Aberrant formation of fused-in sarcoma (FUS) protein condensates in the cytoplasm is implicated in the pathology of amyotrophic lateral sclerosis and frontotemporal lobal degeneration. The molecular mechanisms that connect the formation of such condensates to biological malfunction are incompletely understood. Here, we develop an approach to determine the viscosity of condensates in live mammalian cells and find that disease-related mutants of FUS form more viscous condensates when compared to wild-type (WT-)FUS, with ALS-associated P525L-FUS forming the most viscous condensates. Moreover, P525L-FUS causes severe changes in the mechanoproperties of cells by affecting the levels of actin and tubulin polymerisation, both of which have an impact on cytoskeletal stiffness, and increases euchromatin formation. We further show that some of the main cellular organelles, such as lysosomes, mitochondria, and the tubular endoplasmic reticulum (ER) network, are significantly impaired in the presence of FUS. These might be related to defects in the tubulin network, as the latter facilitates organelle transport but also their formation, fission and fusion. We also observe significant increases in the biogenesis, size and pH of lysosomes, suggesting that autophagy pathways may be upregulated. Over-expression of FUS in cells significantly increases the cytoplasmic-to-nuclear of the transcription factor EB (TFEB), i.e., the master gene for inducing autophagy. Despite TFEB translocation and subsequent increased lysosomal biogenesis, however, increased autophagy needed for protein aggregate clearance is not observed to occur. Our study reveals that the formation of highly viscous FUS condensates significantly impacts cytoskeletal/organelle function and cellular homeostasis. The latter are closely associated with cell ageing, hence poses the question whether mutant FUS induces early cellular senescence.
Gabriele Kaminski Schierle is Professor of Molecular Neuroscience at the University of Cambridge, UK, where she leads the Molecular Neuroscience group (https://www.ceb-mng.org/). Her group has pioneered optical techniques for the study of the molecular causes underlying neurodegenerative diseases, such as Alzheimer’s and Parkinson’s. She is the director of MPhil in Biotechnology at the department of Chemical Engineering and Biotechnology and senior fellow at the higher education academy.
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