G protein-coupled receptor

From Scholarpedia

Author: Dr. Bertil Hille, University of Washington School of Medicine, Seattle, WA

Dr. Bertil Hille accepted the invitation on 18 September 2008 (self-imposed deadline: 18 February 2009).

G-protein coupled receptors (GPCRs) [1]are plasma membrane proteins that transduce signals from extracellular ligands to signals in intracellular heterotrimeric GTP binding proteins (G proteins). By coupling to many downstream effectors, the G proteins initiate pleiotropic changes in many targets. Thus, the extracellular signal is typically amplified to produce robust, cell-specific responses.

GPCRs are quite distinct from growth-factor receptors, which signal through tyrosine kinases, do not use heterotrimeric G proteins, and typically regulate gene expression.

Contents 1 Diversity 2 Signaling from GPCRs 2.1 Downstream coupling of Gα-GTP 2.1.1 Five signaling pathways 2.2 Downstream coupling of Gβγ 2.3 Termination of GPCR signaling 3 Specificity of GPCR signaling 4 Structure 5 History 6 Neurotransmission by GPCR agonists 7 References

Diversity

The GPCR gene family is one of the largest gene families with more than a thousand homologous genes in man. In the central nervous system, the GPCRs include receptors for neurotransmitters, hormones, and transduction of several sensory inputs, as well as receptors for chemokines [2] and wnt signaling. Common ligands are:

• Monoamines: adrenaline, noradrenaline, serotonin, dopamine
• Other small neurotransmitters: Acetylcholine (mACh), gamma aminobutyric acid (GABA_B), glutamate (metabotropic mGluR), cannabinoids
• Many peptide neurotransmitters and hormones: opiates, somatostatin, NPY, kinins, releasing hormones, and more.
• Sensory modalities: light (rhodopsin), odorants, some tastetants including sweet, bitter, and umami

For most of these extracellular stimuli there are multiple different receptors and the same extracellular signal can give rise to different intracellular responses depending on the receptor subtypes expressed.

Signaling from GPCRs

GPCRs couple to heterotrimeric G proteins [3] that consist of Gα, Gβ, and Gγ subunits (Fig. 1). At rest these three subunits are assembled into a complex, Gαβγ. Since Gβ and Gγ are inseparable once coassembled, it is customary to talk about a resting GαGβγ complex. Both Gα and Gβγ are held at the inner leaflet of the plasma membrane because they have hydrophobic lipid modifications.

Gα is a flexible signaling protein. In the resting (inactive) complex, the Gα subunit is bound to the nucleoside phosphate guanosine diphosphate (GDP), but when a receptor is activated, the receptor can catalyze nucleotide exchange reactions on the Gα subunit. GDP leaves and guanosine triphosphate GTP enters instead. This makes an active G protein. In classical teaching, the Gα-GTP-Gβγ complex is unstable so that Gα-GTP and Gβγ separate from one another and from the receptor as well, but they remain attached to the plasma membrane by their lipid anchors. However, in some examples it is believed that the quaternary and active G-protein-GTP complex remains undissociated, and in some other examples the Gβγ dimer may leave the membrane and go into the cytoplasm or to other membranes. Gα-GTP and Gβγ each signal to downstream effectors. Because the activated G proteins are generally membrane associated, the next step involves interaction with a membrane-associated effector proteins or recruitment of cytoplasmic effector proteins to the membrane.

Downstream coupling of Gα-GTP

Five signaling pathways

The downstream effects of stimulating a GPCR depend on which G protein type(s) it couples to. Heterotrimeric G proteins are named by the type of α subunit they contain, and there are almost 20 genes encoding Gα subunits. However, for understanding most signaling it suffices to consider five broad Gα families and the five signaling pathways that they typically activate:

1. Gα_s : activates membrane adenylyl cyclases, increasing cellular cyclic AMP (cAMP), which stimulates phosphorylation by cAMP-dependent protein kinase.
2. Gα_i , Gα_o : inhibit most adenylyl cyclases, decreasing cellular cAMP
3. Gα_q , Gα₁₁ : activate phospholipase Cβ (PLCβ), cleaving certain phosphoinositide lipids of the membrane and creating several second messengers that release Ca²⁺ from intracellular stores and activate phosphorylation by protein kinase C.
4. Gα₁₂ , Gα₁₃ : enhance Rho kinase and gene expression
5. Gα_transducin: activates cyclic GMP (cGMP) phosphodiesterase that cleaves and depletes cytoplasmic cGMP (retina only)

Each of these pathways involves second messengers and effector enzymes. The long cascade of signaling may take up to tens of seconds to be completed. However in a few cases, such as vision using rhodopsin and transducin, a very tight compartmentalization and miniaturization of the geometry has allowed responses that take only tens of milliseconds.

Downstream coupling of Gβγ

The Gβγ subunits also are potent signals. They bind to several effectors. They open G-protein coupled inwardly rectifying K⁺ (GIRK) channels. They inhibit opening of several voltage-gated Ca²⁺ channels. They bind to the SNARE complex of the exocytotic machinery in synapses and reduce exocytosis of neurotransmitters. In neurobiology, the latter two signaling actions provide a major component of presynaptic inhibition by reducing Ca²⁺ entry and by blocking exocytosis of transmitter. In addition Gβγ dimers act directly on at least two more downstream effectors, stimulating PLC β and phosphoinositide 3-kinaseγ (PI3Kγ).

Although there are numerous Gβ and Gγ genes, to a first approximation, the Gβγ populations associated with all Gα subunits are similar. By mass action, the strength of Gβγ actions on effectors is probably greatest when launched by the most abundant Gα subunits (e.g. G_o in neurons) or when derived from types of Gα subunits that dissociate most readily from their Gβγ partners.

Termination of GPCR signaling

Termination of signaling requires turning off activated receptors, turning off activated G-proteins, and return of second messenger levels, protein phosphorylation levels, and other changed metabolites to their original values. Consider the first two. Receptors are quickly deactivated upon removal of agonist ligand. They are also inactivated by other processes even while ligand is still present, mechanisms that prevent over stimulation. In one canonical shutdown pathway (Gainetdinov et al., 2004), activated receptors are recognized and phosphorylated by G-protein coupled receptor kinases (GRKs). Phosphorylated receptors can then be turned off by binding of [[arrestins] [4]] at the plasma membrane. The arrestin-receptor complex may be unable to couple to downstream effectors or it may be endocytosed, removing the receptor entirely from the cell surface, a true downregulation of receptor protein.

Signaling by activated Gα-GTP is terminated by GTP hydrolysis, a reaction catalyzed by the Gα subunit itself that yields the inactive form, Gα-GDP. Gα-GDP in turn is a scavenger that binds any free Gβγ dimers, reforming the inactive heterotrimeric G protein GαβγGDP. Thus activated G-proteins have an intrinsic self timer function that terminates their action. The speed of hydrolysis can be accelerated by proteins that act as GTPase acceleratory proteins (GAPs). Sometimes the effector proteins are GAPS so that activated G-proteins become inactivated more rapidly if they make productive interactions with effectors. In addition, the GTPase activity is speeded by another class of cytoplasmic proteins called regulators of G-protein signaling (RGS proteins) (Hollinger and Hepler, 2002).

Specificity of GPCR signaling

Most GPCRs couple primarily to Gα subunits of only one of the five signaling families listed earlier. Some receptors, termed promiscuous, couple to several. In either case, there would seem to be a large loss of specificity if 1000 types of receptors can couple only to 5 signaling pathways. However, specificity is achieved in several other ways.

1. Each cell expresses only a subset of the available receptors (perhaps 20 or 30 kinds of receptors), meaning that each agonist speaks to specific appropriate cells and not to others. In this way, light stimulates photoreceptors and GnRH stimulates pituitary gonadotropes. #Each cell expresses a specific subset of downstream protein targets that are responsive to the second messengers that GPCRs regulate. Therefore, with the same second messenger each cell has a different response. In response to cAMP, Leydig cells of the testis make testosterone, horizontal cells of the retina decrease their electrical coupling, and cardiac pacemaker cells speed the cardiac beat rate.
2. GPCRs may be localized to certain parts of the plasma membrane, although less strongly than for many other membrane proteins. In that way it would be possible in principle for dendrites, cell bodies, axons, and nerve terminals to give different responses to the same set of GPCR agonists. Evidence for strong localization is mostly lacking except in special cases such as the localization of rhodopsin to discs of vertebrate photoreceptors.

As many GPCR agonists are released at nerve terminals and varicosities, one might suppose that the corresponding receptors would be highly localized to immediately opposite “postsynaptic” membranes of a "target" cell. This concept generalizes from the organization of typical fast chemical synapses where presynaptic ACh, glutamate, GABA, or glycine release talks to postsynaptic receptors within nanometers of the release site and opens an ion channel in one postsynaptic neuron within a fraction of a millisecond. Agonist action stops in a few milliseconds because agonist is quickly removed from the synaptic cleft. However, GPCR signaling is fundamentally different because GPCR agonists typically have an extracellular lifetime of 200 ms to several minutes. In this time, the agonist spreads by diffusion and acts on many cells. The signal acts in a volume rather than conveying a point-to-point message. Correspondingly, GPCRs are found more diffusely over the cell surface with less special reference to "postsynaptic" sites.

Structure

All GPCRs have a membrane topology with seven helical transmembrane segments, N-terminus outside and C-terminus inside (Fig. 1). X-ray crystal structures, available for only a few GPCRs, show that the transmembrane segments are broken helices that cross that membrane at various angles (Fig. 2). The binding site for small agonists lies nestled between helices and part way across the membrane. Biochemical experiments ahow that heterotrimeric G-proteins interact with the third intracellular loop (between helices 5 and 6) and with the cytoplasmic C-terminus of receptors (Fig. 1). These interactions determine which G-proteins each receptor will couple to, and they transmit the message from activated receptor to G-protein that initiates nucleotide exchange.

Since 1998, there is growing evidence that many GPCRs can form dimers, two receptors in one complex (Angers et al., 2002; Pin et al., 2006). Notable examples are mgluR5 or GABA_B receptors. Both homodimers and heterodimers are formed. The structure of such complexes is not known, but it is certain that some receptor dimers are active in signaling and sometimes even are obligatory for signaling (GABA_BR). The consequences, generality, and significance of receptor dimerization need further investigation. Potentially, dimerization can alter agonist and antagonist specificity, G-protein coupling, and membrane trafficking and recycling, giving dimeric receptors properties that the monomeric forms did not have (Pin et al., 2006).

History

Neurotransmission by GPCR agonists

References

Angers S, Salahpour A, Bouvier M. Dimerization: an emerging concept for G protein-coupled receptor ontogeny and function. Annu Rev Pharmacol Toxicol. 2002;42:409-35.

Jean-Philippe Pin, Richard Neubig, Michel Bouvier, Lakshmi Devi, Marta Filizola, Jonathan A. Javitch, Martin J. Lohse, Graeme Milligan, Krzysztof Palczewski, Marc Parmentier and Michael Spedding. International Union of Basic and Clinical Pharmacology. LXVII. Recommendations for the Recognition and Nomenclature of G Protein-Coupled Receptor Heteromultimers. Pharmacol Rev 59:5-13, 2007

Gainetdinov RR, Premont RT, Bohn LM, Lefkowitz RJ, Caron MG. Desensitization of G protein-coupled receptors and neuronal functions. Annu Rev Neurosci. 2004;27:107-44.

Hollinger S, Hepler JR. Cellular regulation of RGS proteins: modulators and integrators of G protein signaling. Pharmacol Rev. 2002 Sep;54(3):527-59.

Invited by:

Dr. Eugene M. Izhikevich, Editor-in-Chief of Scholarpedia, the peer-reviewed open-access encyclopedia