The major components of neuritic plaques found in Alzheimer disease (AD) are peptides known as amyloid β‐peptides (Aβ), which derive from the proteolitic cleavage of the amyloid precursor proteins. In vitro Aβ may undergo a conformational transition from a soluble form to aggregated, fibrillary β‐sheet structures, which seem to be neurotoxic. Alternatively, it has been suggested that an α‐helical form can be involved in a process of membrane poration, which would then trigger cellular death. Conformational studies on these peptides in aqueous solution are complicated by their tendency to aggregate, and only recently NMR structures of Aβ‐(1–40) and Aβ‐(1–42) have been determined in aqueous trifluoroethanol or in SDS micelles. All these studies hint to the presence of two helical regions, connected through a flexible kink, but it proved difficult to determine the length and position of the helical stretches with accuracy and, most of all, to ascertain whether the kink region has a preferred conformation. In the search for a medium which could allow a more accurate structure determination, we performed an exhaustive solvent scan that showed a high propensity of Aβ‐(1–42) to adopt helical conformations in aqueous solutions of fluorinated alcohols. The 3D NMR structure of Aβ‐(1–42) shows two helical regions encompassing residues 8–25 and 28–38, connected by a regular type I β‐turn. The surprising similarity of this structure, as well as the sequence of the C‐terminal moiety, with those of the fusion domain of influenza hemagglutinin suggests a direct mechanism of neurotoxicity.
Current views of the role of beta-amyloid (Abeta) peptide fibrils range from regarding them as the cause of Alzheimer's pathology to having a protective function. In the last few years, it has also been suggested that soluble oligomers might be the most important toxic species. In all cases, the study of the conformational properties of Abeta peptides in soluble form constitutes a basic approach to the design of molecules with "antiamyloid" activity. We have experimentally investigated the conformational path that can lead the Abeta-(1-42) peptide from the native state, which is represented by an alpha helix embedded in the membrane, to the final state in the amyloid fibrils, which is characterized by beta-sheet structures. The conformational steps were monitored by using CD and NMR spectroscopy in media of varying polarities. This was achieved by changing the composition of water and hexafluoroisopropanol (HFIP). In the presence of HFIP, beta conformations can be observed in solutions that have very high water content (up to 99 % water; v/v). These can be turned back to alpha helices simply by adding the appropriate amount of HFIP. The transition of Abeta-(1-42) from alpha to beta conformations occurs when the amount of water is higher than 80 % (v/v). The NMR structure solved in HFIP/H2O with high water content showed that, on going from very apolar to polar environments, the long N-terminal helix is essentially retained, whereas the shorter C-terminal helix is lost. The complete conformational path was investigated in detail with the aid of molecular-dynamics simulations in explicit solvent, which led to the localization of residues that might seed beta conformations. The structures obtained might help to find regions that are more affected by environmental conditions in vivo. This could in turn aid the design of molecules able to inhibit fibril deposition or revert oligomerization processes.
The design of molecules able to interact with the amyloid peptides either as inhibitors of fibril formation or as inhibitors of amyloid membrane pore formation represents one of the most relevant approaches in the development of anti-Alzheimer therapies. Abeta-(25-35), sequence GSNKGAIIGLM, is a highly toxic synthetic derivative of amyloid beta-peptides (Abeta-peptides), which forms fibrillary aggregates. Here, we report the NMR and CD investigation of Abeta-(25-35) in a membrane-mimicking environment and in isotropic mixtures of water and fluoro-alcohols to scan its conformational properties as a function of the medium. The analysis of the 3D structures in the mentioned conditions indicates a propensity of the peptide to behave as a typical transmembrane helix in the lipidic environment. In media characterized by different polarity, it loses the structural regularity at specific points of the sequence as a function of the environment. Furthermore, a comparison with the solution structure of full-length amyloid peptides suggests a role for the 25-27 kink region, which appears to be a general feature of all peptides under the solution conditions explored.
Dmt-Tic opioidmimetic peptides represent a highly potent class of opioid peptide antagonists with greater potency than the nonopioid delta antagonist naltrindole and have potential application as clinical and therapeutic compounds.
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