Voltage Control of G Protein-Coupled Receptor Signals
Voltage Control of G Protein-Coupled Receptor Signals: Comment on Bolton & Zholos, Sci. STKE 2003, pe52 (2003)
Martyn P. Mahaut-Smith* and Juan Martinez-Pinna Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG
*Corresponding author. E-mail, firstname.lastname@example.org
The Perspective by Bolton and Zholos (1) raises awareness of new evidence for an extremely important concept: that signaling through G protein-coupled receptors (GPCRs) can be directly controlled by changes in the cell membrane potential. GPCRs represent the largest family of surface receptors; therefore their regulation by action potentials or other electrophysiological events could have far reaching consequences. Bolton and Zholos focus on a report by Ben Chaim et al. (2) that suggests that activation of K+ channels by muscarinic receptors is voltage-dependent and discuss the results in light of work from their own laboratory. As stated by Bolton and Zholos, the idea that GPCR signaling may be directly dependent upon the membrane potential has been around for sometime yet few articles have examined this principle in any depth.
There are two specific examples (3, 4) not discussed in the context of the Perspective that give clear evidence for bipolar voltage control of GPCRs coupled to phospholipase C and, thus, inositol trisphosphate (IP3)-dependent Ca2+ mobilization. In coronary artery smooth muscle cells, Ganitkevich and Isenberg (4) reported that Ca2+ release mediated by stimulation of endogenous muscarinic receptors is potentiated by cellular depolarization and inhibited by hyperpolarization. Their conclusion, based upon the fact that the effect could also be induced by intracellular dialysis of a nonhydrolyzable analog of guanosine triphosphate (GTPγS) or disturbance of inositol lipid metabolism with lithium, was that the voltage sensor lies downstream of the receptor.
In our studies of signaling in the nonexcitable megakaryocyte, we have also observed a robust voltage-dependence of Ca2+ signals evoked by either purinergic P2Y or thromboxane A2 receptors (3, 5, 6). This platelet precursor cell is a useful model system for studies of GPCR-dependent Ca2+ release due to the absence of functional voltage-gated Ca2+ channels and ryanodine receptors. Thus, in resting cells, changes in membrane potential do not affect the intracellular calcium concentration ([Ca2+]i). However, in the presence of adenosine diphosphate (ADP) or the stable thromboxane A2 analog U46619, depolarization enhances and hyperpolarization inhibits Ca2+ mobilization. These effects of voltage are still observed in Ca2+-free medium and require functional IP3 receptors (3, 5, 6), therefore involve IP3-dependent Ca2+ release. The response during activation of P2Y receptors is sufficiently robust that small amplitude (<5mV, 5s duration) or short duration (25ms, 135mV) depolarizations can repeatedly evoke [Ca2+]i increases (7). Thus, as expected from these properties, application of a voltage clamp waveform in the form of a cardiac action potential potentiates Ca2+ release during exposure to ADP (7).
The voltage sensor responsible for controlling IP3-dependent Ca2+ release in the megakaryocyte is unknown. Three theories can explain the data: (i) The activity of the receptor, the G protein, or phospholipase C are directly controlled by the transmembrane voltage field; (ii) Binding of the agonist or a polar substrate (for example, phosphatidylinositol 4,5-bisphosphate) is affected by membrane potential; or (iii) Configurational coupling exists between a voltage sensor in the plasma membrane and IP3 receptors on the endoplasmic reticulum. More than one voltage sensor may exist, and we are currently investigating the relative contribution of different components of the P2Y receptor signal transduction pathway to the voltage-dependent response. Two pieces of evidence indicate that IP3 production is voltage-dependent, rather than voltage sensitivity being achieved through a configurational coupling model. First, ADP and depolarization evoke Ca2+ waves with indistinguishable spatiotemporal characteristics (8). Second, there is a delay of several hundred milliseconds between depolarization and the first detectable Ca2+ increase, which is slightly reduced by increasing the temperature (7). In neurons, the binding of Gβγ subunits to voltage-gated Ca2+ channels is inhibited by depolarization (9-11). However a similar modulation of Gβγ subunits is unlikely to account for the voltage-dependent Ca2+ release phenomenon in megakaryocytes as a reduction in Gβγ interaction with phospholipase-Cβ would lead to a decrease in IP3 production (12). Future work will hopefully clarify the location of the voltage sensor(s) in GPCR pathways leading to IP3-dependent Ca2+ release in the megakaryocyte and coronary artery smooth muscle. Another important question is the extent to which voltage control of GPCRs is a ubiquitous phenomenon because it would have important consequences for many physiological processes; synaptic efficacy and muscle excitability being just two obvious examples.
1. T. B. Bolton, A. V. Zholos, Potential synergy: voltage-driven steps in receptor-G protein coupling and beyond. Sci. STKE 2003, pe52 (2003).
2. Y. Ben Chaim, O. Tour, N. Dascal, I. Parnas, H. Parnas, The M2 muscarinic G-protein-coupled receptor is voltage-sensitive. J. Biol.Chem. 278, 22482-22491 (2003).
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4. V. Y. Ganitkevich, G. Isenberg, Membrane potential modulates inositol 1,4,5-trisphosphate-mediated Ca2+ transients in guinea-pig coronary myocytes. J. Physiol. 470, 35-44 (1993).
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13. Acknowledgement: Work within the authors’ laboratory is supported by the British Heart Foundation and Medical Research Council.
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