Orphan GPCR GPRC5C Facilitates Angiotensin II-Induced Smooth Muscle Contraction

BACKGROUND

GPCRs (G-protein-coupled receptors) play a central role in the regulation of smooth muscle cell (SMC) contractility, but the function of SMC-expressed orphan GPCR class C group 5 member C (GPRC5C) is unclear.

OBJECTIVE

The aim of this project is to define the role of GPRC5C in SMC in vitro and in vivo.

METHODS AND RESULTS

We studied the role of GPRC5C in the regulation of SMC contractility and differentiation in human and murine SMC in vitro, as well as in tamoxifen-inducible, SMC-specific GPRC5C knockout mice under basal conditions and in vascular disease in vivo. Mesenteric arteries from tamoxifen-inducible, SMC-specific GPRC5C knockout mice showed ex vivo significantly reduced angiotensin II (Ang II)-dependent calcium mobilization and contraction, whereas responses to other relaxant or contractile factors were normal. In vitro, the knockdown of GPRC5C in human aortic SMC resulted in diminished Ang II-dependent inositol phosphate production and lower myosin light chain phosphorylation. In line with this, tamoxifen-inducible, SMC-specific GPRC5C knockout mice showed reduced Ang II-induced arterial hypertension, and acute inactivation of GPRC5C was able to ameliorate established arterial hypertension. Mechanistically, we show that GPRC5C and the Ang II receptor AT1 dimerize, and knockdown of GPRC5C resulted in reduced binding of Ang II to AT1 receptors in HEK293 cells, human and murine SMC, and arteries from tamoxifen-inducible, SMC-specific GPRC5C knockout mice.

CONCLUSIONS

Our data show that GPRC5C regulates Ang II-dependent vascular contraction by facilitating AT1 receptor-ligand binding and signaling.

DP2 receptor activity sensor suited for antagonist screening and measurement of receptor dynamics in real-time

The DP2 receptor is a G-protein coupled receptor involved in allergic inflammation and is the target of recently developed antagonists already being tested in clinics. To get insights into DP2 receptor dynamics and to study its pharmacology on the level of the receptor, we constructed a fluorescence resonance energy transfer-based conformation sensor. The sensor reflects the selectivity profile of the DP2 receptor-wt and is suited for screening of agonists and antagonists due to its robust response. Furthermore, the sensor enables the direct measurement of DP2 receptor dynamics in real-time and revealed markedly distinct on- and off-rates of prostaglandin D2 between DP2 and DP1 receptors, suggesting a different mechanism of ligand receptor interaction.

Differential interaction patterns of opioid analgesics with µ opioid receptors correlate with ligand-specific voltage sensitivity

The µ opioid receptor (MOR) is the key target for analgesia, but the application of opioids is accompanied by several issues. There is a wide range of opioid analgesics, differing in their chemical structure and their properties of receptor activation and subsequent effects. A better understanding of ligand-receptor interactions and the resulting effects is important. Here, we calculated the respective binding poses for several opioids and analyzed interaction fingerprints between ligand and receptor. We further corroborated the interactions experimentally by cellular assays. As MOR was observed to display ligand-induced modulation of activity due to changes in membrane potential, we further analyzed the effects of voltage sensitivity on this receptor. Combining in silico and in vitro approaches, we defined discriminating interaction patterns responsible for ligand-specific voltage sensitivity and present new insights into their specific effects on activation of the MOR.

EP4 Receptor Conformation Sensor Suited for Ligand Screening and Imaging of Extracellular Prostaglandins

Prostaglandins are important lipid mediators with a wide range of functions in the human body. They act mainly via plasma membrane localized prostaglandin receptors, which belong to the G-protein coupled receptor class. Due to their localized formation and short lifetime, it is important to be able to measure the distribution and abundance of prostaglandins in time and/or space. In this study, we present a Foerster resonance energy transfer (FRET)-based conformation sensor of the human prostaglandin E receptor subtype 4 (EP4 receptor), which was capable of detecting prostaglandin E2 (PGE2)-induced receptor activation in the low nanomolar range with a good signal-to-noise ratio. The sensor retained the typical selectivity for PGE2 among arachidonic acid products. Human embryonic kidney cells stably expressing the sensor did not produce detectable amounts of prostaglandins making them suitable for a coculture approach allowing us, over time, to detect prostaglandin formation in Madin-Darby canine kidney cells and primary mouse macrophages. Furthermore, the EP4 receptor sensor proved to be suited to detect experimentally generated PGE2 gradients by means of FRET-microscopy, indicating the potential to measure gradients of PGE2 within tissues. In addition to FRET-based imaging of prostanoid release, the sensor allowed not only for determination of PGE2 concentrations, but also proved to be capable of measuring ligand binding kinetics. The good signal-to-noise ratio at a commercial plate reader and the ability to directly determine ligand efficacy shows the obvious potential of this sensor interest for screening and characterization of novel ligands of the pharmacologically important human EP4 receptor. SIGNIFICANCE STATEMENT: The authors present a biosensor based on the prostaglandin E receptor subtype 4, which is well suited to measure extracellular prostaglandin E2 (PGE2) concentration with high temporal and spatial resolution. It can be used for the imaging of PGE2 levels and gradients by means of Foerster resonance energy transfer microscopy, and for determining PGE2 release of primary cells as well as for screening purposes in a plate reader setting.

Differential effects of glucose-dependent insulinotropic polypeptide receptor/glucagon-like peptide-1 receptor heteromerization on cell signaling when expressed in HEK-293 cells

The incretin hormones: glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important regulators of many aspects of metabolism including insulin secretion. Their receptors (GIPR and GLP-1R) are closely related members of the secretin class of G-protein-coupled receptors. As both receptors are expressed on pancreatic β-cells there is at least the hypothetical possibility that they may form heteromers. In the present study, we investigated GIPR/GLP-1R heteromerization and the impact of GIPR on GLP-1R-mediated signaling and vice versa in HEK-293 cells. Real-time fluorescence resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) saturation experiments confirm that GLP-1R and GIPR form heteromers. Stimulation with 1 μM GLP-1 caused an increase in both FRET and BRET ratio, whereas stimulation with 1 μM GIP caused a decrease. The only other ligand tested to cause a significant change in BRET signal was the GLP-1 metabolite, GLP-1 (9-36). GIPR expression had no significant effect on mini-G_s recruitment to GLP-1R but significantly inhibited GLP-1 stimulated mini-G_q and arrestin recruitment. In contrast, the presence of GLP-1R improved GIP stimulated mini-G_s and mini-G_q recruitment to GIPR. These data support the hypothesis that GIPR and GLP-1R form heteromers with differential consequences on cell signaling.

Residency time of agonists does not affect the stability of GPCR-arrestin complexes

BACKGROUND AND PURPOSE

The interaction of arrestins with G-protein-coupled receptors (GPCRs) desensitizes agonist-dependent receptor responses and often leads to receptor internalization. GPCRs that internalize without arrestin have been classified as "class A" GPCRs whereas "class B" GPCRs co-internalize with arrestin into endosomes. The interaction of arrestins with GPCRs requires both agonist activation and receptor phosphorylation. Here we ask the question whether agonists with very slow off-rate can cause the formation of particularly stable receptor-arrestin complexes.

EXPERIMENTAL APPROACH

The stability of GPCR-arrestin-3 complexes at two class A GPCRs, the β₂ -adrenergic receptor and the μ-opioid receptor, was assessed using two different techniques, fluorescence resonance energy transfer (FRET) and fluorescence recovery after photobleaching (FRAP) employing several ligands with very different off-rates. Arrestin trafficking was determined by confocal microscopy.

KEY RESULTS

Upon agonist washout, GPCR-arrestin-3 complexes showed markedly different dissociation rates in single-cell FRET experiments. In FRAP experiments, however, all full agonists led to the formation of receptor-arrestin complexes of identical stability whereas the complex between the μ-opioid receptor and arrestin-3 induced by the partial agonist morphine was less stable. Agonists with very slow off-rates could not mediate the co-internalization of arrestin-3 with class A GPCRs into endosomes.

CONCLUSIONS AND IMPLICATIONS

Agonist off-rates do not affect the stability of GPCR-arrestin complexes but phosphorylation patterns do. Our results imply that orthosteric agonists will not be able to pharmacologically convert class A into class B GPCRs.

Disentangling bias between Gq, GRK2, and arrestin3 recruitment to the M3 muscarinic acetylcholine receptor

G protein-coupled receptors (GPCRs) transmit extracellular signals to the inside by activation of intracellular effector proteins. Different agonists can promote differential receptor-induced signaling responses – termed bias – potentially by eliciting different levels of recruitment of effector proteins. As activation and recruitment of effector proteins might influence each other, thorough analysis of bias is difficult. Here, we compared the efficacy of seven agonists to induce G protein, G protein-coupled receptor kinase 2 (GRK2), as well as arrestin3 binding to the muscarinic acetylcholine receptor M₃ by utilizing FRET-based assays. In order to avoid interference between these interactions, we studied GRK2 binding in the presence of inhibitors of G_i and G_q proteins and analyzed arrestin3 binding to prestimulated M₃ receptors to avoid differences in receptor phosphorylation influencing arrestin recruitment. We measured substantial differences in the agonist efficacies to induce M₃R-arrestin3 versus M₃R-GRK2 interaction. However, the rank order of the agonists for G protein- and GRK2-M₃R interaction was the same, suggesting that G protein and GRK2 binding to M₃R requires similar receptor conformations, whereas requirements for arrestin3 binding to M₃R are distinct.

Structures in G proteins important for subtype selective receptor binding and subsequent activation

G protein-coupled receptors (GPCRs) selectively couple to specific heterotrimeric G proteins comprised of four subfamilies in order to induce appropriate physiological responses. However, structural determinants in Gα subunits responsible for selective recognition by approximately 800 human GPCRs have remained elusive. Here, we directly compare the influence of subtype-specific Gα structures on the stability of GPCR-G protein complexes and the activation by two Gq-coupled receptors. We used FRET-assays designed to distinguish multiple Go and Gq-based Gα chimeras in their ability to be selectively bound and activated by muscarinic M₃ and histaminic H₁ receptors. We identify the N-terminus including the αN/β1-hinge, the β2/β3-loop and the α5 helix of Gα to be key selectivity determinants which differ in their impact on selective binding to GPCRs and subsequent activation depending on the specific receptor. Altogether, these findings provide new insights into the molecular basis of G protein-coupling selectivity even beyond the Gα C-terminus.

Pharmacological Characterization of the Stick Insect Carausius morosus Allatostatin-C Receptor with Its Endogenous Agonist

G protein-coupled receptors (GPCRs) play a pivotal role in regulating key physiological events in all animal species. Recent advances in collective analysis of genes and proteins revealed numerous potential neuropeptides and GPCRs from insect species, allowing for the characterization of peptide-receptor pairs. In this work, we used fluorescence resonance energy transfer (FRET)-based genetically encoded biosensors in intact mammalian cells to study the pharmacological features of the cognate GPCR of the type-C allatostatin (AST-C) peptide from the stick insect, Carausius morosus. Analysis of multiple downstream pathways revealed that AST-C can activate the human Gi2 protein, and not Gs or Gq, through AST-C receptor (AlstRC). Activated AlstRC recruits β-arrestin2 independent of the Gi protein but stimulates ERK phosphorylation in a Gi protein-dependent manner. Identification of Gαi-, arrestin-, and GRK-like transcripts from C. morosus revealed high evolutionary conservation at the G protein level, while β-arrestins and GRKs displayed less conservation. In conclusion, our study provides experimental and homology-based evidence on the functionality of vertebrate G proteins and downstream signaling biosensors to characterize early signaling steps of an insect GPCR. These results may serve as a scaffold for developing assays to characterize pharmacological and structural aspects of other insect GPCRs and can be used in deorphanization and pesticide studies.

The Effect of Cell Surface Expression and Linker Sequence on the Recruitment of Arrestin to the GIP Receptor

The glucose-dependent insulinotropic polypeptide (GIP) and the glucagon-like peptide-1 (GLP-1) receptor are important targets in the treatment of both type 2 diabetes mellitus (T2DM) and obesity. Originally identified for their role in desensitization, internalization and recycling of G protein-coupled receptors (GPCRs), arrestins have since been shown to act as scaffolding proteins that allow GPCRs to signal in a G protein-independent manner. While GLP-1R has been reported to interact with arrestins, this aspect of cell signaling remains controversial for GIPR. Using a (FRET)-based assay we have previously shown that yellow fluorescent protein (YFP)-labeled GIPR does not recruit arrestin. This GIPR-YFP construct contained a 10 amino acid linker between the receptor and a XbaI restriction site upstream of the YFP. This linker was not present in the modified GIPR-SYFP2 used in subsequent FRET and bioluminescence resonance energy transfer (BRET) assays. However, its removal results in the introduction of a serine residue adjacent to the end of GIPR’s C-terminal tail which could potentially be a phosphorylation site. The resulting receptor was indeed able to recruit arrestin. To find out whether the serine/arginine (SR) coded by the XbaI site was indeed the source of the problem, it was substituted with glycine/glycine (GG) by site-directed mutagenesis. This substitution abolished arrestin recruitment in the BRET assay but only significantly reduced it in the FRET assay. In addition, we show that the presence of a N-terminal FLAG epitope and influenza hemagglutinin signal peptide were also required to detect arrestin recruitment to the GIPR, most likely by increasing receptor cell surface expression. These results demonstrate how arrestin recruitment assay configuration can dramatically alter the result. This becomes relevant when drug discovery programs aim to identify ligands with “biased agonist” properties.

Voltage modulates the effect of μ-receptor activation in a ligand-dependent manner

BACKGROUND AND PURPOSE

Various GPCRs have been described as being modulated in a voltage-dependent manner. Opioid analgesics act via activation of μ receptors in various neurons. As neurons are exposed to large changes in membrane potential, we were interested in studying the effects of depolarization on μ receptor signalling.

EXPERIMENTAL APPROACH

We investigated potential voltage sensitivity of μ receptors in heterologous expression systems (HEK293T cells) using electrophysiology in combination with Förster resonance energy transfer-based assays. Depolarization-induced changes in signalling were also tested in physiological rat tissue containing locus coeruleus neurons. We applied depolarization steps across the physiological range of membrane potentials.

KEY RESULTS

Studying μ receptor function and signalling in cells, we discovered that morphine-induced signalling was strongly dependent on the membrane potential (V_M ). This became apparent at the level of G-protein activation, G-protein coupled inwardly rectifying potassium channel (K_ir 3.X) currents and binding of GPCR kinases and arrestin3 to μ receptors by a robust increase in signalling upon membrane depolarization. The pronounced voltage sensitivity of morphine-induced μ receptor activation was also observed at the level of K_ir 3.X currents in rat locus coeruleus neurons. The efficacy of peptide ligands to activate μ receptors was not (Met-enkephalin) or only moderately ([D-Ala² , N-Me-Phe⁴ , Gly⁵ -ol]-enkephalin) enhanced upon depolarization. In contrast, depolarization reduced the ability of the analgesic fentanyl to activate μ receptors.

CONCLUSION AND IMPLICATIONS

Our results indicate a strong ligand-dependent modulation of μ receptor activity by the membrane potential, suggesting preferential activity of morphine in neurons with high neuronal activity.

The N-termini of GRK2 and GRK3 simulate the stimulating effects of RKIP on β-adrenoceptors

The Raf kinase inhibitor protein (RKIP) activates β-adrenoceptors (β-AR) and thereby induces a well-tolerated cardiac contractility and prevents heart failure in mice. Different to RKIP-mediated β-AR activation, chronic activation of β-AR by catecholamines was shown to be detrimental for the heart. RKIP is an endogenous inhibitor of G protein coupled receptor kinase 2 (GRK2); it binds GRK2 and thereby inhibits GRK2 mediated β-AR phosphorylation and desensitization. Here, we evaluate RKIP-mediated effects on β-AR to explore new strategies for β-AR modulation. Co-immunoprecipitation assays and pull-down assays revealed subtype specificity of RKIP for the cardiac GRK isoforms GRK2 and GRK3 - not GRK5 - as well as several RKIP binding sites within their N-termini (GRK2^1-185 and GRK3^1-185). Overexpression of these N-termini prevented β₂-AR phosphorylation and internalization, subsequently increased receptor signaling in HEK293 cells and cardiomyocyte contractility. Co-immunoprecipitation assays of β₂-AR with these N-terminal GRK fragments revealed a direct interaction suggesting a steric interference of the fragments with the functional GRK-receptor interaction. Altogether, N-termini of GRK2 and GRK3 efficiently simulate RKIP effects on β-AR signaling in HEK293 cells and in cardiomyocytes by their binding to β₂-AR and, thus, provide important insights for the development of new strategies to modulate β₂-AR signaling.

Comparative Docking to Distinct G Protein-Coupled Receptor Conformations Exclusively Yields Ligands with Agonist Efficacy

G protein-coupled receptors exist in a whole spectrum of conformations that are stabilized by the binding of ligands with different efficacy or intracellular effector proteins. Here, we investigate whether three-dimensional structures of receptor conformations in different states of activation can be used to enrich ligands with agonist behavior in prospective docking calculations. We focused on the β ₂-adrenergic receptor, as it is currently the receptor with the highest number of active-state crystal structures. Comparative docking calculations to distinct conformations of the receptor were used for the in silico prediction of ligands with agonist efficacy. The pharmacology of molecules selected based on these predictions was characterized experimentally, resulting in a hit rate of 37% ligands, all of which were agonists. The ligands furthermore contain a pyrazole moiety that has previously not been described for β ₂-adrenergic receptor ligands, and one of them shows an intrinsic efficacy comparable to salbutamol. SIGNIFICANCE STATEMENT: Structure-based ligand design for G protein-coupled receptors crucially depends on receptor conformation and, hence, their activation state. We explored the influence of using multiple active-conformation X-ray structures on the hit rate of docking calculations to find novel agonists, and how to predict the most fruitful strategy to apply. The results suggest that aggregating the ranks of molecules across docking calculations to more than one active-state structure exclusively yields agonists.

Direct targeting of Gαq and Gα11 oncoproteins in cancer cells

Somatic gain-of-function mutations of GNAQ and GNA11, which encode α subunits of heterotrimeric Gαq/11 proteins, occur in about 85% of cases of uveal melanoma (UM), the most common cancer of the adult eye. Molecular therapies to directly target these oncoproteins are lacking, and current treatment options rely on radiation, surgery, or inhibition of effector molecules downstream of these G proteins. A hallmark feature of oncogenic Gαq/11 proteins is their reduced intrinsic rate of hydrolysis of guanosine triphosphate (GTP), which results in their accumulation in the GTP-bound, active state. Here, we report that the cyclic depsipeptide FR900359 (FR) directly interacted with GTPase-deficient Gαq/11 proteins and preferentially inhibited mitogenic ERK signaling rather than canonical phospholipase Cβ (PLCβ) signaling driven by these oncogenes. Thereby, FR suppressed the proliferation of melanoma cells in culture and inhibited the growth of Gαq-driven UM mouse xenografts in vivo. In contrast, FR did not affect tumor growth when xenografts carried mutated B-RafV600E as the oncogenic driver. Because FR enabled suppression of malignant traits in cancer cells that are driven by activating mutations at codon 209 in Gαq/11 proteins, we envision that similar approaches could be taken to blunt the signaling of non-Gαq/11 G proteins.

Potentiation of the activation of cholinergic receptors by multivalent presentation of ligands supported on gold nanoparticles

Gold nanoparticles (NP) with a functionalized ligand shell offer the possibility to potentiate the action of agonists at the receptor site by multivalency. In order to find out whether this can be realized for the pharmacologically important class of cholinergic receptors known to be involved in the regulation of most organ functions, carbachol-functionalized gold NPs (Au-MUDA-CCh) with an average diameter of 14 nm were synthesized. As functional read-out, cholinergic agonist-induced anion secretion was measured as increase in short-circuit current (Isc) across rat proximal colon in Ussing chambers. Similarly to the corresponding native agonist acetylcholine, Au-MUDA-CCh induced a concentration-dependent increase in Isc, which represents chloride secretion across the epithelium. This response was inhibited by atropine and hexamethonium indicating the activation of muscarinic and nicotinic receptors by the functionalized NPs. A strong potentiation of ligand-receptor interaction was a key benefit of functionalized NPs over native agonists. This was observed with physiological approaches as measurements of changes in Isc revealed a nearly equivalent response evoked by 1 pM Au-MUDA-CCh and 500 nM native CCh. To better determine this potentiation at the receptor level, pharmacological approaches based on the signaling cascade of ACh-induced activation of muscarinic receptors were used. FRET (Förster Resonance Energy Transfer) measurements performed on HEK293T cells transiently transfected with M3-R, Gαq-YFP, Gβ1-wt and CFP-Gγ2, revealed that both Au-MUDA-CCh and native CCh activated G-proteins with EC50 amounting to 127 ± 0.44 fM and 224 ± 7.12 nM, respectively. Thus, the functionalization of the NPs with CCh yields a potentiation by over 106, a property that could find usage in specific targeting, activation and compensation of pathologically reduced expression of receptors of interest.

Multisite phosphorylation is required for sustained interaction with GRKs and arrestins during rapid μ-opioid receptor desensitization

G protein receptor kinases (GRKs) and β-arrestins are key regulators of μ-opioid receptor (MOR) signaling and trafficking. We have previously shown that high-efficacy opioids such as DAMGO stimulate a GRK2/3-mediated multisite phosphorylation of conserved C-terminal tail serine and threonine residues, which facilitates internalization of the receptor. In contrast, morphine-induced phosphorylation of MOR is limited to Ser³⁷⁵ and is not sufficient to drive substantial receptor internalization. We report how specific multisite phosphorylation controlled the dynamics of GRK and β-arrestin interactions with MOR and show how such phosphorylation mediated receptor desensitization. We showed that GRK2/3 was recruited more quickly than was β-arrestin to a DAMGO-activated MOR. β-Arrestin recruitment required GRK2 activity and MOR phosphorylation, but GRK recruitment also depended on the phosphorylation sites in the C-terminal tail, specifically four serine and threonine residues within the ³⁷⁰TREHPSTANT³⁷⁹ motif. Our results also suggested that other residues outside this motif participated in the initial and transient recruitment of GRK and β-arrestins. We identified two components of high-efficacy agonist desensitization of MOR: a sustained component, which required GRK2-mediated phosphorylation and a potential soluble factor, and a rapid component, which was likely mediated by GRK2 but independent of receptor phosphorylation. Elucidating these complex receptor-effector interactions represents an important step toward a mechanistic understanding of MOR desensitization that leads to the development of tolerance and dependence.

Lifetime of muscarinic receptor-G-protein complexes determines coupling efficiency and G-protein subtype selectivity

G-protein-coupled receptors (GPCRs) are essential for the detection of extracellular stimuli by cells and transfer the encoded information via the activation of functionally distinct subsets of heterotrimeric G proteins into intracellular signals. Despite enormous achievements toward understanding GPCR structures, major aspects of the GPCR-G-protein selectivity mechanism remain unresolved. As this can be attributed to the lack of suitable and broadly applicable assays, we set out to develop a quantitative FRET-based assay to study kinetics and affinities of G protein binding to activated GPCRs in membranes of permeabilized cells in the absence of nucleotides. We measured the association and dissociation kinetics of agonist-induced binding of Gi/o, Gq/11, Gs, and G12/13 proteins to muscarinic M1, M2, and M3 receptors in the absence of nucleotides between fluorescently labeled G proteins and receptors expressed in mammalian cells. Our results show a strong quantitative correlation between not the on-rates of G-protein-M3-R interactions but rather the affinities of Gq and Go proteins to M3-Rs, their GPCR-G-protein lifetime and their coupling efficiencies determined in intact cells, suggesting that the G-protein subtype-specific affinity to the activated receptor in the absence of nucleotides is, in fact, a major determinant of the coupling efficiency. Our broadly applicable FRET-based assay represents a fast and reliable method to quantify the intrinsic affinity and relative coupling selectivity of GPCRs toward all G-protein subtypes.

The allosteric site regulates the voltage sensitivity of muscarinic receptors

Muscarinic receptors (M-Rs) for acetylcholine (ACh) belong to the class A of G protein-coupled receptors. M-Rs are activated by orthosteric agonists that bind to a specific site buried in the M-R transmembrane helix bundle. In the active conformation, receptor function can be modulated either by allosteric modulators, which bind to the extracellular receptor surface or by the membrane potential via an unknown mechanism. Here, we compared the modulation of M₁-Rs and M₃-Rs induced by changes in voltage to their allosteric modulation by chemical compounds. We quantified changes in receptor signaling in single HEK 293 cells with a FRET biosensor for the G_q protein cycle. In the presence of ACh, M₁-R signaling was potentiated by voltage, similarly to positive allosteric modulation by benzyl quinolone carboxylic acid. Conversely, signaling of M₃-R was attenuated by voltage or the negative allosteric modulator gallamine. Because the orthosteric site is highly conserved among M-Rs, but allosteric sites vary, we constructed "allosteric site" M₃/M₁-R chimeras and analyzed their voltage dependencies. Exchanging the entire allosteric sites eliminated the voltage sensitivity of ACh responses for both receptors, but did not affect their modulation by allosteric compounds. Furthermore, a point mutation in M₃-Rs caused functional uncoupling of the allosteric and orthosteric sites and abolished voltage dependence. Molecular dynamics simulations of the receptor variants indicated a subtype-specific crosstalk between both sites, involving the conserved tyrosine lid structure of the orthosteric site. This molecular crosstalk leads to receptor subtype-specific voltage effects.

Anti-nociceptive action of peripheral mu-opioid receptors by G-beta-gamma protein-mediated inhibition of TRPM3 channels

Opioids, agonists of µ-opioid receptors (µORs), are the strongest pain killers clinically available. Their action includes a strong central component, which also causes important adverse effects. However, µORs are also found on the peripheral endings of nociceptors and their activation there produces meaningful analgesia. The cellular mechanisms downstream of peripheral µORs are not well understood. Here, we show in neurons of murine dorsal root ganglia that pro-nociceptive TRPM3 channels, present in the peripheral parts of nociceptors, are strongly inhibited by µOR activation, much more than other TRP channels in the same compartment, like TRPV1 and TRPA1. Inhibition of TRPM3 channels occurs via a short signaling cascade involving Gβγ proteins, which form a complex with TRPM3. Accordingly, activation of peripheral µORs in vivo strongly attenuates TRPM3-dependent pain. Our data establish TRPM3 inhibition as important consequence of peripheral µOR activation indicating that pharmacologically antagonizing TRPM3 may be a useful analgesic strategy.

There are very few treatments available for people suffering from strong or long-lasting pain. Currently, substances called opioids – which include the well-known drug morphine – are the strongest painkillers. However, these drugs also cause harmful side effects, which makes them less useful.

Like all drugs, opioids mediate their effects by interacting with molecules in the body. In the case of opioids, these interacting molecules belong to a group of receptor proteins called G-protein coupled receptors (or GPCRs for short). These opioid receptors are widely distributed in the nerve cells and brain regions that detect and transmit pain signals. It was poorly understood how activation of opioid receptors reduces the activity of pain-sensing nerve cells, however several lines of evidence had suggested that a protein called TRPM3 might be involved.

TRPM3 is a channel protein that allows sodium and calcium ions to enter into nerve cells by forming pores in cell membranes, and mice that lack this protein are less sensitive to certain kinds of pain. Dembla, Behrendt et al. now show that activating opioid receptors on nerve cells from mice, with morphine and a similar substance, rapidly reduces the flow of calcium ions through TRPM3 channels. Further experiments confirmed that activating opioid receptors in a mouse’s paw also reduced the pain caused when TRPM3 proteins are activated.

GPCRs interact with a group of small proteins called G-proteins that, when activated by the receptor, split into two subunits. Based on studies with human kidney cells, Dembla, Behrendt et al. found the so-called G-beta-gamma subunit then carries the signal from the opioid receptor to TRPM3. Two independent studies by Quallo et al. and Badheka, Yudin et al. also report similar findings.

These new findings show that drugs already used in the treatment of pain can indirectly alter how TRPM3 works in a dramatic way. These results might help scientists to find drugs that work in a more direct way to dial down the activity of TRPM3 and to combat pain with fewer side effects. Though first it will be important to confirm these new findings in human nerve cells.

Potentiation of receptor responses induced by prolonged binding of Gα13 and leukemia-associated RhoGEF

Diverse cellular functions are controlled by RhoA-GTPases, which are activated by trimeric G proteins via RhoGEFs, among others. In this study, we focused on the signaling from GPCRs to RhoA via Gα₁₃ and leukemia-associated RhoGEF (LARG). The activation of Gα₁₃ was elucidated in living cells with high temporal and spatial resolution by means of FRET. The inactivation after agonist withdrawal occurred in the same range (t_1/2 = 25.3 ± 2.2 s; mean ± sem; n = 22) as described for other Gα proteins. The interaction of Gα₁₃ and LARG and the thereby-induced LARG translocation to the plasma membrane were at least 1 order of magnitude more stable after agonist withdrawal, exceeding Gα₁₃ deactivation in the absence of LARG several fold. Consequently, we observed an almost 100-fold higher agonist sensitivity of the Gα₁₃ LARG interaction compared to the Gα₁₃ activation in the absence of LARG.-Bodmann, E.-L., Krett, A.-L., Bünemann, M. Potentiation of receptor responses induced by prolonged binding of Gα₁₃ and leukemia-associated RhoGEF.

SK2 channels regulate mitochondrial respiration and mitochondrial Ca2+ uptake

Mitochondrial calcium ([Ca²⁺]_m) overload and changes in mitochondrial metabolism are key players in neuronal death. Small conductance calcium-activated potassium (SK) channels provide protection in different paradigms of neuronal cell death. Recently, SK channels were identified at the inner mitochondrial membrane, however, their particular role in the observed neuroprotection remains unclear. Here, we show a potential neuroprotective mechanism that involves attenuation of [Ca²⁺]_m uptake upon SK channel activation as detected by time lapse mitochondrial Ca²⁺ measurements with the Ca²⁺-binding mitochondria-targeted aequorin and FRET-based [Ca²⁺]_m probes. High-resolution respirometry revealed a reduction in mitochondrial respiration and complex I activity upon pharmacological activation and overexpression of mitochondrial SK2 channels resulting in reduced mitochondrial ROS formation. Overexpression of mitochondria-targeted SK2 channels enhanced mitochondrial resilience against neuronal death, and this effect was inhibited by overexpression of a mitochondria-targeted dominant-negative SK2 channel. These findings suggest that SK channels provide neuroprotection by reducing [Ca²⁺]_m uptake and mitochondrial respiration in conditions, where sustained mitochondrial damage determines progressive neuronal death.

A reverse signaling pathway downstream of Sema4A controls cell migration via Scrib

Semaphorins comprise a large family of ligands that regulate key cellular functions through their receptors, plexins. In this study, we show that the transmembrane semaphorin 4A (Sema4A) can also function as a receptor, rather than a ligand, and transduce signals triggered by the binding of Plexin-B1 through reverse signaling. Functionally, reverse Sema4A signaling regulates the migration of various cancer cells as well as dendritic cells. By combining mass spectrometry analysis with small interfering RNA screening, we identify the polarity protein Scrib as a downstream effector of Sema4A. We further show that binding of Plexin-B1 to Sema4A promotes the interaction of Sema4A with Scrib, thereby removing Scrib from its complex with the Rac/Cdc42 exchange factor βPIX and decreasing the activity of the small guanosine triphosphatase Rac1 and Cdc42. Our data unravel a role for Plexin-B1 as a ligand and Sema4A as a receptor and characterize a reverse signaling pathway downstream of Sema4A, which controls cell migration.

The mode of agonist binding to a G protein-coupled receptor switches the effect that voltage changes have on signaling

Signaling by many heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) is either enhanced or attenuated by changes in plasma membrane potential. To identify structural correlates of the voltage sensitivity of GPCR signaling, we chose muscarinic acetylcholine receptors (the M1, M3, and M5 isoforms) as a model system. We combined molecular docking analysis with Förster resonance energy transfer (FRET)-based assays that monitored receptor activity under voltage clamp conditions. When human embryonic kidney (HEK) 293 cells expressing the individual receptors were stimulated with the agonist carbachol, membrane depolarization enhanced signaling by the M1 receptor but attenuated signaling by the M3 and M5 receptors. Furthermore, whether membrane depolarization enhanced or inhibited receptor signaling depended on the type of agonist. Membrane depolarization attenuated M3 receptor signaling when the receptor was bound to carbachol or acetylcholine, whereas depolarization enhanced signaling when the receptor was bound to either choline or pilocarpine. Docking calculations predicted that there were two distinct binding modes for these ligands, which were associated with the effect of depolarization on receptor function. From these calculations, we identified a residue in the M3 receptor that, when mutated, would alter the binding mode of carbachol to resemble that of pilocarpine in silico. Introduction of this mutated M3 receptor into cells confirmed that the membrane depolarization enhanced, rather than attenuated, signaling by the carbachol-bound receptor. Together, these data suggest that the directionality of the voltage sensitivity of GPCR signaling is defined by the specific binding mode of each ligand to the receptor.

Science Signaling Podcast for 3 November 2015: Ligands control GPCR voltage sensitivity

How a ligand binds to a G protein–coupled receptor determines whether changes in membrane potential enhance or attenuate signaling.

Membrane Potential Controls the Efficacy of Catecholamine-induced β1-Adrenoceptor Activity

G protein-coupled receptors (GPCRs) are membrane-located proteins and, therefore, are exposed to changes in membrane potential (V(M)) in excitable tissues. These changes have been shown to alter receptor activation of certain Gi-and Gq-coupled GPCRs. By means of a combination of whole-cell patch-clamp and Förster resonance energy transfer (FRET) in single cells, we demonstrate that the activation of the Gs-coupled β1-adrenoreceptor (β1-AR) by the catecholamines isoprenaline (Iso) and adrenaline (Adr) is regulated by V(M). This voltage-dependence is also transmitted to G protein and arrestin 3 signaling. Voltage-dependence of β2-AR activation, however, was weak compared with β1-AR voltage-dependence. Drug efficacy is a major target of β1-AR voltage-dependence as depolarization attenuated receptor activation, even under saturating concentrations of agonists, with significantly faster kinetics than the deactivation upon agonist withdrawal. Also the efficacy of the endogenous full agonist adrenaline was reduced by depolarization. This is a unique finding since reports of natural full agonists at other voltage-dependent GPCRs only show alterations in affinity during depolarization. Based on a Boltzmann function fit to the relationship of V(M) and receptor-arrestin 3 interaction we determined the voltage-dependence with highest sensitivity in the physiological range of membrane potential. Our data suggest that under physiological conditions voltage regulates the activity of agonist-occupied β1-adrenoceptors on a very fast time scale.

Dynamics of G protein effector interactions and their impact on timing and sensitivity of G protein-mediated signal transduction

G protein coupled receptors regulate numerous cellular functions primarily via coupling to heterotrimeric G proteins and subsequent regulation of effector proteins such as ion channels, enzymes or GTP exchange factors for small G proteins. The dynamics of interactions between G protein subunits and effectors have been difficult to study particularly in a cellular context. The introduction of Förster resonance energy transfer methods into the field of GPCR research led to interesting insights into the temporal patterns of interactions between G protein subunits and their effectors. In this review we specifically focus on the interaction of Gαi subunits with adenylyl cyclases and of Gαq subunits with p63RhoGEF or G protein coupled receptor kinases type 2. Comparing the dynamics of these interactions revealed remarkable differences between different G protein effectors regarding the ability to be modulated by members of the regulator of G protein signalling protein family as well as the sensitivity towards receptor activation.

Adenylyl cyclases 5 and 6 underlie PIP3-dependent regulation

Many different neurotransmitters and hormones control intracellular signaling by regulating the production of the second messenger cAMP. The function of the broadly expressed adenylyl cyclases (ACs) 5 and 6 is regulated by either stimulatory or inhibitory G proteins. By analyzing a well-known rebound stimulation phenomenon after withdrawal of Gi protein in atrial myocytes, we discovered that AC5 and -6 are tightly regulated by the second messenger PIP3. By monitoring cAMP levels in real time by means of Förster resonance energy transfer (FRET)-based biosensors, we reproduced the rebound stimulation in a heterologous expression system specifically for AC5 or -6. Strikingly, this cAMP rebound stimulation was completely blocked by the PI3K inhibitor wortmannin, both in atrial myocytes and in transfected human embryonic kidney cells. Similar effects were observed by heterologous expression of the PIP3 phosphatase and tensin homolog (PTEN). However, general kinase inhibitors or inhibitors of Akt had no effect, suggesting a PIP3-dependent mechanism. These findings demonstrate the existence of a novel general pathway for regulation of AC5 and -6 activity via PIP3 that leads to pronounced alterations of cytosolic cAMP levels.

Interhelical interaction and receptor phosphorylation regulate the activation kinetics of different human β1-adrenoceptor variants

G protein-coupled receptors represent the largest class of drug targets, but genetic variation within G protein-coupled receptors leads to variable drug responses and, thereby, compromises their therapeutic application. One of the most intensely studied examples is a hyperfunctional variant of the human β1-adrenoceptor that carries an arginine at position 389 in helix 8 (Arg-389-ADRB1). However, the mechanism underlying the higher efficacy of the Arg-389 variant remained unclear to date. Despite its hyperfunctionality, we found the Arg-389 variant of ADRB1 to be hyperphosphorylated upon continuous stimulation with norepinephrine compared with the Gly-389 variant. Using ADRB1 sensors to monitor activation kinetics by fluorescence resonance energy transfer, Arg-389-ADRB1 exerted faster activation speed and arrestin recruitment than the Gly-389 variant. Both activation speed and arrestin recruitment depended on phosphorylation of the receptor, as shown by knockdown of G protein-coupled receptor kinases and phosphorylation-deficient ADRB1 mutants. Structural modeling of the human β1-adrenoceptor suggested interaction of the side chain of Arg-389 with opposing amino acid residues in helix 1. Site-directed mutagenesis of Lys-85 and Thr-86 in helix 1 revealed that this interaction indeed determined ADRB1 activation kinetics. Taken together, these findings indicate that differences in interhelical interaction regulate the different activation speed and efficacy of ADRB1 variants.

Diacylglycerol mediates regulation of TASK potassium channels by Gq-coupled receptors

The two-pore domain potassium (K2P) channels TASK-1 (KCNK3) and TASK-3 (KCNK9) are important determinants of background K(+) conductance and membrane potential. TASK-1/3 activity is regulated by hormones and transmitters that act through G protein-coupled receptors (GPCR) signalling via G proteins of the Gαq/11 subclass. How the receptors inhibit channel activity has remained unclear. Here, we show that TASK-1 and -3 channels are gated by diacylglycerol (DAG). Receptor-initiated inhibition of TASK required the activity of phospholipase C, but neither depletion of the PLC substrate PI(4,5)P2 nor release of the downstream messengers IP3 and Ca(2+). Attenuation of cellular DAG transients by DAG kinase or lipase suppressed receptor-dependent inhibition, showing that the increase in cellular DAG-but not in downstream lipid metabolites-mediates channel inhibition. The findings identify DAG as the signal regulating TASK channels downstream of GPCRs and define a novel role for DAG that directly links cellular DAG dynamics to excitability.

Engineered hyperphosphorylation of the β2-adrenoceptor prolongs arrestin-3 binding and induces arrestin internalization

G protein-coupled receptor phosphorylation plays a major role in receptor desensitization and arrestin binding. It is, however, unclear how distinct receptor phosphorylation patterns may influence arrestin binding and subsequent trafficking. Here we engineer phosphorylation sites into the C-terminal tail of the β2-adrenoceptor (β2AR) and demonstrate that this mutant, termed β2AR(SSS), showed increased isoprenaline-stimulated phosphorylation and differences in arrestin-3 affinity and trafficking. By measuring arrestin-3 recruitment and the stability of arrestin-3 receptor complexes in real time using fluorescence resonance energy transfer and fluorescence recovery after photobleaching, we demonstrate that arrestin-3 dissociated quickly and almost completely from the β2AR, whereas the interaction with β2AR(SSS) was 2- to 4-fold prolonged. In contrast, arrestin-3 interaction with a β2-adrenoceptor fused to the carboxyl-terminal tail of the vasopressin type 2 receptor was nearly irreversible. Further analysis of arrestin-3 localization revealed that by engineering phosphorylation sites into the β2-adrenoceptor the receptor showed prolonged interaction with arrestin-3 and colocalization with arrestin in endosomes after internalization. This is in contrast to the wild-type receptor that interacts transiently with arrestin-3 at the plasma membrane. Furthermore, β2AR(SSS) internalized more efficiently than the wild-type receptor, whereas recycling was very similar for both receptors. Thus, we show how the interaction between arrestins and receptors can be increased with minimal receptor modification and that relatively modest increases in receptor-arrestin affinity are sufficient to alter arrestin trafficking.

Influence of gαq on the dynamics of m3-acetylcholine receptor-g-protein-coupled receptor kinase 2 interaction

G-protein-coupled receptor kinase 2 (GRK2) is a serine/threonine kinase with an important function in the desensitization of G-protein-coupled receptors. Based on its ability to bind G-protein βγ subunits as well as activated Gαq subunits, it can be considered as an effector for G-proteins. The recruitment of GRK2 to activated receptors is well known to be mediated by Gβγ together with negatively charged membrane phospholipids. In the current study, we address the role of Gαq on the interaction of GRK2 with activated Gq-protein-coupled receptors. Therefore, we established new Förster resonance energy transfer (FRET)-based assays to study the interaction of GRK2 with the M3-acetylcholine (M3-ACh) receptor as well as Gq-protein subunits with high spatiotemporal resolution in single living human embryonic kidney 293T cells. M3-ACh receptor stimulation with 10 µM acetylcholine resulted in distinct changes in FRET, which reflects interaction of the respective proteins. GRK2 mutants with reduced binding affinity toward Gαq [GRK2(D110A)] and Gβγ [GRK2(R587Q)] were used to determine the specific role of Gq-protein-binding by GRK2. Comparison of absolute FRET amplitudes demonstrated that Gαq enhances the extent and stability of the GRK2-M3-ACh receptor interaction, and that not only Gβγ but also Gαq can target GRK2 to the membrane. This reveals an important role of Gαq in efficient recruitment of GRK2 to M3-ACh receptors. Furthermore, interactions between Gαq and GRK2 were associated with a prolongation of the interaction between GRK2 and the M3-ACh receptor and enhanced arrestin recruitment by these receptors, indicating that Gαq influences signaling and desensitization.

The GIP receptor displays higher basal activity than the GLP-1 receptor but does not recruit GRK2 or arrestin3 effectively

Background and Objectives

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are important regulators of insulin secretion, and their functional loss is an early characteristic of type 2 diabetes mellitus (T2DM). Pharmacological levels of GLP-1, but not GIP, can overcome this loss. GLP-1 and GIP exert their insulinotropic effects through their respective receptors expressed on pancreatic β-cells. Both the GLP-1 receptor (GLP-1R) and the GIP receptor (GIPR) are members of the secretin family of G protein-coupled receptors (GPCRs) and couple positively to adenylate cyclase. We compared the signalling properties of these two receptors to gain further insight into why GLP-1, but not GIP, remains insulinotropic in T2DM patients.

Methods

GLP-1R and GIPR were transiently expressed in HEK-293 cells, and basal and ligand-induced cAMP production were investigated using a cAMP-responsive luciferase reporter gene assay. Arrestin3 (Arr3) recruitment to the two receptors was investigated using enzyme fragment complementation, confocal microscopy and fluorescence resonance energy transfer (FRET).

Results

GIPR displayed significantly higher (P<0.05) ligand-independent activity than GLP-1R. Arr3 displayed a robust translocation to agonist-stimulated GLP-1R but not to GIPR. These observations were confirmed in FRET experiments, in which GLP-1 stimulated the recruitment of both GPCR kinase 2 (GRK2) and Arr3 to GLP-1R. These interactions were not reversed upon agonist washout. In contrast, GIP did not stimulate recruitment of either GRK2 or Arr3 to its receptor. Interestingly, arrestin remained at the plasma membrane even after prolonged (30 min) stimulation with GLP-1. Although the GLP-1R/arrestin interaction could not be reversed by agonist washout, GLP-1R and arrestin did not co-internalise, suggesting that GLP-1R is a class A receptor with regard to arrestin binding.

Conclusions

GIPR displays higher basal activity than GLP-1R but does not effectively recruit GRK2 or Arr3.

Comparison of the activation kinetics of the M3 acetylcholine receptor and a constitutively active mutant receptor in living cells

Activation of G-protein-coupled receptors is the first step of the signaling cascade triggered by binding of an agonist. Here we compare the activation kinetics of the G(q)-coupled M(3) acetylcholine receptor (M(3)-AChR) with that of a constitutively active mutant receptor (M(3)-AChR-N514Y) using M(3)-AChR constructs that report receptor activation by changes in the fluorescence resonance energy transfer (FRET) signal. We observed a leftward shift in the concentration-dependent FRET response for acetylcholine and carbachol with M(3)-AChR-N514Y. Consistent with this result, at submaximal agonist concentrations, the activation kinetics of M(3)-AChR-N514Y were significantly faster, whereas at maximal agonist concentrations the kinetics of receptor activation were identical. Receptor deactivation was significantly faster with carbachol than with acetylcholine and was significantly delayed by the N514Y mutation. Receptor-G-protein interaction was measured by FRET between M(3)-AChR-yellow fluorescent protein (YFP) and cyan fluorescent protein (CFP)-Gγ(2). Agonist-induced receptor-G-protein coupling was of a time scale similar to that of receptor activation. As observed for receptor deactivation, receptor-G-protein dissociation was slower for acetylcholine than that for carbachol. Acetylcholine-stimulated increases in receptor-G-protein coupling of M(3)-AChR-N514Y reached only 12% of that of M(3)-AChR and thus cannot be kinetically analyzed. G-protein activation was measured using YFP-tagged Gα(q) and CFP-tagged Gγ(2). Activation of G(q) was significantly slower than receptor activation and indistinguishable for the two agonists. However, G(q) deactivation was significantly prolonged for acetylcholine compared with that for carbachol. Consistent with decreased agonist-stimulated coupling to G(q), agonist-stimulated G(q) activation by M(3)-AChR-N514Y was not detected. Taken together, these results indicate that the N514Y mutation produces constitutive activation of M(3)-AChR by decreasing the rate of receptor deactivation, while having minimal effect on receptor activation.

A polymorphism-specific "memory" mechanism in the β(2)-adrenergic receptor

Signaling through G protein (heterotrimeric guanosine triphosphate-binding protein)-coupled receptors is affected by polymorphisms in receptor-encoding genes. Using fluorescence resonance energy transfer, we found that the β(2)-adrenergic receptor (β(2)AR) responded to repeated activation with altered activation kinetics. Polymorphic variants of the β(2)AR displayed divergent changes of β(2)AR activation kinetics that closely mimicked their different efficacies to generate cyclic adenosine 3',5'-monophosphate. More efficacious variants became faster in their activation kinetics, whereas less efficacious variants became slower, compared to their initial activation. These differences depended on phosphorylation of the receptor by G protein-coupled receptor kinases. Our findings suggest an intrinsic, polymorphism-specific property of the β(2)AR that alters activation kinetics upon continued stimulation and that may account for individual drug responses.

Temporally resolved cAMP monitoring in endothelial cells uncovers a thrombin-induced [cAMP] elevation mediated via the Ca²+-dependent production of prostacyclin

The barrier function of the endothelium is controlled by the second messengers Ca2+ and cAMP that differentially regulate the permeability of endothelial cells. The Ca2+-elevating agent thrombin has been demonstrated to increase endothelial permeability and to decrease cAMP levels as detected via enzyme immunoassays. To study the effects of thrombin on cAMP with high temporal resolution, we utilised the FRET-based cAMP sensor Epac1-camps in single intact human umbilical vein endothelial cells (HUVECs). In these cells, thrombin induced a delayed increase in [cAMP], initiating after about 40 s, with maximum cAMP levels after 130 s of thrombin application. This increase of cAMP levels was Ca2+-dependent, but did not require calmodulin (CaM). Pharmacological approaches revealed that phospholipase A2 (PLA2) activity and cyclooxygenase (COX)-mediated synthesis of prostaglandins was required for the thrombin-induced elevation of [cAMP]. Furthermore, preincubation of HUVECs with a prostacyclin-receptor antagonist significantly reduced the thrombin-induced increase in [cAMP]. We conclude that thrombin causes the synthesis of prostacyclin in endothelial cells and that the subsequent stimulation of Gs-coupled prostacyclin receptors then results in an increase in [cAMP].

G proteins in reverse mode: receptor-mediated GTP release inhibits G protein and effector function

Active G protein-coupled receptors activate heterotrimeric Galphabetagamma proteins by catalyzing the exchange of GDP by GTP at the Galpha subunit. A paradoxical attenuation of G protein-activated inwardly rectifying potassium channels (GIRK) upon stimulation of native cells with high concentrations of agonist is known. However, a deactivation of activated G proteins by active receptors has not been experimentally studied in intact cells. We monitored GIRK currents and G(o) protein activation by means of fluorescence resonance energy transfer (FRET) in parallel. The results suggested that GIRK currents were paradoxically attenuated due to an inactivation of G(o) proteins by active alpha(2A)-adrenergic receptors. To study the mechanisms, G protein activation and receptor-G protein interactions were analyzed as a function of nucleotide type and nucleotide concentrations by means of FRET, while controlling intracellular nucleotides upon permeabilization of the cell membrane. Results suggested a receptor-catalyzed dissociation of GTP from activated heterotrimeric Galphabetagamma. Consequently, nucleotide-free G proteins were sequestrated in heterotrimeric conformation at the active receptor, thus attenuating downstream signaling in an agonist-dependent manner.

Gq-mediated Ca2+ signals inhibit adenylyl cyclases 5/6 in vascular smooth muscle cells

cAMP and Ca(2+) are antagonistic intracellular messengers for the regulation of vascular smooth muscle tone; rising levels of Ca(2+) lead to vasoconstriction, whereas an increase of cAMP induces vasodilatation. Here we investigated whether Ca(2+) interferes with cAMP signaling by regulation of phophodiesterases (PDEs) or adenylyl cyclases (ACs). We studied regulation of cAMP concentrations by Ca(2+) signals evoked by endogenous purinergic receptors in vascular smooth muscle cells (VSMCs). The fluorescence resonance energy transfer (FRET)-based cAMP sensor Epac1-camps allowed the measurement of cAMP levels in single-living VSMCs with subsecond temporal resolution. Moreover, in vitro calibration of Epac1-camps enabled us to estimate the absolute cytosolic cAMP concentrations. Stimulation of purinergic receptors decreased cAMP levels in the presence of the beta-adrenergic agonist isoproterenol. Simultaneous imaging of cAMP with Epac1-camps and of Ca(2+) with Fura 2 revealed a rise of intracellular Ca(2+) in response to purinergic stimulation followed by a decline of cAMP. Chelation of intracellular Ca(2+) and overexpression of Ca(2+)-independent AC4 antagonized this decline of cAMP, whereas pharmacological inhibition of Ca(2+)-activated PDE1 had no effect. AC assays with VSMC membranes revealed a significant attenuation of isoproterenol-stimulated cAMP production by the presence of 2 muM Ca(2+). Furthermore, small interfering RNA (siRNA) knockdown of AC5 and AC6 (the two ACs known to be inhibited by Ca(2+)), significantly reduced the decrease of cAMP upon purinergic stimulation of isoproterenol-prestimulated VSMCs. Taken together, these results implicate a Ca(2+)-mediated inhibition of AC5 and 6 as an important mechanism of purinergic receptor-induced decline of cAMP and show a direct cross talk of these signaling pathways in VSMCs.

Role of Rac 1 and cAMP in endothelial barrier stabilization and thrombin-induced barrier breakdown

Barrier stabilizing effects of cAMP as well as of the small GTPase Rac 1 are well established. Moreover, it is generally believed that permeability-increasing mediators such as thrombin disrupt endothelial barrier functions primarily via activation of Rho A. In this study, we provide evidence that decrease of both cAMP levels and of Rac 1 activity contribute to thrombin-mediated barrier breakdown. Treatment of human dermal microvascular endothelial cells (HDMEC) with Rac 1-inhibitor NSC-23766 decreased transendothelial electrical resistance (TER) and caused intercellular gap formation. These effects were reversed by addition of forskolin/rolipram (F/R) to increase intracellular cAMP but not by the cAMP analogue 8-pCPT-2'-O-Methyl-cAMP (O-Me-cAMP) which primarily stimulates protein kinase A (PKA)-independent signaling via Epac/Rap 1. However, both F/R and O-Me-cAMP did not increase TER above control levels in the presence of NSC-23766 in contrast to experiments without Rac 1 inhibition. Because Rac 1 was required for maintenance of barrier functions as well as for cAMP-mediated barrier stabilization, we tested the role of Rac 1 and cAMP in thrombin-induced barrier breakdown. Thrombin-induced drop of TER and intercellular gap formation were paralleled by a rapid decrease of cAMP as revealed by fluorescence resonance energy transfer (FRET). The efficacy of F/R or O-Me-cAMP to block barrier-destabilizing effects of thrombin was comparable to Y27632-induced inhibition of Rho kinase but was blunted when Rac 1 was inactivated by NSC-23766. Taken together, these data indicate that decrease of cAMP and Rac 1 activity may be an important step in inflammatory barrier disruption.

Real-time monitoring of cAMP levels in living endothelial cells: thrombin transiently inhibits adenylyl cyclase 6

The crosstalk between Ca(2+) and cAMP signals plays a significant role for the regulation of the endothelial barrier function. The Ca(2+)-elevating agent thrombin was demonstrated to increase endothelial permeability and to decrease cAMP levels. Since Ca(2+) and cAMP signals are highly dynamic, we aimed to study the temporal resolution between thrombin-evoked Ca(2+) signals and subsequent changes of cAMP levels. Here we conduct the first real-time monitoring of thrombin-mediated regulation of cAMP signals in intact human umbilical vein endothelial cells (HUVECs) by utilising the Ca(2+)-sensitive dye Fluo-4 and the fluorescence resonance energy transfer (FRET)-based cAMP sensor Epac1-camps. We calibrated in vitro FRET responses of Epac1-camps to [cAMP] in order to estimate changes in intracellular [cAMP] evoked by thrombin treatment of HUVECs. After increasing [cAMP] to 1.2 +/- 0.2 microm by stimulation of HUVECs with isoproterenol (isoprenaline), we observed a transient decrease of cAMP levels by 0.4 +/- 0.1 microm which reached a minimum value 30 s after thrombin application and 15 s after the thrombin-evoked Ca(2+) peak. This transient decrease in [cAMP] was Ca(2+)-dependent and independent of a G(i)-mediated inhibition of adenylyl cyclases (ACs). Instead the knock down of the predominant subtype AC6 in HUVECs provided the first direct evidence that the Ca(2+)-mediated inhibition of AC6 accounts for the thrombin-induced decrease in cAMP levels.

Analysis of receptor oligomerization by FRAP microscopy

Here we describe an approach to investigate di- or oligomerization of transmembrane receptors in living cells with fluorescence recovery after photobleaching (FRAP). We immobilized a defined fraction of receptors with antibodies and then measured lateral mobility of the nonimmobilized fraction by FRAP. We validated this approach with CD86 and CD28 as monomeric and dimeric reference proteins, respectively. Di- or oligomerization of G protein-coupled receptors is strongly debated. We studied human beta-adrenergic receptors as prototypical G protein-coupled receptors and found that beta(1)-AR shows transient interactions whereas beta(2)-AR can form stable oligomers. We propose that this FRAP method can be widely applied to study di- or oligomerization of cell-surface proteins.

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.