Our results left open the possibility that one or two factors might be able to induce functional neuronal properties in MEFs. Thus, we tested smaller subsets of the BAM pool to determine their functionality. In many Ascl1-induced cells, current injection elicited action potentials, but their properties appeared to be immature, consistent with their simple neurite morphology (Fig. 5i, Supplementary Fig. 2b). MEFs infected with Ascl1 and Brn2 or Myt1l generated more mature action potentials and displayed more complex neuronal morphologies. In contrast, the majority of BAM-iN cells exhibited repetitive action potentials with more mature characteristics, and displayed the most complex neuronal morphologies. Thus, it appears likely that Ascl1 alone is sufficient to induce some neuronal traits, such as expression of functional voltage-dependent channel proteins that are necessary for the generation of APs, however, co-infection of additional factors is necessary to facilitate neuronal conversion and maturation.
