Drosophila DEG/ENaC pickpocket genes are expressed in the tracheal system, where they may be involved in liquid clearance

Abstract

The Drosophila tracheal system and mammalian airways are branching networks of tubular epithelia that deliver oxygen to the organism. In mammals, the epithelial Na ؉ channel (ENaC) helps clear liquid from airways at the time of birth and removes liquid from the airspaces in adults. We tested the hypothesis that related Drosophila degenerin (DEG)͞ENaC family members might play a similar role in the fly. Among 16 Drosophila DEG͞ENaC genes, called pickpocket (PPK) genes, we found 9 expressed in the tracheal system. By in situ hybridization, expression appeared in late-stage embryos after tracheal tube formation, with individual PPK genes showing distinct temporal and spatial expression patterns as development progressed. Promoters for several PPK genes drove reporter gene expression in the larval and adult tracheal systems. Adding the DEG͞ENaC channel blocker amiloride to the medium inhibited liquid clearance from the trachea of first instar larvae. Moreover, when RNA interference was used to silence PPK4 and PPK11, larvae failed to clear tracheal liquid. These data suggest substantial molecular diversity of DEG͞ENaC channel expression in the Drosophila tracheal system where the PPK proteins likely play a role in Na ؉ absorption. Extensive similarities between Drosophila and mammalian airways offer opportunities for genetic studies that may decipher further the structure and function of DEG͞ENaC proteins and development of the airways.trachea ͉ Na ϩ channel ͉ absorption ͉ epithelia ͉ amiloride C omplex multicellular organisms require an elaborate oxygen delivery system to meet tissue metabolic demands. In mammalian lung, a network of branching epithelial tubes carries oxygen to the alveoli, where it diffuses into blood to be distributed to the tissues. Drosophila melanogaster also possesses a network of branching epithelial tubes through which oxygen diffuses to reach cells throughout the organism (1). In addition to their functional similarity, the growth factors and transcription factors controlling branching morphogenesis in mammalian airways and the fly tracheal system share numerous features (2). Moreover, at specific points in development, both systems must convert from liquid-to air-filled tubules. In mammals, this conversion occurs at birth, when the epithelium actively absorbs salt and hence liquid from the airways. In Drosophila, this conversion occurs 2 h before larvae emerge, also by active epithelial absorption (1).In mammalian airways, the epithelial Na ϩ channel (ENaC), composed of ␣-, ␤-, and ␥-ENaC subunits, contributes to salt and liquid absorption (3, 4). Striking evidence for this role came from mice bearing targeted disruptions of ENaC genes. Mice with a disrupted ␣-ENaC gene died shortly after birth, because they failed to clear lung liquid, and mice with disrupted ␤-or ␥-ENaC genes showed delayed lung liquid clearance (5-7). In humans, impaired ENaC activity may contribute to neonatal respiratory distress syndrome (8). ENaC-dependent salt and liquid absorption may also help clear pulmonary ed...

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