Sensitivity limits for voltage control of P2Y receptor-evoked Ca2+ mobilization in the rat megakaryocyte

Charge Movements and Conformational Changes: Biophysical Properties and Physiology of Voltage-Dependent GPCRs

G protein-coupled receptors (GPCRs) regulate multiple cellular functions and represent important drug targets. More than 20 years ago, it was noted that GPCR activation (agonist binding) and signaling (G protein activation) are dependent on the membrane potential (VM). While it is now proven that many GPCRs display an intrinsic voltage dependence, the molecular mechanisms of how GPCRs sense depolarization of the plasma membrane are less well defined. This review summarizes the current knowledge of voltage-dependent signaling in GPCRs. We describe how voltage dependence was discovered in muscarinic receptors, present an overview of GPCRs that are regulated by voltage, and show how biophysical properties of GPCRs led to the discovery of voltage-sensing mechanisms in those receptors. Furthermore, we summarize physiological functions that have been shown to be regulated by voltage-dependent GPCR signaling of endogenous receptors in excitable tissues, such as the nervous system or the heart. Finally, we discuss challenges that remain in analyzing voltage-dependent signaling of GPCRs in vivo and present an outlook on experimental applications of the interesting concept of GPCR signaling.

The voltage sensitivity of G-protein coupled receptors: Unraveling molecular mechanisms and physiological implications

In the landscape of proteins controlled by membrane voltage (Vm), like voltage-gated ionotropic channels, the emergence of the voltage sensitivity within the vast family of G-protein coupled receptors (GPCRs) marked a significant milestone at the onset of the 21st century. Since its discovery, extensive research has been devoted to understanding the intricate relationship between Vm and GPCRs. Approximately 30 GPCRs out of a family comprising more than 800 receptors have been implicated in Vm-dependent positive and negative regulation. GPCRs stand out as the quintessential regulators of synaptic transmission in neurons, where they encounter substantial variations in Vm. However, the molecular mechanism underlying the Vm sensor of GPCRs remains enigmatic, hindered by the scarcity of mutant GPCRs insensitive to Vm yet functionally intact, impeding a comprehensive understanding of this unique property in physiology. Nevertheless, two decades of dedicated research have furnished numerous insights into the molecular aspects of GPCR Vm-sensing, accompanied by recently proposed physiological roles as well as pharmacological potential, which we encapsulate in this review. The Vm sensitivity of GPCRs emerges as a pivotal attribute, shedding light on previously unforeseen roles in synaptic transmission and extending beyond, underscoring its significance in cellular signaling and physiological processes.

Voltage dependence of M2 muscarinic receptor antagonists and allosteric modulators

Muscarinic receptors are G protein-coupled receptors (GPCRs) that play a role in various physiological functions. Previous studies have shown that these receptors, along with other GPCRs, are voltage-sensitive; both their affinity toward agonists and their activation are regulated by membrane potential. To our knowledge, whether the effect of antagonists on these receptors is voltage-dependent has not yet been studied. In this study, we used Xenopus oocytes expressing the M2 muscarinic receptor (M2R) to investigate this question. Our results indicate that the potencies of two M2R antagonists, atropine and scopolamine, are voltage-dependent; they are more effective at resting potential than under depolarization. In contrast, the M2R antagonist AF-DX 386 did not exhibit voltage-dependent potency.Furthermore, we discovered that the voltage dependence of M2R activation by acetylcholine remains unchanged in the presence of two allosteric modulators, the negative modulator gallamine and the positive modulator LY2119620. These findings enhance our understanding of GPCRs' voltage dependence and may have pharmacological implications.

Functional consequences of a rare human serotonergic 5-HT1A receptor variant

Serotonin (5-HT) plays a central role in various brain functions via the activation of a family of receptors, most of them G protein coupled receptors (GPCRs). 5-HT1A receptor, the most abundant 5-HT receptors, was implicated in many brain dysfunctions and is a major target for drug discovery. Several genetic polymorphisms within the 5-HT1A receptor gene were identified and linked to different conditions, including anxiety and depression. Here, we used Xenopus oocytes to examine the effects of one of the functional polymorphism, Arg220Leu, on the function of the receptor. We found that the mutated receptor shows normal activation of G protein and normal 5-HT binding. On the other hand, the mutated receptor shows impaired desensitization, probably due to impairment in activation of β arrestin-dependent pathway. Furthermore, while the 5-HT1A receptor was shown to exhibit voltage dependent activation by serotonin and by buspirone, the mutated receptor was voltage-independent. Our results suggest a pronounced effect of the mutation on the function of the 5-HT1A receptor and add to our understanding of the molecular mechanism of its voltage dependence. Moreover, the findings of this study may suggest a functional explanation for the possible link between this variant and brain pathologies.

G Protein-Coupled Receptors Regulated by Membrane Potential

G protein-coupled receptors (GPCRs) are involved in a vast majority of signal transduction processes. Although they span the cell membrane, they have not been considered to be regulated by the membrane potential. Numerous studies over the last two decades have demonstrated that several GPCRs, including muscarinic, adrenergic, dopaminergic, and glutamatergic receptors, are voltage regulated. Following these observations, an effort was made to elucidate the molecular basis for this regulatory effect. In this review, we will describe the advances in understanding the voltage dependence of GPCRs, the suggested molecular mechanisms that underlie this phenomenon, and the possible physiological roles that it may play.

Voltage dependence of the cannabinoid CB1 receptor

Cannabinoids produce their characteristic effects mainly by binding to two types of G-protein coupled receptors (GPCRs), the CB1 and CB2 cannabinoid receptors. The CB1 receptor is the main cannabinoid receptor in the central nervous system, and it participates in many brain functions. Recent studies showed that membrane potential may serve as a novel modulatory modality of many GPCRs. Here, we used Xenopus oocytes as an expression system to examine whether membrane potential modulates the activity of the CB1 receptor. We found that the potencies of the endocannabinoid 2-AG and the phytocannabinoid THC in activating the receptor are voltage dependent; depolarization enhanced the potency of these agonists and decreased their dissociation from the receptor. This voltage dependence appears to be agonist dependent as the potency of the endocannabinoid anandamide (AEA) was voltage independent. The finding of this agonist-specific modulatory factor for the CB1 receptor may contribute to our future understanding of various physiological functions mediated by the endocannabinoid system.

The activity of the serotonergic 5-HT1A receptor is modulated by voltage and sodium levels

G protein–coupled receptors are known to play a key role in many cellular signal transduction processes, including those mediating serotonergic signaling in the nervous system. Several factors have been shown to regulate the activity of these receptors, including membrane potential and the concentration of sodium ions. Whether voltage and sodium regulate the activity of serotonergic receptors is unknown. Here, we used Xenopus oocytes as an expression system to examine the effects of voltage and of sodium ions on the potency of one subtype of serotonin (5-hydroxytryptamine [5-HT]) receptor, the 5-HT_1A receptor. We found that the potency of 5-HT in activating the receptor is voltage dependent and that it is higher at resting potential than under depolarized conditions. Furthermore, we found that removal of extracellular Na⁺ resulted in a decrease of 5-HT potency toward the 5-HT_1A receptor and that a conserved aspartate in transmembrane domain 2 is crucial for this effect. Our results suggest that this allosteric effect of Na⁺ does not underlie the voltage dependence of this receptor. We propose that the characterization of modulatory factors that regulate this receptor may contribute to our future understanding of various physiological functions mediated by serotonergic transmission.

Structure and Dynamics of GPCRs in Lipid Membranes: Physical Principles and Experimental Approaches

Over the past decade, the vast amount of information generated through structural and biophysical studies of GPCRs has provided unprecedented mechanistic insight into the complex signalling behaviour of these receptors. With this recent information surge, it has also become increasingly apparent that in order to reproduce the various effects that lipids and membranes exert on the biological function for these allosteric receptors, in vitro studies of GPCRs need to be conducted under conditions that adequately approximate the native lipid bilayer environment. In the first part of this review, we assess some of the more general effects that a membrane environment exerts on lipid bilayer-embedded proteins such as GPCRs. This is then followed by the consideration of more specific effects, including stoichiometric interactions with specific lipid subtypes. In the final section, we survey a range of different membrane mimetics that are currently used for in vitro studies, with a focus on NMR applications.

ATP triggers a robust intracellular [Ca2+ ]-mediated signalling pathway in human synovial fibroblasts

NEW FINDINGS

What is the central question of this study? What are the main [Ca²⁺ ]_i signalling pathways activated by ATP in human synovial fibroblasts? What is the main finding and its importance? In human synovial fibroblasts ATP acts through a linked G-protein (G_q ) and phospholipase C signalling mechanism to produce IP₃ , which then markedly enhances release of Ca²⁺ from the endoplasmic reticulum. These results provide new information for the detection of early pathophysiology of arthritis.

ABSTRACT

In human articular joints, synovial fibroblasts (HSFs) have essential physiological functions that include synthesis and secretion of components of the extracellular matrix and essential articular joint lubricants, as well as release of paracrine substances such as ATP. Although the molecular and cellular processes that lead to a rheumatoid arthritis (RA) phenotype are not fully understood, HSF cells exhibit significant changes during this disease progression. The effects of ATP on HSFs were studied by monitoring changes in intracellular Ca²⁺ ([Ca²⁺ ]_i ), and measuring electrophysiological properties. ATP application to HSF cell populations that had been enzymatically released from 2-D cell culture revealed that ATP (10-100 μm), or its analogues UTP or ADP, consistently produced a large transient increase in [Ca²⁺ ]_i . These changes (i) were initiated by activation of the P₂ Y purinergic receptor family, (ii) required G_q -mediated signal transduction, (iii) did not involve a transmembrane Ca²⁺ influx, but instead (iv) arose almost entirely from activation of endoplasmic reticulum (ER)-localized inositol 1,4,5-trisphosphate (IP₃ ) receptors that triggered Ca²⁺ release from the ER. Corresponding single cell electrophysiological studies revealed that these ATP effects (i) were insensitive to [Ca²⁺ ]_o removal, (ii) involved an IP₃ -mediated intracellular Ca²⁺ release process, and (iii) strongly turned on Ca²⁺ -activated K⁺ current(s) that significantly hyperpolarized these cells. Application of histamine produced very similar effects in these HSF cells. Since ATP is a known paracrine agonist and histamine is released early in the inflammatory response, these findings may contribute to identification of early steps/defects in the initiation and progression of RA.

Intracellular Transfer of Na+ in an Active-State G-Protein-Coupled Receptor

Playing a central role in cell signaling, G-protein-coupled receptors (GPCRs) are the largest superfamily of membrane proteins and form the majority of drug targets in humans. How extracellular agonist binding triggers the activation of GPCRs and associated intracellular effector proteins remains, however, poorly understood. Structural studies have revealed that inactive class A GPCRs harbor a conserved binding site for Na+ ions in the center of their transmembrane domain, accessible from the extracellular space. Here, we show that the opening of a conserved hydrated channel in the activated state receptors allows the Na+ ion to egress from its binding site into the cytosol. Coupled with protonation changes, this ion movement occurs without significant energy barriers, and can be driven by physiological transmembrane ion and voltage gradients. We propose that Na+ ion exchange with the cytosol is a key step in GPCR activation. Further, we hypothesize that this transition locks receptors in long-lived active-state conformations.

Membrane potentials regulating GPCRs: insights from experiments and molecular dynamics simulations

G-protein coupled receptors (GPCRs) form the largest class of membrane proteins in humans and the targets of most present drugs. Membrane potential is one of the defining characteristics of living cells. Recent work has shown that the membrane voltage, and changes thereof, modulates signal transduction and ligand binding in GPCRs. As it may allow differential signalling patterns depending on tissue, cell type, and the excitation status of excitable cells, GPCR voltage sensitivity could have important implications for their pharmacology. This review summarises recent experimental insights on GPCR voltage regulation and the role of molecular dynamics simulations in identifying the structural basis of GPCR voltage-sensing. We discuss the potential significance for drug design on GPCR targets from excitable and non-excitable cells.

Structural Mechanisms of Voltage Sensing in G Protein-Coupled Receptors

G-protein-coupled receptors (GPCRs) form the largest superfamily of membrane proteins and one-third of all drug targets in humans. A number of recent studies have reported evidence for substantial voltage regulation of GPCRs. However, the structural basis of GPCR voltage sensing has remained enigmatic. Here, we present atomistic simulations on the δ-opioid and M2 muscarinic receptors, which suggest a structural and mechanistic explanation for the observed voltage-induced functional effects. The simulations reveal that the position of an internal Na(+) ion, recently detected to bind to a highly conserved aqueous pocket in receptor crystal structures, strongly responds to voltage changes. The movements give rise to gating charges in excellent agreement with previous experimental recordings. Furthermore, free energy calculations show that these rearrangements of Na(+) can be induced by physiological membrane voltages. Due to its role in receptor function and signal bias, the repositioning of Na(+) has important general implications for signal transduction in GPCRs.

Calcium Signalling through Ligand-Gated Ion Channels such as P2X1 Receptors in the Platelet and other Non-Excitable Cells

Ligand-gated ion channels on the cell surface are directly activated by the binding of an agonist to their extracellular domain and often referred to as ionotropic receptors. P2X receptors are ligand-gated non-selective cation channels with significant permeability to Ca(2+) whose principal physiological agonist is ATP. This chapter focuses on the mechanisms by which P2X1 receptors, a ubiquitously expressed member of the family of ATP-gated channels, can contribute to cellular responses in non-excitable cells. Much of the detailed information on the contribution of P2X1 to Ca(2+) signalling and downstream functional events has been derived from the platelet. The underlying primary P2X1-generated signalling event in non-excitable cells is principally due to Ca(2+) influx, although Na(+) entry will also occur along with membrane depolarization. P2X1 receptor stimulation can lead to additional Ca(2+) mobilization via a range of routes such as amplification of G-protein-coupled receptor-dependent Ca(2+) responses. This chapter also considers the mechanism by which cells generate extracellular ATP for autocrine or paracrine activation of P2X1 receptors. For example cytosolic ATP efflux can result from opening of pannexin anion-permeable channels or following damage to the cell membrane. Alternatively, ATP stored in specialised secretory vesicles can undergo quantal release via the process of exocytosis. Examples of physiological or pathophysiological roles of P2X1-dependent signalling in non-excitable cells are also discussed, such as thrombosis and immune responses.

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.

Ca2+ influx through P2X1 receptors amplifies P2Y1 receptor-evoked Ca2+ signaling and ADP-evoked platelet aggregation

Many cells express both P2X cation channels and P2Y G-protein-coupled receptors that are costimulated by nucleotides released during physiologic or pathophysiologic responses. For example, during hemostasis and thrombosis, ATP-gated P2X1 channels and ADP-stimulated P2Y1 and P2Y12 G-protein coupled receptors play important roles in platelet activation. It has previously been reported that P2X1 receptors amplify P2Y1-evoked Ca(2+) responses in platelets, but the underlying mechanism and influence on function is unknown. In human platelets, we show that maximally activated P2X1 receptors failed to stimulate significant aggregation but could amplify the aggregation response to a submaximal concentration of ADP. Costimulation of P2X1 and P2Y1 receptors generated a superadditive Ca(2+) increase in both human platelets and human embryonic kidney 293 (HEK293) cells via a mechanism dependent on Ca(2+) influx rather than Na(+) influx or membrane depolarization. The potentiation, due to an enhanced P2Y1 response, was observed if ADP was added up to 60 seconds after P2X1 activation. P2X1 receptors also enhanced Ca(2+) responses when costimulated with type 1 protease activated and M1 muscarinic acetylcholine receptors. The P2X1-dependent amplification of Gq-coupled [Ca(2+)]i increase was mimicked by ionomycin and was not affected by inhibition of protein kinase C, Rho-kinase, or extracellular signal-regulated protein kinase 1/2, which suggests that it results from potentiation of inositol 1,4,5-trisphosphate receptors and/or phospholipase C. We conclude that Ca(2+) influx through P2X1 receptors amplifies Ca(2+) signaling through P2Y1 and other Gq-coupled receptors. This represents a general form of co-incidence detection of ATP and coreleased agonists, such as ADP at sites of vascular injury or synaptic transmitters acting at metabotropic Gq-coupled receptors.

ATP inhibits Ins(1,4,5)P3-evoked Ca2+ release in smooth muscle via P2Y1 receptors

Adenosine 5′-triphosphate (ATP) mediates a variety of biological functions following nerve-evoked release, via activation of either G-protein-coupled P2Y- or ligand-gated P2X receptors. In smooth muscle, ATP, acting via P2Y receptors (P2YR), may act as an inhibitory neurotransmitter. The underlying mechanism(s) remain unclear, but have been proposed to involve the production of inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] by phospholipase C (PLC), to evoke Ca²⁺ release from the internal store and stimulation of Ca²⁺-activated potassium (K_Ca) channels to cause membrane hyperpolarization. This mechanism requires Ca²⁺ release from the store. However, in the present study, ATP evoked transient Ca²⁺ increases in only ~10% of voltage-clamped single smooth muscle cells. These results do not support activation of K_Ca as the major mechanism underlying inhibition of smooth muscle activity. Interestingly, ATP inhibited Ins(1,4,5)P₃-evoked Ca²⁺ release in cells that did not show a Ca²⁺ rise in response to purinergic activation. The reduction in Ins(1,4,5)P₃-evoked Ca²⁺ release was not mimicked by adenosine and therefore, cannot be explained by hydrolysis of ATP to adenosine. The reduction in Ins(1,4,5)P₃-evoked Ca²⁺ release was, however, also observed with its primary metabolite, ADP, and blocked by the P2Y₁R antagonist, MRS2179, and the G protein inhibitor, GDPβS, but not by PLC inhibition. The present study demonstrates a novel inhibitory effect of P2Y₁R activation on Ins(1,4,5)P₃-evoked Ca²⁺ release, such that purinergic stimulation acts to prevent Ins(1,4,5)P₃-mediated increases in excitability in smooth muscle and promote relaxation.

Depolarization induces a conformational change in the binding site region of the M2 muscarinic receptor

G protein-coupled receptors play a central role in signal transduction and were only known to be activated by agonists. Recently it has been shown that membrane potential also affects the activity of G protein-coupled receptors. For the M(2) muscarinic receptor, it was further shown that depolarization induces charge movement. A tight correlation was found between the voltage-dependence of the charge movement and the voltage-dependence of the agonist binding. Here we examine whether depolarization-induced charge movement causes a conformational change in the M(2) receptor that may be responsible for the voltage-dependence of agonist binding. Using site-directed fluorescence labeling we show a voltage-dependent fluorescence signal, reflecting a conformational change, which correlates with the voltage-dependent charge movement. We further show that selected mutations in the orthosteric site abolish the fluorescence signal and concomitantly, the voltage-dependence of the agonist binding. Surprisingly, mutations in the allosteric site also abolished the voltage-dependence of agonist binding but did not reduce the fluorescence signal. Finally, we show that treatments, which reduced the charge movement or hindered the coupling between the charge movement and the voltage-dependent binding, also reduced the fluorescence signal. Our results demonstrate that depolarization-induced conformational changes in the orthosteric binding site underlie the voltage-dependence of agonist binding. Our results are also unique in suggesting that the allosteric site is also involved in controlling the voltage-dependent agonist binding.

Intestinal sensing of nutrients

Ingestion of a meal triggers a range of physiological responses both within and outside the gut, and results in the remote modulation of appetite and glucose homeostasis. Luminal contents are sensed by specialised chemosensitive cells scattered throughout the intestinal epithelium. These enteroendocrine and tuft cells make direct contact with the gut lumen and release a range of chemical mediators, which can either act in a paracrine fashion interacting with neighbouring cells and nerve endings or as classical circulating hormones. At the molecular level, the chemosensory machinery involves multiple and complex signalling pathways including activation of G-protein-coupled receptors and solute carrier transporters. This chapter will discuss our current knowledge of the molecular mechanisms underlying intestinal chemosensation with a particular focus on the relatively well-characterised nutrient-triggered secretion from the enteroendocrine system.

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

Lysophosphatidic acid (LPA) G-protein-coupled receptors (GPCRs) play important roles in a variety of physiological and pathophysiological processes, including cell proliferation, angiogenesis, central nervous system development and carcinogenesis. Whilst many ion channels and transporters are recognized to be controlled by a change in cell membrane potential, little is known about the voltage dependence of other proteins involved in cell signalling. Here, we show that the InsP(3)-mediated Ca(2+) response stimulated by the endogenous LPA GPCR in Xenopus oocytes is potentiated by membrane depolarization. Depolarization was able to repetitively stimulate transient [Ca(2+)](i) increases after the initial agonist-evoked response. In addition, the initial rate and amplitude of the LPA-dependent Ca(2+) response were significantly modulated by the steady holding potential over the physiological range, such that the response to LPA was potentiated at depolarized potentials and inhibited at hyperpolarized potentials. Enhancement of LPA receptor-evoked Ca(2+) mobilization by membrane depolarization was observed over a wide range of agonist concentrations. Importantly, the amplitude of the depolarization-evoked intracellular Ca(2+) increase displayed an inverse relationship with agonist concentration such that the greatest effect of voltage was observed at near-threshold levels of agonist. Voltage-dependent Ca(2+) release was not induced by direct elevation of InsP(3) or by activation of heterotrimeric G-proteins in the absence of agonist, indicating that the LPA GPCR itself represents the primary site of action of membrane voltage. This novel modulation of LPA signalling by membrane potential may have important consequences for control of Ca(2+) signals both in excitable and non-excitable tissues.

ATP released by electrical stimuli elicits calcium transients and gene expression in skeletal muscle

ATP released from cells is known to activate plasma membrane P2X (ionotropic) or P2Y (metabotropic) receptors. In skeletal muscle cells, depolarizing stimuli induce both a fast calcium signal associated with contraction and a slow signal that regulates gene expression. Here we show that nucleotides released to the extracellular medium by electrical stimulation are partly involved in the fast component and are largely responsible for the slow signals. In rat skeletal myotubes, a tetanic stimulus (45 Hz, 400 1-ms pulses) rapidly increased extracellular levels of ATP, ADP, and AMP after 15 s to 3 min. Exogenous ATP induced an increase in intracellular free Ca(2+) concentration, with an EC(50) value of 7.8 +/- 3.1 microm. Exogenous ADP, UTP, and UDP also promoted calcium transients. Both fast and slow calcium signals evoked by tetanic stimulation were inhibited by either 100 mum suramin or 2 units/ml apyrase. Apyrase also reduced fast and slow calcium signals evoked by tetanus (45 Hz, 400 0.3-ms pulses) in isolated mouse adult skeletal fibers. A likely candidate for the ATP release pathway is the pannexin-1 hemichannel; its blockers inhibited both calcium transients and ATP release. The dihydropyridine receptor co-precipitated with both the P2Y(2) receptor and pannexin-1. As reported previously for electrical stimulation, 500 mum ATP significantly increased mRNA expression for both c-fos and interleukin 6. Our results suggest that nucleotides released during skeletal muscle activity through pannexin-1 hemichannels act through P2X and P2Y receptors to modulate both Ca(2+) homeostasis and muscle physiology.

A role for membrane potential in regulating GPCRs?

G-protein-coupled receptors (GPCRs) have ubiquitous roles in transducing extracellular signals into cellular responses. Therefore, the concept that members of this superfamily of surface proteins are directly modulated by changes in membrane voltage could have widespread consequences for cell signalling. Although several studies have indicated that GPCRs can be voltage dependent, particularly P2Y(1) receptors in the non-excitable megakaryocyte, the evidence has been mostly indirect. Recent work on muscarinic receptors has stimulated substantial interest in this field by reporting the first voltage-dependent charge movements for a GPCR. An underlying mechanism is proposed whereby a voltage-induced conformational change in the receptor alters its ability to couple to the G protein and thereby influences its affinity for an agonist. We discuss the strength of the evidence behind this hypothesis and include suggestions for future work. We also describe other examples in which direct voltage control of GPCRs can account for effects of membrane potential on downstream signals and highlight the possible physiological consequences of this phenomenon.

Novel consequences of voltage-dependence to G-protein-coupled P2Y1 receptors

BACKGROUND AND PURPOSE

Emerging evidence suggests that activation of G-protein-coupled receptors (GPCRs) can be directly regulated by membrane voltage. However, the physiological and pharmacological relevance of this effect remains unclear. We have further examined this phenomenon for P2Y1 receptors in the non-excitable megakaryocyte using a range of agonists and antagonists.

EXPERIMENTAL APPROACH

Simultaneous whole-cell patch clamp and fura-2 fluorescence recordings of rat megakaryocytes, which lack voltage-gated Ca2+ influx, were used to examine the voltage-dependence of P2Y1 receptor-evoked IP3-dependent Ca2+ mobilization.

RESULTS

Depolarization transiently and repeatedly enhanced P2Y1 receptor-evoked Ca2+ mobilization across a wide concentration range of both weak, partial and full, potent agonists. Moreover, the amplitude of the depolarization-evoked [Ca2+]i increase displayed an inverse relationship with agonist concentration, such that the greatest potentiating effect of voltage was observed at near-threshold levels of agonist. Unexpectedly, depolarization also stimulated an [Ca2+]i increase in the absence of agonist during exposure to the competitive antagonists A3P5PS and MRS2179, or the allosteric enhancer 2,2'-pyridylisatogen tosylate. A further effect of some antagonists, particularly suramin, was to enhance the depolarization-evoked Ca2+ responses during co-application of an agonist. Of several P2Y1 receptor inhibitors, only SCH202676, which has a proposed allosteric mechanism of action, could block ADP-induced voltage-dependent Ca2+ release.

CONCLUSIONS AND IMPLICATIONS

The ability of depolarization to potentiate GPCRs at near-threshold agonist concentrations represents a novel mechanism for coincidence detection. Furthermore, the induction and enhancement of voltage-dependent GPCR responses by antagonists has implications for the design of therapeutic compounds.

Glutamate mediates platelet activation through the AMPA receptor

Glutamate is an excitatory neurotransmitter that binds to the kainate receptor, the N-methyl-D-aspartate (NMDA) receptor, and the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR). Each receptor was first characterized and cloned in the central nervous system (CNS). Glutamate is also present in the periphery, and glutamate receptors have been identified in nonneuronal tissues, including bone, heart, kidney, pancreas, and platelets. Platelets play a central role in normal thrombosis and hemostasis, as well as contributing greatly to diseases such as stroke and myocardial infarction. Despite the presence of glutamate in platelet granules, the role of glutamate during hemostasis is unknown. We now show that activated platelets release glutamate, that platelets express AMPAR subunits, and that glutamate increases agonist-induced platelet activation. Furthermore, we demonstrate that glutamate binding to the AMPAR increases intracellular sodium concentration and depolarizes platelets, which are important steps in platelet activation. In contrast, platelets treated with the AMPAR antagonist CNQX or platelets derived from GluR1 knockout mice are resistant to AMPA effects. Importantly, mice lacking GluR1 have a prolonged time to thrombosis in vivo. Our data identify glutamate as a regulator of platelet activation, and suggest that the AMPA receptor is a novel antithrombotic target.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.

The chemical synapse goes electric: Ca2+- and voltage-sensitive GPCRs control neurotransmitter release

It is widely believed that the initiation of transmitter release in fast synapses is triggered by rapid Ca2+ entry and that the termination of release is governed by removal of Ca2+ from below the release sites. We argue that, although Ca2+ is essential for release, fast-entry kinetics render Ca2+ incapable of being the limiting factor for the initiation of release, and the relatively slow removal of Ca2+ cannot be the limiting factor for the termination of release. We suggest, and provide supporting evidence for, a novel general mechanism for control of fast transmitter release (in the range of milliseconds) from nerve terminals. According to this mechanism, two factors control release: Ca2+ and voltage-sensitive presynaptic inhibitory G-protein-coupled receptors (GPCRs). Inhibitory autoreceptors are known to mediate slow feedback inhibition of transmitter release. We discuss the evidence showing that these receptors also control the initiation and termination of transmitter release by directly interacting with core proteins in the exocytotic machinery. This novel mechanism has important implications for understanding the regulation of transmitter release, synaptic plasticity and neuronal circuit properties.

Modulation of Gq-protein-coupled inositol trisphosphate and Ca2+ signaling by the membrane potential

Gq-protein-coupled receptors (GqPCRs) are widely distributed in the CNS and play fundamental roles in a variety of neuronal processes. Their activation results in phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and Ca2+ release from intracellular stores via the phospholipase C (PLC)-inositol 1,4,5-trisphosphate (IP3) signaling pathway. Because early GqPCR signaling events occur at the plasma membrane of neurons, they might be influenced by changes in membrane potential. In this study, we use combined patch-clamp and imaging methods to investigate whether membrane potential changes can modulate GqPCR signaling in neurons. Our results demonstrate that GqPCR signaling in the human neuronal cell line SH-SY5Y and in rat cerebellar granule neurons is directly sensitive to changes in membrane potential, even in the absence of extracellular Ca2+. Depolarization has a bidirectional effect on GqPCR signaling, potentiating thapsigargin-sensitive Ca2+ responses to muscarinic receptor activation but attenuating those mediated by bradykinin receptors. The depolarization-evoked potentiation of the muscarinic signaling is graded, bipolar, non-inactivating, and with no apparent upper limit, ruling out traditional voltage-gated ion channels as the primary voltage sensors. Flash photolysis of caged IP3/GPIP2 (glycerophosphoryl-myo-inositol 4,5-bisphosphate) places the voltage sensor before the level of the Ca2+ store, and measurements using the fluorescent bioprobe eGFP-PH(PLCdelta) (enhanced green fluorescent protein-pleckstrin homology domain-PLCdelta) directly demonstrate that voltage affects muscarinic signaling at the level of the IP3 production pathway. The sensitivity of GqPCR IP3 signaling in neurons to voltage itself may represent a fundamental mechanism by which ionotropic signals can shape metabotropic receptor activity in neurons and influence processes such as synaptic plasticity in which the detection of coincident signals is crucial.

Movement of 'gating charge' is coupled to ligand binding in a G-protein-coupled receptor

Activation by agonist binding of G-protein-coupled receptors (GPCRs) controls most signal transduction processes. Although these receptors span the cell membrane, they are not considered to be voltage sensitive. Recently it was shown that both the activity of GPCRs and their affinity towards agonists are regulated by membrane potential. However, it remains unclear whether GPCRs intrinsically respond to changes in membrane potential. Here we show that two prototypical GPCRs, the m2 and m1 muscarinic receptors (m2R and m1R), display charge-movement-associated currents analogous to 'gating currents' of voltage-gated channels. The gating charge-voltage relationship of m2R correlates well with the voltage dependence of the affinity of the receptor for acetylcholine. The loop that couples m2R and m1R to their G protein has a crucial function in coupling voltage sensing to agonist-binding affinity. Our data strongly indicate that GPCRs serve as sensors for both transmembrane potential and external chemical signals.

Differential requirements for the activation domain and FOG-interaction surface of GATA-1 in megakaryocyte gene expression and development

GATA1 is mutated in patients with 2 different disorders. First, individuals with a GATA1 mutation that blocks the interaction between GATA-1 and its cofactor Friend of GATA-1 (FOG-1) suffer from dyserythropoietic anemia and thrombocytopenia. Second, children with Down syndrome who develop acute megakaryoblastic leukemia harbor mutations in GATA1 that lead to the exclusive expression of a shorter isoform named GATA-1s. To determine the effect of these patient-specific mutations on GATA-1 function, we first compared the gene expression profile between wild-type and GATA-1-deficient megakaryocytes. Next, we introduced either GATA-1s or a FOG-binding mutant (V205G) into GATA-1-deficient megakaryocytes and assessed the effect on differentiation and gene expression. Whereas GATA-1-deficient megakaryocytes failed to undergo terminal differentiation and proliferated excessively in vitro, GATA-1s-expressing cells displayed proplatelet formation and other features of terminal maturation, but continued to proliferate aberrantly. In contrast, megakaryocytes that expressed V205G GATA-1 exhibited reduced proliferation, but failed to undergo maturation. Examination of the expression of megakaryocyte-specific genes in the various rescued cells correlated with the observed phenotypic differences. These studies show that GATA-1 is required for both normal regulation of proliferation and terminal maturation of megakaryocytes, and further, that these functions can be uncoupled by mutations in GATA1.

Potentiation of P2Y receptors by physiological elevations of extracellular K+ via a mechanism independent of Ca2+ influx

Many physiological and pathophysiological situations generate a significant increase in extracellular K+ concentration. This is known to influence a number of membrane conductances and exchangers, whereas direct effects of K+ on the activation of G protein-coupled receptors have not been reported. We now show that Ca2+ release evoked by P2Y1 receptors expressed in 1321-N1 astrocytoma cells is markedly potentiated by small increases in external K+ concentration. This effect was blocked by the phospholipase-C inhibitor U-73122 (1-[6-[[17 beta]-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione), but not by its analog U-73343 (1-[6-[[17 beta]-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-2,5-pyrrolidinedione), and not by nifedipine, Ni2+, Cd2+, or Gd3+. Thus, K+ enhances d-myo-inositol 1,4,5-trisphosphate-dependent Ca2+ release without a requirement for Ca2+ influx. The cation dependence of this effect displayed the order K+ > Rb+ > N-methyl-D-glucamine+, and Cs+ and choline+ were ineffective. The potentiation by K+ is half-maximal at an increase of 2.6 mM (total K+ of 7.6 mM). K+ caused a reduction in EC50 (2.7-fold for a 29 mM increase) without a change of slope; thus, the greatest effect was observed at near-threshold agonist levels. The response to K+ can be explained in part by depolarization-dependent potentiation of P2Y1 receptors [J Physiol (Lond) 555:61-70, 2004]. However, electrophysiological recordings of 1321-N1 cells and megakaryocytes demonstrated that K+ also amplifies ADP-evoked Ca2+ responses independently of changes in membrane potential. Elevated K+ also amplified endogenous UTP-dependent Ca2+ responses in human embryonic kidney 293 cells, suggesting that other P2Y receptors are K(+)-dependent. P2Y receptors display a widespread tissue distribution; therefore, their modulation by small changes in extracellular K+ may represent a novel means of autocrine and paracrine regulation of cellular activity.

Emerging roles for P2X1 receptors in platelet activation

The platelet surface membrane possesses three P2 receptors activated by extracellular adenosine nucleotides; one member of the ionotropic receptor family (P2X(1)) and two members of the G-protein-coupled receptor family (P2Y(1) and P2Y(12)). P2Y(1) and P2Y(12) receptors have firmly established roles in platelet activation during thrombosis and haemostasis, whereas the importance of the P2X(1) receptor has been more controversial. However, recent studies have demonstrated that P2X(1) receptors can generate significant functional platelet responses alone and in synergy with other receptor pathways. In addition, studies in transgenic animals indicate an important role for P2X(1) receptors in platelet activation, particularly under conditions of shear stress and thus during arterial thrombosis. This review discusses the background behind discovery of P2X(1) receptors in platelets and their precursor cell, the megakaryocyte, and how signalling via these ion channels may participate in platelet activation.

Direct voltage control of signaling via P2Y1 and other Galphaq-coupled receptors

Emerging evidence suggests that Ca2+ release evoked by certain G-protein-coupled receptors can be voltage-dependent; however, the relative contribution of different components of the signaling cascade to this response remains unclear. Using the electrically inexcitable megakaryocyte as a model system, we demonstrate that inositol 1,4,5-trisphosphate-dependent Ca2+ mobilization stimulated by several agonists acting via Galphaq-coupled receptors is potentiated by depolarization and that this effect is most pronounced for ADP. Voltage-dependent Ca2+ release was not induced by direct elevation of inositol 1,4,5-trisphosphate, by agents mimicking diacylglycerol actions, or by activation of phospholipase Cgamma-coupled receptors. The response to voltage did not require voltage-gated Ca2+ channels as it persisted in the presence of nifedipine and was only weakly affected by the holding potential. Strong predepolarizations failed to affect the voltage-dependent Ca2+ increase; thus, an alteration of G-protein betagamma subunit binding is also not involved. Megakaryocytes from P2Y1(-/-) mice lacked voltage-dependent Ca2+ release during the application of ADP but retained this response after stimulation of other Galphaq-coupled receptors. Although depolarization enhanced Ca2+ mobilization resulting from GTPgammaS dialysis and to a lesser extent during AlF4- or thimerosal, these effects all required the presence of P2Y1 receptors. Taken together, the voltage dependence to Ca2+ release via Galphaq-coupled receptors is not due to control of G-proteins or down-stream signals but, rather, can be explained by a voltage sensitivity at the level of the receptor itself. This effect, which is particularly robust for P2Y1 receptors, has wide-spread implications for cell signaling.