What is new in the nucleolus?: workshop on the nucleolus: new perspectives.
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Animals↗
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[The size of the nucleolus- and the nucleolus-karyoplasma relation as an expression of synthetic activities. Autoradiographic studies on liver cells of normal and thioacetamide-treated rats].
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Amides↗
[The nucleolus of lymphocytes; I. Fixation and physiology of nucleolus].
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Cell Nucleolus↗
Changes in nucleus, nucleolus and cell size accompanying somatic embryogenesis of Theobroma cacao L. I. Relationship between DNA and total protein content and size of nucleus, nucleolus and cell.
There was a linear relation between an increase in DNA content and size of nuclei, nucleoli and cells in callus and proembryos (Theobroma cacao L.). In callus the increase of DNA content was accompanied by proportional increase in nuclear size whereas in proembryos the increase in nuclear size did not match the increasing amount of DNA. The stimulation of embryogenesis by 10(-2) mg/l 2,4-D was associated with increase in nuclear and nucleolar size and with decrease in cell sizes. Inhibition of embryogenesis by 1.0 mg/l 2,4-D+10% coconut water did not change nuclear size, but increased cell size in relation to the control. The process of embryo formation was accompanied by changes in relationship between nuclear, nucleolar and cell size and the total (DNFB-stained) proteins content. In callus as well as in proembryo the increase in total protein content in nucleus was not equivalent to the increasing sizes of nuclei which leads to the decrease in nuclear protein concentration. Similar situation was observed for nucleoli. Differences were found in the concentration of cytoplasmic proteins between the callus and proembryo cells. The stimulation of embryogenesis by low concentration of 2,4-D resulted in decrease in concentration of total proteins in nuclei and nucleoli and the increase in cytoplasm.
Cacao↗
The topographic organization of repetitive DNA in the human nucleolus.
The nucleolus is a highly specialized nuclear domain where ribosomal DNA (rDNA) is transcribed and preribosomes are assembled. We investigated the molecular organization of the human lymphocyte nucleolus by fluorescence in situ hybridization and confocal laser scanning microscopy and found that transcribed rDNA and nontranscribed ribosomal intergenic spacer (IGS) sequences colocalized to discrete regions frequently on the nucleolar periphery of phytohemagglutinin-stimulated cells. The 5S rDNA gene cluster located on the long arm of chromosome 1 was not regularly associated with the nucleolus. Short interspersed (SINE) Alu elements detected by BLUR 11 were distributed diffusely throughout the nucleus but were severely underrepresented in the nucleolus, whereas an Alu element subcloned from the IGS detected sequences enriched in the nucleolus but sparsely represented in the remainder of the nucleus. In contrast, long interspersed (LINE) Kpn elements, which were located at the nucleolus, were not found in rDNA but were identified outside the ribosomal gene complex on the short arm of at least one acrocentric chromosome. A human chromosome 21-derived alphoid sequence that hybridized to the centromere was localized outside but near the nucleolus, and nonribosomal DNA consisting of a tandemly repeated simple sequence cluster derived from the short arm of chromosome 15 was organized in a compact fashion in the nucleolus. Our study provides new insight into the content and structure of the human nucleolus and illustrates that the unique organization of repetitive DNA on the acrocentric chromosome short arms is reflected in the topographic organization of the nucleolus.
Blotting, Southern↗
The plurifunctional nucleolus.
The nucleolus of eukaryotic cells was first described in the early 19th century and was discovered in the 1960s to be the seat of ribosome synthesis. Although rRNA transcription, rRNA processing and ribosome assembly have been clearly established as major functions of the nucleolus, recent studies suggest that the nucleolus participates in many other aspects of gene expression as well. Thus, the nucleolus has been implicated in the processing or nuclear export of certain mRNAs. In addition, new results indicate that biosyntheses of signal recognition particle RNA and telomerase RNA involve a nucleolar stage and that the nucleolus is also involved in processing of U6 RNA, one of the spliceosomal small nuclear RNAs. Interestingly, these three nucleolus-associated small nuclear RNAs (signal recognition particle RNA, telomerase RNA and U6 RNA) are components of catalytic ribonucleoprotein machines. Finally, recent work has also suggested that some transfer RNA precursors are processed in the nucleolus. The nucleolus may have evolutionarily descended from a proto-eukaryotic minimal genome that was spatially linked to vicinal RNA processing and ribonucleoprotein assembly events involved in gene read-out. The nucleolus of today's eukaryotes, now surrounded by the chromatin of over 2 billion years of genome expansion, may still perform these ancient functions, in addition to ribosome biosynthesis. The plurifunctional nucleolus concept has a strong footing in contemporary data and adds a new perspective to our current picture of the spatial-functional design of the cell nucleus.
Cell Nucleolus↗
The nucleolus: reviewing oldies to have new understandings.
The nucleolus is the most prominent compartment in the nucleus and known as the site for ribosome biogenesis in eucaryotes. In contrast, there is no such equivalent structure for ribosome synthesis in procaryotes. This raises two concerns that how does the nucleolus evolve and that whether the nucleolus remains playing a single role in ribosome biogenesis along the evolution. Increasing data support new nucleolus functions, including signal recognition particle assembly, small RNA modification, telomerase maturation, cell-cycle and aging control, and cell stress sensor. Multiple functions of the nucleolus possibly result from the plurifunctionality of nucleolar proteins, such as nucleolin and Nopp140. Proteomic analyses of human and Arabidopsis nucleolus lead a remarkable progress in understanding the evolution and new functions of nucleoli. In this review, we present a brief history of nucleolus research and new concepts and unresolved questions. Also, we introduce hepatitis D virus for studying the communication between the nucleolus and other subnuclear compartments, and Caenorhabditis elegans for the role of nucleolus in the development and the epistatic control of nucleologenesis.
Animals↗
Meiosis and embryo technology: renaissance of the nucleolus.
The nucleolus is the site of rRNA and ribosome production. This organelle presents an active fibrillogranular ultrastructure in the oocyte during the growth of the gamete but, at the end of the growth phase, the nucleolus is transformed into an inactive remnant that is dissolved when meiosis is resumed at germinal vesicle breakdown. Upon meiosis, structures resembling the nucleolar remnant, now referred to as nucleolus precursor bodies (NPBs), are established in the pronuclei. These entities harbour the development of fibrillogranular nucleoli and re-establishment of nucleolar function in conjunction with the major activation of the embryonic genome. This so-called nucleologenesis occurs at a species-specific time of development and can be classified into two different models: one where nucleolus development occurs inside the NPBs (e.g. cattle) and one where the nucleolus is formed on the surface of the NPBs (e.g. pigs). A panel of nucleolar proteins with functions during rDNA transcription (topoisomerase I, RNA polymerase I and upstream binding factor) and early (fibrillarin) or late rRNA processing (nucleolin and nucleophosmin) are localised to specific compartments of the oocyte nucleolus and those engaged in late processing are, to some degree, re-used for nucleologenesis in the embryo, whereas the others require de novo embryonic transcription in order to be allocated to the developing nucleolus. In the oocyte, inactivation of the nucleolus coincides with the acquisition of full meiotic competence, a parameter that may be of importance in relation to in vitro oocyte maturation. In embryo, nucleologenesis may be affected by technological manipulations: in vitro embryo production apparently has no impact on this process in cattle, whereas in the pig this technology results in impaired nucleologenesis. In cattle, reconstruction of embryos by nuclear transfer results in profound disturbances in nucleologenesis. In conclusion, the nucleolus is an organelle of great importance for the developmental competence of oocytes and embryos and may serve as a morphological marker for the completion of oocyte growth and normality of activation of the embryonic genome.
Animals↗
Signal recognition particle components in the nucleolus.
The signal recognition particle (SRP) is a ribonucleoprotein composed of an Alu domain and an S domain. The S domain contains unique sequence SRP RNA and four SRP proteins: SRP19, SRP54, SRP68, and SRP72. SRP interacts with ribosomes to bring translating membrane and secreted proteins to the endoplasmic reticulum (ER) for proper processing. Additionally, SRP RNA is a member of a family of small nonribosomal RNAs found recently in the nucleolus, suggesting that the nucleolus is more plurifunctional than previously realized. It was therefore of interest to determine whether other SRP components localize to this intranuclear site. In transfected rat fibroblasts, green fluorescent protein fusions of SRP19, SRP68, and SRP72 localized to the nucleolus, as well as to the cytoplasm, as expected. SRP68 also accumulated in the ER, consistent with its affinity for the ER-bound SRP receptor. SRP54 was detected in the cytoplasm as a green fluorescent protein fusion and in immunofluorescence studies, but was not detected in the nucleolus. In situ hybridization experiments also revealed endogenous SRP RNA in the nucleolus. These results demonstrate that SRP RNA and three SRP proteins visit the nucleolus, suggesting that partial SRP assembly, or another unidentified activity of the SRP components, occurs at the nucleolus. SRP54 apparently interacts with nascent SRP beyond the nucleolus, consistent with in vitro reconstitution experiments showing that SRP19 must bind to SRP RNA before SRP54 binds. Our findings support the notion that the nucleolus is the site of assembly and/or interaction between the family of ribonucleoproteins involved in protein synthesis, in addition to ribosomes themselves.
Animals↗
[The Sertoli cell nucleolus in domestic and wild animals].
Sertoli cells produce special microenvironment for developing germ cells; therefore it is assumed that they play primary role in the onset and control of spermatogenesis. In this connection we extended our previous study on the ultrastructure of Sertoli cells in different domestic and wild animals with special regard to nucleolus. Sertoli cells of domestic and wild ruminants possess the typical vesicular nucleolus except for fallow deer, in this species no vesicular nucleolus occurs in Sertoli cells even during the rut. In roe.buck, another wild ruminant with seasonal spermatogenesis, cyclic changes were found in the nucleolus of Sertoli cells. If no spermatogenesis is present, the Sertoli cells have a reticular nucleolus. Membranous vesicles appear in the nucleolus of Sertoli cells of roe-buck at the onset of spermatogenesis 1-2 months before rut. In domestic ruminants with continuous spermatogenesis the vesicular nucleolus in Sertoli cells is present permanently. During postnatal development of bull and ram the vesicular nucleolus appears in Sertoli cells just before the onset of spermatogenesis. In experimental cryptorchidism of bulls a vesicular nucleolus is found in the Sertoli cells. Our observations and experiments support a hypothesis that Sertoli cells have primary role at the onset and the maintenance of spermatogenesis.
Animals↗
The nucleolus.
Nucleoli are the sites of biosynthesis of the ribosomal precursors. They contain may copies of the genes for the main rRNAs (18S- and 28 S-rRNA) in the form of tandemly arranged repeats at the chromosomal nucleolar organizer regions (NORs). They also contain the small rRNA (5S-rRNA) that is synthesized outside the nucleolus, specific nucleolar proteins, among them the factors and enzymes necessary for transcription and transcript processing, and the precursor units of the ribosomes. In man as in may vertebrate species, three main components of nucleoli, besides chromatin, can be detected: fibrillar centres (FC), dense fibrillar component (DCF), and granular component (GC). Within a nucleolus the FCs are in many cases situated in its central region. The DFc forms a network of strands surrounding the FCs, but may sometimes reach for out towards the periphery of the nucleolus. The GC is usually situated in the peripheral regions of the nucleolus. In cells with a low level of ribosomal biosynthesis the nucleoli are small, usually with a single FC and little surrounding DFC and GC ("ring-shaped nucleolus"). In active cells the DFC forms a large network enclosing several, sometimes up to hundreds of FCs, and the GC covers a large area in the periphery ("compact nucleoli"). In cells at the onset of a new stimulation, the DFC is very prominent whereas the FCs are few and small, and the GC is also not very extensive ("reticulate nucleoli"). In some special cell types that are very active other arrangements of the structural components are found. In Sertoli cells, for instance, only one nucleolus is found, or occasionally two, each with a single large FC and a distinct area of GC, both areas being engulfed by DFC intermingled with some peripheral GC. Immunocytological and in situ hybridization studies to localize the rRNA genes within the nucleolus have so far led to divergent results. Both fibrillar components, the FCs and the DFC, have been claimed as the most probable candidates. Transcription of rDNA and the subsequent early steps of ribosome biosynthesis are localized in the DFC, whereas later steps (mature rRNA, preribosomes) are localized in the GC. The FCs may also serve as sites for the preparation of the rDNA for transcription, and as a store for certain nucleolar proteins. During mitosis, parts of the nucleolar proteins remain at the NORs. A direct contact between the nucleolus and the nuclear envelope is frequently observed but is not dependent on nucleolar activity.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Elimination of the nucleolus from the nucleus of a living cell by centrifugation (a literature review).
The following effects involving the nucleolus take place during centrifugation of living cells at centrifugal forces of several thousand g to several hundred thousand g: settling of the nucleolus in centrifugal direction on the nuclear envelope; pulling the latter as a long stalk with the nucleolus at its end (or alternatively an easy perforation of the nuclear envelope by the nucleolus); release of the nucleolus into the cytoplasm or its expulsion out of the cell; occasional stratification of the nucleolus in the nucleus; fusion of many nucleoli together under centrifugal pressure. The asymmetric topography of the nuclear envelope is considered to be one of the causes of its different resistance to the penetration of the nucleolus. Elimination of the nucleolus from cancer cell nuclei to test the nucleolar contribution to cell malignancy is suggested as one conceivable application of the centrifugal technique of cell enucleolation.
Animals↗
Identification of nucleolus localization signal of betanodavirus GGNNV protein alpha.
Betanodavirus greasy grouper (Epinephelus tauvina) nervous necrosis viruses (GGNNV) protein alpha, a virus capsid protein, was detected in both nucleolus and cytoplasm of infected cells of Asian sea bass (SB) and transfected cells of SB and Cos-7 with pcDNA3.1/RNA2. To study its subcellular localization, ORF of protein alpha with 338 aa was fused with enhanced green fluorescent protein (EGFP) gene and was detected in transfected cells in the absence of other viral proteins. In both SB and Cos-7 cells, protein alpha was found to localize EGFP to the nucleolus and cytoplasm. Deletion mutants of protein alpha indicated that N-terminal 43 amino acid residues were required to import EGFP-alpha protein into the nucleolus. Further deletions within the 43 amino acid backbone, EGFP/33aa(1-33) and EGFP/30aa(14-43), localized to the nucleolus, suggesting that the 20 amino acids from 14 to 33 of protein alpha were the domain of nucleolus localization. To further determine the nucleolus targeting sequence, deletion mutations within the 20 amino acids of protein alpha were constructed. It was found that the deletion of (23)RRR(25), (29)RRR(31), or (23)RRRANNRRR(31) prevented the accumulation of EGFP fusion proteins into the nucleolus, demonstrating that (23)RRRANNRRR(31) contain the signal required for nucleolar localization. A similar distribution pattern of localization of protein alpha and its deletion mutants in SB and Cos-7 cells suggested that N-terminal residues of protein alpha (23)RRRANNRRR(31) constitute a nucleolus localization signal that functions in both fish and mammalian cells.
Amino Acid Sequence↗
The functional organization of the nucleolus in proliferating plant cells.
The nucleolus is a prominent nuclear organelle which morphologically expresses all functional steps necessary for the synthesis of ribosomes, from transcription of rRNA genes to the assembly and maturation of preribosomal particles and their transport to the cytoplasm. Structurally, the nucleolus contains some basic components common to practically all cell types, namely fibrillar centers (FCs), the dense fibrillar component (DFC), and the granular component (GC); however, the organization and distribution of these components is highly variable, depending on cell identity and functional status. The different steps of ribosome biogenesis are not strictly correlated with the structural components of the nucleolus. Thus, FCs are most likely the anchoring sites for the accumulation of rDNA, and the sites where the assembly of transcription complexes takes place, but transcription of rRNA genes actually occurs at discrete points in the transition zone between FCs and the DFC. The DFC is a structurally homogeneous, but functionally heterogeneous component in which transcription and some early and advanced steps of pre-rRNA processing develop successively in a gradual fashion, from transition with FCs to transition with the GC. Finally, the GC is the site of the later steps of preribosomal processing, including the final assembly of ribosomal proteins for the export of mature particles to the cytoplasm. The rate of ribosome biogenesis, as well as the structure of the nucleolus, are highly influenced by the proliferation status of the cell, and by factors regulating cell cycle progression. These factors are nucleolar proteins, such as nucleolin, which are targets of signal transduction mechanisms, being at the same time regulators of key steps in preribosome synthesis and processing. Thus, many features of the nucleolus, such as the structural organization of its components, the level and distribution of certain nucleolar proteins and, in general, the rate of ribosome biogenesis, show profound variations throughout cell cycle periods. Particularly interesting is the behavior of the nucleolus during mitosis, in which its structure is disorganized and its activity is stopped, even though the individual transcription and processing complexes are not disassembled, but carried from one cell generation to the next one in such a way that the daughter-cell nucleoli are built with materials coming from the parent-cell nucleolus. Transcription complexes remain assembled at the chromosomal nucleolar organizer in which the rRNA genes are clustered, and processing complexes are carried at the chromosome periphery, and then they are organized into discrete entities called prenucleolar bodies, whose fusion, together with the resumption of transcription and processing, originates the new nucleolus.
Animals↗
[Electron microscopic study of endopolyploid nuclei in rat trophoblast giant cells. III. Nucleolus and its fibrillary center at different stages of the endoreplication cycle].
The fine structure of the nucleolus and nucleolus-forming chromaosomes has been studied in the course of the endomitotic cycle of the trophoblast giant cells on the 12th and 13th days of the foetus development. The nucleolar ultrastructure was regarded in relation to the degree of chromatin condensation in the nucleus. In the nucleus with dispersed chromatin (endointerphase), the nucleoli have the nucleoneme structure and involve the four components: fibro-granular threads and accumulations (with granules 20 nm in diameter), round zones of fibres with moderate electron density surrounded by dense fibrillar material (fibrillar centres), and lacunar spaces. In contact with the nucleolus are thin chromatinous fibres of the karyoplasm which penetrate into the open lacunae on the periphery of the nucleus to be running, presumably, through the whole nucleus. The fibrillar centres either run deep into the lacunar spaces of the nucleolus, or attach to it. In the nuclei with condensed chromtin (endoprophase), the nucleoli are surrounded with a well developed layer of perinucleolar chromatin running along lacunae deep into the nucleolus. The nucleoli here made of the same components as nucleoli in the nuclei with dispersed chromatin, but are the prominent electron dense fibrillar component is getting more obvious, the number of granules in the fibro-granular component is reduced, it lacunae display regions of the nucleolus-organizer not only as fine scattered chromatinous fibres, but also as small compact blocks. A question is discussed of the fibrillar centres of the nucleolus as being places of the nucleolus organizer, and of its changes within the cycle of mitosis and endomitosis.
Animals↗
Insights into the evolution of the nucleolus by an analysis of its protein domain repertoire.
Recently, the first investigation of nucleoli using mass spectrometry led to the identification of 271 proteins. This represents a rich resource for a comprehensive investigation of nucleolus evolution. We applied a protocol for the identification of known and novel conserved protein domains of the nucleolus, resulting in the identification of 115 known and 91 novel domain profiles. The phyletic distribution of nucleolar protein domains in a collection of complete proteomes of selected organisms from all domains of life confirms the archaebacterial origin of the core machinery for ribosome maturation and assembly, but also reveals substantial eubacterial and eukaryotic contributions to nucleolus evolution. We predict that, in different phases of nucleolus evolution, protein domains with different biochemical functions were recruited to the nucleolus. We suggest a model for the late and continuous evolution of the nucleolus in early eukaryotes and argue against an endosymbiotic origin of the nucleolus and the nucleus. Supplementary material for this article can be found on the BioEssays website at http://www.interscience.wiley.com/jpages/0265-9247/suppmat/index.html.
Archaeal Proteins↗
Segregation of the nucleolus during mitosis in budding and fission yeast.
The segregation of the nucleolus during mitosis was examined in Saccharomyces cerevisiae and Schizosaccharomyces pombe by indirect immunofluorescence using antibodies directed to highly conserved anti-nucleolus antigens. In mitotic S. pombe cells, the nucleolus appears to trail the bulk of the DNA. In wild-type cells of S. cerevisiae, the nucleolus segregates alongside the bulk of the genomic DNA. Based on its distance from the centromere, we would expect the rDNA in both organisms to segregate behind the majority of the genomic DNA, if telomeric regions trail centromeric regions as in other eukaryotes. We therefore suggest that in S. cerevisiae the nucleolus is attached to other parts of the nucleus which enable it to segregate along with the bulk of the DNA. The segregation of the nucleolus in topoisomerase mutants and nuclear division mutants of S. cerevisiae was also investigated. In cdc14 mutants which arrest at late anaphase, the vast majority of the DNA is separated, but the nucleolar antigens remain extended between the mother and daughter cells. Thus, the CDC14 gene of S. cerevisiae appears to be important for the separation of the nucleolus at mitosis.
Autoimmunity↗
A chromosome rearrangement in Neurospora that produces segmental aneuploid progeny containing only part of the nucleolus organizer.
In translocation T (IL leads to VL) OY321 of Neurospora crassa a distal portion of the nucleolus organizer chromosome, including ribosomal DNA sequences and the nucleolus satellite, is interchanged with a long terminal segment of IL. When OY321 is crossed by Normal sequence, one-fourth of the meiotic products are segmental aneuploids that contain two copies of the long IL segment and that are deficient for the distal portion of the organizer. Each such product forms a nucleolus and is viable. The complementary aneuploid products are deficient for the IL segment and are therefore inviable. - In crosses of OY321 X OY321, each product is capable of making two nucleoli; nucleoli formed by the separated nucleolus organizer parts usually fuse, but most 8-spored asci contain some nuclei in which two separate nucleoli can be seen. One nucleolus is then terminal on its chromosome while the second is interstitial and somewhat smaller. - In crosses of OY321 X Normal, half of the meiotic products are capable of making two nucleoli. However, only about 15% of 8-spored asci have one or more nuclei containing separate nucleoli. At pachytene and later in prophase I, the single fusion nucleolus is associated with three bivalent chromosome segments. Each nucleus of every ascus contains at least one nucleolus, even in asci where some nuclei display two nucleoli. - Crosses of Aneuploid X Normal are usually semibarren, producing a reduced number of ascospores, some of which are inviable. Some aneuploid cultures become fully fertile by reverting to a quasinormal sequence lacking a satellite. In some crosses of Aneuploid X Normal, individual asci may show at prophase I either complete loss, partial loss, or pycnosis of the translocated IL segment. This observation of pycnosis suggests chromosome inactivation. - Growth from aneuploid ascospores is initially slow, but can accelerate to the wild-type rate.
Aneuploidy↗