Atypical heterochromatin organization and replication are rapidly acquired by somatic cells following fusion-mediated reprogramming by mouse ESCs.

We recently reported that mouse embryonic stem cells (ESCs) in S/G 2 are more efficient at reprogramming somatic cells than ESCs at other stages of the cell cycle. We also provided evidence that DNA replication is induced in the nuclei of somatic partners upon fusion with ESC partners, and showed that this was critical for their conversion toward a pluripotent state. (1) Here we have used counterflow centrifugal elutriation to enrich for ESCs at different cell cycle phases, so as to examine in detail the properties of S/G 2 phase cells. This revealed that the replication and organization of DAPI-intense heterochromatin in ESCs is unusual in two respects. First, replication of heterochromatin occurred earlier during S phase and was associated with precocious H3S10 phosphorylation. Second, heterochromatin protein 1 α (HP1α), which invariably marks DAPI-intense and H3K9me3-enriched pericentromeric domains in mouse somatic cells, (2) was not necessarily associated with these H3K9me3-enriched domains in undifferentiated ESCs. These data, which complement recent replication timing (3) and electron spectroscopic imaging (ESI) analyses, (4) suggest that heterochromatin is atypical in ESCs. Interestingly, as these unusual features were rapidly acquired by somatic nuclei upon ESC fusion-mediated reprogramming, our results suggest that fundamental changes in cell cycle structure and heterochromatin dynamics may be important for conferring pluripotency.