Relation of nucleolar structure and position to the cytoplasmic microtubule system inDictyostelium

The matrix environmental and cell mechanical properties regulate cell migration and contribute to the invasive phenotype of cancer cells

The minimal structural unit of a solid tumor is a single cell or a cellular compartment such as the nucleus. A closer look inside the cells reveals that there are functional compartments or even structural domains determining the overall properties of a cell such as the mechanical phenotype. The mechanical interaction of these living cells leads to the complex organization such as compartments, tissues and organs of organisms including mammals. In contrast to passive non-living materials, living cells actively respond to the mechanical perturbations occurring in their microenvironment during diseases such as fibrosis and cancer. The transformation of single cancer cells in highly aggressive and hence malignant cancer cells during malignant cancer progression encompasses the basement membrane crossing, the invasion of connective tissue, the stroma microenvironments and transbarrier migration, which all require the immediate interaction of the aggressive and invasive cancer cells with the surrounding extracellular matrix environment including normal embedded neighboring cells. All these steps of the metastatic pathway seem to involve mechanical interactions between cancer cells and their microenvironment. The pathology of cancer due to a broad heterogeneity of cancer types is still not fully understood. Hence it is necessary to reveal the signaling pathways such as mechanotransduction pathways that seem to be commonly involved in the development and establishment of the metastatic and mechanical phenotype in several carcinoma cells. We still do not know whether there exist distinct metastatic genes regulating the progression of tumors. These metastatic genes may then be activated either during the progression of cancer by themselves on their migration path or in earlier stages of oncogenesis through activated oncogenes or inactivated tumor suppressor genes, both of which promote the metastatic phenotype. In more detail, the adhesion of cancer cells to their surrounding stroma induces the generation of intracellular contraction forces that deform their microenvironments by alignment of fibers. The amplitude of these forces can adapt to the mechanical properties of the microenvironment. Moreover, the adhesion strength of cancer cells seems to determine whether a cancer cell is able to migrate through connective tissue or across barriers such as the basement membrane or endothelial cell linings of blood or lymph vessels in order to metastasize. In turn, exposure of adherent cancer cells to physical forces, such as shear flow in vessels or compression forces around tumors, reinforces cell adhesion, regulates cell contractility and restructures the ordering of the local stroma matrix that leads subsequently to secretion of crosslinking proteins or matrix degrading enzymes. Hence invasive cancer cells alter the mechanical properties of their microenvironment. From a mechanobiological point-of-view, the recognized physical signals are transduced into biochemical signaling events that guide cellular responses such as cancer progression after the malignant transition of cancer cells from an epithelial and non-motile phenotype to a mesenchymal and motile (invasive) phenotype providing cellular motility. This transition can also be described as the physical attempt to relate this cancer cell transitional behavior to a T1 phase transition such as the jamming to unjamming transition. During the invasion of cancer cells, cell adaptation occurs to mechanical alterations of the local stroma, such as enhanced stroma upon fibrosis, and therefore we need to uncover underlying mechano-coupling and mechano-regulating functional processes that reinforce the invasion of cancer cells. Moreover, these mechanisms may also be responsible for the awakening of dormant residual cancer cells within the microenvironment. Physicists were initially tempted to consider the steps of the cancer metastasis cascade as single events caused by a single mechanical alteration of the overall properties of the cancer cell. However, this general and simple view has been challenged by the finding that several mechanical properties of cancer cells and their microenvironment influence each other and continuously contribute to tumor growth and cancer progression. In addition, basement membrane crossing, cell invasion and transbarrier migration during cancer progression is explained in physical terms by applying physical principles on living cells regardless of their complexity and individual differences of cancer types. As a novel approach, the impact of the individual microenvironment surrounding cancer cells is also included. Moreover, new theories and models are still needed to understand why certain cancers are malignant and aggressive, while others stay still benign. However, due to the broad variety of cancer types, there may be various pathways solely suitable for specific cancer types and distinct steps in the process of cancer progression. In this review, physical concepts and hypotheses of cancer initiation and progression including cancer cell basement membrane crossing, invasion and transbarrier migration are presented and discussed from a biophysical point-of-view. In addition, the crosstalk between cancer cells and a chronically altered microenvironment, such as fibrosis, is discussed including the basic physical concepts of fibrosis and the cellular responses to mechanical stress caused by the mechanically altered microenvironment. Here, is highlighted how biophysical approaches, both experimentally and theoretically, have an impact on classical hallmarks of cancer and fibrosis and how they contribute to the understanding of the regulation of cancer and its progression by sensing and responding to the physical environmental properties through mechanotransduction processes. Finally, this review discusses various physical models of cell migration such as blebbing, nuclear piston, protrusive force and unjamming transition migration modes and how they contribute to cancer progression. Moreover, these cellular migration modes are influenced by microenvironmental perturbances such as fibrosis that can induce mechanical alterations in cancer cells, which in turn may impact the environment. Hence, the classical hallmarks of cancer need to be refined by including biomechanical properties of cells, cell clusters and tissues and their microenvironment to understand mechano-regulatory processes within cancer cells and the entire organism.

Proteins of the Nucleolus of Dictyostelium discoideum: Nucleolar Compartmentalization, Targeting Sequences, Protein Translocations and Binding Partners

The nucleoli of Dictyostelium discoideum have a comparatively unique, non-canonical, localization adjacent to the inner nuclear membrane. The verified nucleolar proteins of this eukaryotic microbe are detailed while other potential proteins are introduced. Heat shock protein 32 (Hsp32), eukaryotic translation initiation factor 6 (eIF6), and tumour necrosis factor receptor-associated protein 1 (TRAP1) are essential for cell survival. NumA1, a breast cancer type 1 susceptibility protein-C Terminus domain-containing protein linked to cell cycle, functions in the regulation of nuclear number. The cell cycle checkpoint kinase 2 homologue forkhead-associated kinase A (FhkA) and BRG1-associated factor 60a homologue Snf12 are also discussed. While nucleoli appear homogeneous ultrastructurally, evidence for nucleolar subcompartments exists. Nucleolar localization sequences (NoLS) have been defined that target proteins to either the general nucleolar area or to a specific intranucleolar domain. Protein translocations during mitosis are protein-specific and support the multiple functions of the Dictyostelium nucleolus. To enrich the picture, binding partners of NumA1, the most well-characterized nucleolar protein, are examined: nucleolar Ca²⁺-binding protein 4a (CBP4a), nuclear puromycin-sensitive aminopeptidase A (PsaA) and Snf12. The role of Dictyostelium as a model for understanding the contribution of nucleolar proteins to various diseases and cellular stress is discussed throughout the review.

Inside the Cell: Integrins as New Governors of Nuclear Alterations?

Cancer cell migration is a complex process that requires coordinated structural changes and signals in multiple cellular compartments. The nucleus is the biggest and stiffest organelle of the cell and might alter its physical properties to allow cancer cell movement. Integrins are transmembrane receptors that mediate cell-cell and cell-extracellular matrix interactions, which regulate numerous intracellular signals and biological functions under physiological conditions. Moreover, integrins orchestrate changes in tumor cells and their microenvironment that lead to cancer growth, survival and invasiveness. Most of the research efforts have focused on targeting integrin-mediated adhesion and signaling. Recent exciting data suggest the crucial role of integrins in controlling internal cellular structures and nuclear alterations during cancer cell migration. Here we review the emerging role of integrins in nuclear biology. We highlight increasing evidence that integrins are critical for changes in multiple nuclear components, the positioning of the nucleus and its mechanical properties during cancer cell migration. Finally, we discuss how integrins are integral proteins linking the plasma membrane and the nucleus, and how they control cell migration to enable cancer invasion and infiltration. The functional connections between these cell receptors and the nucleus will serve to define new attractive therapeutic targets.

Early nucleolar disorganization in Dictyostelium cell death

Cell death occurs in all eukaryotes, but it is still not known whether some core steps of the cell death process are conserved. We investigated this using the protist Dictyostelium. The dissection of events in Dictyostelium vacuolar developmental cell death was facilitated by the sequential requirement for two distinct exogenous signals. An initial exogenous signal (starvation and cAMP) recruited some cells into clumps. Only within these clumps did subsequent cell death events take place. Contrary to our expectations, already this initial signal provoked nucleolar disorganization and irreversible inhibition of rRNA and DNA synthesis, reflecting marked cell dysfunction. The initial signal also primed clumped cells to respond to a second exogenous signal (differentiation-inducing factor-1 or c-di-GMP), which led to vacuolization and synthesis of cellulose encasings. Thus, the latter prominent hallmarks of developmental cell death were induced separately from initial cell dysfunction. We propose that (1) in Dictyostelium vacuolization and cellulose encasings are late, organism-specific, hallmarks, and (2) on the basis of our observations in this protist and of similar previous observations in some cases of mammalian cell death, early inhibition of rRNA synthesis and nucleolar disorganization may be conserved in some eukaryotes to usher in developmental cell death.

Src1 is a Protein of the Inner Nuclear Membrane Interacting with the Dictyostelium Lamin NE81

The nuclear envelope (NE) consists of the outer and inner nuclear membrane (INM), whereby the latter is bound to the nuclear lamina. Src1 is a Dictyostelium homologue of the helix-extension-helix family of proteins, which also includes the human lamin-binding protein MAN1. Both endogenous Src1 and GFP-Src1 are localized to the NE during the entire cell cycle. Immuno-electron microscopy and light microscopy after differential detergent treatment indicated that Src1 resides in the INM. FRAP experiments with GFP-Src1 cells suggested that at least a fraction of the protein could be stably engaged in forming the nuclear lamina together with the Dictyostelium lamin NE81. Both a BioID proximity assay and mis-localization of soluble, truncated mRFP-Src1 at cytosolic clusters consisting of an intentionally mis-localized mutant of GFP-NE81 confirmed an interaction of Src1 and NE81. Expression GFP-Src1^1–646, a fragment C-terminally truncated after the first transmembrane domain, disrupted interaction of nuclear membranes with the nuclear lamina, as cells formed protrusions of the NE that were dependent on cytoskeletal pulling forces. Protrusions were dependent on intact microtubules but not actin filaments. Our results indicate that Src1 is required for integrity of the NE and highlight Dictyostelium as a promising model for the evolution of nuclear architecture.

Dictyostelium discoideum has a highly Q/N-rich proteome and shows an unusual resilience to protein aggregation

Many protein-misfolding diseases are caused by proteins carrying prion-like domains. These proteins show sequence similarity to yeast prion proteins, which can interconvert between an intrinsically disordered and an aggregated prion state. The natural presence of prions in yeast has provided important insight into disease mechanisms and cellular proteostasis. However, little is known about prions in other organisms, and it is not yet clear whether the findings in yeast can be generalized. Using bioinformatics tools, we show that Dictyostelium discoideum has the highest content of prion-like proteins of all organisms investigated to date, suggesting that its proteome has a high overall aggregation propensity. To study mechanisms regulating these proteins, we analyze the behavior of several well-characterized prion-like proteins, such as an expanded version of human huntingtin exon 1 (Q103) and the prion domain of the yeast prion protein Sup35 (NM), in D. discoideum. We find that these proteins remain soluble and are innocuous to D. discoideum, in contrast to other organisms, where they form cytotoxic cytosolic aggregates. However, when exposed to conditions that compromise molecular chaperones, these proteins aggregate and become cytotoxic. We show that the disaggregase Hsp101, a molecular chaperone of the Hsp100 family, dissolves heat-induced aggregates and promotes thermotolerance. Furthermore, prion-like proteins accumulate in the nucleus, where they are targeted by the ubiquitin-proteasome system. Our data suggest that D. discoideum has undergone specific adaptations that increase the proteostatic capacity of this organism and allow for an efficient regulation of its prion-like proteome.

Evidence for nucleolar subcompartments in Dictyostelium

The nucleolus is a multifunctional nuclear compartment usually consisting of two to three subcompartments which represent stages of ribosomal biogenesis. It is linked to several human diseases including viral infections, cancer, and neurodegeneration. Dictyostelium is a model eukaryote for the study of fundamental biological processes as well as several human diseases however comparatively little is known about its nucleolus. Unlike most nucleoli it does not possess visible subcompartments at the ultrastructural level. Several recently identified nucleolar proteins in Dictyostelium leave the nucleolus after treatment with the rDNA transcription inhibitor actinomycin-D (AM-D). Different proteins exit in different ways, suggesting that previously unidentified nucleolar subcompartments may exist. The identification of nucleolar subcompartments would help to better understand the nucleolus in this model eukaryote. Here, we show that Dictyostelium nucleolar proteins nucleomorphin isoform NumA1 and Bud31 localize throughout the entire nucleolus while calcium-binding protein 4a localizes to only a portion, representing nucleolar subcompartment 1 (NoSC1). SWI/SNF complex member Snf12 localizes to a smaller area within NoSC1 representing a second nucleolar subcompartment, NoSC2. The nuclear/nucleolar localization signal KRKR from Snf12 localized GFP to NoSC2, and thus also appears to function as a nucleolar subcompartment localization signal. FhkA localizes to the nucleolar periphery displaying a similar pattern to that of Hsp32. Similarities between the redistribution patterns of Dictyostelium nucleolar proteins during nucleolar disruption as a result of either AM-D treatment or mitosis support these subcompartments. A model for the AM-D-induced redistribution patterns is proposed.

Functional ultrastructure of the plant nucleolus

Nucleoli are nuclear domains present in almost all eukaryotic cells. They not only specialize in the production of ribosomal subunits but also play roles in many fundamental cellular activities. Concerning ribosome biosynthesis, particular stages of this process, i.e., ribosomal DNA transcription, primary RNA transcript processing, and ribosome assembly proceed in precisely defined nucleolar subdomains. Although eukaryotic nucleoli are conservative in respect of their main function, clear morphological differences between these structures can be noticed between individual kingdoms. In most cases, a plant nucleolus shows well-ordered structure in which four main ultrastructural components can be distinguished: fibrillar centers, dense fibrillar component, granular component, and nucleolar vacuoles. Nucleolar chromatin is an additional crucial structural component of this organelle. Nucleolonema, although it is not always an unequivocally distinguished nucleolar domain, has often been described as a well-grounded morphological element, especially of plant nucleoli. The ratios and morphology of particular subcompartments of a nucleolus can change depending on its metabolic activity which in turn is correlated with the physiological state of a cell, cell type, cell cycle phase, as well as with environmental influence. Precise attribution of functions to particular nucleolar subregions in the process of ribosome biosynthesis is now possible using various approaches. The presented description of plant nucleolar morphology summarizes previous knowledge regarding the function of nucleoli as well as of their particular subdomains not only in the course of ribosome biosynthesis.

Rad53 homologue forkhead-associated kinase A (FhkA) and Ca2+-binding protein 4a (CBP4a) are nucleolar proteins that differentially redistribute during mitosis in Dictyostelium

BACKGROUND

During mitosis most nucleolar proteins redistribute to other locales providing an opportunity to study the relationship between nucleolar protein localization and function. Dictyostelium is a model organism for the study of several fundamental biological processes and human diseases but only two nucleolar proteins have been studied during mitosis: NumA1 and Snf12. Both of them are linked to the cell cycle. To acquire a better understanding of nucleolar protein localization and dynamics in Dictyostelium we studied the nucleolar localization of two additional proteins during mitosis: Snf12-linked forkhead-associated kinase A (FhkA), which is involved in the cell cycle, and Ca2+-binding protein 4a (CBP4a), which is a binding partner of NumA1.

METHODS

Polyclonal antibodies were produced in-house. Cells were fixed and probed with either anti-FhkA or anti-CBP4a in order to determine cellular localization during interphase and throughout the stages of mitosis. Colocalization with DAPI nuclear stain allowed us to determine the location of the nucleus and nucleolus while colocalization with anti-α-tubulin allowed us to determine the cell cycle stage.

RESULTS

Here we verify two novel nucleolar proteins, Rad53 homologue FhkA which localized around the edge of the nucleolus and CBP4a which was detected throughout the entire nucleolus. Treatment with the Ca2+ chelator BAPTA (5 mM) showed that the nucleolar localization of CBP4a is Ca2+-dependent. In response to actinomycin D (0.05 mg/mL) CBP4a disappeared from the nucleolus while FhkA protruded from the nucleus, eventually pinching off as cytoplasmic circles. FhkA and CBP4a redistributed differently during mitosis. FhkA redistributed throughout the entire cell and at the nuclear envelope region from prometaphase through telophase. In contrast, during prometaphase CBP4a relocated to many large, discrete "CBP4a islands" throughout the nucleoplasm. Two larger "CBP4a islands" were also detected specifically at the metaphase plate region.

CONCLUSIONS

FhkA and CBP4a represent the sixth and seventh nucleolar proteins that have been verified to date in Dictyostelium and the third and fourth studied during mitosis. The protein-specific distributions of all of these nucleolar proteins during interphase and mitosis provide unique insight into nucleolar protein dynamics in this model organism setting the stage for future work.

Nucleoplasmic/nucleolar translocation and identification of a nuclear localization signal (NLS) in Dictyostelium BAF60a/SMARCD1 homologue Snf12

Dictyostelium is a model eukaryote for the study of several cellular processes; however, comparatively little is known about its nucleolus. Identification of nucleolar proteins is key to understanding this nuclear subcompartment, but only four have been identified in Dictyostelium. As discussed in this article, a potential relationship between nucleolar NumA1 and BAF60a/SMARCD1 suggested BAF60a may also reside in the nucleolus. Here, we identify BAF60a homologue Snf12 as the fifth nucleolar protein in Dictyostelium. Immunolocalization experiments demonstrate that Snf12 is nucleoplasmic, but translocates to nucleoli upon actinomycin-D-induced transcription inhibition (0.05 mg/mL, 4 h). Translocation was accompanied by a microtubule-independent protrusion of nucleolar Snf12 regions from the nucleus followed by detection of Snf12 in cytoplasmic circles for at least 48 h. Residues (372)KRKR(375) are both necessary and sufficient for nucleoplasmic localization of Snf12 and represent a functional nuclear localization signal (NLS), similar to recently identified NLSs in other Dictyostelium proteins. Since nucleolar and nucleoplasmic proteins redistribute during mitosis, we investigated Snf12 dynamics during this time. Dictyostelium undergoes closed mitosis, meaning its nuclear envelope remains intact. Despite this, during metaphase and anaphase Snf12 redistributed throughout the cytoplasm before reaccumulating in the nucleus during telophase, unlike the previously reported nucleoplasmic redistribution of nucleolar NumA1. The nuclear exit of Snf12 was independent of its putative nuclear export signal and not inhibited by exportin inhibition, suggesting that the redistribution of nuclear proteins during mitosis in Dictyostelium is mediated by other mechanisms. Snf12 is the second Dictyostelium nucleolar protein for which its dynamics during mitosis have been investigated.

Microtubules, centrosomes and intermediate filaments in directed cell movement

Cell movement involves the coordinated interaction of probably hundreds of components. The contractile apparatus based on actin, myosin and their associated proteins is involved in cell protrusion and force generation. Microtubules and intermediate filaments affect the distribution of membranous organelles and are also believed to determine cell shape and cell polarity. This review examines the way in which the distinct polarity of moving cells is influenced by microtubules, the microtubule-organizing centre and intermediate filaments. The observations summarized here suggest a broad spectrum of cell-type-specific differences in how these cytoskeletal components contribute to directional cell movement.

The spindle pole body of the pathogenic yeast Exophiala dermatitidis: variation in morphology and positional relationship to the nucleolus and the bud in interphase cells

The spindle pole body (SPB) in the interphase cell of the pathogenic yeast Exophiala dermatitidis was studied in detail. The SPB was located on the outer nuclear envelope and was 342 +/- 86 nm long in a haploid strain. It consisted of two disk elements that measured 151 +/- 43 nm in diameter and 103 +/- 17 nm in thickness, connected by a rod-shaped midpiece that measured 56 +/- 20 nm in length and 37 +/- 9 nm in diameter. There were considerable variations in size and morphology of interphase SPB. Some disk elements appeared spherical but others were more flattened, and there was variation in electron density. A few SPBs did not have the midpiece. The SPB of a diploid strain was 486 +/- 118 nm long, thus significantly bigger than that of the haploid strain. The SPB tended to be localized away from the nucleolus (110 +/- 48 degrees), but close to the bud (78 +/- 45 degrees). The present study highlights the necessity of observing a large number of micrographs in three-dimensions to describe accurately the ultrastructure of the SPB in yeast.

The formation of actin rods composed of actin tubules in Dictyostelium discoideum spores

A new type of actin rod formed in both the nucleus and the cytoplasm, as well as tyrosine phosphorylation of actin, is implicated in the maintenance of dormancy and viability of Dictyostelium discoideum spores. Here the ultrastructure of the rods and their relationship to the phosphorylation of actin were examined. The rods first appeared in premature spores at the midculmination stage as bundles composed of actin tubules hexagonally cross-linked. The 13-nm-diameter bundles were composed of three actin filaments. Formation of the actin rods begins during the late culmination stage and proceeds until 2 days after completion of fruiting bodies. The physical events occur in the following order; association of several modules of bundles, close packing and decrease in diameter of actin tubules, elongation of rods across the nucleus or the cytoplasm. Actin phosphorylation levels increased at the late culmination stage and reached a maximum level 12 h later. Immediately following activation of spore germination, actin was rapidly dephosphorylated, followed shortly thereafter by the disappearance of rods. Shortened actin tubules once again became arranged in a hexagonal pattern. This hexagonal arrangement of actin tubules is possibly involved in rod formation and disappearance and does not depend upon actin phosphorylation. In contrast, rod-maturation processes may correlate with actin phosphorylation.

Novel actin cytoskeleton: actin tubules

In spores of Dictyostelium discoideum three actin filaments are bundled to form a novel tubular structure and the tubules are then organized into rods. These tubular structures we will term actin tubules. Actin tubules are reconstructed from the supernatant of spore homogenates, while the usual actin filaments were bundled after incubation of supernatants from growing cells. Alpha-actinin, ABP-120 and EF-1alpha are not essential for rod formation. Cofilin is a component of the cytoplasmic rods but few cofilin molecules are included in the nuclear rods. The viability of spores lacking actin rods is very low, and the spore shape is round instead of capsular. The rods can be fragmented by pressure, indicating that the rods may be effective in absorbing physical pressure. The complex organization of actin filaments, actin tubules and rods may be required for spores to achieve complete dormancy and maintain viability.

Hyperosmotic stress-induced reorganization of actin bundles in Dictyostelium cells over-expressing cofilin

BACKGROUND

Cofilin is a low-molecular weight actin-modulating protein, which binds to, severs, and depolymerizes actin filaments in vitro. Aip1, an actin-interacting protein, was recently identified as a product of a gene on a multicopy plasmid which suppresses the temperature-sensitive phenotype of a cofilin mutant in Saccharomyces cerevisiae. Actin cytoskeleton plays an essential role in resistance to hyperosmotic stress in Dictyostelium discoideum. The roles of cofilin and Aip1 in this resistance are not known.

RESULTS

In response to hyperosmotic stress, D. discoideum cells round up. This stress-induced morphological change involves the redistribution of cofilin, together with actin filaments, into cortical contractile portions of the cells, followed by their contraction. Over-expression of cofilin increases and thickens cortical actin bundles in cells. The bundles become tight and are reorganized into a ring-shaped structure in response to hyperosmotic stress. The ring structure of actin bundles had two characteristic bands across them; bright and dark bands, heavily stained and not stained with phalloidin. In the bundles, straight filaments with a diameter of 5.3-nm were aligned parallel by cross-bridge structures. In cells lacking the myosin-II heavy chain, the bundles, which were induced by an over-expression of cofilin, shortened and became straight following hyperosmotic stress, forming a polygonal structure. D. discoideum Aip1/Wrp2 enhanced the severing of actin filaments by cofilin in vitro and colocalized with cofilin in cells, including those that were over-expressing cofilin before and after exposure to hyperosmotic stress.

CONCLUSIONS

Cofilin plays a pivotal role in concert with Aip1/Wrp2 in the reorganization of actin architectures into bundles that contract in a myosin-II-independent manner, in response to hyperosmotic stress.

Nuclear components with microtubule-organizing properties in multicellular eukaryotes: functional and evolutionary considerations

The nucleus and the microtubular cytoskeleton of eukaryotic cells appear to be structurally and functionally interrelated. Together they constitute a "cell body". One of the most important components of this body is a primary microtubule-organizing center (MTOC-I) located on or near the nuclear surface and composed of material that, in addition to constitutive centrosomal material, also comprises some nuclear matrix components. The MTOC-I shares a continuity with the mitotic spindle and, in animal cells, with the centrosome also. Secondary microtubule-organizing centers (MTOC-IIs) are a special feature of walled plant cells and are found at the plasma membrane where they organize arrays of cortical MTs that are essential for ordered cell wall synthesis and hence for cellular morphogenesis. MTOC-IIs are held to be similar in origin to the MTOC-I, but their material has been translocated to the cell periphery, perhaps by MTs organized and radiating from the MTOC-I. Many intranuclear, matrix-related components have been identified to participate in MT organization during mitosis and cytokinesis; some of them also seem to be related to the condensation and decondensation of chromatin during the mitotic chromosome cycle.

Abnormal integrity of the nucleolus associated with cell cycle arrest owing to the temperature-sensitive ubiquitin-activating enzyme E1

A mouse cell mutant, ts85, containing the temperature-sensitive ubiquitin-activating enzyme was arrested in G2 phase at the non-permissive temperature. In the arrested cells, azure C, a nucleolus-specific stain, revealed a U-shaped or ring-shaped arrangement of nucleolar lobes with an unstained region in the center. Silver staining of the nucleolar organizer region (NOR) and fluorescence in situ hybridization (FISH) with rDNA both gave signals in azure C-positive regions. Electron microscopic examination revealed a cloud of unidentified electron-dense particles (diameter approximately 70 nm) in the azure C-negative center space. When the arrested cells were released into M-phase, we observed the association of NOR-bearing chromosomes with a pulverization-like abnormality. FISH with rDNA and NOR silver staining demonstrated that the pulverization-like abnormality was restricted to NORs. The frequent occurrence of persistent nucleolar material in prophase and prometaphase of the stressed cells after release indicated a delayed dissociation of the nucleolus that brought about the abnormal chromosomes in M-phase. ts85 cells transfected with the mouse E1 cDNA recovered growth at the non-permissive temperature and no longer showed abnormal nucleolar morphology. It seems that the ubiquitin system plays a role in the dissolution of the nucleolus, possibly involving the NOR-bearing chromosomes.