Relationship between mitochondrial electron transport chain dysfunction, development, and life extension in Caenorhabditis elegans

Chromatin and epigenetics in aging biology

This book chapter will focus on modifications to chromatin itself, how chromatin modifications are regulated, and how these modifications are deciphered by the cell to impact aging. In this chapter, we will review how chromatin modifications change with age, examine how chromatin-modifying enzymes have been shown to regulate aging and healthspan, discuss how some of these epigenetic changes are triggered and how they can regulate the lifespan of the individual and its naïve descendants, and speculate on future directions for the field.

Biased regulation of protein synthesis and hypoxic death by a conditional raptor mutation

Mechanistic target of rapamycin (mTOR) functions in mTOR complex 1 (mTORC1) with raptor to match metazoan metabolism to available nutrients to regulate multiple cellular, physiological, and pathological processes. Hypoxic cellular injury is influenced by the mTORC1 pathway, but whether its activity promotes or prevents injury is unclear, and which mTORC1-regulated mechanisms control hypoxic injury are obscure. Here, we report the discovery of a hypoxia-resistant, temperature-sensitive raptor mutant in an unbiased forward mutagenesis screen in C. elegans. This raptor mutant is both hypoxia resistant and long lived at intermediate temperatures, while unable to develop at higher temperatures. Temperature-shift experiments show that the conditional hypoxia resistance can be induced in the raptor mutant immediately prior to the hypoxic insult. At these intermediate temperatures, the raptor mutation selectively reduces protein synthesis without affecting autophagy, and epistasis experiments implicate mTOR-targeted translation regulators as components of the hypoxia resistance mechanism. Using the conditional developmental arrest phenotype in a selection for suppressors of raptor loss of function, we isolated multiple second-site raptor missense mutants, whose mutated residue is predicted to interact with RagA, a raptor-binding protein. These suppressor mutations restore normal protein synthesis, hypoxic sensitivity, and lifespan and thereby implicate raptor-RagA interactions as critical to these biological processes.

Conserved components of the macroautophagy machinery in Caenorhabditis elegans

Macroautophagy involves the sequestration of cytoplasmic contents in a double-membrane autophagosome and its subsequent delivery to lysosomes for degradation and recycling. In Caenorhabditis elegans, autophagy participates in diverse processes such as stress resistance, cell fate specification, tissue remodeling, aging, and adaptive immunity. Genetic screens in C. elegans have identified a set of metazoan-specific autophagy genes that form the basis for our molecular understanding of steps unique to the autophagy pathway in multicellular organisms. Suppressor screens have uncovered multiple mechanisms that modulate autophagy activity under physiological conditions. C. elegans also provides a model to investigate how autophagy activity is coordinately controlled at an organismal level. In this chapter, we will discuss the molecular machinery, regulation, and physiological functions of autophagy, and also methods utilized for monitoring autophagy during C. elegans development.

Crassifolin A prolongs lifespan and healthspan in Caenorhabditis elegans via activating autophagy

ETHNOPHARMACOLOGICAL RELEVANCE

The root of Croton crassifolius Geiseler (C. crassifolius), commonly known as "Jiguxiang" in traditional Chinese medicine, is globally recognized for its ethnomedical applications in treating a spectrum of diseases. Crassifolin A (CA), a diterpenoid compound extracted from the roots of C. crassifolius, exhibits anti-herpes simplex virus (HSV), anti-viral and anti-angiogenic properties.

AIM OF THE REVIEW

This study aimed to explore the effects of CA on aging and the mechanisms involved.

MATERIALS AND METHODS

Utilizing Caenorhabditis elegans (C. elegans) as a model organism, we conducted a comprehensive survival analysis and evaluated aging-related phenotypes, including the period of fast body movement and body bending rates. To elucidate the molecular mechanisms of CA's impact on aging, we employed a multifaceted approach, including reverse transcription quantitative polymerase chain reaction (RT-qPCR), western blotting, and fluorescence quantification of transgenic reporter strains.

RESULTS

Our findings demonstrated that CA significantly prolonged both the lifespan and healthspan of C. elegans. The survival benefits conferred by CA were found to correlate with the activation of several key aging-related signaling pathways, including insulin/insulin-like signaling pathway (IIS), dietary restriction (DR) pathway, and germline signaling pathway. Engagement of these pathways led to the activation of transcription factors DAF-16/FOXO, SKN-1/NRF2, HSF-1 and HLH-30/TFEB, as well as the nuclear receptor DAF-12. Consequently, this activation cascade prompted an upregulation of autophagy, a cellular process associated with the maintenance of cellular homeostasis and longevity.

CONCLUSION

Our study delineates novel mechanisms underlying anti-aging strategies, establishing a conceptual framework for the exploitation and advancement of traditional Chinese medicinal herbs as potential therapeutic agents in the fight against aging and its associated pathologies.

Mutations of the Electron Transport Chain Affect Lifespan and ROS Levels in C. elegans

Mutations in highly conserved genes encoding components of the electron transport chain (ETC) provide valuable insights into the mechanisms of oxidative stress and mitochondrial ROS (mtROS) in a wide range of diseases, including cancer, neurodegenerative disorders, and aging. This review explores the structure and function of the ETC in the context of its role in mtROS generation and regulation, emphasizing its dual roles in cellular damage and signaling. Using Caenorhabditis elegans as a model organism, we discuss how ETC mutations manifest as developmental abnormalities, lifespan alterations, and changes in mtROS levels. We highlight the utility of redox sensors in C. elegans for in vivo studies of reactive oxygen species, offering both quantitative and qualitative insights. Finally, we examine the potential of C. elegans as a platform for testing ETC-targeting drug candidates, including OXPHOS inhibitors, which represent promising avenues in cancer therapeutics. This review underscores the translational relevance of ETC research in C. elegans, bridging fundamental biology and therapeutic innovation.

The mitochondrial integrated stress response: A novel approach to anti-aging and pro-longevity

The ISR is a cellular signaling pathway that responds to various physiological changes and types of stimulation. The mitochondrial integrated stress response (ISRmt) is a stress response specific to mitochondria which is initiated by eIF2α phosphorylation and is responsive to mitochondrial stressors. The ISRmt triggers diverse metabolic responses reliant on activating transcription factor 4 (ATF4). The preliminary phases of ISRmt can provoke an adaptive stress response that antagonizes age-related diseases and promotes longevity. In this review, we provide an overview of the molecular mechanisms of the ISRmt, with a particular focus on its potential as a therapeutic target for age-related disease and the promotion of longevity.

Organ-specific electrophile responsivity mapping in live C. elegans

Proximity labeling technologies are limited to indexing localized protein residents. Such data-although valuable-cannot inform on small-molecule responsivity of local residents. We here bridge this gap by demonstrating in live C. elegans how electrophile-sensing propensity in specific organs can be quantitatively mapped and ranked. Using this method, >70% of tissue-specific responders exhibit electrophile responsivity, independent of tissue-specific abundance. One responder, cyp-33e1-for which both human and worm orthologs are electrophile responsive-marshals stress-dependent gut functions, despite manifesting uniform abundance across all tissues studied. Cyp-33e1's localized electrophile responsivity operates site specifically, triggering multifaceted responses: electrophile sensing through the catalytic-site cysteine results in partitioning between enzyme inhibition and localized production of a critical metabolite that governs global lipid availability, whereas rapid dual-cysteine site-specific sensing modulates gut homeostasis. Beyond pinpointing chemical actionability within local proteomes, organ-specific electrophile responsivity mapping illuminates otherwise intractable locale-specific metabolite signaling and stress response programs influencing organ-specific decision-making.

Assessment of azithromycin-induced toxicity in Caenorhabditis elegans: Effects on morphology, behavior, and lipid metabolism

Antibiotics are indispensable in modern healthcare, playing a critical role in mitigating bacterial infections. Azithromycin is used to fight upper respiratory tract infections, however has potential toxic effects that remain inadequately understood. In our present study, azithromycin exposure to Caenorhabditis elegans led to significant physiological and behavioral change, with pronounced effects observed at the studied concentration. The study employs an N2 wild-type strain to examine key physiological and behavioral parameters within the worm. C.elegans were exposed to two concentrations of azithromycin (0.0038 and 0.00038 mg/ml) from the embryonic stage to the L4 stage for 48 hours. The study assessed key endpoints including body length, thrashing behavior, brood size, embryonic viability, lipid accumulation via Nile red staining, pharyngeal pumping rate, and response to 1-Nonanol (which assesses neurotransmitter function). Results showed that at 0.0038 mg/ml, azithromycin significantly reduced body length, increased progeny production, altered lipid deposition, delayed response to 1-Nonanol, and decreased feeding rates. Even at the lowest concentration (0.00038 mg/ml), changes in body length and lipid accumulation were observed. These findings suggest that the toxicity of azithromycin in C.elegans is dose-dependent and varies with exposure duration and developmental stage. Further research is needed to elucidate the molecular mechanisms underlying these toxic effects, particularly at environmentally relevant concentrations of azithromycin.

Investigating impacts of the mycothiazole chemotype as a chemical probe for the study of mitochondrial function and aging

Small molecule inhibitors of the mitochondrial electron transport chain (ETC) hold significant promise to provide valuable insights to the field of mitochondrial research and aging biology. In this study, we investigated two molecules: mycothiazole (MTZ) - from the marine sponge C. mycofijiensis and its more stable semisynthetic analog 8-O-acetylmycothiazole (8-OAc) as potent and selective chemical probes based on their high efficiency to inhibit ETC complex I function. Similar to rotenone (Rote), MTZ, a newly employed ETC complex I inhibitor, exhibited higher cytotoxicity against cancer cell lines compared to certain non-cancer cell lines. Interestingly, 8-OAc demonstrated greater selectivity for cancer cells when compared to both MTZ and Rote, which has promising potential for anticancer therapeutic development. Furthermore, in vivo experiments with these small molecules utilizing a C. elegans model demonstrate their unexplored potential to investigate aging studies. We observed that both molecules have the ability to induce a mitochondria-specific unfolded protein response (UPRMT) pathway, that extends lifespan of worms when applied in their adult stage. We also found that these two molecules employ different pathways to extend lifespan in worms. Whereas MTZ utilizes the transcription factors ATFS-1 and HSF1, which are involved in the UPRMT and heat shock response (HSR) pathways respectively, 8-OAc only required HSF1 and not ATFS-1 to mediate its effects. This observation underscores the value of applying stable, potent, and selective next generation chemical probes to elucidate an important insight into the functional roles of various protein subunits of ETC complexes and their regulatory mechanisms associated with aging.

Comprehensive characterization of mitochondrial bioenergetics at different larval stages reveals novel insights about the developmental metabolism of Caenorhabditis elegans

Mitochondrial bioenergetic processes are fundamental to development, stress responses, and health. Caenorhabditis elegans is widely used to study developmental biology, mitochondrial disease, and mitochondrial toxicity. Oxidative phosphorylation generally increases during development in many species, and genetic and environmental factors may alter this normal trajectory. Altered mitochondrial function during development can lead to both drastic, short-term responses including arrested development and death, and subtle consequences that may persist throughout life and into subsequent generations. Understanding normal and altered developmental mitochondrial biology in C. elegans is currently constrained by incomplete and conflicting reports on how mitochondrial bioenergetic parameters change during development in this species. We used a Seahorse XFe24 Extracellular Flux (XF) Analyzer to carry out a comprehensive analysis of mitochondrial and non-mitochondrial oxygen consumption rates (OCR) throughout larval development in C. elegans. We optimized and describe conditions for analysis of basal OCR, basal mitochondrial OCR, ATP-linked OCR, spare and maximal respiratory capacity, proton leak, and non-mitochondrial OCR. A key consideration is normalization, and we present and discuss results as normalized per individual worm, protein content, worm volume, mitochondrial DNA (mtDNA) count, nuclear DNA (ncDNA) count, and mtDNA:ncDNA ratio. Which normalization process is best depends on the question being asked, and differences in normalization explain some of the discrepancies in previously reported developmental changes in OCR in C. elegans. Broadly, when normalized to worm number, our results agree with previous reports in showing dramatic increases in OCR throughout development. However, when normalized to total protein, worm volume, or ncDNA or mtDNA count, after a significant 2-3-fold increase from L1 to L2 stages, we found small or no changes in most OCR parameters from the L2 to the L4 stage, other than a marginal increase at L3 in spare and maximal respiratory capacity. Overall, our results indicate an earlier cellular shift to oxidative metabolism than suggested in most previous literature.

Selection for Late Reproduction Leads to Loss of Complex I Mitochondrial Capacity and Associated Increased Longevity in Seed Beetles

Mitochondria play a key role in aging. Here, we measured integrated mitochondrial functions in experimentally evolved lines of the seed beetle Acanthoscelides obtectus that were selected for early (E) or late (L) reproduction for nearly 4 decades. The 2 lines have markedly different lifespans (8 days and 13 days in the E and L lines, respectively). The contribution of the NADH pathway to maximal flux was lower in the L compared to the E beetles at young stages, associated with increased control by complex I. In contrast, the contribution of the Succinate pathway was higher in the L than in the E line, whereas the Proline pathway showed no differences between the lines. Our data suggest that selection of age at reproduction leads to a modulation of complex I activity in mitochondria and that mitochondria are a functional link between evolutionary and mechanistic theories of aging.

Comprehensive characterization of mitochondrial bioenergetics at different larval stages reveals novel insights about the developmental metabolism of Caenorhabditis elegans

Mitochondrial bioenergetic processes are fundamental to development, stress responses, and health. Caenorhabditis elegans is widely used to study developmental biology, mitochondrial disease, and mitochondrial toxicity. Oxidative phosphorylation generally increases during development in many species, and genetic and environmental factors may alter this normal trajectory. Altered mitochondrial function during development can lead to both drastic, short-term responses including arrested development and death, and subtle consequences that may persist throughout life and into subsequent generations. Understanding normal and altered developmental mitochondrial biology in C. elegans is currently constrained by incomplete and conflicting reports on how mitochondrial bioenergetic parameters change during development in this species. We used a Seahorse XFe24 Extracellular Flux (XF) Analyzer to carry out a comprehensive analysis of mitochondrial and non-mitochondrial oxygen consumption rates (OCR) throughout larval development in C. elegans. We optimized and describe conditions for analysis of basal OCR, basal mitochondrial OCR, ATP-linked OCR, spare and maximal respiratory capacity, proton leak, and non-mitochondrial OCR. A key consideration is normalization, and we present and discuss results as normalized per individual worm, protein content, worm volume, mitochondrial DNA (mtDNA) count, nuclear DNA (ncDNA) count, and mtDNA:ncDNA ratio. Which normalization process is best depends on the question being asked, and differences in normalization explain some of the discrepancies in previously reported developmental changes in OCR in C. elegans. Broadly, when normalized to worm number, our results agree with previous reports in showing dramatic increases in OCR throughout development. However, when normalized to total protein, worm volume, or ncDNA or mtDNA count, after a significant 2-3-fold increase from L1 to L2 stages, we found small or no changes in most OCR parameters from the L2 to the L4 stage, other than a marginal increase at L3 in spare and maximal respiratory capacity. Overall, our results indicate an earlier cellular shift to oxidative metabolism than suggested in most previous literature.

A Multisystemic Approach Revealed Aminated Polystyrene Nanoparticles-Induced Neurotoxicity

Exposure to plastic nanoparticles has dramatically increased in the last 50 years, and there is evidence that plastic nanoparticles can be absorbed by organisms and cross the blood-brain-barrier (BBB). However, their toxic effects, especially on the nervous system, have not yet been extensively investigated, and most of the knowledge is based on studies using different conditions and systems, thus hard to compare. In this work, physicochemical properties of non-modified polystyrene (PS) and amine-functionalized PS (PS-NH2 ) nanoparticles are initially characterized. Advantage of a multisystemic approach is then taken to compare plastic nanoparticles effects in vitro, through cytotoxic readouts in mammalian cell culture, and in vivo, through behavioral readouts in the nematode Caenorhabditis elegans (C. elegans), a powerful 3R-complying model organism for toxicology studies. In vitro experiments in neuroblastoma cells indicate a specific cytotoxic effect of PS-NH2 particles, including a decreased neuronal differentiation and an increased Amyloid β (Aβ) secretion, a sensitive readout correlating with Alzheimer's disease pathology. In parallel, only in vivo treatments with PS-NH2 particles affect C. elegans development, decrease lifespan, and reveal higher sensitivity of animals expressing human Aβ compared to wild-type animals. In summary, the multisystemic approach discloses a neurotoxic effect induced by aminated polystyrene nanoparticles.

Mitochondrial stress and aging: Lessons from C. elegans

Aging is accompanied by a progressive decline in mitochondrial function, which in turn contributes to a variety of age-related diseases. Counterintuitively, a growing number of studies have found that disruption of mitochondrial function often leads to increased lifespan. This seemingly contradictory observation has inspired extensive research into genetic pathways underlying the mitochondrial basis of aging, particularly within the model organism Caenorhabditis elegans. The complex and antagonistic roles of mitochondria in the aging process have altered the view of mitochondria, which not only serve as simple bioenergetic factories but also as signaling platforms for the maintenance of cellular homeostasis and organismal health. Here, we review the contributions of C. elegans to our understanding of mitochondrial function in the aging process over the past decades. In addition, we explore how these insights may promote future research of mitochondrial-targeted strategies in higher organisms to potentially slow aging and delay age-related disease progression.

Reduced Mitochondrial Protein Translation Promotes Cardiomyocyte Proliferation and Heart Regeneration

BACKGROUND

The importance of mitochondria in normal heart function are well recognized and recent studies have implicated changes in mitochondrial metabolism with some forms of heart disease. Previous studies demonstrated that knockdown of the mitochondrial ribosomal protein S5 (MRPS5) by small interfering RNA (siRNA) inhibits mitochondrial translation and thereby causes a mitonuclear protein imbalance. Therefore, we decided to examine the effects of MRPS5 loss and the role of these processes on cardiomyocyte proliferation.

METHODS

We deleted a single allele of MRPS5 in mice and used left anterior descending coronary artery ligation surgery to induce myocardial damage in these animals. We examined cardiomyocyte proliferation and cardiac regeneration both in vivo and in vitro. Doxycycline treatment was used to inhibit protein translation. Heart function in mice was assessed by echocardiography. Quantitative real-time polymerase chain reaction and RNA sequencing were used to assess changes in transcription and chromatin immunoprecipitation (ChIP) and BioChIP were used to assess chromatin effects. Protein levels were assessed by Western blotting and cell proliferation or death by histology and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assays. Adeno-associated virus was used to overexpress genes. The luciferase reporter assay was used to assess promoter activity. Mitochondrial oxygen consumption rate, ATP levels, and reactive oxygen species were also analyzed.

RESULTS

We determined that deletion of a single allele of MRPS5 in mice results in elevated cardiomyocyte proliferation and cardiac regeneration; this observation correlates with improved cardiac function after induction of myocardial infarction. We identified ATF4 (activating transcription factor 4) as a key regulator of the mitochondrial stress response in cardiomyocytes from Mrps5+/- mice; furthermore, ATF4 regulates Knl1 (kinetochore scaffold 1) leading to an increase in cytokinesis during cardiomyocyte proliferation. The increased cardiomyocyte proliferation observed in Mrps5+/- mice was attenuated when one allele of Atf4 was deleted genetically (Mrps5+/-/Atf4+/-), resulting in the loss in the capacity for cardiac regeneration. Either MRPS5 inhibition (or as we also demonstrate, doxycycline treatment) activate a conserved regulatory mechanism that increases the proliferation of human induced pluripotent stem cell-derived cardiomyocytes.

CONCLUSIONS

These data highlight a critical role for MRPS5/ATF4 in cardiomyocytes and an exciting new avenue of study for therapies to treat myocardial injury.

Investigating impacts of marine sponge derived mycothiazole and its acetylated derivative on mitochondrial function and aging

Small molecule inhibitors of the mitochondrial electron transport chain (ETC) hold significant promise to provide valuable insights to the field of mitochondrial research and aging biology. In this study, we investigated two molecules: mycothiazole (MTZ) - from the marine sponge C. mycofijiensis and its more stable semisynthetic analog 8-O-acetylmycothiazole (8-OAc) as potent and selective chemical probes based on their high efficiency to inhibit ETC complex I function. Similar to rotenone (Rote), a widely used ETC complex I inhibitor, these two molecules showed cytotoxicity to cancer cells but strikingly demonstrate a lack of toxicity to non-cancer cells, a highly beneficial feature in the development of anti-cancer therapeutics. Furthermore, in vivo experiments with these small molecules utilizing C.elegans model demonstrate their unexplored potential to investigate aging studies. We observed that both molecules have the ability to induce a mitochondria-specific unfolded protein response (UPRMT) pathway, that extends lifespan of worms when applied in their adult stage. Interestingly, we also found that these two molecules employ different pathways to extend lifespan in worms. Whereas MTZ utilize the transcription factors ATFS-1 and HSF-1, which are involved in the UPRMT and heat shock response (HSR) pathways respectively, 8-OAc only required HSF-1 and not ATFS-1 to mediate its effects. This observation underscores the value of applying stable, potent, and selective next generation chemical probes to elucidate an important insight into the functional roles of various protein subunits of ETC complexes and their regulatory mechanisms associated with aging.

Biological resilience and aging: Activation of stress response pathways contributes to lifespan extension

While aging was traditionally viewed as a stochastic process of damage accumulation, it is now clear that aging is strongly influenced by genetics. The identification and characterization of long-lived genetic mutants in model organisms has provided insights into the genetic pathways and molecular mechanisms involved in extending longevity. Long-lived genetic mutants exhibit activation of multiple stress response pathways leading to enhanced resistance to exogenous stressors. As a result, lifespan exhibits a significant, positive correlation with resistance to stress. Disruption of stress response pathways inhibits lifespan extension in multiple long-lived mutants representing different pathways of lifespan extension and can also reduce the lifespan of wild-type animals. Combined, this suggests that activation of stress response pathways is a key mechanism by which long-lived mutants achieve their extended longevity and that many of these pathways are also required for normal lifespan. These results highlight an important role for stress response pathways in determining the lifespan of an organism.

Influence of Asafoetida Extract on the Virulence of the Entomopathogenic Nematode Steinernema carpocapsae and Its Symbiotic Bacterium Xenorhabdus nematophila in the Host Pyrrhocoris apterus

Nematode-microbe symbiosis plays a key role in determining pathogenesis against pests. The modulation of symbiotic bacteria may affect the virulence of entomopathogenic nematodes (EPNs) and the biological management of pests. We tested the influence of asafoetida (ASF) extract on the virulence of Steinernema carpocapsae and its symbiotic bacterium, Xenorhabdus nematophila, in Pyrrhocoris apterus. A total of 100 mg of ASF killed 30% of EPNs in 48 h, while P. apterus remained unaffected. The EPNs pre-treated with 100 mg of ASF influenced P. apterus's mortality by 24-91.4% during a period of 24 to 72 h. The topical application of ASF acted as a deterrent to S. carpocapsae, lowering host invasion to 70% and delaying infectivity with 30% mortality for 168 h. Interestingly, Steinernema's symbiotic bacterium, Xenorhabdus, remained unaffected by ASF. An in vitro turbidity test containing 100 mg of ASF in a medium increased the growth rate of Xenorhabdus compared to a control. A disc diffusion assay confirmed the non-susceptibility of Xenorhabdus to ASF compared to a positive control, streptomycin. Pro-phenol oxidase (PPO) and phenol oxidase (PO) upregulation showed that ASF influences immunity, while EPN/ASF showed a combined immunomodulatory effect in P. apterus. We report that ASF modulated the virulence of S. carpocapsae but not that of its symbiotic bacterium, X. nematophila, against P. apterus.

Mitochondria hormesis delays aging and associated diseases in Caenorhabditis elegans impacting on key ferroptosis players

Excessive iron accumulation or deficiency leads to a variety of pathologies in humans and developmental arrest in the nematode Caenorhabditis elegans. Instead, sub-lethal iron depletion extends C. elegans lifespan. Hypoxia preconditioning protects against severe hypoxia-induced neuromuscular damage across species but it has low feasible application. In this study, we assessed the potential beneficial effects of genetic and chemical interventions acting via mild iron instead of oxygen depletion. We show that limiting iron availability in C. elegans through frataxin silencing or the iron chelator bipyridine, similar to hypoxia preconditioning, protects against hypoxia-, age-, and proteotoxicity-induced neuromuscular deficits. Mechanistically, our data suggest that the beneficial effects elicited by frataxin silencing are in part mediated by counteracting ferroptosis, a form of non-apoptotic cell death mediated by iron-induced lipid peroxidation. This is achieved by impacting on different key ferroptosis players and likely via gpx-independent redox systems. We thus point to ferroptosis inhibition as a novel potential strategy to promote healthy aging.

Exercise Improves the Coordination of the Mitochondrial Unfolded Protein Response and Mitophagy in Aging Skeletal Muscle

The mitochondrial unfolded protein response (UPRmt) and mitophagy are two mitochondrial quality control (MQC) systems that work at the molecular and organelle levels, respectively, to maintain mitochondrial homeostasis. Under stress conditions, these two processes are simultaneously activated and compensate for each other when one process is insufficient, indicating mechanistic coordination between the UPRmt and mitophagy that is likely controlled by common upstream signals. This review focuses on the molecular signals regulating this coordination and presents evidence showing that this coordination mechanism is impaired during aging and promoted by exercise. Furthermore, the bidirectional regulation of reactive oxygen species (ROS) and AMPK in modulating this mechanism is discussed. The hierarchical surveillance network of MQC can be targeted by exercise-derived ROS to attenuate aging, which offers a molecular basis for potential therapeutic interventions for sarcopenia.

Optogenetic rejuvenation of mitochondrial membrane potential extends C. elegans lifespan

Mitochondrial dysfunction plays a central role in aging but the exact biological causes are still being determined. Here, we show that optogenetically increasing mitochondrial membrane potential during adulthood using a light-activated proton pump improves age-associated phenotypes and extends lifespan in C. elegans. Our findings provide direct causal evidence that rescuing the age-related decline in mitochondrial membrane potential is sufficient to slow the rate of aging and extend healthspan and lifespan.

Mitochondrial Complex I, a Possible Sensible Site of cAMP Pathway in Aging

In mammals during aging, reactive oxygen species (ROS), produced by the mitochondrial respiratory chain, cause oxidative damage of macromolecules leading to respiratory chain dysfunction, which in turn increases ROS mitochondrial production. Many efforts have been made to understand the role of oxidative stress in aging and age-related diseases. The complex I of the mitochondrial respiratory chain is the major source of ROS production and its dysfunctions have been associated with several forms of neurodegeneration, other common human diseases and aging. Complex I-ROS production and complex I content have been proposed as the major determinants for longevity. The cAMP signal has a role in the regulation of complex I activity and the decrease of ROS production. In the last years, an increasing number of studies have attempted to activate cAMP signaling to treat age-related diseases associated with mitochondrial dysfunctions and ROS production. This idea comes from a long-line of studies showing a main role of cAMP signal in the memory consolidation mechanism and in the regulation of mitochondrial functions. Here, we discuss several evidences on the possible connection between complex I and cAMP pathway in the aging process.

Brain-protective mechanisms of autophagy associated circRNAs: Kick starting self-cleaning mode in brain cells via circRNAs as a potential therapeutic approach for neurodegenerative diseases

Altered autophagy is a hallmark of neurodegeneration but how autophagy is regulated in the brain and dysfunctional autophagy leads to neuronal death has remained cryptic. Being a key cellular waste-recycling and housekeeping system, autophagy is implicated in a range of brain disorders and altering autophagy flux could be an effective therapeutic strategy and has the potential for clinical applications down the road. Tight regulation of proteins and organelles in order to meet the needs of complex neuronal physiology suggests that there is distinct regulatory pattern of neuronal autophagy as compared to non-neuronal cells and nervous system might have its own separate regulator of autophagy. Evidence has shown that circRNAs participates in the biological processes of autophagosome assembly. The regulatory networks between circRNAs, autophagy, and neurodegeneration remains unknown and warrants further investigation. Understanding the interplay between autophagy, circRNAs and neurodegeneration requires a knowledge of the multiple steps and regulatory interactions involved in the autophagy pathway which might provide a valuable resource for the diagnosis and therapy of neurodegenerative diseases. In this review, we aimed to summarize the latest studies on the role of brain-protective mechanisms of autophagy associated circRNAs in neurodegenerative diseases (including Alzheimer's disease, Parkinson's disease, Huntington's disease, Spinal Muscular Atrophy, Amyotrophic Lateral Sclerosis, and Friedreich's ataxia) and how this knowledge can be leveraged for the development of novel therapeutics against them. Autophagy stimulation might be potential one-size-fits-all therapy for neurodegenerative disease as per considerable body of evidence, therefore future research on brain-protective mechanisms of autophagy associated circRNAs will illuminate an important feature of nervous system biology and will open the door to new approaches for treating neurodegenerative diseases.

Mitochondria-originated redox signalling regulates KLF-1 to promote longevity in Caenorhabditis elegans

Alternations of redox metabolism have been associated with the extension of lifespan in roundworm Caenorhabditis elegans, caused by moderate mitochondrial dysfunction, although the underlying signalling cascades are largely unknown. Previously, we identified transcriptional factor Krüppel-like factor-1 (KLF-1) as the main regulator of cytoprotective longevity-assurance pathways in the C. elegans long-lived mitochondrial mutants. Here, we show that KLF-1 translocation to the nucleus and the activation of the signalling cascade is dependent on the mitochondria-derived hydrogen peroxide (H₂O₂) produced during late developmental phases where aerobic respiration and somatic mitochondrial biogenesis peak. We further show that mitochondrial-inducible superoxide dismutase-3 (SOD-3), together with voltage-dependent anion channel-1 (VDAC-1), is required for the life-promoting H₂O₂ signalling that is further regulated by peroxiredoxin-3 (PRDX-3). Increased H₂O₂ release in the cytoplasm activates the p38 MAPK signalling cascade that induces KLF-1 translocation to the nucleus and the activation of transcription of C. elegans longevity-promoting genes, including cytoprotective cytochrome P450 oxidases. Taken together, our results underline the importance of redox-regulated signalling as the key regulator of longevity-inducing pathways in C. elegans, and position precisely timed mitochondria-derived H₂O₂ in the middle of it.

The emergent role of mitochondrial surveillance in cellular health

Mitochondrial dysfunction is one of the primary causatives for many pathologies, including neurodegenerative diseases, cancer, metabolic disorders, and aging. Decline in mitochondrial functions leads to the loss of proteostasis, accumulation of ROS, and mitochondrial DNA damage, which further exacerbates mitochondrial deterioration in a vicious cycle. Surveillance mechanisms, in which mitochondrial functions are closely monitored for any sign of perturbations, exist to anticipate possible havoc within these multifunctional organelles with primitive origin. Various indicators of unhealthy mitochondria, including halted protein import, dissipated membrane potential, and increased loads of oxidative damage, are on the top of the lists for close monitoring. Recent research also indicates a possibility of reductive stress being monitored as part of a mitochondrial surveillance program. Upon detection of mitochondrial stress, multiple mitochondrial stress-responsive pathways are activated to promote the transcription of numerous nuclear genes to ameliorate mitochondrial damage and restore compromised cellular functions. Co-expression occurs through functionalization of transcription factors, allowing their binding to promoter elements to initiate transcription of target genes. This review provides a comprehensive summary of the intricacy of mitochondrial surveillance programs and highlights their roles in our cellular life. Ultimately, a better understanding of these surveillance mechanisms is expected to improve healthspan.

Cobalt chloride has beneficial effects across species through a hormetic mechanism

Severe oxygen and iron deficiencies have evolutionarily conserved detrimental effects, leading to pathologies in mammals and developmental arrest as well as neuromuscular degeneration in the nematode Caenorhabditis elegans. Yet, similar to the beneficial effects of mild hypoxia, non-toxic levels of iron depletion, achieved with the iron chelator bipyridine or through frataxin silencing, extend C. elegans lifespan through hypoxia-like induction of mitophagy. While the positive health outcomes of hypoxia preconditioning are evident, its practical application is rather challenging. Here, we thus test the potential beneficial effects of non-toxic, preconditioning interventions acting on iron instead of oxygen availability. We find that limiting iron availability through the iron competing agent cobalt chloride has evolutionarily conserved dose-dependent beneficial effects: while high doses of cobalt chloride have toxic effects in mammalian cells, iPS-derived neurospheres, and in C. elegans, sub-lethal doses protect against hypoxia- or cobalt chloride-induced death in mammalian cells and extend lifespan and delay age-associated neuromuscular alterations in C. elegans. The beneficial effects of cobalt chloride are accompanied by the activation of protective mitochondrial stress response pathways.

Caenorhabditis elegans as a Model for the Effects of Phytochemicals on Mitochondria and Aging

The study of aging is an important topic in contemporary research. Considering the demographic changes and the resulting shifts towards an older population, it is of great interest to preserve youthful physiology in old age. For this endeavor, it is necessary to choose an appropriate model. One such model is the nematode Caenorhabditis elegans (C. elegans), which has a long tradition in aging research. In this review article, we explore the advantages of using the nematode model in aging research, focusing on bioenergetics and the study of secondary plant metabolites that have interesting implications during this process. In the first section, we review the situation of aging research today. Conventional theories and hypotheses about the ongoing aging process will be presented and briefly explained. The second section focuses on the nematode C. elegans and its utility in aging and nutrition research. Two useful genome editing methods for monitoring genetic interactions (RNAi and CRISPR/Cas9) are presented. Due to the mitochondria's influence on aging, we also introduce the possibility of observing bioenergetics and respiratory phenomena in C. elegans. We then report on mitochondrial conservation between vertebrates and invertebrates. Here, we explain why the nematode is a suitable model for the study of mitochondrial aging. In the fourth section, we focus on phytochemicals and their applications in contemporary nutritional science, with an emphasis on aging research. As an emerging field of science, we conclude this review in the fifth section with several studies focusing on mitochondrial research and the effects of phytochemicals such as polyphenols. In summary, the nematode C. elegans is a suitable model for aging research that incorporates the mitochondrial theory of aging. Its living conditions in the laboratory are optimal for feeding studies, thus enabling bioenergetics to be observed during the aging process.

Oocytes maintain ROS-free mitochondrial metabolism by suppressing complex I

Oocytes form before birth and remain viable for several decades before fertilization1. Although poor oocyte quality accounts for most female fertility problems, little is known about how oocytes maintain cellular fitness, or why their quality eventually declines with age2. Reactive oxygen species (ROS) produced as by-products of mitochondrial activity are associated with lower rates of fertilization and embryo survival3-5. Yet, how healthy oocytes balance essential mitochondrial activity with the production of ROS is unknown. Here we show that oocytes evade ROS by remodelling the mitochondrial electron transport chain through elimination of complex I. Combining live-cell imaging and proteomics in human and Xenopus oocytes, we find that early oocytes exhibit greatly reduced levels of complex I. This is accompanied by a highly active mitochondrial unfolded protein response, which is indicative of an imbalanced electron transport chain. Biochemical and functional assays confirm that complex I is neither assembled nor active in early oocytes. Thus, we report a physiological cell type without complex I in animals. Our findings also clarify why patients with complex-I-related hereditary mitochondrial diseases do not experience subfertility. Complex I suppression represents an evolutionarily conserved strategy that allows longevity while maintaining biological activity in long-lived oocytes.

Differential regulation of degradation and immune pathways underlies adaptation of the ectosymbiotic nematode Laxus oneistus to oxic-anoxic interfaces

Eukaryotes may experience oxygen deprivation under both physiological and pathological conditions. Because oxygen shortage leads to a reduction in cellular energy production, all eukaryotes studied so far conserve energy by suppressing their metabolism. However, the molecular physiology of animals that naturally and repeatedly experience anoxia is underexplored. One such animal is the marine nematode Laxus oneistus. It thrives, invariably coated by its sulfur-oxidizing symbiont Candidatus Thiosymbion oneisti, in anoxic sulfidic or hypoxic sand. Here, transcriptomics and proteomics showed that, whether in anoxia or not, L. oneistus mostly expressed genes involved in ubiquitination, energy generation, oxidative stress response, immune response, development, and translation. Importantly, ubiquitination genes were also highly expressed when the nematode was subjected to anoxic sulfidic conditions, together with genes involved in autophagy, detoxification and ribosome biogenesis. We hypothesize that these degradation pathways were induced to recycle damaged cellular components (mitochondria) and misfolded proteins into nutrients. Remarkably, when L. oneistus was subjected to anoxic sulfidic conditions, lectin and mucin genes were also upregulated, potentially to promote the attachment of its thiotrophic symbiont. Furthermore, the nematode appeared to survive oxygen deprivation by using an alternative electron carrier (rhodoquinone) and acceptor (fumarate), to rewire the electron transfer chain. On the other hand, under hypoxia, genes involved in costly processes (e.g., amino acid biosynthesis, development, feeding, mating) were upregulated, together with the worm's Toll-like innate immunity pathway and several immune effectors (e.g., bactericidal/permeability-increasing proteins, fungicides). In conclusion, we hypothesize that, in anoxic sulfidic sand, L. oneistus upregulates degradation processes, rewires the oxidative phosphorylation and reinforces its coat of bacterial sulfur-oxidizers. In upper sand layers, instead, it appears to produce broad-range antimicrobials and to exploit oxygen for biosynthesis and development.

Pioglitazone Hydrochloride Extends the Lifespan of Caenorhabditis elegans by Activating DAF-16/FOXO- and SKN-1/NRF2-Related Signaling Pathways

Pioglitazone hydrochloride (PGZ), a nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-γ) agonist, is a universally adopted oral agent for the treatment of type 2 diabetes (T2D). Previous studies reported that PGZ could ameliorate the symptoms of aging-related diseases and Alzheimer's disease. However, whether PGZ participates in aging regulation and the underlying mechanism remain undetermined. Here, we found that PGZ significantly prolonged the lifespan and healthspan of Caenorhabditis elegans (C. elegans). We found that a variety of age-related pathways and age-related genes are required for PGZ-induced lifespan extension. The transcription factors DAF-16/FOXO, HSF-1, and SKN-1/NRF2, as well as the nuclear receptors DAF-12 and NHR-49, all functioned in the survival advantage conferred by PGZ. Moreover, our results demonstrated that PGZ induced lifespan extension through the inhibition of insulin/insulin-like signaling (IIS) and reproductive signaling pathways, as well as the activation of dietary restriction- (DR-) related pathways. Additionally, our results also indicated that beneficial longevity mediated by PGZ is linked to its antioxidative activity. Our research may provide a basis for further research on PGZ, as an anti-T2D drug, to interfere with aging and reduce the incidence of age-related diseases in diabetic patients.

Disruption of the MICOS complex leads to an aberrant cristae structure and an unexpected, pronounced lifespan extension in Podospora anserina

Mitochondria are dynamic eukaryotic organelles involved in a variety of essential cellular processes including the generation of adenosine triphosphate (ATP) and reactive oxygen species as well as in the control of apoptosis and autophagy. Impairments of mitochondrial functions lead to aging and disease. Previous work with the ascomycete Podospora anserina demonstrated that mitochondrial morphotype as well as mitochondrial ultrastructure change during aging. The latter goes along with an age-dependent reorganization of the inner mitochondrial membrane leading to a change from lamellar cristae to vesicular structures. Particularly from studies with yeast, it is known that besides the F₁ F_o -ATP-synthase and the phospholipid cardiolipin also the "mitochondrial contact site and cristae organizing system" (MICOS) complex, existing of the Mic60- and Mic10-subcomplex, is essential for proper cristae formation. In the present study, we aimed to understand the mechanistic basis of age-related changes in the mitochondrial ultrastructure. We observed that MICOS subunits are coregulated at the posttranscriptional level. This regulation partially depends on the mitochondrial iAAA-protease PaIAP. Most surprisingly, we made the counterintuitive observation that, despite the loss of lamellar cristae and of mitochondrial impairments, the ablation of MICOS subunits (except for PaMIC12) leads to a pronounced lifespan extension. Moreover, simultaneous ablation of subunits of both MICOS subcomplexes synergistically increases lifespan, providing formal genetic evidence that both subcomplexes affect lifespan by different and at least partially independent pathways. At the molecular level, we found that ablation of Mic10-subcomplex components leads to a mitohormesis-induced lifespan extension, while lifespan extension of Mic60-subcomplex mutants seems to be controlled by pathways involved in the control of phospholipid homeostasis. Overall, our data demonstrate that both MICOS subcomplexes have different functions and play distinct roles in the aging process of P. anserina.

Inhibition of ATR Reverses a Mitochondrial Respiratory Insufficiency

Diseases that affect the mitochondrial electron transport chain (ETC) often manifest as threshold effect disorders, meaning patients only become symptomatic once a certain level of ETC dysfunction is reached. Cells can invoke mechanisms to circumvent reaching their critical ETC threshold, but it is an ongoing challenge to identify such processes. In the nematode Caenorhabditis elegans, severe reduction of mitochondrial ETC activity shortens life, but mild reduction actually extends it, providing an opportunity to identify threshold circumvention mechanisms. Here, we show that removal of ATL-1, but not ATM-1, worm orthologs of ATR and ATM, respectively, key nuclear DNA damage checkpoint proteins in human cells, unexpectedly lessens the severity of ETC dysfunction. Multiple genetic and biochemical tests show no evidence for increased mutation or DNA breakage in animals exposed to ETC disruption. Reduced ETC function instead alters nucleotide ratios within both the ribo- and deoxyribo-nucleotide pools, and causes stalling of RNA polymerase, which is also known to activate ATR. Unexpectedly, atl-1 mutants confronted with mitochondrial ETC disruption maintain normal levels of oxygen consumption, and have an increased abundance of translating ribosomes. This suggests checkpoint signaling by ATL-1 normally dampens cytoplasmic translation. Taken together, our data suggest a model whereby ETC insufficiency in C. elegans results in nucleotide imbalances leading to the stalling of RNA polymerase, activation of ATL-1, dampening of global translation, and magnification of ETC dysfunction. The loss of ATL-1 effectively reverses the severity of ETC disruption so that animals become phenotypically closer to wild type.

Neuroligin-mediated neurodevelopmental defects are induced by mitochondrial dysfunction and prevented by lutein in C. elegans

Complex-I-deficiency represents the most frequent pathogenetic cause of human mitochondriopathies. Therapeutic options for these neurodevelopmental life-threating disorders do not exist, partly due to the scarcity of appropriate model systems to study them. Caenorhabditis elegans is a genetically tractable model organism widely used to investigate neuronal pathologies. Here, we generate C. elegans models for mitochondriopathies and show that depletion of complex I subunits recapitulates biochemical, cellular and neurodevelopmental aspects of the human diseases. We exploit two models, nuo-5/NDUFS1- and lpd-5/NDUFS4-depleted animals, for a suppressor screening that identifies lutein for its ability to rescue animals’ neurodevelopmental deficits. We uncover overexpression of synaptic neuroligin as an evolutionarily conserved consequence of mitochondrial dysfunction, which we find to mediate an early cholinergic defect in C. elegans. We show lutein exerts its beneficial effects by restoring neuroligin expression independently from its antioxidant activity, thus pointing to a possible novel pathogenetic target for the human disease.

Mitochondrial deficiency causes rare incurable disorders. Here, the authors use C. elegans to study these diseases and find that the natural compound lutein prevents neurodevelopmental deficits, thus pointing to a possible therapeutic target for the human diseases.

CEST-2.2 overexpression alters lipid metabolism and extends longevity of mitochondrial mutants

Mitochondrial dysfunction can either extend or decrease Caenorhabditis elegans lifespan, depending on whether transcriptionally regulated responses can elicit durable stress adaptation to otherwise detrimental lesions. Here, we test the hypothesis that enhanced metabolic flexibility is sufficient to circumvent bioenergetic abnormalities associated with the phenotypic threshold effect, thereby transforming short‐lived mitochondrial mutants into long‐lived ones. We find that CEST‐2.2, a carboxylesterase mainly localizes in the intestine, may stimulate the survival of mitochondrial deficient animals. We report that genetic manipulation of cest‐2.2 expression has a minor lifespan impact on wild‐type nematodes, whereas its overexpression markedly extends the lifespan of complex I‐deficient gas‐1(fc21) mutants. We profile the transcriptome and lipidome of cest‐2.2 overexpressing animals and show that CEST‐2.2 stimulates lipid metabolism and fatty acid beta‐oxidation, thereby enhancing mitochondrial respiratory capacity through complex II and LET‐721/ETFDH, despite the inherited genetic lesion of complex I. Together, our findings unveil a metabolic pathway that, through the tissue‐specific mobilization of lipid deposits, may influence the longevity of mitochondrial mutant C. elegans.

SGPL1 stimulates VPS39 recruitment to the mitochondria in MICU1 deficient cells

Objective

Mitochondrial “retrograde” signaling may stimulate organelle biogenesis as a compensatory adaptation to aberrant activity of the oxidative phosphorylation (OXPHOS) system. To maintain energy-consuming processes in OXPHOS deficient cells, alternative metabolic pathways are functionally coupled to the degradation, recycling and redistribution of biomolecules across distinct intracellular compartments. While transcriptional regulation of mitochondrial network expansion has been the focus of many studies, the molecular mechanisms promoting mitochondrial maintenance in energy-deprived cells remain poorly investigated.

Methods

We performed transcriptomics, quantitative proteomics and lifespan assays to identify pathways that are mechanistically linked to mitochondrial network expansion and homeostasis in Caenorhabditis elegans lacking the mitochondrial calcium uptake protein 1 (MICU-1/MICU1). To support our findings, we carried out biochemical and image analyses in mammalian cells and mouse-derived tissues.

Results

We report that micu-1(null) mutations impair the OXPHOS system and promote C. elegans longevity through a transcriptional program that is independent of the mitochondrial calcium uniporter MCU-1/MCU and the essential MCU regulator EMRE-1/EMRE. We identify sphingosine phosphate lyase SPL-1/SGPL1 and the ATFS-1-target HOPS complex subunit VPS-39/VPS39 as critical lifespan modulators of micu-1(null) mutant animals. Cross-species investigation indicates that SGPL1 upregulation stimulates VPS39 recruitment to the mitochondria, thereby enhancing mitochondria-lysosome contacts. Consistently, VPS39 downregulation compromises mitochondrial network maintenance and basal autophagic flux in MICU1 deficient cells. In mouse-derived muscles, we show that VPS39 recruitment to the mitochondria may represent a common signature associated with altered OXPHOS system.

Conclusions

Our findings reveal a previously unrecognized SGPL1/VPS39 axis that stimulates intracellular organelle interactions and sustains autophagy and mitochondrial homeostasis in OXPHOS deficient cells.

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micu-1(null) nematodes are long-lived mitochondrial mutants.

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MICU-1/MICU1 deficiency stimulates VPS-39/VPS39 and SPL-1/SGPL1 upregulation.

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VPS-39 sustains mitochondrial network expansion in micu-1(null) nematodes.

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VPS39 and SGPL1 expression influences mitochondria-lysosome contact sites in MICU1 deficient cells.

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VPS39/SGPL1 signaling may be a common signature of mitochondrial deficient cells.

Reduced expression of mitochondrial complex I subunit Ndufs2 does not impact healthspan in mice

Aging in mammals leads to reduction in genes encoding the 45-subunit mitochondrial electron transport chain complex I. It has been hypothesized that normal aging and age-related diseases such as Parkinson’s disease are in part due to modest decrease in expression of mitochondrial complex I subunits. By contrast, diminishing expression of mitochondrial complex I genes in lower organisms increases lifespan. Furthermore, metformin, a putative complex I inhibitor, increases healthspan in mice and humans. In the present study, we investigated whether loss of one allele of Ndufs2, the catalytic subunit of mitochondrial complex I, impacts healthspan and lifespan in mice. Our results indicate that Ndufs2 hemizygous mice (Ndufs2^+/−) show no overt impairment in aging-related motor function, learning, tissue histology, organismal metabolism, or sensitivity to metformin in a C57BL6/J background. Despite a significant reduction of Ndufs2 mRNA, the mice do not demonstrate a significant decrease in complex I function. However, there are detectable transcriptomic changes in individual cell types and tissues due to loss of one allele of Ndufs2. Our data indicate that a 50% decline in mRNA of the core mitochondrial complex I subunit Ndufs2 is neither beneficial nor detrimental to healthspan.

Hijacking Cellular Stress Responses to Promote Lifespan

Organisms are constantly exposed to stress both from the external environment and internally within the cell. To maintain cellular homeostasis under different environmental and physiological conditions, cell have adapted various stress response signaling pathways, such as the heat shock response (HSR), unfolded protein responses of the mitochondria (UPR^MT), and the unfolded protein response of the endoplasmic reticulum (UPR^ER). As cells grow older, all cellular stress responses have been shown to deteriorate, which is a major cause for the physiological consequences of aging and the development of numerous age-associated diseases. In contrast, elevated stress responses are often associated with lifespan extension and amelioration of degenerative diseases in different model organisms, including C. elegans. Activating cellular stress response pathways could be considered as an effective intervention to alleviate the burden of aging by restoring function of essential damage-clearing machinery, including the ubiquitin-proteosome system, chaperones, and autophagy. Here, we provide an overview of newly emerging concepts of these stress response pathways in healthy aging and longevity with a focus on the model organism, C. elegans.

The Unfolded Protein Responses in Health, Aging, and Neurodegeneration: Recent Advances and Future Considerations

Aging and age-related neurodegeneration are both associated with the accumulation of unfolded and abnormally folded proteins, highlighting the importance of protein homeostasis (termed proteostasis) in maintaining organismal health. To this end, two cellular compartments with essential protein folding functions, the endoplasmic reticulum (ER) and the mitochondria, are equipped with unique protein stress responses, known as the ER unfolded protein response (UPR ER ) and the mitochondrial UPR (UPR mt ), respectively. These organellar UPRs play roles in shaping the cellular responses to proteostatic stress that occurs in aging and age-related neurodegeneration. The loss of adaptive UPR ER and UPR mt signaling potency with age contributes to a feed-forward cycle of increasing protein stress and cellular dysfunction. Likewise, UPR ER and UPR mt signaling is often altered in age-related neurodegenerative diseases; however, whether these changes counteract or contribute to the disease pathology appears to be context dependent. Intriguingly, altering organellar UPR signaling in animal models can reduce the pathological consequences of aging and neurodegeneration which has prompted clinical investigations of UPR signaling modulators as therapeutics. Here, we review the physiology of both the UPR ER and the UPR mt , discuss how UPR ER and UPR mt signaling changes in the context of aging and neurodegeneration, and highlight therapeutic strategies targeting the UPR ER and UPR mt that may improve human health.

Mitochondrial protein import determines lifespan through metabolic reprogramming and de novo serine biosynthesis

Sustained mitochondrial fitness relies on coordinated biogenesis and clearance. Both processes are regulated by constant targeting of proteins into the organelle. Thus, mitochondrial protein import sets the pace for mitochondrial abundance and function. However, our understanding of mitochondrial protein translocation as a regulator of longevity remains enigmatic. Here, we targeted the main protein import translocases and assessed their contribution to mitochondrial abundance and organismal physiology. We find that reduction in cellular mitochondrial load through mitochondrial protein import system suppression, referred to as MitoMISS, elicits a distinct longevity paradigm. We show that MitoMISS triggers the mitochondrial unfolded protein response, orchestrating an adaptive reprogramming of metabolism. Glycolysis and de novo serine biosynthesis are causatively linked to longevity, whilst mitochondrial chaperone induction is dispensable for lifespan extension. Our findings extent the pro-longevity role of UPR^mt and provide insight, relevant to the metabolic alterations that promote or undermine survival and longevity.

Mitochondrial function is linked to lifespan. Here the authors show that inhibition of mitochondrial protein import leads to a reduction in mitochondrial abundance and extends lifespan in Caenorhabditis elegans via activation of glycolysis and de novo serine biosynthesis.

Mitochondrial complex I as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD), the most prominent form of dementia in the elderly, has no cure. Strategies focused on the reduction of amyloid beta or hyperphosphorylated Tau protein have largely failed in clinical trials. Novel therapeutic targets and strategies are urgently needed. Emerging data suggest that in response to environmental stress, mitochondria initiate an integrated stress response (ISR) shown to be beneficial for healthy aging and neuroprotection. Here, we review data that implicate mitochondrial electron transport complexes involved in oxidative phosphorylation as a hub for small molecule-targeted therapeutics that could induce beneficial mitochondrial ISR. Specifically, partial inhibition of mitochondrial complex I has been exploited as a novel strategy for multiple human conditions, including AD, with several small molecules being tested in clinical trials. We discuss current understanding of the molecular mechanisms involved in this counterintuitive approach. Since this strategy has also been shown to enhance health and life span, the development of safe and efficacious complex I inhibitors could promote healthy aging, delaying the onset of age-related neurodegenerative diseases.

Mitochondrial ion channels in aging and related diseases

Transport of materials and information across cellular boundaries, such as plasma, mitochondrial and nuclear membranes, happens mainly through varieties of ion channels and pumps. Various biophysical and biochemical processes play vital roles. The underlying mechanisms and associated phenomenological lipid membrane transports are linked directly or indirectly to the cell health condition. Mitochondrial membranes (mitochondrial outer membrane (MOM) and mitochondrial inner membrane (MIM)) host crucial cellular processes. Their malfunction is often found responsible for the rise of cell-originated diseases, including cancer, Alzheimer's, neurodegenerative disease, etc. A large number of ion channels active across MOM and MIM are known to belong to vital cell-based structures found to be linked directly to cellular signaling. Hence their malfunctions are often found to contribute to abnormalities in intracellular communication, which may even be associated with the rise of various diseases. In this article, the aim is to pinpoint ion channels that are directly or indirectly linked to especially aging and related abnormalities in health conditions. An attempt has been made to address the natural structures of these channels, their mutated conditions, and the ways we may cause interventions in their malfunctioning. The malfunction of ion channel subunits, including especially various proteins, involved directly in channel formation and/or indirectly in channel stabilization, leads to the rise of various channel-specific diseases, which are known as channelopathies. Channelopathies in aging will be discussed briefly. This mini-review may be found as an important reference for drug discovery scientists dealing with aging-related diseases.

High-Content C. elegans Screen Identifies Natural Compounds Impacting Mitochondria-Lipid Homeostasis and Promoting Healthspan

The aging process is concurrently shaped by genetic and extrinsic factors. In this work, we screened a small library of natural compounds, many of marine origin, to identify novel possible anti-aging interventions in Caenorhabditis elegans, a powerful model organism for aging studies. To this aim, we exploited a high-content microscopy platform to search for interventions able to induce phenotypes associated with mild mitochondrial stress, which is known to promote animal's health- and lifespan. Worms were initially exposed to three different concentrations of the drugs in liquid culture, in search of those affecting animal size and expression of mitochondrial stress response genes. This was followed by a validation step with nine compounds on solid media to refine compounds concentration, which led to the identification of four compounds (namely isobavachalcone, manzamine A, kahalalide F and lutein) consistently affecting development, fertility, size and lipid content of the nematodes. Treatment of Drosophila cells with the four hits confirmed their effects on mitochondria activity and lipid content. Out of these four, two were specifically chosen for analysis of age-related parameters, kahalalide F and lutein, which conferred increased resistance to heat and oxidative stress and extended animals' healthspan. We also found that, out of different mitochondrial stress response genes, only the C. elegans ortholog of the synaptic regulatory proteins neuroligins, nlg-1, was consistently induced by the two compounds and mediated lutein healthspan effects.

Cross-species screening platforms identify EPS-8 as a critical link for mitochondrial stress and actin stabilization

The dysfunction of mitochondria is associated with the physiological consequences of aging and many age-related diseases. Therefore, critical quality control mechanisms exist to protect mitochondrial functions, including the unfolded protein response of the mitochondria (UPRMT). However, it is still unclear how UPRMT is regulated in mammals with mechanistic discrepancies between previous studies. Here, we reasoned that a study of conserved mechanisms could provide a uniquely powerful way to reveal previously uncharacterized components of the mammalian UPRMT. We performed cross-species comparison of genetic requirements for survival under—and in response to—mitochondrial stress between karyotypically normal human stem cells and the nematode Caenorhabditis elegans. We identified a role for EPS-8/EPS8 (epidermal growth factor receptor pathway substrate 8), a signaling protein adaptor, in general mitochondrial homeostasis and UPRMT regulation through integrin-mediated remodeling of the actin cytoskeleton. This study also highlights the use of cross-species comparisons in genetic screens to interrogate cellular pathways.

Deletion of Neuronal CuZnSOD Accelerates Age-Associated Muscle Mitochondria and Calcium Handling Dysfunction That Is Independent of Denervation and Precedes Sarcopenia

Skeletal muscle suffers atrophy and weakness with aging. Denervation, oxidative stress, and mitochondrial dysfunction are all proposed as contributors to age-associated muscle loss, but connections between these factors have not been established. We examined contractility, mitochondrial function, and intracellular calcium transients (ICTs) in muscles of mice throughout the life span to define their sequential relationships. We performed these same measures and analyzed neuromuscular junction (NMJ) morphology in mice with postnatal deletion of neuronal Sod1 (i-mn-Sod1^-/- mice), previously shown to display accelerated age-associated muscle loss and exacerbation of denervation in old age, to test relationships between neuronal redox homeostasis, NMJ degeneration and mitochondrial function. In control mice, the amount and rate of the decrease in mitochondrial NADH during contraction was greater in middle than young age although force was not reduced, suggesting decreased efficiency of NADH utilization prior to the onset of weakness. Declines in both the peak of the ICT and force were observed in old age. Muscles of i-mn-Sod1^-/- mice showed degeneration of mitochondrial and calcium handling functions in middle-age and a decline in force generation to a level not different from the old control mice, with maintenance of NMJ morphology. Together, the findings support the conclusion that muscle mitochondrial function decreases during aging and in response to altered neuronal redox status prior to NMJ deterioration or loss of mass and force suggesting mitochondrial defects contribute to sarcopenia independent of denervation.

Green tea catechins EGCG and ECG enhance the fitness and lifespan of Caenorhabditis elegans by complex I inhibition

Green tea catechins are associated with a delay in aging. We have designed the current study to investigate the impact and to unveil the target of the most abundant green tea catechins, epigallocatechin gallate (EGCG) and epicatechin gallate (ECG).

Experiments were performed in Caenorhabditis elegans to analyze cellular metabolism, ROS homeostasis, stress resistance, physical exercise capacity, health- and lifespan, and the underlying signaling pathways. Besides, we examined the impact of EGCG and ECG in isolated murine mitochondria.

A concentration of 2.5 μM EGCG and ECG enhanced health- and lifespan as well as stress resistance in C. elegans. Catechins hampered mitochondrial respiration in C. elegans after 6–12 h and the activity of complex I in isolated rodent mitochondria. The impaired mitochondrial respiration was accompanied by a transient drop in ATP production and a temporary increase in ROS levels in C. elegans. After 24 h, mitochondrial respiration and ATP levels got restored, and ROS levels even dropped below control conditions. The lifespan increases induced by EGCG and ECG were dependent on AAK-2/AMPK and SIR-2.1/SIRT1, as well as on PMK-1/p38 MAPK, SKN-1/NRF2, and DAF-16/FOXO. Long-term effects included significantly diminished fat content and enhanced SOD and CAT activities, required for the positive impact of catechins on lifespan.

In summary, complex I inhibition by EGCG and ECG induced a transient drop in cellular ATP levels and a temporary ROS burst, resulting in SKN-1 and DAF-16 activation. Through adaptative responses, catechins reduced fat content, enhanced ROS defense, and improved healthspan in the long term.

The mitochondrial permeability transition pore activates the mitochondrial unfolded protein response and promotes aging

Mitochondrial activity determines aging rate and the onset of chronic diseases. The mitochondrial permeability transition pore (mPTP) is a pathological pore in the inner mitochondrial membrane thought to be composed of the F-ATP synthase (complex V). OSCP, a subunit of F-ATP synthase, helps protect against mPTP formation. How the destabilization of OSCP may contribute to aging, however, is unclear. We have found that loss OSCP in the nematode Caenorhabditis elegans initiates the mPTP and shortens lifespan specifically during adulthood, in part via initiation of the mitochondrial unfolded protein response (UPR^mt). Pharmacological or genetic inhibition of the mPTP inhibits the UPR^mt and restores normal lifespan. Loss of the putative pore-forming component of F-ATP synthase extends adult lifespan, suggesting that the mPTP normally promotes aging. Our findings reveal how an mPTP/UPR^mt nexus may contribute to aging and age-related diseases and how inhibition of the UPR^mt may be protective under certain conditions.

Allele-specific mitochondrial stress induced by Multiple Mitochondrial Dysfunctions Syndrome 1 pathogenic mutations modeled in Caenorhabditis elegans

Multiple Mitochondrial Dysfunctions Syndrome 1 (MMDS1) is a rare, autosomal recessive disorder caused by mutations in the NFU1 gene. NFU1 is responsible for delivery of iron-sulfur clusters (ISCs) to recipient proteins which require these metallic cofactors for their function. Pathogenic variants of NFU1 lead to dysfunction of its target proteins within mitochondria. To date, 20 NFU1 variants have been reported and the unique contributions of each variant to MMDS1 pathogenesis is unknown. Given that over half of MMDS1 individuals are compound heterozygous for different NFU1 variants, it is valuable to investigate individual variants in an isogenic background. In order to understand the shared and unique phenotypes of NFU1 variants, we used CRISPR/Cas9 gene editing to recreate exact patient variants of NFU1 in the orthologous gene, nfu-1 (formerly lpd-8), in C. elegans. Five mutant C. elegans alleles focused on the presumptive iron-sulfur cluster interaction domain were generated and analyzed for mitochondrial phenotypes including respiratory dysfunction and oxidative stress. Phenotypes were variable between the mutant nfu-1 alleles and generally presented as an allelic series indicating that not all variants have lost complete function. Furthermore, reactive iron within mitochondria was evident in some, but not all, nfu-1 mutants indicating that iron dyshomeostasis may contribute to disease pathogenesis in some MMDS1 individuals.

Functional mitochondria are essential to life in eukaryotes, but they can be perterbured by inherent dysfunction of important proteins or stressors. Mitochondrial dysfunction is the root cause of dozens of diseases many of which involve complex phenotypes. One such disease is Multiple Mitochondrial Dysfunctions Syndrome 1, a pediatric-fatal disease that is poorly understood in part due to the lack of clarity about how mutations in the causative gene, NFU1, affect protein function and phenotype development and severity. Here we employ the power of CRISPR/Cas9 gene editing in the small nematode Caenorhabditis elegans to recreate five patient-specific mutations known to cause Multiple Mitochondrial Dysfunctions Syndrome 1. We are able to analyze each of these mutations individually, evaluate how mitochondrial dysfunction differs between them, and whether or not the phenotypes can be improved. We find that there are meaningful differences between each mutation which not only effects the types of stress that develop, but also the ability to rescue deleterious phenotypes. This work thus provides insight into disease pathogenesis and establishes a foundation for potential future therapeutic intervention.