Aberrant phosphorylation of the microtubule associated protein tau (tau) is associated with multiple neurodegenerative diseases where it is a contributes to neurotoxicity. We have observed that phosphorylation at Thr175 tau (pThr175 tau) exerts toxicity when expressed as a pseudophosphorylated tau construct (Thr175Asp) in vitro. To determine whether pThr175 tau can induce tau pathology in vivo with an accompanying clinical phenotype, we used a recombinant adenoviral expression vector (rAAV9) to express a GFP-tagged Thr175Asp tau protein construct in adult female Sprague-Dawley rat hippocampus. Ten rats per group were injected with rAAV9 vectors encoding either GFP, wild type GFP-tagged tau protein, Thr175Ala tau or Thr175Asp tau. 12 months postinjection, all rats were investigated by immunohistochemistry for GFP (extent of vector expression), pThr231 tau protein, activated GSK3β, and caspase-3 cleavage. Vector expression was primarily localized to hippocampal CA2 subregion. Tau protein pathology restricted to the CA2 region in the form of axonal beading, fibrils, and neurofibrillary tangles was observed in Thr175Asp tau inoculated brains and included colocalization with pThr231 tau and caspase-3 cleavage in this group only. Although no behavioral or imaging phenotype was observed, our results demonstrate that pThr175 tau protein is capable of exerting neuronal toxicity in vivo.

Although it has been suggested that the co-expression of multiple pathological proteins associated with neurodegeneration may act synergistically to induce more widespread neuropathology, experimental evidence of this is sparse. We have previously shown that the expression of Thr175Asp-tau (tauT175D) using somatic gene transfer with a stereotaxically-injected recombinant adeno-associated virus (rAAV9) vector induces tau pathology in rat hippocampus. In this study, we have examined whether the co-expression of human tauT175D with mutant human TDP-43 (TDP-43M337V) will act synergistically. Transgenic female Sprague-Dawley rats that inducibly express mutant human TDP-43M337V using the choline acetyltransferase (ChAT) tetracycline response element (TRE) driver with activity modulating tetracycline-controlled transactivator (tTA) were utilized in these studies. Adult rats were injected with GFP-tagged tau protein constructs in a rAAV9 vector through bilateral stereotaxic injection into the hippocampus. Injected tau constructs were: wild-type GFP-tagged 2N4R human tau (tauWT; n = 8), GFP-tagged tauT175D 2N4R human tau (tauT175D, pseudophosphorylated, toxic variant, n = 8), and GFP (control, n = 8). Six months post-injection, mutant TDP-43M337V expression was induced for 30 days. Behaviour testing identified motor deficits within 3 weeks after TDP-43 expression irrespective of tau expression, though social behaviour and sensorimotor gating remained unchanged. Increased tau pathology was observed in the hippocampus of both tauWT and tauT175D expressing rats and tauT175D pathology was increased in the presence of cholinergic neuronal expression of human TDP-43M337V. These data indicate that co-expression of pathological TDP-43 and tau protein exacerbate the pathology associated with either individual protein.