Comparative analysis of human and Dutch-type alzheimer \u3b2-amyloid peptides by infrared spectroscopy and circular dichroism

The 42 amino acid \u3b2A4 peptide is the major constituent of the senile plaques, one of the hallmark neuropathological lesions of Alzheimer\u2032s disease. While C-terminally truncated variants were shown to be present in normal body fluids, a single Glu \u2192 Gln change in the 39 amino acid form of \u3b2A4 results in accelerated fibril formation in the brains of patients with Dutch-type hereditary cerebral hemorrhage with amyloidosis. In this study we used Fourier-transform infrared and circular dichroism spectroscopies on synthetic peptides to demonstrate that this mutation results in altered secondary structure in membrane mimicking solvents, characterized by a considerably higher \u3b2-structure content for the mutant peptide. Moreover, extreme high and low pH were less effective in eliminating the \u3b2-conformation for the Dutch-variant than for the normal human sequence.Peer reviewed: YesNRC publication: Ye

Infrared spectroscopic characterization of synthetic Alzheimer peptides

We have used IR spectroscopy to study the conformational properties of synthetic analogues of peptides known to be involved in the formation of neuritic plaques in the brains of victims of Alzheimer's disease. We have also investigated the influence of naturally occurring point mutations and amino acid isomerization upon the tendency of the peptides to aggregate which is related to the amount of plaque found.Peer reviewed: YesNRC publication: Ye

Human and rodent Alzheimer \u3b2-amyloid peptides acquire distinct conformations in membrane-mimicking solvents

NRC publication: Ye

Synthetic post-translationally modified human A\u3b2 peptide exhibits a markedly increased tendency to form \u3b2-pleated sheets in vitro

The \u3b2-amyloid peptide (A\u3b2) is the major constituent of senile plaques, one of the hallmark neuropathological lesions of Alzheimer's disease. Recently a post-translationally modified analogue of the human \u3b2-amyloid peptide, which contains isoaspartatic residues in positions 1 and 7, was isolated from parenchyma and leptomeningeal microvasculature of Alzheimer's disease patients [Roher, A. E., Lowenson, JD., Clarke, S., Wolkow, C., Wang, R., Cotter, R. J., Reardon, I. M., Z\ufcrcher-Neely, H. A., Heinrikson, R. L., Ball, M. J. & Greenberg, B. D. (1993) J. Biol. Chem. 268, 3072\u20133083]. We used circular dichroism and Fourier-transform infrared spectroscopy to characterize the conformational changes on human A\u3b2 upon substitution of Asp1 and Asp7 to isoaspartic residues. We found that the intermolecular \u3b2-pleated-sheet content is markedly increased for the post-translationally modified peptide compared to that in the corresponding unmodified human or rodent A\u3b2 sequences both in aqueous solutions in the pH 7\u201312 range, and in membrane-mimicking solvents (such as aqueous octyl-\u3b2-d-glucoside or aqueous acetonitrile solutions). These findings underline the importance of the originally \u3b1-helical N-terminal regions of the unmodified A\u3b2 peptides in defining its secondary structure and may offer an explanation for the selective aggregation and retention of the isomerized A\u3b2 peptide in Alzheimer's-disease-affected brains.Peer reviewed: YesNRC publication: Ye

Infrared spectroscopic characterization of alzheimer plaques

Neuritic plaques in the brains of victims of Alzheimer's disease are primarily composed of a 42 amino acid polypeptide, the \u3b2-amyloid peptide (\u3b2A4), the neurotoxicity of which is related to its aggregation. Fourier transform infrared spectroscopy has been used to study the conformational properties of two synthetic analogues of \u3b2A4 peptides known to be involved in the formation of the neuritic plaques formed in patients with Alzheimer's disease and the influence of a single, naturally occurring point mutation upon the tendency of the peptide to aggregate. Peptides from both \u201cnormal\u201d Alzheimer's and the more severe Dutch variant of the disease were found to form aggregated antiparallel strands. However, the replacement of a single, negatively charged amino acid (glutamic acid) with an uncharged amino acid (glutamine) in the Dutch-type peptide results in significant differences in the strength and stability of these aggregates and the microenvironment of a number of amino acids. The differences in the strength and the stability of the aggregates are attributed to the presence of varying (small) proportions of the classical secondary structures and differences in net charge. Environmental (solvent) effects were shown to significantly affect the strength of the inter-molecular hydrogen bonding in the aggregates, solvent systems mimicking the membrane/water interface resulting in more strongly hydrogen bonded aggregates. Infrared spectra of material from autopsied human Alzheimer's brains show spectral features indicative of the formation of similar aggregates, which may be related to plaque formation. This observation suggests that IR spectroscopic methods may in the future be useful in the diagnosis and monitoring of Alzheimer's disease.Peer reviewed: YesNRC publication: Ye

Aspartate-bond isomerization affects the major conformations of synthetic peptides

The aspartic acid bond changes to an \u3b2-aspartate bond frequently as a side-reaction during peptide synthesis and often as a post-translational modification of proteins. The formation of \u3b2-aspartate bonds is reported to play a major role not only in protein metabolism, activation and deactivation, but also in pathological processes such as deposition of the neuritic plaques of Alzheimer's disease. Recently, we reported how conformational changes following the aspartic-acid-bond isomerization may help the selective aggregation and retention of the amyloid \u3b2 peptide in affected brains (Fabian et al., 1994). In the current study we used circular dichroism, Fourier-transform infrared spectroscopy, and molecular modeling to characterize the general effect of the \u3b2 aspartate-bond formation on the conformation of five sets of synthetic model peptides. Each of the non-modified, parent peptides has one of the major secondary structures as the dominant spectro-scopically determined conformation: a type I \u3b2 turn, a type II \u3b2 turn, short segments of \u3b1 or 310 helices, or extended \u3b2 strands. We found that both types of turn structures are stabilized by the aspartic acid-bond isomerization. The isomerization at a terminal position did not affect the helix propensity, but placing it in mid-chain broke both the helix and the \u3b2-pleated sheet with the formation of reverse turns. The alteration of the geometry of the lowest energy reverse turn was also supported by molecular dynamics calculations. The tendency of the aspartic acid-bond isomerization to stabilize turns is very similar to the effect of incorporating sugars into synthetic peptides and suggests a common feature of these post-translational modifications in defining the secondary structure of protein fragments.Peer reviewed: YesNRC publication: Ye