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

Abstract

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

Figure 1. Characterization of acute kidney injury (AKI) in rats

AKI and Sham rats were killed at 1, 2, and 7 days after ischemia–reperfusion injury (IRI) and blood and urine were collected. (a) PCr, plasma creatinine; (b) Cl_Cr, creatinine clearance; (c) U/Posm, urine-to-plasma osmolality; and (d) FE_Na, fractional excretion of Na⁺. Values are expressed as means±s.e.m. of six rats in each group. Statistical differences for a, b, c, and d on days 0, 1, 2, and 7 were analyzed by analysis of variance (ANOVA) followed by Student–Newman–Keuls test, and significant differences were accepted when *P<0.05, **P<0.01 vs Sham group; ^#P<0.05; ^##P<0.01 vs AKI rats at day 0. Kidneys were harvested for hematoxylin and eosin (H and E) staining (e, top panel) on days 1, 2, and 7 after IRI. Kidney histological scores (e, bottom panel) from H and E staining (e, top panel) were obtained by a renal pathologist blinded to the experimental conditions using a semi-quantitative pathological scoring system as described in Methods section. Scores are expressed as mean±s.e. of four samples in each group. Statistical differences were analyzed by ANOVA followed by Student–Newman–Keuls test, and significant differences were accepted when P<0.05 or less between two groups. A, AKI; S, Sham.

Figure 2. Klotho levels in acute kidney injury (AKI) rats from 1 to 7 days post-ischemia–reperfusion injury (IRI)

(a) Representative immunohistochemistry for Klotho protein in kidneys from three rats in each group on days 1, 2, and 7 post-IRI. Four μm frozen kidney sections were stained for Klotho with rabbit anti-Klotho (KMDC) (1/50 dilution) and for β-actin. (b) Representative immunoblot (30 μg protein per lane) for Klotho in total kidney lysate from AKI and Sham rats at days 1, 2, and 7 after IRI with rat anti-Klotho antibody (KM2076). The upper panel is a representative blot. The lower panel is a summary of densitometric analysis of all samples from Sham and AKI rats. (c) Klotho transcripts in kidney. One μg of total RNA from kidneys of AKI and Sham rats on day 1, 2, and 7 after IRI was used to generate complementary DNA using oligo-dT primers and real-time PCR for Klotho and cyclophilin was performed. Quantification of Klotho transcripts was calculated as described in Methods section. (d) Representative immunoblot for immunoprecipitated plasma Klotho. One hundred μl of heparinized blood from AKI and Sham rats on days 1, 2, and 7 were immunoprecipitated (IP) with 4 μl of rabbit anti-Klotho antibody (KMDC), and the immune complex was immunoblotted with rat anti-Klotho antibody (KM2076, 1/3000 dilution). The band of immunoglobulin heavy chain (IgG-HC) was shown for IP control. The upper panel is a representative blot. The lower panel is summary of densitometric analysis of all samples from Sham and AKI rats. Values are expressed as means±s.e.m. of six rats in each group. Statistical differences were analyzed by analysis of variance (ANOVA) followed by Student–Newman–Keuls test and significant differences were appreciated when **P<0.01 vs Sham group; ^#P<0.05; ^##P<0.01 vs AKI group at day 1; ^£P<0.05; ^££P<0.01 vs AKI group at day 2. (e) Representative immunoblot for urinary Klotho from rats a volume of fresh urine containing 10 μg of creatinine was immunoblotted with rat anti-Klotho antibody (KM2076). Lane 1 was loaded with 20 fmoles of recombinant mouse Klotho protein (rMKl). The top panel is a representative blot for Klotho in urine samples of AKI rats at days 0, 1, 2, and 7. The bottom panel is summary of densitometric quantification of data from immunoblot. (f) Representative immunoblot for Klotho protein in urine samples of normal subjects (open circle) and of AKI patients (closed circle). A volume of concentrated human urine containing 50 μg of creatinine was loaded. Top panel shows membrane blotted with rat anti-Klotho antibody. Values for individual subjects are shown in bottom panel. Values are expressed as means±s.e.m. of six rats in each group. Statistical differences for b, c, and e were analyzed by ANOVA followed by Student–Newman–Keuls test, and significant differences were accepted when P<0.05 or less between two groups.

Figure 3. Klotho and neutrophil gelatinase-associated lipocalin (NGAL) in blood and kidney in acute kidney injury (AKI) rats from 1 to 24 h

(a) Representative immunoblot for immunoprecipitated (IP) Klotho (upper panel) in blood of AKI and Sham rats at 1, 3, 5, and 24 h post-reperfusion (three rats at each time point); immunoglobulin G (IgG) heavy chain (IgG-HC) band is shown for IP control. One hundred μl of heparinized blood was immunoprecipitated with rabbit anti-human Klotho antibody (KMDC) and immunoblotted for Klotho protein with rat anti-Klotho antibody (KM2076). (b) Representative immunoblot for Klotho in total kidney lysates of AKI and Sham rats at 1, 3, 5, and 24 h post-reperfusion (three rats at each time point). Thirty μg of protein were loaded in each lane and subjected to SDS-polyacrylamide gel electrophoresis (PAGE) followed by immoblotting with rat anti-Klotho antibodies. (c) Representative immunoblot for NGAL in blood of AKI and Sham rats at 1, 3, 5, 24 h post-reperfusion (three rats at each time point). Four μl of heparinized blood from rats were mixed with Laemle’s buffer and subjected to immunoblot with goat anti-NGAL antibody (1/200 dilution) (Santa Cruz Biotechnology, Santa Cruz, CA, USA). (d) Representative immunoblot for NGAL in total kidney lysates of AKI and Sham rats at 1, 3, 5, and 24 h post-reperfusion (three rats at each time point). Thirty μg of protein were loaded in each lane and subjected to SDS-PAGE followed by immoblotting with goat anti-NGAL antibody.

Figure 4. Klotho protein expression in acute kidney injury (AKI) mice with different genetic background

(a) Representative immunoblot for Klotho in total kidney lysate from four mouse kidneys each group at day 2 post-ischemia–reperfusion injury (IRI) (upper panel). Thirty μg of protein was immunoblotted for Klotho protein with rat anti-Klotho antibody. Lower panel is a summary of densitometric quantification of the data. (b) Representative fluorescent immunohistochemistry for Klotho in kidney sections of four mice in each group at day 2 post-IRI. Four μm of frozen kidney sections were stained for Klotho with rat anti-Klotho antibody. (c) Plasma Klotho levels. One hundred μl of heparinized blood from mice on day 2 were incubated with rabbit anti-Klotho antibody to precipitate Klotho from the blood. The immune complex was subjected to immunoblot with rat anti-Klotho antibody. Upper panel is a representative blot for Klotho and immunoglobulin G heavy chain (IgG-HC). Lower panel is summary of densitometric analysis of all samples. Values are expressed as means±s.e.m. of four mice in each group. Statistical analysis was conducted similar to that shown in (a). (d) Representative immunoblot for Klotho in urine from four mice in each group. On day 2, a volume of fresh urine containing 10 μg of creatinine was blotted using rat anti-Klotho antibody (upper panel). Lower panel is summary of densitometric analysis of immunoblots for urine Klotho from four mice in each group. (e) Kidney histological scores. Scores are expressed as mean±s.e. of six samples in each group. Statistical differences on day 2 were analyzed by analysis of variance followed by Student–Newman–Keuls test, and significant differences were accepted when P<0.05 or less between groups. Kl^+/−, Klotho haploinsufficiency; WT, wild type; and Tg-Kl, transgenic overexpression of Klotho.

Figure 5. Klotho administration post-ischemia–reperfusion injury (IRI) in acute kidney injury (AKI) rats

AKI and Sham rats treated with Klotho (Kl) or vehicle (Veh) 30 min post-reperfusion were killed at 0, 1, 2, and 7 days after IRI and blood and urine were collected for plasma creatinine (Cr) (a), blood urea nitrogen (BUN) (b), Cl_Cr (Cr clearance) (c), urine excretion rate (d), and urinary osmolality (e). Data are expressed as mean±s.e.m. (n = 4) in each group. Statistical significance was analyzed by analysis of variance (ANOVA) followed by Student–Newman–Keuls test, for data presented in a–e and significant differences were accepted when *P<0.05, **P<0.01 vs Sham-vehicle rats; ^#P<0.05; ^##P<0.01 vs AKI-vehicle rats; ^§P<0.05, ^§§P<0.01 vs Sham-Klotho rats. Sprague–Dawley rats were killed on days 1, 2, and 7 after IRI and the kidneys were harvested for hematoxylin and eosin staining, and kidney histological scores (f) were obtained. Data are expressed as mean±s.e.m. (n = 4) in each group. Statistical significance was analyzed by ANOVA followed by Student–Newman–Keuls test, and significant differences were accepted when P<0.05 or less between two groups. A, AKI rats; S, Sham rats.

Figure 6. Neutrophil gelatinase-associated lipocalin (NGAL) and Klotho response to ischemia–reperfusion injury (IRI) in acute kidney injury (AKI) rats: effect of Klotho treatment.<

br>Representative immunoblot for NGAL protein in total kidney lysate from four rats in each group on days 1, 2, and 7 (a). Thirty μg of protein was immunoblotted for NGAL with goat anti-NGAL antibody (Santa Cruz Biotechnology). (b) Abundance of NGAL transcript in kidney. Total RNA was extracted from rat kidney on days 1, 2, and 7 after IRI. Complementary DNA was generated. Relative quantification of NGAL transcripts was calculated as 2^−(ΔΔCt) by normalization to cyclophilin followed by ratio to Sham-vehicle rats. Relative NGAL transcripts in treated rats vs Sham-vehicle rats are expressed as mean±s.e.m. of four samples in each group. Statistical significance was assessed by analysis of variance (ANOVA) followed by Student–Newman–Keuls test, and significant differences were accepted when P<0.05 or less between two groups. Immunoblot for Klotho protein in total kidney lysate from rats on days 1, 2, and 7. Representative immunoblot for Klotho and β-actin in total kidney lysate of six rats in each group (c, upper panel). A summary of densitometric quantification of all samples is shown in lower panel. Results are expressed as mean±s.e.m. of six rats in each group. Statistical significance was assessed by ANOVA followed by Student–Newman–Keuls test, and significant differences were accepted when *P<0.05, **P<0.01 vs Sham-vehicle rats; ^#P<0.05; ^##P<0.01 vs AKI-vehicle rats; ^§P<0.05,^§§P<0.01 vs Sham-Klotho rats. Infrared dye-labeled Klotho (IR-Kl) in blood and urine of Sham or AKI rats (d). Rats were intravenously injected once with IR-Kl and 4 μl of plasma at 5, 10, 30, 60 min, and at 24 and 48 h post-injection and of fresh spot urine at 20 min and 24 and 48 h post-injection were collected and fractionated on 7.5% SDS-polyacrylamide gel electrophoresis and scanned by Odyssey Infrared Imaging System (Lincoln, NE, USA). IR-Klotho in the kidneys of Sham and AKI rats (e): After a single bonus intravenous injection of IR-Klotho, rats were killed at the indicated time points for examination of labeled IR-Klotho in kidneys. Four μm cryosections of non-fixed kidneys were prepared and scanned directly using Odyssey Infrared Imaging System. A, AKI; S, Sham rats.