Corrupted ER‐mitochondrial calcium homeostasis promotes the collapse of proteostasis

Aging and age‐related diseases are associated with a decline of protein homeostasis (proteostasis), but the mechanisms underlying this decline are not clear. In particular, decreased proteostasis is a widespread molecular feature of neurodegenerative diseases, such as Alzheimer's disease (AD). Familial AD is largely caused by mutations in the presenilin encoding genes; however, their role in AD is not understood. In this study, we investigate the role of presenilins in proteostasis using the model system Caenorhabditis elegans. Previously, we found that mutations in C. elegans presenilin cause elevated ER to mitochondria calcium signaling, which leads to an increase in mitochondrial generated oxidative stress. This, in turn, promotes neurodegeneration. To understand the cellular mechanisms driving neurodegeneration, using several molecular readouts of protein stability in C. elegans, we find that presenilin mutants have widespread defects in proteostasis. Markedly, we demonstrate that these defects are independent of the protease activity of presenilin and that reduction in ER to mitochondrial calcium signaling can significantly prevent the proteostasis defects observed in presenilin mutants. Furthermore, we show that supplementing presenilin mutants with antioxidants suppresses the proteostasis defects. Our findings indicate that defective ER to mitochondria calcium signaling promotes proteostatic collapse in presenilin mutants by increasing oxidative stress.     

Mitochondrial thioredoxin reductase 2 is elevated in long‐lived primate as well as rodent species and extends fly mean lifespan

In a survey of enzymes related to protein oxidation and cellular redox state, we found activity of the redox enzyme thioredoxin reductase (TXNRD) to be elevated in cells from long‐lived species of rodents, primates, and birds. Elevated TXNRD activity in long‐lived species reflected increases in the mitochondrial form, TXNRD2, rather than the cytosolic forms TXNRD1 and TXNRD3. Analysis of published RNA‐Seq data showed elevated TXNRD2 mRNA in multiple organs of longer‐lived primates, suggesting that the phenomenon is not limited to skin‐derived fibroblasts. Elevation of TXNRD2 activity and protein levels was also noted in liver of three different long‐lived mutant mice, and in normal male mice treated with a drug that extends lifespan in males. Overexpression of mitochondrial TXNRD2 in Drosophila melanogaster extended median (but not maximum) lifespan in female flies with a small lifespan extension in males; in contrast, overexpression of the cytosolic form, TXNRD1, did not produce a lifespan extension.     

Antioxidants reveal an inverted U‐shaped dose‐response relationship between reactive oxygen species levels and the rate of aging in Caenorhabditis elegans

Reactive oxygen species (ROS) are potentially toxic, but they are also signaling molecules that modulate aging. Recent observations that ROS can promote longevity have to be reconciled with the numerous claims about the benefits of antioxidants on lifespan. Here, three antioxidants [N‐acetylcysteine (NAC), vitamin C, and resveratrol (RSV)] were tested on Caenorhabditis elegans mutants that alter drug uptake, mitochondrial function, and ROS metabolism. We observed that like pro‐oxidants, antioxidants can both lengthen and shorten lifespan, dependent on concentration, genotypes, and conditions. The effects of antioxidants thus reveal an inverted U‐shaped dose–response relationship between ROS levels and lifespan. In addition, we observed that RSV can act additively to both NAC and paraquat, to dramatically increase lifespan. This suggests that the effect of compounds that modulate ROS levels can be additive when their loci of action or mechanisms of action are sufficiently distinct.     

Mitochondrial metabolites extend lifespan

Disruption of mitochondrial respiration in the nematode Caenorhabditis elegans can extend lifespan. We previously showed that long‐lived respiratory mutants generate elevated amounts of α‐ketoacids. These compounds are structurally related to α‐ketoglutarate, suggesting they may be biologically relevant. Here, we show that provision of several such metabolites to wild‐type worms is sufficient to extend their life. At least one mode of action is through stabilization of hypoxia‐inducible factor‐1 (HIF‐1). We also find that an α‐ketoglutarate mimetic, 2,4‐pyridinedicarboxylic acid (2,4‐PDA), is alone sufficient to increase the lifespan of wild‐type worms and this effect is blocked by removal of HIF‐1. HIF‐1 is constitutively active in isp‐1(qm150) Mit mutants, and accordingly, 2,4‐PDA does not further increase their lifespan. Incubation of mouse 3T3‐L1 fibroblasts with life‐prolonging α‐ketoacids also results in HIF‐1α stabilization. We propose that metabolites that build up following mitochondrial respiratory dysfunction form a novel mode of cell signaling that acts to regulate lifespan.     

Quantification of in vivo oxidative damage in Caenorhabditis elegans during aging by endogenous F3‐isoprostane measurement

Oxidative damage is thought to be a major cause in development of pathologies and aging. However, quantification of oxidative damage is methodologically difficult. Here, we present a robust liquid chromatography–tandem mass spectrometry (LC‐MS/MS) approach for accurate, sensitive, and linear in vivo quantification of endogenous oxidative damage in the nematode Caenorhabditis elegans, based on F3‐isoprostanes. F3‐isoprostanes are prostaglandin‐like markers of oxidative damage derived from lipid peroxidation by Reactive Oxygen Species (ROS). Oxidative damage was quantified in whole animals and in multiple cellular compartments, including mitochondria and peroxisomes. Mutants of the mitochondrial electron transport proteins mev‐1 and clk‐1 showed increased oxidative damage levels. Furthermore, analysis of Superoxide Dismutase (sod) and Catalase (ctl) mutants uncovered that oxidative damage levels cannot be inferred from the phenotype of resistance to pro‐oxidants alone and revealed high oxidative damage in a small group of chemosensory neurons. Longitudinal analysis of aging nematodes revealed that oxidative damage increased specifically with postreproductive age. Remarkably, aging of the stress‐resistant and long‐lived daf‐2 insulin/IGF‐1 receptor mutant involved distinct daf‐16‐dependent phases of oxidative damage including a temporal increase at young adulthood. These observations are consistent with a hormetic response to ROS.     

A novel kinase regulates dietary restriction‐mediated longevity in Caenorhabditis elegans

Although dietary restriction (DR) is known to extend lifespan across species, from yeast to mammals, the signalling events downstream of food/nutrient perception are not well understood. In Caenorhabditis elegans, DR is typically attained either by using the eat‐2 mutants that have reduced pharyngeal pumping leading to lower food intake or by feeding diluted bacterial food to the worms. In this study, we show that knocking down a mammalian MEKK3‐like kinase gene, mekk‐3 in C. elegans, initiates a process similar to DR without compromising food intake. This DR‐like state results in upregulation of beta‐oxidation genes through the nuclear hormone receptor NHR‐49, a HNF‐4 homolog, resulting in depletion of stored fat. This metabolic shift leads to low levels of reactive oxygen species (ROS), potent oxidizing agents that damage macromolecules. Increased beta‐oxidation, in turn, induces the phase I and II xenobiotic detoxification genes, through PHA‐4/FOXA, NHR‐8 and aryl hydrocarbon receptor AHR‐1, possibly to purge lipophilic endotoxins generated during fatty acid catabolism. The coupling of a metabolic shift with endotoxin detoxification results in extreme longevity following mekk‐3 knock‐down. Thus, MEKK‐3 may function as an important nutrient sensor and signalling component within the organism that controls metabolism. Knocking down mekk‐3 may signal an imminent nutrient crisis that results in initiation of a DR‐like state, even when food is plentiful.     

Mitochondrial dysfunction, oxidative stress, regulation of exocytosis and their relevance to neurodegenerative diseases

A common feature in the early stages of many neurodegenerative diseases lies in mitochondrial dysfunction, oxidative stress, and reduced levels of synaptic transmission. Many genes associated with neurodegenerative diseases are now known to regulate either mitochondrial function, redox state, or the exocytosis of neurotransmitters. Mitochondria are the primary source of reactive oxygen species and ATP and control apoptosis. Mitochondria are concentrated in synapses and significant alterations to synaptic mitochondrial localization, number, morphology, or function can be detrimental to synaptic transmission. Mitochondrial by-products are capable of regulating various steps of neurotransmission and mitochondrial dysfunction and oxidative stress occur in the early stages of many neurodegenerative diseases. This mini-review will highlight the prospect that mitochondria regulates synaptic exocytosis by controlling synaptic ATP and reactive oxygen species levels and that dysfunctional exocytosis caused by mitochondrial abnormalities may be a common underlying phenomenon in the initial stages of some human neurodegenerative diseases.     

Oxidative status and stress during highly energetic life‐history stages of Chinstrap Penguins: breeding versus molting

Life‐history stages such as reproduction and molt are energetically costly. Reproductive costs include those associated not only with offspring production, but also protecting and provisioning young. Costs typically associated with molting include decreased thermoregulatory and locomotive performance, and increased metabolic and nutritional costs. Energetic demands may disrupt homeostasis, particularly in terms of its maintenance (e.g., oxidative stress and immunity). Few investigators have explored the relationship between effort (increased metabolic rate) and oxidative status and stress by comparing life‐history stages with different energetic demands. However, comparative studies are crucial for understanding the processes of energy allocation and their consequences for different physiological functions. Our objective was to determine how two highly demanding life‐history stages, breeding and molting, affected oxidative balance in Chinstrap Penguins (Pygoscelis antarcticus), a species where these two activities do not overlap. We found that the heterophil/lymphocyte (H/L) ratio was significantly higher during breeding than molting; oxidative damage was also higher during breeding. In contrast, we found no significant differences between these stages in total antioxidant capacity. We also found sex differences, with males having greater oxidative damage than females. Our results suggest that breeding is more stressful and more demanding for Chinstrap Penguins than molting, and provide further support for the relationship between effort, in terms of increased metabolic rate, and oxidative balance.