During early mouse development, inner cell mass cells differentiate into epiblast or primitive endoderm cells. Before this fate decision occurs, the expression levels of several factors, including the transcription factor NANOG, show significant heterogeneity among cells, suggesting that stochastic gene expression can determine cell fate. Mouse embryonic stem (ES) cells are similar to the inner cell mass and show a significant heterogeneity in gene expression levels of several genes including the transcription factor NANOG. NANOG is an important factor in the acquisition of pluripotency, and inner cell mass cells with high NANOG expression levels differentiate into epiblasts in the future. Although negative feedback between transcription factors and cell-to-cell interaction are thought to be involved in inducing intercellular heterogeneity in gene expression in inner cell mass cells and mouse ES cells, the details are still unclear because the heterogeneity in expression is induced even before these regulatory mechanisms act.
It is interesting to note that the Nanog gene is often transcribed from only one allele during early mouse development and in mouse ES cells. This suggests that the Nanog gene exhibits a pronounced transcriptional bursting, which contribute to the cell-to-cell heterogeneity in NANOG expression levels.
Using live transcriptional imaging and single-molecular fluorescent in situ hybridization (smFISH) analysis, we found that intrinsic noise caused by the transcriptional bursting significantly contributes to the diversity of NANOG gene expression in mouse ES cells. On the other hand, it has not been clarified to what extent the transcriptional bursting contributes to the expression diversity of many other genes in mouse ES cells, and how the transcriptional burst is regulated.
We comprehensively identified factors that could contribute to the transcription bursting in mouse ES cells by single-cell RNA-seq analysis and CRISPR library screening. On the other hand, there is no “master regulator” which controls the transcriptional bursting in the cell, and it was proven that the transcriptional bursting control mechanism differs by the gene.
Currently, focusing on specific genes in mouse ES cells, we are trying to elucidate the detailed transcriptional burst mechanism using live imaging and DNA/RNA-seqFISH.
We are also interested in how random X-chromosome inactivation is induced using female mouse ES cells, and the fate determination mechanism from the inner cell mass to the primitive endoderm and epiblast during early mouse development.