Norepinephrine stimulates the epithelial Na+ channel in cortical collecting duct cells via α2-adrenoceptors

Dynamic changes in renal sodium handling during sympathetic stimulation in healthy human males

The temporal response of changes in renal sodium reabsorption during increased renal sympathetic nerve activity has not been investigated. Central hypovolemia by application of lower-body negative-pressure (LBNP) elicits baroreceptor mediated sympathetic reflexes to maintain arterial blood pressure. We hypothesized, that during 90 min LBNP, the renal sodium retention would increase rapidly, remain increased during intervention, and return to baseline immediately after end of intervention.

METHODS

30 young, healthy, sodium loaded, non-obese males were exposed to -15 mmHg LBNP, -30 mmHg LBNP, -15 mmHg LBNP + renin blockade or time-control (0 mmHg LBNP) for 90 min. Urine was collected every 15 min during 90 min of intervention and 60 min of recovery to identify a possible relation between time of intervention and renal response.

RESULTS

All intervention groups exhibited a comparable reduction in distal sodium excretion at the end of the intervention (P = 0.46 between groups; -15 mmHg: -3.1 ± 0.9 %, -30 mmHg: -2.9 ± 0.6 %, -15 mmHg + aslikiren: -1.8 ± 0.6 %). -15 mmHg+Aliskiren resulted in a slower onset, but all groups exhibited a continued reduction in sodium excretion after 1 h of recovery despite return to baseline of renin, aldosterone, diuresis and cardiovascular parameters.

CONCLUSION

Sympathetic stimulation for 90 min via LBNP at -30 mmHg LBNP compared to -15 mmHg did not result in a greater response in fractional Na+ excretion, suggesting that additional baroreceptor unloading did not cause further increases in renal sodium reabsorption. Changes in distal Na+ excretion were linear with respect to time (dose) of intervention, but seem to exhibit a saturation-like effect at a level around 4 %. The lack of recovery after 1 h is also a new finding that warrants further investigation.

Mapping the Transcriptome Underpinning Acute Corticosteroid Action within the Cortical Collecting Duct

Key Points

We report the transcriptomes associated with acute corticosteroid regulation of ENaC activity in polarized mCCDcl1 collecting duct cells. Nine genes were regulated by aldosterone (ALDO), 0 with corticosterone alone, and 151 with corticosterone when 11βHSD2 activity was inhibited. We validated three novel ALDO-induced genes, Rasd1 , Sult1d1 , and Gm43305 , in primary cells isolated from a novel principal cell reporter mouse.

Background

Corticosteroids regulate distal nephron and collecting duct (CD) Na+ reabsorption, contributing to fluid-volume and blood pressure homeostasis. The transcriptional landscape underpinning the acute stimulation of the epithelial sodium channel (ENaC) by physiological concentrations of corticosteroids remains unclear.

Methods

Transcriptomic profiles underlying corticosteroid-stimulated ENaC activity in polarized mCCDcl1 cells were generated by coupling electrophysiological measurements of amiloride-sensitive currents with RNAseq. Generation of a principal cell-specific reporter mouse line, mT/mG -Aqp2Cre, enabled isolation of primary CD principal cells by FACS, and ENaC activity was measured in cultured primary cells after acute application of corticosteroids. Expression of target genes was assessed by qRT-PCR in cultured cells or freshly isolated cells after the acute elevation of steroid hormones in mT/mG -Aqp2Cre mice.

Results

Physiological relevance of the mCCDcl1 model was confirmed with aldosterone (ALDO)-specific stimulation of SGK1 and ENaC activity. Corticosterone (CORT) only modulated these responses at supraphysiological concentrations or when 11βHSD2 was inhibited. When 11βHSD2 protection was intact, CORT caused no significant change in transcripts. We identified a small number of ALDO-induced transcripts associated with stimulated ENaC activity in mCCDcl1 cells and a much larger number with CORT in the absence of 11βHSD2 activity. Principal cells isolated from mT/mG -Aqp2Cre mice were validated and assessment of identified ALDO-induced genes revealed that Sgk1 , Zbtbt16 , Sult1d1 , Rasd1 , and Gm43305 are acutely upregulated by corticosteroids both in vitro and in vivo .

Conclusions

This study reports the transcriptome of mCCDcl1 cells and identifies a small number of ALDO-induced genes associated with acute stimulation of ENaC, including three previously undescribed genes.

Two adjacent phosphorylation sites in the C-terminus of the channel's α-subunit have opposing effects on epithelial sodium channel (ENaC) activity

How phosphorylation of the epithelial sodium channel (ENaC) contributes to its regulation is incompletely understood. Previously, we demonstrated that in outside-out patches ENaC activation by serum- and glucocorticoid-inducible kinase isoform 1 (SGK1) was abolished by mutating a serine residue in a putative SGK1 consensus motif RXRXX(S/T) in the channel’s α-subunit (S621 in rat). Interestingly, this serine residue is followed by a highly conserved proline residue rather than by a hydrophobic amino acid thought to be required for a functional SGK1 consensus motif according to invitro data. This suggests that this serine residue is a potential phosphorylation site for the dual-specificity tyrosine phosphorylated and regulated kinase 2 (DYRK2), a prototypical proline-directed kinase. Its phosphorylation may prime a highly conserved preceding serine residue (S617 in rat) to be phosphorylated by glycogen synthase kinase 3 β (GSK3β). Therefore, we investigated the effect of DYRK2 on ENaC activity in outside-out patches of Xenopus laevis oocytes heterologously expressing rat ENaC. DYRK2 included in the pipette solution significantly increased ENaC activity. In contrast, GSK3β had an inhibitory effect. Replacing S621 in αENaC with alanine (S621A) abolished the effects of both kinases. A S617A mutation reduced the inhibitory effect of GKS3β but did not prevent ENaC activation by DYRK2. Our findings suggest that phosphorylation of S621 activates ENaC and primes S617 for subsequent phosphorylation by GSK3β resulting in channel inhibition. In proof-of-concept experiments, we demonstrated that DYRK2 can also stimulate ENaC currents in microdissected mouse distal nephron, whereas GSK3β inhibits the currents.

Prostaglandin E2 stimulates the epithelial sodium channel (ENaC) in cultured mouse cortical collecting duct cells in an autocrine manner

Prostaglandin E₂ (PGE₂) is the most abundant prostanoid in the kidney, affecting a wide range of renal functions. Conflicting data have been reported regarding the effects of PGE₂ on tubular water and ion transport. The amiloride-sensitive epithelial sodium channel (ENaC) is rate limiting for transepithelial sodium transport in the aldosterone-sensitive distal nephron. The aim of the present study was to explore a potential role of PGE₂ in regulating ENaC in cortical collecting duct (CCD) cells. Short-circuit current (I_SC) measurements were performed using the murine mCCD_cl1 cell line known to express characteristic properties of CCD principal cells and to be responsive to physiological concentrations of aldosterone and vasopressin. PGE₂ stimulated amiloride-sensitive I_SC via basolateral prostaglandin E receptors type 4 (EP₄) with an EC₅₀ of ∼7.1 nM. The rapid stimulatory effect of PGE₂ on I_SC resembled that of vasopressin. A maximum response was reached within minutes, coinciding with an increased abundance of β-ENaC at the apical plasma membrane and elevated cytosolic cAMP levels. The effects of PGE₂ and vasopressin were nonadditive, indicating similar signaling cascades. Exposing mCCD_cl1 cells to aldosterone caused a much slower (∼2 h) increase of the amiloride-sensitive I_SC. Interestingly, the rapid effect of PGE₂ was preserved even after aldosterone stimulation. Furthermore, application of arachidonic acid also increased the amiloride-sensitive I_SC involving basolateral EP₄ receptors. Exposure to arachidonic acid resulted in elevated PGE₂ in the basolateral medium in a cyclooxygenase 1 (COX-1)–dependent manner. These data suggest that in the cortical collecting duct, locally produced and secreted PGE₂ can stimulate ENaC-mediated transepithelial sodium transport.

Loss of Adam10 Disrupts Ion Transport in Immortalized Kidney Collecting Duct Cells

The kidney cortical collecting duct (CCD) comprises principal cells (PCs), intercalated cells (IC), and the recently discovered intermediate cell type. Kidney pathology in a mouse model of the syndrome of apparent aldosterone excess revealed plasticity of the CCD, with altered PC:intermediate cell:IC ratio. The self-immortalized mouse CCD cell line, mCCDcl1, shows functional characteristics of PCs, but displays a range of cell types, including intermediate cells, making it ideal to study plasticity. We knocked out Adam10, a key component of the Notch pathway, in mCCDcl1 cells, using CRISPR-Cas9 technology, and isolated independent clones, which exhibited severely affected sodium transport capacity and loss of aldosterone response. Single-cell RNA sequencing revealed significantly reduced expression of major PC-specific markers, such as Scnn1g (γ-ENaC) and Hsd11b2 (11βHSD2), but no significant changes in transcription of components of the Notch pathway were observed. Immunostaining in the knockout clone confirmed the decrease in expression of γ-ENaC and importantly, showed an altered, diffuse distribution of PC and IC markers, suggesting altered trafficking in the Adam10 knockout clone as an explanation for the loss of polarization.

Mineralocorticoid receptors in the pathogenesis of insulin resistance and related disorders: from basic studies to clinical disease

Aldosterone is a steroid hormone that regulates blood pressure and cardiovascular function by acting on renal and vascular mineralocorticoid receptors (MRs) to promote sodium retention and modulate endothelial function. Indeed, MRs are expressed in endothelial cells, vascular smooth muscle cells, adipocytes, immune cells, skeletal muscle cells, and cardiomyocytes. Excessive aldosterone and associated MR activation impair insulin secretion, insulin metabolic signaling to promote development of diabetes, and the related cardiometabolic syndrome. These adverse effects of aldosterone are mediated, in part, via increased inflammation, oxidative stress, dyslipidemia, and ectopic fat deposition. Therefore, inhibition of MR activation may have a beneficial effect in prevention of impaired insulin metabolic signaling, type 2 diabetes, and cardiometabolic disorders. This review highlights findings from the recent surge in research regarding MR-related cardiometabolic disorders as well as our contemporary understanding of the detrimental effects of excess MR activation on insulin metabolic signaling.

Role of α2-Adrenoceptors in Hypertension: Focus on Renal Sympathetic Neurotransmitter Release, Inflammation, and Sodium Homeostasis

The kidney is extensively innervated by sympathetic nerves playing an important role in the regulation of blood pressure homeostasis. Sympathetic nerve activity is ultimately controlled by the central nervous system (CNS). Norepinephrine, the main sympathetic neurotransmitter, is released at prejunctional neuroeffector junctions in the kidney and modulates renin release, renal vascular resistance, sodium and water handling, and immune cell response. Under physiological conditions, renal sympathetic nerve activity (RSNA) is modulated by peripheral mechanisms such as the renorenal reflex, a complex interaction between efferent sympathetic nerves, central mechanism, and afferent sensory nerves. RSNA is increased in hypertension and, therefore, critical for the perpetuation of hypertension and the development of hypertensive kidney disease. Renal sympathetic neurotransmission is not only regulated by RSNA but also by prejunctional α2-adrenoceptors. Prejunctional α2-adrenoceptors serve as autoreceptors which, when activated by norepinephrine, inhibit the subsequent release of norepinephrine induced by a sympathetic nerve impulse. Deletion of α2-adrenoceptors aggravates hypertension ultimately by modulating renal pressor response and sodium handling. α2-adrenoceptors are also expressed in the vasculature, renal tubules, and immune cells and exert thereby effects related to vascular tone, sodium excretion, and inflammation. In the present review, we highlight the role of α2-adrenoceptors on renal sympathetic neurotransmission and its impact on hypertension. Moreover, we focus on physiological and pathophysiological functions mediated by non-adrenergic α2-adrenoceptors. In detail, we discuss the effects of sympathetic norepinephrine release and α2-adrenoceptor activation on renal sodium transporters, on renal vascular tone, and on immune cells in the context of hypertension and kidney disease.

Targeting zonulin and intestinal epithelial barrier function to prevent onset of arthritis

Gut microbial dysbiosis is associated with the development of autoimmune disease, but the mechanisms by which microbial dysbiosis affects the transition from asymptomatic autoimmunity to inflammatory disease are incompletely characterized. Here, we identify intestinal barrier integrity as an important checkpoint in translating autoimmunity to inflammation. Zonulin family peptide (zonulin), a potent regulator for intestinal tight junctions, is highly expressed in autoimmune mice and humans and can be used to predict transition from autoimmunity to inflammatory arthritis. Increased serum zonulin levels are accompanied by a leaky intestinal barrier, dysbiosis and inflammation. Restoration of the intestinal barrier in the pre-phase of arthritis using butyrate or a cannabinoid type 1 receptor agonist inhibits the development of arthritis. Moreover, treatment with the zonulin antagonist larazotide acetate, which specifically increases intestinal barrier integrity, effectively reduces arthritis onset. These data identify a preventive approach for the onset of autoimmune disease by specifically targeting impaired intestinal barrier function.

Intestinal dysbiosis is associated with an ever-growing list of autoimmune diseases. Here the authors show that both mice and humans with autoimmune arthritis can have dysbiosis and barrier leakiness prior to major signs of inflammatory arthritis, and treatment of mice with a zonulin antagonist can limit collagen-induced arthritis.

α2A-Adrenoceptors Modulate Renal Sympathetic Neurotransmission and Protect against Hypertensive Kidney Disease

BACKGROUND

Increased nerve activity causes hypertension and kidney disease. Recent studies suggest that renal denervation reduces BP in patients with hypertension. Renal NE release is regulated by prejunctional α2A-adrenoceptors on sympathetic nerves, and α2A-adrenoceptors act as autoreceptors by binding endogenous NE to inhibit its own release. However, the role of α2A-adrenoceptors in the pathogenesis of hypertensive kidney disease is unknown.

METHODS

We investigated effects of α2A-adrenoceptor-regulated renal NE release on the development of angiotensin II-dependent hypertension and kidney disease. In uninephrectomized wild-type and α2A-adrenoceptor-knockout mice, we induced hypertensive kidney disease by infusing AngII for 28 days.

RESULTS

Urinary NE excretion and BP did not differ between normotensive α2A-adrenoceptor-knockout mice and wild-type mice at baseline. However, NE excretion increased during AngII treatment, with the knockout mice displaying NE levels that were significantly higher than those of wild-type mice. Accordingly, the α2A-adrenoceptor-knockout mice exhibited a systolic BP increase, which was about 40 mm Hg higher than that found in wild-type mice, and more extensive kidney damage. In isolated kidneys, AngII-enhanced renal nerve stimulation induced NE release and pressor responses to a greater extent in kidneys from α2A-adrenoceptor-knockout mice. Activation of specific sodium transporters accompanied the exaggerated hypertensive BP response in α2A-adrenoceptor-deficient kidneys. These effects depend on renal nerves, as demonstrated by reduced severity of AngII-mediated hypertension and improved kidney function observed in α2A-adrenoceptor-knockout mice after renal denervation.

CONCLUSIONS

Our findings reveal a protective role of prejunctional inhibitory α2A-adrenoceptors in pathophysiologic conditions with an activated renin-angiotensin system, such as hypertensive kidney disease, and support the concept of sympatholytic therapy as a treatment.

Trichostatin A blocks aldosterone-induced Na+ transport and control of serum- and glucocorticoid-inducible kinase 1 in cortical collecting duct cells

BACKGROUND AND PURPOSE

Aldosterone stimulates epithelial Na⁺ channel (ENaC)-dependent Na⁺ retention in the cortical collecting duct (CCD) of the kidney by activating mineralocorticoid receptors that promote expression of serum and glucocorticoid-inducible kinase 1 (SGK1). This response is critical to BP homeostasis. It has previously been suggested that inhibiting lysine deacetylases (KDACs) can post-transcriptionally disrupt this response by promoting acetylation of the mineralocorticoid receptor. The present study critically evaluates this hypothesis.

EXPERIMENTAL APPROACH

Electrometric and molecular methods were used to define the effects of a pan-KDAC inhibitor, trichostatin A, on the responses to a physiologically relevant concentration of aldosterone (3 nM) in murine mCCD_cl1 cells.

KEY RESULTS

Aldosterone augmented ENaC-induced Na⁺ absorption and increased SGK1 activity and abundance, as expected. In the presence of trichostatin A, these responses were suppressed. Trichostatin A-induced inhibition of KDAC was confirmed by increased acetylation of histone H3, H4, and α-tubulin. Trichostatin A did not block the electrometric response to insulin, a hormone that activates SGK1 independently of increased transcription, indicating that trichostatin A has no direct effect upon the SGK1/ENaC pathway.

CONCLUSIONS AND IMPLICATIONS

Inhibition of lysine de-acetylation suppresses aldosterone-dependent control over the SGK1-ENaC pathway but does not perturb post-transcriptional signalling, providing a physiological basis for the anti-hypertensive action of KDAC inhibition seen in vivo.

Renal denervation improves sodium excretion in rats with chronic heart failure: effects on expression of renal ENaC and AQP2

Previously we have shown that increased expression of renal epithelial sodium channels (ENaC) may contribute to the renal sodium and water retention observed during chronic heart failure (CHF). The goal of this study was to examine whether renal denervation (RDN) changed the expressions of renal sodium transporters ENaC, sodium-hydrogen exchanger-3 proteins (NHE3), and water channel aquaporin 2 (AQP2) in rats with CHF. CHF was produced by left coronary artery ligation in rats. Four weeks after ligation surgery, surgical bilateral RDN was performed. The expression of ENaC, NHE3, and AQP2 in both renal cortex and medulla were measured. As a functional test for ENaC activation, diuretic and natriuretic responses to ENaC inhibitor benzamil were monitored in four groups of rats (Sham, Sham+RDN, CHF, CHF+RDN). Western blot analysis indicated that RDN (1 wk later) significantly reduced protein levels of α-ENaC, β-ENaC, γ-ENaC, and AQP2 in the renal cortex of CHF rats. RDN had no significant effects on the protein expression of kidney NHE3 in both Sham and CHF rats. Immunofluorescence studies of kidney sections confirmed the reduced signaling of ENaC and AQP2 in the CHF+RDN rats compared with the CHF rats. There were increases in diuretic and natriuretic responses to ENaC inhibitor benzamil in rats with CHF. RDN reduced the diuretic and natriuretic responses to benzamil in CHF rats. These findings suggest a critical role for renal nerves in the enhanced expression of ENaC and AQP2 and subsequent pathophysiology of renal sodium and water retention associated with CHF.NEW & NOTEWORTHY This is the first study to show in a comprehensive way that renal denervation initiated after a period of chronic heart failure reduces the expression of epithelial sodium channels and aquaporin 2 leading to reduced epithelial sodium channel function and sodium retention.

mCCDcl1 cells show plasticity consistent with the ability to transition between principal and intercalated cells

The cortical collecting duct of the mammalian kidney plays a critical role in the regulation of body volume, sodium pH, and osmolarity and is composed of two distinct cells types, principal cells and intercalated cells. Each cell type is detectable in the kidney by the localization of specific transport proteins such as aquaporin 2 (Aqp2) and epithelial sodium channel (ENaC) in principal cells and V-ATPase B1 and connexin 30 (Cx30) in intercalated cells. mCCD_cl1 cells have been widely used as a mouse principal cell line on the basis of their physiological characteristics. In this study, the mCCD_cl1 parental cell line and three sublines cloned from isolated single cells (Ed1, Ed2, and Ed3) were grown on filters to assess their transepithelial resistance, transepithelial voltage, equivalent short circuit current and expression of the cell-specific markers Aqp2, ENaC, V-ATPaseB1, and Cx30. The parental mCCD_cl1 cell line presented amiloride-sensitive electrogenic sodium transport indicative of principal cell function; however, immunocytochemistry and RT-PCR showed that some cells expressed the intercalated cell-specific markers V-ATPase B1 and Cx30, including a subset of cells also positive for Aqp2 and ENaC. The three subclonal lines contained cells that were positive for both intercalated and principal cell-specific markers. The vertical transmission of both principal and intercalated cell characteristics via single cell cloning reveals the plasticity of mCCD_cl1 cells and a direct lineage relationship between these two physiologically important cell types and is consistent with mCCD_cl1 cells being precursor cells.

ISN Forefronts Symposium 2015: The Evolution of Hypertension-Old Genes, New Concepts

Hypertension is known as the “silent killer,” driving the global public health burden of cardiovascular and renal disease. Blood pressure homeostasis is intimately associated with sodium balance and the distribution of sodium between fluid compartments and within tissues. On a population level, most societies consume 10 times more salt that the 0.5 g required by physiological need. This high salt intake is strongly linked to hypertension and to the World Health Organization targeting a ∼30% relative reduction in mean population salt intake to arrest the global mortality due to cardiovascular disease. But how does a habitually high-salt diet cause blood pressure to rise? In this focused review, we discuss 2 “evolutionary medicine” concepts, presented at the ISN Forefront Meeting “Immunomodulation of Cardio-renal Function.” We first examine how ancestral variants in genes that conferred a selection advantage during early human development are now maladaptive. We then discuss the conservation of “renal” sodium transport processes across multiple organ systems, including the brain. These systems influence sodium appetite and can exert an often-overlooked effect on long-term blood pressure control.

New insights into sodium transport regulation in the distal nephron: Role of G-protein coupled receptors

The renal handling of Na⁺ balance is a major determinant of the blood pressure (BP) level. The inability of the kidney to excrete the daily load of Na⁺ represents the primary cause of chronic hypertension. Among the different segments that constitute the nephron, those present in the distal part (i.e., the cortical thick ascending limb, the distal convoluted tubule, the connecting and collecting tubules) play a central role in the fine-tuning of renal Na⁺ excretion and are the target of many different regulatory processes that modulate Na⁺ retention more or less efficiently. G-protein coupled receptors (GPCRs) are crucially involved in this regulation and could represent efficient pharmacological targets to control BP levels. In this review, we describe both classical and novel GPCR-dependent regulatory systems that have been shown to modulate renal Na⁺ absorption in the distal nephron. In addition to the multiplicity of the GPCR that regulate Na⁺ excretion, this review also highlights the complexity of these different pathways, and the connections between them.

Harvest and primary culture of the murine aldosterone-sensitive distal nephron

The aldosterone-sensitive distal nephron (ASDN) exhibits axial heterogeneity in structure and function from the distal convoluted tubule to the medullary collecting duct. Ion and water transport is primarily divided between the cortex and medulla of the ASDN, respectively. Transcellular transport in this segment is highly regulated in health and disease and is integrated across different cell types. We currently lack an inexpensive, high-yield, and tractable technique to harvest and culture cells for the study of gene expression and physiological properties of mouse cortical ASDN. To address this need, we harvested tubules bound to Dolichos biflorus agglutinin lectin-coated magnetic beads from the kidney cortex and characterized these cell preparations. We determined that these cells are enriched for markers of distal convoluted tubule, connecting tubule, and cortical collecting duct, including principal and intercalated cells. In primary culture, these cells develop polarized monolayers with high resistance (1,000-1,500 Ω * cm(2)) and maintain expression and activity of key channels. These cells demonstrate an amiloride-sensitive short-circuit current that can be enhanced with aldosterone and maintain measurable potassium and anion secretion. Our method can be easily adopted to study the biology of the ASDN and to investigate phenotypic differences between wild-type and transgenic mouse models.