Regulation of mammalian ribosomal gene transcription by RNA polymerase I

Synthesis of the ribosomal RNA precursor in human cells: mechanisms, factors and regulation

The ribosomal RNA precursor (pre-rRNA) comprises three of the four ribosomal RNAs and is synthesized by RNA polymerase (Pol) I. Here, we describe the mechanisms of Pol I transcription in human cells with a focus on recent insights gained from structure-function analyses. The comparison of Pol I-specific structural and functional features with those of other Pols and with the excessively studied yeast system distinguishes organism-specific from general traits. We explain the organization of the genomic rDNA loci in human cells, describe the Pol I transcription cycle regarding structural changes in the enzyme and the roles of human Pol I subunits, and depict human rDNA transcription factors and their function on a mechanistic level. We disentangle information gained by direct investigation from what had apparently been deduced from studies of the yeast enzymes. Finally, we provide information about how Pol I mutations may contribute to developmental diseases, and why Pol I is a target for new cancer treatment strategies, since increased rRNA synthesis was correlated with rapidly expanding cell populations.

Ribosomal proteins and human diseases: molecular mechanisms and targeted therapy

Ribosome biogenesis and protein synthesis are fundamental rate-limiting steps for cell growth and proliferation. The ribosomal proteins (RPs), comprising the structural parts of the ribosome, are essential for ribosome assembly and function. In addition to their canonical ribosomal functions, multiple RPs have extra-ribosomal functions including activation of p53-dependent or p53-independent pathways in response to stress, resulting in cell cycle arrest and apoptosis. Defects in ribosome biogenesis, translation, and the functions of individual RPs, including mutations in RPs have been linked to a diverse range of human congenital disorders termed ribosomopathies. Ribosomopathies are characterized by tissue-specific phenotypic abnormalities and higher cancer risk later in life. Recent discoveries of somatic mutations in RPs in multiple tumor types reinforce the connections between ribosomal defects and cancer. In this article, we review the most recent advances in understanding the molecular consequences of RP mutations and ribosomal defects in ribosomopathies and cancer. We particularly discuss the molecular basis of the transition from hypo- to hyper-proliferation in ribosomopathies with elevated cancer risk, a paradox termed "Dameshek's riddle." Furthermore, we review the current treatments for ribosomopathies and prospective therapies targeting ribosomal defects. We also highlight recent advances in ribosome stress-based cancer therapeutics. Importantly, insights into the mechanisms of resistance to therapies targeting ribosome biogenesis bring new perspectives into the molecular basis of cancer susceptibility in ribosomopathies and new clinical implications for cancer therapy.

Reduced rDNA transcription diminishes skeletal muscle ribosomal capacity and protein synthesis in cancer cachexia

Muscle wasting in cancer is associated with deficits in protein synthesis, yet, the mechanisms underlying this anabolic impairment remain poorly understood. The capacity for protein synthesis is mainly determined by the abundance of muscle ribosomes, which is in turn regulated by transcription of the ribosomal (r)RNA genes (rDNA). In this study, we investigated whether muscle loss in a preclinical model of ovarian cancer is associated with a reduction in ribosomal capacity and was a consequence of impaired rDNA transcription. Tumor bearing resulted in a significant loss in gastrocnemius muscle weight and protein synthesis capacity, and was consistent with a significant reduction in rDNA transcription and ribosomal capacity. Despite the induction of the ribophagy receptor NUFIP1 mRNA and the loss of NUFIP1 protein, in vitro studies revealed that while inhibition of autophagy rescued NUFIP1, it did not prevent the loss of rRNA. Electrophoretic analysis of rRNA fragmentation from both in vivo and in vitro models showed no evidence of endonucleolytic cleavage, suggesting that rRNA degradation may not play a major role in modulating muscle ribosome abundance. Our results indicate that in this model of ovarian cancer-induced cachexia, the ability of skeletal muscle to synthesize protein is compromised by a reduction in rDNA transcription and consequently a lower ribosomal capacity. Thus, impaired ribosomal production appears to play a key role in the anabolic deficits associated with muscle wasting in cancer cachexia.

Cyclin-Dependent Kinases (CDK) and Their Role in Diseases Development-Review

Cyclin-dependent kinases (CDKs) are involved in many crucial processes, such as cell cycle and transcription, as well as communication, metabolism, and apoptosis. The kinases are organized in a pathway to ensure that, during cell division, each cell accurately replicates its DNA, and ensure its segregation equally between the two daughter cells. Deregulation of any of the stages of the cell cycle or transcription leads to apoptosis but, if uncorrected, can result in a series of diseases, such as cancer, neurodegenerative diseases (Alzheimer’s or Parkinson’s disease), and stroke. This review presents the current state of knowledge about the characteristics of cyclin-dependent kinases as potential pharmacological targets.

Variation in ribosomal DNA copy number is associated with lung cancer risk in a prospective cohort study

Disruption of ribosomal DNA (rDNA) has been linked to a variety of diseases in humans, including carcinogenesis. To evaluate the associations between rDNA copy number (CN) and risk of lung cancer, we measured 5.8S and 18S rDNA CN in the peripheral blood of 229 incident lung cancer cases and 1:1 matched controls from a nested case-control study within a prospective cohort of male smokers. There was a dose-response relationship between quartiles of both 18S and 5.8S rDNA CN and risk of lung cancer (odds ratio [OR], 95% confidence interval [CI]: 18S: 1.0 [ref]; 1.2 [0.6-2.1]; 1.8 [1.0-3.4]; 2.3 [1.3-4.1; Ptrend = 0.0002; 5.8S: 1.0 [ref]; 1.6 [0.8-2.9]; 2.2 [1.1-4.2]; 2.6 [1.3-5.1]; Ptrend = 0.0001). The associations between rDNA CN and lung cancer risk were similar when excluding cases diagnosed within 5 years of follow-up, and when stratifying by heavy (>20 cigarettes per day) and light smokers (≤20 cigarettes per day). We are the first to report that rDNA CN may be associated with future risk of lung cancer. To further elucidate the relationship between rDNA and lung cancer, replication studies are needed in additional populations, particularly those that include non-smokers.

Root of the Tree: The Significance, Evolution, and Origins of the Ribosome

The ribosome is an ancient molecular fossil that provides a telescope to the origins of life. Made from RNA and protein, the ribosome translates mRNA to coded protein in all living systems. Universality, economy, centrality and antiquity are ingrained in translation. The translation machinery dominates the set of genes that are shared as orthologues across the tree of life. The lineage of the translation system defines the universal tree of life. The function of a ribosome is to build ribosomes; to accomplish this task, ribosomes make ribosomal proteins, polymerases, enzymes, and signaling proteins. Every coded protein ever produced by life on Earth has passed through the exit tunnel, which is the birth canal of biology. During the root phase of the tree of life, before the last common ancestor of life (LUCA), exit tunnel evolution is dominant and unremitting. Protein folding coevolved with evolution of the exit tunnel. The ribosome shows that protein folding initiated with intrinsic disorder, supported through a short, primitive exit tunnel. Folding progressed to thermodynamically stable β-structures and then to kinetically trapped α-structures. The latter were enabled by a long, mature exit tunnel that partially offset the general thermodynamic tendency of all polypeptides to form β-sheets. RNA chaperoned the evolution of protein folding from the very beginning. The universal common core of the ribosome, with a mass of nearly 2 million Daltons, was finalized by LUCA. The ribosome entered stasis after LUCA and remained in that state for billions of years. Bacterial ribosomes never left stasis. Archaeal ribosomes have remained near stasis, except for the superphylum Asgard, which has accreted rRNA post LUCA. Eukaryotic ribosomes in some lineages appear to be logarithmically accreting rRNA over the last billion years. Ribosomal expansion in Asgard and Eukarya has been incremental and iterative, without substantial remodeling of pre-existing basal structures. The ribosome preserves information on its history.

Transient inhibition of rDNA transcription in donor cells improves ribosome biogenesis and preimplantation development of embryos derived from somatic cell nuclear transfer

Ribosomal DNA (rDNA) transcription is a limiting step in ribosome biogenesis, crucial for protein synthesis and cell growth-especially at the early stages of embryonic development-and is regulated in a mammalian target of rapamycin (mTOR)-dependent manner. Our previous report demonstrated that treatment with mTOR inhibitors during artificial embryonic activation improved the development of embryos derived from somatic cell nuclear transfer (SCNT). We hypothesize that inhibition of ribosome biogenesis in somatic cells facilitates reactivation of embryonic nucleolar establishment and ribosome biogenesis in SCNT embryos. Herein, we show that mTOR inhibitors suppressed ribosome biogenesis in somatic cells, and more importantly, improved development potential of SCNT embryos (blastocyst rate, 34% vs 24%). SCNT embryos derived from drug-treated somatic cells exhibited higher levels of 47S, 18S, and 5S rRNAs, upstream binding factor (UBF) mRNA, ribosomal protein S6; they also improved the rebuilding of the nucleolar ultrastructure. In addition, treatment of donor cells with the RNA polymerase I (Pol I) inhibitor cx5461 caused similar effects on SCNT embryos. These results indicated that transient inhibition of rDNA transcription in donor cells facilitated the establishment of functional nucleoli and improved preimplantation development of SCNT embryos.

Metastasis-related methyltransferase 1 (Merm1) represses the methyltransferase activity of Dnmt3a and facilitates RNA polymerase I transcriptional elongation

Stimulatory regulators for DNA methyltransferase activity, such as Dnmt3L and some Dnmt3b isoforms, affect DNA methylation patterns, thereby maintaining gene body methylation and maternal methylation imprinting, as well as the methylation landscape of pluripotent cells. Here we show that metastasis-related methyltransferase 1 (Merm1), a protein deleted in individuals with Williams-Beuren syndrome, acts as a repressive regulator of Dnmt3a. Merm1 interacts with Dnmt3a and represses its methyltransferase activity with the requirement of the binding motif for S-adenosyl-L-methionine. Functional analysis of gene regulation revealed that Merm1 is capable of maintaining hypomethylated rRNA gene bodies and co-localizes with RNA polymerase I in the nucleolus. Dnmt3a recruits Merm1, and in return, Merm1 ensures the binding of Dnmt3a to hypomethylated gene bodies. Such interplay between Dnmt3a and Merm1 facilitates transcriptional elongation by RNA polymerase I. Our findings reveal a repressive factor for Dnmt3a and uncover a molecular mechanism underlying transcriptional elongation of rRNA genes.

The Repetitive DNA Composition in the Natural Pesticide Producer Tanacetum cinerariifolium: Interindividual Variation of Subtelomeric Tandem Repeats

Dalmatian pyrethrum (Tanacetum cinerariifolium (Trevir.) Sch. Bip.), a plant species endemic to the east Adriatic coast, is used worldwide for production of the organic insecticide, pyrethrin. Most studies concerning Dalmatian pyrethrum have focused on its morphological and biochemical traits relevant for breeding. However, little is known about the chromosomal evolution and genome organization of this species. Our study aims are to identify, classify, and characterize repetitive DNA in the T. cinerariifolium genome using clustering analysis of a low coverage genomic dataset. Repetitive DNA represents about 71.63% of the genome. T. cinerariifolium exhibits linked 5S and 35S rDNA configuration (L-type). FISH reveals amplification of interstitial telomeric repeats (ITRs) in T. cinerariifolium. Of the three newly identified satellite DNA families, TcSAT1 and TcSAT2 are located subterminally on most of T. cinerariifolium chromosomes, while TcSAT3 family is located intercalary within the longer arm of two chromosome pairs. FISH reveals high levels of polymorphism of the TcSAT1 and TcSAT2 sites by comparative screening of 28 individuals. TcSAT2 is more variable than TcSAT1 regarding the number and position of FISH signals. Altogether, our data highlights the dynamic nature of DNA sequences associated with subtelomeres in T. cinerariifolium and suggests that subtelomeres represent one of the most dynamic and rapidly evolving regions in eukaryotic genomes.

Direct visualization of nucleolar G-quadruplexes in live cells by using a fluorescent light-up probe

BACKGROUND

Direct detection of G-quadruplexes in human cells has become an important issue due to the vital role of G-quadruplex related to biological functions. Despite several probes have been developed for detection of the G-quadruplexes in cytoplasm or whole cells, the probe being used to monitor the nucleolar G-quadruplexes is still lacking.

METHODS

Formation of the nucleolar G-quadruplex structures was confirmed by using circular dichroism (CD) spectroscopy. The binding affinity and selectivity of Thioflavin T (ThT) towards various DNA/RNA motifs in solution and gel system were measured by using fluorescence spectroscopy and polyacrylamide gel electrophoresis (PAGE), respectively. G-quadruplex imaging in live cells was directly captured by using confocal laser scanning microscopy (CLSM).

RESULTS

Formation of the rDNA and rRNA G-quadruplex structures is demonstrated in vitro. ThT is found to show much higher affinity and selectivity towards these G-quadruplex structures versus other nucleic acid motifs either in solution or in gel system. The nucleolar G-quadruplexes in living cells are visualized by using ThT as a fluorescent probe. G-quadruplex-ligand treatments in live cells lead to sharp decrease of ThT signal.

CONCLUSIONS

The natural existence of the G-quadruplexes structure in the nucleoli of living cells is directly visualized by using ThT as an indicator.

GENERAL SIGNIFICANCE

The research provides substantive evidence for formation of the rRNA G-quadruplex structures, and also offers an effective probe for direct visualization of the nucleolar G-quadruplexes in living cells.

Ribosomal DNA copy number amplification and loss in human cancers is linked to tumor genetic context, nucleolus activity, and proliferation

Ribosomal RNAs (rRNAs) are transcribed from two multicopy DNA arrays: the 5S ribosomal DNA (rDNA) array residing in a single human autosome and the 45S rDNA array residing in five human autosomes. The arrays are among the most variable segments of the genome, exhibit concerted copy number variation (cCNV), encode essential components of the ribosome, and modulate global gene expression. Here we combined whole genome data from >700 tumors and paired normal tissues to provide a portrait of rDNA variation in human tissues and cancers of diverse mutational signatures, including stomach and lung adenocarcinomas, ovarian cancers, and others of the TCGA panel. We show that cancers undergo coupled 5S rDNA array expansion and 45S rDNA loss that is accompanied by increased estimates of proliferation rate and nucleolar activity. These somatic changes in rDNA CN occur in a background of over 10-fold naturally occurring rDNA CN variation across individuals and cCNV of 5S-45S arrays in some but not all tissues. Analysis of genetic context revealed associations between cancer rDNA CN amplification or loss and the presence of specific somatic alterations, including somatic SNPs and copy number gain/losses in protein coding genes across the cancer genome. For instance, somatic inactivation of the tumor suppressor gene TP53 emerged with a strong association with coupled 5S expansion / 45S loss in several cancers. Our results uncover frequent and contrasting changes in the 5S and 45S rDNA along rapidly proliferating cell lineages with high nucleolar activity. We suggest that 5S rDNA amplification facilitates increased proliferation, nucleolar activity, and ribosomal synthesis in cancer, whereas 45S rDNA loss emerges as a byproduct of transcription-replication conflict in rapidly replicating tumor cells. The observations raise the prospects of using the rDNA arrays as re-emerging targets for the design of novel strategies in cancer therapy.

The 45S and 5S ribosomal DNA (rDNA) arrays contain hundreds of rDNA copies, with substantial variability across individuals in human populations. Although physically unlinked, the arrays also exhibit joint variation across individual genotypes. However, whether this co-variation in copy number (CN) is universally observed across all tissues is unknown. It also remains unknown if rDNA CN might vary across tissues and in cancer lineages. Here we showed that most cancers undergo coupled 5S rDNA array amplification and 45S rDNA loss, and concerted 5S-45S CN variation in some but not all tissues. The coupled 5S amplification and 45S loss is associated with the presence of certain somatic genetic alterations, as well as increased estimates of cancerous cell proliferation rate and nucleolar activity. Our research uncovers frequent and contrasting changes in rDNA CN in cancers of diverse tissue origin and associated with diverse mutational contexts of tumor suppressors and oncogenes. The observations raise the prospects of using the rDNA arrays as re-emerging targets in novel strategies for cancer therapy.

A new marker sequence for systematics of medically important fungi based on amino acid sequence of the largest subunit of RNA polymerase I

Molecular evolution has dominated taxonomic studies for decades, replacing traditional methods for identification and classification. However, there is a need for better markers to resolve the problems that have limited their usefulness. In this report, we introduce the protein tag (Ptag) sequence, a highly polymorphic amino acid sequence within the C-terminal region of the largest subunit of RNA polymerase I, as a new systematic tag sequence for delineating the evolutionary history of medically important fungi. As Ptag sequences are highly polymorphic between species and low within species, 42 fungal species representing the main taxonomic groups in the phyla Ascomycota and Basidiomycota were tested. The phylogenetic tree inferred from the Ptag sequences showed high consistency with the accepted classification of the Assembling the Fungal Tree of Life (AFTOL) project. Moreover, it could resolve the interspecies phylogenetic relationships of the tested taxa. In contrast, the phylogeny inferred from the nucleotide tag (Ntag) sequence, encoding the Ptag peptide, displayed lesser discriminatory power in resolving the phylogenetic relationships among distantly related taxa. In the case of closely related fungal species, the phylogenetic trees for Ptag and Ntag sequences were consistent with the tree for ITS1 sequences of 11 dermatophytic species. Taken together, the Ptag sequences should contribute to inferring phylogeny among species whereas the Ntag sequences should be useful to analyze variations among closely related species to resolve taxonomic issues in fungi.

mTOR signaling regulates myotube hypertrophy by modulating protein synthesis, rDNA transcription, and chromatin remodeling

Ribosome production is an early event during skeletal muscle hypertrophy and precedes muscle protein accretion. Signaling via mTOR is crucial for ribosome production and hypertrophy; however, the mechanisms by which it regulates these processes remain to be identified. Herein, we investigated the activation of mTOR signaling in hypertrophying myotubes and determined that mTOR coordinates various aspects of gene expression important for ribosome production. First, inhibition of translation with cycloheximide had a more potent effect on protein synthesis than rapamycin indicating that mTOR function during hypertrophy is not on general, but rather on specific protein synthesis. Second, blocking Pol II transcription had a similar effect as Rapamycin and, unexpectedly, revealed the necessity of Pol II transcription for Pol I transcription, suggesting that mTOR may regulate ribosome production also by controlling Class II genes at the transcriptional level. Third, Pol I activity is essential for rDNA transcription and, surprisingly, for protein synthesis as selective Pol I inhibition blunted rDNA transcription, protein synthesis, and the hypertrophic response of myotubes. Finally, mTOR has nuclear localization in muscle, which is not sensitive to rapamycin. Inhibition of mTOR signaling by rapamycin disrupted mTOR-rDNA promoter interaction and resulted in altered histone marks indicative of repressed transcription and formation of higher-order chromatin structure. Thus mTOR signaling appears to regulate muscle hypertrophy by affecting protein synthesis, Class I and II gene expression, and chromatin remodeling.

Effects of permissible maximum-contamination levels of VOC mixture in water on total DNA, antioxidant gene expression, and sequences of ribosomal DNA of Drosophila melanogaster

In this study, we aimed to investigate the mutagenic and carcinogenic potential of a volatile organic compound (VOC) mixture with references to the response of D.melanogaster using selected antioxidant gene expressions, RAPD assay and base-pair change of ribosomal 18S, and the internal transcribed spacer, ITS2 rDNA gene sequences. For this purpose, Drosophila melanogaster Oregon R, reared under controlled conditions on artificial diets, were treated with the mixture of thirteen VOCs, which are commonly found in water in concentrations of 10, 20, 50, and 75 ppb for 1 and 5 days. In the random amplified polymorphic DNA (RAPD) assay, band changes were clearly detected, especially at the 50 and 75 ppb exposure levels, for both treatment periods, and the band profiles exhibited clear differences between the treated and untreated flies with changes in band intensity and the loss/appearance of bands. Quantitative real-time PCR (qRT-PCR) analysis of Mn-superoxide dismutase (Mn-SOD), catalase (CAT) and glutathione-synthetase (GS) expressions demonstrated that these markers responded significantly to VOC-induced oxidative stress. Whilst CAT gene expressions increased linearly with increasing concentrations of VOCs and treatment times, the 50- and 75-ppb treatments caused decreases in GS expressions compared to the control at 5 days. Treatment with VOCs at both exposure times, especially in high doses, caused gene mutation of the 18S and the ITS2 ribosomal DNA. According to this research, we thought that the permissible maximum-contamination level of VOCs can cause genotoxic effect especially when mixed.

The nucleolus: a raft adrift in the nuclear sea or the keystone in nuclear structure?

The nucleolus is a prominent nuclear structure that is the site of ribosomal RNA (rRNA) transcription, and hence ribosome biogenesis. Cellular demand for ribosomes, and hence rRNA, is tightly linked to cell growth and the rRNA makes up the majority of all the RNA within a cell. To fulfill the cellular demand for rRNA, the ribosomal RNA (rDNA) genes are amplified to high copy number and transcribed at very high rates. As such, understanding the rDNA has profound consequences for our comprehension of genome and transcriptional organization in cells. In this review, we address the question of whether the nucleolus is a raft adrift the sea of nuclear DNA, or actively contributes to genome organization. We present evidence supporting the idea that the nucleolus, and the rDNA contained therein, play more roles in the biology of the cell than simply ribosome biogenesis. We propose that the nucleolus and the rDNA are central factors in the spatial organization of the genome, and that rapid alterations in nucleolar structure in response to changing conditions manifest themselves in altered genomic structures that have functional consequences. Finally, we discuss some predictions that result from the nucleolus having a central role in nuclear organization.

Identification and characterization of nuclear and nucleolar localization signals in 58-kDa microspherule protein (MSP58)

BACKGROUND

MSP58 is a nucleolar protein associated with rRNA transcription and cell proliferation. Its mechanism of translocation into the nucleus or the nucleolus, however, is not entirely known. In order to address this lack, the present study aims to determine a crucial part of this mechanism: the nuclear localization signal (NLS) and the nucleolar localization signal (NoLS) associated with the MSP58 protein.

RESULTS

We have identified and characterized two NLSs in MSP58. The first is located between residues 32 and 56 (NLS1) and constitutes three clusters of basic amino acids (KRASSQALGTIPKRRSSSRFIKRKK); the second is situated between residues 113 and 123 (NLS2) and harbors a monopartite signal (PGLTKRVKKSK). Both NLS1 and NLS2 are highly conserved among different vertebrate species. Notably, one bipartite motif within the NLS1 (residues 44-56) appears to be absolutely necessary for MSP58 nucleolar localization. By yeast two-hybrid, pull-down, and coimmunoprecipitation analysis, we show that MSP58 binds to importin α1 and α6, suggesting that nuclear targeting of MSP58 utilizes a receptor-mediated and energy-dependent import mechanism. Functionally, our data show that both nuclear and nucleolar localization of MSP58 are crucial for transcriptional regulation on p21 and ribosomal RNA genes, and context-dependent effects on cell proliferation.

CONCLUSIONS

Results suggest that MSP58 subnuclear localization is regulated by two nuclear import signals, and that proper subcellular localization of MSP58 is critical for its role in transcriptional regulation. Our study reveals a molecular mechanism that controls nuclear and nucleolar localization of MSP58, a finding that might help future researchers understand the MSP58 biological signaling pathway.

Transcriptional Repressor TrmBL2 from Thermococcus kodakarensis Forms Filamentous Nucleoprotein Structures and Competes with Histones for DNA Binding in a Salt- and DNA Supercoiling-dependent Manner

Architectural DNA proteins play important roles in the chromosomal DNA organization and global gene regulation in living cells. However, physiological functions of some DNA-binding proteins from archaea remain unclear. Recently, several abundant DNA-architectural proteins including histones, Alba, and TrmBL2 have been identified in model euryarchaeon Thermococcus kodakarensis. Although histones and Alba proteins have been previously characterized, the DNA binding properties of TrmBL2 and its interplay with the other major architectural proteins in the chromosomal DNA organization and gene transcription regulation remain largely unexplored. Here, we report single-DNA studies showing that at low ionic strength (<300 mM KCl), TrmBL2 binds to DNA largely in non-sequence-specific manner with positive cooperativity, resulting in formation of stiff nucleoprotein filamentous patches, whereas at high ionic strength (>300 mM KCl) TrmBL2 switches to more sequence-specific interaction, suggesting the presence of high affinity TrmBL2-filament nucleation sites. Furthermore, in vitro assays indicate the existence of DNA binding competition between TrmBL2 and archaeal histones B from T. kodakarensis, which can be strongly modulated by DNA supercoiling and ionic strength of surrounding solution. Overall, these results advance our understanding of TrmBL2 DNA binding properties and provide important insights into potential functions of architectural proteins in nucleoid organization and gene regulation in T. kodakarensis.

Coordinated Ribosomal L4 Protein Assembly into the Pre-Ribosome Is Regulated by Its Eukaryote-Specific Extension

Eukaryotic ribosome biogenesis requires nuclear import and hierarchical incorporation of ∼80 ribosomal proteins (RPs) into the ribosomal RNA core. In contrast to prokaryotes, many eukaryotic RPs possess long extensions that interdigitate in the mature ribosome. RpL4 is a prime example, with an ∼80-residue-long surface extension of unknown function. Here, we identify assembly chaperone Acl4 that initially binds the universally conserved internal loop of newly synthesized RpL4 via its superhelical TPR domain, thereby restricting RpL4 loop insertion at its cognate nascent rRNA site. RpL4 release from Acl4 is orchestrated with pre-ribosome assembly, during which the eukaryote-specific RpL4 extension makes several distinct interactions with the 60S surface, including a co-evolved site on neighboring RpL18. Consequently, mutational inactivation of this contact site, on either RpL4 or RpL18, impairs RpL4-Acl4 disassembly and RpL4 pre-ribosome incorporation. We propose that hierarchical ribosome assembly can be achieved by eukaryotic RP extensions and dedicated assembly chaperones.

Altered Sirtuin 7 Expression is Associated with Early Stage Breast Cancer

BACKGROUND

To evaluate sirtuin-7 (SirT7) mRNA expression status in breast cancer patients with different metastatic stages and survey SirT7 mRNA expression status in eight different types of cancer.

METHODS

The expression of SirT7 in the commercially available TissueScan qPCR Breast Cancer Disease cDNA arrays containing 16 normal, 23 Stage I, 36 IIA, 22 IIB, 8 IIIA, 23 IIIA, 6 IIIB, 13 IIIC, and 5 IV were evaluated by quantitative real-time polymerase chain reaction (qRT-PCR) assay. Similar analysis was performed in TissueScan qPCR Cancer Survey cDNA array, which includes breast, colon, kidney, liver, lung, ovarian, prostate, and thyroid specimens.

RESULTS

The mRNA expression levels of SirT7 were significantly higher in breast cancer samples compared to normal breast specimens (P < 0.001). Stratification of patients into groups according to metastatic stages indicated statistically significantly higher levels of SirT7 mRNA in CS-I, CS-II, and CS-III when compared to normal breast tissue (P < 0.05). Notably, SirT7 mRNA levels were higher in CS-I, CS-IIA, CS-IIB, and CS-IIIA (P < 0.05). Additionally, there were significantly lower SirT7 mRNA levels in thyroid carcinoma when compared to their corresponding normal tissue (P < 0.05).

CONCLUSIONS

Our results indicate an increase in the mRNA expression level of SirT7 in breast cancer, particularly in CS-I, CS-IIA, CS-IIB, and CS-IIIA. The relationship of altered SirT7 with breast cancer progression and patient survival should be prospectively explored in future studies.

Ribonucleic acid (RNA) biosynthesis in human cancer

In many respects, the most remarkable chemical substances within the genome of eukaryotic cells are remarkable proteins which are the critical structural and functional units of living cells. The specifications for everything that goes in the cell are natural digital-to-digital decoding process in an archive sequence by deoxyribonucleic acid (DNA) and an articulate construction by ribonucleic acid (RNA). The products of DNA transcription are long polymers of ribonucleotides rather than deoxyribonucleotides and are termed ribonucleic acids. Certain deoxyribonucleotide sequences, or genes, give rise to transfer RNA (tRNA) and other ribosomal RNA (rRNA) when transcribed. The ribonucleotide sequences fold extensively and rRNA is associated with specific proteins to yield the essential cell components, ribosomes. Transcription of other special sequences yields messenger RNAs (mRNAs) that contain ribonucleotide sequences that will be ultimately translated into new types of amino acid sequences of functional cellular protein molecules. This switch to a different variety of cellular molecular sequences is complex, but each sequence of the three ribonucleotides specifies the insertion of one particular amino acid into the polypeptide chain under production. Whilst mRNA is considered the vehicle by which genetic information is transmitted from the genome and allocated in the appropriate cytoplasmic sites for translation into protein via cap-dependent mechanism, the actual translation depends also on the presence of other so-called household and luxury protein molecules. Recent evidence suggests RNA species are required at initiation, because treatment of cells with antibiotics or drugs that inhibit RNA synthesis cause a decrease in protein synthesis. The rRNA is necessary as a structural constituent of the ribosomes upon which translation takes place, whereas tRNA is necessary as an adaptor in amino acid activation and elongation protein chains to ribosomes. In this article, we review malignant tumor, with stem like properties, and recent technical advances into the phenomenon of micro-particles and micro-vesicles containing cell-free nucleic acids that circulate plasma. New areas of research have been opened into screening tumor telomerase progression, prognosis of aptamers targeting cell surface, monitoring the efficacy of anticancer therapies, oncogenic transformation of host cell, and RNA polymerases role in the cell cycle progression and differentiation.

A novel role for the Pol I transcription factor UBTF in maintaining genome stability through the regulation of highly transcribed Pol II genes

Mechanisms to coordinate programs of highly transcribed genes required for cellular homeostasis and growth are unclear. Upstream binding transcription factor (UBTF, also called UBF) is thought to function exclusively in RNA polymerase I (Pol I)-specific transcription of the ribosomal genes. Here, we report that the two isoforms of UBTF (UBTF1/2) are also enriched at highly expressed Pol II-transcribed genes throughout the mouse genome. Further analysis of UBTF1/2 DNA binding in immortalized human epithelial cells and their isogenically matched transformed counterparts reveals an additional repertoire of UBTF1/2-bound genes involved in the regulation of cell cycle checkpoints and DNA damage response. As proof of a functional role for UBTF1/2 in regulating Pol II transcription, we demonstrate that UBTF1/2 is required for recruiting Pol II to the highly transcribed histone gene clusters and for their optimal expression. Intriguingly, lack of UBTF1/2 does not affect chromatin marks or nucleosome density at histone genes. Instead, it results in increased accessibility of the histone promoters and transcribed regions to micrococcal nuclease, implicating UBTF1/2 in mediating DNA accessibility. Unexpectedly, UBTF2, which does not function in Pol I transcription, is sufficient to regulate histone gene expression in the absence of UBTF1. Moreover, depletion of UBTF1/2 and subsequent reduction in histone gene expression is associated with DNA damage and genomic instability independent of Pol I transcription. Thus, we have uncovered a novel role for UBTF1 and UBTF2 in maintaining genome stability through coordinating the expression of highly transcribed Pol I (UBTF1 activity) and Pol II genes (UBTF2 activity).

Reconstitution of human rRNA gene transcription in mouse cells by a complete SL1 complex

An important characteristic of the transcription of a ribosomal RNA gene (rDNA) mediated by DNA-dependent RNA polymerase (Pol) I is its stringent species specificity. SL1/TIF-IB is a key complex for species specificity, but its functional complex has not been reconstituted. Here, we established a novel and highly sensitive monitoring system for Pol I transcription to reconstitute the SL1 activity in which a transcript harboring a reporter gene synthesized by Pol I is amplified and converted into translatable mRNA by the influenza virus RNA-dependent RNA polymerase. Using this monitoring system, we reconstituted Pol I transcription from the human rDNA promoter in mouse cells by expressing four human TATA-binding protein (TBP)-associated factors (TAFIs) in the SL1 complex. The reconstituted SL1 also re-activated human rDNA transcription in mouse A9 cells carrying an inactive human chromosome 21 that contains the rDNA cluster. Chimeric SL1 complexes containing human and mouse TAFIs could be formed, but these complexes were inactive for human rDNA transcription. We conclude that four human TAFIs are necessary and sufficient to overcome the barrier of species specificity for human rDNA transcription in mouse cells.

Bent bone dysplasia syndrome reveals nucleolar activity for FGFR2 in ribosomal DNA transcription

Fibroblast growth factor receptor 2 (FGFR2) promotes osteoprogenitor proliferation and differentiation during bone development, yet how the receptor elicits these distinct cellular responses remains unclear. Analysis of the FGFR2-skeletal disorder bent bone dysplasia syndrome (BBDS) demonstrates that FGFR2, in addition to its canonical signaling activities at the plasma membrane, regulates bone formation from within the nucleolus. Previously, we showed that the unique FGFR2 mutations that cause BBDS reduce receptor levels at the plasma membrane and diminish responsiveness to extracellular FGF2. In this study, we find that these mutations, despite reducing canonical signaling, enhance nucleolar occupancy of FGFR2 at the ribosomal DNA (rDNA) promoter. Nucleolar FGFR2 activates rDNA transcription via interactions with FGF2 and UBF1 by de-repressing RUNX2. An increase in the nucleolar activity of FGFR2 in BBDS elevates levels of ribosomal RNA in the developing bone, consequently promoting osteoprogenitor cell proliferation and decreasing differentiation. Identifying FGFR2 as a transcriptional regulator of rDNA in bone unexpectedly reveals a nucleolar route for FGF signaling that allows for independent regulation of osteoprogenitor cell proliferation and differentiation.

A hierarchical model for assembly of eukaryotic 60S ribosomal subunit domains

Despite having structures for eukaryotic large ribosomal subunits, it has been unclear how these ribonucleoprotein complexes are constructed in living cells. Here, Gamalinda et al. used proteomics to assay changes in the levels of ribosomal protein and assembly factors in preribosomes upon depletion of ribosomal proteins that function in 60S assembly. The results show that the eukaryotic 60S ribosomal subunits are assembled in a hierarchical fashion. This study further reveals striking differences and similarities between bacterial and eukaryotic large ribosomal subunits, providing insights into the evolution of RNA–protein particles.

Despite having high-resolution structures for eukaryotic large ribosomal subunits, it remained unclear how these ribonucleoprotein complexes are constructed in living cells. Nevertheless, knowing where ribosomal proteins interact with ribosomal RNA (rRNA) provides a strategic platform to investigate the connection between spatial and temporal aspects of 60S subunit biogenesis. We previously found that the function of individual yeast large subunit ribosomal proteins (RPLs) in precursor rRNA (pre-rRNA) processing correlates with their location in the structure of mature 60S subunits. This observation suggested that there is an order by which 60S subunits are formed. To test this model, we used proteomic approaches to assay changes in the levels of ribosomal proteins and assembly factors in preribosomes when RPLs functioning in early, middle, and late steps of pre-60S assembly are depleted. Our results demonstrate that structural domains of eukaryotic 60S ribosomal subunits are formed in a hierarchical fashion. Assembly begins at the convex solvent side, followed by the polypeptide exit tunnel, the intersubunit side, and finally the central protuberance. This model provides an initial paradigm for the sequential assembly of eukaryotic 60S subunits. Our results reveal striking differences and similarities between assembly of bacterial and eukaryotic large ribosomal subunits, providing insights into how these RNA–protein particles evolved.

Genomic characterization of the mouse ribosomal DNA locus

The transcription of rRNA is critical to all living cells and is tightly controlled at the level of chromatin structure. Although the widespread adoption of genomic technologies including chromatin immunoprecipitation with massively parallel short-read sequencing (ChIP-seq) has allowed for the interrogation of chromatin structure on a genome-wide scale, until recently rDNA has not been analyzed by this technique. We extended genomic analysis of rDNA to mouse (Mus musculus), in which rDNA is similar in structure but highly divergent in sequence compared with human rDNA. Comparison of rDNA histone marks between mouse embryonic stem cells (mESCs) and more differentiated mouse cell types revealed differences between pluripotent and differentiated states. We also observed substantial divergence in rDNA histone modification patterns between mESCs and human embryonic stem cells (hESCs). Surprisingly, we found that the pluripotency factor OCT4 was bound to rDNA in similar patterns in mESCs and hESCs. Extending this analysis, we found that an additional 17 pluripotency-associated factors were bound to rDNA in mESCs, suggesting novel modes of rDNA regulation in pluripotent cells. Taken together, our results provide a detailed view of rDNA chromatin structure in an important model system and enable high-resolution comparison of rDNA regulation between mouse and human.

Directed polymerase evolution

Polymerases evolved in nature to synthesize DNA and RNA, and they underlie the storage and flow of genetic information in all cells. The availability of these enzymes for use at the bench has driven a revolution in biotechnology and medicinal research; however, polymerases did not evolve to function efficiently under the conditions required for some applications and their high substrate fidelity precludes their use for most applications that involve modified substrates. To circumvent these limitations, researchers have turned to directed evolution to tailor the properties and/or substrate repertoire of polymerases for different applications, and several systems have been developed for this purpose. These systems draw on different methods of creating a pool of randomly mutated polymerases and are differentiated by the process used to isolate the most fit members. A variety of polymerases have been evolved, providing new or improved functionality, as well as interesting new insight into the factors governing activity.

The emerging role of RNA polymerase I transcription machinery in human malignancy: a clinical perspective

Ribosome biogenesis – the complex and highly coordinated cellular process leading to the production of ribosomes – is strictly dependent on the activity of RNA polymerase I (Pol I) transcriptional machinery. Pol I activity is continually increased in proliferating cells to sustain the increased demand for ribosome production and protein synthesis, which are necessary for appropriate cell growth and division. The integrity of the process of ribosome biogenesis represents an important sensor of cellular stress: when this process is altered, a tumor suppressor response is triggered, which leads to proliferative arrest. The present review focuses on the possible implications of Pol I targeting in the treatment of human malignancies.

Genome-wide identification of cancer-related polyadenylated and non-polyadenylated RNAs in human breast and lung cell lines

Eukaryotic mRNAs consist of two forms of transcripts: poly(A)+ and poly(A)-, based on the presence or absence of poly(A) tails at the 3' end. Poly(A)+ mRNAs are mainly protein coding mRNAs, whereas the functions of poly(A)- mRNA are largely unknown. Previous studies have shown that a significant proportion of gene transcripts are poly(A)- or bimorphic (containing both poly(A)+ and poly(A)- transcripts). We compared the expression levels of poly(A)- and poly(A)+ RNA mRNAs in normal and cancer cell lines. We also investigated the potential functions of these RNA transcripts using an integrative workflow to explore poly(A)+ and poly(A)- transcriptome sequences between a normal human mammary gland cell line (HMEC) and a breast cancer cell line (MCF-7), as well as between a normal human lung cell line (NHLF) and a lung cancer cell line (A549). The data showed that normal and cancer cell lines differentially express these two forms of mRNA. Gene ontology (GO) annotation analyses hinted at the functions of these two groups of transcripts and grouped the differentially expressed genes according to the form of their transcript. The data showed that cell cycle-, apoptosis-, and cell death-related functions corresponded to most of the differentially expressed genes in these two forms of transcripts, which were also associated with the cancers. Furthermore, translational elongation and translation functions were also found for the poly(A)- protein-coding genes in cancer cell lines. We demonstrate that poly(A)- transcripts play an important role in cancer development.

Mechanistic studies on the synergistic cytotoxicity of the nucleoside analogs gemcitabine and clofarabine in multiple myeloma: relevance of p53 and its clinical implications

Hematopoietic stem cell transplantation (HSCT) is an established treatment for multiple myeloma (MM), a plasma cell malignancy. To identify an improved pretransplant conditioning regimen, we investigated the cytotoxicity of gemcitabine (Gem) and clofarabine (Clo) combinations toward MM cell lines and patient cell samples. A strong synergism of the two nucleoside analogs, when combined at their approximate IC10 concentrations, was observed. This synergism could be partly due to the observed Gem-mediated phosphorylation and activation of deoxycytidine kinase, resulting in enhanced phosphorylation of Gem and Clo. Their cytotoxicity correlated with a robust activation of the DNA damage response pathway. [Gem+Clo] decreased the mitochondrial membrane potential with a concomitant release of proapoptotic factors into the cytoplasm and nucleus and the activation of apoptosis. Exposure of MM cells to [Gem+Clo] also decreased the level of ribosomal RNA (rRNA), which might have resulted in nucleolar stress, as reported previously, and caused a p53-dependent cell death. A reduction by approximately 50% in the cytotoxicity of Gem and Clo was observed in the presence of pifithrin α, a p53 inhibitor. Furthermore, MM cell lines with mutant p53 exhibited greater resistance to Gem and Clo, supporting a role for the p53 protein in these cytotoxic responses. Our results provide a rationale for clinical trials incorporating [Gem+Clo] combinations as part of conditioning therapy for high-risk patients with MM undergoing HSCT.

Nucleophosmin mutations in acute myeloid leukemia: a tale of protein unfolding and mislocalization

Nucleophosmin (NPM1) is an abundant, ubiquitously expressed protein mainly localized at nucleoli but continuously shuttling between nucleus and cytoplasm. NPM1 plays a role in several cellular functions, including ribosome biogenesis and export, centrosome duplication, chromatin remodeling, DNA repair, and response to stress stimuli. Much of the interest in this protein arises from its relevance in human malignancies. NPM1 is frequently overexpressed in solid tumors and is the target of several chromosomal translocations in hematologic neoplasms. Notably, NPM1 has been characterized as the most frequently mutated gene in acute myeloid leukemia (AML). Mutations alter the C-terminal DNA-binding domain of the protein and result in its aberrant nuclear export and stable cytosolic localization. In this review, we focus on the leukemia-associated NPM1 C-terminal domain and describe its structure, function, and the effect exerted by leukemic mutations. Finally, we discuss the possibility to target NPM1 for the treatment of cancer and, in particular, of AML patients with mutated NPM1 gene.

Nucleophosmin mutations alter its nucleolar localization by impairing G-quadruplex binding at ribosomal DNA

Nucleophosmin (NPM1) is an abundant nucleolar protein implicated in ribosome maturation and export, centrosome duplication and response to stress stimuli. NPM1 is the most frequently mutated gene in acute myeloid leukemia. Mutations at the C-terminal domain led to variant proteins that aberrantly and stably translocate to the cytoplasm. We have previously shown that NPM1 C-terminal domain binds with high affinity G-quadruplex DNA. Here, we investigate the structural determinants of NPM1 nucleolar localization. We show that NPM1 interacts with several G-quadruplex regions found in ribosomal DNA, both in vitro and in vivo. Furthermore, the most common leukemic NPM1 variant completely loses this activity. This is the consequence of G-quadruplex–binding domain destabilization, as mutations aimed at refolding the leukemic variant also result in rescuing the G-quadruplex–binding activity and nucleolar localization. Finally, we show that treatment of cells with a G-quadruplex selective ligand results in wild-type NPM1 dislocation from nucleoli into nucleoplasm. In conclusion, this work establishes a direct correlation between NPM1 G-quadruplex binding at rDNA and its nucleolar localization, which is impaired in the acute myeloid leukemia-associated protein variants.

Basics of Molecular Biology

Molecular biology is the study of biology on molecular level. The field overlaps with areas of biology and chemistry, particularly genetics and biochemistry. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interactions between DNA (deoxyribonucleic acid), RNA (Ribonucleic acid) and protein biosynthesis as well as learning how these interactions are regulated^[1].

RNA polymerase backtracking in gene regulation and genome instability

RNA polymerase is a ratchet machine that oscillates between productive and backtracked states at numerous DNA positions. Since its first description 15 years ago, backtracking--the reversible sliding of RNA polymerase along DNA and RNA--has been implicated in many critical processes in bacteria and eukaryotes, including the control of transcription elongation, pausing, termination, fidelity, and genome instability.

Caveolae, caveolin, and cavins: potential targets for the treatment of cardiac disease

Caveolae are omega-shaped membrane invaginations present in essentially all cell types of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes, and fibroblasts. Numerous functions have been ascribed to this omega-shaped structure. Caveolae are enriched with different signaling molecules and ion channel regulatory proteins and function both in protein trafficking and signal transduction in these cell types. Caveolins are the structural proteins that are necessary for the formation of caveola membrane domains. Mechanistically, caveolins interact with a variety of downstream signaling molecules, as, for example, Src-family tyrosine kinase, p42/44 mitogen-activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS) and hold the signal transducers in the inactive condition until activated with proper stimulus. Caveolae are gradually acquiring increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy, atherosclerosis, and heart failure. At present, very little is known about the role of caveolae in cardiac function and dysfunction, although recent studies with caveolin knock-out mouse have shown that caveolae and caveolins play a pivotal role in various human pathobiological conditions. This review will discuss the possible role and mechanism of action of caveolae and caveolins in different cardiac diseases.

Expression of 5 S rRNA genes linked to 35 S rDNA in plants, their epigenetic modification and regulatory element divergence

BACKGROUND

In plants, the 5 S rRNA genes usually occur as separate tandems (S-type arrangement) or, less commonly, linked to 35 S rDNA units (L-type). The activity of linked genes remains unknown so far. We studied the homogeneity and expression of 5 S genes in several species from family Asteraceae known to contain linked 35 S-5 S units. Additionally, their methylation status was determined using bisulfite sequencing. Fluorescence in situ hybridization was applied to reveal the sub-nuclear positions of rDNA arrays.

RESULTS

We found that homogenization of L-type units went to completion in most (4/6) but not all species. Two species contained major L-type and minor S-type units (termed L(s)-type). The linked genes dominate 5 S rDNA expression while the separate tandems do not seem to be expressed. Members of tribe Anthemideae evolved functional variants of the polymerase III promoter in which a residing C-box element differs from the canonical angiosperm motif by as much as 30%. On this basis, a more relaxed consensus sequence of a plant C-box: (5'-RGSWTGGGTG-3') is proposed. The 5 S paralogs display heavy DNA methylation similarly as to their unlinked counterparts. FISH revealed the close association of 35 S-5 S arrays with nucleolar periphery indicating that transcription of 5 S genes may occur in this territory.

CONCLUSIONS

We show that the unusual linked arrangement of 5 S genes, occurring in several plant species, is fully compatible with their expression and functionality. This extraordinary 5 S gene dynamics is manifested at different levels, such as variation in intrachromosomal positions, unit structure, epigenetic modification and considerable divergence of regulatory motifs.

Where no RNA polymerase has gone before: novel functional transcripts derived from the ribosomal intergenic spacer

The nucleolus is organized around a scaffolding of rDNA tandem repeats. These repeats, known as ribosomal cassettes, are each composed of ribosomal RNA (rRNA) genes preceding a long intergenic spacer (IGS) that has been classically perceived to be transcriptionally silent. Recent study of the IGS has contradicted the dogma that these spacers are merely inert regions of the genome, instead suggesting they are biologically significant, complex and plurifunctional transcriptional units that appear central to proper cellular functioning. Through the timely induction of various ribosomal IGS noncoding RNA (IGS RNA) transcripts, the cell is capable of both regulating rRNA synthesis and sequestering large numbers of proteins, thereby modulating essential molecular networks. Here we discuss our current understanding of the organization and function of the IGS.

A RUNX2-HDAC1 co-repressor complex regulates rRNA gene expression by modulating UBF acetylation

The osteogenic and oncogenic transcription factor RUNX2 downregulates the RNA polymerase I (RNA Pol I)-mediated transcription of rRNAs and changes histone modifications associated with the rDNA repeat. However, the mechanisms by which RUNX2 suppresses rRNA transcription are not well understood. RUNX2 cofactors such as histone deacetylases (HDACs) play a key role in chromatin remodeling and regulation of gene transcription. Here, we show that RUNX2 recruits HDAC1 to the rDNA repeats in osseous cells. This recruitment alters the histone modifications associated with active rRNA-encoding genes and causes deacetylation of the protein upstream binding factor (UBF, also known as UBTF). Downregulation of RUNX2 expression reduces the localization of HDAC1 to the nucleolar periphery and also decreases the association between HDAC1 and UBF. Functionally, depletion of HDAC1 relieves the RUNX2-mediated repression of rRNA-encoding genes and concomitantly increases cell proliferation and global protein synthesis in osseous cells. Our findings collectively identify a RUNX2-HDAC1-dependent mechanism for the regulation of rRNA-encoding genes and suggest that there is plasticity to RUNX2-mediated epigenetic control, which is mediated through selective mitotic exclusion of co-regulatory factors.

Signaling mechanisms in the regulation of renal matrix metabolism in diabetes

Renal hypertrophy and accumulation of extracellular matrix proteins are among cardinal manifestations of diabetic nephropathy. TGF beta system has been implicated in the pathogenesis of these manifestations. Among signaling pathways activated in the kidney in diabetes, mTOR- (mammalian target of rapamycin-)regulated pathways are pivotal in orchestrating high glucose-induced production of ECM proteins leading to functional and structural changes in the kidney culminating in adverse outcomes. Understanding signaling pathways that influence individual matrix protein expression could lead to the development of new interventional strategies. This paper will highlight some of the diverse components of the signaling network stimulated by hyperglycemia with an emphasis on extracellular matrix protein metabolism in the kidney in diabetes.

Caveolae and the regulation of endocytosis

Although clathrin-mediated endocytosis constitutes the main pathway for internalization of extracellular ligands and plasma membrane components it has generally been accepted that other uptake mechanisms-caveolae-mediated and noncaveolar raft-dependent endocytosis-also exist. During the last 20 years many papers have been published about caveolar endocytosis. These studies have fundamentally changed our view about the endocytotic role of caveolae. Views that caveolae are permanently static structures1 have been extensively considered and rejected. Although the initial steps leading to the pinching off of caveolae from the plasma membrane have been studied in details, there are still contradictory data about the intracellular trafficking of caveolae. It is still not entirely clear whether caveolar endocytosis represents an uptake pathway with distinct cellular compartments to avoid lysosomal degradation or ligands taken up by caveolae can also be targeted to late endosomes/lysosomes.In this chapter, we summarize the data available about caveolar endocytosis focusing on the intracellular route of caveolae and we provide data supporting that caveolar endocytosis can join the classical endocytotic pathway.

Identification of DHX33 as a mediator of rRNA synthesis and cell growth

In this report, we employed a lentiviral RNA interference screen to discover nucleolar DEAD/DEAH-box helicases involved in RNA polymerase I (Pol I)-mediated transcriptional activity. Our screen identified DHX33 as an important modulator of 47S rRNA transcription. We show that DHX33 is a cell cycle-regulated nucleolar protein that associates with ribosomal DNA (rDNA) loci, where it interacts with the RNA Pol I transcription factor upstream binding factor (UBF). DHX33 knockdown decreased the association of Pol I with rDNA and caused a dramatic decrease in levels of rRNA synthesis. Wild-type DHX33 overexpression, but not a DNA binding-defective mutant, enhanced 47S rRNA synthesis by promoting the association of RNA polymerase I with rDNA loci. In addition, an NTPase-defective DHX33 mutant (K94R) acted as a dominant negative mutant, inhibiting endogenous rRNA synthesis. Moreover, DHX33 deficiency in primary human fibroblasts triggered a nucleolar p53 stress response, resulting in an attenuation of proliferation. Thus, we show the mechanistic importance of DHX33 in rRNA transcription and proliferation.

Identification and characterization of a novel ubiquitous nucleolar protein 'NARR' encoded by a gene overlapping the rab34 oncogene

There are only few reports on protein products originating from overlapping mammalian genes even though computational predictions suggest that an appreciable fraction of mammalian genes could potentially overlap. Mass spectrometry-based proteomics has now acquired the tools to probe proteins in an unbiased manner, providing direct evidence of the output of the genomic and gene expression machinery. In particular, proteomics can refine gene predictions and discover novel gene-processing events and gene arrangements. Here, we report the mass spectrometric discovery and biochemical validation of the novel protein encoded by a gene overlapping rab34 oncogene. The novel protein is highly conserved in mammals. In humans, it contains 13 distinct Nine-Amino acid Residue-Repeats (NARR) with the consensus sequence PRVIV(S/T)PR in which the serine or threonine residues are phosphorylated during M-phase. NARR is ubiquitously expressed and resides in nucleoli where it colocalizes with ribosomal DNA (rDNA) gene clusters. Its distribution only partially overlaps with upstream binding factor, one of the main regulators of RNA Polymerase I activity, and is entirely uncoupled from it in mitotic cells and upon inhibition of transcription. NARR only partially colocalizes with fibrillarin, the pre-ribosomal RNA-processing protein, positioning NARR in a separate niche within the rDNA cluster.

Single-molecule studies of transcription: from one RNA polymerase at a time to the gene expression profile of a cell

Single-molecule techniques have emerged as powerful tools for deciphering mechanistic details of transcription and have yielded discoveries that would otherwise have been impossible to make through the use of more traditional biochemical and/or biophysical techniques. Here, we provide a brief overview of single-molecule techniques most commonly used for studying RNA polymerase and transcription. We then present specific examples of single-molecule studies that have contributed to our understanding of key mechanistic details for each different stage of the transcription cycle. Finally, we discuss emerging single-molecule approaches and future directions, including efforts to study transcription at the single-molecule level in living cells.

Ribosomal biogenesis induction by high glucose requires activation of upstream binding factor in kidney glomerular epithelial cells

Diabetes promotes protein synthesis to induce kidney hypertrophy and increase renal matrix proteins. Increased capacity for mRNA translation by way of ribosomal biogenesis facilitates sustained stimulation of protein synthesis. We tested the hypothesis that high glucose induces ribosomal biogenesis as indicated by an increase in rRNA synthesis in the setting of augmented protein synthesis. High glucose (30 mM) increased global protein synthesis, expression of matrix proteins, laminin γ1 and fibronectin, and rDNA transcription in glomerular epithelial cells (GECs) compared with 5 mM glucose. High glucose induced Ser388 phosphorylation of upstream binding factor (UBF), an rDNA transcription factor, along with increased phosphorylation of Erk and p70S6 kinase. Inactivation of Erk and p70S6 kinase either by their respective chemical inhibitors or by expression of their inactive mutant constructs blocked high-glucose-induced UBF phosphorylation. High glucose reduced nuclear content of p19ARF and promoted dissolution of inactive UBF-p19ARF complex. High glucose also promoted association of UBF with RPA194, a subunit of RNA polymerase I. Inhibition of Erk, p70S6 kinase, and UBF1 by transfecting GECs with their respective inactive mutants abolished laminin γ1 synthesis, protein synthesis, and rDNA transcription. Renal cortex from type 1 diabetic rats and type 2 diabetic db/db mice showed increased phosphorylation of UBF, Erk, and p70S6 kinase coinciding with renal hypertrophy and onset of matrix accumulation. Our data suggest that augmented ribosome biogenesis occurs in an UBF-dependent manner during increased protein synthesis induced by high glucose in the GECs that correlates with UBF activation and renal hypertrophy in rodents with type 1 and type 2 diabetes.

Three-dimensional reconstruction of nucleolar components by electron microscope tomography

The nucleus is a complex volume constituted of numerous subcompartments in which specific functions take place due to a specific spatial organization of their molecular components. To understand how these molecules are spatially organized within these machineries, it is necessary to investigate their three-dimensional organization at high resolution. To reach this goal, electron tomography appears to be a method of choice; it can generate tomograms with a resolution of a few nanometers by using multiple projections of a tilted section several hundred to several thousand nanometers in thickness imaged by transmission electron microscopy (TEM).Specific identification of molecules of interest contained within such thick sections requires their specific immunocytochemical labelling using electron-dense markers. We recently demonstrated that electron tomography of proteins immunostained with nanogold particles before embedding, and subsequently amplified with silver, was very fruitful due to the inherently high spatial resolution of the medium-voltage scanning and transmission electron microscope (STEM). Here we describe this approach, which is very efficient for tracing the 3D organization of proteins within complex machineries by using antibodies raised against one of the proteins, or against GFP to analyse GFP-tagged proteins.

Nuclear-localized calcineurin homologous protein CHP1 interacts with upstream binding factor and inhibits ribosomal RNA synthesis

Calcineurin homologous protein 1 (CHP1) is a widely expressed, 22-kDa myristoylated EF-hand Ca(2+)-binding protein that shares a high degree of similarity with the regulatory B subunit of calcineurin (65%) and with calmodulin (59%). CHP1 localizes to the plasma membrane, the Golgi apparatus, and the nucleus and functions to regulate trafficking of early secretory vesicles, activation of T cells, and expression and transport of the Na-H exchanger NHE1. Although CHP1 contains nuclear export signals, whether its nuclear and cytoplasmic localization is regulated and has distinct functions remain unknown. We show that CHP1 is predominantly in the nucleus in quiescent fibroblasts, is translocated to cytoplasmic compartments with growth medium, and that translocation is inhibited by mutations in the nuclear export motifs. In a screen for proteins co-precipitating with CHP1 in quiescent cells we identified the upstream binding factor UBF, a DNA-binding protein and component of the RNA polymerase I complex regulating RNA synthesis. The CHP1-UBF interaction is restricted to the nucleus and inhibited by Ca(2+). Nuclear retention of CHP1 attenuates the abundance of UBF in the nucleolus and inhibits RNA synthesis when quiescent cells are transferred to growth medium. These data show UBF as a newly identified CHP1-binding protein and regulation of RNA synthesis as a newly identified function for nuclear-localized CHP1, which is distinct from CHP1 functions in the cytosol.

A comparison between ribo-minus RNA-sequencing and polyA-selected RNA-sequencing

To compare the two RNA-sequencing protocols, ribo-minus RNA-sequencing (rmRNA-seq) and polyA-selected RNA-sequencing (mRNA-seq), we acquired transcriptomic data-52 and 32 million alignable reads of 35 bases in length-from the mouse cerebrum, respectively. We found that a higher proportion, 44% and 25%, of the uniquely alignable rmRNA-seq reads, is in intergenic and intronic regions, respectively, as compared to 23% and 15% from the mRNA-seq dataset. Further analysis made an additional discovery of transcripts of protein-coding genes (such as Histone, Heg1, and Dux), ncRNAs, snoRNAs, snRNAs, and novel ncRNAs as well as repeat elements in rmRNA-seq dataset. This result suggests that rmRNA-seq method should detect more polyA- or bimorphic transcripts. Finally, through comparative analyses of gene expression profiles among multiple datasets, we demonstrated that different RNA sample preparations may result in significant variations in gene expression profiles.

The RNA polymerase-associated factor 1 complex (Paf1C) directly increases the elongation rate of RNA polymerase I and is required for efficient regulation of rRNA synthesis

The rate of ribosome synthesis is proportional to the rate of cell proliferation; thus, transcription of rRNA by RNA polymerase I (Pol I) is an important target for the regulation of this process. Most previous investigations into mechanisms that regulate the rate of ribosome synthesis have focused on the initiation step of transcription by Pol I; however, recent studies in yeast and mammals have identified factors that influence transcription elongation by Pol I. The RNA polymerase-associated factor 1 complex (Paf1C) is a transcription elongation factor with known roles in Pol II transcription. We previously identified a role for Paf1C in transcription elongation by Pol I. In this study, genetic interactions between genes for Paf1C and Pol I subunits confirm this conclusion. In vitro studies demonstrate that purified Paf1C directly increases the rate of transcription elongation by Pol I. Finally, we show that Paf1C function is required for efficient control of Pol I transcription in response to target of rapamycin (TOR) signaling or amino acid limitation. These studies demonstrate that Paf1C plays an important direct role in cellular control of rRNA expression.