Large size fibrillar bundles of the Alzheimer amyloid beta-protein.

Self-assembly of amyloid beta-protein (Abeta) and its deposition into senile plaques are distinctive features of Alzheimer's disease. Abeta forms typical linear aggregates known as amyloid fibrils, with a diameter of a few tens of nanometers and a length spanning from hundreds of nanometers to micrometers. Fibrils eventually assemble into large size clusters and precipitate in vivo in the brain deposits. Here, we study the late stage of aggregation of Abeta(1-40) in vitro at pH 3.1. We characterize the structure of fibrillar aggregates by a combined use of different experimental techniques. Small angle light scattering, heterodyne near field scattering, large angle light scattering, ultra small angle X-ray scattering and small angle X-ray scattering measurements have been performed to highlight the structural features of amyloid bundles over several lengthscales, from nanometers to tens of micrometers. Phase contrast optical microscopy has been used to complement scattering measurements and directly visualize some morphological details. We show that elongated fibrils of Abeta with a diameter of a few nanometers are packed into large size compact bundles having a typical size of tens of micrometers. The linear morphology of fibrils is reflected in the elongated shape of bundles.