PROJECT SUMMARY/ABSTRACT Tauopathies are currently the most prevalent neurodegenerative diseases in our country and represent a public health crisis for our rapidly aging demographic. People living with tauopathy, including Alzheimer's disease (AD) and frontotemporal lobar degeneration with tau inclusions (FTLD-tau), are afflicted with severe and devastating memory loss. There are few treatments available for people suffering from cognitive decline and there is no cure. Memory impairments in AD are highly correlated with synapse loss in the brain, suggesting that synaptic function is particularly vulnerable and may be the most promising target for a successful therapeutic outcome. However, the pathogenic events that lead to synapse decline in tauopathy are not well understood. The accumulation of tau, a microtubule-associated protein, in the brain is a hallmark of tauopathy that coincides with progressive neurodegeneration. How tau contributes to synaptic decline is unclear. The main objectives of the proposed research are to determine how tau affects synaptic function in human induced pluripotent stem cell (iPSC)- derived neurons and to identify tau-dependent mechanisms that modulate synapses. This research will apply cutting-edge techniques including targeted genome editing of iPSCs and NGN2-induced iPSC differentiation into neurons. CRISPR-based genome editing of tauopathy patient derived iPSCs with FTLD-tau mutations will generate isogenic iPSCs with corrected mutations. A wild-type tau iPSC line will also be genetically modified using CRISPR to generate isogenic iPSCs carrying mutations that cause familial FTLD-tau. These iPSC lines will be used to determine how tau affects synaptic function in human neuronal models of FTLD-tau (Aim 1). APEX2-based proximity-dependent biotin labeling and quantitative mass spectrometry analyses will be used to identify tau-dependent mechanisms that promote synaptic dysfunction (Aim 2). Finally, tauopathy patient iPSC- derived neurons will be transplanted into mouse brain to determine how tau alters excitatory and inhibitory synaptic transmission in vivo (Aim 3). Further scientific training will enable the candidate to delineate mechanisms linking pathogenic tau and synaptic toxicity in a human disease model. A team of co-mentors and advisory committee members will provide guidance and support for the candidate's research proposal and career advancement. The vibrant scientific community and abundant resources at the Gladstone Institutes and the University of California, San Francisco will enhance the candidate's training. The short-term goals of the candidate are to 1) acquire additional techniques to investigate tauopathy using human neuronal models 2) acquire leadership, mentoring and networking skills for career advancement 3) obtain funding for research as a junior investigator. The long-term goals of the candidate are to make major contributions in tauopathy research by uncovering how tau triggers pathogenic events that lead to neuronal dysfunction and to establish novel strategies for recovering synaptic function in the brain to treat cognitive decline.