Project Summary Alzheimer's disease (AD) is the most common dementia, currently affecting 10% of the US population over age of 65. The strongest genetic risk factor for AD is the presence of apolipoprotein (apo) E4, which is found in 50–75% of AD cases. ApoE4 exacerbates the classic molecular pathologies of Alzheimer's disease: accumulation of extracellular amyloid-beta (Aβ) plaques and intraneuronal neurofibrillary tangles composed of hyperphosphorylated tau (p-tau). Increasing evidence connects apoE4 toxicity to enhancing tau pathology. Despite its prevalence in patients and decades of research on the molecular pathogenesis of AD, the mechanisms of apoE4's detrimental effects remain incompletely understood. However, a recent discovery further highlights the important role apoE plays in tau pathogenesis in AD. A patient with PSEN1-E280A, a strongly penetrant causal mutation for early onset Alzheimer's disease, was found to be protected from its clinical effects for over 20 years. Significantly, she displayed minimal tau pathology in spite of having very high Aβ burden. The resistance to clinical effects were attributed to the presence of a rare variant of apoE, apoE3-R136S. One notable functional feature of the AD-protective apoE3-R136S variant is a reduced binding to heparin sulfate proteoglycans (HSPGs), a key receptor necessary for tau internalization. This is unsurprising as rare apoE variants with decreasing affinity for HSPGs, such as apoE2, also appear to confer increasing protection against this risk of developing Alzheimer's disease. Understanding the protective mechanisms of the R136S mutation will address a fundamental unknown: whether the R136S mutation protects against the toxic effects of apoE4, which affect the majority of Alzheimer's disease patients. Growing evidence confirms the role of apoE receptors in pathological tau uptake and spread. Therefore, it is important to investigate how this R136S mutation in the receptor-binding region of apoE can rescue the toxicity and dysfunction of the apoE4 isoform by limiting the level of pathological tau burden in neurons. The central hypothesis of this proposal is that the R136S mutation in the apoE receptor-binding region is protective against apoE4-driven tau pathology through modulating tau phosphorylation and/or p-tau uptake by neurons via specific apoE receptor pathways. Experiments proposed in Aim 1 will determine if apoE4-R136S alters the balance of kinases and phosphatases, at the transcriptomic and protein levels, responsible for the phosphorylation state of tau in human induced pluripotent stem cell (hiPSC)-derived neurons, and if so, in a cell-autonomous or non-cell-autonomous manner. Aim 2 will investigate whether the R136S mutation modulates p-tau uptake by hiPSC-derived neurons via specific apoE receptors. The outcomes of the proposed studies will reveal how the R136S mutation confers the resistance of apoE4 to tau pathology in late- onset AD and potentially identify cel...