Genes Dev. 2016 Nov 15;30(22):2538-2550.

A role for mitotic bookmarking of SOX2 in pluripotency and differentiation.

Deluz C1, Friman ET1, Strebinger D1, Benke A1,2, Raccaud M1, Callegari A2, Leleu M3,4, Manley S2, Suter DM1.

1UPSUTER, The Institute of Bioengineering (IBI), School of Life Sciences, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland.
2Institute of Physics, Laboratory of Experimental Biophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
3Bioinformatics and Biostatistics Core Facility, School of Life Sciences, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland.
4Swiss Institute of Bioinformatics, 1015 Lausanne, Switzerland.

 

Abstract

Mitotic bookmarking transcription factors remain bound to chromosomes during mitosis and were proposed to regulate phenotypic maintenance of stem and progenitor cells at the mitosis-to-G1 (M-G1) transition. However, mitotic bookmarking remains largely unexplored in most stem cell types, and its functional relevance for cell fate decisions remains unclear. Here we screened for mitotic chromosome binding within the pluripotency network of embryonic stem (ES) cells and show that SOX2 and OCT4 remain bound to mitotic chromatin through their respective DNA-binding domains. Dynamic characterization using photobleaching-based methods and single-molecule imaging revealed quantitatively similar specific DNA interactions, but different nonspecific DNA interactions, of SOX2 and OCT4 with mitotic chromatin. Using ChIP-seq (chromatin immunoprecipitation [ChIP] combined with high-throughput sequencing) to assess the genome-wide distribution of SOX2 on mitotic chromatin, we demonstrate the bookmarking activity of SOX2 on a small set of genes. Finally, we investigated the function of SOX2 mitotic bookmarking in cell fate decisions and show that its absence at the M-G1 transition impairs pluripotency maintenance and abrogates its ability to induce neuroectodermal differentiation but does not affect reprogramming efficiency toward induced pluripotent stem cells. Our study demonstrates the mitotic bookmarking property of SOX2 and reveals its functional importance in pluripotency maintenance and ES cell differentiation.© 2016 Deluz et al.; Published by Cold Spring Harbor Laboratory Press.

KEYWORDS:

embryonic stem cells; mitotic bookmarking; neuroectoderm; pluripotency

PMID: 27920086

 

Significance 

During cell division, transcription stops until chromatin decondenses to allow gene expression to resume. Since the identity of a cell is essentially determined by the genes it expresses, proper restoration of gene expression after cell division in crucial to maintain the cellular phenotypic state. This is particularly important in stem cells, which have the ability to divide and decide whether they remain in the same state or differentiate. A number of mechanisms have been proposed to allow cells to “remember” their gene expression program during mitosis, involving the maintenance of histone marks and DNA-binding proteins on mitotic chromatin. A minority of so-called “mitotic bookmarking” transcription factors have been shown to retain the ability to bind mitotic chromosomes, and these have been speculated to play a role in proper gene expression restoration and maintenance of cell fate during cell division.

Embryonic stem (ES) cells are derived from the inner cell mass of the pre-implantation embryo and can be maintained in a pluripotent state in vitro, thus retaining the ability to differentiate towards any somatic cell type. SOX2 is a transcription factor that is required for pluripotency maintenance of ES cells and reprogramming of differentiated cells towards induced pluripotent stem cells. It also plays a role in fostering neuroectodermal differentiation upon pluripotency exit. In this work we demonstrated that SOX2 is retained on mitotic chromosomes through its DNA binding domain (Fig.1). SOX2 displays specific DNA binding events as assessed by single molecule imaging and ChIP-seq, but its global retention on mitotic chromosomes suggests that SOX2 also non-specifically associates with mitotic chromatin. Most importantly, we demonstrated that SOX2 plays an essential role during mitotic exit to regulate cell fate decisions – this is the first reported evidence of the phenotypic relevance of a transcription factor during cell division for cell fate decisions. Future work will be required to understand the molecular mechanisms by which SOX2 acts in this context.

 

Figure 1: SOX2 was fused to a yellow fluorescent protein and expressed in a mitotic mouse cell. A strong fluorescent signal can be seen on condensed mitotic chromosomes.