To further determine the detrimental effects of apoE4 on GABAergic interneurons, we adapted a protocol of differentiating hiPSCs to medial ganglionic eminence (MGE) cells, which are GABAergic progenitors, and then to mature GABAergic interneurons in culture26–28. The purity of MGE cells and GABAergic interneurons were 96.03 ± 1.25% and 93.15 ± 0.99 (mean ± SEM), respectively, as determined by anti-NKX2.1 (a MGE cell marker) and anti-GABA immunostaining and flow cytometry analysis (Supplementary Fig. 5a–d). Anti-p-tau western blot (AT8 and PHF1) revealed significantly increased p-tau levels in apoE4/4-GABAergic neurons compared to apoE3/3-GABAergic neurons at 5 weeks of culture, while there were no significant differences at 1 and 2 weeks of culture (Fig. 3i–k). Furthermore, immunostaining with p-tau-specific antibodies (AT8 and PHF1) also revealed that 70–100% more GABAergic neurons derived from apoE4/4-MGE cells than from apoE3/3-MGE cells were immunopositive for p-tau (Supplementary Fig. 5e,f). Importantly, p-tau accumulated in degenerating (beading) axons, as determined by anti-p-tau (PHF1) and anti-total tau double immunostaining, in apoE4/4-GABAergic neurons, but not in apoE3/3-GABAergic neurons at 5 weeks of culture (Fig. 3l,m). As in AD brains, p-tau also
