Immunocytochemical analysis of the coiled body in the cell cycle and during cell proliferation

Is the sphere organelle/coiled body a universal nuclear component?

We present evidence for the essential homology of four nuclear organelles that have previously been described under four different names: coiled bodies in mammalian somatic nuclei, prenucleolar bodies in nuclei assembled in vitro in Xenopus egg extract, sphere organelles in amphibian germinal vesicles (GVs), and Binnenkörper in insect GVs. Each of these organelles contains coilin or a coilin-related protein plus a variety of small nuclear ribonucleoproteins. We suggest that the sphere organelle/coiled body is a "universal" nuclear component in the sense that it is involved in common nuclear processes and hence will be found in one form or another in most eukaryotic cells. We postulate that it functions in the assembly and sorting of snRNP complexes for three RNA processing pathways: pre-mRNA splicing, rRNA processing, and histone pre-mRNA 3' end formation. Specifically, the sphere organelle/coiled body may be the initial site for assembly of processing complexes, which are then sorted to other places in the nucleus, where the actual RNA processing takes place.

Structure, expression and chromosomal localization of human p80-coilin gene

Coiled bodies (CBs) are non-capsular nuclear bodies with a diameter of 0.3-1 micron and appear to be composed of coiled fibrils. Human autoantibodies to CBs recognize an 80-kD nuclear protein highly enriched in CBs, and this protein has been named p80-coilin. CBs are known to assemble and disassemble during the cell cycle, with the highest number of CBs occurring at mid to late G1 where p80-coilin is assembled into several small nuclear body-like structures. In S and G2 phases, CBs become larger and their number decreases and often they are undetectable during mitosis. Using a human autoantibody as a probe for expression cloning, we initially isolated a partial cDNA encoding p80-coilin. In this report, the 5' end of the complete cDNA for p80-coilin was obtained using the 5'-RACE (rapid amplification of cDNA ends) methodology. The size of the reconstructed full-length cDNA corresponds to the 2.7-kb mRNA detected in Northern blot analysis. The complete p80-coilin protein consists of 576 amino acids with a predicted molecular mass of 62,608. A putative p80-coilin pseudogene was also detected during the rescreening of p80-coilin cDNA. To confirm the validity of the cDNA sequence, three overlapping genomic DNA clones representing the human p80-coilin gene were selected for further analysis. The complete gene for p80-coilin contains 7 exons spanning approximately 25kb. Sequence analysis of exons 1 and 2 in genomic DNA clones confirmed the accuracy of the 5' cDNA sequence derived from the 5'-RACE procedure. Furthermore, the human p80-coilin gene was localized to chromosome 17q22-23 by fluorescence in situ hybridization.

Human p80-coilin is targeted to sphere organelles in the amphibian germinal vesicle

Cultured vertebrate cells often display one or more coiled bodies in their nuclei. These are spherical structures approximately 0.5-1.0 micron in diameter that contain high concentrations of small nuclear ribonucleoproteins (snRNPs); they are distinct from nuclear speckles and nucleoli, the other major sites of snRNP concentration. Coiled bodies in human cells contain a unique protein, p80-coilin, that has an M(r) = 80 kDa. Cloned p80-coilin cDNA encodes 576 amino acids with a calculated molecular weight of 62.6 kDa. To determine which of several snRNP-containing structures in the amphibian germinal vesicle (GV) might be the homologue of coiled bodies, we injected myc-tagged transcripts of full-length human p80-coilin into the cytoplasm of Xenopus oocytes and followed the fate of the translated proteins with an antibody specific for the tag. Western blots of GV proteins showed rapid appearance of both full-length and truncated p80-coilin in the nucleus. Immunofluorescent staining of spread GV contents demonstrated specific uptake of p80-coilin by the sphere organelle within 1 h after injection. Similar experiments were performed with a series of deletion constructs that lacked progressively longer segments from the carboxy terminus. A construct that contained only the first 102 amino acids (18% of the molecule) was specifically targeted to the sphere organelle. Conversely, a construct lacking the first 92 amino acids failed to localize, although it was imported into the GV. Thus, a relatively short region at the amino terminus of human p80-coilin is both necessary and sufficient for localization in the sphere organelle. Sphere organelles in the GV and coiled bodies in somatic nuclei are clearly related in composition. We suggest that they are homologous organelles with similar functions in preassembly and sorting of RNA processing components. Differences in their composition suggest functional specialization in the two cell types.

An RNase-sensitive particle containing Drosophila melanogaster DNA topoisomerase II

Most DNA topoisomerase II (topo II) in cell-free extracts of 0-2-h old Drosophila embryos appears to be nonnuclear and remains in the supernatant after low-speed centrifugation (10,000 g). Virtually all of this apparently soluble topo II is particulate with a sedimentation coefficient of 67 S. Similar topo II-containing particles were detected in Drosophila Kc tissue culture cells, 16-19-h old embryos and extracts of progesterone-matured oocytes from Xenopus. Drosophila topo II-containing particles were insensitive to EDTA, Triton X-100 and DNase I, but could be disrupted by incubation with 0.3 M NaCl or RNase A. After either disruptive treatment, topo II sedimented at 9 S. topo II-containing particles were also sensitive to micrococcal nuclease. Results of chemical cross-linking corroborated those obtained by centrifugation. Immunoblot analyses demonstrated that topo II-containing particles lacked significant amounts of lamin, nuclear pore complex protein gp210, proliferating cell nuclear antigen, RNA polymerase II subunits, histones, coilin, and nucleolin. Northern blot analyses demonstrated that topo II-containing particles lacked U RNA. Thus, current data support the notion that nonnuclear Drosophila topo II-containing particles are composed largely of topo II and an unknown RNA molecule(s).

Nucleologenesis: U3 snRNA-containing prenucleolar bodies move to sites of active pre-rRNA transcription after mitosis

We have investigated the distribution of U3 snRNA and rRNA in HeLa cells and normal rat kidney cells during interphase and mitosis. U3 snRNA, known to be involved in pre-rRNA processing, was detected in nucleoli and coiled bodies during interphase, whereas rRNA was distributed in the nucleoli and throughout the cytoplasm. By comparison, ribosomal protein S6 was detected in nucleoli, coiled bodies, and in the cytoplasm. During nucleologenesis, pre-rRNA was observed in newly forming nucleoli during late telophase but not in prenucleolar bodies (PNBs), whereas U3 snRNA was detected in forming nucleoli and PNBs. Similar findings to those reported here for the localization of U3 snRNA have been reported previously for the U3 small nuclear ribonucleoprotein fibrillarin. These results suggest that components involved in pre-rRNA processing localize to discrete PNBs at the end of mitosis. The nucleolus is formed at specific telophase domains (nucleolar organizing regions) and the PNBs, containing factors essential for pre-rRNA processing, are recruited to these sites of rRNA transcription and processing.

In vivo analysis of the stability and transport of nuclear poly(A)+ RNA

We have studied the distribution of poly(A)+ RNA in the mammalian cell nucleus and its transport through nuclear pores by fluorescence and electron microscopic in situ hybridization. Poly(A)+ RNA was detected in the nucleus as a speckled pattern which includes interchromatin granule clusters and perichromatin fibrils. When cells are fractionated by detergent and salt extraction as well as DNase I digestion, the majority of the nuclear poly(A)+ RNA was found to remain associated with the nonchromatin RNP-enriched fraction of the nucleus. After inhibition of RNA polymerase II transcription for 5-10 h, a stable population of poly(A)+ RNA remained in the nucleus and was reorganized into fewer and larger interchromatin granule clusters along with pre-mRNA splicing factors. This stable population of nuclear RNA may play an important role in nuclear function. Furthermore, we have observed that, in actively transcribing cells, the regions of poly(A)+ RNA which reached the nuclear pore complexes appeared as narrow concentrations of RNA suggesting a limited or directed pathway of movement. All of the observed nuclear pores contained poly(A)+ RNA staining suggesting that they are all capable of exporting RNA. In addition, we have directly visualized, for the first time in mammalian cells, the transport of poly(A)+ RNA through the nuclear pore complexes.

Differential interaction of splicing snRNPs with coiled bodies and interchromatin granules during mitosis and assembly of daughter cell nuclei

In the interphase nucleus of mammalian cells the U1, U2, U4/U6, and U5 small nuclear ribonucleoproteins (snRNPs), which are subunits of spliceosomes, associate with specific subnuclear domains including interchromatin granules and coiled bodies. Here, we analyze the association of splicing snRNPs with these structures during mitosis and reassembly of daughter nuclei. At the onset of mitosis snRNPs are predominantly diffuse in the cytoplasm, although a subset remain associated with remnants of coiled bodies and clusters of mitotic interchromatin granules, respectively. The number and size of mitotic coiled bodies remain approximately unchanged from metaphase to early telophase while snRNP-containing clusters of mitotic interchromatin granules increase in size and number as cells progress from anaphase to telophase. During telophase snRNPs are transported into daughter nuclei while the clusters of mitotic interchromatin granules remain in the cytoplasm. The timing of nuclear import of splicing snRNPs closely correlates with the onset of transcriptional activity in daughter nuclei. When transcription restarts in telophase cells snRNPs have a diffuse nucleoplasmic distribution. As cells progress to G1 snRNP-containing clusters of interchromatin granules reappear in the nucleus. Coiled bodies appear later in G1, although the coiled body antigen, p80 coilin, enters early into telophase nuclei. After inhibition of transcription we still observe nuclear import of snRNPs and the subsequent appearance of snRNP-containing clusters of interchromatin granules, but not coiled body formation. These data demonstrate that snRNP associations with coiled bodies and interchromatin granules are differentially regulated during the cell division cycle and suggest that these structures play distinct roles connected with snRNP structure, transport, and/or function.

Cytochemical and immunocytochemical study of coiled bodies in different cultured cell lines

We analyzed by different cytochemical and immunocytochemical approaches the biochemical compositon of coiled bodies in three different cultured cell lines. Coiled bodies are stained by the AgNOR staining method and by the EDTA regressive staining method preferential for ribonucleoprotein (RNP). Using the in situ polyadenylate nucleotidyl transferase-immunogold technique or anti-RNA antibodies, we decisively demonstrated the presence of appreciable amounts of RNA in coiled bodies. Neither the in situ terminal deoxynucleotidyl transferase-immunogold technique nor anti-DNA antibodies revealed any DNA in coiled bodies. Coiled bodies thus appear as distinct regions of cell nuclei involved in some steps of RNA metabolism but not directly in RNA synthesis. Their relationships with the dense fibrillar component of the nucleolus and with interchromatin granule clusters are discussed.

A serine kinase regulates intracellular localization of splicing factors in the cell cycle

Small nuclear ribonucleoprotein particles (snRNPs) and non-snRNP splicing factors containing a serine/arginine-rich domain (SR proteins) concentrate in 'speckles' in the nucleus of interphase cells. It is believed that nuclear speckles act as storage sites for splicing factors while splicing occurs on nascent transcripts. Splicing factors redistribute in response to transcription inhibition or viral infection, and nuclear speckles break down and reform as cells progress through mitosis. We have now identified and cloned a kinase, SRPK1, which is regulated by the cell cycle and is specific for SR proteins; this kinase is related to a Caenorhabditis elegans kinase and to the fission yeast kinase Dsk1 (ref. 7). SRPK1 specifically induces the disassembly of nuclear speckles, and a high level of SRPK1 inhibits splicing in vitro. Our results indicate that SRPK1 may have a central role in the regulatory network for splicing, controlling the intranuclear distribution of splicing factors in interphase cells, and the reorganization of nuclear speckles during mitosis.

In vitro assembly of coiled bodies in Xenopus egg extract

When demembranated sperm nuclei are placed in a Xenopus egg extract, they become surrounded by a nuclear envelope and then swell to form morphologically typical pronuclei. Granules ranging from < 1.0 to approximately 3.0 microns in diameter appear within such nuclei. Bell et al. identified four nucleolar proteins in these "prenucleolar bodies" by immunofluorescent staining (fibrillarin, nucleolin, B23/NO38, 180-kDa nucleolar protein). By in situ hybridization we show that these bodies also contain U3 and U8 small nuclear RNAs (snRNAs), known to be involved in pre-rRNA processing. Moreover, they contain all the snRNAs involved in pre-mRNA splicing (U1, U2, U4, U5, and U6), as well as U7, which is required for histone pre-mRNA 3' end formation. In addition to the nucleolar antigens previously identified, we demonstrated staining with antibodies against the Sm epitope, trimethylguanosine, and coilin. Because the composition of these prenucleolar bodies is closer to that of coiled bodies than to nucleoli, we propose that they be referred to as coiled bodies. The existence of large coiled bodies in transcriptionally inactive pronuclei suggests that they may play a role in the import, assembly, and storage of RNA processing components but are not themselves sites of processing. In transcriptionally active nuclei coiled bodies could serve as sites for initial preassembly and distribution of snRNP complexes for the three major RNA processing pathways: pre-mRNA splicing, pre-rRNA processing, and histone pre-mRNA 3' end formation.

Cytostellin distributes to nuclear regions enriched with splicing factors

Cytostellin, a approximately 240 kDa phosphoprotein found in all cells examined from human to yeast, is predominantly intranuclear in interphase mammalian cells and undergoes continuous redistribution during the cell cycle. Here, mammalian cytostellin is shown to localize to intranuclear regions enriched with multiple splicing proteins, including spliceosome assembly factor, SC-35. Cytostellin and the splicing proteins also co-localize to discrete foci (called ‘dots’), which are distributed throughout the cell during mitosis and part of G1. The cytostellin that is localized to these dots resists extraction by Triton X-100, indicating that it is tightly associated with insoluble cell structures. All immunostainable cytostellin reappears in the nucleus before S-phase. Although cytostellin and the splicing proteins co-localize in interphase and dividing cells, cytostellin is not detected in purified spliceosomes, and it associates with six unidentified proteins, forming a macromolecular complex that is biochemically distinct from the proteins that comprise spliceosomes. This macromolecular complex is detected at constant levels throughout the cell cycle, and the level of cytostellin protein remains constant during the cell cycle. Nevertheless, intranuclear cytostellin immunostaining fluctuates markedly during the cell cycle. The monoclonal antibody (mAb) H5 epitope of cytostellin is ‘masked’ in serum-starved cells, but 60 minutes after serum stimulation intense cytostellin immunoreactivity appears in the nuclear speckles. This rapid induction of cytostellin immunoreactivity in subnuclear regions enriched with many splicing factors, as well as accumulations of RNA polymerase II (Pol II) transcripts, suggests that cytostellin may have a function related to mRNA biogenesis.

Coiled bodies in the nucleolus of breast cancer cells

Coiled bodies are a special type of small round nuclear body, composed of coiled fibers and granules, especially prominent in the nucleoplasm of highly active cells (Brasch and Ochs (1992) Exp. Cell Res. 202, 211–223). Although no specific function has been assigned to coiled bodies, they contain spliceosome snRNAs and proteins, as well as the nucleolar U3 RNA-associated protein fibrillarin. In the present study, we have used antibodies to the coiled body-specific protein p80-coilin, together with double-label immunofluorescence, confocal microscopy and immunoelectron microscopy, to examine the distribution of coiled bodies in a number of different breast cancer cell lines. By immunofluorescence, all cell lines had prominent coiled bodies in the nucleoplasm and several cell lines appeared to have coiled bodies within the nucleolus itself. Double-label immunofluorescence and confocal laser scanning microscopy confirmed the nucleolar localization of coiled bodies. Besides containing p80-coilin, nucleoplasmic and nucleolar coiled bodies contained fibrillarin and Sm proteins. By conventional and immunoelectron microscopy, nucleolar coiled bodies appeared as discrete structures within the nucleolus in a number of different morphotypes, distinct from the normal nucleolar domains of granular component, dense fibrillar component, and fibrillar centers. While the significance of finding coiled bodies in the nucleolus of certain breast cancer cell lines is at present unknown, this represents the first report of coiled bodies and Sm staining in the nucleolus of mammalian cells.

Evidence for a nuclear compartment of transcription and splicing located at chromosome domain boundaries

The nuclear topography of splicing snRNPs, mRNA transcripts and chromosome domains in various mammalian cell types are described. The visualization of splicing snRNPs, defined by the Sm antigen, and coiled bodies, revealed distinctly different distribution patterns in these cell types. Heat shock experiments confirmed that the distribution patterns also depend on physiological parameters. Using a combination of fluorescence in situ hybridization and immunodetection protocols, individual chromosome domains were visualized simultaneously with the Sm antigen or the transcript of an integrated human papilloma virus genome. Three-dimensional analysis of fluorescence-stained target regions was performed by confocal laser scanning microscopy. RNA transcripts and components of the splicing machinery were found to be generally excluded from the interior of the territories occupied by the individual chromosomes. Based on these findings we present a model for the functional compartmentalization of the cell nucleus. According to this model the space between chromosome domains, including the surface areas of these domains, defines a three-dimensional network-like compartment, termed the interchromosome domain (ICD) compartment, in which transcription and splicing of mRNA occurs.

Nucleoplasmic organization of small nuclear ribonucleoproteins in cultured human cells

The organization of eight small nuclear ribonucleoproteins (the U1, U2, U4, U5, and U6 RNAs previously studied by others and three additional snRNAs, U11, U12, and 7SK) has been investigated in cultured human cells by fluorescence in situ hybridization with antisense DNA and 2'-O-Me RNA oligonucleotides. Using highly sensitive digital imaging microscopy we demonstrate that all of these snRNAs are widespread throughout the nucleoplasm, but they are excluded from the nucleoli. In addition, the U2, U4, U5, U6, and U12 snRNAs are concentrated in discrete nuclear foci, known as coiled bodies, but U1 and 7SK are not. In addition to coiled bodies, a classic speckled pattern was observed in the nucleoplasm of monolayer-grown HeLa cells, whereas suspension-grown HeLa cells revealed a more diffuse nucleoplasmic labeling. Immunofluorescence staining using various snRNP-specific antisera shows complete agreement with that of their antisense snRNA oligonucleotide counterparts. Although U2 RNA is concentrated in coiled bodies, quantitation of the fluorescence signals from the U2 antisense probe reveals that the bulk of the U2 snRNP is located in the nucleoplasm. Furthermore, simultaneous visualization of the U2 snRNAs and the tandemly repeated U2 genes demonstrates that coiled bodies are not the sites of U2 transcription.