Pharmaco-genetic evidence of presymptomatic lysosomal dysfunction coupled to detrimental DPR secretion in Drosophila Models of C9orf72-linked ALS/FTD.

Abstract

Hexanucleotide repeat expansions in C9orf72, the most common genetic cause of Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), drive neurodegeneration through both loss of gene function and toxic gain-of-function mechanisms, including transcription of repeat RNA and repeat-associated non-AUG (RAN) translation into dipeptide repeat proteins (DPRs). Both disease mechanisms have been linked to disruption of the autophagy-lysosome pathway (ALP), though how this contributes to disease progression remains unclear. By expressing toxic C9orf72 repeats in Drosophila melanogaster, we observed presymptomatic lysosome impairment, coupled with largely preserved autophagosome formation. Pharmacological treatment with valproic acid (VPA) partially prevented neurodegenerative and locomotor phenotypes in expressing animals in presence of Mitf, which is required for lysosomal biogenesis. Genetic inhibition of residual lysosomal degradation enhanced neurotoxicity, whereas downregulation of autophagy and secretory autophagy genes suppressed toxicity, uncovering a previously unrecognized contribution of secretion to disease. Consistent with this, in vitro experiments indicated that disabling lysosome function, but not autophagy induction, leads to an increase in neuronal release of autophagy-associated proteins and DPRs, which can be found be uptaken by nearby neurons and glia in vivo. We propose that repeat-driven presymptomatic lysosomal dysfunction, prevented by pharmacologic treatment, promotes secretory autophagy, which, in turn facilitates extracellular release and intercellular propagation of neurotoxic DPRs.