Constitutively active G protein-coupled receptors: potential mechanisms of receptor activation

Analysis of the structure and dynamics of human serum albumin

Human serum albumin (HSA) is a biologically relevant protein that binds a variety of drugs and other small molecules. No less than 50 structures are deposited in the RCSB Protein Data Bank (PDB). Based on these structures, we first performed a clustering analysis. Despite the diversity of ligands, only two well defined conformations are detected, with a deviation of 0.46 nm between the average structures of the two clusters, while deviations within each cluster are smaller than 0.08 nm. Those two conformations are representative of the apoprotein and the HSA-myristate complex already identified in previous literature. Considering the structures within each cluster as a representative sample of the dynamical states of the corresponding conformation, we scrutinize the structural and dynamical differences between both conformations. Analysis of the fluctuations within each cluster set reveals that domain II is the most rigid one and better matches both structures. Then, taking this domain as reference, we show that the structural difference between both conformations can be expressed in terms of twist and hinge motions of domains I and III, respectively. We also characterize the dynamical difference between conformations by computing correlations and principal components for each set of dynamical states. The two conformations display different collective motions. The results are compared with those obtained from the trajectories of short molecular dynamics simulations, giving consistent outcomes. Let us remark that, beyond the relevance of the results for the structural and dynamical characterization of HAS conformations, the present methodology could be extended to other proteins in the PDB archive.

Investigation of the influence of external factors on the conformational dynamics of rhodopsin-like receptors by means of molecular dynamics simulation

The study reports about the influence of binding of orthosteric ligands on the conformational dynamics of β-2-adrenoreceptor. Using molecular dynamics (MD) simulation, we found that there was a little fraction of active states of the receptor in its apo (ligand-free) ensemble. Analysis of MD trajectories indicated that such spontaneous activation of the receptor is accompanied by the motion in intracellular part of its alpha-helices. Thus, receptor's constitutive activity directly results from its conformational dynamics. On the other hand, the binding of a full agonist resulted in a significant shift of the initial equilibrium towards its active state. Finally, the binding of the inverse agonist stabilized the receptor in its inactive state. It is likely that the binding of inverse agonists might be a universal way of constitutive activity inhibition in vivo. Our results indicate that ligand binding redistribute pre-existing conformational degrees of freedom (in accordance to the Monod-Wyman-Changeux Model) of the receptor rather than cause induced fit in it. Therefore, the ensemble of biologically relevant receptor conformations is encoded in its spatial structure, and individual conformations from that ensemble might be used by the cell in conformity with the physiological behavior.

Dynamics of heme complexed with human serum albumin: a theoretical approach

Human serum albumin (HSA) is the most abundant protein in the blood serum. It binds several ligands and has an especially strong affinity for heme, hence becoming a natural candidate for oxygen transport. In order to analyze the interaction of HSA-heme, molecular dynamics simulations of HSA with bound heme were performed. Based on the results of X-ray diffraction, the binding site of the heme, localized in subdomain IB, was considered. We analyzed the fluctuations and their correlations along trajectories to detect collective motions. The role of H bonds and salt bridges in the stabilization of heme in its pocket was also investigated. Complementarily, the localization of water molecules in the hydrophobic pocket and the interaction with heme were discussed.

Dynamics may significantly influence the estimation of interatomic distances in biomolecular X-ray structures

Atomic positions obtained by X-ray crystallography are time and space averages over many molecules in the crystal. Importantly, interatomic distances, calculated between such average positions and frequently used in structural and mechanistic analyses, can be substantially different from the more appropriate time-average and ensemble-average interatomic distances. Using crystallographic B-factors, one can deduce corrections, which have so far been applied exclusively to small molecules, to obtain correct average distances as a function of the type of atomic motion. Here, using 4774 high-quality protein X-ray structures, we study the significance of such corrections for different types of atomic motion. Importantly, we show that for distances shorter than 5 Å, corrections greater than 0.5 Å may apply, especially for noncorrelated or anticorrelated motion. For example, 14% of the studied structures have at least one pair of atoms with a correction of ≥ 0.5 Å in the case of noncorrelated motion. Using molecular dynamics simulations of villin headpiece, ubiquitin, and SH3 domain unit cells, we demonstrate that the majority of average interatomic distances in these proteins agree with noncorrelated corrections, suggesting that such deviations may be truly relevant. Importantly, we demonstrate that the corrections do not significantly affect stereochemistry and the overall quality of final refined X-ray structures, but can provide marked improvements in starting unrefined models obtained from low-resolution X-ray data. Finally, we illustrate the potential mechanistic and biological significance of the calculated corrections for KcsA ion channel and show that they provide indirect evidence that motions in its selectivity filter are highly correlated.

Mu opioid receptor mRNA expression, binding, and functional coupling to G-proteins in human epileptic hippocampus

Mu opioid receptors (MOR) are known to be involved in seizure activity. The main goal of the present study was to characterize the MOR mRNA expression, binding, as well as G protein activation mediated by these receptors in epileptic hippocampus of patients with pharmacoresistant mesial temporal lobe epilepsy (TLE). In contrast with autopsy samples, hippocampus obtained from patients with mesial TLE demonstrated enhanced MOR mRNA expression (116%). Saturation binding experiments revealed significantly higher (60%) B(max) values for the mesial TLE group, whereas the K(d) values were not statistically different. Although mesial TLE group demonstrated high levels of basal binding for the G proteins (136%), DAMGO-stimulated [(35)S]GTPγS binding did not demonstrate significant alterations. In conclusion, our present data provide strong evidence that the epileptic hippocampus of patients with pharmacoresistant mesial TLE presents significant alterations in MOR. Such changes may represent adaptive mechanisms to compensate for other as yet unknown alterations.

Cholinergic receptor signaling modulates spontaneous firing of sinoatrial nodal cells via integrated effects on PKA-dependent Ca(2+) cycling and I(KACh)

Prior studies indicate that cholinergic receptor (ChR) activation is linked to beating rate reduction (BRR) in sinoatrial nodal cells (SANC) via 1) a G(i)-coupled reduction in adenylyl cyclase (AC) activity, leading to a reduction of cAMP or protein kinase A (PKA) modulation of hyperpolarization-activated current (I(f)) or L-type Ca(2+) currents (I(Ca,L)), respectively; and 2) direct G(i)-coupled activation of ACh-activated potassium current (I(KACh)). More recent studies, however, have indicated that Ca(2+) cycling by the sarcoplasmic reticulum within SANC (referred to as a Ca(2+) clock) generates rhythmic, spontaneous local Ca(2+) releases (LCR) that are AC-PKA dependent. LCRs activate Na(+)-Ca(2+) exchange (NCX) current, which ignites the surface membrane ion channels to effect an AP. The purpose of the present study was to determine how ChR signaling initiated by a cholinergic agonist, carbachol (CCh), affects AC, cAMP, and PKA or sarcolemmal ion channels and LCRs and how these effects become integrated to generate the net response to a given intensity of ChR stimulation in single, isolated rabbit SANC. The threshold CCh concentration ([CCh]) for BRR was approximately 10 nM, half maximal inhibition (IC(50)) was achieved at 100 nM, and 1,000 nM stopped spontaneous beating. G(i) inhibition by pertussis toxin blocked all CCh effects on BRR. Using specific ion channel blockers, we established that I(f) blockade did not affect BRR at any [CCh] and that I(KACh) activation, evidenced by hyperpolarization, first became apparent at [CCh] > 30 nM. At IC(50), CCh reduced cAMP and reduced PKA-dependent phospholamban (PLB) phosphorylation by approximately 50%. The dose response of BRR to CCh in the presence of I(KACh) blockade by a specific inhibitor, tertiapin Q, mirrored that of CCh to reduced PLB phosphorylation. At IC(50), CCh caused a time-dependent reduction in the number and size of LCRs and a time dependent increase in LCR period that paralleled coincident BRR. The phosphatase inhibitor calyculin A reversed the effect of IC(50) CCh on SANC LCRs and BRR. Numerical model simulations demonstrated that Ca(2+) cycling is integrated into the cholinergic modulation of BRR via LCR-induced activation of NCX current, providing theoretical support for the experimental findings. Thus ChR stimulation-induced BRR is entirely dependent on G(i) activation and the extent of G(i) coupling to Ca(2+) cycling via PKA signaling or to I(KACh): at low [CCh], I(KACh) activation is not evident and BRR is attributable to a suppression of cAMP-mediated, PKA-dependent Ca(2+) signaling; as [CCh] increases beyond 30 nM, a tight coupling between suppression of PKA-dependent Ca(2+) signaling and I(KACh) activation underlies a more pronounced BRR.

Conformational movement of F251 contributes to the molecular mechanism of constitutive activation in the C5a receptor

The activation mechanism of G-protein-coupled receptors triggered upon binding of a ligand represents a very important 'conformational switch' in the biological array of signal transduction. However, the molecular and functional details for this activation switch remain unknown. Random saturation mutagenesis data on the complement factor 5a receptor has provided a large data set of mutants including several constitutively active mutants. In the present study, we employed computational modeling to rationalize the constitutive activity for two constitutively active mutants, NQ (I124N/L127Q) and F251A, and we then made predictions for a series of mutants that either promote or constrain constitutive activity. Biological testing of the site-directed mutants confirmed most of the predictions of the computational modeling. These results support a molecular mechanism of constitutive activity in complement factor 5a receptor mutants that is associated with conformational changes in a network of residues neighboring F251 as the focal point of origin.

Common structural requirements for heptahelical domain function in class A and class C G protein-coupled receptors

G protein-coupled receptors (GPCRs) are key players in cell communication. Several classes of such receptors have been identified. Although all GPCRs possess a heptahelical domain directly activating G proteins, important structural and sequence differences within receptors from different classes suggested distinct activation mechanisms. Here we show that highly conserved charged residues likely involved in an interaction network between transmembrane domains (TM) 3 and 6 at the cytoplasmic side of class C GPCRs are critical for activation of the gamma-aminobutyric acid type B receptor. Indeed, the loss of function resulting from the mutation of the conserved lysine residue into aspartate or glutamate in the TM3 of gamma-aminobutyric acid type B(2) can be partly rescued by mutating the conserved acidic residue of TM6 into either lysine or arginine. In addition, mutation of the conserved lysine into an acidic residue leads to a nonfunctional receptor that displays a high agonist affinity. This is reminiscent of a similar ionic network that constitutes a lock stabilizing the inactive state of many class A rhodopsin-like GPCRs. These data reveal that despite their original structure, class C GPCRs share with class A receptors at least some common structural feature controlling G protein activation.

Generalized correlation for biomolecular dynamics

Correlated motions in biomolecules are often essential for their function, e.g., allosteric signal transduction or mechanical/thermodynamic energy transport. Because correlated motions in biomolecules remain difficult to access experimentally, molecular dynamics (MD) simulations are particular useful for their analysis. The established method to quantify correlations from MD simulations via calculation of the covariance matrix, however, is restricted to linear correlations and therefore misses part of the correlations in the atomic fluctuations. Herein, we propose a general statistical mechanics approach to detect and quantify any correlated motion from MD trajectories. This generalized correlation measure is contrasted with correlations obtained from covariance matrices for the B1 domain of protein G and T4 lysozyme. The new method successfully quantifies correlations and provides a valuable global overview over the functionally relevant collective motions of lysozyme. In particular, correlated motions of helix 1 together with the two main lobes of lysozyme are detected, which are not seen by the conventional covariance matrix. Overall, the established method misses more than 50% of the correlation. This failure is attributed to both, an interfering and unnecessary dependence on mutual orientations of the atomic fluctuations and, to a lesser extent, attributed to nonlinear correlations. Our generalized correlation measure overcomes these problems and, moreover, allows for an improved understanding of the conformational dynamics by separating linear and nonlinear contributions of the correlation.

Constitutive activity of endogenous receptors by inducible Gq overexpression

We have developed an inducible cell line that transiently expresses Gq alpha G protein subunits in response to doxycycline. HEK293/Tet-On pBI(Gq alpha) cells worked consistently, achieving high and tightly regulated levels of Gq alpha overexpression (38-fold increase compared with non-induced cells). We investigated the possibility of using an inducible system to increase the proportion of constitutively active endogenously expressed G protein-coupled receptors (GPCRs) by overexpressing Gq alpha. Not only did we observe an increase in basal activity following doxycycline treatment, but also increased intrinsic activity of agonists such as carbachol, endothelin, lysophosphatidic acid (LPA), and bradykinin. Furthermore, carbachol and LPA potency increased following Gq alpha overexpression, as did the intrinsic activity of the partial agonist pilocarpine, observations indicative of constitutive activity. An inducible cell line, transiently expressing G proteins, can therefore be employed to induce constitutive activity of endogenously expressed GPCRs. This model system could be used to identify clinically important inverse agonists.

Spinophilin regulates Ca2+ signalling by binding the N-terminal domain of RGS2 and the third intracellular loop of G-protein-coupled receptors

Signalling by G proteins is controlled by the regulator of G-protein signalling (RGS) proteins that accelerate the GTPase activity of Galpha subunits and act in a G-protein-coupled receptor (GPCR)-specific manner. The conserved RGS domain accelerates the G subunit GTPase activity, whereas the variable amino-terminal domain participates in GPCR recognition. How receptor recognition is achieved is not known. Here, we show that the scaffold protein spinophilin (SPL), which binds the third intracellular loop (3iL) of several GPCRs, binds the N-terminal domain of RGS2. SPL also binds RGS1, RGS4, RGS16 and GAIP. When expressed in Xenopus laevis oocytes, SPL markedly increased inhibition of alpha-adrenergic receptor (alphaAR) Ca2+ signalling by RGS2. Notably, the constitutively active mutant alphaAR(A293E) (the mutation being in the 3iL) did not bind SPL and was relatively resistant to inhibition by RGS2. Use of betaAR-alphaAR chimaeras identified the 288REKKAA293 sequence as essential for the binding of SPL and inhibition of Ca2+ signalling by RGS2. Furthermore, alphaAR-evoked Ca2+ signalling is less sensitive to inhibition by SPL in rgs2-/- cells and less sensitive to inhibition by RGS2 in spl-/- cells. These findings provide a general mechanism by which RGS proteins recognize GPCRs to confer signalling specificity.

Development of a semi-quantitative plate-based ?-galactosidase gene reporter forSchizosaccharomyces pombe and its use to isolate a constitutively active Mam2

To extend the tools available for biochemical and genetical analysis in the fission yeast Schizosaccharomyces pombe we have investigated the development of gene reporter systems using the secreted alpha-galactosidase encoded by the Sz. pombe ORF SPAC869.07c (CAB60017), which we propose naming Mel1p to reflect its structural and functional similarity to MEL1p in Saccharomyces cerevisiae. The alpha-galactosidase activity can be monitored in liquid assays and converted the colourless substrate 5-bromo-4-chloro-3-indolyl-alpha-D-galactopyranoside (X-alpha-gal) into an insoluble blue product that was suitable for semi quantitative plate-based assays; colonies expressing the highest levels of alpha-galactosidase developed the most intense blue colour. Unlike assays based on beta-galactosidase, the Sz. pombe colonies develop the blue colouration under normal growth conditions, avoiding the need to replicate colonies to fresh plates for analysis. It is therefore suitable for screening large numbers of colonies. To illustrate the use of mel1 as a reporter we linked expression to the sxa2 gene promoter to provide a convenient readout for signalling through the pheromone response pathway. The sxa2 > mel1 strain identified constitutively active Mam2 pheromone receptors from a randomly mutagenised library. There was an approximate correlation between the intensity of the blue colour developed by each mutant colony and its level of constitutive activity and we identified a subset of mutants with low constitutive activity that could not have been isolated by a previous screen using nutritional selection. The mel1 alpha-galactosidase activity identified and characterised in this study can be easily adapted to provide a gene reporter for many biological processes and is a new addition to the research tools available in Sz. pombe.

Toward the active conformations of rhodopsin and the beta2-adrenergic receptor

Using sets of experimental distance restraints, which characterize active or inactive receptor conformations, and the X-ray crystal structure of the inactive form of bovine rhodopsin as a starting point, we have constructed models of both the active and inactive forms of rhodopsin and the beta2-adrenergic G-protein coupled receptors (GPCRs). The distance restraints were obtained from published data for site-directed crosslinking, engineered zinc binding, site-directed spin-labeling, IR spectroscopy, and cysteine accessibility studies conducted on class A GPCRs. Molecular dynamics simulations in the presence of either "active" or "inactive" restraints were used to generate two distinguishable receptor models. The process for generating the inactive and active models was validated by the hit rates, yields, and enrichment factors determined for the selection of antagonists in the inactive model and for the selection of agonists in the active model from a set of nonadrenergic GPCR drug-like ligands in a virtual screen using ligand docking software. The simulation results provide new insights into the relationships observed between selected biochemical data, the crystal structure of rhodopsin, and the structural rearrangements that occur during activation.

Selective subunit antagonists suggest an inhibitory relationship between NR2B and NR2A-subunit containing N-methyl-d-aspartate receptors in hippocampal slices

Glutamate receptors responding to N-methyl-D: -aspartate (NMDA) are involved in neural development, excitotoxicity and neuronal plasticity. Each receptor includes at least two NR2 subunits. Here, we have examined the effects of selective antagonists of NR2A and NR2B subunits (NVP-AAM07 and Ro25-6981 respectively) on the effects of NMDA in the CA1 field of rat hippocampal slices. We have observed that Ro25-6981 potentiates, rather than blocks, the effects of NMD on field EPSPs and paired-pulse interactions (indicators of presynaptic effects) and on postsynaptic depolarisation in hippocampal slices. The NR2A subunit antagonist NVP-AAM077 blocks the effects of NMDA alone, or after potentiation by Ro25-6981. The potentiation of NMDA by Ro25-6981 was not prevented by staurosporine (protein kinase inhibitor), okadaic acid (an inhibitor of serine/threonine protein phosphatases) or anisomycin (protein synthesis inhibitor), but was prevented by cyclosporin A, which inhibits Ca2+/calmodulin-dependent phosphatase 2B [calcineurin]. NMDA-dependent long-term potentiation (LTP) induced by electrical stimulation was not prevented by Ro25-6981 but was prevented by selective blockade of the NR2A subunit. The results suggest that, at both presynaptic and postsynaptic sites in the rat hippocampus, NR2B-subunit-containing receptors limit NMDA receptor function by inhibitory restraint over NR2A-subunit-containing receptors, via calcineurin activation, and that LTP induction critically involves primarily receptors containing the NR2A subunit. Endogenous factors or drugs that modify this NR2B/NR2A interaction could have a major influence on synaptic transmission and plasticity in the brain.

Ligand-Binding Modes in Cationic Biogenic Amine Receptors

The binding site in G protein-coupled cationic biogenic amine receptors is formed in the cleft of the seven transmembrane segments. Upon binding the ligand, the receptors are activated or inactivated through the conformational changes of the transmembrane segments. G protein-coupled receptors bind four functionally distinct ligands; inverse agonists, antagonists, partial agonists, and full agonists. Hence, putative structural models for biogenic amine receptors corresponding to the ligand function (inverse agonist-, antagonist-, partial agonist-, and full agonist-bound receptor models) were built by using photointermediate models in the rhodopsin photocascade (M. Ishiguro et al. ChemBioChem. 2004, 5, 298-310). The ligand-receptor recognition of each was examined by modeling receptor-ligand complexes with functional ligands. The complex models suggested that each functional ligand binds the corresponding receptor structure and that ligand-specific interactions contribute to stabilization of the corresponding receptor structure.

The human formyl peptide receptor as model system for constitutively active G-protein-coupled receptors

According to the two-state model of G-protein-coupled receptor (GPCR) activation, GPCRs isomerize from an inactive (R) state to an active (R*) state. In the R* state, GPCRs activate G-proteins. Agonist-independent R/R* isomerization is referred to as constitutive activity and results in an increase in basal G-protein activity, i.e. GDP/GTP exchange. Agonists stabilize the R* state and further increase, whereas inverse agonists stabilize the R state and decrease, basal G-protein activity. Constitutive activity is observed in numerous wild-type GPCRs and disease-causing GPCR mutants with increased constitutive activity. The human formyl peptide receptor (FPR) exists in several isoforms (FPR-26, FPR-98 and FPR-G6) and activates chemotaxis and cytotoxic cell functions of phagocytes through G(i)-proteins. Studies in HL-60 leukemia cell membranes demonstrated inhibitory effects of Na(+) and pertussis toxin on basal G(i)-protein activity, suggesting that the FPR is constitutively active. However, since HL-60 cells express several constitutively active chemoattractant receptors, analysis of constitutive FPR activity was difficult. Sf9 insect cells do not express chemoattractant receptors and G(i)-proteins and provide a sensitive reconstitution system for FPR/G(i)-protein coupling. Such expression studies showed that FPR-26 is much more constitutively active than FPR-98 and FPR-G6 as assessed by the relative inhibitory effects of Na(+) and of the inverse agonist cyclosporin H on basal G(i)-protein activity. Site-directed mutagenesis studies suggest that the E346A exchange in the C-terminus critically determines dimerization and constitutive activity of FPR. Moreover, N-glycosylation of the N-terminus seems to be important for constitutive FPR activity. Finally, we discuss some future directions of research.

Random mutagenesis of the M3 muscarinic acetylcholine receptor expressed in yeast. Identification of point mutations that "silence" a constitutively active mutant M3 receptor and greatly impair receptor/G protein coupling

The M3 muscarinic receptor is a prototypical member of the class I family of G protein-coupled receptors (GPCRs). To facilitate studies on the structural mechanisms governing M3 receptor activation, we generated an M3 receptor-expressing yeast strain (Saccharomyces cerevisiae) that requires agonist-dependent M3 receptor activation for cell growth. By using receptor random mutagenesis followed by a genetic screen in yeast, we initially identified a point mutation at the cytoplasmic end of transmembrane domain (TM) VI (Q490L) that led to robust agonist-independent M3 receptor signaling in both yeast and mammalian cells. To explore further the molecular mechanisms by which point mutations can render GPCRs constitutively active, we subjected a region of the Q490L mutant M3 receptor that included TM V-VII to random mutagenesis. We then applied a yeast genetic screen to identify second-site mutations that could suppress the activating effects of the Q490L mutation and restore wild-type receptor-like function to the Q490L mutant receptor. This analysis led to the identification of 12 point mutations that allowed the Q490L mutant receptor to function in a fashion similar to the wild-type receptor. These amino acid substitutions mapped to two distinct regions of the M3 receptor, the exofacial segments of TM V and VI and the cytoplasmic ends of TM V-VII. Strikingly, in the absence of the activating Q490L mutation, all recovered point mutations severely reduced the efficiency of receptor/G protein coupling, indicating that the targeted residues play important roles in receptor activation and/or receptor/G protein coupling. This strategy should be generally applicable to identify sites in GPCRs that are critically involved in receptor function.

Insight into the mechanism of dopamine D1-like receptor activation. Evidence for a molecular interplay between the third extracellular loop and the cytoplasmic tail

A chimeric D1A dopaminergic receptor harboring the cytoplasmic tail (CT) of the D1B subtype (D1A-CTB) has been used previously to show that CT imparts high dopamine (DA) affinity and constitutive activity to the D1B receptors. However, the D1A-CTB chimera, unlike the D1B subtype, exhibits a significantly lower DA potency for stimulating adenylyl cyclase and a drastically lower maximal binding capacity (Bmax). Here, using a functional complementation of chimeric D1-like receptors, we have identified the human D1B receptor regions regulating the intramolecular relationships that lead to an increased DA potency and contribute to Bmax. We demonstrate that the addition of variant residues of the third extracellular loop (EL3) of the human D1B receptor into D1A-CTB chimera leads to a constitutively active mutant receptor displaying an increased DA affinity, potency, and Bmax. These results strongly suggest that constitutively active D1-like receptors can adopt multiple active conformations, notably one that confers increased DA affinity with decreased DA potency and Bmax and another that imparts increased DA affinity with a strikingly increased DA potency and Bmax. Overall, we show that a novel molecular interplay between EL3 and CT regulates multiple active conformations of D1-like receptors and may have potential implications for other G protein-coupled receptor classes.

Regulated and constitutive activation of specific signalling pathways by the human S1P5 receptor

1 We tested the hypothesis, whether G Protein-coupled receptors (GPCRs) may differentially regulate specific signalling pathways by constitutive and agonist-induced activation using the human sphingosine 1-phosphate receptor S1P(5) as a model. 2 S1P(5) receptor-expressing HEK293 cells exhibited a high degree of basal activity for both inhibition of adenylyl cyclase and extracellular signal regulated kinase (ERK) when cultured in serum, which contains high levels of sphingosine 1-phosphate (S1P). However, basal activity was independent of the presence of S1P: (i) constitutive activity remained when cells were cultured in delipidated serum, (ii) addition of S1P to delipidated serum did not increase basal S1P(5) receptor signalling. 3 Conversely, constitutive inhibition of forskolin-stimulated adenylyl cyclase was further enhanced by S1P in S1P(5)-HEK293 cells. 4 Transfection of several mammalian cell lines (CHO-K1, HEK293, NIH-3T3, RH7777) with the S1P(5) receptor induced cell rounding, which was more pronounced in the presence of S1P-containing serum. Rounded cell morphology did not correlate with apoptotic cell death, but led to detachment of cells. 5 Cell surface ELISA assays showed that a fraction of plasma membrane S1P(5) receptors were dose-dependently internalized with S1P. 6 These data reveal that intrinsic inhibition of unstimulated adenylyl cyclase or ERK activity by the S1P(5) receptor is insensitive to ligand modulation. Conversely, effects on forskolin-stimulated adenylyl cyclase, cell morphology and internalization can be further augmented with S1P. Our results suggest that different signal transduction pathways are not equally activated through constitutively active GPCRs with promiscuous signalling characteristics.

Measurement of agonist-dependent and -independent signal initiation of alpha(1b)-adrenoceptor mutants by direct analysis of guanine nucleotide exchange on the G protein galpha(11)

Immunoprecipitation of a fusion protein between the alpha(1b)-adrenoceptor and Galpha(11) following a [(35)S]GTPgammaS [guanosine-5'-O-(3-thio)triphosphate] binding assay resulted in incorporation of low levels of nucleotide. The agonist phenylephrine increased incorporation some 30-fold. Agonist-induced binding represented 1.0 mol of [(35)S]GTPgammaS/mol of fusion protein. This was to the G protein linked to the receptor rather than endogenous Galpha(q)/Galpha(11) as a fusion protein containing the alpha(1b)-adrenoceptor and a form of Galpha(11) (G(208)A) unable to exchange guanine nucleotides effectively, bound [(35)S]GTPgammaS very poorly. Fusion proteins between A(293)E, D(142)A, and 3CAM mutants of the alpha(1b)-adrenoceptor and Galpha(11) bound substantially greater levels of [(35)S]GTPgammaS in the absence of agonist than the fusion incorporating the wild-type receptor. Constitutive binding of the nucleotide induced by these mutants was only 20% of the level achieved by phenylephrine. These mutant receptors thus do not provide an accurate mimic of the agonist-occupied state. Phentolamine reduced the binding of [(35)S]GTPgammaS and acted as a partial inverse agonist for each of the constitutively active mutants. [(35)S]GTPgammaS binding to Galpha(11) was elevated by phenylephrine in both wild-type and constitutively active mutant forms of the fusion proteins, but agonist potency and binding affinity were 50 times higher for the fusions containing the mutated receptors. These studies provide the first direct demonstration of the capacity of constitutively active mutants of a receptor to stimulate guanine nucleotide exchange on the alpha subunit of a G(q) family G protein and defines a strategy potentially suitable for any receptor that couples to these G proteins.

Generation and analysis of constitutively active and physically destabilized mutants of the human beta(1)-adrenoceptor

Constitutive activity of wild-type and mutant forms of human beta(1)- and beta(2)-adrenoceptors was measured by guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding assays using fusion proteins between these receptors and G(s)alpha. Constitutive activity of the beta(1)-adrenoceptor is enhanced by mutation of Leu(322). The ability of ligands to suppress receptor instability and produce up-regulation is often associated with constitutively active mutants. Leu(322)Lysbeta(1)-adrenoceptor, but not wild type, was up-regulated by exposure to the beta(1)-adrenoceptor selective blocker betaxolol. More extensive sequence alterations of the beta(1)-adrenoceptor were generated to mimic the initially described constitutively active mutant (CAM) of the beta(2)-adrenoceptor that is up-regulated strongly by betaxolol. Substitution of amino acids 316 to 324 of the beta(1)-adrenoceptor with the equivalent alpha(1b)-adrenoceptor sequence did not result in up-regulation by betaxolol. However, these forms of both beta(1)- and beta(2)-adrenoceptors displayed substantial and equivalent constitutive activity. The addition of the Leu(322)Lys mutation into the alpha(1b)-adrenoceptor substituted beta(1)-adrenoceptor to produce the CAMKbeta(1)-adrenoceptor allowed substantially greater levels of up-regulation by betaxolol without enhancement of constitutive [(35)S]GTPgammaS binding. Arg(156)Alabeta(1)-adrenoceptor was up-regulated strongly by betaxolol but displayed lower constitutive activity than did other mutants. Binding of [(35)S]GTPgammaS binding to all the fusion proteins was increased substantially by isoprenaline. Despite the ability of betaxolol to cause up-regulation of many mutants, only for the CAMbeta(2)-adrenoceptor-G(s)alpha and CAMKbeta(1)-adrenoceptor-G(s)alpha fusion proteins was the basal binding of [(35)S]GTPgammaS decreased by betaxolol. Clear resolution between receptor constitutive activity and ligand suppression of receptor instability can be obtained for mutant beta-adrenoceptors, and potential inverse agonists do not function equally at phenotypically apparently equivalent CAM receptors.

Enhanced detection of receptor constitutive activity in the presence of regulators of G protein signaling: applications to the detection and analysis of inverse agonists and low-efficacy partial agonists

Fusion proteins between the human 5-hydroxytryptamine (5-HT)(1A) receptor and either wild type or certain pertussis toxin-resistant forms of G(o1)alpha and G(i1)alpha display constitutive GTPase activity that can be inhibited by the inverse agonist spiperone. Addition of recombinant regulator of G protein signaling (RGS) 1 or RGS16 to membranes expressing these fusion proteins resulted in elevation of this constitutive GTPase activity without significantly altering the binding affinity of antagonist/inverse agonist ligands. For a 5-HT(1A) receptor-(Cys(351)Ile)G(o1)alpha fusion protein the increase in basal GTPase activity was greater than 4-fold. Enzyme kinetic analysis demonstrated that the effect of RGS1 was as a GTPase-activating protein for the fusion construct. In the presence of the RGS proteins, both agonists and inverse agonists produced much more robust regulation of high-affinity GTPase activity than in their absence. This allowed detection of the partial agonist nature of WAY100635, which has been described previously as a neutral antagonist at the 5-HT(1A) receptor. Of a range of ligands studied, only haloperidol functioned as a neutral ligand in the presence of RGS1. These studies show that addition of a recombinant RGS protein provides a simple and novel means to elevate the fraction of basal membrane GTPase activity contributed by the constitutive activity of a receptor. By so doing, it also greatly enhances the ability to detect and analyze the effects of inverse agonists and to discriminate between neutral ligands and those with low levels of positive intrinsic efficacy.

The cytoplasmic end of transmembrane domain 3 regulates the activity of the Saccharomyces cerevisiae G-protein-coupled alpha-factor receptor

The binding of alpha-factor to its receptor (Ste2p) activates a G-protein-signaling pathway leading to conjugation of MATa cells of the budding yeast S. cerevisiae. We conducted a genetic screen to identify constitutively activating mutations in the N-terminal region of the alpha-factor receptor that includes transmembrane domains 1-5. This approach identified 12 unique constitutively activating mutations, the strongest of which affected polar residues at the cytoplasmic ends of transmembrane domains 2 and 3 (Asn84 and Gln149, respectively) that are conserved in the alpha-factor receptors of divergent yeast species. Targeted mutagenesis, in combination with molecular modeling studies, suggested that Gln149 is oriented toward the core of the transmembrane helix bundle where it may be involved in mediating an interaction with Asn84. These residues appear to play specific roles in maintaining the inactive conformation of the protein since a variety of mutations at either position cause constitutive receptor signaling. Interestingly, the activity of many mammalian G-protein-coupled receptors is also regulated by conserved polar residues (the E/DRY motif) at the cytoplasmic end of transmembrane domain 3. Altogether, the results of this study suggest a conserved role for the cytoplasmic end of transmembrane domain 3 in regulating the activity of divergent G-protein-coupled receptors.

Constitutive activation of the neurotensin receptor 1 by mutation of Phe(358) in Helix seven

1. The neurotensin receptor 1, NTS1, is a G protein-coupled receptor with seven transmembrane domains (TM) that mediates most of the known effects of the neuropeptide. Our previous studies have pointed to extracellular loop 3 and adjacent TM7 as being potentially involved in agonist-induced activation of the NTS1. 2. Here we investigated residues in these domains that might be involved in transconformational activation of the rat NTS1. Single amino acid mutated receptors were expressed in COS cells and inositol phosphate (IP) and cyclic AMP productions were studied. 3. The F358A mutation in TM7 resulted in a time- and receptor concentration-dependent increase in spontaneous IP production. At expression levels of 12 pmol mg(-1), agonist-independent IP production was increased 10 fold over basal for the F358A mutant receptor whereas the wild type NTS1 exhibited virtually no spontaneous activity at expression levels of 7.5 pmol mg(-1). 4. Neurotensin remained agonist on the F358A mutant receptor with a maximal effect that amounted to greater than twice basal IP levels. SR 48692 was inverse agonist at the mutant receptor, reversing IP production almost back to the levels measured in wild type NTS1-transfected cells. 5. Cyclic AMP production was not constitutively activated with the F358A mutant receptor but was stimulated by neurotensin with the same concentration dependence as that observed with the wild type NTS1. 6. This is the first report, to our knowledge, of a constitutively active mutant of the NTS1. The data are consistent with TM7 being involved in the transconformational changes that lead to agonist-induced coupling of the NTS1 to Gq.

Mutational and computational analysis of the alpha(1b)-adrenergic receptor. Involvement of basic and hydrophobic residues in receptor activation and G protein coupling

To investigate their role in receptor coupling to G(q), we mutated all basic amino acids and some conserved hydrophobic residues of the cytosolic surface of the alpha(1b)-adrenergic receptor (AR). The wild type and mutated receptors were expressed in COS-7 cells and characterized for their ligand binding properties and ability to increase inositol phosphate accumulation. The experimental results have been interpreted in the context of both an ab initio model of the alpha(1b)-AR and of a new homology model built on the recently solved crystal structure of rhodopsin. Among the twenty-three basic amino acids mutated only mutations of three, Arg(254) and Lys(258) in the third intracellular loop and Lys(291) at the cytosolic extension of helix 6, markedly impaired the receptor-mediated inositol phosphate production. Additionally, mutations of two conserved hydrophobic residues, Val(147) and Leu(151) in the second intracellular loop had significant effects on receptor function. The functional analysis of the receptor mutants in conjunction with the predictions of molecular modeling supports the hypothesis that Arg(254), Lys(258), as well as Leu(151) are directly involved in receptor-G protein interaction and/or receptor-mediated activation of the G protein. In contrast, the residues belonging to the cytosolic extensions of helices 3 and 6 play a predominant role in the activation process of the alpha(1b)-AR. These findings contribute to the delineation of the molecular determinants of the alpha(1b)-AR/G(q) interface.

Effect of fenoterol-induced constitutive beta(2)-adrenoceptor activity on contractile receptor function in airway smooth muscle

In the present study, we investigated the effect of fenoterol-induced constitutive beta(2)-adrenoceptor activity on muscarinic receptor agonist- and histamine-induced bovine tracheal smooth muscle contractions. Bovine tracheal smooth muscle strips were incubated with 10 microM fenoterol or vehicle for various periods of time (5, 30 min, 18 h) at 37 degrees C. After extensive washout (3 h, 37 degrees C), isometric contractions were measured to the full muscarinic receptor agonist methacholine, the partial muscarinic receptor agonist 4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium (McN-A-343) and histamine. Fenoterol treatment significantly reduced the sensitivity (pEC(50)) to methacholine in a time-dependent manner, without affecting maximal contraction (E(max)). Fenoterol treatment similarly reduced the pEC(50) of McN-A-343 and histamine; however, E(max) values were also reduced, to approximately 70% of control after 18-h treatment. The inverse agonist timolol, having no effect on control preparations, consistently restored the reduced pEC(50) and E(max) values of the contractile agonists. Remarkably, in the presence of timolol the pEC(50) values of McN-A-343 and histamine in fenoterol-treated airways were significantly enhanced compared to controls. In conclusion, fenoterol-induced constitutive beta(2)-adrenoceptor activity reduces muscarinic receptor agonist- and histamine-induced contractions of bovine tracheal smooth muscle, which can be reversed by the inverse agonist timolol. Moreover, after beta(2)-adrenoceptor agonist treatment, inverse agonism by beta-adrenoceptor antagonists may cause enhanced airway reactivity to contractile mediators.

Functional role of a conserved motif in TM6 of the rat mu opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp

Mutations within the "X1BBX2X3B" motif or its variants in the junction of the third intracellular (i3) loop and the sixth transmembrane domain (TM6) have been shown to lead to constitutive activation of several G protein-coupled receptors (GPCRs). In this study, T6.34(279) at the X3 locus of the rat mu opioid receptor was mutated to Lys and Asp, and the mutants were examined for binding and signaling properties. The T6.34(279)K mutant was poorly expressed, and pretreatment with naloxone greatly enhanced its expression. This construct exhibited properties identified previously with constitutive activation: (1) compared with the wild type, it produced much higher agonist-independent [35S]GTPgammaS binding, which was abolished by pertussis toxin treatment; (2) it displayed an enhanced affinity for the agonist DAMGO similar to that of the high-affinity state of the wild type, which was not altered by GTPgammaS, while having unchanged affinity for the antagonist diprenorphine. The T6.34(279)K mutant displayed a higher intracellular receptor pool than the wild type. Naloxone inhibited the basal [35S]GTPgammaS binding of the T6.34(279)K mutant, demonstrating inverse agonist activity at this mutant receptor. In contrast, the T6.34(279)D substitution did not increase basal [35S]GTPgammaS binding, greatly reduced agonist-promoted [35S]GTPgammaS binding, and markedly decreased affinity for DAMGO. Thus, the T6.34(279)D mutant adopts conformations corresponding to inactive states of the receptor. The results were interpreted in the structural context of a model for the mu opioid receptor that incorporates the information from the crystal structure of rhodopsin. The interaction of T6.34(279) with R3.50(165) in the mu opioid receptor is considered to stabilize the inactive conformations. The T6.34(279)K substitution would then disrupt this interaction and support agonist-free activation, while T6.34(279)D mutation should strengthen this interaction which keeps the receptor in inactive states. T6.34(279) may, in addition, interact with the neighboring R6.35(280) to help constrain the receptor in inactive states, and T6.34(279)K and T6.34(279)D mutations would affect this interaction by disrupting or strengthening it, respectively. To the best of our knowledge, the results presented here represent the first structurally rationalized demonstration that mutations of this locus can lead to dramatically different properties of a GPCR.

Inverse agonist up-regulates the constitutively active D3.49(164)Q mutant of the rat mu-opioid receptor by stabilizing the structure and blocking constitutive internalization and down-regulation

We demonstrated previously that D3.49(164) mutations resulted in constitutive activation of the rat mu-opioid receptor and abolished receptor expression unless cells were pretreated with naloxone, an inverse agonist. In this study, we investigated the properties of the D3.49(164)Q mutant and the mechanisms underlying the effect of naloxone. Naloxone pretreatment up-regulated [(3)H]diprenorphine binding and protein expression of the D3.49(164)Q mutant in a time- and dose-dependent manner without affecting its mRNA level. After naloxone removal, binding and protein expression of the mutant declined with time with no effect on its mRNA level. Naloxone methiodide (a quaternary ammonium analog) caused a maximal up-regulation about 50% of the naloxone effect, indicating that naloxone acts extracellularly and intracellularly. Expression of the mutant was enhanced by inverse agonists, a neutral antagonist, and agonists, with inverse agonists being most effective. In membranes, the mutant was structurally less stable than the wild type upon incubation at 37 degrees C, and naloxone and [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin stabilized the mutant. Coexpression of the dominant-negative mutants GRK2-K220R, arrestin-2(319-418), dynamin I-K44A, rab5A-N133I or rab7-N125I partially prevented the decline in binding of the mutant after naloxone removal. Chloroquine or proteasome inhibitor I reduced the down-regulation of the mutant. These results indicate that the D3.49(164)Q mutant is constitutively internalized via G protein coupled-receptor kinase-, arrestin-2-, dynamin-, rab5-, and rab7-dependent pathways and probably trafficked through early and late endosomes into lysosomes and degraded by lysosomes and proteasomes. Naloxone up-regulates the D3.49(164)Q mutant by stabilizing the mutant protein and blocking its constitutive internalization and down-regulation. To the best of our knowledge, this represents the first comprehensive analysis of the mechanisms involved in up-regulation of constitutively active mutants by an inverse agonist.

Constitutive activation of the mu opioid receptor by mutation of D3.49(164), but not D3.32(147): D3.49(164) is critical for stabilization of the inactive form of the receptor and for its expression

The roles of conserved aspartates in the third transmembrane domain of the rat mu opioid receptor (RMOR) were explored with mutations of D3.32(147) and D3.49(164). D3.49(164) in the highly conserved DRY motif was mutated to 13 amino acids. Except for the D3.49(164)E mutant, each mutant displayed little or no detectable [(3)H]diprenorphine binding, and pretreatment with naloxone greatly enhanced binding. D3.49(164)H, -Q, -Y, -M, and -E mutants were further studied. D3.32(147) was substituted with A or N. All seven mutants exhibited similar binding affinities for the antagonist [(3)H]diprenorphine as the wild-type. The D3.49(164)H, -Q, -Y, and -M mutants, but not the D3.49(164)E and D3.32(147) mutants, exhibited enhanced basal [(35)S]GTPgammaS binding which was comparable to the maximally activated level of the wild-type and was related to expression levels. Naloxone, naltrexone, and naloxone methiodide significantly inhibited the basal [(35)S]GTPgammaS binding of the D3.49(164) mutants, indicating inverse agonist activities. Treatment of the D3.49(164)Y mutant with pertussis toxin greatly reduced the basal [(35)S]GTPgammaS binding, demonstrating constitutive activation of Galpha(i)/Galpha(o). The D3.49(164)H, -Y, -M, and -Q mutants had higher affinities for DAMGO than the wild-type, which were not significantly lowered by GTPgammaS. Thus, mutation of D3.49(164) to H, Y, M, or Q in RMOR resulted in receptor assuming activated conformations. In contrast, the D3.49(164)E mutant displayed significantly lower basal [(35)S]GTPgammaS binding and reduced affinity for DAMGO. Upon incubation of membranes at 37 degrees C, the constitutively active D3.49(164)Y mutant was structurally less stable, whereas the inactivated D3.49(164)E mutant was more stable, than the wild-type. Computational simulations showed that the E3.49 side chain interacted strongly with the conserved R3.50 in the DRY motif and stabilized the inactive form of the receptor. Taken together, these results indicate that D3.49 plays an important role in constraining the receptor in inactive conformations.

Sensitization of adenylate cyclase induced by a dopamine D2 receptor mutant: inverse agonism by D2 receptor antagonists

Mutations of residues in the third intracellular loops of several G-protein coupled receptors have been shown to confer constitutive activation. The authors investigated the effects of one such mutation in the dopamine D2 receptor. Compared to the wild type D2, the mutant D2 receptor (D2T344K) showed a substantial increase in agonist affinity with affinity for antagonists unchanged. The increased agonist affinity was unaffected by pertussis toxin treatment, indicating it is an intrinsic property of the mutant receptor. The potency of dopamine for acute inhibition of forskolin-stimulated cAMP production in stably expressing Chinese Hamster Ovary (CHO) cells was higher for the mutant than the wild type receptor. CHO cells stably expressing D2T344K displayed enhanced responses to forskolin-stimulated adenylate cyclase activity compared with cells stably expressing the wild type D2 receptor. The increased forskolin responsiveness of adenylate cyclase is similar to the sensitization previously observed with wild type D2 receptor after agonist treatment. Adenylate cyclase responsiveness of CHO cells stably expressing D2T344K receptor was not further increased by agonist treatment. Sensitization was blocked by pertussis toxin and D2 receptor antagonists haloperidol, butaclamol, and clozapine, indicating inverse agonist activity of these compounds at D2T344K. Inverse agonist activity was further demonstrated by the finding that overnight treatment with these compounds drastically increased the density of the mutant receptor but had minimal effect on the density of the wild type receptor. Taken together, these results suggest the authors have generated a constitutively active dopamine D2 receptor capable of sensitizing adenylate cyclase in the absence of agonist activation.

Detection of receptor ligands by monitoring selective stabilization of a Renilla luciferase-tagged, constitutively active mutant, G-protein-coupled receptor

The wild-type beta2-adrenoceptor and a constitutively active mutant of this receptor were C-terminally tagged with luciferase from the sea pansy Renilla reniformis. C-terminal addition of Renilla luciferase did not substantially alter the levels of expression of either form of the receptor, the elevated constitutive activity of the mutant beta2-adrenoceptor nor the capacity of isoprenaline to elevate cyclic AMP levels in intact cells expressing these constructs. Treatment of cells expressing constitutively active mutant beta2-adrenoceptor-Renilla luciferase with antagonist/inverse agonist ligands resulted in upregulation of levels of this polypeptide which could be monitored by the elevated luciferase activity. The pEC50 for ligand-induced luciferase upregulation and ligand affinity to bind the receptor were highly correlated. Similar upregulation could be observed following sustained treatment with agonist ligands. These effects were only observed at a constitutively active mutant of the beta2-adrenoceptor. Co-expression of the wild-type beta2-adrenoceptor C-terminally tagged with the luciferase from Photinus pyralis did not result in ligand-induced upregulation of the levels of activity of this luciferase. Co-expression of the constitutively active mutant beta2-adrenoceptor-Renilla luciferase and an equivalent mutant of the alpha1b-adrenoceptor C-terminally tagged with green fluorescent protein allowed pharmacological selectivity of adrenoceptor antagonists to be demonstrated. This approach offers a sensitive and convenient means, which is amenable to high throughput analysis, to monitor ligand binding to a constitutively active mutant receptor. As no prior knowledge of receptor ligands is required this approach may be suitable to identify ligands at orphan G protein-coupled receptors.

Cloning of the mouse 5-HT6 serotonin receptor and mutagenesis studies of the third cytoplasmic loop

We have cloned the mouse 5-HT6 serotonin receptor and examined structure-function relationships in the C-terminal end of the third cytoplasmic (CIII) loop, introducing point mutations by site-directed mutagenesis at positions 264 to 268. We examined the ability of 5-HT6 wild type and receptor mutants to activate a cAMP responsive reporter gene when transiently expressed in JEG-3 or COS-7 cells. The wild type 5-HT6 receptor showed strong constitutive activity even when expressed at very low levels and which increased in proportion to the amount of receptor cDNA transfected. Three of the five mutants investigated (K264I, K267A and A268R) showed reduction in constitutive activity compared to wild type. These data suggest that constitutive activity may be important to 5-HT6 receptor activity in vivo and that, unlike some other G-protein coupled receptors, alteration in the BBXXB CIII-loop motif reduces rather than further activates basal activity of the murine 5-HT6 receptor.

The human B1 bradykinin receptor exhibits high ligand-independent, constitutive activity. Roles of residues in the fourth intracellular and third transmembrane domains

The B1 bradykinin (BK) receptor (B1R) is a seven-transmembrane domain, G protein-coupled receptor that is induced by injury and important in inflammation and nociception. Here, we show that the human B1R exhibits a high level of ligand-independent, constitutive activity. Constitutive activity was identified by the increase in basal cellular phosphoinositide hydrolysis as a function of the density of the receptors in transiently transfected HEK293 cells. Several B1R peptide antagonists were neutral antagonists or very weakly efficacious inverse agonists. Constitutive B1R activity was further increased by alanine mutation of Asn(121) in the third transmembrane domain of the receptor (B1A(121)). This mutant resembled the agonist-preferred receptor state since it also exhibited increased agonist affinity and decreased agonist responsiveness. A dramatic loss of constitutive activity occurred when the fourth intracellular C-terminal domain (IC-IV) of the human B2 BK receptor subtype (B2R), which exhibits minimal constitutive activity, was substituted in either B1R or B1A(121) to make B1(B2ICIV) and B1(B2ICIV)A(121), respectively. Activity was partially recovered by subsequent alanine mutation of a cluster of two serines and two threonines in IC-IV of either B1(B2ICIV) or B1(B2ICIV)A(121), a cluster that is important for B2R desensitization. The ligand-independent, constitutive activity of B1R therefore depends on epitopes in both transmembrane and intracellular domains. We propose that the activity is primarily due to the lack of critical epitopes in IC-IV that regulate such activity.

Ligand induced conformational states of the 5-HT(1A) receptor

It has been shown that G-protein coupled receptors have seven transmembrane alpha-helices, but the structural changes occurring in a G-protein coupled receptor as a response on agonist stimulus and the molecular events leading to blockade of the signal transduction by antagonists are not well understood. In the present study, the AMBER 5.0 force field was used for comparative molecular dynamics simulations of a 5-HT(1A) receptor model in the absence of ligand, in complex with a 5-HT(1A) receptor agonist (R)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-8-OH-DPAT], in complex with a selective 5-HT(1A) receptor antagonist (S)-N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl ]-2-phenylpropanamide [(S)-WAY100135], and in complex with the partial agonist, buspirone. In the simulations, the agonist induced larger conformational changes into transmembrane helix 3 and 6 than into the other helices, while the main conformational differences between the agonist bound receptor and the antagonist bound receptor were in transmembrane helix 5 and 6. During the simulations, all the three ligands constrained the helical movements compared to those observed in the receptor without any ligand.

Charged residues at the intracellular boundary of transmembrane helices 2 and 3 independently affect constitutive activity of Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor

Because charged residues at the intracellular ends of transmembrane helix (TMH) 2 and TMH3 of G protein-coupled receptors (GPCRs) affect signaling, we performed mutational analysis of these residues in the constitutively signaling Kaposi's sarcoma-associated herpesvirus GPCR (KSHV-GPCR). KSHV-GPCR contains the amino acid sequence Val-Arg-Tyr rather than the Asp/Glu-Arg-Tyr ((D/E)RY) motif at the intracellular end of TMH3. Mutation of Arg-143 to Ala (R143A) or Gln (R143Q) abolished constitutive signaling whereas R143K exhibited 50% of the basal activity of KSHV-GPCR. R143A was not stimulated by agonist, whereas R143Q was stimulated by growth-related oncogene-alpha, and R143K, similar to KSHV-GPCR, was stimulated further. These findings show that Arg-143 is critical for signal generation in KSHV-GPCR. In other GPCRs, Arg in this position may act as a signaling switch by movement of its sidechain from a hydrophilic pocket in the TMH bundle to a position outside the bundle. In rhodopsin, the Arg of Glu-Arg-Tyr interacts with the adjacent Asp to constrain Arg outside the TMH bundle. V142D was 70% more active than KSHV-GPCR, suggesting that an Arg residue, which is constrained outside the bundle by interacting with Asp-142, leads to a receptor that signals more actively. Because the usually conserved Asp in the middle of TMH2 is not present in KSHV-GPCR, we tested whether Asp-83 at the intracellular end of TMH2 was involved in signaling. D83N and D83A were 110 and 190% more active than KSHV-GPCR, respectively. The double mutant D83A/V142D was 510% more active than KSHV-GPCR. That is, cosubstitutions of Asp-83 by Ala and Val-142 by Asp act synergistically to increase basal signaling. A model of KSHV-GPCR predicts that Arg-143 interacts with residues in the TMH bundle and that the sidechain of Asp-83 does not interact with Arg-143. These data are consistent with the hypothesis that Arg-143 and Asp-83 independently affect the signaling activity of KSHV-GPCR.

Genetic mapping of the human C5a receptor. Identification of transmembrane amino acids critical for receptor function

Many hormones and sensory stimuli signal through a superfamily of seven transmembrane-spanning receptors to activate heterotrimeric G proteins. How the seven transmembrane segments of the receptors (a molecular architecture of bundled alpha-helices conserved from yeast to man) work as "on/off" switches remains unknown. Previously, we used random saturation mutagenesis coupled with a genetic selection in yeast to determine the relative importance of amino acids in four of the seven transmembrane segments of the human C5a receptor (Baranski, T. J., Herzmark, P., Lichtarge, O., Gerber, B. O., Trueheart, J., Meng, E. C., Iiri, T., Sheikh, S. P., and Bourne, H. R. (1999) J. Biol. Chem. 274, 15757-15765). In this study, we evaluate helices I, II, and IV, thereby furnishing a complete mutational map of the seven transmembrane helices of the human C5a receptor. Our analysis identified 19 amino acid positions resistant to non-conservative substitutions. When combined with the 25 essential residues previously identified in helices III and V-VII, they delineate two distinct components of the receptor switch: a ligand-binding surface at or near the extracellular surface of the helix bundle and a core cluster in the cytoplasmic half of the bundle. In addition, we found critical amino acids in the first and second helices that are predicted to face the lipid membrane. These residues form an extended surface that might mediate interactions with lipids and other membrane proteins or function as an oligomerization domain with other receptors.

Gain of function mutants: ion channels and G protein-coupled receptors

Many ion channels and receptors display striking phenotypes for gain-of-function mutations but milder phenotypes for null mutations. Gain of molecular function can have several mechanistic bases: selectivity changes, gating changes including constitutive activation and slowed inactivation, elimination of a subunit that enhances inactivation, decreased drug sensitivity, changes in regulation or trafficking of the channel, or induction of apoptosis. Decreased firing frequency can occur via increased function of K+ or Cl- channels. Channel mutants also cause gain-of-function syndromes at the cellular and circuit level; of these syndromes, the cardiac long-QT syndromes are explained in a more straightforward way than are the epilepsies. G protein-coupled receptors are also affected by activating mutations.

Resolution of inverse agonist-induced up-regulation from constitutive activity of mutants of the alpha(1b)-adrenoceptor

Constitutively active forms of the hamster alpha(1b)-adrenoceptor can be produced from the point mutations Asp(142)Ala or Ala(293)Glu or exchange of a small segment of the third intracellular loop with the equivalent region of the beta(2)-adrenoceptor. Green fluorescent protein (GFP)-tagged forms of each of these mutants and of the wild type alpha(1b)-adrenoceptor were expressed stably in HEK293 cells. The wild type alpha(1b)-adrenoceptor-GFP was expressed both at the plasma membrane and with a distinctly perinuclear punctate pattern. Sustained treatment with a range of antagonist/inverse agonist ligands failed to modulate the cellular distribution or levels of expression of this construct. The form of the alpha(1b)-adrenoceptor containing the beta(2)-adrenoceptor sequence substitution was predominantly located in punctate intracellular vesicles and sustained challenge with the same series of antagonists/inverse agonists produced a 5-fold up-regulation of protein levels with elevation of both plasma membrane and intracellular receptor. Quantification of these effects could be produced by spectrofluorometric analysis of cells grown in a 96-well microtiter plate. In contrast, both the Asp(142)Ala and Ala(293)Glu forms of the alpha(1b)-adrenoceptor-GFP were located predominantly at the plasma membrane. Levels of these two point mutants were not increased by any of the antagonist/inverse agonist ligands tested, although the sequence substitution mutation encompasses codon 293. Resolution of constitutive activity and ligand-induced up-regulation was further exemplified by a mutant lacking eight serine residues in the C-terminal tail that displayed little constitutive activity but was up-regulated by sustained ligand challenge. These results demonstrate the nonequivalence of mutations in their regulation by antagonist/inverse agonist ligands.

Systematic identification of mutations that constitutively activate the angiotensin II type 1A receptor by screening a randomly mutated cDNA library with an original pharmacological bioassay

The constitutive activation of G-protein-coupled receptors is a major new approach to investigating their physiopathology and pharmacology. A large number of spontaneous and site-directed mutations resulting in constitutive activity have been identified, but systematic mapping of the amino acids involved for a given receptor would be extremely useful for complete elucidation of the molecular mechanisms underlying its activation. We carried out such mapping for the angiotensin II type 1A (AT(1A)) receptor by screening a randomly mutated cDNA library after expressing the mutated clones in eukaryotic cells. To test the AT(1A) mutants generated, we developed an original, specific, and highly sensitive assay based on the properties of CGP42112A. This classical AT(2) agonist is a weak partial agonist of the wild-type AT(1A) receptor and becomes a full agonist for constitutively active AT(1A) mutants, as shown experimentally and in allostery-based theoretical models. Activation of the mutated receptors by CGP42112A was monitored by using the bioluminescent protein aequorin, a very sensitive and specific sensor of intracellular calcium mobilization. The screening of 4,800 clones, providing an exhaustive coverage of all of the mutations generated, led to the identification of 16 mutations in sequences encoding the transmembrane domains that were responsible for high sensitivity to CGP42112A. The constitutive activity was confirmed by agonist-independent production of inositol phosphates, which showed that at least half of the clones had significantly increased basal activity. These data demonstrate that this new type of approach is very efficient for the systematic identification of constitutively active mutants of G-protein-coupled receptors.

The human dopamine D2(Longer) receptor has a high-affinity state and inhibits adenylyl cyclase

Brain dopamine D2 receptors are the main targets for antipsychotic and anti-Parkinsonian drugs. The dopamine D2 receptor has three forms, D2(Short), D2(Long) and D2(Longer). D2(Longer) is a newly found splice variant which contains two additional amino acids (valine and glutamine) in the third cytoplasmic loop of the receptor. To determine whether D2(Longer) was functional, the cDNA was transfected into CHO cells. D2(Longer) revealed a high-affinity state for dopamine ( approximately 1.5 nM), and mediated dopamine-inhibited adenylyl cyclase.

Distinct function of the cytoplasmic tail in human D1-like receptor ligand binding and coupling

To delineate the role of the cytoplasmic tail in the distinct binding and coupling properties of human dopamine D1-like receptors, chimeric receptors were generated in which the entire tail region of wild-type human D1A (or D1) and D1B (or D5) receptors was exchanged. The hD1A-D1BT, but not hD1B-D1AT, receptor expression was dramatically reduced compared with wild-type receptor expression. Swapping the cytoplasmic tail resulted in a full switch of dopamine binding affinity and constitutive activity, while dopamine potency decreased and agonist-mediated maximal activation of adenylyl cyclase increased for both chimeras. Hence, the cytoplasmic tail plays a crucial role in D1-like receptor expression, agonist binding affinity and constitutive activation but regulates in a distinct fashion the formation of D1A and D1B receptor active states upon dopamine binding.

New dopamine receptor, D2(Longer), with unique TG splice site, in human brain

Brain dopamine receptor agonists alleviate the signs of Parkinson's disease, while dopamine receptor antagonists alleviate hallucinations and delusions in psychosis. The dopamine type 2 receptor (or D2) is blocked by antipsychotic drugs, including even the "atypical" drugs such as clozapine or remoxipride, in direct relation to their clinical potencies. Compared to the long form of the D2 receptor (D2(Long)), the short form (D2(Short)) may be three times more sensitive to benzamide antipsychotic drugs. Hence, it is essential to identify additional variants of dopamine receptors for which more selective antipsychotic drugs can be found. Although no family linkage has been found between the D2 receptor and schizophrenia, there can be brain region abnormalities in the RNA transcript expression of dopamine receptors. Therefore, in order to identify variant dopamine D2 receptors, we searched for mutations in the RNA transcripts for the dopamine D2 receptor in the striatum of post-mortem brains from individuals who died with psychosis, including schizophrenia. A new splice variant of the D2 receptor, D2(Longer), with a unique TG splice site, was found in one control brain and in two psychotic brains.

Theoretical study on mutation-induced activation of the luteinizing hormone receptor

Here, three-dimensional model building and molecular dynamics simulations of the luteinizing hormone receptor have been employed to generate hypotheses about the molecular mechanisms underlying the activation of the receptor induced by naturally occurring activating mutations. The comparative analysis of the wild-type receptor and of 16 constitutively active or inactive mutants has been instrumental in inferring the structural/dynamic features which could characterize the inactive and the active forms of the receptor. These features have been also employed for predicting the functional behavior of new receptor mutants. The results of this study might provide a structural framework to interpret the pathological effects induced by mutations of the luteinizing hormone receptor. In addition, the proposed theoretical model could be useful for engineering new mutations or ligands able to modulate receptor function.

Characterization of chimeric prostacyclin/prostaglandin D(2) receptors

The functional activity of two chimeric mouse prostacyclin/prostaglandin D(2) (IP/DP) receptors, in which the carboxyl-terminal region of the IP receptor was progressively replaced by that of the DP receptor, was examined in Chinese hamster ovary (CHO) cells. The order of potency of prostaglandin D(2), prostaglandin E(2) and the IP receptor agonists cicaprost, iloprost and BMY 45778 (3-[4-(4, 5-diphenyl-2-oxazolyl)-5-oxazolyl]phenoxy]acetic acid) to stimulate cyclic AMP production was identical for the IP(N-VII)/DP(C), IP(N-V)/DP(VI-C) and wild-type IP receptors. IP(N-VII)/DP(C) receptor-expressing cells showed increases in basal adenylate cyclase activity, agonist potency and coupling efficiency. In addition, the intrinsic activity of the partial IP receptor agonists BMY 45778 and PGE(2) was significantly increased in IP(N-VII)/DP(C) receptor-expressing cells. Therefore, substitution of just the carboxyl-terminal tail of the IP receptor by that of the DP receptor appears to result in a chimeric IP/DP receptor with all the properties of a constitutively-active receptor.

Spontaneous human B2 bradykinin receptor activity determines the action of partial agonists as agonists or inverse agonists. Effect of basal desensitization

In this report, we show that desensitization regulates ligand-independent, spontaneous activity of the human B2 bradykinin (BK) receptor, and the level of spontaneous receptor activity determines the action of the BK antagonists and partial receptor agonists NPC17731 and HOE140 as agonists or inverse agonists. Spontaneous receptor activity was monitored by measuring basal cellular phosphoinositide (PI) hydrolysis as a function of the density of the receptor in transiently transfected HEK293 cells. Minimal spontaneous activity of the wild-type B2 receptor was detected in these cells. Mutating a cluster of serines and threonines within the fourth intracellular domain of the receptor, which is critical for agonist-promoted desensitization, significantly increased the spontaneous receptor activity. BK, the natural B2 receptor ligand and, consequently, a full agonist, stimulated PI hydrolysis at high and low levels of spontaneous receptor activity. On the other hand, the partial agonists NPC17731 and HOE140 were stimulatory, or agonists, at the lower level of receptor activity but inhibitory, or inverse agonists, at the higher level of activity. These results show that receptors are desensitized in response to their spontaneous activity. Furthermore, these results, which refute traditional theories, show that the capacity of a drug to modulate a receptor response is not intrinsic to the drug but is also dependent on the cellular environment in which the drug acts.

Constitutively active mutants of the beta1-adrenergic receptor

We provide the first evidence that point mutations can constitutively activate the beta(1)-adrenergic receptor (AR). Leucine 322 of the beta(1)-AR in the C-terminal portion of its third intracellular loop was replaced with seven amino acids (I, T, E, F, C, A and K) differing in their physico-chemical properties. The beta(1)-AR mutants expressed in HEK-293 cells displayed various levels of constitutive activity which could be partially inhibited by some beta-blockers. The results of this study might have interesting implications for future studies aiming at elucidating the activation process of the beta(1)-AR as well as the mechanism of action of beta-blockers.

Qualitative and quantitative assessment of relative agonist efficacy

Historically, the ability of a ligand to bind to its receptor and the ability to subsequently activate that receptor have been described as the properties of affinity and intrinsic efficacy, respectively. These properties were originally believed to be independent of one another; both are possessed by ligands classed as "agonists," and they have served as the quantitative foundation of the drug and receptor classification process. Although affinity has been interpreted readily in physicochemical terms, equivalent molecular models for efficacy remain elusive. In recent times, there has been a significant paradigm shift in our understanding of the interrelationship between affinity and intrinsic efficacy, particularly on theoretical grounds, yet the actual methods available to measure these parameters remain largely operational. Nevertheless, a number of approaches, based on both functional measurements and radioligand binding studies, are available to quantify agonist efficacy on a relative scale and, to date, these remain the most practical. This commentary discusses the most common of these methods, their advantages and limitations, the dependence of the expression of agonism on the chosen assay system, and the impact of recent biochemical and molecular biological advances on the study of efficacy. Additionally, some of the more contemporary theories regarding the molecular nature of efficacy are briefly discussed, as well as the caveats that always must be borne in mind when any determinations of relative agonist efficacy are made.