Characterization of M2 muscarinic receptor activation of different G protein subtypes

Fusion with Promiscuous Gα16 Subunit Reveals Signaling Bias at Muscarinic Receptors

A complex evaluation of agonist bias at G-protein coupled receptors at the level of G-protein classes and isoforms including non-preferential ones is essential for advanced agonist screening and drug development. Molecular crosstalk in downstream signaling and a lack of sufficiently sensitive and selective methods to study direct coupling with G-protein of interest complicates this analysis. We performed binding and functional analysis of 11 structurally different agonists on prepared fusion proteins of individual subtypes of muscarinic receptors and non-canonical promiscuous α-subunit of G₁₆ protein to study agonist bias. We have demonstrated that fusion of muscarinic receptors with Gα₁₆ limits access of other competitive Gα subunits to the receptor, and thus enables us to study activation of Gα₁₆ mediated pathway more specifically. Our data demonstrated agonist-specific activation of G₁₆ pathway among individual subtypes of muscarinic receptors and revealed signaling bias of oxotremorine towards Gα₁₆ pathway at the M₂ receptor and at the same time impaired Gα₁₆ signaling of iperoxo at M₅ receptors. Our data have shown that fusion proteins of muscarinic receptors with α-subunit of G-proteins can serve as a suitable tool for studying agonist bias, especially at non-preferential pathways.

BRET- and fluorescence anisotropy-based assays for real-time monitoring of ligand binding to M2 muscarinic acetylcholine receptors

BRET and fluorescence anisotropy (FA) are two fluorescence-based techniques used for the characterization of ligand binding to G protein-coupled receptors (GPCRs) and both allow monitoring of ligand binding in real time. In this study, we present the first direct comparison of BRET-based and FA-based binding assays using the human M₂ muscarinic acetylcholine receptor (M₂R) and two TAMRA (5-carboxytetramethylrhodamine)-labeled fluorescent ligands as a model system. The determined fluorescent ligand affinities from both assays were in good agreement with results obtained from radioligand competition binding experiments. The assays yielded real-time kinetic binding data revealing differences in the mechanism of binding for the investigated fluorescent probes. Furthermore, the investigation of various unlabeled M₂R ligands yielded pharmacological profiles in accordance with earlier reported data. Taken together, this study showed that BRET- and FA-based binding assays represent valuable alternatives to radioactivity-based methods for screening purposes and for a precise characterization of binding kinetics supporting the exploration of binding mechanisms.

The constitutive activity of melanocortin-4 receptors in cAMP pathway is allosterically modulated by zinc and copper ions

Melanocortin-4 receptors (MC₄ R) are unique among G-protein-coupled receptors (GPCRs) as they have endogenous ligands that can exhibit inverse agonistic properties in the case of elevated basal activity. It is known that the constitutive activity of GPCRs strongly affects the ligand-dependent physiological responses, but little is known about these regulatory mechanisms. Since several metal ions have been shown to be important modulators of the signal transduction of GPCRs, we hypothesized that metal ions regulate the basal activity of MC₄ Rs. Implementation of a fluorescence anisotropy assay and novel redshifted fluorescent peptides enabled kinetic characterization of ligand binding to MC₄ R expressed on budded baculoviruses. We show that Ca²⁺ is required for high-affinity ligand binding, but Zn²⁺ and Cu²⁺ in the presence of Ca²⁺ behave as negative allosteric modulators of ligand binding to MC₄ R. FRET-based cAMP biosensor was used to measure the activation of MC₄ R stably expressed in CHO-K1 cells. At low micromolar concentrations, Zn²⁺ caused MC₄ R-dependent activation of the cAMP pathway, whereas Cu²⁺ reduced the activity of MC₄ R even below the basal level. These findings indicate that at physiologically relevant concentrations can Zn²⁺ and Cu²⁺ function as MC₄ R agonists or inverse agonists, respectively. This means that depending on the level of constitutive activity induced by Zn²⁺ ions, the pharmacological effect of orthosteric ligands of MC₄ R can be switched from a partial to an inverse agonist. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. More information about the Open Science badges can be found at https://cos.io/our-services/open-science-badges/.

Characterization of 5-HT₁A receptors and their complexes with G-proteins in budded baculovirus particles using fluorescence anisotropy of Bodipy-FL-NAN-190

Bodipy-FL-NAN-190 was found to be well suited for characterization of ligand binding to 5-HT1A receptors expressed in budded baculovirus particles, as binding is accompanied by large increases in fluorescence intensity and anisotropy. This ligand appears to bind rapidly (t1/2,ass<1 min), reversibly (t1/2,diss∼6 min) and has high affinity (Kd=0.30 ± 0.13 nM). This fluorescence anisotropy assay based on Bodipy-FL-NAN-190 binding to baculovirus particles was also a suitable assay system for the pharmacological characterization of non-labelled serotonergic ligands, as well as being sensitive to the presence of G-proteins and guanine nucleotides. Coexpression of αi subunits of human G-proteins in baculovirus particles resulted in the appearance of significantly greater proportion of nucleotide sensitive high affinity agonist binding sites. There were no significant differences between αi1 and αi3 subtypes, while ligand binding in the presence of αi2 had higher sensitivity to GDP and Mn(2+).

Biarsenical ligands bind to endogenous G-protein α-subunits and enable allosteric sensing of nucleotide binding

BACKGROUND

Heterotrimeric G-proteins relay extracellular signals to intracellular effector proteins. Multiple methods have been developed to monitor their activity; including labeled nucleotides and biosensors based on genetically engineered G-proteins. Here we describe a method for monitoring unlabeled nucleotide binding to endogenous G-proteins α-subunits in a homogeneous assay based on the interaction of 4',5'-bis(1,2,3-dithioarsolan-2-yl)-2',7'-difluorofluorescein (F2FlAsH) with G-protein α-subunits.

RESULTS

The biarsenic fluorescent ligand F2FlAsH binds to various wild-type G-protein α-subunits (αi1, αi2, αi3, αslong, αsshort, αolf, αq, α13) via high affinity As-cysteine interactions. This allosteric label enables real time monitoring of the nucleotide bound states of α-subunits via changes in fluorescence anisotropy and intensity of their F2FlAsH-complexes. We have found that different α-subunits displayed different signal amplitudes when interacting with F2FlAsH, being more sensitive to nucleotide binding to αi, αs, αolf and αq than to α13. Addition of nucleotides to F2FlAsH-labeled α-subunits caused concentration-dependent effects on their fluorescence anisotropy. pEC50 values of studied nucleotides depended on the subtype of the α-subunit and were from 5.7 to 8.2 for GTPγS, from 5.4 to 8.1 for GppNHp and from 4.8 to 8.2 for GDP and lastly up to 5.9 for GMP. While GDP and GMP increased the fluorescence anisotropy of F2FlAsH complexes with αi-subunits, they had the opposite effect on the other αβγM complexes studied.

CONCLUSIONS

Biarsenical ligands interact allosterically with endogenous G-protein α-subunits in a nucleotide-sensitive manner, so the presence or absence of guanine nucleotides has an effect on the fluorescence anisotropy, intensity and lifetime of F2FlAsH-G-protein complexes.

Millimolar Mn2+ influences agonist binding to 5-HT1A receptors by inhibiting guanosine nucleotide binding to receptor-coupled G-proteins

Manganese is an essential trace element but its overexposure causes poisoning (called manganism) that shares several symptoms with Parkinson's disease, but with a mechanism that is still not well understood: in addition to involvement of the dopaminergic system, both serotonergic and peptiergic systems have been implicated. In the present report we have studied the influence of Mn(2+) on 5-HT(1A) receptor signaling complexes in rat brain and found that Mn(2+) in millimolar concentration caused an increase of high-affinity agonist binding to rat hippocampal membranes in comparison with experiments in the presence of Mg(2+), but not in rat cortical membranes and in Sf9 cell membranes expressing 5-HT(1A) receptors and G(i1) heterotrimers. Activation of G proteins with 30μM GTPγS turned all 5-HT(1A) receptors in these preparations into a low-affinity state for agonist binding in the presence of 1mM Mg(2+), but not in the presence of 1mM Mn(2+) in rat hippocampal membranes. However, if 1μM GTPγS was used for G protein activation, a substantial amount of high affinity agonist binding was detected in the presence of Mn(2+) also in cortical membranes and Sf9 cells, but not with Mg(2+) or EDTA. Comparison of the abilities of GDP and GTPγS to modulate high affinity agonist binding to 5-HT(1A) receptors indicated that both nucleotides were almost 10-fold less potent in the presence of MnCl(2) compared to MgCl(2). This means that by inhibiting guanosine nucleotide binding to G proteins in complex with 5-HT(1A) receptors, Mn(2+) acts as an enhancer for agonist binding and signal transduction. As the influence of Mn(2+) resembles the hypersensitivity of dopaminergic system in Parkinsonial models, it can be proposed that at least some symptoms of manganism are connected with a change of signal transduction complex caused by manganese-nucleotide complexes.

Functional characterization of heterotrimeric G-proteins in rat diaphragm muscle

Seven-transmembrane receptors mediate diverse skeletal muscle responses for a wide variety of stimuli, via activation of heterotrimeric G-proteins. Herein we evaluate the expression and activation of rat diaphragm or cultured skeletal muscle G-proteins using [(35)S]GTPγS. Total membrane Gα subunit content was 4-7 times higher in rat primary cultured myotubes and L6 cell line than in diaphragm (32.6±1.2fmol/mg protein) and 7-27% of them were in the active conformational state. Immunoprecipitation assay showed equal expression of diaphragm Gαs, Gαq and Gαi/o. Addition of GDP allowed the measurement of G-protein activation by different GPCR, including adrenoceptor, adenosine, melatonin and muscarinic receptors. Diaphragm denervation resulted in a marked increase in both total and active state G-protein levels. Together, the results show that [(35)S]GTPγS binding assay is a sensitive and valuable method to evaluate GPCR activity in skeletal muscle cells, which is of particular interest for pharmacological analysis of drugs with potential use in the management of respiratory muscle failure.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

Alpha 2A-adrenoceptor-specific stimulation of [35S]GTP gamma S binding to membrane preparations of rat frontal cortex

Functional activation of alpha 2A adrenergic receptors in the crude membranes from rat frontal cortex was studied by a [35S]-guanosine 5'-O-(gamma-thiotriphosphate) ([35S]GTP gamma S) binding assay. alpha 2A agonists UK14304 and guanfacine decreased the ability of GDP to compete with [35S]GTP gamma S binding to the membranes and 0.1 mM GDP was found to be optimal for the following functional experiments. However, even after careful optimization of experimental conditions the specificity of ligands for rat alpha 2 adrenoceptors were not sufficient, as agonists as well as antagonists became activators of other signal transduction systems before achieving their maximal effect in the alpha 2A-adrenergic system. Only using compromising concentration of agonist (up to 1 microM UK14304) and antagonist (up to 1 microM RS79948) to inhibit agonist's effect, allowed us to filtrate out alpha 2A specific effect for characterization of signal transduction in rat frontal cortex membranes for the comparison efficacies of this system for different animals from behavioral experiments.

Modulation of dopamine D1 receptor signaling by adenosine A1 receptors in Sf9 cells requires expression of Gi proteins

There are several evidences that some functions of D1 dopamine receptors can be modulated by colocalized adenosine A1 receptors. To elucidate the role of particular components of the receptor complex in the ligand binding and second messenger activation level we have used Sf9 cell expression system. The expression of D1 and A1 receptors was confirmed by proper binding of specific radioligands [3H]SCH23390 (Kd=1.1+/-0.1 nM, Bmax=2.2+/-0.1 pmol/mg protein) and [3H]DPCPX (Kd=2.1+/-0.8nM, Bmax=2.9+/-0.4 pmol/mg protein), respectively. The kinetics of [3H]SCH23390 binding corresponded to the simplest reversible bimolecular binding reaction of complex formation, with k(on)=0.20+/-0.02 min(-1)nM(-1) and k(off)=0.13+/-0.01 min(-1). Dopaminergic agonists increased the accumulation of cAMP in the transfected cells in concentration-dependent manner, indicating a correct coupling of receptor to second messenger system. The coupling of the A1 receptor to Gi proteins was confirmed by both GTPgammaS dependent agonist binding and inhibition of cAMP accumulation by N-cyclopentyladenosine (NCPA). Activation of the A1 receptor by NCPA had no significant influence on neither affinities of dopaminergic ligands nor the radioligand binding kinetics to the co-exprssed D1 receptors in Sf9 cell membranes. On the other hand, the activation of the A1 receptors inhibited the D1 receptor-specific accumulation of cAMP, but only in cells where Gi proteins were expressed with the receptors.

Characteristics of Binding of [3H]WAY100635 to Rat Hippocampal Membranes

Kinetic analysis of binding of [(3)H][N-[2-[4-(2-[O-methyl-(3)H]methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexane carboxamide ([(3)H]WAY100635) to 5-HT(1A) receptors in rat hippocampal membranes has revealed complex regulation mechanism for this radioligand. Saturation binding experiments revealed that [(3)H]WAY100635 binds to a single class of receptors with very high apparent affinity (K (D) = 87 +/- 4 pM, B (max) = 15.1 +/- 0.2 fmol/mg protein). The binding was almost irreversible, as the dissociation rate constant obtained k (off) = (7.8 +/- 1.1) x 10(-3) min(-1), means that equilibrium with this radioligand cannot be achieved before 7.5 h incubation at 25 degrees C. Systematic association kinetic studies of [(3)H]WAY100635 binding revealed sharp reaction acceleration at higher radioligand concentration, proposing mechanism of positive cooperativity. The affinities of antagonists determined from competition with [(3)H]WAY100635 did not coincide with their abilities to inhibit 5-HT-dependent activation of [(35)S]GTPgammaS binding probably due to the ligand's kinetic peculiarities. Thus, [(3)H]WAY100635 appears to be an excellent tool for determining receptor binding sites, but its applicability in equilibrium studies is strongly limited.

Constitutive activity of muscarinic acetylcholine receptors

We review the literature describing constitutive activity of the five muscarinic acetylcholine receptors in native and recombinant systems and discuss the effect of constitutive activity on muscarinic pharmacology in the context of modern models of receptor activation. We include a summary of mutations found to cause constitutive activity and discuss the implications of these data for the structure, function, and activation mechanism of muscarinic receptors. Finally, we discuss the possible physiological significance of constitutive activity of muscarinic receptors, incorporating information provided by targeted deletion of each of the muscarinic subtypes.

An investigation of whether agonist-selective receptor conformations occur with respect to M2 and M4 muscarinic acetylcholine receptor signalling via Gi/o and Gs proteins

1. A range of muscarinic acetylcholine (mACh) receptor agonists (methacholine (MCh), oxotremorine-M (OXO-M), oxotremorine (OXO), arecoline (AREC), bethanechol (BETH), pilocarpine (PILO)) have been investigated with respect to their binding to, and activation of, M(2) and M(4) mACh receptors, recombinantly expressed in Chinese hamster ovary cells, to explore the possibility that these agonists may differentially affect mACh receptor-G(i/o) and -G(s) coupling. 2. M(2)/M(4) mACh receptor coupling to the adenylyl cyclase/cyclic AMP signalling pathway has been explored in intact cells. G(i/o)-mediated negative coupling to adenylyl cyclase was explored functionally by assessing the ability of the mACh receptor agonists to inhibit forskolin-stimulated enzymic activity. Following pertussis toxin treatment (100 ng ml(-1), 18-20 h) to inactivate G(i/o) proteins, each agonist caused a G(s)-mediated enhancement of forskolin-stimulated adenylyl cyclase activity. 3. At both M(2) and M(4) mACh receptors, all agonists tested were more potent in mediating G(i/o)- versus G(s)-coupled responses. This difference (determined as the pIC(50) (G(i/o) coupling) minus pEC(50) (G(s) coupling) value) was greatest for AREC (65-75-fold) and least for BETH and PILO (</=10-fold). 4. Using apparent binding affinities (pK(B)), and potency (EC(50)/IC(50)) and responsiveness (E(max)/I(max)) estimates, relative efficacy (e(rel)) values for each agonist with respect to M(2) and M(4) mACh receptor coupling to G(i/o)- and G(s)-mediated signalling were also calculated. While the e(rel) values obtained for MCh and OXO-M in CHO-m2 cells were similar, OXO-M behaved as a 'super-agonist' at the M(4) mACh receptor giving greater e(rel) values for both G(i/o) and G(s) coupling relative to MCh. 5. The experimental data indicate that while interesting differences between agonists with respect to M(2)/M(4) mACh receptor activation and receptor-G(i/o) and -G(s) coupling can be discerned, no clear examples of agonist trafficking of signal have emerged.