Effects of ethidium bromide on development and aging of Drosophila: implications for the free radical theory of aging

Temperature-dependent trade-offs between longevity and fertility in the Drosophila mutant, methuselah

Single gene, hypomorphic mutations which extend the life spans of cold-blooded animals, such as the methuselah (mth) mutation in the fruit fly, Drosophila melanogaster, may have additional, deleterious effects on overall fitness. The hypotheses tested here were: (i) that the extension of life span by mth might be temperature-dependent, and (ii) that it might be associated with depression of reproductive output, physical activity, or the rate of metabolism. The effect of mth on life span was smaller in magnitude than reported previously, and it was both sex-specific and temperature-dependent. Female longevity was increased only at 29 degrees C, whereas for male flies the extension of mean life span diminished progressively from 15-25% 25-29 degrees C to 2% at 18 degrees C, and the survival time at 4 degrees C was decreased by 22-39%. Conversely, the lifetime reproductive output of mth mutants was decreased at 29 degrees C, but increased at 18-22 degrees C. The walking speed of mth flies was significantly elevated, but mth had no effect on the rate of oxygen consumption at 25 degrees C. Collectively, the results demonstrate that where the life span is extended, there is an offsetting effect on reproductive output, suggesting that mth induces trade-off effects and is not a direct, mechanistic regulator of the aging process.

Alternative pathways as mechanism for the negative effects associated with overexpression of superoxide dismutase

One of the most important antioxidant enzymes is superoxide dismutase (SOD), which catalyses the dismutation of superoxide radicals to hydrogen peroxide. The enzyme plays an important role in diseases like trisomy 21 and also in theories of the mechanisms of aging. But instead of being beneficial, intensified oxidative stress is associated with the increased expression of SOD and also studies on bacteria and transgenic animals show that high levels of SOD actually lead to increased lipid peroxidation and hypersensitivity to oxidative stress. Using mathematical models we investigate the question how overexpression of SOD can lead to increased oxidative stress, although it is an antioxidant enzyme. We consider the following possibilities that have been proposed in the literature: (i) Reaction of H(2)O(2) with CuZnSOD leading to hydroxyl radical formation. (ii) Superoxide radicals might reduce membrane damage by acting as radical chain breaker. (iii) While detoxifying superoxide radicals SOD cycles between a reduced and oxidized state. At low superoxide levels the intermediates might interact with other redox partners and increase the superoxide reductase (SOR) activity of SOD. This short-circuiting of the SOD cycle could lead to an increased hydrogen peroxide production. We find that only one of the proposed mechanisms is under certain circumstances able to explain the increased oxidative stress caused by SOD. But furthermore we identified an additional mechanism that is of more general nature and might be a common basis for the experimental findings. We call it the alternative pathway mechanism.

Mechanisms of aging: an appraisal of the oxidative stress hypothesis

The main purpose of this article is to provide a critical overview of the currently available evidence bearing on the validity of the oxidative stress hypothesis of aging, which postulates that senescence-associated attenuations in physiological functions are caused by molecular oxidative damage. Several lines of correlative evidence support the predictions of the hypothesis, e.g., macromolecular oxidative damage increases with age and tends to be associated with life expectancy of organisms. Nevertheless, a direct link between oxidative stress and aging has not as yet been established. Single gene mutations have been reported to extend the life spans of lower organisms, such as nematodes and insects; however, such prolongations of chronological clock time survival are usually associated with decreases in the rate of metabolism and reproductive output without affecting the metabolic potential, i.e., the total amount of energy consumed during life. Studies on genetic manipulations of the aging process have often been conducted on relatively short-lived strains that are physiologically weak, whereby life-span extensions can not be unambiguously assigned to a slowing effect on the rate of aging. It is concluded that although there is considerable evidence implicating oxidative stress in the aging process, additional evidence is needed to clearly define the nature of the involvement.

Energetic stress induces premature aging of diploid human fibroblasts (Wi-38) in vitro

Oxidative phosphorylation is the main endogenous source for the generation of reactive oxygen species (ROS). In order to investigate the influence of enhanced ROS production on the in vitro senescence of Wi-38 fibroblasts, cells were cultivated in medium with elevated (hypertonic) NaCl concentrations. The number of active Na(+)/K(+)-ATPase molecules per cell was found to be increased. A rise in both respiration and glycolysis as evidenced by the increases in oxygen and glucose consumption and lactate production was revealed. Cells stayed alive in medium with NaCl concentrations of up to 0.30 M and could be adapted to growth under these hypertonic conditions (high-NaCl tolerant cells). These cells exhibited an increased cell size and protein content. A growing number of cells showed stress fibers and granulation. The proliferation rate and the maximum number of cumulative population doublings of these high-NaCl tolerant cultures were reduced and saturation density was decreased. Thus, these cells under energetic stress due to increased energy requirements for active ion transport expressed features typical for aging in vitro. We conclude therefore that energetic stress induces premature aging in human diploid fibroblasts.

Current issues concerning the role of oxidative stress in aging: a perspective

The main tenet of the oxidative stress hypothesis of aging is that accrual of molecular oxidative damage is the principal causal factor in the senescence-related loss of ability to maintain homeostasis. This hypothesis has garnered a considerable amount of supportive correlational evidence, which is now being extended experimentally in transgenic Drosophila over-expressing antioxidative defense enzymes. Some of these studies have reported extensions of life span, while others have not. Interpretation of life spans in poikilotherms is complicated by a number of factors, including the interrelationship between metabolic rate and longevity. The life spans of poikilotherms can be extended multi-fold by reducing the metabolic rate but without affecting the metabolic potential, i.e., the total amount of energy expended during life. A hypometabolic state in poikilotherms also enhances stress resistance and activities of antioxidative enzymes. It is emphasized that extension of life span without simultaneously increasing metabolic potential is of questionable biological significance.

The effect of antioxidants and dietary restriction on mortality curves

The full exploitation of information contained in mortality curves offers a tool for direct verification of theories of aging. We have analyzed the behavior of mortality curves for middle and high age groups and have proposed a mathematical model of mortality correlated with the rate of aging. The model offers an explanation for the mutual relationships between mortality curves and suggests potential methodologies for determining experimental modification of the rate of aging. The applicability of this theory is demonstrated by analysis of the changes in mortality as it is influenced by dietary antioxidants or by a calorie restricted diet. Survival data taken from a variety of studies were used as primary information, with parameters of mortality curves being determined by computer-assisted analysis of the curves. These analyses support the hypothesis that a dominant role for free radicals exists in the control of aging in Drosophila. However, in mammals-mice and rats, the effects of antioxidants as well as caloric restriction on mortality curves do not indicate that these treatments alter the rates of aging.

Age-related activity of catalase in different genotypes of Drosophila melanogaster

The enzyme catalase protects aerobic organisms from oxygen free radical damage by converting hydrogen peroxide to molecular oxygen and water before it can decompose to form the highly reactive hydroxyl radical. This damage has been implicated in the increased risk of disease and death associated with aging. In order to study the age-specific activity of catalase in male D. melanogaster, three different genotypes (Oregon w.t., ebony mutant, and the F1 hybrids of the two), whose mean life spans are about 55, 40, and 63 days, respectively, were used in the experiments. As the mean life span of the mutant is the shortest the enzyme activity was measured until the 43rd day, whereas in Oregon w.t., it continued until the 72nd day and in hybrids until the 79th day (longest-lived group). Although the enzyme activity in mutant flies increased sharply till the 9th day (and attained the highest level), later it declined sharply. In the other genotypes, the enzyme activity increased gradually until the 20-25th days, and then declined steadily in comparison to that of the ebony mutant. We found that higher catalase activity is associated with reduced life span for ebony mutant. It is obvious that some relationship exists between life span and antioxidant enzymes; however, a review of the literature does not at the moment allow as to understand the underlying mechanism involved in aging.

Role of oxidative stress in Drosophila aging

We review the role that oxidative damage plays in regulating the lifespan of the fruit fly, Drosophila melanogaster. Results from our laboratory show that the lifespan of Drosophila is inversely correlated to its metabolic rate. The consumption of oxygen by adult insects is related to the rate of damage induced by oxygen radicals, which are purported to be generated as by-products of respiration. Moreover, products of activated oxygen species such as hydrogen peroxide and lipofuscin are higher in animals kept under conditions of increased metabolic rate. In order to understand the in vivo relationship between oxidative damage and the production of the superoxide radical, we generated transgenic strains of Drosophila melanogaster that synthesize excess levels of enzymatically active superoxide dismutase. This was accomplished by P-element transformation of Drosophila melanogaster with the bovine cDNA for CuZn superoxide dismutase, an enzyme that catalyzes the dismutation of the superoxide radical to hydrogen peroxide and water. Adult flies that express the bovine SOD in addition to native Drosophila SOD are more resistant to oxidative stresses and have a slight but significant increase in their mean lifespan. Thus, resistance to oxidative stress and lifespan of Drosophila can be manipulated by molecular genetic intervention. In addition, we have examined the ability of adult flies to respond to various environmental stresses during senescence. Resistance to oxidative stress, such as that induced by heat shock, is drastically reduced in senescent flies. This loss of resistance is correlated with the increase in protein damage generated in old flies by thermal stress and by the insufficient protection from cellular defense systems which includes the heat shock proteins as well as the oxygen radical scavenging enzymes. Collectively, results from our laboratory demonstrate that oxidative damage plays a role in governing the lifespan of Drosophila during normal metabolism and under conditions of environmental stress.

An integrated theory of aging as the result of mitochondrial-DNA mutation in differentiated cells

We maintain that aging of humans and animals derives from a mutation or inactivation (probably followed by endonuclease digestion) of the mitochondrial genome of differentiated cells. This extranuclear somatic mutation hypothesis of aging is based on the finding that mitochondrial DNA (mtDNA) synthesis takes place at the inner mitochondrial membrane near the sites of formation of highly reactive oxygen species and their products, such as lipoperoxides and malonaldehyde. The mtDNA may be unable to counteract the damage inflicted by those by-products of respiration because, in contrast to the nuclear genome, it lacks histone protection and scission repair. Since the mitochondrial genome controls the synthesis of several hydrophobic proteins of the inner mitochondrial membrane, the postulated mutation, inactivation or loss of mtDNA will prevent the replication of the organelles. Thus deprived of the ability to regenerate their mitochondrial populations, the cells will sustain an irreversible decline in their bioenergetic ability, with concomitant senescent loss of physiological performance and eventual death. The above hypothesis is integrated with the concepts of Minot, Pearl and others in order to offer a more comprehensive view of aging.

The fine structural morphology of the midgut of aged Drosophila: a morphometric analysis

The midguts of 1-day and 72 day-old fruitflies were examined morphometrically at the electron microscopic level. The major alterations noted were that the number of supranuclear mitochondria decreased by approximately 50%, while the volume of individual mitochondria doubled as a function of age. Moreover, approximately 29% of the nuclear volume of old flies, was occupied by inclusion bodies as was 19% of the supranuclear cytoplasmic volume. Additionally, the surface density of rough endoplasmic reticulum was reduced to more than half that of young flies. It is suggested that the functional capability of the parenchymal cells become debilitated due to the presence of these inclusion bodies, and that the cell's ability to manufacture proteins and produce energy are seriously hindered by the mitochondrial alterations and reduction in the surface density of the rough endoplasmic reticulum.

Role of metabolic rate and DNA-repair in Drosophila aging: implications for the mitochondrial mutation theory of aging

The respective roles of respiration rate and DNA repair in determining the life-span of Drosophila melanogaster have been investigated in three wild strains (Oregon R, Domodedov 32, & Swedish C) and two mutants (w/w & w/mei-41 D5). In agreement with the rate of living theory, there was an inverse relation between oxygen consumption and median life-span in flies having normal DNA repair. In contrast, w/mei-41 D5, showed an abnormally low life-span as compared to the controls. The median life-spans for these flies were: Oregon R, 49; Domodedov 32, 46; Swedish C, 35; w/w, 33; and w/mei-41 D5, 22. Furthermore, this mutant also showed significant deficiency in mating fitness and a depressed metabolic rate. These data suggest that the short life-span of the w/mei-41 D5 is a consequence of a specific syndrome unrelated to normal aging and is not the expression of accelerated senescence. It is proposed that DNA repair mechanisms influence adult life-span by their effect on actively dividing progenitor cells during development rather than by modulating senescence of the resulting differentiated cells of the imago during adulthood.