Renal denervation and CD161a immune ablation prevent cholinergic hypertension and renal sodium retention.

Cholinergic receptor activation leads to premature development of hypertension and infiltration of proinflammatory CD161a+/CD68+ M1 macrophages into the renal medulla. Renal inflammation is implicated in renal sodium retention and the development of hypertension. Renal denervation is known to decrease renal inflammation. The objective of this study was to determine the role of CD161a+/CD68+ macrophages and renal sympathetic nerves in cholinergic-hypertension and renal sodium retention. Bilateral renal nerve denervation (RND) and immune ablation of CD161a+ immune cells were performed in young prehypertensive spontaneously hypertensive rat (SHR) followed by infusion of either saline or nicotine (15 mg·kg-1·day-1) for 2 wk. Immune ablation was conducted by injection of unconjugated azide-free antibody targeting rat CD161a+. Blood pressure was monitored by tail cuff plethysmography. Tissues were harvested at the end of infusion. Nicotine induced premature hypertension, renal expression of the sodium-potassium chloride cotransporter (NKCC2), increases in renal sodium retention, and infiltration of CD161a+/CD68+ macrophages into the renal medulla. All of these effects were abrogated by RND and ablation of CD161a+ immune cells. Cholinergic activation of CD161a+ immune cells with nicotine leads to the premature development of hypertension in SHR. The effects of renal sympathetic nerves on chemotaxis of CD161a+ macrophages to the renal medulla, increased renal expression of NKCC2, and renal sodium retention contribute to cholinergic hypertension. The CD161a+ immune cells are necessary and essential for this prohypertensive nicotine-mediated inflammatory response.NEW & NOTEWORTHY This is the first study that describes a novel integrative physiological interaction between the adrenergic, cholinergic, and renal systems in the development of hypertension, describing data for the role of each in a genetic model of essential hypertension. Noteworthy findings include the prevention of nicotine-mediated hypertension following successful immune ablation of CD161a+ immune cells and the necessary role these cells play in the overexpression of the sodium-potassium-chloride cotransporter (NKCC2) in the renal medulla and renal sodium retention. Renal infiltration of these cells is demonstrated to be dependent on the presence of renal adrenergic innervation. These data offer a fertile ground of therapeutic potential for the treatment of hypertension as well as open the door for further investigation into the mechanism involved in inflammation-mediated renal sodium transporter expression. Taken together, these findings suggest immune therapy, renal denervation, and, possibly, other new molecular targets as having a potential role in the development and maintenance of essential hypertension.

Acetylation stimulates the epithelial sodium channel by reducing its ubiquitination and degradation.

The epithelial Na(+) channel (ENaC) functions as a pathway for Na(+) absorption in the kidney and lung, where it is crucial for Na(+) homeostasis and blood pressure regulation. ENaC is regulated in part through signaling pathways that control the ubiquitination state of ENaC lysines. A defect in ubiquitination causes Liddle syndrome, an inherited form of hypertension. Here we determined that α-, ß-, and γENaC are also substrates for lysine acetylation. Trichostatin A (TSA), a histone deacetylase inhibitor, enhanced ENaC acetylation and increased ENaC abundance in the total cell lysate and at the cell surface. Moreover, TSA increased ENaC current in Fischer rat thyroid and kidney collecting duct epithelia. We found that HDAC7 is expressed in the kidney collecting duct, supporting a potential role for this histone deacetylase in ENaC regulation. HDAC7 overexpression reduced ENaC abundance and ENaC current, whereas ENaC abundance and current were increased by silencing of HDAC7. ENaC and HDAC7 form a complex, as detected by coimmunoprecipitation. We observed a reciprocal relationship between acetylation and ubiquitination; TSA reduced ENaC ubiquitination, whereas HDAC7 increased ubiquitination. By reducing ENaC ubiquitination, TSA decreased the rate of ENaC degradation. Thus, acetylation increases epithelial Na(+) absorption by antagonizing ENaC ubiquitination. This stabilizes ENaC, and hence, increases its abundance at the cell surface.

Ubiquitin-specific peptidase 8 (USP8) regulates endosomal trafficking of the epithelial Na+ channel.

Ubiquitination plays a key role in trafficking of the epithelial Na(+) channel (ENaC). Previous work indicated that ubiquitination enhances ENaC endocytosis and sorting to lysosomes for degradation. Moreover, a defect in ubiquitination causes Liddle syndrome, an inherited form of hypertension. In this work, we identified a role for USP8 in the control of ENaC ubiquitination and trafficking. USP8 increased ENaC current in Xenopus oocytes and collecting duct epithelia and enhanced ENaC abundance at the cell surface in HEK 293 cells. This resulted from altered endocytic sorting; USP8 abolished ENaC degradation in the endocytic pathway, but it had no effect on ENaC endocytosis. USP8 interacted with ENaC, as detected by co-immunoprecipitation, and it deubiquitinated ENaC. Consistent with a functional role for deubiquitination, mutation of the cytoplasmic lysines of ENaC reduced the effect of USP8 on ENaC cell surface abundance. In contrast to USP8, USP2-45 increased ENaC surface abundance by reducing endocytosis but not degradation. Thus, USP8 and USP2-45 selectively modulate ENaC trafficking at different steps in the endocytic pathway. Together with previous work, the data indicate that the ubiquitination state of ENaC is critical for the regulation of epithelial Na(+) absorption.

Regulation of cardiac ATP-sensitive potassium channel surface expression by calcium/calmodulin-dependent protein kinase II.

Cardiac ATP-sensitive potassium (K(ATP)) channels are key sensors and effectors of the metabolic status of cardiomyocytes. Alteration in their expression impacts their effectiveness in maintaining cellular energy homeostasis and resistance to injury. We sought to determine how activation of calcium/calmodulin-dependent protein kinase II (CaMKII), a central regulator of calcium signaling, translates into reduced membrane expression and current capacity of cardiac K(ATP) channels. We used real-time monitoring of K(ATP) channel current density, immunohistochemistry, and biotinylation studies in isolated hearts and cardiomyocytes from wild-type and transgenic mice as well as HEK cells expressing wild-type and mutant K(ATP) channel subunits to track the dynamics of K(ATP) channel surface expression. Results showed that activation of CaMKII triggered dynamin-dependent internalization of K(ATP) channels. This process required phosphorylation of threonine at 180 and 224 and an intact (330)YSKF(333) endocytosis motif of the K(ATP) channel Kir6.2 pore-forming subunit. A molecular model of the µ2 subunit of the endocytosis adaptor protein, AP2, complexed with Kir6.2 predicted that µ2 docks by interaction with (330)YSKF(333) and Thr-180 on one and Thr-224 on the adjacent Kir6.2 subunit. Phosphorylation of Thr-180 and Thr-224 would favor interactions with the corresponding arginine- and lysine-rich loops on µ2. We concluded that calcium-dependent activation of CaMKII results in phosphorylation of Kir6.2, which promotes endocytosis of cardiac K(ATP) channel subunits. This mechanism couples the surface expression of cardiac K(ATP) channels with calcium signaling and reveals new targets to improve cardiac energy efficiency and stress resistance.

N-glycosylation of acid-sensing ion channel 1a regulates its trafficking and acidosis-induced spine remodeling.

Acid-sensing ion channel-1a (ASIC1a) is a potential therapeutic target for multiple neurological diseases. We studied here ASIC1a glycosylation and trafficking, two poorly understood processes pivotal in determining the functional outcome of an ion channel. We found that most ASIC1a in the mouse brain was fully glycosylated. Inhibiting glycosylation with tunicamycin reduced ASIC1a surface trafficking, dendritic targeting, and acid-activated current density. N-glycosylation of the two glycosylation sites, Asn393 and Asn366, has differential effects on ASIC1a biogenesis. Maturation of Asn393 increased ASIC1a surface and dendritic trafficking, pH sensitivity, and current density. In contrast, glycosylation of Asn366 was dispensable for ASIC1a function and may be a rate-limiting step in ASIC1a biogenesis. In addition, we revealed that acidosis reduced the density and length of dendritic spines in a time- and ASIC1a-dependent manner. ASIC1a N366Q, which showed increased glycosylation and dendritic targeting, potentiated acidosis-induced spine loss. Conversely, ASIC1a N393Q, which had diminished dendritic targeting and inhibited ASIC1a current dominant-negatively, had the opposite effect. These data tie N-glycosylation of ASIC1a with its trafficking. More importantly, by revealing a site-specific effect of acidosis on dendritic spines, our findings suggest that these processes have an important role in regulating synaptic plasticity and determining long-term consequences in diseases that generate acidosis.

Acid-sensing ion channels (ASICs) are differentially modulated by anions dependent on their subunit composition.

Acid-sensing ion channels (ASICs) are sodium channels gated by extracellular protons. ASIC1a channels possess intersubunit Cl(-)-binding sites in the extracellular domain, which are highly conserved between ASIC subunits. We previously found that anions modulate ASIC1a gating via these sites. Here we investigated the effect of anion substitution on native ASICs in rat sensory neurons and heterologously expressed ASIC2a and ASIC3 channels by whole cell patch clamp. Similar to ASIC1a, anions modulated the kinetics of desensitization of other ASIC channels. However, unlike ASIC1a, anions also modulated the pH dependence of activation. Moreover, the order of efficacy of different anions to modulate ASIC2a and -3 was very different from that of ASIC1a. More surprising, mutations of conserved residues that form an intersubunit Cl(-)-binding site in ASIC1a only partially abrogated the effects of anion modulation of ASIC2a and had no effect on anion modulation of ASIC3. The effects of anions on native ASICs in rat dorsal root ganglion neurons mimicked those in heterologously expressed ASIC1a/3 heteromeric channels. Our data show that anions modulate a variety of ASIC properties and are dependent on the subunit composition, and the mechanism of modulation for ASIC2a and -3 is distinct from that of ASIC1a. We speculate that modulation of ASIC gating by Cl(-) is a novel mechanism to sense shifts in extracellular fluid composition.

Regulation of epithelial sodium channel trafficking by proprotein convertase subtilisin/kexin type 9 (PCSK9).

The epithelial Na(+) channel (ENaC) is critical for Na(+) homeostasis and blood pressure control. Defects in its regulation cause inherited forms of hypertension and hypotension. Previous work found that ENaC gating is regulated by proteases through cleavage of the extracellular domains of the α and γ subunits. Here we tested the hypothesis that ENaC is regulated by proprotein convertase subtilisin/kexin type 9 (PCSK9), a protease that modulates the risk of cardiovascular disease. PCSK9 reduced ENaC current in Xenopus oocytes and in epithelia. This occurred through a decrease in ENaC protein at the cell surface and in the total cellular pool, an effect that did not require the catalytic activity of PCSK9. PCSK9 interacted with all three ENaC subunits and decreased their trafficking to the cell surface by increasing proteasomal degradation. In contrast to its previously reported effects on the LDL receptor, PCSK9 did not alter ENaC endocytosis or degradation of the pool of ENaC at the cell surface. These results support a role for PCSK9 in the regulation of ENaC trafficking in the biosynthetic pathway, likely by increasing endoplasmic reticulum-associated degradation. By reducing ENaC channel number, PCSK9 could modulate epithelial Na(+) absorption, a major contributor to blood pressure control.

Identification of extracellular domain residues required for epithelial Na+ channel activation by acidic pH.

A growing body of evidence suggests that the extracellular domain of the epithelial Na(+) channel (ENaC) functions as a sensor that fine tunes channel activity in response to changes in the extracellular environment. We previously found that acidic pH increases the activity of human ENaC, which results from a decrease in Na(+) self-inhibition. In the current work, we identified extracellular domain residues responsible for this regulation. We found that rat ENaC is less sensitive to pH than human ENaC, an effect mediated in part by the γ subunit. We identified a group of seven residues in the extracellular domain of γENaC (Asp-164, Gln-165, Asp-166, Glu-292, Asp-335, His-439, and Glu-455) that, when individually mutated to Ala, decreased proton activation of ENaC. γ(E455) is conserved in ßENaC (Glu-446); mutation of this residue to neutral amino acids (Ala, Cys) reduced ENaC stimulation by acidic pH, whereas reintroduction of a negative charge (by MTSES modification of Cys) restored pH regulation. Combination of the seven γENaC mutations with ß(E446A) generated a channel that was not activated by acidic pH, but inhibition by alkaline pH was intact. Moreover, these mutations reduced the effect of pH on Na(+) self-inhibition. Together, the data identify eight extracellular domain residues in human ß- and γENaC that are required for regulation by acidic pH.

Regulation of the delta and alpha epithelial sodium channel (ENaC) by ubiquitination and Nedd8.

The δ epithelial sodium channel (δENaC) is a proton-activated, sodium-selective, amiloride-sensitive ion channel in the ENaC/degenerin family of ion channels involved in blood pressure regulation and mechanosensation. Other ENaC family members are subject to ubiquitin modification leading to internalization from the cell surface, and degradation of the channel. Here, we show that δENaC is also modified by ubiquitin on three intracellular lysine residues. Absence of these lysines abolished ubiquitin modification of δENaC and increased cell surface levels of δENaC. Although the HECT-domain ubiquitin ligase Nedd4-2 reduced amiloride-sensitive current generated by δßγENaC-containing channels, δENaC does not contain a binding site for Nedd4-2; therefore, this effect is probably mediated by the ßγENaC subunits. Nedd8, a ubiquitin-like protein that regulates RING-domain E3 ubiquitin ligases, promoted δENaC ubiquitination, decreased both the intracellular and cell surface δENaC populations, and decreased δßγENaC amiloride-sensitive short circuit current (Isc -amiloride) in a mammalian epithelium. Nedd8 also promoted α- and γENaC ubiquitination, decreased the cell surface pools, and decreased αßγENaC Isc -amiloride. Conversely, XIAP, a single subunit RING E3 ligase, decreased ubiquitinated δENaC, increased the δENaC cell surface pool and increased δßγENaC Isc -amiloride. Therefore δ- and α - ßγENaC channel function may be influenced by RING-domain E3 ubiquitin ligases.

Identification of epithelial Na+ channel (ENaC) intersubunit Cl- inhibitory residues suggests a trimeric alpha gamma beta channel architecture.

The extracellular domain of the epithelial Na(+) channel (ENaC) is exposed to a wide range of anion concentrations in the kidney. We have previously demonstrated that extracellular Cl(-) inhibits ENaC activity. To identify sites involved in Cl(-) inhibition, we mutated residues in the extracellular domain of α-, ß-, and γENaC that are homologous to the Cl(-) binding site in acid-sensing ion channel 1a and tested the effect of Cl(-) on the activity of ENaC expressed in Xenopus oocytes. We identified two Cl(-) inhibitory sites in ENaC. One is formed by residues in the thumb domain of αENaC and the palm domain of ßENaC. Mutation of residues at this interface decreased Cl(-) inhibition and decreased Na(+) self-inhibition. The second site is formed by residues at the interface of the thumb domain of ßENaC and the palm domain of γENaC. Mutation of these residues also decreased Cl(-) inhibition yet had no effect on Na(+) self-inhibition. In contrast, mutations in the thumb domain of γENaC and palm of αENaC had little or no effect on Cl(-) inhibition or Na(+) self-inhibition. The data demonstrate that Cl(-) inhibits ENaC activity by two distinct Na(+)-dependent and Na(+)-independent mechanisms that correspond to the two functional Cl(-) inhibitory sites. Furthermore, based on the effects of mutagenesis on Cl(-) inhibition, the additive nature of mutations, and on differences in the mechanisms of Cl(-) inhibition, the data support a model in which ENaC subunits assemble in an αÎ³ß orientation (listed clockwise when viewed from the top).

Mutations of KCNJ10 together with mutations of SLC26A4 cause digenic nonsyndromic hearing loss associated with enlarged vestibular aqueduct syndrome.

Mutations in SLC26A4 cause nonsyndromic hearing loss associated with an enlarged vestibular aqueduct (EVA, also known as DFNB4) and Pendred syndrome (PS), the most common type of autosomal-recessive syndromic deafness. In many patients with an EVA/PS phenotype, mutation screening of SLC26A4 fails to identify two disease-causing allele variants. That a sizable fraction of patients carry only one SLC26A4 mutation suggests that EVA/PS is a complex disease involving other genetic factors. Here, we show that mutations in the inwardly rectifying K(+) channel gene KCNJ10 are associated with nonsyndromic hearing loss in carriers of SLC26A4 mutations with an EVA/PS phenotype. In probands from two families, we identified double heterozygosity in affected individuals. These persons carried single mutations in both SLC26A4 and KCNJ10. The identified SLC26A4 mutations have been previously implicated in EVA/PS, and the KCNJ10 mutations reduce K(+) conductance activity, which is critical for generating and maintaining the endocochlear potential. In addition, we show that haploinsufficiency of Slc26a4 in the Slc26a4(+/-) mouse mutant results in reduced protein expression of Kcnj10 in the stria vascularis of the inner ear. Our results link KCNJ10 mutations with EVA/PS and provide further support for the model of EVA/PS as a multigenic complex disease.

Oxidant regulated inter-subunit disulfide bond formation between ASIC1a subunits.

The acid-sensing ion channel-1a (ASIC1a) is composed of 3 subunits and is activated by a decrease in extracellular pH. It plays an important role in diseases associated with a reduced pH and production of oxidants. Previous work showed that oxidants reduce ASIC1a currents. However, the effects on channel structure and composition are unknown. We found that ASIC1a formed inter-subunit disulfide bonds and the oxidant H(2)O(2) increased this link between subunits. Cys-495 in the ASIC1a C terminus was particularly important for inter-subunit disulfide bond formation, although other C-terminal cysteines contributed. Inter-subunit disulfide bonds also produced some ASIC1a complexes larger than trimers. Inter-subunit disulfide bond formation reduced the proportion of ASIC1a located on the cell surface and contributed to the H(2)O(2)-induced decrease in H(+)-gated current. These results indicate that channel function is controlled by disulfide bond formation between intracellular residues on distinct ASIC1a subunits. They also suggest a mechanism by which the redox state can dynamically regulate membrane protein activity by forming intracellular bridges.

Hrs controls sorting of the epithelial Na+ channel between endosomal degradation and recycling pathways.

Epithelial Na(+) absorption is regulated by Nedd4-2, an E3 ubiquitin ligase that reduces expression of the epithelial Na(+) channel (ENaC) at the cell surface. Defects in this regulation cause Liddle syndrome, an inherited form of hypertension. Previous work found that Nedd4-2 functions through two distinct effects on trafficking, enhancing both ENaC endocytosis and ENaC degradation in lysosomes. To investigate the mechanism by which Nedd4-2 targets ENaC to lysosomes, we tested the role of hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), a component of the endosomal sorting complexes required for transport (ESCRT)-0 complex. We found that α-, ß-, and γENaC each interact with Hrs. These interactions were enhanced by Nedd4-2 and were dependent on the catalytic function of Nedd4-2 as well as its WW domains. Mutation of ENaC PY motifs, responsible for inherited hypertension (Liddle syndrome), decreased Hrs binding to ENaC. Moreover, binding of ENaC to Hrs was reduced by dexamethasone/serum- and glucocorticoid-inducible kinase and cAMP, which are signaling pathways that inhibit Nedd4-2. Nedd4-2 bound to Hrs and catalyzed Hrs ubiquitination but did not alter Hrs protein levels. Expression of a dominant negative Hrs lacking its ubiquitin-interacting motif (Hrs-ΔUIM) increased ENaC surface expression and current. This occurred through reduced degradation of the cell surface pool of proteolytically activated ENaC, which enhanced its recycling to the cell surface. In contrast, Hrs-ΔUIM had no effect on degradation of uncleaved inactive channels. The data support a model in which Nedd4-2 induces binding of ENaC to Hrs, which mediates the sorting decision between ENaC degradation and recycling.

Paradoxical Potentiation of Acid-Sensing Ion Channel 3 (ASIC3) by Amiloride via Multiple Mechanisms and Sites Within the Channel.

Acid-Sensing Ion Channels (ASICs) are proton-gated sodium-selective cation channels that have emerged as metabolic and pain sensors in peripheral sensory neurons and contribute to neurotransmission in the CNS. These channels and their related degenerin/epithelial sodium channel (DEG/ENaC) family are often characterized by their sensitivity to amiloride inhibition. However, amiloride can also cause paradoxical potentiation of ASIC currents under certain conditions. Here we characterized and investigated the determinants of paradoxical potentiation by amiloride on ASIC3 channels. While inhibiting currents induced by acidic pH, amiloride potentiated sustained currents at neutral pH activation. These effects were accompanied by alterations in gating properties including (1) an alkaline shift of pH-dependent activation, (2) inhibition of pH-dependent steady-state desensitization (SSD), (3) prolongation of desensitization kinetics, and (4) speeding of recovery from desensitization. Interestingly, extracellular Ca2+ was required for paradoxical potentiation and it diminishes the amiloride-induced inhibition of SSD. Site-directed mutagenesis within the extracellular non-proton ligand-sensing domain (E79A, E423A) demonstrated that these residues were critical in mediating the amiloride-induced inhibition of SSD. However, disruption of the purported amiloride binding site (G445C) within the channel pore blunted both the inhibition and potentiation of amiloride. Together, our results suggest that the myriad of modulatory and blocking effects of amiloride are the result of a complex competitive interaction between amiloride, Ca2+, and protons at probably more than one site in the channel.

Extracellular chloride regulates the epithelial sodium channel.

The extracellular domain of the epithelial sodium channel ENaC is exposed to a wide range of Cl(-) concentrations in the kidney and in other epithelia. We tested whether Cl(-) alters ENaC activity. In Xenopus oocytes expressing human ENaC, replacement of Cl(-) with SO4(2-), H2PO4(-), or SCN(-) produced a large increase in ENaC current, indicating that extracellular Cl(-) inhibits ENaC. Extracellular Cl(-) also inhibited ENaC in Na+-transporting epithelia. The anion selectivity sequence was SCN(-) < SO4(2-) < H2PO4(-) < F(-) < I(-) < Cl(-) < Br(-). Crystallization of ASIC1a revealed a Cl(-) binding site in the extracellular domain. We found that mutation of corresponding residues in ENaC (alpha(H418A) and beta(R388A)) disrupted the response to Cl(-), suggesting that Cl(-) might regulate ENaC through an analogous binding site. Maneuvers that lock ENaC in an open state (a DEG mutation and trypsin) abolished ENaC regulation by Cl(-). The response to Cl(-) was also modulated by changes in extracellular pH; acidic pH increased and alkaline pH reduced ENaC inhibition by Cl(-). Cl(-) regulated ENaC activity in part through enhanced Na+ self-inhibition, a process by which extracellular Na+ inhibits ENaC. Together, the data indicate that extracellular Cl(-) regulates ENaC activity, providing a potential mechanism by which changes in extracellular Cl(-) might modulate epithelial Na+ absorption.

A pharmacist intervention for monitoring and treating hypertension using bidirectional texting: PharmText BP.

BACKGROUND: New approaches are needed to better monitor blood pressure (BP) between physician visits, especially for patients in rural areas or for those who lack transportation. We have developed a custom-built bi-directional texting platform for home BP measurements that can then be managed by clinical pharmacists located remotely. The purpose of this study is to evaluate whether the BP texting approach combined with a pharmacist-based intervention improves BP management and to determine if the approach is cost effective. METHODS: This study is a randomized, prospective trial in four primary care offices that serve patients in rural areas. Subjects will receive standardized research BP measurements at baseline, 6 and 12 months. The primary outcome will be differences between the intervention and control group in mean systolic BP at 12 months. Secondary outcomes will include systolic BP at 6 months; diastolic BP at 6 and 12 months, number of medication changes and costs. CONCLUSIONS: This study plans to enroll subjects through 2022, follow-up will be completed in 2023 and results will be available in 2024. This study will provide information on whether a combined approach using texting of home BP values and a pharmacist-based telehealth services can improve BP control.

ASICs are required for immediate exercise-induced muscle pain and are downregulated in sensory neurons by exercise training.

Exercise training is an effective therapy for many pain-related conditions, and trained athletes have lower pain perception compared with unconditioned people. Some painful conditions, including strenuous exercise, are associated with elevated levels of protons, metabolites, and inflammatory factors, which may activate receptors and/or ion channels, including acid-sensing ion channels (ASICs), on nociceptive sensory neurons. We hypothesized that ASICs are required for immediate exercise-induced muscle pain (IEIP) and that exercise training diminishes IEIP by modulating ASICs within muscle afferents. We found high-intensity interval training (HIIT) reduced IEIP in C57BL/6 mice and diminished ASIC mRNA levels in lumber dorsal root ganglia, and this downregulation of ASICs correlated with improved exercise capacity. Additionally, we found that ASIC3 -/- mice did not develop IEIP; however, the exercise capacity of ASIC3 -/- was similar to wild-type mice. These results suggest that ASICs are required for IEIP and that diminishment of IEIP after exercise training correlates with downregulation of ASICs in sensory neurons.NEW & NOTEWORTHY Exercise performance can be limited by the sensations of muscle fatigue and pain transmitted by muscle afferents. It has been proposed that exercise training abrogates these negative feedback signals. We found that acid-sensing ion channels (ASICs) are required for immediate exercise-induced muscle pain (IEIP). Moreover, exercise training prevented IEIP and was correlated with downregulation of ASICs in sensory neurons.

The epithelial Na+ channel: cell surface insertion and retrieval in Na+ homeostasis and hypertension.

The epithelial Na+ channel (ENaC) forms the pathway for Na+ absorption in the kidney collecting duct and other epithelia. Dominant gain-of-function mutations cause Liddle's syndrome, an inherited form of hypertension resulting from excessive renal Na+ absorption. Conversely, loss-of-function mutations cause pseudohypoaldosteronism type I, a disorder of salt wasting and hypotension. Thus, ENaC has a critical role in the maintenance of Na+ homeostasis and blood pressure control. Altered Na+ absorption in the lung may also contribute to the pathogenesis of cystic fibrosis. Epithelial Na+ absorption is regulated in large part by mechanisms that control the expression of ENaC at the cell surface. Nedd4, a ubiquitin protein ligase, binds to ENaC and targets the channel for endocytosis and degradation. Liddle's syndrome mutations disrupt the interaction between ENaC and Nedd4, resulting in an increase in the number of ENaC channels at the cell surface. Aldosterone and vasopressin also regulate Na+ absorption to defend against hypotension and hypovolemia. Both hormones increase the expression of ENaC at the cell surface. The goal of this review is to summarize recent data on the regulation of ENaC expression at the cell surface.

Minireview: regulation of epithelial Na+ channel trafficking.

The epithelial Na(+) channel (ENaC) is a pathway for Na(+) transport across epithelia, including the kidney collecting duct, lung, and distal colon. ENaC is critical for Na(+) homeostasis and blood pressure control; defects in ENaC function and regulation are responsible for inherited forms of hypertension and hypotension and may contribute to the pathogenesis of cystic fibrosis and other lung diseases. An emerging theme is that epithelial Na(+) transport is regulated in large part through trafficking mechanisms that control ENaC expression at the cell surface. ENaC trafficking is regulated at multiple steps. Delivery of channels to the cell surface is regulated by aldosterone (and corticosteroids) and vasopressin, which increase ENaC synthesis and exocytosis, respectively. Conversely, endocytosis and degradation is controlled by a sequence located in the C terminus of alpha, beta, and gammaENaC (PPPXYXXL). This sequence functions as an endocytosis motif and as a binding site for Nedd4-2, an E3 ubiquitin protein ligase that targets ENaC for degradation. Mutations that delete or disrupt this motif cause accumulation of channels at the cell surface, resulting in Liddle's syndrome, an inherited form of hypertension. Nedd4-2 is a central convergence point for ENaC regulation by aldosterone and vasopressin; both induce phosphorylation of a common set of three Nedd4-2 residues, which blocks Nedd4-2 binding to ENaC. Thus, aldosterone and vasopressin regulate epithelial Na(+) transport in part by altering ENaC trafficking to and from the cell surface.