Yeast as a model system to study tau biology

Abstract

Hyperphosphorylated and aggregated human protein tau constitutes a hallmark of a multitude of neurodegenerative diseases called tauopathies, exemplified by Alzheimer's disease. In spite of an enormous amount of research performed on tau biology, several crucial questions concerning the mechanisms of tau toxicity remain unanswered. In this paper we will highlight some of the processes involved in tau biology and pathology, focusing on tau phosphorylation and the interplay with oxidative stress. In addition, we will introduce the development of a human tau-expressing yeast model, and discuss some crucial results obtained in this model, highlighting its potential in the elucidation of cellular processes leading to tau toxicity.

Figure 1

Schematic representation of the six isoforms of tau, present in the central nervous system, and their amino acid lengths. The isoforms are generated by alternative splicing of exons 2, 3, and 10. As shown for the longest isoforms (2N4R), tau can be divided into the projection domain and the assembly domain, based on the cleavage by chymotrypsin after Tyr197 [25]. While tau binds to MT via the microtubule-binding domain (MTBD) in the assembly domain, sequences in the projection domain regulate, among others, the spacing between MT. In an alternative description, tau is subdivided into 4 domains: an N-terminal acidic region, followed by the proline-rich region, the MTBD, and the C-terminal tail.

Figure 2

Chain-of-events involved in the onset and propagation of tau pathology. Several upstream events have been shown to lead to tau malfunctioning, such as Aβ-mediated effects in Alzheimer's disease. Tau hyperphosphorylation and truncations are thought to constitute early and crucial modifications involved in tau pathology, and may mediate conformational changes leading to oligomerization and aggregation into higher-order aggregates, such as paired-helical filaments (PHF) and end-stage neurofibrillary tangles (NFT). Different forms or tau, especially hyperphosphorylated, oligomeric species, are thought to mediate toxicity via multiple mechanisms, including both loss of normal functions and gain of toxic functions. Note that some consequences of tau toxicity, such as stimulation of oxidative stress and neuroinflammation, reinforce further tau malfunctioning, creating a detrimental, self-sustaining cycle that propagates tau pathology throughout the brain. See text for details.

Figure 3

Phosphoepitope mapping of human protein tau (2N4R isoform) in yeast. Western blotting with indicated monoclonal antibodies of total protein extracts from wild-type (lanes 1), mds1Δ (lanes 2), and pho85Δ (lanes 3) yeast strains. The arrowhead on the right denotes a slow-mobility, hyperphosphorylated tau species. Adapted from [84] with permission from the publisher and the authors.

Figure 4

Determination of soluble (SolT) and sarkosyl-insoluble (SinT) fractions from wild-type tau or the synthetic mutants tau-S409A and tau-S409E, as obtained in WT cells (a) or pho85Δ cells. (b) Western blot with tau5 of a representative experiment is shown on the left, and quantifications of SinT levels for each experiment are shown on the right. Adapted from [86] with permission from the publisher and the authors.

Figure 5

Western blot analysis with the indicated antibodies, of total protein extracts isolated from tau-expressing WT and pho85Δ cells grown in a medium without (−) or with supplementation of 20 mM FeSO₄ (+). See Figure 3 for antibody specificity. Adapted from [86] with permission from the publisher and the authors.