SDMA attenuates renal tubulointerstitial fibrosis through inhibition of STAT4

BACKGROUND

Renal tubulointerstitial fibrosis is the hallmark of various chronic kidney diseases. Symmetric dimethylarginine (SDMA) is an independent cardiovascular risk factor in patients with chronic kidney diseases, which is mostly excreted through renal tubules. However, the effect of SDMA on kidneys in a pathological condition is currently unknown. In this study, we investigated the role of SDMA in renal tubulointerstitial fibrosis and explored its underlying mechanisms.

METHODS

Mouse unilateral ureteral obstruction (UUO) and unilateral ischemia-reperfusion injury (UIRI) models were established to study renal tubulointerstitial fibrosis. SDMA was injected into kidneys through ureter retrogradely. TGF-β stimulated human renal epithelial (HK2) cells were used as an in vitro model and treated with SDMA. Signal transducer and activator of transcription-4 (STAT4) was inhibited by berbamine dihydrochloride or siRNA or overexpressed by plasmids in vitro. Masson staining and Western blotting were performed to evaluate renal fibrosis. Quantitative PCR was performed to validate findings derived from RNA sequencing analysis.

RESULTS

We observed that SDMA (from 0.01 to 10 µM) dose-dependently inhibited the expression of pro-fibrotic markers in TGF-β stimulated HK2 cells. Intrarenal administration of SDMA (2.5 µmol/kg or 25 µmol/kg) dose-dependently attenuated renal fibrosis in UUO kidneys. A significant increase in SDMA concentration (from 19.5 to 117.7 nmol/g, p < 0.001) in mouse kidneys was observed after renal injection which was assessed by LC-MS/MS. We further showed that intrarenal administration of SDMA attenuated renal fibrosis in UIRI induced mouse fibrotic kidneys. Through RNA sequencing analysis, we found that the expression of STAT4 was reduced by SDMA in UUO kidneys, which was further confirmed by quantitative PCR and Western blotting analysis in mouse fibrotic kidneys and renal cells. Inhibition of STAT4 by berbamine dihydrochloride (0.3 mg/ml or 3.3 mg/ml) or siRNA reduced the expression of pro-fibrotic markers in TGF-β stimulated HK2 cells. Furthermore, blockage of STAT4 attenuated the anti-fibrotic effect of SDMA in TGF-β stimulated HK2 cells. Conversely, overexpression of STAT4 reversed the anti-fibrotic effect of SDMA in TGF-β stimulated HK2 cells.

CONCLUSION

Taken together, our study indicates that renal SDMA ameliorates renal tubulointerstitial fibrosis through inhibition of STAT4.

Novel in vivo and ex vivo hybrid in vivo imaging system (IVIS) imaging offers a convenient and precise way to measure the glomerular filtration rate in conscious mice

INTRODUCTION

In pharmacology and toxicology studies, the glomerular filtration rate (GFR) is the gold standard for the assessment of renal function, and the renal clearance of inulin in blood measured by photometers is known as a filtration marker for the determination of GFR. Preclinically, a non-invasive GFR measurement method was recently developed in which near-infrared fluorescently labelled inulin (GFR-Vivo 680) was scanned with fluorescence molecular tomography (FMT). However, measurement of GFR using FMT has major disadvantages and technical challenges, such as requiring experienced skills in animal handling and rapid and precise time management. Additionally, fur and skin pigmentation may severely compromise imaging due to tissue fluorescence absorption. To overcome these drawbacks of FMT imaging, we have developed an in- and ex vivo hybrid method for measuring GFR using the in vivo imaging system (IVIS).

METHODS

An IVIS-based imaging method was tested to determine the clearance kinetics of plasma GFR-Vivo 680 after a single bolus injection in conscious C57BL/6 mice administered vehicle or cyclosporine A (CsA, 80 mg/kg) for 14 days.

RESULTS

Based on a two-compartment model fitting, the estimated GFR was 235 ± 53 and 189 ± 19 μL/min in vehicle-treated and CsA-treated male mice, respectively (p < 0.01). Our assay revealed the decreased GFR, similar to the sensitivity of FMT imaging, which yielded comparable GFR values (229 ± 61 and 151 ± 35 μL/min in vehicle-treated and CsA-treated mice, respectively, p < 0.01), and to those previously reported in the literature.

DISCUSSION

These studies demonstrate the feasibility of IVIS imaging measurement of inulin clearance in untreated, vehicle-treated and cyclosporine A-treated mice. We propose this new method as an alternative, simple, and versatile way to measure GFR in vivo and ex vivo in pharmacological and toxicological studies.

A Review of Specific Biomarkers of Chronic Renal Injury and Their Potential Application in Nonclinical Safety Assessment Studies

A host of novel renal biomarkers have been developed over the past few decades which have enhanced monitoring of renal disease and drug-induced kidney injury in both preclinical studies and in humans. Since chronic kidney disease (CKD) and acute kidney injury (AKI) share similar underlying mechanisms and the tubulointerstitial compartment has a functional role in the progression of CKD, urinary biomarkers of AKI may provide predictive information in chronic renal disease. Numerous studies have explored whether the recent AKI biomarkers could improve upon the standard clinical biomarkers, estimated glomerular filtration rate (eGFR), and urinary albumin to creatinine ratio, for predicting outcomes in CKD patients. This review is an introduction to alternative assays that can be utilized in chronic (>3 months duration) nonclinical safety studies to provide information on renal dysfunction and to demonstrate specific situations where these assays could be utilized in nonclinical drug development. Novel biomarkers such as symmetrical dimethyl arginine, dickkopf homolog 3, and cystatin C predict chronic renal injury in animals, act as surrogates for GFR, and may predict changes in GFR in patients over time, ultimately providing a bridge from preclinical to clinical renal monitoring.

Enhanced dimethylarginine degradation improves coronary flow reserve and exercise tolerance in Duchenne muscular dystrophy carrier mice

Duchenne muscular dystrophy (DMD) is an X-linked disease caused by null mutations in dystrophin and characterized by muscle degeneration. Cardiomyopathy is common and often prevalent at similar frequency in female DMD carriers irrespective of whether they manifest skeletal muscle disease. Impaired muscle nitric oxide (NO) production in DMD disrupts muscle blood flow regulation and exaggerates postexercise fatigue. We show that circulating levels of endogenous methylated arginines including asymmetric dimethylarginine (ADMA), which act as NO synthase inhibitors, are elevated by acute necrotic muscle damage and in chronically necrotic dystrophin-deficient mice. We therefore hypothesized that excessive ADMA impairs muscle NO production and diminishes exercise tolerance in DMD. We used transgenic expression of dimethylarginine dimethylaminohydrolase 1 (DDAH), which degrades methylated arginines, to investigate their contribution to exercise-induced fatigue in DMD. Although infusion of exogenous ADMA was sufficient to impair exercise performance in wild-type mice, transgenic DDAH expression did not rescue exercise-induced fatigue in dystrophin-deficient male mdx mice. Surprisingly, DDAH transgene expression did attenuate exercise-induced fatigue in dystrophin-heterozygous female mdx carrier mice. Improved exercise tolerance was associated with reduced heart weight and improved cardiac β-adrenergic responsiveness in DDAH-transgenic mdx carriers. We conclude that DDAH overexpression increases exercise tolerance in female DMD carriers, possibly by limiting cardiac pathology and preserving the heart's responses to changes in physiological demand. Methylated arginine metabolism may be a new target to improve exercise tolerance and cardiac function in DMD carriers or act as an adjuvant to promote NO signaling alongside therapies that partially restore dystrophin expression in patients with DMD.NEW & NOTEWORTHY Duchenne muscular dystrophy (DMD) carriers are at risk for cardiomyopathy. The nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) is released from damaged muscle in DMD and impairs exercise performance. Transgenic expression of dimethylarginine dimethylaminohydrolase to degrade ADMA prevents cardiac hypertrophy, improves cardiac function, and improves exercise tolerance in DMD carrier mice. These findings highlight the relevance of ADMA to muscular dystrophy and have important implications for therapies targeting nitric oxide in patients with DMD and DMD carriers.

Sexual Dimorphism of Metabolomic Profile in Arterial Hypertension

Metabolomic studies have demonstrated the existence of biological signatures in blood of patients with arterial hypertension, but no study has hitherto reported the sexual dimorphism of these signatures. We compared the plasma metabolomic profiles of 28 individuals (13 women and 15 men) with essential arterial hypertension with those of a healthy control group (18 women and 18 men), using targeted metabolomics. Among the 188 metabolites explored, 152 were accurately measured. Supervised OPLS-DA (orthogonal partial least squares-discriminant analysis) showed good predictive performance for hypertension in both sexes (Q²cum = 0.59 in women and 0.60 in men) with low risk of overfitting (p-value-CV ANOVA = 0.004 in women and men). Seventy-five and 65 discriminant metabolites with a VIP (variable importance for the projection) greater than 1 were evidenced in women and men, respectively. Both sexes showed a considerable increase in phosphatidylcholines, a decrease in C16:0 with an increase in C28:1 lysophosphatidylcholines, an increase in sphingomyelins, as well as an increase of symmetric dimethylarginine (SDMA), acetyl-ornithine and hydroxyproline. Twenty-nine metabolites, involved in phospholipidic and cardiac remodeling, arginine/nitric oxide pathway and antihypertensive and insulin resistance mechanisms, discriminated the metabolic sexual dimorphism of hypertension. Our results highlight the importance of sexual dimorphism in arterial hypertension.

Arginine Derivatives in Cerebrovascular Diseases: Mechanisms and Clinical Implications

The amino acid L-arginine serves as substrate for the nitric oxide synthase which is crucial in vascular function and disease. Derivatives of arginine, such as asymmetric (ADMA) and symmetric dimethylarginine (SDMA), are regarded as markers of endothelial dysfunction and have been implicated in vascular disorders. While there is a variety of studies consolidating ADMA as biomarker of cerebrovascular risk, morbidity and mortality, SDMA is currently emerging as an interesting metabolite with distinct characteristics in ischemic stroke. In contrast to dimethylarginines, homoarginine is inversely associated with adverse events and mortality in cerebrovascular diseases and might constitute a modifiable protective risk factor. This review aims to provide an overview of the current evidence for the pathophysiological role of arginine derivatives in cerebrovascular ischemic diseases. We discuss the complex mechanisms of arginine metabolism in health and disease and its potential clinical implications in diverse aspects of ischemic stroke.

Asymmetric and Symmetric Dimethylarginine Predict Outcomes in Patients With Atrial Fibrillation: An ARISTOTLE Substudy

BACKGROUND

There is little mechanistic information on factors predisposing atrial fibrillation (AF) patients to thromboembolism or bleeding, but generation of nitric oxide (NO) might theoretically contribute to both.

OBJECTIVES

The authors tested the hypothesis that plasma levels of the methylated arginine derivatives asymmetric and symmetric dimethylarginine (ADMA/SDMA), which inhibit NO generation, might be associated with outcomes in AF.

METHODS

Plasma samples were obtained from 5,004 patients with AF at randomization to warfarin or apixaban in the ARISTOTLE (Apixaban for Reduction in Stroke and Other Thromboembolic Events in Atrial Fibrillation) trial. ADMA and SDMA concentrations were measured by high-performance liquid chromatography. Relationships to clinical characteristics were evaluated by multivariable analyses. Associations with major outcomes, during a median of 1.9 years follow-up, were evaluated by adjusted Cox proportional hazards models.

RESULTS

Both ADMA and SDMA plasma concentrations at study entry increased significantly with patients' age, female sex, renal impairment, permanent AF, or congestive heart failure. ADMA and SDMA increased (p < 0.001) with both increased CHA₂DS₂-VASc and HAS-BLED scores, but decreased in the presence of diabetes. On multivariable analysis adjusting for established risk factors and treatment, tertile groups of ADMA concentrations were significantly associated with stroke/systemic embolism (p = 0.034), and death (p < 0.0001), whereas tertile groups of SDMA were associated with major bleeding and death (p < 0.001 for both). Incorporating ADMA and SDMA into CHA₂DS₂-VASc or HAS-BLED predictive models improved C-indices for those outcomes. Neither ADMA nor SDMA predicted differential responses to warfarin or apixaban.

CONCLUSIONS

In anticoagulated patients with AF, elevated ADMA levels are weakly associated with thromboembolic events, elevated SDMA levels with bleeding events and both are strongly associated with increased mortality. These findings suggest that disturbances of NO function modulate both thrombotic and hemorrhagic risk in anticoagulated patients with AF. (Apixaban for Reduction in Stroke and Other Thromboembolic Events in Atrial Fibrillation [ARISTOTLE]; NCT00412984).

Inhibition of PAI-1 attenuates perirenal fat inflammation and the associated nephropathy in high-fat diet-induced obese mice

Plasminogen activator inhibitor-1 (PAI-1) is increasingly recognized as a mediator in extracellular matrix (ECM) accumulation in diabetic nephropathy. Previous studies have implicated PAI-1 in adipose tissue (AT) expansion, while also contributing to insulin resistance. As inflammation is also known to occur in perirenal AT during obesity, we hypothesized that in a high-fat diet (HFD)-induced obese mouse model, PAI-1 contributes to macrophage-mediated inflammation and diabetic nephropathy. The HFD mice showed increased expression of PAI-1 in perirenal fat and also displayed increased fat weight and macrophage numbers. We found that the macrophage polarization, proinflammatory macrophage-M1-phenotype, including CD11c, IL-6, and monocyte chemoattractant protein-1, were increased by an HFD and decreased by either the genetic depletion of PAI-1 or treatment with the PAI-1 inhibitor, PAI-039. Similarly, an enhanced anti-inflammatory M2-phenotype, including CD206 and IL-10, was accompanied by either the genetic deletion of PAI-1 or PAI-039 treatment. Furthermore, the inhibition of PAI-1 reduced HFD-induced renal histological lesions and abated profibrotic/extracellular-matrix protein. Collectively, our findings provide support that PAI-1 contributes to the development of inflammation in perirenal fat and correlates with the development of diabetic nephropathy in HFD-induced obesity.

Reliability of symmetric dimethylarginine in dogs with myxomatous mitral valve disease as kidney biomarker

The most common cause of heart failure in the canine population is myxomatous mitral valve disease, sometimes complicated by chronic kidney disease. Many studies have been done on the use of symmetric dimethylarginine as biomarker of renal impairment in dogs affected by chronic kidney disease, few studies have examined his reliability as biomarker in dogs affected by heart diseases. Aim of this study was to evaluate symmetric dimethylarginine in dogs affected by mitral valve disease in order to assess his reliability in heart diseases. This was a retrospective case-control study on a clinical population of dogs affected by mitral valve disease (cases) vs healthy dogs (controls). Both groups underwent a complete physical evaluation, echocardiographic examination, complete blood count, biochemical panel, including serum creatinine and urea and urine analysis with protein-to-creatinine ratio. Serum was frozen and sent to IDEXX laboratories for symmetric dimethylarginine determination. General linear model was applied to data. A total number of 24 cases and 7 controls were included. Symmetric dimethylarginine value was in the reference value in the 75% (n=18) of cases, and in the 43% (n=3) of controls. Once set symmetric dimethylarginine as dependent variable, no statistical significant differences were found for each variable considered (breed, age, sex, weight, class of cardiomyopathy, presence/absence of valvular disease, presence/absence of congestive heart failure, pharmacological therapy, creatinine and urea concentration). Blood concentration of SDMA resulted not influenced by the variables mentioned above, so it could be considered a reliable marker of early renal impairment in dogs affected by mitral valve disease.

Symmetric Dimethylarginine in Cats with Hypertrophic Cardiomyopathy and Diabetes Mellitus

Background

Symmetric dimethylarginine (SDMA) has been increasingly used as a marker of early chronic kidney disease (CKD) in cats, but little is known about the influence of comorbidities on SDMA in this species.

Hypothesis

Hypertrophic cardiomyopathy (HCM) and diabetes mellitus (DM), independently of CKD, are associated with changes in serum SDMA.

Animals

Ninety‐four cats (17 with CKD, 40 with HCM, 17 with DM, and 20 healthy controls).

Methods

Case‐control study. Clinical examination, echocardiography, ECG, blood pressure, CBC, biochemistry, thyroxine, and SDMA measurement were performed. Urinalysis was performed in controls and cats with CKD and DM. Analysis of variance was used to compare overall differences in the log‐transformed SDMA data among groups. A random forest algorithm was applied to explore which clinical and other factors influenced serum SDMA.

Results

Median (range) serum SDMA for the renal group (positive control) was 19 (10–93) μg/dL, whereas for the control group (negative control), it was 10 (5–15) μg/dL. For the cardiac and diabetic groups, serum SDMA was 9 (4–24) μg/dL and 7 (3–11) μg/dL, respectively. The renal group had significantly higher SDMA concentrations and the diabetic group significantly lower SDMA concentrations compared to all other groups.

Conclusions and Clinical Importance

Serum SDMA concentrations in cats with HCM were not significantly different from those of healthy control cats. Cats with DM, however, had significantly lower SDMA concentrations than controls, a finding that needs further investigation and should be kept in mind when evaluating renal function of cats with this endocrinopathy.

Serum Asymmetric and Symmetric Dimethylarginine and Morbidity and Mortality in Hemodialysis Patients

BACKGROUND

Asymmetric (ADMA) and symmetric dimethylarginine (SDMA) are putative uremic toxins that may exert toxicity by a number of mechanisms, including impaired nitric oxide synthesis and generation of reactive oxygen species. The study goal was to determine the association between these metabolites and cardiovascular outcomes in hemodialysis patients.

STUDY DESIGN

Post hoc analysis of the Hemodialysis (HEMO) Study.

SETTING & PARTICIPANTS

1,276 prevalent hemodialysis patients with available samples 3 to 6 months after randomization.

PREDICTOR

ADMA and SDMA measured in stored specimens.

OUTCOMES

Cardiac death, sudden cardiac death, first cardiovascular event, and any-cause death. Association with predictors analyzed using Cox regression adjusted for potential confounders (including demographics, clinical characteristics, comorbid conditions, albumin level, and residual kidney function).

RESULTS

Mean age of patients was 57±14 (SD) years, 63% were black, and 57% were women. Mean ADMA (0.9±0.2μmol/L) and SDMA levels (4.3±1.4μmol/L) were moderately correlated (r=0.418). Higher dialysis dose or longer session length were not associated with lower predialysis ADMA or SDMA concentrations. In fully adjusted models, each doubling of ADMA level was associated with higher risk (HR per 2-fold higher concentration; 95% CI) of cardiac death (1.83; 1.29-2.58), sudden cardiac death (1.79; 1.19-2.69), first cardiovascular event (1.50; 1.20-1.87), and any-cause death (1.44; 1.13-1.83). Compared to the lowest ADMA quintile (<0.745 μmol/L), the highest ADMA quintile (≥1.07μmol/L) was associated with higher risk (HR; 95% CI) of cardiac death (2.10; 1.44-3.05), sudden cardiac death (2.06; 1.46-2.90), first cardiovascular event (1.75; 1.35-2.27), and any-cause death (1.56; 1.21-2.00). SDMA level was associated with higher risk for cardiac death (HR, 1.40; 95% CI, 1.03-1.92), but this was no longer statistically significant after adjusting for ADMA level (HR, 1.20; 95% CI, 0.86-1.68).

LIMITATIONS

Single time-point measurement of ADMA and SDMA.

CONCLUSIONS

ADMA and, to a lesser extent, SDMA levels are associated with cardiovascular outcomes in hemodialysis patients.

Kidney Disease and the Nexus of Chronic Kidney Disease and Acute Kidney Injury: The Role of Novel Biomarkers as Early and Accurate Diagnostics

Chronic kidney disease (CKD) and acute kidney injury (AKI) are interconnected and the presence of one is a risk for the other. CKD is an important predictor of AKI after exposure to nephrotoxic drugs or major surgery, whereas persistent or repetitive injury could result in the progression of CKD. This brings new perspectives to the diagnosis and monitoring of kidney diseases highlighting the need for a panel of kidney-specific biomarkers that reflect functional as well as structural damage and recovery, predict potential risk and provide prognosis. This article discusses the kidney-specific biomarkers, symmetric dimethylarginine (SDMA), clusterin, cystatin B, and inosine.

Arginine dimethylation products in pediatric patients with chronic kidney disease

Background

arginine and its metabolites have been linked to pediatric chronic kidney disease (CKD). We aimed to estimate serum levels of argninine (Arg), asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) in pediatric CKD patients and its relation to altered kidney function.

Patients and methods

132 pediatric patients with CKD and 120 healthy age and sex matched controls were compared regarding; serum Arg, ADMA and SDMA levels.

Results

In comparison to their values in control subjects, serum Arg levels were significantly lower; serum ADMA levels were non-significantly higher, but serum SDMA levels were significantly higher in CKD patients (p values: < 0.000; = 0.054; <0.000, respectively).

Calculated Arg/ADMA and Arg/SDMA ratios were significantly higher in patients compared to controls (p values: 0.001, and <0.000, respectively). However ADMA/SDMA ratio was significantly lower in patients compared to controls (p = 0.001. Serum Arg levels showed positive significant correlation, while serum ADMA and SDMA levels showed negative significant correlation with eGFR. Moreover, Arg/ADMA ratio showed negative significant correlation, while ADMA/SDMA ratio showed positive significant correlation with eGFR of patients. Regression analysis defined high serum SDMA level as persistently significant predictor for low eGFR.

Conclusion

Disturbed serum levels of arginine and its dimethyl derivatives may underlie development and/or progression of CKD. Elevated serum SDMA level is strongly correlated with impaired kidney functions and could be considered as a predictor for kidney functions deterioration and CKD progression.

•

The underlying pathogenic mechanisms for pediatric CKD are multiple and interlocking.

•

Disturbed serum levels of Arg and its dimethyl derivatives may underlie development and/or progression of CKD.

•

Elevated serum SDMA level is strongly correlated with impaired kidney functions.

•

Elevated SDMA level can be a predictor for kidney functions deterioration and CKD progression.

Serum Concentrations of Symmetric Dimethylarginine and Creatinine in Dogs with Naturally Occurring Chronic Kidney Disease

Background

Serum concentrations of symmetric dimethylarginine (SDMA) detected chronic kidney disease (CKD) in cats an average of 17.0 months before serum creatinine (Cr) concentrations increased above the reference interval.

Objectives

To report on the utility of measuring serum SDMA concentrations in dogs for detection of CKD before diagnosis by measurement of serum Cr.

Animals

CKD dogs (n = 19) included those persistently azotemic for ≥3 months (n = 5), dogs that were azotemic at the time of death (n = 4), and nonazotemic dogs (n = 10). CKD dogs were compared with healthy control dogs (n = 20).

Methods

Retrospective study, whereby serum Cr concentrations were determined by enzymatic colorimetry and serum SDMA concentrations were determined by liquid chromatography‐mass spectrometry in dogs with necropsy confirmed CKD.

Results

Serum SDMA increased before serum Cr in 17 of 19 dogs (mean, 9.8 months; range, 2.2–27.0 months). Duration of elevations in serum SDMA concentrations before the dog developed azotemia (N = 1) or before the dog died (N = 1) was not determined. Serum SDMA and Cr concentrations were linearly related (r = 0.84; P < .001). Serum SDMA (r = −0.80) and serum Cr (r = −0.89) concentrations were significantly related to glomerular filtration rate (both P < .001).

Conclusion and Clinical Importance

Using serum SDMA as a biomarker for CKD allows earlier detection of kidney dysfunction in dogs than does measurement of serum Cr. Earlier detection might be desirable for initiating renoprotective interventions that slow progression of kidney disease.

Positive Impact of Nutritional Interventions on Serum Symmetric Dimethylarginine and Creatinine Concentrations in Client-Owned Geriatric Dogs

A prospective study was conducted in client-owned geriatric dogs to evaluate the short-term effects of a test food on serum symmetric dimethylarginine (SDMA) and creatinine (Cr) concentrations. Test food contained functional lipids (fish oil), antioxidants (lipoic acid, vitamins C and E), L-carnitine, botanicals (fruits and vegetables), controlled sodium concentration, and high quality protein sources (high bioavailability and an ideal amino acid composition). Dogs (n = 210) were fed either test food or owner's-choice foods (non-nutritionally controlled cohort). Dogs were included based on age and body weight: small (6.8 to 11.4 kg) and medium dogs (11.5 to 22.7 kg) were ≥ 9 years, whereas dogs >22.7 kg were ≥ 7 years at baseline. At baseline, all dogs had to have serum Cr concentrations within the reference interval and be free of chronic disease. Renal function biomarkers and urinalysis results at baseline, and after consuming test food or owner's-choice foods for 3 and 6 months, were evaluated. Only dogs consuming test food showed significant decreases in serum SDMA and Cr concentrations (both P ≤ 0.05) across time. At baseline or during the 6-month feeding trial, 18 dogs (8.6%) had increased serum SDMA, but normal serum Cr, consistent with IRIS Stage 1 chronic kidney disease. This included 9 dogs fed test food and 9 dogs fed owner's-choice foods. Compared with baseline, after feeding 9 dogs test food for 6 months, serum SDMA decreased in 8 dogs and increased in 1 dog. After feeding 9 dogs owner's-choice foods for 6 months, serum SDMA decreased in 4 dogs and increased in 4 dogs (remained stable in 1 dog). The decreases in serum SDMA and Cr concentrations were significant (both P = 0.03) only for dogs fed test food. These results suggest that nonazotemic dogs with elevated serum SDMA (early renal insufficiency) when fed a test food designed to promote healthy aging are more likely to demonstrate improved renal function compared with dogs fed owner's-choice foods.

Positive Impact of Nutritional Interventions on Serum Symmetric Dimethylarginine and Creatinine Concentrations in Client-Owned Geriatric Cats

A prospective study was conducted in client-owned geriatric cats to evaluate the short- term effects of a test food on serum symmetric dimethylarginine (SDMA) and creatinine (Cr) concentrations. Test food contained functional lipids (fish oil), antioxidants (vitamins C and E), L-carnitine, botanicals (vegetables), highly bioavailable protein, and amino acid supplements. Cats (n = 80) were fed either test food or owner's-choice foods (non-nutritionally controlled cohort). Cats were included based on age (≥ 9 years), indoor only, neutered, and free of chronic disease. At baseline, all cats had serum Cr concentrations within the reference interval. Renal function biomarkers and urinalysis results at baseline and after consuming test food or owner's-choice foods for 3 and 6 months were evaluated. Cats consuming test food showed significant decreases in serum Cr and BUN concentrations across time. Overall, cats consuming owner's-choice foods showed significant increases in serum SDMA concentrations at 3 and 6 months compared with baseline (P ≤ 0.05), whereas in cats consuming test food serum SDMA concentrations did not change. At baseline or during the 6-month feeding trial, 23 (28.8%) cats had increased serum SDMA, but normal serum Cr consistent with IRIS Stage 1 chronic kidney disease. This included 6 cats fed test food and 17 cats fed owner's-choice foods. In the 6 cats fed test food, serum SDMA decreased in 3 cats and remained stable in 1 cat, whereas in the 17 cats fed owner's-choice foods, serum SDMA increased in 13 cats and decreased or remained stable in 4 cats. The increase in serum SDMA concentration was significant (P = 0.02) only for cats fed owner's-choice foods. These results suggest that nonazotemic cats with elevated serum SDMA (early renal insufficiency) when fed a food designed to promote healthy aging are more likely to demonstrate stable renal function compared with cats fed owner's-choice foods. Cats fed owner's-choice foods were more likely to demonstrate progressive renal insufficiency.

Acetylation of asymmetric and symmetric dimethylarginine: an undercharacterized pathway of metabolism of endogenous methylarginines

BACKGROUND

Increased levels of asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) are associated with cardiovascular and renal diseases. We and others have shown that both ADMA and SDMA can be Nα-acetylated to form asymmetric and symmetric Nα-acetyldimethylarginine (Ac-ADMA and Ac-SDMA). The current study further investigated this undercharacterized metabolic pathway.

METHODS

ADMA and SDMA were infused in C57/BL6 mice for 3 days using osmotic minipumps. Half of the mice underwent bilateral nephrectomy 24 h before completion of the infusion. Plasma and tissue levels of Ac-ADMA and Ac-SDMA were detected by liquid chromatography-tandem mass spectrometry.

RESULTS

ADMA and SDMA infusion resulted in a 3.6-fold increase in plasma Ac-ADMA and a 21-fold increase in plasma Ac-SDMA levels, respectively. Plasma Ac-ADMA and Ac-SDMA levels were dramatically increased after bilateral nephrectomy. The highest baseline tissue concentrations of Ac-ADMA and Ac-SDMA in wild-type mice were detected in the liver, kidney, small intestine, pancreas and spleen. Incubation of the tissue lysates with ADMA and SDMA resulted in increased levels of the corresponding Nα-acetylated products only in the liver, kidney and small intestine.

CONCLUSIONS

Our results show that overload of ADMA or SDMA leads to an increase in plasma Ac-ADMA and Ac-SDMA levels. This observation is consistent with the hypothesis that Ac-ADMA and Ac-SDMA are formed directly from ADMA and SDMA in vivo. The increase in plasma Ac-ADMA and Ac-SDMA concentrations after bilateral nephrectomy suggests that both compounds are predominantly eliminated via the kidneys. We demonstrated that acetylation of ADMA and SDMA occurs primarily in the liver, kidney and small intestine.

A Murine Hypertrophic Cardiomyopathy Model: The DBA/2J Strain

Familial hypertrophic cardiomyopathy (HCM) is attributed to mutations in genes that encode for the sarcomere proteins, especially Mybpc3 and Myh7. Genotype-phenotype correlation studies show significant variability in HCM phenotypes among affected individuals with identical causal mutations. Morphological changes and clinical expression of HCM are the result of interactions with modifier genes. With the exceptions of angiotensin converting enzyme, these modifiers have not been identified. Although mouse models have been used to investigate the genetics of many complex diseases, natural murine models for HCM are still lacking. In this study we show that the DBA/2J (D2) strain of mouse has sequence variants in Mybpc3 and Myh7, relative to widely used C57BL/6J (B6) reference strain and the key features of human HCM. Four-month-old of male D2 mice exhibit hallmarks of HCM including increased heart weight and cardiomyocyte size relative to B6 mice, as well as elevated markers for cardiac hypertrophy including β-myosin heavy chain (MHC), atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and skeletal muscle alpha actin (α1-actin). Furthermore, cardiac interstitial fibrosis, another feature of HCM, is also evident in the D2 strain, and is accompanied by up-regulation of type I collagen and α-smooth muscle actin (SMA)-markers of fibrosis. Of great interest, blood pressure and cardiac function are within the normal range in the D2 strain, demonstrating that cardiac hypertrophy and fibrosis are not secondary to hypertension, myocardial infarction, or heart failure. Because D2 and B6 strains have been used to generate a large family of recombinant inbred strains, the BXD cohort, the D2 model can be effectively exploited for in-depth genetic analysis of HCM susceptibility and modifier screens.

Symmetric Dimethylarginine Assay Validation, Stability, and Evaluation as a Marker for the Early Detection of Chronic Kidney Disease in Dogs

BACKGROUND

Symmetric dimethylarginine (SDMA) is a small molecule formed by methylation of arginine, and released into blood during protein degradation. SDMA is primarily eliminated by renal excretion and is a promising endogenous marker of glomerular filtration rate (GFR).

OBJECTIVES

To validate an assay for SDMA measurement, determine stability of SDMA in blood, and compare SDMA with serum creatinine concentration (sCr) and GFR for early detection of decreasing kidney function in dogs with chronic kidney disease (CKD).

ANIMALS

Eight male dogs affected with X-linked hereditary nephropathy and 4 unaffected male littermates.

METHODS

Prospective study validating SDMA measurement using liquid chromatography-mass spectrometry, assessing stability of SDMA in serum and plasma, and serially determining sCr, SDMA, and GFR (using iohexol clearance) in dogs during progression from preclinical disease to end-stage renal failure. Correlations were determined using linear regression. Timepoints at which sCr, SDMA, and GFR identified decreased renal function were compared using defined cutoffs, trending in an individual dog, and comparison with unaffected littermates.

RESULTS

Symmetric dimethylarginine was highly stable in serum and plasma, and the assay demonstrated excellent analytical performance. In unaffected dogs, SDMA remained unchanged whereas in affected dogs, SDMA increased during disease progression, correlating strongly with an increase in sCr (r = 0.95) and decrease in GFR (r = -0.95). Although trending improved sCr's sensitivity, SDMA identified, on average, <20% decrease in GFR, which was earlier than sCr using any comparison method.

CONCLUSIONS AND CLINICAL IMPORTANCE

Symmetric dimethylarginine is useful for both early identification and monitoring of decreased renal function in dogs with CKD.

Uraemic toxins and new methods to control their accumulation: game changers for the concept of dialysis adequacy

The current concept of an adequate dialysis based only on the dialysis process itself is rather limited. We now have considerable knowledge of uraemic toxicity and improved tools for limiting uraemic toxin accumulation. It is time to make use of these. A broader concept of adequacy that focusses on uraemic toxicity is required. As discussed in the present review, adequacy could be achieved by many different methods in combination with, or instead of, dialysis. These include preservation of renal function, dietary intake, reducing uraemic toxin generation rate and intestinal absorption, isolated ultrafiltration and extracorporeal adsorption of key uraemic toxins. A better measure of the quality of dialysis treatment would quantify the uraemic state in the patient using levels of a panel of key uraemic toxins. Treatment would focus on controlling uraemic toxicity while reducing harm or inconvenience to the patient. Delivering more dialysis might not be the best way to achieve this.

Relationship between lean body mass and serum renal biomarkers in healthy dogs

Background

Symmetric dimethylarginine (SDMA) is an accurate and precise biomarker for estimating glomerular filtration rate (GFR) in humans and cats. Serum creatinine (sCr) also correlates with GFR, but has limitations as a biomarker of renal function because nonrenal factors can influence its concentration.

Hypothesis

Differences in lean body mass (LBM) influence sCr, but not serum SDMA concentrations.

Animals

Forty‐one healthy Beagles, mean age 9.9 years (range: 3.1–14.8 years), were studied over a 6 month period.

Methods

Serum biomarkers of renal function were measured prospectively at baseline, and 1, 3, and 6 months. SDMA concentrations were measured by liquid chromatography‐mass spectroscopy and sCr concentrations by enzymatic colorimetry. Body composition was determined by dual energy x‐ray absorptiometry.

Results

LBM (P < .001) and age (P = .006) were significant explanatory variables for sCr concentration (R² = 0.38), but not SDMA concentration. Time on food was the only significant explanatory variable for SDMA concentration (R² = 0.49). SDMA concentrations decreased across time (P < .001). LBM was affected by sex (males > females; P = .02). Mature adult dogs (<8 years) had greater LBM compared with geriatric dogs (≥8 years; P < .001).

Conclusion and Clinical Importance

sCr concentrations, but not SDMA concentrations, are influenced by LBM, which limits sCr utility as a biomarker for monitoring renal function in dogs with decreased LBM. Reductions in LBM can lower sCr concentration and overestimate GFR. SDMA concentrations, but not sCr concentrations were influenced by time on food. SDMA could have clinical advantages over sCr in monitoring response to nutritional interventions.

Role of alanine:glyoxylate aminotransferase 2 in metabolism of asymmetric dimethylarginine in the settings of asymmetric dimethylarginine overload and bilateral nephrectomy

BACKGROUND

Asymmetric and symmetric dimethylarginines (ADMA and SDMA) predict complications and mortality in cardiovascular and renal diseases. Alanine:glyoxylate aminotransferase 2 (AGXT2) can metabolize both ADMA and SDMA; however, this metabolic pathway is still poorly understood. The goal of our study was to test the hypothesis that AGXT2 is compensatory upregulated in the settings of ADMA overload and bilateral nephrectomy.

METHODS

ADMA was infused for 3 days using osmotic minipumps in mice. Half of the mice underwent bilateral nephrectomy 24 h before the end of the infusion.

RESULTS

Infusion of ADMA caused a 3- to 4-fold increase in plasma and urine ADMA levels and a 2- to 3-fold increase in plasma and urine levels of the ADMA-specific metabolite of AGXT2 α-keto-δ-(N,N-dimethylguanidino)valeric acid (DMGV). Bilateral nephrectomy led to an ∼4-fold increase of plasma SDMA levels, but did not change plasma ADMA levels. Interestingly, plasma levels of DMGV were elevated 32-fold in the mice, which underwent bilateral nephrectomy. Neither bilateral nephrectomy nor ADMA infusion caused upregulation of AGXT2 expression or activity.

CONCLUSIONS

Our data demonstrate that short-term elevation of systemic levels of ADMA leads to a dramatic increase of DMGV formation without upregulation of AGXT2 expression or activity, which suggests that AGXT2-mediated pathway of ADMA metabolism is not saturated under normal conditions and may play a major role in the maintenance of ADMA homeostasis in the setting of local or systemic elevation of ADMA levels.

Time course of asymmetric dimethylarginine and symmetric dimethylarginine levels after successful renal transplantation

BACKGROUND

Although renal transplantation (Tx) improves the outcome of patients with renal disease, cardiovascular (CV) risk remains high. Recently, it was demonstrated that asymmetric dimethylarginine (ADMA) levels predict CV events and graft survival in renal transplant recipients (RTRs). Little is known about the impact of renal Tx on the plasma levels of ADMA and symmetric dimethylarginine (SDMA). The present study aimed to define the time course of ADMA and SDMA after Tx.

METHODS

We prospectively followed 167 incident RTRs with visits at the time of Tx and 3 and 12 months thereafter. At all visits, demographics and relevant biochemistry were recorded and blood was sampled for analysis of ADMA and SDMA (high-performance liquid chromatography). Eighty-four patients had an additional sampling in the immediate postoperative period. In a case-controlled substudy (n = 31), we compared ADMA and SDMA levels between RTRs and chronic kidney disease (CKD) patients, matched for glomerular filtration rate, gender, age, CV history and diabetes.

RESULTS

Overall, plasma ADMA and SDMA levels decreased after Tx. The decline of SDMA was more pronounced and paralleled the recovery of renal function. Interestingly, the decline of ADMA was preceded by an increase in the immediate postoperative period. In the case-controlled substudy, SDMA levels were similar, whereas ADMA levels were significantly higher in RTRs compared with the CKD counterparts (P = 0.003).

CONCLUSION

ADMA levels follow a biphasic pattern after successful renal Tx with a transient rise in the immediate postoperative period followed by a decline. Levels remain elevated compared with CKD patients, matched for age, gender, diabetes, CV history and renal function.

Dimethylarginines ADMA and SDMA: the real water-soluble small toxins?

Uremia occurs if the kidney loses the ability to eliminate toxic compounds at a sufficient rate into the urine. In 1970, N-N, N-G- and N-G,N׳-G-dimethyl-arginine (asymmetric dimethylarginine [ADMA] and symmetric dimethylarginine) were isolated from human urine. It was anticipated that both substances might be important in the pathophysiology and for the diagnosis of various pathologic states. It took 22 years, however, before this idea materialized when it was found that ADMA, which is increased in hemodialysis patients, inhibits the synthesis of the endothelial-derived relaxing factor, identified as nitric oxide. ADMA correlates with traditional and nontraditional cardiovascular risk factors and is a strong predictor of cardiovascular events and death in both patients with chronic kidney disease and in the general population. It also seems to mediate adverse cardiovascular effects of drugs such as proton pump inhibitors. To date, we have no specific pharmacologic therapy at hand to neutralize the deleterious effects of ADMA, curbing the enthusiasm for this marker and mediator of cardiovascular disease.

Thrombospondin 1 mediates renal dysfunction in a mouse model of high-fat diet-induced obesity

Obesity is prevalent worldwide and is a major risk factor for many diseases including renal complications. Thrombospondin 1 (TSP1), a multifunctional extracellular matrix protein, plays an important role in diabetic kidney diseases. However, whether TSP1 plays a role in obesity-related kidney disease is unknown. In the present studies, the role of TSP1 in obesity-induced renal dysfunction was determined by using a diet-induced obese mouse model. The results demonstrated that TSP1 was significantly upregulated in the kidney from obese mice. The increased TSP1 was localized in the glomerular mesangium as well as in the tubular system from obese wild-type mice. Obese wild-type mice developed renal hypertrophy and albuminuria, which was associated with increased kidney macrophage infiltration, augmented kidney inflammation, and activated transforming growth factor (TGF)-β signaling and renal fibrosis. In contrast, obese TSP1-deficient mice did not develop these kidney damages. Furthermore, in vitro studies demonstrated that leptin treatment stimulated the expression of TSP1, TGF-β1, fibronectin, and collagen type IV in mesangial cells isolated from wild-type mice. These leptin-stimulated effects were abolished in TSP1-deficient mesangial cells. Taken together, these data suggest that TSP1 is an important mediator for obesity- or hyperleptinemia-induced kidney dysfunction.