Supported by the National Institutes of Health, the research was done in collaboration with scientists at Eli Lilly and Company and the Washington University School of Medicine. "Because AFM provides three-dimensional topographical information at the nanoscale, it could prove important in assessing the potential usefulness of molecules like antibodies to effectively inhibit protein aggregation associated with Alzheimer's Disease," said Tomasz Kowalewski, Ph.D., assistant professor of chemistry at Carnegie Mellon and senior author on the paper.
Alzheimer's Disease belongs to a class of disorders called conformational diseases, which are caused by changes in a protein's physical state. Another widely known example is prion-based 'Mad Cow' disease. Tools used to understand these disorders are targeted primarily to biochemical processes, noted Kowalewski, who works at the Mellon College of Science. "Because AFM probes the physical state of proteins, it could really assist in understanding conformational diseases, which traditionally have been difficult to fight," said Kowalewski.
A hallmark of Alzheimer's Disease is the presence of insoluble plaques in the brains of disease victims. These plaques are made of beta amyloid (Aß), a peptide derived from a precursor protein found in cell membranes. Once cleaved from its parent protein, the Aß peptide readily aggregates into fibrils that are rich in insoluble stacked structures called beta sheets.
These fibrils then congeal into tangled plaques. For the past few years, scientists in the field have been focusing considerable efforts on developing molecules that would inhibit Aß aggregation or perhaps even break up existing plaques. In this quest, they have recently turned their attention to antibodies that bind to different portions of the Aß peptide.
In their study, Kowalewski and a graduate student, Justin Legleiter, incubated the Aß peptide in solution with two different monoclonal antibodies: m3D6, which binds near one end of the peptide called the amino terminus, and m266.2, which binds to the peptide's central portion.
Over several days, the researchers placed drops of the sample solutions on mica sheets and observed the degree of protein aggregation using AFM. The m266.2 antibody proved much better than m3D6 at preventing the formation of amyloid fibrils.
"Interestingly, we found that both antibodies interfere with the formation of protofibrils, or fibril precursors," noted Kowalewski. "In solutions of Aß alone, numerous protofibrils were present under our experimental conditions as early as the third day, whereas in the presence of m3D6 they grew at a slower rate. We found that m266.2 completely inhibited protofibril formation."
The scientists believe that the two antibodies differ in their ability to inhibit fibril formation due to the way they bind to the Aß peptide. Binding at one end of Aß, m3D6 does not inhibit the formation of extended beta sheets, which are the major structural feature of mature fibrils. Because m266.2 binds to the center of Aß, it blocks the formation of these extended beta sheets, the investigators surmised.
In AFM, a tiny lever ending with an ultra-sharp tip is scanned across a surface from side-to-side and top-to-bottom, much as a cursor moves across a computer screen. A laser beam reflected off the lever's end monitors its vertical motion. Perturbation of the lever motion by nanoscale features of a sample's surface topography is used to reconstruct a detailed three-dimensional map of the surface. In the study, the research team used custom-written software to process and to analyze their AFM images.
AFM is an appealing tool, according to Kowalewski, because it provides three-dimensional nanoscale images and, unlike some other techniques, it does not require specialized preparation or contrasting agents that could introduce artifacts. In addition, it allows to study the samples in liquids, under nearly physiological conditions, according to Kowalewski, who is now pursuing this kind of AFM studies to understand the interaction of antibodies and lipoproteins, with the Aß peptide.
Collaborating investigators include Dan L. Czilli, Bruce Gitter and Ronald B. DeMattos (Eli Lilly and Company) and David M. Holtzman (Washington University).