István Czikora, Abdel Alli, Hui-Fang Bao, David Kaftan, Supriya Sridhar, Hans-Jürgen Apell, Boris Gorshkov, Richard White, Astrid Zimmermann, Albrecht Wendel, Meike Pauly-Evers, Jürg Hamacher, Irène Garcia-Gabay, Bernhard Fischer, Alexander Verin, Zsolt Bagi, Jean Francois Pittet, Waheed Shabbir, Rosa Lemmens-Gruber, Trinad Chakraborty, Ahmed Lazrak, Michael A. Matthay, Douglas C. Eaton, and Rudolf Lucas Am. J. Resp. Crit. Care Med. Sep 1, 2014, vol. 190, no. 5: 522-532
Rationale: Alveolar liquid clearance is regulated by Na+ uptake through the apically expressed epithelial sodium channel (ENaC) and basolaterally localized Na+-K+-ATPase in type II alveolar epithelial cells. Dysfunction of these Na+ transporters during pulmonary inflammation can contribute to pulmonary edema.
Objectives: In this study, we sought to determine the precise mechanism by which the TIP peptide, mimicking the lectin-like domain of tumor necrosis factor (TNF), stimulates Na+ uptake in a homologous cell system in the presence or absence of the bacterial toxin pneumolysin (PLY).
Methods: We used a combined biochemical, electrophysiological, and molecular biological in vitro approach and assessed the physiological relevance of the lectin-like domain of TNF in alveolar liquid clearance in vivo by generating triple-mutant TNF knock-in mice that express a mutant TNF with deficient Na+ uptake stimulatory activity.
Measurements and Main Results: TIP peptide directly activates ENaC, but not the Na+-K+-ATPase, upon binding to the carboxy-terminal domain of the α subunit of the channel. In the presence of PLY, a mediator of pneumococcal-induced pulmonary edema, this binding stabilizes the ENaC-PIP2-MARCKS complex, which is necessary for the open probability conformation of the channel and preserves ENaC-α protein expression, by means of blunting the protein kinase C-α pathway. Triple-mutant TNF knock-in mice are more prone than wild-type mice to develop edema with low-dose intratracheal PLY, correlating with reduced pulmonary ENaC-α subunit expression.
Conclusions: These results demonstrate a novel TNF-mediated mechanism of direct ENaC activation and indicate a physiological role for the lectin-like domain of TNF in the resolution of alveolar edema during inflammation.
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