The Greenland shark: A new challenge for the oxidative stress theory of ageing?

Upstream migrant sea lamprey (Petromyzon marinus) show signs of increasing oxidative stress but maintain aerobic capacity with age

Following the parasitic juvenile phase of their life cycle, sea lamprey (Petromyzon marinus) mature into a reproductive but rapidly aging and deteriorating adult, and typically die shortly after spawning in May or June. However, pre-spawning upstream migrant sea lamprey can be maintained for several months beyond their natural lifespan when held in cold water (∼4-8 °C) under laboratory conditions. We exploited this feature to investigate the interactions between senescence, oxidative stress, and metabolic function in this phylogenetically ancient fish. We investigated how life history traits and mitochondria condition, as indicated by markers of oxidative stress (catalase activity, lipid peroxidation) and aerobic capacity (citrate synthase activity), changed in adult sea lamprey from June to December after capture during their upstream spawning migration. Body mass but not liver mass declined with age, resulting in an increase in hepatosomatic index. Both effects were most pronounced in males, which also tended to have larger livers than females. Lamprey experienced greater oxidative stress with age, as reflected by increasing activity of the antioxidant enzyme catalase and increasing levels of lipid peroxidation in liver mitochondrial isolates over time. Surprisingly, the activity of citrate synthase also increased with age in both sexes. These observations implicate mitochondrial dysfunction and oxidative stress in the senescence of sea lamprey. Due to their unique evolutionary position and the technical advantage of easily delaying the onset of senescence in lampreys using cold water, these animals could represent an evolutionary unique and tractable model to investigate senescence in vertebrates.

Elasmobranchs as bioindicators of pollution in the marine environment

Bioindicator species are increasingly valuable in environmental pollution monitoring, and elasmobranch species include many suitable candidates for that role. By measuring contaminants and employing biomarkers of effect in relevant elasmobranch species, scientists may gain important insights about the impacts of pollution in marine ecosystems. This review compiles biomarkers applied in elasmobranchs to assess the effect of pollutants (e.g., metals, persistent organic pollutants, and plastics), and the environmental changes induced by anthropogenic activities (e.g., shifts in marine temperature, pH, and oxygenation). Over 30 biomarkers measured in more than 12 species were examined, including biotransformation biomarkers (e.g., cytochrome P450 1A), oxidative stress-related biomarkers (e.g., superoxide anion, lipid peroxidation, catalase, and vitamins), stress proteins (e.g., heat shock protein 70), reproductive and endocrine biomarkers (e.g., vitellogenin), osmoregulation biomarkers (e.g., trimethylamine N-oxide, Na⁺/K⁺-ATPase, and plasma ions), energetic and neurotoxic biomarkers (e.g., lactate dehydrogenase, lactate, and cholinesterases), and histopathological and morphologic biomarkers (e.g., tissue lesions and gross indices).

Lifespan Extension in Long-Lived Vertebrates Rooted in Ecological Adaptation

Contemporary studies on aging and longevity have largely overlooked the role that adaptation plays in lifespan variation across species. Emerging evidence indicates that the genetic signals of extended lifespan may be maintained by natural selection, suggesting that longevity could be a product of organismal adaptation. The mechanisms of adaptation in long-lived animals are believed to account for the modification of physiological function. Here, we first review recent progress in comparative biology of long-lived animals, together with the emergence of adaptive genetic factors that control longevity and disease resistance. We then propose that hitchhiking of adaptive genetic changes is the basis for lifespan changes and suggest ways to test this evolutionary model. As individual adaptive or adaptation-linked mutations/substitutions generate specific forms of longevity effects, the cumulative beneficial effect is largely nonrandom and is indirectly favored by natural selection. We consider this concept in light of other proposed theories of aging and integrate these disparate ideas into an adaptive evolutionary model, highlighting strategies in decoding genetic factors of lifespan control.

Alternative Animal Models of Aging Research

Most research on mechanisms of aging is being conducted in a very limited number of classical model species, i.e., laboratory mouse (Mus musculus), rat (Rattus norvegicus domestica), the common fruit fly (Drosophila melanogaster) and roundworm (Caenorhabditis elegans). The obvious advantages of using these models are access to resources such as strains with known genetic properties, high-quality genomic and transcriptomic sequencing data, versatile experimental manipulation capabilities including well-established genome editing tools, as well as extensive experience in husbandry. However, this approach may introduce interpretation biases due to the specific characteristics of the investigated species, which may lead to inappropriate, or even false, generalization. For example, it is still unclear to what extent knowledge of aging mechanisms gained in short-lived model organisms is transferable to long-lived species such as humans. In addition, other specific adaptations favoring a long and healthy life from the immense evolutionary toolbox may be entirely missed. In this review, we summarize the specific characteristics of emerging animal models that have attracted the attention of gerontologists, we provide an overview of the available data and resources related to these models, and we summarize important insights gained from them in recent years. The models presented include short-lived ones such as killifish (Nothobranchius furzeri), long-lived ones such as primates (Callithrix jacchus, Cebus imitator, Macaca mulatta), bathyergid mole-rats (Heterocephalus glaber, Fukomys spp.), bats (Myotis spp.), birds, olms (Proteus anguinus), turtles, greenland sharks, bivalves (Arctica islandica), and potentially non-aging ones such as Hydra and Planaria.

What Animal Cancers teach us about Human Biology

Cancers in animals present a large, underutilized reservoir of biomedical information with critical implication for human oncology and medicine in general. Discussing two distinct areas of tumour biology in non-human hosts, we highlight the importance of these findings for our current understanding of cancer, before proposing a coordinated strategy to harvest biomedical information from non-human resources and translate it into a clinical setting. First, infectious cancers that can be transmitted as allografts between individual hosts, have been identified in four distinct, unrelated groups, dogs, Tasmanian devils, Syrian hamsters and, surprisingly, marine bivalves. These malignancies might hold the key to improving our understanding of the interaction between tumour cell and immune system and, thus, allow us to devise novel treatment strategies that enhance anti-cancer immunosurveillance, as well as suggesting more effective organ and stem cell transplantation strategies. The existence of these malignancies also highlights the need for increased scrutiny when considering the existence of infectious cancers in humans. Second, it has long been understood that no linear relationship exists between the number of cells within an organism and the cancer incidence rate. To resolve what is known as Peto's Paradox, additional anticancer strategies within different species have to be postulated. These naturally occurring idiosyncrasies to avoid carcinogenesis represent novel potential therapeutic strategies.

Hydrogen sulfide in longevity and pathologies: Inconsistency is malodorous

Hydrogen sulfide (H₂S) is one of the biologically active gases (gasotransmitters), which plays an important role in various physiological processes and aging. Its production in the course of methionine and cysteine catabolism and its degradation are finely balanced, and impairment of H₂S homeostasis is associated with various pathologies. Despite the strong geroprotective action of exogenous H₂S in C. elegans, there are controversial effects of hydrogen sulfide and its donors on longevity in other models, as well as on stress resistance, age-related pathologies and aging processes, including regulation of senescence-associated secretory phenotype (SASP) and senescent cell anti-apoptotic pathways (SCAPs). Here we discuss that the translation potential of H₂S as a geroprotective compound is influenced by a multiplicity of its molecular targets, pleiotropic biological effects, and the overlapping ranges of toxic and beneficial doses. We also consider the challenges of the targeted delivery of H₂S at the required dose. Along with this, the complexity of determining the natural levels of H₂S in animal and human organs and their ambiguous correlations with longevity are reviewed.

Nontraditional systems in aging research: an update

Research on the evolutionary and mechanistic aspects of aging and longevity has a reductionist nature, as the majority of knowledge originates from experiments on a relatively small number of systems and species. Good examples are the studies on the cellular, molecular, and genetic attributes of aging (senescence) that are primarily based on a narrow group of somatic cells, especially fibroblasts. Research on aging and/or longevity at the organismal level is dominated, in turn, by experiments on Drosophila melanogaster, worms (Caenorhabditis elegans), yeast (Saccharomyces cerevisiae), and higher organisms such as mice and humans. Other systems of aging, though numerous, constitute the minority. In this review, we collected and discussed a plethora of up-to-date findings about studies of aging, longevity, and sometimes even immortality in several valuable but less frequently used systems, including bacteria (Caulobacter crescentus, Escherichia coli), invertebrates (Turritopsis dohrnii, Hydra sp., Arctica islandica), fishes (Nothobranchius sp., Greenland shark), reptiles (giant tortoise), mammals (blind mole rats, naked mole rats, bats, elephants, killer whale), and even 3D organoids, to prove that they offer biogerontologists as much as the more conventional tools. At the same time, the diversified knowledge gained owing to research on those species may help to reconsider aging from a broader perspective, which should translate into a better understanding of this tremendously complex and clearly system-specific phenomenon.

Viviparity in the longest-living vertebrate, the Greenland shark (Somniosus microcephalus)

BACKGROUND

The Greenland shark is renowned for its great longevity, yet little is known about its reproduction.

METHODS

We supplemented the sparse information on this species by extrapolation from observations on other members of the sleeper shark family and the order Squaliformes.

RESULTS AND CONCLUSION

The Greenland shark is viviparous and a single observation suggests a litter size of about ten. The gestation period is unknown, but embryos reach a length of around 40 cm at birth. Nutrition is derived from the yolk sac with minimal histotrophy. The surface area of the uterus is increased by villi that presumably increase in length with advancing gestation. These villi are not likely to be secretory but play a key role in the oxygen supply to the embryo. We argue that the ability of the uterus to supply oxygen is a limiting factor for litter size, which is not likely to exceed the small number reported in this and other sleeper sharks.

Exploring the role of host specialisation and oxidative stress in interspecific lifespan variation in subtropical tephritid flies

In herbivorous insects, the degree of host specialisation may be one ecological factor that shapes lifespan. Because host specialists can only exploit a limited number of plants, their lifecycle should be synchronised with host phenology to allow reproduction when suitable hosts are available. For species not undergoing diapause or dormancy, one strategy to achieve this could be evolving long lifespans. From a physiological perspective, oxidative stress could explain how lifespan is related to degree of host specialisation. Oxidative stress caused by Reactive Oxygen Species (ROS) might help underpin ageing (the Free Radical Theory of Aging (FRTA)) and mediate differences in lifespan. Here, we investigated how lifespan is shaped by the degree of host specialisation, phylogeny, oxidative damage accumulation and antioxidant protection in eight species of true fruit flies (Diptera: Tephritidae). We found that lifespan was not constrained by species relatedness or oxidative damage (arguing against the FRTA); nevertheless, average lifespan was positively associated with antioxidant protection. There was no lifespan difference between generalist and specialist species, but most of the tephritids studied had long lifespans in comparison with other dipterans. Long lifespan may be a trait under selection in fruit-feeding insects that do not use diapause.

Long-lived animals with negligible senescence: clues for ageing research

Among several theories to explain the complicated process of human ageing, the mitochondrial oxidative stress hypothesis has received recent attention. Considering that lifespan and ageing rates vary considerably across taxa, a better understanding of factors that lead to negligible or extremely rapid senescence in mammals may generate novel approaches to target human ageing. Several species, such as naked mole rats, ocean quahog, rockfish and Greenland shark, have been identified that exhibit negligible senescence and superior resistance to age-related diseases. Considering that the available literature suggests that their outstanding stress resistance is linked to maintenance of protein homeostasis and robust mitochondrial functions, treatments that target protein modification and upregulation of matrix antioxidants may have implications for extending human health span.

Of Mice, Whales, Jellyfish and Men: In Pursuit of Increased Longevity

The quest for increased human longevity has been a goal of mankind throughout recorded history. Recent molecular studies are now providing potentially useful insights into the aging process which may help to achieve at least some aspects of this quest. This chapter will summarize the main findings of these studies with a focus on long-lived mutant mice and worms, and the longest living natural species including Galapagos giant tortoises, bowhead whales, Greenland sharks, quahog clams and the immortal jellyfish.

Blood pressure in the Greenland shark as estimated from ventral aortic elasticity

We conducted in vitro inflations of freshly excised ventral aortas of the Greenland shark, Somniosus microcephalus, and used pressure-diameter data to estimate the point of transition from high to low compliance, which has been shown to occur at the mean blood pressure in other vertebrates including fishes. We also determined the pressure at which the modulus of elasticity of the aorta reached 0.4 MPa, as occurs at the compliance transition in other species. From these analyses, we predict the average ventral aortic blood pressure in S. microcephalus to be about 2.3-2.8 kPa, much lower than reported for other sharks. Our results support the idea that this species is slow moving and has a relatively low aerobic metabolism. Histological investigation of the ventral aorta shows that elastic fibres are present in relatively low abundance and loosely connected, consistent with this aorta having high compliance at a relatively low blood pressure.