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

CRF1 Receptor Signaling via the ERK1/2-MAP and Akt Kinase Cascades: Roles of Src, EGF Receptor, and PI3-Kinase Mechanisms

In the present study, we determined the cellular regulators of ERK1/2 and Akt signaling pathways in response to human CRF₁ receptor (CRF₁R) activation in transfected COS-7 cells. We found that Pertussis Toxin (PTX) treatment or sequestering Gβγ reduced CRF₁R-mediated activation of ERK1/2, suggesting the involvement of a G_i-linked cascade. Neither G_s/PKA nor G_q/PKC were associated with ERK1/2 activation. Besides, CRF induced EGF receptor (EGFR) phosphorylation at Tyr¹⁰⁶⁸, and selective inhibition of EGFR kinase activity by AG1478 strongly inhibited the CRF₁R-mediated phosphorylation of ERK1/2, indicating the participation of EGFR transactivation. Furthermore, CRF-induced ERK1/2 phosphorylation was not altered by pretreatment with batimastat, GM6001, or an HB-EGF antibody indicating that metalloproteinase processing of HB-EGF ligands is not required for the CRF-mediated EGFR transactivation. We also observed that CRF induced Src and PYK2 phosphorylation in a Gβγ-dependent manner. Additionally, using the specific Src kinase inhibitor PP2 and the dominant-negative-SrcYF-KM, it was revealed that CRF-stimulated ERK1/2 phosphorylation depends on Src activation. PP2 also blocked the effect of CRF on Src and EGFR (Tyr⁸⁴⁵) phosphorylation, further demonstrating the centrality of Src. We identified the formation of a protein complex consisting of CRF₁R, Src, and EGFR facilitates EGFR transactivation and CRF₁R-mediated signaling. CRF stimulated Akt phosphorylation, which was dependent on G_i/βγ subunits, and Src activation, however, was only slightly dependent on EGFR transactivation. Moreover, PI3K inhibitors were able to inhibit not only the CRF-induced phosphorylation of Akt, as expected, but also ERK1/2 activation by CRF suggesting a PI3K dependency in the CRF₁R ERK signaling. Finally, CRF-stimulated ERK1/2 activation was similar in the wild-type CRF₁R and the phosphorylation-deficient CRF₁R-Δ386 mutant, which has impaired agonist-dependent β-arrestin-2 recruitment; however, this situation may have resulted from the low β-arrestin expression in the COS-7 cells. When β-arrestin-2 was overexpressed in COS-7 cells, CRF-stimulated ERK1/2 phosphorylation was markedly upregulated. These findings indicate that on the base of a constitutive CRF₁R/EGFR interaction, the G_i/βγ subunits upstream activation of Src, PYK2, PI3K, and transactivation of the EGFR are required for CRF₁R signaling via the ERK1/2-MAP kinase pathway. In contrast, Akt activation via CRF₁R is mediated by the Src/PI3K pathway with little contribution of EGFR transactivation.

Molecular Modeling of Structures and Interaction of Human Corticotropin-Releasing Factor (CRF) Binding Protein and CRF Type-2 Receptor

The corticotropin-releasing factor (CRF) system is a key mediator of the stress response and addictive behavior. The CRF system includes four peptides: The CRF system includes four peptides: CRF, urocortins I-III, CRF binding protein (CRF-BP) that binds CRF with high affinity, and two class B G-protein coupled receptors CRF₁R and CRF₂R. CRF-BP is a secreted protein without significant sequence homology to CRF receptors or to any other known class of protein. Recently, it has been described a potentiation role of CRF-BP over CRF signaling through CRF₂R in addictive-related neuronal plasticity and behavior. In addition, it has been described that CRF-BP is capable to physically interact specifically with the α isoform of CRF₂R and acts like an escort protein increasing the amount of the receptor in the plasma membrane. At present, there are no available structures for CRF-BP or for full-length CRFR. Knowing and studying the structure of these proteins could be beneficial in order to characterize the CRF-BP/CRF_2αR interaction. In this work, we report the modeling of CRF-BP and of full-length CRF_2αR and CRF_2βR based on the recently solved crystal structures of the transmembrane domains of the human glucagon receptor and human CRF₁R, in addition with the resolved N-terminal extracellular domain of CRFRs. These models were further studied using molecular dynamics simulations and protein-protein docking. The results predicted a higher possibility of interaction of CRF-BP with CRF_2αR than CRF_2βR and yielded the possible residues conforming the interacting interface. Thus, the present study provides a framework for further investigation of the CRF-BP/CRF_2αR interaction.

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.

Alcohol-preferring rats show decreased corticotropin-releasing hormone-2 receptor expression and differences in HPA activation compared to alcohol-nonpreferring rats

BACKGROUND

Corticotropin-releasing hormone (CRH) and urocortins (UCNs) bind to corticotropin-releasing hormone type 2 receptor (CRF2 receptor ), a Gs protein-coupled receptor that plays an important role in modulation of anxiety and stress responses. The Crhr2 gene maps to a quantitative trait locus (QTL) for alcohol preference on chromosome 4 previously identified in inbred alcohol-preferring (iP) and-nonpreferring (iNP) F2 rats.

METHODS

Real-time polymerase chain reaction was utilized to screen for differences in Crhr2 mRNA expression in the central nervous system (CNS) of male iP and iNP rats. DNA sequence analysis was then performed to screen for polymorphism in Crhr2 in order to identify genetic variation, and luciferase reporter assays were then applied to test their functional significance. Next, binding assays were used to determine whether this polymorphism affected CRF2 receptor binding affinity as well as CRF2 receptor density in the CNS. Finally, social interaction and corticosterone levels were measured in the P and NP rats before and after 30-minute restraint stress.

RESULTS

Crhr2 mRNA expression studies found lower levels of Crhr2 mRNA in iP rats compared to iNP rats. In addition, DNA sequencing identified polymorphisms in the promoter region, coding region, and 3'-untranslated region between the iP and iNP rats. A 7 bp insertion in the Crhr2 promoter of iP rats altered expression in vitro as measured by reporter assays, and we found that CRF2 receptor density was lower in the amygdala of iP as compared to iNP rats. Male P rats displayed decreased social interaction and significantly higher corticosterone levels directly following 30-minute restraint when compared to male NP rats.

CONCLUSIONS

This study identified Crhr2 as a candidate gene of interest underlying the chromosome 4 QTL for alcohol consumption that was previously identified in the P and NP model. Crhr2 promoter polymorphism is associated with reduced mRNA expression in certain brain regions, particularly the amygdala, and lowered the density of CRF2 receptor in the amygdala of iP compared to iNP rats. Together, these differences between the animals may contribute to the drinking disparity as well as the anxiety differences of the P and NP rats.

Ro 32-0432 attenuates mecamylamine-precipitated nicotine withdrawal syndrome in mice

G protein-coupled receptor kinase 5 is noted to mediate a number of signal transduction cascades involved in the causation of nicotine withdrawal syndrome. Therefore, the present study investigated the effect of Ro 32-0432, a G protein-coupled receptor kinase 5 inhibitor, on propagation of nicotine dependence and resultant withdrawal signs in subchronic nicotine mouse model. Our experimental protocol consisted of administration of nicotine, (2.5 mg/kg, subcutaneously), four times daily for 7 days. In order to precipitate nicotine withdrawal, mice were given one injection of mecamylamine (3 mg/kg, intraperitoneally) 1 h after the last nicotine injection on the test day (day 8). Behavioral observations were made for a period of 30 min immediately after mecamylamine treatment. Withdrawal syndrome was quantitated in terms of a composite withdrawal severity score, jumping frequency, nicotine-induced hyperalgesia by tail flick method, and withdrawal syndrome-related anxiety was assessed by elevated plus maze test results. Ro 32-0432 dose dependently attenuated mecamylamine-induced nicotine withdrawal syndrome in mice. It is concluded that Ro 32-0432 attenuates the propagation of nicotine dependence and reduce withdrawal signs possibly by G protein-coupled receptor kinase 5 activation-linked mechanisms.

Structural basis for hormone recognition by the Human CRFR2{alpha} G protein-coupled receptor

The mammalian corticotropin releasing factor (CRF)/urocortin (Ucn) peptide hormones include four structurally similar peptides, CRF, Ucn1, Ucn2, and Ucn3, that regulate stress responses, metabolism, and cardiovascular function by activating either of two related class B G protein-coupled receptors, CRFR1 and CRFR2. CRF and Ucn1 activate both receptors, whereas Ucn2 and Ucn3 are CRFR2-selective. The molecular basis for selectivity is unclear. Here, we show that the purified N-terminal extracellular domains (ECDs) of human CRFR1 and the CRFR2α isoform are sufficient to discriminate the peptides, and we present three crystal structures of the CRFR2α ECD bound to each of the Ucn peptides. The CRFR2α ECD forms the same fold observed for the CRFR1 and mouse CRFR2β ECDs but contains a unique N-terminal α-helix formed by its pseudo signal peptide. The CRFR2α ECD peptide-binding site architecture is similar to that of CRFR1, and binding of the α-helical Ucn peptides closely resembles CRF binding to CRFR1. Comparing the electrostatic surface potentials of the ECDs suggests a charge compatibility mechanism for ligand discrimination involving a single amino acid difference in the receptors (CRFR1 Glu104/CRFR2α Pro-100) at a site proximate to peptide residue 35 (Arg in CRF/Ucn1, Ala in Ucn2/3). CRFR1 Glu-104 acts as a selectivity filter preventing Ucn2/3 binding because the nonpolar Ala-35 is incompatible with the negatively charged Glu-104. The structures explain the mechanisms of ligand recognition and discrimination and provide a molecular template for the rational design of therapeutic agents selectively targeting these receptors.

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).

Exploring the binding site crevice of a family B G protein-coupled receptor, the type 1 corticotropin releasing factor receptor

Family B of G protein-coupled receptors (GPCRs) is composed of receptors that bind peptides, such as secretin, glucagon, parathyroid hormone, and corticotropin releasing factor (CRF), which play critical physiological roles. These receptors, like all GPCRs, share a common structural motif of seven membrane-spanning segments, which have been proposed to bind small ligands, such as antalarmin, a nonpeptide antagonist of the type 1 receptor for CRF (CRF(1)). This leads to the hypothesis that as for family A GPCRs, the binding sites of small ligands for family B GPCRs are on the surface of a water-accessible crevice, the binding-site crevice, which is formed by the membrane-spanning segments and extends from the extracellular surface of the receptor into the plane of the membrane. To test this hypothesis we have begun to obtain structural information about family B GPCRs, using as a prototype the CRF(1), by determining the ability of sulfhydryl-specific methanethiosulfonate derivatives, such as the methanethiosulfonate-ethylammonium (MTSEA), to react with CRF(1) and thus irreversibly inhibit (125)I-Tyr(0)-sauvagine binding. We found that MTSEA inhibited (125)I-Tyr(0)-sauvagine binding to CRF(1) and that antalarmin protected against this irreversible inhibition. To identify the susceptible cysteine(s), we mutated, one at a time, four endogenous cysteines to serine. Mutation to serine of Cys211, Cys233, or Cys364 decreased the susceptibility of sauvagine binding to irreversible inhibition by MTSEA. Thus, Cys211, Cys233, and Cys364 at the cytoplasmic ends of the third, fourth, and seventh membrane-spanning segments, respectively, are exposed in the binding site crevice of CRF(1).

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.

Astressin-amide and astressin-acid are structurally different in dimethylsulfoxide

The C-terminally amidated CRF antagonist astressin binds to CRF-R1 or CRF-R2 receptors with low nanomolar affinity while the corresponding astressin-acid has >100 times less affinity. To understand the role of the amide group in binding, the conformations of astressin-amide and astressin-acid were studied in DMSO using NMR techniques. The 3D NMR structures show that the backbones of both analogs prefer an alpha-helical conformation, with a small kink around Gln(26). However, astressin-amide has a well-defined helical structure from Leu(27) to Ile(41) and a conformation very similar to the bioactive conformation reported by our group (Grace et al., Proc Natl Acad Sci USA 2007, 104, 4858-4863). In contrast, astressin-acid has an irregular helical conformation from Arg(35) onward, including a rearrangement of the side chains in that region. This structural difference highlights the crucial role of the C-terminal amidation for stabilization of astressin's bioactive conformation.

Carboxyl-terminal and intracellular loop sites for CRF1 receptor phosphorylation and beta-arrestin-2 recruitment: a mechanism regulating stress and anxiety responses

The primary goal was to test the hypothesis that agonist-induced corticotropin-releasing factor type 1 (CRF(1)) receptor phosphorylation is required for beta-arrestins to translocate from cytosol to the cell membrane. We also sought to determine the relative importance to beta-arrestin recruitment of motifs in the CRF(1) receptor carboxyl terminus and third intracellular loop. beta-Arrestin-2 translocated significantly more rapidly than beta-arrestin-1 to agonist-activated membrane CRF(1) receptors in multiple cell lines. Although CRF(1) receptors internalized with agonist treatment, neither arrestin isoform trafficked with the receptor inside the cell, indicating that CRF(1) receptor-arrestin complexes dissociate at or near the cell membrane. Both arrestin and clathrin-dependent mechanisms were involved in CRF(1) receptor internalization. To investigate molecular determinants mediating the robust beta-arrestin-2-CRF(1) receptor interaction, mutagenesis was performed to remove potential G protein-coupled receptor kinase phosphorylation sites. Truncating the CRF(1) receptor carboxyl terminus at serine-386 greatly reduced agonist-dependent phosphorylation but only partially impaired beta-arrestin-2 recruitment. Removal of a serine/threonine cluster in the third intracellular loop also significantly reduced CRF(1) receptor phosphorylation but did not alter beta-arrestin-2 recruitment. Phosphorylation was abolished in a CRF(1) receptor possessing both mutations. Surprisingly, this mutant still recruited beta-arrestin-2. These mutations did not alter membrane expression or cAMP signaling of CRF(1) receptors. Our data reveal the involvement of at least the following two distinct receptor regions in beta-arrestin-2 recruitment: 1) a carboxyl-terminal motif in which serine/threonine residues must be phosphorylated and 2) an intracellular loop motif configured by agonist-induced changes in CRF(1) receptor conformation. Deficient beta-arrestin-2-CRF(1) receptor interactions could contribute to the pathophysiology of affective disorders by inducing excessive CRF(1) receptor signaling.

Structure of the N-terminal domain of a type B1 G protein-coupled receptor in complex with a peptide ligand

The corticotropin releasing factor (CRF) family of ligands and their receptors coordinate endocrine, behavioral, autonomic, and metabolic responses to stress and play additional roles within the cardiovascular, gastrointestinal, and other systems. The actions of CRF and the related urocortins are mediated by activation of two receptors, CRF-R1 and CRF-R2, belonging to the B1 family of G protein-coupled receptors. The short-consensus-repeat fold (SCR) within the first extracellular domain (ECD1) of the CRF receptor(s) comprises the major ligand binding site and serves to dock a peptide ligand via its C-terminal segment, thus positioning the N-terminal segment to interact with the receptor's juxtamembrane domains to activate the receptor. Here we present the 3D NMR structure of ECD1 of CRF-R2beta in complex with astressin, a peptide antagonist. In the structure of the complex the C-terminal segment of astressin forms an amphipathic helix, whose entire hydrophobic face interacts with the short-consensus-repeat motif, covering a large intermolecular interface. In addition, the complex is characterized by intermolecular hydrogen bonds and a salt bridge. These interactions are quantitatively weighted by an analysis of the effects on the full-length receptor affinities using an Ala scan of CRF. These structural studies identify the major determinants for CRF ligand specificity and selectivity and support a two-step model for receptor activation. Furthermore, because of a proposed conservation of the fold for both the ECD1s and ligands, this structure can serve as a model for ligand recognition for the entire B1 receptor family.

Evolution of secretin family GPCR members in the metazoa

Background

Comparative approaches using protostome and deuterostome data have greatly contributed to understanding gene function and organismal complexity. The family 2 G-protein coupled receptors (GPCRs) are one of the largest and best studied hormone and neuropeptide receptor families. They are suggested to have arisen from a single ancestral gene via duplication events. Despite the recent identification of receptor members in protostome and early deuterostome genomes, relatively little is known about their function or origin during metazoan divergence. In this study a comprehensive description of family 2 GPCR evolution is given based on in silico and expression analyses of the invertebrate receptor genes.

Results

Family 2 GPCR members were identified in the invertebrate genomes of the nematodes C. elegans and C. briggsae, the arthropods D. melanogaster and A. gambiae (mosquito) and in the tunicate C. intestinalis. This suggests that they are of ancient origin and have evolved through gene/genome duplication events. Sequence comparisons and phylogenetic analyses have demonstrated that the immediate gene environment, with regard to gene content, is conserved between the protostome and deuterostome receptor genomic regions. Also that the protostome genes are more like the deuterostome Corticotrophin Releasing Factor (CRF) and Calcitonin/Calcitonin Gene-Related Peptide (CAL/CGRP) receptors members than the other family 2 GPCR members. The evolution of family 2 GPCRs in deuterostomes is characterised by acquisition of new family members, with SCT (Secretin) receptors only present in tetrapods. Gene structure is characterised by an increase in intron number with organismal complexity with the exception of the vertebrate CAL/CGRP receptors.

Conclusion

The family 2 GPCR members provide a good example of gene duplication events occurring in tandem with increasing organismal complexity during metazoan evolution. The putative ancestral receptors are proposed to be more like the deuterostome CAL/CGRP and CRF receptors and this may be associated with their fundamental role in calcium regulation and the stress response, both of which are essential for survival.

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.

Novel effects of CRF on visuomotor behavior and autonomic function in anuran amphibians

Administration of corticotropin-releasing factor (CRF) or exposure to stressors inhibits feeding in anuran amphibians. Since most amphibians rely on visual cues for feeding, these findings have led to the hypothesis that CRF may modulate visuomotor pathways involved in prey detection and predator avoidance. The inhibitory effects of CRF on feeding and prey capture are rapid, and do not appear to require the pituitary-adrenal axis in the short term. CRF neurons are located in key visuomotor processing areas of the anuran brain. Corticotropin-releasing factor also has potent stimulatory effects on sympathetic nervous system activity, a key regulatory system involved in both prey capture and predator avoidance. In this review I will discuss the unique model that amphibian species provide for investigating CRF effects on visual perception and visuomotor processing, and will summarize the data suggesting a role for CRF in visuomotor behavior and autonomic function in amphibians.

Molecular cloning and functional expression of the mouse CRF2(a) receptor splice variant

The mouse corticotropin-releasing factor (CRF) type 2a receptor (CRF2(a)) splice variant was cloned by a PCR-based approach. The corresponding cDNA was found to encode a 411-amino acid polypeptide with highest sequence homology to the rat CRF2(a) receptor. By semiquantitative reverse transcriptase PCR (RT-PCR) analysis, the CRF2(b) mRNA was mainly found in the heart and skeletal muscle with only low level expression in the brain. In contrast, CRF2(a) mRNA was restricted to the brain with major expression sites in the cortex, hippocampus, hypothalamus and telencephalon. Binding and cyclic AMP stimulation studies showed a similar ligand selective profile for both mCRF2 receptor splice variants. A notable exception however, was urotensin I which displayed a approximately 3-fold higher affinity for the CRF2(a) receptor and also stimulated cyclic AMP production in mCRF2(a)-transfected cells with a approximately 3-fold higher potency than in mCRF2(b)-transfected cells. These data show that the mouse like other mammalian species expresses two ligand-selective CRF2 receptor splice variants and that the mCRF2(a) receptor is the predominant central CRF2 receptor in the mouse.

Peptide ligand binding properties of the corticotropin-releasing factor (CRF) type 2 receptor: pharmacology of endogenously expressed receptors, G-protein-coupling sensitivity and determinants of CRF2 receptor selectivity

The CRF2 receptor is involved in stress responses, cardiovascular function and gastric motility. Endogenous agonists (urocortin (UCN) 2, UCN 3) and synthetic antagonists (astressin2-B, antisauvagine-30) are selective for CRF2 over the CRF1 receptor. Peptide ligand binding properties of the CRF2 receptor require further investigation, including ligand affinity for endogenously expressed receptors, the effect of receptor-G-protein coupling on ligand affinity, and the molecular basis of ligand selectivity. Ligand affinity for rat CRF(2a) in olfactory bulb and CRF(2b) in A7r5 cells was similar to that for the cloned human CRF(2a) receptor (within three-fold), except for oCRF (9.4- and 5.4-fold higher affinity in olfactory bulb and A7r5 cells, respectively). Receptor-G-protein uncoupling reduced agonist affinity only 1.2- to 6.5-fold (compared with 92-1300-fold for the CRF1 receptor). Ligand selectivity mechanisms were investigated using chimeric CRF2/CRF1 receptors. The juxtamembrane receptor domain determined selectivity of antisauvagine-30, the N-terminal-extracellular domain contributed to selectivity of UCN 3, and both domains contributed to selectivity of UCN 2 and astressin2-B. Therefore ligands differ in the contribution of receptor domains to their selectivity, and CRF2-selective antagonists bind the juxtamembrane domain. These findings will be important for identifying the CRF2 receptor in tissues and for developing ligands targeting the receptor, both of which will be useful in identifying the emerging physiological functions of the CRF2 receptor.

Binding differences of human and amphibian corticotropin-releasing factor type 1 (CRF(1)) receptors: identification of amino acids mediating high-affinity astressin binding and functional antagonism

The corticotropin-releasing factor (CRF) type 1 receptors (CRF(1)) from human (hCRF(1)) and Xenopus (xCRF(1)) differ from one another by their agonist- and antagonist-binding preference. While the agonist-binding site of the xCRF(1) receptor has been mapped, the amino acids that mediate binding of the potent peptide antagonist astressin are unknown. By constructing receptor chimeras followed by site-directed mutagenesis, the astressin-binding site of the xCRF(1) receptor was located between residues 76 and 83. This region partially overlaps with the agonist-selective domain of the xCRF(1) receptor (residues 76-89). Mutagenesis of the amphibian residues Gln(76), Gly(81) and Val(83) to the human sequence (Arg(76)Asn(81)Gly(83)) generated a receptor mutant that bound astressin with even higher affinity than the native hCRF(1) receptor. An amino acid doublet (Glu(70)Tyr(71)) that is conserved in the xCRF(1) and hCRF(2(a)) receptor after incorporation into the hCRF(1) receptor sequence was found to facilitate antagonist binding up to 15-fold higher. In agreement with the binding data, astressin was a more potent functional antagonist at receptors expressing the Glu(70)Tyr(71) motif. These data show that the agonist- and antagonist-binding sites of the hCRF(1) receptor partially overlap and that two amino acids within the N terminus of the hCRF(1) receptor negatively influence binding and functional antagonism of astressin.

Corticotropin-releasing factor receptors CRF1 and CRF2 exert both additive and opposing influences on defensive startle behavior

The corticotropin-releasing factor (CRF) receptors (CRF1 and CRF2) are crucial mediators of physiological and behavioral responses to stress. In animals, CRF1 appears to primarily mediate CRF-induced anxiety-like responses, but the role of CRF2 during stress is still unclear. Here we report the effects of CRF1 and CRF2 on the magnitude and plasticity of defensive startle responses in mice. Startle plasticity is measured by inhibition of startle by sensory stimuli, i.e., prepulse inhibition (PPI), and is disrupted in patients with panic or posttraumatic stress disorders in which CRF neurotransmission may be overactive. Pharmacological blockade of CRF1 reversed both CRF-induced increases in startle and CRF-induced deficits in PPI. CRF2 blockade attenuated high-dose but not low-dose CRF-induced increases in startle and reduced PPI. Conversely, activation of CRF2 enhanced PPI. CRF had no effect on startle and increased PPI in CRF1 knock-out mice. These data indicate that CRF receptors act in concert to increase the magnitude of defensive startle yet in opposition to regulate the flexibility of startle. These data support a new model of respective CRF receptor roles in stress-related behavior such that, although both receptors enhance the magnitude of defensive responses, CRF1 receptors contravene, whereas CRF2 receptors enhance, the impact of sensory information on defensive behavior. We hypothesize that excessive CRF1 activation combined with reduced CRF2 signaling may contribute to information processing deficits seen in panic and posttraumatic stress disorder patients and support CRF1-specific pharmacotherapy.

Monkey corticotropin-releasing factor1 receptor: Complementary DNA cloning and pharmacological characterization

The full-length complementary DNA (cDNA) of monkey corticotropin-releasing factor type 1 (CRF1) receptor was isolated from a rhesus monkey (Macaca mulatta) amygdala cDNA library. The cloned monkey CRF1 receptor cDNA has 2,374 bp with an open reading frame encoding a 415-amino acid protein. The sequence of the monkey CRF1 receptor cDNA showed a high degree of sequence identity with other species of CRF1 receptors, and being 99.5% identical to human CRF1 receptors. When monkey CRF1 was expressed into COS-7 cells, high specific binding of [125I]-ovine CRF was observed. CRF and CRF-related peptides inhibited [125I]-ovine CRF binding in a concentration-dependent manner. IC50 values of ovine CRF, human/rat CRF, sauvagine and urotensin I were 23.5 +/- 7.4, 22.7 +/- 10.8, 27.5 +/- 12.3 and 14.2 +/- 7.0 nM, respectively. CRF1 receptor specific antagonists, such as CP-154,526, SC241 and CRA1000, also inhibited the [125I]-ovine CRF binding, with IC50 values of 3.9 +/- 0.4, 43.5 +/- 8.0 and 19.8 +/- 2.0 nM, respectively. GTP and its nonhydrolyzed analogue, GTPgammaS, reduced [125I]-ovine CRF binding, while ATP had a negligible effect, thereby indicating that the monkey CRF1 receptor belongs to a family of G-protein coupled receptors. CRF and its related peptides increased cyclic AMP formation concentration-dependently in COS-7 cells transiently expressing the monkey CRF1 receptor. Monkey CRF1 was expressed abundantly in the pituitary, cerebral cortex, hippocampus, amygdala and cerebellum. Thus the monkey CRF1 receptor and the human CRF1 receptor have similar molecular and pharmacological characteristics.

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.

GRK3 regulation during CRF- and urocortin-induced CRF1 receptor desensitization

The EC(50) values for concentration-dependent stimulation of cAMP accumulation by CRF (1.3nM) and urocortin (1.0nM) were equivalent in human retinoblastoma Y79 cells. The time course and magnitude of CRF- and urocortin-induced CRF(1) receptor desensitization were similar. A significant 3-fold increase in GRK3, but not GRK2, mRNA levels accompanied the emergence of CRF(1) receptor desensitization in Y79 cells exposed to CRF. In preliminary experiments, retinoblastoma GRK3 protein expression became upregulated during a 48-h CRF exposure. Neither GRK3 nor GRK2 expression increased in Y79 cells exposed to urocortin for 10 min to 48 h. We hypothesize that GRK3 upregulation may be a cellular negative feedback process directed at maximizing CRF(1) receptor desensitization by heightening GRK3 phosphorylating capacity during prolonged exposure to high CRF. Regulation of GRK expression associated with urocortin- and CRF-induced CRF(1) receptor desensitization appears to differ, despite a similar level of signaling via the cAMP-protein kinase A pathway.

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.

Five amino acids of the Xenopus laevis CRF (corticotropin-releasing factor) type 2 receptor mediate differential binding of CRF ligands in comparison with its human counterpart

The ligand selectivity of human (hCRF(2A)) and Xenopus laevis (xCRF(2)) forms of the corticotropin-releasing factor type 2 (CRF(2)) receptor differs. The purpose of this study was to identify amino acids in these two CRF(2) receptors conferring these differences. An amino acid triplet in the third extracellular domain (Asp(262)Leu(263)Val(264) in hCRF(2A) or Lys(264)Tyr(265)Ile(266) in xCRF(2)) was found to diverge between both receptors. When binding and signaling characteristics of receptor mutants hR2KYI and xR2DLV were assessed, the tri-amino acid motif replacement produced receptors with binding properties resembling the xCRF(2) receptor. The converse mutation created a mutant receptor with a binding pharmacology identical to the profile of the hCRF(2A) receptor. This effect was most notable for xR2DLV, which possessed a binding affinity for astressin approximately 15-fold greater for astressin than sauvagine. In contrast, the binding profiles of the hCRF(2A) receptor and hR2KYI did not differ. These data indicate that another domain of the xCRF(2) receptor mediated low-affinity binding of astressin. Two amino acids in the first extracellular domain differ in xCRF(2) (Asp(69)Ser(70)) and hCRF(2A) (Glu(66)Tyr(67)) receptors. The hCRF(2A) receptor mutant (hR2DS-KYI) bound astressin with a low affinity indistinguishable from the xCRF(2) receptor. Therefore, these data demonstrate that ligand selectivity differences between amphibian and human forms of the CRF(2A) receptor are governed by these two motifs of the extracellular domains of the xCRF(2) receptor.

Cloning and functional pharmacology of two corticotropin-releasing factor receptors from a teleost fish

Although it is well established that fish possess corticotropin-releasing factor (CRF) and a CRF-like peptide, urotensin I, comparatively little is known about the pharmacology of their cognate receptors. Here we report the isolation and functional expression of two complementary DNAs (cDNAs), from the chum salmon Oncorhynchus keta, which encode orthologues of the mammalian and amphibian CRF type 1 (CRF(1)) and type 2 (CRF(2)) receptors. Radioligand competition binding experiments have revealed that the salmon CRF(1) and CRF(2) receptors bind urotensin I with approximately 8-fold higher affinity than rat/human CRF. These two peptides together with two related CRF-like peptides, namely, sauvagine and urocortin, were also tested in cAMP assays; for cells expressing the salmon CRF(1) receptor, EC(50) values for the stimulation of cAMP production were between 4.5+/-1.8 and 15.3+/-3.1 nM. For the salmon CRF(2) receptor, the corresponding values were: rat/human CRF, 9.4+/-0.4 nM; urotensin I, 21.2+/-2.1 nM; sauvagine, 0.7+/-0.1 nM; and urocortin, 2.2+/-0.7 nM. We have also functionally coupled the O. keta CRF(1) receptor, in Xenopus laevis oocytes, to the endogenous Ca(2+)-activated chloride conductance by co-expression with the G-protein alpha subunit, G(alpha16). The EC(50) value for channel activation by rat/human CRF (11.2+/-2.6 nM) agrees well with that obtained in cAMP assays (15.3+/-3.1 nM). We conclude that although sauvagine is 13- and 30-fold more potent than rat/human CRF and urotensin I, respectively, in activating the salmon CRF(2) receptor, neither receptor appears able to discriminate between the native ligands CRF and urotensin I.

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.

Molecular biology of the CRH receptors-- in the mood

Dysfunctioning of corticotropin-releasing hormone (CRH) and its receptors (CRH(1) and CRH(2)) has been linked to the development of stress-related disorders, such as mood and eating disorders. The molecular characterization of CRH(1) and CRH(2) receptors and their splice variants has generated detailed information on their pharmacology, tissue distribution and physiology. While mammalian CRH(1) receptors nonselectively bind CRH analogs, the ligand specificity of CRH(2) is narrower. CRH(1) receptors are predominantly expressed in the brain and pituitary, whereas CRH(2) receptor expression is limited to particular brain areas and to some peripheral organs. Molecular approaches to block CRH(1) receptor expression in the brain argue in favor of its involvement in the regulation of some aspects of the stress response. The CRH(2alpha) receptor may be more important for motivational types of behavior essential for survival, such as feeding and defense.(1)

GRK3 mediates desensitization of CRF1 receptors: a potential mechanism regulating stress adaptation

Potential G protein-coupled receptor kinase (GRK) and protein kinase A (PKA) mediation of homologous desensitization of corticotropin-releasing factor type 1 (CRF1) receptors was investigated in human retinoblastoma Y-79 cells. Inhibition of PKA activity by PKI(5-22) or H-89 failed to attenuate homologous desensitization of CRF1 receptors, and direct activation of PKA by forskolin or dibutyryl cAMP failed to desensitize CRF-induced cAMP accumulation. However, treatment of permeabilized Y-79 cells with heparin, a nonselective GRK inhibitor, reduced homologous desensitization of CRF1 receptors by approximately 35%. Furthermore, Y-79 cell uptake of a GRK3 antisense oligonucleotide (ODN), but not of a random or mismatched ODN, reduced GRK3 mRNA expression by approximately 50% without altering GRK2 mRNA expression and inhibited homologous desensitization of CRF1 receptors by approximately 55%. Finally, Y-79 cells transfected with a GRK3 antisense cDNA construct exhibited an approximately 50% reduction in GRK3 protein expression and an ~65% reduction in homologous desensitization of CRF1 receptors. We conclude that GRK3 contributes importantly to the homologous desensitization of CRF1 receptors in Y-79 cells, a brain-derived cell line.

G-protein-coupled receptor kinase 3- and protein kinase C-mediated desensitization of the PACAP receptor type 1 in human Y-79 retinoblastoma cells

Pituitary adenylyl cyclase-activating polypeptide (PACAP) receptor type 1 (PAC(1)) signaling and desensitization were investigated in human retinoblastoma Y-79 cells. Concentration-dependent stimulation of cAMP accumulation was observed in Y-79 cells incubated for 30 min with PACAP38, PACAP27, or VIP (10(-12) to 10(-6) M). The following EC(50) values were calculated: PACAP38, 24+/-3 pM; PACAP27, 99+/-8 pM; and VIP, 29+/-3 nM. Homologous desensitization of PAC(1) receptors in Y-79 cells pretreated with 10 nM PACAP38 or PACAP27 for 60 min was characterized by a 30-50% reduction in PACAP-stimulated cAMP accumulation (p<0.0001) and a two- to fivefold rightward shift in EC(50) values (p<0.0001). PAC(1) receptor desensitization was not accompanied by a reduction in PAC(1) mRNA expression. We concluded that the desensitizing effect of PACAP38 was homologous because neither corticotropin-releasing factor- nor (-)-isoproterenol-stimulated cAMP accumulation was altered by PACAP38 preincubation. Pretreating Y-79 cells with the protein kinase A (PKA) inhibitor H89 failed to inhibit homologous PAC(1) receptor desensitization. Similarly, pretreating Y-79 cells with the protein kinase C (PKC) inhibitors staurosporine or bisindolylmaleimide failed to alter homologous PAC(1) receptor desensitization. Although activation of PKA by dibutyryl cAMP or forskolin did not desensitize PAC(1) receptors, direct activation of PKC by PMA heterologously desensitized PAC(1) receptors, reducing cAMP accumulation 34.2+/-2.2% (p<0.001). Using RT-PCR, mRNA levels for G-protein-coupled receptor kinase 3 (GRK3), but not GRK2, were found to increase 2.2- to 4.8-fold in Y-79 cells exposed to PACAP38 for 10 min to 24 h (p<0.001). PAC(1) receptor desensitization decreased 72.5+/-4.3% (p<0.001) in Y-79 cells transfected with a GRK3 antisense cDNA construct that also reduced GRK3 protein expression 48.5+/-7.9% (p<0.0005). These experiments demonstrate that GRK3 plays an important role in the homologous desensitization of retinoblastoma PAC(1) receptors, whereas PKC, but not PKA, contributes to the heterologous desensitization of retinoblastoma PAC(1) receptors.

125I-Antisauvagine-30: a novel and specific high-affinity radioligand for the characterization of corticotropin-releasing factor type 2 receptors

Corticotropin-releasing factor (CRF) receptors type 1 (CRF(1)) and type 2 (CRF(2)) differ from each other in their pharmacological properties. The human and ovine CRF versions bind to CRF(1) receptors with significantly higher affinity than to CRF(2) receptors. Recently antisauvagine-30, an N-terminally truncated version of the CRF analog sauvagine, was characterized as a specific antagonist to mouse CRF(2B). We have synthesized the radiolabeled version (125)I-antisauvagine-30 and tested it for its affinity at human CRF(1) (hCRF(1)), hCRF(2A), Xenopus CRF(1) (xCRF(1)) and xCRF(2) receptors. In control binding studies (125)I-labeled hCRF, sauvagine and astressin were also bound to these receptors. (125)I-antisauvagine-30 exclusively bound to hCRF(2A) and xCRF(2) but not to hCRF(1) and xCRF(1) receptors. (125)I-antisauvagine-30 binding to hCRF(2A) and xCRF(2) receptors was saturable and of high affinity (hCRF(2A): K(d)=125 pM; xCRF(2): K(d)=1.1 nM). In displacement binding experiments using (125)I-antisauvagine-30 as radioligand several CRF analogs bound to hCRF(2A) and xCRF(2) receptors with similar rank orders as reported with other CRF radioligands. Finally, preliminary studies using (125)I-antisauvagine-30 binding to membrane homogenates prepared from different rat brain structures showed that the peptide bound specifically to brain areas expressing CRF(2) receptors. These data demonstrate that (125)I-antisauvagine-30 is the first high-affinity ligand to specifically label CRF(2) receptors.

Juxtamembrane region of the amino terminus of the corticotropin releasing factor receptor type 1 is important for ligand interaction

The functional properties of the amino terminus (NT) of the corticotropin releasing factor (CRF) receptor type 1 (R1) were studied by use of murine (m) CRFR1 and rat (r) parathyroid hormone (PTH)/parathyroid hormone-related peptide receptor (PTH1R) chimeras. The chimeric receptor CXP, in which the NT of mCRFR1 was annealed to the TMs of PTH1R, and the reciprocal hybrid, PXC, bound radiolabeled analogues of sauvagine and PTH(3--34), respectively. Neither hybrid bound radiolabeled CRF or PTH(1--34). CRF and PTH(1--34) weakly stimulated intracellular cAMP accumulation in COS-7 cells transfected with PXC and CXP, respectively. Thus the NT is required for ligand binding and the TMs are required for agonist-stimulated cAMP accumulation. Replacing individual intercysteine segments of PXC with their mCRFR1 counterparts did not rescue CRF or sauvagine radioligand binding or stimulation of cAMP accumulation. Replacement of residues 1--31 of mCRFR1 with their PTH1R counterparts resulted in a chimeric receptor, PEC, which had normal CRFR1 functional properties. In addition, a series of chimeras (F1PEC--F6PEC) were generated by replacement of the NT intercysteine residues of PEC with their PTH1R counterparts. Only F1PEC, F2PEC, and F3PEC showed detectable CRF and sauvagine radioligand binding. All of the PEC chimeras except F5PEC increased cAMP accumulation. These data indicate that the Cys(68)(-)Glu(109) domain is important for binding and that the Cys(87)(-)Cys(102) region plays an important role in CRFR1 activation.

Evidence for the abundant expression of arginine 185 containing human CRF(2alpha) receptors and the role of position 185 for receptor-ligand selectivity

The abundance of a histidine residue at position 185 (His(185)) of the human corticotropin-releasing factor (CRF) type 2 alpha receptor (hCRF(2alpha)) was investigated. His(185) has only been reported in hCRF(2); CRF(2) proteins from other species and all CRF(1) receptors encode an arginine (Arg(185)) at the corresponding position. Cloning of partial and full-length hCRF(2) cDNAs from a variety of neuronal and peripheral tissues revealed the existence of receptor molecules encoding Arg(185) only. Sequence analysis of the hCRF(2) gene verified the existence of Arg(185) also on genomic level. Full-length cDNAs encoding either the His(185) (R2H(185)) or the Arg(185) (R2R(185)) variants of hCRF(2alpha) were stably expressed in HEK293 cells and tested for ligand binding properties. In displacement studies R2H(185) and R2R(185) displayed a similar substrate specificity, human and rat urocortin, and the peptide antagonists astressin and alpha-helical CRF((9-41)) were bound with high affinity whereas human and ovine CRF were low-affinity ligands. Significant differences were observed for sauvagine and urotensin I, which bound with 3-fold (sauvagine) and 9-fold (urotensin I) higher affinity to R2R(185). These data indicate that hCRF(2), like all vertebrate CRF(1) and CRF(2) proteins encodes an arginine residue at the junction between extracellular domain 2 and transmembrane domain 3 and that this amino acid plays a role for the discrimination of some CRF peptide ligands.

Functional characterization of corticotropin-releasing factor type 1 receptor endogenously expressed in human embryonic kidney 293 cells

The endogenous expression in human embryonic kidney 293 (HEK293) cells of corticotropin-releasing factor (CRF) receptors was detected. High-affinity binding sites for human CRF (K(i)=3.6 nM), ovine CRF (K(i)=4.6 nM), rat urocortin (K(i)=2.2 nM), sauvagine (K(i)=2.4 nM) and astressin (K(i)=4.3 nM) with the pharmacological characteristics for CRF type 1 (CRF(1)) receptors and B(max) values of approximately 30 fmol/mg protein were determined. The four CRF receptor agonists nonselectively stimulated cAMP production in HEK293 cells at low agonist concentrations, whereas the antagonist astressin shifted the dose-response curve for ovine CRF significantly rightward. Transfection of the pcDNA3 vector into HEK293 cells strongly reduced the expression of the endogenous CRF receptor. Northern blot analysis revealed the expression of a CRF(1) transcript in human neuronal tissues, HEK293, human NTera-2 (NT2) carcinoma, Y-79 retinoblastoma and African green monkey kidney (COS-7) cells. Neither by Northern blot analysis nor by reverse transcriptase PCR (RT-PCR), the expression of CRF(2) could be detected. In cAMP stimulation experiments, functional CRF receptors were detected in these cell lines. These data show that HEK293 and other cell lines endogenously express CRF(1) receptors.

Rapid agonist-induced phosphorylation of the human CRF receptor, type 1: a potential mechanism for homologous desensitization

Agonist-induced phosphorylation of the human corticotropin-releasing factor type 1 receptor (hCRF(1)-R) was investigated using an influenza hemagglutinin (HA) epitope-tagged receptor transiently expressed in COS-7 cells. The HA-hCRF(1)-R migrated as a broad band (M(r) 60,000-70,000) in SDS-PAGE and showed increased mobility (M(r) approximately 48,000) after enzymatic deglycosylation with peptide-N-glycosidase F, consistent with the predicted size (47 kDa) of the nonglycosylated HA-hCRF(1)-R protein. A marked increase in HA-hCRF(1)-R phosphorylation was observed in HA-hCRF(1)-R-expressing COS-7 cells exposed to 1 microM ovine CRF for 5 min, whereas activation of protein kinase A (PKA) by 50 microM forskolin, or of Ca(2+)/calmodulin (CaM)-dependent kinases by 10 microM ionomycin, had little effect. These findings are consistent with preliminary data suggesting that CRF(1)-R phosphorylation mediated by G protein receptor kinase 3 (GRK3), but not by PKA or CaM-dependent kinases, has an important role in the homologous desensitization of brain CRF(1)-Rs.