Signaling, desensitization, and trafficking of G protein-coupled receptors revealed by green fluorescent protein conjugates

Cardiac voltage gated calcium channels and their regulation by β-adrenergic signaling

Voltage-gated calcium channels (VGCCs) are the predominant source of calcium influx in the heart leading to calcium-induced calcium release and ultimately excitation-contraction coupling. In the heart, VGCCs are modulated by the β-adrenergic signaling. Signaling through β-adrenergic receptors (βARs) and modulation of VGCCs by β-adrenergic signaling in the heart are critical signaling and changes to these have been significantly implicated in heart failure. However, data related to calcium channel dysfunction in heart failure is divergent and contradictory ranging from reduced function to no change in the calcium current. Many recent studies have highlighted the importance of functional and spatial microdomains in the heart and that may be the key to answer several puzzling questions. In this review, we have briefly discussed the types of VGCCs found in heart tissues, their structure, and significance in the normal and pathological condition of the heart. More importantly, we have reviewed the modulation of VGCCs by βARs in normal and pathological conditions incorporating functional and structural aspects. There are different types of βARs, each having their own significance in the functioning of the heart. Finally, we emphasize the importance of location of proteins as it relates to their function and modulation by co-signaling molecules. Its implication on the studies of heart failure is speculated.

β2 adrenergic receptor fluorescent protein fusions traffic to the plasma membrane and retain functionality

Green fluorescent protein (GFP) has proven useful for the study of protein interactions and dynamics for the last twenty years. A variety of new fluorescent proteins have been developed that expand the use of available excitation spectra. We have undertaken an analysis of seven of the most useful fluorescent proteins (XFPs), Cerulean (and mCerulean3), Teal, GFP, Venus, mCherry and TagRFP657, as fusions to the archetypal G-protein coupled receptor, the β₂ adrenergic receptor (β₂AR). We have characterized these β₂AR::XFP fusions in respect to membrane trafficking and G-protein activation. We noticed that in the mouse neural cell line, OP 6, that membrane bound β₂AR::XFP fusions robustly localized in the filopodia identical to gap::XFP fusions. All β₂AR::XFP fusions show responses indistinguishable from each other and the non-fused form after isoprenaline exposure. Our results provide a platform by which G-protein coupled receptors can be dissected for their functionality.

Structural and biophysical characterisation of G protein-coupled receptor ligand binding using resonance energy transfer and fluorescent labelling techniques

The interaction between ligands and the G protein-coupled receptors (GPCRs) to which they bind has long been the focus of intensive investigation. The signalling cascades triggered by receptor activation, due in most cases to ligand binding, are of great physiological and medical importance; indeed, GPCRs are targeted by in excess of 30% of small molecule therapeutic medicines. Attempts to identify further pharmacologically useful GPCR ligands, for receptors with known and unknown endogenous ligands, continue apace. In earlier days direct assessment of such interactions was restricted largely to the use of ligands incorporating radioactive isotope labels as this allowed detection of the ligand and monitoring its interaction with the GPCR. This use of such markers has continued with the development of ligands labelled with fluorophores and their application to the study of receptor-ligand interactions using both light microscopy and resonance energy transfer techniques, including homogenous time-resolved fluorescence resonance energy transfer. Details of ligand-receptor interactions via X-ray crystallography are advancing rapidly as methods suitable for routine production of substantial amounts and stabilised forms of GPCRs have been developed and there is hope that this may become as routine as the co-crystallisation of serine/threonine kinases with ligands, an approach that has facilitated widespread use of rapid structure-based ligand design. Conformational changes involved in the activation of GPCRs, widely predicted by biochemical and biophysical means, have inspired the development of intramolecular FRET-based sensor forms of GPCRs designed to investigate the events following ligand binding and resulting in a signal propagation across the cell membrane. Finally, a number of techniques are emerging in which ligand-GPCR binding can be studied in ways that, whilst indirect, are able to monitor its results in an unbiased and integrated manner. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

Fluorogen activating proteins in flow cytometry for the study of surface molecules and receptors

The use of fluorescent proteins, particularly when genetically fused to proteins of biological interest, have greatly advanced many flow cytometry research applications. However, there remains a major limitation to this methodology in that only total cellular fluorescence is measured. Commonly used fluorescent proteins (e.g., EGFP and its variants) are fluorescent whether the fusion protein exists on the surface or in sub-cellular compartments. A flow cytometer cannot distinguish between these separate sources of fluorescence. This can be of great concern when using flow cytometry, plate readers or microscopy to quantify cell surface receptors or other surface proteins genetically fused to fluorescent proteins. Recently developed fluorogen activating proteins (FAPs) solve many of these issues by allowing the selective visualization of only those cell surface proteins that are exposed to the extracellular milieu. FAPs are GFP-sized single chain antibodies that specifically bind to and generate fluorescence from otherwise non-fluorescent dyes ('activate the fluorogen'). Like the fluorescent proteins, FAPs can be genetically fused to proteins of interest. When exogenously added fluorogens bind FAPs, fluorescence immediately increases by as much as 20,000-fold, rendering the FAP fusion proteins highly fluorescent. Moreover, since fluorogens can be made membrane impermeant, fluorescence can be limited to only those receptors expressed on the cell surface. Using cells expressing beta-2 adrenergic receptor (β2AR) fused at its N-terminus to a FAP, flow cytometry based receptor internalization assays have been developed and characterized. The fluorogen/FAP system is ideally suited to the study of cell surface proteins by fluorescence and avoids drawbacks of using receptor/fluorescent protein fusions, such as internal accumulation. We also briefly comment on extending FAP-based technologies to the study of events occurring inside of the cell as well.

Detection and quantification of beta2AR internalization in living cells using FAP-based biosensor technology

Ligand-dependent receptor internalization is a feature of numerous signaling systems. In this article, the authors describe a new kind of live-cell biosensor of receptor internalization that takes advantage of fluorogen-activating protein (FAP) technology. Recombinant genes that express the human beta2 adrenergic receptor (beta2AR) with FAP domains at their extracellular N-termini were transduced into mammalian cells. Exposure of the cells to membrane-impermeant fluorogens led to a strong fluorescent signal from the cell surface. Agonist-dependent translocation of the receptor from the surface to the cell interior was readily observed and quantified by fluorescence microscopy or flow cytometry in a homogeneous format without wash or separation steps. The approach described here is generalizable to other receptors and cell surface proteins and is adaptable to a variety of fluorescence-based high-throughput screening platforms.

Regulating the total level of a signaling protein can vary its dynamics in a range from switch like ultrasensitivity to adaptive responses

Biological signaling networks can exhibit rich response dynamics including ultrasensitivity, adaptation to persistent stimuli and oscillations. Previous modeling efforts have considered the proteins in these networks as two-state entities and their total levels as fixed quantities. However, inside the cell, most molecules are in constant flux because of various processes such as degradation, synthesis, binding of scaffold proteins and release from vesicles. The resulting freedom in the amount of signaling protein that is available for signaling has not been explored. Here, we analyze the response dynamics of a signaling protein when it enters the signaling pool in one state (modified or unmodified) and exits in both states. When the exit rates of these two states are comparable, a persistent stimulus results in step responses and can produce ultrasensitivity, as shown previously. However, we find that when the exit rates are imbalanced, the signaling protein gives transient responses to persistent stimuli even though the system does not have any explicit feedback. Further, these rates determine the signal range over which the system is responsive. Building small networks from signaling proteins with different exit rates, we show that these systems can exhibit rich behavior. Taken together, these findings indicate that altering the total level of signaling proteins can significantly change their response and provide additional richness in system dynamics. We discuss relevant biological examples in which regulating total protein levels could be exploited to alter signaling behavior.

C5L2 is a functional receptor for acylation-stimulating protein

C5L2 binds acylation-stimulating protein (ASP) with high affinity and is expressed in ASP-responsive cells. Functionality of C5L2 has not yet been demonstrated. Here we show that C5L2 is expressed in human subcutaneous and omental adipose tissue in both preadipocytes and adipocytes. In mice, C5L2 is expressed in all adipose tissues, at levels comparable with other tissues. Stable transfection of human C5L2 cDNA into HEK293 cells results in ASP stimulation of triglyceride synthesis (TGS) (193 +/- 33%, 5 microM ASP, p < 0.001, where basal = 100%) and glucose transport (168 +/- 21%, 10 microM ASP, p < 0.001). C3a similarly stimulates TGS (163 +/- 12%, p < 0.001), but C5a and C5a des-Arg have no effect. The ASP mechanism is to increase Vmax of glucose transport (149%) and triglyceride (TG) synthesis activity (165%) through increased diacylglycerolacyltransferase activity (200%). Antisense oligonucleotide down-regulation of C5L2 in human skin fibroblasts decreases cell surface C5L2 (down to 54 +/- 4% of control, p < 0.001, comparable with nonimmune background). ASP response is coordinately lost (basal TGS = 14.6 +/- 1.6, with ASP = 21.0 +/- 1.4 (144%), with ASP + oligonucleotides = 11.0 +/- 0.8 pmol of TG/mg of cell protein, p < 0.001). In mouse 3T3-L1 preadipocytes, antisense oligonucleotides decrease C5L2 expression to 69.5 +/- 0.5% of control, p < 0.001 (comparable with nonimmune) with a loss of ASP stimulation (basal TGS = 22.4 +/- 2.9, with ASP = 39.6 +/- 8.8 (177%), with ASP + oligonucleotides = 25.3 +/- 3.0 pmol of TG/mg of cell protein, p < 0.001). C5L2 down-regulation and decreased ASP response correlate (r = 0.761, p < 0.0001 for HSF and r = 0.451, p < 0.05 for 3T3-L1). In HEK-hC5L2 expressing fluorescently tagged beta-arrestin, ASP induced beta-arrestin translocation to the plasma membrane and formation of endocytic complexes concurrently with increased phosphorylation of C5L2. This is the first demonstration that C5L2 is a functional receptor, mediating ASP triglyceride stimulation.

Applications of bioluminescence- and fluorescence resonance energy transfer to drug discovery at G protein-coupled receptors

Bioluminescence (BRET)- and fluorescence resonance energy transfer (FRET) techniques have become integral approaches in studies of protein-protein interactions in living cells. They rely on non-radiative transfer of energy between donor and acceptor species that can be appended to the proteins of interest. These techniques display exquisite dependence on distance and orientation between the energy transfer partners. This means they are well suited to measure both small conformational changes in response to ligand binding between partner proteins that remain within a complex or more extensive translocations of proteins between cellular compartments that occur in response to cellular challenge. Introduction of both energy donor and acceptor into a single polypeptide can also allow the detection of ligand-induced conformational switches in monomeric proteins in the millisecond time scale. Many of these approaches are amenable to high throughput screening and the drug discovery process. G protein-coupled receptors (GPCRs) represent a key drug target class. Specific applications of resonance energy transfer techniques to the identification of ligands for this class of protein are highlighted to illustrate general principles.

The role of phosphorylation in D1 dopamine receptor desensitization: evidence for a novel mechanism of arrestin association

Homologous desensitization of D(1) dopamine receptors is thought to occur through their phosphorylation leading to arrestin association which interdicts G protein coupling. In order to identify the relevant domains of receptor phosphorylation, and to determine how this leads to arrestin association, we created a series of mutated D(1) receptor constructs. In one mutant, all of the serine/threonine residues within the 3rd cytoplasmic domain were altered (3rdTOT). A second construct was created in which only three of these serines (serines 256, 258, and 259) were mutated (3rd234). We also created four truncation mutants of the carboxyl terminus (T347, T369, T394, and T404). All of these constructs were comparable with the wild-type receptor with respect to expression and adenylyl cyclase activation. In contrast, both of the 3rd loop mutants exhibited attenuated agonist-induced receptor phosphorylation that was correlated with an impaired desensitization response. Sequential truncation of the carboxyl terminus of the receptor resulted in a sequential loss of agonist-induced phosphorylation. No phosphorylation was observed with the most severely truncated T347 mutant. Surprisingly, all of the truncated receptors exhibited normal desensitization. The ability of the receptor constructs to promote arrestin association was evaluated using arrestin-green fluorescent protein translocation assays and confocal fluorescence microscopy. The 3rd234 mutant receptor was impaired in its ability to induce arrrestin translocation, whereas the T347 mutant was comparable with wild type. Our data suggest a model in which arrestin directly associates with the activated 3rd cytoplasmic domain in an agonist-dependent fashion; however, under basal conditions, this is sterically prevented by the carboxyl terminus of the receptor. Receptor activation promotes the sequential phosphorylation of residues, first within the carboxyl terminus and then the 3rd cytoplasmic loop, thereby dissociating these domains and allowing arrestin to bind to the activated 3rd loop. Thus, the role of receptor phosphorylation is to allow access of arrestin to its receptor binding domain rather than to create an arrestin binding site per se.

The rat gonadotropin-releasing hormone receptor internalizes via a beta-arrestin-independent, but dynamin-dependent, pathway: addition of a carboxyl-terminal tail confers beta-arrestin dependency

This study examined the mechanism underlying the rat GnRH receptor (GnRH-R) internalization pathway by investigating the role of added/extended C-terminal tails and the effect of beta-arrestins and dynamin. The internalization of the wild-type (WT) rat GnRH-R, stop codon mutants, GnRH-R/TRH receptor (TRH-R) chimera, rat TRH-R, and catfish GnRH-R was examined using radioligand binding assay. Overexpression of beta-arrestin in COS-7 cells expressing each of the receptor constructs substantially increased endocytosis rate constants (k(e)) of the TRH-R, catfish GnRH-R, and GnRH-R/TRH-R chimera, but not of the WT rat GnRH-R and stop codon mutants. The beta-arrestin-promoted increase in the k(e) value was diminished by cotransfecting cells with the dominant negative beta-arrestin-(319-418) mutant, whereas WT GnRH-R and stop codon mutant internalization were unaffected. Additionally, confocal microscopy showed that activated GnRH-Rs failed to induce time-dependent redistribution of either beta-arrestin-1- or beta-arrestin-2-green fluorescent protein conjugate to the plasma membrane. However, the dominant negative dynamin (DynK44A) mutant impaired internalization of all of the receptors regardless of their beta-arrestin dependency, indicating that they internalize via a clathrin-mediated pathway. We conclude that the mammalian GnRH-R uses a beta-arrestin-independent, dynamin-dependent internalization mechanism distinct from that employed by the other receptors studied.