Inhibition of Focal Adhesion Kinase Signaling by Integrin α6β1 Supports Human Pluripotent Stem Cell Self-Renewal

Self-renewal of human embryonic stem cells and human induced pluripotent stem cells (hiPSCs)known as pluripotent stem cells (PSC)is influenced by culture conditions, including the substrate on which they are grown. However, details of the molecular mechanisms interconnecting the substrate and self-renewal of these cells remain unclear. We describe a signaling pathway in hPSCs linking self-renewal and expression of pluripotency transcription factors to integrin 61 and inactivation of focal adhesion kinase (FAK). Disruption of this pathway results in hPSC differentiation. In hPSCs, 61 is the dominant integrin and FAK is not phosphorylated at Y397, and thus, it is inactive. During differentiation, integrin 6 levels diminish and Y397 FAK is phosphorylated and activated. During reprogramming of fibroblasts into iPSCs, integrin 6 is upregulated and FAK is inactivated. Knockdown of integrin 6 and activation of 1 integrin lead to FAK phosphorylation and reduction of Nanog, Oct4, and Sox2, suggesting that integrin 6 functions in inactivation of integrin 1 and FAK signaling and prevention of hPSC differentiation. The N-terminal domain of FAK, where Y397 is localized, is in the nuclei of hPSCs interacting with Oct4 and Sox2, and this immunolocalization is regulated by Oct4. hPSCs remodel the extracellular microenvironment and deposit laminin 5, the primary ligand of integrin 61. Knockdown of laminin 5 resulted in reduction of integrin 6 expression, phosphorylation of FAK and decreased Oct4. In conclusion, hPSCs promote the expression of integrin 61, and nuclear localization and inactivation of FAK to supports stem cell self-renewal. Stem Cells2016;34:1753-1764