Three papers in this issue demonstrate the production of functional induced neuronal (iN) cells from human fibroblasts, a procedure that holds great promise for regenerative medicine. Pang et al. show that a combination of the three transcription factors Ascl1 (also known as Mash1), Brn2 (or Pou3f2) and Myt1l greatly enhances the neuronal differentiation of human embryonic stem cells. When combined with the basic helix–loop–helix transcription factor NeuroD1, these factors can also convert fetal and postnatal human fibroblasts into iN cells. Caiazzo et al. use a cocktail of three transcription factors to convert prenatal and adult mouse and human fibroblasts into functional dopaminergic neurons. The three are Mash1, Nurr1 (or Nr4a2) and Lmx1a. Conversion is direct with no reversion to a progenitor cell stage, and it occurs in cells from Parkinson's disease patients as well as from healthy donors. Yoo et al. use an alternative approach. They show that microRNAs can have an instructive role in neural fate determination. Expression of miR-9/9* and miR-124 in human fibroblasts induces their conversion into functional neurons, and the process is facilitated by the addition of some neurogenic transcription factors.
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