Repeat expansions in the C9orf72 gene cause amyotrophic lateral sclerosis and frontotemporal dementia characterized by dipeptide-repeat protein (DPR) inclusions. The toxicity associated with two of these DPRs, poly-GR and poly-PR, has been associated with nucleocytoplasmic transport. To investigate the causal role of poly-GR or poly-PR on active nucleocytoplasmic transport, we measured nuclear import and export in poly-GR or poly-PR expressing Hela cells, neuronal-like SH-SY5Y cells and iPSC-derived motor neurons. Our data strongly indicate that poly-GR and poly-PR do not directly impede active nucleocytoplasmic transport.

TDP-43 is the major component of pathological inclusions in most ALS patients and in up to 50% of patients with frontotemporal dementia (FTD). Heterozygous missense mutations in TARDBP, the gene encoding TDP-43, are one of the common causes of familial ALS. In this study, we investigate TDP-43 protein behavior in induced pluripotent stem cell (iPSC)-derived motor neurons from three ALS patients with different TARDBP mutations, three healthy controls and an isogenic control. TARDPB mutations induce several TDP-43 changes in spinal motor neurons, including cytoplasmic mislocalization and accumulation of insoluble TDP-43, C-terminal fragments, and phospho-TDP-43. By generating iPSC lines with allele-specific tagging of TDP-43, we find that mutant TDP-43 initiates the observed disease phenotypes and has an altered interactome as indicated by mass spectrometry. Our findings also indicate that TDP-43 proteinopathy results in a defect in mitochondrial transport. Lastly, we show that pharmacological inhibition of histone deacetylase 6 (HDAC6) restores the observed TDP-43 pathologies and the axonal mitochondrial motility, suggesting that HDAC6 inhibition may be an interesting therapeutic target for neurodegenerative disorders linked to TDP-43 pathology.

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases characterized by the progressive loss of specific groups of neurons. Due to clinical, genetic and pathological overlap, both diseases are considered as the extremes of one disease spectrum and in a number of ALS and FTD patients, fused in sarcoma (FUS) aggregates are present. Even in families with a monogenetic disease cause, a striking variability is observed in disease presentation. This suggests the presence of important modifying genes. The identification of disease-modifying genes will contribute to defining clear therapeutic targets and to understanding the pathways involved in motor neuron death. In this study, we established a novel in vivo screening platform in which new modifying genes of FUS toxicity can be identified. Expression of human FUS induced the selective apoptosis of crustacean cardioactive peptide (CCAP) neurons from the ventral nerve cord of fruit flies. No defects in the development of these neurons were observed nor were the regulatory CCAP neurons from the brain affected. We used the number of CCAP neurons from the ventral nerve cord as an in vivo read-out for FUS toxicity in neurons. Via a targeted screen, we discovered a potent modifying role of proteins involved in nucleocytoplasmic transport. Downregulation of Nucleoporin 154 and Exportin1 (XPO1) prevented FUS-induced neurotoxicity. Moreover, we show that XPO1 interacted with FUS. Silencing XPO1 significantly reduced the propensity of FUS to form inclusions upon stress. Taken together, our findings point to an important role of nucleocytoplasmic transport proteins in FUS-induced ALS/FTD.