Coilin shuttles between the nucleus and cytoplasm in Xenopus oocytes

The Impact of p70S6 Kinase-Dependent Phosphorylation of Gemin2 in UsnRNP Biogenesis

The survival motor neuron (SMN) complex is a multi-megadalton complex involved in post-transcriptional gene expression in eukaryotes via promotion of the biogenesis of uridine-rich small nuclear ribonucleoproteins (UsnRNPs). The functional center of the complex is formed from the SMN/Gemin2 subunit. By binding the pentameric ring made up of the Sm proteins SmD1/D2/E/F/G and allowing for their transfer to a uridine-rich short nuclear RNA (UsnRNA), the Gemin2 protein in particular is crucial for the selectivity of the Sm core assembly. It is well established that post-translational modifications control UsnRNP biogenesis. In our work presented here, we emphasize the crucial role of Gemin2, showing that the phospho-status of Gemin2 influences the capacity of the SMN complex to condense in Cajal bodies (CBs) in vivo. Additionally, we define Gemin2 as a novel and particular binding partner and phosphorylation substrate of the mTOR pathway kinase ribosomal protein S6 kinase beta-1 (p70S6K). Experiments using size exclusion chromatography further demonstrated that the Gemin2 protein functions as a connecting element between the 6S complex and the SMN complex. As a result, p70S6K knockdown lowered the number of CBs, which in turn inhibited in vivo UsnRNP synthesis. In summary, these findings reveal a unique regulatory mechanism of UsnRNP biogenesis.

Distinctive actions of connexin 46 and connexin 50 in anterior pituitary folliculostellate cells

Folliculostellate cell gap junctions establish a network for the transmission of information within the anterior pituitary. Connexins make up gap junction channels. Changes in connexin (Cx) turnover modify gap junction-mediated intercellular communication. We have reported that cytokines and hormones influence Cx43 turnover and coupling in folliculostellate cells and in the folliculostellate cell line TtT/GF. In addition, the expression of different connexins alters intercellular communication and connexins may have functions besides cell coupling. Here we assessed the expression, turnover and subcellular localization of Cx46 and Cx50 in the anterior pituitary and TtT/GF cells. Then, we assessed the impact of various natural (lactation, annual reproductive cycle, bFGF) and pathological (autoimmune orchitis, diabetes/obesity) conditions associated with altered anterior pituitary hormone secretion on Cx46 and Cx50. Anterior pituitary Cx46 and Cx50 expression and subcellular distribution were cell-dependent. Cx46 was expressed by folliculostellate, TtT/GF and endocrine cells. In the cytoplasm, Cx46 was chiefly associated with lysosomes. Variously sized Cx46 molecules were recovered exclusively in the TtT/GF cell nuclear fraction. In the nucleus, Cx46 co-localized with Nopp-140, a nucleolar factor involved in rRNA processing. Neither cytoplasmic nor nuclear Cx46 and Cx43 co-localized. Cx50 localized to folliculostellate and TtT/GF cells, and to the walls of blood capillaries, not to endocrine cells. Cx50 was cytoplasmic and associated with the cell membrane, not nuclear. Cx50 did not co-localize with Cx46 but it co-localized in the cytoplasm and co-immunoprecipitated with Cx43. Cx46 and Cx50 responses to various physiological and pathological challenges were different, often opposite. Cx46 and Cx43 expression and phosphorylation profiles differed in the anterior pituitary, whereas Cx50 and Cx43 were similar. The data suggest that Cx46 participates to cellular growth and proliferation and that Cx50, together with Cx43, contributes to folliculostellate cell coupling.

Phosphoregulation of the human SMN complex

The survival motor neuron (SMN) complex is a macromolecular machine comprising 9 core proteins: SMN, Gemins2-8 and unrip in vertebrates. It performs tasks in RNA metabolism including the cytoplasmic assembly of spliceosomal small nuclear ribonucleoprotein particles (snRNPs). The SMN complex also localizes to the nucleus, where it accumulates in Cajal Bodies (CB) and may function in transcription and/or pre-mRNA splicing. The SMN complex is subject to extensive phosphorylation. Detailed understanding of SMN complex regulation necessitates a comprehensive analysis of these post-translational modifications. Here, we report on the first comprehensive phosphoproteome analysis of the intact human SMN complex, which identify 48 serine/threonine phosphosites in the complex. We find that 7 out of 9 SMN components of the intact complex are phosphoproteins and confidently place 29 phosphorylation sites, 12 of them in SMN itself. By the generation of multi non-phosphorylatable or phosphomimetic variants of SMN, respectively, we address to which extent phosphorylation regulates SMN complex function and localization. Both phosphomimetic and non-phosphorylatable variants assemble into intact SMN complexes and can compensate the loss of endogenous SMN in snRNP assembly at least to some extent. However, they partially or completely fail to target to nuclear Cajal bodies. Moreover, using a mutant of SMN, which cannot be phosphorylated on previously reported tyrosine residues, we provide first evidence that this PTM regulates SMN localization and nuclear accumulation. Our data suggest complex regulatory cues mediated by phosphorylation of serine/threonine and tyrosine residues, which control the subcellular localization of the SMN complex and its accumulation in nuclear CB.

Microinjected antibodies interfere with protein nucleocytoplasmic shuttling by distinct molecular mechanisms

The observation that some antibodies can enter the nucleus after their microinjection into the cytoplasm established the principle of protein nucleocytoplasmic shuttling. Here, we introduce the concept of stationary antibodies for studying nuclear transport, particularly of native proteins. Contrary to the aforementioned translocating immunoglobulins, stationary antibodies do not cross the nuclear envelope. They are distinguished by their ability to trigger the nucleocytoplasmic redistribution of their antigen. What determines these apparently contradictory outcomes has not been explored. We studied a stationary STAT1 antibody and a translocating importin-beta antibody. The stationary phenotype resulted from the inhibition of carrier-independent transport. This was not due to crosslinking or precipitation of antigen, because the antigen-antibody complex remained highly mobile. Rather, decoration with stationary antibody precluded actual nuclear pore passage of antigen. In addition, both antibodies inhibited the carrier-dependent translocation via importin-alpha, but by diverse mechanisms. The translocating antibody blocked the association with importin-alpha, whereas the stationary antibody prevented the phosphorylation of its antigen, and thus functioned upstream of the importin-alpha binding step. We identified a stationary antibody to green-fluorescent protein (GFP) and probed the translocation of GFP fusions of STAT1, thyroid hormone receptor and histones, demonstrating general application of this approach. Our results provide an experimental rationale for the use of antibodies as unique tools for dissecting protein nuclear translocation. As the microinjection of stationary antibodies extends to analyses of native proteins, this method can complement and validate results obtained with fluorescent-labeled derivatives.

The assembly of a spliceosomal small nuclear ribonucleoprotein particle

The U1, U2, U4, U5 and U6 small nuclear ribonucleoprotein particles (snRNPs) are essential elements of the spliceosome, the enzyme that catalyzes the excision of introns and the ligation of exons to form a mature mRNA. Since their discovery over a quarter century ago, the structure, assembly and function of spliceosomal snRNPs have been extensively studied. Accordingly, the functions of splicing snRNPs and the role of various nuclear organelles, such as Cajal bodies (CBs), in their nuclear maturation phase have already been excellently reviewed elsewhere. The aim of this review is, then, to briefly outline the structure of snRNPs and to synthesize new and exciting developments in the snRNP biogenesis pathways.

The nuclear phenotypic plasticity observed in fish during rRNA regulation entails Cajal bodies dynamics

Cajal bodies (CBs) are small mobile organelles found throughout the nucleoplasm of animal and plant cells. The dynamics of these organelles involves interactions with the nucleolus. The later has been found to play a substantial role in the compensatory response that evolved in eurythermal fish to adapt to the cyclic seasonal habitat changes, i.e., temperature and photoperiod. Contrary to being constitutive, rRNA synthesis is dramatically regulated between summer and winter, thus affecting ribosomal biogenesis which plays a central role in the acclimatization process. To examine whether CBs, up to now, never described in fish, were also sustaining the phenotypic plasticity observed in nuclei of fish undergoing seasonal acclimatization, we identified these organelles both, by transmission electronic microscopy and immunodetection with the marker protein p80-coilin. We found transcripts in all tissues analyzed. Furthermore we assessed that p80-coilin gene expression was always higher in summer-acclimatized fish when compared to that adapted to the cold season, indicating that p80-coilin expression is modulated upon seasonal acclimatization. Concurrently, CBs were more frequently found in summer-acclimatized carp which suggests that the organization of CBs is involved in adaptive processes and contribute to the phenotypic plasticity of fish cell nuclei observed concomitantly with profound reprogramming of nucleolar components and regulation of ribosomal rRNAs.

A sequence motif conserved in diverse nuclear proteins identifies a protein interaction domain utilised for nuclear targeting by human TFIIS

The three structural domains of transcription elongation factor TFIIS are conserved from yeast to human. Although the N-terminal domain is not needed for transcriptional activity, a similar sequence has been identified previously in other transcription factors. We found this conserved sequence, the LW motif, in another three human proteins that are predominantly nuclear localized. We investigated two examples to determine whether the LW motif is actually a dedicated nuclear targeting signal. However, in one of the newly identified proteins, hIWS1 (human Iws1), a region containing classic nuclear localization signals (NLS) rather than the LW motif was necessary and sufficient for nuclear targeting in HeLa cells. In contrast, human TFIIS does not possess an NLS and only constructs containing the LW motif were efficiently targeted to nuclei. Moreover, mutations in the motif could cause cytoplasmic accumulation of TFIIS and enabled a structure/function assay for the domain based on the efficiency of nuclear targeting. Finally, GST pull-down assays showed that the LW motif is part of a protein-binding domain. We suggest that the targeting role the LW motif plays in TFIIS arises from its more general function as a protein interaction domain, enabling TFIIS to bind a carrier protein(s) that accomplishes nuclear import.

Colocalization of coilin and nucleolar proteins in Cajal body-like structures of micronucleated PtK2 cells

Cajal bodies (CB) are ubiquitous nuclear structures involved in the biogenesis of small nuclear ribonucleoproteins and show narrow association with the nucleolus. To identify possible relationships between CB and the nucleolus, the localization of coilin, a marker of CB, and of a set of nucleolar proteins was investigated in cultured PtK2 cells undergoing micronucleation. Nocodazol-induced micronucleated cells were examined by double indirect immunofluorescence with antibodies against coilin, fibrillarin, NOR-90/hUBF, RNA polymerase I, PM/Scl, and To/Th. Cells were imaged on a BioRad 1024-UV confocal system attached to a Zeiss Axiovert 100 microscope. Since PtK2 cells possess only one nucleolus organizer region, micronucleated cells presented only one or two micronuclei containing nucleolus. By confocal microscopy we showed that in most micronuclei lacking a typical nucleolus a variable number of round structures were stained by antibodies against fibrillarin, NOR-90/hUBF protein, and coilin. These bodies were regarded as CB-like structures and were not stained by anti-PM/Scl and anti-To/Th antibodies. Anti-RNA polymerase I antibodies also reacted with CB-like structures in some micronuclei lacking nucleolus. The demonstration that a set of proteins involved in RNA/RNP biogenesis, namely coilin, fibrillarin, NOR-90/hUBF, and RNA polymerase I gather in CB-like structures present in nucleoli-devoid micronuclei may contribute to shed some light into the understanding of CB function.

Nuclear distribution of Oct-4 transcription factor in transcriptionally active and inactive mouse oocytes and its relation to RNA polymerase II and splicing factors

The intranuclear distribution of the transcription factor Oct-4, which is specifically expressed in totipotent mice stem and germ line cells, was studied in mouse oocytes using immunogold labeling/electron microscopy and immunofluorescence/confocal laser scanning microcopy. The localization of Oct-4 was studied in transcriptionally active (uni/bilaminar follicles) and inactive (antral follicles) oocytes. Additionally, the Oct-4 distribution was examined relative to that of the unphosphorylated form of RNA polymerase II (Pol II) and splicing factor (SC 35) in the intranuclear entities such as perichromatin fibrils (PFs), perichromatin granules (PGs), interchromatin granule clusters (IGCs), Cajal bodies (CBs), and nucleolus-like bodies (NLBs). It was shown that: (i) Oct-4 is localized in PFs, IGCs, and in the dense fibrillar component (DFC) of the nucleolus at the transcriptionally active stage of the oocyte nucleus; (ii) Oct-4 present in PFs and IGCs colocalizes with Pol II and SC 35 at the transcriptionally active stage; (iii) Oct-4 accumulates in NLBs, CBs, and PGs at the inert stage of the oocyte. The results confirm the previous suggestion that PFs represent the major nucleoplasmic structural domain involved in active pre-mRNA transcription/processing. The colocalization of Oct-4 with Pol II in both IGCs and PFs in active oocytes (uni/bilaminar follicles) suggests that Oct-4 is intimately associated with the Pol II holoenzyme before and during transcription. The colocalization of Oct-4, Pol II, and SC 35 with coilin-containing structures such as NLBs and CBs at the inert stage (antral follicles) suggests that the latter may represent storage sites for the transcription/splicing machinery during the decline of transcription.

Association of the nuclear matrix component NuMA with the Cajal body and nuclear speckle compartments during transitions in transcriptional activity in lens cell differentiation

The transcriptional status of cells can be deduced from the staining pattern of various nuclear markers such as the Cajal body, nucleolus and nuclear speckles. In this study we have used these markers to correlate transcriptional status with cell differentiation in the lens. As a closed system with no cell loss and with each stage being spatially preserved, it is particularly well suited to such studies. To confirm that the nuclear markers in lens cells follow the same trends as in other cells, primary bovine lens epithelial cells were cultured and then treated with actinomycin D to inhibit transcription. This reduced the Cajal body markers to one or two foci per nucleus and the nucleoli became compacted as revealed by fibrillarin staining. The nuclear speckles, containing snRNPs (e.g. Sm) and the splicing factor, SC35, also became larger and more numerous while the signal for trimethylguanine (TMG) decreased suggesting a role hierarchy for the various speckle factors during transcriptional shutdown. The signal for survival of motor neurones gene product (SMN) also decreased at this point. In the lens epithelium, postmitotic cells near the equatorial region had one or two Cajal bodies per nucleus, indicating these cells had only basal levels of transcription. Sm was also present as large foci in these cells. Interestingly, both the speckles and Cajal bodies were NuMA-positive in these post-mitotic cells. At the epithelial-fibre cell transition, Cajal body number increased, while their size decreased indicative of increased transcriptional activity. Fibrillarin adopted the open floret pattern indicating increased transcriptional activity. The nuclear speckles adopted a more diffuse nucleoplasmic pattern, although some spots were still observed. All NuMA colocalisation with the Cajal bodies and nuclear speckles was lost at this stage of lens cell differentiation. Transcriptional shutdown occurs at a later stage in fibre cell differentiation, prior to programmed nuclear destruction. In the lens, both the Cajal bodies and nuclear speckles again became NuMA-positive, although separate NuMA spots were also formed during transcriptional shutdown. These data suggest the nuclear matrix is important in the concentration of Cajal body and speckle components into large, distinct spots in transcriptionally inactive nuclei and also suggest a new role for NuMA in post-mitotic cells to assist in these sub-nuclear reorganisations.

Nup98 is a mobile nucleoporin with transcription-dependent dynamics

Nucleoporin 98 (Nup98), a glycine-leucine-phenylalanine-glycine (GLFG) amino acid repeat-containing nucleoporin, plays a critical part in nuclear trafficking. Injection of antibodies to Nup98 into the nucleus blocks the export of most RNAs. Nup98 contains binding sites for several transport factors; however, the mechanism by which this nucleoporin functions has remained unclear. Multiple subcellular localizations have been suggested for Nup98. Here we show that Nup98 is indeed found both at the nuclear pore complex and within the nucleus. Inside the nucleus, Nup98 associates with a novel nuclear structure that we term the GLFG body because the GLFG domain of Nup98 is required for targeting to this structure. Photobleaching of green fluorescent protein-Nup98 in living cells reveals that Nup98 is mobile and moves between these different localizations. The rate of recovery after photobleaching indicates that Nup98 interacts with other, less mobile, components in the nucleoplasm. Strikingly, given the previous link to nuclear export, the mobility of Nup98 within the nucleus and at the pore is dependent on ongoing transcription by RNA polymerases I and II. These data give rise to a model in which Nup98 aids in direction of RNAs to the nuclear pore and provide the first potential mechanism for the role of a mobile nucleoporin.

RNA helicase p54 (DDX6) is a shuttling protein involved in nuclear assembly of stored mRNP particles

Previously, we showed that an integral component of stored mRNP particles in Xenopus oocytes, Xp54, is a DEAD-box RNA helicase with ATP-dependent RNA-unwinding activity. Xp54 belongs to small family of helicases (DDX6) that associate with mRNA molecules encoding proteins required for progress through meiosis. Here we describe the nucleocytoplasmic translocation of recombinant Xp54 in microinjected oocytes and in transfected culture cells. We demonstrate that Xp54 is present in oocyte nuclei, its occurrence in both soluble and particle-bound forms and its ability to shuttle between nucleus and cytoplasm. Translocation of Xp54 from the nucleus to the cytoplasm appears to be dependent on the presence of a leucine-rich nuclear export signal (NES) and is blocked by leptomycin B, a specific inhibitor of the CRM1 receptor pathway. However, the C-terminal region of Xp54 can act to retain the protein in the cytoplasm of full-grown oocytes and culture cells. Cytoplasmic retention of Xp54 is overcome by activation of transcription. That Xp54 interacts directly with nascent transcripts is shown by immunostaining of the RNP matrix of lampbrush chromosome loops and co-immunoprecipitation with de novo-synthesized RNA. However, we are unable to show that nuclear export of this RNA is affected by either treatment with leptomycin B or mutation of the NES. We propose that newly synthesized Xp54 is regulated in its nucleocytoplasmic distribution: in transcriptionally quiescent oocytes it is largely restricted to the cytoplasm and, if imported into the nucleus, it is rapidly exported again by the CRM1 pathway. In transcriptionally active oocytes, it binds to a major set of nascent transcripts, accompanies mRNA sequences to the cytoplasm by an alternative export pathway and remains associated with masked mRNA until the time of translation activation at meiotic maturation and early embryonic cell division.

Nopp 140 involvement in nucleologenesis of mouse preimplantation embryos

As it was shown earlier, resumption of rRNA transcription in early mouse embryo is localized in the peripheral region of nucleolus precursor body/NPB/during the two-cell stage. Recently, nucleolar phosphoprotein Nopp140 was presented to shuttle between the nucleolus and cytoplasm as chaperone of snoRNPs. Nopp140 interacts with RNA polymerase I in nucleolus and also accumulates in CBs, suggesting a pathway between the two organelles. The aim of the study was to describe the changing location of Nopp140 during the first cleavage stages of mouse embryos and its re-location after inhibition of rRNA synthesis with actinomycin D. Light microscope immunocytochemical staining showed Nopp140 in the periphery of NPBs before activation of rDNA transcription and in addition confirmed its localization in CBs. Immunolabelling with antibodies against RNA Pol I and UBF gave co-localization of these proteins, implicating that Nopp140 may actively participate to rDNA transcription. We suggest that fundamental differences in molecular organization of rDNA synthesis and postranscriptional processes between cycling somatic and pre-implantation embryonic cells may be in selective transport of transcription and/or processing-complexes of proteins to the nucleolar organizer regions (NOR). Mol. Reprod. Dev. 59:277-284, 2001.

Functional consequences of Rett syndrome mutations on human MeCP2

The neurodevelopmental disorder known as Rett syndrome has recently been linked to the methyl-CpG-binding transcriptional repressor, MeCP2. In this report we examine the consequences of these mutations on the function of MeCP2. The ability to bind specifically to methylated DNA and the transcription repression capabilities are tested, as well as the stability of proteins in vivo. We find that all missense mutations (R106W, R133C, F155S, T158M) within the methyl-binding domain impair selectivity for methylated DNA, and that all nonsense mutations (L138X, R168X, E235X, R255X, R270X, V288X, R294X) that truncate all or some of the transcriptional repression domain (TRD) affect the ability to repress transcription and have decreased levels of stability in vivo. Two missense mutations, one in the TRD (R306C) and one in the C-terminus (E397K), had no noticeable effects on MeCP2 function. Together, these results provide evidence of how Rett syndrome mutations can affect distinct functions of MeCP2 and give insight into these mutations that may contribute to the disease.

Coilin, more than a molecular marker of the cajal (coiled) body

The Cajal (coiled) body is a discrete nuclear organelle that was first described in mammalian neurons in 1903. Because the molecular composition, structure, and function of Cajal bodies were unknown, these enigmatic structures were largely ignored for most of the last century. The Cajal body has now regained the interest of biologists, due to the isolation of a protein marker, coilin. Despite current widespread use of coilin to identify Cajal bodies in various cell types, its structure and function are still little understood. Here, I would like to discuss what we have learned about coilin and suggest a possible role for coilin in RNA processing and cellular trafficking, especially in relation to Cajal bodies and nucleoli. Although coilin has been investigated primarily in somatic cells, I will emphasize the advantages of using the amphibian oocyte to study nuclear proteins and organelles.

Review: dynamic stability of the interphase nucleus in health and disease

Ongoing export of newly synthesized RNAs, as well as control of transcriptional activity, involves dynamic nucleocytoplasmic transport of proteins. Some proteins that shuttle reside primarily in the nucleus while others are concentrated in the cytoplasm. Moreover, some proteins shuttle continuously, while others shuttle only once. A third group is stimulated to relocate either into or out of the nucleus as a result of interruption of shuttling. In addition to these protein-specific events, several physiological stimuli have global effects on nucleocytoplasmic transport. In related events, selected proteins move between distinct sites in the nucleoplasm, others enter and leave the nucleolus, and still others transit between the nuclear envelope and cytoplasmic membranes. These multiple dynamic distributions provide numerous opportunities for precise communication between spatially distant sites in the cell.