Importin-β Proteins Expression Modulation in Embryonic Stem Cells and Embryonic Fibroblasts of Mouse

Percival P. Sangel*

Institute of Animal Science, College of Agriculture and Food Science
University of the Philippines Los Baños, College 4031 Laguna, Philippines

*Corresponding Author: This email address is being protected from spambots. You need JavaScript enabled to view it.




Importin-β proteins are transport proteins important in the shuttling of cargo proteins by binding to either nuclear localization signal (NLS) or nuclear export signal (NES). This study investigated the in vitro expression modulation of selected importin-βs. Specifically, this study characterized the culture behavior of mouse embryonic stem cells after knockdown of selected importin-β proteins like Cse1L, IPO7, KPNB1, RanBP16, RanBP17, or XPO4. Also, this study assessed the effects of overexpressing RanBP17 or IPO7 during cellular reprogramming of mouse embryonic fibroblasts (MEFs). Results showed that Cse1L and KPNB1 are essential for the viability of mouse embryonic stem cells since knockdown of either one of these proteins resulted in the death of mouse embryonic stem cells. Meanwhile, the growth characteristics of RanBP17, XPO4, IPO7, or RanBP16 knockdown mouse embryonic stem cells were comparable with the control. Aside from round colonies, the appearance of flat cells and spreading growth characteristics in some colonies were observed, which indicated early signs of differentiation. On the other hand, the number of colonies with overexpressed Oct4, Sox2, Klf4, cMYC (OSKM) + RanBP17, or OSKM+ IPO7 was comparable to OSKM+Flag or OSKM (controls). This suggests that RanBP17 or IPO7 has limited application in the generation of induced pluripotent stem cells.




Many intricate trafficking pathways involve the transport of various substances in a living cell. In the case of the eukaryotic cell, its biomembrane creates compartmentalization, giving rise to the different organelles and affecting the transport of all cargo molecules. Central to these transport processes is the nucleocytoplasmic transport system. This regulates the shuttle of the different cargo proteins in and out of the nucleus as they pass through the nuclear pore complex (NPC) of the nuclear membrane. The major key players mediating most cargo protein transport via this system are small proteins collectively called importins (alpha and beta). Between the two importins, there are more reports available for importin-α contributing to a better understanding of its roles as a transporter. The previous works of Sekimoto et al. (1997) lead to the identification of the different subtypes of importin-α. This opened a research platform to identify and analyze respective NLS in different cargo proteins and elucidate the specific mechanisms involved in their nuclear transport. The study of Yasuhara et al. (2007) showed that functional switching of the importin-α subtype may lead to neural lineage differentiation of embryonic stem cells. Their study also demonstrated significance in the regulation of the activity of importin-α subtypes, which influence mostly the transport of transcription factors as protein cargoes. Moreover, the same report indicated the role of importin-αs as indicators of cell lineage determination. . . . read more




CHOOK YM, BLOBEL G. 2001. Karyopherin and nuclear import. Current Opinion in Structural Biology 11: 703–715.
JACKSON SA, SCHIESSER J, STANLEY EG, ELEFANTY AG. 2010. Differentiating embryonic stem cells pass through ‘temporal window’ that mark responsiveness to exogenous and paracrine mesoendoderm inducing signals. PLoS ONE 5: e10706.
MAHERALI N, SRIDHARAN R, XIE W, UTIKAL J, EMINLI S, ARNOLD K, STADTFELD M,  YACHECHKO R, TCHIEU J, JAENISCH R, PLATH K, HOCHEDLINGE K. 2007. Directly reprogrammed fibroblasts show global epigenetic remodeling and widespread tissue contribution. Cell Stem Cell 1: 55–70.
MOSAMMAPARAST M, PEMBERTON LF. 2004. Karyopherins: from nuclear-transport mediators to nuclear-function regulators. Trends Cell Biol 14: 547–556.
NIWA H, BURDON T, CHAMBERS I, SMITH A. 1998. Self-renewal of pluripotent embryonic stem cells is mediated via activation of STAT3. Genes and Dev 12: 2048–2060.
NIWA H, MASUI S, CHAMBERS I, SMITH AG, MIYAZAKI J. 2002. Phenotypic complementation establishes requirements for specific POU domain and generic transactivation function of Oct-3/4 in embryonic stem cells. Mol Cell Biol 22: 1526–1536.
OKADA M, OKA M, YONEDA Y. 2010. Effective culture conditions for the induction of pluripotent stem cells. Biochem Biophys Acta 1800: 956–963.
OGRYZKO VV, BRINKMANN E, HOWARD BH, PASTAN I, BRINKMANN U. 1997. Antisense inhibition of CAS, the human homologue of the yeast chromosome segregation gene CSE, interferes with mitosis in HeLa cells. Biochemistry 36(31): 9493–9500.
QUENSEL C, FRIEDRICH B, SOMMER T, HARTMANN E, KOHLER M. 2004. In Vivo Analysis of Importin α Proteins Reveals Cellular Proliferation Inhibition and Substrate Specificity. Mol Cell Biol 24(23): 10246–10255.
SANGEL P, OKA M, YONEDA Y. 2014. The role of Importin-βs in the maintenance and lineage commitment of mouse embryonic stem cells. FEBS Open Bio 4(1): 112–120.
SEKIMOTO T, IMAMOTO N, NAKAJIMA K, HIRANO T, YONEDA Y. 1997. Extracellular signal-dependent nuclear import of Stat1 is mediated by nuclear pore-targeting complex formation with NPI-1, but not Rch1. Embo J 23: 7067–7077.
STRÖM A-C, WEIS K. 2001. Importin-beta-like nuclear transport receptors. Genome Biol 2: reviews3008.1.
THOMSON M, LIU SJ, ZOU LN, SMITH Z, MEISSNER A, RAMANATHAN S. 2011. Pluripotency factors in embryonic stem cells regulate differentiation into germ layers. Cell 145: 875–889.
YASUHARA N, SHIBAZAKI N, TANAKA S, NAGAI M, KAMIKAWA Y, OE S, ASALLY M, KAMACHI Y, KONDOH H, YONEDA Y. 2007. Triggering neural differentiation of ES cells by subtype switching of importin-alpha. Nat Cell Biol 9: 72–79.
YUH MC AND SUEL KE. 2011. Nuclear import by karyopherin-βs: recognition and inhibition. Biochemica et Biophysica Acta 1813: 1593–1606.