The homologous putative GTPases Grn1p from fission yeast and the human GNL3L are required for growth and play a role in processing of nucleolar pre-rRNA

GNL3L exhibits pro-tumor activities via NF-κB pathway as a poor prognostic factor in acute myeloid leukemia

Acute myeloid leukemia (AML) is the leukemia with the worst prognosis, and current knowledge of AML pathogenesis and available therapies for AML remain limited. 40% of AML patients exhibit elevated nuclear factor kappa B (NF-κB) activity, which provides a compelling rationale for targeting the NF-κB pathway in AML. Guanine nucleotide-binding protein-like 3-like protein (GNL3L) is a recently identified pro-oncogene that promotes NF-κB activation in a variety of malignancies. For the first time, we comprehensively examined GNL3L expression in AML, reporting GNL3L as a poor prognostic factor in three independent AML cohorts. GNL3L enhanced RELA activity, activated NF-κB, promoted AML cell proliferation, resisted apoptosis, and encouraged cytarabine resistance in AML. In conclusion, these data suggest a role for GNL3L in the malignant process of AML and as a promising therapeutic target.

A connection between the ribosome and two S. pombe tRNA modification mutants subject to rapid tRNA decay

tRNA modifications are crucial in all organisms to ensure tRNA folding and stability, and accurate translation. In both the yeast Saccharomyces cerevisiae and the evolutionarily distant yeast Schizosaccharomyces pombe, mutants lacking certain tRNA body modifications (outside the anticodon loop) are temperature sensitive due to rapid tRNA decay (RTD) of a subset of hypomodified tRNAs. Here we show that for each of two S. pombe mutants subject to RTD, mutations in ribosomal protein genes suppress the temperature sensitivity without altering tRNA levels. Prior work showed that S. pombe trm8Δ mutants, lacking 7-methylguanosine, were temperature sensitive due to RTD, and that one class of suppressors had mutations in the general amino acid control (GAAC) pathway, which was activated concomitant with RTD, resulting in further tRNA loss. We now find that another class of S. pombe trm8Δ suppressors have mutations in rpl genes, encoding 60S subunit proteins, and that suppression occurs with minimal restoration of tRNA levels and reduced GAAC activation. Furthermore, trm8Δ suppression extends to other mutations in the large or small ribosomal subunit. We also find that S. pombe tan1Δ mutants, lacking 4-acetylcytidine, are temperature sensitive due to RTD, that one class of suppressors have rpl mutations, associated with minimal restoration of tRNA levels, and that suppression extends to other rpl and rps mutations. However, although S. pombe tan1Δ temperature sensitivity is associated with some GAAC activation, suppression by an rpl mutation only modestly inhibits GAAC activation. We propose a model in which ribosomal protein mutations result in reduced ribosome concentrations, leading to both reduced ribosome collisions and a reduced requirement for tRNA, with these effects having different relative importance in trm8Δ and tan1Δ mutants. This model is consistent with our results in S. cerevisiae trm8Δ trm4Δ mutants, known to undergo RTD, fueling speculation that this model applies across eukaryotes.

A connection between the ribosome and two S. pombe tRNA modification mutants subject to rapid tRNA decay

tRNA modifications are crucial in all organisms to ensure tRNA folding and stability, and accurate translation in the ribosome. In both the yeast Saccharomyces cerevisiae and the evolutionarily distant yeast Schizosaccharomyces pombe, mutants lacking certain tRNA body modifications (outside the anticodon loop) are temperature sensitive due to rapid tRNA decay (RTD) of a subset of hypomodified tRNAs. Here we show that for each of two S. pombe mutants subject to RTD, mutations in ribosomal protein genes suppress the temperature sensitivity without altering tRNA levels. Prior work showed that S. pombe trm8Δ mutants, lacking 7-methylguanosine, were temperature sensitive due to RTD and that one class of suppressors had mutations in the general amino acid control (GAAC) pathway, which was activated concomitant with RTD, resulting in further tRNA loss. We now find that another class of S. pombe trm8Δ suppressors have mutations in rpl genes, encoding 60S subunit proteins, and that suppression occurs with minimal restoration of tRNA levels and reduced GAAC activation. Furthermore, trm8Δ suppression extends to other mutations in the large or small ribosomal subunit. We also find that S. pombe tan1Δ mutants, lacking 4-acetylcytidine, are temperature sensitive due to RTD, that one class of suppressors have rpl mutations, associated with minimal restoration of tRNA levels, and that suppression extends to other rpl and rps mutations. However, although S. pombe tan1Δ temperature sensitivity is associated with some GAAC activation, suppression by an rpl mutation does not significantly inhibit GAAC activation. These results suggest that ribosomal protein mutations suppress the temperature sensitivity of S. pombe trm8Δ and tan1Δ mutants due to reduced ribosome concentrations, leading to both a reduced requirement for tRNA, and reduced ribosome collisions and GAAC activation. Results with S. cerevisiae trm8Δ trm4Δ mutants are consistent with this model, and fuel speculation that similar results will apply across eukaryotes.

GNL3 is an evolutionarily conserved stem cell gene influencing cell proliferation, animal growth and regeneration in the hydrozoan Hydractinia

Nucleostemin (NS) is a vertebrate gene preferentially expressed in stem and cancer cells, which acts to regulate cell cycle progression, genome stability and ribosome biogenesis. NS and its paralogous gene, GNL3-like (GNL3L), arose in the vertebrate clade after a duplication event from their orthologous gene, G protein Nucleolar 3 (GNL3). Research on invertebrate GNL3, however, has been limited. To gain a greater understanding of the evolution and functions of the GNL3 gene, we have performed studies in the hydrozoan cnidarian Hydractinia symbiolongicarpus, a colonial hydroid that continuously generates pluripotent stem cells throughout its life cycle and presents impressive regenerative abilities. We show that Hydractinia GNL3 is expressed in stem and germline cells. The knockdown of GNL3 reduces the number of mitotic and S-phase cells in Hydractinia larvae of different ages. Genome editing of Hydractinia GNL3 via CRISPR/Cas9 resulted in colonies with reduced growth rates, polyps with impaired regeneration capabilities, gonadal morphological defects, and low sperm motility. Collectively, our study shows that GNL3 is an evolutionarily conserved stem cell and germline gene involved in cell proliferation, animal growth, regeneration and sexual reproduction in Hydractinia, and sheds new light into the evolution of GNL3 and of stem cell systems.

Antagonistic fungal enterotoxins intersect at multiple levels with host innate immune defences

Animals and plants need to defend themselves from pathogen attack. Their defences drive innovation in virulence mechanisms, leading to never-ending cycles of co-evolution in both hosts and pathogens. A full understanding of host immunity therefore requires examination of pathogen virulence strategies. Here, we take advantage of the well-studied innate immune system of Caenorhabditis elegans to dissect the action of two virulence factors from its natural fungal pathogen Drechmeria coniospora. We show that these two enterotoxins have strikingly different effects when expressed individually in the nematode epidermis. One is able to interfere with diverse aspects of host cell biology, altering vesicle trafficking and preventing the key STAT-like transcription factor STA-2 from activating defensive antimicrobial peptide gene expression. The second increases STA-2 levels in the nucleus, modifies the nucleolus, and, potentially as a consequence of a host surveillance mechanism, causes increased defence gene expression. Our results highlight the remarkably complex and potentially antagonistic mechanisms that come into play in the interaction between co-evolved hosts and pathogens.

High expression of guanine nucleotide-binding protein-like-3-like is associated with poor prognosis in esophageal cancer

Guanine nucleotide-binding protein-like-3-like (GNL3L) is required for processing ribosomal pre-rRNA and cell proliferation and is upregulated in many types of cancer. This study is aimed to investigate the clinical significance of GNL3L in esophageal cancer. The mRNA and protein expression levels of GNL3L were determined by using quantitative real-time polymerase chain reaction and immunohistochemistry, respectively. GNL3L was localized in both cytoplasm and nucleus. The expression levels of GNL3L in esophageal cancer tissues were significantly higher than those in adjacent nonmalignant tissues. High GNL3L expression was associated with pathologic type and poor differentiation. Patients with high GNL3L expression had shorter overall survival (OS) than those with low GNL3L expression. Multivariate Cox regression analysis revealed that GNL3L expression was an independently predictive factor for the OS of patient with esophageal cancer. The Gene Expression Profiling Interactive Analysis (GEPIA) databases also showed that GNL3L was upregulated in esophageal cancer, which was closely associated with an unfavorable prognosis of patients with esophageal cancer. Taken together, our findings suggest that GNL3L is upregulated in esophageal cancer, which is linked to the progression of the disease. As a result, GNL3L could be used as a biomarker for esophageal cancer.

Therapeutic strategies for targeting telomerase in cancer

Telomere and telomerase play important roles in abnormal cell proliferation, metastasis, stem cell maintenance, and immortalization in various cancers. Therefore, designing of drugs targeting telomerase and telomere is of great significance. Over the past two decades, considerable knowledge regarding telomere and telomerase has been accumulated, which provides theoretical support for the design of therapeutic strategies such as telomere elongation. Therefore, the development of telomere-based therapies such as nucleoside analogs, non-nucleoside small molecules, antisense technology, ribozymes, and dominant negative human telomerase reverse transcriptase are being prioritized for eradicating a majority of tumors. While the benefits of telomere-based therapies are obvious, there is a need to address the limitations of various therapeutic strategies to improve the possibility of clinical applications. In this study, current knowledge of telomere and telomerase is discussed, and therapeutic strategies based on recent research are reviewed.

Candidate susceptibility variants in angioimmunoblastic T-cell lymphoma

Angioimmunoblastic T-cell lymphoma (AITL) is a subtype of peripheral T-cell lymphoma with a poor prognosis: the 5-year survival rate is approximately 30%. Somatic driver mutations have been found in TET2, IDH2, DNMT3A, RHOA, FYN, PLCG1, and CD28, whereas germline susceptibility to AITL has to our knowledge not been studied. The homogenous Finnish population is well suited for studies on genetic predisposition. Here, we performed an exome-wide rare variant analysis in 23 AITL patients. No germline mutations were found in the driver genes, implying that they are not frequently involved in genetic AITL predisposition. Potentially pathogenic variants present in at least two patients and showing significant (p < 0.01) enrichment in our sample set were found in ten genes: POLK, PRKCB, ZNF676, PRRC2B, PCDHGB6, GNL3L, TTC36, OTOG, OSGEPL1, and RASSF9. The most significantly enriched variants, causing p.Lys469Ter in a splice variant of POLK and p.Pro588His in PRKCB, are intriguing candidates as Polk deficient mice display a spontaneous mutator phenotype, whereas PRKCB was recently shown to be somatically mutated in 33% of another peripheral T-cell lymphoma, adult T-cell lymphoma. If validated, our findings would provide new insight into the pathogenesis of AITL, as well as tools for early detection in susceptible individuals.

Interplay between human nucleolar GNL1 and RPS20 is critical to modulate cell proliferation

Human Guanine nucleotide binding protein like 1 (GNL1) belongs to HSR1_MMR1 subfamily of nucleolar GTPases. Here, we report for the first time that GNL1 promotes cell cycle and proliferation by inducing hyperphosphorylation of retinoblastoma protein. Using yeast two-hybrid screening, Ribosomal protein S20 (RPS20) was identified as a functional interacting partner of GNL1. Results from GST pull-down and co-immunoprecipitation assays confirmed that interaction between GNL1 and RPS20 was specific. Further, GNL1 induced cell proliferation was altered upon knockdown of RPS20 suggesting its critical role in GNL1 function. Interestingly, cell proliferation was significantly impaired upon expression of RPS20 interaction deficient GNL1 mutant suggest that GNL1 interaction with RPS20 is critical for cell growth. Finally, the inverse correlation of GNL1 and RPS20 expression in primary colon and gastric cancers with patient survival strengthen their critical importance during tumorigenesis. Collectively, our data provided evidence that cross-talk between GNL1 and RPS20 is critical to promote cell proliferation.

Leucine Zipper Down-regulated in Cancer-1 (LDOC1) interacts with Guanine nucleotide binding protein-like 3-like (GNL3L) to modulate Nuclear Factor-kappa B (NF-κB) signaling during cell proliferation

Guanine nucleotide binding protein-like 3-like (GNL3L) is an evolutionarily conserved putative nucleolar GTPase belonging to the HSR1-MMR1 family. In the present study, using protein-protein interaction assays, we show that Leucine Zipper Down-regulated in Cancer-1 (LDOC1) is a novel interacting partner of GNL3L. Furthermore, our results reveal that ectopic expression of LDOC1 destabilizes endogenous GNL3L levels and down modulates GNL3L-induced cell proliferation, in contrast, the knockdown of LDOC1 potentiates cell proliferation upon GNL3L expression. Interestingly, GNL3L upregulates NF-κB dependent transcriptional activity by modulating the expression of NF-κB subunit p65, which is reversed upon co-expression of LDOC1 with GNL3L. GNL3L also potentiates TNF-α mediated NF-κB activity. In addition, anti-apoptotic function of GNL3L is impaired upon p65 knockdown, suggesting its critical role in GNL3L mediated cell proliferation/survival. An inverse correlation of GNL3L and LDOC1 expression profiles in various tumor tissues from BioXpress database indicate their critical role in cancer. Collectively, our data provides evidence that GNL3L-LDOC1 interplay regulates cell proliferation through the modulation of NF-κB pathway during tumorigenesis.

New prospects for targeting telomerase beyond the telomere

Telomerase activity is responsible for the maintenance of chromosome end structures (telomeres) and cancer cell immortality in most human malignancies, making telomerase an attractive therapeutic target. The rationale for targeting components of the telomerase holoenzyme has been strengthened by accumulating evidence indicating that these molecules have extra-telomeric functions in tumour cell survival and proliferation. This Review discusses current knowledge of the biogenesis, structure and multiple functions of telomerase-associated molecules intertwined with recent advances in drug discovery approaches. We also describe the fertile ground available for the pursuit of next-generation small-molecule inhibitors of telomerase.

The nucleolar GTPase nucleostemin-like 1 plays a role in plant growth and senescence by modulating ribosome biogenesis

Nucleostemin is a nucleolar GTP-binding protein that is involved in stem cell proliferation, embryonic development, and ribosome biogenesis in mammals. Plant nucleostemin-like 1 (NSN1) plays a role in embryogenesis, and apical and floral meristem development. In this study, a nucleolar function of NSN1 in the regulation of ribosome biogenesis was identified. Green fluorescent protein (GFP)-fused NSN1 localized to the nucleolus, which was primarily determined by its N-terminal domain. Recombinant NSN1 and its N-terminal domain (NSN1-N) bound to RNA in vitro. Recombinant NSN1 expressed GTPase activity in vitro. NSN1 silencing in Arabidopsis thaliana and Nicotiana benthamiana led to growth retardation and premature senescence. NSN1 interacted with Pescadillo and EBNA1 binding protein 2 (EBP2), which are nucleolar proteins involved in ribosome biogenesis, and with several ribosomal proteins. NSN1, NSN1-N, and EBP2 co-fractionated primarily with the 60S ribosomal large subunit in vivo. Depletion of NSN1 delayed 25S rRNA maturation and biogenesis of the 60S ribosome subunit, and repressed global translation. NSN1-deficient plants exhibited premature leaf senescence, excessive accumulation of reactive oxygen species, and senescence-related gene expression. Taken together, these results suggest that NSN1 plays a crucial role in plant growth and senescence by modulating ribosome biogenesis.

GNL3L Is a Nucleo-Cytoplasmic Shuttling Protein: Role in Cell Cycle Regulation

GNL3L is an evolutionarily conserved high molecular weight GTP binding nucleolar protein belonging to HSR1-MMR1 subfamily of GTPases. The present investigation reveals that GNL3L is a nucleo-cytoplasmic shuttling protein and its export from the nucleus is sensitive to Leptomycin B. Deletion mutagenesis reveals that the C-terminal domain (amino acids 501–582) is necessary and sufficient for the export of GNL3L from the nucleus and the exchange of hydrophobic residues (M567, L570 and 572) within the C-terminal domain impairs this process. Results from the protein-protein interaction analysis indicate that GNL3L interaction with CRM1 is critical for its export from the nucleus. Ectopic expression of GNL3L leads to lesser accumulation of cells in the ‘G2/M’ phase of cell cycle whereas depletion of endogenous GNL3L results in ‘G2/M’ arrest. Interestingly, cell cycle analysis followed by BrdU labeling assay indicates that significantly increased DNA synthesis occurs in cells expressing nuclear export defective mutant (GNL3L^∆NES) compared to the wild type or nuclear import defective GNL3L. Furthermore, increased hyperphosphorylation of Rb at Serine 780 and the upregulation of E2F1, cyclins A2 and E1 upon ectopic expression of GNL3L^∆NES results in faster ‘S’ phase progression. Collectively, the present study provides evidence that GNL3L is exported from the nucleus in CRM1 dependent manner and the nuclear localization of GNL3L is important to promote ‘S’ phase progression during cell proliferation.

Nucleolar stress with and without p53

A veritable explosion of primary research papers within the past 10 years focuses on nucleolar and ribosomal stress, and for good reason: with ribosome biosynthesis consuming ~80% of a cell’s energy, nearly all metabolic and signaling pathways lead ultimately to or from the nucleolus. We begin by describing p53 activation upon nucleolar stress resulting in cell cycle arrest or apoptosis. The significance of this mechanism cannot be understated, as oncologists are now inducing nucleolar stress strategically in cancer cells as a potential anti-cancer therapy. We also summarize the human ribosomopathies, syndromes in which ribosome biogenesis or function are impaired leading to birth defects or bone narrow failures; the perplexing problem in the ribosomopathies is why only certain cells are affected despite the fact that the causative mutation is systemic. We then describe p53-independent nucleolar stress, first in yeast which lacks p53, and then in other model metazoans that lack MDM2, the critical E3 ubiquitin ligase that normally inactivates p53. Do these presumably ancient p53-independent nucleolar stress pathways remain latent in human cells? If they still exist, can we use them to target >50% of known human cancers that lack functional p53?

Nucleolar GTP-binding Protein-1 (NGP-1) Promotes G1 to S Phase Transition by Activating Cyclin-dependent Kinase Inhibitor p21 Cip1/Waf1

Nucleolar GTP-binding protein (NGP-1) is overexpressed in various cancers and proliferating cells, but the functional significance remains unknown. In this study, we show that NGP-1 promotes G1 to S phase transition of cells by enhancing CDK inhibitor p21(Cip-1/Waf1) expression through p53. In addition, our results suggest that activation of the cyclin D1-CDK4 complex by NGP-1 via maintaining the stoichiometry between cyclin D1-CDK4 complex and p21 resulted in hyperphosphorylation of retinoblastoma protein at serine 780 (p-RB(Ser-780)) followed by the up-regulation of E2F1 target genes required to promote G1 to S phase transition. Furthermore, our data suggest that ribosomal protein RPL23A interacts with NGP-1 and abolishes NGP-1-induced p53 activity by enhancing Mdm2-mediated p53 polyubiquitination. Finally, reduction of p-RB(Ser-780) levels and E2F1 target gene expression upon ectopic expression of RPL23a resulted in arrest at the G1 phase of the cell cycle. Collectively, this investigation provides evidence that NGP-1 promotes cell cycle progression through the activation of the p53/p21(Cip-1/Waf1) pathway.

Turning a new page on nucleostemin and self-renewal

A quintessential trait of stem cells is embedded in their ability to self-renew without incurring DNA damage as a result of genome replication. One key self-renewal factor is the nucleolar GTP-binding protein nucleostemin (also known as guanine-nucleotide-binding protein-like 3, GNL3, in invertebrate species). Several studies have recently pointed to an unexpected role of nucleostemin in safeguarding the genome integrity of stem and cancer cells. Since its discovery, the predominant presence of nucleostemin in the nucleolus has led to the notion that it might function in the card-carrying event of the nucleolus--the biogenesis of ribosomes. As tantalizing as this might be, a ribosomal role of nucleostemin is refuted by evidence from recent studies, which argues that nucleostemin depletion triggers a primary event of DNA damage in S phase cells that then leads to ribosomal perturbation. Furthermore, there have been conflicting reports regarding the p53 dependency of nucleostemin activity and the cell cycle arrest profile of nucleostemin-depleted cells. In this Commentary, I propose a model that explains how the many contradictory observations surrounding nucleostemin can be reconciled and suggest that this protein might not be as multi-tasking as has been previously perceived. The story of nucleostemin highlights the complexity of the underlying molecular events associated with the appearance of any cell biological phenotype and also signifies a new understanding of the genome maintenance program in stem cells.

Connecting the nucleolus to the cell cycle and human disease

Long known as the center of ribosome synthesis, the nucleolus is connected to cell cycle regulation in more subtle ways. One is a surveillance system that reacts promptly when rRNA synthesis or processing is impaired, halting cell cycle progression. Conversely, the nucleolus also acts as a first-responder to growth-related stress signals. Here we review emerging concepts on how these "infraribosomal" links between the nucleolus and cell cycle progression operate in both forward and reverse gears. We offer perspectives on how new cancer therapeutic designs that target this infraribosomal mode of cell growth control may shape future clinical progress.

Concomitant upregulation of nucleostemin and downregulation of Sox2 and Klf4 in gastric adenocarcinoma

Nucleostemin (NS) is a nucleolar protein involved in stem cell (SC) self-renewal by controlling cell cycle progression. In addition to SCs, NS is also expressed in some highly proliferating cells including several adult stem cells and cancer cell lines. NS knock-down in different cell lines demonstrated its cell type-dependent function in arresting cell cycle in either G1 or G2/M phases. Here, we have evaluated the expression of NS and iPS genes in 36 gastric cancer and their matched marginal nontumor tissues by means of real-time polymerase chain reaction (RT-PCR). We have also examined a potential causative role of NS in gastric tumorigenesis by suppressing its expression in a gastric cancer cell line, AGS. Our data revealed that NS expression level is much higher in tumor tissues (p = 0.046), especially in high-grade ones (p < 0.001), whereas the expression of Klf4 and Sox2 is downregulated in tumor tissues compared to marginal nontumor samples (p < 0.001). Furthermore, NS suppression in the AGS cell line caused some morphological alterations, a cell cycle arrest at G1 phase, and an upregulation of iPS genes: Nanog, Sox2, and Klf4. Based on our results, NS overexpression seems to have a causative role in gastric tumorigenesis and/or progression, and it could be considered as a potential tumor marker for diagnosis, molecular classification, and molecular therapy of gastric adenocarcinoma.

Nucleostemin and GNL3L exercise distinct functions in genome protection and ribosome synthesis, respectively

The mammalian nucleolar proteins nucleostemin and GNL3-like (GNL3L) are encoded by paralogous genes that arose from an ancestral invertebrate gene, GNL3. Invertebrate GNL3 has been implicated in ribosome biosynthesis, as has its mammalian descendent, GNL3L. The paralogous mammalian nucleostemin protein has, instead, been implicated in cell renewal. Here, we found that depletion of nucleostemin in a human breast carcinoma cell line triggers prompt and significant DNA damage in S-phase cells without perturbing the initial step of ribosomal (r)RNA synthesis and only mildly affects the total ribosome production. By contrast, GNL3L depletion markedly impairs ribosome production without inducing appreciable DNA damage. These results indicate that, during vertebrate evolution, GNL3L retained the role of the ancestral gene in ribosome biosynthesis, whereas the paralogous nucleostemin acquired a novel genome-protective function. Our results provide a coherent explanation for what had seemed to be contradictory findings about the functions of the invertebrate versus vertebrate genes and are suggestive of how the nucleolus was fine-tuned for a role in genome protection and cell-cycle control as the vertebrates evolved.

Evolutionary conservation and expression of human RNA-binding proteins and their role in human genetic disease

RNA-binding proteins (RBPs) are effectors and regulators of posttranscriptional gene regulation (PTGR). RBPs regulate stability, maturation, and turnover of all RNAs, often binding thousands of targets at many sites. The importance of RBPs is underscored by their dysregulation or mutations causing a variety of developmental and neurological diseases. This chapter globally discusses human RBPs and provides a brief introduction to their identification and RNA targets. We review RBPs based on common structural RNA-binding domains, study their evolutionary conservation and expression, and summarize disease associations of different RBP classes.

Regulation of ribosome biogenesis by nucleostemin 3 promotes local and systemic growth in Drosophila

Nucleostemin 3 (NS3) is an evolutionarily conserved protein with profound roles in cell growth and viability. Here we analyze cell-autonomous and non-cell-autonomous growth control roles of NS3 in Drosophila and demonstrate its GTPase activity using genetic and biochemical assays. Two null alleles of ns3, and RNAi, demonstrate the necessity of NS3 for cell autonomous growth. A hypomorphic allele highlights the hypersensitivity of neurons to lowered NS3 function. We propose that NS3 is the functional ortholog of yeast and human Lsg1, which promotes release of the nuclear export adapter from the large ribosomal subunit. Release of the adapter and its recycling to the nucleus are essential for sustained production of ribosomes. The ribosome biogenesis role of NS3 is essential for proper rates of translation in all tissues and is necessary for functions of growth-promoting neurons.

A nucleostemin-like GTPase required for normal apical and floral meristem development in Arabidopsis

Stem cell activities have to be terminated during flower development. A nucleostemin-like 1 gene, NSN1, is described in Arabidopsis. NSN1 is a midsize nucleolar GTPase required for proper termination of stem cell activities during flower development.

Mammalian nucleostemin (NS) is preferentially expressed in stem cells and acts to promote cell cycle progression. In plants, stem cell activities have to be terminated during flower development, and this process requires the activation of AGAMOUS (AG) gene expression. Here, a nucleostemin-like 1 gene, NSN1, is shown to be required for flower development in Arabidopsis. The NSN1 mRNA was found in the inflorescence meristem and floral primordia, and its protein was localized to the nucleoli. Both heterozygous and homozygous plants developed defective flowers on inflorescences that were eventually terminated by the formation of carpelloid flowers. Overexpression of NSN1 resulted in loss of apical dominance and formation of defective flowers. Expression of the AG gene was found to be up-regulated in nsn1. The carpelloid flower defect of nsn1 was suppressed by the ag mutation in the nsn1 ag double mutant, whereas double mutants of nsn1 apetala2 (ap2) displayed enhanced defective floral phenotypes. These results suggest that in the delicately balanced regulatory network, NSN1 acts to repress AG and plays an additive role with AP2 in floral organ specification. As a midsize nucleolar GTPase, NSN1 represents a new class of regulatory proteins required for flower development in Arabidopsis.

Signals and pathways regulating nucleolar retention of novel putative nucleolar GTPase NGP-1(GNL-2)

NGP-1(GNL-2) is a putative GTPase, overexpressed in breast carcinoma and localized in the nucleolus. NGP-1 belongs to the MMR1-HSR1 family of large GTPases that are emerging as crucial coordinators of signaling cascades in different cellular compartments. The members of this family share very closely related G-domains, but the signals and pathways regulating their subcellular localization and their functional relevance remain unknown. To improve our understanding of the nuclear transport mechanism of NGP-1, we have identified two nucleolar localization signals (NoLS) that are independently shown to translocate NGP-1 as well the heterologous protein to the nucleolus. Site-specific mutagenesis and immunofluorescence studies suggest that the tandem repeats of positively charged amino acids are critical for NGP-1 NoLS function. Interestingly, amino-terminal (NGP-1(1-100)) and carboxyl-terminal (NGP-1(661-731)) signals independently interact with receptors importin-β and importin-α, respectively. This investigation, for the first time, provides evidence that the interaction of importin-α with C-terminal NoLS (NGP-1(661-731)) was able to target the heterologous protein to the nucleolar compartment. Structural modeling analysis and alanine scanning mutagenesis of conserved G-domains suggest that G4 and G5 motifs are critical for GTP binding of NGP-1 and further show that the nucleolar localization of NGP-1 is regulated by a GTP gating-mediated mechanism. In addition, our data suggest that an ongoing transcription is essential for efficient localization of NGP-1 to the nucleolus. We have observed a high level of NGP-1 expression in the mitogen-activated primary human peripheral blood mononuclear cells (hPBMC) as well as in human fetal brain-derived neural precursor cells (hNPCs) in comparison to cells undergoing differentiation. Overall, the results suggest that multiple mechanisms are involved in the localization of NGP-1 to the nucleolus for the regulation of nucleolar function in cell growth and proliferation.

Nuclear/nucleolar GTPase 2 proteins as a subfamily of YlqF/YawG GTPases function in pre-60S ribosomal subunit maturation of mono- and dicotyledonous plants

The YlqF/YawG families are important GTPases involved in ribosome biogenesis, cell proliferation, or cell growth, however, no plant homologs have yet to be characterized. Here we isolated rice (Oryza sativa) and Arabidopsis nuclear/nucleolar GTPase 2 (OsNug2 and AtNug2, respectively) that belong to the YawG subfamily and characterized them for pre-60S ribosomal subunit maturation. They showed typical intrinsic YlqF/YawG family GTPase activities in bacteria and yeasts with k(cat) values 0.12 ± 0.007 min(-1) (n = 6) and 0.087 ± 0.002 min(-1) (n = 4), respectively, and addition of 60S ribosomal subunits stimulated their activities in vitro. In addition, OsNug2 rescued the lethality of the yeast nug2 null mutant through recovery of 25S pre-rRNA processing. By yeast two-hybrid screening five clones, including a putative one of 60S ribosomal proteins, OsL10a, were isolated. Subcellular localization and pulldown assays resulted in that the N-terminal region of OsNug2 is sufficient for nucleolar/nuclear targeting and association with OsL10a. OsNug2 is physically associated with pre-60S ribosomal complexes highly enriched in the 25S, 5.8S, and 5S rRNA, and its interaction was stimulated by exogenous GTP. Furthermore, the AtNug2 knockdown mutant constructed by the RNAi method showed defective growth on the medium containing cycloheximide. Expression pattern analysis revealed that the distribution of AtNug2 mainly in the meristematic region underlies its potential role in active plant growth. Finally, it is concluded that Nug2/Nog2p GTPase from mono- and didicotyledonous plants is linked to the pre-60S ribosome complex and actively processed 27S into 25S during the ribosomal large subunit maturation process, i.e. prior to export to the cytoplasm.

Nucleolar modulation of TRF1: a dynamic way to regulate telomere and cell cycle by nucleostemin and GNL3L

Chromosomal ends are protected by a high-order structure called telomere. Maintenance of correct telomere length and structure is critically important for the viability of both dividing and non-dividing cells. Notably, targeted deletion of a component of the multi-protein telomere-capping complex, TRF1 (telomeric repeat binding factor 1), causes lethality at embryonic day 5-6 without apparent telomere deficiency, raising the possibility that TRF1 may also moonlight outside the telomere. Further reinforcing the extra-telomeric tie of TRF1, two studies from our group have reported the findings that TRF1 can be bound and modulated by two nucleolar GTP-binding proteins, nucleostemin (NS) and guanine nucleotide binding protein-like 3-like (GNL3L), which exhibit apparently opposite effects on the protein degradation of TRF1. In particular, GNL3L is able to stabilize TRF1 protein during mitosis and promote the metaphase-to-anaphase transition. This manuscript extends the discussion on how this GNL3L-mediated TRF1 regulation creates a novel dynamic control on telomere and cell cycle, and extrapolates its evolutionary significance by contrasting the activities of NS and GNL3L.

Knockdown of the Drosophila GTPase nucleostemin 1 impairs large ribosomal subunit biogenesis, cell growth, and midgut precursor cell maintenance

Mammalian nucleostemin (NS) is a nucleolar guanosine triphosphate-binding protein implicated in cell cycle progression, stem cell proliferation, and ribosome assembly. Drosophila melanogaster contains a four-member nucleostemin family (NS1-4). NS1 is the closest orthologue to human NS; it shares 33% identity and 67% similarity with human NS. We show that NS1 has intrinsic GTPase and ATPase activity and that it is present within nucleoli of most larval and adult cells. Endogenous NS1 and lightly expressed green fluorescent protein (GFP)-NS1 enrich within the nucleolar granular regions as expected, whereas overexpressed GFP-NS1 localized throughout the nucleolus and nucleoplasm, and to several transcriptionally active interbands of polytene chromosomes. Severe overexpression correlated with the appearance of melanotic tumors and larval/pupal lethality. Depletion of 60% of NS1 transcripts also lead to larval and pupal lethality. NS1 protein depletion>95 correlated with the loss of imaginal island (precursor) cells in the larval midgut and to an apparent block in the nucleolar release of large ribosomal subunits in terminally differentiated larval midgut polyploid cells. Ultrastructural examination of larval Malpighian tubule cells depleted for NS1 showed a loss of cytoplasmic ribosomes and a concomitant appearance of cytoplasmic preautophagosomes and lysosomes. We interpret the appearance of these structures as indicators of cell stress response.

Nucleostemin: a latecomer with new tricks

Nucleostemin was first identified in neural stem cells and has become a focus of research in cell cycle control, tumorigenesis and cellular senescence. As the biology of nucleostemin begins to be unveiled in multiple species, an ensuing task is to resolve the apparent differences between the functions of mammalian and invertebrate nucleostemin and its homologues, an issue of pressing interest given the role of nucleostemin in stem cell self-renewal and tissue regeneration. A genome-wide search reveals that nucleostemin and its closest homologue, GNL3L, only emerge as separate genes in vertebrates and possess conserved protein sequences as evolution proceeded to the Mammalia. The invertebrate orthologue of nucleostemin and GNL3L resembles GNL3L more than it does nucleostemin in function, raising the idea that nucleostemin acquires new properties while GNL3L inherits an evolutionarily fixed role, and that the birth of nucleostemin may signify the appearance of new functional features in the vertebrate lineage.

Critical role of nucleostemin in pre-rRNA processing

Nucleostemin is a nucleolar protein widely expressed in proliferating cells. Nucleostemin is involved in the regulation of cell proliferation, and both depletion and overexpression of nucleostemin induce cell cycle arrest through the p53 signaling pathway. Although the presence of p53-independent functions of nucleostemin has been previously suggested, the identities of these additional functions remained to be investigated. Here, we show that nucleostemin has a novel role as an integrated component of ribosome biogenesis, particularly pre-rRNA processing. Nucleostemin forms a large protein complex (>700 kDa) that co-fractionates with the pre-60 S ribosomal subunit in a sucrose gradient. This complex contains proteins related to pre-rRNA processing, such as Pes1, DDX21, and EBP2, in addition to several ribosomal proteins. We show that the nucleolar retention of DDX21 and EBP2 is dependent on the presence of nucleostemin in the nucleolus. Furthermore, the knockdown of nucleostemin delays the processing of 32 S pre-rRNA into 28 S rRNA. This is accompanied by a substantial decrease of protein synthesis as well as the levels of rRNAs and some mRNAs. In addition, overexpressed nucleostemin significantly promotes the processing of 32 S pre-rRNA. Collectively, these biochemical and functional studies demonstrate a novel role of nucleostemin in ribosome biogenesis. This is a key aspect of the role of nucleostemin in regulating cell proliferation.

Serotonin and insulin signaling team up to control growth in Drosophila

Neuroendocrine signaling pathways play a central role in modulating animal body size in response to environmental signals. Little is known, however, regarding how these neuroendocrine circuits are controlled. An important advance in this area is reported in this issue of Genes & Development by Kaplan and colleagues (pp. 1877-1893), who show that serotonergic neurons regulate the growth of peripheral tissues in Drosophila through the insulin/IGF pathway.

C. elegans nucleostemin is required for larval growth and germline stem cell division

The nucleolus has shown to be integral for many processes related to cell growth and proliferation. Stem cells in particular are likely to depend upon nucleolus-based processes to remain in a proliferative state. A highly conserved nucleolar factor named nucleostemin is proposed to be a critical link between nucleolar function and stem-cell–specific processes. Currently, it is unclear whether nucleostemin modulates proliferation by affecting ribosome biogenesis or by another nucleolus-based activity that is specific to stem cells and/or highly proliferating cells. Here, we investigate nucleostemin (nst-1) in the nematode C. elegans, which enables us to examine nst-1 function during both proliferation and differentiation in vivo. Like mammalian nucleostemin, the NST-1 protein is localized to the nucleolus and the nucleoplasm; however, its expression is found in both differentiated and proliferating cells. Global loss of C. elegans nucleostemin (nst-1) leads to a larval arrest phenotype due to a growth defect in the soma, while loss of nst-1 specifically in the germ line causes germline stem cells to undergo a cell cycle arrest. nst-1 mutants exhibit reduced levels of rRNAs, suggesting defects in ribosome biogenesis. However, NST-1 is generally not present in regions of the nucleolus where rRNA transcription and processing occurs, so this reduction is likely secondary to a different defect in ribosome biogenesis. Transgenic studies indicate that NST-1 requires its N-terminal domain for stable expression and both its G1 GTPase and intermediate domains for proper germ line function. Our data support a role for C. elegans nucleostemin in cell growth and proliferation by promoting ribosome biogenesis.

Stem cells are carefully poised between the alternate fates of proliferation and differentiation. The regulation of this choice is a complex one that occurs on many different levels. One major influence controlling this choice derives signals emanating from the nucleolus, which serves dual roles as the site of ribosome biogenesis and as a repository for sequestered key regulatory factors. The nucleolar GTPase nucleostemin has recently been identified as a potential link between stem cell proliferation and nucleolar function, but its exact role in the nucleolus has not been directly addressed in a metazoan. Here, we use the model organism C. elegans to investigate the function of nucleostemin in both differentiated cells and proliferating stem cells. We show that nucleostemin probably acts to regulate ribosome biogenesis, and through this process controls cell proliferation. We also suggest that, at least in C. elegans, the function of nucleostemin is not restricted to proliferating stem cells, but that it also functions in differentiated cells to control cell growth. Our study highlights the complexity of the role of the nucleolus in regulation of cell growth and division.

Purification of human telomerase complexes identifies factors involved in telomerase biogenesis and telomere length regulation

The identities and roles of proteins associated with human telomerase remain poorly defined. To gain insight, we undertook an affinity purification of endogenously assembled human telomerase complexes. We show that specific subsets of H/ACA, Sm, and hnRNP proteins associate with active and inactive telomerase RNPs, while two NTPase proteins associate preferentially with active enzyme. All three core H/ACA-motif binding proteins are telomerase holoenzyme components essential for RNP accumulation. On the other hand, telomerase RNPs lacking interaction with Sm proteins or hnRNP C remain fully functional for telomere elongation. Curiously, overexpression of either associated hnRNP protein (hnRNP C and hnRNP U) or either NTPase protein (NAT10 and GNL3L) induced telomere shortening. Our findings suggest that endogenous human telomerase complexes are more heterogeneous than those of single-celled eukaryotes, have predominantly shared rather than telomerase-specific proteins, and make numerous regulatory interactions.

GNL3L inhibits activity of estrogen-related receptor gamma by competing for coactivator binding

Guanine nucleotide binding protein-like 3 (GNL3L) is the closest homologue of a stem cell-enriched factor nucleostemin in vertebrates. They share the same yeast orthologue, Grn1p, but only GNL3L can rescue the growth-deficient phenotype in Grn1-null yeasts. To determine the unique function of GNL3L, we identified estrogen-related receptor gamma (ERRgamma) as a GNL3L-specific binding protein. GNL3L and ERRgamma are coexpressed in the eye, kidney and muscle, and co-reside in the nucleoplasm. The interaction between GNL3L and ERRgamma requires the intermediate domain of GNL3L and the AF2-domain of ERRgamma. Gain-of- and loss-of-function experiments show that GNL3L can inhibit the transcriptional activities of ERR genes in a cell-based reporter system, which does not require the nucleolar localization of GNL3L. We further demonstrate that GNL3L is able to reduce the steroid receptor coactivator (SRC) binding and the SRC-mediated transcriptional coactivation of ERRgamma. This work reveals a novel mechanism that negatively regulates the transcriptional function of ERRgamma by GNL3L through coactivator competition.

Multiple conserved domains of the nucleoporin Nup124p and its orthologs Nup1p and Nup153 are critical for nuclear import and activity of the fission yeast Tf1 retrotransposon

The nucleoporin Nup124p is a host protein required for the nuclear import of both, retrotransposon Tf1-Gag as well as the retroviral HIV-1 Vpr in fission yeast. The human nucleoporin Nup153 and the Saccharomyces cerevisiae Nup1p were identified as orthologs of Nup124p. In this study, we show that all three nucleoporins share a large FG/FXFG-repeat domain and a C-terminal peptide sequence, GRKIxxxxxRRKx, that are absolutely essential for Tf1 retrotransposition. Though the FXFG domain was essential, the FXFG repeats themselves could be eliminated without loss of retrotransposon activity, suggesting the existence of a common element unrelated to FG/FXFG motifs. The Nup124p C-terminal peptide, GRKIAVPRSRRKR, was extremely sensitive to certain single amino acid changes within stretches of the basic residues. On the basis of our comparative study of Nup124p, Nup1p, and Nup153 domains, we have developed peptides that specifically knockdown retrotransposon activity by disengaging the Tf1-Gag from its host nuclear transport machinery without any harmful consequence to the host itself. Our results imply that those domains challenged a specific pathway affecting Tf1 transposition. Although full-length Nup1p or Nup153 does not complement Nup124p, the functionality of their conserved domains with reference to Tf1 activity suggests that these three proteins evolved from a common ancestor.

The essential GTPase YqeH is required for proper ribosome assembly in Bacillus subtilis

Recent work with bacteria and eukaryotes has shown that GTPases play important roles in ribosome assembly. Here we show that the essential GTPase YqeH is required for proper 70S ribosome formation and 30S subunit assembly/stability in Bacillus subtilis.

Evolutionarily conserved role of nucleostemin: controlling proliferation of stem/progenitor cells during early vertebrate development

Nucleostemin (NS) is a putative GTPase expressed preferentially in the nucleoli of neuronal and embryonic stem cells and several cancer cell lines. Transfection and knockdown studies indicated that NS controls the proliferation of these cells by interacting with the p53 tumor suppressor protein and regulating its activity. To assess the physiological role of NS in vivo, we generated a mutant mouse line with a specific gene trap event that inactivates the NS allele. The corresponding NS(-/-) embryos died around embryonic day 4. Analyses of NS mutant blastocysts indicated that NS is not required to maintain pluripotency, nucleolar integrity, or survival of the embryonic stem cells. However, the homozygous mutant blastocysts failed to enter S phase even in the absence of functional p53. Haploid insufficiency of NS in mouse embryonic fibroblasts leads to decreased cell proliferation. NS also functions in early amphibian development to control cell proliferation of neural progenitor cells. Our results show that NS has a unique ability, derived from an ancestral function, to control the proliferation rate of stem/progenitor cells in vivo independently of p53.

Nucleostemin delays cellular senescence and negatively regulates TRF1 protein stability

Nucleostemin (NS) encodes a nucleolar GTP-binding protein highly enriched in the stem cells and cancer cells. To determine its biological activity in vivo, we generated NS loss- and gain-of-function mouse models. The embryogenesis of homozygous NS-null (NS(-/-)) mice was aborted before the blastula stage. Although the growth and fertility of heterozygous NS-null (NS(+/-)) mice appeared normal, NS(+/-) mouse embryonic fibroblasts (MEFs) had fewer NS proteins, a lower population growth rate, and higher percentages of senescent cells from passage 5 (P5) to P7 than their wild-type littermates. Conversely, transgenic overexpression of NS could rescue the NS(-/-) embryo in a dose-dependent manner, increase the population growth rate, and reduce the senescent percentage of MEFs. Cell cycle analyses revealed increased pre-G1 percentages in the late-passage NS(+/-) MEF cultures compared to the wild-type cultures. We demonstrated that NS could interact with telomeric repeat-binding factor 1 (TRF1) and enhance the degradation but not the ubiquitination of the TRF1 protein, which negatively regulates telomere length and is essential for early embryogenesis. This work demonstrates the roles of NS in establishing early embryogenesis and delaying cellular senescence of MEFs and reveals a mechanism of a NS-regulated degradation of TRF1.

A novel lysine-rich domain and GTP binding motifs regulate the nucleolar retention of human guanine nucleotide binding protein, GNL3L

A variety of G-proteins and GTPases are known to be involved in nucleolar function. We describe here a new evolutionarily conserved putative human GTPase, guanine nucleotide binding protein-like 3-like (GNL3L). Genes encoding proteins related to GNL3L are present in bacteria and yeast to metazoa and suggests its critical role in development. Conserved domain search analysis revealed that the GNL3L contains a circularly permuted G-motif described by a G5-G4-G1-G2-G3 pattern similar to the HSR1/MMR1 GTP-binding protein subfamily. Highly conserved and critical residues were identified from a three-dimensional structural model obtained for GNL3L using the crystal structure of an Ylqf GTPase from Bacillus subtilis. We demonstrate here that GNL3L is transported into the nucleolus by a novel lysine-rich nucleolar localization signal (NoLS) residing within 1-50 amino acid residues. NoLS identified here is necessary and sufficient to target the heterologous proteins to the nucleolus. We show for the first time that the lysine-rich targeting signal interacts with the nuclear transport receptor, importin-beta and transports GNL3L into the nucleolus. Interestingly, depletion of intracellular GTP blocks GNL3L accumulation into the nucleolar compartment. Furthermore, mutations within the G-domains alter the GTP binding ability of GNL3L and abrogate wild-type nucleolar retention even in the presence of functional NoLS, suggesting that the efficient nucleolar retention of GNL3L involves activities of both basic NoLS and GTP-binding domains. Collectively, these data suggest that GNL3L is composed of distinct modules, each of which plays a specific role in molecular interactions for its nucleolar retention and subsequent function(s) within the nucleolus.

The NUG1 GTPase reveals and N-terminal RNA-binding domain that is essential for association with 60 S pre-ribosomal particles

The putative yeast GTPase Nug1, which is associated with several pre-60 S particles in the nucleolus and nucleoplasm, consists of an N-terminal domain, which is found only in eukaryotic orthologues, and middle and C-terminal domains that are conserved throughout eukaryotes, bacteria, and archaea. Here, we analyzed the role of the eukaryote-specific Nug1 N-domain (Nug1-N). We show that the essential Nug1-N is sufficient and necessary for nucle(ol)ar targeting and association with pre-60 S particles. Nug1-N exhibits RNA binding activity and is genetically linked in an allele-specific way to the pre-60 S factors Noc2, Noc3, and Dbp10. In contrast, the middle domain, which exhibits a circularly permuted GTPase fold and an intrinsic GTP hydrolysis activity in vitro, is not essential for cell growth. The conserved Nug1 C-domain, which has a yet uncharacterized fold, is also essential for ribosome biogenesis. Our findings suggest that Nug1 associates with pre-60 S subunits via its essential N-terminal RNA-binding domain and exerts a non-essential regulative role in pre-60 S subunit biogenesis via its central GTPase domain.