To evaluate whether iN cells could also be derived from postnatal cells, we isolated tail-tip fibroblasts (TTFs) from three-day-old TauEGFP and Rosa26-rtTA mice 28. Similar to our MEF cultures, we could not detect preexisting neurons, glia, or neural progenitor cells (Supplementary Fig. 1a). Twelve days after infecting TTFs with the 5F pool, Tuj1-positive iN cells with a complex, neuronal morphology could be readily detected (Fig. 2a). TTF-iN cells expressed the pan-neuronal markers NeuN, MAP2, and synapsin (Fig. 2b–c, f). Electrophysiological recordings twelve days after infection demonstrated an average RMP of ~ −57 mV (range: −35 to −70mV, n=11), firing of APs (81.8%, n=11) (Fig 2d), and expression of functional voltage-gated membrane channel proteins (Fig. 2e, Supplemental Table 2,3). We were also able to detect vGLUT1- as well as GABA-positive cells (Fig. 2g–h). Despite extensive screening (>1,000 iN cells analyzed), we were unable to detect tyrosine hydroxylase, choline-acetyltransferase, or serotonin expression. Individual iN cells exhibited peripherin-positive filaments (Supplementary Fig. 3d).
