Nucleolus

About: Nucleolus is a research topic. Over the lifetime, 5873 publications have been published within this topic receiving 232435 citations. The topic is also known as: GO:0005730 & cell nucleolus.

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

Abstract: 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.

Activation of an Endogenous Suicide Response after Perturbation of rRNA Synthesis Leads to Neurodegeneration in Mice

Abstract: Transcription of rRNA genes is essential for maintaining nucleolar integrity, a hallmark for the healthy state and proliferation rate of a cell. Inhibition of rRNA synthesis leads to disintegration of the nucleolus, elevated levels of p53, and induction of cell suicide, identifying the nucleolus as a critical stress sensor. Whether deregulation of rRNA synthesis is causally involved in neurodegeneration by promoting cell death and/or by inhibiting cellular growth has however not been addressed. The transcription factor TIF-IA plays a central role in mammalian rRNA synthesis, regulating the transcriptional activity of RNA polymerase I. To investigate the consequences of nucleolar perturbation in the nervous system, we have chosen to specifically ablate the gene encoding the transcription factor TIF-IA in two different contexts: neural progenitors and hippocampal neurons. Here, we show that ablation of TIF-IA leads to impaired nucleolar activity and results in increased levels of the proapoptotic transcription factor p53 in both neural progenitors and hippocampal neurons but induces rapid apoptosis only in neural progenitors. Nondividing cells of the adult hippocampus are more refractory to loss of rRNA transcription and face a protracted degeneration. Our study provides an unexploited strategy to initiate neurodegeneration based on perturbation of nucleolar function and underscores a novel perspective to study the cellular and molecular changes involved in the neurodegenerative processes.

Organization of nucleoli and nuclear bodies in osmotically stimulated supraoptic neurons of the rat.

Abstract: This study has analyzed variations in the number of nucleoli and nuclear bodies, as well as in their ultrastructural and cytochemical organization, after the osmotically induced activation of supraoptic nucleus (SON) neurons of the rat. The number of nucleoli and nuclear bodies and also the nucleolar size were determined on smear preparations of previously block-impregnated SON. The mean number of nucleoli per cell was 1.35 +/- 0.6 (mean +/- SDM) in control rats. No significant variations in this value were registered either in dehydrated or rehydrated rats. The mean nucleolar volume and the total nucleolar volume per cell showed a significant increase in dehydrated rats with respect to the controls, whereas these two parameters tended to return to control values in rats rehydrated after dehydration. The mean number of nuclear bodies per cell increased significantly from 0.56 +/- 0.50 (mean +/- SDM) in control rats to 1.54 +/- 1.1 after 6 days of dehydration. By electron microscopy, SON neurons displayed a reticulated nucleolar configuration. After the osmotically induced neuronal activation, there was an increase in the proportion of the total nucleolar area occupied by the granular component, and also a reduction in the mean fibrillar-center area. The most characteristic nucleolar features in rehydrated rats were the tendency for the granular component to be segregated and the occurrence of intranucleolar vacuoles. Ultrastructural cytochemistry with a specific silver method revealed a selective silver reaction on the coiled threads of the nuclear bodies--identified as "coiled bodies"--and on the nucleolar fibrillar components in all animal groups studied. Since nucleoli play a major role in ribosome biogenesis, a relationship between these nucleolar changes and the level of cellular activity of SON neurons is proposed. Furthermore, the response of nuclear "coiled bodies" to neuronal activation suggests their participation in the processing and transport of rRNA precursors.

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

Abstract: 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.

Nucleophosmin mutations alter its nucleolar localization by impairing G-quadruplex binding at ribosomal DNA.

Abstract: Nucleophosmin (NPM1) is an abundant nucleolar protein implicated in ribosome maturation and export, centrosome duplication and response to stress stimuli. NPM1 is the most frequently mutated gene in acute myeloid leukemia. Mutations at the C-terminal domain led to variant proteins that aberrantly and stably translocate to the cytoplasm. We have previously shown that NPM1 C-terminal domain binds with high affinity G-quadruplex DNA. Here, we investigate the structural determinants of NPM1 nucleolar localization. We show that NPM1 interacts with several G-quadruplex regions found in ribosomal DNA, both in vitro and in vivo. Furthermore, the most common leukemic NPM1 variant completely loses this activity. This is the consequence of G-quadruplex-binding domain destabilization, as mutations aimed at refolding the leukemic variant also result in rescuing the G-quadruplex-binding activity and nucleolar localization. Finally, we show that treatment of cells with a G-quadruplex selective ligand results in wild-type NPM1 dislocation from nucleoli into nucleoplasm. In conclusion, this work establishes a direct correlation between NPM1 G-quadruplex binding at rDNA and its nucleolar localization, which is impaired in the acute myeloid leukemia-associated protein variants.