Elevated Levels of Soluble Total and Hyperphosphorylated Tau Result in Early Behavioral Deficits and Distinct Changes in Brain Pathology in a New Tau Transgenic Mouse Model

Tauopathies, characterized by hyperphosphorylation and aggregation of tau protein, include frontotemporal dementias and Alzheimer's disease. To explore disease mechanisms and investigate potential treatments, we generated a transgenic (tg) mouse line overexpressing human tau441 with V337M and R406W mutations. Biochemical characterization of these TMHT (Thy-1 mutated human tau) mice showed a significant increase in human transgene expression relative to endogenous murine tau by Western blot and multi-array immunosorbent assay. Only soluble total tau and phosphorylated tau (ptau at residue Thr 181 , Ser 199 , Thr 231 and Thr 235 ), but not insoluble total tau and ptau were increased. Application of the Phospho-Tau SRM assay revealed that phosphorylation at Ser 396 and Ser 404 in soluble tau in the presence of the R406W mutation was at baseline levels in the cortex of TMHT mice compared to non-tg littermates. Histological analyses showed a progressive increase in human tau protein in the amygdala over age, while hippocampal tau levels remained constant from 2 months onwards. Behavioral testing of TMHT mice in the Morris water maze revealed a distinct progressive spatial learning impairment starting already at 5 months of age. Furthermore, we showed that the TMHT mice have early olfactory deficits. These impairments are unbiased by any motor disturbance or lack of motivation. Our results prove that combination of the V337M and R406W mutations of tau accelerates human tau phosphorylation and induces tau pathology as well as cognitive deficits, making this model a suitable tool for basic research on tau as well as in vivo drug testing.