This proposal seeks to determine how TDP-43 protein aggregates dysregulate P-body function in neurons and subsequently produce neurotoxicity. Cytoplasmic aggregation of TDP-43 has been reported in nearly every age-dependent neurodegenerative disease, including in >40% of frontotemporal dementia (FTD), in the hippocampal neurons of Alzheimer’s disease (AD) patients, in >90% of ALS. It also defines a recently recognized AD-like dementia in the oldest elderly, an AD-like syndrome named Limbic-predominant Age- related TDP-43 Encephalopathy (LATE). We have identified that TDP-43 cytoplasmic aggregates regulate the liquid-liquid phase separation (LLPS) of RNA processing bodies (P-bodies) in neuron-like cells and postmortem spinal cord motor neurons in ALS patients. P-bodies are cytoplasmic membraneless ribonucleoprotein (RNP) granules composed of RNAs and protein complexes involved in translational repression and mRNA decay. Neurons carry a high number of P-bodies in the soma. We hypothesize that TDP-43 aggregation causes neuronal toxicity by disrupting the morphology and function of P-bodies. Our proposal is highly innovative because how TDP-43 proteinopathy regulates the LLPS of other membraneless organelles has not been reported in vivo. We propose to use cutting-edge imaging, proteomic, and sequencing approaches to determine the protein and RNA composition of neuronal P-bodies and how it changes in response to TDP-43 aggregation in vivo. We will first determine how TDP-43 cytoplasmic aggregates initiate P-body disassembly and then determine the RNA metabolism change in neurons carrying TDP-43 aggregates or defective P-bodies. Lastly and importantly, we will determine whether P-body proteins and RNA can serve as pathological markers for AD-related dementia, such as LATE.