Reestablishment of the inactive X chromosome to the ground state through cell fusion-induced reprogramming

Reprogramming lineage identity through cell-cell fusion

The conversion of differentiated cells to a pluripotent state through somatic cell nuclear transfer provided the first unequivocal evidence that differentiation was reversible. In more recent times, introducing a combination of key transcription factors into terminally differentiated mammalian cells was shown to drive their conversion to induced pluripotent stem cells (iPSCs). These discoveries were transformative, but the relatively slow speed (2-3 weeks) and low efficiency of reprogramming (0.1-1%) made deciphering the underlying molecular mechanisms difficult and complex. Cell fusion provides an alternative reprogramming approach that is both efficient and tractable, particularly when combined with modern multi-omics analysis of individual cells. Here we review the history and the recent advances in cell-cell fusion that are enabling a better understanding cell fate conversion, and we discuss how this knowledge could be used to shape improved strategies for regenerative medicine.

Generation of Mouse Parthenogenetic Epiblast Stem Cells and Their Imprinting Patterns

Pluripotent stem cells can be established from parthenogenetic embryos, which only possess maternal alleles with maternal-specific imprinting patterns. Previously, we and others showed that parthenogenetic embryonic stem cells (pESCs) and parthenogenetic induced pluripotent stem cells (piPSCs) progressively lose the bimaternal imprinting patterns. As ESCs and iPSCs are naïve pluripotent stem cells, parthenogenetic primed pluripotent stem cells have not yet been established, and thus, their imprinting patterns have not been studied. Here, we first established parthenogenetic epiblast stem cells (pEpiSCs) from 7.5 dpc parthenogenetic implantation embryos and compared the expression patterns and DNA methylation status of the representative imprinted genes with biparental EpiSCs. We found that there were no striking differences between pEpiSCs and biparental EpiSCs with respect to morphology, pluripotency gene expression, and differentiation potential, but there were differences in the expression and DNA methylation status of imprinted genes (H19, Igf2, Peg1, and Peg3). Moreover, pEpiSCs displayed a different DNA methylation pattern compared with that of parthenogenetic neural stem cells (pNSCs), which showed a typical bimaternal imprinting pattern. These results suggest that both naïve pluripotent stem cells and primed pluripotent stem cells have an unstable imprinting status.

Reactivation of inactive X chromosome and post-transcriptional reprogramming of Xist in induced pluripotent stem cells

Direct reprogramming of somatic cells to pluripotent stem cells entails the obliteration of somatic cell memory and the reestablishment of epigenetic events. Induced pluripotent stem (iPS) cells have been created by reprogramming somatic cells through the transduction of reprogramming factors. During cell reprogramming, female somatic cells must overcome at least one more barrier than male somatic cells in order to enter a pluripotent state, as they must reactivate an inactive X chromosome (Xi). In this study, we investigated whether the sex of somatic cells affects reprogramming efficiency, differentiation potential, and the post-transcriptional processing of Xist RNA after reprogramming. There were no differences between male and female iPS cells with respect to reprogramming efficiency or their differentiation potential in vivo. However, reactivating Xi took longer than reactivating pluripotency-related genes. We also found that direct reprogramming leads to gender appropriate posttranscriptional reprogramming: like male embryonic stem (ES) cells, male iPS cells expressed only the long Xist isoform, whereas female iPS cells, like female ES cells, expressed both the long and short isoforms.