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

Does Effective Population Size Govern Evolutionary Differences in Telomere Length?

Lengths of telomeres vary by an order of magnitude across mammalian species. Similarly, age- and sex-standardized telomere lengths differ by up to 1 kb (14%) across human populations. How to explain these differences? Telomeres play a central role in senescence and aging, and genes that affect telomere length are likely under weak selection (i.e. telomere length is a trait that is subject to nearly neutral evolution). Importantly, natural selection is more effective in large populations than in small populations. Here, we propose that observed differences in telomere length across species and populations are largely due to differences in effective population sizes. In this perspective, we present preliminary evolutionary genetic evidence supporting this hypothesis and highlight the need for more data.

Transcriptional coupling of telomeric retrotransposons with the cell cycle

Instead of employing telomerases to safeguard chromosome ends, dipteran species maintain their telomeres by transposition of telomeric-specific retrotransposons (TRs): in Drosophila , these are HeT-A , TART , and TAHRE . Previous studies have shown how these TRs create tandem repeats at chromosome ends, but the exact mechanism controlling TR transcription has remained unclear. Here we report the identification of multiple subunits of the transcription cofactor Mediator complex and transcriptional factors Scalloped (Sd, the TEAD homolog in flies) and E2F1-Dp as novel regulators of TR transcription and telomere length in Drosophila . Depletion of multiple Mediator subunits, Dp, or Sd increased TR expression and telomere length, while over-expressing E2F1-Dp or knocking down the E2F1 regulator Rbf1 (Retinoblastoma-family protein 1) stimulated TR transcription, with Mediator and Sd affecting TR expression through E2F1-Dp. The CUT&RUN analysis revealed direct binding of CDK8, Dp, and Sd to telomeric repeats. These findings highlight the essential role of the Mediator complex in maintaining telomere homeostasis by regulating TR transcription through E2F1-Dp and Sd, revealing the intricate coupling of TR transcription with the host cell-cycle machinery, thereby ensuring chromosome end protection and genomic stability during cell division.

Genomic Instability and Epigenetic Changes during Aging

Aging is considered the deterioration of physiological functions along with an increased mortality rate. This scientific review focuses on the central importance of genomic instability during the aging process, encompassing a range of cellular and molecular changes that occur with advancing age. In particular, this revision addresses the genetic and epigenetic alterations that contribute to genomic instability, such as telomere shortening, DNA damage accumulation, and decreased DNA repair capacity. Furthermore, the review explores the epigenetic changes that occur with aging, including modifications to histones, DNA methylation patterns, and the role of non-coding RNAs. Finally, the review discusses the organization of chromatin and its contribution to genomic instability, including heterochromatin loss, chromatin remodeling, and changes in nucleosome and histone abundance. In conclusion, this review highlights the fundamental role that genomic instability plays in the aging process and underscores the need for continued research into these complex biological mechanisms.

Identification of the Telomere elongation Mutation in Drosophila

Telomeres in Drosophila melanogaster, which have inspired a large part of Sergio Pimpinelli work, are similar to those of other eukaryotes in terms of their function. Yet, their length maintenance relies on the transposition of the specialized retrotransposons Het-A, TART, and TAHRE, rather than on the activity of the enzyme telomerase as it occurs in most other eukaryotic organisms. The length of the telomeres in Drosophila thus depends on the number of copies of these transposable elements. Our previous work has led to the isolation of a dominant mutation, Tel1, that caused a several-fold elongation of telomeres. In this study, we molecularly identified the Tel1 mutation by a combination of transposon-induced, site-specific recombination and next-generation sequencing. Recombination located Tel1 to a 15 kb region in 92A. Comparison of the DNA sequence in this region with the Drosophila Genetic Reference Panel of wild-type genomic sequences delimited Tel1 to a 3 bp deletion inside intron 8 of Ino80. Furthermore, CRISPR/Cas9-induced deletions surrounding the same region exhibited the Tel1 telomere phenotype, confirming a strict requirement of this intron 8 gene sequence for a proper regulation of Drosophila telomere length.

The Annoying Flaws of Gerontological Research

Gerontological research has accelerated dramatically in the last few decades. However, despite increased public interest, federal funding, an army of researchers, and many notable discoveries and high-impact publications, the goal of achieving even a modest extension of human lifespan seems to be as far away as ever or, at best, remains within the realm of lifestyle and diet optimization efforts. Humanity has already benefited from a lifespan revolution in the first half of 20^th Century, which was brought about by improved sanitation and hygiene, clean water, and our successful war on infectious diseases. Thanks to all these developments, in which gerontologists played no part, our expected lifespan increased by about 40% and our primary causes of death decidedly shifted from extrinsic to intrinsic causality. The next step is not that simple as it implies tackling intrinsic mechanisms of aging, and the lack of working human-specific antiaging solutions likely stems from flawed research strategies.

Taming active transposons at Drosophila telomeres: The interconnection between HipHop's roles in capping and transcriptional silencing

Drosophila chromosomes are elongated by retrotransposon attachment, a process poorly understood. Here we characterized a mutation affecting the HipHop telomere-capping protein. In mutant ovaries and the embryos that they produce, telomere retrotransposons are activated and transposon RNP accumulates. Genetic results are consistent with that this hiphop mutation weakens the efficacy of HP1-mediated silencing while leaving piRNA-based mechanisms largely intact. Remarkably, mutant females display normal fecundity suggesting that telomere de-silencing is compatible with germline development. Moreover, unlike prior mutants with overactive telomeres, the hiphop stock does not over-accumulate transposons for hundreds of generations. This is likely due to the loss of HipHop’s abilities both to silence transcription and to recruit transposons to telomeres in the mutant. Furthermore, embryos produced by mutant mothers experience a checkpoint activation, and a further loss of maternal HipHop leads to end-to-end fusion and embryonic arrest. Telomeric retroelements fulfill an essential function yet maintain a potentially conflicting relationship with their Drosophila host. Our study thus showcases a possible intermediate in this arm race in which the host is adapting to over-activated transposons while maintaining genome stability. Our results suggest that the collapse of such a relationship might only occur when the selfish element acquires the ability to target non-telomeric regions of the genome. HipHop is likely part of this machinery restricting the elements to the gene-poor region of telomeres. Lastly, our hiphop mutation behaves as a recessive suppressor of PEV that is mediated by centric heterochromatin, suggesting its broader effect on chromatin not limited to telomeres.

Transposons are selfish elements that multiply by inserting extra copies of themselves into the host genome. Active transposons thus threaten the stability of the host genome, while the host responses by transcriptionally silencing the selfish elements or targeting their insertions towards gene-poor regions of the genome. Chromosome ends (telomeres) in the fruit fly Drosophila are elongated by active transposition of retrotransposons. Although much is known about how these elements are silenced, little is known about the remarkable accuracy by which they are targeted to telomeres. Prime candidates through which the host mounts such defenses are members of the protein complexes that protect telomeres. Here we characterized a hypomorphic mutation of the HipHop protein, and showed that active telomeric transcription in the mutant germline persists for generations without leading to runaway telomere elongation, that embryos laid by the mutant female suffer rampant end-to-end fusions, and that telomeric targeting of the transposon machinery is defective in the mutant soma. Collectively our data suggest that HipHop is essential for preventing telomere fusions, silencing telomeric transposons, and recruiting transposon machinery to telomeres. Our study thus identifies a factor essential for the host control over active transposons and a paradigm for studying such control mechanisms.

Potential Antiaging Effects of DLBS1649, a Centella asiatica Bioactive Extract

Purpose

Centella asiatica is a traditional medicinal plant, especially for wound healing and as a neuroprotective agent. DLBS1649 is a bioactive extract from C. asiatica, and was studied to investigate its benefits as an antiaging agent.

Methods

DLBS1649 was administered to HEK293 and 3T3L1 mammalian cells cultured in a time- or dose-dependent manner. Telomere length analysis was performed. TERT, CMYC, SIRT1, SIRT2, and KL expression were observed using reverse-transcription qPCR. Telomerase protein was studied with ELISA, while calorie restriction was observed using Oil Red O. In vivo study was conducted using Drosophila melanogaster with restricted mean survival time as the statistical method of analysis.

Results

DLBS1649 50 µg/mL showed an effect in the prevention of telomere shortening by 50% and decrease in telomerase activity by 28% compared to the controls (70% and 40%, respectively) in the HEK293 cell cultures. TERT-, CMYC-, SIRT1-, SIRT2-, and KL-expression degression was also reduced (29%, 9%, 18%, 25%, 9%, and 30%, respectively) compared to the controls (46%, 40%, 56%, 44%, and 46%, respectively) after ten serial passages. Calorie-restriction activity from DLBS1649 50 µg/mL was seen, with lower fat droplet counts being detected in the treated samples (37%) than the controls (28%) in 3T3L1 cells. DLBS1649 2 mg/mL increased restricted mean survival time in male and female D. melanogaster (23.87% [p<0.05] and 12.58%, respectively).

Conclusion

The results revealed DLBS1649’s potential as an antiaging agent based on telomere-length preservation, decreased expression of aging-related genes, increased calorie restriction in vitro, and mortality reduction in D. melanogaster in vivo.

The Power of Stress: The Telo-Hormesis Hypothesis

Adaptative response to stress is a strategy conserved across evolution to promote survival. In this context, the groundbreaking findings of Miroslav Radman on the adaptative value of changing mutation rates opened new avenues in our understanding of stress response. Inspired by this work, we explore here the putative beneficial effects of changing the ends of eukaryotic chromosomes, the telomeres, in response to stress. We first summarize basic principles in telomere biology and then describe how various types of stress can alter telomere structure and functions. Finally, we discuss the hypothesis of stress-induced telomere signaling with hormetic effects.

Transposable Element Landscape in Drosophila Populations Selected for Longevity

Transposable elements (TEs) inflict numerous negative effects on health and fitness as they replicate by integrating into new regions of the host genome. Even though organisms employ powerful mechanisms to demobilize TEs, transposons gradually lose repression during aging. The rising TE activity causes genomic instability and was implicated in age-dependent neurodegenerative diseases, inflammation, and the determination of lifespan. It is therefore conceivable that long-lived individuals have improved TE silencing mechanisms resulting in reduced TE expression relative to their shorter-lived counterparts and fewer genomic insertions. Here, we test this hypothesis by performing the first genome-wide analysis of TE insertions and expression in populations of Drosophila melanogaster selected for longevity through late-life reproduction for 50–170 generations from four independent studies. Contrary to our expectation, TE families were generally more abundant in long-lived populations compared with nonselected controls. Although simulations showed that this was not expected under neutrality, we found little evidence for selection driving TE abundance differences. Additional RNA-seq analysis revealed a tendency for reducing TE expression in selected populations, which might be more important for lifespan than regulating genomic insertions. We further find limited evidence of parallel selection on genes related to TE regulation and transposition. However, telomeric TEs were genomically and transcriptionally more abundant in long-lived flies, suggesting improved telomere maintenance as a promising TE-mediated mechanism for prolonging lifespan. Our results provide a novel viewpoint indicating that reproduction at old age increases the opportunity of TEs to be passed on to the next generation with little impact on longevity.

Rapid evolution at the Drosophila telomere: transposable element dynamics at an intrinsically unstable locus

Drosophila telomeres have been maintained by three families of active transposable elements (TEs), HeT-A, TAHRE, and TART, collectively referred to as HTTs, for tens of millions of years, which contrasts with an unusually high degree of HTT interspecific variation. While the impacts of conflict and domestication are often invoked to explain HTT variation, the telomeres are unstable structures such that neutral mutational processes and evolutionary tradeoffs may also drive HTT evolution. We leveraged population genomic data to analyze nearly 10,000 HTT insertions in 85  Drosophila melanogaster genomes and compared their variation to other more typical TE families. We observe that occasional large-scale copy number expansions of both HTTs and other TE families occur, highlighting that the HTTs are, like their feral cousins, typically repressed but primed to take over given the opportunity. However, large expansions of HTTs are not caused by the runaway activity of any particular HTT subfamilies or even associated with telomere-specific TE activity, as might be expected if HTTs are in strong genetic conflict with their hosts. Rather than conflict, we instead suggest that distinctive aspects of HTT copy number variation and sequence diversity largely reflect telomere instability, with HTT insertions being lost at much higher rates than other TEs elsewhere in the genome. We extend previous observations that telomere deletions occur at a high rate, and surprisingly discover that more than one-third do not appear to have been healed with an HTT insertion. We also report that some HTT families may be preferentially activated by the erosion of whole telomeres, implying the existence of HTT-specific host control mechanisms. We further suggest that the persistent telomere localization of HTTs may reflect a highly successful evolutionary strategy that trades away a stable insertion site in order to have reduced impact on the host genome. We propose that HTT evolution is driven by multiple processes, with niche specialization and telomere instability being previously underappreciated and likely predominant.

Adaptive evolution of an essential telomere protein restricts telomeric retrotransposons

Essential, conserved cellular processes depend not only on essential, strictly conserved proteins but also on essential proteins that evolve rapidly. To probe this poorly understood paradox, we exploited the rapidly evolving Drosophila telomere-binding protein, cav/HOAP, which protects chromosomes from lethal end-to-end fusions. We replaced the D. melanogaster HOAP with a highly diverged version from its close relative, D. yakuba. The D. yakuba HOAP ('HOAP[yak]') localizes to D. melanogaster telomeres and protects D. melanogaster chromosomes from fusions. However, HOAP[yak] fails to rescue a previously uncharacterized HOAP function: silencing of the specialized telomeric retrotransposons that, instead of telomerase, maintain chromosome length in Drosophila. Whole genome sequencing and cytogenetics of experimentally evolved populations revealed that HOAP[yak] triggers telomeric retrotransposon proliferation, resulting in aberrantly long telomeres. This evolution-generated, separation-of-function allele resolves the paradoxical observation that a fast-evolving essential gene directs an essential, strictly conserved function: telomeric retrotransposon containment, not end-protection, requires evolutionary innovation at HOAP.

Telomeric TART elements target the piRNA machinery in Drosophila

Coevolution between transposable elements (TEs) and their hosts can be antagonistic, where TEs evolve to avoid silencing and the host responds by reestablishing TE suppression, or mutualistic, where TEs are co-opted to benefit their host. The TART-A TE functions as an important component of Drosophila telomeres but has also reportedly inserted into the Drosophila melanogaster nuclear export factor gene nxf2. We find that, rather than inserting into nxf2, TART-A has actually captured a portion of nxf2 sequence. We show that TART-A produces abundant Piwi-interacting small RNAs (piRNAs), some of which are antisense to the nxf2 transcript, and that the TART-like region of nxf2 is evolving rapidly. Furthermore, in D. melanogaster, TART-A is present at higher copy numbers, and nxf2 shows reduced expression, compared to the closely related species Drosophila simulans. We propose that capturing nxf2 sequence allowed TART-A to target the nxf2 gene for piRNA-mediated repression and that these 2 elements are engaged in antagonistic coevolution despite the fact that TART-A is serving a critical role for its host genome.

Co-evolution between transposable elements (TEs) and their hosts can be antagonistic, where TEs evolve to avoid silencing and the host responds by re-establishing TE suppression, or mutualistic, where TEs are co-opted to benefit their host. This study shows that a specialized Drosophila retrotransposon that functions as a telomere has captured a portion of a host piRNA gene which may allow it to evade silencing.

The Beginning of the End: A Chromosomal Assembly of the New World Malaria Mosquito Ends with a Novel Telomere

Chromosome level assemblies are accumulating in various taxonomic groups including mosquitoes. However, even in the few reference-quality mosquito assemblies, a significant portion of the heterochromatic regions including telomeres remain unresolved. Here we produce a de novo assembly of the New World malaria mosquito, Anopheles albimanus by integrating Oxford Nanopore sequencing, Illumina, Hi-C and optical mapping. This 172.6 Mbps female assembly, which we call AalbS3, is obtained by scaffolding polished large contigs (contig N50 = 13.7 Mbps) into three chromosomes. All chromosome arms end with telomeric repeats, which is the first in mosquito assemblies and represents a significant step toward the completion of a genome assembly. These telomeres consist of tandem repeats of a novel 30-32 bp Telomeric Repeat Unit (TRU) and are confirmed by analyzing the termini of long reads and through both chromosomal in situ hybridization and a Bal31 sensitivity assay. The AalbS3 assembly included previously uncharacterized centromeric and rDNA clusters and more than doubled the content of transposable elements and other repetitive sequences. This telomere-to-telomere assembly, although still containing gaps, represents a significant step toward resolving biologically important but previously hidden genomic components. The comparison of different scaffolding methods will also inform future efforts to obtain reference-quality genomes for other mosquito species.

Telomere dynamic in humans and animals: Review and perspectives in environmental toxicology

Telomeres (TLs) play major roles in stabilizing the genome and are usually shortened with ageing. The maintenance of TLs is ensured by two mechanisms involving telomerase (TA) enzyme and alternative lengthening telomeres (ALT). TL shortening and/or TA inhibition have been related to health effects on organisms (leading to reduced reproductive lifespan and survival), suggesting that they could be key processes in toxicity mechanisms (at molecular and cellular levels) and relevant as an early warning of exposure and effect of chemicals on human health and animal population dynamics. Consequently, a critical analysis of knowledge about relationships between TL dynamic and environmental pollution is essential to highlight the relevance of TL measurement in environmental toxicology. The first objective of this review is to provide a survey on the basic knowledge about TL structure, roles, maintenance mechanisms and causes of shortening in both vertebrates (including humans) and invertebrates. Overall, TL length decreases with ageing but some unexpected exceptions are reported (e.g., in species with different lifespans, such as the nematode Caenorhabditis elegans or the crustacean Homarus americanus). Inconsistent results reported in various biological groups or even between species of the same genus (e.g., the microcrustacean Daphnia sp.) indicate that the relation usually proposed between TL shortening and a decrease in TA activity cannot be generalized and depends on the species, stage of development or lifespan. Although the scientific literature provides evidence of the effect of ageing on TL shortening, much less information on the relationships between shortening, maintenance of TLs, influence of other endogenous and environmental drivers, including exposure to chemical pollutants, is available, especially in invertebrates. The second objective of this review is to connect knowledge on TL dynamic and exposure to contaminants. Most of the studies published on humans rely on correlative epidemiological approaches and few in vitro experiments. They have shown TL attrition when exposed to contaminants, such as polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), pesticides and metallic elements (ME). In other vertebrates, the studies we found deals mainly with birds and, overall, report a disturbance of TL dynamic consecutively to exposure to chemicals, including metals and organic compounds. In invertebrates, no data are available and the potential of TL dynamic in environmental risk assessment remains to be explored. On the basis of the main gaps identified some research perspectives (e.g., impact of endogenous and environmental drivers, dose response effects, link between TL length, TA activity, longevity and ageing) are proposed to better understand the potential of TL and TA measurements in humans and animals in environmental toxicology.

Social behavior and aging: A fly model

The field of behavioral genetics has recently begun to explore the effect of age on social behaviors. Such studies are particularly important, as certain neuropsychiatric disorders with abnormal social interactions, like autism and schizophrenia, have been linked to older parents. Appropriate social interaction can also have a positive impact on longevity, and is associated with successful aging in humans. Currently, there are few genetic models for understanding the effect of aging on social behavior and its potential transgenerational inheritance. The fly is emerging as a powerful model for identifying the basic molecular mechanisms underlying neurological and neuropsychiatric disorders. In this review, we discuss these recent advancements, with a focus on how studies in Drosophila melanogaster have provided insight into the effect of aging on aspects of social behavior, including across generations.

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.

Sex Differences in Aging: Genomic Instability

Aging is characterized by decreasing physiological integration, reduced function, loss of resilience, and increased risk of death. Paradoxically, although women live longer, they suffer greater morbidity particularly late in life. These sex differences in human lifespan and healthspan are consistently observed in all countries and during every era for which reliable data exist. While these differences are ubiquitous in humans, evidence of sex differences in longevity and health for other species is more equivocal. Among fruit flies, nematodes, and mice, sex differences in lifespan vary depending on strain and treatment. In this review, we focus on sex differences in age-related alterations in DNA damage and mutation rates, telomere attrition, epigenetics, and nuclear architecture. We find that robust sex differences exist, eg, the higher incidence of DNA damage in men compared to women, but sex differences are not often conserved between species. For most mechanisms reviewed here, there are insufficient data to make a clear determination regarding the impact of sex, largely because sex differences have not been analyzed. Overall, our findings reveal an urgent need for well-designed studies that explicitly examine sex differences in molecular drivers of aging.

Ectothermic telomeres: it's time they came in from the cold

We review the evolutionary ecology and genetics of telomeres in taxa that cannot elevate their body temperature to a preferred level through metabolism but do so by basking or seeking out a warm environment. This group of organisms contains all living things on earth, apart from birds and mammals. One reason for our interest in this synthetic group is the argument that high, stable body temperature increases the risk of malignant tumours if long, telomerase-restored telomeres make cells 'live forever'. If this holds true, ectotherms should have significantly lower cancer frequencies. We discuss to what degree there is support for this 'anti-cancer' hypothesis in the current literature. Importantly, we suggest that ectothermic taxa, with variation in somatic telomerase expression across tissue and taxa, may hold the key to understanding ongoing selection and evolution of telomerase dynamics in the wild. We further review endotherm-specific effects of growth on telomeres, effects of autotomy ('tail dropping') on telomere attrition, and costs of maintaining sexual displays measured in telomere attrition. Finally, we cover plant ectotherm telomeres and life histories in a separate 'mini review'.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

Genomic Instabilities, Cellular Senescence, and Aging: In Vitro, In Vivo and Aging-Like Human Syndromes

As average life span and elderly people prevalence in the western world population is gradually increasing, the incidence of age-related diseases such as cancer, heart diseases, diabetes, and dementia is increasing, bearing social and economic consequences worldwide. Understanding the molecular basis of aging-related processes can help extend the organism’s health span, i.e., the life period in which the organism is free of chronic diseases or decrease in basic body functions. During the last few decades, immense progress was made in the understanding of major components of aging and healthy aging biology, including genomic instability, telomere attrition, epigenetic changes, proteostasis, nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and intracellular communications. This progress has been made by three spear-headed strategies: in vitro (cell and tissue culture from various sources), in vivo (includes diverse model and non-model organisms), both can be manipulated and translated to human biology, and the study of aging-like human syndromes and human populations. Herein, we will focus on current repository of genomic “senescence” stage of aging, which includes health decline, structural changes of the genome, faulty DNA damage response and DNA damage, telomere shortening, and epigenetic alterations. Although aging is a complex process, many of the “hallmarks” of aging are directly related to DNA structure and function. This review will illustrate the variety of these studies, done in in vitro, in vivo and human levels, and highlight the unique potential and contribution of each research level and eventually the link between them.

Proliferation Cycle Causes Age Dependent Mitochondrial Deficiencies and Contributes to the Aging of Stem Cells

In addition to chronological aging, stem cells are also subject to proliferative aging during the adult life span. However, the consequences of proliferative cycle and their contributions to stem cells aging have not been well investigated. Using Drosophila female germ line stem cells as a model, we found that the replication cycle leads to the age dependent decline of female fecundity, and is a major factor causing developmental abnormalities in the progeny of old females. The proliferative aging does not cause telomere shortening, but causes an accumulation of mitochondrial DNA (mtDNA) mutations or rearrangements at the control region. We propose that damaging mutations on mtDNA caused by accumulation of proliferation cycles in aged stem cells may disrupt mitochondrial respiration chain and impair mtDNA replication and represent a conserved mechanism underlying stem cell aging.

Telomeres, Aging and Exercise: Guilty by Association?

Telomeres are repetitive tandem DNA sequences that cap chromosomal ends protecting genomic DNA from enzymatic degradation. Telomeres progressively shorten with cellular replication and are therefore assumed to correlate with biological and chronological age. An expanding body of evidence suggests (i) a predictable inverse association between telomere length, aging and age-related diseases and (ii) a positive association between physical activity and telomere length. Both hypotheses have garnered tremendous research attention and broad consensus; however, the evidence for each proposition is inconsistent and equivocal at best. Telomere length does not meet the basic criteria for an aging biomarker and at least 50% of key studies fail to find associations with physical activity. In this review, we address the evidence in support and refutation of the putative associations between telomere length, aging and physical activity. We finish with a brief review of plausible mechanisms and potential future research directions.

Epigenetic factors Polycomb (Pc) and Suppressor of zeste (Su(z)2) negatively regulate longevity in Drosophila melanogaster

The process of aging is a hallmark of the natural life span of all organisms and individuals within a population show variability in the measures of age related performance. Longevity and the rate of aging are influenced by several factors such as genetics, nutrition, stress, and environment. Many studies have focused on the genes that impact aging and there is increasing evidence that epigenetic factors regulate these genes to control life span. Polycomb (PcG) and trithorax (trxG) protein complexes maintain the expression profiles of developmentally important genes and regulate many cellular processes. Here, we report that mutations of PcG and trxG members affect the process of aging in Drosophila melanogaster, with perturbations mostly associated with retardation in aging. We find that mutations in polycomb repressive complex (PRC1) components Pc and Su(z)2 increase fly survival. Using an inducible UAS-GAL4 system, we show that this effect is tissue-specific; knockdown in fat body, but not in muscle or brain tissues, enhances life span. We hypothesize that these two proteins influence life span via pathways independent of their PRC1 functions, with distinct effects on response to oxidative stress. Our observations highlight the role of global epigenetic regulators in determining life span.

dAdd1 and dXNP prevent genome instability by maintaining HP1a localization at Drosophila telomeres

Telomeres are important contributors to genome stability, as they prevent linear chromosome end degradation and contribute to the avoidance of telomeric fusions. An important component of the telomeres is the heterochromatin protein 1a (HP1a). Mutations in Su(var)205, the gene encoding HP1a in Drosophila, result in telomeric fusions, retrotransposon regulation loss and larger telomeres, leading to chromosome instability. Previously, it was found that several proteins physically interact with HP1a, including dXNP and dAdd1 (orthologues to the mammalian ATRX gene). In this study, we found that mutations in the genes encoding the dXNP and dAdd1 proteins affect chromosome stability, causing chromosomal aberrations, including telomeric defects, similar to those observed in Su(var)205 mutants. In somatic cells, we observed that dXNP and dAdd1 participate in the silencing of the telomeric HTT array of retrotransposons, preventing anomalous retrotransposon transcription and integration. Furthermore, the lack of dAdd1 results in the loss of HP1a from the telomeric regions without affecting other chromosomal HP1a binding sites; mutations in dxnp also affected HP1a localization but not at all telomeres, suggesting a specialized role for dAdd1 and dXNP proteins in locating HP1a at the tips of the chromosomes. These results place dAdd1 as an essential regulator of HP1a localization and function in the telomere heterochromatic domain.

Natural variation of piRNA expression affects immunity to transposable elements

In the Drosophila germline, transposable elements (TEs) are silenced by PIWI-interacting RNA (piRNA) that originate from distinct genomic regions termed piRNA clusters and are processed by PIWI-subfamily Argonaute proteins. Here, we explore the variation in the ability to restrain an alien TE in different Drosophila strains. The I-element is a retrotransposon involved in the phenomenon of I-R hybrid dysgenesis in Drosophila melanogaster. Genomes of R strains do not contain active I-elements, but harbour remnants of ancestral I-related elements. The permissivity to I-element activity of R females, called reactivity, varies considerably in natural R populations, indicating the existence of a strong natural polymorphism in defense systems targeting transposons. To reveal the nature of such polymorphisms, we compared ovarian small RNAs between R strains with low and high reactivity and show that reactivity negatively correlates with the ancestral I-element-specific piRNA content. Analysis of piRNA clusters containing remnants of I-elements shows increased expression of the piRNA precursors and enrichment by the Heterochromatin Protein 1 homolog, Rhino, in weak R strains, which is in accordance with stronger piRNA expression by these regions. To explore the nature of the differences in piRNA production, we focused on two R strains, weak and strong, and showed that the efficiency of maternal inheritance of piRNAs as well as the I-element copy number are very similar in both strains. At the same time, germline and somatic uni-strand piRNA clusters generate more piRNAs in strains with low reactivity, suggesting the relationship between the efficiency of primary piRNA production and variable response to TE invasions. The strength of adaptive genome defense is likely driven by naturally occurring polymorphisms in the rapidly evolving piRNA pathway proteins. We hypothesize that hyper-efficient piRNA production is contributing to elimination of a telomeric retrotransposon HeT-A, which we have observed in one particular transposon-resistant R strain.

Transposon activity in the germline is suppressed by the PIWI-interacting RNA (piRNA) pathway. The resistance of natural Drosophila strains to transposon invasion varies considerably, but the nature of this variability is unknown. We discovered that natural variation in the efficiency of primary piRNA production in the germline causes dramatic differences in the susceptibility to expansion of a newly invaded transposon. A high level of piRNA production in the germline is achieved by increased expression of piRNA precursors. In one of the most transposon-resistant strains, increased content of primary piRNA is observed in both the germline and ovarian somatic cells. We suggest that polymorphisms in piRNA pathway factors are responsible for increased piRNA production. piRNA pathway proteins have been shown to be evolving rapidly under selective pressure. Our data are the first to describe a phenotype that might be caused by this kind of polymorphism. We also demonstrate a likely explanation as to why an overly active piRNA pathway can cause more harm than good in Drosophila: Highly efficient piRNA processing leads to elimination of domesticated telomeric retrotransposons essential for telomere elongation, an effect which has been observed in a natural strain that is extremely resistant to transposon invasion.

Recurrent Innovation at Genes Required for Telomere Integrity in Drosophila

Telomeres are nucleoprotein complexes at the ends of linear chromosomes. These specialized structures ensure genome integrity and faithful chromosome inheritance. Recurrent addition of repetitive, telomere-specific DNA elements to chromosome ends combats end-attrition, while specialized telomere-associated proteins protect naked, double-stranded chromosome ends from promiscuous repair into end-to-end fusions. Although telomere length homeostasis and end-protection are ubiquitous across eukaryotes, there is sporadic but building evidence that the molecular machinery supporting these essential processes evolves rapidly. Nevertheless, no global analysis of the evolutionary forces that shape these fast-evolving proteins has been performed on any eukaryote. The abundant population and comparative genomic resources of Drosophila melanogaster and its close relatives offer us a unique opportunity to fill this gap. Here we leverage population genetics, molecular evolution, and phylogenomics to define the scope and evolutionary mechanisms driving fast evolution of genes required for telomere integrity. We uncover evidence of pervasive positive selection across multiple evolutionary timescales. We also document prolific expansion, turnover, and expression evolution in gene families founded by telomeric proteins. Motivated by the mutant phenotypes and molecular roles of these fast-evolving genes, we put forward four alternative, but not mutually exclusive, models of intra-genomic conflict that may play out at very termini of eukaryotic chromosomes. Our findings set the stage for investigating both the genetic causes and functional consequences of telomere protein evolution in Drosophila and beyond.

The short-lived African turquoise killifish: an emerging experimental model for ageing

Human ageing is a fundamental biological process that leads to functional decay, increased risk for various diseases and, ultimately, death. Some of the basic biological mechanisms underlying human ageing are shared with other organisms; thus, animal models have been invaluable in providing key mechanistic and molecular insights into the common bases of biological ageing. In this Review, we briefly summarise the major applications of the most commonly used model organisms adopted in ageing research and highlight their relevance in understanding human ageing. We compare the strengths and limitations of different model organisms and discuss in detail an emerging ageing model, the short-lived African turquoise killifish. We review the recent progress made in using the turquoise killifish to study the biology of ageing and discuss potential future applications of this promising animal model.

Questioning causal involvement of telomeres in aging

Multiple studies have demonstrated that telomere length predicts mortality and that telomeres shorten with age. Although rarely acknowledged these associations do not dictate causality. I review telomerase knockout and overexpression studies and find little support that telomeres cause aging. In addition, the causality hypothesis assumes that there is a critical telomere length at which senescence is induced. This generates the prediction that variance in telomere length decreases with age. In contrast, using meta-analysis of human data, I find no such decline. Inferring the causal involvement of telomeres in aging from current knowledge is therefore speculative and could hinder scientific progress.

Functional diversification of yeast telomere associated protein, Rif1, in higher eukaryotes

Background

Telomeres are nucleoprotein complexes at the end of linear eukaryotic chromosomes which maintain the genome integrity by regulating telomere length, preventing recombination and end to end fusion events. Multiple proteins associate with telomeres and function in concert to carry out these functions. Rap1 interacting factor 1 (Rif1), was identified as a protein involved in telomere length regulation in yeast. Rif1 is conserved upto mammals but its function has diversified from telomere length regulation to maintenance of genome integrity.

Results

We have carried out detailed bioinformatic analyses and identified Rif1 homologues in 92 organisms from yeast to human. We identified Rif1 homologues in Drosophila melanogaster, even though fly telomeres are maintained by a telomerase independent pathway. Our analysis shows that Drosophila Rif1 (dRif1) sequence is phylogenetically closer to the one of vertebrates than yeast and has identified a few Rif1 specific motifs conserved through evolution. This includes a Rif1 family specific conserved region within the HEAT repeat domain and a motif involved in protein phosphatase1 docking. We show that dRif1 is nuclear localized with a prominent heterochromatin association and unlike human Rif1, it does not respond to DNA damage by localizing to damaged sites. To test the evolutionary conservation of dRif1 function, we expressed the dRif1 protein in yeast and HeLa cells. In yeast, dRif1 did not perturb yeast Rif1 (yRif1) functions; and in HeLa cells it did not colocalize with DNA damage foci.

Conclusions

Telomeres are maintained by retrotransposons in all Drosophila species and consequently, telomerase and many of the telomere associated protein homologues are absent, including Rap1, which is the binding partner of Rif1. We found that a homologue of yRif1 protein is present in fly and dRif1 has evolutionarily conserved motifs. Functional studies show that dRif1 responds differently to DNA damage, implying that dRif1 may have a different function and this may be conserved in other organisms as well.

Taurine reduces ER stress in C. elegans

Background

ER stress is a strong indicator of whether or not a cell is undergoing physiological stress. C. elegans is a practical system of characterizing the effect of ER stress at the in vivo or organismal level.

Methods

This study characterized taurine’s anti-ER stress potential employing western blotting on ER stress markers and assays of motility, lifespan comparison, and fecundity measurement.

Results

When treated with tunicamycin, C. elegans showed the typical ER stress symptoms. It showed a higher expression of hsp-70 and skn-1 than the non-treated control. Survivorship significantly decreased under tunicamycin treatment, and the offspring number also decreased. During the synchronized culture under ER stress conditions, the C. elegans showed early signs of aging especially between L3 and L4 within their life span, along with lowered motility. The worms, however, showed a positive response to the taurine treatment under ER stress conditions.

Conclusions

When C. elegans were treated with taurine before or after the tunicamycin treatment, they showed a less severe level of ER stress, including an enhanced survivorship, increased motility, and augmented fecundity. Taken together, these results strongly indicate that taurine works positively to cope with ER stress from the organismal perspective.

A 'higher order' of telomere regulation: telomere heterochromatin and telomeric RNAs

Protection of chromosome ends from DNA repair and degradation activities is mediated by specialized protein complexes bound to telomere repeats. Recently, it has become apparent that epigenetic regulation of the telomric chromatin template critically impacts on telomere function and telomere-length homeostasis from yeast to man. Across all species, telomeric repeats as well as the adjacent subtelomeric regions carry features of repressive chromatin. Disruption of this silent chromatin environment results in loss of telomere-length control and increased telomere recombination. In turn, progressive telomere loss reduces chromatin compaction at telomeric and subtelomeric domains. The recent discoveries of telomere chromatin regulation during early mammalian development, as well as during nuclear reprogramming, further highlights a central role of telomere chromatin changes in ontogenesis. In addition, telomeres were recently shown to generate long, non-coding RNAs that remain associated to telomeric chromatin and will provide new insights into the regulation of telomere length and telomere chromatin. In this review, we will discuss the epigenetic regulation of telomeres across species, with special emphasis on mammalian telomeres. We will also discuss the links between epigenetic alterations at mammalian telomeres and telomere-associated diseases.

Telomere recombination accelerates cellular aging in Saccharomyces cerevisiae

Telomeres are nucleoprotein structures located at the linear ends of eukaryotic chromosomes. Telomere integrity is required for cell proliferation and survival. Although the vast majority of eukaryotic species use telomerase as a primary means for telomere maintenance, a few species can use recombination or retrotransposon-mediated maintenance pathways. Since Saccharomyces cerevisiae can use both telomerase and recombination to replicate telomeres, budding yeast provides a useful system with which to examine the evolutionary advantages of telomerase and recombination in preserving an organism or cell under natural selection. In this study, we examined the life span in telomerase-null, post-senescent type II survivors that have employed homologous recombination to replicate their telomeres. Type II recombination survivors stably maintained chromosomal integrity but exhibited a significantly reduced replicative life span. Normal patterns of cell morphology at the end of a replicative life span and aging-dependent sterility were observed in telomerase-null type II survivors, suggesting the type II survivors aged prematurely in a manner that is phenotypically consistent with that of wild-type senescent cells. The shortened life span of type II survivors was extended by calorie restriction or TOR1 deletion, but not by Fob1p inactivation or Sir2p over-expression. Intriguingly, rDNA recombination was decreased in type II survivors, indicating that the premature aging of type II survivors was not caused by an increase in extra-chromosomal rDNA circle accumulation. Reintroduction of telomerase activity immediately restored the replicative life span of type II survivors despite their heterogeneous telomeres. These results suggest that telomere recombination accelerates cellular aging in telomerase-null type II survivors and that telomerase is likely a superior telomere maintenance pathway in sustaining yeast replicative life span.

Telomeres are the specialized structures at the ends of eukaryotic linear chromosomes. The simple guanine-rich DNA repeats at telomeres and their associated proteins are important for chromosome stability. Most eukaryotic species have evolved an enzyme named telomerase to replicate their telomeric DNA. Telomerase usually contains a protein catalytic subunit and a RNA template subunit. A few eukaryotic species can use either telomere recombination or retrotransposon-mediated transposition to accomplish telomere elongation. Interestingly, the baker's yeast Saccharomyces cerevisiae can use both telomerase and recombination to replicate telomeres. In this study, we utilize this unique eukaryotic model system to compare the efficiency of these two mechanisms in the maintenance of cellular function and life span. Telomerase-null cells that used recombination to elongate telomeres were able to maintain relatively stable chromosomes; however, they exhibited a shortened replicative life span which may represent a novel aging pathway. Reintroduction of telomerase inhibited telomere recombination and restored the replicative life span of these cells, implying that telomerase is superior to telomere recombination in the regulation of yeast replicative life span.

Regulation of telomere length in Drosophila

Telomeres in all organisms must perform the same vital functions to ensure cell viability: to act as a protective chromosome cap that distinguishes natural chromosome ends from DNA double strand breaks, and to balance the loss of DNA from the chromosome end due to incomplete DNA replication. Most eukaryotes rely on a specialized reverse transcriptase, telomerase, to generate short repeats at the chromosome end to maintain chromosome length. Drosophila, however, uses retrotransposons that target telomeres. Transposition of these elements may be controlled by small RNAs and spreading of silent chromatin from the telomere associated sequence, both of which limit the retrotransposon expression level. Proteins binding to the retrotransposon array, such as HP1 and PROD, may also modulate transcription. It is not clear however, that simply increasing transcript levels of the telomeric retrotransposons is sufficient to increase transposition. The chromosome cap may control the ability of the telomere-specific elements to attach to chromosome ends. As in other organisms, chromosomes can be elongated by gene conversion. Although the mechanism is not known, HP1, a component of the cap, and the Ku proteins are key components in this pathway.

Drosophila telomeres: an exception providing new insights

Drosophila telomeres comprise DNA sequences that differ dramatically from those of other eukaryotes. Telomere functions, however, are similar to those found in telomerase-based telomeres, even though the underlying mechanisms may differ. Drosophila telomeres use arrays of retrotransposons to maintain chromosome length, while nearly all other eukaryotes rely on telomerase-generated short repeats. Regardless of the DNA sequence, several end-binding proteins are evolutionarily conserved. Away from the end, the Drosophila telomeric and subtelomeric DNA sequences are complexed with unique combinations of proteins that also modulate chromatin structure elsewhere in the genome. Maintaining and regulating the transcriptional activity of the telomeric retrotransposons in Drosophila requires specific chromatin structures and, while telomeric silencing spreads from the terminal repeats in yeast, the source of telomeric silencing in Drosophila is the subterminal arrays. However, the subterminal arrays in both species may be involved in telomere-telomere associations and/or communication.

Gene expression profiling during Forskolin induced differentiation of BeWo cells by differential display RT-PCR

The differentiation of cytotrophoblasts into syncytiotrophoblasts in the placenta has been employed as a model to investigate stage specific expression as well as regulation of genes during this process. While the cytotrophoblasts are highly invasive and proliferative with relatively less capacity to synthesize pregnancy related proteins, the multinucleated syncytiotrophoblasts are non-proliferative and non-invasive. However, syncytiotrophoblasts are the site of synthesis of a variety of protein, peptide and steroid hormones as well as several growth factors. Both the freshly isolated cytotrophoblasts from human placenta as well as the BeWo cell, a choriocarcinoma cell line model which retain several characteristic of cytotrophoblasts has been employed by us to study regulation of differentiation. In the present study, we have employed the differential display RT-PCR analysis (DD-RT-PCR) to evaluate gene expression changes during Forskolin induced in vitro differentiation of BeWo cells. We have identified several genes which are differentially expressed during differentiation and the differential expression of 10 transcripts was confirmed by Northern blot analysis. Based on the identity of the transcripts an attempt has been made to relate the known function of the gene products, to changes observed during differentiation. Of the several transcripts, one of the transcripts, namely Secretory Leukocyte Protease Inhibitor (SLPI) which is known to have multiple functions was found to increase 15-fold in the syntiotrophoblast.

Genome-wide analysis reveals differences in brain gene expression patterns associated with caste and reproductive status in honey bees (Apis mellifera)

A key characteristic of eusocial species is reproductive division of labour. Honey bee colonies typically have a single reproductive queen and thousands of sterile workers. Adult queens differ dramatically from workers in anatomy, physiology, behaviour and lifespan. Young female workers can activate their ovaries and initiate egg laying; these 'reproductive' workers differ from sterile workers in anatomy, physiology, and behaviour. These differences, however, are on a much smaller scale than those observed between the queen and worker castes. Here, we use microarrays to monitor expression patterns of several thousand genes in the brains of same-aged virgin queens, sterile workers, and reproductive workers. We found large differences in expression between queens and both worker groups (~2000 genes), and much smaller differences between sterile and reproductive workers (221 genes). The expression patterns of these 221 genes in reproductive workers are more queen-like, and may represent a core group of genes associated with reproductive physiology. Furthermore, queens and reproductive workers preferentially up-regulate genes associated with the nurse bee behavioural state, which supports the hypothesis of an evolutionary link between worker division of labour and molecular pathways related to reproduction. Finally, several functional groups of genes associated with longevity in other species are significantly up-regulated in queens. Identifying the genes that underlie the differences between queens, sterile workers, and reproductive workers will allow us to begin to characterize the molecular mechanisms underlying the evolution of social behaviour and large-scale remodelling of gene networks associated with polyphenisms.