Epigenetic regulation in embryonic stem cells

Epigenetic regulation in embryonic stem cells

In mammals, fertilization of an oocyte by the sperm is followed by epigenetic reprogramming, which involves de novo acquisition of chromatin signatures in the two parental genomes. The molecular determinants underlying such reprogramming are not fully understood. In particular, the formation of heterochromatin de novo and the silencing of specific genomic loci is thought to be essential to ensure the subsequent organization of the embryonic epigenome and embryonic development.

Embryonic stem cell (ES cell) lines were first generated by culturing mouse inner cell mass (ICM) explants on feeder layers in 1981, and are being used since then as the model system to study pluripotency. They are unique among primary cells in that they can give rise to all cell types of the body and have a very high self-renewing capacity.

In my research, I study the various epigenetic properties of ES cells and their changes during differentiation.

Chromatin has recently emerged as a key-contributing factor to embryonic stem cell identity and plasticity. There is evidence that both unique features of ES cells - self renewal and pluripotency - are regulated to a large extent by epigenetic modifications. We aim to identify, characterize and functionally study epigenetic factors and chromatin dynamics that govern pluripotency in both human and murine ES cells.

I- Elucidating histone turnover rate in the genome of pluripotent and differentiated cells. While histones are generally among the most stably associated DNA-binding proteins known, a subset of histones exhibit dynamic exchange with the soluble pool of nucleoplasmic histones. In all eukaryotes studied, histone H3 exchange is most rapid at promoters, and is generally slowest over heterochromatic regions. Dynamic histone turnover is linked to a variety of key aspects of chromatin biology. In ES cells this dynamic interaction is enhanced, thus maintaining a hyper-dynamic chromatin state in pluripotent ES cells. This hyper-dynamic state has been proposed to maintain the ES cell genome accessible as a relatively permissive ground state that becomes ‘‘locked down’’ during the process of lineage commitment and subsequent differentiation. Surprisingly, FRAP analysis of the H3 variants show that H3.1 is indeed hyper-dynamic in ES cells, while H3.3 has slow turnover rate in both embryonic and differentiated cells. This is somewhat counterintuitive as H3.3 is specifically enriched at transcriptionally active genes and regulatory elements, where a more dynamic state would be anticipated. What is the functional link between H3 turnover rate and expression levels in ES cells? And what is the meaning of the correlation between hyper-dynamic epigenetic state and heterogeneous (or stochastic/noisy) gene expression50 observed often in ES cells?