In order to model tau pathology in mice and explore behavioral changes induced by tau overexpression, we injected mouse neonates in the lateral ventricles (ICV injection, postnatal day 2) with AAV2/6-pgk vectors either encoding the WT form of human 4R0N tau (AAV-WT), or encoding the P301S mutant tau associated with frontotemporal dementia (AAV-P301S) (Fig. 1a). To modulate the propensity of tau to form β-sheets, we generated a vector encoding another form of the protein (2P tau) with amino acid substitutions in the domain that controls binding to microtubules (Fig. 1a). Two Ile-to-Pro mutations were introduced between R1 and R2 (I277P), and between R2 and R3 (I308P), two regions of the microtubule-binding domain that are critical for the transition towards a β-sheet conformation. These mutations have been previously reported to act as β-structure breakers in the highly aggregating ΔK280 mutant tau19. As a control group, mouse neonates were injected with the same dose of a vector encoding the fluorescent maxFP-Green protein (AAV-maxFP). This technique of AAV injection has been reported to efficiently transduce widespread regions of the mouse forebrain20. Indeed, at three months post-injection, overexpression of human tau was detected by immunohistochemistry on histological brain sections using an antibody specific for human tau (HT7). The human tau protein was broadly expressed in neuronal cells throughout the mouse forebrain (Fig. 1b), the highest expression being observed in the cortex and hippocampus (Fig. 1c). Expression was most prominent in large-sized neurons, such as pyramidal cells in cortical layers II/III and V, or hippocampal CA1 region (Fig. 1b and c). Regardless of the form of tau overexpressed (WT or P301S), the distribution of the protein was found to be both somatodendritic and axonal (Fig. 1c). As expected, there was no HT7 signal in mice injected with AAV-maxFP (Fig. 1c). Following ICV injection of AAV-2P, HT7 immunostaining showed similar expression of the tau protein in both the mouse cortex and hippocampus (Fig. 1c).Figure 1Intracerebroventricular injection of AAV-tau leads to widespread expression of human tau in the mouse forebrain. (a) Schema of the three tau-encoding sequences with amino acid substitutions. (b) Representative immunolabeling of human tau (HT7 antibody) in sagittal brain section, 3 months after injection of AAV-WT shows broad expression of human tau, mainly in cortex and hippocampus. (c) Representative HT7 immunolabeling in sagittal sections shows human tau-positive neurons in cortex and hippocampus (CA1) of mice injected with either AAV-WT, AAV-P301S or AAV-2P, 3 and 7 months after vector injection. Negative HT7 staining in a control mouse injected with AAV-maxFP is shown for comparison. Scale: 100 µm.