18 November 2005. Current interest in the toxic mechanisms of oligomeric amyloid-β proteins notwithstanding, the larger fibrils this protein forms also are neurotoxic in their own right. Recent structural studies on prions and other amyloid-forming proteins have laid out the basic architectural underpinnings of the initiation and propagation of toxic fibrils (see ARF related news story). But for the Alzheimer amyloid, the devil truly is in the details, and gaining a molecular-level understanding of how the Aβ1-42 peptide stacks up is needed to answer some pressing questions: How exactly do fibrils form? How can they be stopped? How are they toxic to neurons?
To get at these questions, researchers in the lab of Roland Riek and collaborators at the Salk Institute in San Diego and in Lausanne and Basel, Switzerland, combined their own NMR, mutagenesis, and cryoelectron microscopy studies with existing data to determine the 3D structure of the Aβ1-42 fibril. Their results, published this week in PNAS online, show how peptide sequence and conformation drive Aβ1-42 fibril growth, and explain how one fibrillization inhibitor may stop it.
All amyloids form a cross-β structure, in which two parallel β sheets run along the fiber axis. Using NMR, first author Thorsten Lührs and colleagues showed that the two β sheets in the Alzheimer amyloid form from β1 and β2 strands spanning amino acids 18-26 and 31-42, respectively. Joined by a turn, the two strands form a U shape, and multiple peptides stack to form the fiber, held together by hydrogen bonds between in-register aligned β strands. By modeling and mutagenesis, the researchers determined that between the two β sheets there are three specific side-chain interactions. One is a salt bridge between the β1 strand and the linking region, and the other two are hydrophobic interactions between residues in the β1 and β2 strands.
A set of experiments that followed fiber formation using a variety of mutated Aβ1-42 peptides suggested that the contacts between β strands spanned two adjacent peptides. In particular, the modeling suggests that the D23 residue of a β1 strand reaches down to make a salt bridge with the K28 of the hinge region of the peptide underneath it in the stack. The two side-chain-specific hydrophobic interactions occur between the β1 strand of one peptide and the β2 strand of the peptide below it, relative to the direction of fibril growth. From this, the authors concluded that the Aβ1-42 fibrils are stabilized by intermolecular domain swapping interactions similar to those seen in other amyloids (see ARF related news story).
The directional intermolecular contact pattern exposes a hydrophobic groove on the growing end of the fiber where new Aβ1-42 monomers could bind. This groove also makes a welcome landing site for peptide analogs of residues 17-21 that have been shown to inhibit fiber growth (Gordon et al., 2001). The identification of this site may aid the design of additional inhibitors, the authors write.
The Aβ mutants used in these studies were observed to form a variety of structures, and the authors decided to test whether they could see a structure-toxicity relationship using a cultured neuronal cell line. Despite having long or short fibrils, or seed-like morphologies, all of the variants displayed neurotoxicity at approximately similar concentrations as did a wild-type control peptide (Aβ25-35). The most toxic mixtures contained fibrils, but enhanced toxicity was also seen with some oligomers, providing further evidence that harmful amyloid comes in several guises.—Pat McCaffrey.
Lührs T, Ritter C, Adrian M, Riek-Loher D, Bohrmann B, Döbeli H, Schubert D, Riek R. 3D structure of Alzheimer's amyloid-(1-42) fibrils. Proc Natl Acad Sci U S A. 2005 Nov 29;102(48):17342-7. Epub 2005 Nov 17. Abstract