Direct conversion of adult human retinal pigmented epithelium cells to neurons with photoreceptor properties

Degeneration of photoreceptors is a major cause of blindness. Identifying new methods for the generation of photoreceptors offers valuable options for a cell replacement therapy of blindness. Here, we show that primary adult human retinal pigmented epithelium (hRPE) cells were directly converted to postmitotic neurons with various properties of photoreceptors by the neurogenic transcription factor ASCL1 and microRNA124. At Day 8 after the induction of ASCL1 and miRNA124 expression in hRPE cells, 91% of all cells were Tuj1⁺, and 83% of all cells were MAP2⁺ neurons. The cone photoreceptor marker L/M-opsin, the rod photoreceptor marker rhodopsin, and the generic photoreceptor marker recoverin were expressed in 76%, 86%, and 92% of all cells, respectively. Real-time quantitative PCR measurements showed significant and continuous increases in the expression of photoreceptor markers phosducin and recoverin, rod cell markers phosphodiesterases 6 b and arrestin S-antigen, and cone cell markers L/M-opsin and S-opsin in three independent lines of primary hRPE cells at different days of transdifferentiation. Transmission electron microscopy of converted neurons showed disc-like structures similar to those found in photoreceptors. While the converted neurons had voltage-dependent Na⁺, K⁺, and Ca²⁺ currents, light-induced change in membrane potential was not detected. The study demonstrates the feasibility of rapid and efficient transdifferentiation of adult hPRE cells to neurons with many properties of photoreceptors. It opens up a new possibility in cell replacement therapy of blindness caused by photoreceptor degeneration.