Zn2+ and H+ are coactivators of acid-sensing ion channels.

Acid-sensing ion channels (ASICs) are cationic channels activated by extracellular protons. They are expressed in sensory neurons, where they are thought to be involved in pain perception associated with tissue acidosis. They are also expressed in brain. A number of brain regions, like the hippocampus, contain large amounts of chelatable vesicular Zn(2+). This paper shows that Zn(2+) potentiates the acid activation of homomeric and heteromeric ASIC2a-containing channels (i.e. ASIC2a, ASIC1a+2a, ASIC2a+3), but not of homomeric ASIC1a and ASIC3. The EC(50) for Zn(2+) potentiation is 120 and 111 microm for the ASIC2a and ASIC1a+2a current, respectively. Zn(2+) shifts the pH dependence of activation of the ASIC1a+2a current from a pH(0.5) of 5.5 to 6.0. Systematic mutagenesis of the 10 extracellular histidines of ASIC2a leads to the identification of two residues (His-162 and His-339) that are essential for the Zn(2+) potentiating effect. Mutation of another histidine residue, His-72, abolishes the pH sensitivity of ASIC2a. This residue, which is located just after the first transmembrane domain, seems to be an essential component of the extracellular pH sensor of ASIC2a.

Isolation of a tarantula toxin specific for a class of proton-gated Na+ channels.

Acid sensing is associated with nociception, taste transduction, and perception of extracellular pH fluctuations in the brain. Acid sensing is carried out by the simplest class of ligand-gated channels, the family of H(+)-gated Na(+) channels. These channels have recently been cloned and belong to the acid-sensitive ion channel (ASIC) family. Toxins from animal venoms have been essential for studies of voltage-sensitive and ligand-gated ion channels. This paper describes a novel 40-amino acid toxin from tarantula venom, which potently blocks (IC(50) = 0.9 nm) a particular subclass of ASIC channels that are highly expressed in both central nervous system neurons and sensory neurons from dorsal root ganglia. This channel type has properties identical to those described for the homomultimeric assembly of ASIC1a. Homomultimeric assemblies of other members of the ASIC family and heteromultimeric assemblies of ASIC1a with other ASIC subunits are insensitive to the toxin. The new toxin is the first high affinity and highly selective pharmacological agent for this novel class of ionic channels. It will be important for future studies of their physiological and physio-pathological roles.

Cloning and functional expression of a novel degenerin-like Na+ channel gene in mammals.

1. A degenerate polymerase chain reaction (PCR) homology screening procedure was applied to rat brain cDNA in order to identify novel genes belonging to the amiloride-sensitive Na+ channel and degenerin (NaC/DEG) family of ion channels. A single gene was identified that encodes a protein related to but clearly different from the already cloned members of the family (18-30 % amino acid sequence identity). Phylogenetic analysis linked this protein to the group of ligand-gated channels that includes the mammalian acid-sensing ion channels and the Phe-Met-Arg-Phe-amide (FMRFamide)-activated Na+ channel. 2. Expression of gain-of-function mutants after cRNA injection into Xenopus laevis oocytes or transient transfection of COS cells induced large constitutive currents. The activated channel was amiloride sensitive (IC50, 1.31 microM) and displayed a low conductance (9-10 pS) and a high selectivity for Na+ over K+ (ratio of the respective permeabilities, PNa+/PK+ >= 10), all of which are characteristic of NaC/DEG channel behaviour. 3. Northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed a predominant expression of its mRNA in the small intestine, the liver (including hepatocytes) and the brain. This channel has been called the brain-liver-intestine amiloride-sensitive Na+ channel (BLINaC). 4. Corresponding gain-of-function mutations in Caenorhabditis elegans degenerins are responsible for inherited neurodegeneration in the nematode. Besides the BLINaC physiological function that remains to be established, mutations in this novel mammalian degenerin-like channel might be of pathophysiological importance in inherited neurodegeneration and liver or intestinal pathologies.

H(+)-gated cation channels.

H(+)-gated cation channels are members of a new family of ionic channels, which includes the epithelial Na+ channel and the FMRFamide-activated Na+ channel. ASIC, the first member of the H(+)-gated Na+ channel subfamily, is expressed in brain and dorsal root ganglion cells (DRGs). It is activated by pHe variations below pH 7. The presence of this channel throughout the brain suggests that the H+ might play an essential role as a neurotransmitter or neuromodulator. The ASIC channel is also present in dorsal root ganglion cells, as is its homolog DRASIC, which is specifically present in DRGs and absent in the brain. Since external acidification is a major factor in pain associated with inflammation, hematomas, cardiac or muscle ischemia, or cancer, these two channel proteins are potentially central players in pain perception. ASIC activates and inactivates rapidly, while DRASIC has both a fast and sustained component. Other members of this family such as MDEG1 and MDEG2 are either H(+)-gated Na+ channels by themselves (MDEG1) or modulators of H(+)-gated channels formed by ASIC and DRASIC. MDEG1 is of particular interest because the same mutations that produce selective neurodegeneration in C. elegans mechanosensitive neurons, when introduced in MDEG1, also produce neurodegeneration. MDEG2 is selectively expressed in DRGs, where it assembles with DRASIC to radically change its biophysical properties, making it similar to the native H(+)-gated channel, which is presently the best candidate for pain perception.

Mutations causing neurodegeneration in Caenorhabditis elegans drastically alter the pH sensitivity and inactivation of the mammalian H+-gated Na+ channel MDEG1.

The mammalian degenerin MDEG1 belongs to the nematode degenerin/epithelial Na+ channel superfamily. It is constitutively activated by the same mutations that cause gain-of-function of the Caenorhabditis elegans degenerins and neurodegeneration. ASIC and DRASIC, which were recently cloned, are structural homologues of MDEG1 and behave as H+-gated cation channels. MDEG1 is also a H+-activated Na+ channel, but it differs from ASIC in its lower pH sensitivity and slower kinetics. In addition to the generation of a constitutive current, mutations in MDEG1 also alter the properties of the H+-gated current. Replacement of Gly-430 in MDEG1 by bulkier amino acids, such as Val, Phe, or Thr, drastically increases the H+ sensitivity of the channel (half-maximal pH (pHm) approximately 4.4 for MDEG1, pHm approximately 6.7 for the different mutants). Furthermore, these replacements completely suppress the inactivation observed with the wild-type channel and increase the sensitivity of the H+-gated channel to blockade by amiloride by a factor of 10 without modification of its conductance and ionic selectivity. These results as well as those obtained with other mutants clearly indicate that the region surrounding Gly-430, situated just before the second transmembrane segment, is essential for pH sensitivity and gating.

dGNaC1, a gonad-specific amiloride-sensitive Na+ channel.

Amiloride-sensitive sodium channels have been implicated in reproductive and early developmental processes of several species. These include the fast block of polyspermy in Xenopus oocytes that follows the sperm binding to the egg or blastocoel expansion in mammalian embryo. We have now identified a gene called dGNaC1 that is specifically expressed in the gonads and early embryo in Drosophila melanogaster. The corresponding protein belongs to the superfamily of cationic channels blocked by amiloride that includes Caenorhabditis elegans degenerins, the Helix aspersa FMRF-amide ionotropic receptor (FaNaC), the mammalian epithelial Na+ channel (ENaC), and acid-sensing ionic channels (ASIC, DRASIC, and MDEG). Expression of dGNaC1 in Xenopus oocytes generates a constitutive current that does not discriminate between Na+ and Li+, but is selective for Na+ over K+. This current is blocked by amiloride (IC50 = 24 microM), benzamil (IC50 = 2 microM), and ethylisopropyl amiloride (IC50 = 49 microM). These properties are clearly different from those obtained after expression of the previously cloned members of this family, including ENaC and the human alphaENaC-like subunit, deltaNaC. Interestingly, the pharmacology of dGNaC1 is not very different from that found for the Na+ channel characterized in rabbit preimplantation embryos. We postulate that this channel may participate in gametogenesis and early embryonic development in Drosophila.

Genotype-phenotype analysis of a newly discovered family with Liddle's syndrome.

OBJECTIVE: To investigate the clinical, biologic, and molecular abnormalities in a family with Liddle's syndrome and analyze the short- and long-term efficacies of amiloride treatment. PATIENTS: The pedigree consisted of one affected mother and four children, of whom three suffered from early-onset and moderate-to-severe hypertension. METHODS: In addition to the biochemical and hormonal measurements, genetic analysis of the carboxy terminus of the beta subunit of the epithelial sodium channel (beta ENaC) was conducted through single-strand conformation analysis and direct sequencing. The functional properties of the mutation were analyzed using the Xenopus expression system and compared with one mutation affecting the proline-rich sequence of the beta ENaC. RESULTS: Mild hypokalemia and suppressed levels of plasma renin and aldosterone were observed in all affected subjects. Administration of 10 mg/day amiloride for 2 months normalized the blood pressure and plasma potassium levels of all of the affected subjects, whereas their plasma and urinary aldosterone levels remained surprisingly low. A similar pattern was observed after 11 years of follow-up, but a fivefold increase in plasma aldosterone was observed under treatment with 20 mg/day amiloride for 2 weeks. Genetic analysis of the beta ENaC revealed a deletion of 32 nucleotides that had modified the open reading frame and introduced a stop codon at position 582. Expression of this beta 579del32 mutant caused a 3.7 +/- 0.3-fold increase in the amiloride-sensitive sodium current, without modification of the unitary properties of the channel. A similar increase was elicited by one mutation affecting the carboxy terminus of the beta ENaC. CONCLUSIONS: This new mutation leading to Liddle's syndrome highlights the importance of the carboxy terminus of the beta ENaC in the activity of the epithelial sodium channel. Small doses of amiloride are able to control the blood pressure on a long-term basis in this monogenic form of hypertension.

The amiloride-sensitive Na+ channel: from primary structure to function.

Three homologous subunits of the amiloride-sensitive Na+ channel, entitled alpha, beta, and gamma, have been cloned either from distal colon of a steroid-treated rat or from human lung. The alpha, beta, and gamma subunits have similarities with degenerins, a family of proteins found in the mechanosensory neurons of the nematode Caenorhabditis elegans. All these proteins are characterized by the presence of a large extracellular domain, located between two transmembrane alpha-helices, and by short NH2 and COOH terminal cytoplasmic segments. They constitute the first members of a new gene super-family of ionic channels. The epithelial Na+ channel is specifically expressed at the apical membrane of Na(+)-reabsorbing epithelial cells. Its activity is controlled by several distinct hormones, especially by corticosteroids. These hormones act either transcriptionally (such as aldosterone in distal colon, or glucocorticoids in lung) and/or post-transcriptionally (such as aldosterone in kidney). Recent works have provided new insights in the function of that important osmoregulatory system.

The acid-sensitive ionic channel subunit ASIC and the mammalian degenerin MDEG form a heteromultimeric H+-gated Na+ channel with novel properties.

Proton-gated cation channels are acid sensors that are present in both sensory neurons and in neurons of the central nervous system. One of these acid-sensing ion channels (ASIC) has been recently cloned. This paper shows that ASIC and the mammalian degenerin MDEG, which are colocalized in the same brain regions, can directly associate with each other. Immunoprecipitation of MDEG causes coprecipitation of ASIC. Moreover, coexpression of ASIC and MDEG subunits in Xenopus oocytes generates an amiloride-sensitive H+-gated Na+ channel with novel properties (different kinetics, ionic selectivity, and pH sensitivity). In addition, coexpression of MDEG with mutants of the ASIC subunit can create constitutively active channels that become completely nonselective for Na+ versus K+ and H+-gated channels that have a drastically altered pH sensitivity compared with MDEG. These data clearly show that ASIC and MDEG can form heteromultimeric assemblies with novel properties. Heteromultimeric assembly is probably used for creating a diversity of H+-gated cation channels acting as neuronal acid sensors in different pH ranges.

Molecular cloning of a non-inactivating proton-gated Na+ channel specific for sensory neurons.

We have cloned and expressed a novel proton-gated Na+ channel subunit that is specific for sensory neurons. In COS cells, it forms a Na+ channel that responds to a drop of the extracellular pH with both a rapidly inactivating and a sustained Na+ current. This biphasic kinetic closely resembles that of the H+-gated current described in sensory neurons of dorsal root ganglia (1). Both the abundance of this novel H+-gated Na+ channel subunit in sensory neurons and the kinetics of the channel suggest that it is part of the channel complex responsible for the sustained H+-activated cation current in sensory neurons that is thought to be important for the prolonged perception of pain that accompanies tissue acidosis (1, 2).

A proton-gated cation channel involved in acid-sensing.

Acid-sensing is associated with both nociception and taste transduction. Stimulation of sensory neurons by acid is of particular interest, because acidosis accompanies many painful inflammatory and ischaemic conditions. The pain caused by acids is thought to be mediated by H+-gated cation channels present in sensory neurons. We have now cloned a H+-gated channel (ASIC, for acid-sensing ionic channel) that belongs to the amiloride-sensitive Na+ channel/degenerin family of ion channels. Heterologous expression of ASIC induces an amiloride-sensitive cation (Na+ > Ca2+ > K+) channel which is transiently activated by rapid extracellular acidification. The biophysical and pharmacological properties of the ASIC channel closely match the H+-gated cation channel described in sensory neurons. ASIC is expressed in dorsal root ganglia and is also distributed widely throughout the brain. ASIC appears to be the simplest of ligand-gated channels.

Lack of effect of Presenilin 1, betaAPP and their Alzheimer's disease-related mutated forms on Xenopus oocytes membrane currents.

The effect of the microinjection of Xenopus oocytes with various cRNAs coding for Presenilin 1 and four mutated presenilins linked to early onset familial forms of Alzheimer's disease was examined. These cRNAs were injected either alone or in combination with the cRNA encoding betaAPP751 and the Swedish mutated form of betaAPP751 known to produce exacerbated amount of Abeta. Current-voltage relationships generated by voltage step were recorded. None of the cRNA injected alone or in combination displayed the ability to modify the current recorded with naive cells. Altogether, this study shows that Presenilin 1 does not mediate membrane currents and is more likely involved in the physiopathological maturation of betaAPP.

The mammalian degenerin MDEG, an amiloride-sensitive cation channel activated by mutations causing neurodegeneration in Caenorhabditis elegans.

Mutations of the degenerins (deg-1, mec-4, mec-10) are the major known causes of hereditary neurodegeneration in the nematode Caenorhabditis elegans. We cloned a neuronal degenerin (MDEG) from human and rat brain. MDEG is an amiloride-sensitive cation channel permeable for Na+, K+, and Li+. This channel is activated by the same mutations which cause neurodegeneration in C. elegans. Like the hyperactive C. elegans degenerin mutants, constitutively active mutants of MDEG cause cell death, suggesting that gain of function of this novel neuronal ion channel might be involved in human forms of neurodegeneration.

[The amiloride sensitive sodium channel]. / Le canal Na+ sensible à l'amiloride.

The amiloride-sensitive epithelial Na+ channel is formed by the assembly of three homologous subunits alpha, beta and gamma. The channel is characterized by its sensitivity to amiloride and to some amiloride derivatives, such as phenamil and benzamil, by its small unitary conductance (approximately 5pS), by its high selectivity for lithium and sodium, and by its slow kinetics. The alpha, beta, and gamma proteins share significant identity with degenerins, a family of proteins found in the mechanosensory neurons and interneurons of the nematode Caenorhabditis elegans. They are also homologous to FaNaCh, a protein from Helix aspersa nervous tissues, which corresponds to a neuronal ionotropic receptor for the Phe-Met-Arg-Phe-amide peptide. All these proteins contain a large extracellular loop, located between two transmembrane alpha-helices. The NH2 and COOH terminal segments are cytoplasmic, and contain potential regulatory segments that are able to modulate the activity of the channel. In Liddle syndrome, in which patients develop a form of genetic hypertension, mutations within the cytoplasmic COOH terminal of the beta and gamma chains of the epithelial Na+ channel lead to a hyper-activity of the channel. Epithelial Na+ channel activity is tightly controlled by several distinct hormonal systems, including corticosteroids and vasopressin. In kidney and colon, aldosterone is the major sodium-retaining hormone, acting, by stimulation of Na+ reabsorption through the epithelium. In the distal colon from steroid-treated animals, a large increase of the beta and gamma subunits transcription is observed, whereas the alpha subunit remains constitutively transcribed. In kidney, RNA levels of the three subunits are not significantly altered by aldosterone, suggesting that other mechanisms control Na+ channel activity in that tissue. In lung, the glucocorticoids are the positive regulators of the channel activity, especially around birth, and act via an increased transcription of the three subunits.

Cloning of the amiloride-sensitive FMRFamide peptide-gated sodium channel.

The peptide Phe-Met-Arg-Phe-NH2 (FMRFamide) and structurally related peptides are present both in invertebrate and vertebrate nervous systems. Although they constitute a major class of invertebrate peptide neurotransmitters, the molecular structure of their receptors has not yet been identified. In neurons of the snail Helix aspersa, as well as in Aplysia bursting and motor neurons, FMRFamide induces a fast excitatory depolarizing response due to direct activation of an amiloride-sensitive Na+ channel. We have now isolated a complementary DNA from Helix nervous tissue; when expressed in Xenopus oocytes, it encodes an FMRFamide-activated Na+ channel (FaNaCh) that can be blocked by amiloride. The corresponding protein shares a very low sequence identity with the previously cloned epithelial Na+ channel subunits and Caenorhabditis elegans degenerins, but it displays the same overall structural organization. To our knowledge, this is the first characterization of a peptide-gated ionotropic receptor.

Molecular cloning and functional expression of a novel amiloride-sensitive Na+ channel.

We have isolated a cDNA for a novel human amiloride-sensitive Na+ channel isoform (called delta) which is expressed mainly in brain, pancreas, testis, and ovary. When expressed in Xenopus oocytes, it generates an amiloride-sensitive Na+ channel with biophysical and pharmacological properties distinct from those of the epithelial Na+ channel, a multimeric assembly of alpha, beta, and gamma subunits. The Na+ current produced by the new delta isoform is increased by two orders of magnitude after coexpression of the beta and gamma subunit of the epithelial Na+ channel showing that delta can associate with other subunits and is part of a novel multisubunit ion channel.

A change in gating mode leading to increased intrinsic Cl- channel activity compensates for defective processing in a cystic fibrosis mutant corresponding to a mild form of the disease.

The effects of the mild cystic fibrosis (CF) mutation P574H were analysed and compared with those of three severe ones (delta I507, delta F508 and R560T). Immunochemical and functional analyses indicate that the rank order of CFTR expression at the cell surface is: wild type CFTR > P574H >> delta F508 >> R560T approximately 0. Patch-clamp analysis indicates that the open probability of P574H Cl- channels is almost twice as high as that of the wild type CFTR-Cl- channel. This increased intrinsic activity of individual P574H CFTR-Cl- channels compensates for the lower number of P574H CFTR-Cl- channels reaching the cell surface, and probably explains the milder form of CF associated with the P574H mutation. NS004, a recently described activator, restores near normal CFTR activity in cells expressing the P574H-CFTR channel. The P574H mutation modifies the gating mode of the channel with a large increase (approximately x 7) in the mean channel open time. Proline 574 might play an important role in the process connecting ATP hydrolysis at the nucleotide binding domain and opening and closing events of the CFTR-Cl- channel.

Functional degenerin-containing chimeras identify residues essential for amiloride-sensitive Na+ channel function.

The highly selective, amiloride-sensitive Na+ channel is formed of three homologous subunits termed alpha, beta, and gamma. The three subunits exhibit similarities with Caenorhabditis elegans proteins called degenerins involved in sensory touch transduction and, when mutated, in neurodegeneration. Swelling of neurons observed in neurodegeneration suggests an involvement of ion transport, but the channel function of degenerins has not yet been demonstrated. We used chimeras to study the functional relationship between the epithelial sodium channel and the degenerin Mec-4. Exchange of the hydrophobic domains of the Na+ channel alpha subunit by those of Mec-4 results in a functional ion channel with changed pharmacology for amiloride and benzamil and changed selectivity, conductance, gating, and voltage dependence. All of these differences were also obtained by exchanging Ser-589 and Ser-593 in the second transmembrane region by the corresponding residues of Mec-4, suggesting that these two residues are essential for the ionic pore function of the channel.

Different homologous subunits of the amiloride-sensitive Na+ channel are differently regulated by aldosterone.

Long term regulation of the amiloride-sensitive Na+ channel activity by steroid hormones occurs via de novo protein synthesis. The messenger level of RCNaCh1, previously shown by expression cloning to be a component of this channel, was measured in colons from rats fed with a low sodium diet. After 1 week of this diet, the channel activity was increased in an all-or-none fashion, whereas the level of RCNaCh1 messenger remained constant. A cDNA coding for another subunit of the Na+ channel was obtained by polymerase chain reaction. The 650-amino acid protein, entitled RCNaCh2, is 58% homologous to RCNaCh1 and displays a similar structure. It had no intrinsic activity when expressed alone in Xenopus oocytes, but its co-expression with RCNaCh1 increased the channel activity 18 +/- 5-fold. The increase in messenger level for RCNaCh2 during the time course of the diet is likely to explain the positive regulation of the rat colon Na+ channel by steroids. Immunocytochemical localization of the RCNaCh1 subunit revealed an apical labeling in colon from sodium-depleted rats. No labeling was observed in colon from control animals. These results suggest that oligomerization is needed for the proper expression of RCNaCh1 at the cell surface.

Regulation of expression of the lung amiloride-sensitive Na+ channel by steroid hormones.

Molecular cloning of the amiloride-sensitive Na+ channel has permitted analysis of the mechanisms of its stimulation by steroids. In rat lung cells in primary culture, where its mRNA has been detected, the activity of an amiloride-sensitive channel, highly selective for Na+, is controlled by corticosteroids. Dexamethasone (0.1 microM) or aldosterone (1 microM) induced, after a minimum 10 h treatment, a large increase of the amiloride-induced hyperpolarization and of the amiloride-sensitive current. A parallel increase in the amount of the mRNA was observed. The corresponding gene is thus a target for steroid action. Using synthetic specific agonists and antagonists for mineralo- and glucocorticoid receptors, it has been shown that the steroid action on Na+ channel expression is mediated via glucocorticoid receptors. Triiodothyronine, known to modulate steroid action in several tissues, had no effect on both the amiloride-sensitive Na+ current and the level of the mRNA for the Na+ channel protein, but potentiates the stimulatory effect of dexamethasone. The increase in Na+ channel activity observed in the lung around birth can thus be explained by a direct increase in transcription of the Na+ channel gene.