Human CRF2 alpha and beta splice variants: pharmacological characterization using radioligand binding and a luciferase gene expression assay

Differentiation of Keratinocytes Modulates Skin HPA Analog

It is well established, that epidermal keratinocytes express functional equivalent of hypothalamus-pituitary-adrenal axis (HPA) in order to respond to changing environment and maintain internal homeostasis. We are presenting data indicating that differentiation of primary neonatal human keratinocytes (HPEKp), induced by prolonged incubation or calcium is accompanied by significant changes in the expression of the elements of skin analog of HPA (sHPA). Expression of CRF, UCN1-3, POMC, ACTH, CRFR1, CRFR2, MC1R, MC2R, and GR (coded by NR3C1 gene) were observed on gene/protein levels along differentiation of keratinocytes in culture with similar pattern seen by immunohistochemistry on full thickness skin biopsies. Expression of CRF was more pronounced in less differentiated keratinocytes, which corresponded to the detection of CRF immunoreactivity preferentially in the stratum basale. POMC expression was enhanced in more differentiated keratinocytes, which corresponded to detection of ACTH immunoreactivity, predominantly in the stratum spinosum and stratum granulosum. Expression of urocortins was also affected by induction of HPEKp differentiation. Immunohistochemical studies showed high prevalence of CRFR1 in well differentiated keratinocytes, while smaller keratinocytes showed predominantly CRFR2 immunoreactivity. MC2R mRNA levels were elevated from days 4 to 8 of in vitro incubation, while MC2R immunoreactivity was the highest in the upper layers of epidermis. Similar changes in mRNA/protein levels of sHPA elements were observed in HPEKp keratinocytes treated with calcium. Summarizing, preferential expression of CRF and POMC (ACTH) by populations of keratinocytes on different stage of differentiation resembles organization of central HPA axis suggesting their distinct role in physiology and pathology of the epidermis. J. Cell. Physiol. 232: 154-166, 2017. © 2016 Wiley Periodicals, Inc.

Characterization of a Pachymedusa dacnicolor-Sauvagine analog as a new high-affinity radioligand for corticotropin-releasing factor receptor studies

The corticotropin-releasing factor (CRF) peptide family comprises the mammalian peptides CRF and the urocortins as well as frog skin sauvagine and fish urophyseal urotensin. Advances in understanding the roles of the CRF ligand family and associated receptors have often relied on radioreceptor assays using labeled CRF ligands. These assays depend on stable, high-affinity CRF analogs that can be labeled, purified, and chemically characterized. Analogs of several of the native peptides have been used in this context, most prominently including sauvagine from the frog Phyllomedusa sauvageii (PS-Svg). Because each of these affords both advantages and disadvantages, new analogs with superior properties would be welcome. We find that a sauvagine-like peptide recently isolated from a different frog species, Pachymedusa dacnicolor (PD-Svg), is a high-affinity agonist whose radioiodinated analog, [(125)ITyr(0)-Glu(1), Nle(17)]-PD-Svg, exhibits improved biochemical properties over those of earlier iodinated agonists. Specifically, the PD-Svg radioligand binds both CRF receptors with comparably high affinity as its PS-Svg counterpart, but detects a greater number of sites on both type 1 and type 2 receptors. PD-Svg is also ∼10 times more potent at stimulating cAMP accumulation in cells expressing the native receptors. Autoradiographic localization using the PD-Svg radioligand shows robust specific binding to rodent brain and peripheral tissues that identifies consensus CRF receptor-expressing sites in a greater number and/or with greater sensitivity than its PS-Svg counterpart. We suggest that labeled analogs of PD-Svg may be useful tools for biochemical, structural, pharmacological, and anatomic studies of CRF receptors.

Role of Corticotropin-releasing Factor Signaling in Stress-related Alterations of Colonic Motility and Hyperalgesia

The corticotropin-releasing factor (CRF) signaling systems encompass CRF and the structurally related peptide urocortin (Ucn) 1, 2, and 3 along with 2 G-protein coupled receptors, CRF₁ and CRF₂. CRF binds with high and moderate affinity to CRF₁ and CRF₂ receptors, respectively while Ucn1 is a high-affinity agonist at both receptors, and Ucn2 and Ucn3 are selective CRF₂ agonists. The CRF systems are expressed in both the brain and the colon at the gene and protein levels. Experimental studies established that the activation of CRF₁ pathway in the brain or the colon recaptures cardinal features of diarrhea predominant irritable bowel syndrome (IBS) (stimulation of colonic motility, activation of mast cells and serotonin, defecation/watery diarrhea, and visceral hyperalgesia). Conversely, selective CRF₁ antagonists or CRF₁/CRF₂ antagonists, abolished or reduced exogenous CRF and stress-induced stimulation of colonic motility, defecation, diarrhea and colonic mast cell activation and visceral hyperalgesia to colorectal distention. By contrast, the CRF₂ signaling in the colon dampened the CRF₁ mediated stimulation of colonic motor function and visceral hyperalgesia. These data provide a conceptual framework that sustained activation of the CRF₁ system at central and/or peripheral sites may be one of the underlying basis of IBS-diarrhea symptoms. While targeting these mechanisms by CRF₁ antagonists provided a relevant novel therapeutic venue, so far these promising preclinical data have not translated into therapeutic use of CRF₁ antagonists. Whether the existing or newly developed CRF₁ antagonists will progress to therapeutic benefits for stress-sensitive diseases including IBS for a subset of patients is still a work in progress.

Sex- and region-specific pubertal maturation of the corticotropin-releasing factor receptor system in the rat

One of the most reliable findings in psychiatry is in the incidence of anxiety and depression. Beginning at puberty, women develop mood disorders twice as often as men. Because corticotropin-releasing factor (CRF) receptors are implicated, we compared CRF receptor binding in pre- and postpubertal rats. In each brain area, CRF receptor binding was sexually dimorphic, but no two areas were alike in the way the sexes differed. In the nucleus accumbens and olfactory tubercle, CRF1 binding was initially the same in juveniles, but became greater in adult females. In piriform cortex, CRF1 binding increased in females and decreased in males, again becoming sexually dimorphic. CRF1 binding in the anterior cingulate was greater in females than in males at both ages. In CA3, CRF1 binding was greater in males before puberty but decreased during puberty, abolishing the sex difference. CRF2 binding in the posterior bed nucleus of the stria terminalis was greater in males irrespective of age. In contrast, in each of three subdivisions of the lateral septum, females had greater CRF2 binding than males as juveniles, or, as juveniles and as adults. CRF2 binding in the ventromedial hypothalamus was the same in juveniles, but binding levels increased in males, leading to an adult sex difference. Thus, eight CRF receptor-expressing areas displayed eight distinct sex differences. These results show that sex differences pervade the CRF receptor system in juvenile and adult rats, and the mechanisms that control them are likely to be sex-, region-, and subtype-specific.

Corticotropin-releasing factor-binding protein (CRF-BP) inhibits CRF- and urotensin-I-mediated activation of CRF receptor-1 and -2 in common carp

Corticotropin-releasing factor-binding protein (CRF-BP) is considered a key determinant for CRF receptor (CRF-R) activation by CRF and several related peptides. Earlier studies have shown that the CRF system is highly conserved in gene structures throughout evolution, yet little is known about the evolutionary conservation of its biological functions. Therefore, we address the functional properties of CRF-BP and CRF-Rs in a teleost fish (common carp; Cyprinus carpio L.). We report the finding of two similar, yet distinct, genes for both CRF-R1 and CRF-R2 in this species. The four receptors are differentially responsive to CRF, urotensin-I (UI), sauvagine, and urocortin-2 (Ucn-2) and -3 (Ucn-3) as shown by luciferase assays. In vitro, carp CRF-BP inhibits CRF- and UI-mediated activation of the newfound CRF-Rs, but its potency to do so varies between receptor and peptide ligand. This is the first paper to establish the functionality and physiological interplay between CRF-BP, CRF-Rs and CRF-family peptides in a teleostean species.

CRF and urocortin peptides as modulators of energy balance and feeding behavior during stress

Early on, corticotropin-releasing factor (CRF), a hallmark brain peptide mediating many components of the stress response, was shown to affect food intake inducing a robust anorexigenic response when injected into the rodent brain. Subsequently, other members of the CRF signaling family have been identified, namely urocortin (Ucn) 1, Ucn 2, and Ucn 3 which were also shown to decrease food intake upon central or peripheral injection. However, the kinetics of feeding suppression was different with an early decrease following intracerebroventricular injection of CRF and a delayed action of Ucns contrasting with the early onset after systemic injection. CRF and Ucns bind to two distinct G-protein coupled membrane receptors, the CRF1 and CRF2. New pharmacological tools such as highly selective peptide CRF1 or CRF2 agonists or antagonists along with genetic knock-in or knock-out models have allowed delineating the primary role of CRF2 involved in the anorexic response to exogenous administration of CRF and Ucns. Several stressors trigger behavioral changes including suppression of feeding behavior which are mediated by brain CRF receptor activation. The present review will highlight the state-of-knowledge on the effects and mechanisms of action of CRF/Ucns-CRF1/2 signaling under basal conditions and the role in the alterations of food intake in response to stress.

Consequences of splice variation on Secretin family G protein-coupled receptor function

The Secretin family of GPCRs are endocrine peptide hormone receptors that share a common genomic organization and are the subject of a wide variety of alternative splicing. All GPCRs contain a central seven transmembrane domain responsible for transducing signals from the outside of the cell as well as extracellular amino and intracellular carboxyl termini. Members of the Secretin receptor family have a relatively large N-terminus and a variety of lines of evidence support a common mode of ligand binding and a common ligand binding fold. These receptors are best characterized as coupling to intracellular signalling pathways via G(αs) and G(αq) but are also reported to couple to a multitude of other signalling pathways. The intracellular loops are implicated in regulating the interaction between the receptor and heterotrimeric G protein complexes. Alternative splicing of exons encoding both the extracellular N-terminal domain as well as the extracellular loops of some family members has been reported and as expected these splice variants display altered ligand affinity as well as differential activation by endogenous ligands. Various forms of alternative splicing have also been reported to alter intracellular loops 1 and 3 as well as the C-terminus and as one might expect these display differences in signalling bias towards downstream effectors. These diverse pharmacologies require that the physiological role of these splice variants be addressed but should provide unique opportunities for drug design and development.

Activation of corticotropin-releasing factor receptor 2 mediates the colonic motor coping response to acute stress in rodents

BACKGROUND & AIMS

Corticotropin-releasing factor receptor-1 (CRF(1)) mediates the stress-induced colonic motor activity. Less is known about the role of CRF(2) in the colonic response to stress.

METHODS

We studied colonic contractile activity in rats and CRF(2)-/-, CRF-overexpressing, and wild-type mice using still manometry; we analyzed defecation induced by acute partial-restraint stress (PRS), and/or intraperitoneal injection of CRF ligands. In rats, we monitored activation of the colonic longitudinal muscle myenteric plexus (LMMP) neurons and localization of CRF(1) and CRF(2) using immunohistochemical and immunoblot analyses. We measured phosphorylation of extracellular signal-regulated kinase 1/2 by CRF ligands in primary cultures of LMMP neurons (PC-LMMPn) and cyclic adenosine monophosphate (cAMP) production in human embryonic kidney-293 cells transfected with CRF(1) and/or CRF(2).

RESULTS

In rats, a selective agonist of CRF(2) (urocortin 2) reduced CRF-induced defecation (>50%), colonic contractile activity, and Fos expression in the colonic LMMP. A selective antagonist of CRF(2) (astressin(2)-B) increased these responses. Urocortin 2 reduced PRS-induced colonic contractile activity in wild-type and CRF-overexpressing mice, whereas disruption of CRF(2) increased PRS-induced colonic contractile activity and CRF-induced defecation. CRF(2) colocalized with CRF(1) and neuronal nitric oxide synthase in the rat colon, LMMP, and PC-LMMPn. CRF-induced phosphorylation of extracellular signal-regulated kinase in PC-LMMPn; this was inhibited or increased by a selective antagonist of CRF(1) (NBI35965) or astressin(2)-B, respectively. The half maximal effective concentration, EC(50), for the CRF-induced cAMP response was 8.6 nmol/L in human embryonic kidney-293 cells that express only CRF(1); this response was suppressed 10-fold in cells that express CRF(1) and CRF(2).

CONCLUSIONS

In colon tissues of rodents, CRF(2) activation inhibits CRF(1) signaling in myenteric neurons and the stress-induced colonic motor responses. Disruption of CRF(2) function impairs colonic coping responses to stress.

Activation of corticotropin-releasing factor receptor 2 mediates the colonic motor coping response to acute stress in rodents

BACKGROUND & AIMS

Corticotropin-releasing factor receptor-1 (CRF(1)) mediates the stress-induced colonic motor activity. Less is known about the role of CRF(2) in the colonic response to stress.

METHODS

We studied colonic contractile activity in rats and CRF(2)-/-, CRF-overexpressing, and wild-type mice using still manometry; we analyzed defecation induced by acute partial-restraint stress (PRS), and/or intraperitoneal injection of CRF ligands. In rats, we monitored activation of the colonic longitudinal muscle myenteric plexus (LMMP) neurons and localization of CRF(1) and CRF(2) using immunohistochemical and immunoblot analyses. We measured phosphorylation of extracellular signal-regulated kinase 1/2 by CRF ligands in primary cultures of LMMP neurons (PC-LMMPn) and cyclic adenosine monophosphate (cAMP) production in human embryonic kidney-293 cells transfected with CRF(1) and/or CRF(2).

RESULTS

In rats, a selective agonist of CRF(2) (urocortin 2) reduced CRF-induced defecation (>50%), colonic contractile activity, and Fos expression in the colonic LMMP. A selective antagonist of CRF(2) (astressin(2)-B) increased these responses. Urocortin 2 reduced PRS-induced colonic contractile activity in wild-type and CRF-overexpressing mice, whereas disruption of CRF(2) increased PRS-induced colonic contractile activity and CRF-induced defecation. CRF(2) colocalized with CRF(1) and neuronal nitric oxide synthase in the rat colon, LMMP, and PC-LMMPn. CRF-induced phosphorylation of extracellular signal-regulated kinase in PC-LMMPn; this was inhibited or increased by a selective antagonist of CRF(1) (NBI35965) or astressin(2)-B, respectively. The half maximal effective concentration, EC(50), for the CRF-induced cAMP response was 8.6 nmol/L in human embryonic kidney-293 cells that express only CRF(1); this response was suppressed 10-fold in cells that express CRF(1) and CRF(2).

CONCLUSIONS

In colon tissues of rodents, CRF(2) activation inhibits CRF(1) signaling in myenteric neurons and the stress-induced colonic motor responses. Disruption of CRF(2) function impairs colonic coping responses to stress.

Corticotrophin-releasing factor, related peptides, and receptors in the normal and inflamed gastrointestinal tract

Corticotrophin-releasing factor (CRF) is mainly known for its role in the stress response in the hypothalamic–pituitary–adrenal axis. However, increasing evidence has revealed that CRF receptor signaling has additional peripheral effects. For instance, activation of CRF receptors in the gastrointestinal tract influences intestinal permeability and motility. These receptors, CRF1 and CRF2, do not only bind CRF, but are also activated by urocortins. Most interestingly, CRF-related signaling also assumes an important role in inflammatory bowel diseases in that it influences inflammatory processes, such as cytokine secretion and immune cell activation. These effects are characterized by an often contrasting function of CRF1 and CRF2. We will review the current data on the expression of CRF and related peptides in the different regions of the gastrointestinal tract, both in normal and inflamed conditions. We next discuss the possible functional roles of CRF signaling in inflammation. The available data clearly indicate that CRF signaling significantly influences inflammatory processes although there are important species and inflammation model differences. Although further research is necessary to elucidate this apparently delicately balanced system, it can be concluded that CRF-related peptides and receptors are (certainly) important candidates in the modulation of gastrointestinal inflammation.

Glucocorticoids differentially regulate the expression of CRFR1 and CRFR2α in MIN6 insulinoma cells and rodent islets

Urocortin 3 (Ucn 3), member of the corticotropin-releasing factor (CRF) family of peptide hormones, is released from β-cells to potentiate insulin secretion. Ucn 3 activates the CRF type-2 receptor (CRFR2) but does not activate the type-1 receptor (CRFR1), which was recently demonstrated on β-cells. While the direct actions of Ucn 3 on insulin secretion suggest the presence of cognate receptors within the islet microenvironment, this has not been established. Here we demonstrate that CRFR2α is expressed by MIN6 insulinoma cells and by primary mouse and human islets, with no detectable expression of CRFR2β. Furthermore, stimulation of MIN6 cells or primary mouse islets in vitro or in vivo with glucocorticoids (GCs) robustly and dose-dependently increases the expression of CRFR2α, while simultaneously inhibiting the expression of CRFR1 and incretin receptors. Luciferase reporters driven by the mouse CRFR1 or CRFR2α promoter in MIN6 cells confirm these differential effects of GCs. In contrast, GCs inhibit CRFR2α promoter activity in HEK293 cells and inhibit the expression of CRFR2β in A7r5 rat aortic smooth muscle cells and differentiated C2C12 myotubes. These findings suggest that the GC-mediated increase of CRFR2α depends on the cellular context of the islet and deviates from the GC-mediated suppression of CRFR1 and incretin receptors. Furthermore, GC-induced increases in CRFR2α expression coincide with increased Ucn 3-dependent activation of cAMP and MAPK pathways. We postulate that differential effect of GCs on the expression of CRFR1 and CRFR2α in the endocrine pancreas represent a mechanism to shift sensitivity from CRFR1 to CRFR2 ligands.

Neuroendocrine mechanisms in pregnancy and parturition

The complex mechanisms controlling human parturition involves mother, fetus, and placenta, and stress is a key element activating a series of physiological adaptive responses. Preterm birth is a clinical syndrome that shares several characteristics with term birth. A major role for the neuroendocrine mechanisms has been proposed, and placenta/membranes are sources for neurohormones and peptides. Oxytocin (OT) is the neurohormone whose major target is uterine contractility and placenta represents a novel source that contributes to the mechanisms of parturition. The CRH/urocortin (Ucn) family is another important neuroendocrine pathway involved in term and preterm birth. The CRH/Ucn family consists of four ligands: CRH, Ucn, Ucn2, and Ucn3. These peptides have a pleyotropic function and are expressed by human placenta and fetal membranes. Uterine contractility, blood vessel tone, and immune function are influenced by CRH/Ucns during pregnancy and undergo major changes at parturition. Among the others, neurohormones, relaxin, parathyroid hormone-related protein, opioids, neurosteroids, and monoamines are expressed and secreted from placental tissues at parturition. Preterm birth is the consequence of a premature and sustained activation of endocrine and immune responses. A preterm birth evidence for a premature activation of OT secretion as well as increased maternal plasma CRH levels suggests a pathogenic role of these neurohormones. A decrease of maternal serum CRH-binding protein is a concurrent event. At midgestation, placental hypersecretion of CRH or Ucn has been proposed as a predictive marker of subsequent preterm delivery. While placenta represents the major source for CRH, fetus abundantly secretes Ucn and adrenal dehydroepiandrosterone in women with preterm birth. The relevant role of neuroendocrine mechanisms in preterm birth is sustained by basic and clinic implications.

The CRF system, stress, depression and anxiety-insights from human genetic studies

A concatenation of findings from preclinical and clinical studies support a preeminent function for the corticotropin-releasing factor (CRF) system in mediating the physiological response to external stressors and in the pathophysiology of anxiety and depression. Recently, human genetic studies have provided considerable support to several long-standing hypotheses of mood and anxiety disorders, including the CRF hypothesis. These data, reviewed in this report, are congruent with the hypothesis that this system is of paramount importance in mediating stress-related psychopathology. More specifically, variants in the gene encoding the CRF(1) receptor interact with adverse environmental factors to predict risk for stress-related psychiatric disorders. In-depth characterization of these variants will likely be important in furthering our understanding of the long-term consequences of adverse experience.

From Hans Selye's discovery of biological stress to the identification of corticotropin-releasing factor signaling pathways: implication in stress-related functional bowel diseases

Selye pioneered the concept of biological stress in 1936, culminating in the identification of the corticotropin-releasing factor (CRF) signaling pathways by Vale's group in the last two decades. The characterization of the 41 amino-acid CRF and other peptide members of the mammalian CRF family, urocortin 1, urocortin 2, and urocortin 3, and the cloning of CRF(1) and CRF(2) receptors, which display distinct affinity for CRF ligands, combined with the development of selective CRF receptor antagonists enable us to unravel the importance of CRF(1) receptor in the stress-related endocrine (activation of pituitary-adrenal axis), behavioral (anxiety/depression, altered feeding), autonomic (activation of sympathetic nervous system), and immune responses. The activation of CRF(1) receptors is also one of the key mechanisms through which various stressors impact the gut to stimulate colonic propulsive motor function and to induce hypersensitivity to colorectal distension as shown by the efficacy of the CRF(1) receptor antagonists in blunting these stress-related components. The importance of CRF(1) signaling pathway in the visceral response to stress in experimental animals provided new therapeutic approaches for treatment of functional bowel disorder such as irritable bowel syndrome, a multifactor functional disorder characterized by altered bowel habits and visceral pain, for which stress has been implicated in the pathophysiology and is associated with anxiety-depression in a subset of patients.

Molecular genetics of the stress-responsive adrenocortical axis

Stress-responsive adrenocortical function is the final physiological response to the cascade of events that occurs when the interaction between individuals and their environment takes place. Glucocorticoids are produced in response to perturbance of homeostasis and are necessary for the energy required to restore this homeostasis. Genetics contributes to the individual variation in basal and stimulated plasma glucocorticoid levels and also to adrenal gland mass that increases in response to prolonged adrenal stimulation. This review briefly describes regulation of the adrenocortical axis, summarizes the linkage studies carried out so far in humans and in model organisms, and discusses the potential candidate genes that might contribute to the variation. The significance of individual variations in the glucocorticoid stress-responsiveness, with particular attention to their potential role in the recent explosion of obesity and the prevalence of metabolic syndrome X, is commented upon.

Identification and characterization of multiple corticotropin-releasing factor type 2 receptor isoforms in the rat esophagus

The rat esophagus shares some cellular features with skin squamous epithelium and striated muscle that express high levels of corticotropin-releasing factor type 2 (CRF2) receptors or their cognate ligand urocortin (Ucn) 1, 2, and 3. We investigated the expression and cell signaling of CRF2 receptors and ligands in the rat esophagus and lower esophageal sphincter (LES) by RT-PCR and quantitative PCR in normal and corticosterone-treated whole esophageal tissue, laser capture microdissected layers, and isolated esophageal cells. The expression of CRF2 receptor protein and intracellular cAMP and ERK1/2 responses to CRF agonists and CRF2 antagonist were determined in cultured esophageal cells and HEK-293 cells transfected with CRF2b receptors. CRF2 was abundantly expressed in the mucosa and longitudinal muscle layers of the esophagus and LES, whereas CRF1 expression was scarce. CRF2b wild-type transcript was predominantly expressed in the esophagus, and in addition, several new CRF2 splice variants including six CRF2a isoforms were identified. Expression of Ucn 1, Ucn 2, and to a smaller extent Ucn 3, but not CRF mRNA, was detected in the esophagus and LES. Ucn 1 and Ucn 2 stimulated dose-dependent cAMP production and ERK1/2 phosphorylation in the esophageal cells, whereas CRF and CRF1 agonist, cortagine, had less potent effects. In addition, Ucn 2-stimulated cAMP and ERK responses were blocked by the CRF2 antagonist, astressin2-B. These data established the presence of a prominent CRF2 signaling system in the esophagus and LES-encompassing multiple CRF2 receptor variants and Ucn, suggesting a functional role in secretomotor activity and epithelial and muscle cell proliferation.

Radioligand binding and functional responses of ligands for human recombinant adenosine A(3) receptors

The binding and functional properties of adenosine receptor ligands were compared in Chinese hamster ovary cells transfected with human adenosine A(3) receptors. Inhibition of [(125)I]-aminobenzyl-5'-N-methylcarboamidoadenosine ([(125)I]-AB-MECA) binding by adenosine receptor ligands was examined in membrane preparations. Inhibition of forskolin-induced cAMP accumulation by agonists was measured using a cAMP enzyme immunoassay. The rank order of agonist potency for both assays was N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) > 5'-N-ethylcarboxamidoadenosine (NECA) > (-)-N(6)-[(R)-phenylisopropyl] adenosine (R-PIA) > 4-aminobenzyl-5'-N-methylcarboxamidoadenosine (AB-MECA) > N(6)-cyclopentyl adenosine (CPA) > adenosine. The radioligand binding rank order of antagonist potency was N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide (MRS1220) > 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) > 8-phenyltheophylline (8-PT) > 8-(p-sulfophenyl)-theophylline (8-SPT). MRS1220 competitively inhibited the effect of IB-MECA on cAMP production, with a K(B) value of 0.35 nm. These data are characteristic of adenosine A(3) receptors. The absence of Mg(2+) and presence of guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) significantly reduced agonist binding inhibition potency, indicating binding to high- and low-affinity states. The IB-MECA, NECA and R-PIA IC(50) values were greater for the cAMP assay than for radioligand binding, suggesting an efficient stimulus-response transduction pathway.

Time- and dose-dependent effects of corticotropin releasing factor on cerebral glucose metabolism in rats

The time course and the relation to dose of locomotor activity and of the regional cerebral metabolic rates for glucose (rCMRglc) were measured in freely moving Sprague-Dawley rats after intracerebroventricular administration of ovine corticotropin releasing factor (oCRF). Motor activity was determined using a familiar photocage cell. rCMRglc was measured, using the quantitative autoradiographic [(14)C]2-deoxyglucose procedure, in 73 brain regions at 10, 30, 90 and 180 min after administration of oCRF 10 microg and at 90 min after oCRF 0.1, 1 and 100 microg. oCRF 10 microg increased motor activity in a sustained fashion and increased rCMRglc with different time courses throughout brain regions. In cerebellar regions rCMRglc increases peaked at 90 min and were sustained up to 180 min. In non-cerebellar regions rCMRglc increases peaked at 90 min but declined thereafter. At lower doses (0.1 and 1 microg) oCRF increased rCMRglc in fewer brain regions (1 and 5 regions affected, average increases 1% and 7%) including cerebellar areas and brainstem sensory nuclei and decreased rCMRglc in medial prefrontal cortex. At the highest dose (100 microg) oCRF induced large and widespread rCMRglc increases in cerebellar, brainstem, hypothalamic, limbic and neocortical areas (40 brain regions affected, average increase 32%). The findings indicate that cerebellar areas and brainstem nuclei are highly sensitive to oCRF and may mediate oCRF autonomic and behavioral effects.

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.

Cell-type specific calcium signaling by corticotropin-releasing factor type 1 (CRF1) and 2a (CRF2(a)) receptors: phospholipase C-mediated responses in human embryonic kidney 293 but not SK-N-MC neuroblastoma cells

The human corticotropin-releasing factor (hCRF) receptors CRF1 and CRF2(a) couple to the Gs protein. It has been postulated that CRF receptors may also signal through phospholipase C (PLC). To test this hypothesis, binding and signaling properties were determined for both receptor subtypes stably expressed in human embryonic kidney 293 (HEK293) and human SK-N-MC neuroblastoma cells. CRF receptors were highly expressed and strongly coupled to Gs in HEK293 and SK-N-MC cells. However, when the calcium mobilization pathway was investigated, marked differences were observed. In SK-N-MC cells, neither CRF receptor stimulated calcium mobilization in the fluorometric imaging plate reader (FLIPR) assay, whereas activation of orexin type 1 and 2 receptors stably expressed in SK-N-MC cells revealed robust calcium responses. In contrast, intracellular calcium was strongly mobilized by agonist stimulation of hCRF1 and hCRF2(a) receptors in HEK293 cells. In HEK293 cells, potency rank orders for calcium and cAMP responses were identical for both receptors, despite a rightward shift of the dose-response curves. Complete inhibition of calcium signaling of both hCRF1 and hCRF2(a) receptors was observed in the presence of the PLC inhibitor U-73,122 whereas ryanodine, an inhibitor of calcium release channels and the protein kinase A inhibitor Rp-cAMPS were ineffective. Finally, CRF agonists produced a small but significant stimulation of inositol 1,4,5-triphosphate (IP3) accumulation in hCRF1-and hCRF2(a)-transfected HEK293 cells. These data clearly show that phospholipase C-mediated signaling of CRF receptors is dependent upon the cellular background and that in HEK293 cells human CRF receptors robustly respond in the FLIPR format.

CRF1 receptor signaling pathways are involved in stress-related alterations of colonic function and viscerosensitivity: implications for irritable bowel syndrome

1. The characterization of corticotropin releasing factor (CRF) and, more recently, the discovery of additional CRF-related ligands, urocortin 1, urocortin 2 and urocortin 3, the cloning of two distinct CRF receptor subtypes, 1 (CRF(1)) and 2 (CRF(2)), and the development of selective CRF receptor antagonists provided new insight to unravel the mechanisms of stress. Activation of brain CRF(1) receptor signaling pathways is implicated in stress-related endocrine response and the development of anxiety-like behaviors. 2. Compelling evidence in rodents showed also that both central and peripheral injection of CRF and urocortin 1 mimic acute stress-induced colonic response (stimulation of motility, transit, defecation, mucus and watery secretion, increased ionic permeability and occurrence of diarrhea) in rodents. Central CRF enhances colorectal distention-induced visceral pain in rats. Peripheral CRF reduced pain threshold to colonic distention and increased colonic motility in humans. 3. Nonselective CRF(1)/CRF(2) antagonists and selective CRF(1) antagonists inhibit exogenous (central or peripheral) CRF- and acute stress-induced activation of colonic myenteric neurons, stimulation of colonic motor function and visceral hyperalgesia while selective CRF(2) antagonists have no effect. None of the CRF antagonists influence basal or postprandial colonic function in nonstressed animals. 4. These findings implicate CRF(1) receptors in stress-related stimulation of colonic function and hypersensitivity to colorectal distention. Targeting CRF(1)-dependent pathways may have potential benefit against stress or anxiety-/depression-related functional bowel disorders.

Genomic regions controlling corticosterone levels in rats

BACKGROUND

The identification of genetic factors controlling stress-responsiveness should advance the understanding of susceptibility to psychiatric illness.

METHODS

Rat strains, F344/NHsd and LEW/NHsd, which differ in measures of stress-responsiveness and behaviors modeling psychiatric disorders, were bred to generate F2 progeny that were used in a quantitative trait loci (QTL) analysis to identify genomic regions influencing late-afternoon corticosterone levels.

RESULTS

Regions on chromosomes 4 and 10 previously identified as influencing autoimmune phenomena were the most significant QTL observed, reaching suggestive significance at the genome-wide level. Congenic animals targeting these regions with F344/NHsd deoxyribonucleic acid on a DA/Bkl genomic background demonstrated corticosterone levels approximating those of F344/NHsd rats and differing significantly from DA/Bkl rats.

CONCLUSIONS

Specific genomic regions influence both corticosterone levels and stress-related disease susceptibility. These findings not only represent the first identification of QTL controlling corticosterone levels but also suggest a mechanism underlying genetic differences in stress-responsiveness.

Differential profile of CRF receptor distribution in the rat stomach and duodenum assessed by newly developed CRF receptor antibodies

Peripheral corticotropin-releasing factor (CRF) receptor ligands inhibit gastric acid secretion and emptying while stimulating gastric mucosal blood flow in rats. Endogenous CRF ligands are expressed in the upper gastrointestinal (GI) tissues pointing to local expression of CRF receptors. We mapped the distribution of CRF receptor type 1 (CRF1) and 2 (CRF2) in the rat upper GI. Polyclonal antisera directed against the C-terminus of the CRF receptor protein were generated in rabbits and characterized by western blotting and immunofluorescence using CRF1- and CRF2-transfected cell lines and in primary cultured neurons from rat brain cortex. A selective anti-CRF1 antiserum (4467a-CRF1) was identified and used in parallel with another antiserum recognizing both CRF1 and CRF2 (4392a-CRF1&2) to immunostain gastric tissue sections. Antiserum 4467a-CRF1 demonstrated specific immunostaining in a narrow zone in the upper oxyntic gland within the stomach corpus. Conversely, 4392a-CRF1&2 labeled cells throughout the oxyntic gland and submucosal blood vessels. Pre-absorption with the specific antigen peptide blocked immunostaining in all experiments. Doublestaining showed co-localization of 4392a-CRF1&2 but not 4467a-CRF1 immunoreactivity with H/K-ATPase and somatostatin immunostaining in parietal and endocrine cells of the oxyntic gland. No specific staining was observed in the antrum with either antisera, whereas only antiserum 4392a-CRF1&2 showed modest immunoreactivity in the duodenal mucosa. Finally, co-localization of CRF2 and urocortin immunoreactivity was found in the gastric glands. These results indicate that both CRF receptor subtypes are expressed in the rat upper GI tissues with a distinct pattern and regional differences suggesting differential function.

Mechanisms of relaxation by urocortin in renal arteries from male and female rats

Urocortin is a peptide recently identified, which is structurally related to the corticotropin-releasing factor (CRF). To analyze the mechanisms that could be involved in its effect on renal arteries from male and female rats, the response to urocortin was studied in isolated segments, 2 mm long, of renal arteries from male and female rats. In renal artery segments precontracted with endothelin-1 (1 nm), urocortin (1 pm-10 nm) produced concentration-dependent relaxation, which was similar in the arteries from male and female rats. This relaxation was reduced by the antagonists of urocortin receptors astressin (1 microM) and alpha-helical CRF(9-41) (1 microM) in arteries from both male and female rats. In renal arteries from female rats, the relaxation to urocortin was reduced by the inhibitor of adenyl cyclase SQ22536 (300 microM), by 8-bromo-cyclic-ADP-ribose (cADPR; 30 microM), an antagonist of the endogenous activator of sarcoplasmic Ca2+ channel cADPR and by ryanodine (1 microM), which produces depletion of sarcoplasmic Ca2+. In renal arteries from male rats, the relaxation to urocortin was increased by ryanodine, and was not modified by SQ22536 or 8-bromo-cADPR. These results suggest that the mechanisms involved in the relaxation to urocortin in renal arteries differ between female and male rats. In female rats, this relaxation may be mediated by the production of cyclic AMP (cAMP), synthesis of cADPR and release of sarcoplasmic Ca2+, whereas in male rats it is not mediated by cAMP.

The effect of urocortin on ingestive behaviours and brain Fos immunoreactivity in mice

The influence of urocortin (UCN) on ingestive behaviours and brain neural activity, as measured immunohistochemically by the presence of Fos protein, was determined in mice. Rat UCN was administered by continuous intracerebroventricular (ICV) or subcutaneous (SC) infusion. ICV infusion of UCN (100 ng/h, 14 days) transiently reduced daily food and water intakes (days 1-4) but body weight was reduced from day 2 into the post-infusion period. Sodium intake was reduced from day 3 to the end of infusion. SC infusion of UCN caused similar but smaller reductions in food and water intakes and body weight, without change in sodium intake. In separate experiments, Fos immunoreactivity was increased in several brain nuclei known to be involved in the control of body fluid and energy homeostasis, e.g. central nucleus of the amygdala, median preoptic nucleus, bed nucleus of the stria terminalis and arcuate nucleus. Increased Fos expression was similar for ICV and SC infusions when measured on days 2-3 or 6-7 of infusion. In conclusion, increases of brain activity by UCN may be associated with stimulation of adrenocorticotrophic hormone release and sympathetic nervous activity, but increases may also indicate suppression of ingestive behaviours by stimulating central inhibitory mechanisms located in areas known to control body fluid and energy homeostasis.

Potent and long-acting corticotropin releasing factor (CRF) receptor 2 selective peptide competitive antagonists

We present evidence that members of the corticotropin releasing factor (CRF) family assume distinct structures when interacting with the CRF(1) and CRF(2) receptors. Predictive methods, physicochemical measurements, and structure-activity relationship studies have suggested that CRF, its family members, and competitive antagonists such as astressin [cyclo(30-33)[DPhe(12),Nle(21),Glu(30),Lys(33),Nle(38)]hCRF((12-41))] assume an alpha-helical conformation when interacting with their receptors. We had shown that alpha-helical CRF((9-41)) and sauvagine showed some selectivity for CRF receptors other than that responsible for ACTH secretion(1) and later for CRF2.(2) More recently, we suggested the possibility of a helix-turn-helix motif around a turn encompassing residues 30-33(3) that would confer high affinity for both CRF(1) and CRF(2)(2,4) in agonists and antagonists of all members of the CRF family.(3) On the other hand, the substitutions that conferred ca. 100-fold CRF(2) selectivity to the antagonist antisauvagine-30 [[DPhe(11),His(12)]sauvagine((11-40))] did not confer such property to the corresponding N-terminally extended agonists. We find here that a Glu(32)-Lys(35) side chain to side chain covalent lactam constraint in hCRF and the corresponding Glu(31)-Lys(34) side chain to side chain covalent lactam constraint in sauvagine yield potent ligands that are selective for CRF(2). Additionally, we introduced deletions and substitutions known to increase duration of action to yield antagonists such as cyclo(31-34)[DPhe(11),His(12),C(alpha)MeLeu(13,39),Nle(17),Glu(31),Lys(34)]Ac-sauvagine((8-40)) (astressin(2)-B) with CRF(2) selectivities greater than 100-fold. CRF receptor autoradiography was performed in rat tissue known to express CRF(2) and CRF(1) in order to confirm that astressin(2)-B could indeed bind to established CRF(2) but not CRF(1) receptor-expressing tissues. Extended duration of action of astressin(2)-B vs that of antisauvagine-30 is demonstrated in the CRF(2)-mediated animal model whereby the inhibition of gastric emptying of a solid meal in mice by urocortin administered intraperitoneally at time zero is antagonized by the administration of astressin(2)-B but not by antisauvagine-30 at times -3 and -6 h while both peptides are effective when given 10 min before urocortin.

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.

N-glycosylation of CRF receptor type 1 is important for its ligand-specific interaction

The corticotropin-releasing factor (CRF) receptor type 1 (CRFR1) contains five potential N-glycosylation sites: N38, N45, N78, N90, and N98. Cells expressing CRFR1 were treated with tunicamycin to block receptor glycosylation. The nonglycosylated receptor did not bind the radioligand and had a decreased cAMP stimulation potency in response to CRF. To determine which of the polysaccharide chain(s) is/are involved in ligand interaction, the polysaccharide chains were deleted using site-directed mutagenesis of the glycosylation consensus, N-X-S/T. Two sets of mutations were performed for each glycosylation site: N to Q and S/T to A, respectively. The single mutants Q38, Q45, Q78, Q90, Q98, A40, A47, A80, A92, and A100 and the double mutants A40/A47 and A80/A100 were well expressed, bound CRF, sauvagine (SVG), and urotensin-I (UTS-I) with a normal affinity, and increased cAMP accumulation with a high efficiency. In contrast, the combined mutations A80/A92/A100, A40/A80/A92/A100, and A40/A47/A80/A92/A100 had low levels of expression, did not bind the radioligand, and had a decreased cAMP stimulation. These data indicate the requirement for three or more polysaccharide chains for normal CRFR1 function.

Peripheral urocortin inhibits gastric emptying and food intake in mice: differential role of CRF receptor 2

Intraperitoneal urocortin inhibits gastric emptying and food intake in mice. We investigated corticotropin-releasing factor receptor (CRF-R) subtypes involved in intraperitoneal urocortin actions using selective CRF-R antagonists. Gastric emptying was measured 2 h after a chow meal, and food intake was measured hourly after an 18-h fast in mice. Urocortin (3 microg/kg ip) inhibited gastric emptying by 88%. The CRF-R1/CRF-R2 antagonist astressin B (30 microg/kg ip) and the selective CRF-R2 antagonist antisauvagine-30 (100 microg/kg ip) completely antagonized urocortin action, whereas the selective CRF-R1 antagonist CP-154,526 (10 mg/kg ip) had no effect. Urocortin (1-10 microg/kg ip) dose dependently decreased the 2-h cumulative food intake by 30-62%. Urocortin (3 microg/kg)-induced hypophagia was completely antagonized by astressin B (30 microg/kg ip) and partially (35 and 31%) by antisauvagine-30 (100 or 200 microg/kg ip). The CRF-R1 antagonists CP-154,526 or DMP904 (10 mg/kg ip) had no effect. Capsaicin did not alter urocortin-inhibitory actions while blocking the satiety effect of intraperitoneal CCK. These data indicate that intraperitoneal urocortin-induced decrease in feeding is only partly mediated by CRF-R2, whereas urocortin action to delay gastric emptying of a meal involves primarily CRF-R2.

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)

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.

Peripheral corticotropin-releasing factor and stress-stimulated colonic motor activity involve type 1 receptor in rats

BACKGROUND & AIMS

Corticotropin-releasing factor (CRF) exerts its action through CRF receptors 1 and 2 (CRF-R1 and CRF-R2). CRF has preferential affinity for CRF-R1, whereas urocortin displays high affinity for both. We investigated changes in colonic motor function after intraperitoneal (IP) injection of CRF-related peptides.

METHODS

Colonic motility was recorded in vivo in conscious rats equipped with electrodes chronically implanted in the cecum and proximal colon or in vitro in distal colon; fecal output was monitored in naive rats.

RESULTS

Rat CRF, rat urocortin, and amphibian sauvagine (10 microg/kg, IP) induced a new pattern of cecocolonic myoelectric activity characterized by clustered spike bursts of long duration; the percentage of occurrence was highest after CRF. The rank order of potency to increase fecal pellet output after IP peptide injection (0.3-10 microg/kg, IP) was CRF > urocortin = sauvagine. The CRF-R1/R2 antagonist astressin (33 microg/kg, IP) and the CRF-R1 antagonist CP-154,526 (20 mg/kg, subcutaneously) inhibited IP CRF-induced changes in cecocolonic myoelectric activity and IP CRF- and water avoidance stress-induced fecal output. In vitro, CRF injected into the inferior mesenteric artery increased distal colonic myoelectric activity compared with saline injection.

CONCLUSIONS

These results demonstrate that CRF acts peripherally to stimulate colonic motility and that CRF-R1 is primarily involved in mediating IP CRF/urocortin- and water avoidance stress-induced colonic motor response.

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.

Use of a dual firefly and Renilla luciferase reporter gene assay to simultaneously determine drug selectivity at human corticotrophin releasing hormone 1 and 2 receptors

The aim of this study was to investigate and validate the use of a dual glow-signal luciferase reporter gene assay to simultaneously evaluate drug activity at two different seven-transmembrane receptor subtypes. Stable cell lines (CHO) transfected with either human corticotrophin releasing hormone 1 (hCRH1) receptors and a firefly luciferase reporter gene or hCRH2 and a Renilla luciferase reporter gene were created to provide different luciferase readouts for CRH1 and CRH2 receptors, respectively. Cells were combined for stimulation and measurement of luciferase luminescence in a 96-well plate format assay. The nonselective CRH agonists rat/human CRH and sauvagine caused concentration-dependent increases in luminescence via activation of CRH1 (firefly luciferase; pEC50 = 8.40 +/- 0.06 and 8.39 +/- 0.08, respectively, n = 8) and CRH2 (Renilla luciferase; pEC50 = 8.89 +/- 0.14 and 8.92 +/- 0.13, respectively, n = 8) receptors. The nonselective CRH antagonist astressin blocked these agonist-induced increases in luciferase at both CRH1 and CRH2 receptors. The selective CRH1 antagonist CP154,526 blocked r/hCRH- and sauvagine-induced increases in luciferase at CRH1 receptors only. These data report the expected pharmacology for CRH1 and CRH2 receptors. This assay enabled two receptor subtypes to be studied simultaneously in the same 96-well plate and generated robust data with low variability. It has the potential advantage of significant time and cost savings, with application to both basic research and compound screening.

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.

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.

Isolation and pharmacological characterization of two functional splice variants of corticotropin-releasing factor type 2 receptor from Tupaia belangeri

From brain, heart and muscle tissue of the tree shrew (Tupaia belangeri), a higher order mammal, cDNA clones were isolated that encoded two functional splice variants of the corticotropin-releasing factor (CRF) type 2 receptor (CRF-R2). The first, full-length splice variant, amplified from brain and heart tissue, encoded a CRF receptor protein that is 410 amino acids in length and approximately 96% homologous to human CRF-R2alpha. The second, full-length splice variant, derived from skeletal muscle tissue, encoded a 437-amino acid CRF receptor protein that is approximately 92% homologous to human CRF-R2beta. Semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) amplifications and RNase protection analyses, showed that tree shrew CRF-R2alpha (tCRF-R2alpha) and tree shrew CRF-R2beta (tCRF-R2beta) were coexpressed in brain tissue but not in heart and skeletal muscle tissue. Finally, human embryonic kidney 293 (HEK293) cells stably transfected with tCRF-R2alpha and tCRF-R2beta were used to demonstrate that the CRF analogs urocortin and sauvagine bind with significantly greater affinity (21- to 140-fold) to these two CRF-R2 splice variants than do human/rat and ovine CRF analogs. In keeping with these results of our CRF binding studies, EC50 values were substantially lower for urocortin-and sauvagine-stimulated than for h/rCRF-and oCRF-stimulated cyclic AMP accumulation in HEK293 cells stably transfected with tCRF-R2alpha or tCRF-R2beta cDNAs. The tree shrew therefore constitutes an important animal model in which to investigate the role of CRF receptor subtypes in the stress response.