We compared the differential expression results from our data to other transcriptomic datasets in ALS, obtained from the Gene Expression Omnibus (GEO). Raw Affymetrix array data (.CEL files) were downloaded for dataset GSE56504, and preprocessed using a standard exon array pipeline implemented using the R Bioconductor package oligo. For GSE56504, only the laser-capture microdissection samples were included/ Differential expression was calculated using the R Bioconductor package limma. RNA-seq counts data was obtained for dataset GSE67196. For GSE67196, only the frontal cortex samples were included. Normalization and differential expression analysis were performed using DESeq2.

Biotinylation of plasma membrane localized glutamate receptors was performed using the Piece™ Cell Surface Protein Isolation Kit (Thermo Fisher Scientific) following the manufacturer’s instructions. Briefly, Dox-NIL iMNs were incubated with 0.25mg/ml Sulfo-NHS-SS-Biotin in cold room for 1~2 hrs with end-to-end shaking. After quenching, cells were harvested by scraping and lysed with lysis buffer from the Piece™ Cell Surface Protein Isolation Kit or the M-PER™ mammilian protein extraction buffer (Thermo Fisher Scientific). Cell lysate was incubated with High Capacity NeutrAvidin™ agorase beads (Thermo Fisher Scientific), and the bound protein was eluted in 2X SDS-PAGE sample buffer supplemented with 50mM DTT for 1 hr at room temperature with end-to-end rotation, and further analyzed by western blot.
Postsynaptic density extraction was done following a protocol published previously 63. Briefly, mouse spinal cord tissue or human cortical tissue was homogenized in cold Sucrose Buffer (320 mM Sucrose, 10 mM HEPES pH 7.4, 2 mM EDTA, 30 mM NaF, 40 mM β-Glycerophosphate, 10 mM Na3VO4, and protease inhibitor cocktail (Roche)) using a tissue grinder and then spun down at 500 g for 6 min at 4℃. The supernatant was re-centrifuged at 10,000 g for 10 min at 4℃. The supernatant was collected as the “Total” fraction, and the pellet was resuspended in cold Triton buffer (50 mM HEPES pH 7.4, 2 mM EDTA, 50 mM NaF, 40 mM β-Glycerophosphate, 10 mM Na3VO4, 1% Triton X-100 and protease inhibitor cocktail (Roche)) and then spun down at 30,000 RPM using a Beckman rotor MLA-130 for 40 min at 4℃. The supernantant was collected as the “Triton” fraction and the pellet was resuspended in DOC buffer (50 mM HEPES pH 9.0, 50 mM NaF, 40 mM β-Glycerophosphate, 10 mM Na3VO4, 20 uM ZnCl2, 1% Sodium Deoxycholate and protease inhibitor cocktail (Roche)) and collected as the “DOC”, PSD-enriched fraction. Collected samples were boiled with SDS-PAGE sample buffer and analyzed by western blot. Purity of the PSD preps was analyzed by immunoblotting for PSD-95 (PSD), p53 (non-PSD), and synaptophysin (non-PSD).

To measure the effect of dipeptide repeat protein expression on iMN survival, PR50 and GR50 were cloned into the pHAGE lentiviral vector as fusions with GFP to allow tracking of protein expression. iMN cultures were transduced with PR50 and GR50 lentiviruses at day 17 of reprogramming and longitudinal survival analysis was started the same day. 10 ng/ml of GDNF, BDNF, and CNTF was maintained throughout the experiment, and glutamate treatment was not performed. To measure PR50 turnover, PR50 was cloned into the pHAGE lentiviral vector as a fusion with Dendra2 (Addgene). iPSC-derived fibroblasts were generated according to Daley and colleagues64. Briefly, when C9ORF72−/− iPSC cultures reached 80% confluence, the medium was switched from mTeSR1 (Stem Cell Technologies) to human fibroblast medium containing DMEM (Life Technologies), 10% fetal bovine serum (FBS)(Thermo Fisher Scientific), and 1% penicillin/streptomycin (Life Technologies). Cells were passaged 2 to 3 times using Accutase (Life Technologies) before use in experiments. iPSC-derived fibroblasts were transduced with either pMXs-eGFP or pMXs-C9ORF72 isoform B-T2A-eGFP retrovirus and treated with 10 μg/ml mitomycin C for 3 hrs to inhibit cell proliferation. The cells were then transduced with the PR50–Dendra2 lentivirus and exposed to blue light for 1.5 sec using a lumencor LED light source to initiate photoconversion. The amount of decay (as a fraction of the starting level) of the red fluorescent punctae was monitored by longitudinal time lapse imaging in a Molecular Devices ImageExpress and analyzed using SVCell 2.0 (DRVision Technologies). Fluorescence was quantified at t = 0 and 12 hours after photoconversion. Distinct photoconverted punctae were treated as discrete objects for analysis (n = 20 each for +eGFP and +C9ORF72-T2A-eGFP). For each object, background fluorescence was subtracted and fluorescence was normalized according to object size. The fractional decay was statistically analyzed by two-tailed Student’s t-test. ** - p<.01.