Microglia release activators of neuronal proliferation mediated by activation of mitogen‐activated protein kinase, phosphatidylinositol‐3‐kinase/Akt and delta–Notch signalling cascades

Microglia, the resident macrophage of the brain, can release substances that aid neuronal development, differentiation and survival. We have investigated the effects of non‐activated microglia on the survival of cultured rat cerebellar granule neurones. Microglial‐conditioned medium, collected from primary rat microglial cultures, was used to treat 7‐day‐in‐vitro neurones, and neuronal viability and proliferation was assessed following a further 1 or 7 days in culture. Microglial‐conditioned medium enhanced neuronal survival by up to 50% compared with untreated neurones and this effect was completely abated by pretreatment of the microglia with l‐leucine methyl ester. The expression of the proliferation marker Ki‐67 increased in neuronal cultures treated with microglial‐conditioned medium suggesting enhanced proliferation of precursor neurones. Microglial‐induced neuronal proliferation could be attenuated by specific inhibition of mitogen‐activated protein kinase or phosphatidylinositol‐3‐kinase/Akt signalling pathways, and by selective fractionation and immunodepletion of the microglial‐conditioned medium. Activation of the Notch pathway was enhanced as antibody against the Notch ligand, delta‐1, prevented the microglial‐induced neuronal proliferation. These results show that microglia release stable neurotrophic factors that can promote neuronal precursor cell proliferation.