Newly assembled snRNPs associate with coiled bodies before speckles, suggesting a nuclear snRNP maturation pathway.

BACKGROUND: Small nuclear ribonucleoproteins (snRNPs), which are essential components of the mRNA splicing machinery, comprise small nuclear RNAs, each complexed with a set of proteins. An early event in the maturation of snRNPs is the binding of the core proteins - the Sm proteins - to snRNAs in the cytoplasm followed by nuclear import. Immunolabelling with antibodies against Sm proteins shows that splicing snRNPs have a complex steady-state localisation within the nucleus, the result of the association of snRNPs with several distinct subnuclear structures. These include speckles, coiled bodies and nucleoli, in addition to a diffuse nucleoplasmic compartment. The reasons for snRNP accumulation in these different structures are unclear. RESULTS: When mammalian cells were microinjected with plasmids encoding the Sm proteins B, D1 and E, each tagged with either the green fluorescent protein (GFP) or yellow-shifted GFP (YFP), a pulse of expression of the tagged proteins was observed. In each case, the newly synthesised GFP/YFP-labelled snRNPs accumulated first in coiled bodies and nucleoli, and later in nuclear speckles. Mature snRNPs localised immediately to speckles upon entering the nucleus after cell division. CONCLUSIONS: The complex nuclear localisation of splicing snRNPs results, at least in part, from a specific pathway for newly assembled snRNPs. The data demonstrate that the distribution of snRNPs between coiled bodies and speckles is directed and not random.     

The Sm Core Domain Mediates Targeting of U1 snRNP to Subnuclear Compartments Involved in Transcription and Splicing

In the mammalian cell nucleus pre-mRNA splicing factors such as U snRNPs are concentrated in distinct subnuclear compartments named perichromatin fibrils (PFs), interchromatin granules (IGs), interchromatin granule-associated zones (IG-associated zones), and coiled bodies (CBs). The structural requirement for the localization of U snRNPs to these domains was investigated by microinjection of digoxygenin-labeled in vitro-reconstituted U1 snRNPs and mutants thereof and subsequent analysis by immunoelectron microscopy. Wild-type U1 snRNP was targeted, after injection into the cytoplasm, to the nucleus and localized in PFs, IGs, IG-associated zones, and CBs. Thus, microinjected U1 snRNP particles exhibited a subnuclear localization similar to that previously observed for endogenous U1 snRNPs. Specific U snRNP proteins were shown not to be essential for subnuclear targeting since U1 snRNP mutants that did not bind to 70K, A, or C peptides were distributed in the cell nucleus in a pattern indistinguishable from that of wild-type U1 snRNP. Moreover, the Sm core domain, common to all spliceosomal U snRNPs, was shown to be sufficient for appropriate subnuclear distribution. Thus, these observations indicate that the Sm core domain, previously shown to be essential for nuclear import of spliceosomal U1 snRNPs, is also important for mediating the targeting to distinct nuclear subcompartments.     

The spinal muscular atrophy disease gene product, SMN: A link between snRNP biogenesis and the Cajal (coiled) body

The spliceosomal snRNAs U1, U2, U4, and U5 are synthesized in the nucleus, exported to the cytoplasm to assemble with Sm proteins, and reimported to the nucleus as ribonucleoprotein particles. Recently, two novel proteins involved in biogenesis of small nuclear ribonucleoproteins (snRNPs) were identified, the Spinal muscular atrophy disease gene product (SMN) and its associated protein SIP1. It was previously reported that in HeLa cells, SMN and SIP1 form discrete foci located next to Cajal (coiled) bodies, the so-called "gemini of coiled bodies" or "gems." An intriguing feature of gems is that they do not appear to contain snRNPs. Here we show that gems are present in a variable but small proportion of rapidly proliferating cells in culture. In the vast majority of cultured cells and in all primary neurons analyzed, SMN and SIP1 colocalize precisely with snRNPs in the Cajal body. The presence of SMN and SIP1 in Cajal bodies is confirmed by immunoelectron microscopy and by microinjection of antibodies that interfere with the integrity of the structure. The association of SMN with snRNPs and coilin persists during cell division, but at the end of mitosis there is a lag period between assembly of new Cajal bodies in the nucleus and detection of SMN in these structures, suggesting that SMN is targeted to preformed Cajal bodies. Finally, treatment of cells with leptomycin B (a drug that blocks export of U snRNAs to the cytoplasm and consequently import of new snRNPs into the nucleus) is shown to deplete snRNPs (but not SMN or SIP1) from the Cajal body. This suggests that snRNPs flow through the Cajal body during their biogenesis pathway.     

Mutational analysis of p80 coilin indicates a functional interaction between coiled bodies and the nucleolus

Coiled bodies are conserved subnuclear domains found in both plant and animal cells. They contain a subset of splicing snRNPs and several nucleolar antigens, including Nopp140 and fibrillarin. In addition, autoimmune patient sera have identified a coiled body specific protein, called p80 coilin. In this study we show that p80 coilin is ubiquitously expressed in human tissues. The full-length human p80 coilin protein correctly localizes in coiled bodies when exogenously expressed in HeLa cells using a transient transfection assay. Mutational analysis identifies separate domains in the p80 coilin protein that differentially affect its subnuclear localization. The data show that p80 coilin has a nuclear localization signal, but this is not sufficient to target the protein to coiled bodies. The results indicate that localization in coiled bodies is not determined by a simple motif analogous to the NLS motifs involved in nuclear import. A specific carboxy-terminal deletion in p80 coilin results in the formation of pseudo-coiled bodies that are unable to recruit splicing snRNPs. This causes a loss of endogenous coiled bodies. A separate class of mutant coilin proteins are shown to localize in fibrillar structures that surround nucleoli. These mutants also lead to loss of endogenous coiled bodies, produce a dramatic disruption of nucleolar architecture and cause a specific segregation of nucleolar antigens. The structural change in nucleoli is accompanied by the loss of RNA polymerase I activity. These data indicate that p80 coilin plays an important role in subnuclear organization and suggest that there may be a functional interaction between coiled bodies and nucleoli.     

Differential expression of tropomyosin forms in the microfilaments isolated from normal and transformed rat cultured cells

Using a newly developed method for microfilament isolation (Matsumura, F., Yamashiro-Matsumura, S. and Lin, J. J.-C. (1983) J. Biol. Chem. 258, 6636-6644), we have analyzed protein composition of microfilaments in "normal" and transformed rat tissue culture cells. They include REF-52 (an established rat embryo cell line) cells, REF-52 transformed by DNA viruses (SV40 or adenovirus type 5), normal rat kidney cells, and normal rat kidney cells transformed by RNA viruses (Kirsten or Rous sarcoma virus). Microfilaments from normal rat culture cells contain three major tropomyosins (apparent Mr = 40,000, 36,500, and 32,400) and two relatively minor tropomyosins (apparent Mr = 35,000 and 32,000). In transformed cells the levels of one or two of the major tropomyosins (Mr = 40,000 and 36,500) are decreased and the levels of one or both of the minor tropomyosins (Mr = 35,000 and 32,000) are increased. These changes in tropomyosin patterns were also observed in temperature shift experiments with rat-1 cells transformed with a Rous sarcoma virus mutant, temperature-sensitive for transformation. Cell-free translation of whole cell mRNA generated similar tropomyosin patterns on two-dimensional gels, suggesting that changes in the pattern of tropomyosin expression were largely effected at the level of RNA rather than by post-translational modification. Such changes in the tropomyosin composition of microfilaments were consistently found to accompany the various morphological alterations associated with transformation. We suggest that alterations in the pattern of tropomyosin expression are involved in, or cause, rearrangement of stress fibers and that this may be responsible (in part) for morphological transformation.     

Isolation and characterization of tropomyosin-containing microfilaments from cultured cells

We have developed a new method for the rapid isolation of tropomyosin-containing microfilaments from cultured cells using anti-tropomyosin monoclonal antibodies. Anti-tropomyosin monoclonal antibodies induce the bundle formation of microfilaments, which can be easily collected by low speed centrifugation. Electron microscopic studies of the isolated microfilaments show periodic localization of tropomyosin along the microfilaments of nonmuscle cells with a 33-34 nm repeat. Furthermore, the isolated microfilaments have the ability to activate the Mg2+-ATPase activity of skeletal muscle myosin to almost the same extent as skeletal muscle F-actin (filamentous actin). This microfilament isolation method is applicable to a variety of cell types, including REF-52 cells (an established rat embryo line), L6 myoblasts, 3T3 fibroblasts, Chinese hamster ovary cells, baby hamster kidney (BHK-21) cells, mouse neuroblastoma cells, gerbil fibroma cells, and chicken embryo fibroblasts. Sodium dodecyl sulfate-polyacrylamide gel analysis shows that, in addition to actin, microfilaments isolated from REF-52 cells contain five species of tropomyosin with apparent Mr = 40,000, 36,500, 35,000, 32,400, and 32,000, alpha-actinin, and as yet unknown proteins with apparent Mr = 83,000 and 37,000. The molar ratio of total tropomyosin (dimer) to actin in the isolated microfilaments is 1:8. The patterns of these multiple forms of tropomyosin were found to change when REF-52 cells were transformed with SV40 or adenovirus type 5.     

The relationship between SMN, the spinal muscular atrophy protein, and nuclear coiled bodies in differentiated tissues and cultured cells

The spinal muscular atrophy protein, SMN, is a cytoplasmic protein that is also found in distinct nuclear structures called "gems." Gems are closely associated with nuclear coiled bodies and both may have a direct role in snRNP maturation and pre-RNA splicing. There has been some controversy over whether gems and coiled bodies colocalize or form adjacent/independent structures in HeLa and other cultured cells. Using a new panel of antibodies against SMN and antibodies against coilin-p80, a systematic and quantitative study of adult differentiated tissues has shown that gems always colocalize with coiled bodies. In some tissues, a small proportion of coiled bodies (<10%) had no SMN, but independent or adjacent gems were not found. The most striking observation, however, was that many cell types appear to have neither gems nor coiled bodies (e.g., cardiac and smooth muscle, blood vessels, stomach, and spleen) and this expression pattern is conserved across human, rabbit, and pig species. This shows that assembly of distinct nuclear bodies is not essential for RNA splicing and supports the view that they may be storage sites for reserves of essential proteins and snRNPs. Overexpression of SMN in COS-7 cells produced supernumerary nuclear bodies, most of which also contained coilin-p80, confirming the close relationship between gems and coiled bodies. However, when SMN is reduced to very low levels in type I SMA fibroblasts, coiled bodies are still formed. Overall, the data suggest that gem/coiled body formation is not determined by high cytoplasmic SMN concentrations or high metabolic activity alone and that a differentiation-specific factor may control their formation.     

Association between the nucleolus and the coiled body

By means of light and electron microscopic immunocytochemistry, we have localized p80-coilin, a specific protein marker for coiled bodies, in mammalian cell lines as well as in primary rat neuron cultures. p80-coilin-stained nuclear bodies, which also contained fibrillarin, could be subsequently silver stained by a method specific for the visualization of nucleolar organizer regions. In cycling cells, most coiled bodies were not associated with nucleoli, whereas in rat neurons such as association was frequent. The treatment of cycling cells with actinomycin D or 5,6-dichloro-1-beta-D-ribo furanosyl-benzimidazole led to nucleolar segregation and/or disintegration, and to an association of p80-coilin staining structures with nucleoli. p80-coilin-positive structures contained fibrillarin in both untreated and treated cells. These results support the opinion that there might be a special association between coiled bodies and nucleoli, particularly in neuronal cells.     

The immunolocalization of small nuclear ribonucleoprotein particles in testicular cells during the cycle of the seminiferous epithelium of the adult rat

The objective of this study was to determine the cellular and subcellular distribution of small nuclear ribonucleoprotein particles (snRNPs) in the adult rat testis in relation to the different cell types at the various stages of the cycle of the seminiferous epithelium. The distribution of snRNPs in the nucleus and cytoplasm of germ cells was quantitated in an attempt to correlate RNA processing with morphological and functional changes occurring during the development of these cells. Light-microscopic immunoperoxidase staining of rat testes with polyclonal anti-Sm and monoclonal anti-Y12 antibodies localized spliceosome snRNPs in the nuclei and cytoplasm of germ cells up to step 10 spermatids. Nuclear staining was intense in Sertoli cells, spermatogonia, spermatocytes, and in the early steps of round spermatid development. Although comparatively weaker, cytoplasmic staining for snRNPs was strongest in mid and late pachytene spermatocytes and early round spermatids. Quantitative electron-microscopic immunogold labeling of Lowicryl embedded testicular sections confirmed the light-microscopic observations but additionally showed that the snRNP content peaked in the cytoplasm of midpachytene spermatocytes and in the nuclei of late pachytene spermatocytes. The immunogold label tended to aggregate into distinct loci over the nuclear chromatin. The chromatoid body of spermatids and spermatocytes and the finely granular material in the interstices of mitochondrial aggregates of spermatocytes were found to be additional sites of snRNP localization and were intensely labeled. This colocalization suggests that these dense cytoplasmic structures may be functionally related. Anti-U1 snRNP antibodies applied to frozen sections showed the same LM localization pattern as spliceosome snRNPs. Anti-U3 snRNP antibodies applied to frozen sections stained nucleoli of germ cells where pre-rRNA is spliced.     

Coiled bodies without coilin.

Nuclei assembled in vitro in Xenopus egg extract contain coiled bodies that have components from three different RNA processing pathways: pre-mRNA splicing, pre-rRNA processing, and histone pre-mRNA 3'-end formation. In addition, they contain SPH-1, the Xenopus homologue of p80-coilin, a protein characteristic of coiled bodies. To determine whether coilin is an essential structural component of the coiled body, we removed it from the egg extract by immunoprecipitation. We showed that nuclei with bodies morphologically identical to coiled bodies (at the light microscope level) formed in such coilin-depleted extract. As expected, these bodies did not stain with antibodies against coilin. Moreover, they failed to stain with an antibody against the Sm proteins, although Sm proteins associated with snRNAs were still present in the extract. Staining of the coilin- and Sm-depleted coiled bodies was normal with antibodies against two nucleolar proteins, fibrillarin and nucleolin. Similar results were observed when Sm proteins were depleted from egg extract: staining of the coiled bodies with antibodies against the Sm proteins and coilin was markedly reduced but bright nucleolin and fibrillarin staining remained. These immunodepletion experiments demonstrate an interdependence between coilin and Sm snRNPs and suggest that neither is essential for assembly of nucleolar components in coiled bodies. We propose that coiled bodies are structurally heterogeneous organelles in which the components of the three RNA processing pathways may occur in separate compartments.