Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects millions of people worldwide. Current pharmacological treatments for AD are symptomatic and ineffective, and clinical trials of therapies based on anti-A? monoclonal antibodies (mAbs) or secretase inhibitors have been disappointing. One reason for the recent failures of anti-A? therapies, even those begun presymptomatically, is that the A? assemblies that accumulate in AD are conformationally diverse, and currently available mAbs do not target the primary neurotoxic species. Therefore, it is critical to thoroughly characterize the molecular mechanisms by which therapeutic mAbs interact with A? and influence its assembly, structure, and toxicity. It is presumed that AD pathology starts by the binding of neurotoxic A? oligomers (A?o) to receptor proteins or lipids on the surface of neurons, resulting ultimately in synaptic dysfunction and degeneration. In previous studies, we have used super-resolution microscopy to directly visualize ?-receptor interactions at the nanometer scale. We find that one documented A? receptor, the cellular prion protein, PrPC, specifically inhibits the polymerization of A? fibrils via a unique mechanism in which it binds specifically to the rapidly growing end of each fibril, thereby blocking polarized elongation at that end. PrPC binds neurotoxic oligomers and protofibrils in a similar fashion, suggesting that it may recognize a common, end-specific, structural motif on all of these assemblies. Additional experiments suggest that two other previously described A? receptors (Fc?RIIb, and LilrB2) act in a similar fashion. Taken together, our results suggest that neurotoxic signaling by several different receptors may be activated by common molecular interactions with both fibrillar and oligomeric A? ligands. The experimental approach used in these studies opens up the possibility of probing the mechanism of action of other agents that affect A? polymerization or toxicity, in particular anti-A? mAbs such as those currently undergoing extensive testing in clinical trials. In this application, we propose to characterize the mechanism of action of four clinical stage antibodies (aducanumab, gantenerumab, bapineuzumab and solanezumab), as well as a panel of conformation-dependent antibodies that recognize oligomeric, pre-fibrillar, and fibrillar forms of A?. First, we will measure the effect of these mAbs on A? aggregation processes using biochemical assays. Then, we will take advantage of single molecule, SRM to determine the localization of the mAbs on the individual A? aggregates. In addition, we will directly measure fibril elongation rates, as well as primary and secondary nucleation processes, in the presence and absence of each mAb. Finally, we will determine how these mAbs affect the interaction between A? and three of its documented receptor proteins; PrPC, Fc?RIIb, and LilrB2. This proposal will lay the groundwork for the design of improved mAbs that selectively alter specific steps in the A? assembly process, and that block the interaction of oligomeric aggregates with cell surface receptors that transduce their neurotoxic effects.
Alzheimer's disease is the most common cause of memory loss in the elderly population, currently affecting 5 million individuals in the U.S., a number that is expected to triple by 2050 as the population ages. To date, immunotherapeutics to prevent or treat AD have yielded limited clinical success for most patients. This application will employ a multi-disciplinary experimental approach to understand how toxic proteins in the brain cause degeneration of neuronal connections in Alzheimer's disease, thereby laying the groundwork for devising new therapies.
