Differential compartmental processing and phosphorylation of pathogenic human tau and native mouse tau in the line 66 model of frontotemporal dementia

Abstract

Synapse loss is associated with motor and cognitive decline in multiple neurodegenerative disorders, and the cellular redistribution of tau is related to synaptic impairment in tauopathies, such as Alzheimer’s disease and frontotemporal dementia. Here, we examined the cellular distribution of tau protein species in human tau overexpressing line 66 mice, a transgenic mouse model akin to genetic variants of frontotemporal dementia. Line 66 mice express intracellular tau aggregates in multiple brain regions and exhibit sensorimotor and motor learning deficiencies. Using a series of anti-tau antibodies, we observed, histologically, that non-phosphorylated transgenic human tau is enriched in synapses, whereas phosphorylated tau accumulates predominantly in cell bodies and axons. Subcellular fractionation confirmed that human tau is highly enriched in insoluble cytosolic and synaptosomal fractions, while endogenous mouse tau is virtually absent from synapses. Cytosolic tau was resistant to solubilisation with urea and Triton X-100, indicating the formation of larger tau aggregates. By contrast, synaptic tau was partially soluble after Triton X-100 treatment and most likely represents aggregates of smaller size. Mass spectrometry corroborated that synaptosomal tau is non-phosphorylated. Tau enriched in the synapse of line 66 mice, therefore, appears to be in an oligomeric and non-phosphorylated state, and one that could have a direct impact on cognitive function.

Proteomic Analysis of Hydromethylthionine in the Line 66 Model of Frontotemporal Dementia Demonstrates Actions on Tau-Dependent and Tau-Independent Networks

“…We have recently confirmed the enrichment of P301S tau in the synaptic compartment of L66 mice [41]. Further, our pathway analyses suggests that overexpression of tau leads to changes in synapses, and that these are modifiable by treatment with LMTM (Table 1, middle section).…”

“…These mice express early onset, high tau load in the brain globally and their genetics, behavioural and histopathological phenotypes are reminiscent of frontotemporal dementia. A detailed characterisation of these transgenic mice was reported earlier and male and female mice show similar behavioural and pathological phenotypes [40][41][42].…”

“…These findings correlated with the amount of tau load in neurons ( [39]; this study) and are clearly in line with clinical investigations underpinning the suitability of P301L/P301S mutant mice as a model for FTD. A presynaptic location for these changes is further indicated by a decrease in amphiphysin-1 levels [85], a protein involved in clathrinmediated endocytosis [86][87][88], and in L66 mice were most likely due to the accumulation of non-phosphorylated tau in the pre-synapse [41]. A complementary study on RNA profiling conducted in young (2 months) and old (12 months) P301S mice found that most of the differentially regulated genes were associated with synaptic signalling [75].…”

“…Given our recent observation that oligomeric tau is enriched in vesicular fractions derived from L66 mice [41], we explored mechanisms involved in release and recycling of synaptic vesicles. Proteins of interest included clathrin, dynamins (DYN1, DYN2 and DYN3), dynamin-like proteins (OPA1 and DNM1L), calcineurin (CANB1) and adaptor protein complex 2 (AP-2 or AP2M1), which were all differentially expressed between L66 and wild-type mice.…”

“…We selected the line 66 (L66) mouse and compared whole brain tissue with age-matched wild-type mice using two-dimensional electrophoresis (2-DE) followed by nano liquid chromatography tandem mass spectrometry (nano-LC-ESI-MS/MS). L66 has been extensively characterised behaviourally [40], with respect to tau expression and localisation [41], in terms of cholinergic status and inflammation [42] and in synaptic release of glutamate [43]. L66 mice overexpress the largest human 2N4R isoform (441 amino acid residues) carrying aggregation-promoting mutations P301S and G335D [40,44].…”