The Drosophila melanogaster Cajal body

Cajal bodies (CBs) are nuclear organelles that are usually identified by the marker protein p80-coilin. Because no orthologue of coilin is known in Drosophila melanogaster, we identified D. melanogaster CBs using probes for other components that are relatively diagnostic for CBs in vertebrate cells. U85 small CB-specific RNA, U2 small nuclear RNA, the survival of motor neurons protein, and fibrillarin occur together in a nuclear body that is closely associated with the nucleolus. Based on its similarity to CBs in other organisms, we refer to this structure as the D. melanogaster CB. Surprisingly, the D. melanogaster U7 small nuclear RNP resides in a separate nuclear body, which we call the histone locus body (HLB). The HLB is invariably colocalized with the histone gene locus. Thus, canonical CB components are distributed into at least two nuclear bodies in D. melanogaster. The identification of these nuclear bodies now permits a broad range of questions to be asked about CB structure and function in a genetically tractable organism.     

Characterization of human myocardial fibroblasts immortalized by HPV16 E6--E7 genes

SMN, the affected protein in spinal muscular atrophy (SMA), is a cytoplasmic protein that also occurs in nuclear structures called "gems" and is involved in snRNP maturation. Coilin-p80 is a marker protein for nuclear Cajal bodies (coiled bodies; CBs) which are also involved in snRNP maturation, storage or transport. We now show that gems and CBs are present in all fetal tissues, even those that lack gems/CBs in the adult. Most gems and CBs occur as separate nuclear structures in fetal tissues, but their colocalization increases with fetal age and is almost complete in the adult. In adult tissues, up to half of all gems/CBs are inside the nucleolus, whereas in cultured cells they are almost exclusively nucleoplasmic. The nucleolar SMN is often more diffusely distributed, compared with nucleoplasmic gems. Up to 30% of cells in fetal tissues have SMN distributed throughout the nucleolus, instead of forming gems in the nucleoplasm. The results suggest a function for gems distinct from Cajal bodies in fetal nuclei and a nucleolar function for SMN. Spinal cord, the affected tissue in SMA, behaves differently in several respects. In both fetal and adult motor neurons, many gems/CBs occur as larger bodies closely associated with the nucleolar perimeter. Uniquely in motor neurons, gems/CBs are more numerous in adult than in fetal stages and colocalization of gems and CBs occurs earlier in development. These unusual features of motor neurons may relate to their special sensitivity to reduced SMN levels in SMA patients.     

Assembly of snRNP-containing coiled bodies is regulated in interphase and mitosis--evidence that the coiled body is a kinetic nuclear structure

Coiled bodies (CBs) are nuclear organelles in which splicing snRNPs concentrate. While CBs are sometimes observed in association with the nucleolar periphery, they are shown not to contain 5S or 28S rRNA or the U3 snoRNA. This argues against CBs playing a role in rRNA maturation or transport as previously suggested. We present evidence here that CBs are kinetic structures and demonstrate that the formation of snRNP-containing CBs is regulated in interphase and mitosis. The coiled body antigen, p80 coilin, was present in all cell types studied, even when CBs were not prominent. Striking changes in the formation of CBs could be induced by changes in cellular growth temperature without a concomitant change in the intracellular p80 coilin level. During mitosis, CBs disassemble, coinciding with a mitotic-specific phosphorylation of p80 coilin. Coilin is shown to be a phosphoprotein that is phosphorylated on at least two additional sites during mitosis. CBs reform in daughter nuclei after a lag period during which they are not detected. CBs are thus, dynamic nuclear organelles and we propose that cycling interactions of splicing snRNPs with CBs may be important for their participation in the processing or transport of pre-mRNA in mammalian cells.     

In vivo kinetics of Cajal body components

Cajal bodies (CBs) are subnuclear domains implicated in small nuclear ribonucleoprotein (snRNP) biogenesis. In most cell types, CBs coincide with nuclear gems, which contain the survival of motor neurons (SMN) complex, an essential snRNP assembly factor. Here, we analyze the exchange kinetics of multiple components of CBs and gems in living cells using photobleaching microscopy. We demonstrate differences in dissociation kinetics of CB constituents and relate them to their functions. Coilin and SMN complex members exhibit relatively long CB residence times, whereas components of snRNPs, small nucleolar RNPs, and factors shared with the nucleolus have significantly shorter residence times. Comparison of the dissociation kinetics of these shared proteins from either the nucleolus or the CB suggests the existence of compartment-specific retention mechanisms. The dynamic properties of several CB components do not depend on their interaction with coilin because their dissociation kinetics are unaltered in residual nuclear bodies of coilin knockout cells. Photobleaching and fluorescence resonance energy transfer experiments demonstrate that coilin and SMN can interact within CBs, but their interaction is not the major determinant of their residence times. These results suggest that CBs and gems are kinetically independent structures.     

SUMO-1 transiently localizes to Cajal bodies in mammalian neurons

Cajal bodies (CBs) are nuclear organelles involved in the maturation of small nuclear ribonucleoproteins required for the processing of pre-mRNAs. They concentrate coilin, splicing factors and the survival of motor neuron protein (SMN). By using immunocytochemistry and transfection experiments with GFP–SUMO-1, DsRed1-Ubc9, GFP–coilin and GFP–SMN constructs we demonstrate the presence of SUMO-1 and the SUMO conjugating enzyme (Ubc9) in a subset of CBs in undifferentiated neuron-like UR61 cells. Furthermore, SUMO-1 is transiently localized into neuronal CBs from adult nervous tissue in response to osmotic stress or inhibition of methyltransferase activity. SUMO-1-positive CBs contain coilin, SMN and small nuclear ribonucleoproteins, suggesting that they are functional CBs involved in pre-mRNA processing. Since coilin and SMN have several putative motifs of SUMO-1 modification, we suggest that the sumoylation of coilin and/or SMN might play a role in the molecular reorganization of CBs during the neuronal differentiation or stress–response.     

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.

Coiled bodies (CBs) are nuclear organelles whose morphological structure and molecular composition have been conserved from plants to animals. Furthermore, CBs are often found to co-localize with specific DNA loci in both mammalian somatic nuclei and amphibian oocytes. Much as rDNA sequences are called nucleolus organizers, we term these coiled body-associated sequences 'coiled body organizers' (CBORs). The only sequences that have been shown to be CBORs in human cells are the U1, U2 and histone gene loci. We wanted to determine whether other snRNA genes might also act as CBORs. In this paper we show that human U3 genes (the RNU3 locus) preferentially associate with CBs in interphase cells. In addition, we have analyzed the genomic organization of the RNU3 locus by constructing a BAC and P1 clone contig. We found that, unlike the RNU1 and RNU2 loci, U3 genes are not tandemly repeated. Rather, U3 genes are clustered on human chromosome 17p11.2, with evidence for large inverted duplications within the cluster. Thus all of the CBORs identified to date are composed of either tandemly repeated or tightly clustered genes. The evolutionary and cell biological consequences of this type of organization are discussed.     

The Movement of Coiled Bodies Visualized in Living Plant Cells by the Green Fluorescent Protein

Coiled bodies are nuclear organelles that contain components of at least three RNA-processing pathways: pre-mRNA splicing, histone mRNA 3'- maturation, and pre-rRNA processing. Their function remains unknown. However, it has been speculated that coiled bodies may be sites of splicing factor assembly and/or recycling, play a role in histone mRNA 3'-processing, or act as nuclear transport or sorting structures. To study the dynamics of coiled bodies in living cells, we have stably expressed a U2B"-green fluorescent protein fusion in tobacco BY-2 cells and in Arabidopsis plants. Time-lapse confocal microscopy has shown that coiled bodies are mobile organelles in plant cells. We have observed movements of coiled bodies in the nucleolus, in the nucleoplasm, and from the periphery of the nucleus into the nucleolus, which suggests a transport function for coiled bodies. Furthermore, we have observed coalescence of coiled bodies, which suggests a mechanism for the decrease in coiled body number during the cell cycle. Deletion analysis of the U2B" gene construct has shown that the first RNP-80 motif is sufficient for localization to the coiled body.     

Universal nuclear domains of somatic and germ cells: some lessons from oocyte interchromatin granule cluster and Cajal body structure and molecular composition

It is now clear that two prominent nuclear domains, interchromatin granule clusters (IGCs) and Cajal bodies (CBs), contribute to the highly ordered organization of the extrachromosomal space of the cell nucleus. These functional domains represent structurally stable but highly dynamic nuclear organelles enriched in factors that are required for different nuclear activities, especially RNA biogenesis. IGCs are considered to be the main sites for storage, assembly, and/or recycling of the essential spliceosome components. CBs are involved in the biogenesis of several classes of small RNPs as well as the modification of newly assembled small nuclear RNA. We have summarized data on the molecular composition, structure, and functional roles of IGCs and CBs in the nuclei of mammalian somatic cells and oocytes of some animals with a special focus on insects. We have focused on similarities and differences between the IGCs and CBs of oocytes and the well‐studied CBs and IGCs of cultured mammalian somatic cells. We have shown the heterogeneous character of oocyte IGCs and CBs, both in structure and molecular content. We have also demonstrated the unique capacity of oocytes to form close structural interactions between IGC and CB components. We proposed to consider these joint structures as integrated entities, sharing the features of both IGCs and CBs.     

Identification of coiled body-like structures in meristematic cells of Pisum sativum cotyledonary buds

This study focused on two types of nuclear bodies visible in plant cells that were previously thought to be similar to the coiled bodies (CBs) of animal cells: the nucleolus-associated body (NAB) and dense body (DB). We show that both NABs and DBs share common features with animal CBs: they consist of ribonucleoproteins, are silver-stainable, and lack DNA. Immunoelectron microscopy shows that only the NABs are rich in snRNAs and fibrillarin, two markers characteristic of animal CBs. This suggests that NABs rather than DBs are the plant counterparts of the CBs of animal cells. These structures appear most frequently in cells blocked in G0–1, their frequency gradually declining with resumption of the cell cycle and nucleolar activity. During this reactivation period, NABs are released from the nucleolus to the nucleoplasm, suggesting that they may act as nuclear transport or sorting structures. Received: 2 November 1998; in revised form: 18 January 1999 / Accepted: 1 February 1999     

Of coiled bodies,gems, and salmon

Coiled bodies (CBs) are nuclear organelles whose morphology and composition have been conserved from plants to animals. They are highly enriched in components of three different RNA processing pathways. Small nuclear RNAs (snRNAs) involved in pre-mRNA splicing, rRNA processing, and histone mRNA 3′ end maturation all take up residence in CBs. However, CB function(s) remain obscure. This review will focus on recent developments in several aspects of CB structure and function, including exciting new results on their twin organelles, called gems. In particular, the reader will be introduced to a novel hypothesis called the “salmon theory of snRNP biogenesis.” Questions arising from and experiments necessary to test this hypothesis will be discussed. J. Cell. Biochem. 70:181–192, 1998. © 1998 Wiley-Liss, Inc.     

The exonuclease ISG20 mainly localizes in the nucleolus and the Cajal (Coiled) bodies and is associated with nuclear SMN protein-containing complexes

We have previously shown that ISG20, an interferon (IFN)-induced gene, encodes a 3' to 5' exoribonuclease member of the DEDD superfamily of exonucleases. ISG20 specifically degrades single-stranded RNA. In this report, using immunofluorescence analysis, we demonstrate that in addition to a diffuse cytoplasmic and nucleoplasmic localization, the endogenous ISG20 protein was present in the nucleus both in the nucleolus and in the Cajal bodies (CBs). In addition, we show that the ectopic expression of the CBs signature protein, coilin, fused to the red fluorescent protein (coilin-dsRed) increased the number of nuclear dots containing both ISG20 and coilin-dsRed. Using electron microcopy analysis, ISG20 appeared principally concentrated in the dense fibrillar component of the nucleolus, the major site for rRNA processing. We also present evidences that ISG20 was associated with survival of motor neuron (SMN)-containing macromolecular nuclear complexes required for the biogenesis of various small nuclear ribonucleoproteins. Finally, we demonstrate that ISG20 was associated with U1 and U2 snRNAs, and U3 snoRNA. The accumulation of ISG20 in the CBs after IFN treatment strongly suggests its involvement in a new route for IFN-mediated inhibition of protein synthesis by modulating snRNA and rRNA maturation.