Regeneration of the damaged central nervous system through reprogramming technology: Basic concepts and potential application for cell replacement therapy

Neural stem cell (NSC) transplantation provides a new approach for the repair of damage to the central nervous system (CNS), including that resulting from cerebral infarction and spinal cord injury (SCI). In the past, there were no reputable means of converting non-neural somatic cells into neural cells. This status was overturned by the establishment of induced pluripotent stem (iPS) cells, which have pluripotency akin to that of embryonic stem (ES) cells and can differentiate into most cells of the three germ layers. If differentiated somatic cells could be reprogrammed into iPS cells, and if these iPS cells could be induced to differentiate once again, it would be theoretically possible to obtain a large number of neural cells. However, this is not yet feasible due to the limitations of existing stem cell technology. Induction of neural cells from iPS cells is currently hindered by two distinct problems: 1) the preparation of specific types of targeted neural cells requires extensive cell culture, and 2) tumors are likely to form due to the presence of residual undifferentiated cells following transplantation of the induced cells. By contrast, direct induction methods permit the generation of target cells from somatic cells without the transitional iPS cell stage. This review outlines the present-day status of research surrounding the direct induction of NSCs from somatic cells, as well as the perspectives for the future clinical application of this technique for cell replacement therapy following CNS injury.