Modulation of Tau Dysfunction In Vitro

Abstract

The microtubule associated protein tau is a causative factor in a class of neurodegenerative diseases termed tauopathies. Alzheimer's disease (AD) is the most prevalent tauopathy. In AD, natively unfolded, tau becomes hyperphosphorylated, and undergoes a conformational change allowing hexapeptide regions near microtubule binding repeat region 2 to aggregate into fibers. Tau exists in neurons as six alternatively spliced isoforms. The expression levels of tau are not changed in many tauopathies; however, various levels of each isoform can be included in aggregation depending on the disease. This indicates post-translational modifications or interactions with other proteins causes certain tau isoforms to be included in or excluded from these insoluble inclusions. Tau functions by assembling and stabilizing microtubules and is regulated by phosphorylation. In AD, the phosphorylation state of tau is altered so the affinity for microtubules is reduced; however, how changes in tau phosphorylation affect polymerization of tau isoforms is not fully understood. Abnormal aggregation of protein in a cell is mediated by molecular chaperones. A class of molecular chaperones, heat shock proteins, is upregulated in response to cellular stress. The most widely involved is heat shock protein 70 (Hsp70), which has upregulated and inversely proportional protein levels to tau aggregation in hippocampal neurons. Hsp70 can increase tau solubility in cells; however, it is unknown if Hsp70 can act directly on tau to prevent its dysfunction. This dissertation explores how phosphorylation with GSK-3β, the major kinase believed to be involved in tau hyperphosphorylation, and interactions with Hsp70, affect tau dysfunction (polymerization) and function (microtubule binding and assembly). Tau isoforms were found to be differentially affected by phosphorylation, as indicated by differences in their overall polymerization and various morphologies. Likewise, tau phosphorylation differentially affected their affinity for stabilized microtubules. Tau isoform polymerization was inhibited by Hsp70, with various concentrations of Hsp70 needed for complete inhibition. Finally, while Hsp70 altered the microtubule assembly properties of tau isoforms, each individual isoform was able to assemble microtubules robustly. Taken together, this dissertation shows tau isoforms respond differently to modifications and interaction with Hsp70, indicating each isoform could play a specific role in the progression of tauopathies.