, but renal phosphate excretion was normal. Thus, the polycystic kidney produces FGF23 but is resistant to its action.

Fibroblast growth factor 23 and 25-hydroxyvitamin D levels are associated with estimated glomerular filtration rate decline

The combination of serum 25-hydroxyvitamin D (25D) and fibroblast growth factor 23 (FGF23) levels predict hard renal outcomes in patients with chronic kidney disease (CKD), independent of classical markers of mineral and bone disorders, including serum phosphorus, parathyroid hormone, 1,25-dihydroxyvitamin D levels, and active vitamin D therapy. In a prospective cohort study of 738 Japanese pre-dialysis outpatients with CKD, we examined potentially non-linear associations between 25D and FGF23 levels and estimated glomerular filtration rate (eGFR) changes in 727 patients with at least a 6-month observation period and no history of admission by acute kidney injury. We used multiple regression analyses with restricted cubic spline functions using annualized eGFR decline as a dependent variable. A significantly non-linear positive relationship between 25D and eGFR changes was observed. The annualized eGFR decline was greater in patients with 25D concentrations <25 and 23 ng/ml in univariate and multivariate analyses, respectively. Above this threshold, the eGFR decline plateaued. FGF23 showed a linear negative association with eGFR changes. After dividing the patients into four groups according to median 25D and FGF23 levels, the annualized eGFR changes in the Low FGF23-Low 25D, High FGF23-High 25D, and High FGF23-Low 25D groups were 0.49 (95% confidence intervals: -2.83 to 3.81), -1.24 (-5.00 to 2.52), -4.77 (-8.85 to -0.69), respectively, relative to the Low FGF23-High 25D group (P for trend, 0.02). Thus, combined use of FGF23 and 25D is useful to predict eGFR change in patients with CKD as well as hard renal outcomes.

Growth hormone and Klotho

Acromegaly is characterized by excessively high GH and IGF1 levels. Recent data suggest that soluble Klotho (sKlotho) is also elevated in patients with active acromegaly. sKlotho decreases towards normal following removal of the GH-producing pituitary adenoma. The Klotho gene was identified in mice following its accidental disruption by ectopic DNA. It is an ageing suppressor gene of restricted expression (mainly in kidneys, brain, and parathyroid and pituitary glands) encoding a transmembrane protein, mKlotho. mKlotho serves as a co-receptor in fibroblast growth factor 23 (FGF23) signalling. FGF23 promotes urinary phosphate excretion and inhibits the synthesis of calcitriol. The ectodomain of mKlotho is enzymatically released to result in a humoral factor, sKlotho, which exerts systemic effects (on ion channels and signalling pathways), possibly by working as an enzyme that modifies glycans of cell surface glycoproteins. GH enhances renal phosphate reabsorption and calcitriol production, i.e. exerts effects in the proximal tubule opposing those attributed to mKlotho, and attenuates calciuria in the distal tubule similar to sKlotho. sKlotho can be measured in extracellular fluids (serum, urine and cerebrospinal fluid (CSF)) by an ELISA. In line with predominant expression of Klotho in kidneys and choroid plexus, concentrations of sKlotho are particularly high in urine and CSF. Determination of sKlotho in serum and urine (both presumably reflecting GH action on the kidneys) could be used as a supplementary tool in the diagnosis and follow-up of patients with acromegaly. The question arises whether GH exerts selected actions via modifying activities of Klotho.

From bone abnormalities to mineral metabolism dysregulation in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic cause of kidney failure. It is a systemic disorder, not only affecting the kidneys, but also associated with cyst formation in other organs such as the liver, spleen, pancreas, and seminal vesicles. Other extra-renal symptoms may consist of intracranial arterial aneurysms, cardiac valvular defects, abdominal and inguinal hernias and colonic diverticulosis. Very little is known regarding bone involvement in ADPKD; however, recent evidence has revealed the potential role of fibroblast growth factor 23 (FGF23). FGF23 is an endocrine fibroblast growth factor acting in the kidney as a phosphaturic hormone and a suppressor of active vitamin D with key effects on the bone/kidney/parathyroid axis, and has been shown to increase in patients with ADPKD, even with normal renal function. The aim of this review is to provide an overview of bone and mineral abnormalities found in experimental models and in patients with ADPKD, and to discuss the possible role of FGF23 in this disease.

Infection and smoking are associated with decreased plasma concentration of the anti-aging protein, α-klotho

OBJECTIVE

The objective of this study was to determine whether maternal plasma concentrations of soluble α-klotho are different between women with microbial invasion of the intra-amniotic cavity (MIAC) and those without MIAC among preterm labor and intact membranes (PTL) or preterm prelabor rupture of membranes (pPROM).

METHODS

A cross-sectional study was conducted to include women in the following groups: i) PTL with MIAC (n=14); ii) PTL without MIAC (n=79); iii) pPROM with MIAC (n=30); and iv) pPROM without MIAC (n=33). MIAC was defined as a positive amniotic fluid culture for microorganisms (aerobic/anaerobic bacteria or genital mycoplasmas). Amniotic fluid samples were obtained within 48 h of maternal blood collection. Plasma concentration of soluble α-klotho was determined by ELISA.

RESULTS

i) The median plasma concentration (pg/mL) of soluble α-klotho was significantly lower in patients with MIAC than in those without MIAC (787.0 vs. 1117.8; P<0.001). ii) Among patients with PTL, those with MIAC had a lower median plasma concentration (pg/mL) of soluble α-klotho than those without MIAC (787.0 vs. 1138.9; P=0.007). iii) Among patients with pPROM, those with MIAC had a lower median plasma concentration (pg/mL) of soluble α-klotho than those without MIAC (766.4 vs. 1001.6; P=0.045). iv) There was no significant difference in the median plasma concentration of soluble α-klotho between PPROM without MIAC and PTL without MIAC (1001.6 pg/mL vs. 1138.9 pg/mL, respectively; P=0.5). v) After adjustment for potential confounders (maternal age, tobacco use, gestational age at venipuncture), soluble α-klotho remained significantly associated with MIAC (P=0.02); and vi) Among patients without MIAC, smoking was significantly associated with a lower median plasma concentration soluble α-klotho than in non-smokers (794.2 pg/mL vs. 1382.0 pg/mL, respectively; P<0.001); however, this difference was not observed in patients with MIAC.

CONCLUSIONS

Intra-amniotic infection occurring at preterm gestations (regardless of membrane status) was associated with a decrease in maternal plasma concentrations of soluble α-klotho. Moreover, among patients without infection, the plasma concentration of α-klotho was lower in smokers.

Laboratory aspects of circulating α-Klotho

BACKGROUND

α-Klotho is a protein mainly produced in the kidney. Its circulating form has been suggested to link renal damage and distant tissue pathology. As three assays to measure α-Klotho became commercially available, we performed an evaluation of these commercially available Klotho assays.

METHODS

We studied within-run variation, between-run variation, matrix effects, linearity, and recovery of added recombinant human Klotho in the α-Klotho assays of IBL (IBL International GmbH, Hamburg, Germany), Cusabio (Cusabio Biotech, Wuhan, China) and USCN (USCN life Science, Inc., Wuhan, China) using both serum and ethylenediaminetetraacetic acid plasma.

RESULTS

Within run variation was 4, 13 and 32% for the IBL, Cusabio and USCN assay, respectively. Agreement between serum and EDTA plasma was good in the IBL assay, but poor in the USCN and Cusabio assays however improved after modifications in the Cusabio assay. Standardization and agreement between assays was poor.

CONCLUSIONS

The commercially available methods for the measurement of α-Klotho differ in quality. Some of the manufacturers should improve their assays in order to produce accurate results so that reliable conclusions can be drawn from studies in which these assays are used.

Circulating α-klotho levels in CKD and relationship to progression

BACKGROUND

α-Klotho is reported to have protective effects against kidney injury, and its renal expression is decreased in many experimental models of kidney disease. However, circulating α-klotho levels in human chronic kidney disease (CKD) and the relationship to progression are unknown.

STUDY DESIGN

Post hoc analysis of a prospective cohort study.

SETTING & PARTICIPANTS

243 of 301 participants from a CKD cohort at our institution between January 2006 and December 2011 were eligible for the study.

PREDICTOR

Baseline α-klotho levels.

OUTCOMES

Primary outcome was the composite of doubling of baseline serum creatinine concentration, end-stage renal disease, or death. End-stage renal disease was defined as onset of treatment by renal replacement therapy.

MEASUREMENTS

Serum α-klotho and fibroblast growth factor 23 (FGF-23) were measured using enzyme-linked immunosorbent assay.

RESULTS

Lower serum α-klotho levels were associated with more severe CKD stage in the cross-sectional analysis of the baseline data (P for trend < 0.001). In the adjusted multivariable linear regression model, log(α-klotho) was associated independently with estimated glomerular filtration rate (β = 0.154; P = 0.001). Cox regression analysis showed that baseline α-klotho level independently predicted the composite outcome after adjustment for age, diabetes, blood pressure, estimated glomerular filtration rate, proteinuria, parathyroid hormone level, and FGF-23 level (HR per 10-pg/mL increase, 0.96; 95% CI, 0.94-0.98; P < 0.001). When patients were categorized into 2 groups according to baseline median α-klotho value, 43 (35.2%) patients with α-klotho levels ≤396.3 pg/mL reached the primary composite outcome compared with 19 (15.7%) with α-klotho levels >396.3 pg/mL (HR, 2.03; 95% CI, 1.07-3.85; P = 0.03).

LIMITATIONS

Uncontrolled dietary phosphorus intake and use of frozen samples.

CONCLUSIONS

This observational study showed that low circulating α-klotho levels were associated with adverse kidney disease outcome, suggesting that α-klotho is a novel biomarker for CKD progression. More data from larger prospective longitudinal studies are required to validate our findings.

Fibroblast growth factor 23 and Klotho: physiology and pathophysiology of an endocrine network of mineral metabolism

The metabolically active and perpetually remodeling calcium phosphate-based endoskeleton in terrestrial vertebrates sets the demands on whole-organism calcium and phosphate homeostasis that involves multiple organs in terms of mineral flux and endocrine cross talk. The fibroblast growth factor (FGF)-Klotho endocrine networks epitomize the complexity of systems biology, and specifically, the FGF23-αKlotho axis highlights the concept of the skeleton holding the master switch of homeostasis rather than a passive target organ as hitherto conceived. Other than serving as a coreceptor for FGF23, αKlotho circulates as an endocrine substance with a multitude of effects. This review covers recent data on the physiological regulation and function of the complex FGF23-αKlotho network. Chronic kidney disease is a common pathophysiological state in which FGF23-αKlotho, a multiorgan endocrine network, is deranged in a self-amplifying vortex resulting in organ dysfunction of the utmost severity that contributes to its morbidity and mortality.

The emerging role of Klotho in clinical nephrology

Klotho is highly expressed in the kidney and a soluble form of Klotho functions as an endocrine substance that exerts multiple actions including the modulation of renal solute transport and the protection of the kidney from a variety of insults in experimental models. At present, the Klotho database is still largely preclinical, but the anticipated forthcoming impact on clinical nephrology can be immense. This manuscript puts these potentials into perspective for the clinician. There is renal and systemic Klotho deficiency in both acute kidney injury (AKI) and chronic kidney disease (CKD). Klotho plummets very early and severely in AKI and represents a pathogenic factor that exacerbates acute kidney damage. In CKD, Klotho deficiency exerts a significant impact on progression of renal disease and extra renal complications. In AKI, soluble Klotho levels in plasma and/or urine may serve as an early biomarker for kidney parenchymal injury. Restoration by exogenous supplementation or stimulation of endogenous Klotho may prevent and/or ameliorate kidney injury and mitigate CKD development. In CKD, Klotho levels may be an indicator of early disease and predict the rate of progression, and presence and severity of soft tissue calcification. The correction of Klotho deficiency may delay progression and forestall development of extra renal complications in CKD. Rarely does one find a molecule with such broad potential applications in nephrology. Klotho can possibly emerge on the horizon as a candidate for an unprecedented sole biomarker and intervention. Nephrologists should monitor the progress of the preclinical studies and the imminently emerging human database.