Telomere variation in Xenopus laevis

Telomeres and telomerase: active but complex players in life-history decisions

Studies on human telomeres have established that telomeres exert a significant influence on lifespan and health of organisms. However, recent research has indicated that the original idea that telomeres affect lifespan in a universal and central manner across all eukaryotic species is an oversimplification. Indeed, findings from a variety of animal species revealed that the role of telomere biology in aging is more subtle and intricate than previously recognized. Here, we show how telomere biology varies depending on the taxon. We also show how telomere biology corresponds to basic life history traits and affects the life table of a species and investments in growth, body size, reproduction, and lifespan; telomeres are hypothesized to shape evolutionary perspectives for species in an active but complex manner. Our evaluation is based on telomere biology data from many examples from throughout the animal kingdom that vary according to the degree of organismal complexity and life history strategies.

The evolution of metazoan shelterin

In this study, Myler et al. investigated the evolutionary origins of shelterin complex, which is comprised of TRF1, TRF2, Rap1, TIN2, TPP1, and POT1; blocks the DNA damage response at chromosome ends; and interacts with telomerase and the CST complex to regulate telomere length. They describe the evolution of metazoan shelterin, showing that TRF1 emerged in vertebrates upon duplication of a TRF2-like ancestor, and providing insights into the biology of shelterin and its evolution from ancestral telomeric DNA-binding proteins.

The mammalian telomeric shelterin complex—comprised of TRF1, TRF2, Rap1, TIN2, TPP1, and POT1—blocks the DNA damage response at chromosome ends and interacts with telomerase and the CST complex to regulate telomere length. The evolutionary origins of shelterin are unclear, partly because unicellular organisms have distinct telomeric proteins. Here, we describe the evolution of metazoan shelterin, showing that TRF1 emerged in vertebrates upon duplication of a TRF2-like ancestor. TRF1 and TRF2 diverged rapidly during vertebrate evolution through the acquisition of new domains and interacting factors. Vertebrate shelterin is also distinguished by the presence of an HJRL domain in the split C-terminal OB fold of POT1, whereas invertebrate POT1s carry inserts of variable nature. Importantly, the data reveal that, apart from the primate and rodent POT1 orthologs, all metazoan POT1s are predicted to have a fourth OB fold at their N termini. Therefore, we propose that POT1 arose from a four-OB-fold ancestor, most likely an RPA70-like protein. This analysis provides insights into the biology of shelterin and its evolution from ancestral telomeric DNA-binding proteins.

Enzymatically Generated CRISPR Libraries for Genome Labeling and Screening

CRISPR-based technologies have emerged as powerful tools to alter genomes and mark chromosomal loci, but an inexpensive method for generating large numbers of RNA guides for whole genome screening and labeling is lacking. Using a method that permits library construction from any source of DNA, we generated guide libraries that label repetitive loci or a single chromosomal locus in Xenopus egg extracts and show that a complex library can target the E. coli genome at high frequency.

Telomere-targeted retrotransposons in the rice blast fungus Magnaporthe oryzae: agents of telomere instability

The fungus Magnaporthe oryzae is a serious pathogen of rice and other grasses. Telomeric restriction fragments in Magnaporthe isolates that infect perennial ryegrass (prg) are hotspots for genomic rearrangement and undergo frequent, spontaneous alterations during fungal culture. The telomeres of rice-infecting isolates are very stable by comparison. Sequencing of chromosome ends from a number of prg-infecting isolates revealed two related non-LTR retrotransposons (M. oryzae Telomeric Retrotransposons or MoTeRs) inserted in the telomere repeats. This contrasts with rice pathogen telomeres that are uninterrupted by other sequences. Genetic evidence indicates that the MoTeR elements are responsible for the observed instability. MoTeRs represent a new family of telomere-targeted transposons whose members are found exclusively in fungi.

Telomere biology in Metazoa

In this review we present critical overview of some of the available literature on the fundamental biology of telomeres and telomerase in Metazoan. With the exception of Nematodes and Arthropods, the (TTAGGG)(n) sequence is conserved in most Metazoa. Available data show that telomerase-based end maintenance is a very ancient mechanism in unicellular and multicellular organisms. In invertebrates, fish, amphibian, and reptiles persistent telomerase activity in somatic tissues might allow the maintenance of the extensive regenerative potentials of these species. Telomerase repression among birds and many mammals suggests that, as humans, they may use replicative aging as a tumor protection mechanism.

Origin-dependent initiation of DNA replication within telomeric sequences

Replication of telomeres requires the action of telomerase, the semi-conservative replication machinery and the stabilization of the replication fork during passage through telomeric DNA. Whether vertebrate telomeres support initiation of replication has not been experimentally addressed. Using Xenopus cell free extracts we established a system to study replication initiation within linear telomeric DNA substrates. We show binding of TRF2 to telomeric DNA, indicating that exogenous DNA exclusively composed of telomeric repeats is recognized by shelterin components. Interaction with telomere binding proteins is not sufficient to prevent a DNA damage response. Notably, we observe regulated assembly of the pre-replicative complex proteins ORC2, MCM6 and Cdc6 to telomeric DNA. Most importantly, we detect origin-dependent replication of telomeric substrates under conditions that inhibit checkpoint activation. These results indicate that pre-replicative complexes assemble within telomeric DNA and can be converted into functional origins.

Site-specific self-cleavage of G-quadruplexes formed by human telemetric repeats

It is demonstrated in our investigations that certain G-quadruplex structures formed by human telomeric repeats could perform self-cleaving actions. Our further studies verify that these reactions are site-specific and undergo hydrolytic pathways.

Engineered telomeres in transgenic Xenopus laevis

The expanding roles of telomeres in epigenetic gene regulation, nuclear organization, and human disease have necessitated the establishment of model organisms in which to study telomere function under normal developmental conditions. We present an efficient system for generating numerous vertebrate animals containing engineered telomeres using a Xenopus laevis transgenesis technique. Our results indicate Xenopus zygotes efficiently recognize telomeric repeats at chromosome break points and form telomeric complexes thus generating a new telomere. The resulting transgenic animals progress through normal development and successfully metamorphose into froglets despite the chromosome breakage. Overall, this presents an efficient mechanism for generating engineered telomeres in a vertebrate system and provides an opportunity to investigate epigenetic aspects of telomere function during normal vertebrate development.

Do telomere dynamics link lifestyle and lifespan?

Identifying and understanding the processes that underlie the observed variation in lifespan within and among species remains one of the central areas of biological research. Questions directed at how, at what rate and why organisms grow old and die link disciplines such as evolutionary ecology to those of cell biology and gerontology. One process now thought to have a key role in ageing is the pattern of erosion of the protective ends of chromosomes, the telomeres. Here, we discuss what is currently known about the factors influencing telomere regulation, and how this relates to fundamental questions about the relationship between lifestyle and lifespan.

A truncated acidic domain in Xenopus TRF1

Telomere function is mediated by a complex of proteins bound to double-stranded and single-stranded telomeric repeats. A key player in this complex is TRF1, which binds to duplex TTAGGG repeats and acts as a negative regulator of telomere length. This protein's domain structure, as defined by studies with mammalian orthologs, consists of an N-terminal acidic domain, a dimerization domain, and a C-terminal Myb DNA binding domain. TRF1 from Xenopus laevis was cloned and sequenced, and the encoded protein found to have a similar structure but with a very short acidic domain. This short acidic domain was confirmed in Xenopus tropicalis, a true diploid, by cloning of cDNA sequences by RACE and analysis of the genomic locus. The TRF1 transcript is expressed in developing and adult frogs. Compared to the mammalian orthologs, the Xenopus genes are the most distantly related vertebrate examples characterized to date. Since adult Xenopus ubiquitously express somatic telomerase activity, proteins that regulate telomerase access to the chromosome ends are important in regulating telomere length in normal somatic tissue. The structure of Xenopus TRF1 has implications for its regulation by tankyrase.

Localization of repetitive DNA sequences on in vitro Xenopus laevis chromosomes by primed in situ labeling (PRINS)

Xenopus laevis is an important reference model organism used in many vertebrate studies. Gene mapping in X. laevis, in comparison to other reference organisms, is in its early stages. Few studies have been conducted to localize DNA sequences on X. laevis chromosomes. Primed in situ labeling (PRINS) is a recently developed innovative tool that has been used to locate specific DNA sequences in various organisms. PRINS has been reported to have increased sensitivity compared to other in situ hybridization techniques. In the present study, PRINS was first used to label the location of telomeres at the ends of in vitro X. laevis chromosomes. The terminal location was as expected from in vivo reports, however, the overall amount seemed to decrease in the in vitro chromosomes. Once the PRINS technique was optimized, this technique was used to determine the chromosomal location of the satellite 1 repetitive sequence, which is an important sequence in X. laevis development. The sequence was observed on the interstitial regions of the majority of the chromosomes similar to the in vivo locations reported. In contrast to the telomeric sequence, the amount of sequence appeared to increase in the satellite 1 sequence. PRINS was found to be useful in the localization of repetitive DNA sequences in the X. laevis genome.

Multiple origins of tetraploid taxa in the Eurasian Bufo viridis subgroup

We used Q-banding and analyzed nucleolar organizing regions (NORs) to study the cytogenetic evolution of tetraploids within the Palearctic Bufo viridis subgroup, the only known amphibian complex comprising di-, tri- and tetraploid bisexually reproducing taxa. We examined three diploid (2n) nominal taxa (Bufo viridis viridis, B. v. turanensis, B. v. kermanensis) from five Eurasian localities and six tetraploid (4n) nominal taxa (B. oblongus, B. o. danatensis, B. pewzowi pewzowi, B. p. taxkorensis, B. p. unicolor, B. p. strauchi) from eight Central Asian localities. Homeologous chromosomes of 2n and 4n toads exhibit a similar morphology. Silver-staining and in situ hybridization revealed terminal NORs in the long arms of chromosomes 6 in all 2n but in only two out of four chromosomes 6 in all 4n taxa. Q-banding and a rapidly evolving mitochondrial marker suggest at least two origination events for Asian 4n toads: "Western Central Asian tetraploids" (B. oblongus Nikolsky, 1896) exhibit distinct differences within some chromosome quartets, which are divisible into pairs of chromosomes and may be allopolyploid. In contrast, "Central Asian tetraploids" (B. pewzowi Bedriaga, 1898) showed homogenous Q-banding patterns within each quartet, suggesting autopolyploidy. In Northeastern Iran, we discovered a zone of either common ancestry or hybridization of 2n and Western Central Asian 4n toads. This raises intriguing questions about how diploid and tetraploid taxa may evolve by exchanging genetic material.

Telomeres and Telomerase: A Modern Fountain of Youth?

Since ageing is a universal human feature, it is not surprising that, from the Babylonian epic of Gilgamesh to Ponce de Leon seeking the "Fountain of Youth," countless people have dreamed of finding a way to avoid ageing, to no avail. Yet the search continues. In this review, we present one of the latest candidates: the enzyme telomerase, capable of elongating the tips of chromosomes, the telomeres. Research into the causes of cellular ageing established the telomeres as the molecular clock that counts the number of times cells divide and triggers cellular senescence. Herein, we review arguments both in favor and against the use of telomerase as an anti-ageing therapy. The importance of the telomeres in cellular ageing, the low or non-existent levels of telomerase activity in human tissues, and the ability of telomerase to immortalize human cells suggest that telomerase can be used as an anti-ageing therapy. On the other hand, recent experiments in mice have raised doubts whether telomerase affects organismal ageing. Results from human cells expressing telomerase have also suggested telomerase may promote tumorigenesis. We conclude that, though telomerase may be used in regenerative medicine and to treat specific diseases, it is unlikely to become a source of anti-ageing therapies.

Telomeres and telomerase

Telomeres are the protective DNA-protein complexes found at the ends of eukaryotic chromosomes. Telomeric DNA consists of tandem repeats of a simple, often G-rich, sequence specified by the action of telomerase, and complete replication of telomeric DNA requires telomerase. Telomerase is a specialized cellular ribonucleoprotein reverse transcriptase. By copying a short template sequence within its intrinsic RNA moiety, telomerase synthesizes the telomeric DNA strand running 5' to 3' towards the distal end of the chromosome, thus extending it. Fusion of a telomere, either with another telomere or with a broken DNA end, generally constitutes a catastrophic event for genomic stability. Telomerase acts to prevent such fusions. The molecular consequences of telomere failure, and the molecular contributors to telomere function, with an emphasis on telomerase, are discussed here.

Telomeres in the chicken: genome stability and chromosome ends

Telomeres are the complex nucleoprotein structures at the termini of linear chromosomes. Telomeric DNA consists of a highly conserved hexanucleotide arranged in tandem repeats. Telomerase, a ribonucleoprotein of the reverse transcriptase family, specifies the sequence of telomeric DNA and maintains telomere array length. Numerous studies in model organisms established the significance of telomere structure and function in regulating genome stability, cellular aging, and oncogenesis. Our overall research objectives are to understand the organization of the telomere arrays in chicken in the context of the unusual organization and specialized features of this higher vertebrate genome (which include a compact genome, numerous microchromosomes, and high recombination rate) and to elucidate the role telomeres play in genome stability impacting cell function and life span. Recent studies found that the chicken genome contains three overlapping size classes of telomere arrays that differ in location and age-related stability: Class I 0.5 to 10 kb, Class II 10 to 40 kb, and Class III 40 kb to 2 Mb. Some notable features of chicken telomere biology are that the chicken genome contains ten times more telomeric DNA than the human genome and the Class III telomere arrays are the largest described for any vertebrate species. In vivo, chicken telomeres (Class II) shorten in an age-related fashion and telomerase activity is high in early stage embryos and developing organs but down-regulates during late embryogenesis or postnatally in most somatic tissues. In vitro, chicken cells down-regulate telomerase activity unless transformed. Knowledge of chicken telomere biology contributes information relevant to present and future biotechnology applications of chickens in vivo and chicken cells in vitro.

Telomerase activity is widespread in adult somatic tissues of Xenopus

Chromosome ends, or telomeres, are maintained by telomerase. Work in selected vertebrates has implied that telomerase is often repressed in differentiated cells, and telomere erosion results in senescence of cultured cells. Tissues from mature Xenopus laevis frogs were examined for telomerase enzymatic activity with the TRAP (telomere repeat amplification protocol) assay. All tissues contained active telomerase, most abundantly in testis, spleen, liver, and embryos; activity was less abundant but still readily detectable with < 100 ng of protein extract from brain and muscle tissues. Activity in somatic tissues of the diploid Xenopus tropicalis suggests this condition is not limited to the polyploid members of the genus, and that extensive differentiation-linked telomerase repression does not occur in Xenopus.

Formation of extrachromosomal circles from telomeric DNA in Xenopus laevis

Instability and plasticity of telomeric DNA, which includes extrachromosomal DNA, are usually correlated with the absence of telomerase and with abnormal growth of mammalian cells. Here, we show the formation of extrachromosomal circular DNA of telomeric repeats (tel-eccDNA) during the development of Xenopus laevis. Tel-eccDNA is double-stranded relaxed circles composed of the vertebrate consensus telomeric repeats [TTAGGG](n). Its size varies from <2 to >20 kb and it comprises up to 10% of the total cellular telomere content of the early embryo (pre-MBT stage). The amount of tel-eccDNA is reduced in later developmental stages and in adult tissues. Using a cell-free system derived from Xenopus egg extracts, we show that tel-eccDNA can be formed de novo from the telomere chromosomal tracts of sperm nuclei and naked DNA in a replication-independent manner. These results reveal an unusual plasticity of telomeric DNA during normal development of Xenopus.

Identification and analyses of the Xenopus TERT gene that encodes the catalytic subunit of telomerase

The Xenopus telomerase catalytic component gene, xTERT (Xenopus telomerase reverse transcriptase), has been cloned. The production of xTERT recombinant protein together with the proposed Xenopus telomerase RNA (xTR) (Chen et al., 2000. Cell 100, 503-514) in a rabbit reticulocyte lysate system led to the reconstitution of active telomerase, indicating that both products are functional telomerase components. Both xTERT expression and telomerase activity are high from the early to the late blastula stage. However, they are decreased at the gastrula stage and thereafter, suggesting that the xTERT expression level is the primary mechanism for regulating telomerase activity in Xenopus development. This is the first report of a non-mammalian vertebrate TERT gene. Sequence comparison of xTERT with human and mouse TERTs has uncovered four regions conserved in the amino-terminal halves of vertebrate TERT proteins, the functions of which will be discussed herein.

Telomerase expression and telomere length in immortal leukocyte lines from channel catfish

Normal channel catfish leukocytes readily undergo spontaneous in vitro immortalization yielding functionally active diploid cell lines. Since telomerase activation appears to be a critical step in the establishment of immortal mammalian cells, studies were undertaken to determine if and when telomerase expression occurs during the in vitro immortalization process of channel catfish leukocytes. To this end, freshly isolated peripheral blood leukocytes (PBL) from normal fish were shown to exhibit low to undetectable levels of telomerase activity and within four days after culture initiation showed dramatic increases in telomerase activity which typically remained high for at least four weeks. This activity then declined, concomitant with decreases in cellular proliferation and increases in cell death. Cells which escaped this culture "crisis" re-expressed high levels of telomerase activity indefinitely. Although telomerase activity was expressed early in the immortalization process, clonal cell lines derived from these cultures had relatively short telomeres. These results suggest that telomerase expression in catfish leukocytes is activation-induced, and its expression does not necessarily stabilize telomere length until a critically, albeit ill-defined, short length is reached.