Direct Conversion of Human Fibroblasts into Neuronal Restricted Progenitors*

Background: Neuronal restricted progenitors have not been generated from fibroblasts by transdifferentiation. Results: Human induced neuronal restricted progenitors (hiNRPs) were efficiently generated from fibroblasts by transfection of three defined factors: Sox2, c-Myc, and either Brn2 or Brn4. Conclusion: Unipotent neuronal restricted progenitors can be rapidly and efficiently produced from fibroblasts. Significance: This novel method will provide a new source of neurons for cellular replacement therapy of human neurodegenerative diseases. Neuronal restricted progenitors (NRPs) represent a type of transitional intermediate cells that lie between multipotent neural progenitors and terminal differentiated neurons during neurogenesis. These NRPs have the ability to self-renew and differentiate into neurons, but not into glial cells, which is considered an advantage for cellular therapy of human neurodegenerative diseases. However, difficulty in the extraction of highly purified NRPs from normal nervous tissue prevents further studies and applications. In this study, we report the conversion of human fetal fibroblasts into human induced NRPs (hiNRPs) in 11 days by using just three defined factors: Sox2, c-Myc, and either Brn2 or Brn4. The hiNRPs exhibited distinct neuronal characteristics, including cell morphology, multiple neuronal marker expression, self-renewal capacity, and a genome-wide transcriptional profile. Moreover, hiNRPs were able to differentiate into various terminal neurons with functional membrane properties but not glial cells. Direct generation of hiNRPs from somatic cells will provide a new source of cells for cellular replacement therapy of human neurodegenerative diseases.