Role of Structure and Stability of β-Amyloid Oligomers and Fibrils On Intracellular Pathways Associated with Alzheimer's Disease

The detection of β-amyloid (Aβ), a 39 to 43 amino acid peptide, in the plaques of the human brain is one of the hallmarks of the Alzheimer's disease. While there is no consensus on the mechanism by which Aβ causes neuron death, most researchers believe that Aβ is toxic when aggregated and the most toxic species are certain oligomers formed along the aggregation pathway from monomer to fibril¹. While there is some evidence that oligomers have different molecular structure than fibrils², solid state NMR studies reveal that oligomers have similar molecular structure as fibrils³.

We believe that along with the molecular structure, the stability of the aggregates is an important factor that may contribute to the differences in toxicity of different Aβ aggregate species. It is interesting to observe that among the two prevalent Aβ peptide isoforms, Aβ(1-42) is more toxic compared to Aβ(1-40)⁴. In the present study, we prepared oligomers and fibrils of both Aβ(1-40) and Aβ(1-42), the two most prevalent Aβ peptides associated with Alzheimer's disease. We confirmed the structures of both the oligomers and the fibrils using transmission electron microscopy. We used circular Dichroism to monitor changes in the secondary structure of both fibrils and oligomers of Aβ(1-40) and Aβ(1-42) as a function of guanidine hydrochloride concentration. We fit the data to two and three state unfolding models and estimated free energies of unfolding for all species. Our results indicate that while both fibrils and oligomers were rich in β-sheet structure, oligomers were less stable compared to fibrils. Our results also provide an explanation for the higher toxicity of aggregates of Aβ(1-42) compared to that of  Aβ(1-40).

We then examined how both oligomers and fibrils affect intracellular signaling pathways, focusing particularly on the ubiquitin proteasome pathway. We are in the process of using a combination of immunoprecipitation and proteomic techniques to identify proteins that are uniquely targeted for degradation by the ubiquitin proteasome pathway. We hypothesize that the targeted proteins may be associated with neurotransmission which might provide insights into loss of learning and memory in Alzheimer's disease.

References:

1. Lambert et al., (1998) Proc. Natl. Acad. Sci.U. S. A. 95, 6448–6453

2. Kayed, R et al. (2003) Science 300, 486–489

3. Chimon S et al., (2007) Nature Struc. and Mol. Bio. 14, 1157-1164

4. Omar et al., (2000) Biochem. and Biophy. Res. Comm. 273, 1003–1007