Given that the presence of RNA foci in C9ORF72 patient‐derived motorneurons is associated with changes in excitability 47 and maturation of mutant SOD1 oligodendrocytes is severely impaired in mouse 3, we next tested whether functional maturation of PSC‐derived oligodendrocyte‐lineage cells was affected by the presence of C9ORF72 mutation. Whole‐cell capacitance and input resistance measurements did not reveal impairments in passive membrane properties versus controls (Supporting Information Fig. 1E, 1F). The percentage depression of rectification indices for week 3 O4+‐oligodendrocytes in both iPSC9 lines compared with those obtained from PDGFRα+‐OPCs showed no differences compared with control lines (iPSC91, p > 0.30; iPSC92, p > 0.27, Fig. 7G). Maturation to week 3 O4+‐oligodendrocytes was associated with an equivalent strong functional downregulation of I A and I K conductances but an increase in K ir‐channel expression (Supporting Information Fig. 1G–1I). Na V‐channel expression was present in all PDGFRα+‐OPCs (iPSC91/iPSC92, n = 5/5, N = 1/1), but not oligodendrocytes (iPSC91/iPSC92, n = 5/4, N = 1/1). In addition, an equivalent reduction in AMPAR unitary conductance (Fig. 7H) as they mature from OPCs to oligodendrocytes
