Study of regeneration of peripheral nervous system with adenoviral vectors

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For regeneration of peripheral nerves, the elongation of axons and their myelination are essential. Schwann cells are the myelinating glia of the peripheral nervous system and their development is regulated by various growth factors. However, the mechanisms of intracellular signaling pathways following these ligands stimuli in Schwann cells differentiation remain elusive. Here, we demonstrated that, in cultured Schwann cells, neuregulin and PDGF suppressed the expression of myelin associated protein markers, while IGF-I promoted it. Although these ligands activated common downstream signaling pathways, i.e., extracellular signal-regulated kinase (Erk) and phosphatidylinositol-3-kinase (PI3K)/Akt pathways, the profiles of activation varied among ligands. To elucidate the function of these pathways and the mechanisms underlying Schwann cell differentiation, we used adenoviral vectors to selectively activate or inactivate these pathways. In rat pheocromocytoma cell line PC12 cells, CA-MEK virus infection induced sustained activation of ERKs and stimulated neurite outgrowth in the absence of neurotrophic factors. We found that the selective activation of Erk pathways suppressed Schwann cell differentiation, while that of PI3K pathways promoted it. Selective activation of PI3K pathways in Schwann cells by gene transfer further demonstrated increased myelination in in vitro Schwann cell-DRG neuron cocultures and in vivo allogenic nerve graft experiment. We conclude that signals mediated by PI3K/Akt are crucial for initiation of myelination and the effects of growth factors are largely dependent on the balance between Erk and PI3K/Akt activation. Our results also propose the possibilities to augment Schwann cell functions by modulating intracellular signals in the light of future cell therapies.