Direct conversion of somatic cells to neuronal precursors: Problems and outlooks

The generation of multipotent patient-specific neural precursors from human fibroblasts is one of the challenges to regenerative medicine. A recently proposed novel approach allows direct conversion of human and mouse fibroblasts into induced neuronal precursor (iNP) cells by overexpression of a single transcription factor, Sox2. In this work, we have analyzed the Sox2-induced iNP cells and evaluated the possibility of its medical application. Both mouse and human fibroblasts had pronounced morphological changes after lentiviral-mediated Sox2 overexpression. The resulting mouse and human iNP cell cultures are morphologically similar to the neuronal precursors (NPs) derived from the mouse embryonic brain. Both human and mouse iNP cells express NP molecular markers, however, differentiation of mouse iNP cells fails to produce various types of neural cells. Especially, these cells are unable to differentiate into mature neurons. In addition to Sox2, we have treated human fibroblasts with c-Myc in combination with either Ascl or Brn2. One of the produced cell lines displayed a low proliferative potential, while another intensely divided; however, cytogenetic analysis revealed their aberrant karyotype. The observed specific features of both the human and mouse iNP cells produced according to this protocol make them of limited therapeutic use. Thus, these iNP cell cultures are not completely equivalent to natural NPs. Correspondingly, we assume that the published protocol for iNP generation via Sox2 exogenous expression is poorly reproducible.