Do mechanisms similar to those we defined in relatively immature human neurons operate in more mature mouse neurons? To address this question, we examined dissociated cultures of neurons and glia from mouse cortex or hippocampus. ApoE had no effect on APP levels or Aβ-synthesis in these cultures, which is expected since ApoE and other factors are abundantly produced by their constituent glia (Fig. 6A, S7A, S7B). However, DLK knockdowns decreased the steady-state levels of MKK7 and ERK1/2 phosphorylation, APP mRNA and APP protein levels, and Aβ40 and Aβ42 secretion in these neuron/glia cultures, suggesting that the same DLK→MKK7→ERK1/2→APP signaling pathway controls App transcription in these neurons (Fig. 6B–D, S7C, S7D). Conversely, DLK and MKK7 overexpression in rescue experiments on the background of the DLK knockdown caused the opposite effect, with 2-to 4-fold increases in the steady-state levels of MKK7 and ERK1/2 phosphorylation, APP mRNA and protein levels, and Aβ40 and Aβ42 secretion (Fig. 6B–D, S7D). Thus, glial factors likely stimulate neuronal APP and Aβ synthesis by the same pathway in dissociated mouse neuron/glia cultures as in human neurons.
