Assessment of a novel tau propagation pathway from layer II medial entorhinal cortical neurons to CA1 pyramidal neurons as an early BRAAK stage mouse model

Background Tau pathology correlates with cognitive impairment and is thought to propagate along neuroanatomical pathways. Tau accumulation is first observed in layer II of the entorhinal cortex (ECII), and then in CA1/2 of the hippocampus. The mechanism and nature of this propagation remains unclear and at present, no animal model recapitulates predominant tau propagation from ECII to CA1. Wolframin‐1‐immunoreactive (Wfs1+) neurons in the mouse ECII project to CA1 pyramidal neurons directly, or indirectly via interneurons in the S. lacunosum‐moleculare. We hypothesized that ECII Wfs1+ neurons mediate tau propagation to the CA1 pyramidal cells, making our model relevant for mimicking early stages of tau pathology in Alzheimer’s disease (AD). Method To specifically express mutant tau in Wfs1+ neurons, Wfs1‐Cre mice at 4‐6 months of age were injected with Cre‐inducible AAV2/6‐Flex‐P301Ltau expressing human P301L tau mutant or AAV2/6‐Flex‐Tdtomato in ECII, and euthanized at 4 weeks post‐injection for immunohistochemistry, electrophysiology and electron microscopy. To label and trace ECII‐CA1 connections, AAV9‐hSyn‐FLEX‐TVA‐P2A‐GFP‐2A‐oG and EnvA‐G‐deleted Rabies‐mCherry were injected into the CA1 pyramidal cells. The functional effects of tau on these connections were assessed by the multielectrode array to evaluate light‐evoked CA1 neuronal firing responses after the optogenetic stimulation of Wfs1+ ECII axons and also by measuring chemogenetic activation of CA1 pyramidal cells. Result Wfs1‐Cre mice injected in ECII with AAV2/6‐Flex‐P301Ltau exhibited significant human tau+ cells in the CA1 but not dentate gyrus at 4 weeks post‐injection. Monosynaptic tracing and electron microscopy revealed a direct synaptic connection between ECII Wfs1+ axons and CA1 pyramidal dendrites, and confirmed the presence of human tau in the pre‐ and post‐synaptic elements. Whole cell patch clamp recordings of CA1 pyramidal cells demonstrated reduction in measures of excitability. Optogenetic stimulation of Wfs1+ axons in the hippocampus coupled with multielectrode array‐recordings revealed a reduction in CA1 neuronal firing to light stimulation after AAV2/6‐Flex‐P301Ltau injection. Chemogenetic activation of CA1 neurons showed a reduction in c‐fos+ cells in CA1 after AAV2/6‐Flex‐P301Ltau injection compared to AAV2/6‐Flex‐Tdtomato controls. Conclusion Expression of P301Ltau in Wfs1+ neurons in the ECII induces the spread of human tau specifically to CA1 pyramidal neurons, which as