NMR structure of the first extracellular domain of corticotropin-releasing factor receptor 1 (ECD1-CRF-R1) complexed with a high affinity agonist.

The corticotropin-releasing factor (CRF) peptide hormone family members coordinate endocrine, behavioral, autonomic, and metabolic responses to stress and play important roles within the cardiovascular, gastrointestinal, and central nervous systems, among others. The actions of the peptides are mediated by activation of two G-protein-coupled receptors of the B1 family, CRF receptors 1 and 2 (CRF-R1 and CRF-R2α,ß). The recently reported three-dimensional structures of the first extracellular domain (ECD1) of both CRF-R1 and CRF-R2ß (Pioszak, A. A., Parker, N. R., Suino-Powell, K., and Xu, H. E. (2008) J. Biol. Chem. 283, 32900-32912; Grace, C. R., Perrin, M. H., Gulyas, J., Digruccio, M. R., Cantle, J. P., Rivier, J. E., Vale, W. W., and Riek, R. (2007) Proc. Natl. Acad. Sci. U.S.A. 104, 4858-4863) complexed with peptide antagonists provided a starting point in understanding the binding between CRF ligands and receptors at a molecular level. We now report the three-dimensional NMR structure of the ECD1 of human CRF-R1 complexed with a high affinity agonist, α-helical cyclic CRF. In the structure of the complex, the C-terminal residues (23-41) of α-helical cyclic CRF bind to the ECD1 of CRF-R1 in a helical conformation mainly along the hydrophobic face of the peptide in a manner similar to that of the antagonists in their corresponding ECD1 complex structures. Unique to this study is the observation that complex formation between an agonist and the ECD1-CRF-R1 promotes the helical conformation of the N terminus of the former, important for receptor activation (Gulyas, J., Rivier, C., Perrin, M., Koerber, S. C., Sutton, S., Corrigan, A., Lahrichi, S. L., Craig, A. G., Vale, W., and Rivier, J. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 10575-10579).

[Pacemaker in MR: absolute contraindication?]. / Pacemaker az MR-ben: abszolút kontraindikáció?

Due to developments in pacemaker technology, implanted pacemakers do not mean an absolute contraindication for MRI examination. However, there are several aspects of MRI examinations that should be considered for safety reasons in pacemaker patients. Based on literature data and own experiments, the safety protocol of MRI examination in pacemaker-implanted patients is described. The interaction of pacemakers--frequently implanted in Hungary--with MR scanners of 0,35 and 1,5 T was studied in vitro. In addition, the cardiac MRI examination of two pacemaker patients is presented. -ICD pacemakers showed strong interaction with static and changing magnetic field that affected pacemaker performance significantly. MRI examination can be safely performed in pacemaker-independent patients. Based on our in vitro and in vivo measurements, MRI examination is still contraindicated in pacemaker-dependent patients. In pacemaker-independent patients blood pressure, ECG monitoring and pulsoximetry are absolutely necessary, in addition, equipment for resuscitation should be available. Pacemaker should be specifically programmed before MRI examination and parameters and functionality should be checked in details afterwards.

Stressin1-A, a potent corticotropin releasing factor receptor 1 (CRF1)-selective peptide agonist.

The potencies and selectivity of peptide CRF antagonists is increased through structural constraints, suggesting that the resulting ligands assume distinct conformations when interacting with CRF1 and CRF2 receptors. To develop selective CRF receptor agonists, we have scanned the sequence -Gln-Ala-His-Ser-Asn-Arg- (residues 30-35 of [DPhe12,Nle21,38]Ac-hCRF4-41) with an i-(i+3) bridge consisting of the Glui-Xaa-Xbb-Lysi+3 scaffold, where residues i=30, 31, and 32. When i=31, stressin1-A, a potent CRF1 receptor-selective agonist was generated. In vitro, stressin1-A was equipotent to h/rCRF to release ACTH. Astressin1-A showed a low nanomolar affinity for CRF1 receptor (Ki=1.7 nM) and greater than 100-fold selectivity versus CRF2 receptor (Ki=222 nM). Stressin1-A released slightly less ACTH than oCRF in adult adrenal-intact male rats, with increased duration of action. Stressin1-A, injected intraperitoneally in rats, induced fecal pellet output (a CRF1 receptor-mediated response) and did not influence gastric emptying and blood pressure (CRF2 receptor-mediated responses).

Characterization of Multisubstituted Corticotropin Releasing Factor (CRF) Peptide Antagonists (Astressins).

CRF mediates numerous stress-related endocrine, autonomic, metabolic, and behavioral responses. We present the synthesis and chemical and biological properties of astressin B analogues {cyclo(30-33)[D-Phe(12),Nle(21,38),C(α)MeLeu(27,40),Glu(30),Lys(33)]-acetyl-h/r-CRF(9-41)}. Out of 37 novel peptides, 17 (2, 4, 6-8, 10, 11, 16, 17, 27, 29, 30, 32-36) and 16 (3, 5, 9, 12-15, 18, 19, 22-26, 28, 31) had k(i) to CRF receptors in the high picomolar and low nanomole ranges, respectively. Peptides 1, 2, and 11 inhibited h/rCRF and urocortin 1-induced cAMP release from AtT20 and A7r5 cells. When Astressin C 2 was administered to adrenalectomized rats at 1.0 mg subcutaneously, it inhibited ACTH release for >7 d. Additional rat data based on the inhibitory effect of (2) on h/rCRF-induced stimulation of colonic secretory motor activity and urocortin 2-induced delayed gastric emptying also indicate a safe and long-lasting antagonistic effect. The overall properties of selected analogues may fulfill the criteria expected from clinical candidates.

Iterative approach to the discovery of novel degarelix analogues: substitutions at positions 3, 7, and 8. Part II.

Degarelix (FE200486, Ac-d-2Nal(1)-d-4Cpa(2)-d-3Pal(3)-Ser(4)-4Aph(l-Hor)(5)-d-4Aph(Cbm)(6)-Leu(7)-ILys(8)-Pro(9)-d-Ala(10)-NH(2)) is a potent and very long acting antagonist of gonadotropin-releasing hormone (GnRH) after subcutaneous administration in mammals including humans. Analogues of degarelix were synthesized, characterized, and screened for the antagonism of GnRH-induced response in a reporter gene assay in HEK-293 cells expressing the human GnRH receptor. The duration of action was also determined in the castrated male rat assay to measure the extent (efficacy and duration of action) of inhibition of luteinizing hormone (LH) release. Structurally, this series of analogues has novel substitutions at positions 3, 7, and 8 and N(alpha)-methylation at positions 6, 7, and 8 in the structure of degarelix. These substitutions were designed to probe the spatial limitations of the receptor's cavity and to map the steric and ionic boundaries. Some functional groups were introduced that were hypothesized to influence the phamacokinetic properties of the analogues such as bioavailability, solubility, intra- or intermolecular hydrogen bond forming capacity, and ability to bind carrier proteins. Substitutions at positions 3 ([N(beta)-(2-pyridyl-methyl)d-Dap(3)]degarelix, IC(50) = 2.71 nM) (5), 7 ([Pra(7)]degarelix, IC(50) = 2.11 nM) (16), and 8 ([N(delta)-(IGly)Orn(8)]degarelix, IC(50) = 1.38 nM) (20) and N-methylation ([N(alpha)-methyl-Leu(7)]degarelix, IC(50) = 1.47 nM) (32) yielded analogues that were equipotent to degarelix (2) in vitro (IC(50) = 1.64 nM) but shorter acting in vivo. Out of the 33 novel analogues tested for the duration of action in this series, two analogues ([N(epsilon)-cyclohexyl-Lys(8)]degarelix, IC(50) = 1.50 nM) (23) and ([N(beta)-(IbetaAla)Dap(8)]degarelix, IC(50) = 1.98 nM) (26) had antagonist potencies and duration of action similar to that of azaline B {inhibited LH (>80%) release for >72 h after sc injection to castrated male rats at a standard dose of 50 mug/rat in 5% mannitol}. Under similar conditions analogues ([N(gamma)-(IGly)Dab(8)]degarelix, IC(50) = 1.56 nM) (21) and ([IOrn(8)]degarelix, IC(50) = 1.72 nM) (18) had a longer duration of action {inhibited LH (>96 h) release} than azaline B; however they were shorter acting than degarelix. Hydrophilicity of these analogues, a potential measure of their ability to be formulated for sustained release, was determined using RP-HPLC at neutral pH yielding analogues with shorter as well as longer retention times. No correlation was found between retention times and antagonist potency or duration of action.

AlphaA-Conotoxin OIVA defines a new alphaA-conotoxin subfamily of nicotinic acetylcholine receptor inhibitors.

The venoms of cone snails are rich in multiply disulfide-crosslinked peptides, the conotoxins. Conotoxins are grouped into families on the basis of shared cysteine patterns and homologous molecular targets. For example, both the kappaA- and alphaA-conotoxin families share the same Class IV Cys pattern (-CC-C-C-C-C-), but differ in their molecular targets. The kappaA-conotoxins are excitatory toxins that purportedly block potassium channels, while the alphaA-conotoxins are paralytic conotoxins that inhibit nicotinic acetylcholine receptors (nAChRs). In this work, we describe the isolation and characterization of a novel Conus peptide from venom milked from Hawaiian specimens of Conus obscurus. This peptide shares the Class IV Cys pattern but differs from both previously characterized alphaA- and kappaA-conotoxins in the spacing of amino acids between Cys resides. However, the peptide is similar to previously characterized alphaA-conotoxins in its paralytic effects on fish and its antagonist activity on the neuromuscular nAChR. Unexpectedly, the peptide differs in its disulfide bonding from alphaA-conotoxin PIVA. We have named this unique peptide alphaA-conotoxin OIVA, and we consider it the defining member of a subfamily of alphaA-conotoxins that we designate the alphaA(1-3)-conotoxins to identify them by their unique disulfide bonding framework. These results indicate that the alphaA-conotoxin family is both more structurally diverse and broadly distributed than previously believed.

Mammalian neuronal sodium channel blocker µ-conotoxin BuIIIB has a structured N-terminus that influences potency.

Among the µ-conotoxins that block vertebrate voltage-gated sodium channels (VGSCs), some have been shown to be potent analgesics following systemic administration in mice. We have determined the solution structure of a new representative of this family, µ-BuIIIB, and established its disulfide connectivities by direct mass spectrometric collision induced dissociation fragmentation of the peptide with disulfides intact. The major oxidative folding product adopts a 1-4/2-5/3-6 pattern with the following disulfide bridges: Cys5-Cys17, Cys6-Cys23, and Cys13-Cys24. The solution structure reveals that the unique N-terminal extension in µ-BuIIIB, which is also present in µ-BuIIIA and µ-BuIIIC but absent in other µ-conotoxins, forms part of a short α-helix encompassing Glu3 to Asn8. This helix is packed against the rest of the toxin and stabilized by the Cys5-Cys17 and Cys6-Cys23 disulfide bonds. As such, the side chain of Val1 is located close to the aromatic rings of Trp16 and His20, which are located on the canonical helix that displays several residues found to be essential for VGSC blockade in related µ-conotoxins. Mutations of residues 2 and 3 in the N-terminal extension enhanced the potency of µ-BuIIIB for NaV1.3. One analogue, [d-Ala2]BuIIIB, showed a 40-fold increase, making it the most potent peptide blocker of this channel characterized to date and thus a useful new tool with which to characterize this channel. On the basis of previous results for related µ-conotoxins, the dramatic effects of mutations at the N-terminus were unanticipated and suggest that further gains in potency might be achieved by additional modifications of this region.

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

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

Discovery, synthesis, and structure activity of a highly selective alpha7 nicotinic acetylcholine receptor antagonist.

Nicotinic acetylcholine receptors (nAChRs) that contain an alpha7 subunit are widely distributed in neuronal and nonneuronal tissue. These receptors are implicated in the release of neurotransmitters such as glutamate and in functions ranging from thought processing to inflammation. Currently available ligands for alpha7 nAChRs have substantial affinity for one or more other nAChR subtypes, including those with an alpha1, alpha3, alpha6, and/or alpha9 subunit. An alpha-conotoxin gene was cloned from Conus arenatus. Predicted peptides were synthesized and found to potently block alpha3-, alpha6-, and alpha7-containing nAChRs. Structure-activity information regarding conotoxins from distantly related Conus species was employed to modify the C. arenatus derived toxin into a novel, highly selective alpha7 nAChR antagonist. This ligand, alpha-CtxArIB[V11L,V16D], has low nanomolar affinity for rat alpha7 homomers expressed in Xenopus laevis oocytes, and antagonism is slowly reversible. Kinetic analysis provided insight into the mechanism of antagonism. alpha-CtxArIB interacts with five ligand binding sites per alpha7 receptor, and occupation of a single site is sufficient to block function. The peptide was also shown to be highly selective in competition binding assays in rat brain membranes. alpha-CtxArIB[V11L,V16D] is the most selective ligand yet reported for alpha7 nAChRs.

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

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

Definition and characterization of the short alphaA-conotoxins: a single residue determines dissociation kinetics from the fetal muscle nicotinic acetylcholine receptor.

We report the definition and characterization of a conotoxin subfamily, designated the short alphaA-conotoxins (alphaA(S)) and demonstrate that all of these share the unique property of selectively antagonizing the fetal subtype of the mammalian neuromuscular nicotinic acetylcholine receptor (nAChR). We have characterized newly identified alphaA(S)-conotoxins from Conus pergrandis and have conducted a more detailed characterization of alphaA-conotoxins previously reported from additional Conus species. Among the results, the characterization of the short alphaA-conotoxins revealed diverse kinetics of a block of the fetal muscle nAChR, particularly in dissociation rates. The structure-function relationships of native alphaA(S)-conotoxins and some analogues revealed a single amino acid locus (alternatively either His or Pro in native peptides) that is a critical determinant of the dissociation kinetics. The unprecedented binding selectivity for the fetal muscle nAChR, coupled with the kinetic diversity, should make alphaA(S)-conotoxins useful ligands for a diverse set of studies. The rapidly reversible peptides may be most suitable for electrophysiological studies, while the relatively irreversible peptides should be most useful for binding and localization studies.

Synthesis, in vivo and in vitro biological activity of novel azaline B analogs.

Several azaline B analogs (2-10) were synthesized and evaluated for their ability to antagonize GnRH in vitro and for duration of action in inhibiting luteinizing hormone secretion in a castrated male rat assay in vivo. Analogs, 8 (IC(50) = 1.85 nM), and 9 (IC(50) = 1.78 nM), are equipotent with azaline B (1, IC(50) = 1.36 nM) in vitro. Whereas 9 is short acting, 8 is as long acting as azaline B. Other analogs have IC(50) greater than 2.0 nM and are all short acting.

KappaM-conotoxin RIIIK, structural and functional novelty in a K+ channel antagonist.

Venomous organisms have evolved a variety of structurally diverse peptide neurotoxins that target ion channels. Despite the lack of any obvious structural homology, unrelated toxins that interact with voltage-activated K(+) channels share a dyad motif composed of a lysine and a hydrophobic amino acid residue, usually a phenylalanine or a tyrosine. kappaM-Conotoxin RIIIK (kappaM-RIIIK), recently characterized from the cone snail Conus radiatus, blocks Shaker and TSha1 K(+) channels. The functional and structural study presented here reveals that kappaM-conotoxin RIIIK blocks voltage-activated K(+) channels with a novel pharmacophore that does not comprise a dyad motif. Despite the quite different amino acid sequence and no overlap in the pharmacological activity, we found that the NMR solution structure of kappaM-RIIIK in the C-terminal half is highly similar to that of mu-conotoxin GIIIA, a specific blocker of the skeletal muscle Na(+) channel Na(v)1.4. Alanine substitutions of all non-cysteine residues indicated that four amino acids of kappaM-RIIIK (Leu1, Arg10, Lys18, and Arg19) are key determinants for interaction with K(+) channels. Following the hypothesis that Leu1, the major hydrophobic amino acid determinant for binding, serves as the hydrophobic partner of a dyad motif, we investigated the effect of several mutations of Leu1 on the biological function of kappaM-RIIIK. Surprisingly, both the structural and mutational analysis suggested that, uniquely among well-characterized K(+) channel-targeted toxins, kappaM-RIIIK blocks voltage-gated K(+) channels with a pharmacophore that is not organized around a lysine-hydrophobic amino acid dyad motif.