We and others have reported the successful conversion of human fibroblasts into functional induced neuronal (iN) cells; however the reprogramming efficiencies were very low. Robust reprogramming methods must be developed before iN cells can be used for translational applications such as disease modeling or transplantation-based therapies. Here, we describe a novel approach in which we significantly enhance iN cell conversion efficiency of human fibroblast cells by reprogramming under hypoxic conditions (5% O₂). Fibroblasts were derived under high (21%) or low (5%) oxygen conditions and reprogrammed into iN cells using a combination of the four transcription factors BRN2, ASCL1, MYT1L and NEUROD1. An increase in Map2 immunostaining was only observed when fibroblasts experienced an acute drop in O₂ tension upon infection. Interestingly, cells derived and reprogrammed under hypoxic conditions did not produce more iN cells. Approximately 100% of patched cells fired action potentials in low O₂ conditions compared to 50% under high O₂ growth conditions, confirming the beneficial aspect of reprogramming under low O₂. Further characterization showed no significant difference in the intrinsic properties of iN cells reprogrammed in either condition. Surprisingly, the acute drop in oxygen tension did not affect cell proliferation or cell survival and was not synergistic with the blockade of GSK3β and Smad-mediated pathways. Our results showed that lowering the O₂ tension at the initiation of reprogramming is a simple and efficient strategy to enhance the production of iN cells which will facilitate their use for basic discovery and regenerative medicine.
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