G protein-coupled receptors: too many dimers?

Dimerization of β2-adrenergic receptor is responsible for the constitutive activity subjected to inverse agonism

Dimerization of beta 2-adrenergic receptor (β₂-AR) has been observed across various physiologies. However, the function of dimeric β₂-AR is still elusive. Here, we revealed that dimerization of β₂-AR is responsible for the constitutive activity of β₂-AR generating inverse agonism. Using a co-immunoimmobilization assay, we found that transient β₂-AR dimers exist in a resting state, and the dimer was disrupted by the inverse agonists. A Gα_s preferentially interacts with dimeric β₂-AR, but not monomeric β₂-AR, in a resting state, resulting in the production of a resting cAMP level. The formation of β₂-AR dimers requires cholesterol on the plasma membrane. The cholesterol did not interfere with the agonist-induced activation of monomeric β₂-AR, unlike the inverse agonists, implying that the cholesterol is a specific factor regulating the dimerization of β₂-AR. Our model not only shows the function of dimeric β₂-AR but also provides a molecular insight into the mechanism of the inverse agonism of β₂-AR.

Determination of G-protein-coupled receptor oligomerization by molecular brightness analyses in single cells

Oligomerization of membrane proteins has received intense research interest because of their importance in cellular signaling and the large pharmacological and clinical potential this offers. Fluorescence imaging methods are emerging as a valid tool to quantify membrane protein oligomerization at high spatial and temporal resolution. Here, we provide a detailed protocol for an image-based method to determine the number and oligomerization state of fluorescently labeled prototypical G-protein-coupled receptors (GPCRs) on the basis of small out-of-equilibrium fluctuations in fluorescence (i.e., molecular brightness) in single cells. The protocol provides a step-by-step procedure that includes instructions for (i) a flexible labeling strategy for the protein of interest (using fluorescent proteins, small self-labeling tags or bio-orthogonal labeling) and the appropriate controls, (ii) performing temporal and spatial brightness image acquisition on a confocal microscope and (iii) analyzing and interpreting the data, excluding clusters and intensity hot-spots commonly observed in receptor distributions. Although specifically tailored for GPCRs, this protocol can be applied to diverse classes of membrane proteins of interest. The complete protocol can be implemented in 1 month.

Molecular details of dimerization kinetics reveal negligible populations of transient µ-opioid receptor homodimers at physiological concentrations

Various experimental and computational techniques have been employed over the past decade to provide structural and thermodynamic insights into G Protein-Coupled Receptor (GPCR) dimerization. Here, we use multiple microsecond-long, coarse-grained, biased and unbiased molecular dynamics simulations (a total of ~4 milliseconds) combined with multi-ensemble Markov state models to elucidate the kinetics of homodimerization of a prototypic GPCR, the µ-opioid receptor (MOR), embedded in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/cholesterol lipid bilayer. Analysis of these computations identifies kinetically distinct macrostates comprising several different short-lived dimeric configurations of either inactive or activated MOR. Calculated kinetic rates and fractions of dimers at different MOR concentrations suggest a negligible population of MOR homodimers at physiological concentrations, which is supported by acceptor photobleaching fluorescence resonance energy transfer (FRET) experiments. This study provides a rigorous, quantitative explanation for some conflicting experimental data on GPCR oligomerization.

Single-Molecule Imaging Demonstrates Ligand Regulation of the Oligomeric Status of CXCR4 in Living Cells

The role of dimerization and oligomerization of G-protein-coupled receptors in their signal transduction is highly controversial. Delineating this issue can greatly facilitate rational drug design. With single-molecule imaging, we show that chemokine receptor CXCR4 exists mainly as a monomer in normal mammalian living cells and forms dimers and higher-order oligomers at a high expression level, such as in cancer cells. Chemotaxis tests demonstrate that the signal transduction activity of CXCR4 does not depend only on its expression level, indicating a close relation with the oligomeric status of CXCR4. Moreover, binding ligands can effectively upregulate or downregulate the oligomeric level of CXCR4, which suggests that binding ligands may realize their pivotal roles by regulating the oligomeric status of CXCR4 rather than by simply inducing conformational changes.

Cane Toad Skin Extract-Induced Upregulation and Increased Interaction of Serotonin 2A and D2 Receptors via Gq/11 Signaling Pathway in CLU213 Cells

Recent evidences show that activation of serotonin 2A receptors (5-HT_2A R) by agonists is significant in improving therapeutic activity of disease conditions, such as obsessive-compulsive disorder (OCD). Though the exact molecular mechanism is still not well understood, it is thought to involve agonist-driven, enhanced expression of 5-HT_2A R in certain areas of brain, such as the pre-frontal cortex (PFC). Several other reports have also demonstrated association of OCD with lower dopamine receptor (D₂ R) availability, primarily in the striatum of the brain along with dysfunction of 5-HT_2A R-D₂ R heteromer regulation. We thus hypothesized that compound(s) interacting with this molecular mechanism could be developed as drugs for long-term beneficial effects against OCD. In the present study, we have obtained experimental evidence in cultured neuronal cells (CLU213) that aqueous extract (AE, 50 μg/mL, P < 0.05) of the Australian cane toad skin significantly increased the levels of 5-HT_2A R and D₂ R protein and mRNA expression. AE was also found to enhance the interaction between 5-HT_2A R and D₂ R and formation of expression of 5-HT_2A R-D₂ R heteromer using co-immunoprecipitation and Western blot. Further investigation showed the involvement of classical signaling pathway (G_q/11 -PLCβ) along with c-FOS transcription factor preferentially in 5-HT_2A -mediated agonist activation. These results obtained demonstrated that AE upregulates 5-HT_2A R by a mechanism that appears to involve G_q/11 -PLCβ signaling pathway and c-FOS transcription factor activation. We indicate this enhanced 5-HT_2A R and D₂ R expression and their interaction to induce increased 5-HT_2A R-D₂ R heteromer formation by exposure to AE might provide a molecular mechanism to develop potential novel drug candidates to ameliorate OCD symptoms. J. Cell. Biochem. 118: 979-993, 2017. © 2016 Wiley Periodicals, Inc.

Homo- and hetero-oligomerization of β2-adrenergic receptor in receptor trafficking, signaling pathways and receptor pharmacology

The β2-adrenergic receptor (β2AR) is the prototypic member of G protein-coupled receptors (GPCRs) involved in the production of physiological responses to adrenaline and noradrenaline. Research done in the past few years vastly demonstrated that β2AR can form homo- and hetero-oligomers. Despite the fact that currently this phenomenon is widely accepted, the spread and relevance of β2AR oligomerization are still a matter of debate. This review considers the progress achieved in the field of β2AR oligomerization with focus on the implications of the receptor-receptor interactions to β2AR trafficking, pharmacology and downstream signal transduction pathways.

Cannabinoid-induced enhanced interaction and protein levels of serotonin 5-HT(2A) and dopamine D₂ receptors in rat prefrontal cortex

Recent evidence suggests that non-selective cannabinoid receptor agonists may regulate serotonin 2A (5-HT(2A)) receptor neurotransmission in brain. The molecular mechanisms of this regulation are unknown, but could involve cannabinoid-induced enhanced interaction between 5-HT(2A) and dopamine D2 (D₂) receptors. Here, we present experimental evidence that Sprague-Dawley rats treated with a non-selective cannabinoid receptor agonist (CP55,940, 50 µg/kg, 7 days, i.p.) showed enhanced co-immunoprecipitation of 5-HT(2A) and D₂ receptors and enhanced membrane-associated expression of D₂ and 5-HT(2A) receptors in prefrontal cortex (PFCx). Furthermore, 5-HT(2A) receptor mRNA levels were increased in PFCx, suggesting a cannabinoid-induced upregulation of 5-HT(2A) receptors. To date, two cannabinoids receptors have been found in brain, CB1 and CB2 receptors. We used selective cannabinoid agonists in a neuronal cell line to study mechanisms that could mediate this 5-HT(2A) receptor upregulation. We found that selective CB2 receptor agonists upregulate 5-HT(2A) receptors by a mechanism that seems to involve activation of Gα(i) G-proteins, ERK1/2, and AP-1 transcription factor. We hypothesize that the enhanced cannabinoid-induced interaction between 5-HT(2A) and D₂ receptors and in 5-HT(2A) and D₂ receptors protein levels in the PFCx might provide a molecular mechanism by which activation of cannabinoid receptors might be contribute to the pathophysiology of some cognitive and mood disorders.

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.

Optical techniques to analyze real-time activation and signaling of G-protein-coupled receptors

The activation of G-protein-coupled receptors (GPCRs) is traditionally measured either by monitoring downstream physiological events or by membrane-based biochemical assays. Neither of these approaches permits detailed kinetic or spatial analysis of receptor activation and signaling. Recently, several optical techniques have been developed to monitor receptor activation either by using purified reconstituted GPCRs or by observing GPCRs, G proteins and second messengers in intact cells. These techniques are providing, literally, new views on both the mechanistic basis of the signaling process and the kinetic and spatial properties of GPCR-mediated signals. They suggest that agonists can activate GPCRs within milliseconds, that different compounds can induce distinct active conformations of GPCRs, that G-protein activation is the rate-limiting step in GPCR signaling, and that cellular signals can be temporally and spatially confined. They are also raising controversial issues, such as whether or not receptors and G proteins are pre-coupled and whether G proteins dissociate during activation.

Transmembrane segment IV contributes a functionally important interface for oligomerization of the Class II G protein-coupled secretin receptor

Oligomerization of the Class II G protein-coupled secretin receptor has been reported, but the molecular basis for this and its functional significance have not been determined. In the current work, we have examined the possible contribution of each of the transmembrane (TM) segments of this receptor to its homo-oligomerization, using the method of competitive disruption screening for inhibition of receptor bioluminescence resonance energy transfer signal. TM IV was the only segment that was found to disrupt receptor bioluminescence resonance energy transfer. Evaluation of predicted interhelical and lipid-exposed faces of this TM segment demonstrated that its lipid-exposed face represented the determinant for oligomerization. This was further confirmed by mutagenesis of the intact secretin receptor. Morphological FRET was utilized to demonstrate that secretin receptor oligomerization occurred at the cell surface and that this oligomerization was disrupted by mutating Gly(243) and Ile(247), key residues within the lipid-exposed face of TM IV. Although disruption of the receptor oligomerization interface had no effect on secretin binding parameters, it reduced the ability of secretin to stimulate intracellular cAMP. This supports a clear functional effect of oligomerization of this receptor. Such an effect might be particularly relevant to clinical situations in which this receptor is overexpressed, such as in certain neoplasms.