Novel isoforms of the beta and gamma subunits of the Xenopus epithelial Na channel provide information about the amiloride binding site and extracellular sodium sensing

Physiological and morphological correlates of extreme acid tolerance in larvae of the acidophilic amphibian Litoria cooloolensis

The Cooloola sedgefrog (Litoria cooloolensis) is one of a number of frog species endemic to the coastal sandy lowlands of east Australia exhibiting remarkable tolerance to dilute waters of low pH (< pH 3.5). To investigate the physiological and morphological underpinnings of acid tolerance in L. cooloolensis larvae, we compared Na⁺ balance, uptake and efflux rates, and gill and skin morphology in larvae reared in circum-neutral (pH 6.5) and pH 3.5 water. We hypothesised that L. cooloolensis larvae would be more resistant to ionregulatory disturbance and epithelial damage at low pH relative to acid-sensitive species. Net Na⁺ flux rates were not significantly different from zero in L. cooloolensis larvae reared at pH 3.5 and in acid-naïve animals maintained in pH 6.5 water. Animals reared at pH 6.5 and acutely exposed to pH 3.5, however, exhibited a net loss of Na⁺ due to inhibition of Na⁺ uptake. In contrast, L. cooloolensis larvae reared at pH 3.5 maintained Na⁺ balance at pH 3.5 and did not exhibit inhibition of Na⁺ uptake at this pH. Investigation of Na⁺ transport kinetics and the morphology of the gills and integument suggests tolerance of L. cooloolensis larvae to low pH may be attributed to a high capacity for branchial Na⁺ uptake, increased tight junction length and elevated mucus production at the gills and integument. These factors confer resistance to acid damage and disruption of ionic homeostasis which would otherwise result in the death of amphibian larvae exposed to waters of pH 4.0 and less.

Mutations of the serine protease CAP1/Prss8 lead to reduced embryonic viability, skin defects, and decreased ENaC activity

CAP1/Prss8 is a membrane-bound serine protease involved in the regulation of several different effectors, such as the epithelial sodium channel ENaC, the protease-activated receptor PAR2, the tight junction proteins, and the profilaggrin polypeptide. Recently, the V170D and the G54-P57 deletion mutations within the CAP1/Prss8 gene, identified in mouse frizzy (fr) and rat hairless (fr(CR)) animals, respectively, have been proposed to be responsible for their skin phenotypes. In the present study, we analyzed those mutations, revealing a change in the protein structure, a modification of the glycosylation state, and an overall reduction in the activation of ENaC of the two mutant proteins. In vivo analyses demonstrated that both fr and fr(CR) mutant animals present analogous reduction of embryonic viability, similar histologic aberrations at the level of the skin, and a significant decrease in the activity of ENaC in the distal colon compared with their control littermates. Hairless rats additionally had dehydration defects in skin and intestine and significant reduction in the body weight. In conclusion, we provided molecular and functional evidence that CAP1/Prss8 mutations are accountable for the defects in fr and fr(CR) animals, and we furthermore demonstrate a decreased function of the CAP1/Prss8 mutant proteins. Therefore, fr and fr(CR) animals are suitable models to investigate the consequences of CAP1/Prss8 action on its target proteins in the whole organism.

Ubiquitin-specific protease 2-45 (Usp2-45) binds to epithelial Na+ channel (ENaC)-ubiquitylating enzyme Nedd4-2

Regulation of the epithelial Na(+) channel (ENaC) by ubiquitylation is controlled by the activity of two counteracting enzymes, the E3 ubiquitin-protein ligase Nedd4-2 (mouse ortholog of human Nedd4L) and the ubiquitin-specific protease Usp2-45. Previously, Usp2-45 was shown to decrease ubiquitylation and to increase surface function of ENaC in Xenopus laevis oocytes, whereas the splice variant Usp2-69, which has a different N-terminal domain, was inactive toward ENaC. It is shown here that the catalytic core of Usp2 lacking the N-terminal domain has a reduced ability relative to Usp2-45 to enhance ENaC activity in Xenopus oocytes. In contrast, its catalytic activity toward the artificial substrate ubiquitin-AMC is fully maintained. The interaction of Usp2-45 with ENaC exogenously expressed in HEK293 cells was tested by coimmunoprecipitation. The data indicate that different combinations of ENaC subunits, as well as the α-ENaC cytoplasmic N-terminal but not C-terminal domain, coprecipitate with Usp2-45. This interaction is decreased but not abolished when the cytoplasmic ubiquitylation sites of ENaC are mutated. Importantly, coimmunoprecipitation in HEK293 cells and GST pull-down of purified recombinant proteins show that both the catalytic domain and the N-terminal tail of Usp2-45 physically interact with the HECT domain of Nedd4-2. Taken together, the data support the conclusion that Usp2-45 action on ENaC is promoted by various interactions, including through binding to Nedd4-2 that is suggested to position Usp2-45 favorably for ENaC deubiquitylation.

Heterodimeric amino acid transporter glycoprotein domains determining functional subunit association

The heteromeric amino acid transporter glycoprotein subunits rBAT and 4F2hc (heavy chains) form, with different catalytic subunits (light chains), functional heterodimers that are covalently stabilized by a disulphide bridge. Whereas rBAT associates with b(0,+)AT to form the cystine and cationic amino acid transporter defective in cystinuria, 4F2hc associates with other homologous light chains, for instance with LAT1 to form a system L neutral amino acid transporter. To identify within the heavy chains the domain(s) involved in recognition of and functional interaction with partner light chains, chimaeric and truncated forms of rBAT and 4F2hc were co-expressed in Xenopus laevis oocytes with b(0,+)AT or LAT1. Heavy chain-light chain association was analysed by co-immunoprecipitation, and transport function was tested by tracer uptake experiments. The results indicate that the cytoplasmic tail and transmembrane domain of rBAT together play a dominant role in selective functional interaction with b(0,+)AT, whereas the extracellular domain of rBAT appears to facilitate specifically L-cystine uptake. For 4F2hc, functional interaction with LAT1 was mediated by the N-terminal part, comprising cytoplasmic tail, transmembrane segment and neck, even in the absence of the extracellular domain. Alternatively, functional association with LAT1 was also supported by the extracellular part of 4F2hc comprising neck and glycosidase-like domain linked to the complementary part of rBAT. In conclusion, the cytoplasmic tail and the transmembrane segment together play a determinant role for the functional interaction of rBAT with b(0,+)AT, whereas either cytoplasmic or extracellular glycosidase-like domains are dispensable for the functional interaction of 4F2hc with LAT1.

Aromatic amino acid transporter AAT-9 of Caenorhabditis elegans localizes to neurons and muscle cells

The Caenorhabditis elegans genome encodes nine homologues of mammalian glycoprotein-associated amino acid transporters. Two of these C. elegans proteins (AAT-1 and AAT-3) have been shown to function as catalytic subunits (light chains) of heteromeric amino acid transporters. These proteins need to associate with a glycoprotein heavy chain subunit (ATG-2) to reach the cell surface in a manner similar to that of their mammalian homologues. AAT-1 and AAT-3 contain a cysteine residue in the second putative extracellular loop through which a disulfide bridge can form with a heavy chain. In contrast, six C. elegans members of this family (AAT-4 to AAT-9) lack such a cysteine residue. We show here that one of these transporter proteins, AAT-9, reaches the cell surface in Xenopus oocytes without an exogenous heavy chain and that it functions as an exchanger of aromatic amino acids. Two-electrode voltage clamp experiments demonstrate that AAT-9 displays a substrate-activated conductance. Immunofluorescence shows that it is expressed close to the pharyngeal bulbs within C. elegans neurons. The selective expression of an aat-9 promoter-green fluorescent protein construct in several neurons of this region and in wall muscle cells around the mouth supports and extends these localization data. Taken together, the results show that AAT-9 is expressed in excitable cells of the nematode head and pharynx in which it may provide a pathway for aromatic amino acid transport.

A naturally occurring human Nedd4-2 variant displays impaired ENaC regulation in Xenopus laevis oocytes

The epithelial Na(+) channel (ENaC) is regulated by the ubiquitin-protein ligase Nedd4-2 via interaction with ENaC PY-motifs. These PY-motifs are mutated/deleted in Liddle's syndrome, resulting in elevated Na(+) reabsorption and hypertension explained partly by impaired ENaC-Nedd4-2 interaction. We hypothesized that Nedd4-2 is a susceptibility gene for hypertension and screened 856 renal patients and healthy controls for mutations in a subset of exons of the human Nedd4-2 gene that are relevant for ENaC regulation by PCR/single-strand conformational polymorphism. Several variants were identified, and one nonsynonymous mutation (Nedd4-2-P355L) was further characterized. This mutation next to the 3' donor site of exon 15 does not affect in vitro splicing of Nedd4-2 mRNA. However, in the Xenopus oocyte expression system, Nedd4-2-P355L-dependent ENaC inhibition was weaker compared with the wild type (Nedd4-2-WT), and this difference depended on the presence of intact PY-motifs on ENaC. This could not be explained by the amount of wild type or mutant Nedd4-2 coimmunoprecipitating with ENaC. When the phosphorylation level of human Nedd4-2 Ser(448) (known to be phosphorylated by the Sgk1 kinase) was determined with a specific anti-pSer(448) antibody, we observed stronger basal phosphorylation of Nedd4-2-P355L. Both the phosphorylation level and the accompanying amiloride-sensitive Na(+) currents could be further enhanced to approximately the same levels by coexpressing Sgk1. In addition, the role of the two other putative Sgk1 phosphorylation sites (S342 and T367) appears also to be affected by the P355L mutation. The differential phosphorylation status between wild-type and mutant Nedd4-2 provides an explanation for the different potential to inhibit ENaC activity.

Stimulation of transepithelial Na(+) current by extracellular Gd(3+) in Xenopus laevis alveolar epithelium

In the present study we investigated the effect of extracellular gadolinium on amiloride-sensitive Na(+) current across Xenopus alveolar epithelium by Ussing chamber experiments and studied its direct effect on epithelial Na(+) channels with the patch-clamp method. As observed in various epithelia, the short-circuit current ( I(sc)) and the amiloride-sensitive Na(+) current ( I(ami)) across Xenopus alveolar epithelium was downregulated by high apical Na(+) concentrations. Apical application of gadolinium (Gd(3+)) increased I(sc) in a dose-dependent manner ( EC(50) = 23.5 microM). The effect of Gd(3+) was sensitive to amiloride, which indicated the amiloride-sensitive transcellular Na(+) transport to be upregulated. Benz-imidazolyl-guanidin (BIG) and p-hydroxy-mercuribenzonic-acid (PHMB) probably release apical Na(+) channels from Na(+)-dependent autoregulating mechanisms. BIG did not stimulate transepithelial Na(+) currents across Xenopus lung epithelium but, interestingly, it prevented the stimulating effect of Gd(3+) on transepithelial Na(+) transport. PHMB increased I(sc) and this stimulation was similar to the effect of Gd(3+). Co-application of PHMB and Gd(3+) had no additive effects on I(sc). In cell-attached patches on Xenopus oocytes extracellular Gd(3+) increased the open probability ( NP(o)) of Xenopus epithelial sodium channels (ENaC) from 0.72 to 1.79 and decreased the single-channel conductance from 5.5 to 4.6 pS. Our data indicate that Xenopus alveolar epithelium exhibits Na(+)-dependent non-hormonal control of transepithelial Na(+) transport and that the earth metal gadolinium interferes with these mechanisms. The patch-clamp experiments indicate that Gd(3+) directly modulates the activity of ENaCs.

Participation of the ubiquitin-conjugating enzyme UBE2E3 in Nedd4-2-dependent regulation of the epithelial Na+ channel

The epithelial Na+ channel (ENaC) is a heteromeric protein complex playing a fundamental role in Na+ homeostasis and blood pressure regulation. Specific mutations inactivating PY motifs in ENaC C termini cause Liddle's syndrome, an inherited form of hypertension. Previously we showed that these PY motifs serve as binding sites for the E3 enzyme Nedd4-2, implying ubiquitination as a regulatory mechanism of ENaC. Ubiquitination involves the sequential action of E1, E2, and E3 enzymes. Here we identify the E2 enzyme UBE2E3, which acts in concert with Nedd4-2, and show by coimmunoprecipitation that UBE2E3 and Nedd4-2 interact together. In Xenopus laevis oocytes, UBE2E3 reduces ENaC activity marginally, consistent with Nedd4-2 being the rate-limiting factor in this process, whereas a catalytically inactive mutant of UBE2E3 (UBE2E3-CS) causes elevated ENaC activity by increasing cell surface expression. No additive effect is observed when UBE2E3-CS is coexpressed with an inactive Nedd4-2 mutant, and the stimulatory role of UBE2E3-CS depends on the integrity of the PY motifs (Nedd4-2 binding sites) and the ubiquitination sites on ENaC. In renal mpkCCD(cl4) cells, displaying ENaC-dependent transepithelial Na+ transport, Nedd4-2 and UBE2E3 can be coimmunoprecipitated and overexpression of UBE2E3 affects Na+ transport, corroborating the concept of a concerted action of UBE2E3 and Nedd4-2 in ENaC regulation.

Functional characterization of Caenorhabditis elegans heteromeric amino acid transporters

Mammalian heteromeric amino acid transporters (HATs) are composed of a multi-transmembrane spanning catalytic protein covalently associated with a type II glycoprotein (e.g. 4F2hc, rBAT) through a disulfide bond. Caenorhabditis elegans has nine genes encoding close homologues of the HAT catalytic proteins. Three of these genes (designated AAT-1 to AAT-3) have a much higher degree of similarity to the mammalian homologues than the other six, including the presence of a cysteine residue at the position known to form a disulfide bridge to the glycoprotein partner in mammalian HATs. C. elegans also has two genes encoding homologues of the heteromeric amino acid transporter type II glycoprotein subunits (designated ATG-1 and ATG-2). Both ATG, and/or AAT-1, -2, -3 proteins were expressed in Xenopus oocytes and tested for amino acid transport function. This screen revealed that AAT-1 and AAT-3 facilitate amino acid transport when expressed together with ATG-2 but not with ATG-1 or the mammalian type II glycoproteins 4F2hc and rBAT. AAT-1 and AAT-3 covalently bind to both C. elegans ATG glycoproteins, but only the pairs with ATG-2 traffic to the oocyte surface. Both of these functional, surface-expressed C. elegans HATs transport most neutral amino acids and display the highest transport rate for l-Ala and l-Ser (apparent K(m) 100 microm range). Similar to their mammalian counterparts, the C. elegans HATs function as (near) obligatory amino acid exchangers. Taken together, this study demonstrates that the heteromeric structure and the amino acid exchange function of HATs have been conserved throughout the evolution of nematodes to mammals.

Channels as taste receptors in vertebrates

Taste reception is fundamental for proper selection of food and beverages. Chemicals detected as taste stimuli by vertebrates include a large variety of substances, ranging from inorganic ions (e.g., Na(+), H(+)) to more complex molecules (e.g., sucrose, amino acids, alkaloids). Specialized epithelial cells, called taste receptor cells (TRCs), express specific membrane proteins that function as receptors for taste stimuli. Classical view of the early events in chemical detection was based on the assumption that taste substances bind to membrane receptors in TRCs without permeating the tissue. Although this model is still valid for some chemicals, such as sucrose, it does not hold for small ions, such as Na(+), that actually diffuse inside the taste tissue through ion channels. Electrophysiological, pharmacological, biochemical, and molecular biological studies have provided evidence that indeed TRCs use ion channels to reveal the presence of certain substances in foodstuff. In this review, we focus on the functional and molecular properties of ion channels that serve as receptors in taste transduction.

A new subunit of the epithelial Na+ channel identifies regions involved in Na+ self-inhibition

The epithelial Na+ channel (ENaC) is the apical entry pathway for Na+ in many Na+-reabsorbing epithelia. ENaC is a heterotetrameric protein composed of homologous alpha, beta, and gamma subunits. Mutations in ENaC cause severe hypertension or salt wasting in humans; and consequently, ENaC activity is tightly controlled. According to the concept of Na+ self-inhibition, the extracellular Na+ ion itself can reduce ENaC activity. The molecular basis for Na+ self-inhibition is unknown. Here, we describe cloning of a new ENaC subunit from Xenopus laevis (epsilonxENaC). epsilonxENaC can replace alphaxENaC and formed functional, highly selective, amiloride-sensitive Na+ channels when coexpressed with betaxENaC and gammaxENaC. Channels containing epsilonxENaC showed strong inhibition by extracellular Na+. This Na+ self-inhibition was significantly slower than for alphaxENaC-containing channels. Using site-directed mutagenesis, we show that the proximal part of the large extracellular domain controls the speed of self-inhibition. This suggests that this region is involved in conformational changes during Na+ self-inhibition.

Sensitivity of oocyte-expressed epithelial Na+ channel to glibenclamide

The effect of glibenclamide on heterologously expressed amiloride-sensitive sodium channels (ENaCs) was investigated in Xenopus oocytes. The ENaC is a heteromer and consists of alpha-, beta- and gamma-subunits and the alpha- and beta-subunits have previously been shown to confer sensitivity to glibenclamide. We coexpressed either colonic rat alpha- (ralpha) or guinea-pig alpha-subunit (gpalpha) with Xenopus betagamma-subunits. The gpalphaxbetagamma was significantly stimulated by glibenclamide (100 microM) (184+/-15%), whereas the ralpha-combination was slightly down-regulated by the sulfonylurea (79+/-4%). The stimulating effect did not interfere with Na(+)-self-inhibition resulting from intracellular accumulation of Na(+)-ions. We exchanged cytosolic termini between both orthologs but the gpalpha-chimera with the termini from rat retained sensitivity to glibenclamide. The effect of glibenclamide on Xenopus ENaC (xENaC) was inhibited by ADP-beta-S but not by ATP-gamma-S, when applied intracellularly. Intracellular loading with Na(+)-ions after inhibition of Na(+)/K(+)-ATPases with ouabain prevented an up-regulation of ENaC activity by glibenclamide. Pretreatment of oocytes expressing xENaC with edelfosine (ET-18-OCH(3)) slightly reduced stimulation of I(ami) (118+/-12%; control: 132+/-9%) while phosphatidylinositol-4,5-biphosphate (PIP(2)) significantly reduced the effect of glibenclamide to 101+/-3%.

Phosphorylation of Nedd4-2 by Sgk1 regulates epithelial Na(+) channel cell surface expression

The epithelial Na(+) channel (ENaC) plays an essential role in the regulation of whole body Na(+) balance and blood pressure. The cell surface expression of this channel, a complex of three subunits (alpha, beta and gamma ENaC), has been shown to be regulated by hormones such as aldosterone and vasopressin and by intracellular signaling, including ubiquitylation and/or phosphorylation. However, the molecular mechanisms involving phosphorylation in the regulation of ENaC are unclear. Here we show by expression studies in Xenopus laevis oocytes that the aldosterone-induced Sgk1 kinase interacts with the ubiquitin protein ligase Nedd4-2 in a PY motif-dependent manner and phosphorylates Nedd4-2 on Ser444 and, to a lesser extent, Ser338. Such phosphorylation reduces the interaction between Nedd4-2 and ENaC, leading to elevated ENaC cell surface expression. These data show that phosphorylation of an enzyme involved in the ubiquitylation cascade (Nedd4-2) controls cell surface density of ENaC and propose a paradigm for the control of ion channels. Moreover, they suggest a novel and complete signaling cascade for aldosterone-dependent regulation of ENaC.

Distinct characteristics of two human Nedd4 proteins with respect to epithelial Na(+) channel regulation

The epithelial Na(+) channel (ENaC) is regulated via PY motif-WW domain interaction by the mouse (m) ubiquitin-protein ligase mNedd4-2 but not by its close relative mNedd4-1. Whereas mNedd4-1 is composed of one C2, three WW, and one HECT domain, mNedd4-2 comprises four WW domains and one HECT domain. Both proteins have human (h) homologs, hNedd4-1 and hNedd4-2; however, both of them include four WW domains. Therefore, we characterized hNedd4-1 and hNedd4-2 in Xenopus laevis oocytes with respect to ENaC binding and interaction. We found that hNedd4-2 binds to and abrogates ENaC activity, whereas hNedd4-1 does not coimmunoprecipitate with ENaC and has only modest effects on ENaC activity. Structure-function studies revealed that the C2 domain of hNedd4-1 prevents this protein from downregulating ENaC and that WW domains 3 and 4, involved in interaction with ENaC, do not by themselves provide specificity for ENaC recognition. Taken together, our data demonstrate that hNedd4-2 inhibits ENaC, implying that this protein is a modulator of salt homeostasis, whereas hNedd4-1 is not primarily involved in ENaC regulation.

A novel mouse Nedd4 protein suppresses the activity of the epithelial Na+ channel

Liddle's syndrome is a form of inherited hypertension linked to mutations in the genes encoding the epithelial Na+ channel (ENaC). These mutations alter or delete PY motifs involved in protein-protein interactions with a ubiquitin-protein ligase, Nedd4. Here we show that Na+ transporting cells, derived from mouse cortical collecting duct, express two Nedd4 proteins with different structural organization and characteristics of ENaC regulation: 1) the classical Nedd4 (herein referred to as Nedd4-1) containing one amino-terminal C2, three WW, and one HECT-ubiquitin protein ligase domain and 2) a novel Nedd4 protein (Nedd4-2), homologous to Xenopus Nedd4 and comprising four WW, one HECT, yet lacking a C2 domain. Nedd4-2, but not Nedd4-1, inhibits ENaC activity when coexpressed in Xenopus oocytes and this property correlates with the ability to bind to ENaC, as only Nedd4-2 coimmunoprecipitates with ENaC. Furthermore, this interaction depends on the presence of at least one PY motif in the ENaC complex and on WW domains 3 and 4 in Nedd4-2. Thus, these results suggest that the novel suppressor protein Nedd4-2 is the regulator of ENaC and hence a potential susceptibility gene for arterial hypertension.

Physiology of renal sodium transport

A wealth of studies performed with a spectrum of methods spanning simple clearance studies to the molecular identification of ion transporters has increased our understanding of how approximately 1.7 kg of NaCl and 180 L of H2O are absorbed by renal tubules in man and how the urinary excretion is fine-tuned to meet homeostatic requirements. This review will summarize our current understanding. In the proximal nephron, approximately 60 to 70% of the filtered Na+ and H2O is absorbed together with approximately 90% of the filtered HCO3-. The exact quantities are determined by many regulatory factors, such as glomerulotubular balance, angiotensin II, endothelin, sympathetic innervation, parathyroid hormone, dopamine, acid base status and others. The essential components of absorption are luminal membrane Na+/H+ exchange and the basolateral (Na+ + K+)-ATPase. In the thick ascending limb of the loop of Henle, 20 to 30% of the filtered NaCl is absorbed via Na+2Cl-K+ cotransport driven by the basolateral (Na+ + K+)-ATPase. No H2O is absorbed at this nephron site. The transport rate is determined by the Na+ load and by several hormones and neurotransmitters, including prostaglandins, parathyroid hormone, glucagon, calcitonin, arginine vasopressin (AVP), and adrenaline. In the distal tubule, some 5 to 10% of the filtered load is absorbed via Na+Cl- cotransport in the luminal membrane driven by the basolateral (Na+ + K+)-ATPase. The rate of transport is again determined by the delivered load and by several hormones and neurotransmitters. One of the tasks of the collecting duct is to control the absorption of approximately 10 to 15% of the filtered H2O, regulated by AVP, and just a few percent of the filtered Na+, controlled by aldosterone and natriuretic hormone. The water absorption proceeds through the luminal membrane via aquaporin 2 and through the basolateral membrane via aquaporin 3 channels and is driven by the osmotic gradient built up by the counter current concentrating system. The Na+ absorption occurs via Na+ channels present in the luminal membrane driven by the basolateral (Na+ + K+)-ATPase. With no pharmacological interference, urinary excretion of Na+ can vary between less than 0.1% and no more than 3% of the filtered load, and that of H2O can vary between 0.3 and 15%.

LAT2, a new basolateral 4F2hc/CD98-associated amino acid transporter of kidney and intestine

Glycoprotein-associated amino acid transporters (gpaAT) are permease-related proteins that require heterodimerization to express their function. So far, four vertebrate gpaATs have been shown to associate with 4F2hc/CD98 for functional expression, whereas one gpaAT specifically associates with rBAT. In this study, we characterized a novel gpaAT, LAT2, for which mouse and human cDNAs were identified by expressed sequence tag data base searches. The encoded ortholog proteins are 531 and 535 amino acids long and 92% identical. They share 52 and 48% residues with the gpaATs LAT1 and y(+)LAT1, respectively. When mouse LAT2 and human 4F2hc cRNAs were co-injected into Xenopus oocytes, disulfide-linked heterodimers were formed, and an L-type amino acid uptake was induced, which differed slightly from that produced by LAT1-4F2hc: the apparent affinity for L-phenylalanine was higher, and L-alanine was transported at physiological concentrations. In the presence of an external amino acid substrate, LAT2-4F2hc also mediated amino acid efflux. LAT2 mRNA is expressed mainly in kidney and intestine, whereas LAT1 mRNA is expressed widely. Immunofluorescence experiments showed colocalization of 4F2hc and LAT2 at the basolateral membrane of kidney proximal tubules and small intestine epithelia. In conclusion, LAT2 forms with LAT1 a subfamily of L-type gpaATs. We propose that LAT1 is involved in cellular amino acid uptake, whereas LAT2 plays a role in epithelial amino acid (re)absorption.

Luminal heterodimeric amino acid transporter defective in cystinuria

Mutations of the glycoprotein rBAT cause cystinuria type I, an autosomal recessive failure of dibasic amino acid transport (b(0,+) type) across luminal membranes of intestine and kidney cells. Here we identify the permease-like protein b(0,+)AT as the catalytic subunit that associates by a disulfide bond with rBAT to form a hetero-oligomeric b(0,+) amino acid transporter complex. We demonstrate its b(0,+)-type amino acid transport kinetics using a heterodimeric fusion construct and show its luminal brush border localization in kidney proximal tubule. These biochemical, transport, and localization characteristics as well as the chromosomal localization on 19q support the notion that the b(0,+)AT protein is the product of the gene defective in non-type I cystinuria.

Terbutaline increases open channel density of epithelial sodium channel (ENaC) in distal lung

Neonatal and adult vertebrate respiration is facilitated by alveolar fluid and sodium (Na+) absorption driven by apical sodium channels (ENaC). ENaC are characterized in Xenopus laevis lung (XLL) epithelia using voltage clamping and fluctuation analysis to non-invasively examine macroscopic transepithelial current and resistance (I(SC), R(T)), single channel current (i(Na)) and total channel density (N(T)) responses to a beta adrenergic agonist (Terbutaline). Terbutaline addition to the basolateral bath of XLL increased Na entry to > 200% of control reflecting a doubling of open channel density (N(o). These data are consistent with the notion that XLL can serve as a useful model for investigation of distal lung ENaC response to agents of physiological interest.

Epithelial sodium channel regulated by aldosterone-induced protein sgk

Sodium homeostasis in terrestrial and freshwater vertebrates is controlled by the corticosteroid hormones, principally aldosterone, which stimulate electrogenic Na+ absorption in tight epithelia. Although aldosterone is known to increase apical membrane Na+ permeability in target cells through changes in gene transcription, the mechanistic basis of this effect remains poorly understood. The predominant early effect of aldosterone is to increase the activity of the epithelial sodium channel (ENaC), although ENaC mRNA and protein levels do not change initially. Rather, the open probability and/or number of channels in the apical membrane are greatly increased by unknown modulators. To identify hormone-stimulated gene products that modulate ENaC activity, a subtracted cDNA library was generated from A6 cells, a stable cell line of renal distal nephron origin, and the effect of candidates on ENaC activity was tested in a coexpression assay. We report here the identification of sgk (serum and glucocorticoid-regulated kinase), a member of the serine-threonine kinase family, as an aldosterone-induced regulator of ENaC activity. sgk mRNA and protein were strongly and rapidly hormone stimulated both in A6 cells and in rat kidney. Furthermore, sgk stimulated ENaC activity approximately 7-fold when they were coexpressed in Xenopus laevis oocytes. These data suggest that sgk plays a central role in aldosterone regulation of Na+ absorption and thus in the control of extracellular fluid volume, blood pressure, and sodium homeostasis.

Amino acid transport of y+L-type by heterodimers of 4F2hc/CD98 and members of the glycoprotein-associated amino acid transporter family

Amino acid transport across cellular membranes is mediated by multiple transporters with overlapping specificities. We recently have identified the vertebrate proteins which mediate Na+-independent exchange of large neutral amino acids corresponding to transport system L. This transporter consists of a novel amino acid permease-related protein (LAT1 or AmAT-L-lc) which for surface expression and function requires formation of disulfide-linked heterodimers with the glycosylated heavy chain of the h4F2/CD98 surface antigen. We show that h4F2hc also associates with other mammalian light chains, e.g. y+LAT1 from mouse and human which are approximately 48% identical with LAT1 and thus belong to the same family of glycoprotein-associated amino acid transporters. The novel heterodimers form exchangers which mediate the cellular efflux of cationic amino acids and the Na+-dependent uptake of large neutral amino acids. These transport characteristics and kinetic and pharmacological fingerprints identify them as y+L-type transport systems. The mRNA encoding my+LAT1 is detectable in most adult tissues and expressed at high levels in kidney cortex and intestine. This suggests that the y+LAT1-4F2hc heterodimer, besides participating in amino acid uptake/secretion in many cell types, is the basolateral amino acid exchanger involved in transepithelial reabsorption of cationic amino acids; hence, its defect might be the cause of the human genetic disease lysinuric protein intolerance.

Cell localization and ontogeny of sodium transport pathways in the distal nephron: perspectives in function and failure

The expression and function of ion co-transporters/exchangers/channels in the distal nephron have recently been defined. The role of cation-chloride co-transporters and proteins implicated in aldosterone target cell function are reported in the adult and during ontogeny. Volume disorders can currently be related to identified gene products acting in defined nephron sites.

Ras pathway activates epithelial Na+ channel and decreases its surface expression in Xenopus oocytes

The small G protein K-Ras2A is rapidly induced by aldosterone in A6 epithelia. In these Xenopus sodium reabsorbing cells, aldosterone rapidly activates preexisting epithelial Na+ channels (XENaC) via a transcriptionally mediated mechanism. In the Xenopus oocytes expression system, we tested whether the K-Ras2A pathway impacts on XENaC activity by expressing XENaC alone or together with XK-Ras2A rendered constitutively active (XK-Ras2AG12V). As a second control, XENaC-expressing oocytes were treated with progesterone, a sex steroid that induces maturation of the oocytes similarly to activated Ras. Progesterone or XK-Ras2AG12V led to oocyte maturation characterized by a decrease in surface area and endogenous Na+ pump function. In both conditions, the surface expression of exogenous XENaC's was also decreased; however, in comparison with progesterone-treated oocytes, XK-ras2AG12V-coinjected oocytes expressed a fivefold higher XENaC-mediated macroscopic Na+ current that was as high as that of control oocytes. Thus, the Na+ current per surface-expressed XENaC was increased by XK-Ras2AG12V. The chemical driving force for Na+ influx was not changed, suggesting that XK-Ras2AG12V increased the mean activity of XENaCs at the oocyte surface. These observations raise the possibility that XK-Ras2A, which is the first regulatory protein known to be transcriptionally induced by aldosterone, could play a role in the control of XENaC function in aldosterone target cells.

Epithelial sodium channel regulatory proteins identified by functional expression cloning

We describe here our current strategy for identifying and cloning proteins involved in the regulation of the epithelial sodium channel (ENaC). We have set up a complementation functional assay in the Xenopus laevis oocyte expression system. Using this assay, we have been able to identify a channel-activating protease (CAP-1) that can increase ENaC activity threefold. We propose a novel extracellular signal transduction pathway controlling ionic channels of the ENaC gene family that include genes involved in mechanotransduction (degenerins), in peptide-gated channels involved in neurotransmission (FaNaCh), in proton-gated channels involved in pH sensing (ASIC) or pain sensation (DRASIC).

Amiloride-sensitive Na channels

The emerging epithelial Na channel/degenerin family of sodium channels is rapidly expanding, in particular with new members expressed in mammalian neurons and potentially involved in pain transmission. Experimental evidence supports a four-subunit stoichiometry for these channels (although this is still controversial), and basic functional elements (pore and selectivity filter, amiloride binding site, gating) have started to be attributed to specific domains of the protein. Although much remains to be done, in the past year progress has been made in the understanding of several regulatory mechanisms: the control of epithelial Na channel translation by mineralocorticoid hormones, the role of endocytosis and ubiquitination for degradation in the control of the channel density and the role of extracellular proteases.

Molecular cloning of a DEG/ENaC sodium channel cDNA from human testis

The number of members of the recently defined DEG/ENaC sodium channel superfamily is increasing. Their importance in Na transport, taste perception, acid sensing, and mechanotransduction has been implicated. We have cloned a new member of this superfamily from human testis, which was named hTNaC1 (for human testis sodium channel 1). The hTNaC1 has 532 amino acid residues with two hydrophobic transmembrane domains. It has the highest identity (82%) with a rat H(+)-gated Na channel specific for sensory neurons (DRASIC) and a low identity (29%) with an epithelial isoform (alpha-ENaC) of this superfamily. Northern blot of human tissues revealed its selective expression in testis (7 kb) and absence in other tissues. The identification of a new member of Na channel specifically expressed in testis will expand the role of this channel family to the reproduction physiology.