Tau-directed drug discovery for Alzheimer's disease and related tauopathies: a focus on tau assembly inhibitors

Abstract

The microtubule-associated protein tau forms insoluble filaments that deposit as neurofibrillary tangles (NFTs) in the brains of those with Alzheimer's disease (AD) and other related neurodegenerative disorders. The presence of both NFTs and amyloid beta (Abeta)-containing senile plaques within the brain is required to confirm the diagnosis of AD. However, the demonstration that familial AD can be caused by mutations that result in increased Abeta production has resulted in AD drug discovery strategies that are largely focused on reducing brain Abeta levels, with substantially less emphasis on tau-directed approaches. This trend may be changing, as there are an increasing number of research programs that are exploring ways to reduce NFTs in AD and related tauopathies. We briefly review recent advances in tau-based drug discovery, with an emphasis on the identification of compounds that inhibit the assembly of tau into multimers and fibrils.

Figure 1. Inhibitors of tau aggregation

¹Wischik et al., 1996; ²Taniguchi et al., 2005; ³Necula et al., 2005; ⁴Pickhardt et al., 2005; ⁵Khlistunova et al., 2006; ⁶Pickhardt et al., 2007; ⁷Bulic et al., 2007; ⁸Crowe et al., 2007.

Figure 2. Characterization of a tau fibrillization inhibitor

A unique quinoxaline (113F08) identified from HTS of ~51,000 compounds (Crowe et al., 2007) underwent A) a full-dose response analysis in a K18 tau fibrillization reaction as monitored by ThT fluorescence. B) K18 fibrillization reaction mixtures that were incubated with increasing concentrations of 113F08, as indicated, were subjected to centrifugation and the supernatant and pellet fractions were subjected to SDS-PAGE followed by densitometry to ascertain the amount of K18 in each fraction. The black and white bar graph for each concentration represents the percentage of K18 that was in the pellet and supernatant fractions, respectively. Much more soluble K18 was found in the samples containing 2.5 and 20 µM compound, and the pellet fraction from these samples had significantly fewer K18 fibrils as judged by EM than the samples containing lower concentrations of 113F08.

Figure 3. Schematic representation of a FP-based tau assembly assay and the kinetics of tau fibrillization

A FP assay was utilized to monitor tau assembly in which Alexa Red-labeled K18PL was mixed at a 1:62.5 molar ratio with unlabeled K18PL. A) Upon addition of heparin and initiation of tau assembly, Alexa Red-labeled K18PL is incorporated into tau multimers, thereby decreasing the rotational freedom of the fluorescent label with a resulting increase in FP signal. B) FP signal as a function of time in a K18PL fibrillization reaction.