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Objective Liddle syndrome (LS) is a rare autosomal dominant condition secondary to a gain-of-function mutation affecting the epithelial sodium channels (ENaCs) in the distal nephron. It presents with early-onset hypertension, hypokalemia, and metabolic alkalosis in the face of hyporeninemia and hypoaldosteronism. We report a novel mutation affecting the ENaCs in a normotensive adolescent with LS. Methods We describe a pediatric case of LS with a novel mutation and review the condition's presentation and management. To date, 31 different mutations in the β- or γ-subunit of ENaCs have been reported as associated with LS. Results We describe a 16-year-old girl presenting with muscle cramps with a strong family history of hypertension and hypokalemia. Initial investigations revealed hypokalemia together with hypoaldosteronism and hyporeninemia. Subsequent genetic testing revealed a novel mutation in SCNN1B (deletion: c.1713delC), leading to the premature termination of the sodium channel epithelial 1 subunit-β protein and the LS phenotype. Treatment with triamterene (50 mg, twice daily) and potassium chloride (20 mEq, once daily) normalized the serum potassium and led to resolution of her muscle cramps. Conclusion It is essential to consider investigating the presence of rare genetic syndromes, like LS, when a patient presents with hypokalemia. Further studies are needed to understand the variable presentation of this condition.

Airway and alveolar epithelial cells express a variety of K+ channels that are specifically localized to either the apical or basolateral membrane depending upon their physiological function. These channels play important roles in maintenance of membrane potential, control of cell volume, regulation of cell proliferation, and support of transepithelial electrolyte and nutrient transport. K+ channels present in the apical membrane of alveolar epithelial cells, for example, facilitate K+ secretion, thus contributing to the elevated K+ concentration observed in alveolar fluid. K+ channels localized to the basolateral membrane of airway epithelial cells contribute to transepithelial electrolyte and fluid transport by sustaining the electrical driving force necessary for anion efflux or cation influx and limit fluctuations in intracellular K+ concentration associated with changes in Na+–K+ ATPase activity. Signaling molecules that control the absorptive and secretory functions of lung epithelia have been shown to regulate specific K+ channel subtypes through the mobilization of intracellular second messengers such as Ca2+ and cAMP. In this review, the diversity of K+ channel expression and the functional role of these channels in transepithelial electrolyte and fluid transport in lung epithelia are addressed.

Summary: A controversial hypothesis pertaining to cystic fibrosis (CF) lung disease is that the CF transmembrane conductance regulator (CFTR) channel fails to inhibit the epithelial Na+ channel (ENaC), yielding increased Na+ reabsorption and airway dehydration. We use a non-invasive self-referencing Na+-selective microelectrode technique to measure Na+ transport across individual folds of distal airway surface epithelium preparations from CFTR−/− (CF) and wild-type (WT) swine. We show that, under unstimulated control conditions, WT and CF epithelia exhibit similar, low rates of Na+ transport that are unaffected by the ENaC blocker amiloride. However, in the presence of the cyclic AMP (cAMP)-elevating agents forskolin+IBMX (isobutylmethylxanthine), folds of WT tissues secrete large amounts of Na+, while CFTR−/− tissues absorb small, but potentially important, amounts of Na+. In cAMP-stimulated conditions, amiloride inhibits Na+ absorption in CFTR−/− tissues but does not affect secretion in WT tissues. Our results are consistent with the hypothesis that ENaC-mediated Na+ absorption may contribute to dehydration of CF distal airways.

Mechanosensory transduction (MST) in proprioceptors, and other low threshold mechanosensory nerve terminals (LTMT), has been debated intensely for decades. MST in muscle spindles produces a receptor potential that encodes stimulus speed and duration, is predominantly due to Na+, a little Ca2+, plus some transient, non-mechanically-gated K+ ion fluxes. The abundant, multiple Na+-selective DEG/ENaC channel isoforms present in all LTMTs seemed obvious Na+ sources, perhaps supplemented with Ca2+-selective TRPs, and Ca2+-activated K+ channels. However, genetic deletions of even multiple DEG/ENaC genes produces only mild functional perturbation. Conversely, deleting the more recently discovered Piezo2 mechanosensory protein profoundly impairs LTMT responses, including in muscle spindles. Yet its transient opening, non-Na+-selectivity and its pharmacology do not reflect known receptor potential and response properties. A Ca2+-dependent recycling vesicle pool that we have shown is essential for mechanosensitivity, plus other recent DEG/ENaC discoveries, may reconcile these conflicting observations. We propose the abundance of axolemmal MST complexes, comprising untested DEG/ENaC combinations, is controlled by Piezo2-gated Ca2+ influx that regulates their vesicular insertion and retrieval.

Epithelial sodium channel (ENaC) is a protein that has a central role in the development of hypertension. ENaC is a transmembrane protein that overexpressed in arterial hypertension patients. The expression of ENaC is proportional to sodium intake and clinically related to hypertension. The concentration of ENaC excreted in the urine of patients can be used as a biomarker of hypertension. Herein, we report a gold nanoparticle-bioconjugate based electrochemical immunosensor for the detection of ENaC. The bioconjugate (AuNP/HS-PEG-COOH/Anti-ENaC) was immobilized on the surface of a gold-screen printed carbon electrode treated with cysteamine (SPCE/Au-cys) via covalent bonds between the -NH2 group of the cysteamines and –COOH terminal of the bioconjugates. The interaction of ENaC with anti-ENaC on the bioconjugates was detected by differential pulse voltammetry using a ferricyanide redox system. The assay was optimized using a Box-Behnken experimental design. The result shows that SPCE/Au increased the peak current signal of [Fe(CN)6]4−/3- by 10-fold compared to SPCE bare. Scanning electron microscope analysis revealed that the surface of the modified electrodes has become rougher. The detection and quantification limit of the immunosensors were 8.4 × 10−2 ng/mL and 2.8 × 10−1 ng/mL, respectively, within the range of 9.375 × 10−2 to 1.0 ng/mL of ENaC. Detection of ENaC in urine samples showed high recovery, suggesting that the designed electrochemical immunosensor can be used as a simple and fast alternative method to determine the ENaC in urine samples.

The mineralocorticoid receptor (MR) has classically been studied in the renal epithelium for its role in regulating sodium and water balance and, subsequently, blood pressure. However, the MR also plays a critical role in the microvasculature by regulating ion channel expression and function. Activation of the MR by its endogenous agonist aldosterone results in translocation of the MR into the nucleus, where it can act as a transcription factor. Although most of the actions of the aldosterone can be attributed to its genomic activity though MR activation, it can also act by nongenomic mechanisms. Activation of this ubiquitous receptor increases the expression of epithelial sodium channels (ENaC) in both the endothelium and smooth muscle cells of peripheral and cerebral vessels. MR activation also regulates activity of calcium channels, calcium-activated potassium channels, and various transient receptor potential (TRP) channels. Modification of these ion channels results in a myriad of negative consequences, including impaired endothelium-dependent vasodilation, alterations in generation of myogenic tone, and increased inflammation and oxidative stress. Taken together, these studies demonstrate the importance of studying the impact of the MR on ion channel function in the vasculature. While research in this area has made advances in recent years, there are still many large gaps in knowledge that need to be filled. Crucial future directions of study include defining the molecular mechanisms involved in this interaction, as well as elucidating the potential sex differences that may exist, as these areas of understanding are currently lacking.

This review focuses on epithelial ion channels and their regulation by hormones. The channels discussed are located in the apical or basolateral membranes of epithelial cells. The epithelial channels covered are the following: epithelial sodium channel (ENaC), renal outer medullary potassium ion channel (ROMK), potassium channels activated by calcium (BK), magnesium channels (TRPM6 and TRPM7), epithelial basolateral sodium/potassium pump (Na+/K+ ATPase), apical brush border renal phosphate channels (SLC34 transporters, NPT2 and NPT3), aquaporins, epithelial calcium transport channels (TRPN5 and TRPV6), chloride transport, cystic fibrosis transmembrane conductance regulator (CFTR), pendrin (apical iodide transporting channel in thyroid follicular cells (thyrocytes), sodium/iodide (Na+/I− symport (NIS) in thyrocyte basolateral membrane, and finally, the multidrug resistance channel, P-glycoprotein (MDR). Much of the signal pathways regulating channels, channel activities, mechanisms, and structures are presented in figures.

Hypertension (HTN) is a complex multi-factorial disease and is considered one of the foremost modifiable risk factors for stroke, heart failure, ischemic heart disease and renal dysfunction. Over the past century, salt and its linkage to HTN and cardiovascular (CV) mortality has been the subject of intense scientific scrutiny. There is now consensus that different individuals have different susceptibilities to blood pressure (BP)-raising effects of salt and this susceptiveness is called as salt sensitivity. Several renal and extra-renal mechanisms are believed to play a role. Blunted activity of the renin–angiotensin–aldosterone system (RAAS), adrenal Rac1-MR-Sgk1-NCC/ENaC pathway, renal SNS-GR-WNK4-NCC pathway, defect of membrane ion transportation, inflammation and abnormalities of Na+/Ca2+ exchange have all been implicated as pathophysiological basis for salt sensitive HTN. While salt restriction is definitely beneficial recent observation suggests that treatment with Azilsartan may improve salt sensitivity by selectively reducing renal proximal tubule Na+/H+ exchange. This encourages the future potential benefits of recognizing and therapeutically addressing the salt sensitive phenotype in humans. Keywords: Hypertension, Salt sensitivity, Patho-physiological mechanisms, Azilsartan

The homeostatic control of presynaptic neurotransmitter release stabilizes information transfer at synaptic connections in the nervous system of organisms ranging from insect to human. Presynaptic homeostatic signaling centers upon the regulated membrane insertion of an amiloride-sensitive degenerin/epithelial sodium (Deg/ENaC) channel. Elucidating the subunit composition of this channel is an essential step toward defining the underlying mechanisms of presynaptic homeostatic plasticity (PHP). Here, we demonstrate that the ppk1 gene encodes an essential subunit of this Deg/ENaC channel, functioning in motoneurons for the rapid induction andmaintenance of PHP. We provide genetic and biochemical evidence that PPK1 functions together with PPK11 and PPK16 as a presynaptic, hetero-trimeric Deg/ENaC channel. Finally, we highlight tight control of Deg/ENaC channel expression and activity, showing increased PPK1 protein expression during PHP and evidence for signaling mechanisms that fine tune the level of Deg/ENaC activity during PHP.

Since its discovery, aldosterone and ion modulation have been entwined. While scientific investigations throughout the decades have emphasized aldosterone's connection to Na+, K+, and H+ homeostasis, more recent research has demonstrated a relationship between aldosterone and Mg2+, Ca2+, and Cl− homeostasis. The mechanisms connecting aldosterone to ion regulation frequently involve ion channels; the membrane localized proteins containing at least one aqueous pore for ion conduction. In order to precisely control intracellular or intraorganelle ion concentrations, ion channels have evolved highly specific regions within the conduction pore that select ions by charge, size, and/or dehydration energy requirement, meaning aldosterone must be able to modulate multiple ion channels to regulate the many ions described above. The list of ion channels presently connected to aldosterone includes ENaC (Na+), ROMK/BK (K+), TRPV4/5/6 (Ca2+), TRPM7/6 (Mg2+), and ClC-K/CFTR (Cl−), among others. This list is only expected to grow over time, as the promiscuity of aldosterone becomes more understood.

Type 1 and 2 diabetes mellitus are major medical epidemics affecting millions of patients worldwide. Diabetes mellitus is the leading cause of a form of chronic kidney disease known as Diabetic Kidney Disease (DKD), which is the most common cause of end-stage renal disease (ESRD). DKD is associated with significant changes in renal hemodynamics and electrolyte transport. Alterations in renal ion transport, triggered by pathophysiological conditions in diabetes, can exacerbate hypertension, accelerate renal injury, and are integral to the development of DKD. Renal ion transporters and electrolyte homeostasis play a fundamental role in functional changes and injury to the kidney during DKD. With the large number of ion transporters involved in DKD, understanding the roles of individual transporters as well as the complex cascades through which they interact is essential in the development of effective treatments for patients suffering from this disease. This chapter aims to gather current knowledge of the major renal ion transporters with altered expression and activity under diabetic conditions, and provide a comprehensive overview of their interactions and collective function in DKD.

Liddle's syndrome (LS) is a rare hereditary condition, transmitted with an autosomal-dominant pattern, due to gain-of-function mutations in either SCNN1A or SCNN1B or SCNN1G genes, which code for the three subunits (α, β, γ) of the epithelial sodium channel (ENaC). Main clinical features include early-onset salt-sensitive hypertension, increased incidence of premature cardiovascular events, hypokalemic metabolic alkalosis, suppression of both renin and aldosterone secretion, and unresponsiveness to mineralocorticoid receptor (MR) antagonists, as opposed to ENaC blockers responsiveness.

The adrenocortical steroid hormone, aldosterone, is the primary mineralocorticoid in many species, the main function of which is to conserve sodium. Aldosterone-mediated sodium homeostasis is tightly coupled to its role in potassium and hydrogen ion homeostasis. Aldosterone is one effector arm of the renin–angiotensin aldosterone system in that it regulates sodium, potassium, and hydrogen ion vectorial transport in the epithelial cells of the distal nephron and distal colon. The receptor for aldosterone, the mineralocorticoid receptor (MR), also responds to the physiological glucocorticoids, cortisol (or corticosterone in rodents). In epithelial cells cortisol access to the MR is precluded by the enzyme 11β hydroxysteroid dehydrogenase type 2, which converts cortisol/corticosterone to their receptor inactive metabolites. This section will cover the classic sodium-retaining actions of aldosterone and the nonsodium cell signaling actions of aldosterone in tissues of the cardiovascular system, the brain, adipose tissue, and the immune systems among others.

Aldosterone-sensitive distal nephron (ASDN) including the distal convoluted tubule (DCT), connecting tubule (CNT) and collecting duct (CD) plays an important role in the regulation of hormone-dependent Na+ reabsorption and dietary K+-intake dependent K+ excretion. The major Na+ transporters in the ASDN are thiazide-sensitive Na-Cl cotransporter (NCC), epithelial Na+ channel (ENaC), pendrin/Na+-dependent Cl−-bicarbonate exchanger (NDCBE). Whereas major K+ channels in the ASDN are Kir4.1 and Kir5.1 in the basolateral membrane; and Kir1.1 (ROMK) and Ca2 + activated big conductance K+ channel (BK) in the apical membrane. Although a variety of in vitro cell lines of the ASDN is available and these cell models have been employed for studying Na+ and K+ channels, the biophysical properties and the regulation of Na+ and K+ channels in vitro cell models may not be able to recapitulate those in vivo conditions. Thus, the studies performed in the native ASDN are essential for providing highly physiological relevant information and for understanding the Na+ and K+ transport in the ASDN. Here we provide a detailed methodology describing how to perform the electrophysiological measurement in the native DCT, CNT and cortical collecting duct (CCD).

Skin affections after sulfur mustard (SM) exposure include erythema, blister formation and severe inflammation. An antidote or specific therapy does not exist. Anti-inflammatory compounds as well as substances counteracting SM-induced cell death are under investigation. In this study, we investigated the benzylisoquinoline alkaloide berberine (BER), a metabolite in plants like berberis vulgaris, which is used as herbal pharmaceutical in Asian countries, against SM toxicity using a well-established in vitro approach. Keratinocyte (HaCaT) mono-cultures (MoC) or HaCaT/THP-1 co-cultures (CoC) were challenged with 100, 200 or 300 mM SM for 1 h. Post-exposure, both MoC and CoC were treated with 10, 30 or 50 mu M BER for 24 h. At that time, supernatants were collected and analyzed both for interleukine (IL) 6 and 8 levels and for content of adenylate-kinase (AK) as surrogate marker for cell necrosis. Cells were lysed and nucleosome formation as marker for late apoptosis was assessed. In parallel, AK in cells was determined for normalization purposes. BER treatment did not influence necrosis, but significantly decreased apoptosis. Anti-inflammatory effects were moderate, but also significant, primarily in CoC. Overall, BER has protective effects against SM toxicity in vitro. Whether this holds true should be evaluated in future in vivo studies.

Both transient tachypnea of the newborn and neonatal respiratory distress syndrome have been associated with changes in gene expression of aquaporine-5 (AQP5) and the β subunit of the epithelial sodium channel (β-ENaC) in the respiratory epithelium. Gastric aspirate (GA) obtained immediately after birth could represent a new source for gene expression analysis for these respiratory diseases. The aims of this study were to determine the feasibility of estimating AQP5 and β-ENaC gene expression in exfoliated respiratory epithelial cells from the GA of term neonates, and to compare the values with those found in scraped nasal epithelial cells, previously validated as a surrogate for distal lung epithelium in terms of ionic channel activity. The study had a cross-sectional, proof-of-concept design. Immediately after birth, we obtained GA and nasal mucous membrane scrapings from term newborns, in which total RNA and RT-qPCR assays for AQP5 and β-ENaC genes were performed. AQP5 gene expression was greater in GA than in nasal scrapings, and β-ENaC gene expression was at least as great in GA as that obtained in nasal scrapings. Amplification of samples from the two sites was comparable. AQP5 gene expression was greater in babies delivered by cesarean section; β-ENaC gene expression was greater in babies delivered vaginally, but only in the nasal samples. Quantitation of the expression of AQP5 and of β-ENaC genes in GA, obtained shortly after birth from term newborns is feasible. If confirmed in preterm neonates, this approach could aid in the differential diagnosis of neonatal respiratory diseases.

The primary sites of action of aldosterone in the kidney are in the epithelial cells of the distal portions of the nephron. Aldosterone binds and activates the mineralocorticoid receptor (MR), a member of the ligand-dependent family of nuclear transcription regulators to initiate the transcription of genes encoding multiple proteins that enable and enhance the vectorial transport of sodium and water from the filtrate in the lumen to the extracellular space. Key among these are proteins that increase the number of functional epithelial sodium channels intercalated into the luminal membrane of the tubular epithelial cell. One of the first and most prominent genes to be transcribed is SGK1, a serine-threonine kinase. A prominent function of SGK1 is the phosphorylation of Nedd4-2, a ubiquitin ligase, thereby inhibiting the ubiquitylation of the PY domains of the β and γ epithelial sodium channel (ENaC) subunits, leading to proteosomal degradation of the channel. Phosphorylation of the repressor molecule AF9 by SGK1 impairs its ability to hypermethylate H3 histone, allowing the MR to bind the ENaCα promoter and initiate α subunit transcription. Aldosterone also has important rapid nongenomic effects on cell signaling cascades that participate in the nephron in sodium transport.

This chapter summarizes the role of epithelial ion (K+, Na+, and Cl−) channels in the gastrointestinal (GI) tract. Both cAMP- and Ca2 +-activated K+ channels are localized on both apical and basolateral membranes of the entire GI tract. Apical K+ channels regulate H+,K+-ATPase-mediated acid (i.e., H+) secretion is stomach, while it regulates active K+ secretion in colon. Basolateral K+ channels regulate active Cl− secretion by maintaining cell membrane potential. Epithelial Na+ channels (ENaC), which are present in the apical membrane of colon, mediates active Na+ absorption. Cystic fibrosis transmembrane regulator (CFTR) Cl− channels that regulate electrogenic Cl− secretion are present on the apical membranes of the entire intestine, while Slc26a9 Cl− channels are strongly expressed in the upper GI tract. TMEM16 Ca2 +-activated Cl− channels are important for the increase in intracellular Ca2 + concentration during purinergic and cholinergic signaling.

Summary The sense of taste plays a major role in evaluating the quality of food components in the oral cavity. Sweet, salty, umami, sour and bitter taste are generally accepted as five basic taste qualities. Among them, salty taste is attractive to animals and influences sodium intake. Angiotensin II (ANG II) and aldosterone (ALDO, which is stimulated by ANG II) are key hormones that regulate sodium homeostasis and water balance. At the peripheral gustatory organs, it has been reported that ALDO increases the amiloride-sensitivity of the rat gustatory neural responses to NaCl in a time course of several hours. A recent study demonstrated that ANG II suppresses amiloride-sensitivity of the mouse gustatory and behavioral responses to NaCl via its receptor AT1 within an hour. Moreover, ANG II enhances sweet taste sensitivity without affecting umami, sour and bitter tastes. These results suggest that the reciprocal and sequential regulatory mechanisms by ANG II (as an acute suppressor) together with ALDO (as a slow enhancer) on the salt taste sensitivity may exist in peripheral taste organs, contribute to salt intake, and play an important role in sodium homeostasis. Furthermore, the linkage between salty and sweet taste modulations via the ANG II signaling may optimize sodium and calorie intakes.

The chapter begins with an overview, showing how the regulation of water permeability of the distal nephron, combined with the osmotic gradient established in the kidney interstitium, allows the kidney to concentrate or dilute the urine. It then shows how the kidney establishes this osmotic gradient by using the counter-current flow of the loop of Henle and collecting ducts, and the spatial separation of transport mechanisms, to make a gradient greater than that achievable by the transport mechanisms themselves. The essential role of the vasa recta is discussed. The contribution of urea to the osmotic gradient in the inner medulla is explained. The function of ADH in allowing the contribution of urea is also discussed. The mechanism of osmotic diuresis is explained.

© 2014 The Authors. A major gap in our understanding of sensation is how a single sensory neuron can differentially respond to a multitude of different stimuli (polymodality), such as propio- or nocisensation. The prevailing hypothesis is that different stimuli are transduced through ion channels with diverse properties and subunit composition. In a screen for ion channel genes expressed in polymodal nociceptive neurons, we identified Ppk26, a member of the trimeric degenerin/epithelial sodium channel (DEG/ENaC) family, as being necessary for proper locomotion behavior in Drosophila larvae in a mutually dependent fashion with coexpressed Ppk1, another member of the same family. Mutants lacking Ppk1 and Ppk26 were defective in mechanical, but not thermal, nociception behavior. Mutants of Piezo, a channel involved in mechanical nociception in the same neurons, did not show a defect in locomotion, suggesting distinct molecular machinery for mediating locomotor feedback and mechanical nociception.

Background The T594M variant of the β-subunit of the sodium epithelial channel ( ENaC ) gene may contribute to hypertension in individuals of Indo-Aryan origin. Methods Present study was performed to assess the role of the ENaC gene variant as an independent risk factor for hypertension in subjects of Indo-Aryan ancestry. A total of 150 patients of recently detected essential hypertension and 150 matched controls were genotyped for the T594M polymorphism of the ENaC gene by PCR–RFLP method. Results β-T594M mutation was found to be non-polymorphic. There was major genotype call in both the groups i.e. cases and controls. Other phenotypic parameters like age, sex and body mass index were also similar among hypertensive patients and controls ( P > 0.05). Hypertensive patients had significantly higher total cholesterol and triglycerides compared with controls ( P Conclusion These results do not suggest an important role for the T594M variant of the ENaC gene contributing to either the development or severity of hypertension in subjects of Indo-Aryan ancestry.

When an animal first samples a food, it engages the gustatory system not only to assess the presence of critical nutrients but also to provide an early warning about potential toxins or other noxious compounds. Taste stimuli vary in their size from complex molecules such as carbohydrates (sweet taste) or amino acids (umami taste) to small ions like Na + (salty taste) or the protons of aversive acids (sour taste). The principal gustatory detector of sodium salts has been identified as the epithelial sodium channel, but the molecular mechanisms by which acids and nonsodium salts are transduced in the taste buds remains unclear. This chapter will review much of what we know, and what we do not, of the mechanisms by which the mammalian gustatory system detects salts and acids.

Lung injury especially acute respiratory distress syndrome (ARDS) can be triggered by diverse stimuli, including fatty acids. ARDS affects thousands of people worldwide each year, presenting a high mortality rate and impacting the economy, because most ARDS patients need intensive care units treatment. The lung injury hallmark is edema formation with alveoli flooding. Animals models are used to study mechanisms and deepen the understanding of lung injury. Oleic acid-induced lung injury is a relevant model sharing numerous features with human disease. Oleic acid is an n-9 monounsaturated fatty acid obtained from the diet or produced endogenously. The oleic acid has metabolic/physiologic and deleterious effects on the body. In this chapter we address some basic concepts about fatty acids (mainly oleic acid), discussing its involvement in metabolic and inflammatory diseases and focusing on the importance of oleic acid to lung injury. We briefly discuss lung structure; pulmonary type I and II cells functions, and molecules, such as sodium channel and Na/K-ATPase, involved in those functions. We discuss what is currently known about lung injury and mechanisms involved in ARDS physiopathology and putative targets for its treatment. We further discuss mechanisms likely involved in oleic acid-induced lung injury, and we also bring about the lung injury animal models and their significance for ARDS understanding. Finally, we briefly discuss the importance of ARDS models, especially oleic acid-induced lung injury model, to identify and test potential targets in therapeutic options for ARDS.

Drosophila has served as the primary model system in studies on insect taste. Early electrophysiological and behavioral analyses have identified four basic taste modalities in the fly: the appetitive taste modalities of sweet, low concentration of salt and water, and the aversive taste of high concentration of salt. Extension of behavioral analyses and identification and expression studies of two distinct families of taste receptors genes, the gustatory receptor and ionotropic receptor genes, have expanded insect taste to include fatty acid, amino acids, and carboxylic acids, all of which are appetitive or appetitive enhancing, and aversive bitter taste. Molecular genetic investigations have now firmly associated sweet and bitter taste with specific gustatory receptor subfamilies. Pickpocket (PPK) 28, a degenerin/epithelial sodium channel family protein, is essential for water sensing. Future research will focus on the ionotropic receptor genes and their potential role in the other insect taste modalities.

Mucociliary clearance in the lung epithelium is a critical protective function and is essential for the clearance of respiratory pathogens. Maintenance of the periciliary layer requires the careful coordination of Na+ absorption through epithelial Na+ (ENaC) and cation channels and Cl− secretion through the cystic fibrosis transmembrane conductance regulator (CFTR). In cystic fibrosis (CF), defective CFTR expression and activity lead to chronic bacterial respiratory infections, airway obstruction, bronchiectasis, and respiratory failure. Two recent models, the Beta ENaC overexpressing mouse and the CF pig, have shed light on the underlying mechanisms that cause dysfunction in CF. Altered fluid regulation in the airway epithelium is consistently observed in many respiratory infections. Influenza infection can lead to respiratory symptoms such as pneumonia, pulmonary edema, and acute respiratory distress syndrome in severe cases. Alteration of ENaC by influenza occurs through protein kinase C activation initiated by hemagglutinin binding during virus attachment. Influenza also decreases fluid clearance in vivo by nucleotide/nucleoside signaling well before changes in lung morphology occur. Finally, the matrix protein 2, an ion channel expressed during influenza infection, inhibits both ENaC and CFTR activity.

Kinins, such as Bradykinin (BK), are peptide hormones of the kallikrein-kinin system. Apart from being a vasodilator, BK also increases urinary sodium excretion to reduce systemic blood pressure. It is becoming appreciated that BK modulates function of the epithelial Na(+) channel in the distal part of the renal nephron to affect tubular sodium reabsorption. In this chapter, we outline the molecular details, as well as discuss the physiological relevance of this regulation for the whole organism sodium homeostasis and setting chronic blood pressure.

Lung pathology in cystic fibrosis is linked to dehydration of the airways epithelial surface which in part results from inappropriately raised sodium reabsorption through the epithelial sodium channel (ENaC). To identify a small-interfering RNA (siRNA) which selectively inhibits ENaC expression, chemically modified 21-mer siRNAs targeting human ENaCα were designed and screened. GSK2225745, was identified as a potent inhibitor of ENaCα mRNA (EC(50) (half maximal effective concentration) = 0.4 nmol/l, maximum knockdown = 85%) and protein levels in A549 cells. Engagement of the RNA interference (RNAi) pathway was confirmed using 5' RACE. Further profiling was carried out in therapeutically relevant human primary cells. In bronchial epithelial cells, GSK2225745 elicited potent suppression of ENaCα mRNA (EC(50) = 1.6 nmol/l, maximum knockdown = 82%). In human nasal epithelial cells, GSK2225745 also produced potent and long-lasting (≥72 hours) suppression of ENaCα mRNA levels which was associated with significant inhibition of ENaC function (69% inhibition of amiloride-sensitive current in cells treated with GSK2225745 at 10 nmol/l). GSK2225745 showed no evidence for potential to stimulate toll-like receptor (TLR)3, 7 or 8. In vivo, topical delivery of GSK2225745 in a lipid nanoparticle formulation to the airways of mice resulted in significant inhibition of the expression of ENaCα in the lungs. In conclusion, GSK2225745 is a potent inhibitor of ENaCα expression and warrants further evaluation as a potential novel inhaled therapeutic for cystic fibrosis.Molecular Therapy - Nucleic Acids (2013) 2, e65; doi:10.1038/mtna.2012.57; published online 15 January 2013.

Aldosterone, a steroid hormone secreted by the zona glomerulosa of the adrenal, has two principal functions: to prevent hyperkalemia and to protect against sodium loss. Aldosterone binds to a specific nuclear receptor, the mineralocorticoid receptor, in the distal nephron. The ligand-receptor complex serves as a transcription factor. The resulting new protein synthesis ultimately results in a prolonged residency time for the epithelial sodium channel (ENaC) on the apical surface of principal cells in collecting duct. Although only a small fraction of the filtered load of sodium is reabsorbed by ENaC (~2%), it is a major determinant of sodium homeostasis; dysregulation of ENaC function can result in either hypo- or hypertension. Autonomous over secretion of aldosterone, known as primary aldosteronism, accounts for approximately 10% of cases of hypertension. In patients with resistant hypertension (elevated blood pressure despite treatment with three or more drugs), nearly half have excess aldosterone since treatment with spironolactone or eplerenone effectively reduces blood pressure. There is increasing appreciation for the role played by aldosterone in the development of hypertension.

All animals use a sophisticated array of receptor proteins to sense their external and internal environments. Major advances have been made in recent years in understanding the molecular and genetic bases for sensory transduction in diverse modalities, indicating that both metabotropic and ionotropic pathways are important in sensory functions. Here, I review the historical background and recent advances in understanding the roles of a relatively newly discovered family of receptors, the degenerin/epithelial sodium channels (DEG/ENaC). These animal-specific cation channels show a remarkable sequence and functional diversity in different species and seem to exert their functions in diverse sensory modalities. Functions for DEG/ENaC channels have been implicated in mechanosensation as well as chemosensory transduction pathways. In spite of overall sequence diversity, all family members share a unique protein topology that includes just two transmembrane domains and an unusually large and highly structured extracellular domain, that seem to be essential for both their mechanical and chemical sensory functions. This review will discuss many of the recent discoveries and controversies associated with sensory function of DEG/ENaC channels in both vertebrate and invertebrate model systems, covering the role of family members in taste, mechanosensation, and pain.

Publisher Summary This chapter discusses several ion regulatory mechanisms in freshwater (FW) fish. In the mammalian kidney, different types of ion-transporting cells (ionocytes) in the various segments exhibit specific ion transporter expression patterns and functions. With respect to ion uptake and acid–base regulatory mechanisms, the FW fish gill is in some respects analogous to the mammalian kidney and thus it is not surprising that the gill also possesses various types of ionocytes to achieve a variety of distinct ion transport functions. Zebrafish possess the highest affinity Cl- uptake mechanism of any species thus far examined with an affinity constant (Km) of only 8 μM in fish acclimated to ion-poor water. It is the combination of high Cl- uptake affinity and transport capacity (JMAX) that allows zebrafish to inhabit extremely dilute environments. Two pathways have been proposed for the apical inward transport of Na+ and outward secretion of H+ in fish gill cells: (1) an apical V-type H+–ATPase electrically linked to Na+ absorption via the epithelial Na+ channel (ENaC) or structural analog, and (2) an electroneutral exchange of Na+ and H+ via an apical Na+/H+ exchanger (NHE).

Publisher Summary 14-3-3 proteins are highly conserved and ubiquitously expressed proteins, which are found in all eukaryotic cells, and not in bacteria. There are seven isoforms (β, γ, ɛ, σ, ς, τ/θ, and η, with α and δ corresponding to the phosphorylated forms of β and γ respectively) in mammals, each encoded by a distinct gene. The binding of 14-3-3 proteins is primarily phosphorylation dependent; therefore 14-3-3 interactions are largely regulated by the kinases and phosphatases that modulate the phosphorylation state of a target protein. Binding partners include signaling proteins that are involved in various pathways leading to the control or modulation of cell survival, growth, migration, and differentiation. One of the interventions of 14-3-3 proteins in cell signaling is the regulation of the serine/threonine kinase Raf1. 14-3-3 proteins can stimulate the transformation between the inactive and active forms of Raf1 and mediate the association of Raf1 with other proteins that activate Raf1, such as protein kinase C (PKC), Bcr, and the KSR kinase suppressor of Ras. 14-3-3 proteins are involved in controlling the function and localization of related ion channels such as the γ and ɛ forms can regulate TWIK-related spinal cord K(+) channel (TRESK) and the regulation of epithelial sodium channel by aldosterone requires the β and δ forms of 14-3-3ɛ. This ability to regulate ion channels is of crucial importance in the nervous system, in which 14-3-3 proteins participate in synaptic transmission and plasticity.

Publisher Summary The molecular genetic discovery that mutations in WNK1 and WNK4 cause pseudohypoaldosteronism type II (PHAII), has unraveled the complex mechanism by which the kidney regulates the balance between salt reabsorption and potassium secretion. Logical targets include the sodium and chloride reabsorption pathways mediated by Na-Cl cotransporter (NCC) and epithelial sodium channel (ENaC), the mediators and regulators of distal nephron potassium secretion, including renal outer medullary potassium channel (ROMK), the BK potassium channel, and ENaC, the paracellular chloride flux pathway, and the apical H+-ATPase involved in distal proton secretion. The marked impairment in renal potassium secretion is the most consistent feature of PHAII and suggests several potential targets. The potassium channel ROMK is the major mediator of renal potassium secretion in the connecting tubule (CNT)/CCD under normal circumstances and is one possible target of WNK4. ENaC, the major pathway for electrogenic sodium reabsorption in the CNT and CCD, is another potential target of the WNKs. ENaC activity produces the lumen-negative potential and is required for normal potassium secretion, as revealed by inherited and acquired alteration in ENaC activity. States featuring increased ENaC activity often cause hypokalemia owing to the increased lumen-negative potential but mutations that result in loss of ENaC function lead to impaired production of a lumen-negative potential and often profound hyperkalemia.

The mineralocorticoid receptor (MR) responds not only to the physiological mineralocorticoids, aldosterone and deoxycorticosterone, but also to the physiological glucocorticoid, cortisol. The MR is a member of the nuclear receptor superfamily that acts as a ligand-dependent transcription factor. The structural determinants of MR function are described and discussed. The MR mediates its effects not only through a transcriptional mechanism but also through nongenomic mechanisms and perhaps through transrepression. The nature of these responses is both ligand and tissue dependent. The MR is expressed in a number of tissues beyond the traditional epithelial targets for aldosterone, including the central nervous and the cardiovascular systems. Recent clinical studies have demonstrated the importance of the MR as a therapeutic target.

Publisher Summary This chapter sheds light on the genetic, physiological, and biochemical studies of Liddle’s syndrome (pseudoaldosteronism) providing a new understanding of the cellular and molecular basis of the regulation of blood pressure and volume by the kidney. Morphological and functional studies on rodent and human kidneys indicate that at least three successive portions of the distal nephron, i.e., the late portion of the distal convoluted tubule (DCT), the connecting tubule (CNT), and the collecting duct (CD), contribute to the ASDN. Although these segments have distinct structural and functional features, they have in common the expression of the epithelial sodium channel (ENaC), the mineralocorticoid receptor (MR), and the 11-βhydroxysteroid dehydrogenase type II (11-βHSD2) proteins. Analysis of the β ENaC gene of subject with Liddle’s syndrome showed different types of mutations, all located in the cytosolic C-terminus end of the β or γ ENaC subunit (exon XIII of SCNN1B and SCNN1G). These mutations include premature stop codons, frameshift or missense mutations that delete or change a conserved proline-rich sequence. Liddle’s syndrome also arises from mutations in the γ ENaC subunit at corresponding positions in the cytosolic C-terminus of the subunit (exon XIII). All the mutations associated with Liddle syndrome delete or modify the sequence of a conserved proline-rich motif xPPxY (x being any amino acid, P proline, and Y tyrosine) in the C-terminus of the β and γ ENaC subunits. In addition to Liddle’s syndrome, hereditary renal disorders associated with elevated blood pressure, expansion of the extracellular fluid volume due to excessive distal Na+ absorption include the apparent mineralocorticoid excess (AME), the hypertension exacerbated in pregnancy, or the pseudohypoaldosteronism type II.

Alveolar epithelial cells of mammalian lungs actively transport sodium (Na+) and chloride (Cl−) ions and these processes are important in fluid homeostasis. Sodium ions enter the apical membranes of both type I and type II alveolar epithelial cells through sodium selective, cation and cyclic nucleotide gated ion channels and are extruded across the basolateral membrane by the ouabain-sensitive Na,K-ATPase. This vectorial transport of Na+ ions (and concomitant movement of Cl− ions to maintain electroneutrality) creates an oncotic force leading to the reabsorption of fluid across both normal and damaged lungs. Nitric oxide and reactive oxygen nitrogen intermediates (formed by the reactions of nitric oxide with partially reduced oxygen species), generated in close proximity of epithelial cell membranes by activated inflammatory cells, modulate the activity of sodium channels via signal transduction mechanisms (such as increasing PKG and PKC) or by post-translational oxidative modifications of sodium channel proteins and their chaperons. Channel activity (and vectorial sodium transport) may be either increased or decreased depending on levels of reactive intermediates and length of exposure. Better understanding of these interactions would offer considerable new insights into the mechanisms leading to pulmonary edema in a number of pathological conditions.

There are two classes of salt taste receptors. One class is sensitive specifically to sodium (Na+) salts, the other responds to a variety of cations. This chapter reviews the cumulative evidence that the Na+-specific taste receptor is the amiloride-sensitive epithelial Na+ channel (ENaC). This is accomplished by first presenting a description of what is known currently regarding the function, structure, and regulation of ENaC in Na+-transporting epithelia including taste bud cells. The regulatory factors discussed include: hormones, cyclic AMP, pH, calcium, osmolarity, temperature, proteases, and Na+ self-inhibition. Data are then presented that demonstrate the action of these regulatory factors on the sodium chloride (NaCl) responses of the rat chorda tympani taste nerves. The cumulative physiological and behavioral data leave little doubt that ENaC is the Na+-specific taste receptor in mammals.

Epithelial Na + channels (ENaCs) have a key role in the regulation of extracellular fluid volume and blood pressure. Over the two decades since the initial characterization of ENaCs at a molecular level, investigators have identified key structural features within channel subunits and have defined mechanisms by which hormones and other factors regulate ENaC activity. Mutations within ENaC subunits have been identified that result in a gain or loss of function, which have profound effects on blood pressure. The identification of Liddle syndrome mutations led to the elucidation of a network of proteins, including Sgk1, GILZ, and Nedd4-2, which have important roles in the regulation of ENaC activity. The high-resolution structure of ASIC1, in combination with functional studies, has provided new insights regarding the role of the extracellular region in the regulation of ENaC gating.

Lingual taste cells are specialized sensory epithelial cells gathered together in groups of 50–100 to make up taste buds. Taste cells are responsive to chemically diverse compounds including salts, sugars, amino acids, organic and inorganic acids, alkaloids, and other nonvolatile molecules. The initial detection of sapid compounds by taste cells depends upon specific ion channels and G-protein-coupled receptors. Activation of taste ion channels, receptors, and downstream signaling cascades typically leads to taste cell membrane depolarization, action potential generation, and release of neurotransmitter from taste cells to the afferent gustatory nerves. Concerted studies using molecular, physiological, and transgenic approaches have led to the identification of the taste detection and transducing elements underlying the initial steps for each taste quality, as well as the neurotransmitters involved in cell-to-cell communication and signaling to afferent nerve fibers.

Publisher Summary Mechanotransduction influences many aspects of biological function. In the cardiovascular system, mechanotransduction has a significant impact on vascular function. Multiple signal transduction pathways participate in the transmission of biomechanical forces into cellular signals, including integrins, adhesion molecules, cytoskeleton, and activation of membrane-bound transporters and ion channels. This chapter discusses the potential role of a novel family of ion channels with evolutionarily conserved involvement in mechanotransduction, the degenerin/epithelial Na + channel (DEG/ENaC) family. Members of the family are identified in a diverse range of species and tissue types. Most members of the DEG/ENaC family form cation selective ion channels, some of which are believed to form mechanically gated ion channels. While most research has focused on degenerins in sensory neuron mechanotransduction, emerging evidence suggests ENaC proteins may also participate in vascular-smooth-muscle mechanotransduction. The chapter describes the potential role and physiological importance of ENaC proteins as mechanosensors in vascular smooth muscle.