Kir7.1 knockdown and inhibition alter renal electrolyte handling but not the development of hypertension in Dahl salt-sensitive rats

High potassium supplementation is correlated with a lower risk of the composite of death, major cardiovascular events, and ameliorated blood pressure, but the exact mechanisms have not been established. Inwardly rectifying potassium (K_ir) channels expressed in the basolateral membrane of the distal nephron play an essential role in maintaining electrolyte homeostasis. Mutations in this channel family have been shown to result in strong disturbances in electrolyte homeostasis, among other symptoms. K_ir7.1 is a member of the ATP-regulated subfamily of K_ir channels. However, its role in renal ion transport and its effect on blood pressure has yet to be established. Our results indicate the localization of K_ir7.1 to the basolateral membrane of the aldosterone-sensitive distal nephron cells. To examine the physiological implications of K_ir7.1, we generated a knockout of K_ir7.1 (Kcnj13) in Dahl salt-sensitive (SS) rats and deployed chronic infusion of a specific K_ir7.1 inhibitor, ML418, in wild-type Dahl SS strain. Knockout of Kcnj13^-/- resulted in embryonic lethality. Heterozygous Kcnj13^+/- rats revealed an increase in potassium excretion on a normal salt diet but did not exhibit a difference in blood pressure development or plasma electrolytes after three weeks of a high salt diet. Wild-type SS rats exhibited increased renal K_ir7.1 expression when dietary potassium was increased. Potassium supplementation also demonstrated that Kcnj13^+/- rats excreted more potassium on normal salt. The development of hypertension was not different when challenged with high salt for three weeks, though Kcnj13^+/- rats excrete less sodium. Interestingly, chronic infusion of ML418 significantly increased sodium and chloride excretion after 14 days of high salt but did not alter salt-induced hypertension development. Here we find that reduction of K_ir7.1 function, either through genetic ablation or pharmacological inhibition, can influence renal electrolyte excretion but not to a sufficient degree to impact the development of SS hypertension.

The circadian clock protein PER1 is important in maintaining endothelin axis regulation in Dahl salt-sensitive rats

Endothelin-1 (ET-1) is a peptide hormone that acts on its receptors to regulate sodium handling in the kidney's collecting duct. Dysregulation of the endothelin axis is associated with various diseases, including salt-sensitive hypertension and chronic kidney disease. Previously, our lab has shown that the circadian clock gene PER1 regulates ET-1 levels in mice. However, the regulation of ET-1 by PER1 has never been investigated in rats. Therefore, we used a novel model where knockout of Per1 was performed in Dahl salt-sensitive rat background (SS Per1 -/-) to test a hypothesis that PER1 regulates the ET-1 axis in this model. Here, we show increased renal ET-1 peptide levels and altered endothelin axis gene expression in several tissues, including the kidney, adrenal glands, and liver in SS Per1 -/- compared with control SS rats. Edn1 antisense lncRNA Edn1-AS, which has previously been suggested to be regulated by PER1, was also altered in SS Per1 -/- rats compared with control SS rats. These data further support the hypothesis that PER1 is a negative regulator of Edn1 and is important in the regulation of the endothelin axis in a tissue-specific manner.

Knockout of the Circadian Clock Protein PER1 (Period1) Exacerbates Hypertension and Increases Kidney Injury in Dahl Salt-Sensitive Rats

BACKGROUND

Circadian rhythms play an essential role in physiological function. The molecular clock that underlies circadian physiological function consists of a core group of transcription factors, including the protein PER1 (Period1). Studies in mice show that PER1 plays a role in the regulation of blood pressure and renal sodium handling; however, the results are dependent on the strain being studied. Using male Dahl salt-sensitive (SS) rats with global knockout of PER1 (SSPer1-/-), we aim to test the hypothesis that PER1 plays a key role in the regulation of salt-sensitive blood pressure.

METHODS

The model was generated using CRISPR/Cas9 and was characterized using radiotelemetry and measures of renal function and circadian rhythm.

RESULTS

SSPer1-/- rats had similar mean arterial pressure when fed a normal 0.4% NaCl diet but developed augmented hypertension after three weeks on a high-salt (4% NaCl) diet. Despite being maintained on a normal 12:12 light:dark cycle, SSPer1-/- rats exhibited desynchrony mean arterial pressure rhythms on a high-salt diet, as evidenced by increased variability in the time of peak mean arterial pressure. SSPer1-/- rats excrete less sodium after three weeks on the high-salt diet. Furthermore, SSPer1-/- rats exhibited decreased creatinine clearance, a measurement of renal function, as well as increased signs of kidney tissue damage. SSPer1-/- rats also exhibited higher plasma aldosterone levels.

CONCLUSIONS

Altogether, our findings demonstrate that loss of PER1 in Dahl SS rats causes an array of deleterious effects, including exacerbation of the development of salt-sensitive hypertension and renal damage.

Abstract 029: Loss Of The Circadian Protein Per1 In The Dahl Salt-sensitive Rat Causes Exacerbation Of Diurnal Blood Pressure And Disruption Of Electrolyte Homeostasis

Circadian rhythm is the 24-hr cycle experienced by most organisms which influences many physiological processes. Rhythms in physiological function are governed by a molecular circadian clock. Disruption of the circadian rhythm has been linked to deleterious cardiovascular outcomes, including the exacerbation of hypertension. The circadian clock is maintained by a group of transcription factors, including the protein Period 1 (encoded by PER1 gene), which regulate the expression of many different target genes in a tissue-specific manner. Previous studies in mice have shown that knockout of PER1 affects their ability to maintain proper sodium and blood pressure homeostasis. We hypothesized that the global knockout of PER1 on the Dahl salt-sensitive background (SS Per1-/- ) will affect the development of salt-sensitive blood pressure. Following the high salt diet, SS Per1-/- rats experienced reduced weight gain (344 ± 7 vs 280 ± 9 g, p<0.001), excreted less sodium (17.3 ± 1.1 vs 12.1 ± 1.0 Na + /Cre, p=0.021), and exhibited reduced plasma potassium (3.6 ± 0.1 vs 3.0 ± 0.1 mM, p=0.005) and creatinine clearance (126.0 ± 23.8 vs 64.7 ± 5.9 mL/hr, p=0.008). Additionally, the SS Per1-/- group displayed increased renal tissue damage quantified by cortical fibrosis (0.9 ± 0.4 vs 6.2 ± 1.6 % area, p=0.011) and medullary protein casts (7.4 ± 0.9 vs 24.3 ± 4.0 % area, p=0.003). Examining the diurnal mean arterial pressure revealed no difference between SS and SS Per1-/- during their inactive day/light cycle (121.0 ± 1.8 vs 122.0 ± 1.9 mmHg, p=0.792), or their active night/dark cycle (127.2 ± 2.2 vs 127.6 ± 1.4 mmHg, p=0.883) when on a normal salt diet. However, after three weeks on a high salt 4% NaCl diet, SS Per1-/- animals experienced an exacerbated blood pressure during both the light (167.5 ± 3.7 vs 190.9 ± 7.2 mmHg, p=0.020) and dark (177.7 ± 3.4 vs 195.4 ± 4.5 mmHg, p=0.014) cycles. In the following analysis, we compared tissues, plasma, and electrolytes from SS Per1-/- and SS rats collected at 2 am and 2 pm. The conducted analysis revealed a number of pathways differently regulated during day and night cycles. In summary, our data indicate that global knockout of PER1 exacerbates salt-sensitive blood pressure development, disrupts electrolyte homeostasis, and aggravates renal damage.

Modulation of blood pressure regulatory genes in the Agtrap-Plod1 locus associated with a deletion in Clcn6

The AGTRAP-PLOD1 locus is a conserved gene cluster containing several blood pressure regulatory genes, including CLCN6, MTHFR, NPPA, and NPPB. Previous work revealed that knockout of Clcn6 on the Dahl Salt-Sensitive (SS) rat background (SS-Clcn6) resulted in lower diastolic blood pressure compared to SS-WT rats. Additionally, a recent study found sickle cell anemia patients with mutations in CLCN6 had improved survival and reduced stroke risk. We investigated whether loss of Clcn6 would delay the mortality of Dahl SS rats on an 8% NaCl (HS) diet. No significant difference in survival was found. The ability of Clcn6 to affect mRNA expression of nearby Mthfr, Nppa, and Nppb genes was also tested. On normal salt (0.4% NaCl, NS) diets, renal Mthfr mRNA and protein expression were significantly increased in the SS-Clcn6 rats. MTHFR reduces homocysteine to methionine, but no differences in circulating homocysteine levels were detected. Nppa mRNA levels in cardiac tissue from SS-Clcn6 rat in both normotensive and hypertensive conditions were significantly reduced compared to SS-WT. Nppb mRNA expression in SS-Clcn6 rats on a NS diet was also substantially decreased. Heightened Mthfr expression would be predicted to be protective; however, diminished Nppa and Nppb expression could be deleterious and by preventing or blunting vasodilation, natriuresis, and diuresis that ought to normally occur to offset blood pressure increases. The conserved nature of this genetic locus in humans and rats suggests more studies are warranted to understand how mutations in and around these genes may be influencing the expression of their neighbors.

Lack of xanthine dehydrogenase leads to a remarkable renal decline in a novel hypouricemic rat model

Uric acid (UA) is the final metabolite in purine catabolism in humans. Previous studies have shown that the dysregulation of UA homeostasis is detrimental to cardiovascular and kidney health. The Xdh gene encodes for the Xanthine Oxidoreductase enzyme group, responsible for producing UA. To explore how hypouricemia can lead to kidney damage, we created a rat model with the genetic ablation of the Xdh gene on the Dahl salt-sensitive rat background (SS^Xdh−/−). SS^Xdh−/− rats lacked UA and exhibited impairment in growth and survival. This model showed severe kidney injury with increased interstitial fibrosis, glomerular damage, crystal formation, and an inability to control electrolyte balance. Using a multi-omics approach, we highlighted that lack of Xdh leads to increased oxidative stress, renal cell proliferation, and inflammation. Our data reveal that the absence of Xdh leads to kidney damage and functional decline by the accumulation of purine metabolites in the kidney and increased oxidative stress.

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A novel rat model of hypouricemia was created by the gene ablation of the Xdh gene

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The SS^Xdh-/- rat showed a failure to thrive, kidney injury, and functional decline

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Multi-omics revealed increased inflammation and oxidative stress in SS^Xdh-/- rats

Effects of elevation of ANP and its deficiency on cardiorenal function

Atrial natriuretic peptide (ANP), encoded by Nppa, is a vasodilatory hormone that promotes salt excretion. Genome-wide association studies identified Nppa as a causative factor of blood pressure development, and in humans, ANP levels were suggested as an indicator of salt sensitivity. This study aimed to provide insights into the effects of ANP on cardiorenal function in salt-sensitive hypertension. To address this question, hypertension was induced in SSNPPA-/- (knockout of Nppa in the Dahl Salt-Sensitive (SS) rat background) or SSWT (wild type Dahl SS) rats by a high salt diet challenge (HS, 4% NaCl for 21 days). Chronic infusion of ANP in SSWT rats attenuated the increase in blood pressure and cardiorenal damage. Overall, SSNPPA-/- strain demonstrated higher blood pressure and intensified cardiac fibrosis (with no changes in ejection fraction) compared to SSWT rats. Furthermore, SSNPPA-/- rats exhibited kidney hypertrophy and higher glomerular injury scores, reduced diuresis, and lower sodium and chloride excretion than SSWT when fed a HS diet. Additionally, the activity of epithelial Na+ channel (ENaC) was found to be increased in the collecting ducts of the SSNPPA-/- rats. Taken together, these data show promise for the therapeutic benefits of ANP and ANP-increasing drugs for treating salt-sensitive hypertension.

Crosstalk between ENaC and basolateral Kir 4.1/Kir 5.1 channels in the cortical collecting duct

BACKGROUND AND PURPOSE

Inwardly rectifying K⁺ (K_ir ) channels located on the basolateral membrane of epithelial cells of the distal nephron play a crucial role in K⁺ handling and blood pressure control, making these channels an attractive target for the treatment of hypertension. The purpose of the present study was to determine how the inhibition of basolateral K_ir 4.1/K_ir 5.1 heteromeric K⁺ channel affects epithelial sodium channel (ENaC)-mediated Na⁺ transport in the principal cells of cortical collecting duct (CCD).

EXPERIMENTAL APPROACH

The effect of fluoxetine, amitriptyline, and recently developed K_ir inhibitor, VU0134992, on the activity of K_ir 4.1, K_ir 4.1/K_ir 5.1, and ENaC were tested using electrophysiological approaches in Chinese hamster ovary (CHO) cells transfected with respective channel subunits, cultured polarized epithelial mCCD_cl1 cells, and freshly isolated rat and human CCD tubules. To test the effect of pharmacological K_ir 4.1/K_ir 5.1 inhibition on electrolyte homeostasis in vivo and corresponding changes in distal tubule transport, Dahl salt-sensitive rats were injected with amitriptyline (15 mg kg^-1 day^-1 ) for three days.

KEY RESULTS

We found that inhibition of K_ir 4.1/K_ir 5.1, but not K_ir 4.1 channel, depolarizes cell membrane, induces the elevation of intracellular Ca²⁺ concentration, and suppresses ENaC activity. Furthermore, we demonstrate that amitriptyline administration leads to a significant drop in plasma K⁺ level, triggering sodium excretion and diuresis.

CONCLUSION AND IMPLICATIONS

Present data uncovers a specific role of the K_ir 4.1/K_ir 5.1 channel in the modulation of ENaC activity and emphasizes the potential for using K_ir 4.1/K_ir 5.1 inhibitors to regulate electrolyte homeostasis and blood pressure.

Abstract P242: The Role Of Kappa Opioid Receptors In The Development Of Hypertension And Kidney Injury In Sprague-Dawley Rats

The extensive use of opioid-based pain management strongly correlates with poor cardiovascular and cardiorenal outcomes. Our recent studies suggest that treatment with kappa opioid receptor (KOR) agonist BRL 52537 leads to the progression of chronic kidney disease (CKD) and aggravation of salt-sensitive hypertension. We hypothesize that stimulation of KORs leads to blood pressure elevation, albuminuria, and kidney damage in healthy Sprague-Dawley (SD) rats. To characterize the effect of the KOR agonist BRL 52537 on the development of blood pressure and kidney function in vivo , SD rats were treated with a daily i.v. bolus infusion of BRL 52537 or a corresponding vehicle. To test the contribution of KOR stimulation on calcium homeostasis in podocytes, BRL 52537 was used on freshly isolated glomeruli from SD rats. Single-channel analysis was applied to assess the effect of KORs stimulation on TRPC6 channel activity in the human immortalized podocytes. Chronic treatment with BRL 52537 leads to increased mean arterial pressure (88±1 vs 101±4 mmHg, vehicle vs treated, p<0.05), podocyte basal calcium (90±12 vs 216±16 a.u., vehicle vs treated, p<0.05), and GFB impairment in SD rats which is reflected by a transient increase in albumin excretion (Alb/cre ratio 0.35±0.1 vs 0.72±0.2, vehicle vs treated, p<0.05). Cumulative probability distribution analysis of the glomerular injury score revealed a rightward shift toward a high glomerular injury score in the group treated with BRL 52537 (p<0.05). Angiotensin II level was higher in a BRL-treated group (156±17 vs 232±59 pmol, vehicle vs treated, p=0.065); however, it did not reach a statistical difference. Acute application of BRL 52537 resulted in sustained calcium response (0.23±0.01 a.u., Fluo4/FuraRed, maximum calcium response) in freshly isolated glomeruli from SD rats. Furthermore, patch-clamp experiments in human immortalized podocytes (cell-attached configuration) revealed that BRL 52537 activated TRPC6 channels. Taken together, these data support the hypothesis that administration of opioids in SD rats leads to activation of the KOR/TRPC6 pathway, which in turn led to glomerular filtration barrier impairment, increased glomerular damage, and blood pressure elevation.

Abstract P382: The Role of Atrial Natriuretic Peptide in Cardiac Damage During Salt Sensitive Hypertension

Atrial natriuretic peptide (ANP), encoded by the Nppa gene, is an osmoregulatory hormone that promotes salt excretion. GWAS identified Nppa to be associated with hypertension; some studies suggest varying ANP levels as an indicator of salt-sensitivity. The aim of the current project was to assess the effects of ANP deficiency on cardiac damage and function using a knockout of Nppa in the Dahl Salt-Sensitive (SS) rat background (SS NPPA-/- ). A combination of in vivo techniques with ex vivo and biochemical methods were used to test the role of ANP in the development of SS hypertension in male and female rats. IHC analysis was employed to quantify tissue damage; echocardiography was used to test heart function. SS NPPA-/- rats demonstrated higher blood pressure compared to SS controls when fed a high salt (HS, 4% NaCl) diet: on day 21 of diet MAP was 185.8 ± 9 mmHg in SS NPPA-/- rats compared to 144.6 ± 4 mmHg in SS controls. Heart rate was found to be on average 40 bpm lower in SS NPPA-/- rats until day 8 of the HS challenge, when it rose up to the rate of wild type counterparts and continued to decline at the same pace. SS NPPA-/- rats showed exacerbated kidney damage, reduced diuresis and lower sodium excretion when fed a HS diet. SS NPPA-/- rats exhibited intensified cardiac damage compared to SS controls, as demonstrated by heart to body weight ratio after HS (3.9 ± 0.13 in wild type vs 5.5 ± 0.09 in KO rats), elevated cardiac fibrosis (up to 6% of fibrotic cardiac tissue area in SS NPPA-/- vs less than 3% in wild types), and a significantly increased cardiac vessel media thickness. Fibrosis and heart hypertrophy were attenuated in female rats. Echocardiography revealed that although SS NPPA-/- rats show heart remodeling, ejection fraction is preserved and they do not exhibit heart failure. Chronic i.v. infusion of ANP in SS rats (100 ng/kg/day) attenuated the HS-induced increase in blood pressure and renal damage, and resulted in less cardiac hypertrophy and fibrosis (1% fibrosis in ANP-infused animals vs 3% in vehicle-treated group). Therefore, ANP deficiency aggravates SS hypertension and cardiac damage; further work is needed to reveal if ANP deficiency causes heart failure of a HF/PEF phenotype, and what the interplay between heart and kidney is in this setting.