REST suppression mediates neural conversion of adult human fibroblasts via microRNA dependent and independent pathways

Direct conversion of human fibroblasts into mature and functional neurons, termed induced neurons (iNs) was achieved for the first time five years ago. This technology offers a promising shortcut for obtaining patient and disease specific neurons for disease modeling, drug screening and other biomedical applications. However, fibroblasts from adult donors do not reprogram as easily as fetal donors and no current reprogramming approach is sufficiently efficient to allow the use of this technology using patient derived material for large scale applications. Here, we investigate the difference in reprogramming requirements between fetal and adult human fibroblasts and identify REST as a major reprogramming barrier in adult fibroblasts. Via functional experiments where we overexpress and knockdown the REST-controlled neuron specific microRNAs miR-9 and miR-124, we show that the effect of RESTi is only partially mediated via microRNA up-regulation. Transcriptional analysis confirmed that RESTi activates an overlapping subset of neuronal genes as microRNA overexpression and also a distinct set of neuronal genes not activated via microRNA overexpression. Based in this, we developed an optimized one-step method to efficiently reprogram dermal fibroblasts from elderly individuals using a single vector system and demonstrate that it is possible to obtain iNs of high yield and purity from aged individuals with a range of familial and sporadic neurodegenerative disorders including Parkinson's, Huntington's as well as Alzheimer's disease. Overall design: mRNA profiles of fetal and adult fibroblasts 5 days after transduction of vectors expressing neural reprogramming factors were generated using deep sequencing in triplicates, using Illumina HiSeq 2000