Mitochondria and aging: dilution is the solution

NFYB-1 regulates mitochondrial function and longevity via lysosomal prosaposin

Mitochondria are multidimensional organelles whose activities are essential to cellular vitality and organismal longevity, yet underlying regulatory mechanisms spanning these different levels of organization remain elusive^1-5. Here we show that Caenorhabditis elegans nuclear transcription factor Y, beta subunit (NFYB-1), a subunit of the NF-Y transcriptional complex^6-8, is a crucial regulator of mitochondrial function. Identified in RNA interference (RNAi) screens, NFYB-1 loss leads to perturbed mitochondrial gene expression, reduced oxygen consumption, mitochondrial fragmentation, disruption of mitochondrial stress pathways, decreased mitochondrial cardiolipin levels and abolition of organismal longevity triggered by mitochondrial impairment. Multi-omics analysis reveals that NFYB-1 is a potent repressor of lysosomal prosaposin, a regulator of glycosphingolipid metabolism. Limiting prosaposin expression unexpectedly restores cardiolipin production, mitochondrial function and longevity in the nfyb-1 background. Similarly, cardiolipin supplementation rescues nfyb-1 phenotypes. These findings suggest that the NFYB-1-prosaposin axis coordinates lysosomal to mitochondria signalling via lipid pools to enhance cellular mitochondrial function and organismal health.

Genetic inhibition of an ATP synthase subunit extends lifespan in C. elegans

Mild inhibition of mitochondrial respiration leads to longevity. Disruption of mitochondrial respiratory components extends lifespan in Caenorhabditis elegans, but the effects appear to be complex and the underlying mechanism for lifespan regulation by mitochondrial respiratory genes is still not fully understood. Here, we investigated the role of Y82E9BR.3, a worm homolog of the ATP synthase subunit C, in modulating longevity in C. elegans. We found that the Y82E9BR.3 protein is localized in mitochondria and expressed in various tissues throughout development. RNAi knockdown of Y82E9BR.3 extends lifespan, decreases the accumulation of lipofuscin, and affects various physiological processes, including development delay, reproduction impairment and slow behavior. Further tissue-specific RNAi analysis showed that the intestine is a crucial organ for the longevity effects conferred by Y82E9BR.3 RNAi. Moreover, we demonstrated that lifespan extension by Y82E9BR.3 RNAi is associated with reduced mitochondrial function, as well as the suppression of complex I activity in mitochondria. Unexpectedly, Y82E9BR.3 RNAi knock down did not influence the whole-worm ATP level. Our findings first reveal the crucial role of Y82E9BR.3 in mitochondrial function and the underlying mechanism of how Y82E9BR.3 regulates lifespan in C. elegans.

Biphasic Modeling of Mitochondrial Metabolism Dysregulation during Aging

Organismal aging is classically viewed as a gradual decline of cellular functions and a systemic deterioration of tissues that leads to an increased mortality rate in older individuals. According to the prevailing theory, aging is accompanied by a continuous and progressive decline in mitochondrial metabolic activity in cells. However, the most robust approaches to extending healthy lifespan are frequently linked with reduced energy intake or with lowering of mitochondrial activity. While these observations appear contradictory, recent work and technological advances demonstrate that metabolic deregulation during aging is potentially biphasic. In this Opinion we propose a novel framework where middle-age is accompanied by increased mitochondrial activity that subsequently declines at advanced ages.

Mitochondrial responsibility in ageing process: innocent, suspect or guilty

Ageing is accompanied by the accumulation of damaged molecules in cells due to the injury produced by external and internal stressors. Among them, reactive oxygen species produced by cell metabolism, inflammation or other enzymatic processes are considered key factors. However, later research has demonstrated that a general mitochondrial dysfunction affecting electron transport chain activity, mitochondrial biogenesis and turnover, apoptosis, etc., seems to be in a central position to explain ageing. This key role is based on several effects from mitochondrial-derived ROS production to the essential maintenance of balanced metabolic activities in old organisms. Several studies have demonstrated caloric restriction, exercise or bioactive compounds mainly found in plants, are able to affect the activity and turnover of mitochondria by increasing biogenesis and mitophagy, especially in postmitotic tissues. Then, it seems that mitochondria are in the centre of metabolic procedures to be modified to lengthen life- or health-span. In this review we show the importance of mitochondria to explain the ageing process in different models or organisms (e.g. yeast, worm, fruitfly and mice). We discuss if the cause of aging is dependent on mitochondrial dysfunction of if the mitochondrial changes observed with age are a consequence of events taking place outside the mitochondrial compartment.

Reduced IGF signaling prevents muscle cell death in a Caenorhabditis elegans model of muscular dystrophy

Duchenne muscular dystrophy, a fatal degenerative muscle disease, is caused by mutations in the dystrophin gene. Loss of dystrophin in the muscle cell membrane causes muscle fiber necrosis. Previously, loss-of-function mutations in dys-1, the Caenorhabditis elegans dystrophin ortholog, were shown to cause a contractile defect and mild fiber degeneration in striated body wall muscle. Here, we show that loss of dystrophin function in C. elegans results in a shorter lifespan and stochastic, age-dependent muscle-cell death. Reduction of dystrophin function also accelerated age-dependent protein aggregation in muscle cells, suggesting a defect in proteostasis. Both muscle cell death and protein aggregation showed wide variability among the muscle cells. These observations suggest that muscle cell death in dys-1 mutants is greatly influenced by cellular environments. Thus, the manipulation of the cellular environment may provide an opportunity to thwart the cell death initiated by the loss of dystrophin. We found that reduced insulin-like growth factor (IGF) signaling, which rejuvenates the cellular environment to protect cells from a variety of age-dependent pathologies, prevented muscle cell death in the dys-1 mutants in a daf-16-dependent manner. Our study suggests that manipulation of the IGF signaling pathways in muscle cells could be a potent intervention for muscular dystrophy.

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.

A mitochondrial superoxide signal triggers increased longevity in Caenorhabditis elegans

The study of long-lived C. elegans mutants suggests that mitochondrial oxidants can actually help reduce aging by acting as stress signals, rather than acting solely as toxic molecules.

The nuo-6 and isp-1 genes of C. elegans encode, respectively, subunits of complex I and III of the mitochondrial respiratory chain. Partial loss-of-function mutations in these genes decrease electron transport and greatly increase the longevity of C. elegans by a mechanism that is distinct from that induced by reducing their level of expression by RNAi. Electron transport is a major source of the superoxide anion (O^⋅–), which in turn generates several types of toxic reactive oxygen species (ROS), and aging is accompanied by increased oxidative stress, which is an imbalance between the generation and detoxification of ROS. These observations have suggested that the longevity of such mitochondrial mutants might result from a reduction in ROS generation, which would be consistent with the mitochondrial oxidative stress theory of aging. It is difficult to measure ROS directly in living animals, and this has held back progress in determining their function in aging. Here we have adapted a technique of flow cytometry to directly measure ROS levels in isolated mitochondria to show that the generation of superoxide is elevated in the nuo-6 and isp-1 mitochondrial mutants, although overall ROS levels are not, and oxidative stress is low. Furthermore, we show that this elevation is necessary and sufficient to increase longevity, as it is abolished by the antioxidants NAC and vitamin C, and phenocopied by mild treatment with the prooxidant paraquat. Furthermore, the absence of effect of NAC and the additivity of the effect of paraquat on a variety of long- and short-lived mutants suggest that the pathway triggered by mitochondrial superoxide is distinct from previously studied mechanisms, including insulin signaling, dietary restriction, ubiquinone deficiency, the hypoxic response, and hormesis. These findings are not consistent with the mitochondrial oxidative stress theory of aging. Instead they show that increased superoxide generation acts as a signal in young mutant animals to trigger changes of gene expression that prevent or attenuate the effects of subsequent aging. We propose that superoxide is generated as a protective signal in response to molecular damage sustained during wild-type aging as well. This model provides a new explanation for the well-documented correlation between ROS and the aged phenotype as a gradual increase of molecular damage during aging would trigger a gradually stronger ROS response.

An unequivocal demonstration that mitochondria are important for lifespan comes from studies with the nematode Caenorhabditis elegans. Mutations in mitochondrial proteins such as ISP-1 and NUO-6, which function directly in mitochondrial electron transport, lead to a dramatic increase in the lifespan of this organism. One theory proposes that toxicity of mitochondrial reactive oxygen species (ROS) is the cause of aging and predicts that the generation of the ROS superoxide should be low in these mutants. Here we have measured superoxide generation in these mutants and found that it is in fact elevated, rather than reduced. Furthermore, we found that this elevation is necessary and sufficient for longevity, as it is abolished by antioxidants and induced by mild treatment with oxidants. This suggests that superoxide can act as a signal triggering cellular changes that attenuate the effects of aging. This idea suggests a new model for the well-documented correlation between ROS and the aged phenotype. We propose that a gradual increase of molecular damage during aging triggers a concurrent, gradually intensifying, protective superoxide response.