Discovery and characterization of taspoglutide, a novel analogue of human glucagon-like peptide-1, engineered for sustained therapeutic activity in type 2 diabetes.

AIM: Glucagon-like peptide-1 (GLP-1) receptor agonists for the treatment of type 2 diabetes are administered by daily injection because of short plasma half-lives, which result partly from the biochemical instability of these peptides. There is a medical need for GLP-1 analogues that can be administered less frequently for patient convenience. METHODS: We synthesized a series of human GLP-1 (hGLP-1(7-36)NH(2) ) derivatives containing α-aminoisobutyric acid (Aib) substitutions, analysed their enzymatic stabilities and evaluated their secondary structures using circular dichroism (CD) and nuclear magnetic resonance (NMR). RESULTS: Plasma stability experiments showed that only the analogue containing Aib substitutions in both the N-terminus (position 8) and the C-terminus (position 35), [Aib8(,)³5]hGLP-1(7-36)NH2 (BIM-51077), was fully resistant to enzymatic cleavage. Incubation with human plasma kallikrein or plasmin confirmed that the Aib substitution at position 35 prevented protease cleavage around this residue, which contributes to the significantly enhanced plasma stability and increased plasma half-life. CD revealed increased C-terminal α-helicity in Aib³5-substituted analogues compared with both hGLP-1(7-36)NH2 and analogues containing only Aib8 substitutions. Based on NMR studies, the secondary structure of BIM-51077 is similar to hGLP-1(7-36)NH2 with a slight increase in α-helicity in the C-terminus. Compared with hGLP-1(7-36)NH2, BIM-51077 had similar binding affinity for the human GLP-1 receptor and activated this receptor with similar potency. CONCLUSIONS: We have discovered an Aib8(,)³5-substituted analogue of native hGLP-1(7-36)NH2 (BIM-51077) that retains the structure of the native peptide, and has similar activity and enhanced stability. A sustained-release formulation of this molecule (taspoglutide) is in phase-3 clinical development.

Structure of cobra cardiotoxin CTX I as derived from nuclear magnetic resonance spectroscopy and distance geometry calculations.

The structure of cardiotoxin CTX I from Naja naja atra has been investigated by NMR spectroscopy. Sequence specific resonance assignments have been obtained for all backbone protons as well as for most side-chain protons. Distance geometry calculations were carried out using a metric matrix DG program. A total of 715 NOE constraints, 27 phi angle constraints and a list of the hydrogen bond donors were used for the metric matrix DG calculations and refinement. The average pairwise r.m.s.d. of the resulting structures was 1.01 A for the backbone heavy atoms, and 1.69 A for all heavy atoms. The protein is rich in beta structure and consists of a large triple-stranded, antiparallel beta sheet as well as a short double-stranded, antiparallel beta sheet. Non-regular hydrogen bonding is found between side-chains of the carboxy-terminal end and the rest of the core region. The structure is discussed in terms of evolutionary aspects as well as recent investigations about the biological function and active site.

Molecular structure of the cyanobacterial tumor-promoting microcystins.

The three-dimensional structure of the two hepatotoxic microcystins LR and LY has been determined by two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy and distance geometry calculations. For the microcystin LY a single family of highly convergent structures was obtained. This family is characterized by a relatively compact boat-like ring structure with the large side chain of the Adda residue protruding from the concave side, in close proximity to the Tyr side chain. Conversely, for the microcystin LR the calculations result in three conformational families characterized by an even more compact ring structure. The Adda and Arg side chains protrude from the ring distal from one another caused by the repulsion between the guanido function of Arg and the hydrophobic Adda. The lower toxicity of the LY microcystin could result from the restricted access of the Adda side chain, an essential residue for activity, which results from the close proximity of the aromatic Tyr residue. A significant enthalpic cost would be expected for disturbance of this hydrophobic collapse and correspondingly lower binding affinity to receptor molecules would be predicted. From the structures of the two related microcystins, and homology with other known toxins, we propose a working hypothesis of the Adda side chain interacting with a hydrophobic pore of the phosphatases while the rest of the microcystin acts as a scaffold to help stabilize the interdigitation of the Adda with additional intermolecular interactions.

Parathyroid hormone and parathyroid hormone-related protein: model systems for the development of an osteoporosis therapy.

The parathyroid hormone (PTH) plays a vital role in the homeostasis of calcium within the blood stream. Given its unique ability to increase bone density, an understanding of the molecular mechanism by which the hormone is recognized and binds to its receptor should provide targets for the development of PTH-based, anabolic agents for the treatment of osteoporosis. Parathyroid hormone related protein (PTHrP), a genetically and structurally distinct hormone which displays similar binding and activation profiles as PTH, has greatly facilitated the effort to establish a structure-biological function relationship by allowing for direct comparisons. In an analogous manner, the presence of two receptors, PTH/PTHrP (PTH1) and PTH2, which differ in their ligand selectivity (PTH2 is activated by PTH, not PTHrP) has provided a unique vehicle for probing the structural motifs of the receptor required for ligand recognition and binding. Recent photo-affinity cross-linking studies of PTH and PTHrP binding to PTH1 have produced direct points of contact between the ligand and receptor. Here, we review each of the components involved in this important hormone system, with particular emphasis on the structural features of each: the ligands (PTH and PTHrP), the receptors (PTH1 and PTH2), and the interaction between ligand and receptor. Although the current understanding of the molecular mechanism of ligand binding and receptor activation does not allow for the rational design of drug candidates, and indeed contains much conjecture, significant strides have been made towards this end.

Threonine6-bradykinin: molecular dynamics simulations in a biphasic membrane mimetic.

The natural peptide [Thr6]-bradykinin, Arg1-Pro2-Pro3-Gly4-Phe5-Thr6-Pro7-Phe8-Arg9, has been conformationally examined by molecular dynamics simulations using a two-phase box consisting of H2O and CCl4 to mimic the micellar environment utilized in the 1H-NMR studies. The different conformations generated from distance geometry calculations were refined with extensive molecular dynamics simulations. The resulting conformations provide additional structural insight into the differing biological activities of native bradykinin and [Thr6]-bradykinin, produced by the one conservative substitution Thr6 for Ser6. In addition, the simulations give some indication of the interaction of the peptide with the biphasic, hydrophilic/hydrophobic environment of the micelle. Such information is vital given the accumulating data indicating that the peptide first interacts with the membrane before the membrane-bound receptor. The structures of membrane-bound [Thr6]-bradykinin developed here provide experimental support for the interaction of residues 7 and 8 with the core of the membrane-bound receptor and the N-terminus and C-terminal arginine interacting with the extracellular portion of the receptor.

Vancomycin: conformational consequences of the sugar substituent.

High-resolution, three-dimensional structures of vancomycin and aglyco-vancomycin in DMSO were determined by nuclear magnetic resonance, metric matrix distance geometry, and molecular dynamics calculations. Conformational flexibility fast on the NMR time scale was examined by ensemble-based calculations which apply the experimentally derived restraints as an ensemble average. Two families of conformations of vancomycin, differing in the positioning of the vancosamine substituent, were observed. In contrast, the aglyco-vancomycin adopts only one conformation in solution. The conformations of vancomycin and the aglyco-vancomycin differ in the alignment of the amide protons which participate in the hydrogen-bonding network with the cell-wall precursor and orientation of the aromatic rings relative to the backbone. Therefore, the high-resolution structural characterization provides insight into a possible role of glycosylation on the activity of this important family of antibiotics.

Probing the topological arrangement of the N- and C-terminal residues of bradykinin for agonist activity at the B1 receptor.

The conformational features of H-Lys-Arg-Ado-Ser-Pro-Phe-OH (Ado = 12-aminododecanoic acid), a des-Arg(9) analogue of Lys-bradykinin, have been determined by high-resolution NMR in the presence of a zwitterionic lipid environment. The analogue is the most active member of a series of analogues designed to probe the topological arrangement of the N- and C-termini required for agonistic activity at the B1 kinin receptor. A novel computational procedure for the utilization of NOE constraints from cis and trans configurational isomers is illustrated. Only with this computational methodology could the structural features of the N-terminus of the peptide be determined. Using radical-induced relaxation of the (1)H NMR signals, we measured the topological orientation of the peptide with respect to the zwitterionic lipid interface. The results indicate that the long, alkyl chain of the Ado amino acid imbeds into the lipid surface. The structural features of the C-terminus of the B1-selective analogue consist of a well-defined turn. Although removed from a standard beta-turn, required for activity at the B2 kinin receptor, the topological orientation of the side chains of the des-Arg(9) compound are surprisingly similar to those previously observed for beta-turn-containing bradykinin analogues. Therefore, we attribute the high B1 receptor selectivity, observed upon removal of Arg(9) from bradykinin, solely to the loss of a charged amino acid and not to altered structural features.

Threonine6-bradykinin: structural characterization in the presence of micelles by nuclear magnetic resonance and distance geometry.

The conformation of the natural peptide [Thr6]-bradykinin, Arg1-Pro2-Pro3-Gly4-Phe5-Thr6-Pro7-Phe8-Arg9, is investigated by NMR spectroscopy and computer simulations in an aqueous solution of sodium dodecyl sulfate micelles. The structural analysis of the peptide is of particular interest since it displays a different biological profile from bradykinin despite the high sequence homology (only one conservative substitution: Ser6/Thr6) and the fact that both peptides bind and activate common receptors. The SDS micelles provide a model system for the membrane-interface environment the peptide experiences when interacting with the membrane-embedded receptor and allow for the conformational examination of the peptide using high-resolution NMR techniques. The NMR spectra show that the micellar system induces a secondary structure in the otherwise inherently flexible peptide (as observed in benign aqueous solution). The distance geometry calculations indicate a beta-turn of type I about residues 7-8 as the preferred conformation. The results of ensemble calculations reveal conformational changes occurring rapidly on the NMR time scale and allow for the identification of three different families of conformations that average to reproduce the NMR observables. The three families differ in the type of conformation adopted at the C-terminus: type I beta-turn, type II beta-turn and a third conformation, intermediate between the two beta-turns. The structural results support the hypothesis of the determining role of the C-terminal conformation for biological activity and can provide an explanation of the different activities observed for bradykinin and [Thr6]-bradykinin.

Conformational studies of RS-66271, an analog of parathyroid hormone-related protein with pronounced bone anabolic activity.

Both the parathyroid hormone (PTH) and the functionally similar parathyroid hormone-related protein (PTHrP) have served as templates for the development of novel bone anabolic agents for the treatment of osteoporosis. The PTHrP analog RS-66271 (Vickery, B. H.; Avnur, Z.; Cheng Y.; Chiou, S.-S.; Leaffer, D.; Caulfield, J. P.; Kimmel, D. B.; Ho, T.; Krstenansky, J. L. J. Bone Miner. Res. 1996, 11, 1943-1951), in which the amino acids 22-31 have been substituted by the sequence E22-L-L-E-K-L-L-E-K-L31 (a model amphiphilic peptide), is a potent bone anabolic agent in vivo. Therefore, RS-66271 is a good candidate for structural analysis with the aim of developing a structure-activity relationship. The structural characterization described here was carried out in aqueous solution employing circular dichroism and nuclear magnetic resonance spectroscopy. We find that the incorporated amphiphilic decapeptide is indeed helical. In addition, it induces the adjacent residues, up to residue 16, to adopt the helical conformation. The helical domain, including residues 16-32, incorporates most of the previously identified principal receptor binding domain PTHrP(25-34). We discuss the relevance of the distinct and extensive helicity in light of the reduced in vitro receptor affinity/ activity and the enhanced in vivo bone anabolic efficacy of RS-66271.

Comparison of the conformation of active and nonactive backbone cyclic analogs of substance P as a tool to elucidate features of the bioactive conformation: NMR and molecular dynamics in DMSO and water.

The conformations of two backbone-cyclized substance P analogs as derived from 1H NMR and molecular dynamics simulations carried out in DMSO and water are described. The method of floating chiralities is used in the simulations to facilitate the diastereotopic assignment of methylene protons. One of the analogs, cyclo-[-(CH2)3-NH-CO-(CH2)4-Arg-Phe-Phe-N-]-CH2-CO-Leu-Met-NH2, is a highly active, selective agonist for the NK-receptor, while the other, cyclo[-(CH2)2-NH-CO-(CH2)2-Gly-Arg-Phe-Phe-N-]-CH2-CO-Leu-Met-NH2, is inactive. Both analogs contain cyclic ring systems of the same size, varying in only the number of amide linkages. From the conformational analysis, the lack of activity can be attributed to the introduction of too much constraint into the ring system. This has an effect on the topological array of the important residues Arg-Phe-Phe. The results presented here are compared with biologically active analogs previously examined. The differences between conformations of active and inactive compounds are used to develop insight into the conformational requirements for biological activity.

Agonist activity at the kinin B1 receptor: structural requirements of the central tetrapeptide.

A series of analogues of desArg(9)-Lys-bradykinin (BK), Lys-Arg-X-Ac(n)c-X-Ser-Pro-Phe, in which the spacer X-Ac(n)c-X replaces the central tetrapeptide Pro-Pro-Gly-Phe of BK, have been synthesized and functionally characterized at the B1 receptor. The 1-aminocycloalkane-1-carboxylic acids (Ac(6)c, Ac(7)c, Ac(8)c, Ac(9)c, Ac(12)c) were incorporated to impart conformational constraint and probe the importance of the hydrophobicity of the residue in the central position. The linker is varied in length (X = Gly, betaAla, gammaAbu) to examine the optimal distance between the biologically important residues at the N- and C-termini. The biological assays indicate that the optimal length is obtained with X = Gly, with reduced activities for the longer linkers. Although the size of the central cyclic amino acid does not significantly alter the biological activity, the hydrophobic residue Ac(n)c which may tether the peptide in the membrane environment is required (Lys-Arg-Gly-Gly-Gly-Ser-Pro-Phe is inactive). Two of the analogues, Lys-Arg-Gly-Ac(7)c-Gly-Ser-Pro-Phe and Lys-Arg-gammaAbu-Ac(7)c-gammaAbu-Ser-Pro-Phe, have been structurally characterized in the presence of a zwitterionic lipid environment by high-resolution NMR. Both compounds have similar structural features, differing greatest in the distance between the termini (9 and 15 A for the Gly- and gammaAbu-containing analogues, respectively). The correlation of the smaller distance with activity at the B1 receptor is in complete accord with the results from our previous examination of Lys-Arg-NH-(CH(2))(11)-CO-Ser-Pro-Phe. With the results from this series of compounds we are beginning to define some of the molecular descriptors important for activity at the B1 BK receptor.

Characterization of the molecular motions of constitutively active G protein-coupled receptors for parathyroid hormone.

The molecular mechanism of constitutive activity of the G protein-coupled receptor for human parathyroid hormone (PTH1) has been examined by molecular dynamics (MD) simulations. The single point mutations H223R, T410P, and I458R, of the PTH1 receptor result in ligand-independent receptor activation. Extensive MD simulations indicate that each of the mutations, through different mechanisms, lead to very similar conformational changes of the third intracellular loop. The structural changes, centered on K405 in the C-terminus of the third intracellular loop, can be traced back to the single-point mutations by calculation of the forces and torques responsible for the collective motions of the receptor. This analysis indicates a direct correlation between the conformational preferences of the cytoplasmic loop and the mutations in different locations of the receptor: TM2 (H223R), TM6 (T410P), and TM7 (1458R). Given the pivotal role of the third intracellular loop of PTH1 in coupling to the G proteins, the structural changes induced by these single-point mutations may be responsible for the ligand-free activation of the receptor. These results coupled with the high-resolution structure of the third cytoplasmic loop of PTH1, previously determined in our laboratory, provide unique insight into the mechanism of ligand free activation of the PTH1 receptor.

Structural characterization of the parathyroid hormone receptor domains determinant for ligand binding.

Over the years, the association of peptide ligands to Family B GPCRs (G-protein coupled receptors) has been characterized by a number of experimental and theoretical techniques. For the PTH (parathyroid hormone) ligand-receptor system, important insight has been provided by photoaffinity labelling experiments and the elucidation of direct contact points between ligand and receptor. Our research has focused on the structural elucidation of the receptor domains shown to be involved in the binding of PTH. Employing a combination of carefully designed receptor domains, solution-state NMR carried out in the presence of membrane mimetics and extensive computer simulations, we have obtained a well-resolved model of the ligand-receptor complex for PTH. Here, we review the development of this model and highlight some inherent limitations of the methods employed and their consequences on interpretation of the ligand-receptor model.

Characterization of interactions of Na+/H+ exchanger regulatory factor-1 with the parathyroid hormone receptor and phospholipase C.

The Na(+)/H(+) exchanger regulatory factor-1 (NHERF1) is a molecular scaffold important for the signaling of the G-protein coupled receptor for the parathyroid hormone (PTH1R). The two PDZ (PSD-95, Discs-large, ZO1) domains of NHERF1 through association with the C-termini of PTH1R and phospholipase C enhance the signaling pathway associated with PTH. To examine these interactions, we have produced the individual PDZ1 and PDZ2 domains as well as the tandem PDZ1-PDZ2 domains (PDZ12) of NHERF1 and have characterized the binding affinities of the C-terminal motifs of PTH1R and PLCbeta using fluorescence anisotropy. Circular dichroism indicates that the PDZ1 and PDZ2 are properly folded. Based on fluorescence anisotropy we find that the C-terminus of PTH1R, containing ETVM, has similar affinities (approximately 10 microm) for both PDZ1 and PDZ2. The PTH1R displayed reduced binding affinity for the tandem PDZ12 (16 microm) compared with the individual domains or a solution of equal molar concentrations of PDZ1 and PDZ2 (5.8 microm), suggesting negative cooperativity between the PDZ domains or intervening region. The C-termini of PLCbeta (both beta1 and beta2 isozymes were examined, containing DTPL and ESRL, respectively) displayed a diminished affinity for PDZ2 (approximately 30 microm) over that of PDZ1 (approximately 8 microm). Finally, we demonstrate trans PDZ1-PDZ2 association that is enhanced in the presence of the C-terminus of PTH1R or PLCbeta, suggesting oligomerization of NHERF as a mode for enhancing the signaling associated with PTH.

Conformational analysis by NMR and distance geometry techniques of a peptide mimetic of the third helix of the Antennapedia homeodomain.

The Antennapedia homeodomain structure consists of four helices. The helices II and III are connected by a tripeptide that forms a turn, and constitute the well-known helix-turn-helix motif. The recognition helix penetrates the DNA major groove, gives specific protein-DNA contacts and forms direct, or water-mediated, intermolecular hydrogen bonds. It was suggested that helix III (and perhaps also helix IV) might represent the recognition helix of Antennapedia homeodomain, which makes contact with the surface of the major groove of the DNA. In an attempt to clarify the helix III capabilities of assuming an helical conformation when separated from the rest of the protein, we carried out the structural determination of the recognition helix III in different solvent media. The conformational study of fragments 42-53, where residues W48 and F49, not involved in the protein-DNA interaction, were substituted by two alanines, was conducted in sodium dodecyl sulfate (SDS), trifluoroethanol (TFE) and TFE/water, using circular dichroism, nuclear magnetic resonance (NMR) and distance geometry (DG) techniques. The fragment assumes a well-defined secondary structure in TFE and in TFE/water (90/10, v/v) with an alpha-helix encompassing residues 4-9, while in TFE/water (70/30, v/v) a less regular structure was found. The DG results in the micellar system evidence the presence of a distorted alpha-helical conformation involving residues 4-8. Our results reveal that the isolated Antennapedia recognition helix III tend to preserve in solution the alpha-helical conformation even if separated from the rest of the molecule.

Combined use of homo- and heteronuclear coupling constants as restraints in molecular dynamics simulations.

A penalty function for scalar coupling constants has been applied in molecular dynamics simulations as an experimental constraint. The function is based on the difference between the coupling constant calculated from the dihedral angle and the experimentally measured coupling constant. The method is illustrated on a model cyclic pentapeptide for which 3JHN-H alpha and 3JHN-C beta, both about the phi backbone dihedral angle, have been measured. The function is efficient in producing structures consistent with the scalar couplings, but removed from the conformation observed in solution. This arises from the lack of J restraints for the psi dihedral angle. Simulations with both nuclear Overhauser effect (NOE) and J-coupling restraints illustrates small but significant differences from simulations using only NOEs.

Improved molecular dynamics simulations for the determination of peptide structures.

In this article a few methods or modifications proven to be useful in the conformational examination of peptides and related molecules by molecular dynamics are illustrated. The first is the explicit use of organic solvents in the simulations. For many cases such solvents are appropriate since the nmr measurements (or other experimental observations) were carried out in the same solvent. Here, the use of dimethylsulfoxide and chloroform in molecular dynamics is described, with some advantages of the use of these solvents high-lighted. A constant allowing for the scaling of the nonbonded interactions of the force field, an idea previously employed in distance geometry and simulated annealing, has been implemented. The usefulness of this method is that when the nonbonded term is turned to zero, atoms can pass through each other, while the connectivity of the molecule is maintained. It will be shown that such simulations, if a sufficient driving force is present (i.e., nuclear Overhauser effects restraints), can produce the correct stereoconfiguration (i.e., chiral center) as well as configurational isomer (i.e., cis/trans isomers). Lastly, a penalty term for coupling constants directly related to the Karplus curve has been implemented into the potential energy force field. The advantages of this method over the commonly used dihedral angle restraining are discussed. In particular, it is shown that with more than one coupling constant about a dihedral angle a great reduction of the allowed conformational space is obtained.

Peptide hormone binding to G-protein-coupled receptors: structural characterization via NMR techniques.

G-protein-coupled receptors (GPCRs) allow cells to respond to calcium, hormones, and neurotransmitters. Not surprisingly, they currently make up the largest family of validated drug targets. Rational drug design for molecular regulators targeting GPCRs has been limited to theoretical-based computational approaches. X-ray crystallography of intact GPCRs has provided the topological orientation of the seven transmembrane helices, but limited structural information of the extracellular and intracellular loops and protein termini. In this review we detail an NMR-based approach which provides the high-resolution structural features on the extracellular domains of GPCRs and the ligand/receptor complexes formed upon titration of the peptide hormone. The results provide important contact points and a high-resolution description of the ligand/receptor interactions, which may be useful for the rational design of therapeutic agents targeting GPCRs. Recent results from our investigation of the cholecystokinin peptide hormone system are used to highlight this approach.

Intermolecular interactions between cholecystokinin-8 and the third extracellular loop of the cholecystokinin A receptor.

The interaction of the C-terminal octapeptide of cholecystokinin, CCK-8, with the third extracellular loop of human cholecystokinin-A receptor, CCK(A)-R(329-357), has been probed by high-resolution NMR and extensive computer simulations. The structure of CCK(A)-R(329-357) in the presence of dodecylphosphocholine micelles consists of three alpha-helices, with the first and third corresponding to the extracellular ends of transmembrane (TM) helices 6 and 7. The central helix, residues W335-R345, is found to lie on the zwitterionic surface. Titration with CCK-8 produces a stable complex with a number of intermolecular NOEs between the C-terminus of the ligand (Trp(30), Met(31), Asp(32)) and the interface of TM6 and the third extracellular loop (N333, A334, Y338) of the receptor fragment. The mode of ligand binding based on these intermolecular NOEs is in agreement with a number of published findings from receptor mutagenesis and photoaffinity cross-linking. Utilizing these ligand/receptor points of interaction, the structural features of CCK(A)-R(329-357), and also the structures of CCK-8 and CCK(A)-R(1-47) previously determined, extensive molecular dynamics simulations of the CCK-8/CCK(A)-R complex were carried out. The results provide unique insight into the molecular interactions and forces important for the binding of CCK-8 to CCK(A)-R.

Structural characterization of peptide hormone/receptor interactions by NMR spectroscopy.

The structural characterization of peptide hormones and their interaction with G-protein (guanine nucleotide-binding regulatory protein) coupled receptors by high-resolution nmr is described. The general approaches utilized can be categorized into three different classes based on their target: the ligand, the receptor, and the ligand/receptor complex. Examples of these different approaches, aimed at facilitating the rational design of peptides and peptidomimetics with improved pharmacological profiles, based on work carried out in our own laboratory, are given. In the ligand-based approach, the high-resolution structures of bradykinin analogues allowing for the development of a structure-activity relationship for activation of the B1 receptor are described. Studies targeting the receptor are to a large extent theoretical, based on computational molecular modeling. However, experimentally based structural features provided by high-resolution nmr can be used to great advantage, providing insight into the mechanism of receptor function, as illustrated here with results from parathyroid hormone. A similar combination of theoretical methods, supplemented by high-resolution structures from nmr has been utilized to probe the formation and stabilization of the ligand/receptor complex both for parathyroid hormone and cholecystokinin. In each of these three approaches, the importance of well-designed peptide mimetics and accurate structural analysis by high-resolution nmr, will be highlighted.