Ran-GTP coordinates regulation of microtubule nucleation and dynamics during mitotic-spindle assembly

Nucleocytoplasmic transport rates are regulated by cellular processes that modulate GTP availability

Nucleocytoplasmic transport (NCT), the facilitated diffusion of cargo molecules between the nucleus and cytoplasm through nuclear pore complexes (NPCs), enables numerous fundamental eukaryotic cellular processes. Ran GTPase uses cellular energy in the direct form of GTP to create a gradient across the nuclear envelope (NE) that drives the majority of NCT. We report here that changes in GTP availability resulting from altered cellular physiology modulate the rate of NCT, as monitored using synthetic and natural cargo, and the dynamics of Ran itself. Cell migration, cell spreading, and/or modulation of the cytoskeleton or its connection to the nucleus alter GTP availability and thus rates of NCT, regulating RNA export and protein synthesis. These findings support a model in which changes in cellular physiology that alter GTP availability can regulate the rate of NCT, impacting fundamental cellular processes that extensively utilize NCT.

Nucleocytoplasmic transport rates are regulated by cellular processes that modulate GTP availability

Nucleocytoplasmic transport (NCT), the facilitated diffusion of cargo molecules between the nucleus and cytoplasm through nuclear pore complexes (NPCs), enables numerous fundamental eukaryotic cellular processes. Ran GTPase uses cellular energy in the direct form of GTP to create a gradient across the nuclear envelope (NE) that drives the majority of NCT. We report here that changes in GTP availability resulting from altered cellular physiology modulate the rate of NCT, as monitored using synthetic and natural cargo, and the dynamics of Ran itself. Cell migration, cell spreading and/or modulation of the cytoskeleton or its connection to the nucleus alter GTP availability and thus rates of NCT, regulating RNA export and protein synthesis. These findings support a model in which changes in cellular physiology that alter GTP availability can regulate the rate of NCT, impacting fundamental cellular processes that extensively utilize NCT.

Targeting small GTPases: emerging grasps on previously untamable targets, pioneered by KRAS

Small GTPases including Ras, Rho, Rab, Arf, and Ran are omnipresent molecular switches in regulating key cellular functions. Their dysregulation is a therapeutic target for tumors, neurodegeneration, cardiomyopathies, and infection. However, small GTPases have been historically recognized as "undruggable". Targeting KRAS, one of the most frequently mutated oncogenes, has only come into reality in the last decade due to the development of breakthrough strategies such as fragment-based screening, covalent ligands, macromolecule inhibitors, and PROTACs. Two KRAS^G12C covalent inhibitors have obtained accelerated approval for treating KRAS^G12C mutant lung cancer, and allele-specific hotspot mutations on G12D/S/R have been demonstrated as viable targets. New methods of targeting KRAS are quickly evolving, including transcription, immunogenic neoepitopes, and combinatory targeting with immunotherapy. Nevertheless, the vast majority of small GTPases and hotspot mutations remain elusive, and clinical resistance to G12C inhibitors poses new challenges. In this article, we summarize diversified biological functions, shared structural properties, and complex regulatory mechanisms of small GTPases and their relationships with human diseases. Furthermore, we review the status of drug discovery for targeting small GTPases and the most recent strategic progress focused on targeting KRAS. The discovery of new regulatory mechanisms and development of targeting approaches will together promote drug discovery for small GTPases.

Morphological growth dynamics, mechanical stability, and active microtubule mechanics underlying spindle self-organization

The spindle is a dynamic intracellular structure self-organized from microtubules and microtubule-associated proteins. The spindle’s bipolar morphology is essential for the faithful segregation of chromosomes during cell division, and it is robustly maintained by multifaceted mechanisms. However, abnormally shaped spindles, such as multipolar spindles, can stochastically arise in a cell population and cause chromosome segregation errors. The physical basis of how microtubules fail in bipolarization and occasionally favor nonbipolar assembly is poorly understood. Here, using live fluorescence imaging and quantitative shape analysis in Xenopus egg extracts, we find that spindles of varied shape morphologies emerge through nonrandom, bistable self-organization paths, one leading to a bipolar and the other leading to a multipolar phenotype. The bistability defines the spindle’s unique morphological growth dynamics linked to each shape phenotype and can be promoted by a locally distorted microtubule flow that arises within premature structures. We also find that bipolar and multipolar spindles are stable at the steady-state in bulk but can infrequently switch between the two phenotypes. Our microneedle-based physical manipulation further demonstrates that a transient force perturbation applied near the assembled pole can trigger the phenotypic switching, revealing the mechanical plasticity of the spindle. Together with molecular perturbation of kinesin-5 and augmin, our data propose the physical and molecular bases underlying the emergence of spindle-shape variation, which influences chromosome segregation fidelity during cell division.

Dual control of formin-nucleated actin assembly by the chromatin and ER in mouse oocytes

The first asymmetric meiotic cell divisions in mouse oocytes are driven by formin 2 (FMN2)-nucleated actin polymerization around the spindle. In this study, we investigated how FMN2 is recruited to the spindle peripheral ER and how its activity is regulated in mouse meiosis I (MI) oocytes. We show that this process is regulated by the Ran GTPase, a conserved mediator of chromatin signal, and the ER-associated protein VAPA. FMN2 contains a nuclear localization sequence (NLS) within a domain (SLD) previously shown to be required for FMN2 localization to the spindle periphery. FMN2 NLS is bound to the importin α1/β complex, and the disruption of this interaction by RanGTP is required for FMN2 accumulation in the area proximal to the chromatin and the MI spindle. The importin-free FMN2 is then recruited to the surface of ER around the spindle through the binding of the SLD with the ER-membrane protein VAPA. We further show that FMN2 is autoinhibited through an intramolecular interaction between the SLD with the C-terminal formin homology 2 (FH2) domain that nucleates actin filaments. VAPA binding to SLD relieves the autoinhibition of FMN2, leading to localized actin polymerization. This dual control of formin-mediated actin assembly allows actin polymerization to initiate the movement of the meiotic spindle toward the cortex, an essential step in the maturation of the mammalian female gamete.

Non-centrosomal microtubules at kinetochores promote rapid chromosome biorientation during mitosis in human cells

Proper segregation of chromosomes during mitosis depends on "amphitelic attachments"-load-bearing connections of sister kinetochores to the opposite spindle poles via bundles of microtubules, termed as the "K-fibers." Current models of spindle assembly assume that K-fibers arise largely from stochastic capture of microtubules, which occurs at random times and locations and independently at sister kinetochores. We test this assumption by following the movements of all kinetochores in human cells and determine that most amphitelic attachments form synchronously at a specific stage of spindle assembly and within a spatially distinct domain. This biorientation domain is enriched in bundles of antiparallel microtubules, and perturbation of microtubule bundling changes the temporal and spatial dynamics of amphitelic attachment formation. Structural analyses indicate that interactions of kinetochores with microtubule bundles are mediated by non-centrosomal short microtubules that emanate from most kinetochores during early prometaphase. Computational analyses suggest that momentous molecular motor-driven interactions with antiparallel bundles rapidly convert these short microtubules into nascent K-fibers. Thus, load-bearing connections to the opposite spindle poles form simultaneously on sister kinetochores. In contrast to the uncoordinated sequential attachments of sister kinetochores expected in stochastic models of spindle assembly, our model envisions the formation of amphitelic attachments as a deterministic process in which the chromosomes connect with the spindle poles synchronously at a specific stage of spindle assembly and at a defined location determined by the spindle architecture. Experimental analyses of changes in the kinetochore behavior in cells with perturbed activity of molecular motors CenpE and dynein confirm the predictive power of the model.

A celebration of the 25th anniversary of chromatin-mediated spindle assembly

Formation of a bipolar spindle is required for the faithful segregation of chromosomes during cell division. Twenty-five years ago, a transformative insight into how bipolarity is achieved was provided by Rebecca Heald, Eric Karsenti, and colleagues in their landmark publication characterizing a chromatin-mediated spindle assembly pathway in which centrosomes and kinetochores were dispensable. The discovery revealed that bipolar spindle assembly is a self-organizing process where microtubules, which possess an intrinsic polarity, polymerize around chromatin and become sorted by mitotic motors into a bipolar structure. On the 25^th anniversary of this seminal paper, we discuss what was known before, what we have learned since, and what may lie ahead in understanding the bipolar spindle.

Primary Cilia and Centrosomes in Neocortex Development

During mammalian brain development, neural stem and progenitor cells generate the neurons for the six-layered neocortex. The proliferative capacity of the different types of progenitor cells within the germinal zones of the developing neocortex is a major determinant for the number of neurons generated. Furthermore, the various modes of progenitor cell divisions, for which the orientation of the mitotic spindle of progenitor cells has a pivotal role, are a key parameter to ensure the appropriate size and proper cytoarchitecture of the neocortex. Here, we review the roles of primary cilia and centrosomes of progenitor cells in these processes during neocortical development. We specifically focus on the apical progenitor cells in the ventricular zone. In particular, we address the alternating, dual role of the mother centriole (i) as a component of one of the spindle poles during mitosis, and (ii) as the basal body of the primary cilium in interphase, which is pivotal for the fate of apical progenitor cells and their proliferative capacity. We also discuss the interactions of these organelles with the microtubule and actin cytoskeleton, and with junctional complexes. Centriolar appendages have a specific role in this interaction with the cell cortex and the plasma membrane. Another topic of this review is the specific molecular composition of the ciliary membrane and the membrane vesicle traffic to the primary cilium of apical progenitors, which underlie the ciliary signaling during neocortical development; this signaling itself, however, is not covered in depth here. We also discuss the recently emerging evidence regarding the composition and roles of primary cilia and centrosomes in basal progenitors, a class of progenitors thought to be of particular importance for neocortex expansion in development and evolution. While the tight interplay between primary cilia and centrosomes makes it difficult to allocate independent roles to either organelle, mutations in genes encoding ciliary and/or centrosome proteins indicate that both are necessary for the formation of a properly sized and functioning neocortex during development. Human neocortical malformations, like microcephaly, underpin the importance of primary cilia/centrosome-related processes in neocortical development and provide fundamental insight into the underlying mechanisms involved.

Multiple pools of PP2A regulate spindle assembly, kinetochore attachments and cohesion in Drosophila oocytes

Meiosis in female oocytes lacks centrosomes, the microtubule-organizing centers. In Drosophila oocytes, meiotic spindle assembly depends on the chromosomal passenger complex (CPC). To investigate the mechanisms that regulate Aurora B activity, we examined the role of protein phosphatase 2A (PP2A) in Drosophila oocyte meiosis. We found that both forms of PP2A, B55 and B56, antagonize the Aurora B spindle assembly function, suggesting that a balance between Aurora B and PP2A activity maintains the oocyte spindle during meiosis I. PP2A-B56, which has a B subunit encoded by two partially redundant paralogs, wdb and wrd, is also required for maintenance of sister chromatid cohesion, establishment of end-on microtubule attachments, and metaphase I arrest in oocytes. WDB recruitment to the centromeres depends on BUBR1, MEI-S332 and kinetochore protein SPC105R. Although BUBR1 stabilizes microtubule attachments in Drosophila oocytes, it is not required for cohesion maintenance during meiosis I. We propose at least three populations of PP2A-B56 regulate meiosis, two of which depend on SPC105R and a third that is associated with the spindle.

Confinement size determines the architecture of Ran-induced microtubule networks

The organization of microtubules (MTs) is critical for cells during interphase and mitosis. During mitotic spindle assembly, MTs are made and organized around chromosomes in a process regulated by RanGTP. The role of RanGTP has been explored in Xenopus egg extracts, which are not limited by a cell membrane. Here, we investigated whether cell-sized confinements affect the assembly of RanGTP-induced MT networks in Xenopus egg extracts. We used microfluidics to encapsulate extracts within monodisperse extract-in-oil droplets. Importantly, we find that the architecture of Ran-induced MT networks depends on the droplet diameter and the Ran concentration, and differs from structures formed in bulk extracts. Our results highlight that both MT nucleation and physical confinement play critical roles in determining the spatial organization of the MT cytoskeleton.

Mitotic poleward flux: Finding balance between microtubule dynamics and sliding

Continuous poleward motion of microtubules in metazoan mitotic spindles has been fascinating generations of cell biologists over the last several decades. In human cells, this so-called poleward flux was recently shown to be driven by the coordinated action of four mitotic kinesins. The sliding activities of kinesin-5/EG5 and kinesin-12/KIF15 are sequentially supported by kinesin-7/CENP-E at kinetochores and kinesin-4/KIF4A on chromosome arms, with the individual contributions peaking during prometaphase and metaphase, respectively. Although recent data elucidate the molecular mechanism underlying this cellular phenomenon, the functional roles of microtubule poleward flux during cell division remain largely elusive. Here, we discuss potential contribution of microtubule flux engine to various essential processes at different stages of mitosis.

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.

The metaphase spindle at steady state - Mechanism and functions of microtubule poleward flux

The mitotic spindle is a bipolar cellular structure, built from tubulin polymers, called microtubules, and interacting proteins. This macromolecular machine orchestrates chromosome segregation, thereby ensuring accurate distribution of genetic material into the two daughter cells during cell division. Powered by GTP hydrolysis upon tubulin polymerization, the microtubule ends exhibit a metastable behavior known as the dynamic instability, during which they stochastically switch between the growth and shrinkage phases. In the context of the mitotic spindle, dynamic instability is furthermore regulated by microtubule-associated proteins and motor proteins, which enables the spindle to undergo profound changes during mitosis. This highly dynamic behavior is essential for chromosome capture and congression in prometaphase, as well as for chromosome alignment to the spindle equator in metaphase and their segregation in anaphase. In this review we focus on the mechanisms underlying microtubule dynamics and sliding and their importance for the maintenance of shape, structure and dynamics of the metaphase spindle. We discuss how these spindle properties are related to the phenomenon of microtubule poleward flux, highlighting its highly cooperative molecular basis and role in keeping the metaphase spindle at a steady state.

Role of spatial patterns and kinetochore architecture in spindle morphogenesis

Mitotic spindle is a self-assembling macromolecular machine responsible for the faithful segregation of chromosomes during cell division. Assembly of the spindle is believed to be governed by the 'Search & Capture' (S&C) principle in which dynamic microtubules explore space in search of kinetochores while the latter capture microtubules and thus connect chromosomes to the spindle. Due to the stochastic nature of the encounters between kinetochores and microtubules, the time required for incorporating all chromosomes into the spindle is profoundly affected by geometric constraints, such as the size and shape of kinetochores as well as their distribution in space at the onset of spindle assembly. In recent years, several molecular mechanisms that control these parameters have been discovered. It is now clear that stochastic S&C takes place in structured space, where components are optimally distributed and oriented to minimize steric hindrances. Nucleation of numerous non-centrosomal microtubules near kinetochores accelerates capture, while changes in the kinetochore architecture at various stages of spindle assembly promote proper connection of sister kinetochores to the opposite spindle poles. Here we discuss how the concerted action of multiple facilitating mechanisms ensure that the spindle assembles rapidly yet with a minimal number of errors.

Ca2+-binding protein from Entamoeba histolytica (EhCaBP6) is a novel GTPase

GTPases are molecular switches, which regulate a variety of cellular processes such as cell polarity, gene transcription, microtubule dynamics, cell-cycle etc. In this paper, we characterize a Ca²⁺-binding protein from Entamoeba histolytica (EhCaBP6) as a novel GTPase. We locate the active site for GTP hydrolysis within the C-terminal domain of EhCaBP6, although it requires full length protein for its complete range of activity. Using NMR studies, we observe that GTP binding induces conformational change in EhCaBP6. The identification of this novel and unusual Ca²⁺-dependent GTPase is important to elucidate the unconventional cell cycle of E. histolytica.

Tight junction protein occludin regulates progenitor Self-Renewal and survival in developing cortex

Occludin (OCLN) mutations cause human microcephaly and cortical malformation. A tight junction component thought absent in neuroepithelium after neural tube closure, OCLN isoform-specific expression extends into corticogenesis. Full-length and truncated isoforms localize to neuroprogenitor centrosomes, but full-length OCLN transiently localizes to plasma membranes while only truncated OCLN continues at centrosomes throughout neurogenesis. Mimicking human mutations, full-length OCLN depletion in mouse and in human CRISPR/Cas9-edited organoids produce early neuronal differentiation, reduced progenitor self-renewal and increased apoptosis. Human neural progenitors were more severely affected, especially outer radial glial cells, which mouse embryonic cortex lacks. Rodent and human mutant progenitors displayed reduced proliferation and prolonged M-phase. OCLN interacted with mitotic spindle regulators, NuMA and RAN, while full-length OCLN loss impaired spindle pole morphology, astral and mitotic microtubule integrity. Thus, early corticogenesis requires full-length OCLN to regulate centrosome organization and dynamics, revealing a novel role for this tight junction protein in early brain development.

Structural genomics applied to the rust fungus Melampsora larici-populina reveals two candidate effector proteins adopting cystine knot and NTF2-like protein folds

Rust fungi are plant pathogens that secrete an arsenal of effector proteins interfering with plant functions and promoting parasitic infection. Effectors are often species-specific, evolve rapidly, and display low sequence similarities with known proteins. How rust fungal effectors function in host cells remains elusive, and biochemical and structural approaches have been scarcely used to tackle this question. In this study, we produced recombinant proteins of eleven candidate effectors of the leaf rust fungus Melampsora larici-populina in Escherichia coli. We successfully purified and solved the three-dimensional structure of two proteins, MLP124266 and MLP124017, using NMR spectroscopy. Although both MLP124266 and MLP124017 show no sequence similarity with known proteins, they exhibit structural similarities to knottins, which are disulfide-rich small proteins characterized by intricate disulfide bridges, and to nuclear transport factor 2-like proteins, which are molecular containers involved in a wide range of functions, respectively. Interestingly, such structural folds have not been reported so far in pathogen effectors, indicating that MLP124266 and MLP124017 may bear novel functions related to pathogenicity. Our findings show that sequence-unrelated effectors can adopt folds similar to known proteins, and encourage the use of biochemical and structural approaches to functionally characterize effector candidates.

REV7 has a dynamic adaptor region to accommodate small GTPase RAN/Shigella IpaB ligands, and its activity is regulated by the RanGTP/GDP switch

REV7, also termed mitotic arrest-deficient 2-like 2 (MAD2L2 or MAD2B), acts as an interaction module in a broad array of cellular pathways, including translesion DNA synthesis, cell cycle control, and nonhomologous end joining. Numerous REV7 binding partners have been identified, including the human small GTPase Ras-associated nuclear protein (RAN), which acts as a potential upstream regulator of REV7. Notably, the Shigella invasin IpaB hijacks REV7 to disrupt cell cycle control to prevent intestinal epithelial cell renewal and facilitate bacterial colonization. However, the structural details of the REV7-RAN and REV7-IpaB interactions are mostly unknown. Here, using fusion protein and rigid maltose-binding protein tagging strategies, we determined the crystal structures of these two complexes at 2.00-2.35 Å resolutions. The structures revealed that both RAN and IpaB fragments bind the "safety belt" region of REV7, inducing rearrangement of the C-terminal β-sheet region of REV7, conserved among REV7-related complexes. Of note, the REV7-binding motifs of RAN and IpaB each displayed some unique interactions with REV7 despite sharing consensus residues. Structural alignments revealed that REV7 has an adaptor region within the safety belt region that can rearrange secondary structures to fit a variety of different ligands. Our structural and biochemical results further indicated that REV7 preferentially binds GTP-bound RAN, implying that a GTP/GDP-bound transition of RAN may serve as the molecular switch that controls REV7's activity. These results provide insights into the regulatory mechanism of REV7 in cell cycle control, which may help with the development of small-molecule inhibitors that target REV7 activity.

Spatiotemporal control of spindle disassembly in fission yeast

Maintenance of genomic stability during cell division is one of the most important cellular tasks, and it critically depends on the faithful replication of the genetic material and its equal partitioning into daughter cells, gametes, or spores in the case of yeasts. Defective mitotic spindle assembly and disassembly both result in changes in cellular ploidy that ultimately impinge proliferation fitness and might increase tumor malignancy. Although a great progress has been made in understanding how spindles are assembled to orchestrate chromosome segregation, much less is known about how they are disassembled once completed their function. Here, we review two recently uncovered mechanisms of spindle disassembly that operate at different stages of the fission yeast life cycle.

Search-and-capture model of cytoneme-mediated morphogen gradient formation

Morphogen protein gradients play an essential role in the spatial regulation of patterning during embryonic development. The most commonly accepted mechanism of protein gradient formation involves the diffusion and degradation of morphogens from a localized source. Recently, an alternative mechanism has been proposed, which is based on cell-to-cell transport via thin actin-rich cellular extensions known as cytonemes. Very little is currently known about the precise nature of the contacts between cytonemes and their target cells. Important unresolved issues include how cytoneme tips find their targets, how they are stabilized at their contact sites, and how vesicles are transferred to a receiving cell and subsequently internalized. It has been hypothesized that cytonemes find their targets via a random search process based on alternating periods of retraction and growth, perhaps mediated by some chemoattractant. This is an actin-based analog of the search-and-capture model of microtubules of the mitotic spindle searching for cytochrome binding sites (kinetochores) prior to separation of cytochrome pairs. In this paper we develop a search-and-capture model of cytoneme-based morphogenesis, in which nucleating cytonemes from a source cell dynamically grow and shrink along the surface of a one-dimensional array of target cells until making contact with one of the target cells. We analyze the first-passage-time problem for making contact and then use this to explore the formation of morphogen gradients under the mechanism proposed for Wnt in vertebrates. That is, we assume that morphogen is localized at the tip of a growing cytoneme, which is delivered as a "morphogen burst" to a target cell when the cytoneme makes temporary contact with a target cell before subsequently retracting. We show how multiple rounds of search-and-capture, morphogen delivery, cytoneme retraction, and nucleation events lead to the formation of a morphogen gradient. We proceed by formulating the morphogen bursting model as a queuing process, analogous to the study of translational bursting in gene networks. In order to analyze the expected times for cytoneme contact, we introduce an efficient method for solving first-passage-time problems in the presence of sticky boundaries, which exploits some classical concepts from probability theory, namely, stopping times and the strong Markov property. We end the paper by demonstrating how this method simplifies previous analyses of a well-studied problem in cell biology, namely, the search-and-capture model of microtubule-kinetochore attachment. Although the latter is completely unrelated to cytoneme-based morphogenesis from a biological perspective, it shares many of the same mathematical elements.

Aurora A Protein Kinase: To the Centrosome and Beyond

Accurate chromosome segregation requires the perfect spatiotemporal rearrangement of the cellular cytoskeleton. Isolated more than two decades ago from Drosophila, Aurora A is a widespread protein kinase that plays key roles during cell division. Numerous studies have described the localisation of Aurora A at centrosomes, the mitotic spindle, and, more recently, at mitotic centromeres. In this review, we will summarise the cytoskeletal rearrangements regulated by Aurora A during cell division. We will also discuss the recent discoveries showing that Aurora A also controls not only the dynamics of the cortical proteins but also regulates the centromeric proteins, revealing new roles for this kinase during cell division.

Animal Female Meiosis: The Challenges of Eliminating Centrosomes

Sexual reproduction requires the generation of gametes, which are highly specialised for fertilisation. Female reproductive cells, oocytes, grow up to large sizes when they accumulate energy stocks and store proteins as well as mRNAs to enable rapid cell divisions after fertilisation. At the same time, metazoan oocytes eliminate their centrosomes, i.e., major microtubule-organizing centres (MTOCs), during or right after the long growth phases. Centrosome elimination poses two key questions: first, how can the centrosome be re-established after fertilisation? In general, metazoan oocytes exploit sperm components, i.e., the basal body of the sperm flagellum, as a platform to reinitiate centrosome production. Second, how do most metazoan oocytes manage to build up meiotic spindles without centrosomes? Oocytes have evolved mechanisms to assemble bipolar spindles solely around their chromosomes without the guidance of pre-formed MTOCs. Female animal meiosis involves microtubule nucleation and organisation into bipolar microtubule arrays in regulated self-assembly under the control of the Ran system and nuclear transport receptors. This review summarises our current understanding of the molecular mechanism underlying self-assembly of meiotic spindles, its spatio-temporal regulation, and the key players governing this process in animal oocytes.

Autocatalytic microtubule nucleation determines the size and mass of Xenopus laevis egg extract spindles

Regulation of size and growth is a fundamental problem in biology. A prominent example is the formation of the mitotic spindle, where protein concentration gradients around chromosomes are thought to regulate spindle growth by controlling microtubule nucleation. Previous evidence suggests that microtubules nucleate throughout the spindle structure. However, the mechanisms underlying microtubule nucleation and its spatial regulation are still unclear. Here, we developed an assay based on laser ablation to directly probe microtubule nucleation events in Xenopus laevis egg extracts. Combining this method with theory and quantitative microscopy, we show that the size of a spindle is controlled by autocatalytic growth of microtubules, driven by microtubule-stimulated microtubule nucleation. The autocatalytic activity of this nucleation system is spatially regulated by the limiting amounts of active microtubule nucleators, which decrease with distance from the chromosomes. This mechanism provides an upper limit to spindle size even when resources are not limiting.

Dynamic Reorganization of the Cytoskeleton during Apoptosis: The Two Coffins Hypothesis

During apoptosis, cells undergo characteristic morphological changes in which the cytoskeleton plays an active role. The cytoskeleton rearrangements have been mainly attributed to actinomyosin ring contraction, while microtubule and intermediate filaments are depolymerized at early stages of apoptosis. However, recent results have shown that microtubules are reorganized during the execution phase of apoptosis forming an apoptotic microtubule network (AMN). Evidence suggests that AMN is required to maintain plasma membrane integrity and cell morphology during the execution phase of apoptosis. The new “two coffins” hypothesis proposes that both AMN and apoptotic cells can adopt two morphological patterns, round or irregular, which result from different cytoskeleton kinetic reorganization during the execution phase of apoptosis induced by genotoxic agents. In addition, round and irregular-shaped apoptosis showed different biological properties with respect to AMN maintenance, plasma membrane integrity and phagocyte responses. These findings suggest that knowing the type of apoptosis may be important to predict how fast apoptotic cells undergo secondary necrosis and the subsequent immune response. From a pathological point of view, round-shaped apoptosis can be seen as a physiological and controlled type of apoptosis, while irregular-shaped apoptosis can be considered as a pathological type of cell death closer to necrosis.

Noncovalent interaction-assisted drug delivery system with highly efficient uptake and release of paclitaxel for anticancer therapy

An effective drug delivery system requires efficient drug uptake and release inside cancer cells. Here, we report a novel drug delivery system, in which paclitaxel (PTX) interacts with a novel cell penetrating peptide (CPP) through noncovalent interaction designed based on molecular simulations. This CPP/PTX complex confers high efficiency in delivering PTX into cancer cells not by endocytosis but by an energy-independent pathway. Once inside cells, the noncovalent interaction between PTX and the CPP may allow fast release of PTX within cells due to the direct translocation of CPP/PTX. This drug delivery system exhibits strong capacity for inhibition of tumor growth and offers a new avenue for the development of advanced drug delivery systems for anticancer therapy.

Myosin efflux promotes cell elongation to coordinate chromosome segregation with cell cleavage

Chromatid segregation must be coordinated with cytokinesis to preserve genomic stability. Here we report that cells clear trailing chromatids from the cleavage site by undergoing two phases of cell elongation. The first phase relies on the assembly of a wide contractile ring. The second phase requires the activity of a pool of myosin that flows from the ring and enriches the nascent daughter cell cortices. This myosin efflux is a novel feature of cytokinesis and its duration is coupled to nuclear envelope reassembly and the nuclear sequestration of the Rho-GEF Pebble. Trailing chromatids induce a delay in nuclear envelope reassembly concomitant with prolonged cortical myosin activity, thus providing forces for the second elongation. We propose that the modulation of cortical myosin dynamics is part of the cellular response triggered by a “chromatid separation checkpoint” that delays nuclear envelope reassembly and, consequently, Pebble nuclear sequestration when trailing chromatids are present at the midzone.

Chromatid segregation must be coordinated with cytokinesis to preserve genomic stability. Here the authors show that cells clear trailing chromatids from the cleavage site in a two-step cell elongation and demonstrate the role of myosin efflux in the second phase.

Rotavirus replication is correlated with S/G2 interphase arrest of the host cell cycle

In infected cells rotavirus (RV) replicates in viroplasms, cytosolic structures that require a stabilized microtubule (MT) network for their assembly, maintenance of the structure and perinuclear localization. Therefore, we hypothesized that RV could interfere with the MT-breakdown that takes place in mitosis during cell division. Using synchronized RV-permissive cells, we show that RV infection arrests the cell cycle in S/G2 phase, thus favoring replication by improving viroplasms formation, viral protein translation, and viral assembly. The arrest in S/G2 phase is independent of the host or viral strain and relies on active RV replication. RV infection causes cyclin B1 down-regulation, consistent with blocking entry into mitosis. With the aid of chemical inhibitors, the cytoskeleton network was linked to specific signaling pathways of the RV-induced cell cycle arrest. We found that upon RV infection Eg5 kinesin was delocalized from the pericentriolar region to the viroplasms. We used a MA104-Fucci system to identify three RV proteins (NSP3, NSP5, and VP2) involved in cell cycle arrest in the S-phase. Our data indicate that there is a strong correlation between the cell cycle arrest and RV replication.

Error-prone meiotic division and subfertility in mice with oocyte-conditional knockdown of pericentrin

Mouse oocytes lack canonical centrosomes and instead contain unique acentriolar microtubule-organizing centers (aMTOCs). To test the function of these distinct aMTOCs in meiotic spindle formation, pericentrin (Pcnt), an essential centrosome/MTOC protein, was knocked down exclusively in oocytes by using a transgenic RNAi approach. Here, we provide evidence that disruption of aMTOC function in oocytes promotes spindle instability and severe meiotic errors that lead to pronounced female subfertility. Pcnt-depleted oocytes from transgenic (Tg) mice were ovulated at the metaphase-II stage, but show significant chromosome misalignment, aneuploidy and premature sister chromatid separation. These defects were associated with loss of key Pcnt-interacting proteins (γ-tubulin, Nedd1 and Cep215) from meiotic spindle poles, altered spindle structure and chromosome-microtubule attachment errors. Live-cell imaging revealed disruptions in the dynamics of spindle assembly and organization, together with chromosome attachment and congression defects. Notably, spindle formation was dependent on Ran GTPase activity in Pcnt-deficient oocytes. Our findings establish that meiotic division is highly error-prone in the absence of Pcnt and disrupted aMTOCs, similar to what reportedly occurs in human oocytes. Moreover, these data underscore crucial differences between MTOC-dependent and -independent meiotic spindle assembly.

Roles of actin binding proteins in mammalian oocyte maturation and beyond

Actin nucleation factors, which promote the formation of new actin filaments, have emerged in the last decade as key regulatory factors controlling asymmetric division in mammalian oocytes. Actin nucleators such as formin-2, spire, and the ARP2/3 complex have been found to be important regulators of actin remodeling during oocyte maturation. Another class of actin-binding proteins including cofilin, tropomyosin, myosin motors, capping proteins, tropomodulin, and Ezrin-Radixin-Moesin proteins are thought to control actin cytoskeleton dynamics at various steps of oocyte maturation. In addition, actin dynamics controlling asymmetric-symmetric transitions after fertilization is a new area of investigation. Taken together, defining the mechanisms by which actin-binding proteins regulate actin cytoskeletons is crucial for understanding the basic biology of mammalian gamete formation and pre-implantation development.

Ahead of the Curve: New Insights into Microtubule Dynamics

Microtubule dynamics are fundamental for many aspects of cell physiology, but their mechanistic underpinnings remain unclear despite 40 years of intense research. In recent years, the continued union of reconstitution biochemistry, structural biology, and modeling has yielded important discoveries that deepen our understanding of microtubule dynamics. These studies, which we review here, underscore the importance of GTP hydrolysis-induced changes in tubulin structure as microtubules assemble, and highlight the fact that each aspect of microtubule behavior is the output of complex, multi-step processes. Although this body of work moves us closer to appreciating the key features of microtubule biochemistry that drive dynamic instability, the divide between our understanding of microtubules in isolation versus within the cellular milieu remains vast. Bridging this gap will serve as fertile grounds of cytoskeleton-focused research for many years to come.

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.

The GTPase RAN regulates multiple steps of the centrosome life cycle

Growing lines of evidence implicate the small GTPase RAN, its regulators and effectors--predominantly, nuclear transport receptors--in practically all aspects of centrosome biology in mammalian cells. These include duplication licensing, cohesion, positioning, and microtubule-nucleation capacity. RAN cooperates with the protein nuclear export vector exportin 1/CRM1 to recruit scaffolding proteins containing nuclear export sequences that play roles in the structural organization of centrosomes. Together, they also limit centrosome reduplication by regulating the localization of key "licensing" proteins during the centrosome duplication cycle. In parallel, RAN also regulates the capacity of centrosomes to nucleate and organize functional microtubules, and this predominanlty involves importin vectors: many factors regulating microtubule nucleation or function harbor nuclear localization sequences that interact with importin molecules and such interaction inhibits their activity. Active RANGTP binding to importin molecules removes the inhibition and releases microtubule regulatory factors in the free productive form. A dynamic scenario emerges, in which RAN is pivotal in linking spatiotemporal control of centrosome regulators to the cell cycle machinery.

Proteins associated with heat-induced leaf senescence in creeping bentgrass as affected by foliar application of nitrogen, cytokinins, and an ethylene inhibitor

Heat stress causes premature leaf senescence in cool-season grass species. The objective of this study was to identify proteins regulated by nitrogen, cytokinins, and ethylene inhibitor in relation to heat-induced leaf senescence in creeping bentgrass (Agrostis stolonifera). Plants (cv. Penncross) were foliar sprayed with 18 mM carbonyldiamide (N source), 25 μM aminoethoxyvinylglycine (AVG, ethylene inhibitor), 25 μM zeatin riboside (ZR, cytokinin), or a water control, and then exposed to 20/15°C (day/night) or 35/30°C (heat stress) in growth chambers. All treatments suppressed heat-induced leaf senescence, as shown by higher turf quality and chlorophyll content, and lower electrolyte leakage in treated plants compared to the untreated control. A total of 49 proteins were responsive to N, AVG, or ZR under heat stress. The abundance of proteins in photosynthesis increased, with ribulose-1,5-bisphosphate carboxylase/oxygenase affected by all three treatments, chlorophyll a/b-binding protein by AVG and N or Rubisco activase by AVG. Proteins for amino acid metabolism were upregulated, including alanine aminotransferase by three treatments and ferredoxin-dependent glutamate synthase by AVG and N. Upregulated proteins also included catalase by AVG and N and heat shock protein by ZR. Exogenous applications of AVG, ZR, or N downregulated proteins in respiration (enolase, glyceraldehyde 3-phosphate dehydrogenase, and succinate dehygrogenase) under heat stress. Alleviation of heat-induced senescence by N, AVG, or ZR was associated with enhanced protein abundance in photosynthesis and amino acid metabolism and stress defense systems (heat shock protection and antioxidants), as well as suppression of those imparting respiration metabolism.

The nucleoporin MEL-28 promotes RanGTP-dependent γ-tubulin recruitment and microtubule nucleation in mitotic spindle formation

The GTP-bound form of the Ran GTPase (RanGTP), produced around chromosomes, drives nuclear envelope and nuclear pore complex (NPC) re-assembly after mitosis. The nucleoporin MEL-28/ELYS binds chromatin in a RanGTP-regulated manner and acts to seed NPC assembly. Here we show that, upon mitotic NPC disassembly, MEL-28 dissociates from chromatin and re-localizes to spindle microtubules and kinetochores. MEL-28 directly binds microtubules in a RanGTP-regulated way via its C-terminal chromatin-binding domain. Using Xenopus egg extracts, we demonstrate that MEL-28 is essential for RanGTP-dependent microtubule nucleation and spindle assembly, independent of its function in NPC assembly. Specifically, MEL-28 interacts with the γ-tubulin ring complex and recruits it to microtubule nucleation sites. Our data identify MEL-28 as a RanGTP target that functions throughout the cell cycle. Its cell cycle-dependent binding to chromatin or microtubules discriminates MEL-28 functions in interphase and mitosis, and ensures that spindle assembly occurs only after NPC breakdown.

Acentrosomal Microtubule Assembly in Mitosis: The Where, When, and How

In mitosis the cell assembles the bipolar spindle, a microtubule (MT)-based apparatus that segregates the duplicated chromosomes into two daughter cells. Most animal cells enter mitosis with duplicated centrosomes that provide an active source of dynamic MTs. However, it is now established that spindle assembly relies on the nucleation of acentrosomal MTs occurring around the chromosomes after nuclear envelope breakdown, and on pre-existing microtubules. Where chromosome-dependent MT nucleation occurs, when MT amplification takes place and how the two pathways function are still key questions that generate some controversies. We reconcile the data and present an integrated model accounting for acentrosomal microtubule assembly in the dividing cell.

Emerging roles of apoptotic microtubules during the execution phase of apoptosis

Apoptosis is a genetically programmed energy-dependent process of cell demise, characterized by specific morphological and biochemical events in which the activation of caspases has an essential role. During apoptosis the cytoskeleton participates actively in characteristic morphological rearrangements of the dying cell. This reorganisation has been assigned mainly to actinomyosin ring contraction, while microtubule and intermediate filaments are depolymerized at early stages of apoptosis. However, recent reports have showed that microtubules are reformed during the execution phase of apoptosis organizing an apoptotic microtubule network (AMN). AMN is organized behind plasma membrane, forming a cortical structure. Apoptotic microtubules repolymerization takes place in many cell types and under different apoptotic inducers. It has been hypothesized that AMN is critical for maintaining plasma membrane integrity and cell morphology during the execution phase of apoptosis. AMN disorganization leads apoptotic cells to secondary necrosis and the release of potential toxic molecules which can damage neighbor cells and promotes inflammation. Therefore, AMN formation during physiological apoptosis or in pathological apoptosis induced by anti-cancer treatments is essential for tissue homeostasis and the prevention of additional cell damage and inflammation.

RanGTP aids anaphase entry through Ubr5-mediated protein turnover

Cells sense the overall chromosome congression state in metaphase to promote anaphase entry using RanGTP, which induces the reduction of two spindle assembly checkpoint proteins, BuGZ and Bub3, via the E3 ligase Ubr5.

RanGTP is known to regulate the spindle assembly checkpoint (SAC), but the underlying molecular mechanism is unclear. BuGZ stabilizes SAC protein Bub3 through direct interaction and facilitates its mitotic function. Here we show that RanGTP promotes the turnover of BuGZ and Bub3 in metaphase, which in turn facilitates metaphase-to-anaphase transition. BuGZ and Bub3 interact with either importin-β or an E3 ubiquitin ligase, Ubr5. RanGTP promotes the dissociation of importin-β from BuGZ and Bub3 in metaphase. This results in increased binding of BuGZ and Bub3 to Ubr5, leading to ubiquitination and subsequent turnover of both proteins. We propose that elevated metaphase RanGTP levels use Ubr5 to couple overall chromosome congression to SAC silencing.

Spatiotemporal Regulation of Nuclear Transport Machinery and Microtubule Organization

Spindle microtubules capture and segregate chromosomes and, therefore, their assembly is an essential event in mitosis. To carry out their mission, many key players for microtubule formation need to be strictly orchestrated. Particularly, proteins that assemble the spindle need to be translocated at appropriate sites during mitosis. A small GTPase (hydrolase enzyme of guanosine triphosphate), Ran, controls this translocation. Ran plays many roles in many cellular events: nucleocytoplasmic shuttling through the nuclear envelope, assembly of the mitotic spindle, and reorganization of the nuclear envelope at the mitotic exit. Although these events are seemingly distinct, recent studies demonstrate that the mechanisms underlying these phenomena are substantially the same as explained by molecular interplay of the master regulator Ran, the transport factor importin, and its cargo proteins. Our review focuses on how the transport machinery regulates mitotic progression of cells. We summarize translocation mechanisms governed by Ran and its regulatory proteins, and particularly focus on Ran-GTP targets in fission yeast that promote spindle formation. We also discuss the coordination of the spatial and temporal regulation of proteins from the viewpoint of transport machinery. We propose that the transport machinery is an essential key that couples the spatial and temporal events in cells.

Human oocytes. Error-prone chromosome-mediated spindle assembly favors chromosome segregation defects in human oocytes

Aneuploidy in human eggs is the leading cause of pregnancy loss and several genetic disorders such as Down syndrome. Most aneuploidy results from chromosome segregation errors during the meiotic divisions of an oocyte, the egg's progenitor cell. The basis for particularly error-prone chromosome segregation in human oocytes is not known. We analyzed meiosis in more than 100 live human oocytes and identified an error-prone chromosome-mediated spindle assembly mechanism as a major contributor to chromosome segregation defects. Human oocytes assembled a meiotic spindle independently of either centrosomes or other microtubule organizing centers. Instead, spindle assembly was mediated by chromosomes and the small guanosine triphosphatase Ran in a process requiring ~16 hours. This unusually long spindle assembly period was marked by intrinsic spindle instability and abnormal kinetochore-microtubule attachments, which favor chromosome segregation errors and provide a possible explanation for high rates of aneuploidy in human eggs.

Emergent Properties of the Metaphase Spindle

A metaphase spindle is a complex structure consisting of microtubules and a myriad of different proteins that modulate microtubule dynamics together with chromatin and kinetochores. A decade ago, a full description of spindle formation and function seemed a lofty goal. Here, we describe how work in the last 10 years combining cataloging of spindle components, the characterization of their biochemical activities using single-molecule techniques, and theory have advanced our knowledge. Taken together, these advances suggest that a full understanding of spindle assembly and function may soon be possible.

A sensitised RNAi screen reveals a ch-TOG genetic interaction network required for spindle assembly

How multiple spindle assembly pathways are integrated to drive bipolar spindle assembly is poorly understood. We performed an image-based double RNAi screen to identify genes encoding Microtubule-Associated Proteins (MAPs) that interact with the highly conserved ch-TOG gene to regulate bipolar spindle assembly in human cells. We identified a ch-TOG centred network of genetic interactions which promotes ensures centrosome-mediated microtubule polymerisation, leading to the incorporation of microtubules polymerised by all pathways into a bipolar structure. Our genetic screen also reveals that ch-TOG maintains a dynamic microtubule population, in part, through modulating HSET activity. ch-TOG ensures that spindle assembly is robust to perturbation but sufficiently dynamic such that spindles can explore a diverse shape space in search of structures that can align chromosomes.

Purification of nuclear localization signal-containing proteins and its application to investigation of the mechanisms of the cell division cycle

The GTP bound form of the Ran GTPase (RanGTP) in the nucleus promotes nuclear import of the proteins bearing nuclear localization signals (NLS). When nuclear envelopes break down during mitosis, RanGTP is locally produced around chromosomes and drives the assembly of the spindle early in mitosis and the nuclear envelope (NE) later. RanGTP binds to the heterodimeric nuclear transport receptor importin α/β and releases NLS proteins from the receptor. Liberated NLS proteins around chromosomes have been shown to play distinct, essential roles in spindle and NE assembly. Here we provide a highly specific protocol to purify NLS proteins from crude cell lysates. The pure NLS fraction is an excellent resource to investigate the NLS protein function and identify new mitotic regulators, uncovering fundamental mechanisms of the cell division cycle. It takes 2–3 days to obtain the NLS fraction.

Microtubule nucleation in mitosis by a RanGTP-dependent protein complex

BACKGROUND

The γ-tubulin ring complex (γTuRC) is a multisubunit complex responsible for microtubule (MT) nucleation in eukaryotic cells. During mitosis, its spatial and temporal regulation promotes MT nucleation through different pathways. One of them is triggered around the chromosomes by RanGTP. Chromosomal MTs are essential for functional spindle assembly, but the mechanism by which RanGTP activates MT nucleation has not yet been resolved.

RESULTS

We used a combination of Xenopus egg extracts and in vitro experiments to dissect the mechanism by which RanGTP triggers MT nucleation. In egg extracts, NEDD1-coated beads promote MT nucleation only in the presence of RanGTP. We show that RanGTP promotes a direct interaction between one of its targets, TPX2, and XRHAMM that defines a specific γTuRC subcomplex. Through depletion/add-back experiments using mutant forms of TPX2 and NEDD1, we show that the activation of MT nucleation by RanGTP requires both NEDD1 phosphorylation on S405 by the TPX2-activated Aurora A and the recruitment of the complex through a TPX2-dependent mechanism.

CONCLUSIONS

The XRHAMM-γTuRC complex is the target for activation by RanGTP that promotes an interaction between TPX2 and XRHAMM. The resulting TPX2-RHAMM-γTuRC supracomplex fulfills the two essential requirements for the activation of MT nucleation by RanGTP: NEDD1 phosphorylation on S405 by the TPX2-activated Aurora A and the recruitment of the complex onto a TPX2-dependent scaffold. Our data identify TPX2 as the only direct RanGTP target and NEDD1 as the only Aurora A substrate essential for the activation of the RanGTP-dependent MT nucleation pathway.

Chromosomal attachments set length and microtubule number in the Saccharomyces cerevisiae mitotic spindle

Altering the number of kinetochores revealed that chromosomal attachments set the length of the metaphase spindle and the number of microtubules within it. Reducing the number of kinetochores increases length, whereas adding extra kinetochores shortens it, suggesting that kinetochore-generated inward forces help set spindle length in budding yeast.

The length of the mitotic spindle varies among different cell types. A simple model for spindle length regulation requires balancing two forces: pulling, due to micro­tubules that attach to the chromosomes at their kinetochores, and pushing, due to interactions between microtubules that emanate from opposite spindle poles. In the budding yeast Saccharomyces cerevisiae, we show that spindle length scales with kinetochore number, increasing when kinetochores are inactivated and shortening on addition of synthetic or natural kinetochores, showing that kinetochore–microtubule interactions generate an inward force to balance forces that elongate the spindle. Electron microscopy shows that manipulating kinetochore number alters the number of spindle microtubules: adding extra kinetochores increases the number of spindle microtubules, suggesting kinetochore-based regulation of microtubule number.

Selective impairment of a subset of Ran-GTP-binding domains of ran-binding protein 2 (Ranbp2) suffices to recapitulate the degeneration of the retinal pigment epithelium (RPE) triggered by Ranbp2 ablation

Retinal pigment epithelium (RPE) degeneration underpins diseases triggered by disparate genetic lesions, noxious insults, or both. The pleiotropic Ranbp2 controls the expression of intrinsic and extrinsic pathological stressors impinging on cellular viability. However, the physiological targets and mechanisms controlled by Ranbp2 in tissue homeostasis, such as RPE, are ill defined. We show that mice, RPE-cre::Ranbp2(-/-), with selective Ranbp2 ablation in RPE develop pigmentary changes, syncytia, hypoplasia, age-dependent centrifugal and non-apoptotic degeneration of the RPE, and secondary leakage of choriocapillaris. These manifestations are accompanied by the development of F-actin clouds, metalloproteinase-11 activation, deregulation of expression or subcellular localization of critical RPE proteins, atrophic cell extrusions into the subretinal space, and compensatory proliferation of peripheral RPE. To gain mechanistic insights into what Ranbp2 activities are vital to the RPE, we performed genetic complementation analyses of transgenic lines of bacterial artificial chromosomes of Ranbp2 harboring loss of function of selective Ranbp2 domains expressed in a Ranbp2(-/-) background. Among the transgenic lines produced, only Tg(RBD2/3*-HA)::RPE-cre::Ranbp2(-/-)-expressing mutations, which selectively impair binding of RBD2/3 (Ran-binding domains 2 and 3) of Ranbp2 to Ran-GTP, recapitulate RPE degeneration, as observed with RPE-cre::Ranbp2(-/-). By contrast, Tg(RBD2/3*-HA) expression rescues the degeneration of cone photoreceptors lacking Ranbp2. The RPE of RPE-cre::Ranbp2(-/-) and Tg(RBD2/3*-HA)::RPE-cre::Ranbp2(-/-) share proteostatic deregulation of Ran GTPase, serotransferrin, and γ-tubulin and suppression of light-evoked electrophysiological responses. These studies unravel selective roles of Ranbp2 and its RBD2 and RBD3 in RPE survival and functions. We posit that the control of Ran GTPase by Ranbp2 emerges as a novel therapeutic target in diseases promoting RPE degeneration.

TPX2: of spindle assembly, DNA damage response, and cancer

For more than 15 years, TPX2 has been studied as a factor critical for mitosis and spindle assembly. These functions of TPX2 are attributed to its Ran-regulated microtubule-associated protein properties and to its control of the Aurora A kinase. Overexpressed in cancers, TPX2 is being established as marker for the diagnosis and prognosis of malignancies. During interphase, TPX2 resides preferentially in the nucleus where its function had remained elusive until recently. The latest finding that TPX2 plays a role in amplification of the DNA damage response, combined with the characterization of TPX2 knockout mice, open new perspectives to understand the biology of this protein. This review provides an historic overview of the discovery of TPX2 and summarizes its cytoskeletal and signaling roles with relevance to cancer therapies. Finally, the review aims to reconcile discrepancies between the experimental and pathological effects of TPX2 overexpression and advances new roles for compartmentalized TPX2.

CDK-dependent phosphorylation of Alp7-Alp14 (TACC-TOG) promotes its nuclear accumulation and spindle microtubule assembly

As cells transition from interphase to mitosis, the microtubule cytoskeleton is reorganized to form the mitotic spindle. In the closed mitosis of fission yeast, a microtubule-associated protein complex, Alp7-Alp14 (transforming acidic coiled-coil-tumor overexpressed gene), enters the nucleus upon mitotic entry and promotes spindle formation. However, how the complex is controlled to accumulate in the nucleus only during mitosis remains elusive. Here we demonstrate that Alp7-Alp14 is excluded from the nucleus during interphase using the nuclear export signal in Alp14 but is accumulated in the nucleus during mitosis through phosphorylation of Alp7 by the cyclin-dependent kinase (CDK). Five phosphorylation sites reside around the nuclear localization signal of Alp7, and the phosphodeficient alp7-5A mutant fails to accumulate in the nucleus during mitosis and exhibits partial spindle defects. Thus our results reveal one way that CDK regulates spindle assembly at mitotic entry: CDK phosphorylates the Alp7-Alp14 complex to localize it to the nucleus.