γ-Aminobutyric acid (GABA) is an important neurotransmitter that, through the subtype A GABA receptor (GABAAR), induces inhibition in the adult brain. Here we show that an excitatory, rather than inhibitory, GABAergic system exists in airway epithelial cells. Both GABAARs and the GABA synthetic enzyme glutamic acid decarboxylase (GAD) are expressed in pulmonary epithelial cells. Activation of GABAARs depolarized these cells. The expression of GAD in the cytosol and GABAARs in the apical membranes of airway epithelial cells increased markedly when mice were sensitized and then challenged with ovalbumin, an approach for inducing allergic asthmatic reactions. Similarly, GAD and GABAARs in airway epithelial cells of humans with asthma increased after allergen inhalation challenge. Intranasal application of selective GABAAR inhibitors suppressed the hyperplasia of goblet cells and the overproduction of mucus induced by ovalbumin or interleukin-13 in mice. These findings show that a previously unknown epithelial GABAergic system has an essential role in asthma.
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We thank M. Matthay (University of California) for isolated primary human type II epithelial cells; J. MacDonald, Y.T. Wang, M. Post, D. Bieger and M. Jackson for comments on the manuscript; B. Han for comments on some experiments; M. Zhuo and L. Wu for assistance in analysis of confocal microscopy; and J. Wattie and R. Ellis for assistance in the AHR experiment. This study was supported by grant MOP74653 (to W.-Y.L.) and grant MT-14680 (to X.Y.) from the Canadian Institutes of Health Research (CIHR). X.Y. is Canada Research Chair in Infection and Immunity. W.-Y.L is CIHR New Investigator.
The authors are filing a patent application for the role of the epithelial GABAA receptor signaling system in mucus production.
RT-PCR assays of GAD and GABAAR subunits in BEAS-2B cells. (PDF 82 kb)
The selective GABAAR antagonist bicuculline blocks the GABA-induced current in pulmonary EC cells. (PDF 24 kb)
The intracellular and extracellular alcian blue staining increase in the SAEC after GABA treatment. (PDF 45 kb)
The expression of GAD and GABAAR subunits increases in the lung of OVA-treated mice. (PDF 76 kb)
GABAAR β2 and β3 subunits are not expressed in the airway smooth muscle cells. (PDF 115 kb)
The expression levels of GAD and GABAARs do not increase in the airway epithelial cells in OVA-challenged IL-13 knockout mice. (PDF 115 kb)
GABAAR inhibitor does not affect inflammatory cells infiltration to the lung. (PDF 30 kb)
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