(485af) Morphological and Kinetic Changes of Aβ(1-42) Aggregation in Contact with Osmolytes

Alzheimer's disease is characterized by the formation of plaques in the brains of its victims that are composed primarily of the beta amyloid peptide (Aβ). These plaques are often found in the extracellular matrix of the neural cortex and are surrounded by patches of dead neurons. From this observation, it was long thought that the formation of plaques was the cause of neural death, yet recent observations have shown that small oligomeric species that precede fibril formation may be the toxic agents that cause neuronal death. The Aβ peptide has been found in vivo in a variety of lengths, from 39-43 amino acids. The primary component of the amyloid plaques is Aβ(1-42), and this species has been found to quickly aggregate under mild conditions. To better understand the chemical stresses that may enhance or retard the aggregation pathway, we have studied the kinetic and morphological changes of Aβ(1-42) in contact with different osmolytes. The rate of oligomer and fibril formation has been monitored with circular dichroism and thioflavin T fluorescence. The addition of 300 mM sugars and urea has had little impact on the rate of aggregation. But the addition of 300 mM guanidine HCl has shown little beta sheet formation, but aggregation has been demonstrated by an increase in thioflavin T fluorescence. When different concentrations of guanidine HCl (0 ? 3 M) were added prior to aggregation, there was a maximum fluorescence signal between 150 ? 300 mM and then as more guanidine HCl was added, the fluorescence signal decreased. The aggregates formed in the presence of guanidine HCl were also less stable than aggregates made in the presence of other osmolytes because they could be disintegrated by SDS. This shows that guanidine HCl is able to change the aggregation pathway for Aβ(1-42).

The morphology of the fibrils made while in contact with different osmolytes was monitored by atomic force microscopy (AFM). The morphology was different for many of the osmolytes, forming either short fibrils, small round aggregates, or large aggregates. Even though there was no change in the rate of aggregate formation, the pathway taken, as shown by the different morphologies, was most likely different for the osmolytes. Contact of the Aβ(1-42) peptide with different osmolytes during aggregation did little to change the rate of aggregation, but changed the morphology of the structures formed. As we study more about the aggregation of Aβ(1-42) in contact with these different chemical species, we can better predict the formation of Aβ plaques that are produced in the brains of Alzheimer's patients. This will hopefully lead to better treatments of this horrible disease.