Generation of mouse chimeras with high contribution of tetraploid embryonic stem cells and embryonic stem cell-fibroblast hybrid cells

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.

Alternative dominance of the parental genomes in hybrid cells generated through the fusion of mouse embryonic stem cells with fibroblasts

For the first time, two types of hybrid cells with embryonic stem (ES) cell-like and fibroblast-like phenotypes were produced through the fusion of mouse ES cells with fibroblasts. Transcriptome analysis of 2,848 genes differentially expressed in the parental cells demonstrated that 34-43% of these genes are expressed in hybrid cells, consistent with their phenotypes; 25-29% of these genes display intermediate levels of expression, and 12-16% of these genes maintained expression at the parental cell level, inconsistent with the phenotype of the hybrid cell. Approximately 20% of the analyzed genes displayed unexpected expression patterns that differ from both parents. An unusual phenomenon was observed, namely, the illegitimate activation of Xist expression and the inactivation of one of two X-chromosomes in the near-tetraploid fibroblast-like hybrid cells, whereas both Xs were active before and after in vitro differentiation of the ES cell-like hybrid cells. These results and previous data obtained on heterokaryons suggest that the appearance of hybrid cells with a fibroblast-like phenotype reflects the reprogramming, rather than the induced differentiation, of the ES cell genome under the influence of a somatic partner.