UC San Diego

The mammalian nervous system is comprised of an unknown, but recognizably large, number of diverse neuronal subtypes. Recently, direct reprogramming (also known as transdifferentiation) has become an established method to rapidly produce “induced” neurons of numerous different subtypes directly from fibroblasts by overexpressing specific combinations of transcription factors and/or microRNAs. This technique not only provides the means to study various neuronal subtype populations that are not easily accessible, particularly in humans, but it also serves as a tool to interrogate the transcriptional codes that regulate neuronal subtype identity and maintenance. Both in vivo studies and direct reprogramming protocols have demonstrated that basic helix-loop-helix (bHLH) and Pit-Oct-Unc (POU) transcription factors can aid in the specification of distinct neuronal subtypes. Therefore, we set out to comprehensively and systematically address whether first, additional bHLH and POU factor pairings could reprogram fibroblasts into functional neurons and second, dissect out the discrete and synergistic roles of these factors in neuronal subtype specification. We discovered over 70 novel pairs of bHLH and POU (and non-POU) transcription factors sufficient to generate candidate induced neurons (iNs) from mouse embryonic fibroblasts. Transcriptomic analysis of 35 of these candidate iN populations revealed gene expression profiles similar to those of endogenous neuronal populations. Additionally, differences between iN populations were observed at both a transcriptional and functional level.