Mitochondrial changes in ageing Caenorhabditis elegans--what do we learn from superoxide dismutase knockouts?

Making heads or tails of mitochondrial membranes in longevity and aging: a role for comparative studies

Mitochondria play vital roles in metabolic energy transduction, intermediate molecule metabolism, metal ion homeostasis, programmed cell death and regulation of the production of reactive oxygen species. As a result of their broad range of functions, mitochondria have been strongly implicated in aging and longevity. Numerous studies show that aging and decreased lifespan are also associated with high reactive oxygen species production by mitochondria, increased mitochondrial DNA and protein damage, and with changes in the fatty acid composition of mitochondrial membranes. It is possible that the extent of fatty acid unsaturation of the mitochondrial membrane determines susceptibility to lipid oxidative damage and downstream protein and genome toxicity, thereby acting as a determinant of aging and lifespan. Reviewing the vast number of comparative studies on mitochondrial membrane composition, metabolism and lifespan reveals some evidence that lipid unsaturation ratios may correlate with lifespan. However, we caution against simply relating these two traits. They may be correlative but have no functional relation. We discuss an important methodology for body mass and phylogenetic correction in comparative studies.

Influence of oral supplementation with sesamin on longevity of Caenorhabditis elegans and the host defense

PURPOSE

Nutritional control has been proposed as a potential therapy for slowing the senescence of immune function and decreasing mortality. This study investigated whether sesamin could modify host defense systems and extend the lifespan of the nematode Caenorhabditis elegans.

METHODS

Nematodes were fed standard food (the bacterium Escherichia coli strain OP50) supplemented with various doses of sesamin/γ-cyclodextrin inclusion compounds starting from young adulthood. The mean lifespan, muscle function, lipofuscin accumulation, protein carbonyl content, and stress resistance of the worms were examined. Then, C. elegans mutants harboring loss-of-function lesions in longevity- and host defense-related signaling pathways were supplemented with sesamin to identify the genes involved in the longevity effects.

RESULTS

Worms supplemented with sesamin displayed higher locomotion and prolongevity and produced offspring at levels similar to unsupplemented control animals. The growth curves of nematodes were similar to those of controls, suggesting that sesamin did not induce prolongevity effects through dietary restriction. Notably, sesamin made the worms more resistant to infection by Legionella pneumophila and more resistant to oxidative stressors such as paraquat and hydrogen peroxide and prolonged the lifespan of a mev-1 mutant that produces abundant superoxide anions. However, the accumulation of protein carbonyls and lipofuscin was similar in sesamin-exposed and control worms, suggesting that sesamin is unlikely to work simply as an antioxidant. Sesamin supplementation failed to extend the lifespan of loss-of-function mutants of daf-2, daf-16, pmk-1, and skn-1.

CONCLUSIONS

Sesamin enhances the host defense of C. elegans and increases the average lifespan via activation of both skn-1 (encoding a component of the p38 MAPK pathway) and daf-16 (encoding a component of the IGF-1 pathway).

Effects of lithium on age-related decline in mitochondrial turnover and function in Caenorhabditis elegans

Aging has been associated with the accumulation of damages in molecules and organelles in cells, particularly mitochondria. The rate of damage accumulation is closely tied to the turnover of the affected cellular components. Perturbing mitochondrial turnover has been shown to significantly affect the rate of deterioration of mitochondrial function with age and to alter lifespan of model organisms. In this study, we investigated the effects of upregulating autophagy using lithium in Caenorhabditis elegans. We found that lithium treatment increased both the lifespan and healthspan of C. elegans without any significant change in the mortality rate and oxidative damages to proteins. The increase in healthspan was accompanied by improved mitochondrial energetic function. In contrast, mitochondrial DNA copy number decreased faster with age under lithium. To better understand the interactions among mitochondrial turnover, damage, and function, we created a mathematical model that described the dynamics of functional and dysfunctional mitochondria population. The combined analysis of model and experimental observations showed how preferential (selective) autophagy of dysfunctional mitochondria could lead to better mitochondrial functionality with age, despite a lower population size. However, the results of model analysis suggest that the benefit of increasing autophagy for mitochondrial function is expected to diminish at higher levels of upregulation due to a shrinking mitochondrial population.

Manganese superoxide dismutase in Meloidogyne incognita isolates selected for virulence on Mi-1-carrying tomato: gene expression and enzyme activity

Root knot nematodes (Meloidogyne spp.) are important pests of a wide range of crops, including tomato. Resistance of tomato to root-knot nematodes is conferred by the single dominant gene Mi-1, which currently is present in all commercially available resistant tomato cultivars. However, several resistance-breaking populations are being collected worldwide. Two isolates coming from the same standard population of Meloidogyne incognita, one selected for virulence against Mi-1 (SM1) and the other left avirulent (avr1), have been used in this study. qRT-PCR was used to detect transcript levels of the manganese superoxide dismutase (Mn-SOD) gene from (a)virulent pre-parasitic second-stage juveniles (J2). Over-expression of the Mn-SOD gene was found in the virulent isolate compared with the avirulent counterpart. The enzyme activity of membrane-bound mitochondrial Mn-SOD was assayed in J2 and adult females as the fraction of total SOD activity insensitive to hydrogen peroxide (H2O2). J2 from SM1 showed about a two-fold higher enzyme activity than J2 from avr1; conversely, no difference was found when adult females were tested. Proteins of J2 extracts were separated by n-PAGE on special mini-gels and stained for SOD. One slow migrating and three fast migrating bands were stained. SOD activity of the slow migrating band was H2O2-insensitive and enriched by treatment with the detergent Triton X-100. J2 survival was monitored in suspensions provided or not provided with the cell oxygen radical generator paraquat, at high concentration. Virulent J2 responded to paraquat treatment by increasing life extension with respect to control conditions; by contrast, avirulent J2 suffered major mortality in the presence of paraquat. Mn-SOD gene expression is discussed in relation to nematode fitness in oxidative stresses and vir phenotype.

Maintenance of cellular ATP level by caloric restriction correlates chronological survival of budding yeast

The free radical theory of aging emphasizes cumulative oxidative damage in the genome and intracellular proteins due to reactive oxygen species (ROS), which is a major cause for aging. Caloric restriction (CR) has been known as a representative treatment that prevents aging; however, its mechanism of action remains elusive. Here, we show that CR extends the chronological lifespan (CLS) of budding yeast by maintaining cellular energy levels. CR reduced the generation of total ROS and mitochondrial superoxide; however, CR did not reduce the oxidative damage in proteins and DNA. Subsequently, calorie-restricted yeast had higher mitochondrial membrane potential (MMP), and it sustained consistent ATP levels during the process of chronological aging. Our results suggest that CR extends the survival of the chronologically aged cells by improving the efficiency of energy metabolism for the maintenance of the ATP level rather than reducing the global oxidative damage of proteins and DNA.

The antioxidant paradox: less paradoxical now?

The term 'antioxidant paradox' is often used to refer to the observation that oxygen radicals and other reactive oxygen species are involved in several human diseases, but giving large doses of dietary antioxidant supplements to human subjects has, in most studies, demonstrated little or no preventative or therapeutic effect. Why should this be? First, the role of reactive oxygen species in the origin and/or progression of most human diseases is unclear, although they are probably important in cancer, neurodegenerative diseases and perhaps some others. Second, the endogenous antioxidant defences in the human body are complex, interlocking and carefully regulated. The body's 'total antioxidant capacity' seems unresponsive to high doses of dietary antioxidants, so that the amount of oxidative damage to key biomolecules is rarely changed. Indeed, manipulation of endogenous antioxidant levels (e.g. by supplying weak pro-oxidants) may be a more useful approach to treatment and prevention of diseases in which reactive oxygen species are important than is consumption of large doses of dietary antioxidants.

Senescence-associated superoxide dismutase influences mitochondrial gene expression in budding tunicates

A recent study has shown that in the budding tunicate Polyandrocarpa misakiensis, the mitochondrial respiratory chain (MRC) dramatically attenuates the gene activity during senescence. In this study, we examined the possible involvement of superoxide dismutase (SOD) in the attenuation of gene expression of cytochrome c oxidase subunit 1 (COX1) in aged zooids. By RT-PCR and in situ hybridization, Cu/Zn-SOD (SOD1) was found to be expressed in most cells and tissues of buds and juvenile zooids but showed a conspicuous decline in senescent adult zooids, except in the gonad tissue in which the cytoplasm of juvenile oocytes was stained heavily. This expression pattern of SOD1 was similar to that of COX1. In contrast to SOD1, Mn-SOD (SOD2) was expressed constitutively in both somatic and germline tissues of buds, juvenile zooids, and senescent adult zooids. Knockdown of SOD1 by RNAi diminished the gene activity of not only SOD1 but also of COX1. The resultant zooids had transient deficiencies in growth and budding, and they recovered from these deficiencies approximately 1 month later. Our results indicate that in P. misakiensis, SOD1 is a senescence-associated nuclear gene and that the experimental decline in SOD1 gene expression accompanies the attenuation of MRC gene activity. Although it is uncertain how SOD1 is downregulated during tunicate senescence, the decreased SOD1 activity could be one of the main causes of MRC gene attenuation during normal senescence.

Robustness and aging--a systems-level perspective

The theory of robustness describes a system level property of evolutionary systems, which predicts tradeoffs of great interest for the systems biology of aging, such as accumulation of non-heritable damage, occurrence of fragilities and limitations in performance, optimized allocation of restricted resources and confined redundancies. According to the robustness paradigm cells and organisms evolved into a state of highly optimized tolerance (HOT), which provides robustness to common perturbations, but causes tradeoffs generally characterized as "robust yet fragile". This raises the question whether the ultimate cause of aging is more than a lack of adaptation, but an inherent fragility of complex evolutionary systems. Since robustness connects to evolutionary designs, consideration of this theory provides a deeper connection between evolutionary aspects of aging, mathematical models and experimental data. In this review several mechanisms influential for aging are re-evaluated in support of robustness tradeoffs. This includes asymmetric cell division improving performance and specialization with limited capacities to prevent and repair age-related damage, as well as feedback control mechanisms optimized to respond to acute stressors, but unable to halt nor revert aging. Improvement in robustness by increasing efficiencies through cellular redundancies in larger organisms alleviates some of the damaging effects of cellular specialization, which can be expressed in allometric relationships. The introduction of the robustness paradigm offers unique insights for aging research and provides novel opportunities for systems biology endeavors.

Mitochondria-targeted antioxidants and metabolic modulators as pharmacological interventions to slow ageing

Populations in many nations today are rapidly ageing. This unprecedented demographic change represents one of the main challenges of our time. A defining property of the ageing process is a marked increase in the risk of mortality and morbidity with age. The incidence of cancer, cardiovascular and neurodegenerative diseases increases non-linearly, sometimes exponentially with age. One of the most important tasks in biogerontology is to develop interventions leading to an increase in healthy lifespan (health span), and a better understanding of basic mechanisms underlying the ageing process itself may lead to interventions able to delay or prevent many or even all age-dependent conditions. One of the putative basic mechanisms of ageing is age-dependent mitochondrial deterioration, closely associated with damage mediated by reactive oxygen species (ROS). Given the central role that mitochondria and mitochondrial dysfunction play not only in ageing but also in apoptosis, cancer, neurodegeneration and other age-related diseases there is great interest in approaches to protect mitochondria from ROS-mediated damage. In this review, we explore strategies of targeting mitochondria to reduce mitochondrial oxidative damage with the aim of preventing or delaying age-dependent decline in mitochondrial function and some of the resulting pathologies. We discuss mitochondria-targeted and -localized antioxidants (e.g.: MitoQ, SkQ, ergothioneine), mitochondrial metabolic modulators (e.g. dichloroacetic acid), and uncouplers (e.g.: uncoupling proteins, dinitrophenol) as well as some alternative future approaches for targeting compounds to the mitochondria, including advances from nanotechnology.

ROS in aging Caenorhabditis elegans: damage or signaling?

Many insights into the mechanisms and signaling pathways underlying aging have resulted from research on the nematode Caenorhabditis elegans. In this paper, we discuss the recent findings that emerged using this model organism concerning the role of reactive oxygen species (ROS) in the aging process. The accrual of oxidative stress and damage has been the predominant mechanistic explanation for the process of aging for many years, but reviewing the recent studies in C. elegans calls this theory into question. Thus, it becomes more and more evident that ROS are not merely toxic byproducts of the oxidative metabolism. Rather it seems more likely that tightly controlled concentrations of ROS and fluctuations in redox potential are important mediators of signaling processes. We therefore discuss some theories that explain how redox signaling may be involved in aging and provide some examples of ROS functions and signaling in C. elegans metabolism. To understand the role of ROS and the redox status in physiology, stress response, development, and aging, there is a rising need for accurate and reversible in vivo detection. Therefore, we comment on some methods of ROS and redox detection with emphasis on the implementation of genetically encoded biosensors in C. elegans.

Function and Regulation of Lipid Biology in Caenorhabditis elegans Aging

Rapidly expanding aging populations and a concomitant increase in the prevalence of age-related diseases are global health problems today. Over the past three decades, a large body of work has led to the identification of genes and regulatory networks that affect longevity and health span, often benefiting from the tremendous power of genetics in vertebrate and invertebrate model organisms. Interestingly, many of these factors appear linked to lipids, important molecules that participate in cellular signaling, energy metabolism, and structural compartmentalization. Despite the putative link between lipids and longevity, the role of lipids in aging remains poorly understood. Emerging data from the model organism Caenorhabditis elegans suggest that lipid composition may change during aging, as several pathways that influence aging also regulate lipid metabolism enzymes; moreover, some of these enzymes apparently play key roles in the pathways that affect the rate of aging. By understanding how lipid biology is regulated during C. elegans aging, and how it impacts molecular, cellular, and organismal function, we may gain insight into novel ways to delay aging using genetic or pharmacological interventions. In the present review we discuss recent insights into the roles of lipids in C. elegans aging, including regulatory roles played by lipids themselves, the regulation of lipid metabolic enzymes, and the roles of lipid metabolism genes in the pathways that affect aging.

The redox stress hypothesis of aging

The main objective of this review is to examine the role of endogenous reactive oxygen/nitrogen species (ROS) in the aging process. Until relatively recently, ROS were considered to be potentially toxic by-products of aerobic metabolism, which, if not eliminated, may inflict structural damage on various macromolecules. Accrual of such damage over time was postulated to be responsible for the physiological deterioration in the postreproductive phase of life and eventually the death of the organism. This "structural damage-based oxidative stress" hypothesis has received support from the age-associated increases in the rate of ROS production and the steady-state amounts of oxidized macromolecules; however, there are increasing indications that structural damage alone is insufficient to satisfactorily explain the age-associated functional losses. The level of oxidative damage accrued during aging often does not match the magnitude of functional losses. Although experimental augmentation of antioxidant defenses tends to enhance resistance to induced oxidative stress, such manipulations are generally ineffective in the extension of life span of long-lived strains of animals. More recently, in a major conceptual shift, ROS have been found to be physiologically vital for signal transduction, gene regulation, and redox regulation, among others, implying that their complete elimination would be harmful. An alternative notion, advocated here, termed the "redox stress hypothesis," proposes that aging-associated functional losses are primarily caused by a progressive pro-oxidizing shift in the redox state of the cells, which leads to the overoxidation of redox-sensitive protein thiols and the consequent disruption of the redox-regulated signaling mechanisms.