Detection of heteromerization of more than two proteins by sequential BRET-FRET

Metabotropic glutamate type 5, dopamine D2 and adenosine A2a receptors form higher-order oligomers in living cells

G protein-coupled receptors are known to form homo- and heteromers at the plasma membrane, but the stoichiometry of these receptor oligomers are relatively unknown. Here, by using bimolecular fluorescence complementation, we visualized for the first time the occurrence of heterodimers of metabotropic glutamate mGlu(5) receptors (mGlu(5)R) and dopamine D(2) receptors (D(2)R) in living cells. Furthermore, the combination of bimolecular fluorescence complementation and bioluminescence resonance energy transfer techniques, as well as the sequential resonance energy transfer technique, allowed us to detect the occurrence receptor oligomers containing more than two protomers, mGlu(5)R, D(2)R and adenosine A(2A) receptor (A(2A)R). Interestingly, by using high-resolution immunoelectron microscopy we could confirm that the three receptors co-distribute within the extrasynaptic plasma membrane of the same dendritic spines of asymmetrical, putative glutamatergic, striatal synapses. Also, co-immunoprecipitation experiments in native tissue demonstrated the existence of an association of mGlu(5)R, D(2)R and A(2A)R in rat striatum homogenates. Overall, these results provide new insights into the molecular composition of G protein-coupled receptor oligomers in general and the mGlu(5)R/D(2)R/A(2A)R oligomer in particular, a receptor oligomer that might constitute an important target for the treatment of some neuropsychiatric disorders.

Modelling the interdependence between the stoichiometry of receptor oligomerization and ligand binding for a coexisting dimer/tetramer receptor system

Many G protein-coupled receptors have been shown to exist as oligomers, but the oligomerization state and the effects of this on receptor function are unclear. For some G protein-coupled receptors, in ligand binding assays, different radioligands provide different maximal binding capacities. Here we have developed mathematical models for co-expressed dimeric and tetrameric species of receptors. We have considered models where the dimers and tetramers are in equilibrium and where they do not interconvert and we have also considered the potential influence of the ligands on the degree of oligomerization. By analogy with agonist efficacy, we have considered ligands that promote, inhibit or have no effect on oligomerization. Cell surface receptor expression and the intrinsic capacity of receptors to oligomerize are quantitative parameters of the equations. The models can account for differences in the maximal binding capacities of radioligands in different preparations of receptors and provide a conceptual framework for simulation and data fitting in complex oligomeric receptor situations.

G-protein-coupled receptor heteromers or how neurons can display differently flavoured patterns in response to the same neurotransmitter

It is becoming accepted that G-protein-coupled receptors (GPCRs) arrange in the neuronal membrane into homo- and hetero-oligomers and, therefore, these complexes mediate neurotransmission. New models are then needed to understand GPCR operation and predict the consequences of GPCR homo- or hetero-oligomerization. Although there is not any unifying theory addressing how hetero-oligomerization occurs, recent models have been devised to understand the thermodynamics of binding of neurotransmitters to GPCRs and the allosteric protomer-protomer interactions involved in neurotransmitter-mediated activation of GPCRs. Although a model to predict how signalling is produced via homo- or hetero-oligomerization is lacking, functional data show that receptor oligomers exist to produce a variety of effects in neurons in response to a single neurotransmitter.

Brain receptor mosaics and their intramembrane receptor-receptor interactions: molecular integration in transmission and novel targets for drug development

The concept of intramembrane receptor-receptor interactions and evidence for their existence was introduced by Agnati and Fuxe in 1980/81 suggesting the existence of heteromerization of receptors. In 1982, they proposed the existence of aggregates of multiple receptors in the plasma membrane and coined the term receptor mosaics (RM). In this way, cell signaling becomes a branched process beginning at the level of receptor recognition at the plasma membrane where receptors can directly modify the ligand recognition and signaling capacity of the receptors within a RM. Receptor-receptor interactions in RM are classified as operating either with classical cooperativity, when consisting of homomers or heteromers of similar receptor subtypes having the same transmitter, or non-classical cooperativity, when consisting of heteromers. It has been shown that information processing within a RM depends not only on its receptor composition, but also on the topology and the order of receptor activation determined by the concentrations of the ligands and the receptor properties. The general function of RM has also been demonstrated to depend on allosteric regulators (e.g., homocysteine) of the receptor subtypes present. RM as integrative nodes for receptor-receptor interactions in conjunction with membrane associated proteins may form horizontal molecular networks in the plasma membrane coordinating the activity of multiple effector systems modulating the excitability and gene expression of the cells. The key role of electrostatic epitope-epitope interactions will be discussed for the formation of the RM. These interactions probably represent a general molecular mechanism for receptor-receptor interactions and, without a doubt, indicate a role for phosphorylation-dephosphorylation events in these interactions. The novel therapeutic aspects given by the RMs will be discussed in the frame of molecular neurology and psychiatry and combined drug therapy appears as the future way to go.

Building a new conceptual framework for receptor heteromers

Receptor heteromers constitute a new area of research that is reshaping our thinking about biochemistry, cell biology, pharmacology and drug discovery. In this commentary, we recommend clear definitions that should facilitate both information exchange and research on this growing class of transmembrane signal transduction units and their complex properties. We also consider research questions underlying the proposed nomenclature, with recommendations for receptor heteromer identification in native tissues and their use as targets for drug development.

Heterodimers and receptor mosaics of different types of G-protein-coupled receptors

Through an assembly of interacting GPCRs, heterodimers and high-order heteromers (termed receptor mosaics) are formed and lead to changes in the agonist recognition, signaling, and trafficking of participating receptors via allosteric mechanisms, sometimes involving the appearance of cooperativity. This field has now become a major research area, and this review deals with their physiology being integrators of receptor signaling in the CNS and their use as targets for novel drug development based on their unique pharmacology.

Biosensing and imaging based on bioluminescence resonance energy transfer

Bioluminescence resonance energy transfer (BRET) operates with biochemical energy generated by bioluminescent proteins to excite fluorophores and offers additional advantages over fluorescence energy transfer (FRET) for in vivo imaging and biosensing. While fluorescent proteins are frequently used as BRET acceptors, both small molecule dyes and nanoparticles can also serve as acceptor fluorophores. Semiconductor fluorescent nanocrystals or quantum dots (QDs) are particularly well suited for use as BRET acceptors due to their high quantum yields, large Stokes shifts and long wavelength emission. This review examines the potential of QDs for BRET-based bioassays and imaging, and highlights examples of QD-BRET for biosensing and imaging applications. Future development of new BRET acceptors should further expand the multiplexing capability of BRET and improve its applicability and sensitivity for in vivo imaging applications.

GPCR and G proteins: drug efficacy and activation in live cells

Many biochemical pathways are driven by G protein-coupled receptors, cell surface proteins that convert the binding of extracellular chemical, sensory, and mechanical stimuli into cellular signals. Their interaction with various ligands triggers receptor activation that typically couples to and activates heterotrimeric G proteins, which in turn control the propagation of secondary messenger molecules (e.g. cAMP) involved in critically important physiological processes (e.g. heart beat). Successful transfer of information from ligand binding events to intracellular signaling cascades involves a dynamic interplay between ligands, receptors, and G proteins. The development of Förster resonance energy transfer and bioluminescence resonance energy transfer-based methods has now permitted the kinetic analysis of initial steps involved in G protein-coupled receptor-mediated signaling in live cells and in systems as diverse as neurotransmitter and hormone signaling. The direct measurement of ligand efficacy at the level of the receptor by Förster resonance energy transfer is also now possible and allows intrinsic efficacies of clinical drugs to be linked with the effect of receptor polymorphisms.

Fluorescent and bioluminescent protein-fragment complementation assays in the study of G protein-coupled receptor oligomerization and signaling

Most cellular functions, including signaling by G protein-coupled receptors (GPCRs), are mediated by protein-protein interactions, making the identification and localization of protein complexes key to the understanding of cellular processes. In complement to traditional biochemical techniques, noninvasive resonance energy transfer (RET) and protein-fragment complementation assays (PCAs) now allow protein interactions to be detected in the context of living cells. In this review, fluorescent and bioluminescent PCAs are discussed and their application illustrated with studies on GPCR signaling. Newly developed techniques combining PCA and RET assays for the detection of ternary and quaternary protein complexes are also presented.

Adenosine A(2A) receptors assemble into higher-order oligomers at the plasma membrane

Oligomerization of G protein-coupled receptors (GPCRs) is known to play important roles in regulating receptor pharmacology and function. Whereas many bivalent GPCR interactions have been described, the stoichiometry and localization of GPCR oligomers are largely unknown. We have used bimolecular fluorescence complementation (BiFC) to study adenosine A(2A) receptor (A(2A)R) oligomerization. The data suggest specificity of the A(2A)R/A(2A)R interaction monitored by BiFC and proper sub-cellular localization of tagged receptors. Moreover, using a novel approach combining fluorescence resonance energy transfer and BiFC, we found that at least three A(2A) receptors assemble into higher-order oligomers at the plasma membrane in Cath.A differentiated neuronal cells.

Neurotransmitter receptor heteromers in neurodegenerative diseases and neural plasticity

Metabotropic receptors for neurotransmitters on the plasma membrane of neurons are forming homo- hetero- dimers and even homo- or hetero-oligomers. Neurotransmission has been studied assuming that these G-protein-coupled receptors were monomers. Then, on considering receptor dimers, we are entering a new era for the understanding how neurotransmitter receptors decode signals originating at the nervous system. At the moment it is becoming clear that receptor homo and hetero-oligomers provide signaling diversity, help to understand synaptic plasticity and open new therapeutic potential as targets for neurodegenerative and neuropsychiatric diseases.

Direct comparison of bioluminescence-based resonance energy transfer methods for monitoring of proteolytic cleavage

Bioluminescence resonance energy transfer (BRET) is a powerful tool for the study of protein-protein interactions and conformational changes within proteins. Two common implementations of BRET are BRET(1) with Renilla luciferase (RLuc) and coelenterazine h (CLZ, lambda(em) approximately 475 nm) and BRET(2) with the substrate coelenterazine 400a (CLZ400A substrate, lambda(em)=395 nm) as the respective donors. For BRET(1) the acceptor is yellow fluorescent protein (YFP) (lambda(em) approximately 535 nm), a mutant of green fluorescent protein (GFP), and for BRET(2) it is GFP(2) (lambda(em) approximately 515 nm). It is not clear from previous studies which of these systems has superior signal-to-background characteristics. Here we directly compared BRET(1) and BRET(2) by placing two different protease-specific cleavage sequences between the donor and acceptor domains. The intact proteins simulate protein-protein association. Proteolytic cleavage of the peptide linker simulates protein dissociation and can be detected as a change in the BRET ratios. Complete cleavage of its target sequence by thrombin changed the BRET(2) ratio by a factor of 28.9+/-0.2 (relative standard deviation [RSD], n=3) and changed the BRET(1) ratio by a factor of 3.05+/-0.07. Complete cleavage of a caspase-3 target sequence resulted in the BRET ratio changes by factors of 15.45+/-0.08 for BRET(2) and 2.00+/-0.04 for BRET(1). The BRET(2) assay for thrombin was 2.9 times more sensitive compared with the BRET(1) version. Calculated detection limits (blank signal+3sigma(b), where sigma(b)=standard deviation [SD] of blank signal) were 53 pM (0.002 U) thrombin with BRET(1) and 15 pM (0.0005 U) thrombin with BRET(2). The results presented here suggest that BRET(2) is a more suitable system than BRET(1) for studying protein-protein interactions and as a potential sensor for monitoring protease activity.

Nucleostemin: a multiplex regulator of cell-cycle progression

Nucleostemin (NS) is a protein concentrated in the nucleolus of most stem cells and also in many tumor cells, which has been implicated in cell-cycle progression owing to its ability to modulate p53. Depletion of NS causes G(1) cell-cycle arrest, but its overexpression does so as well. Recently, this paradox has been clarified. NS overexpression causes a sequestration of murine double minute 2 (MDM2), preventing the destruction of p53. A recent study has demonstrated that loss of NS promotes the interaction of L5 and L11 ribosomal proteins with MDM2 and, thus, also prevents p53 degradation. This new finding expands our understanding of the multiple modes of NS action and reinforces the concept that the nucleolus has key roles in cell-cycle progression.

Looking for the role of cannabinoid receptor heteromers in striatal function

The introduction of two concepts, "local module" and "receptor heteromer", facilitates the understanding of the role of interactions between different neurotransmitters in the brain. In artificial cell systems, cannabinoid CB(1) receptors form receptor heteromers with dopamine D2, adenosine A2A and mu opioid receptors. There is indirect but compelling evidence for the existence of the same CB1 receptor heteromers in striatal local modules centered in the dendritic spines of striatal GABAergic efferent neurons, particularly at a postsynaptic location. Their analysis provides new clues for the role of endocannabinoids in striatal function, which cannot only be considered as retrograde signals that inhibit neurotransmitter release. Recent studies using a new method to detect heteromerization of more than two proteins, which consists of sequential BRET-FRET (SRET) analysis, has demonstrated that CB1, D2 and A2A receptors can form heterotrimers in transfected cells. It is likely that functional CB1-A2A-D2 receptor heteromers can be found where they are highly co-expressed, in the dendritic spines of GABAergic enkephalinergic neurons. The functional properties of these multiple receptor heteromers and their role in striatal function need to be determined.