Localization of ligand-binding domains of human corticotropin-releasing factor receptor: a chimeric receptor approach

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.

Antiproliferative Activity of Non-Calcemic Vitamin D Analogs on Human Melanoma Lines in Relation to VDR and PDIA3 Receptors

Vitamin D is a precursor for secosteroidal hormones, which demonstrate pleiotropic biological activities, including the regulation of growth and the differentiation of normal and malignant cells. Our previous studies have indicated that the inhibition of melanoma proliferation by a short side-chain, low calcemic analog of vitamin D—21(OH)pD is not fully dependent on the expression of vitamin D receptor (VDR). We have examined the effects of classic vitamin D metabolites, 1,25(OH)₂D₃ and 25(OH)D₃, and two low calcemic vitamin D analogs, (21(OH)pD and calcipotriol), on proliferation, mRNA expression and vitamin D receptor (VDR) translocation in three human melanoma cell lines: WM98, A375 and SK-MEL-188b (subline b of SK-MEL-188, which lost responsiveness to 1,25(OH)₂D₃ and became VDR^−/−CYP27B1^−/−). All tested compounds efficiently inhibited the proliferation of WM98 and A375 melanoma cells except SK-MEL-188b, in which only the short side-chain vitamin D analog—21(OH)pD was effective. Overall, 21(OH)pD was the most potent compound in all three melanoma cell lines in the study. The lack of responsiveness of SK-MEL-188b to 1,25(OH)₂D₃, 25(OH)D₃ and calcipotriol is explained by a lack of characteristic transcripts for the VDR, its splicing variants as well as for vitamin D-activating enzyme CYP27B1. On the other hand, the expression of VDR and its splicing variants and other vitamin D related genes (RXR, PDIA3, CYP3A4, CYP2R1, CYP27B1, CYP24A1 and CYP11A1) was detected in WM98 and A375 melanomas with the transcript levels being modulated by vitamin D analogs. The expression of VDR isoforms in WM98 cells was stimulated strongly by calcipotriol. The antiproliferative activities of 21(OH)pD appear not to require VDR translocation to the nucleus, which explains the high efficacy of this noncalcemic pregnacalciferol analog in SK-MEL-188b melanoma, that is, VDR^−/−. Therefore, we propose that 21(OH)pD is a good candidate for melanoma therapy, although the mechanism of its action remains to be defined.

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.

Defining the role of corticotropin releasing factor binding protein in alcohol consumption

The corticotropin releasing factor (CRF) exerts its effects by acting on its receptors and on the binding protein (CRFBP), and has been implicated in alcohol use disorder (AUD). Therefore, identification of the exact contribution of each protein that mediates CRF effects is necessary to design effective therapeutic strategies for AUD. A series of in vitro/in vivo experiments across different species were performed to define the biological discrete role of CRFBP in AUD. First, to establish the CRFBP role in receptor signaling, we developed a novel chimeric cell-based assay and showed that CFRBP full length can stably be expressed on the plasma membrane. We discovered that only CRFBP(10 kD) fragment is able to potentiate CRF-intracellular Ca²⁺ release. We provide evidence that CRHBP gene loss increased ethanol consumption in mice. Then, we demonstrate that selective reduction of CRHBP expression in the center nucleus of the amygdala (CeA) decreases ethanol consumption in ethanol-dependent rats. CRFBP amygdalar downregulation, however, does not attenuate yohimbine-induced ethanol self-administration. This effect was associated with decreased hemodynamic brain activity in the CRFBP-downregulated CeA and increased hemodynamic activity in the caudate putamen during yohimbine administration. Finally, in alcohol-dependent patients, genetic variants related to the CRFBP(10 kD) fragment were associated with greater risk for alcoholism and anxiety, while other genetic variants were associated with reduced risk for anxiety. Taken together, our data provide evidence that CRFBP may possess both inhibitory and excitatory roles and may represent a novel pharmacological target for the treatment of AUD.

Transmembrane signal transduction by peptide hormones via family B G protein-coupled receptors

Although family B G protein-coupled receptors (GPCRs) contain only 15 members, they play key roles in transmembrane signal transduction of hormones. Family B GPCRs are drug targets for developing therapeutics for diseases ranging from metabolic to neurological disorders. Despite their importance, the molecular mechanism of activation of family B GPCRs remains largely unexplored due to the challenges in expression and purification of functional receptors to the quantity for biophysical characterization. Currently, there is no crystal structure available of a full-length family B GPCR. However, structures of key domains, including the extracellular ligand binding regions and seven-helical transmembrane regions, have been solved by X-ray crystallography and NMR, providing insights into the mechanisms of ligand recognition and selectivity, and helical arrangements within the cell membrane. Moreover, biophysical and biochemical methods have been used to explore functions, key residues for signaling, and the kinetics and dynamics of signaling processes. This review summarizes the current knowledge of the signal transduction mechanism of family B GPCRs at the molecular level and comments on the challenges and outlook for mechanistic studies of family B GPCRs.

Conformational states of the full-length glucagon receptor

Class B G protein-coupled receptors are composed of an extracellular domain (ECD) and a seven-transmembrane (7TM) domain, and their signalling is regulated by peptide hormones. Using a hybrid structural biology approach together with the ECD and 7TM domain crystal structures of the glucagon receptor (GCGR), we examine the relationship between full-length receptor conformation and peptide ligand binding. Molecular dynamics (MD) and disulfide crosslinking studies suggest that apo-GCGR can adopt both an open and closed conformation associated with extensive contacts between the ECD and 7TM domain. The electron microscopy (EM) map of the full-length GCGR shows how a monoclonal antibody stabilizes the ECD and 7TM domain in an elongated conformation. Hydrogen/deuterium exchange (HDX) studies and MD simulations indicate that an open conformation is also stabilized by peptide ligand binding. The combined studies reveal the open/closed states of GCGR and suggest that glucagon binds to GCGR by a conformational selection mechanism.

Exploration of the antagonist CP-376395 escape pathway for the corticotropin-releasing factor receptor 1 by random acceleration molecular dynamics simulations

Corticotropin-releasing factor receptor 1 (CRF1R), a member of class B G-protein-coupled receptors (GPCRs), plays an important role in the treatment of osteoporosis, diabetes, depression, migraine and anxiety. To explore the escape pathway of the antagonist CP-376395 in the binding pocket of CRF1R, molecular dynamics (MD) simulations, dynamical network analysis, random acceleration molecular dynamics (RAMD) simulations and adaptive biasing force (ABF) calculations were performed on the crystal structure of CRF1R in complex with CP-376395. The results of dynamical network analysis show that TM7 of CRF1R has the strongest edges during MD simulation. The bent part of TM7 forms a V-shape pocket with Gly356(7.50). Asn283(5.50) has high hydrogen bond occupancy during 100 ns MD simulations and is the key interaction residue with the antagonist in the binding pocket of CRF1R. RAMD simulation has identified three possible pathways (PW1, PW2 and PW3) for CP-376395 to escape from the binding pocket of CRF1R. The PW3 pathway was proved to be the most likely escape pathway for CP-376395. The free energy along the PW3 pathway was calculated by using ABF simulations. Two energy barriers were found along the reaction coordinates. Residues Leu323(6.49), Asn283(5.50) and Met206(3.47) contribute to the steric hindrance for the first energy barrier. Residues His199(3.40) and Gln355(7.49) contribute to the second energy barrier through the hydrogen bonding interaction between CP-376395 and CRF1R. The results of our study can not only provide useful information to understand the interaction mechanism between CP-376395 and CRF1R, but also provide the details about the possible escape pathway and the free energy profile of CP-376395 in the pocket of CRF1R.

An insight into antagonist binding and induced conformational dynamics of class B GPCR corticotropin-releasing factor receptor 1

The binding of small-molecule antagonists, CP-376395 and MTIP, would induce conformational dynamics behaviors of CRF₁R.

Structural and functional insights into the juxtamembranous amino-terminal tail and extracellular loop regions of class B GPCRs

Class B guanine nucleotide-binding protein GPCRs share heptahelical topology and signalling via coupling with heterotrimeric G proteins typical of the entire superfamily of GPCRs. However, they also exhibit substantial structural differences from the more extensively studied class A GPCRs. Even their helical bundle region, most conserved across the superfamily, is predicted to differ from that of class A GPCRs. Much is now known about the conserved structure of the amino-terminal domain of class B GPCRs, coming from isolated NMR and crystal structures, but the orientation of that domain relative to the helical bundle is unknown, and even less is understood about the conformations of the juxtamembranous amino-terminal tail or of the extracellular loops linking the transmembrane segments. We now review what is known about the structure and function of these regions of class B GPCRs. This comes from indirect analysis of structure-function relationships elucidated by mutagenesis and/or ligand modification and from the more direct analysis of spatial approximation coming from photoaffinity labelling and cysteine trapping studies. Also reviewed are the limited studies of structure of some of these regions. No dominant theme was recognized for the structures or functional roles of distinct regions of these juxtamembranous portions of the class B GPCRs. Therefore, it is likely that a variety of molecular strategies can be engaged for docking of agonist ligands and for initiation of conformational changes in these receptors that would be expected to converge to a common molecular mechanism for activation of intracellular signalling cascades.

Insights into the structure of class B GPCRs

The secretin-like (class B) family of G protein-coupled receptors (GPCRs) are key players in hormonal homeostasis and are interesting drug targets for the treatment of several metabolic disorders (such as type 2 diabetes, osteoporosis, and obesity) and nervous system diseases (such as migraine, anxiety, and depression). The recently solved crystal structures of the transmembrane domains of the human glucagon receptor and human corticotropin-releasing factor receptor 1 have opened up new opportunities to study the structure and function of class B GPCRs. The current review shows how these structures offer more detailed explanations to previous biochemical and pharmacological studies of class B GPCRs, and provides new insights into their interactions with ligands.

Genetically encoded chemical probes in cells reveal the binding path of urocortin-I to CRF class B GPCR

Molecular determinants regulating the activation of class B G-protein-coupled receptors (GPCRs) by native peptide agonists are largely unknown. We have investigated here the interaction between the corticotropin releasing factor receptor type 1 (CRF1R) and its native 40-mer peptide ligand Urocortin-I directly in mammalian cells. By incorporating unnatural amino acid photochemical and new click-chemical probes into the intact receptor expressed in the native membrane of live cells, 44 intermolecular spatial constraints have been derived for the ligand-receptor interaction. The data were analyzed in the context of the recently resolved crystal structure of CRF1R transmembrane domain and existing extracellular domain structures, yielding a complete conformational model for the peptide-receptor complex. Structural features of the receptor-ligand complex yield molecular insights on the mechanism of receptor activation and the basis for discrimination between agonist and antagonist function.

Insights into mechanisms of corticotropin-releasing hormone receptor signal transduction

During evolution, mammals have developed remarkably similar molecular mechanisms to respond to external challenges and maintain survival. Critical regulators of these mechanisms are the family of 'stress'-peptides that consists of the corticotropin-releasing hormone (CRH) and urocortins (Ucns). These neuropeptides 'fine-tune' integration of an intricate series of physiological responses involving the autonomic, endocrine, immune, cardiovascular and reproductive systems, which induce a spectrum of behavioural and homeostatic changes. CRH and Ucns exert their actions by activating two types of CRH receptors (CRH-R), CRH-R1 and CRH-R2, which belong to the class-B1 family of GPCRs. The CRH-Rs exhibit signalling promiscuity facilitated by their ability to couple to multiple G-proteins and regulate diverse intracellular networks that involve intracellular effectors such as cAMP and an array of PKs in an agonist and tissue-specific manner, a property that allows them to exert unique roles in the integration of homeostatic mechanisms. We only now begin to unravel the plethora of CRH-R biological actions and the transcriptional and post-translational mechanisms such as alternative mRNA splicing or phosphorylation-mediated desensitization developed to tightly control CRH-Rs biological activity and regulate their physiological actions. This review summarizes the current understanding of CRH-R signalling complexity and regulatory mechanisms that underpin cellular responses to CRH and Ucns.

Second extracellular loop of human glucagon-like peptide-1 receptor (GLP-1R) has a critical role in GLP-1 peptide binding and receptor activation

The glucagon-like peptide-1 receptor (GLP-1R) is a therapeutically important family B G protein-coupled receptor (GPCR) that is pleiotropically coupled to multiple signaling effectors and, with actions including regulation of insulin biosynthesis and secretion, is one of the key targets in the management of type II diabetes mellitus. However, there is limited understanding of the role of the receptor core in orthosteric ligand binding and biological activity. To assess involvement of the extracellular loop (ECL) 2 in ligand-receptor interactions and receptor activation, we performed alanine scanning mutagenesis of loop residues and assessed the impact on receptor expression and GLP-1(1-36)-NH(2) or GLP-1(7-36)-NH(2) binding and activation of three physiologically relevant signaling pathways as follows: cAMP formation, intracellular Ca(2+) (Ca(2+)(i)) mobilization, and phosphorylation of extracellular signal-regulated kinases 1 and 2 (pERK1/2). Although antagonist peptide binding was unaltered, almost all mutations affected GLP-1 peptide agonist binding and/or coupling efficacy, indicating an important role in receptor activation. However, mutation of several residues displayed distinct pathway responses with respect to wild type receptor, including Arg-299 and Tyr-305, where mutation significantly enhanced both GLP-1(1-36)-NH(2)- and GLP-1(7-36)-NH(2)-mediated signaling bias for pERK1/2. In addition, mutation of Cys-296, Trp-297, Asn-300, Asn-302, and Leu-307 significantly increased GLP-1(7-36)-NH(2)-mediated signaling bias toward pERK1/2. Of all mutants studied, only mutation of Trp-306 to alanine abolished all biological activity. These data suggest a critical role of ECL2 of the GLP-1R in the activation transition(s) of the receptor and the importance of this region in the determination of both GLP-1 peptide- and pathway-specific effects.

Second extracellular loop of human glucagon-like peptide-1 receptor (GLP-1R) differentially regulates orthosteric but not allosteric agonist binding and function

The glucagon-like peptide-1 receptor (GLP-1R) is a prototypical family B G protein-coupled receptor that exhibits physiologically important pleiotropic coupling and ligand-dependent signal bias. In our accompanying article (Koole, C., Wootten, D., Simms, J., Miller, L. J., Christopoulos, A., and Sexton, P. M. (2012) J. Biol. Chem. 287, 3642-3658), we demonstrate, through alanine-scanning mutagenesis, a key role for extracellular loop (ECL) 2 of the receptor in propagating activation transition mediated by GLP-1 peptides that occurs in a peptide- and pathway-dependent manner for cAMP formation, intracellular (Ca(2+)(i)) mobilization, and phosphorylation of extracellular signal-regulated kinases 1 and 2 (pERK1/2). In this study, we examine the effect of ECL2 mutations on the binding and signaling of the peptide mimetics, exendin-4 and oxyntomodulin, as well as small molecule allosteric agonist 6,7-dichloro-2-methylsulfonyl-3-tert-butylaminoquinoxaline (compound 2). Lys-288, Cys-296, Trp-297, and Asn-300 were globally important for peptide signaling and also had critical roles in governing signal bias of the receptor. Peptide-specific effects on relative efficacy and signal bias were most commonly observed for residues 301-305, although R299A mutation also caused significantly different effects for individual peptides. Met-303 was more important for exendin-4 and oxyntomodulin action than those of GLP-1 peptides. Globally, ECL2 mutation was more detrimental to exendin-4-mediated Ca(2+)i release than GLP-1(7-36)-NH(2), providing additional evidence for subtle differences in receptor activation by these two peptides. Unlike peptide activation of the GLP-1R, ECL2 mutations had only limited impact on compound 2 mediated cAMP and pERK responses, consistent with this ligand having a distinct mechanism for receptor activation. These data suggest a critical role of ECL2 of the GLP-1R in the activation transition of the receptor by peptide agonists.

Corticotropin releasing factor receptor antagonists for major depressive disorder

INTRODUCTION

Major depressive disorder is a serious and common psychiatric illness, and many of the depressive patients benefit from pharmacological treatment. Available antidepressants produce remission in only about 30 -- 40% of the patients. Therefore, new concepts are being explored for the development of innovative antidepressants with higher efficacy.

AREAS COVERED

The use of corticotropin releasing factor type 1 (CRF1) receptor antagonists for depression is supported by abundant evidence of target validation, the availability of in vitro and in vivo assays and specific small ligands. Some of these compounds have advanced to clinical studies, with discouraging results so far in depression. This review covers the development of CRF1 receptor antagonists at different stages of the development pipeline of the pharmaceutical industry and its bottlenecks. Most of the available CRF1 receptor antagonists known so far share a common chemical scaffold. We present possible strategies to overcome obstacles in the discovery and development process at the levels of library screenings and clinical studies to find more diverse compounds.

EXPERT OPINION

CRF1 receptor antagonists are expected to be beneficial only for those patients with CRF overexpression and the need for tests to identify these individuals is discussed. New technical developments and diagnostic tools might eventually lead to a more successful treatment of major depression with CRF1 receptor antagonists.

Refinement of glucagon-like peptide 1 docking to its intact receptor using mid-region photolabile probes and molecular modeling

The glucagon-like peptide 1 (GLP1) receptor is an important drug target within the B family of G protein-coupled receptors. Its natural agonist ligand, GLP1, has incretin-like actions and the receptor is a recognized target for management of type 2 diabetes mellitus. Despite recent solution of the structure of the amino terminus of the GLP1 receptor and several close family members, the molecular basis for GLP1 binding to and activation of the intact receptor remains unclear. We previously demonstrated molecular approximations between amino- and carboxyl-terminal residues of GLP1 and its receptor. In this work, we study spatial approximations with the mid-region of this peptide to gain insights into the orientation of the intact receptor and the ligand-receptor complex. We have prepared two new photolabile probes incorporating a p-benzoyl-l-phenylalanine into positions 16 and 20 of GLP1(7-36). Both probes bound to the GLP1 receptor specifically and with high affinity. These were each fully efficacious agonists, stimulating cAMP accumulation in receptor-bearing CHO cells in a concentration-dependent manner. Each probe specifically labeled a single receptor site. Protease cleavage and radiochemical sequencing identified receptor residue Leu(141) above transmembrane segment one as its site of labeling for the position 16 probe, whereas the position 20 probe labeled receptor residue Trp(297) within the second extracellular loop. Establishing ligand residue approximation with this loop region is unique among family members and may help to orient the receptor amino-terminal domain relative to its helical bundle region.

Dihydropyrrole[2,3-d]pyridine derivatives as novel corticotropin-releasing factor-1 antagonists: mapping of the receptor binding pocket by in silico docking studies

In an effort to discover novel CRF-1 receptor antagonists exhibiting improved physicochemical properties, a dihydropirrole[2,3]pyridine scaffold was designed and explored in terms of the SAR of the substitution at the pendent phenyl ring and the nature of the heterocyclic moieties present in the upper region of the molecule. Selective and potent compounds have been discovered endowed with reduced ClogP with respect to compounds known in the literature. Of particular relevance was the finding that the in vitro affinity of the series was maintained by reducing the overall lipophilicity. The results achieved by this exploration enabled the formulation of a novel hypothesis on the nature of the receptor binding pocket of this class of CRF-1 receptor antagonists, making use of in silico docking studies of the putative nonpeptidic antagonist binding site set up in house by homology modeling techniques.

Drugability of extracellular targets: discovery of small molecule drugs targeting allosteric, functional, and subunit-selective sites on GPCRs and ion channels

Beginning with the discovery of the structure of deoxyribose nucleic acid in 1953, by James Watson and Francis Crick, the sequencing of the entire human genome some 50 years later, has begun to quantify the classes and types of proteins that may have relevance to human disease with the promise of rapidly identifying compounds that can modulate these proteins so as to have a beneficial and therapeutic outcome. This so called 'drugable space' involves a variety of membrane-bound proteins including the superfamily of G-protein-coupled receptors (GPCRs), ion channels, and transporters among others. The recent number of novel therapeutics targeting membrane-bound extracellular proteins that have reached the market in the past 20 years however pales in magnitude when compared, during the same timeframe, to the advancements made in the technologies available to aid in the discovery of these novel therapeutics. This review will consider select examples of extracellular drugable targets and focus on the GPCRs and ion channels highlighting the corticotropin releasing factor (CRF) type 1 and gamma-aminobutyric acid receptors, and the Ca(V)2.2 voltage-gated ion channel. These examples will elaborate current technological advancements in drug discovery and provide a prospective framework for future drug development.

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.

Allosteric approaches to the targeting of G-protein-coupled receptors for novel drug discovery: A critical assessment

In recent years, the concept of allosteric modulation of G-protein-coupled receptors (GPCRs) has matured and now represents an increasingly viable approach to drug discovery. This is evident in the fact that allosteric modulators have been reported for every class of GPCR, and several are currently in clinical trials with one drug example approved and launched. The allosteric approach has been highlighted for the potential of identifying highly selective compounds with a minimal propensity to produce adverse effect. While much has been written regarding the promises of this approach, important challenges, caveats, and pitfalls exist that are often overlooked. Therefore, a balanced overview of the field that describes both the promises and the challenges of discovering allosteric modulators of GPCRs as novel drugs is presented.

Modulation of T-type Ca2+ channels by corticotropin-releasing factor through protein kinase C pathway in MN9D dopaminergic cells

Corticotrophin-releasing factor (CRF) is the main regulator of the body's stress axis and its signal is translated through G-protein-coupled CRF receptors (CRF-R1, CRF-R2). Even though CRF receptors are present in the midbrain dopamine neurons, the cellular mechanism of CRF action is not clear yet. Since voltage-dependent Ca(2+) channels are highly expressed and important in dopamine neuronal functions, we tested the effect of CRF on voltage-dependent Ca(2+) channels in MN9D cells, a model of dopamine neurons. The application of CRF-related peptide, urocortin 1, reversibly inhibited T-type Ca(2+) currents, which was a major Ca(2+) channel in the cells. The effect of urocortin was abolished by specific CRF-R1 antagonist and was mimicked by protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate. PKC inhibitors abolished the effect of urocortin. These results suggest that urocortin modulates T-type Ca(2+) channel by interacting with CRF-R1 via the activation of PKC signal pathway in MN9D cells.

The corticotropin-releasing factor receptor: a novel target for the treatment of depression and anxiety-related disorders

The treatment of mood disorders has been the subject of intense study for more than half a century and has resulted in the discovery and availability of a number of compounds that have seen tremendous success in the management of major depression and anxiety-related disorders. In spite of this success, these drugs have not provided a complete therapeutic solution for all patients and this has revitalised the need for a greater understanding of the underlying molecular mechanisms and targets involved in these disorders. Elucidation of these novel targets will enable the development of a better class of compounds which could benefit a greater majority of the patient population and be devoid of the current side effect liabilities. Towards that end, this review examines, in detail, the prospect of one such target, the corticotropin-releasing factor system, as having an enhanced therapeutic profile with the potential of a broader range of efficacy with reduced side effect liabilities.

Corticotropin releasing hormone and the skin

Cotricotropin-releasing hormone (CRH) and related peptides are produced in skin that is dependent on species and anatomical location. Local peptide production is regulated by ultraviolet radiation (UVR), glucocorticoids and phase of the hair cycle. The skin also expresses the corresponding receptors (CRH-R1 and CRH-R2), with CRH-R1 being the major receptor in humans. CRH-R1 is expressed in epidermal and dermal compartments, and CRH-R2 predominantly in dermal structures. The gene coding for CRH-R1 generates multiple isoforms through a process modulated by UVR, cyclic adenosine monophosphate (cAMP) and phorbol 12-myristate 13-acetate. The phenotypic effects of CRH in human skin cells are largely mediated by CRH-R1alpha through increases in concentrations of cAMP, inositol triphosphate (IP3), or Ca2+ with subsequent activation of protein kinases A (PKA) and C (PKC) dependent pathways. CRH also modulates the activity of nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-kappaB), activator protein 1 (AP-1) and cAMP responsive element binding protein (CREB). The cellular functions affected by CRH depend on cell type and nutritional status and include modulation of differentiation program(s), proliferation, viability and immune activity. The accumulated evidence indicates that cutaneous CRH is also a component of a local structure organized similarly to the hypothalamo-pituitary-adrenal axis.

Corticotropin releasing factor (CRF) receptor signaling in the central nervous system: new molecular targets

Corticotropin-releasing factor (CRF) and the related urocortin peptides mediate behavioral, cognitive, autonomic, neuroendocrine and immunologic responses to aversive stimuli by activating CRF(1) or CRF(2) receptors in the central nervous system and anterior pituitary. Markers of hyperactive central CRF systems, including CRF hypersecretion and abnormal hypothalamic-pituitary-adrenal axis functioning, have been identified in subpopulations of patients with anxiety, stress and depressive disorders. Because CRF receptors are rapidly desensitized in the presence of high agonist concentrations, CRF hypersecretion alone may be insufficient to account for the enhanced CRF neurotransmission observed in these patients. Concomitant dysregulation of mechanisms stringently controlling magnitude and duration of CRF receptor signaling also may contribute to this phenomenon. While it is well established that the CRF(1) receptor mediates many anxiety- and depression-like behaviors as well as HPA axis stress responses, CRF(2) receptor functions are not well understood at present. One hypothesis holds that CRF(1) receptor activation initiates fear and anxiety-like responses, while CRF(2) receptor activation re-establishes homeostasis by counteracting the aversive effects of CRF(1) receptor signaling. An alternative hypothesis posits that CRF(1) and CRF(2) receptors contribute to opposite defensive modes, with CRF(1) receptors mediating active defensive responses triggered by escapable stressors, and CRF(2) receptors mediating anxiety- and depression-like responses induced by inescapable, uncontrollable stressors. CRF(1) receptor antagonists are being developed as novel treatments for affective and stress disorders. If it is confirmed that the CRF(2) receptor contributes importantly to anxiety and depression, the development of small molecule CRF(2) receptor antagonists would be therapeutically useful.

Single amino acid residue determinants of non-peptide antagonist binding to the corticotropin-releasing factor1 (CRF1) receptor

The molecular interactions between non-peptide antagonists and the corticotropin-releasing factor type 1 (CRF1) receptor are poorly understood. A CRF1 receptor mutation has been identified that reduces binding affinity of the non-peptide antagonist NBI 27914 (M276I in transmembrane domain 5). We have investigated the mechanism of the mutation's effect using a combination of peptide and non-peptide ligands and receptor mutations. The M276I mutation reduced binding affinity of standard non-peptide antagonists 5-75-fold while having no effect on peptide ligand binding. We hypothesized that the side chain of isoleucine, beta-branched and so rotationally constrained when within an alpha-helix, introduces a barrier to non-peptide antagonist binding. In agreement with this hypothesis, mutation of M276 to the rotationally constrained valine produced similar reductions of affinity as M276I mutation, whereas mutation to leucine (with an unbranched beta-carbon) minimally affected non-peptide antagonist affinity. Mutation to alanine did not appreciably affect non-peptide antagonist affinity, implying the methionine side chain does not contribute directly to binding. Three observations suggested M276I/V mutations interfere with binding of the heterocyclic core of the compounds: (1) all compounds affected by M276I/V mutations possess a planar heterocyclic core. (2) None of the M276 mutations affected binding of an acylic compound. (3) The mutations differentially affected affinity of two compounds that differ only by core methylation. These findings imply that non-peptide antagonists, and specifically the heterocyclic core of such molecules, bind in the vicinity of M276 of the CRF1 receptor. M276 mutations did not affect peptide ligand binding and this residue is distant from determinants of peptide binding (predominantly in the extracellular regions), providing molecular evidence for non-overlapping (allosteric) binding sites for peptide and non-peptide ligands within the CRF1 receptor.

The molecular mechanisms underlying the regulation of the biological activity of corticotropin-releasing hormone receptors: implications for physiology and pathophysiology

The CRH receptor (CRH-R) is a member of the secretin family of G protein-coupled receptors. Wide expression of CRH-Rs in the central nervous system and periphery ensures that their cognate agonists, the family of CRH-like peptides, are capable of exerting a wide spectrum of actions that underpin their critical role in integrating the stress response and coordinating the activity of fundamental physiological functions, such as the regulation of the cardiovascular system, energy balance, and homeostasis. Two types of mammal CRH-R exist, CRH-R1 and CRH-R2, each with unique splicing patterns and remarkably distinct pharmacological properties, but similar signaling properties, probably reflecting their distinct and sometimes contrasting biological functions. The regulation of CRH-R expression and activity is not fully elucidated, and we only now begin to fully understand the impact on mammalian pathophysiology. The focus of this review is the current and evolving understanding of the molecular mechanisms controlling CRH-R biological activity and functional flexibility. This shows notable tissue-specific characteristics, highlighted by their ability to couple to distinct G proteins and activate tissue-specific signaling cascades. The type of activating agonist, receptor, and target cell appears to play a major role in determining the overall signaling and biological responses in health and disease.

Distinct Conformations of the Corticotropin Releasing Factor Type 1 Receptor Adopted following Agonist and Antagonist Binding Are Differentially Regulated

The corticotropin releasing factor (CRF) type 1 receptor (CRF1) is a class B family G protein-coupled receptor that regulates the hypothalamic-pituitary-adrenal stress axis. Astressin is an amino-terminal truncated analog of CRF that retains high affinity binding to the extracellular domain of the receptor and is believed to act as a neutral competitive antagonist of receptor activation. Here we show that despite being unable to activate the CRF1 receptor, astressin binding results in the internalization of the receptor. Furthermore, entirely different pathways of internalization of CRF1 receptors are utilized following CRF and astressin binding. CRF causes the receptor to be phosphorylated, recruit beta-arrestin2, and to be internalized rapidly, likely through clathrin-coated pits. Astressin, however, fails to induce receptor phosphorylation or beta-arrestin2 recruitment, and internalization is slow and occurs through a pathway that is insensitive to inhibitors of clathrin-coated pits and caveolae. The fate of the internalized receptors also differs because only CRF-induced internalization results in receptor down-regulation. Furthermore, we present evidence that for astressin to induce internalization it must interact with both the extracellular amino terminus and the juxtamembrane domain of the receptor. Astressin binds with 6-fold higher affinity to full-length CRF1 receptors than to a chimeric protein containing only the extracellular domain attached to the transmembrane domain of the activin IIB receptor, yet two 12-residue analogs of astressin have similar affinities for both proteins but are unable to induce receptor internalization. These data demonstrate that agonists and antagonists for CRF1 receptors promote distinct conformations, which are then differentially regulated.

Corticosteroid pharmacogenetics: association of sequence variants in CRHR1 with improved lung function in asthmatics treated with inhaled corticosteroids

Corticosteroids mediate a variety of immunological actions and are commonly utilized in the treatment of a wide range of diseases. Unfortunately, therapy with this class of medications is associated with a large proportion of non-responders and significant side effects. Inhaled corticosteroids are the most commonly used asthma controller therapy. However, asthmatic response to corticosteroids also varies widely between individuals. We investigated the genetic contribution to the variation in response to inhaled corticosteroid therapy in asthma. The association of longitudinal change in lung function and single nucleotide polymorphisms from candidate genes crucial to the biologic actions of corticosteroids were evaluated in three independent asthmatic clinical trial populations utilizing inhaled corticosteroids as the primary therapy in at least one treatment arm. Variation in one gene, corticotropin-releasing hormone receptor 1 (CRHR1) was consistently associated with enhanced response to therapy in each of our three populations. Individuals homozygous for the variants of interest manifested a doubling to quadrupling of the lung function response to corticosteroids compared with lack of the variants (P-values ranging from 0.006 to 0.025 for our three asthmatic populations). As the primary receptor mediating the release of adrenocorticotropic hormone, which regulates endogenous cortisol levels, CRHR1 plays a pivotal, pleiotropic role in steroid biology. These data indicate that genetic variants in CRHR1 have pharmacogenetic effects influencing asthmatic response to corticosteroids, provide a rationale for predicting therapeutic response in asthma and other corticosteroid-treated diseases, and suggests this gene pathway as a potential novel therapeutic target.

Genetic variation in the corticotrophin-releasing factor receptors: identification of single-nucleotide polymorphisms and association studies with obesity in UK Caucasians

OBJECTIVE

To investigate whether genetic variation at the loci encoding the corticotropin-releasing factor receptors-1 and -2 (CRF-R1 and CRF-R2) contributes to human obesity.

DESIGN

The coding region of the CRF-R1 and CRF-R2 genes was screened in 51 severely obese children (body mass index (BMI)>4 kg/m(2) standard deviations above the age-related mean) using denaturing high-performance liquid chromatography and direct nucleotide sequencing. Common polymorphisms that were identified were typed from a UK Caucasian population-based cohort by a PCR-based forced restriction digestion. A repeated measures analysis was used to determine associations between the C861T and G1047A genotypes and anthropometric and biochemical indices relevant to obesity.

RESULTS

In subjects with extreme early-onset obesity, four missense mutations were found, each in a single individual: CRF-R1 (Val161Met) and CRF-R2 (Glu220Asp, Val240Ile and Val411Met). However, none of these missense mutations clearly cosegregated with obesity in family studies. Two common single-nucleotide polymorphisms, C861T (Cys287Cys) in CRF-R1 and G1047A (Ser349Ser) in CRF-R2, were also detected. G1047A did not associate with any obesity-related phenotype. In contrast, carriers of the CRF-R1 polymorphism, C861T, had a significantly higher body mass index (BMI).

CONCLUSION

Mutations in the coding sequence of the CRF-R1 and CRF-R2 genes are unlikely to be a common monogenic cause of early-onset obesity. In an adult UK Caucasian population, the CRF-R1 C861T polymorphism is associated with increased BMI.

Identification of a contact site for residue 19 of parathyroid hormone (PTH) and PTH-related protein analogs in transmembrane domain two of the type 1 PTH receptor

Recent functional studies have suggested that position 19 in PTH interacts with the portion of the PTH-1 receptor (P1R) that contains the extracellular loops and seven transmembrance helices (TMs) (the J domain). We tested this hypothesis using the photoaffinity cross-linking approach. A PTHrP(1-36) analog and a conformationally constrained PTH(1-21) analog, each containing para-benzoyl-l-phenylalanine (Bpa) at position 19, each cross-linked efficiently to the P1R expressed in COS-7 cells, and digestive mapping analysis localized the cross-linked site to the interval (Leu232-Lys240) at the extracellular end of TM2. Point mutation analysis identified Ala234, Val235, and Lys240 as determinants of cross-linking efficiency, and the Lys240-->Ala mutation selectively impaired the binding of PTH(1-21) and PTH(1-19) analogs, relative to that of PTH(1-15) analogs. The findings support the hypothesis that residue 19 of the receptor-bound ligand contacts, or is close to, the P1R J domain-specifically, Lys240 at the extracellular end of TM2. The findings also support a molecular model in which the 1-21 region of PTH binds to the extracellular face of the P1R J domain as an alpha-helix.

A soluble form of the first extracellular domain of mouse type 2beta corticotropin-releasing factor receptor reveals differential ligand specificity

The heptahelical receptors for corticotropin-releasing factor (CRF), CRFR1 and CRFR2, display different specificities for CRF family ligands: CRF and urocortin I bind to CRFR1 with high affinity, whereas urocortin II and III bind to this receptor with very low affinities. In contrast, all the urocortins bind with high affinities, and CRF binds with lower affinity to CRFR2. The first extracellular domain (ECD1) of CRFR1 is important for ligand recognition. Here, we characterize a bacterially expressed soluble protein, ECD1-CRFR2beta, corresponding to the ECD1 of mouse CRFR2beta. The K(i) values for binding to ECD1-CRFR2beta are: astressin = 10.7 (5.4-21.1) nm, urocortin I = 6.4 (4.7-8.7) nm, urocortin II = 6.9 (5.8-8.3) nm, CRF = 97 (22-430) nm, urocortin III = sauvagine >200 nm. These affinities are similar to those for binding to a chimeric receptor in which the ECD1 of CRFR2beta replaces the ECD of the type 1B activin receptor (ALK4). The ECD1-CRFR2beta possesses a disulfide arrangement identical to that of the ECD1 of CRFR1, namely Cys(45)-Cys(70), Cys(60)-Cys(103), and Cys(84)-Cys(118). As determined by circular dichroism, ECD1-CRFR2beta undergoes conformational changes upon binding astressin. These data reinforce the importance of the ECD1 of CRF receptors for ligand recognition and raise the interesting possibility that different ligands having similar affinity for the full-length receptor may, nevertheless, have different affinities for microdomains of the receptor.

Sauvagine cross-links to the second extracellular loop of the corticotropin-releasing factor type 1 receptor

Contact sites between the corticotropin-releasing factor receptor type 1 (CRFR1), the sauvagine (SVG) radioligands [Tyr(0),Gln(1)]SVG ((125)I-YQS) and [Tyr(0),Gln(1), Leu(17)]SVG ((125)I-YQLS) were examined. (125)I-YQLS or (125)I-YQS was cross-linked to CRFR1 using the chemical cross-linker, disuccinimidyl suberate (DSS), which cross-links the epsilon amino groups of lysine residues that have a molecular distance of 11.4 A. DSS specifically and efficiently cross-linked (125)I-YQLS and (125)I-YQS to CRFR1. CRFR1 contains 5 putative extracellular lysine residues (Lys(110), Lys(111), Lys(113), Lys(257), and Lys(262)) that can cross-link to the 4 lysine residues (Lys(16), Lys(22), Lys(25), and Lys(27)) of the radioligands. Identification of the CNBr-cleaved fragments of CRFR1 cross-linked to (125)I-YQLS or (125)I-YQS established that the second extracellular loop of CRFR1 cross-links to Lys(16) of YQS. Additionally, site-directed mutagenesis (changing Lys to Arg in CRFR1 individually and in combination) revealed that Lys(257) in the second extracellular loop of CRFR1 is an important cross-linking site. In conclusion, it was shown that in SVG-bound CRFR1, Lys(257) of CRFR1 lies in close proximity (11.4 A) to Lys(16) of SVG.

Alternative splicing of CRH-R1 receptors in human and mouse skin: identification of new variants and their differential expression

We identified four new isoforms of human CRH-R1 (e-h) and three of mouse (mCRH-R1c, e, and f). In all new forms exon 6 was missing. Human CRH-R1e was characterized by the deletion of exons 3 and 4; exon 12 from CRH-R1f; exon 11, 27 base pairs (bp) of exon 10 and 28 bp of exon 12 from CRH-R1g and CRH-R1h by the addition of a cryptic exon. In mouse CRH-R1c exon 3 was spliced out; in mCRH-R1e exons 3 and 4 and in mCRH-R1f exon 11 were spliced from mRNA. CRH-R1 was expressed in all skin specimens in patterns dependent on the cell type, physiological status, and presence of pathology. CRH-R1a, the most prevalent form, was detected in almost all samples. Ultraviolet radiation (UV) changed the splicing pattern and induced or increased expression of CRH-R1a in cultured skin cells. Continuing UV treatment of succeeding generations of cells resulted in a progressive increase in the number of CRH-R1 isoforms, which suggests that receptor heterogeneity might favor cell survival. TPA (phorbol 12-myristate 13-acetate), forskolin, dbcAMP (N6, 2'-O-dibutyryladenosine 3':5'-cyclic monophospate sodium), and IBMX (3-isobutyl-1-methylxanthine) also changed the splicing pattern. We suggest that a polymorphism of CRH-R1 expression is related to anatomic location, skin physiological or pathologic status, specific cell type, and external stress (UV), and that cAMP-dependent pathways and TPA may regulate CRH-R1.

Expression, purification, and characterization of a soluble form of the first extracellular domain of the human type 1 corticotropin releasing factor receptor

The first extracellular domain (ECD-1) of the corticotropin releasing factor (CRF) type 1 receptor, (CRFR1), is important for binding of CRF ligands. A soluble protein, mNT-CRFR1, produced by COS M6 cells transfected with a cDNA encoding amino acids 1--119 of human CRFR1 and modified to include epitope tags, binds a CRF antagonist, astressin, in a radioreceptor assay using [(125)I-d-Tyr(0)]astressin. N-terminal sequencing of mNT-CRFR1 showed the absence of the first 23 amino acids of human CRFR1. This result suggests that the CRFR1 protein is processed to cleave a putative signal peptide corresponding to amino acids 1--23. A cDNA encoding amino acids 24--119 followed by a FLAG tag, was expressed as a thioredoxin fusion protein in Escherichia coli. Following thrombin cleavage, the purified protein (bNT-CRFR1) binds astressin and the agonist urocortin with high affinity. Reduced, alkylated bNT-CRFR1 does not bind [(125)I-D-Tyr(0)]astressin. Mass spectrometric analysis of photoaffinity labeled bNT-CRFR1 yielded a 1:1 complex with ligand. Analysis of the disulfide arrangement of bNT-CRFR1 revealed bonds between Cys(30) and Cys(54), Cys(44) and Cys(87), and Cys(68) and Cys(102). This arrangement is similar to that of the ECD-1 of the parathyroid hormone receptor (PTHR), suggesting a conserved structural motif in the N-terminal domain of this family of receptors.

CRF and urocortin decreased brain stimulation reward in the rat: reversal by a CRF receptor antagonist

The present studies were designed to investigate the effects of corticotropin-releasing factor (CRF) receptor activation and antagonism on intracranial self-stimulation (ICSS) reward using a discrete-trial current-intensity threshold procedure. Bipolar electrodes were implanted in the lateral hypothalamus, and cannula guides were implanted above the lateral ventricle of male Wistar rats. Dose-effect functions were established for the effects on ICSS of the competitive CRF receptor agonist h/rCRF (0-5.0 microg, i.c.v. ), the CRF receptor agonist urocortin (0-5.0 microg, i.c.v.), and the CRF receptor antagonist [D-Phe(12), Nle(21,38), C(alpha) MeLeu(37)] h/rCRF(12-41) (0-5.0 microg, i.c.v.). Administration of h/rCRF or urocortin dose-dependently elevated ICSS thresholds without altering performance measures (latencies to respond to stimulation, extra and time-out responses). CRF was more potent than urocortin in terms of threshold dose-effects on ICSS thresholds compared to vehicle. Despite these apparent potency differences, percent effect sizes on ICSS thresholds were comparable at the highest doses of both peptides. In contrast to the significant threshold elevation effects of CRF and urocortin, the competitive CRF antagonist D-Phe CRF(12-41) had no effect on ICSS thresholds or performance measures. To determine the neuropharmacological specificity of the effect of CRF on brain stimulation reward, D-Phe CRF(12-41) was used to antagonize CRF-induced threshold elevations. Pretreatment with either the 5.0- or 10.0-microg doses of D-Phe CRF(12-41) effectively blocked CRF-induced reward threshold elevations (3.0 microg) without affecting other ICSS performance measures. These results indicate that CRF neurotransmission can modulate ICSS reward processes.

A novel spliced variant of the type 1 corticotropin-releasing hormone receptor with a deletion in the seventh transmembrane domain present in the human pregnant term myometrium and fetal membranes

CRH exerts its actions via activation of specific G protein-coupled receptors, which exist in two types, CRH-R1 and CRH-R2, and arise from different genes with multiple spliced variants. RT-PCR amplification of CRH receptor sequences from human myometrium and fetal membranes yielded cDNAs that encode a novel CRH-R type 1 spliced variant. This variant (CRH-R1d) is present in the human pregnant myometrium at term only, which suggests a physiologically important role at the end of human pregnancy and labor. The amino acid sequence of CRH-R1d is identical to the CRH-R1alpha receptor except that it contains an exon deletion resulting in the absence of 14 amino acids in the predicted seventh transmembrane domain. Binding studies in HEK-293 cells stably expressing the CRH-R1d or CRH-R1alpha receptors revealed that the deletion does not change the binding characteristics of the variant receptor. In contrast, studies on the G protein activation demonstrated that CRH-R1d is not well coupled to the four subtypes of G proteins (G(s), G(i), G(o), G(q)) that CRH-R1alpha can activate. These data suggest that although the deleted segment is not important for CRH binding, it plays a crucial role in CRH receptor signal transduction. Second messenger studies of the variant receptor showed that CRH and CRH-like peptides can stimulate the adenylate cyclase system, with reduced sensitivity and potency by 10-fold compared with the CRH-R1alpha. Furthermore, CRH failed to stimulate inositol trisphosphate production. Coexpression studies between the CRH-R1d or CRH-R1alpha showed that this receptor does not play a role as a dominant negative receptor for CRH.

Corticotropin-releasing factor antagonists in affective disorders

Following a search lasting nearly three decades, corticotropin-releasing factor (CRF), a 41 amino acid-containing peptide, was isolated and characterised in 1981. In the preceding 18 years, a concatenation was developed that appears to show that CRF integrates not only the endocrine, but also the autonomic, immunologic and behavioural responses of mammalian organisms to stress. Direct CNS administration of CRF to laboratory animals produces actions similar to those observed after exposure to stress. Moreover, CNS administration of peptidergic CRF antagonists blocks many of the behavioural responses to stress. Since both early untoward life events as well as recently experienced stress have been implicated in the pathophysiology of affective disorders, and because there is substantial evidence for CRF neuronal hyperactivity in patients with affective disorders, small molecule, lipophilic CRF antagonists have been hypothesised to possess antidepressant and/or anxiolytic activity. Within the last few years, a number of pharmaceutical companies have developed selective, small molecule CRF(1) receptor antagonists. These compounds block the effects of CRF both in vitro and in vivo. There is also evidence that these agents possess anxiolytic and antidepressant activity in animal behavioural models. Compounds that act upon the CRF system have been hypothesised to be of value not only for certain psychiatric disorders, but also in neurodegenerative and inflammatory disorders. Some of these CRF(1) receptor antagonists are currently undergoing clinical trials to determine their efficacy and tolerability in patients with mood and anxiety disorders.

Corticotropin releasing factor receptors and their ligand family

The CRF receptors belong to the VIP/GRF/PTH family of G-protein coupled receptors whose actions are mediated through activation of adenylate cyclase. Two CRF receptors, encoded by distinct genes, CRF-R1 and CRF-R2, and that can exist in two alternatively spliced forms, have been cloned. The type-1 receptor is expressed in many areas of the rodent brain, as well as in the pituitary, gonads, and skin. In the rodent, one splice variant of the type-2 receptor, CRF-R2 alpha, is expressed mainly in the brain, whereas the other variant, CRF-R2 beta, is found not only in the CNS, but also in cardiac and skeletal muscle, epididymis, and the gastrointestinal tract. The poor correlation between the sites of expression of CRF-R2 and CRF, as well as the relatively low affinity of CRF for CRF-R2, suggested the presence of another ligand, whose existence was confirmed in our cloning of urocortin. This CRF-like peptide is found not only in brain, but also in peripheral sites, such as lymphocytes. The broad tissue distribution of CRF receptors and their ligands underscores the important role of this system in maintenance of homeostasis. Functional studies of the two receptor types reveal differences in the specificity for CRF and related ligands. On the basis of its greater affinity for urocortin, in comparison with CRF, as well as its brain distribution, CRF-R2 may be the cognate receptor for urocortin. Mutagenesis studies of CRF receptors directed toward understanding the basis for their specificity, provide insight into the structural determinants for hormone-receptor recognition and signal transduction.

The ligand-selective domains of corticotropin-releasing factor type 1 and type 2 receptor reside in different extracellular domains: generation of chimeric receptors with a novel ligand-selective profile

The nonselective human corticotropin-releasing factor (hCRF) receptor 1 (hCRFR1) and the ligand-selective Xenopus CRFR1 (xCRFR1), xCRFR2, and hCRFR2alpha were compared. To understand the interactions of hCRF, ovine CRF (oCRF), rat urocortin (rUcn), and sauvagine, ligands with different affinities for type 1 and type 2 CRFRs, chimeric and mutant receptors of hCRFR1, xCRFR1, hCRFR2alpha, and xCRFR2 were constructed. In cyclic AMP stimulation and CRF-binding assays, it was established that different extracellular regions of CRFR1 and CRFR2 conferred their ligand selectivities. The ligand selectivity of xCRFR1 resided in five N-terminal amino acids, whereas the N-terminus of both CRFR2 proteins did not contribute to their ligand selectivities. Chimeric receptors in which the first extracellular domain of hCRFR1 replaced that of hCRFR2alpha or xCRFR2 showed a similar pharmacological profile to the two parental CRFR2 molecules. Chimeric receptors carrying the N-terminal domain of xCRFR1 linked to hCRFR2alpha or xCRFR2 displayed a novel pharmacological profile. hCRF, rUcn, and sauvagine were bound with high affinity, whereas oCRF was bound with low affinity. Furthermore, when three or five residues of xCRFR1 (Gln76, Gly81, Val83, His88, Leu89; or Gln76, Gly81, Val83) were introduced into receptor chimeras carrying the N-terminus of hCRFR1 linked to xCRFR2, the same novel pharmacology was observed. These data indicate a compensation mechanism of two differentially selecting regions located in different domains of both xCRFR1 and CRFR2.

Cloning and characterization of a short variant of the corticotropin-releasing factor receptor subtype from rat amygdala

We have identified and characterized a cDNA encoding a novel isoform of the corticotropin-releasing factor (CRF) receptor, referred to as CRF2alpha-tr, from the rat amygdala cDNA library. The nucleotide sequence of the cloned cDNA has a structure of an alternatively spliced form of the CRF2alpha receptor, which contains unspliced introns 6 and 7 in the message, and encodes a 236-amino-acid truncated protein that comprises three unique transmembrane domains. Northern blot analysis shows that the CRF2alpha-tr receptor is more strongly expressed in the rat amygdala, thalamus, and hypothalamus than the intact CRF2alpha receptor. Western blot analysis also reveals that the CRF2alpha-tr protein can be expressed in transfected COS-7 cells as well as CRF2alpha. Furthermore, this receptor binds rat/human CRF with almost the same low affinity (Kd = 12.7 nM) as the CRF2alpha and without accumulation of intracellular cAMP. Interestingly, it does not bind sauvagine or rat urocortin. These findings suggest that this truncated CRF receptor is the major isoform of CRF2alpha receptor mRNA transcripts in the amygdala and would mediate some functions of CRF pathways in the central nervous system.

The role of the fourth extracellular domain of the rat corticotropin-releasing factor receptor type 1 in ligand binding

The role of the fourth extracellular loop (e4) of rat corticotropin-releasing factor (CRF) receptor, type 1, in ligand binding was investigated using chimeric receptor molecules. e4 of CRF receptor, type 1, was replaced by the corresponding domains of two other G protein-coupled receptors, the rat glucagon receptor or the human pituitary adenylate cyclase activating polypeptide (PACAP) receptor. Both chimeras were transported properly to the cell membranes of transfected chinese hamster ovary cells as indicated by immunocytochemical analysis. Ovine CRF (oCRF) was bound specifically, but with low affinity (Kd = 2-5 microm). Cyclic AMP was not accumulated intracellularly in response to increasing concentrations of oCRF. Based on these data, it is concluded that e4 of rat CRF receptor, type 1, is involved in ligand binding. To confirm the importance of e4 in binding CRF, three negatively charged amino acids of e4, Glu336, Asp337 and Glu338, were replaced by Gln, Asn and Gln, respectively. No effect on ligand binding and cyclic AMP accumulation was observed (Kd = 5 nm; EC50 = 1.5 nm). However, when Tyr346, Phe347 and Asn348 of e4 were changed to three alanine residues, ligand binding affinity as well as efficacy in cyclic AMP accumulation were significantly decreased (Kd = 64 nm; EC50 = 32 nm).

Identification of amino acids in the N-terminal domain of corticotropin-releasing factor receptor 1 that are important determinants of high-affinity ligand binding

The aim of the present study was to identify the N-terminal regions of human corticotropin-releasing factor (CRF) receptor type 1 (hCRF-R1) that are crucial for ligand binding. Mutant receptors were constructed by replacing specific residues in hCRF-R1 with amino acids from the corresponding position in the N-terminal region of the human vasoactive intestinal peptide receptor type 2 (hVIP-R2). In cyclic AMP stimulation and CRF binding assays, it was established that two regions within the N-terminal domain were crucial for the binding of CRF receptor agonists and antagonists: one region mapping to amino acids 43-50 and a second amino acid sequence extending from position 76 to 84 of hCRF-R1. Recently, it was found that the latter sequence plays a very important role in determining the high ligand selectivity of the Xenopus CRF-R1 (xCRF-R1). Replacement of amino acids 76-84 of hCRF-R1 with residues from the same segment of the hVIP-R2 N terminus markedly reduced the binding affinity of CRF ligands. Mutation of Arg76 or Asn81 but not Gly83 of hCRF-R1 to the corresponding amino acids of xCRF-R1 or hVIP-R2 resulted in 100-1,000-fold lower affinities for human/rat CRF, rat urocortin, and astressin. These data underline the importance of the N-terminal domain of CRF-R1 in high-affinity ligand binding.

Identification of binding domains of the growth hormone-releasing hormone receptor by analysis of mutant and chimeric receptor proteins

The hypothalamic peptide GH-releasing hormone (GHRH) stimulates the release of GH from the pituitary through binding and activation of the GHRH receptor, which belongs to the family of G protein-coupled receptors. The objective of this study was to identify regions of the receptor critical for interaction with the ligand by expressing and analyzing truncated and chimeric epitope-tagged GHRH receptors. Two truncated receptors, GHRHdeltaN, in which part of the N-terminal domain between the putative signal sequence and the first transmembrane domain was deleted, and GHRHdeltaC, which was truncated downstream of the first intracellular loop, were generated. Both the receptors were deficient in ligand binding, indicating that neither the N-terminal extracellular domain (N terminus) nor the membrane-spanning domains with the associated extracellular loops (C terminus) are alone sufficient for interaction with GHRH. In subsequent studies, chimeric proteins between the receptors for GHRH and vasoactive intestinal peptide (VIP) or secretin were generated, using the predicted start of the first transmembrane domain as the junction for the exchange of the N terminus between receptors. The chimeras having the N terminus of the GHRH receptor and the C terminus of either the VIP or secretin receptor (GNVC and GNSC) did not bind GHRH or activate adenylate cyclase after GHRH treatment. The reciprocal chimeras having the N terminus of either the VIP or secretin receptors and the C terminus of the GHRH receptor (VNGC and SNGC) bound GHRH and stimulated cAMP accumulation after GHRH treatment. These results suggest that although the N-terminal extracellular domain is essential for ligand binding, the transmembrane domains and associated extracellular loop regions of the GHRH receptor provide critical information necessary for specific interaction with GHRH.

Mapping of the ligand-selective domain of the Xenopus laevis corticotropin-releasing factor receptor 1: implications for the ligand-binding site

The nonselective human corticotropin-releasing factor receptor 1 (hCRF-R1) and the ligand-selective Xenopus CRF-R1 (xCRF-R1) were compared. To understand the interactions of sauvagine and ovine CRF, both high-affinity ligands for hCRF-R1 but surprisingly weak ligands for xCRF-R1, chimeric receptors of hCRF-R1 and xCRF-R1 followed by double or multiple point mutations were constructed. Binding studies and cAMP assays demonstrated that the N-terminal domain exhibited the complete ligand selectivity of xCRF-R1. The important region was mapped between amino acids 70 and 89; replacement of amino acids Arg76, Asn81, Gly83, Leu88, and Ala89 in hCRF-R1 with the corresponding amino acids of xCRF-R1 (Gln76, Gly81, Val83, His88, and Leu89) resulted in a receptor that had approximately 30-fold higher affinity for human/rat CRF than for sauvagine. Mutagenesis of these amino acids in xCRF-R1 to the human sequence completely abolished the ligand selectivity of xCRF-R1. Mutagenesis of amino acids 88 and 89 in hCRF-R1 or xCRF-R1 had only a minor (approximately 2.5-fold) effect on the ligand selectivity of the mutant receptor. Substitution of Arg76, Asn81, and Gly83 in hCRF-R1 with the corresponding sequence of xCRF-R1 (Gln76, Gly81, and Val83) resulted in a receptor with approximately 11-fold higher affinity for human/rat CRF compared with ovine CRF or sauvagine. When only two of these three amino acids were mutated, no effect on the ligand selectivity was observed. On the basis of these data, it is suggested that amino acids 70-89 of CRF-R1 are important for the ligand binding site.

Localization of agonist- and antagonist-binding domains of human corticotropin-releasing factor receptors

The CRF receptors, CRFR1 and CRFR2, are members of the G protein-coupled receptor superfamily. Despite their considerable sequence similarity, CRFR1 and CRFR2 have quite different affinities for the peptide ligand rat/human CRF. Previous studies using chimeric receptors between human CRFR1 and CRFR2 have identified three potentially important regions in the second and third extracellular domains of CRF receptor for the binding of rat/human CRF. The present report further demonstrates that these same three regions also affect the binding of urocortin and sauvagine, two other members of the CRF peptide family, albeit to different extents. We also show that a fourth region in the third extracellular domain, Asp254, has been identified to be important for sauvagine but not CRF or urocortin binding. Thus, the three peptide ligands not only interact with a different set of regions on CRFR1 and CRFR2 but also differentially interact with some of the same regions. These data could, at least in part, account for the much higher affinity of CRFR2 for urocortin and sauvagine compared with rat/human CRF. We have also identified two amino acid residues, His199 in the third transmembrane domain and Met276 in the fifth transmembrane domain, that are important for binding the non-peptide high-affinity CRFR1 antagonist NBI 27914. Mutations of His199 and Met276 to the corresponding amino acids in CRFR2 each decreased the binding affinity of NBI 27914 for CRFR1 by 40- and 200-fold, respectively. This suggests that the transmembrane regions are critically important in forming the binding pocket for the nonpeptide antagonist.