Rapid translocation and insertion of the epithelial Na+ channel in response to RhoA signaling

Small G-protein RhoA is a potential inhibitor of cardiac fast sodium current

Small G-proteins of Rho family modulate the activity of several classes of ion channels, including K⁺ channels Kv1.2, Kir2.1, and ERG; Ca²⁺ channels; and epithelial Na⁺ channels. The present study was aimed to check the RhoA potential regulatory effects on Na⁺ current (I_Na) transferred by Na⁺ channel cardiac isoform Na_V1.5 in heterologous expression system and in native rat cardiomyocytes. Whole-cell patch-clamp experiments showed that coexpression of Na_V1.5 with the wild-type RhoA in CHO-K1 cell line caused 2.7-fold decrease of I_Na density with minimal influence on steady-state activation and inactivation. This effect was reproduced by the coexpression with a constitutively active RhoA, but not with a dominant negative RhoA. In isolated ventricular rat cardiomyocytes, a 5-h incubation with the RhoA activator narciclasine (5 × 10^-6 M) reduced the maximal I_Na density by 38.8%. The RhoA-selective inhibitor rhosin (10^-5 M) increased the maximal I_Na density by 25.3%. Experiments with sharp microelectrode recordings in isolated right ventricular wall preparations showed that 5 × 10^-6 M narciclasine induced a significant reduction of action potential upstroke velocity after 2 h of incubation. Thus, RhoA might be considered as a potential negative regulator of sodium channels cardiac isoform Na_V1.5.

Neuroendocrine Mechanisms Governing Sex Differences in Hyperalgesic Priming Involve Prolactin Receptor Sensory Neuron Signaling

Many clinical and preclinical studies report higher prevalence and severity of chronic pain in females. We used hyperalgesic priming with interleukin 6 (IL-6) priming and PGE₂ as a second stimulus as a model for pain chronicity. Intraplantar IL-6 induced hypersensitivity was similar in magnitude and duration in both males and females, while both paw and intrathecal PGE₂ hypersensitivity was more persistent in females. This difference in PGE₂ response was dependent on both circulating estrogen and translation regulation signaling in the spinal cord. In males, the duration of hypersensitivity was regulated by testosterone. Since the prolactin receptor (Prlr) is regulated by reproductive hormones and is female-selectively activated in sensory neurons, we evaluated whether Prlr signaling contributes to hyperalgesic priming. Using ΔPRL, a competitive Prlr antagonist, and a mouse line with ablated Prlr in the Nav1.8 sensory neuronal population, we show that Prlr in sensory neurons is necessary for the development of hyperalgesic priming in female, but not male, mice. Overall, sex-specific mechanisms in the initiation and maintenance of chronic pain are regulated by the neuroendocrine system and, specifically, sensory neuronal Prlr signaling.SIGNIFICANCE STATEMENT Females are more likely to experience chronic pain than males, but the mechanisms that underlie this sex difference are not completely understood. Here, we demonstrate that the duration of mechanical hypersensitivity is dependent on circulating sex hormones in mice, where estrogen caused an extension of sensitivity and testosterone was responsible for a decrease in the duration of the hyperalgesic priming model of chronic pain. Additionally, we demonstrated that prolactin receptor expression in Nav1.8⁺ neurons was necessary for hyperalgesic priming in female, but not male, mice. Our work demonstrates a female-specific mechanism for the promotion of chronic pain involving the neuroendrocrine system and mediated by sensory neuronal prolactin receptor.

Potassium and Calcium Channel Complexes as Novel Targets for Cancer Research

The intracellular Ca²⁺ concentration is mainly controlled by Ca²⁺ channels. These channels form complexes with K⁺ channels, which function to amplify Ca²⁺ flux. In cancer cells, voltage-gated/voltage-dependent Ca²⁺ channels and non-voltage-gated/voltage-independent Ca²⁺ channels have been reported to interact with K⁺ channels such as Ca²⁺-activated K⁺ channels and voltage-gated K⁺ channels. These channels are activated by an increase in cytosolic Ca²⁺ concentration or by membrane depolarization, which induces membrane hyperpolarization, increasing the driving force for Ca²⁺ flux. These complexes, composed of K⁺ and Ca²⁺ channels, are regulated by several molecules including lipids (ether lipids and cholesterol), proteins (e.g. STIM), receptors (e.g. S1R/SIGMAR1), and peptides (e.g. LL-37) and can be targeted by monoclonal antibodies, making them novel targets for cancer research.

Interactive Actions of Aldosterone and Insulin on Epithelial Na+ Channel Trafficking

Epithelial Na+ channel (ENaC) participates in renal epithelial Na+ reabsorption, controlling blood pressure. Aldosterone and insulin elevate blood pressure by increasing the ENaC-mediated Na+ reabsorption. However, little information is available on the interactive action of aldosterone and insulin on the ENaC-mediated Na+ reabsorption. In the present study, we tried to clarify if insulin would modify the aldosterone action on the ENaC-mediated Na+ reabsorption from a viewpoint of intracellular ENaC trafficking. We measured the ENaC-mediated Na+ transport as short-circuit currents using a four-state mathematical ENaC trafficking model in renal A6 epithelial cells with or without aldosterone treatment under the insulin-stimulated and -unstimulated conditions. We found that: (A) under the insulin-stimulated condition, aldosterone treatment (1 µM for 20 h) significantly elevated the ENaC insertion rate to the apical membrane ( k I ) 3.3-fold and the ENaC recycling rate ( k R ) 2.0-fold, but diminished the ENaC degradation rate ( k D ) 0.7-fold without any significant effect on the ENaC endocytotic rate ( k E ); (B) under the insulin-unstimulated condition, aldosterone treatment decreased k E 0.5-fold and increased k R 1.4-fold, without any significant effect on k I or k D . Thus, the present study indicates that: (1) insulin masks the well-known inhibitory action of aldosterone on the ENaC endocytotic rate; (2) insulin induces a stimulatory action of aldosterone on ENaC apical insertion and an inhibitory action of aldosterone on ENaC degradation; (3) insulin enhances the aldosterone action on ENaC recycling; (4) insulin has a more effective action on diminution of ENaC endocytosis than aldosterone.

Membrane trafficking pathways regulating the epithelial Na+ channel

Renal Na⁺ reabsorption, facilitated by the epithelial Na⁺ channel (ENaC), is subject to multiple forms of control to ensure optimal body blood volume and pressure through altering both the ENaC population and activity at the cell surface. Here, the focus is on regulating the number of ENaCs present in the apical membrane domain through pathways of ENaC synthesis and targeting to the apical membrane as well as ENaC removal, recycling, and degradation. Finally, the mechanisms by which ENaC trafficking pathways are regulated are summarized.

Prolactin Regulates Pain Responses via a Female-Selective Nociceptor-Specific Mechanism

Many clinical and preclinical studies report an increased prevalence and severity of chronic pain among females. Here, we identify a sex-hormone-controlled target and mechanism that regulates dimorphic pain responses. Prolactin (PRL), which is involved in many physiologic functions, induces female-specific hyperalgesia. A PRL receptor (Prlr) antagonist in the hind paw or spinal cord substantially reduced hyperalgesia in inflammatory models. This effect was mimicked by sensory neuronal ablation of Prlr. Although Prlr mRNA is expressed equally in female and male peptidergic nociceptors and central terminals, Prlr protein was found only in females and PRL-induced excitability was detected only in female DRG neurons. PRL-induced excitability was reproduced in male Prlr⁺ neurons after prolonged treatment with estradiol but was prevented with addition of a translation inhibitor. We propose a novel mechanism for female-selective regulation of pain responses, which is mediated by Prlr signaling in sensory neurons via sex-dependent control of Prlr mRNA translation.

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Local or spinal PRL injection induces hyperalgesia in a female-selective manner

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Sensory neuron Prlr regulates tissue injury-induced pain only in females

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PRL regulates excitability in Prlr⁺ neurons depending on sex and estrogen

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Regulation of Prlr translation defines female-selective neuronal excitability

Druggable targets in the Rho pathway and their promise for therapeutic control of blood pressure

The prevalence of high blood pressure (also known as hypertension) has steadily increased over the last few decades. Known as a silent killer, hypertension increases the risk for cardiovascular disease and can lead to stroke, heart attack, kidney failure and associated sequela. While numerous hypertensive therapies are currently available, it is estimated that only half of medicated patients exhibit blood pressure control. This signifies the need for a better understanding of the underlying cause of disease and for more effective therapies. While blood pressure homeostasis is very complex and involves the integrated control of multiple body systems, smooth muscle contractility and arterial resistance are important contributors. Strong evidence from pre-clinical animal models and genome-wide association studies indicate that smooth muscle contraction and BP homeostasis are governed by the small GTPase RhoA and its downstream target, Rho kinase. In this review, we summarize the signaling pathways and regulators that impart tight spatial-temporal control of RhoA activity in smooth muscle cells and discuss current therapeutic strategies to target these RhoA pathway components. We also discuss known allelic variations in the RhoA pathway and consider how these polymorphisms may affect genetic risk for hypertension and its clinical manifestations.

β1Pix exchange factor stabilizes the ubiquitin ligase Nedd4-2 and plays a critical role in ENaC regulation by AMPK in kidney epithelial cells

Our previous work has established that the metabolic sensor AMP-activated protein kinase (AMPK) inhibits the epithelial Na⁺ channel (ENaC) by promoting its binding to neural precursor cell-expressed, developmentally down-regulated 4-2, E3 ubiquitin protein ligase (Nedd4-2). Here, using MS analysis and in vitro phosphorylation, we show that AMPK phosphorylates Nedd4-2 at the Ser-444 (Xenopus Nedd4-2) site critical for Nedd4-2 stability. We further demonstrate that the Pak-interacting exchange factor β₁Pix is required for AMPK-mediated inhibition of ENaC-dependent currents in both CHO and murine kidney cortical collecting duct (CCD) cells. Short hairpin RNA-mediated knockdown of β₁Pix expression in CCD cells attenuated the inhibitory effect of AMPK activators on ENaC currents. Moreover, overexpression of a β₁Pix dimerization-deficient mutant unable to bind 14-3-3 proteins (Δ602-611) increased ENaC currents in CCD cells, whereas overexpression of WT β₁Pix had the opposite effect. Using additional immunoblotting and co-immunoprecipitation experiments, we found that treatment with AMPK activators promoted the binding of β₁Pix to 14-3-3 proteins in CCD cells. However, the association between Nedd4-2 and 14-3-3 proteins was not consistently affected by AMPK activation, β₁Pix knockdown, or overexpression of WT β₁Pix or the β₁Pix-Δ602-611 mutant. Moreover, we found that β₁Pix is important for phosphorylation of the aforementioned Nedd4-2 site critical for its stability. Overall, these findings elucidate novel molecular mechanisms by which AMPK regulates ENaC. Specifically, they indicate that AMPK promotes the assembly of β₁Pix, 14-3-3 proteins, and Nedd4-2 into a complex that inhibits ENaC by enhancing Nedd4-2 binding to ENaC and its degradation.

RhoA increases ASIC1a plasma membrane localization and calcium influx in pulmonary arterial smooth muscle cells following chronic hypoxia

Increases in pulmonary arterial smooth muscle cell (PASMC) intracellular Ca²⁺ levels and enhanced RhoA/Rho kinase-dependent Ca²⁺ sensitization are key determinants of PASMC contraction, migration, and proliferation accompanying the development of hypoxic pulmonary hypertension. We previously showed that acid-sensing ion channel 1a (ASIC1a)-mediated Ca²⁺ entry in PASMC is an important constituent of the active vasoconstriction, vascular remodeling, and right ventricular hypertrophy associated with hypoxic pulmonary hypertension. However, the enhanced ASIC1a-mediated store-operated Ca²⁺ entry in PASMC from pulmonary hypertensive animals is not dependent on an increase in ASIC1a protein expression, suggesting that chronic hypoxia (CH) stimulates ASIC1a function through other regulatory mechanism(s). RhoA is involved in ion channel trafficking, and levels of activated RhoA are increased following CH. Therefore, we hypothesize that activation of RhoA following CH increases ASIC1a-mediated Ca²⁺ entry by promoting ASIC1a plasma membrane localization. Consistent with our hypothesis, we found greater plasma membrane localization of ASIC1a following CH. Inhibition of RhoA decreased ASIC1a plasma membrane expression and largely diminished ASIC1a-mediated Ca²⁺ influx, whereas activation of RhoA had the opposite effect. A proximity ligation assay revealed that ASIC1a and RhoA colocalize in PASMC and that the activation state of RhoA modulates this interaction. Together, our findings show a novel interaction between RhoA and ASIC1a, such that activation of RhoA in PASMC, both pharmacologically and via CH, promotes ASIC1a plasma membrane localization and Ca²⁺ entry. In addition to enhanced RhoA-mediated Ca²⁺ sensitization following CH, RhoA can also activate a Ca²⁺ signal by facilitating ASIC1a plasma membrane localization and Ca²⁺ influx in pulmonary hypertension.

30 YEARS OF THE MINERALOCORTICOID RECEPTOR: Nongenomic effects via the mineralocorticoid receptor

The mineralocorticoid receptor (MR) belongs to the steroid hormone receptor family and classically functions as a ligand-dependent transcription factor. It is involved in water-electrolyte homeostasis and blood pressure regulation but independent from these effects also furthers inflammation, fibrosis, hypertrophy and remodeling in cardiovascular tissues. Next to genomic effects, aldosterone elicits very rapid actions within minutes that do not require transcription or translation and that occur not only in classical MR epithelial target organs like kidney and colon but also in nonepithelial tissues like heart, vasculature and adipose tissue. Most of these effects can be mediated by classical MR and its crosstalk with different signaling cascades. Near the plasma membrane, the MR seems to be associated with caveolin and striatin as well as with receptor tyrosine kinases like EGFR, PDGFR and IGF1R and G protein-coupled receptors like AT1 and GPER1, which then mediate nongenomic aldosterone effects. GPER1 has also been named a putative novel MR. There is a close interaction and functional synergism between the genomic and the nongenomic signaling so that nongenomic signaling can lead to long-term effects and support genomic actions. Therefore, understanding nongenomic aldosterone/MR effects is of potential relevance for modulating genomic aldosterone effects and may provide additional targets for intervention.

RhoA signaling and blood pressure: The consequence of failing to “Tone it Down”

Uncontrolled high blood pressure is a major risk factor for heart attack, stroke, and kidney failure and contributes to an estimated 25% of deaths worldwide. Despite numerous treatment options, estimates project that reasonable blood pressure (BP) control is achieved in only about half of hypertensive patients. Improvements in the detection and management of hypertension will undoubtedly be accomplished through a better understanding of the complex etiology of this disease and a more comprehensive inventory of the genes and genetic variants that influence BP regulation. Recent studies (primarily in pre-clinical models) indicate that the small GTPase RhoA and its downstream target, Rho kinase, play an important role in regulating BP homeostasis. Herein, we summarize the underlying mechanisms and highlight signaling pathways and regulators that impart tight spatial-temporal control of RhoA activity. We also discuss known allelic variations in the RhoA pathway and consider how these polymorphisms may affect genetic risk for hypertension and its clinical manifestations. Finally, we summarize the current (albeit limited) clinical data on the efficacy of targeting the RhoA pathway in hypertensive patients.

Role of epithelial sodium channels in the regulation of lung fluid homeostasis

In utero, fetal lung epithelial cells actively secrete Cl(-) ions into the lung air spaces while Na(+) ions follow passively to maintain electroneutrality. This process, driven by an electrochemical gradient generated by the Na(+)-K(+)-ATPase, is responsible for the secretion of fetal fluid that is essential for normal lung development. Shortly before birth, a significant upregulation of amiloride-sensitive epithelial channels (ENaCs) on the apical side of the lung epithelial cells results in upregulation of active Na(+) transport. This process is critical for the reabsorption of fetal lung fluid and the establishment of optimum gas exchange. In the adult lung, active Na(+) reabsorption across distal lung epithelial cells limits the degree of alveolar edema in patients with acute lung injury and cardiogenic edema. Cl(-) ions are transported either paracellularly or transcellularly to preserve electroneutrality. An increase in Cl(-) secretion across the distal lung epithelium has been reported following an acute increase in left atrial pressure and may result in pulmonary edema. In contrast, airway epithelial cells secrete Cl(-) through apical cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels and absorb Na(+). Thus the coordinated action of Cl(-) secretion and Na(+) absorption is essential for maintenance of the volume of epithelial lining fluid that, in turn, maximizes mucociliary clearance and facilitates clearance of bacteria and debris from the lungs. Any factor that interferes with Na(+) or Cl(-) transport or dramatically upregulates ENaC activity in airway epithelial cells has been associated with lung diseases such as cystic fibrosis or chronic obstructive lung disease. In this review we focus on the role of the ENaC, the mechanisms involved in ENaC regulation, and how ENaC dysregulation can lead to lung pathology.

Deletion of PDK1 causes cardiac sodium current reduction in mice

Background

The AGC protein kinase family regulates multiple cellular functions. 3-phosphoinositide-dependent protein kinase-1 (PDK1) is involved in the pathogenesis of arrhythmia, and its downstream factor, Forkhead box O1 (Foxo1), negatively regulates the expression of the cardiac sodium channel, Nav1.5. Mice are known to die suddenly after PDK1 deletion within 11 weeks, but the underlying electrophysiological bases are unclear. Thus, the aim of this study was to investigate the potential mechanisms between PDK1 signaling pathway and cardiac sodium current.

Methods and Results

Using patch clamp and western blotting techniques, we investigated the role of the PDK1-Foxo1 pathway in PDK1 knockout mice and cultured cardiomyocytes. We found that PDK1 knockout mice undergo slower heart rate, prolonged QRS and QTc intervals and abnormal conduction within the first few weeks of birth. Furthermore, the peak sodium current is decreased by 33% in cells lacking PDK1. The phosphorylation of Akt (308T) and Foxo1 (24T) and the expression of Nav1.5 in the myocardium of PDK1-knockout mice are decreased, while the nuclear localization of Foxo1 is increased. The role of the PDK1-Foxo1 pathway in regulating Nav1.5 levels and sodium current density was verified using selective PDK1, Akt and Foxo1 inhibitors and isolated neonatal rat cardiomyocytes.

Conclusion

These results indicate that PDK1 participates in the dysregulation of electrophysiological basis by regulating the PDK1-Foxo1 pathway, which in turn regulates the expression of Nav1.5 and cardiac sodium channel function.

Involvement of Rho GTPases and their regulators in the pathogenesis of hypertension

Proper regulation of arterial blood pressure is essential to allow permanent adjustment of nutrient and oxygen supply to organs and tissues according to their need. This is achieved through highly coordinated regulation processes controlling vascular resistance through modulation of arterial smooth muscle contraction, cardiac output, and kidney function. Members of the Rho family of small GTPases, in particular RhoA and Rac1, have been identified as key signaling molecules playing important roles in several different steps of these regulatory processes. Here, we review the current state of knowledge regarding the involvement of Rho GTPase signaling in the control of blood pressure and the pathogenesis of hypertension. We describe how knockout models in mouse, genetic, and pharmacological studies in human have been useful to address this question.

Role of Rho GDP dissociation inhibitor α in control of epithelial sodium channel (ENaC)-mediated sodium reabsorption

The epithelial sodium channel (ENaC) is expressed in the aldosterone-sensitive distal nephron where it performs sodium reabsorption from the lumen. We have recently shown that ENaC activity contributes to the development of salt-induced hypertension as a result of deficiency of EGF level. Previous studies revealed that Rho GDP-dissociation inhibitor α (RhoGDIα) is involved in the control of salt-sensitive hypertension and renal injury via Rac1, which is one of the small GTPases activating ENaC. Here we investigated the intracellular mechanism mediating the involvement of the RhoGDIα/Rac1 axis in the control of ENaC and the effect of EGF on ENaC in this pathway. We demonstrated that RhoGDIα is highly expressed in the cortical collecting ducts of mice and rats, and its expression is down-regulated in Dahl salt-sensitive rats fed a high salt diet. Knockdown of RhoGDIα in cultured cortical collecting duct principal cells increased ENaC subunits expression and ENaC-mediated sodium reabsorption. Furthermore, RhoGDIα deficiency causes enhanced response to EGF treatment. Patch clamp analysis reveals that RhoGDIα significantly decreases ENaC current density and prevents its up-regulation by RhoA and Rac1. Inhibition of Rho kinase with Y27632 had no effects on ENaC response to EGF either in control or RhoGDIα knocked down cells. However, EGF treatment increased levels of active Rac1, which was further enhanced in RhoGDIα-deficient cells. We conclude that changes in the RhoGDIα-dependent pathway have a permissive role in the Rac1-mediated enhancement of ENaC activity observed in salt-induced hypertension.

Rapid aldosterone actions on epithelial sodium channel trafficking and cell proliferation

Aldosterone regulates blood pressure through its effects on the kidney and the cardiovascular system. Dysregulation of aldosterone signalling can result in hypertension which in turn can lead to chronic pathologies of the kidney such as renal fibrosis and nephropathy. Aldosterone acts by binding to the mineralocorticoid receptor (MR), which acts as a ligand-dependent transcription factor in target tissues such as segments of the distal nephron including the connecting tubule and cortical collecting duct (CCD). Aldosterone also promotes the activation of protein kinase signalling cascades that are coupled to growth factor receptors and act directly on specific substrates in the cell membrane or cytoplasm. The rapid actions of aldosterone can also modulate gene expression through the phosphorylation of transcription factors. Aldosterone is a key regulator of Na(+) conservation in the distal nephron, largely through multiple mechanisms that modulate the activity of the epithelial Na(+) channel (ENaC). Aldosterone transcriptionally up-regulates the ENaCα subunit and also up regulates serum and glucocorticoid-regulated kinase-1 (SGK1) that indirectly regulates the ubiquitination of ENaC subunits. Aldosterone promotes the activation of protein kinase D1 (PKD1) which can modify the activity of ENaC and other transporters through effects on sub-cellular trafficking. In M1-CCD cells, early sub-cellular trafficking causes the redistribution of ENaC subunits within minutes of treatment with aldosterone. ENaC subunits can also interact directly with phosphatidylinositide signalling intermediates in the membrane and the mechanism by which PKD isoforms regulate protein trafficking is through the control of vesicle fission from the trans Golgi network by activation of phosphatidylinositol 4-kinaseIIIβ (PI4KIIIβ).

Dysfunction of vascular smooth muscle and vascular remodeling by simvastatin

Statins, inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, are widely prescribed for hypercholesterolemia. With the increasing use of statins, numerous reports demonstrated that statins can cause damage to skeletal muscles. However, the toxicities of statins on vascular smooth muscle, which are essential to cardiovascular homeostasis, have not been previously described. Here, we examined the effects of simvastatin on the contractile function and the integrity of vascular smooth muscle in isolated rat thoracic aortic rings, primary cultured vascular smooth muscle cells (VSMCs) in vitro and rats in vivo. In aortic rings, simvastatin suppressed the normal agonist-induced contractile responses in time- and concentration-dependent manners (0.86 g ± 0.11 at 10 μM simvastatin for 24 h compared with 1.89 g ± 0.11 at control). The suppression persisted in the endothelium-denuded aortic rings and was irreversible even after wash-out of simvastatin. Simvastatin suppressed the contraction induced by Bay K8644, an activator of voltage-operated Ca²⁺ channel (VOCC) in rat aortic rings and abolished agonist-induced intracellular Ca²⁺ increase in VSMCs. The simvastatin-induced contractile dysfunction was reversed by the supplementation of mevalonate and geranylgeranylpyrophosphate, precursors for protein isoprenylation. Consistently, activation of RhoA, a representative isoprenylated protein, was disrupted by simvastatin in VSMCs and RhoA-mediated phosphorylation of MYPT1 and CPI-17, and tonic tension were also suppressed. Notably, prolonged treatment of simvastatin up to 48 h induced apoptosis of vascular smooth muscle in aortic rings. Most importantly, simvastatin treatment in vivo significantly attenuated the agonist-induced vasoconstriction in rats ex vivo and induced a decrease in luminal area of the vascular wall. Collectively, these results demonstrate that simvastatin can impair the normal vascular contractility by disturbing Ca²⁺ influx and RhoA activity, ultimately leading to apoptosis and structural remodeling.

Role of ASIC1 in the development of chronic hypoxia-induced pulmonary hypertension

Chronic hypoxia (CH) associated with respiratory disease results in elevated pulmonary vascular intracellular Ca(2+) concentration, which elicits enhanced vasoconstriction and promotes vascular arterial remodeling and thus has important implications in the development of pulmonary hypertension (PH). Store-operated Ca(2+) entry (SOCE) contributes to this elevated intracellular Ca(2+) concentration and has also been linked to acute hypoxic pulmonary vasoconstriction (HPV). Since our laboratory has recently demonstrated an important role for acid-sensing ion channel 1 (ASIC1) in mediating SOCE, we hypothesized that ASIC1 contributes to both HPV and the development of CH-induced PH. To test this hypothesis, we examined responses to acute hypoxia in isolated lungs and assessed the effects of CH on indexes of PH, arterial remodeling, and vasoconstrictor reactivity in wild-type (ASIC1(+/+)) and ASIC1 knockout (ASIC1(-/-)) mice. Restoration of ASIC1 expression in pulmonary arterial smooth muscle cells from ASIC1(-/-) mice rescued SOCE, confirming the requirement for ASIC1 in this response. HPV responses were blunted in lungs from ASIC1(-/-) mice. Both SOCE and receptor-mediated Ca(2+) entry, along with agonist-dependent vasoconstrictor responses, were diminished in small pulmonary arteries from control ASIC(-/-) mice compared with ASIC(+/+) mice. The effects of CH to augment receptor-mediated vasoconstrictor and SOCE responses in vessels from ASIC1(+/+) mice were not observed after CH in ASIC1(-/-) mice. In addition, ASIC1(-/-) mice exhibited diminished right ventricular systolic pressure, right ventricular hypertrophy, and arterial remodeling in response to CH compared with ASIC1(+/+) mice. Taken together, these data demonstrate an important role for ASIC1 in both HPV and the development of CH-induced PH.

Involvement of PtdIns(4,5)P2 in the regulatory mechanism of small intestinal P-glycoprotein expression

Previously, we reported that repeated oral administration of etoposide (ETP) activates the ezrin/radixin/moesin (ERM) scaffold proteins for P-glycoprotein (P-gp) via Ras homolog gene family member A (RhoA)/Rho-associated coiled-coil containing protein kinase (ROCK) signaling, leading to increased ileal P-gp expression. Recent studies indicate that phosphatidyl inositol 4,5-bisphosphate [PtdIns(4,5)P2] regulates the plasma-membrane localization of certain proteins, and its synthase, the type I phosphatidyl inositol 4-phosphate 5-kinase (PI4P5K), is largely controlled by RhoA/ROCK. Here, we examined whether PtdIns(4,5)P2 and PI4P5K are involved in the increased expression of ileal P-gp following the ERM activation by ETP treatment. Male ddY mice (4-week-old) were treated with ETP (10 mg/kg/day, per os, p.o.) for 5 days. Protein-expression levels were measured by either western blot or dot blot analysis and molecular interactions were assessed using immunoprecipitation assays. ETP treatment significantly increased PI4P5K, ERM, and P-gp expression in the ileal membrane. This effect was suppressed following the coadministration of ETP with rosuvastatin (a RhoA inhibitor) or fasudil (a ROCK inhibitor). Notably, the PtdIns(4,5)P2 expression in the ileal membrane, as well as both P-gp and ERM levels coimmunoprecipitated with anti-PtdIns(4,5)P2 antibody, were increased by ETP treatment. PtdIns(4,5)P2 and PI4P5K may contribute to the increase in ileal P-gp expression observed following the ETP treatment, possibly through ERM activation via the RhoA/ROCK pathway.

Small G Proteins in the Cardiovascular System: Physiological and Pathological Aspects

Small G proteins exist in eukaryotes from yeast to human and constitute the Ras superfamily comprising more than 100 members. This superfamily is structurally classified into five families: the Ras, Rho, Rab, Arf, and Ran families that control a wide variety of cell and biological functions through highly coordinated regulation processes. Increasing evidence has accumulated to identify small G proteins and their regulators as key players of the cardiovascular physiology that control a large panel of cardiac (heart rhythm, contraction, hypertrophy) and vascular functions (angiogenesis, vascular permeability, vasoconstriction). Indeed, basal Ras protein activity is required for homeostatic functions in physiological conditions, but sustained overactivation of Ras proteins or spatiotemporal dysregulation of Ras signaling pathways has pathological consequences in the cardiovascular system. The primary object of this review is to provide a comprehensive overview of the current progress in our understanding of the role of small G proteins and their regulators in cardiovascular physiology and pathologies.

The Epac1 signaling pathway regulates Cl- secretion via modulation of apical KCNN4c channels in diarrhea

The apical membrane of intestinal epithelia expresses intermediate conductance K(+) channel (KCNN4), which provides the driving force for Cl(-) secretion. However, its role in diarrhea and regulation by Epac1 is unknown. Previously we have established that Epac1 upon binding of cAMP activates a PKA-independent mechanism of Cl(-) secretion via stimulation of Rap2-phospholipase Cε-[Ca(2+)]i signaling. Here we report that Epac1 regulates surface expression of KCNN4c channel through its downstream Rap1A-RhoA-Rho-associated kinase (ROCK) signaling pathway for sustained Cl(-) secretion. Depletion of Epac1 protein and apical addition of TRAM-34, a specific KCNN4 inhibitor, significantly abolished cAMP-stimulated Cl(-) secretion and apical K(+) conductance (IK(ap)) in T84WT cells. The current-voltage relationship of basolaterally permeabilized monolayers treated with Epac1 agonist 8-(4-chlorophenylthio)-2'-O- methyladenosine 3',5'-cyclic monophosphate showed the presence of an inwardly rectifying and TRAM-34-sensitive K(+) channel in T84WT cells that was absent in Epac1KDT84 cells. Reconstructed confocal images in Epac1KDT84 cells revealed redistribution of KCNN4c proteins into subapical intracellular compartment, and a biotinylation assay showed ∼83% lower surface expression of KCNN4c proteins compared with T84WT cells. Further investigation revealed that an Epac1 agonist activates Rap1 to facilitate IK(ap). Both RhoA inhibitor (GGTI298) and ROCK inhibitor (H1152) significantly reduced cAMP agonist-stimulated IK(ap), whereas the latter additionally reduced colocalization of KCNN4c with the apical membrane marker wheat germ agglutinin in T84WT cells. In vivo mouse ileal loop experiments showed reduced fluid accumulation by TRAM-34, GGTI298, or H1152 when injected together with cholera toxin into the loop. We conclude that Rap1A-dependent signaling of Epac1 involving RhoA-ROCK is an important regulator of intestinal fluid transport via modulation of apical KCNN4c channels, a finding with potential therapeutic value in diarrheal diseases.

Cholinergic regulation of epithelial sodium channels in rat alveolar type 2 epithelial cells

We and others have shown that epithelial Na(+) channels (ENaC) in alveolar type 2 (AT2) cells are activated by β2 agonists, steroid hormones, elevated oxygen tension, and by dopamine. Although acetylcholine receptors (AChRs) have been previously described in the lung, there are few reports of whether cholinergic agonists alter sodium transport in the alveolar epithelium. Therefore, we investigated how cholinergic receptors regulate ENaC activity in primary cultures of rat AT2 cells using cell-attached patch-clamp recordings to assess ENaC activity. We found that the muscarinic agonists, carbachol (CCh) and oxotremorine, activated ENaC in a dose-dependent manner but that nicotine did not. CCh-induced activation of ENaC was blocked by atropine. Western blotting and immunohistochemistry suggested that muscarinic M2 and M3 receptors (mAChRs) but not nicotinic receptors were present in AT2 cells. Endogenous RhoA and GTP-RhoA increased in response to CCh and the increase was reduced by pretreatment with atropine. We showed that Y-27632, an inhibitor of Rho-associated protein kinase (ROCK), abolished endogenous ENaC activity and inhibited the activation of ENaC by CCh. We also showed that ROCK signaling was necessary for ENaC stability in 2F3 cells, a model for AT2 cells. Our results showed that muscarinic agonists activated ENaC in rat AT2 cells through M2 and/or M3 mAChRs probably via a RhoA/ROCK signaling pathway.

Role of βPix in the Kidney

Small GTPases function as molecular switches in cell signaling, alternating between an inactive, GDP-bound state, and active GTP-bound state. βPix is one of guanine nucleotide exchange factors (GEFs) that catalyze the exchange of bound GDP for ambient GTP. The central goal of this review article is to summarize recent findings on βPix and the role it plays in kidney pathology and physiology. Recent studies shed new light on several key questions concerning the signaling mechanisms mediated by βPix. This manuscript provides a review of the various mechanisms whereby βPix has been shown to function within the kidney through a wide range of actions. Both canonical GEF activity and non-canonical signaling pathways mediated by βPix are discussed. Distribution patterns of βPix in the kidney will be also covered. Much has yet to be discerned, but it is clear that βPix plays a significant role in the kidney.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Non-genomic actions of aldosterone: from receptors and signals to membrane targets

In tissues which express the mineralocorticoid receptor (MR), aldosterone modulates the expression of membrane targets such as the subunits of the epithelial Na(+) channel, in combination with important signalling intermediates such as serum and glucocorticoid-regulated kinase-1. In addition, the rapid 'non-genomic' activation of protein kinases and secondary messenger signalling cascades has also been detected in aldosterone-sensitive tissues of the nephron, distal colon and cardiovascular system. These rapid actions are variously described as being coupled to MR or to an as yet unidentified, membrane-associated aldosterone receptor. The rapidly activated signalling cascades add a level of fine-tuning to the activity of aldosterone-responsive membrane transporters and also modulate the aldosterone-induced changes in gene expression through receptor and transcription factor phosphorylation.

Contribution of Rho-kinase to membrane excitability of murine colonic smooth muscle

BACKGROUND AND PURPOSE

The Rho-kinase pathway regulates agonist-induced contractions in several smooth muscles, including the intestine, urinary bladder and uterus, via dynamic changes in the Ca(2+) sensitivity of the contractile apparatus. However, there is evidence that Rho-kinase also modulates other cellular effectors such as ion channels.

EXPERIMENTAL APPROACH

We examined the regulation of colonic smooth muscle excitability by Rho-kinase using conventional microelectrode recording, isometric force measurements and patch-clamp techniques.

KEY RESULTS

The Rho-kinase inhibitors, Y-27632 and H-1152, decreased nerve-evoked on- and off-contractions elicited at a range of frequencies and durations. The Rho-kinase inhibitors decreased the spontaneous contractions and the responses to carbachol and substance P independently of neuronal inputs, suggesting Y-27632 acts directly on smooth muscle. The Rho-kinase inhibitors significantly reduced the depolarization in response to carbachol, an effect that cannot be due to regulation of Ca(2+) sensitization. Patch-clamp experiments showed that Rho-kinase inhibitors reduce GTPγS-activated non-selective cation currents.

CONCLUSIONS AND IMPLICATIONS

The Rho-kinase inhibitors decreased contractions evoked by nerve stimulation, carbachol and substance P. These effects were not solely due to inhibition of the Ca(2+) sensitization pathway, as the Rho-kinase inhibitors also inhibited the non-selective cation conductances activated by excitatory transmitters. Thus, Rho-kinase may regulate smooth muscle excitability mechanisms by regulating non-selective cation channels as well as changing the Ca(2+) sensitivity of the contractile apparatus.

Mechanisms underlying rapid aldosterone effects in the kidney

The steroid hormone aldosterone is a key regulator of electrolyte transport in the kidney and contributes to both homeostatic whole-body electrolyte balance and the development of renal and cardiovascular pathologies. Aldosterone exerts its action principally through the mineralocorticoid receptor (MR), which acts as a ligand-dependent transcription factor in target tissues. Aldosterone also stimulates the activation of protein kinases and secondary messenger signaling cascades that act independently on specific molecular targets in the cell membrane and also modulate the transcriptional action of aldosterone through MR. This review describes current knowledge regarding the mechanisms and targets of rapid aldosterone action in the nephron and how aldosterone integrates these responses into the regulation of renal physiology.

Cortical actin binding protein cortactin mediates ENaC activity via Arp2/3 complex

Epithelial Na(+) channel (ENaC) activity is regulated, in part, by the cortical cytoskeleton. Here we demonstrate that cortactin is highly expressed in the kidney cortex and polarized epithelial cells, and is localized to the cortical collecting duct. Coexpression of cortactin with ENaC decreases ENaC activity, as measured in patch-clamp experiments. Biotinylation experiments and single-channel analysis reveal that cortactin decreases ENaC activity via affecting channel open probability (P(o)). Knockdown of cortactin in mpkCCD(c14) principal cells results in an increase in ENaC activity and sodium reabsorption. Coimmunoprecipitation analysis shows direct interactions between cortactin and all three ENaC subunits in cultured and native cells. To address the question of what mechanism underlies the action of cortactin on ENaC activity, we assayed the effects of various mutants of cortactin. The data show that only a cortactin mutant unable to bind Arp2/3 complex does not influence ENaC activity. Furthermore, inhibitor of the Arp2/3 complex CK-0944666 precludes the effect of cortactin. Depolymerization of the actin microfilaments and inhibition of the Arp2/3 complex does not result in the loss of association between ENaC and cortactin. Thus, these results indicate that cortactin is functionally important for ENaC activity and that Arp2/3 complex is involved in this mechanism.

Novel role of Rac1/WAVE signaling mechanism in regulation of the epithelial Na+ channel

The epithelial Na(+) channel (ENaC) is an essential channel responsible for Na(+) reabsorption in the aldosterone-sensitive distal nephron. Consequently, ENaC is a major effector impacting systemic blood volume and pressure. We have shown recently that Rac1 increases ENaC activity, whereas Cdc42 fails to change channel activity. Here we tested whether Rac1 signaling plays a physiological role in modulating ENaC in native tissue and polarized epithelial cells. We found that Rac1 inhibitor NSC23766 markedly decreased ENaC activity in freshly isolated collecting ducts. Knockdown of Rac1 in native principal cells decreased ENaC-mediated sodium reabsorption and the number of channels at the apical plasma membrane. Members of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in the control of the actin cytoskeleton. N-WASP functions downstream of Cdc42, whereas WAVEs are effectors of Rac1 activity. N-WASP and all 3 isoforms of WAVE significantly increased ENaC activity when coexpressed in Chinese hamster ovary cells. However, wiskostatin, an inhibitor of N-WASP, had no effect on ENaC activity. Immunoblotting demonstrated the presence of WAVE1 and WAVE2 and absence of N-WASP and WAVE3 in mpkCCD(c14) and M-1 principal cells. Immunohistochemistry analysis also revealed localization of WAVE1 and WAVE2 but not N-WASP in the cortical collecting duct of Sprague-Dawley rat kidneys. Moreover, patch clamp analysis revealed that Rac1 and WAVE1/2 are parts of the same signaling pathway with respect to activation of ENaC. Thus, our findings suggest that Rac1 is essential for ENaC activity and regulates the channel via WAVE proteins.

Combined phosphoinositide and Ca2+ signals mediating receptor specificity toward neuronal Ca2+ channels

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) regulates Ca(2+) (I(Ca)) and M-type K(+) currents in superior cervical ganglion sympathetic neurons. In those cells, M(1) muscarinic and AT(1) angiotensin types do not elicit Ca(2+)(i) signals and suppress both currents via depletion of PIP(2), whereas the B(2) bradykinin and P2Y purinergic types elicit robust IP(3)-mediated [Ca(2+)](i) rises and neither deplete PIP(2) nor inhibit I(Ca). We have suggested that this specificity arises from differential Ca(2+)(i) signals underlying receptor-specific stimulation of PIP(2) synthesis by phosphatidylinositol (PI) 4-kinase. Here, we investigate which PI 4-kinase isoform underlies this signal, whether stimulation of PI 4-phosphate 5-kinase is also required, and the origin of receptor-specific Ca(2+)(i) signals. Recordings of I(Ca) were used as a PIP(2) "biosensor." In control, stimulation of M(1), but not B(2) or P2Y, receptors robustly suppressed I(Ca). However, when PI 4-kinase IIIβ, diacylglycerol kinase, Rho, or Rho-kinase was blocked, agonists of all three receptors robustly suppressed I(Ca). Overexpression of exogenous M(1) receptors yielded large [Ca(2+)](i) rises by muscarinic agonist, and transfection of wild-type IRBIT decreased Ca(2+)(i) signals, whereas dominant negative IRBIT-S68A had little effect on B(2) or P2Y responses but greatly increased muscarinic responses. We conclude that overlaid on microdomain organization is IRBIT, setting a "threshold" for [IP(3)], assisting in fidelity of receptor specificity.

Epidermal growth factor-mediated proliferation and sodium transport in normal and PKD epithelial cells

Members of the epidermal growth factor (EGF) family bind to ErbB (EGFR) family receptors which play an important role in the regulation of various fundamental cell processes including cell proliferation and differentiation. The normal rodent kidney has been shown to express at least three members of the ErbB receptor family and is a major site of EGF ligand synthesis. Polycystic kidney disease (PKD) is a group of diseases caused by mutations in single genes and is characterized by enlarged kidneys due to the formation of multiple cysts in both kidneys. Tubule cells proliferate, causing segmental dilation, in association with the abnormal deposition of several proteins. One of the first abnormalities described in cell biological studies of PKD pathogenesis was the abnormal mislocalization of the EGFR in cyst lining epithelial cells. The kidney collecting duct (CD) is predominantly an absorptive epithelium where electrogenic Na(+) entry is mediated by the epithelial Na(+) channel (ENaC). ENaC-mediated sodium absorption represents an important ion transport pathway in the CD that might be involved in the development of PKD. A role for EGF in the regulation of ENaC-mediated sodium absorption has been proposed. However, several investigations have reported contradictory results indicating opposite effects of EGF and its related factors on ENaC activity and sodium transport. Recent advances in understanding how proteins in the EGF family regulate the proliferation and sodium transport in normal and PKD epithelial cells are discussed here. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Protein kinase D1 modulates aldosterone-induced ENaC activity in a renal cortical collecting duct cell line

Aldosterone treatment of M1-CCD cells stimulated an increase in epithelial Na(+) channel (ENaC) alpha-subunit expression that was mainly localized to the apical membrane. PKD1-suppressed cells constitutively expressed ENaCalpha at low abundance, with no increase after aldosterone treatment. In the PKD1-suppressed cells, ENaCalpha was mainly localized proximal to the basolateral surface of the epithelium both before and after aldosterone treatment. Apical membrane insertion of ENaCbeta in response to aldosterone treatment was also sensitive to PKD1 suppression as was the aldosterone-induced rise in the amiloride-sensitive, trans-epithelial current (I(TE)). The interaction of the mineralocorticoid receptor (MR) with specific elements in the promoters of aldosterone responsive genes is stabilized by ligand interaction and phosphorylation. PKD1 suppression inhibited aldosterone-induced SGK-1 expression. The nuclear localization of MR was also blocked by PKD1 suppression and MEK antagonism implicating both these kinases in MR nuclear stabilization. PKD1 thus modulates aldosterone-induced ENaC activity through the modulation of sub-cellular trafficking and the stabilization of MR nuclear localization.

The actin cytoskeleton and small G protein RhoA are not involved in flow-dependent activation of ENaC

Background

Epithelial cells are exposed to a variety of mechanical stimuli. Epithelial Na⁺ channels (ENaC) mediate sodium transport across apical membranes of epithelial cells that line the distal nephron, airway and alveoli, and distal colon. Early investigations into stretch sensitivity of ENaC were controversial. However, recent studies are supportive of ENaC's mechanosensitivity. This work studied whether flow-dependent activation of ENaC is modulated by changes in the state of the actin cytoskeleton and whether small GTPase RhoA is involved in flow-mediated increase of ENaC activity.

Findings

Pretreatment with Cytochalasin D and Latrunculin B for 20 min and 1-2 hrs to disassemble F-actin had no effect on flow-mediated increase of amiloride-sensitive current. Overexpression of ENaC with constitutively active (G14V) or dominant negative (T19N) RhoA similarly had no effect on flow-dependent activation of ENaC activity. In addition, we did not observe changes when we inhibited Rho-kinase with Y27632.

Conclusions

Our results suggest that the flow-dependent activation of ENaC is not influenced by small GTPase RhoA and modifications in the actin cytoskeleton.

Endothelin-1 inhibits the epithelial Na+ channel through betaPix/14-3-3/Nedd4-2

Epithelial Na+ channels (ENaCs) mediate sodium reabsorption in the cortical collecting duct (CCD), but the regulatory pathways that modulate the activity of these channels are incompletely understood. Here, we observed that endothelin-1 (ET-1) attenuates ENaC activity acutely by reducing the channel's open probability and chronically by decreasing the number of channels in the plasma membrane. To investigate whether beta1Pix, a signaling protein activated by ET-1, mediates ENaC activity, we reconstituted ENaC in CHO cells with or without coexpressed beta1Pix and found that beta1Pix negatively regulates ENaC. Knockdown of betaPix in native principal cells abolished the ET-1-induced decrease in ENaC channel number. Furthermore, we found that betaPix does not decrease ENaC activity through its guanine nucleotide exchange factor (GEF) activity for Rac1 and Cdc42. Instead, coexpression of beta1Pix mutant constructs revealed that beta1Pix affects ENaC activity through binding 14-3-3 proteins. Coimmunoprecipitation experiments supported a physical interaction between beta1Pix and 14-3-3beta in cultured principal cells. Coexpression of 14-3-3beta increased ENaC activity in CHO cells, but concomitant expression of beta1Pix attenuated this increase. Recruitment of 14-3-3beta by beta1Pix impaired the interaction of 14-3-3beta with the ubiquitin ligase Nedd4-2, thereby promoting ubiquitination and degradation of ENaC. Taken together, these results suggest that the inhibitory effects of chronic ET-1 on ENaC result from betaPix interacting with the 14-3-3/Nedd4-2 pathway.

EGF and its related growth factors mediate sodium transport in mpkCCDc14 cells via ErbB2 (neu/HER-2) receptor

Amiloride-sensitive sodium entry, via the epithelial sodium channel (ENaC), is the rate-limiting step for Na(+) absorption. Epidermal growth factor (EGF) is involved in the regulation of Na(+) transport and ENaC activity. However it is still controversial exactly how EGF regulates ENaC and Na(+) absorption. The aim of the present study was to characterize the EGF regulation of Na(+) transport in cultured mouse renal collecting duct principal mpkCCD(c14) cells, a highly differentiated cell line which retains many characteristics of the cortical collecting duct (CCD). EGF dose dependently regulates basal transepithelial Na(+) transport in two phases: an acute phase (<4 h) and a chronic phase (>8 h). Similar effects were observed with TGF-alpha, HB-EGF, and amphiregulin which also belong to the EGF-related peptide growth factor family. Inhibition of MEK1/2 by PD98059 or U0126 increased acute effects and disrupted chronic effects of EGF on Na(+) reabsorption. Inhibition of PI3-kinase with LY294002 abolished acute effect of EGF. As assessed by Western blotting, ErbB2 is the most predominant member of the ErbB family detected in mpkCCD(c14) cells. Immunohistochemistry analysis revealed localization of ErbB2 in the CCD in Sprague-Dawley rat kidneys. Both acute and long-term effects of EGF were abolished when cells were treated with tyrphostin AG-825 and ErbB2 inhibitor II, chemically dissimilar selective inhibitors of the ErbB2 receptor. Thus, we conclude that EGF and its related growth factors are important for maintaining transepithelial Na(+) transport and that EGF biphasically modulates sodium transport in mpkCCD(c14) cells via the ErbB2 receptor.

Intact cytoskeleton is required for small G protein dependent activation of the epithelial Na+ channel

BACKGROUND

The Epithelial Na(+) Channel (ENaC) plays a central role in control of epithelial surface hydration and vascular volume. Similar to other ion channels, ENaC activity is regulated, in part, by cortical cytoskeleton. Besides, the cytoskeleton is an established target for small G proteins signaling. Here we studied whether ENaC activity is modulated by changes in the state of the cytoskeleton and whether cytoskeletal elements are involved in small G protein mediated increase of ENaC activity.

METHODS AND FINDINGS

First, the functional importance of the cytoskeleton was established with whole-cell patch clamp experiments recording ENaC reconstituted in CHO cells. Pretreatment with Cytochalasin D (CytD; 10 microg/ml; 1-2 h) or colchicine (500 microM; 1-3 h) to disassembly F-actin and destroy microtubules, respectively, significantly decreased amiloride sensitive current. However, acute application of CytD induced rapid increase in macroscopic current. Single channel measurements under cell-attached conditions revealed similar observations. CytD rapidly increased ENaC activity in freshly isolated rat collecting duct, polarized epithelial mouse mpkCCD(c14) cells and HEK293 cells transiently transfected with ENaC subunits. In contrast, colchicine did not have an acute effect on ENaC activity. Small G proteins RhoA, Rac1 and Rab11a markedly increase ENaC activity. 1-2 h treatment with colchicine or CytD abolished effects of these GTPases. Interestingly, when cells were coexpressed with ENaC and RhoA, short-term treatment with CytD decreased ENaC activity.

CONCLUSIONS

We conclude that cytoskeleton is involved in regulation of ENaC and is necessary for small G protein mediated increase of ENaC activity.

Regulation of Ca2+ entry by inositol lipids in mammalian cells by multiple mechanisms

Increased phosphoinositide turnover was first identified as an early signal transduction event initiated by cell surface receptors that were linked to calcium signaling. Subsequently, the generation of inositol 1,4,5-trisphosphate by phosphoinositide-specific phospholipase C enzymes was defined as the major link between inositide turnover and the cytosolic Ca(2+) rise in response to external stimulation. However, in the last decades, phosphoinositides have been emerging as major regulatory lipids involved in virtually every membrane-associated signaling process. Phosphoinositides regulate both the activity and the trafficking of almost all ion channels and transporters contributing to the maintenance of the ionic gradients that are essential for the proper functioning of all eukaryotic cells. Here we summarize the various means by which phosphoinositides affect ion channel functions with special emphasis on Ca(2+) signaling and outline the principles that govern the highly compartmentalized roles of these regulatory lipids.

Acquisition of brain Na sensitivity contributes to salt-induced sympathoexcitation and cardiac dysfunction in mice with pressure overload

In animal models of salt-sensitive hypertension, high salt augments sympathetic outflow via central mechanisms. It is not known, however, whether pressure overload affects salt sensitivity, thereby modifying central sympathetic outflow and cardiac function. We induced left ventricular hypertrophy with aortic banding in mice. Four weeks after aortic banding (AB-4), the left ventricle wall thickness was increased without changing the percentage fractional shortening. AB-4 mice were then fed either a high-salt (8%) diet or regular-salt diet for additional 4 weeks. Cardiac dysfunction, wall thickness, and 24-hour urinary catecholamine excretion were increased with high-salt diet compared with regular-salt diet. We then examined brain Na sensitivity. Intracerebroventricular infusion of high-Na (0.2 mol/L) artificial cerebrospinal fluid into AB-4 mice and mice Sham-4 increased urinary catecholamine excretion, arterial pressure, and heart rate more in AB-4 mice than in Sham-4 mice. Intracerebroventricular infusion of an epithelial Na channel blocker (benzamil) into mice with high-salt diet significantly decreased urinary catecholamine excretion and improved cardiac function. Infusion of either an angiotensin II type 1 receptor blocker or a Rho-kinase inhibitor also attenuated the salt-induced sympathetic hyperactivation and cardiac dysfunction in mice with high-salt diet. The levels of angiotensin II type 1 receptor and phosphorylated moesin, a substrate of Rho-kinase, were significantly greater in AB-4 mice than in Sham-4 mice. These results suggest that mice with pressure overload acquire brain Na sensitivity because of the activation of epithelial Na channel via Rho-kinase and angiotensin II, and this mechanism contributes to salt-induced sympathetic hyperactivation, further pressure overload, and cardiac dysfunction.

Paracrine regulation of the epithelial Na+ channel in the mammalian collecting duct by purinergic P2Y2 receptor tone

Growing evidence implicates a key role for extracellular nucleotides in cellular regulation, including of ion channels and renal function, but the mechanisms for such actions are inadequately defined. We investigated purinergic regulation of the epithelial Na+ channel (ENaC) in mammalian collecting duct. We find that ATP decreases ENaC activity in both mouse and rat collecting duct principal cells. ATP and other nucleotides, including UTP, decrease ENaC activity via apical P2Y2 receptors. ENaC in collecting ducts isolated from mice lacking this receptor have blunted responses to ATP. P2Y2 couples to ENaC via PLC; direct activation of PLC mimics ATP action. Tonic regulation of ENaC in the collecting duct occurs via locally released ATP; scavenging endogenous ATP and inhibiting P2 receptors, in the absence of other stimuli, rapidly increases ENaC activity. Moreover, ENaC has greater resting activity in collecting ducts from P2Y2-/- mice. Loss of collecting duct P2Y2 receptors in the knock-out mouse is the primary defect leading to increased ENaC activity based on the ability of direct PLC stimulation to decrease ENaC activity in collecting ducts from P2Y2-/- mice in a manner similar to ATP in collecting ducts from wild-type mice. These findings demonstrate that locally released ATP acts in an autocrine/paracrine manner to tonically regulate ENaC in mammalian collecting duct. Loss of this intrinsic regulation leads to ENaC hyperactivity and contributes to hypertension that occurs in P2Y2 receptor-/- mice. P2Y2 receptor activation by nucleotides thus provides physiologically important regulation of ENaC and electrolyte handling in mammalian kidney.

Regulation of ENaC expression at the cell surface by Rab11

The epithelial Na(+) channel (ENaC) is an essential channel responsible for Na(+) reabsorption. Coexpression of Rab11a and Rab3a small G proteins with ENaC results in a significant increase in channel activity. In contrast, coexpression of Rab5, Rab27a, and Arf-1 had no effect or slightly decreased ENaC activity. Inhibition of MEK with PD98059, Rho-kinase with Y27632 or PI3-kinase with LY294002 had no effect on ENaC activity in Rab11a-transfected CHO cells. Fluorescence imaging methods demonstrate that Rab11a colocalized with ENaC. Rab11a increases ENaC activity in an additive manner with dominant-negative dynamin, which is a GTPase responsible for endocytosis. Brefeldin A, an inhibitor of intracellular protein translocation, blocked the stimulatory action of Rab11a on ENaC activity. We conclude that ENaC channels, present on the apical plasma membrane, are being exchanged with channels from the intracellular pool in a Rab11-dependent manner.

[Expression and regulation of renal sodium-cotransporters and -antiporters, and related-transport proteins]

The authors' researches have been focused on pathogenic, physiological and biochemical mechanisms in hypertension and diabetes. Studies on hypertension were performed using salt-sensitive hypertensive Dahl rats as compared with the corresponding normotensive rats. Especially, implication with mobilization of electrolytes such as sodium, potassium, calcium and magnesium in hypertension gave rise to provocative to the author. Furthermore, complications of diabetes with hypertension were themes for the authors' researches. Thus, sodium-dependent glucose transport has been studied on sodium-dependent glucose transporters such as SGLT1 and SGLT2 using cell lines of porcelain renal cell, LLC-PK(1), and murine renal cell, NRK-52E. Relationship between magnesium mobilization and NO in hypertension has been explored using renal epithelial cell-lines and salt-sensitive hypertensive Dahl rats in the latter half of the author's research life.

Ion channel regulation by Ras, Rho, and Rab small GTPases

Regulation of ion channels by heterotrimeric guanosine triphosphatases (GTPases), activated by heptathelical membrane receptors, has been the focus of several recent reviews. In comparison, regulation of ion channels by small monomeric G proteins, activated by cytoplasmic guanine nucleotide exchange factors, has been less well reviewed. Small G proteins, molecular switches that control the activity of cellular and membrane proteins, regulate a wide variety of cell functions. Many upstream regulators and downstream effectors of small G proteins now have been isolated. Their modes of activation and action are understood. Recently, ion channels were recognized as physiologically important effectors of small GTPases. Recent advances in understanding how small G proteins regulate the intracellular trafficking and activity of ion channels are discussed here. We aim to provide critical insight into physiological control of ion channel function and the biological consequences of regulation of these important proteins by small, monomeric G proteins.

Aldosterone regulates rapid trafficking of epithelial sodium channel subunits in renal cortical collecting duct cells via protein kinase D activation

Aldosterone elicits rapid physiological responses in target tissues such as the distal nephron through the stimulation of cell signaling cascades. We identified protein kinase D (PKD1) as an early signaling response to aldosterone treatment in the M1-cortical collecting duct (M1-CCD) cell line. PKD1 activation was blocked by the PKC inhibitor chelerythrine chloride and by rottlerin, a specific inhibitor of PKCdelta. The activation of PKCdelta and PKCepsilon coincided with PKD1 activation and while a complex was formed between PKD1 and PKCepsilon after aldosterone treatment, there was a concurrent reduction in PKD1 association with PKCdelta. A stable PKD1 knockdown M1-CCD-derrived clone was developed in which PKD1 expression was 90% suppressed by gene silencing with a PKD1-specific siRNA. The effect of aldosterone treatment on the subcellular distribution of enhanced cyan fluorescent protein (eCFP)-tagged epithelial sodium channel (ENaC) subunits in wild type (WT) and PKD1 suppressed cells was examined using confocal microscopy. In an untreated confluent monolayer of M1-CCD cells, alpha, beta, and gamma ENaC subunits were evenly distributed throughout the cytoplasm of WT and PKD1-suppressed cells. After 2 min treatment, aldosterone stimulated the localization of each of the ENaC subunits to discrete regions within the cytoplasm of WT cells. The translocation of eCFP-ENaC subunits in WT cells was inhibited by rottlerin and the mineralocorticoid receptor (MR) antagonist spironolactone. No subcellular translocation of eCFP-ENaC subunits was observed in PKD1-suppressed cells treated with aldosterone. These data demonstrate the involvement of a novel MR/PKCdelta /PKD1 signaling cascade in the earliest ENaC subunit intracellular trafficking events that follow aldosterone treatment.

Molecular determinants of PI(4,5)P2 and PI(3,4,5)P3 regulation of the epithelial Na+ channel

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃) are physiologically important second messengers. These molecules bind effector proteins to modulate activity. Several types of ion channels, including the epithelial Na⁺ channel (ENaC), are phosphoinositide effectors capable of directly interacting with these signaling molecules. Little, however, is known of the regions within ENaC and other ion channels important to phosphoinositide binding and modulation. Moreover, the molecular mechanism of this regulation, in many instances, remains obscure. Here, we investigate modulation of ENaC by PI(3,4,5)P₃ and PI(4,5)P₂ to begin identifying the molecular determinants of this regulation. We identify intracellular regions near the inner membrane interface just following the second transmembrane domains in β- and γ- but not α-ENaC as necessary for PI(3,4,5)P₂ but not PI(4,5)P₂ modulation. Charge neutralization of conserved basic amino acids within these regions demonstrated that these polar residues are critical to phosphoinositide regulation. Single channel analysis, moreover, reveals that the regions just following the second transmembrane domains in β- and γ-ENaC are critical to PI(3,4,5)P₃ augmentation of ENaC open probability, thus, defining mechanism. Unexpectedly, intracellular domains within the extreme N terminus of β- and γ-ENaC were identified as being critical to down-regulation of ENaC activity and P_o in response to depletion of membrane PI(4,5)P₂. These regions of the channel played no identifiable role in a PI(3,4,5)P₃ response. Again, conserved positive-charged residues within these domains were particularly important, being necessary for exogenous PI(4,5)P₂ to increase open probability. We conclude that β and γ subunits bestow phosphoinositide sensitivity to ENaC with distinct regions of the channel being critical to regulation by PI(3,4,5)P₃ and PI(4,5)P₂. This argues that these phosphoinositides occupy distinct ligand-binding sites within ENaC to modulate open probability.

Phosphatidylinositol 4-phosphate 5-kinase reduces cell surface expression of the epithelial sodium channel (ENaC) in cultured collecting duct cells

Ubiquitination of ENaC subunits has been shown to negatively regulate the cell surface expression of ENaC channels. We have previously demonstrated that epsin links ubiquitinated ENaC to clathrin adaptors for clathrin-mediated endocytosis. Epsin is thought to directly modify the curvature of membranes upon binding to phosphatidylinositol 4,5-bisphosphate (PIP2) where it recruits clathrin and stimulates lattice assembly. Murine phosphatidylinositol 4-phosphate 5-kinase alpha (PI5KIalpha) has been shown to enhance endocytosis in a PIP2-dependent manner. We tested the hypothesis that PI5KIalpha-mediated PIP2 production would negatively regulate ENaC current by enhancing epsin-mediated endocytosis of the channel. Expression of PI5KIalpha decreased ENaC currents in Xenopus oocytes by 80%, entirely because of a decrease in cell surface ENaC levels. Catalytically inactive mutants of PI5Kalpha had no effect on ENaC activity. Expression of the PIP2 binding region of epsin increased ENaC current in oocytes, an effect completely reversed by co-expression of PI5KIalpha. Overexpression of epsin reduced amiloride-sensitive current in CCD cells. Overexpression of PI5KIalpha enhanced membrane PIP2 levels and reduced apical surface expression of ENaC in CCD cells, down-regulating amiloride-sensitive current. Knockdown of PI5KIalpha with isoform-specific siRNA resulted in a 4-fold enhancement of ENaC activity. PI5KIalpha localized exclusively to the apical plasma membrane domain when overexpressed in mouse CCD cells, consistent for a role in regulating PIP2 production at the apical plasma membrane. We conclude that membrane turnover events regulating ENaC surface expression and activity in oocytes and CCD cells can be regulated by PI5KIalpha.

Complex roles of PIP2 in the regulation of ion channels and transporters

The regulation of ion channels and transporters by phosphoinositides has received much attention over the past 10 years. There are multiple potential mechanisms for regulation of ion channels and transporters by PIP(2), including a direct binding of PIP(2) to the target proteins, alterations of membrane insertion, and retrieval. Added to the complexities of multiple potential mechanisms is how cells use PIP(2) to regulate so many different processes. Here, I briefly review several past and recent studies to illustrate the complexities and raise outstanding questions for future studies.

Purinergic control of apical plasma membrane PI(4,5)P2 levels sets ENaC activity in principal cells

Activity of the epithelial sodium channel (ENaC) is limiting for Na(+) reabsorption at the distal nephron. Phosphoinositides, such as phosphatidylinositol 4,5-biphosphate [PI(4,5)P(2)] modulate the activity of this channel. Activation of purinergic receptors triggers multiple events, including activation of PKC and PLC, with the latter depleting plasma membrane PI(4,5)P(2). Here, we investigate regulation of ENaC in renal principal cells by purinergic receptors via PLC and PI(4,5)P(2). Purinergic signaling rapidly decreases ENaC open probability and apical membrane PI(4,5)P(2) levels with similar time courses. Moreover, inhibiting purinergic signaling with suramin rescues ENaC activity. The PLC inhibitor U73122, but not U73343, its inactive analog, recapitulates the action of suramin. In contrast, modulating PKC signaling failed to affect purinergic regulation of ENaC. Unexpectedly, inhibiting either purinergic receptors or PLC in resting cells dramatically increased ENaC activity above basal levels, indicating tonic activation of purinergic signaling in these polarized renal epithelial cells. Increased ENaC activity was associated with elevation of apical membrane PI(4,5)P(2) levels. Subsequent treatment with ATP in the presence of inhibited purinergic signaling failed to decrease ENaC activity and apical membrane PI(4,5)P(2) levels. Dwell-time analysis reveals that depletion of PI(4,5)P(2) forces ENaC toward a closed state. In contrast, increasing PI(4,5)P(2) levels above basal values locks the channel in an open state interrupted by brief closings. Thus our results suggest that purinergic control of apical membrane PI(4,5)P(2) levels is a major regulator of ENaC activity in renal epithelial cells.