Induction Of Chronic Stress Reveals An Interplay Of Stress Granules And TDP-43 Pathological Aggregates In Human ALS Fibroblasts And iPSC-Neurons

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are fatal neurodegenerative diseases characterized by the presence of neuropathological aggregates of phosphorylated TDP-43 (P-TDP-43). The RNA-binding protein TDP-43 is also a component of stress granules (SG), cytoplasmic foci forming to arrest translation under sub-lethal stress conditions. Although commonly considered as distinct structures, a link between SG and pathological TDP-43 inclusions may occur despite evidence that TDP-43 pathology directly arises from SG is still under debate. Primary fibroblasts and iPSC-derived neurons (iPSC-N) from ALS patients carrying mutations in TARDBP (n=3) and C9ORF72 (n=3) genes and from healthy controls (n=3) were exposed to oxidative stress by sodium arsenite. SG formation and cell response to stress was evaluated and quantified by immunofluorescence and electron microscopy analyses. We found that, not only an acute, but also a chronic oxidative insult, better mimicking a persistent condition of stress as in neurodegeneration, is able to induce SG formation in primary fibroblasts and iPSC-N. Importantly, only upon chronic stress, we observed TDP-43 recruitment into SG and the formation of distinct P-TDP-43 aggregates, very similar to the abnormal inclusions observed in ALS/FTD autoptic brains. Moreover, in fibroblasts, cell response to stress was different in control compared with mutant ALS cells, probably due to their different vulnerability. A quantitative analysis revealed also differences in terms of number of SG-forming cells and SG size, suggesting a different composition of foci in acute and chronic stress. In condition of prolonged stress, SG and P-TDP-43 aggregate formation was concomitant with p62 increase and autophagy dysregulation in both ALS fibroblasts and iPSC-N, as confirmed by immunofluorescence and ultrastructural analyses. We found that exposure to a chronic oxidative insult promotes the formation of both SG and P-TDP-43 aggregates in patient-derived cells, reinforcing the idea that SG fail to properly disassemble, interfering with the protein quality control system. Moreover, we obtained a disease cell model recapitulating ALS/FTD P-TDP-43 aggregates, which represents an invaluable bioassay to study TDP-43 pathology and develop therapeutic strategies aimed at disaggregating or preventing the formation of pathological inclusions.