Low Focal Adhesion Signaling Promotes Ground State Pluripotency of Mouse Embryonic Stem Cells

Mouse embryonic stem cells (mESCs) can be maintained in a pluripotent state when cultured with 2 inhibitors (2i) of extracellular signal-regulated kinase (MEK) and glycogen synthase kinase-3 (GSK3), and Royan 2 inhibitors (R2i) of FGF4 and TGFβ. The molecular mechanisms that control ESC self-renewal and pluripotency are more important for translating stem cell technologies to clinical applications. In this study, we used the shotgun proteomics technique to compare the proteome of the ground state condition (R2i- and 2i-grown cells) to that of serum. Out of 1749 proteins identified, 171 proteins were differentially expressed (p < 0.05) in the 2i, R2i, and serum samples. Gene ontology (GO) analysis of differentially abundant proteins showed that the focal adhesion signaling pathway significantly down-regulated under ground state conditions. mESCs had highly adhesive attachment under the serum condition, whereas in the 2i and R2i culture conditions, a loss of adhesion was observed and the cells were rounded and grew in compact colonies on gelatin. Quantitative RT-PCR showed reduced expression of the integrins family in the 2i and R2i conditions. The serum culture had more prominent phosphorylation of focal adhesion kinase (FAK) compared to 2i and R2i cultures. Activity of the extracellular signal-regulated kinase (ERK)­1/2 decreased in the 2i and R2i cultures compared to serum. Activation of integrins by Mn2+ in the 2i and R2i cultures resulted in reduced Nanog and increased the expression of lineage marker genes. In this study, we demonstrated that reduced focal adhesion enabled mESCs to be maintained in an undifferentiated and pluripotent state.