Receptor-mediated airway smooth muscle (ASM) contraction via Gαq, and relaxation via Gαs, underlie the bronchospastic features of asthma and its treatment. Asthma models show increased ASM Gαi expression, considered the basis for the proasthmatic phenotypes of enhanced bronchial hyperreactivity to contraction mediated by M3-muscarinic receptors and diminished relaxation mediated by β2-adrenergic receptors (β2ARs). A causal effect between Gi expression and phenotype has not been established, nor have mechanisms whereby Gi modulates Gq/Gs signaling. To delineate isolated effects of altered Gi, transgenic mice were generated overexpressing Gαi2 or a Gαi2 peptide inhibitor in ASM. Unexpectedly, Gαi2 overexpression decreased contractility to methacholine, while Gαi2 inhibition enhanced contraction. These opposite phenotypes resulted from different crosstalk loci within the Gq signaling network: decreased phospholipase C and increased PKCα, respectively. Gαi2 overexpression decreased β2AR-mediated airway relaxation, while Gαi2 inhibition increased this response, consistent with physiologically relevant coupling of this receptor to both Gs and Gi. IL-13 transgenic mice (a model of asthma), which developed increased ASM Gαi, displayed marked increases in airway hyperresponsiveness when Gαi function was inhibited. Increased Gαi in asthma is therefore a double-edged sword: a compensatory event mitigating against bronchial hyperreactivity, but a mechanism that evokes β-agonist resistance. By selective intervention within these multipronged signaling modules, advantageous Gs/Gq activities could provide new asthma therapies.
Dennis W. McGraw, Jean M. Elwing, Kevin M. Fogel, Wayne C.H. Wang, Clare B. Glinka, Kathryn A. Mihlbachler, Marc E. Rothenberg, Stephen B. Liggett