Behaviour of telomere and telomerase during aging and regeneration in zebrafish

Aluminum chloride and D-galactose induced a zebrafish model of Alzheimer's disease with cognitive deficits and aging

Alzheimer's disease (AD) is an age-related neurodegenerative disorder. Transgenic and pharmacological AD models are extensively studied to understand AD mechanisms and drug discovery. However, they are time-consuming and relatively costly, which hinders the discovery of potential anti-AD therapeutics. Here, we established a new model of AD in larval zebrafish by co-treatment with aluminum chloride (AlCl3) and D-galactose (D-gal) for 72 h. In particular, exposure to 150 μM AlCl3 + 40 mg/mL D-gal, 200 μM AlCl3 + 30 mg/mL D-gal, or 200 μM AlCl3 + 40 mg/mL D-gal successfully induced AD-like symptoms and aging features. Co-treatment with AlCl3 and D-gal caused significant learning and memory deficits, as well as impaired response ability and locomotor capacity in the plus-maze and light/dark test. Moreover, increased acetylcholinesterase and β-galactosidase activities, β-amyloid 1-42 deposition, reduced telomerase activity, elevated interleukin 1 beta mRNA expression, and enhanced reactive oxygen species production were also observed. In conclusion, our zebrafish model is simple, rapid, effective and affordable, incorporating key features of AD and aging, thus may become a unique and powerful tool for high-throughput screening of anti-AD compounds in vivo.

Cardiac regeneration in goldfish (Carassius auratus) associated with increased expression of key extracellular matrix molecules

Cardiac regeneration is a natural phenomenon that occurs in many species outside of humans. The goldfish (Carassius auratus) is an understudied model of cardiac wound response, despite its ubiquity as pets as well as its relationship to the better-studied zebrafish. In this study, we examined the response of the goldfish heart to a resection injury. We found that by 70 days post-injury, goldfish scarlessly heal cardiac wounds under a certain size, with local cardiomyocyte proliferation driving the restoration of the myocardial layer. We also found the upregulation of extracellular matrix components related to cardiac regeneration in the injury site. This upregulation correlated with the level of cardiomyocyte proliferation occurring in the injury site, indicating an association between the two that warrants further exploration.

Transcript level of telomerase reverse-transcriptase (TERT) gene in the rainbow trout (Oncorhynchus mykiss) eggs with different developmental competence for gynogenesis

Expression of the telomerase reverse-transcriptase (TERT) gene and activity of telomerase have been reported in the somatic tissues and gonads in fish irrespective of their age and size. Nevertheless, little is known about TERT expression in the fish eggs. In the current study, the presence of the TERT transcripts was confirmed in the rainbow trout ovulated eggs before and after activation with nonirradiated and UV-irradiated (gynogenesis) sperm. Eggs originating from eight females had high and comparable quality expressed by similar hatching rates. However, survival of the gynogenetic larvae that hatched from eggs activated with UV-irradiated sperm and further exposed to the high hydrostatic pressure (HHP) shock for duplication of the maternal chromosomes varied between females from 2.1 ± 0.4 to 40.5 ± 2.2%. Increased level of TERT transcripts was observed in eggs originating from two females, and gametes from only one of them showed improved competence for gynogenesis (27.3 ± 1.9%). In turn, eggs from the female that exhibited the highest survival after gynogenetic activation were characterized by the lowest expression of the TERT gene. Telomerase in rainbow trout eggs may compensate erosion of the telomeres during early embryonic development; however, its upregulation does not assure better development after gynogenetic activation.

Immunolocalization of telomerases in human hairs identifies proliferating cells in the bulb matrix and outer root sheath

Telomerase is present in cells with numerous or even un-limited replicative cycles, and some studies suggest it is a stemness marker. In order to determine whether this is the case for the human hair bulbs, an immunohistochemical and ultrastructural study has been carried out using antibodies against telomerase and PCNA (a cell proliferation marker). The observed labeling is similar for the two antibodies here utilized and is mainly nuclear. More frequent telomerase-positive cells are seen in the matrix epithelium of anagen hair bulbs but sparse labeled cells are also seen in the outer root sheath. In late catagen and also in telogen hair follicles only sparse labeled cells are present in the outer root sheath and few cells also in the secondary germinal epithelium formed at the base of the hair bulb in telogen. Electron microscopic immunogold shows a prevalent nuclear distribution and a lower cytoplasmic distribution in sparse cells of anagen bulb matrix that contain few keratin bundles. The nuclear localization is generally seen over the euchromatin or in areas occupied by more compact chromatin that may indicate an activity of telomerase in chromatin assemblage or dis-assemblage. The study concludes that the localization of telomerase is present in cells undergoing proliferation, namely transit amplifying cells of the outer root sheath that are sparsely detected in the lowermost secondary germinal hair bulb also in telogen.

Directional selection, not the direction of selection, affects telomere length and copy number at ribosomal RNA loci

Many fisheries exert directional selection on traits such as body size and growth rate. Whether directional selection impacts regions of the genome associated with traits related to growth is unknown. To address this issue, we characterised copy number variation in three regions of the genome associated with cell division, (1) telomeric DNA, (2) loci transcribed as ribosomal RNA (rDNA), and (3) mitochondrial DNA (mtDNA), in three selection lines of zebrafish reared at three temperatures (22 °C, 28 °C, and 34 °C). Selection lines differed in (1) the direction of selection (two lines experienced directional selection for large or small body size) and (2) whether they experienced any directional selection itself. Lines that had experienced directional selection were smaller, had lower growth rate, shorter telomeres, and lower rDNA copy number than the line that experiencing no directional selection. Neither telomere length nor rDNA copy number were affected by temperature. In contrast, mtDNA content increased at elevated temperature but did not differ among selection lines. Though directional selection impacts rDNA and telomere length, direction of such selection did not matter, whereas mtDNA acts as a stress marker for temperature. Future work should examine the consequences of these genomic changes in natural fish stocks.

Aging in zebrafish is associated with reduced locomotor activity and strain dependent changes in bottom dwelling and thigmotaxis

Aging is associated with a wide range of physiological and behavioral changes in many species. Zebrafish, like humans, rodents, and birds, exhibits gradual senescence, and thus may be a useful model organism for identifying evolutionarily conserved mechanisms related to aging. Here, we compared behavior in the novel tank test of young (6-month-old) and middle aged (12-month-old) zebrafish from two strains (TL and TU) and both sexes. We find that this modest age difference results in a reduction in locomotor activity in male fish. We also found that background strain modulated the effects of age on predator avoidance behaviors related to anxiety: older female TL fish increased bottom dwelling whereas older male TU fish decreased thigmotaxis. Although there were no consistent effects of age on either short-term (within session) or long-term (next day) habituation to the novel tank, strain affected the habituation response. TL fish tended to increase their distance from the bottom of the tank whereas TU fish had no changes in bottom distance but instead tended to increase thigmotaxis. Our findings support the use of zebrafish for the study of how age affects locomotion and how genetics interacts with age and sex to alter exploratory and emotional behaviors in response to novelty.

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.

Zebrafish-A Suitable Model for Rapid Translation of Effective Therapies for Pediatric Cancers

Pediatric cancers are the leading cause of disease-related deaths in children and adolescents. Most of these tumors are difficult to treat and have poor overall survival. Concerns have also been raised about drug toxicity and long-term detrimental side effects of therapies. In this review, we discuss the advantages and unique attributes of zebrafish as pediatric cancer models and their importance in targeted drug discovery and toxicity assays. We have also placed a special focus on zebrafish models of pediatric brain cancers-the most common and difficult solid tumor to treat.

Harnessing Genetics to Extend Lifespan and Healthspan: Current Progress and Future Directions

Aging is inevitable, but the lifespan (duration of life) and healthspan (healthy aging) vary greatly among individuals and across species. Unlocking the secrets behind these differences has captivated scientific curiosity for ages. This review presents relevant recent advances in genetics and cell biology that are shedding new light by untangling how subtle changes in conserved genes, pathways, and epigenetic factors influence organismal senescence and associated declines. Biogerontology is a complex and rapidly growing field aimed at elucidating genetic modifications that extend lifespan and healthspan. This review explores gerontogenes, genes influencing lifespan and healthspan across species. Though subtle differences exist, long-lived individuals such as centenarians demonstrate extended healthspans, and numerous studies confirm the heritability of longevity/healthspan genes. Importantly, genes and gerontogenes are directly and indirectly involved in DNA repair, insulin/IGF-1 and mTOR signaling pathways, long non-coding RNAs, sirtuins, and heat shock proteins. The complex interactions between genetics and epigenetics are teased apart. While more research into optimizing healthspan is needed, conserved gerontogenes offer synergistic potential to forestall aging and age-related diseases. Understanding complex longevity genetics brings closer the goal of extending not only lifespan but quality years of life. The primary aim of human Biogerontology is to enhance lifespan and healthspan, but the question remains: are current genetic modifications effectively promoting healthy aging? This article collates the advancements in gerontogenes that enhance lifespan and improve healthspan alongside their potential challenges.

Telomere length and dynamics in Astyanax mexicanus cave and surface morphs

Background

Telomeres are non-coding DNA repeats at the chromosome ends and their shortening is considered one of the major causes of aging. However, they also serve as a biomarker of environmental exposures and their length and attrition is affected by various stressors. In this study, we examined the average telomere length in Astyanax mexicanus, a species that has both surface-dwelling and cave-adapted populations. The cave morph descended from surface ancestors and adapted to a markedly different environment characterized by specific biotic and abiotic stressors, many of which are known to affect telomere length. Our objective was to explore whether telomere length differs between the two morphs and whether it serves as a biological marker of aging or correlates with the diverse environments the morphs are exposed to.

Methods

We compared telomere length and shortening between laboratory-reared Pachón cavefish and Rio Choy surface fish of A. mexicanus across different tissues and ages.

Results

Astyanax mexicanus surface fish exhibited longer average telomere length compared to cavefish. In addition, we did not observe telomere attrition in either cave or surface form as a result of aging in adults up to 9 years old, suggesting that efficient mechanisms prevent telomere-mediated senescence in laboratory stocks of this species, at least within this time frame. Our results suggest that telomere length in Astyanax may be considered a biomarker of environmental exposures. Cavefish may have evolved shorter and energetically less costly telomeres due to the absence of potential stressors known to affect surface species, such as predator pressure and ultra-violet radiation. This study provides the first insights into telomere dynamics in Astyanax morphs and suggests that shorter telomeres may have evolved as an adaptation to caves.

Influence of EE2 exposure, age and sex on telomere length in European long-snouted seahorse (Hippocampus guttulatus)

After a Telomere Lengthening in juvenile stage, a progressive telomere shortening occurs with age despite higher telomerase level. Telomere Length (TL) may also reflect past physiological state such as a chronic chemical stress. Several studies have revealed a correlation between TL, ageing and/or sex in vertebrates, including teleosts; however, the patterns of telomere dynamics with telomerase mRNA expression, sex, lifespan or chemical stress in teleosts are unclear. The first aim of this study is to verify if telomere length is age and sex-dependent. The second aim is to consider if TL is a useful indicator of stress response in European long-snouted seahorse, Hippocampus guttulatus, an ectothermic and non-model system. We showed that after telomere lengthening during the juvenile stage, a telomeric attrition became significant in sexually mature individuals (p = 0.042). TL decreased in older seahorses despite the presence of somatic telomerase mRNA expression at all life stages studied. There was no difference in TL between males and females, but telomerase mRNA expression was consistently higher in females than males. Exposure to EE2 had no effect on TL in young seahorses, but was correlated with a significant increase in telomerase mRNA expression and various physiological disruptions. Here, a growth retardation of -10 % for body length (p = 0.016) and approximately -45 % for mass (p = 0.001) compared to healthy juvenile seahorses was observed. Our data suggest that telomere dynamics alone should not be used as a marker of EE2 exposure in juvenile seahorses.

Biological clocks as age estimation markers in animals: a systematic review and meta-analysis

Various biological attributes associated with individual fitness in animals change predictably over the lifespan of an organism. Therefore, the study of animal ecology and the work of conservationists frequently relies upon the ability to assign animals to functionally relevant age classes to model population fitness. Several approaches have been applied to determining individual age and, while these methods have proved useful, they are not without limitations and often lack standardisation or are only applicable to specific species. For these reasons, scientists have explored the potential use of biological clocks towards creating a universal age-determination method. Two biological clocks, tooth layer annulation and otolith layering have found universal appeal. Both methods are highly invasive and most appropriate for post-mortem age-at-death estimation. More recently, attributes of cellular ageing previously explored in humans have been adapted to studying ageing in animals for the use of less-invasive molecular methods for determining age. Here, we review two such methods, assessment of methylation and telomere length, describing (i) what they are, (ii) how they change with age, and providing (iii) a summary and meta-analysis of studies that have explored their utility in animal age determination. We found that both attributes have been studied across multiple vertebrate classes, however, telomere studies were used before methylation studies and telomere length has been modelled in nearly twice as many studies. Telomere length studies included in the review often related changes to stress responses and illustrated that telomere length is sensitive to environmental and social stressors and, in the absence of repair mechanisms such as telomerase or alternative lengthening modes, lacks the ability to recover. Methylation studies, however, while also detecting sensitivity to stressors and toxins, illustrated the ability to recover from such stresses after a period of accelerated ageing, likely due to constitutive expression or reactivation of repair enzymes such as DNA methyl transferases. We also found that both studied attributes have parentally heritable features, but the mode of inheritance differs among taxa and may relate to heterogamy. Our meta-analysis included more than 40 species in common for methylation and telomere length, although both analyses included at least 60 age-estimation models. We found that methylation outperforms telomere length in terms of predictive power evidenced from effect sizes (more than double that observed for telomeres) and smaller prediction intervals. Both methods produced age correlation models using similar sample sizes and were able to classify individuals into young, middle, or old age classes with high accuracy. Our review and meta-analysis illustrate that both methods are well suited to studying age in animals and do not suffer significantly from variation due to differences in the lifespan of the species, genome size, karyotype, or tissue type but rather that quantitative method, patterns of inheritance, and environmental factors should be the main considerations. Thus, provided that complex factors affecting the measured trait can be accounted for, both methylation and telomere length are promising targets to develop as biomarkers for age determination in animals.

Why and when should organisms elongate their telomeres? Elaborations on the 'excess resources elongation' and 'last resort elongation' hypotheses

Telomere length and telomere shortening are thought to be critical cellular attributes and processes that are related to an individual's life span and fitness. The general pattern across most taxa is that after birth telomere length gradually decreases with age. Telomere protection and restoration mechanisms are usually assumed to reduce the rate of shortening or at most keep telomere length constant. However, here we have compiled a list of 26 articles showing that there is an increasing number of studies reporting apparent elongation of telomeres (i.e., a net increase in TL from timet to timet+1) often in a considerable proportion of the individuals studied. Moreover, the few studies which have studied telomere elongation in detail show that increases in telomere length are unlikely to be due to measurement error alone. In this article, we argue that episodes of telomere elongation deserve more attention as they could reflect individual strategies to optimise life histories and maximise fitness, which may not be reflected in the overall telomere dynamics patterns. We propose that patterns of telomere (net) elongation may be partly determined by other factors than those causing telomere shortening, and therefore deserve analyses specifically targeted to investigate the occurrence of telomere elongation. We elaborate on two ecological hypotheses that have been proposed to explain patterns of telomere elongation (the 'excess resources elongation' and the 'last resort elongation' hypothesis) and we discuss the current evidence for (or against) these hypotheses and propose ways to test them.

Thermal Preconditioning Alters the Stability of Hump-Snout Whitefish (Coregonus fluviatilis) and Its Hybrid Form, Showing Potential for Aquaculture

One of the little-studied ways that climate warming or temperature increases in aquaculture could affect aquatic animals is through accelerated aging. This study is dedicated to understanding the principles of molecular and cellular aging in the target tissues of juvenile whitefishes (Yenisei hump-snout whitefish and its hybrid) under the influence of acute heat stress (up to 26 °C), and the effects of thermal preconditioning as pre-adaptation. Non-adapted stressed hump-snout whitefish showed a higher induction threshold for functionally active mitochondria in the blood and a decrease in telomerase activity in the liver after heat shock exposure as a long-term compensatory response to prevent telomere shortening. However, we observed heat-induced telomere shortening in non-adapted hybrids, which can be explained by a decrease in mitochondrial membrane stability and a gradual increase in energy demand, leading to a decrease in protective telomerase activity. The pre-adapted groups of hump-snout whitefish and hybrids showed a long-term or delayed response of telomerase activity to heat shock, which served as a therapeutic mechanism against telomere shortening. We concluded that the telomerase and telomere responses to thermal stress demonstrate plasticity of tolerance limits and greater stability in hump-snout whitefish compared with hybrids.

Cell differentiation and matrix organization are differentially affected during bone formation in osteogenesis imperfecta zebrafish models with different genetic defects impacting collagen type I structure

Osteogenesis imperfecta (OI) is a family of rare heritable skeletal disorders associated with dominant mutations in the collagen type I encoding genes and recessive defects in proteins involved in collagen type I synthesis and processing and in osteoblast differentiation and activity. Historically, it was believed that the OI bone phenotype was only caused by abnormal collagen type I fibrils in the extracellular matrix, but more recently it became clear that the altered bone cell homeostasis, due to mutant collagen retention, plays a relevant role in modulating disease severity in most of the OI forms and it is correlated to impaired bone cell differentiation. Despite in vitro evidence, in vivo data are missing. To better understand the physiopathology of OI, we used two zebrafish models: Chihuahua (Chi/+), carrying a dominant p.G736D substitution in the α1 chain of collagen type I, and the recessive p3h1-/-, lacking prolyl 3-hydroxylase (P3h1) enzyme. Both models share the delay of collagen type I folding, resulting in its overmodification and partial intracellular retention. The regeneration of the bony caudal fin of Chi/+ and p3h1-/- was employed to investigate the impact of abnormal collagen synthesis on bone cell differentiation. Reduced regenerative ability was evident in both models, but it was associated to impaired osteoblast differentiation and osteoblastogenesis/adipogenesis switch only in Chi/+. On the contrary, reduced osteoclast number and activity were found in both models during regeneration. The dominant OI model showed a more detrimental effect in the extracellular matrix organization. Interestingly, the chemical chaperone 4-phenylbutyrate (4-PBA), known to reduce cellular stress and increase collagen secretion, improved bone formation only in p3h1-/- by favoring caudal fin growth without affecting bone cell markers expression. Taken together, our in vivo data proved the negative impact of structurally abnormal collagen type I on bone formation but revealed a gene mutation-specific effect on bone cell differentiation and matrix organization in OI. These, together with the distinct ability to respond to the chaperone treatment, underline the need for precision medicine approaches to properly treat the disease.

Telomerase Activity in Somatic Tissues and Ovaries of Diploid and Triploid Rainbow Trout (Oncorhynchus mykiss) Females

Telomerase activity has been found in the somatic tissues of rainbow trout. The enzyme is essential for maintaining telomere length but also assures homeostasis of the fish organs, playing an important role during tissue regeneration. The unique morphological and physiological characteristics of triploid rainbow trout, when compared to diploid specimens, make them a promising model for studies concerning telomerase activity. Thus, in this study, we examined the expression of the Tert gene in various organs of subadult and adult diploid and triploid rainbow trout females. Upregulated Tert mRNA transcription was observed in all the examined somatic tissues sampled from the triploid fish when compared to diploid individuals. Contrastingly, Tert expression in the ovaries was significantly decreased in the triploid specimens. Within the diploids, the highest expression of Tert was observed in the liver and in the ovaries of the subadult individuals. In the triploids, Tert expression was increased in the somatic tissues, while the ovaries exhibited lower activity of telomerase compared to other organs and decreased compared to the ovaries in the diploids. The ovaries of triploid individuals were underdeveloped, consisting of only a few oocytes. The lack of germ cells, which are usually characterized by high Tert expression, might be responsible for the decrease in telomerase activity in the triploid ovaries. The increase in Tert expression in triploid somatic tissues suggests that they require higher telomerase activity to cope with environmental stress and maintain internal homeostasis.

Causes and consequences of telomere lengthening in a wild vertebrate population

Telomeres have been advocated to be important markers of biological age in evolutionary and ecological studies. Telomeres usually shorten with age and shortening is frequently associated with environmental stressors and increased subsequent mortality. Telomere lengthening - an apparent increase in telomere length between repeated samples from the same individual - also occurs. However, the exact circumstances, and consequences, of telomere lengthening are poorly understood. Using longitudinal data from the Seychelles warbler (Acrocephalus sechellensis), we tested whether telomere lengthening - which occurs in adults of this species - is associated with specific stressors (reproductive effort, food availability, malarial infection and cooperative breeding) and predicts subsequent survival. In females, telomere shortening was observed under greater stress (i.e., low food availability, malaria infection), while telomere lengthening was observed in females experiencing lower stress (i.e., high food availability, assisted by helpers, without malaria). The telomere dynamics of males were not associated with the key stressors tested. These results indicate that, at least for females, telomere lengthening occurs in circumstances more conducive to self-maintenance. Importantly, both females and males with lengthened telomeres had improved subsequent survival relative to individuals that displayed unchanged, or shortened, telomeres - indicating that telomere lengthening is associated with individual fitness. These results demonstrate that telomere dynamics are bidirectionally responsive to the level of stress that an individual faces, but may poorly reflect the accumulation of stress over an individuals lifetime.

Habitat restoration weakens negative environmental effects on telomere dynamics

Habitat quality can have far-reaching effects on organismal fitness, an issue of concern given the current scale of habitat degradation. Many temperate upland streams have reduced nutrient levels due to human activity. Nutrient restoration confers benefits in terms of invertebrate food availability and subsequent fish growth rates. Here we test whether these mitigation measures also affect the rate of cellular ageing of the fish, measured in terms of the telomeres that cap the ends of eukaryotic chromosomes. We equally distributed Atlantic salmon eggs from the same 30 focal families into 10 human-impacted oligotrophic streams in northern Scotland. Nutrient levels in five of the streams were restored by simulating the deposition of a small number of adult Atlantic salmon Salmo salar carcasses at the end of the spawning period, while five reference streams were left as controls. Telomere lengths and expression of the telomerase reverse transcriptase (TERT) gene that may act to lengthen telomeres were then measured in the young fish when 15 months old. While TERT expression was unrelated to any of the measured variables, telomere lengths were shorter in salmon living at higher densities and in areas with a lower availability of the preferred substrate (cobbles and boulders). However, the adverse effects of these habitat features were much reduced in the streams receiving nutrients. These results suggest that adverse environmental pressures are weakened when nutrients are restored, presumably because the resulting increase in food supply reduces levels of both competition and stress.

Telomerase activity in ecological studies: What are its consequences for individual physiology and is there evidence for effects and trade-offs in wild populations

Increasing evidence at the cellular level is helping to provide proximate explanations for the balance between investment in growth, reproduction and somatic maintenance in wild populations. Studies of telomere dynamics have informed researchers about the loss and gain of telomere length both on a seasonal scale and across the lifespan of individuals. In addition, telomere length and telomere rate of loss seems to have evolved differently among taxonomic groups, and relate differently to organismal diversity of lifespan. So far, the mechanisms behind telomere maintenance remain elusive, although many studies have inferred a role for telomerase, an enzyme/RNA complex known to induce telomere elongation from laboratory studies. Exciting further work is also emerging that suggests telomerase (and/or its individual component parts) has a role in fitness that goes beyond the maintenance of telomere length. Here, we review the literature on telomerase biology and examine the evidence from ecological studies for the timing and extent of telomerase activation in relation to life history events associated with telomere maintenance. We suggest that the underlying mechanism is more complicated than originally anticipated, possibly involves several complimentary pathways, and is probably associated with high energetic costs. Potential pathways for future research are numerous and we outline what we see as the most promising prospects to expand our understanding of individual differences in immunity or reproduction efficiency.

Telomerase Activity in Androgenetic Rainbow Trout with Growth Deficiency and in Normally Developed Individuals

Role of telomerase in specimens with retarded growth (dwarfs) has not been thoroughly examined to date. Considering that some of the fish species show correlation between somatic growth and activity of telomerase, it has been tempting to assume that pattern of telomerase activity in specimens with retarded growth and these with normal growth rate may vary. In the present research, telomerase activity has been examined in liver, skin, and muscles in the androgenetic rainbow trout (Oncorhynchus mykiss) with growth deficiency and their normally developed siblings. Among the examined organs, the liver showed the highest telomerase activity in all studied fish, what may be linked to the enormous regeneration capacity of the liver tissue. Although dwarf specimens examined here displayed significantly lower body size and weight they did not exhibit any significant differences in the telomerase activity measured in liver and muscle when compared to the rainbow trout without growth deficiency. In turn, telomerase activity in skin was significantly upregulated in the normally developed androgenotes. The present study indicates that dwarfism in the androgenetic rainbow trout is neither associated with ceased telomerase activity nor its decrease throughout the ontogenetic development.

Age-Dependent Effects of Chronic Stress on Zebrafish Behavior and Regeneration

Stress can have a significant impact on many aspects of an organism’s physiology and behavior. However, the relationship between stress and regeneration, and how this relationship changes with age remains poorly understood. Here, we subjected young and old zebrafish to a chronic stress protocol and evaluated the impact of stress exposure on multiple measures of zebrafish behavior, specifically thigmotaxis (open field test) and scototaxis (light/dark preference test), and on regeneration ability after partial tail amputation. We found evidence that young and older adult fish are differentially impacted by stress. Only young fish showed a significant change in anxiety-like behaviors after being exposed to chronic stress, while their regeneration ability was not affected by the stress protocol. On the other hand, older fish regenerated their caudal fin significantly slower compared to young fish, but their behavior remained unaffected after being exposed to stress. We further investigated the expression of two candidate genes (nlgn1 and sam2) expressed in the central nervous system, and known to be associated with stress and anxiety-like behavior. The expression of stress-related gene candidate sam2 increased in the brain of older individuals exposed to stress. Our results suggest there is a close relationship between chronic stress, regeneration, and behavior in zebrafish (Danio rerio), and that the impact of stress is age-dependent.

An Identification and Characterization of the Axolotl (Ambystoma mexicanum, Amex) Telomerase Reverse Transcriptase (Amex TERT)

The Mexican axolotl is one of the few vertebrates that is able to replace its lost body parts during lifespan. Due to its remarkable regenerative abilities, the axolotl emerged as a model organism especially for limb regeneration. Telomeres and the telomerase enzyme are crucial for regeneration and protection against aging processes and degenerating diseases. Despite its relevance for regeneration, the axolotl telomerase and telomere length have not yet been investigated. Therefore, in the present paper, we reveal the sequence of the axolotl telomerase reverse transcriptase gene (Tert) and protein (TERT). Multiple sequence alignment (MSA) showed the known conserved RT- and TERT-specific motifs and residues found in other TERTs. In addition, we establish methods to determine the Tert expression (RT-PCR) and telomerase activity (Q-TRAP) of adult axolotl and blastema tissues. We found that both differentiated forelimb tissue and regenerating blastema tissue express Tert and show telomerase activity. Furthermore, blastema tissue appears to exhibit a higher Tert expression and telomerase activity. The presence of active telomerase in adult somatic cells is a decisive difference to somatic cells of non-regenerating vertebrates, such as humans. These findings indicate that telomere biology may play a key role in the regenerative abilities of cells.

Long-read sequencing of the zebrafish genome reorganizes genomic architecture

BACKGROUND

Nanopore sequencing technology has revolutionized the field of genome biology with its ability to generate extra-long reads that can resolve regions of the genome that were previously inaccessible to short-read sequencing platforms. Over 50% of the zebrafish genome consists of difficult to map, highly repetitive, low complexity elements that pose inherent problems for short-read sequencers and assemblers.

RESULTS

We used long-read nanopore sequencing to generate a de novo assembly of the zebrafish genome and compared our assembly to the current reference genome, GRCz11. The new assembly identified 1697 novel insertions and deletions over one kilobase in length and placed 106 previously unlocalized scaffolds. We also discovered additional sites of retrotransposon integration previously unreported in GRCz11 and observed the expression of these transposable elements in adult zebrafish under physiologic conditions, implying they have active mobility in the zebrafish genome and contribute to the ever-changing genomic landscape.

CONCLUSIONS

We used nanopore sequencing to improve upon and resolve the issues plaguing the current zebrafish reference assembly, GRCz11. Zebrafish is a prominent model of human disease, and our corrected assembly will be useful for studies relying on interspecies comparisons and precise linkage of genetic events to disease phenotypes.

Genome biology of the Darkedged Splitfin, Girardinichthys multiradiatus, and the evolution of sex chromosomes and placentation

Viviparity evolved independently about 150 times in vertebrates and more than 20 times in fish. Several lineages added to the protection of the embryo inside the body of the mother, the provisioning of nutrients, and physiological exchange. This often led to the evolution of a placenta. Among fish, one of the most complex systems serving the function of the placenta is the embryonal trophotaenia/ovarian luminal epithelium of the goodeid fishes. For a better understanding of this feature and others of this group of fishes, high-quality genomic resources are essential. We have sequenced the genome of the darkedged splitfin, Girardinichthys multiradiatus. The assembly is chromosome level and includes the X and Y Chromosomes. A large male-specific region on the Y was identified covering 80% of Chromosome 20, allowing some first inferences on the recent origin and a candidate male sex determining gene. Genome-wide transcriptomics uncovered sex-specific differences in brain gene expression with an enrichment for neurosteroidogenesis and testis genes in males. The expression signatures of the splitfin embryonal and maternal placenta showed overlap with homologous tissues including human placenta, the ovarian follicle epithelium of matrotrophic poeciliid fish species and the brood pouch epithelium of the seahorse. Our comparative analyses on the evolution of embryonal and maternal placenta indicate that the evolutionary novelty of maternal provisioning development repeatedly made use of genes that already had the same function in other tissues. In this way, preexisting modules are assembled and repurposed to provide the molecular changes for this novel trait.

Aging Activates the Immune System and Alters the Regenerative Capacity in the Zebrafish Heart

Age-associated organ failure and degenerative diseases have a major impact on human health. Cardiovascular dysfunction has an increasing prevalence with age and is one of the leading causes of death. In contrast to humans, zebrafish have extraordinary regeneration capacities of complex organs including the heart. In addition, zebrafish has recently become a model organism in research on aging. Here, we have compared the ventricular transcriptome as well as the regenerative capacity after cryoinjury of old and young zebrafish hearts. We identified the immune system as activated in old ventricles and found muscle organization to deteriorate upon aging. Our data show an accumulation of immune cells, mostly macrophages, in the old zebrafish ventricle. Those immune cells not only increased in numbers but also showed morphological and behavioral changes with age. Our data further suggest that the regenerative response to cardiac injury is generally impaired and much more variable in old fish. Collagen in the wound area was already significantly enriched in old fish at 7 days post injury. Taken together, these data indicate an ‘inflammaging’-like process in the zebrafish heart and suggest a change in regenerative response in the old.

Role and relevance of fish cell lines in advanced in vitro research

Introduction

Cell line derived from fish has been established as a promising tool for studying many key issues of aquaculture covering fish growth, disease, reproduction, genetics, and biotechnology. In addition, fish cell lines are very useful in vitro models for toxicological, pathological, and immunological studies. The easier maintenance of fish cell lines in flexible temperature regimes and hypoxic conditions make them preferable in vitro tools over mammalian cell lines. Great excitement has been observed in establishing and characterizing new fish cell lines representing diverse fish species and tissue types. The well-characterized and authenticated cell lines are of utmost essential as these represent cellular functions very similar to in vivo state of an organism otherwise it would affect the reproducibility of scientific research.

Conclusion

The fish cell lines have exhibited encouraging results in several key aspects of in vitro research in aquaculture including virology, nutrition and metabolism, production of vaccines, and transgenic fish production. The review paper reports the cell lines developed from fish, their characterization, and biobanking along with their potential applications and challenges in in vitro research.

Monitoring Telomere Maintenance During Regeneration of Annelids

Telomere shortening is a hallmark of aging and eventually constrains the proliferative capacity of cells. The protocols discussed here are used for monitoring telomeres comprehensively in Aeolosoma viride, a model system for regeneration studies. We present methods for analyzing the activity of telomerase enzyme in regenerating tissue by telomeric repeat amplification protocol (TRAP) assay, for comparing telomere length between existing tissue and newly regenerated tissue by telomere restriction fragment (TRF) assay, as well as for visualizing telomeres by fluorescence in situ hybridization (FISH).

Telomere length variation does not correspond with the growth disturbances in the rainbow trout (Oncorhynchus mykiss)

Somatic growth is considered to affect pace of the telomere attrition in vertebrates. As normally developed and dwarf fish differ in the body size we have decided to compare telomere length in the rainbow trout (Oncorhynchus mykiss) with normal growth and with growth reduced due to the dwarf condition. Examined 1-year-old fish with normal and dwarf appearance were siblings originated from androgenetic fully homozygous doubled haploid (DH) line of rainbow trout. Particular dwarf individuals had body deformities such as humpback, kyphosis, and lordosis. Somatic cells of examined rainbow trout had an average telomere length between 17 and 20 kb, comparable in females and males. Dwarf rainbow trout exhibited significantly lower body length and weight than their normally developed siblings even though no differences in the telomere length were found between these fishes. Statistical analysis did not exhibit any correlation between body size and the telomere length. Equal length of telomeres observed in the studied normal and dwarf rainbow trout suggests morphological and physiological differences in fish with different growth rates do not affect dynamics of telomeric DNA. Or any variation in the telomere length might have been levelled by telomerase that in rainbow trout is active in all tissues irrespective of the individual developmental stage.

Development in the European flounder (Platichthys flesus) of a q-PCR assay for the measurement of telomere length, a potential biomarker of pollutant effects for biomonitoring studies

Telomeres protect the coding sequence of chromosome ends and Telomere Length (TL) has been proposed as a biomarker of cellular aging, cumulative stress exposure and life-span in humans. With the aim to propose new biomarkers, a q-PCR protocol was adapted for the measurement of TL in the European flounder Platichthys flesus. The protocol was then applied in 2-year-old flounders from the Seine Estuary. The absolute TL in the flounder is 54 ± 13 kbp per genome (mean ± standard error). Considering relative or absolute TL, no correlation was observed with DNA damage and any of the measured contaminant concentrations (trace elements, metabolites of polycyclic aromatic hydrocarbons, polychlorobiphenyls, organochlorinated pesticides, polybrominated diphenyl ethers, perfluoroalkyl substances). Because sampling was limited, further investigations are required to state a possible impact of chemical pollution on flatfish telomeres. This is motivated by correlations observed with organochlorinated compounds when decreasing statistical significance (p ≤ 0.10).

Telomerase RNA recruits RNA polymerase II to target gene promoters to enhance myelopoiesis

Dyskeratosis congenita (DC) is a rare inherited bone marrow failure and cancer predisposition syndrome caused by mutations in telomerase or telomeric proteins. Here, we report that zebrafish telomerase RNA (terc) binds to specific DNA sequences of master myeloid genes and controls their expression by recruiting RNA Polymerase II (Pol II). Zebrafish terc harboring the CR4-CR5 domain mutation found in DC patients hardly interacted with Pol II and failed to regulate myeloid gene expression in vivo and to increase their transcription rates in vitro. Similarly, TERC regulated myeloid gene expression and Pol II promoter occupancy in human myeloid progenitor cells. Strikingly, induced pluripotent stem cells derived from DC patients with a TERC mutation in the CR4-CR5 domain showed impaired myelopoiesis, while those with mutated telomerase catalytic subunit differentiated normally. Our findings show that TERC acts as a transcription factor, revealing a target for therapeutic intervention in DC patients.

Individual telomere dynamics and their links to life history in a viviparous lizard

Emerging patterns suggest telomere dynamics and life history are fundamentally linked in endotherms through life-history traits that mediate the processes underlying telomere attrition. Unlike endotherms, ectotherms maintain the ability to lengthen somatic telomeres throughout life and the link between life-history strategies and ectotherm telomere dynamics is unknown. In a well-characterized model system (Niveoscincus ocellatus), we used long-term longitudinal data to study telomere dynamics across climatically divergent populations. We found longer telomeres in individuals from the cool highlands than those from the warm lowlands at birth and as adults. The key determinant of adult telomere length across populations was telomere length at birth, with population-specific effects of age and growth on adult telomere length. The reproductive effort had no proximate effect on telomere length in either population. Maternal factors influenced telomere length at birth in the warm lowlands but not the cool highlands. Our results demonstrate that life-history traits can have pervasive and context-dependent effects on telomere dynamics in ectotherms both within and between populations. We argue that these telomere dynamics may reflect the populations' different life histories, with the slow-growing cool highland population investing more into telomere lengthening compared to the earlier-maturing warm lowland population.

Aging delays epimorphic regeneration in mice

Epimorphic regeneration is a multi-tissue regeneration process where amputation does not lead to scarring, but blastema formation and patterned morphogenesis for which cell plasticity and concerted cell-cell interactions are pivotal. Tissue regeneration declines with aging, yet if and how aging impairs epimorphic regeneration is unknown. Here we show for the first time that aging derails the spatiotemporal regulation of epimorphic regeneration in mammals, first, by exacerbating tissue histolysis and delaying wound closure, and second, by impairing blastema differentiation and skeletal regrowth. Surprisingly, aging did not limit stem cell availability in the blastema, but reduced osteoblast-dependent bone formation. Our data suggest that aging delays regeneration not by stem cell exhaustion, but functional defects of differentiated cells that may be driven by an aged wound environment and alterations in the spatiotemporal regulation of regeneration events. Our findings emphasize the importance of accurate timing of signaling events for regeneration, and highlight the need for carefully timed interventions in regenerative medicine.

No evidence of physiological declines with age in an extremely long-lived fish

Although the pace of senescence varies considerably, the physiological systems that contribute to different patterns of senescence are not well understood, especially in long-lived vertebrates. Long-lived bony fish (i.e., Class Osteichthyes) are a particularly useful model for studies of senescence because they can readily be aged and exhibit some of the longest lifespans among vertebrates. In this study we examined the potential relationship between age and multiple physiological systems including: stress levels, immune function, and telomere length in individuals ranging in age from 2 to 99 years old in bigmouth buffalo (Ictiobus cyprinellus), the oldest known freshwater teleost fish. Contrary to expectation, we did not find any evidence for age-related declines in these physiological systems. Instead, older fish appeared to be less stressed and had greater immunity than younger fish, suggesting age-related improvements rather than declines in these systems. There was no significant effect of age on telomeres, but individuals that may be more stressed had shorter telomeres. Taken together, these findings suggest that bigmouth buffalo exhibit negligible senescence in multiple physiological systems despite living for nearly a century.

Modeling Neuroregeneration and Neurorepair in an Aging Context: The Power of a Teleost Model

Aging increases the risk for neurodegenerative disease and brain trauma, both leading to irreversible and multifaceted deficits that impose a clear societal and economic burden onto the growing world population. Despite tremendous research efforts, there are still no treatments available that can fully restore brain function, which would imply neuroregeneration. In the adult mammalian brain, neuroregeneration is naturally limited, even more so in an aging context. In view of the significant influence of aging on (late-onset) neurological disease, it is a critical factor in future research. This review discusses the use of a non-standard gerontology model, the teleost brain, for studying the impact of aging on neurorepair. Teleost fish share a vertebrate physiology with mammals, including mammalian-like aging, but in contrast to mammals have a high capacity for regeneration. Moreover, access to large mutagenesis screens empowers these teleost species to fill the gap between established invertebrate and rodent models. As such, we here highlight opportunities to decode the factor age in relation to neurorepair, and we propose the use of teleost fish, and in particular killifish, to fuel new research in the neuro-gerontology field.

Appendage Regeneration in Vertebrates: What Makes This Possible?

The ability to regenerate amputated or injured tissues and organs is a fascinating property shared by several invertebrates and, interestingly, some vertebrates. The mechanism of evolutionary loss of regeneration in mammals is not understood, yet from the biomedical and clinical point of view, it would be very beneficial to be able, at least partially, to restore that capability. The current availability of new experimental tools, facilitating the comparative study of models with high regenerative ability, provides a powerful instrument to unveil what is needed for a successful regeneration. The present review provides an updated overview of multiple aspects of appendage regeneration in three vertebrates: lizard, salamander, and zebrafish. The deep investigation of this process points to common mechanisms, including the relevance of Wnt/β-catenin and FGF signaling for the restoration of a functional appendage. We discuss the formation and cellular origin of the blastema and the identification of epigenetic and cellular changes and molecular pathways shared by vertebrates capable of regeneration. Understanding the similarities, being aware of the differences of the processes, during lizard, salamander, and zebrafish regeneration can provide a useful guide for supporting effective regenerative strategies in mammals.

Relative Telomere Length and Telomerase Reverse Transcriptase (TERT) Expression Are Associated with Age in Almond (Prunus dulcis [Mill.] D.A.Webb)

While all organisms age, our understanding of how aging occurs varies among species. The aging process in perennial plants is not well-defined, yet can have implications on production and yield of valuable fruit and nut crops. Almond exhibits an age-related disorder known as non-infectious bud failure (BF) that affects vegetative bud development, indirectly affecting kernel yield. This species and disorder present an opportunity to address aging in a commercially relevant and vegetatively propagated perennial crop. The hypothesis tested in this study was that relative telomere length and/or telomerase reverse transcriptase (TERT) expression can serve as biomarkers of aging in almond. Relative telomere lengths and expression of TERT, a subunit of the enzyme telomerase, were measured via qPCR methods using bud and leaf samples collected from distinct age cohorts over a two-year period. Results from this work show a marginal but significant association between both relative telomere length and TERT expression, and age, suggesting that as almonds age, telomeres shorten and TERT expression decreases. This work provides information on potential biomarkers of perennial plant aging, contributing to our knowledge of this process. In addition, these results provide opportunities to address BF in almond breeding and nursery propagation.

Stress reactivity elicits a tissue-specific reduction in telomere length in aging zebrafish (Danio rerio)

Individual differences in personality are associated with variation in healthy aging. Health behaviours are often cited as the likely explanation for this association; however, an underlying biological mechanism may also exist. Accelerated leukocyte telomere shortening is implicated in multiple age-related diseases and is associated with chronic activation of the hypothalamus-pituitary-adrenal (HPA) axis, providing a link between stress-related personality differences and adverse health outcomes. However, the effects of the HPA axis are tissue specific. Thus, leukocyte telomere length may not accurately reflect telomere length in disease-relevant tissues. Here, we examined the correlation between stress reactivity and telomere length in heart and brain tissue in young (6-9 month) and aging (18 month) zebrafish. Stress reactivity was assessed by tank diving and through gene expression. Telomere length was assessed using quantitative PCR. We show that aging zebrafish have shorter telomeres in both heart and brain. Telomere length was inversely related to stress reactivity in heart but not brain of aging individuals. These data support the hypotheses that an anxious predisposition contributes to accelerated telomere shortening in heart tissue, which may have important implications for our understanding of age-related heart disease, and that stress reactivity contributes to age-related telomere shortening in a tissue-specific manner.

Behavioral responses of three freshwater planaria species to light, visual and olfactory stimuli: Setting the stage for further ecotoxicological studies

Planarians are freshwater flatworms commonly used as environmental bioindicator due to their sensitivity of response and their ease of culturing in lab. Nevertheless, to date, very few studies describing their behavior have been led. This work aims to fill the literature gap by providing preliminary results through six behavioral challenges (locomotion, exploration, light stress, planarian light/dark test, shoaling and foraging) conducted with three different species Dugesia tigrina, Schmidtea mediterranea and Schmidtea polychroa. The behavioral responses of every species in each of these six assays were recorded and differences between species were highlighted, depending on the assays and conditions. Schmidtea polychroa is less active than the two others and had the highest light aversion. Reactions observed in response to diverse and realistic stimuli helped us to select the most suitable tests and choose the species that seem the most appropriate for future ecotoxicological and neurophysiological tests. Four tests - out of the six tested- seem reliable in order to standardize planarian behavioral tests.

Flexibility Is Costly: Hidden Physiological Damage From Seasonal Phenotypic Transitions in Heterothermic Species

Heterothermy allows organisms to cope with fluctuating environmental conditions. The use of regulated hypometabolism allows seasonal heterothermic species to cope with annual resource shortages and thus to maximize survival during the unfavorable season. This comes with deep physiological remodeling at each seasonal transition to allow the organism to adjust to the changing environment. In the wild, this adaptation is highly beneficial and largely overcomes potential costs. However, researchers recently proposed that it might also generate both ecological and physiological costs for the organism. Here, we propose new perspectives to be considered when analyzing adaptation to seasonality, in particular considering these costs. We propose a list of putative costs, including DNA damage, inflammatory response to fat load, brain and cognitive defects, digestive malfunction and immunodeficiency, that should receive more attention in future research on physiological seasonality. These costs may only be marginal at each transition event but accumulate over time and therefore emerge with age. In this context, studies in captivity, where we have access to aging individuals with limited extrinsic mortality (e.g., predation), could be highly valuable to experimentally assess the costs of physiological flexibility. Finally, we offer new perspectives, which should be included in demographic models, on how the adaptive value of physiological flexibility could be altered in the future in the context of global warming.

Anti-Proliferative and Pro-Apoptotic Effects of Short-Term Inhibition of Telomerase In Vivo and in Human Malignant B Cells Xenografted in Zebrafish

Besides its canonical role in stabilizing telomeres, telomerase reverse transcriptase (TERT) may promote tumor growth/progression through extra-telomeric functions. Our previous in vitro studies demonstrated that short-term TERT inhibition by BIBR1532 (BIBR), an inhibitor of TERT catalytic activity, negatively impacts cell proliferation and viability via telomeres’ length-independent mechanism. Here we evaluate the anti-proliferative and pro-apoptotic effects of short-term telomerase inhibition in vivo in wild-type (wt) and tert mutant (tert^{hu3430/hu3430}; tert−/−) zebrafish embryos, and in malignant human B cells xenografted in casper zebrafish embryos. Short-term Tert inhibition by BIBR in wt embryos reduced cell proliferation, induced an accumulation of cells in S-phase and ultimately led to apoptosis associated with the activation of DNA damage response; all these effects were unrelated to telomere shortening/dysfunction. BIBR treatment showed no effects in tert−/− embryos. Xenografted untreated malignant B cells proliferated in zebrafish embryos, while BIBR pretreated cells constantly decreased and were significantly less than those in the controls from 24 to up to 72 h after xenotransplantation. Additionally, xenografted tumor cells, treated with BIBR prior- or post-transplantation, displayed a significant higher apoptotic rate compared to untreated control cells. In conclusion, our data demonstrate that short-term telomerase inhibition impairs proliferation and viability in vivo and in human malignant B cells xenografted in zebrafish, thus supporting therapeutic applications of TERT inhibitors in human malignancies.

Telomere Dynamics in the Diploid and Triploid Rainbow Trout (Oncorhynchus mykiss) Assessed by Q-FISH Analysis

Changes of telomere length with age were assessed in diploid and triploid rainbow trout (Oncorhynchus mykiss) females in the cross-sectional study using Q-FISH technique. Triploid trout as sterile do not invest an energy in gametogenesis and continue to grow, whereas fertile diploid individuals suffer from declines in growth and survival during sexual maturation. However, triploid and diploid specimens exhibited similar patterns of telomere dynamics. Telomere length in the embryos, larvae and one-year-old juveniles did not change significantly. In the second year after hatching, subadults exhibited substantially shortened telomeres, while significant increase of the telomere length was reported in the three-year-old adults. On the other hand, correlation between telomere length and body size was observed in the triploid, but not in the diploid rainbow trout. Telomere shortening observed in two-year-old subadults may have been associated with the premature period of the fast growth in rainbow trout. Similar pattern of the telomere dynamics reported in the fertile diploids and sterile triploids indicated processes related to reproduction did not affect telomere dynamics in this species. Unexpected increase of the telomere length reported during the third year of life confirmed that in rainbow trout telomeric DNA shortens and lengthens, depending on the developmental stage.

Ambiguities in using telomere length for age determination in two North American bat species

The age of an animal, determined by time (chronological age) as well as genetic and environmental factors (biological age), influences the likelihood of mortality and reproduction and thus the animal’s contribution to population growth. For many long-lived species, such as bats, a lack of external and morphological indicators has made determining age a challenge, leading researchers to examine genetic markers of age for application to demographic studies. One widely studied biomarker of age is telomere length, which has been related both to chronological and biological age across taxa, but only recently has begun to be studied in bats. We assessed telomere length from the DNA of known-age and minimum known-age individuals of two bat species using a quantitative PCR assay. We determined that telomere length was quadratically related to chronological age in big brown bats (Eptesicus fuscus), although it had little predictive power for accurate age determination of unknown-age individuals. The relationship was different in little brown bats (Myotis lucifugus), where telomere length instead was correlated with biological age, apparently due to infection and wing damage associated with white-nose syndrome. Furthermore, we showed that wing biopsies currently are a better tissue source for studying telomere length in bats than guano and buccal swabs; the results from the latter group were more variable and potentially influenced by storage time. Refinement of collection and assessment methods for different non-lethally collected tissues will be important for longitudinal sampling to better understand telomere dynamics in these long-lived species. Although further work is needed to develop a biomarker capable of determining chronological age in bats, our results suggest that biological age, as reflected in telomere length, may be influenced by extrinsic stressors such as disease.

Opposing p53 and mTOR/AKT promote an in vivo switch from apoptosis to senescence upon telomere shortening in zebrafish

Progressive telomere shortening during lifespan is associated with restriction of cell proliferation, genome instability and aging. Apoptosis and senescence are the two major outcomes upon irreversible cellular damage. Here, we show a transition of these two cell fates during aging of telomerase deficient zebrafish. In young telomerase mutants, proliferative tissues exhibit DNA damage and p53-dependent apoptosis, but no senescence. However, these tissues in older animals display loss of cellularity and senescence becomes predominant. Tissue alterations are accompanied by a pro-proliferative stimulus mediated by AKT signaling. Upon AKT activation, FoxO transcription factors are phosphorylated and translocated out of the nucleus. This results in reduced SOD2 expression causing an increase of ROS and mitochondrial dysfunction. These alterations induce p15/16 growth arrest and senescence. We propose that, upon telomere shortening, early apoptosis leads to cell depletion and insufficient compensatory proliferation. Following tissue damage, the mTOR/AKT is activated causing mitochondrial dysfunction and p15/16-dependent senescence.

Changes in the Expression of Pre-Replicative Complex Genes in hTERT and ALT Pediatric Brain Tumors

Background: The up-regulation of a telomere maintenance mechanism (TMM) is a common feature of cancer cells and a hallmark of cancer. Routine methods for detecting TMMs in tumor samples are still missing, whereas telomerase targeting treatments are becoming available. In paediatric cancers, alternative lengthening of telomeres (ALT) is found in a subset of sarcomas and malignant brain tumors. ALT is a non-canonical mechanism of telomere maintenance developed by cancer cells with no-functional telomerase. Methods: To identify drivers and/or markers of ALT, we performed a differential gene expression analysis between two zebrafish models of juvenile brain tumors, that differ only for the telomere maintenance mechanism adopted by tumor cells: one is ALT while the other is telomerase-dependent. Results: Comparative analysis of gene expression identified five genes of the pre-replicative complex, ORC4, ORC6, MCM2, CDC45 and RPA3 as upregulated in ALT. We searched for a correlation between telomerase levels and expression of the pre-replicative complex genes in a cohort of paediatric brain cancers and identified a counter-correlation between telomerase expression and the genes of the pre-replicative complex. Moreover, the analysis of ALT markers in a group of 20 patients confirmed the association between ALT and increased RPA and decreased H3K9_me3 localization at telomeres. Conclusions: Our study suggests that telomere maintenance mechanisms may act as a driver of telomeric DNA replication and chromatin status in brain cancers and identifies markers of ALT that could be exploited for precise prognostic and therapeutic purposes.

Telomeres and Telomerase in Heart Ontogenesis, Aging and Regeneration

The main purpose of the review article is to assess the contributions of telomere length and telomerase activity to the cardiac function at different stages of development and clarify their role in cardiac disorders. It has been shown that the telomerase complex and telomeres are of great importance in many periods of ontogenesis due to the regulation of the proliferative capacity of heart cells. The review article also discusses the problems of heart regeneration and the identification of possible causes of dysfunction of telomeres and telomerase.

Expression of tert Prevents ALT in Zebrafish Brain Tumors

The activation of a telomere maintenance mechanism (TMM) is an essential step in cancer progression to escape replicative senescence and apoptosis. Alternative lengthening of telomeres (ALT) is found in a subset of malignant brain tumors with poor outcomes. Here, we describe a model of juvenile zebrafish brain tumor that progressively develops ALT. We discovered that reduced expression of tert, linked to a widespread hypomethylation of the tert promoter and increase in Terra expression precedes ALT development. Surprisingly, expression of tert during juvenile brain tumor development led to reduced proliferation of tumor cells and prolonged survival. Most importantly, expression of tert reverted all ALT features and normalizes TERRA expression, promoted heterochromatin formation at telomeres, and attenuated telomeric DNA damage. These data suggest that the activity of telomerase goes beyond telomere maintenance and has profound consequences on genome stability.

Maternal predator-exposure affects offspring size at birth but not telomere length in a live-bearing fish

The perception of predation risk could affect prey phenotype both within and between generations (via parental effects). The response to predation risk could involve modifications in physiology, morphology, and behavior and can ultimately affect long-term fitness. Among the possible modifications mediated by the exposure to predation risk, telomere length could be a proxy for investigating the response to predation risk both within and between generations, as telomeres can be significantly affected by environmental stress. Maternal exposure to the perception of predation risk can affect a variety of offspring traits but the effect on offspring telomere length has never been experimentally tested. Using a live-bearing fish, the guppy (Poecilia reticulata), we tested if the perceived risk of predation could affect the telomere length of adult females directly and that of their offspring with a balanced experimental setup that allowed us to control for both maternal and paternal contribution. We exposed female guppies to the perception of predation risk during gestation using a combination of both visual and chemical cues and we then measured female telomere length after the exposure period. Maternal effects mediated by the exposure to predation risk were measured on offspring telomere length and body size at birth. Contrary to our predictions, we did not find a significant effect of predation-exposure neither on female nor on offspring telomere length, but females exposed to predation risk produced smaller offspring at birth. We discuss the possible explanations for our findings and advocate for further research on telomere dynamics in ectotherms.

Changes in the physicochemical properties of fish cell membranes during cellular senescence

Fish cell lines are widely used for the studies of developmental biology, virology, biology of aging, and nutrition physiology. However, little is known about their physicochemical properties. Here, we report the phospholipid compositions and mechanical properties of cell membranes derived from freshwater, anadromous and marine fish species. Biophysical analyses revealed that fish cell lines have highly deformable cell membranes with significantly low membrane tensions and Young's moduli compared with those of mammalian cell lines. The induction of cellular senescence by DNA demethylation using 5-Aza-2'-deoxycytidine significantly reduced the deformability of fish cell membrane, but hydrogen peroxide-induced oxidative stress did not affect the deformability. Mass spectrometry analysis of phospholipids revealed that the level of phosphatidylethanolamine molecules containing polyunsaturated fatty acids significantly increased during the 5-Aza-2'-deoxycytidine-induced cellular senescence. Fish cell lines provide a useful model system for studying the changes in the physicochemical properties of cell membranes during cellular senescence.Abbreviations: 2D-TLC: two-dimensional thin layer chromatography; 5-Aza-dC: 5-Aza-2'-deoxycytidine; DHA: docosahexaenoic acid; EPA: eicosapentaenoic acid; FBS: fetal bovine serum; PC: phosphatidylcholine; PE: phosphatidylethanolamine; PI: phosphatidylinositol; PS: phosphatidylserine; PUFA: polyunsaturated fatty acid; SA-β-gal: senescence-associated beta-galactosidase; SM: sphingomyelin.