Coiled Bodies and Gems: Janus or Gemini?
1998; Elsevier BV; Volume: 63; Issue: 2 Linguagem: Inglês
10.1086/301992
ISSN1537-6605
AutoresA. Gregory Matera, Mark R. Frey,
Tópico(s)Congenital Anomalies and Fetal Surgery
ResumoCoiled bodies (CBs) were discovered around the turn of the century by Santiago Ramon y Cajal, 1903Ramon y Cajal SR Un sencillo metodo de coloracion selectiva del reticulo protoplasmico y sus efectos en los diversos organos nerviosos de vertebrados y invertebrados.Trab Lab Invest Biol (Madrid). 1903; 2: 129-221Google Scholar, a true pioneer of cytology and neuroscience. In 1906, Cajal and Camillo Golgi shared the Nobel Prize in Physiology/Medicine for their descriptions of the architecture of the vertebrate nervous system. As an almost ancillary part of their studies, each investigator discovered a new cellular organelle: the internal reticular apparatus and the accessory body. Cajal originally termed his structure the “accessory body” because, like its larger cousin, the nucleolus, it was easily stained by silver nitrate. Golgi's organelle now bears his name, and we have since learned a great deal about its function. However, insight into the function of Cajal's organelle has been particularly opaque. In fact, it was not until the late 1960s that accessory bodies were identified in the electron microscope (Monneron and Bernhard, 1969Monneron A Bernhard W Fine structural organization of the interphase nucleus in some mammalian cells.J Ultrastruct Res. 1969; 27: 266-288Crossref PubMed Scopus (505) Google Scholar). Microscopists have since preferred the name “coiled bodies,” since they appear to be composed of a tangle of coiled, electron-dense threads, ∼0.5 μm in diameter. It was not until the 1990s that a molecular characterization of CBs began (Andrade et al., 1991Andrade LEC Chan EKL Raska I Peebles CL Roos G Tan EM Human autoantibody to a novel protein of the nuclear coiled body: immunological characterization and cDNA cloning of p80 coilin.J Exp Med. 1991; 173: 1407-1419Crossref PubMed Scopus (317) Google Scholar; Raska et al., 1991Raska I Andrade LEC Ochs RL Chan EKL Chang C-M Roos G Tan EM Immunological and ultrastructural studies of the nuclear coiled body with autoimmune antibodies.Exp Cell Res. 1991; 195: 27-37Crossref PubMed Scopus (287) Google Scholar; Carmo-Fonseca et al., 1992Carmo-Fonseca M Pepperkok R Carvalho MT Lamond AI Transcription-dependent colocalization of the U1, U2, U4/U6 and U5 snRNPs in coiled bodies.J Cell Biol. 1992; 117: 1-14Crossref PubMed Scopus (339) Google Scholar; Andrade et al., 1993Andrade LEC Tan EM Chan EKL Immunocytochemical analysis of the coiled body in the cell cycle and during cell proliferation.Proc Natl Acad Sci USA. 1993; 90: 1947-1951Crossref PubMed Scopus (184) Google Scholar; Matera and Ward, 1993Matera AG Ward DC Nucleoplasmic organization of small nuclear ribonucleoproteins in cultured human cells.J Cell Biol. 1993; 121: 715-727Crossref PubMed Scopus (134) Google Scholar). Despite these discoveries, functional data on these organelles has not been forthcoming. A number of more recent studies, the focus of this review, suggest that CBs participate in the biogenesis of small nuclear ribonucleoproteins (snRNPs), the ubiquitous mediators of posttranscriptional RNA processing. It also appears that, at least in most cell types, CBs are indistinguishable from nuclear structures known as “gems” and that these Janus-faced nuclear organelles may be of central importance in the etiology of spinal muscular atrophy (SMA). Patient autoantisera recognizing an 80-kD protein strongly and specifically label CBs (Andrade et al., 1991Andrade LEC Chan EKL Raska I Peebles CL Roos G Tan EM Human autoantibody to a novel protein of the nuclear coiled body: immunological characterization and cDNA cloning of p80 coilin.J Exp Med. 1991; 173: 1407-1419Crossref PubMed Scopus (317) Google Scholar; Raska et al., 1991Raska I Andrade LEC Ochs RL Chan EKL Chang C-M Roos G Tan EM Immunological and ultrastructural studies of the nuclear coiled body with autoimmune antibodies.Exp Cell Res. 1991; 195: 27-37Crossref PubMed Scopus (287) Google Scholar). Immunofluorescence studies reveal that coilin is a nuclear protein, localizing diffusely throughout the nucleoplasm and concentrating in a few bright foci (fig. 1). Anti-coilin antibodies stain similar structures in a wide variety of species from vertebrates to plants (Tuma et al., 1993Tuma RS Stolk JA Roth MB Identification and characterization of a sphere organelle protein.J Cell Biol. 1993; 122: 767-773Crossref PubMed Scopus (89) Google Scholar; Beven et al., 1995Beven AF Simpson GG Brown JWS Shaw PJ The organization of spliceosomal components in the nuclei of higher plants.J Cell Sci. 1995; 108: 509-518Crossref PubMed Google Scholar; Gall et al., 1995Gall JG Tsvetkov A Wu Z Murphy C Is the sphere organelle/coiled body a universal nuclear component?.Dev Genet. 1995; 16: 25-35Crossref PubMed Scopus (156) Google Scholar). CBs are dynamic structures; they disassemble during mitosis, and they reassemble in mid G1 after nucleologenesis and the resumption of transcription (Andrade et al., 1993Andrade LEC Tan EM Chan EKL Immunocytochemical analysis of the coiled body in the cell cycle and during cell proliferation.Proc Natl Acad Sci USA. 1993; 90: 1947-1951Crossref PubMed Scopus (184) Google Scholar; Ferreira et al., 1994Ferreira JA Carmo-Fonseca M Lamond AI Differential interaction of splicing snRNPs with coiled bodies and interchromatin granules during mitosis and assembly of daughter cell nuclei.J Cell Biol. 1994; 126: 11-23Crossref PubMed Scopus (94) Google Scholar). Since identification of the CB signature protein, p80 coilin, the list of macromolecules that accumulate within CBs has grown steadily (for reviews see Lamond and Earnshaw, 1998Lamond AI Earnshaw WC Structure and function in the nucleus.Science. 1998; 280: 547-553Crossref PubMed Scopus (756) Google Scholar; Matera, 1998Matera AG Of coiled bodies, gems, and salmon.J Cell Biochem. 1998; 70: 181-192Crossref PubMed Scopus (69) Google Scholar). CBs are highly enriched in components of three major RNA processing pathways: pre-mRNA splicing, histone mRNA 3′ maturation, and pre-rRNA processing (Gall et al., 1995Gall JG Tsvetkov A Wu Z Murphy C Is the sphere organelle/coiled body a universal nuclear component?.Dev Genet. 1995; 16: 25-35Crossref PubMed Scopus (156) Google Scholar). Despite a lack of ongoing transcription within CBs, they also contain elements of the basal transcription machinery, as well as cell cycle–control proteins (Grande et al., 1997Grande M van der Kraan I de Jong L van Driel R Nuclear distribution of transcription factors in relation to sites of transcription and RNA polymerase II.J Cell Sci. 1997; 110: 1781-1791Crossref PubMed Google Scholar; Jordan et al., 1997Jordan P Cunha C Carmo-Fonseca M The cdk7-cyclin H-MAT1 complex associated with TFIIH is localized in coiled bodies.Mol Biol Cell. 1997; 8: 1207-1217Crossref PubMed Scopus (53) Google Scholar; Schul et al., 1998Schul W van Driel R de Jong L Coiled bodies and U2 snRNA genes adjacent to coiled bodies are enriched in factors required for snRNA transcription.Mol Biol Cell. 1998; 9: 1025-1036Crossref PubMed Scopus (75) Google Scholar). These observations raise the possibility that CBs may provide an interface through which these different cellular machineries interact. The large size and molecular complexity of CBs have made it tempting to speculate on their roles, but several seemingly attractive models of CB function can now be excluded. First, experiments in a variety of cells, using tritiated uridine (Fakan and Bernhard, 1971Fakan S Bernhard W Localisation of rapidly and slowly labelled nuclear RNA as visualized by high resolution autoradiography.Exp Cell Res. 1971; 67: 129-141Crossref PubMed Scopus (179) Google Scholar; Callan and Gall, 1991Callan HG Gall JG Association of RNA with the B and C snurposomes of Xenopus oocyte nuclei.Chromosoma. 1991; 101: 69-82Crossref PubMed Scopus (27) Google Scholar) or Br-UTP incorporation (Jordan et al., 1997Jordan P Cunha C Carmo-Fonseca M The cdk7-cyclin H-MAT1 complex associated with TFIIH is localized in coiled bodies.Mol Biol Cell. 1997; 8: 1207-1217Crossref PubMed Scopus (53) Google Scholar; Schul et al., 1998Schul W van Driel R de Jong L Coiled bodies and U2 snRNA genes adjacent to coiled bodies are enriched in factors required for snRNA transcription.Mol Biol Cell. 1998; 9: 1025-1036Crossref PubMed Scopus (75) Google Scholar), show that CBs become labeled only slowly and inefficiently, which indicates that CBs are probably not sites of transcription per se. Second, the absence of non-snRNP splicing factors such as SC-35 and U2AF (Gama-Carvalho et al., 1997Gama-Carvalho M Krauss R Chiang L Valcarcel J Green M Carmo-Fonseca M Targeting of U2AF65 to sites of active splicing in the nucleus.J Cell Biol. 1997; 137: 975-987Crossref PubMed Scopus (107) Google Scholar and references therein), coupled with the lack of heterogeneous nuclear RNPs and poly A+ RNA, makes it unlikely that CBs are directly involved in splicing. Rather, although it seems ever more likely that CBs play a part in a number of cellular functions, the high concentration of snRNPs suggests that they participate in snRNP maturation. There is little doubt that small ribonucleoprotein particles play central roles in pre-mRNA splicing, pre-rRNA processing, histone mRNA 3′ end maturation, and pre-tRNA processing. Because CBs are enriched in components that carry out at least the first three of these pathways, these structures might represent supply centers for the various factors required for transcription and processing of nearby genes and gene products. Viewed in this light, nucleoplasmic CBs may play a role analogous to nucleolar fibrillar centers (Hozák, 1995Hozák P Catching RNA polymerase I in flagranti: ribosomal genes are transcribed in the dense fibrillar component of the nucleolus.Exp Cell Res. 1995; 216: 285-289Crossref PubMed Scopus (53) Google Scholar and references therein). Recent evidence demonstrating that CBs associate with snRNA genes in interphase human cells is consistent with this idea (Frey and Matera, 1995Frey MR Matera AG Coiled bodies contain U7 small nuclear RNA and associate with specific DNA sequences in interphase cells.Proc Natl Acad Sci USA. 1995; 92: 5915-5919Crossref PubMed Scopus (222) Google Scholar; Smith et al., 1995Smith K Carter K Johnson C Lawrence J U2 and U1 snRNA gene loci associate with coiled bodies.J Cell Biochem. 1995; 59: 473-485Crossref PubMed Scopus (91) Google Scholar; Gao et al., 1997Gao L Frey MR Matera AG Human genes encoding U3 snRNA associate with coiled bodies in interphase cells and are clustered on chromosome 17p11.2 in a complex inverted repeat structure.Nucleic Acids Res. 1997; 25: 4740-4747Crossref PubMed Scopus (75) Google Scholar). However, although CBs often colocalize with specific snRNA genes, they do not appear to contain nascent transcripts. Rather, epitopes present on mature snRNPs (e.g., Sm proteins and trimethylguanosine [TMG] cap structures) are highly enriched within CBs. After transcription, most of the splicing or “Sm” class snRNAs are exported to the cytoplasm. Assembly into snRNP particles, cap hypermethylation, and 3′ end trimming takes place in the cytoplasm, followed by import back into the nucleus (Mattaj et al., 1993Mattaj IW Boelens W Izaurralde E Jarmolowski A Kambach C Nucleocytoplasmic transport and snRNP assembly.Mol Biol Rep. 1993; 18: 79-83Crossref PubMed Scopus (31) Google Scholar). The paths taken by newly assembled snRNPs once they reenter the nucleus are unknown. However, at least a fraction of them transit through CBs. In effect, mature (or nearly mature) snRNP particles return to CBs that are located next to the genes that spawned them. This salmonlike behavior of snRNPs returning to the sites of their synthesis provides the cell with a plausible means to effect feedback regulation and gene dosage compensation (Frey and Matera, 1995Frey MR Matera AG Coiled bodies contain U7 small nuclear RNA and associate with specific DNA sequences in interphase cells.Proc Natl Acad Sci USA. 1995; 92: 5915-5919Crossref PubMed Scopus (222) Google Scholar; Matera, 1998Matera AG Of coiled bodies, gems, and salmon.J Cell Biochem. 1998; 70: 181-192Crossref PubMed Scopus (69) Google Scholar). Other hints that CBs and snRNP import may be coupled come from in vitro experiments in Xenopus egg extract. Addition of demembranated sperm to amphibian egg extracts results in formation of typical pronuclei (Lohka and Masui, 1983Lohka MJ Masui Y Formation in vitro of sperm pronuclei and mitotic chromosomes induced by amphibian ooplasmic components.Science. 1983; 220: 719-721Crossref PubMed Scopus (448) Google Scholar). Bauer and Gall, 1997Bauer DW Gall JG Coiled bodies without coilin.Mol Biol Cell. 1997; 8: 73-82Crossref PubMed Scopus (48) Google Scholar have used this procedure to demonstrate that extracts immunodepleted of coilin still form CB-like structures. However, the CBs thus formed do not contain Sm snRNPs (Bauer and Gall, 1997Bauer DW Gall JG Coiled bodies without coilin.Mol Biol Cell. 1997; 8: 73-82Crossref PubMed Scopus (48) Google Scholar). Additional links between CBs and Sm snRNP trafficking come from Lamond and coworkers, who showed that coilin's phosphorylation state is important for its localization (Lyon et al., 1997Lyon CE Bohmann K Sleeman J Lamond AI Inhibition of protein dephosphorylation results in the accumulation of splicing snRNPs and coiled bodies within the nucleolus.Exp Cell Res. 1997; 230: 84-93Crossref PubMed Scopus (105) Google Scholar; Sleeman et al., in pressSleeman J, Lyon CE, Platani M, Kreivi J-P, Lamond AI. Dynamic interactions between splicing snRNPs, coiled bodies and nucleoli revealed using snRNP protein fusions to the green fluorescent protein. Exp Cell Res (in press)Google Scholar). Inhibition of Ser/Thr dephosphorylation or transfection of a coilin point mutation that mimics a constitutively phosphorylated protein results in accumulation of p80 coilin and splicing snRNPs within the nucleolus (Lyon et al., 1997Lyon CE Bohmann K Sleeman J Lamond AI Inhibition of protein dephosphorylation results in the accumulation of splicing snRNPs and coiled bodies within the nucleolus.Exp Cell Res. 1997; 230: 84-93Crossref PubMed Scopus (105) Google Scholar). On the surface, it would seem that Sm snRNPs and nucleoli make strange bedfellows, yet the nucleolus may well play a central role in intranuclear trafficking of other RNAs and proteins besides ribosomal ones. Additional evidence implicating CBs in snRNP biogenesis comes from the discovery that CBs have twins. Gemini of coiled bodies, or gems, are nuclear structures that have size and shape similar to those of CBs but that do not contain snRNPs (Liu and Dreyfuss, 1996Liu Q Dreyfuss G A novel nuclear structure containing the survival of motor neurons protein.EMBO J. 1996; 15: 3555-3565Crossref PubMed Scopus (622) Google Scholar). Instead, gems contain high concentrations of the survival motor neuron protein, SMN. The SMN gene is duplicated on human chromosome 5q13 (Lefebvre et al., 1995Lefebvre S Burglen L Reboullet S Clermont O Burlet P Viollet L Benichou B et al.Identification and characterization of a spinal muscular atrophy–determining gene.Cell. 1995; 80: 155-165Abstract Full Text PDF PubMed Scopus (2746) Google Scholar) but is an essential, single-copy locus in mice (Schrank et al., 1997Schrank B Gotz R Gunnersen JM Ure JM Toyka KV Smith AG Sendtner M Inactivation of the survival motor neuron gene, a candidate gene for human spinal muscular atrophy, leads to massive cell death in early mouse embryos.Proc Natl Acad Sci USA. 1997; 94: 9920-9925Crossref PubMed Scopus (522) Google Scholar). Deletion of the telomeric copy of the human gene (SMN1) leads to an autosomal recessive disorder, SMA, in which spinal motor neurons degenerate, causing progressive paralysis and muscular atrophy (Lefebvre et al., 1995Lefebvre S Burglen L Reboullet S Clermont O Burlet P Viollet L Benichou B et al.Identification and characterization of a spinal muscular atrophy–determining gene.Cell. 1995; 80: 155-165Abstract Full Text PDF PubMed Scopus (2746) Google Scholar). SMA is the most common genetic cause of infant mortality (Crawford and Pardo, 1996Crawford TO Pardo CA The neurobiology of childhood spinal muscular atrophy.Neurobiol Dis. 1996; 3: 97-110Crossref PubMed Scopus (375) Google Scholar). There is a strong inverse correlation between the severity of the disease and the SMN protein level (Coovert et al., 1997Coovert D Le T McAndrew P Strasswimmer J Crawford T Mendell J Coulson S et al.The survival motor neuron protein in spinal muscular atrophy.Hum Mol Genet. 1997; 6: 1205-1214Crossref PubMed Scopus (556) Google Scholar; Lefebvre et al., 1997Lefebvre S Burlet P Liu Q Bertrandy S Clermont O Munnich A Dreyfuss G et al.Correlation between severity and SMN protein level in spinal muscular atrophy.Nat Genet. 1997; 16: 265-269Crossref PubMed Scopus (822) Google Scholar). The predicted protein products of the duplicate human SMN genes are identical except for several silent codon changes (Burghes, 1997Burghes A When is a deletion not a deletion? When it is converted.Am J Hum Genet. 1997; 61: 9-15Abstract Full Text PDF PubMed Scopus (228) Google Scholar; Melki, 1997Melki J Spinal muscular atrophy.Curr Opin Neurol. 1997; 10: 381-385Crossref PubMed Scopus (82) Google Scholar). However, most transcripts of the centromeric copy of SMN (called SMN2) are spliced to generate an isoform of the protein that fails to self-assemble (Lorson et al., 1998Lorson CL Strasswimmer J Yao JM Baleja JD Hahnen E Wirth B Le T et al.SMN oligomerization defect correlates with spinal muscular atrophy severity.Nat Genet. 1998; 19: 63-66Crossref PubMed Scopus (397) Google Scholar and references therein). It is possible that the oligomerization domain encoded by exons 6 and 7 is required for proper association of SMN with other components of the snRNP biogenesis pathway (Lorson et al., 1998Lorson CL Strasswimmer J Yao JM Baleja JD Hahnen E Wirth B Le T et al.SMN oligomerization defect correlates with spinal muscular atrophy severity.Nat Genet. 1998; 19: 63-66Crossref PubMed Scopus (397) Google Scholar). Moreover, these oligomerization-defective SMN proteins may somehow impair gem formation. Indeed, cells derived from patients with the most severe forms of the disease display fewer gems than do those from less severely affected patients (Coovert et al., 1997Coovert D Le T McAndrew P Strasswimmer J Crawford T Mendell J Coulson S et al.The survival motor neuron protein in spinal muscular atrophy.Hum Mol Genet. 1997; 6: 1205-1214Crossref PubMed Scopus (556) Google Scholar; Lefebvre et al., 1997Lefebvre S Burlet P Liu Q Bertrandy S Clermont O Munnich A Dreyfuss G et al.Correlation between severity and SMN protein level in spinal muscular atrophy.Nat Genet. 1997; 16: 265-269Crossref PubMed Scopus (822) Google Scholar). SMN protein is localized throughout the cytoplasm, but its nuclear staining is restricted to gems (fig. 1). SMN directly interacts with several snRNP core factors, including Sm proteins (Liu et al., 1997Liu Q Fischer U Wang F Dreyfuss G The spinal muscular atrophy disease gene product, SMN, and its associated protein SIP1 are in a complex with spliceosomal snRNP proteins.Cell. 1997; 90: 1013-1021Abstract Full Text Full Text PDF PubMed Scopus (514) Google Scholar). These polypeptides form a complex, along with the SMN interacting protein 1 (SIP1), that is >300 kD (Liu et al., 1997Liu Q Fischer U Wang F Dreyfuss G The spinal muscular atrophy disease gene product, SMN, and its associated protein SIP1 are in a complex with spliceosomal snRNP proteins.Cell. 1997; 90: 1013-1021Abstract Full Text Full Text PDF PubMed Scopus (514) Google Scholar). SIP1 and SMN colocalize in the nucleus and the cytoplasm (Liu et al., 1997Liu Q Fischer U Wang F Dreyfuss G The spinal muscular atrophy disease gene product, SMN, and its associated protein SIP1 are in a complex with spliceosomal snRNP proteins.Cell. 1997; 90: 1013-1021Abstract Full Text Full Text PDF PubMed Scopus (514) Google Scholar). Most important, SIP1 has been shown to play an essential role in spliceosomal snRNP biogenesis (Fischer et al., 1997Fischer U Liu Q Dreyfuss G The SMN-SIP1 complex has an essential role in spliceosomal snRNP biogenesis.Cell. 1997; 90: 1023-1029Abstract Full Text Full Text PDF PubMed Scopus (544) Google Scholar). When injected into the cytoplasm of Xenopus oocytes, anti-SIP1 antibodies inhibit Sm core-particle assembly and transport (Fischer et al., 1997Fischer U Liu Q Dreyfuss G The SMN-SIP1 complex has an essential role in spliceosomal snRNP biogenesis.Cell. 1997; 90: 1023-1029Abstract Full Text Full Text PDF PubMed Scopus (544) Google Scholar). SIP1 is thought to be the mammalian homolog of a yeast protein, called Brr1p, that is also involved in snRNP particle assembly (Noble and Guthrie, 1996Noble SM Guthrie C Transcriptional pulse-chase analysis reveals a role for a novel snRNP-associated protein in the manufacture of spliceosomal snRNPs.EMBO J. 1996; 15: 4368-4379Crossref PubMed Google Scholar; Liu et al., 1997Liu Q Fischer U Wang F Dreyfuss G The spinal muscular atrophy disease gene product, SMN, and its associated protein SIP1 are in a complex with spliceosomal snRNP proteins.Cell. 1997; 90: 1013-1021Abstract Full Text Full Text PDF PubMed Scopus (514) Google Scholar). One intriguing question is how a defect in a general cellular function such as snRNP biogenesis can have such a tissue-specific effect. Clearly the facts that mice have only a single copy of the gene (DiDonato et al., 1997DiDonato CJ Chen XN Noya D Korenberg JR Nadeau JH Simard LR Cloning, characterization, and copy number of the murine survival motor neuron gene: homolog of the spinal muscular atrophy-determining gene.Genome Res. 1997; 7: 339-352Crossref PubMed Scopus (100) Google Scholar) and humans have two copies (Lefebvre et al., 1995Lefebvre S Burglen L Reboullet S Clermont O Burlet P Viollet L Benichou B et al.Identification and characterization of a spinal muscular atrophy–determining gene.Cell. 1995; 80: 155-165Abstract Full Text PDF PubMed Scopus (2746) Google Scholar) offer some clues to the pathogenesis in humans. In this regard, gene conversion events within the SMN-inverted duplication can create alleles of the gene that are particularly telling (Campbell et al., 1997Campbell L Potter A Ignatius J Dubowitz V Davies K Genomic variation and gene conversion in spinal muscular atrophy: implications for disease process and clinical phenotype.Am J Hum Genet. 1997; 61: 40-50Abstract Full Text PDF PubMed Scopus (263) Google Scholar). A combination of two severe-SMA alleles results in type I SMA, whereas other combinations are less severe (Burghes, 1997Burghes A When is a deletion not a deletion? When it is converted.Am J Hum Genet. 1997; 61: 9-15Abstract Full Text PDF PubMed Scopus (228) Google Scholar). It is well known that neural tissues also express a wide variety of alternatively spliced messages. Thus, neurons may be particularly sensitive to perturbations in snRNP biogenesis. In the absence of the telomeric SMN1 gene, the fact that the centromeric SMN2 gene tends to produce transcripts lacking the SMN oligomerization domain (Lorson et al., 1998Lorson CL Strasswimmer J Yao JM Baleja JD Hahnen E Wirth B Le T et al.SMN oligomerization defect correlates with spinal muscular atrophy severity.Nat Genet. 1998; 19: 63-66Crossref PubMed Scopus (397) Google Scholar) is prima facie evidence that the protein products from the two genes are not equivalent. Furthermore, motor neuron survival plausibly depends not only on expression of SMN protein but on its ability to assemble. The original identification of CBs was in neurons; indeed, some stages of neuronal development are accompanied by a burst in overall transcriptional activity and coilin production (Santama et al., 1996Santama N Dotti C Lamond A Neuronal differentiation in the rat hippocampus involves a stage-specific reorganization of subnuclear structure both in vivo and in vitro.Eur J Neurosci. 1996; 8: 892-905Crossref PubMed Scopus (39) Google Scholar). This increase in transcription would therefore require large quantities of snRNPs. Thus, the emerging view is that gem formation (and presumably snRNP assembly) is impaired in SMA patients by deletion or mutation of the SMN1 gene. As implied by their name, Gemini bodies are most often found in tight association with CBs. But are they really separate entities? The answer to this question awaits confirmation in the electron microscope; however, as shown in figure 1, antibodies against p80 coilin (red) and SMN (green) stain structures that are often indistinguishable in the light microscope. We screened several different human cell lines and found that, in most of them, CBs and gems were invariably associated (authors' unpublished observations), even when incubated at low temperatures (Liu and Dreyfuss, 1996Liu Q Dreyfuss G A novel nuclear structure containing the survival of motor neurons protein.EMBO J. 1996; 15: 3555-3565Crossref PubMed Scopus (622) Google Scholar). This finding agrees with published findings that show that SMN and coilin have very similar localization patterns in neurons, including nucleolar cap staining (Francis et al., 1998Francis JW Sandrock AW Bhide PG Vonsattel JP Brown RHJ Heterogeneity of subcellular localization and electrophoretic mobility of survival motor neuron (SMN) protein in mammalian neural cells and tissues.Proc Natl Acad Sci USA. 1998; 95: 6492-6497Crossref PubMed Scopus (59) Google Scholar). For example, there appears to be good concordance if one compares the coilin staining in “stage 5” neurons (Santama et al., 1996Santama N Dotti C Lamond A Neuronal differentiation in the rat hippocampus involves a stage-specific reorganization of subnuclear structure both in vivo and in vitro.Eur J Neurosci. 1996; 8: 892-905Crossref PubMed Scopus (39) Google Scholar) with the nucleolar SMN staining noted above (Francis et al., 1998Francis JW Sandrock AW Bhide PG Vonsattel JP Brown RHJ Heterogeneity of subcellular localization and electrophoretic mobility of survival motor neuron (SMN) protein in mammalian neural cells and tissues.Proc Natl Acad Sci USA. 1998; 95: 6492-6497Crossref PubMed Scopus (59) Google Scholar). Curiously, two different strains of HeLa cells displayed two different staining patterns: HeLa strain PV (a gift of G. Dreyfuss) indeed displayed distinct SMN and coilin foci (data not shown), whereas HeLa strain ATCC (fig. 1) did not. Other human cell lines (e.g., HT-1080) fail to show the separation phenotype, suggesting that, in most tissues, CBs are inseparable from gems. CBs, as judged by the presence of p80 coilin, seemingly contain all the various CB components. However, evidence that CB-like structures can be formed in Xenopus egg extracts depleted of coilin (Bauer and Gall, 1997Bauer DW Gall JG Coiled bodies without coilin.Mol Biol Cell. 1997; 8: 73-82Crossref PubMed Scopus (48) Google Scholar) may provide insight into the possible identity of CBs and gems. The nuclear bodies thus formed are devoid of coilin and Sm snRNPs. Whether or not these CBs contain SMN is an open question. Perhaps the structures detected in HeLa-PV cells, which contain SMN but lack both coilin and splicing snRNPs, are similar to Bauer and Gall's in vitro CBs. Alternatively, if gems are indeed separate structures, perhaps they associate with different chromosomal loci that have been scrambled in the two aneuploid HeLa strains. The onus is upon the electron microscopists to answer this question definitively, but it may well be that CBs and gems are just two different faces of the same structure. The Janus hypothesis raises some interesting questions. Not only would the unity of gems and CBs strengthen putative roles for the organelle in snRNP biogenesis but would suggest that defects in other parts of the pathway (e.g., in the gene for p80 coilin) could also have neurodegenerative phenotypes. Although our laboratory is currently using both genetic and cell culture systems to address this question, it is clear that additional structural studies are in order. For example, do other CB components besides SMN and SIP1 localize in coilin-negative CBs (i.e., gems)? Are there factors other than SMN and SIP1 that fail to localize in CBs from SMA patient–derived cells? These and other questions should fuel investigation of the structure and function of CBs well into the next century. We thank E. Chan and G. Dreyfuss for antibodies against p80 coilin and SMN, respectively. We are grateful to many colleagues, including A. Weiner, J. Gall, A. Lamond, G. Dreyfuss, W. Schul, and M. Carmo-Fonseca for many stimulating discussions on CBs and their functions. Thanks also to J. Ashkenas for editorial assistance. This work was supported by NIH grant GM53034 (to A.G.M.).
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