Adding to the ends: what makes telomerase processive and how important is it?

Leishmania major telomerase RNA knockout: From altered cell proliferation to decreased parasite infectivity

This study focuses on the biological impacts of deleting the telomerase RNA from Leishmania major (LeishTER), a parasite responsible for causing leishmaniases, for which no effective treatment or prevention is available. TER is a critical player in the telomerase ribonucleoprotein complex, containing the template sequence copied by the reverse transcriptase component during telomere elongation. The success of knocking out both LeishTER alleles was confirmed, and no off-targets were detected. LmTER-/- cells share similar characteristics with other TER-depleted eukaryotes, such as altered growth patterns and partial G0/G1 cell cycle arrest in early passages, telomere shortening, and elevated TERRA expression. They also exhibit increased γH2A phosphorylation, suggesting that the loss of LeishTER induces DNA damage signaling. Moreover, pro-survival autophagic signals and mitochondrion alterations were shown without any detectable plasma membrane modifications. LmTER-/- retained the ability to transform into metacyclics, but their infectivity capacity was compromised. Furthermore, the overexpression of LeishTER was also deleterious, inducing a dominant negative effect that led to telomere shortening and growth impairments. These findings highlight TER's vital role in parasite homeostasis, opening discussions about its potential as a drug target candidate against Leishmania.

Rnr1, but not Rnr3, facilitates the sustained telomerase-dependent elongation of telomeres

Ribonucleotide reductase (RNR) provides the precursors for the generation of dNTPs, which are required for DNA synthesis and repair. Here, we investigated the function of the major RNR subunits Rnr1 and Rnr3 in telomere elongation in budding yeast. We show that Rnr1 is essential for the sustained elongation of short telomeres by telomerase. In the absence of Rnr1, cells harbor very short, but functional, telomeres, which cannot become elongated by increased telomerase activity or by tethering of telomerase to telomeres. Furthermore, we demonstrate that Rnr1 function is critical to prevent an early onset of replicative senescence and premature survivor formation in telomerase-negative cells but dispensable for telomere elongation by Homology-Directed-Repair. Our results suggest that telomerase has a "basal activity" mode that is sufficient to compensate for the “end-replication-problem” and does not require the presence of Rnr1 and a different "sustained activity" mode necessary for the elongation of short telomeres, which requires an upregulation of dNTP levels and dGTP ratios specifically through Rnr1 function. By analyzing telomere length and dNTP levels in different mutants showing changes in RNR complex composition and activity we provide evidence that the Mec1^ATR checkpoint protein promotes telomere elongation by increasing both dNTP levels and dGTP ratios through Rnr1 upregulation in a mechanism that cannot be replaced by its homolog Rnr3.

Telomeres protect the ends of eukaryotic chromosomes and as such determine the replicative capacity of a cell. In budding yeast and approximately 80% of human tumors the enzyme telomerase maintains telomere length by adding newly synthesized repeats to telomeres using dNTPs generated by Ribonucleotide reductase (RNR) complexes. Similarly, telomerase activity can restore telomere length after more severe telomere shortenings that result from collapsed replication forks or lead to telomere over-elongation in the absence of negative regulators of telomerase. Here we provide evidence for two activity modes of telomerase that differentially depend on the major RNR subunit Rnr1. We demonstrate that telomere maintenance and a compensation of the "end-replication-problem" is possible under conditions where Rnr1 activity is absent but that a sustained elongation of short telomeres fully depends on Rnr1 activity. We show that the Rnr1-homolog, Rnr3, cannot compensate for this telomeric function of Rnr1 even when overall cellular dNTP values are restored.

Human MLH1 suppresses the insertion of telomeric sequences at intra-chromosomal sites in telomerase-expressing cells

Aberrant formation of interstitial telomeric sequences (ITSs) promotes genome instabilities. However, it is unclear how aberrant ITS formation is suppressed in human cells. Here, we report that MLH1, a key protein involved in mismatch repair (MMR), suppresses telomeric sequence insertion (TSI) at intra-chromosomal regions. The frequency of TSI can be elevated by double-strand break (DSB) inducer and abolished by ATM/ATR inhibition. Suppression of TSI requires MLH1 recruitment to DSBs, indicating that MLH1's role in DSB response/repair is important for suppressing TSI. Moreover, TSI requires telomerase activity but is independent of the functional status of p53 and Rb. Lastly, we show that TSI is associated with chromosome instabilities including chromosome loss, micronuclei formation and chromosome breakage that are further elevated by replication stress. Our studies uncover a novel link between MLH1, telomerase, telomere and genome stability.

The protein subunit of telomerase displays patterns of dynamic evolution and conservation across different metazoan taxa

BACKGROUND

Most animals employ telomerase, which consists of a catalytic subunit known as the telomerase reverse transcriptase (TERT) and an RNA template, to maintain telomere ends. Given the importance of TERT and telomere biology in core metazoan life history traits, like ageing and the control of somatic cell proliferation, we hypothesised that TERT would have patterns of sequence and regulatory evolution reflecting the diverse life histories across the Animal Kingdom.

RESULTS

We performed a complete investigation of the evolutionary history of TERT across animals. We show that although TERT is almost ubiquitous across Metazoa, it has undergone substantial sequence evolution within canonical motifs. Beyond the known canonical motifs, we also identify and compare regions that are highly variable between lineages, but show conservation within phyla. Recent data have highlighted the importance of alternative splice forms of TERT in non-canonical functions and although animals may share some conserved introns, we find that the selection of exons for alternative splicing appears to be highly variable, and regulation by alternative splicing appears to be a very dynamic feature of TERT evolution. We show that even within a closely related group of triclad flatworms, where alternative splicing of TERT was previously correlated with reproductive strategy, we observe highly diverse splicing patterns.

CONCLUSIONS

Our work establishes that the evolutionary history and structural evolution of TERT involves previously unappreciated levels of change and the emergence of lineage specific motifs. The sequence conservation we describe within phyla suggests that these new motifs likely serve essential biological functions of TERT, which along with changes in splicing, underpin diverse functions of TERT important for animal life histories.

TERT promoter mutations in telomere biology

Telomere repeats at chromosomal ends, critical to genome integrity, are maintained through an elaborate network of proteins and pathways. Shelterin complex proteins shield telomeres from induction of DNA damage response to overcome end protection problem. A specialized ribonucleic protein, telomerase, maintains telomere homeostasis through repeat addition to counter intrinsic shortcomings of DNA replication that leads to gradual sequence shortening in successive mitoses. The biogenesis and recruitment of telomerase composed of telomerase reverse transcriptase (TERT) subunit and an RNA component, takes place through the intricate machinery that involves an elaborate number of molecules. The synthesis of telomeres remains a controlled and limited process. Inherited mutations in the molecules involved in the process directly or indirectly cause telomeropathies. Telomerase, while present in stem cells, is deactivated due to epigenetic silencing of the rate-limiting TERT upon differentiation in most of somatic cells with a few exceptions. However, in most of the cancer cells telomerase reactivation remains a ubiquitous process and constitutes one of the major hallmarks. Discovery of mutations within the core promoter of the TERT gene that create de novo binding sites for E-twenty-six (ETS) transcription factors provided a mechanism for cancer-specific telomerase reactivation. The TERT promoter mutations occur mainly in tumors from tissues with low rates of self-renewal. In melanoma, glioma, hepatocellular carcinoma, urothelial carcinoma and others, the promoter mutations have been shown to define subsets of patients with adverse disease outcomes, associate with increased transcription of TERT, telomerase reactivation and affect telomere length; in stem cells the mutations inhibit TERT silencing following differentiation into adult cells. The TERT promoter mutations cause an epigenetic switch on the mutant allele along with recruitment of pol II following the binding of GABPA/B1 complex that leads to mono-allelic expression. Thus, the TERT promoter mutations hold potential as biomarkers as well as future therapeutic targets.

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.

Potential Risks in the Paradigm of Basic to Translational Research: A Critical Evaluation of qPCR Telomere Size Techniques

Real time qPCR has become the method of choice for rapid large-scale telomere length measurements. Large samples sizes are critical for clinical trials, and epidemiological studies. QPCR has become such routine procedure that it is often used with little critical analysis. With proper controls, the mean telomere size can be derived from the data and even the size can be estimated. But there is a need for more consistent and reliable controls that will provide closer to the actual mean size can be obtained with uniform consensus controls. Although originating at the level of basic telomere research, many researchers less familiar with telomeres often misunderstand the source and significance of the qPCR metric. These include researchers and clinicians who are interested in having a rapid tool to produce exciting results in disease prognostics and diagnostics than in the multiple characteristics of telomeres that form the basis of the measurement. But other characteristics of the non-bimodal and heterogeneous telomeres as well as the complexities of telomere dynamics are not easily related to qPCR mean telomere values. The qPCR metric does not reveal the heterogeneity and dynamics of telomeres. This is a critical issue since mutations in multiple genes including telomerase can cause telomere dysfunction and a loss of repeats. The smallest cellular telomere has been shown to arrest growth of the cell carrying the dysfunction telomere. A goal for the future is a simple method that takes into account the heterogeneity by measuring the highest and lowest values as part of the scheme to compare. In the absence of this technique, Southern blots need to be performed in a subset of qPCR samples for both mean telomere size and the upper and lower extremes of the distribution. Most importantly, there is a need for greater transparency in discussing the limitations of the qPCR data. Given the potentially exciting qPCR telomere size results emerging from clinical studies that relate qPCR mean telomere size estimates to disease states, the current ambiguities have become urgent issues to validate the findings and to set the right course for future clinical investigations.

Telomerase activity and telomere length in Daphnia

Telomeres, comprised of short repetitive sequences, are essential for genome stability and have been studied in relation to cellular senescence and aging. Telomerase, the enzyme that adds telomeric repeats to chromosome ends, is essential for maintaining the overall telomere length. A lack of telomerase activity in mammalian somatic cells results in progressive shortening of telomeres with each cellular replication event. Mammals exhibit high rates of cell proliferation during embryonic and juvenile stages but very little somatic cell proliferation occurs during adult and senescent stages. The telomere hypothesis of cellular aging states that telomeres serve as an internal mitotic clock and telomere length erosion leads to cellular senescence and eventual cell death. In this report, we have examined telomerase activity, processivity, and telomere length in Daphnia, an organism that grows continuously throughout its life. Similar to insects, Daphnia telomeric repeat sequence was determined to be TTAGG and telomerase products with five-nucleotide periodicity were generated in the telomerase activity assay. We investigated telomerase function and telomere lengths in two closely related ecotypes of Daphnia with divergent lifespans, short-lived D. pulex and long-lived D. pulicaria. Our results indicate that there is no age-dependent decline in telomere length, telomerase activity, or processivity in short-lived D. pulex. On the contrary, a significant age dependent decline in telomere length, telomerase activity and processivity is observed during life span in long-lived D. pulicaria. While providing the first report on characterization of Daphnia telomeres and telomerase activity, our results also indicate that mechanisms other than telomere shortening may be responsible for the strikingly short life span of D. pulex.

Coordinated DNA dynamics during the human telomerase catalytic cycle

The human telomerase reverse transcriptase (hTERT) utilizes a template within the integral RNA subunit (hTR) to direct extension of telomeres. Telomerase exhibits repeat addition processivity (RAP) and must therefore translocate the nascent DNA product into a new RNA:DNA hybrid register to prime each round of telomere repeat synthesis. Here we use single-molecule FRET and nuclease protection assays to monitor telomere DNA structure and dynamics during the telomerase catalytic cycle. DNA translocation during RAP proceeds through a previously uncharacterized kinetic sub-step during which the 3′-end of the DNA substrate base pairs downstream within the hTR template. The rate constant for DNA primer re-alignment reveals this step is not rate-limiting for RAP, suggesting a second slow conformational change repositions the RNA:DNA hybrid into the telomerase active site and drives the extrusion of the 5′-end of the DNA primer out of the enzyme complex.

Telomere and telomerase biology

Telomeres are the physical ends of eukaryotic linear chromosomes. Telomeres form special structures that cap chromosome ends to prevent degradation by nucleolytic attack and to distinguish chromosome termini from DNA double-strand breaks. With few exceptions, telomeres are composed primarily of repetitive DNA associated with proteins that interact specifically with double- or single-stranded telomeric DNA or with each other, forming highly ordered and dynamic complexes involved in telomere maintenance and length regulation. In proliferative cells and unicellular organisms, telomeric DNA is replicated by the actions of telomerase, a specialized reverse transcriptase. In the absence of telomerase, some cells employ a recombination-based DNA replication pathway known as alternative lengthening of telomeres. However, mammalian somatic cells that naturally lack telomerase activity show telomere shortening with increasing age leading to cell cycle arrest and senescence. In another way, mutations or deletions of telomerase components can lead to inherited genetic disorders, and the depletion of telomeric proteins can elicit the action of distinct kinases-dependent DNA damage response, culminating in chromosomal abnormalities that are incompatible with life. In addition to the intricate network formed by the interrelationships among telomeric proteins, long noncoding RNAs that arise from subtelomeric regions, named telomeric repeat-containing RNA, are also implicated in telomerase regulation and telomere maintenance. The goal for the next years is to increase our knowledge about the mechanisms that regulate telomere homeostasis and the means by which their absence or defect can elicit telomere dysfunction, which generally results in gross genomic instability and genetic diseases.

Specific features of telomerase RNA from Hansenula polymorpha

Telomerase, a ribonucleoprotein, is responsible for the maintenance of eukaryotic genome integrity by replicating the ends of chromosomes. The core enzyme comprises the conserved protein TERT and an RNA subunit (TER) that, in contrast, displays large variations in size and structure. Here, we report the identification of the telomerase RNA from thermotolerant yeast Hansenula polymorpha (HpTER) and describe its structural features. We show further that the H. polymorpha telomerase reverse transcribes the template beyond the predicted boundary and adds a nontelomeric dT in vitro. Sequencing of the chromosomal ends revealed that this nucleotide is specifically present as a terminal nucleotide at the 3' end of telomeres. Mutational analysis of HpTER confirmed that the incorporation of dT functions to limit telomere length in this species.

Structure-function relationship and biogenesis regulation of the human telomerase holoenzyme

Telomeres are nucleoprotein structures found at the ends of linear chromosomes. Telomeric DNA shortens with each cell division, effectively restricting the proliferative capacity of human cells. Telomerase, a specialized reverse transcriptase, is responsible for de novo synthesis of telomeric DNA, and is the major physiological means by which mammalian cells extend telomere length. Telomerase activity in human soma is developmentally regulated according to cell type. Failure to tightly regulate telomerase has dire consequences: dysregulated telomerase activity is observed in more than 90% of human cancers, while haplo-insufficient expression of telomerase components underlies several inherited premature aging syndromes. Over the past decade, we have significantly improved our understanding of the structure-activity relationships between the two core telomerase components: telomerase reverse transcriptase and telomerase RNA. Genetic screening for telomerase deficiency syndromes has identified new partners in the biogenesis of telomerase and its catalytic functions. These data revealed a level of regulation complexity that is unexpected when compared with the other cellular polymerases. In this review, we summarize current knowledge on the structure-activity relationships of telomerase reverse transcriptase and telomerase RNA, and discuss how the biogenesis of telomerase provides additional regulation of its actions.

A translocation-defective telomerase with low levels of activity and processivity stabilizes short telomeres and confers immortalization

Short, repetitive, G-rich telomeric sequences are synthesized by telomerase, a ribonucleoprotein consisting of telomerase reverse transcriptase (TERT) and an integrally associated RNA. Human TERT (hTERT) can repetitively reverse transcribe its RNA template, acting processively to add multiple telomeric repeats onto the same substrate. We investigated whether certain threshold levels of telomerase activity and processivity are required to maintain telomere function and immortalize human cells with limited lifespan. We assessed hTERT variants with mutations in motifs implicated in processivity and interaction with DNA, namely the insertion in fingers domain (V791Y), and the E primer grip motif (W930F). hTERT-W930F and hTERT-V791Y reconstitute reduced levels of DNA synthesis and processivity compared with wild-type telomerase. Of interest, hTERT-W930F is more defective in translocation than hTERT-V791Y. Nonetheless, hTERT-W930F, but not hTERT-V791Y, immortalizes limited-lifespan human cells. Both hTERT-W930F- and hTERT-V791Y-expressing cells harbor short telomeres, measured as signal free ends (SFEs), yet SFEs persist only in hTERT-V791Y cells, which undergo apoptosis, likely as a consequence of a defect in recruitment of hTERT-V791Y to telomeres. Our study is the first to demonstrate that low levels of DNA synthesis--on the order of 20% of wild-type telomerase levels--and extension of as few as three telomeric repeats are sufficient to maintain functional telomeres and immortalize limited-lifespan human cells.

The TEL patch of telomere protein TPP1 mediates telomerase recruitment and processivity

Human chromosome ends are capped by shelterin, a protein complex that protects the natural ends from being recognized as sites of DNA damage and also regulates the telomere-replicating enzyme, telomerase^1–3. Shelterin includes the heterodimeric POT1-TPP1 protein, which binds the telomeric single-stranded DNA tail^4–9. TPP1 has been implicated both in recruiting telomerase to telomeres and in stimulating telomerase processivity (the addition of multiple DNA repeats after a single primer-binding event)^9–14. Determining the mechanisms of these activities has been difficult, especially because genetic perturbations also tend to affect the essential chromosome end-protection function of TPP1^15–17. Here we identify separation-of-function mutants of TPP1 that retain full telomere-capping function in vitro and in vivo, yet are defective in binding telomerase. The seven separation-of-function mutations map to a patch of amino acids on the surface of TPP1, the TEL patch, that both recruits telomerase to telomeres and promotes high-processivity DNA synthesis, indicating that these two activities are manifestations of the same molecular interaction. Given that the interaction between telomerase and TPP1 is required for telomerase function in vivo, the TEL patch of TPP1 provides a new target for anti-cancer drug development.

Evolutionary patterns of non-coding RNAs

A plethora of new functions of non-coding RNAs (ncRNAs) have been discovered in past few years. In fact, RNA is emerging as the central player in cellular regulation, taking on active roles in multiple regulatory layers from transcription, RNA maturation, and RNA modification to translational regulation. Nevertheless, very little is known about the evolution of this "Modern RNA World" and its components. In this contribution, we attempt to provide at least a cursory overview of the diversity of ncRNAs and functional RNA motifs in non-translated regions of regular messenger RNAs (mRNAs) with an emphasis on evolutionary questions. This survey is complemented by an in-depth analysis of examples from different classes of RNAs focusing mostly on their evolution in the vertebrate lineage. We present a survey of Y RNA genes in vertebrates and study the molecular evolution of the U7 snRNA, the snoRNAs E1/U17, E2, and E3, the Y RNA family, the let-7 microRNA (miRNA) family, and the mRNA-like evf-1 gene. We furthermore discuss the statistical distribution of miRNAs in metazoans, which suggests an explosive increase in the miRNA repertoire in vertebrates. The analysis of the transcription of ncRNAs suggests that small RNAs in general are genetically mobile in the sense that their association with a hostgene (e.g. when transcribed from introns of a mRNA) can change on evolutionary time scales. The let-7 family demonstrates, that even the mode of transcription (as intron or as exon) can change among paralogous ncRNA.

The human telomerase catalytic subunit and viral telomerase RNA reconstitute a functional telomerase complex in a cell-free system, but not in human cells

The minimal vertebrate telomerase enzyme is composed of a protein component (telomerase reverse transcriptase, TERT) and an RNA component (telomerase RNA, TR). Expression of these two subunits is sufficient to reconstitute telomerase activity in vitro, while the formation of a holoenzyme comprising telomerase-associated proteins is necessary for proper telomere length maintenance. Previous reports demonstrated the high processivity of the human telomerase complex and the interspecies compatibility of human TERT (hTERT). In this study, we tested the function of the only known viral telomerase RNA subunit (vTR) in association with human telomerase, both in a cell-free system and in human cells. When vTR is assembled with hTERT in a cell-free environment, it is able to interact with hTERT and to reconstitute telomerase activity. However, in human cells, vTR does not reconstitute telomerase activity and could not be detected in the human telomerase complex, suggesting that vTR is not able to interact properly with the proteins constituting the human telomerase holoenzyme.

[The influence of compound aITEL1296 on telomerase activity and the growth of cancer cells]

Telomerase is a ribonucleoprotein that synthesizes telomeric repeats and identified as a promising target for anticancer therapy. Here we describe a new compound aITEL1296 as a potent telomerase inhibitor. Its inhibitory activity was a bit higher (IC50 = 0,19 +/- 0,02 ng/ml) than that of BIBR1532, one of the most potent telomerase inhibitors known to date. Besides telomerase inhibition aITEL1296 activated apoptotic mechanisms and effectively suppressed proliferation of tumor cell lines (GI50 = 5,0 +/- 0,2 ng/ml for most sensitive cell line LnCap) but not normal fibroblast cell line.

ATR cooperates with CTC1 and STN1 to maintain telomeres and genome integrity in Arabidopsis

Telomeres protect chromosome ends from DNA damage. CTC1/STN1/TEN1 (CST), a core telomere-capping complex in plant and vertebrates, suppresses an ATR-dependent DNA damage response in Arabidopsis. Protracted ATR inactivation inhibits telomerase, hastening the onset of telomere dysfunction in CST mutants.

The CTC1/STN1/TEN1 (CST) complex is an essential constituent of plant and vertebrate telomeres. Here we show that CST and ATR (ataxia telangiectasia mutated [ATM] and Rad3-related) act synergistically to maintain telomere length and genome stability in Arabidopsis. Inactivation of ATR, but not ATM, temporarily rescued severe morphological phenotypes associated with ctc1 or stn1. Unexpectedly, telomere shortening accelerated in plants lacking CST and ATR. In first-generation (G1) ctc1 atr mutants, enhanced telomere attrition was modest, but in G2 ctc1 atr, telomeres shortened precipitously, and this loss coincided with a dramatic decrease in telomerase activity in G2 atr mutants. Zeocin treatment also triggered a reduction in telomerase activity, suggesting that the prolonged absence of ATR leads to a hitherto-unrecognized DNA damage response (DDR). Finally, our data indicate that ATR modulates DDR in CST mutants by limiting chromosome fusions and transcription of DNA repair genes and also by promoting programmed cell death in stem cells. We conclude that the absence of CST in Arabidopsis triggers a multifaceted ATR-dependent response to facilitate maintenance of critically shortened telomeres and eliminate cells with severe telomere dysfunction.

Regulation of telomere length and homeostasis by telomerase enzyme processivity

Telomerase is a ribonucleoprotein consisting of a catalytic subunit, the telomerase reverse transcriptase, TERT, and an integrally associated RNA, TR, which contains a template for the synthesis of short repetitive G-rich DNA sequences at the ends of telomeres. Telomerase can repetitively reverse transcribe its short RNA template, acting processively to add multiple telomeric repeats onto the same DNA substrate. The contribution of enzyme processivity to telomere length regulation in human cells is not well characterized. In cancer cells, under homeostatic telomere length-maintenance conditions, telomerase acts processively, while under nonequilibrium conditions, telomerase acts distributively on the shortest telomeres. To investigate the role of increased telomerase processivity on telomere length regulation in human cells with limited lifespan that are dependent on human TERT (hTERT) for lifespan extension and immortalization, we mutated the leucine at position 866 in the reverse transcriptase C motif of hTERT to a tyrosine (L866Y), which is the amino acid found at a similar position in HIV-1 reverse transcriptase. We report that, similar to the previously reported ‘gain of function’ Tetrahymena telomerase mutant (L813Y), the human telomerase variant displays increased processivity. hTERT-L866Y, like wild-type hTERT can immortalize and extend the lifespan of limited lifespan cells. Moreover, hTERT-L866Y expressing cells display heterogenous telomere lengths, telomere elongation, multiple telomeric signals indicative of fragile sites and replicative stress, and an increase in short telomeres, which is accompanied by telomere trimming events. Our results suggest that telomere length and homeostasis in human cells may be regulated by telomerase enzyme processivity.

A mutation in the catalytic subunit of yeast telomerase alters primer-template alignment while promoting processivity and protein-DNA binding

Telomerase is a ribonucleoprotein complex that is required for maintenance of linear chromosome ends (telomeres). In yeast, the Est2 protein reverse transcribes a short template region of the TLC1 RNA using the chromosome terminus to prime replication. Yeast telomeres contain heterogeneous G(1-3)T sequences that arise from incomplete reverse transcription of the TLC1 template and alignment of the DNA primer at multiple sites within the template region. We have previously described mutations in the essential N-terminal TEN domain of Est2p that alter telomere sequences. Here, we demonstrate that one of these mutants, glutamic acid 76 to lysine (est2-LT(E76K)), restricts possible alignments between the DNA primer and the TLC1 template. In addition, this mutant exhibits increased processivity in vivo. Within the context of the telomerase enzyme, the Est2p TEN domain is thought to contribute to enzyme processivity by mediating an anchor-site interaction with the DNA primer. We show that binding of the purified TEN domain (residues 1-161) to telomeric DNA is enhanced by the E76K mutation. These results support the idea that the anchor-site interaction contributes to telomerase processivity and suggest a role for the anchor site of yeast telomerase in mediating primer-template alignment within the active site.

RNA/DNA hybrid binding affinity determines telomerase template-translocation efficiency

Telomerase synthesizes telomeric DNA repeats onto chromosome termini from an intrinsic RNA template. The processive synthesis of DNA repeats relies on a unique, yet poorly understood, mechanism whereby the telomerase RNA template translocates and realigns with the DNA primer after synthesizing each repeat. Here, we provide evidence that binding of the realigned RNA/DNA hybrid by the active site is an essential step for template translocation. Employing a template-free human telomerase system, we demonstrate that the telomerase active site directly binds to RNA/DNA hybrid substrates for DNA polymerization. In telomerase processivity mutants, the template-translocation efficiency correlates with the affinity for the RNA/DNA hybrid substrate. Furthermore, the active site is unoccupied during template translocation as a 5 bp extrinsic RNA/DNA hybrid effectively reduces the processivity of the template-containing telomerase. This suggests that strand separation and template realignment occur outside the active site, preceding the binding of realigned hybrid to the active site. Our results provide new insights into the ancient RNA/DNA hybrid binding ability of telomerase and its role in template translocation.

Telomerase: structure, functions, and activity regulation

Telomerase is the enzyme responsible for maintenance of the length of telomeres by addition of guanine-rich repetitive sequences. Telomerase activity is exhibited in gametes and stem and tumor cells. In human somatic cells proliferation potential is strictly limited and senescence follows approximately 50-70 cell divisions. In most tumor cells, on the contrary, replication potential is unlimited. The key role in this process of the system of the telomere length maintenance with involvement of telomerase is still poorly studied. No doubt, DNA polymerase is not capable to completely copy DNA at the very ends of chromosomes; therefore, approximately 50 nucleotides are lost during each cell cycle, which results in gradual telomere length shortening. Critically short telomeres cause senescence, following crisis, and cell death. However, in tumor cells the system of telomere length maintenance is activated. Besides catalytic telomere elongation, independent telomerase functions can be also involved in cell cycle regulation. Inhibition of the telomerase catalytic function and resulting cessation of telomere length maintenance will help in restriction of tumor cell replication potential. On the other hand, formation of temporarily active enzyme via its intracellular activation or due to stimulation of expression of telomerase components will result in telomerase activation and telomere elongation that can be used for correction of degenerative changes. Data on telomerase structure and function are summarized in this review, and they are compared for evolutionarily remote organisms. Problems of telomerase activity measurement and modulation by enzyme inhibitors or activators are considered as well.

Telomerase regulatory subunit Est3 in two Candida species physically interacts with the TEN domain of TERT and telomeric DNA

The yeast telomerase regulatory protein Est3 is required for telomere maintenance in vivo, and shares intriguing structural and functional similarities with the mammalian telomeric protein TPP1. Here we report our physical and functional characterizations of Est3 homologues from Candida parapsilosis and Lodderomyces elongisporus, which bear unique N- and C-terminal tails in addition to a conserved central OB fold. We show that these Est3 homologues form stable complexes with the TEN domain of telomerase reverse transcriptase. Efficient complex formation requires both the N- and C-terminal tails, as well as conserved OB fold residues of Est3. Other Est3 homologues devoid of the tails failed to interact strongly with the cognate TEN domains. Remarkably, the C. parapsilosis Est3 alone exhibits no appreciable DNA-binding activity, but can be crosslinked to telomeric DNA in the presence of the TEN domain. A conserved basic residue on the putative DNA-binding surface of CpEst3 is required for efficient crosslinking. Mutating the equivalent residue in Candida albicans Est3 caused telomere attrition. We propose that interaction with the TEN domain unmasks a functionally important nucleic acid-binding activity in Est3. Our findings provide insights on the mechanisms and evolution of a widely conserved and functionally critical telomeric/telomerase component.

Human telomerase model shows the role of the TEN domain in advancing the double helix for the next polymerization step

Telomerases constitute a group of specialized ribonucleoprotein enzymes that remediate chromosomal shrinkage resulting from the "end-replication" problem. Defects in telomere length regulation are associated with several diseases as well as with aging and cancer. Despite significant progress in understanding the roles of telomerase, the complete structure of the human telomerase enzyme bound to telomeric DNA remains elusive, with the detailed molecular mechanism of telomere elongation still unknown. By application of computational methods for distant homology detection, comparative modeling, and molecular docking, guided by available experimental data, we have generated a three-dimensional structural model of a partial telomerase elongation complex composed of three essential protein domains bound to a single-stranded telomeric DNA sequence in the form of a heteroduplex with the template region of the human RNA subunit, TER. This model provides a structural mechanism for the processivity of telomerase and offers new insights into elongation. We conclude that the RNADNA heteroduplex is constrained by the telomerase TEN domain through repeated extension cycles and that the TEN domain controls the process by moving the template ahead one base at a time by translation and rotation of the double helix. The RNA region directly following the template can bind complementarily to the newly synthesized telomeric DNA, while the template itself is reused in the telomerase active site during the next reaction cycle. This first structural model of the human telomerase enzyme provides many details of the molecular mechanism of telomerase and immediately provides an important target for rational drug design.

Telomerase-based immortalization modifies the angiogenic/inflammatory responses of human coronary artery endothelial cells

Telomerase reverse transcriptase (TERT) is fundamental in determining the life span by regulating telomere length of chromosomes. To address the question whether the enhancement of the proliferative potential hampers cell differentiation, we generated TERT-over-expressing endothelial cells (ECs) and analyzed in vitro their (1) barrier function; (2) low-density lipoprotein uptake; (3) expression pattern of six selected marker proteins; (4) angiogenic potential in four assays; and (5) inflammatory responses. In contrast to investigations with focus on other cell parameters, we demonstrate that immortalization of ECs by over-expression of TERT resulted in different angiogenic and inflammatory behavior in comparison to cells with low native telomerase levels.

A modified model for translocation events of processive nucleotide and repeat additions by the recombinant telomerase

Telomerase is a unique reverse transcriptase that extends the single-stranded 3' overhangs of telomeres by copying a short template sequence within the integral RNA component of the enzyme. It shows processive nucleotide and repeat addition activities, which are realized via two types of movements: translocation of the DNA:RNA hybrid away from the active site following each nucleotide addition and translocation of the 3' end of the DNA primer relative to the RNA template after each round of repeat synthesis. Here, a model is presented to describe these two types of translocation events by the recombinant Tetrahymena telomerase, via the modification of the model that has been proposed recently. Using the present model, the dynamics of the dissociation of the DNA primer from the telomerase and the dynamics of the disruption of the DNA:RNA hybrid and then repositioning of the product 3' end to the beginning of the template are studied quantitatively. Their effects on the repeat addition processivity are theoretically studied. The theoretical results are in agreement with the available experimental data.

Tetrahymena telomerase protein p65 induces conformational changes throughout telomerase RNA (TER) and rescues telomerase reverse transcriptase and TER assembly mutants

The biogenesis of the Tetrahymena telomerase ribonucleoprotein particle (RNP) is enhanced by p65, a La family protein. Single-molecule and biochemical studies have uncovered a hierarchical assembly of the RNP, wherein the binding of p65 to stems I and IV of telomerase RNA (TER) causes a conformational change that facilitates the subsequent binding of telomerase reverse transcriptase (TERT) to TER. We used purified p65 and variants of TERT and TER to investigate the conformational rearrangements that occur during RNP assembly. Nuclease protection assays and mutational analysis revealed that p65 interacts with and stimulates conformational changes in regions of TER beyond stem IV. Several TER mutants exhibited telomerase activity only in the presence of p65, revealing the importance of p65 in promoting the correct RNP assembly pathway. In addition, p65 rescued TERT assembly mutants but not TERT activity mutants. Taken together, these results suggest that p65 stimulates telomerase assembly and activity in two ways. First, by sequestering stems I and IV, p65 limits the ensemble of structural conformations of TER, thereby presenting TERT with the active conformation of TER. Second, p65 acts as a molecular buttress within the assembled RNP, mutually stabilizing TER and TERT in catalytically active conformations.

Does telomerase reverse transcriptase induce functional de-differentiation of human endothelial cells?

By counteracting the shortening of chromosome telomeres, telomerase reverse transcriptase (hTERT) prevents senescence and age-related cell death. Embryonic cells display a high telomerase activity that declines rapidly with cell differentiation. Conversely, de-differentiated tumor cells tend to re-express telomerase. In view of the controversial data on the reciprocal correlation between cell proliferation and differentiation, we questioned whether telomerase overexpression and the resulting immortalization would affect the functional phenotype of human endothelial cells. Our comparative analysis addressed (1) distinct cell adhesion to different ECM-proteins analyzed on miniaturized multisubstrate arrays (MSA), (2) protein expression of diverse markers, (3) the uptake of DiI-Ac-LDL, (4) the inflammatory response based on upregulation of ICAM-1, (5) tube formation, and (6) the barrier properties of cell monolayers in transfilter cultures. Our results, based on some 40 data sets, demonstrate that immortalization of primary endothelial cells by hTERT maintains the typical endothelial characteristics without any sign of functional de-differentiation.

Multiple mechanisms for elongation processivity within the reconstituted tetrahymena telomerase holoenzyme

To maintain telomeres, telomerase evolved a unique biochemical activity: the use of a single-stranded RNA template for the synthesis of single-stranded DNA repeats. High repeat addition processivity (RAP) of the Tetrahymena telomerase holoenzyme requires association of the catalytic core with the telomere adaptor subcomplex (TASC) and an RPA1-related subunit (p82 or Teb1). Here, we used DNA binding and holoenzyme reconstitution assays to investigate the mechanism by which Teb1 and TASC confer high RAP. We show that TASC association with the recombinant telomerase catalytic core increases enzyme activity. Subsequent association of the Teb1 C-terminal domain with TASC confers the capacity for high RAP even though the Teb1 C-terminal domain does not provide a high-affinity DNA interaction site. Efficient RAP also requires suppression of nascent product folding mediated by the central Teb1 DNA-binding domains (DBDs). These sequence-specific high-affinity DBDs of Teb1 can be functionally substituted by the analogous DBDs of Tetrahymena Rpa1 to suppress nascent product folding but only if the Rpa1 high-affinity DBDs are physically tethered into holoenzyme context though the Teb1 C-terminal domain. Overall, our findings reveal multiple mechanisms and multiple surfaces of protein-DNA and protein-protein interaction that give rise to elongation processivity in the synthesis of a single-stranded nucleic acid product.

POT1-TPP1 enhances telomerase processivity by slowing primer dissociation and aiding translocation

Telomerase contributes to chromosome end replication by synthesizing repeats of telomeric DNA, and the telomeric DNA-binding proteins protection of telomeres (POT1) and TPP1 synergistically increase its repeat addition processivity. To understand the mechanism of increased processivity, we measured the effect of POT1-TPP1 on individual steps in the telomerase reaction cycle. Under conditions where telomerase was actively synthesizing DNA, POT1-TPP1 bound to the primer decreased primer dissociation rate. In addition, POT1-TPP1 increased the translocation efficiency. A template-mutant telomerase that synthesizes DNA that cannot be bound by POT1-TPP1 exhibited increased processivity only when the primer contained at least one POT1-TPP1-binding site, so a single POT1-TPP1-DNA interaction is necessary and sufficient for stimulating processivity. The POT1-TPP1 effect is specific, as another single-stranded DNA-binding protein, gp32, cannot substitute. POT1-TPP1 increased processivity even when substoichiometric relative to the DNA, providing evidence for a recruitment function. These results support a model in which POT1-TPP1 enhances telomerase processivity in a manner markedly different from the sliding clamps used by DNA polymerases.

The N-terminus of hTERT contains a DNA-binding domain and is required for telomerase activity and cellular immortalization

Telomerase defers the onset of telomere damage-induced signaling and cellular senescence by adding DNA onto chromosome ends. The ability of telomerase to elongate single-stranded telomeric DNA depends on the reverse transcriptase domain of TERT, and also relies on protein:DNA contacts outside the active site. We purified the N-terminus of human TERT (hTEN) from Escherichia coli, and found that it binds DNA with a preference for telomeric sequence of a certain length and register. hTEN interacted with the C-terminus of hTERT in trans to reconstitute enzymatic activity in vitro. Mutational analysis of hTEN revealed that amino acids Y18 and Q169 were required for telomerase activity in vitro, but not for the interaction with telomere DNA or the C-terminus. These mutants did not reconstitute telomerase activity in cells, maintain telomere length, or extend cellular lifespan. In addition, we found that T116/T117/S118, while dispensable in vitro, were required for cellular immortalization. Thus, the interactions of hTEN with telomere DNA and the C-terminus of hTERT are functionally separable from the role of hTEN in telomere elongation activity in vitro and in vivo, suggesting other roles for the protein and nucleic acid interactions of hTEN within, and possibly outside, the telomerase catalytic core.

The telomerase-specific T motif is a restrictive determinant of repetitive reverse transcription by human telomerase

The central hallmark of telomerases is repetitive copying of a short, defined sequence within its integral RNA subunit. We sought to identify structural determinants of this unique activity in the catalytic protein subunit telomerase reverse transcriptase (TERT) of telomerase. Residues within the highly conserved telomerase-specific T motif of human TERT were mutationally probed, leading to variant telomerases with increased repeat extension rates and wild-type processivity. The extension rate increases were independent of template sequence composition and only moderately correlated to telomerase RNA (TR) binding. Importantly, analysis of substrate primer elongation showed that the extension rate increases primarily resulted from increases in the repeat (type II) translocation rate. Our findings indicate a participatory role for the T motif in repeat translocation, an obligatory event for repetitive telomeric DNA synthesis. Thus, the T motif serves as a restrictive determinant of repetitive reverse transcription.

Structures of telomerase subunits provide functional insights

BACKGROUND

Telomerase continues to generate substantial attention both because of its pivotal roles in cellular proliferation and aging and because of its unusual structure and mechanism. By replenishing telomeric DNA lost during the cell cycle, telomerase overcomes one of the many hurdles facing cellular immortalization. Functionally, telomerase is a reverse transcriptase, and it shares structural and mechanistic features with this class of nucleotide polymerases. Telomerase is a very unusual reverse transcriptase because it remains stably associated with its template and because it reverse transcribes multiple copies of its template onto a single primer in one reaction cycle.

SCOPE OF REVIEW

Here, we review recent findings that illuminate our understanding of telomerase. Even though the specific emphasis is on structure and mechanism, we also highlight new insights into the roles of telomerase in human biology.

GENERAL SIGNIFICANCE

Recent advances in the structural biology of telomerase, including high resolution structures of the catalytic subunit of a beetle telomerase and two domains of a ciliate telomerase catalytic subunit, provide new perspectives into telomerase biochemistry and reveal new puzzles.

Antitumor activity of small double-stranded oligodeoxynucleotides targeting telomerase RNA in malignant melanoma cells

Human telomerase RNA (hTR) is an intrinsic component of telomerase enzyme. Small interfering RNAs (siRNAs) and single-stranded antisense oligonucleotides have been used previously for silencing of the hTR. The objective of this study was to investigate the effect of partially double-stranded oligodeoxynucleotides (ODNs), in vitro and in vivo in comparison to single-stranded antisense ODNs and siRNAs. ODNs were designed on the basis of structural properties of an ODN from previous studies on HIV, to target the hTR in the human cervical carcinoma HeLa cell line and mouse telomerase RNA (mTR) in the murine metastatic melanoma B16-F10 cell line, respectively. Our results indicate that ODNs were able to inhibit the hTR by 68% and the mTR by 81% in the respective cell lines. This correlated with ODN-mediated rapid inhibition of cell proliferation and induction of apoptosis excluding slow effects on telomerase function. The inhibition of the hTR was decreased by knock-down of the cellular RNases H suggesting their contribution. Furthermore, we showed a reduction in numbers of metastases by 70% after intravenous administration of ODN-transfected B16-F10 cells in C57BL/6 mice. Our study demonstrates the potential utility of these hairpin-loop-structured ODNs as a different group of nucleic acids for telomerase-based antiproliferative strategies.

A possible mechanism of processive nucleotide and repeat additions by the telomerase

Telomerase is a specialized cellular ribonucleoprotein complex that can synthesize long stretches of a DNA primer by using an intrinsic RNA template sequence. This requires that the telomerase must be able to carry out both nucleotide and repeat additions. Here, based on available structures and experimental data, a model is presented to describe these two addition activities. In the model, the forward movement of the polymerase active site along the template during the processive nucleotide addition is rectified through the incorporation of a matched base, via the Brownian ratchet mechanism. The unpairing of the DNA:RNA hybrid and then repositioning of product 3'-end after each round of repeat synthesis, which are prerequisites for the processive repeat addition, are caused by a force acting on the primer. The force results from the conformational transition of the stem III pseudoknot, which is mechanically induced by the rotation of TERT fingers together with stem IV loop towards the polymerase active site upon a nucleotide binding. Based on the model, the dynamics of processive nucleotide and repeat additions by recombinant Tetrahymena telomerase is studied analytically, which gives good quantitative explanations to the previous experimental results. Moreover, some predicted results are presented. In particular, it is shown that the repeat addition processivity is mainly determined by the difference between the free-energy change required to disrupt the DNA:RNA hybrid and that required to unfold the stem III pseudoknot. A large difference in free energy corresponds to a low repeat addition processivity while a small difference in free energy corresponds to a high repeat addition processivity.

Down-regulation of epidermal growth factor receptor-dependent signaling by Porphyromonas gingivalis lipopolysaccharide in life-expanded human gingival fibroblasts

BACKGROUND AND OBJECTIVE

Human gingival fibroblasts exhibit proliferative responses following epidermal growth factor exposure, which are thought to enhance periodontal regeneration in the absence of bacterial products such as lipopolysacharide. However, lipopolysaccharide challenge activates human gingival fibroblasts to release several inflammatory mediators that contribute to the immune response associated with periodontitis and attenuate wound repair. We tested the hypothesis that Porphyromonas gingivalis lipopolysaccharide-activated signaling pathways down-regulate epidermal growth factor receptor-dependent events.

MATERIAL AND METHODS

To study lipopolysaccharide/epidermal growth factor interactions in human gingival fibroblasts, we introduced the catalytic subunit of human telomerase into human gingival fibroblasts, thereby generating a more long-lived cellular model. These cells were characterized and evaluated for lipopolysaccharide/epidermal growth factor responsiveness and regulation of epidermal growth factor-dependent pathways.

RESULTS

Comparison of human telomerase-transduced gingival fibroblasts with human gingival fibroblasts revealed that both cell lines exhibit a spindle-like morphology and express similar levels of epidermal growth factor receptor, CD14 and Toll-like receptors 2 and 4. Importantly, human telomerase-transduced gingival fibroblasts proliferation rates are increased 5-9 fold over human gingival fibroblasts and exhibit a longer life span in culture. In addition, human telomerase-transduced gingival fibroblasts and human gingival fibroblasts exhibit comparable profiles of mitogen-activated protein kinase kinase (extracellular signal-regulated kinase 1/2) activation upon epidermal growth factor or P. gingivalis lipopolysaccharide administration. Interestingly, treatment with P. gingivalis lipopolysaccharide leads to a down-regulation of epidermal growth factor-dependent extracellular signal-regulated kinase 1/2, p38 and cyclic-AMP response element binding protein phosphorylation in both cell types.

CONCLUSION

These studies demonstrate that human telomerase-transduced gingival fibroblasts exhibit an extended life span and recapitulate human gingival fibroblasts biology. Moreover, this system has allowed for the first demonstration of lipopolysaccharide down-regulation of epidermal growth factor activated pathways in human gingival fibroblasts and should facilitate the analysis of signaling events relevant to the pathogenesis and treatment of periodontitis.

Analysis of human telomerase activity and function by two color single molecule coincidence fluorescence spectroscopy

Telomerase is a nonclassical DNA polymerase that uses its integral RNA as a template to synthesize telomeric repeats onto chromosome ends. The molecular mechanism of telomerase is unique and involves a translocation step after the synthesis of each telomeric repeat. To directly measure the enzymatic turnover of substrate and the efficiency of the translocation step we have extended our two-color single molecule fluorescence coincidence method (Anal.Chem. 2003, 75, 1664-1670). The method employs Cy5-dATP incorporation into a DNA primer that has been prelabeled with a reference fluorophore. Measurements are performed in the single molecule regime and products, which necessarily have both fluorophores, are excited by two independent lasers, and give rise to coincident events. By counting the number of coincident events and using the coincidence detection efficiency, it is possible to determine the number of the extended products generated by attomole quantities of telomerase, without separation or the use of PCR or radioactivity. Histograms of the logarithms of the ratios of the Cy5 to the reference fluorophore fluorescence can be used to determine the length distribution of the products and hence the enzyme processivity. The mean processivity obtained from the single molecule fluorescence coincidence assay is 0.32 +/- 0.04, in good agreement with the value of 0.37 +/- 0.05 derived from the direct radioactive assay approach. The function of the alignment domain of human telomerase RNA in sustaining catalytic activity in vitro has been reevaluated using this method. Together with our previous results (Nucleic Acids Res. 2002, 30, 4470-4480) these experiments identify the essential residues in the alignment domain of human telomerase RNA that contribute to the activity and processivity of telomerase.

Tools and methods for genetic analysis of Saccharomyces castellii

The budding yeast species Saccharomyces castellii has provided important new insights into molecular evolution when incorporated in comparative genomics studies and studies of mitochondrial inheritage. Although it shows some diversity in the specific molecular details, several analyses have shown that it contains many genetic pathways similar to those of S. cerevisiae. Here we have investigated the possibility of performing genetic analyses in S. castellii. We optimized the LiAc transformation protocol to achieve 200-300 transformants/microg plasmid DNA. We found that the commonly used plasmids for S. cerevisiae are stably maintained in S. castellii under selective conditions. Surprisingly, both 2micro and CEN/ARS plasmids are kept at a high copy number. Moreover, the kanMX cassette can be used as a resistance marker against the selective drug geneticin (G418). Finally, we determined that the S. cerevisiae GAL1 promoter can be used for the activation of transcription in S. castellii, thus enabling the controlled overexpression of genes when galactose is present in the medium. The availability of these tools provides the possibility of performing genetic analyses in S. castellii, and makes it a promising new model system in which hypotheses derived from bioinformatics studies can be experimentally tested.

Modeling and structure function analysis of the putative anchor site of yeast telomerase

Telomerase is a ribonucleoprotein reverse transcriptase responsible for extending one strand of the telomere terminal repeats. Unique among reverse transcriptases, telomerase is thought to possess a DNA-binding domain (known as anchor site) that allows the enzyme to add telomere repeats processively. Previous crosslinking and mutagenesis studies have mapped the anchor site to an N-terminal region of TERT, and the structure of this region of Tetrahymena TERT was recently determined at atomic resolutions. Here we use a combination of homology modeling, electrostatic calculation and site-specific mutagenesis analysis to identify a positively charged, functionally important surface patch on yeast TERT. This patch is lined by both conserved and non-conserved residues, which when mutated, caused loss of telomerase processivity in vitro and telomere shortening in vivo. In addition, we demonstrate that a point mutation in this domain of yeast TERT simultaneously enhanced the repeat addition processivity of telomerase and caused telomere elongation. Our data argue that telomerase anchor site has evolved species-specific residues to interact with species-specific telomere repeats. The data also reinforce the importance of telomerase processivity in regulating telomere length.

Induction of parallel human telomeric G-quadruplex structures by Sr(2+)

Human telomeric DNA forms G-quadruplex secondary structures, which can inhibit telomerase activity and are targets for anti-cancer drugs. Here we show that Sr(2+) can induce human telomeric DNA to form both inter- and intramolecular structures having characteristics consistent with G-quadruplexes. Unlike Na(+) or K(+), Sr(2+) facilitated intermolecular structure formation for oligonucleotides with 2 to 5 5'-d(TTAGGG)-3' repeats. Longer 5'-d(TTAGGG)-3' oligonucleotides formed exclusively intramolecular structures. Altering the 5'-d(TTAGGG)-3' to 5'-d(TTAGAG)-3' in the 1st, 3rd, or 4th repeats of 5'-d(TTAGGG)(4)-3' stabilized the formation of intermolecular structures. However, a more compact, intramolecular structure was still observed when the 2nd repeat was altered. Circular dichroism spectroscopy results suggest that the structures were parallel-stranded, distinguishing them from similar DNA sequences in Na(+) and K(+). This study shows that Sr(2+), promotes parallel-stranded, inter- and intramolecular G-quadruplexes that can serve as models to study DNA substrate recognition by telomerase.

Aging Schwann cells in vitro

Schwann cells (SCs) can support the regeneration of lesioned fiber tracts of the peripheral and central nervous system and have been transplanted alone or in combination with synthetic nerve guides. For neuronal tissue engineering purposes, the cells must be isolated from small biopsies and expanded in vitro. In this study we analyze the impact of cell expansion on 9 different cell parameters, comparing short- and long-term cultured rat SCs, which we refer to as 'young' and 'old' or 'aged' cells, respectively. In comparison to young SCs, old SCs doubled the axonal outgrowth from dorsal root ganglion explants and displayed only one-third as much adhesion to the gray and white matter of spinal cord cryosections. In a 3-dimensional extracellular matrix the two cell populations showed very different cellular responses with regard to cell morphology and cell-cell adhesion. Cell proliferation of old SCs was independent of serum components and was not hampered by contact inhibition. In addition, population doubling times were reduced by a factor of almost three compared to those of young SCs. Despite considerable karyotype changes, with an average of 68.7 chromosomes versus 42 in native rat cells, old SCs did not show any increase in telomerase activity and loss of anchorage dependence--characteristics that are typical of tumor cells. The data also provide biological insights into which cell characteristics (proliferation and adhesion, for example) are functionally clustered and either change or remain constant with aging in vitro. Though the data indicate a lack of tumorigenic transformation coupled with increased neurite outgrowth-promoting activity after extensive SC expansion in vitro, thus suggesting better regeneration qualities, we strongly recommend that in vitro aged rat SCs (>11 passages) should not be employed for tissue engineering.

The structure and function of telomerase reverse transcriptase

The structure and integrity of telomeres are essential for genome stability. Telomere dysregulation can lead to cell death, cell senescence, or abnormal cell proliferation. The maintenance of telomere repeats in most eukaryotic organisms requires telomerase, which consists of a reverse transcriptase (RT) and an RNA template that dictates the synthesis of the G-rich strand of telomere terminal repeats. Structurally, telomerase reverse transcriptase (TERT) contains unique and variable N- and C-terminal extensions that flank a central RT-like domain. The enzymology of telomerase includes features that are both similar to and distinct from those characteristic of other RTs. Two distinguishing features of TERT are its stable association with the telomerase RNA and its ability to repetitively reverse transcribe the template segment of RNA. Here we discuss TERT structure and function; its regulation by RNA-DNA, TERT-DNA, TERT-RNA, TERT-TERT interactions, and TERT-associated proteins; and the relationship between telomerase enzymology and telomere maintenance.

The biogenesis and regulation of telomerase holoenzymes

Chromosome stability requires a dynamic balance of DNA loss and gain in each terminal tract of telomeric repeats. Repeat addition by a specialized reverse transcriptase, telomerase, has an important role in maintaining this equilibrium. Insights that have been gained into the cellular pathways for biogenesis and regulation of telomerase ribonucleoproteins raise new questions, particularly concerning the dynamic nature of this unique polymerase.

Structural study of elements of Tetrahymena telomerase RNA stem-loop IV domain important for function

Tetrahymena telomerase RNA (TER) contains several regions in addition to the template that are important for function. Central among these is the stem-loop IV domain, which is involved in both catalysis and RNP assembly, and includes binding sites for both the holoenzyme assembly protein p65 and telomerase reverse transcriptase (TERT). Stem-loop IV contains two regions with high evolutionary sequence conservation: a central GA bulge between helices, and a terminal loop. We solved the solution structure of loop IV and modeled the structure of the helical region containing the GA bulge, using NMR and residual dipolar couplings. The central GA bulge with flanking C-G base pairs induces a approximately 50 degrees semi-rigid bend in the helix. Loop IV is highly structured, and contains a conserved C-U base pair at the top of the helical stem. Analysis of new and previous biochemical data in light of the structure provides a rationale for some of the sequence conservation in this region of TER. The results suggest that during holoenzyme assembly the protein p65 recognizes a bend in stem IV, and this binding to central stem IV helps to position the structured loop IV for interaction with TERT and other region(s) of TER.