Phenylalanine 138 in the second intracellular loop of human thromboxane receptor is critical for receptor-G-protein coupling

Preparing to strike: Acute events in signaling by the serpentine receptor for thromboxane A2

Over the last two decades, awareness of the (patho)physiological roles of thromboxane A₂ signaling has been greatly extended. From humble beginnings as a short-lived stimulus that activates platelets and causes vasoconstriction to a dichotomous receptor system involving multiple endogenous ligands capable of modifying tissue homeostasis and disease generation in almost every tissue of the body. Thromboxane A₂ receptor (TP) signal transduction is associated with the pathogenesis of cancer, atherosclerosis, heart disease, asthma, and host response to parasitic infection amongst others. The two receptors mediating these cellular responses (TPα and TPβ) are derived from a single gene (TBXA2R) through alternative splicing. Recently, knowledge about the mechanism(s) of signal propagation by the two receptors has undergone a revolution in understanding. Not only have the structural relationships associated with G-protein coupling been established but the modulation of that signaling by post-translational modification to the receptor has come sharply into focus. Moreover, the signaling of the receptor unrelated to G-protein coupling has become a burgeoning field of endeavor with over 70 interacting proteins currently identified. These data are reshaping the concept of TP signaling from a mere guanine nucleotide exchange factors for Gα activation to a nexus for the convergence of diverse and poorly characterized signaling pathways. This review summarizes the advances in understanding in TP signaling, and the potential for new growth in a field that after almost 50 years is finally coming of age.

Assembly of a GPCR-G Protein Complex

The activation of G proteins by G protein-coupled receptors (GPCRs) underlies the majority of transmembrane signaling by hormones and neurotransmitters. Recent structures of GPCR-G protein complexes obtained by crystallography and cryoelectron microscopy (cryo-EM) reveal similar interactions between GPCRs and the alpha subunit of different G protein isoforms. While some G protein subtype-specific differences are observed, there is no clear structural explanation for G protein subtype-selectivity. All of these complexes are stabilized in the nucleotide-free state, a condition that does not exist in living cells. In an effort to better understand the structural basis of coupling specificity, we used time-resolved structural mass spectrometry techniques to investigate GPCR-G protein complex formation and G-protein activation. Our results suggest that coupling specificity is determined by one or more transient intermediate states that serve as selectivity filters and precede the formation of the stable nucleotide-free GPCR-G protein complexes observed in crystal and cryo-EM structures.

Recent advances in computational studies of GPCR-G protein interactions

Protein-protein interactions are key in cellular signaling. G protein-coupled receptors (GPCRs), the largest superfamily of human membrane proteins, are able to transduce extracellular signals (e.g., hormones and neurotransmitters) to intracellular proteins, in particular the G proteins. Since GPCRs serve as primary targets of ~1/3 of currently marketed drugs, it is important to understand mechanisms of GPCR signaling in order to design selective and potent drug molecules. This chapter focuses on recent advances in computational studies of the GPCR-G protein interactions using bioinformatics, protein-protein docking and molecular dynamics simulation approaches.

Methods for the Development of In Silico GPCR Models

The Reggio group has constructed computer models of the inactive and G-protein-activated states of the cannabinoid CB1 and CB2 receptors, as well as, several orphan receptors that recognize a subset of cannabinoid compounds, including GPR55 and GPR18. These models have been used to design ligands, mutations, and covalent labeling studies. The resultant second-generation models have been used to design ligands with improved affinity, efficacy, and subtype selectivity. Herein, we provide a guide for the development of GPCR models using the most recent orphan receptor studied in our lab, GPR3. GPR3 is an orphan receptor that belongs to the Class A family of G-protein-coupled receptors. It shares high sequence similarity with GPR6, GPR12, the lysophospholipid receptors, and the cannabinoid receptors. GPR3 is predominantly expressed in mammalian brain and oocytes and it is known as a Gαs-coupled receptor activated constitutively in cells. GPR3 represents a possible target for the treatment of different pathological conditions such as Alzheimer's disease, oocyte maturation, or neuropathic pain. However, the lack of potent and selective GPR3 ligands is delaying the exploitation of this promising therapeutic target. In this context, we aim to develop a homology model that helps us to elucidate the structural determinants governing ligand-receptor interactions at GPR3. In this chapter, we detail the methods and rationale behind the construction of the GPR3 active-and inactive-state models. These homology models will enable the rational design of novel ligands, which may serve as research tools for further understanding of the biological role of GPR3.

Functional importance of two conserved residues in intracellular loop 1 and transmembrane region 2 of Family A GPCRs: insights from ligand binding and signal transduction responses of D1 and D5 dopaminergic receptor mutants

For many G protein-coupled receptors (GPCRs), the role of the first intracellular loop (IL1) and its connections with adjacent transmembrane (TM) regions have not been investigated. Notably, these regions harbor several polar residues such as Ser and Thr. To begin uncovering how these polar residues may contribute to the structural basis for GPCR functionality, we have designed human D1-class receptor mutants (hD1-ST1 and hD5-ST1) whereby all Ser and Thr of IL1 and IL1/TM2 juncture have been replaced by Ala and Val, respectively. Both ST1 mutants exhibited a loss of dopamine affinity but similar binding properties for inverse agonists compared to their parent receptors. As well, these mutations diminished receptor activation for both subtypes, as indicated by an ablated constitutive activity and a pronounced decrease in dopamine potency. Interestingly, both mutants exhibited enhanced dopamine-mediated maximal stimulation (Emax) of adenylyl cyclase that was at least two-fold higher than wild-type. Point mutations for hD1R revealed that the loss in dopamine affinity and potency was attributed to Thr59, while the enhanced Emax of adenylyl cyclase was directly influenced by Ser65. These two residues are conserved among many Family A GPCRs and have recurring molecular interactions among crystallized structures. As such, their functional roles for IL1 and its transition into TM2 reported herein may also be applicable to other GPCRs. Our work thus potentially highlights a structural role of Thr59 and Ser65 in the formation of critical intramolecular interactions for ligand binding and signal transduction of D1-class dopaminergic receptors.

Conformational flexibility and structural dynamics in GPCR-mediated G protein activation: a perspective

Structure and dynamics of G proteins and their cognate receptors, both alone and in complex, are becoming increasingly accessible to experimental techniques. Understanding the conformational changes and timelines that govern these changes can lead to new insights into the processes of ligand binding and associated G protein activation. Experimental systems may involve the use of, or otherwise stabilize, non-native environments. This can complicate our understanding of structural and dynamic features of processes such as the ionic lock, tryptophan toggle, and G protein flexibility. While elements in the receptor's transmembrane helices and the C-terminal α5 helix of Gα undergo well-defined structural changes, regions subject to conformational flexibility may be important in fine-tuning the interactions between activated receptors and G proteins. The pairing of computational and experimental approaches will continue to provide powerful tools to probe the conformation and dynamics of receptor-mediated G protein activation.

New insights into structural determinants for prostanoid thromboxane A2 receptor- and prostacyclin receptor-G protein coupling

G protein-coupled receptors (GPCRs) interact with heterotrimeric G proteins and initiate a wide variety of signaling pathways. The molecular nature of GPCR-G protein interactions in the clinically important thromboxane A2 (TxA(2)) receptor (TP) and prostacyclin (PGI(2)) receptor (IP) is poorly understood. The TP activates its cognate G protein (Gαq) in response to the binding of thromboxane, while the IP signals through Gαs in response to the binding of prostacyclin. Here, we utilized a combination of approaches consisting of chimeric receptors, molecular modeling, and site-directed mutagenesis to precisely study the specificity of G protein coupling. Multiple chimeric receptors were constructed by replacing the TP intracellular loops (ICLs) with the ICL regions of the IP. Our results demonstrate that both the sequences and lengths of ICL2 and ICL3 influenced G protein specificity. Importantly, we identified a precise ICL region on the prostanoid receptors TP and IP that can switch G protein specificities. The validities of the chimeric technique and the derived molecular model were confirmed by introducing clinically relevant naturally occurring mutations (R60L in the TP and R212C in the IP). Our findings provide new molecular insights into prostanoid receptor-G protein interactions, which are of general significance for understanding the structural basis of G protein activation by GPCRs in basic health and cardiovascular disease.

Role of the ERC motif in the proximal part of the second intracellular loop and the C-terminal domain of the human prostaglandin F2alpha receptor (hFP-R) in G-protein coupling control

The human FP-R (F2alpha prostaglandin receptor) is a Gq-coupled heptahelical ectoreceptor, which is of significant medical interest, since it is a potential target for the treatment of glaucoma and preterm labour. On agonist exposure, it mediates an increase in intracellular inositol phosphate formation. Little is known about the structures that govern the agonist-dependent receptor activation. In other prostanoid receptors, the C-terminal domain has been inferred in the control of agonist-dependent receptor activation. A DRY motif at the beginning of the second intracellular loop is highly conserved throughout the G-protein-coupled receptor family and appears to be crucial for controlling agonist-dependent receptor activation. It is replaced by an ERC motif in the FP-R and no evidence for the relevance of this motif in ligand-dependent activation of prostanoid receptors has been provided so far. The aim of the present study was to elucidate the potential role of the C-terminal domain and the ERC motif in agonist-controlled intracellular signalling in FP-R mutants generated by site-directed mutagenesis. It was found that substitution of the acidic Glu(132) in the ERC motif by a threonine residue led to full constitutive activation, whereas truncation of the receptor's C-terminal domain led to partial constitutive activation of all three intracellular signal pathways that had previously been shown to be activated by the FP-R, i.e. inositol trisphosphate formation, focal adhesion kinase activation and T-cell factor signalling. Inositol trisphosphate formation and focal adhesion kinase phosphorylation were further enhanced by ligand binding in cells expressing the truncation mutant but not the E132T (Glu132-->Thr) mutant. Thus C-terminal truncation appeared to result in a receptor with partial constitutive activation, whereas substitution of Glu132 by threonine apparently resulted in a receptor with full constitutive activity.

Disease-causing mutation in GPR54 reveals the importance of the second intracellular loop for class A G-protein-coupled receptor function

The G-protein-coupled receptor (GPCR) GPR54 is essential for the development and maintenance of reproductive function in mammals. A point mutation (L148S) in the second intracellular loop (IL2) of GPR54 causes idiopathic hypogonadotropic hypogonadism, a disorder characterized by delayed puberty and infertility. Here, we characterize the molecular mechanism by which the L148S mutation causes disease and address the role of IL2 in Class A GPCR function. Biochemical, immunocytochemical, and pharmacological analysis demonstrates that the mutation does not affect the expression, ligand binding properties, or protein interaction network of GPR54. In contrast, diverse GPR54 functional responses are markedly inhibited by the L148S mutation. Importantly, the leucine residue at this position is highly conserved among class A GPCRs. Indeed, mutating the corresponding leucine of the alpha(1A)-AR recapitulates the effects observed with L148S GPR54, suggesting the critical importance of this hydrophobic IL2 residue for Class A GPCR functional coupling. Interestingly, co-immunoprecipitation studies indicate that L148S does not hinder the association of Galpha subunits with GPR54. However, fluorescence resonance energy transfer analysis strongly suggests that L148S impairs the ligand-induced catalytic activation of Galpha. Combining our data with a predictive Class A GPCR/Galpha model suggests that IL2 domains contain a conserved hydrophobic motif that, upon agonist stimulation, might stabilize the switch II region of Galpha. Such an interaction could promote opening of switch II of Galpha to facilitate GDP-GTP exchange and coupling to downstream signaling responses. Importantly, mutations that disrupt this key hydrophobic interface can manifest as human disease.

Assembling NMR structures for the intracellular loops of the human thromboxane A2 receptor: implication of the G protein-coupling pocket

It has been reported that the multiple intracellular loops (iLPs) of the thromboxane A(2) receptor (TP) are involved in the receptor G protein coupling. In this study, a high-resolution 2D NMR technique was used to determine the 3D structures of the first, second, and third iLPs of the TP using synthetic peptides constrained into the loop structures. 2D (1)H NMR spectra, TOCSY and NOESY were obtained for the two peptides from proton NMR experiments. The NMR data was processed and assigned through the Felix 2000 program. Standard methods were used to acquire sequence-specific assignments. Structure calculations were processed through DGII and NMR refinement programs within the Insight II program. We were able to calculate and use the NOE constraints to obtain the superimposed structure of 10 structures for each iLP peptide. The NMR-determined structures of the iLP peptides were used to refine a homology model of the TP. A 3D G-protein-binding cavity, formed by the three intracellular loops, was predicted by the docking of the C-terminal domain of the Galphaq. Based on the structural model and the previous mutagenesis studies, the residues, R130, R60, C223, F138, L360, V361, E358 and Y359, which are important for interaction with the G protein, were further highlighted. These results reveal the possibly important molecular mechanisms in TP signaling and provide structural information to characterize other prostanoid receptor signalings.

Mutational analysis of the highly conserved ERY motif of the thromboxane A2 receptor: alternative role in G protein-coupled receptor signaling

The presence of highly conserved amino acid stretches in G protein-coupled receptors (GPCRs) usually predicts an important role in receptor function. Considerable attention has therefore been focused on the involvement of the highly conserved Glu/Asp-Arg-Tyr (E/DRY) motif at the cytoplasmic end of transmembrane domain 3 in the regulation of GPCR conformational states and/or the mediation of G protein activation. In the present study, we investigated the role of Glu129 and Arg130 in the ERY of thromboxane A2 receptor alpha (TPalpha) in transfected human embryonic kidney 293 cells. We show that no conservative or nonconservative substitutions of Glu129 and Arg130 generated a constitutively active TPalpha mutant, but a nonconservative mutation of Arg130 (R130V) yielded a mutant receptor with significantly impaired 9,11-dideoxy-9alpha,11alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid (U46619)-induced accumulation of inositol phosphates (IPs). This loss-of-function phenotype seems to be caused by the uncoupling of the TPalpha receptor from Gq, as demonstrated by the loss of high-affinity agonist binding, and not by receptor internalization, as shown by localization studies with the R130V-green fluorescent protein fusion protein. It is interesting to note that U46619-induced activation of the nonconservative E129V mutant stimulated the production of IPs with a approximately 10-fold lower EC50 and a approximately 2-fold higher Emax than in the wild-type receptor. Collectively, these data demonstrate that, unlike other GPCRs, mutations of Glu129 do not induce constitutive activity, whereas Arg130 is involved in G protein coupling or recognition, and they suggest the existence within class A GPCRs of at least two different subclasses that make different uses of the highly conserved E/DRY motif.

Structural and functional characterization of the first intracellular loop of human thromboxane A2 receptor

The conformation of a constrained peptide mimicking the putative first intracellular domain (iLP1) of thromboxane A(2) receptor (TP) was determined by (1)H 2D NMR spectroscopy. Through completed assignments of TOCSY, DQF-COSY, and NOESY spectra, a NMR structure of the peptide showed a beta-turn in residues 56-59 and a short helical structure in the residues 63-66. It suggests that residues 63-66 may be part of the second transmembrane domain (TM), and that Arg60, in an exposed position on the outer surface of the loop, may be involved in signaling through charge contact with Gq protein. The sequence alignment of Lys residue in the same position of other prostanoid receptors mediates different G protein couplings, suggesting that the chemical properties of Arg and Lys may also affect the receptor signaling activity. These hypotheses were supported by mutagenesis studies, in which the mutant of Arg60Leu completely lost activity in increasing intracellular calcium level through Gq coupling, and the mutant of Arg60Lys retained only about 35% signaling activity. The difference between the side chain functions of Lys and Arg in effecting the signaling was discussed.

Hydrogen bond interactions of a series of N-substituted TXA2 receptor antagonists

A series of N-substituted sulphonamide based thromboxane A2 (TXA2) receptor antagonists were synthesised with the objective to explore the role of hydrogen bond donation properties in the binding of these ligands to the TXA2 receptor. Pharmacological evaluation of these compounds revealed that the binding affinity decreased significantly with the removal of the hydrogen bond donor. This indicates that a hydrogen bond donor is important for the binding of these antagonists to the TXA2 receptor.

Importance of amino acids of the central portion of the second intracellular loop of the gastrin-releasing Peptide receptor for phospholipase C activation, internalization, and chronic down-regulation

Little is known about the function of the central portion of the second intracellular loop (i2 loop) of peptide receptors in activation of downstream pathways and receptor modulatory processes such as receptor internalization or chronic down-regulation (DR). Recent data suggest a role for i2 loop hydrophobic amino acids in these processes. We used site-directed mutagenesis to address these issues with the gastrin-releasing peptide receptor (GRP-R). Each i2 loop residue from 142 to 148 was mutated and the receptors were expressed in Balb 3T3 cells. Two mutants showed a minimal (<2-fold) decrease in affinity. Five mutants showed decreased efficacy for activating phospholipase C (PLC). Two double mutants (IM143.147AA and VM144.147AA) showed a minimal decrease in affinity but had a decreased ability to fully activate PLC. Only the IM double mutation had decreased maximal internalization, whereas the R145A single mutant showed an increase, suggesting a tonic inhibitory role for Arg-145 in internalization. Three single and both double mutants showed decreases in receptor DR. There was a weak correlation between the extent of GRP-R internalization and the maximal PLC activation, whereas changes in the maximal PLC activation were significantly (p = 0.008) coupled to receptor DR. This study shows that amino acids of the i2 loop of the GRP-R are important in activation of PLC, internalization and down-regulation, but not for affinity. Our results support the proposal that internalization and chronic down-regulation have differing dependence on PLC and are largely independent processes, because some mutants showed no changes in internalization, but significant alterations in down-regulation.

Induction of pro-angiogenic signaling by a synthetic peptide derived from the second intracellular loop of S1P3 (EDG3)

The G-protein-coupled receptors of the endothelial differentiation gene (EDG) family mediate pro-angiogenic activities, such as endothelial cell proliferation, chemotaxis, and vessel morphogenesis. We synthesized and tested the effects of a 9-amino acid peptide (KRX-725), derived from the second intracellular loop of S1P3 (EDG3). KRX-725 mimics the effects of sphingosine 1-phosphate (S1P), the natural ligand of S1P3, by triggering a Gi-dependent MEK-ERK (mitogen-activated protein kinase kinase and extracellular signal-regulated kinase) signal transduction pathway. Using aortic rings as an ex vivo model of angiogenesis, vascular sprouting was assessed in the presence of KRX-725 or S1P. KRX-725 induced extensive and dense vascular sprouts, which contain an elaborated organization of endothelial and smooth muscle layers, including lumen formation. When KRX-725 or S1P was combined with proangiogenic factors, such as basic fibroblast growth factor (bFGF), stem cell factor, or vascular endothelial growth factor, the effect was synergistic, leading to further enhancement of vascular sprouting. KRX-725 also initiated neovascularization in a mouse corneal pocket assay in vivo and showed synergism with bFGF. The specificity of KRX-725 was demonstrated via peptide-induced receptor internalization of S1P3 but not S1P1. The ability of a short peptide to stimulate extensive angiogenesis and to synergize with pro-angiogenic factors suggests that KRX-725 may serve as a useful agent in treating pathologic conditions such as peripheral vascular disease, cardiac ischemia, or tissue grafts.

Identification by site-directed mutagenesis of amino acids contributing to ligand-binding specificity or signal transduction properties of the human FP prostanoid receptor

Prostanoid receptors belong to the class of heptahelical plasma membrane receptors. For the five prostanoids, eight receptor subtypes have been identified. They display an overall sequence similarity of roughly 30%. Based on sequence comparison, single amino acids in different subtypes of different species have previously been identified by site-directed mutagenesis or in hybrid receptors that appear to be essential for ligand binding or G-protein coupling. Based on this information, a series of mutants of the human FP receptor was generated and characterized in ligand-binding and second-messenger-formation studies. It was found that mutation of His-81 to Ala in transmembrane domain 2 and of Arg-291 to Leu in transmembrane domain 7, which are putative interaction partners for the prostanoid's carboxyl group, abolished ligand binding. Mutants in which Ser-263 in transmembrane domain 6 or Asp-300 in transmembrane domain 7 had been replaced by Ala or Gln, respectively, no longer discriminated between prostaglandins PGF(2alpha) and PGD(2). Thus distortion of the topology of transmembrane domains 6 and 7 appears to interfere with the cyclopentane ring selectivity of the receptor. PGF(2alpha)-induced inositol formation was strongly reduced in the mutant Asp-300Gln, inferring a role for this residue in agonist-induced G-protein activation.

Human prostacyclin receptor

Prostacyclin, a member of the eicosanoid family of lipid mediators, is the major product of arachidonic acid metabolism formed in the marcovascular endothelium. It is a potent vasodilator, antithrombotic, and antiplatelet agent that mediates it effects through a membrane-associated receptor termed the IP. Cloning of the cDNA for IP, from human and other species, indicated its membership of the G protein-coupled receptor superfamily and has allowed detailed examination of the signaling and regulatory pathways utilized by this receptor. This article examines the current state of knowledge of the IP, its signaling and regulation, and its biological role in vivo and examines the possible existence of multiple PGI2 receptor sites.