Using ApoE3 as a standard ApoE isoform, we probed the mechanism of ApoE-induced ERK1/2 activation. RAP, a protein that prevents binding of ApoE to surface receptors (Herz et al., 1991), blocked ApoE-triggered phosphorylation of ERK1/2 (Fig. 2B, S2A, S2B), suggesting that ApoE acts by binding to ApoE-receptors. The MAP-kinase kinase inhibitors U0126 and PD98059 (which primarily but not exclusively target MEK1/2) also blocked ApoE-stimulated ERK1/2 phosphorylation. In contrast, inhibitors of the PI3-kinase Akt (Wortmannin), cJun N-terminal kinase (JNK; SP600125), and src kinase (PP2) had no effect (Fig. 2B, S2A, S2B). Therefore, ApoE activates a MAP-kinase signaling pathway in human neurons by binding to ApoE receptors, and does so with a potency rank order of ApoE4>ApoE3>ApoE2 similar to Aβ production.
