Probing differences among Aß oligomers with two triangular trimers derived from Aß.

The assembly of the ß-amyloid peptide (Aß) to form oligomers and fibrils is closely associated with the pathogenesis and progression of Alzheimer's disease. Aß is a shape-shifting peptide capable of adopting many conformations and folds within the multitude of oligomers and fibrils the peptide forms. These properties have precluded detailed structural elucidation and biological characterization of homogeneous, well-defined Aß oligomers. In this paper, we compare the structural, biophysical, and biological characteristics of two different covalently stabilized isomorphic trimers derived from the central and C-terminal regions Aß. X-ray crystallography reveals the structures of the trimers and shows that each trimer forms a ball-shaped dodecamer. Solution-phase and cell-based studies demonstrate that the two trimers exhibit markedly different assembly and biological properties. One trimer forms small soluble oligomers that enter cells through endocytosis and activate capase-3/7-mediated apoptosis, while the other trimer forms large insoluble aggregates that accumulate on the outer plasma membrane and elicit cellular toxicity through an apoptosis-independent mechanism. The two trimers also exhibit different effects on the aggregation, toxicity, and cellular interaction of full-length Aß, with one trimer showing a greater propensity to interact with Aß than the other. The studies described in this paper indicate that the two trimers share structural, biophysical, and biological characteristics with oligomers of full-length Aß. The varying structural, assembly, and biological characteristics of the two trimers provide a working model for how different Aß trimers can assemble and lead to different biological effects, which may help shed light on the differences among Aß oligomers.

A Hexamer of a Peptide Derived from Aß16-36.

The absence of high-resolution structures of amyloid oligomers constitutes a major gap in our understanding of amyloid diseases. A growing body of evidence indicates that oligomers of the ß-amyloid peptide Aß are especially important in the progression of Alzheimer's disease. In many Aß oligomers, the Aß monomer components are thought to adopt a ß-hairpin conformation. This paper describes the design and study of a macrocyclic ß-hairpin peptide derived from Aß16-36. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size exclusion chromatography studies show that the Aß16-36 ß-hairpin peptide assembles in solution to form hexamers, trimers, and dimers. X-ray crystallography reveals that the peptide assembles to form a hexamer in the crystal state and that the hexamer is composed of dimers and trimers. Lactate dehydrogenase release assays show that the oligomers formed by the Aß16-36 ß-hairpin peptide are toxic toward neuronally derived SH-SY5Y cells. Replica-exchange molecular dynamics demonstrates that the hexamer can accommodate full-length Aß. These findings expand our understanding of the structure, solution-phase behavior, and biological activity of Aß oligomers and may offer insights into the molecular basis of Alzheimer's disease.

X-ray Crystallographic Structures of a Trimer, Dodecamer, and Annular Pore Formed by an Aß17-36 ß-Hairpin.

High-resolution structures of oligomers formed by the ß-amyloid peptide Aß are needed to understand the molecular basis of Alzheimer's disease and develop therapies. This paper presents the X-ray crystallographic structures of oligomers formed by a 20-residue peptide segment derived from Aß. The development of a peptide in which Aß17-36 is stabilized as a ß-hairpin is described, and the X-ray crystallographic structures of oligomers it forms are reported. Two covalent constraints act in tandem to stabilize the Aß17-36 peptide in a hairpin conformation: a δ-linked ornithine turn connecting positions 17 and 36 to create a macrocycle and an intramolecular disulfide linkage between positions 24 and 29. An N-methyl group at position 33 blocks uncontrolled aggregation. The peptide readily crystallizes as a folded ß-hairpin, which assembles hierarchically in the crystal lattice. Three ß-hairpin monomers assemble to form a triangular trimer, four trimers assemble in a tetrahedral arrangement to form a dodecamer, and five dodecamers pack together to form an annular pore. This hierarchical assembly provides a model, in which full-length Aß transitions from an unfolded monomer to a folded ß-hairpin, which assembles to form oligomers that further pack to form an annular pore. This model may provide a better understanding of the molecular basis of Alzheimer's disease at atomic resolution.