Functional expression of the human serotonin 5-HT1A receptor in Escherichia coli. Ligand binding properties and interaction with recombinant G protein alpha-subunits

Engineered Chimera Protein Constructs to Facilitate the Production of Heterologous Transmembrane Proteins in E. coli

To delve into the structure-function relationship of transmembrane proteins (TMPs), robust protocols are needed to produce them in a pure, stable, and functional state. Among all hosts that express heterologous TMPs, E. coli has the lowest cost and fastest turnover. However, many of the TMPs expressed in E. coli are misfolded. Several strategies have been developed to either direct the foreign TMPs to E. coli's membrane or retain them in a cytosolic soluble form to overcome this deficiency. Here, we summarize protein engineering methods to produce chimera constructs of the desired TMPs fused to either a signal peptide or precursor maltose binding protein (pMBP) to direct the entire construct to the periplasm, therefore depositing the fused TMP in the plasma membrane. We further describe strategies to produce TMPs in soluble form by utilizing N-terminally fused MBP without a signal peptide. Depending on its N- or C-terminus location, a fusion to apolipoprotein AI can either direct the TMP to the membrane or shield the hydrophobic regions of the TMP, maintaining the soluble form. Strategies to produce G-protein-coupled receptors, TMPs of Mycobacterium tuberculosis, HIV-1 Vpu, and other TMPs are discussed. This knowledge could increase the scope of TMPs' expression in E. coli.

Protein Design Strategies for the Structural–Functional Studies of G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are an important family of membrane proteins responsible for many physiological functions in human body. High resolution GPCR structures are required to understand their molecular mechanisms and perform rational drug design, as GPCRs play a crucial role in a variety of diseases. That is difficult to obtain for the wild-type proteins because of their low stability. In this review, we discuss how this problem can be solved by using protein design strategies developed to obtain homogeneous stabilized GPCR samples for crystallization and cryoelectron microscopy.

Cholesterol Is a Dose-Dependent Positive Allosteric Modulator of CCR3 Ligand Affinity and G Protein Coupling

Cholesterol as an allosteric modulator of G protein-coupled receptor (GPCR) function is well documented. This quintessential mammalian lipid facilitates receptor–ligand interactions and multimerization states. Functionally, this introduces a complicated mechanism for the homeostatic modulation of GPCR signaling. Chemokine receptors are Class A GPCRs responsible for immune cell trafficking through the binding of endogenous peptide ligands. CCR3 is a CC motif chemokine receptor expressed by eosinophils and basophils. It traffics these cells by transducing the signal stimulated by the CC motif chemokine primary messengers 11, 24, and 26. These behaviors are close to the human immunoresponse. Thus, CCR3 is implicated in cancer metastasis and inflammatory conditions. However, there is a paucity of experimental evidence linking the functional states of CCR3 to the molecular mechanisms of cholesterol–receptor cooperativity. In this vein, we present a means to combine codon harmonization and a maltose-binding protein fusion tag to produce CCR3 from E. coli. This technique yields ∼2.6 mg of functional GPCR per liter of minimal media. We leveraged this protein production capability to investigate the effects of cholesterol on CCR3 function in vitro. We found that affinity for the endogenous ligand CCL11 increases in a dose-dependent manner with cholesterol concentration in both styrene:maleic acid lipid particles (SMALPs) and proteoliposomes. This heightened receptor activation directly translates to increased signal transduction as measured by the GTPase activity of the bound G-protein α inhibitory subunit 3 (Gα_i3). This work represents a critical step forward in understanding the role of cholesterol-GPCR allostery in regulation of signal transduction.

Efficient production of a functional G protein-coupled receptor in E. coli for structural studies

G protein-coupled receptors (GPCRs) are transmembrane signal transducers which regulate many key physiological process. Since their discovery, their analysis has been limited by difficulties in obtaining sufficient amounts of the receptors in high-quality, functional form from heterologous expression hosts. Albeit highly attractive because of its simplicity and the ease of isotope labeling for NMR studies, heterologous expression of functional GPCRs in E. coli has proven particularly challenging due to the absence of the more evolved protein expression and folding machinery of higher eukaryotic hosts. Here we first give an overview on the previous strategies for GPCR E. coli expression and then describe the development of an optimized robust protocol for the E. coli expression and purification of two mutants of the turkey β₁-adrenergic receptor (β₁AR) uniformly or selectively labeled in ¹⁵N or ²H,¹⁵N. These mutants had been previously optimized for thermal stability using insect cell expression and used successfully in crystallographic and NMR studies. The same sequences were then used for E. coli expression. Optimization of E. coli expression was achieved by a quantitative analysis of losses of receptor material at each step of the solubilization and purification procedure. Final yields are 0.2-0.3 mg receptor per liter culture. Whereas both expressed mutants are well folded and competent for orthosteric ligand binding, the less stable YY-β₁AR mutant also comprises the two native tyrosines Y^5.58 and Y^7.53, which enable G protein binding. High-quality ¹H-¹⁵N TROSY spectra were obtained for E. coli-expressed YY-β₁AR in three different functional states (antagonist, agonist, and agonist + G protein-mimicking nanobody-bound), which are identical to spectra obtained of the same forms of the receptor expressed in insect cells. NdeI and AgeI restriction sites introduced into the expression plasmid allow for the easy replacement of the receptor gene by other GPCR genes of interest, and the provided quantitative workflow analysis may guide the respective adaptation of the purification protocol.

Expression and purification of recombinant G protein-coupled receptors: A review

Given their extensive role in cell signalling, GPCRs are significant drug targets; despite this, many of these receptors have limited or no available prophylaxis. Novel drug design and discovery significantly rely on structure determination, of which GPCRs are typically elusive. Progress has been made thus far to produce sufficient quantity and quality of protein for downstream analysis. As such, this review highlights the systems available for recombinant GPCR expression, with consideration of their advantages and disadvantages, as well as examples of receptors successfully expressed in these systems. Additionally, an overview is given on the use of detergents and the styrene maleic acid (SMA) co-polymer for membrane solubilisation, as well as purification techniques.

Expression and Purification of a Functional E. coli 13CH3-Methionine-Labeled Thermostable Neurotensin Receptor 1 Variant for Solution NMR Studies

Escherichia coli (E. coli) is the most widely used expression host for recombinant proteins due to high expression yields and straightforward molecular cloning. Directed evolution of G protein-coupled receptors (GPCRs) has made several of these difficult to express membrane proteins amenable to prokaryotic expression. Here, we describe a protocol for near complete ¹³CH₃-methionine labeling of a thermostable neurotensin receptor 1 (enNTS₁) variant in E. coli for solution NMR-based dynamics studies. Our expression strategy utilizes methionine biosynthesis pathway inhibition forcing E. coli to incorporate exogenous methionine with 96% efficiency at expression levels of 2.6 mg enNTS₁ per liter of expression culture containing 50 mg of ¹³CH₃-methionine. We also provide a 3-step purification protocol that produces final yields of 0.6 mg of functional Apo-state enNTS₁.

Role of hippocampal 5-HT1A receptors in the antidepressant-like phenotype of mice expressing RGS-insensitive Gαi2 protein

A single base mutation in the Gα_i2 protein (G184S) renders this Gα subunit insensitive to the negative modulatory effects of Regulator of G-protein Signaling (RGS) proteins. Mice expressing this RGS insensitive (RGSi) variant of Gα_i2 (RGSi Gα_i2) display a spontaneous antidepressant-like phenotype that is reversible by treatment with the 5-HT1A receptor (5-HT1AR) antagonist WAY100635. Here we test the hypothesis that increased activity of 5-HT1ARs in the hippocampus of RGSi Gα_i2 knock-in mice is responsible for the expression of the observed antidepressant-like behavior. We administered the 5-HT1AR antagonist WAY100635 or the agonist 8-OH-DPAT via bilateral intra-hippocampal infusion cannulae and evaluated antidepressant-like behavior using the tail suspension test (TST). WAY100635 reversed the antidepressant-like phenotype of the RGSi Gα_i2 knock-in mice and 8-OH-DPAT produced an antidepressant-like response in wild type mice that was blocked by systemic WAY100635. Furthermore, intra-hippocampal infusion of the RGS19/4 inhibitor CCG-203769 produced an antidepressant-like effect in female mice. Ex-vivo slice recording confirmed the 5-HT1AR-mediated decrease in hippocampal CA1 pyramidal neuron excitability was enhanced in the RGSi Gα_i2 knock-in mice. There was no change in hippocampal 5-HT1AR expression as measured by ligand binding but there was a compensatory reduction in Gαi proteins. The findings demonstrate that RGS protein control of hippocampal 5-HT1AR signaling is necessary and sufficient to account for the antidepressant-like phenotype in the RGSi Gα_i2 knock-in mice and that RGS proteins highly expressed in the hippocampus should be investigated as targets for novel antidepressant therapies.

Relax, Cool Down and Scaffold: How to Restore Surface Expression of Folding-Deficient Mutant GPCRs and SLC6 Transporters

Many diseases arise from mutations, which impair protein folding. The study of folding-deficient variants of G protein-coupled receptors and solute carrier 6 (SLC6) transporters has shed light on the folding trajectory, how it is monitored and how misfolding can be remedied. Reducing the temperature lowers the energy barrier between folding intermediates and thereby eliminates stalling along the folding trajectory. For obvious reasons, cooling down is not a therapeutic option. One approach to rescue misfolded variants is to use membrane-permeable orthosteric ligands. Antagonists of GPCRs are—in many instances—effective pharmacochaperones: they restore cell surface expression provided that they enter cells and bind to folding intermediates. Pharmacochaperoning of SLC6 transporters is less readily achieved because the ionic conditions in the endoplasmic reticulum (ER) are not conducive to binding of typical inhibitors. The second approach is to target the heat-shock protein (HSP) relay, which monitors the folding trajectory on the cytosolic side. Importantly, orthosteric ligands and HSP-inhibitors are not mutually exclusive. In fact, pharmacochaperones and HSP-inhibitors can act in an additive or synergistic manner. This was exemplified by rescuing disease-causing, folding-deficient variants of the human dopamine transporters with the HSP70 inhibitor pifithrin-μ and the pharmacochaperone noribogaine in Drosophila melanogaster.

Recombinant Microorganisms as Tools for High Throughput Screening for Nonantibiotic Compounds

Microorganisms were among the first tools used for the discovery of biologically active compounds. Their utility reached a zenith during the era of antibiotic development in the 1950s and 1960s, then declined. Subsequently, a substantial role for microorganisms in the pharmaceutical industry developed with the realization that microbial fermentations were intriguing sources of nonantibiotic natural products. From recombinant DNA technology emerged another important role for microorganisms in pharmaceutical research: the expression of heterologous proteins for therapeutic products or for in vitro high throughput screens (HTSs). Recent developments in cloning, genetics, and expression systems have opened up new applications for recombinant microorganisms in screening for nonantibiotic compounds in HTSs. These screens employ microorganisms that depend upon the function of a heterologous protein for survival under defined nutritional conditions. Compounds that specifically target the heterologous protein can be identified by measuring viability of the microorganism under different nutrient selection. Advantages of this approach include a built-in selection for target selectivity, an easily measured end point that can be used for a multitude of different targets, and compatibility with automation required for HTSs. Mechanism-based HTSs using recombinant microorganisms can also address drug targets that are not readily approachable in other HTS formats, including certain enzymes; ion channels and transporters; and protein::protein, protein::DNA, and protein::RNA interactions.

Large-scale production and protein engineering of G protein-coupled receptors for structural studies

Structural studies of G protein-coupled receptors (GPCRs) gave insights into molecular mechanisms of their action and contributed significantly to molecular pharmacology. This is primarily due to technical advances in protein engineering, production and crystallization of these important receptor targets. On the other hand, NMR spectroscopy of GPCRs, which can provide information about their dynamics, still remains challenging due to difficulties in preparation of isotopically labeled receptors and their low long-term stabilities. In this review, we discuss methods used for expression and purification of GPCRs for crystallographic and NMR studies. We also summarize protein engineering methods that played a crucial role in obtaining GPCR crystal structures.

How much do we know about the coupling of G-proteins to serotonin receptors?

Serotonin receptors are G-protein-coupled receptors (GPCRs) involved in a variety of psychiatric disorders. G-proteins, heterotrimeric complexes that couple to multiple receptors, are activated when their receptor is bound by the appropriate ligand. Activation triggers a cascade of further signalling events that ultimately result in cell function changes. Each of the several known G-protein types can activate multiple pathways. Interestingly, since several G-proteins can couple to the same serotonin receptor type, receptor activation can result in induction of different pathways. To reach a better understanding of the role, interactions and expression of G-proteins a literature search was performed in order to list all the known heterotrimeric combinations and serotonin receptor complexes. Public databases were analysed to collect transcript and protein expression data relating to G-proteins in neural tissues. Only a very small number of heterotrimeric combinations and G-protein-receptor complexes out of the possible thousands suggested by expression data analysis have been examined experimentally. In addition this has mostly been obtained using insect, hamster, rat and, to a lesser extent, human cell lines. Besides highlighting which interactions have not been explored, our findings suggest additional possible interactions that should be examined based on our expression data analysis.

Expression and functional characterization of membrane-integrated mammalian corticotropin releasing factor receptors 1 and 2 in Escherichia coli

Corticotropin-Releasing Factor Receptors (CRFRs) are class B1 G-protein-coupled receptors, which bind peptides of the corticotropin releasing factor family and are key mediators in the stress response. In order to dissect the receptors' binding specificity and enable structural studies, full-length human CRFR1α and mouse CRFR2β as well as fragments lacking the N-terminal extracellular domain, were overproduced in E. coli. The characteristics of different CRFR2β-PhoA gene fusion products expressed in bacteria were found to be in agreement with the predicted ones in the hepta-helical membrane topology model. Recombinant histidine-tagged CRFR1α and CRFR2β expression levels and bacterial subcellular localization were evaluated by cell fractionation and Western blot analysis. Protein expression parameters were assessed, including the influence of E. coli bacterial hosts, culture media and the impact of either PelB or DsbA signal peptide. In general, the large majority of receptor proteins became inserted in the bacterial membrane. Across all experimental conditions significantly more CRFR2β product was obtained in comparison to CRFR1α. Following a detergent screen analysis, bacterial membranes containing CRFR1α and CRFR2β were best solubilized with the zwitterionic detergent FC-14. Binding of different peptide ligands to CRFR1α and CRFR2β membrane fractions were similar, in part, to the complex pharmacology observed in eukaryotic cells. We suggest that our E. coli expression system producing functional CRFRs will be useful for large-scale expression of these receptors for structural studies.

Recruitment of a cytoplasmic chaperone relay by the A2A adenosine receptor

The adenosine A_2A receptor is a prototypical rhodopsin-like G protein-coupled receptor but has several unique structural features, in particular a long C terminus (of >120 residues) devoid of a palmitoylation site. It is known to interact with several accessory proteins other than those canonically involved in signaling. However, it is evident that many more proteins must interact with the A_2A receptor, if the trafficking trajectory of the receptor is taken into account from its site of synthesis in the endoplasmic reticulum (ER) to its disposal by the lysosome. Affinity-tagged versions of the A_2A receptor were expressed in HEK293 cells to identify interacting partners residing in the ER by a proteomics approach based on tandem affinity purification. The receptor-protein complexes were purified in quantities sufficient for analysis by mass spectrometry. We identified molecular chaperones (heat-shock proteins HSP90α and HSP70-1A) that interact with and retain partially folded A_2A receptor prior to ER exit. Complex formation between the A_2A receptor and HSP90α (but not HSP90β) and HSP70-1A was confirmed by co-affinity precipitation. HSP90 inhibitors also enhanced surface expression of the receptor in PC12 cells, which endogenously express the A_2A receptor. Finally, proteins of the HSP relay machinery (e.g. HOP/HSC70-HSP90 organizing protein and P23/HSP90 co-chaperone) were recovered in complexes with the A_2A receptor. These observations are consistent with the proposed chaperone/coat protein complex II exchange model. This posits that cytosolic HSP proteins are sequentially recruited to folding intermediates of the A_2A receptor. Release of HSP90 is required prior to recruitment of coat protein complex II components. This prevents premature ER export of partially folded receptors.

Application of HaloTag technology to expression and purification of cannabinoid receptor CB2

Expression of milligram quantities of functional, stable G protein-coupled receptors (GPCR) for high-resolution structural studies remains a challenging task. The goal of this work was to evaluate the usefulness of the HaloTag system (Promega) for expression and purification of the human cannabinoid receptor CB(2), an important target for development of drugs for treatment of immune disorders, inflammation, and pain. Here we investigated expression in Escherichia coli cells of the integral membrane receptor CB(2) as a fusion with the 34 kDa HaloTag at N- or C-terminal location, either in the presence or in the absence of the N-terminal maltose-binding protein (MBP). The CB(2) was flanked at both ends by the tobacco etch virus (TEV) protease cleavage sites to allow for subsequent removal of expression partners. Expression by induction with either IPTG (in E. coli BL21(DE3) cell cultures) or by auto-induction (in E. coli KRX cells) were compared. While the N-terminal location of the HaloTag resulted in high levels of expression of the fusion CB(2), the recombinant receptor was not functional. However, when the HaloTag was placed in the C-terminal location, a fully active receptor was produced irrespective of induction method or bacterial strain used. For purification, the fusion protein was captured onto HaloLink resin in the presence of detergents. Treatment with specific TEV protease released the CB(2) upon washing. To our knowledge, this study represents the first example of expression, surface immobilization and purification of a functional GPCR using HaloTag technology.

Reengineering the collision coupling and diffusion mode of the A2A-adenosine receptor: palmitoylation in helix 8 relieves confinement

The A(2A)-adenosine receptor undergoes restricted collision coupling with its cognate G protein G(s) and lacks a palmitoylation site at the end of helix 8 in its intracellular C terminus. We explored the hypothesis that there was a causal link between the absence of a palmitoyl moiety and restricted collision coupling by introducing a palmitoylation site. The resulting mutant A(2A)-R309C receptor underwent palmitoylation as verified by both mass spectrometry and metabolic labeling. In contrast to the wild type A(2A) receptor, the concentration-response curve for agonist-induced cAMP accumulation was shifted to the left with increasing expression levels of A(2A)-R309C receptor, an observation consistent with collision coupling. Single particle tracking of quantum dot-labeled receptors confirmed that wild type and mutant A(2A) receptor differed in diffusivity and diffusion mode; agonist activation resulted in a decline in mean square displacement of both receptors, but the drop was substantially more pronounced for the wild type receptor. In addition, in the agonist-bound state, the wild type receptor was frequently subject to confinement events (estimated radius 110 nm). These were rarely seen with the palmitoylated A(2A)-R309C receptor, the preferred diffusion mode of which was a random walk in both the basal and the agonist-activated state. Taken together, the observations link restricted collision coupling to diffusion limits imposed by the absence of a palmitoyl moiety in the C terminus of the A(2A) receptor. The experiments allowed for visualizing local confinement of an agonist-activated G protein-coupled receptor in an area consistent with the dimensions of a lipid raft.

Ice breaking in GPCR structural biology

G-protein-coupled receptors (GPCRs) are one of the most challenging targets in structural biology. To successfully solve a high-resolution GPCR structure, several experimental obstacles must be overcome, including expression, extraction, purification, and crystallization. As a result, there are only a handful of unique structures reported from this protein superfamily, which consists of over 800 members. In the past few years, however, there has been an increase in the amount of solved GPCR structures, and a few high-impact structures have been determined: the peptide receptor CXCR4, the agonist bound receptors, and the GPCR-G protein complex. The dramatic progress in GPCR structural studies is not due to the development of any single technique, but a combination of new techniques, new tools and new concepts. Here, we summarize the progress made for GPCR expression, purification, and crystallization, and we highlight the technical advances that will facilitate the future determination of GPCR structures.

ER-bound steps in the biosynthesis of G protein-coupled receptors

The polypeptide of a G protein-coupled receptor is inserted into the membrane of the endoplasmic reticulum while being translated and this process by itself may be sufficient to establish the proper receptor fold. X-ray structures reveal a common polypeptide topology with little variation in the alignment and orientation of the seven transmembrane segments, the proximal carboxyl terminus (C-tail) and parts of the extracellular loops. These define a structural core the stability of which probably represents a major criterion for the receptor to pass endoplasmic reticulum (ER) quality control; point mutations affecting the structure of the core have an extraordinary chance of causing receptor retention. In contrast, cytoplasmic loops 2 and 3 and the distal C-tail are poorly ordered at least in the absence of an interaction partner. Similarly, the amino terminal tail of rhodopsin-related receptors (but not of receptor subtypes where ligand binding requires a stable fold of the N-tail) is unlikely to establish a stable fold. These segments can cause ER retention when mutated to inappropriately expose hydrophobic peptide patches; to prevent protein aggregation chaperone molecules attach to them thus initiating selection for ER-associated degradation. It is less clear however if there are additional mechanisms to specifically survey the transmembrane core at the level of the lipid bilayer or if insufficient packing is detected due to misalignment of the cytoplasmic or extracellular face of the receptor.

Reprogramming chaperone pathways to improve membrane protein expression in Escherichia coli

Because membrane proteins are difficult to express, our understanding of their structure and function is lagging. In Escherichia coli, α-helical membrane protein biogenesis usually involves binding of a nascent transmembrane segment (TMS) by the signal recognition particle (SRP), delivery of the SRP-ribosome nascent chain complexes (RNC) to FtsY, a protein that serves as SRP receptor and docks to the SecYEG translocon, cotranslational insertion of the growing chain into the translocon, and lateral transfer, packing and folding of TMS in the lipid bilayer in a process that may involve chaperone YidC. Here, we explored the feasibility of reprogramming this pathway to improve the production of recombinant membrane proteins in exponentially growing E. coli with a focus on: (i) eliminating competition between SRP and chaperone trigger factor (TF) at the ribosome through gene deletion; (ii) improving RNC delivery to the inner membrane via SRP overexpression; and (iii) promoting substrate insertion and folding in the lipid bilayer by increasing YidC levels. Using a bitopic histidine kinase and two heptahelical rhodopsins as model systems, we show that the use of TF-deficient cells improves the yields of membrane-integrated material threefold to sevenfold relative to the wild type, and that whereas YidC coexpression is beneficial to the production of polytopic proteins, higher levels of SRP have the opposite effect. The implications of our results on the interplay of TF, SRP, YidC, and SecYEG in membrane protein biogenesis are discussed.

Agonist-dependent modulation of G-protein coupling and transduction of 5-HT1A receptors in rat dorsal raphe nucleus

5-HT1A receptors couple to different Go/Gi proteins in order to mediate a wide range of physiological actions. While activation of post-synaptic 5-HT1A receptors is mainly related to inhibition of adenylyl cyclase activity, functionality of autoreceptors located in raphe nuclei has been classically ascribed to modifications of the activity of potassium and calcium channels. In order to evaluate the possible existence of agonist-directed trafficking for 5-HT1A autoreceptors in the rat dorsal raphe nucleus, we studied their activation by two agonists with a different profile of efficacy [(+)8-OH-DPAT and buspirone], addressing simultaneously the identification of the specific Galpha subtypes ([35S]GTPgammaS labelling and immunoprecipitation) involved and the subsequent changes in cAMP formation. A significant increase (32%, p<0.05) in (+)8-OH-DPAT-induced [35S]GTPgammaS labelling of immunoprecipitates was obtained with anti-Galphai3 antibodies but not with anti-Galphao, anti-Galphai1, anti-Galphai2, anti-Galphaz or anti-Galphas antibodies. In contrast, in the presence of buspirone, significant [35S]GTPgammaS labelling of immunoprecipitates was obtained with anti-Galphai3 (50%, p<0.01), anti-Galphao (32%, p<0.01) and anti-Galphai2 (29%, p<0.05) antibodies, without any labelling with anti-Galphai1, anti-Galphaz or anti-Galphas. The selective 5-HT1A antagonist WAY 100635 blocked the labelling induced by both agonists. Furthermore, (+)8-OH-DPAT failed to modify forskolin-stimulated cAMP accumulation, while buspirone induced a dose-dependent, WAY 100635-sensitive, inhibition of this response (Imax 30.8+/-4.9, pIC50 5.95+/-0.46). These results demonstrate the existence of an agonist-dependency pattern of G-protein coupling and transduction for 5-HT1A autoreceptors in native brain tissue. These data also open new perspectives for the understanding of the differential profiles of agonist efficacy in pre- vs. post-synaptic 5-HT1A receptor-associated responses.

Structure determination of the seven-helix transmembrane receptor sensory rhodopsin II by solution NMR spectroscopy

Seven-helix membrane proteins represent a challenge for structural biology. Here we report the first NMR structure determination of a detergent-solubilized seven-helix transmembrane (7TM) protein, the phototaxis receptor sensory rhodopsin II (pSRII) from Natronomonas pharaonis, as a proof of principle. The overall quality of the structure ensemble is good (backbone r.m.s. deviation of 0.48 A) and agrees well with previously determined X-ray structures. Furthermore, measurements in more native-like small phospholipid bicelles indicate that the protein structure is the same as in detergent micelles, suggesting that environment-specific effects are minimal when using mild detergents. We use our case study as a platform to discuss the feasibility of similar solution NMR studies for other 7TM proteins, including members of the family of G protein-coupled receptors.

Tuning microbial hosts for membrane protein production

The last four years have brought exciting progress in membrane protein research. Finally those many efforts that have been put into expression of eukaryotic membrane proteins are coming to fruition and enable to solve an ever-growing number of high resolution structures. In the past, many skilful optimization steps were required to achieve sufficient expression of functional membrane proteins. Optimization was performed individually for every membrane protein, but provided insight about commonly encountered bottlenecks and, more importantly, general guidelines how to alleviate cellular limitations during microbial membrane protein expression. Lately, system-wide analyses are emerging as powerful means to decipher cellular bottlenecks during heterologous protein production and their use in microbial membrane protein expression has grown in popularity during the past months.

This review covers the most prominent solutions and pitfalls in expression of eukaryotic membrane proteins using microbial hosts (prokaryotes, yeasts), highlights skilful applications of our basic understanding to improve membrane protein production. Omics technologies provide new concepts to engineer microbial hosts for membrane protein production.

Strategies for the cloning and expression of membrane proteins

Despite the determination of thousands of high-resolution structures of soluble proteins, many features of integral membrane proteins render them difficult targets for the structural biologist. Among these, the most important challenge is in expressing sufficient quantities of active protein to support downstream purification and structure determination efforts. Over 190 unique membrane protein structures have now been solved, and noticeable trends in successful expression strategies are beginning to emerge. A number of groups have also explored high-throughput (HTP) methods for membrane protein expression, with varying degrees of success. Here we review the current state of expressing membrane proteins for functional and structural studies. We first survey successful methods that have already yielded levels of membrane protein expression sufficient for structure determination. HTP methods are also examined since these aim to explore large numbers of targets and can predict reasonable starting points for many membrane proteins. Since HTP techniques may fail, particularly for certain classes of eukaryotic targets, detailed strategies for the expression of two prominent classes of eukaryotic protein families, G-protein-coupled receptors and ion channels, are also summarized.

Europium-labeled GTP as a general nonradioactive substitute for [(35)S]GTPgammaS in high-throughput G protein studies

[(35)S]GTPgammaS, the nonhydrolyzable radioactive GTP analog, has been a powerful tool in G protein studies and has set the standards in this field of research. However, its radioactive nature imposes clear limitations to its use in regular laboratory practice and in high-throughput experimentation. The europium-labeled GTP analog (Eu-GTP) has been used as an alternative in the analysis of G protein activation by G protein-coupled receptors in cellular membrane preparations. Here we expand the usage of Eu-GTP and show that it can be applied in other types of assays where [(35)S]GTPgammaS has been previously utilized. We demonstrate the applicability of the modified Eu-GTP binding technology to analysis of heterotrimeric and monomeric G proteins of natural and recombinant sources, from different organisms, in assays with soluble proteins and membrane-containing assays of a high-throughput format. The deci-nanomolar K(D) of Eu-GTP for the tested G proteins is similar to that of other fluorescent-modified GTP analogs, while the sensitivity achieved in time-resolved fluorescence analysis of Eu-GTP exceeds that of the radioactive measurements. Overall, the results of our modified Eu-GTP binding assay present Eu-GTP as a general nonradioactive alternative for G protein studies, especially attractive in high-throughput experiments.

Comprehensive proteomic mass spectrometric characterization of human cannabinoid CB2 receptor

The CB1 and CB2 cannabinoid receptors belong to the GPCR superfamily and are associated with a variety of physiological and pathophysiological processes. Both receptors, with several lead compounds at different phases of development, are potentially useful targets for drug discovery. For this reason, fully elucidating the structural features of these membrane-associated proteins would be extremely valuable in designing more selective, novel therapeutic drug molecules. As a first step toward obtaining information on the structural features of the drug-receptor complex, we describe the full mass spectrometric (MS) analysis of the recombinant human cannabinoid CB2 receptor. This first complete proteomic characterization of a GPCR protein beyond rhodopsin was accomplished by a combination of several LC/MS approaches involving nanocapillary liquid chromatography, coupled with either a quadrupole-linear ion trap or linear ion trap-FTICR mass spectrometer. The CB2 receptor, with incorporated N-terminal FLAG and C-terminal HIS6 epitope tags, was functionally expressed in baculovirus cells and purified using a single step of anti-FLAG M2 affinity chromatography. To overcome the difficulties involved with in-gel digestion, due to the highly hydrophobic nature of this membrane-associated protein, we conducted in-solution trypsin and chymotrypsin digestions of purified and desalted samples in the presence of a low concentration of CYMAL5. This was followed by nanoLC peptide separation and analysis using a nanospray ESI source operated in the positive mode. The results can be reported confidently, based on the overlapping sequence data obtained using the highly mass accurate LTQ-FT and the 4000 Q-Trap mass spectrometers. Both instruments gave very similar patterns of identified peptides, with full coverage of all transmembrane helices, resulting in the complete characterization of the cannabinoid CB2 receptor. Mass spectrometric identification of all amino acid residues in the cannabinoid CB2 receptor is a key step toward the "Ligand Based Structural Biology" approach developed in our laboratory for characterizing ligand binding sites in GPCRs using a variety of covalent cannabinergic ligands.

Heterologous GPCR expression: a bottleneck to obtaining crystal structures

G protein-coupled receptors (GPCRs) are an important, medically relevant class of integral membrane proteins. Laboratories throughout all disciplines of science devote time and energy into developing practical methods for the discovery, isolation, and characterization of these proteins. Since the crystal structure of rhodopsin was solved 6 years ago, the race to determine high-resolution structures of more GPCRs has gained momentum. Since certain GPCRs are currently produced at sufficient levels for X-ray crystallography trials, it is speculated that heterologous expression of GPCRs may no longer be a bottleneck in obtaining crystal structures. This Review focuses on the current approaches in heterologous expression of GPCRs and explores the problems associated with obtaining crystal structures from GPCRs expressed in different systems. Although milligram amounts of certain GPCRs are attainable, the majority of GPCRs are still either produced at very low levels or not at all. Developing reliable expression techniques for GPCRs is still a major priority for the structural characterization of GPCRs.

Use of dual affinity tags for expression and purification of functional peripheral cannabinoid receptor

The human peripheral cannabinoid receptor (CB2) was expressed as a fusion with the maltose-binding protein (at the N-terminus), thioredoxin A (at the C-terminus) and two small affinity tags (a Strep-tag and a polyhistidine tag). Expression levels of the recombinant receptor in Escherichia coli BL21(DE3) cells were dependent on location and type of tags in the expression construct, and were as high as 1-2mg per liter of bacterial culture. The recombinant receptor was ligand binding-competent, and activated cognate G-proteins in an in vitro coupled assay. The fusion CB2-125 protein was purified by immobilized metal affinity chromatography on a Ni-NTA resin. Maltose-binding protein, thioredoxin and a decahistidine tag were removed from the fusion by treatment with Tobacco etch virus (Tev) protease. Purification to over 90% homogeneity of the resulting CB2, containing an N-terminal Strep-tag was achieved by affinity chromatography on a StrepTactin resin. Circular dichroism spectroscopy indicated an alpha-helical content of the purified recombinant protein of approximately 54%. The expression and purification protocol allows for production of large (milligram) quantities of functional peripheral cannabinoid receptor, suitable for subsequent structural characterization. Preliminary results of reconstitution experiments indicate that the CB2 has retained its ligand-binding properties.

Large-scale expression and purification of a G-protein-coupled receptor for structure determination -- an overview

Structure determination of G-protein-coupled receptors and other applications, such as nuclear magnetic resonance studies, require milligram quantities of purified, functional receptor protein on a regular basis. We present an overview on expression and purification studies with a receptor for neurotensin. Functional expression in Escherichia coli and an automated two-column purification routine allow ongoing crystallization experiments and studies on receptor-bound ligands.

Regional differences in the coupling of 5-hydroxytryptamine-1A receptors to G proteins in the rat brain

Numerous data showed that 5-hydroxytryptamine-1A (5-HT1A) receptors couple to Galpha(o)/alpha(i) proteins for signal transduction. However, the alpha subunit isoforms really involved in 5-HT1A receptor coupling in brain remain to be identified. Moreover, regional differences in the functional characteristics of brain 5-HT1A receptors have been evidenced repeatedly. Because such differences could be due to variations in G proteins interacting with the same receptor, relevant approaches were used for identifying alpha subunits physically coupled to 5-HT1A receptors in different regions of the rat brain. Using immunoaffinity chromatography coupled to Western blot detection, 5-HT1A receptors were found to interact equally with Galpha(o) and Galpha(i3) in the cerebral cortex, mainly with Galpha(o) and weakly with Galpha(i3) in the hippocampus and exclusively with Galpha(i3) in the anterior raphe area. In the hypothalamus, 5-HT(1A) receptors seemed to be coupled to the latter two G proteins plus Galpha(i1) and Galpha(z). Complementary experiments based on an antibody capture technique coupled to both classic radioactivity and scintillation proximity assay detections showed that hippocampal 5-HT1A receptor stimulation induced 5'-O-(3-[35S]thio)triphosphate binding to immunoprecipitates with Galpha(i3) and Galpha(o) antisera. In the anterior raphe, such 5-HT1A receptor-mediated effect was obtained with Galpha(i3) antiserum only. These results demonstrated the existence of regional differences in the coupling of 5-HT1A receptors to G proteins in the rat brain. In the anterior raphe, 5-HT1A receptors seem to interact specifically with Galpha(i3), whereas in the hippocampus, they are mainly coupled to Galpha(o) proteins. Such a disparity in G-protein coupling might explain regional differences in adaptive regulations of brain 5-HT1A receptors.

Characterization of 16 human G protein-coupled receptors expressed in baculovirus-infected insect cells

Understanding the three-dimensional structure of G protein-coupled receptors (GPCRs) has been limited by the technical challenges associated with expression, purification, and crystallization of membrane proteins, and their low abundance in native tissue. In the first large-scale comparative study of GPCR protein production using recombinant baculovirus, we report the characterization of 16 human receptors. The GPCRs were produced in three insect cell lines and functional protein levels monitored over 72 h using radioligand binding assays. Different GPCRs exhibited widely different expression levels, ranging from less than 1 pmol receptor/mg protein to more than 250 pmol/mg. No single set of conditions was suitable for all GPCRs, and large differences were seen for the expression of individual GPCRs in different cell lines. Closely related GPCRs did not share similar expression profiles; however, high expression (greater than 20 pmol/mg) was achieved for over half the GPCRs in our study. Overall, the levels of protein production compared favourably to other published systems.

Isolation and Characterization of the B-Cell Marker CD20

The integral membrane protein CD20 has been identified as an important therapeutic target in the treatment of non-Hodgkin's lymphoma (NHL). CD20 binding of many antibodies including the therapeutic antibody, rituximab, has been shown to be critically dependent upon the conformation of a loop structure between the third and fourth helical transmembrane regions. In this work, human and murine CD20 proteins expressed in Escherichia coli are shown to be localized with the cell membrane and are purified in nondenaturing detergent solutions. The purified human and murine CD20 proteins have a substantial helical structure as measured by circular dichroism spectroscopy. Only small changes in the secondary structure are observed following the reduction of CD20, with the addition of SDS, or after heating. The rituximab antibody is shown to bind to purified human CD20 with nanomolar affinity. Rituximab binding is abolished by reduction and alkylation of CD20, with data consistent with the proposed antibody epitope being within the disulfide-bonded loop formed between cysteine residues 167 and 183. Disulfide-bond-dependent antibody binding is partially recovered following reoxidation of reduced CD20. Antibody binding is unaffected by mutations of cysteines proposed to be in the intracellular domain of CD20. The affinities of intact rituximab and its Fab fragment to the isolated and purified CD20 are similar to the observed affinity of rituximab Fab for CD20 on the surface of B cells. However, the intact rituximab antibody shows much higher affinity for CD20 on B cells. This suggests that B cells display CD20 in such a way that allows for marked avidity effects to be observed, perhaps through cross-linking of CD20 monomers into lipid rafts, which limits receptor diffusion in the membrane. Such cross-linking may play a role in partitioning CD20 into lipid rafts and in enhancing antibody-dependent B-cell depletion activities of rituximab and other therapeutic anti-CD20 antibodies.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

In vitro profile of the antidepressant candidate OPC-14523 at rat and human 5-HT1A receptors

This study determined the in vitro functional profile of 1-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-methoxy-3,4-dihydro-2-quinolinone monomethanesulfonate (OPC-14523) at rat and human serotonin (5-HT) 5-HT1A receptors and binding affinity of OPC-14523 at human frontocortical 5-HT1A receptors. OPC-14523 (1 microM) increased guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS) binding to 5-HT1A receptor-containing regions of rat brain tissue sections (approximately 53% of the effect of 1 microM (+)8-hydroxy-2-(di-n-propylamino)tetralin ((+)8-OH-DPAT) that were blocked by the selective 5-HT1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY-100635). OPC-14523 also behaved as a partial agonist in its stimulation of [35S]GTPgammaS binding to membranes from rat hippocampus (pEC50=7.60+/-0.23, Emax=41.1% of the effect of 10 microM (+)8-OH-DPAT), human frontal cortex (pEC50=7.89+/-0.08; Emax=64% of the effect of 10 microM (+)8-OH-DPAT), and Chinese Hamster Ovary cells expressing cloned human 5-HT1A receptors (pEC50=8.0+/-0.11; Emax=85.5% of the effect of 10 microM 5-HT), and all of these effects of OPC-14523 were blocked by WAY-100635. Taken together, these data support the development of OPC-14523 as an antidepressant whose mechanism of action involves potent partial agonist activity at 5-HT1A receptors.

Cloned human 5-HT1A receptor pharmacology determined using agonist binding and measurement of cAMP accumulation

Twenty agonists and nine antagonists were evaluated for their ability to compete for [3H]-8-hydroxy-2-(di-n-propylamino)tetralin ([3H]-8-OH-DPAT) binding to the cloned human serotonin-1A (ch-5-HT1A) receptor expressed in Chinese hamster ovary cells and for their ability to alter adenylyl cyclase activity in the same cells. The most potent full agonists of high affinity included N,N-dipropyl-5-carboxamidotryptamine (pEC50=9.6 +/- 0.1), MDL 73005EF (pEC50=9.3 +/- 0.2), 5-methyl-urapidil (pEC50=9.2 +/- 0.1), 5-carboxamidotryptamine (pEC50=9.1 +/- 0.2), R(+)-8-OH-DPAT (pEC50=8.6 +/- 0.1) and BMY-7378 (pEC50=8.6 +/- 0.1). WB-4101 (pEC50=8.3 +/- 0.2; IA=79%), clozapine (pEC50=8.1 +/- 0.3; IA=29%), (buspirone (pEC50=7.6 +/- 0.2; IA=79%), quipazine (pEC50 <5; IA=45%) and R-DOI (pEC50 < 5; IA=31%) were weaker agonists with partial agonist properties. The most potent antagonists were WAY-100,635 (pKi=10.2 +/- 0.1), methiothepin (pKi=8.8 +/- 0.2), spiperone (pKi=8.7 +/- 0.2) and NAN-190 (pKi=8.5 +/- 0.2). The receptor affinities and functional potencies were well correlated (r=0.88; P <0.0001). Our binding data correlated well with the pharmacology of endogenous 5-HT1A receptors in the rabbit iris-ciliary body (r=0.91; P <0.001) and rat hippocampus (r=0.93, P <0.0001). Our functional cAMP data correlated well with other cAMP accumulation data (r=0.8, P <0.01 vs calf hippocampus) but less so with [35S]-GTPgammaS binding to the ch-5-HT(1A) receptor as a functional activity read-out (r=0.58, P <0.05). The present study provides a detailed pharmacological characterization of the ch-5-HT1A receptor using binding and functional assays.

Antibody capture assay reveals bell-shaped concentration-response isotherms for h5-HT(1A) receptor-mediated Galpha(i3) activation: conformational selection by high-efficacy agonists, and relationship to trafficking of receptor signaling

Although serotonin 5-HT(1A) receptors couple to several Gi/o G-protein subtypes, little is known concerning their differential activation patterns. In this study, in membranes of Chinese hamster ovary cells expressing h5-hydroxytryptamine(1A) receptors (CHO-h5-HT(1A)), isotherms of 5-HT-stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding were biphasic, suggesting coupling to multiple G-protein subtypes. The high potency component was abolished by preincubation with an antibody recognizing Galpha(i3) subunits and was resistant to induction of [(35)S]GTPgammaS dissociation by unlabeled GTPgammaS, thus yielding a bell-shaped concentration-response isotherm. To directly investigate Galpha(i3) activation, we adopted an antibody-capture/scintillation proximity assay. 5-HT and other high-efficacy agonists yielded bell-shaped [(35)S]GTPgammaS binding isotherms, with peaks at nanomolar concentrations. As drug concentrations increased, Galpha(i3) stimulation progressively returned to basal values. In contrast, the partial agonists (-)-pindolol and 4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) displayed sigmoidal stimulation isotherms, whereas spiperone and other inverse agonists sigmoidally inhibited [(35)S]GTPgammaS binding. Agonist-induced stimulation and inverse agonist-induced inhibition of Galpha(i3) activation were i) abolished by pretreatment of CHO-h5-HT(1A) cells with pertussis toxin; ii) reversed by the selective 5-HT(1A) antagonist (N-[2-[4-(2-methoxy-phenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-cyclohexane-carboxamide) fumarate (WAY100,635), and iii) absent in nontransfected CHO cell membranes. 5-HT isotherms could be modified by altering sodium concentration; only stimulatory actions were observed at 300mM NaCl, whereas only inhibitory actions were seen at 10 mM NaCl. Furthermore, bell-shaped isotherms were not detected at short incubation times, suggesting time-dependent changes in receptor/Galpha(i3) coupling. Taken together, these data show that low but not high concentrations of high-efficacy 5-HT(1A) agonists direct receptor signaling to Galpha(i3). In contrast, partial agonists favor h5-HT(1A) receptor signaling to Galpha(i3) over a wide concentration range, whereas inverse agonists inhibit constitutive Galpha(i3) activation.

Receptor signaling and structure: insights from serotonin-1 receptors

Studies of the serotonin (5-HT) receptors have illustrated several important concepts in G-protein-mediated signaling. These concepts include G-protein specificity and cellular specificity of signaling; mechanisms of transactivation; receptor states and constitutive receptor activity; and the structural basis of coupling. The 5-HT1 receptors couple via specific G(i)/G(o) proteins to inhibitory pathways [inhibition of adenylyl cyclase (AC) activity and regulation of ion channels], but also to stimulate phospholipase C, ACII, and the mitogen-activated protein kinase (MAPK) growth-signaling pathway. 5-HT1 receptors initiate novel endocytotic and Ca(2+)-dependent pathways to activate MAPK acutely, but can downregulate MAPK on chronic activation. These pathways are often mediated via distinct G(i)/G(o)-protein subtypes. Desensitization by multiple protein kinases via receptor phosphorylation is pathway selective. Structural determination of 5-HT1 receptor and G-protein domains that mediate G-protein-specific coupling and desensitization could lead to the development of highly selective ligands that directly regulate receptor-G-protein coupling.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Recombinant Semliki Forest virus for over-expression and pharmacological characterisation of the histamine H(2) receptor in mammalian cells

We describe the use of recombinant Semliki Forest virus (SFV) vectors for efficient expression of the rat histamine H(2) (rH(2)) receptor in COS-7 (African green monkey kidney cells) cells. Recombinant SFV-infected COS-7 cells express the histamine rH(2) receptor in a time-dependent fashion with a maximum expression level of 50 pmol mg(-1) after 40 h. SFV-mediated histamine rH(2) receptor expression shows similar pharmacological properties as the receptor expressed transiently or stably in mammalian cells. In addition, we demonstrate the pharmacological and functional characterisation of the D(115)N mutated histamine rH(2) receptor. It has been shown that the D(115)N mutation renders the receptor constitutively active and structurally unstable. The rapid onset of and high maximal expression levels obtained from SFV-infected COS-7 cells enabled us to characterise this mutant receptor. We prove that recombinant SFV vectors are powerful tools for heterologous expression of G-protein-coupled receptors and that one can achieve both the high-level gene expression described for baculovirus-infected insect cells and the use of mammalian cells as hosts.

Functional coupling with Galpha(q) and Galpha(i1) protein subunits promotes high-affinity agonist binding to the neurotensin receptor NTS-1 expressed in Escherichia coli

To analyze the coupling of Galpha subunits to the rat neurotensin receptor NTS-1 (NTR), fusion proteins were expressed in Escherichia coli with various Galpha subunits covalently linked to the receptor C-terminus. The presence of Galpha(q) or Galpha(i/q), in which the six C-terminal residues of Galpha(i1) were replaced with those from Galpha(q), increased the percentage of receptors in the agonist high-affinity state. This effect was less pronounced for wild-type Galpha(i1) and not observed for Galpha(i/s). Functional coupling of neurotensin receptor to Galpha was demonstrated by neurotensin-induced [(35)S]GTPgammaS binding for the Galpha(q), Galpha(i/q) and Galpha(i1) subunits, but not for Galpha(i/s). Our results extend previous findings of the dual coupling of NTR to pertussis toxin-sensitive and -insensitive G-proteins in Chinese hamster ovary cells with preference for the latter.

Heuristic Statistical Analysis of Fluorescence Fluctuation Data with Bright Spikes: Application to Ligand Binding to the Human Serotonin Receptor Expressed in Escherichia coli Cells

A statistical method for the analysis of fluorescence fluctuation amplitudes including bright spikes is presented. This situation arises e. g. when fluorescent ligands interact with receptors carrying multiple binding sites. The technique gives information on the amount of bound ligand in solution, making it a complementary technique to fluorescence correlation spectroscopy analysis, which cannot be applied in this situation. Two simple statistical tests are proposed that can discriminate between fluorescence intensities originating from free ligands or complexes. The performance of the two tests is evaluated and compared on mixtures of a fluorophore and fluorophore coated beads that mimic the behaviour of multiliganded complexes. An application to ligand binding to the serotonin receptor, expressed on Escherichia coli cells, is also provided. Specific binding of a fluorophore to this receptor, as well as competition with several ligands, is assessed.

Estrogen desensitizes 5-HT(1A) receptors and reduces levels of G(z), G(i1) and G(i3) proteins in the hypothalamus

The present study investigated whether estrogen would desensitize hypothalamic serotonin(1A) (5-HT(1A)) receptors by examining the neuroendocrine response to 8-OH-DPAT, a 5-HT(1A) agonist. Rats were ovariectomized, allowed to recover for 5 days, then given 2 daily injections of estradiol benzoate or vehicle (10 microg/day, s.c.). Twenty-four hours after the second injection, rats were challenged with a sub-maximal dose of 8-OH-DPAT (50 microg/kg, sc) or saline 15 min prior to sacrifice. 8-OH-DPAT produced a significant increase in plasma oxytocin, ACTH and corticosterone levels in ovariectomized rats. While estrogen treatment for 2 days did not alter basal hormone levels, it did significantly reduce the magnitude of oxytocin, ACTH and corticosterone responses to 8-OH-DPAT. The reduction in hormone responses was accompanied by a significant reduction in hypothalamic levels of G(z), G(i1) and G(i3) proteins (by 50%, 30% and 50%, respectively). These findings suggest that a reduction in these G proteins may contribute to the mechanisms underlying estrogen-induced desensitization of 5-HT(1A) receptors. The desensitization of 5-HT(1A) receptors has been suggested to underlie the therapeutic effects of antidepressant 5-HT uptake inhibitors (SSRIs). Thus, the present results suggest that estrogen or estrogen-like substances in combination with SSRIs may prove effective in developing novel therapeutic strategies for neuropsychiatric disorders in women.

Constitutive G(i2)-dependent activation of adenylyl cyclase type II by the 5-HT1A receptor. Inhibition by anxiolytic partial agonists

The 5-HT1A receptor is implicated in depression and anxiety. This receptor couples to G(i) proteins to inhibit adenylyl cyclase (AC) activity but can stimulate AC in tissues (e.g. hippocampus) that express ACII. The role of ACII in receptor-mediated stimulation of cAMP formation was examined in HEK-293 cells transfected with the 5-HT1A receptor, which mediated inhibition of basal and G(s)-induced cAMP formation in the absence of ACII. In cells cotransfected with 5-HT1A receptor and ACII plasmids, 5-HT1A agonists induced a 1. 5-fold increase in cAMP level. Cotransfection of 5-HT1A receptor, ACII, and Galpha(i2), but not Galpha(i1), Galpha(i3), or Galpha(o), resulted in an agonist-independent 6-fold increase in the basal cAMP level, suggesting that G(i2) preferentially coupled the receptor to ACII. The 5-HT1B receptor also constitutively activated ACII. Constitutive activity of the 5-HT1A receptor was blocked by pertussis toxin and the Gbetagamma antagonist, betaCT, suggesting an important role for Gbetagamma-mediated activation of ACII. The Thr-149 --> Ala mutation in the second intracellular domain of the 5-HT1A receptor disrupted Gbetagamma-selective activation of ACII. Spontaneous 5-HT1A receptor activity was partially attenuated by 5-HT1A receptor partial agonists with anxiolytic activity (e.g. buspirone and flesinoxan) but was not altered by full agonists or antagonists. Thus, anxiolytic activity may involve inhibition of spontaneous 5-HT1A receptor activity.

Quantitative analysis of formyl peptide receptor coupling to g(i)alpha(1), g(i)alpha(2), and g(i)alpha(3)

The human formyl peptide receptor (FPR) is a prototypical G(i) protein-coupled receptor, but little is known about quantitative aspects of FPR-G(i) protein coupling. To address this issue, we fused the FPR to G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) and expressed the fusion proteins in Sf9 insect cells. Fusion of a receptor to Galpha ensures a defined 1:1 stoichiometry of the signaling partners. By analyzing high affinity agonist binding, the kinetics of agonist- and inverse agonist-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding and GTP hydrolysis and photolabeling of Galpha, we demonstrate highly efficient coupling of the FPR to fused G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) without cross-talk of the receptor to insect cell G proteins. The FPR displayed high constitutive activity when coupled to all three G(i)alpha isoforms. The K(d) values of high affinity agonist binding were approximately 100-fold lower than the EC(50) (concentration that gives half-maximal stimulation) values of agonist for GTPase activation. Based on the B(max) values of agonist saturation binding and ligand-regulated GTPgammaS binding, it was previously proposed that the FPR activates G proteins catalytically, i.e. one FPR activates several G(i) proteins. Analysis of agonist saturation binding, ligand-regulated GTPgammaS saturation binding and quantitative immunoblotting with membranes expressing FPR-G(i)alpha fusion proteins and nonfused FPR now reveals that FPR agonist binding greatly underestimates the actual FPR expression level. Our data show the following: (i) the FPR couples to G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) with similar efficiency; (ii) the FPR can exist in a state of low agonist affinity that couples efficiently to G proteins; and (iii) in contrast to the previously held view, the FPR appears to activate G(i) proteins linearly and not catalytically.

The 5HT(1A) receptor ligand, S15535, antagonises G-protein activation: a [35S]GTPgammaS and [3H]S15535 autoradiography study

4-(Benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) is a highly selective ligand at 5-HT(1A) receptors. The present study compared its autoradiographic labelling of rat brain sections with its functional actions, visualised by guanylyl-5'-[gamma-thio]-triphosphate ([35S]GTPgammaS) autoradiography, which affords a measure of G-protein activation. [3H]S15535 binding was highest in hippocampus, frontal cortex, entorhinal cortex, lateral septum, interpeduncular nucleus and dorsal raphe, consistent with specific labelling of 5-HT(1A) receptors. In functional studies, S15535 (10 microM) did not markedly stimulate G-protein activation in any brain region, but abolished the activation induced by the selective 5-HT(1A) agonist, (+)-8-hydroxy-dipropyl-aminotetralin ((+)-8-OH-DPAT, 1 microM), in structures enriched in [3H]S15535 labelling. S15535 did not block 5-HT-stimulated activation in caudate nucleus or substantia nigra, regions where (+)-8-OH-DPAT was ineffective and [3H]S15535 binding was absent. Interestingly, S15535 attenuated (+)-8-OH-DPAT and 5-HT-stimulated G-protein activation in dorsal raphe, a region in which S15535 is known to exhibit agonist properties in vivo [Lejeune, F., Millan, M.J., 1998. Induction of burst firing in ventral tegmental area dopaminergic neurons by activation of serotonin (5-HT)(1A) receptors: WAY100,635-reversible actions of the highly selective ligands, flesinoxan and S15535. Synapse 30, 172-180.]. The present data show that (i) [3H]S15535 labels pre- and post-synaptic populations of 5-HT(1A) sites in rat brain sections, (ii) S15535 exhibits antagonist properties at post-synaptic 5-HT(1A) receptors in corticolimbic regions, and (iii) S15535 also attenuates agonist-stimulated G-protein activation at raphe-localised 5-HT(1A) receptors.