Lipid and membrane interactions of neuropeptide Y

Orexin A, an amphipathic α-helical neuropeptide involved in pleiotropic functions in the nervous and immune systems: Synthetic approach and biophysical studies of the membrane-bound state

This research reports on the membrane interactions of orexin A (OXA), an α-helical and amphipathic neuropeptide that contains 33 residues and two disulfide bonds in the N-terminal region. OXA, which activates the orexins 1 and 2 receptors in neural and immune cell membranes, has essential pleiotropic physiological effects, including at the levels of arousal, sleep/wakefulness, energy balance, neuroprotection, lipid signaling, the inflammatory response, and pain. As a result, the orexin system has become a prominent target to treat diseases such as sleep disorders, drug addiction, and inflammation. While the high-resolution structure of OXA has been investigated in water and bound to micelles, there is a lack of information about its conformation bound to phospholipid membranes and its receptors. NMR is a powerful method to investigate peptide structures in a membrane environment. To facilitate the NMR structural studies of OXA exposed to membranes, we present a novel synthetic scheme, leading to the production of isotopically-labeled material at high purity. A receptor activation assay shows that the ¹⁵N-labeled peptide is biologically active. Biophysical studies are performed using surface plasmon resonance, circular dichroism, and NMR to investigate the interactions of OXA with phospholipid bilayers. The results demonstrate a strong interaction between the peptide and phospholipids, an increase in α-helical content upon membrane binding, and an in-plane orientation of the C-terminal region critical to function. This new knowledge about structure-activity relationships in OXA could inspire the design of novel therapeutics that leverage the anti-inflammatory and neuro-protective functions of OXA, and therefore could help address neuroinflammation, a major issue associated with neurological disorders such as Alzheimer's disease.

The interaction of neuropeptide Y with negatively charged and zwitterionic phospholipid membranes

The interaction of the 36 amino acid neuropeptide Y (NPY) with liposomes was studied using the intrinsic tyrosine fluorescence of NPY and an NPY fragment comprising amino acids 18-36. The vesicular membranes were composed of phosphatidylcholine and phosphatidylserine at varying mixing ratios. From the experimentally measured binding curves, the standard Gibbs free energy for the peptide transfer from aqueous solution to the lipid membrane was calculated to be around -30 kJ/mol for membrane mixtures containing physiological amounts of acidic lipids at pH 5. The effective charge of the peptide depends on the pH of the buffer and is about half of its theoretical net charge. The results were confirmed using the fluorescence of the NPY analogue [Trp(32)]-NPY. Further, the position of NPY's alpha-helix in the membrane was estimated from the intrinsic tyrosine fluorescence of NPY, from quenching experiments with spin-labelled phospholipids using [Trp(32)]-NPY, and from (1)H magic-angle spinning NMR relaxation measurements using spin-labelled [Ala(31), TOAC(32)]-NPY. The results suggest that the immersion depth of NPY into the membrane is triggered by the membrane composition. The alpha-helix of NPY is located in the upper chain region of zwitterionic membranes but its position is shifted to the glycerol region in negatively charged membranes. For membranes composed of phosphatidylcholine and phosphatidylserine, an intermediate position of the alpha-helix is observed.

Self-regulation of agonist activity at the Y receptors

Neuropeptide Y (NPY) is one of the most abundant neuropeptides, and is likely to be present at nanomolar levels over extended periods in the synaptic space of many forebrain areas. This might be linked to an evolved generalized toning activity through a number of other peptide receptors that use C-terminally amidated agonists (with LHRH and orexin receptors and GIR as examples). However, the Y1 and Y2 receptors (which constitute the bulk of Y receptors active in the neural matrix) possess subnanomolar affinities that, at saturating NPY levels, could produce excessive signaling, as well as receptor losses via repeated endocytosis. The related Y4 receptor shows an even higher agonist affinity, and faces the same problem in visceral and neural locations accessible to pancreatic polypeptide (PP). An examination of agonist peptide interaction with Y receptors shows that Y1 and Y4 receptors in particular (as located on either the intact cells, or on particulates derived from various cell types) develop a blockade dependent on ligand concentration, with the blocking ranks of [NPY]>>[peptide YY] (PYY) for the Y1, and [human PP]>>>[PYY-related Y4 agonist] for the Y4 receptor. This blockade is also echoed in a concentration-related reduction in biological activity of primary agonists (NPY and PP), resembling a partial agonism, and is influenced especially by the allosteric interactivity of agonists. With the Y2 receptor, the blocking by agonists is less pronounced, but the signaling by NPY-related peptides is apparently less than with PYY-related agonists. The extended occupancy and self-attenuation of primary agonist activity at Y receptors could represent an evolutionary solution contributing to a balancing of metabolic signaling, agonist clearance and receptor conservation.

Membrane interaction of neuropeptide Y detected by EPR and NMR spectroscopy

Neuropeptide Y (NPY) is one of the most abundant peptides in the central nervous system of mammals. It belongs to the best-conserved peptides in nature, i.e., the amino acid sequences of even evolutionary widely separated species are very similar to each other. Using porcine NPY, which differs from human NPY only at position 17 (a leucine residue exchanged for a methionine), labeled with a TOAC spin probe at the 2nd, 32nd, or 34th positions of the peptide backbone, the membrane binding and penetration of NPY was determined using EPR and NMR spectroscopy. The vesicular membranes were composed of phosphatidylcholine and phosphatidylserine at varying mixing ratios. From the analysis of the EPR line shapes, the spectral contributions of free, dimerized, and membrane bound NPY could be separated. This analysis was further supported by quenching experiments, which selected the contributions of the bound NPY fraction. The results of this study give rise to a model where the alpha-helical part of NPY (amino acids 13-36) penetrates the membrane interface. The unstructured N-terminal part (amino acids 1-12) extends into the aqueous phase with occasional contacts with the lipid headgroup region. Besides the mixing ratio of zwitterionic and negatively charged phospholipid species, the electrostatic peptide membrane interactions are influenced by the pH value, which determines the net charge of the peptide resulting in a modified membrane binding affinity. The results of these variations indicate that NPY binding to phospholipid membranes depends strongly on the electrostatic interactions. An estimation of the transfer energy of the peptide from aqueous solution to the membrane interface DeltaG supports the preferential interaction of NPY with negatively charged membranes.

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.

Structural transitions associated with the interaction of Alzheimer beta-amyloid peptides with gangliosides

Alzheimer's disease is characterized pathologically by the presence of neurofibrillary tangles and amyloid plaques. The principal component of the plaque is the beta-amyloid peptide (Abeta), a 39-43-residue peptide. The conformational change required for the conversion of soluble peptide into amyloid fibrils is modulated by pH, Abeta concentration, addition of kinetic and thermodynamic enhancers, and alterations in the primary sequence of Abeta. We report here the ability of gangliosides to induce an alpha-helical structure in Abeta and thereby diminish fibrillogenesis. Circular dichroism and a fluorescence dye release assay data indicate that gangliosides interact with and induce alpha-helix formation in Abeta. We find that the sialic acid moiety of gangliosides is necessary for the induction of alpha-helical structure. Differences in the amount and the position of the sialic acid on the carbohydrate backbone also affect the conformational switch. The Abeta-ganglioside interaction at pH 7.0, monitored by CD, is stable over time and resistant to high concentrations of NaCl. The induction of alpha-helical structure is greater with Abeta1-40 than Abeta1-42. The ability of gangliosides to sequester Abeta from fibril formation was also evaluated by electron microscopy.

Characterization of the interactions of Alzheimer beta-amyloid peptides with phospholipid membranes

Increasing evidence suggests that Alzheimer beta-amyloid peptides (AAPbeta) may be toxic agents in Alzheimer disease. We investigated the possibility that the toxicity may be the result of peptide-lipid interactions, involving either the cell membrane or the intracellular vesicular system. The interaction of the AAPbeta-(1-40), AAPbeta-(1-42), AAPbeta-(9-25) and AAPbeta-(25-35)-peptides with acidic and zwitterionic phospholipids was investigated by means of circular dichroism, vesicle disruption and lipid-aggregation assays. These studies were undertaken at peptide concentrations approaching in vivo levels and at physiological salt concentrations. Circular-dichroism studies demonstrate that acidic phospholipids induce a conformational change from random coil to beta structure in AAPbeta-(1-40)-peptide and AAPbeta-(1-42)-peptide at pH 6.0. In contrast, at pH 7.0, only AAPbeta-(1-42)-peptide was induced to adopt beta structure. Phosphatidylinositol was the most efficient inducer of beta structure in AAPbeta-(1-42)-peptide. To further investigate the peptide-lipid interactions, we examined the ability of the AAPbeta peptides to disrupt and/or aggregate phospholipid vesicles. These properties were found to be mediated predominantly through electrostatic interactions with the phospholipid headgroup. The data presented in this paper have implications for AAPbeta toxicity and senile-plaque formation.

An irregularity in the transmembrane domain helix correlates with the rate of insulin receptor internalization

Internalization of insulin and its receptor via receptor-mediated endocytosis is an important step in insulin-induced signal transduction. To investigate the structural determinants underlying the enhanced internalization rate observed for the insulin receptor transmembrane mutant Gly933-Pro934-->Ala-Ala (GP-->AA), we have designed and chemically synthesized two peptides, IR(TM)-GP and IR-(TM)-AA, corresponding respectively to the N-terminal portion of the wild-type and the mutant insulin receptor TM segment containing these sites. Conformational studies by circular dichroism (CD) spectroscopy on these two peptides in their monomeric states revealed that peptide IR(TM)-GP forms an irregular helix in the membrane-mimetic environments of sodium dodecyl sulfate (SDS) micelles with a possible "kink" in the helix imposed by its Gly-Pro sequence, while peptide IR(TM)-AA assumes largely classical alpha-helical structure under corresponding conditions. The helical pattern of peptide IR(TM)-AA was maintained at elevated temperatures, while the shape of the CD curve for peptide IR(TM)-GP was found to alter as a function of temperature. At higher concentrations, both peptides formed high molecular weight aggregates in SDS micelles, as demonstrated by SDS-PAGE gels, but peptide IR(TM)-AA was shown to aggregate more readily and more extensively than peptide IR(TM)-GP. Fluorescent dye-leakage experiments indicated that peptide IR(TM)-GP produces an enhanced disruption of the membrane bilayer in phosphatidylglycerol vesicles vs that induced by IR(TM)-AA.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y inhibits vasoactive intestinal peptide-stimulated adenylyl cyclase in rat ventral prostate

Neuropeptide Y (NPY), a peptide present in the prostate gland, was found to inhibit vasoactive intestinal peptide (VIP)-stimulated cyclic AMP accumulation in isolated rat prostatic epithelial cells as well as VIP-stimulated adenylyl cyclase activity in rat prostatic membranes. The inhibitory effect of NPY was selective for the VIP receptor/effector system since it was also observed when using pituitary adenylyl cyclase activating peptide (PACAP-27) which presumably recognizes VIP receptors in this gland, but not when using unrelated substances such as isoproterenol or forskolin. NPY did not modify either the general lipid membrane microviscosity or the VIP-receptor binding. The inhibitory effect of VIP was blocked by pretreatment of the prostatic membranes with pertussis toxin. These results suggest the presence of NPY receptors in rat ventral prostate coupled in an inhibitory manner to adenylyl cyclase through a guanine nucleotide regulatory Gi protein.

High-performance liquid chromatography of amino acids, peptides and proteins. CXXVIII. Effect of D-amino acid substitutions on the reversed-phase high-performance liquid chromatography retention behaviour of neuropeptide Y[18-36] analogues

The reversed-phase high-performance liquid chromatographic (RP-HPLC) gradient elution behaviour of a series of peptides related to Neuropeptide Y (NPY) has been investigated. The peptides studied included NPY, NPY[13-36], NPY[18-36] and a series of 16 analogues of NPY[18-36], each with a single D-amino acid substitution. Chromatographic parameters which relate to the interactive contact area and the binding affinity have been evaluated with two different stationary phase ligands and two organic modifiers. The results demonstrate that D-amino acid substitutions in the sequence region encompassing amino acid residues NPY[27-31] of these NPY[18-36] peptides significantly influence the interactive behaviour of these peptides relative to the unsubstituted NPY[18-36] molecule, while substitutions in the N- and C-terminal regions had little effect. Further, these results indicate that, in hydrophobic environments, NPY[18-36] adopts a significant degree of secondary structure which is severely disrupted by the presence of the D-amino acids in the central portion of the molecule.

Promotion of beta-structure by interaction of diabetes associated polypeptide (amylin) with phosphatidylcholine

The interaction of the diabetes associated polypeptide (amylin) with dimyristoylphosphatidylcholine (DMPC) was assessed by measurements of turbidity (absorbance at 400 nm) and secondary structure by CD spectroscopy. In trifluoroethanol, human amylin adopts a highly alpha-helical conformation while the rat peptide is less structured. In water, the rat peptide is largely disordered and the human peptide exhibits a combination of alpha- and beta-structures. Mixtures of DMPC and the rat peptide have no effect on either the turbidity of the DMPC or the CD spectrum of the peptide. By contrast, mixtures of the human peptide with DMPC form relatively clear mixtures similar to those observed with amphipathic alpha-helical peptides, but the structure adopted, based on the CD spectrum, is largely beta. These data demonstrate that fundamental differences in the structures adopted by amylins from human and rat species exist in mixtures with DMPC and suggest that these differences may be related to the formation of amyloid fibrils in the human amylin peptide which are not observed in the rat peptide.