PolyGA targets the ER stress-adaptive response by impairing GRP75 function at the MAM in C9ORF72-ALS/FTD.

"Declarations Conflict of interestThe authors declare that there is no conflict of interest. Conflict of interest The authors declare that there is no conflict of interest."

"Funding Open access funding provided by University of Bern. This study was funded by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement #725825), Swiss National Science Foundation, Swiss Foundation for Research on Muscle Diseases, SAND: Marie Skłodowska-Curie actions (MSCA) Innovative Training Network (ITN) under the H2020 program of the European Union and E-rare grant (CALSER) to S.S. K.M. was funded by the Young Investigator Development Award from the Muscular Dystrophy Association. Neuropathology work by J.W. and A.G. was supported by the German Research Foundation (DFG; WE 1406/16–1), and the German Society for Muscle Diseases, DGM. E.A. is supported by an ALS Stitching grant (The Dutch ALS Tissue Bank). S.C was supported by the Euan MacDonald Centre for Motor Neurone Disease Research, and the UK Dementia Research Institute (DRI), which receives its funding from UK DRI Ltd, funded by the MRC, Alzheimer's Society and Alzheimer's Research UK. BTS is a Rowling-DRI Fellow, administered by Anne Rowling Regenerative Neurology clinic."