Hypertension as a mitochondrial and metabolic disease

Transcriptomic changes in glomeruli in response to a high salt challenge in the Dahl SS rat

Salt sensitivity impacts a significant portion of the population and is an important contributor to the development of chronic kidney disease. One of the significant early predictors of salt-induced damage is albuminuria, which reflects the deterioration of the renal filtration barrier: the glomerulus. Despite significant research efforts, there is still a gap in knowledge regarding the molecular mechanisms and signaling networks contributing to instigating and/or perpetuating salt-induced glomerular injury. To address this gap, we used 8-wk-old male Dahl salt-sensitive rats fed a normal-salt diet (0.4% NaCl) or challenged with a high-salt diet (4% NaCl) for 3 wk. At the end of the protocol, a pure fraction of renal glomeruli obtained by differential sieving was used for next-generation RNA sequencing and comprehensive semi-automatic transcriptomic data analyses, which revealed 149 differentially expressed genes (107 and 42 genes were downregulated and upregulated, respectively). Furthermore, a combination of predictive gene correlation networks and computational bioinformatic analyses revealed pathways impacted by a high salt dietary challenge, including renal metabolism, mitochondrial function, apoptotic signaling and fibrosis, cell cycle, inflammatory and immune responses, circadian clock, cytoskeletal organization, G protein-coupled receptor signaling, and calcium transport. In conclusion, we report here novel transcriptomic interactions and corresponding predicted pathways affecting glomeruli under salt-induced stress.NEW & NOTEWORTHY Our study demonstrated novel pathways affecting glomeruli under stress induced by dietary salt. Predictive gene correlation networks and bioinformatic semi-automatic analysis revealed changes in the pathways relevant to mitochondrial function, inflammatory, apoptotic/fibrotic processes, and cell calcium transport.

Fumarate and its downstream signalling pathways in the cardiorenal system: Recent insights and novel expositions in the etiology of hypertension

Hypertension, a risk factor for cardiorenal disease has a huge global health impact. Hence, there is a continuous search for new therapeutic targets and putative antihypertensive ligands. This search has transcended into the realm of mitochondrial metabolism which has been reported to underline the etiology of certain diseases, including hypertension. Recently, genetic alterations in the tricarboxylic acid (TCA) cycle enzyme, fumarase, which converts fumarate to malate, reportedly worsened salt-sensitive hypertension. These novel expositions shifted focus into the activity of TCA in the pathogenesis of hypertension. There is now evidence to show that a mechanistic link exists between blood pressure regulation and intermediaries in the TCA cycle involving fumarate metabolism. Fumarate has been reported to mediate the actions of endogenous ligands such as nitric oxide (NO), and hypoxia inducible factor (HIF)-1α. Similarly, there has been upregulation of protective genes such as nuclear erythroid factor 2 (Nrf2) and reduction in the expression of certain markers like kidney injury molecule 1 (KIM-1). There are reports of interactions with endogenous enzymes such as catalase (CAT) and renin via the activation of GPR91. Fumarate has also been shown to modulate the actions of renal ion channels and by extension, natriuresis. These actions of fumarate have conferred a reno- and cardio-protective effect in hypertension. This review evaluates the role of the TCA cycle, its mechanistic links, and significant contribution to blood pressure regulation with a view to understanding the possibility of a new pathological axis which may be involved in the pathogenesis of hypertension.

Effects of fumarate on renal vascular reactivity and the modulation of blood pressure in normotensive rats: Possible contribution of the nitric oxide synthase-nitric oxide system

Introduction: Fumarate, the tricarboxylic acid (TCA) cycle intermediary, has been linked to nitric oxide (NO) production. NO plays a prominent role in the physiological regulation of blood pressure and renal hemodynamics. This study is aimed to investigate any contribution of fumarate to blood pressure and renal hemodynamics in normotensive rats with a possible link to the nitrergic system.

Materials and methods: Fumarate (1, 3 and 10 µmol) was injected into isolated perfused kidneys, pre-constricted with epinephrine (30 µM). The fumarase inhibitor, pyromellitic acid (PMA) (1, 3 and 10 µM), was used to perfuse the isolated kidney and perfusate was collected for nitric oxide and fumarate assays. An acute blood pressure study involved the injection of bolus doses of fumarate (0.1, 0.3 and 1 µg/kg, iv) or PMA (1, 3 and 10 µg/kg, iv) to normotensive rats in the presence of N(ω)-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg, iv) or PMA (1, 3 and 10 µg/kg).

Results and discussion: Fumarate reduced perfusion pressure and elicited a peak reduction at the highest dose. Perfusing the kidney with PMA caused a paradoxical increase in perfusion pressure (70%, p<0.05), compared to baseline. Bolus doses of fumarate reduced blood pressure (-29.3±6.2 mmHg, p<0.05), cortical blood flow (CBF) and increased medullary blood flow (MBF). L-NAME did not abolish the vasodilatory effect of fumarate, but reduced the magnitude of response (50%, p<0.05). PMA did not significantly affect the vasodilatory effect of fumarate (p>0.05).

Conclusion: These data suggest that fumarate exerts a vasodilatory effect on renal and systemic hemodynamics that may partly involve the nitric oxide signaling.

Graphical abstract:

Study design, general characteristics of participants, and preliminary findings from the metabolome, microbiome, and dietary salt intervention study (MetaSalt)

Background

High sodium intake is an important risk factor for hypertension and cardiovascular disease. However, the association between gut microbiota composition and metabolomic profiles with dietary sodium intake and blood pressure (BP) is not well-understood. The metabolome, microbiome, and dietary salt intervention (MetaSalt) study aimed to investigate microbial and metabolomic profiles related to dietary sodium intake and BP regulation.

Methods

This family-based intervention study was conducted in four communities across three provinces in rural northern China in 2019. Probands with untreated prehypertension or stage-1 hypertension were identified through community-based BP screening, and family members including siblings, offspring, spouses, and parents were subsequently included. All participants participated in a 3-day baseline examination with usual diet consumption, followed by a 10-day low-salt diet (3 g/d of salt or 51.3 mmol/d of sodium) and a 10-day high-salt diet (18 g/d of salt or 307.8 mmol/d of sodium). Differences in mean BP levels were compared according to the intervention phases using a paired Student's t-test.

Results

A total of 528 participants were included in this study, with a mean age of 48.1 years, 36.7% of whom were male, 76.8% had a middle school (69.7%) or higher (7.1%) diploma, 23.4% had a history of smoking, and 24.4% were current drinkers. The mean arterial pressure at baseline was 97.2 ± 10.5 mm Hg for all participants, and significantly decreased during the low-salt intervention (93.8 ± 9.3, P < 0.0001) and subsequently increased during the high-salt intervention (96.4 ± 10.0, P < 0.0001).

Conclusions

Our dietary salt intervention study has successfully recruited participants and will facilitate to evaluate the effects of gut microbiota and metabolites on BP regulation in response to sodium burden, which will provide important evidence for investigating the underlying mechanisms in the development of hypertension and subsequent cardiovascular diseases.

Trial registration

The study was registered in the Chinese Clinical Trial Registry database (ChiCTR1900025171).

Fumarate exerted an antihypertensive effect and reduced kidney injury molecule (KIM)-1 expression in deoxycorticosterone acetate-salt hypertension

Background: The tricarboxylic (TCA) acid cycle provides the energy needed for regulatory functions in the cardio-renal system. Recently, a genetic defect in the TCA cycle enzyme, fumarase hydratase, altered L-arginine metabolism and exacerbated hypertension in salt-sensitive rats. This study evaluated the effect of fumarate and its possible link to L-arginine metabolism in deoxycorticosterone (DOCA)-salt hypertension, a non-genetic model of hypertension.Method: Hypertension was induced with DOCA (25 mg/kg s.c, twice weekly) + 1% NaCL in uninephrectomised rats placed on fumarate (1 g/L, ad libitum). Blood pressure was measured in conscious rats via carotid cannulation. Biochemical and western blot analyses were carried out on kidney fractions.Results: Fumarate reduced mean blood pressure (198 ± 5 vs 167 ± 7 mmHg, p < .01), increased nitric oxide levels in the renal cortex (36.1 ± 2 vs 61.3 ± 4 nM/µg) and medulla (27.4 ± 1 vs 54.1 ± 2 nM/µg) of DOCA-salt rats (p < .01). Consistent with this, arginase activity was reduced (threefold) in the renal medulla but not cortex of DOCA-salt rats. Fumarate increased superoxide dismutase activity in the medulla (p < .001) of DOCA-hypertensive rats. However, catalase activity was exacerbated by fumarate in both renal cortex (4.5 ± 1 vs 11.2 ± 1) and medulla (3.7 ± 1 vs 16.3 ± 1 units/mg) of DOCA-salt rats (p < .001). Proteinuria (64.6%), kidney injury molecule-1 expression and kidney weight were reduced in DOCA-hypertensive rats treated with fumarate (p< .05). However, there was a paradoxical increase in TGF-β expression in fumarate-treated DOCA-salt rats. Conclusion: These data show that fumarate attenuated hypertension, renal injury and improved the redox state of the kidney in DOCA/salt hypertension by mechanisms involving selective reduction of L-arginine metabolism.

Renal metabolism and hypertension

Hypertension is a leading risk factor for disease burden worldwide. The kidneys, which have a high specific metabolic rate, play an essential role in the long-term regulation of arterial blood pressure. In this review, we discuss the emerging role of renal metabolism in the development of hypertension. Renal energy and substrate metabolism is characterized by several important and, in some cases, unique features. Recent advances suggest that alterations of renal metabolism may result from genetic abnormalities or serve initially as a physiological response to environmental stressors to support tubular transport, which may ultimately affect regulatory pathways and lead to unfavorable cellular and pathophysiological consequences that contribute to the development of hypertension.

Mitochondrial tRNA Mutations Associated With Essential Hypertension: From Molecular Genetics to Function

Essential hypertension (EH) is one of the most common cardiovascular diseases worldwide, entailing a high level of morbidity. EH is a multifactorial disease influenced by both genetic and environmental factors, including mitochondrial DNA (mtDNA) genotype. Previous studies identified mtDNA mutations that are associated with maternally inherited hypertension, including tRNA^Ile m.4263A>G, m.4291T>C, m.4295A>G, tRNA^Met m.4435A>G, tRNA^Ala m.5655A>G, and tRNA^Met/tRNA^Gln m.4401A>G, et al. These mtDNA mutations alter tRNA structure, thereby leading to metabolic disorders. Metabolic defects associated with mitochondrial tRNAs affect protein synthesis, cause oxidative phosphorylation defects, reduced ATP synthesis, and increase production of reactive oxygen species. In this review we discuss known mutations of tRNA genes encoded by mtDNA and the potential mechanisms by which these mutations may contribute to hypertension.

Differential metabolic profile associated with the condition of normoalbuminuria in the hypertensive population

BACKGROUND AND AIM

Albuminuria is an indicator of sub-clinical organ damage and a marker of cardiovascular risk and renal disease. A percentage of hypertensive patients develop albuminuria despite being under chronic suppression of the renin-angiotensin system (RAS). We previously identified urinary metabolites associated with the development of albuminuria. In this study, we searched for metabolic alterations which reflect different levels within the condition of normoalbuminuria.

PATIENTS, MATERIALS AND METHODS

Urine from 48 hypertensive patients under chronic RAS suppression was analysed. They were classified according to the albumin/creatinine ratio (ACR) into 3groups: Normoalbuminuria (<10mg/g); high-normal (10-30mg/g in men, or 20-40mg/g in women); and moderately high albuminuria (microalbuminuria, 30-200mg/g or 40-300mg/g, respectively). The metabolome was analysed by mass spectrometry and a correlation analysis was performed between altered metabolite levels and ACR.

RESULTS

Oxaloacetate, 3-ureidopropionate, guanidoacetate and malate show significant variation between the normo and micro groups. Additionally, these metabolites are able to differentiate between patients in the normo and high-normal range. A significant correlation between metabolites and ACR was found. Observed variations point to alterations in the energy metabolism already in patients with albuminuria in the high-normal range.

CONCLUSIONS

The association between the molecular panel consisting of 3-ureidopropionate, oxaloacetate, malate and guanidoacetate and different levels of albuminuria is confirmed. A metabolic fingerprint was also identified showing variations within the condition of normoalbuminuria allowing an earlier molecular stratification of patients.

Fumarase Overexpression Abolishes Hypertension Attributable to endothelial NO synthase Haploinsufficiency in Dahl Salt-Sensitive Rats

Human blood pressure salt sensitivity is associated with changes in urinary metabolites related to fumarase (Fh) and nitric oxide (NO) metabolism, and fumarase promotes NO production through an arginine regeneration pathway. We examined the role of the fumarase-NO pathway in the development of hypertension using genetically engineered rat models. Dahl salt-sensitive (SS) rats with heterozygous mutation of eNOS (endothelial NO synthase or Nos3; SS-Nos3^+/-) were bred with SS rats with a hemizygous Fh transgene. SS-Nos3^+/- rats without the Fh transgene (SS-Nos3^+/-/Fh^0/0) developed substantial hypertension with a mean arterial pressure of 134.2±3.7 mm Hg on a 0.4% NaCl diet and 178.0±3.5 mm Hg after 14 days on a 4% NaCl diet. Mean arterial pressure decreased remarkably to 123.1±1.4 mm Hg on 0.4% NaCl, and 143.3±1.5 mm Hg on 4% NaCl in SS-Nos3^+/- rats with a Fh transgene (SS-Nos3^+/-/Fh^0/1), and proteinuria, renal fibrosis, and tubular casts were attenuated in SS-Nos3^+/-/Fh^0/1 rats compared with SS-Nos3^+/-/Fh^0/0 rats. eNOS protein abundance decreased in rats with the Nos3 heterozygous mutation, which was not influenced by Fh overexpression in rats on the 0.4% NaCl diet. However, the decrease in NO metabolite in the renal outer medulla of SS-Nos3^+/-/Fh^0/0 rats on the 0.4% NaCl diet was reversed in SS-Nos3^+/-/Fh^0/1 rats, and levels of L-arginine, but not the other 12 amino acids analyzed, were significantly higher in SS-Nos3^+/-/Fh^0/1 rats than in SS-Nos3^+/+/Fh^0/0 rats. In conclusion, fumarase has potent effects in restoring NO production and blunting the development of hypertension attributable to eNOS haploinsufficiency.

Regulation of DNA methylation and 2-OG/TET signaling by choline alleviated cardiac hypertrophy in spontaneously hypertensive rats

DNA methylation is a well-defined epigenetic modification that regulates gene transcription. However, the role of DNA methylation in the cardiac hypertrophy seen in hypertension is unclear. This study was performed to investigate genome-wide DNA methylation profiles in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto rats (WKY), and the cardioprotective effect of choline. Eight-week-old male SHRs received intraperitoneal injections of choline (8 mg/kg/day) for 8 weeks. SHRs showed aberrant methylation distribution on chromosomes and genome regions, with decreased methylation levels at CHG and CHH sites. A total of 91,559 differentially methylated regions (DMRs) were detected between SHRs and WKY rats, of which 28,197 were demethylated and 63,362 were methylated. Choline treatment partly restored the DMRs in SHRs, which were related to 131 genes. Gene ontology analysis and Kyoto Encyclopedia of Genes and Genomes analysis of DMRs suggested that choline partly reversed the dysfunctions of biological processes, cellular components and molecular functions in SHRs. Moreover, the inhibition of 2-oxoglutarate accumulation by choline, thereby inhibiting excessive activation of ten-eleven translocation methylcytosine dioxygenase enzymes, may correlate with the beneficial effects of choline on methylation levels, cardiac hypertrophy and cardiac function of SHRs, as indicated by decreased heart rate and blood pressure, and increased ejection fraction and fractional shortening. This study provides the first genome-wide DNA methylation profile of the hypertrophic myocardium of SHRs and suggests a novel role for this epigenetic modification in hypertension. Choline treatment may represent a promising approach for modification of DNA methylation and optimization of the epigenetic profile for antihypertensive therapy.

Urinary Metabolites Associated with Blood Pressure on a Low- or High-Sodium Diet

Dietary salt intake has significant effects on arterial blood pressure and the development of hypertension. Mechanisms underlying salt-dependent changes in blood pressure remain poorly understood, and it is difficult to assess blood pressure salt-sensitivity clinically. Methods: We examined urinary levels of metabolites in 103 participants of the Dietary Approaches to Stop Hypertension (DASH)-Sodium trial after nearly 30 days on a defined diet containing high sodium (targeting 150 mmol sodium intake per day) or low sodium (50 mmol per day). Targeted chromatography/mass spectrometry analysis was performed in 24 h urine samples for 47 amino metabolites and 10 metabolites related to the tricarboxylic acid cycle. The effect of an identified metabolite on blood pressure was examined in Dahl salt-sensitive rats. Results: Urinary metabolite levels improved the prediction of classification of blood pressure salt-sensitivity based on race, age and sex. Random forest and generalized linear mixed model analyses identified significant (false discovery rate <0.05) associations of 24 h excretions of β-aminoisobutyric acid, cystine, citrulline, homocysteine and lysine with systolic blood pressure and cystine with diastolic blood pressure. The differences in homocysteine levels between low- and high-sodium intakes were significantly associated with the differences in diastolic blood pressure. These associations were significant with or without considering demographic factors. Treatment with β-aminoisobutyric acid significantly attenuated high-salt-induced hypertension in Dahl salt-sensitive rats. Conclusion: These findings support the presence of new mechanisms of blood pressure regulation involving metabolic intermediaries, which could be developed as markers or therapeutic targets for salt-sensitive hypertension.

Citric Acid Metabolism in Resistant Hypertension: Underlying Mechanisms and Metabolic Prediction of Treatment Response

Resistant hypertension (RH) affects 9% to 12% of hypertensive adults. Prolonged exposure to suboptimal blood pressure control results in end-organ damage and cardiovascular risk. Spironolactone is the most effective drug for treatment, but not all patients respond and side effects are not negligible. Little is known on the mechanisms responsible for RH. We aimed to identify metabolic alterations in urine. In addition, a potential capacity of metabolites to predict response to spironolactone was investigated. Urine was collected from 29 patients with RH and from a group of 13 subjects with pseudo-RH. For patients, samples were collected before and after spironolactone administration and were classified in responders (n=19) and nonresponders (n=10). Nuclear magnetic resonance was applied to identify altered metabolites and pathways. Metabolites were confirmed by liquid chromatography-mass spectrometry. Citric acid cycle was the pathway most significantly altered (P<0.0001). Metabolic concentrations were quantified and ranged from ng/mL malate to μg/mL citrate. Citrate and oxaloacetate increased in RH versus pseudoresistant. Together with α-ketoglutarate and malate, they were able to discriminate between responders and nonresponders, being the 4 metabolites increased in nonresponders. Combined as a prediction panel, they showed receiver operating characteristiccurve with area under the curve of 0.96. We show that citric acid cycle and deregulation of reactive oxygen species homeostasis control continue its activation after hypertension was developed. A metabolic panel showing alteration before spironolactone treatment and predicting future response of patients is shown. These molecular indicators will contribute optimizing the rate of control of RH patients with spironolactone.

Elevation of fumarase attenuates hypertension and can result from a nonsynonymous sequence variation or increased expression depending on rat strain

The activity of fumarase, an enzyme in the tricarboxylic acid cycle, is lower in Dahl salt-sensitive SS rats compared with SS.13^BN rats. SS.13^BN rats have a Brown Norway (BN) allele of fumarase and exhibit attenuated hypertension. The SS allele of fumarase differs from the BN allele by a K481E sequence variation. It remains unknown whether higher fumarase activities can attenuate hypertension and whether the mechanism is relevant without the K481E variation. We developed SS-TgFh1 transgenic rats overexpressing fumarase on the background of the SS rat. Hypertension was attenuated in SS-TgFh1 rats. Mean arterial pressure in SS-TgFh1 rats was 20 mmHg lower than transgene-negative SS littermates after 12 days on a 4% NaCl diet. Fumarase overexpression decreased H₂O₂, while fumarase knockdown increased H₂O₂ Ectopically expressed BN form of fumarase had higher specific activity than the SS form. However, sequencing of more than a dozen rat strains indicated most rat strains including salt-insensitive Sprague-Dawley (SD) rats had the SS allele of fumarase. Despite that, total fumarase enzyme activity in the renal medulla was still higher in SD rats than in SS rats, which was associated with higher expression of fumarase in SD. H₂O₂ can suppress the expression of fumarase. Renal medullary interstitial administration of fumarase siRNA in SD rats resulted in higher blood pressure on the high-salt diet. These findings indicate elevation of total fumarase activity attenuates the development of hypertension and can result from a nonsynonymous sequence variation in some rat strains and higher expression in other rat strains.

Hypertensive patients exhibit an altered metabolism. A specific metabolite signature in urine is able to predict albuminuria progression

Hypertension (HTN) is increasing in prevalence, and albuminuria is a strong indicator of cardiovascular risk and renal damage progression. Despite blood pressure control with chronic treatment, a relevant subgroup of patients develop albuminuria. However, the biological factors responsible for albuminuria development and progression are underexplored. We aimed to identify key metabolic targets and biological pathways involved in the negative progression of cardiovascular and renal damage in hypertensives undergoing chronic treatment. A series of 1533 patients were followed for 5 years to investigate the evolution of albuminuria. Patients were classified as: (1) patients with persistent normoalbuminuria; (2) patients developing de novo albuminuria; and (3) patients with maintained albuminuria. At the end of follow-up, urine from 30 nonhypertensive subjects (control group) and a representative cohort of 118 patients was collected for metabolomic analysis. Metabolic patterns of interest were identified in a first discovery phase by nuclear magnetic resonance and further confirmed by liquid chromatography-mass spectrometry. Metabolites corresponding to HTN or albuminuria were measured in a prospective study carried out in 35 individuals still in normoalbuminuria, to evaluate their potential as predictors of albuminuria development. Nine metabolites were significantly altered, linking β-alanine metabolism, arginine and proline metabolism, and tricarboxylic acid cycle. The prospective study revealed a panel composed of guanidinoacetate, glutamate, and pantothenate, which was able to predict development of albuminuria. These metabolic signatures open new possibilities in hypertensive therapy and cardiovascular risk control, providing prompt and more efficient intervention, particularly in patients with worse cardiovascular prognosis.

Reconstruction and analysis of correlation networks based on GC-MS metabolomics data for young hypertensive men

The awareness, treatment, and control rates of hypertension for young adults are much lower than average. It is urgently needed to explore the variances of metabolic profiles for early diagnosis and treatment of hypertension. In current study, we applied a GC-MS based metabolomics platform coupled with a network approach to analyze plasma samples from young hypertensive men and age-matched healthy controls. Our findings confirmed distinct metabolic footprints of young hypertensive men. The significantly altered metabolites between two groups were enriched for the biological module of amino acids biosynthesis. The correlations of GC-MS metabolomics data were then visualized as networks based on Pearson correlation coefficient (threshold=0.6). The plasma metabolites identified by GC-MS and the significantly altered metabolites (P<0.05) between patients and controls were respectively included as nodes of a network. Statistical and topological characteristics of the networks were studied in detail. A few amino acids, glycine, lysine, and cystine, were screened as hub metabolites with higher values of degree (k), and also obtained highest scores of three centrality indices. The short average path lengths and high clustering coefficients of the networks revealed a small-world property, indicating that variances of these amino acids have a major impact on the metabolic change in young hypertensive men. These results suggested that disorders of amino acid metabolism might play an important role in predisposing young men to developing hypertension. The combination of metabolomics and network methods would provide another perspective on expounding the molecular mechanism underlying complex diseases.

Analysis of metabolites in plasma reveals distinct metabolic features between Dahl salt-sensitive rats and consomic SS.13(BN) rats

Salt-sensitive hypertension is a major risk factor for cardiovascular disorders. Our previous proteomic study revealed substantial differences in several proteins between Dahl salt-sensitive (SS) rats and salt-insensitive consomic SS.13(BN) rats. Subsequent experiments indicated a role of fumarase insufficiency in the development of hypertension in SS rats. In the present study, a global metabolic profiling study was performed using gas chromatography/mass spectrometry (GC/MS) in plasma of SS rats (n=9) and SS.13(BN) rats (n=8) on 0.4% NaCl diet, designed to gain further insights into the relationship between alterations in cellular intermediary metabolism and predisposition to hypertension. Principal component analysis of the data sets revealed a clear clustering and separation of metabolic profiles between SS rats and SS.13(BN) rats. 23 differential metabolites were identified (P<0.05). Higher levels of five TCA cycle metabolites, fumarate, cis-aconitate, isocitrate, citrate and succinate, were observed in SS rats. Pyruvate, which connects TCA cycle and glycolysis, was also increased in SS rats. Moreover, lower activity levels of fumarase, aconitase, α-ketoglutarate dehydrogenase and succinyl-CoA synthetase were detected in the heart, liver or skeletal muscles of SS rats. The distinct metabolic features in SS and SS.13(BN) rats indicate abnormalities of TCA cycle in SS rats, which may play a role in predisposing SS rats to developing salt-sensitive hypertension.

Genomic and metabolomic profile associated to microalbuminuria

To identify factors related with the risk to develop microalbuminuria using combined genomic and metabolomic values from a general population study. One thousand five hundred and two subjects, Caucasian, more than 18 years, representative of the general population, were included. Blood pressure measurement and albumin/creatinine ratio were measured in a urine sample. Using SNPlex, 1251 SNPs potentially associated to urinary albumin excretion (UAE) were analyzed. Serum metabolomic profile was assessed by 1H NMR spectra using a Brucker Advance DRX 600 spectrometer. From the total population, 1217 (mean age 54 ± 19, 50.6% men, ACR>30 mg/g in 81 subjects) with high genotyping call rate were analysed. A characteristic metabolomic profile, which included products from mitochondrial and extra mitochondrial metabolism as well as branched amino acids and their derivative signals, were observed in microalbuminuric as compare to normoalbuminuric subjects. The comparison of the metabolomic profile between subjects with different UAE status for each of the genotypes associated to microalbuminuria revealed two SNPs, the rs10492025_TT of RPH3A gene and the rs4359_CC of ACE gene, with minimal or no statistically significant differences. Subjects with and without microalbuminuria, who shared the same genotype and metabolomic profile, differed in age. Microalbuminurics with the CC genotype of the rs4359 polymorphism and with the TT genotype of the rs10492025 polymorphism were seven years older and seventeen years younger, respectively as compared to the whole microalbuminuric subjects. With the same metabolomic environment, characteristic of subjects with microalbuminuria, the TT genotype of the rs10492025 polymorphism seems to increase and the CC genotype of the rs4359 polymorphism seems to reduce risk to develop microalbuminuria.

Characterization of biological pathways associated with a 1.37 Mbp genomic region protective of hypertension in Dahl S rats

The goal of the present study was to narrow a region of chromosome 13 to only several genes and then apply unbiased statistical approaches to identify molecular networks and biological pathways relevant to blood-pressure salt sensitivity in Dahl salt-sensitive (SS) rats. The analysis of 13 overlapping subcongenic strains identified a 1.37 Mbp region on chromosome 13 that influenced the mean arterial blood pressure by at least 25 mmHg in SS rats fed a high-salt diet. DNA sequencing and analysis filled genomic gaps and provided identification of five genes in this region, Rfwd2, Fam5b, Astn1, Pappa2, and Tnr. A cross-platform normalization of transcriptome data sets obtained from our previously published Affymetrix GeneChip dataset and newly acquired RNA-seq data from renal outer medullary tissue provided 90 observations for each gene. Two Bayesian methods were used to analyze the data: 1) a linear model analysis to assess 243 biological pathways for their likelihood to discriminate blood pressure levels across experimental groups and 2) a Bayesian graphical modeling of pathways to discover genes with potential relationships to the candidate genes in this region. As none of these five genes are known to be involved in hypertension, this unbiased approach has provided useful clues to be experimentally explored. Of these five genes, Rfwd2, the gene most strongly expressed in the renal outer medulla, was notably associated with pathways that can affect blood pressure via renal transcellular Na(+) and K(+) electrochemical gradients and tubular Na(+) transport, mitochondrial TCA cycle and cell energetics, and circadian rhythms.

Ultrastructure of mitochondria and the endoplasmic reticulum in renal tubules of Dahl salt-sensitive rats

Metabolic and functional abnormalities in the kidney precede or coincide with the initiation of overt hypertension in the Dahl salt-sensitive (SS) rat. However, renal histological injury in SS rats is mild before the development of overt hypertension. We performed electron microscopy analysis in 7-wk-old SS rats and salt-insensitive consomic SS.13(BN) rats and Sprague-Dawley (SD) rats fed a 4% NaCl diet for 7 days. Long mitochondria (>2 μm) accounted for a significantly smaller fraction of mitochondria in medullary thick ascending limbs in SS rats (4% ± 1%) than in SS.13(BN) rats (8% ± 1%, P < 0.05 vs. SS rats) and SD rats (9% ± 1%, P < 0.01 vs. SS rats), consistent with previous findings of mitochondrial functional insufficiency in the medulla of SS rats. Long mitochondria in proximal tubules, however, were more abundant in SS rats than in SS.13(BN) and SD rats. The width of the endoplasmic reticulum, an index of endoplasmic reticulum stress, was significantly greater in medullary thick ascending limbs of SS rats (107 ± 1 nm) than in SS.13(BN) rats (95 ± 2 nm, P < 0.001 vs. SS rats) and SD rats (74 ± 3 nm, P < 0.01 vs. SS or SS.13(BN) rats). The tubules examined were indistinguishable between rat strains under light microscopy. These data indicate that ultrastructural abnormalities occur in the medullary thick ascending limbs of SS rats before the development of histological injury in renal tubules, providing a potential structural basis contributing to the subsequent development of overt hypertension.

Base-resolution maps of 5-methylcytosine and 5-hydroxymethylcytosine in Dahl S rats: effect of salt and genomic sequence

Analysis of 5-hydroxymethylcytosine (5hmC) at single-base resolution has been largely limited to studies of stem cells or developmental stages. Given the potential importance of epigenetic events in hypertension, we have analyzed 5hmC and 5-methylcytosine (5mC) at single-base resolution in the renal outer medulla of the Dahl salt-sensitive rat and examined the effect of disease-relevant genetic or environmental alterations on 5hmC and 5mC patterns. Of CpG sites that fell within CpG islands, 11% and 1% contained significant 5mC and 5hmC, respectively. 5mC levels were substantially higher for genes with lower mRNA abundance and showed a prominent nadir around the transcription start site. In contrast, 5hmC levels were higher in genes with higher expression. Substitution of a 12.9-Mbp region of chromosome 13, which attenuates the hypertensive and renal injury phenotypes in salt-sensitive rats, or exposure to a high-salt diet, which accelerates the disease phenotypes, was associated with differential 5mC or 5hmC in several hundred CpG islands. Nearly 80% of the CpG islands that were differentially methylated in response to salt and associated with differential mRNA abundance were intragenic CpG islands. The substituted genomic segment had significant cis effects on mRNA abundance but not on DNA methylation. The study established base-resolution maps of 5mC and 5hmC in an in vivo model of disease and revealed several characteristics of 5mC and 5hmC important for understanding the role of epigenetic modifications in the regulation of organ systems function and complex diseases.

Increased proliferative cells in the medullary thick ascending limb of the loop of Henle in the Dahl salt-sensitive rat

Studies of transcriptome profiles have provided new insights into mechanisms underlying the development of hypertension. Cell type heterogeneity in tissue samples, however, has been a significant hindrance in these studies. We performed a transcriptome analysis in medullary thick ascending limbs of the loop of Henle isolated from Dahl salt-sensitive rats. Genes differentially expressed between Dahl salt-sensitive rats and salt-insensitive consomic SS.13(BN) rats on either 0.4% or 7 days of 8.0% NaCl diet (n=4) were highly enriched for genes located on chromosome 13, the chromosome substituted in the SS.13(BN) rat. A pathway involving cell proliferation and cell cycle regulation was identified as one of the most highly ranked pathways based on differentially expressed genes and by a Bayesian model analysis. Immunofluorescent analysis indicated that just 1 week of a high-salt diet resulted in a severalfold increase in proliferative medullary thick ascending limb cells in both rat strains, and that Dahl salt-sensitive rats exhibited a significantly greater proportion of medullary thick ascending limb cells in a proliferative state than in SS.13(BN) rats (15.0±1.4% versus 10.1±0.6%; n=7-9; P<0.05). The total number of cells per medullary thick ascending limb section analyzed was not different between the 2 strains. The study revealed alterations in regulatory pathways in Dahl salt-sensitive rats in tissues highly enriched for a single cell type, leading to the unexpected finding of a greater increase in the number of proliferative medullary thick ascending limb cells in Dahl salt-sensitive rats on a high-salt diet.

Mitochondrial proteomic analysis reveals deficiencies in oxygen utilization in medullary thick ascending limb of Henle in the Dahl salt-sensitive rat

The renal medullary thick ascending limb (mTAL) of the Dahl salt-sensitive (SS) rat is the site of enhanced NaCl reabsorption and excess superoxide production. In the present studies we isolated mitochondria from mTAL of SS and salt-resistant control strain SS.13(BN) rats on 0.4 and 8% salt diet for 7 days and performed a proteomic analysis. Purity of mTAL and mitochondria isolations exceeded 93.6 and 55%, respectively. Using LC/MS spectral analysis techniques we identified 96 mitochondrial proteins in four biological mTAL mitochondria samples, run in duplicate, as defined by proteins with a false discovery rate <5% and scan count ≥2. Seven of these 96 proteins, including IDH2, ACADM, SCOT, Hsp60, ATPA, EFTu, and VDAC2 were differentially expressed between the two rat strains. Oxygen consumption and high-resolution respirometry analyses showed that mTAL cells and the mitochondria in the outer medulla of SS rats fed high-salt diet exhibited lower rates of oxygen utilization compared with those from SS.13(BN) rats. These studies advance the conventional proteomic paradigm of focusing exclusively upon whole tissue homogenates to a focus upon a single cell type and specific subcellular organelle. The results reveal the importance of a largely unexplored role for deficiencies of mTAL mitochondrial metabolism and oxygen utilization in salt-induced hypertension and renal medullary oxidative stress.