Klotho May Ameliorate Proteinuria by Targeting TRPC6 Channels in Podocytes

Systemic immunoinflammatory indexes in albuminuric adults are negatively associated with α-klotho: evidence from NHANES 2007-2016

BACKGROUND

Systemic Immune-Inflammation Index (SII) is a novel inflammatory biomarker closely associated with the inflammatory response and chronic kidney disease. Klotho is implicated as a pathogenic factor in the progression of kidney disease, and supplementation of Klotho may delay the progression of chronic kidney disease by inhibiting the inflammatory response. Our aim is to investigate the potential relationship between SII and Klotho in adult patients in the United States and explore the differences in the populations with and without albuminuria.

METHODS

We conducted a cross-sectional study recruiting adult participants with complete data on SII, Klotho, and urine albumin-to-creatinine ratio (ACR) from the National Health and Nutrition Examination Survey from 2007 to 2016. SII was calculated as platelet count × neutrophil count/lymphocyte count, with abnormal elevation defined as values exceeding 330 × 10^9/L. Albuminuria was defined as ACR >30 mg/g. Weighted multivariable regression analysis and subgroup analysis were employed to explore the independent relationship between SII and Klotho.

RESULTS

Our study included a total of 10,592 individuals. In all populations, non-albuminuria population, and proteinuria population with ACR ≥ 30, participants with abnormally elevated SII levels, as compared to those with SII less than 330 × 10^9/L, showed a negative correlation between elevated SII levels and increased Klotho, which persisted after adjusting for covariates.

CONCLUSIONS

There is a negative correlation between SII and Klotho in adult patients in the United States. This finding complements previous research but requires further analysis through large prospective studies.

Lessons from Klotho mouse models to understand mineral homeostasis

AIM

Klotho, a key component of the endocrine fibroblast growth factor receptor-fibroblast growth factor axis, is a multi-functional protein that impacts renal electrolyte handling. The physiological significance of Klotho will be highlighted in the regulation of calcium, phosphate, and potassium metabolism.

METHODS

In this review, we compare several murine models with different renal targeted deletions of Klotho and the insights into the molecular and physiological function that these models offer.

RESULTS

In vivo, Klotho deficiency is associated with severely impaired mineral metabolism, with consequences on growth, longevity and disease development. Additionally, we explore the perspectives of Klotho in renal pathology and vascular events, as well as potential Klotho treatment options.

CONCLUSION

This comprehensive review emphasizes the use of Klotho to shed light on deciphering the renal molecular in vivo mechanisms in electrolyte handling, as well as novel therapeutic interventions.

ADAM10 as a major activator of reactive oxygen species production and klotho shedding in podocytes under diabetic conditions

Early stages of diabetes are characterized by elevations of insulin and glucose concentrations. Both factors stimulate reactive oxygen species (ROS) production, leading to impairments in podocyte function and disruption of the glomerular filtration barrier. Podocytes were recently shown to be an important source of αKlotho (αKL) expression. Low blood Klotho concentrations are also associated with an increase in albuminuria, especially in patients with diabetes. We investigated whether ADAM10, which is known to cleave αKL, is activated in glomeruli and podocytes under diabetic conditions and the potential mechanisms by which ADAM10 mediates ROS production and disturbances of the glomerular filtration barrier. In cultured human podocytes, high glucose increased ADAM10 expression, shedding, and activity, NADPH oxidase activity, ROS production, and albumin permeability. These effects of glucose were inhibited when cells were pretreated with an ADAM10 inhibitor or transfected with short-hairpin ADAM10 (shADAM10) or after the addition soluble Klotho. We also observed increases in ADAM10 activity, NOX4 expression, NADPH oxidase activity, and ROS production in αKL-depleted podocytes. This was accompanied by an increase in albumin permeability in shKL-expressing podocytes. The protein expression and activity of ADAM10 also increased in isolated glomeruli and urine samples from diabetic rats. Altogether, these results reveal a new mechanism by which hyperglycemia in diabetes increases albumin permeability through ADAM10 activation and an increase in oxidative stress via NOX4 enzyme activation. Moreover, αKlotho downregulates ADAM10 activity and supports redox balance, consequently protecting the slit diaphragm of podocyteσ under hyperglycemic conditions.

CD36-mediated podocyte lipotoxicity promotes foot process effacement

Background

Lipid metabolism disorders lead to lipotoxicity. The hyperlipidemia-induced early stage of renal injury mainly manifests as podocyte damage. CD36 mediates fatty acid uptake and the subsequent accumulation of toxic lipid metabolites, resulting in podocyte lipotoxicity.

Methods

Male Sprague-Dawley rats were divided into two groups: the normal control group and the high-fat diet group (HFD). Podocytes were cultured and treated with palmitic acid (PA) and sulfo-N-succinimidyl oleate (SSO). Protein expression was measured by immunofluorescence and western blot analysis. Boron-dipyrromethene staining and Oil Red O staining was used to analyze fatty acid accumulation.

Results

Podocyte foot process (FP) effacement and marked proteinuria occurred in the HFD group. CD36 protein expression was upregulated in the HFD group and in PA-treated podocytes. PA-treated podocytes showed increased fatty acid accumulation, reactive oxygen species (ROS) production, and actin cytoskeleton rearrangement. However, pretreatment with the CD36 inhibitor SSO decreased lipid accumulation and ROS production and alleviated actin cytoskeleton rearrangement in podocytes. The antioxidant N-acetylcysteine suppressed PA-induced podocyte FP effacement and ROS generation.

Conclusions

CD36 participated in fatty acid-induced FP effacement in podocytes via oxidative stress, and CD36 inhibitors may be helpful for early treatment of kidney injury.

Klotho Stabilizes the Podocyte Actin Cytoskeleton in Idiopathic Membranous Nephropathy through Regulating the TRPC6/CatL Pathway

INTRODUCTION

The aim of this study was to explore the renoprotective effects of Klotho on podocyte injury mediated by complement activation and autoantibodies in idiopathic membranous nephropathy (IMN).

METHODS

Rat passive Heymann nephritis (PHN) was induced as an IMN model. Urine protein levels, serum biochemistry, kidney histology, and podocyte marker levels were assessed. In vitro, sublytic podocyte injury was induced by C5b-9. The expression of Klotho, transient receptor potential channel 6 (TRPC6), and cathepsin L (CatL); its substrate synaptopodin; and the intracellular Ca2+ concentration were detected via immunofluorescence. RhoA/ROCK pathway activity was measured by an activity quantitative detection kit, and the protein expression of phosphorylated-LIMK1 (p-LIMK1) and p-cofilin in podocytes was detected via Western blotting. Klotho knockdown and overexpression were performed to evaluate its role in regulating the TRPC6/CatL pathway.

RESULTS

PHN rats exhibited proteinuria, podocyte foot process effacement, decreased Klotho and Synaptopodin levels, and increased TRPC6 and CatL expression. The RhoA/ROCK pathway was activated by the increased phosphorylation of LIMK1 and cofilin. Similar changes were observed in C5b-9-injured podocytes. Klotho knockdown exacerbated podocyte injury, while Klotho overexpression partially ameliorated podocyte injury.

CONCLUSION

Klotho may protect against podocyte injury in IMN patients by inhibiting the TRPC6/CatL pathway. Klotho is a potential target for reducing proteinuria in IMN patients.

Histone deacetylase 9 exacerbates podocyte injury in hyperhomocysteinemia through epigenetic repression of Klotho

Although hyperhomocysteinemia (hHcys) has been recognized as an important independent risk factor in the progression of end-stage renal disease and the development of cardiovascular complications related to end-stage renal disease, the mechanisms triggering pathogenic actions of hHcys are not fully understood. The present study was mainly designed to investigate the role of HDACs in renal injury induced by hHcys. Firstly, we identified the expression patterns of HDACs and found that, among zinc-dependent HDACs, HDAC9 was preferentially upregulated in the kidney from mice with hHcys. Deficiency or pharmacological inhibition of HDAC9 ameliorated renal injury in mice with hHcys. Moreover, podocyte-specific deletion of HDAC9 significantly attenuated podocyte injury and proteinuria. In vitro, gene silencing of HDAC9 attenuated podocyte injury by inhibiting apoptosis, reducing oxidative stress and maintaining the expressions of podocyte slit diaphragm proteins. Mechanically, we proved for the first time that HDAC9 reduced the acetylation level of H3K9 in the promoter of Klotho, then inhibited gene transcription of Klotho, finally aggravating podocyte injury in hHcys. In conclusion, our results indicated that targeting of HDAC9 might be an attractive therapeutic strategy for the treatment of renal injury induced by hHcys.

Advances in the pharmacological study of Chinese herbal medicine to alleviate diabetic nephropathy by improving mitochondrial oxidative stress

Diabetic nephropathy (DN) is one of the serious complications of diabetes mellitus, primarily arising from type 2 diabetes (T2DM), and can progress to chronic kidney disease (CKD) and end stage renal disease (ESRD). The pathogenesis of DN involves various factors such as hemodynamic changes, oxidative stress, inflammatory response, and lipid metabolism disorders. Increasing attention is being given to DN caused by oxidative stress in the mitochondrial pathway, prompting researchers to explore drugs that can regulate these target pathways. Chinese herbal medicine, known for its accessibility, rich historical usage, and remarkable efficacy, has shown promise in ameliorating renal injury caused by DN by modulating oxidative stress in the mitochondrial pathway. This review aims to provide a reference for the prevention and treatment of DN. Firstly, we outline the mechanisms by which mitochondrial dysfunction impairs DN, focusing on outlining the damage to mitochondria by oxidative stress. Subsequently, we describe the process by which formulas, herbs and monomeric compounds protect the kidney by ameliorating oxidative stress in the mitochondrial pathway. Finally, the rich variety of Chinese herbal medicine, combined with modern extraction techniques, has great potential, and as we gradually understand the pathogenesis of DN and research techniques are constantly updated, there will be more and more promising therapeutic targets and herbal drug candidates. This paper aims to provide a reference for the prevention and treatment of DN.

The roles of phospholipase C-β related signals in the proliferation, metastasis and angiogenesis of malignant tumors, and the corresponding protective measures

PLC-β is widely distributed in eukaryotic cells and is the key enzyme in phosphatidylinositol signal transduction pathway. The cellular functions regulated by its four subtypes (PLC-β1, PLC-β2, PLC-β3, PLC-β4) play an important role in maintaining homeostasis of organism. PLC-β and its related signals can promote or inhibit the occurrence and development of cancer by affecting the growth, differentiation and metastasis of cells, while targeted intervention of PLC-β1-PI3K-AKT, PLC-β2/CD133, CXCR2-NHERF1-PLC-β3, Gαq-PLC-β4-PKC-MAPK and so on can provide new strategies for the precise prevention and treatment of malignant tumors. This paper reviews the mechanism of PLC-β in various tumor cells from four aspects: proliferation and differentiation, invasion and metastasis, angiogenesis and protective measures.

Astragaloside IV Attenuates High-Glucose-Induced Impairment in Diabetic Nephropathy by Increasing Klotho Expression via the NF-κB/NLRP3 Axis

Objective

Deficiencies in klotho are implicated in various kidney dysfunctions including diabetic nephropathy (DN) related to inflammatory responses. Klotho is closely related to inflammatory responses and is a potential target for ameliorating kidney failure. Pyroptosis, an inflammatory form of programmed cell death, is reported to take part in DN pathogenesis recently. This study is aimed at exploring whether and how klotho inhibited podocyte pyroptosis and whether astragaloside IV (AS-IV) protect podocyte through the regulation of klotho.

Materials and Methods

SD rat model of DN and conditionally immortalized mouse podocytes exposed to high glucose were treated with AS-IV. Biochemical assays and morphological examination, cell viability assay, cell transfection, phalloidin staining, ELISA, LDH release assay, SOD and MDA detection, MMP assay, ROS level detection, flow cytometry analysis, TUNEL staining assay, PI/Hoechst 33342 staining, immunofluorescence assay, and western blot were performed to elucidate podocyte pyroptosis and to observe the renal morphology.

Results

The treatment of AS-IV can improve renal function and protect podocytes exposed to high glucose. Klotho was decreased, and AS-IV increased klotho levels in serum and kidney tissue of DN rats as well as podocytes exposed to high glucose. AS-IV can inhibit DN glomeruli pyroptosis in vivo. In vitro, overexpressed klotho and treatment with AS-IV inhibited pyroptosis of podocytes cultured in high glucose. Klotho knockdown promoted podocyte pyroptosis, and treatment with AS-IV reversed this effect. Furthermore, the overexpression of klotho and AS-IV reduces oxidative stress levels and inhibited NF-κB activation and NLRP3-mediated podocytes' pyroptosis which was abolished by klotho knockdown. In addition, both the ROS inhibitor NAC and the NF-κB pathway inhibitor PDTC can inhibit NLRP3 inflammasome activation. NLRP3 inhibitor MCC950 can inhibit pyroptosis of podocytes exposed to high glucose.

Conclusion

Altogether, our results demonstrate that the protective effect of AS-IV in upregulating klotho expression in diabetes-induced podocyte injury is associated with the inhibition of NLRP3-mediated pyroptosis via the NF-κB signaling pathway.

Soluble Klotho protects against glomerular injury through regulation of ER stress response

αKlotho (Klotho) has well established renoprotective effects; however, the molecular pathways mediating its glomerular protection remain incompletely understood. Recent studies have reported that Klotho is expressed in podocytes and protects glomeruli through auto- and paracrine effects. Here, we examined renal expression of Klotho in detail and explored its protective effects in podocyte-specific Klotho knockout mice, and by overexpressing human Klotho in podocytes and hepatocytes. We demonstrate that Klotho is not significantly expressed in podocytes, and transgenic mice with either a targeted deletion or overexpression of Klotho in podocytes lack a glomerular phenotype and have no altered susceptibility to glomerular injury. In contrast, mice with hepatocyte-specific overexpression of Klotho have high circulating levels of soluble Klotho, and when challenged with nephrotoxic serum have less albuminuria and less severe kidney injury compared to wildtype mice. RNA-seq analysis suggests an adaptive response to increased endoplasmic reticulum stress as a putative mechanism of action. To evaluate the clinical relevance of our findings, the results were validated in patients with diabetic nephropathy, and in precision cut kidney slices from human nephrectomies. Together, our data reveal that the glomeruloprotective effects of Klotho is mediated via endocrine actions, which increases its therapeutic potential for patients with glomerular diseases.

Association between Serum Soluble α-Klotho and Urinary Albumin Excretion in Middle-Aged and Older US Adults: NHANES 2007-2016

(1) Background: Preclinical and clinical studies on the anti-aging effect of α-Klotho are emerging. Urinary albumin excretion (UAE) is a well-known biomarker of kidney injury and generalized damage in the cardiovascular system. However, the potential relationship between α-Klotho and UAE is limited and controversial. This study aimed to quantify this relationship in the general middle-aged and elderly population from the National Health and Nutrition Survey (NHANES) 2007-2016. (2) Methods: Serum α-Klotho was measured by enzyme-linked immunosorbent assay. UAE was assessed by the albumin-to-creatinine ratio (ACR). After adjusting for several confounding variables, the relationship between α-Klotho and ACR was analyzed by weighted multivariable logistic regression, subgroup analysis, and interaction tests. A generalized additive model (GAM) with smooth functions using the two-piecewise linear regression model was used to examine the potential nonlinear relationship between α-Klotho and ACR. (3) Results: Among 13,584 participants aged 40-79 years, we observed an independent and significant negative correlation between α-Klotho and ACR (β = -12.22; 95% CI, -23.91, -0.53, p = 0.0448) by multivariable logistic regression analysis, especially in those with age ≥ 60 years, pulse pressure (PP) ≥ 60 mmHg, hypertension or diabetes. We further discovered the nonlinear relationship between α-Klotho and ACR by GAM, revealing the first negative and then positive correlations with an inflection point of 9.91 pg/mL between α-Klotho and ACR. (4) Conclusions: A dose-response relationship between α-Klotho and ACR was demonstrated, and the negative correlation therein indicated that α-Klotho has potential as a serum marker and prophylactic or therapeutic agent despite its metabolic and effective mechanisms needing to be further explored.

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

CHILKBP protects against podocyte injury by preserving ZO-1 expression

Glomerular diseases afflict millions of people and impose an enormous burden on public healthcare costs worldwide. Identification of potential therapeutic targets for preventing glomerular diseases is of considerable clinical importance. CHILKBP is a focal adhesion protein and modulates a wide array of biological functions. However, little is known about the role of CHILKBP in glomerular diseases. To investigate the function of CHILKBP in maintaining the structure and function of podocytes in a physiologic setting, a mouse model (CHILKBP cKO) was generated in which CHILKBP gene was conditionally deleted in podocytes using the Cre-LoxP system. Ablation of CHILKBP in podocytes resulted in massive proteinuria and kidney failure in mice. Histologically, typical podocyte injury including podocyte loss, foot process effacement, and glomerulosclerosis was observed in CHILKBP cKO mice. Mechanistically, we identified ZO-1 as a key junctional protein that interacted with CHILKBP. Loss of CHILKBP in podocytes exhibited a significant reduction of ZO-1 expression, leading to abnormal actin organization, aberrant slit diaphragm protein expression and compromised podocyte filtration capacity. Restoration of CHILKBP or ZO-1 in CHILKBP-deficient podocytes effectively alleviated podocyte injury induced by the loss of CHILKBP in vitro and in vivo. Finally, we showed the glomerular expression of CHILKBP and ZO-1 was decreased in patients with proteinuric kidney diseases. Our findings reveal a novel signaling pathway consisting of CHILKBP and ZO-1 that plays an essential role in maintaining podocyte homeostasis and suggest novel therapeutic approaches to alleviate glomerular diseases.

Emerging role of extracellular vesicles in kidney diseases

Many types of renal disease eventually progress to end-stage renal disease, which can only be maintained by renal replacement therapy. Therefore, kidney diseases now contribute significantly to the health care burden in many countries. Many new advances and strategies have been found in the research involving kidney diseases; however, there is still no efficient treatment. Extracellular vesicles (EVs) are cell-derived membrane structures, which contains proteins, lipids, and nucleic acids. After internalization by downstream cells, these components can still maintain functional activity and regulate the phenotype of downstream cells. EVs drive the information exchange between cells and tissues. Majority of the cells can produce EVs; however, its production, contents, and transportation may be affected by various factors. EVs have been proved to play an important role in the occurrence, development, and treatment of renal diseases. However, the mechanism and potential applications of EVs in kidney diseases remain unclear. This review summarizes the latest research of EVs in renal diseases, and provides new therapeutic targets and strategies for renal diseases.

Dexmedetomidine Alleviates Neuropathic Pain via the TRPC6-p38 MAPK Pathway in the Dorsal Root Ganglia of Rats

Purpose

Neuropathic pain is a chronic intractable disease characterized by allodynia and hyperalgesia. Effective treatments are unavailable because of the complicated mechanisms of neuropathic pain. Transient receptor potential canonical 6 (TRPC6) is a nonselective calcium (Ca2+)-channel protein related to hyperalgesia. Dexmedetomidine (Dex) is an alpha-2 (α2) adrenoreceptor agonist that mediates intracellular Ca2+ levels to alleviate pain. However, the relationship between TRPC6 and Dex is currently unclear. We speculated that the α2 receptor agonist would be closely linked to the TRPC6 channel. We aimed to investigate whether Dex relieves neuropathic pain by the TRPC6 pathway in the dorsal root ganglia (DRG).

Methods

The chronic constriction injury (CCI) model was established in male rats, and we evaluated the mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL). The expression of TRPC6 and Iba-1 in the DRG were analyzed using quantitative real-time polymerase chain reaction, Western blot, and immunofluorescence assay. The levels of inflammatory cytokines were measured using an enzyme-linked immunosorbent assay.

Results

Compared with the CCI normal saline group, both the MWT and TWL were significantly improved after 7 days of Dex administration. Results demonstrated that TRPC6 expression was increased in the DRG following CCI but was suppressed by Dex. In addition, multiple administrations of Dex inhibited the phosphorylation level of p38 mitogen-activated protein kinase and the upregulation of neuroinflammatory factors.

Conclusion

The results of this study demonstrated that Dex exhibits anti-nociceptive and anti-inflammatory properties in a neuropathic pain model. Moreover, our findings of the CCI model suggested that Dex has an inhibitory effect on TRPC6 expression in the DRG by decreasing the phosphorylation level of p38 in the DRG.

Novel Markers in Diabetic Kidney Disease—Current State and Perspectives

Diabetic kidney disease (DKD) is a leading cause of end-stage renal disease. Along with the increasing prevalence of diabetes, DKD is expected to affect a higher number of patients. Despite the major progress in the therapy of DKD and diabetes mellitus (DM), the classic clinical diagnostic tools in DKD remain insufficient, delaying proper diagnosis and therapeutic interventions. We put forward a thesis that there is a need for novel markers that will be early, specific, and non-invasively obtained. The ongoing investigations uncover new molecules that may potentially become new markers of DKD—among those are: soluble α-Klotho and proteases (ADAM10, ADAM17, cathepsin, dipeptidyl peptidase 4, caspase, thrombin, and circulating microRNAs). This review summarizes the current clinical state-of-the-art in the diagnosis of DKD and a selection of potential novel markers, based on up-to-date literature.

The Role of Alterations in Alpha-Klotho and FGF-23 in Kidney Transplantation and Kidney Donation

Cardiovascular disease and mineral bone disorders are major contributors to morbidity and mortality among patients with chronic kidney disease and often persist after renal transplantation. Ongoing hormonal imbalances after kidney transplant (KT) are associated with loss of graft function and poor outcomes. Fibroblast growth factor 23 (FGF-23) and its co-receptor, α-Klotho, are key factors in the underlying mechanisms that integrate accelerated atherosclerosis, vascular calcification, mineral disorders, and osteodystrophy. On the other hand, kidney donation is also associated with endocrine and metabolic adaptations that include transient increases in circulating FGF-23 and decreases in α-Klotho levels. However, the long-term impact of these alterations and their clinical relevance have not yet been determined. This manuscript aims to review and summarize current data on the role of FGF-23 and α-Klotho in the endocrine response to KT and living kidney donation, and importantly, underscore specific areas of research that may enhance diagnostics and therapeutics in the growing population of KT recipients and kidney donors.

Renal Protective Effects of Inonotus obliquus on High-Fat Diet/Streptozotocin-Induced Diabetic Kidney Disease Rats: Biochemical, Color Doppler Ultrasound and Histopathological Evidence

Diabetic kidney disease (DKD) is the current leading cause of end-stage renal disease. Inonotus obliquus (chaga), a medicinal fungus, has been used in treatment of diabetes. Here, we aim to identify the renal protective effects of chaga extracts on a DKD rat model which was induced by a high-fat diet and streptozotocin injection. During the total 17-weeks experiment, the biological parameters of serum and urine were examined, and the color Doppler ultrasound of renal artery, the periodic acid-Schiff staining, and electron microscopy of kidney tissue were performed. The compositions of chaga extracts were analyzed and the intervention effects of the extracts were also observed. Compared with the normal control group, the biochemical research showed that insulin resistance was developed, blood glucose and total cholesterol were elevated, urinary protein excretion and serum creatinine levels were significantly increased in the DKD model. The ultrasound examinations confirmed the deteriorated blood flow parameters of the left renal interlobar artery in the rat models. Finally, histopathological data supported renal injury on the thickened glomerular basement membrane and fusion of the foot processes. 8 weeks intervention of chaga improved the above changes significantly, and the 100 mg/kg/d chaga group experienced significant effects compared with the 50 mg/kg/d in some parameters. Our findings suggested that Doppler ultrasound examinations guided with biochemical indicators played important roles in evaluating the renal injury as an effective, noninvasive, and repeatable method in rats. Based on biochemical, ultrasound, and histopathological evidence, we confirmed that chaga had pharmacodynamic effects on diabetes-induced kidney injury and the aforementioned effects may be related to delaying the progression of DKD.

Klotho Ameliorates Podocyte Injury through Targeting TRPC6 Channel in Diabetic Nephropathy

Podocyte damage is vital for the etiopathogenesis of diabetic nephropathy (DN). Klotho (KL), a multifunctional protein, has been demonstrated to have renoprotective effects; nevertheless, the mechanism for protective effect has not been completely elucidated. Transient receptor potential cation channel subfamily C, member 6 (TRPC6), a potential target of KL, is implicated in glomerular pathophysiology. Here, we sought to determine whether KL could protect against podocyte injury through inhibiting TRPC6 in DN. We found that high glucose (HG) triggered podocyte injury as manifested by actin cytoskeleton damage along with the downregulation of KL and Synaptopodin and the upregulation of TRPC6. KL overexpression reversed HG-induced podocytes injury, whereas cotreatment with TRPC6 activator flufenamic acid (FFA) significantly abrogated the beneficial effects conferred by KL. Moreover, KL knockdown in podocytes resulted in actin cytoskeleton impairment, decreased Synaptopodin expression, and increased TRPC6 expression. In db/db mice, KL overexpression inhibited TRPC6 expression and attenuated diabetes-induced podocyte injury, which was accompanied by decreased albuminuria and ameliorated glomerulosclerosis. Our data provided novel mechanistic insights for KL against DN and highlighted TRPC6 as a new target for KL in podocytes to prevent DN.

Klotho overexpression protects against renal aging along with suppression of transforming growth factor-β1 signaling pathways

Klotho is an antiaging protein reported to suppress transforming growth factor-β1 (TGF-β1) signaling. Aging kidneys are characterized by interstitial fibrosis, accumulation of cell cycle-arrested cells, and increased levels of oxidative stress. TGF-β1 signaling is involved in these processes. In this study, we investigated whether klotho overexpression improves these features in the kidneys of aging mice and examined the inhibitory effect of klotho on signaling molecules related to transforming growth of TGF-β1. Klotho transgenic (KLTG) and wild-type (WT) mice were used, and 8-wk-old and 24-mo-old mice were defined as young and aging, respectively. We found that klotho expression was decreased in aging WT mice, but it was maintained in aging KLTG mice. Klotho overexpression improved the survival of 24-mo-old mice. Although the serum Ca²⁺ level was significantly lower in aging KLTG mice than in aging WT mice, the serum phosphate level did not differ between these mice. Klotho overexpression attenuated the increases in blood pressure, serum blood urea nitrogen level, and serum creatinine level in aging mice. Interstitial fibrosis, accumulation of cell cycle-arrested cells, and oxidative stress did not differ between young KLTG and WT mice, but they were significantly suppressed in aging KLTG mice compared with aging WT mice. Furthermore, the expression of TGF-β1-related signaling molecules was increased in aging WT mice, whereas it was inhibited in aging KLTG mice. These data suggest that klotho overexpression protects against kidney aging along with suppression of TGF-β1 signaling pathways.NEW & NOTEWORTHY Klotho is considered as an antiaging protein, and its overexpression may be a candidate therapy for protection against kidney damage with advanced aging. Although multiple factors are involved in the aging process, we showed that klotho overexpression inhibited interstitial fibrosis, accumulation of cell cycle-arrested cells, and increased levels of oxidative stress in the kidneys of aging mice, suppressing transforming growth factor-β1-related signaling pathways. The present data showed that klotho overexpression protects against age-associated kidney damage.

Insulin-activated store-operated Ca2+ entry via Orai1 induces podocyte actin remodeling and causes proteinuria

Podocyte, the gatekeeper of the glomerular filtration barrier, is a primary target for growth factor and Ca²⁺ signaling whose perturbation leads to proteinuria. However, the effects of insulin action on store-operated Ca²⁺ entry (SOCE) in podocytes remain unknown. Here, we demonstrated that insulin stimulates SOCE by VAMP2-dependent Orai1 trafficking to the plasma membrane. Insulin-activated SOCE triggers actin remodeling and transepithelial albumin leakage via the Ca²⁺-calcineurin pathway in podocytes. Transgenic Orai1 overexpression in mice causes podocyte fusion and impaired glomerular filtration barrier. Conversely, podocyte-specific Orai1 deletion prevents insulin-stimulated SOCE, synaptopodin depletion, and proteinuria. Podocyte injury and albuminuria coincide with Orai1 upregulation at the hyperinsulinemic stage in diabetic (db/db) mice, which can be ameliorated by the suppression of Orai1-calcineurin signaling. Our results suggest that tightly balanced insulin action targeting podocyte Orai1 is critical for maintaining filter integrity, which provides novel perspectives on therapeutic strategies for proteinuric diseases, including diabetic nephropathy.

Perturbations of Ca²⁺ signaling in podocytes may deteriorate kidney function and eventually lead to proteinuria. Here the authors show that insulin can affect the function of the calcium regulator Ora1 in podocytes, which is critical for maintaining kidney filter integrity.

Role of Klotho in Hyperglycemia: Its Levels and Effects on Fibroblast Growth Factor Receptors, Glycolysis, and Glomerular Filtration

Hyperglycemic conditions (HG), at early stages of diabetic nephropathy (DN), cause a decrease in podocyte numbers and an aberration of their function as key cells for glomerular plasma filtration. Klotho protein was shown to overcome some negative effects of hyperglycemia. Klotho is also a coreceptor for fibroblast growth factor receptors (FGFRs), the signaling of which, together with a proper rate of glycolysis in podocytes, is needed for a proper function of the glomerular filtration barrier. Therefore, we measured levels of Klotho in renal tissue, serum, and urine shortly after DN induction. We investigated whether it influences levels of FGFRs, rates of glycolysis in podocytes, and albumin permeability. During hyperglycemia, the level of membrane-bound Klotho in renal tissue decreased, with an increase in the shedding of soluble Klotho, its higher presence in serum, and lower urinary excretion. The addition of Klotho increased FGFR levels, especially FGFR1/FGFR2, after their HG-induced decrease. Klotho also increased levels of glycolytic parameters of podocytes, and decreased podocytic and glomerular albumin permeability in HG. Thus, we found that the decrease in the urinary excretion of Klotho might be an early biomarker of DN and that Klotho administration may have several beneficial effects on renal function in DN.

NAicotinamide retains Klotho expression and ameliorates rhabdomyolysis-induced acute kidney injury

OBJECTIVES

Acute kidney injury (AKI) is a severe complication of rhabdomyolysis that significantly increases mortality. Unfortunately, the therapeutic approach is limited. Inflammation plays a critical role in the pathogenesis of rhabdomyolysis-induced AKI, which is a potential therapeutic target. Nicotinamide, a form of vitamin B3 and a precursor of nicotinamide adenine dinucleotide, has been shown to have potent antiinflammation effects. Klotho is a tubular highly expressed renoprotective protein. Therefore, we explored the effect of nicotinamide on rhabdomyolysis-induced AKI and the underlying mechanisms.

METHODS

We intramuscularly injected glycerol to induce rhabdomyolysis, and intraperitoneally administrated nicotinamide to observe the effect on kidney injury. Interleukin-1 beta, tumor necrosis factor alpha, nuclear factor kappa B (NF-κB), and Klotho were determined by Western blot. Chromatin immunoprecipitation was used to assess the interaction of NF-κB, nuclear receptor corepressor, and histone deacetylase 1 with Klotho promoters. Small interfering RNA was used to evaluate the role of Klotho in nicotinamide-related renoprotection.

RESULTS

The results showed that nicotinamide attenuated renal pathologic morphology, kidney functional abnormalities, and kidney inflammatory response in rhabdomyolysis. Moreover, nicotinamide effectively blocked the recruitment of NF-κB, nuclear receptor corepressor, and histone deacetylase 1 to the promoter of Klotho, and preserved Klotho expression. More importantly, the renoprotection effect of nicotinamide was abrogated when Klotho was knocked down by small interfering RNA in rhabdomyolysis mice.

CONCLUSIONS

Our study demonstrated that Klotho preservation is essential for the renoprotection effect of nicotinamide, and provides a new preventive strategy for rhabdomyolysis-induced AKI.

Exploiting the neuroprotective effects of α-klotho to tackle ageing- and neurodegeneration-related cognitive dysfunction

Cognitive dysfunction is a key symptom of ageing and neurodegenerative disorders, such as Alzheimer's disease (AD). Strategies to enhance cognition would impact the quality of life for a significant proportion of the ageing population. The α-klotho protein may protect against cognitive decline through multiple mechanisms: such as promoting optimal synaptic function via activation of N-methyl-d-aspartate (NMDA) receptor signalling; stimulating the antioxidant defence system; reducing inflammation; promoting autophagy and enhancing clearance of amyloid-β. However, the molecular and cellular pathways by which α-klotho mediates these neuroprotective functions have yet to be fully elucidated. Key questions remain unanswered: which form of α-klotho (transmembrane, soluble or secreted) mediates its cognitive enhancing properties; what is the neuronal receptor for α-klotho and which signalling pathways are activated by α-klotho in the brain to enhance cognition; how does peripherally administered α-klotho mediate neuroprotection; and what is the molecular basis for the beneficial effect of the VS variant of α-klotho? In this review, we summarise the recent research on neuronal α-klotho and discuss how the neuroprotective properties of α-klotho could be exploited to tackle age- and neurodegeneration-associated cognitive dysfunction.

Protective effects of klotho on palmitate-induced podocyte injury in diabetic nephropathy

The anti-aging gene, klotho, has been identified as a multi-functional humoral factor and is implicated in multiple biological processes. However, the effects of klotho on podocyte injury in diabetic nephropathy are poorly understood. Thus, the current study aims to investigate the renoprotective effects of klotho against podocyte injury in diabetic nephropathy. We examined lipid accumulation and klotho expression in the kidneys of diabetic patients and animals. We stimulated cultured mouse podocytes with palmitate to induce lipotoxicity-mediated podocyte injury with or without recombinant klotho. Klotho level was decreased in podocytes of lipid-accumulated obese diabetic kidneys and palmitate-treated mouse podocytes. Palmitate-treated podocytes showed increased apoptosis, intracellular ROS, ER stress, inflammation, and fibrosis, and these were significantly attenuated by klotho administration. Klotho treatment restored palmitate-induced downregulation of the antioxidant molecules, Nrf2, Keap1, and SOD1. Klotho inhibited the phosphorylation of FOXO3a, promoted its nuclear translocation, and then upregulated MnSOD expression. In addition, klotho administration attenuated palmitate-induced cytoskeleton changes, decreased nephrin expression, and increased TRPC6 expression, eventually improving podocyte albumin permeability. These results suggest that klotho administration prevents palmitate-induced functional and morphological podocyte injuries, and this may indicate that klotho is a potential therapeutic agent for the treatment of podocyte injury in obese diabetic nephropathy.

Dysfunction of the Klotho-miR-30s/TRPC6 axis confers podocyte injury

Klotho deficiency was observed in virtually all kinds of kidney disease and is thought to play a critical role in podocyte injury. However, the underline mechanisms involved in podocyte injury remain unknown. miRNAs have diverse regulatory roles, and miR-30 family members were essential for podocyte homeostasis. Our study revealed that Klotho and miR-30s were downregulated in PAN-treated podocytes. The ectopic expression of Klotho ameliorates PAN induced podocyte apoptosis through upregulating miR-30a and downregulating Ppp3ca, Ppp3cb, Ppp3r1, and Nfact3 expression, which are the known targets of miR-30s. We also found that Klotho regulates TRPC6 via miR-30a to activate calcium/calcineurin signaling. Further, glucocorticoid (Dexamethasone, DEX) was found to sustain Klotho and miR-30a levels during PAN treatment in vitro. Eventually, in rats, PAN treatment substantially downregulated Klotho and miR-30a levels, lead to podocyte injury and increased proteinuria. The transfer of exogenous Klotho to podocytes of PAN-treated rats could increase miR-30a expression, reduce TRPC6 expression, and also ameliorated podocyte injury and proteinuria. In conclusion, Klotho, acting on miR-30s, which directly regulates its target genes, contributes to podocyte apoptosis induced by PAN. It is a novel mechanism underlying PAN-induced podocyte injury.

Effects of TRPC6 Inactivation on Glomerulosclerosis and Renal Fibrosis in Aging Rats

Canonical transient receptor potential 6 (TRPC6) channels have been implicated in familial and acquired forms of focal and segmental glomerulosclerosis (FSGS) in patients and animal models, as well as in renal fibrosis following ureteral obstruction in mice. Aging also evokes declines in renal function owing to effects on almost every renal compartment in humans and rodents. Here, we have examined the role of TRPC6 in driving inflammation and fibrosis during aging in Sprague-Dawley rats. This was assessed in rats with non-functional TRPC6 channels owing to CRISPR-Cas9 deletion of a portion of the ankyrin repeat domain required for the assembly of functional TRPC6 channels (Trpc6^del/del rats). Wild-type littermates (Trpc6^wt/wt rats) were used as controls. Animals were evaluated at 2 months and 12 months of age. There was no sign of kidney disease at 2 months of age, regardless of genotype. However, by 12 months of age, all rats examined showed declines in renal function associated with albuminuria, azotemia and increased urine excretion of β2-microglobulin, a marker for proximal tubule pathology. These changes were equally severe in Trpc6^wt/wt and Trpc6^del/del rats. We also observed age-related increases in renal cortical expression of markers of fibrosis (α-smooth muscle actin and vimentin) and inflammation (NLRP3 and pro-IL-1β), and there was no detectable protective effect of TRPC6 inactivation. Tubulointerstitial fibrosis assessed from histology also appeared equally severe in Trpc6^wt/wt and Trpc6^del/del rats. By contrast, glomerular pathology, blindly scored from histological sections, suggested a significant protective effect of TRPC6 inactivation, but only within the glomerular compartment.

Antiinflammatory Actions of Klotho: Implications for Therapy of Diabetic Nephropathy

Klotho was initially introduced as an antiaging molecule. Klotho deficiency significantly reduces lifespan, and its overexpression extends it and protects against various pathological phenotypes, especially renal disease. It was shown to regulate phosphate and calcium metabolism, protect against oxidative stress, downregulate apoptosis, and have antiinflammatory and antifibrotic properties. The course of diabetes mellitus and diabetic nephropathy resembles premature cellular senescence and causes the activation of various proinflammatory and profibrotic processes. Klotho was shown to exert many beneficial effects in these disorders. The expression of Klotho protein is downregulated in early stages of inflammation and diabetic nephropathy by proinflammatory factors. Therefore, its therapeutic effects are diminished in this disorder. Significantly lower urine levels of Klotho may serve as an early biomarker of renal involvement in diabetes mellitus. Recombinant Klotho administration and Klotho overexpression may have immunotherapeutic potential for the treatment of both diabetes and diabetic nephropathy. Therefore, the current manuscript aims to characterize immunopathologies occurring in diabetes and diabetic nephropathy, and tries to match them with antiinflammatory actions of Klotho. It also gives reasons for Klotho to be used in diagnostics and immunotherapy of these disorders.

Astragaloside IV protects against podocyte apoptosis by inhibiting oxidative stress via activating PPARγ-Klotho-FoxO1 axis in diabetic nephropathy

AIMS

Podocyte apoptosis plays an important role in the pathogenesis of diabetic nephropathy (DN). Astragaloside IV (AS-IV) has been shown to protect against podocyte apoptosis. Here we aim to investigate the mechanism responsible for the protective effects of AS-IV.

MAIN METHODS

Diabetic db/db mice and high glucose (HG)-cultured podocytes were treated with AS-IV. Renal function and histopathological changes were measured to evaluate the therapeutic effects of AS-IV against DN. Adenovirus-mediated Klotho overexpression, Klotho siRNA, and PPARγ inhibitor were applied in vitro to investigate the potential mechanism. The expression levels of mRNA and proteins were analyzed by qRT-PCR, western blot or immunofluorescence. Intracellular ROS and mitochondrial superoxide were detected by DHE and MitoSOx Red, respectively. Cell apoptosis was evaluated by TUNEL staining and flow cytometry.

KEY FINDINGS

AS-IV improved renal function and ameliorated podocyte injury in db/db mice accompanied with enhanced Klotho expression in glomerular podocytes. In vitro, AS-IV inhibited HG-induced podocyte apoptosis and restored HG-inhibited Klotho expression, whereas Klotho knockdown abrogated the anti-apoptosis action of AS-IV. Further study showed that adenovirus-mediated Klotho overexpression enhanced Forkhead transcription factor O1 (FoxO1)-dependent antioxidant activity and attenuated HG-evoked oxidative stress and apoptosis. AS-IV prevented HG-induced FoxO1 inhibition and oxidative stress, whereas Klotho knockdown reversed these effects. Cotreatment with PPARγ inhibitor T0070907 abolished AS-IV-induced Klotho expression and anti-apoptosis action.

SIGNIFICANCE

These data suggested that AS-IV attenuated podocyte apoptosis presumably by inhibiting oxidative stress via activating PPARγ-Klotho-FoxO1 signaling pathway, thereby ameliorating DN. This study provided new insights into the molecular mechanisms of AS-IV against DN.

Soluble αKlotho downregulates Orai1-mediated store-operated Ca2+ entry via PI3K-dependent signaling

αKlotho is a type 1 transmembrane anti-aging protein. αKlotho-deficient mice have premature aging phenotypes and an imbalance of ion homeostasis including Ca²⁺ and phosphate. Soluble αKlotho is known to regulate multiple ion channels and growth factor-mediated phosphoinositide-3-kinase (PI3K) signaling. Store-operated Ca²⁺ entry (SOCE) mediated by pore-forming subunit Orai1 and ER Ca²⁺ sensor STIM1 is a ubiquitous Ca²⁺ influx mechanism and has been implicated in multiple diseases. However, it is currently unknown whether soluble αKlotho regulates Orai1-mediated SOCE via PI3K-dependent signaling. Among the Klotho family, αKlotho downregulates SOCE while βKlotho or γKlotho does not affect SOCE. Soluble αKlotho suppresses serum-stimulated SOCE and Ca²⁺ release-activated Ca²⁺ (CRAC) channel currents. Serum increases the cell-surface abundance of Orai1 via stimulating vesicular exocytosis of the channel. The serum-stimulated SOCE and cell-surface abundance of Orai1 are inhibited by the preincubation of αKlotho protein or PI3K inhibitors. Moreover, the inhibition of SOCE and cell-surface abundance of Orai1 by pretreatment of brefeldin A or tetanus toxin or PI3K inhibitors prevents further inhibition by αKlotho. Functionally, we further show that soluble αKlotho ameliorates serum-stimulated SOCE and cell migration in breast and lung cancer cells. These results demonstrate that soluble αKlotho downregulates SOCE by inhibiting PI3K-driven vesicular exocytosis of the Orai1 channel and contributes to the suppression of SOCE-mediated tumor cell migration.

Epigenetic and non-epigenetic regulation of Klotho in kidney disease

Klotho is a novel renoprotective anti-aging protein available in membrane-bound or soluble form. Klotho is expressed in brain, pancreas, and other solid organs but shows highest expression levels in the kidney. Klotho sustains normal kidney physiology but Klotho regulation also contributes to the progression of kidney disease. Systemic and intrarenal levels of Klotho fall drastically during acute kidney injury, kidney fibrosis, diabetic nephropathy, and other forms of chronic kidney disease, etc. Moreover, exogenous supplementation or overexpression of endogenous Klotho attenuates kidney disease. The regulation of endogenous Klotho expression involves epigenetic as well as non-epigenetic mechanisms. The epigenetic modifications such as DNA methylation, post-translational histone modifications, miRNAs regulate the change in Klotho expression in kidney disease. Non-epigenetic mechanisms such as ER stress, Wnt signaling, activation of the renin angiotensin system (RAS), excessive reactive oxygen species and cytokine generation, albumin overload, and PPAR-γ signaling also contribute to Klotho regulation. Evolving evidence highlight the capacity of natural products to regulate Klotho expression in kidney disease. All these preclinical data suggest that Klotho could be a novel biomarker as well as therapeutic target. Here we review the different mechanisms of Klotho regulation in the context of Klotho as a biomarker and potential therapeutic agent.

The Use of Genomics to Drive Kidney Disease Drug Discovery and Development

As opposed to diseases such as cancer, autoimmune disease, and diabetes, identifying drugs to treat CKD has proven significantly more challenging. Over the past 2 decades, new potential therapeutic targets have been identified as genetically altered proteins involved in rare monogenetic kidney diseases. Other possible target genes have been implicated through common genetic polymorphisms associated with CKD in the general population. Significant challenges remain before translating these genetic insights into clinical therapies for CKD. This paper will discuss how genetic variants may be leveraged to develop drugs and will especially focus on those genes associated with CKD to exemplify the value and challenges in including genetic information in the drug development pipeline.

Mediation of the relationship between proteinuria and serum phosphate: Insight from the KNOW-CKD study

Proteinuria and hyperphosphatemia are risk factors for cardiovascular disease in patients with chronic kidney disease (CKD). Although the interaction between proteinuria and the serum phosphate level is well established, the mechanistic link between the two, particularly the extent to which this interaction is mediated by phosphate-regulating factors, remains poorly understood. In this study, we examined the association between proteinuria and the serum phosphate level, as well as potential mediators, including circulating fibroblast growth factor (FGF23)/klotho, the 24-h urinary phosphate excretion rate to glomerular filtration rate ratio (EP/GFR), and the 24-h tubular phosphate reabsorption rate to GFR ratio (TRP/GFR). The analyses were performed with data from 1793 patients in whom 24-h urine protein and phosphate, serum phosphate, FGF23, and klotho levels were measured simultaneously, obtained from the KoreaN cohort study for Outcome in patients With Chronic Kidney Disease (KNOW-CKD). Multivariable linear regression and mediation analyses were performed. Total, direct, and indirect effects were also estimated. Patients with high serum phosphate levels were found to be more likely to exhibit greater proteinuria, higher FGF23 levels, and lower klotho levels. The 24-h EP/GFR increased and the 24-h TRP/GFR decreased with increasing proteinuria and CKD progression. Simple mediation analyses showed that 15.4% and 67.9% of the relationship between proteinuria and the serum phosphate level were mediated by the FGF23/klotho ratio and 24-h EP/GFR, respectively. Together, these two factors accounted for 73.1% of the relationship between serum markers. These findings suggest that proteinuria increases the 24-h EP/GFR via the FGF23/klotho axis as a compensatory mechanism for the increased phosphate burden well before the reduction in renal function is first seen.

P2X7 siRNA targeted to the kidneys increases klotho and delays the progression of experimental diabetic nephropathy

Previous studies in our laboratory have suggested that P2X7 could contribute to the progression of diabetic nephropathy and modulated klotho expression. The aim of this study was to investigate if P2X7 receptor is related to the expression of klotho in the onset of diabetic nephropathy in rats. Seven-week-old male Wistar rats weighing 210 g were all uninephrectomized; two-third of the animals were induced to diabetes with 60 mg/kg streptozotocin i.v., and one-third received its vehicle (control rats). At 4th day of the fifth week of the protocol, half of the diabetic rats received a small interfering RNA targeting for P2X7 mRNA, and the other half received its vehicle. Euthanasia was made at the eighth week. Diabetic animals reproduced all classic symptoms of the disease; besides, they showed reduced renal function and low NO bioavailability; also, SOD1, SOD2, and catalase were increased, probably due to the oxidative stress which was elevated in this situation. Metabolic data of diabetic rats did not change by silencing P2X7 receptor. For the other hand, silencing P2X7 was able to contribute to balance oxidative and nitrosative profile, ultimately improving the renal function and increasing plasma and membrane forms of klotho. These findings suggest that the management of P2X7 receptor can benefit the kidneys with diabetic nephropathy. Further studies are needed to show the therapeutic potential of this receptor inhibition to provide a better quality of life for the diabetic patient.

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.

Klotho deficiency aggravates diabetes-induced podocyte injury due to DNA damage caused by mitochondrial dysfunction

Diabetic nephropathy (DN) is a progressive disease, the main pathogeny of which is podocyte injury inducing glomerular filtration barrier and proteinuria. The occurrence and development of DN could be partly attributed to the reactive oxygen species (ROS) generated by mitochondria. However, research on how mitochondrial dysfunction (MtD) ultimately causes DNA damage is poor. Here, we investigated the influence of Klotho deficiency on high glucose (HG)-induced DNA damage in vivo and in vitro. First, we found that the absence of Klotho aggravated diabetic phenotypes indicated by podocyte injury accompanied by elevated urea albumin creatinine ratio (UACR), creatinine and urea nitrogen. Then, we further confirmed that Klotho deficiency could significantly aggravate DNA damage by increasing 8-OHdG and reducing OGG1. Finally, we demonstrated Klotho deficiency may promote MtD to promote 8-OHdG-induced podocyte injury. Therefore, we came to a conclusion that Klotho deficiency may promote diabetes-induced podocytic MtD and aggravate 8-OHdG-induced DNA damage by affecting OOG1.

The Klotho proteins in health and disease

The Klotho proteins, αKlotho and βKlotho, are essential components of endocrine fibroblast growth factor (FGF) receptor complexes, as they are required for the high-affinity binding of FGF19, FGF21 and FGF23 to their cognate FGF receptors (FGFRs). Collectively, these proteins form a unique endocrine system that governs multiple metabolic processes in mammals. FGF19 is a satiety hormone that is secreted from the intestine on ingestion of food and binds the βKlotho-FGFR4 complex in hepatocytes to promote metabolic responses to feeding. By contrast, under fasting conditions, the liver secretes the starvation hormone FGF21, which induces metabolic responses to fasting and stress responses through the activation of the hypothalamus-pituitary-adrenal axis and the sympathetic nervous system following binding to the βKlotho-FGFR1c complex in adipocytes and the suprachiasmatic nucleus, respectively. Finally, FGF23 is secreted by osteocytes in response to phosphate intake and binds to αKlotho-FGFR complexes, which are expressed most abundantly in renal tubules, to regulate mineral metabolism. Growing evidence suggests that the FGF-Klotho endocrine system also has a crucial role in the pathophysiology of ageing-related disorders, including diabetes, cancer, arteriosclerosis and chronic kidney disease. Therefore, targeting the FGF-Klotho endocrine axes might have therapeutic benefit in multiple systems; investigation of the crystal structures of FGF-Klotho-FGFR complexes is paving the way for the development of drugs that can regulate these axes.

Urinary Extracellular Vesicles Carrying Klotho Improve the Recovery of Renal Function in an Acute Tubular Injury Model

Acute kidney injury, defined by a rapid deterioration of renal function, is a common complication in hospitalized patients. Among the recent therapeutic options, the use of extracellular vesicles (EVs) is considered a promising strategy. Here we propose a possible therapeutic use of renal-derived EVs isolated from normal urine (urine-derived EVs [uEVs]) in a murine model of acute injury generated by glycerol injection. uEVs accelerated renal recovery, stimulating tubular cell proliferation, reducing the expression of inflammatory and injury markers, and restoring endogenous Klotho loss. When intravenously injected, labeled uEVs localized within injured kidneys and transferred their microRNA cargo. Moreover, uEVs contained the reno-protective Klotho molecule. Murine uEVs derived from Klotho null mice lost the reno-protective effect observed using murine EVs from wild-type mice. This was regained when Klotho-negative murine uEVs were reconstituted with recombinant Klotho. Similarly, ineffective fibroblast EVs acquired reno-protection when engineered with human recombinant Klotho. Our results reveal a novel potential use of uEVs as a new therapeutic strategy for acute kidney injury, highlighting the presence and role of the reno-protective factor Klotho.

Disrupted tubular parathyroid hormone/parathyroid hormone receptor signaling and damaged tubular cell viability possibly trigger postsurgical kidney injury in patients with advanced hyperparathyroidism

BACKGROUND

Parathyroidectomy (PTX) that alleviates clinical manifestations of advanced hyperparathyroidism, including hypercalcemia and hypophosphatemia, is considered the best protection from calcium overload in the kidney. However, little is known about the relationship between postsurgical robust parathyroid hormone (PTH) reduction and perisurgical renal tubular cell viability. Post-PTX kidney function is still a crucial issue for primary hyperparathyroidism (PHPT) and tertiary hyperparathyroidism after kidney transplantation (THPT).

METHODS

As a clinical study, we examined data from 52 consecutive patients (45 with PHPT, 7 with THPT) who underwent PTX in our center between 2015 and 2017 to identify post-PTX kidney injury. Their clinical data, including urinary liver-type fatty acid-binding protein (L-FABP), a tubular biomarker for acute kidney injury (AKI), were obtained from patient charts. An absolute change in serum creatinine level of 0.3 mg/dL (26.5 µmol/L) on Day 2 after PTX defines AKI. Post-PTX calcium supplement dose adjustment was performed to strictly maintain serum calcium at the lower half of the normal range. To mimic post-PTX-related kidney status, a unique parathyroidectomized rat model was produced as follows: 13-week-old rats underwent thyroparathyroidectomy (TPTX) and/or 5/6 subtotal nephrectomy (NX). Indicated TPTX rats were given continuous infusion of a physiological level of 1-34 PTH using a subcutaneously implanted osmotic minipump. Immunofluorescence analyses were performed by polyclonal antibodies against PTH receptor (PTHR) and a possible key modulator of kidney injury, Klotho.

RESULTS

Patients' estimated glomerular filtration rate (eGFR) did not have any clinically relevant change (62.5 ± 22.0 versus 59.4 ± 21.9 mL/min/1.73 m2, NS), whereas serum calcium (2.7 ± 0.18 versus 2.2 ± 0.16 mmol/L, P < 0.0001) and phosphorus levels (0.87 ± 0.19 versus 1.1 ± 0.23 mmol/L, P < 0.0001) were normalized and PTH decreased robustly (181 ± 99.1 versus 23.7 ± 16.8 pg/mL, P < 0.0001) after successful PTX. However, six patients who met postsurgical AKI criteria had lower eGFR and greater L-FABP than those without AKI. Receiver operating characteristics (ROC) analysis revealed eGFR <35 mL/min/1.73 m2 had 83% accuracy. Strikingly, L-FABP >9.8 µg/g creatinine had 100% accuracy in predicting post-PTX-related AKI. Rat kidney PTHR expression was lower in TPTX. PTH infusion (+PTH) restored tubular PTHR expression in rats that underwent TPTX. Rats with TPTX, +PTH and 5/6 NX had decreased PTHR expression compared with those without 5/6 NX. 5/6 NX partially cancelled tubular PTHR upregulation driven by +PTH. Tubular Klotho was modestly expressed in normal rat kidneys, whereas enhanced patchy tubular expression was identified in 5/6 NX rat kidneys. This Klotho and expression and localization pattern was absolutely canceled in TPTX, suggesting that PTH indirectly modulated the Klotho expression pattern. TPTX +PTH recovered tubular Klotho expression and even triggered diffusely abundant Klotho expression. 5/6 NX decreased viable tubular cells and eventually downregulated tubular Klotho expression and localization.

CONCLUSIONS

Preexisting tubular damage is a potential risk factor for AKI after PTX although, overall patients with hyperparathyroidism are expected to keep favorable kidney function after PTX. Patients with elevated tubular cell biomarker levels may suffer post-PTX kidney impairment even though calcium supplement is meticulously adjusted after PTX. Our unique experimental rat model suggests that blunted tubular PTH/PTHR signaling may damage tubular cell viability and deteriorate kidney function through a Klotho-linked pathway.

Klotho inhibits PKCα/p66SHC-mediated podocyte injury in diabetic nephropathy

Diabetic nephropathy (DN) is a progressive disease, the main pathogeny of which is podocyte injury. As a calcium-dependent serine/threonine protein kinase involved in podocyte injury, protein kinase C isoform α (PKCα) was reported to regulate the phosphorylation of p66SHC. However, the role of PKCα/p66SHC in DN remains unknown. Klotho, an anti-aging protein with critical roles in protecting kidney, is expressed predominantly in the kidney and secreted in the blood. Nonetheless, the mechanism underlying amelioration of podocyte injury by Klotho in DN remains unclear. Our data showed that Klotho was decreased in STZ-treated mice and was further declined in diabetic KL ± mice. As expected, Klotho deficiency aggravated diabetes-induced proteinuria and podocyte injury, accompanied by the activation of PKCα and p66SHC. In contrast, overexpression of Klotho partially ameliorated PKCα/p66SHC-mediated podocyte injury and proteinuria. In addition, in vitro experiments showed that activation of PKCα and subsequently increased intracellular reactive oxygen species (ROS) was involved in podocytic apoptosis induced by high glucose (HG), which could be partially reversed by Klotho. Hence, we conclude that Klotho might inhibit PKCα/p66SHC-mediated podocyte injury in diabetic nephropathy.

Fibroblast Growth Factor 23 and Klotho in AKI

Acute kidney injury (AKI) is associated with many of the same mineral metabolite abnormalities that are observed in chronic kidney disease. These include increased circulating levels of the osteocyte-derived, vitamin D-regulating hormone, fibroblast growth factor 23 (FGF23), and decreased renal expression of klotho, the co-receptor for FGF23. Recent data have indicated that increased FGF23 and decreased klotho levels in the blood and urine could serve as novel predictive biomarkers of incident AKI, or as novel prognostic biomarkers of adverse outcomes in patients with established AKI. In addition, because FGF23 and klotho exert numerous classic as well as off-target effects on a variety of organ systems, targeting their dysregulation in AKI may represent a unique opportunity for therapeutic intervention. We review the pathophysiology, kinetics, and regulation of FGF23 and klotho in animal and human studies of AKI, and we discuss the challenges and opportunities involved in targeting FGF23 and klotho therapeutically.

Angiotensin II-mediated MYH9 downregulation causes structural and functional podocyte injury in diabetic kidney disease

MYH9, a widely expressed gene encoding nonmuscle myosin heavy chain, is also expressed in podocytes and is associated with glomerular pathophysiology. However, the mechanisms underlying MYH9-related glomerular diseases associated with proteinuria are poorly understood. Therefore, we investigated the role and mechanism of MYH9 in diabetic kidney injury. MYH9 expression was decreased in glomeruli from diabetic patients and animals and in podocytes treated with Ang II in vitro. Ang II treatment and siRNA-mediated MYH9 knockdown in podocytes resulted in actin cytoskeleton reorganization, reduced cell adhesion, actin-associated protein downregulation, and increased albumin permeability. Ang II treatment increased NOX4 expression and ROS generation. The Ang II receptor blocker losartan and the ROS scavenger NAC restored MYH9 expression in Ang II-treated podocytes, attenuated disrupted actin cytoskeleton and decreased albumin permeability. Furthermore, MYH9 overexpression in podocytes restored the effects of Ang II on the actin cytoskeleton and actin-associated proteins. Ang II-mediated TRPC6 activation reduced MYH9 expression. These results suggest that Ang II-mediated MYH9 depletion in diabetic nephropathy may increase filtration barrier permeability by inducing structural and functional podocyte injury through TRPC6-mediated Ca²⁺ influx by NOX4-mediated ROS generation. These findings reveal a novel MYH9 function in maintaining urinary filtration barrier integrity. MYH9 may be a potential target for treating diabetic nephropathy.

WNK1 promotes renal tumor progression by activating TRPC6-NFAT pathway

Deregulation of Ca²⁺ signaling has been regarded as one of the key features of cancer progression. Lysine-deficient protein kinase 1 (WNK1), a major regulator of renal ion transport, regulates Ca²⁺ signaling through stimulating the phosphatidylinositol 4-kinase IIIα (PI4KIIIα) to activate Gαq-coupled receptor/PLC-β signaling. However, the contribution of WNK1-mediated Ca²⁺ signaling in the development of clear-cell renal-cell carcinoma (ccRCC) is yet unknown. We found that the canonical transient receptor potential channel (TRPC)6 was widely expressed in ccRCC tissues and functioned as a primary Ca²⁺ influx mechanism. We further identified that the expressions of WNK1, PI4KIIIα, TRPC6, and the nuclear factor of activated T cells cytoplasmic 1 (NFATc1) were elevated in the tumor tissues compared with the adjacent normal tissues. WNK1 expression was directly associated with the nuclear grade of ccRCC tissues. Functional experiments showed that WNK1 activated TRPC6-mediated Ca²⁺ influx and current by stimulating PI4KIIIα. Notably, the inhibition of WNK1-mediated TRPC6 activation and its downstream substrate calcineurin attenuated NFATc1 activation and the subsequent migration and proliferation of ccRCC. These findings revealed a novel perspective of WNK1 signaling in targeting the TRPC6-NFATc1 pathway as a therapeutic potential for renal-cell carcinoma.-Kim, J.-H., Hwang, K.-H., Eom, M., Kim, M., Park, E. Y., Jeong, Y., Park, K.-S., Cha, S.-K. WNK1 promotes renal tumor progression by activating TRPC6-NFAT pathway.

TRPC channels: Regulation, dysregulation and contributions to chronic kidney disease

Mutations in the gene encoding canonical transient receptor potential-6 (TRPC6) channels result in severe nephrotic syndromes that typically lead to end-stage renal disease. Many but not all of these mutations result in a gain in the function of the resulting channel protein. Since those observations were first made, substantial work has supported the hypothesis that TRPC6 channels can also contribute to progression of acquired (non-genetic) glomerular diseases, including primary and secondary FSGS, glomerulosclerosis during autoimmune glomerulonephritis, and possibly in type-1 diabetes. Their regulation has been extensively studied, especially in podocytes, but also in mesangial cells and other cell types present in the kidney. More recent evidence has implicated TRPC6 in renal fibrosis and tubulointerstitial disease caused by urinary obstruction. Consequently TRPC6 is being extensively investigated as a target for drug discovery. Other TRPC family members are present in kidney. TRPC6 can form a functional heteromultimer with TRPC3, and it has been suggested that TRPC5 may also play a role in glomerular disease progression, although the evidence on this is contradictory. Here we review literature on the expression and regulation of TRPC6, TRPC3 and TRPC5 in various cell types of the vertebrate kidney, the evidence that these channels are dysregulated in disease models, and research showing that knock-out or pharmacological inhibition of these channels can reduce the severity of kidney disease. We also summarize several areas that remain controversial, and some of the large gaps of knowledge concerning the fundamental role of these proteins in regulation of renal function.

TRPC6 inactivation does not affect loss of renal function in nephrotoxic serum glomerulonephritis in rats, but reduces severity of glomerular lesions

Canonical transient receptor potential-6 (TRPC6) channels have been implicated in a variety of chronic kidney diseases including familial and acquired forms of focal and segmental glomerulosclerosis (FSGS) and renal fibrosis following ureteral obstruction. Here we have examined the role of TRPC6 in progression of inflammation and fibrosis in the nephrotoxic serum (NTS) model of crescentic glomerulonephritis. This was assessed in rats with non-functional TRPC6 channels due to genomic disruption of an essential domain in TRPC6 channels (Trpc6^del/del rats) and wild-type littermates (Trpc6^wt/wt rats). Administration of NTS evoked albuminuria and proteinuria observed 4 and 28 days later that was equally severe in Trpc6^wt/wt and Trpc6^del/del rats. By 28 days, there were dense deposits of complement and IgG within glomeruli in both genotypes, accompanied by severe inflammation and fibrosis readily observed by standard histological methods, and also by increases in renal cortical expression of multiple markers (α-smooth muscle actin, vimentin, NLRP3, and CD68). Tubulointerstitial fibrosis appeared equally severe in Trpc6^wt/wt and Trpc6^del/del rats. TRPC6 inactivation did not protect against the substantial declines in renal function (increases in blood urea nitrogen, serum creatinine and kidney:body weight ratio) in NTS-treated animals, and increases in a urine maker of proximal tubule pathology (β2-macroglobulin) were actually more severe in Trpc6^del/del animals. By contrast, glomerular pathology, blindly scored from histology, and from renal cortical expression of podocin suggested a partial but significant protective effect of TRPC6 inactivation within the glomerular compartment, at least during the autologous phase of the NTS model.

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TRPC6 inactivation in rats does not affect declines in overall renal function in an autoimmune model of rapidly progressing glomerulonephritis.

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TRPC6 inactivation does not reduce renal fibrosis or tubulointerstitial disease in autoimmune glomerulonephritis, and may exacerbate proximal tubule dysfunction in this model.

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TRPC6 inactivation reduces glomerulosclerosis and podocyte loss in autoimmune glomerulonephritis in rats.

New Understanding on the Role of Proteinuria in Progression of Chronic Kidney Disease

Proteinuria is identified as an important marker and risk factor of progression in chronic kidney disease. However, the precise mechanism of action in the progress of chronic kidney disease is still unclear. Mesangial toxicity from specific filtered compounds such as albumin-bound fatty acids and transferrin/iron, tubular overload and hyperplasia, and induction of proinflammatory molecules such as MCP-1 and inflammatory cytokines are some of the proposed mechanisms. Reversing intraglomerular hypertension with protein restriction or antihypertensive therapy may be beneficial both by diminishing hemodynamic injury to the glomeruli and by reducing protein filtration. Therefore, understanding proteinuria and its role in renal tubular interstitial inflammation and fibrosis is of great significance for the study of renal protective therapy, such as antiproteinuric treatments, and delaying the progression of chronic renal disease.

The Role of the Anti-Aging Protein Klotho in IGF-1 Signaling and Reticular Calcium Leak: Impact on the Chemosensitivity of Dedifferentiated Liposarcomas

By inhibiting Insulin-Like Growth Factor-1-Receptor (IGF-1R) signaling, Klotho (KL) acts like an aging- and tumor-suppressor. We investigated whether KL impacts the aggressiveness of liposarcomas, in which IGF-1R signaling is frequently upregulated. Indeed, we observed that a higher KL expression in liposarcomas is associated with a better outcome for patients. Moreover, KL is downregulated in dedifferentiated liposarcomas (DDLPS) compared to well-differentiated tumors and adipose tissue. Because DDLPS are high-grade tumors associated with poor prognosis, we examined the potential of KL as a tool for overcoming therapy resistance. First, we confirmed the attenuation of IGF-1-induced calcium (Ca²⁺)-response and Extracellular signal-Regulated Kinase 1/2 (ERK1/2) phosphorylation in KL-overexpressing human DDLPS cells. KL overexpression also reduced cell proliferation, clonogenicity, and increased apoptosis induced by gemcitabine, thapsigargin, and ABT-737, all of which are counteracted by IGF-1R-dependent signaling and activate Ca²⁺-dependent endoplasmic reticulum (ER) stress. Then, we monitored cell death and cytosolic Ca²⁺-responses and demonstrated that KL increases the reticular Ca²⁺-leakage by maintaining TRPC6 at the ER and opening the translocon. Only the latter is necessary for sensitizing DDLPS cells to reticular stressors. This was associated with ERK1/2 inhibition and could be mimicked with IGF-1R or MEK inhibitors. These observations provide a new therapeutic strategy in the management of DDLPS.

WNK1 kinase is essential for insulin-stimulated GLUT4 trafficking in skeletal muscle

With‐no‐lysine 1 (WNK1) kinase is a substrate of the insulin receptor/Akt pathway. Impaired insulin signaling in skeletal muscle disturbs glucose transporter 4 (GLUT4) translocation associated with the onset of type 2 diabetes (T2D). WNK1 is highly expressed in skeletal muscle. However, it is currently unknown how insulin signaling targeting WNK1 regulates GLUT4 trafficking in skeletal muscle, and whether this regulation is perturbed in T2D. Hereby, we show that insulin phosphorylates WNK1 at its activating site via a phosphatidylinositol 3‐kinase‐dependent mechanism. WNK1 promotes the cell surface abundance of GLUT4 via regulating TBC1D4. Of note, we observed insulin resistance and decreased WNK1 phosphorylation in T2D db/db mice as compared to the control mice. These results provide a new perspective on WNK1 function in the pathogenesis of hyperglycemia in T2D.

Klotho plays a protective role against glomerular hypertrophy in a cell cycle-dependent manner in diabetic nephropathy

There are few studies on the effect of klotho on podocytes in diabetic nephropathy. Thus, we tested whether klotho exerts a protective effect against glomerular injury in diabetes. Mouse podocytes were cultured in media containing 5.6 or 30 mM glucose(HG) with or without 200 pM of recombinant klotho (rKL). Additionally, 32 mice were injected intraperitoneally with either diluent( n = 16, C) or with streptozotocin ( n = 16, DM). Control and diabetic mice underwent sham operation and unilateral nephrectomy, respectively. Eight mice from each control and DM group were treated daily with 10 μg·kg^-1·day^-1 of rKL, using an osmotic minipump. Klotho was expressed in podocytes, and its expression was dependent on peroxisome proliferator-activateed receptor-γ (PPARγ). HG treatment increased the expression of cell cycle-related and apoptotic markers, and these were significantly attenuated by rKL; rKL inhibited the extracellular signal-regulated protein kinase-1/2 and p38 signaling pathways in HG-induced podocyte injury. However, siRNA against klotho gene in HG-treated podocytes failed to aggravate cell cycle arrest and apoptosis. When HG-treated podocytes were incubated in the high-klotho-conditioned medium from tubular epithelial cells, cell injury was significantly attenuated. This effect was not observed when klotho was inhibited by siRNA. In vivo, the expressions of cell cycle-related and apoptotic markers were increased in diabetic mice compared with controls, which were significantly decreased by rKL. Glomerular hypertrophy (GH) and increased profibrotic markers were significantly alleviated after rKL administration. These results showed that klotho was expressed in glomerular podocytes that and its expression was regulated by PPARγ. Additionally, administration of rKL attenuated GH via a cell cycle-dependent mechanism and decreased apoptosis.