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PubMed2007

RNA viruses: hijacking the dynamic nucleolus.

The nucleolus is a dynamic subnuclear structure with roles in ribosome subunit biogenesis, mediation of cell-stress responses and regulation of cell growth. The proteome and structure of the nucleolus are constantly changing in response to metabolic conditions. RNA viruses interact with the nucleolus to usurp host-cell functions and recruit nucleolar proteins to facilitate virus replication. Investigating the interactions between RNA viruses and the nucleolus will facilitate the design of novel anti-viral therapies, such as recombinant vaccines and therapeutic molecular interventions, and also contribute to a more detailed understanding of the cell biology of the nucleolus.

Animals↗
PubMed2006

Ultrastructural changes of the carp (Cyprinus carpio) hepatocyte nucleolus during seasonal acclimatization.

BACKGROUND INFORMATION: The eurythermal fish carp (Cyprinus carpio) adjusts to the seasonal changes in the temperature and photoperiod of its habitat through diverse cellular and molecular mechanisms. We have observed that ribosomal biogenesis is modulated during the acclimatization process and correlates with profound phenotypic changes, reflecting a seasonal-dependent ultrastructural appearance of the nucleolar components. Previous studies using classical techniques showed that in winter-adapted carp the nucleolus appears to be segregated. In the present work, we have reassessed the nucleolar ultrastructural organization of the carp in summer- and winter-adapted fish by using more specific cytochemical and immunocytological techniques. RESULTS: The acetylation method provided evidence that the nucleolar organization is different between winter- and summer-adapted carp. In winter-adapted fish the fibrillar component appears as a unique mass surrounded by several granular caps, whereas in summer-adapted carp the fibrillar component forms few cordons surrounded by granular masses. The nucleolar structure and distribution of the condensed chromatin observed varies upon seasonal acclimatization. In winter the nucleolar chromatin is densely packed in masses that surround the nucleolus, whereas during summer it displays a rather looser organization formed by filaments that not only surround the nucleolus, but also go through the nucleolar body. Using the TdT (terminal deoxynucleotidyl transferase)-immunogold labelling technique, we detected condensed and decondensed nucleolar chromatin, and found some labelling of fibrillar components in both seasons. When liver tissue from summer-adapted carp was treated with AMD (actinomycin D), we observed that the rearrangement of the nucleolar components and condensed chromatin were similar to that found in winter-adapted fish, with differences in the distribution of the perinucleolar chromatin. CONCLUSIONS: The acetylation and TdT-immunogold labelling experiments indicated that the rearrangement of the nucleolar components of winter-adapted carp is very similar to the AMD-treated summer-adapted carp nucleolus, with the latter representing the repression of the ribosomal biogenesis that occurs during the cold season. Nevertheless, the distribution of the condensed perinucleolar chromatin in winter-adapted carp compared with AMD-treated cells suggests that the transcription of rRNA genes in winter-adapted fish is less strongly inhibited and does not lead to the classical segregation of the nucleolus of that described after AMD treatment. In addition, we have confirmed that carp hepatocyte nucleoli comprise only two main structural compartments: a fibrillar component and a granular component. Fibrillar centres were not observed.

Acclimatization↗
PubMed2004

In vivo HIV-1 Rev multimerization in the nucleolus and cytoplasm identified by fluorescence resonance energy transfer.

Nuclear export of intron-containing human immunodeficiency virus type 1 RNA is mediated by the viral Rev protein. Rev is a nucleocytoplasmic transport protein that directly binds to its cis-acting Rev-responsive element RNA. Rev function depends on its ability to multimerize. The in vivo dynamics and the subcellular dependence of this process are still largely unexplored. To visualize and quantitatively analyze the mechanism of Rev multimeric assembly in live cells, we used high resolution in vivo fluorescence resonance energy transfer (FRET) and fluorescence recovery after photobleaching. By using two different dynamic FRET approaches (acceptor photobleaching and donor bleaching time measurements), we observed a strong Rev-Rev interaction in the nucleoli of living cells. Most interestingly, we could also detect Rev multimerization in the cytoplasm; however, FRET efficiency in the cytoplasm was significantly lower than in the nucleolus. By using fluorescence recovery after photobleaching, we investigated the mobility of Rev within the nucleolus. Mathematical modeling of the fluorescence recovery after photobleaching recoveries enabled us to extract relative association and dissociation constants and the diffusion coefficient of Rev in the nucleolus. Our results show that Rev multimerizes in the nucleolus of living cells, suggesting an important role of the nucleolus in nucleocytoplasmic transport.

Cell Nucleolus↗
PubMed1969

Sites of nucleolus production in cultured Chinese hamster cells.

Chinese hamster cell strains in the early passages in culture display wide variation in number of nucleolus-like bodies per cell, though such strains are characteristically euploid. A variety of criteria indicate that the nucleolus-like bodies are true nucleoli. Their Azure B- and fast green-staining properties indicate the presence of RNA and protein; they have typical nucleolar fine structure, including both fibrous and granular components; radioautography reveals that their patterns of uptake of uridine-(3)H into RNA are similar to those reported for nucleoli of other cell types; actinomycin D, at a level which selectively inhibits ribosomal RNA synthesis, greatly reduces their RNA synthesis and also causes segregation of fibrous and granular nucleolar components. Colchicine was used to experimentally fragment the nuclei of these cells into a number of separate karyomeres, each presumably containing some, or only one, of the chromosomes of the complement. Almost all the karyomeres contain nucleolus-like bodies which, by the same criteria applied to the multiple nucleolus-like bodies of uninuclear cells, appear to be true nucleoli. The nucleoli of individual karyomeres of the same cell often differ from each other in fine structure while the multiple nucleoli of a uninuclear cell generally resemble each other. The evidence presented in this study indicates that Chinese hamster cells contain many nucleolus-producing sites scattered through the genome.

Animals↗
PubMed2001

RNA relocation and persistence of nucleolus-like bodies at mitosis in benzo[a]pyrene-transformed human breast epithelial cells after microcell-mediated transfer of chromosomes 11 and 17.

RNA relocation and the incidence of nucleolus-like bodies accumulated during mitosis were studied cytochemically in benzo[a]pyrene (BP)-transformed human breast epithelial MCF-10F cells after microcell-mediated transfer of normal chromosomes 11 and 17. The changes resulting from the transfer of these two chromosomes in tumorigenic MCF-10F cells (BP1-E cell line) were examined, since alterations in these chromosomes are involved in the expression of the transformed and tumorigenic phenotypes in the MCF-10F cell series. In addition, the frequency of nucleolus-like bodies decreases drastically with transformation and tumorigenicity in MCF-10F cells, thus being conceivable that it would be affected in presence of normal chromosomes 11 or 17. The pattern of RNA relocation associated with the mitotic spindle did not vary in the cell lines analyzed. The introduction of chromosome 17 in BP1-E cells either decreased or did not affect the frequency of persistent nucleolus-like bodies. In contrast, in cells which received a normal chromosome 11, the frequency of nucleolus-like bodies was closer to that of non-transformed MCF-10F cells. These results suggest that a normal chromosome 11 but not chromosome 17 contributes to the maintenance of an RNA surplus which accumulates in nucleolus-like bodies during cell division of the human breast epithelial cells, at least in vitro. Some loci which were retained in the BP1-E cells which received a normal chromosome 11 are probably involved with the control of RNA transcript production. Figure 1 on http://www.esacp.org/acp/2001/23-3,4/mello.htm

Benzo(a)pyrene↗
PubMed1998

Dynamics of the sub-nuclear distribution of Modulo and the regulation of position-effect variegation by nucleolus in Drosophila.

modulo belongs to the class of Drosophila genes named 'suppressor of position-effect variegation', suggesting the involvement of the encoded protein in chromatin compaction/relaxation processes. Using complementary procedures of cell fractionation, immunolocalisation on mitotic and polytene chromosomes and cross-linking/immunoprecipitation of genomic DNA targets, we have analysed the sub-nuclear distribution of Modulo. While actually associated to condensed chromatin and heterochromatin sites, the protein is also abundantly found at nucleolus. From a comparison of Modulo pattern on chromosomes of different cell types and mutant lines, we propose a model in which the nucleolus balances the Modulo protein available for chromatin compaction and PEV modification. At a molecular level, repetitive elements instead of rDNA constitute Modulo DNA targets, indicating that the protein directly contacts DNA in heterochromatin but not at the nucleolus. Consistent with a role for Modulo in nucleolus activity and protein synthesis capacity, somatic clones homozygous for a null mutation express a cell-autonomous phenotype consisting of growth alteration and short slender bristles, characteristic traits of Minute mutations, which are known to affect ribosome biogenesis. The results provide evidence suggesting that Modulo participates in distinct molecular networks in the nucleolus and heterochromatin and has distinct functions in the two compartments.

Animals↗
PubMed1977

The nucleolus and meiosis during microsporogenesis in Endymion non-scriptus (L.).

Stages of meiosis from the bluebell Endymion non-scriptus (L.) were studied by electron microscopy. The segregated components of the nucleolus at meiotic prophase underwent fragmentation and dissolution at pachytene-diplotene. Nucleoli were absent during both meiotic divisions and reformed on the nucleolus organizer into a fibrillar mass from scattered fibrillar components at the dyad and tetrad stages. Ti is argued that the fibrillar region shows continuity through nuclear division though undergoing structural transformations in the process. Nucleolar reformation occurs on condensed nucleolus organizers. Processing of the ribosomal precursors and the resumption of RNA synthesis is discussed in relation to the dispersal of the nucleolus organizer into the fibrillar region of the reformed nucleolus.

Cell Nucleolus↗
PubMed1981

Ultrastructural organization, sites of transcription and distribution of fibrillar centres in the nucleolus of the mouse oocyte.

The emergence of newly formed nucleoli and their development have been studied in mouse oocytes from pachytene to diplotene stages. At mid-pachytene, the nucleolus first appears as a fibrillar centre surrounded by a layer of electron-dense fibrils and penetrated by chromatin fibres emanating from the secondary constriction region of the nucleolar bivalent. Since this bivalent contains 2 paired nucleolar organizers, 2 nucleoli are formed in a symmetrical fashion. At advanced pachytene, the nucleoli are extended by strands of fibrillar component which become fibrillogranular distally. The 2 nucleoli fuse together at late pachytene. At diplotene, the nucleolus becomes large and reticulated. The development of the nucleolonema coincides with the appearance of numerous secondary fibrillar centres. Three-dimensional reconstruction of the reticulated nucleolus shows that the number of fibrillar centres largely exceeds that of nucleolar organizers. Radioautography after [3H]uridine incorporation demonstrates that during the first step of nucleologenesis the labelling is limited to the layer of electron-dense fibrils surrounding the fibrillar centre. Study of the time course of tritiated uridine incorporation from pachytene to diplotene shows that the labelling extends with the extending strands of fibrillar component. In the fully developed nucleolus, all fibrillar strands are labelled and contain, therefore, actively transcribed rDNA. These observations suggest that the rDNA, which is initially compacted in the primary fibrillar centre at the onset of nucleogenesis, progressively unravels and becomes distributed throughout the fibrillar parts of the nucleolonema. The lack of labelling of the secondary fibrillar centres suggests that they are zones of inactivity of the ribosomal genes where the rDNA remains locally compacted. A model of the ultrastructural organization of the nucleolus is proposed based on our observations.

Animals↗
PubMed1985

Chronobiological studies on the nucleolus.

Chronobiological studies on the nucleolus were performed, using stereological analysis at the electron microscopic level, on different cell types in permanent interphasic state, in rats submitted to various lighting regimen. During the dark span (12L/12D): (1) in sympathetic neurons of superior cervical ganglion circadian changes in nucleolar organization were characterized by an increase in volumes of the nucleolus and each of its components, namely, fibrillar centres, dense fibrillar, granular and vacuolar components; (2) concerning the fibrillar centres, regarded as the interphasic counterpart of nucleolar organizing regions (NORs), the most striking fact, only observed in sympathetic neurons, is the occurrence of a single large-type fibrillar centre, accompanied by small-type fibrillar centres which are present throughout the 24-hr period; (3) the overall increase in volume of fibrillar centres was shown to correspond to a marked drop in the number (up to 4 fold) of small-type fibrillar centres, the unit volume of which (0.01 mum3) remaining unchanged over the 24-hr period and to an increase in size of a large-type fibrillar centre, the volume of which is 100 fold greater than the latter and (4) cytochemical studies showed that the Ag-NOR proteins exhibit a marked increase in amount, suggesting a circadian rhythmicity of these nucleolar proteins. These results, discussed in the light of our current understanding of the nucleolus, briefly summarized in this paper, suggest that the circadian rhythm of the nucleolus and of its components is correlated with circadian rhythms in both transcriptional activity and processing of preribosomes. Analogies between sympathetic neurons and the two other cell types studied, namely, chromaffin cells of adrenal medulla and vagal sensory neurons of nodose ganglion, led to the conclusion that rhythmicity is a fundamental characteristic of the nuclear structure devoted to ribosome biogenesis. Attention is focused on the superior cervical ganglion in which amplitude of nucleolar rhythms are greater and in which fibrillar centres exhibit a particular pattern. These results are discussed with regard to the role played by this sympathetic paravertebral ganglion which is known to regulate circadian rhythmic activities of the rat pineal gland. The persistence of these nucleolar rhythms in continuous lighting, as demonstrated in sympathetic superior cervical ganglion neurons, provide evidence that they are endogenously generated. The intrinsic factors underlying these rhythms in morpho-functional organization of the nucleolus are yet unknown.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenal Medulla↗
PubMed1986

Ultrastructural and cytochemical study of the pinealocyte nucleolus in rats.

The pinealocyte nucleolus has practically not been studied, although many studies have been carried out on this particular cell. In order to study the pinealocyte nucleolus, we have used conventional electron microscopy techniques, as well as cytochemical methods: silver nitrate, phosphotungstic acid (PTA) and ethylenediaminetetraacetic acid (EDTA). The pinealocyte nucleolus shows a reticular structure formed by a dense fibrillar component and closely packed granules (granular component), as well as electron lucent spaces which contain nucleoplasm. There are also places of light fibrillar material (fibrillar centers). Using the Ag-staining we have observed spherical aggregates of argyrophylic granules densely packed (fibrillar centers) and other granules loosely packed in its periphery (dense fibrillar component). PTA stains preferentially the fibrillar centers. We have identified as heterochromatin a dense granular aggregate that appears associated to the nucleolus. We present the pinealocyte nucleolus as an excellent model for the study of the functional meaning of nucleolar components.

Animals↗
PubMed1980

Effect of low temperature on the nucleolus of larkspur (Delphinium Ajacis L.) meiocytes.

In comparison with stage specific modifications of nucleolus in the early stages of meiotic prophase in Delphinium Ajacis meiocytes, the effect of low temperature is described. On the contrary to control leptotene nucleolus, exhibiting intermingled fibrillar and granular components, cold induced a disappearance of fibrillar material from these organelles. Disappearance of fibrillar material was accompanied by marked diminishment of 3H-uridine incorporation. NOR in control as well as in cold treated material was present. Zygotene nucleolus being in eccentric position displayed segregation of granular and fibrillar components, especially well visible in cold treated cells. In the area occupied in control material by NOR, cold treated cells were found to contain the numerous dark granules. The application of Bernhard's EDTA technique and enzymatic digestion with RNase, DNase and pepsine have proved that the dark granules are composed of RNA. Although pachytene nucleolus still pressed to nuclear envelope exhibited compact structure, composed probably of fibrillar and granular elements, in cold treated meiocytes it showed only the presence of granular component. Moreover, in cold treated cells some dark granules of RNA character were visible at the surface of nucleolus. No was absent. In addition to these changes, in the cytoplasm of cold treated cells in pachytene stage a material of nucleolar origin has appeared. The meiotic irregularities described in the present paper are discussed in relation to the partial male sterility that occurs in Delphinium meiocytes growing in a low temperature.

Cell Nucleolus↗
PubMed1996

Heat shock disassembles the nucleolus and inhibits nuclear protein import and poly(A)+ RNA export.

Heat shock causes major positive and negative changes in gene expression, drastically alters the appearance of the nucleolus and inhibits rRNA synthesis. We here show that it causes many yeast nucleolar proteins, including the fibrillarin homolog Nop1p, to relocate to the cytoplasm. Relocation depends on several proteins implicated in mRNA transport (Mtrps) and is reversible. Two observations indicate, surprisingly, that disassembly results from a reduction in Ssa protein (Hsp70) levels: (i) selective depletion of Ssa1p leads to disassembly of the nucleolus; (ii) preincubation at 37 degrees C protects the nucleolus against disassembly by heat shock, unless expression of Ssa proteins is specifically inhibited. We observed that heat shock or reduction of Ssa1p levels inhibits protein import into the nucleus and therefore we propose that inhibition of import leads to disassembly of the nucleolus. These observations provide a simple explanation of the effects of heat shock on the anatomy of the nucleolus and rRNA transcription. They also extend understanding of the path of nuclear export. Since a number of nucleoplasmic proteins also relocate upon heat shock, these observations can provide a general mechanism for regulation of gene expression. Relocation of the hnRNP-like protein Mtr13p (= Npl3p, Nop3p), explains the heat shock sensitivity of export of average poly(A)+ RNA. Strikingly, Hsp mRNA export appears not to be affected.

Cell Nucleolus↗
PubMed2003

Natural variation in nucleolar dominance reveals the relationship between nucleolus organizer chromatin topology and rRNA gene transcription in Arabidopsis.

In genetic hybrids, nucleolus formation on chromosomes inherited from only one parent is the epigenetic phenomenon, nucleolar dominance. By using Arabidopsis suecica, the allotetraploid hybrid of Arabidopsis thaliana and Arabidopsis arenosa, natural variation in nucleolar dominance was found to occur, providing a unique opportunity to examine homologous nucleolus organizer regions (NORs) in their active and inactive states. In A. suecica strain LC1, NORs derived from A. arenosa are active, whereas A. thaliana-derived NORs are silenced. In A. suecica strain 9502, NORs of both parental species are active. When active, NORs are partially, but not fully, decondensed. Both active and inactive LC1 NORs colocalize with the nucleolus, contradicting the long-standing assumption that rRNA gene transcription drives nucleolus association. Collectively, these observations clarify the relationships among NOR chromatin topology, rRNA gene transcription, and NOR-nucleolus associations. A. suecica strains LC1 and 9502 have each lost one pair of A. thaliana NORs during evolution, and amplified fragment-length polymorphism analysis further indicates that these strains are genetically very similar. These data suggest that nucleolar dominance can result from subtle genetic or epigenetic variation but is not a trait fundamental to a given interspecies hybrid combination.

Arabidopsis↗
PubMed2006

The nucleolus: a model for the organization of nuclear functions.

Nucleoli are the prominent contrasted structures of the cell nucleus. In the nucleolus, ribosomal RNAs (rRNAs) are synthesized, processed and assembled with ribosomal proteins. The size and organization of the nucleolus are directly related to ribosome production. The organization of the nucleolus reveals the functional compartmentation of the nucleolar machineries that depends on nucleolar activity. When this activity is blocked, disrupted or impossible, the nucleolar proteins have the capacity to interact independently of the processing activity. In addition, nucleoli are dynamic structures in which nucleolar proteins rapidly associate and dissociate with nucleolar components in continuous exchanges with the nucleoplasm. At the time of nucleolar assembly, the processing machineries are recruited in a regulated manner in time and space, controlled by different kinases and form intermediate structures, the prenucleolar bodies. The participation of stable pre-rRNAs in nucleolar assembly was demonstrated after mitosis and during development but this is an intriguing observation since the role of these pre-rRNAs is presently unknown. A brief report on the nucleolus and diseases is proposed as well as of nucleolar functions different from ribosome biogenesis.

Cell Cycle↗
PubMed1985

Ultrastructural and functional variations in the spermatid nucleolus during spermiogenesis in the mouse.

The localization, structure, and activity of the nucleolus-organizers (NORs) were studied during spermiogenesis in the mouse by light and electron microscopy procedures including NOR-silver-staining and actinomycin D treatment. After the two meiotic divisions the NORs resume their activity during the Golgi phase of spermatid differentiation (steps 1-3), and the nucleolus displays a specific 'padlock' structure containing the fibrillar components of an active nucleolus. This activity drops during the cap phase (steps 4-7) during which the nucleolus undergoes a segregation process of its components. No nucleolar structure is visible during the acrosomal and maturation phases of spermatid differentiation.

Animals↗
PubMed2004

Behaviour of nucleolus organizing regions (NORs) and nucleoli during mitotic and meiotic divisions in budding yeast.

Spatial organization and segregation behaviour in mitosis and meiosis of nucleoli and NORs of Saccharomyces cerevisiae were examined by fluorescence in situ hybridization (FISH) and immunostaining, and integrated with previous ultrastructural studies. Our observations suggest that in interphase the NOR-bearing chromosome arm forms a hairpin-shaped loop inside the nucleolus, unlike the other arms which adopt a Rabl-like orientation. Prior to mitosis and meiosis, the appearance of the NOR changes from puffed to thread-shaped. In mitosis, it is stretched between the mother and daughter nuclei and seems to be among the last regions where chromatids separate. The nucleoli remain intact and split at the end of anaphase. Similarly, during meiosis I, intact nucleoli trail behind the separating homologous NORs and are partitioned equally to the two half-nuclei. During the second meiotic division, however, the nucleolus, together with a major portion of the nucleoplasm and the nuclear pore complexes, are not included in the spores. The behaviour of nucleoli in meiotic mutants and a strain with extrachromosomal rDNA suggests that they are not actively extruded but rather are lost due to their detachment from the separating chromosomes. We discuss the possibility that the exclusion of the nucleolus from the spores serves the disposal of agents that resort to the nucleolus, and has a role in spore dormancy or rejuvenation.

Cell Nucleolus↗
PubMed2002

Signal recognition particle RNA localization within the nucleolus differs from the classical sites of ribosome synthesis.

The nucleolus is the site of ribosome biosynthesis, but is now known to have other functions as well. In the present study we have investigated how the distribution of signal recognition particle (SRP) RNA within the nucleolus relates to the known sites of ribosomal RNA synthesis, processing, and nascent ribosome assembly (i.e., the fibrillar centers, the dense fibrillar component (DFC), and the granular component). Very little SRP RNA was detected in fibrillar centers or the DFC of the nucleolus, as defined by the RNA polymerase I-specific upstream binding factor and the protein fibrillarin, respectively. Some SRP RNA was present in the granular component, as marked by the protein B23, indicating a possible interaction with ribosomal subunits at a later stage of maturation. However, a substantial portion of SRP RNA was also detected in regions of the nucleolus where neither B23, UBF, or fibrillarin were concentrated. Dual probe in situ hybridization experiments confirmed that a significant fraction of nucleolar SRP RNA was not spatially coincident with 28S ribosomal RNA. These results demonstrate that SRP RNA concentrates in an intranucleolar location other than the classical stations of ribosome biosynthesis, suggesting that there may be nucleolar regions that are specialized for other functions.

Animals↗
PubMed1965

THE FINE STRUCTURE OF THE NUCLEOLUS DURING MITOSIS IN THE GRASSHOPPER NEUROBLAST CELL.

The behavior of the nucleolus during mitosis was studied by electron microscopy in neuroblast cells of the grasshopper embryo, Chortophaga viridifasciata. Living neuroblast cells were observed in the light microscope, and their mitotic stages were identified and recorded. The cells were fixed and embedded; alternate thick and thin sections were made for light and electron microscopy. The interphase nucleolus consists of two fine structural components arranged in separate zones. Concentrations of 150 A granules form a dense peripheral zone, while the central regions are composed of a homogeneous background substance. Observations show that nucleolar dissolution in prophase occurs in two steps with a preliminary loss of the background substance followed by a dispersal of the granules. Nucleolar material reappears at anaphase as small clumps or layers at the chromosome surfaces. These later form into definite bodies, which disappear as the nucleolus grows in telophase. Evidence suggests both a collecting and a synthesizing role for the nucleolus-associated chromatin. The final, mature nucleolar form is produced by a rearrangement of the fine structural components and an increase in their mass.

Anaphase↗