Ligand based structural studies of the CB1 cannabinoid receptor

In Silico Infrared Characterization of Synthetic Cannabinoids by Quantum Chemistry and Chemometrics

The concept of forensic sciences as mere trace analysis has been modified by the idea of forensic intelligence, which entails applying data to make decisions within the investigative process. Many countries are engaged in combating drug trafficking and drug use because they are related to public health and safety issues. Prohibiting the consumption of traditional drugs has led new psychoactive substances (NPSs) to emerge. NPSs consist of compounds that resemble the initially banned substance and which aim to mimic the traditional drug recreational effects while circumventing drug legislation. For example, synthetic cannabinoids are sprayed on herbal products to reproduce the cannabis recreational effects. According to the United Nations Office on Drugs and Crime (UNODC), the toxic effects of synthetic cannabis types are unknown, and harm and fatalities associated with the use of these drugs have been reported. Information on the characterization related to these species is lacking. The rate at which NPSs appear poses a significant challenge because employing conventional methods to understand the characteristics of these compounds may require time and be costly. This work uses in silico practices as an alternative to understand how NPSs related to cannabis behave. We apply quantum chemistry methods to evaluate several synthetic cannabinoids recognized in forensic samples. More specifically, we generate infrared spectra that can be employed as a benchmark for NPSs. We apply a multivariate classification to evaluate the results. We conclude that in silico methods are an alternative that provide information about the spectra of undetected substances. This information can help to identify new drugs, to increase knowledge about them, and to feed information procedures.

Development of Covalent Ligands for G Protein-Coupled Receptors: A Case for the Human Adenosine A3 Receptor

The development of covalent ligands for G protein-coupled receptors (GPCRs) is not a trivial process. Here, we report a streamlined workflow thereto from synthesis to validation, exemplified by the discovery of a covalent antagonist for the human adenosine A₃ receptor (hA₃AR). Based on the 1 H,3 H-pyrido[2,1- f]purine-2,4-dione scaffold, a series of ligands bearing a fluorosulfonyl warhead and a varying linker was synthesized. This series was subjected to an affinity screen, revealing compound 17b as the most potent antagonist. In addition, a nonreactive methylsulfonyl derivative 19 was developed as a reversible control compound. A series of assays, comprising time-dependent affinity determination, washout experiments, and [³⁵S]GTPγS binding assays, then validated 17b as the covalent antagonist. A combined in silico hA₃AR-homology model and site-directed mutagenesis study was performed to demonstrate that amino acid residue Y265^7.36 was the unique anchor point of the covalent interaction. This workflow might be applied to other GPCRs to guide the discovery of covalent ligands.

An Affinity-Based Probe for the Human Adenosine A2A Receptor

Using activity-based protein profiling (ABPP), functional proteins can be interrogated in their native environment. Despite their pharmaceutical relevance, G protein-coupled receptors (GPCRs) have been difficult to address through ABPP. In the current study, we took the prototypical human adenosine A_2A receptor (hA_2AR) as the starting point for the construction of a chemical toolbox allowing two-step affinity-based labeling of GPCRs. First, we equipped an irreversibly binding hA_2AR ligand with a terminal alkyne to serve as probe. We showed that our probe irreversibly and concentration-dependently labeled purified hA_2AR. Click-ligation with a sulfonated cyanine-3 fluorophore allowed us to visualize the receptor on SDS-PAGE. We further demonstrated that labeling of the purified hA_2AR by our probe could be inhibited by selective antagonists. Lastly, we showed successful labeling of the receptor in cell membranes overexpressing hA_2AR, making our probe a promising affinity-based tool compound that sets the stage for the further development of probes for GPCRs.

Chemical Tools for Studying Lipid-Binding Class A G Protein-Coupled Receptors

Cannabinoid, free fatty acid, lysophosphatidic acid, sphingosine 1-phosphate, prostanoid, leukotriene, bile acid, and platelet-activating factor receptor families are class A G protein-coupled receptors with endogenous lipid ligands. Pharmacological tools are crucial for studying these receptors and addressing the many unanswered questions surrounding expression of these receptors in normal and diseased tissues. An inherent challenge for developing tools for these lipid receptors is balancing the often lipophilic requirements of the receptor-binding pharmacophore with favorable physicochemical properties to optimize highly specific binding. In this study, we review the radioligands, fluorescent ligands, covalent ligands, and antibodies that have been used to study these lipid-binding receptors. For each tool type, the characteristics and design rationale along with in vitro and in vivo applications are detailed.

Molecular signatures of human melanocortin receptors for ligand binding and signaling

Human melanocortin receptors (hMCRs) belong to the seven-transmembrane (TM) domain proteins. There are five hMCR subtypes and each of these receptor subtypes has different patterns of tissue expression and physiological function. The endogenous agonists for hMCRs are α-, β-, and γ-MSH and ACTH and endogenous antagonists are Agouti and AGRP which are the only known naturally occurring antagonists for the receptors. These peptides have their own profiles regarding the relative potency for specific hMCR subtype. Extensive studies have been performed to examine the molecular basis of the hMCRs for different ligand binding affinity and potency. Studies indicate that natural ligand α-MSH utilizes conserved amino acid residues for MCR specific binding (orthosteric binding) while synthetic ligands utilize non-conserved amino acid residues for receptor subtype specific binding (allosteric binding). ACTH is the only endogenous agonist for hMC2R and more amino acid residues at hMC2R are required for ACTH binding and signaling. HMCR computer modeling provides the detailed information of ligand and MCR interaction. This review provides the latest understanding of the molecular basis of the hMCRs for ligand binding and signaling. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Guidelines for the Synthesis of Small-Molecule Irreversible Probes Targeting G Protein-Coupled Receptors

Irreversible probes have been proven to be useful pharmacological tools in the study of structural and functional features in drug receptor pharmacology. They have been demonstrated to be particularly valuable for the isolation and purification of receptors. Furthermore, irreversible probes are helpful tools for the identification and characterization of binding sites, thereby supporting the advancement of rational drug design. In this Minireview, we provide insight into universal strategies and guidelines to successfully synthesize irreversible probes that target G protein-coupled receptors (GPCRs). We provide an overview of commonly used chemoreactive and photoreactive groups, and make a comparison of their properties and potential applications. Furthermore, there is a particular focus on synthetic approaches to introduce these reactive groups based on commercially available reagents.

Covalent agonists for studying G protein-coupled receptor activation

Structural studies on G protein-coupled receptors (GPCRs) provide important insights into the architecture and function of these important drug targets. However, the crystallization of GPCRs in active states is particularly challenging, requiring the formation of stable and conformationally homogeneous ligand-receptor complexes. Native hormones, neurotransmitters, and synthetic agonists that bind with low affinity are ineffective at stabilizing an active state for crystallogenesis. To promote structural studies on the pharmacologically highly relevant class of aminergic GPCRs, we here present the development of covalently binding molecular tools activating Gs-, Gi-, and Gq-coupled receptors. The covalent agonists are derived from the monoamine neurotransmitters noradrenaline, dopamine, serotonin, and histamine, and they were accessed using a general and versatile synthetic strategy. We demonstrate that the tool compounds presented herein display an efficient covalent binding mode and that the respective covalent ligand-receptor complexes activate G proteins comparable to the natural neurotransmitters. A crystal structure of the β2-adrenoreceptor in complex with a covalent noradrenaline analog and a conformationally selective antibody (nanobody) verified that these agonists can be used to facilitate crystallogenesis.

2012 Division of medicinal chemistry award address. Trekking the cannabinoid road: a personal perspective

My involvement with the field of cannabinoids spans close to 3 decades and covers a major part of my scientific career. It also reflects the robust progress in this initially largely unexplored area of biology. During this period of time, I have witnessed the growth of modern cannabinoid biology, starting from the discovery of its two receptors and followed by the characterization of its endogenous ligands and the identification of the enzyme systems involved in their biosynthesis and biotransformation. I was fortunate enough to start at the beginning of this new era and participate in a number of the new discoveries. It has been a very exciting journey. With coverage of some key aspects of my work during this period of "modern cannabinoid research," this Award Address, in part historical, intends to give an account of how the field grew, the key discoveries, and the most promising directions for the future.

Novel insights into CB1 cannabinoid receptor signaling: a key interaction identified between the extracellular-3 loop and transmembrane helix 2

Activation of the cannabinoid CB1 receptor (CB1) is modulated by aspartate residue D2.63(176) in transmembrane helix (TMH) 2. Interestingly, D2.63 does not affect the affinity for ligand binding at the CB1 receptor. Studies in class A G protein-coupled receptors have suggested an ionic interaction between residues of TMH2 and 7. In this report, modeling studies identified residue K373 in the extracellular-3 (EC-3) loop in charged interactions with D2.63. We investigated this possibility by performing reciprocal mutations and biochemical studies. D2.63(176)A, K373A, D2.63(176)A-K373A, and the reciprocal mutant with the interacting residues juxtaposed D2.63(176)K-K373D were characterized using radioligand binding and guanosine 5'-3-O-(thio)triphosphate functional assays. None of the mutations resulted in a significant change in the binding affinity of N-(piperidiny-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A) or (-)-3cis -[2-hydroxyl-4-(1,1-dimethyl-heptyl)phenyl]-trans-4-[3-hydroxyl-propyl] cyclohexan-1-ol (CP55,940). Modeling studies indicated that binding-site interactions and energies of interaction for CP55,940 were similar between wild-type and mutant receptors. However, the signaling of CP55,940, and (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl)-methanone mesylate (WIN55,212-2) was impaired at the D2.63(176)A-K373A and the single-alanine mutants. In contrast, the reciprocal D2.63(176)K-K373D mutant regained function for both CP55,940 and WIN55,212-2. Computational results indicate that the D2.63(176)-K373 ionic interaction strongly influences the conformation(s) of the EC-3 loop, providing a structure-based rationale for the importance of the EC-3 loop to signal transduction in CB1. The putative ionic interaction results in the EC-3 loop pulling over the top (extracellular side) of the receptor; this EC-3 loop conformation may serve protective and mechanistic roles. These results suggest that the ionic interaction between D2.63(176) and K373 is important for CB1 signal transduction.

Mass spectrometry-based GPCR proteomics: comprehensive characterization of the human cannabinoid 1 receptor

The human cannabinoid 1 receptor (hCB1), a ubiquitous G protein-coupled receptor (GPCR), transmits cannabinergic signals that participate in diverse (patho)physiological processes. Pharmacotherapeutic hCB1 targeting is considered a tractable approach for treating such prevalent diseases as obesity, mood disorders, and drug addiction. The hydrophobic nature of the transmembrane helices of hCB1 presents a formidable difficulty to its direct structural analysis. Comprehensive experimental characterization of functional hCB1 by mass spectrometry (MS) is essential to the targeting of affinity probes that can be used to define directly hCB1 binding domains using a ligand-assisted experimental approach. Such information would greatly facilitate the rational design of hCB1-selective agonists/antagonists with therapeutic potential. We report the first high-coverage MS analysis of the primary sequence of the functional hCB1 receptor, one of the few such comprehensive MS-based analyses of any GPCR. Recombinant C-terminal hexa-histidine-tagged hCB1 (His6-hCB1) was expressed in cultured insect (Spodoptera frugiperda) cells, solubilized by a procedure devised to enhance receptor purity following metal-affinity chromatography, desalted by buffer exchange, and digested in solution with (chymo)trypsin. "Bottom-up" nanoLC-MS/MS of the (chymo)tryptic digests afforded a degree of overall hCB1 coverage (>94%) thus far reported for only two other GPCRs. This MS-compatible procedure devised for His6-hCB1 sample preparation, incorporating in-solution (chymo)trypsin digestion in the presence of a low concentration of CYMAL-5 detergent, may be applicable to the MS-based proteomic characterization of other GPCRs. This work should help enable future ligand-assisted structural characterization of hCB1 binding motifs at the amino-acid level using rationally designed and targeted covalent cannabinergic probes.

Comparative docking study of anibamine as the first natural product CCR5 antagonist in CCR5 homology models

Anibamine, a novel pyridine quaternary alkaloid recently isolated from Aniba sp., has been found to effectively bind to the chemokine receptor CCR5 with an IC(50) at 1 microM in competition with (125)I-gp120, an HIV viral envelope protein binding to CCR5 with high affinity. Since CCR5, a G-protein-coupled receptor, is an essential coreceptor for the human immunodeficiency virus type I (HIV-1) entry to host cells, a CCR5 antagonist that inhibits the cellular entry of HIV-1 provides a new therapy choice for the treatment of HIV. Anibamine provides a novel structural skeleton that is remarkably different from all lead compounds previously identified as CCR5 antagonists. Here, we report comparative docking studies of anibamine with several other known CCR5 antagonists in two CCR5 homology models built based on the crystal structures of bovine rhodopsin and human beta(2)-adrenergic receptor. The binding pocket of anibamine has some common features shared with other high affinity CCR5 antagonists, suggesting that they may bind in similar binding sites and/or modes. At the same time, several unique binding features of anibamine were identified, and it will likely prove beneficial in future molecular design of novel CCR5 antagonists based on the anibamine scaffold.

Ligand-binding architecture of human CB2 cannabinoid receptor: evidence for receptor subtype-specific binding motif and modeling GPCR activation

The extensive physiological influence of transmission through the CB2 cannabinoid receptor makes this G protein-coupled receptor (GPCR) a promising therapeutic target for treating neuropathic pain, inflammation, and immune disorders. However, there is little direct structural information pertaining to either GPCR or CB2-receptor ligand recognition and activation. The present work helps characterize experimentally the ligand-binding interactions of the human CB2 (hCB2) receptor. This study illustrates how our overall experimental approach, "ligand-assisted protein structure" (LAPS), affords direct determination of the requirements for ligand binding to the hCB2 receptor and discrimination among the binding motifs for ligands that activate therapeutically relevant GPCRs.

Synthesis and Cannabinoid Activity of a Variety of 2,3-Substituted 1-Benzo[b]thiophen Derivatives and 2,3-Substituted Benzofuran: Novel Agonists for the CB1 Receptor

An exploratory chemical effort has been undertaken to develop a novel series of compounds as selective CB1 agonists. It is hoped that compounds of this type will have clinical utility in pain control and cerebral ischaemia following stroke or traumatic head injury. We report here medicinal chemistry studies directed towards the investigation of several classes of 1-benzo[b]thiophen and benzofuran derivatives as novel CB1 agonists. We have discovered a novel series of compounds, which contain a 1-benzo[b]thiophen or a benzofuran group as the central aromatic group. Our investigation of this series of compounds has enhanced our understanding of the importance of binding sites within the CB1 receptor for favourable CB1 potency. Our understanding of these factors allowed us to modify the structure of a 1-benzothiophen derivative and improve its potency at the CB1 receptor.

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.

Mutation studies of Ser7.39 and Ser2.60 in the human CB1 cannabinoid receptor: evidence for a serine-induced bend in CB1 transmembrane helix 7

Ligands of structurally diverse natures are able to bind at the CB(1) cannabinoid receptor, suggesting the existence of multiple binding sites on the receptor. Modeling studies have implicated Ser2.60(173) and Ser7.39(383) as possible interaction site(s) for CB(1) agonists. To test the importance of these residues for receptor recognition, recombinant human CB(1) receptors, stably expressed in human embryonic kidney 293 cells, were used to investigate the consequences of mutating Ser2.60 (to S2.60A) or Ser7.39 (to S7.39A) in radioligand binding and guanosine 5'-3-O-(thio)triphosphate functional assays. The S7.39A mutant resulted in a total ablation of [(3)H](-)-3-[2-hydroxyl-4-(1,1-dimethylheptyl)phenyl]-4-[3-hydroxylpropyl] cyclohexan-1-ol (CP55,940) high-affinity binding. However, [(3)H](R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl)methanone (WIN55,212-2) binding properties at S7.39A were comparable with those of the wild-type (WT) receptor. The binding affinity of (-)-11beta-hydroxy-3-(1',1'-dimethylheptyl)hexahydrocannabinol (AM4056) and (-)-11-hydroxydimethylheptyl-Delta(8)-tetrahydrocannabinol (HU210) were drastically reduced (50- to 100-fold) at the S7.39A mutant. Likewise, the EC(50) for HU210 and AM4056-mediated activation of the S7.39A receptor was increased by >200-fold. In contrast, the binding affinity and potency of WIN55,212-2, CP55,940, HU210, and AM4056 were unaltered at the S2.60A mutant compared with WT human CB(1) receptors. These results clearly suggest that Ser7.39, but not Ser2.60, plays a crucial role in mediating ligand specific interactions for CP55,940, HU210, and AM4056 at the human CB(1) receptor. Our modeling studies predict that Ser7.39 in a g-chi1 conformation may induce a helix bend in TMH7 that provides docking space for CP55,940 binding; the S7.39A mutation may alter this binding space, precluding CP55,940 binding.

Irt>t schedule controlled behavior in ‘learned-helpless’ rats: Effects from a cannabinoid agonist

Human depression is partly a congenital disorder. Aspects of the behavior accompanying depression can be magnified by genetic manipulation of bred animal species. Learned Helplessness (LH) is a trait-mark behavior that successfully breeds in rodents. Here, 'congenital' LH (cLH) rats were trained to recognize and respond to 12s long interval cues (irt>12s schedule). Rats compliant to an irt>t schedule will space responses evenly and respond rhythmically. Irt>t schedule derived data are plotted in histograms showing irt (interresponse time) frequencies. A pause response peak emerges, for outbred rats, at irt values approximating the minimum interval for reinforcement. cLH rats [n=9] complied poorly to schedule contingencies when diluent (vehicle) was injected before testing. Moderate and high dose injections of a CB 1 receptor selective agonist drug (AM 411), however, increased operant schedule compliance and normalized the cLH rats' irt>t histogram distributions. Performance indicators for cLH rats are presented alongside coordinate measures from a comparison group [n=5] of normally bred Sprague-Dawley (SD) rats. In both cLH and SD rats, treatment session histograms revealed shifts of the pause response peak not accompanied by a change in motor responsiveness. The irt>12s histogram shifts were absent when AM 411 dosages were arranged to follow pre-medication injections of a CB 1 receptor selective antagonist drug (AM 251). In short, AM 411 increased timing acuity in rats prone to behavioral despair but had opposite timing effects in normally bred SD rats.

High affinity electrophilic and photoactivatable covalent endocannabinoid probes for the CB1 receptor

We have designed and synthesized the first two high affinity covalent anandamide probes for the CB1 receptor by introducing either an electrophilic isothiocyanato or a photoactivatable azido group at the terminal carbon of the arachidonic acid moiety. The headgroup of these anandamide analogues was optimized by using a cyclopropylamide substituent to impart optimal CB1 affinity. Both 20-isothiocyanato-eicosa-5,8,11,14-tetraenoic acid cyclopropylamide (1, AM3677) and 20-azido-eicosa-5,8,11,14-tetraenoic acid cyclopropylamide (2, AM3661) exhibited high selectivities for the CB1 receptor with K(i) values of 1.3 and 0.9 nM, respectively. Using suitable experimental conditions, both ligands were shown to covalently label the CB1 receptor with high efficiency. These two covalent probes for the endocannabinoid CB1 binding site open the door for exploring the ligand binding motifs involved in the activation of the CB1 receptor by its endogenous ligand, anandamide.

Cysteine residues in the human cannabinoid receptor: only C257 and C264 are required for a functional receptor, and steric bulk at C386 impairs antagonist SR141716A binding

The human neuronal cannabinoid receptor (CB1) is a G-protein-coupled receptor (GPCR) triggered by the psychoactive ingredients in marijuana, as well as endogenous cannabinoids produced in the brain. As with most GPCRs, the mechanism of CB1 activation is poorly understood. In this work, we have assessed the role of cysteine residues in CB1 ligand binding and activation, and demonstrate a method for mapping key determinants in CB1 structure and function. Through mutational analysis, we find that only two cysteines, C257 and C264, are required for high-level expression and receptor function. In addition, through cysteine reactivity studies, we find that a cysteine in transmembrane helix seven, C386 (C7.42), is reactive toward methanethiosulfonate (MTS) sulfhydryl labeling agents, and is thus solvent accessible. Interestingly, steric bulk introduced at this site, either through MTS labeling or by mutation, inhibits binding of the antagonist drug SR141716A (also known as Rimonabant or Accomplia), but does not affect the binding of the agonist CP55940. Our subsequent modeling studies suggest this effect is caused by steric clash of the modified C386 residue with the piperidine ring of SR141716A and/or disruption of an aromatic microdomain in the binding pocket. On the basis of these results, we hypothesize that bound SR141716A inhibits the ability of transmembrane helix 6 to move during formation of the functionally active receptor state.

(-)-7′-Isothiocyanato-11-hydroxy-1′,1′-dimethylheptylhexahydrocannabinol (AM841), a High-Affinity Electrophilic Ligand, Interacts Covalently with a Cysteine in Helix Six and Activates the CB1 Cannabinoid Receptor

The CB1 cannabinoid receptor has been shown to play important physiological roles in the central nervous system, as well as peripherally, and is a target for development of therapeutic medications. To gain insight on the ligand binding site(s) and structural features of activation, we designed and synthesized (-)-7'-isothiocyanato-11-hydroxy-1',1'-dimethylheptylhexahydrocannabinol (AM841), a classical cannabinoid affinity label that incorporates an isothiocyanate substituent as an electrophilic reactive group capable of interacting irreversibly with a suitably located and properly oriented nucleophilic amino acid residue at or near the binding site. To obtain evidence for the site of covalent attachment of AM841, C6.47, identified in part by interactive ligand docking, was mutated to serine, alanine, and leucine to reduce or eliminate the nucleophilic character. Wild-type (WT) and mutant CB1 receptors were evaluated for their abilities to recognize a series of cannabinergic ligands. Each bound comparably to WT, excluding C6.47L, which displayed a reduced affinity for 3H-labeled (1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl)cyclohexan-1-ol (CP55940), AM841, 11-hydroxy-1',1'-dimethylheptylhexahydrocannabinol (AM4056), and (-)-7'-bromo-11-hydroxy-1',1'-dimethylheptylhexahydrocannabinol (AM4043) and an improvement in affinity for (-)-trans-delta9-tetrahydrocannabinol (delta9-THC). The affinity of 3H-labeled [2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazin-6-yl](naphthyl)methanone (WIN55212-2) was unchanged across all mutants. It is noteworthy that AM841 was shown to bind irreversibly to WT CB1 but exhibited no covalent attachment with the mutants and behaved as an agonist suggesting irreversible attachment to C6.47 maintains CB1 in its active state. The evidence presented identifies C6.47 as the site of covalent bond formation with AM841 and combined with the binding data fully supports the molecular modeling. These studies present the first report of tandem applications of affinity labeling, site-directed mutagenesis, and interactive ligand docking for CB1.

Purification and mass spectroscopic analysis of human CB2 cannabinoid receptor expressed in the baculovirus system

The cannabinergic system is present in a variety of organs and tissues that perform a wide range of essential physiologic functions making it an inherently important therapeutic target for drug discovery. In order to augment our knowledge regarding the interactions between cannabinoid receptors (CBs) and their ligands, efficient and effective tools are essential for robust expression and purification of these membrane-bound proteins. In this report, we describe a suitable method for purification of the human cannabinoid receptor 2 (CB2) to a qualitative and quantitative level sufficient for mass spectral analysis. We utilized a baculovirus expression system, incorporating several epitope tags to facilitate purification and to ameliorate the effect the tags have on CB2 expression and function. Expressed protein encoded by a carboxy (C)-terminal His-tagged CB2 construct displayed a B(max) value of 9.3 pmol/mg with a K(D) of 7.30 nM using [3(H)]CP-55(940), a standard cannabinoid radioligand, and was selected for subsequent purification experiments. Western blot analysis of purified membrane protein yielded several forms of CB2, the most abundant being a 41 kDa peptide. A second protein species was observed with an apparent molecular weight of 46 kDa representing a glycosylated form of CB2. In addition, a CB2 homodimer was also identified. The purified receptor was subjected to mass spectroscopic analysis to confirm its identity and purity. Mass spectra corresponding to the intracellular, extracellular and transmembrane domains were obtained. These experiments exemplify the importance of high-level expression systems when developing membrane-bound protein purification strategies. This work will aid in the identification of receptor-ligand binding sites, the characterization of molecular features involved in receptor activation, and the elucidation of the CB2 receptor tertiary structure.

Structural requirements for cannabinoid receptor probes

The discovery and cloning of CB1 and CB2, the two known G(i/o) protein-coupled cannabinoid receptors, as well as the isolation and characterization of two families of endogenous cannabinergic ligands represented by arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol (2-AG), have opened new horizons in this newly discovered field of biology. Furthermore, a considerable number of cannabinoid analogs belonging to structurally diverse classes of compounds have been synthesized and tested, thus providing substantial information on the structural requirements for cannabinoid receptor recognition and activation. Experiments with site-directed mutated receptors and computer modeling studies have suggested that these diverse classes of ligands may interact with the receptors through different binding motifs. The information about the exact binding site may be obtained with the help of suitably designed molecular probes. These ligands either interact with the receptors in a reversible fashion (reversible probes) or alternatively attach at or near the receptor active site with the formation of covalent bonds (irreversible probes). This review focuses on structural requirements of cannabinoid receptor ligands and highlights their pharmacological and therapeutic potential.