TPX2 is required for postmitotic nuclear assembly in cell-free Xenopus laevis egg extracts

An Analysis Regarding the Association Between the Nuclear Pore Complex (NPC) and Hepatocellular Carcinoma (HCC)

Background

The nuclear pore complex (NPC) is the main mediator of nuclear and cytoplasmic communication, and delaying or blocking nuclear RNA export and protein shuttling can inhibit cell proliferation and induce apoptosis. Although NPC is a research hotspot in structural biology, relevant studies in hepatocellular carcinoma are scarce, especially in terms of translation into clinical practice.

Methods

This study used a bioinformatics approach combining validation experiments to investigate the biological mechanisms that may be related with NPC. A series of experiments performed to explore the function of the Targeting protein for Xenopus kinesin-like protein 2 (TPX2) in HCC.

Results

Patients with HCC can be divided into two NPC clusters. Patients with high NPC levels (C1) had a shorter survival time than those with low NPC levels (C2) and are characterised by high levels of proliferative signals. We demonstrated that TPX2 regulates HCC growth and inhibits apoptosis in an NPC-dependent manner and contributes to the maintenance of HCC stemness. We developed the NPCScore to predict the prognosis and degree of differentiation in HCC patients.

Conclusion

NPC plays an important role in the malignant proliferation of HCC. Assessing NPC expression patterns could help enhance our understanding of tumor cell proliferation and could guide more effective chemotherapeutic strategies.

The Cytoskeleton and Its Roles in Self-Organization Phenomena: Insights from Xenopus Egg Extracts

Self-organization of and by the cytoskeleton is central to the biology of the cell. Since their introduction in the early 1980s, cytoplasmic extracts derived from the eggs of the African clawed-frog, Xenopus laevis, have flourished as a major experimental system to study the various facets of cytoskeleton-dependent self-organization. Over the years, the many investigations that have used these extracts uniquely benefited from their simplified cell cycle, large experimental volumes, biochemical tractability and cell-free nature. Here, we review the contributions of egg extracts to our understanding of the cytoplasmic aspects of self-organization by the microtubule and the actomyosin cytoskeletons as well as the importance of cytoskeletal filaments in organizing nuclear structure and function.

HPV Infection Significantly Accelerates Glycogen Metabolism in Cervical Cells with Large Nuclei: Raman Microscopic Study with Subcellular Resolution

Using Raman microscopy, we investigated epithelial cervical cells collected from 96 women with squamous cell carcinoma (SCC) or belonging to groups I, IIa, IIID-1 and IIID-2 according to Munich III classification (IIID-1 and IIID-2 corresponding to Bethesda LSIL and HSIL groups, respectively). All women were tested for human papillomavirus (HPV) infection using PCR. Subcellular resolution of Raman microscopy enabled to understand phenotypic differences in a heterogeneous population of cervical cells in the following groups: I/HPV⁻, IIa/HPV⁻, IIa/HPV⁻, LSIL/HPV⁻, LSIL/HPV⁺, HSIL/HPV⁻, HSIL/HPV⁺ and cancer cells (SCC/HPV⁺). We showed for the first time that the glycogen content in the cytoplasm decreased with the nucleus size of cervical cells in all studied groups apart from the cancer group. For the subpopulation of large-nucleus cells HPV infection resulted in considerable glycogen depletion compared to HPV negative cells in IIa, LSIL (for both statistical significance, ca. 45%) and HSIL (trend, 37%) groups. We hypothesize that accelerated glycogenolysis in large-nucleus cells may be associated with the increased protein metabolism for HPV positive cells. Our work underlines unique capabilities of Raman microscopy in single cell studies and demonstrate potential of Raman-based methods in HPV diagnostics.

Excess TPX2 Interferes with Microtubule Disassembly and Nuclei Reformation at Mitotic Exit

The microtubule-associated protein TPX2 is a key mitotic regulator that contributes through distinct pathways to spindle assembly. A well-characterised function of TPX2 is the activation, stabilisation and spindle localisation of the Aurora-A kinase. High levels of TPX2 are reported in tumours and the effects of its overexpression have been investigated in cancer cell lines, while little is known in non-transformed cells. Here we studied TPX2 overexpression in hTERT RPE-1 cells, using either the full length TPX2 or a truncated form unable to bind Aurora-A, to identify effects that are dependent-or independent-on its interaction with the kinase. We observe significant defects in mitotic spindle assembly and progression through mitosis that are more severe when overexpressed TPX2 is able to interact with Aurora-A. Furthermore, we describe a peculiar, and Aurora-A-interaction-independent, phenotype in telophase cells, with aberrantly stable microtubules interfering with nuclear reconstitution and the assembly of a continuous lamin B1 network, resulting in daughter cells displaying doughnut-shaped nuclei. Our results using non-transformed cells thus reveal a previously uncharacterised consequence of abnormally high TPX2 levels on the correct microtubule cytoskeleton remodelling and G1 nuclei reformation, at the mitosis-to-interphase transition.

Use of Xenopus cell-free extracts to study size regulation of subcellular structures

Striking size variations are prominent throughout biology, at the organismal, cellular, and subcellular levels. Important fundamental questions concern organelle size regulation and how organelle size is regulated relative to cell size, also known as scaling. Uncovering mechanisms of organelle size regulation will inform the functional significance of size as well as the implications of misregulated size, for instance in the case of nuclear enlargement in cancer. Xenopus egg and embryo extracts are powerful cell-free systems that have been utilized extensively for mechanistic and functional studies of various organelles and subcellular structures. The open biochemical nature of the extract permits facile manipulation of its composition, and in recent years extract approaches have illuminated mechanisms of organelle size regulation. This review largely focuses on in vitro Xenopus studies that have identified regulators of nuclear and spindle size. We also discuss potential relationships between size scaling of the nucleus and spindle, size regulation of other subcellular structures, and extract experiments that have clarified developmental timing mechanisms. We conclude by offering some future prospects, notably the integration of Xenopus extract with microfluidic technology.

Gamma-tubulin coordinates nuclear envelope assembly around chromatin

The cytosolic role of γ-tubulin as a microtubule organizer has been studied thoroughly, but its nuclear function is poorly understood. Here, we show that γ-tubulin is located throughout the chromatin of demembranated Xenopus laevis sperm and, as the nucleus is formed, γ-tubulin recruits lamin B3 and nuclear membranes. Immunodepletion of γ-tubulin impairs X. laevis assembly of both the lamina and the nuclear membrane. During nuclear formation in mammalian cell lines, γ-tubulin establishes a cellular protein boundary around chromatin that coordinates nuclear assembly of the daughter nuclei. Furthermore, expression of a γ-tubulin mutant that lacks the DNA-binding domain forms chromatin-empty nuclear like structures and demonstrate that a constant interplay between the chromatin-associated and the cytosolic pools of γ-tubulin is required and, when the balance between pools is impaired, aberrant nuclei are formed. We therefore propose that the nuclear protein meshwork formed by γ-tubulin around chromatin coordinates nuclear formation in eukaryotic cells.

New Insights into Mechanisms and Functions of Nuclear Size Regulation

Nuclear size is generally maintained within a defined range in a given cell type. Changes in cell size that occur during cell growth, development, and differentiation are accompanied by dynamic nuclear size adjustments in order to establish appropriate nuclear-to-cytoplasmic volume relationships. It has long been recognized that aberrations in nuclear size are associated with certain disease states, most notably cancer. Nuclear size and morphology must impact nuclear and cellular functions. Understanding these functional implications requires an understanding of the mechanisms that control nuclear size. In this review, we first provide a general overview of the diverse cellular structures and activities that contribute to nuclear size control, including structural components of the nucleus, effects of DNA amount and chromatin compaction, signaling, and transport pathways that impinge on the nucleus, extranuclear structures, and cell cycle state. We then detail some of the key mechanistic findings about nuclear size regulation that have been gleaned from a variety of model organisms. Lastly, we review studies that have implicated nuclear size in the regulation of cell and nuclear function and speculate on the potential functional significance of nuclear size in chromatin organization, gene expression, nuclear mechanics, and disease. With many fundamental cell biological questions remaining to be answered, the field of nuclear size regulation is still wide open.

Dynein light intermediate chains maintain spindle bipolarity by functioning in centriole cohesion

Cytoplasmic dynein light intermediate chains are required for the maintenance of centriole cohesion and the formation of a bipolar spindle in both human cells and Xenopus embryos.

Cytoplasmic dynein 1 (dynein) is a minus end–directed microtubule motor protein with many cellular functions, including during cell division. The role of the light intermediate chains (LICs; DYNC1LI1 and 2) within the complex is poorly understood. In this paper, we have used small interfering RNAs or morpholino oligonucleotides to deplete the LICs in human cell lines and Xenopus laevis early embryos to dissect the LICs’ role in cell division. We show that although dynein lacking LICs drives microtubule gliding at normal rates, the LICs are required for the formation and maintenance of a bipolar spindle. Multipolar spindles with poles that contain single centrioles were formed in cells lacking LICs, indicating that they are needed for maintaining centrosome integrity. The formation of multipolar spindles via centrosome splitting after LIC depletion could be rescued by inhibiting Eg5. This suggests a novel role for the dynein complex, counteracted by Eg5, in the maintenance of centriole cohesion during mitosis.

Sizing and shaping the nucleus: mechanisms and significance

The size and shape of the nucleus are tightly regulated, indicating the physiological significance of proper nuclear morphology, yet the mechanisms and functions of nuclear size and shape regulation remain poorly understood. Correlations between altered nuclear morphology and certain disease states have long been observed, most notably many cancers are diagnosed and staged based on graded increases in nuclear size. Here we review recent studies investigating the mechanisms regulating nuclear size and shape, how mitotic events influence nuclear morphology, and the role of nuclear size and shape in subnuclear chromatin organization and cancer progression.

Mechanisms of nuclear size regulation in model systems and cancer

Changes in nuclear size have long been used by cytopathologists as an important parameter to diagnose, stage, and prognose many cancers. Mechanisms underlying these changes and functional links between nuclear size and malignancy are largely unknown. Understanding mechanisms of nuclear size regulation and the physiological significance of proper nuclear size control will inform the interplay between altered nuclear size and oncogenesis. In this chapter we review what is known about molecular mechanisms of nuclear size control based on research in model experimental systems including yeast, Xenopus, Tetrahymena, Drosophila, plants, mice, and mammalian cell culture. We discuss how nuclear size is influenced by DNA ploidy, nuclear structural components, cytoplasmic factors and nucleocytoplasmic transport, the cytoskeleton, and the extracellular matrix. Based on these mechanistic insights, we speculate about how nuclear size might impact cell physiology and whether altered nuclear size could contribute to cancer development and progression. We end with some outstanding questions about mechanisms and functions of nuclear size regulation.

Overexpressed TPX2 causes ectopic formation of microtubular arrays in the nuclei of acentrosomal plant cells

TPX2 performs multiple roles in microtubule organization. Previously, it was shown that plant AtTPX2 binds AtAurora1 kinase and colocalizes with microtubules in a cell cycle-specific manner. To elucidate the function of TPX2 further, this work analysed Arabidopsis cells overexpressing AtTPX2-GFP. Distinct arrays of bundled microtubules, decorated with AtTPX2-GFP, were formed in the vicinity of the nuclear envelope and in the nuclei of overexpressing cells. The microtubular arrays showed reduced sensitivity to anti-microtubular drugs. TPX2-mediated formation of nuclear/perinuclear microtubular arrays was not specific for the transition to mitosis and occurred independently of Aurora kinase. The fibres were not observed in cells with detectable programmed cell death and, in this respect, they differed from TPX2-dependent microtubular assemblies functioning in mammalian apoptosis. Colocalization and co-purification data confirmed the interaction of importin with AtTPX2-GFP. In cells with nuclear foci of overexpressed AtTPX2-GFP, strong nuclear signals for Ran and importin diminished when microtubular arrays were assembled. This observation suggests that TPX2-mediated microtubule formation might be triggered by a Ran cycle. Collectively, the data suggest that in the acentrosomal plant cell, in conjunction with importin, overexpressed AtTPX2 reinforces microtubule formation in the vicinity of chromatin and the nuclear envelope.

Mitotic Stress and Chromosomal Instability in Cancer: The Case for TPX2

Cell cycle deregulation is a common motif in human cancer, and multiple therapeutic strategies are aimed to prevent tumor cell proliferation. Whereas most current therapies are designed to arrest cell cycle progression either in G1/S or in mitosis, new proposals include targeting the intrinsic chromosomal instability (CIN, an increased rate of gain or losses of chromosomes during cell division) or aneuploidy (a genomic composition that differs from diploid) that many tumor cells display. Why tumors cells are chromosomally unstable or aneuploid and what are the consequences of these alterations are not completely clear at present. Several mitotic regulators are overexpressed as a consequence of oncogenic alterations, and they are likely to alter the proper regulation of chromosome segregation in cancer cells. In this review, we propose the relevance of TPX2, a mitotic regulator involved in the formation of the mitotic spindle, in oncogene-induced mitotic stress. This protein, as well as its partner Aurora-A, is frequently overexpressed in human cancer, and its deregulation may participate not only in chromosome numeric aberrations but also in other forms of genomic instability in cancer cells.

The distribution of TPX2 in dividing leaf cells of the fern Asplenium nidus

Plant cell division requires the dynamic organisation of several microtubule arrays. The mechanisms of regulation of the above arrays are under rigorous research. Among several factors that are involved in plant microtubule dynamics, the Targeting Protein for Xklp2 (TPX2) has been found to play a role in spindle organisation, in combination with Aurora kinases, in dividing cells of angiosperms. Microtubule organisation in dividing cells of ferns exhibits certain peculiarities. Accordingly, the presence and distribution of a TPX2 homologue might be helpful in understanding the patterns and regulatory mechanisms of microtubule arrays in this plant group. In this study, a putative TPX2 homologue was identified using Western blotting in the fern Asplenium nidus. It was found, using immunostaining and CLSM, that it is co-localised with perinuclear preprophase microtubules and the prophase spindle, and follows the microtubule pattern during metaphase/anaphase and telophase. During cytokinesis, while in angiosperms TPX2 is degraded, in A. nidus the TPX2 signal persists, co-localising with the phragmoplast. In early post-cytokinetic cells, a TPX2 signal is present on the nuclear surface facing the daughter cell wall and, thereafter it is co-localised with the fern-specific microtubule aggregation that lines the new wall, which is possibly involved in cortical microtubule assembly.

Urochordate ascidians possess a single isoform of Aurora kinase that localizes to the midbody via TPX2 in eggs and cleavage stage embryos

Aurora kinases are key proteins found throughout the eukaryotes that control mitotic progression. Vertebrate Aurora-A and B kinases are thought to have evolved from a single Aurora-kinase isoform closest to that found in present day urochordates. In urochordate ascidians Aurora binds both TPX2 (a vertebrate AURKA partner) and INCENP (a vertebrate AURKB partner) and localizes to centrosomes and spindle microtubules as well as chromosomes and midbody during both meiosis and mitosis. Ascidian Aurora also displays this localization pattern during mitosis in echinoderms, strengthening the idea that non-vertebrate deuterostomes such as the urochordates and echinoderms possess a single form of Aurora kinase that has properties of vertebrate Aurora-kinase A and B. In the ascidian, TPX2 localizes to the centrosome and the spindle poles also as in vertebrates. However, we were surprised to find that TPX2 also localized strongly to the midbody in ascidian eggs and embryos. We thus examined more closely Aurora localization to the midbody by creating two separate point mutations of ascidian Aurora predicted to perturb binding to TPX2. Both forms of mutated Aurora behaved as predicted: neither localized to spindle poles where TPX2 is enriched. Interestingly, neither form of mutated Aurora localized to the midbody where TPX2 is also enriched, suggesting that ascidian Aurora midbody localization required TPX2 binding in ascidians. Functional analysis revealed that inhibition of Aurora kinase with a pharmacological inhibitor or with a dominant negative kinase dead form of Aurora caused cytokinesis failure and perturbed midbody formation during polar body extrusion. Our data support the view that vertebrate Aurora-A and B kinases evolved from a single non-vertebrate deuterostome ancestor. Moreover, since TPX2 localizes to the midbody in ascidian eggs and cleavage stage embryos it may be worthwhile re-assessing whether Aurora A kinase or TPX2 localize to the midbody in eggs and cleavage stage embryos.

Embryonic and adult isoforms of XLAP2 form microdomains associated with chromatin and the nuclear envelope

Laminin-associated polypeptide 2 (LAP2) proteins are alternatively spliced products of a single gene; they belong to the LEM domain family and, in mammals, locate to the nuclear envelope (NE) and nuclear lamina. Isoforms lacking the transmembrane domain also locate to the nucleoplasm. We used new specific antibodies against the N-terminal domain of Xenopus LAP2 to perform immunoprecipitation, identification and localization studies during Xenopus development. By immunoprecipitation and mass spectrometry (LC/MS/MS), we identified the embryonic isoform XLAP2γ, which was downregulated during development similarly to XLAP2ω. Embryonic isoforms XLAP2ω and XLAP2γ were located in close association with chromatin up to the blastula stage. Later in development, both embryonic isoforms and the adult isoform XLAP2β were localized in a similar way at the NE. All isoforms colocalized with lamin B2/B3 during development, whereas XLAP2β was colocalized with lamin B2 and apparently with the F/G repeat nucleoporins throughout the cell cycle in adult tissues and culture cells. XLAP2β was localized in clusters on chromatin, both at the NE and inside the nucleus. Embryonic isoforms were also localized in clusters at the NE of oocytes. Our results suggest that XLAP2 isoforms participate in the maintenance and anchoring of chromatin domains to the NE and in the formation of lamin B microdomains.

The plant TPX2 protein regulates prospindle assembly before nuclear envelope breakdown

The Targeting Protein for Xklp2 (TPX2) is a central regulator of spindle assembly in vertebrate cells. The absence or excess of TPX2 inhibits spindle formation. We have defined a TPX2 signature motif that is present once in vertebrate sequences but twice in plants. Plant TPX2 is predominantly nuclear during interphase and is actively exported before nuclear envelope breakdown to initiate prospindle assembly. It localizes to the spindle microtubules but not to the interdigitating polar microtubules during anaphase or to the phragmoplast as it is rapidly degraded during telophase. We characterized the Arabidopsis thaliana TPX2-targeting domains and show that the protein is able to rescue microtubule assembly in TPX2-depleted Xenopus laevis egg extracts. Injection of antibodies to TPX2 into living plant cells inhibits the onset of mitosis. These results demonstrate that plant TPX2 already functions before nuclear envelope breakdown. Thus, plants have adapted nuclear-cytoplasmic shuttling of TPX2 to maintain proper spindle assembly without centrosomes.

Myosin-10 and actin filaments are essential for mitotic spindle function

Mitotic spindles are microtubule-based structures responsible for chromosome partitioning during cell division. Although the roles of microtubules and microtubule-based motors in mitotic spindles are well established, whether or not actin filaments (F-actin) and F-actin-based motors (myosins) are required components of mitotic spindles has long been controversial. Based on the demonstration that myosin-10 (Myo10) is important for assembly of meiotic spindles, we assessed the role of this unconventional myosin, as well as F-actin, in mitotic spindles. We find that Myo10 localizes to mitotic spindle poles and is essential for proper spindle anchoring, normal spindle length, spindle pole integrity, and progression through metaphase. Furthermore, we show that F-actin localizes to mitotic spindles in dynamic cables that surround the spindle and extend between the spindle and the cortex. Remarkably, although proper anchoring depends on both F-actin and Myo10, the requirement for Myo10 in spindle pole integrity is F-actin independent, whereas F-actin and Myo10 actually play antagonistic roles in maintenance of spindle length.

The BRCA1/BARD1 heterodimer modulates ran-dependent mitotic spindle assembly

The heterodimeric tumor-suppressor complex BRCA1/BARD1 exhibits E3 ubiquitin ligase activity and participates in cell proliferation and chromosome stability control by incompletely defined mechanisms. Here we show that, in both mammalian cells and Xenopus egg extracts, BRCA1/BARD1 is required for mitotic spindle-pole assembly and for accumulation of TPX2, a major spindle organizer and Ran target, on spindle poles. This function is centrosome independent, operates downstream of Ran GTPase, and depends upon BRCA1/BARD1 E3 ubiquitin ligase activity. Xenopus BRCA1/BARD1 forms endogenous complexes with three spindle-pole proteins, TPX2, NuMA, and XRHAMM--a known TPX2 partner--and specifically attenuates XRHAMM function. These observations reveal a previously unrecognized function of BRCA1/BARD1 in mitotic spindle assembly that likely contributes to its role in chromosome stability control and tumor suppression.