Mechanisms of Adsorption and Surface-Mediated Aggregation of Intrinsically Disordered Protein Tau at Model Surfaces

Abstract

The adsorption and aggregation of an intrinsically disordered soluble protein, tau, into insoluble filaments is a defining hallmark of many neurodegenerative diseases, commonly referred to as tauopathies. In its native state, the protein tau’s function is to promote the assembly, and aid in the stabilization of microtubules. The microtubules allow for material transport through the axon, to and from the neuron. While the presence of aggregated tau protein fibrils are hypothesized to accelerate neuronal degradation, possibly by destabilizing microtubules, or disrupting cell membranes, more recent research has established the presence of soluble oligomeric species as being cytotoxic. These results necessitate a complete fundamental understanding of the governing principles that modulate the initial steps in the mechanisms of tau protein aggregation. The macromolecular environment, including the presence of surfaces such as the cell membrane, and the presence of macromolecules in a crowded environment, has been implicated in the aggregation of tau protein. However, the exact role of surfaces in modulating Tau protein aggregation has not been explored in detail. We hypothesize that Tau protein aggregation at model surfaces is modulated by two factors, the physicochemical properties of the surfaces, as well as the biochemistry of the protein molecules. The work presented in this thesis project employs a combination of biophysical techniques to study the adsorption and aggregation of a wild type and several mutations of tau protein at model surfaces. A Quartz Crystal Microbalance with Dissipation (QCM-D) was used to monitor the adsorption of different tau species at nanomolar concentrations, mimicking the in vivo situation, to surfaces with different surface charge, wettability and softness, while Atomic Force Microscopy (AFM) was utilized to obtain direct visualization of the proteins at these different surfaces. Our results indicate that the hydrophobic amino acid sequence in the microtubule binding region was the leading force driving the adsorption of tau proteins to different surfaces. Further, AFM images provided direct evidence of the presence of oligomeric tau species at the interfaces, establishing that the solid surface did in fact provide a template for the tau protein to form aggregates. Adsorption of different tau protein mutations to phospholipid covered surfaces of different fluidity indicated that tau protein oligomers can also cause destabilization or disintegration of lipid bilayers. Such disintegration may well be the cause of observed cell death in several tauopathies. In summary, this thesis establishes that both protein biochemistry and the physicochemical properties of the surface modulate surface mediated aggregation. The work described in this thesis also provides a foundation for further research focused on the role of surfaces as templates that mediate tau aggregation pathway in vivo. A complete understanding of the mechanisms of tau aggregation will ultimately lead to strategies for therapeutic solutions for neurodegenerative diseases.