Repopulation of the postmitotic nucleolus by preformed RNA

Sharing the mitotic pre-ribosomal particles between daughter cells

Ribosome biogenesis is a fundamental multistep process initiated by the synthesis of 90S pre-ribosomal particles in the nucleoli of higher eukaryotes. Even though synthesis of ribosomes stops during mitosis while nucleoli disappear, mitotic pre-ribosomal particles persist as observed in prenucleolar bodies (PNBs) during telophase. To further understand the relationship between the nucleolus and the PNBs, the presence and the fate of the mitotic pre-ribosomal particles during cell division was investigated. We demonstrate that the recently synthesized 45S precursor ribosomal RNAs (pre-rRNAs) but also the 32S and 30S pre-rRNAs are maintained during mitosis and associated with the chromosome periphery together with pre-rRNA processing factors. Maturation of the mitotic pre-ribosomal particles, as assessed by the stability of the mitotic pre-rRNAs, is transiently arrested during mitosis by a cyclin-dependent kinase (CDK)1-cyclin B-dependent mechanism and may be restored by CDK inhibitor treatments. At the M/G1 transition, the resumption of mitotic pre-rRNA processing in PNBs does not induce the disappearance of PNBs that only occurs when functional nucleoli reform. Strikingly, during their maturation process, mitotic pre-rRNAs localize in reforming nucleoli.

Topographic changes in a heterochromatic chromosome block in humans (15P) during formation of the nucleolus

Fluorescence in situ hybridization and multispectral confocal laser scanning microscopy revealed a highly dynamic nucleolar-associated chromosome 15 satellite III heterochromatin cluster in humans. This nucleolar-associated DNA was highly decondensed at the metaphase plate compared with its topography at interphase and appeared to act as a centre for the post-mitotic reorganization of the nucleolus. Our data showed unexpected trans-mitotic changes in the topography of this nucleolar-associated satellite III DNA that suggest that this locus-specific heterochromatin superstructure may be involved in nucleolar organization.

Nucleolus-like morphology produced during the in vitro reassociation of nucleolar components

Nucleoli, the sites of rRNA synthesis, rRNA processing, and the assembly of ribosomes, are dynamic organelles that, in most cells, disperse and reform during mitosis. The mechanisms that regulate nucleolar formation are unknown as is the relationship between nucleolar morphology and the pathway of ribosome biogenesis. In this report we describe the in vitro formation of nucleolus-like particles (NLPs) from soluble extracts of nucleoli. NLPs, which reached sizes comparable to nucleoli (1-3 microns), were found to contain 40% of the nucleolar DNA, RNA, and protein. The ultrastructure of NLPs resembled that of a number of in vivo structures including compact nucleoli, prenucleolar bodies, and pseudonucleoli. The particles were composed of two morphologically distinct regions. The core resembled the dense fibrillar component (DFC) of nucleoli while the cortex resembled the granular component (GC) of nucleoli. The cortex of NLPs contained numerous 15-20 nm osmophilic granules that resembled the preribosomes found in the GC of nucleoli. The distribution of nucleolar proteins in NLPs also resembled that in nucleoli. BN46/51, a component of the GC of nucleoli, was restricted to the GC-like cortex of NLPs. A mAb that bound to the DFC of nucleoli, bound only to the DFC-like core of NLPs while a second mAb that bound to both the DFC and GC of nucleoli, bound to both the core and cortex of NLPs. Thus solubilized components of nucleoli can reassociate in vitro to produce particles that resemble nucleoli in their size, ultrastructure, and protein distribution.

Assigning functions to nucleolar structures

Nucleoli provide the fascinating possibility of linking morphologically distinct structures such as those seen in the electron microscope with biochemical features of the formation and stepwise maturation of ribosomes. Localization of proteins by immunocytochemistry and of rRNA genes and their transcripts by in situ hybridization has greatly improved our understanding of the structural-functional relationships of the nucleolus. The present review describes some recent results obtained by electron microscopic in situ hybridization and argues that this approach has the potential to correlate each step of the complex pre-rRNA maturation pathway with nucleolar structures. Evidence is accumulating that the nucleolus-specific U3 snRNPs (small nuclear ribonucleoprotein particles) participate in rRNA processing events, similar to the role played by the nucleoplasmic snRNPs in mRNA maturation. The intranucleolar distribution of U3 snRNA is consistent with the view that it is involved in both early and late stages of pre-rRNA processing.

Inhibition of nucleolar reformation after microinjection of antibodies to RNA polymerase I into mitotic cells

The formation of daughter nuclei and the reformation of nucleolar structures was studied after microinjection of antibodies to RNA polymerase I into dividing cultured cells (PtK2). The fate of several nucleolar proteins representing the three main structural subcomponents of the nucleolus was examined by immunofluorescence and electron microscopy. The results show that the RNA polymerase I antibodies do not interfere with normal mitotic progression or the early steps of nucleologenesis, i.e., the aggregation of nucleolar material into prenucleolar bodies. However, they inhibit the telophasic coalescence of the prenucleolar bodies into the chromosomal nucleolar organizer regions, thus preventing the formation of new nucleoli. These prenucleolar bodies show a fibrillar organization that also compositionally resembles the dense fibrillar component of interphase nucleoli. We conclude that during normal nucleologenesis the dense fibrillar component forms from preformed entities around nucleolar organizer regions, and that this association seems to be dependent on the presence of an active form of RNA polymerase I.

Localization of ribosomal protein S1 in the granular component of the interphase nucleolus and its distribution during mitosis

Using antibodies to various nucleolar and ribosomal proteins, we define, by immunolocalization in situ, the distribution of nucleolar proteins in the different morphological nucleolar subcompartments. In the present study we describe the nucleolar localization of a specific ribosomal protein (S1) by immunofluorescence and immunoelectron microscopy using a monoclonal antibody (RS1-105). In immunoblotting experiments, this antibody reacts specifically with the largest and most acidic protein of the small ribosomal subunit (S1) and shows wide interspecies cross-reactivity from amphibia to man. Beside its localization in cytoplasmic ribosomes, this protein is found to be specifically localized in the granular component of the nucleolus and in distinct granular aggregates scattered over the nucleoplasm. This indicates that ribosomal protein S1, in contrast to reports on other ribosomal proteins, is not bound to nascent pre-rRNA transcripts but attaches to preribosomes at later stages of rRNA processing and maturation. This protein is not detected in the residual nucleolar structures of cells inactive in rRNA synthesis such as amphibian and avian erythrocytes. During mitosis, the nucleolar material containing ribosomal protein S1 undergoes a remarkable transition and shows a distribution distinct from that of several other nucleolar proteins. In prophase, the nucleolus disintegrates and protein S1 appears in numerous small granules scattered throughout the prophase nucleus. During metaphase and anaphase, a considerable amount of this protein is found in association with the surfaces of all chromosomes and finely dispersed in the cell plasm. In telophase, protein S1-containing material reaccumulates in granular particles in the nucleoplasm of the newly formed nuclei and, finally, in the re-forming nucleoli. These observations indicate that the nucleolus-derived particles containing ribosomal protein S1 are different from cytoplasmic ribosomes and, in the living cell, are selectively recollected after mitosis into the newly formed nuclei and translocated into a specific nucleolar subcompartment, i.e., the granular component. The nucleolar location of ribosomal protein S1 and its rearrangement during mitosis is discussed in relation to the distribution of other nucleolar proteins.

Repopulation of postmitotic nucleoli by preformed RNA. II. Ultrastructure

The reconstruction of the nucleolus after mitosis was analyzed by electron microscopy in cultured mammalian (L929) cells in which nucleolar RNA synthesis was inhibited for a 3 h period either after or before mitosis. When synchronized mitotic cells were plated into a concentration of actinomycin D sufficient to block nucleolar RNA synthesis preferentially, nucleoli were formed at telophase as usual. 3 h after mitosis, these nucleoli had fibrillar and particulate components and possessed the segregated appearance characteristic of nucleoli of actinomycin D-treated cells. Cells in which actinomycin D was present for the last 3 h preceding mitosis did not form nucleoli by 3 h after mitosis though small fibrillar prenucleolar bodies were detectable at this time. These bodies subsequently grew in size and eventually acquired a particulate component. It took about a full cell cycle before nucleoli of these cells were completely normal in appearance. Thus, nucleolar RNA synthesis after mitosis is not necessary for organization of nucleoli after mitosis. However, inhibition of nucleolar RNA synthesis before mitosis renders the cell incapable of forming nucleoli immediately after mitosis. If cells are permitted to resume RNA synthesis after mitosis, they eventually regain nucleoli of normal morphology.