Receptor Species-dependent Desensitization Controls KCNQ1/KCNE1 K+ Channels as Downstream Effectors of Gq Protein-coupled Receptors

Abstract

Activation of G q protein-coupled receptors (G q PCRs) might induce divergent cellular responses, related to receptor-specific activation of different branches of the G q signaling pathway. Receptor-specific desensitization provides a mechanism of effector modulation by restricting the spatiotemporal activation of signaling components downstream of G q . We quantified signaling events downstream of G q PCR activation with FRETbased biosensors in CHO and HEK 293 cells. KCNQ1/KCNE1 channels (I Ks ) were measured as a functional readout of receptor-specific activation. Activation of muscarinic M 1 receptors (M 1 -Rs) caused robust and reversible inhibition of I Ks. In contrast, activation of 1B -adrenergic receptors ( 1B -ARs) induced transient inhibition of I Ks , which turned into delayed facilitation after agonist withdrawal. As a novel finding, we demonstrate that G q PCR-specific kinetics of I Ks modulation are determined by receptor-specific desensitization, evident at the level of G q activation, phosphatidylinositol 4,5-bisphosphate (PIP 2 ) depletion, and diacylglycerol production. Sustained I Ks inhibition during M 1 -R stimulation is attributed to robust membrane PIP 2 depletion, whereas the rapid desensitization of 1B -AR delimits PIP 2 reduction and augments current activation by protein kinase C (PKC). Overexpression of Ca 2 -independent PKC did not affect the time course of 1B -AR-induced diacylglycerol formation, excluding a contribution of PKC to 1B -AR desensitization. Pharmacological inhibition of Ca 2 -dependent PKC isoforms abolished fast 1B receptor desensitization and augmented I Ks reduction, but did not affect I Ks facilitation. These data indicate a contribution of Ca 2 -dependent PKCs to 1B -AR desensitization, whereas I Ks facilitation is induced by Ca 2 -independent PKC isoforms. In contrast, neither inhibition of Ca 2 -dependent/Ca 2 -independent isoforms nor overexpression of PKC induced M 1 receptor desensitization, excluding a contribution of PKC to M 1 -R-induced I Ks modulation.