Early Reprogramming Intermediates Enable Direct Neuronal Conversion via NGN2

The generation of engineered neurons via Neurogenin-2 (NGN2) overexpression, starting from human induced pluripotent stem cells (hiPSCs), is a powerful tool for modeling neurological diseases. However, using stabilized hiPSCs as a starting point significantly increases the time required to obtain a valuable human neuronal model in vitro. Here, we demonstrated that as little as 3 days of transient expression of reprogramming factors in human fibroblasts can unlock the ability of these cells to transdifferentiate into neurons upon overexpression of NGN2. We used single-cell transcriptomic data to dissect the distinct cell identities that emerge during reprogramming. We identified three distinct reprogramming intermediate populations responsive to NGN2-mediated neuronal induction and found that partial reprogramming for only 3 days is sufficient to mediate NGN2 neuronal conversion of human fibroblasts. Moreover, we found that the efficiency in neuronal fate acquisition mediated by NGN2 overexpression is strictly correlated with the stage of reprogramming used as a starting point.