Overexpression of serotonergic transcription factors for transdifferentiation of human fibroblasts. (a) Schematic shows the transdifferentiation strategy and timeline for inducing human fibroblasts to neurons using two neuronal transcription factors (AN: ASCL1, NGN2) and to serotonergic neurons with four additional transcription factors (S4F: NKX2.2, FEV, GATA2 and LMX1B). (b) Representative bright-field images of fibroblasts transdifferentiating to serotonergic neurons, from 0 to 8 weeks (Ast, astrocyte) are shown. (c) Images of induced neurons (iNs) 3 weeks post conversion show efficient conversion (60%) of MAP2ab+ (white, left panel), BetaIII-tubulin+ (white, right panel) and NeuN+ (red, right panel) cells over DAPI counterstained nuclei (blue); quantification shown in (e). (d) Panel shows that 3 weeks after conversion of human fibroblasts overexpressing neuronal transcription factors (AN, top panel) compared with serotonergic transcription factors (S4F, lower panel) there was a significantly higher percentage (f) of TPH+/MAP2ab+ neurons in the induced serotonergic neuron (iSN) group as compared with the control iN group; gray dots represent individual data points from triplicate conversions from fibroblasts derived from three individuals. All other data are derived from triplicates of neurons derived from fibroblasts of two individuals. Data are represented as mean±s.e.m., *P-value<0.05. Arrowheads point to double positive cells. Scale bars: (c) left, 100 μm; right, 11.75 μm; (d) 50 μm. TPH, tryptophan hydroxylase.