Signaling properties of mouse and human corticotropin-releasing factor (CRF) receptors: decreased coupling efficiency of human type II CRF receptor

miR-375 mediates the CRF signaling pathway to regulate catecholamine biosynthesis by targeting Sp1 in porcine adrenal gland

Corticotropin-releasing-factor (CRF) is a key regulator of catecholamines (CATs) biosynthesis in the adrenal gland. Furthermore, miR-375 has been confirmed to be localized in the mouse adrenal gland. However, the relationships between miR-375 and CRF in regulating CATs biosynthesis remain to be established. This study was designed to investigate the relationship between CRF and miR-375 in the regulation of CATs biosynthesis in the porcine adrenal gland. Eight adult female pigs (four controls; four injected intracerebroventricularly with 50 μg of CRF) were used for the in vivo experiments in this study. The results showed that miR-375 was exclusively localized in porcine adrenal medullary cells. Functional studies showed that miR-375 negatively regulated CATs synthesis in primary cells by affecting the expression of the CATs synthetases tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT). CRF up-regulated the expression of CATs synthetase in primary adrenal medullary cells under basal conditions and upon endogenous miR-375 inhibition; the enhanced effects vanished when cellular miR-375 was overexpressed by transfecting miR-375-mic. CRF decreased the expression of miR-375 both in vivo and in vitro. Our in vitro results showed that CRF significantly decreased the expression of miR-375, perhaps by binding to CRFR1. miR-375 functions by directly binding to the 3'-UTR region of specificity protein 1 (Sp1), which is involved in regulating Th and Dbh expression. These data collectively indicate that miR-375 plays an important role in regulating CATs synthesis and mediates the CRF signaling pathway in porcine adrenal medullary cells.

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

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

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.

Insights into mechanisms of corticotropin-releasing hormone receptor signal transduction

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

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

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

Structural domains determining signalling characteristics of the CRH-receptor type 1 variant R1beta and response to PKC phosphorylation

Mammalian adaptive mechanisms to stressful stimuli involve release of corticotropin-releasing hormone (CRH) and downstream activation of specific G-protein-coupled 7 transmembrane domain receptors. These CRH receptors (CRH-R) are expressed as multiple mRNA spliced variants. In contrast to other mammals, the human type 1 CRH-R gene contains an additional exon (exon 6) that needs to be spliced out in order to generate the fully active CRH-R1alpha. Transcription of all 14 exons results in a CRH-R1 variant (CRH-R1beta) with an extended 1st intracellular loop (IC1); this sequence modification impairs signalling activity and alters receptor responsiveness to PKC-induced phosphorylation that leads to signalling desensitization and receptor endocytosis. To elucidate structure-function relationships and delineate sequences involved in CRH-R1beta properties, site directed mutagenesis was used to introduce a number of specific mutations into IC1 of CRH-R1beta as well as replace specific phospho-acceptor residues within the aminoacid sequence of CRH-R1alpha and CRH-R1beta. Mutant receptors were transiently expressed in human embryonic kidney (HEK293) cells and tested for their abilities to increase intracellular cAMP and their response to PKC-induced phosphorylation. Results identified a penta-aminoacid cassette within the 29-aminoacid insert of CRH-R1beta, which contains multiple positive charged aminoacids (F170-R174), as an important structural determinant for the impaired cAMP response. Furthermore, serine at position 408 in the carboxy-terminus appears to be important for mediating CRH-R1alpha resistance, but not CRH-R1beta susceptibility, to PKC-induced desensitization and internalization. These findings provide new insights about the structural determinants of CRH-R1 coupling to Gs proteins and response to protein kinase phosphorylation.

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.

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

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

Discovery of corticotropin releasing factor 2 receptor selective sauvagine analogues for treatment of skeletal muscle atrophy

The corticotropin release factor 2 receptor (CRF2R) has many biological activities including modulation of the stress response. Recently, we have demonstrated that CRF2R activation functions to prevent skeletal muscle wasting resulting from a variety of physiological stimuli. Thus we are interested in identifying CRF2R selective agonists with optimal pharmacological properties for use in treating muscle wasting diseases. Several CRF2R agonists are known including the frog peptide sauvagine (Svg), which display superior pharmacological properties compared to other CRF2R agonists. Unfortunately sauvagine is a nonselective CRFR agonist, thus making it of less utility due to side effects resulting from corticotropin release factor 1 receptor (CRF1R) activation. Because our initial modifications of Svg at position 11 improved CRF2R selectivity, we investigated the role of amino acids at positions 12 and 13 in Svg. We observed that phenylalanine, leucine, isoleucine, threonine, glutamine, histidine, and tyrosine at the 12th position were the strongest promoters of CRF2R selectivity whereas phenylalanine, glutamine, trytophane, tyrosine, valine, isoleucine, leucine, and 2-naphthylalanine were the preferred residues at the 13th position. Selective sauvagine peptides demonstrated improved antiatrophy effects in a mouse-casting model when compared to sauvagine itself. Thus, we demonstrate that the CRF2R selectivity can be improved by optimizing amino acids at positions 12 and 13 (all with proline at position 11) and that the selective sauvagine analogues demonstrate better in vivo efficacy than sauvagine itself.

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.

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

OBJECTIVE

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

DESIGN

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

RESULTS

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

CONCLUSION

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

Activation of the CRF 2 receptor modulates skeletal muscle mass under physiological and pathological conditions

Two receptors activated by the corticotropin-releasing factor (CRF) family of peptides have been identified, the CRF 1 receptor (CRF1R) and the CRF 2 receptor (CRF2R). Of these, the CRF2R is expressed in skeletal muscle. To understand the role of the CRF2R in skeletal muscle, we utilized CRFR knockout mice and CRF2R-selective agonists to modulate nerve damage and corticosteroid- and disuse-induced skeletal muscle atrophy in mice. These analyses demonstrated that activation of the CRF2R decreased nerve damage and corticosteroid- and disuse-induced skeletal muscle mass and function loss. In addition, selective activation of the CRF2R increased nonatrophy skeletal muscle mass. Thus we describe for the first time a novel activity of the CRF2R, modulation of skeletal muscle mass.

An artificial peptide with corticotropin-releasing factor receptor-2 (CRF-R2) selective properties: the role of primary structure in the induction of signal transduction pathways

Considerable plasticity can occur within the amino acid sequence of amphiphilic peptide hormones. This is particularly evident within the corticotropin-releasing factor (CRF) family of peptides where, despite less than 15% sequence similarity among the four paralogous lineages, all are capable of acting as high affinity ligands to members of the CRF receptor family. This suggests that these peptides could undergo many mutational changes and remain as high affinity ligands to their receptors as long as the functional motifs do not change radically. Because paralogous peptide lineages are a product of genome duplications, additional genes encoding peptide-like sequences, which through mutation have lost their functional integrity, may exist. Function to these sequences may be restored if the appropriate motifs are reinserted into the primary structure. We screened rat genomic DNA with highly degenerate polymerase chain reaction (PCR) primers targeted to hybridize with the termini of CRF-related sequences. One set of sauvagine-based primers hybridized with a 120-bp sequence. The theoretical peptide sequence (SV4) showed similarity to the CRF family of peptides at the primary structure level. The encoded sequence was prepared by solid-phase synthesis and its activity assayed against mouse R1 and human R1/R2 receptors. SV4 did not bind to either mouse or human variants of the R1 receptor, but did bind to the R2 receptor with an affinity comparable to human CRF. SV4 exhibited a similar efficacy of cellular activation as CRF in trials quantifying the acidification rate of human R2alpha-transfected Chinese hamster ovary (CHO) cells, but not R1-transfected cells. SV4 utilizes adenylate cyclase as the principal secondary messenger of R2 signal transduction but, unlike urocortin or sauvagine, does not activate guanylate cyclase-, calcium- or mitogen-activated protein (MAP) kinase-mediated pathways. These data suggest that this artificial peptide may be useful to understand the cyclic adenosine monophosphate (cAMP)-dependent component of the CRF-R2 signal transduction cascade, and that additional sequences in the genome may be used to engineer bioactive peptides.

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.

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.

The neurobiology of urocortin

Urocortin (UCN) is a recently isolated 40 amino acid-containing neuropeptide that is the second endogenous mammalian ligand for the corticotropin-releasing factor (CRF) receptors. While UCN and CRF both display a similar high affinity for the CRF(1) receptor, the affinity of UCN for the CRF(2) receptor is more than 10-fold higher than that of rat/human CRF. UCN mRNA expression is highest in the Edinger-Westphal nucleus and lateral superior olive, with the most prominent terminal fields found in the lateral septum. Because of the higher relative affinity of UCN for the CRF(2) receptor and the corresponding neuroanatomical distribution of the highest density of UCN expression and innervation to brain regions preferentially expressing the CRF(2) receptor subtype, it has been hypothesized that UCN is the preferred endogenous ligand for the CRF(2) receptor. Following central administration, UCN has been demonstrated to produce behavioral and physiological effects that are qualitatively similar to CRF. Quantitatively, however, UCN appears to be a more potent suppressor of ingestive behavior (food and water intake) and a less potent inducer of anxiogenic behavior than CRF.

Signal transduction characteristics of the corticotropin-releasing hormone receptors in the feto-placental unit

Placentally derived CRH plays a major role in the mechanisms controlling human pregnancy and parturition. In this study, we sought to investigate the signal transduction mechanisms of CRH Type-1 receptors in the feto-placental unit. To clarify the signal transduction components in placenta and fetal membranes, we investigated the expression of G proteins and adenylate cyclase. Using the nonhydrolysable photoreactive analog [alpha-32P] GTP-azidoanilide and peptide antisera raised against G protein alpha-subunits, we studied coupling of CRH receptors to G proteins in both placental and fetal membranes. Treatment of placental membranes with human CRH (100 nM) increased the labeling of Gq, Go, and Gz but not Gi and Gs. Treatment of fetal membranes with human CRH (100 nM) increased the labeling of Go and Gq but not Gi, Gs, and Gz. These results were supported by experiments that showed that CRH failed to activate adenylate cyclase in these tissues, but induced an increase in inositol phosphates instead. These findings provide new insights into the components of the signal transduction machinery in both fetal and placental membranes and suggest that CRH Type-1 receptors can couple to different G proteins in different tissues. The physiological significance of these observations remains to be elucidated.

N-terminal splice variants of the type I PACAP receptor: isolation, characterization and ligand binding/selectivity determinants

Three full-length cDNAs encoding functional splice variants of the pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor (PAC1) were isolated from Y-79 retinoblastoma cells and human cerebellum. Although the third intracellular loops of the three splice variants were identical, their N-terminal extracellular domains differed. The first full-length PAC1 variant, PAC1normal (PAC1n), encoded the entire N-terminus, whereas the second variant named PAC1short (PAC1s) was deleted by 21 amino acids (residues 89-109). Finally, the third variant, named PAC1very short (PAC1vs), was deleted by 57 amino acids (residues 53-109). Using semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis, it was established that all three variants were expressed in neuronal tissues. Binding- and cAMP studies using human embryonic kidney 293 (HEK293) cells stably transfected with PAC1n, PAC1s and PAC1vs showed significant differences in the affinities and selectivities towards PACAP38, PACAP27 and VIP. PAC1n bound PACAP38 and PACAP27 with affinities in the low nanomolar range whereas VIP was bound with up to 400-fold lower affinity. PAC1vs preferentially bound PACAP38 (Ki=121 nM) and PACAP27 (Ki=129 nM) over VIP (Ki>1000 nM) but with 100-fold lower affinity than PAC1n. Surprisingly, PAC1s unselectively bound all three ligands with high affinity. These data indicate that residues 53-88 within the N-terminal domain of the PAC1 are important for high affinity ligand binding, whereas residues 89-109 determine the receptor's ligand selectivity.

Corticotropin-releasing factor plays a permissive role in cerebellar long-term depression

This study of rat cerebellar slices yielded two lines of evidence indicating that the corticotropin-releasing factor (CRF) found in climbing fibers (CFs) is critical for the induction of long-term depression (LTD) at the parallel fiber (PF) synapses of Purkinje cells (PCs) by their conjunctive activation with either stimulation of CFs or depolarization of PCs. First, LTD induction was effectively blocked by specific CRF receptor antagonists, alpha-helical CRF-(9-41) (alpha-h CRF) and astressin; and second, LTD was no longer observed in CF-deprived cerebella but was restored by CRF replenishment. The data obtained in this study suggest that these effects are mediated by protein kinase C (PKC) and not by Ca2+ signaling or cyclic GMP (cGMP) production.

Epitope tag mapping of the extracellular and cytoplasmic domains of the rat parathyroid hormone (PTH)/PTH-related peptide receptor

The PTH/PTH-related peptide (PTHrP) receptor is predicted to span the plasma membrane seven times with an amino-terminal extracellular extension and a cytoplasmic carboxyl-terminal tail. To assess this prediction, we inserted 10- or 9-amino acid epitope tags from c-myc or hemophilus influenza hemaglutinin (HA), which are recognized by the monoclonal antibodies 9E10 and 12Ca5, respectively, in different extracellular and cytoplasmic regions of the receptor and examined the immunoreactivity of the epitopes in intact and permeabilized cells. The data show that the epitopes were well tolerated when introduced into the E2 region of the extracellular amino-terminus (E2-myc and E2-HA), in the first extracellular loop (EL1), in the second and third cytoplasmic loops (CL2c and CL3), or in the carboxyl-terminal tail (T-myc). Receptors tagged at these locations were well expressed, bound PTH with high affinity, and increased cAMP accumulation with a good efficiency. Receptors tagged in the second and third extracellular loops (EL2c and EL3c) or the first cytoplasmic loop (CL1c) bound the PTH radioligand with a low affinity, stimulated cAMP accumulation with a low efficiency, and had low expression levels. The receptors tagged on presumed extracellular regions, E2-myc, E2-HA, EL1, EL2c, and EL3c, were readily detected on the surface of intact cells with the monoclonal antibody against the epitope tag. In contrast, receptors tagged with the c-myc epitope in the cytoplasmic loops (CL1c, CL2c, and CL3) or in the carboxyl-terminal tail (T-myc) did not show any 9E10 binding in intact cells. These receptors, however, were well expressed on the cell surface, as detected by the binding of the monoclonal antibody, 12Ca5, to the HA tag that was introduced into the E2 region of these constructs. The c-myc epitopes, however, became accessible after permeabilization of the cell membrane. In conclusion, these data provide experimental evidence for the sidedness of the extracellular and cytoplasmic domains of the PTH/PTHrP receptor.

Direct regulation of GnRH transcription by CRF-like peptides in an immortalized neuronal cell line

The existence of a CRF-dependent inhibition of GnRH transcription was investigated using a neuronal GnRH-expressing cell line (Gn11) stably transfected with mouse (-611 bp) or chicken (-3000 bp) GnRH promoter/luciferase reporter constructs. The presence of the CRF-R1 receptor was established using a specific CRF-R1 antiserum. After 7 h of incubation, urotensin-I and sauvagine increased the mouse GnRH-reporter bioluminescence by 1.3- and 1.2-fold, respectively, compared with control cells. Subsequently, CRF, urotensin-I and sauvagine decreased luciferase reporter activity to about 60% of the control values after 14 h. Similar trends occurred with the chicken GnRH promoter with UI increasing reporter gene activity 2.4-fold over the controls after 14 h incubation. These data provide additional evidence for the direct regulation of GnRH transcription by CRF-like peptides.

Corticotropin-releasing factor (CRF) agonists stimulate testosterone production in mouse leydig cells through CRF receptor-1

The influence of CRF on testosterone production in primary mouse Leydig cell cultures was studied, and the type of CRF receptor (CRF-R) involved in this activity was determined. CRF directly stimulated testosterone production in mouse Leydig cells, but did not influence the maximum human (h)CG-induced testosterone production. The effect was time- and dose-dependent, saturable with an EC50 of 2.84 nM for hCRF, antagonized by the CRF antagonist alpha-helical CRF9-41, and accompanied by intracellular cAMP elevation. The rank order of potency of the natural CRF agonists, hCRF, ovine CRF, sauvagine, and urotensin, corresponded to that of their activities on CRF-R1 in rat pituitary cells and also to that reported for this receptor, but not for CRF-R2, when transfected into various cell lines. Furthermore, the difference in response of mouse Leydig cells to [11-D-Thr,12-D-Phe]- and [13-D-His,14-D-Leu]-ovine CRF corresponded to that measured when COS cells expressing CRF-R1 were activated, but was considerably smaller than that observed for activation of COS cells expressing CRF-R2alpha or -R2beta. The messenger RNA encoding the mouse CRF-R1 was detected by RT-PCR in mouse Leydig cell preparations. In contrast to mouse Leydig cells, CRF agonists had no influence on the basal testosterone and cAMP production by rat Leydig cells, nor did the agonists or antagonist change the hCG-stimulated testosterone and cAMP production by these cells. It is concluded that mouse Leydig cells express CRF-R1, mediating elevation of testosterone production by CRF agonists through cAMP. Because potencies of CRF agonists in activating mouse Leydig cells were more than 10-fold lower compared with their potencies in stimulating rat pituitary cells, it is suggested that the coupling of the CRF-R1 to intracellular signaling in Leydig cells is different from that in corticotropic pituitary cells, at least in quantitative terms.

First intracellular loop of the human cholecystokinin-A receptor is essential for cyclic AMP signaling in transfected HEK-293 cells

Cholecystokinin (CCK)-A and CCK-B receptors are highly homologous members of the seven transmembrane domain G-protein-coupled receptor superfamily. Genes of both receptors contain five exons and share a similar exon-intron organization. To determine the structural basis of CCK-A receptor (CCK-AR) functionally coupled to Gs, a series of chimeric mutants were constructed by replacing exons of human CCK-B receptor (CCK-BR), from the second to the fifth (last) exon, with human CCK-AR counterparts. Binding and signal transduction properties of wild-type and chimeric receptors were examined in stably transfected HEK-293 cells. Chimeric receptors that maintained high affinity binding to CCK exhibited dose-dependent increases in intracellular calcium mobilization similar to both wild-type receptors. However, only the wild-type CCK-AR and chimeric mutants containing the second exon of CCK-AR were able to mediate significantly greater increases in intracellular cAMP content and adenylyl cyclase activity compared with wild-type CCK-BR. A CCK-BR mutant was further constructed by replacing five amino acids, Gly-Leu-Ser-Arg-(Arg)-Leu, in the first intracellular loop with the corresponding five CCK-AR specific amino acids, Ile-Arg-Asn-Lys-(Arg)-Met. The resultant receptor maintained high affinity binding to both CCK and gastrin and dose-dependent calcium responses similar to wild-type CCK-BR. However, this first intracellular loop mutant also gained positive cAMP responses to both sulfated CCK-8 and gastrin-17 with EC50 values of 8.5 +/- 1 nM and 23 +/- 7 nM, respectively. These data suggest that the first intracellular loop of CCK-AR is essential for coupling to Gs and activation of adenylyl cyclase signal transduction cascade.

Rapid generation of stable cell lines expressing corticotropin-releasing hormone receptor for drug discovery

Human HEK293 cells that stably express the Epstein Barr nuclear antigen 1 (EBNA1) support the episomal replication of plasmids containing the Epstein Barr virus origin of replication (EBV oriP). A 293EBNA (293E) cell line expressing the human corticotropin-releasing hormone receptor subtype I (CRHR1) from an episomal plasmid was generated (293CR1s), analyzed, adapted to spinner culture, and scaled-up for production in less than 6 weeks. Forty-seven stable CHO cell lines transfected with CRHR1 were also isolated. Expression of the receptor in the best of these lines (as judged by CRH-induced cAMP production), CHO-R22, was compared to that in 293CR1s cells. Results indicate that the CRHR1 episomal expression vector in 293E cells (1) rapidly generates stable cell lines suitable for scale-up; (2) is stably maintained during 3 months in culture; (3) expresses high levels of CRHR1 mRNA; and (4) expresses significantly more CRHR1 than the CHO-R22 line. Coexpression of additional G protein alpha subunit (G alpha s) with CRHR1 in 293E cells converts a higher percentage of receptor to the agonist high-affinity G-protein-coupled state. Our data support the idea that using the EBV oriP-driven episomal system for gene expression results in greater production of protein in a relatively short period of time.

A single-point slight alteration set as a tool for structure-activity relationship studies of ovine corticotropin releasing factor

In order to determine which amino acid side chains of ovine corticotropin releasing factor (oCRF) are most sensitive to alterations with respect to receptor binding and activation, we synthesized a single-point replacement set by replacing each residue by a similar, preferably proteinogenic amino acid, maintaining a minimal change of character at each position (Ser by Thr, Gln by Asn, Glu by Asp, Arg by Lys, and vice versa, Pro by N-MeAla, Ile by Leu, Leu by Nle, Phe by Trp, His by Ala, Val by Leu, Met by Nle, Ala by Leu). In general, any loss in the biological potency by a single-point substitution in oCRF parallels a decrease in receptor binding, indicating that, in contrast to previous suggestions, there is no specific side chain in the peptide that is more responsible for receptor activation than for receptor binding. In addition to Arg(16), Ala(31), and Arg(35), amino acid residues in the N-terminal sequence (5-14) were found to be sensitive to alteration, demonstrating their particular importance for the receptor interaction of CRF agonists. Most of the analogs tested exhibited agonistic potencies in an in vitro pituitary cell culture assay at a concentration of 0.3 nM, and all analogs showed full agonistic potency at 1 microM. In contrast to the results of an alanine replacement study, the strongest decrease in receptor binding and biological potency was observed for analogs with substitutions of hydrophilic amino acids Ser(7), Arg(16), Glu(17), or Asn(34). In the case of Ser(7) and Arg(16), side chain specific interactions with the receptor may be required for high affinity. Alanine replacements at positions 17 or 34 resulted in analogs that were as potent as oCRF, while replacement of Glu(17) by Asp or Asn(34) by Gln caused a dramatic loss in potency, thereby suggesting an important effect at sterically or conformationally sensitive positions. In contrast to corresponding alanine analogs which exhibited a significant loss in biological potency, slight alterations of lipophilic side chains at positions 6, 12, or 38 did not cause a significant reduction of receptor binding and activation, indicating that it is not specific side chains but rather lipophilicity which is essential at these positions. Indeed, replacement of Phe(12) by Trp provides an agonist with significantly increased receptor binding and biological potency.

The genomic structure of the rat corticotropin releasing factor receptor. A member of the class II G protein-coupled receptors

Isolation and structural characterization of the rat corticotropin releasing factor receptor (CRFR) gene was performed to determine the exon/intron organization of the coding region and the potential for splice variants. The CRFR gene contains 13 exons and 12 introns, and the positions of the exon/intron junctions are similar to those of other Class II G protein-coupled receptor genes including the parathyroid hormone and glucagon receptors. The promoter resides within 593 base pairs of the initiation codon and the major transcriptional start site at nucleotide -238. This domain does not possess a TATA box but contains multiple Sp1 and AP-2 sites upstream and downstream of the major transcriptional start site. Intron junctions were identified in the extracellular, transmembrane (TM), and cytoplasmic (C) domains of the CRFR, giving the potential for differential signal transduction by splice variants. CRFR cDNAs derived from rat Leydig cell mRNA included the pituitary Form A, which spans exons 1-13, and two splice variants with deletion of exon 3 or exons 7, 11, and 12. An evolutionary link between the intronless TM/C module of the glycoprotein hormone receptors and the intron-containing TM/C module of the CRFR is suggested by the common position of the luteinizing hormone receptor Form D alternate acceptor splice site and the CRFR intron 12.