Pluripotent embryonic stem (ES) and germ (EG) cells differ from other stem cells in several respects. They display a characteristically ‘compressed’ cell cycle in which G1 is shortened, contain a high proportion of cells in DNA synthesis (S)-phase and lack several cell cycle checkpoints. This unusual cell cycle signature is important for maintaining their identity since cell cycle arrest, or lengthening, results in irreversible differentiation, and somatic cells assume this unusual signature when successfully reprogrammed.
We have shown that ES and EG cells rapidly convert somatic cells towards pluripotency in transient heterokaryons, and this can be enhanced by genetic modification or using ES cells enriched at G2-phase. On the other hand, mouse epiblast stem (EpiS) and ES cells that lack specific repressor activities (such as Polycomb) although pluripotent, do notdominantly convert somatic cells and show increased doubling times. To determine the importance of cell cycle control for pluripotent self-renewal, we have optimised elutriation to allow the isolation of ES, EG and EpiS cells at progressive stages of the cell cycle. Pilot studies show changes in the levels of modified histones as ES cells transit the cell cycle, and increased levels of specific reprogramming factors during G2-phase. We will extend these analyses genome wide (using ChIP and RNA Seq, and high throughput proteomics) and use fluorescent microscopy to document changes in chromatin dynamics during ES cell cycle, and during somatic cell reprogramming. This will be achieved using novel micro-fluidic approaches to generate heterokaryons between ES, EG, EpiS and lymphocytes. The importance of cell cycle stage and its relevance during early events in successful reprogramming will be tested in heterokaryon and hybrid assays using conditional mutant cells, RNAi-based approaches and cell cycle inhibitors to block critical components.
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