Nuclear localized C9orf72-associated arginine-containing dipeptides exhibit age-dependent toxicity in C. elegans

A hexanucleotide repeat expansion mutation in the C9orf72 gene represents a prevalent genetic cause of several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. Non-canonical translation of this repeat gives rise to several distinct dipeptide protein species that could play pathological roles in disease. Here, we show in the model system Caenorhabditis elegans that expression of the arginine-containing dipeptides, but not alanine-containing dipeptides, produces toxic phenotypes in multiple cellular contexts, including motor neurons. Expression of either (PR)(50) or (GR)(50) during development caused a highly penetrant developmental arrest, while post-developmental expression caused age-onset paralysis. Both (PR)(50)- and (GR)(50)-green fluorescent protein tagged dipeptides were present in the nucleus and nuclear localization was necessary and sufficient for their toxicity. Using an inducible expression system, we discovered that age-onset phenotypes caused by (PR)(50) required both continual (PR)(50) expression and an aged cellular environment. The toxicity of (PR)(50) was modified by genetic mutations that uncouple physiological aging from chronological aging. However, these same mutations failed to modify the toxicity of (GR)(50), suggesting that (PR)(50) and (GR)(50) exert their toxicity through partially distinct mechanism(s). Changing the rate of physiological aging also mitigates toxicity in other C. elegans models of ALS, suggesting that the (PR)(50) dipeptide might engage similar toxicity mechanisms as other ALS disease-causing proteins.