Nup98 regulates bipolar spindle assembly through association with microtubules and opposition of MCAK

Mechanobiology of the nucleus during the G2-M transition

Cellular behavior is continuously influenced by mechanical forces. These forces span the cytoskeleton and reach the nucleus, where they trigger mechanotransduction pathways that regulate downstream biochemical events. Therefore, the nucleus has emerged as a regulator of cellular response to mechanical stimuli. Cell cycle progression is regulated by cyclin-CDK complexes. Recent studies demonstrated these biochemical pathways are influenced by mechanical signals, highlighting the interdependence of cellular mechanics and cell cycle regulation. In particular, the transition from G2 to mitosis (G2-M) shows significant changes in nuclear structure and organization, ranging from nuclear pore complex (NPC) and nuclear lamina disassembly to chromosome condensation. The remodeling of these mechanically active nuclear components indicates that mitotic entry is particularly sensitive to forces. Here, we address how mechanical forces crosstalk with the nucleus to determine the timing and efficiency of the G2-M transition. Finally, we discuss how the deregulation of nuclear mechanics has consequences for mitosis.

The pediatric leukemia oncoprotein NUP98-KDM5A induces genomic instability that may facilitate malignant transformation

Pediatric Acute Myeloid Leukemia (AML) is a rare and heterogeneous disease characterized by a high prevalence of gene fusions as driver mutations. Despite the improvement of survival in the last years, about 50% of patients still experience a relapse. It is not possible to improve prognosis only with further intensification of chemotherapy, as come with a severe cost to the health of patients, often resulting in treatment-related death or long-term sequels. To design more effective and less toxic therapies we need a better understanding of pediatric AML biology. The NUP98-KDM5A chimeric protein is exclusively found in a particular subgroup of young pediatric AML patients with complex karyotypes and poor prognosis. In this study, we investigated the impact of NUP98-KDM5A expression on cellular processes in human Pluripotent Stem Cell models and a patient-derived cell line. We found that NUP98-KDM5A generates genomic instability through two complementary mechanisms that involve accumulation of DNA damage and direct interference of RAE1 activity during mitosis. Overall, our data support that NUP98-KDM5A promotes genomic instability and likely contributes to malignant transformation.

Establishment of a repertoire of fertility associated sperm proteins and their differential abundance in buffalo bulls (Bubalus bubalis) with contrasting fertility

Sperm harbours a wide range of proteins regulating their functions and fertility. In the present study, we made an effort to characterize and quantify the proteome of buffalo bull spermatozoa, and to identify fertility associated sperm proteins through comparative proteomics. Using high-throughput mass spectrometry platform, we identified 1305 proteins from buffalo spermatozoa and found that these proteins were mostly enriched in glycolytic process, mitochondrial respiratory chain, tricarboxylic acid cycle, protein folding, spermatogenesis, sperm motility and sperm binding to zona pellucida (p < 7.74E-08) besides metabolic (p = 4.42E-31) and reactive oxygen species (p = 1.81E-30) pathways. Differential proteomic analysis revealed that 844 proteins were commonly expressed in spermatozoa from both the groups while 77 and 52 proteins were exclusively expressed in high- and low-fertile bulls, respectively. In low-fertile bulls, 75 proteins were significantly (p < 0.05) upregulated and 176 proteins were significantly (p < 0.05) downregulated; these proteins were highly enriched in mitochondrial respiratory chain complex I assembly (p = 2.63E-07) and flagellated sperm motility (p = 7.02E-05) processes besides oxidative phosphorylation pathway (p = 6.61E-15). The down regulated proteins in low-fertile bulls were involved in sperm motility, metabolism, sperm-egg recognition and fertilization. These variations in the sperm proteome could be used as potential markers for the selection of buffalo bulls for fertility.

The Retinoblastoma Tumor Suppressor Is Required for the NUP98-HOXA9-Induced Aberrant Nuclear Envelope Phenotype

Chromosomal translocations involving the nucleoporin NUP98 gene are recurrently identified in leukemia; yet, the cellular defects accompanying NUP98 fusion proteins are poorly characterized. NUP98 fusions cause changes in nuclear and nuclear envelope (NE) organization, in particular, in the nuclear lamina and the lamina associated polypeptide 2α (LAP2α), a regulator of the tumor suppressor retinoblastoma protein (RB). We demonstrate that, for NUP98-HOXA9 (NHA9), the best-studied NUP98 fusion protein, its effect(s) on nuclear architecture largely depend(s) on RB. Morphological alterations caused by the expression of NHA9 are largely diminished in the absence of RB, both in human cells expressing the human papillomavirus 16 E7 protein and in mouse embryonic fibroblasts lacking RB. We further show that NHA9 expression associates with distinct histone modification. Moreover, the pattern of trimethylation of histone H3 lysine-27 is affected by NHA9, again in an RB-dependent manner. Our results pinpoint to an unexpected interplay between NUP98 fusion proteins and RB, which may contribute to leukemogenesis.

Exportins can inhibit major mitotic assembly events in vitro: membrane fusion, nuclear pore formation, and spindle assembly

Xenopus

egg extracts are a powerful in vitro tool for studying complex biological processes, including nuclear reconstitution, nuclear membrane and pore assembly, and spindle assembly. Extracts have been further used to demonstrate a moonlighting regulatory role for nuclear import receptors or importins on these cell cycle assembly events. Here we show that exportins can also play a role in these events. Addition of Crm1, Exportin-t, or Exportin-5 decreased nuclear pore assembly in vitro. RanQ69L-GTP, a constitutively active form of RanGTP, ameliorated inhibition. Both Crm1 and Exportin-t inhibited fusion of nuclear membranes, again counteracted by RanQ69L-GTP. In mitotic extracts, Crm1 and Exportin-t negatively impacted spindle assembly. Pulldowns from the extracts using Crm1- or Exportin-t-beads revealed nucleoporins known to be essential for both nuclear pore and spindle assembly, with RanQ69L-GTP decreasing a subset of these target interactions. This study suggests a model where exportins, like importins, can regulate major mitotic assembly events.

From the cytoskeleton to the nucleus: An integrated view on early spindle assembly

Accurate chromosome segregation requires a complete restructuring of cellular organization. Microtubules remodel to assemble a mitotic spindle and the actin cytoskeleton rearranges to form a stiff actomyosin cortex. These cytoplasmic events must be spatially and temporally coordinated with mitotic chromosome condensation and nuclear envelope permeabilization, in order to ensure mitotic timing and fidelity. Here, we discuss the main cytoskeletal and nuclear events that occur during mitotic entry in proliferating animal cells, focusing on their coordinated contribution for early mitotic spindle assembly. We will also explore recent progress in understanding their regulatory biochemical and mechanical pathways.

Differential turnover of Nup188 controls its levels at centrosomes and role in centriole duplication

NUP188 encodes a scaffold component of the nuclear pore complex (NPC) and has been implicated as a congenital heart disease gene through an ill-defined function at centrioles. Here, we explore the mechanisms that physically and functionally segregate Nup188 between the pericentriolar material (PCM) and NPCs. Pulse-chase fluorescent labeling indicates that Nup188 populates centrosomes with newly synthesized protein that does not exchange with NPCs even after mitotic NPC breakdown. In addition, the steady-state levels of Nup188 are controlled by the sensitivity of the PCM pool, but not the NPC pool, to proteasomal degradation. Proximity-labeling and super-resolution microscopy show that Nup188 is vicinal to the inner core of the interphase centrosome. Consistent with this, we demonstrate direct binding between Nup188 and Cep152. We further show that Nup188 functions in centriole duplication at or upstream of Sas6 loading. Together, our data establish Nup188 as a component of PCM needed to duplicate the centriole with implications for congenital heart disease mechanisms.

Two Nucleoporin98 homologous genes jointly participate in the regulation of starch degradation to repress senescence in Arabidopsis

BACKGROUND

Starch is synthesized during daylight for temporary storage in leaves and then degraded during the subsequent night to support plant growth and development. Impairment of starch degradation leads to stunted growth, even senescence and death. The nuclear pore complex is involved in many cellular processes, but its relationship with starch degradation has been unclear until now. We previously identified that two Nucleoporin98 genes (Nup98a and Nup98b) redundantly regulate flowering via the CONSTANS (CO)-independent pathway in Arabidopsis thaliana. The double mutant also shows severe senescence phenotypes.

RESULTS

We find that Nucleoporin 98 participates in the regulation of sugar metabolism in leaves and is also involved in senescence regulation in Arabidopsis. We show that Nup98a and Nup98b function redundantly at different stages of starch degradation. The nup98a-1 nup98b-1 double mutant accumulates more starch, showing a severe early senescence phenotype compared to wild type plants. The expression of marker genes related to starch degradation is impaired in the nup98a-1 nup98b-1 double mutant, and marker genes of carbon starvation and senescence express their products earlier and in higher abundance than in wild type plants, suggesting that abnormalities in energy metabolism are the main cause of senescence in the double mutant. Addition of sucrose to the growth medium rescues early senescence phenotypes of the nup98a-1 nup98b-1 mutant.

CONCLUSIONS

Our results provide evidence for a novel role of the nuclear pore complex in energy metabolism related to growth and development, in which Nup98 functions in starch degradation to control growth regulation in Arabidopsis.

The Biology of the Nuclear Envelope and Its Implications in Cancer Biology

The formation of the nuclear envelope and the subsequent compartmentalization of the genome is a defining feature of eukaryotes. Traditionally, the nuclear envelope was purely viewed as a physical barrier to preserve genetic material in eukaryotic cells. However, in the last few decades, it has been revealed to be a critical cellular component in controlling gene expression and has been implicated in several human diseases. In cancer, the relevance of the cell nucleus was first reported in the mid-1800s when an altered nuclear morphology was observed in tumor cells. This review aims to give a current and comprehensive view of the role of the nuclear envelope on cancer first by recapitulating the changes of the nuclear envelope during cell division, second, by reviewing the role of the nuclear envelope in cell cycle regulation, signaling, and the regulation of the genome, and finally, by addressing the nuclear envelope link to cell migration and metastasis and its use in cancer prognosis.

NUP98 - a novel predictor of response to anthracycline-based chemotherapy in triple negative breast cancer

Background

Triple Negative breast cancer (TNBC) is a poor outcome subgroup of breast cancer defined based on the absence of expression of ERα and PR and HER2 amplification. These hard to treat cancers lack targeted treatment options and are therefore treated with a standard of care (SoC) generic cocktail of DNA damaging chemotherapy, with a wide range of clinical responses. While a subset of TNBC patients respond very well to this treatment, others receive no clinical benefit and die from their disease within a short time period. We currently lack biomarkers to prospectively identify patients likely to relapse and we lack alternate treatment options.

Methods

NUP98 protein expression was investigated in patient samples using two independent tissue microarrays (TMAs), as well as a normal breast TMA. Correlation with pathological response to various chemotherapy regimens was investigated.

Results

We have shown that high NUP98 is significantly associated with poor outcome in TNBC patient samples both by gene expression and IHC-based protein analysis. While trends linking NUP98 expression with poorer outcomes were observed in breast cancer overall (and more specifically in the LuminalB Her2- subgroup), significant correlations were observed in TNBC. This appeared to be specific to anthracycline based regimens as the association between NUP98 and response was not observed in patients treated with taxane-based chemotherapy.

Conclusions

We have identified a novel biomarker, NUP98, that can predict response to anthracycline based chemotherapy in TNBC. The ability to prospectively identify patients who are less likely to respond to SoC chemotherapy is a vital step in improving the overall survival of these patients.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5407-9) contains supplementary material, which is available to authorized users.

The Arabidopsis nucleoporin NUP1 is essential for megasporogenesis and early stages of pollen development

Loss-of-function of nucleoporin NUP1 in Arabidopsis causes defect in both male and female gametogenesis. Its ovules are arrested during meiosis, and its pollen grains are aborted at mitosis I. Nuclear pore complex (NPC) plays crucial roles in nucleocytoplasmic trafficking of proteins and RNAs. The NPC contains approximately 30 different proteins termed nucleoporins (NUPs). So far, only a few of plant NUPs have been characterized. The Arabidopsis NUP1 was identified as an ortholog of the yeast NUP1 and animal NUP153. Loss-of-function of NUP1 in Arabidopsis caused fertility defect; however, the molecular mechanism of this defect remains unknown. Here, we found that both male and female gametogenesis of the nup1 mutants were defective. nup1 ovules were arrested from the meiosis stage onward; only approximately 6.7% and 3% ovules of the nup1-1 and nup1-4 mutants developed up to the FG7 stage, respectively. Pollen development of the nup1 mutants was arrested during the first mitotic division. In addition, enlarged pollen grains with increased DNA content were observed in the nup1 mutant. RNA-sequencing showed that expression levels of genes involved in pollen development or regulation of cell size were reduced dramatically in nup1 compared with wild type. These results suggest that NUP1 plays an important role in gametogenesis.

Proteomics of phosphorylation and protein dynamics during fertilization and meiotic exit in the Xenopus egg

Fertilization releases the meiotic arrest and initiates the events that prepare the egg for the ensuing developmental program. Protein degradation and phosphorylation are known to regulate protein activity during this process. However, the full extent of protein loss and phosphoregulation is still unknown. We examined absolute protein and phosphosite dynamics of the fertilization response by mass spectrometry-based proteomics in electroactivated eggs. To do this, we developed an approach for calculating the stoichiometry of phosphosites from multiplexed proteomics that is compatible with dynamic, stable, and multisite phosphorylation. Overall, the data suggest that degradation is limited to a few low-abundance proteins. However, this degradation promotes extensive dephosphorylation that occurs over a wide range of abundances during meiotic exit. We also show that eggs release a large amount of protein into the medium just after fertilization, most likely related to the blocks to polyspermy. Concomitantly, there is a substantial increase in phosphorylation likely tied to calcium-activated kinases. We identify putative degradation targets and components of the slow block to polyspermy. The analytical approaches demonstrated here are broadly applicable to studies of dynamic biological systems.

GTSE1 tunes microtubule stability for chromosome alignment and segregation by inhibiting the microtubule depolymerase MCAK

The microtubule depolymerase MCAK influences chromosomal instability (CIN), but what controls its activity remains unclear. Bendre et al. show that GTSE1, a protein found overexpressed in tumors, regulates microtubule stability and chromosome alignment during mitosis by inhibiting MCAK. High levels of GTSE1 are linked to chromosome missegregation and CIN.

The dynamic regulation of microtubules (MTs) during mitosis is critical for accurate chromosome segregation and genome stability. Cancer cell lines with hyperstabilized kinetochore MTs have increased segregation errors and elevated chromosomal instability (CIN), but the genetic defects responsible remain largely unknown. The MT depolymerase MCAK (mitotic centromere-associated kinesin) can influence CIN through its impact on MT stability, but how its potent activity is controlled in cells remains unclear. In this study, we show that GTSE1, a protein found overexpressed in aneuploid cancer cell lines and tumors, regulates MT stability during mitosis by inhibiting MCAK MT depolymerase activity. Cells lacking GTSE1 have defects in chromosome alignment and spindle positioning as a result of MT instability caused by excess MCAK activity. Reducing GTSE1 levels in CIN cancer cell lines reduces chromosome missegregation defects, whereas artificially inducing GTSE1 levels in chromosomally stable cells elevates chromosome missegregation and CIN. Thus, GTSE1 inhibition of MCAK activity regulates the balance of MT stability that determines the fidelity of chromosome alignment, segregation, and chromosomal stability.

Expression of Leukemia-Associated Nup98 Fusion Proteins Generates an Aberrant Nuclear Envelope Phenotype

Chromosomal translocations involving the nucleoporin NUP98 have been described in several hematopoietic malignancies, in particular acute myeloid leukemia (AML). In the resulting chimeric proteins, Nup98's N-terminal region is fused to the C-terminal region of about 30 different partners, including homeodomain (HD) transcription factors. While transcriptional targets of distinct Nup98 chimeras related to immortalization are relatively well described, little is known about other potential cellular effects of these fusion proteins. By comparing the sub-nuclear localization of a large number of Nup98 fusions with HD and non-HD partners throughout the cell cycle we found that while all Nup98 chimeras were nuclear during interphase, only Nup98-HD fusion proteins exhibited a characteristic speckled appearance. During mitosis, only Nup98-HD fusions were concentrated on chromosomes. Despite the difference in localization, all tested Nup98 chimera provoked morphological alterations in the nuclear envelope (NE), in particular affecting the nuclear lamina and the lamina-associated polypeptide 2α (LAP2α). Importantly, such aberrations were not only observed in transiently transfected HeLa cells but also in mouse bone marrow cells immortalized by Nup98 fusions and in cells derived from leukemia patients harboring Nup98 fusions. Our findings unravel Nup98 fusion-associated NE alterations that may contribute to leukemogenesis.

The RanGTP Pathway: From Nucleo-Cytoplasmic Transport to Spindle Assembly and Beyond

The small GTPase Ran regulates the interaction of transport receptors with a number of cellular cargo proteins. The high affinity binding of the GTP-bound form of Ran to import receptors promotes cargo release, whereas its binding to export receptors stabilizes their interaction with the cargo. This basic mechanism linked to the asymmetric distribution of the two nucleotide-bound forms of Ran between the nucleus and the cytoplasm generates a switch like mechanism controlling nucleo-cytoplasmic transport. Since 1999, we have known that after nuclear envelope breakdown (NEBD) Ran and the above transport receptors also provide a local control over the activity of factors driving spindle assembly and regulating other aspects of cell division. The identification and functional characterization of RanGTP mitotic targets is providing novel insights into mechanisms essential for cell division. Here we review our current knowledge on the RanGTP system and its regulation and we focus on the recent advances made through the characterization of its mitotic targets. We then briefly review the novel functions of the pathway that were recently described. Altogether, the RanGTP system has moonlighting functions exerting a spatial control over protein interactions that drive specific functions depending on the cellular context.

Chromatin-prebound Crm1 recruits Nup98-HoxA9 fusion to induce aberrant expression of Hox cluster genes

The nucleoporin Nup98 is frequently rearranged to form leukemogenic Nup98-fusion proteins with various partners. However, their function remains largely elusive. Here, we show that Nup98-HoxA9, a fusion between Nup98 and the homeobox transcription factor HoxA9, forms nuclear aggregates that frequently associate with facultative heterochromatin. We demonstrate that stable expression of Nup98-HoxA9 in mouse embryonic stem cells selectively induces the expression of Hox cluster genes. Genome-wide binding site analysis revealed that Nup98-HoxA9 is preferentially targeted and accumulated at Hox cluster regions where the export factor Crm1 is originally prebound. In addition, leptomycin B, an inhibitor of Crm1, disassembled nuclear Nup98-HoxA9 dots, resulting in the loss of chromatin binding of Nup98-HoxA9 and Nup98-HoxA9-mediated activation of Hox genes. Collectively, our results indicate that highly selective targeting of Nup98-fusion proteins to Hox cluster regions via prebound Crm1 induces the formation of higher order chromatin structures that causes aberrant Hox gene regulation.

Formation of Nup98-containing nuclear bodies in HeLa sublines is linked to genomic rearrangements affecting chromosome 11

Nup98 is an important component of the nuclear pore complex (NPC) and also a rare but recurrent target for chromosomal translocation in leukaemogenesis. Nup98 contains multiple cohesive Gly-Leu-Phe-Gly (GLFG) repeats that are critical notably for the formation of intranuclear GLFG bodies. Previous studies have reported the existence of GLFG bodies in cells overexpressing exogenous Nup98 or in a HeLa subline (HeLa-C) expressing an unusual elevated amount of endogenous Nup98. Here, we have analysed the presence of Nup98-containing bodies in several human cell lines. We found that HEp-2, another HeLa subline, contains GLFG bodies that are distinct from those identified in HeLa-C. Rapid amplification of cDNA ends (RACE) revealed that HEp-2 cells express additional truncated forms of Nup98 fused to a non-coding region of chromosome 11q22.1. Cytogenetic analyses using FISH and array-CGH further revealed chromosomal rearrangements that were distinct from those observed in leukaemic cells. Indeed, HEp-2 cells feature a massive amplification of juxtaposed NUP98 and 11q22.1 loci on a chromosome marker derived from chromosome 3. Unexpectedly, minor co-amplifications of NUP98 and 11q22.1 loci were also observed in other HeLa sublines, but on rearranged chromosomes 11. Altogether, this study reveals that distinct genomic rearrangements affecting NUP98 are associated with the formation of GLFG bodies in specific HeLa sublines.

The nucleoporin ALADIN regulates Aurora A localization to ensure robust mitotic spindle formation

The nucleoporin ALADIN, which is mutated in patients with triple A syndrome, is necessary for proper spindle formation. Without ALADIN, active Aurora A moves away from centrosomes. The relocalization of active Aurora A leads to a redistribution of specific spindle assembly factors that make spindles less stable and slows their formation.

The formation of the mitotic spindle is a complex process that requires massive cellular reorganization. Regulation by mitotic kinases controls this entire process. One of these mitotic controllers is Aurora A kinase, which is itself highly regulated. In this study, we show that the nuclear pore protein ALADIN is a novel spatial regulator of Aurora A. Without ALADIN, Aurora A spreads from centrosomes onto spindle microtubules, which affects the distribution of a subset of microtubule regulators and slows spindle assembly and chromosome alignment. ALADIN interacts with inactive Aurora A and is recruited to the spindle pole after Aurora A inhibition. Of interest, mutations in ALADIN cause triple A syndrome. We find that some of the mitotic phenotypes that we observe after ALADIN depletion also occur in cells from triple A syndrome patients, which raises the possibility that mitotic errors may underlie part of the etiology of this syndrome.

Reprint of "Nuclear transport factors: global regulation of mitosis"

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator – the γ-TuRC complex – and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

Nuclear transport factors: global regulation of mitosis

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator - the γ-TuRC complex - and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

Nuclear pore proteins and the control of genome functions

Nuclear pore complexes (NPCs) are composed of several copies of ∼30 different proteins called nucleoporins (Nups). NPCs penetrate the nuclear envelope (NE) and regulate the nucleocytoplasmic trafficking of macromolecules. Beyond this vital role, NPC components influence genome functions in a transport-independent manner. Nups play an evolutionarily conserved role in gene expression regulation that, in metazoans, extends into the nuclear interior. Additionally, in proliferative cells, Nups play a crucial role in genome integrity maintenance and mitotic progression. Here we discuss genome-related functions of Nups and their impact on essential DNA metabolism processes such as transcription, chromosome duplication, and segregation.

Nup2 requires a highly divergent partner, NupA, to fulfill functions at nuclear pore complexes and the mitotic chromatin region

Among nuclear pore proteins, Nup2 is unique because it transfers to the mitotic chromatin region to fulfill unknown functions. Analysis of Nup2 and a novel targeting partner, NupA, shows that they are required for normal anaphase and nucleokinesis. Their functions also involve an import pathway for Mad1 but apparently not general nuclear protein import.

Chromatin and nuclear pore complexes (NPCs) undergo dramatic changes during mitosis, which in vertebrates and Aspergillus nidulans involves movement of Nup2 from NPCs to the chromatin region to fulfill unknown functions. This transition is shown to require the Cdk1 mitotic kinase and be promoted prematurely by ectopic expression of the NIMA kinase. Nup2 localizes with a copurifying partner termed NupA, a highly divergent yet essential NPC protein. NupA and Nup2 locate throughout the chromatin region during prophase but during anaphase move to surround segregating DNA. NupA function is shown to involve targeting Nup2 to its interphase and mitotic locations. Deletion of either Nup2 or NupA causes identical mitotic defects that initiate a spindle assembly checkpoint (SAC)–dependent mitotic delay and also cause defects in karyokinesis. These mitotic problems are not caused by overall defects in mitotic NPC disassembly–reassembly or general nuclear import. However, without Nup2 or NupA, although the SAC protein Mad1 locates to its mitotic locations, it fails to locate to NPCs normally in G1 after mitosis. Collectively the study provides new insight into the roles of Nup2 and NupA during mitosis and in a surveillance mechanism that regulates nucleokinesis when mitotic defects occur after SAC fulfillment.

Cancer driver candidate genes AVL9, DENND5A and NUPL1 contribute to MDCK cystogenesis

AVL9, DENND5A and NUPL1 are among the cancer driver candidate genes previously identified via dog-human comparison, and may function in epithelial cell polarity as indicated by bioinformatics analysis. To better understand their cellular functions and roles in cancer, we knocked down each gene in MDCKII cells through shRNA and performed three-dimensional culture. Compared to the control, the knockdown clones developed significantly more abnormal cysts, e.g., cysts with the lumen harboring dead and/or live cells, or cysts having multiple lumens. Further analysis revealed that abnormalities initiated at the first cell division and persisted throughout the entire cystogenesis process. For NUPL1-knockdown cells, abnormal cytogenesis largely arose from faulty cell divisions, notably monopolar spindles or spindles with poorly separated poles. For AVL9- or DENND5A-knockdown cells, abnormalities originated from both aberrant intracellular trafficking and defective mitosis. Moreover, while all knockdown clones displayed an accelerated rate of both cell proliferation and death, only AVL9- and DENND5A-knockdowns, but not NUPL1-knockdown, promoted cell migration. These observations indicate that NUPL1 contributes to bipolar spindle formation, whereas AVL9 and DENND5A participate in both intracellular trafficking and cell cycle progression. Our study shed lights on these genes' normal cellular functions and on how their alteration contributes to carcinogenesis.

Nucleoporin MOS7/Nup88 is required for mitosis in gametogenesis and seed development in Arabidopsis

Angiosperm reproduction is characterized by alternate diploid sporophytic and haploid gametophytic generations. Gametogenesis shares similarities with that of animals except for the formation of the gametophyte, whereby haploid cells undergo several rounds of postmeiotic mitosis to form gametes and the accessory cells required for successful reproduction. The mechanisms regulating gametophyte development in angiosperms are incompletely understood. Here, we show that the nucleoporin Nup88-homolog MOS7 (Modifier of Snc1,7) plays a crucial role in mitosis during both male and female gametophyte formation in Arabidopsis thaliana. Using a mutagenesis screen, we identify the mos7-5 mutant allele, which causes ovule and pollen abortion in MOS7/mos7-5 heterozygous plants, and preglobular stage embryonic lethality in homozygous mos7-5 seeds. During interphase, we show that MOS7 is localized to the nuclear membrane but, like many nucleoporins, is associated with the spindle apparatus during mitosis. We detect interactions between MOS7 and several nucleoporins known to control spindle dynamics, and find that in pollen from MOS7/mos7-5 heterozygotes, abortion is accompanied by a failure of spindle formation, cell fate specification, and phragmoplast activity. Most intriguingly, we show that following gamete formation by MOS7/mos7-5 heterozygous spores, inheritance of either the MOS7 or the mos7-5 allele by a given gamete does not correlate with its respective survival or abortion. Instead, we suggest a model whereby MOS7, which is highly expressed in the Pollen- and Megaspore Mother Cells, enacts a dosage-limiting effect on the gametes to enable their progression through subsequent mitoses.

Nucleoporin Gene Fusions and Hematopoietic Malignancies

Nuclear pore complexes (NPCs) are the sole gateways between the nucleus and the cytoplasm of eukaryotic cells and they mediate all macromolecular trafficking between these cellular compartments. Nucleocytoplasmic transport is highly selective and precisely regulated and as such an important aspect of normal cellular function. Defects in this process or in its machinery have been linked to various human diseases, including cancer. Nucleoporins, which are about 30 proteins that built up NPCs, are critical players in nucleocytoplasmic transport and have also been shown to be key players in numerous other cellular processes, such as cell cycle control and gene expression regulation. This review will focus on the three nucleoporins Nup98, Nup214, and Nup358. Common to them is their significance in nucleocytoplasmic transport, their multiple other functions, and being targets for chromosomal translocations that lead to haematopoietic malignancies, in particular acute myeloid leukaemia. The underlying molecular mechanisms of nucleoporin-associated leukaemias are only poorly understood but share some characteristics and are distinguished by their poor prognosis and therapy outcome.

Nucleoporins and nucleocytoplasmic transport in hematologic malignancies

Hematologic malignancies are often associated with chromosomal rearrangements that lead to the expression of chimeric fusion proteins. Rearrangements of the genes encoding two nucleoporins, NUP98 and NUP214, have been implicated in the pathogenesis of several types of hematologic malignancies, particularly acute myeloid leukemia. NUP98 rearrangements result in fusion of an N-terminal portion of NUP98 to one of numerous proteins. These rearrangements often follow treatment with topoisomerase II inhibitors and tend to occur in younger patients. They have been shown to induce leukemia in mice and to enhance proliferation and disrupt differentiation in primary human hematopoietic precursors. NUP214 has only a few fusion partners. DEK-NUP214 is the most common NUP214 fusion in AML; it tends to occur in younger patients and is usually associated with FLT3 internal tandem duplications. The leukemogenic activity of NUP214 fusions is less well characterized. Normal nucleoporins, including NUP98 and NUP214, have important functions in nucleocytoplasmic transport, transcription, and mitosis. These functions and their disruptions by oncogenic nucleoporin fusions are discussed.

Transportin acts to regulate mitotic assembly events by target binding rather than Ran sequestration

Transportin-specific molecular tools are used to show that the mitotic cell contains importin β and transportin “global positioning system” pathways that are mechanistically parallel. Transportin works to control where the spindle, nuclear membrane, and nuclear pores are formed by directly affecting assembly factor function.

The nuclear import receptors importin β and transportin play a different role in mitosis: both act phenotypically as spatial regulators to ensure that mitotic spindle, nuclear membrane, and nuclear pore assembly occur exclusively around chromatin. Importin β is known to act by repressing assembly factors in regions distant from chromatin, whereas RanGTP produced on chromatin frees factors from importin β for localized assembly. The mechanism of transportin regulation was unknown. Diametrically opposed models for transportin action are as follows: 1) indirect action by RanGTP sequestration, thus down-regulating release of assembly factors from importin β, and 2) direct action by transportin binding and inhibiting assembly factors. Experiments in Xenopus assembly extracts with M9M, a superaffinity nuclear localization sequence that displaces cargoes bound by transportin, or TLB, a mutant transportin that can bind cargo and RanGTP simultaneously, support direct inhibition. Consistently, simple addition of M9M to mitotic cytosol induces microtubule aster assembly. ELYS and the nucleoporin 107–160 complex, components of mitotic kinetochores and nuclear pores, are blocked from binding to kinetochores in vitro by transportin, a block reversible by M9M. In vivo, 30% of M9M-transfected cells have spindle/cytokinesis defects. We conclude that the cell contains importin β and transportin “global positioning system”or “GPS” pathways that are mechanistically parallel.

Wnt signaling proteins associate with the nuclear pore complex: implications for cancer

Several components of the Wnt signaling pathway have in recent years been linked to the nuclear pore complex. β-catenin, the primary transducer of Wnt signals from the plasma membrane to the nucleus, has been shown to transiently associate with different FG-repeat containing nucleoporins (Nups) and to translocate bidirectionally through pores of the nuclear envelope in a manner independent of classical transport receptors and the Ran GTPase. Two key regulators of β-catenin, IQGAP1 and APC, have also been reported to bind specific Nups or to locate at the nuclear pore complex. The interaction between these Wnt signaling proteins and different Nups may have functional implications beyond nuclear transport in cellular processes that include mitotic regulation, centrosome positioning and cell migration, nuclear envelope assembly/disassembly, and the DNA replication checkpoint. The broad implications of interactions between Wnt signaling proteins and Nups will be discussed in the context of cancer.

New mitotic regulators released from chromatin

Faithful action of the mitotic spindle segregates duplicated chromosomes into daughter cells. Perturbations of this process result in chromosome mis-segregation, leading to chromosomal instability and cancer development. Chromosomes are not simply passengers segregated by spindle microtubules but rather play a major active role in spindle assembly. The GTP bound form of the Ran GTPase (RanGTP), produced around chromosomes, locally activates spindle assembly factors. Recent studies have uncovered that chromosomes organize mitosis beyond spindle formation. They distinctly regulate other mitotic events, such as spindle maintenance in anaphase, which is essential for chromosome segregation. Furthermore, the direct function of chromosomes is not only to produce RanGTP but, in addition, to release key mitotic regulators from chromatin. Chromatin-remodeling factors and nuclear pore complex proteins, which have established functions on chromatin in interphase, dissociate from mitotic chromatin and function in spindle assembly or maintenance. Thus, chromosomes actively organize their own segregation using chromatin-releasing mitotic regulators as well as RanGTP.

Viral subversion of nucleocytoplasmic trafficking

Trafficking of proteins and RNA into and out of the nucleus occurs through the nuclear pore complex (NPC). Because of its critical function in many cellular processes, the NPC and transport factors are common targets of several viruses that disrupt key constituents of the machinery to facilitate viral replication. Many viruses such as poliovirus and severe acute respiratory syndrome (SARS) virus inhibit protein import into the nucleus, whereas viruses such as influenza A virus target and disrupt host mRNA nuclear export. Current evidence indicates that these viruses may employ such strategies to avert the host immune response. Conversely, many viruses co‐opt nucleocytoplasmic trafficking to facilitate transport of viral RNAs. As viral proteins interact with key regulators of the host nuclear transport machinery, viruses have served as invaluable tools of discovery that led to the identification of novel constituents of nuclear transport pathways. This review explores the importance of nucleocytoplasmic trafficking to viral pathogenesis as these studies revealed new antiviral therapeutic strategies and exposed previously unknown cellular mechanisms. Further understanding of nuclear transport pathways will determine whether such therapeutics will be useful treatments for important human pathogens.

Nuclear imprisonment: viral strategies to arrest host mRNA nuclear export

Viruses possess many strategies to impair host cellular responses to infection. Nuclear export of host messenger RNAs (mRNA) that encode antiviral factors is critical for antiviral protein production and control of viral infections. Several viruses have evolved sophisticated strategies to inhibit nuclear export of host mRNAs, including targeting mRNA export factors and nucleoporins to compromise their roles in nucleo-cytoplasmic trafficking of cellular mRNA. Here, we present a review of research focused on suppression of host mRNA nuclear export by viruses, including influenza A virus and vesicular stomatitis virus, and the impact of this viral suppression on host antiviral responses.

Shaping the p53 response with nucleoporins

In this issue, Singer et al. (2012) reveal that the nucleoporin Nup98 supports adaptation to genotoxic stress by protecting specific p53-induced mRNAs from exosome-dependent degradation, suggesting that wild-type Nup98 may possess tumor suppressor function.

A NUP98-HOXD13 leukemic fusion gene leads to impaired class switch recombination and antibody production

Myelodysplastic syndrome is a clonal process characterized by ineffective hematopoiesis and progression to acute leukemia. Although many myelodysplastic syndrome and leukemic patients have compromised immunity, the role of underlying mutations in regulating immune function is poorly understood. Recent studies show that NUP98-HOXD13 (NHD13) fusion gene results in myelodysplastic syndrome and impairs lymphocyte differentiation in transgenic mice. In our studies, we sought to elucidate the mechanism by which NHD13 affects B-lymphocyte development and function. Based on our preliminary findings that transgenic mice had increased levels of IgM and reduced IgG1 and IgE, we hypothesized that the fusion gene might impair class switch recombination (CSR). Mice were immunologically challenged with dinitrophenol. NHD13 mice showed a marked reduction in B-lymphocyte differentiation in their bone marrow and spleen following dinitrophenol stimulation and had reduced production of dinitrophenol-specific antibodies. Spleen follicles from these mice were small and hypocellular, indicating failure of clonal expansion. When isolated NHD13 B lymphocytes were stimulated in vitro using Escherichia coli lipopolysaccharide or lipopolysaccharide + interleukin-4, they failed to undergo sufficient CSR and proliferation. Taken together, our findings show that expression of NUP98-HOXD13 impairs CSR and reduces the antibody-mediated immune response, in addition to its role in leukemia. Further delineation of the NUP98-HOXD13 transgene may reveal novel pathways involved in CSR.

MCAK activity at microtubule tips regulates spindle microtubule length to promote robust kinetochore attachment

The kinesin MCAK binds to end-binding proteins and antagonizes centrosome separation and promotes robust kinetochore attachments to spindle microtubules.

Mitotic centromere-associated kinesin (MCAK) is a microtubule-depolymerizing kinesin-13 member that can track with polymerizing microtubule tips (hereafter referred to as tip tracking) during both interphase and mitosis. MCAK tracks with microtubule tips by binding to end-binding proteins (EBs) through the microtubule tip localization signal SKIP, which lies N terminal to MCAK’s neck and motor domain. The functional significance of MCAK’s tip-tracking behavior during mitosis has never been explained. In this paper, we identify and define a mitotic function specific to the microtubule tip–associated population of MCAK: negative regulation of microtubule length within the assembling bipolar spindle. This function depends on MCAK’s ability to bind EBs and track with polymerizing nonkinetochore microtubule tips. Although this activity antagonizes centrosome separation during bipolarization, it ultimately benefits the dividing cell by promoting robust kinetochore attachments to the spindle microtubules.

Microtubule-associated nuclear envelope proteins in interphase and mitosis

The LINC (linker of nucleoskeleton and cytoskeleton) complex forms a transcisternal bridge across the NE (nuclear envelope) that connects the cytoskeleton with the nuclear interior. This enables some proteins of the NE to communicate with the centrosome and the microtubule cytoskeleton. The position of the centrosome relative to the NE is of vital importance for many cell functions, such as cell migration and division, and centrosomal dislocation is a frequent phenotype in laminopathic disorders. Also in mitosis, a small group of transmembrane NE proteins associate with microtubules when they concentrate in a specific membrane domain associated with the mitotic spindle. The present review discusses structural and functional aspects of microtubule association with NE proteins and how this association may be maintained over the cell cycle.

The nuclear envelope environment and its cancer connections

Because of the association between aberrant nuclear structure and tumour grade, nuclear morphology is an indispensible criterion in the current pathological assessment of cancer. Components of the nuclear envelope environment have central roles in many aspects of cell function that affect tumour development and progression. As the roles of the nuclear envelope components, including nuclear pore complexes and nuclear lamina, are being deciphered in molecular detail there are opportunities to harness this knowledge for cancer therapeutics and biomarker development. In this Review, we summarize the progress that has been made in our understanding of the nuclear envelope and the implications of changes in this environment for cancer biology.

Evolution: functional evolution of nuclear structure

The evolution of the nucleus, the defining feature of eukaryotic cells, was long shrouded in speculation and mystery. There is now strong evidence that nuclear pore complexes (NPCs) and nuclear membranes coevolved with the endomembrane system, and that the last eukaryotic common ancestor (LECA) had fully functional NPCs. Recent studies have identified many components of the nuclear envelope in living Opisthokonts, the eukaryotic supergroup that includes fungi and metazoan animals. These components include diverse chromatin-binding membrane proteins, and membrane proteins with adhesive lumenal domains that may have contributed to the evolution of nuclear membrane architecture. Further discoveries about the nucleoskeleton suggest that the evolution of nuclear structure was tightly coupled to genome partitioning during mitosis.

Domain topology of nucleoporin Nup98 within the nuclear pore complex

Nuclear pore complexes (NPCs) facilitate selective transport of macromolecules across the nuclear envelope in interphase eukaryotic cells. NPCs are composed of roughly 30 different proteins (nucleoporins) of which about one third are characterized by the presence of phenylalanine-glycine (FG) repeat domains that allow the association of soluble nuclear transport receptors with the NPC. Two types of FG (FG/FxFG and FG/GLFG) domains are found in nucleoporins and Nup98 is the sole vertebrate nucleoporin harboring the GLFG-type repeats. By immuno-electron microscopy using isolated nuclei from Xenopus oocytes we show here the localization of distinct domains of Nup98. We examined the localization of the C- and N-terminal domain of Nup98 by immunogold-labeling using domain-specific antibodies against Nup98 and by expressing epitope tagged versions of Nup98. Our studies revealed that anchorage of Nup98 to NPCs through its C-terminal autoproteolytic domain occurs in the center of the NPC, whereas its N-terminal GLFG domain is more flexible and is detected at multiple locations within the NPC. Additionally, we have confirmed the central localization of Nup98 within the NPC using super resolution structured illumination fluorescence microscopy (SIM) to position Nup98 domains relative to markers of cytoplasmic filaments and the nuclear basket. Our data support the notion that Nup98 is a major determinant of the permeability barrier of NPCs.

The nucleoskeleton as a genome-associated dynamic 'network of networks'

In the cytosol, actin polymers, intermediate filaments and microtubules can anchor to cell surface adhesions and interlink to form intricate networks. This cytoskeleton is anchored to the nucleus through LINC (links the nucleoskeleton and cytoskeleton) complexes that span the nuclear envelope and in turn anchor to networks of filaments in the nucleus. The metazoan nucleoskeleton includes nuclear pore-linked filaments, A-type and B-type lamin intermediate filaments, nuclear mitotic apparatus (NuMA) networks, spectrins, titin, 'unconventional' polymers of actin and at least ten different myosin and kinesin motors. These elements constitute a poorly understood 'network of networks' that dynamically reorganizes during mitosis and is responsible for genome organization and integrity.

The role of nuclear pore complex in tumor microenvironment and metastasis

One of the main reasons for cancer mortality is caused by the highly invasive behavior of cancer cells, which often due to aggressive metastasis. Metastasis is mediated by various growth factors and cytokines, operating through numerous signaling pathways. Remarkably, all these metastatic signaling pathways must enter the nucleus through a single gatekeeper, the nuclear pore complex (NPC). NPCs are the only gateway between the cytoplasm and the nucleus. NPCs are among the largest proteinaceous assemblies in the cell and are composed of multiple copies of around 30 different proteins called nucleoporins. Here, we review what is currently known about the NPC, and its role in the mechanisms of tumor progression. We will also explore potential strategies to target metastatic pathways by manipulating the karyopherins (importins/exportins) of nucleocytoplasmic traffic through NPCs.

Coordinating postmitotic nuclear pore complex assembly with abscission timing

Cells divide and accurately inherit genomic and cellular content through synchronized changes in cellular organization and chromosome dynamics. Although DNA segregation, nuclear reformation, and cytokinesis/abscission temporally overlap, little is known about how these distinct events are coordinated to ensure accurate cell division. Recently, we found that disruption of postmitotic nuclear pore complex assembly, an essential aspect of the newly forming nuclear envelope, triggers an Aurora B-dependent delay in abscission. This delay is further characterized by mislocalized, aberrantly active Aurora B in the cytoplasm of midbody-stage cells. These results support a model in which an Aurora B-mediated abscission checkpoint provides surveillance of nuclear pore complex formation to ensure that elements of nuclear architecture are fully formed before daughter cells are physically separated. Here we discuss the process of nuclear pore complex assembly, describe potential mechanisms that may explain how this process could be coordinated with abscission, and postulate why such a checkpoint mechanism may exist.

Nucleoporins: leaving the nuclear pore complex for a successful mitosis

The nuclear envelope (NE) separates the cytoplasm and the cell nucleus of interphase eukaryotic cells and nuclear pore complexes (NPCs) mediate the macromolecular exchange between these two compartments. The NE and the NPCs of vertebrate cells disassemble during prophase and the nuclear pore proteins (nucleoporins) are distributed within the mitotic cytoplasm. For an increasing number of them active mitotic functions have been assigned over the past few years. Nucleoporins are participating in spindle assembly, kinetochore organisation, and the spindle assembly checkpoint, all processes that control chromosome segregation and are important for maintenance of genome integrity. But nucleoporins are also engaged in early and late mitotic events, such as centrosome positioning and cytokinesis. Here we will highlight recent progress in deciphering the roles for nucleoporins in the distinct steps of mitosis.