We first asked whether the three different human genetic ApoE variants that are differentially linked to AD –ApoE2, ApoE3, and ApoE4– differentially stimulate Aβ-synthesis in human neurons. When we incubated neurons cultured on MEFs with recombinant human ApoE2, ApoE3, or ApoE4 (all at 10 µg/ml for 48 h), we found that all three ApoE isoforms robustly enhanced Aβ-synthesis, but with strikingly different efficacy (ApoE4>ApoE3>ApoE2). This differential efficacy was observed for total Aβ, Aβ40, or Aβ42, and for secreted as well as cellular Aβ (Fig. 1E, S1C, S1D), and was unrelated to α-or β-secretase expression (Fig. S1E). Moreover, the differential efficacy of ApoE2, ApoE3, or ApoE4 was independently reproduced with human neurons generated from different iPS cell lines (Fig. S1F, S1G). When we added ApoE to neurons co-cultured with glia, however, we observed no additional increase in Aβ-production, presumably because ApoE and other glial factors already maximally stimulate Aβ synthesis (Fig. 1D, S1H). Control experiments confirmed that our Aβ measurements monitored exclusively human neuronal Aβ synthesis, and were not contaminated by Aβ from other sources (Fig. S1I, S1J). Thus, ApoE secreted
