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

Angiotensin receptors and α1B-adrenergic receptors regulate native IK(ACh) and phosphorylation-deficient GIRK4 (S418A) channels through different PKC isoforms

Signaling of G protein-activated inwardly rectifying K+ (GIRK) channels is an important mechanism of the parasympathetic regulation of the heart rate and cardiac excitability. GIRK channels are inhibited during stimulation of Gq-coupled receptors (GqPCRs) by depletion of phosphatidyl-4,5-bisphosphate (PIP2) and/or channel phosphorylation by protein kinase C (PKC). The GqPCR-dependent modulation of GIRK currents in terms of specific PKC isoform activation was analyzed in voltage-clamp experiments in rat atrial myocytes and in CHO or HEK 293 cells. By using specific PKC inhibitors, we identified the receptor-activated PKC isoforms that contribute to phenylephrine- and angiotensin-induced GIRK channel inhibition. We demonstrate that the cPKC isoform PKCα significantly contributes to GIRK inhibition during stimulation of wildtype α1B-adrenergic receptors (α1B-ARs). Deletion of the α1B-AR serine residues S396 and S400 results in a preferential regulation of GIRK activity by PKCβ. As a novel finding, we report that the AT1-receptor-induced GIRK inhibition depends on the activation of the nPKC isoform PKCε whereas PKCα and PKCβ do not mainly participate in the angiotensin-mediated GIRK reduction. Expression of the dominant negative (DN) PKCε prolonged the onset of GIRK inhibition and significantly reduced AT1-R desensitization, indicating that PKCε regulates both GIRK channel activity and the strength of the receptor signal via a negative feedback mechanism. The serine residue S418 represents an important phosphorylation site for PKCε in the GIRK4 subunit. To analyze the functional impact of this PKC phosphorylation site for receptor-specific GIRK channel modulation, we monitored the activity of a phosphorylation-deficient (GIRK4 (S418A)) GIRK4 channel mutant during stimulation of α1B-ARs or AT1-receptors. Mutation of S418 did not impede α1B-AR-mediated GIRK inhibition, suggesting that S418 within the GIRK4 subunit is not subject to PKCα-induced phosphorylation. Furthermore, activation of angiotensin receptors induced pronounced GIRK4 (S418A) channel inhibition, excluding that this phosphorylation site contributes to the AT1-R-induced GIRK reduction. Instead, phosphorylation of S418 has a facilitative effect on GIRK activity that was abolished in the GIRK4 (S418A) mutant. To summarize, the present study shows that the receptor-dependent regulation of atrial GIRK channels is attributed to the GqPCR-specific activation of different PKC isoforms. Receptor-specific activated PKC isoforms target distinct phosphorylation sites within the GIRK4 subunit, resulting in differential regulation of GIRK channel activity with either facilitative or inhibitory effects on GIRK currents.

Signalling of Adrenoceptors: Canonical Pathways and New Paradigms

The concept of G protein-coupled receptors initially arose from studies of the β-adrenoceptor, adenylyl cyclase, and cAMP signalling pathway. Since then both canonical G protein-coupled receptor signalling pathways and emerging paradigms in receptor signalling have been defined by experiments focused on adrenoceptors. Here, we discuss the evidence for G protein coupling specificity of the nine adrenoceptor subtypes. We summarise the ability of each of the adrenoceptors to activate proximal signalling mediators including cAMP, calcium, mitogen-activated protein kinases, and protein kinase C pathways. Finally, we highlight the importance of precise spatial and temporal control of adrenoceptor signalling that is controlled by the localisation of receptors at intracellular membranes and in larger protein complexes.

Modulation of IKs channel-PIP2 interaction by PRMT1 plays a critical role in the control of cardiac repolarization

Recent studies have shown that protein arginine methyltransferase 1 (PRMT1) is highly expressed in the human heart, and loss of PRMT1 contributes to cardiac remodeling in the heart failure. However, the functional importance of PRMT1 in cardiac ion channels remains uncertain. The slow activating delayed rectifier K⁺ (I_Ks) channel is a cardiac K⁺ channel composed of KCNQ1 and KCNE1 subunits and is a new therapeutic target for treating lethal arrhythmias in many cardiac pathologies, especially heart failure. Here, we demonstrate that PRMT1 is a critical regulator of the I_Ks channel and cardiac rhythm. In the guinea pig ventricular myocytes, treatment with furamidine, a PRMT1‐specific inhibitor, prolonged the action potential duration (APD). We further show that this APD prolongation was attributable to I_Ks reduction. In HEK293T cells expressing human KCNQ1 and KCNE1, inhibiting PRMT1 via furamidine reduced I_Ks and concurrently decreased the arginine methylation of KCNQ1, a pore‐forming α‐subunit. Evidence presented here indicates that furamidine decreased I_Ks mainly by lowering the affinity of I_Ks channels for the membrane phospholipid, phosphatidylinositol 4,5‐bisphosphate (PIP₂), which is crucial for pore opening. Finally, applying exogenous PIP₂ to cardiomyocytes prevented the furamidine‐induced I_Ks reduction and APD prolongation. Taken together, these results indicate that PRMT1 positively regulated I_Ks activity through channel–PIP₂ interaction, thereby restricting excessive cardiac action potential.

PKC-isoform specific regulation of receptor desensitization and KCNQ1/KCNE1 K+ channel activity by mutant α1B-adrenergic receptors

Activation of a specific protein kinase C (PKC) isoform during stimulation of G_q protein-coupled receptors (G_qPCRs) is determined by homologous receptor desensitization that controls the spatiotemporal formation of downstream G_q signalling molecules. Furthermore, G_qPCR-activated PKC isoforms specifically regulate receptor activity via a negative feedback mechanism. In the present study, we investigated the contribution of several phosphorylation sites in the α_1B-adrenergic receptor (α_1B-AR) for PKC and G protein coupled receptor kinase 2 (GRK2) to homologous receptor desensitization and effector modulation. We analyzed signalling events downstream to human wildtype α_1B-ARs and α_1B-ARs lacking PKC or GRK2 phosphorylation sites (Δ391-401, α_1B-ΔPKC-AR and Δ402-520, α_1B-ΔGRK-AR) by means of FRET-based biosensors in HEK293 that served as online-assays of receptor activity. K⁺ currents through KCNQ1/KCNE1 channels (I_Ks), which are regulated by both phosphatidylinositol 4,5-bisphosphate (PIP₂)-depletion and/or phosphorylation by PKC, were measured as a functional readout of wildtype and mutant α_1B-AR receptor activity. As a novel finding, we provide evidence that deletion of PKC and GRK2 phosphorylation sites in α_1B-ARs abrogates the contribution of PKCα to homologous receptor desensitization. Instead, the time course of mutant receptor activity was specifically modulated by PKCβ. Mutant α_1B-ARs displayed pronounced homologous receptor desensitization that was abolished by PKCβ-specific pharmacological inhibitors. I_Ks modulation during stimulation of wildtype and mutant α_1B-ARs displayed transient inhibition and current facilitation after agonist withdrawal with reduced capability of mutant α_1B-ARs to induce I_Ks inhibition. Pharmacological inhibition of the PKCβ isoform did not augment I_Ks reduction by mutant α_1B-ARs, but shifted I_Ks modulation towards current facilitation. Coexpression of an inactive (dominant-negative) PKCδ isoform (DN-PKCδ) abolished I_Ks facilitation in α_1B-ΔGRK-AR-expressing cells, but not in α_1B-ΔPKC-AR-expressing cells. The data indicate that the differential modulation of I_Ks activity by α_1B-ΔGRK- and α_1B-ΔPKC-receptors is attributed to the activation of entirely distinct novel PKC isoforms. To summarize, specific phosphorylation sites within the wildtype and mutant α_1B-adrenergic receptors are targeted by different PKC isoforms, resulting in differential regulation of receptor desensitization and effector function.

Phosphatidylinositol 4,5-bisphosphate is regenerated by speeding of the PI 4-kinase pathway during long PLC activation

The dynamic metabolism of membrane phosphoinositide lipids involves several cellular compartments including the ER, Golgi, and plasma membrane. There are cycles of phosphorylation and dephosphorylation and of synthesis, transfer, and breakdown. The simplified phosphoinositide cycle comprises synthesis of phosphatidylinositol in the ER, transport, and phosphorylation in the Golgi and plasma membranes to generate phosphatidylinositol 4,5-bisphosphate, followed by receptor-stimulated hydrolysis in the plasma membrane and return of the components to the ER for reassembly. Using probes for specific lipid species, we have followed and analyzed the kinetics of several of these events during stimulation of M₁ muscarinic receptors coupled to the G-protein G_q. We show that during long continued agonist action, polyphosphorylated inositol lipids are initially depleted but then regenerate while agonist is still present. Experiments and kinetic modeling reveal that the regeneration results from gradual but massive up-regulation of PI 4-kinase pathways rather than from desensitization of receptors. Golgi pools of phosphatidylinositol 4-phosphate and the lipid kinase PI4KIIIα (PI4KA) contribute to this homeostatic regeneration. This powerful acceleration, which may be at the level of enzyme activity or of precursor and product delivery, reveals strong regulatory controls in the phosphoinositide cycle.

Neurohormonal Regulation of IKs in Heart Failure: Implications for Ventricular Arrhythmogenesis and Sudden Cardiac Death

Heart failure (HF) results in sustained alterations in neurohormonal signaling, including enhanced signaling through the sympathetic nervous system and renin-angiotensin-aldosterone system pathways. While enhanced sympathetic nervous system and renin-angiotensin-aldosterone system activity initially help compensate for the failing myocardium, sustained signaling through these pathways ultimately contributes to HF pathophysiology. HF remains a leading cause of mortality, with arrhythmogenic sudden cardiac death comprising a common mechanism of HF-related death. The propensity for arrhythmia development in HF occurs secondary to cardiac electrical remodeling that involves pathological regulation of ventricular ion channels, including the slow component of the delayed rectifier potassium current, that contribute to action potential duration prolongation. To elucidate a mechanistic explanation for how HF-mediated electrical remodeling predisposes to arrhythmia development, a multitude of investigations have investigated the specific regulatory effects of HF-associated stimuli, including enhanced sympathetic nervous system and renin-angiotensin-aldosterone system signaling, on the slow component of the delayed rectifier potassium current. The objective of this review is to summarize the current knowledge related to the regulation of the slow component of the delayed rectifier potassium current in response to HF-associated stimuli, including the intracellular pathways involved and the specific regulatory mechanisms.

G protein αq exerts expression level-dependent distinct signaling paradigms

G protein αq-coupled receptors (Gq-GPCRs) primarily signal through GαqGTP mediated phospholipase Cβ (PLCβ) stimulation and the subsequent hydrolysis of phosphatidylinositol 4, 5 bisphosphate (PIP2). Though Gq-heterotrimer activation results in both GαqGTP and Gβγ, unlike Gi/o-receptors, it is unclear if Gq-coupled receptors employ Gβγ as a major signal transducer. Compared to Gi/o- and Gs-coupled receptors, we observed that most cell types exhibit a limited free Gβγ generation upon Gq-pathway and Gαq/11 heterotrimer activation. We show that cells transfected with Gαq or endogenously expressing more than average-levels of Gαq/11 compared to Gαs and Gαi exhibit a distinct signaling regime primarily characterized by recovery-resistant PIP2 hydrolysis. Interestingly, the elevated Gq-expression is also associated with enhanced free Gβγ generation and signaling. Furthermore, the gene GNAQ, which encodes for Gαq, has recently been identified as a cancer driver gene. We also show that GNAQ is overexpressed in tumor samples of patients with Kidney Chromophobe (KICH) and Kidney renal papillary (KIRP) cell carcinomas in a matched tumor-normal sample analysis, which demonstrates the clinical significance of Gαq expression. Overall, our data indicates that cells usually express low Gαq levels, likely safeguarding cells from excessive calcium as wells as from Gβγ signaling.