A conserved motif in the membrane proximal C-terminal tail of human muscarinic m1 acetylcholine receptors affects plasma membrane expression

Specific motifs mediate post-synaptic and surface transport of G protein-coupled receptors

G protein-coupled receptors (GPCRs) are key regulators of synaptic functions. However, their targeted trafficking to synapses after synthesis is poorly understood. Here, we demonstrate that multiple motifs mediate α2B-adrenergic receptor transport to the dendritic and post-synaptic compartments in primary hippocampal neurons, with a single leucine residue on the first intracellular loop being specifically involved in synaptic targeting. The N-terminally located tyrosine-serine motif operates differently in neuronal and non-neuronal cells. We further show that the highly conserved dileucine (LL) motif in the C-terminus is required for the dendritic and post-synaptic traffic of all GPCRs studied. The LL motif also directs the export from the endoplasmic reticulum of a chimeric GPCR and confers its transport ability to vesicular stomatitis virus glycoprotein in cell lines. Collectively, these data reveal the intrinsic structural determinants for the synaptic targeting of nascent GPCRs and their cell-type-specific trafficking along the biosynthetic pathways.

Differential barcoding of opioid receptors trafficking

Over the past several years, studies have highlighted the δ-opioid receptor (DOPr) as a promising therapeutic target for chronic pain management. While exhibiting milder undesired effects than most currently prescribed opioids, its specific agonists elicit effective analgesic responses in numerous animal models of chronic pain, including inflammatory, neuropathic, diabetic, and cancer-related pain. However, as compared with the extensively studied μ-opioid receptor, the molecular mechanisms governing its trafficking remain elusive. Recent advances have denoted several significant particularities in the regulation of DOPr intracellular routing, setting it apart from the other members of the opioid receptor family. Although they share high homology, each opioid receptor subtype displays specific amino acid patterns potentially involved in the regulation of its trafficking. These precise motifs or "barcodes" are selectively recognized by regulatory proteins and therefore dictate several aspects of the itinerary of a receptor, including its anterograde transport, internalization, recycling, and degradation. With a specific focus on the regulation of DOPr trafficking, this review will discuss previously reported, as well as potential novel trafficking barcodes within the opioid and nociceptin/orphanin FQ opioid peptide receptors, and their impact in determining distinct interactomes and physiological responses.

Mechanisms of the anterograde trafficking of GPCRs: Regulation of AT1R transport by interacting proteins and motifs

Anterograde cell surface transport of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum (ER) through the Golgi apparatus represents a crucial checkpoint to control the amount of the receptors at the functional destination and the strength of receptor activation-elicited cellular responses. However, as compared with extensively studied internalization and recycling processes, the molecular mechanisms of cell surface trafficking of GPCRs are relatively less defined. Here, we will review the current advances in understanding the ER-Golgi-cell surface transport of GPCRs and use angiotensin II type 1 receptor as a representative GPCR to discuss emerging roles of receptor-interacting proteins and specific motifs embedded within the receptors in controlling the forward traffic of GPCRs along the biosynthetic pathway.

Role of Rab GTPases in the export trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) constitute a superfamily of cell surface receptors that regulate a variety of cell functions. As the cell surface is the functional destination for most GPCRs, the cell surface targeting process represents a crucial checkpoint in controlling the functionality of the receptors. However, the molecular mechanisms underlying the cell surface delivery of newly synthesized GPCRs remain poorly understood. In this review, we will highlight the role of Rab GTPases in GPCR cell surface transport, particularly post-Golgi traffic, and discuss the underlying molecular mechanisms.

Atropine augments cardiac contractility by inhibiting cAMP-specific phosphodiesterase type 4

Atropine is a clinically relevant anticholinergic drug, which blocks inhibitory effects of the parasympathetic neurotransmitter acetylcholine on heart rate leading to tachycardia. However, many cardiac effects of atropine cannot be adequately explained solely by its antagonism at muscarinic receptors. In isolated mouse ventricular cardiomyocytes expressing a Förster resonance energy transfer (FRET)-based cAMP biosensor, we confirmed that atropine inhibited acetylcholine-induced decreases in cAMP. Unexpectedly, even in the absence of acetylcholine, after G-protein inactivation with pertussis toxin or in myocytes from M₂- or M_1/3-muscarinic receptor knockout mice, atropine increased cAMP levels that were pre-elevated with the β-adrenergic agonist isoproterenol. Using the FRET approach and in vitro phosphodiesterase (PDE) activity assays, we show that atropine acts as an allosteric PDE type 4 (PDE4) inhibitor. In human atrial myocardium and in both intact wildtype and M₂ or M_1/3-receptor knockout mouse Langendorff hearts, atropine led to increased contractility and heart rates, respectively. In vivo, the atropine-dependent prolongation of heart rate increase was blunted in PDE4D but not in wildtype or PDE4B knockout mice. We propose that inhibition of PDE4 by atropine accounts, at least in part, for the induction of tachycardia and the arrhythmogenic potency of this drug.

Beta2-adrenergic receptor homodimers: Role of transmembrane domain 1 and helix 8 in dimerization and cell surface expression

Even though there are hundreds of reports in the published literature supporting the hypothesis that G protein-coupled receptors (GPCR) form and function as dimers this remains a highly controversial area of research and mechanisms governing homodimer formation are poorly understood. Crystal structures revealing homodimers have been reported for many different GPCR. For adrenergic receptors, a potential dimer interface involving transmembrane domain 1 (TMD1) and helix 8 (H8) was identified in crystal structures of the beta₁-adrenergic (β₁-AR) and β₂-AR. The purpose of this study was to investigate a potential role for TMD1 and H8 in dimerization and plasma membrane expression of functional β₂-AR. Charged residues at the base of TMD1 and in the distal portion of H8 were replaced, singly and in combination, with non-polar residues or residues of opposite charge. Wild type and mutant β₂-AR, tagged with YFP and expressed in HEK293 cells, were evaluated for plasma membrane expression and function. Homodimer formation was evaluated using bioluminescence resonance energy transfer, bimolecular fluorescence complementation, and fluorescence correlation spectroscopy. Amino acid substitutions at the base of TMD1 and in the distal portion of H8 disrupted homodimer formation and caused receptors to be retained in the endoplasmic reticulum. Mutations in the proximal region of H8 did not disrupt dimerization but did interfere with plasma membrane expression. This study provides biophysical evidence linking a potential TMD1/H8 interface with ER export and the expression of functional β₂-AR on the plasma membrane. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Regulation of α2B-Adrenergic Receptor Cell Surface Transport by GGA1 and GGA2

The molecular mechanisms that control the targeting of newly synthesized G protein-coupled receptors (GPCRs) to the functional destinations remain poorly elucidated. Here, we have determined the role of Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding proteins 1 and 2 (GGA1 and GGA2) in the cell surface transport of α_2B-adrenergic receptor (α_2B-AR), a prototypic GPCR, and studied the underlying mechanisms. We demonstrated that knockdown of GGA1 and GGA2 by shRNA and siRNA significantly reduced the cell surface expression of inducibly expressed α_2B-AR and arrested the receptor in the perinuclear region. Knockdown of each GGA markedly inhibited the dendritic expression of α_2B-AR in primary cortical neurons. Consistently, depleting GGA1 and GGA2 attenuated receptor-mediated signal transduction measured as ERK1/2 activation and cAMP inhibition. Although full length α_2B-AR associated with GGA2 but not GGA1, its third intracellular loop was found to directly interact with both GGA1 and GGA2. More interestingly, further mapping of interaction domains showed that the GGA1 hinge region and the GGA2 GAE domain bound to multiple subdomains of the loop. These studies have identified an important function and revealed novel mechanisms of the GGA family proteins in the forward trafficking of a cell surface GPCR.

G protein-coupled receptors as promising cancer targets

G protein-coupled receptors (GPCRs) regulate an array of fundamental biological processes, such as growth, metabolism and homeostasis. Specifically, GPCRs are involved in cancer initiation and progression. However, compared with the involvement of the epidermal growth factor receptor in cancer, that of GPCRs have been largely ignored. Recent findings have implicated many GPCRs in tumorigenesis, tumor progression, invasion and metastasis. Moreover, GPCRs contribute to the establishment and maintenance of a microenvironment which is permissive for tumor formation and growth, including effects upon surrounding blood vessels, signaling molecules and the extracellular matrix. Thus, GPCRs are considered to be among the most useful drug targets against many solid cancers. Development of selective ligands targeting GPCRs may provide novel and effective treatment strategies against cancer and some anticancer compounds are now in clinical trials. Here, we focus on tumor related GPCRs, such as G protein-coupled receptor 30, the lysophosphatidic acid receptor, angiotensin receptors 1 and 2, the sphingosine 1-phosphate receptors and gastrin releasing peptide receptor. We also summarize their tissue distributions, activation and roles in tumorigenesis and discuss the potential use of GPCR agonists and antagonists in cancer therapy.

GGA3 Interacts with a G Protein-Coupled Receptor and Modulates Its Cell Surface Export

Molecular mechanisms governing the anterograde trafficking of nascent G protein-coupled receptors (GPCRs) are poorly understood. Here, we have studied the regulation of cell surface transport of α2-adrenergic receptors (α2-ARs) by GGA3 (Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding protein 3), a multidomain clathrin adaptor protein that sorts cargo proteins at the trans-Golgi network (TGN) to the endosome/lysosome pathway. By using an inducible system, we demonstrated that GGA3 knockdown significantly inhibited the cell surface expression of newly synthesized α2B-AR without altering overall receptor synthesis and internalization. The receptors were arrested in the TGN. Furthermore, GGA3 knockdown attenuated α2B-AR-mediated signaling, including extracellular signal-regulated kinase 1/2 (ERK1/2) activation and cyclic AMP (cAMP) inhibition. More interestingly, GGA3 physically interacted with α2B-AR, and the interaction sites were identified as the triple Arg motif in the third intracellular loop of the receptor and the acidic motif EDWE in the VHS domain of GGA3. In contrast, α2A-AR did not interact with GGA3 and its cell surface export and signaling were not affected by GGA3 knockdown. These data reveal a novel function of GGA3 in export trafficking of a GPCR that is mediated via a specific interaction with the receptor.

Regulation of α2B-Adrenerigc Receptor Export Trafficking by Specific Motifs

Intracellular trafficking and precise targeting to specific locations of G protein-coupled receptors (GPCRs) control the physiological functions of the receptors. Compared to the extensive efforts dedicated to understanding the events involved in the endocytic and recycling pathways, the molecular mechanisms underlying the transport of the GPCR superfamily from the endoplasmic reticulum (ER) through the Golgi to the plasma membrane are relatively less well defined. Over the past years, we have used α(2B)-adrenergic receptor (α(2B)-AR) as a model to define the factors that control GPCR export trafficking. In this chapter, we will review specific motifs identified to mediate the export of nascent α(2B)-AR from the ER and the Golgi and discuss the possible underlying mechanisms. As these motifs are highly conserved among GPCRs, they may provide common mechanisms for export trafficking of these receptors.

Intracellular localization of M1 muscarinic acetylcholine receptor through clathrin-dependent constitutive internalization via a C-terminal tryptophan-based motif

The M1 muscarinic acetylcholine receptor (M1-mAChR) is a G-protein-coupled membrane receptor that is activated by extracellular cholinergic stimuli. Recent investigations have revealed the intracellular localization of M1-mAChRs. In this study, we observed constitutive internalization of M1-mAChRs in mouse neuroblastoma N1E-115 cells without agonist stimulation. Constitutive internalization depends on dynamin, clathrin, and the adaptor protein-2 (AP-2) complex. A WXXI motif in the M1-mAChR C-terminus is essential for its constitutive internalization, since alanine mutations of W442 or I445 abolished constitutive internalization. This WXXI motif resembles YXXΦ, which is the canonical binding motif for the μ2 subunit of the AP-2 complex. The M1-mAChR C-terminal WXXI motif interacted with AP-2 μ2. W442A and I445A mutants of the M1-mAChR C-terminal sequence lost AP-2 μ2 binding activity, while the W442Y mutant bound more effectively than wild-type. Consistent with these results, W442A and I445A M1-mAChR mutants selectively localized to the cell surface. Conversely, the W442Y receptor mutant was found only at intracellular sites. Our data indicate that the cellular distribution of M1-mAChRs is governed by the C-terminal tryptophan-based motif that mediates constitutive internalization.

G-protein-coupled receptor interaction with small GTPases

In addition to heterotrimeric G-proteins, Ras-like small GTPases are also involved in regulating physiological functions of the G-protein-coupled receptor (GPCR) superfamily. In particular, Rab and ARF GTPases function either as "traffic cops" to coordinate receptor targeting to specific locations or as "signal transducers" to directly mediate receptor signal propagation. As revealed in protein-protein interaction assays, GPCRs may use specific motifs to physically interact with small GTPases, providing important insights into the underlying molecular mechanisms. In this chapter, we describe coimmunoprecipitation and GST fusion protein pull-down approaches to study the GPCR-small GTPase interaction, by focusing on the interaction of α(2B)- and β(2)-adrenegic receptors with the small GTPases Rab8 and ARF1.

A single lys residue on the first intracellular loop modulates the endoplasmic reticulum export and cell-surface expression of α2A-adrenergic receptor

Export from the endoplasmic reticulum (ER) represents an initial step in intracellular trafficking of G protein-coupled receptors (GPCRs). However, the underlying molecular mechanisms remain poorly understood. We have previously demonstrated that a highly conserved Leu residue on the first intracellular loop (ICL1) is required for exit of several GPCRs from the ER. Here we found that, in addition to Leu64 residue in the ICL1, the neighboring positively charged residue Lys65also modulates the cell-surface transport of α_2A-adrenergic receptor (α_2A-AR). Mutation of Lys65 to Ala, Glu and Gln significantly attenuated, whereas mutation of Lys65 to Arg strongly augmented α_2A-AR expression at the cell surface. Consistent with the effects on the cell-surface expression of α_2A-AR, mutation of Lys65 to Ala and Arg produced opposing effects on α_2A-AR-mediated ERK1/2 activation. Furthermore, confocal microscopy revealed that the α_2A-AR mutant K65A displayed a strong intracellular expression pattern and was extensively co-localized with the ER marker DsRed2-ER, suggestive of ER accumulation. These data provide the first evidence indicating an important function for a single Lys residue on the ICL1 in the ER export and cell-surface expression of α_2A-AR. These data also suggest that the ICL1 may possess multiple signals that control the cell-surface targeting of GPCRs via distinct mechanisms.

Regulation of post-Golgi traffic of G protein-coupled receptors

Anterograde trafficking of newly synthesized G protein-coupled -receptors (GPCRs) from the endoplasmic reticulum to the cell surface represents a crucial checkpoint in controlling the amount of the functional receptors at the cell surface and the strength of signaling initiated by the receptors. In contrast to the extensively studied, well-understood endocytic and recycling pathways, the molecular mechanisms underlying the cell-surface targeting of the receptors remain poorly defined. In this chapter, I will discuss current advances in understanding post-Golgi transport of GPCRs by focusing on specific motifs or sequences that may function as sorting signals regulating export from the Golgi and subsequent transport to the plasma membrane of GPCRs.

Small GTPase regulation of GPCR anterograde trafficking

The physiological functions of heterotrimeric G protein-coupled receptors (GPCRs) are dictated by their intracellular trafficking and precise targeting to the functional destinations. Over the past decades, most studies on the trafficking of GPCRs have focused on the events involved in endocytosis and recycling. By contrast, the molecular mechanisms underlying anterograde transport of newly synthesized GPCRs from the endoplasmic reticulum (ER) to the cell surface have only now begun to be revealed. In this review we discuss current advances in understanding the role of Ras-like GTPases, specifically the Rab and Sar1/ARF subfamilies, in regulating cell-surface transport of GPCRs en route from the ER and the Golgi.

Helix 8 of the M1 muscarinic acetylcholine receptor: scanning mutagenesis delineates a G protein recognition site

We have used alanine-scanning mutagenesis followed by functional expression and molecular modeling to analyze the roles of the 14 residues, Asn422 to Cys435, C-terminal to transmembrane (TM) helix 7 of the M(1) muscarinic acetylcholine receptor. The results suggest that they form an eighth (H8) helix, associated with the cytoplasmic surface of the cell membrane in the active state of the receptor. We suggest that the amide side chain of Asn422 may act as a cap to the C terminus of TM7, stabilizing its junction with H8, whereas the side chain of Phe429 may restrict the relative movements of H8 and the C terminus of TM7 in the inactive ground state of the receptor. We have identified four residues, Phe425, Arg426, Thr428, and Leu432, which are important for G protein binding and signaling. These may form a docking site for the C-terminal helix of the G protein α subunit, and collaborate with G protein recognition residues elsewhere in the cytoplasmic domain of the receptor to form a coherent surface for G protein binding in the activated state of the receptor.