The exceptional longevity of the naked mole‐rat may be explained by mitochondrial antioxidant defenses

Naked mole‐rats (NMRs) are mouse‐sized mammals that exhibit an exceptionally long lifespan (>30 vs. <4 years for mice), and resist aging‐related pathologies such as cardiovascular and pulmonary diseases, cancer, and neurodegeneration. However, the mechanisms underlying this exceptional longevity and disease resistance remain poorly understood. The oxidative stress theory of aging posits that (a) senescence results from the accumulation of oxidative damage inflicted by reactive oxygen species (ROS) of mitochondrial origin, and (b) mitochondria of long‐lived species produce less ROS than do mitochondria of short‐lived species. However, comparative studies over the past 28 years have produced equivocal results supporting this latter prediction. We hypothesized that, rather than differences in ROS generation, the capacity of mitochondria to consume ROS might distinguish long‐lived species from short‐lived species. To test this hypothesis, we compared mitochondrial production and consumption of hydrogen peroxide (H₂O₂; as a proxy of overall ROS metabolism) between NMR and mouse skeletal muscle and heart. We found that the two species had comparable rates of mitochondrial H₂O₂ generation in both tissues; however, the capacity of mitochondria to consume ROS was markedly greater in NMRs. Specifically, maximal observed consumption rates were approximately two and fivefold greater in NMRs than in mice, for skeletal muscle and heart, respectively. Our results indicate that differences in matrix ROS detoxification capacity between species may contribute to their divergence in lifespan.     

Stress profiling of longevity mutants identifies Afg3 as a mitochondrial determinant of cytoplasmic mRNA translation and aging

Although environmental stress likely plays a significant role in promoting aging, the relationship remains poorly understood. To characterize this interaction in a more comprehensive manner, we examined the stress response profiles for 46 long‐lived yeast mutant strains across four different stress conditions (oxidative, ER, DNA damage, and thermal), grouping genes based on their associated stress response profiles. Unexpectedly, cells lacking the mitochondrial AAA protease gene AFG3 clustered strongly with long‐lived strains lacking cytosolic ribosomal proteins of the large subunit. Similar to these ribosomal protein mutants, afg3Δ cells show reduced cytoplasmic mRNA translation, enhanced resistance to tunicamycin that is independent of the ER unfolded protein response, and Sir2‐independent but Gcn4‐dependent lifespan extension. These data demonstrate an unexpected link between a mitochondrial protease, cytoplasmic mRNA translation, and aging.     

A metabolic signature for long life in the Caenorhabditis elegans Mit mutants

Mit mutations that disrupt function of the mitochondrial electron transport chain can, inexplicably, prolong Caenorhabditis elegans lifespan. In this study we use a metabolomics approach to identify an ensemble of mitochondrial‐derived α‐ketoacids and α‐hydroxyacids that are produced by long‐lived Mit mutants but not by other long‐lived mutants or by short‐lived mitochondrial mutants. We show that accumulation of these compounds is dependent on concerted inhibition of three α‐ketoacid dehydrogenases that share dihydrolipoamide dehydrogenase (DLD) as a common subunit, a protein previously linked in humans with increased risk of Alzheimer's disease. When the expression of DLD in wild‐type animals was reduced using RNA interference we observed an unprecedented effect on lifespan – as RNAi dosage was increased lifespan was significantly shortened, but, at higher doses, it was significantly lengthened, suggesting that DLD plays a unique role in modulating length of life. Our findings provide novel insight into the origin of the Mit phenotype.     

Artificial selection,pre‐release diet,and gut symbiont inoculation effects on sterile male longevity for area‐wide fruit‐fly management

Longevity is an important life‐history trait for successful and cost‐effective application of the sterile insect technique. Furthermore, it has been shown that females of some species – e.g., Anastrepha ludens (Loew) (Diptera: Tephritidae) – preferentially copulate with ‘old’, sexually experienced males, rather than younger and inexperienced males. Long‐lived sterile males may therefore have greater opportunity to find and mate with wild females than short‐lived males, and be more effective in inducing sterility into wild populations. We explored the feasibility of increasing sterile male lifespan through selection of long‐lived strains and provision of pre‐release diets with added protein, and inoculated with bacterial symbionts recovered from cultures of the gut of wild Anastrepha obliqua (Macquart). Artificial selection for long‐lived A. ludens resulted in a sharp drop of fecundity levels for F1 females. Nevertheless, the cross of long‐lived males with laboratory females produced a female F1 progeny with fecundity levels comparable to those of females in the established colony. However, the male progeny of long‐lived males*laboratory females did not survive in higher proportions than laboratory males. Provision of sugar to A. obliqua adults resulted in increased survival in comparison to adults provided only with water, whereas the addition of protein to sugar‐only diets had no additional effect on longevity. Non‐irradiated males lived longer than irradiated males, and supplying a generic probiotic diet produced no noticeable effect in restoring irradiated male longevity of A. obliqua. We discuss the need to evaluate the time to reach sexual maturity and survival under stress for long‐lived strains, and the inclusion of low amounts of protein and specific beneficial bacteria in pre‐release diets to increase sterile male performance and longevity in the field.     

Mitochondrial metabolites extend lifespan

Disruption of mitochondrial respiration in the nematode Caenorhabditis elegans can extend lifespan. We previously showed that long‐lived respiratory mutants generate elevated amounts of α‐ketoacids. These compounds are structurally related to α‐ketoglutarate, suggesting they may be biologically relevant. Here, we show that provision of several such metabolites to wild‐type worms is sufficient to extend their life. At least one mode of action is through stabilization of hypoxia‐inducible factor‐1 (HIF‐1). We also find that an α‐ketoglutarate mimetic, 2,4‐pyridinedicarboxylic acid (2,4‐PDA), is alone sufficient to increase the lifespan of wild‐type worms and this effect is blocked by removal of HIF‐1. HIF‐1 is constitutively active in isp‐1(qm150) Mit mutants, and accordingly, 2,4‐PDA does not further increase their lifespan. Incubation of mouse 3T3‐L1 fibroblasts with life‐prolonging α‐ketoacids also results in HIF‐1α stabilization. We propose that metabolites that build up following mitochondrial respiratory dysfunction form a novel mode of cell signaling that acts to regulate lifespan.     

Anti‐aging drugs reduce hypothalamic inflammation in a sex‐specific manner

Aging leads to hypothalamic inflammation, but does so more slowly in mice whose lifespan has been extended by mutations that affect GH/IGF‐1 signals. Early‐life exposure to GH by injection, or to nutrient restriction in the first 3 weeks of life, also modulate both lifespan and the pace of hypothalamic inflammation. Three drugs extend lifespan of UM‐HET3 mice in a sex‐specific way: acarbose (ACA), 17‐α‐estradiol (17αE2), and nordihydroguaiaretic acid (NDGA), with more dramatic longevity increases in males in each case. In this study, we examined the effect of these anti‐aging drugs on neuro‐inflammation in hypothalamus and hippocampus. We found that age‐associated hypothalamic inflammation is reduced in males but not in females at 12 months of age by ACA and 17αE2 and at 22 months of age in NDGA‐treated mice. The three drugs blocked indices of hypothalamic reactive gliosis associated with aging, such as Iba‐1‐positive microglia and GFAP‐positive astrocytes, as well as age‐associated overproduction of TNF‐α. This effect was not observed in drug‐treated female mice or in the hippocampus of the drug‐treated animals. On the other hand, caloric restriction (CR; an intervention that extends the lifespan in both sexes) significantly reduced hypothalamic microglia and TNF‐α in both sexes at 12 months of age. Together, these results suggest that the extent of drug‐induced changes in hypothalamic inflammatory processes is sexually dimorphic in a pattern that parallels the effects of these agents on mouse longevity and that mimics the changes seen, in both sexes, of long‐lived nutrient restricted or mutant mice.     

Autophagy compensates impaired energy metabolism in CLPXP‐deficient Podospora anserina strains and extends healthspan

The degradation of nonfunctional mitochondrial proteins is of fundamental relevance for maintenance of cellular homeostasis. The heteromeric CLPXP protein complex in the mitochondrial matrix is part of this process. In the fungal aging model Podospora anserina, ablation of CLPXP leads to an increase in healthy lifespan. Here, we report that this counterintuitive increase depends on a functional autophagy machinery. In PaClpXP mutants, autophagy is involved in energy conservation and the compensation of impairments in respiration. Strikingly, despite the impact on mitochondrial function, it is not mitophagy but general autophagy that is constitutively induced and required for longevity. In contrast, in another long‐lived mutant ablated for the mitochondrial PaIAP protease, autophagy is neither induced nor required for lifespan extension. Our data provide novel mechanistic insights into the capacity of different forms of autophagy to compensate impairments of specific components of the complex mitochondrial quality control network and about the biological role of mitochondrial CLPXP in the control of cellular energy metabolism.     

Low hydrogen peroxide production in mitochondria of the long‐lived Arctica islandica: underlying mechanisms for slow aging

The observation of an inverse relationship between lifespan and mitochondrial H₂O₂ production rate would represent strong evidence for the disputed oxidative stress theory of aging. Studies on this subject using invertebrates are surprisingly lacking, despite their significance in both taxonomic richness and biomass. Bivalve mollusks represent an interesting taxonomic group to challenge this relationship. They are exposed to environmental constraints such as microbial H₂S, anoxia/reoxygenation, and temperature variations known to elicit oxidative stress. Their mitochondrial electron transport system is also connected to an alternative oxidase that might improve their ability to modulate reactive oxygen species (ROS) yield. Here, we compared H₂O₂ production rates in isolated mantle mitochondria between the longest‐living metazoan—the bivalve Arctica islandica—and two taxonomically related species of comparable size. In an attempt to test mechanisms previously proposed to account for a reduction of ROS production in long‐lived species, we compared oxygen consumption of isolated mitochondria and enzymatic activity of different complexes of the electron transport system in the two species with the greatest difference in longevity. We found that A. islandica mitochondria produced significantly less H₂O₂ than those of the two short‐lived species in nearly all conditions of mitochondrial respiration tested, including forward, reverse, and convergent electron flow. Alternative oxidase activity does not seem to explain these differences. However, our data suggest that reduced complex I and III activity can contribute to the lower ROS production of A. islandica mitochondria, in accordance with previous studies. We further propose that a lower complex II activity could also be involved.     

Contrasting Demographic Structure of Short‐ and Long‐lived Pioneer Tree Species on Amazonian Forest Edges

Although tropical forests have been rapidly converted into human‐modified landscapes, tree species response to forest edges remains poorly examined. In this study, we addressed four pioneer tree species to document demographic shifts experienced by this key ecological group and make inferences about pioneer response to forest edges. All individuals with dbh ≥ 1 cm of two short‐lived (Bellucia grossularioides and Cecropia sciadophylla) and two long‐lived species (Goupia glabra and Laetia procera) were sampled in 20 1‐ha forest edge plots and 20 1‐ha forest interior plots in Oiapoque and Manaus, Northeast and Central Amazon, respectively. As expected, pioneer stem density with dbh ≥ 1 cm increased by around 10–17‐fold along forest edges regardless of species, lifespan, and study site. Edge populations of long‐lived pioneers presented 84–94 percent of their individuals in sapling/subadult size classes, whereas edge populations of short‐lived pioneers showed 56–97 percent of their individuals in adult size classes. These demographic biases were associated with negative and positive net adult recruitment of long‐ and short‐lived pioneers, respectively. Our population‐level analyses support three general statements: (1) native pioneer tree species proliferate along forest edges (i.e., increased density), at least in terms of non‐reproductive individuals; (2) pioneer response to edge establishment is not homogeneous as species differ in terms of demographic structure and net adult recruitment; and (3) some pioneer species, particularly long‐lived ones, may experience population decline due to adult sensitivity to edge‐affected habitats.     

Effects of diet restriction on life history in a sexually cannibalistic spider

Diet restriction increases longevity while reducing fecundity in a broad range of organisms. However, there are exceptions to this rule, and the causes of these exceptions remain unclear. One hypothesis is that short‐lived, semelparous organisms gain no benefit from increased longevity regardless of nutritional resources. Another hypothesis is that organisms may alter their behaviour to compensate for nutrient deficiencies. We examined these hypotheses in the colonial orb‐weaving spider Cyrtophora citricola. Sexual cannibalism is frequent in this species so that females are long lived and interoparous while males are semelparous. Because of these differing sexual strategies, we predicted that the common pattern of increased longevity under diet restriction would hold for females but not for males. We also investigated in a semi‐natural setting whether spiders could compensate for diet restriction by altering their feeding behaviour. Diet‐restricted females produced fewer offspring but lived longer than well‐fed females, while diet had no effect on male longevity. Despite being semelparous, virgin males were quite long‐lived, suggesting that potential lifespan is relatively unimportant in determining the effects of diet restriction. Contrary to our predictions, females were unable to compensate for their restricted diet by altering their foraging behaviour. Instead, semi‐natural conditions increased the differences between spiders on high and low diets, suggesting that the effects of diet restriction can be pervasive under natural conditions.     

Within‐season increase in parental investment in a long‐lived bird species: investment shifts to maximize successful reproduction?

In nest‐building species predation of nest contents is a main cause of reproductive failure and parents have to trade off reproductive investment against antipredatory behaviours. While this trade‐off is modified by lifespan (short‐lived species prioritize current reproduction; long‐lived species prioritize future reproduction), it may vary within a breeding season, but this idea has only been tested in short‐lived species. Yet, life history theory does not make any prediction how long‐lived species should trade off current against future reproductive investment within a season. Here, we investigated this trade‐off through predator‐exposure experiments in a long‐lived bird species, the brown thornbill. We exposed breeding pairs that had no prior within‐season reproductive success to the models of a nest predator and a predator of adults during their first or second breeding attempt. Overall, parents reduced their feeding rate in the presence of a predator, but parents feeding second broods were more risk sensitive and almost ceased feeding when exposed to both types of predators. However, during second breeding attempts, parents had larger clutches and a higher feeding rate in the absence of predators than during first breeding attempts and approached both types of predators closer when mobbing. Our results suggest that the trade‐off between reproductive investment and risk‐taking can change in a long‐lived species within a breeding season depending on both prior nest predation and renesting opportunities. These patterns correspond to those in short‐lived species, raising the question of whether a within‐season shift in reproductive investment trade‐offs is independent of lifespan.     

Hybrid crosses and the genetic basis of interspecific divergence in lifespan in Pristionchus nematodes

Characterizing the genetic basis of among‐species variation in lifespan is a major goal of evolutionary gerontology research, but the very feature that defines separate species – the inability to interbreed – makes achieving this goal impractical, if not impossible, for most taxa. Pristionchus nematodes provide an intriguing system for tackling this problem, as female lifespan varies among species that can be crossed to form viable (although infertile) hybrids. By conducting reciprocal crosses among three species – two dioecious (long‐lived Pristionchus exspectatus and short‐lived Pristionchus arcanus) and one androdioecious (short‐lived Pristionchus pacificus) – we found that female lifespan was long for all hybrids, consistent with the hypothesis that the relatively short lifespans seen for P. pacificus hermaphrodites and P. arcanus females are caused by independent, recessive alleles that are masked in hybrid genomes. Cross‐direction had a small effect on survivorship for crosses involving P. exspectatus, indicating that nuclear–mitochondrial interactions may also influence Pristionchus longevity. Our findings suggest that long lifespan in P. exspectatus reflects the realization of an ancestral potential for extended longevity in the P. pacificus species complex. This work demonstrates the utility of interspecific hybrids for ageing research and provides a foundation for future work on the genetic architecture of interspecific lifespan variation.     

IGF‐I regulates the age‐dependent signaling peptide humanin

Aging is influenced by endocrine pathways including the growth hormone/insulin‐like growth factor‐1 (GH/IGF) axis. Mitochondrial function has also been linked to the aging process, but the relevant mitochondrial signals mediating the effects of mitochondria are poorly understood. Humanin is a novel signaling peptide that acts as a potent regulator of cellular stress responses and protects from a variety of in vitro and in vivo toxic and metabolic insults. The circulating levels of humanin decline with age in mice and humans. Here, we demonstrate a negative correlation between the activity of the GH‐IGF axis and the levels of humanin, as well as a positive correlation between humanin and lifespan in mouse models with altered GH/IGF‐I axis. Long‐lived, GH‐deficient Ames mice displayed elevated humanin levels, while short‐lived GH‐transgenic mice have reduced humanin levels. Furthermore, treatment with GH or IGF‐I reduced circulating humanin levels in both mice and human subjects. Our results indicate that GH and IGF are potent regulators of humanin levels and that humanin levels correlate with lifespan in mice. This suggests that humanin represents a circulating mitochondrial signal that participates in modulating the aging process, adding a coordinated mitochondrial element to the endocrine regulation of aging.     

Movement decline across lifespan of Caenorhabditis elegans mutants in the insulin/insulin‐like signaling pathway

Research in aging biology has identified several pathways that are molecularly conserved across species that extend lifespan when mutated. The insulin/insulin‐like signaling (IIS) pathway is one of the most widely studied of these. It has been assumed that extending lifespan also extends healthspan (the period of life with minimal functional loss). However, data supporting this assumption conflict and recent evidence suggest that life extension may, in and of itself, extend the frail period. In this study, we use Caenorhabditis elegans to further probe the link between lifespan and healthspan. Using movement decline as a measure of health, we assessed healthspan across the entire lifespan in nine IIS pathway mutants. In one series of experiments, we studied healthspan in mass cultures, and in another series, we studied individuals longitudinally. We found that long‐lived mutants display prolonged mid‐life movement and do not prolong the frailty period. Lastly, we observed that early‐adulthood movement was not predictive of late‐life movement or survival, within identical phenotypes. Overall, these observations show that extending lifespan does not prolong the period of frailty. Both genotype and a stochastic component modulate aging, and movement late in life is more variable than early‐life movement.     

Detection of barriers to dispersal is masked by long lifespans and large population sizes

Population genetic analyses of species inhabiting fragmented landscapes are essential tools for conservation. Occasionally, analyses of fragmented populations find no evidence of isolation, even though a barrier to dispersal is apparent. In some cases, not enough time may have passed to observe divergence due to genetic drift, a problem particularly relevant for long‐lived species with overlapping generations. Failing to consider this quality during population structure analyses could result in incorrect conclusions about the impact of fragmentation on the species. We designed a model to explore how lifespan and population size influence perceived population structure of isolated populations over time. This iterative model tracked how simulated populations of variable lifespan and population size were affected by drift alone, using a freshwater mussel, Quadrula quadrula (mapleleaf), as a model system. In addition to exhibiting dramatic lifespan variability among species, mussels are also highly imperiled and exhibit fragmentation by dams throughout the range of many species. Results indicated that, unless population size was small (<50 individuals) or lifespan short (<22 years), observing genetic divergence among populations was unlikely. Even if wild populations are isolated, observing population structure in long‐lived mussels from modern damming practices is unlikely because it takes longer for population structure to develop in these species than most North American dams have existed. Larger population sizes and longer lifespans increase the time needed for significant divergence to occur. This study helps illuminate the factors that influence genetic responses by populations to isolation and provides a useful model for conservation‐oriented research.     

Redox regulation by TXNRD3 during epididymal maturation underlies capacitation-associated mitochondrial activity and sperm motility in mice

During epididymal transit, redox remodeling protects mammalian spermatozoa, preparing them for survival in the subsequent journey to fertilization. However, molecular mechanisms of redox regulation in sperm development and maturation remain largely elusive. In this study, we report that thioredoxin-glutathione reductase (TXNRD3), a thioredoxin reductase family member particularly abundant in elongating spermatids at the site of mitochondrial sheath formation, regulates redox homeostasis to support male fertility. Using Txnrd3^−/− mice, our biochemical, ultrastructural, and live cell imaging analyses revealed impairments in sperm morphology and motility under conditions of TXNRD3 deficiency. We find that mitochondria develop more defined cristae during capacitation in wildtype sperm. Furthermore, we show that absence of TXNRD3 alters thiol redox status in both the head and tail during sperm maturation and capacitation, resulting in defective mitochondrial ultrastructure and activity under capacitating conditions. These findings provide insights into molecular mechanisms of redox homeostasis and bioenergetics during sperm maturation, capacitation, and fertilization.