Amyloid-beta (A-beta) aggregates are tangles of proteins most notably associated with neurodegenerative diseases like Alzheimer’s. Despite its constant stint in the limelight, however, researchers have been unable to get a good understanding of how A-beta comes together and breaks apart.
“The way A-beta behaves in a variety of environments, including the human brain, is elusive,” said Brian Sun, an electrical systems and engineering alumnus of Washington University in St. Louis who is now an MD/PhD student in the School of Medicine.
“There’s an understanding of growth and decay that isn’t fully fleshed out,” he added.
That’s going to change, thanks to research recently published by Sun with colleagues in Associate Professor Matthew Lew’s lab at the Preston M. Green Department of Electrical and Systems Engineering (ESE) in WashU’s McKelvey School of Engineering.
In their work, Sun and colleagues were able to make measurements of amyloid fibril beta-sheet assemblies, the underlying girders of the protein conglomeration, while they were changing. Previous high-resolution microscopy studies have only gotten static shots.
“We wanted to look specifically at dynamics of the underlying structure of A-beta that could be responsible for the changes we’re seeing, not just changes in the overall shape,” Sun said.
Lew uses Lego as an analogy, noting that current imaging technology shows you the full Lego building but not any look at how each individual brick is organized.
“The individual proteins are always changing in response to their environment,” Lew said. “It is like having certain Lego bricks causing other bricks to change their shape. The changing architecture of the proteins and the assembled aggregates together leads to the complexity of neurogenerative disease.”
The Lew lab has developed a new type of imaging tech that allows them to see the orientation and other minute details in nanostructures of biological systems that were previously invisible. Their technique — single-molecule orientation–localization microscopy (SMOLM) — uses the flashes of light from chemical probes to visualize the sheets of peptides underlying Aβ42, one kind of A-beta peptide.
Using SMOLM lets them look at individual orientation of the underlying beta-sheets to see the relationship between their organization and how that relates to the overall structure of the amyloid protein.
Aβ42 is constantly changing, and step one is to try to find a method to the madness, a model or pattern of action to predict the protein’s behavior.
Now that the Lew lab can make these measurements, they made some intuitive observations and found some surprises hidden in the amyloid-beta architecture.
As can be expected, stable Aβ42 structures tend to retain stable underlying beta-sheets; growing structures have underlying beta-sheets that become more defined and rigid as the growth continues. Decaying structures exhibit increasingly disordered and less rigid beta-sheets. But they also found more than one way that Aβ42 can renovate.
The researchers also discovered that Aβ42 can grow and decay in ways that defy expectations. For example, Aβ42 can grow and decay in ways that preserve the underlying structure; sometimes there’s growth where the peptides just pile on, but the underlying beta-sheet orientations don’t change. In other cases, Aβ42 undergoes “stable decay,” where the opposite happens, i.e., peptides leave, but beta-sheet structure remains. Finally, Aβ42’s beta-sheets sometimes reorganize and change orientations without immediate accompanying changes to the overall shape. These nano-structural reorganizations can predispose to future large-scale remodeling.
“Because SMOLM can track Aβ42’s underlying organization and not just its shape, we can see different kinds of subtypes of remodeling that aren’t visible to diffraction-limited, non-orientation imaging modalities,” Sun said.
