Analysis of isoform-specific tau aggregates suggests a common toxic mechanism involving similar pathological conformations and axonal transport inhibition

Abstract

Misfolded tau proteins are characteristic of tauopathies, but the isoform composition of tau inclusions varies by tauopathy. Using aggregates of the longest tau isoform (containing four microtubule-binding repeats, 4-repeat tau), we recently described a direct mechanism of toxicity that involves exposure of the N-terminal phosphatase-activating domain (PAD) in tau, which triggers a signaling pathway that disrupts axonal transport. However, the impact of aggregation on PAD exposure for other tau isoforms was unexplored. Here, results from immunochemical assays indicate that aggregation-induced increases in PAD exposure and oligomerization are common features among all tau isoforms. The extent of PAD exposure and oligomerization was larger for tau aggregates composed of 4-repeat isoforms compared to those made of 3-repeat isoforms. Importantly, aggregates of all isoforms exhibited enough PAD exposure to significantly impair axonal transport in the squid axoplasm. We also show that PAD exposure and oligomerization represent common pathological characteristics in multiple tauopathies. Collectively, these results suggest a mechanism of toxicity common to each tau isoform that likely contributes to degeneration in different tauopathies.

Production of recombinant tau oligomers in vitro

“…Several alterations to various parameters within the standard optimized protocol can result in modifications to filament morphology, including the enhanced formation of oligomeric species. Some of these modifications include changes to the tau protein itself, such as changing the tau isoform content or inserting specific point mutations (Combs & Gamblin, 2012; Cox et al, 2016; King, Gamblin, Kuret, & Binder, 2000). Increasing the arachidonic acid to tau ratio can result in a nearly homogenous population of tau oligomers (Carlson et al, 2007).…”

“…Here, we provide a quick and simple method for imaging tau aggregates generated in vitro with recombinant tau protein. The protocol below describes TEM using unfixed samples, but glutaraldehyde fixation can be used as an alternative approach as described before (Cox et al, 2016). Using these standard methods tau oligomers are easily visualized with TEM and typically range from6 to 16nm in diameter (mean = ~12nm; Fig.…”

“…Importantly, toxic effects can be observed in animal models prior to the formation of tangles, indicating a toxic form of tau precedes them (Andorfer et al, 2005). This toxic species may be oligomeric tau, a nonfibrillar, multimeric, soluble form of the protein that has been identified in a variety of tauopathies, including Alzheimer’s disease, corticobasal degeneration, progressive supranuclear palsy, Pick’s disease, and chronic traumatic encephelopathy (Cox et al, 2016; Gerson et al, 2014; Kanaan et al, 2016; Lasagna-Reeves et al, 2012; Maeda et al, 2006; Patterson et al, 2011). The common presence of tau oligomers suggests that they could act as a toxic molecule across all tauopathies through a similar mechanism.…”

Regulation of motor proteins, axonal transport deficits and adult-onset neurodegenerative diseases

“…Relevant to AD pathogenesis, multiple independent reports documented increased GSK3 activity in AD, as well as in various tauopathies (Cox et al, 2016; Crews and Masliah, 2010; Hernandez and Avila, 2008; Hernandez et al, 2010). Several different pathways may contribute to increased GSK3 activity in AD.…”

“…Further, increased GSK3 activation and concomitant deficits in AT were also found in mutant APP or presenilin 1-based cellular (Pigino et al, 2003) and mouse-based models (Lazarov et al, 2007). Highlighting the disease relevance of these findings, immunohistochemical analysis of post mortem AD and various tauopathy brain samples demonstrated increased PAD exposure as an early pathogenic event common to all these diseases (Cox et al, 2016). …”