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-Aβ monoclonal antibodies (mAbs) or secretase inhibitors have been disappointing. One reason for the recent failures of anti-Aβ therapies, even those begun presymptomatically, is that the Aβ 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 Aβ and influence its assembly, structure, and toxicity. It is presumed that AD pathology starts by the binding of neurotoxic Aβ oligomers (Aβo) 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 β-receptor interactions at the nanometer scale. We find that one documented Aβ receptor, the cellular prion protein, PrPC, specifically inhibits the polymerization of Aβ 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 Aβ receptors (FcγRIIb, 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 Aβ ligands. The experimental approach used in these studies opens up the possibility of probing the mechanism of action of other agents that affect Aβ polymerization or toxicity, in particular anti-Aβ 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 Aβ. First, we will measure the effect of these mAbs on Aβ aggregation processes using biochemical assays. Then, we will take advantage of single molecule, SRM to determine the localization of the mAbs on the individual Aβ 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 Aβ and three of its documented receptor proteins; PrPC, FcγRIIb, and LilrB2. This proposal will lay the groundwork for the design of i...