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      Epithelial Sodium Channel: A Ligand-Gated Channel?

      a , b

      Nephron Physiology

      S. Karger AG

      Amiloride, Epithelial sodium channel, Protease, Self-inhibition

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          The epithelial sodium channel (ENaC) is a key component of the transepithelial Na<sup>+</sup> transport. In epithelia, it is responsible for the maintenance of Na<sup>+</sup> balance (which in turn controls extracellular fluid volume and arterial blood pressure) and the regulation of airway surface fluid. While the regulation of channel synthesis and surface density have been well described, the control of channel opening is still poorly understood. The channel has a large extracellular domain of as yet unknown function; a number of extracellular factors have been shown to modulate ENaC activity, including extracellular Na<sup>+</sup> itself (through a phenomenon called ‘self-inhibition’), several other organic or inorganic cations, which seem to interfere with self-inhibition, and serine proteases. Although a direct interaction with the extracellular domain of ENaC has not yet been demonstrated for each of these modulators, the available data strongly suggest that ENaC behaves as a ligand-gated channel similar to several other members of the ENaC/degenerin family.

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          Most cited references 13

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          The mec-4 gene is a member of a family of Caenorhabditis elegans genes that can mutate to induce neuronal degeneration.

          Three dominant mutations of mec-4, a gene needed for mechanosensation, cause the touch-receptor neurons of Caenorhabditis elegans to degenerate. With deg-1, another C. elegans gene that can mutate to induce neuronal degeneration and that is similar in sequence, mec-4 defines a new gene family. Cross-hybridizing sequences are detectable in other species, raising the possibility that degenerative conditions in other organisms may be caused by mutations in similar genes. All three dominant mec-4 mutations affect the same amino acid. Effects of amino-acid substitutions at this position suggest that steric hindrance may induce the degenerative state.
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            An epithelial serine protease activates the amiloride-sensitive sodium channel.

            Sodium balance, and ultimately blood pressure and extracellular fluid volume, is maintained by precise regulation of the activity of the epithelial sodium channel (ENaC). In a Xenopus kidney epithelial cell line (A6), exposure of the apical membrane to the protease inhibitor aprotinin reduces transepithelial sodium transport. Sodium-channel activity can be restored by subsequent exposure to the nonspecific protease trypsin. Using A6 cells and a functional complementation assay to detect increases in ENaC activity, we have cloned a 329-residue protein belonging to the serine protease family. We show that coexpression of this protein with ENaC in Xenopus oocytes increases the activity of the sodium channel by two- to threefold. This channel-activating protease (CAP1) is expressed in kidney, gut, lung, skin and ovary. Sequence analysis predicts that CAP1 is a secreted and/or glycosylphosphatidylinositol-anchored protein: ENaC activity would thus be regulated by the activity of a protease expressed at the surface of the same cell. This previously undiscovered mechanism for autocrine regulation may apply to other ion channels, in particular to members of the ENaC family that are present in neurons and epithelial cells.
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              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.

                Author and article information

                Nephron Physiol
                Nephron Physiology
                S. Karger AG
                February 2004
                27 February 2004
                : 96
                : 2
                : p37-p41
                aInstitut de Pharmacologie et de Toxicologie, Université de Lausanne, Lausanne, Switzerland; bFaculté de Médecine, Département de Physiologie et Biophysique, Université de Sherbrooke, Sherbrooke, Canada
                76406 Nephron Physiol 2004;96:p37–p41
                © 2004 S. Karger AG, Basel

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                Page count
                Figures: 1, References: 24, Pages: 1
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                Cardiovascular Medicine, Nephrology

                Self-inhibition, Amiloride, Protease, Epithelial sodium channel


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