Hexanucleotide repeat expansions in C9orf72 are the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD) (c9ALS/FTD). Unconventional translation of these repeats produces dipeptide repeat proteins (DPRs) that may cause neurodegeneration. We performed a modifier screen in Drosophila and discovered a critical role for importins and exportins, Ran-GTP cycle regulators, nuclear pore components, and arginine methylases in mediating DPR toxicity. These findings provide evidence for an important role for nucleocytoplasmic transport in the pathogenic mechanism of c9ALS/FTD.

Influx of Ca 2؉ ions through ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors contributes to neuronal damage in stroke, epilepsy, and neurodegenerative disorders such as ALS. The Ca 2؉ permeability of AMPA receptors is largely determined by the glutamate receptor 2 (GluR2) subunit, receptors lacking GluR2 being permeable to Ca 2؉ ions. We identified a difference in GluR2 expression in motor neurons from two rat strains, resulting in a difference in vulnerability to AMPA receptor-mediated excitotoxicity both in vitro and in vivo. Astrocytes from the ventral spinal cord were found to mediate this difference in GluR2 expression in motor neurons. The presence of ALS-causing mutant superoxide dismutase 1 in astrocytes abolished their GluR2-regulating capacity and thus affected motor neuron vulnerability to AMPA receptormediated excitotoxicity. These results reveal a mechanism through which astrocytes influence neuronal functioning in health and disease.amyotrophic lateral sclerosis ͉ ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor ͉ neurodegeneration ͉ mutant superoxide dismutase 1

The exact mechanism underlying amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) associated with the GGG GCC repeat expansion in C9orf72 is still unclear. Two gain-of-function mechanisms are possible: repeat RNA toxicity and dipeptide repeat protein (DPR) toxicity. We here dissected both possibilities using a zebrafish model for ALS. Expression of two DPRs, glycine-arginine and proline-arginine, induced a motor axonopathy. Similarly, expanded sense and antisense repeat RNA also induced a motor axonopathy and formed mainly cytoplasmic RNA foci. However, DPRs were not detected in these conditions. Moreover, stop codon-interrupted repeat RNA still induced a motor axonopathy and a synergistic role of low levels of DPRs was excluded. Altogether, these results show that repeat RNA toxicity is independent of DPR formation. This RNA toxicity, but not the DPR toxicity, was attenuated by the RNA-binding protein Pur-alpha and the autophagy-related protein p62. Our findings demonstrate that RNA toxicity, independent of DPR toxicity, can contribute to the pathogenesis of C9orf72-associated ALS/FTD.