We have been investigating the physiological and pathological roles of stem cells and progenitor cells in the central nervous system using multimodal imaging methods, including positron emission tomography (PET), in vivo optical imaging, and light as well as electron microscopy. Furthermore, we generated transgenic rats for selective ablation of these cells. Imaging studies have demonstrated the proliferation and dynamics of neural stem cells in neurogenic regions and glial progenitor cells expressing a chondroitin sulfate proteoglycan (neuron-glial antigen 2; NG2) in the brain of adult rodents. Glial progenitor cells change their direction of differentiation into mature oligodendrocytes or astrocytes by neural activity following their proliferation. This phenomenon was thought to control the local tissue structure for maintenance of moderate neural activity. Furthermore, selective ablation of glial progenitor cells in the brain induced defects of neurons via neuroinflammation with microglial activation and proinflammatory cytokine production in the region. Thus, we have proposed a novel concept that glial progenitor cells regulate the neuro-immune system in the central nervous system, in addition to their role as germinal cells, giving rise to mature glial cells. Neuroinflammation is associated with the onset and progression of depression, chronic fatigue syndrome, and neurodegenerative diseases, including Alzheimer's disease. Anti-inflammatory effects of glial progenitor cells might bring about the possibility of these cells as the new therapeutic targets for such neurological disorders.
