As expected, neurons grown in the absence of glia exhibited decreased synaptogenesis. Interestingly, however, neurons co-cultured with glia secreted 2–3 fold more Aβ40 and Aβ42 than neurons co-cultured with MEFs, suggesting that a secreted glial signaling factor stimulates neuronal Aβ-synthesis. Indeed, adding conditioned medium from glial cultures to human neurons significantly stimulated Aβ40 and Aβ42 production (Fig. 1D). We thus asked which glial factor(s) might be responsible. Based on RNAseq data from glia, we selected 24 abundant secreted glial proteins that we produced in transfected HEK293 cells. Adding these factors to neurons cultured on MEFs revealed that three proteins, ApoE, Igf2, and IgfBP2, significantly increased Aβ-production (Fig. 1D). Since ApoE is an abundant secreted glial proteins that is critically important for AD pathogenesis (Holtzman et al., 2012) and is nearly undetectable in neurons or MEFs (Fig. S1B), the induction of Aβ-secretion by ApoE was intriguing, prompting us to focus on studying its mechanism of action.
