Bidirectional communication between the innate immune and nervous systems for homeostatic neurogenesis in the adult hippocampus.

A population of proliferating neural stem/progenitor cells located in the subgranular zone of the adult hippocampal dentate gyrus (DG) gives rise to new neurons continuously throughout life, and this process is referred to as adult hippocampal neurogenesis. To date, it has generally been accepted that impairments of adult hippocampal neurogenesis resulting from pathological conditions such as stress, ischemia and epilepsy lead to deficits in hippocampus-dependent learning and memory tasks. Recently, we have discovered that microglia, the major immune cells in the brain, attenuate seizure-induced aberrant hippocampal neurogenesis to withstand cognitive decline and recurrent seizure. In that study, we further showed that Toll-like receptor 9, known as a pathogen-sensing receptor for innate immune system activation, recognizes self-DNA derived from degenerating neurons to induce TNF-α production in the microglia after seizure, resulting in inhibition of seizure-induced aberrant neurogenesis. Our findings provide new evidence that interaction between the innate immune and nervous systems ensures homeostatic neurogenesis in the adult hippocampus and should pave the way for the development of new therapeutic strategies for neurological diseases including epilepsy.