Ionotropic GABA receptor expression in the lung during development

Abstract

Cl − transport is essential for lung development. Because gamma-aminobutyric acid (GABA) receptors allow the flow of negatively-charged Cl − ions across the cell membrane, we hypothesized that the expression of ionotropic GABA receptors are regulated in the lungs during development. We identified 17 GABA receptor subunits in the lungs by real-time PCR. These subunits were categorized into 4 groups: Group 1 had high mRNA expression during fetal stages and low in adults; Group 2 had steady expression to adult stages with a slight up-regulation at birth; Group 3 showed an increasing expression from fetal to adult lungs; and Group 4 displayed irregular mRNA fluctuations. The protein levels of selected subunits were also determined by Western blots and some subunits had protein levels that corresponded to mRNA levels. Further studied subunits were primarily localized in epithelial cells in the developing lung with differential mRNA expression between isolated cells and whole lung tissues. Our results add to the knowledge of GABA receptor expression in the lung during development. Keywordslungs; ionotropic GABA receptor; fetal lung development; chloride homeostasis; lung fluid transport; real time PCR; gene expression; alveolar epithelial cells RESULTS AND DISCUSSION SignificanceCl − channels play an important role in the lung throughout an animal's life. During the fetal stages, the lung is a fluid-secreting organ. The main driving force for fluid secretion is the active secretion of Cl − into the developing lungs. Therefore, Cl − channels are indispensable for fetal lung development and morphogenesis (Folkesson et al., 1998;Olver et al., 2004;Bland and Nielson, 1992). On the other hand, the adult lung is air-filled and maintains a "dry" state. The apical surface is covered by a thin layer of alveolar surface fluid. This layer of fluid is required for the proper air exchange and airway defenses. Cl − channels are also essential for maintaining the composition and volume of the alveolar surface fluid (Brochiero et al., 2004 Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Several Cl − channels have previously been studied in the lung. The cystic fibrosis transmembrane conductance regulator (CFTR) (Harris et al., 1991), the voltage-gated Cl − channels, ClC2 (Thiemann et al., 1992), ClC3 (Lamb et al., 2001), and ClC5 (Edmonds et al., 2002, and Na + -K + -Cl − cotransporter (NKCC) (Rochelle et al., 2000) have been detected in fetal lungs, distal airway epithelial cells and alveolar epithelial cells. Some of the channels have been shown to be involved in fluid homeostasis i...

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