Cell stress pathways in ALS The most common genetic cause of both Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS) is an intronic GGGGCC (G4C2) hexanucleotide repeat expansion in the gene C9orf72 (C9 FTD/ALS). This repeat likely elicits neurodegeneration at least in part through repeat associated non AUG initiated (RAN) translation of the repeats into dipeptide repeat proteins. Our own group has studied the mechanisms underlying RAN translation at C9orf72 repeats (C9 RAN). This work reveals that C9 RAN is very sensitive to alterations in cellular stress pathways. During the integrated stress response, insults such as impaired proteostasis and viral infections trigger initiation factor eIF2α phosphorylation (p-eIF2α), which impairs initiator tRNA (tRNAiMet) incorporation into pre-initiation complexes, blocks global protein synthesis and triggers stress granule formation. In contrast, p-eIF2α enhances C9 RAN by selectively favoring the use of non-AUG start codons and IRES mediated translation. Moreover, G4C2 repeats trigger Stress granule formation and impair global protein translation through this same pathway. The functional interplay between cellular stress pathways, stress granule formation, nucleotide repeats, and neurodegeration are poorly understood but potentially central to the pathophysiology of ALS and FTD. Our preliminary data suggests that C9 RAN and cellular stress pathways participate in a feed-forward loop capable of causing neurodegeneration. The pathways that mediate this vicious cycle are thus potential therapeutic targets. Our central hypothesis is that aberrant activation of C9 RAN in response to cellular stress drives neurodegeneration in C9 FTD/ALS. Our goals are to determine how cellular stress pathways activate RAN translation, how G4C2 repeats activate and maintain cellular stress, define whether SG formation is central to repeat elicited neurodegeneration, and evaluate whether inhibition of selective cellular stress pathways or RAN translation alleviate G4C2 repeat toxicity and alter TDP-43 pathology. Our long-term objective is to define robust therapeutic targets in C9 FTD/ALS and other neurodegenerative disorders.