At autopsy, intraneuronal neurofibrillary tangles (NFTs) are one of the major AD pathological hallmarks, and tau pathology shows a significant correlation to clinical presentations in AD. Excessively phosphorylated tau (p-tau) is a main component of NFTs, but how physiological tau becomes pathological tau in aging neurons remains to be elucidated. The ubiquitin-proteasome system (UPS) is a major protein quality control system in cells and can regulate tau degradation. Proteasome impairment has been reported in several studies of AD human brains and tau transgenics. However, it is unclear if impaired proteasome in the aged brain is causal to tau pathology in AD. Advances in iPSC-derived neuron (iN) technologies allow modeling of disease biology directly in patient-derived neurons in a well-controlled environment and provide a readily manipulable system to study the initial phase of AD cellular processes. We have integrated iPSC technology with the datasets from the Religious Order Study and Memory and Aging Project (ROSMAP) cohorts, in which human subjects’ AD clinical, pathological, and genetic information is well-characterized. This has allowed us to identify a significant association between global cognition and different forms of p-tau in iNs from the same subjects. Unbiased proteomic analyses of these iNs further reveal correlations between the expression of proteasome components and p-tau. Our preliminary studies have also found a reduced proteasome activity in iNs derived from AD patients. I hypothesize that AD neurons are susceptible to proteasome impairment in the initial stage of AD, and this impairment leads to p-tau aggregation and, ultimately, neurodegeneration in AD. In the ROSMAP cohorts, I precisely define proteasome composition and activity in iNs and brains across the clinical and pathological spectrum of aging by immunostaining and proteasome activity assays. Next, I will use in vivo fluorescent reporters and live-cell imaging to determine proteasome activity in living iNs in culture. Importantly, to directly test mechanistic connections between impaired proteasome and tau proteostasis, I will modulate the proteasome genetically and pharmacologically, comparing AD and control neurons. I will also characterize the proteasome in iNs with abnormal, toxic tau species. Longitudinal assessments of the proteasome and tau expression in iNs from AD subjects will determine the onset of impaired proteasome and abnormally accumulated tau. Completing these studies will further our understanding of the UPS and tau proteostasis in human neurons and suggest a promising therapeutic path for AD.