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PKC activation and PIP(2) depletion underlie biphasic regulation of IKs by Gq-coupled receptors.

ABSTRACT: KCNQ1 is co-assembled with KCNE1 subunits in the heart to form the cardiac delayed rectifier K(+) current (IKs), which is one of the main currents responsible for myocyte repolarization. The most commonly inherited form of cardiac arrhythmias, long-QT syndrome type 1 (LQT1), is due to mutations on KCNQ1. Gq-coupled receptors (GqPCRs) are known to mediate positive inotropism in human ventricular myocardium. The mechanism of IKs current modulation by GqPCRs remains incompletely understood. Here we studied the molecular mechanisms underlying Gq regulation of the IKs channel. Heterologously expressed IKs (human KCNQ1/KCNE1 subunits) was measured in Xenopus oocytes, expressed together with GqPCRs. Our data from several GqPCRs shows that IKs is regulated in a biphasic manner, showing both an activation and an inhibition phase. Receptor-mediated inhibition phase was irreversible when recycling of agonist-sensitive pools of phosphatidylinositol-4,5-bisphosphate (PIP2) was blocked by the lipid kinase inhibitor wortmannin. In addition, stimulation of PIP(2) production, by overexpression of phosphatidylinositol-4-phosphate-5-kinase (PIP5-kinase), decreased receptor-mediated inhibition. The receptor-mediated activation phase was inhibited by the PKC inhibitor calphostin C and by a mutation in a putative PKC phosphorylation site in the KCNE1 subunit. Our results indicate that the depletion of membrane PIP(2) underlies receptor-mediated inhibition of IKs and that phosphorylation by PKC of the KCNE1 subunit underlies the GqPCR-mediated channel activation.

SUBMITTER: Matavel A

PROVIDER: S-EPMC2668609 | biostudies-literature | 2009 May

REPOSITORIES: biostudies-literature

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PKC activation and PIP(2) depletion underlie biphasic regulation of IKs by Gq-coupled receptors.

Matavel Alessandra A Lopes Coeli M B CM

Journal of molecular and cellular cardiology 20090220 5

KCNQ1 is co-assembled with KCNE1 subunits in the heart to form the cardiac delayed rectifier K(+) current (IKs), which is one of the main currents responsible for myocyte repolarization. The most commonly inherited form of cardiac arrhythmias, long-QT syndrome type 1 (LQT1), is due to mutations on KCNQ1. Gq-coupled receptors (GqPCRs) are known to mediate positive inotropism in human ventricular myocardium. The mechanism of IKs current modulation by GqPCRs remains incompletely understood. Here we ...[more]

PMID: 19233191

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Project description:In superior cervical ganglion (SCG) neurons, stimulation of M(1) receptors (M(1)Rs) produces a distinct pattern of modulation of N-type calcium (N-) channel activity, enhancing currents elicited with negative test potentials and inhibiting currents elicited with positive test potentials. Exogenously applied arachidonic acid (AA) reproduces this profile of modulation, suggesting AA functions as a downstream messenger of M(1)Rs. In addition, techniques that diminish AA's concentration during M(1)R stimulation minimize N-current modulation. However, other studies suggest depletion of phosphatidylinositol-4,5-bisphosphate during M(1)R stimulation suffices to elicit modulation. In this study, we used an expression system to examine the physiological mechanisms regulating modulation. We found the beta subunit (Ca(V)beta) acts as a molecular switch regulating whether modulation results in enhancement or inhibition. In human embryonic kidney 293 cells, stimulation of M(1)Rs or neurokinin-1 receptors (NK-1Rs) inhibited activity of N channels formed by Ca(V)2.2 and coexpressed with Ca(V)beta1b, Ca(V)beta3, or Ca(V)beta4 but enhanced activity of N channels containing Ca(V)beta2a. Exogenously applied AA produced the same pattern of modulation. Coexpression of Ca(V)beta2a, Ca(V)beta3, and Ca(V)beta4 recapitulated the modulatory response previously seen in SCG neurons, implying heterogeneous association of Ca(V)beta with Ca(V)2.2. Further experiments with mutated, chimeric Ca(V)beta subunits and free palmitic acid revealed that palmitoylation of Ca(V)beta2a is essential for loss of inhibition. The data presented here fit a model in which Ca(V)beta2a blocks inhibition, thus unmasking enhancement. Our discovery that the presence or absence of palmitoylated Ca(V)beta2a toggles M(1)R- or NK-1R-mediated modulation of N current between enhancement and inhibition identifies a novel role for palmitoylation. Moreover, these findings predict that at synapses, modulation of N-channel activity by M(1)Rs or NK-1Rs will fluctuate between enhancement and inhibition based on the presence of palmitoylated Ca(V)beta2a.

Project description:Activation of Gq protein-coupled receptors (GqPCRs) might induce divergent cellular responses, related to receptor-specific activation of different branches of the Gq signaling pathway. Receptor-specific desensitization provides a mechanism of effector modulation by restricting the spatiotemporal activation of signaling components downstream of Gq We quantified signaling events downstream of GqPCR activation with FRET-based biosensors in CHO and HEK 293 cells. KCNQ1/KCNE1 channels (IKs) were measured as a functional readout of receptor-specific activation. Activation of muscarinic M1 receptors (M1-Rs) caused robust and reversible inhibition of IKs. In contrast, activation of α1B-adrenergic receptors (α1B-ARs) induced transient inhibition of IKs, which turned into delayed facilitation after agonist withdrawal. As a novel finding, we demonstrate that GqPCR-specific kinetics of IKs modulation are determined by receptor-specific desensitization, evident at the level of Gαq activation, phosphatidylinositol 4,5-bisphosphate (PIP2) depletion, and diacylglycerol production. Sustained IKs inhibition during M1-R stimulation is attributed to robust membrane PIP2 depletion, whereas the rapid desensitization of α1B-AR delimits PIP2 reduction and augments current activation by protein kinase C (PKC). Overexpression of Ca2+-independent PKCδ did not affect the time course of α1B-AR-induced diacylglycerol formation, excluding a contribution of PKCδ to α1B-AR desensitization. Pharmacological inhibition of Ca2+-dependent PKC isoforms abolished fast α1B receptor desensitization and augmented IKs reduction, but did not affect IKs facilitation. These data indicate a contribution of Ca2+-dependent PKCs to α1B-AR desensitization, whereas IKs facilitation is induced by Ca2+-independent PKC isoforms. In contrast, neither inhibition of Ca2+-dependent/Ca2+-independent isoforms nor overexpression of PKCδ induced M1 receptor desensitization, excluding a contribution of PKC to M1-R-induced IKs modulation.

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PKC activation and PIP(2) depletion underlie biphasic regulation of IKs by Gq-coupled receptors.

Publications

PKC activation and PIP(2) depletion underlie biphasic regulation of IKs by Gq-coupled receptors.

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