Amyloid beta protein does not interact with tachykinin receptors coupled to inositol phospholipid hydrolysis in human astrocytoma cells

Assemblies of Alzheimer's peptides A beta 25-35 and A beta 31-35: reverse-turn conformation and side-chain interactions revealed by X-ray diffraction

Alzheimer's beta amyloid protein (A beta) is a 39 to 43 amino acid peptide that is a major component in the neuritic plaques of Alzheimer's disease (AD). The assemblies constituted from residues 25-35 (A beta 25-35), which is a sequence homologous to the tachykinin or neurokinin class of neuropeptides, are neurotoxic. We used X-ray diffraction and electron microscopy to investigate the structure of the assemblies formed by A beta 25-35 peptides and of various length sequences therein, and of tachykinin-like analogues. Most solubilized peptides after subsequent drying produced diffraction patterns characteristic of beta-sheet structure. Moreover, the peptides A beta 31-35 (Ile-Ile-Gly-Leu-Met) and tachykinin analogue A beta(Phe(31))31-35 (Phe-Ile-Gly-Leu-Met) gave powder diffraction patterns to 2.8A Bragg spacing. The observed reflections were indexed by an orthogonal unit cell having dimensions of a=9.36 A, b=15.83 A, and c=20.10 A for the native A beta 31-35 peptide, and a=9.46 A, b=16.22 A, and c=11.06 A for the peptide having the Ile31Phe substitution. The initial model was a beta strand where the hydrogen bonding, chain, and intersheet directions were placed along the a, b, and c axes. An atomic model was fit to the electron density distribution, and subsequent refinement resulted in R factors of 0.27 and 0.26, respectively. Both peptides showed a reverse turn at Gly33 which results in intramolecular hydrogen bonding between the antiparallel chains. Based on previous reports that antagonists for the tachykinin substance P require a reverse turn, and that A beta is cytotoxic when it is oligomeric or fibrillar, we propose that the tachykinin-like A beta 31-35 domain is a turn exposed at the A beta oligomer surface where it could interact with the ligand-binding site of the tachykinin G-protein-coupled receptor.

Amyloid beta peptide membrane perturbation is the basis for its biological effects

Experimental studies have indicated that the mechanisms offered for explaining the neurotoxicity of amyloid beta peptide (AbetaP) are diverse, and include altered enzyme activities, disrupted calcium homeostasis, and increased free radical formation. AbetaP appears to interact at the cell membrane with a multitude of receptor sites and also inserts physically into the membrane matrix. This membrane insertion affects the membrane fluidity and potentially influences the function of resident membrane proteins. We propose a unifying hypothesis to explain the experimental observations of the diverse cellular responses to AbetaP. The indiscriminate physical insertion of AbetaP into the cell membrane unspecifically activates a host of membrane processes by perturbation of the membrane proteins. This recurrent activation of membrane processes eventually culminates in neuronal cell death. We recommend that successful therapeutic interventions should be directed at reducing or preventing the interaction of AbetaP with neuronal cell membranes.

Comparative studies on peptides representing the so-called tachykinin-like region of the Alzheimer Abeta peptide [Abeta(25-35)]

In an attempt to answer the question of whether or not the so-called tachykinin-like region of the Alzheimer beta-amyloid protein [Abeta(25-35)] can act as a tachykinin, the sequences Abeta(25-35), Abeta(25-35)amide and their norleucine-35 and phenylalanine-31 analogues were synthesized. These peptides were examined with ligand binding studies, electron microscopy, CD and NMR. In all cases some differences were found between the Abeta(25-35) analogue and the corresponding Phe31 peptide. In addition, in ligand displacement studies on tachykinin NK1 receptors, only the Phe31 analogue showed activity comparable to that of genuine tachykinins. We conclude that peptides based on Abeta(25-35) but with a Phe residue at position 31 do display properties typical of a tachykinin, but that peptides with Ile at this position do not.

Cellular MTT reduction distinguishes the mechanism of action of beta-amyloid from that of tachykinin receptor peptides

Inhibition of the reduction of the redox dye 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide by rat phaeochromocytoma PC12 cells is a specific, early response to nanomolar concentrations of the beta-amyloid peptide fragment beta (25-35), and appears to be a reliable indicator of the mechanism of beta-amyloid toxicity. Neither selective tachykinin receptor agonists, nor tachykinin receptor peptide and non-peptide antagonists elicited such a response. Furthermore, tachykinin receptor peptides did not block the effects of beta-amyloid in PC12 cells, or in two other beta-amyloid-sensitive cell lines. These experimental model systems allow the mechanism of action of beta-amyloid to be distinguished from that of tachykinin receptor peptides, and prove that the neurotoxic action of beta-amyloid, as measured by the inhibition of cellular 3-[4,5-dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide reduction is not mediated by an interaction with tachykinin receptors.

Inhibitory effects of beta-amyloid peptides on nicotine-induced Ca2+ influx in PC12h cells in culture

Synthetic beta-amyloid peptides and the neuropeptide substance P (SP) were examined for their ability to modulate nicotinic response in PC12h cells, a subclone of PC12 cells, SP, beta A1-40 and its peptide fragment beta A25-35-NH2 significantly inhibited an increase in cytoplasmic calcium concentrations ([Ca2+]i) induced by nicotine in a dose-dependent manner. Furthermore, beta A1-40 was found to inhibit the [Ca2+]i increase induced by depolarization with a high concentration of potassium. These findings show that both beta A1-40 and beta A25-35-NH2 may mimic the function of SP on inhibition of nicotinic response through different mechanisms.

Amyloid beta protein-induced neuronal cell death: neurotoxic properties of aggregated amyloid beta protein

The neurotoxic effects of soluble and aggregated synthetic amyloid beta protein (A beta P) have been investigated in rat primary cultures. Freshly solubilized beta(1-40) was neurotoxic not to immature, but to mature hippocampal neurons. On the other hand, aggregated beta(1-40) was neurotoxic to both. Neurotoxicity induced by aggregated beta(1-40) was 10-fold more potent than soluble beta(1-40) and was not prevented by substance P. The neurotoxicity of aggregated beta(1-40) to cultured neurons depended on the peptide concentration and the duration of exposure to it. Cerebral cortical and hippocampal neurons were significantly susceptible to aggregated beta(1-40) than cerebellar granular cells, and cultured astrocytes were not vulnerable to aggregated beta(1-40) even at high concentrations.