The varicella-zoster virus ORF16 protein promotes both the nuclear transport and the protein abundance of the viral DNA polymerase subunit ORF28

Although all herpesviruses utilize a highly conserved replication machinery to amplify their viral genomes, different members may have unique strategies to modulate the assembly of their replication components. Herein, we characterize the subcellular localization of seven essential replication proteins of varicella-zoster virus (VZV) and show that several viral replication enzymes such as the DNA polymerase subunit ORF28, when expressed alone, are localized in the cytoplasm. The nuclear import of ORF28 can be mediated by the viral DNA polymerase processivity factor ORF16. Besides, ORF16 could markedly enhance the protein abundance of ORF28. Noteworthily, an ORF16 mutant that is defective in nuclear transport still retained the ability to enhance ORF28 abundance. The low abundance of ORF28 in transfected cells was due to its rapid degradation mediated by the ubiquitin-proteasome system. We additionally reveal that radicicol, an inhibitor of the chaperone Hsp90, could disrupt the interaction between ORF16 and ORF28, thereby affecting the nuclear entry and protein abundance of ORF28. Collectively, our findings imply that the cytoplasmic retention and rapid degradation of ORF28 may be a key regulatory mechanism for VZV to prevent untimely viral DNA replication, and suggest that Hsp90 is required for the interaction between ORF16 and ORF28.

Nuclear transport of phosphorylated LanCL2 promotes invadopodia formation and tumor progression of glioblastoma by activating STAT3/Cortactin signaling

INTRODUCTION

Our previous study showed that the abscisic acid receptor lanthionine synthetase C-like 2 (LanCL2) is a significant prognostic factor for overall survival in young glioblastoma patients. However, the role of LanCL2 in glioblastoma remains unclear yet.

OBJECTIVES

This study aims to investigate the role of LanCL2 in regulating in-vitro cell invasion and in-vivo tumor progression of glioblastoma and its underlying mechanism.

METHODS

Tyrosine 198 or 295 residue of LanCL2 was mutated using site-directed mutagenesis to block its phosphorylation. The role of LanCL2 in glioblastoma was investigated using transwell or 3D invasion assay, matrix degradation assay and intracranial xenograft model.

RESULTS

This study showed that nuclear transport of LanCL2 was enhanced by overexpression of LanCL2 or its ligand abscisic acid in glioblastoma cells. Knockdown of LanCL2 suppressed migration, invasion and invadopodia formation of glioblastoma cells, whereas overexpression of wild-type LanCL2 enhanced them. Blocking of Tyr295 residue phosphorylation of LanCL2 impeded its nuclear transport, retarded glioblastoma cell motility and invadopodia formation, and deceased the phosphorylation of Cortactin and STAT3. c-Met was identified as the upstream tyrosine kinase of Tyr295 residue of LanCL2, and inhibition of c-Met markedly suppressed the nuclear transport of LanCL2. Moreover, overexpression of wild-type LanCL2 significantly promoted orthotopic tumor growth of glioblastoma in vivo and led to poor survival of mice with median survival time of 33.5 days, whereas Tyr295 mutation rescued it with median survival time of 49 days.

CONCLUSION

Our findings suggested that Tyr295 phosphorylation is crucial to the activation and nuclear transport of LanCL2, as well as invadopodia formation and tumor progression of glioblastoma, providing the evidence of a novel signaling axis c-Met/LanCL2/STAT3/Cortactin and the first observation of the importance of Tyr295 phosphorylation to LanCL2.

Structural basis of nuclear transport for NEIL DNA glycosylases mediated by importin-alpha

NEIL glycosylases, including NEIL1, NEIL2, and NEIL3, play a crucial role in the base excision DNA repair pathway (BER). The classical importin pathway mediated by importin α/β and cargo proteins containing nuclear localization sequences (NLS) is the most common transport mechanism of DNA repair proteins to the nucleus. Previous studies have identified putative NLSs located at the C-terminus of NEIL3 and NEIL1. Crystallographic, bioinformatics, calorimetric (ITC), and fluorescence assays were used to investigate the interaction between NEIL1 and NEIL3 putative NLSs and importin-α (Impα). Our findings showed that NEIL3 contains a typical cNLS, with medium affinity for the major binding site of Impα. In contrast, crystallographic analysis of NEIL1 NLS revealed its binding to Impα, but with high B-factors and a lack of electron density at the linker region. ITC and fluorescence assays indicated no detectable affinity between NEIL1 NLS and Impα. These data suggest that NEIL1 NLS is a non-classical NLS with low affinity to Impα. Additionally, we compared the binding mode of NEIL3 and NEIL1 with Mus musculus Impα to human isoforms HsImpα1 and HsImpα3, which revealed interesting binding differences for HsImpα3 variant. NEIL3 is a classical medium affinity monopartite NLS, while NEIL1 is likely to be an unclassical low-affinity bipartite NLS. The base excision repair pathway is one of the primary systems involved in repairing DNA. Thus, understanding the mechanisms of nuclear transport of NEIL proteins is crucial for comprehending the role of these proteins in DNA repair and disease development.

APC mutations disrupt β-catenin destruction complex condensates organized by Axin phase separation

The Wnt/β-catenin pathway is critical to maintaining cell fate decisions. Recent study showed that liquid-liquid-phase separation (LLPS) of Axin organized the β-catenin destruction complex condensates in a normal cellular state. Mutations inactivating the APC gene are found in approximately 80% of all human colorectal cancer (CRC). However, the molecular mechanism of the formation of β-catenin destruction complex condensates organized by Axin phase separation and how APC mutations impact the condensates are still unclear. Here, we report that the β-catenin destruction complex, which is constructed by Axin, was assembled condensates via a phase separation process in CRC cells. The key role of wild-type APC is to stabilize destruction complex condensates. Surprisingly, truncated APC did not affect the formation of condensates, and GSK 3β and CK1α were unsuccessfully recruited, preventing β-catenin phosphorylation and resulting in accumulation in the cytoplasm of CRCs. Besides, we propose that the phase separation ability of Axin participates in the nucleus translocation of β-catenin and be incorporated and concentrated into transcriptional condensates, affecting the transcriptional activity of Wnt signaling pathway.

Discovering the nuclear localization signal of Werner Helicase Interacting Protein 1

Our study maps the classic nuclear localization signal (cNLS) domain within WRNIP that directs the protein's nuclear positioning.

Bacopa monnieri reduces Tau aggregation and Tau-mediated toxicity in cells

Alzheimer's disease is a neurodegenerative disease characterized by progressive memory loss and behavioral impairments. In the present study, the ethanolic extract of Bacopa monnieri was studied for its potency to inhibit Tau aggregation and rescuing of the viability of Tau-stressed cells. Bacopa monnieri was observed to inhibit the Tau aggregation in vitro. The cells exposed to Bacopa monnieri were also observed to have a low level of ROS and caspase-3 activity. The immunoblot and immunofluorescence analysis showed that Bacopa monnieri acts as an antioxidant and restored the Nrf2 levels in Neuro2a cells. Bacopa monnieri treatment to Neuro2a cells was observed to reduce the phospho-Tau load in formaldehyde-stressed cells. Furthermore, the treatment of Bacopa monnieri reduced the phosphorylation of GSK-3β in formaldehyde-stressed cells. Ran and NUP358 are the key proteins involved in nuclear transport. It was observed that formaldehyde treatment impaired the nuclear transport by missorting the NUP358 arrangement in Neuro2a cells. On the contrary, Bacopa monnieri treatment restored the NUP358 arrangement in cells. The overall results of the present study suggested that Bacopa monnieri could be considered a potent herb against Tau phosphorylation and Tau aggregation, which projects it as a promising formulation for Alzheimer's disease.

Importin alpha family NAAT/IBB domain: Functions of a pleiotropic long chameleon sequence

Nuclear transport is essential for eukaryotic cell survival and regulates the movement of functional molecules in and out of the nucleus via the nuclear pore. Transport is facilitated by protein-protein interactions between cargo and transport receptors, which contribute to the expression and regulation of downstream genetic information. This chapter focuses on the molecular basis of the multifunctional nature of the importin α family, the representative transport receptors that bring proteins into the nucleus. Importin α performs multiple functions during the nuclear transport cycle through interactions with multiple molecules by a single domain called the IBB domain. This domain is a long chameleon sequence, which can change its conformation and binding mode depending on the interaction partners. By considering the evolutionarily conserved biochemical/physicochemical propensities of the amino acids constituting the functional complex interfaces, together with their structural properties, the mechanisms of switching between multiple complexes formed via IBB and the regulation of downstream functions are examined in detail. The mechanism of regulation by IBB indicates that the time has come for a paradigm shift in the way we view the molecular mechanisms by which proteins regulate downstream functions through their interactions with other molecules.

Olfactory marker protein contains a leucine-rich domain in the Ω-loop important for nuclear export

Olfactory marker protein (OMP) is a cytosolic protein expressed in mature olfactory receptor neurons (ORNs). OMP modulates cAMP signalling and regulates olfactory sensation and axonal targeting. OMP is a small soluble protein, and passive diffusion between nucleus and cytoplasm is expected. However, OMP is mostly situated in the cytosol and is only sparsely detected in the nuclei of a subset of ORNs, hypothalamic neurons and heterologously OMP-expressing cultured cells. OMP can enter the nucleus in association with transcription factors. However, how OMP is retained in the cytosol at rest is unclear. Because OMP is proposed to affect cell differentiation, it is important to understand how OMP is distributed between cytoplasm and nucleus. To elucidate the structural profile of OMP, we applied several bioinformatics methods to a multiple sequence alignment (MSA) of OMP protein sequences and ranked the evolutionarily conserved residues. In addition to the previously reported cAMP-binding domain, we identified a leucine-rich domain in the Ω-loop of OMP. We introduced mutations into the leucine-rich region and heterologously expressed the mutant OMP in HEK293T cells. Mutations into alanine increased the nuclear distribution of OMP quantified by immunocytochemistry and western blotting. Therefore, we concluded that OMP contains a leucine-rich domain important for nuclear transport.

The secondary pocket of Cryptochrome 2 is important for the regulation of its stability and localization

Human clock-gene variations contribute to the phenotypic differences observed in various behavioral and physiological processes such as diurnal preference, sleep, metabolism, mood regulation, addiction, and fertility. However, little is known about the possible effects of identified variations at the molecular level. In this study, we performed a functional characterization at the cellular level of rare CRYPTOCHROME 2 (CRY2) missense variations that were identified from the Ensembl database. Our structural studies revealed that three variations (p.Pro123Leu, p.Asp406His, p.Ser410Ile) are located at the rim of the secondary pocket of CRY2. We show these variants were unable to repress CLOCK/BMAL1-driven transcription in a cell-based reporter assay and had reduced affinity to CLOCK/BMAL1. Furthermore, our biochemical studies indicated that the variants were less stable than the wild-type CRY2, which could be rescued in the presence of Period 2 (PER2), another core clock protein. Finally, we found these variants were unable to properly localize to the nucleus, and thereby were unable to rescue the circadian rhythm in a Cry1^-/-Cry2^-/- double-knockout mouse embryonic fibroblast cell line. Collectively, our data suggest that the rim of the secondary pocket of CRY2 plays a significant role in its nuclear localization independently of PER2 and in the intact circadian rhythm at the cellular level.

Using Single Molecule RNA FISH to Determine Nuclear Export and Transcription Phenotypes in Drosophila Tissues

Single molecule RNA fluorescence in situ hybridization (smRNA FISH) is a widely used method for examining cellular localization of RNA and assessing gene expression outputs. The Nuclear Pore Complex (NPC) is a nuclear macro-complex known to both mediate nucleocytoplasmic transport and influence transcription via interactions with chromatin. Consequently, depletion of NPC proteins can result in defects in either transcription or nuclear export of mRNA. To distinguish between these two different functions of NPC components, it is preferable to analyze transcription and mRNA export simultaneously or in the same cell. Here, we present a smRNA FISH protocol with downstream custom MATLAB image analysis for application in Drosophila larval salivary gland tissues. This method can detect both nuclear export and transcriptional phenotypes in the same cell and as a single assay, and can be adapted to many other cell types and organisms.

Impaired nuclear transport induced by juvenile ALS causing P525L mutation in NLS domain of FUS: A molecular mechanistic study

Amyotrophic lateral sclerosis (ALS) and fronto-temporal lobar degeneration (FTLD) are progressive neurological disorders affecting motor neurons. Cellular aggregates of fused in sarcoma (FUS) protein are found in cytoplasm of ALS and FTLD patients. Nuclear localisation signal (NLS) domain of FUS binds to Karyopherin β2 (Kapβ2), which drives nuclear transport of FUS from cytoplasm. Several pathogenic mutations are reported in FUS NLS, which are associated with its impaired nuclear transport and cytoplasmic mis-localisation. P525L mutation in NLS is most commonly found in cases of juvenile ALS (jALS), which affects individuals below 25 years of age. jALS progresses aggressively causing death within a year of its onset. This study elucidates the molecular mechanism behind jALS-causing P525L mutation hindering nuclear transport of FUS. We perform multiple molecular dynamics simulations in aqueous and hydrophobic solvent to understand the effect of the mutation at molecular level. Dynamics of Kapβ2-FUS complex is better captured in hydrophobic solvent compared to aqueous solvent. P525 and Y526 (PY-motif) of NLS exhibit fine-tuned stereochemical arrangement, which is essential for optimum Kapβ2 binding. P525L causes loss of several native contacts at interface leading to weaker binding, which promotes self-aggregation of FUS in cytoplasm. Native complex samples closed conformation, while mutant complex exhibits open conformation exposing hydrophilic residues of Kapβ2 to hydrophobic solvent. Mutant complex also fails to exhibit spring-like motion essential for its transport through nuclear pore complex. This study provides a mechanistic insight of binding affinity between NLS and Kapβ2 that inhibits self-aggregation of FUS preventing the disease condition.

The effects of glipizide on DNA damage and nuclear transport in differentiated 3T3-L1 adipocytes

BACKGROUND

Despite commonly use for treatment of type II diabetes, possible effects of glipizide on nuclear transport and DNA damage in cells are unknown. Since clinical response of glipizide may change with aging, the aim of the study was to investigate the effect of glipizide by comparing mature and senescent adipocytes.

METHODS AND RESULTS

The effects of glipizide were investigated in 3T3-L1 adipocytes. Effective and lethal doses were determined by real-time monitoring iCELLigence system. Comet assay was performed to determine DNA damage and quantitative PCR was conducted to detect gene expression levels. RAN expressions were found to be up regulated in mature 180 µM glipizide treated adipocytes compared to control group (p < 0.05); whereas down regulated in senescent 180 µM glipizide treated adipocytes compared to their control adipocytes (p < 0.05). Olive Tail Moment values were significantly higher in mature 180 µM glipizide treated adipocytes (MTG) and senescent 180 µM glipizide treated adipocytes (STG) comparing their untreated controls (p < 0.001 and p < 0.001 respectively). Also class 5 comets that shows severe DNA damage were found to be higher in both MTG and STG groups than their controls (p < 0.001 and p < 0.001, respectively). OTM values were higher in STG than MTG (p < 0.001).

CONCLUSIONS

This is the first study that reports glipizide caused DNA damage increasing with senescence in adipocytes. As a response to glipizide treatment Ran gene expression increased in mature; and decreased in senescent adipocytes. Further studies are needed to reveal the effect of glipizide on DNA and nuclear interactions in molecular level.

The LINC Complex Assists the Nuclear Import of Mechanosensitive Transcriptional Regulators

Mechanical forces play pivotal roles in directing cell functions and fate. To elicit gene expression, either intrinsic or extrinsic mechanical information are transmitted into the nucleus beyond the nuclear envelope via at least two distinct pathways, possibly more. The first and well-known pathway utilizes the canonical nuclear transport of mechanoresponsive transcriptional regulators through the nuclear pore complex, which is an exclusive route for macromolecular trafficking between the cytoplasm and nucleoplasm. The second pathway depends on the linker of the nucleoskeleton and cytoskeleton (LINC) complex, which is a molecular bridge traversing the nuclear envelope between the cytoskeleton and nucleoskeleton. This protein complex is a central component in mechanotransduction at the nuclear envelope that transmits mechanical information from the cytoskeleton into the nucleus to influence the nuclear structure, nuclear stiffness, chromatin organization, and gene expression. Besides the mechanical force transducing function, recent increasing evidence shows that the LINC complex plays a role in controlling nucleocytoplasmic transport of mechanoresponsive transcriptional regulators. Here we discuss recent findings regarding the contribution of the LINC complex to the regulation of intracellular localization of the most-notable mechanosensitive transcriptional regulators, β-catenin, YAP, and TAZ.

Bimolecular Fluorescence Complementation: Quantitative Analysis of In Cell Interaction of Nuclear Transporter Importin α with Cargo Proteins

Bimolecular fluorescence complementation utilizes the ability of two complementary nonfluorescent fragments to reconstitute and emit fluorescence when brought together through specific interaction of attached protein fragments of interest. It has been used in several different contexts to study protein-protein interaction. Here we apply the method for the first time to study interaction of the nuclear transporter importin α and its cargoes in a cellular context. By using image analysis to quantify the extent of nuclear complexation, it is possible to gain insight into the strength of interaction in cells.

Localizing Total mRNA in Plant Cells

Visualizing the location of the total cellular mRNA pool can be important to understand how different genes affect cellular physiology. Over the past decade researchers investigating RNA processing, nuclear transport and the function of the nuclear pore complex have used in situ hybridization protocol to visualize and quantify the accumulation of the total mRNA pool within the plant cell nucleus.

Nuclear transport genes recurrently duplicate by means of RNA intermediates in Drosophila but not in other insects

Background

The nuclear transport machinery is involved in a well-known male meiotic drive system in Drosophila. Fast gene evolution and gene duplications have been major underlying mechanisms in the evolution of meiotic drive systems, and this might include some nuclear transport genes in Drosophila. So, using a comprehensive, detailed phylogenomic study, we examined 51 insect genomes for the duplication of the same nuclear transport genes.

Results

We find that most of the nuclear transport duplications in Drosophila are of a few classes of nuclear transport genes, RNA mediated and fast evolving. We also retrieve many pseudogenes for the Ran gene. Some of the duplicates are relatively young and likely contributing to the turnover expected for genes under strong but changing selective pressures. These duplications are potentially revealing what features of nuclear transport are under selection. Unlike in flies, we find only a few duplications when we study the Drosophila duplicated nuclear transport genes in dipteran species outside of Drosophila, and none in other insects.

Conclusions

These findings strengthen the hypothesis that nuclear transport gene duplicates in Drosophila evolve either as drivers or suppressors of meiotic drive systems or as other male-specific adaptations circumscribed to flies and involving a handful of nuclear transport functions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08170-4.

Nuclear import receptors and hnRNPK mediates nuclear import and stress granule localization of SIRLOIN

The majority of lncRNAs and a small fraction of mRNAs localize in the cell nucleus to exert their functions. A SIRLOIN RNA motif was previously reported to drive its nuclear localization by the RNA-binding protein hnRNPK. However, the underlying mechanism remains unclear. Here, we report crystal structures of hnRNPK in complex with SIRLOIN, and with the nuclear import receptor (NIR) Impα1, respectively. The protein hnRNPK bound to SIRLOIN with multiple weak interactions, and interacted Impα1 using an independent high-affinity site. Forming a complex with hnRNPK and Impα1 was essential for the nuclear import and stress granule localization of SIRLOIN in semi-permeabilized cells. Nuclear import of SIRLOIN enhanced with increasing NIR concentrations, but its stress granule localization peaked at a low NIR concentration. Collectively, we propose a mechanism of SIRLOIN localization, in which NIRs functioned as drivers/regulators, and hnRNPK as an adaptor.

Advances in targeting the WNT/β-catenin signaling pathway in cancer

WNT/β-catenin signaling orchestrates various physiological processes, including embryonic development, growth, tissue homeostasis, and regeneration. Abnormal WNT/β-catenin signaling is associated with various cancers and its inhibition has shown effective antitumor responses. In this review, we discuss the pathway, potential targets for the development of WNT/β-catenin inhibitors, available inhibitors, and their specific molecular interactions with the target proteins. We also discuss inhibitors that are in clinical trials and describe potential new avenues for therapeutically targeting the WNT/β-catenin pathway. Furthermore, we introduce emerging strategies, including artificial intelligence (AI)-assisted tools and technology-based actionable approaches, to translate WNT/β-catenin inhibitors to the clinic for cancer therapy.

Selinexor decreases HIF-1α via inhibition of CRM1 in human osteosarcoma and hepatoma cells associated with an increased radiosensitivity

BACKGROUND

The nuclear pore complexes (NPCs) are built of about 30 different nucleoporins and act as key regulators of molecular traffic between the cytoplasm and the nucleus for sizeable proteins (> 40 kDa) which must enter the nucleus. Various nuclear transport receptors are involved in import and export processes of proteins through the nuclear pores. The most prominent nuclear export receptor is chromosome region maintenance 1 (CRM1), also known as exportin 1 (XPO1). One of its cargo proteins is the prolyl hydroxylase 2 (PHD2) which is involved in the initiation of the degradation of hypoxia-inducible factors (HIFs) under normoxia. HIFs are proteins that regulate the cellular adaptation under hypoxic conditions. They are involved in many aspects of cell viability and play an important role in the hypoxic microenvironment of cancer. In cancer, CRM1 is often overexpressed thus being a putative target for the development of new cancer therapies. The newly FDA-approved pharmaceutical Selinexor (KPT-330) selectively inhibits nuclear export via CRM1 and is currently tested in additional Phase-III clinical trials. In this study, we investigated the effect of CRM1 inhibition on the subcellular localization of HIF-1α and radiosensitivity.

METHODS

Human hepatoma cells Hep3B and human osteosarcoma cells U2OS were treated with Selinexor. Intranuclear concentration of HIF-1α protein was measured using immunoblot analysis. Furthermore, cells were irradiated with 2-8 Gy after treatment with Selinexor compared to untreated controls.

RESULTS

Selinexor significantly reduced the intranuclear level of HIF-1α protein in human hepatoma cells Hep3B and human osteosarcoma cells U2OS. Moreover, we demonstrated by clonogenic survival assays that Selinexor leads to dose-dependent radiosensitization in Hep3B-hepatoma and U2OS-osteosarcoma cells.

CONCLUSION

Targeting the HIF pathway by Selinexor might be an attractive tool to overcome hypoxia-induced radioresistance.

KPNA4 is involved in cataract formation via the nuclear import of p53

KPNA4 (also called importin-α3) belongs to the importin α adaptor proteins family, which orchestrates classical nuclear transport processes, importin-α/importin-β1 pathway, and involves in cellular homeostasis. Disruption of balanced transport pathways may result in ectopic nuclear proteins and eventually cause diseases, mainly under the situation of cellular stress, such as oxidative stress. Little evidence is available on its cellular functions for high specific expression in lens. We firstly studied the role of KPNA4 in cataract formation. Lens defects were observed at an early age in kpna4 gene knockout zebrafish, generated by the CRISPR/Cas9 system. Those phenotype, including cloudy center part of the lens, via bright field microscopy, and the thinning of the LE layer, wider space between the adjacent LE and LF cells, irregular cells morphology and the increased number of holes inside the LE cells, which were detected by transmission electron microscopy, recapitulate the clinical features of cataract patients. As the p53-specific adaptor of the nuclear import, KPNA4 upregulated with the same pattern of p53 in hydrogen peroxide-induced apoptosis in human lens epithelia cells. Furthermore, the loss of Kpna4 resulted in the accumulation of p53 in the center of lens. Taken together, we showed that KPNA4 was involved in the formation of cataract, likely by mediating p53 nuclear transport.

Nuclear Envelope and Nuclear Pore Complexes in Neurodegenerative Diseases-New Perspectives for Therapeutic Interventions

Transport of proteins, transcription factors, and other signaling molecules between the nucleus and cytoplasm is necessary for signal transduction. The study of these transport phenomena is particularly challenging in neurons because of their highly polarized structure. The bidirectional exchange of molecular cargoes across the nuclear envelope (NE) occurs through nuclear pore complexes (NPCs), which are aqueous channels embedded in the nuclear envelope. The NE and NPCs regulate nuclear transport but are also emerging as relevant regulators of chromatin organization and gene expression. The alterations in nuclear transport are regularly identified in affected neurons associated with human neurodegenerative diseases. This review presents insights into the roles played by nuclear transport defects in neurodegenerative disease, focusing primarily on NE proteins and NPCs. The subcellular mislocalization of proteins might be a very desirable means of therapeutic intervention in neurodegenerative disorders.

Molecular profiling of nucleocytoplasmic transport factor genes in breast cancer

Transport of functional molecules across the nuclear membrane of a eukaryotic cell is regulated by a dedicated set of transporter proteins that carry molecules into the nucleus or out of the nucleus to the cytoplasm for homeostasis of the cell. One of the categories of cargo molecules these transporters carry are the molecules for cell cycle regulation. Therefore, their role is critical in terms of cancer development. Any misregulation of the transport factors would means aberrant abundance of cell cycle regulators and might have consequences in cell cycle progression. While earlier studies have focussed on individual transport related molecules, a collective overview of how these molecules may be dysregulated in breast cancer is lacking.

Using genomic and transcriptomic datasets from TCGA (The Cancer Genome Atlas) and microarray platforms, we carried out bioinformatic analysis and provide a genetic and molecular profile of all the molecules directly related to nucleocytoplasmic shuttling of proteins and RNAs. Interestingly, we identified that many of these molecules are either mutated or have dysregulated expression in breast cancer. Strikingly, some of the molecules, namely, KPNA2, KPNA3, KPNA5, IPO8, TNPO1, XPOT, XPO7 and CSE1L were correlated with poor patient survival. This study provides a comprehensive genetic and molecular landscape of nucleocytoplasmic factors in breast cancer and points to the important roles of various nucleocytoplasmic factors in cancer progression. This data might have implications in prognosis and therapeutic targeting in breast cancer.

Cargo transport through the nuclear pore complex at a glance

Bidirectional transport of macromolecules across the nuclear envelope is a hallmark of eukaryotic cells, in which the genetic material is compartmentalized inside the nucleus. The nuclear pore complex (NPC) is the major gateway to the nucleus and it regulates nucleocytoplasmic transport, which is key to processes including transcriptional regulation and cell cycle control. Accordingly, components of the nuclear transport machinery are often found to be dysregulated or hijacked in diseases. In this Cell Science at a Glance article and accompanying poster, we provide an overview of our current understanding of cargo transport through the NPC, from the basic transport signals and machinery to more emerging aspects, all from a 'cargo perspective'. Among these, we discuss the transport of large cargoes (>15 nm), as well as the roles of different cargo properties to nuclear transport, from size and number of bound nuclear transport receptors (NTRs), to surface and mechanical properties.

GEF-independent Ran activation shifts a fraction of the protein to the cytoplasm and promotes cell proliferation

Ran (Ras-related nuclear protein) plays several important roles in nucleo-cytoplasmic transport, mitotic spindle formation, nuclear envelope/nuclear pore complex assembly, and other functions in the cytoplasm, as well as in cellular transformation when switched on. Unlike other members of the GTPase superfamily, Ran binds more tightly to GDP than to GTP due to the presence of an auto-inhibitory C-terminal tail. Multiple missense mutations in the C-terminus of Ran occur in cancers, but their biological significance remains unclear. Here, the quantitative GDP/GTP binding preference of four engineered mutations with unstable C-termini was analyzed using a devised mant-GDP dissociation assay. The results showed that the impact of different C-terminal mutations depends on multiple factors. Although these mutants were more GTP-loaded in human cells, they were shown to be more cytoplasmic, and to support nuclear transport with minimally or partially reduced efficiency. Further, several Ran cancer mutants were compromised in autoinhibition, slightly more GTP-bound, more cytoplasmic, and enhanced the proliferation of A549 and HeLa cells in vitro. Thus, our work reveals a new route of Ran activation independent of guanine nucleotide exchange factor (GEF), which may account for the hyper-proliferation induced by Ran cancer mutations.

A nuclear transport-related gene signature combined with IDH mutation and 1p/19q codeletion better predicts the prognosis of glioma patients

Background

The nuclear transport system has been proposed to be indispensable for cell proliferation and invasion in cancers. Prognostic biomarkers and molecular targets in nuclear transport systems have been developed. However, no systematic analysis of genes related to nuclear transport in gliomas has been performed. An integrated prognostic classification involving mutation and nuclear transport gene signatures has not yet been explored.

Methods

In the present study, we analyzed gliomas from a training cohort (TCGA dataset, n = 660) and validation cohort (CGGA dataset, n = 668) to develop a prognostic nuclear transport gene signature and generate an integrated classification system. Gene set enrichment analysis (GSEA) showed that glioblastoma (GBM) was mainly enriched in nuclear transport progress compared to lower-grade glioma (LGG). Then, we developed a nuclear transport risk score (NTRS) for gliomas with a training cohort. NTRS was significantly correlated with clinical and genetic characteristics, including grade, age, histology, IDH status and 1p/19q codeletion, in the training and validation cohorts.

Results

Survival analysis revealed that patients with a higher NTRS exhibited shorter overall survival. NTRS showed better prognostic value compared to classical molecular markers, including IDH status and 1p/19q codeletion. Furthermore, univariate and multivariate analyses indicated that NTRS was an independent prognostic factor for gliomas. Enrichment map and Gene Ontology analysis demonstrated that signaling pathways related to the cell cycle were enriched in the NTRS^High group. Subgroup survival analysis revealed that NTRS could differentiate the outcomes of low- and high-risk patients with wild-type IDH or mutant IDH and 1p/19q non-codeletion.

Conclusions

NTRS is associated with poor outcomes and could be an independent prognostic marker in diffuse gliomas. Prognostic classification combined with IDH mutation, 1p/19q codeletion and NTRS could better predict the survival of glioma patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-020-07552-3.

African Swine Fever Virus MGF360-12L Inhibits Type I Interferon Production by Blocking the Interaction of Importin α and NF-κB Signaling Pathway

African swine fever (ASF) is an infectious transboundary disease of domestic pigs and wild boar and spreading throughout Eurasia. There is no vaccine and treatment available. Complex immune escape strategies of African swine fever virus (ASFV) are crucial factors affecting immune prevention and vaccine development. MGF360 genes have been implicated in the modulation of the IFN-I response. The molecular mechanisms contributing to innate immunity are poorly understood. In this study, we demonstrated that ASFV MGF360-12L (MGF360 families 12L protein) significantly inhibited the mRNA transcription and promoter activity of IFN-β and NF-κB, accompanied by decreases of IRF3, STING, TBK1, ISG54, ISG56 and AP-1 mRNA transcription. Also, MGF360-12L might suppress the nuclear localization of p50 and p65 mediated by classical nuclear localization signal (NLS). Additionally, MGF360-12L could interact with KPNA2, KPNA3, and KPNA4, which interrupted the interaction between p65 and KPNA2, KPNA3, KPNA4. We further found that MGF360-12L could interfere with the NF-κB nuclear translocation by competitively inhibiting the interaction between NF-κB and nuclear transport proteins. These findings suggested that MGF360-12L could inhibit the IFN-I production by blocking the interaction of importin α and NF-κB signaling pathway, which might reveal a novel strategy for ASFV to escape the host innate immune response.

Nuclear receptor tyrosine kinase transport and functions in cancer

Signaling functions of plasma membrane-localized receptor tyrosine kinases (RTKs) have been extensively studied after they were first described in the mid-1980s. Plasma membrane RTKs are activated by extracellular ligands and cellular stress stimuli, and regulate cellular responses by activating the downstream effector proteins to initiate a wide range of signaling cascades in the cells. However, increasing evidence indicates that RTKs can also be transported into the intracellular compartments where they phosphorylate traditional effector proteins and non-canonical substrate proteins. In general, internalization that retains the RTK's transmembrane domain begins with endocytosis, and endosomal RTK remains active before being recycled or degraded. Further RTK retrograde transport from endosome-Golgi-ER to the nucleus is primarily dependent on membranes vesicles and relies on the interaction with the COP-I vesicle complex, Sec61 translocon complex, and importin. Internalized RTKs have non-canonical substrates that include transcriptional co-factors and DNA damage response proteins, and many nuclear RTKs harbor oncogenic properties and can enhance cancer progression. Indeed, nuclear-localized RTKs have been shown to positively correlate with cancer recurrence, therapeutic resistance, and poor prognosis of cancer patients. Therefore, understanding the functions of nuclear RTKs and the mechanisms of nuclear RTK transport will further improve our knowledge to evaluate the potential of targeting nuclear RTKs or the proteins involved in their transport as new cancer therapeutic strategies.

CHMPions of repair: Emerging perspectives on sensing and repairing the nuclear envelope barrier

Understanding how the integrity of the nuclear membranes is protected against internal and external stresses is an emergent challenge. Work reviewed here investigated the mechanisms by which losses of nuclear-cytoplasmic compartmentalization are sensed and ameliorated. Fundamental to these is spatial control over interactions between the endosomal sorting complexes required for transport machinery and LAP2-emerin-MAN1 family inner nuclear membrane proteins, which together promote nuclear envelope sealing in interphase and at the end of mitosis. We suggest that the size of the nuclear envelope hole dictates the mechanism of its repair, with larger holes requiring barrier-to-autointegration factor and the potential triggering of a postmitotic nuclear envelope reassembly pathway in interphase. We also consider why these mechanisms fail at ruptured micronuclei. Together, this work re-emphasizes the need to understand how membrane flow and local lipid metabolism help ensure that the nuclear envelope is refractory to mechanical rupture yet fluid enough to allow its essential dynamics.

Karyopherins and condensates

Several aggregation-prone RNA-binding proteins, including FUS, EWS, TAF15, hnRNP A1, hnRNP A2, and TDP-43, are mutated in neurodegenerative diseases. The nuclear-cytoplasmic distribution of these proteins is controlled by proteins in the karyopherin family of nuclear transport factors (Kaps). Recent studies have shown that Kaps not only transport these proteins but also inhibit their self-association/aggregation, acting as molecular chaperones. This chaperone activity is impaired for disease-causing mutants of the RNA-binding proteins. Here, we review physical data on the mechanisms of self-association of several disease-associated RNA-binding proteins, through liquid-liquid phase separation and amyloid fiber formation. In each case, we relate these data to biophysical, biochemical, and cell biological data on the inhibition of self-association by Kaps. Our analyses suggest that Kaps may be effective chaperones because they contain large surfaces with diverse physical properties that enable them to engage multiple different regions of their cargo proteins, blocking self-association.

Passive diffusion through nuclear pore complexes regulates levels of the yeast SAGA and SLIK coactivator complexes

Nuclear pore complexes (NPCs) control gene expression by regulating the bi-directional exchange of proteins and RNAs between nuclear and cytoplasmic compartments, including access of transcriptional regulators to the nucleoplasm. Here, we show that the yeast (Saccharomyces cerevisiae) nucleoporin Nup170, in addition to binding and silencing subtelomeric genes, supports transcription of genes regulated by the SAGA transcriptional activator complex. Specifically, we show that a lower amount of SAGA complex is bound to target genes in the absence of Nup170. Consistent with this observation, levels of the SAGA complex are decreased in cells lacking Nup170, while those of the SAGA-related SLIK complexes are increased. This change in the ratio of SAGA to SLIK complexes is due to increased nuclear activity of Pep4, a protease responsible for production of the SLIK complex. Further analyses of various nucleoporin mutants revealed that the increased nuclear entry of Pep4 observed in the nup170Δ mutant likely occurs as the consequence of an increase in the sieving limits of the NPC diffusion channel. On the basis of these results, we propose that changes in passive diffusion rates represent a mechanism for regulating SAGA- and SLIK complex-mediated transcriptional events.

Karyopherin α2-dependent import of E2F1 and TFDP1 maintains protumorigenic stathmin expression in liver cancer

Background

Members of the karyopherin superfamily serve as nuclear transport receptors/adaptor proteins and provide exchange of macromolecules between the nucleo- and cytoplasm. Emerging evidence suggests a subset of karyopherins to be dysregulated in hepatocarcinogenesis including karyopherin-α2 (KPNA2). However, the functional and regulatory role of KPNA2 in liver cancer remains incompletely understood.

Methods

Quantitative proteomics (LC-MS/MS, ~ 1750 proteins in total) was used to study changes in global protein abundance upon siRNA-mediated KPNA2 knockdown in HCC cells. Functional and mechanistic analyses included colony formation and 2D migration assays, co-immunoprecipitation (CoIP), chromatin immunoprecipitation (ChIP), qRT-PCR, immmunblotting, and subcellular fractionation. In vitro results were correlated with data derived from a murine HCC model and HCC patient samples (3 cohorts, n > 600 in total).

Results

The proteomic approach revealed the pro-tumorigenic, microtubule (MT) interacting protein stathmin (STMN1) among the most downregulated proteins upon KPNA2 depletion in HCC cells. We further observed that KPNA2 knockdown leads to reduced tumor cell migration and colony formation of HCC cells, which could be phenocopied by direct knockdown of stathmin. As the underlying regulatory mechanism, we uncovered E2F1 and TFDP1 as transport substrates of KPNA2 being retained in the cytoplasm upon KPNA2 ablation, thereby resulting in reduced STMN1 expression. Finally, murine and human HCC data indicate significant correlations of STMN1 expression with E2F1/TFPD1 and with KPNA2 expression and their association with poor prognosis in HCC patients.

Conclusion

Our data suggest that KPNA2 regulates STMN1 by import of E2F1/TFDP1 and thereby provide a novel link between nuclear transport and MT-interacting proteins in HCC with functional and prognostic significance.

Novel anti-flavivirus drugs targeting the nucleolar distribution of core protein

The risk of infectious diseases caused by Flavivirus is increasing globally. Here, we developed a novel high-throughput screening (HTS) system to evaluate the inhibitory effects of compounds targeting the nuclear localization of the flavivirus core protein. We screened 4000 compounds based on their ability to inhibit the nuclear localization of the core protein, and identified over 20 compounds including inhibitors for cyclin dependent kinase and glycogen synthase kinase. The efficacy of the identified compounds to suppress viral growth was validated in a cell-based infection system. Remarkably, the nucleolus morphology was affected by the treatment with the compounds, suggesting that the nucleolus function is critical for viral propagation. The present HTS system provides a useful strategy for the identification of antivirals against flavivirus by targeting the nucleolar localization of the core protein.

Drosophila Ref1/ALYREF regulates transcription and toxicity associated with ALS/FTD disease etiologies

RNA-binding proteins (RBPs) are associated with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), but the underlying disease mechanisms remain unclear. In an unbiased screen in Drosophila for RBPs that genetically interact with TDP-43, we found that downregulation of the mRNA export factor Ref1 (fly orthologue to human ALYREF) mitigated TDP-43 induced toxicity. Further, Ref1 depletion also reduced toxicity caused by expression of the C9orf72 GGGGCC repeat expansion. Ref1 knockdown lowered the mRNA levels for these related disease genes and reduced the encoded proteins with no effect on a wild-type Tau disease transgene or a control transgene. Interestingly, expression of TDP-43 or the GGGGCC repeat expansion increased endogenous Ref1 mRNA levels in the fly brain. Further, the human orthologue ALYREF was upregulated by immunohistochemistry in ALS motor neurons, with the strongest upregulation occurring in ALS cases harboring the GGGGCC expansion in C9orf72. These data support ALYREF as a contributor to ALS/FTD and highlight its downregulation as a potential therapeutic target that may affect co-existing disease etiologies.

ERK1/2 phosphorylates HIF-2α and regulates its activity by controlling its CRM1-dependent nuclear shuttling

Hypoxia-inducible factor 2 (HIF-2) is a principal component of the cellular response to oxygen deprivation (hypoxia). Its inducible subunit, HIF-2α (also known as EPAS1), is controlled by oxygen-dependent as well as oxygen-independent mechanisms, such as phosphorylation. We show here that HIF-2α is phosphorylated under hypoxia (1% O2) by extracellular signal-regulated protein kinases 1 and 2 (ERK1/2; also known as MAPK3 and MAPK1, respectively) at serine residue 672, as identified by in vitro phosphorylation assays. Mutation of this site to an alanine residue or inhibition of the ERK1/2 pathway decreases HIF-2 transcriptional activity and causes HIF-2α to mislocalize to the cytoplasm without changing its protein expression levels. Localization, reporter gene and immunoprecipitation experiments further show that HIF-2α associates with the exportin chromosomal maintenance 1 (CRM1, also known as XPO1) in a phosphorylation-sensitive manner and identify two critical leucine residues as part of an atypical CRM1-dependent nuclear export signal (NES) neighboring serine 672. Inhibition of CRM1 or mutation of these residues restores nuclear accumulation and activity of HIF-2α lacking the ERK1/2-mediated modification. In summary, we reveal a novel regulatory mechanism of HIF-2, involving ERK1/2-dependent phosphorylation of HIF-2α, which controls its nucleocytoplasmic shuttling and the HIF-2 transcriptional activity.This article has an associated First Person interview with the first author of the paper.

Characterization of the Nucleocytoplasmic Transport Mechanisms of Epstein-Barr Virus BFLF2

BACKGROUND/AIMS

Epstein-Barr virus (EBV) BFLF2, the homologue of herpes simplex virus 1 (HSV-1) UL31, is crucial for the efficient viral DNA packaging and primary egress across the nuclear membrane. However, we still do not know its subcellular transport mechanisms.

METHODS

Interspecies heterokaryon assays were utilized to detect the nucleocytoplasmic shuttling of BFLF2, and mutation analysis, plasmid transfection and fluorescence microscopy assays were performed to identify the functional nuclear localization sequence (NLS) and nuclear export sequence (NES) of BFLF2 in live cells. Furthermore, the nuclear import and export of BFLF2 were assessed by confocal microscopy, co-immunoprecipitation and immunoblot assays.

RESULTS

BFLF2 was confirmed to shuttle between the nucleus and cytoplasm. Two predicted NESs were shown to be nonfunctional, yet we proved that the nuclear export of BFLF2 was mediated through transporter associated with antigen processing (TAP), but not chromosomal region maintenance 1 (CRM1) dependent pathway. Furthermore, one functional NLS, 22RRLMHPHHRNYTASKASAH40, was identified, and the aa22-23, aa22-25, aa28-30 and aa37-40 had an important role in the nuclear localization of BFLF2. Besides, the nuclear import of BFLF2 was demonstrated through Ran-, importin α7-, importin β1- and transportin-1-dependent mechanism that does not require importin α1, α3 and α5.

CONCLUSION

These works are of significance for the further study of the functions of BFLF2 during EBV infection, as well as for further insights into the design of new antiviral drug target and vaccine development against EBV.

FUS causes synaptic hyperexcitability in Drosophila dendritic arborization neurons

Mutations in the nuclear localization signal of the RNA binding protein FUS cause both Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). These mutations result in a loss of FUS from the nucleus and the formation of FUS-containing cytoplasmic aggregates in patients. To better understand the role of cytoplasmic FUS mislocalization in the pathogenesis of ALS, we identified a population of cholinergic neurons in Drosophila that recapitulate these pathologic hallmarks. Expression of mutant FUS or the Drosophila homolog, Cabeza (Caz), in class IV dendritic arborization neurons results in cytoplasmic mislocalization and axonal transport to presynaptic terminals. Interestingly, overexpression of FUS or Caz causes the progressive loss of neuronal projections, reduction of synaptic mitochondria, and the appearance of large calcium transients within the synapse. Additionally, we find that overexpression of mutant but not wild type FUS results in a reduction in presynaptic Synaptotagmin, an integral component of the neurotransmitter release machinery, and mutant Caz specifically disrupts axonal transport and induces hyperexcitability. These results suggest that FUS/Caz overexpression disrupts neuronal function through multiple mechanisms, and that ALS-causing mutations impair the transport of synaptic vesicle proteins and induce hyperexcitability.

Nuclear distribution of the Trypanosoma cruzi RNA Pol I subunit RPA31 during growth and metacyclogenesis, and characterization of its nuclear localization signal

Trypanosoma cruzi is the aetiologic agent of Chagas disease. Our research group studies ribosomal RNA (rRNA) gene transcription and nucleolus dynamics in this species of trypanosomes. RPA31 is an essential subunit of RNA polymerase I (Pol I) whose presence is apparently restricted to trypanosomes. Using fluorescent-tagged versions of this protein (TcRPA31-EGFP), we describe its nuclear distribution during growth and metacyclogenesis. Our findings indicate that TcRPA31-EGFP alters its nuclear presence from concentrated nucleolar localization in exponentially growing epimastigotes to a dispersed granular distribution in the nucleoplasm of stationary epimastigotes and metacyclic trypomastigotes. These changes likely reflect a structural redistribution of the Pol I transcription machinery in quiescent cellular stages where downregulation of rRNA synthesis is known to occur. In addition, and related to the nuclear internalization of this protein, the presence of a classical bipartite-type nuclear localization signal was identified towards its C-terminal end. The functionality of this motif was demonstrated by its partial or total deletion in recombinant versions of the tagged fluorescent protein. Moreover, ivermectin inhibited the nuclear localization of the labelled chimaera, suggesting the involvement of the importin α/β transport system.

Prolines in the α-helix confer the structural flexibility and functional integrity of importin-β

The karyopherin family of nuclear transport receptors is composed of a long array of amphiphilic α-helices and undergoes flexible conformational changes to pass through the hydrophobic crowding barrier of the nuclear pore. Here, we focused on the characteristic enrichment of prolines in the middle of the outer α-helices of importin-β. When these prolines were substituted with alanine, nuclear transport activity was reduced drastically in vivo and in vitro, and caused a severe defect in mitotic progression. These mutations did not alter the overall folding of the helical repeat or affect its interaction with cargo or the regulatory factor Ran. However, in vitro and in silico analyses revealed that the mutant lost structural flexibility and could not undergo rapid conformational changes when transferring from a hydrophilic to hydrophobic environment or vice versa. These findings reveal the essential roles of prolines in ensuring the structural flexibility and functional integrity of karyopherins.

Nuclear Protein Transport in Digitonin Permeabilized Cells

The high concentration of cholesterol in the plasma membrane relative to the endomembranes of eukaryotic cells allows the selective permeabilization of the plasma membrane with the glycoside digitonin leaving the intracellular membrane bound organelles intact. In this chapter, we describe the basic method to use digitonin permeabilized cells to reconstitute the transport of proteins containing nuclear localization signals into the nucleus. The assay requires only a target cell line that can be permeabilized with digitonin, a source of soluble transport factors, typically provided by the cytosol fraction of cultured cells, and a cargo protein of interest. No other specialized equipment is required other than a fluorescence microscope. The assay can be used to identify transport factors required to transport specific proteins, to study the regulation of protein transport, or to study nuclear protein transport under different conditions.

Alterations of the nuclear transport system in hepatocellular carcinoma - New basis for therapeutic strategies

Hepatocellular carcinoma (HCC) is among the most prevalent human malignancies worldwide with rising incidence in industrialised countries, few therapeutic options and poor prognosis. To expand and improve therapeutic strategies, identification of drug targets involved in several liver cancer-related pathways is crucial. Virtually all signal transduction cascades cross the nuclear envelope and therefore require components of the nuclear transport system (NTS), including nuclear transport receptors (e.g. importins and exportins) and the nuclear pore complex. Accordingly, members of the NTS represent promising targets for therapeutic intervention. Selective inhibitors of nuclear export have already entered clinical trials for various malignancies. Herein, we review the current knowledge regarding alterations of the NTS and their potential for targeted therapy in HCC.

The molecular mechanism for nuclear transport and its application

Transportation between the cytoplasm and the nucleoplasm is critical for many physiological and pathophysiological processes including gene expression, signal transduction, and oncogenesis. So, the molecular mechanism for the transportation needs to be studied not only to understand cell physiological processes but also to develop new diagnostic and therapeutic targets. Recent progress in the research of the nuclear transportation (import and export) via nuclear pore complex and four important factors affecting nuclear transport (nucleoporins, Ran, karyopherins, and nuclear localization signals/nuclear export signals) will be discussed. Moreover, the clinical significance of nuclear transport and its application will be reviewed. This review will provide some critical insight for the molecular design of therapeutics which need to be targeted inside the nucleus.

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.

The role of nuclear localization signal in parvovirus life cycle

Parvoviruses are small, non-enveloped viruses with an approximately 5.0 kb, single-stranded DNA genome. Usually, the parvovirus capsid gene contains one or more nuclear localization signals (NLSs), which are required for guiding the virus particle into the nucleus through the nuclear pore. However, several classical NLSs (cNLSs) and non-classical NLSs (ncNLSs) have been identified in non-structural genes, and the ncNLSs can also target non-structural proteins into the nucleus. In this review, we have summarized recent research findings on parvovirus NLSs. The capsid protein of the adeno-associated virus has four potential nuclear localization sequences, named basic region 1 (BR), BR2, BR3 and BR4. BR3 was identified as an NLS by fusing it with green fluorescent protein. Moreover, BR3 and BR4 are required for infectivity and virion assembly. In Protoparvovirus, the canine parvovirus has a common cNLS located in the VP1 unique region, similar to parvovirus minute virus of mice (MVM) and porcine parvovirus. Moreover, an ncNLS is found in the C-terminal region of MVM VP1/2. Parvovirus B19 also contains an ncNLS in the C-terminal region of VP1/2, which is essential for the nuclear transport of VP1/VP2. Approximately 1 or 2 cNLSs and 1 ncNLS have been reported in the non-structural protein of bocaviruses. Understanding the role of the NLS in the process of parvovirus infection and its mechanism of nuclear transport will contribute to the development of therapeutic vaccines and novel antiviral medicines.

Diversity of actin architecture in human osteoclasts: network of curved and branched actin supporting cell shape and intercellular micrometer-level tubes

Osteoclasts are multinucleated bone-resorbing cells with a dynamic actin cytoskeleton. Osteoclasts are derived from circulating mononuclear precursors. Confocal and stimulated emission depletion (STED) super-resolution microscopy was used to investigate peripheral blood-derived human osteoclasts cultured on glass surfaces. STED and confocal microscopy demonstrated that the actin was curved and branched, for instance, in the vicinity of membrane ruffles. The overall architecture of the curved actin network extended from the podosomes to the top of the cell. The other novel finding was that a micrometer-level tube containing actin bridged the osteoclasts well above the level of the culture glass. The actin filaments of the tubes originated from the network of curved actin often surrounding a group of nuclei. Furthermore, nuclei were occasionally located inside the tubes. Our findings demonstrated the accumulation of c-Src, cortactin, cofilin, and actin around nuclei suggesting their role in nuclear processes such as the locomotion of nuclei. ARP2/3 labeling was abundant at the substratum level of osteoclasts and in the branched actin network, where it localized to the branching points. We speculate that the actin-containing tubes of osteoclasts may provide a means of transportation of nuclei, e.g., during the fusion of osteoclasts. These novel findings can pave the way for future studies aiming at the elucidation of the differentiation of multinucleated osteoclasts.

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.

Nucleocytoplasmic trafficking of Nipah virus W protein involves multiple discrete interactions with the nuclear import and export machinery

Paramyxoviruses replicate in the cytoplasm with no obvious requirement to interact with the nucleus. Nevertheless, the W protein of the highly lethal bat-borne paramyxovirus Nipah virus (NiV) is known to undergo specific targeting to the nucleus, mediated by a single nuclear localisation signal (NLS) within the C-terminal domain. Here, we report for the first time that additional sites modulate nucleocytoplasmic localisation of W. We show that the N-terminal domain interacts with importin α1 and contributes to nuclear accumulation of W, indicative of a novel N-terminal NLS. We also find that W undergoes exportin-1 mediated nuclear export, dependent on a leucine at position 174. Together, these data enable significant revision of the generally accepted model of W trafficking, with implications for understanding of the mechanisms of NiV immune evasion.

Chitosan DNA nanoparticles for oral gene delivery

Gene therapy is a promising technology with potential applications in the treatment of medical conditions, both congenital and acquired. Despite its label as breakthrough technology for the 21^st century, the simple concept of gene therapy - the introduction of a functional copy of desired genes in affected individuals - is proving to be more challenging than expected. Oral gene delivery has shown intriguing results and warrants further exploration. In particular, oral administration of chitosan DNA nanoparticles, one the most commonly used formulations of therapeutic DNA, has repeatedly demonstrated successful in vitro and in vivo gene transfection. While oral gene therapy has shown immense promise as treatment options in a variety of diseases, there are still significant barriers to overcome before it can be considered for clinical applications. In this review we provide an overview of the physiologic challenges facing the use of chitosan DNA nanoparticles for oral gene delivery at both the extracellular and intracellular level. From administration at the oral cavity, chitosan nanoparticles must traverse the gastrointestinal tract and protect its DNA contents from significant jumps in pH levels, various intestinal digestive enzymes, thick mucus layers with high turnover, and a proteinaceous glycocalyx meshwork. Once these extracellular barriers are overcome, chitosan DNA nanoparticles must enter intestinal cells, escape endolysosomes, and disassociate from genetic material at the appropriate time allowing transport of genetic material into the nucleus to deliver a therapeutic effect. The properties of chitosan nanoparticles and modified nanoparticles are discussed in this review. An understanding of the barriers to oral gene delivery and how to overcome them would be invaluable for future gene therapy development.

Inside out: the role of nucleocytoplasmic transport in ALS and FTLD

Neurodegenerative diseases are characterized by the presence of protein inclusions with a different protein content depending on the type of disease. Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are no exceptions to this common theme. In most ALS and FTLD cases, the predominant pathological species are RNA-binding proteins. Interestingly, these proteins are both depleted from their normal nuclear localization and aggregated in the cytoplasm. This key pathological feature has suggested a potential dual mechanism with both nuclear loss of function and cytoplasmic gain of function being at play. Yet, why and how this pathological cascade is initiated in most patients, and especially sporadic cases, is currently unresolved. Recent breakthroughs in C9orf72 ALS/FTLD disease models point at a pivotal role for the nuclear transport system in toxicity. To address whether defects in nuclear transport are indeed implicated in the disease, we reviewed two decades of ALS/FTLD literature and combined this with bioinformatic analyses. We find that both RNA-binding proteins and nuclear transport factors are key players in ALS/FTLD pathology. Moreover, our analyses suggest that disturbances in nucleocytoplasmic transport play a crucial initiating role in the disease, by bridging both nuclear loss and cytoplasmic gain of functions. These findings highlight this process as a novel and promising therapeutic target for ALS and FTLD.

Nucleo-cytoplasmic shuttling of the endonuclease ankyrin repeats and LEM domain-containing protein 1 (Ankle1) is mediated by canonical nuclear export- and nuclear import signals

BACKGROUND

Ankyrin repeats and LEM domain containing protein 1 (Ankle1) belongs to the LEM protein family, whose members share a chromatin-interacting LEM motif. Unlike most other LEM proteins, Ankle1 is not an integral protein of the inner nuclear membrane but shuttles between the nucleus and the cytoplasm. It contains a GIY-YIG-type nuclease domain, but its function is unknown. The mammalian genome encodes only one other GIY-YIG domain protein, termed Slx1. Slx1 has been described as a resolvase that processes Holliday junctions during homologous recombination-mediated DNA double strand break repair. Resolvase activity is regulated in a spatial and temporal manner during the cell cycle. We hypothesized that Ankle1 may have a similar function and its nucleo-cytoplasmic shuttling may contribute to the regulation of Ankle1 activity. Hence, we aimed at identifying the domains mediating Ankle1 shuttling and investigating whether cellular localization is affected during DNA damage response.

RESULTS

Sequence analysis predicts the presence of two canonical nuclear import and export signals in Ankle1. Immunofluorescence microscopy of cells expressing wild-type and various mutated Ankle1-fusion proteins revealed a C-terminally located classical monopartite nuclear localization signal and a centrally located CRM1-dependent nuclear export signal that mediate nucleo-cytoplasmic shuttling of Ankle1. These sequences are also functional in heterologous proteins. The predominant localization of Ankle1 in the cytoplasm, however, does not change upon induction of several DNA damage response pathways throughout the cell cycle.

CONCLUSIONS

We identified the domains mediating nuclear import and export of Ankle1. Ankle1's cellular localization was not affected following DNA damage.

Zinc fingers 1, 2, 5 and 6 of transcriptional regulator, PRDM4, are required for its nuclear localisation

PRDM4 is a member of the PRDM family of transcriptional regulators which control various aspects of cellular differentiation and proliferation. PRDM proteins exert their biological functions both in the cytosol and the nucleus of cells. All PRDM proteins are characterised by the presence of two distinct structural motifs, the PR/SET domain and the zinc finger (ZF) motifs. We previously observed that deletion of all six zinc fingers found in PRDM4 leads to its accumulation in the cytosol, whereas overexpressed full length PRDM4 is found predominantly in the nucleus. Here, we investigated the requirements for single zinc fingers in the nuclear localisation of PRDM4. We demonstrate that ZF's 1, 2, 5 and 6 contribute to the accumulation of PRDM4 in the nucleus. Their effect is additive as deleting either ZF1-2 or ZF 5-6 redistributes PRDM4 protein from being almost exclusively nuclear to cytosolic and nuclear. We investigated the potential mechanism of nuclear shuttling of PRDM4 via the importin α/β-mediated pathway and find that PRDM4 nuclear targeting is independent of α/β-mediated nuclear import.