Cellular Stress Induces Nucleocytoplasmic Transport Deficits Independent of Stress Granules.

Stress granules are non-membrane bound granules temporarily forming in the cytoplasm in response to stress. Proteins of the nucleocytoplasmic transport machinery were found in these stress granules and it was suggested that stress granules contribute to the nucleocytoplasmic transport defects in several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). The aim of this study was to investigate whether there is a causal link between stress granule formation and nucleocytoplasmic transport deficits. Therefore, we uncoupled stress granule formation from cellular stress while studying nuclear import. This was carried out by preventing cells from assembling stress granules despite being subjected to cellular stress either by knocking down both G3BP1 and G3BP2 or by pharmacologically inhibiting stress granule formation. Conversely, we induced stress granules by overexpressing G3BP1 in the absence of cellular stress. In both conditions, nuclear import was not affected demonstrating that stress granule formation is not a direct cause of stress-induced nucleocytoplasmic transport deficits.

The Role of Nucleocytoplasmic Transport Defects in Amyotrophic Lateral Sclerosis.

There is ample evidence that nucleocytoplasmic-transport deficits could play an important role in the pathology of amyotrophic lateral sclerosis (ALS). However, the currently available data are often circumstantial and do not fully clarify the exact causal and temporal role of nucleocytoplasmic transport deficits in ALS patients. Gaining this knowledge will be of great significance in order to be able to target therapeutically nucleocytoplasmic transport and/or the proteins involved in this process. The availability of good model systems to study the nucleocytoplasmic transport process in detail will be especially crucial in investigating the effect of different mutations, as well as of other forms of stress. In this review, we discuss the evidence for the involvement of nucleocytoplasmic transport defects in ALS and the methods used to obtain these data. In addition, we provide an overview of the therapeutic strategies which could potentially counteract these defects.

HDAC6 inhibition restores TDP-43 pathology and axonal transport defects in human motor neurons with TARDBP mutations.

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.

Quantitative Nucleocytoplasmic Transport Assays in Cellular Models of Neurodegeneration.

Nucleocytoplasmic transport deficits are suggested to play a role in neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). Given the importance and complexity of this process, understanding when these aberrations occur and which pathways are involved is of great importance. Here, we make use of CRISPR-Cas9 technology to design cell lines stably expressing fluorophore proteins shuttling between the nucleus and cytoplasm by karyopherins of choice. To validate this protocol, we measured an ALS-associated nucleocytoplasmic transport pathway in the presence of the disease-associated peptide poly-PR. This technique allows measuring a particular active nucleocytoplasmic transport pathway in intact cells in a neurodegenerative disease-associated context. Moreover, these experiments can be performed without the need for expensive equipment and have the potential to be upscaled for high-throughput screening purposes.

C9orf72-generated poly-GR and poly-PR do not directly interfere with nucleocytoplasmic transport.

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.

FUS-induced neurotoxicity in Drosophila is prevented by downregulating nucleocytoplasmic transport proteins.

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.