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Association of membrane cisternae and macrotubules in vinblastine-treated histiocytes.

Resting histiocytes within the dermis and epidermis of the cichlid fish Pterophyllum scalare exhibit an unusual association between stacks of lamellar cisternae and tubular structures greater in diameter than ordinary microtubules (macrotubules) after treatment with 2 x 10(-4) M vinblastine. Macrotubules are found in the intercisternal spaces in a regular side-by-side orientation and extend the entire length of the cisternae. The lamellae seem to be formed as a result of increased membrane proliferation of the outer nuclear membrane; in some instances, cisternae have been observed to be continuous with the nuclear envelope. Since blebbing of the outer nuclear membrane is also observed in untreated histiocytes, lamellae formation is envisaged as hypertrophy of a normally occurring process. The association of lamellae with macrotubules possibly indicates a relationship between membranes and microtubule proteins.

Animals↗

Contribution of noncentrosomal microtubules to spindle assembly in Drosophila spermatocytes.

Previous data suggested that anastral spindles, morphologically similar to those found in oocytes, can assemble in a centrosome-independent manner in cells that contain centrosomes. It is assumed that the microtubules that build these acentrosomal spindles originate over the chromatin. However, the actual processes of centrosome-independent microtubule nucleation, polymerisation, and sorting have not been documented in centrosome-containing cells. We have identified two experimental conditions in which centrosomes are kept close to the plasma membrane, away from the nuclear region, throughout meiosis I in Drosophila spermatocytes. Time-lapse confocal microscopy of these cells labelled with fluorescent chimeras reveals centrosome-independent microtubule nucleation, growth, and sorting into a bipolar spindle array over the nuclear region, away from the asters. The onset of noncentrosomal microtubule nucleation is significantly delayed with respect to nuclear envelope breakdown and coincides with the end of chromosome condensation. It takes place in foci that are close to the membranes that ensheath the nuclear region, not over the condensed chromosomes. Metaphase plates are formed in these spindles, and, in a fraction of them, some degree of polewards chromosome segregation takes place. In these cells that contain both membrane-bound asters and an anastral spindle, the orientation of the cytokinesis furrow correlates with the position of the asters and is independent of the orientation of the spindle. We conclude that the fenestrated nuclear envelope may significantly contribute to the normal process of spindle assembly in Drosophila spermatocytes. We also conclude that the anastral spindles that we have observed are not likely to provide a robust back-up able to ensure successful cell division. We propose that these anastral microtubule arrays could be a constitutive component of wild-type spindles, normally masked by the abundance of centrosome-derived microtubules and revealed when asters are kept away. These observations are consistent with a model in which centrosomal and noncentrosomal microtubules contribute to the assembly and are required for the robustness of the cell division spindle in cells that contain centrosomes.

Animals↗

A freeze-fracture study of the soma membranes of monoaminergic neurons in the hypothalamic nucleus recessi posterioris of the rainbow trout (Salmo gairdneri Rich.).

Monoaminergic neurons in the hypothalamic nucleus recessi posterioris of the rainbow trout were studied by freeze-fracturing. The size and distribution of intramembrane particles in the nuclear and plasma membranes and the density of nuclear pores in the nuclear envelope were determined. The density of nuclear pores in these cells was significantly greater than in the non-granulated neurons of this nucleus. The density of IMPs in the P face of the inner nuclear membrane was significantly greater than in the P face of the outer nuclear membrane; however in the E faces the density is greater in the outer nuclear membrane but the differences are not statistically significant. In both membranes, the proportion of large particles (greater than 9.6 nm) was greater in the E than in the P faces. The coefficient of partition between the plasma membrane faces was 1.97 +/- 0.24 while in non-granulated neurons was 4.45 +/- 0.51. There are significant differences among these values. Interestingly, the coefficient of partition of the plasma membrane in monoaminergic neurons was lesser than in some other types of neurons. Postsynaptic areas or other membrane specializations were not found on the cell body plasma membrane, which corroborates the results of conventional transmission electron microscopy.

Animals↗

Differences in the nuclear matrix phosphoproteins of a wild-type and nitrogen mustard-resistant rat breast carcinoma cell line.

Using polyclonal antibodies raised against a rat liver nuclear envelope protein, lamin protein A, the nuclear matrix proteins of a Walker 256 rat mammary carcinoma wild-type (WS) and a selected cell line with acquired resistance to nitrogen mustards (WR) were found to possess antigenic determinants which were recognized by the antibodies. In one-dimensional immunoblotting analysis, the nuclear matrix protein fractions of both cell lines revealed a common band at Mr 75,000; however only the WS nuclear matrix protein fraction contained a broad band at approximately Mr 70,000. Two-dimensional gel blotting studies of these proteins showed that this Mr 70,000 WS protein had a pI of approximately 7.5. Immunoprecipitation analysis revealed that the altered mobility of this protein could be a function of phosphorylation. The nuclear matrix proteins from both WS and WR cells were shown to bind 3':5'-cyclic adenylic acid (cAMP), as judged by photoaffinity labeling and gel electrophoresis studies. The WS nuclear matrix proteins showed a quantitatively greater level of cAMP binding compared to WR, with predominant binding to proteins with molecular weights of 45,000, 55,000, and 70,000. In WR cells, there was no cAMP binding in the Mr 70,000 region. These data indicate that the Mr 70,000 nuclear matrix lamin proteins are antigenically similar in WS and WR but differ in that the WR protein is hypophosphorylated and does not bind cAMP.

Animals↗

Nucleolar ultrastructure in bovine nuclear transfer embryos.

Nuclear transfer experiments in mammals have attempted to reprogram a donor nucleus to a state equivalent to the zygotic one. Reprogramming of the donor nucleus is, among other features, indicated by a synthesis of ribosomal RNA (rRNA). The initiation of rRNA synthesis is simultaneously reflected in nuclear morphology as a transformation of the nucleolus precursor body into a functional rRNA synthesising nucleolus with a characteristic ultrastructure. We examined nucleolar ultrastructure in bovine in vitro produced (control) embryos and in nuclear transfer embryos reconstructed from a MII phase (nonactivated) or S phase (activated) cytoplasts. Control embryos were fixed at the two-, four-, early eight- and late eight-cell stages; nuclear transfer embryos were fixed at 1 and 3 hr post fusion and at the two-, four-, and eight-cell stages. Control embryos possessed a nucleolar precursor body throughout all three cell cycles. In the eight-cell stage embryo, a primary vacuole appeared as an electron lucid area originating in the centre of the nucleolar precursor body. In nuclear transfer embryos reconstructed from nonactivated cytoplasts, the nuclear envelope was fragmented or completely broken down at 1 hr after fusion and, by 3 hr after fusion, it was restored again. At this time, the reticulated fibrillo-granular nucleolus had an almost round shape. The nucleolar precursor body seen in the two-cell stage nuclear transfer embryos consisted of intermingled filamentous components and secondary vacuoles. A nucleolar precursor body typical for the two-cell stage control embryos was never observed. None of the reconstructed embryos of this group reached the eight-cell stage. Nuclear transfer embryos reconstructed from activated cytoplasts, in contrast, exhibited a complete nuclear envelope at all time intervals after fusion. In the two-cell stage nuclear transfer embryo, the originally reticulated nucleolus of the donor blastomere had changed into a typical nucleolar precursor body consisting of a homogeneous fibrillar structure. A primary vacuole appeared in the four-cell stage nuclear transfer embryos, which was one cell cycle earlier than in control embryos. Only nuclear transfer embryos reconstructed from activated cytoplasts underwent complete remodelling of the nucleolus. The reorganisation of the donor nucleolar architecture into a functionally active nucleolus was observed as early as in the four-cell stage nuclear transfer embryo. These ultrastructural observations were correlated with our autoradiographic data on the initiation of RNA synthesis in nuclear transfer embryos.

Animals↗

A biochemical and immunological comparison of nuclear and cytoplasmic pore complexes.

Pore complexes are not confined to the nuclear envelope but can also be found in the cytoplasm of numerous cell types in the form of annulate lamellae (AL). We have induced formation of AL by exposure of rat cells (line RV) to sublethal doses of the antimitotic drug vinblastine sulfate, and compared the distribution of several nuclear pore complex proteins (nucleoporins) in the nuclear envelope and AL by immunocytochemistry, cytochemical lectin binding studies and immunoblot analyses of nuclear and AL-enriched fractions. All the antibodies used yielded punctate nuclear surface staining in immunofluorescence microscopy which is characteristic for nuclear pore complex components. When we applied antibodies against the nucleoporin p62, AL were visualized as numerous cytoplasmic dot-like structures. Immunogold electron microscopy confirmed the correspondence of the cytoplasmic bodies with stacks of AL. Antibodies to constituents of the cytoplasmic (nup180) and nucleoplasmic (nup153) filaments extending from both sides of nuclear pore complexes also stained the AL, indicating that pore complexes are intrinsically asymmetric assemblies independent of their specific intracellular topology. By contrast, AL were negative with five different antibodies against the transmembrane nuclear pore glycoprotein gp210 and the lectin concanavalin A (ConA) known to bind to the oligosaccharide side chains of gp210. Similarly, there was no staining of the AL with antibodies to the other nuclear pore membrane protein so far known in higher eukaryotes, POM121. Immunoblot analyses confirmed the presence of p62, nup180 and nup153 in both the nuclear and AL fractions and the absence of gp210 and POM121 from AL. Our results do not support the generally held view that gp210 and POM121 function in anchoring the pore complex scaffold to the pore membrane. Rather, they point to a role for these proteins in transport processes through the nuclear pore complexes. Since AL are not involved in nucleocytoplasmic transport processes they may lack components of the transport machinery.

Animals↗

O-linked glycoproteins of the nuclear pore complex interact with a cytosolic factor required for nuclear protein import.

Mediated import of proteins into the nucleus requires cytosolic factors and can be blocked by reagents that bind to O-linked glycoproteins of the nuclear pore complex. To investigate whether a cytosolic transport factor directly interacts with these glycoproteins, O-linked glycoproteins from rat liver nuclear envelopes were immobilized on Sepharose beads via wheat germ agglutinin or specific antibodies. When rabbit reticulocyte lysate (which provides cytosolic factors required for in vitro nuclear import) was incubated with the immobilized glycoproteins, the cytosol was found to be inactivated by up to 80% in its ability to support mediated protein import in permeabilized mammalian cells. Inactivation of the import capacity of cytosol, which was specifically attributable to the glycoproteins, involves stoichiometric interactions and is likely to involve binding and depletion of a required factor from the cytosol. This factor is distinct from an N-ethylmaleimide-sensitive receptor for nuclear localization sequences characterized recently since it is insensitive to N-ethylmaleimide. Cytosol inactivation is suggested to be caused by at least two proteins of the glycoprotein fraction, although substantial capacity for inactivation can be attributed to protein bound by the RL11 antibody, consisting predominantly of a 180-kD glycosylated polypeptide. Considered together, these experiments identify a novel cytosolic factor required for nuclear protein import that directly interacts with O-linked glycoproteins of the pore complex, and provide a specific assay for isolation of this component.

Animals↗

Synaptonemal complexes and XY behavior in two species of Argentinian armadillos: Chaetophractus villosus and Dasypus hybridus (Xenarthra, Dasypodidae).

Spermatocytes from the two armadillo species, C. villosus and D. hybridus were studied in microspreads for synaptonemal complexes (SCs) and in thin sections for electron microscopy (EM). The complete SC karyotype generally agrees with previous reports on mitotic chromosomes, except for the sex chromosomes. The X chromosome is submetacentric in both species and the Y is the shortest one in C. villosus and the second shortest in D. hybridus, and an extremely acrocentric one. A SC is formed along the total length of the Y chromosome, and this SC persists along all the pachytene substages. A single recombination nodule (RN) is located in the region of the SC nearest to the attachment to the nuclear envelope. The lateral element (LE) of the X axis in the SC shows a wavy aspect in most of the SC length distant from the nuclear envelope. Nucleoli are attached to acrocentric or submetacentric bivalents, are visibly double in some cells, and in thin sections show an elaborate nucleolonema. Some differences in the XY are species-specific, as the higher degree of tangling and stronger heteropycnosis in D. hybridus. The effective, single crossover of the XY pair is highly localized, despite the permanence of a long tract of SC.

Animals↗

Spermatozoon ultrastructure of Platichthys flesus.

The ultrastructure of the spermatozoon of Platichthys flesus (flounder) has been determined. The cell is of the primitive type and lacks an acrosome. The bullet-shaped, homogeneously electron-dense nucleus has a deep, caudal articular fossa containing proximal and distal centrioles. Both centrioles are embedded in pericentriolar material which is intimately associated with the nuclear envelope. Around the proximal centriole the material is organised into a dense ring which bears nine fibres associated with the triplets, and a large fibrous body, all connected to the nuclear envelope. Connections exist also between the pericentriolar material of the proximal and distal centrioles. The material around the distal centriole is organised into a complex collar. The anterior surfaces of eight spherical mitochondria lie within shallow depressions of the nuclear caudal surface. They form a ring surrounding the posterior opening of the articular fossa and the proximal region of the 9 + 2 flagellar axoneme.

Animals↗

Intranuclear microtubules are hallmarks of an unusual form of cell death in cisplatin-treated C6 glioma cells.

We describe an unusual form of non-accidental cell death marked by ectopic microtubules in the nucleus of a subpopulation of cisplatin-treated C6 glioma astrocytes in culture. At electron microscopy, the perinuclear condensed chromatin did not completely adhere to the nuclear envelope of these cells being separated by single or loosely bundled 20-nm-thick microtubules located in an electron-lucid slit-like zone; the presence of alpha-tubulin lining the inner membrane of the nuclear envelope was confirmed by immunolabeling at confocal microscopy. Since tufts of microfilaments-like fibers also occurred in their central nuclear areas, these cells are referred to as CIMMs (Cells with Intranuclear Microtubules and Microfilaments). The nuclear reorganization of CIMMs also involved nucleolar segregation and formation of heterogeneous ectopic ribonucleoprotein (RNP)-derived structures, indicating disruption of the RNP-based transcription machinery. The cytoplasmic organelles of CIMMs were structurally intact, and propidium iodide did not accumulate intracellularly under vital conditions while the plasma membrane was often Annexin V-positive. All these findings suggest that CIMMs were lethally damaged and committed to an atypical programmed cell death resembling early apoptosis (this is also supported by the presence of a limited number of TUNEL-positive CIMMs). CIMMs appeared well before the main cisplatin-induced cycling arrest of the cell population (G2/M block at 72 h) and had mostly G1 DNA content: this suggests that they may represent the cohort of cells which passed cisplatin-altered mitoses with intranuclear retention of microtubules from an incompletely disassembled mitotic spindle.

Actin Cytoskeleton↗

Somatic linker histone H1 is present throughout mouse embryogenesis and is not replaced by variant H1 degrees.

A striking feature of early embryogenesis in a number of organisms is the use of embryonic linker histones or high mobility group proteins in place of somatic histone H1. The transition in chromatin composition towards somatic H1 appears to be correlated with a major increase in transcription at the activation of the zygotic genome. Previous studies have supported the idea that the mouse embryo essentially follows this pattern, with the significant difference that the substitute linker histone might be the differentiation variant H1 degrees, rather than an embryonic variant. We show that histone H1 degrees is not a major linker histone during early mouse development. Instead, somatic H1 was present throughout this period. Though present in mature oocytes, somatic H1 was not found on maternal metaphase II chromatin. Upon formation of pronuclear envelopes, somatic H1 was rapidly incorporated onto maternal and paternal chromatin, and the amount of somatic H1 steadily increased on embryonic chromatin through to the 8-cell stage. Microinjection of somatic H1 into oocytes, and nuclear transfer experiments, demonstrated that factors in the oocyte cytoplasm and the nuclear envelope, played central roles in regulating the loading of H1 onto chromatin. Exchange of H1 from transferred nuclei onto maternal chromatin required breakdown of the nuclear envelope and the extent of exchange was inversely correlated with the developmental advancement of the donor nucleus.

Animals↗

The synaptonemal complexes of caenorhabditis elegans: comparison of wild-type and mutant strains and pachytene karyotype analysis of wild-type.

Normal synaptonemal complexes (SCs), consisting of two lateral elements and a central element, are present in wild-type, him-4 and him-8 mutant strains in both hermaphrodites and males of caenorhabditis elegans. Thus, the increase in rate of nondisjunction in the him mutants is not related to aberrant SC morphology. The wild-type hermaphrodite has six SCs, as determined from 3-D reconstruction analysis of serial section from electron microscopy. Thus, n=6 and this confirms early reports based on cytological studies with the light microscope. Only one end of the SC is attached to the nuclear envelope while the other end is free in the nucleoplasm and there is no apparent bouquet formation. Either end of the SC can attach to the nuclear envelope. The pairing behavior of the XX bivalent is normal and occurs synchronously with the autosomes. Electron dense bodies, or knobs, are associated with the SC via the central element and displace the chromatin for a distance of 200 nm. Each pachytene nucleus of the wild-type hermaphrodite has six such structures that are randomly dispersed along the bivalents such that some SCs have one or two knobs while others have none. Their function is unknown.

Animals↗

Interaction between rat brain microtubule associated proteins (MAPs) and free ribosomes from Xenopus oocyte: a possible mechanism for the in ovo distribution of MAPs.

The binding of microtubule associated proteins (MAPs) to free 80 S ribosomes isolated from Xenopus laevis oocytes inhibits in vitro tubulin assembly (Jessus et al., 1984). The inhibition of tubulin polymerisation was shown to be dependent upon GTP. The dose of GTP needed to induce 50% of the maximal effect was 0.5 mM. Furthermore, the inhibition is enhanced by pretreatment of the ribosomes with ATP-gamma-S, and partially abolished after phosphatase treatment, which strongly suggests that protein phosphorylation regulated the inhibitory effect. When fluorescent purified MAPs are microinjected into Xenopus laevis oocyte, they cap 1 h later the basal nuclear envelope; in contrast, when the fluorescent MAPs-ribosome complex is injected, the fluorescent MAPs remain in the cytoplasm and never reach the region underlying the nuclear envelope.

Animals↗

Corona radiata density as a non-invasive marker of bovine cumulus-corona-oocyte complexes selected for in vitro embryo production.

The density of the corona radiata as a marker for the quality of cumulus-corona-oocyte complexes (CCOC's) for in vitro embryo production was tested. The CCOC's in which the corona radiata appeared as a dark rim surrounding the zona pellucida (Group 1) and CCOC's in which the corona had the same density as the rest of the cumulus investment (Group 2) were assessed with respect to nuclear ultrastructure, corona-cumulus expansion and capacity for sustaining embryonic development in vitro. An intermediate Group 3 with characteristics between Groups 1 and 2 was also assessed for in vitro development capacity. The CCOC's in Group 1 were typically meiotically unactivated and presented a nonundulating nuclear envelope. More than half of the CCOC's in Group 2 showed some degree of meiotic activation, and those that were nonactivated displayed "holes" in the nuclear envelope or dilatations of the perinuclear cisterna. The CCOC's in Group 2 were characterized by partial corona-cumulus expansion already at collection. The CCOC's from Groups 1 and 3 sustained embryonic development in vitro at a significantly higher rate than CCOC's from Group 2. It is concluded that CCOC's in which the corona radiata has the same density as the rest of the cumulus investment are less competent candidates for in vitro embryo production.

Journal Article↗

Live imaging of telomeres: yKu and Sir proteins define redundant telomere-anchoring pathways in yeast.

BACKGROUND: The positioning of chromosomal domains within interphase nuclei is thought to facilitate transcriptional repression in yeast. Although this is particularly well characterized for telomeres, the molecular basis of their specific subnuclear organization is poorly understood. The use of live fluorescence imaging overcomes limitations of in situ staining on fixed cells and permits the analysis of chromatin dynamics in relation to stages of the cell cycle. RESULTS: We have characterized the dynamics of yeast telomeres and their associated domains of silent chromatin by using rapid time-lapse microscopy. In interphase, native telomeres are highly dynamic but remain within a restricted volume adjacent to the nuclear envelope. This constraint is lost during mitosis. A quantitative analysis of selected mutants shows that the yKu complex is necessary for anchoring some telomeres at the nuclear envelope (NE), whereas the myosin-like proteins Mlp1 and Mlp2 are not. We are able to correlate increased telomeric repression with increased anchoring and show that silent chromatin is tethered to the NE in a Sir-dependent manner in the absence of the yKu complex. Sir-mediated anchoring is S phase specific, while the yKu-mediated pathway functions throughout interphase. Subtelomeric elements of yeast telomere structure influence the relative importance of the yKu- and Sir-dependent mechanisms. CONCLUSIONS: Interphase positioning of telomeres can be achieved through two partially redundant mechanisms. One requires the heterodimeric yKu complex, but not Mlp1 and Mlp2. The second requires Silent information regulators, correlates with transcriptional repression, and is specific to S phase.

Cell Nucleus↗

Characterization of a membrane-associated 3,3',5-triiodo-L-thyronine binding protein by use of monoclonal antibodies.

Four mouse hybridoma cell lines have been isolated which secrete antibodies to the membrane-associated thyroid hormone binding protein (Mr 55,000) from human epidermoid carcinoma A431 cells. J6 is rat specific; J2 is human and monkey specific; J8 and J9 have a wider specificity and react with similar thyroid hormone binding proteins (p55) from human, monkey, rat, and hamster. None of these antibodies reacts with mouse cells. J2, J6, and J9 are of the IgG1k class, and J8 is an IgAk antibody. p55 was characterized by using these monoclonal antibodies. It is not posttranslationally processed by glycosylation, phosphorylation, or sulfation. It has a cellular degradation rate t1/2 approximately equal to 3.2 h. Using immunofluorescence and electron microscopic immunocytochemistry, p55 was found to be associated with the lumenal face of the endoplasmic reticulum and nuclear envelope. When cell homogenates were prepared, significant amounts of p55 were released into the 110000g supernatant, indicating that p55 is loosely associated with the endoplasmic reticulum and nuclear envelope.

Animals↗

Determinants for intracellular sorting of cytoplasmic and nuclear intermediate filaments.

The mechanism by which nuclear and cytoplasmic filaments are sorted in vivo was studied by examining which lamin sequences are required to target an otherwise cytoplasmic IF protein, the small neurofilament subunit (NF-L), to the nuclear lamina. By swapping corresponding domains between NF-L and lamin A, nuclear envelope targeting of NF-L was shown to require the presence of the "head" domain, a 42-amino acid sequence unique to lamin rod domains, a nuclear localization signal and the CAAX motif. Replacement of the entire COOH-terminal tail of lamin A with that of NF-L had no discernible effect on nuclear localization of lamin A, provided the substituted NF-L tail contained a NLS and a CAAX motif. This chimeric protein exhibited characteristics more typical of lamin B than that of the parental lamin A. With regard to cytoplasmic assembly properties, substitution of the head domain of lamin A for that of NF-L did not substantially affect the ability of NF-L to coassemble with vimentin in the cytoplasm. In contrast, insertion of a 42-amino acid sequence unique to lamin rod domains into NF-L profoundly affected NF-L coassembly with vimentin indicating that the 42-amino acid insertion in lamins may be important for sorting lamins from cytoplasmic IF proteins.

Amino Acid Sequence↗

Specificity of expression and effects of eicosanoid mediators in normal physiology and human diseases.

The eicosanoids are a family of oxygenated arachidonic acid derivatives that potently mediate diverse physiological and pathophysiological processes. Recent research on eicosanoids has revealed novel pathways of synthesis, a family of related cell membrane receptors, and distinctive roles in cellular functions. There are two cyclooxygenases that convert arachidonic acid to thromboxane and prostaglandins, one of which is localized in the endoplasmic reticulum and the other in the nuclear envelope. The cyclooxygenases differ in their susceptibility to inhibition by nonsteroidal antiinflammatory drugs. The leukotriene-generating pathway consists of a cytosolic perinuclear 5-lipoxygenase, two integral nuclear envelope proteins, termed 5-lipoxygenase-activating protein and LTC4 synthase, and a cytosolic LTA4 hydrolase. Each protein of the leukotriene synthetic pathway is a target for specific pharmacological intervention. Cellular recognition and effects of eicosanoids are mediated by at least 12 different G protein-associated primary receptors, which differ in tissue distribution, signaling mechanisms, and cellular behavior, as well as binding specificity. Transient localized increases in tissue concentrations of eicosanoids and the concurrent upregulation of complementary receptors influence differentiation, migration, and specific activities of cells in immunity and other integrated physiological responses.

Eicosanoids↗

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