A comparison between C9ORF72-derived poly-GA DPRs at different stages of protein aggregation in neurons and glia

The polymorphic hexanucleotide repeat expansion (HRE) in the C9orf72 gene is the major cause of both familial and sporadic amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD). The pathogenic HRE consists of hundreds to thousands of GGGGCC (G4C2) repeats, located in the first intron of the C9orf72 gene. A crucial driver of C9orf72-mediated ALS/FTD pathology is the unconventional repeat-associated non-AUG (RAN) translation of the HRE into five toxic dipeptide repeat (DPR) species. Among the five different DPRs generated by RAN translation, poly-GA appears to be the most abundantly detected as well as one of the most toxic DPR species. This thesis focuses on the role of poly-GA DPRs in disease spread. We first illustrate the process of poly-GA oligomer coalescence into solid-like species. Importantly, poly-GA oligomers show a distinct ability to form fibrillar species of amyloid nature with characteristic β-sheets in vitro. To dissect the process of cell-to-cell DPR transmission, we closely follow the fate of poly-GA DPRs in either their oligomeric or fibrillized form after administration in the cell culture medium. We observe that poly-GA DPRs are taken up via dynamin-dependent and -independent endocytosis, eventually converging at the lysosomal compartment and leading to spherical axonal structures in neurons. We then use a co-culture system to demonstrate astrocyte-to-motor neuron DPR propagation, showing that astrocytes may internalise and release aberrant peptides in disease pathogenesis. Overall, our results shed light on the mechanisms of poly-GA cellular uptake and cell-to-cell propagation, suggesting lysosomal impairment as a possible feature underlying the cellular pathogenicity of these DPR species.