Comparison of an agonist, urocortin, and an antagonist, astressin, as radioligands for characterization of corticotropin-releasing factor receptors

The characteristics of a high-affinity antagonist radioligand are compared with those a high-affinity agonist in binding to the cloned corticotropin-releasing factor receptor type 1 (CRF-R1) and type 2 (CRF-R2) and to the native receptors that exist in rat cerebellum and brain stem. The relative potencies of CRF antagonists and agonists to the two types of cloned CRF receptors overexpressed stably in Chinese hamster ovary cells are determined using the antagonist radioligand 125I- [DTyr1]astressin (Ast*), and the agonist radioligand, 125I -[Tyr0]rat urocortin (Ucn*). The inhibitory binding constants (Ki) of astressin and urocortin are 1 to 2 nM for all receptors and are independent of which radioligand is employed. Astressin binds with high affinity to the native cerebellar/brain stem receptor and relative potencies of selected CRF analogs determined with Ast* on the native receptor are similar to those obtained for the cloned CRF-R1. The specific binding of Ast* to endogenous brain receptors is greater than that of Ucn*, resulting in more sites being detected by the antagonist than by the agonist. In contrast to another CRF agonist, the binding of Ucn* to the cloned receptors is relatively insensitive to guanyl nucleotides at both 20 degreesC and 37 degreesC; however, its binding to the native receptor is displaced by guanyl nucleotides at 37 degreesC and, to a lesser degree, at 20 degreesC. As expected, the binding of the antagonist Ast* is not affected by guanyl nucleotides. Because it is a high-affinity, specific CRF antagonist, astressin is eminently suitable as a ligand for detection and characterization of both endogenous and cloned CRF receptors.

CRF type I receptor-deficient mice exhibit a pronounced pituitary-adrenal response to local inflammation

Recent studies indicate that the regulation of adrenocorticotropin (ACTH) secretion by corticotropin-releasing factor (CRF) is mediated predominantly by the type I CRF receptor (CRF-R1). Indeed, CRF-R1-deficient (CRF-R1 -/-) mice show marked impairment of the pituitary-adrenal axis. However, the plasma ACTH concentrations of unstressed CRF-R1 -/- mice are similar to those in wild-type mice. We show here that arginine vasopressin (AVP) is a major ACTH secretagogue in CRF-R1 -/- mice in resting conditions, since administration of anti-AVP serum, but not anti-CRF serum, markedly reduced (by 60%) resting plasma ACTH concentrations in these mutants. We also investigated the pituitary-adrenal response to turpentine-induced local inflammation in CRF-R1 -/- mice. Administration of turpentine into the hind-limb of CRF-R1 -/- mice produced a slightly (15-25%) smaller swelling of the limb, but a 10 fold greater rise in plasma IL-6 levels, compared to CRF-R1 +/+ controls. Turpentine-induced local inflammation produced pronounced elevations in the plasma concentrations of both ACTH and corticosterone in both CRF-R1 -/- and wild-type mice, but ACTH secretion could be inhibited by anti-CRF and anti-AVP sera only in wild-type mice. These data indicate that resting ACTH secretion in CRF-R1 -/- mice is in part attributable to AVP-dependent mechanisms. Furthermore, while in normal mice the pituitary-adrenal response to local inflammation is mediated largely via CRF-dependent mechanisms, mice deficient in CRF-R1 are still able to mount a pituitary-adrenal response via mechanisms that do not depend critically on either CRF or AVP action.

Urocortin is not a significant regulator of intermittent electrofootshock-induced adrenocorticotropin secretion in the intact male rat

Urocortin (Ucn) is a newly identified mammalian member of the CRF family of peptides. Ucn activates CRF receptors (both CRF-R1 and CRF-R2) with greater potency than CRF itself, suggesting that Ucn may play an endogenous role in eliciting (at least some) CRF receptor-mediated events. Because the most characterized physiological function of CRF receptors is the activation of pituitary ACTH secretion, we have compared the effects and potential endogenous roles of CRF and Ucn in regulating plasma ACTH concentrations in intact male rats. Synthetic rat Ucn injected i.v. (0.09-9.0 nmol/kg) elicited ACTH secretion in a dose-dependent manner, causing greater ACTH secretion than CRF at each dose tested. The increases in plasma ACTH concentrations produced by CRF or Ucn were virtually abolished by pretreatment with the CRF receptor antagonist, astressin (3 mg/kg), and were partially attenuated (by 27-37%) by an antiarginine vasopressin serum. These data indicate that both Ucn and CRF elicit ACTH secretion via CRF receptor-dependent mechanisms, and that the ACTH-releasing activities of both CRF and Ucn are potentiated by endogenous arginine vasopressin. Intravenous administration of rabbit anti-Ucn serum, which inhibited ACTH secretion produced by Ucn, but not CRF, had no statistically significant effect on either resting (midday) plasma ACTH concentrations or the rise in ACTH levels elicited by 30 min of intermittent electrofootshocks. By contrast, treatment with a rabbit anti-CRF serum that specifically inhibited the ACTH response to CRF lowered plasma concentrations in control unstressed rats and largely prevented the plasma ACTH response to electrofootshocks. These data indicate that although Ucn is a more potent ACTH secretagogue than CRF in the intact male rat, it is not a major endogenous regulator of pituitary ACTH secretion under basal (midday) conditions or during acute footshock stress.

Three-finger toxin fold for the extracellular ligand-binding domain of the type II activin receptor serine kinase

The transforming growth factor beta (TGFbeta) superfamily of cytokines elicit diverse biological responses by interacting with two distinct, but structurally related transmembrane receptor serine kinases (type I and type II). The binding of these dimeric ligands to the type II receptor is the first event in transmembrane signaling for this family. Here we report the 1.5 A resolution crystal structure of the extracellular ligand-binding domain of the type II activin receptor (ActRII-ECD), which reveals a fold similar to that of a class of toxins known as three-finger toxins. This fold is primarily dictated by disulfide bonds formed by eight conserved cysteines, with a characteristic spacing, and thus is likely to be shared by most of the type I and II receptors for the TGFbeta family. Sequence comparison with an evolutionarily distant activin binding-protein identifies several conserved residues, including two hydrophobic clusters that may form binding surfaces for activin and the type I receptor.

Evolutionarily conserved, "acatalytic" carbonic anhydrase-related protein XI contains a sequence motif present in the neuropeptide sauvagine: the human CA-RP XI gene (CA11) is embedded between the secretor gene cluster and the DBP gene at 19q13.3

Conserved amino acid motifs are found in numerous expressed genes. Proteins and peptides with functional relationships may be identified using probes designed to hybridize with these motifs. An oligonucleotide probe was prepared to match the sequence of the expected active region of a frog corticotropin-releasing factor-like peptide sauvagine and used to screen a sheep brain cDNA library. A novel 1331-bp cDNA encoding a putative 328-residue protein with a theoretical mass of 36 kDa was identified. The presence of a strong signal sequence indicates that it is a secreted protein. The amino- and carboxy-terminal regions are characterized by several potential phosphorylation sites and binding motifs, suggesting a role in intracellular signal transduction. Although the protein possesses a 7-residue sequence identical to that found in sauvagine, its overall primary structure most closely resembles those of the alpha-carbonic anhydrases (alpha-CAs). Moreover, the detection of the human and mouse orthologues in the EST databases, together with an evolutionary analysis, indicates that the protein represents a new member of the alpha-CA gene family, which we designate carbonic anhydrase-related protein XI (CA-RP XI), encoded by CA11 (human) and Car11 (mouse, rat). The human CA11 gene appears to be located between the secretor type alpha(1,2)-fucosyltransferase gene cluster (FUT1-FUT2-FUT2P) and the D-site binding protein gene (DBP) on chromosome 19q13.3. Despite potentially inactivating changes in the active-site residues, CA-RP XI is evolving very slowly in mammals, a property indicative of an important function, which has also been observed in the two other "acatalytic" CA isoforms, CA-RP VIII and CA-RP X, whose functions are unknown.

Interleukin-1beta regulates pituitary follistatin and inhibin/activin betaB mRNA levels and attenuates FSH secretion in response to activin-A

Activins and follistatins regulate all levels of the reproductive axis, including the pituitary where they stimulate and inhibit FSH production, respectively. Gonadotropes are known to express inhibin/activin betaB and activin-B (betaBbetaB) functions as an autocrine modulator of FSH production. By contrast, the mRNA for the activin-binding protein, follistatin, is present in most pituitary cells and folliculo-stellate cells may be the major source of the protein secreted by the anterior pituitary. Interleukin-1beta (IL-1beta) is one of several cytokines known to also influence the reproductive axis. IL-1beta inhibits the hypothalamo-pituitary-gonadal (HPG) axis by suppressing GnRH and gonadal steroid production. Because several pituitary cell types, including follistatin-producing folliculo-stellate cells, are targets of IL-1beta, cytokine effects on gonadotrope function were evaluated using cultured rat anterior pituitary cells. Activin-A (0.01 to 1 nM; 24h) increased basal FSH secretion approximately 2-fold. IL-1beta (0.005 to 0.5 nM) by itself had no effect on basal FSH secretion. However, IL-1beta attenuated FSH secretion in response to all concentrations of activin-A. These results suggest that the cytokine might stimulate the local production of a factor, such as follistatin, that antagonizes the action of activin-A. RNase protection analysis indicated that IL-1beta (0.005 to 5 nM) stimulated follistatin and inhibin/activin betaB mRNA accumulation in a time-dependent manner. These in vitro effects of IL-1beta were blocked by the specific IL-1 receptor antagonist (IL-lra) and were not mimicked by either rhIL-6 or lipopolysaccharide (LPS). Treatment of intact male rats with LPS (50 microg, i.v.), which increases plasma IL-1beta and induces IL-1beta expression in many tissues, including the pituitary, produced similar time-dependent increases in pituitary follistatin and inhibin/activin subunit mRNA levels. These results suggest that IL-1beta can modulate gonadotrope responses to activins by influencing the local balance of activin-B and follistatin within the pituitary.

Ectopic expression of the CRF-binding protein: minor impact on HPA axis regulation but induction of sexually dimorphic weight gain

Corticotrophin-releasing factor (CRF) and urocortin possess a high-affinity binding protein. Although the CRF binding protein (BP) can sequester these ligands and inhibit their activity, the endogenous activity of this protein is not understood. Therefore, transgenic mouse lines that over-express the CRF-BP were created. The transgene was constructed by ligating rat CRF-BP cDNA (1.1 kb) between a mouse metallothionein-I promoter (1.8 kb) and a nonfunctional human growth hormone gene sequence (2.1 kb) in a modified pBR322 plasmid and microinjecting the transgene into C57BL/6 x SJL hybrid ova. The transgene was expressed in 50% in both male and female progeny. All transgenic lines were maintained by crossing transgenic animals with wild-type C57BL/6 mates. Reverse-transcriptase (RT) PCR of the CRF-BP transgene showed that it is widely expressed not only in the brain and pituitary, but also peripheral tissues including the liver, kidney and spleen. Transgenic animals of both sexes showed significant increases in weight gain as established by analysis of variance; however, the weight gain profiles for each sex were distinct. High levels of circulating CRF-BP were detected in the transgenic animals, but the basal ACTH and corticosterone levels were not significantly decreased compared to wild-type littermates. The hypothalamopituitary-adrenal (HPA) axis was stimulated by systemic inflammation induced with lipopolysaccharide (LPS). An expected increase in transgene expression was observed and was accompanied by a significant attenuation of ACTH secretion at 3 h after LPS injection in the transgenic males but not the females. These data suggest that HPA axis regulation is significantly affected only with very high circulating levels of CRF-BP. Moreover, this work supports previous studies that implicate CRF and urocortin in the regulation of appetite and the binding protein expression may play a sexually dimorphic role in regulating this and other responses.

The structures of the mouse and human urocortin genes (Ucn and UCN)

The mouse and human urocortin genes (Ucn and UCN, respectively) have been isolated, characterized, and found to have very similar structures. Each has two exons, and the entire coding region is located in the second exon, as is the case for the gene of the related peptide, corticotropin-releasing factor. Several putative transcription factor-binding sites were identified in each of the urocortin promoters, including a TATA box, a cyclic AMP response element (CRE), GATA-binding sites, and a C/EBP-binding site as well as a Brn-2-binding site(s). Sequence analyses of the mouse and human genes also revealed the presence of a previously identified gene, Mpv17, in the 5' region upstream of the urocortin gene. Functional studies following transient transfection of urocortin reporter plasmids in PC12 cells revealed that the urocortin promoter is controlled by both positive and negative elements; the CRE is important for basal activity as well as responsiveness to forskolin stimulation.

Human growth hormone-releasing hormone hGHRH(1-29)-NH2: systematic structure-activity relationship studies

Two complete and two partial structure-activity relationship scans of the active fragment of human growth hormone-releasing hormone, [Nle27]-hGHRH(1-29)-NH2, have identified potent agonists in vitro. Single-point replacement of each amino acid by alanine led to the identification of [Ala8]-, [Ala9]-, [Ala15]- (Felix et al. Peptides 1986 1986, 481), [Ala22]-, and [Ala28, Nle27]-hGHRH(1-29)-NH2 as being 2-6 times more potent than hGHRH(1-40)-OH (standard) in vitro. Nearly complete loss of potency was seen for [Ala1], [Ala3], [Ala5], [Ala6], [Ala10], [Ala11], [Ala13], [Ala14], and [Ala23], whereas [Ala16], [Ala18], [Ala24], [Ala25], [Ala26], and [Ala29] yielded equipotent analogues and [Ala7], [Ala12], [Ala17], [Ala20], [Ala21], and [Ala27] gave weak agonists with potencies 15-40% that of the standard. The multiple-alanine-substituted peptides [MeTyr1,Ala15,22,Nle27]-hGHRH(1-29)-NH2 (29) and [MeTyr1,Ala8,9,15,22,28,Nle 27]-hGHRH(1-29)-NH2 (30) released growth hormone 26 and 11 times, respectively, more effectively than the standard in vitro. Individual substitution of the nine most potent peptides identified from the Ala series with the helix promoter alpha-aminoisobutyric acid (Aib) produced similar results, except for [Aib8] (doubling vs [Ala8]), [Aib9] (having vs [Ala9]), and [Aib15] (10-fold decrease vs [Ala15]). A series of cyclic analogues was synthesized having the general formula cyclo(25-29)[MeTyr1,-Ala15,Xaa25,Nle27,Yaa29+ ++]-GHRH(1-29)-NH2, where Xaa and Yaa represent the bridgehead residues of a side-chain cystine or [i-(i + 4)] lactam ring. The ring size, bridgehead amino acid chirality, and side-chain amide bond location were varied in this partial series in an attempt to maximize potency. Application of lactam constraints in the C-terminus of GHRH(1-29)-NH2 identified cyclo(25-29)[MeTyr1,Ala15,DAsp25,Nle27,Orn29+ ++]-hGHRH(1-29)-NH2 (46) as containing the optimum bridging element (19-membered ring) in this region of the molecule. This analogue (46) was 17 times more potent than the standard. Equally effective was an [i-(i + 3)] constraint yielding the 18-membered ring cyclo(25-28)[MeTyr1,Ala15,Glu25,Nle,27Lys28]- hGHRH-(1-29)-NH2 (51) which was 14 times more potent than the standard. A complete [i-(i + 3)] scan of cyclo(i,i + 3)[MeTyr1,Ala15,Glui,Lys(i + 3),Nle27]-hGHRH(1-29)-NH2 was then produced in order to test the effects of a Glu-to-Lys lactam bridge at all points in the peptide. Of the 26 analogues in the series, 11 had diminished potencies of less than 10% that of the agonist standard, 4 were weak agonists (15-40% relative potency), and 4 analogues were equipotent to the standard. The 7 most potent analogues ranged in potency from 3 to 14 times greater than that of the standard and contained the [i-(i + 3)] cycles between residues 4-7, 5-8, 9-12, 16-19, 21-24, 22-25, and 25-28. The combined results from these systematic studies allowed for an analysis of structural features in the native peptide that are important for receptor activation. Reinforcement of the characteristics of amphiphilicity, helicity, and peptide dipolar effects, using recognized medicinal chemistry approaches including introduction of conformational constraints, has resulted in several potent GHRH analogues.

Displacement of insulin-like growth factors from their binding proteins as a potential treatment for stroke

Insulin-like growth factors I and II (IGF-I and IGF-II) play an important role in normal growth and brain development and protect brain cells from several forms of injury. The effects of IGFs are mediated by type-I and type-II receptors and modulated by potentially six specific binding proteins that form high-affinity complexes with IGFs in blood and cerebrospinal fluid (CSF) and under most circumstances inactivate them. Because brain injury is commonly associated with increases in IGFs and their associated binding proteins, we hypothesized that displacement of this large "pool" of endogenous IGF from the binding proteins would elevate "free" IGF levels to elicit neuroprotective effects comparable to those produced by administration of exogenous IGF. A human IGF-I analog [(Leu24, 59, 60, Ala31)hIGF-I] with high affinity to IGF-binding proteins (Ki = 0.3-3.9 nM) and no biological activity at the IGF receptors (Ki = >10,000 nM) increased the levels of "free, bioavailable" IGF-I in the CSF. Intracerebroventricular administration of this analog up to 1h after an ischemic insult to the rat brain had a potent neuroprotective action comparable to IGF-I. This novel strategy for increasing "free" IGF levels in the brain may be useful for the treatment of stroke and other neurodegenerative diseases.

The first extracellular domain of corticotropin releasing factor-R1 contains major binding determinants for urocortin and astressin

The CRF receptors are members of a 7-transmembrane receptor family that includes GH-releasing hormone (GRF), calcitonin, vasoactive intestinal peptide (VIP), secretin, and PTH receptors. To determine the structural features of the CRF receptor that may influence ligand recognition, a series of mutant receptors was analyzed for binding to astressin, a CRF antagonist, and to urocortin, a CRF agonist. Mutant receptors included chimeras between the CRF-R1 and GRF-R or Activin IIB-R, a single membrane spanning receptor serine/threonine kinase. Binding to the mutant receptors was assessed using 125I-[DTyr1] astressin (Ast*) and 125I-[Tyr0]-rat urocortin (Ucn*). There was no binding to a chimeric receptor in which the first extracellular domain (E1c) (i.e. the N-terminal region) of the CRF-R1 was replaced by that of the GRF-R. The complementary chimera in which E1 domain of the GRF-R was replaced by that of the CRF-R1 bound astressin and urocortin with Ki values approximately 10 nM, compared with inhibitory binding dissociation constant (Ki) values of approximately 2-4 nM for the wild-type CRF-R1. The chimera in which E1 of the activin IIB receptor was replaced by E1 of the CRF-R1 bound astressin with a Ki approximately 4 nM. A chimera in which both the first and fourth extracellular domains of the CRF-R1 replaced the corresponding domains of the GRF-R bound astressin with Ki approximately 4 nM and urocortin with a Ki approximately 2 nM. A chimera in which all four extracellular domains of the CRF receptor replaced those of the GRF-R bound astressin and urocortin with Ki values approximately 4 nM and approximately 1 nM, respectively. In conclusion, the major determinants for high affinity binding of CRF agonists and antagonists to CRF-R1 are found in the first extracellular domain of the receptor.

Anti-sexual and anxiogenic behavioral consequences of corticotropin-releasing factor overexpression are centrally mediated

Corticotropin-releasing factor (CRF) acts as a neurotransmitter in brain to promote behavioral responses such as flight and immobility, which have adaptive value in the context of exposure to environmental stressors. CRF also suppresses behavioral repertoires such as mating, which are incompatible with such threat-related coping responses. In this study, we employed transgenic (Tg) mice which overexpress CRF in brain and exhibit a constitutive and persistent phenotype of emotionality in order to determine the consequences of long-term CRF excess on indices of reproductive success, male sexual performance and female sexual receptivity. Sexual performance of CRF Tg males was relatively intact, whereas female receptivity was masked in CRF Tg mice by active rejection of sexually experienced male counterparts. This impairment in social interaction was only partially normalized by the serotonin antagonist, methysergide, which enhanced olfactory exploration of the still non-receptive CRF Tg females. Moreover, the anxiogenic-like character of CRF Tg mice is likely to be centrally mediated, since attenuation of hypercorticosteronemia by adrenalectomy did not alter either impaired sexual receptivity or fear-like behavior in an animal model of anxiety. Thus, overexpression of CRF in the brain results in a variety of adverse consequences including diminished social interactions.

Activin and inhibin have antagonistic effects on ligand-dependent heteromerization of the type I and type II activin receptors and human erythroid differentiation

Activins and inhibins belong to the transforming growth factor beta (TGF-beta)-like superfamily and exert their effects on a broad range of cellular targets by modulating cell differentiation and proliferation. Members of this family interact with two structurally related classes of receptors (type I and type II), both containing a serine/threonine kinase domain. When expressed alone, the type II but not the type I activin receptor can bind activin. However, the presence of a type I receptor is required for signaling. For TGF-beta1, ligand binding to the type II receptor results in the recruitment and transphosphorylation of the type I receptor. Transient overexpression of the two types of activin receptor results in ligand-independent receptor heteromerization and activation. Nevertheless, activin addition to the transfected cells increased complex formation between the two receptors, suggesting a mechanism of action similar to that observed for the TGF-beta receptor. In the present study, we generated a stable cell line, overexpressing the two types of activin receptor upon induction, in the human erythroleukemia cell line K562. We demonstrate here that activin specifically induces heteromer formation between the type I and type II receptors in a time-dependent manner. Using this stable line, we analyzed the effects of activin and inhibin on human erythroid differentiation. Our results indicate that activin signal transduction mediated through its type I and type II receptors results in an increase in the hemoglobin content of the cells and limits their proliferation. Finally, using cell lines that can be induced to overexpress ActRII and ActRIB or ActRIB only, we show that the inhibin antagonistic effects on activin-induced biological responses are mediated through a competition for the type II activin receptor but also require the presence of an inhibin-specific binding component.

Corticotropin-releasing factor-binding protein ligand inhibitor blunts excessive weight gain in genetically obese Zucker rats and rats during nicotine withdrawal

Elevation of the neuropeptide corticotropin-releasing factor (CRF) in the brain is associated with a reduction of food intake and body weight gain in normal and obese animals. A protein that binds CRF and the related peptide, urocortin, with high affinity, CRF-binding protein (CRF-BP), may play a role in energy homeostasis by inactivating members of this peptide family in ingestive and metabolic regulatory brain regions. Intracerebroventricular administration in rats of the high-affinity CRF-BP ligand inhibitor, rat/human CRF (6-33), which dissociates CRF or urocortin from CRF-BP and increases endogenous brain levels of "free" CRF or urocortin significantly blunted exaggerated weight gain in Zucker obese subjects and in animals withdrawn from chronic nicotine. Chronic administration of CRF suppressed weight gain nonselectively by 60% in both Zucker obese and lean control rats, whereas CRF-BP ligand inhibitor treatment significantly reduced weight gain in obese subjects, without altering weight gain in lean control subjects. Nicotine abstinent subjects, but not nicotine-naive controls, experienced a 35% appetite suppression and a 25% weight gain reduction following acute and chronic administration, respectively, of CRF-BP ligand inhibitor. In marked contrast to the effects of a CRF-receptor agonist, the CRF-BP ligand inhibitor did not stimulate adrenocorticotropic hormone secretion or elevate heart rate and blood pressure. These results provide support for the hypothesis that the CRF-BP may function within the brain to limit selected actions of CRF and/or urocortin. Furthermore, CRF-BP may represent a novel and functionally selective target for the symptomatic treatment of excessive weight gain associated with obesity of multiple etiology.

Pituitary follistatin and inhibin subunit messenger ribonucleic acid levels are differentially regulated by local and hormonal factors

Follistatins, activins, and inhibins are expressed in a wide range of tissues where they function as autocrine and/or paracrine factors. Activin B (beta B beta B) and inhibin B (alpha beta B) are the predominant forms expressed in the rat anterior pituitary. This study was designed to evaluate the regulation of the messenger RNAs (mRNAs) for inhibin alpha and beta B, and follistatin, relative to each other, using cultured rat anterior pituitary cells. Activin A stimulated follistatin (a maximal 4-fold stimulation by 6 h) and beta B (a maximal 1.7-fold stimulation after 2 h) mRNA levels. Although inhibin A dramatically decreased follistatin mRNA levels (34% of the control value after 24 h), it only marginally affected those of beta B (86% of the control value after 2 h). Follistatin inhibited the accumulation of its own mRNA (46% of the control value after 6 h), but had no statistically significant effect on beta B or alpha mRNA levels. Inhibin A was the only treatment that had an effect on alpha mRNA levels, causing a slight decrease (82% of the control value by 24 h). The effects of activin A and inhibin A on follistatin and beta B mRNA levels were dose dependent. Moreover, follistatin and inhibin A blocked the effects of activin A. Immunoneutralization experiments were performed to determine whether locally secreted activin B regulates the expression of these three mRNAs. A monoclonal antibody to activin B reduced follistatin and beta B mRNA levels (37% and 73% of the control value, respectively) and enhanced the stimulatory effect of exogenous activin A on these mRNAs (840% vs. 300% and 170% vs. 145% of the control value, respectively); there was no change in alpha mRNA accumulation. GnRH and activators of the protein kinase A (forskolin) and protein kinase C (12-O-tetradecanoylphorbol acetate) pathways also had differential effects on follistatin, beta B, and alpha mRNA levels. GnRH stimulated follistatin mRNA levels, but suppressed those of beta B. 12-O-Tetraphorbol acetate had no effect on beta B, but stimulated follistatin mRNA levels to the same extent as forskolin. Of these agents, only forskolin produced a marginal inhibitory effect on alpha mRNA accumulation. Testosterone decreased both follistatin and beta B mRNA levels without affecting those of alpha. The results of this study demonstrate that the local production of rat anterior pituitary follistatin, activin B, and inhibin B is regulated by hypothalamic, peripheral, and local factors in such a way that the ratios between activin B and its two inactivators, follistatin and inhibin B, are very tightly maintained.

125I-Tyro-sauvagine: a novel high affinity radioligand for the pharmacological and biochemical study of human corticotropin-releasing factor 2 alpha receptors

Corticotropin-releasing factor (CRF) receptors encoded by two distinct genes have recently been identified and termed CRF1 and CRF2. CRF and the non-mammalian-related peptide sauvagine bind to and activate CRF1 receptors with high affinity and equal potency. Although CRF is significantly weaker at the CRF2 receptor, sauvagine retains its high affinity interactions with this receptor subtype. We expressed the human CRF1 and CRF2 receptor subtypes in stable cell lines and characterized 125I-Tyr0-sauvagine, a high affinity radiolabel suitable for the pharmacological and functional profiles of these proteins. 125I-Tyr0-sauvagine has high affinity (200-400 PM) for CRF1 receptors and demonstrates a pharmacological profile identical to that of 125I-Tyr0-ovine CRF-labeled CRF1 receptors. 125I-Tyr0-sauvagine binding to human CRF2 alpha receptors is saturable and of high affinity (KD = 100-300 PM) and demonstrates guanine nucleotide sensitivity typical of agonist binding to receptors. The pharmacological profile of 125I-Tyr0-sauvagine binding to CRF2 alpha receptors with respect to inhibition by CRF-related analogs is similar to the agonist profile of potencies obtained by measurements of cAMP production stimulated by these analogs in CRF2 alpha expressing cell lines and distinct from the profile of the CRF1 receptor subtype. Thus, the related nonmammalian peptides sauvagine and urotensin have high affinity and rat/ human CRF and ovine CRF have lower affinity for CRF2 receptors labeled with 125I-Tyr0-sauvagine. Because the distribution of CRF1 and CRF2 alpha receptors has been demonstrated to be distinct, suggesting selective functional roles for each receptor subtype, the ability to label CRF2 alpha receptors with 125I-Tyr0-sauvagine in vitro represents a unique opportunity for the discovery of subtype-selective nonpeptide ligands, which would presumably target different aspects of CRF-mediated disorders. We have thus identified and characterized a novel high affinity radioligand for the labeling of CRF2 receptors.

Learning impairment in transgenic mice with central overexpression of corticotropin-releasing factor

The present studies were designed to test the learning and memory capacities of transgenic mice with central overexpression of corticotropin-releasing factor in a forced alternation water T-maze task and in the Morris water maze. In T-maze testing, littermate control mice reached a criterion of 70% correct responses after five days of trials, while the performance of transgenic subjects was still random after the same training. In Morris maze testing, control subjects reached the submerged platform significantly faster (F(1.48) = 4.51, P < 0.05) after three days of trials, while the performance of transgenic mice was unimproved over the same period. The deficit in Morris maze performance in transgenic mice was reversed when the platform was visible above the surface of the water. Pre-test administration of the benzodiazepine anxiolytic, chlordiazepoxide (10 mg/kg), before acquisition training also produced a significant (F(4.40) = 16.61, P < 0.001) and persistent improvement in Morris maze performance in transgenic mice when compared to vehicle-treated transgenic litter mates. Finally, there was no evidence of hippocampal cell loss in transgenic brains. The results suggest that corticotropin-releasing factor-overexpressing mice exhibit a profound learning deficit without sensory or motor-related impairments, and that memory plasticity can be restored by anxiolytic pre-treatment. Thus, constitutive overabundance of brain corticotropin-releasing factor may produce hyperemotionality that interferes with learned behaviors. Stress-related disorders characterized by co-morbid deficits in learning/memory may benefit from pharmacological normalization of brain corticotropin-releasing factor systems.

Molecular cloning of a novel type I receptor serine/threonine kinase for the TGF beta superfamily from rat brain

Growth factors belonging to the TGF beta superfamily bind to and signal through a receptor complex comprising two transmembrane serine/threonine kinases, called type I and type II. Each receptor is responsible for the signaling of the individual TGF beta superfamily members. So far, five type II and six type I receptors have been cloned from mammalian sources. We report here the molecular cloning of a novel type I receptor serine/threonine kinase, ALK7 (activin receptor-like kinase 7), from rat brain. ALK7 shows a significant sequence similarity with TGF beta RI and ActRIB in the intracellular kinase domain and is quite distinct from other type I receptors in the extracellular domain. ALK7 mRNA is expressed in embryonic and in adult rat brain, where it was localized in superficial layers of the forebrain, the CA3 pyramidal subfield of hippocampus, the basal ganglia, the thalamus, and the cerebellar cortex. The functionality of the receptor was demonstrated by the identification of a constitutively active point mutant of ALK7 that activates the TGF beta/activin-responsive reporter without any ligand stimulation. Although the endogenous ligand for ALK7 has yet to be identified, its extensive anatomic distribution in brain, gut, spleen, and lung suggests important roles for this orphan receptor.

Cloning and characterization of human urocortin

Urocortin, a new member of the CRF peptide family which also includes urotensin I and sauvagine, was recently cloned from the rat midbrain. The synthetic replicate of urocortin was found to bind with high affinity to type 1 and type 2 CRF receptors and, based upon its anatomic localization within the brain, was proposed to be a natural ligand for the type 2 CRF receptors. Using a genomic library, we have cloned the human counterpart of rat urocortin and localized it to human chromosome 2. Human and rat urocortin share 95% identity within the mature peptide region. Synthetic human urocortin binds with high affinity to CRF receptor types 1, 2 alpha, and 2 beta, stimulates cAMP accumulation from cells stably transfected with these receptors, and acts in vitro to release ACTH from dispersed rat anterior pituitary cells. In addition, the CRF-binding protein binds human urocortin with high affinity and can prevent urocortin-stimulated ACTH secretion in vitro. The inhibitory effect of the CRF-binding protein on human urocortin can be blocked by biologically inactive CRF fragments, such as CRF(9-33).

Characterization of a sheep brain corticotropin releasing factor binding protein

We report here the identification, purification and cDNA cloning of a corticotropin releasing factor (CRF) binding protein(s) (CRF-BP) from sheep brain. Native sheep and rat brain CRF-BP and recombinant rat CRF-BP were shown to be N-glycosylated. Two membrane associated forms of brain CRF-BPs of 33 and 35 kDa were purified from sheep brain homogenates after solubilization in the presence of detergent. N-Terminal sequence analysis revealed that the 35 kDa protein is proteolytically cleaved near the N-terminus giving rise to an 18 amino acid peptide and a 33 kDa CRF-BP. Both the purified 33 and 35 kDa ovine CRF-BPs could be specifically cross linked to ovine [125I]CRF and human [125I]CRF. In contrast, recombinant rat CRF-BP can only be cross-linked to human [125I]CRF. A 1.7 kb cDNA clone (Basil 7) encoding an open reading frame for a 324 amino acid CRF-BP precursor was cloned from a sheep brain lambda gtlO cDNA library and was shown to have 85% and 87% amino acid homology to the rat and human proteins, respectively. Competitive binding analysis of the recombinant sheep CRF-BP (Basil 7) expressed in CHO cells revealed that it binds human and ovine CRF with high affinity. However, the recombinant sheep CRF-BP (Basil 7) had approximately 50-fold higher affinity for human CRF than for the ovine peptide. These data present the first biochemical proof that CRF-BP is in the brain and provides evidence for the existence of different forms of CRF-BP which have evolved across species to regulate CRF.

Corticotropin-releasing factor (CRF) and glucocorticoids modulate the expression of type 1 CRF receptor messenger ribonucleic acid in rat anterior pituitary cell cultures

Previous studies involving radioreceptor and functional assays have shown that CRF and glucocorticoids are able to modulate CRF receptors of the brain and anterior pituitary. In this study, we analyzed the effects of CRF, vasopressin (AVP), dexamethasone (DEX), and corticosterone on the regulation of CRF receptor (CRF-R1) messenger RNA (mRNA) levels in cultured rat anterior pituitary cells. CRF decreased CRF-R1 mRNA levels in a time- and concentration-dependent manner. In the presence of 10 nM CRF, CRF-R1 mRNA levels decreased within 1 h (to 65 +/- 3% of the control value; P < 0.01) with a maximal effect after 3 h (to 28 +/- 1% of the control value; P < 0.001). The concentration dependence of the inhibitory effect of CRF at 3 h correlated with that required for ACTH secretion (half-maximal at approximately 0.03 nM). Treatment with a maximal (100 nM) dose of AVP or a submaximal (0.1 nM) dose of CRF for 3 h reduced CRF-R1 mRNA levels to 66 +/- 3% and 53 +/- 6% of the control value, respectively. In the presence of both AVP and CRF, CRF-R1 mRNA levels were 32 +/- 3% of the control value. The incubation of cells for 3 h with 10 microM forskolin to activate adenylate cyclase or with 20 nM 12-0-tetradecanoylphorbol-13-acetate to activate protein kinase C resulted in a decrease in receptor mRNA levels to 40 +/- 9% (P < 0.01) and 28 +/- 8% (P < 0.001) of the control value, respectively, suggesting that the effects of CRF and AVP may be mediated by these pathways. DEX (20 nM) also caused a dose- and time-dependent decrease in mRNA levels. Maximal inhibition was observed after 3 h (to 31 +/- 6% of the control value; P < 0.001), with a partial recovery of mRNA levels at 24 or 48 h. Corticosterone similarly inhibited the accumulation of CRF-R1 mRNA in a dose- and time-dependent manner, but, in contrast to DEX, CRF-R1 mRNA levels returned almost to control levels after 24 h. These results indicate that the ability of CRF, AVP, and glucocorticoids to modulate the responses of corticotropes to CRF may be due in part to the actions of these agents on CRF-R1 mRNA accumulation.

Activin increases phosphorylation and decreases stability of the transcription factor Pit-1 in MtTW15 somatotrope cells

Activin is a polypeptide growth factor which exerts endocrine, paracrine, and autocrine effects in a variety of tissues. In the pituitary somatotrope, activin represses proliferation and growth hormone (GH) biosynthesis and secretion. We previously demonstrated that decreases in GH biosynthesis in MtTW15 somatotrope cells are due at least in part to decreased binding of the tissue-specific transcription factor, Pit-1, to the GH promoter, resulting in decreased transcription of the GH gene. The objective of the current study was to determine the extent to which activin-mediated decreases in GH transcription were the result of decreased Pit-1 activity and/or decreased Pit-1 protein content in MtTW15 cells. Activin caused rapid increases in Pit-1 phosphorylation, which were temporally correlated with decreases in GH DNA binding. Pit-1 phosphorylation preceded marked decreases in steady-state levels of Pit-1 protein. The rate of Pit-1 synthesis was only moderately decreased by activin, with a time-course similar to that observed for decreases in GH biosynthesis. However, Pit-1 stability was markedly decreased after more than 4 h of activin treatment. These data demonstrate that activin decreases GH expression in MtTW15 cells through multilevel regulation of Pit-1, which may represent a more general mechanism whereby activin and other transforming growth factor beta family members modulate gene expression through regulation of transcription factor activity as well as content.