Purified epithelial Na+ channel complex contains the pertussis toxin-sensitive G alpha i-3 protein

Aldosterone-induced expression of ENaC-α is associated with activity of p65/p50 in renal epithelial cells

The amiloride-sensitive epithelial sodium channel (ENaC), located in the apical membrane in the cortical collecting duct of the kidney, mediates the fine-tuned regulation of external Na⁺ balance. Expression of the alpha-subunit of ENaC (ENaC-α) is regulated by a number of factors in the lung, including transcription factor nuclear factor kappa B (NF-κB). In the present study, we examined the effect of IKKβ/p65/p50 on ENaC-α in a murine cortical collecting duct cell line that endogenously expresses ENaC, mpkCCDc14 (CCD) cells. Aldosterone exposure led to up-regulation of ENaC-α and IKKβ, and nuclear p65 and p50. Knockdown of IKKβ or p65 exhibited >60 % reduction of aldosterone-induced ENaC-α mRNA levels. Chromatin immunoprecipitation and electrophoretic mobility shift assays demonstrated a specific interaction between p65/p50 and ENaC-α gene promoter, which was further confirmed using luciferase reporter-gene vectors transiently transfected into CCD cells. Taken together these data support an important role for p65/p50 in the direct regulation of ENaC-α transcription and have important implications for understanding the role of NF-κB in the regulation of renal function.

The inhibitory effect of Gβγ and Gβ isoform specificity on ENaC activity

Epithelial Na(+) channel (ENaC) activity, which determines the rate of renal Na(+) reabsorption, can be regulated by G protein-coupled receptors. Regulation of ENaC by Gα-mediated downstream effectors has been studied extensively, but the effect of Gβγ dimers on ENaC is unclear. A6 cells endogenously contain high levels of Gβ1 but low levels of Gβ3, Gβ4, and Gβ5 were detected by Q-PCR. We tested Gγ2 combined individually with Gβ1 through Gβ5 expressed in A6 cells, after which we recorded single-channel ENaC activity. Among the five β and γ2 combinations, β1γ2 strongly inhibits ENaC activity by reducing both ENaC channel number (N) and open probability (Po) compared with control cells. In contrast, the other four β-isoforms combined with γ2 have no significant effect on ENaC activity. By using various inhibitors to probe Gβ1γ2 effects on ENaC regulation, we found that Gβ1γ2-mediated ENaC inhibition involved activation of phospholipase C-β and its enzymatic products that induce protein kinase C and ERK1/2 signaling pathways.

On the segregation of protein ionic residues by charge type

Based on ubiquitous presence of large ionic motifs and clusters in proteins involved in gene transcription and protein synthesis, we analyzed the distribution of ionizable sidechains in a broad selection of proteins with regulatory, metabolic, structural and adhesive functions, in agonist, antagonist, toxin and antimicrobial peptides, and in self-excising inteins and intron-derived proteins and sequence constructs. All tested groups, regardless of taxa or sequence size, show considerable segregation of ionizable sidechains into same type charge (homoionic) tracts. These segments in most cases exceed half of the sequence length and comprise more than two-thirds of all ionizable sidechains. This distribution of ionic residues apparently reflects a fundamental advantage of sorted electrostatic contacts in association of sequence elements within and between polypeptides, as well as in interaction with polynucleotides. While large ionic densities are encountered in highly interactive proteins, the average ionic density in most sets does not change appreciably with size of the homoionic segments, which supports the segregation as a modular feature favoring association.

Scaffold protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3) coordinates assembly of a multiprotein epithelial sodium channel (ENaC)-regulatory complex

Hormone regulation of ion transport in the kidney tubules is essential for fluid and electrolyte homeostasis in vertebrates. A large body of evidence has suggested that transporters and channels exist in multiprotein regulatory complexes; however, relatively little is known about the composition of these complexes or their assembly. The epithelial sodium channel (ENaC) in particular is tightly regulated by the salt-regulatory hormone aldosterone, which acts at least in part by increasing expression of the serine-threonine kinase SGK1. Here we show that aldosterone induces the formation of a 1.0-1.2-MDa plasma membrane complex, which includes ENaC, SGK1, and the ENaC inhibitor Nedd4-2, a key target of SGK1. We further show that this complex contains the PDZ domain-containing protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3). CNK3 physically interacts with ENaC, Nedd4-2, and SGK1; enhances the interactions among them; and stimulates ENaC function in a PDZ domain-dependent, aldosterone-induced manner. These results strongly suggest that CNK3 is a molecular scaffold, which coordinates the assembly of a multiprotein ENaC-regulatory complex and hence plays a central role in Na(+) homeostasis.

Regulation of the epithelial sodium channel by phosphatidylinositides: experiments, implications, and speculations

Recent studies suggest that the activity of epithelial sodium channels (ENaC) is increased by phosphatidylinositides, especially phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)). Stimulation of phospholipase C by either adenosine triphosphate (ATP)-activation of purinergic P2Y receptors or epidermal growth factor (EGF)-activation of EGF receptors reduces membrane PI(4,5)P(2), and consequently decreases ENaC activity. Since ATP and EGF may be trapped in cysts formed by the distal tubule, it is possible that ENaC inhibition induced by ATP and EGF facilitates cyst formation in polycystic kidney diseases (PKD). However, some results suggest that ENaC activity is increased in PKD. In contrast to P2Y and EGF receptors, stimulation of insulin-like growth factor-1 (IGF-1) receptor by aldosterone or insulin produces PI(3,4,5)P(3), and consequently increases ENaC activity. The acute effect of aldosterone on ENaC activity through PI(3,4,5)P(3) possibly accounts for the initial feedback for blood volume recovery after hypovolemic hypotension. PI(4,5)P(2) and PI(3,4,5)P(3), respectively, interacts with the N terminus of beta-ENaC and the C terminus of gamma-ENaC. However, whether ENaC selectively binds to PI(4,5)P(2) and PI(3,4,5)P(3) over other anionic phospholipids remains unclear.

Characterization of Na+-permeable cation channels in LLC-PK1 renal epithelial cells

In this study, the presence of Na(+)-permeable cation channels was determined and characterized in LLC-PK1 cells, a renal tubular epithelial cell line with proximal tubule characteristics derived from pig kidney. Patch-clamp analysis under cell-attached conditions indicated the presence of spontaneously active Na(+)-permeable cation channels. The channels displayed nonrectifying single channel conductance of 11 pS, substates, and an approximately 3:1 Na(+)/K(+) permeability-selectivity ratio. The Na(+)-permeable cation channels were inhibited by pertussis toxin and reactivated by G protein agonists. Cation channel activity was observed in quiescent cell-attached patches after vasopressin stimulation. The addition of protein kinase A and ATP to excised patches also induced Na(+) channel activity. Spontaneous and vasopressin-induced Na(+) channel activity were inhibited by extracellular amiloride. To begin assessing potential molecular candidates for this cation channel, both reverse transcription-PCR and immunocytochemical analyses were conducted in LLC-PK1 cells. Expression of porcine orthologs of the alphaENaC and ApxL genes were found in LLC-PK1 cells. The expression of both gene products was confirmed by immunocytochemical analysis. Although alphaENaC labeling was mostly intracellular, ApxL labeled to both the apical membrane and cytoplasmic compartments of subconfluent LLC-PK1 cells. Vasopressin stimulation had no effect on alphaENaC immunolabeling but modified the cellular distribution of ApxL, consistent with an increased membrane-associated ApxL. The data indicate that proximal tubular LLC-PK1 renal epithelial cells express amiloride-sensitive, Na(+)-permeable cation channels, which are regulated by the cAMP pathway, and G proteins. This channel activity may implicate previously reported epithelial channel proteins, although this will require further experimentation. The evidence provides new clues as to potentially relevant Na(+) transport mechanisms in the mammalian proximal nephron.

Non-hydrolyzable analog of GTP induces activity of Na+ channels via disassembly of cortical actin cytoskeleton

The role of G proteins in regulation of non-voltage-gated Na+ channels in human myeloid leukemia K562 cells was studied by inside-out patch-clamp method. Na+ channels were activated by non-hydrolyzable analog of guanosine triphosphate (GTP), GTPgammaS, known to activate both heterotrimeric and small G proteins. Channel activity was not affected by aluminum fluoride that indiscriminately activates heterotrimeric G proteins. The effect of GTPgammaS was prevented by phalloidin and by G-actin, both interfering with actin disassembly, which indicates that GTPgammaS-induced channel activation was likely due to microfilament disruption. GTPgammaS-activated channels were inactivated by polymerizing actin. These data show, for the first time, that small G proteins can regulate Na+ channels, and an intracellular mechanism mediating their effect involves actin cytoskeleton rearrangements.

Phosphatidylinositol 4,5-bisphosphate (PIP2) stimulates epithelial sodium channel activity in A6 cells

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a membrane lipid found in all eukaryotic cells, which regulates many important cellular processes, including ion channel activity. In this study, we used inside-out patch clamp technique, immunoprecipitation, and Western blot analysis to investigate the effect of PIP(2) on epithelial sodium channel activity in A6 cells. A6 cells were cultured in media supplemented with 1.5 microm aldosterone. Single sodium channel activity in excised, inside-out patches was increased by perfusion of the bath solution with 30 microm PIP(2) plus 100 microm GTP (NP(o) = 1.34 +/- 0.14) compared with the paired control (NP(o) = 0.09 +/- 0.02). However, neither 30 microm PIP(2) (NP(o) = 0.11 +/- 0.02) nor 100 microm GTP (NP(o) = 0.10 +/- 0.02) alone stimulated the sodium channels. The PIP(2)-stimulated channel activity was abolished by application of 10 nm G protein betagamma subunits (NP(o) = 0.14 +/- 0.05). However, 10 nm Galpha(i-3) + 30 microm PIP(2) increased both NP(o) and P(o). The stimulating effect of 10 nm Galpha(i-3) + 30 microm PIP(2) is similar to that of 30 microm PIP(2) plus 100 microm GTP. Immunoprecipitation and Western blot analysis show that both Gi(alpha-3) and PIP(2) bind beta and gamma epithelial Na(+) channels (ENaC), but not alpha ENaC. These results indicate that PIP(2) increases ENaC activity by direct interaction with beta or gamma xENaC in the presence of Galpha(i-3).

Mechanisms of TNF-alpha stimulation of amiloride-sensitive sodium transport across alveolar epithelium

Because tumor necrosis factor (TNF)-alpha can upregulate alveolar fluid clearance (AFC) in pneumonia or septic peritonitis, the mechanisms responsible for the TNF-alpha-mediated increase in epithelial fluid transport were studied. In rats, 5 microg of TNF-alpha in the alveolar instillate increased AFC by 67%. This increase was inhibited by amiloride but not by propranolol. We also tested a triple-mutant TNF-alpha that is deficient in the lectinlike tip portion of the molecule responsible for its membrane conductance effect; the mutant also has decreased binding affinity to both TNF-alpha receptors. The triple-mutant TNF-alpha did not increase AFC. Perfusion of human A549 cells, patched in the whole cell mode, with TNF-alpha (120 ng/ml) resulted in a sustained increase in Na(+) currents from 82 +/- 9 to 549 +/- 146 pA (P < 0.005; n = 6). The TNF-alpha-elicited Na(+) current was inhibited by amiloride, and there was no change when A549 cells were perfused with the triple-mutant TNF-alpha or after preincubation with blocking antibodies to the two TNF-alpha receptors before perfusion with TNF-alpha. In conclusion, although TNF- alpha can initiate acute inflammation and edema formation in the lung, TNF-alpha can also increase AFC by an amiloride-sensitive, cAMP-independent mechanism that enhances the resolution of alveolar edema in pathological conditions by either binding to its receptors or activating Na(+) channels by means of its lectinlike domain.

Antibody-induced activation of beta1 integrin receptors stimulates cAMP-dependent migration of breast cells on laminin-5

The beta1 integrin-stimulating antibody TS2/16 induces cAMP-dependent migration of MCF-10A breast cells on the extracellular matrix protein laminin-5. TS2/16 stimulates a rise in intracellular cAMP within 20 min after plating. Pertussis toxin, which inhibits both antibody-induced migration and cAMP accumulation, targets the Galphai3 subunit of heterotrimeric G proteins in these cells, suggesting that Galphai3 may link integrin activation and migration via a cAMP signaling pathway.

Identification of Gbetagamma binding sites in the third intracellular loop of the M(3)-muscarinic receptor and their role in receptor regulation

Gbetagamma binds directly to the third intracellular (i3) loop subdomain of the M(3)-muscarinic receptor (MR). In this report, we identified the Gbetagamma binding motif and G-protein-coupled receptor kinase (GRK2) phosphorylation sites in the M(3)-MR i3 loop via a strategy of deletional and site-directed mutagenesis. The Gbetagamma binding domain was localized to Cys(289)-His(330) within the M(3)-MR-Arg(252)-Gln(490) i3 loop, and the binding properties (affinity, influence of ionic strength) of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain were similar to those observed for the entire i3 loop. Site-directed mutagenesis of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain indicated that Phe(312), Phe(314), and a negatively charged region (Glu(324)-Asp(329)) were required for interaction with Gbetagamma. Generation of the full-length M(3)-MR-Arg(252)-Gln(490) i3 peptides containing the F312A mutation were also deficient in Gbetagamma binding and exhibited a reduced capacity for phosphorylation by GRK2. A similar, parallel strategy resulted in identification of major residues ((331)SSS(333) and (348)SASS(351)) phosphorylated by GRK2, which were just downstream of the Gbetagamma binding motif. Full-length M(3)-MR constructs lacking the 42-amino acid Gbetagamma binding domain (Cys(289)-His(330)) or containing the F312A mutation exhibited ligand recognition properties similar to wild type receptor and also effectively mediated agonist-induced increases in intracellular calcium following receptor expression in Chinese hamster ovary and/or COS 7 cells. However, the M(3)-MRDeltaCys(289)-His(330) and M(3)-MR(F312A) constructs were deficient in agonist-induced sequestration, indicating a key role for the Gbetagamma-M(3)-MR i3 loop interaction in receptor regulation and signal processing.

Feedback inhibition of epithelial Na(+) channels in Xenopus oocytes does not require G(0) or G(i2) proteins

Regulation of amiloride-sensitive epithelial Na(+) channels (ENaC) is a prerequisite for coordination of electrolyte transport in epithelia. Downregulation of Na(+) conductance occurs when the intracellular Na(+) concentration is increased during reabsorption of electrolytes, known as feedback inhibition. Recent studies have demonstrated the involvement of alphaG(0) and alphaG(i2) proteins in the feedback control of ENaC in mouse salivary duct cells. In this report, we demonstrate that Na(+) feedback inhibition is also present in Xenopus oocytes after expression of rat alpha,beta, gamma-ENaC. Interfering with intracellular alphaG(0) or alphaG(i2) signaling by coexpression of either constitutively active alphaG(0)/alphaG(i2) or dominant negative alphaG(0)/alphaG(i2) and by coinjecting sense or antisense oligonucleotides for alphaG(0) had no impact on Na(+) feedback. Moreover, no evidence for involvement of the intracellular G protein cascade was found in experiments in which a regulator of G protein signaling (RGS3) or beta-adrenergic receptor kinase (betaARK) was coexpressed together with alpha,beta, gamma-ENaC. Although some experiments suggest the presence of an intracellular Na(+) receptor, we may conclude that Na(+) feedback in Xenopus oocytes is different from that described for salivary duct cells in that it does not require G protein signaling.

Perinatal PTX-sensitive G-protein expression and regulation of conductive 22Na+ transport in lung apical membrane vesicles

Using apical membrane vesicles (AMV) prepared from mature foetal and early neonatal guinea pig lung we show that pertussis toxin (PTX)-sensitive G-protein regulation of conductive 22Na+ uptake undergoes rapid changes following birth. Thus, G-protein activation by intravesicular incorporation of 100 microM GTPgammaS into vesicles resuspended in NaCl, which in late gestation stimulated uptake, consistently induced inhibition of conductive Na+ uptake into AMV prepared from neonatal lung at 4 days of age (N4) (52+/-9%, n=8, P<0.05). This response was not significantly different in the presence of the relatively impermeant anion isethionate (Ise-) (69+/-9%, n=7, P<0.05). Changes in the regulation of uptake were already detectable on the day of birth (N0) in AMV resuspended in NaCl, with GTPgammaS inducing both stimulatory and inhibitory responses. These data indicate that the processes by which 22Na+ uptake into AMV is regulated by G-proteins undergoes a change at birth and by 4 days of age, G-protein regulation of uptake occurs predominantly via modulation of co-localised Na+ channels. Intravesicular incorporation of GDPbetaS or pre-treatment with PTX did not significantly alter conductive 22Na+ uptake in the presence of NaCl or NaIse suggesting that constitutively active G-proteins are not involved in this process. Pre-treatment of AMV with PTX prevented the inhibition of conductive 22Na+ uptake by GTPgammaS (105+/-16% n=7) indicating that a PTX-sensitive G-protein mediates the inhibition of channels in neonatal AMV. Western blotting demonstrated enrichment of Gialpha1, Gialpha2, Gialpha3 and Goalpha in the apical membrane preparations. We also show that there is a significant rise in the levels of Gialpha3 during the early neonatal period providing a potential candidate for the G-protein mediated changes in regulation of conductive 22Na+ uptake in neonatal AMV.

Sodium channels in alveolar epithelial cells: molecular characterization, biophysical properties, and physiological significance

At birth, fetal distal lung epithelial (FDLE) cells switch from active chloride secretion to active sodium (Na+) reabsorption. Sodium ions enter the FDLE and alveolar type II (ATII) cells mainly through apical nonselective cation and Na(+)-selective channels, with conductances of 4-26 pS (picoSiemens) in FDLE and 20-25 pS in ATII cells. All these channels are inhibited by amiloride with a 50% inhibitory concentration of < 1 microM, and some are also inhibited by [N-ethyl-N-isopropyl]-2'-4'-amiloride (50% inhibitory concentration of < 1 microM). Both FDLE and ATII cells contain the alpha-, beta-, and gamma-rENaC (rat epithelial Na+ channels) mRNAs; reconstitution of an ATII cell Na(+)-channel protein into lipid bilayers revealed the presence of 25-pS Na+ single channels, inhibited by amiloride and [N-ethyl-N-isopropyl]-2'-4'-amiloride. A variety of agents, including cAMP, oxygen, glucocorticoids, and in some cases Ca2+, increased the activity and/or rENaC mRNA levels. The phenotypic properties of these channels differ from those observed in other Na(+)-absorbing epithelia. Pharmacological blockade of alveolar Na+ transport in vivo, as well as experiments with newborn alpha-rENaC knock-out mice, demonstrate the importance of active Na+ transport in the reabsorption of fluid from the fetal lung and in reabsorbing alveolar fluid in the injured adult lung. Indeed, in a number of inflammatory diseases, increased production of reactive oxygen-nitrogen intermediates, such as peroxynitrite (ONOO-), may damage ATII and FDLE Na+ channels, decrease Na+ reabsorption in vivo, and thus contribute to the formation of alveolar edema.

Intracellular H+ regulates the alpha-subunit of ENaC, the epithelial Na+ channel

Protons regulate electrogenic sodium absorption in a variety of epithelia, including the cortical collecting duct, frog skin, and urinary bladder. Recently, three subunits (alpha, beta, gamma) coding for the epithelial sodium channel (ENaC) were cloned. However, it is not known whether pH regulates Na+ channels directly by interacting with one of the three ENaC subunits or indirectly by interacting with a regulatory protein. As a first step to identifying the molecular mechanisms of proton-mediated regulation of apical membrane Na+ permeability in epithelia, we examined the effect of pH on the biophysical properties of ENaC. To this end, we expressed various combinations of alpha-, beta-, and gamma-subunits of ENaC in Xenopus oocytes and studied ENaC currents by the two-electrode voltage-clamp and patch-clamp techniques. In addition, the effect of pH on the alpha-ENaC subunit was examined in planar lipid bilayers. We report that alpha,beta,gamma-ENaC currents were regulated by changes in intracellular pH (pHi) but not by changes in extracellular pH (pHo). Acidification reduced and alkalization increased channel activity by a voltage-independent mechanism. Moreover, a reduction of pHi reduced single-channel open probability, reduced single-channel open time, and increased single-channel closed time without altering single-channel conductance. Acidification of the cytoplasmic solution also inhibited alpha, beta-ENaC, alpha,gamma-ENaC, and alpha-ENaC currents. We conclude that pHi but not pHo regulates ENaC and that the alpha-ENaC subunit is regulated directly by pHi.

Guanine nucleotide binding proteins in cultured renal epithelia: studies with pertussis toxin and aldosterone

The GTP-binding proteins from cultured A6 epithelia were examined in isolated membrane preparations. Binding of [35S]GTPgammaS revealed a class of binding sites with an apparent Kd value of 100 nM and a Bmax of 220 pmol/mg protein. Short-term aldosterone treatment of the cells did not modify the binding kinetics, whereas pertussis toxin (PTX) decreased Bmax by 50%. The mRNA levels for Galphai-3, Galpha0, Galphas, and Galphaq were not increased after aldosterone. The patterns of small Mr G proteins and of PTX-ribosylated proteins were identical in membranes of both control and aldosterone-treated cells. Cross-linking of [alpha-32P]GTP, in control membranes, showed either no labeling or a faint band of Mr 59.5 kDa. This protein became prominent after aldosterone, and its labeling decreased with spironolactone. Thus short-term aldosterone does not promote increased expression of known heterotrimeric G proteins in epithelial membranes but activates resident PTX-sensitive Gi proteins and stimulates the expression of a specific GTP-binding protein of Mr 59.5 kDa.

CFTR is a conductance regulator as well as a chloride channel

CFTR Is a Conductance Regulator as well as a Chloride Channel. Physiol. Rev. 79, Suppl.: S145-S166, 1999. - Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter gene family. Although CFTR has the structure of a transporter that transports substrates across the membrane in a nonconductive manner, CFTR also has the intrinsic ability to conduct Cl- at much higher rates, a function unique to CFTR among this family of ABC transporters. Because Cl- transport was shown to be lost in cystic fibrosis (CF) epithelia long before the cloning of the CF gene and CFTR, CFTR Cl- channel function was considered to be paramount. Another equally valid perspective of CFTR, however, derives from its membership in a family of transporters that transports a multitude of different substances from chemotherapeutic drugs, to amino acids, to glutathione conjugates, to small peptides in a nonconductive manner. Moreover, at least two members of this ABC transporter family (mdr-1, SUR) can regulate other ion channels in the membrane. More simply, ABC transporters can regulate somehow the function of other cellular proteins or cellular functions. This review focuses on a plethora of studies showing that CFTR also regulates other ion channel proteins. It is the hope of the authors that the reader will take with him or her the message that CFTR is a conductance regulator as well as a Cl- channel.

Differential regulation of Na+ and Cl- conductances by PTX-sensitive G proteins in fetal lung apical membrane vesicles

In apical membrane vesicles (AMV) prepared from late gestation fetal guinea pig lung we show that conductive 22Na+ uptake is modulated by at least two pathways involving pertussis toxin (PTX)-sensitive G proteins. Intravesicular incorporation of 100 microM GTPgammaS into vesicles resuspended in NaCl caused a significant stimulation (P<0. 05) of conductive Na+ uptake in AMV to 150+/-10% (n=10) of control, whereas GDPbetaS reduced uptake to 65+/-9% (n=4) of control. This contrasting response to GTPgammaS and GDPbetaS is characteristic of a G protein mediated pathway. GTPgammaS induced a significantly smaller stimulation, 125+/-8% (n=5) of control, in the presence of the relatively impermeant anion isethionate (Ise-). Taken together, these data indicate modulation of both Na+ and Cl- channels in the apical membrane by co-localised G protein(s). Treatment with PTX stimulated conductive 22Na+ uptake to 171+/-20% (n=13) of control in AMV resuspended in NaCl, but did not have a significant effect, 94+/-19% of control, in the presence of NaIse indicating the existence of tonic activation of Cl- channels in these AMV under resting conditions. As the combined effects of PTX and GTPgammaS diminished uptake, we propose that the G protein(s) responsible for Na+ channel activation in response to GTPgammaS is PTX-sensitive and that additional PTX-insensitive G proteins might also modulate 22Na+ uptake in these AMV. The presence of Gialpha1, Gialpha2, Gialpha3 and Goalpha in this apical membrane preparation was confirmed by PTX catalysed [32P]ADP-dependent ribosylation and Western blotting. Incubation of AMV with 200 microM DTT caused an inhibition of conductive Na+ uptake in AMV resuspended in NaCl or NaIse to 66+/-8% (n=11) and 64+/-8% (n=6) of control respectively. Pre-treatment with DTT did not affect the ability of GTPgammaS to stimulate conductive Na+ uptake suggesting that the regulation of 22Na+ uptake in late gestation guinea pig fetal lung AMV is unlikely to involve an associated regulatory protein.

Cytochemical localization of adenylate cyclase in cultured renal epithelial (A6) cells

To characterize the vasopressin-adenylate cyclase (AC) signaling pathway in control of Na+ reabsorption in cultured renal (A6) cells, we determined the distribution of AC with a cytochemical technique using 5'-adenylylimidodiphosphate as substrate and cerium chloride as capturing agent. The addition of forskolin to the medium to stimulate AC activity increased the production of reaction deposits at the enzyme sites. To ensure that the cells were close to their physiological states, cytochemical reactions were performed on unfixed tissues. Subsequent postfixation adequately preserved the morphological features of the cells. AC was mainly restricted to the lateral folds of the cells while the apical membranes were devoid of any deposits. This result provided evidence that the V2-AC pathway is not present in the apical membrane and, hence, any vasopressin action on apical Na+ channels from the luminal side of the cell must involve other signaling pathways. The cytochemical results provided further morphological evidence of the functional coupling between the basolateral and apical membranes of renal cells. We examined the idea that highly variable basal rates of Na+ transport in young differentiating cell cultures may be related to the degree of AC activity. Cytochemical results apparently revealed highly variable amounts of deposits in these cells, but by quantitative analysis of AC activity we could find no significant differences between cells of 6, 14, and 21 days.

Microtubule disruption inhibits AVT-stimulated Cl- secretion but not Na+ reabsorption in A6 cells

The effects of microtubule disruption on arginine vasotocin (AVT)-stimulated Na+ and Cl- transport were studied in A6 cells by measuring short-circuit currents (Isc) across cell layers grown in tissue culture on permeable supports. Microtubule disruption inhibited an AVT-stimulated secretory Cl- current but did not prevent activation of amiloride-sensitive Na+ transport. This AVT-stimulated secretory Cl- current was significantly inhibited by glibenclamide, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR). Reverse transcription of RNA isolated from A6 cells followed by polymerase chain reaction (PCR) using primers designed to amplify a portion of the R-domain of CFTR cloned from Xenopus laevis skin and immunocytochemistry demonstrated the presence of CFTR in A6 cells and an apparent recruitment of cytoplasmic CFTR to the apical cell surface after AVT stimulation. In contrast, indirect immunofluorescent labeling of Na+ channels using a polyclonal antibody raised against a biochemically isolated Na+ channel complex from bovine renal medulla labeled the apical plasma membrane but failed to demonstrate intracellular labeling of Na+ channels (except in subconfluent cells) or recruitment of Na+ channels to the apical membrane region after AVT stimulation.

Protein prenylation is required for aldosterone-stimulated Na+ transport

Aldosterone stimulation of transcellular Na+ flux in polarized epithelial cells is dependent on at least one transmethylation reaction, but the substrate of this signaling step is unknown. Because it is clear that the majority of cellular protein methylation occurs in conjunction with protein prenylation, we examined the importance of prenylation to aldosterone-stimulated Na+ transport in the A6 cell line. Lovastatin, an inhibitor of the first committed step of the mevalonate pathway, inhibits the natriferic effect of aldosterone but does not inhibit insulin-stimulated Na+ flux. The addition of a farnesyl group does not appear to be involved in aldosterone's action. Neither alpha-hydroxyfarne-sylphosphonic acid, an inhibitor of farnesyl:protein transferase, nor N-acetyl-S-farnesyl-L-cysteine, an inhibitor of farnesylated protein methylation, inhibits the hormone-induced increase in Na+ transport. In contrast, N-acetyl-S-geranyl-geranyl-L-cysteine, an inhibitor of geranylgeranyl protein methylation, completely abolishes the aldosterone-induced increase in Na+ flux with no effect on insulin-mediated Na+ transport or cellular protein content. These data indicate that methylation of a geranylgeranylated protein is involved in aldosterone's natriferic action.

Protein kinase A phosphorylation and G protein regulation of type II pneumocyte Na+ channels in lipid bilayers

Protein kinase A (PKA)- and G protein-mediated regulation of immunopurified adult rabbit alveolar epithelial type II (ATII) cell proteins that exhibit amiloride-sensitive Na+ channel activity was studied in planar lipid bilayers and freshly isolated ATII cells. Addition of the catalytic subunit of PKA + ATP increased single channel open probability from 0.42 +/- 0.05 to 0.82 +/- 0.07 in a voltage-independent manner, without affecting unitary conductance. This increase in open probability of the channels was mainly due to a decrease in the time spent by the channel in its closed state. The apparent inhibition constant for amiloride increased from 8.0 +/- 1.8 microM under control conditions to 15 +/- 3 microM after PKA-induced phosphorylation; that for ethylisopropylamiloride increased from 1.0 +/- 0.4 to 2.0 +/- 0.5 microM. Neither pertussis toxin (PTX) nor guanosine 5'-O-(3-thiotriphosphate) affected ATII Na+ channel activity in bilayers. Moreover, PTX failed to affect amiloride-inhibitable 22Na+ uptake in freshly isolated ATII cells. In vitro, ADP ribosylation induced by PTX revealed the presence of a specifically ribosylated band at 40-45 kDa in the total solubilized ATII cell protein fraction, but not in the immunopurified fraction. Moreover, the immunopurified channel was downregulated in response to guanosine 5'-O-(3-thiotriphosphate)-mediated activation of the exogenous G alpha(i-2), but not G(oA), G alpha(i-1), or G alpha(i-3), protein added to the channel. This effect occurred only in the presence of actin. These results suggest that amiloride-sensitive Na+ channels in adult alveolar epithelia regulated by PKA-mediated phosphorylation also retain the ability to be regulated by G alpha([i-2), but not G alpha([i-1) or G alpha(i-3), protein.

Molecular modeling of mechanotransduction in the nematode Caenorhabditis elegans

Genetic and molecular studies of touch avoidance in the nematode Caenorhabditis elegans have resulted in a molecular model for a mechanotransducing complex. mec-4 and mec-10 encode proteins hypothesized to be subunits of a mechanically gated ion channel that are related to subunits of the vertebrate amiloride-sensitive epithelial Na+ channel. Products of mec-5, a novel collagen, and mec-9, a protein that includes multiple Kunitz-type protease inhibitor repeats and EGF repeats, may interact with the channel in the extracellular matrix. Inside the cell, specialized 15-protofilament microtubules composed of mec-12 alpha-tubulin and mec-7 beta-tubulin may be linked to the mechanosensitive channel by stomatin-homologous MEC-2. MEC-4 and MEC-10 are members of a large family of C. elegans proteins, the degenerins. Two other degenerins, UNC-8 and DEL-1, are candidate components of a stretch-sensitive channel in motor neurons. Implications for advancing understanding of mechanotransduction in other systems are discussed.

Diversity and regulation of amiloride-sensitive Na+ channels

Amiloride-sensitive Na+ channels play a vital role in many important physiological processes such as delineation of the final urine composition, sensory transduction, and whole-body Na+ homeostasis. These channels display a wide range of biophysical properties, and are regulated by cAMP-mediated second messenger systems. The first of these channels has recently been cloned. This cloned amiloride-sensitive Na+ channel is termined ENaC (Epithelial Na+ Channel) and, in heterologous cellular expression systems, displays a single channel conductance of 4 to 7 pS, a high PNa/PK (> 10), a high amiloride sensitivity (Ki(amil) = 150 nM), and relatively long open and closed times. ENaC may form the core conduction element of many of these functionally diverse forms of Na+ channel. The kinetic and regulatory differences between these channels may be due, in large measure, to unique polypeptides that associate with the core element, forming a functional channel unit.

G-protein regulation of outwardly rectified epithelial chloride channels incorporated into planar bilayer membranes

Experiments were designed to test if immunopurified outwardly rectified chloride channels (ORCCs) and the cystic fibrosis transmembrane conductance regulator (CFTR) incorporated into planar lipid bilayers are regulated by G-proteins. pertussis toxin (PTX) (100 ng/ml) + NAD (1 mM) + ATP (1 mM) treatment of ORCC and CFTR in bilayers resulted in a 2-fold increase in single channel open probability (Po) of ORCC but not of CFTR. Neither PTX, NAD, nor ATP alone affected the biophysical properties of either channel. Further, PTX conferred a linearity to the ORCC current-voltage curve, with a slope conductance of 80 +/- 3 picosiemens (pS) in the +/- 100 mV range of holding potentials. PKA-mediated phosphorylation of these PTX + NAD-treated channels further increased the Po of the linear 80-pS channels from 0.66 +/- 0.05 to >0.9, and revealed the presence of a small (16 +/- 2 pS) linear channel in the membrane. PTX treatment of a CFTR-immunodepleted protein preparation incorporated into bilayer membranes resulted in a similar increase in the Po of the larger conductance channel and restored PKA-sensitivity that was lost after CFTR immunodepletion. The addition of guanosine 5'-3-O-(thio)triphosphate (100 mum) to the cytoplasmic bathing solutions decreased the activity of the ORCC and increased its rectification at both negative and positive voltages. ADP-ribosylation of immunopurified material revealed the presence of a 41-kDa protein. These results demonstrate copurification of a channel-associated G-protein that is involved in the regulation of ORCC function.

Regulation of a sodium channel-associated G-protein by aldosterone

The action of aldosterone to increase apical membrane permeability in responsive epithelia is thought to be due to activation of sodium channels. This channel is regulated, in part, by G-proteins, but it is not known if this mechanism is regulated by aldosterone. We report that aldosterone stimulates the expression of the 41-kDa alphai3 subunit of the heterotrimeric GTP-binding proteins in A-6 cells. Both mRNA and the total amount of this protein are increased by aldosterone. The G-protein is palmitoylated in response to the steroid, and the newly synthesized subunit is found to co-localize with the sodium channel. Aldosterone stimulation of sodium transport is significantly inhibited by inhibition of palmitoylation. These results suggest that aldosterone regulates sodium channel activity in epithelia through stimulation of the expression and post-translational targeting of a channel regulatory G-protein subunit.

Peptide block of constitutively activated Na+ channels in Liddle's disease

Hypertension is a multifactorial disorder that results in an increased risk of cardiovascular and end-stage renal disease. Liddle's disease represents a specific hypertensive disease and expresses itself in the human population as an autosomal dominant trait. Recent experimental evidence indicates that patients with Liddle's disease have constitutively active amiloride-sensitive Na+ channels and that these channels are phenotypically expressed in lymphocytes obtained from normal and affected members of the original Liddle's kindred. Linkage analysis indicates that this disease results from a deletion of the carboxy-terminal region of the beta-subunit of a recently cloned epithelial Na+ channel (ENaC). We report the successful immunopurification and reconstitution of both normal and constitutively active lymphocyte Na+ channels into planar lipid bilayers. These channels display all of the characteristics typical of renal Na+ channels, including sensitivity to protein kinase A phosphorylation. We demonstrate that gating of normal Na+ channels is removed by cytoplasmic trypsin digestion and that the constitutively active Liddle's Na+ channels are blocked by a beta- or gamma-ENaC carboxy-terminal peptide in a GTP-dependent fashion.

Effects of phosphorylation on ion channel function

A fundamental property of ion channels is their ability to be modulated by intracellular second messenger systems acting via covalent modifications of the channel protein itself. One such important biochemical reaction is phosphorylation on serine, threonine, and tyrosine residues. Ion channels in the kidney are no exception. Moreover, many ion channels, including many amiloride-sensitive epithelial Na+ channels, are subject to modulation by a multiplicity of inputs. For example, renal Na+ channels are not gated by voltage in their unphosphorylated state. However, upon phosphorylation by PKA plus ATP, these channels become voltage-dependent as well as having their open probability increased. Phosphorylation by PKC inhibits channel activity regardless of whether the channel was previously phosphorylated by PKA. Likewise, Na+ channel ADP-ribosylation by PTX overrides the actions of cAMP-dependent phosphorylation. Consistent with this idea is the fact that the phosphorylation sites for PKA and PKC and the ADP-ribosylation sites occur on different polypeptides comprising the channel complex. Epithelial Na+ channel activity is also regulated by methylation, arachidonic acid metabolites, and by interactions with cytoskeletal components. An exciting new age in understanding renal Na+ channel function has begun. Canessa and collaborators [103, 104] and Lingueglia et al [105] have, for the first time, identified by expression cloning an amiloride-sensitive Na+ channel from rat distal colon. The messenger RNA encoding the subunits comprising this channel are expressed in the distal tubule and cortical collecting tubule of the kidney (Rossier, unpublished observations). In addition, our laboratory has successfully cloned a mammalian homologue of this same channel from bovine renal papillary collecting ducts [106].(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the actin cytoskeleton on epithelial Na+ channel regulation

The regulatory role of actin filament organization on epithelial Na+ channel activity is reviewed in this report. The actin cytoskeleton, consisting of actin filaments and associated actin-binding proteins, is essential to various cellular events including the maintenance of cell shape, the onset of cell motility, and the distribution and stability of integral membrane proteins. Functional interactions between the actin cytoskeleton and specific membrane transport proteins are, however, not as well understood. Recent studies from our laboratory have determined that dynamic changes in the actin cytoskeletal organization may represent a novel signaling mechanism in the regulation of ion transport in epithelia. This report summarizes work conducted in our laboratory leading to an understanding of the molecular steps associated with the regulatory role of the actin-based cytoskeleton on epithelial Na+ channel function. The basis of this interaction lies on the regulation by actin-binding proteins and adjacent structures, of actin filament organization which in turn, modulates ion channel activity. The scope of this interaction may extend to such relevant cellular events as the vasopressin response in the kidney.

Whole cell sodium conductance of principal cells freshly isolated from rat cortical collecting duct

Cortical collecting duct fragments were manually dissected from 6-wk-old Sprague-Dawley rats. The fragments were enzymatically digested (collagenase A) into single cells, washed, and resuspended in serum-free RPMI 1640. Individual cells were examined electrophysiologically using the whole cell patch-clamp technique. Two morphologically distinct cell types were present in the cell suspension. Small round cells that had a capacitance of 7 pF and larger oval cells with a capacitance of 29 pF were consistently observed. Whole cell electrophysiological examination revealed that the small round cells had virtually no plasma membrane ionic conductance, whereas both inward and outward currents were observed in the larger oval-type cells. Also, superfusion of 250 pM arginine vasopressin specifically increased the inward conductance of only the larger cells. The effect could be completely inhibited by 2 microM amiloride or 100 mumol of the Rp diastereomer of 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (a specific adenosine 3',5'-cyclic monophosphate inhibitor). These findings are consistent with the hypothesis that the larger cells are principal cells and the smaller cells are intercalated cells and directly demonstrate that an amiloride-sensitive whole cell conductance is readily observable in freshly isolated cortical collecting duct cells. Thus the whole cell configuration of the patch-clamp technique appears to be well suited for assessing cellular mechanisms that regulate the ionic conductances of cortical collecting duct cells.

CFTR as a cAMP-dependent regulator of sodium channels

Cystic fibrosis transmembrane regulator (CFTR), the gene product that is mutated in cystic fibrosis (CF) patients, has a well-recognized function as a cyclic adenosine 3',5'-monophosphate (cAMP)-regulated chloride channel, but this property does not account for the abnormally high basal rate and cAMP sensitivity of sodium ion absorption in CF airway epithelia. Expression of complementary DNAs for rat epithelial Na+ channel (rENaC) alone in Madin Darby canine kidney (MDCK) epithelial cells generated large amiloride-sensitive sodium currents that were stimulated by cAMP, whereas coexpression of human CFTR with rENaC generated smaller basal sodium currents that were inhibited by cAMP. Parallel studies that measured regulation of sodium permeability in fibroblasts showed similar results. In CF airway epithelia, the absence of this second function of CFTR as a cAMP-dependent regulator likely accounts for abnormal sodium transport.

G protein G alpha i-2 inhibits outwardly rectifying chloride channels in human airway epithelial cells

Previously we demonstrated that the heterotrimeric G protein, G alpha i-2, inhibits cystic fibrosis transmembrane conductance regulator (CFTR) chloride (Cl-) channels in human airway epithelial cells (E. M. Schwiebert, F. Gesek, L. Ercolani, C. Wjasow, D. C. Gruenert, and B. A. Stanton. Am. J. Physiol. 267 (Cell Physiol. 36): C272-C281, 1994, and E. M. Schwiebert, N. L. Kizer, D. C. Gruenert, and B. A. Stanton. Proc. Natl. Acad. Sci. USA 89: 10623-10627, 1992). The goal of the present study was to determine if G proteins also regulate outwardly rectifying Cl- channels (ORCC), a distinct class of Cl- channels regulated defectively by protein kinase A (PKA) in cystic fibrosis (CF). To this end, we used the patch-clamp technique to study ORCC in a normal human airway epithelial cell line (9HTEo-) that expresses CFTR and ORCC. Stimulation of G proteins with GTP and GTP gamma S decreased the single-channel open probability (Po) of ORCC, whereas inhibition of G proteins by GDP beta S increased the Po. Moreover, pertussis toxin (PTX), an uncoupler of Gi and G(o) subclasses of heterotrimeric G proteins, also increased the Po. Purified G alpha i-2 decreased the Po. In contrast, other PTX-sensitive G proteins, G alpha i-1, G alpha i-3, and G alpha o, had no effect on Po. We propose that G alpha i-2 couples to a receptor whose agonist negatively regulates ORCC in human airway epithelial cells.

Factors determining specificity of signal transduction by G-protein-coupled receptors. Regulation of signal transfer from receptor to G-protein

Among subfamilies of G-protein-coupled receptors, agonists initiate several cell signaling events depending on the receptor subtype (R) and the type of G-protein (G) or effector molecule (E) expressed in a particular cell. Determinants of signaling specificity/efficiency may operate at the R-G interface, where events are influenced by cell architecture or accessory proteins found in the receptor's microenvironment. This issue was addressed by characterizing signal transfer from R to G following stable expression of the alpha 2A/D adrenergic receptor in two different membrane environments (NIH-3T3 fibroblasts and the pheochromocytoma cell line, PC-12). Receptor coupling to endogenous G-proteins in both cell types was eliminated by pertussis toxin pretreatment and R-G signal transfer restored by reconstitution of cell membranes with purified brain G-protein. Thus, the receptor has access to the same population of G-proteins in the two different environments. In this signal restoration assay, agonist-induced activation of G was 3-9-fold greater in PC-12 as compared with NIH-3T3 alpha 2-adrenergic receptor transfectants. The cell-specific differences in signal transfer were observed over a range of receptor densities or G-protein concentration. The augmented signal transfer in PC-12 versus NIH-3T3 transfectants occurred despite a 2-3-fold lower level of receptors existing in the R-G-coupled state (high affinity, guanyl-5'-yl imidodiphosphate-sensitive agonist binding), suggesting the existence of other membrane factors that influence the nucleotide binding behavior of G-protein in the two cell types. Detergent extraction of PC-12 but not NIH-3T3 membranes yielded a heat-sensitive, macromolecular entity that increased 35S-labeled guanosine 5'-O-(thiotriphosphate) binding to brain G-protein in a concentration-dependent manner. These data indicate that the transfer of signal from R to G is regulated by a cell type-specific, membrane-associated protein that enhances the agonist-induced activation of G.

G-protein regulation of ion channels

Even though the vast majority of ion channels are regulated by voltage, extracellular ligands, phosphorylation, intracellular ions, or a combination of these influences, probably only a handful of ion channels are regulated by direct interaction with activated G proteins. Although results from electrophysiological studies of some channels are consistent with the hypothesis of regulation via direct physical interactions with G proteins, strong biochemical evidence for such interactions is still lacking. In most cases, such evidence has been difficult to obtain because ion channels are present at very low abundances in cell membranes, or because the molecular identity of the channel is unknown. The recent cloning of members of the inwardly rectifying K+ channel and voltage-gated Ca2+ channel families should facilitate the rigorous study of the putative interactions between G proteins and ion channels.

Aldosterone stimulation of GTP hydrolysis in membranes from renal epithelia

Specific hydrolysis of GTP catalyzed by membranes prepared from A6 epithelial cells grown on porous supports was measured. Aldosterone treatment of the cells for 4 h increased Na+ transport and stimulated GTP hydrolysis by apical membranes in vitro more than twofold over basal levels. This stimulation was attributed to an increase in maximum velocity with little change in Michaelis-Menten constant values. Na+ transport rate and GTP hydrolysis were linearly correlated after aldosterone. This relationship was maintained when aldosterone's response was blunted by various inhibitors. Spironolactone decreased both the hormone-stimulated guanosinetriphosphatase (GTPase) and the Na+ transport rate. Pertussis toxin, which exerted minimal effects on basal rates, reduced the increase of Na+ current normally observed after aldosterone and the hormone stimulation of GTPase activity. The expression of classical Gi/Go-type G proteins was not increased after hormone treatment. When A6 cells were grown on nonporous plastic dishes, aldosterone neither stimulated GTPase activity nor increased amiloride-blockable 22Na+ fluxes. We propose that activation of one or more G proteins in the apical membrane of A6 cells is directly involved in the natriferic action of aldosterone.

Amiloride-sensitive and HCO3(-)-dependent ion transport activated by aldosterone and vasotocin in A6 cells

We studied the effects of aldosterone (Aldo) and arginine vasotocin (AVT) on ion transport of renal epithelial cell line (A6) by measuring short-circuit current (Isc). AVT induced a rapid, transient increase in Isc, followed by a decrease toward the baseline in cells untreated with Aldo. In cells treated with Aldo, Isc showed a biphasic response to AVT, i.e., both transient and sustained increases over 40 min after addition of AVT. The transient increase was composed only of amiloride-insensitive Isc regardless of Aldo treatment, whereas the sustained increase contained both amiloride-sensitive and amiloride-insensitive components. The main part of the amiloride-insensitive, sustained Isc depended on HCO3(-). In cells treated with Aldo for 1 day, removal of HCO3(-) in the bathing solution enhanced the amiloride-sensitive component and decreased the amiloride-insensitive one. These data suggest that 1) Aldo treatment is necessary for an AVT-induced sustained increase of Isc and 2) a HCO3(-)-dependent Isc mainly contributes to the sustained increase in amiloride-insensitive Isc.

Antidiuretic hormone-responding nonselective cation channel in distal nephron epithelium (A6)

Arginine vasotocin (AVT, 70 mU/ml) added from the basolateral side transiently activated a nonselective cation (NSC) channel with a single-channel conductance of 28.5 pS and almost identical selectivity for Na+ and K+ in the apical membrane of distal nephron cells (A6) cultured on permeable supports for 10-12 days in media containing 10% fetal bovine serum without supplemental aldosterone. The open probability (Po) of the NSC channel at the apical resting membrane potential in cell-attached patches was approximately 0.09 and increased when the apical membrane depolarized. The Po of the NSC channel was decreased by a rise in cytosolic Ca2+ concentration within a range of 30 nM-1 microM but not affected by cytosolic pH within a range of 6-8. The channel was activated by the application of negative pressure (10-60 cmH2O) into the patch pipette. Gadolinium (2 microM), an inhibitor of stretch-activated channels, decreased the Po by 40%. This blocking action of gadolinium was more effective after the channel was activated by stretch, i.e., 2 microM gadolinium decreased the Po by 70% when a negative pressure (60 cmH2O) was applied into the patch pipette. Amiloride (10 and 100 microM) showed a blocking action on the channel only when the NSC channel was activated by stretch.

Enhancement of gustatory nerve fibers to NaCl and formation of ion channels by commercial novobiocin

Single fibers of the rat chorda tympani nerve were used to study the mechanism of action of the antibiotic novobiocin on salt taste transduction. In the rat, novobiocin selectively enhanced the responses of sodium-specific and amiloride-sensitive chorda tympani nerve fibers (N type) without affecting more broadly responsive cation-sensitive and amiloride-insensitive fibers (E type). In the presence of amiloride, novobiocin was ineffective at enhancing the response of N-type fibers toward sodium chloride. Novobiocin also increased the conductance of bilayers formed from neutral lipids by forming nonrectifying ion channels with low conductance (approximately 7 pS in 110 mM NaCl), long open times (several seconds and longer), and high cation selectivity. Amiloride did not alter either the conductance or kinetics of these novobiocin channels. These observations suggest that even though novobiocin is able to form cation channels in lipid bilayers, and possibly in cell membranes as well, its action on the salt-taste response is through modulation of existing amiloride-sensitive sodium channels.

Targeting of chimeric G alpha i proteins to specific membrane domains

Heterotrimeric guanine nucleotide-regulatory (G) proteins are associated with a variety of intracellular membranes and specific plasma membrane domains. In polarized epithelial LLC-PK1 cells we have shown previously that endogenous G alpha i-2 is localized on the basolateral plasma membrane, whereas G alpha i-3 is localized on Golgi membranes. The targeting of these highly homologous G alpha i proteins to distinct membrane domains was studied by the transfection and expression of chimeric G alpha i proteins in LLC-PK1 cells. Chimeric cDNAs were constructed from the cDNAs for G alpha i-3 and G alpha i-2 and introduced into a pMXX eukaryotic expression vector containing a mouse metallothionein-I promoter. Stably transfected cell lines were produced that expressed either G alpha i-2/3 or G alpha i-3/2 chimeric proteins. Chimeric and endogenous G alpha i proteins were detected in cells using specific carboxy-terminal peptide antibodies. Immunofluorescence staining was used to localize endogenous and chimeric G alpha i proteins in LLC-PK1 cells. The staining of chimeric proteins was detected as an increased intensity of staining on membranes containing endogenous G alpha i proteins. Using confocal microscopy and image analysis we localized G alpha i-2 to a specific sub-domain of the lateral membrane of polarized cells, the chimeric G alpha i-3/2 protein was then shown to colocalize with endogenous G alpha i-2 in the same lateral plasma membrane domain. The chimeric G alpha i-2/3 protein colocalized with endogenous G alpha i-3 on Golgi membranes in LLC-PK1 cells. These results show that chimeric G alpha i proteins were targeted to the same membrane domains as endogenous G alpha i proteins and the specificity of their membrane targeting was conferred by the carboxy-terminal end of the proteins. These data provide the first evidence for specific targeting information contained in the carboxy termini of G alpha i proteins, which appears to be independent of amino-terminal membrane attachment sites in these proteins.

Amiloride-sensitive epithelial Na+ channel is made of three homologous subunits

The amiloride-sensitive epithelial sodium channel constitutes the rate-limiting step for sodium reabsorption in epithelial cells that line the distal part of the renal tubule, the distal colon, the duct of several exocrine glands, and the lung. The activity of this channel is upregulated by vasopressin and aldosterone, hormones involved in the maintenance of sodium balance, blood volume and blood pressure. We have identified the primary structure of the alpha-subunit of the rat epithelial sodium channel by expression cloning in Xenopus laevis oocytes. An identical subunit has recently been reported. Here we identify two other subunits (beta and gamma) by functional complementation of the alpha-subunit of the rat epithelial Na+ channel. The ion-selective permeability, the gating properties and the pharmacological profile of the channel formed by coexpressing the three subunits in oocytes are similar to that of the native channel.