title: Expression of ENaC and other transport proteins in Xenopus oocytes is modulated by intracellular Na+.
abstract: The expression of the epithelial Na+ channel (ENaC) is tissue-specific and dependent on a variety of mediators and interacting proteins. Here we examined the role of intracellular Na+ ([Na+](i)) as a modulator of the expression of rat ENaC in Xenopus laevis oocytes. We manipulated [Na+](i) of ENaC-expressing oocytes in the range of 0-20 mM by incubating in extracellular solutions of different [Na+](o). Electrophysiological, protein biochemical and fluorescence optical methods were used to determine the effects of different [Na+]i on ENaC expression and membrane abundance. In voltage-clamp experiments we found that amiloride-sensitive ENaC current (Iami) and conductance (Gami) peak at a [Na+](i) of approximately 10 mM Na+, but were significantly reduced in 5 mM and 20 mM [Na+](i). Fluorescence intensity of EGFP-ENaC-expressing oocytes also followed a bell-shaped curve with a maximum at approximately 10 mM [Na+](i). In Western blot experiments with specific anti-ENaC antibodies the highest protein expression was found in ENaC-expressing oocytes with [Na+](i) of 10-15 mM. Since ENaC is also highly permeable for Li+, we incubated ENaC-expressing oocytes in different Li+ concentrations and found a peak of Iami and Gami with 5 mM Li+. The influence of [Na+](i) on the expression is not ENaC-specific, since expression of a Cl(-) channel (CFTR) and a Na+/glucose cotransporter (SGLT1) showed the same dependence on [Na+](i). We conclude that specific concentrations of Na+ and Li+ influence the expression and abundance of ENaC and other transport proteins in the plasma membrane in Xenopus laevis oocytes. Furthermore, we suggest the existence of a general mechanism dependent on monovalent cations that optimizes the expression of membrane proteins.
authors.affiliation: Institute of Animal Physiology, Westphalian Wilhelms-University Muenster, Germany.
authors.forename: Kristina
authors.initials: K
authors.lastname: Kusche-Vihrog
authors.forename: Andrei
authors.initials: A
authors.lastname: Segal
authors.forename: Ryszard
authors.initials: R
authors.lastname: Grygorczyk
authors.forename: Nadine
authors.initials: N
authors.lastname: Bangel-Ruland
authors.forename: Willy
authors.initials: W
authors.lastname: Van Driessche
authors.forename: Wolf-Michael
authors.initials: WM
authors.lastname: Weber
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000235:genetics / Q000378:metabolism; D004594:Electrophysiology; D053503:Epithelial Sodium Channels / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression / Q000235:genetics / Q000502:physiology; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D008954:Models, Biological; D009865:Oocytes / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D051269:Sodium-Glucose Transporter 1 / Q000235:genetics / Q000378:metabolism; D029867:Xenopus Proteins / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis / Q000235:genetics / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D053503:Epithelial Sodium Channels; D051269:Sodium-Glucose Transporter 1; D029867:Xenopus Proteins; C485184:enhanced green fluorescent protein; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D049452:Green Fluorescent Proteins; D012964:Sodium