Pseudophosphorylation of tau at S422 enhances SDS-stable dimer formation and impairs both anterograde and retrograde fast axonal transport

Abstract

In Alzheimer's disease (AD), tau undergoes numerous modifications, including increased phosphorylation at serine-422 (pS422). In the human brain, pS422 tau protein is found in prodromal AD, correlates well with cognitive decline and neuropil thread pathology, and appears associated with increased oligomer formation and exposure of the N-terminal phosphatase-activating domain (PAD). However, whether S422E phosphorylation contributes to toxic mechanisms associated with disease-related forms of tau remains unknown. Here, we report that S422-pseudophosphorylated tau (S422E) lengthens the nucleation phase of aggregation without altering the extent of aggregation or the types of aggregates formed. When compared to unmodified tau aggregates, the S422E modification significantly increased the amount of SDS-stable tau dimers, despite similar levels of immunoreactivity with an oligomer-selective antibody (TOC1) and another antibody that reports PAD exposure (TNT1). Vesicle motility assays in isolated squid axoplasm further revealed that S422E tau monomers inhibited anterograde, kinesin-1 dependent fast axonal transport (FAT). Unexpectedly, and unlike unmodified tau aggregates, which selectively inhibit anterograde FAT, aggregates composed of S422E tau were found to inhibit both anterograde and retrograde FAT. Highlighting the relevance of these findings to human disease, pS422 tau was found to colocalize with tau oligomers and with a fraction of tau showing increased PAD exposure in the human AD brain. This study identifies novel effects of pS422 on tau biochemical properties, including prolonged nucleation and enhanced dimer formation, which correlate with a distinct inhibitory effect on FAT. Taken together, these findings identify a novel mechanistic basis by which pS422 confers upon tau a toxic effect that may directly contribute to axonal dysfunction in AD and other tauopathies.

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

“…While these potential regulatory events are not well understood, phosphorylation is important in modulating tau conformations. Previous studies have shown that pseudophosphorylation at the AT8 site alters tau folding (Jeganathan, et al, 2008) and significantly impairs anterograde FAT as a monomeric protein (Kanaan, et al, 2011), and more recently we found that pseudophosphorylation at Ser 422 similarly impairs anterograde transport as a monomer and is toxic to retrograde transport when aggregated (Tiernan, et al, 2016). Conversely, phosphorylation of tyrosine 18 within PAD appears to mitigate the deleterious effects of pathological forms of tau (Kanaan, et al, 2012), and might be involved in the normal regulation of PAD-dependent effects on transport (Kanaan, et al, 2015).…”

“…The recent development of tau antibodies that detect PAD exposure (Combs, et al, 2016,Kanaan, et al, 2011) or oligomeric species (Castillo-Carranza, et al, 2014b,Lasagna-Reeves, et al, 2012,Patterson, et al, 2011a,Ward, et al, 2013) has facilitated additional insight into the pathological conformations adopted by tau. More recently, we showed that several disease-related modifications of tau cause conformational display of the PAD in the amino terminus of tau and oligomeric tau aggregates impaired axonal transport confirming a link between pathological conformations and tau toxicity (Kanaan, et al, 2011,LaPointe, et al, 2009,Patterson, et al, 2011a,Patterson, et al, 2011b,Tiernan, et al, 2016). Moreover, many other studies suggest oligomeric tau is toxic to neurons in culture and in vivo (Castillo-Carranza, et al, 2014a,Castillo-Carranza, et al, 2014b,Fa, et al, 2016,Gerson, et al, 2016,Lasagna-Reeves, et al, 2010,Tian, et al, 2013,Usenovic, et al, 2015).…”

“…Importantly, aggregates of all six isoforms significantly impaired anterograde FAT, thus extending prior findings from experiments using the longest 4R tau isoform (Kanaan, et al, 2012,Kanaan, et al, 2011,LaPointe, et al, 2009). Although the longest 3R isoform (hT39) was found to impair retrograde FAT slightly, a finding not seen with any other unmodified tau construct evaluated to date (Kanaan, et al, 2012,Kanaan, et al, 2011,LaPointe, et al, 2009,Morfini, et al, 2007,Patterson, et al, 2011a), and while this may be due to a slight increase in retrograde rates with monomeric hT39 we recently found that pseudophosphorylated Ser 422 tau aggregates also impair retrograde FAT through an unknown mechanism (Tiernan, et al, 2016). Either way, this is intriguing and may merit further characterization.…”

Pretangle pathology within cholinergic nucleus basalis neurons coincides with neurotrophic and neurotransmitter receptor gene dysregulation during the progression of Alzheimer's disease

“…However, TrkA expression is under positive feedback from NGF (Holtzman et al, 1992; Li et al, 1995), and this pathway may be disrupted during the progression of dementia by reduced retrograde transport of cortical NGF to nbM consumer neurons (Mufson et al, 1995; Scott et al, 1995). While aggregated wild-type tau is a known inhibitor of anterograde fast axonal transport (Kanaan et al, 2012, 2011; LaPointe et al, 2009), there is some evidence to suggest that phosphorylation at S422 impairs retrograde transport, as well (Tiernan et al, 2016a). Thus, phosphorylation of tau at S422 and the resulting inhibition of retrograde transport may impair positive feedback from NGF leading to a loss of TrkA expression on the nbM neuronal cell surface.…”

Tau Structures

“…Finally, truncation of aspartic 421 facilitates tau aggregation and toxicity (García-Sierra et al, 2008) and phosphorylation at serine 422 could enhance SDS-stable dimer formation (Tiernan et al, 2016). …”