Investigation into the mechanisms of mesendoderm specification during ES cell differentiation

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There are over two hundred different cell types in a human body. All cells are derived from one fertilized egg, therefore all cells have the same genome, which encode all information to make all different type of cells. However each type of cells has different character and different function to constitute individual tissues. This is caused by different usage of genes, different gene expression. In general, terminally differentiated cells do not divide, and therefore most of tissues are supplied cells to be maintained and renewed by lineage-committed stem cells and/or progenitor cells. The Lineage-committed stem cells continuously supply specific type of cells, but not cells in different lineage by keeping similar gene expression profiles, as hematopoietic stem cells do not give rise to muscles. During embryogenesis there are pluripotent cells, which can give rise to any type of cells in a body until periimplantation stage. However, during development those cells gradually lose the ability to give rise different type of cells. Some part of this loss of plasticity is explained by lineage specific transcription factors, which promote the lineage commitment or repress gene expression important for other lineages. In addition, there is a mechanism called ‘epigenetic modification‘, e. g. DNA methylation followed by accumulation of many other silencing molecules, and modification of histone, which acts as spools around which DNA winds, such as acetylation, methylation, phosphorylation. Epigenetic modification is inheritable active or silent marks on chromatin and function to restrict plasticity. These marks on chromatin are recognized by other molecules and/or change chromatin structure, and resulting in gene activation or silencing. Although these modifications do not introduce any changing in genomic sequence, they are copied in newly synthesized chromatin during replication with unknown mechanism and inherited by daughter cells when cells are divided. Therefore epigenetic modifications are important to maintain cell type specific gene expression and restrict cell fate. Embryonic stem (ES) cells, which are derived from a pluripotent population, inner cell mass (ICM) of blastocyst, inherit the pluripotency and possess indefinite self-renewal ability in a certain condition. Because of their ability to give rise any type of cells in a body use of ES cells is expected as the most widely applicable strategy for regenerative medicine. To obtain certain type of cells from ES cells, specific culture conditions with specific cytokines, which allow ES cells go out from self-renewal and promote lineage commitment, are often used. Recently efficiency to generate endoderm, mesoderm and neuroectoderm has been improved, from which pancreas and hurt, muscle and cartridge, neuron for transplantation could be generated in future (1, 2). However, ES cells have a disadvantage in addition to this advantage, pluripotency, for the transplantation. Undifferentiated ES cells are tumorgenic. Little amount contamination of undifferentiated ES cells in transplanted tissue can be a high risk of tumor (3). To avoid this risk, understanding the mechanism of an initial step of ES cell differentiation and controlling undifferentiated-differentiated state is indispensable. Mbd3 is a one of components of nucleosome remodeling and histone deacetylase (NuRD) complex, which possess activity to change chromatin structure and remove active epigenetic mark on histone, resulting in silence of target genes. Recently we found that ES cells disrupted Mbd3 gene by gene targeting have severe defect in their differentiation ability (4). Wild-type ES cells lose expression of undifferentiated cell specific genes and generate lineage committed differentiated cells in the absence of LIF, a cytokine which promotes self-renewal and prevents ES cell differentiation, therefore supplied in a culture medium to maintain undifferentiated ES cells. However, Mbd3-null ES cells stop to