In one project, Dr. Rosenberry and his laboratory colleagues are studying β-amyloid (Aβ), a component from human brain that may play a role in Alzheimer's disease (AD). Aβ corresponds to a number of closely related peptides of varying length, with 40- and 42-residue peptides being the most predominant. The brains of patients with AD contain large numbers of Aβ fibrils in the form of senile plaques. It now appears likely that Aβ peptides and fibrillar and nonfibrillar Aβ aggregates play an important role in the development of AD, and therapeutic strategies that prevent aggregate formation are attractive. These strategies can be enhanced by structural information about the aggregates. Aβ fibrils have been shown to consist of several closely related β-strand conformations that arrange to form in-register parallel β-sheets. Nonfibrillar Aβ aggregates, often denoted oligomers, have eluded structural characterization because of their low levels in AD brains. We have synthesized Aβ-42 and generated oligomers on anionic micelles. These oligomers are sufficiently homogenous to allow structural characterization by solid-state NMR. We have only partly completed the structural analysis, but it is clear that the structure of these oligomers is quite different from that of the Aβ fibrils.
In a second project, Dr. Rosenberry and his colleagues focus on acetylcholinesterase (AChE), an enzyme that 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 like organophosphates, and this can lead to neuromuscular paralysis and death. Individuals risk exposure to organophosphates in certain pesticides and in chemical warfare agents, and we 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 1.5-2.0 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 we have clarified its role in AChE function. We 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. We then found that substrates like acetylthiocholine itself transiently bind to the peripheral site to accelerate their entry into the acylation site. We also observed conformational interaction between the peripheral and acylation sites. These studies have led us to embark on the discovery or design of new peripheral site ligands that may offer protection against organophosphate toxicity.