The role of RNA binding proteins (RBPs) and disruption of RNA biology is strongly implicated in pathogenesis of several neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Their role is highlighted by the fact that the key neuropathological signature in 97% of all ALS and 50% of FTD cases is nuclear depletion, cytoplasmic mislocalization, and aggregation of the mainly nuclear TDP-43 protein. Other RBPs that have been implicated besides TDP-43 include FUS, hnRNPA1, hnRNPA2B1, TAF15, EWS, and MATR3. Abnormal TDP-43 has recently emerged as a critical player in other neurodegenerative diseases including Alzheimer’s disease (AD) and limbic-predominant age-related TDP-43 encephalopathy (LATE). RBPs have unique low complexity domains, allowing phase changes, aggregation and propagation, key aspects of disease pathogenesis. We have recently re-analyzed our published transcriptome profiles generated from laser captured motor neurons from ALS nervous systems and identified that the RBP ELAVL3 is one of the most downregulated genes in the ALS profiles. Since this is a relatively unrecognized RBP in the field of neurodegenerations, we pursued this candidate neuropathologically with immunofluorescence. Strikingly, we find nuclear depletion, and accumulation in cytoplasm and axons of many motor neurons that is even more prevalent than TDP-43 abnormalities. All neurons that have TDP-43 abnormalities also have ELAVL3 abnormalities, but many neurons with ELAVL3 abnormalities do not have TDP-43 abnormalities. Strikingly, preliminary study of ALS nervous systems from SOD1 mutant ALS and FTLD-tau, neither of which have TDP-43 abnormalities, also seem to have ELAVL3 abnormalities. Based on these findings, we therefore believe that ELAVL3 is critically involved in these and maybe other neurodegenerations. In this grant, we have three main aims. In Aim 1, we will determine if ELAV3 ALS neuropathology also occurs in other neurodegenerations, including FTD, AD and LATE. In Aim 2, we will seek to characterize nuclear loss of function (LOF) in cellular model of human neuron-like cells by genome editing the RNA binding domain and reading out a variety of candidates, cell viability and RNA-seq. In addition, we will use eCLIP-seq to determine the RNA binding targets of ELAVL3 and the possible downstream disruptions created by LOF. In Aim 3, we will seek to characterize cytoplasmic toxicity through by genome editing ELAVL3’s nuclear localization signal in our cellular model and reading out a variety of candidates, cell viability and RNA-seq. In addition, we will characterize stress granule formation and autophagy, two highly implicated pathways in ALS. We will compare LOF and GOF using bioinformatics of the RNA-seq and eCLIP-seq profiles and validate. By the end of these studies, we will know the significance of this under-recognized RBP in neurodegenerations and have hypotheses with which to test in more mechanistic stud...