Mineralocorticoid receptors/epithelial Na+ channels in the choroid plexus are involved in hypertensive mechanisms in stroke-prone spontaneously hypertensive rats

Esaxerenone: blood pressure reduction and cardiorenal protection without reflex sympathetic activation in salt-loaded stroke-prone spontaneously hypertensive rats

Mineralocorticoid receptor (MR) is involved in the mechanisms of blood pressure elevation, organ fibrosis, and inflammation. MR antagonists have been used in patients with hypertension, heart failure, or chronic kidney disease. Esaxerenone, a recently approved MR blocker with a nonsteroidal structure, has demonstrated a strong blood pressure-lowering effect. However, blood pressure reduction may lead to sympathetic activation through the baroreflex. The effect of esaxerenone on the sympathetic nervous system remains unclear. We investigated the effect of esaxerenone on organ damage and the sympathetic nervous system in salt-loaded stroke-prone spontaneously hypertensive rats (SHRSP), a well-established model of essential hypertension with sympathoexcitation and organ damage. Three-week administration of esaxerenone or hydralazine successfully attenuated the blood pressure elevation. Both esaxerenone and hydralazine comparably suppressed left ventricular hypertrophy and urinary albumin excretion. However, renal fibrosis and glomerular sclerosis were suppressed by esaxerenone but not hydralazine. Furthermore, plasma norepinephrine level, a parameter of systemic sympathetic activity, was significantly increased by hydralazine but not by esaxerenone. Consistent with these findings, the activity of the control centers of sympathetic nervous system, the parvocellular region of the paraventricular nucleus in the hypothalamus and the rostral ventrolateral medulla, was enhanced by hydralazine but remained unaffected by esaxerenone. These results suggest that esaxerenone effectively lowers blood pressure without inducing reflex sympathetic nervous system activation. Moreover, the organ-protective effects of esaxerenone appear to be partially independent of its blood pressure-lowering effect. In conclusion, esaxerenone demonstrates a blood pressure-lowering effect without concurrent sympathetic activation and exerts organ-protective effects in salt-loaded SHRSP. Esaxerenone has antihypertensive and cardiorenal protective effects without reflex sympathetic activation in salt-loaded stroke-prone spontaneously hypertensive rats.

Cycle-dependent sex differences in expression of membrane proteins involved in cerebrospinal fluid secretion at rat choroid plexus

BACKGROUND

Female sex is a known risk factor of brain disorders with raised intracranial pressure (ICP) and sex hormones have been suggested to alter cerebrospinal fluid (CSF) dynamics, thus impairing ICP regulation in CSF disorders such as idiopathic intracranial hypertension (IIH). The choroid plexus (CP) is the tissue producing CSF and it has been hypothesized that altered hormonal composition could affect the activity of transporters involved in CSF secretion, thus affecting ICP. Therefore, we aimed to investigate if expression of various transporters involved in CSF secretion at CP were different between males and females and between females in different estrous cycle states. Steroid levels in serum was also investigated.

METHODS

Female and male rats were used to determine sex-differences in the genes encoding for the transporters Aqp1 and 4, NKCC1, NBCe2, NCBE; carbonic anhydrase enzymes II and III (CA), subunits of the Na+/K+-ATPase including Atp1a1, Atp1b1 and Fxyd1 at CP. The estrous cycle stage metestrus (MET) and estrous (ES) were determined before euthanasia. Serum and CP were collected and subjected to RT-qPCR analysis and western blots. Serum was used to measure steroid levels using liquid chromatography tandem mass spectrometry (LC-MS/MS).

RESULTS

Significant differences in gene expression and steroid levels between males and ES females were found, while no differences were found between male and MET females. During ES, expression of Aqp1 was lower (p < 0.01) and NKCC1 was higher in females compared to males. CAII was lower while CAIII was higher in ES females (p < 0.0001). Gene expression of Atp1a1 was lower in ES compared to male (p = 0.0008). Several of these choroidal genes were also significantly different in MET compared to females in ES. Differences in gene expression during the estrus cycle were correlated to serum level of steroid hormones. Protein expression of AQP1 (p = 0.008) and CAII (p = 0.035) was reduced in ES females compared to males.

CONCLUSIONS

This study demonstrates for the first time that expression at CP is sex-dependent and markedly affected by the estrous cycle in female rats. Further, expression was related to hormone levels in serum. This opens a completely new avenue for steroid regulation of the expression of CSF transporters and the close link to the understanding of CSF disorders such as IIH.

Effects of a high-salt diet on MAP and expression levels of renal ENaCs and aquaporins in SHR

An increase in blood pressure by a high-salt (HS) diet may change the expression levels of renal epithelial sodium channels (ENaCs) and aquaporins (AQPs). Spontaneously hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats were exposed to HS and regular-salt (RS) diets for 6 weeks. Mean arterial pressure (MAP) and plasma atrial natriuretic peptide (ANP), angiotensin II (Ang II), aldosterone, and arginine vasopressin (AVP) levels were determined. Expression of mRNA levels of ENaCs and AQPs were quantified by real-time PCR. The MAP was higher in SHRs on the HS diet. Plasma Ang II and aldosterone levels were low while plasma ANP level was high in both strains of rats. Renal expression of mRNA levels of α-, β-, and γ-ENaCs was lowered in SHRs on the HS diet. Meanwhile, renal AQP1, AQP2, and AQP7 mRNA expression levels were lowered in both strains of rats on the HS diet. Suppression of mRNA expression levels of ENaC and AQP subunits suggests that the high-salt-induced increase in the MAP of SHR may not be solely due to renal sodium and water retention.

Mineralocorticoid receptor blockade improved gut microbiota dysbiosis by reducing gut sympathetic tone in spontaneously hypertensive rats

Microbiota has a crucial role in the host blood pressure (BP) regulation. The present study analyzes whether the mineralocorticoid receptor antagonist spironolactone ameliorates the dysbiosic state in a genetic model of neurogenic hypertension. Twenty-week-old male Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were randomly allocated into three groups: untreated WKY, untreated SHR, and SHR treated with spironolactone for 5 weeks. Spironolactone restored the Firmicutes/Bacteroidetes proportion, and acetate-producing bacteria populations to WKY levels. Spironolactone reduced the percentage of intestinal aerobic bacteria. The amelioration of gut dysbiosis was linked to a reduction in the gut pathology, an enhanced colonic integrity, a reduced gut permeability and an attenuated sympathetic drive in the gut. Spironolactone was unable to reduce neuroinflammation and oxidative stress in the paraventricular nuclei in the hypothalamus. Spironolactone reduced the higher Th17 cells proportion in mesenteric lymph nodes and Th17 infiltration in aorta, improved aortic endothelial function and reduced systolic BP. This study demonstrates for the first time that spironolactone reduces gut dysbiosis in SHR. This effect could be related to its capability to improve gut integrity and pathology due to reduced sympathetic drive in the gut.

Choroid plexus epithelium and its role in neurological diseases

Choroid plexus epithelial cells can secrete cerebrospinal fluid into the ventricles, serving as the major structural basis of the selective barrier between the neurological system and blood in the brain. In fact, choroid plexus epithelial cells release the majority of cerebrospinal fluid, which is connected with particular ion channels in choroid plexus epithelial cells. Choroid plexus epithelial cells also produce and secrete a number of essential growth factors and peptides that help the injured cerebrovascular system heal. The pathophysiology of major neurodegenerative disorders like Alzheimer's disease, Parkinson's disease, as well as minor brain damage diseases like hydrocephalus and stroke is still unknown. Few studies have previously connected choroid plexus epithelial cells to the etiology of these serious brain disorders. Therefore, in the hopes of discovering novel treatment options for linked conditions, this review extensively analyzes the association between choroid plexus epithelial cells and the etiology of neurological diseases such as Alzheimer's disease and hydrocephalus. Finally, we review CPE based immunotherapy, choroid plexus cauterization, choroid plexus transplantation, and gene therapy.

Aldosterone alleviates lipopolysaccharide-induced acute lung injury by regulating epithelial sodium channel through PI3K/Akt/SGK1 signaling pathway

Reduced alveolar fluid clearance (AFC) is a major pathological feature of acute lung injury (ALI). Epithelial sodium channel (ENaC) plays a key role in regulating the transport of Na⁺ and clearing alveolar edema fluid effectively. ENaC has been reported to be regulated by aldosterone in the distal collecting tube of the kidney. We hypothesized whether aldosterone regulated ENaC in alveolar epithelium and correspondingly played a role in ALI. In this study we found that the expression of aldosterone synthesis encoding gene, CYP11B2, and ENaC were decreased in the lung tissue of LPS-induced ALI mice. Furthermore, aldosterone alleviated ALI by increasing the expression of ENaC-α and relieving pulmonary edema. Besides, we found that aldosterone upregulated ENaC-α through PI3K/Akt/SGK1 pathway. In conclusion, our study demonstrated that aldosterone attenuated pulmonary edema by upregulating ENaC-α through the PI3K/Akt/SGK1 pathway in LPS-induced ALI, indicating that aldosterone might be a promising adjuvant drug for ALI treatment.

Epithelial Sodium Channel and Salt-Sensitive Hypertension

The development of high blood pressure is influenced by genetic and environmental factors, with high salt intake being a known environmental contributor. Humans display a spectrum of sodium-sensitivity, with some individuals displaying a significant blood pressure rise in response to increased sodium intake while others experience almost no change. These differences are, in part, attributable to genetic variation in pathways involved in sodium handling and excretion. ENaC (epithelial sodium channel) is one of the key transporters responsible for the reabsorption of sodium in the distal nephron. This channel has an important role in the regulation of extracellular fluid volume and consequently blood pressure. Herein, we review the role of ENaC in the development of salt-sensitive hypertension, and present mechanistic insights into the regulation of ENaC activity and how it may accelerate sodium-induced damage and dysfunction. We discuss the traditional role of ENaC in renal sodium reabsorption and review work addressing ENaC expression and function in the brain, vasculature, and immune cells, and how this has expanded the implications for its role in the initiation and progression of salt-sensitive hypertension.

Sympathetic Activation in Hypertension: Importance of the Central Nervous System

The sympathetic nervous system plays a critical role in the pathogenesis of hypertension. The central nervous system (CNS) organizes the sympathetic outflow and various inputs from the periphery. The brain renin-angiotensin system has been studied in various regions involved in controlling sympathetic outflow. Recent progress in cardiovascular research, particularly in vascular biology and neuroscience, as well as in traditional physiological approaches, has advanced the field of the neural control of hypertension in which the CNS plays a vital role. Cardiovascular research relating to hypertension has focused on the roles of nitric oxide, oxidative stress, inflammation, and immunity, and the network among various organs, including the heart, kidney, spleen, gut, and vasculature. The CNS mechanisms are similarly networked with these factors and are widely studied in neuroscience. In this review, I describe the development of the conceptual flow of this network in the field of hypertension on the basis of several important original research articles and discuss potential future breakthroughs leading to clinical precision medicine.

Review of the health benefits of habitual consumption of miso soup: focus on the effects on sympathetic nerve activity, blood pressure, and heart rate

High salt intake increases blood pressure, and dietary salt intake has been clearly demonstrated to be associated with hypertension incidence. Japanese people consume higher amounts of salt than Westerners. It has been reported that miso soup was one of the major sources of daily salt intake in Japanese people. Adding salt is indispensable to make miso, and therefore, in some cases, refraining from miso soup is recommended to reduce dietary salt intake. However, recent studies using salt-sensitive hypertensive models have revealed that miso lessens the effects of salt on blood pressure. In other word, the intake of miso dose not increase the blood pressure compared to the equivalent intake of salt. In addition, many clinical observational studies have demonstrated the absence of a relationship between the frequency of miso soup intake and blood pressure levels or hypertension incidence. The mechanism of this phenomenon seen in the subjects with miso soup intake has not been fully elucidated yet. However, in basic studies, it was found that the ingredients of miso attenuate sympathetic nerve activity, resulting in lowered blood pressure and heart rate. Therefore, this review focused on the differences between the effects of miso intake and those of the equivalent salt intake on sympathetic nerve activity, blood pressure, and heart rate.

High salt diet elevates the mean arterial pressure of SLC14α1 gene depletion mice

AIMS

Urea transporter B (UTB) is encoded by the SLC14α1 gene, and exerts its activity in the choroid plexus (CP) by regulating [Na+] in the cerebrospinal fluid (CSF) and maintaining normal blood pressure in mice fed on high salt diet. The aim of this study is to investigate the effect of high salt diet on the mean arterial pressure (MAP) in SLC14α1 depletion mice and its possible molecular mechanism.

MAIN METHODS

Adult male mice were divided into four groups: 1) UTB+/+(wild type) mice + normal salt diet (0.3% NaCl, NS); 2) UTB+/+ mice + high salt diet (8% NaCl, HS); 3) UTB-/- (SLC14α1 knockout) mice + NS; 4) UTB-/- mice + HS, each group consisted of 6 mice. The MAP of mice was measured by non-invasive detection method after HS diet for 4 weeks, followed by euthanization for brain and blood collection.

KEY FINDINGS

HS significantly elevated the MAP and CSF [Na+] in UTB-/- mice in comparison with wild type mice; however, NS didn't alter the MAP and CSF [Na+] in either wild type mice or UTB-/- mice. HS also induced the expression of ENaC-α and α1-Na+-K+-ATPase in UTB-/- mice as confirmed by RT-PCR and Western blot.

SIGNIFICANCE

These results suggest that the depletion of SLC14α1 gene in mice may contribute to the HS-induced abnormality of sodium transportation in the CSF, and lead to the elevation of MAP, which eventually promote the development of salt-sensitive hypertension.

Advances in the neurophysiology of magnocellular neuroendocrine cells

Hypothalamic magnocellular neuroendocrine cells have unique electrical properties and a remarkable capacity for morphological and synaptic plasticity. Their large somatic size, their relatively uniform and dense clustering in the supraoptic and paraventricular nuclei, and their large axon terminals in the neurohypophysis make them an attractive target for direct electrophysiological interrogation. Here, we provide a brief review of significant recent findings in the neuroplasticity and neurophysiological properties of these neurones that were presented at the symposium "Electrophysiology of Magnocellular Neurons" during the 13th World Congress on Neurohypophysial Hormones in Ein Gedi, Israel in April 2019. Magnocellular vasopressin (VP) neurones respond directly to hypertonic stimulation with membrane depolarisation, which is triggered by cell shrinkage-induced opening of an N-terminal-truncated variant of transient receptor potential vanilloid type-1 (TRPV1) channels. New findings indicate that this mechanotransduction depends on actin and microtubule cytoskeletal networks, and that direct coupling of the TRPV1 channels to microtubules is responsible for mechanical gating of the channels. Vasopressin neurones also respond to osmostimulation by activation of epithelial Na⁺ channels (ENaC). It was shown recently that changes in ENaC activity modulate magnocellular neurone basal firing by generating tonic changes in membrane potential. Both oxytocin and VP neurones also undergo robust excitatory synapse plasticity during chronic osmotic stimulation. Recent findings indicate that new glutamate synapses induced during chronic salt loading express highly labile Ca²⁺ -permeable GluA1 receptors requiring continuous dendritic protein synthesis for synapse maintenance. Finally, recordings from the uniquely tractable neurohypophysial terminals recently revealed an unexpected property of activity-dependent neuropeptide release. A significant fraction of the voltage-dependent neurohypophysial neurosecretion was found to be independent of Ca²⁺ influx through voltage-gated Ca²⁺ channels. Together, these findings provide a snapshot of significant new advances in the electrophysiological signalling mechanisms and neuroplasticity of the hypothalamic-neurohypophysial system, a system that continues to make important contributions to the field of neurophysiology.

Association of SGK1 Polymorphisms With Susceptibility to Coronary Heart Disease in Chinese Han Patients With Comorbid Depression

There is a strong link between heart disease and depression, both of which are closely related to lifetime stress exposure. Serum/glucocorticoid-regulated kinase 1 (SGK1) is a stress-responsive gene with a pivotal role in both the heart and brain. To determine the role of SGK1 polymorphisms (rs2758151, rs1743963, rs9493857, rs1763509, rs9376026, and rs9389154) in susceptibility to comorbid coronary heart disease (CHD) and depression, we conducted a hospital-based case–control study involving 257 CHD cases (including 69 cases with depression and 188 cases without depression) and 107 controls in a Chinese Han population. Six single-nucleotide polymorphisms (SNPs) in the SGK1 gene were successfully genotyped by polymerase chain reaction–ligase detection reaction (PCR-LDR) assay. Our results showed no significant differences in SGK1 genetic polymorphisms between CHD patients and controls, whereas significant associations were observed between SGK1 SNPs (rs1743963 and rs1763509) and the development of depression in CHD patients (P = 0.018 by genotype, P = 0.032 by allele; P = 0.017 by genotype, P = 0.003 by allele, respectively). However, none of these associations remained significant after Bonferroni correction (P = 0.054 for rs1743963; P = 0.051 for rs1763509). Interestingly, both the GG genotype of SGK1 rs1743963 and AA genotype of SGK1 rs1763509 were associated with a higher risk of depression in CHD patients; for rs1763509, the Patient Health Questionnaire-9 (PHQ-9) scores in the carriers of the risk genotype for comorbid depression, AA, were significantly higher than in GG and AG carriers (P = 0.008). Notably, haplotype analysis indicated that haplotype GGA significantly increased the risk of depression in CHD patients (P = 0.011, odds ratio (OR) = 1.717, 95% confidence interval (CI) = 1.132–2.605), whereas haplotype AAG may be a protective factor for CHD patients with comorbid depression (P = 0.038, OR = 0.546, 95% CI = 0.307–0.972). It should be noted that only the significance of haplotype GGA survived after Bonferroni adjustment (P = 0.044) and that no significant differences were found for other SGK1 SNPs (rs2758151, rs9493857, rs9376026, and rs9389154) between CHD patients with and without depression. These findings, for the first time, elucidate the important role of SGK1 variants in the comorbidity of CHD and depression.

Brain perivascular macrophages contribute to the development of hypertension in stroke-prone spontaneously hypertensive rats via sympathetic activation

Hypertension is associated with systemic inflammation. The activation of the sympathetic nervous system is critically involved in the pathogenesis of hypertension. Brain perivascular macrophages (PVMs) can be affected by circulating inflammatory cytokines, and the contribution of brain PVMs to sympathoexcitation has been demonstrated in a heart failure model. We thus investigated whether brain PVMs contribute to the development of hypertension through sympathoexcitation. Stroke-prone spontaneously hypertensive rats (SHRSP) developed hypertension over an 8-week period from 4 to 12 weeks of age. The number of brain PVMs and plasma interleukin-1β levels significantly increased at the ages of 8 and 12 weeks in SHRSP compared with normotensive Wistar-Kyoto rats (WKY). To determine the contribution of brain PVMs to blood pressure elevation, we intracerebroventricularly injected liposome-encapsulated clodronate, which eliminates macrophages by inducing apoptosis, into 8-week-old rats; we then assessed its effects in 10-week-old rats. Clodronate treatment attenuated the increase in mean blood pressure in SHRSP but not in WKY. Clodronate treatment reduced the depressor effect of hexamethonium, an index of sympathetic activity; it also reduced neuronal activity in sympathetic regulatory nuclei such as the hypothalamic paraventricular nucleus and rostral ventrolateral medulla and reduced the expression of cyclooxygenase-2 and prostaglandin E2, a downstream pathway in activated macrophages, in SHRSP but not in WKY. Furthermore, clodronate treatment attenuated the increase in blood pressure and renal sympathetic nerve activity in response to an acute intravenous injection of interleukin-1β in WKY. In conclusion, brain PVMs contribute to the development of hypertension via sympathetic activation. PVMs may be activated by increased levels of circulating interleukin-1β.

Aldosterone-induced protein kinase signalling and the control of electrolyte balance

Aldosterone acts through the mineralocorticoid receptor (MR) to modulate gene expression in target tissues. In the kidney, the principal action of aldosterone is to promote sodium conservation in the distal nephron and so indirectly enhance water conservation under conditions of hypotension. Over the last twenty years the rapid activation of protein kinase signalling cascades by aldosterone has been described in various tissues. This review describes the integration of rapid protein kinase D signalling responses with the non-genomic actions of aldosterone and transcriptional effects of MR activation.

Two Liters a Day Keep the Doctor Away? Considerations on the Pathophysiology of Suboptimal Fluid Intake in the Common Population

Suboptimal fluid intake may require enhanced release of antidiuretic hormone (ADH) or vasopressin for the maintenance of adequate hydration. Enhanced copeptin levels (reflecting enhanced vasopressin levels) in 25% of the common population are associated with enhanced risk of metabolic syndrome with abdominal obesity, type 2 diabetes, hypertension, coronary artery disease, heart failure, vascular dementia, cognitive impairment, microalbuminuria, chronic kidney disease, inflammatory bowel disease, cancer, and premature mortality. Vasopressin stimulates the release of glucocorticoids which in turn up-regulate the serum- and glucocorticoid-inducible kinase 1 (SGK1). Moreover, dehydration upregulates the transcription factor NFAT5, which in turn stimulates SGK1 expression. SGK1 is activated by insulin, growth factors and oxidative stress via phosphatidylinositide-3-kinase, 3-phosphoinositide-dependent kinase PDK1 and mTOR. SGK1 is a powerful stimulator of Na+/K+-ATPase, carriers (e.g. the Na+,K+,2Cl- cotransporter NKCC, the NaCl cotransporter NCC, the Na+/H+ exchanger NHE3, and the Na+ coupled glucose transporter SGLT1), and ion channels (e.g. the epithelial Na+ channel ENaC, the Ca2+ release activated Ca2+ channel Orai1 with its stimulator STIM1, and diverse K+ channels). SGK1 further participates in the regulation of the transcription factors nuclear factor kappa-B NFκB, p53, cAMP responsive element binding protein (CREB), activator protein-1, and forkhead transcription factor FKHR-L1 (FOXO3a). Enhanced SGK1 activity fosters the development of hypertension, obesity, diabetes, thrombosis, stroke, inflammation including inflammatory bowel disease and autoimmune disease, cardiac fibrosis, proteinuria, renal failure as well as tumor growth. The present brief review makes the case that suboptimal fluid intake in the common population may enhance vasopressin and glucocorticoid levels thus up-regulating SGK1 expression and favouring the development of SGK1 related pathologies.

Mineralocorticoid receptor associates with pro-inflammatory bias in the hippocampus of spontaneously hypertensive rats

Damage observed in the hippocampus of the adult spontaneously hypertensive rat (SHR) resembles the neuropathology of mineralocorticoid-induced hypertension, supporting a similar endocrine dysfunction in both entities. In the present study, we tested the hypothesis that increased expression of the hippocampal mineralocorticoid receptor (MR) in SHR animals is associated with a prevalent expression of pro-inflammatory over anti-inflammatory factors. Accordingly, in the hippocampus, we measured mRNA expression and immunoreactivity of the MR and glucocorticoid receptor (GR) using a quantitative polymerase chain reaction and histochemistry. We also measured serum-glucocorticoid-activated kinase 1 (Sgk1 mRNA), the number and phenotype of Iba1+ microglia, as well as mRNA expression levels of the pro-inflammatory factors cyclo-oxygenase 2 (Cox2), Nlrp3 inflammasome and tumour necrosis factor α (Tnfα). Expression of anti-inflammatory transforming growth factor (Tgf)β mRNA and the NADPH-diaphorase activity of nitric oxide synthase (NOS) were also determined. The results showed that, in the hippocampus of SHR rats, expression of MR and the number of immunoreactive MR/GR co-expressing cells were increased compared to Wistar-Kyoto control animals. Expression of Sgk1, Cox2, Nlrp3 and the number of ramified glia cells positive for Iba1+ were also increased, whereas Tgfβ mRNA expression and the NADPH-diaphorase activity of NOS were decreased. We propose that, in the SHR hippocampus, increased MR expression causes a bias towards a pro-inflammatory phenotype characteristic for hypertensive encephalopathy.

Transport across the choroid plexus epithelium

The choroid plexus epithelium is a secretory epithelium par excellence. However, this is perhaps not the most prominent reason for the massive interest in this modest-sized tissue residing inside the brain ventricles. Most likely, the dominant reason for extensive studies of the choroid plexus is the identification of this epithelium as the source of the majority of intraventricular cerebrospinal fluid. This finding has direct relevance for studies of diseases and conditions with deranged central fluid volume or ionic balance. While the concept is supported by the vast majority of the literature, the implication of the choroid plexus in secretion of the cerebrospinal fluid was recently challenged once again. Three newer and promising areas of current choroid plexus-related investigations are as follows: 1) the choroid plexus epithelium as the source of mediators necessary for central nervous system development, 2) the choroid plexus as a route for microorganisms and immune cells into the central nervous system, and 3) the choroid plexus as a potential route for drug delivery into the central nervous system, bypassing the blood-brain barrier. Thus, the purpose of this review is to highlight current active areas of research in the choroid plexus physiology and a few matters of continuous controversy.

The Role of CNS in the Effects of Salt on Blood Pressure

Sympathetic nerve activity is involved in the pathogenesis of salt-sensitive hypertension. The central nervous system, which regulates sympathetic nerve activity and blood pressure, plays a pivotal role. Central sympathoexcitation is deeply involved in the pathogenesis of salt-sensitive hypertension, although the precise mechanisms have not been fully elucidated because of their complexity. The role of brain oxidative stress in sympathoexcitation has been suggested in some types of hypertensive animal models. We have shown that increased brain oxidative stress may elevate arterial pressure through central sympathoexcitation in salt-sensitive hypertension. Several other factors such as mineralocorticoid receptors, aldosterone, corticosterone, epithelial sodium channels, and angiotensin II also play important roles in central sympathetic activation, some of which can be associated with brain oxidative stress. Furthermore, brain paraventricular nucleus Gαi2-protein-mediated transduction has been recently reported as a candidate for the molecular mechanism countering the development of salt-sensitive hypertension.

Lnc-SGK1 induced by Helicobacter pylori infection and highsalt diet promote Th2 and Th17 differentiation in human gastric cancer by SGK1/Jun B signaling

Serum and glucocorticoid-inducible kinase (SGK) 1can be triggered in several malignancies. Most research on SGK1has focused on its role in cancer cells, and we sought to investigate its potential upstream non-coding RNA nominated as Lnc-SGK1, and their expression and diagnostic value in T cells in human gastric cancer (GC). Excessive expression of Lnc-SGK1 and SGK1 were observed in T cell either within the tumor or peripheral T cells, and furthermore associated with Helicobacter pylori infection and high-salt diet (HSD). Within T cells, Helicobacter pylori (Hp) infection and high-salt dietcan up-regulated SGK1 expression and in turn enhance expression of Lnc-SGK1 through JunB activation. And expression of Lnc-SGK1 can further enhance transcription of SGK1 through cis regulatory mode. Lnc-SGK1 can induce Th2 and Th17 and reduce Th1 differentiation via SGK1/JunB signaling. Serum Lnc-SGK1 expression in combination with H. pylori infection and/or HSD in T cells was associated with poor prognosis of GC patients, and could be an ideal diagnostic index in human GC.

Conditional Deletion of Hsd11b2 in the Brain Causes Salt Appetite and Hypertension

Supplemental Digital Content is available in the text.

Background—

The hypertensive syndrome of Apparent Mineralocorticoid Excess is caused by loss-of-function mutations in the gene encoding 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2), allowing inappropriate activation of the mineralocorticoid receptor by endogenous glucocorticoid. Hypertension is attributed to sodium retention in the distal nephron, but 11βHSD2 is also expressed in the brain. However, the central contribution to Apparent Mineralocorticoid Excess and other hypertensive states is often overlooked and is unresolved. We therefore used a Cre-Lox strategy to generate 11βHSD2 brain-specific knockout (Hsd11b2.BKO) mice, measuring blood pressure and salt appetite in adults.

Methods and Results—

Basal blood pressure, electrolytes, and circulating corticosteroids were unaffected in Hsd11b2.BKO mice. When offered saline to drink, Hsd11b2.BKO mice consumed 3 times more sodium than controls and became hypertensive. Salt appetite was inhibited by spironolactone. Control mice fed the same daily sodium intake remained normotensive, showing the intrinsic salt resistance of the background strain. Dexamethasone suppressed endogenous glucocorticoid and abolished the salt-induced blood pressure differential between genotypes. Salt sensitivity in Hsd11b2.BKO mice was not caused by impaired renal sodium excretion or volume expansion; pressor responses to phenylephrine were enhanced and baroreflexes impaired in these animals.

Conclusions—

Reduced 11βHSD2 activity in the brain does not intrinsically cause hypertension, but it promotes a hunger for salt and a transition from salt resistance to salt sensitivity. Our data suggest that 11βHSD2-positive neurons integrate salt appetite and the blood pressure response to dietary sodium through a mineralocorticoid receptor–dependent pathway. Therefore, central mineralocorticoid receptor antagonism could increase compliance to low-sodium regimens and help blood pressure management in cardiovascular disease.

Aldosterone and Salt Loading Independently Exacerbate the Exercise Pressor Reflex in Rats

The sympathetic and pressor responses to exercise are exaggerated in hypertension. Evidence suggests that an overactive exercise pressor reflex (EPR) contributes to this abnormal responsiveness. The mechanisms underlying this EPR overactivity are poorly understood. An increasing body of evidence suggests that aldosterone and excessive salt intake play a role in regulating resting sympathetic activity and blood pressure in hypertension. Therefore, each is a good candidate for the generation of EPR dysfunction in this disease. The purpose of this study was to examine whether excessive salt intake and chronic administration of aldosterone potentiate EPR function. Changes in mean arterial pressure and renal sympathetic nerve activity induced by EPR stimulation were examined in vehicle and aldosterone-treated (4 weeks via osmotic mini-pump) Sprague-Dawley rats given either water or saline (elevated salt load) to drink. When compared with vehicle/water-treated rats, stimulation of the EPR by muscle contraction evoked significantly greater increases in mean arterial pressure in vehicle/saline, aldosterone/water, and aldosterone/saline-treated animals (14±3, 29±3, 37±6, and 44±7 mm Hg/kg, respectively; P<0.01). A similar renal sympathetic nerve activity response profile was likewise produced (39±11%, 87±15%, 110±20%, and 151±25%/kg, respectively; P<0.01). The pressor and sympathetic responses to the individual activation of the mechanically and chemically sensitive components of the EPR were also augmented by both saline and aldosterone. These data provide the first direct evidence that both aldosterone and high salt intake elicit EPR overactivity. As such, each represents a potential mechanism by which sympathetic activity and blood pressure are augmented during exercise in hypertension.

Regulation of the epithelial Na+ channel by the mTORC2/SGK1 pathway

The epithelial Na(+) channel (ENaC) is decisive for sodium reabsorption by the aldosterone-sensitive distal nephron (ASDN) of the kidney. ENaC is regulated by the serum- and glucocorticoid-inducible kinase 1 (SGK1), a kinase genomically upregulated by several hormones including glucocorticoids and mineralocorticoids. SGK1 is activated by the serine/threonine kinase mammalian target of rapamycin (mTOR) isoform mTORC2. SGK1 knockout (sgk1(-/-) mice) impairs renal Na(+) retention during salt depletion. The mTOR catalytic site inhibitor, PP242, but not mTORC1 inhibitor rapamycin, inhibits ENaC, decreases Na(+) flux in isolated perfused tubules and induces natriuresis in wild-type mice. PP242 does not lead to further impairment of Na(+) reabsorption in sgk1(-/-) mice. The mTORC2/SGK1 sensitive renal Na(+) retention leads to extracellular volume expansion with increase of blood pressure. A SGK1 gene variant (prevalence ∼ 3-5% in Caucasians, ∼ 10% in Africans) predisposes to hypertension, stroke, obesity and type 2 diabetes. Future studies will be required to define the role of mTORC2 in the regulation of further SGK1 sensitive transport proteins, such as further ion channels, carriers and the Na(+)/K(+)-ATPase. Moreover, studies are required disclosing the impact of mTORC2 on SGK1 sensitive disorders, such as hypertension, obesity, diabetes, thrombosis, stroke, inflammation, autoimmune disease, fibrosis and tumour growth.

High salt-diet reduces SLC14A1 gene expression in the choroid plexus of Dahl salt sensitive rats

Elevated Na(+) concentration ([Na(+)]) in the cerebrospinal fluid (CSF) contributes to the development of salt-sensitive hypertension. CSF is formed by the choroid plexus (CP) in cerebral ventricles, and [Na(+)] in CSF is controlled by transporters in CP. Here, we examined the effect of high salt diet on the expression of urea transporters (UTs) in the CP of Dahl S vs Dahl R rats using real time PCR. High salt intake (8%, for 2 weeks) did not alter the mRNA levels of UT-A (encoded by SLC14A2 gene) in the CP of either Dahl S or Dahl R rats. In contrast, the mRNA levels of UT-B (encoded by SLC14A1 gene) were significantly reduced in the CP of Dahl S rats on high salt diet as compared with Dahl R rats or Dahl S rats on normal salt diet. Reduced UT-B expression was associated with increased [Na(+)] in the CSF and elevated mean arterial pressure (MAP) in Dahl S rats treated with high salt diet, as measured by radiotelemetry. High salt diet-induced reduction in UT-B protein expression in the CP of Dahl S rats was confirmed by Western blot. Immunohistochemistry using UT-B specific antibodies demonstrated that UT-B protein was expressed on the epithelial cells in the CP. These data indicate that high salt diet induces elevations in CSF [Na(+)] and in MAP, both of which are associated with reduced UT-B expression in the CP of Dahl S rats, as compared with Dahl R rats. The results suggest that altered UT-B expression in the CP may contribute to an imbalance of water and electrolytes in the CSF of Dahl S rats on high salt diet, thereby leading to alterations in MAP.

Blockade of ENaCs by amiloride induces c-Fos activation of the area postrema

Epithelial sodium channels (ENaCs) are strongly expressed in the circumventricular organs (CVOs), and these structures may play an important role in sensing plasma sodium levels. Here, the potent ENaC blocker amiloride was injected intraperitoneally in rats and 2h later, the c-Fos activation pattern in the CVOs was studied. Amiloride elicited dose-related activation in the area postrema (AP) but only ~10% of the rats showed c-Fos activity in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO). Tyrosine hydroxylase-immunoreactive (catecholamine) AP neurons were activated, but tryptophan hydroxylase-immunoreactive (serotonin) neurons were unaffected. The AP projects to FoxP2-expressing neurons in the dorsolateral pons which include the pre-locus coeruleus nucleus and external lateral part of the parabrachial nucleus; both cell groups were c-Fos activated following systemic injections of amiloride. In contrast, another AP projection target--the aldosterone-sensitive neurons of the nucleus tractus solitarius which express the enzyme 11-β-hydroxysteriod dehydrogenase type 2 (HSD2) were not activated. As shown here, plasma concentrations of amiloride used in these experiments were near or below the IC50 level for ENaCs. Amiloride did not induce changes in blood pressure, heart rate, or regional vascular resistance, so sensory feedback from the cardiovascular system was probably not a causal factor for the c-Fos activity seen in the CVOs. In summary, amiloride may have a dual effect on sodium homeostasis causing a loss of sodium via the kidney and inhibiting sodium appetite by activating the central satiety pathway arising from the AP.

Mineralocorticoid-induced sodium appetite and renal salt retention: evidence for common signaling and effector mechanisms

An increase in renal sodium chloride (salt) retention and an increase in sodium appetite are the body's responses to salt restriction or depletion in order to restore salt balance. Renal salt retention and increased sodium appetite can also be maladaptive and sustain the pathophysiology in conditions like salt-sensitive hypertension and chronic heart failure. Here we review the central role of the mineralocorticoid aldosterone in both the increase in renal salt reabsorption and sodium appetite. We discuss the working hypothesis that aldosterone activates similar signaling and effector mechanisms in the kidney and brain, including the mineralocorticoid receptor, the serum- and glucocorticoid-induced kinase SGK1, the ubiquitin ligase NEDD4-2, and the epithelial sodium channel ENaC. The latter also mediates the gustatory salt sensing in the tongue, which is required for the manifestation of increased salt intake. Effects of aldosterone on both the brain and kidney synergize with the effects of angiotensin II. Thus, mineralocorticoids appear to induce similar molecular pathways in the kidney, brain, and possibly tongue, which could provide opportunities for more effective therapeutic interventions. Inhibition of renal salt reabsorption is compensated by stimulation of salt appetite and vice versa; targeting both mechanisms should be more effective. Inhibiting the arousal to consume salty food may improve a patient's compliance to reducing salt intake. While a better understanding of the molecular mechanisms is needed and will provide new therapeutic options, current pharmacological interventions that target both salt retention and sodium appetite include mineralocorticoid receptor antagonists and potentially inhibitors of angiotensin II and ENaC.

Steroids and the blood-brain barrier: therapeutic implications

Steroids have a wide spectrum of impact, serving as fundamental regulators of nearly every physiological process within the human body. Therapeutic applications of steroids are equally broad, with a diverse range of medications and targets. Within the central nervous system (CNS), steroids influence development, memory, behavior, and disease outcomes. Moreover, steroids are well recognized as to their impact on the vascular endothelium. The blood-brain barrier (BBB) at the level of the brain microvascular endothelium serves as the principle interface between the peripheral circulation and the brain. Steroids have been identified to impact several critical properties of the BBB, including cellular efflux mechanisms, nutrient uptake, and tight junction integrity. Such actions not only influence brain homeostasis but also the delivery of CNS-targeted therapeutics. A greater understanding of the respective steroid-BBB interactions may shed further light on the differential treatment outcomes observed across CNS pathologies. In this chapter, we examine the current therapeutic implications of steroids respective to BBB structure and function, with emphasis on glucocorticoids and estrogens.

Salt, the renin-angiotensin-aldosterone system and resistant hypertension

High salt intake is a risk for developing resistant hypertension, and even under triple therapy with diuretics, an angiotensin-converting enzyme inhibitor/angiotensin receptor blocker and a calcium channel blocker, the volume is occasionally not controlled. In such cases, a mineralocorticoid receptor (MR) antagonist additively lowers the circulating blood volume and blood pressure despite the lower circulating aldosterone level. This mechanism may be explained by the increase in the number of MR under some conditions or the activation of these receptors independent of aldosterone. Future diagnostic tools to evaluate receptor activity may be valuable for the proper diagnosis and choice of therapy. Additionally, basic research has suggested that oxidative stress and the renin-angiotensin-aldosterone system in the brain represent new targets for the treatment of resistant hypertension.

The role of CNS in salt-sensitive hypertension

The role of sympathetic nerve activity in hypertension is currently receiving increased attention, because catheter-based renal denervation was recently shown to reduce blood pressure safely in patients with treatment-resistant hypertension. The central nervous system, which regulates sympathetic nerve activity and blood pressure, is pivotal. Central sympathoexcitation has been shown to be deeply involved in the pathogenesis of salt-sensitive hypertension, although its precise mechanisms have not yet been fully elucidated due to their complexity. Recently, a role for brain oxidative stress in sympathoexcitation has been suggested in some hypertensive animal models. We have demonstrated that increased brain oxidative stress may elevate arterial pressure through central sympathoexcitation in salt-sensitive hypertension. Several factors other than oxidative stress have also been shown to play important roles in central sympathetic activation. In the future, strategies may be developed to elicit a sympathetic inhibition by modulating these factors to prevent and manage salt-sensitive hypertension.

Blood pressure regulation via the epithelial sodium channel: from gene to kidney and beyond

The epithelial sodium channel (ENaC) has long been recognized as playing a vital role in blood pressure (BP) regulation due to its involvement in fluid balance. The genes encoding the three ENaC subunits are likewise important contributors to hypertension, both in rare monogenic diseases and in the general population. The unusually high numbers of genetic variants associated with complex traits, including BP, that are located in non-coding areas suggest an involvement of these variants in regulatory functions. This may involve differential regulation of expression in different tissues. Emerging evidence indicates that the ENaC plays an important role in BP determination not only via its actions in the kidney, but also in other tissues commonly involved in BP regulation. The ENaC in the central nervous system is proposed to regulate BP via sympathetic nervous system activity. Recent evidence suggests that the ENaC contributes to vascular function and the myogenic response. Additional roles potentially include initiation of the baroreceptor reflex via ENaC in the baroreceptors and driving high salt intake with a 'taste for salt' via ENaC in the tongue. The present review describes the involvement of the ENaC in the determination of BP at a genetic and physiological level, detailing recent evidence for its role in the kidney and in other pertinent tissues.

Regulation of transport across cell membranes by the serum- and glucocorticoid-inducible kinase SGK1

The serum- and glucocorticoid-inducible kinase 1 (SGK1) is genomically upregulated by cell stress including energy depletion and hyperosmotic shock as well as a variety of hormones including glucocorticoids, mineralocorticoids and TGFβ. SGK1 is activated by insulin, growth factors and oxidative stress via phosphatidylinositide-3-kinase, 3-phosphoinositide-dependent kinase PDK1 and mTOR. SGK1 is a powerful stimulator of Na(+)/K(+)-ATPase, carriers (e.g., NCC, NKCC, NHE1, NHE3, SGLT1, several amino acid transporters) and ion channels (e.g., ENaC, SCN5A, TRPV4-6, ORAI1/STIM1, ROMK, KCNE1/KCNQ1, GluR6, CFTR). Mechanisms employed by SGK1 in transport regulation include direct phosphorylation of target transport proteins, phosphorylation and thus activation of other transport regulating kinases, stabilization of membrane proteins by phosphorylation and thus inactivation of the ubiquitin ligase NEDD4-2, as well as stimulation of transport protein expression by upregulation transcription factors (e.g., nuclear factor kappa-B [NFκB]) and by fostering of protein translation. SGK1 sensitivity of pump, carrier and channel activities participate in the regulation of epithelial transport, cardiac and neuronal excitability, degranulation, platelet function, migration, cell proliferation and apoptosis. SGK1-sensitive functions do not require the presence of SGK1 but are markedly upregulated by SGK1. Accordingly, the phenotype of SGK1 knockout mice is mild. The mice are, however, less sensitive to excessive activation of transport by glucocorticoids, mineralocorticoids, insulin and inflammation. Moreover, excessive SGK1 activity contributes to the pathophysiology of hypertension, obesity, diabetes, thrombosis, stroke, inflammation, autoimmune disease, fibrosis and tumor growth.