Nucleolin functions in nucleolus formation and chromosome congression

Aptamer-Loop DNA Nanoflower Recognition and Multicolor Fluorescent Carbon Quantum Dots Labeling System for Multitarget Living Cell Imaging

Visualization of multiple targets in living cells is important for understanding complex biological processes, but it still faces difficulties, such as complex operation, difficulty in multiplexing, and expensive equipment. Here, we developed a nanoplatform integrating a nucleic acid aptamer and DNA nanotechnology for living cell imaging. Aptamer-based recognition probes (RPs) were synthesized through rolling circle amplification, which were further self-assembled into DNA nanoflowers encapsulated by an aptamer loop. The signal probes (SPs) were obtained by conjugation of multicolor emission carbon quantum dots with oligonucleotides complementary to RPs. Through base pairing, RPs and SPs were hybridized to generate aptamer sgc8-, AS1411-, and Apt-based imaging systems. They were used for individual/simultaneous imaging of cellular membrane protein PTK7, nucleolin, and adenosine triphosphate (ATP) molecules. Fluorescence imaging and intensity analysis showed that the living cell imaging system can not only specifically recognize and efficiently bind their respective targets but also provide a 5-10-fold signal amplification. Cell-cycle-dependent distribution of nucleolin and concentration-dependent fluorescence intensity of ATP demonstrated the utility of the system for tracking changes in cellular status. Overall, this system shows the potential to be a simple, low-cost, highly selective, and sensitive living cell imaging platform.

U3 snoRNA inter-regulates with DDX21 in the perichromosomal region to control mitosis

U3 snoRNA is essential for ribosome biogenesis during interphase. Upon mitotic onset, the nucleolus disassembles and U3 snoRNA relocates to the perichromosomal region (PR) to be considered as a chromosome passenger. Whether U3 controls mitosis remains unknown. Here, we demonstrate that U3 snoRNA is required for mitotic progression. We identified DDX21 as the predominant U3-binding protein during mitosis and confirmed that U3 snoRNA colocalizes with DDX21 in the PR. DDX21 knockdown induces mitotic catastrophe and similar mitotic defects caused by U3 snoRNA depletion. Interestingly, the uniform PR distribution of U3 snoRNA and DDX21 is interdependent. DDX21 functions in mitosis depending on its PR localization. Mechanistically, U3 snoRNA regulates DDX21 PR localization through maintaining its mobility. Moreover, Cy5-U3 snoRNA downsizes the fibrous condensates of His-DDX21 at proper molecular ratios in vitro. This work highlights the importance of the equilibrium between U3 snoRNA and DDX21 in PR formation and reveals the potential relationship between the PR assembly and mitotic regulation.

The Wnt-dependent master regulator NKX1-2 controls mouse pre-implantation development

Embryo size, specification, and homeostasis are regulated by a complex gene regulatory and signaling network. Here we used gene expression signatures of Wnt-activated mouse embryonic stem cell (mESC) clones to reverse engineer an mESC regulatory network. We identify NKX1-2 as a novel master regulator of preimplantation embryo development. We find that Nkx1-2 inhibition reduces nascent RNA synthesis, downregulates genes controlling ribosome biogenesis, RNA translation, and transport, and induces severe alteration of nucleolus structure, resulting in the exclusion of RNA polymerase I from nucleoli. In turn, NKX1-2 loss of function leads to chromosome missegregation in the 2- to 4-cell embryo stages, severe decrease in blastomere numbers, alterations of tight junctions (TJs), and impairment of microlumen coarsening. Overall, these changes impair the blastocoel expansion-collapse cycle and embryo cavitation, leading to altered lineage specification and developmental arrest.

Macromolecular condensation organizes nucleolar sub-phases to set up a pH gradient

Nucleoli are multicomponent condensates defined by coexisting sub-phases. We identified distinct intrinsically disordered regions (IDRs), including acidic (D/E) tracts and K-blocks interspersed by E-rich regions, as defining features of nucleolar proteins. We show that the localization preferences of nucleolar proteins are determined by their IDRs and the types of RNA or DNA binding domains they encompass. In vitro reconstitutions and studies in cells showed how condensation, which combines binding and complex coacervation of nucleolar components, contributes to nucleolar organization. D/E tracts of nucleolar proteins contribute to lowering the pH of co-condensates formed with nucleolar RNAs in vitro. In cells, this sets up a pH gradient between nucleoli and the nucleoplasm. By contrast, juxta-nucleolar bodies, which have different macromolecular compositions, featuring protein IDRs with very different charge profiles, have pH values that are equivalent to or higher than the nucleoplasm. Our findings show that distinct compositional specificities generate distinct physicochemical properties for condensates.

The chromatin-associated lncREST ensures effective replication stress response by promoting the assembly of fork signaling factors

Besides the well-characterized protein network involved in the replication stress response, several regulatory RNAs have been shown to play a role in this critical process. However, it has remained elusive whether they act locally at the stressed forks. Here, by investigating the RNAs localizing on chromatin upon replication stress induced by hydroxyurea, we identified a set of lncRNAs upregulated in S-phase and controlled by stress transcription factors. Among them, we demonstrate that the previously uncharacterized lncRNA lncREST (long non-coding RNA REplication STress) is transcriptionally controlled by p53 and localizes at stressed replication forks. LncREST-depleted cells experience sustained replication fork progression and accumulate un-signaled DNA damage. Under replication stress, lncREST interacts with the protein NCL and assists in engaging its interaction with RPA. The loss of lncREST is associated with a reduced NCL-RPA interaction and decreased RPA on chromatin, leading to defective replication stress signaling and accumulation of mitotic defects, resulting in apoptosis and a reduction in tumorigenic potential of cancer cells. These findings uncover the function of a lncRNA in favoring the recruitment of replication proteins to sites of DNA replication.

Loss of function of ribosomal protein L13a blocks blastocyst formation and reveals a potential nuclear role in gene expression

Ribosomal proteins play diverse roles in development and disease. Most ribosomal proteins have canonical roles in protein synthesis, while some exhibit extra-ribosomal functions. Previous studies in our laboratory revealed that ribosomal protein L13a (RPL13a) is involved in the translational silencing of a cohort of inflammatory proteins in myeloid cells. This prompted us to investigate the role of RPL13a in embryonic development. Here we report that RPL13a is required for early development in mice. Crosses between Rpl13a+/- mice resulted in no Rpl13a-/- offspring. Closer examination revealed that Rpl13a-/- embryos were arrested at the morula stage during preimplantation development. RNA sequencing analysis of Rpl13a-/- morulae revealed widespread alterations in gene expression, including but not limited to several genes encoding proteins involved in the inflammatory response, embryogenesis, oocyte maturation, stemness, and pluripotency. Ex vivo analysis revealed that RPL13a was localized to the cytoplasm and nucleus between the two-cell and morula stages. RNAi-mediated depletion of RPL13a phenocopied Rpl13a-/- embryos and knockdown embryos exhibited increased expression of IL-7 and IL-17 and decreased expression of the lineage specifier genes Sox2, Pou5f1, and Cdx2. Lastly, a protein-protein interaction assay revealed that RPL13a is associated with chromatin, suggesting an extra ribosomal function in transcription. In summary, our data demonstrate that RPL13a is essential for the completion of preimplantation embryo development. The mechanistic basis of the absence of RPL13a-mediated embryonic lethality will be addressed in the future through follow-up studies on ribosome biogenesis, global protein synthesis, and identification of RPL13a target genes using chromatin immunoprecipitation and RNA-immunoprecipitation-based sequencing.

Nucleolin is required for multiple centrosome-associated functions in early vertebrate mitosis

Nucleolin is a multifunctional RNA-binding protein that resides predominantly not only in the nucleolus, but also in multiple other subcellular pools in the cytoplasm in mammalian cells, and is best known for its roles in ribosome biogenesis, RNA stability, and translation. During early mitosis, nucleolin is required for equatorial mitotic chromosome alignment prior to metaphase. Using high resolution fluorescence imaging, we reveal that nucleolin is required for multiple centrosome-associated functions at the G2-prophase boundary. Nucleolin depletion led to dissociation of the centrosomes from the G2 nuclear envelope, a delay in the onset of nuclear envelope breakdown, reduced inter-centrosome separation, and longer metaphase spindles. Our results reveal novel roles for nucleolin in early mammalian mitosis, establishing multiple important functions for nucleolin during mammalian cell division.

Increased recombinant adeno-associated virus production by HEK293 cells using small molecule chemical additives

Recombinant adeno-associated virus (rAAV) has established itself as a highly efficacious gene delivery vector with a well characterised safety profile allowing broad clinical application. Recent successes in rAAV-mediated gene therapy clinical trials will continue to drive demand for improved rAAV production processes to reduce costs. Here, we demonstrate that small molecule bioactive chemical additives can significantly increase recombinant AAV vector production by human embryonic kidney (HEK) cells up to three-fold. Nocodazole (an anti-mitotic agent) and M344 (a selective histone deacetylase inhibitor) were identified as positive regulators of rAAV8 genome titre in a microplate screening assay. Addition of nocodazole to triple-transfected HEK293 suspension cells producing rAAV arrested cells in G2/M phase, increased average cell volume and reduced viable cell density relative to untreated rAAV producing cells at harvest. Final crude genome vector titre from nocodazole treated cultures was >2-fold higher compared to non-treated cultures. Further investigation showed nocodazole addition to cultures to be time critical. Genome titre improvement was found to be scalable and serotype independent across two distinct rAAV serotypes, rAAV8 and rAAV9. Furthermore, a combination of M344 and nocodazole produced a positive additive effect on rAAV8 genome titre, resulting in a three-fold increase in genome titre compared to untreated cells.

CCDC86 is a novel Ki-67-interacting protein important for cell division

The chromosome periphery is a network of proteins and RNAs that coats the outer surface of mitotic chromosomes. Despite the identification of new components, the functions of this complex compartment are poorly characterised. In this study, we identified a novel chromosome periphery-associated protein, CCDC86 (also known as cyclon). Using a combination of RNA interference, microscopy and biochemistry, we studied the functions of CCDC86 in mitosis. CCDC86 depletion resulted in partial disorganisation of the chromosome periphery with alterations in the localisation of Ki-67 (also known as MKI67) and nucleolin (NCL), and the formation of abnormal cytoplasmic aggregates. Furthermore, CCDC86-depleted cells displayed errors in chromosome alignment, altered spindle length and increased apoptosis. These results suggest that, within the chromosome periphery, different subcomplexes that include CCDC86, nucleolin and B23 (nucleophosmin or NPM1) are required for mitotic spindle regulation and correct kinetochore-microtubule attachments, thus contributing to chromosome segregation in mitosis. Moreover, we identified CCDC86 as a MYCN-regulated gene, the expression levels of which represent a powerful marker for prognostic outcomes in neuroblastoma.

The role of the nucleolus in regulating the cell cycle and the DNA damage response

The nucleolus has long been perceived as the site for ribosome biogenesis, but numerous studies suggest that the nucleolus carefully sequesters crucial proteins involved in multiple cellular functions. Among these, the role of nucleolus in cell cycle regulation is the most evident. The nucleolus is the first responder of growth-related signals to mediate normal cell cycle progression. The nucleolus also senses different cellular stress insults by activating diverse pathways that arrest the cell cycle, promote DNA repair, or initiate apoptosis. Here, we review the emerging concepts on how the ribosomal and nonribosomal nucleolar proteins mediate such cellular effects.

Insights on the Role of PGRMC1 in Mitotic and Meiotic Cell Division

During mitosis, chromosome missegregation and cytokinesis defects have been recognized as hallmarks of cancer cells. Cytoskeletal elements composing the spindle and the contractile ring and their associated proteins play crucial roles in the faithful progression of mitotic cell division. The hypothesis that PGRMC1, most likely as a part of a yet-to-be-defined complex, is involved in the regulation of spindle function and, more broadly, the cytoskeletal machinery driving cell division is particularly appealing. Nevertheless, more than ten years after the preliminary observation that PGRMC1 changes its localization dynamically during meiotic and mitotic cell division, this field of research has remained a niche and needs to be fully explored. To encourage research in this fascinating field, in this review, we will recap the current knowledge on PGRMC1 function during mitotic and meiotic cell division, critically highlighting the strengths and limitations of the experimental approaches used so far. We will focus on known interacting partners as well as new putative associated proteins that have recently arisen in the literature and that might support current as well as new hypotheses of a role for PGRMC1 in specific spindle subcompartments, such as the centrosome, kinetochores, and the midzone/midbody.

Micro-Topographies Induce Epigenetic Reprogramming and Quiescence in Human Mesenchymal Stem Cells

Biomaterials can control cell and nuclear morphology. Since the shape of the nucleus influences chromatin architecture, gene expression and cell identity, surface topography can control cell phenotype. This study provides fundamental insights into how surface topography influences nuclear morphology, histone modifications, and expression of histone-associated proteins through advanced histone mass spectrometry and microarray analysis. The authors find that nuclear confinement is associated with a loss of histone acetylation and nucleoli abundance, while pathway analysis reveals a substantial reduction in gene expression associated with chromosome organization. In light of previous observations where the authors found a decrease in proliferation and metabolism induced by micro-topographies, they connect these findings with a quiescent phenotype in mesenchymal stem cells, as further shown by a reduction of ribosomal proteins and the maintenance of multipotency on micro-topographies after long-term culture conditions. Also, this influence of micro-topographies on nuclear morphology and proliferation is reversible, as shown by a return of proliferation when re-cultured on a flat surface. The findings provide novel insights into how biophysical signaling influences the epigenetic landscape and subsequent cellular phenotype.

Heterozygous De Novo Truncating Mutation of Nucleolin in an ASD Individual Disrupts Its Nucleolar Localization

Nucleolin (NCL/C23; OMIM: 164035) is a major nucleolar protein that plays a critical role in multiple processes, including ribosome assembly and maturation, chromatin decondensation, and pre-rRNA transcription. Due to its diverse functions, nucleolin has frequently been implicated in pathological processes, including cancer and viral infection. We recently identified a de novo frameshifting indel mutation of NCL, p.Gly664Glufs*70, through whole-exome sequencing of autism spectrum disorder trios. Through the transfection of constructs encoding either a wild-type human nucleolin or a mutant nucleolin with the same C-terminal sequence predicted for the autism proband, and by using co-localization with the nucleophosmin (NPM; B23) protein, we have shown that the nucleolin mutation leads to mislocalization of the NCL protein from the nucleolus to the nucleoplasm. Moreover, a construct with a nonsense mutation at the same residue, p.Gly664*, shows a very similar effect on the location of the NCL protein, thus confirming the presence of a predicted nucleolar location signal in this region of the NCL protein. Real-time fluorescence recovery experiments show significant changes in the kinetics and mobility of mutant NCL protein in the nucleoplasm of HEK293Tcells. Several other studies also report de novoNCL mutations in ASD or neurodevelopmental disorders. The altered mislocalization and dynamics of mutant NCL (p.G664Glufs*70/p.G664*) may have relevance to the etiopathlogy of NCL-related ASD and other neurodevelopmental phenotypes.

Chromosome clustering in mitosis by the nuclear protein Ki-67

Ki-67 is highly expressed in proliferating cells, a characteristic that made the protein a very important proliferation marker widely used in the clinic. However, the molecular functions and properties of Ki-67 remained quite obscure for a long time. Only recently important discoveries have shed some light on its function and shown that Ki-67 has a major role in the formation of mitotic chromosome periphery compartment, it is associated with protein phosphatase one (PP1) and regulates chromatin function in interphase and mitosis. In this review, we discuss the role of Ki-67 during cell division. Specifically, we focus on the importance of Ki-67 in chromosome individualisation at mitotic entry (prometaphase) and its contribution to chromosome clustering and nuclear remodelling during mitotic exit.

Relocation of phosphofructokinases within epithelial cells is a novel event preceding breast cancer recurrence that accurately predicts patient outcomes

Although recurrent cancers are often aggressive, little is known about the intracellular events required for cancer recurrences. Due to this lack of mechanistic information, there is no test to predict cancer recurrences or non-recurrences during early stages of disease. In this retrospective study, we use ductal carcinoma in situ (DCIS) of the breast as a framework to better understand the mechanism of cancer recurrences using patient outcomes as the physiological observable. Conventional pathology slides were labeled with anti-phosphofructokinase type L (PFKL) and anti-phosphofructokinase/fructose-2,6-bisphosphatase type 4 (PFKFB4) reagents. PFKL and PFKFB4 were found in ductal epithelial cell nucleoli from DCIS samples of women who did not experience a cancer recurrence. In contrast, PFKL and PFKFB4 may be found near the plasma membrane in samples from patients who will develop recurrent cancer. Using machine learning to predict patient outcomes, holdout studies of individual patient micrographs for the three biomarkers PFKL, PFKFB4, and phosphorylated GLUT1 demonstrated 38.6% true negatives, 49.5% true positives, 11.9% false positives and 0% false negatives (N=101). A sub-population of recurrent samples demonstrated PFKL, PFKFB4, and phosphorylated glucose transporter 1 accumulation at the apical surface of epithelial cells, suggesting that carbohydrates can be harvested from the ducts' luminal spaces as an energy source. We suggest that PFK isotype patterns are metabolic switches representing key mechanistic steps of recurrences. Furthermore, PFK enzyme patterns within epithelial cells contribute to an accurate diagnostic test to classify DCIS patients as high or low recurrence risk.

Expression of transduced nucleolin promotes the clearance of accumulated α-synuclein in rodent cells and animal model

Alpha-synuclein (αSyn) is a major component of Lewy bodies, which are a known pathogenic marker of Parkinson's disease (PD). The dysfunction of protein degradation machinery causes αSyn accumulation. The reinforcement of αSyn degradation is a potential therapeutic target for PD because accumulated αSyn is responsible for the pathogenesis of PD. Nucleolin (NCL) is essential in the formation of the nucleolar structure. The function of NCL is correlated with oxidative stress-mediated cell death. A previous study demonstrated that NCL overexpression alleviated rotenone-induced neurotoxic effects, whereas knockdown of NCL had the opposite effect. These results suggest that NCL malfunction would exacerbate PD pathology. Thus, it was hypothesized that the introduction of ectopic NCL could rescue α-synucleinopathy in PD. This study investigated whether the ectopic expression of NCL facilitates αSyn clearance. Ectopic expression of NCL was accomplished via the transfection of green fluorescent protein (GFP) or GFP-NCL in mouse embryonic fibroblasts (MEF) or transduction of GFP or GFP-NCL using lentivirus in rat primary cortical neurons and mouse substantia nigra. NCL overexpression enhanced the clearance of accumulated or aggregated αSyn in MEFs and rat primary cortical neurons. The activity of the autophagy-lysosome pathway was enhanced by NCL expression. NCL transduction in the substantia nigra, which was co-injected with αSyn fibrils, rescued PD manifestation. The elevation of NCL levels may reflect a therapeutic strategy for α-synucleinopathy in PD.

Cell surface nucleolin as active bait for nanomedicine in cancer therapy: a promising option

Conventional chemotherapy used against cancer is mostly limited due to their non-targeted nature, affecting normal tissue and causing undesirable toxic effects to the affected tissue. With the aim of improving these treatments both therapeutically and in terms of their safety, numerous studies are currently being carried out using nanoparticles (NPs) as a vector combining tumor targeting and carrying therapeutic tools. In this context, it appears that nucleolin, a molecule over-expressed on the surface of tumor cells, is an interesting therapeutic target. Several ligands, antagonists of nucleolin of various origins, such as AS1411, the F3 peptide and the multivalent pseudopeptide N6L have been developed and studied as therapeutic tools against cancer. Over the last ten years or so, numerous studies have been published demonstrating that these antagonists can be used as tumor targeting agents with NPs from various origins. Focusing on nucleolin ligands, the aim of this article is to review the literature recently published or under experimentation in our research team to evaluate the efficacy and future development of these tools as anti-tumor agents.

Cancer Stem Cells and Nucleolin as Drivers of Carcinogenesis

Cancer, one of the most mortal diseases worldwide, is characterized by the gain of specific features and cellular heterogeneity. Clonal evolution is an established theory to explain heterogeneity, but the discovery of cancer stem cells expanded the concept to include the hierarchical growth and plasticity of cancer cells. The activation of epithelial-to-mesenchymal transition and its molecular players are widely correlated with the presence of cancer stem cells in tumors. Moreover, the acquisition of certain oncological features may be partially attributed to alterations in the levels, location or function of nucleolin, a multifunctional protein involved in several cellular processes. This review aims at integrating the established hallmarks of cancer with the plasticity of cancer cells as an emerging hallmark; responsible for tumor heterogeneity; therapy resistance and relapse. The discussion will contextualize the involvement of nucleolin in the establishment of cancer hallmarks and its application as a marker protein for targeted anticancer therapies

Nucleolin regulates 14-3-3ζ mRNA and promotes cofilin phosphorylation to induce tunneling nanotube formation

Tunneling nanotubes (TNTs) mediate intercellular communication between animal cells in health and disease, but the mechanisms of their biogenesis and function are poorly understood. Here we report that the RNA-binding protein (RBP) nucleolin, which interacts with the known TNT-inducing protein MSec, is essential for TNT formation in mammalian cells. Nucleolin, through its RNA-binding domains (RBDs), binds to and maintains the cytosolic levels of 14-3-3ζ mRNA, and is, therefore, required for TNT formation. A specific region of the 3'-untranslated region (UTR) of the 14-3-3ζ mRNA is likely to be involved in its regulation by nucleolin. Functional complementation experiments suggest that nucleolin and 14-3-3ζ form a linear signaling axis that promotes the phosphorylation and inactivation of the F-actin depolymerization factor cofilin to induce TNT formation. MSec also similarly inactivates cofilin, but potentiates TNT formation independent of the nucleolin-14-3-3ζ axis, despite biochemically interacting with both proteins. We show that 14-3-3ζ and nucleolin are required for the formation of TNTs between primary mouse neurons and astrocytes and in multiple other mammalian cell types. We also report that the Caenorhabditis elegans orthologs of 14-3-3ζ and MSec regulate the size and architecture of the TNT-like cellular protrusions of the distal tip cell (DTC), the germline stem cell niche in the gonad. Our study demonstrates a novel and potentially conserved mRNA-guided mechanism of TNT formation through the maintenance of cellular 14-3-3ζ mRNA levels by the RBP nucleolin.

The Importance of Earth Reference Controls in Spaceflight -Omics Research: Characterization of Nucleolin Mutants from the Seedling Growth Experiments

Understanding plant adaptive responses to the space environment is a requisite for enabling space farming. Spaceflight produces deleterious effects on plant cells, particularly affecting ribosome biogenesis, a complex stress-sensitive process coordinated with cell division and differentiation, known to be activated by red light. Here, in a series of ground studies, we have used mutants from the two Arabidopsis nucleolin genes (NUC1 and NUC2, nucleolar regulators of ribosome biogenesis) to better understand their role in adaptive response mechanisms to stress on Earth. Thus, we show that nucleolin stress-related gene NUC2 can compensate for the environmental stress provided by darkness in nuc1 plants, whereas nuc2 plants are not able to provide a complete response to red light. These ground control findings, as part of the ESA/NASA Seedling Growth spaceflight experiments, will determine the basis for the identification of genetic backgrounds enabling an adaptive advantage for plants in future space experiments.

Nucleolin and nucleophosmin expression patterns in pulmonary adenocarcinoma invading the pleura and in pleural malignant mesothelioma

BACKGROUND

Visceral pleural invasion (VPI) in adenocarcinoma of the lung is considered a poor prognostic factor. The purpose of this study was to analyze nucleolin and nucleophosmin expression in pulmonary adenocarcinoma (PA) with VPI and in pleural malignant mesothelioma.

METHODS

The study was conducted on the basis of 19 pathologically-confirmed cases of adenocarcinoma of the lung and 29 cases of epithelioid malignant mesothelioma. The nucleolin and nucleophosmin expression was assessed immunohistochemically and analyzed with image analysis software.

RESULTS

Nucleolin expression was lower while nucleophosmin was higher in pleural invasion of pulmonary adenocarcinoma than in the central part of the tumor. Differences in subpopulations of cells with different expression of proteins studied were also found. Malignant mesothelioma showed lower nucleolin expression than adenocarcinoma of the lung but no differences in nucleophosmin expression were found.

CONCLUSIONS

The results of our study suggested that lower nucleolin and higher nucleophosmin expression may be related to higher invasiveness of adenocarcinoma of the lung. Differences in nucleolin expression between pulmonary adenocarcinoma and malignant mesothelioma indicate another aspect of biology of these pleura-invading cancers that requires further study.

KEY POINTS

SIGNIFICANT FINDINGS OF THE STUDY: Differences in nucleolin and nucleophosmin expression in pleura invading pulmonary adenocarcinoma indicate the involvement of these proteins in its locoregional spread while differences in nucleolin expression between pulmonary adenocarcinoma and malignant mesothelioma suggest another aspect of biology of these cancers.

WHAT THIS STUDY ADDS

This is the first study on nucleolin and nucleophosmin expression in pleural malignant mesothelioma and pleura-invading pulmonary adenocarcinoma. Our findings may assist in understanding the mechanisms of locoregional spread of adenocarcinoma and differences between these two pleura-invading cancers.

Prevention of dsRNA-induced interferon signaling by AGO1x is linked to breast cancer cell proliferation

Translational readthrough, i.e., elongation of polypeptide chains beyond the stop codon, was initially reported for viral RNA, but later found also on eukaryotic transcripts, resulting in proteome diversification and protein‐level modulation. Here, we report that AGO1x, an evolutionarily conserved translational readthrough isoform of Argonaute 1, is generated in highly proliferative breast cancer cells, where it curbs accumulation of double‐stranded RNAs (dsRNAs) and consequent induction of interferon responses and apoptosis. In contrast to other mammalian Argonaute protein family members with primarily cytoplasmic functions, AGO1x exhibits nuclear localization in the vicinity of nucleoli. We identify AGO1x interaction with the polyribonucleotide nucleotidyltransferase 1 (PNPT1) and show that the depletion of this protein further augments dsRNA accumulation. Our study thus uncovers a novel function of an Argonaute protein in buffering the endogenous dsRNA‐induced interferon responses, different than the canonical function of AGO proteins in the miRNA effector pathway. As AGO1x expression is tightly linked to breast cancer cell proliferation, our study thus suggests a new direction for limiting tumor growth.

Trnp1 organizes diverse nuclear membrane-less compartments in neural stem cells

TMF1‐regulated nuclear protein 1 (Trnp1) has been shown to exert potent roles in neural development affecting neural stem cell self‐renewal and brain folding, but its molecular function in the nucleus is still unknown. Here, we show that Trnp1 is a low complexity protein with the capacity to phase separate. Trnp1 interacts with factors located in several nuclear membrane‐less organelles, the nucleolus, nuclear speckles, and condensed chromatin. Importantly, Trnp1 co‐regulates the architecture and function of these nuclear compartments in vitro and in the developing brain in vivo. Deletion of a highly conserved region in the N‐terminal intrinsic disordered region abolishes the capacity of Trnp1 to regulate nucleoli and heterochromatin size, proliferation, and M‐phase length; decreases the capacity to phase separate; and abrogates most of Trnp1 protein interactions. Thus, we identified Trnp1 as a novel regulator of several nuclear membrane‐less compartments, a function important to maintain cells in a self‐renewing proliferative state.

NOL12 Repression Induces Nucleolar Stress-Driven Cellular Senescence and Is Associated with Normative Aging

The nucleolus is a subnuclear compartment with key roles in rRNA synthesis and ribosome biogenesis, complex processes that require hundreds of proteins and factors. Alterations in nucleolar morphology and protein content have been linked to the control of cell proliferation and stress responses and, recently, further implicated in cell senescence and ageing. In this study, we report the functional role of NOL12 in the nucleolar homeostasis of human primary fibroblasts. NOL12 repression induces specific changes in nucleolar morphology, with increased nucleolar area but reduced nucleolar number, along with nucleolar accumulation and increased levels of fibrillarin and nucleolin. Moreover, NOL12 repression leads to stabilization and activation of p53 in an RPL11-dependent manner, which arrests cells at G₂ phase and ultimately leads to senescence. Importantly, we found NOL12 repression in association with nucleolar stress-like responses in human fibroblasts from elderly donors, disclosing it as a biomarker in human chronological aging.

Downregulation of NOP53 Ribosome Biogenesis Factor Leads to Abnormal Nuclear Division and Chromosomal Instability in Human Cervical Cancer Cells

NOP53 ribosome biogenesis factor (NOP53) is a nucleolar protein involved in oncogenesis/tumor suppression, cell cycle regulation, and cell death. Here, we investigated the role of NOP53 in the maintenance of normal nuclear shape and chromosomal stability. Depletion of NOP53 by shRNA caused abnormal nuclear morphology, including large nucleus, irregular nucleus, and multinucleated cells, and chromosomal instability resulting in micronucleus or nuclear bud formation. The abnormal nuclear shape and chromosomal instability were restored by re-expression of NOP53. We further showed that NOP53 was involved in chromosome congression in metaphase. Downregulation of NOP53 induced aberrant chromosome congression and spindle checkpoint activation, resulting in delayed mitosis and mitotic arrest. Thus, our findings demonstrated that the nucleolar protein NOP53 participated in mitotic progression and that dysregulated NOP53 expression caused chromosomal instability in cancer cells.

AML-associated mutation of nucleophosmin compromises its interaction with nucleolin

C-terminal mutations of the nucleolar protein nucleophosmin (NPM) are the most frequent genetic aberration detected in acute myeloid leukemia (AML) with normal karyotype. The mutations cause aberrant cytoplasmic localization of NPM and lead to loss of functions associated with NPM nucleolar localization, e.g. in ribosome biogenesis or DNA-damage repair. NPM has many interaction partners and some of them were proved to interact also with the mutated form (NPMmut) and due to this interaction thereby to be withdrawn from their site of action. We analyzed the impact of the mutation on NPM interaction with nucleolin (NCL) which is also prevalently localized into the nucleolus and cooperates with wild-type NPM (NPMwt) in many cellular processes. We revealed that the NCL-NPM complex formation is completely abolished by the mutation and that the presence/absence of the interaction is not affected by drugs causing genotoxic stress or differentiation. Deregulation resulting from changes of NCL/NPMwt ratio may contribute to leukemogenesis.

Nucleolin modulates compartmentalization and dynamics of histone 2B-ECFP in the nucleolus

Eukaryotic cells have 2 to ​3 discrete nucleoli required for ribosome synthesis. Nucleoli are phase separated nuclear sub-organelles. Here we examined the role of nuclear Lamins and nucleolar factors in modulating the compartmentalization and dynamics of histone 2B (H2B-ECFP) in the nucleolus. Live imaging and Fluorescence Recovery After Photobleaching (FRAP) of labelled H2B, showed that the depletion of Lamin B1, Fibrillarin (FBL) or Nucleostemin (GNL3), enhances H2B-ECFP mobility in the nucleolus. Furthermore, Nucleolin knockdown significantly decreases H2B-ECFP compartmentalization in the nucleolus, while H2B-ECFP residence and mobility in the nucleolus was prolonged upon Nucleolin overexpression. Co-expression of N-terminal and RNA binding domain (RBD) deletion mutants of Nucleolin or inhibiting 45S rRNA synthesis reduces the sequestration of H2B-ECFP in the nucleolus. Taken together, these studies reveal a crucial role of Nucleolin-rRNA complex in modulating the compartmentalization, stability and dynamics of H2B within the nucleolus.

Nucleolar integrity during interphase supports faithful Cdk1 activation and mitotic entry

The nucleolus is a dynamic nuclear body that has been demonstrated to disassemble at the onset of mitosis; the relationship between cell cycle progression and nucleolar integrity, however, remains poorly understood. We studied the role of nucleolar proteins in mitosis by performing a global analysis using small interfering RNAs specific to nucleolar proteins; we focused on nucleolar protein 11 (NOL11), with currently unknown mitotic functions. Depletion of NOL11 delayed entry into the mitotic phase owing to increased inhibitory phosphorylation of cyclin-dependent kinase 1 (Cdk1) and aberrant accumulation of Wee1, a kinase that phosphorylates and inhibits Cdk1. In addition to effects on overall mitotic phenotypes, NOL11 depletion reduced ribosomal RNA (rRNA) levels and caused nucleolar disruption during interphase. Notably, mitotic phenotypes found in NOL11-depleted cells were recapitulated when nucleolar disruption was induced by depletion of rRNA transcription factors or treatment with actinomycin D. Furthermore, delayed entry into the mitotic phase, caused by the depletion of pre-rRNA transcription factors, was attributable to nucleolar disruption rather than to G₂/M checkpoint activation or reduced protein synthesis. Our findings therefore suggest that maintenance of nucleolar integrity during interphase is essential for proper cell cycle progression to mitosis via the regulation of Wee1 and Cdk1.

Visualization of human karyopherin beta-1/importin beta-1 interactions with protein partners in mitotic cells by co-immunoprecipitation and proximity ligation assays

Karyopherin beta-1/Importin beta-1 is a conserved nuclear transport receptor, acting in protein nuclear import in interphase and as a global regulator of mitosis. These pleiotropic functions reflect its ability to interact with, and regulate, different pathways during the cell cycle, operating as a major effector of the GTPase RAN. Importin beta-1 is overexpressed in cancers characterized by high genetic instability, an observation that highlights the importance of identifying its partners in mitosis. Here we present the first comprehensive profile of importin beta-1 interactors from human mitotic cells. By combining co-immunoprecipitation and proteome-wide mass spectrometry analysis of synchronized cell extracts, we identified expected (e.g., RAN and SUMO pathway factors) and novel mitotic interactors of importin beta-1, many with RNA-binding ability, that had not been previously associated with importin beta-1. These data complement interactomic studies of interphase transport pathways. We further developed automated proximity ligation assay (PLA) protocols to validate selected interactors. We succeeded in obtaining spatial and temporal resolution of genuine importin beta-1 interactions, which were visualized and localized in situ in intact mitotic cells. Further developments of PLA protocols will be helpful to dissect importin beta-1-orchestrated pathways during mitosis.

Lamin B2 Modulates Nucleolar Morphology, Dynamics, and Function

The nucleolus is required for ribosome biogenesis. Human cells have 2 or 3 nucleoli associated with nucleolar organizer region (NOR)-bearing chromosomes. An increase in number and altered nucleolar morphology define cancer cells. However, the mechanisms that modulate nucleolar morphology and function are unclear. Here we show that in addition to localizing at the nuclear envelope, lamin B2 localizes proximal to nucleolin at the granular component (GC) of the nucleolus and associates with the nucleolar proteins nucleolin and nucleophosmin. Lamin B2 knockdown severely disrupted the nucleolar morphology, which was rescued to intact and discrete nucleoli upon lamin B2 overexpression. Furthermore, two mutually exclusive lamin B2 deletion mutants, ΔHead and ΔSLS, rescued nuclear and nucleolar morphology defects, respectively, induced upon lamin B2 depletion, suggesting independent roles for lamin B2 at the nucleolus and nuclear envelope. Lamin B2 depletion increased nucleolin aggregation in the nucleoplasm, implicating lamin B2 in stabilizing nucleolin within the nucleolus. Lamin B2 knockdown upregulated nucleolus-specific 45S rRNA and upstream intergenic sequence (IGS) transcripts. The IGS transcripts colocalized with aggregates of nucleolin speckles, which were sustained in the nucleoplasm upon lamin B2 depletion. Taken together, these studies uncover a novel role for lamin B2 in modulating the morphology, dynamics, and function of the nucleolus.

Multifaceted Nucleolin Protein and Its Molecular Partners in Oncogenesis

Discovered in 1973, nucleolin is one of the most abundant phosphoproteins of the nucleolus. The ability of nucleolin to be involved in many cellular processes is probably related to its structural organization and its capability to form many different interactions with other proteins. Many functions of nucleolin affect cellular processes involved in oncogenesis-for instance: in ribosome biogenesis; in DNA repair, remodeling, and genome stability; in cell division and cell survival; in chemokine and growth factor signaling pathways; in angiogenesis and lymphangiogenesis; in epithelial-mesenchymal transition; and in stemness. In this review, we will describe the different functions of nucleolin in oncogenesis through its interaction with other proteins.

Ki-67 and the Chromosome Periphery Compartment in Mitosis

The chromosome periphery is a complex network of proteins and RNA molecules (many derived from nucleoli) that covers the outer surface of chromosomes and whose function remains mysterious. Although it was first described over 130 years ago, technological advances and the recent discovery that Ki-67 acts as an organiser of this region have allowed the chromosome periphery to be dissected in previously unattainable detail, leading to a revival of interest in this obscure chromosomal compartment. Here, we review the most recent advances into the composition, structure and function of the chromosome periphery, discuss possible roles of Ki-67 during mitosis and consider why this structure is likely to remain the focus of ongoing attention in the future.

The roles of RRP15 in nucleolar formation, ribosome biogenesis and checkpoint control in human cells

The nucleolus controls ribosome biogenesis and its perturbation induces nucleolar stress that inhibits cell cycle progression and activates checkpoint responses. Here, we investigate the roles of ribosomal RNA processing protein, RRP15, in nucleolar formation, ribosome biogenesis, cell cycle progression and checkpoint control in human cells. RRP15 is localized in the nucleolus and required for nucleolar formation. In contrast to the budding yeast Rrp15p that was reported as a component of pre-60S subunits, RRP15 is found in both pre-40S and pre-60S subunits and involved in regulating rRNA transcription and ribosome biogenesis. Perturbation of RRP15 induces nucleolar stress that activates RPL5/RPL11/5S rRNA (RP)-Mdm2-p53 axis checkpoint response and arrests cells at G1-G1/S in p53-proficient non-transformed RPE1 cells but not in p53-deficient HeLa and MCF7 tumor cells. Instead, p53-deficient HeLa and MCF7 cells with RRP15-dependent nucleolar stress enter S-phase with S-phase perturbation that activates ATR-Chk1- γH2AX axis DNA replication/damage checkpoint response, delaying S-G2/M progression and, ultimately, causing cell death. The selective checkpoint response, cell cycle inhibition and/or cytotoxicity induced by RRP15-dependent nucleolar stress in p53-proficient non-transformed cells and p53-deficient tumor cells suggest that RRP15 might be a potential target for cancer therapy.

Regulation of nucleolus assembly by non-coding RNA polymerase II transcripts

The nucleolus is a nuclear subcompartment for tightly regulated rRNA production and ribosome subunit biogenesis. It also acts as a cellular stress sensor and can release enriched factors in response to cellular stimuli. Accordingly, the content and structure of the nucleolus change dynamically, which is particularly evident during cell cycle progression: the nucleolus completely disassembles during mitosis and reassembles in interphase. Although the mechanisms that drive nucleolar (re)organization have been the subject of a number of studies, they are only partly understood. Recently, we identified Alu element-containing RNA polymerase II transcripts (aluRNAs) as important for nucleolar structure and rRNA synthesis. Integrating these findings with studies on the liquid droplet-like nature of the nucleolus leads us to propose a model on how RNA polymerase II transcripts could regulate the assembly of the nucleolus in response to external stimuli and during cell cycle progression.

Mechanisms of Chromosome Congression during Mitosis

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called "direct congression" pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call "peripheral congression", is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called "tubulin code" might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

The p150N domain of chromatin assembly factor-1 regulates Ki-67 accumulation on the mitotic perichromosomal layer

Chromatin assembly factor 1 (CAF-1) deposits histones during DNA synthesis. The p150 subunit of human CAF-1 contains an N-terminal domain (p150N) that is dispensable for histone deposition but promotes the localization of specific loci (nucleolar-associated domains [NADs]) and proteins to the nucleolus during interphase. One of the p150N-regulated proteins is proliferation antigen Ki-67, whose depletion also decreases the nucleolar association of NADs. Ki-67 is also a fundamental component of the perichromosomal layer (PCL), a sheath of proteins surrounding condensed chromosomes during mitosis. We show here that a subset of p150 localizes to the PCL during mitosis and that p150N is required for normal levels of Ki-67 accumulation on the PCL. This activity requires the sumoylation-interacting motif within p150N, which is also required for the nucleolar localization of NADs and Ki-67 during interphase. In this manner, p150N coordinates both interphase and mitotic nuclear structures via Ki67.

Expression of Nucleolin Affects Microtubule Dynamics

Nucleolin is present in diverse cellular compartments and is involved in a variety of cellular processes from nucleolar structure and function to intracellular trafficking, cell adhesion and migration. Recently, nucleolin has been localized at the mature centriole where it is involved in microtubule nucleation and anchoring. Although this new function of nucleolin linked to microtubule regulation has been identified, the global effects of nucleolin on microtubule dynamics have not been addressed yet. In the present study, we analyzed the roles of nucleolin protein levels on global microtubule dynamics by tracking the EB3 microtubule plus end binding protein in live cells. We have found that during microtubule growth phases, nucleolin affects both the speed and life time of polymerization and by analyzing catastrophe events, we showed that nucleolin reduces catastrophe frequency. This new property of nucleolin was then confirmed in a cold induced microtubule depolymerization experiment in which we have found that cold resistant microtubules were totally destabilized in nucleolin depleted cells. Altogether, our data demonstrate a new function of nucleolin on microtubule stabilization, thus bringing novel insights into understanding the multifunctional properties of nucleolin in healthy and cancer cells.

Association Between Human Hair Loss and the Expression Levels of Nucleolin, Nucleophosmin, and UBTF Genes

AIMS

Nucleolar organizer regions, also known as argyrophilic nucleolar organizer regions, are associated with ribosomal genes. The main function of the nucleolus is the rapid production of ribosomal subunits, a process that must be highly regulated to provide the appropriate levels for cellular proliferation and cell growth. There are no studies in the literature addressing the expression and function of nucleolar component proteins, including nucleophosmin, nucleolin and the upstream binding transcription factor (UBTF), in human follicular hair cells.

METHODS

Nineteen healthy males who had normal and sufficient hair follicles on the back of the head, but exhibited hair loss on the frontal/vertex portions of the head and 14 healthy males without hair loss were included in the current study. Gene expression levels were measured by relative quantitative real time polymerase chain reaction.

RESULTS

In the individuals suffering from alopecia, the total expression levels of nucleolin, nucleophosmin, and UBTF were lower in normal sites than in hair loss sites. Strong expression level correlations were detected between: nucleophosmin and nucleolin; nucleophosmin and UBTF, and nucleolin and UBTF for both groups.

CONCLUSIONS

There was an association between human hair loss and the expression levels of nucleolin, nucleophosmin, and UBTF genes.

Centromere Protein (CENP)-W Interacts with Heterogeneous Nuclear Ribonucleoprotein (hnRNP) U and May Contribute to Kinetochore-Microtubule Attachment in Mitotic Cells

Background

Recent studies have shown that heterogeneous nuclear ribonucleoprotein U (hnRNP U), a component of the hnRNP complex, contributes to stabilize the kinetochore-microtubule interaction during mitosis. CENP-W was identified as an inner centromere component that plays crucial roles in the formation of a functional kinetochore complex.

Results

We report that hnRNP U interacts with CENP-W, and the interaction between hnRNP U and CENP-W mutually increased each other’s protein stability by inhibiting the proteasome-mediated degradation. Further, their co-localization was observed chiefly in the nuclear matrix region and at the microtubule-kinetochore interface during interphase and mitosis, respectively. Both microtubule-stabilizing and microtubule-destabilizing agents significantly decreased the protein stability of CENP-W. Furthermore, loss of microtubules and defects in microtubule organization were observed in CENP-W-depleted cells.

Conclusion

Our data imply that CENP-W plays an important role in the attachment and interaction between microtubules and kinetochore during mitosis.

Identification of a novel RNA giant nuclear body in cancer cells

Constitutive synthesis of oncogenic mRNAs is essential for maintaining the uncontrolled growth of cancer cells. However, little is known about how these mRNAs are exported from the nucleus to the cytoplasm. Here, we report the identification of a RNA giant nuclear body (RNA-GNB) that is abundant in cancer cells but rare in normal cells. The RNA-GNB contains a RNA core surrounded by a protein shell. We identify 782 proteins from cancer-associated RNA-GNBs, 40% of which are involved in the nuclear mRNA trafficking. RNA-GNB is required for cell proliferation, and its abundance is positively associated with tumor burden and outcome of therapies. Our findings suggest that the RNA-GNB is a novel nuclear RNA trafficking organelle that may contribute to the nuclear mRNA exporting and proliferation of cancer cells.

Sharing the mitotic pre-ribosomal particles between daughter cells

Ribosome biogenesis is a fundamental multistep process initiated by the synthesis of 90S pre-ribosomal particles in the nucleoli of higher eukaryotes. Even though synthesis of ribosomes stops during mitosis while nucleoli disappear, mitotic pre-ribosomal particles persist as observed in prenucleolar bodies (PNBs) during telophase. To further understand the relationship between the nucleolus and the PNBs, the presence and the fate of the mitotic pre-ribosomal particles during cell division was investigated. We demonstrate that the recently synthesized 45S precursor ribosomal RNAs (pre-rRNAs) but also the 32S and 30S pre-rRNAs are maintained during mitosis and associated with the chromosome periphery together with pre-rRNA processing factors. Maturation of the mitotic pre-ribosomal particles, as assessed by the stability of the mitotic pre-rRNAs, is transiently arrested during mitosis by a cyclin-dependent kinase (CDK)1-cyclin B-dependent mechanism and may be restored by CDK inhibitor treatments. At the M/G1 transition, the resumption of mitotic pre-rRNA processing in PNBs does not induce the disappearance of PNBs that only occurs when functional nucleoli reform. Strikingly, during their maturation process, mitotic pre-rRNAs localize in reforming nucleoli.

The relationship between the nucleolus and cancer: Current evidence and emerging paradigms

The nucleolus is the most prominent nuclear substructure assigned to produce ribosomes; molecular machines that are responsible for carrying out protein synthesis. To meet the increased demand for proteins during cell growth and proliferation the cell must increase protein synthetic capacity by upregulating ribosome biogenesis. While larger nucleolar size and number have been recognized as hallmark features of many tumor types, recent evidence has suggested that, in addition to overproduction of ribosomes, decreased ribosome biogenesis as well as qualitative changes in this process could also contribute to tumor initiation and cancer progression. Furthermore, the nucleolus has become the focus of intense attention for its involvement in processes that are clearly unrelated to ribosome biogenesis such as sensing and responding to endogenous and exogenous stressors, maintenance of genome stability, regulation of cell-cycle progression, cellular senescence, telomere function, chromatin structure, establishment of nuclear architecture, global regulation of gene expression and biogenesis of multiple ribonucleoprotein particles. The fact that dysregulation of many of these fundamental cellular processes may contribute to the malignant phenotype suggests that normal functioning of the nucleolus safeguards against the development of cancer and indicates its potential as a therapeutic approach. Here we review the recent advances made toward understanding these newly-recognized nucleolar functions and their roles in normal and cancer cells, and discuss possible future research directions.

Nucleolus-tethering system (NoTS)

Multiple biological processes are regulated by complicated interaction networks formed by protein-protein or protein-RNA interactions. Nuclear bodies (NBs) are a class of membrane-less subnuclear structures, acting as reaction sites, storage and modification sites, or transcription regulating sites involved in signaling transduction. Biochemical and fluorescence-based methods are widely used to study protein-protein interactions, but false-positive results are a major issue, especially for some fluorescence-based methods. Moreover, these methods fail to be applied to study the formation of NBs, which were characterized by a popular bacterial Lac operator and/or repressor (LacO/LacI) system in mammalian cells. Methods investigating assembly of plant NBs are not available. We have recently developed a nucleolar marker protein nucleolin2 (Nuc2)-based method named Nucleolus-tethering System (NoTS) and showed its application in interaction assay among nucleoplasmic proteins and initiation of plant specific NBs, photobodies. In this extraview, we will compare NoTS with the traditional methods and discuss the assembly mechanisms of NBs, in addition to advantages, limitations, and perspectives about the application of NoTS.

HAUSP-nucleolin interaction is regulated by p53-Mdm2 complex in response to DNA damage response

HAUSP (herpes virus-associated ubiquitin specific protease, known as ubiquitin specific protease 7), one of DUBs, regulates the dynamics of the p53 and Mdm2 network in response to DNA damage by deubiquitinating both p53 and its E3 ubiquitin ligase, Mdm2. Its concerted action increases the level of functional p53 by preventing proteasome-dependent degradation of p53. However, the protein substrates that are targeted by HAUSP to mediate DNA damage responses in the context of the HAUSP-p53-Mdm2 complex are not fully identified. Here, we identified nucleolin as a new substrate for HAUSP by proteomic analysis. Nucleolin has two HAUSP binding sites in its N- and C-terminal regions, and the mutation of HAUSP interacting peptides on nucleolin disrupts their interaction and it leads to the increased level of nucleolin ubiquitination. In addition, HAUSP regulates the stability of nucleolin by removing ubiquitin from nucleolin. Nucleolin exists as a component of the HAUSP-p53-Mdm2 complex, and both Mdm2 and p53 are required for the interaction between HAUSP and nucleolin. Importantly, the irradiation increases the HAUSP-nucleolin interaction, leading to nucleolin stabilization significantly. Taken together, this study reveals a new component of the HAUSP-p53-Mdm2 complex that governs dynamic cellular responses to DNA damage.

Nucleolar Translocation of Histone Deacetylase 2 Is Involved in Regulation of Transcriptional Silencing in the Cat Germinal Vesicle

Histone deacetylase 2 (HDAC2) is a key transcriptional coregulator that is suspected to play a role during oogenesis. It is known that RNA transcription in the cat germinal vesicle (GV) stops during folliculogenesis at the late antral follicle stage and is unrelated to histone deacetylation or chromatin condensation. The objective of the present study was to determine if and how HDAC2 participates in transcription regulation in the cat GV. Spatiotemporal HDAC2 protein expression was examined by immunostaining oocytes from primary to large antral follicles. HDAC2 was detected in the majority of GVs within oocytes from early, small, and large antral follicles. At early and small antral stages, HDAC2 was found primarily in the GV's nucleoplasm. There then was a significant shift in HDAC2 localization into the nucleolus, mostly in oocytes from large antral follicles. Assessments revealed that transcription was active in oocytes that contained nucleoplasm-localized HDAC2, whereas nucleolar-bound HDAC2 was associated with loss of both global transcription and ribosomal RNA presence at all antral stages. When oocytes were exposed to the HDAC inhibitor valproic acid, results indicated that HDAC regulated transcriptional activity in the nucleoplasm, but not in the nucleolus. Collective results suggest that nucleolar translocation of HDAC2 is associated with transcriptional silencing in the GV, thereby likely contributing to an oocyte's acquisition of competence.

IGFBP-5 Promotes Fibrosis Independently of Its Translocation to the Nucleus and Its Interaction with Nucleolin and IGF

BACKGROUND

Insulin-like growth factor binding protein (IGFBP)-5 levels are increased in systemic sclerosis (SSc) skin and lung. We previously reported that IGFBP-5 is a pro-fibrotic factor that induces extracellular matrix (ECM) production and deposition. Since IGFBP-5 contains a nuclear localization signal (NLS) that facilitates its nuclear translocation, we sought to examine the role of nuclear translocation on the fibrotic activity of IGFBP-5 and identify IGFBP-5 binding partners relevant for its nuclear compartmentalization.

METHODS

We generated functional wild type IGFBP-5 and IGFBP-5 with a mutated NLS or a mutated IGF binding site. Abrogation of nuclear translocation in the NLS mutant was confirmed using immunofluorescence and immunoblotting of nuclear and cytoplasmic cellular extracts. Abrogation of IGF binding was confirmed using western ligand blot. The fibrotic activity of wild type and mutant IGFBP-5 was examined in vitro in primary human fibroblasts and ex vivo in human skin. We identified IGFBP-5 binding partners using immunoprecipitation and mass spectrometry. We examined the effect of nucleolin on IGFBP-5 localization and function via sequence-specific silencing in primary human fibroblasts.

RESULTS

Our results show that IGFBP-5-induced ECM production in vitro in primary human fibroblasts is independent of its nuclear translocation. The NLS-mutant also induced fibrosis ex vivo in human skin, thus confirming and extending the in vitro findings. Similar findings were obtained with the IGF-binding mutant. Nucleolin, a nucleolar protein that can serve as a nuclear receptor, was identified as an IGFBP-5 binding partner. Silencing nucleolin reduced IGFBP-5 translocation to the nucleus but did not block the ability of IGFBP-5 to induce ECM production and a fibrotic phenotype.

CONCLUSIONS

IGFBP-5 transport to the nucleus requires an intact NLS and nucleolin. However, nuclear translocation is not necessary for IGFBP-5 fibrotic activity; neither is IGF binding. Our data provide further insights into the role of cellular compartmentalization in IGFBP-5-induced fibrosis.

The roles of nucleolin subcellular localization in cancer

Nucleolin (NCL) is one of the most abundant non ribosomal protein of the nucleolus where it plays a central role in polymerase I transcription. NCL is also found outside of the nucleolus, in the nucleoplasm, cytoplasm as well as on the cell membrane. It acts in all cell compartments to control cellular homeostasis and therefore each cellular pool of NCL can play a different role in cancer development. NCL overexpression and its increased localization at the cell membrane is a common feature of several tumor cells. In cancer cells, NCL overexpression influences cell survival, proliferation and invasion through its action on different cellular pathways. In this review, we describe how the multiple functions of NCL that are associated to its multiple cellular localization can participate to the development of cancer.

Phosphorylation of targeting protein for Xenopus kinesin-like protein 2 (TPX2) at threonine 72 in spindle assembly

The human ortholog of the targeting protein for Xenopus kinesin-like protein 2 (TPX2) is a cytoskeletal protein that plays a major role in spindle assembly and is required for mitosis. During spindle morphogenesis, TPX2 cooperates with Aurora A kinase and Eg5 kinesin to regulate microtubule organization. TPX2 displays over 40 putative phosphorylation sites identified from various high-throughput proteomic screenings. In this study, we characterize the phosphorylation of threonine 72 (Thr(72)) in human TPX2, a residue highly conserved across species. We find that Cdk1/2 phosphorylate TPX2 in vitro and in vivo. Using homemade antibodies specific for TPX2 phosphorylated at Thr(72), we show that this phosphorylation is cell cycle-dependent and peaks at M phase. Endogenous TPX2 phosphorylated at Thr(72) does not associate with the mitotic spindle. Furthermore, ectopic GFP-TPX2 T72A preferentially concentrates on the spindle, whereas GFP-TPX2 WT distributes to both spindle and cytosol. The T72A mutant also increases the proportion of cells with multipolar spindles phenotype. This effect is associated with increased Aurora A activity and abnormally elongated spindles, indicative of higher Eg5 activity. In summary, we propose that phosphorylation of Thr(72) regulates TPX2 localization and impacts spindle assembly via Aurora A and Eg5.

Nucleolin: dual roles in rDNA chromatin transcription

Nucleolin is a major nucleolar protein conserved in all eukaryotic organisms. It is a multifunctional protein involved in different cellular aspects like chromatin organization and stability, DNA and RNA metabolism, assembly of ribonucleoprotein complexes, cytokinesis, cell proliferation and stress response. The multifunctionality of nucleolin is linked to its tripartite structure, post-translational modifications and its ability of shuttling from and to the nucleolus/nucleoplasm and cytoplasm. Nucleolin has been now studied for many years and its activities and properties have been described in a number of excellent reviews. Here, we overview the role of nucleolin in RNA polymerase I (RNAPI) transcription and describe recent results concerning its functional interaction with rDNA chromatin organization. For a long time, nucleolin has been associated with rRNA gene expression and pre-rRNA processing. However, the functional connection between nucleolin and active versus inactive rRNA genes is still not fully understood. Novel evidence indicates that the nucleolin protein might be required for controlling the transcriptional ON/OFF states of rDNA chromatin in both mammals and plants.