Crm1 is a mitotic effector of Ran-GTP in somatic cells

Translocation Biosensors - Cellular System Integrators to Dissect CRM1-Dependent Nuclear Export by Chemicogenomics

Fluorescent protein biosensors are powerful cellular systems biology tools for dissecting the complexity of cellular processes with high spatial and temporal resolution. As regulated nucleo-cytoplasmic transport is crucial for the modulation of numerous (patho)physiological cellular responses, a detailed understanding of its molecular mechanism would open up novel options for a rational manipulation of the cell. In contrast to genetic approaches, we here established and employed high-content cellular translocation biosensors applicable for dissecting nuclear export by chemicogenomics. A431 cell lines, stably expressing a translocation biosensor composed of glutathione S-transferase, GFP and a rational combination of nuclear import and export signals, were engineered by antibiotic selection and flow cytometry sorting. Using an optimized nuclear translocation algorithm, the translocation response could be robustly quantified on the Cellomics Arrayscan^® VTI platform. Subsequent to assay optimization, the assay was developed into a higher density 384-well format high-content assay and employed for the screening of the 17K ChemBioNet compound collection. This library was selected on the basis of a genetic algorithm used to identify maximum common chemical substructures in a database of annotated bioactive molecules and hence, is well-placed in the chemical space covered by bioactive compounds. Automated multiparameter data analysis combined with visual inspection allowed us to identify and to rationally discriminate true export inhibitors from false positives, which included fluorescent compounds or cytotoxic substances that dramatically affected the cellular morphology. A total of 120 potential hit compounds were selected for Cellomics Arrayscan^® VTI based rescreening. The export inhibitory activity of 20 compounds effective at concentrations < 25 μM were confirmed by fluorescence microscopy in several cell lines. Interestingly, kinetic analysis allowed the identification of inhibitors capable to interfere with the export receptor CRM1-mediated nuclear export not only in an irreversible, but also in a reversible fashion. In sum, exploitation of biosensor based screening allows the identification of chemicogenomic tools applicable for dissecting nucleo-cytoplasmic transport in living cells.

Orchestrating nuclear envelope disassembly and reassembly during mitosis

Cell division in eukaryotes requires extensive architectural changes of the nuclear envelope (NE) to ensure that segregated DNA is finally enclosed in a single cell nucleus in each daughter cell. Higher eukaryotic cells have evolved 'open' mitosis, the most extreme mechanism to solve the problem of nuclear division, in which the NE is initially completely disassembled and then reassembled in coordination with DNA segregation. Recent progress in the field has now started to uncover mechanistic and molecular details that underlie the changes in NE reorganization during open mitosis. These studies reveal a tight interplay between NE components and the mitotic machinery.

The nuclear export factor Xpo1p targets Mad1p to kinetochores in yeast

Nuclear pore complexes (NPCs) mediate all nucleocytoplasmic traffic and provide docking sites for the spindle assembly checkpoint (SAC) protein Mad1p. Upon SAC activation, Mad1p is recruited onto kinetochores and rapidly cycles between NPCs and kinetochores. We examined the mechanism of Mad1p movement onto kinetochores and show that it is controlled by two components of the nuclear transport machinery, the exportin Xpo1p and Ran-guanosine triphosphate (GTP). Mad1p contains a nuclear export signal (NES) that is recognized by Xpo1p. The NES, Xpo1p, and RanGTP are all required for Mad1p recruitment onto kinetochores in checkpoint-activated cells. Consistent with this function, Xpo1p also accumulates on kinetochores after SAC activation. We have also shown that Xpo1p and RanGTP are required for the dynamic cycling of Mad1p between NPCs and kinetochores in checkpoint-arrested cells. These results reveal an important function for Xpo1p in mediating intranuclear transport events and identify a signaling pathway between kinetochores and NPCs.

Inhibition of the CRM1-mediated nucleocytoplasmic transport by N-azolylacrylates: structure-activity relationship and mechanism of action

CRM1-mediated nucleocytoplasmic transport plays an important role in many cellular processes and diseases. To investigate the structural basis required for the inhibition of the CRM1-mediated nuclear export we have synthesized analogs of a previously identified small molecule lead compound and monitored their activity against the Rev function of the human immunodeficiency virus. Microscopy studies show that the active congeners of this series inhibit the nucleocytoplasmic transport of Rev and the co-localization between Rev and CRM1 in living cells. Mechanism of action studies show their interaction with the Cys528 residue of CRM1 involving a Michael-addition type of reaction. However, structure-activity relationship demonstrates strict constraints to the structure of the inhibitors, and shows that activity is not solely correlated to Michael-addition suggesting a more complex mechanism of action. Our results are suggestive for the existence of a well-defined interaction at the CRM1-NES binding site. In addition, the most selective congener inhibited the HIV-1 production in latently infected cells. These specific CRM1 inhibitors are of interest as tool for analyzing the mechanisms of post-transcriptional control of gene expression and provide insight in the design of new agents.

Exportin1 genes are essential for development and function of the gametophytes in Arabidopsis thaliana

Gametes are produced in plants through mitotic divisions in the haploid gametophytes. We investigated the role of EXPORTIN1 (XPO1) genes during the development of both female and male gametophytes of Arabidopsis. Exportins exclude target proteins from the nucleus and are also part of a complex recruited at the kinetochores during mitosis. Here we show that double mutants in Arabidopsis XPO1A and XPO1B are gametophytic defective. In homozygous-heterozygous plants, 50% of the ovules were arrested at different stages according to the parental genotype. Double-mutant female gametophytes of xpo1a-3/+; xpo1b-1/xpo1b-1 plants failed to undergo all the mitotic divisions or failed to complete embryo sac maturation. Double-mutant female gametophytes of xpo1a-3/xpo1a-3; xpo1b-1/+ plants had normal mitotic divisions and fertilization occurred; in most of these embryo sacs the endosperm started to divide but an embryo failed to develop. Distortions in male transmission correlated with the occurrence of smaller pollen grains, poor pollen germination, and shorter pollen tubes. Our results show that mitotic divisions are possible without XPO1 during the haploid phase, but that XPO1 is crucial for the maternal-to-embryonic transition.

Going green: plants' alternative way to position the Ran gradient

Ran is a multi-functional small GTPase of the Ras super-family involved in nucleocytoplasmic transport, mitotic spindle assembly, cell cycle control and nuclear envelope (NE) formation. Its roles are accomplished by the asymmetric distribution of its GTP- and GDP-bound forms, enabled by the specific localization of Ran accessory proteins, the Ran GTPase-activating protein RanGAP and the nucleotide exchange factor RCC1. Mammalian RanGAP1 is targeted to the NE during interphase and to the spindle and kinetochores during mitosis via a SUMOylated C-terminal domain and interaction with the nucleoporin Nup358/RanBP2. Arabidopsis RanGAP1 (AtRanGAP1) lacks the SUMOylated C-terminal domain of vertebrate RanGAP, but contains a plant-specific N-terminal domain (WPP domain), which is necessary and sufficient for its targeting to the NE in interphase. AtRanGAP1 has a mitotic trafficking pattern uniquely different from that of vertebrate RanGAP, which includes targeting to the outward-growing rim of the cell plate. The WPP domain is necessary and sufficient for this targeting. Now, a novel family of plant-specific, nuclear pore-associated proteins has been identified in Arabidopsis, which is essential for anchoring RanGAP to the Arabidopsis nuclear envelope at the root meristem. This suggests that RanGAP anchoring to the nuclear pore has been solved in two fundamentally different ways in animals and plants. These findings support a separate evolution of RanGAP targeting mechanisms in different kingdoms, possibly related to different functional geometries of the Ran gradient in animal and higher plant cell division.

Nuclear export receptor Xpo1/Crm1 is physically and functionally linked to the spindle pole body in budding yeast

The spindle pole body (SPB) represents the microtubule organizing center in the budding yeast Saccharomyces cerevisiae. It is a highly structured organelle embedded in the nuclear membrane, which is required to anchor microtubules on both sides of the nuclear envelope. The protein Spc72, a component of the SPB, is located at the cytoplasmic face of this organelle and serves as a receptor for the gamma-tubulin complex. In this paper we show that it is also a binding partner of the nuclear export receptor Xpo1/Crm1. Xpo1 binds its cargoes in a Ran-dependent fashion via a short leucine-rich nuclear export signal (NES). We show that binding of Spc72 to Xpo1 depends on Ran-GTP and a functional NES in Spc72. Mutations in this NES have severe consequences for mitotic spindle morphology in vivo. This is also the case for xpo1 mutants, which show a reduction in cytoplasmic microtubules. In addition, we find a subpopulation of Xpo1 localized at the SPB. Based on these data, we propose a functional link between Xpo1 and the SPB and discuss a role for this exportin in spindle biogenesis in budding yeast.

Twenty years of cell-cycle conferences in Roscoff

At the end of April 2008, the cell-cycle community celebrated the twentieth anniversary of the Jacques-Monod cell-cycle conferences in Roscoff (France), where the discovery of Cdk1 as a key cell-cycle regulator was first discussed in 1988.

A survivin-ran complex regulates spindle formation in tumor cells

Aberrant cell division is a hallmark of cancer, but the molecular circuitries of this process in tumor cells are not well understood. Here, we used a high-throughput proteomics screening to identify novel molecular partners of survivin, an essential regulator of mitosis overexpressed in cancer. We found that survivin associates with the small GTPase Ran in an evolutionarily conserved recognition in mammalian cells and Xenopus laevis extracts. This interaction is regulated during the cell cycle, involves Ran-GTP, requires a discrete binding interface centered on Glu65 in survivin, and is independent of the Ran effector Crm1. Disruption of a survivin-Ran complex does not affect the assembly of survivin within the chromosomal passenger complex or its cytosolic accumulation, but it inhibits the delivery of the Ran effector molecule TPX2 to microtubules. In turn, this results in aberrant mitotic spindle formation and chromosome missegregation in tumor, but not normal, cells. Therefore, survivin is a novel effector of Ran signaling, and this pathway may be preferentially exploited for spindle assembly in tumor cells.

In vivo dynamics of Drosophila nuclear envelope components

Nuclear pore complexes (NPCs) are multisubunit protein entities embedded into the nuclear envelope (NE). Here, we examine the in vivo dynamics of the essential Drosophila nucleoporin Nup107 and several other NE-associated proteins during NE and NPCs disassembly and reassembly that take place within each mitosis. During both the rapid mitosis of syncytial embryos and the more conventional mitosis of larval neuroblasts, Nup107 is gradually released from the NE, but it remains partially confined to the nuclear (spindle) region up to late prometaphase, in contrast to nucleoporins detected by wheat germ agglutinin and lamins. We provide evidence that in all Drosophila cells, a structure derived from the NE persists throughout metaphase and early anaphase. Finally, we examined the dynamics of the spindle checkpoint proteins Mad2 and Mad1. During mitotic exit, Mad2 and Mad1 are actively imported back from the cytoplasm into the nucleus after the NE and NPCs have reformed, but they reassociate with the NE only later in G1, concomitantly with the recruitment of the basket nucleoporin Mtor (the Drosophila orthologue of vertebrate Tpr). Surprisingly, Drosophila Nup107 shows no evidence of localization to kinetochores, despite the demonstrated importance of this association in mammalian cells.

Spatial and temporal coordination of mitosis by Ran GTPase

The small nuclear GTPase Ran controls the directionality of macromolecular transport between the nucleus and the cytoplasm. Ran also has important roles during mitosis, when the nucleus is dramatically reorganized to allow chromosome segregation. Ran directs the assembly of the mitotic spindle, nuclear-envelope dynamics and the timing of cell-cycle transitions. The mechanisms that underlie these functions provide insights into the spatial and temporal coordination of the changes that occur in intracellular organization during the cell-division cycle.

The RanGTP gradient - a GPS for the mitotic spindle

The GTPase Ran has a key role in nuclear import and export, mitotic spindle assembly and nuclear envelope formation. The cycling of Ran between its GTP- and GDP-bound forms is catalyzed by the chromatin-bound guanine nucleotide exchange factor RCC1 and the cytoplasmic Ran GTPase-activating protein RanGAP. The result is an intracellular concentration gradient of RanGTP that equips eukaryotic cells with a ;genome-positioning system' (GPS). The binding of RanGTP to nuclear transport receptors (NTRs) of the importin beta superfamily mediates the effects of the gradient and generates further downstream gradients, which have been elucidated by fluorescence resonance energy transfer (FRET) imaging and computational modeling. The Ran-dependent GPS spatially directs many functions required for genome segregation by the mitotic spindle during mitosis. Through exportin 1, RanGTP recruits essential centrosome and kinetochore components, whereas the RanGTP-induced release of spindle assembly factors (SAFs) from importins activates SAFs to nucleate, bind and organize nascent spindle microtubules. Although a considerable fraction of cytoplasmic SAFs is active and RanGTP induces only partial further activation near chromatin, bipolar spindle assembly is robustly induced by cooperativity and positive-feedback mechanisms within the network of Ran-activated SAFs. The RanGTP gradient is conserved, although its roles vary among different cell types and species, and much remains to be learned regarding its functions.

Localized RanGTP accumulation promotes microtubule nucleation at kinetochores in somatic mammalian cells

Centrosomes are the major sites for microtubule nucleation in mammalian cells, although both chromatin- and kinetochore-mediated microtubule nucleation have been observed during spindle assembly. As yet, it is still unclear whether these pathways are coregulated, and the molecular requirements for microtubule nucleation at kinetochore are not fully understood. This work demonstrates that kinetochores are initial sites for microtubule nucleation during spindle reassembly after nocodazole. This process requires local RanGTP accumulation concomitant with delocalization from kinetochores of the hydrolysis factor RanGAP1. Kinetochore-driven microtubule nucleation is also activated after cold-induced microtubule disassembly when centrosome nucleation is impaired, e.g., after Polo-like kinase 1 depletion, indicating that dominant centrosome activity normally masks the kinetochore-driven pathway. In cells with unperturbed centrosome nucleation, defective RanGAP1 recruitment at kinetochores after treatment with the Crm1 inhibitor leptomycin B activates kinetochore microtubule nucleation after cold. Finally, nascent microtubules associate with the RanGTP-regulated microtubule-stabilizing protein HURP in both cold- and nocodazole-treated cells. These data support a model for spindle assembly in which RanGTP-dependent abundance of nucleation/stabilization factors at centrosomes and kinetochores orchestrates the contribution of the two spindle assembly pathways in mammalian cells. The complex of RanGTP, the export receptor Crm1, and nuclear export signal-bearing proteins regulates microtubule nucleation at kinetochores.

Resolution of sister centromeres requires RanBP2-mediated SUMOylation of topoisomerase IIalpha

RanBP2 is a nucleoporin with SUMO E3 ligase activity that functions in both nucleocytoplasmic transport and mitosis. However, the biological relevance of RanBP2 and the in vivo targets of its E3 ligase activity are unknown. Here we show that animals with low amounts of RanBP2 develop severe aneuploidy in the absence of overt transport defects. The main chromosome segregation defect in cells from these mice is anaphase-bridge formation. Topoisomerase IIalpha (Topo IIalpha), which decatenates sister centromeres prior to anaphase onset to prevent bridges, fails to accumulate at inner centromeres when RanBP2 levels are low. We find that RanBP2 sumoylates Topo IIalpha in mitosis and that this modification is required for its proper localization to inner centromeres. Furthermore, mice with low amounts of RanBP2 are highly sensitive to tumor formation. Together, these data identify RanBP2 as a chromosomal instability gene that regulates Topo IIalpha by sumoylation and suppresses tumorigenesis.

Expression of the nuclear export protein chromosomal region maintenance/exportin 1/Xpo1 is a prognostic factor in human ovarian cancer

BACKGROUND

The human nuclear export protein chromosomal region maintenance/exportin 1/Xpo1 (CRM1) mediates the nuclear export of proteins and messenger RNAs and, thus, is an important regulator of subcellular distribution of key molecules. Whereas cell-biologic studies have suggested a fundamental role for CRM1 in the regulation of mitosis, the expression of this protein in human tumor tissue has not been investigated to date.

METHODS

In this study, the expression of CRM1 was analyzed in a cohort of 88 ovarian tumors and 12 ovarian cell lines for the first time to the authors' knowledge.

RESULTS

Immunohistochemistry revealed increased nuclear (52.7%) and cytoplasmic (56.8%) expression of CRM1 in 74 carcinomas compared with the expression revealed in borderline tumors and benign lesions. Similarly, CRM1 expression was increased in ovarian cancer cell lines compared with human ovarian surface epithelial cells. Cytoplasmic CRM1 expression was related significantly to advanced tumor stage (P= .043), poorly differentiated carcinomas (P= .011), and higher mitotic rate (P= .008). Nuclear CRM1 was associated significantly with cyclooxygenase-2 (COX-2) expression (P= .002) and poor overall survival (P= .01). Because it was demonstrated previously that blocking of CRM1 by leptomycin B (LMB) contributes to the inhibition of nuclear export, the authors used a set of mechanistic assays to study the effects of CRM1 inhibition in cancer cells. Treatment of OVCAR-3 cells with LMB revealed a significant reduction of cell proliferation and increased apoptosis as well as suppressed interleukin-1beta-induced COX-2 expression.

CONCLUSIONS

The current results indicated that CRM1 is expressed in a subpopulation of ovarian carcinomas with aggressive behavior and is related to poor patient outcome. A correlation also was demonstrated between CRM1 and COX-2 expression in ovarian cancer tissue. Furthermore, the treatment of ovarian cancer cells with LMB revealed a reduction in COX-2 expression. Therefore, the authors suggest that CRM1 may be an interesting biomarker for the assessment of patient prognosis and a molecular target for anticancer treatment.

Tumor cell dependence on Ran-GTP-directed mitosis

Deregulated cell division is a hallmark of cancer, but whether tumor cells become dependent on specific mitotic mechanisms is not known. Here, we show that the small GTPase Ran, a regulator of mitotic spindle formation, is differentially overexpressed in human cancer as compared with normal tissues, in vivo. Acute silencing of Ran in various tumor cell types causes aberrant mitotic spindle formation, mitochondrial dysfunction, and apoptosis. This pathway does not require p53, Bax, or Smac, but is controlled by survivin as a novel Ran target in cancer. Conversely, loss of Ran in normal cells is well tolerated and does not result in mitotic defects or loss of cell viability. Therefore, tumor cells can become dependent on Ran signaling for cell division, and targeting this pathway may provide a novel and selective anticancer strategy.

Ran-binding protein 3 phosphorylation links the Ras and PI3-kinase pathways to nucleocytoplasmic transport

The major participants of the Ras/ERK and PI3-kinase (PI3K) pathways are well characterized. The cellular response to activation of these pathways, however, can vary dramatically. How differences in signal strength, timing, spatial location, and cellular context promote specific cell-fate decisions remains unclear. Nuclear transport processes can have a major impact on the determination of cell fate; however, little is known regarding how nuclear transport is regulated by or regulates these pathways. Here we show that RSK and Akt, which are activated downstream of Ras/ERK and PI3K, respectively, modulate the Ran gradient and nuclear transport by interacting with, phosphorylating, and regulating Ran-binding protein 3 (RanBP3) function. Our findings highlight an important link between two major cell-fate determinants: nuclear transport and the Ras/ERK/RSK and PI3K/Akt signaling pathways.

RanBP2: a tumor suppressor with a new twist on TopoII, SUMO, and centromeres

In vertebrate cells, the small ubiquitin-like modifier SUMO plays a poorly defined role in targeting DNA topoisomerase II (TopoII) to centromeres (CENs) during mitosis, presumably to facilitate the untangling of sister chromatids as cells transition into anaphase. A new study by Dawlaty in the April 4 issue of Cell identifies the nucleoporin RanBP2 as a novel tumor suppressor that acts as a SUMO ligase for TopoII. Analysis of this interaction reveals TopoII recruitment to CENs is likely to play an important role in preventing chromosome segregation errors that lead to cancer.

Importin-beta and the small guanosine triphosphatase Ran mediate chromosome loading of the human chromokinesin Kid

Nucleocytoplasmic transport factors mediate various cellular processes, including nuclear transport, spindle assembly, and nuclear envelope/pore formation. In this paper, we identify the chromokinesin human kinesin-like DNA binding protein (hKid) as an import cargo of the importin-alpha/beta transport pathway and determine its nuclear localization signals (NLSs). Upon the loss of its functional NLSs, hKid exhibited reduced interactions with the mitotic chromosomes of living cells. In digitonin-permeabilized mitotic cells, hKid was bound only to the spindle and not to the chromosomes themselves. Surprisingly, hKid bound to importin-alpha/beta was efficiently targeted to mitotic chromosomes. The addition of Ran-guanosine diphosphate and an energy source, which generates Ran-guanosine triphosphate (GTP) locally at mitotic chromosomes, enhanced the importin-beta-mediated chromosome loading of hKid. Our results indicate that the association of importin-beta and -alpha with hKid triggers the initial targeting of hKid to mitotic chromosomes and that local Ran-GTP-mediated cargo release promotes the accumulation of hKid on chromosomes. Thus, this study demonstrates a novel nucleocytoplasmic transport factor-mediated mechanism for targeting proteins to mitotic chromosomes.

Emerging roles of the SUMO pathway in mitosis

SUMO proteins are small ubiquitin-like modifiers found in all eukaryotes that become covalently conjugated to other cellular proteins. The SUMO conjugation pathway is biochemically similar to ubiquitin conjugation, although the enzymes within the pathway act exclusively on SUMO proteins. This post-translational modification controls many processes. Here, I will focus on evidence that SUMOylation plays a critical role(s) in mitosis: Early studies showed a genetic requirement for SUMO pathway components in the process of cell division, while later findings implicated SUMOylation in the control of mitotic chromosome structure, cell cycle progression, kinetochore function and cytokinesis. Recent insights into the targets of SUMOylation are likely to be extremely helpful in understanding each of these aspects. Finally, growing evidence suggests that SUMOylation is a downstream target of regulation through Ran, a small GTPase with important functions in both interphase nuclear trafficking and mitotic spindle assembly.

Centrin 2 localizes to the vertebrate nuclear pore and plays a role in mRNA and protein export

Centrins in vertebrates have traditionally been associated with microtubule-nucleating centers such as the centrosome. Unexpectedly, we found centrin 2 to associate biochemically with nucleoporins, including the Xenopus laevis Nup107-160 complex, a critical subunit of the vertebrate nuclear pore in interphase and of the kinetochores and spindle poles in mitosis. Immunofluorescence of Xenopus cells and in vitro reconstituted nuclei indeed revealed centrin 2 localized at the nuclear pores. Use of the mild detergent digitonin in immunofluorescence also allowed centrin 2 to be clearly visualized at the nuclear pores of human cells. Disruption of nuclear pores using RNA interference of the pore assembly protein ELYS/MEL-28 resulted in a specific decrease of centrin 2 at the nuclear rim of HeLa cells. Functionally, excess expression of either the N- or C-terminal calcium-binding domains of human centrin 2 caused a dominant-negative effect on both mRNA and protein export, leaving protein import intact. The mRNA effect mirrors that found for the Saccharomyes cerevisiae centrin Cdc31p at the yeast nuclear pore, a role until now thought to be unique to yeast. We conclude that in vertebrates, centrin 2 interacts with major subunits of the nuclear pore, exhibits nuclear pore localization, and plays a functional role in multiple nuclear export pathways.

Mechanisms of mitotic spindle assembly and function

The mitotic spindle is the macromolecular machine that segregates chromosomes to two daughter cells during mitosis. The major structural elements of the spindle are microtubule polymers, whose intrinsic polarity and dynamic properties are critical for bipolar spindle organization and function. In most cell types, spindle microtubule nucleation occurs primarily at two centrosomes, which define the spindle poles, but microtubules can also be generated by the chromosomes and within the spindle itself. Many associated factors help organize the spindle, including molecular motors and regulators of microtubule dynamics. The past decade has provided a wealth of information on the molecular players that are critical for spindle assembly as well as a high-resolution view of the intricate movements and dynamics of the spindle microtubules and the chromosomes. In this chapter we provide a historical account of the key observations leading to current models of spindle assembly, as well as an up-to-date status report on this exciting field.

Biology and biophysics of the nuclear pore complex and its components

Nucleocytoplasmic exchange of proteins and ribonucleoprotein particles occurs via nuclear pore complexes (NPCs) that reside in the double membrane of the nuclear envelope (NE). Significant progress has been made during the past few years in obtaining better structural resolution of the three-dimensional architecture of NPC with the help of cryo-electron tomography and atomic structures of domains from nuclear pore proteins (nucleoporins). Biophysical and imaging approaches have helped elucidate how nucleoporins act as a selective barrier in nucleocytoplasmic transport. Nucleoporins act not only in trafficking of macromolecules but also in proper microtubule attachment to kinetochores, in the regulation of gene expression and signaling events associated with, for example, innate and adaptive immunity, development and neurodegenerative disorders. Recent research has also been focused on the dynamic processes of NPC assembly and disassembly that occur with each cell cycle. Here we review emerging results aimed at understanding the molecular arrangement of the NPC and how it is achieved, defining the roles of individual nucleoporins both at the NPC and at other sites within the cell, and finally deciphering how the NPC serves as both a barrier and a conduit of active transport.

The nucleoporin Nup358 associates with and regulates interphase microtubules

The nucleoporin Nup358 resides on the cytoplasmic face of the interphase nuclear pore complex (NPC). During metaphase, its recruitment to kinetochores is important for correct microtubule-kinetochore attachment. Here, we report that a fraction of endogenous Nup358 interacts with interphase microtubules through its N-terminal region (BPN). Cells overexpressing the microtubule targeting domain of Nup358 displayed dramatic alteration in the microtubule organization including increased microtubule bundling and stability. Ectopic expression of BPN and full-length Nup358 exhibited significantly higher levels of acetylated microtubules that were resistant to nocodazole, a microtubule depolymerizing agent. Furthermore, RNAi mediated depletion of Nup358 affected polarized stabilization of microtubules during directed cell migration, confirming the in vivo role of Nup358 in regulating interphase microtubules.

RANBP1 localizes a subset of mitotic regulatory factors on spindle microtubules and regulates chromosome segregation in human cells

The GTPase RAN has an established role in spindle assembly and in mitotic progression, although not all mechanisms are fully understood in somatic cells. Here, we have downregulated RAN-binding protein 1 (RANBP1), a RAN partner that has highest abundance in G2 and mitosis, in human cells. RANBP1-depleted cells underwent prolonged prometaphase delay often followed by apoptosis. Cells that remained viable assembled morphologically normal spindles; these spindles, however, were hyperstable and failed to recruit cyclin B1 or to restrict the localization of HURP (DLG7), a microtubule-stabilizing factor, to plus-ends. RANBP1 depletion did not increase the frequency of unattached chromosomes; however, RANBP1-depleted cells frequently showed lagging chromosomes in anaphase, suggesting that merotelic attachments form and are not efficiently resolved. These data indicate that RANBP1 activity is required for the proper localization of specific factors that regulate microtubule function; loss of this activity contributes to the generation of aneuploidy in a microtubule-dependent manner.

Brinkley-fest of mitosis

"Mitosis: Spindle Assembly and Function," a conference in honor of Dr. Bill R. Brinkley, brought together many researchers to discuss progress in the field and celebrate the many contributions that Dr. Brinkley has made.

Biochemical characterization of a family of proteins that stabilizes a plant Ran protein in its GTP-bound conformation

Ran-binding proteins (RanBP) are a group of proteins that bind to Ran (Ras-related nuclear small G-protein) and thus control the GTP/GDP-bound states of the Ran and couple the Ran GTPase cycle to cellular processes. In an effort to identify potential downstream effectors for PsRan1-dependent cellular processes, we detected a group of pea Ran (PsRan1)-binding proteins and characterized their biochemical activities. A Ran overlay assay using [(32)P-GTP]-labeled PsRan1 revealed three PsRan1-binding proteins (33, 45, and 85kDa in size) from total protein extracts of dark-grown pea plumules. These proteins bound preferentially to the Ran-GTP over Ran-GDP conformation and subsequently stabilized its GTP-bound status. We propose that they are a family of proteins that maintain the Ran protein in the active conformation and are potential downstream mediators for PsRan1-dependent cellular processes. Our report provides the basis for characterizing and dissecting Ran downstream targets and Ran-mediated events, and it thus facilitates our understanding about the roles played by Ran/RanBP signaling pathways during plant growth and development.

The Ndc80 complex: hub of kinetochore activity

Kinetochores are protein scaffolds coordinating the process of chromosome segregation in mitosis. Kinetochore components are organized in functionally and topologically distinct domains that are designed to connect the sister chromatids to the mitotic spindle. The inner kinetochore proteins are in direct contact with the centromeric DNA, whilst the outer kinetochore proteins are responsible for binding to spindle microtubules. The conserved Ndc80 complex is implicated in several essential outer kinetochore functions, including microtubule binding and control of a safety device known as the spindle assembly checkpoint. Here, we describe how current work is contributing to unravel the complex endeavors of this essential kinetochore complex.

Dynamic intracellular survivin in oral squamous cell carcinoma: underlying molecular mechanism and potential as an early prognostic marker

Survivin functions as an apoptosis inhibitor and a regulator of cell division in many tumours. The intracellular localization of survivin in tumours has been suggested as a prognostic marker. However, current reports are inconsistent and the underlying molecular mechanisms are not understood. The present study has examined the localization and prognostic value of nuclear and cytoplasmic survivin in the pre-therapeutic biopsies from 71 oral and oropharyngeal squamous carcinoma (OSCC) patients. Statistical analysis indicated that preferential nuclear versus cytoplasmic survivin correlated with favourable versus unfavourable disease outcome. Uni- and multi-variate analysis showed that in contrast to total survivin expression, the difference between nuclear and cytoplasmic survivin was a strong predictor for relapse-free survival (p=0.0003). As a potential underlying molecular mechanism, it is shown in OSCC cell lines that predominantly cytoplasmic survivin mediates protection against chemo- and radio-therapy-induced apoptosis. Importantly, the cytoplasmic localization of survivin is regulated by its nuclear export signal (NES), and export-deficient nuclear survivin is not cytoprotective. This study suggests that the difference between cytoplasmic and nuclear survivin is an indicator for survivin activity in tumour cells. Thus, this difference may serve as a predictive marker of outcome in OSCC patients undergoing multi-modality therapy. The pharmacogenetic interference with survivin's cytoplasmic localization is also to be pursued as a potential therapeutic strategy.

CRM1-mediated nuclear export: to the pore and beyond

CRM1 (chromosome region maintenance 1; also referred to as exportin1 or Xpo1) is a member of the importin beta superfamily of nuclear transport receptors, recognizing proteins bearing a leucine-rich nuclear export sequence. CRM1 is the major receptor for the export of proteins out of the nucleus and is also required for transport of many RNAs. Besides its established role in nuclear export, CRM1 is also implicated in various steps during mitosis, widening its functional spectrum within the cell.

The human Nup107-160 nuclear pore subcomplex contributes to proper kinetochore functions

We previously demonstrated that a fraction of the human Nup107-160 nuclear pore subcomplex is recruited to kinetochores at the onset of mitosis. However, the molecular determinants for its kinetochore targeting and the functional significance of this localization were not investigated. Here, we show that the Nup107-160 complex interacts with CENP-F, but that CENP-F only moderately contributes to its targeting to kinetochores. In addition, we show that the recruitment of the Nup107-160 complex to kinetochores mainly depends on the Ndc80 complex. We further demonstrate that efficient depletion of the Nup107-160 complex from kinetochores, achieved either by combining siRNAs targeting several of its subunits excluding Seh1, or by depleting Seh1 alone, induces a mitotic delay. Further analysis of Seh1-depleted cells revealed impaired chromosome congression, reduced kinetochore tension and kinetochore-microtubule attachment defects. Finally, we show that the presence of the Nup107-160 complex at kinetochores is required for the recruitment of Crm1 and RanGAP1-RanBP2 to these structures. Together, our data thus provide the first molecular clues underlying the function of the human Nup107-160 complex at kinetochores.

Nuclear export is essential for the tumor-promoting activity of survivin

Survivin appears to function as an apoptosis inhibitor and a regulator of cell division during development and tumorigenesis. Here we report the molecular characterization of the nucleocytoplasmic transport of survivin and its potential implications for tumorigenesis. We identified an evolutionary conserved Crm1-dependent nuclear export signal (NES) in survivin. In dividing cells, the NES is essential for tethering survivin and the survivin/Aurora-B kinase complex to the mitotic machinery, which in turn appears to be essential for proper cell division. In addition, export seems to be required for the cytoprotective activity of survivin, as export-deficient survivin fails to protect tumor cells against chemo- and radiotherapy-induced apoptosis. These findings appear to be clinically relevant since preferential nuclear localization of survivin correlated with enhanced survival in colorectal cancer patients. Targeting survivin's nuclear export by the application of NES-specific antibodies promoted its nuclear accumulation and inhibited its cytoprotective function. We demonstrate that nuclear export is essential for the biological activity of survivin and promote the identification of molecular decoys to specifically interfere with survivin's nuclear export as potential anticancer therapeutics.

The karyopherin Kap95 regulates nuclear pore complex assembly into intact nuclear envelopes in vivo

Nuclear pore complex (NPC) assembly in interphase cells requires that new NPCs insert into an intact nuclear envelope (NE). Our previous work identified the Ran GTPase as an essential component in this process. We proposed that Ran is required for targeting assembly factors to the cytoplasmic NE face via a novel, vesicular intermediate. Although the molecular target was not identified, Ran is known to function by modulating protein interactions for karyopherin (Kap) beta family members. Here we characterize loss-of-function Saccharomyces cerevisiae mutants in KAP95 with blocks in NPC assembly. Similar to defects in Ran cycle mutants, nuclear pore proteins are no longer localized properly to the NE in kap95 mutants. Also like Ran cycle mutants, the kap95-E126K mutant displayed enhanced lethality with nic96 and nup170 mutants. Thus, Kap95 and Ran are likely functioning at the same stage in assembly. However, although Ran cycle mutants accumulate small cytoplasmic vesicles, cells depleted of Kap95 accumulated long stretches of cytoplasmic membranes and had highly distorted NEs. We conclude that Kap95 serves as a key regulator of NPC assembly into intact NEs. Furthermore, both Kap95 and Ran may provide spatial cues necessary for targeting of vesicular intermediates in de novo NPC assembly.

The Survivin-Crm1 interaction is essential for chromosomal passenger complex localization and function

The chromosomal passenger complex (CPC) of Aurora-B, Borealin, INCENP (inner centromere protein) and Survivin coordinates essential chromosomal and cytoskeletal events during mitosis. Here, we show that the nuclear export receptor Crm1 is crucially involved in tethering the CPC to the centromere by interacting with a leucine-rich nuclear export signal (NES), evolutionarily conserved in all mammalian Survivin proteins. We show that inhibition of the Survivin-Crm1 interaction by treatment with leptomycin B or by RNA-interference-mediated Crm1 depletion prevents centromeric targeting of Survivin. The genetic inactivation of the Survivin-Crm1 interaction by mutation of the NES affects the correct localization and function of Survivin and the CPC during mitosis. By contrast, CPC assembly does not seem to require the Survivin-Crm1 interaction. Our report shows the functional significance of the Survivin-Crm1 interface and provides a novel link between the mitotic effector Crm1 and the CPC.

Nucleocytoplasmic shuttling and the biological activity of mouse survivin are regulated by an active nuclear export signal

Survivin appears to function as a regulator of cell division and as an apoptosis inhibitor in many species. Here, we characterized the nucleocytoplasmic transport of mouse survivin(140), and its splice variants survivin(121) and survivin(40). We show that the dynamic intracellular localization of survivin(140) is mediated by a Crm1-dependent nuclear export signal (NES) present also in survivin(121), but absent in survivin(40). In contrast, neither survivin nor survivin splice variants contain an active nuclear import signal and seem to enter the nucleus by passive diffusion. The activity of the NES is required for survivin-mediated protection against cell death inducing stimuli and influences protein degradation. During mitosis, NES-deficient survivin variants fail to correctly localize to the mitotic machinery and promote proper cell division. In vivo and in vitro protein interaction assays show that survivin(140) and survivin(121) as well as their export-deficient mutants are able to form homo- as well as heterodimers. The trans-dominant negative phenotype observed upon expression of export-deficient survivin appears, therefore, to be mediated by the formation of inactive survivin heterodimers. The survivin-Crm1 axis is essential for the biological activities of murine survivin, and mouse models will allow investigating its functional implications during development and tumorigenesis.

Ran at kinetochores

The Ran GTPase controls many cellular functions, including nucleocytoplasmic trafficking, spindle assembly, nuclear assembly and cell-cycle progression. Considerable evidence suggests that diffusible Ran-GTP near mitotic chromatin facilitates the release of critical factors from nuclear transport receptors, thereby promoting organization of mitotic spindles with respect to chromosomes. In addition to this role of soluble Ran-GTP, Ran has two important but less understood roles at mitotic kinetochores. Namely, it is essential for regulation of the spindle assembly checkpoint and for assembly of microtubule fibres that attach kinetochores to spindle poles. Here, I will briefly summarize evidence for these kinetochore-associated functions and mention some of the issues that remain to be addressed regarding them.

HTLV-1 Tax: centrosome amplification and cancer

During interphase, each cell contains a single centrosome that acts as a microtubule organizing center for cellular functions in interphase and in mitosis. Centrosome amplification during the S phase of the cell cycle is a tightly regulated process to ensure that each daughter cell receives the proper complement of the genome. The controls that ensure that centrosomes are duplicated exactly once in the cell cycle are not well understood. In solid tumors and hematological malignancies, centrosome abnormalities resulting in aneuploidy is observed in the majority of cancers. These phenotypes are also observed in cancers induced by viruses, including adult T cell lymphoma which is caused by the human T cell lymphotrophic virus Type 1 (HTLV-1). Several reports have indicated that the HTLV-1 transactivator, Tax, is directly responsible for the centrosomal abnormalities observed in ATL cells. A recent paper in Nature Cell Biology by Ching et al. has shed some new light into how Tax may be inducing centrosome abnormalities. The authors demonstrated that 30% of ATL cells contained more than two centrosomes and expression of Tax alone induced supernumerary centrosomes. A cellular coiled-coil protein, Tax1BP2, was shown to interact with Tax and disruption of this interaction led to failure of Tax to induce centrosome amplification. Additionally, down-regulation of Tax1BP2 led to centrosome amplification. These results suggest that Tax1BP2 may be an important block to centrosome re-duplication that is observed in normal cells. Presently, a specific cellular protein that prevents centrosome re-duplication has not been identified. This paper has provided further insight into how Tax induces centrosome abnormalities that lead to ATL. Lastly, additional work on Tax1BP2 will also provide insight into how the cell suppresses centrosome re-duplication during the cell cycle and the role that Tax1BP2 plays in this important cellular pathway.

HURP is a Ran-importin beta-regulated protein that stabilizes kinetochore microtubules in the vicinity of chromosomes

BACKGROUND

Formation of a bipolar mitotic spindle in somatic cells requires the cooperation of two assembly pathways, one based on kinetochore capture by centrosomal microtubules, the other on RanGTP-mediated microtubule organization in the vicinity of chromosomes. How RanGTP regulates kinetochore-microtubule (K-fiber) formation is not presently understood.

RESULTS

Here we identify the mitotic spindle protein HURP as a novel target of RanGTP. We show that HURP is a direct cargo of importin beta and that in interphase cells, it shuttles between cytoplasm and nucleus. During mitosis, HURP localizes predominantly to kinetochore microtubules in the vicinity of chromosomes. Overexpression of importin beta or RanT24N (resulting in low RanGTP) negatively regulates its spindle localization, whereas overexpression of RanQ69L (mimicking high RanGTP) enhances HURP association with the spindle. Thus, RanGTP levels control HURP localization to the mitotic spindle in vivo, a conclusion supported by the analysis of tsBN2 cells (mutant in RCC1). Upon depletion of HURP, K-fiber stabilization is impaired and chromosome congression is delayed. Nevertheless, cells eventually align their chromosomes, progress into anaphase, and exit mitosis. HURP is able to bundle microtubules and, in vitro, this function is abolished upon complex formation with importin beta and regulated by Ran. These data indicate that HURP stabilizes K-fibers by virtue of its ability to bind and bundle microtubules.

CONCLUSIONS

Our study identifies HURP as a novel component of the Ran-importin beta-regulated spindle assembly pathway, supporting the conclusion that K-fiber formation and stabilization involves both the centrosome-dependent microtubule search and capture mechanism and the RanGTP pathway.

The Nup107-160 nucleoporin complex is required for correct bipolar spindle assembly

The Nup107-160 complex is a critical subunit of the nuclear pore. This complex localizes to kinetochores in mitotic mammalian cells, where its function is unknown. To examine Nup107-160 complex recruitment to kinetochores, we stained human cells with antisera to four complex components. Each antibody stained not only kinetochores but also prometaphase spindle poles and proximal spindle fibers, mirroring the dual prometaphase localization of the spindle checkpoint proteins Mad1, Mad2, Bub3, and Cdc20. Indeed, expanded crescents of the Nup107-160 complex encircled unattached kinetochores, similar to the hyperaccumulation observed of dynamic outer kinetochore checkpoint proteins and motors at unattached kinetochores. In mitotic Xenopus egg extracts, the Nup107-160 complex localized throughout reconstituted spindles. When the Nup107-160 complex was depleted from extracts, the spindle checkpoint remained intact, but spindle assembly was rendered strikingly defective. Microtubule nucleation around sperm centrosomes seemed normal, but the microtubules quickly disassembled, leaving largely unattached sperm chromatin. Notably, Ran-GTP caused normal assembly of microtubule asters in depleted extracts, indicating that this defect was upstream of Ran or independent of it. We conclude that the Nup107-160 complex is dynamic in mitosis and that it promotes spindle assembly in a manner that is distinct from its functions at interphase nuclear pores.

Stimulated expression of mRNAs in activated T cells depends on a functional CRM1 nuclear export pathway

In metazoans, the nuclear export of bulk mRNAs is mediated by the export receptor TAP, together with its binding partner p15. A number of viral mRNAs, including the unspliced and partially spliced mRNA species of the human immunodeficiency virus (HIV), however, use an alternative export route via the importin beta-related export receptor CRM1. This raises the question of whether a subset of cellular mRNAs might be exported by CRM1 as well. To identify such mRNAs, we performed a systematic screen in different cell lines, using representational difference analyses of cDNA (cDNA-RDA). In HeLa and Cl-4 cells no cellular transcripts could be identified as exported via CRM1. In contrast, we found a number of CRM1-dependent mRNAs in Jurkat T cells, most of which are induced during a T cell response. One of the identified gene products, the dendritic cell marker CD83, was analyzed in detail. CD83 expression depends on a functional CRM1 pathway in activated Jurkat T cells as well as in a heterologous expression system, independent of activation. Our results point to an important role of the CRM1-dependent export pathway for the expression of CD83 and other genes under conditions of T cell activation.

Analysis of a RanGTP-regulated gradient in mitotic somatic cells

The RanGTPase cycle provides directionality to nucleocytoplasmic transport, regulating interactions between cargoes and nuclear transport receptors of the importin-beta family. The Ran-importin-beta system also functions in mitotic spindle assembly and nuclear pore and nuclear envelope formation. The common principle underlying these diverse functions throughout the cell cycle is thought to be anisotropy of the distribution of RanGTP (the RanGTP gradient), driven by the chromatin-associated guanine nucleotide exchange factor RCC1 (refs 1, 4, 5). However, the existence and function of a RanGTP gradient during mitosis in cells is unclear. Here we examine the Ran-importin-beta system in cells by conventional and fluorescence lifetime microscopy using a biosensor, termed Rango, that increases its fluorescence resonance energy transfer signal when released from importin-beta by RanGTP. Rango is predominantly free in mitotic cells, but is further liberated around mitotic chromatin. In vitro experiments and modelling show that this localized increase of free cargoes corresponds to changes in RanGTP concentration sufficient to stabilize microtubules in extracts. In cells, the Ran-importin-beta-cargo gradient kinetically promotes spindle formation but is largely dispensable once the spindle has been established. Consistent with previous reports, we observe that the Ran system also affects spindle pole formation and chromosome congression in vivo. Our results demonstrate that conserved Ran-regulated pathways are involved in multiple, parallel processes required for spindle function, but that their relative contribution differs in chromatin- versus centrosome/kinetochore-driven spindle assembly systems.

Subcellular localization and nucleocytoplasmic transport of the chromosomal passenger proteins before nuclear envelope breakdown

As mitosis progresses, the chromosomal passenger proteins (CPPs) Survivin, Aurora B, INCENP and Borealin dynamically colocalize to mitotic structures. Chromosomal passenger proteins are already expressed during G2, whereas the nuclear envelope is only disassembled at the end of prophase. However, the mechanisms that modulate their nucleocytoplasmic localization before nuclear envelope breakdown (NEB) are poorly characterized. Using epitope-tagged proteins, we show that Aurora B, like Survivin, undergoes CRM1-mediated nucleocytoplasmic shuttling, although both proteins lack identifiable 'classical' nuclear transport signals. On the other hand, INCENP resides more stably in the nucleus and contains multiple nuclear localization signals. Finally, Borealin was detected in the nucleolus and the cytoplasm, but its cytoplasmic localization is not directly regulated by CRM1. Coexpression experiments indicate that the nuclear localization of Aurora B, but not of Survivin, is modulated by INCENP and that Survivin prevents the nucleolar accumulation of Borealin. Our data reveal that, in contrast to their closely related localization during mitosis, the nucleocytoplasmic localization of the CPPs before NEB is largely unrelated. Furthermore, the specific effect of INCENP and Survivin on the localization of Aurora B and Borealin, respectively, suggests that different complexes of CPPs may exist not only during mitosis, as recently reported, but also before NEB.

Ran is required before metaphase for spindle assembly and chromosome alignment and after metaphase for chromosome segregation and spindle midbody organization

The Ran pathway has been shown to have a role in spindle assembly. However, the extent of the role of the Ran pathway in mitosis in vivo is unclear. We report that perturbation of the Ran pathway disrupted multiple steps of mitosis in syncytial Drosophila embryos and uncovered new mitotic processes that are regulated by Ran. During the onset of mitosis, the Ran pathway is required for the production, organization, and targeting of centrosomally nucleated microtubules to chromosomes. However, the role of Ran is not restricted to microtubule organization, because Ran is also required for the alignment of chromosomes at the metaphase plate. In addition, the Ran pathway is required for postmetaphase events, including chromosome segregation and the assembly of the microtubule midbody. The Ran pathway mediates these mitotic events, in part, by facilitating the correct targeting of the kinase Aurora A and the kinesins KLP61F and KLP3A to spindles.

The nuclear envelope: form and reformation

The membrane system that encloses genomic DNA is referred to as the nuclear envelope. However, with emerging roles in signaling and gene expression, these membranes clearly serve as more than just a physical barrier separating the nucleus and cytoplasm. Recent progress in our understanding of nuclear envelope architecture and composition has also revealed an intriguing connection between constituents of the nuclear envelope and human disease, providing further impetus to decipher this cellular structure and the dramatic remodeling process it undergoes with each cell division.