The Role of Ataxin-2 in Drosophila Models of C9orf72-mediated Frontotemporal Dementia and Amyotrophic Lateral Sclerosis

Abstract

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are two incredibly devastating, debilitating and currently incurable, early-onset neurodegenerative diseases, which share a clinical and pathological spectrum of disease. The most common genetic cause of both conditions is a hexanucleotide (GGGGCC) repeat expansion within the chromosome 9 open reading frame 72 (C9orf72) gene. Bidirectional sense and antisense translation of C9orf72 repeat RNA produces five dipeptide repeat proteins (DPRs) which are now known to be the most potent driver of neurodegeneration in C9orf72-FTD/ALS. The precise mechanisms by which they induce toxicity are unclear, however, they are known to disrupt a range of cellular processes including stress granule dynamics. In neurodegenerative disease, it is hypothesised that under conditions of chronic cellular stress, stress granule dysregulation could augment the concentration of aggregation-prone proteins, such as TDP- 43, leading to the formation of pathological inclusions, characteristic of FTD and ALS. In addition, inhibition of stress granule assembly through Ataxin-2 knockdown has been shown to be protective in yeast, fly and mouse TDP-43 models. Previously, using Drosophila, we have established the first, and currently only, in vivo models expressing disease-causing DPRs of a pathologically and physiologically relevant repeat length. Combining primary neuronal cultures and in vivo analysis of these models we aimed to investigate the role of Ataxin-2 in DPR toxicity, by delineating the mechanistic implications of the DPRs on stress granule accumulation and the integrated stress response. In select Drosophila DPR models we identify a role for Ataxin-2 in DPR-dependent toxicity and observe stress granule accumulation, which can be alleviated through Ataxin-2 knockdown. We also show that expression of select DPRs activate the integrated stress response and that modulation of the eIF2a kinase, GCN2, can mitigate DPR toxicity. As GCN2 is activated upon nutrient deprivation, we demonstrate that poly-GR broadly depletes the availability of free amino acids, and that through arginine supplementation, poly-GR toxicity can be alleviated. Comprehensive characterisation of the mechanism by which DPR toxicity affects the integrated stress response and stress granule dynamics, is essential to establish therapeutic targets, such as Ataxin-2, for C9orf72-mediated FTD and ALS.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ryan West (Supervisor) & Andreas Prokop (Supervisor)