Tetrahymena mutants with short telomeres

Evaluation of oxidative stress mechanisms and the effects of phytotherapic extracts on Parkinson's disease Drosophila PINK1B9 model

Oxidative stress is commonly observed in both idiopathic and genetic cases of Parkinson's disease (PD). It plays an important role in the degeneration of dopaminergic neurons, and it has been associated with altered telomere length (TL). There is currently no cure for PD, and extracts of antioxidative plant, such as Mucuna pruriens and Withania somnifera, are commonly used in Ayurveda to treat patients with PD. In this study, we evaluated 2 enzymatic markers of oxidative stress, glutathione (GSH) system and superoxide dismutase (SOD), and TL in a Drosophila melanogaster model for PD [phosphatase and tensin homolog-induced putative kinase 1 (PINK1)^B9]. This evaluation was also performed after treatment with the phytoextracts. PINK1^B9 mutants showed a decrease in GSH amount and SOD activity and unexpected longer telomeres compared with wild-type flies. M. pruriens treatment seemed to have a beneficial effect on the oxidative stress conditions. On the other hand, W. somnifera treatment did not show any improvements in the studied oxidative stress mechanisms and even seemed to favor the selection of flies with longer telomeres. In summary, our study suggests the importance of testing antioxidant phytoextracts in a PINK1^B9 model to identify beneficial effects for PD.-Baroli, B., Loi, E., Solari, P., Kasture, A., Moi, L., Muroni, P., Kasture, S., Setzu, M. D., Liscia, A., Zavattari, P. Evaluation of oxidative stress mechanisms and the effects of phytotherapic extracts on Parkinson's disease Drosophila PINK1^B9 model.

Tetrahymena as a Unicellular Model Eukaryote: Genetic and Genomic Tools

Tetrahymena thermophila is a ciliate model organism whose study has led to important discoveries and insights into both conserved and divergent biological processes. In this review, we describe the tools for the use of Tetrahymena as a model eukaryote, including an overview of its life cycle, orientation to its evolutionary roots, and methodological approaches to forward and reverse genetics. Recent genomic tools have expanded Tetrahymena's utility as a genetic model system. With the unique advantages that Tetrahymena provide, we argue that it will continue to be a model organism of choice.

Enforced telomere elongation increases the sensitivity of human tumour cells to ionizing radiation

More than 85% of all human cancers possess the ability to maintain chromosome ends, or telomeres, by virtue of telomerase activity. Loss of functional telomeres is incompatible with survival, and telomerase inhibition has been established in several model systems to be a tractable target for cancer therapy. As human tumour cells typically maintain short equilibrium telomere lengths, we wondered if enforced telomere elongation would positively or negatively impact cell survival. We found that telomere elongation beyond a certain length significantly decreased cell clonogenic survival after gamma irradiation. Susceptibility to irradiation was dosage-dependent and increased at telomere lengths exceeding 17kbp despite the fact that all chromosome ends retained telomeric DNA. These data suggest that an optimal telomere length may promote human cancer cell survival in the presence of genotoxic stress.

Brh2 and Rad51 promote telomere maintenance in Ustilago maydis, a new model system of DNA repair proteins at telomeres

Recent studies implicate a number of DNA repair proteins in mammalian telomere maintenance. However, because several key repair proteins in mammals are missing from the well-studied budding and fission yeast, their roles at telomeres cannot be modeled in standard fungi. In this report, we explored the dimorphic fungus Ustilago maydis as an alternative model for telomere research. This fungus, which belongs to the phylum Basidiomycota, has a telomere repeat unit that is identical to the mammalian repeat, as well as a constellation of DNA repair proteins that more closely mimic the mammalian collection. We showed that the two core components of homology-directed repair (HDR) in U. maydis, namely Brh2 and Rad51, both promote telomere maintenance in telomerase positive cells, just like in mammals. In addition, we found that Brh2 is localized to telomeres in vivo, suggesting that it acts directly at chromosome ends. We surveyed a series of mutants with DNA repair defects, and found many of them to have short telomeres. Our results indicate that factors involved in DNA repair are probably also needed for optimal telomere maintenance in U. maydis, and that this fungus is a useful alternative model system for telomere research.

Haploinsufficiency and telomere length homeostasis

In humans, autosomal dominant or X-linked disease can arise through a phenomenon termed haploinsufficiency, where one remaining wild-type allele is insufficient for function. In model organisms, the impact of heterozygosity can be tested directly with engineered mutant alleles or in a hemizygous state where the expression of one allele is abrogated completely. This review will focus on haploinsufficiency as it relates to telomerase and telomere length maintenance and, citing selected examples in various model organisms, it will discuss how the problem of gene dosage relates to telomere function in normal and diseased states.

Effects of telomere length in Drosophila melanogaster on life span, fecundity, and fertility

Chromosome length in Drosophila is maintained by targeted transposition of three non-long terminal repeat retrotransposons, HeT-A, TART, and TAHRE, to the chromosome ends. The length and composition of these retrotransposon arrays can vary significantly between chromosome tips and between fly stocks, but the significance and consequences of these length differences are not understood. A dominant genetic factor, Tel, has been described, which causes a severalfold elongation of the retrotransposon arrays at all telomeres. We used this strain to assess possible affects of extended telomeres on the organism. While we found no effect on life span of the adults, we could demonstrate a correlation between long telomeres and reduced fertility and fecundity in individual females, which is also reflected in abnormal oocyte development.

Positive and negative regulation of Tetrahymena telomerase holoenzyme

Telomerase replenishes the telomeric repeats that cap eukaryotic chromosome ends. To perform DNA synthesis, the active site of telomerase reverse transcriptase (TERT) copies a template within the integral telomerase RNA (TER). In vivo, TERT and TER and additional subunits form a telomerase holoenzyme capable of telomere elongation. We previously purified epitope-tagged Tetrahymena thermophila TERT and characterized two of the associated proteins. Here we characterize the remaining two proteins that were enriched by TERT purification. The primary sequence of the p75 polypeptide lacks evident homology with other proteins, whereas the p20 polypeptide is the Tetrahymena ortholog of a conserved multifunctional protein, Skp1. Genetic depletion of p75 induced telomere shortening without affecting the accumulation of TER or TERT, suggesting that p75 promotes telomerase function at the telomere. Affinity purification of p75 coenriched telomerase activity and each other known telomerase holoenzyme protein. On the other hand, genetic depletion of Skp1p induced telomere elongation, suggesting that this protein plays a negative regulatory role in the maintenance of telomere length homeostasis. Affinity purification of Skp1p did not detectably enrich active telomerase but did copurify ubiquitin ligase machinery. These studies reveal additional complexity in the positive and negative regulation of Tetrahymena telomerase function.

Tetrahymena POT1a regulates telomere length and prevents activation of a cell cycle checkpoint

The POT1/TEBP telomere proteins are a group of single-stranded DNA (ssDNA)-binding proteins that have long been assumed to protect the G overhang on the telomeric 3' strand. We have found that the Tetrahymena thermophila genome contains two POT1 gene homologs, POT1a and POT1b. The POT1a gene is essential, but POT1b is not. We have generated a conditional POT1a cell line and shown that POT1a depletion results in a monster cell phenotype and growth arrest. However, G-overhang structure is essentially unchanged, indicating that POT1a is not required for overhang protection. In contrast, POT1a is required for telomere length regulation. After POT1a depletion, most telomeres elongate by 400 to 500 bp, but some increase by up to 10 kb. This elongation occurs in the absence of further cell division. The growth arrest caused by POT1a depletion can be reversed by reexpression of POT1a or addition of caffeine. Thus, POT1a is required to prevent a cell cycle checkpoint that is most likely mediated by ATM or ATR (ATM and ATR are protein kinases of the PI-3 protein kinase-like family). Our findings indicate that the essential function of POT1a is to prevent a catastrophic DNA damage response. This response may be activated when nontelomeric ssDNA-binding proteins bind and protect the G overhang.

Biological and biochemical functions of RNA in the tetrahymena telomerase holoenzyme

Telomerase extends chromosome ends by the synthesis of tandem simple-sequence repeats. Studies of minimal recombinant telomerase ribonucleoprotein (RNP) reconstituted in vitro have revealed sequences within the telomerase RNA subunit (TER) that are required to establish its internal template and other unique features of enzyme activity. Here we test the significance of these motifs following TER assembly into telomerase holoenzyme in vivo. We established a method for stable expression of epitope-tagged TER and TER variants in place of wild-type Tetrahymena TER. We found that sequence substitutions in nontemplate regions of TER altered telomere length maintenance in vivo, with an increase or decrease in the set point for telomere length homeostasis. We also characterized the in vitro activity of the telomerase holoenzymes reconstituted with TER variants, following RNA-based RNP affinity purification from cell extracts. We found that nontemplate sequence substitutions imposed specific defects in the fidelity and processivity of template use. These findings demonstrate nontemplate functions of TER that are critical for the telomerase holoenzyme catalytic cycle and for proper telomere length maintenance in vivo.

Modulation of telomere length dynamics by the subtelomeric region of tetrahymena telomeres

Tetrahymena telomeres usually consist of approximately 250 base pairs of T(2)G(4) repeats, but they can grow to reach a new length set point of up to 900 base pairs when kept in log culture at 30 degrees C. We have examined the growth profile of individual macronuclear telomeres and have found that the rate and extent of telomere growth are affected by the subtelomeric region. When the sequence of the rDNA subtelomeric region was altered, we observed a decrease in telomere growth regardless of whether the GC content was increased or decreased. In both cases, the ordered structure of the subtelomeric chromatin was disrupted, but the effect on the telomeric complex was relatively minor. Examination of the telomeres from non-rDNA chromosomes showed that each telomere exhibited a unique and characteristic growth profile. The subtelomeric regions from individual chromosome ends did not share common sequence elements, and they each had a different chromatin structure. Thus, telomere growth is likely to be regulated by the organization of the subtelomeric chromatin rather than by a specific DNA element. Our findings suggest that at each telomere the telomeric complex and subtelomeric chromatin cooperate to form a unique higher order chromatin structure that controls telomere length.

Holoenzyme proteins required for the physiological assembly and activity of telomerase

Many proteins have been implicated in the physiological function of telomerase, but specific roles of telomerase-associated proteins other than telomerase reverse transcriptase (TERT) remain ambiguous. To gain a more comprehensive understanding of catalytically active enzyme composition, we performed affinity purification of epitope-tagged, endogenously assembled Tetrahymena telomerase. We identified and cloned genes encoding four telomerase proteins in addition to TERT. We demonstrate that both of the two new proteins characterized in detail, p65 and p45, have essential roles in the maintenance of telomere length as part of a ciliate telomerase holoenzyme. The p65 subunit contains an La motif characteristic of a family of direct RNA-binding proteins. We find that p65 in cell extract is associated specifically with telomerase RNA, and that genetic depletion of p65 reduces telomerase RNA accumulation in vivo. These findings demonstrate that telomerase holoenzyme proteins other than TERT play critical roles in RNP biogenesis and function.

Loss of cap structure causes mitotic defect in Tetrahymena thermophila telomerase mutants

Mutation of the telomeric repeat sequence has severe cellular consequences in a variety of systems. A Tetrahymena thermophila telomerase template mutant, ter1-43AA, displays an acute mitotic chromosome segregation defect. In the study described here we investigated the molecular basis for this lethality. Although cloned ter1-43AA macronuclear telomeres had long tracts of wild-type G4T2 repeats, they were capped by a mixture of G4T3 repeats, shown previously to be non-lethal, and G4T4 repeats, the telomeric sequence normally found in hypotrichous ciliates such as Oxytricha. To test further the functionality of the G4T4 repeat sequence in T. thermophila, we devised a new template mutation, ter1-44+AA, that resulted in more uniform synthesis of this sequence at telomere caps in vivo. The ter1-44+AA mutant displayed the most severe mitotic defect reported to date, with up to 85% of the population having micronuclei in anaphase, providing firm evidence that the hypotrich repeat sequence is not functional in Tetrahymena. Surprisingly, in spite of the telomeric sequence mutation, neither the ter1-43AA nor ter1-44+AA mutant displayed any significant loss of telomere length regulation. These results demonstrate that loss of telomere cap integrity, rather than length regulation, leads to the anaphase defect.

Replicative senescence of hematopoietic stem cells during serial transplantation: does telomere shortening play a role?

Hematopoietic stem cells (HSC) have a finite proliferative lifespan, based upon the limited number of times they can be serially transplanted in mice. Telomeres have been shown to shorten during the division of many normal somatic cells in humans, and the attrition of telomeres has been shown to ultimately cause replicative senescence in vitro for a number of different human cell strains. Whereas most human cell types have little to no detectable levels of telomerase activity, hematopoietic cells, including HSC, express low to moderate levels of telomerase, and yet telomeres shorten considerably during replicative aging of these cells. Here we consider the role telomerase may play in the hematopoietic system as well as the effect that over-expression of telomerase reverse transcriptase may have on the replicative capacity of hematopoietic stem cells during transplantation.

A mutant of Tetrahymena telomerase reverse transcriptase with increased processivity

The protein catalytic subunit of telomerase (TERT) is a reverse transcriptase (RT) that utilizes an internal RNA molecule as a template for the extension of chromosomal DNA ends. In all retroviral RTs there is a conserved tyrosine two amino acids preceding the catalytic aspartic acids in motif C, a motif that is critical for catalysis. In TERTs, however, this position is a leucine, valine, or phenylalanine. We developed and characterized a robust in vitro reconstitution system for Tetrahymena telomerase and tested the effects of amino acid substitutions on activity. Substitution of the retroviral-like tyrosine in motif C did not change overall enzymatic activity but increased processivity. This increase in processivity correlated with an increased affinity for telomeric DNA primer. Substitution of an alanine did not increase processivity, while substitution of a phenylalanine had an intermediate effect. The data suggest that this amino acid is involved in interactions with the primer in telomerase as in other RTs, and show that mutating an amino acid to that conserved in retroviral RTs makes telomerase more closely resemble these other RTs.

Ciliate telomerase biochemistry

Telomerase is a cellular reverse transcriptase specialized for use of a template carried within the RNA component of the enzyme ribonucleoprotein complex. Substrates for telomerase are single-stranded oligonucleotides in vitro and chromosome ends in vivo. In vitro, a bound substrate is extended by an initial round of DNA synthesis on the internal RNA template and in some cases by multiple rounds of template copying before product dissociation. In vivo, de novo synthesis of one strand of a telomeric repeat sequence by telomerase balances the sequence loss resulting from incomplete replication of linear chromosome ends by RNA primer-requiring DNA polymerases. Telomerase biochemistry has been studied extensively by using partially purified cell extracts. Telomerase components are being identified and beginning to be produced in recombinant form. This review focuses on the enzyme mechanism of telomerases from ciliate species, thus far the most intensively studied systems.