Next, we assessed AD-related pathologies in neurons derived from the isogenic apoE3/3-hiPSC line. The conversion from apoE4 to apoE3 had striking phenotypic effects. It increased the levels of full-length apoE (Fig. 4a), decreased the level of apoE fragments in neuronal lysates (Fig. 4b) and the secretion of Aβ40 and Aβ42 into the culture medium (Fig. 4c,d). P-tau levels decreased (Fig. 4e–g), and there were fewer p-tau-positive neurons (Fig. 4i–k and Supplementary Fig. 7a–c). The conversion also rescued the degeneration/loss of GABAergic neurons in apoE4/4-hiPSC neuronal cultures, as shown by significant increases in GAD67 levels (Fig. 4e,h) and in the number of GABA-positive neurons (Fig. 4l–n). The isogenic apoE3/3-hiPSC cultures also had significantly fewer p-tau-positive GABAergic neurons (Supplementary Fig. 7d–i) than parental apoE4/4-hiPSC cultures. Likewise, GABAergic neurons generated from the isogenic apoE3/3-hiPSC-derived MGE cells also had decreased p-tau levels and axonal degeneration compared to GABAergic neurons derived from the parental apoE4/4-hiPSC-derived MGE cells (Supplementary Fig. 8). In sum, conversion of apoE4 to apoE3 by gene editing abolished the detrimental effects of apoE4 and resulted in cellular phenotypes similar to those of apoE3, strongly suggesting that the AD-related pathologies in apoE4/4-neurons are induced specifically by apoE4.
