Telomerase is required for zebrafish lifespan

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.

Lack of telomerase reduces cancer incidence and increases lifespan of zebrafish tp53M214K mutants

Telomerase activity is restricted in humans and telomere attrition occurs in several tissues accompanying natural aging. Critically short telomeres trigger DNA damage responses and activate p53 which leads to apoptosis or replicative senescence. These processes reduce cell proliferation and disrupt tissue homeostasis, thus contributing to systemic aging. Similarly, zebrafish have restricted telomerase expression, and telomeres shorten to critical length during their lifespan. Telomerase-deficient zebrafish (tert -/-) is a premature model of aging that anticipates aging phenotypes due to early telomere shortening. tert -/- zebrafish have impaired cell proliferation, accumulation of DNA damage markers and p53 response. These cellular defects lead to disruption of tissue homeostasis, resulting in premature infertility, gastrointestinal atrophy, sarcopenia and kyphosis. Such consequences contribute to its premature death. Here we reveal a genetic interdependence between tp53 and telomerase function. Mutation of tp53 abrogates premature aging of tert -/- zebrafish, prolonging male fertility and lifespan. However, it does not fully rescue healthspan. tp53mut tert -/- zebrafish retain high levels of inflammation and increased spontaneous cancer incidence. Conversely, loss of telomerase prolongs the lifespan of tp53mut single mutants. Lack of telomerase reduces two-fold the cancer incidence in double mutants and increases lifetime survival. Thus, we observe a reciprocal rescue of tp53mut and tert -/- that ameliorates lifespan but not spontaneous cancer incidence of tp53mut, likely due to higher levels of inflammation.

Telomere length is an epigenetic trait - Implications for the use of telomerase-deficient organisms to model human disease

Telomere length, unlike most genetic traits, is epigenetic, in the sense that it is not fully coded by the genome. Telomeres vary in length and randomly assort to the progeny leaving some individuals with longer and others with shorter telomeres. Telomerase activity counteracts this by extending telomeres in the germline and during embryogenesis but sizeable variances remain in telomere length. This effect is exacerbated by the absence of fully active telomerase. Telomerase heterozygous animals (tert+/-) have reduced telomerase activity and their telomeres fail to be elongated to wild-type average length, meaning that - with every generation - they decrease. After a given number of successive generations of telomerase-insufficient crosses, telomeres become critically short and cause organismal defects that, in humans, are known as telomere biology disorders. Importantly, these defects also occur in wild-type (tert+/+) animals derived from such tert+/- incrosses. Despite these tert+/+ animals being proficient for telomerase, they have shorter than average telomere length and, although milder, develop phenotypes that are similar to those of telomerase mutants. Here, we discuss the impact of this phenomenon on human pathologies associated with telomere length, provide a brief overview of telomere biology across species and propose specific measures for working with telomerase-deficient zebrafish.

Senescence-associated ß-galactosidase staining over the lifespan differs in a short- and a long-lived fish species

During the aging process, cells can enter cellular senescence, a state in which cells leave the cell cycle but remain viable. This mechanism is thought to protect tissues from propagation of damaged cells and the number of senescent cells has been shown to increase with age. The speed of aging determines the lifespan of a species and it varies significantly in different species. To assess the progress of cellular senescence during lifetime, we performed a comparative longitudinal study using histochemical detection of the senescence-associated beta-galactosidase as senescence marker to map the staining patterns in organs of the long-lived zebrafish and the short-lived turquoise killifish using light- and electron microscopy. We compared age stages corresponding to human stages of newborn, childhood, adolescence, adult and old age. We found tissue-specific but conserved signal patterns with respect to organ distribution. However, we found dramatic differences in the onset of tissue staining. The stained zebrafish organs show little to no signal at newborn age followed by a gradual increase in signal intensity, whereas the organs of the short-lived killifish show an early onset of staining already at newborn stage, which remains conspicuous at all age stages. The most prominent signal was found in liver, intestine, kidney and heart, with the latter showing the most prominent interspecies divergence in onset of staining and in staining intensity. In addition, we found staining predominantly in epithelial cells, some of which are post-mitotic, such as the intestinal epithelial lining. We hypothesize that the association of the strong and early-onset signal pattern in the short-lived killifish is consistent with a protective mechanism in a fast growing species. Furthermore, we believe that staining in post-mitotic cells may play a role in maintaining tissue integrity, suggesting different roles for cellular senescence during life.

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.

Telomeres as hotspots for innate immunity and inflammation

Aging is marked by the gradual accumulation of deleterious changes that disrupt organ function, creating an altered physiological state that is permissive for the onset of prevalent human diseases. While the exact mechanisms governing aging remain a subject of ongoing research, there are several cellular and molecular hallmarks that contribute to this biological process. This review focuses on two factors, namely telomere dysfunction and inflammation, which have emerged as crucial contributors to the aging process. We aim to discuss the mechanistic connections between these two distinct hallmarks and provide compelling evidence highlighting the loss of telomere protection as a driver of pro-inflammatory states associated with aging. By reevaluating the interplay between telomeres, innate immunity, and inflammation, we present novel perspectives on the etiology of aging and its associated diseases.

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.

Taurine deficiency as a driver of aging

Aging is associated with changes in circulating levels of various molecules, some of which remain undefined. We find that concentrations of circulating taurine decline with aging in mice, monkeys, and humans. A reversal of this decline through taurine supplementation increased the health span (the period of healthy living) and life span in mice and health span in monkeys. Mechanistically, taurine reduced cellular senescence, protected against telomerase deficiency, suppressed mitochondrial dysfunction, decreased DNA damage, and attenuated inflammaging. In humans, lower taurine concentrations correlated with several age-related diseases and taurine concentrations increased after acute endurance exercise. Thus, taurine deficiency may be a driver of aging because its reversal increases health span in worms, rodents, and primates and life span in worms and rodents. Clinical trials in humans seem warranted to test whether taurine deficiency might drive aging in humans.

Telomere DNA length regulation is influenced by seasonal temperature differences in short-lived but not in long-lived reef-building corals

Telomeres are environment-sensitive regulators of health and aging. Here,we present telomere DNA length analysis of two reef-building coral genera revealing that the long- and short-term water thermal regime is a key driver of between-colony variation across the Pacific Ocean. Notably, there are differences between the two studied genera. The telomere DNA lengths of the short-lived, more stress-sensitive Pocillopora spp. colonies were largely determined by seasonal temperature variation, whereas those of the long-lived, more stress-resistant Porites spp. colonies were insensitive to seasonal patterns, but rather influenced by past thermal anomalies. These results reveal marked differences in telomere DNA length regulation between two evolutionary distant coral genera exhibiting specific life-history traits. We propose that environmentally regulated mechanisms of telomere maintenance are linked to organismal performances, a matter of paramount importance considering the effects of climate change on health.

A p21-GFP zebrafish model of senescence for rapid testing of senolytics in vivo

Senescence drives the onset and severity of multiple ageing-associated diseases and frailty. As a result, there has been an increased interest in mechanistic studies and in the search for compounds targeting senescent cells, known as senolytics. Mammalian models are commonly used to test senolytics and generate functional and toxicity data at the level of organs and systems, yet this is expensive and time consuming. Zebrafish share high homology in genes associated with human ageing and disease. They can be genetically modified relatively easily. In larvae, most organs develop within 5 days of fertilisation and are transparent, which allows tracking of fluorescent cells in vivo in real time, testing drug off-target toxicity and assessment of cellular and phenotypic changes. Here, we have generated a transgenic zebrafish line that expresses green fluorescent protein (GFP) under the promoter of a key senescence marker, p21. We show an increase in p21:GFP+ cells in larvae following exposure to ionising radiation and with natural ageing. p21:GFP+ cells display other markers of senescence, including senescence-associated β-galactosidase and IL6. The observed increase in senescent cells following irradiation is associated with a reduction in the thickness of muscle fibres and mobility, two important ageing phenotypes. We also show that quercetin and dasatinib, two senolytics currently in clinical trials, reduce the number of p21:GFP+ cells, in a rapid 5-day assay. This model provides an important tool to study senescence in a living organism, allowing the rapid selection of senolytics before moving to more expensive and time-consuming mammalian systems.

Short-Term TERT Inhibition Impairs Cellular Proliferation via a Telomere Length-Independent Mechanism and Can Be Exploited as a Potential Anticancer Approach

Telomerase reverse transcriptase (TERT), the catalytic component of telomerase, may also contribute to carcinogenesis via telomere-length independent mechanisms. Our previous in vitro and in vivo studies demonstrated that short-term telomerase inhibition by BIBR1532 impairs cell proliferation without affecting telomere length. Here, we show that the impaired cell cycle progression following short-term TERT inhibition by BIBR1532 in in vitro models of B-cell lymphoproliferative disorders, i.e., Epstein-Barr virus (EBV)-immortalized lymphoblastoid cell lines (LCLs), and B-cell malignancies, i.e., Burkitt's lymphoma (BL) cell lines, is characterized by a significant reduction in NF-κB p65 nuclear levels leading to the downregulation of its target gene MYC. MYC downregulation was associated with increased expression and nuclear localization of P21, thus promoting its cell cycle inhibitory function. Consistently, treatment with BIBR1532 in wild-type zebrafish embryos significantly decreased Myc and increased p21 expression. The combination of BIBR1532 with antineoplastic drugs (cyclophosphamide or fludarabine) significantly reduced xenografted cells' proliferation rate compared to monotherapy in the zebrafish xenograft model. Overall, these findings indicate that short-term inhibition of TERT impairs cell growth through the downregulation of MYC via NF-κB signalling and supports the use of TERT inhibitors in combination with antineoplastic drugs as an efficient anticancer strategy.

Gut-specific telomerase expression counteracts systemic aging in telomerase-deficient zebrafish

Telomere shortening is a hallmark of aging and is counteracted by telomerase. As in humans, the zebrafish gut is one of the organs with the fastest rate of telomere decline, triggering early tissue dysfunction during normal zebrafish aging and in prematurely aged telomerase mutants. However, whether telomere-dependent aging of an individual organ, the gut, causes systemic aging is unknown. Here we show that tissue-specific telomerase expression in the gut can prevent telomere shortening and rescues premature aging of tert^-/-. Induction of telomerase rescues gut senescence and low cell proliferation, while restoring tissue integrity, inflammation and age-dependent microbiota dysbiosis. Averting gut aging causes systemic beneficial impacts, rescuing aging of distant organs such as reproductive and hematopoietic systems. Conclusively, we show that gut-specific telomerase expression extends the lifespan of tert^-/- by 40%, while ameliorating natural aging. Our work demonstrates that gut-specific rescue of telomerase expression leading to telomere elongation is sufficient to systemically counteract aging in zebrafish.

Tetrahydroxystilbene Glucoside Attenuates Oxidative Stress-Induced Aging by Regulating Oxidation Resistance and Inflammation in Larval Zebrafish

Population aging is a global problem worldwide, and the discovery of antiaging drugs and knowledge of their potential molecular mechanisms are research hotspots in biomedical field. Tetrahydroxystilbene glucoside (TSG) is a natural component isolated from Heshouwu (Polygonum multiflorum Thunb.). It has been widely used to treat various chronic diseases for its remarkable biological activities. In this study, we successfully established aging larval zebrafish by exposing larvae to 2 mM hydrogen peroxide (H2O2). Using this aging model, we assessed the antiaging effect of TSG with different concentrations (25-100 μg/mL). After being treated with H2O2, zebrafish showed the obvious aging-associated phenotypes characterized by higher senescence-associated β-galactosidase activity, significantly downregulated expression of sirtuin 1 (sirt1) and telomerase reverse transcriptase (tert), and upregulated serpine1 mRNA level compared to the control group. TSG pretreatment delayed the aging process of oxidative stress-induced zebrafish, indicative of the reduced positive rate of senescence-associated β-galactosidase, improved swimming velocity, and stimulus-response capacity. Further studies proved that TSG could suppress reactive oxygen species production and enhance the activity of antioxidant enzymes superoxide dismutase and catalase. TSG also inhibited the H2O2-induced expressions of inflammation-related genes il-1β, il-6, cxcl-c1c, and il-8 in aging zebrafish, but it did not affect apoptosis-related genes (bcl-2, bax, and caspase-3) of aging zebrafish. In conclusion, TSG can protect against aging by regulating the antioxidative genes and enzyme activity, as well as inflammation in larval zebrafish, providing insight into the application of TSG for clinical treatment of aging or aging-related diseases.

Learning from Zebrafish Hematopoiesis

Hematopoiesis is a complex process that tightly regulates the generation, proliferation, differentiation, and maintenance of hematopoietic cells. Disruptions in hematopoiesis can lead to various diseases affecting both hematopoietic and non-hematopoietic systems, such as leukemia, anemia, thrombocytopenia, rheumatoid arthritis, and chronic granuloma. The zebrafish serves as a powerful vertebrate model for studying hematopoiesis, offering valuable insights into both hematopoietic regulation and hematopoietic diseases. In this chapter, we present a comprehensive overview of zebrafish hematopoiesis, highlighting its distinctive characteristics in hematopoietic processes. We discuss the ontogeny and modulation of both primitive and definitive hematopoiesis, as well as the microenvironment that supports hematopoietic stem/progenitor cells. Additionally, we explore the utility of zebrafish as a disease model and its potential in drug discovery, which not only advances our understanding of the regulatory mechanisms underlying hematopoiesis but also facilitates the exploration of novel therapeutic strategies for hematopoietic diseases.

Zebrafish - The Neurobehavioural Model in Trend

Zebrafish (Danio rerio) is currently in vogue as a prevalently used experimental model for studies concerning neurobehavioural disorders and associated fields. Since the 1960s, this model has succeeded in breaking most barriers faced in the hunt for an experimental model. From its appearance to its high parity with human beings genetically, this model renders itself as an advantageous experimental lab animal. Neurobehavioural disorders have always posed an arduous task in terms of their detection as well as in determining their exact etiology. They are still, in most cases, diseases of interest for inventing or discovering novel pharmacological interventions. Thus, the need for a harbinger experimental model for studying neurobehaviours is escalating. Ensuring the same model is used for studying several neuro-studies conserves the results from inter-species variations. For this, we need a model that satisfies all the pre-requisite conditions to be made the final choice of model for neurobehavioural studies. This review recapitulates the progress of zebrafish as an experimental model with its most up-to-the-minute advances in the area. Various tests, assays, and responses employed using zebrafish in screening neuroactive drugs have been tabulated effectively. The tools, techniques, protocols, and apparatuses that bolster zebrafish studies are discussed. The probable research that can be done using zebrafish has also been briefly outlined. The various breeding and maintenance methods employed, along with the information on various strains available and most commonly used, are also elaborated upon, supplementing Zebrafish's use in neuroscience.

Lifespan and telomere length variation across populations of wild-derived African killifish

Telomeres and telomerase prevent the continuous erosion of chromosome-ends caused by lifelong cell division. Shortened telomeres are associated with age-related pathologies. While short telomere length is positively correlated with increased lethality at the individual level, in comparisons across species short telomeres are associated with long (and not short) lifespans. Here, we tested this contradiction between individual and evolutionary patterns in telomere length using African annual killifish. We analysed lifespan and telomere length in a set of captive strains derived from well-defined wild populations of Nothobranchius furzeri and its sister species, N. kadleci, from sites along a strong gradient of aridity which ultimately determines maximum natural lifespan. Overall, males were shorter-lived than females, and also had shorter telomeres. Male lifespan (measured in controlled laboratory conditions) was positively associated with the amount of annual rainfall in the site of strain origin. However, fish from wetter climates had shorter telomeres. In addition, individual fish which grew largest over the juvenile period possessed shorter telomeres at the onset of adulthood. This demonstrates that individual condition and environmentally-driven selection indeed modulate the relationship between telomere length and lifespan in opposite directions, validating the existence of inverse trends within a single taxon. Intraindividual heterogeneity of telomere length (capable to detect very short telomeres) was not associated with mean telomere length, suggesting that the shortest telomeres are controlled by regulatory pathways other than those that determine mean telomere length. The substantial variation in telomere length between strains from different environments identifies killifish as a powerful system in understanding the adaptive value of telomere length.

Inbreeding effects on telomeres in hatchling sand lizards (Lacerta agilis): An optimal family affair?

Telomeres are nucleotide-protein caps, predominantly at the ends of Metazoan linear chromosomes, showing complex dynamics with regard to their lengthening and shortening through life. Their complexity has entertained the idea that net telomere length and attrition could be valuable biomarkers of phenotypic and genetic quality of their bearer. Intuitively, those individuals could be more heterozygous and, hence, less inbred. However, some inbred taxa have longer, not shorter, telomeres. To understand the role of inbreeding in this complex scenario we need large samples across a range of genotypes with known maternity and paternity in telomere-screened organisms under natural conditions. We assessed the effects of parental and hatchling inbreeding on telomere length in >1300 offspring from >500 sires and dams in a population of sand lizards (Lacerta agilis). Maternal and paternal ID and their interactions predict hatchling telomere length at substantial effect sizes (R²  > .50). Deviation from mean maternal heterozygosity statistically predicts shorter offspring telomeres but this only when sibship is controlled for by paternal ID, and then is still limited (R²  = .06). Raw maternal heterozygosity scores, ignoring absolute deviation from the mean, explained 0.07% of the variance in hatchling telomere length. In conclusion, inbreeding is not a driver of telomere dynamics in the sand lizard (Lacerta agilis) study system.

Zebrafish models of COVID-19

Although COVID-19 has only recently appeared, research studies have already developed and implemented many animal models for deciphering the secrets of the disease and provided insights into the biology of SARS-CoV-2. However, there are several major factors that complicate the study of this virus in model organisms, such as the poor infectivity of clinical isolates of SARS-CoV-2 in some model species, and the absence of persistent infection, immunopathology, severe acute respiratory distress syndrome, and, in general, all the systemic complications which characterize COVID-19 clinically. Another important limitation is that SARS-CoV-2 mainly causes severe COVID-19 in older people with comorbidities, which represents a serious problem when attempting to use young and immunologically naïve laboratory animals in COVID-19 testing. We review here the main animal models developed so far to study COVID-19 and the unique advantages of the zebrafish model that may help to contribute to understand this disease, in particular to the identification and repurposing of drugs to treat COVID-19, to reveal the mechanism of action and side-effects of Spike-based vaccines, and to decipher the high susceptibility of aged people to COVID-19.

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.

A subset of gut leukocytes has telomerase-dependent "hyper-long" telomeres and require telomerase for function in zebrafish

BACKGROUND

Telomerase, the enzyme capable of elongating telomeres, is usually restricted in human somatic cells, which contributes to progressive telomere shortening with cell-division and ageing. T and B-cells cells are somatic cells that can break this rule and can modulate telomerase expression in a homeostatic manner. Whereas it seems intuitive that an immune cell type that depends on regular proliferation outbursts for function may have evolved to modulate telomerase expression it is less obvious why others may also do so, as has been suggested for macrophages and neutrophils in some chronic inflammation disease settings. The gut has been highlighted as a key modulator of systemic ageing and is a key tissue where inflammation must be carefully controlled to prevent dysfunction. How telomerase may play a role in innate immune subtypes in the context of natural ageing in the gut, however, remains to be determined.

RESULTS

Using the zebrafish model, we show that subsets of gut immune cells have telomerase-dependent"hyper-long" telomeres, which we identified as being predominantly macrophages and dendritics (mpeg1.1+ and cd45+mhcII+). Notably, mpeg1.1+ macrophages have much longer telomeres in the gut than in their haematopoietic tissue of origin, suggesting that there is modulation of telomerase in these cells, in the gut. Moreover, we show that a subset of gut mpeg1.1+ cells express telomerase (tert) in young WT zebrafish, but that the relative proportion of these cells decreases with ageing. Importantly, this is accompanied by telomere shortening and DNA damage responses with ageing and a telomerase-dependent decrease in expression of autophagy and immune activation markers. Finally, these telomerase-dependent molecular alterations are accompanied by impaired phagocytosis of E. coli and increased gut permeability in vivo.

CONCLUSIONS

Our data show that limiting levels of telomerase lead to alterations in gut immunity, impacting on the ability to clear pathogens in vivo. These are accompanied by increased gut permeability, which, together, are likely contributors to local and systemic tissue degeneration and increased susceptibility to infection with ageing.

Whole-mount Senescence-Associated Beta-Galactosidase (SA-β-GAL) Activity Detection Protocol for Adult Zebrafish

Senescence-associated beta-galactosidase (SA-β-GAL) is an enzyme that accumulates in the lysosomes of senescent cells, where it hydrolyses β-galactosides. With p16, it represents a well-recognized biomarker used to assess senescence both in vivo and in cell culture. The use of a chromogenic substrate, such as 5-bromo-4-chloro-3-indoyl-β-d-galactopyranoside (X-Gal), allows the detection of SA-β-GAL activity at pH 6.0 by the release of a visible blue product. Senescence occurs during aging and is part of the aging process itself. We have shown that prematurely aged zebrafish accumulate senescent cells detectable by SA-β-GAL staining in different tissues, including testis and gut. Here, we report a detailed protocol to perform an SA-β-GAL assay to detect senescent cell accumulation across the entire adult zebrafish organism ( Danio rerio ). We also identify previously unreported organs that show increased cell senescence in telomerase mutants, including the liver and the spinal cord.

An evolutionary perspective on stress responses, damage and repair

Variation in stress responses has been investigated in relation to environmental factors, species ecology, life history and fitness. Moreover, mechanistic studies have unravelled molecular mechanisms of how acute and chronic stress responses cause physiological impacts ('damage'), and how this damage can be repaired. However, it is not yet understood how the fitness effects of damage and repair influence stress response evolution. Here we study the evolution of hormone levels as a function of stressor occurrence, damage and the efficiency of repair. We hypothesise that the evolution of stress responses depends on the fitness consequences of damage and the ability to repair that damage. To obtain some general insights, we model a simplified scenario in which an organism repeatedly encounters a stressor with a certain frequency and predictability (temporal autocorrelation). The organism can defend itself by mounting a stress response (elevated hormone level), but this causes damage that takes time to repair. We identify optimal strategies in this scenario and then investigate how those strategies respond to acute and chronic exposures to the stressor. We find that for higher repair rates, baseline and peak hormone levels are higher. This typically means that the organism experiences higher levels of damage, which it can afford because that damage is repaired more quickly, but for very high repair rates the damage does not build up. With increasing predictability of the stressor, stress responses are sustained for longer, because the animal expects the stressor to persist, and thus damage builds up. This can result in very high (and potentially fatal) levels of damage when organisms are exposed to chronic stressors to which they are not evolutionarily adapted. Overall, our results highlight that at least three factors need to be considered jointly to advance our understanding of how stress physiology has evolved: (i) temporal dynamics of stressor occurrence; (ii) relative mortality risk imposed by the stressor itself versus damage caused by the stress response; and (iii) the efficiency of repair mechanisms.

Müller Glia maintain their regenerative potential despite degeneration in the aged zebrafish retina

Ageing is a significant risk factor for degeneration of the retina. Müller glia cells (MG) are key for neuronal regeneration, so harnessing the regenerative capacity of MG in the retina offers great promise for the treatment of age-associated blinding conditions. Yet, the impact of ageing on MG regenerative capacity is unclear. Here, we show that the zebrafish retina undergoes telomerase-independent, age-related neurodegeneration but that this is insufficient to stimulate MG proliferation and regeneration. Instead, age-related neurodegeneration is accompanied by MG morphological aberrations and loss of vision. Mechanistically, yes-associated protein (Yap), part of the Hippo signalling, has been shown to be critical for the regenerative response in the damaged retina, and we show that Yap expression levels decline with ageing. Despite this, morphologically and molecularly altered aged MG retain the capacity to regenerate neurons after acute light damage, therefore, highlighting key differences in the MG response to high-intensity acute damage versus chronic neuronal loss in the zebrafish retina.

A high-resolution genome of an euryhaline and eurythermal rhinogoby (Rhinogobius similis Gill 1895)

Rhinogobius similis is distributed in East and Southeast Asia. It is an amphidromous species found mostly in freshwater and sometimes brackish waters. We have obtained a high-resolution assembly of the R. similis genome using nanopore sequencing, high-throughput chromosome conformation capture (Hi-C), and transcriptomic data. The assembled genome was 890.10 Mb in size and 40.15% in GC content. Including 1373 contigs with contig N50 is 1.54 Mb, and scaffold N50 is 41.51 Mb. All of the 1373 contigs were anchored on 22 pairs of chromosomes. The BUSCO evaluation score was 93.02% indicating high quality of genome assembly. The repeat sequences accounted for 34.92% of the whole genome, with retroelements (30.13%), DNA transposons (1.64%), simple repeats (2.34%), and so forth. A total of 31,089 protein-coding genes were predicted in the genome and functionally annotated using Maker, of those genes, 26,893 (86.50%) were found in InterProScan5. There were 1910 gene families expanded in R. similis, 1171 gene families contracted and 170 gene families rapidly evolving. We have compared one rapidly change gene family (PF05970) commonly found in four species (Boleophthalmus pectinirostris, Neogobius melanostomus, Periophthalmus magnuspinnatus, and R. similis), which was found probably related to the lifespan of those species. During 400-10 Ka, the period of the Guxiang Ice Age, the population of R. similis decreased drastically, and then increased gradually following the last interglacial period. A high-resolution genome of R. similis should be useful to study taxonomy, biogeography, comparative genomics, and adaptive evolution of the most speciose freshwater goby genus, Rhinogobius.

Telomere dysfunction in ageing and age-related diseases

Ageing organisms accumulate senescent cells that are thought to contribute to body dysfunction. Telomere shortening and damage are recognized causes of cellular senescence and ageing. Several human conditions associated with normal ageing are precipitated by accelerated telomere dysfunction. Here, we systematize a large body of evidence and propose a coherent perspective to recognize the broad contribution of telomeric dysfunction to human pathologies.

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.

Systems Analysis Reveals Ageing-Related Perturbations in Retinoids and Sex Hormones in Alzheimer's and Parkinson's Diseases

Neurodegenerative diseases, including Alzheimer's (AD) and Parkinson's diseases (PD), are complex heterogeneous diseases with highly variable patient responses to treatment. Due to the growing evidence for ageing-related clinical and pathological commonalities between AD and PD, these diseases have recently been studied in tandem. In this study, we analysed transcriptomic data from AD and PD patients, and stratified these patients into three subclasses with distinct gene expression and metabolic profiles. Through integrating transcriptomic data with a genome-scale metabolic model and validating our findings by network exploration and co-analysis using a zebrafish ageing model, we identified retinoids as a key ageing-related feature in all subclasses of AD and PD. We also demonstrated that the dysregulation of androgen metabolism by three different independent mechanisms is a source of heterogeneity in AD and PD. Taken together, our work highlights the need for stratification of AD/PD patients and development of personalised and precision medicine approaches based on the detailed characterisation of these subclasses.

Zebrafish Cancer Predisposition Models

Cancer predisposition syndromes are rare, typically monogenic disorders that result from germline mutations that increase the likelihood of developing cancer. Although these disorders are individually rare, resulting cancers collectively represent 5-10% of all malignancies. In addition to a greater incidence of cancer, affected individuals have an earlier tumor onset and are frequently subjected to long-term multi-modal cancer screening protocols for earlier detection and initiation of treatment. In vivo models are needed to better understand tumor-driving mechanisms, tailor patient screening approaches and develop targeted therapies to improve patient care and disease prognosis. The zebrafish (Danio rerio) has emerged as a robust model for cancer research due to its high fecundity, time- and cost-efficient genetic manipulation and real-time high-resolution imaging. Tumors developing in zebrafish cancer models are histologically and molecularly similar to their human counterparts, confirming the validity of these models. The zebrafish platform supports both large-scale random mutagenesis screens to identify potential candidate/modifier genes and recently optimized genome editing strategies. These techniques have greatly increased our ability to investigate the impact of certain mutations and how these lesions impact tumorigenesis and disease phenotype. These unique characteristics position the zebrafish as a powerful in vivo tool to model cancer predisposition syndromes and as such, several have already been created, including those recapitulating Li-Fraumeni syndrome, familial adenomatous polyposis, RASopathies, inherited bone marrow failure syndromes, and several other pathogenic mutations in cancer predisposition genes. In addition, the zebrafish platform supports medium- to high-throughput preclinical drug screening to identify compounds that may represent novel treatment paradigms or even prevent cancer evolution. This review will highlight and synthesize the findings from zebrafish cancer predisposition models created to date. We will discuss emerging trends in how these zebrafish cancer models can improve our understanding of the genetic mechanisms driving cancer predisposition and their potential to discover therapeutic and/or preventative compounds that change the natural history of disease for these vulnerable children, youth and adults.

Long-lived termite kings and queens activate telomerase in somatic organs

Kings and queens of termites, like queens of other advanced eusocial insects, are endowed with admirable longevity, which dramatically exceeds the life expectancies of their non-reproducing nest-mates and related solitary insects. In the quest to find the mechanisms underlying the longevity of termite reproductives, we focused on somatic maintenance mediated by telomerase. This ribonucleoprotein is well established for pro-longevity functions in vertebrates, thanks primarily to its ability of telomere extension. However, its participation in lifespan regulation of insects, including the eusocial taxa, remains understudied. Here, we report a conspicuous increase of telomerase abundance and catalytic activity in the somatic organs of primary and secondary reproductives of the termite Prorhinotermes simplex and confirm a similar pattern in two other termite species. These observations stand in contrast with the telomerase downregulation characteristic for most adult somatic tissues in vertebrates and also in solitary insects and non-reproducing castes of termites. At the same time, we did not observe caste-specific differences in telomere lengths that might explain the differential longevity of termite castes. We conclude that although the telomerase activation in termite reproductives is in line with the broadly assumed association between telomerase and longevity, its direct phenotypic impact remains to be elucidated.

Short telomeres drive pessimistic judgement bias in zebrafish

The role of telomerase reverse transcriptase has been widely investigated in the contexts of ageing and age-related diseases. Interestingly, decreased telomerase activities (and accelerated telomere shortening) have also been reported in patients with emotion-related disorders, opening the possibility for subjective appraisal of stressful stimuli playing a key role in stress-driven telomere shortening. In fact, patients showing a pessimistic judgement bias have shorter telomeres. However, in humans the evidence for this is correlational and the causal directionality between pessimism and telomere shortening has not been established experimentally yet. We have developed and validated a judgement bias experimental paradigm to measure subjective evaluations of ambiguous stimuli in zebrafish. This behavioural assay allows classification of individuals in an optimistic-pessimistic dimension (i.e. from individuals that consistently evaluate ambiguous stimuli as negative to others that perceive them as positive). Using this behavioural paradigm we found that telomerase-deficient zebrafish (tert-/-) were more pessimistic in response to ambiguous stimuli than wild-type zebrafish. The fact that individuals with constitutive shorter telomeres have pessimistic behaviours demonstrates for the first time in a vertebrate model a genetic basis of judgement bias.

Loss of the glucocorticoid receptor causes accelerated ovarian ageing in zebrafish

Reproductive decline in mid-adult females is an established phenotype of the ageing process. Stress and the rise in glucocorticoids (GCs) accelerate reproductive ageing, but little is known about the mechanisms involved. During stress, GCs activate the glucocorticoid receptor (GR), a ubiquitously expressed, ligand-bound transcription factor, to elicit physiological changes for restoring homeostasis. Here, we tested the hypothesis that GC-GR signalling is essential for accelerating reproductive ageing. To test this, we used a ubiquitous GR knockout (GRKO) zebrafish, which is inherently hypercortisolemic, to delineate the role of high cortisol and GR signalling on reproductive ageing. The loss of GR led to premature ovarian ageing, including high frequency of typical and atypical follicular atresia in vitellogenic oocytes, yolk liquefaction and large inflammatory infiltrates. The reduction in oocyte quality was also associated with a decline in ovarian tert expression in the adult GRKO fish compared to the early adult GRKO and adult wild-type zebrafish. Accelerated ovarian ageing also impacted the progeny, including lower breeding success, fecundity, egg fertilization rate and delayed somitogenesis and embryo survival in the adult GRKO fish. We adduce that GR signalling is essential for prolonging the reproductive lifespan and improving the egg quality and embryo viability in zebrafish.

Preliminary Identification of Candidate Genes Related to Survival of Gynogenetic Rainbow Trout (Oncorhynchus mykiss) Based on Comparative Transcriptome Analysis

In the present research, the eggs from four rainbow trout females were used to provide four groups of gynogenetic doubled haploids (DHs). The quality of the eggs from different clutches was comparable, however, interclutch differences were observed in the gynogenetic variants of the experiment and the survival of DH specimens from different groups varied from 3% to 57% during embryogenesis. Transcriptome analysis of the eggs from different females exhibited inter-individual differences in the maternal genes' expression. Eggs originating from females whose gynogenetic offspring had the highest survival showed an increased expression of 46 genes when compared to the eggs from three other females. Eggs with the highest survival of gynogenetic embryos showed an up-regulation of genes that are associated with cell survival, migration and differentiation (tyrosine-protein kinase receptor TYRO3-like gene), triglyceride metabolism (carnitine O-palmitoyltransferase 1 gene), biosynthesis of polyunsaturated fat (3-oxoacyl-acyl-carrier-protein reductase gene), early embryogenic development (protein argonaute-3 gene, leucine-rich repeat-containing protein 3-like gene), 5S RNA binding (ribosome biogenesis regulatory protein homolog) as well as senescence and aging (telomerase reverse transcriptase, TERT gene), among others. Positive correlation between the genotypic efficiency and egg transcriptome profiles indicated that at least some of the differentially expressed genes should be considered as potential candidate genes for the efficiency of gynogenesis in rainbow trout.

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 shortening produces an inflammatory environment that increases tumor incidence in zebrafish

Cancer incidence increases exponentially with age when human telomeres are shorter. Similarly, telomerase reverse transcriptase (tert) mutant zebrafish have premature short telomeres and anticipate cancer incidence to younger ages. However, because short telomeres constitute a road block to cell proliferation, telomere shortening is currently viewed as a tumor suppressor mechanism and should protect from cancer. This conundrum is not fully understood. In our current study, we report that telomere shortening promotes cancer in a noncell autonomous manner. Using zebrafish chimeras, we show increased incidence of invasive melanoma when wild-type (WT) tumors are generated in tert mutant zebrafish. Tissues adjacent to melanoma lesions (skin) and distant organs (intestine) in tert mutants exhibited higher levels of senescence and inflammation. In addition, we transferred second generation (G2) tert blastula cells into WT to produce embryo chimeras. Cells with very short telomeres induced increased tumor necrosis factor1-α (TNF1-α) expression and senescence in larval tissues in a noncell autonomous manner, creating an inflammatory environment. Considering that inflammation is protumorigenic, we transplanted melanoma-derived cells into G2 tert zebrafish embryos and observed that tissue environment with short telomeres leads to increased tumor development. To test if inflammation was necessary for this effect, we treated melanoma transplants with nonsteroid anti-inflammatory drugs and show that higher melanoma dissemination can be averted. Thus, apart from the cell autonomous role of short telomeres in contributing to genome instability, we propose that telomere shortening with age causes systemic chronic inflammation leading to increased tumor incidence.

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.

Nephrotoxic Effects in Zebrafish after Prolonged Exposure to Aristolochic Acid

With the aim to explore the possibility to generate a zebrafish model of renal fibrosis, in this study the fibrogenic renal effect of aristolochic acid I (AAI) after immersion was assessed. This compound is highly nephrotoxic able to elicit renal fibrosis after exposure of rats and humans. Our results reveal that larval zebrafish at 15 days dpf (days post-fertilization) exposed for 8 days to 0.5 µM AAI showed clear signs of AKI (acute kidney injury). The damage resulted in the relative loss of the functional glomerular filtration barrier. Conversely, we did not observe any deposition of collagen, nor could we immunodetect α-SMA, a hallmark of myofibroblasts, in the tubules. In addition, no increase in gene expression of fibrogenesis biomarkers after whole animal RNA extraction was found. As zebrafish have a high capability for tissue regeneration possibly impeding fibrogenic processes, we also used a tert^−/− zebrafish line exhibiting telomerase deficiency and impaired tissue homeostasis. AAI-treated tert^−/− larvae displayed an increased sensitivity towards 0.5 µM AAI. Importantly, after AAI treatment a mild collagen deposition could be found in the tubules. The outcome implies that sustained AKI induced by nephrotoxic compounds combined with defective tert^−/− stem cells can produce a fibrotic response.

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.

celsr1a is essential for tissue homeostasis and onset of aging phenotypes in the zebrafish

The use of genetics has been invaluable in defining the complex mechanisms of aging and longevity. Zebrafish, while a prominent model for vertebrate development, have not been used systematically to address questions of how and why we age. In a mutagenesis screen focusing on late developmental phenotypes, we identified a new mutant that displays aging phenotypes at young adult stages. We find that the phenotypes are due to loss-of-function in the non-classical cadherin celsr1a. The premature aging is not associated with increased cellular senescence or telomere length but is a result of a failure to maintain progenitor cell populations. We show that celsr1a is essential for maintenance of stem cell progenitors in late stages. Caloric restriction can ameliorate celsr1a aging phenotypes. These data suggest that celsr1a function helps to mediate stem cell maintenance during maturation and homeostasis of tissues and thus regulates the onset or expressivity of aging phenotypes.

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.

Rag1 immunodeficiency-induced early aging and senescence in zebrafish are dependent on chronic inflammation and oxidative stress

In mammals, recombination activating gene 1 (RAG1) plays a crucial role in adaptive immunity, generating a vast range of immunoglobulins. Rag1^−/− zebrafish (Danio rerio) are viable and reach adulthood without obvious signs of infectious disease in standard nonsterile conditions, suggesting that innate immunity could be enhanced to compensate for the lack of adaptive immunity. By using microarray analysis, we confirmed that the expression of immunity‐ and apoptosis‐related genes was increased in the rag1^−/− fish. This tool also allows us to notice alterations of the DNA repair and cell cycle mechanisms in rag1^−/− zebrafish. Several senescence and aging markers were analyzed. In addition to the lower lifespan of rag1^−/− zebrafish compared to their wild‐type (wt) siblings, rag1^−/− showed a higher incidence of cell cycle arrest and apoptosis, a greater amount of phosphorylated histone H2AX, oxidative stress and decline of the antioxidant mechanisms, an upregulated expression and activity of senescence‐related genes and senescence‐associated β‐galactosidase, respectively, diminished telomere length, and abnormal self‐renewal and repair capacities in the retina and liver. Metabolomic analysis also demonstrated clear differences between wt and rag1^−/− fish, as was the deficiency of the antioxidant metabolite l‐acetylcarnitine (ALCAR) in rag1^−/− fish. Therefore, Rag1 activity does not seem to be limited to V(D)J recombination but is also involved in senescence and aging. Furthermore, we confirmed the senolytic effect of ABT‐263, a known senolytic compound and, for the first time, the potential in vivo senolytic activity of the antioxidant agent ALCAR, suggesting that this metabolite is essential to avoid premature aging.

The protective function of non-coding DNA in DNA damage accumulation with age and its roles in age-related diseases

Aging is a progressive decline of physiological function in tissue and organ accompanying both accumulation of DNA damage and reduction of non-coding DNA. Peripheral non-coding DNA/heterochromatin has been proposed to protect the genome and centrally-located protein-coding sequences in soma and male germ cells against radiation and the invasion of exogenous nucleic acids. Therefore, this review summarizes the reduction of non-coding DNA/heterochromatin (including telomeric DNA and rDNA) and DNA damage accumulation during normal physiological aging and in various aging-related diseases. Based on analysis of data, it is found that DNA damage accumulation is roughly negatively correlated with the reduction of non-coding DNA and therefore speculated that DNA damage accumulation is likely due to the reduction of non-coding DNA protection in genome defense during aging. Therefore, it is proposed here that means to increase the total amount of non-coding DNA and/or heterochromatin prior to the onset of these diseases could potentially better protect the genome and protein-coding DNA, reduce the incidence of aging-related diseases, and thus lead to better health during aging.

Zebrafish Carrying pycr1 Gene Deficiency Display Aging and Multiple Behavioral Abnormalities

Aging is a natural process that internal gene control and external stimuli mediate. Clinical data pointed out that homozygotic or heterozygotic mutation in the pyrroline-5-carboxylate reductase 1 (PYCR1) gene in humans caused cutis laxa (ARCL) disease, with progeroid appearance, lax and wrinkled skin, joint laxity, osteopenia, and mental retardation phenotypes. In this study, we aimed to generate pycr1 knockout (KO) zebrafish and carried out biochemical characterizations and behavior analyses. Marked apoptosis and senescence were detected in pycr1 KO zebrafish, which started from embryos/larvae stage. Biochemical assays showed that adult pycr1 KO fish have significantly reduced proline and extracellular matrix contents, lowered energy, and diminished superoxide dismutase (SOD) and telomerase activity when compared to the wild type fish, which suggested the pycr1 KO fish may have dysfunction in mitochondria. The pycr1 KO fish were viable; however, displayed progeria-like phenotype from the 4 months old and reach 50% mortality around six months old. In adult stage, we found that pycr1 KO fish showed reduced locomotion activity, aggression, predator avoidance, social interaction interest, as well as dysregulated color preference and circadian rhythm. In summary, we have identified multiple behavioral alterations in a novel fish model for aging with pycr1 gene loss-of-function by behavioral tests. This animal model may not only provide a unique vertebrate model to screen potential anti-aging drugs in the future, but also be an excellent in vivo model towards a better understanding of the corresponding behavioral alterations that accompany aging.

Loss of atrx cooperates with p53-deficiency to promote the development of sarcomas and other malignancies

The SWI/SNF-family chromatin remodeling protein ATRX is a tumor suppressor in sarcomas, gliomas and other malignancies. Its loss of function facilitates the alternative lengthening of telomeres (ALT) pathway in tumor cells, while it also affects Polycomb repressive complex 2 (PRC2) silencing of its target genes. To further define the role of inactivating ATRX mutations in carcinogenesis, we knocked out atrx in our previously reported p53/nf1-deficient zebrafish line that develops malignant peripheral nerve sheath tumors and gliomas. Complete inactivation of atrx using CRISPR/Cas9 was lethal in developing fish and resulted in an alpha-thalassemia-like phenotype including reduced alpha-globin expression. In p53/nf1-deficient zebrafish neither peripheral nerve sheath tumors nor gliomas showed accelerated onset in atrx+/- fish, but these fish developed various tumors that were not observed in their atrx+/+ siblings, including epithelioid sarcoma, angiosarcoma, undifferentiated pleomorphic sarcoma and rare types of carcinoma. These cancer types are included in the AACR Genie database of human tumors associated with mutant ATRX, indicating that our zebrafish model reliably mimics a role for ATRX-loss in the early pathogenesis of these human cancer types. RNA-seq of p53/nf1- and p53/nf1/atrx-deficient tumors revealed that down-regulation of telomerase accompanied ALT-mediated lengthening of the telomeres in atrx-mutant samples. Moreover, inactivating mutations in atrx disturbed PRC2-target gene silencing, indicating a connection between ATRX loss and PRC2 dysfunction in cancer development.

Somatic mutations in genes coding for epigenetic regulators such as ATRX are found across a diverse group of cancer types, suggesting their broad relevance in tumor induction and progression. However, tumors that have been linked to these chromatin remodelers can arise in many different molecular and cellular contexts, requiring studies with new experimental models to understand the extent and mechanisms of tumor development mediated by these regulatory proteins. Thus, we analyzed the tumor suppressive role of atrx in zebrafish that already harbored inactivating mutations of p53 and nf1. Homozygous deletion of atrx was lethal in developing fish, whereas the partial loss of this gene (atrx+/-) within the p53/nf1-deficient background led to a diverse spectrum of tumors not observed in animals that were wildtype for atrx, including epithelioid sarcoma, angiosarcoma, and rare carcinomas. Most of the cancer types we identified correspond to human tumors in the ATRX-mutant tumor sample cohort within the AACR Genie database, attesting to the relevance of our findings to human cancer. Further analysis revealed downregulation of telomerase during the lengthening of the telomeres through the ALT pathway, and disturbed function of the polycomb repressive complex 2 as key mechanistic components underlying atrx-linked tumorigenesis. These results demonstrate how a p53/nf1 compromised genetic background combined with ATRX haploinsufficiency leads to a broad spectrum of sarcomas and carcinomas associated with loss of this chromatin modulator.

Resident Immunity in Tissue Repair and Maintenance: The Zebrafish Model Coming of Age

The zebrafish has emerged as an exciting vertebrate model to study different aspects of immune system development, particularly due to its transparent embryonic development, the availability of multiple fluorescent reporter lines, efficient genetic tools and live imaging capabilities. However, the study of immunity in zebrafish has largely been limited to early larval stages due to an incomplete knowledge of the full repertoire of immune cells and their specific markers, in particular, a lack of cell surface antibodies to detect and isolate such cells in living tissues. Here we focus on tissue resident or associated immunity beyond development, in the adult zebrafish. It is our view that, with our increasing knowledge and the development of improved tools and protocols, the adult zebrafish will be increasingly appreciated for offering valuable insights into the role of immunity in tissue repair and maintenance, in both health and disease throughout the lifecourse.

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'.

The Zebrafish as an Emerging Model to Study DNA Damage in Aging, Cancer and Other Diseases

Cancer is a disease of the elderly, and old age is its largest risk factor. With age, DNA damage accumulates continuously, increasing the chance of malignant transformation. The zebrafish has emerged as an important vertebrate model to study these processes. Key mechanisms such as DNA damage responses and cellular senescence can be studied in zebrafish throughout its life course. In addition, the zebrafish is becoming an important resource to study telomere biology in aging, regeneration and cancer. Here we review some of the tools and resources that zebrafish researchers have developed and discuss their potential use in the study of DNA damage, cancer and aging related diseases.

Peering through zebrafish to understand inherited bone marrow failure syndromes

Inherited bone marrow failure syndromes are experiments of nature characterized by impaired hematopoiesis with cancer and leukemia predisposition. The mutations associated with inherited bone marrow failure syndromes affect fundamental cellular pathways, such as DNA repair, telomere maintenance, or proteostasis. How these disturbed pathways fail to produce sufficient blood cells and lead to leukemogenesis are not understood. The rarity of inherited cytopenias, the paucity of affected primary human hematopoietic cells, and the sometime inadequacy of murine or induced pluripotential stem cell models mean it is difficult to acquire a greater understanding of them. Zebrafish offer a model organism to study gene functions. As vertebrates, zebrafish share with humans many orthologous genes involved in blood disorders. As a model organism, zebrafish provide advantages that include rapid development of transparent embryos, high fecundity (providing large numbers of mutant and normal siblings), and a large collection of mutant and transgenic lines useful for investigating the blood system and other tissues during development. Importantly, recent advances in genomic editing in zebrafish can speedily validate the new genes or novel variants discovered in clinical investigation as causes for marrow failure. Here we review zebrafish as a model organism that phenocopies Fanconi anemia, Diamond-Blackfan anemia, dyskeratosis congenita, Shwachman-Diamond syndrome, congenital amegakaryocytic thrombocytopenia, and severe congenital neutropenia. Two important insights, provided by modeling inherited cytopenias in zebrafish, widen understanding of ribosome biogenesis and TP53 in mediating marrow failure and non-hematologic defects. They suggest that TP53-independent pathways contribute to marrow failure. In addition, zebrafish provide an attractive model organism for drug development.