Ligand-induced endocytosis and nuclear localization of angiotensin II receptors expressed in CHO cells

A construct (AT1R-NF) containing a "Flag" sequence added to the N-terminus of the rat AT1 receptor was stably expressed in Chinese hamster ovary cells and quantified in the cell membrane by confocal microscopy after reaction with a fluorescein-labeled anti-Flag monoclonal antibody. Angiotensin II bound to AT1R-NF and induced endocytosis with a half-time of 2 min. After 60-90 min, fluorescence accumulated around the cell nucleus, suggesting migration of the ligand-receptor complex to the nuclear membrane. Angiotensin antagonists also induced endocytosis, suggesting that a common step in the transduction signal mechanism occurring after ligand binding may be responsible for the ligand-receptor complex internalization.

Constitutive interaction of the P2Y2 receptor with the hematopoietic cell-specific G protein G(alpha16) and evidence for receptor oligomers

Hematopoietic cells uniquely express G(alpha16), a G protein alpha-subunit of the G(q)-type. G(alpha16) is obligatory for P2Y2 receptor-dependent Ca2+-mobilization in human erythroleukemia cells and induces hematopoietic cell differentiation. We tested whether P2Y2 receptors physically interact with G(alpha16). Receptor and G protein were fused to cyan (CFP) and yellow (YFP) variants of the green fluorescent protein (GFP), respectively. When expressed in K562 leukemia cells, the fusion proteins were capable of triggering a Ca2+-signal upon receptor stimulation, demonstrating their functional integrity. In fluorescence resonance energy transfer (FRET) measurements using confocal microscopy, a strong FRET signal from the plasma membrane region of fixed, resting cells was detected when the receptor was co-expressed with the G protein as the FRET acceptor, as well as when the CFP-tagged receptor was co-expressed with receptor fused to YFP. We conclude that, under resting conditions, G(alpha16) and P2Y2 receptors form constitutive complexes, and that the P2Y2 receptor is present as an oligomer.

Direct Visualization of the Human Estrogen Receptor α Reveals a Role for Ligand in the Nuclear Distribution of the Receptor

The human estrogen receptor α (ER α) has been tagged at its amino terminus with the S65T variant of the green fluorescent protein (GFP), allowing subcellular trafficking and localization to be observed in living cells by fluorescence microscopy. The tagged receptor, GFP-ER, is functional as a ligand-dependent transcription factor, responds to both agonist and antagonist ligands, and can associate with the nuclear matrix. Its cellular localization was analyzed in four human breast cancer epithelial cell lines, two ER+ (MCF7 and T47D) and two ER− (MDA-MB-231 and MDA-MB-435A), under a variety of ligand conditions. In all cell lines, GFP-ER is observed only in the nucleus in the absence of ligand. Upon the addition of agonist or antagonist ligand, a dramatic redistribution of GFP-ER from a reticular to punctate pattern occurs within the nucleus. In addition, the full antagonist ICI 182780 alters the nucleocytoplasmic compartmentalization of the receptor and causes partial accumulation in the cytoplasm in a process requiring continued protein synthesis. GFP-ER localization varies between cells, despite being cultured and treated in a similar manner. Analysis of the nuclear fluorescence intensity for variation in its frequency distribution helped establish localization patterns characteristic of cell line and ligand. During the course of this study, localization of GFP-ER to the nucleolar region is observed for ER− but not ER+ human breast cancer epithelial cell lines. Finally, our work provides a visual description of the “unoccupied” and ligand-bound receptor and is discussed in the context of the role of ligand in modulating receptor activity.

Dynamics of a Chemoattractant Receptor in Living Neutrophils during ChemotaxisV⃞

Persistent directional movement of neutrophils in shallow chemotactic gradients raises the possibility that cells can increase their sensitivity to the chemotactic signal at the front, relative to the back. Redistribution of chemoattractant receptors to the anterior pole of a polarized neutrophil could impose asymmetric sensitivity by increasing the relative strength of detected signals at the cell’s leading edge. Previous experiments have produced contradictory observations with respect to receptor location in moving neutrophils. To visualize a chemoattractant receptor directly during chemotaxis, we expressed a green fluorescent protein (GFP)-tagged receptor for a complement component, C5a, in a leukemia cell line, PLB-985. Differentiated PLB-985 cells, like neutrophils, adhere, spread, and polarize in response to a uniform concentration of chemoattractant, and orient and crawl toward a micropipette containing chemoattractant. Recorded in living cells, fluorescence of the tagged receptor, C5aR–GFP, shows no apparent increase anywhere on the plasma membrane of polarized and moving cells, even at the leading edge. During chemotaxis, however, some cells do exhibit increased amounts of highly folded plasma membrane at the leading edge, as detected by a fluorescent probe for membrane lipids; this is accompanied by an apparent increase of C5aR–GFP fluorescence, which is directly proportional to the accumulation of plasma membrane. Thus neutrophils do not actively concentrate chemoattractant receptors at the leading edge during chemotaxis, although asymmetrical distribution of membrane may enrich receptor number, relative to adjacent cytoplasmic volume, at the anterior pole of some polarized cells. This enrichment could help to maintain persistent migration in a shallow gradient of chemoattractant.

Impaired beta-adrenergic response and decreased L-type calcium current of hypertrophied left ventricular myocytes in postinfarction heart failure

Infarct-induced heart failure is usually associated with cardiac hypertrophy and decreased ß-adrenergic responsiveness. However, conflicting results have been reported concerning the density of L-type calcium current (I Ca(L)), and the mechanisms underlying the decreased ß-adrenergic inotropic response. We determined I Ca(L) density, cytoplasmic calcium ([Ca2+]i) transients, and the effects of ß-adrenergic stimulation (isoproterenol) in a model of postinfarction heart failure in rats. Left ventricular myocytes were obtained by enzymatic digestion 8-10 weeks after infarction. Electrophysiological recordings were obtained using the patch-clamp technique. [Ca2+]i transients were investigated via fura-2 fluorescence. ß-Adrenergic receptor density was determined by [³H]-dihydroalprenolol binding to left ventricle homogenates. Postinfarction myocytes showed a significant 25% reduction in mean I Ca(L) density (5.7 ± 0.28 vs 7.6 ± 0.32 pA/pF) and a 19% reduction in mean peak [Ca2+]i transients (0.13 ± 0.007 vs 0.16 ± 0.009) compared to sham myocytes. The isoproterenol-stimulated increase in I Ca(L) was significantly smaller in postinfarction myocytes (Emax: 63.6 ± 4.3 vs 123.3 ± 0.9% in sham myocytes), but EC50 was not altered. The isoproterenol-stimulated peak amplitude of [Ca2+]i transients was also blunted in postinfarction myocytes. Adenylate cyclase activation through forskolin produced similar I Ca(L) increases in both groups. ß-Adrenergic receptor density was significantly reduced in homogenates from infarcted hearts (Bmax: 93.89 ± 20.22 vs 271.5 ± 31.43 fmol/mg protein in sham myocytes), while Kd values were similar. We conclude that postinfarction myocytes from large infarcts display reduced I Ca(L) density and peak [Ca2+]i transients. The response to ß-adrenergic stimulation was also reduced and was probably related to ß-adrenergic receptor down-regulation and not to changes in adenylate cyclase activity.

A time-resolved fluorescence resonance energy transfer assay suitable for high-throughput screening for inhibitors of immunoglobulin E-receptor interactions

The interaction of immunoglobulin E (IgE) antibodies with the high-affinity receptor, FcεRI, plays a central role in initiating most allergic reactions. The IgE-receptor interaction has been targeted for treatment of allergic diseases, and many high-affinity macromolecular inhibitors have been identified. Small molecule inhibitors would offer significant advantages over current anti-IgE treatment, but no candidate compounds have been identified and fully validated. Here, we report the development of a time-resolved fluorescence resonance energy transfer (TR-FRET) assay for monitoring the IgE-receptor interaction. The TR-FRET assay measures an increase in fluorescence intensity as a donor lanthanide fluorophore is recruited into complexes of site-specific Alexa Fluor 488-labeled IgE-Fc and His-tagged FcεRIα proteins. The assay can readily monitor classic competitive inhibitors that bind either IgE-Fc or FcεRIα in equilibrium competition binding experiments. Furthermore, the TR-FRET assay can also be used to follow the kinetics of IgE-Fc-FcεRIα dissociation and identify inhibitory ligands that accelerate the dissociation of preformed complexes, as demonstrated for an engineered DARPin (designed ankyrin repeat protein) inhibitor. The TR-FRET assay is suitable for high-throughput screening (HTS), as shown by performing a pilot screen of the National Institutes of Health (NIH) Clinical Collection Library in a 384-well plate format.

Development of an affinity pair “Tag-Receptor” for recombinant protein expression and purification

Dissertação apresentada para a obtenção do Grau de Mestre em Biotecnologia, pela Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia

Oligomerization of the alpha 1a- and alpha 1b-adrenergic receptor subtypes. Potential implications in receptor internalization.

We combined biophysical, biochemical, and pharmacological approaches to investigate the ability of the alpha 1a- and alpha 1b-adrenergic receptor (AR) subtypes to form homo- and hetero-oligomers. Receptors tagged with different epitopes (hemagglutinin and Myc) or fluorescent proteins (cyan and green fluorescent proteins) were transiently expressed in HEK-293 cells either individually or in different combinations. Fluorescence resonance energy transfer measurements provided evidence that both the alpha 1a- and alpha 1b-AR can form homo-oligomers with similar transfer efficiency of approximately 0.10. Hetero-oligomers could also be observed between the alpha 1b- and the alpha 1a-AR subtypes but not between the alpha 1b-AR and the beta2-AR, the NK1 tachykinin, or the CCR5 chemokine receptors. Oligomerization of the alpha 1b-AR did not require the integrity of its C-tail, of two glycophorin motifs, or of the N-linked glycosylation sites at its N terminus. In contrast, helix I and, to a lesser extent, helix VII were found to play a role in the alpha 1b-AR homo-oligomerization. Receptor oligomerization was not influenced by the agonist epinephrine or by the inverse agonist prazosin. A constitutively active (A293E) as well as a signaling-deficient (R143E) mutant displayed oligomerization features similar to those of the wild type alpha 1b-AR. Confocal imaging revealed that oligomerization of the alpha1-AR subtypes correlated with their ability to co-internalize upon exposure to the agonist. The alpha 1a-selective agonist oxymetazoline induced the co-internalization of the alpha 1a- and alpha 1b-AR, whereas the alpha 1b-AR could not co-internalize with the NK1 tachykinin or CCR5 chemokine receptors. Oligomerization might therefore represent an additional mechanism regulating the physiological responses mediated by the alpha 1a- and alpha 1b-AR subtypes.

Fluorescence imaging reveals the nuclear behavior of peroxisome proliferator-activated receptor/retinoid X receptor heterodimers in the absence and presence of ligand.

In a global approach combining fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), and fluorescence resonance energy transfer (FRET), we address the behavior in living cells of the peroxisome proliferator-activated receptors (PPARs), a family of nuclear receptors involved in lipid and glucose metabolism, inflammation control, and wound healing. We first demonstrate that unlike several other nuclear receptors, PPARs do not form speckles upon ligand activation. The subnuclear structures that may be observed under some experimental conditions result from overexpression of the protein and our immunolabeling experiments suggest that these structures are subjected to degradation by the proteasome. Interestingly and in contrast to a general assumption, PPARs readily heterodimerize with retinoid X receptor (RXR) in the absence of ligand in living cells. PPAR diffusion coefficients indicate that all the receptors are engaged in complexes of very high molecular masses and/or interact with relatively immobile nuclear components. PPARs are not immobilized by ligand binding. However, they exhibit a ligand-induced reduction of mobility, probably due to enhanced interactions with cofactors and/or chromatin. Our study draws attention to the limitations and pitfalls of fluorescent chimera imaging and demonstrates the usefulness of the combination of FCS, FRAP, and FRET to assess the behavior of nuclear receptors and their mode of action in living cells.

Étude de l'oligomérisation et de la fonction de canaux ioniques par spectroscopie de fluorescence et fluorométrie en voltage imposé

La fonction des canaux ioniques est finement régulée par des changements structuraux de sites clés contrôlant l’ouverture du pore. Ces modulations structurales découlent de l’interaction du canal avec l’environnement local, puisque certains domaines peuvent être suffisamment sensibles à des propriétés physico-chimiques spécifiques. Les mouvements engendrés dans la structure sont notamment perceptibles fonctionnellement lorsque le canal ouvre un passage à certains ions, générant ainsi un courant ionique mesurable selon le potentiel électrochimique. Une description détaillée de ces relations structure-fonction est cependant difficile à obtenir à partir de mesures sur des ensembles de canaux identiques, puisque les fluctuations et les distributions de différentes propriétés individuelles demeurent cachées dans une moyenne. Pour distinguer ces propriétés, des mesures à l’échelle de la molécule unique sont nécessaires. Le but principal de la présente thèse est d’étudier la structure et les mécanismes moléculaires de canaux ioniques par mesures de spectroscopie de fluorescence à l’échelle de la molécule unique. Les études sont particulièrement dirigées vers le développement de nouvelles méthodes ou leur amélioration. Une classe de toxine formeuse de pores a servi de premier modèle d’étude. La fluorescence à l’échelle de la molécule unique a aussi été utilisée pour l’étude d’un récepteur glutamate, d’un récepteur à la glycine et d’un canal potassique procaryote. Le premier volet porte sur l’étude de la stœchiométrie par mesures de photoblanchiment en temps résolu. Cette méthode permet de déterminer directement le nombre de monomères fluorescents dans un complexe isolé par le décompte des sauts discrets de fluorescence suivant les événements de photoblanchiment. Nous présentons ici la première description, à notre connaissance, de l’assemblage dynamique d’une protéine membranaire dans un environnement lipidique. La toxine monomérique purifiée Cry1Aa s’assemble à d’autres monomères selon la concentration et sature en conformation tétramérique. Un programme automatique est ensuite développé pour déterminer la stœchiométrie de protéines membranaires fusionnées à GFP et exprimées à la surface de cellules mammifères. Bien que ce système d’expression soit approprié pour l’étude de protéines d’origine mammifère, le bruit de fluorescence y est particulièrement important et augmente significativement le risque d’erreur dans le décompte manuel des monomères fluorescents. La méthode présentée permet une analyse rapide et automatique basée sur des critères fixes. L’algorithme chargé d’effectuer le décompte des monomères fluorescents a été optimisé à partir de simulations et ajuste ses paramètres de détection automatiquement selon la trace de fluorescence. La composition de deux canaux ioniques a été vérifiée avec succès par ce programme. Finalement, la fluorescence à l’échelle de la molécule unique est mesurée conjointement au courant ionique de canaux potassiques KcsA avec un système de fluorométrie en voltage imposé. Ces enregistrements combinés permettent de décrire la fonction de canaux ioniques simultanément à leur position et densité alors qu’ils diffusent dans une membrane lipidique dont la composition est choisie. Nous avons observé le regroupement de canaux KcsA pour différentes compositions lipidiques. Ce regroupement ne paraît pas être causé par des interactions protéine-protéine, mais plutôt par des microdomaines induits par la forme des canaux reconstitués dans la membrane. Il semble que des canaux regroupés puissent ensuite devenir couplés, se traduisant en ouvertures et fermetures simultanées où les niveaux de conductance sont un multiple de la conductance « normale » d’un canal isolé. De plus, contrairement à ce qui est actuellement suggéré, KcsA ne requiert pas de phospholipide chargé négativement pour sa fonction. Plusieurs mesures indiquent plutôt que des lipides de forme conique dans la phase cristalline liquide sont suffisants pour permettre l’ouverture de canaux KcsA isolés. Des canaux regroupés peuvent quant à eux surmonter la barrière d’énergie pour s’ouvrir de manière coopérative dans des lipides non chargés de forme cylindrique.

Constitutive oligomerization of human D-2 dopamine receptors expressed in Spodoptera frugiperda 9 (Sf9) and in HEK293 cells - Analysis using co-immunoprecipitation and time-resolved fluorescence resonance energy transfer

Human D-2Long (D-2L) and D-2Short (D-2S) dopamine receptor isoforms were modified at their N-terminus by the addition of a human immunodeficiency virus (HIV) or a FLAG epitope tag. The receptors were then expressed in Spodoptera frugiperda 9 (Sf9) cells using the baculovirus system, and their oligomerization was investigated by means of co-immunoprecipitation and time-resolved fluorescence resonance energy transfer (FRET). [H-3] Spiperone labelled D-2 receptors in membranes prepared from Sf9 cells expressing epitope-tagged D-2L or D-2S receptors, with a pK(d) value of approximate to 10. Co-immunoprecipitation using antibodies specific for the tags showed constitutive homo-oligomerization of D-2L and D-2S receptors in Sf9 cells. When the FLAG-tagged D-2S and HIV-tagged D-2L receptors were co-expressed, co-immunoprecipitation showed that the two isoforms can also form hetero-oligomers in Sf9 cells. Time-resolved FRET with europium and XL665-labelled antibodies was applied to whole Sf9 cells and to membranes from Sf9 cells expressing epitope-tagged D-2 receptors. In both cases, constitutive homo-oligomers were revealed for D-2L and D-2S isoforms. Time-resolved FRET also revealed constitutive homo-oligomers in HEK293 cells expressing FLAG-tagged D-2S receptors. The D-2 receptor ligands dopamine, R-(-) propylnorapomorphine, and raclopride did not affect oligomerization of D-2L and D-2S in Sf9 and HEK293 cells. Human D-2 dopamine receptors can therefore form constitutive oligomers in Sf9 cells and in HEK293 cells that can be detected by different approaches, and D-2 oligomerization in these cells is not regulated by ligands.

The cell surface receptor FGFRL1 forms constitutive dimers that promote cell adhesion

FGFRL1 is a novel member of the fibroblast growth factor (FGF) receptor family. Utilizing the FRET (fluorescence resonance energy transfer) technique, we demonstrate that FGFRL1 forms constitutive homodimers at cell surfaces. The formation of homodimers was verified by co-precipitation of differentially tagged FGFRL1 polypeptides from solution. If overexpressed in cultivated cells, FGFRL1 was found to be enriched at cell-cell contact sites. The extracellular domain of recombinant FGFRL1 promoted cell adhesion, but not cell spreading, when coated on plastic surfaces. Adhesion was mediated by heparan sulfate glycosaminoglycans located at the cell surface. It could specifically be blocked by addition of soluble heparin but not by addition of other glycosaminoglycans. When the amino acid sequence of the putative heparin-binding site was modified by in vitro mutagenesis, the resulting protein exhibited decreased affinity for heparin and reduced activity in the cell-binding assay. Moreover, a synthetic peptide corresponding to the heparin-binding site was able to neutralize the effect of heparin. With its dimeric structure and its adhesion promoting properties, FGFRL1 resembles the nectins, a family of cell adhesion molecules found at cell-cell junctions.

Preassembly of interleukin 2 (IL-2) receptor subunits on resting Kit 225 K6 T cells and their modulation by IL-2, IL-7, and IL-15: A fluorescence resonance energy transfer study

Assembly and mutual proximities of α, β, and γc subunits of the interleukin 2 receptors (IL-2R) in plasma membranes of Kit 225 K6 T lymphoma cells were investigated by fluorescence resonance energy transfer (FRET) using fluorescein isothiocyanate- and Cy3-conjugated monoclonal antibodies (mAbs) that were directed against the IL-2Rα, IL-2Rβ, and γc subunits of IL-2R. The cell-surface distribution of subunits was analyzed at the nanometer scale (2–10 nm) by FRET on a cell-by-cell basis. The cells were probed in resting phase and after coculture with saturating concentrations of IL-2, IL-7, and IL-15. FRET data from donor- and acceptor-labeled IL-2Rβ-α, γ-α, and γ-β pairs demonstrated close proximity of all subunits to each other in the plasma membrane of resting T cells. These mutual proximities do not appear to represent mAb-induced microaggregation, because FRET measurements with Fab fragments of the mAbs gave similar results. The relative proximities were meaningfully modulated by binding of IL-2, IL-7, and IL-15. Based on FRET analysis the topology of the three subunits at the surface of resting cells can be best described by a “triangular model” in the absence of added interleukins. IL-2 strengthens the bridges between the subunits, making the triangle more compact. IL-7 and IL-15 act in the opposite direction by opening the triangle possibly because they associate their private specific α receptors with the β and/or γc subunits of the IL-2R complex. These data suggest that IL-2R subunits are already colocalized in resting T cells and do not require cytokine-induced redistribution. This colocalization is significantly modulated by binding of relevant interleukins in a cytokine-specific manner.