Nucleocytoplasmic shuttling of lgl2 is developmentally regulated in fetal lung

The Nuclear Transporter Importin 13 Can Regulate Stress-Induced Cell Death through the Clusterin/KU70 Axis

The cellular response to environmental stresses, such as heat and oxidative stress, is dependent on extensive trafficking of stress-signalling molecules between the cytoplasm and nucleus, which potentiates stress-activated signalling pathways, eventually resulting in cell repair or death. Although Ran-dependent nucleocytoplasmic transport mediated by members of the importin (IPO) super family of nuclear transporters is believed to be responsible for nearly all macromolecular transit between nucleus and cytoplasm, it is paradoxically known to be significantly impaired under conditions of stress. Importin 13 (IPO13) is a unique bidirectional transporter that binds to and releases cargo in a Ran-dependent manner, but in some cases, cargo release from IPO13 is affected by loading of another cargo. To investigate IPO13's role in stress-activated pathways, we performed cell-based screens to identify a multitude of binding partners of IPO13 from human brain, lung, and testes. Analysis of the IPO13 interactome intriguingly indicated more than half of the candidate binding partners to be annotated for roles in stress responses; these included the pro-apoptotic protein nuclear clusterin (nCLU), as well as the nCLU-interacting DNA repair protein KU70. Here, we show, for the first time, that unlike other IPOs which are mislocalised and non-functional, IPO13 continues to translocate between the nucleus and cytoplasm under stress, retaining the capacity to import certain cargoes, such as nCLU, but not export others, such as KU70, as shown by analysis using fluorescence recovery after photobleaching. Importantly, depletion of IPO13 reduces stress-induced import of nCLU and protects against stress-induced cell death, with concomitant protection from DNA damage during stress. Overexpression/FACS experiments demonstrate that nCLU is dependent on IPO13 to trigger stress-induced cell death via apoptosis. Taken together, these results implicate IPO13 as a novel functional nuclear transporter in cellular stress, with a key role thereby in cell fate decision.

Nuclear Transporter IPO13 Is Central to Efficient Neuronal Differentiation

Molecular transport between the nucleus and cytoplasm of the cell is mediated by the importin superfamily of transport receptors, of which the bidirectional transporter Importin 13 (IPO13) is a unique member, with a critical role in early embryonic development through nuclear transport of key regulators, such as transcription factors Pax6, Pax3, and ARX. Here, we examined the role of IPO13 in neuronal differentiation for the first time, using a mouse embryonic stem cell (ESC) model and a monolayer-based differentiation protocol to compare IPO13^−/− to wild type ESCs. Although IPO13^−/− ESCs differentiated into neural progenitor cells, as indicated by the expression of dorsal forebrain progenitor markers, reduced expression of progenitor markers Pax6 and Nestin compared to IPO13^−/− was evident, concomitant with reduced nuclear localisation/transcriptional function of IPO13 import cargo Pax6. Differentiation of IPO13^−/− cells into neurons appeared to be strongly impaired, as evidenced by altered morphology, reduced expression of key neuronal markers, and altered response to the neurotransmitter glutamate. Our findings establish that IPO13 has a key role in ESC neuronal differentiation, in part through the nuclear transport of Pax6.

Nuclear transporter Importin-13 plays a key role in the oxidative stress transcriptional response

The importin superfamily member Importin-13 is a bidirectional nuclear transporter. To delineate its functional roles, we performed transcriptomic analysis on wild-type and Importin-13-knockout mouse embryonic stem cells, revealing enrichment of differentially expressed genes involved in stress responses and apoptosis regulation. De novo promoter motif analysis on 277 Importin-13-dependent genes responsive to oxidative stress revealed an enrichment of motifs aligned to consensus sites for the transcription factors specificity protein 1, SP1, or Kruppel like factor 4, KLF4. Analysis of embryonic stem cells subjected to oxidative stress revealed that Importin-13-knockout cells were more resistant, with knockdown of SP1 or KLF4 helping protect wild-type embryonic stem cells against stress-induced death. Importin-13 was revealed to bind to SP1 and KLF4 in a cellular context, with a key role in oxidative stress-dependent nuclear export of both transcription factors. The results are integral to understanding stress biology, highlighting the importance of Importin-13 in the stress response.

The nuclear transporter importin 13 is critical for cell survival during embryonic stem cell differentiation

Nuclear transporter Importin (Imp, Ipo) 13 is known to transport various mammalian cargoes into/out of the nucleus, but its role in directing cell-fate is unclear. Here we examine the role of Imp13 in the maintenance of pluripotency and differentiation of embryonic stem cells (ESCs) for the first time, using an embryonic body (EB)-based model. When induced to differentiate, Ipo13^-/- ESCs displayed slow proliferation, reduced EB size, and lower expression of the proliferation marker KI67, concomitant with an increase in the number of TUNEL⁺ nuclei compared to wildtype ESCs. At days 5 and 10 of differentiation, Ipo13^-/- EBs also showed enhanced loss of the pluripotency transcript OCT3/4, and barely detectable clusters of OCT3/4 positive cells. Day 5 Ipo13^-/- EBs further exhibited reduced levels of the mesodermal markers Brachyury and Mixl1, correlating with reduced numbers of haemoglobinised cells generated. Our findings suggest that Imp13 is critical to ESC survival as well as early post-gastrulation differentiation.

Importin 13 promotes NSCLC progression by mediating RFPL3 nuclear translocation and hTERT expression upregulation

Our previous studies have reported that RFPL3 protein exerts its unique function as a transcriptional factor of hTERT promoter after being transported into the lung cancer cell nucleus. However, the detailed mechanism by which RFPL3 undergoes nuclear transport has not been reported yet. Here, we identified RFPL3 as a potential import cargo for IPO13, which was found to be overexpressed in NSCLC cells and tissues. IPO13 interacted with RFPL3 in lung cancer cells, and the knockdown of IPO13 led to the cytoplasmic accumulation of RFPL3, the decreased anchoring of RFPL3 at hTERT promoter, and the downregulation of hTERT expression. Moreover, IPO13 silencing suppressed tumor growth in vitro and in vivo. IHC analysis confirmed the positive correlation between the expression levels of IPO13 and hTERT in the tumor tissues from patients with lung cancer. Furthermore, the mechanistic study revealed that IPO13 recognized RFPL3 via a functional nuclear localization signal (NLS), which is located in the B30.2 domain at the C-terminal region of RFPL3. Of note, the presence of EGFR mutations was significantly related to the increased IPO13 expression. The EGFR-TKI Osimertinib downregulated IPO13 expression level in NSCLC cell lines with EGFR mutations, but not in EGFR wild-type ones. In summary, our data suggest that inhibition of IPO13 transport activity itself might be an alternative and potential therapeutic strategy for NSCLC.

Importin-11 is Essential for Normal Embryonic Development in Mice

Importin-11 (Ipo11) is a novel member of the human importin family of transport receptors (karyopherins), which are known to mediate the nucleocytoplasmic transport of protein and RNA cargos. Despite its role in the transport of protein, we found that knockout of Ipo11 nuclear import factor affects normal embryonic development and govern embryo-lethal phenotypes in mice. In this study, we for the first time produced a mouse line containing null mutation in Ipo11 gene utilized by gene trapping. The Ipo11^-/- embryos showed an embryonic lethal phenotype. The Ipo11^-/- embryos showed a reduced size at embryonic day 10.5 (E10.5) when compared with Ipo11^+/+ or Ipo11^+/- embryos and died by E11.5. Whereas Ipo11^+/- mice were healthy and fertile, and there was no detectable changes in embryonic lethality and phenotype when reviewed. In the X-gal staining with the Ipo11^-/- or Ipo11^+/- embryos, strong X-gal staining positivity was detected systematically in the whole mount embryos at E10.5, although almost no X-gal positivity was detected at E9.5, indicating that the embryos die soon after the process of Ipo11 expression started. These results indicate that Ipo11 is essential for the normal embryonic development in mice.

PKA-site phosphorylation of importin13 regulates its subcellular localization and nuclear transport function

Importin 13 (IPO13) is a key member of the importin β superfamily, which can transport cargoes both into and out of the nucleus to contribute to a variety of important cellular processes. IPO13 is known to undergo phosphorylation, but the impact of this on function has not been investigated. Here, we show for the first time that IPO13 is phosphorylated by cAMP-dependent protein kinase A specifically at serine 193. Results from fluorescence recovery after photobleaching and fluorescence loss in photobleaching approaches establish that negative charge at serine 193 through phosphorylation or point mutation both reduces IPO13 nuclear import and increases its nuclear export. Importantly, phosphorylation also appears to enhance cargo interaction on the part of IPO13, with significant impact on localization, as shown for the Pax6 homeobox-containing transcription partner. This is the first report that IPO13 can be phosphorylated at Ser193 and that this modification regulates IPO13 subcellular localization and nucleocytoplasmic transport function, with important implications for IPO13's role in development and other processes.

Time-resolved biophysical approaches to nucleocytoplasmic transport

Molecules are continuously shuttling across the nuclear envelope barrier that separates the nucleus from the cytoplasm. Instead of being just a barrier to diffusion, the nuclear envelope is rather a complex filter that provides eukaryotes with an elaborate spatiotemporal regulation of fundamental molecular processes, such as gene expression and protein translation. Given the highly dynamic nature of nucleocytoplasmic transport, during the past few decades large efforts were devoted to the development and application of time resolved, fluorescence-based, biophysical methods to capture the details of molecular motion across the nuclear envelope. These methods are here divided into three major classes, according to the differences in the way they report on the molecular process of nucleocytoplasmic transport. In detail, the first class encompasses those methods based on the perturbation of the fluorescence signal, also known as ensemble-averaging methods, which average the behavior of many molecules (across many pores). The second class comprises those methods based on the localization of single fluorescently-labelled molecules and tracking of their position in space and time, potentially across single pores. Finally, the third class encompasses methods based on the statistical analysis of spontaneous fluorescence fluctuations out of the equilibrium or stationary state of the system. In this case, the behavior of single molecules is probed in presence of many similarly-labelled molecules, without dwelling on any of them. Here these three classes, with their respective pros and cons as well as their main applications to nucleocytoplasmic shuttling will be briefly reviewed and discussed.

Secreted phosphoprotein 1 is a determinant of lung function development in mice

Secreted phosphoprotein 1 (Spp1) is located within quantitative trait loci associated with lung function that was previously identified by contrasting C3H/HeJ and JF1/Msf mouse strains that have extremely divergent lung function. JF1/Msf mice with diminished lung function had reduced lung SPP1 transcript and protein during the peak stage of alveologenesis (postnatal day [P]14-P28) as compared with C3H/HeJ mice. In addition to a previously identified genetic variant that altered runt-related transcription factor 2 (RUNX2) binding in the Spp1 promoter, we identified another promoter variant in a putative RUNX2 binding site that increased the DNA protein binding. SPP1 induced dose-dependent mouse lung epithelial-15 cell proliferation. Spp1((-/-)) mice have decreased specific total lung capacity/body weight, higher specific compliance, and increased mean airspace chord length (Lm) compared with Spp1((+/+)) mice. Microarray analysis revealed enriched gene ontogeny categories, with numerous genes associated with lung development and/or respiratory disease. Insulin-like growth factor 1, Hedgehog-interacting protein, wingless-related mouse mammary tumor virus integration site 5A, and NOTCH1 transcripts decreased in the lung of P14 Spp1((-/-)) mice as determined by quantitative RT-PCR analysis. SPP1 promotes pneumocyte growth, and mice lacking SPP1 have smaller, more compliant lungs with enlarged airspace (i.e., increased Lm). Microarray analysis suggests a dysregulation of key lung developmental transcripts in gene-targeted Spp1((-/-)) mice, particularly during the peak phase of alveologenesis. In addition to its known roles in lung disease, this study supports SPP1 as a determinant of lung development in mice.

Expression and subcellular distribution of imp13 are regulated in brain development

Imp13, a member of importin-β superfamily, is found to be one of two bidirectional transport receptors in many nuclear transport activities in mammals. Several cargoes of imp13 have been identified; most of these are essential factors involved in cell cycle and development. The expression and localization of imp13 may influence its cargoes in playing their roles in appropriate time and space. To gain insight into the role of imp13 in brain development, we generated an anti-imp13 polyclonal antibody and investigated the expressions of imp13 in mouse embryonic brains during development, including E13.5, E15.5, E17.5, P0, and adult, at both transcriptional and translational levels. In addition, we performed immunohistochemical analysis and revealed that imp13 tends to be localized in the cytoplasm at the early stages and relocates into the nucleus at the late stages in neuronal cells of mouse brains. These findings suggested that the expression and localization of imp13 in brain tissues are regulated developmentally, which extends our knowledge of the dynamic presence of imp13. These observations also imply that imp13 contributes to the neural cell-specific cargo trafficking and potentially to other functions during brain development.

Importin 13 serves as a potential marker for corneal epithelial progenitor cells

Importin13 (IPO13), the newest member of importin-beta family discovered recently, is a unique nucleus-cytoplasm bidirectional transport receptor protein. In this study, IPO13 expression in human corneal tissue, limbal epithelial primary explant and clonal culture was evaluated by immunostaining and reverse-transcription polymerase chain reasgon. IPO13 function was evaluated in the corneal epithelial culture treated with IPO13 inhibitor, or fetal bovine serum (FBS)-containing Dulbecco's modified Eagle's medium (DMEM) medium by colony-forming efficiency, clone growth capacity, MTT, immunostaining, and Western blotting assay. IPO13 protein was expressed mainly in nuclei of limbal epithelial basal cells, but not in the other cell layers of limbus and full thickness of corneal epithelia. IPO13 was expressed in the majority of epithelial cells in early-stage clones and in the margin of late-stage clones. IPO13 was positively expressed in mouse TKE2 progenitor cells cultured in keratinocyte serum-free defined medium, while it became negative in FBS-containing DMEM, which promoted TKE2 cell differentiation. In the presence of IPO13 inhibitor, IPO13 expression and the proliferative capacity decreased in human limbal epithelial clones and mouse TKE2 cells, which were accompanied with the cell differentiation. In conclusion, our findings demonstrate for the first time that IPO13 is uniquely expressed by human limbal basal epithelial cells, and plays an important role in maintaining the phenotype, high proliferative potential, and less differentiation of corneal epithelial progenitor cells, suggesting that IPO13 could serve as a novel potential marker for corneal epithelial progenitor cells.

Genetic influences on asthma susceptibility in the developing lung

Asthma is the leading serious pediatric chronic illness in the United States, affecting 7.1 million children. The prevalence of asthma in children under 4 years of age has increased dramatically in the last 2 decades. Existing evidence suggests that this increase in prevalence derives from early environmental exposures acting on a pre-existing asthma-susceptible genotype. We studied the origins of asthma susceptibility in developing lung in rat strains that model the distinct phenotypes of airway hyperresponsiveness (Fisher rats) and atopy (brown Norway [BN] rats). Postnatal BN rat lungs showed increased epithelial proliferation and tracheal goblet cell hyperplasia. Fisher pups showed increased lung resistance at age 2 weeks, with elevated neutrophils throughout the postnatal period. Diverse transcriptomic signatures characterized the distinct respiratory phenotypes of developing lung in both rat models. Linear regression across age and strain identified developmental variation in expression of 1,376 genes, and confirmed both strain and temporal regulation of lung gene expression. Biological processes that were heavily represented included growth and development (including the T Box 1 transcription factor [Tbx5], the epidermal growth factor receptor [Egfr], the transforming growth factor beta-1-induced transcript 1 [Tgfbr1i1]), extracellular matrix and cell adhesion (including collagen and integrin genes), and immune function (including lymphocyte antigen 6 (Ly6) subunits, IL-17b, Toll-interacting protein, and Ficolin B). Genes validated by quantitative RT-PCR and protein analysis included collagen III alpha 1 Col3a1, Ly6b, glucocorticoid receptor and Importin-13 (specific to the BN rat lung), and Serpina1 and Ficolin B (specific to the Fisher lung). Innate differences in patterns of gene expression in developing lung that contribute to individual variation in respiratory phenotype are likely to contribute to the pathogenesis of asthma.

Reduced viability of mice with lung epithelial-specific knockout of glucocorticoid receptor

Glucocorticoid (GC)-responsive epithelial-mesenchymal interactions regulate lung development. The GC receptor (GR) mediates GC signaling. Mice lacking GR in all tissues die at birth of respiratory failure. To determine the specific need for epithelial GR in lung development, we bred triple transgenic mice that carry SPC/rtTA, tet-O-Cre, and floxed, but not wild-type, GR genes. When exposed to doxycycline in utero, triple transgenic (GRepi⁻) mice exhibit a Cre-mediated recombination event that inactivates the floxed GR gene in airway epithelial cells. Immunofluorescence confirmed the elimination of GR in Cre-positive airway epithelial cells of late gestation GRepi⁻ mice. Embryonic Day 18.5 pups had a relatively immature appearance with increased lung cellularity and increased pools of glycogen in the epithelium. Postnatal Day 0.5 pups had decreased viability. We used quantitative RT-PCR to demonstrate that specific elimination of epithelial immunoreactive GR in GRepi⁻ mice is associated with reduced mRNA expression for surfactant proteins (SPs) A, B, C, and D; β- and γ-ENaC; T1α; the 10-kD Clara cell protein (CCSP); and aquaporin 5 (AQP5). Western blots confirmed reduced levels of AQP5 protein. No reduction in the levels of the GR transport protein importin (IPO)-13 was observed. Our findings demonstrate a requirement for lung epithelial cell GR in normal lung development. We speculate that impaired epithelial differentiation, leading to decreased SPs, transepithelial Na, and liquid absorption at birth, may contribute to the reduced survival of newborn mice with suppressed lung epithelial GR.

The roles of multiple importins for nuclear import of murine aristaless-related homeobox protein

Nuclear import of proteins with nuclear localization signals (NLSs) is mediated by shuttling carriers, the importins. Some cargoes display more than a single NLS, and among these are homeodomain proteins such as Arx, which is critical for development of multiple tissues. Arx has two functional NLSs. The present studies show that several pathways can import Arx via its NLS2, which is within its DNA binding homeodomain. Using an in vitro nuclear import assay, we show that import of Arx via NLS2 can be mediated by importin beta1, importin 9, or importin 13, with binding being strongest to importin beta1. All binding is sensitive to RanGTP. Experiments based on precise domain deletions indicate that NLS2 binds impbeta1, imp9, and imp13 and includes both an importin binding subdomain and a regulatory subdomain with arginine residues being important for function. Moreover, Arx can be co-precipitated with these importins when NLS2 is present. Although nuclear import of Arx can be mediated by these three importin betas, importin beta1 seems to play the major role judging from in vivo small interfering RNA ablations and the in vitro import assay. This is the first evidence to show the role of importin beta1 in nuclear import of paired-type homeodomain proteins. We propose a novel and possibly quite general mechanism for nuclear import of paired-type homeodomain proteins which is critical for development.

Regulation of nuclear import and export of negative cofactor 2

The negative cofactor 2 (NC2) is a protein complex composed of two subunits, NC2alpha and NC2beta, and plays a key role in transcription regulation. Here we investigate whether each subunit contains a nuclear localization signal (NLS) that permits individual crossing of the nuclear membrane or whether nuclear import of NC2alpha and NC2beta depends on heterodimerization. Our results from in vitro binding studies and transfection experiments in cultured cells show that each subunit contains a classical NLS (cNLS) that is recognized by the importin alpha/beta heterodimer. Regardless of the individual cNLSs the two NC2 subunits are translocated as a preassembled complex as co-transfection experiments with wild-type and cNLS-deficient NC2 subunits demonstrate. Ran-dependent binding of the nuclear export receptor Crm1/exportin 1 confirmed the presence of a leucine-rich nuclear export signal (NES) in NC2beta. In contrast, NC2alpha does not exhibit a NES. Our results from interspecies heterokaryon assays suggest that heterodimerization with NC2alpha masks the NES in NC2beta, which prevents nuclear export of the NC2 complex. A mutation in either one of the two cNLSs decreases the extent of importin alpha/beta-mediated nuclear import of the NC2 complex. In addition, the NC2 complex can enter the nucleus via a second pathway, facilitated by importin 13. Because importin 13 binds exclusively to the NC2 complex but not to the individual subunits this alternative import pathway depends on sequence elements distributed among the two subunits.

Importin 13 mediates nuclear import of histone fold-containing chromatin accessibility complex heterodimers

The histone fold is a structural element that facilitates heterodimerization, and histone fold heterodimers play crucial roles in gene regulation. Here, we investigated the nuclear import of two human histone fold pairs, which belong to the H2A/H2B family: CHRAC-15/CHRAC-17 and p12/CHRAC-17. Our results from in vitro nuclear import assays with permeabilized cells and in vivo cotransfection experiments reveal that importin 13 facilitates nuclear import of both histone fold heterodimers. Using glutathione S-transferase pulldown experiments, we provide evidence that heterodimers are required for efficient binding of importin 13 because the monomers alone do not significantly interact. Mutational analysis shows that stepwise substitution of basic amino acid residues conserved among the histone fold subunits leads to a progressive loss of importin 13 binding and nuclear accumulation of CHRAC-15/CHRAC-17 and p12/CHRAC-17. The distribution of basic amino acid residues among the histone fold subunits essential for nuclear uptake suggests that heterodimerization of the histone fold motif-containing proteins forms an importin 13-specific binding platform.

Multiple regions in dengue virus capsid protein contribute to nuclear localization during virus infection

During infection, the capsid (C) protein of many flaviviruses localizes to the nuclei and nucleoli of several infected cell lines; the underlying basis and significance of C protein nuclear localization remain poorly understood. In this study, double alanine-substitution mutations were introduced into three previously proposed nuclear-localization signals (at positions 6-9, 73-76 and 85-100) of dengue virus C protein, and four viable mutants, c(K6A,K7A), c(K73A,K74A), c(R85A,K86A) and c(R97A,R98A), were generated in a mosquito cell line in which C protein nuclear localization was rarely observed. Indirect immunofluorescence analysis revealed that, whilst C protein was present in the nuclei of PS and Vero cells throughout infection with a dengue serotype 2 parent virus, the substitution mutations in c(K73A,K74A) and c(R85A,K86A) resulted in an elimination of nuclear localization in PS cells and marked reduction in Vero cells. Mutants c(K6A,K7A) and c(R97A,R98A) exhibited reduced nuclear localization at the late period of infection in PS cells only. All four mutants displayed reduced replication in PS, Vero and C6/36 cells, but there was a lack of correlation between nuclear localization and viral growth properties. Distinct dibasic residues within dengue virus C protein, many of which were located on the solvent-exposed side of the C protein homodimer, contribute to its ability to localize to nuclei during virus infection.

Evolutionary and transcriptional analysis of karyopherin beta superfamily proteins

In eukaryotes, karyopherin beta superfamily proteins mediate nucleocytoplasmic transport of macromolecules. We investigated the evolutionary and transcriptional patterns of these proteins using bioinformatics approaches. No obvious homologs were found in prokaryotes, but an extensive set of beta-karyopherin proteins was found in yeast. Among 14 beta-karyopherins of Saccharomyces cerevisiae, eight corresponded to their human orthologs directly without diversification, two were lost, and the remaining four proteins exhibited gene duplications by different mechanisms. We also identified beta-karyopherin orthologs in Caenorhabditis elegans, Drosophila melanogaster, Danio rerio, Xenopus tropicalis, Gallus gallus, and Mus musculus. beta-Karyopherins were ubiquitously but nonuniformly expressed in distinct cells and tissues. In yeast and mice, the titer of some beta-karyopherin transcripts appeared to be regulated both during the cell cycle and during development. Further virtual analysis of promoter binding elements suggested that the transcription factors SP1, NRF-2, HEN-1, RREB-1, and nuclear factor Y regulate expression of most beta-karyopherin genes. These findings emphasize new mechanisms in functional diversification of beta-karyopherins and regulation of nucleocytoplasmic transport.

Interaction between importin 13 and myopodin suggests a nuclear import pathway for myopodin

Importin 13 is a member of the importin beta superfamily of nuclear transport proteins and is expressed in multiple tissues at high levels both in humans and rodents, including fetal lung, brain, and heart. In order to elucidate potential functions of imp13 in the heart, we have used rat imp13 as bait to screen a human heart cDNA library and identified an interaction with the C-terminal peptide of myopodin (a.a. 360-698), an actin-bundling protein, associated with tumor-suppressor activity that localizes to both the cytoplasm and the nucleus. We have used GST-pull down assays and co-immunoprecipitation experiments to demonstrate an interaction between imp13 and full-length myopodin and observed that RanGTP dissociates the myopodin-imp13 complex. In studies of cultured cells, we show that both imp13 siRNA and a C-terminal fragment of imp13 protein prevent nuclear localization of myopodin. We, therefore, conclude that imp13 functions in myopodin import and we suggest that the regulation of these events is critical for normal and abnormal cellular differentiation.

Stage-specific Importin13 activity influences meiosis of germ cells in the mouse

Importin-mediated transport of cargoes is known to be a key mechanism for nucleo-cytoplasmic trafficking of molecules. Ipo13, which is a member of Importin-beta gene family, encodes two transcripts by utilizing different transcription start sites. In the mouse, the full-length transcript (L-Ipo13) is expressed in the primordial germ cells in the embryo and is later expressed predominantly at the pachytene phase of meiosis in both male and female germ cells. The shorter transcript (TS-Ipo13) is only expressed in the germ cells in the adult testis. Activity of L-Ipo13, but not TS-Ipo13, mediates the nuclear accumulation of ubiquitin-conjugating enzyme 9 (UBC9), a cargo of human IPO13. This finding is consistent with the progressive accumulation of UBC9 in the nucleus of the meiotic germ cells after the onset of L-Ipo13 expression. Following siRNA knockdown of IPO13 activity in the fetal ovary, fewer germ cells are found to progress to the late-pachytene stage of meiosis and nuclear accumulation of UBC9 is reduced. Our findings strongly implicate a stage-specific role of IPO13 in nuclear-cytoplasmic translocation of cargoes that accompanies meiotic differentiation of the mouse germ cells.

Importin 13 regulates nuclear import of the glucocorticoid receptor in airway epithelial cells

Antiinflammatory effects of glucocorticoids are critical to treatment of airway inflammation in such common disorders as asthma. There is considerable variation in responsiveness to glucocorticoid, and prolonged exposure can result in glucocorticoid resistance. We cloned LGL2, a glucocorticoid-inducible gene in fetal rat lung. We described the characterization of lgl2 as a nuclear transport protein, classified as importin 13 (IPO13), and demonstrated developmental regulation of IPO13 nucleocytoplasmic shuttling. We now report on the identification of the glucocorticoid receptor (GR) as a cargo substrate for IPO13. Binding of GR and IPO13 was demonstrated by GR-GST pulldown and coimmunoprecipitation. To investigate the role of IPO13 in modulating GR signaling in the lung, we studied IPO13-regulated GR transport in airway epithelial cells. Small interfering RNAs that inhibited IPO13 synthesis prevented nuclear translocation of GR. Silencing of IPO13 also abrogated the ability of cortisol to inhibit synthesis of the inflammatory cytokine IL-8 after stimulation with TNF-alpha. Our findings support a role for IPO13 in promoting nuclear occupancy of GR in a way that strongly potentiates the antiinflammatory effects of glucocorticoids. We speculate that variation in cellular levels of IPO13 and intracellular IPO13 shuttling rates may contribute to glucocorticoid resistance.