Photobleaching fluorescence resonance energy transfer reveals ligand-induced oligomer formation of human somatostatin receptor subtypes

Differential Interaction of Metal Ions with Gold Nanoclusters and Application in Detection of Cobalt and Cadmium

Interest in biosensing platforms using protein fluorescent gold nanoclusters (FGNCs) has grown significantly in the past due to the unique optical properties they offer. This study investigates the interaction of metal ions with FGNCs, and the structural modifications brought about by the interaction resulting in fluorescence changes of the cluster and its successful application in the detection of two heavy metals, cobalt and cadmium. The binding of cobalt and cadmium to FGNCs synthesized from BSA significantly altered the secondary structure of the protein, causing a change in its hydrophobicity. It also resulted in a change in fluorescence properties of FGNCs by intersystem crossing (ICT) and fluorescence resonance energy transfer (FRET). Cobalt and cadmium could successfully be detected in the range of 5-165 ng/mL (R² = 0.95) and 20-1000 ng/ mL (R² = 0.91), respectively, with appreciable sensitivity. The principle was also applied for the detection of Vitamin B₁₂ in commercially available ampoules, validating the proposed method. Graphical Abstract Proposed detection method of cadmium and cobalt using FGNCs.

Fluorescent approaches for understanding interactions of ligands with G protein coupled receptors

G protein coupled receptors are responsible for a wide variety of signaling responses in diverse cell types. Despite major advances in the determination of structures of this class of receptors, the underlying mechanisms by which binding of different types of ligands specifically elicits particular signaling responses remain unclear. The use of fluorescence spectroscopy can provide important information about the process of ligand binding and ligand dependent conformational changes in receptors, especially kinetic aspects of these processes that can be difficult to extract from X-ray structures. We present an overview of the extensive array of fluorescent ligands that have been used in studies of G protein coupled receptors and describe spectroscopic approaches for assaying binding and probing the environment of receptor-bound ligands with particular attention to examples involving yeast pheromone receptors. In addition, we discuss the use of fluorescence spectroscopy for detecting and characterizing conformational changes in receptors induced by the binding of ligands. Such studies have provided strong evidence for diversity of receptor conformations elicited by different ligands, consistent with the idea that GPCRs are not simple on and off switches. This diversity of states constitutes an underlying mechanistic basis for biased agonism, the observation that different stimuli can produce different responses from a single receptor. It is likely that continued technical advances will allow fluorescence spectroscopy to play an important role in continued probing of structural transitions in G protein coupled receptors. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

Modern methods to investigate the oligomerization of glycoprotein hormone receptors (TSHR, LHR, FSHR)

As for other GPCRs, the oligomerization of glycoprotein hormone receptors (GPHRs) appears as critical event for receptor function. By means of modern techniques based on the BRET or FRET principle, GPHR oligomerization has been reported to explain several physiological and pathological conditions. In particular, the presence of oligomers was demonstrated not only in in vitro heterologous systems but also in in vivo tissues, and GPHR homodimerization appears associated with strong negative cooperativity, thus suggesting that one hormone molecule may be sufficient for receptor dimer stimulation. In addition, oligomerization has been reported to occur early during the posttranslational maturation process and to be involved in the dominant negative effect exerted by loss-of-function TSH receptor (TSHR) mutants, that are prevalently retained inside the cell, on the surface expression of wild-type receptors. This molecular mechanism thus explains the dominant inheritance of certain forms of TSH resistance. Here, we provide the description of the methods used in the original BRET, FRET, and HTRF-RET experiments.

Fluorescence/bioluminescence resonance energy transfer techniques to study G-protein-coupled receptor activation and signaling

Fluorescence and bioluminescence resonance energy transfer (FRET and BRET) techniques allow the sensitive monitoring of distances between two labels at the nanometer scale. Depending on the placement of the labels, this permits the analysis of conformational changes within a single protein (for example of a receptor) or the monitoring of protein-protein interactions (for example, between receptors and G-protein subunits). Over the past decade, numerous such techniques have been developed to monitor the activation and signaling of G-protein-coupled receptors (GPCRs) in both the purified, reconstituted state and in intact cells. These techniques span the entire spectrum from ligand binding to the receptors down to intracellular second messengers. They allow the determination and the visualization of signaling processes with high temporal and spatial resolution. With these techniques, it has been demonstrated that GPCR signals may show spatial and temporal patterning. In particular, evidence has been provided for spatial compartmentalization of GPCRs and their signals in intact cells and for distinct physiological consequences of such spatial patterning. We review here the FRET and BRET technologies that have been developed for G-protein-coupled receptors and their signaling proteins (G-proteins, effectors) and the concepts that result from such experiments.

The role of somatostatin and dopamine D2 receptors in endocrine tumors

Somatostatin (SS) and dopamine (DA) receptors have been highlighted as two critical regulators in the negative control of hormonal secretion in a wide group of human endocrine tumors. Both families of receptors belong to the superfamily of G protein-coupled receptors and share a number of structural and functional characteristics. Because of the generally reported high expression of somatostatin receptors (SSTRs) in neuroendocrine tumors (NET), somatostatin analogs (SSA) have a pronounced role in the medical therapy for this class of tumors, especially pituitary adenomas and well-differentiated gastroenteropancreatic NET (GEP NET). Moreover, NET express not only SSTR but also frequently dopamine receptors (DRs), and DA agonists targeting the D(2) receptor (D(2)) have been demonstrated to be effective in controlling hormone secretion and cell proliferation in in vivo and in vitro studies. The treatment with SSAs combined with DA agonists has already been demonstrated efficacious in a subgroup of patients with GH-secreting pituitary adenomas and few reported cases of carcinoids. The recent availability of new selective and universal SSA and DA agonists, as well as the chimeric SS/DA compounds, may shed new light on the potential role of SSTR and D(2) as combined targets for biotherapy in NET. This review provides an overview of the latest studies evaluating the expression of SSTR and DR in NET, focusing on their co-expression and the possible clinical implications of such co-expression. Moreover, the most recent insights in SSTR and D(2) pathophysiology and the future perspectives for treatment with SSA, DA agonists, and SS/DA chimeric compounds are discussed.

Two-state displacement by the kinesin-14 Ncd stalk

The nonprocessive kinesin-14 Ncd motor binds to microtubules and hydrolyzes ATP, undergoing a single displacement before releasing the microtubule. A lever-like rotation of the coiled-coil stalk is thought to drive Ncd displacements or steps along microtubules. Crystal structures and cryoelectron microscopy reconstructions imply that stalk rotation is correlated with ADP release and microtubule binding by the motor. Here we report FRET assays showing that the end of the stalk is more than ~9nm from the microtubule when wild-type Ncd binds microtubules without added nucleotide, but the stalk is within ~6nm of the microtubule surface when the microtubule-bound motor binds an ATP analogue, matching the rotated state observed in crystal structures. We propose that the stalk rotation is initiated when the motor binds to microtubules and releases ADP, and is completed when ATP binds.

GPCR somatostatin receptor extracellular loop 2 is a key ectodomain for making subtype-selective antibodies with agonist-like activities in the pancreatic neuroendocrine tumor BON cell line

OBJECTIVES

The extracellular loop 2 (ECL2) ectodomain of the G protein-coupled receptor class A is thought to function like an inactivation "lid." We created polyclonal somatostatin receptor ECL2 (anti-SSTR ECL2) antibodies to target this lid and to examine if these antibodies can selectively activate the SSTR.

METHODS

Western blots and live-cell immunofluorescence microscopy determined anti-SSTR ECL2 antibody receptor binding selectivity. 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay (MTS assay) and cell cycle assay (fluorescence-activated cell sorting) checked for antibody effect on antiproliferation. Nexin assay examined the antibody's ability to induce apoptosis. LANCE cAMP kit (Perkin Elmer) detected antibody-dependent cAMP decrease. Enzyme-linked immunosorbent assay measured antibody effect on suppressing serotonin secretion. Ligand-receptor binding interference assay with the fluorescent somatostatin (FAM-SST) was used to examine antibody interference to SST-SSTR binding.

RESULTS

Anti-SSTR ECL2 antibodies are SSTR subtype selective and agonist-like, and they suppress cell proliferation via cell cycle arrest and apoptosis. In addition, these antibodies decrease cAMP production and inhibit serotonin secretion. Interestingly, these antibodies do not interfere with SST-SSTR binding.

CONCLUSIONS

The ECL2 is an important ectodomain for G protein-coupled receptor activation and required for ligand binding selectivity. The anti-SSTR2, anti-SSTR3, and anti-SSTR5 ECL2 antibodies independently inhibited BON proliferation and decreased hormone secretion. Unlike octreotide, our antibodies do not interfere with SST-SSTR binding.

Illuminating the life of GPCRs

The investigation of biological systems highly depends on the possibilities that allow scientists to visualize and quantify biomolecules and their related activities in real-time and non-invasively. G-protein coupled receptors represent a family of very dynamic and highly regulated transmembrane proteins that are involved in various important physiological processes. Since their localization is not confined to the cell surface they have been a very attractive "moving target" and the understanding of their intracellular pathways as well as the identified protein-protein-interactions has had implications for therapeutic interventions. Recent and ongoing advances in both the establishment of a variety of labeling methods and the improvement of measuring and analyzing instrumentation, have made fluorescence techniques to an indispensable tool for GPCR imaging. The illumination of their complex life cycle, which includes receptor biosynthesis, membrane targeting, ligand binding, signaling, internalization, recycling and degradation, will provide new insights into the relationship between spatial receptor distribution and function. This review covers the existing technologies to track GPCRs in living cells. Fluorescent ligands, antibodies, auto-fluorescent proteins as well as the evolving technologies for chemical labeling with peptide- and protein-tags are described and their major applications concerning the GPCR life cycle are presented.

C-tail mediated modulation of somatostatin receptor type-4 homo- and heterodimerizations and signaling

Somatostatin receptors show great diversity in response to agonist mediated receptor-specific homo- and heterodimerizations. Here, using photobleaching-fluorescence resonance energy transfer, immunocytochemistry, western blot and co-immunoprecipitation, we investigated dimerization, trafficking, coupling to adenylyl cyclase and signaling of human somatostatin receptor-4 (hSSTR4) in HEK-293 cells. We also determined the role of the C-tail of hSSTR4 on physiological responses of the cells. wt-hSSTR4 exogenously expressed in HEK-293 cells exhibits constitutive dimerization, inhibits forskolin-stimulated cAMP, and displays agonist dependent changes in pERK1/2 and pERK5 expressions. Upon C-tail deletion, the receptor loses membrane expression and ability to dimerize and inhibition of cAMP and pERK5 however, displays several-fold increases in the expression of pERK1/2. Chimeric hSSTR4 with the C-tail of hSSTR5 functions like wt-hSSTR4, in contrast, with the C-tail of hSSTR1 functions like C-tail deleted hSSTR4. hSSTR4 dimerization and signaling are associated with increased cyclin-dependent-kinase p27(kip1) expression and inhibition of the cell proliferation. We also report heterodimerization between hSSTR4/hSSTR5, but not between hSSTR4/hSSTR1, with significant changes in receptor functions. Taken together, these data define a novel mechanism for the role of hSSTR4 in cell proliferation and modulation of signaling pathways.

Cell growth inhibition and functioning of human somatostatin receptor type 2 are modulated by receptor heterodimerization

Somatostatin (SST) analogs have been successfully used in the medical treatment of acromegaly, caused by GH hypersecreting pituitary adenomas. Patients on SST analogs rarely develop tachyphylaxis despite years of continuous administration. It has been recently proposed that a functional association between SST receptor (SSTR) subtypes 2 and 5 exists to account for this behavior; however, a physical interaction has yet to be identified. Using both coimmunoprecipitation and photobleaching fluorescence resonance energy transfer microscopy techniques, we determined that SSTR2 and SSTR5 heterodimerize. Surprisingly, selective activation of SSTR2 and not SSTR5, or their costimulation, modulates the association. The SSTR2-selective agonist L-779,976 is more efficacious at inhibiting adenylate cyclase, activating ERK1/2, and inducing the cyclin-dependent kinase inhibitor p27(Kip1) in cells expressing both SSTR2 and SSTR5 compared with SSTR2 alone. Furthermore, cell growth inhibition by L-779,976 treatment was markedly extended in coexpressing cells. Trafficking of SSTR2 is also affected upon heterodimerization, an attribute corresponding to modifications in beta-arrestin association kinetics. Activation of SSTR2 results in the recruitment and stable association of beta-arrestin, followed by receptor internalization and intracellular receptor pooling. In contrast, heterodimerization increases the recycling rate of internalized SSTR2 by destabilizing its interaction with beta-arrestin. Given that SST analogs show preferential binding to SSTR2, these data provide a mechanism for their effectiveness in controlling pituitary tumors and the absence of tolerance seen in patients undergoing long-term administration.

Structure-activity relationships of urotensin II and URP

Urotensin II (U-II) and urotensin II-related peptide (URP) are the endogenous ligands for the orphan G-protein-coupled receptor GPR14 now renamed UT. At the periphery, U-II and/or URP exert a wide range of biological effects on cardiovascular tissues, airway smooth muscles, kidney and endocrine glands, while central administration of U-II elicits various behavioral and cardiovascular responses. There is also evidence that U-II and/or URP may be involved in a number of pathological conditions including heart failure, atherosclerosis, renal dysfunction and diabetes. Because of the potential involvement of the urotensinergic system in various physiopathological processes, there is need for the rational design of potent and selective ligands for the UT receptor. Structure-activity relationship studies have shown that the minimal sequence required to retain full biological activity is the conserved U-II(4-11) domain, in particular the Cys5 and Cys10 residues involved in the disulfide bridge, and the Phe6, Lys8 and Tyr9 residues. Free alpha-amino group and C-terminal COOH group are not necessary for the biological activity, and modifications of these radicals may even increase the stability of the analogs. Punctual substitution of native amino acids, notably Phe6 and Trp7, by particular residues generates analogs with antagonistic properties. These studies, which provide crucial information regarding the structural and conformational requirements for ligand-receptor interactions, will be of considerable importance for the design of novel UT ligands with increased selectivity, potency and stability, that may eventually lead to the development of innovative drugs.

G Protein-Coupled Receptor Trafficking in Health and Disease: Lessons Learned to Prepare for Therapeutic Mutant Rescue in Vivo

G protein-coupled receptors (GPCR) comprise the largest family of drug targets. This is not surprising as many signaling systems rely on this class of receptor to convert external and internal stimuli to intracellular responses. As is the case with other membrane proteins, GPCRs are subjected to a stringent quality control mechanism at the endoplasmic reticulum, which ensures that only correctly folded proteins enter the secretory pathway. Because of this quality control system, point mutations resulting in protein sequence variations may result in the production of misfolded and disease-causing proteins that are unable to reach their functional destinations in the cell. There is now a wealth of information demonstrating the functional rescue of misfolded mutant receptors by small nonpeptide molecules originally designed to serve as receptor antagonists; these small molecules ("pharmacoperones") serve as molecular templates, promoting correct folding and allowing the mutants to pass the scrutiny of the cellular quality control system and be expressed at the cell surface membrane. Two of these systems are especially well characterized: the gonadotropin-releasing hormone and the vasopressin type 2 receptors, which play important roles in regulating reproduction and water homeostasis, respectively. Mutations in these receptors can lead to well defined diseases that are recognized as being caused by receptor misfolding that may potentially be amenable to treatment with pharmacoperones. This review is focused on protein misfolding and misrouting related to various disease states, with special emphasis on these two receptors, which have proved to be of value for development of drugs potentially useful in regulating GPCR trafficking in healthy and disease states.

Technology Insight: modern methods to monitor protein-protein interactions reveal functional TSH receptor oligomerization

The formation of supramolecular structures (dimers or oligomers) is emerging as an important aspect of G-protein-coupled receptor (GPCR) biology. In some cases, GPCR oligomerization is a prerequisite for membrane targeting or function; in others, the relevance of the phenomenon is presently unknown. Although supramolecular structures of GPCRs were initially documented by classical biochemical techniques such as coimmunoprecipitation, many recent advances in the field of GPCR oligomerization have been prompted by the introduction of two new biophysical assays based on Förster's resonance energy transfer-fluorescence resonance energy transfer and bioluminescence resonance energy transfer. These modern techniques allow the study of protein-protein interaction in intact cells, and can be used to monitor monomer association and dissociation in vivo. Recently, oligomerization has also been reported in the case of the TSH receptor (TSHR). This review will focus on the previously unsuspected implications that oligomerization has in TSHR physiology and pathology. It is now clear that TSHR oligomerization is constitutive, occurs early during post-translational processing, and may be involved in membrane targeting and activation by the hormone or by stimulating antibodies. Oligomerization between inactive mutants and wild-type TSHR provides a molecular explanation for the dominant forms of TSH resistance.

Microspectroscopic fluorescence analysis with prism-based imaging spectrometers: review and current studies

BACKGROUND

Fluorescence imaging spectroscopy is a powerful but under-utilized tool. This article gives perspective on the use of imaging spectroscopy, and provides two examples of imaging spectroscopy done with a prism-based system. The intent is to give insight into the power of imaging spectroscopy when used in combination with other imaging techniques. In particular, studies of intact coral photobleaching and beads designed to show energy transfer are reported. In the bead study, spectroscopic lifetime imaging was performed at each photobleaching step.

RESULTS

Spectroscopic photobleaching of the hard coral, Montastrea annularis, revealed two spectral regions. A region in the red portion of the spectrum bleached rapidly while progressively increasing fluorescence was observed over a wide portion of the spectrum. This behavior is consistent with current theories for the role of fluorescent proteins in corals. Following a photobleaching study of beads designed to exhibit energy transfer with imaging spectroscopic fluorescence lifetime imaging microscopy (ISFLIM) allowed unambiguous assignment of fluorescence resonance energy transfer (FRET). The data in this experiment indicated that most of the commonly used markers of FRET would have been inconclusive. The ability of the ISFLIM to look at all regions of the spectrum, particularly the acceptor region, allowed FRET to be assigned.

CONCLUSIONS

Fluorescence imaging spectroscopy is a rapidly advancing technology, uniquely suited to the flexible detection of dyes over a wide range of wavelengths.

Ghrelin Amplifies Dopamine Signaling by Cross Talk Involving Formation of Growth Hormone Secretagogue Receptor/Dopamine Receptor Subtype 1 Heterodimers

Our objective is to determine the neuromodulatory role of ghrelin in the brain. To identify neurons that express the ghrelin receptor [GH secretagogue receptor (GHS-R)], we generated GHS-R-IRES-tauGFP mice by gene targeting. Neurons expressing the GHS-R exhibit green fluorescence and are clearly evident in the hypothalamus, hippocampus, cortex, and midbrain. Using immunohistochemistry in combination with green fluorescent protein fluorescence, we identified neurons that coexpress the dopamine receptor subtype 1 (D1R) and GHS-R. The potential physiological relevance of coexpression of these two receptors and the direct effect of ghrelin on dopamine signaling was investigated in vitro. Activation of GHS-R by ghrelin amplifies dopamine/D1R-induced cAMP accumulation. Intriguingly, amplification involves a switch in G protein coupling of the GHS-R from Gα11/q to Gαi/o by a mechanism consistent with agonist-dependent formation of GHS-R/D1R heterodimers. Most importantly, these results indicate that ghrelin has the potential to amplify dopamine signaling selectively in neurons that coexpress D1R and GHS-R.

Current methods used to investigate G protein coupled receptor oligomerisation

Classical models of G protein coupled receptor (GPCR) signalling assume that each receptor functions as a single unit. However, evidence is increasing that GPCRs may form functional assemblies of dimeric or oligomeric units. There are several methods that can be used to give evidence of GPCR oligomerisation that will be discussed in this review. These include co-immunoprecipitation and Western blotting, resonance energy transfer methods and transactivation / complementation of partially functional receptors. One definitive method currently does not exist and there are various advantages and disadvantages to each method depending upon the system considered. Although co-immunoprecipitation and Western blot studies require disruption of the cellular environment and require specific antibodies, they are a good starting point to show that receptor oligomerisation occurs in native systems. Resonance energy transfer techniques provide evidence that receptors are in close proximity, are measured in living cells and some formats may be used for imaging applications. Transactivation / complementation requires extensive modification of the GPCR, but provides evidence that the receptors are in physical contact. Despite great advances being made using these techniques, future challenges involve the development of other methodologies to determine the role of receptor complexes in the pharmacology and physiology of native systems.

Calcium-sensing receptor dimerizes in the endoplasmic reticulum: biochemical and biophysical characterization of CASR mutants retained intracellularly

Calcium-sensing receptor (CASR), expressed in parathyroid gland and kidney, is a critical regulator of extracellular calcium homeostasis. This G protein-coupled receptor exists at the plasma membrane as a homodimer, although it is unclear at which point in the biosynthetic pathway dimerization occurs. To address this issue, we have analyzed wild-type and mutant CASRs harboring R66H, R66C or N583X-inactivating mutations identified in familial hypocalciuric hypercalcemia/neonatal severe hyperparathyroid patients, which were transiently expressed in kidney cells. All mutants were deficient in cell signaling responses to extracellular CASR ligands relative to wild-type. All mutants, although as well expressed as wild-type, lacked mature glycosylation, indicating impaired trafficking from the endoplasmic reticulum (ER). Dimerized forms of wild-type, R66H and R66C mutants were present, but not of the N583X mutant. By immunofluorescence confocal microscopy of non-permeabilized cells, although cell surface expression was observed for the wild-type, little or none was seen for the mutants. In permeabilized cells, perinuclear staining was observed for both wild-type and mutants. By colocalization fluorescence confocal microscopy, the mutant CASRs were localized within the ER but not within the Golgi apparatus. By the use of photobleaching fluorescence resonance energy transfer microscopy, it was demonstrated that the wild-type, R66H and R66C mutants were dimerized in the ER, whereas the N583X mutant was not. Hence, constitutive CASR dimerization occurs in the ER and is likely to be necessary, but is not sufficient, for exit of the receptor from the ER and trafficking to the cell surface.

Oligomerization of recombinant and endogenously expressed human histamine H(4) receptors

In this study, we report the homo- and hetero-oligomerization of the human histamine H(4)R by both biochemical (Western blot and immobilized metal affinity chromatography) and biophysical [bioluminescence resonance energy transfer and time-resolved fluorescence resonance energy transfer (tr-FRET)] techniques. The H(4)R receptor is the most recently discovered member of the histamine family of G-protein-coupled receptors. Using specific polyclonal antibodies raised against the C-terminal tail of the H(4)R, we demonstrate the presence of H(4)R oligomers in human embryonic kidney 293 and COS-7 cells heterologously overexpressing H(4)Rs and putative native H(4)R oligomers in human phytohaemagglutinin blasts endogenously expressing H(4)Rs. Moreover, we show that H(4)R homo-oligomers are formed constitutively, are formed at low receptor densities (300 fmol/mg of protein), and are present at the cell surface, as detected by tr-FRET. The formation of these oligomers is independent of N-glycosylation and is not modulated by H(4)R ligands, covering the full spectrum of agonists, neutral antagonists, and inverse agonists. Although we show H(4)R homo-oligomer formation at physiological expression levels, the detection of H(1)R-H(4)R hetero-oligomers was achieved only at higher H(1)R expression levels and are most likely not physiologically relevant.

Dimerization of corticotropin-releasing factor receptor type 1 is not coupled to ligand binding

As described previously, receptor dimerization of G protein-coupled receptors may influence signaling, trafficking, and regulation in vivo. Up to now, most studies aiming at the possible role of receptor dimerization in receptor activation and signal transduction are focused on class A GPCRs. In the present work, the dimerization behavior of the corticotropin-releasing factor receptor type 1 (CRF1R), which belongs to class B of GPCRs and plays an important role in coordination of the immune response, stress, and learning behavior, was investigated by using fluorescence resonance energy transfer (FRET). For this purpose, we generated fusion proteins of CRF1R tagged at their C-terminus to a cyan or yellow fluorescent protein, which can be used as a FRET pair. Binding studies verified that the receptor constructs were able to bind their natural ligands in a manner comparable with the wild-type receptor, whereas cAMP accumulation proved the functionality of the constructs. In microscopic studies, a dimerization of the CRF1R was observed, but the addition of either CRF-related agonists or antagonists did not show any dose-related increase of the observed FRET signal, indicating that the dimer-monomer ratio is not changed on addition of ligand.

Intracellular entrapment of wild-type TSH receptor by oligomerization with mutants linked to dominant TSH resistance

TSH resistance is one of the causes of congenital hypothyroidism with thyroid gland in situ. We recently identified families with dominant transmission of partial TSH resistance due to heterozygous inactivating mutations in TSH receptor (TSHR) gene. Although we documented a poor routing of TSHR mutants to the cell membrane, the mechanism responsible for dominant inheritance of partial TSH resistance remained unexplained. We therefore co-transfected Cos-7 cells with wild-type TSHR and mutant receptors found in these patients. A variable impairment of cAMP response to bTSH stimulation was observed, suggesting that inactive TSHR mutants can exert a dominant negative effect on wild-type TSHR. We then generated chimeric constructs of wild-type or inactive TSHR mutants fused to different reporters. By fluorescence microscopy and immunoblotting, we documented an intracellular entrapment, mainly in the endoplasmic reticulum, and reduced maturation of wild-type TSHR in the presence of inactive TSHR mutants. Finally, fluorescence resonance energy transfer and co-immunoprecipitation experiments were performed to study the molecular interactions between wild-type and mutant TSHRs. The results are in agreement with the presence of oligomers formed by wild-type and mutant receptors in the endoplasmic reticulum. Such physical interaction represents the molecular basis for the dominant negative effect of inactive TSHR mutants. These findings provide an explanation for the dominant transmission of partial TSH resistance. This is the first report linking dominant negative mutations of a G protein-coupled receptor to an abnormal endocrine phenotype in heterozygous patients.

Monitoring the formation of dynamic G-protein-coupled receptor-protein complexes in living cells

GPCRs (G-protein-coupled receptors) play an extremely important role in transducing extracellular signals across the cell membrane with high specificity and sensitivity. They are central to many of the body's endocrine and neurotransmitter pathways, and are consequently a major drug target. It is now clear that GPCRs interact with a range of proteins, including other GPCRs. Identifying and elucidating the function of such interactions will significantly enhance our understanding of cellular function, with the promise of new and improved pharmaceuticals. Biophysical techniques involving resonance energy transfer, namely FRET (fluorescence resonance energy transfer) and BRET (bioluminescence resonance energy transfer), now enable us to monitor the formation of dynamic GPCR-protein complexes in living cells, in real time. Their use has firmly established the concept of GPCR oligomerization, as well as demonstrating GPCR interactions with GPCR kinases, beta-arrestins, adenylate cyclase and a subunit of an inwardly rectifying K+ channel. The present review examines recent technological advances and experimental applications of FRET and BRET, discussing particularly how they have been adapted to extract an ever-increasing amount of information about the nature, specificity, stoichiometry, kinetics and agonist-dependency of GPCR-protein interactions.

Following FRET through five energy transfer steps: spectroscopic photobleaching, recovery of spectra, and a sequential mechanism of FRET

We report the acquisition and analysis of spectrally resolved photobleaching data from a model system designed to exhibit FRET. Spectrally resolved photobleaching can be used to determine the presence of FRET in these systems and to investigate multi-step mechanisms of energy transfer. The model system was a previously described set of fluorescent beads consisting of a system of six fluorophores. In standard photobleaching experiments to determine FRET, bleaching of an acceptor molecule resulting in recovery of donor intensity or changes in photobleaching kinetics are used as indicators of FRET. Here, we use the Bateman equations to model growth and decay in a photobleaching experiment. Linked donor-acceptor growth and decay is used as an indicator of FRET. The apparatus required is relatively simple when compared to lifetime imaging systems. Several data analysis strategies, rigorous model building, global fitting procedures, and error analysis are presented. Using these procedures a five-step sequential mechanism of energy transfer was selected for these beads.

Pharmacologic rescue of conformationally-defective proteins: implications for the treatment of human disease

The process of quality control in the endoplasmic reticulum involves a variety of mechanisms which ensure that only correctly folded proteins enter the secretory pathway. Among these are conformation-screening mechanisms performed by molecular chaperones that assist in protein folding and prevent non-native (or misfolded) proteins from interacting with other misfolded proteins. Chaperones play a central role in the triage of newly formed proteins prior to their entry into the secretion, retention, and degradation pathways. Despite this stringent quality control mechanism, gain- or loss-of-function mutations that affect protein folding in the endoplasmic reticulum can manifest themselves as profound effects on the health of an organism. Understanding the molecular, cellular, and energetic mechanisms of protein routing could prevent or correct the structural abnormalities associated with disease-causing misfolded proteins. Rescue of misfolded, "trafficking-defective", but otherwise functional, proteins is achieved by a variety of physical, chemical, genetic, and pharmacological approaches. Pharmacologic chaperones (or "pharmacoperones") are template molecules that may potentially arrest or reverse diseases by inducing mutant proteins to adopt native-type-like conformations instead of improperly folded ones. Such restructuring leads to a normal pattern of cellular localization and function. This review focuses on protein misfolding and misrouting related to various disease states and describes promising approaches to overcoming such defects. Special attention is paid to the gonadotropin-releasing hormone receptor, since there is a great deal of information about this receptor, which has recently emerged as a particularly instructive model.

Design, Synthesis, and Application of Particle-Based Fluorescence Resonance Energy Transfer Sensors for Carbohydrates and Glycoproteins

This paper describes the development of novel particle-based fluorescence resonance energy transfer (FRET) sensors to quantify the concentration and monitor the binding affinity of carbohydrates and glycoproteins to lectins, which are carbohydrate binding proteins. The sensing approach is based on FRET between fluorescein (donor)-labeled lectin molecules, adsorbed on the surface of micrometric polymeric beads, and polymeric dextran molecules labeled with Texas Red (acceptor). The FRET efficiency of the donor-acceptor pair decreases in the presence of carbohydrates or glycoproteins that compete with the Texas Red-labeled dextran molecules on the lectinic binding sites. The inhibitory effect is concentration and time dependent. The sensing technique enables the discrimination between carbohydrates and glycoproteins based on their binding affinity to the FRET sensing particles as well as quantitative analysis of carbohydrates and glycoproteins in aqueous samples. In the future, the newly developed sensors could enable screening glycoprotein-based drugs for their binding affinity toward selective receptors.

Functional consequences of 7TM receptor dimerization

7TM receptors work as signaling platforms that activate multiple signalling systems at the intracellular face of the plasma membrane. It is an emerging concept that 7TM receptors form homo- and hetero-dimers or -oligomers in vitro and in vivo. Numerous studies suggest dimerization is important for receptor function including agonist/antagonist affinity, efficacy, trafficking, and specificity of signal transduction, yet it remains unknown whether dimerization is a prerequisite for 7TM receptor signaling. The current review provides an overview of the biochemical support for 7TM homodimerization, followed by a discussion of the characteristics of homodimerization, with focus on dimer organization, and the functional consequences of dimerization. Heterodimerization will not generally be discussed in this review although we have included a few examples to illustrate specific points, and a table that summarises the current literature on this subject.

The role of subtype-specific ligand binding and the C-tail domain in dimer formation of human somatostatin receptors

G-protein-coupled receptors (GPCRs) represent the largest and most diverse family of cell surface receptors. Several GPCRs have been documented to dimerize with resulting changes in pharmacology. We have previously reported by means of photobleaching fluorescence resonance energy transfer (pbFRET) microscopy and fluorescence correlation spectroscopic (FCS) analysis in live cells, that human somatostatin receptor (hSSTR) 5 could both homodimerize and heterodimerize with hSSTR1 in the presence of the agonist SST-14. In contrast, hSSTR1 remained monomeric when expressed alone regardless of agonist exposure in live cells. In an effort to elucidate the role of ligand and receptor subtypes in heterodimerization, we have employed both pb-FRET microscopy and Western blot on cells stably co-expressing hSSTR1 and hSSTR5 treated with subtype-specific agonists. Here we provide evidence that activation of hSSTR5 but not hSSTR1 is necessary for heterodimeric assembly. This property was also reflected in signaling as shown by increases in adenylyl cyclase coupling efficiencies. Furthermore, receptor C-tail chimeras allowed for the identification of the C-tail as a determinant for dimerization. Finally, we demonstrate that heterodimerization is subtype-selective involving ligand-induced conformational changes in hSSTR5 but not hSSTR1 and could be attributed to molecular events occurring at the C-tail. Understanding the mechanisms by which GPCRs dimerize holds promise for improvements in drug design and efficacy.

G-protein coupled receptor oligomerization in neuroendocrine pathways

Protein-protein interactions are fundamental processes for many biological systems including those involving the superfamily of G-protein coupled receptors (GPCRs). A growing body of biochemical and functional evidence supports the existence of GPCR-GPCR homo- and hetero-oligomers. In particular, hetero-oligomers can display pharmacological and functional properties distinct from those of the homodimer or oligomer thus adding another level of complexity to how GPCRs are activated, signal and traffick in the cell. Dimerization may also play a role in influencing the activity of agonists and antagonists. We are only beginning to unravel how and why such complexes are formed, the functional implications of which will have an enormous impact on GPCR biology. Future research that studies GPCRs as dimeric or oligomeric complexes will enhance not only our understanding of GPCRs in cellular function but will also be critical for novel drug design and improved treatment of the vast array of GPCR-related conditions.

GPCR dimerisation

The concept that GPCRs exist and potentially function as dimers and/or higher oligomers has progressed recently from hypothesis to being widely accepted. A range of techniques has contributed to this understanding, including co-immunoprecipitation and various forms of fluorescence and bioluminescence resonance energy transfer. Although co-immunoprecipitation studies indicate the capacity of a wide range of GPCRs to form hetero-dimers as well as homo-dimers, this approach is not well suited to examine selectivity of interactions. Both bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET) have been applied to the detection of GPCR dimerisation in intact cells and BRET and FRET have been used to attempt to quantitate the fraction of GPCRs present as dimers. Following heterologous expression, a considerable fraction of many GPCRs is not fully processed and is trafficked to the proteasome or lysosome for destruction. A distinct limitation of both BRET and conventional FRET approaches is that both the energy donor and energy acceptor tags are inside the cell. Time-resolved FRET employing N-terminally epitope-tagged GPCRs has been used to allow detection only of dimers trafficked successfully to the cell surface. Reports indicating the appearance of distinct pharmacology and function following co-expression of two GPCRs are fascinating. Much remains to be examined, however, on the specificity and mechanisms of these interactions and to develop techniques to monitor the function only of hetero-dimers when the corresponding homo-dimers must also be present.

Potent activation of dopamine D3/D2 heterodimers by the antiparkinsonian agents, S32504, pramipexole and ropinirole

Recombinant, human dopamine D3 and D2 receptors form functional heterodimers upon co-expression in COS-7 cells. Herein, actions of the antiparkinsonian agents, S32504, ropinirole and pramipexole, at D3/D2L heterodimers were compared to their effects at the respective monomers and at split, chimeric D3trunk/D2tail and D2trunk/D3tail receptors: the trunk incorporated transmembrane domains (TDs) I-V and the tail TDs VI and VII. In binding assays with the antagonist [3H]nemonapride, all agonists were potent ligands of D3 receptors showing, respectively, 100-, 18- and 56-fold lower affinity at D2L receptors, mimicking the selective D3 receptor antagonist, S33084 (100-fold). At D3trunk/D2tail receptors, except for ropinirole, all drugs showed lower affinities than at D3 sites, whereas for D2trunk/D3tail receptors, affinities of all drugs were higher than at D2L sites. The proportion of high affinity binding sites recognized by S32504, pramipexole and ropinirole in membranes derived from cells co-expressing D3 and D2L sites was higher than in an equivalent mixture of membranes from cells expressing D3 or D2L sites, consistent with the promotion of heterodimer formation. In contrast, the percentage of high and low affinity sites (biphasic isotherms) recognized by S33084 was identical. Functional actions were determined by co-transfection of a chimeric adenylyl cyclase (AC)-V/VI insensitive to D3 receptors. Accordingly, D3 receptor-transfected cells were irresponsive whereas, in D2L receptor-transfected cells, agonists suppressed forskolin-stimulated cAMP production with modest potencies. In cells co-transfected with D3 and D2L receptors, S32504, ropinirole and pramipexole potently suppressed AC-V/VI with EC50s 33-, 19- and 11-fold lower than at D2L receptors, respectively. S32504 also suppressed AC-V/VI activity at split D3trunk/D2tail and D2trunk/D3tail chimeras transfected into COS-7 cells. In conclusion, antiparkinson agents behave as potent agonists at D3/D2'heterodimers', though any role in their actions in vivo remains to be demonstrated.