Experimental induction of prenucleolar bodies (PNBs) in interphase cells: interphase PNBs show similar characteristics as those typically observed at telophase of mitosis in untreated cells

Dynamic chromosomal interactions and control of heterochromatin positioning by Ki-67

Ki-67 is a chromatin-associated protein with a dynamic distribution pattern throughout the cell cycle and is thought to be involved in chromatin organization. The lack of genomic interaction maps has hampered a detailed understanding of its roles, particularly during interphase. By pA-DamID mapping in human cell lines, we find that Ki-67 associates with large genomic domains that overlap mostly with late-replicating regions. Early in interphase, when Ki-67 is present in pre-nucleolar bodies, it interacts with these domains on all chromosomes. However, later in interphase, when Ki-67 is confined to nucleoli, it shows a striking shift toward small chromosomes. Nucleolar perturbations indicate that these cell cycle dynamics correspond to nucleolar maturation during interphase, and suggest that nucleolar sequestration of Ki-67 limits its interactions with larger chromosomes. Furthermore, we demonstrate that Ki-67 does not detectably control chromatin-chromatin interactions during interphase, but it competes with the nuclear lamina for interaction with late-replicating DNA, and it controls replication timing of (peri)centromeric regions. Together, these results reveal a highly dynamic choreography of genome interactions and roles for Ki-67 in heterochromatin organization.

Nuclear Compartments: An Incomplete Primer to Nuclear Compartments, Bodies, and Genome Organization Relative to Nuclear Architecture

This work reviews nuclear compartments, defined broadly to include distinct nuclear structures, bodies, and chromosome domains. It first summarizes original cytological observations before comparing concepts of nuclear compartments emerging from microscopy versus genomic approaches and then introducing new multiplexed imaging approaches that promise in the future to meld both approaches. I discuss how previous models of radial distribution of chromosomes or the binary division of the genome into A and B compartments are now being refined by the recognition of more complex nuclear compartmentalization. The poorly understood question of how these nuclear compartments are established and maintained is then discussed, including through the modern perspective of phase separation, before moving on to address possible functions of nuclear compartments, using the possible role of nuclear speckles in modulating gene expression as an example. Finally, the review concludes with a discussion of future questions for this field.

Live-Cell Imaging and Analysis of Nuclear Body Mobility

The cell nucleus contains different domains and nuclear bodies, whose position relative to each other inside the nucleus can vary depending on the physiological state of the cell. Changes in the three-dimensional organization are associated with the mobility of individual components of the nucleus. In this chapter, we present a protocol for live-cell imaging and analysis of nuclear body mobility. Unlike other similar protocols, our image analysis pipeline includes non-rigid compensation for global motion of the nucleus before particle tracking and trajectory analysis, leading to precise detection of intranuclear movements. The protocol described can be easily adapted to work with most cell lines and nuclear bodies.

Heterochromatin restricts the mobility of nuclear bodies

Nuclear bodies are relatively immobile organelles. Here, we investigated the mechanisms underlying their movement using experimentally induced interphase prenucleolar bodies (iPNBs). Most iPNBs demonstrated constrained diffusion, exhibiting infrequent fusions with other iPNBs and nucleoli. Fusion events were actin-independent and appeared to be the consequence of stochastic collisions between iPNBs. Most iPNBs were surrounded by condensed chromatin, while fusing iPNBs were usually found in a single heterochromatin-delimited compartment ("cage"). The experimentally induced over-condensation of chromatin significantly decreased the frequency of iPNB fusion. Thus, the data obtained indicate that the mobility of nuclear bodies is restricted by heterochromatin.

RNA-dependent disassembly of nuclear bodies

Nuclear bodies are membraneless organelles that play important roles in genome functioning. A specific type of nuclear bodies known as interphase prenucleolar bodies (iPNBs) are formed in the nucleoplasm after hypotonic stress from partially disassembled nucleoli. iPNBs are then disassembled, and the nucleoli are reformed simultaneously. Here, we show that diffusion of B23 molecules (also known as nucleophosmin, NPM1) from iPNBs, but not fusion of iPNBs with the nucleoli, contributes to the transfer of B23 from iPNBs to the nucleoli. Maturation of pre-ribosomal RNAs (rRNAs) and the subsequent outflow of mature rRNAs from iPNBs led to the disassembly of iPNBs. We found that B23 transfer was dependent on the synthesis of pre-rRNA molecules in nucleoli; these pre-rRNA molecules interacted with B23 and led to its accumulation within nucleoli. The transfer of B23 between iPNBs and nucleoli was accomplished through a nucleoplasmic pool of B23, and increased nucleoplasmic B23 content retarded disassembly, whereas B23 depletion accelerated disassembly. Our results suggest that iPNB disassembly and nucleolus assembly might be coupled through RNA-dependent exchange of nucleolar proteins, creating a highly dynamic system with long-distance correlations between spatially distinct processes.

Nucleolar stress with and without p53

A veritable explosion of primary research papers within the past 10 years focuses on nucleolar and ribosomal stress, and for good reason: with ribosome biosynthesis consuming ~80% of a cell’s energy, nearly all metabolic and signaling pathways lead ultimately to or from the nucleolus. We begin by describing p53 activation upon nucleolar stress resulting in cell cycle arrest or apoptosis. The significance of this mechanism cannot be understated, as oncologists are now inducing nucleolar stress strategically in cancer cells as a potential anti-cancer therapy. We also summarize the human ribosomopathies, syndromes in which ribosome biogenesis or function are impaired leading to birth defects or bone narrow failures; the perplexing problem in the ribosomopathies is why only certain cells are affected despite the fact that the causative mutation is systemic. We then describe p53-independent nucleolar stress, first in yeast which lacks p53, and then in other model metazoans that lack MDM2, the critical E3 ubiquitin ligase that normally inactivates p53. Do these presumably ancient p53-independent nucleolar stress pathways remain latent in human cells? If they still exist, can we use them to target >50% of known human cancers that lack functional p53?

Nucleolar localization/retention signal is responsible for transient accumulation of histone H2B in the nucleolus through electrostatic interactions

The majority of known nuclear proteins are highly mobile. The molecular mechanisms by which they accumulate inside stable compartments that are not separated from the nucleoplasm by membranes are obscure. The compartmental retention of some proteins is associated with their biological function; however, some protein interactions within distinct nuclear structures may be non-specific. The non-specific retention may lead to the accumulation of proteins in distinct structural domains, even if the protein does not function inside this domain. In this study, we have shown that histone H2B-EGFP initially accumulated in the nucleolus after ectopic expression, and then gradually incorporated into the chromatin to leave only a small amount of nucleolus-bound histone that was revealed by removing chromatin-bound proteins with DNase I treatment. Nucleolar histone H2B had several characteristics: (i) it preferentially bound to granular component of the nucleolus and interacted with RNA or RNA-containing nucleolar components; (ii) it freely exchanged between the nucleolus and nucleoplasm; (iii) it associated with the nuclear matrix; and (iv) it bound to interphase prenuclear bodies that formed after hypotonic treatment. The region in histone H2B that acts as a nucleolar localization/retention signal (NoRS) was identified. This signal overlapped with a nuclear localization signal (NLS), which appears to be the primary function of this region. The NoRS activity of this region was non-specific, but the molecular mechanism was probably similar to the NoRSs of other nucleolar proteins. All known NoRSs are enriched with basic amino acids, and we demonstrated that positively charged motifs (nona-arginine (R9) and nona-lysine (K9)) were sufficient for the nucleolar accumulation of EGFP. Also, the correlation between measured NoRS activity and the predicted charge was observed. Thus, NoRSs appear to achieve their function through electrostatic interactions with the negatively charged components of the nucleolus. Though these interactions are non-specific, the functionally unrelated retention of a protein can increase the probability of its interaction with specific and functionally related binding sites.

State of oncomarker protein B23/nucleophosmin in HeLa cells

Western blot after SDS-PAGE for protein separation showed two immunoreactive bands corresponding to monomers (38-40 kDa) and oligomers (210-230 kDa) of nucleophosmin in HeLa cell lysates. Decreasing the buffer ionic strength during the incubation of cells and nuclei destabilized these oligomers. We also showed the existence of two B23/nucleophosmin pools in nuclei of HeLa cells with different sensitivity to hypotonic buffer treatment: one extractable from the nucleus and the other non-extractable and tightly bound to the nucleus. A detailed structural analysis of the extractable B23 pool was carried out: two closely related nucleophosmin isoforms (B23.1 and B23.2) were identified as a result of analysis of C-terminal amino acid sequences using carboxypeptidase hydrolysis; the N-termini of both isoforms are blocked by an acetyl group. As a result of sequencing of the deacetylated proteins, it has been established that the N-terminal amino acid sequence of nucleophosmin in these preparations is truncated by nine amino acid residues and the acetylated residue is Ser. The truncated monomer of nucleophosmin (represented only by the extractable part of the protein) on addition of magnesium ions to low ionic strength buffer or increase in buffer ionic strength was shown to form oligomers with molecular weights (210-230 kDa) similar to those revealed in the total cell lysate. It should be noted that the set of oligomers in this case differs from the one in total cell lysate. Our strategy of characterization of B23 forms for HeLa cells can be applied for other tumor cells.

Ultrastructural analysis of the nucleolar aspects at spermiogenesis in triatomines (Heteroptera, Triatominae)

In this study the ultrastructural technique was used to analyze seminiferous tubule cells of the triatomine species Panstrongylus megistus, Rhodnius pallescens and Triatoma infestans. The data obtained provided evidence of the phenomenon known as persistence of the nucleolar material in initial spermatids at early differentiation. Our results confirmed the presence of the nucleolus and its products during spermiogenesis up to the formation of the axoneme and during spermatid elongation in all three species studied, similar to the process that takes place during cell division. In early spermatids, the nucleoli had a reticulate appearance and a well defined nucleolonema in P. megistus; showed a clear distinction between the fibrillar and the granular component in T. infestans; and had a compact aspect in R. pallescens. In this study, ultrastructural analyses at spermiogenesis indicated that these nucleolar products may represent RNP complexes that will probably be needed at early spermiogenesis when important changes such as chromatin condensation and acrosome and flagellum formation take place. Therefore, it was concluded from the ultrastructural analysis that the triatomine nucleolus does not totally disappear but remains as corpuscles that gather to form the next nucleolar cycle that in the case of meiosis, will be completed if fertilization occurs and a zygote is formed.

Meiotic nucleolar cycle and chromatoid body formation during the rat (Rattus novergicus) and mouse (Mus musculus) spermiogenesis

The aims of the present study were to follow the nucleolar cycle in spermiogenesis of the laboratory rodents Rattus novergicus and Mus musculus, to verify the relationship between the nucleolar component and chromatoid body (CB) formation and to investigate the function of this cytoplasmic supramolecular structure in spermatogenic haploid cells. Histological sections of adult seminiferous tubules were analyzed cytochemically by light microscopy and ultrastructural procedures by transmission electron microscopy. The results reveal that in early spermatids, the CB was visualized in association with the Golgi cisterns indicating that this structure may participate in the acrosome formation process. In late spermatids, the CB was observed near the axonema, a fact suggesting that this structure may support the formation of the spermatozoon tail. In conclusion, our data showed that there is disintegration of spermatid nucleoli at the beginning of spermatogenesis and a fraction of this nucleolar material migrates to the cytoplasm, where a specific structure is formed, known as the "chromatoid body", which, apparently, participates in some parts of the rodent spermiogenesis process.

High Resolution Mapping of Ribosomal DNA in Early Mouse Embryos by Fluorescence In Situ Hybridization1

The nucleolar precursor bodies (NPBs) are numerous discrete entities present in the nuclei of early mammalian embryos, which structurally support active rRNA genes. However, whether all rRNA genes, including those not transcribed, are spatially associated with NPBs, and moreover what is the general arrangement of ribosomal DNA (rDNA) in early mouse embryos, still remain unanswered questions. In our study, we examined the localization of rDNA in transcriptionally silent (one-cell and early two-cell) and transcriptionally active (late two-cell) mouse embryos by highly sensitive fluorescence in situ hybridization with probes complementary to mouse rDNA repeats. The results obtained showed that irrespective of the rDNA transcriptional status, one or more NPBs per nucleus were not structurally associated with rDNA. These observations support the idea that NPBs are heterogeneous in their ability to recruit rRNA genes and thus to participate in reassembly of the mature nucleolus. As in somatic cells, and despite the absence of the characteristic nucleoli, the general arrangement of rRNA genes in early mouse embryos reflected the intensity of rDNA transcription. Ribosomal RNA genes were unequally distributed with respect to repeat putative copy numbers between nucleolar organizing region (NOR)-bearing chromosomes at the first cleavage division, and more strikingly, between sister chromatid NORs of a single nucleolar organizing chromosome. The latter indicates that sister chromatids might harbor various numbers of rRNA gene copies, and that the genes might be unequally distributed between the two blastomeres during the first cleavage mitosis.

In vivo dynamics and kinetics of pKi-67: transition from a mobile to an immobile form at the onset of anaphase

A cell proliferation marker protein, pKi-67, distributes to the chromosome periphery during mitosis and nucleolar heterochromatin in the interphase. We report here on the structural domains of pKi-67 that are required for its correct distribution. While both the LR domain and the conserved domain were involved in localization to the nucleolar heterochromatin, both the LR domain and the Ki-67 repeat domain were required for its distribution to the mitotic chromosome periphery. Using in vivo time-lapse microscopy, GFP-pKi-67 was dynamically tracked from the mitotic chromosome periphery to reforming nucleoli via prenucleolar bodies (PNBs). The signals in PNBs then moved towards and fused into the reforming nucleoli with a thin string-like fluorescence during early G1 phase. An analysis of the in vivo kinetics of pKi-67 using photobleaching indicated that the association of pKi-67 with chromatin was progressively altered from "loose" to "tight" after the onset of anaphase. These findings indicate that pKi-67 dynamically alters the nature of the interaction with chromatin structure during the cell cycle, which is closely related to the reformation process of the interphase nucleolar chromatin.

Nuclear bodies and compartments: functional roles and cellular signalling in health and disease

There is much interest in recent years in the possible role of different nuclear compartments and subnuclear domains in the regulation of gene expression, signalling, and cellular functions. The nucleus contains inositol phosphates, actin and actin-binding proteins and myosin isoforms, multiple protein kinases and phosphatases targeting Cdk-1 and Cdk-2, MAPK/SAPK, and Src-related kinases and their substrates, suggesting the implication of several signalling pathways in the intranuclear organization and function of nuclear bodies (NBs). NBs include the well-characterized Cajal bodies (CBs; or coiled bodies), the nucleolus, perinucleolar and perichromatin regions, additional NBs best illustrated by the promyelocytic leukemia nuclear bodies [PML-NBs, also named PML oncogenic dots (PODs), ND10, Kr-bodies] and similar intranuclear foci containing multi-molecular complexes with major role in DNA replication, surveillance, and repair, as well as messenger RNA and ribosomal RNA synthesis and assembly. Chromatin modifying proteins, such as the CBP acetyltransferase and type I histone deacetylase, accumulate at PML-NBs. PML-NBs and Cajal bodies are very dynamic and mobile within the nuclear space and are regulated by cellular stress (heat shock, apoptosis, senescence, heavy metal exposure, viral infection, and DNA damage responses). NBs strongly interact, using signalling mechanisms for the directional and ordered traffic of essential molecular components. NBs organize the delivery and storage of essential RNAs and proteins that play a role in transcription, pre-mRNA biosynthesis and splicing, and the sequestration and/or degradation of regulatory proteins, such as heterogenous nuclear ribonuclear proteins (hnRNPs), p53, Rb1, CBP, STAT3, and others. The objective of this review is to summarize some aspects of these nuclear structures/bodies/domains, including their proposed roles in cellular signalling and in human diseases, mainly neurodegenerative disorders and cancer.

Deletion and site-specific mutagenesis of nucleolin's carboxy GAR domain

Vertebrate nucleolin is an abundant RNA-binding protein in the dense fibrillar component of active nucleoli. Nucleolin is modular in composition. Its amino-terminal third contains alternating acidic and basic domains, its middle section contains four consensus RNA-binding domains (cRBDs), and its carboxy-terminus contains a distinctive glycine/arginine-rich (GAR) domain with several RGG motifs. The arginines within these motifs are asymmetrically dimethylated. Several laboratories have shown that the GAR domain is necessary but not sufficient for the efficient localization of nucleolin to nucleoli. We examined the distribution of endogenous fibrillarin, Nopp140, and B23 when full-length and DeltaGAR nucleolin were expressed exogenously as enhanced green fluorescent protein (EGFP)-tagged fusions. Only B23 redistributed when DeltaGAR-EGFP was expressed at moderate to high levels, suggesting an in vivo interaction between nucleolin and B23. Next we substituted all ten arginines within the GAR domain of Chinese hamster ovary (CHO) nucleolin with lysines to test the hypothesis that methylation of the carboxy GAR domain is necessary for the nucleolar association of nucleolin. The lysine-substituted mutant was not an in vitro substrate for the yeast protein methyltransferase, Hmt1p/Rmt1. It was, however, able to associate properly with interphase nucleoli and with interphase pre-nucleolar bodies upon recovery from hypotonic shock. We conclude, therefore, that although the GAR domain is necessary for the efficient localization of nucleolin to nucleoli, methylation of this domain is not required for proper nucleolar localization.

Maintenance of nucleolar machineries and pre-rRNAs in remnant nucleolus of erythrocyte nuclei and remodeling in Xenopus egg extracts

The nuclear functions in erythrocytes are almost completely extinct. There is no RNA polymerase I transcription, although a remnant nucleolar structure is still present. The remnant nucleolus of Xenopus laevis erythrocytes maintains a morphologically organized structure, nearly exclusively fibrillar. In this inactive nucleolar remnant, we revealed the presence of a modified form of transcription factor UBF. Several proteins of the processing machinery such as fibrillarin, nucleolin and B23/NO38, snoRNAs U3 and U8, and partially processed preribosomal RNAs colocalized in these remnant structures. Attempts to reprogram these erythrocyte nuclei in Xenopus egg extract showed that import of several nucleolar proteins was induced while the nucleolar remnant was disorganized. UBF became abundant and showed a necklace-like distribution on the decondensed ribosomal genes. Fibrillarin, nucleolin, and snoRNAs U3 and U8, also largely imported from the extract, were associated in large prenuclear bodies scattered in the nucleoplasm. B23/NO38 was present in different small bodies formed only in the most decondensed nuclei. In these remodeled erythrocyte nuclei, there was no imported preribosomal RNA and the initial presence of a residual nucleolar structure containing several partners of ribosome biogenesis was not sufficient to promote reassembly of newly imported nucleolar machineries. These nuclei, which reproduce the early events of nucleogenesis are also transcriptionally silent and thus compare to the early embryonic nuclei of Xenopus laevis.

Nucleolar assembly of the rRNA processing machinery in living cells

To understand how nuclear machineries are targeted to accurate locations during nuclear assembly, we investigated the pathway of the ribosomal RNA (rRNA) processing machinery towards ribosomal genes (nucleolar organizer regions [NORs]) at exit of mitosis. To follow in living cells two permanently transfected green fluorescence protein-tagged nucleolar proteins, fibrillarin and Nop52, from metaphase to G1, 4-D time-lapse microscopy was used. In early telophase, fibrillarin is concentrated simultaneously in prenucleolar bodies (PNBs) and NORs, whereas PNB-containing Nop52 forms later. These distinct PNBs assemble at the chromosome surface. Analysis of PNB movement does not reveal the migration of PNBs towards the nucleolus, but rather a directional flow between PNBs and between PNBs and the nucleolus, ensuring progressive delivery of proteins into nucleoli. This delivery appeared organized in morphologically distinct structures visible by electron microscopy, suggesting transfer of large complexes. We propose that the temporal order of PNB assembly and disassembly controls nucleolar delivery of these proteins, and that accumulation of processing complexes in the nucleolus is driven by pre-rRNA concentration. Initial nucleolar formation around competent NORs appears to be followed by regroupment of the NORs into a single nucleolus 1 h later to complete the nucleolar assembly. This demonstrates the formation of one functional domain by cooperative interactions between different chromosome territories.

Association of yeast RNA polymerase I with a nucleolar substructure active in rRNA synthesis and processing

A novel ribonucleoprotein complex enriched in nucleolar proteins was purified from yeast extracts and constituents were identified by mass spectrometry. When isolated from rapidly growing cells, the assembly contained ribonucleic acid (RNA) polymerase (pol) I, and some of its transcription factors like TATA-binding protein (TBP), Rrn3p, Rrn5p, Rrn7p, and Reb1p along with rRNA processing factors, like Nop1p, Cbf5p, Nhp2p, and Rrp5p. The small nucleolar RNAs (snoRNAs) U3, U14, and MRP were also found to be associated with the complex, which supports accurate transcription, termination, and pseudouridylation of rRNA. Formation of the complex did not depend on pol I, and the complex could efficiently recruit exogenous pol I into active ribosomal DNA (rDNA) transcription units. Visualization of the complex by electron microscopy and immunogold labeling revealed a characteristic cluster-forming network of nonuniform size containing nucleolar proteins like Nop1p and Fpr3p and attached pol I. Our results support the idea that a functional nucleolar subdomain formed independently of the state of rDNA transcription may serve as a scaffold for coordinated rRNA synthesis and processing.

The ribosomal RNA processing machinery is recruited to the nucleolar domain before RNA polymerase I during Xenopus laevis development

Transcription and splicing of messenger RNAs are temporally and spatially coordinated through the recruitment by RNA polymerase II of processing factors. We questioned whether RNA polymerase I plays a role in the recruitment of the ribosomal RNA (rRNA) processing machinery. During Xenopus laevis embryogenesis, recruitment of the rRNA processing machinery to the nucleolar domain occurs in two steps: two types of precursor structures called prenucleolar bodies (PNBs) form independently throughout the nucleoplasm; and components of PNBs I (fibrillarin, nucleolin, and the U3 and U8 small nucleolar RNAs) fuse to the nucleolar domain before components of PNBs II (B23/NO38). This fusion process is independent of RNA polymerase I activity, as shown by actinomycin D treatment of embryos and by the lack of detectable RNA polymerase I at ribosomal gene loci during fusion. Instead, this process is concomitant with the targeting of maternally derived pre-rRNAs to the nucleolar domain. Absence of fusion was correlated with absence of these pre-rRNAs in nuclei where RNA polymerase II and III are inhibited. Therefore, during X. laevis embryogenesis, the recruitment of the rRNA processing machinery to the nucleolar domain could be dependent on the presence of pre-rRNAs, but is independent of either zygotic RNA polymerase I transcription or the presence of RNA polymerase I itself.

Ultrastructural methods for nucleic acid detection by immunocytology

In the present review are summarized recent developments in immunocytochemical detection of nucleic acids in biological materials at the ultrastructural level. Not only the approaches using antibodies to natural nucleic acids are described but also the techniques involving the use of antibodies raised against various nucleotide analogs incorporated beforehand into nucleic acids. Special emphasis is placed on each method's potential and limitations. These methods, combined or not with molecular biotechnology, are powerful tools for studying the structure and function of nucleic acids. They can be used to investigate the distribution and topological organization of DNA and RNA molecules or of specialized within these molecules in the cells.

Effects of anti-PM-Scl 100 (Rrp6p exonuclease) antibodies on prenucleolar body dynamics at the end of mitosis

Prenucleolar bodies (PNBs) are transitory structures which serve as building blocks for nucleoli at the transition mitosis/interphase. The assembly of PNBs and their pathway are not clearly understood. To better understand these events, the behavior of the PNB-containing PM-Scl 100 protein was compared with that of other PNB proteins. This nucleolar protein was chosen because its yeast homologue, Rrp6p exonuclease [1], is known to participate in late events in 5.8 S rRNA (ribosomal RNA) processing. There was a heterogeneous distribution of nucleolar proteins in different classes of PNBs. The PM-Scl 100 colocalized predominantly with protein B23. The PM-Scl-100-containing PNBs were translocated at later times to nucleoli as opposed to the fibrillarin-containing PNBs. Microinjections of antibodies directed against PM-Scl 100 during mitosis inhibited targeting of PM-Scl 100 to the nucleolus. However fibrillarin and protein B23 still participated in nucleolar assembly in early G1. We conclude that there are different kinds of PNBs whose translocation to the nucleoli follow ordered kinetics. Interestingly, proteins involved in late steps of processing as PM-Scl 100 are translocated late, suggesting that they are not cotranscriptionally associated with the rRNA precursors.

The nucleolar antigen Nop52, the human homologue of the yeast ribosomal RNA processing RRP1, is recruited at late stages of nucleologenesis

We report the molecular characterization of a novel nucleolar protein, Nop52, and its subcellular distribution during the cell cycle and nucleologenesis. This protein was originally identified with human autoantibodies which were subsequently used to clone its corresponding cDNA. Transfection experiments in mammalian cells have confirmed that this cDNA encodes a nucleolar protein that accumulates in the nucleoli and at the periphery of the chromosomes. Nop52 is the putative human homologue of the yeast ribosomal RNA processing protein RRP1 which is involved in pre-rRNA processing from 27S to 25S and 5.8S. In nucleoli, Nop52 is excluded from the ribosomal RNA transcription sites, accumulates in the granular external domain and mainly colocalizes with nucleolar proteins involved in the late processing step such as hPop1 and protein B23. During the building process of the nucleolus at the end of mitosis, a sequential order was observed in the assembly of nucleolar proteins of early and late processing mainly via the prenucleolar body pathway. The order is the following: fibrillarin, nucleolin, Nop52 together with protein B23 in the prenucleolar bodies, and simultaneously with hPop1, and finally Ki-67. The evolutionary conservation of Nop52 and the lethal effects observed in gene disruption experiments, predict a critical role for Nop52 in the generation of 28S rRNA.

The nucleolar phosphoprotein B23 redistributes in part to the spindle poles during mitosis

B23 is a major phosphoprotein in the interphasic nucleolus where it is involved in the assembly of pre-ribosomes. Using several cultured animal cells, we report that, in addition to the known redistribution of the protein during mitosis, B23 also becomes associated with mitotic spindle poles starting from early prometaphase onwards. Colocalization of B23 with the protein NuMA (Nuclear Mitotic Apparatus protein) was studied in mitotic cells and taxol-arrested cells. During the onset of mitosis, we observed that a fraction of B23 associates with, and dissociates from, the poles later than NuMA. At metaphase, both proteins are colocalized at the poles. The polar redistribution of both B23 and NuMA is mediated by microtubules. In taxol-treated cells, B23 is associated with the microtubule minus ends in the center of mitotic asters together with NuMA. Association of B23 with microtubule minus ends of mitotic asters was further confirmed with an in vitro assay, where B23 was found by western blotting to co-sediment with taxol-induced microtubule asters formed in a mitotic cell extract. Immunolabeling demonstrated that B23 and NuMA were both present at the center of the asters. Furthermore, an additional hyperphosphorylated form of B23 appeared when microtubule asters formed and associated with the asters. Immunodepletion of B23 from the mitotic extract revealed that taxol-induced microtubule asters were still observed in B23-immunodepleted mitotic extract, indicating that the presence of B23 at the poles is unlikely to be essential for spindle formation or stabilisation.

Presence of pre-rRNAs before activation of polymerase I transcription in the building process of nucleoli during early development of Xenopus laevis

During the early development of Xenopus laevis, we followed in individual nuclei the formation of a nucleolus by examining simultaneously its structural organization and its transcriptional competence. Three distinct situations were encountered with different frequencies during development. During the first period of general transcriptional quiescence, the transcription factor UBF of maternal origin, was present in most nuclei at the ribosomal gene loci. In contrast, fibrillarin, a major protein of the processing machinery, was found in multiple prenucleolar bodies (PNBs) whereas nucleolin was dispersed largely in the nucleoplasm. During the second period, for most nuclei these PNBs had fused into two domains where nucleolin concentrated, generating a structure with most features expected from a transcriptionally competent nucleolus. However, RNA polymerase I-dependent transcription was not detected using run-on in situ assays whereas unprocessed ribosomal RNAs were observed. These RNAs were found to derive from a maternal pool. Later, during a third period, an increasing fraction of the nuclei presented RNA polymerase I-dependent transcription. Thus, the structural organization of the nucleolus preceded its transcriptional competence. We conclude that during the early development of X. laevis, the organization of a defined nucleolar structure, is not associated with the transcription process per se but rather with the presence of unprocessed ribosomal RNAs.

Effects of anti-fibrillarin antibodies on building of functional nucleoli at the end of mitosis

During mitosis some nuclear complexes are relocalized at the chromosome periphery and are then reintegrated into the re-forming nuclei in late telophase. To address questions concerning translocation from the chromosome periphery to nuclei, the dynamics of one nucleolar perichromosomal protein which is involved in the ribosomal RNA processing machinery, fibrillarin, was followed. In the same cells, the onset of the RNA polymerase I (RNA pol I) activity and translocation of fibrillarin were simultaneously investigated. In PtK1 cells, RNA pol I transcription was first detected at anaphase B. At the same mitotic stage, fibrillarin formed foci of increasing size around the chromosomes, these foci then gathered into prenucleolar bodies (PNBs) and later PNBs were targeted into the newly formed nucleoli. Electron microscopy studies enabled the visualization of the PNBs forming the dense fibrillar component (DFC) of new nucleoli. Anti-fibrillarin antibodies microinjected at different periods of mitosis blocked fibrillarin translocation at different steps, i.e. the formation of large foci, foci gathering in PNBs or PNB targeting into nucleoli, and thereby modified the ultrastructural organization of the nucleoli as well as of the PNBs. In addition, antibody-bound fibrillarin seemed localized with blocks of condensed chromatin in early G1 nuclei. It has been found that blocking fibrillarin translocation reduced or inhibited RNA pol I transcription. It is postulated that when translocation of proteins belonging to the processing machinery is inhibited or diminished, a negative feed-back effect is induced on nucleolar reassembly and transcriptional activity.