Another important question that remained unclear after our initial publication was whether iN cells could also be generated from human fibroblasts. This issue is important, because potential clinical applications could only be realized with human cells. As the exact same four transcription factors can reprogram both mouse and human fibroblasts, into induced pluripotent cells, one might have expected that converting human fibroblasts to iN cells could be achieved in the same way to that used for mouse fibroblasts. However, when the BAM factors were introduced into human fetal fibroblasts the resulting cells remained immature and failed to generate action potentials when depolarized (Pang et al., 2011). This finding was later confirmed by another group that reported little reprogramming by the BAM factors in human cells in this case attributed to pronounced cell death (Qiang et al., 2011). We therefore screened 20 additional factors in combination with BAM and found that by introducing the bHLH factor NeuroD1, the generation of functional neuronal cells from human fibroblasts can be achieved. These human iN cells expressed a variety of neuronal markers including Tuj1,
