Removal of TTX and TEA during glutamate receptor agonist treatment revealed additional increases in Gcamp6 activation in C9ORF72+/− iMNs compared to controls, suggesting that C9ORF72+/− iMNs also fire action potentials more frequently than controls (Supplementary Fig. 13a), although we did not detect large changes in sodium or potassium current amplitudes in C9ORF72+/− iMNs (Supplementary Fig. 13b, c). To determine if increased neuronal activity due in part to elevated glutamate receptor levels contributes to neurodegeneration in C9ORF72 patient and C9ORF72+/− iMNs, we measured iMN survival in the presence or absence of retigabine. Retigabine is approved by the U.S. Food and Drug Administration for the treatment of epilepsy and reduces neuronal excitability by activating Kv7 potassium channels 48. In the glutamate treatment assay, retigabine increased the survival of C9ORF72 patient (n=2 patients) and C9ORF72-deficient iMNs, but not controls (n=2 controls)(Supplementary Fig. 13d-g).
Live imaging of iMNs expressing a M6PR-GFP fusion protein that localizes to M6PR+ vesicles 44 confirmed that C9ORF72 patient and C9ORF72-deficient iMNs possess increased numbers of M6PR+ vesicle clusters, and that overexpression of C9ORF72 isoform A or B rescues this phenotype (Supplementary Fig. 9c-g and Supplementary Videos 5-9). Clusters did not disperse over the time course of the assay, suggesting that they are relatively stable and not in rapid flux (Supplementary Videos 5-9). In addition, M6PR+ puncta moved with a slower average speed in C9ORF72 patient and C9ORF72+/− iMNs than controls (Supplementary Fig. 9h, i). Thus, reduced C9ORF72 levels lead to fewer lysosomes in motor neurons in vitro and in vivo, and this may be due in part to altered trafficking of M6PR+ vesicles.

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Human lymphocytes from healthy subjects and ALS patients were obtained from the NINDS Biorepository at the Coriell Institute for Medical Research and reprogrammed into iPSCs as previously described using episomal vectors61. Briefly, mammalian expression vectors containing Oct4, Sox2, Klf4, L-Myc, Lin28, and a p53 shRNA were introduced into the lymphocytes using the Adult Dermal Fibroblast Nucleofector™ Kit and Nucleofector™ 2b Device (Lonza) according to the manufacturer’s protocol. The cells were then cultured on mouse feeders until iPSC colonies appeared. The colonies were then expanded and maintained on Matrigel (BD) in mTeSR1 medium (Stem Cell Technologies).