Noninvasive System for Tracking Naïve Induced Pluripotent Stem Cells During Reprogramming

Naïve induced pluripotent stem (iPS) cells are useful for drug development, disease model and regenerative medicine. In female cells, X chromosome reactivation (XCR) is closely linked to the acquisition of naïve iPSCs. To visualize the status of X chromosome in living cells and further analyze XCR mechanism, we established a novel noninvasive detection system of XCR. Here we established female mouse embryonic stem cell (mESC) lines whose allele of both X chromosomes carry each of different fluorescent protein genes [Kusabira Orange (KO) and Enhanced green fluorescent protein (EGFP)]. These mESC clones, initially displaying yellow fluorescence owing to two active X chromosomes, generate chimeric cluster of cells showing either orange or green fluorescence (KO⁺ or EGFP⁺), indicating that X chromosome inactivation (XCI) occurs in these mESC clones during differentiation. When the single colored cells were reprogrammed, the iPS colonies displayed yellow (KO⁺/EGFP⁺) fluorescence, indicating XCR upon reprogramming. Our work reveals generated female KO⁺/EGFP⁺ mESC lines can be used for monitoring X chromosome status during differentiation and/or reprogramming. This novel system is also potential for tracking acquisition of pluripotency of iPSCs during reprogramming and screening for factors or culture conditions which promote this event.