Characterisation of Neuropeptide Y Receptor Subtypes by Synthetic NPY Analogues and by Anti-receptor Antibodies

Pancreatic polypeptide is recognized by two hydrophobic domains of the human Y4 receptor binding pocket

Structural characterization of the human Y4 receptor (hY4R) interaction with human pancreatic polypeptide (hPP) is crucial, not only for understanding its biological function but also for testing treatment strategies for obesity that target this interaction. Here, the interaction of receptor mutants with pancreatic polypeptide analogs was studied through double-cycle mutagenesis. To guide mutagenesis and interpret results, a three-dimensional comparative model of the hY4R-hPP complex was constructed based on all available class A G protein-coupled receptor crystal structures and refined using experimental data. Our study reveals that residues of the hPP and the hY4R form a complex network consisting of ionic interactions, hydrophobic interactions, and hydrogen binding. Residues Tyr(2.64), Asp(2.68), Asn(6.55), Asn(7.32), and Phe(7.35) of Y4R are found to be important in receptor activation by hPP. Specifically, Tyr(2.64) interacts with Tyr(27) of hPP through hydrophobic contacts. Asn(7.32) is affected by modifications on position Arg(33) of hPP, suggesting a hydrogen bond between these two residues. Likewise, we find that Phe(7.35) is affected by modifications of hPP at positions 33 and 36, indicating interactions between these three amino acids. Taken together, we demonstrate that the top of transmembrane helix 2 (TM2) and the top of transmembrane helices 6 and 7 (TM6-TM7) form the core of the peptide binding pocket. These findings will contribute to the rational design of ligands that bind the receptor more effectively to produce an enhanced agonistic or antagonistic effect.

A parallel semisynthetic approach for structure-activity relationship studies of peptide YY

The gut hormone peptide YY (PYY) is postprandially secreted from enteroendocrine L cells and is involved in the regulation of energy homeostasis. The N-terminal truncated version PYY(3-36) decreases food intake and has potential as an anti-obesity agent. The anorectic effect of PYY(3-36) is mediated through Y₂ receptors in the hypothalamus, vagus, and brainstem regions, and it is well known that the C-terminal tetrapeptide sequence of PYY(3-36) is crucial for Y2 receptor activation. The aim of this work was to develop a semisynthetic methodology for the generation of a library of C-terminally modified PYY(3-36) analogues. By using an intein-based expression system, PYY(3-29) was generated as a C-terminal peptide α-thioester. Heptapeptides bearing an N-terminal cysteine and modifications at one of the four C-terminal positions were synthesized in a 96-well plate by parallel solid-phase synthesis. In the plate format, an array of [Ala30]PYY(3-36) analogues were generated by ligation, desulfurization, and subsequent solid-phase extraction. The generated analogues, in which either Arg33, Gln34, Arg35, or Tyr36 had been substituted with proteinogenic or non-proteinogenic amino acids, were tested in a functional Y₂ receptor assay. Generally, substitutions of Tyr36 were better tolerated than modifications of Arg33, Gln34, and Arg35. Two analogues showed significantly improved Y₂ receptor selectivity; therefore, these results could be used to design new drug candidates for the treatment of obesity.

Molecular recognition of the NPY hormone family by their receptors

Many G-protein-coupled receptors belong to families of different receptor subtypes, which are recognized by a variety of distinct ligands. We summarize the current state of the art of the multireceptor/multiligand system of the so-called Y-receptor family. This family consists of four G-protein-coupled Y receptors in humans (hY(1), hY(2), hY(4), and hY(5)) and is activated by the so-called neuropeptide Y hormone family, which consists of three native peptide ligands named neuropeptide Y, pancreatic polypeptide, and peptide YY. We recently reported that one conserved aspartate residue in the third extracellular loop is essential for ligand binding in all four Y receptors, but binds the endogenous ligands in a different mode by interacting with different ligand arginine residues. By combining peptide synthesis to obtain chemically modified neuropeptide Y, peptide YY, and pancreatic polypeptide analogs, receptor mutagenesis, and receptor chimeras, we could trace binding and signaling to a molecular level. The data on the variation of the ligands and an overview of the currently known mutagenesis data are summarized and specific models for the binding mode of the three ligands in all four receptors are provided.

Pyridone dipeptide backbone scan to elucidate structural properties of a flexible peptide segment

Whereas the C-terminal fragment of neuropeptide Y (NPY) has been structurally well-defined both in solution and as membrane-bound, detailed structural information regarding the proline-rich N-terminus is still missing. The systematic variation of each position by a conformationally constrained pyridone dipeptide building block within the amino terminal segment of NPY leads to a systematic receptor subtype selectivity of the neuropeptide. Thereby, the systematic dipeptide scan proved superior to the traditional L-Ala scan because it showed how to modify the N-terminus in order to obtain increasingly more Y1 or Y5 receptor selective ligands. NMR and CD spectroscopic analyses were used to characterize the stepwise rigidification of the N-terminus of NPY when up to three dipeptide building blocks were incorporated by solid-phase peptide synthesis. The pyridone dipeptide increases the hydrophobicity of the amino terminus of NPY, and this allows the tuning of the membrane affinity of NPY. The amphiphilic C-terminal helix of 3-fold-substituted NPY thus becomes visible by selective line broadening in the (1)H NMR. Accordingly, we could structurally characterize protein segments that are too flexible for other methods.

NPY signaling through Y1 receptors modulates thalamic oscillations

Neuropeptide Y is the ligand of a family of G-protein coupled receptors (Y(1) to Y(6)). In the thalamus, exogenous and endogenously released NPY can shorten the duration of thalamic oscillations in brain slices from P13 to P15 rats, an in vitro model of absence seizures. Here, we examine which Y receptors are involved in this modulation. Application of the Y(1) receptor agonist Leu(31)Pro(34)NPY caused a reversible reduction in the duration of thalamic oscillations (-26.6+/-7.8%), while the Y(2) receptor agonist peptideYY((3-36)) and the Y(5) receptor agonist BWX-46 did not exert a significant effect. No Y receptor agonist affected oscillation period. Application of antagonists of Y(1), Y(2) and Y(5) receptors (BIBP3226, BIIE0246 and L152,806, respectively) produced results consistent with those obtained from agonists. BIBP3226 caused a reversible disinhibition, an effect that increases oscillation duration (18.2+/-9.7%) while BIIE0246 and L152,806 had no significant effect. Expression of NPY is limited to neurons in the reticular thalamic nucleus (nRt), but Y(1) receptors are expressed in both nRt and adjacent thalamic relay nuclei. Thus, intra-nRt or nRt to relay nucleus NPY release could cause Y(1) receptor mediated inhibition of thalamic oscillations.

Bovine pancreatic polypeptide (bPP) undergoes significant changes in conformation and dynamics upon binding to DPC micelles

The pancreatic polypeptide (PP), a 36-residue, C-terminally amidated polypeptide hormone is a member of the neuropeptide Y (NPY) family. Here, we have studied the structure and dynamics of bovine pancreatic polypeptide (bPP) when bound to DPC-micelles as a membrane-mimicking model as well as the dynamics of bPP in solution. The comparison of structure and dynamics of bPP in both states reveals remarkable differences. The overall correlation time of 5.08ns derived from the 15N relaxation data proves unambiguously that bPP in solution exists as a dimer. Therein, intermolecular as well as intramolecular hydrophobic interactions from residues of both the amphiphilic helix and of the back-folded N terminus contribute to the stability of the PP fold. The overall rigidity is well-reflected in positive values for the heteronuclear NOE for residues 4-34. The membrane-bound species displays a partitioning into a more flexible N-terminal region and a well-defined alpha-helical region comprising residues 17-31. The average RMSD value for residues 17-31 is 0.22(+/-0.09)A. The flexibility of the N terminus is compatible with negative values of the heteronuclear NOE observed for the N-terminal residues 4-12 and low values of the generalized order parameter S(2). The membrane-peptide interface was investigated by micelle-integrating spin-labels and H,2H exchange measurements. It is formed by those residues which make contacts between the C-terminal alpha-helix and the polyproline helix. In contrast to pNPY, also residues from the N terminus display spatial proximity to the membrane interface. Furthermore, the orientation of the C terminus, that presumably contains residues involved in receptor binding, is different in the two environments. We speculate that this pre-positioning of residues could be an important requirement for receptor activation. Moreover, we doubt that the PP fold is of functional relevance for binding at the Y(4) receptor.

A Y2 receptor mimetic aptamer directed against neuropeptide Y

Neuropeptide Y (NPY) is a 36-amino acid neuropeptide that exerts its activity by at least five different receptor subtypes that belong to the family of G-protein-coupled receptors. We isolated an aptamer directed against NPY from a nuclease-resistant RNA library. Mapping experiments with N-terminally, C-terminally, and centrally truncated analogues of NPY revealed that the aptamer recognizes the C terminus of NPY. Individual replacement of the four arginine residues at positions 19, 25, 33, and 35 by l-alanine showed that arginine 33 is essential for binding. The aptamer does not recognize pancreatic polypeptide, a highly homologous Y4 receptor-specific peptide of the gut. Furthermore, the affinity of the aptamer to the Y5 receptor-selective agonist [Ala(31),Aib(32)]NPY and the Y1/Y5 receptor-binding peptide [Leu(31),Pro(34)]NPY was considerably reduced, whereas Y2 receptor-specific NPY mutants were bound well by the aptamer. Accordingly, the NPY epitope was recognized by the Y2 receptor, and the aptamer was highly similar. This Y2 receptor mimicking effect was further confirmed by competition binding studies. Whereas the aptamer competed with the Y2 receptor for binding of [(3)H]NPY with high affinity, a low affinity displacement of [(3)H]NPY was observed at the Y1 and the Y5 receptors. Consequently, competition at the Y2 receptor occurred with a considerably lower K(i) value compared with the Y1 and Y5 receptors. These results indicate that the aptamer mimics the binding of NPY to the Y2 receptor more closely than to the Y1 and Y5 receptors.