Intermediate Antiparallel Fibrils in Aß40 Dutch Mutant Aggregation: Insights from Nanoscale Infrared Spectroscopy.

Cerebral amyloid angiopathy (CAA), which involves amyloid deposition in blood vessels leading to fatal cerebral hemorrhage and recurring strokes, is present in the majority Alzheimer's disease (AD) cases. Familial mutations in the amyloid ß peptide are correlated to higher risks of CAA and are mostly comprised of mutations at residues 22 and 23. While the structure of the wild-type Aß peptide has been investigated in great detail, less is known about the structure of mutants involved in CAA and evolutions thereof. This is particularly true for mutations at residue 22, for which detailed molecular structures, as typically determined from Nuclear Magnetic Resonance (NMR) spectroscopy or electron microscopy, do not exist. In this report, we have used nanoscale infrared (IR) spectroscopy augmented with atomic force microscopy (AFM-IR) to investigate structural evolution of the Aß Dutch mutant (E22Q) at the single aggregate level. We show that in the oligomeric stage, the structural ensemble is distinctly bimodal, with the two subtypes differing with respect to population of parallel ß sheets. Fibrils on the other hand are structurally homogeneous, with early-stage fibrils distinctly antiparallel in character, which develop parallel ß sheets upon maturation. Furthermore, the antiparallel structure is found to be a persistent feature across different stages of aggregation.

Intermediate antiparallel fibrils in Aß40 Dutch mutant aggregation: nanoscale insights from AFM-IR.

Cerebral Amyloid Angiopathy (CAA), which involves amyloid deposition in blood vessels leading to fatal cerebral hemorrhage and recurring strokes, is present in the majority Alzheimer's disease cases. Familial mutations in the amyloid ß peptide is correlated to higher risks of CAA, and are mostly comprised of mutations at residues 22 and 23. While the structure of the wild type Aß peptide has been investigated in great detail, less is known about the structure of mutants involved in CAA and evolutions thereof. This is particularly true for mutations at residue 22, for which detailed molecular structures, as typically determined from Nuclear Magnetic Resonance (NMR) spectroscopy or electron microscopy, do not exist. In this report, we have used nanoscale infrared (IR) spectroscopy augmented with Atomic Force Microscopy (AFM-IR) to investigate structural evolution of the Aß Dutch mutant (E22Q) at the single aggregate level. We show that that in the oligomeric stage, the structural ensemble is distinctly bimodal, with the two subtypes differing with respect to population of parallel ß-sheets. Fibrils on the other hand are structurally homogeneous, with early-stage fibrils distinctly anti parallel in character, which develop parallel ß-sheets upon maturation. Furthermore, the antiparallel structure is found to be a persistent feature across different stages of aggregation.

Nanoscale Infrared Spectroscopy Identifies Structural Heterogeneity in Individual Amyloid Fibrils and Prefibrillar Aggregates.

Amyloid plaques are one of the central manifestations of Alzheimer's disease pathology. Aggregation of the amyloid beta (Aß) protein from amorphous oligomeric species to mature fibrils has been extensively studied. However, structural heterogeneities in prefibrillar species, and how that affects the structure of later-stage aggregates are not yet well understood. The integration of infrared spectroscopy with atomic force microscopy (AFM-IR) allows for identifying the signatures of individual nanoscale aggregates by spatially resolving spectra. We use AFM-IR to demonstrate that amyloid oligomers exhibit significant structural variations as evidenced in their infrared spectra. This heterogeneity is transmitted to and retained in protofibrils and fibrils. We show that amyloid fibrils do not always conform to their putative ordered structure and structurally different domains exist in the same fibril. We further demonstrate that these structural heterogeneities manifest themselves as a lack of ß sheet structure in amyloid plaques in Alzheimer's tissue using infrared imaging.