The limit to human longevity: an approach through a stress theory of ageing

New Insights into the Genetics and Epigenetics of Aging Plasticity

Biological aging is characterized by irreversible cell cycle blockade, a decreased capacity for tissue regeneration, and an increased risk of age-related diseases and mortality. A variety of genetic and epigenetic factors regulate aging, including the abnormal expression of aging-related genes, increased DNA methylation levels, altered histone modifications, and unbalanced protein translation homeostasis. The epitranscriptome is also closely associated with aging. Aging is regulated by both genetic and epigenetic factors, with significant variability, heterogeneity, and plasticity. Understanding the complex genetic and epigenetic mechanisms of aging will aid the identification of aging-related markers, which may in turn aid the development of effective interventions against this process. This review summarizes the latest research in the field of aging from a genetic and epigenetic perspective. We analyze the relationships between aging-related genes, examine the possibility of reversing the aging process by altering epigenetic age.

Tissue Mechanics in Haired Murine Skin: Potential Implications for Skin Aging

During aging, the skin undergoes changes in architecture and composition. Skin aging phenotypes occur due to accumulated changes in the genome/epigenome, cytokine/cell adhesion, cell distribution/extracellular matrix (ECM), etc. Here we review data suggesting that tissue mechanics also plays a role in skin aging. While mouse and human skin share some similarities, their skin architectures differ in some respects. However, we use recent research in haired murine skin because of the available experimental data. Skin suffers from changes in both its appendages and inter-appendage regions. The elderly exhibit wrinkles and loose dermis and are more likely to suffer from wounds and superficial abrasions with poor healing. They also have a reduction in the number of skin appendages. While telogen is prolonged in aging murine skin, hair follicle stem cells can be rejuvenated to enter anagen if transplanted to a young skin environment. We highlight recent single-cell analyses performed on epidermis and aging human skin which identified new basal cell subpopulations that shift in response to wounding. This may be due to alterations of basement membrane stiffness which would change tissue mechanics in aging skin, leading to altered homeostatic dynamics. We propose that the extracellular matrix (ECM) may play a key role as a chemo-mechanical integrator of the multi-layered senescence-associated signaling pathways, dictating the tissue mechanical landscape of niche microenvironments in aging phenotypes. We show examples where failed chemo-mechanical signaling leads to deteriorating homeostasis during skin aging and suggest potential therapeutic strategies to guide future research to delay the aging processes.

Mitochondrial protein Fus1/Tusc2 in premature aging and age-related pathologies: critical roles of calcium and energy homeostasis

Decreased energy production and increased oxidative stress are considered to be major contributors to aging and aging-associated pathologies. The role of mitochondrial calcium homeostasis has also been highlighted as an important factor affecting different pathological conditions. Here, we present evidence that loss of a small mitochondrial protein Fus1 that maintains mitochondrial homeostasis results in premature aging, aging-associated pathologies, and decreased survival. We showed that Fus1KO mice develop multiple early aging signs including lordokyphosis, lack of vigor, inability to accumulate fat, reduced ability to tolerate stress, and premature death. Other prominent pathological changes included low sperm counts, compromised ability of adult stem cells to repopulate tissues, and chronic inflammation. At the molecular level, we demonstrated that mitochondria of Fus1 KO cells have low reserve respiratory capacity (the ability to produce extra energy during sudden energy demanding situations), and show significantly altered dynamics of cellular calcium response.

Our recent studies on early hearing and memory loss in Fus1 KO mice combined with the new data presented here suggest that calcium and energy homeostasis controlled by Fus1 may be at the core of its aging-regulating activities. Thus, Fus1 protein and Fus1-dependent pathways and processes may represent new tools and targets for anti-aging strategies.

Introduction to skin aging

Cutaneous science has seen considerable development in the last 25 years, in part due to the Omics revolution, and the appreciation that this organ is hardwired into the body's key neuro-immuno-endocrine axes. Moreover, there is greater appreciation of how stratification of skin disorders will permit more targeted and more effective treatments. Against this has been how the remarkable extension in the average human life-span, though in the West at least, this parallels worrying increases in lifestyle-associated conditions like diabetes, skin cancer etc. These demographic trends bring greater urgency to finding clinical solutions for numerous age-related deficits in skin function caused by extrinsic and intrinsic factors. Mechanisms for aging skin include the actions of reactive oxygen species (ROS), mtDNA mutations, and telomere shortening, as well as hormonal changes. We have also significantly improved our understanding of how to harness the skin's considerable regenerative capacity e.g., via its remarkable investment of stem cell subpopulations. In this way we hope to develop new strategies to selectively target the skin's capacity to undergo optimal wound repair and regeneration. Here, the unsung hero of the skin regenerative power may be the humble hair follicle, replete with its compliment of epithelial, mesenchymal, neural and other stem cells. This review introduces the topic of human skin aging, with a focus on how maintenance of function in this complex multi-cell type organ is key for retaining quality of life into old age.

How the effects of aging and stresses of life are integrated in mortality rates: insights for genetic studies of human health and longevity

Increasing proportions of elderly individuals in developed countries combined with substantial increases in related medical expenditures make the improvement of the health of the elderly a high priority today. If the process of aging by individuals is a major cause of age related health declines then postponing aging could be an efficient strategy for improving the health of the elderly. Implementing this strategy requires a better understanding of genetic and non-genetic connections among aging, health, and longevity. We review progress and problems in research areas whose development may contribute to analyses of such connections. These include genetic studies of human aging and longevity, the heterogeneity of populations with respect to their susceptibility to disease and death, forces that shape age patterns of human mortality, secular trends in mortality decline, and integrative mortality modeling using longitudinal data. The dynamic involvement of genetic factors in (i) morbidity/mortality risks, (ii) responses to stresses of life, (iii) multi-morbidities of many elderly individuals, (iv) trade-offs for diseases, (v) genetic heterogeneity, and (vi) other relevant aging-related health declines, underscores the need for a comprehensive, integrated approach to analyze the genetic connections for all of the above aspects of aging-related changes. The dynamic relationships among aging, health, and longevity traits would be better understood if one linked several research fields within one conceptual framework that allowed for efficient analyses of available longitudinal data using the wealth of available knowledge about aging, health, and longevity already accumulated in the research field.

Antidepressant effect of Shudihuang on mice exposed to unpredictable chronic mild stress

AIM OF THE STUDY

Depression is a severe mood disorder. It was treated with Shudihuang, the steamed roots of Rehmannia glutinota Libosch. (SRG), in traditional Chinese medicine. The present paper was designed to verify its antidepressant effect.

MATERIALS AND METHODS

A mouse model of depression was established though unpredictable chronic mild stress (UCMS). Low and high doses of SRG were administered orally. Fur state, body and organ weight, and gastric ulcers were examined. Locomotion was assayed in open field test. Liver antioxidant indexes were measured spectrophotometrically.

RESULTS

Fur state, body and organ weight were found to be insensitive to UCMS. The locomotion reduced by UCMS was restored by low dose of SRG (2.5 g/kg BW) but not by high dose (5 g/kg BW). UCMS resulted in aggravated gastric ulceration, elevated liver malondialdehyde, together with reduced total antioxidant capability, glutathione content, and superoxide dismutase and catalase activities. The alterations were improved by SRG in a dose-dependent manner. The differences in the activity of glutathione peroxidase were statistically nonsignificant among groups. Clomipramine the positive drug was similar to SRG especially in antioxidation.

CONCLUSION

SRG is of therapeutic value for depression-like disorders, and antioxidation may be one of the mechanisms underlying its antidepressant action.

Stress and ageing interactions: A paradox in the context of shared etiological and physiopathological processes

Gerontology has made considerable progress in the understanding of the mechanisms underlying the ageing process and age-related neurodegenerative disorders. However, ways to improve quality of life in the elderly remain to be elucidated. It is now clear that stress and the ageing process share a number of underlying mechanisms bound in a very close, if not indissociable, relationship. The ageing process is regulated by the factors underlying the ability to adjust to stress, whilst stress has an influence on the life span and the quality of ageing. In addition, the ability to cope with stress in adulthood predicts life expectancy and quality of life at senescence. The ageing process and stress also share several common mechanisms, particularly in relation to the energy factor. Stress consumes energy and ageing may be considered as a cost of the energy expended to deal with the stressors to which the body is exposed throughout its lifetime. This suggests that the ageing process is associated with and/or a consequence of a long-lasting activation of the major stress responsive systems. However, despite common features, the interaction between stress and the ageing process gives rise to some paradoxes. Stress can either diminish or exacerbate the ageing process just as the ageing process can worsen or counter the effects of stress. There has been little attempt to understand how ageing and stress might interact to promote "successful" or pathological ageing. A key factor in this respect is the individual's ability to adapt to stress. Viewed from this angle, the quality of life of aged subjects may be improved through therapy designed to improve the tolerance to stress.

Adaptive mechanisms to oxidative stress during aging

Whether or not oxidative stress is the cause of the aging process, as proposed by the oxidative stress theory of aging remains unknown; but accumulated evidence overwhelmingly identifies increased oxidative stress with age as a source of damage to cellular structure and function. From an evolutionary perspective, the utilization of oxygen as a life supporting means makes oxidative stress an inescapable part of an organism's biological system. The inseparability of oxidative stress from the biological system can be viewed as an adaptive response that all aerobic organisms undergo to ward-off the potentially harmful effects of oxygen and its derivatives, including free radicals. The organism's adaptive mechanisms include an intricate network of defenses that regulate and guard against any over-acting oxidative reactions to ensure its survival. This review discusses and illustrates several adaptive responses at various levels (from gene regulation to physical exercise) that organisms use as part of their survival strategy.

Stressors and antistressors: how do they influence life span in HER-2/neu transgenic mice?

The purpose of this study is to investigate possible influences of different stressors (saline injections, light deprivation and constant light regimen) and geroprotectors (Epitalon and melatonin) on survivals of female HER-2/neu transgenic mice. We propose a semi-parametric model of heterogeneous mortality (frailty model) for the analysis of the experimental data. In this model, we assume that treatment influences parameters of both frailty distribution and baseline hazard. The unique design of the experiments makes it possible to compare the effects on survival produced by different treatments in terms of changes in population heterogeneity and underlying hazard. Parameters of the model help to describe the possible influences of various stressors, geroprotectors, and their dosage on the life span of laboratory animals. The proposed model helps to advance our understanding of the effects--such as debilitation, longevity hormesis and incomplete hormesis--which occur in the population as a result of different treatments.

Effects of aging on cardiac and skeletal muscle AMPK activity: basal activity, allosteric activation, and response to in vivo hypoxemia in mice

Although a diminished ability of tissues and organisms to tolerate stress is a clinically important hallmark of normal aging, little is known regarding its biochemical basis. Our goal was to determine whether age-associated changes in AMP-activated protein kinase (AMPK), a key regulator of cellular metabolism during the stress response, might contribute to the poor stress tolerance of aged cardiac and skeletal muscle. Basal AMPK activity and the degree of activation of AMPK by AMP and by in vivo hypoxemia (arterial Po2 of 39 mmHg) were measured in cardiac and skeletal muscle (gastrocnemius) from 5- and 24-mo-old C57Bl/6 mice. In the heart, neither basal AMPK activity nor its allosteric activation by AMP was affected by age. However, after 10 min of hypoxemia, the activity of alpha2-AMPK, but not alpha1-AMPK, was significantly higher in the hearts from old than from young mice (P < 0.005), this difference being due to differences in phosphorylation of alpha2-AMPK. Significant activation of AMPK in the young hearts did not occur until 30 min of hypoxemia (P < 0.01), stress that was poorly tolerated by the old mice (mortality = 67%). In contrast, AMPK activity in gastrocnemius muscle was unaffected by age or hypoxemia. We conclude that the age-associated decline in hypoxic tolerance in cardiac and skeletal muscle is not caused by changes in basal AMPK activity or a blunted AMPK response to hypoxia. Activation of AMPK by in vivo hypoxia is slower and more modest than might be predicted from in vitro and ex vivo experiments.

Heat shock changes the heterogeneity distribution in populations of Caenorhabditis elegans: does it tell us anything about the biological mechanism of stress response?

In this paper we analyze survival data of populations of sterilized nematodes, Caenorhabditis elegans, exposed to heat shocks of different duration at the beginning of their adult lives. There are clear hormesis effects after short exposure to heat and clear debilitation effects after long exposure. Intermediate durations result in a mixture of these two effects. In this latter case, the survival curves for the control and experimental populations intersect. We show that observed effects may be explained by using a model of discrete heterogeneity. According to this model, each population of worms in the experiment is a mixture of subcohorts of frail, normal, and robust individuals; exposure to heat changes the initial proportion of worms in the subcohorts (heterogeneity distribution); and these changes depend on the duration of exposure. In other words, exposure to heat does not influence mortality rates (survival functions) in the subcohorts but does cause individuals to move from one subcohort to another. In a biological interpretation of this finding we hypothesize that, when coping with stress, the organisms of worms use several lines of defense. Switching these lines on and off in response to stress in individual organisms generates the spectrum of observed survival effects at the population level. We discuss possible molecular biological mechanisms of stress response and directions for further research.

Ageing and survival after different doses of heat shock: the results of analysis of data from stress experiments with the nematode worm Caenorhabditis elegans

Stress experiments performed on a population of sterilised nematode worms (Caenorhabditis elegans) show a clear hormesis effect after short exposure and clear debilitation effects after long exposure to heat shock. An intermediate duration of exposure results in a mixture of these two effects. In this latter case the survival curves for populations in the stress and control groups intersect. In this paper we develop an adaptation model of stress and apply it to the analysis of survival data from three such stress experiments. We show that the model can be used to explain empirical age-patterns of mortality and survival observed in these experiments. We discuss possible biological mechanisms involved in stress response and directions for further research.

Stress resistance by caloric restriction for longevity

Hardly an aspect of aging is more important than an organism's ability to withstand stress or to resist both internally and externally imposed insults. We know that as organisms loose their ability to resist these insults, aged organisms suffer more than the young. Therefore, a prime strategy for an organism's survival has been the evolutionarily adapted defense systems that guard against insult. For better survivability, an organism's defense system must be maximized to its full effect through well-coordinated networks of diverse biologically responsive elements. Although terms like stress, resistance, and adaptability have long been used in biology, they remain mechanistically and quantitatively poorly defined. In a gerontological context, stress resistance or susceptibility are often discussed in association with an organism's vulnerability to disease and age-related damage. However, to date, there is no clear molecular delineation of cellular and molecular mechanisms for such complex biological phenomena. The life-prolonging action of caloric restriction (CR) seems to offer an excellent opportunity for investigating the interrelationship between stress and the aging process. As an omnipotent intervention, CR provides a unique opportunity to probe the organism's ability to withstand age-related stress as a survival strategy. In this context, the antiaging action of CR can be viewed as "nutritional stress," because the organism's reduced caloric intake seems to be a stimulatory metabolic response for survivability. Recent gerontologic research has provided sufficient experimental data supporting this antiaging property of CR, of which several pertinent, key examples are discussed below.

Attenuation of NF-kappaB signaling response to UVB light during cellular senescence

The ability of cells to adapt to environmental stresses undergoes a progressive reduction during aging. NF-kappaB-mediated signaling is a major defensive system against various environmental challenges. The aim of this study was to find out whether replicative senescence affects the response of the NF-kappaB signaling pathway to UVB light in human WI-38 and IMR-90 fibroblasts. The exposure of early passage fibroblasts to UVB light inhibited the proliferation and induced a flat phenotype similar to that observed in replicatively senescent fibroblasts not exposed to UVB light. The UVB radiation dose used (153 mJ/cm2) did not induce apoptosis in either early or late passage WI-38 fibroblasts. UVB exposure induced a prominent activation of the NF-kappaB signaling pathway both in early and in late passage WI-38 and IMR-90 fibroblasts. Interestingly, the response to UVB light was significantly attenuated in late passage fibroblasts. This attenuation was most prominent in DNA binding activities of nuclear NF-kappaB complexes. Similar senescence-related attenuation was also observed in the DNA binding activities of nuclear AP-1 and Sp-1 factors after UVB treatment. Immunoblotting and -cytochemistry showed an increase in nuclear localization of p50 and p65 components of NF-kappaB complexes. Supershift experiments showed that the specific NF-kappaB complexes contain p50 and p65 protein components but not p52 and c-Rel proteins. Cytoplasmic IkappaBalpha showed a marked decrease at protein level but an increase in phosphorylation after UVB treatment. Transient transfection assays with TK5-CAT and TK10-CAT plasmids carrying NF-kappaB-responsive sites of the TNFalpha promoter were used to analyze the functional activity of the NF-kappaB complexes. Results showed that UVB exposure induced an increase in NF-kappaB-driven CAT expression both in early and in late passage fibroblasts though the response was significantly stronger in early passage fibroblasts. Our results show that the induction of NF-kappaB-mediated signaling by UVB light is highly attenuated in senescent fibroblasts. This attenuation may reduce the stress resistance during cellular senescence.

Early male initiation (time frame shift): a possible contributor to their shortened lifespan

The shorter life expectancy of human males, as compared with females, was analyzed by utilizing the assumption that the specific rate of change of disease occurrence or mortality was a function of the specific rate of change of age. Separate plots were made of log incidence versus log age, for males and females. In several disorders, the line representing males was displaced toward the origin, as compared with the females. This 'time frame shift' suggested an earlier onset of the disorder or its precursors in males ('early male initiation'). While the physiologic/genetic basis for this has to be fully determined, the earlier male 'initiation' of disorders may be a contributing factor to their shorter life expectancy.

Stress-resistance genotypes, metabolic efficiency and interpreting evolutionary change

Assuming stress levels to which free-living populations are normally exposed, an association between rapid development time, a long life, success in mating and size of sexual ornaments can be predicted. Fitness at one stage of the life cycle should therefore correlate with fitness at other stages under this environmental model. Assuming that stress targets energy carriers, high-energy efficiency underlain by stress-resistance genotypes that are likely to be heterozygous is the basis of this prediction. Stress-resistance genotypes therefore have a role in promoting the energy efficiency required for organisms to accommodate a stressed world. Selection for energy efficiency to utilize heterogenous resources implies that the process of speciation should normally occur rapidly and be rarely observed. It follows that the ecological species concept is primary to other species concepts. The intensity of selection for stress resistance goes from an extreme in the highly disturbed and stressful environments of living fossils to relatively stable abiotic habitats, where specialist diversifications and adaptive radiations are likely. Between these extremes, a punctuated pattern of evolutionary change may occur in perturbed environments during a transient phase of increased resources. In abiotically benign tropical habitats where energy constraints are low, specialization of resource utilization by learning appears possible.