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PubMed1998

Mutational analysis of the structure and localization of the nucleolus in the yeast Saccharomyces cerevisiae.

The nucleolus in Saccharomyces cerevisiae is a crescent-shaped structure that makes extensive contact with the nuclear envelope. In different chromosomal rDNA deletion mutants that we have analyzed, the nucleolus is not organized into a crescent structure, as determined by immunofluorescence microscopy, fluorescence in situ hybridization, and electron microscopy. A strain carrying a plasmid with a single rDNA repeat transcribed by RNA polymerase I (Pol I) contained a fragmented nucleolus distributed throughout the nucleus, primarily localized at the nuclear periphery. A strain carrying a plasmid with the 35S rRNA coding region fused to the GAL7 promoter and transcribed by Pol II contained a rounded nucleolus that often lacked extensive contact with the nuclear envelope. Ultrastructurally distinct domains were observed within the round nucleolus. A similar rounded nucleolar morphology was also observed in strains carrying the Pol I plasmid in combination with mutations that affect Pol I function. In a Pol I-defective mutant strain that carried copies of the GAL7-35S rDNA fusion gene integrated into the chromosomal rDNA locus, the nucleolus exhibited a round morphology, but was more closely associated with the nuclear envelope in the form of a bulge. Thus, both the organization of the rDNA genes and the type of polymerase involved in rDNA expression strongly influence the organization and localization of the nucleolus.

Cell Nucleolus↗

PubMed1982

Changes in the size and structure of the nucleolus of columnar cells during their migration from crypt base to villus top in rat jejunum.

An image analyser was used to measure the area of the nucleolus and its component parts in columnar cells at six levels of the jejunal epithelium, corresponding to stages in cell migration from crypt base to villus top. In columnar cells of crypt base, which function as stem cells for the epithelium, the nucleolus is large (3.1 micron2), irregular and reticulated. As cells migrate up the crypt, divide and differentiate, the nucleolus decreases in size (1.7 micron2) and becomes spherical, but remains reticulated. In the fully differentiated cells of the midvillus, however, the nucleolus becomes small (0.9 micron2) and compact. At the villus top, as the cells display early signs of degeneration, the nucleolus is further compacted (0.5 micron2). Most nucleolar components also decrease in size. Pars fibrosa (about 19% of the nucleolar area in crypt base) and pars granulosa (about 70%) decrease in proportion to the rest of the nucleolus, except in mid-villus and villus top where loss of pars granulosa predominates. In contrast, the total area of fibrillar centres remains constant (about 0.1 micron2), even though individual centres are small and numerous in crypt base, larger and fewer at higher levels, and they coalesce into a single structure in villus top. The other nucleolar components are also segregated into distinct, but adjacent, areas at this level. The changes in size and structure of the nucleolus taking place during the migration of columnar cells can be correlated with the maturation of the cells and the loss of their ability to synthesize ribosomal RNA.

Animals↗

PubMed2006

[The nucleolus in oocytes of multylayer mouse follicles: topography of fibrillarin, RNA polymerase I and coilin].

By electron microscopy, conventional fluorescence and confocal microscopy, some features of structural and molecular organization of the nucleolus in oocyte nucleus from mouse multilayer follicles were examined. The examined nucleolus lacks almost all characteristic nucleolar components, such as fibrillar centers, dense fibrillar and granular components. This nucleolus consists exclusively of a homogenous filamentous material and is penetrated by numerous interstices. Besides, a striking association of the nucleolus with coilin, a marker of Cajal bodies, was observed. We could map the coilin accumulation in three different areas: around, in the periphery, or inside the nucleolus. Additionally, we examined a topological relationship between coilin and two key proteins of nucleolar transcription-processing machinery, RNA polymerase I and fibrillarin. RNA polymerase I rather than fibrillarin was found to be colocalized with coilin. Finally, we propose that data on dynamics of coilin relation with the nucleolus may elucidate a possible role of coilin in functional relationship between the nucleolus and Cajal bodies.

Animals↗

PubMed1984

The association of the nucleolus and the short arm of acrocentric chromosomes with the XY pair in human spermatocytes: its possible role in facilitating sex-chromosome acrocentric translocations.

Sex vesicle-nucleolus association was observed in 12% of zygotene and pachytene human spermatocytes using Giemsa and NOR-silver stained preparations. The silver-positive area of the nucleolus, corresponding to the nucleolus organizer (NOR), was usually close to the XY pair. C-banding frequently showed the terminal chromomere, formed by the condensed short arm of an acrocentric bivalent, attached to the sex vesicle. When a nucleolus produced by transcription of rDNA was connected to the short arm, it seemed to be secondarily associated with the sex vesicle. Non-transcribed ribosomal genes, which did not form a nucleolus, were revealed by in situ hybridization. Autoradiographs showed the rDNA-containing short arm of acrocentric bivalents associated with the sex vesicle in 18% of spermatocytes. The difference with the frequency of nucleolus-XY pair association was partially explained by the presence of inactive ribosomal genes. Moreover, electron microscopy showed that the dimensions of the newly formed nucleoli at early zygotene did not exceed 0.5 micron; they can be missed in light microscope investigations. From early zygotene to late pachytene, close relationships were observed between the sex vesicle chromatin and that of the associated acrocentric bivalent, especially in the short arm region. These relationships might explain the frequent involvement of acrocentrics in Y-autosome and X-autosome translocations occurring during male meiosis.

Cell Nucleolus↗

PubMed2005

XMR, a dual location protein in the XY pair and in its associated nucleolus in mouse spermatocytes.

Xlr and Xmr are sex-specific genes which are expressed during the meiotic prophase I in the mouse. In spermatocytes, XMR concentrates on the asynapsed regions of the XY chromosomes, suggesting that XMR plays a role in sex chromosome condensation and silencing. The present study shows that in the mouse, XMR also concentrates in the nucleolus which is closely associated with the XY chromosome pair. In this species, the formation of a large fibrillo-granular nucleolus signals the activation of the ribosomal genes, but release of pre-ribosomal particles is inhibited. Using laser confocal microscopy we characterized the distribution of XMR in the XY body relative to the XY chromatin and the nucleolus. Immunoelectron microscopy showed that XMR concentrates in the fibrillo-granular component and the granular component (GC) of the nucleolus. In (T[X;16]16H) mouse spermatocytes, the nucleolus displays little or no activity and does not associate with the XY pair. XMR concentrated only on the XY chromosomes in (T[X;16]16H) mouse spermatocytes. These data suggest that XMR could play a role both in the XY pair and the nucleolus associated to the sex chromosomes.

Animals↗

PubMed1976

Chromosomal constitution of nucleolus-associated chromatin in man.

Use of specific stains permits analysis of the frequency of nucleolus-associated heterochromatin in chromosomes 1 and 9 from human fibroblasts. In 81 per cent of interphase nuclei the heterochromatic segment of both No. 1 chromosomes is associated with the nucleolus, while in 19 per cent only one heterochromatic segment shows such an association with the other occupying a random position in the nucleoplasm. The nucleolar association of chromosome 9 heterochromatin is less constant: in 42.3 per cent of the nuclei both segments are associated with the nucleolus, in 39 per cent of the nuclei only one heterochromatic segment presents such an association, and in 18.7 per cent neither of the two heterochromatic segments is in nucleolar association. In 6 per cent of the cells, one or two chromosome 9 heterochromatic segments are in contact with the nuclear membrane. In situ hybridization using tritium-labeled 28S and 18S RNA shows that in the interphase nucleus the acrocentric short arms, carriers of ribosomal cistrons, are associated with the nucleolus. These observations demonstrate the complexity of the nucleolus-associated chromatin which, in addition to segments of chromosomes 1, 9, 13, 14, 15, 21, 22, may include the Y chromosome. They also confirm that the nucleolus constitutes one of the orientation points determining the relative localization of chromosomes in the interphase nucleus.

Cell Nucleolus↗

PubMed2002

The nucleolus--a gateway to viral infection?

A number of viruses and viral proteins interact with a dynamic sub-nuclear structure called the nucleolus. The nucleolus is present during interphase in mammalian cells and is the site of ribosome biogenesis, and has been implicated in controlling regulatory processes such as the cell cycle. Viruses interact with the nucleolus and its antigens; viral proteins co-localise with factors such as nucleolin, B23 and fibrillarin, and can cause their redistribution during infection. Viruses can use these components as part of their replication process, and also use the nucleolus as a site of replication itself. Many of these properties are not restricted to any particular type of virus or replication mechanism, and examples of these processes can be found in DNA, RNA and retroviruses. Evidence suggests that viruses may target the nucleolus and its components to favour viral transcription, translation and perhaps alter the cell cycle in order to promote virus replication. Autoimmunity to nucleolin and fibrillarin have been associated with a number of diseases, and by targeting the nucleolus and displacing nucleolar antigens, virus infection might play a role in the initiation of these conditions.

Amino Acid Sequence↗

PubMed1992

Centromere autoantigens are associated with the nucleolus.

Because of their importance as target antigens in scleroderma and since all other major autoantigens in scleroderma can be localized to the interphase nucleolus, we were interested in a further investigation of the potential relationship between interphase centromeres and the nucleolus. Using human anticentromere autoantibodies (ACA) from patients with the CREST form of scleroderma as probes in indirect immunofluorescence microscopy, we observed nonrandom interphase "clumping" of centromeres in a distribution suggestive of nucleoli. By double-label immunofluorescence comparing the localization of centromeres to nucleolar proteins Ki-67, fibrillarin, or protein B23 (nucleophosmin), interphase centromeres appeared to be localized around and within nucleoli. A number of different ACA sera were tested on HEp-2, HeLa, PtK2, Indian muntjac, 3T3, and NRK cells, all with identical results indicating colocalization between centromeres and nucleoli. Immunoelectron microscopy revealed that interphase centromeres were distributed free in the nucleoplasm, in contact with the nuclear envelope, in contact with and on the periphery of nucleoli, and totally embedded within the confines of the nucleolus itself. Interestingly, actinomycin D treatment dissociated centromeres from localization within the segregated nucleolus. To determine if interphase centromeres were integral components of nucleoli, nucleoli were isolated according to classical methods. By double-label immunofluorescence, immunoelectron microscopy, and Western blotting, it was demonstrated that centromere autoantigens copurified with isolated nucleoli. These studies offer proof that some interphase centromeres can be associated with, and may even be considered part of, the interphase nucleolus. Furthermore, all of the major autoantigens in scleroderma can now be localized to the nucleolus.

Autoantigens↗

PubMed1988

Origin of nucleolus-like bodies found in the nucleoplasm and cytoplasm of Vicia faba meristematic cells.

Cytohistochemical staining and RNase-gold labelling have been applied to root-tip meristematic cells of Vicia faba to study the origin and biological significance of 2 types of inclusions: one seen in the nucleoplasm and the other in the cytoplasm of early telophase cells. They have been termed "dense bodies" and "cytoplasmic nucleolus-like bodies" (NLB), respectively. Both types of inclusions respond positively to silver staining and ribonucleoprotein (RNP) staining in a similar fashion to nucleolus. Interestingly, the dense bodies label heavily with the RNase-gold complex, as does the nucleolus, while the cytoplasmic NLB have no affinity with the label. In most cases, the dense bodies label more heavily than the nucleolus. Light microscope surveys reveal that the dense bodies sometimes appear to be released from the surface of the nucleolus. On the other hand, prenucleolar material showing the same silver staining and RNP preferential staining characteristics as the dense bodies begin to accumulate on the surface of chromosomes in mid-anaphase. This material does not label with RNase-gold. These data are discussed in terms of the hypothesis that the dense bodies are derived from the nucleolus by direct budding or fragmentation, and the cytoplasmic NLB are composed of prenucleolar material that failed to attach to chromosomes.

Cell Nucleolus↗

PubMed1976

Expression of human and suppression of mouse nucleolus organizer activity in mouse-human somatic cell hybrids.

Most mouse-human somatic cell hybrids show preferential loss of human chromosomes, absence of human 28S ribosomal RNA, and suppression of human nucleolus organizer activity, as visualized by the Ag-AS silver histochemical stain. In contrast, the mouse-human hybrids studied here show preferential loss of mouse chromosomes. The hybrids were made by fusion of HT-1080-6TG human fibrosarcoma cells with BALB/c mouse peritoneal macrophages or strain 129 mouse teratocarcinoma cells. The Ag-AS staining method shows nucleolus organizer activity of chromosomes 13, 14, 15, 21 (rarely), and 22 in the human parent and chromosomes 12, 15, 16 (rarely), and 18 in the BALB/c mouse parent. In the hybrid cells the human nucleolus organizer regions are active, as shown by Ag-AS staining and involvement in "satellite association." The mouse nucleolus organizer regions are not stained by the Ag-AS method even though mouse chromosomes 12, 15, and 18 are present in the BALB/c hybrids and at least one copy of each mouse chromosome is present in the teratocarcinoma-derived hybrids. Thus, in these mouse-human hybrids, unlike those that lose human chromosomes, only human nucleolus organizer activity is expressed, and mouse nucleolus organizer activity is suppressed.

Animals↗

PubMed2003

Dynamic association of RNA-editing enzymes with the nucleolus.

ADAR1 and ADAR2 are editing enzymes that deaminate adenosine to inosine in long double stranded RNA duplexes and specific pre-mRNA transcripts. Here, we show that full-length and N-terminally truncated forms of ADAR1 are simultaneously expressed in HeLa and COS7 cells owing to the usage of alternative starting methionines. Because the N-terminus of ADAR1 contains a nuclear export signal, the full-length protein localizes predominantly in the cytoplasm, whereas the N-terminally truncated forms are exclusively nuclear and accumulate in the nucleolus. ADAR2, which lacks a region homologous to the N-terminal domain of ADAR1, localizes exclusively to the nucleus and similarly accumulates in the nucleolus. Within the nucleolus, ADAR1 and ADAR2 co-localize in a novel compartment. Photobleaching experiments demonstrate that, in live cells, ADAR1 and ADAR2 are in constant flux in and out of the nucleolus. When cells express the editing-competent glutamate receptor GluR-B RNA, endogenous ADAR1 and ADAR2 de-localize from the nucleolus and accumulate at sites where the substrate transcripts accumulate. This suggests that ADAR1 and ADAR2 are constantly moving through the nucleolus and might be recruited onto specific editing substrates present elsewhere in the cell.

Adenosine Deaminase↗

PubMed2006

Multiple controls regulate nucleostemin partitioning between nucleolus and nucleoplasm.

Nucleostemin plays an essential role in maintaining the continuous proliferation of stem cells and cancer cells. The movement of nucleostemin between the nucleolus and the nucleoplasm provides a dynamic way to partition the nucleostemin protein between these two compartments. Here, we show that nucleostemin contains two nucleolus-targeting regions, the basic and the GTP-binding domains, that exhibit a short and a long nucleolar retention time, respectively. In a GTP-unbound state, the nucleolus-targeting activity of nucleostemin is blocked by a mechanism that traps its intermediate domain in the nucleoplasm. A nucleostemin-interacting protein, RSL1D1, was identified that contains a ribosomal L1-domain. RSL1D1 co-resides with nucleostemin in the same subnucleolar compartment, unlike the B23 and fibrillarin, and displays a longer nucleolar residence time than nucleostemin. It interacts with both the basic and the GTP-binding domains of nucleostemin through a non-nucleolus-targeting region. Overexpression of the nucleolus-targeting domain of RSL1D1 alone disperses nucleolar nucleostemin. Loss of RSL1D1 expression reduces the compartmental size and amount of nucleostemin in the nucleolus. Our work reveals that the partitioning of nucleostemin employs complex mechanisms involving both nucleolar and nucleoplasmic components, and provides insight into the post-translational regulation of its activity.

Amino Acid Sequence↗

PubMed1980

The nucleolus in telophase, interphase and prophase.

The ultrastructure of telophase to interphase has been followed in a green alga, Spirogyra submargaritata. A series of changes transitional between the late anaphase chromatid, the decondensing chromatid of telophase, and the 'pale fibrillar material' occupying channels in the nucleolus at interphase have been demonstrated. Early stages in the regeneration of the nucleolus are described. It has been shown that the pale fibrillar material in the nucleolus is attached to, and continuous with, the fully condensed (chromocentric) part of the nucleolar-organizing chromosome at interphase. It is also shown that in early prophase, the channels in the nucleolonema of the nucleolus are no longer occupied by pale fibrillar material, but instead a long section of condensed chromosome is present, traversing the nucleolonema. It is contended that these observations taken together constitute evidence that the pale fibrillar material of the nucleolus is the chromatin of the nucleolar-organizing region of the chromosome, expanded for transcription. A model of the nucleolus as it is seen in most electron-microscope sections, and as it can be interpreted in the light of present-day knowledge about it, is presented. A brief review of the relevant literature considers the views supporting the mode, and the contrary views, implicating the use of the term 'nucleolar organizer', that are still current at the present time.

Cell Nucleolus↗

PubMed2005

Nucleolus: from structure to dynamics.

The nucleolus, a large nuclear domain, is the ribosome factory of the cells. Ribosomal RNAs are synthesized, processed and assembled with ribosomal proteins in the nucleolus, and the ribosome subunits are then transported to the cytoplasm. In this review, the structural organization of the nucleolus and the dynamics of the nucleolar proteins are discussed in an attempt to link both information. By electron microscopy, three main nucleolar components corresponding to different steps of ribosome biogenesis are identified and the nucleolar organization reflects its activity. Time-lapse videomicroscopy and fluorescent recovery after photobleaching (FRAP) demonstrate that mobility of GFP-tagged nucleolar proteins is slower in the nucleolus than in the nucleoplasm. Fluorescent recovery rates change with inhibition of transcription, decreased temperature and depletion of ATP, indicating that recovery is correlated with cell activity. At the exit of mitosis, the nucleolar processing machinery is first concentrated in prenucleolar bodies (PNBs). The dynamics of the PNBs suggests a steady state favoring residence of processing factors that are then released in a control- and time-dependent manner. Time-lapse analysis of fluorescence resonance energy transfer demonstrates that processing complexes are formed in PNBs. Finally, the nucleolus appears at the center of several trafficking pathways in the nucleus.

Animals↗

PubMed1985

Morphometric analysis of the nucleolus during the life cycle of human odontoblasts.

Developing first premolars were used as a model system to obtain information on the nucleolar structure of human odontoblasts at several stages of their life cycle. Four stages were defined by their location within the tooth: a) preodontoblasts were located at the growing tip of the root; b) secretory odontoblasts in the apical region; c) transitional odontoblasts in the middle region; and d) aged odontoblasts in the coronal region. Preodontoblasts have a small nucleolus (0.55 micron 2) with few strands of dense fibrillar material radiating from the fibrillar center. Secretory odontoblasts are characterized by a large (1.24 micron 2), irregular, and reticulated nucleolus. The fibrillogranular material, the largest component in all nucleoli, reaches maximal size at this stage (0.88 micron 2). Fibrillar centers occupy about the same area (0.1 micron 2) throughout the odontoblast's life cycle. As the formation of primary dentin is completed, the nucleolus of transitional odontoblasts is reduced in size (0.54 micron 2). Finally, the aged odontoblasts have a small, compact nucleolus (0.39 micron 2), with segregated components. Morphologic analysis and quantification of size and component areas of nucleoli obtained with an image analyzer indicated that secretory odontoblasts had the most active, and aged odontoblasts the least active, nucleolus.

Bicuspid↗

PubMed1989

Higher order structure is present in the yeast nucleus: autoantibody probes demonstrate that the nucleolus lies opposite the spindle pole body.

A panel of sera from 892 autoimmune patients was screened by indirect immunofluorescence on mammalian cells. Seventy-three sera were identified that recognize the nucleolus. Three of these sera appear to stain the nucleolus in yeast, suggesting that they recognize highly conserved antigens. These three sera also immunoprecipitate mammalian U3 snRNA-containing particles, which reside in the nucleolus and have been implicated in rRNA processing. Double immunofluorescence experiments with anti-nucleolus and anti-tubulin antibodies revealed a novel form of non-random nuclear organization in yeast. The spindle pole body and the nucleolus-both of which are associated with the nuclear envelope-preferentially localize at opposite ends of the nucleus. Organization of these and other components into specific regions of the nucleus may be important for optimizing their proper function.

Autoantibodies↗

PubMed1998

The nature of the 'nucleolus precursor body' in early preimplantation embryos: a review of fine-structure cytochemical, immunocytochemical and autoradiographic data related to nucleolar function.

In mammals, the restoration of rRNA transcription after fertilisation is accompanied by a gradual differentiation of the nucleolar structure by a process called embryonic nucleogenesis. During cleavage, the nucleolar components appear sterically related to a class of nuclear bodies already detectable in pronuclei. These structures, due to their apparent function as centres of nucleolus formation, have been designated nucleolus precursor bodies (NPBs). It was found recently not only that the size and morphology of the NPBs differ among mammalian species, but that the pattern of embryonic nucleologenesis and even the molecular composition of different NPB compartments vary from one species to another. Accordingly we assumed that at least two definitely different types of NPBs exist, namely the mouse-type NPB and cow-type NPB. In the mouse-type NPB, the original compact material of the NPB remains detectable in the early functional nucleolus. This NPB core does not contain DNA or typical Ag-NOR nucleolar proteins. At the onset of rRNA transcription, the nucleolonema is formed at the periphery of the NPB. The cow-type NPB shows a homogeneous distribution of typical nucleolar proteins throughout its body from the pronucleolar to the early 8-cell stage. At the beginning of rRNA transcription, the cow-type NPB is penetrated by perinucleolar DNA and rRNA synthesis is detectable deep inside the nucleolus. In this case, the entire NPB is readily transformed into a typical nucleolus. These processes are recognisable using fine-structure analysis of preimplantation mammalian embryos. For this reason this approach is often used as a method of evaluating the state of experimental embryos; in such studies, the species differences must be taken into account.

Animals↗

PubMed2005

Electron microscopy analysis of the nucleolus of Trypanosoma cruzi.

The nucleolus is the main site for synthesis and processing of ribosomal RNA in eukaryotes. In mammals, plants, and yeast the nucleolus has been extensively characterized by electron microscopy, but in the majority of the unicellular eukaryotes no such studies have been performed. Here we used ultrastructural cytochemical and immunocytochemical techniques as well as three-dimensional reconstruction to analyze the nucleolus of Trypanosoma cruzi, which is an early divergent eukaryote of medical importance. In T. cruzi epimastigotes the nucleolus is a spherical intranuclear ribonucleoprotein organelle localized in a relatively central position within the nucleus. Dense fibrillar and granular components but not fibrillar centers were observed. In addition, nuclear bodies resembling Cajal bodies were observed associated to the nucleolus in the surrounding nucleoplasm. Our results provide additional morphological data to better understand the synthesis and processing of the ribosomal RNA in kinetoplastids.

Animals↗