Project Summary The nucleotide repeat expansion (NRE) mutation on chromosome 9 open reading frame 72 (C9orf72) has been identified as the most common genetic mutation link to frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). It has become clear that these two diseases exist in a spectrum of neurological and neuromuscular disorders based on overlapping pathological features identified in patients post-mortem tissue, yet the mechanistic cause for neuropathogenesis is unclear. Age-dependent hyper- and/or hypo-excitability has been a well-described feature of ALS and is now being reported in FTD/ALS patients and patient-derived induced pluripotent stem cells (iPSCs) that carry the C9orf72 NRE mutation. Moreover, aberrant neuronal activity increases with age and can contribute to accelerated aging in animal models. Although the neurospecific contribution for loss-of-function of the C9orf72 protein is not well described, two C9orf72 NRE gain-of-function mechanisms, RNA toxicity from the bidirectional transcription of the NRE and/or dipeptide repeat (DPR) toxicity from the non-AUG-dependent translation of the NRE, have been shown to contribute to neurotoxic mechanisms. We recently identified a potential link between aberrant neuronal activity and increased DPR toxicity, where increased excitotoxic stress or repetitive neuronal activation can drive DPR production. However, further investigation is crucial to understand these in vitro observations and to determine if these may contribute to C9orf72 NRE-linked disease pathogenesis in vivo. Therefore, in this research proposal we hypothesize that abnormal activity increase neurophysiological stress that can modify C9orf72 NRE-linked neuropathogenesis and thereby increase the pathological burden in C9orf72 NRE bacterial artificial chromosome (BAC) disease models, which currently lack overt or robust FTD or ALS phenotypes. Additionally, we postulate that aberrant activity will alter the spatiotemporal gain-of-function toxicity. To test these hypotheses, we will utilize a battery of biochemical to cellular longitudinal assays while utilizing C9orf72 NRE human iPSC and in vivo models to test the causal relationship among aberrant neuronal activity and spatiotemporal dynamics of C9orf72 NRE derived RNA and DPRs using novel patient derived neuronal models and then validate these finds in vivo. Ultimately, we anticipate that this work will establish a crucial foundation for understanding the spatiotemporal causal relationship between neurophysiological stress caused by aberrant neuronal activity and C9-RNE-linked pathogenesis in vitro and in vivo, as well as reveal new therapeutic intervention opportunities to treat neurogenerative disorders linked to the C9-NRE mutation.