Telomerase RNA accumulates in Cajal bodies in human cancer cells

Methods that shaped telomerase research

Telomerase, the ribonucleoprotein (RNP) responsible for telomere maintenance, has a complex life. Complex in that it is made of multiple proteins and an RNA, and complex because it undergoes many changes, and passes through different cell compartments. As such, many methods have been developed to discover telomerase components, delve deep into understanding its structure and function and to figure out how telomerase biology ultimately relates to human health and disease. While some old gold-standard methods are still key for determining telomere length and measuring telomerase activity, new technologies are providing promising new ways to gain detailed information that we have never had access to before. Therefore, we thought it timely to briefly review the methods that have revealed information about the telomerase RNP and outline some of the remaining questions that could be answered using new methodology.

Fyn-mediated phosphorylation of Menin disrupts telomere maintenance in stem cells

Telomeres protect chromosome ends and determine the replication potential of dividing cells. The canonical telomere sequence TTAGGG is synthesized by telomerase holoenzyme, which maintains telomere length in proliferative stem cells. Although the core components of telomerase are well-defined, mechanisms of telomerase regulation are still under investigation. We report a novel role for the Src family kinase Fyn, which disrupts telomere maintenance in stem cells by phosphorylating the scaffold protein Menin. We found that Fyn knockdown prevented telomere erosion in human and mouse stem cells, validating the results with four telomere measurement techniques. We show that Fyn phosphorylates Menin at tyrosine 603 (Y603), which increases Menin's SUMO1 modification, C-terminal stability, and importantly, its association with the telomerase RNA component (TR). Using mass spectrometry, immunoprecipitation, and immunofluorescence experiments we found that SUMO1-Menin decreases TR's association with telomerase subunit Dyskerin, suggesting that Fyn's phosphorylation of Menin induces telomerase subunit mislocalization and may compromise telomerase function at telomeres. Importantly, we find that Fyn inhibition reduces accelerated telomere shortening in human iPSCs harboring mutations for dyskeratosis congenita.

A Keratin 12 Expression-Based Analysis of Stem-Precursor Cells and Differentiation in the Limbal-Corneal Epithelium Using Single-Cell RNA-Seq Data

The corneal epithelium (CE) is spread between two domains, the outer vascularized limbus and the avascular cornea proper. Epithelial cells undergo constant migration from the limbus to the vision-critical central cornea. Coordinated with this migration, the cells undergo differentiation changes where a pool of unique stem/precursor cells at the limbus yields the mature cells that reach the corneal center. Differentiation is heralded by the expression of the corneal-specific Krt12. Processing data acquired by scRNA-Seq showed that the increase in Krt12 expression occurs in four distinct steps within the limbus, plus a single continuous increase in the cornea. Differential gene analysis demonstrated that these domains reflect discreet stages of CE differentiation and yielded extensive information of the genes undergoing down- or upregulation in the sequential transition from less to more differentiate conditions. The approach allowed the identification of multiple gene cohorts, including (a) the genes which have maximal expression in the most primitive, Krt12-negative cell cohort, which is likely to include the stem/precursor cells; (b) the sets of genes that undergo continuous increase or decrease along the whole differentiation path; and (c) the genes showing maximal positive or negative correlation with the changes in Krt12.

Coilin and Cajal bodies

The nucleus of higher eukaryotes contains a number of structures that concentrate specific biomolecules and play distinct roles in nuclear metabolism. In recent years, the molecular mechanisms controlling their formation have been intensively studied. In this brief review, I focus on coilin and Cajal bodies. Coilin is a key scaffolding protein of Cajal bodies that is evolutionarily conserved in metazoans. Cajal bodies are thought to be one of the archetypal nuclear structures involved in the metabolism of several short non-coding nuclear RNAs. Yet surprisingly little is known about the structure and function of coilin, and a comprehensive model to explain the origin of Cajal bodies is also lacking. Here, I summarize recent results on Cajal bodies and coilin and discuss them in the context of the last three decades of research in this field.

Large-scale assessment of pros and cons of autopsy-derived or tumor-matched tissues as the norms for gene expression analysis in cancers

Normal tissues are essential for studying disease-specific differential gene expression. However, healthy human controls are typically available only in postmortal/autopsy settings. In cancer research, fragments of pathologically normal tissue adjacent to tumor site are frequently used as the controls. However, it is largely underexplored how cancers can systematically influence gene expression of the neighboring tissues. Here we performed a comprehensive pan-cancer comparison of molecular profiles of solid tumor-adjacent and autopsy-derived "healthy" normal tissues. We found a number of systemic molecular differences related to activation of the immune cells, intracellular transport and autophagy, cellular respiration, telomerase activation, p38 signaling, cytoskeleton remodeling, and reorganization of the extracellular matrix. The tumor-adjacent tissues were deficient in apoptotic signaling and negative regulation of cell growth including G2/M cell cycle transition checkpoint. We also detected an extensive rearrangement of the chemical perception network. Molecular targets of 32 and 37 cancer drugs were over- or underexpressed, respectively, in the tumor-adjacent norms. These processes may be driven by molecular events that are correlated between the paired cancer and adjacent normal tissues, that mostly relate to inflammation and regulation of intracellular molecular pathways such as the p38, MAPK, Notch, and IGF1 signaling. However, using a model of macaque postmortal tissues we showed that for the 30 min - 24-hour time frame at 4ºC, an RNA degradation pattern in lung biosamples resulted in an artifact "differential" expression profile for 1140 genes, although no differences could be detected in liver. Thus, such concerns should be addressed in practice.

Post-Transcriptional and Post-Translational Modifications in Telomerase Biogenesis and Recruitment to Telomeres

Telomere length is associated with the proliferative potential of cells. Telomerase is an enzyme that elongates telomeres throughout the entire lifespan of an organism in stem cells, germ cells, and cells of constantly renewed tissues. It is activated during cellular division, including regeneration and immune responses. The biogenesis of telomerase components and their assembly and functional localization to the telomere is a complex system regulated at multiple levels, where each step must be tuned to the cellular requirements. Any defect in the function or localization of the components of the telomerase biogenesis and functional system will affect the maintenance of telomere length, which is critical to the processes of regeneration, immune response, embryonic development, and cancer progression. An understanding of the regulatory mechanisms of telomerase biogenesis and activity is necessary for the development of approaches toward manipulating telomerase to influence these processes. The present review focuses on the molecular mechanisms involved in the major steps of telomerase regulation and the role of post-transcriptional and post-translational modifications in telomerase biogenesis and function in yeast and vertebrates.

Emerging Implications of Phase Separation in Cancer

In eukaryotic cells, biological activities are executed in distinct cellular compartments or organelles. Canonical organelles with membrane-bound structures are well understood. Cells also inherently contain versatile membrane-less organelles (MLOs) that feature liquid or gel-like bodies. A biophysical process termed liquid-liquid phase separation (LLPS) elucidates how MLOs form through dynamic biomolecule assembly. LLPS-related molecules often have multivalency, which is essential for low-affinity inter- or intra-molecule interactions to trigger phase separation. Accumulating evidence shows that LLPS concentrates and organizes desired molecules or segregates unneeded molecules in cells. Thus, MLOs have tunable functional specificity in response to environmental stimuli and metabolic processes. Aberrant LLPS is widely associated with several hallmarks of cancer, including sustained proliferative signaling, growth suppressor evasion, cell death resistance, telomere maintenance, DNA damage repair, etc. Insights into the molecular mechanisms of LLPS provide new insights into cancer therapeutics. Here, the current understanding of the emerging concepts of LLPS and its involvement in cancer are comprehensively reviewed.

How Structural Features Define Biogenesis and Function of Human Telomerase RNA Primary Transcript

Telomerase RNA has been uncovered as a component of the telomerase enzyme, which acts as a reverse transcriptase and maintains the length of telomeres in proliferated eukaryotic cells. Telomerase RNA is considered to have major functions as a template for telomeric repeat synthesis and as a structural scaffold for telomerase. However, investigations of its biogenesis and turnover, as well as structural data, have provided evidence of functions of telomerase RNA that are not associated with telomerase activity. The primary transcript produced from the human telomerase RNA gene encodes for the hTERP protein, which presents regulatory functions related to autophagy, cellular proliferation, and metabolism. This review focuses on the specific features relating to the biogenesis and structure of human telomerase RNA that support the existence of an isoform suitable for functioning as an mRNA. We believe that further investigation into human telomerase RNA biogenesis mechanisms will provide more levels for manipulating cellular homeostasis, survival, and transformation mechanisms, and may contribute to a deeper understanding of the mechanisms of aging.

Regulation of Cellular Ribonucleoprotein Granules: From Assembly to Degradation via Post-translational Modification

Cells possess membraneless ribonucleoprotein (RNP) granules, including stress granules, processing bodies, Cajal bodies, or paraspeckles, that play physiological or pathological roles. RNP granules contain RNA and numerous RNA-binding proteins, transiently formed through the liquid–liquid phase separation. The assembly or disassembly of numerous RNP granules is strongly controlled to maintain their homeostasis and perform their cellular functions properly. Normal RNA granules are reversibly assembled, whereas abnormal RNP granules accumulate and associate with various neurodegenerative diseases. This review summarizes current studies on the physiological or pathological roles of post-translational modifications of various cellular RNP granules and discusses the therapeutic methods in curing diseases related to abnormal RNP granules by autophagy.

Molecular Mechanisms of lncRNAs in the Dependent Regulation of Cancer and Their Potential Therapeutic Use

Deep whole genome and transcriptome sequencing have highlighted the importance of an emerging class of non-coding RNA longer than 200 nucleotides (i.e., long non-coding RNAs (lncRNAs)) that are involved in multiple cellular processes such as cell differentiation, embryonic development, and tissue homeostasis. Cancer is a prime example derived from a loss of homeostasis, primarily caused by genetic alterations both in the genomic and epigenetic landscape, which results in deregulation of the gene networks. Deregulation of the expression of many lncRNAs in samples, tissues or patients has been pointed out as a molecular regulator in carcinogenesis, with them acting as oncogenes or tumor suppressor genes. Herein, we summarize the distinct molecular regulatory mechanisms described in literature in which lncRNAs modulate carcinogenesis, emphasizing epigenetic and genetic alterations in particular. Furthermore, we also reviewed the current strategies used to block lncRNA oncogenic functions and their usefulness as potential therapeutic targets in several carcinomas.

Post-transcriptional diversity in riboproteins and RNAs in aging and cancer

Post-transcriptional (PtscM) and post-translational (PtrnM) modifications of nucleotides and amino acids are covalent modifications able to change physio-chemical properties of RNAs and proteins. In the ribosome, the adequate assembly of rRNAs and ribosomal protein subunits in the nucleolus ensures suitable translational activity, with protein synthesis tuned according to intracellular demands of energy production, replication, proliferation, and growth. Disruption in the regulatory control of PtscM and PtrnM can impair ribosome biogenesis and ribosome function. Ribosomal impairment may, in turn, impact the synthesis of proteins engaged in functions as varied as telomere maintenance, apoptosis, and DNA repair, as well as intersect with mitochondria and telomerase activity. These cellular processes often malfunction in carcinogenesis and senescence. Here we discuss regulatory mechanisms of PtscMs and PtrnMs on ribosomal function. We also address chemical modification in rRNAs and their impacts on cellular metabolism, replication control, and senescence. Further, we highlight similarities and differences of PtscMs and PtrnMs in ribosomal intermediates during aging and carcinogenesis. Understanding these regulatory mechanisms may uncover critical steps for the development of more efficient oncologic and anti-aging therapies.

Integrated evaluation of telomerase activation and telomere maintenance across cancer cell lines

In cancer, telomere maintenance is critical for the development of replicative immortality. Using genome sequences from the Cancer Cell Line Encyclopedia and Genomics of Drug Sensitivity in Cancer Project, we calculated telomere content across 1299 cancer cell lines. We find that telomerase reverse transcriptase (TERT) expression correlates with telomere content in lung, central nervous system, and leukemia cell lines. Using CRISPR/Cas9 screening data, we show that lower telomeric content is associated with dependency of CST telomere maintenance genes. Increased dependencies of shelterin members are associated with wild-type TP53 status. Investigating the epigenetic regulation of TERT, we find widespread allele-specific expression in promoter-wildtype contexts. TERT promoter-mutant cell lines exhibit hypomethylation at PRC2-repressed regions, suggesting a cooperative global epigenetic state in the reactivation of telomerase. By incorporating telomere content with genomic features across comprehensively characterized cell lines, we provide further insights into the role of telomere regulation in cancer immortality.

Arabidopsis retains vertebrate-type telomerase accessory proteins via a plant-specific assembly

The recent discovery of the bona-fide telomerase RNA (TR) from plants reveals conserved and unique secondary structure elements and the opportunity for new insight into the telomerase RNP. Here we examine how two highly conserved proteins previously implicated in Arabidopsis telomere maintenance, AtPOT1a and AtNAP57 (dyskerin), engage plant telomerase. We report that AtPOT1a associates with Arabidopsis telomerase via interaction with TERT. While loss of AtPOT1a does not impact AtTR stability, the templating domain is more accessible in pot1a mutants, supporting the conclusion that AtPOT1a stimulates telomerase activity but does not facilitate telomerase RNP assembly. We also show, that despite the absence of a canonical H/ACA binding motif within AtTR, dyskerin binds AtTR with high affinity and specificity in vitro via a plant specific three-way junction (TWJ). A core element of the TWJ is the P1a stem, which unites the 5′ and 3′ ends of AtTR. P1a is required for dyskerin-mediated stimulation of telomerase repeat addition processivity in vitro, and for AtTR accumulation and telomerase activity in vivo. The deployment of vertebrate-like accessory proteins and unique RNA structural elements by Arabidopsis telomerase provides a new platform for exploring telomerase biogenesis and evolution.

Chemical inhibition of PAPD5/7 rescues telomerase function and hematopoiesis in dyskeratosis congenita

Dyskeratosis congenita (DC) is a pediatric bone marrow failure syndrome caused by germline mutations in telomere biology genes. Mutations in DKC1 (the most commonly mutated gene in DC), the 3' region of TERC, and poly(A)-specific ribonuclease (PARN) cause reduced levels of the telomerase RNA component (TERC) by reducing its stability and accelerating TERC degradation. We have previously shown that depleting wild-type DKC1 levels by RNA interference or expression of the disease-associated A353V mutation in the DKC1 gene leads to decay of TERC, modulated by 3'-end oligoadenylation by noncanonical poly(A) polymerase 5 (PAPD5) followed by 3' to 5' degradation by EXOSC10. Furthermore, the constitutive genetic silencing of PAPD5 is sufficient to rescue TERC levels, restore telomerase function, and elongate telomeres in DKC1_A353V mutant human embryonic stem cells (hESCs). Here, we tested a novel PAPD5/7 inhibitor (RG7834), which was originally discovered in screens against hepatitis B viral loads in hepatic cells. We found that treatment with RG7834 rescues TERC levels, restores correct telomerase localization in DKC1 and PARN-depleted cells, and is sufficient to elongate telomeres in DKC1_A353V hESCs. Finally, treatment with RG7834 significantly improved definitive hematopoietic potential from DKC1_A353V hESCs, indicating that the chemical inhibition of PAPD5 is a potential therapy for patients with DC and reduced TERC levels.

Loss of Human TGS1 Hypermethylase Promotes Increased Telomerase RNA and Telomere Elongation

Biogenesis of the human telomerase RNA (hTR) involves a complex series of posttranscriptional modifications, including hypermethylation of the 5' mono-methylguanosine cap to a tri-methylguanosine cap (TMG). How the TMG cap affects hTR maturation is unknown. Here, we show that depletion of trimethylguanosine synthase 1 (TGS1), the enzyme responsible for cap hypermethylation, increases levels of hTR and telomerase. Diminished trimethylation increases hTR association with the cap-binding complex (CBC) and with Sm chaperone proteins. Loss of TGS1 causes an increase in accumulation of mature hTR in both the nucleus and the cytoplasm compared with controls. In TGS1 mutant cells, increased hTR assembles with telomerase reverse transcriptase (TERT) protein to yield elevated active telomerase complexes and increased telomerase activity, resulting in telomere elongation in cultured human cells. Our results show that TGS1-mediated hypermethylation of the hTR cap inhibits hTR accumulation, restrains levels of assembled telomerase, and limits telomere elongation.

Regulation of human telomerase RNA biogenesis and localization

Maintenance of telomeres is essential for genome integrity and replicative capacity in eukaryotic cells. Telomerase, the ribonucleoprotein complex that catalyses telomere synthesis is minimally composed of a reverse transcriptase and an RNA component. The sequence and structural domains of human telomerase RNA (hTR) have been extensively characterized, while the regulation of hTR transcription, maturation, and localization, is not fully understood. Here, we provide an up-to-date review of hTR, with an emphasis on current breakthroughs uncovering the mechanisms of hTR maturation and localization.

Global Transcriptomic Analyses Reveal Genes Involved in Conceptus Development During the Implantation Stages in Pigs

Prenatal mortality remains a significant concern to the pig farming industry around the world. Spontaneous fetal loss ranging from 20 to 45% by term occur after fertilization, with most of the loss happening during the implantation period. Since the factors regulating the high mortality rates of early conceptus during implantation phases are poorly understood, we sought to analyze the overall gene expression changes during this period, and identify the molecular mechanisms involved in conceptus development. This work employed Illumina's next-generation sequencing (RNA-Seq) and quantitative real-time PCR to analyze differentially expressed genes (DEGs). Soft clustering was subsequently used for the cluster analysis of gene expression. We identified 8236 DEGs in porcine conceptus at day 9, 12, and 15 of pregnancy. Annotation analysis of these genes revealed rRNA processing (GO:0006364), cell adhesion (GO:1904874), and heart development (GO:0007507), as the most significantly enriched biological processes at day 9, 12, and 15 of pregnancy, respectively. In addition, we found various genes, such as T-complex 1, RuvB-like AAA ATPase 2, connective tissue growth factor, integrins, interferon gamma, SLA-1, chemokine ligand 9, PAG-2, transforming growth factor beta receptor 1, and Annexin A2, that play essential roles in conceptus morphological development and implantation in pigs. Furthermore, we investigated the function of PAG-2 in vitro and found that PAG-2 can inhibit trophoblast cell proliferation and migration. Our analysis provides a valuable resource for understanding the mechanisms of conceptus development and implantation in pigs.

Folding heterogeneity in the essential human telomerase RNA three-way junction

Telomeres safeguard the genome by suppressing illicit DNA damage responses at chromosome termini. To compensate for incomplete DNA replication at telomeres, most continually dividing cells, including many cancers, express the telomerase ribonucleoprotein (RNP) complex. Telomerase maintains telomere length by catalyzing de novo synthesis of short DNA repeats using an internal telomerase RNA (TR) template. TRs from diverse species harbor structurally conserved domains that contribute to RNP biogenesis and function. In vertebrate TRs, the conserved regions 4 and 5 (CR4/5) fold into a three-way junction (TWJ) that binds directly to the telomerase catalytic protein subunit and is required for telomerase function. We have analyzed the structural properties of the human TR (hTR) CR4/5 domain using a combination of in vitro chemical mapping, secondary structural modeling, and single-molecule structural analysis. Our data suggest the essential P6.1 stem-loop within CR4/5 is not stably folded in the absence of the telomerase reverse transcriptase in vitro. Rather, the hTR CR4/5 domain adopts a heterogeneous ensemble of conformations. Finally, single-molecule FRET measurements of CR4/5 and a mutant designed to stabilize the P6.1 stem demonstrate that TERT binding selects for a structural conformation of CR4/5 that is not the dominant state of the TERT-free in vitro RNA ensemble.

A single-molecule view of telomerase regulation at telomeres

Telomerase plays a key role in the immortalization of cancer cells by maintaining telomeres length. Using single-molecule imaging of telomerase RNA molecules in cancer cells, we recently reported novel insights into the role of Cajal bodies in telomerase biogenesis and the regulation of telomerase recruitment to telomeres.

Co-methylation analysis in lung tissue identifies pathways for fetal origins of COPD

COPD likely has developmental origins; however, the underlying molecular mechanisms are not fully identified. Investigation of lung tissue-specific epigenetic modifications such as DNA methylation using network approaches might facilitate insights linking in utero smoke (IUS) exposure and risk for COPD in adulthood.We performed genome-wide methylation profiling for adult lung DNA from 160 surgical samples and 78 fetal lung DNA samples isolated from discarded tissue at 8-18 weeks of gestation. Co-methylation networks were constructed to identify preserved modules that shared methylation patterns in fetal and adult lung tissues and associations with fetal IUS exposure, gestational age and COPD.Weighted correlation networks highlighted preserved and co-methylated modules for both fetal and adult lung data associated with fetal IUS exposure, COPD and lower adult lung function. These modules were significantly enriched for genes involved in embryonic organ development and specific inflammation-related pathways, including Hippo, phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT), Wnt, mitogen-activated protein kinase and transforming growth factor-β signalling. Gestational age-associated modules were remarkably preserved for COPD and lung function, and were also annotated to genes enriched for the Wnt and PI3K/AKT pathways.Epigenetic network perturbations in fetal lung tissue exposed to IUS and of early lung development recapitulated in adult lung tissue from ex-smokers with COPD. Overlapping fetal and adult lung tissue network modules highlighted putative disease pathways supportive of exposure-related and age-associated developmental origins of COPD.

Composition and Function of Telomerase-A Polymerase Associated with the Origin of Eukaryotes

The canonical DNA polymerases involved in the replication of the genome are unable to fully replicate the physical ends of linear chromosomes, called telomeres. Chromosomal termini thus become shortened in each cell cycle. The maintenance of telomeres requires telomerase—a specific RNA-dependent DNA polymerase enzyme complex that carries its own RNA template and adds telomeric repeats to the ends of chromosomes using a reverse transcription mechanism. Both core subunits of telomerase—its catalytic telomerase reverse transcriptase (TERT) subunit and telomerase RNA (TR) component—were identified in quick succession in Tetrahymena more than 30 years ago. Since then, both telomerase subunits have been described in various organisms including yeasts, mammals, birds, reptiles and fish. Despite the fact that telomerase activity in plants was described 25 years ago and the TERT subunit four years later, a genuine plant TR has only recently been identified by our group. In this review, we focus on the structure, composition and function of telomerases. In addition, we discuss the origin and phylogenetic divergence of this unique RNA-dependent DNA polymerase as a witness of early eukaryotic evolution. Specifically, we discuss the latest information regarding the recently discovered TR component in plants, its conservation and its structural features.

Immunostaining of telomerase in embryonic and juvenile feather follicle of the chick labels proliferating cells for feather formation

Feathers regenerate through proliferation of cells derived from follicle stem cells. Immunoloblotting for telomerase in chick embryonic and juvenile feathers shows immunopositive bands around 100 kDa, 75 and 60 kDa only in embryonic feathers, indicating fragmentation of the protein due to physiological processing or artifacts derived from protein extraction. Immunolabeling for telomerase is present in the cytoplasm and nuclei of cells of the collar epithelium and bulge located in the follicle, and in sparse cells of the dermal papilla. PCNA-immunolabeling indicates that the collar and dermal papilla contain numerous proliferating cells, including the ramogenic zone where barb ridges are formed. Ultrastructural labeling indicates that a telomerase-like protein or its fragment is localized in nucleoli and in sparse nuclear clumps, likely representing Cajal bodies. The cytoplasm shows sparse immune-gold particles, also associated to mitochondria and sparse keratin filaments. An intense labeling is present in some areas of condensing chromosomes in dividing cells. Since telomerase positive cells are also seen in suprabasal layers of the collar epithelium and in the ramogenic zone, it is suggested that they represent dividing cells, most likely transit amplifying cells that give rise to the corneocytes of feathers. The significance of telomerase localization in chromatin is unknown.

Imaging of Telomerase RNA by Single-Molecule Inexpensive FISH Combined with Immunofluorescence

Fluorescent in situ hybridization (FISH) on the RNA moiety of human telomerase (hTR) with 50-mer probes detects hTR RNA accumulated in Cajal bodies. Using both live-cell imaging and single-molecule inexpensive FISH, our published work revealed that only a fraction of hTR localizes to Cajal bodies, with the majority of hTR molecules distributed throughout the nucleoplasm. This protocol is an application guide to the smiFISH method for the dual detection of hTR RNA and telomeres or Cajal bodies by immunofluorescence.

For complete details on the use and execution of this protocol, please refer to Laprade et al. (2020).

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RNA smiFISH with multiple small probes reveals single molecules of hTR RNA in nucleus

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The smiFISH technique is compatible with immunofluorescence for colocalization assay

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Colocalized regions can be mapped in 3D images with the open source 3D ImageJ Suite

Telomerase Biogenesis and Activities from the Perspective of Its Direct Interacting Partners

Telomerase reverse transcriptase (TERT)—the catalytic subunit of telomerase—is reactivated in up to 90% of all human cancers. TERT is observed in heterogenous populations of protein complexes, which are dynamically regulated in a cell type- and cell cycle-specific manner. Over the past two decades, in vitro protein–protein interaction detection methods have discovered a number of endogenous TERT binding partners in human cells that are responsible for the biogenesis and functionalization of the telomerase holoenzyme, including the processes of TERT trafficking between subcellular compartments, assembly into telomerase, and catalytic action at telomeres. Additionally, TERT have been found to interact with protein species with no known telomeric functions, suggesting that these complexes may contribute to non-canonical activities of TERT. Here, we survey TERT direct binding partners and discuss their contributions to TERT biogenesis and functions. The goal is to review the comprehensive spectrum of TERT pro-malignant activities, both telomeric and non-telomeric, which may explain the prevalence of its upregulation in cancer.

Regulation of human telomerase in homeostasis and disease

Telomerase is a ribonucleoprotein complex, the catalytic core of which includes the telomerase reverse transcriptase (TERT) and the non-coding human telomerase RNA (hTR), which serves as a template for the addition of telomeric repeats to chromosome ends. Telomerase expression is restricted in humans to certain cell types, and telomerase levels are tightly controlled in normal conditions. Increased levels of telomerase are found in the vast majority of human cancers, and we have recently begun to understand the mechanisms by which cancer cells increase telomerase activity. Conversely, germline mutations in telomerase-relevant genes that decrease telomerase function cause a range of genetic disorders, including dyskeratosis congenita, idiopathic pulmonary fibrosis and bone marrow failure. In this Review, we discuss the transcriptional regulation of human TERT, hTR processing, assembly of the telomerase complex, the cellular localization of telomerase and its recruitment to telomeres, and the regulation of telomerase activity. We also discuss the disease relevance of each of these steps of telomerase biogenesis.

Radiolabeled Oligonucleotides Targeting the RNA Subunit of Telomerase Inhibit Telomerase and Induce DNA Damage in Telomerase-Positive Cancer Cells

Telomerase is expressed in the majority (>85%) of tumors, but has restricted expression in normal tissues. Long-term telomerase inhibition in malignant cells results in progressive telomere shortening and reduction in cell proliferation. Here we report the synthesis and characterization of radiolabeled oligonucleotides that target the RNA subunit of telomerase, hTR, simultaneously inhibiting enzymatic activity and delivering radiation intracellularly. Oligonucleotides complementary (Match) and noncomplementary (Scramble or Mismatch) to hTR were conjugated to diethylenetriaminepentaacetic dianhydride (DTPA), allowing radiolabeling with the Auger electron-emitting radionuclide indium-111 (111In). Match oligonucleotides inhibited telomerase activity with high potency, which was not observed with Scramble or Mismatch oligonucleotides. DTPA-conjugation and 111In-labeling did not change telomerase inhibition. In telomerase-positive cancer cells, unlabeled Match oligonucleotides had no effect on survival, however, 111In-labeled Match oligonucleotides significantly reduced clonogenic survival and upregulated the DNA damage marker γH2AX. Minimal radiotoxicity and DNA damage was observed in telomerase-negative cells exposed to 111In-Match oligonucleotides. Match oligonucleotides localized in close proximity to nuclear Cajal bodies in telomerase-positive cells. In comparison with Match oligonucleotides, 111In-Scramble or 111In-Mismatch oligonucleotides demonstrated reduced retention and negligible impact on cell survival. This study indicates the therapeutic activity of radiolabeled oligonucleotides that specifically target hTR through potent telomerase inhibition and DNA damage induction in telomerase-expressing cancer cells and paves the way for the development of novel oligonucleotide radiotherapeutics targeting telomerase-positive cancers. SIGNIFICANCE: These findings present a novel radiolabeled oligonucleotide for targeting telomerase-positive cancer cells that exhibits dual activity by simultaneously inhibiting telomerase and promoting radiation-induced genomic DNA damage.

TOE1 acts as a 3' exonuclease for telomerase RNA and regulates telomere maintenance

In human cells, telomeres are elongated by the telomerase complex that contains the reverse transcriptase hTERT and RNA template TERC/hTR. Poly(A)-specific ribonuclease (PARN) is known to trim hTR precursors by removing poly(A) tails. However, the precise mechanism of hTR 3′ maturation remains largely unknown. Target of Egr1 (TOE1) is an Asp-Glu-Asp-Asp (DEDD) domain containing deadenylase that is mutated in the human disease Pontocerebella Hypoplasia Type 7 (PCH7) and implicated in snRNA and hTR processing. We have previously found TOE1 to localize specifically in Cajal bodies, where telomerase RNP complex assembly takes place. In this study, we showed that TOE1 could interact with hTR and the telomerase complex. TOE1-deficient cells accumulated hTR precursors, including oligoadenylated and 3′-extended forms, which was accompanied by impaired telomerase activity and shortened telomeres. Telomerase activity in TOE1-deficient cells could be rescued by wild-type TOE1 but not the catalytically inactive mutant. Our results suggest that hTR 3′ end processing likely involves multiple exonucleases that work in parallel and/or sequentially, where TOE1 may function non-redundantly as a 3′-to-5′ exonuclease in conjunction with PARN. Our study highlights a mechanistic link between TOE1 mutation, improper hTR processing and telomere dysfunction in diseases such as PCH7.

The PinX1/NPM interaction associates with hTERT in early-S phase and facilitates telomerase activation

Background

Telomere maintenance is an important factor in tumorigenesis. PinX1 is a potent telomerase regulator which also involves in telomerase loading to telomeres. Nucleophosmin (NPM) can partially attenuate PinX1 inhibition of telomerase activity and NPM loading to hTERT requires PinX1. However, the role of the PinX1/NPM interaction in telomerase activity is not fully understood.

Method

The long-term effects of PinX1 and NPM down-regulation on telomere length were investigated by TRF assay. The localization of the PinX1/NPM association and the NPM/PinX1/hTERT complex formation were examined by immunofluorescence studies.

Results

Concurrent long-term down-regulation of PinX1 and NPM led to a substantial decrease in telomere length. The interaction with PinX1 was crucial in NPM localization in the nucleolus during the S phase. PinX1 and NPM associated throughout S phase and the NPM/PinX1/hTERT complex formation peaked during the early-S phase. The PinX1/NPM interaction was shown to localize away from Cajal Bodies at the start of S phase.

Conclusion

PinX1/NPM interaction is important in telomerase regulation during catalysis. NPM is recruited to hTERT by PinX1 and is required in the proposed telomerase modulating unit to activate telomerase when telomere extension occurs during S phase.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0306-y) contains supplementary material, which is available to authorized users.

Alteration of 28S rRNA 2'-O-methylation by etoposide correlates with decreased SMN phosphorylation and reduced Drosha levels

The most common types of modification in human rRNA are pseudouridylation and 2'-O ribose methylation. These modifications are performed by small nucleolar ribonucleoproteins (snoRNPs) which contain a guide RNA (snoRNA) that base pairs at specific sites within the rRNA to direct the modification. rRNA modifications can vary, generating ribosome heterogeneity. One possible method that can be used to regulate rRNA modifications is by controlling snoRNP activity. RNA fragments derived from some small Cajal body-specific RNAs (scaRNA 2, 9 and 17) may influence snoRNP activity. Most scaRNAs accumulate in the Cajal body - a subnuclear domain - where they participate in the biogenesis of small nuclear RNPs, but scaRNA 2, 9 and 17 generate nucleolus-enriched fragments of unclear function, and we hypothesize that these fragments form regulatory RNPs that impact snoRNP activity and modulate rRNA modifications. Our previous work has shown that SMN, Drosha and various stresses, including etoposide treatment, may alter regulatory RNP formation. Here we demonstrate that etoposide treatment decreases the phosphorylation of SMN, reduces Drosha levels and increases the 2'-O-methylation of two sites within 28S rRNA. These findings further support a role for SMN and Drosha in regulating rRNA modification, possibly by affecting snoRNP or regulatory RNP activity.

Comparative proteomic study reveals the enhanced immune response with the blockade of interleukin 10 with anti-IL-10 and anti-IL-10 receptor antibodies in human U937 cells

Blocking cytokine interleukin 10 (IL-10) at the time of immunisation enhances vaccine induced T cell responses and improves control of tumour cell growth in vivo. However, the effect of an IL-10 blockade on the biological function of macrophages has not been explored. In the current paper, a macrophage precursor cell line, U937 cells, was selected to investigate the differential expression of proteins and relevant cell signalling pathway changes, when stimulated with lipopolysaccharide (LPS) in the presence of antibodies to IL-10 or IL-10 receptor. We used a quantitative proteomic strategy to investigate variations in protein profiles of U937 cells following the treatments with LPS, LPS plus human anti-IL10 antibody and anti-IL10R antibody in 24hrs, respectively. The LPS treatment significantly activated actin-related cell matrix formation and immune response pathways. The addition of anti-IL10 and anti-IL10R antibody further promoted the immune response and potentially effect macrophage survival through PI3K/AKT signalling; however, the latter appeared to also upregulated oncogene XRCC5 and Cajal body associated processes.

Re-analysis of the coral Acropora digitifera transcriptome reveals a complex lncRNAs-mRNAs interaction network implicated in Symbiodinium infection

BACKGROUND

Being critically important to the ecosystem, the stability of coral reefs is directly related to the marine and surrounding terrestrial environments. However, coral reefs are now undergoing massive and accelerating devastation due to bleaching. The fact that the breakdown of symbiosis between coral and the dinoflagellate, zooxanthellae, has been well elucidated as the main cause of bleaching, implying the establishment of symbiosis with zooxanthellae plays a crucial role in maintaining coral survival. However, the relevant molecular and cellular mechanisms have not been well studied yet. In this study, based on the deep RNA-sequencing data derived from Mohamed, A. R. et al., an integrated transcriptome analysis was performed to deeply investigate global transcriptome changes of the coral Acropora digitifera in response to infection by dinoflagellate of the genus Symbiodinium.

RESULTS

The results revealed that compared to RefTranscriptome_v1.0 (A. digitifera transcriptome assembly v1.0), numerous novel transcripts and isoforms were identified, the Symbiodinium-infected coral larvae at 4 h generated the highest proportion of specific isoforms. Alternative splicing analysis showed that intron retention predominated in all alternative transcripts among six statuses. Additionally, 8117 lncRNAs were predicted via a stringent stepwise filtering pipeline. A complex lncRNAs-mRNAs network including 815 lncRNAs and 6395 mRNAs were established, in which 21 lncRNAs were differentially expressed at 4 h post infection. Functional clustering analysis for those differentially lncRNAs-coexpressed mRNAs demonstrated that several biological processes and pathways related to protein kinase activity, metabolic pathways, mitochondrion, ribosome, etc. were enriched.

CONCLUSIONS

Our study not only refined A. digitifera transcriptome via identification of novel transcripts and isoforms, but also predicted a high-confidence dataset of lncRNAs. Functional study based on the construction of lncRNAs-mRNAs co-expression network has disclosed a complex lncRNA-mediated regulation in response to Symbiodinium infection exhibited in A. digitifera. Once validated, these lncRNAs could be good potential targets for treatment and prevention of bleaching in coral.

The RNA interactome of human telomerase RNA reveals a coding-independent role for a histone mRNA in telomere homeostasis

Telomerase RNA (TR) provides the template for DNA repeat synthesis at telomeres and is essential for genome stability in continuously dividing cells. We mapped the RNA interactome of human TR (hTR) and identified a set of non-coding and coding hTR-interacting RNAs, including the histone 1C mRNA (HIST1H1C). Disruption of the hTR-HIST1H1C RNA association resulted in markedly increased telomere elongation without affecting telomerase enzymatic activity. Conversely, over-expression of HIST1H1C led to telomere attrition. By using a combination of mutations to disentangle the effects of histone 1 RNA synthesis, protein expression, and hTR interaction, we show that HIST1H1C RNA negatively regulates telomere length independently of its protein coding potential. Taken together, our data provide important insights into a surprisingly complex hTR-RNA interaction network and define an unexpected non-coding RNA role for HIST1H1C in regulating telomere length homeostasis, thus offering a glimpse into the mostly uncharted, vast space of non-canonical messenger RNA functions.

Altered dynamics of scaRNA2 and scaRNA9 in response to stress correlates with disrupted nuclear organization

Small Cajal body-specific RNAs (scaRNAs) are part of small Cajal body-specific ribonucleoproteins (scaRNPs) that modify small nuclear RNA (snRNA) in Cajal bodies (CBs). Several scaRNAs (scaRNA 2, 9 and 17) have been found to generate smaller, nucleolus-enriched fragments. We hypothesize that the fragments derived from scaRNA 2, 9 and 17 form regulatory RNPs that influence the level of modifications within rRNA by altering small nucleolar RNP (snoRNP) activity. Here we show that external factors such as DNA damaging agents can alter the scaRNA9 full length to processed fragment ratio. We also show that full-length scaRNA2 levels are likewise impacted by DNA damage, which correlates with the disruption of SMN, coilin and WRAP53 co-localization in CBs. The dynamics of scaRNA9 were also shown to be affected by Drosha levels, which suggests that this protein may participate in the biogenesis and processing of this non-coding RNA. Identification of factors that contribute to scaRNA 2, 9 and 17 processing may facilitate an assessment of how external stress can lead to changes in rRNA modifications.

Phase separated microenvironments inside the cell nucleus are linked to disease and regulate epigenetic state, transcription and RNA processing

Proteins and RNAs inside the cell nucleus are organized into distinct phases, also known as liquid-liquid phase separated (LLPS) droplet organelles or nuclear bodies. These regions exist within the spaces between chromatin-rich regions but their function is tightly linked to gene activity. They include major microscopically-observable structures such as the nucleolus, paraspeckle and Cajal body. The biochemical and assembly factors enriched inside these microenvironments regulate chromatin structure, transcription, and RNA processing, and other important cellular functions. Here, we describe published evidence that suggests nuclear bodies are bona fide LLPS droplet organelles and major regulators of the processes listed above. We also outline an updated "Supply or Sequester" model to describe nuclear body function, in which proteins or RNAs are supplied to surrounding genomic regions or sequestered away from their sites of activity. Finally, we describe recent evidence that suggests these microenvironments are both reflective and drivers of diverse pathophysiological states.

An Activity Switch in Human Telomerase Based on RNA Conformation and Shaped by TCAB1

Ribonucleoprotein enzymes require dynamic conformations of their RNA constituents for regulated catalysis. Human telomerase employs a non-coding RNA (hTR) with a bipartite arrangement of domains-a template-containing core and a distal three-way junction (CR4/5) that stimulates catalysis through unknown means. Here, we show that telomerase activity unexpectedly depends upon the holoenzyme protein TCAB1, which in turn controls conformation of CR4/5. Cells lacking TCAB1 exhibit a marked reduction in telomerase catalysis without affecting enzyme assembly. Instead, TCAB1 inactivation causes unfolding of CR4/5 helices that are required for catalysis and for association with the telomerase reverse-transcriptase (TERT). CR4/5 mutations derived from patients with telomere biology disorders provoke defects in catalysis and TERT binding similar to TCAB1 inactivation. These findings reveal a conformational "activity switch" in human telomerase RNA controlling catalysis and TERT engagement. The identification of two discrete catalytic states for telomerase suggests an intramolecular means for controlling telomerase in cancers and progenitor cells.

Visualization of Human Telomerase Localization by Fluorescence Microscopy Techniques

Human telomerase is a ribonucleoprotein (RNP) that synthesizes DNA repeats at the ends of chromosomes and maintains telomere length and genome stability. The enzyme is comprised of telomerase RNA (hTR) (which provides the template for telomere addition) and several protein subunits including telomerase reverse transcriptase (hTERT) (the catalytic component). Intracellular trafficking of the enzyme has emerged as an important factor in the regulation of telomerase activity. Telomerase trafficking between nuclear Cajal bodies (proposed sites of telomerase biogenesis and regulation) and telomeres (sites of action) is regulated by the cell cycle in concordance with telomere synthesis during S phase. Here, we describe fluorescence microscopy approaches to visualize the subcellular localization of the essential RNA component of telomerase (hTR) relative to Cajal bodies and telomeres in cultured human cells. These methods include fluorescence in situ hybridization (to detect hTR and telomeric DNA) and immunofluorescence (to detect Cajal bodies and telomere binding proteins). Because telomerase localization to telomeres is normally restricted to S phase, we also describe methods to synchronize and analyze cells within this phase of the cell cycle.

Telomerase RNA Imaging in Budding Yeast and Human Cells by Fluorescent In Situ Hybridization

Telomerase, the enzyme that elongates telomeres in most eukaryotes, is a ribonucleoprotein complex composed of a reverse transcriptase catalytic subunit (TERT in human, Est2 in the budding yeast S. cerevisiae), regulatory factors and a noncoding RNA called hTERC (in human) or TLC1 (in budding yeast). Telomerase trafficking is a major process in the biogenesis and regulation of telomerase action at telomeres. Due to its higher signal-to-noise ratio, imaging of the telomerase RNA moiety is frequently used to determine telomerase intracellular localization. Here we describe how to image telomerase RNA in human and yeast cells using fluorescence in situ hybridization.

Assembly and trafficking of box C/D and H/ACA snoRNPs

Box C/D and box H/ACA snoRNAs are abundant non-coding RNAs that localize in the nucleolus and mostly function as guides for nucleotide modifications. While a large pool of snoRNAs modifies rRNAs, an increasing number of snoRNAs could also potentially target mRNAs. ScaRNAs belong to a family of specific RNAs that localize in Cajal bodies and that are structurally similar to snoRNAs. Most scaRNAs are involved in snRNA modification, while telomerase RNA, which contains H/ACA motifs, functions in telomeric DNA synthesis. In this review, we describe how box C/D and H/ACA snoRNAs are processed and assembled with core proteins to form functional RNP particles. Their biogenesis involve several transport factors that first direct pre-snoRNPs to Cajal bodies, where some processing steps are believed to take place, and then to nucleoli. Assembly of core proteins involves the HSP90/R2TP chaperone-cochaperone system for both box C/D and H/ACA RNAs, but also several factors specific for each family. These assembly factors chaperone unassembled core proteins, regulate the formation and disassembly of pre-snoRNP intermediates, and control the activity of immature particles. The AAA+ ATPase RUVBL1 and RUVBL2 belong to the R2TP co-chaperones and play essential roles in snoRNP biogenesis, as well as in the formation of other macro-molecular complexes. Despite intensive research, their mechanisms of action are still incompletely understood.

Neural Differentiation in HDAC1-Depleted Cells Is Accompanied by Coilin Downregulation and the Accumulation of Cajal Bodies in Nucleoli

Cajal bodies (CBs) are important compartments containing accumulated proteins that preferentially regulate RNA-related nuclear events, including splicing. Here, we studied the nuclear distribution pattern of CBs in neurogenesis. In adult brains, coilin was present at a high density, but CB formation was absent in the nuclei of the choroid plexus of the lateral ventricles. Cells of the adult hippocampus were characterized by a crescent-like morphology of coilin protein. We additionally observed a 70 kDa splice variant of coilin in adult mouse brains, which was different to embryonic brains and mouse pluripotent embryonic stem cells (mESCs), characterized by the 80 kDa standard variant of coilin. Here, we also showed that depletion of coilin is induced during neural differentiation and HDAC1 deficiency in mESCs caused coilin accumulation inside the fibrillarin-positive region of the nucleoli. A similar distribution pattern was observed in adult brain hippocampi, characterized by lower levels of both coilin and HDAC1. In summary, we observed that neural differentiation and HDAC1 deficiency lead to coilin depletion and coilin accumulation in body-like structures inside the nucleoli.

Telomerase and telomere biology in hematological diseases: A new therapeutic target

Telomeres are structures confined at the ends of eukaryotic chromosomes. With each cell division, telomeric repeats are lost because DNA polymerases are incapable to fully duplicate the very ends of linear chromosomes. Loss of repeats causes cell senescence, and apoptosis. Telomerase neutralizes loss of telomeric sequences by adding telomere repeats at the 3' telomeric overhang. Telomere biology is frequently associated with human cancer and dysfunctional telomeres have been proved to participate to genetic instability. This review covers the information on telomerase expression and genetic alterations in the most relevant types of hematological diseases. Telomere erosion hampers the capability of hematopoietic stem cells to effectively replicate, clinically resulting in bone marrow failure. Furthermore, telomerase mutations are genetic risk factors for the occurrence of some hematologic cancers. New discoveries in telomere structure and telomerase functions have led to an increasing interest in targeting telomeres and telomerase in anti-cancer therapy.

Life and Death of Yeast Telomerase RNA

Telomerase reverse transcriptase elongates telomeres to overcome their natural attrition and allow unlimited cellular proliferation, a characteristic shared by stem cells and the majority of malignant cancerous cells. The telomerase holoenzyme comprises a core RNA molecule, a catalytic protein subunit, and other accessory proteins. Malfunction of certain telomerase components can cause serious genetic disorders including dyskeratosis congenita and aplastic anaemia. A hierarchy of tightly regulated steps constitutes the process of telomerase biogenesis, which, if interrupted or misregulated, can impede the production of a functional enzyme and severely affect telomere maintenance. Here, we take a closer look at the budding yeast telomerase RNA component, TLC1, in its long lifetime journey around the cell. We review the extensive knowledge on TLC1 transcription and processing. We focus on exciting recent studies on telomerase assembly, trafficking, and nuclear dynamics, which for the first time unveil striking similarities between the yeast and human telomerase ribonucleoproteins. Finally, we identify questions yet to be answered and new directions to be followed, which, in the future, might improve our knowledge of telomerase biology and trigger the development of new therapies against cancer and other telomerase-related diseases.

RNA modification in Cajal bodies

Aside from nucleoli, Cajal bodies (CBs) are the best-characterized organelles of mammalian cell nuclei. Like nucleoli, CBs concentrate ribonucleoproteins (RNPs), in particular, spliceosomal small nuclear RNPs (snRNPs) and small nucleolar RNPs (snoRNPs). In one of the best-defined functions of CBs, most of the snoRNPs are involved in site-specific modification of snRNAs. The two major modifications are pseudouridylation and 2'-O-methylation that are guided by the box H/ACA and C/D snoRNPs, respectively. This review details the modifications, their function, the mechanism of modification, and the machineries involved. We dissect the different classes of noncoding RNAs that meet in CBs, guides and substrates. Open questions and conundrums, often raised and appearing due to experimental limitations, are pointed out and discussed. The emphasis of the review is on mammalian CBs and their function in modification of noncoding RNAs.

Towards an understanding of regulating Cajal body activity by protein modification

The biogenesis of small nuclear ribonucleoproteins (snRNPs), small Cajal body-specific RNPs (scaRNPs), small nucleolar RNPs (snoRNPs) and the telomerase RNP involves Cajal bodies (CBs). Although many components enriched in the CB contain post-translational modifications (PTMs), little is known about how these modifications impact individual protein function within the CB and, in concert with other modified factors, collectively regulate CB activity. Since all components of the CB also reside in other cellular locations, it is also important that we understand how PTMs affect the subcellular localization of CB components. In this review, we explore the current knowledge of PTMs on the activity of proteins known to enrich in CBs in an effort to highlight current progress as well as illuminate paths for future investigation.

The Cajal body and the nucleolus: "In a relationship" or "It's complicated"?

From their initial identification as 'nucleolar accessory bodies' more than a century ago, the relationship between Cajal bodies and nucleoli has been a subject of interest and controversy. In this review, we seek to place recent developments in the understanding of the physical and functional relationships between the 2 structures in the context of historical observations. Biophysical models of nuclear body formation, the molecular nature of CB/nucleolus interactions and the increasing list of joint roles for CBs and nucleoli, predominantly in assembling ribonucleoprotein (RNP) complexes, are discussed.

Telomerase Regulation from Beginning to the End

The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts to its maturity as a complex and function at its site of action, emphasizing new developments and how they contribute to the foundational knowledge of telomerase and telomere biology.

Live-cell imaging of budding yeast telomerase RNA and TERRA

In most eukaryotes, the ribonucleoprotein complex telomerase is responsible for maintaining telomere length. In recent years, single-cell microscopy techniques such as fluorescent in situ hybridization and live-cell imaging have been developed to image the RNA subunit of the telomerase holoenzyme. These techniques are now becoming important tools for the study of telomerase biogenesis, its association with telomeres and its regulation. Here, we present detailed protocols for live-cell imaging of the Saccharomyces cerevisiae telomerase RNA subunit, called TLC1, and also of the non-coding telomeric repeat-containing RNA TERRA. We describe the approach used for genomic integration of MS2 stem-loops in these transcripts, and provide information for optimal live-cell imaging of these non-coding RNAs.

Identification of processing elements and interactors implicate SMN, coilin and the pseudogene-encoded coilp1 in telomerase and box C/D scaRNP biogenesis

Many cellular functions, such as translation, require ribonucleoproteins (RNPs). The biogenesis of RNPs is a multi-step process that, depending on the RNP, can take place in many cellular compartments. Here we examine 2 different RNPs: telomerase and small Cajal body-specific RNPs (scaRNPs). Both of these RNPs are enriched in the Cajal body (CB), which is a subnuclear domain that also has high concentrations of another RNP, small nuclear RNPs (snRNPs). SnRNPs are essential components of the spliceosome, and scaRNPs modify the snRNA component of the snRNP. The CB contains many proteins, including WRAP53, SMN and coilin, the CB marker protein. We show here that coilin, SMN and coilp1, a newly identified protein encoded by a pseudogene in human, associate with telomerase RNA and a subset of scaRNAs. We also have identified a processing element within box C/D scaRNA. Our findings thus further strengthen the connection between the CB proteins coilin and SMN in the biogenesis of telomeras e and box C/D scaRNPs, and reveal a new player, coilp1, that likely participates in this process.

Preferential extension of short telomeres induced by low extracellular pH

The majority of tumor cells overcome proliferative limit by expressing telomerase. Whether or not telomerase preferentially extends the shortest telomeres is still under debate. When human cancer cells are cultured at neutral pH, telomerase extends telomeres in telomere length-independent manner. However, the microenvironment of tumor is slightly acidic, and it is not yet known how this influences telomerase action. Here, we examine telomere length homeostasis in tumor cells cultured at pHe 6.8. The results indicate that telomerase preferentially extends short telomeres, such that telomere length distribution narrows and telomeres become nearly uniform in size. After growth at pHe 6.8, the expression of telomerase, TRF1, TRF2 and TIN2 decreases, and the abundance of Cajal bodies decreases. Therefore, telomerase are insufficient for extending every telomere and shorter telomeres bearing less shelterin proteins are more accessible for telomerase recruitment. The findings support the ‘protein-counting mechanism’ in which extended and unextended state of telomere is determined by the number of associated shelterin proteins and the abundance of telomerase. Decreased expression of telomerase and preferential extension of short telomeres have important implications for tumor cell viability, and generate a strong rationale for research on telomerase-targeted anti-cancer therapeutics.