Distinct effects of importin α2 and α4 on Oct3/4 localization and expression in mouse embryonic stem cells

The cellular repertoire of importin (IMP) proteins that mediates nuclear import of transcription factors and chromatin remodeling agents is critical to processes such as differentiation and transformation. This study identifies for the first time independent roles for specific IMPαs in murine embryonic stem cells (mESCs), showing that mESC differentiation is accompanied by dynamic changes in the levels of transcripts encoding the IMPs, IMPα3, IMPα4, IMPβ1, and IPO5. Of these, only IMPα4 was maintained at higher levels in differentiating mESCs, correlating with the finding that IMPα4 overexpression induced a significant decrease in Oct3/4 protein levels compared to control transfections. In parallel, IMPα4 protein showed a unique and striking shift in subcellular localization from the nucleus to the cytoplasm during differentiation, which is consistent with activation of a role in nuclear import of differentiation factors. Overexpression of a dominant-negative IMPα2 isoform, when assessed against adjacent untransfected or IMPα2 transfected cells, led to both a significant reduction in endogenous Oct3/4 protein levels and inhibition of Oct3/4 nuclear localization, suggesting that IMPα2-mediated delivery of Oct3/4 to the nucleus contributes directly to maintenance of mESC pluripotency. These findings implicate IMPα2 and IMPα4 in specific but distinct roles in the fate choice between pluripotency and commitment to differentiation.