Structure and function analyses of the purified GPCR human vomeronasal type 1 receptor 1

Key aspects of modern GPCR drug discovery

G-protein-coupled receptors (GPCRs) are the largest and most versatile cell surface receptor family with a broad repertoire of ligands and functions. We've learned an enormous amount about discovering drugs of this receptor class since the first GPCR was cloned and expressed in 1986, such that it's now well-recognized that GPCRs are the most successful target class for approved drugs. Here we take the reader through a GPCR drug discovery journey from target to the clinic, highlighting the key learnings, best practices, challenges, trends and insights on discovering drugs that ultimately modulate GPCR function therapeutically in patients. The future of GPCR drug discovery is inspiring, with more desirable drug mechanisms and new technologies enabling the delivery of better and more successful drugs.

Anti-atherosclerotic Effects of Myrtenal in High-Fat Diet-Induced Atherosclerosis in Rats

The major cause of death worldwide is atherosclerosis-related cardiovascular disease (ACD). Myrtenal was studied to determine control rats were given standard diets and a high-fat diet was given to AS model groups. Atherosclerosis-related cardiovascular disease (ACD) is globally attributed to being a predominant cause of mortality. While the beneficial effects of Myrtenal, the monoterpene from natural compounds, are increasingly being acknowledged, its anti-atherosclerotic activity has not been demonstrated clearly. The present study is proposed to determine the anti-atherosclerotic activity of Myrtenal in high-fat diet-induced atherosclerosis (AS) rat models. Control groups were maintained with standard diets, the AS model rats were provided a high-fat diet, two of the experimental groups fed with a high-fat diet were treated with Myrtenal (50 mg/kg and 100 mg/kg), and one experimental group on high-fat diet was treated with simvastatin (10 mg/kg) for 30 days. The levels of inflammatory cytokines were analyzed using kits. The lipoproteins and the lipid profile were estimated using an auto-analyzer. The atherogenic index and marker enzyme activities were also determined. Serum concentrations of 6-keto-prostaglandin F1α (6-keto-PGF1α), thromboxaneB2 (TXB2), endothelin (ET), and nitric oxide (NO) were measured. The AS model groups indicated altered lipid profile, lipoprotein content, atherogenic index, calcium levels, HMG-CoA reductase activity, collagen level, and mild mineralization indicating atherosclerosis, while the AS-induced Myrtenal-treated groups demonstrated anti-atherogenic activity. The Myrtenal-treated groups exhibited a decreased TC, TG, and LDLc levels; increased HDLc levels; and a decline in the inflammatory cytokines such as CRP, IL-1β, IL-8, and IL-18 when compared to the untreated AS rats. Furthermore, Myrtenal decreased ET, TXB2, and 6-keto-PGF1α levels indicating its anti-atherosclerotic activity. The study results thus indicate that Myrtenal modulates the lipid metabolic pathway to exert its anti-atherosclerotic activity.

An original approach to measure ligand/receptor binding affinity in non-purified samples

Several biochemical and biophysical methods are available to determine ligand binding affinities between a biological target and its ligands, most of which require purification, labelling or surface immobilisation. These measurements, however, remain challenging in regards to membrane proteins, as purification processes require their extraction from their native lipid environment, which may in turn impact receptor conformation and functionality. In this study, we have developed a novel experimental procedure using microscale thermophoresis (MST) directly from cell membrane fragments, to determine different ligand binding affinities to a membrane protein, the dopamine D2 receptor (D2R). In order to achieve this, two main challenges had to be overcome: determining the concentration of dopamine D2R in the crude sample; finding ways to minimize or account for non-specific binding of the ligand to cell fragments. Using MST, we were able to determine the D2R concentration in cell membrane fragments to approximately 36.8 ± 2.6 pmol/mg. Next, the doses-responses curves allowed for the determination of K_D, to approximately 5.3 ± 1.7 nM, which is very close to the reported value. Important details of the experimental procedure have been detailed in this paper to allow the application of this novel method to various membrane proteins.

Functional Characterization of the γ-Aminobutyric Acid Transporter from Mycobacterium smegmatis MC2 155 Reveals Sodium-Driven GABA Transport

The spread of multidrug-resistant tuberculosis increases its global health impact in humans. As there is transmission both to and from animals, the spread of the disease also increases its effects in a broad range of animal species.

Characterizing the mycobacterial transporters involved in the uptake and/or catabolism of host-derived nutrients required by mycobacteria may identify novel drug targets against tuberculosis. Here, we identify and characterize a member of the amino acid-polyamine-organocation superfamily, a potential γ-aminobutyric acid (GABA) transport protein, GabP, from Mycobacterium smegmatis. The protein was expressed to a level allowing its purification to homogeneity, and size exclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) analysis of the purified protein showed that it was dimeric. We showed that GabP transported γ-aminobutyric acid both in vitro and when overexpressed in E. coli. Additionally, transport was greatly reduced in the presence of β-alanine, suggesting it could be either a substrate or inhibitor of GabP. Using GabP reconstituted into proteoliposomes, we demonstrated that γ-aminobutyric acid uptake is driven by the sodium gradient and is stimulated by membrane potential. Molecular docking showed that γ-aminobutyric acid binds MsGabP, another Mycobacterium smegmatis putative GabP, and the Mycobacterium tuberculosis homologue in the same manner. This study represents the first expression, purification, and characterization of an active γ-aminobutyric acid transport protein from mycobacteria.

IMPORTANCE The spread of multidrug-resistant tuberculosis increases its global health impact in humans. As there is transmission both to and from animals, the spread of the disease also increases its effects in a broad range of animal species. Identifying new mycobacterial transporters will enhance our understanding of mycobacterial physiology and, furthermore, provides new drug targets. Our target protein is the gene product of msmeg_6196, annotated as GABA permease, from Mycobacterium smegmatis strain MC² 155. Our current study demonstrates it is a sodium-dependent GABA transporter that may also transport β-alanine. As GABA may well be an essential nutrient for mycobacterial metabolism inside the host, this could be an attractive target for the development of new drugs against tuberculosis.

Functional Assembly of Caenorhabditis elegans Cytochrome b-2 (Cecytb-2) into Phospholipid Bilayer Nanodisc with Enhanced Iron Reductase Activity

Among seven homologs of cytochrome b₅₆₁ in a model organism C. elegans, Cecytb-2 was confirmed to be expressed in digestive organs and was considered as a homolog of human Dcytb functioning as a ferric reductase. Cecytb-2 protein was expressed in Pichia pastoris cells, purified, and reconstituted into a phospholipid bilayer nanodisc. The reconstituted Cecytb-2 in nanodisc environments was extremely stable and more reducible with ascorbate than in a detergent-micelle state. We confirmed the ferric reductase activity of Cecytb-2 by analyzing the oxidation of ferrous heme upon addition of ferric substrate under anaerobic conditions, where clear and saturable dependencies on the substrate concentrations following the Michaelis–Menten equation were observed. Further, we confirmed that the ferric substrate was converted to a ferrous state by using a nitroso-PSAP assay. Importantly, we observed that the ferric reductase activity of Cecytb-2 became enhanced in the phospholipid bilayer nanodisc.

G-Protein-Coupled Receptor Expression and Purification

G-protein-coupled receptors (GPCRs) are integral proteins of the cell membrane and are directly involved in the regulation of many biological functions and in drug targeting. However, our knowledge of GPCRs' structure and function remains limited. The first bottleneck in GPCR studies is producing sufficient quantities of soluble, functional, and stable receptors. Currently, GPCR production largely depends on the choice of the host system and the type of detergent used to extract the GPCR from the cell membrane and stabilize the protein outside the membrane bilayer. Here, we present three protocols that we employ in our lab to produce and solubilize stable GPCRs: (1) cell-free in vitro translation, (2) HEK cells, and (3) Escherichia coli. Stable receptors can be purified using immunoaffinity chromatography and gel filtration, and can be analyzed with standard biophysical techniques and biochemical assays.

The G protein coupled receptor CXCR4 designed by the QTY code becomes more hydrophilic and retains cell signaling activity

G protein-coupled receptors (GPCRs) are vital for diverse biological functions, including vision, smell, and aging. They are involved in a wide range of diseases, and are among the most important targets of medicinal drugs. Tools that facilitate GPCR studies or GPCR-based technologies or therapies are thus critical to develop. Here we report using our QTY (glutamine, threonine, tyrosine) code to systematically replace 29 membrane-facing leucine, isoleucine, valine, and phenylalanine residues in the transmembrane α-helices of the GPCR CXCR4. This variant, CXCR4^QTY29, became more hydrophilic, while retaining the ability to bind its ligand CXCL12. When transfected into HEK293 cells, it inserted into the cell membrane, and initiated cellular signaling. This QTY code has the potential to improve GPCR and membrane protein studies by making it possible to design functional hydrophilic receptors. This tool can be applied to diverse α-helical membrane proteins, and may aid in the development of other applications, including clinical therapies.

Self-assembling peptides: From a discovery in a yeast protein to diverse uses and beyond

Well-defined nanofiber scaffold hydrogels made of self-assembling peptides have found their way into various 3D tissue culture and clinical products. I reflect initial puzzlement of the unexpected discovery, gradual understanding of how these peptides undergo self-assembly, to eventually translating designer biological scaffolds into commercial products. Peptides are ubiquitous in nature and useful in many fields. They are found as hormones, pheromones, antibacterial, and antifungal agents in innate immunity systems, toxins, as well anti-inset pesticides. However, the concept of peptides as materials was not recognized until 1990 when a self-assembling peptide as a repeating segment in a yeast protein was serendipitously discovered. The peptide materials have bona fide materials properties and are made from simple amino acids with well-ordered nanostructures under physiological conditions. Some current applications include: (a) Real 3D tissue cell cultures of diverse tissue cells and various stem cells; (b) reparative and regenerative medicine as well as tissue engineering; (c) 3D tissue printing; (d) sustained releases of small molecules, growth factors and monoclonal antibodies; and (e) accelerated wound healing of skin and diabetic ulcers as well as instant hemostasis in surgery. Self-assembling peptide nanobiotechnology will likely continue to expand in many directions in the coming years. I will also briefly introduce my current research using a simple QTY code for membrane protein design. I am greatly honored and humbled to be invited to contribute an Award Winner Recollection of the 2020 Emil Thomas Kaiser Award from the Protein Society.

[Modern studies of the role of the vomeronasal system in the perception of pheromones and their impact on social and sexual behavior]

The vomeronasal system (VNS) provides regulation of a wide range of autonomic and affective functions, behavioral reactions in response to the specific chemical stimuli pheromones secreted by mammals, including humans. The results of experimental studies confirming the existence of VNS and explaining the basic mechanisms of its functioning are presented. The results of studies of healthy volunteers, explaining the effect of pheromones on a number of functions of the human body, are considered.

Modular detergents tailor the purification and structural analysis of membrane proteins including G-protein coupled receptors

Detergents enable the purification of membrane proteins and are indispensable reagents in structural biology. Even though a large variety of detergents have been developed in the last century, the challenge remains to identify guidelines that allow fine-tuning of detergents for individual applications in membrane protein research. Addressing this challenge, here we introduce the family of oligoglycerol detergents (OGDs). Native mass spectrometry (MS) reveals that the modular OGD architecture offers the ability to control protein purification and to preserve interactions with native membrane lipids during purification. In addition to a broad range of bacterial membrane proteins, OGDs also enable the purification and analysis of a functional G-protein coupled receptor (GPCR). Moreover, given the modular design of these detergents, we anticipate fine-tuning of their properties for specific applications in structural biology. Seen from a broader perspective, this represents a significant advance for the investigation of membrane proteins and their interactions with lipids.

Detergents are indispensable reagents in membrane protein structural biology. Here, L. H. Urner and co-workers introduce oligoglycerol detergents (OGDs) and use native mass spectrometry to show how interactions of membrane proteins with native membrane lipids can be preserved during purification.

Discovery of the first self-assembling peptide, study of peptide dynamic behaviors, and G protein-coupled receptors using an Aviv circular dichroism spectropolarimeter

Circular dichroism (CD) spectroscopy is a useful technique to study the structure and dynamics of peptides, proteins and nucleic acids. CD is particularly useful because sample volumes may be as low as 50 μL, it provides high precision and sensitivity, and it achieves a good signal to noise ratio. CD characterizes molecular conformational changes in real time by finely controlling temperature, pH, and titrating urea and guanidine·HCl which is necessary for studying protein folding. Although CD does not provide detailed structure at the atomic level, it provides a global structural framework. Researchers use CD to observe molecular phenomena, namely how macromolecules unfold/refold and their overall self-assembly/disassembly. Using CD to monitor a peptide structure, I serendipitously discovered the self-assembling peptide EAK16 from yeast protein Zuotin. This unusual peptide formed a new type of nanofiber scaffold hydrogel material. The discovery in 1990 opened a new field in the design and study of numerous self-assembling peptides, thereby launching the area of peptide nanobiotechnology. In this review, I reflect on my personal discoveries of several self-assembling peptides, investigations into the dynamic behaviors of peptides, as well as the impact of the work on society. I also describe studies of natural membrane proteins and engineered membrane proteins using CD. Furthermore, I enjoyed numerous and close interactions with Jack Aviv since 1997. He generously supported 10 high impact workshops (Crete and Mikonos) and meetings in various countries around the world that left fond memories of many young researches who later became leading scientists in their respective fields.

Establishment of a novel microscale thermophoresis ligand-binding assay for characterization of SLC solute carriers using oligopeptide transporter PepT1 (SLC15 family) as a model system

INTRODUCTION

Membrane proteins represent roughly one third of the human proteome and many of them serve as targets of therapeutic drugs. An exception is the SLC solute carrier superfamily with only a handful of approved drugs targeting SLCs. Indeed, for many of the SLCs, the natural transport substrates are still unknown. A major limitation for SLCs has been the difficulty to thoroughly characterize these multimembrane spanning proteins. The intrinsic properties of membrane proteins with alternative hydrophobic and hydrophilic domains lead to instability, making the purification tasks even more challenging compared to soluble proteins. This issue also holds true for conventional ligand-binding assays (LBAs) which usually require high-quality, pure and concentrated protein samples. Herein, we report a novel binding assay strategy to overcome these issues, taking advantage of a unique combination of yeast expression and microscale thermophoresis (MST). Following yeast overexpression of SLC15A1/PepT1 ortholog from moss Physcomitrella patens, PepT_Pp, which exhibits remarkable similarity to human PepT1, the approach was validated using dipeptide glycylsarcosine (Gly-Sar) and antiviral prodrug valacyclovir as test substrates.

METHOD

The originality of our approach is based on the comparative analysis of solubilized total membrane preparations with or without expression of the SLC target of interest, using a yeast strain (S. cerevisiae), in which the corresponding endogenous SLC homolog is depleted. MST is a recently developed technique that takes advantage of the properties of biomolecules in solution to migrate along a temperature gradient. Importantly, this migration is affected by substrate binding. It is being monitored by fluorescence using labelled SLC molecules in the presence of different ligand concentrations.

RESULTS

We herein report a novel MST/yeast-based method to characterize binding of ligands to SLCs without the need for a prior SLC-purification step. For validation purposes, we used a close eukaryotic homolog of the human H⁺-coupled oligopeptide transporter PepT1 (SLC15A1) that mediates uptake of di-tripeptides and peptide-like drugs as a test model. This approach allowed the successful confirmation of the binding of Gly-Sar at the mM range and revealed for the first time the K_D of the antiviral prodrug valacyclovir to the PepT1 homolog at around 50 μM.

DISCUSSION

This novel LBA approach is independent of protein purification. It is suitable for drug discovery as it is upscalable to high throughput compound screening. It works well for SLC transporters which are underrepresented targets due to their difficulties to study them. Moreover, this approach could make a significant contribution toward "deorphanization" of SLCs, revealing their transport substrates.

Applications and Advances in Bioelectronic Noses for Odour Sensing

A bioelectronic nose, an intelligent chemical sensor array system coupled with bio-receptors to identify gases and vapours, resembles mammalian olfaction by which many vertebrates can sniff out volatile organic compounds (VOCs) sensitively and specifically even at very low concentrations. Olfaction is undertaken by the olfactory system, which detects odorants that are inhaled through the nose where they come into contact with the olfactory epithelium containing olfactory receptors (ORs). Because of its ability to mimic biological olfaction, a bio-inspired electronic nose has been used to detect a variety of important compounds in complex environments. Recently, biosensor systems have been introduced that combine nanoelectronic technology and olfactory receptors themselves as a source of capturing elements for biosensing. In this article, we will present the latest advances in bioelectronic nose technology mimicking the olfactory system, including biological recognition elements, emerging detection systems, production and immobilization of sensing elements on sensor surface, and applications of bioelectronic noses. Furthermore, current research trends and future challenges in this field will be discussed.

A cell-free platform for rapid synthesis and testing of active oligosaccharyltransferases

Protein glycosylation, or the attachment of sugar moieties (glycans) to proteins, is important for protein stability, activity, and immunogenicity. However, understanding the roles and regulations of site-specific glycosylation events remains a significant challenge due to several technological limitations. These limitations include a lack of available tools for biochemical characterization of enzymes involved in glycosylation. A particular challenge is the synthesis of oligosaccharyltransferases (OSTs), which catalyze the attachment of glycans to specific amino acid residues in target proteins. The difficulty arises from the fact that canonical OSTs are large (>70 kDa) and possess multiple transmembrane helices, making them difficult to overexpress in living cells. Here, we address this challenge by establishing a bacterial cell-free protein synthesis platform that enables rapid production of a variety of OSTs in their active conformations. Specifically, by using lipid nanodiscs as cellular membrane mimics, we obtained yields of up to 420 μg/ml for the single-subunit OST enzyme, "Protein glycosylation B" (PglB) from Campylobacter jejuni, as well as for three additional PglB homologs from Campylobacter coli, Campylobacter lari, and Desulfovibrio gigas. Importantly, all of these enzymes catalyzed N-glycosylation reactions in vitro with no purification or processing needed. Furthermore, we demonstrate the ability of cell-free synthesized OSTs to glycosylate multiple target proteins with varying N-glycosylation acceptor sequons. We anticipate that this broadly applicable production method will advance glycoengineering efforts by enabling preparative expression of membrane-embedded OSTs from all kingdoms of life.

TRPA1-FGFR2 binding event is a regulatory oncogenic driver modulated by miRNA-142-3p

Recent evidence suggests that the ion channel TRPA1 is implicated in lung adenocarcinoma (LUAD), where its role and mechanism of action remain unknown. We have previously established that the membrane receptor FGFR2 drives LUAD progression through aberrant protein–protein interactions mediated via its C-terminal proline-rich motif. Here we report that the N-terminal ankyrin repeats of TRPA1 directly bind to the C-terminal proline-rich motif of FGFR2 inducing the constitutive activation of the receptor, thereby prompting LUAD progression and metastasis. Furthermore, we show that upon metastasis to the brain, TRPA1 gets depleted, an effect triggered by the transfer of TRPA1-targeting exosomal microRNA (miRNA-142-3p) from brain astrocytes to cancer cells. This downregulation, in turn, inhibits TRPA1-mediated activation of FGFR2, hindering the metastatic process. Our study reveals a direct binding event and characterizes the role of TRPA1 ankyrin repeats in regulating FGFR2-driven oncogenic process; a mechanism that is hindered by miRNA-142-3p.

TRPA1 has been reported to contribute lung cancer adenocarcinoma (LUAD), but the mechanisms are unclear. Here the authors propose that TRPA1/FGFR2 interaction is functional in LUAD and show that astrocytes oppose brain metastasis by mediating the downregulation of TRPA1 through exosome-delivered miRNA-142-3p.

Production of G protein-coupled receptors in an insect-based cell-free system

The biochemical analysis of human cell membrane proteins remains a challenging task due to the difficulties in producing sufficient quantities of functional protein. G protein‐coupled receptors (GPCRs) represent a main class of membrane proteins and drug targets, which are responsible for a huge number of signaling processes regulating various physiological functions in living cells. To circumvent the current bottlenecks in GPCR studies, we propose the synthesis of GPCRs in eukaryotic cell‐free systems based on extracts generated from insect (Sf21) cells. Insect cell lysates harbor the fully active translational and translocational machinery allowing posttranslational modifications, such as glycosylation and phosphorylation of de novo synthesized proteins. Here, we demonstrate the production of several GPCRs in a eukaryotic cell‐free system, performed within a short time and in a cost‐effective manner. We were able to synthesize a variety of GPCRs ranging from 40 to 133 kDa in an insect‐based cell‐free system. Moreover, we have chosen the μ opioid receptor (MOR) as a model protein to analyze the ligand binding affinities of cell‐free synthesized MOR in comparison to MOR expressed in a human cell line by “one‐point” radioligand binding experiments. Biotechnol. Bioeng. 2017;114: 2328–2338. © 2017 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.

Purification of family B G protein-coupled receptors using nanodiscs: Application to human glucagon-like peptide-1 receptor

Family B G protein-coupled receptors (GPCRs) play vital roles in hormone-regulated homeostasis. They are drug targets for metabolic diseases, including type 2 diabetes and osteoporosis. Despite their importance, the signaling mechanisms for family B GPCRs at the molecular level remain largely unexplored due to the challenges in purification of functional receptors in sufficient amount for biophysical characterization. Here, we purified the family B GPCR human glucagon-like peptide-1 (GLP-1) receptor (GLP1R), whose agonists, e.g. exendin-4, are used for the treatment of type 2 diabetes mellitus. The receptor was expressed in HEK293S GnTl- cells using our recently developed protocol. The protocol incorporates the receptor into the native-like lipid environment of reconstituted high density lipoprotein (rHDL) particles, also known as nanodiscs, immediately after the membrane solubilization step followed by chromatographic purification, minimizing detergent contact with the target receptor to reduce denaturation and prolonging stabilization of receptor in lipid bilayers without extra steps of reconstitution. This method yielded purified GLP1R in nanodiscs that could bind to GLP-1 and exendin-4 and activate Gs protein. This nanodisc purification method can potentially be a general strategy to routinely obtain purified family B GPCRs in the 10s of microgram amounts useful for spectroscopic analysis of receptor functions and activation mechanisms.

A missense polymorphism in the putative pheromone receptor gene VN1R1 is associated with sociosexual behavior

Pheromones regulate social and reproductive behavior in most mammalian species. These effects are mediated by the vomeronasal and main olfactory systems. Effects of putative pheromones on human neuroendocrine activity, brain activity and attractiveness ratings suggest that humans may communicate via similar chemosignaling. Here we studied two samples of younger and older individuals, respectively, with respect to one nonsynonymous polymorphism in the gene encoding the human vomeronasal type-1 receptor 1, VN1R1, and one nonsynonymous polymorphism in the gene encoding the olfactory receptor OR7D4. Participants in both samples had self-reported their sociosexual behavior using the sociosexual orientation inventory, including questions regarding lifetime number of one-night stands, number of partners last year and expected number of partners the coming 5 years. In women, there was a significant association between the VN1R1 polymorphism and sociosexual behavior in both samples, driven specifically by the question regarding one-night stands. Our results support the hypothesis that human social interaction is modulated by communication via chemosignaling.

Zinc Is Involved in Depression by Modulating G Protein-Coupled Receptor Heterodimerization

5-Hydroxytryptamine 1A receptor and galanin receptor 1 belong to the G protein-coupled receptors superfamily, and they have been described to heterodimerize triggering an anomalous physiological state that would underlie depression. Zinc supplementation has been widely reported to improve treatment against major depressive disorder. Our work has focused on the study and characterization of these receptors and its relationships with zinc both under purified conditions and in cell culture. To this aim, we have designed a strategy to purify the receptors in a conformationally active state. We have used receptors tagged with the monoclonal Rho-1D4 antibody and employed ligand-assisted purification in order to successfully purify both receptors in a properly folded and active state. The interaction between both purified receptors has been analyzed by surface plasmon resonance in order to determine the kinetics of dimerization. Zinc effect on heteromer has also been tested using the same methodology but exposing the 5-hydroxytryptamine 1A receptor to zinc before the binding experiment. These results, combined with Förster resonance energy transfer (FRET) measurements, in the absence and presence of zinc, suggest that this ion is capable of disrupting this interaction. Moreover, molecular modeling suggests that there is a coincidence between zinc-binding sites and heterodimerization interfaces for the serotonin receptor. Our results establish a rational explanation for the role of zinc in the molecular processes associated with receptor-receptor interactions and its relationship with depression, in agreement with previously reported evidence for the positive effects of zinc in depression treatment, and the involvement of our target dimer in the same disease.

The smelling of Hedione results in sex-differentiated human brain activity

A large family of vomeronasal receptors recognizes pheromone cues in many animals including most amphibia, reptiles, rhodents, and other mammals. Humans possess five vomeronasal-type 1 receptor genes (VN1R1-VN1R5), which code for proteins that are functional in recombinant expression systems. We used two different recombinant expression systems and identified Hedione as a ligand for the putative human pheromone receptor VN1R1 expressed in the human olfactory mucosa. Following the ligand identification, we employed functional magnetic resonance imaging (fMRI) in healthy volunteers to characterize the in vivo action of the VN1R1 ligand Hedione. In comparison to a common floral odor (phenylethyl alcohol), Hedione exhibited significantly enhanced activation in limbic areas (amygdala, hippocampus) and elicited a sex-differentiated response in a hypothalamic region that is associated with hormonal release. Utilizing a novel combination of methods, our results indicate that the putative human pheromone receptor VN1R1 is involved in extra-olfactory neuronal activations induced by the odorous substance Hedione. The activation of VN1R1 might play a role in gender-specific modulation of hormonal secretion in humans.

Cell-free expression, purification, and ligand-binding analysis of Drosophila melanogaster olfactory receptors DmOR67a, DmOR85b and DmORCO

Insects transmit numerous devastating diseases, including malaria, dengue fever, and sleeping sickness. Olfactory cues guide insects to their hosts, and are thus responsible for disease transmission. Understanding the molecular basis of insect olfaction could facilitate the development of interventions. The first step is to heterologously overexpress and purify insect olfactory receptors (ORs). This is challenging, as ORs are membrane proteins. Here, we show that insect ORs and their co-receptor can be expressed in an E. coli cell-free system. After immunoaffinity chromatography, the ORs are ~95% pure, and up to 1 mg/10 ml reaction is obtained. Circular dichroism together with microscale thermophoresis indicate that each receptor is properly folded, and can bind its respective ligand. This is the first time insect ORs have been expressed in an E. coli system. The methods described here could facilitate future structure-function studies, which may aid in developments to alleviate the suffering of millions caused by insect-transmitted diseases.

G-protein-coupled receptor expression and purification

G-protein-coupled receptors (GPCRs) are integral proteins of the cell membrane and are directly involved in the regulation of many biological functions and in drug targeting. However, our knowledge of GPCRs' structure and function remains limited. The first bottleneck in GPCR studies is producing sufficient quantities of soluble, functional, and stable receptors. Currently, GPCR production largely depends on the choice of the overexpression host system and the type of detergent used to extract the GPCR from the cell membrane and stabilize the protein outside the membrane bilayer. Here, we present three protocols that we employ in our lab to produce and solubilize stable GPCRs by cell-free in vitro translation systems, HEK cells, and Escherichia coli. Stable receptors can be purified using immunoaffinity chromatography and gel filtration and can be analyzed with standard biophysical techniques and biochemical assays.

Expression, stabilization and purification of membrane proteins via diverse protein synthesis systems and detergents involving cell-free associated with self-assembly peptide surfactants

G-protein coupled receptors (GPCRs) are involved in regulating most of physiological actions and metabolism in the bodies, which have become most frequently addressed therapeutic targets for various disorders and diseases. Purified GPCR-based drug discoveries have become routine that approaches to structural study, novel biophysical and biochemical function analyses. However, several bottlenecks that GPCR-directed drugs need to conquer the problems including overexpression, solubilization, and purification as well as stabilization. The breakthroughs are to obtain efficient protein yield and stabilize their functional conformation which are both urgently requiring of effective protein synthesis system methods and optimal surfactants. Cell-free protein synthesis system is superior to the high yields and post-translation modifications, and early signs of self-assembly peptide detergents also emerged to superiority in purification of membrane proteins. We herein focus several predominant protein synthesis systems and surfactants involving the novel peptide detergents, and uncover the advantages of cell-free protein synthesis system with self-assembling peptide detergents in purification of functional GPCRs. This review is useful to further study in membrane proteins as well as the new drug exploration.

Milligram production and biological activity characterization of the human chemokine receptor CCR3

Human chemokine receptor CCR3 (hCCR3) belongs to the G protein-coupled receptors (GPCRs) superfamily of membrane proteins and plays major roles in allergic diseases and angiogenesis. In order to study the structural and functional mechanism of hCCR3, it is essential to produce pure protein with biological functions on a milligram scale. Here we report the expression of hCCR3 gene in a tetracycline-inducible stable mammalian cell line. A cell clone with high hCCR3 expression was selected from 46 stably transfected cell clones and from this cell line pure hCCR3 on a milligram scale was obtained after two-step purification. Circular dichroism spectrum with a characteristic shape and magnitude for α-helix indicated proper folding of hCCR3 after purification. The biological activity of purified hCCR3 was verified by its high binding affinity with its endogenous ligands CCL11 and CCL24, with K D in the range of 10(-8) M to 10(-6) M.

Transient and stable expression of the neurotensin receptor NTS1: a comparison of the baculovirus-insect cell and the T-REx-293 expression systems

Nowadays, baculovirus-infected insect cells and tetracycline-inducible mammalian cell lines (T-REx-293) are intensively used for G protein-coupled receptor (GPCR) production for crystallography purposes. Here we constructed a suspension T-REx-293 cell line to stably express an engineered neurotensin receptor 1 (NTS1) mutant and we quantitatively compared this cell line with the transient baculovirus-insect cell system throughout a milligram-scale NTS1 expression and purification process. The two systems were comparable with respect to functional NTS1 expression levels and receptor binding affinity for the agonist [³H] neurotensin. However, NTS1 surface display on T-REx-293 cells determined by radio-ligand binding assays was 2.8 fold higher than that on insect cells. This work demonstrates two approaches for preparing milligram quantities of purified NTS1 suitable for structural studies and provides useful input to users in choosing and optimizing an appropriate expression host for other GPCRs.

Overexpression of membrane proteins in mammalian cells for structural studies

The number of structures of integral membrane proteins from higher eukaryotes is steadily increasing due to a number of innovative protein engineering and crystallization strategies devised over the last few years. However, it is sobering to reflect that these structures represent only a tiny proportion of the total number of membrane proteins encoded by a mammalian genome. In addition, the structures determined to date are of the most tractable membrane proteins, i.e., those that are expressed functionally and to high levels in yeast or in insect cells using the baculovirus expression system. However, some membrane proteins that are expressed inefficiently in these systems can be produced at sufficiently high levels in mammalian cells to allow structure determination. Mammalian expression systems are an under-used resource in structural biology and represent an effective way to produce fully functional membrane proteins for structural studies. This review will discuss examples of vertebrate membrane protein overexpression in mammalian cells using a variety of viral, constitutive or inducible expression systems.