Glycosylation of beta(1)-adrenergic receptors regulates receptor surface expression and dimerization

Protein glycosylation in cardiovascular health and disease

Protein glycosylation, which involves the attachment of carbohydrates to proteins, is one of the most abundant protein co-translational and post-translational modifications. Advances in technology have substantially increased our knowledge of the biosynthetic pathways involved in protein glycosylation, as well as how changes in glycosylation can affect cell function. In addition, our understanding of the role of protein glycosylation in disease processes is growing, particularly in the context of immune system function, infectious diseases, neurodegeneration and cancer. Several decades ago, cell surface glycoproteins were found to have an important role in regulating ion transport across the cardiac sarcolemma. However, with very few exceptions, our understanding of how changes in protein glycosylation influence cardiovascular (patho)physiology remains remarkably limited. Therefore, in this Review, we aim to provide an overview of N-linked and O-linked protein glycosylation, including intracellular O-linked N-acetylglucosamine protein modification. We discuss our current understanding of how all forms of protein glycosylation contribute to normal cardiovascular function and their roles in cardiovascular disease. Finally, we highlight potential gaps in our knowledge about the effects of protein glycosylation on the heart and vascular system, highlighting areas for future research.

G Protein-Coupled Receptor Signaling: New Insights Define Cellular Nanodomains

G protein-coupled receptors are the largest and pharmacologically most important receptor family and are involved in the regulation of most cell functions. Most of them reside exclusively at the cell surface, from where they signal via heterotrimeric G proteins to control the production of second messengers such as cAMP and IP3 as well as the activity of several ion channels. However, they may also internalize upon agonist stimulation or constitutively reside in various intracellular locations. Recent evidence indicates that their function differs depending on their precise cellular localization. This is because the signals they produce, notably cAMP and Ca2+, are mostly bound to cell proteins that significantly reduce their mobility, allowing the generation of steep concentration gradients. As a result, signals generated by the receptors remain confined to nanometer-sized domains. We propose that such nanometer-sized domains represent the basic signaling units in a cell and a new type of target for drug development.

The Succinate Receptor SUCNR1 Resides at the Endoplasmic Reticulum and Relocates to the Plasma Membrane in Hypoxic Conditions

The GPCR SUCNR1/GPR91 exerts proangiogenesis upon stimulation with the Krebs cycle metabolite succinate. GPCR signaling depends on the surrounding environment and intracellular localization through location bias. Here, we show by microscopy and by cell fractionation that in neurons, SUCNR1 resides at the endoplasmic reticulum (ER), while being fully functional, as shown by calcium release and the induction of the expression of the proangiogenic gene for VEGFA. ER localization was found to depend upon N-glycosylation, particularly at position N8; the nonglycosylated mutant receptor localizes at the plasma membrane shuttled by RAB11. This SUCNR1 glycosylation is physiologically regulated, so that during hypoxic conditions, SUCNR1 is deglycosylated and relocates to the plasma membrane. Downstream signal transduction of SUCNR1 was found to activate the prostaglandin synthesis pathway through direct interaction with COX-2 at the ER; pharmacologic antagonism of the PGE₂ EP₄ receptor (localized at the nucleus) was found to prevent VEGFA expression. Concordantly, restoring the expression of SUCNR1 in the retina of SUCNR1-null mice renormalized vascularization; this effect is markedly diminished after transfection of the plasma membrane-localized SUCNR1 N8A mutant, emphasizing that ER localization of the succinate receptor is necessary for proper vascularization. These findings uncover an unprecedented physiologic process where GPCR resides at the ER for signaling function.

Stachytine Hydrochloride Improves Cardiac Function in Mice with ISO-Induced Heart Failure by Inhibiting the α-1,6-Fucosylation on N-Glycosylation of β1AR

Background: Cardiovascular diseases have become a major public health problem that seriously threatens human health. The cumulative effects of various cardiovascular events will eventually develop into chronic heart insufficiency and even heart failure, and the β1 adrenergic receptor signal pathway plays an important role in this process. Stachytine hydrochloride is the main active ingredient of Yimucao, which is a traditional Chinese medicine used to treat gynecological diseases. Modern studies have found that stachytine hydrochloride has a good cardioprotective effect, but it is still unclear whether stachytine hydrochloride has an effect on the β1 adrenergic receptor signal pathway. The purpose of this study is to explore the effect of stachytine hydrochloride on the β1 adrenergic receptor signal pathway.

Method: In this study, a continuous infusion of isoproterenol (40 mg/kg/day) was administered to mice and ventricular myocytes explored the potential mechanism of stachytine hydrochloride (12 mg/kg/day) on the β1 adrenergic receptor signal pathway in the heart. Evaluate changes in cardiac morphology and function by echocardiography, cardiac hemodynamics, and histological methods, and detect molecular changes by Western blot and immunofluorescence. Treat primary cultured adult mouse or neonatal rat ventricular myocytes with or without isoproterenol (0.1 μMol), PNGase F (10^–2 units/ml), and stachytine hydrochloride (10 μMol) at different time points. Detect α-1,6-fucosylation on N-glycosylation, calcium transient, contraction, and relaxation function and related signals.

Results: Stachytine hydrochloride reduces cardiac remodeling and modulates hemodynamic parameters during chronic β1 adrenergic receptor activation in vivo. The N-glycosylation of β1 adrenergic receptors decreased after continuous isoproterenol stimulation, while stachytine hydrochloride can increase the N-glycosylation of β1AR in the heart of mice with isoproterenol-induced heart failure. Decreased N-glycosylation of β1 adrenergic receptors will downregulate the cAMP/PKA signal pathway and inhibit myocardial excitation and contraction coupling. Stachytine hydrochloride significantly reduced isoproterenol-induced cardiac N-linked glycoproteins with α-1,6-fucosylation.

Conclusion: Our results show that stachytine hydrochloride inhibits the synthesis of α-1,6-fucosylation on the N-terminal sugar chain by reducing α-1,6-fucosyltransferase (FUT8) and α-1,3-mannosyl-glycoprotein 4-β-N-acetylglucosaminyltransferase A (MGAT4a), upregulating the N-glycosylation level on β1 adrenergic receptors, and maintaining cAMP/PKA signal pathway activation.

N-glycosylation of the human neuropeptide QRFP receptor (QRFPR) is essential for ligand binding and receptor activation

The newly identified pyroglutamylated RFamide peptide (QRFP) signaling system has been shown to be implicated in regulating a variety of physiological processes. G-protein-coupled receptors (GPCRs) are preferentially N-glycosylated on extracellular domains. The human QRFP receptor QRFPR (GPR103) possesses three N-glycosylation consensus sites, two located on the N-terminal domain (N5 and N19) and one on the first extracellular loop (ECL1) (N106); however, to date, their role in QRFPR expression and signaling has not been established. Here, we combined mutants with glutamine substitution of the critical asparagines of the consensus sites with glycosidase PNGase F and N-glycosylation inhibitor tunicamycin to study the effect of N-glycosylation in the regulation of QRFPR cell surface expression and signaling. Western blot analysis performed with site-directed mutagenesis revealed that two asparagines at N19 in the N-terminus and N106 in ECL1, but not N5 in the N-terminus, served as sites for N-glycosylation. Treatment with PNGase F and tunicamycin resulted in a reduction in both two-protein species, ~43 kDa and ~85 kDa in size, by 2-4 kDa. Analysis with confocal microscopy and quantitative ELISA showed that N-glycosylation of QRFPR is not essentially required for targeting the cell membrane. However, further binding assay and functional assays demonstrated that removal of N-glycosylation sequons or treatment with tunicamycin led to significant impairments in the interaction of receptor with QRFP26 and downstream signaling. Thus, our findings suggest that for the human QRFP receptor (QRFPR), N-glycosylation is not important for cell surface expression but is a pre-requisite for ligand binding and receptor activation.

Post-Translational Modifications of G Protein-Coupled Receptors Control Cellular Signaling Dynamics in Space and Time

G protein-coupled receptors (GPCRs) are a large family comprising >800 signaling receptors that regulate numerous cellular and physiologic responses. GPCRs have been implicated in numerous diseases and represent the largest class of drug targets. Although advances in GPCR structure and pharmacology have improved drug discovery, the regulation of GPCR function by diverse post-translational modifications (PTMs) has received minimal attention. Over 200 PTMs are known to exist in mammalian cells, yet only a few have been reported for GPCRs. Early studies revealed phosphorylation as a major regulator of GPCR signaling, whereas later reports implicated a function for ubiquitination, glycosylation, and palmitoylation in GPCR biology. Although our knowledge of GPCR phosphorylation is extensive, our knowledge of the modifying enzymes, regulation, and function of other GPCR PTMs is limited. In this review we provide a comprehensive overview of GPCR post-translational modifications with a greater focus on new discoveries. We discuss the subcellular location and regulatory mechanisms that control post-translational modifications of GPCRs. The functional implications of newly discovered GPCR PTMs on receptor folding, biosynthesis, endocytic trafficking, dimerization, compartmentalized signaling, and biased signaling are also provided. Methods to detect and study GPCR PTMs as well as PTM crosstalk are further highlighted. Finally, we conclude with a discussion of the implications of GPCR PTMs in human disease and their importance for drug discovery. SIGNIFICANCE STATEMENT: Post-translational modification of G protein-coupled receptors (GPCRs) controls all aspects of receptor function; however, the detection and study of diverse types of GPCR modifications are limited. A thorough understanding of the role and mechanisms by which diverse post-translational modifications regulate GPCR signaling and trafficking is essential for understanding dysregulated mechanisms in disease and for improving and refining drug development for GPCRs.

Modulation of Endocannabinoid-Binding Receptors in Human Neuroblastoma Cells by Tunicamycin

Endocannabinoid (eCB)-binding receptors can be modulated by several ligands and membrane environment, yet the effect of glycosylation remains to be assessed. In this study, we used human neuroblastoma SH-SY5Y cells to interrogate whether expression, cellular localization, and activity of eCB-binding receptors may depend on N-linked glycosylation. Following treatment with tunicamycin (a specific inhibitor of N-linked glycosylation) at the non-cytotoxic dose of 1 µg/mL, mRNA, protein levels and localization of eCB-binding receptors, as well as N-acetylglucosamine (GlcNAc) residues, were evaluated in SH-SY5Y cells by means of quantitative real-time reverse transcriptase-polymerase chain reaction (qRT-PCR), fluorescence-activated cell sorting (FACS), and confocal microscopy, respectively. In addition, the activity of type-1 and type-2 cannabinoid receptors (CB₁ and CB₂) was assessed by means of rapid binding assays. Significant changes in gene and protein expression were found upon tunicamycin treatment for CB₁ and CB₂, as well as for GPR55 receptors, but not for transient receptor potential vanilloid 1 (TRPV1). Deglycosylation experiments with N-glycosidase-F and immunoblot of cell membranes derived from SH-SY5Y cells confirmed the presence of one glycosylated form in CB₁ (70 kDa), that was reduced by tunicamycin. Morphological studies demonstrated the co-localization of CB₁ with GlcNAc residues, and showed that tunicamycin reduced CB₁ membrane expression with a marked nuclear localization, as confirmed by immunoblotting. Cleavage of the carbohydrate side chain did not modify CB receptor binding affinity. Overall, these results support N-linked glycosylation as an unprecedented post-translational modification that may modulate eCB-binding receptors’ expression and localization, in particular for CB₁.

N-glycosylation-dependent regulation of hK2P17.1 currents

Two pore-domain potassium (K_2P) channels mediate potassium background currents that stabilize the resting membrane potential and facilitate action potential repolarization. In the human heart, hK_2P17.1 channels are predominantly expressed in the atria and Purkinje cells. Reduced atrial hK_2P17.1 protein levels were described in patients with atrial fibrillation or heart failure. Genetic alterations in hK_2P17.1 were associated with cardiac conduction disorders. Little is known about posttranslational modifications of hK_2P17.1. Here, we characterized glycosylation of hK_2P17.1 and investigated how glycosylation alters its surface expression and activity. Wild-type hK_2P17.1 channels and channels lacking specific glycosylation sites were expressed in Xenopus laevis oocytes, HEK-293T cells, and HeLa cells. N-glycosylation was disrupted using N-glycosidase F and tunicamycin. hK_2P17.1 expression and activity were assessed using immunoblot analysis and a two-electrode voltage clamp technique. Channel subunits of hK_2P17.1 harbor two functional N-glycosylation sites at positions N65 and N94. In hemi-glycosylated hK_2P17.1 channels, functionality and membrane trafficking remain preserved. Disruption of both N-glycosylation sites results in loss of hK_2P17.1 currents, presumably caused by impaired surface expression. This study confirms diglycosylation of hK_2P17.1 channel subunits and its pivotal role in cell-surface targeting. Our findings underline the functional relevance of N-glycosylation in biogenesis and membrane trafficking of ion channels.

Distinct Dopamine D₂ Receptor Antagonists Differentially Impact D₂ Receptor Oligomerization

Dopamine D₂ receptors (D₂R) are known to form transient homodimer complexes, of which the increased formation has already been associated with development of schizophrenia. Pharmacological targeting and modulation of the equilibrium of these receptor homodimers might lead to a better understanding of the critical role played by these complexes in physiological and pathological conditions. Whereas agonist addition has shown to prolong the D₂R dimer lifetime and increase the level of dimer formation, the possible influence of D₂R antagonists on dimerization has remained rather unexplored. Here, using a live-cell reporter assay based on the functional complementation of a split Nanoluciferase, a panel of six D₂R antagonists were screened for their ability to modulate the level of D_2LR dimer formation. Incubation with the D₂R antagonist spiperone decreased the level of D_2LR dimer formation significantly by 40–60% in real-time and after long-term (≥16 h) incubations. The fact that dimer formation of the well-studied A_2a–D_2LR dimer was not altered following incubation with spiperone supports the specificity of this observation. Other D₂R antagonists, such as clozapine, risperidone, and droperidol did not significantly evoke this dissociation event. Furthermore, molecular modeling reveals that spiperone presents specific Tyr199^5.48 and Phe390^6.52 conformations, compared to clozapine, which may determine D₂R homodimerization.

Dynamic States of the Ligand-Free Class A G Protein-Coupled Receptor Extracellular Side

G protein-coupled receptors (GPCRs) make up the largest family of drug targets. The second extracellular loop (ECL2) and extracellular end of the third transmembrane helix (TM3) are basic structural elements of the GPCR ligand binding site. Currently, the disulfide bond between the two conserved cysteines in the ECL2 and TM3 is considered to be a basic GPCR structural feature. This disulfide bond has a significant effect on receptor dynamics and ligand binding. Here, molecular dynamics simulations and experimental results show that the two cysteines are distant from one another in the highest-population conformational state of ligand-free class A GPCRs and do not form a disulfide bond, indicating that the dynamics of the GPCR extracellular side are different from our conventional understanding. These surprising dynamics should have important effects on the drug binding process. On the basis of the two distinct ligand-free states, we suggest two kinetic processes for binding of ligands to GPCRs. These results challenge our commonly held beliefs regarding both GPCR structural features and ligand binding.

Adverse Effects on β-Adrenergic Receptor Coupling: Ischemic Postconditioning Failed to Preserve Long-Term Cardiac Function

BACKGROUND

Ischemic preconditioning (IPC) and ischemic postconditioning (IPoC) are currently among the most efficient strategies protecting the heart against ischemia/reperfusion injury. However, the effect of IPC and IPoC on functional recovery following ischemia/reperfusion is less clear, particularly with regard to the specific receptor-mediated signaling of the postischemic heart. The current article examines the effect of IPC or IPoC on the regulation and coupling of β-adrenergic receptors and their effects on postischemic left ventricular function.

METHODS AND RESULTS

The β-adrenergic signal transduction was analyzed in 3-month-old Wistar rats for each of the intervention strategies (Sham, ischemia/reperfusion, IPC, IPoC) immediately and 7 days after myocardial infarction. Directly after the infarction a cardioprotective potential was demonstrated for both IPC and IPoC: the infarct size was reduced, apoptosis and production of reactive oxygen species were lowered, and the myocardial tissue was preserved. Seven days after myocardial ischemia, only IPC hearts showed significant functional improvement. Along with a deterioration in fractional shortening, IPoC hearts no longer responded adequately to β-adrenergic stimulation. The stabilization of β-adrenergic receptor kinase-2 via increased phosphorylation of Mdm2 (an E3-ubiquitin ligase) was responsible for desensitization of β-adrenergic receptors and identified as a characteristic specific to IPoC hearts.

CONCLUSIONS

Immediately after myocardial infarction, rapid and transient activation of β-adrenergic receptor kinase-2 may be an appropriate means to protect the injured heart from excessive stress. In the long term, however, induction and stabilization of β-adrenergic receptor kinase-2, with the resultant loss of positive inotropic function, leads to the functional picture of heart failure.

β1-adrenergic receptor O-glycosylation regulates N-terminal cleavage and signaling responses in cardiomyocytes

β₁-adrenergic receptors (β₁ARs) mediate catecholamine actions in cardiomyocytes by coupling to both Gs/cAMP-dependent and Gs-independent/growth-regulatory pathways. Structural studies of the β₁AR define ligand-binding sites in the transmembrane helices and effector docking sites at the intracellular surface of the β₁AR, but the extracellular N-terminus, which is a target for post-translational modifications, typically is ignored. This study identifies β₁AR N-terminal O-glycosylation at Ser³⁷/Ser⁴¹ as a mechanism that prevents β₁AR N-terminal cleavage. We used an adenoviral overexpression strategy to show that both full-length/glycosylated β₁ARs and N-terminally truncated glycosylation-defective β₁ARs couple to cAMP and ERK-MAPK signaling pathways in cardiomyocytes. However, a glycosylation defect that results in N-terminal truncation stabilizes β₁ARs in a conformation that is biased toward the cAMP pathway. The identification of O-glycosylation and N-terminal cleavage as novel structural determinants of β₁AR responsiveness in cardiomyocytes could be exploited for therapeutic advantage.

Receptor Reserve Moderates Mesolimbic Responses to Opioids in a Humanized Mouse Model of the OPRM1 A118G Polymorphism

The OPRM1 A118G polymorphism is the most widely studied μ-opioid receptor (MOR) variant. Although its involvement in acute alcohol effects is well characterized, less is known about the extent to which it alters responses to opioids. Prior work has shown that both electrophysiological and analgesic responses to morphine but not to fentanyl are moderated by OPRM1 A118G variation, but the mechanism behind this dissociation is not known. Here we found that humanized mice carrying the 118GG allele (h/mOPRM1-118GG) were less sensitive than h/mOPRM1-118AA littermates to the rewarding effects of morphine and hydrocodone but not those of other opioids measured with intracranial self-stimulation. Reduced morphine reward in 118GG mice was associated with decreased dopamine release in the nucleus accumbens and reduced effects on GABA release in the ventral tegmental area that were not due to changes in drug potency or efficacy in vitro or receptor-binding affinity. Fewer MOR-binding sites were observed in h/mOPRM1-118GG mice, and pharmacological reduction of MOR availability unmasked genotypic differences in fentanyl sensitivity. These findings suggest that the OPRM1 A118G polymorphism decreases sensitivity to low-potency agonists by decreasing receptor reserve without significantly altering receptor function.

Smoothened Regulation: A Tale of Two Signals

The G protein-coupled receptor (GPCR) Smoothened (Smo) is the signal transducer of the developmentally and therapeutically relevant Hedgehog (Hh) pathway. Although recent structural analyses have advanced our understanding of Smo biology, several questions remain. Chief among them are the identity of its natural ligand, the regulatory processes controlling its activation, and the mechanisms by which it signals to downstream effectors. In this review, we discuss recent discoveries from multiple model systems that have set the stage for solving these mysteries. We focus on the roles of distinct Smo functional domains, post-translational modifications, and trafficking, and conclude by discussing their contributions to signal output.

Functional Divergence in the Role of N-Linked Glycosylation in Smoothened Signaling

The G protein-coupled receptor (GPCR) Smoothened (Smo) is the requisite signal transducer of the evolutionarily conserved Hedgehog (Hh) pathway. Although aspects of Smo signaling are conserved from Drosophila to vertebrates, significant differences have evolved. These include changes in its active sub-cellular localization, and the ability of vertebrate Smo to induce distinct G protein-dependent and independent signals in response to ligand. Whereas the canonical Smo signal to Gli transcriptional effectors occurs in a G protein-independent manner, its non-canonical signal employs Gαi. Whether vertebrate Smo can selectively bias its signal between these routes is not yet known. N-linked glycosylation is a post-translational modification that can influence GPCR trafficking, ligand responsiveness and signal output. Smo proteins in Drosophila and vertebrate systems harbor N-linked glycans, but their role in Smo signaling has not been established. Herein, we present a comprehensive analysis of Drosophila and murine Smo glycosylation that supports a functional divergence in the contribution of N-linked glycans to signaling. Of the seven predicted glycan acceptor sites in Drosophila Smo, one is essential. Loss of N-glycosylation at this site disrupted Smo trafficking and attenuated its signaling capability. In stark contrast, we found that all four predicted N-glycosylation sites on murine Smo were dispensable for proper trafficking, agonist binding and canonical signal induction. However, the under-glycosylated protein was compromised in its ability to induce a non-canonical signal through Gαi, providing for the first time evidence that Smo can bias its signal and that a post-translational modification can impact this process. As such, we postulate a profound shift in N-glycan function from affecting Smo ER exit in flies to influencing its signal output in mice.

N-linked glycosylation is a post-translational modification occurring on membrane proteins such as G protein-coupled receptors (GPCR). Smoothened (Smo) is a GPCR that functions as the signal transducer of the Hedgehog (Hh) pathway. We used a mutagenesis approach to assess the role of N-glycans in Smo signaling in two genetic models for Hh pathway activity, Drosophila and mouse. In doing so, we discovered a divergence in glycan function between them. We mapped an essential N-glycan acceptor site that when lost in Drosophila, triggered ER retention, altered Smo protein stability and blunted its signaling capacity. Conversely, ER exit of the murine protein was unaffected by glycan loss, as was its ability to traffic and induce a G protein-independent signal to activate Hh target genes. However, the ability of vertebrate Smo to induce a distinct G protein-dependent signal was lost. This suggests that N-linked glycosylation may influence signal bias of vertebrate Smo to favor one signal output over the other. We therefore propose that the role of this conserved post-translational modification may have been repurposed from governing Smo ER exit in the fly to influencing effector route selection in vertebrates.

Role of post-translational modifications on structure, function and pharmacology of class C G protein-coupled receptors

G protein-coupled receptors are divided into three classes (A, B and C) based on homology of their seven transmembrane domains. Class C is the smallest class with 22 human receptor subtypes including eight metabotropic glutamate (mGlu1-8) receptors, two GABAB receptors (GABAB1 and GABAB2), three taste receptors (T1R1-3), one calcium-sensing (CaS) receptor, one GPCR, class C, group 6, subtype A (GPRC6) receptor, and seven orphan receptors. G protein-coupled receptors undergo a number of post-translational modifications, which regulate their structure, function and/or pharmacology. Here, we review the existence of post-translational modifications in class C G protein-coupled receptors and their regulatory roles, with particular focus on glycosylation, phosphorylation, ubiquitination, SUMOylation, disulphide bonding and lipidation.

N-glycosylation and disulfide bonding affects GPRC6A receptor expression, function, and dimerization

Investigation of post-translational modifications of receptor proteins is important for our understanding of receptor pharmacology and disease physiology. However, our knowledge about post-translational modifications of class C G protein-coupled receptors and how these modifications regulate expression and function is very limited. Herein, we show that the nutrient-sensing class C G protein-coupled receptor GPRC6A carries seven N-glycans and that one of these sites modulates surface expression whereas mutation of another site affects receptor function. GPRC6A has been speculated to form covalently linked dimers through cysteine disulfide linkage in the extracellular amino-terminal domain and here we show that GPRC6A indeed is a homodimer and that a disulfide bridge between the C131 residues is formed.

N-linked Glycosylation on the N-terminus of the dopamine D2 and D3 receptors determines receptor association with specific microdomains in the plasma membrane

Numerous G protein-coupled receptors (GPCRs) are glycosylated at extracellular regions. The regulatory roles of glycosylation on receptor function vary across receptor types. In this study, we used the dopamine D₂and D₃receptors as an experimental model to understand the underlying principles governing the functional roles of glycosylation. We used the pharmacological inhibitor, tunicamycin, to inhibit glycosylation, generated chimeric D₂and D₃receptors by swapping their respective N-termini, and produced the glycosylation site mutant D₂and D₃receptors to study the roles of glycosylation on receptor functions, including cell surface expression, signaling, and internalization through specific microdomains. Our results demonstrate that glycosylation on the N-terminus of the D₃ receptor is involved in the development of desensitization and proper cell surface expression. In addition, glycosylation on the N-terminus mediates the internalization of D₂and D₃receptors within the caveolae and clathrin-coated pit microdomains of the plasma membrane, respectively, by regulating receptor interactions with caveolin-1 and clathrin. In conclusion, this study shows for the first time that glycosylation on the N-terminus of GPCRs is involved in endocytic pathway selection through specific microdomains. These data suggest that changes in the cellular environment that influence posttranslational modification could be an important determinant of intracellular GPCR trafficking.

Identification of a high-mannose ICAM-1 glycoform: effects of ICAM-1 hypoglycosylation on monocyte adhesion and outside in signaling

Endothelial adhesion molecules are critical effectors of inflammation ensuring coordinated interactions that allow leukocytes to home to sites of injury. These adhesion molecules are often extensively modified posttranslationaly by the addition of N-glycans, but if, or how, these modifications contribute to the protein function remains poorly understood. Herein we show that activated endothelial cells express two distinct N-glycoforms of intercellular adhesion molecule 1 (ICAM-1) that comprise a complex N-glycoform with α-2,6 sialic acid present at relatively high levels and a second, less abundant and previously undescribed high-mannose glycoform (HM-ICAM-1). This novel HM-ICAM-1 glycoform was also detected in human coronary artery specimens and moreover appeared to be the dominant glycoform in vivo. Production of exclusively HM-ICAM-1 in cells by α-mannosidase inhibition increased monocyte rolling and adhesion compared with mature ICAM-1 consistent with high-mannose epitopes providing leukocyte ligands. Cross-linking of ICAM-1 transmits outside-in signals that affect endothelial permeability and survival. Interestingly, cell signaling (assessed using ERK, VE-cadherin, and Akt phosphorylation) was maintained after cross-linking of HM-ICAM-1 compared with mature ICAM-1; however, interactions with the actin cytoskeleton were lost with HM-ICAM-1. These findings suggest that specific ICAM-1 N-glycoforms modulate distinct aspects of the inflammatory response and identify HM-ICAM-1 as a new therapeutic target for controlling leukocyte trafficking and endothelial inflammation.

The N-terminal region of the dopamine D2 receptor, a rhodopsin-like GPCR, regulates correct integration into the plasma membrane and endocytic routes

BACKGROUND AND PURPOSE

Functional roles of the N-terminal region of rhodopsin-like GPCR family remain unclear. Using dopamine D(2) and D(3) receptors as a model system, we probed the roles of the N-terminal region in the signalling, intracellular trafficking of receptor proteins, and explored the critical factors that determine the functionality of the N-terminal region.

EXPERIMENTAL APPROACH

The N-terminal region of the D(2) receptor was gradually shortened or switched with that of the D(3) receptor or a non-specific sequence (FLAG), or potential N-terminal glycosylation sites were mutated. Effects of these manipulations on surface expression, internalization, post-endocytic behaviours and signalling were determined.

KEY RESULTS

Shortening the N-terminal region of the D(2) receptor enhanced receptor internalization and impaired surface expression and signalling; ligand binding, desensitization and down-regulation were not affected but their association with a particular microdomain, caveolae, was disrupted. Replacement of critical residues within the N-terminal region with the FLAG epitope failed to restore surface expression but partially restored the altered internalization and signalling. When the N-terminal regions were switched between D(2) and D(3) receptors, cell surface expression pattern of each receptor was switched. Mutations of potential N-terminal glycosylation sites inhibited surface expression but enhanced internalization of D(2) receptors.

CONCLUSIONS AND IMPLICATIONS

Shortening of N-terminus or mutation of glycosylation sites located within the N-terminus enhanced receptor internalization but impaired the surface expression of D(2) receptors. The N-terminal region of the D(2) receptor, in a sequence-specific manner, controls the receptor's conformation and integration into the plasma membrane, which determine its subcellular localization, intracellular trafficking and signalling properties.

Comparison of dynamics of extracellular accesses to the β(1) and β(2) adrenoceptors binding sites uncovers the potential of kinetic basis of antagonist selectivity

From the molecular mechanism of antagonist unbinding in the β(1) and β(2) adrenoceptors investigated by steered molecular dynamics, we attempt to provide further possibilities of ligand subtype and subspecies selectivity. We have simulated unbinding of β(1)-selective Esmolol and β(2)-selective ICI-118551 from both receptors to the extracellular environment and found distinct molecular features of unbinding. By calculating work profiles, we show different preference in antagonist unbinding pathways between the receptors, in particular, perpendicular to the membrane pathway is favourable in the β(1) adrenoceptor, whereas the lateral pathway involving helices 5, 6 and 7 is preferable in the β(2) adrenoceptor. The estimated free energy change of unbinding based on the preferable pathway correlates with the experimental ligand selectivity. We then show that the non-conserved K347 (6.58) appears to facilitate in guiding Esmolol to the extracellular surface via hydrogen bonds in the β(1) adrenoceptor. In contrast, hydrophobic and aromatic interactions dominate in driving ICI-118551 through the easiest pathway in the β(2) adrenoceptor. We show how our study can stimulate design of selective antagonists and discuss other possible molecular reasons of ligand selectivity, involving sequential binding of agonists and glycosylation of the receptor extracellular surface.

Regulation of GIP and GLP1 receptor cell surface expression by N-glycosylation and receptor heteromerization

In response to a meal, Glucose-dependent Insulinotropic Polypeptide (GIP) and Glucagon-like Peptide-1 (GLP-1) are released from gut endocrine cells into the circulation and interact with their cognate G-protein coupled receptors (GPCRs). Receptor activation results in tissue-selective pleiotropic responses that include augmentation of glucose-induced insulin secretion from pancreatic beta cells. N-glycosylation and receptor oligomerization are co-translational processes that are thought to regulate the exit of functional GPCRs from the ER and their maintenance at the plasma membrane. Despite the importance of these regulatory processes, their impact on functional expression of GIP and GLP-1 receptors has not been well studied. Like many family B GPCRs, both the GIP and GLP-1 receptors possess a large extracellular N-terminus with multiple consensus sites for Asn-linked (N)-glycosylation. Here, we show that each of these Asn residues is glycosylated when either human receptor is expressed in Chinese hamster ovary cells. N-glycosylation enhances cell surface expression and function in parallel but exerts stronger control over the GIP receptor than the GLP-1 receptor. N-glycosylation mainly lengthens receptor half-life by reducing degradation in the endoplasmic reticulum. N-glycosylation is also required for expression of the GIP receptor at the plasma membrane and efficient GIP potentiation of glucose-induced insulin secretion from the INS-1 pancreatic beta cell line. Functional expression of a GIP receptor mutant lacking N-glycosylation is rescued by co-expressed wild type GLP1 receptor, which, together with data obtained using Bioluminescence Resonance Energy Transfer, suggests formation of a GIP-GLP1 receptor heteromer.

GPCR oligomerization: contribution to receptor biogenesis

G protein-coupled receptor (GPCR) export to the plasma membrane is considered to follow the default secretory pathway. Several observations indicate that trafficking from the endoplasmic reticulum to the plasma membrane is strictly regulated and involves interactions with specific proteins, such as resident ER chaperones. These interactions help with GPCR folding, but more importantly, they ensure that only properly folded proteins proceed from the ER to the trans-golgi network. The assembly of several GPCRs into a quaternary structure is started in the ER, before cell surface delivery, and helps in the correct expression of the GPCRs. This review will mainly focus on the role of GPCR oligomerization in receptor biogenesis.

Pharmacological consequence of the A118G μ opioid receptor polymorphism on morphine- and fentanyl-mediated modulation of Ca²⁺ channels in humanized mouse sensory neurons

BACKGROUND

The most common functional single nucleotide polymorphism of the human OPRM1 gene, A118G, has been shown to be associated with interindividual differences in opioid analgesic requirements, particularly with morphine, in patients with acute postoperative pain. The purpose of this study was to examine whether this polymorphism would modulate the morphine and fentanyl pharmacological profile of sensory neurons isolated from a humanized mouse model homozygous for either the 118A or 118G allele.

METHODS

The coupling of wild-type and mutant μ opioid receptors to voltage-gated Ca channels after exposure to either ligand was examined by employing the whole cell variant of the patch-clamp technique in acutely dissociated trigeminal ganglion neurons. Morphine-mediated antinociception was measured in mice carrying either the 118AA or 118GG allele.

RESULTS

The biophysical parameters (cell size, current density, and peak current amplitude potential) measured from both groups of sensory neurons were not significantly different. In 118GG neurons, morphine was approximately fivefold less potent and 26% less efficacious than that observed in 118AA neurons. On the other hand, the potency and efficacy of fentanyl were similar for both groups of neurons. Morphine-mediated analgesia in 118GG mice was significantly reduced compared with the 118AA mice.

CONCLUSIONS

This study provides evidence to suggest that the diminished clinical effect observed with morphine in 118G carriers results from an alteration of the receptor's pharmacology in sensory neurons. In addition, the impaired analgesic response with morphine may explain why carriers of this receptor variant have an increased susceptibility to become addicted to opioids.

On the origin of the triplet puzzle of homologies in receptor heteromers: Toll-like receptor triplets in different types of receptors

Based on our theory, we have discovered main triplets of amino acid residues in the GABAB1 receptor and several other neural receptors, which seem to originate from toll-like receptors and appear also as homologies in receptor heteromers. The obtained results strengthen our hypothesis that these triplets may 'guide-and-clasp' receptor-receptor interactions.

GPR37 surface expression enhancement via N-terminal truncation or protein-protein interactions

GPR37, also known as the parkin-associated endothelin-like receptor (Pael-R), is an orphan G-protein-coupled receptor (GPCR) that exhibits poor plasma membrane expression when expressed in most cell types. We sought to find ways to enhance GPR37 trafficking to the cell surface to facilitate studies of GPR37 functional activity in heterologous cells. In truncation studies, we found that removing the GPR37 N-terminus (NT) dramatically enhanced the receptor's plasma membrane insertion. Further studies on sequential NT truncations revealed that removal of the first 210 amino acids increased the level of surface expression nearly as much as removal of the entire NT. In studies examining the effects of coexpression of GPR37 with a variety of other GPCRs, we observed significant increases in the level of GPR37 surface expression when the receptor was coexpressed with adenosine receptor A(2A)R or dopamine receptor D(2)R. Co-immunoprecipitation experiments revealed that full-length GPR37 and, to a greater extent, the truncated GPR37 were capable of robustly associating with D(2)R, resulting in modestly altered D(2)R affinity for both agonists and antagonists. In studies examining potential interactions of GPR37 with PDZ scaffolds, we observed a specific interaction between GPR37 and syntenin-1, which resulted in a dramatic increase in the level of GPR37 surface expression in HEK-293 cells. These findings reveal three independent approaches (N-terminal truncation, coexpression with other receptors, and coexpression with syntenin-1) by which GPR37 surface trafficking in heterologous cells can be greatly enhanced to facilitate functional studies with this orphan receptor.

Monitoring the human beta1, beta2, beta3 adrenergic receptors expression and purification in Pichia pastoris using the fluorescence properties of the enhanced green fluorescent protein

The three beta adrenergic receptor subtypes, beta1-, beta2- and beta3-, were expressed in the methylotrophic yeast Pichia pastoris. These receptors were N-terminally fused to the enhanced green fluorescent protein (EGFP) and the fluorescent properties of EGFP were used: (1) to select the recombinant strains, (2) to monitor the expression of the fluorescent receptors, and (3) to monitor the purification of the receptors by immobilized metal affinity chromatography. We demonstrate here that Pichia pastoris can be an alternative host to express and purify milligram amounts of human beta adrenergic receptors.

Role of key transmembrane residues in agonist and antagonist actions at the two conformations of the human beta1-adrenoceptor

Studies with 4-[3-[(1,1-dimethylethyl)amino]2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one hydrochloride (CGP 12177) at the human beta1-adrenoceptor have provided evidence for two binding modes or conformations that have markedly different pharmacological properties. Here, key transmembrane residues (Asp104, Asp138, Ser228, Ser229, Ser232, Phe341, Asn344 and Asn363) have been mutated to provide structural insights into the nature of these conformations. [(3)H]CGP 12177 binding and cAMP response element-mediated reporter gene studies confirmed that CGP 12177 was a neutral antagonist (log K(D) = -9.18) at the "catecholamine site" and an agonist at the "CGP 12177 site" (log EC(50) = -8.12). Agonist responses to isoprenaline and CGP 12177 had different sensitivities to beta1-antagonists (e.g., CGP 20712A; log K(D) = -8.65 and -7.26, respectively). Site-directed mutagenesis showed that Asn363 and Asp138 were key residues for binding of agonists and antagonists, and they were also essential for the agonist actions of CGP 12177. S228A and S229A in transmembrane-spanning region (TM) 5 reduced the binding of CGP 12177 and had an identical effect on its agonist and antagonist actions. Both N344A and F341A in TM6 abolished the ability of CGP 20712A to discriminate between responses elicited by isoprenaline and CGP 12177. The fact that both Asp138 and Asn363 are absolutely required for CGP 12117 binding in both agonist and antagonist modes leads to the conclusion that the secondary agonist binding site for CGP 12117 must overlap with the catecholamine binding site. Modeling studies provide a basis for these overlapping sites with either the tert-butylamino group or the hydroxyethyloxy and imidazolone portions of CGP 12177 capable of forming polar interactions with Asp138 and Asn363.

Modulation of Ca2+ Channels by Heterologously Expressed Wild-Type and Mutant Human μ-Opioid Receptors (hMORs) Containing the A118G Single-Nucleotide Polymorphism

The most common single-nucleotide polymorphism (SNP) of the human μ-opioid receptor (hMOR) gene occurs at position 118 (A118G) and results in substitution of asparagine to aspartate at the N-terminus. The purpose of the present study was to compare the pharmacological profile of several opioid agonists to heterologously expressed hMOR and N-type Ca2+ channels in sympathetic neurons. cDNA constructs coding for wild-type and mutant hMOR were microinjected in rat superior cervical ganglion neurons and N-type Ca2+ channel modulation was investigated using the whole cell variant of the patch-clamp technique. Concentration–response relationships were generated with the following selective MOR agonists: DAMGO, morphine, morphine-6-glucuronide (M-6-G), and endomorphin I. The estimated maximal inhibition for the agonists ranged from 52 to 64% for neurons expressing either hMOR subtype. The rank order of potencies for estimated EC50 values (nM) in cells expressing wild-type hMOR was: DAMGO (31) ≫ morphine (76) ≅ M-6-G (77) ≅ endomorphin I (86). On the other hand, the rank order in mutant-expressing neurons was: DAMGO (14) ≫ morphine (39) ≫ endomorphin I (74) ≅ M-6-G (82), with a twofold leftward shift for both DAMGO and morphine. The DAMGO-mediated Ca2+ current inhibition was abolished by the selective MOR blocker, CTAP, and by pertussis toxin pretreatment of neurons expressing either hMOR subtype. These results suggest that the A118G variant MOR exhibits an altered signal transduction pathway and may help explain the variability of responses to opiates observed with carriers of the mutant allele.

The functional significance of genetic variation within the beta-adrenoceptor

The beta-1 adrenoceptor is an archetypal G-coupled protein receptor that controls sympathetic responses in the heart, kidney and adipocytes. It has been widely exploited as a drug target with the development of antagonists to treat cardiovascular diseases such as hypertension, angina and heart failure. Signalling through the receptor is modulated by desensitization and beta1- adrenoceptor down-regulation. It is also affected by in vitro substitution of specific amino acid residues within the beta-1 adrenoceptor. Amino acid substitutions also occur naturally due to polymorphic variation within the human beta-1 adrenoceptor gene itself. Since these variants are common (typically being present in > 5% of the population), the pharmacogenetic implications are enormous. A number of these variants have been identified, although two have been the particular focus of recent publications: a serine to glycine substitution at position 49 (49S > G) and an arginine to glycine at position 389 (389R > G). The data on the in vitro behaviour of these two receptor variants is reviewed here, along with the evidence that they may affect both the risk of cardiovascular disease and the therapeutic response to beta-1 adrenoceptor antagonists.

Expression of beta adrenergic receptors in mouse oocytes and preimplantation embryos

Accumulating evidence indicates the role of endogenous catecholamines in mammalian embryogenesis. We searched public databases containing nucleotide sequences derived from mouse preimplantation cDNA libraries and found a partial sequence homology between a cDNA clone from mouse blastocysts and the mouse beta 2-adrenergic receptor sequence. No significant sequence homology was found for other mouse adrenergic and dopamine receptors. Using RT-PCR, we showed that beta 2-adrenoceptor is transcribed not only at blastocyst stage but also at earlier stages of preimplantation development as well as in oocytes. Moreover, we demonstrated that transcripts encoding both isoforms of the beta 3-adrenoceptor (beta 3a- and beta 3b-) are expressed in mouse oocytes and preimplantation embryos as well. We did not detect the beta 1-adrenoceptor transcript either in oocytes or in preimplantation embryos. Using an antibody against the mouse beta 2-adrenergic receptor, we showed that the receptor protein is expressed in oocytes and preimplantation embryos; in blastocysts, the immufluorescence labeling was stronger in the inner cell mass than in throphectodermal cells. The cell number of the in vitro cultured mouse preimplantation embryos exposed to isoproterenol (a potent beta adrenoceptor agonist) was lower than in control embryos, suggesting that activation of beta adrenergic receptors by appropriate agonist concentration can influence cell proliferation in mouse pre-implantation embryos. Thus, our results indicate that beta adrenergic receptors are expressed in mouse oocytes and preimplantation embryos and that ligands for the receptors can affect the mouse embryo even in the very early stages of development.

Hierarchical Regulation of CTLA-4 Dimer-Based Lattice Formation and Its Biological Relevance for T Cell Inactivation

CTLA-4 is an activation-induced, homodimeric inhibitory receptor in T cells. Recent crystallographic reports have suggested that it may form lattice-like arrays on the cell surface upon binding B7.1/B7.2 (CD80, CD86) molecules. To test the biological relevance of these CTLA-4-B7 lattices, we introduced a C122A point mutation in human CTLA-4, because this residue was shown to be essential for dimerization in solution. Surprisingly, we found that up to 35% of C122A CTLA-4 dimerized in human T lymphocytes. Moreover, C122A CTLA-4 partitioned within lipid rafts, colocalized with the TCR in the immunological synapse, and inhibited T cell activation. C122-independent dimerization of CTLA-4 involved N-glycosylation, because further mutation of the N78 and N110 glycosylation sites abrogated dimerization. Despite being monomeric, the N78A/N110A/C122A triple mutant CTLA-4 localized in the immunological synapse and inhibited T cell activation. Such functionality correlated with B7-induced dimerization of these mutant molecules. Based on these data, we propose a model of hierarchical regulation of CTLA-4 oligomerization by which B7 binding ultimately determines the formation of dimer-dependent CTLA-4 lattices that may be necessary for triggering B7-dependent T cell inactivation.

Glycosylation of Eag1 (Kv10.1) potassium channels: intracellular trafficking and functional consequences

N-Linked glycosylation is a common post-translational modification of membrane proteins. Here we report that mature Eag1 potassium channels carry sugar moieties linked to asparagines at positions 388 and 406. Asn-388 seems to undergo only core glycosylation, but complex sugars are bound to Asn-406. Correct complex glycosylation is required for proper trafficking of Eag1 to the plasma membrane but is also crucial for the correct function of channels already inserted in the membrane.

Endoplasmic reticulum stress and N-glycosylation modulate expression of WFS1 protein

Mutations of the WFS1 gene are responsible for two hereditary diseases, Wolfram syndrome and low frequency sensorineural hearing loss. The WFS1 protein is a glycoprotein located in the endoplasmic reticulum (ER) membrane but its function is poorly understood. Herein we show WFS1 mRNA and protein levels in pancreatic islets to be increased with ER-stress inducers, thapsigargin and dithiothreitol. Another ER-stress inducer, the N-glycosylation inhibitor tunicamycin, also raised WFS1 mRNA but not protein levels. Site-directed mutagenesis showed both Asn-663 and Asn-748 to be N-glycosylated in mouse WFS1 protein. The glycosylation-defective WFS1 protein, in which Asn-663 and Asn-748 had been substituted with aspartate, exhibited an increased protein turnover rate. Consistent with this, the WFS1 protein was more rapidly degraded in the presence of tunicamycin. These data indicate that ER-stress and N-glycosylation play important roles in WFS1 expression and stability, and also suggest regulatory roles for this protein in ER-stress induced cell death.

Beta-adrenergic receptors and their interacting proteins

The three subtypes of beta-adrenergic receptor (beta AR) all interact with G proteins as a central aspect of their signaling. The various beta AR subtypes also associate differentially with a variety of other cytoplasmic and transmembrane proteins. These beta AR-interacting proteins play distinct roles in the regulation of receptor signaling and trafficking. The specificity of beta AR associations with various binding partners can help to explain key physiological differences between beta AR subtypes. Moreover, the differential tissue expression patterns of many of the beta AR-interacting proteins may contribute to tissue-specific regulation of beta AR function.

Interaction with Cystic Fibrosis Transmembrane Conductance Regulator-associated Ligand (CAL) Inhibits β1-Adrenergic Receptor Surface Expression

G protein-coupled receptors such as the beta1-adrenergic receptor (beta1AR) must be trafficked to the plasma membrane in order to bind with their extracellular ligands and regulate cellular physiology. By using glutathione S-transferase pull-down techniques, we found that the beta1AR carboxyl terminus directly interacts with the cystic fibrosis transmembrane conductance regulator-associated ligand (CAL; also known as PIST, GOPC, and FIG), a protein known to be primarily localized to the Golgi apparatus. CAL contains two predicted coiled-coil domains and one PSD-95/Discs-large/ZO-1 homology (PDZ) domain. The beta1AR carboxyl terminus (CT) binds to the PDZ domain of CAL, with the last few amino acids (ESKV) of the beta1AR-CT being the key determinants for the interaction. Mutation of the terminal valine residue resulted in markedly reduced association of the beta1AR-CT with CAL. Numerous other mutations to the ESKV motif also impaired the beta1AR-CT/CAL interaction, suggesting that this motif is close to optimal for association with the CAL PDZ domain. In cells, full-length beta1AR robustly associates with CAL, and this interaction is abolished by mutation of the terminal valine to alanine of the receptor (V477A), as determined by co-immunoprecipitation experiments and immunofluorescence co-localization studies. Consistent with observations that CAL is a Golgi-associated protein, overexpression of CAL reduces surface expression of beta1AR. Interaction with CAL promotes retention of beta1AR within the cell, whereas PSD-95, another beta1AR-associated PDZ domain-containing protein, competitively blocks beta1AR association with CAL and promotes receptor trafficking to the cell surface. These data reveal that CAL, a novel beta1AR-binding partner, modulates beta1AR intracellular trafficking, thereby revealing a new mechanism of regulation for beta1AR anterograde trafficking through the endoplasmic reticulum-Golgi complex to the plasma membrane.

Deglycosylation of Fas receptor and chronic morphine treatment up-regulate high molecular mass Fas aggregates in the rat brain

This study was designed to immunodetect and characterize Fas receptor aggregates (oligomerization) in the brain and to assess its possible modulation in opiate addiction. High molecular mass, sodium dodecyl sulfate (SDS)- and beta-mercaptoethanol-resistant Fas aggregates (approximately 110/120 and approximately 203 kDa specific peptides) were immunodetected with a cytoplasmic domain-specific antibody in brain tissue (rat, mouse and human) and SH-SY5Y cells by Western blot analysis. Preincubation of rat cortical membranes with N-ethylmaleimide (NEM; 1 mM for 1 h at 37 degrees C) reduced the immunodensity of approximately 203 kDa Fas aggregates (51%) and increased that of 35 kDa native Fas (172%) and 51/48 kDa glycosylated Fas (47%), indicating that disulfide bonds are involved in Fas dimerization. Enzymatic N-deglycosylation of Fas receptor increased the content of Fas aggregates (approximately 110/120 kDa: five- to sixfold, and approximately 203 kDa: two- to threefold), suggesting that Fas glycosylation is involved in regulating receptor dimerization. Chronic (10-100 mg/kg for 5 days), but not acute (30 mg/kg for 2 h), treatment with morphine (a micro-opioid peptide receptor agonist) induced up-regulation of Fas aggregates in the brain (approximately 110/120 kDa: 39%, and approximately 203 kDa: 89%). The acute and/or chronic treatments with delta- and kappa-opioid peptide receptor agonists and with a sigma1-receptor agonist did not readily alter the content of Fas aggregates in the rat brain. The results indicate that Fas aggregates are natively expressed in the brain and that its density is regulated by the state of Fas glycosylation. These forms of Fas (receptor homodimerization) are functionally relevant because they were up-regulated in the brain of morphine-dependent rats.

Elevated intracellular calcium triggers recruitment of the receptor cross-talk accessory protein calcyon to the plasma membrane

Calcyon is called a "cross-talk accessory protein" because the mechanism by which it enables the typically Gs-linked D1 dopamine receptor to stimulate intracellular calcium release depends on a priming step involving heterologous Gq-linked G-protein-coupled receptor activation. The details of how priming facilitates the D1R calcium response have yet to be precisely elucidated. The present work shows that calcyon is constitutively localized both in vesicular and plasma membrane compartments within HEK293 cells. In addition, surface biotinylation and luminescence assays revealed that priming stimulates a 2-fold increase in the levels of calcyon expressed on the cell surface and that subsequent D1R activation produces further accumulation of the protein in the plasma membrane. The effects of priming and D1R agonists were blocked by nocodazole implicating microtubules in the delivery of calcyon-containing vesicles to the cell surface. Accumulation of calcyon in the plasma membrane correlated well with increased intracellular calcium levels as thapsigargin mimicked, and 2-aminoethoxydiphenylborane abrogated, the effects of priming. KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase II (CaMKII) also blocked the effects of priming and D1R agonists. Furthermore, expression of constitutively active forms of the kinase bypassed the requirement for priming indicating that CaMKII is a key effector in the Ca2+ and microtubule-dependent delivery of calcyon to the cell surface.

Experimental evidence for lack of homodimerization of the G protein-coupled human N-formyl peptide receptor

A large number of G protein-coupled receptors have been shown to form homodimers based on a number of different techniques such as receptor coimmunoprecipitation, cross-linking, and fluorescence resonance energy transfer. In addition, functional assays of cells coexpressing a mutant receptor with a wild-type receptor have shown receptor phenotypes that can best be explained through dimerization. We asked whether the human neutrophil N-formyl peptide receptor (FPR) forms dimers in Chinese hamster ovary cells by coexpressing wild-type FPR with one of two mutants: D71A, which is uncoupled from G protein, and N297A, which has a defect in receptor phosphorylation and endocytosis. Experiments measuring chemotaxis, ligand-induced release of intracellular calcium, and p42/44 mitogen-activated protein kinase activation did not show an inhibitory effect of the coexpressed FPR D71A mutant. Coexpressed wild-type receptor was efficiently internalized, but failed to correct the endocytosis defects of the D71A and the N297A mutants. To explore the possibility that the mutations themselves prevented dimerization, we examined the coimmunoprecipitation of differentially epitope-tagged FPR. Immunoprecipitation of hemagglutinin-tagged FPR failed to coimmunoprecipitate coexpressed c-myc-tagged FPR and vice versa. Together, these data suggest that, unlike many other G protein-coupled receptors, FPR does not form homodimers.

Developmental changes in beta2-adrenergic receptor signaling in ventricular myocytes: the role of Gi proteins and caveolae microdomains

Cardiomyocyte beta2-adrenergic receptors (beta-ARs) provide a source of inotropic support and influence the evolution of heart failure. Recent studies identify distinct mechanisms for beta2-AR actions in neonatal and adult rat cardiomyocytes. This study examines whether ontogenic changes in cardiac beta2-AR actions can be attributed to altered Gi expression or changes in the spatial organization of the beta2-AR complex in membrane subdomains (caveolae). We show that beta2-ARs increase cAMP, calcium, and contractile amplitude in a pertussis toxin (PTX)-insensitive manner in neonatal cardiomyocytes. This is not caused by lack of Gi; Galphai expression is higher in neonatal cardiomyocytes than in those of adult rats. beta2-ARs provide inotropic support without detectably increasing cAMP, in adult cardiomyocytes. This cannot be attributed to dual coupling of beta2-ARs to Gs and Gi, because beta2-ARs do not promote cAMP accumulation in PTX-pretreated adult cardiomyocytes. Spatial segregation of beta2-ARs, Galphas/Galphai, and adenylyl cyclase to distinct membrane subdomains also is not a factor, because all of these proteins copurify in caveolin-3-enriched vesicles isolated from adult cardiomyocytes. However, these studies demonstrate that enzyme-based protocols routinely used to isolate ventricular cardiomyocytes lead to proteolysis of beta-ARs. The functional consequences of this limited beta-AR proteolysis is uncertain, because truncated beta1-ARs promote cAMP accumulation and truncated beta2-ARs provide inotropic support in adult cardiomyocytes. Collectively, these studies indicate that components of the beta2-AR signaling complex compartmentalize to restricted membrane subdomains in adult rat cardiomyocytes. Neither compartmentalization nor changes in Gi expression fully explain the ontogenic changes in beta2-AR responsiveness in the rat ventricle.

Heterodimerization of alpha 2A- and beta 1-adrenergic receptors

beta- and alpha(2)-adrenergic receptors are known to exhibit substantial cross-talk and mutual regulation in tissues where they are expressed together. We have found that the beta(1)-adrenergic receptor (beta(1)AR) and alpha(2A)-adrenergic receptor (alpha(2A)AR) heterodimerize when coexpressed in cells. Immunoprecipitation studies with differentially tagged beta(1)AR and alpha(2A)AR expressed in HEK-293 cells revealed robust co-immunoprecipitation of the two receptors. Moreover, agonist stimulation of alpha(2A)AR was found to induce substantial internalization of coexpressed beta(1)AR, providing further evidence for a physical association between the two receptors in a cellular environment. Ligand binding assays examining displacement of [(3)H]dihydroalprenolol binding to the beta(1)AR by various ligands revealed that beta(1)AR pharmacological properties were significantly altered when the receptor was coexpressed with alpha(2A)AR. Finally, beta(1)AR/alpha(2A)AR heterodimerization was found to be markedly enhanced by a beta(1)AR point mutation (N15A) that blocks N-linked glycosylation of the beta(1)AR as well as by point mutations (N10A/N14A) that block N-linked glycosylation of the alpha(2A)AR. These data reveal an interaction between beta(1)AR and alpha(2A)AR that is regulated by glycosylation and that may play a key role in cross-talk and mutual regulation between these receptors.