The levels of Aβ peptides in neuronal culture media were measured by an ELISA10,22. ApoE4/4 neurons secreted >2-fold more Aβ40 and Aβ42 into the culture medium than apoE3/3 neurons, both in individual lines (Supplementary Fig. 2g,h) and as shown by mean values (Fig. 1n,o). The apoE4/4 neurons also secreted greater amounts of soluble APP (sAPP) into the medium (Fig. 1p,q), suggesting increased processing of APP. Interestingly, the full-length APP protein (Fig. 1r,s) and mRNA (Fig. 1t) levels were similar in apoE4/4 and apoE3/3 neurons, as determined by western blot and qRT-PCR analysis respectively, suggesting that apoE4 does not alter APP expression in human neurons. Importantly, the ratios of Tuj1 to actin were similar in apoE3/3 and apoE4/4 neuronal cultures (Fig. 1r,u), indicative of comparable neuronal populations.
