Neuronal Ca2+ dyshomeostasis has for many years been implicated in Alzheimer's disease (AD), and increased Ca2+ influx has been shown to have a close link to tau pathology, but this connection has not been well examined in animal models. A few prior studies have examined Ca2+ signaling in tauopathy mouse models by two-photon imaging, with variable results, which may relate to different experimental parameters. We recently reported more pronounced neuronal dysfunction in tauopathy mice by examining them by two-photon imaging during enhanced neuronal activity. This deficit could be largely corrected with an acute tau antibody treatment and the benefits were associated with clearance of soluble phospho-tau protein. Those prior studies were in mice expressing neuronal calcium indicator via an intracerebral AAV infection. For a better approach, we recently generated a novel mouse cross by breeding a transgenic model that expresses a calcium indicator in cortical neurons (Tg-Thy-1-GCAMP6) with PS19 tauopathy mice, and show here its neuronal dysfunction at a young age, supporting its use in this proposal. Beside neuronal dysfunction, microglial activation is associated with AD pathogenesis. However, very little is known about how microglia interact with cells accumulating tau and how they respond to tau-antibody complexes. While this can be explored to some extent in culture, it is well known that ex vivo microglia behave very differently from in vivo. By crossing mice expressing GFP or dTomato in microglia with PS19 tauopathy mice, we can for the first time visualize structural dynamics of microglia during tau pathology progression and monitor in real time microglia-mediated phagocytic clearance of tau-antibody complexes. Regarding diagnostic imaging, small molecule PET ligands can image tau lesions in vivo but because these probes are all β-sheet binders, they can recognize various amyloid aggregates. In contrast, whole antibodies are very specific for their target protein but too large to enter the brain in sufficient quantities for PET detection. However, single domain antibodies (sdAbs) retain this specificity while being less than 1/10 the size of whole antibodies, and therefore get into the brain in much larger quantities. To address these important issues, which are of a very high relevance to tauopathies, including several ongoing clinical trials on tau antibody therapies, and well supported by preliminary and published data, we propose the following independent aims: 1) To determine the time course of tau pathology-induced neuronal dysfunction in vivo in awake novel transgenic mouse cross of established tauopathy and neuronal calcium indicator lines; 2) To determine if microglial morphology is influenced by tau pathology in awake mice, and how it responds to therapy with tau antibodies with or without effector function; 3) To examine the interplay between neuronal dysfunction and microglial response during the progression of tau pathology a...