Brain inflammation and adult neurogenesis: The dual role of microglia

In the adult mammalian brain, neurogenesis from neural stem/progenitor cells continues in two regions: the subgranular zone in the dentate gyrus and the subventricular zone lining the lateral ventricles. The generated neuroblasts migrate to their appropriate location and differentiate to mature granule cells and olfactory bulb interneurons, respectively. Following injury such as stroke, neuroblasts generated in the subventricular zone migrate also into areas which are not normally neurogenic, e.g. striatum and cerebral cortex. In the initial studies in rodents, brain inflammation and microglia activation were found to be detrimental for the survival of the new hippocampal neurons early after they had been born. The role of inflammation for adult neurogenesis has, however, turned out to be much more complex. Recent experimental evidence indicates that microglia under certain circumstances can be beneficial and support the different steps in neurogenesis, progenitor proliferation, survival, migration, and differentiation. Here we summarize the current knowledge on the role of inflammation and in particular of microglia in adult neurogenesis in the intact and injured mammalian brain. We conclude that microglia activation, as an indicator of inflammation, is not pro- or antineurogenic per se but the net outcome is dependent on the balance between secreted molecules with pro- and antiinflammatory action.