Temperature affects longevity and age-related locomotor and cognitive decay in the short-lived fish Nothobranchius furzeri

Temperature variations are known to modulate aging and life-history traits in poikilotherms as different as worms, flies and fish. In invertebrates, temperature affects lifespan by modulating the slope of age-dependent acceleration in death rate, which is thought to reflect the rate of age-related damage accumulation. Here, we studied the effects of temperature on aging kinetics, aging-related behavioural deficits, and age-associated histological markers of senescence in the short-lived fish Nothobranchius furzeri. This species shows a maximum captive lifespan of only 3 months, which is tied with acceleration in growth and expression of aging biomarkers. These biological peculiarities make it a very convenient animal model for testing the effects of experimental manipulations on life-history traits in vertebrates. Here, we show that (i) lowering temperature from 25 degrees C to 22 degrees C increases both median and maximum lifespan; (ii) life extension is due to reduction in the slope of the age-dependent acceleration in death rate; (iii) lowering temperature from 25 degrees C to 22 degrees C retards the onset of age-related locomotor and learning deficits; and (iv) lowering temperature from 25 degrees C to 22 degrees C reduces the accumulation of the age-related marker lipofuscin. We conclude that lowering water temperature is a simple experimental manipulation which retards the rate of age-related damage accumulation in this short-lived species.     

Does Aging Have a Purpose?

Ideas of proponents and opponents of programmed aging concerning the expediency of this phenomenon for the evolution of living organisms are briefly considered. We think that evolution has no “gerontological” purpose, because the obligate restriction of cell proliferation during the development of multicellular organisms is a factor that “automatically” triggers aging due to the accumulation of various macromolecular lesions in cells as a result of the suppression, or even complete cessation of emergence of new, intact cells. This leads to the “dilution” of stochastic damage (the most important of which is DNA damage) at the level of the entire cellular population. Some additional arguments in favor of the inexpediency of aging for both species and individuals are also listed.     

Integrating cellular senescence with the concept of damage accumulation in aging: Relevance for clearance of senescent cells

Understanding the aging process and ways to manipulate it is of major importance for biology and medicine. Among the many aging theories advanced over the years, the concept most consistent with experimental evidence posits the buildup of numerous forms of molecular damage as a foundation of the aging process. Here, we discuss that this concept integrates well with recent findings on cellular senescence, offering a novel view on the role of senescence in aging and age‐related disease. Cellular senescence has a well‐established role in cellular aging, but its impact on the rate of organismal aging is less defined. One of the most prominent features of cellular senescence is its association with macromolecular damage. The relationship between cell senescence and damage concerns both damage as a molecular signal of senescence induction and accelerated accumulation of damage in senescent cells. We describe the origin, regulatory mechanisms, and relevance of various damage forms in senescent cells. This view on senescent cells as carriers and inducers of damage puts new light on senescence, considering it as a significant contributor to the rise in organismal damage. Applying these ideas, we critically examine current evidence for a role of cellular senescence in aging and age‐related diseases. We also discuss the differential impact of longevity interventions on senescence burden and other types of age‐related damage. Finally, we propose a model on the role of aging‐related damage accumulation and the rate of aging observed upon senescent cell clearance.     

Importance of individuality in oxidative stress and aging

Tight linkage between aging and oxidative stress is indicated by the observations that reactive oxygen species generated under various conditions of oxidative stress are able to oxidize nucleic acids, proteins, and lipids and that aging is associated with the accumulation of oxidized forms of cellular constituents, and also by the fact that there is an inverse relationship between the maximum life span of organisms and the age-related accumulation of oxidative damage. Nevertheless, validity of the oxidative stress hypothesis of aging is questioned by (i) the failure to establish a causal relationship between aging and oxidative damage and (ii) lack of a consistent correlation between the accumulation of oxidative damage and aging. The present discussion is focused on the complexity of the aging process and suggests that discrepancies between various studies in this area are likely due to the fact that aging is not a single process and that the lack of consistent experimental results is partly explained by individual variations. Even so, there is overwhelming support for a dominant role of oxidative stress in the aging of some individuals.     

Candida albicans, a distinctive fungal model for cellular aging study

The unicellular eukaryotic organisms represent the popular model systems to understand aging in eukaryotes. Candida albicans, a polymorphic fungus, appears to be another distinctive unicellular aging model in addition to the budding yeast Saccharomyces cerevisiae and fission yeast Schizosaccharomyces pombe. The two types of Candida cells, yeast (blastospore) form and hyphal (filamentous) form, have similar replicative lifespan. Taking the advantage of morphologic changes, we are able to obtain cells of different ages. Old Candida cells tend to accumulate glycogen and oxidatively damaged proteins. Deletion of the SIR2 gene causes a decrease of lifespan, while insertion of an extra copy of SIR2 extends lifespan, indicating that like in S. cerevisiae, Sir2 regulates cellular aging in C. albicans. Interestingly, Sir2 deletion does not result in the accumulation of extra-chromosomal rDNA molecules, but influences the retention of oxidized proteins in mother cells, suggesting that the extra-chromosomal rDNA molecules may not be associated with cellular aging in C. albicans. This novel aging model, which allows efficient large-scale isolation of old cells, may facilitate biochemical characterizations and genomics/proteomics studies of cellular aging, and help to verify the aging pathways observed in other organisms including S. cerevisiae.     

Garbage catastrophe theory of aging: imperfect removal of oxidative damage?

Increasing evidence suggests an important role of oxidant-induced damage in the progress of senescent changes, providing support for the free radical theory of aging proposed by Harman in 1956. However, considering that biological organisms continuously renew their structures, it is not clear why oxidative damage should accumulate with age. No strong evidence has been provided in favor of the concept of aging as an accumulation of synthetic errors (e.g. Orgel's 'error-catastrophe' theory and the somatic mutation theory). Rather, we believe that the process of aging may derive from imperfect clearance of oxidatively damaged, relatively indigestible material, the accumulation of which further hinders cellular catabolic and anabolic functions. From this perspective, it might be predicted that: (i) suppression of oxidative damage would enhance longevity; (ii) accumulation of incompletely digested material (e.g. lipofuscin pigment) would interfere with cellular functions and increase probability of death; (iii) rejuvenation during reproduction is mainly provided by dilution of undigested material associated with intensive growth of the developing organism; and (iv) age-related damage starts to accumulate substantially when development is complete, and mainly affects postmitotic, cells and extracellular matrix, not proliferating cells. There is abundant support for all these predictions.     

Redox control of iron regulatory proteins

Abstract

Increasing evidence suggests an important role of oxidant-induced damage in the progress of senescent changes, providing support for the free radical theory of aging proposed by Harman in 1956. However, considering that biological organisms continuously renew their structures, it is not clear why oxidative damage should accumulate with age. No strong evidence has been provided in favor of the concept of aging as an accumulation of synthetic errors (e.g. Orgels `error-catastrophe' theory and the somatic mutation theory). Rather, we believe that the process of aging may derive from imperfect clearance of oxidatively damaged, relatively indigestible material, the accumulation of which further hinders cellular catabolic and anabolic functions. From this perspective, it might be predicted that: (i) suppression of oxidative damage would enhance longevity; (ii) accumulation of incompletely digested material (e.g. lipofuscin pigment) would interfere with cellular functions and increase probability of death; (iii) rejuvenation during reproduction is mainly provided by dilution of undigested material associated with intensive growth of the developing organism; and (iv) age-related damage starts to accumulate substantially when development is complete, and mainly affects postmitotic cells and extracellular matrix, not proliferating cells. There is abundant support for all these predictions.     

Calorie restriction delays lipid oxidative damage in Drosophila melanogaster

The oxidative stress hypothesis predicts that the accumulation of oxidative damage to a variety of macromolecules is the molecular trigger driving the process of aging. Although an inverse relationship between oxidative damage and lifespan has been established in several different species, the precise relationship between oxidative damage and aging is not fully understood. Drosophila melanogaster is a favored model organism for aging research. Environmental interventions such as ambient temperature and calorie restriction can alter adult lifespan to provide an excellent system to examine the relationship between oxidative damage, aging and lifespan. We have developed an enzyme-linked immunosorbent assay (ELISA) using commercially available reagents for measuring 4-hydroxy-2-nonenal (HNE) in proteins, a marker for oxidative damage to lipids, and present data in flies to show that HNE adducts accumulate in an age-dependent manner. With immunohistology, we also find the primary site of HNE accumulation is the pericerebral fat body, where induction of dFOXO was recently shown to retard aging. When subjected to environmental interventions that shorten lifespan, such as elevated ambient temperature, the chronological accumulation of HNE adduct is accelerated. Conversely, interventions that extend lifespan, such as lower ambient temperature or low calorie diets, slow the accumulation of HNE adduct. These studies associate damage from lipid peroxidation with aging and lifespan in Drosophila and show that calorie restriction in flies, as in mammals, slows the accumulation of lipid related oxidative damage.     

Some highlights of research on aging with invertebrates, 2009

This annual review focuses on invertebrate model organisms, which shed light on new mechanisms in aging and provide excellent systems for both genome-wide and in-depth analysis. This year, protein interaction networks have been used in a new bioinformatic approach to identify novel genes that extend replicative lifespan in yeast. In an extended approach, using a new, human protein interaction network, information from the invertebrates was used to identify new, candidate genes for lifespan extension and their orthologues were validated in the nematode Caenorhabditis elegans . Chemosensation of diffusible substances from bacteria has been shown to limit lifespan in C. elegans , while a systematic study of the different methods used to implement dietary restriction in the worm has shown that they involve mechanisms that are partially distinct and partially overlapping, providing important clarification for addressing whether or not they are conserved in other organisms. A new theoretical model for the evolution of rejuvenating cell division has shown that asymmetrical division for either cell size or for damaged cell constituents results in increased fitness for most realistic levels of cellular protein damage. Work on aging-related disease has both refined our understanding of the mechanisms underlying one route to the development of Parkinson's disease and has revealed that in worms, as in mice, dietary restriction is protective against cellular proteotoxicity. Two systematic studies genetically manipulating the superoxide dismutases of C. elegans support the idea that damage from superoxide plays little or no role in aging in this organism, and have prompted discussion of other kinds of damage and other kinds of mechanisms for producing aging-related decline in function.     

Promoting basal levels of autophagy in the nervous system enhances longevity and oxidant resistance in adult Drosophila

Autophagy is involved with the turnover of intracellular components and the management of stress responses. Genetic studies in mice have shown that suppression of neuronal autophagy can lead to the accumulation of protein aggregates and neurodegeneration. However, no study has shown that increasing autophagic gene expression can be beneficial to an aging nervous system. Here we demonstrate that expression of several autophagy genes is reduced in Drosophila neural tissues as a normal part of aging. The age-dependent suppression of autophagy occurs concomitantly with the accumulation of insoluble ubiquitinated proteins (IUP), a marker of neuronal aging and degeneration. Mutations in the Atg8a gene (autophagy-related 8a) result in reduced lifespan, IUP accumulation and increased sensitivity to oxidative stress. In contrast, enhanced Atg8a expression in older fly brains extends the average adult lifespan by 56% and promotes resistance to oxidative stress and the accumulation of ubiquitinated and oxidized proteins. These data indicate that genetic or age-dependent suppression of autophagy is closely associated with the buildup of cellular damage in neurons and a reduced lifespan, while maintaining the expression of a rate-limiting autophagy gene prevents the age-dependent accumulation of damage in neurons and promotes longevity.     

Daughters of the budding yeast from old mothers have shorter replicative lifespans but not total lifespans. Are DNA damage and rDNA instability the factors that determine longevity?

Although a lot of effort has been put into the search for factors responsible for aging in yeast mother cells, our knowledge of cellular changes in daughter cells originating from old mothers is still very limited. It has been shown that an old mother is not able to compensate for all negative changes within its cell and therefore transfers them to the bud. In this paper, we show for the first time that daughter cells of an old mother have a reset lifespan expressed in units of time despite drastic reduction of their budding lifespan, which suggests that a single yeast cell has a fixed programmed longevity regardless of the time point at which it was originated. Moreover, in our study we found that longevity parameters are not correlated with the rDNA level, DNA damage, chromosome structure or aging parameters (budding lifespan and total lifespan).     

Tetz’s theory and law of longevity

Here, we present new theory and law of longevity intended to evaluate fundamental factors that control lifespan. This theory is based on the fact that genes affecting host organism longevity are represented by subpopulations: genes of host eukaryotic cells, commensal microbiota, and non-living genetic elements. Based on Tetz’s theory of longevity, we propose that lifespan and aging are defined by the accumulation of alterations over all genes of macroorganism and microbiome and the non-living genetic elements associated with them. Tetz’s law of longevity states that longevity is limited by the accumulation of alterations to the limiting value that is not compatible with life. Based on theory and law, we also propose a novel model to calculate several parameters, including the rate of aging and the remaining lifespan of individuals. We suggest that this theory and model have explanatory and predictive potential to eukaryotic organisms, allowing the influence of diseases, medication, and medical procedures to be re-examined in relation to longevity. Such estimates also provide a framework to evaluate new fundamental aspects that control aging and lifespan.     

Identification of evolutionarily conserved genetic regulators of cellular aging

To identify new genetic regulators of cellular aging and senescence, we performed genome-wide comparative RNA profiling with selected human cellular model systems, reflecting replicative senescence, stress-induced premature senescence, and distinct other forms of cellular aging. Gene expression profiles were measured, analyzed, and entered into a newly generated database referred to as the GiSAO database. Bioinformatic analysis revealed a set of new candidate genes, conserved across the majority of the cellular aging models, which were so far not associated with cellular aging, and highlighted several new pathways that potentially play a role in cellular aging. Several candidate genes obtained through this analysis have been confirmed by functional experiments, thereby validating the experimental approach. The effect of genetic deletion on chronological lifespan in yeast was assessed for 93 genes where (i) functional homologues were found in the yeast genome and (ii) the deletion strain was viable. We identified several genes whose deletion led to significant changes of chronological lifespan in yeast, featuring both lifespan shortening and lifespan extension. In conclusion, an unbiased screen across species uncovered several so far unrecognized molecular pathways for cellular aging that are conserved in evolution.     

No change in DNA damage or repair of single- and double-strand breaks as human diploid fibroblasts age in vitro

Using the in vitro human diploid fibroblast model, we tested theories of aging which hypothesize that either accumulation of DNA damage or decreased DNA repair capacity is causally related to cellular senescence. Between population doubling level (PDL) 32 and 71, fetal lung-derived normal diploid human fibroblasts (IMR 90) were assayed for both DNA single-strand breaks (SSBs, spontaneous and induced by 6 Gy) and DNA double-strand breaks (DSBs, spontaneous and induced by 100 Gy). After gamma-irradiation cells were kept on ice unless undergoing repair incubation at 37 degrees C for 7.5-120 min or 18-24 h. To assay DNA strand breaks we used the filter elution technique in conjunction with a fluorometric determination of DNA which is not biased in favor of proliferating aging cells as are radioactive labelling methods. We found no change with in vitro age in the accumulation of spontaneous SSBs or DSBs, nor in the kinetics or completeness of DNA strand rejoining after gamma-irradiation. Cells at varying PDLs rejoined approx. 90% of SSBs and DSBs after 60 min repair incubation and 100% after 18-24 h repair incubation. We conclude that aging and senescence as measured by proliferative lifespan in IMR 90 cells are neither accompanied nor caused by accumulation of DNA strand breaks or by diminished capacity to rejoin gamma-radiation-induced SSBs or DSBs in DNA.     

Ensheathing glia promote increased lifespan and healthy brain aging

Glia have an emergent role in brain aging and disease. In the Drosophila melanogaster brain, ensheathing glia function as phagocytic cells and respond to acute neuronal damage, analogous to mammalian microglia. We previously reported changes in glia composition over the life of ants and fruit flies, including a decline in the relative proportion of ensheathing glia with time. How these changes influence brain health and life expectancy is unknown. Here, we show that ensheathing glia but not astrocytes decrease in number during Drosophila melanogaster brain aging. The remaining ensheathing glia display dysregulated expression of genes involved in lipid metabolism and apoptosis, which may lead to lipid droplet accumulation, cellular dysfunction, and death. Inhibition of apoptosis rescued the decline of ensheathing glia with age, improved the neuromotor performance of aged flies, and extended lifespan. Furthermore, an expanded ensheathing glia population prevented amyloid-beta accumulation in a fly model of Alzheimer's disease and delayed the premature death of the diseased animals. These findings suggest that ensheathing glia play a vital role in regulating brain health and animal longevity.     

Long-Term Quiescent Fibroblast Cells Transit into Senescence

Cellular senescence is described to be a consequence of telomere erosion during the replicative life span of primary human cells. Quiescence should therefore not contribute to cellular aging but rather extend lifespan. Here we tested this hypothesis and demonstrate that cultured long-term quiescent human fibroblasts transit into senescence due to similar cellular mechanisms with similar dynamics and with a similar maximum life span as proliferating controls, even under physiological oxygen conditions. Both, long-term quiescent and senescent fibroblasts almost completely fail to undergo apoptosis. The transition of long-term quiescent fibroblasts into senescence is also independent of HES1 which protects short-term quiescent cells from becoming senescent. Most significantly, DNA damage accumulates during senescence as well as during long-term quiescence at physiological oxygen levels. We suggest that telomere-independent, potentially maintenance driven gradual induction of cellular senescence during quiescence is a counterbalance to tumor development.     

Long-Term IGF-I Exposure Decreases Autophagy and Cell Viability

A reduction in IGF-I signaling has been found to increase lifespan in multiple organisms despite the fact that IGF-I is a trophic factor for many cell types and has been found to have protective effects against multiple forms of damage in acute settings. The increase in longevity seen in response to reduced IGF-I signaling suggests that there may be differences between the acute and chronic impact of IGF-I signaling. We have examined the possibility that long-term stimulation with IGF-I may have a negative impact at the cellular level using quiescent human fibroblasts. We find that fibroblast cells exposed to IGF-I for 14 days have reduced long-term viability as judged by colony forming assays, which is accompanied by an accumulation of senescent cells. In addition we observe an accumulation of cells with depolarized mitochondria and a reduction in autophagy in the long-term IGF-I treated cultures. An examination of mice with reduced IGF-I levels reveals evidence of enhanced autophagy and fibroblast cells derived from these mice have a larger mitochondrial mass relative to controls indicating that changes in mitochondrial turnover occurs in animals with reduced IGF-I. The results indicate that chronic IGF-I stimulation leads to mitochondrial dysfunction and reduced cell viability.     

Theories of biological aging: Genes,proteins, and free radicals

Traditional categorization of theories of aging into programmed and stochastic ones is outdated and obsolete. Biological aging is considered to occur mainly during the period of survival beyond the natural or essential lifespan (ELS) in Darwinian terms. Organisms survive to achieve ELS by virtue of genetically determined longevity assuring maintenance and repair systems (MRS). Aging at the molecular level is characterized by the progressive accumulation of molecular damage caused by environmental and metabolically generated free radicals, by spontaneous errors in biochemical reactions, and by nutritional components. Damages in the MRS and other pathways lead to age-related failure of MRS, molecular heterogeneity, cellular dysfunctioning, reduced stress tolerance, diseases and ultimate death. A unified theory of biological aging in terms of failure of homeodynamics comprising of MRS, and involving genes, milieu and chance, is acquiring a definitive shape and wider acceptance. Such a theory also establishes the basis for testing and developing effective means of intervention, prevention and modulation of aging.