The art of forging neurons: direct reprogramming of somatic cells into induced neuronal cells
For millennia, the idea of transmogrifying one being into another has fascinated mankind. Homer told of the ability of the sorceress Circe to transform human beings into pigs using her powers of magic. Likewise, developmental neurobiologists attempt to alter the identity of a given cell in a targeted manner. Here, a certain cellular identity is based on a transcriptional regulatory network which is maintained through positive feedback loops and induces a particular gene expression profile. The resulting (meta-)stable state of gene expression is referred to as a cellular program. By manipulating this program, e.g., via over-expression of regulatory key components such as transcription factors or microRNAs, cells can be maneuvered into an unstable state, which permits the establishment of a new transcriptional regulatory network (. Fig. 1), and as a consequence allows the cell to acquire a new identity: a process which has won fame as cellular reprogramming. In this article we discuss current approaches and developments within the field of direct reprogramming of somatic cells into induced neuronal cells (iNs) and induced neural stem cells (iNSCs).