Genetic analysis of ageing: role of oxidative damage and environmental stresses

Levels of 8-hydroxy-2'-deoxyguanosine in cellular DNA from 12 tissues of young and old Sprague-Dawley rats

The age dependent increase of 8-hydroxy-2'-deoxyguanosine (8-OH-dG) reported in DNA of organs of old rats appears to vary with the strain, age and sex of the animals used for the investigation. Here we report on 8-OH-dG concentrations in the cellular DNA of 12 tissues of male Sprague-Dawley rats aged 5 or 30 months and kept under standard conditions throughout their lives until being killed. DNA from frozen organs was isolated using a Qiagen DNA purification kit. Following digestion (nuclease P1, alkaline nuclease) hydrolysed DNA was applied onto a HPLC column; native nucleosides were monitored at 254 nm and 8-OH-dG by electrochemical detection. 8-OH-dG levels in organs of young rats ranged between 10 and 90 micromol/mol deoxyguanosine (dG). Highest levels (micromol 8-OH-dG /mol dG) were detected in the oesophagus (90), muscle (61), brain (65), liver (59), spleen (57), and testicles (63). 8-OH-dG in DNA from kidney, lung, heart, small and large intestine ranged between 28 and 38 micromol/mol dG. Lowest amounts were found in the glandular stomach (10). DNA of old rats generally contained higher 8-OH-dG levels with significant increases in liver (186%) and kidney (372%); other organs showed no significant decreases (spleen, brain, testicles) or increases up to 69% (heart). These findings are discussed in the context with previously published data on 8-OH-dG levels in organs from young and old rats.

A cost of reproduction: oxidative stress susceptibility is associated with increased egg production in Drosophila melanogaster

The present study tests the hypothesis that reproduction is correlated with decreased oxidative stress resistance. In numerous species, it has been observed that longevity is negatively correlated with reproduction but the physiological basis of this cost is not well understood. In the present study, female egg production was stimulated by adding live yeast to the surface of Drosophila food. After females were held on yeast-supplemented and unmodified medium for 6-12 days, susceptibility to oxidative stress was measured by exposure to methyl viologen. Added yeast was associated with stress susceptibility of fertile females but not of sterile females. The results of the present study suggest that oxidative stress susceptibility is a physiological cost of reproduction.

Age-specific demographic profiles of longevity mutants in Caenorhabditis elegans show segmental effects

Demographic profiles of several single-gene longevity mutants of the nematode Caenorhabditis elegans reveal segmental (age-specific) effects on mortality. The mortality profiles of wild-type worms were examined across multiple replicate cultures containing 100,000 or more nematodes and found to be quite replicable, although clear environmental effects are routinely found. The combined profile of wild type was compared with those of three long-lived mutants to determine how age-specific mortality is altered by mutations in age-1, clk-1, or spe-26. In all four genotypes, death rates fit a two-stage Gompertz model better than a one-stage Gompertz; that is, mortality levels off at later ages. The largest genetic effect on mortality was that of an age-1 mutation, which lowered mortality more than fivefold at most later ages. In contrast, a spe-26 mutant had a tenfold lower mortality until approximately 2 weeks of age but ultimately achieved a higher mortality, whereas clk-1 mutants show slightly higher mortality than wild type during the fertile period, early in life, but ultimately level off at lower mortality. Each mutant thus has a distinctive profile of age-specific mortalities that could suggest the time of action of each gene.

Stress, DNA damage and ageing -- an integrative approach

Ageing is highly complex, involving multiple mechanisms at different levels. Nevertheless, recent evidence suggests that several of the most important mechanisms are linked via endogenous stress-induced DNA damage caused by reactive oxygen species (ROS). Understanding how such damage contributes to age-related changes requires that we explain how these different mechanisms relate to each other and potentially interact. In this article, we review the contributions of stress-induced damage to cellular DNA through (i) the role of damage to nuclear DNA and its repair mediated via the actions of poly(ADP-ribose) polymerase-1, (ii) the role of damage to telomeric DNA and its contribution to telomere-driven cell senescence, and (iii) the role of damage to and the accumulation of mutations in mitochondrial DNA. We describe how an integrative approach to studying these mechanisms, coupled with computational modelling, may be of considerable importance in resolving some of the complexity of cellular ageing.

Loss of stress-induced expression of catalase3 during leaf senescence in Arabidopsis thaliana is restricted to oxidative stress

Different stress conditions can induce changes in the activity of the antioxidant enzymes superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11) and catalase (CAT, EC 1.11.1.6). The enzyme activities of all SOD and APX isoforms detected in young Arabidopsis leaves remained unaffected or slightly decreased after moderate paraquat treatment. While CAT2 activity also remained unaffected under these conditions, CAT3 enzyme activity was enhanced. In contrast to the enzyme activities, mRNA levels of both cat2 and cat3 were enhanced under oxidative stress induced by either paraquat or the fungal toxin cercosporin. This indicates that, with respect to enzyme activity level, CAT3 is the enzyme which is most sensitive to oxidative stress in this developmental stage and that the enzyme activity of CAT2 is possibly regulated at the post-transcriptional level. Interestingly, cat3 mRNA level and CAT3 activity are not elevated by paraquat treatment in senescing leaves. In contrast, the response to other stress conditions, such as water stress induced by flooding of detached leaves and heat stress, is maintained in senescing leaves. Since changes in stress response are not a general phenomenon in leaf senescence but appear to be restricted to oxidative stress, this might be a specific mechanism to promote senescence in Arabidopsis thaliana.

Heat shock protein accumulation is upregulated in a long-lived mutant of Caenorhabditis elegans

We present evidence for elevated levels of heat shock protein 16 (HSP16) in an intrinsically thermotolerant, long-lived strain of Caenorhabditis elegans during and after heat stress. Mutation of the age-1 gene, encoding a phosphatidylinositol 3-kinase catalytic subunit, results in both extended life span (Age) and increased intrinsic thermotolerance (Itt) in adult hermaphrodites. We subjected age-synchronous cohorts of worms to lethal and nonlethal thermal stress and observed the accumulation of a small (16-18 kd) heat-shock-specific polypeptide detected by an antibody raised against C. elegans HSP16. Strains carrying the mutation hx546 consistently accumulated HSP16 to higher levels than a wild-type strain. Significantly, overaccumulation of HSP16 in the age-1(hx546) strain following heat was observed throughout the adult life span. A chimeric transgene containing the Escherichia coli beta-galactosidase gene fused to a C. elegans HSP16-41 transcriptional promoter was introduced into wild-type and age-1(hx546) backgrounds. Heat-inducible expression of the transgene was elevated in the age-1(hx546) strain compared with the wild-type strain under a wide variety of heat shock and recovery conditions. These observations are consistent with a model in which Age mutations exhibit thermotolerance and extended life span as a result of elevated levels of molecular chaperones.

Strategies for mapping susceptibility genes in age-related maculopathy

The prospects for mapping genes conferring susceptibility to common disorders such as age-related maculopathy have increased as a result of advances in high-throughput genotyping using single nucleotide polymorphisms. This should not obscure the magnitude of the task of identifying what are potentially a diverse group of genetic variants with small or intermediate effects on disease susceptibility. Strategies for increasing the odds of success in such an endeavour are discussed.

Mechanisms of ageing

Recent experimental work from a variety of biological systems, ranging from yeast to human beings, lends increasing support to the view that stochastic damage inflicted to biological macromolecules is the driving force for the ageing process. The damage is derived from small reactive molecules, most prominently reactive oxygen intermediates (ROI), that arise during normal cellular metabolism and are associated with important if not essential cellular functions. The major classes of macromolecules at risk are proteins, lipids and DNA, but damage to DNA (both nuclear and mitochondrial) may entail particularly severe consequences. Cellular dysfunction resulting from macromolecular damage can be detected as a variety of expressions, such as genomic instability, inappropriate cell differentiation events or cell death. While for post-mitotic cell types replacement of the dead cell by another cell of the same lineage is not possible, mitotic cell types may initially replace dead cells via cell proliferation. But exhaustion of the self-renewal capacity of the respective lineage, by either replication-associated or damage-associated telomere shortening, will ultimately also lead to loss of parenchymal cell mass and functional impairment of tissues, the latter being a typical feature of ageing of tissues and organs. It has been demonstrated in various experimental systems that the rate ageing of can be retarded by lowering the production of endogenous ROI or by improving cellular anti-oxidative defences. Whether augmentation of cellular DNA repair capacity will have the same effect remains to be seen.

A functional interaction of Ku with Werner exonuclease facilitates digestion of damaged DNA

Werner syndrome (WS) is a premature aging disorder where the affected individuals appear much older than their chronological age. The single gene that is defective in WS encodes a protein (WRN) that has ATPase, helicase and 3'-->5' exonuclease activities. Our laboratory has recently uncovered a physical and functional interaction between WRN and the Ku heterodimer complex that functions in double-strand break repair and V(D)J recombination. Importantly, Ku specifically stimulates the exonuclease activity of WRN. We now report that Ku enables the Werner exonuclease to digest through regions of DNA containing 8-oxoadenine and 8-oxoguanine modifications, lesions that have previously been shown to block the exonuclease activity of WRN alone. These results indicate that Ku significantly alters the exonuclease function of WRN and suggest that the two proteins function concomitantly in a DNA damage processing pathway. In support of this notion we also observed co-localization of WRN and Ku, particularly after DNA damaging treatments.

Gene expression and regulation in the extended longevity phenotypes of Drosophila

We used both selection and single-gene mutagenesis studies to identify the mechanisms underlying the genetic control of longevity in Drosophila. The expression of the extended longevity phenotype (ELP) in our forward-selected strains depends on an early and specific upregulation of the antioxidant defense system (ADS) genes and enzymes, which results in decreased oxidative damage levels and a delayed onset of senescence. This mechanism does not alter metabolic rate and is itself reversed by a reverse selection regime. Single-gene mutational analysis of the regulatory genes controlling ADS gene expression show they are under the positive and negative control of several such genes, each of which can bring about the expression/repression of the ELP. Sister strains with identical ELPs have different patterns of ADS gene expression, showing that phenotypic equivalence does not require molecular equivalence. The organism may have multiple genetic strategies to cope with similar levels of oxidative stress.

BRCA1 gene sequence variation in centenarians

With the ample gene sequence information that has become available with the human genome project virtually completed, it has become possible to identify functional gene variants and their frequencies in elderly populations with different aging-related characteristics. Such a genetic epidemiological approach could lead to new insights with respect to the basic mechanisms of aging and longevity as well as the identification of new targets to prevent or retard some of the late-age adverse effects. Using our recently developed two-dimensional gene scanning (TDGS) technology platform we demonstrate the feasibility of this approach by screening two different populations of centenarians for polymorphic variation in the BRCA1 breast cancer susceptibility gene, one of the many genes involved in genome maintenance. The initial results obtained with this approach suggest differences in BRCA1 genotype frequencies between the centenarian populations and controls.

Chronological lifespan of stationary phase yeast cells; a model for investigating the factors that might influence the ageing of postmitotic tissues in higher organisms

Budding yeast can be considered to have two distinct lifespans: (a) a replicative (budding, non-chronological) lifespan, measured as the number of daughters produced by each actively dividing mother cell; and (ii) a chronological lifespan, measured as the ability of stationary cultures to maintain viability over time. In non-dividing cells, essential components that become damaged cannot be diluted out through cell division but must, of necessity, be turned over and renewed. By elevating stress resistances, many of the activities needed for such renewal should be elevated with commensurate reduction in the steady-state levels of damaged cell components. Therefore, chronological lifespan in particular might be expected to relate to stress resistance. For yeast to attain a full chronological lifespan requires the expression of the general stress response. It is more important, though, that the cells should be efficiently adapted to respiratory maintenance, since it is cultures grown to stationary phase on respiratory media that usually display the longest chronological lifespans. For this reason, respiration-adapted cells potentially provide a better model of chronological ageing than cultures pre-grown on glucose.

Caloric restriction mimetics: metabolic interventions

Caloric restriction (CR) retards diseases and aging in laboratory rodents and is now being tested in nonhuman primates. One way to apply these findings to human health is to identify and test agents that may mimic critical actions of CR. Panel 2 focused on two outcomes of CR, reduction of oxidative stress and improved glucoregulation, for which candidate metabolic mimics exist. It was recommended that studies on oxidative stress should emphasize mitochondrial function and to test the efficacy of nitrone and other antioxidants in mimicking CR's effects. Studies should also focus on the long-term effects of compounds known to lower circulating glucose and insulin concentrations or to increase insulin sensitivity. Also, four other developing areas were identified: intermediary metabolism, response to infection, stress responses, and source of dietary fat. These areas are important because either they hold promise for the discovery of new mimetics or they need to be explored prior to initiation of CR trials in humans. Other recommendations were that transgenic approaches and adult-onset CR should be emphasized in future studies.

Telomeres, replicative senescence and human ageing

Ageing concerns the extracellular environment and cells that are either post-mitotic or capable of division during life. Primary human cells have a finite division capacity in culture before they enter a state of viable cell cycle arrest termed senescence. Cell division occurs during life in many tissues, either as part of normal tissue function or in response to tissue damage. The accumulation of cells at the end of their replicative lifespan in the elderly might contribute to aged tissue either because of a reduced ability to undergo proliferation or because of the known altered gene-expression patterns of senescent cells. This has been illustrated experimentally using a transgenic telomerase-negative mouse, which shows some premature ageing phenotypes. The mechanism whereby cells count divisions uses the gradual erosion of the ends of chromosomes (telomeres) with cell division caused by the repression of the telomere-maintenance enzyme telomerase in most human cells. Telomere erosion ultimately triggers replicative senescence in many cell types; this can be prevented experimentally by forcibly expressing telomerase. This extends the lifespan of normal human cells and those from progeroid syndromes such as Werner's. Telomere-driven senescence did not evolve to cause ageing, but is instead a by-product of a system devised to provide a tumour-suppression function, a concept that fits well with evolutionary arguments regarding trade-offs between somatic maintenance and reproduction. Work in the future will focus on the development of new animal models to critically address the quantitative significance of this ageing mechanism.

Cinnamophilin as a novel antiperoxidative cytoprotectant and free radical scavenger

The antioxidant properties of cinnamophilin were evaluated by studying its ability to react with relevant reactive oxygen species, and its protective effect on cultured cells and biomacromolecules under oxidative stress. Cinnamophilin concentration-dependently suppressed non-enzymatic iron-induced lipid peroxidation in rat brain homogenates with an IC50 value of 8.0+/-0.7 microM and iron ion/ADP/ascorbate-initiated rat liver mitochondrial lipid peroxidation with an IC50 value of 17.7+/-0.2 microM. It also exerted an inhibitory activity on NADPH-dependent microsomal lipid peroxidation with an IC50 value of 3.4+/-0.1 microM without affecting microsomal electron transport of NADPH-cytochrome P-450 reductase. Both 1,1-diphenyl-2-picrylhydrazyl and 2,2'-azo-bis(2-amidinopropane) dihydrochloride-derived peroxyl radical tests demonstrated that cinnamophilin possessed marked free radical scavenging capacity. Cinnamophilin significantly protected cultured rat aortic smooth muscle cells (A7r5) against alloxan/iron ion/H2O2-induced damage resulting in cytoplasmic membranous disturbance and mitochondrial potential decay. By the way, cinnamophilin inhibited copper-catalyzed oxidation of human low-density lipoprotein, as measured by fluorescence intensity and thiobarbituric acid-reactive substance formation in a concentration-dependent manner. On the other hand, it was reactive toward superoxide anions generated by the xanthine/xanthine oxidase system and the aortic segment from aged spontaneously hypertensive rat. Furthermore, cinnamophilin exerted a divergent effect on the respiratory burst of human neutrophil by different stimulators. Our results show that cinnamophilin acts as a novel antioxidant and cytoprotectant against oxidative damage.

Translating basic aging research into geriatric health care

Aging processes are amenable to molecular genetic analyses. Two aspects of such research have been selected for discussion in this paper because of current great interest and their relevance to human aging. Studies on telomeres have revealed new insights on the control of cellular replicative senescence and provided a means to extend the cell's life span during in vitro cultivation. Emerging studies on genetic biomarkers have identified genes that appear to be associated with longevity or with risk factors for aging-related diseases, and raised considerations of ways to reduce disease expression. An interchange between basic scientists and clinicians would encourage new thoughts on the feasibility of translating these fundamental studies into interventions that promote healthier longevity.

Ataxia-telangiectasia: chronic activation of damage-responsive functions is reduced by alpha-lipoic acid

Cells from patients with the genetic disorder ataxia-telangiectasia (A-T) are hypersensitive to ionizing radiation and radiomimetic agents, both of which generate reactive oxygen species capable of causing oxidative damage to DNA and other macromolecules. We describe in A-T cells constitutive activation of pathways that normally respond to genotoxic stress. Basal levels of p53 and p21(WAF1/CIP1), phosphorylation on serine 15 of p53, and the Tyr15-phosphorylated form of cdc2 are chronically elevated in these cells. Treatment of A-T cells with the antioxidant alpha-lipoic acid significantly reduced the levels of these proteins, pointing to the involvement of reactive oxygen species in their chronic activation. These findings suggest that the absence of functional ATM results in a mild but continuous state of oxidative stress, which could account for several features of the pleiotropic phenotype of A-T.

N-acetyl-L-cysteine protects SHSY5Y neuroblastoma cells from oxidative stress and cell cytotoxicity: effects on beta-amyloid secretion and tau phosphorylation

Redox changes within neurones are increasingly being implicated as an important causative agent in brain ageing and neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD). Cells have developed a number of defensive mechanisms to maintain intracellular redox homeostasis, including the glutathione (GSH) system and antioxidant enzymes. Here we examine the effects of N-acetyl-L-cysteine (NAC) on beta-amyloid (A beta) secretion and tau phosphorylation in SHSY5Y neuroblastoma cells after exposure to oxidative stress inducing/cytotoxic compounds (H(2)O(2), UV light and toxic A beta peptides). A beta and tau protein are hallmark molecules in the pathology of AD while the stress factors are implicated in the aetiology of AD. The results show that H(2)O(2), UV light, A beta 1-42 and toxic A beta 25-35, but not the inactive A beta 35-25, produce a significant induction of oxidative stress and cell cytotoxicity. The effects are reversed when cells are pre-treated with 30 mM NAC. Cells exposed to H(2)O(2), UV light and A beta 25-35, but not A beta 35-25, secrete significantly higher amounts of A beta 1-40 and A beta 1-42 into the culture medium. NAC pre-treatment increased the release of A beta 1-40 compared with controls and potentiated the release of both A beta 1-40 and A beta 1-42 in A beta 25-35-treated cells. Tau phosphorylation was markedly reduced by H(2)O(2) and UV light but increased by A beta 25-35. NAC strongly lowered phospho-tau levels in the presence or absence of stress treatment.

Reproductive costs of heat shock protein in transgenic Drosophila melanogaster

Senescence may evolve when genes have antagonistic effects between early reproduction and later age-specific mortality. Although widely consistent with data of quantitative genetics, this model has yet to be validated with the identification of a specific locus presenting such trade-offs. The molecular chaperone hsp70 may be a candidate for such a gene. Heat induced expression of the Hsp70 protein in adults decreases rates of age-specific mortality during normal aging, while maternally experienced heat shock depresses the production of mature progeny. Here we show that maternal heat shock reduces the proportion of egg hatch but not the viability of successfully hatched offspring. To assess whether heat induced maternal expression of hsp70 causes reduced egg hatch, we measured the proportion of eggs that hatch from females engineered to overexpress hsp70 transgenes. We used the same transgenic strains that extend longevity upon hsp70 expression and found that Hsp70 is sufficient to suppress egg hatch. The proportion of egg hatch as a function of hsp70 expression was not reduced in the first eggs laid after maternal heat shock, but appears in later laid eggs, which were at preoogenic and early vitellogenic stages during the maternal expression of hsp70. The contervailing effects of hsp70 upon fecundity and subsequent age-specific mortality exemplify antagonistic pleiotropy, and this trade-off could contribute to the evolution of Drosophila senescence.

Lessons from human progeroid syndromes

A number of human genes have been identified in which mutations can lead to the accelerated emergence of features of senescence. Studies of these genes, and of the functions of their protein products, may lead to a clearer understanding of the nature of senescence, and could provide clues for ways in which ageing might be retarded.

Why do we age?

The evolutionary theory of ageing explains why ageing occurs, giving valuable insight into the mechanisms underlying the complex cellular and molecular changes that contribute to senescence. Such understanding also helps to clarify how the genome shapes the ageing process, thereby aiding the study of the genetic factors that influence longevity and age-associated diseases.

Oxidants, oxidative stress and the biology of ageing

Living in an oxygenated environment has required the evolution of effective cellular strategies to detect and detoxify metabolites of molecular oxygen known as reactive oxygen species. Here we review evidence that the appropriate and inappropriate production of oxidants, together with the ability of organisms to respond to oxidative stress, is intricately connected to ageing and life span.

Carboxyfullerenes protect human keratinocytes from ultraviolet-B-induced apoptosis

Carboxyfullerene, a water-soluble carboxylic acid derivative of a fullerene, which acts as a free-radical scavenger, was investigated as a protective agent against ultraviolet-light-induced damage in human keratinocytes. First, we demonstrate that carboxyfullerene is not cytotoxic for these cells. In addition, this compound significantly reduces the ultraviolet-B-induced inhibition of keratinocyte proliferation and protects keratinocytes from apoptosis caused by ultraviolet B irradiation in a time- and dose-dependent fashion. Furthermore, the percentage of cells with depolarized mitochondria is significantly lower in ultraviolet-B-irradiated keratinocytes pretreated with carboxyfullerene than in cells provided with diluent alone. Carboxyfullerene also protects human keratinocytes from apoptosis induced by exposure to deoxy-D-ribose, a sugar that causes cell death through a pathway involving oxidative stress. On the other hand, ultraviolet B downregulates bcl-2 levels in human keratinocytes, and carboxyfullerene fails to prevent this effect. These results suggest that carboxy- fullerene protects human keratinocytes from ultraviolet B damage possibly via a mechanism interfering with the generation of reactive oxygen species from depolarized mitochondria without the involvement of bcl-2.

Anticipation of oxidative damage decelerates aging in virgin female medflies: hypothesis tested by statistical modeling

Empirical analysis of survival data obtained from large samples of Mediterranean fruit flies shows that the trajectory of the mortality rate for virgin females departs from that for females maintained in mixed sex cages. It increases, decelerates, reaches its maximum, declines and then increases again within the reproductive interval. Non-virgin females, however, display an early-age plateau instead of this dip. We assume that these deviations are produced by the interplay between changes in oxygen consumption associated with reproductive behavior and the antioxidant defense that acts against anticipated oxidative damage caused by reproduction. Since there are no data on antioxidant mechanisms in medflies available that explain the observed patterns of mortality, we develop a model of physiological aging based on oxidative stress theory, which describes age-related changes in oxygen consumption and in antioxidative capacity during the reproductive period. Using this model, we simulate virtual populations of 25,000 virgin and non-virgin flies, calculate the respective mortality rates and show that they practically coincide with those of experimental populations. We show that the hypothesis about the biological support of reproduction used in our model does not contradict experimental data. The model explains how the early-age dip and plateau might arise in the mortality rates of female medflies and why the male mortality pattern does not exhibit such deviations.

Gerontogenes mediate health and longevity in nematodes through increasing resistance to environmental toxins and stressors

More than 40 mutants in Caenorhabditis elegans have been demonstrated to lead to increased life span (a rigorous, operational test for being a gerontogene) of 20% or more ("Age" mutants). Age mutants alter rate-limiting determinants of longevity; moreover, important genes are identified independent of prior hypotheses as to actual mode of gene action in extending longevity and/or "slowing" aging. Age mutants define as many as nine (possibly) distinct pathways and/or modes of action, as defined by primary phenotype. Three well-studied mutants (age-1, clk-1, and spe-26) alter age-specific mortality rates in characteristic fashions; in age-1 mutants, especially, the changes in mortality rates are quite dramatic. All Age mutants (so far without exception) increase response to several (but not all) stresses, including heat, UV, and reactive oxidants. We have used directed strategies, as well as random mutagenesis, to identify novel genes increasing the worm's ability to resist stress. Two genes (daf-16 and old-1) yield over-expression strains that are stress resistant and long-lived. A variety of approaches to assess transcriptional alterations associated with increased longevity are underway. We suggest that the role of the Age genes in both longevity and stress resistance indicates that a major evolutionary determinant of longevity is the ability to respond to stress.

Oxidative stress resistance: a robust correlated response to selection in extended longevity lines of Drosophila melanogaster?

Stress resistance is associated with longevity in Drosophila melanogaster and other model organisms used for genetic research. The present study tests for oxidative stress resistance in one set of lines selected for late-life reproduction and extended longevity. Both females and males from the selected lines were appreciably more resistant to oxidative stress than were flies from the control lines. A relative increase in oxidative stress resistance is a correlated response to selection in this laboratory selection experiment. Increased oxidative stress resistance appears to be a relatively robust correlated response to laboratory selection for late-life reproduction and extended longevity.

Somatic cell mutations: can they provide a link between aging and cancer?

Cancers increase during aging in mammals, and an accumulating body of evidence suggests that mutational events too do likewise. Mutational events are intimately involved in the malignant process. One current view is that mutator phenotypes are required in malignant cells for a sufficient number of critical target genes to be affected. These mutator phenotypes are believed to result from underlying deficiencies in genes necessary to maintain genomic stability. This review will provide a framework for a discussion of cancer and aging by detailing with a pair of wise approach studies that address the relations between aging, cancer, and mutations. Results from these studies will be used to suggest that a mutator phenotype develops in the cells of older individuals in the absence of an underlying genetic deficiency. Instead, it is proposed that a mixture of chromosomal aberrations, DNA damage, and chronic exposure to genotoxic forces, including oxidative stress, provide the basis for this age-accelerated mutator phenotype.

Distinct spectra of somatic mutations accumulated with age in mouse heart and small intestine

Somatic mutation accumulation has been implicated as a major cause of cancer and aging. By using a transgenic mouse model with a chromosomally integrated lacZ reporter gene, mutational spectra were characterized at young and old age in two organs greatly differing in proliferative activity, i.e., the heart and small intestine. At young age the spectra were nearly identical, mainly consisting of G. C to A.T transitions and 1-bp deletions. At old age, however, distinct patterns of mutations had developed. In small intestine, only point mutations were found to accumulate, including G.C to T.A, G.C to C.G, and A.T to C.G transversions and G.C to A.T transitions. In contrast, in heart about half of the accumulated mutations appeared to be large genome rearrangements, involving up to 34 centimorgans of chromosomal DNA. Virtually all other mutations accumulating in the heart appeared to be G.C to A.T transitions at CpG sites. These results suggest that distinct mechanisms lead to organ-specific genome deterioration and dysfunction at old age.

Some new directions for research on the biology of aging

A highly selective, eclectic, and personal view of new directions and new opportunities for research on the biology of aging is briefly outlined. Some concern is raised regarding the present emphasis on the use of centenarians for the definition of genetic loci responsible for unusually robust retention of structure and function. More progress is likely to be made were we to focus on the genetic basis for "elite" aging in middle-aged subjects examined for very specific phenotypes, as these are likely to be far less polygenic. Descriptive gerontology is entering a renaissance, given such new clinical tools as functional MRI and basic science tools such as functional genomics and proteomics. Advances in genomics should expedite answers to such questions as why some avian species have exceptionally long lifespans despite unusual loads of oxidative stress. One hopes to see renewed mechanistic studies, using such tools, at the systems levels. New methodologies are permitting the evaluation of stochastic alterations in gene structure and function in postreplicative cells. The exciting work on molecular misreading should prompt us to reexplore the Orgel hypothesis as it applies to such cell types. Epigenetic shifts in gene expression that occur in association with sexual maturation and the cessation of growth may have deleterious consequences late in the life course. It will therefore be important for gerontologists to investigate the molecular biology of pubescence. Finally, our community should investigate the impact of environmental "gerontogens," agents that accelerate specific processes of aging and specific senescent phenotypes.

Inherited variability of the mitochondrial genome and successful aging in humans

Increasing data indicate that polymorphic variants of nuclear loci can affect rate and quality of aging in humans. However, the mitochondrial genome is another good candidate, because of the central role played by mitochondrial genes in oxidative phosphorylation (OXPHOS) and cell metabolism. A characteristic of the mitochondrial genome (mtDNA) is the high level of interindividual variability that ensues from high mutation rate and unilinear inheritance. Related groups of germline/inherited mtDNA polymorphisms (haplogroups) have been identified as continent-specific sets of stable/ancient/associated restriction fragment length polymorphisms in the mtDNA coding region, representing markers capable of exactly depicting the mtDNA pool of a specific population. The hypothesis can be put forward that mtDNA variants included in a haplogroup may have similar OXPHOS efficiency and therefore act as genetic factors predisposing to individual successful or unsuccessful aging. This idea can be explored by sampling groups of individuals of different ages from a well-defined population and comparing the pools of mtDNA haplogroups between samples. The results obtained by screening mtDNA haplogroups in about 800 Italians of different ages, including more than 200 centenarians, agree with the hypothesis that the inherited variability of the mitochondrial genome is associated with the chance of successful aging and longevity in humans.

Mitochondrial oxidative stress. Physiologic consequences and potential for a role in aging

During the last 10 years, the theory known as the "free radical theory of aging" has achieved prominence as one of the most compelling explanations for many of the degenerative changes associated with aging. Although its appeal derives from a long-standing body of supporting correlative data, the theory was only recently more rigorously tested. Ongoing researches in the study of free radical biochemistry and the genetics of aging have been at the forefront of this work. First, transgenic approaches in invertebrate models with candidate genes such as superoxide dismutase (SOD) involved in the detoxification of reactive oxygen species (ROS) have shown that the endogenous production of ROS due to normal physiologic processes is a major limiter of life span. Genes involved in ROS detoxification are highly conserved among eukaryotes; hence, the physiologic processes that limit life span in invertebrates are likely to be similar in higher eukaryotes. Secondly, transgenic mice deficient in the antioxidant enzyme mitochondrial superoxide dismutase (SOD2) die within their first week of life, demonstrating the importance of limiting endogenous mitochondrial free radicals in mammals. Together, data from studies using transgenic invertebrates and those using sod2 mutant mice demonstrate that modulation of metabolic ROS can have a profound effect on life span. We show here that the effects of mitochondrial ROS can be modulated through appropriate catalytic antioxidant intervention. These catalytic antioxidants are discussed in the context of mitochondrial oxidative stress and their potential role in intervening in mitochondrial oxidative stress and aging.

Molecular genetic mechanisms of life span manipulation in Caenorhabditis elegans

Aging and a limited life span are fundamental biological realities. Recent studies have demonstrated that longevity can be manipulated and have revealed molecular mechanisms underlying longevity control in the soil nematode Caenorhabditis elegans. Signals from both neurons and the gonad appear to negatively regulate longevity. One tissue-specific signal involves an insulin-like phosphatidylinositol 3-OH kinase pathway, dependent upon the DAF-16 forkhead transcription factor. These signals regulate mechanisms determining longevity that include the OLD-1 (formerly referred to as TKR-1) receptor tyrosine kinase. Interestingly, increased resistance to environmental stress shows a strong correlation with life extension.

Caloric restriction and aging: an update

Restricting food intake to 50 to 70% of that eaten by ad lib-fed rats and mice markedly increases longevity, retards age-associated physiological deterioration, and delays and, in some cases, prevents age-associated diseases. These actions are due to the reduced intake of calories, and thus the phenomenon has been called the antiaging action of caloric restriction (CR). This article focuses on the possible biological mechanisms underlying the antiaging action. The following three proposed mechanisms are considered in depth: 1) attenuation of oxidative damage; 2) modulation of glycemia and insulinemia; 3) hormesis. The evolution of the antiaging action of CR is also considered. Based on this consideration, a scenario unifying the above mechanisms is presented.

A causal link between respiration and senescence in Podospora anserina

Senescence, a progressive degenerative process leading to age-related increase in mortality, is found in most eukaryotes. However, the molecular events underlying aging remain largely unknown. Understanding how longevity is regulated is a fundamental problem. Here we demonstrate that the respiratory function is a key factor that contributes to shortening lifespan of the filamentous fungus Podospora anserina. In this organism, senescence is systematically associated with mitochondrial DNA instabilities. We show that inactivation of the nuclear COX5 gene encoding subunit V of the cytochrome c oxidase complex leads to the exclusive use of the alternative respiratory pathway and to a decrease in production of reactive oxygen species. This inactivation results in a striking increase of longevity associated with stabilization of the mitochondrial chromosome. Moreover, accumulation of several senescence-specific mitochondrial DNA molecules is prevented in this nuclear mutant. These findings provide direct evidence of a causal link between mitochondrial metabolism and longevity in Podospora anserina.

The role of metabolism in aging

Metabolism appears to play a significant role in determining the rate of aging. Long-lived mutants and selected stocks of model experimental organisms exhibit characteristic changes in life history and physiological features consistent with altered metabolism. Specifically, these include the accumulation of metabolite pools of glycogen, lipid and polyhedric alcohols, suggesting that prolonged life is associated with restriction of the flow of carbon through glycolysis. Limiting carbon flow by reducing caloric intake is well known to extend life. Studies examining the mRNA expression pattern of functional gene groups generally indicate that nutrient restriction does affect metabolism. One study of Drosophila melanogaster has demonstrated that the flux of carbon through glycolysis is reduced in larvae of selected long-lived populations. Here we propose a new hypothesis describing the interaction between the glycation process, glucose level, damage by free oxygen radicals and chaperonin proteins. Intermediate steps of the Maillard Reaction produce free radicals, similar to those produced during respiration, that also damage lipids, proteins and DNA. Antioxidant enzymes themselves can be inactivated by glycation. This establishes a positive feed-back between the rate of metabolism, glucose availability and damage by free oxygen radicals. The high levels of available glucose in ad libitum feeding should add to production of radicals, reduce levels of protective antioxidants and proportionally increase damage by free radicals. Thus, caloric restriction should lower available glucose, increase the level of antioxidants and thereby slow the rate of aging. Chaperonins act to block glycation, reducing its negative effect on antioxidant enzymes release and contribution of free radicals.

Forward and reverse selection for longevity in Drosophila is characterized by alteration of antioxidant gene expression and oxidative damage patterns

Patterns of antioxidant gene expression and of oxidative damage were measured throughout the adult life span of a selected long-lived strain (La) of Drosophila melanogaster and compared to that of their normal-lived progenitor strain (Ra). Extended longevity in the La strain is correlated with enhanced antioxidant defense system gene expression, accumulation of CuZnSOD protein, and an increase in ADS enzyme activities. Extended longevity is strongly associated with a significantly increased resistance to oxidative stress. Reverse-selecting this long-lived strain for shortened longevity (RevLa strain) yields a significant decrease in longevity accompanied by reversion to normal levels of its antioxidant defense system gene expression patterns and antioxidant enzyme patterns. The significant effects of forward and reverse selection in these strains seem limited to the ADS enzymes; 11 other enzymes with primarily metabolic functions show no obvious effect of selection on their activity levels whereas six other enzymes postulated to play a role in flux control may actually be involved in NADPH reoxidation and thus support the enhanced activities of the ADS enzymes. Thus, alterations in the longevity of these Drosophila strains are directly correlated with corresponding alterations in; 1) the mRNA levels of certain antioxidant defense system genes; 2) the protein level of at least one antioxidant defense system gene; 3) the activity levels of the corresponding antioxidant defense system enzymes, and 4) the ability of the organism to resist the biological damage arising from oxidative stress.

Hormesis: an adaptive expectation with emphasis on ionizing radiation

Non-linear fitness gradients with maxima between extremes are expected for any environmental variable to which free-living populations are exposed. For exceedingly toxic agents, including ionizing radiation, such deviations from linearity are close to zero exposure and are conventionally called hormesis. Accordingly, hormesis is an extreme version of the non-linear fitness gradients for general environmental stresses such as temperature fluctuations, for which maximum fitness occurs at the moderate temperature fluctuations to which free-living populations are most commonly exposed. Some metabolic reserves should occur under moderate temperature stresses because of the need for pre-adaptation enabling survival during exposure to occasional periods of more extreme stress, especially at species borders where selection for stress resistance is likely to be most intense. Because heat shock proteins are induced by all stresses, adaptation to extreme temperatures should translate into adaptation to other stresses. Consequently, metabolic reserves from adaptation to extreme temperatures in the past should translate into protection from correlated abiotic stresses, especially in human populations where modern cultural processes are now ameliorating exposure to extreme stresses. Ambient and man-made radiations of non-catastrophic dimensions should therefore lead to stress-derived radiation hormesis. Other stresses can, in principle, be incorporated into this model. Accordingly, evolutionary and ecological considerations suggest two components of hormesis in relation to ionizing radiation: background radiation hormesis based upon the background exposure to which all organisms on earth are subjected; and stress-derived radiation hormesis. Exposure under stress-derived radiation hormesis is considerably larger than under background radiation hormesis, so significant deleterious effects from non-catastrophic radiation normally may be impossible to detect. Suggestions are provided for testing such postulated deviations from the commonly assumed linear no-threshold (LNT) hypothesis for the biological consequences of exposure to radiation.

When lysosomes get old

Changes in the lysosomes of senescent tissues and organisms are common and have been used as biomarkers of aging. Lysosomes are responsible for the degradation of many macromolecules, including proteins. At least five different pathways for the delivery of substrate proteins to lysosomes are known. Three of these pathways decline with age, and the molecular explanations for these deficiencies are currently being studied. Other aspects of lysosomal proteolysis increase or do not change with age in spite of marked changes in lysosomal morphology and biochemistry. Age-related changes in certain lysosomal pathways of proteolysis remain to be studied. This area of research is important because abnormalities in lysosomal protein degradation pathways may contribute to several characteristics and pathologies associated with aging.

The effect of age on cyclooxygenase-2 gene expression: NF-kappaB activation and IkappaBalpha degradation

Increased oxidative stress resulting in the activation of NF-kappaB is thought to play a crucial role in the expression of the cyclooxygenase-2 (COX-2), which is the key enzyme in proinflammatory prostanoid synthesis. In the current study, we investigated whether the aging process affects the status of the redox-sensitive NF-kappaB in rat kidney, and how this age-related modulation is related to COX-2 gene expression and COX-derived reactive oxygen species (ROS). We found that the aging process strongly enhanced the activation of NF-kappaB and its DNA-binding activity with an increased ROS status. Accompanied with the change in the NF-kappaB activity was a decreased IkappaBalpha as confirmed by the increased nuclear p65 protein. Thus, these data strongly indicated that the aging process increases NF-kappaB activity by downregulating IkappaBalpha. A closer examination further revealed that age-related oxidative status correlated with the increased COX-derived prostanoid biosynthetic process is mediated by the increased NF-kappaB-regulated COX activity. This increase in NF-kappaB activity was accompanied by the increased COX-2 mRNA and protein levels. Based on these data, we concluded that the age-related increase in redox-sensitive NF-kappaB translocation and binding activities are associated with increased ROS, and further that this transactivation was modulated by the age-related decrease of IkappaBalpha.