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The arginine-rich dipeptide repeats (DPRs) are highly toxic products from the C9orf72 repeat expansion mutations, which are the most common causes of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). However, the effects of DPRs in the synaptic regulation and excitotoxicity remain elusive, and how they contribute to the development of FTD is primarily unknown. By expressing DPRs with different toxicity strength in various neuronal populations in a Drosophila model, we unexpectedly found that Glycine-Arginine/Proline-Arginine (GR/PR) with 36 repeats could lead to neurodegenerative phenotypes only when they were expressed in glutamatergic neurons, including motor neurons. We detected increased extracellular glutamate and intracellular calcium levels in GR/PR-expressing larval ventral nerve cord and/or adult brain, accompanied by significant increase of synaptic boutons and active zones in larval neuromuscular junctions. Inhibiting the vesicular glutamate transporter expression or blocking the NMDA receptor in presynaptic glutamatergic motor neurons could effectively rescue the motor deficits and shortened life span caused by poly GR/PR, thus indicating a cell-autonomous excitotoxicity mechanism. Therefore, our results have revealed a novel mode of synaptic regulation by arginine-rich C9 DPRs expressed at more physiologically relevant toxicity levels and provided a mechanism that could contribute to the development of C9-related ALS and FTD. SIGNIFICANCE STATEMENT C9orf72 dipeptide repeats (DPRs) are key toxic species causing ALS/FTD, but their roles in synaptic regulation and excitotoxicity are unclear. Using C9orf72 DPRs with various toxicity strength, we have found that the arginine-rich DPRs cause selective degeneration in Drosophila glutamatergic neurons and revealed an NMDA receptor-dependent cell-autonomous excitotoxicity mechanism. Therefore, this study has advanced our understanding of C9orf72 DPR functions in synaptic regulation and excitotoxicity and provided a new mechanism that could contribute to the development of C9-related ALS and FTD.

C9orf72 Dipeptide Repeats Cause Selective Neurodegeneration and Cell-Autonomous Excitotoxicity in Drosophila Glutamatergic Neurons