The evolution of postreproductive life span as an insurance against indeterminacy

Postreproductive life span remains a puzzle for evolutionary biologists. The explanation of increased inclusive fitness through parental care after reproduction that applies for humans is unrealistic for many species. We propose a new selective mechanism, independent of parental care, which relies on the hypothesis that postreproductive life span can evolve as an insurance against indeterminacy: longer life expectancy reduces the risk of dying by chance before the cessation of reproductive activity. We demonstrate numerically that the duration of evolved postreproductive life span is indeed expected to increase with variability in life span duration. An unprecedented assay of 11 strains of the collembola Folsomia candida shows the existence of (1) postreproductive life span in the absence of parental care; (2) genetic variability in mean postreproductive life span and postreproductive life span variability itself; (3) strong genetic correlation between latter traits. This new explanation brings along the novel idea that loose canalization of a trait (here, somatic life span) can itself act as a selective pressure on other traits.     

Increased life span due to calorie restriction in respiratory-deficient yeast

A model for replicative life span extension by calorie restriction (CR) in yeast has been proposed whereby reduced glucose in the growth medium leads to activation of the NAD⁺–dependent histone deacetylase Sir2. One mechanism proposed for this putative activation of Sir2 is that CR enhances the rate of respiration, in turn leading to altered levels of NAD⁺ or NADH, and ultimately resulting in enhanced Sir2 activity. An alternative mechanism has been proposed in which CR decreases levels of the Sir2 inhibitor nicotinamide through increased expression of the gene coding for nicotinamidase, PNC1. We have previously reported that life span extension by CR is not dependent on Sir2 in the long-lived BY4742 strain background. Here we have determined the requirement for respiration and the effect of nicotinamide levels on life span extension by CR. We find that CR confers robust life span extension in respiratory-deficient cells independent of strain background, and moreover, suppresses the premature mortality associated with loss of mitochondrial DNA in the short-lived PSY316 strain. Addition of nicotinamide to the medium dramatically shortens the life span of wild type cells, due to inhibition of Sir2. However, even in cells lacking both Sir2 and the replication fork block protein Fob1, nicotinamide partially prevents life span extension by CR. These findings (1) demonstrate that respiration is not required for the longevity benefits of CR in yeast, (2) show that nicotinamide inhibits life span extension by CR through a Sir2-independent mechanism, and (3) suggest that CR acts through a conserved, Sir2-independent mechanism in both PSY316 and BY4742.     

Relief of oxidative stress by ascorbic acid delays cellular senescence of normal human and Werner syndrome fibroblast cells

Primary human cells have a definite life span and enter into cellular senescence before ceasing cell growth. Oxidative stress produced by aerobic metabolism has been shown to accelerate cellular senescence. Here, we demonstrated that ascorbic acid, used as an antioxygenic reagent, delayed cellular senescence in a continuous culture of normal human embryonic cells, human adult skin fibroblast cells, and Werner syndrome (WS) cells. The results using human embryonic cells showed that treatment with ascorbic acid phospholic ester magnesium salt (APM) decreased the level of oxidative stress, and extended the replicative life span. The effect of APM to extend the replicative life span was also shown in normal human adult cells and WS cells. To understand the mechanism of extension of cellular life span, we determined the telomere lengths of human embryonic cells, both with and without APM treatment, and demonstrated that APM treatment reduced the rate of telomere shortening. The present results indicate that constitutive oxidative stress plays a role in determining the replicative life span and that suppression of oxidative stress by an antioxidative agent, APM, extends the replicative life span by reducing the rate of telomere shortening.     

High osmolarity extends life span in Saccharomyces cerevisiae by a mechanism related to calorie restriction

Calorie restriction (CR) extends life span in many different organisms, including mammals. We describe here a novel pathway that extends the life span of Saccharomyces cerevisiae mother cells but does not involve a reduction in caloric content of the media, i.e., there is growth of yeast cells in the presence of a high concentration of external osmolytes. Like CR, this longevity-promoting response to high osmolarity requires SIR2, suggesting a common mechanism of life span regulation. Genetic and microarray analysis indicates that high osmolarity extends the life span by activating Hog1p, leading to an increase in the biosynthesis of glycerol from glycolytic intermediates. This metabolic shift likely increases NAD levels, thereby activating Sir2p and promoting longevity.     

Autophagy-mediated longevity is modulated by lipoprotein biogenesis

Autophagy-dependent longevity models in C. elegans display altered lipid storage profiles, but the contribution of lipid distribution to life-span extension is not fully understood. Here we report that lipoprotein production, autophagy and lysosomal lipolysis are linked to modulate life span in a conserved fashion. We find that overexpression of the yolk lipoprotein VIT/vitellogenin reduces the life span of long-lived animals by impairing the induction of autophagy-related and lysosomal genes necessary for longevity. Accordingly, reducing vitellogenesis increases life span via induction of autophagy and lysosomal lipolysis. Life-span extension due to reduced vitellogenesis or enhanced lysosomal lipolysis requires nuclear hormone receptors (NHRs) NHR-49 and NHR-80, highlighting novel roles for these NHRs in lysosomal lipid signaling. In dietary-restricted worms and mice, expression of VIT and hepatic APOB (apolipoprotein B), respectively, are significantly reduced, suggesting a conserved longevity mechanism. Altogether, our study demonstrates that lipoprotein biogenesis is an important mechanism that modulates aging by impairing autophagy and lysosomal lipolysis.     

Control of the replicative life span of human fibroblasts by p16 and the polycomb protein Bmi-1

The polycomb protein Bmi-1 represses the INK4a locus, which encodes the tumor suppressors p16 and p14(ARF). Here we report that Bmi-1 is downregulated when WI-38 human fibroblasts undergo replicative senescence, but not quiescence, and extends replicative life span when overexpressed. Life span extension by Bmi-1 required the pRb, but not p53, tumor suppressor protein. Deletion analysis showed that the RING finger and helix-turn-helix domains of Bmi-1 were required for life span extension and suppression of p16. Furthermore, a RING finger deletion mutant exhibited dominant negative activity, inducing p16 and premature senescence. Interestingly, presenescent cultures of some, but not all, human fibroblasts contained growth-arrested cells expressing high levels of p16 and apparently arrested by a p53- and telomere-independent mechanism. Bmi-1 selectively extended the life span of these cultures. Low O(2) concentrations had no effect on p16 levels or life span extension by Bmi-1 but reduced expression of the p53 target, p21. We propose that some human fibroblast strains are more sensitive to stress-induced senescence and have both p16-dependent and p53/telomere-dependent pathways of senescence. Our data suggest that Bmi-1 extends the replicative life span of human fibroblasts by suppressing the p16-dependent senescence pathway.     

A molecular mechanism of chronological aging in yeast

The molecular mechanisms that cause organismal aging are a topic of intense scrutiny and debate. Dietary restriction extends the life span of many organisms, including yeast, and efforts are underway to understand the biochemical and genetic pathways that regulate this life span extension in model organisms. Here we describe the mechanism by which dietary restriction extends yeast chronological life span, defined as the length of time stationary yeast cells remain viable in a quiescent state. We find that aging under standard culture conditions is the result of a cell-extrinsic component that is linked to the pH of the culture medium. We identify acetic acid as a cell-extrinsic mediator of cell death during chronological aging, and demonstrate that dietary restriction, growth in a non-fermentable carbon source, or transferring cells to water increases chronological life span by reducing or eliminating extracellular acetic acid. Other life span extending environmental and genetic interventions, such as growth in high osmolarity media, deletion of SCH9 or RAS2, increase cellular resistance to acetic acid. We conclude that acetic acid induced morality is the primary mechanism of chronological aging in yeast under standard conditions.     

Variation in endocrine signaling underlies variation in social life history

Variation in endocrine pathways can be a major mechanism underlying life-history evolution. Yet it is unclear whether this insight, derived primarily from solitary species, explains the origins of complex life-history traits in highly social taxa. Thus, we here document and study variation in social life-history syndromes of female fecundity, behavior, and life span in selectively bred honeybee (Apis mellifera) strains. Associated variation in endocrine signaling was uncovered by RNA interference (RNAi) silencing of the juvenile hormone (JH) suppressor gene vitellogenin. High versus low endocrine reactivity in response to vitellogenin knockdown consistently correlated with rapid social behavioral ontogeny and short life span versus slow social behavioral ontogeny and long life span. Variation in JH reactivity, furthermore, was a function of variation in fecundity (ovary size and follicle development). A JH-mediated pleiotropy of female life-history traits, including fecundity, behavior, and life span, characterizes the distantly related solitary insect Drosophila. For the first time, we document a similar regulatory principle in a highly social species where most females are alloparental helpers (workers) that seldom reproduce. We conclude that variation in endocrine pathways of solitary origin can underlie variation and evolvability of complex social life-history traits.     

Long-lived worms and aging

Several investigators have generated long-lived nematode worms (Caenorhabditis elegans) in the past decade by mutation of genes in the organism in order to study the genetics of aging and longevity. Dozens of longevity assurance genes (LAG) that dramatically increase the longevity of this organism have been identified. All long-lived mutants of C. elegans are also resistant to environmental stress, such as high temperature, reactive oxygen species (ROS), and ultraviolet irradiation. Double mutations of some LAGs further extended life span up to 400%, providing more insight into cellular mechanisms that put limits on the life span of organisms. With the availability of the LAG mutants and the combined DNA microarray and RNAi technology, the understanding of actual biochemical processes that determine life span is within reach: the downstream signal transduction pathway may regulate life span by up-regulating pro-longevity genes such as those that encode antioxidant enzymes and/or stress-response proteins, and down-regulating specific life-shortening genes. Furthermore, longevity could be modified through chemical manipulation. Results from these studies further support the free radical theory of aging, suggest that the molecular mechanism of aging process may be shared in all organisms, and provide insight for therapeutic intervention in age-related diseases.     

Long-lived alphaMUPA transgenic mice exhibit pronounced circadian rhythms

Robust biological rhythms have been shown to affect life span. Biological clocks can be entrained by two feeding regimens, restricted feeding (RF) and caloric restriction (CR). RF restricts the time of food availability, whereas CR restricts the amount of calories with temporal food consumption. CR is known to retard aging and extend life span of animals via yet-unknown pathways. We hypothesize that resetting the biological clock could be one possible mechanism by which CR extends life span. Because it is experimentally difficult to uncouple calorie reduction from temporal food consumption, we took advantage of the murine urokinase-like plasminogen activator (alphaMUPA) transgenic mice overexpressing a serine protease implicated in brain development and plasticity; they exhibit spontaneously reduced eating and increased life span. Quantitative real-time PCR analysis revealed that alphaMUPA mice exhibit robust expression of the clock genes mPer1, mPer2, mClock, and mCry1 but not mBmal1 in the liver. We also found changes in the circadian amplitude and/or phase of clock-controlled output systems, such as feeding behavior, body temperature, and enteric cryptdin expression. A change in the light-dark regimen led to modified clock gene expression and abrogated circadian patterns of food intake in wild-type (WT) and alphaMUPA mice. Consequently, food consumption of WT mice increased, whereas that of alphaMUPA mice remained the same, indicating that reduced food intake occurs upstream and independently of the biological clock. Thus we surmise that CR could lead to pronounced and synchronized biological rhythms. Because the biological clock controls mitochondrial, hormonal, and physiological parameters, system synchronicity could lead to extended life span.     

Juvenile Hormone and Insulin Regulate Trehalose Homeostasis in the Red Flour Beetle,Tribolium castaneum

Insulin/IGF-1 signaling (IIS) has been well studied for its role in the control of life span extension and resistance to a variety of stresses. The Drosophila melanogaster insulin-like receptor (InR) mutant showed extended life span due to reduced juvenile hormone (JH) levels. However, little is known about the mechanism of cross talk between IIS and JH in regulation of life span extension and resistance to starvation. In the current study, we investigated the role of IIS and JH signaling in regulation of resistance to starvation. Reduction in JH biosynthesis, JH action, or insulin-like peptide 2 (ILP2) syntheses by RNA interference (RNAi)-aided knockdown in the expression of genes coding for juvenile hormone acid methyltransferase (JHAMT), methoprene-tolerant (Met), or ILP2 respectively decreased lipid and carbohydrate metabolism and extended the survival of starved beetles. Interestingly, the extension of life span could be restored by injection of bovine insulin into JHAMT RNAi beetles but not by application of JH III to ILP2 RNAi beetles. These data suggest that JH controls starvation resistance by regulating synthesis of ILP2. More importantly, JH regulates trehalose homeostasis, including trehalose transport and metabolism, and controls utilization of stored nutrients in starved adults.     

Mitochondrial electron transport is a key determinant of life span in Caenorhabditis elegans.

Increased protection from reactive oxygen species (ROS) is believed to increase life span. However, it has not been clearly demonstrated that endogenous ROS production actually limits normal life span. We have identified a mutation in the Caenorhabditis elegans iron sulfur protein (isp-1) of mitochondrial complex III, which results in low oxygen consumption, decreased sensitivity to ROS, and increased life span. Furthermore, combining isp-1(qm150) with a mutation (daf-2) that increases resistance to ROS does not result in any significant further increase in adult life span. These findings indicate that both isp-1 and daf-2 mutations increase life span by lowering oxidative stress and result in the maximum life span increase that can be produced in this way.     

Reproduction and longevity: secrets revealed by C. elegans

What is the relationship between reproduction and longevity? Evolutionary biology suggests that reproduction exacts a cost in somatic maintenance, a cost that reduces longevity. The frequent occurrence of this tradeoff between life span and fecundity, both due to experimental manipulations as well as natural variation, suggest that the mechanism might be conserved during evolution. Until recently, little was known about the mechanistic details of how reproduction might regulate life span. Here we discuss recent advances in our understanding of the regulation of life span by reproductive signaling, focusing on studies using Caenorhabditis elegans.     

Mammal aging: Active and passive mechanisms and their medical implications

This article compares two hypotheses regarding the mechanisms responsible for aging in humans and other mammals. In the passive mechanism, aging is the result of inadequacies in maintenance and repair functions that act to prevent or repair damage from fundamental deteriorative processes. In the active mechanism, a life span management system purposely limits life span by deactivating maintenance and repair processes beyond a species-specific age.As described here, the active mechanism provides a much better fit to observational evidence while the passive mechanism provides a much better fit to traditional evolutionary mechanics theory. However, there are many other observations that conflict with traditional theory and consequently a number of alternative evolutionary mechanics theories have been developed since 1962. Several of these alternatives support active life span management and aging theories providing a rationale for active life span management have been developed based on each of those alternatives.This issue is very important to our ability to treat age-related diseases and conditions. If indeed the passive mechanism is correct, then efforts should continue to be exerted to find treatments for each different manifestation of aging, independently of the others. If the active concept is valid it is clear that there are, in addition, substantial opportunities for finding agents that generally delay aging and simultaneously ameliorate multiple manifestations of aging.In the past, the evolutionary issues have been used as essentially the entire justification for summarily rejecting active theories. Given the public health considerations and the increasing number of issues surrounding evolutionary mechanics theory, this is no longer a reasonable or responsible path.     

Enhanced Energy Metabolism Contributes to the Extended Life Span of Calorie-restricted Caenorhabditis elegans

Caloric restriction (CR) markedly extends life span and improves the health of a broad number of species. Energy metabolism fundamentally contributes to the beneficial effects of CR, but the underlying mechanisms that are responsible for this effect remain enigmatic. A multidisciplinary approach that involves quantitative proteomics, immunochemistry, metabolic quantification, and life span analysis was used to determine how CR, which occurs in the Caenorhabditis elegans eat-2 mutants, modifies energy metabolism of the worm, and whether the observed modifications contribute to the CR-mediated physiological responses. A switch to fatty acid metabolism as an energy source and an enhanced rate of energy metabolism by eat-2 mutant nematodes were detected. Life span analyses validated the important role of these previously unknown alterations of energy metabolism in the CR-mediated longevity of nematodes. As observed in mice, the overexpression of the gene for the nematode analog of the cytosolic form of phosphoenolpyruvate carboxykinase caused a marked extension of the life span in C. elegans, presumably by enhancing energy metabolism via an altered rate of cataplerosis of tricarboxylic acid cycle anions. We conclude that an increase, not a decrease in fuel consumption, via an accelerated oxidation of fuels in the TCA cycle is involved in life span regulation; this mechanism may be conserved across phylogeny.     

Identification of a Gene That Shortens Clonal Life Span of Paramecium Tetraurelia

We have isolated a Paramecium tetraurelia mutant that divides slowly in daily reisolation cultures and repeats short clonal life spans after successive autogamies. Here we show, using breeding analysis, that a recessive mutation is responsible for the low fission rate and that this low rate is closely related to the short clonal life span. We conclude that a single pleiotropic gene controls these traits and have named it jumyo. In an attempt to further characterize the jumyo mutant, we have revealed that it has a culture life span similar to that of the wild-type cells and that, when mass cultured, it can divide as rapidly as wild-type cells. There was strong evidence that the mutant cells excreted into culture medium some substance that promotes their cell division. These findings may not only present supporting evidence for the hypothesis that the cellular life span is genetically programmed but also give a material basis for the study of the controlling mechanism of cell division in relation to the clonal life span.