Rational design of cannabinoid type-1 receptor allosteric modulators: Org27569 and PSNCBAM-1 hybrids

Allosteric modulation offers an alternate approach to target the cannabinoid type-1 receptor (CB₁) for therapeutic benefits. Examination of the two widely studied prototypic CB₁ negative allosteric modulators (NAMs) Org27569 and PSNCBAM-1 revealed structural resemblance and similar structure-activity relationships (SARs). In silico docking and dynamics simulation studies using the crystal structure of CB₁ co-bound with CP55,940 and Org27569 suggested that Org27569 and PSNCBAM-1 occupied the same binding pocket and several common interactions were present in both series with the CB₁ receptor. A new scaffold was therefore designed by merging the key structural features from the two series and the hybrids retained these binding features in the in silico docking studies. In addition, one such hybrid displayed similar functions to Org27569 in dynamic simulations by preserving a key R214^3.50-D338^6.30 salt bridge and maintaining an antagonist-like Helix3-Helix6 interhelical distance. Based on these results, a series of hybrids were synthesized and assessed in calcium mobilization, [³⁵S]GTPγS binding and cAMP assays. Several compounds displayed comparable potencies to Org27569 and PSNCBAM-1 in these assays. This work offers new insight of the SAR requirement at the allosteric site of the CB₁ receptor and provides a new scaffold that can be optimized for the development of future CB₁ allosteric modulators.

Insight into Evolution and Conservation Patterns of B1-Subfamily Members of GPCR

The diverse, evolutionary architectures of proteins can be regarded as molecular fossils, tracing a historical path that marks important milestones across life. The B1-subfamily of GPCRs (G-protein-coupled receptors) are medically significant proteins that comprise 15 transmembrane receptor proteins in Homo sapiens. These proteins control the intracellular concentration of cyclic AMP as well as various vital processes in the body. However, little is known about the evolutionary correlation and conservational blueprint of this GPCR subfamily. We performed a comprehensive analysis to understand the evolutionary architecture among 13 members of the B1-subfamily. Multiple sequence alignment analysis exhibited six multiple sequence aligned blocks and five highly aligned blocks. Molecular phylogenetics indicated that CRHR1 and CRHR2 share a typical ancestral relationship and are siblings in 100% bootstrap replications with a total of 24 nodes observed in the cladogram. CRHR2 has the maximum number of extremely conserved amino acids followed by ADCYAP1R1. The longest continuous number sequence logos (74) were found between sequence location 349 and 423, and consequently, the maximum and minimum logo height recorded was 3.6 bits and 0.18 bits, respectively. Finally, to understand the model and pattern of evolutionary relatedness, the conservation blueprint, and the diversification among the members of a protein family, GPCR distribution from several species throughout the animal kingdom was analysed. Together, the study provides an evolutionary insight and offers a rapid method to explore the potential of depicting the evolutionary relationship, conservation blueprint, and diversification among the B1-subfamily of GPCRs using bioinformatics, algorithm analysis, and mathematical models.

The Corticotropin-Releasing Factor Family: Physiology of the Stress Response

The physiological stress response is responsible for the maintenance of homeostasis in the presence of real or perceived challenges. In this function, the brain activates adaptive responses that involve numerous neural circuits and effector molecules to adapt to the current and future demands. A maladaptive stress response has been linked to the etiology of a variety of disorders, such as anxiety and mood disorders, eating disorders, and the metabolic syndrome. The neuropeptide corticotropin-releasing factor (CRF) and its relatives, the urocortins 1–3, in concert with their receptors (CRFR1, CRFR2), have emerged as central components of the physiological stress response. This central peptidergic system impinges on a broad spectrum of physiological processes that are the basis for successful adaptation and concomitantly integrate autonomic, neuroendocrine, and behavioral stress responses. This review focuses on the physiology of CRF-related peptides and their cognate receptors with the aim of providing a comprehensive up-to-date overview of the field. We describe the major molecular features covering aspects of gene expression and regulation, structural properties, and molecular interactions, as well as mechanisms of signal transduction and their surveillance. In addition, we discuss the large body of published experimental studies focusing on state-of-the-art genetic approaches with high temporal and spatial precision, which collectively aimed to dissect the contribution of CRF-related ligands and receptors to different levels of the stress response. We discuss the controversies in the field and unravel knowledge gaps that might pave the way for future research directions and open up novel opportunities for therapeutic intervention.

Current understanding of the structure and function of family B GPCRs to design novel drugs

Family B of G-protein-coupled receptors (GPCRs) and their ligands play a central role in a number of homeostatic mechanisms in the endocrine, gastrointestinal, skeletal, immune, cardiovascular and central nervous systems. Alterations in family B GPCR-regulated homeostatic mechanisms may cause a variety of potentially life-threatening conditions, signifying the necessity to develop novel ligands targeting these receptors. Obtaining structural and functional information on family B GPCRs will accelerate the development of novel drugs to target these receptors. Family B GPCRs are proteins that span the plasma membrane seven times, thus forming seven transmembrane domains (TM1-TM7) which are connected to each other by three extracellular (EL) and three intracellular (IL) loops. In addition, these receptors have a long extracellular N-domain and an intracellular C-tail. The upper parts of the TMs and ELs form the J-domain of receptors. The C-terminal region of peptides first binds to the N-domain of receptors. This 'first-step' interaction orients the N-terminal region of peptides towards the J-domain of receptors, thus resulting in a 'second-step' of ligand-receptor interaction that activates the receptor. Activation-associated structural changes of receptors are transmitted through TMs to their intracellular regions and are responsible for their interaction with the G proteins and activation of the latter, thus resulting in a biological effect. This review summarizes the current information regarding the structure and function of family B GPCRs and their physiological and pathophysiological roles.

An activating mutation in the CRHR1 gene is rarely associated with pituitary-dependent hyperadrenocorticism in poodles

OBJECTIVES

Pituitary-dependent hyperadrenocorticism is the most common cause of naturally occurring hypercortisolism in dogs. CRHR1 expression in human and dog corticotrophinomas suggested that this gene affects pituitary tumorigenesis. The present study aimed to investigate mutations in the CRHR1 coding region in poodles with pituitary-dependent hyperadrenocorticism.

METHODS

Fifty poodles with pituitary-dependent hyperadrenocorticism and 50 healthy poodles were studied. Genomic DNA was amplified by PCR and analyzed by Sanger sequencing.

RESULTS

The novel CRHR1 p.V97M mutation was identified in one dog. This valine residue, located in the amino-terminal extracellular domain, exhibits high affinity for its corticotropin-releasing hormone (CRH) ligand. Bioinformatic analysis revealed structural rearrangements in the mutant protein, with a 17% increase in the surface binding affinity between CRHR1 and CRH. In vitro functional studies showed that mutant CRHR1 induced higher ACTH secretion than the wild type after stimulation with human CRH.

CONCLUSION

These results suggest that germline activating mutations in CRHR1 may be a rare cause of pituitary hyperadrenocorticism in poodles.

Incretin hormones and the expanding families of glucagon-like sequences and their receptors

Peptide hormones encoded by the proglucagon (Gcg) and glucose-dependent insulinotropic polypeptide (Gip) genes are evolutionarily related glucagon-like sequences and act through a subfamily of G-protein-coupled receptors. A better understanding of the evolutionary history of these hormones and receptors should yield insight into their biological functions. The availability of a large number of near-complete vertebrate genome sequences is a powerful resource to address questions concerning the evolution of sequences; here, we utilize these resources to examine the evolution of glucagon-like sequences and their receptors. These studies led to the discovery of novel genes for a glucagon receptor-like receptor (Grlr) and a glucagon-like sequence (exendin) in vertebrates. Both exendin and GRLR have ancient origins, early in vertebrate evolution, but have been lost on the ancestral lineage leading to extant mammals. We also show that exendin and GRLR are both expressed in the brain of the chicken and Xenopus tropicals, results that suggest that the products of these genes function in this tissue. The lack of exendin or Grlr genes in mammals suggests that other genes may have acquired the functions of exendin and Grlr during mammalian evolution.

NMR structure of the first extracellular domain of corticotropin-releasing factor receptor 1 (ECD1-CRF-R1) complexed with a high affinity agonist

The corticotropin-releasing factor (CRF) peptide hormone family members coordinate endocrine, behavioral, autonomic, and metabolic responses to stress and play important roles within the cardiovascular, gastrointestinal, and central nervous systems, among others. The actions of the peptides are mediated by activation of two G-protein-coupled receptors of the B1 family, CRF receptors 1 and 2 (CRF-R1 and CRF-R2α,β). The recently reported three-dimensional structures of the first extracellular domain (ECD1) of both CRF-R1 and CRF-R2β (Pioszak, A. A., Parker, N. R., Suino-Powell, K., and Xu, H. E. (2008) J. Biol. Chem. 283, 32900-32912; Grace, C. R., Perrin, M. H., Gulyas, J., Digruccio, M. R., Cantle, J. P., Rivier, J. E., Vale, W. W., and Riek, R. (2007) Proc. Natl. Acad. Sci. U.S.A. 104, 4858-4863) complexed with peptide antagonists provided a starting point in understanding the binding between CRF ligands and receptors at a molecular level. We now report the three-dimensional NMR structure of the ECD1 of human CRF-R1 complexed with a high affinity agonist, α-helical cyclic CRF. In the structure of the complex, the C-terminal residues (23-41) of α-helical cyclic CRF bind to the ECD1 of CRF-R1 in a helical conformation mainly along the hydrophobic face of the peptide in a manner similar to that of the antagonists in their corresponding ECD1 complex structures. Unique to this study is the observation that complex formation between an agonist and the ECD1-CRF-R1 promotes the helical conformation of the N terminus of the former, important for receptor activation (Gulyas, J., Rivier, C., Perrin, M., Koerber, S. C., Sutton, S., Corrigan, A., Lahrichi, S. L., Craig, A. G., Vale, W., and Rivier, J. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 10575-10579).

Mutational and cysteine scanning analysis of the glucagon receptor N-terminal domain

The glucagon receptor belongs to the B family of G-protein coupled receptors. Little structural information is available about this receptor and its association with glucagon. We used the substituted cysteine accessibility method and three-dimensional molecular modeling based on the gastrointestinal insulinotropic peptide and glucagon-like peptide 1 receptor structures to study the N-terminal domain of this receptor, a central element for ligand binding and specificity. Our results showed that Asp(63), Arg(116), and Lys(98) are essential for the receptor structure and/or ligand binding because mutations of these three residues completely disrupted or markedly impaired the receptor function. In agreement with these data, our models revealed that Asp(63) and Arg(116) form a salt bridge, whereas Lys(98) is engaged in cation-π interactions with the conserved tryptophans 68 and 106. The native receptor could not be labeled by hydrophilic cysteine biotinylation reagents, but treatment of intact cells with [2-(trimethylammonium)ethyl]methanethiosulfonate increased the glucagon binding site density. This result suggested that an unidentified protein with at least one free cysteine associated with the receptor prevented glucagon recognition and that [2-(trimethylammonium)ethyl]methanethiosulfonate treatment relieved this inhibition. The substituted cysteine accessibility method was also performed on 15 residues selected using the three-dimensional models. Several receptor mutants, despite a relatively high predicted cysteine accessibility, could not be labeled by specific reagents. The three-dimensional models show that these mutated residues are located on one face of the protein. This could be part of the interface between the receptor and the unidentified inhibitory protein, making these residues inaccessible to biotinylation compounds.

VPAC1 receptor binding site: contribution of photoaffinity labeling approach

The vasoactive intestinal peptide (VIP) is a prominent 28 aminoacid neuropeptide with wide distribution in both central and peripheral nervous systems, where it plays important regulatory role in many physiological processes. VIP has a large spectrum of biological functions including exocrine secretions, hormone release, foetal development, immune response and also exerts beneficial effect in neuro-degenerative and inflammatory diseases. Few years ago, it has been shown that VIP can be a promising anti-inflammatory agent. VIP mechanisms of action implicate two sub-types of receptors (VPAC1 and VPAC2) which are members of class B receptors belonging to the super-family of G protein-coupled receptor (GPCR). Because, VPAC1 receptor plays an important role in the modulation of the ant-inflammatory response and represent an archetype of class B GPCR, we have extensively studied the structure-function relationship of this receptor, which allowed us to define the molecular basis of that receptor in term of affinity, specificity, desensitization and coupling to adenylyl cyclase. Those studies showed the crucial role of the N-terminal ectodomain (N-ted) of VPAC1 receptor in VIP binding. Using different techniques including photoaffinity labeling, NMR, molecular modeling and molecular dynamic simulation, it has been possible to define how VIP interacts with its receptor. We have shown that most of the VIP molecule, 1-28 (alpha-helix) sequence, tightly binds the N-ted part of the receptor which is himself structured as a <<Sushi>> domain. In contrast, the N-terminal part of the specific antagonist PG97-269 is in physical contact with the N-ted but in different region. These studies define the molecular mechanism implicated in the activation of class B VPAC1 receptor and should allow the development of new VIP pharmacology using rational synthesis of agonist molecules.

Passing the baton in class B GPCRs: peptide hormone activation via helix induction?

G-protein-coupled receptors (GPCRs) represent the largest constellation of validated drug targets. Crystal structures of class A GPCRs have facilitated major advances in understanding the principles underlying GPCR activation. By contrast, relatively little is known about class B GPCRs, a family of receptors for a variety of therapeutically relevant peptide hormones. Encouraging progress has recently been made through the structural elucidation of several extracellular hormone-binding domains of class B GPCRs in complex with their natural ligands or synthetic analogues. The structures reveal similar modes of ligand binding, with concomitant alpha-helical structuring of the ligand. The latter suggests an attractive mechanical model for class B GPCR activation.

Molecular recognition of corticotropin-releasing factor by its G-protein-coupled receptor CRFR1

The bimolecular interaction between corticotropin-releasing factor (CRF), a neuropeptide, and its type 1 receptor (CRFR1), a class B G-protein-coupled receptor (GPCR), is crucial for activation of the hypothalamic-pituitary-adrenal axis in response to stress, and has been a target of intense drug design for the treatment of anxiety, depression, and related disorders. As a class B GPCR, CRFR1 contains an N-terminal extracellular domain (ECD) that provides the primary ligand binding determinants. Here we present three crystal structures of the human CRFR1 ECD, one in a ligand-free form and two in distinct CRF-bound states. The CRFR1 ECD adopts the alpha-beta-betaalpha fold observed for other class B GPCR ECDs, but the N-terminal alpha-helix is significantly shorter and does not contact CRF. CRF adopts a continuous alpha-helix that docks in a hydrophobic surface of the ECD that is distinct from the peptide-binding site of other class B GPCRs, thereby providing a basis for the specificity of ligand recognition between CRFR1 and other class B GPCRs. The binding of CRF is accompanied by clamp-like conformational changes of two loops of the receptor that anchor the CRF C terminus, including the C-terminal amide group. These structural studies provide a molecular framework for understanding peptide binding and specificity by the CRF receptors as well as a template for designing potent and selective CRFR1 antagonists for therapeutic applications.

Residue 17 of sauvagine cross-links to the first transmembrane domain of corticotropin-releasing factor receptor 1 (CRFR1)

Corticotropin-releasing factor receptor 1 (CRFR1) mediates the physiological actions of corticotropin-releasing factor in the anterior pituitary gland and the central nervous system. Using chemical cross-linking we have previously reported that residue 16 of sauvagine (SVG) is in a close proximity to the second extracellular loop of CRFR1. Here we introduced p-benzoylphenylalanine (Bpa) at position 17 of a sauvagine analog, [Tyr0, Gln1, Bpa17]SVG, to covalently label CRFR1 and characterize the cross-linking site. Using a combination of receptor mutagenesis, peptide mapping, and N-terminal sequencing, we identified His117 within the first transmembrane domain (TM1) of CRFR1 as the cross-linking site for Bpa17 of 125I-[Tyr0, Gln1, Bpa17]SVG. These data indicate that, within the SVG-CRFR1 complex, residue 17 of the ligand lies within a 9 angstroms distance from residue 117 of the TM1 of CRFR1. The molecular proximity between residue 17 of the ligand and TM1 of CRFR1 described here and between residue 16 of the ligand and the CRFR1 second extracellular loop described previously provides useful molecular constraints for modeling ligand-receptor interaction in mammalian cells expressing CRFR1.