Mutations in fused in sarcoma (FUS) are a cause of familial amyotrophic lateral sclerosis (fALS). Patients carrying point mutations in the C-terminus of FUS show neuronal cytoplasmic FUS-positive inclusions, whereas in healthy controls, FUS is predominantly nuclear. Cytoplasmic FUS inclusions have also been identified in a subset of frontotemporal lobar degeneration (FTLD-FUS). We show that a non-classical PY nuclear localization signal (NLS) in the C-terminus of FUS is necessary for nuclear import. The majority of fALS-associated mutations occur within the NLS and impair nuclear import to a degree that correlates with the age of disease onset. This presents the first case of disease-causing mutations within a PY-NLS. Nuclear import of FUS is dependent on Transportin, and interference with this transport pathway leads to cytoplasmic redistribution and recruitment of FUS into stress granules. Moreover, proteins known to be stress granule markers co-deposit with inclusions in fALS and FTLD-FUS patients, implicating stress granule formation in the pathogenesis of these diseases. We propose that two pathological hits, namely nuclear import defects and cellular stress, are involved in the pathogenesis of FUS-opathies.

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Neuronal and glial deposition of misfolded, proteolytically processed, polyubiquitinated and abnormally phosphorylated C‐terminal fragments (CTFs) of the TAR DNA binding protein‐43 (TDP‐43) is a pathological hallmark of frontotemporal lobar degeneration with ubiquitin positive inclusions (FTLD‐U) and certain cases of amyotrophic lateral sclerosis. We demonstrate that TDP‐43 can be proteolytically processed by caspases upon induction of apoptosis to a major 35 kDa and a minor 25 kDa CTF. These fragments are initially soluble, but over time they accumulate as insoluble and pathologically phosphorylated derivatives. However, proteolytic processing appears not to be absolutely required for the deposition of insoluble TDP‐43 species, since a caspase resistant mutant of TDP‐43 is also converted into insoluble species. Phosphorylation at S409/410 apparently occurs late during the conversion of soluble to insoluble TDP‐43, suggesting that phosphorylation is not a prerequisite for aggregation. Loss of function of the progranulin (PGRN) gene causes FTLD‐U with TDP‐43 positive inclusions and has been suggested to lead to caspase activation and subsequent TDP‐43 processing. However, siRNA‐mediated knockdown of PGRN in cell culture as well as a PGRN gene knockout in mice failed to cause the formation of the disease characterizing CTFs of TDP‐43. Our findings therefore suggest that caspase‐mediated processing generates CTFs of similar biochemical properties as those occurring in nuclear and cytoplasmic deposits of FTLD‐U patients independent of PGRN levels.

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