Direct Reprogramming Into Corneal Epithelial Cells Using a Transcriptional Network Comprising PAX6, OVOL2, and KLF4.

In its early stages, an embryo polarizes to form cell subpopulations that subsequently produce specific organ cell types. These cell subpopulations are defined by transcription factors (TFs) that activate or repress specific genes. Although an embryo comprises thousands of TFs, surprisingly few are needed to determine the fate of a given cell. The ectoderm divides into the neuroectoderm and surface ectoderm, the latter of which gives rise to epidermal keratinocytes and corneal epithelial cells (CECs). Meanwhile, neuroectoderm cells give rise to other parts of the eye such as the corneal endothelium and retina. To investigate the regulatory role of TFs in CECs, we overexpressed the "core TFs" (PAX6, OVOL2, and KLF4) in human fibroblasts and found that the cells adopted a CEC-like quality. OVOL2 overexpression was even able to directly induce cells with a neuroectoderm fate toward a surface ectoderm fate, designated "direct reprogramming." Conversely, suppression of OVOL2 or PAX6 expression induced CECs to show qualities consistent with neural lineage cells or epidermal keratinocytes, respectively. This suggests that these core TFs can maintain the CEC phenotype through reciprocal gene regulation. Direct reprogramming has important implications for cell therapies. The potential benefits of cells derived by direct reprogramming compared with induced pluripotent stem cells include the fact that it requires less time than reprogramming a cell back to the pluripotent state and then to another cell type. Further understanding of the reciprocally repressive mechanism of action for core TFs could lead to alternative treatments for regenerative medicine not requiring cell transplantation.