Effects of hunger on neuronal histone modifications slow aging in Drosophila.

Hunger is an ancient drive, yet the molecular nature of pressures of this sort and how they modulate physiology are unknown. We find that hunger modulates aging in Drosophila. Limitation of branched-chain amino acids (BCAAs) or activation of hunger-promoting neurons induced a hunger state that extended life span despite increased feeding. Alteration of the neuronal histone acetylome was associated with BCAA limitation, and preventing these alterations abrogated the effect of BCAA limitation to increase feeding and extend life span. Hunger acutely increased feeding through usage of the histone variant H3.3, whereas prolonged hunger seemed to decrease a hunger set point, resulting in beneficial consequences for aging. Demonstration of the sufficiency of hunger to extend life span reveals that motivational states alone can be deterministic drivers of aging.

The propensity for consuming ethanol in Drosophila requires rutabaga adenylyl cyclase expression within mushroom body neurons.

Alcohol activates reward systems through an unknown mechanism, in some cases leading to alcohol abuse and dependence. Herein, we utilized a two-choice Capillary Feeder assay to address the neural and molecular basis for ethanol self-administration in Drosophila melanogaster. Wild-type Drosophila shows a significant preference for food containing between 5% and 15% ethanol. Preferred ethanol self-administration does not appear to be due to caloric advantage, nor due to perceptual biases, suggesting a hedonic bias for ethanol exists in Drosophila. Interestingly, rutabaga adenylyl cyclase expression within intrinsic mushroom body neurons is necessary for robust ethanol self-administration. The expression of rutabaga in mushroom bodies is also required for both appetitive and aversive olfactory associative memories, suggesting that reinforced behavior has an important role in the ethanol self-administration in Drosophila. However, rutabaga expression is required more broadly within the mushroom bodies for the preference for ethanol-containing food than for olfactory memories reinforced by sugar reward. Together these data implicate cAMP signaling and behavioral reinforcement for preferred ethanol self-administration in D. melanogaster.

Chronological aging-independent replicative life span regulation by Msn2/Msn4 and Sod2 in Saccharomyces cerevisiae.

Mutations in RAS2, CYR1, and SCH9 extend the chronological life span in Saccharomyces cerevisiae by activating stress-resistance transcription factors and mitochondrial superoxide dismutase (Sod2). Here we show that mutations in CYR1 and SCH9 also extend the replicative life span of individual yeast mother cells. However, the triple deletion of stress-resistance genes MSN2/MSN4 and RIM15, which causes a major decrease in chronological life span, extends replicative life span. Similarly, the overexpression of superoxide dismutases, which extends chronological survival, shortens the replicative life span and prevents budding in 30-40% of virgin mother cells. These results suggest that stress-resistance transcription factors Msn2/Msn4 negatively regulate budding and the replicative life span in part by increasing SOD2 expression. The role of superoxide dismutases and of other stress-resistance proteins in extending the chronological life span of yeast, worms, and flies indicates that the negative effect of Sod2, Msn2/Msn4/Rim15 on the replicative life span of S. cerevisiae is independent of aging.

Regulation of longevity and stress resistance by Sch9 in yeast.

The protein kinase Akt/protein kinase B (PKB) is implicated in insulin signaling in mammals and functions in a pathway that regulates longevity and stress resistance in Caenorhabditis elegans. We screened for long-lived mutants in nondividing yeast Saccharomyces cerevisiae and identified mutations in adenylate cyclase and SCH9, which is homologous to Akt/PKB, that increase resistance to oxidants and extend life-span by up to threefold. Stress-resistance transcription factors Msn2/Msn4 and protein kinase Rim15 were required for this life-span extension. These results indicate that longevity is associated with increased investment in maintenance and show that highly conserved genes play similar roles in life-span regulation in S. cerevisiae and higher eukaryotes.

Statistical models for estimating the genetic basis of repeated measures and other function-valued traits.

The genetic analysis of characters that are best considered as functions of some independent and continuous variable, such as age, can be a complicated matter, and a simple and efficient procedure is desirable. Three methods are common in the literature: random regression, orthogonal polynomial approximation, and character process models. The goals of this article are (i) to clarify the relationships between these methods; (ii) to develop a general extension of the character process model that relaxes correlation stationarity, its most stringent assumption; and (iii) to compare and contrast the techniques and evaluate their performance across a range of actual and simulated data. We find that the character process model, as described in 1999 by Pletcher and Geyer, is the most successful method of analysis for the range of data examined in this study. It provides a reasonable description of a wide range of different covariance structures, and it results in the best models for actual data. Our analysis suggests genetic variance for Drosophila mortality declines with age, while genetic variance is constant at all ages for reproductive output. For growth in beef cattle, however, genetic variance increases linearly from birth, and genetic correlations are high across all observed ages.

Why do life spans differ? Partitioning mean longevity differences in terms of age-specific mortality parameters.

Populations typically differ in mean life spans because of genetic, environmental, or experimental factors. In this paper methods are presented that clarify the relationship between differences in the longevity of two populations and differences in their underlying age-specific patterns of mortality. Data are examined from rodent and fruit fly (Drosophila melanogaster) experiments that investigated the longevity effects of a variety of environmental and genetic manipulations, including temperature, dietary restriction, laboratory selection for increased longevity, and severe inbreeding. Analyses suggest that longevity differences mediated by temperature and dietary restriction result predominantly from differences in the rate of increase in mortality with age. Increases in longevity through laboratory selection result primarily from a reduction in baseline mortality and not a slowing of the rate of aging. Although the methods are applied primarily in the context of simple mathematical models of mortality (e.g., the Gompertz model), they are quite general and can be applied to mortality models of arbitrary complexity. Mathematica protocols ("notebooks") and computer software have been developed to perform all the analyses discussed and are available from the first author.

The influence of environmentally induced heterogeneity on age-specific genetic variance for mortality rates.

Using parametric models that describe the increase in mortality rates with age, we demonstrate that environmentally induced heterogeneity among genetically identical individuals is sufficient to generate biased estimates of age-specific genetic variance. Although the magnitude of the bias may change with age, one general trend emerges: the true genetic variance at the oldest ages is likely to be dramatically underestimated. Our results are robust to different manifestations of heterogeneity and suggest that such a bias is a general feature of these models. We note that age-dependent estimates of genetic variance for characters that are correlated with mortality (either genetically or environmentally) can be expected to be similarly affected. The results are independent of sample size and suggest that the bias may be more widespread in the literature than is currently appreciated. Our results are discussed with reference to existing data on mortality variance in Drosophila melanogaster.

The genetic analysis of age-dependent traits: modeling the character process.

The extension of classical quantitative genetics to deal with function-valued characters (also called infinite-dimensional characters) such as growth curves, mortality curves, and reaction norms, was begun by Kirkpatrick and co-workers. In this theory, the analogs of variance components for single traits are covariance functions for function-valued traits. In the approach presented here, we employ a variety of parametric models for covariance functions that have a number of desirable properties: the functions (1) are positive definite, (2) can be estimated using procedures like those currently used for single traits, (3) have a small number of parameters, and (4) allow simple hypotheses to be easily tested. The methods are illustrated using data from a large experiment that examined the effects of spontaneous mutations on age-specific mortality rates in Drosophila melanogaster. Our methods are shown to work better than a standard multivariate analysis, which assumes the character value at each age is a distinct character. Advantages over existing methods that model covariance functions as a series of orthogonal polynomials are discussed.

The evolution of age-specific mortality rates in Drosophila melanogaster: genetic divergence among unselected lines.

Age-specific effects of spontaneous mutations on mortality rates in Drosophila are inferred from three large demographic experiments. Data were collected from inbred lines that were allowed to accumulate spontaneous mutations for 10, 19, and 47 generations. Estimates of age-specific mutational variance for mortality were based on data from all three experiments, totalling approximately 225,000 flies, using a model developed for genetic analysis of age-dependent traits (the character process model). Both within- and among-generation analyses suggest that the input of genetic variance is greater for early life mortality rates than for mortality at older ages. In females, age-specific mutational variances ranged over an order of magnitude from 5.96 x 10(-3) at 2 wk posteclosion to 0.02 x 10(-3) at 7 wk. The male data show a similar pattern. Age-specific genetic variances were substantially less at generation 47 than at generation 19-an unexplained observation that is likely due to block effects. Mutational correlations among mortality rates at different ages tend to increase with the accumulation of new mutations. Comparison of the mutation-accumulation lines at generations 19 and 47 with their respective control lines suggests little age-specific mutational bias.

The fractionation experiment: reducing heterogeneity to investigate age-specific mortality in Drosophila.

Age-specific mortality rates decelerate at older ages in both genetically homogenous and heterogeneous populations of Drosophila. One explanation proposed for deceleration is population heterogeneity. This hypothesis suggests that a population consists of sub-populations that differ in mortality characteristics and that the deceleration is the result of selective survival of stronger individuals. Here we describe an experiment that fractionates populations into several sub-populations without changing the physiological characteristics of the post-fractionated populations. Through a careful process of selection of Drosophila eggs, larvae, pupae and adults, we attempt to reduce as much as possible the degree of pre-adult, environmentally induced heterogeneity among individuals of a genetically identical cohort. We then ask whether such cohorts, when compared to non-fractionated populations, exhibit a lesser degree of mortality deceleration at advanced ages. From a total of 106 fractionated and control populations, consisting of 51331 individuals, 101 populations (93% of the fractionated populations and 100% of the control populations) exhibit a significant amount of mortality deceleration late in life. These observations suggest that environmental heterogeneity accrued during larval development is not a major factor contributing to mortality deceleration at older ages.

Age-specific properties of spontaneous mutations affecting mortality in Drosophila melanogaster.

An analysis of the effects of spontaneous mutations affecting age-specific mortality was conducted using 29 lines of Drosophila melanogaster that had accumulated spontaneous mutations for 19 generations. Divergence among the lines was used to estimate the mutational variance for weekly mortality rates and the covariance between weekly mortality rates at different ages. Significant mutational variance was observed in both males and females early in life (up to approximately 30 days of age). Mutational variance was not significantly different from zero for mortality rates at older ages. Mutational correlations between ages separated by 1 or 2 wk were generally positive, but they declined monotonically with increasing separation such that mutational effects on early-age mortality were uncorrelated with effects at later ages. Analyses of individual lines revealed several instances of mutation-induced changes in mortality over a limited range of ages. Significant age-specific effects of mutations were identified in early and middle ages, but surprisingly, mortality rates at older ages were essentially unaffected by the accumulation procedure. Our results provide strong evidence for the existence of a class of polygenic mutations that affect mortality rates on an age-specific basis. The patterns of mutational effects measured here relate directly to recently published estimates of standing genetic variance for mortality in Drosophila, and they support mutation accumulation as a viable mechanism for the evolution of senescence.

Heat-induced longevity extension in Drosophila. I. Heat treatment, mortality, and thermotolerance.

Survival data were collected on a total of 28,000 Drosophila melanogaster adults in order to investigate mortality patterns and induced physiological responses after a mild thermal stress. A brief, nonlethal heat treatment extends adult life span at normal temperatures by an average of 2 days (64), compared to nontreated controls of the same genotypes. Life expectancy is extended as a demographic consequence of reduced age-specific mortality over a period of up to several weeks after the heat treatment. Heat treatment also increases tolerance to subsequent, more severe thermal stress. Observations on single-sex populations suggest that heat-induced longevity extension is independent of the suppression of reproductive activity.

Age-specific mortality costs of exposure to inbred Drosophila melanogaster in relation to longevity selection.

This article address the hypotheses that selection for early- or late-life fitness changes patterns of reproductive behavior, that this behavior may be dependent on the genetic makeup of the females, and that patterns of male mortality are strongly dependent on the type of females to which they are exposed. Flies selected for late-life reproduction and their associated stocks selected for early reproduction were exposed to flies of the opposite sex from either the same stock or a highly inbred stock. Males of both long- and short-lived stocks showed an increase in early mortality when exposed to inbred females. In addition, when males were exposed to inbred females early in life they showed a lower age-specific mortality rate late in life than males exposed to females from their own stock. Interestingly, females exposed to inbred males showed a significant reduction in mean longevity. Analysis of age-specific mortality revealed that this reduction was brought about as a result of increased early mortality. Interpretation of the results from an analysis of mean longevity not only fails to identify important information--as shown from a demographic analysis of age-specific mortality--but also presents a misleading description of mortality costs.

Selection for increased longevity in Drosophila melanogaster: a response to Baret and Lints.

Baret and Lints [Gerontology 1993;39:252-259] have questioned the interpretation of artificial selection experiments for increased longevity in Drosophila. They suggest that such experiments cannot demonstrate the genetic determination of longevity, because line differences in mean longevity are confounded with erratic temporal variations in life span. Using 15,000 flies from selected and control lines developed by Luckinbill and Clare [Heredity 1985;55:9-19], we show here that when lines are tested simultaneously in a carefully controlled environment, they exhibit markedly different average life spans: selected males live 20 days longer than controls, and selected females live 10 days longer. These and other observations leave no doubt about the existence of heritable variation influencing longevity in Drosophila.

Genetic variation and aging.

Life span is subject to genetic modification in yeasts, nematodes, fruit flies, mice, humans, and other vertebrates and invertebrates. There are a few single-gene mutants known that extend life span in yeast and nematodes; in other experimental systems the character is treated quantitatively, and generally has a low to moderate heritability. Life span responds to artificial selection in Drosophila and Caenorhabditis. There are many candidate genes presently under investigation, including the anti-oxidizing enzymes and heat-shock proteins. The main evolutionary models of senescence are antagonistic pleiotropy and mutation accumulation, neither of which has substantial experimental support. The incorporation of analytical techniques from demography is playing an increasing role in research on aging.