No one knows if these "ß -amyloid plaques" cause the disease, or are merely a symptom. But as America ages - people over 85 are the fastest-growing segment of the population, and 30 to 40 percent of them will contract the disease - every clue may help medical science slow or stop the coming epidemic.
"The three-dimensional structure of the entire ß-amyloid fibril is the largest noncrystalline structure ever characterized," said Argonne researcher P. Thiyagarajan, one of the project's principal investigators.
Previous research in the field had shown that each plaque is a tangle of millions of ribbon-like peptide chains called ß-amyloid fibrils. Peptides are chains of amino acids, simple organic compounds that form the building blocks of proteins. Inside the body's cells, amino acids are used for growth, maintenance and repair. Some peptides are needed for physiological processes; others have antibacterial properties.
Amyloid peptides are chains of 40 to 42 amino acid residues. Due to their unique chemical architecture, consisting of water-loving and water-avoiding amino acid sequences, Alzheimer's peptides "self-assemble" to form tangles of fibrils in the brains of persons with Alzheimer's Disease.
Alzheimer's peptides seem to associate laterally, stacking on top of the other to form ribbon-like structures - the beginnings of a fibril. The ribbon-like structures further associate to form thick twisting fibrils. Inside the brain, these fibrils can further associate and form insoluble plaques.
The insolubility of these large peptides prevented researchers from carrying out studies of their self-assembly. Due to the lack of detailed structural information, little progress had been made on understanding fibril structure and self-assembly - crucial to identifying targets for potential drug candidates.
But now, a breakthrough in this field has come from the collaborative work of Argonne scientists Robert Botto and Thiyagarajan with two University of Chicago groups - David Lynn and his students Tim Burkoth and David Morgan from the Chemistry Department, and Stephen Meredith and student T.L.S. Benzinger from the Pathology Department.
Through a careful analysis of the Alzheimer's peptide the researchers found the strategic sites where amino acids can be removed such that the "truncated peptide" still would form fibrils just like the whole peptide.
"This finding makes it possible to study the actual assembly process of the whole peptide," said Thiyagarajan, one of the project's principal investigators. Since the truncated peptide is less complex than the whole peptide, it would be possible to analyze its self-assembly process and the fibril structure with spectroscopic and scattering techniques.
To prevent the individual fibrils from sticking together, researchers added polyethylene glycol - a water-soluble polymer - to the hydrophobic (water-avoiding) side of each peptide. "This served as an extremely crucial step," Thiyagarajan said, "as it allowed us to isolate and study individual fibrils, which was instrumental to determining their structure."
In addition to electron microscopy studies on the fibrils at the University of Chicago, solid-state nuclear magnetic resonance (NMR) studies were performed at Botto's lab at Argonne. The researchers replaced specific amino acids in the truncated peptide, one at a time, with those containing carbon-13 isotope, enabling them to measure the distances between carbon atoms with a precision down to two-tenths of an Angstrom (one hundred-millionth of a centimeter). NMR gave the local structure and organization of the peptides in the fibril. "Having the NMR facility here at Argonne was an important part of an excellent collaboration," Thiyagarajan said.
Once the researchers deduced the atomic structure and the orientation of the peptide chains, they began to investigate the fibril's structural hierarchy as well as its self-assembly mechanism. Small-angle neutron scattering at Argonne's Intense Pulsed Neutron Source let them monitor the fibril's initiation and growth in solution. These studies revealed that the fibrils form faster when the solution's pH is higher.
Results on the radius and the molecular weight from the small-angle neutron scattering analyses showed that six peptide ribbons are laminated together, 10 Angstroms apart, by weak bonding between corresponding hydrogen atoms along each peptide molecule. These strands gently twist in a clockwise direction to form the helical structures seen in electron micrographs.
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