Terrone L. Rosenberry, Ph.D., and colleagues in his Protein Biochemistry and Neuroenzymology Laboratory study two components from human brain that may play roles in Alzheimer's disease (AD): amyloid-beta (Aβ) and acetylcholinesterase (AChE).
Aβ. The brains of patients with AD contain large amounts of 40- and 42-residue amyloid-beta (Aβ) peptides that have aggregated to form fibrils in amyloid plaques. However, these peptides generate a variety of fibrils and smaller oligomers, some of which play important roles in the development of AD. Dr. Rosenberry and colleagues have produced a 150-kDa oligomer of the 42-residue form of Aβ. It is formed at a particular concentration of dilute anionic micelles that is slightly higher than their critical concentration. In a collaborative effort, researchers in Dr. Rosenberry's lab have obtained the oligomer's structure using solid-state NMR and cryo-electron microscopy (cryo-EM).
In contrast to fibrils, which form β-sheets with parallel N- and C-strands, the 150-kDa oligomer has two β-strands simultaneously organized into both parallel and antiparallel β-sheets. Cryo-EM reconstruction reveals a four-fold symmetry and a central pore. Researchers in the lab will refine this structure to obtain a higher resolution and test the hypothesis that formation of the antiparallel C-strands occurs first in the assembly pathway by examining aggregation of Aβ(20-42). Studies will also examine Aβ aggregates formed with other micelles to determine whether they, too, form oligomers.
AChE. This enzyme controls communication between nerve cells by the neurotransmitter acetylcholine. This communication is disrupted by the death of nerve cells in patients with AD, and inhibitors of AChE are approved as drugs to elevate acetylcholine and aid neuronal function in these patients. AChE is also important in another context: It is inactivated by toxic agents such as organophosphates, and this can lead to neuromuscular paralysis and death. Individuals risk exposure to organophosphates in certain pesticides and in chemical warfare agents. Researchers in Dr. Rosenberry's lab are pursuing new therapeutic strategies to protect against organophosphate poisoning.
The active site gorge of AChE contains two sites of ligand binding: an acylation site near the base of the gorge and a peripheral site at the mouth of the gorge some 1.5 to 2.0 nm from the acylation site. This peripheral site is an attractive target for the design of new drugs that might selectively protect against organophosphate reaction at the acylation site, and work in Dr. Rosenberry's lab has clarified the role of this site in AChE function. Lab researchers first showed that small ligands that bind selectively to the peripheral site inhibit AChE by slowing the rates at which substrates enter and exit the acylation site. They then found that substrates such as acetylthiocholine itself transiently bind to the peripheral site to accelerate their entry into the acylation site. Researchers also observed conformational interaction between the peripheral and the acylation sites. These studies have led Dr. Rosenberry and colleagues to embark on the design of new peripheral site ligands that may block the access of organophosphates but not acetylcholine to the acylation site, thereby offering protection against organophosphate toxicity.
Significance to patient care
Determining which Aβ structures form in the brain and what controls their assembly there are long-term goals of Dr. Rosenberry's research. Preparing monoclonal antibodies to the antiparallel C-strand β-sheets may allow researchers to survey for these structures in vivo. Determining whether the pore observed in the 150-kDa species has sufficient conductivity to provide toxic effects is an additional research goal in Dr. Rosenberry's lab. His AChE work is directed toward selectively blocking irreversible organophosphate inhibition, an important goal in preventing large-scale nerve agent toxicity.