Direct voltage control of endogenous lysophosphatidic acid G-protein-coupled receptors in Xenopus oocytes

Membrane depolarization is required for pressure-dependent pulmonary arterial tone but not enhanced vasoconstriction to endothelin-1 following chronic hypoxia

Enhanced vasoconstriction is increasingly identified as an important contributor to the development of pulmonary hypertension. Chronic hypoxia results in enhanced Rho kinase mediated Ca²⁺ sensitization contributing to pressure-dependent pulmonary arterial tone as well as augmented vasoconstriction to endothelin-1 and depolarizing stimuli. We sought to investigate the interaction between these vasoconstrictor stimuli in isolated, pressurized, pulmonary arteries. We used the K⁺ ionophore, valinomycin, to clamp membrane potential (V_m) to investigate the role of membrane depolarization in endothelin-1 and pressure-dependent constriction, and endothelin-1 receptor inhibitors to determine whether membrane depolarization or stretch signal through endothelin-1 receptors. Clamping V_m prevented pressure-dependent tone, but not enhanced vasoconstriction to endothelin-1 following chronic hypoxia. Furthermore, endothelin-1 receptor inhibition had no effect on either pressure-dependent tone or vasoconstriction to KCl. As Src kinases contribute to both pressure-dependent tone and enhanced endothelin-1 vasoconstriction following chronic hypoxia, we further investigated their role in depolarization-induced vasoconstriction. Inhibition of Src kinases attenuated enhanced vasoconstriction to KCl. We conclude that membrane depolarization contributes to pressure-dependent tone but not enhanced vasoconstriction to ET-1, and that Src kinases serve as upstream mediators facilitating enhanced Rho kinase-dependent vasoconstriction following chronic hypoxia.

Voltage modulates the effect of μ-receptor activation in a ligand-dependent manner

BACKGROUND AND PURPOSE

Various GPCRs have been described as being modulated in a voltage-dependent manner. Opioid analgesics act via activation of μ receptors in various neurons. As neurons are exposed to large changes in membrane potential, we were interested in studying the effects of depolarization on μ receptor signalling.

EXPERIMENTAL APPROACH

We investigated potential voltage sensitivity of μ receptors in heterologous expression systems (HEK293T cells) using electrophysiology in combination with Förster resonance energy transfer-based assays. Depolarization-induced changes in signalling were also tested in physiological rat tissue containing locus coeruleus neurons. We applied depolarization steps across the physiological range of membrane potentials.

KEY RESULTS

Studying μ receptor function and signalling in cells, we discovered that morphine-induced signalling was strongly dependent on the membrane potential (V_M ). This became apparent at the level of G-protein activation, G-protein coupled inwardly rectifying potassium channel (K_ir 3.X) currents and binding of GPCR kinases and arrestin3 to μ receptors by a robust increase in signalling upon membrane depolarization. The pronounced voltage sensitivity of morphine-induced μ receptor activation was also observed at the level of K_ir 3.X currents in rat locus coeruleus neurons. The efficacy of peptide ligands to activate μ receptors was not (Met-enkephalin) or only moderately ([D-Ala² , N-Me-Phe⁴ , Gly⁵ -ol]-enkephalin) enhanced upon depolarization. In contrast, depolarization reduced the ability of the analgesic fentanyl to activate μ receptors.

CONCLUSION AND IMPLICATIONS

Our results indicate a strong ligand-dependent modulation of μ receptor activity by the membrane potential, suggesting preferential activity of morphine in neurons with high neuronal activity.

Beyond voltage-gated ion channels: Voltage-operated membrane proteins and cellular processes

Voltage-gated ion channels were believed to be the only voltage-sensitive proteins in excitable (and some non-excitable) cells for a long time. Emerging evidence indicates that the voltage-operated model is shared by some other transmembrane proteins expressed in both excitable and non-excitable cells. In this review, we summarize current knowledge about voltage-operated proteins, which are not classic voltage-gated ion channels as well as the voltage-dependent processes in cells for which single voltage-sensitive proteins have yet to be identified. Particularly, we will focus on the following. (1) Voltage-sensitive phosphoinositide phosphatases (VSP) with four transmembrane segments homologous to the voltage sensor domain (VSD) of voltage-gated ion channels; VSPs are the first family of proteins, other than the voltage-gated ion channels, for which there is sufficient evidence for the existence of the VSD domain; (2) Voltage-gated proton channels comprising of a single voltage-sensing domain and lacking an identified pore domain; (3) G protein coupled receptors (GPCRs) that mediate the depolarization-evoked potentiation of Ca²⁺ mobilization; (4) Plasma membrane (PM) depolarization-induced but Ca²⁺ -independent exocytosis in neurons. (5) Voltage-dependent metabolism of phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P₂ , PIP₂ ) in the PM. These recent discoveries expand our understanding of voltage-operated processes within cellular membranes.

The mode of agonist binding to a G protein-coupled receptor switches the effect that voltage changes have on signaling

Signaling by many heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) is either enhanced or attenuated by changes in plasma membrane potential. To identify structural correlates of the voltage sensitivity of GPCR signaling, we chose muscarinic acetylcholine receptors (the M1, M3, and M5 isoforms) as a model system. We combined molecular docking analysis with Förster resonance energy transfer (FRET)-based assays that monitored receptor activity under voltage clamp conditions. When human embryonic kidney (HEK) 293 cells expressing the individual receptors were stimulated with the agonist carbachol, membrane depolarization enhanced signaling by the M1 receptor but attenuated signaling by the M3 and M5 receptors. Furthermore, whether membrane depolarization enhanced or inhibited receptor signaling depended on the type of agonist. Membrane depolarization attenuated M3 receptor signaling when the receptor was bound to carbachol or acetylcholine, whereas depolarization enhanced signaling when the receptor was bound to either choline or pilocarpine. Docking calculations predicted that there were two distinct binding modes for these ligands, which were associated with the effect of depolarization on receptor function. From these calculations, we identified a residue in the M3 receptor that, when mutated, would alter the binding mode of carbachol to resemble that of pilocarpine in silico. Introduction of this mutated M3 receptor into cells confirmed that the membrane depolarization enhanced, rather than attenuated, signaling by the carbachol-bound receptor. Together, these data suggest that the directionality of the voltage sensitivity of GPCR signaling is defined by the specific binding mode of each ligand to the receptor.

Membrane Potential Controls the Efficacy of Catecholamine-induced β1-Adrenoceptor Activity

G protein-coupled receptors (GPCRs) are membrane-located proteins and, therefore, are exposed to changes in membrane potential (V(M)) in excitable tissues. These changes have been shown to alter receptor activation of certain Gi-and Gq-coupled GPCRs. By means of a combination of whole-cell patch-clamp and Förster resonance energy transfer (FRET) in single cells, we demonstrate that the activation of the Gs-coupled β1-adrenoreceptor (β1-AR) by the catecholamines isoprenaline (Iso) and adrenaline (Adr) is regulated by V(M). This voltage-dependence is also transmitted to G protein and arrestin 3 signaling. Voltage-dependence of β2-AR activation, however, was weak compared with β1-AR voltage-dependence. Drug efficacy is a major target of β1-AR voltage-dependence as depolarization attenuated receptor activation, even under saturating concentrations of agonists, with significantly faster kinetics than the deactivation upon agonist withdrawal. Also the efficacy of the endogenous full agonist adrenaline was reduced by depolarization. This is a unique finding since reports of natural full agonists at other voltage-dependent GPCRs only show alterations in affinity during depolarization. Based on a Boltzmann function fit to the relationship of V(M) and receptor-arrestin 3 interaction we determined the voltage-dependence with highest sensitivity in the physiological range of membrane potential. Our data suggest that under physiological conditions voltage regulates the activity of agonist-occupied β1-adrenoceptors on a very fast time scale.

Enhanced depolarization-induced pulmonary vasoconstriction following chronic hypoxia requires EGFR-dependent activation of NAD(P)H oxidase 2

AIMS

Chronic hypoxia (CH) enhances depolarization-induced myofilament Ca(2+) sensitization and resultant pulmonary arterial constriction through superoxide (O(2)(-))-dependent stimulation of RhoA. Because NAD(P)H oxidase (NOX) has been implicated in the development of pulmonary hypertension, we hypothesized that vascular smooth muscle (VSM) depolarization increases NOX-derived O(2)(-) production leading to myofilament Ca(2+) sensitization and augmented vasoconstrictor reactivity following CH. As epidermal growth factor receptor (EGFR) mediates Rac1-dependent NOX activation in renal mesangial cells, we further sought to examine the role EGFR plays in this response.

RESULTS

Vasoconstrictor responses to depolarizing concentrations of KCl were greater in lungs isolated from CH (4 wk, 0.5 atm) rats compared to normoxic controls, and this effect of CH was abolished by the general NOX inhibitor, apocynin. CH similarly augmented KCl-induced vasoconstriction and O(2)(-) generation (assessed using the fluorescent indicator, dihydroethidium) in Ca(2+)-permeabilized, pressurized small pulmonary arteries. These latter responses to CH were prevented by general inhibition of NOX isoforms (apocynin, diphenylene iodonium), and by selective inhibition of NOX 2 (gp91ds-tat), Rac1 (NSC 23766), and EGFR (AG 1478). Consistent with these observations, CH increased KCl-induced EGFR phosphorylation, and augmented depolarization-induced Rac1 activation in an EGFR-dependent manner.

INNOVATION

This study establishes a novel signaling axis in VSM linking membrane depolarization to contraction that is independent of Ca(2+) influx, and which mediates myofilament Ca(2+) sensitization in the hypertensive pulmonary circulation.

CONCLUSION

CH augments membrane depolarization-induced pulmonary VSM Ca(2+) sensitization and vasoconstriction through EGFR-dependent stimulation of Rac1 and NOX 2.

Voltage regulates adrenergic receptor function

The present study demonstrates that agonist-mediated activation of α2A adrenergic receptors (α(2A)AR) is voltage-dependent. By resolving the kinetics of conformational changes of α(2A)AR at defined membrane potentials, we show that negative membrane potentials in the physiological range promote agonist-mediated activation of α(2A)AR. We discovered that the conformational change of α(2A)AR by voltage is independent from receptor-G protein docking and regulates receptor signaling, including β-arrestin binding, activation of G proteins, and G protein-activated inwardly rectifying K(+) currents. Comparison of the dynamics of voltage-dependence of clonidine- vs. norepinephrine-activated receptors uncovers interesting mechanistic insights. For norepinephrine, the time course of voltage-dependent deactivation reflected the deactivation kinetics of the receptor after agonist withdrawal and was strongly attenuated at saturating concentrations. In contrast, clonidine-activated α(2A)AR were switched by voltage even under fully saturating concentrations, and the kinetics of this switch was notably faster than dissociation of clonidine from α(2A)AR, indicating voltage-dependent regulation of the efficacy. We conclude that adrenergic receptors exhibit a unique, agonist-dependent mechanism of voltage-sensitivity that modulates downstream receptor signaling.

G-protein-coupled receptors in intestinal chemosensation

Food intake is detected by the chemical senses of taste and smell and subsequently by chemosensory cells in the gastrointestinal tract that link the composition of ingested foods to feedback circuits controlling gut motility/secretion, appetite, and peripheral nutrient disposal. G-protein-coupled receptors responsive to a range of nutrients and other food components have been identified, and many are localized to intestinal chemosensory cells, eliciting hormonal and neuronal signaling to the brain and periphery. This review examines the role of G-protein-coupled receptors as signaling molecules in the gut, with a particular focus on pathways relevant to appetite and glucose homeostasis.

Agonist-specific voltage sensitivity at the dopamine D2S receptor--molecular determinants and relevance to therapeutic ligands

Voltage sensitivity has been demonstrated for some GPCRs. At the dopamine D(2S) receptor, this voltage sensitivity is agonist-specific; some agonists, including dopamine, exhibit decreased potency at depolarized potentials, whereas others are not significantly affected. In the present study, we examined some of the receptor-agonist interactions contributing to these differences, and investigated how dopamine D(2S) receptor voltage sensitivity affects clinically used dopamine agonists. GIRK channel activation in voltage-clamped Xenopus oocytes was used as readout of receptor activation. Structurally distinct agonists and complementary site-directed mutagenesis of the receptor's binding site were used to investigate the role of agonist-receptor interactions. We also confirmed that the depolarization-induced decrease of dopamine potency in GIRK activation is correlated by decreased binding of radiolabeled dopamine, and by decreased potency in G protein activation. In the mutagenesis experiments, a conserved serine residue as well as the conserved aspartate in the receptor's binding site were found to be important for voltage sensitive potency of dopamine. Furthermore, the voltage sensitivity of the receptor had distinct effects on different therapeutic D(2) agonists. Depolarization decreased the potency of several compounds, whereas for others, efficacy was reduced. For some agonists, both potency and efficacy were diminished, whereas for others still, neither parameter was significantly altered. The present work identifies some of the ligand-receptor interactions which determine agonist-specific effects of voltage at the dopamine D(2S) receptor. The observed differences between therapeutic agonists might be clinically relevant, and make them potential tools for investigating the roles of dopamine D(2) receptor voltage sensitivity in native tissue.