Cell-based Models To Investigate Tau Aggregation

Abstract

Accumulation of abnormal tau aggregates in neuron is an important pathological signature in multiple neurodegenerative disorders including Alzheimer's disease. Tau is a neuron specific microtubule-associated protein that regulates microtubule stability, which is critical for axonal outgrowth and synaptic plasticity. In a pathological condition, tau dissociates from microtubules and forms insoluble aggregates called neurofibrillary tangles (NFTs). The accumulation of NFTs in neuron directly correlates with microtubule dysfunction and neuronal degeneration. Due to the pathophysiological importance of tau, great efforts have been made to understand tau aggregation processes and find therapeutics to halt or reverse the processes. However, progress has been slow due to the lack of a suitable method for monitoring tau aggregation. In this mini-review, we will review the conventional methods for studying tau aggregation, and introduce recent cell-based sensor approaches that allow monitoring tau aggregation in living cells.

Keywords: BiFC; Cell-based sensor; In vitro assay; Tau oligomerization; Tau phosphorylation; Tauopathy.

Fig. 1

Tau aggregation and neuronal degeneration . (a) In a healthy neuron, tau stabilizes microtubules promoting axonal outgrowth and synaptic vesicle transport. (b) When tau goes bad, tau becomes neurotoxic aggregates and microtubules become dissociates.

Fig. 2

Diagrammatic representation of tau aggregation .

Fig. 3

Illustration represents human tau isoforms and truncated repeat domains. Human tau has six isoforms resulting from alternative splicing. The truncated tau repeat domains (K18 or K19) are known to facilitate tau aggregation.

Fig. 4

Tau phosphorylation. (a) Initiation of tau phosphorylation by GSK3β. (b) Putative tau phosphorylation residues. The red colored residues indicate GSK3β mediated tau phosphorylation sites.

Fig. 5

Cell-based sensors for tau aggregation. (a) FRET-based sensor. Tau protein is fused to CFP or YFP. In the system, the FRET sensor is activated only when tau assembles. (b) BiFC/GFP turn-off sensor. Tau is fused to a smaller fragment of GFP (GFP11) and co-expressed with a large GFP fragments (GFP1–10). When tau exists as a monomer, GFP1–10 freely binds to GFP11 giving the strong GFP fluorescence. When tau aggregates, GFP1–10 cannot access to GFP11, resulting in the decrease of GFP fluorescence intensity. (c) Tau is fused to non-fluorescent N- or C-terminal fragment of Venus fluorescence protein (VN173 or VC155). The Venus fluorescence turns on only when tau assembles together.