Three distinct classes of the alpha-subunit of the nuclear pore-targeting complex (importin-alpha) are differentially expressed in adult mouse tissues

Karyopherin α2 is a maternal effect gene required for early embryonic development and female fertility in mice

The nuclear transport of proteins plays an important role in mediating the transition from egg to embryo and distinct karyopherins have been implicated in this process. Here, we studied the impact of KPNA2 deficiency on preimplantation embryo development in mice. Loss of KPNA2 results in complete arrest at the 2cell stage and embryos exhibit the inability to activate their embryonic genome as well as a severely disturbed nuclear translocation of Nucleoplasmin 2. Our findings define KPNA2 as a new maternal effect gene.

Comparative iTRAQ proteomics identified proteins associated with sperm maturation between yak and cattleyak epididymis

Background

During maturation, spermatozoa acquire motility and fertilizing capacity as they transit through the epididymis. In recent years, two-dimensional gel electrophoresis has been employed in proteomics studies conducted in rat, boar and human. However, there has not been a complete information regarding the proteins associated with sperm maturation in the epididymis. In this study, we employed iTRAQ proteomics to investigate proteins associated with sperm maturation between yak and cattleyak epididymis.

Results

After a successful sampling and protein extraction, the iTRAQ coupled with LC-MS/MS mass spectrometry and bioinformatics analysis were performed. We identified 288 differentially abundant proteins (DAPs) between yak and cattleyak epididymis; 151 were up-regulated while 137 were down-regulated in cattleyak relative to yak. Gene Ontology analysis identified that down-regulated DAPs in cattleyak were mostly enriched in the acetylation of protein component, along with negative and positive regulatory activities. iTRAQ proteomics data showed that the top up-regulated DAPs were mainly enriched in cell communication, cell adhesion, cytoskeleton organization, stress response, post-translational modifications and metabolic functions while the down-regulated DAPs were predominantly associated with sperm maturation, long-term sperm storage, sperm forward motility, sperm-oocyte fusion and regulatory functions.

Conclusion

These results provide insight into the molecular mechanisms underlying male cattleyak sterility.

Chromatin remodeling in mammalian embryos

The mammalian embryo undergoes a dramatic amount of epigenetic remodeling during the first week of development. In this review, we discuss several epigenetic changes that happen over the course of cleavage development, focusing on covalent marks (e.g., histone methylation and acetylation) and non-covalent remodeling (chromatin remodeling via remodeling complexes; e.g., SWI/SNF-mediated chromatin remodeling). Comparisons are also drawn between remodeling events that occur in embryos from a variety of mammalian species.

Importin α: functions as a nuclear transport factor and beyond

Nucleocytoplasmic transport is an essential process in eukaryotes. The molecular mechanisms underlying nuclear transport that involve the nuclear transport receptor, small GTPase Ran, and the nuclear pore complex are highly conserved from yeast to humans. On the other hand, it has become clear that the nuclear transport system diverged during evolution to achieve various physiological functions in multicellular eukaryotes. In this review, we first summarize the molecular mechanisms of nuclear transport and how these were elucidated. Then, we focus on the diverse functions of importin α, which acts not merely an import factor but also as a multi-functional protein contributing to a variety of cellular functions in higher eukaryotes.

Reprint of: Importins in the maintenance and lineage commitment of ES cells

The nucleus of a eukaryotic cell is separated from the cytoplasm by a nuclear envelope, and nuclear pores within the envelope facilitate nucleocytoplasmic transport and the exchange of information. Gene regulation is a key component of biological activity regulation in the cell. Transcription factors control the expression levels of various genes that are necessary for the maintenance or conversion of cellular states during animal development. Because transcription factor activities determine the extent of transcription of target genes, the number of active transcription factors must be tightly regulated. In this regard, the nuclear translocation of a transcription factor is an important determinant of its activity. Therefore, it is becoming clear that the nucleocytoplasmic transport machinery is involved in cell differentiation and organism development. This review examines the regulation of transcription factors by the nucleocytoplasmic transport machinery in ES cells.

Importins in the maintenance and lineage commitment of ES cells

The nucleus of a eukaryotic cell is separated from the cytoplasm by a nuclear envelope, and nuclear pores within the envelope facilitate nucleocytoplasmic transport and the exchange of information. Gene regulation is a key component of biological activity regulation in the cell. Transcription factors control the expression levels of various genes that are necessary for the maintenance or conversion of cellular states during animal development. Because transcription factor activities determine the extent of transcription of target genes, the number of active transcription factors must be tightly regulated. In this regard, the nuclear translocation of a transcription factor is an important determinant of its activity. Therefore, it is becoming clear that the nucleocytoplasmic transport machinery is involved in cell differentiation and organism development. This review examines the regulation of transcription factors by the nucleocytoplasmic transport machinery in ES cells.

Identification of the nuclear localisation signal of O-GlcNAc transferase and its nuclear import regulation

Nucleocytoplasmic O-GlcNAc transferase (OGT) attaches a single GlcNAc to hydroxyl groups of serine and threonine residues. Although the cellular localisation of OGT is important to regulate a variety of cellular processes, the molecular mechanisms regulating the nuclear localisation of OGT is unclear. Here, we characterised three amino acids (DFP; residues 451-453) as the nuclear localisation signal of OGT and demonstrated that this motif mediated the nuclear import of non-diffusible β-galactosidase. OGT bound the importin α5 protein, and this association was abolished when the DFP motif of OGT was mutated or deleted. We also revealed that O-GlcNAcylation of Ser389, which resides in the tetratricopeptide repeats, plays an important role in the nuclear localisation of OGT. Our findings may explain how OGT, which possesses a NLS, exists in the nucleus and cytosol simultaneously.

Brain Corticosteroid Receptor Dynamics and Trafficking: Implications from Live Cell Imaging

Adrenal corticosteroids (cortisol in humans or corticosterone in rodents) exert numerous effects in the central nervous system that regulate the stress response, mood, learning and memory, and various neuroendocrine functions. Corticosterone actions in the brain are mediated by two corticosteroid receptors, glucocorticoid receptor (GR) and mineralocorticoid receptor (MR), and they show a high degree of colocalization in the hippocampal region. These receptors predominantly reside in the cytoplasm without ligand and are translocated into the nucleus upon ligand binding to act as transcriptional factors. Thus, their subcellualr localizations are an important component of their biological activity. Given the differential action of MR and GR in the central nervous system, it is important to elucidate how the trafficking of these receptors between the cytoplasm and the nucleus and their interactions are regulated by ligand or other molecules to exert transcriptional activity. In this review, the authors focus on the nucleocytoplasmic and subnuclear trafficking of GR and MR in neural cells and nonneural cells and discuss various factors affecting the dynamics of these receptors.

Dynamic changes in nuclear import of a nuclear localisation signal-bearing substrate in 8-cell stage porcine embryos

Coordinated intracellular trafficking is critically important for proper timing of major cellular events during embryogenesis. Nuclear import mediated by the karyopherin α/β (importin α/β) heterodimer is perhaps the best characterised nuclear trafficking system in eukaryotic cells. Seven karyopherin α subtypes have been identified in the domestic pig, and although each karyopherin α subtype transports proteins bearing classical nuclear localisation signals (NLSs), individual karyopherin α subtypes have been shown to preferentially transport specific cargoes. The aim of the present study was to determine the mechanism by which BRN2, a transcription factor previously reported to be transported by the karyopherin α/β heterodimer, gains access to the nucleus in porcine oocytes and embryos. Using a combination of in vivo and in vitro assays, we tested the hypothesis that discrete karyopherin α subtypes transport BRN2 into the nuclei of porcine oocytes and cleavage stage embryos. Our results show that ectopically expressed BRN2 adopts a nuclear localisation in all nuclei through the 4-cell stage of development, whereas only a subset of blastomeres in 8-cell stage embryos possess nuclear BRN2. This pattern is unique to BRN2 because another ectopically expressed NLS-containing protein is able to adopt a nuclear localisation in all blastomeres of 8-cell stage embryos.

Importin alpha subtypes determine differential transcription factor localization in embryonic stem cells maintenance

We recently demonstrated that the expression of the importin α subtype is switched from α2 to α1 during neural differentiation in mouse embryonic stem cells (ESCs) and that this switching has a major impact on cell differentiation. In this study, we report a cell-fate determination mechanism in which importin α2 negatively regulates the nuclear import of certain transcription factors to maintain ESC properties. The nuclear import of Oct6 and Brn2 was inhibited via the formation of a transport-incompetent complex of the cargo bound to a nuclear localization signal binding site in importin α2. Unless this dominant-negative effect was downregulated upon ESC differentiation, inappropriate cell death was induced. We propose that although certain transcription factors are necessary for differentiation in ESCs, these factors are retained in the cytoplasm by importin α2, thereby preventing transcription factor activity in the nucleus until the cells undergo differentiation.

Identification of cargo proteins specific for importin-β with importin-α applying a stable isotope labeling by amino acids in cell culture (SILAC)-based in vitro transport system

The human importin (Imp)-β family consists of 21 nucleocytoplasmic transport carrier proteins, which transport thousands of proteins (cargoes) across the nuclear envelope through nuclear pores in specific directions. To understand the nucleocytoplasmic transport in a physiological context, the specificity of cargoes for their cognate carriers should be determined; however, only a limited number of nuclear proteins have been linked to specific carriers. To address this biological question, we recently developed a novel method to identify carrier-specific cargoes. This method includes the following three steps: (i) the cells are labeled by stable isotope labeling by amino acids in cell culture (SILAC); (ii) the labeled cells are permeabilized, and proteins in the unlabeled cell extracts are transported into the nuclei of the permeabilized cells by a particular carrier; and (iii) the proteins in the nuclei are quantitatively identified by LC-MS/MS. The effectiveness of this method was demonstrated by the identification of transportin (Trn)-specific cargoes. Here, we applied this method to identify cargo proteins specific for Imp-β, which is a predominant carrier that exclusively utilizes Imp-α as an adapter for cargo binding. We identified candidate cargoes, which included previously reported and potentially novel Imp-β cargoes. In in vitro binding assays, most of the candidate cargoes bound to Imp-β in one of three binding modes: directly, via Imp-α, or via other cargoes. Thus, our method is effective for identifying a variety of Imp-β cargoes. The identified Imp-β and Trn cargoes were compared, ensuring the carrier specificity of the method and illustrating the complexity of these transport pathways.

Identification of Novel Stress Granule Components That Are Involved in Nuclear Transport

Background

Importin-α1 belongs to a subfamily of nuclear transport adaptors and participates in diverse cellular functions. Best understood for its role in protein transport, importin-α1 also contributes to other biological processes. For instance, arsenite treatment causes importin-α1 to associate with cytoplasmic stress granules (SGs) in mammalian cells. These stress-induced compartments contain translationally arrested mRNAs, small ribosomal subunits and numerous proteins involved in mRNA transport and metabolism. At present, it is not known whether members of all three importin-α subfamilies locate to SGs in response to stress.

Results

Here, we demonstrate that the oxidant diethyl maleate (DEM), arsenite and heat shock, promote the formation of cytoplasmic SGs that contain nuclear transport factors. Specifically, importin-α1, α4 and α5, which belong to distinct subfamilies, and importin-β1 were targeted by all of these stressors to cytoplasmic SGs, but not to P-bodies. Importin-α family members have been implicated in transcriptional regulation, which prompted us to analyze their ability to interact with poly(A)-RNA in growing cells. Our studies show that importin-α1, but not α4, α5, importin-β1 or CAS, associated with poly(A)-RNA under nonstress conditions. Notably, this interaction was significantly reduced when cells were treated with DEM. Additional studies suggest that importin-α1 is likely connected to poly(A)-RNA through an indirect interaction, as the adaptor did not bind homopolymer RNA specifically in vitro.

Significance

Our studies establish that members of three importin-α subfamilies are bona fide SG components under different stress conditions. Furthermore, importin-α1 is unique in its ability to interact with poly(A)-RNA in a stress-dependent fashion, and in vitro experiments indicate that this association is indirect. Collectively, our data emphasize that nuclear transport factors participate in a growing number of cellular activities that are modulated by stress.

The nuclear import factor importin α4 can protect against oxidative stress

The importin (IMP) superfamily of nuclear transport proteins is essential to key developmental pathways, including in the murine testis where expression of the 6 distinct IMPα proteins is highly dynamic. Present predominantly from the spermatocyte stage onwards, IMPα4 is unique in showing a striking nuclear localization, a property we previously found to be linked to maintenance of pluripotency in embryonic stem cells and to the cellular stress response in cultured cells. Here we examine the role of IMPα4 in vivo for the first time using a novel transgenic mouse model in which we overexpress an IMPα4-EGFP fusion protein from the protamine 1 promoter to recapitulate endogenous testicular germ cell IMPα4 expression in spermatids. IMPα4 overexpression did not affect overall fertility, testis morphology/weight or spermatogenic progression under normal conditions, but conferred significantly (>30%) increased resistance to oxidative stress specifically in the spermatid subpopulation expressing the transgene. Consistent with a cell-specific role for IMPα4 in protecting against oxidative stress, haploid germ cells from IMPα4 null mice were significantly (c. 30%) less resistant to oxidative stress than wild type controls. These results from two unique and complementary mouse models demonstrate a novel protective role for IMPα4 in stress responses specifically within haploid male germline cells, with implications for male fertility and genetic integrity.

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.

Nuclear pore complex composition: a new regulator of tissue-specific and developmental functions

Nuclear pore complexes (NPCs) are multiprotein aqueous channels that penetrate the nuclear envelope connecting the nucleus and the cytoplasm. NPCs consist of multiple copies of roughly 30 different proteins known as nucleoporins (NUPs). Due to their essential role in controlling nucleocytoplasmic transport, NPCs have traditionally been considered as structures of ubiquitous composition. The overall structure of the NPC is indeed conserved in all cells, but new evidence suggests that the protein composition of NPCs varies among cell types and tissues. Moreover, mutations in various nucleoporins result in tissue-specific diseases. These findings point towards a heterogeneity in NPC composition and function. This unexpected heterogeneity suggests that cells use a combination of different nucleoporins to assemble NPCs with distinct properties and specialized functions.

Identification of a karyopherin β1/β2 proline-tyrosine nuclear localization signal in huntingtin protein

Among the known pathways of protein nuclear import, the karyopherin β2/transportin pathway is only the second to have a defined nuclear localization signal (NLS) consensus. Huntingtin, a 350-kDa protein, has defined roles in the nucleus, as well as a CRM1/exportin-dependent nuclear export signal; however, the NLS and exact pathway of import have remained elusive. Here, using a live cell assay and affinity chromatography, we show that huntingtin has a karyopherin β2-dependent proline-tyrosine (PY)-NLS in the amino terminus of the protein. This NLS comprises three consensus components: a basic charged sequence, a downstream conserved arginine, and a PY sequence. Unlike the classic PY-NLS, which has an unstructured intervening sequence between the consensus components, we show that a β sheet structured region separating the consensus elements is critical for huntingtin NLS function. The huntingtin PY-NLS is also capable of import through the importin/karyopherin β1 pathway but was not functional in all cell types tested. We propose that this huntingtin PY-NLS may comprise a new class of multiple import factor-dependent NLSs with an internal structural component that may regulate NLS activity.

Identification of karyopherin α1 and α7 interacting proteins in porcine tissue

Specialized trafficking systems in eukaryotic cells serve a critical role in partitioning intracellular proteins between the nucleus and cytoplasm. Cytoplasmic proteins (including chromatin remodeling enzymes and transcription factors) must gain access to the nucleus to exert their functions to properly program fundamental cellular events ranging from cell cycle progression to gene transcription. Knowing that nuclear import mediated by members of the karyopherin α family of transport receptors plays a critical role in regulating development and differentiation, we wanted to determine the identity of proteins that are trafficked by this karyopherin α pathway. To this end, we performed a GST pull-down assay using porcine orthologs of karyopherin α1 (KPNA1) and karyopherin α7 (KPNA7) and prey protein derived from porcine fibroblast cells and used a liquid chromatography and tandem mass spectrometry (LC-MS/MS) approach to determine the identity of KPNA1 and KPNA7 interacting proteins. Our screen revealed that the proteins that interact with KPNA1 and KPNA7 are generally nuclear proteins that possess nuclear localization signals. We further validated two candidate proteins from this screen and showed that they are able to be imported into the nucleus in vivo and also interact with members of the karyopherin α family of proteins in vitro. Our results also reveal the utility of using a GST pull-down approach coupled with LC-MS/MS to screen for protein interaction partners in a non-traditional model system.

Regulated nucleocytoplasmic transport during gametogenesis

Gametogenesis is the process by which sperm or ova are produced in the gonads. It is governed by a tightly controlled series of gene expression events, with some common and others distinct for males and females. Nucleocytoplasmic transport is of central importance to the fidelity of gene regulation that is required to achieve the precisely regulated germ cell differentiation essential for fertility. In this review we discuss the physiological importance for gamete formation of the molecules involved in classical nucleocytoplasmic protein transport, including importins/karyopherins, Ran and nucleoporins. To address what functions/factors are conserved or specialized for these developmental processes between species, we compare knowledge from mice, flies and worms. The present analysis provides evidence of the necessity for and specificity of each nuclear transport factor and for nucleoporins during germ cell differentiation. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

Long-distance signaling from synapse to nucleus via protein messengers

The communication between synapses and the cell nucleus has attracted considerable interest for many years. This interest is largely fueled by the idea that synapse-to-nucleus signaling might specifically induce the expression of genes that make long-term memory "stick." However, despite many years of research, it is still essentially unclear how synaptic signals are conveyed to the nucleus, and it remains to a large degree enigmatic how activity-induced gene expression feeds back to synaptic function. In this chapter, we will focus on the activity-dependent synapto-nuclear trafficking of protein messengers and discuss the underlying mechanisms of their retrograde transport and their supposed functional role in neuronal plasticity.

KPNA7, an oocyte- and embryo-specific karyopherin?subtype, is required for porcine embryo development

Coordinated partitioning of intracellular cargoes between nuclear and cytoplasmic compartments is critical for cell survival and differentiation. The karyopherin α/β heterodimer functions to import cytoplasmic proteins that possess classical nuclear localisation signals into the nucleus. Seven karyopherin α subtypes have been identified in mammals. The aim of this study was to determine the relative abundance of transcripts encoding seven karyopherin α subtypes in porcine oocytes and embryos at discrete stages of cleavage development, and to determine the developmental requirements of karypopherin α 7 (KPNA7), an oocyte and cleavage stage embryo-specific karyopherin α subtype. We hypothesised that knockdown of KPNA7 would negatively affect porcine cleavage development. To test this hypothesis, in vitro matured and fertilised porcine oocytes were injected with a double-stranded interfering RNA molecule that targeted KPNA7; nuclei were counted in all embryos 6 days after fertilisation. Embryos injected with KPNA7-interfering RNAs possessed significantly lower cell numbers than their respective control groups (P < 0.05). In vitro binding assays also suggest that KPNA7 may transport intracellular proteins that possess unique nuclear localisation signals. Our data suggest that embryos have differential requirements for individual karyopherin α subtypes and that these karyopherin α subtypes differentially transport intracellular cargo during cleavage development.

Regulation of Nuclear Import During Differentiation; The IMP alpha Gene Family and Spermatogenesis

Access to nuclear genes in eukaryotes is provided by members of the importin (IMP) superfamily of proteins, which are of α- or β-types, the best understood nuclear import pathway being mediated by a heterodimer of an IMP α and IMP β1. IMP α recognises specific targeting signals on cargo proteins, while IMP β1 mediates passage into, and release within, the nucleus by interacting with other components of the transport machinery, including the monomeric guanine nucleotide binding protein Ran. In this manner, hundreds of different proteins can be targeted specifically into the nucleus in a tightly regulated fashion. The IMP α gene family has expanded during evolution, with only a single IMP α (Srp1p) gene in budding yeast, and three (IMP α1, 2/pendulin and 3) and five (IMP α1, -2, -3, -4 and -6) IMP α genes in Drosophila melanogaster and mouse respectively, which fall into three phylogenetically distinct groups. The fact that IMP α3 and IMP α2 are only present in metazoans implies that they emerged during the evolution of multicellular animals to perform specialised roles in particular cells and tissues. This review describes what is known of the IMP α gene family in mouse and in D. melanogaster, including a comparitive examination of their mRNA expression profiles in a highly differentiated tissue, the testis. The clear implication of their highly regulated synthesis during the course of spermatogenesis is that the different IMP αs have distinct expression patterns during cellular differentiation, implying tissue/cell type-specific roles.

Distinct roles for classical nuclear import receptors in the growth of multinucleated muscle cells

Proper muscle function is dependent on spatial and temporal control of gene expression in myofibers. Myofibers are multinucleated cells that are formed, repaired and maintained by the process of myogenesis in which progenitor myoblasts proliferate, differentiate and fuse. Gene expression is dependent upon proteins that require facilitated nuclear import, however little is known about the regulation of nucleocytoplasmic transport during the formation of myofibers. We analyzed the role of karyopherin alpha (KPNA), a key classical nuclear import receptor, during myogenesis. We established that five karyopherin alpha paralogs are expressed by primary mouse myoblasts in vitro and that their steady-state levels increase in multinucleated myotubes, suggesting a global increase in demand for classical nuclear import during myogenesis. We used siRNA-mediated knockdown to identify paralog-specific roles for KPNA1 and KPNA2 during myogenesis. KPNA1 knockdown increased myoblast proliferation, whereas KPNA2 knockdown decreased proliferation. In contrast, no proliferation defect was observed with KPNA4 knockdown. Only knockdown of KPNA2 decreased myotube growth. These results identify distinct pathways involved in myoblast proliferation and myotube growth that rely on specific nuclear import receptors suggesting that regulation of classical nuclear import pathways likely plays a critical role in controlling gene expression in skeletal muscle.

Importin α7 is essential for zygotic genome activation and early mouse development

Importin α is involved in the nuclear import of proteins. It also contributes to spindle assembly and nuclear membrane formation, however, the underlying mechanisms are poorly understood. Here, we studied the function of importin α7 by gene targeting in mice and show that it is essential for early embryonic development. Embryos lacking importin α7 display a reduced ability for the first cleavage and arrest completely at the two-cell stage. We show that the zygotic genome activation is severely disturbed in these embryos. Our findings indicate that importin α7 is a new member of the small group of maternal effect genes.

Targeted disruption of one of the importin α family members leads to female functional incompetence in delivery

Importin α mediates the nuclear import of proteins through nuclear pore complexes in eukaryotic cells, and is common to all eukaryotes. Previous reports identified at least six importin α family genes in mice. Although these isoforms show differential binding to various import cargoes in vitro, the in vivo physiological roles of these mammalian importin α isoforms remain unknown. Here, we generated and examined importin α5 knockout (impα5(-/-)) mice. These mice developed normally, and showed no gross histological abnormalities in most major organs. However, the ovary and uterus of impα5(-/-) female mice exhibited hypoplasia. Furthermore, we found that impα5(-/-) female mice had a 50% decrease in serum progesterone levels and a 57% decrease in progesterone receptor mRNA levels in the ovary. Additionally, impα5(-/-) uteruses that were treated with exogenous gonadotropins displayed hypertrophy, similarly to progesterone receptor-deficient mice. Although these mutant female mice could become pregnant, the total number of pups was significantly decreased, and some of the pups were dead at birth. These results suggest that importin α5 has essential roles in the mammalian female reproductive organs.

Expression of nucleocytoplasmic transport machinery: clues to regulation of spermatogenic development

Spermatogenesis is one example of a developmental process which requires tight control of gene expression to achieve normal growth and sustain function. This review is based on the principle that events in spermatogenesis are controlled by changes in the distribution of proteins between the nuclear and cytoplasmic compartments. Through analysis of the regulated production of nucleocytoplasmic transport machinery in mammalian spermatogenesis, this review addresses the concept that access to the nucleus is tightly controlled to enable and prevent differentiation. A broad review of nuclear transport components is presented, outlining the different categories of machinery required for import, export and non-nuclear functions. In addition, the complexity of nomenclature is addressed by the provision of a concise yet comprehensive listing of information that will aid in comparative studies of different transport proteins and the genes which encode them. We review a suite of existing transcriptional analyses which identify common and distinct patterns of transport machinery expression, showing how these can be linked with key events in spermatogenic development. The additional importance of this for human fertility is considered, in light of data that identify which importin and nuclear transport machinery components are present in testicular cancer specimens, while also providing an indication of how their presence (and absence) may be considered as potential mediators of oncogenesis. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Regulation of nucleocytoplasmic transport in skeletal muscle

Proper skeletal muscle function is dependent on spatial and temporal control of gene expression in multinucleated myofibers. In addition, satellite cells, which are tissue-specific stem cells that contribute critically to repair and maintenance of skeletal muscle, are also required for normal muscle physiology. Gene expression in both myofibers and satellite cells is dependent upon nuclear proteins that require facilitated nuclear transport. A unique challenge for myofibers is controlling the transcriptional activity of hundreds of nuclei in a common cytoplasm yet achieving nuclear selectivity in transcription at specific locations such as neuromuscular synapses and myotendinous junctions. Nucleocytoplasmic transport of macromolecular cargoes is regulated by a complex interplay among various components of the nuclear transport machinery, namely nuclear pore complexes, nuclear envelope proteins, and various soluble transport receptors. The focus of this review is to highlight what is known about the nuclear transport machinery and its regulation in skeletal muscle and to consider the unique challenges that multinucleated muscle cells as well as satellite cells encounter in regulating nucleocytoplasmic transport during cell differentiation and tissue adaptation. Understanding how regulated nucleocytoplasmic transport controls gene expression in skeletal muscle may lead to further insights into the mechanisms contributing to muscle growth and maintenance throughout the lifespan of an individual.

Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication

HIV-1 employs the cellular nuclear import machinery to actively transport its preintegration complex (PIC) into the nucleus for integration of the viral DNA. Several viral karyophilic proteins and cellular import factors have been suggested to contribute to HIV-1 PIC nuclear import and replication. However, how HIV interacts with different cellular machineries to ensure efficient nuclear import of its preintegration complex in dividing and nondividing cells is still not fully understood. In this study, we have investigated different importin alpha (Impalpha) family members for their impacts on HIV-1 replication, and we demonstrate that short hairpin RNA (shRNA)-mediated Impalpha3 knockdown (KD) significantly impaired HIV infection in HeLa cells, CD4(+) C8166 T cells, and primary macrophages. Moreover, quantitative real-time PCR analysis revealed that Impalpha3-KD resulted in significantly reduced levels of viral 2-long-terminal repeat (2-LTR) circles but had no effect on HIV reverse transcription. All of these data indicate an important role for Impalpha3 in HIV nuclear import. In an attempt to understand how Impalpha3 participates in HIV nuclear import and replication, we first demonstrated that the HIV-1 karyophilic protein integrase (IN) was able to interact with Impalpha3 both in a 293T cell expression system and in HIV-infected CD4(+) C8166 T cells. Deletion analysis suggested that a region (amino acids [aa] 250 to 270) in the C-terminal domain of IN is involved in this viral-cellular protein interaction. Overall, this study demonstrates for the first time that Impalpha3 is an HIV integrase-interacting cofactor that is required for efficient HIV-1 nuclear import and replication in both dividing and nondividing cells.

Common importin alpha specificity for papillomavirus E2 proteins

Papillomaviruses infect keratinocytes and their reproduction is tied to differentiation of the skin. The E2 protein of papillomaviruses is a multifunctional early protein that binds specifically to the viral DNA to regulate genome transcription, replication, and segregation. All of these are nuclear events that require specific transport of E2 into the host nucleus. Nuclear localization signal (NLS) sequences have been mapped for several E2 proteins, and these sequences resemble motifs that interact with cellular transport adaptor molecules termed alpha importins. To determine which importins could carry E2 proteins, in vitro binding studies were performed with three different E2 proteins and the five ubiquitous alpha importins. The E2 proteins preferentially interacted with alpha importins 3 and 5, and showed very weak or no interaction with the other three widely expressed alpha importins (alpha1, alpha 4, and alpha 7). While all five alpha importins appear to be constitutively expressed in keratinocytes, during differentiation of a human keratinocyte line (HaCaT) we observed a specific increase in expression of alphas 3 and 5. This differentiation-specific increase in alpha 3 and alpha 5 expression suggests that preferential usage of these two importins by E2 may facilitate E2 nuclear uptake during terminal differentiation.

A single acute pharmacological dose of γ-hydroxybutyrate modifies multiple gene expression patterns in rat hippocampus and frontal cortex

γ-Hydroxybutyrate (GHB) is a natural brain neuromodulator that has its own enzymatic machinery for synthesis and degradation, release, and transport systems and several receptors that belong to the G protein-coupled receptor (GPCR) family. Targeting of this system with exogenous GHB is used in therapy to induce sleep and anesthesia and to reduce alcohol withdrawal syndrome. GHB is also popular as a recreational drug for its anxiolytic and mild euphoric effects. However, in both cases, GHB must be administered at high doses in order to maintain GHB concentrations in brain of ∼800-1,000 μM. These high concentrations are thought to be necessary for interactions with low-affinity sites on GABA(B) receptor, but the molecular targets and cellular mechanisms modulated by GHB remain poorly characterized. Therefore, to provide new insights into the elucidation of GHB mechanisms of action and open new tracks for future investigations, we explored changes of GHB-induced transcriptomes in rat hippocampus and prefrontal cortex by using DNA microarray studies. We demonstrate that a single acute anesthetic dose of 1 g/kg GHB alters a large number of genes, 121 in hippocampus and 53 in prefrontal cortex; 16 genes were modified simultaneously in both brain regions. In terms of molecular functions, the majority of modified genes coded for proteins or nucleotide binding sites. In terms of Gene Ontology (GO) functional categories, the largest groups were involved in metabolic processing for hippocampal genes and in biological regulation for prefrontal cortex genes. The majority of genes modified in both structures were implicated in cell communication processes. Western blot and immunohistochemical studies carried out on eight selected proteins confirmed the microarray findings.

Mechanisms and signals for the nuclear import of proteins

In eukaryotes, the nuclear membrane provides a physical barrier to the passive diffusion of macromolecules from and into the cytoplasm. Nucleocytoplasmic traffic occurs through highly specialized structures known as nuclear pores, and involves the participation of a special class of transport proteins. Active transport across the nuclear pores is an energy-dependent process that relies on the activity of Ran-GTPases both in the nuclear and cytoplasmic compartments. Nuclear import of proteins is an essential step in regulating gene expression and the replication cycle of several viruses. In this review, the key mechanisms, pathways, and models underlying the transport of proteins across nuclear pores are analysed.

The role of the nuclear transport system in cell differentiation

The eukaryotic cell nuclear transport system selectively mediates molecular trafficking to facilitate the regulation of cellular processes. The components of this system include diverse transport factors such as importins and nuclear pore components that are precisely organized to coordinate cellular events. A number of studies have demonstrated that the nuclear transport system is indispensible in many types of cellular responses. In particular, the nuclear transport machinery has been shown to be an important regulator of development, organogenesis, and tissue formation, wherein altered nuclear transport of key transcription factors can lead to disease. Importantly, precise switching between distinct forms of importin alpha is central to neural lineage specification, consistent with the hypothesis that importin expression can be a key mediator of cell differentiation.

Evolution of the metazoan-specific importin alpha gene family

Importin alphas are import receptors for nuclear localization signal-containing proteins. Most animal importin alphas assort into alpha1, alpha2, and alpha3 groups. Studies in Drosophila melanogaster, Caenorhabditis elegans, and mouse suggest that the animal importin alpha gene family evolved from ancestral plant-like genes to serve paralog-specific roles in gametogenesis. To explore this hypothesis we extended the phylogenetic analysis of the importin alpha gene family to nonbilateral animals and investigated whether animal-like genes occur in premetazoan taxa. Maximum likelihood analysis suggests that animal-like importin alpha genes occur in the Choanoflaggelate Monosiga brevicollis and the amoebozoan Dictyostelium; however, both of these results are caused by long-branch attraction effects. The absence of animal-like alpha genes in premetazoan taxa is consistent with the hypothesis that they duplicated and then specialized to function in animal gametogenesis. The gene structures of the importin alphas provide insight into how the animal importin alpha gene family may have evolved from the most likely ancestral gene. Interestingly, animal alpha1s are more similar to plant and fungal alpha1-like sequences than they are to animal alpha2s or alpha3s. We show that animal alpha1 genes share most of their introns with plant alpha1-like genes, and alpha2s and alpha3s share many more intron positions with each other than with the alpha1s. Together, phylogenetics and gene structure analysis suggests a parsimonious path for the evolution of the mammalian importin alpha gene family from an ancestral alpha1-like progenitor. Finally, these results establish a rational basis for a unified nomenclature of the importin alpha gene family.

Regional distribution of importin subtype mRNA expression in the nervous system: study of early postnatal and adult mouse

Importin-alpha and beta1 mediate the translocation of macromolecules bearing nuclear localization signals across the nuclear pore complex. Five importin-alpha isoforms have been identified in mice and six in human. Some of these importins play an important role in neural activity such as long term potentiation, but the functional differences of each isoform in the CNS are still unclear. We performed in situ hybridization (ISH) using non-isotopic probes to clarify the expression patterns of importin-alpha subtypes (alpha5, alpha7, alpha1, alpha4, alpha3) and importin-beta1 in the mouse CNS of adult and early postnatal stages. The mRNAs of the importin-alpha subtypes and importin beta1 were expressed throughout the CNS with specific patterns; importin-alpha5, alpha7, alpha3, and beta1 showed moderate to high expression levels throughout the brain and spinal cord; importin-alpha4 showed a lack of expression in limited regions; and importin-alpha1 showed a low expression level throughout the brain and spinal cord but with a moderate expression level in the olfactory bulb and reticular system. We also demonstrated that importin-alphas and beta1 mRNAs were predominantly expressed in neurons in the adult mouse brain by using double-labeling fluorescence ISH and immunohistochemistry. Moreover, importin-alphas and beta1 mRNAs were detected throughout the CNS of postnatal mice and were highly expressed in the external granule layer of the cerebellar cortex on postnatal days 0, 4, and 10. This is the first report of importin-alphas and beta1 expression throughout the CNS of adult mice, as well as in the developing brain, including cell type specific localization.

Drosophila importin alpha1 performs paralog-specific functions essential for gametogenesis

Importin alpha's mediate nuclear transport by linking nuclear localization signal (NLS)-containing proteins to importin beta1. Animal genomes encode three conserved groups of importin alpha's, alpha1's, alpha2's, and alpha3's, each of which are competent to bind classical NLS sequences. Using Drosophila melanogaster we describe the isolation and phenotypic characterization of the first animal importin alpha1 mutant. Animal alpha1's are more similar to ancestral plant and fungal alpha1-like genes than to animal alpha2 and alpha3 genes. Male and female importin alpha1 (Dalpha1) null flies developed normally to adulthood (with a minor wing defect) but were sterile with defects in gametogenesis. The Dalpha1 mutant phenotypes were rescued by Dalpha1 transgenes, but not by Dalpha2 or Dalpha3 transgenes. Genetic interactions between the ectopic expression of Dalpha1 and the karyopherins CAS and importin beta1 suggest that high nuclear levels of Dalpha1 are deleterious. We conclude that Dalpha1 performs paralog-specific activities that are essential for gametogenesis and that regulation of subcellular Dalpha1 localization may affect cell fate decisions. The initial expansion and specialization of the animal importin alpha-gene family may have been driven by the specialized needs of gametogenesis. These results provide a framework for studies of the more complex mammalian importin alpha-gene family.

Novel nuclear import of Vpr promoted by importin alpha is crucial for human immunodeficiency virus type 1 replication in macrophages

Monocytes/macrophages are major targets of human immunodeficiency virus type 1 (HIV-1) infection. The viral preintegration complex (PIC) of HIV-1 enters the nuclei of monocyte-derived macrophages, but very little PIC migrates into the nuclei of immature monocytes. Vpr, one of the accessory gene products of HIV-1, is essential for the nuclear import of PIC in these cells, although the role of Vpr in the entry mechanism of PIC remains to be clarified. We have shown previously that Vpr is targeted to the nuclear envelope and then transported into the nucleus by importin alpha alone, in an importin beta-independent manner. Here we demonstrate that the nuclear import of Vpr is strongly promoted by the addition of cytoplasmic extract from macrophages but not of that from monocytes and that the nuclear import activity is lost with immunodepletion of importin alpha from the cytoplasmic extract. Immunoblot analysis and real-time PCR demonstrate that immature monocytes express importin alpha at low levels, whereas the expression of three major importin alpha isoforms markedly increases upon their differentiation into macrophages, indicating that the expression of importin alpha is required for nuclear import of Vpr. Furthermore, interaction between importin alpha and the N-terminal alpha-helical domain of Vpr is indispensable, not only for the nuclear import of Vpr but also for HIV-1 replication in macrophages. This study suggests the possibility that the binding of Vpr to importin alpha, preceding a novel nuclear import process, is a potential target for therapeutic intervention.

Importin alpha1 is involved in the nuclear localization of Zac1 and the induction of p21WAF1/CIP1 by Zac1

Zac1, a novel seven-zinc-finger transcription factor, preferentially binds GC-rich DNA elements and has intrinsic transactivation activity. To date, the NLS (nuclear localization signal) of Zac1 has not been empirically determined. We generated a series of EGFP (enhanced green fluorescence protein)-tagged deletion mutants of Zac1 and examined their subcellular localization, from which we defined two NLSs within the DNA-binding (or zinc-finger) domain. Fusion proteins consisting of the two EGFP-tagged zinc-finger clusters (zinc finger motifs 1-3 and 4-7) were located exclusively in the nucleus, demonstrating that each of the zinc-finger clusters is sufficient for nuclear localization. Physical interactions between these two zinc-finger clusters and importin alpha1 were demonstrated using an in vitro glutathione S-transferase pull-down assay. Finally, our results indicate that the association of Zac1 with importin alpha1 is also involved in regulating the transactivation activity of Zac1 on the p21WAF1/CIP1 gene and protein expression.

Topoisomerase II binds importin alpha isoforms and exportin/CRM1 but does not shuttle between the nucleus and cytoplasm in proliferating cells

Resistance to anticancer drugs that target DNA topoisomerase II (topo II) isoforms alpha and/or beta is associated with decreased nuclear and increased cytoplasmic topo IIalpha. Earlier studies have confirmed that functional nuclear localization and export signal sequences (NLS and NES) are present in both isoforms. In this study, we show that topo II alpha and beta bind and are imported into the nucleus by importin alpha1, alpha3, and alpha5 in conjunction with importin beta. Topo IIalpha also binds exportin/CRM1 in vitro. However, wild-type topo IIalpha has only been observed in the cytoplasm of cells that are entering plateau phase growth. This suggests that topo IIalpha may shuttle between the nucleus and the cytoplasm with the equilibrium towards the nucleus in proliferating cells but towards the cytoplasm in plateau phase cells. The CRM1 inhibitor Leptomycin B increases the nuclear localization of GFP-tagged topo IIalpha with a mutant NLS, suggesting that its export is being inhibited. However, homokaryon shuttling experiments indicate that fluorescence-tagged wild-type topo II alpha and beta proteins do not shuttle in proliferating Cos-1 or HeLa cells. We conclude that topo II alpha and beta nuclear export is inhibited in proliferating cells so that these proteins do not shuttle.

Nuclear import of bovine papillomavirus type 1 E1 protein is mediated by multiple alpha importins and is negatively regulated by phosphorylation near a nuclear localization signal

Papillomavirus DNA replication occurs in the nucleus of infected cells and requires the viral E1 protein, which enters the nuclei of host epithelial cells and carries out enzymatic functions required for the initiation of viral DNA replication. In this study, we investigated the pathway and regulation of the nuclear import of the E1 protein from bovine papillomavirus type 1 (BPV1). Using an in vitro binding assay, we determined that the E1 protein interacted with importins alpha3, alpha4, and alpha5 via its nuclear localization signal (NLS) sequence. In agreement with this result, purified E1 protein was effectively imported into the nucleus of digitonin-permeabilized HeLa cells after incubation with importin alpha3, alpha4, or alpha5 and other necessary import factors. We also observed that in vitro binding of E1 protein to all three alpha importins was significantly decreased by the introduction of pseudophosphorylation mutations in the NLS region. Consistent with the binding defect, pseudophosphorylated E1 protein failed to enter the nucleus of digitonin-permeabilized HeLa cells in vitro. Likewise, the pseudophosphorylation mutant showed aberrant intracellular localization in vivo and accumulated primarily on the nuclear envelope in transfected HeLa cells, while the corresponding alanine replacement mutant displayed the same cellular location pattern as wild-type E1 protein. Collectively, our data demonstrate that BPV1 E1 protein can be transported into the nucleus by more than one importin alpha and suggest that E1 phosphorylation by host cell kinases plays a regulatory role in modulating E1 nucleocytoplasmic localization. This phosphoregulation of nuclear E1 protein uptake may contribute to the coordination of viral replication with keratinocyte proliferation and differentiation.

Regulation of Stat3 nuclear import by importin α5 and importin α7 via two different functional sequence elements

Regulated import of STAT proteins into the nucleus through the nuclear pores is a vital event. We previously identified Arg214/215 in the coiled-coil domain and Arg414/417 in the DNA binding domain involved in the ligand-induced nuclear translocation of Stat3. In this study, we investigated the mechanism for Stat3 nuclear transport. We report here that among five ubiquitously expressed human importin alphas, importin alpha5 and alpha7, but not importin alpha1, alpha3, and alpha4, bind to Stat3 upon cytokine stimulation. Similar results were observed for Stat1, but not for Stat5a and 5b, which were unable to interact with any of the importin alphas. The C-terminus of importin alpha5 is necessary but not sufficient for Stat3 binding. Truncation mutant of Stat3 (aa1-320) that contains Arg214/215 exhibits specific binding to importin alpha5, and an exclusive nuclear localization. Point mutations of Arg214/215 in this mutant destroy importin alpha5 binding and its nuclear localization. In contrast, the truncation mutant (aa320-770) including Arg414/417 fails to interact with importin alpha5 and is localized in the cytoplasm. However, both sequence elements are necessary for the full-length Stat3's interaction with importin alpha5. These results suggest that Arg214/215 is likely the binding site for importin alpha5, whereas Arg414/417 may not be involved in the direct binding, but necessary for maintaining the proper conformation of Stat3 dimer for importin binding. A model for Stat3 nuclear translocation is proposed based on these data.

Coordinated intracellular translocation of phosphoinositide-specific phospholipase C-δ with the cell cycle

The delta family phosphoinositide (PI)-specific phospholipase C (PLC) are most fundamental forms of eukaryotic PI-PLCs. Despite the presence of lipid targeting domains such as the PH domain and C2 domain, the isoforms are also found in the cytoplasm and nucleus as well as at the plasma membrane. The isoforms have sequences or regions that can serve as a nuclear localization signal (NLS) and a nuclear export signal (NES). Their intracellular localization differs from one isoform to another, presumably due to the difference in the transport equilibrium balanced by the strength of the two signals of each isoform. Even for a particular isoform, its intracellular localization seems to vary during the cell cycle. As an example, PLCdelta(1), which is generally found at the plasma membrane and in the cytoplasm of quiescent cells, localizes to discrete nuclear structures in the G(1)/S boundary of the cell cycle. This may be at least partly due to an increase in intracellular Ca(2+), since Ca(2+) facilitates the formation of a nuclear transport complex comprised of PLCdelta(1) and importin beta1, a carrier molecule for the nuclear import. PLCdelta(1) as well as PLCdelta(4) may play a pivotal role in controlling the initiation of DNA synthesis in S phase. Spatio-temporal changes in the levels of PtdIns(4,5)P(2) seem to be another major determinant for the localization and regulation of the delta isoforms. High nuclear PtdIns(4,5)P(2) levels are associated with the G(1)/S phases. After entering M phase, PtdIns(4,5)P(2) synthesis at sites of cell division occurs and PLCs seem to localize to the cleavage furrow during cytokinesis. Coordinated translocation of PLCs with the cell cycle or with stress responses may result in changes in intra-nuclear environments and local membrane architectures that modulate proliferation and differentiation. In this review, recent findings regarding the molecular machineries and mechanisms of the nucleocytoplasmic shuttling as well as roles in the cell cycle progression of the delta isoforms of PLC will be discussed.

Importin alpha mRNAs have distinct expression profiles during spermatogenesis

Importin proteins control access to the cell nucleus by mediating the nuclear transport of specific cargoes. We hypothesized that developmental regulation of gene expression may be partially effected by changes in the nuclear transport machinery complement, manifested as regulated expression of importin alpha family genes. We first clarified the identity of the five known mouse importin alpha genes relative to those for human and then determined their expression throughout postnatal rodent testis using PCR and in situ hybridization. Distinct expression patterns were observed for each. At 10 dpp, all importin alpha mRNAs were detected in spermatogonia. In the adult mouse testis, importins alpha1 and alpha3 were detected in spermatogonia and early pachytene spermatocytes. Importin alpha4 mRNA was identified in pachytene spermatocytes, alpha6 mRNA in round spermatids, and alpha2 mRNA in both of these. The distinct importin alpha expression patterns are consistent with their having specific roles and transport cargoes during spermatogenesis.

The cellular localization pattern of Varicella-Zoster virus ORF29p is influenced by proteasome-mediated degradation

Varicella-zoster virus (VZV) open reading frame 29 (ORF29) encodes a single-stranded DNA binding protein. During lytic infection, ORF29p is localized primarily to infected-cell nuclei, whereas during latency it appears in the cytoplasm of infected neurons. Following reactivation, ORF29p accumulates in the nucleus. In this report, we analyze the cellular localization patterns of ORF29p during VZV infection and during autonomous expression. Our results demonstrate that ORF29p is excluded from the nucleus in a cell-type-specific manner and that its cellular localization pattern may be altered by subsequent expression of VZV ORF61p or herpes simplex virus type 1 ICP0. In these cases, ORF61p and ICP0 induce nuclear accumulation of ORF29p in cell lines where it normally remains cytoplasmic. One cellular system utilized by ICP0 to influence protein abundance is the proteasome degradation pathway. Inhibition of the 26S proteasome, but not heat shock treatment, resulted in accumulation of ORF29p in the nucleus, similar to the effect of ICP0 expression. Immunofluorescence microscopy and pulse-chase experiments reveal that stabilization of ORF29p correlates with its nuclear accumulation and is dependent on a functional nuclear localization signal. ORF29p nuclear translocation in cultured enteric neurons and cells derived from an astrocytoma is reversible, as the protein's distribution and stability revert to the previous states when the proteasomal activity is restored. Thus, stabilization of ORF29p leads to its nuclear accumulation. Although proteasome inhibition induces ORF29p nuclear accumulation, this is not sufficient to reactivate latent VZV or target the immediate-early protein ORF62p to the nucleus in cultured guinea pig enteric neurons.

The view from Awaji island: past, present, and future of RCC1 and the Ran GTPase system

The International Symposium on Ran and the Cell Cycle was held on October 1-4, 2005, at the Awaji Island Resort near Osaka, to celebrate the career and scientific achievements of Professor Takeharu Nishimoto. One hundred of his former lab members, collaborators and other scientific colleagues from around the world attended the symposium organized by Mary Dasso (National Institutes of Health) and Yoshihiro Yoneda (Osaka University). The program was divided into sessions on cell cycle and chromosomes, nuclear import and export of proteins and RNA, nuclear envelope and the nuclear pore complex, and RCC1 and chromatin. Dr. Nishimoto's retirement from Kyushu University is a perfect time to look back at the history of Ran and RCC1, assess the current state of the field, and discuss the challenges that remain in order to unravel the complexities of the Ran GTPase system.

Nucleocytoplasmic shuttling of phospholipase C-δ1: A link to Ca2+

Phosphoinositides (PIs) and proteins involved in the PI signaling pathway are distributed in the nucleus as well as at the plasma membrane and in the cytoplasm, although their nuclear localization mechanisms have not been clarified in detail. Generally, proteins that shuttle between the cytoplasm and nucleus contain nuclear localization signal (NLS) and nuclear export signal (NES) sequences for nuclear import and export, respectively. They bind to specific carrier proteins of the importin/exportin family and are transported to and from the nucleus. Thus there is a steady state shuttling of the cargo molecules to and from the nucleus, and the shift in equilibrium determines their nuclear or cytoplasmic localization. Our previous studies have shown that phospholipase C (PLC)-delta1, regarded as having cytoplasmic- or plasma membrane-bound localization, accumulates in the nucleus when its NES sequence is disrupted. In addition, a cluster of positively charged residues on the surface of the catalytic barrel is important for nuclear import. In quiescent cells, the shuttling equilibrium seems to be shifted to the nuclear export of PLCdelta1. In this review, recent findings regarding the molecular machineries and mechanisms of the nucleocytoplasmic shuttling of PLCdelta1 will be discussed. It is important to know when and how they are regulated. A shift in the equilibrium in a certain stage of the cell cycle or by external stimuli is possible and resulting changes in the intra-nuclear environments (or architectures) may alter proliferation and differentiation patterns. Evidences support the idea that an increase in the levels of intracellular Ca2+ shifts the equilibrium to the nuclear import of PLCdelta1. A myriad of external stimuli have also been reported to change the nuclear PI metabolism following accelerated accumulation in the nucleus of other phospholipases such as phospholipase A2 and phospholipase D in addition to PLC isoforms such as PLCbeta1 and PLCgamma1. The consequence of the nuclear accumulation of PLC is also discussed.

Cloning and characterization of rat importin 9: implication for its neuronal function

We describe the structure of the rat importin 9 gene, together with its transcripts and the encoded protein with its putative functional domains. The importin 9 gene contains 24 exons in a genomic region spanning >52,000 bp. It is transcribed into two mRNAs, generated by means of alternative polyadenylation site usage arranged in tandem. Both transcripts possess the same noncanonical polyadenylation signal (AGUAAA) in rat, this hexamer being conserved in all vertebrates examined. Additionally, intron 8 is bordered by AT-AC dinucleotides. Importin 9 is expressed throughout adult rat tissues, but the 114-kDa Importin 9 protein was detected only in the brain. The localization of the Importin 9 protein was examined by immunohistochemistry in both adult rat tissues and primary hippocampal cell cultures. The strongest labeling was detected in vivo in areas populated by neurons in high density and also in the dendritic processes emanating from these cells. This protein was clearly concentrated in the nuclei of these cells, although their cytoplasms too were heavily labeled. Strong cytoplasmic and very strong nuclear staining was found in a vast majority of the cells with neuronal morphology in vitro. Cultured cells with glial morphology generally exhibited a weaker cytoplasmic labeling. In these cells, the signal decorated the nuclear envelope without nuclear staining and gradually dwindled toward the cell periphery. These results hint at the cell- or tissue-type specific functions of this type of importin protein.