Structure activity studies of mast cell activation and hypotension induced by neuropeptide Y (NPY), centrally truncated and C-terminal NPY analogues

1. Neuropeptide-induced histamine release is thought to occur via receptor-independent mechanisms, with net charge and lipophilicity being important factors. 2. In this study, the histamine releasing ability of neuropeptide Y (NPY), two C-terminal segments of NPY and 13 centrally truncated NPY analogues was examined. These results were compared with the ability of the peptides to bind to the Y2 receptor in the rabbit kidney membrane model and with their hypotensive actions in the anaesthetized-rat model. 3. All analogues tested, with the exception of [Glu4,25,33,35]-NPY(1-4)-Ahx-(25-36) and [Asp4,25,33,35]NPY(1-4)-Ahx-(25-36) which were devoid of histamine releasing activity, evoked a dose-dependent histamine release but there were marked differences between the peptides. The native peptide was the least active. 4. Histamine release was not linked to the ability of the peptides to displace NPY from Y2 receptors. There was a statistical correlation between the hypotensive effects expressed as ED10 values (mumol kg-1, which induced a blood pressure decrease of 10 mmHg) and the EC25 for histamine release (r = 0.62, P = 0.04), although histamine release may not be the sole determinant of the alterations in blood pressure. 5. There was a strong negative correlation between EC25 for histamine release and net positive charge (r = -0.93, P = 5.7 x 10(-7), i.e. increasing the net positive charge caused greater histamine release. However, there was a 12 fold difference in activity amongst the most positively charged analogues (+5). Helicity did not correlate with histamine releasing ability. 6. In the development of NPY-related drugs the avoidance of compounds with net positive charge is recommended.

Inhibition of Ca2+ channel currents in human neuroblastoma (SH-SY5Y) cells by neuropeptide Y and a novel cyclic neuropeptide Y analogue

Whole-cell Ca2+ channel currents were recorded in human neuroblastoma (SH-SY5Y) cells, using conventional and perforated-patch techniques. Neuropeptide Y (NPY, 10-1000 nM) caused concentration-dependent inhibition of Ca2+ channel current amplitudes which was pertussis toxin-sensitive, voltage-dependent and associated with slowing of channel activation kinetics, regardless of which recording configuration was used. Inhibition was observed in all cells tested. Similar current inhibitions were observed with NPY (18-36) and peptide YY, but not with [Leu31, Pro34]NPY, indicating that the effects were mediated by Y2 receptors. Pancreatic polypeptide (100 nM) was without effect on Ca2+ channel currents. The effects of NPY were additive with nifedipine (at a supramaximal concentration of 5 microM), suggesting that NPY predominantly inhibits N-type Ca2+ channels present in these cells, and not L-type. The effects of NPY were mimicked by a novel, cyclic analogue of NPY which is 40-fold more selective for Y2 receptors than other commonly used Y2-selective peptides. The cyclic analogue was also more potent than NPY itself, causing maximal current inhibition at approx 300 nM, whereas the response to NPY was not fully saturated at 1 microM. Our results indicate that SH-SY5Y cells represent an excellent model system for studying the coupling of Y2 receptors to N-type channel inhibition. Furthermore, in the absence of selective antagonists for NPY receptor subtypes, the highly selective Y2 agonist cyclic NPY derivative may prove a useful tool for probing the various roles of Y2 receptors in the nervous system.

A rational approach for the development of reduced-size analogues of neuropeptide Y with high affinity to the Y1 receptor

Four sets of centrally truncated analogues of neuropeptide Y have been synthesized. In each series the N-terminal part was constant, while the C-terminal segment was systematically varied in length. The C- and N-terminal parts were linked by 6-aminohexanoic acid. The affinity to the Y1 receptor was investigated on human neuroblastoma cells SK-N-MC. Significant differences were found between the series of peptides as well as within each set. Remarkably, the affinity did not solely depend on the length of the segment, and with increasing numbers of residues the IC50 values were not always decreased. With a given N-terminal segment, only one optimal length of the C-terminal segment was found, which suggests that it is not the amino acids themselves but their 3D arrangement and orientation that is important for high receptor affinity.

Role of a hydrophobic pocket of the human Y1 neuropeptide Y receptor in ligand binding

We are investigating the nature of the chemical interactions between the neuropeptide Y (NPY) and its cell surface receptor (Y1). A previous study involving site-directed mutagenesis and computer-aided modelling (Walker et al., 1994) suggested that the C-terminal Tyr36 of NPY, known to be a key residue for receptor binding, might dock at a pocket formed by hydrophobic amino acids of transmembrane domains (TM) 1, 2, 6 and 7 of the Y1 receptor. To investigate which residues were required for ligand binding, we mutated the sequences encoding F41, L43, F96, Y100, F286 and H298 of the human Y1 receptor. The mutant cDNAs were transiently expressed in Hela cells and the ability of the encoded proteins to bind NPY was evaluated. Replacing F41, L43 or F96 with alanines had no effect on NPY binding. On the contrary, Y100, F286 and H298 appeared to be residues critical for ligand binding. In particular, the removal of the hydroxyl group of Y100 (Tyr100-->Phe100 mutation) yielded a protein devoid of affinity for the ligand. The level of expression and the presence on the cell surface of mutants lacking NPY binding activity was assessed by immunological techniques. In addition, we tested the ability of synthetic analogues of neuropeptide Y with substitutions at position 36 to bind to the Y1 receptor. To get spatial insight into the relative positions of the above mentioned residues we constructed a molecular model of the interaction between NPY:Y36 and the elements of the hydrophobic pocket surrounding this residue.

Low potency inhibition of Ca2+ channel currents in human neuroblastoma (SH-SY5Y) cells by [Ala31]NPY, an L-alanine substituted analogue of neuropeptide Y

Whole-cell Ca2+ channel currents were recorded in human neuroblastoma (SH-SY5Y) cells, using the perforated-patch technique with 10 mM Ba2+ as charge carrier. Neuropeptide Y (NPY; 10 nM to 1 microM) caused concentration-dependent inhibition of Ca2+ channel currents which were associated with a slowing in current activation kinetics. [Ala31]NPY, a residue 31 L-alanine substituted analogue of NPY, also inhibited Ca2+ channel currents and caused slowing of activation kinetics, but with approximately 6-fold lower potency. In the presence of 100 nM [Ala31]NPY (which itself had little or no effect on currents), the actions of NPY were similar in magnitude to its effects in the absence of the analogue. Our results suggest that substitution of isoleucine for alanine at residue 31 results in a NPY analogue which is a full agonist but with lower affinity for Y2 receptors.

Synthetic S-(2,3-dihydroxypropyl)-cysteinyl peptides derived from the N-terminus of the cytochrome subunit of the photoreaction centre of Rhodopseudomonas viridis enhance murine splenocyte proliferation

Various lipopeptides representing the N-terminal part of the cytochrome subunit of the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas virdis were prepared by solid-phase peptide synthesis. These lipopeptides consisted of a S-[2,3-dihydroxypropyl]-cysteinyl (Dhc) residue N-terminally coupled to the nonapeptide FEPPPATTT. Different numbers of palmitoyl (Pam) chains were attached to Dhc via ester and/or amide bonds. The lipopeptide Dhc(Pam)2-FEPPPATTT containing two ester-bonded palmitoyl residues and a free N-terminus was a potent polyclonal activator of murine (BALB/c) spleen cells at subnanomolar concentrations. The lipopeptide Pam-Dhc(Pam)2-FEPPPATTT containing three palmitoyl residues, the two-chain lipopeptide Pam-Dhc(Pam)-FEPPPATTT containing one amide- and one ester-bonded palmitoyl residue, and the N-terminally elongated lipopeptide SLVAG-Dhc(Pam)2-FEPPPATTT were less active. The nonapeptide FEPPPATTT and the decapeptide Dhc-FEPPPATTT were only marginal splenocyte activators, even at concentrations as high as 1 microM. Thus, lipopeptide Dhc(Pam)2-FEPPPATTT constitutes the first potent splenocyte stimulation Dhc-lipopeptide described so far that contains only two fatty acid residues.

Anchor-linked intermediates in peptide amide synthesis are caused by dimeric anchors on the solid supports

Cleavage and kinetic studies have been carried out using commercially obtained H-Tyr(tBu)-5-(4'-aminomethyl-3',5'-dimethoxyphenoxy)valeric acid-TentaGelS (H-Tyr(tBu)-4-ADPV-TentaGelS) and H-Tyr (tBu)-4-ADPV-Ala-aminomethyl-resin (H-Tyr(tBu)-4-ADPV-AM-resin) prepared from commercially available resin and loaded with commercially available Fmoc-4-ADPV-OH amide anchor. Cleavage with pure trifluoroacetic acid (TFA) gave the intermediate H-Tyr-4-ADPV-NH2, which was then degraded to H-Tyr-NH2, and cleavage with TFA/dichloromethane (1:9) yielded H-Tyr-4-ADPV-NH2 which could be isolated in preparative amounts. Cleavage reactions with 15N-labelled H-Ala-4-ADPV-(15N)-Gly-AM-resin yielded the intermediate H-Ala-4-ADPV-NH2, which contained no 15N as demonstrated by 1H-NMR. The analysis of the commercial Fmoc-4-ADPV-OH amide anchor showed the presence of Fmoc-4-ADPV-4-ADPV-OH as an impurity in high amounts. This dimeric anchor molecule is the cause of formation of the anchor-linked peptide intermediate obtained during the cleavage from the resin. The particularly high acid-lability of the amide bond between the two ADPV moieties was utilized to synthesize sidechain and C-terminally 4-ADPV protected pentagastrin on a double-anchor resin, and to cleave it using 5% trifluoroacetic acid in dichloromethane. This method may offer a new way for the synthesis of protected peptide amides with improved solubility to be used in fragment condensation.

Secondary structure prediction of adenylate kinase by circular dichroism spectroscopy of synthetic peptides

The synthesis of 13 segments of porcine adenylate kinase (AK 1) consisting each of 15 residues and covering the whole sequence of AK 1 is reported. The peptides were obtained by multiple peptide synthesis applying Fmoc/tert. butyl strategy. For the conformational analysis by circular dichroism spectroscopy different mixtures of phosphate buffer (pH 7), trifluoroethanol and methanol were used. Ten peptides showed alpha-helical conformation in all solvents except of pure buffer. The conformation of these peptides correlated well with the secondary structure obtained by homology modelling and even more striking with the X-ray structure of adenylate kinase, which was published recently. In contrast, three peptides, which were suggested to adopt no helical conformation did not show any content of alpha-helicity in any of the solvents that we investigated. One peptide, which contains a beta-sheet according to the X-ray analysis revealed a high content of beta-conformation by CD in buffer and in mixtures with trifluoroethanol. The two other peptides revealed mainly random coil in all solvents which we investigated. Therefore, we conclude that synthetic peptides are suitable tools to investigate the secondary structure of protein segments and that this conformation is frequently preserved in the intact protein.

Synthesis, receptor binding, and crosslinking of photoactive analogues of neuropeptide Y

Five photoactive analogues of porcine neuropeptide Y (NPY), a 36 amino acid hormone of the pancreatic polypeptide family, have been synthesized by solid phase peptide synthesis method, Fmoc/tBu strategy and carefully characterized. The analogues contain the photoactivatable amino acid 4'-(3-trifluoromethyl)-3H-diazirine-3-yl-phenyl-alanine ((Tmd)Phe) individually at different positions (1, 20, 21, 27 or 36) instead of tyrosine in the wildtype sequence. Affinity to membranes prepared from SMS-KAN cells, which stably express the Y2 receptor has been investigated by measuring the displacement of 125I-Bolton Hunter-NPY. After incubation of the membranes with different concentrations of the crosslinker and subsequent photolysis, the specific binding of 125I-Bolton Hunter-NPY at those membranes was tested in order to quantify the crosslinking efficiency. Whereas [(Tmd)Phe20] NPY, [(Tmd)Phe21] NPY and [(Tmd)Phe27] NPY revealed highest affinity to the Y2 receptor, crosslinking was most efficient when Tyr36 was replaced by (Tmd)Phe. This is in good agreement with the previously suggested C-terminal binding site of neuropeptide Y.

Structure-activity relationships of neuropeptide Y analogues with respect to Y1 and Y2 receptors

Secondary structure investigations, affinities, and activities of neuropeptide Y analogues with respect to the Y1 and the Y2 receptor are reviewed. The results are discussed with respect to the different prerequisites for affinities to both receptor subtypes. The results from a systematic scanning of the hormone using L-alanine and from a large variety of discontinuous and cyclic analogs suggest that two different conformations of neuropeptide Y are adopted at the Y1 and Y2 receptors. Whereas a C-terminal turn structure is suggested for Y1 receptor affinity, an alpha-helical conformation of the C-terminus is afforded for good binding to the Y2 receptor.

The first highly potent and selective non-peptide neuropeptide Y Y1 receptor antagonist: BIBP3226

The design and subsequent in vitro and in vivo biological characterisation of the first potent and selective non-peptide neuropeptide Y Y1 receptor antagonist, BIBP3226 ((R)-N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-argininami de) is reported. BIBP3226 displaced 125I-labelled neuropeptide Y with high affinity (Ki = 7 nM) from the human neuropeptide Y Y1 receptor and proved to be highly selective. BIBP3226 displayed potent antagonistic properties both in in vitro and in vivo models and thus represents the first selective non-peptide neuropeptide Y Y1 receptor antagonist.

Complete L-alanine scan of neuropeptide Y reveals ligands binding to Y1 and Y2 receptors with distinguished conformations

The synthesis of more than fifty 36-residue oligopeptide analogs of neuropeptide Y (NPY) and their affinity to human Y1 and Y2 receptors is described. Each amino acid of the natural sequence was replaced by L-alanine, the four alanine residues at position 12, 14, 18 and 23 were replaced by glycine. Additional residues were exchanged to closely related ones in order to characterize the prerequisites for binding. A combination of automated single and multiple peptide synthesis using fluoren-9-ylmethoxycarbonyl/tert-butoxy strategy was applied. The purified peptides were characterized by electrospray mass spectrometry, analytical HPLC and amino acid analysis. Binding was investigated by displacement of 125I-labelled neuropeptide Y from human neuroblastoma cell lines SK-N-MC and SMS-KAN. Whereas Pro2 and the integrity of the neuropeptide Y loop is important for the binding to the Y1 receptor, exchanges within the C-terminal helix affect the affinity to the Y2 receptor. The C-terminal pentapeptide amide is important for both receptors and probably represents the binding site. However, Arg33 and Arg35 may not be exchanged by L-alanine in the Y1 system, whereas Arg35 and Tyr36 are the most susceptible residues in the Y2 system. In order to distinguish between conformational effects and direct hormone/receptor interaction via the side chains of neuropeptide Y, circular dichroic studies of the alanine-containing peptides were performed and structure affinity relationships are discussed. Comparing the affinities of the neuropeptide Y analogs to Y1 and Y2 receptors significant differences were found for the two binding sites, which suggests a different active conformation of neuropeptide Y at the two subtypes of receptors. Using molecular dynamics calculations, two distinct conformations were identified which are in good agreement with the data obtained by structure/affinity investigations.

Differential vascular effects of neuropeptide Y(NPY) selective receptor agonists

Neuropeptide Y (NPY) increases blood pressure either directly or indirectly by potentiating the effect of various vasoconstrictors. Only one (the Y1-receptor) of two subtypes of receptors (Y1 and Y2) is thought to mediate the vascular smooth muscle contraction. To test this hypothesis we challenged isolated rat mesenteric arteries that had a functional endothelium with (1-36) NPY and with specific Y1-receptor ([Leu31, Pro34] NPY) and Y2-receptor ([Ahx5-24, gamma-Glu2-epsilon-Lys30] NPY) agonists. The Y1-receptor agonist elicited a contractile response similar to that of NPY, whereas the Y2-receptor agonist had no effect on wall tension. We also found that the presence of a functional endothelium has no influence on the contractile response to NPY. From these data we conclude that the direct contractile effect of NPY in the mesenteric artery is mediated by stimulation of Y1-receptors and is not endothelium-dependent.

ESAG 6 and 7 products of Trypanosoma brucei form a transferrin binding protein complex

In Trypanosoma brucei, the gene for the expressed variant surface glycoprotein (VSG) is preceded by a series of open reading frames designated expression site associated genes (ESAGs), which together with the VSG gene form a polycistronic transcription unit. It is shown that the products derived from two ESAGs (ESAG 6 and 7 in the nomenclature of Pays, E., et al. Cell 57, 835-845 (1989)) form a complex, which binds transferrin with high affinity. Transferrin affinity chromatography yields heterodimers or higher order heteroligomers composed of the products of ESAG 6 and ESAG 7. The former is a heterogeneously glycosylated protein of 50 to 60 kDa modified by a glycosylphosphatidylinositol membrane anchor at the COOH-terminus, while the latter is the previously identified 42 kDa glycoprotein carrying an unmodified COOH-terminus (Schell, D., et al. EMBO J. 10, 1061-1066 (1991) and Schell, D., et al. EMBO J. 12, 2990 (1993)). When isolated from trypanosomes grown in rodents, the complex is in part free and in part associated with transferrin. Also, the complex is present both in the membrane fraction and the soluble fraction of cell lysates. As shown by immunoelectron microscopy, both transferrin and ESAG 6/7-derived proteins can be demonstrated in the lumen of the flagellar pocket, an invagination of the plasma membrane serving as the sole site for endocytotic uptake of macromolecular nutrients. Weak labeling is also obtained on the flagellar pocket membrane and in intracellular vesicles. The possibility that the binding protein complex serves as a receptor for the uptake of transferrin in T. brucei is discussed.

Mapping of antigenic and immunogenic sites of Haemophilus influenzae outer membrane protein P6 using synthetic lipopeptides

The immune response to the P6 protein of Haemophilus influenzae was characterized with 24 synthetic icosapeptides and 45 dodecapeptides conjugated to the immune stimulator N-palmitoyl-S-[2,3-(bispalmitoyloxy)-(2RS)-propyl]-(R)-cysteinyl-( S)-serine . The antigenicity of these lipopeptides was investigated by enzyme-linked immunosorbent assays with rabbit anti-P6 serum. The epitopes of P6 protein were identified and localized within residues 31-46 and 59-70 and in the C-terminal part of the P6 protein. Mice were immunized with lipoicosapeptides without using additional adjuvants or carriers and the antibody titers were measured with isolated P6 protein and lipopeptides in a dot blot assay. Lipopeptides containing the sequence pattern QILDAHAA (P6 47-54) and the mouse B cell epitope GEYV (P6 43-46) induced high titers of anti P6 antibodies. These murine antibodies were able to neutralize the intact bacterium Haemophilus influenzae.

Antibodies to the protein core of the small cell lung cancer workshop antigen cluster-w4 and to the leucocyte workshop antigen CD24 recognize the same short protein sequence leucine-alanine-proline

We recently described the identity of the small cell lung cancer (SCLC) cluster-w4 antigen and the human B cell differentiation marker CD24, a glycosylphosphatidylinositol (GPI)-anchored, highly glycosylated surface molecule of only 31-35 amino acids [15]. The specificities of three anti-cluster-w4 and of eleven anti-CD24 MoAbs have been investigated with respect to their binding capacity to the protein core of cluster-w4/CD24 antigen. Four overlapping peptides spanning this protein core were synthesized. MoAbs shown to bind to two overlapping peptides by antibody binding inhibition using the cluster-w4/CD24-positive SCLC cell line SW2 and by direct peptide binding detected in an ELISA were investigated in more detail. To determine the exact epitopes recognized by these MoAbs, an epitope mapping assay using peptides synthesized onto polyethylene pins was established. The three anti-cluster-w4 MoAbs SWA11, SWA21 and SWA22 and the anti-CD24 MoAbs OKB2 and ALB9 recognized the same short leucine-alanine-proline (LAP) sequence in an area without potential glycosylation sites close to the GPI anchor of the protein core of the cluster-w4/CD24 antigen.

Epitope mapping: synthetic approaches to the understanding of molecular recognition in the immune system

Progress in the field of immunochemistry is rapidly increasing due to very efficient methods of epitope mapping. Experimental results on the allele-specific sequence motifs of MHC-binding peptides allow the exact forecast of T-cell epitopes and, in combination with B-cell prediction methods and synthetic adjuvant systems, fully synthetic vaccines may be constructed. Methods of multiple peptide synthesis are of particular use for such constructs and for the fine mapping of monoclonal antibodies or sera of patients. Peptide libraries, containing hundred thousands of different oligopeptides are made available for novel screening procedures. These techniques and their applications in various fields are summarized and discussed with respect to efficiency and productivity.

Optimization of capillary electrophoresis of mixtures of basic peptides and comparison with HPLC

Fourteen synthetic, strongly basic hexadeca- to octadecapeptides, which differ only in one amino acid residue, were synthesized and characterized. In order to investigate the application of capillary electrophoresis for the separation of mixtures of closely related basic peptides, seven different mixtures, which represent possible byproducts during solid-phase synthesis, were analyzed by HPLC and different techniques of capillary electrophoresis. Peaks within the mixture were identified by single peak injection or on-line CE/MS. Utilization of micellar electrokinetic capillary electrophoresis offered the separation of complex mixtures. Comparing the electrophoretic mobilities with calculated correlations, we found that the separation of peptides of this length is influenced not only by the different size and charge but also by hydrophobicity and the orientation of the charge due to different conformations.

Cyclopeptide analogs for characterization of the neuropeptide Y Y2-receptor

A discontinuous 17-amino acid peptide analog of neuropeptide Y (NPY), NPY 1-4-Ahx-25-36 containing 6-aminohexanoic acid instead of the residues 5 to 24, was found to bind preferentially to Y2 subtypes of NPY receptors. In order to further characterize the binding site, three different types of cyclic analogs were synthesized. Firstly lactamisation between residues 2 and 30 led to the most selective Y2-agonist, secondly lactamisation between the N-terminus and residue 31 reduced binding significantly. Thirdly, any cyclization including the C-terminus led to an inactive compound. Circular dichroism revealed different conformations for the three analogs with reduced alpha-helical content in comparison to the linear ana-log. The different conformation of the peptides has been confirmed by molecular dynamics simulations. A model for peptide-receptor interaction is suggested.

Further characterization of neuropeptide Y receptor subtypes using centrally truncated analogs of neuropeptide Y: evidence for subtype-differentiating effects on affinity and intrinsic efficacy

Previous attempts to classify neuropeptide Y receptor subtypes suffered from relying only on carboxyl-terminal analogs and fragments of neuropeptide Y. We have tested the potency and affinity of chemically different compounds, i.e., centrally truncated analogs of neuropeptide Y, in three Y1-like (Ca2+ mobilization in HEL cells, blood pressure increases in pithed rats, and 125I-neuropeptide Y binding in SK-N-MC cells) and two Y2-like (125I-neuropeptide Y binding to rabbit kidney membranes and presynaptic inhibition in rat vas deferens) model systems of neuropeptide Y receptors. Our data confirm the concept of two major subclasses of neuropeptide Y receptors, with some centrally truncated neuropeptide Y analogs having high affinity for Y2-like and low affinity for Y1-like neuropeptide Y receptors. Some of the truncated neuropeptide Y analogs are antagonists at Y1-like receptors and (possibly partial) agonists at Y2-like receptors. Our data also indicate that amino acid residues distal from the amino- and carboxyl-terminal ends of the peptide may subtype-selectively affect affinity and intrinsic efficacy of peptide agonists at neuropeptide Y receptors.