Changes of mitochondrial ultrastructure and function during ageing in mice and Drosophila

Mitochondrial fatty acid oxidation drives senescence

Cellular senescence is a stress-induced irreversible cell cycle arrest involved in tumor suppression and aging. Many stresses, such as telomere shortening and oncogene activation, induce senescence by damaging nuclear DNA. However, the mechanisms linking DNA damage to senescence remain unclear. Here, we show that DNA damage response (DDR) signaling to mitochondria triggers senescence. A genome-wide small interfering RNA screen implicated the outer mitochondrial transmembrane protein BNIP3 in senescence induction. We found that BNIP3 is phosphorylated by the DDR kinase ataxia telangiectasia mutated (ATM) and contributes to an increase in the number of mitochondrial cristae. Stable isotope labeling metabolomics indicated that the increase in cristae enhances fatty acid oxidation (FAO) to acetyl-coenzyme A (acetyl-CoA). This promotes histone acetylation and expression of the cyclin-dependent kinase inhibitor p16INK4a. Notably, pharmacological activation of FAO alone induced senescence both in vitro and in vivo. Thus, mitochondrial energy metabolism plays a critical role in senescence induction and is a potential intervention target to control senescence.

The Roles of Mitochondria in Human Being's Life and Aging

The universe began 13.8 billion years ago, and Earth was born 4.6 billion years ago. Early traces of life were found as soon as 4.1 billion years ago; then, ~200,000 years ago, the human being was born. The evolution of life on earth was to become individual rather than cellular life. The birth of mitochondria made this possible to be the individual life. Since then, individuals have had a limited time of life. It was 1.4 billion years ago that a bacterial cell began living inside an archaeal host cell, a form of endosymbiosis that is the development of eukaryotic cells, which contain a nucleus and other membrane-bound compartments. The bacterium started to provide its host cell with additional energy, and the interaction eventually resulted in a eukaryotic cell, with both archaeal (the host cell) and bacterial (mitochondrial) origins still having genomes. The cells survived high concentrations of oxygen producing more energy inside the cell. Further, the roles of mitochondria in human being's life and aging will be discussed.

Aging mitochondria in the context of SARS-CoV-2: exploring interactions and implications

The coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has presented global challenges with a diverse clinical spectrum, including severe respiratory complications and systemic effects. This review explores the intricate relationship between mitochondrial dysfunction, aging, and obesity in COVID-19. Mitochondria are vital for cellular energy provision and resilience against age-related macromolecule damage accumulation. They manage energy allocation in cells, activating adaptive responses and stress signals such as redox imbalance and innate immunity activation. As organisms age, mitochondrial function diminishes. Aging and obesity, linked to mitochondrial dysfunction, compromise the antiviral response, affecting the release of interferons, and worsening COVID-19 severity. Furthermore, the development of post-acute sequelae of SARS-CoV-2 infection (PASC), also known as long COVID has been associated with altered energy metabolism, and chronic immune dysregulation derived from mitochondrial dysfunction. Understanding the interplay between mitochondria, aging, obesity, and viral infections provides insights into COVID-19 pathogenesis. Targeting mitochondrial health may offer potential therapeutic strategies to mitigate severe outcomes and address long-term consequences in infected individuals.

An evolving roadmap: using mitochondrial physiology to help guide conservation efforts

The crucial role of aerobic energy production in sustaining eukaryotic life positions mitochondrial processes as key determinants of an animal's ability to withstand unpredictable environments. The advent of new techniques facilitating the measurement of mitochondrial function offers an increasingly promising tool for conservation approaches. Herein, we synthesize the current knowledge on the links between mitochondrial bioenergetics, ecophysiology and local adaptation, expanding them to the wider conservation physiology field. We discuss recent findings linking cellular bioenergetics to whole-animal fitness, in the current context of climate change. We summarize topics, questions, methods, pitfalls and caveats to help provide a comprehensive roadmap for studying mitochondria from a conservation perspective. Our overall aim is to help guide conservation in natural populations, outlining the methods and techniques that could be most useful to assess mitochondrial function in the field.

PKM2 regulates metabolic flux and oxidative stress in the murine heart

Cardiac metabolism ensures a continuous ATP supply, primarily using fatty acids in a healthy state and favoring glucose in pathological conditions. Pyruvate kinase muscle (PKM) controls the final step of glycolysis, with PKM1 being the main isoform in the heart. PKM2, elevated in various heart diseases, has been suggested to play a protective role in cardiac stress, but its function in basal cardiac metabolism remains unclear. We examined hearts from global PKM2 knockout (PKM2-/-) mice and found reduced intracellular glucose. Isotopic tracing of U-13C glucose revealed a shift to biosynthetic pathways in PKM2-/- cardiomyocytes. Total ATP content was two-thirds lower in PKM2-/- hearts, and functional analysis indicated reduced mitochondrial oxygen consumption. Total reactive oxygen species (ROS) and mitochondrial superoxide were also increased in PKM2-/- cardiomyocytes. Intriguingly, PKM2-/- hearts had preserved ejection fraction compared to controls. Mechanistically, increased calcium/calmodulin-dependent kinase II activity and phospholamban phosphorylation may contribute to higher sarcoendoplasmic reticulum calcium ATPase 2 pump activity in PKM2-/- hearts. Loss of PKM2 led to altered glucose metabolism, diminished mitochondrial function, and increased ROS in cardiomyocytes. These data suggest that cardiac PKM2 acts as an important rheostat to maintain ATP levels while limiting oxidative stress. Although loss of PKM2 did not impair baseline contractility, its absence may make hearts more sensitive to environmental stress or injury.

Quantitative assessment of morphological changes in lipid droplets and lipid-mito interactions with aging in brown adipose

The physical characteristics of brown adipose tissue (BAT) are defined by the presence of multilocular lipid droplets (LDs) within the brown adipocytes and a high abundance of iron-containing mitochondria, which give it its characteristic color. Normal mitochondrial function is, in part, regulated by organelle-to-organelle contacts. For example, the contact sites that mediate mitochondria-LD interactions are thought to have various physiological roles, such as the synthesis and metabolism of lipids. Aging is associated with mitochondrial dysfunction, and previous studies show that there are changes in mitochondrial structure and the proteins that modulate organelle contact sites. However, how mitochondria-LD interactions change with aging has yet to be fully clarified. Therefore, we sought to define age-related changes in LD morphology and mitochondria-lipid interactions in BAT. We examined the three-dimensional morphology of mitochondria and LDs in young (3-month) and aged (2-year) murine BAT using serial block face-scanning electron microscopy and the Amira program for segmentation, analysis, and quantification. Our analyses showed reductions in LD volume, area, and perimeter in aged samples in comparison to young samples. Additionally, we observed changes in LD appearance and type in aged samples compared to young samples. Notably, we found differences in mitochondrial interactions with LDs, which could implicate that these contacts may be important for energetics in aging. Upon further investigation, we also found changes in mitochondrial and cristae structure for the mitochondria interacting with LDs. Overall, these data define the nature of LD morphology and organelle-organelle contacts during aging and provide insight into LD contact site changes that interconnect biogerontology with mitochondrial function, metabolism, and bioactivity in aged BAT.

Effects of High-Intensity Swimming Interval Training on Area, Perimeter, Circularity Index and Phenotype of Cardiac Mitochondrial Ultrastructure in Sprague Dawley Rats

Physical inactivity impairs health by increasing morbidity. In childhood, modifiable risk factors associated with cardiovascular pathologies and related to mitochondrial function and structure are initiated by physical inactivity. The objective of this study was to analyze the effect of high-intensity swimming interval training (HIIT-swim) on cardiac mitochondrial ultrastructure in young Sprague Dawley rats compared with a sedentary group. Five-week-old Sprague Dawley rats (n = 18) were divided into a control group (C) (n = 6), a sedentary group (S) (n = 6) and an HIIT-swim group (H-s) (n = 6), the last of which performed HIIT-swim for 4 weeks. A mitochondrial ultrastructural evaluation was performed using transmission electron microscopy. In the H-s rats, mitochondrial areas and perimeters were found to be statistically significantly different from those of the C and S rats. In addition, no predominant intramitochondrial multifragmentation was observed in the mitochondria of H-s rats, but multifragmentation was evident in the mitochondria of S rats.

The MICOS Complex Regulates Mitochondrial Structure and Oxidative Stress During Age-Dependent Structural Deficits in the Kidney

The kidney filters nutrient waste and bodily fluids from the bloodstream, in addition to secondary functions of metabolism and hormone secretion, requiring an astonishing amount of energy to maintain its functions. In kidney cells, mitochondria produce adenosine triphosphate (ATP) and help maintain kidney function. Due to aging, the efficiency of kidney functions begins to decrease. Dysfunction in mitochondria and cristae, the inner folds of mitochondria, is a hallmark of aging. Therefore, age-related kidney function decline could be due to changes in mitochondrial ultrastructure, increased reactive oxygen species (ROS), and subsequent alterations in metabolism and lipid composition. We sought to understand if there is altered mitochondrial ultrastructure, as marked by 3D morphological changes, across time in tubular kidney cells. Serial block facing-scanning electron microscope (SBF-SEM) and manual segmentation using the Amira software were used to visualize murine kidney samples during the aging process at 3 months (young) and 2 years (old). We found that 2-year mitochondria are more fragmented, compared to the 3-month, with many uniquely shaped mitochondria observed across aging, concomitant with shifts in ROS, metabolomics, and lipid homeostasis. Furthermore, we show that the mitochondrial contact site and cristae organizing system (MICOS) complex is impaired in the kidney due to aging. Disruption of the MICOS complex shows altered mitochondrial calcium uptake and calcium retention capacity, as well as generation of oxidative stress. We found significant, detrimental structural changes to aged kidney tubule mitochondria suggesting a potential mechanism underlying why kidney diseases occur more readily with age. We hypothesize that disruption in the MICOS complex further exacerbates mitochondrial dysfunction, creating a vicious cycle of mitochondrial degradation and oxidative stress, thus impacting kidney health.

Characterization of dUTPase expression in mouse postnatal development and adult neurogenesis

The enzyme dUTPase has an essential role in maintaining genomic integrity. In mouse, nuclear and mitochondrial isoforms of the enzyme have been described. Here we present the isoform-specific mRNA expression levels in different murine organs during development using RT-qPCR. In this study, we analyzed organs of 14.5-day embryos and of postnatal 2-, 4-, 10-week- and 13-month-old mice. We demonstrate organ-, sex- and developmental stage-specific differences in the mRNA expression levels of both isoforms. We found high mRNA expression level of the nuclear isoform in the embryo brain, and the expression level remained relatively high in the adult brain as well. This was surprising, since dUTPase is known to play an important role in proliferating cells, and mass production of neural cells is completed by adulthood. Thus, we investigated the pattern of the dUTPase protein expression specifically in the adult brain with immunostaining and found that dUTPase is present in the germinative zones, the subventricular and the subgranular zones, where neurogenesis occurs and in the rostral migratory stream where neuroblasts migrate to the olfactory bulb. These novel findings suggest that dUTPase may have a role in cell differentiation and indicate that accurate dTTP biosynthesis can be vital, especially in neurogenesis.

Disruption of the mitochondrial network in a mouse model of Huntington's disease visualized by in-tissue multiscale 3D electron microscopy

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an expanded CAG repeat in the coding sequence of huntingtin protein. Initially, it predominantly affects medium-sized spiny neurons (MSSNs) of the corpus striatum. No effective treatment is still available, thus urging the identification of potential therapeutic targets. While evidence of mitochondrial structural alterations in HD exists, previous studies mainly employed 2D approaches and were performed outside the strictly native brain context. In this study, we adopted a novel multiscale approach to conduct a comprehensive 3D in situ structural analysis of mitochondrial disturbances in a mouse model of HD. We investigated MSSNs within brain tissue under optimal structural conditions utilizing state-of-the-art 3D imaging technologies, specifically FIB/SEM for the complete imaging of neuronal somas and Electron Tomography for detailed morphological examination, and image processing-based quantitative analysis. Our findings suggest a disruption of the mitochondrial network towards fragmentation in HD. The network of interlaced, slim and long mitochondria observed in healthy conditions transforms into isolated, swollen and short entities, with internal cristae disorganization, cavities and abnormally large matrix granules.

Senescent glia link mitochondrial dysfunction and lipid accumulation

Senescence is a cellular state linked to ageing and age-onset disease across many mammalian species1,2. Acutely, senescent cells promote wound healing3,4 and prevent tumour formation5; but they are also pro-inflammatory, thus chronically exacerbate tissue decline. Whereas senescent cells are active targets for anti-ageing therapy6-11, why these cells form in vivo, how they affect tissue ageing and the effect of their elimination remain unclear12,13. Here we identify naturally occurring senescent glia in ageing Drosophila brains and decipher their origin and influence. Using Activator protein 1 (AP1) activity to screen for senescence14,15, we determine that senescent glia can appear in response to neuronal mitochondrial dysfunction. In turn, senescent glia promote lipid accumulation in non-senescent glia; similar effects are seen in senescent human fibroblasts in culture. Targeting AP1 activity in senescent glia mitigates senescence biomarkers, extends fly lifespan and health span, and prevents lipid accumulation. However, these benefits come at the cost of increased oxidative damage in the brain, and neuronal mitochondrial function remains poor. Altogether, our results map the trajectory of naturally occurring senescent glia in vivo and indicate that these cells link key ageing phenomena: mitochondrial dysfunction and lipid accumulation.

Mitochondrial dysfunction precedes hippocampal IL-1β transcription and cognitive impairments after low-dose lipopolysaccharide injection in aged mice

Acute cognitive impairments termed delirium often occur after inflammatory insults in elderly patients. While previous preclinical studies suggest mitochondria as a target for reducing neuroinflammation and cognitive impairments after LPS injection, fewer studies have evaluated the effects of a low-grade systemic inflammation in the aged brain. Thus, to identify the significance of mitochondrial dysfunction after a clinically relevant systemic inflammatory stimulus, we injected old-aged mice (18-20 months) with low-dose lipopolysaccharide (LPS, 0.04 mg/kg). LPS injection reduced mitochondrial respiration in the hippocampus 24 h after injection (respiratory control ratio [RCR], state3u/state4o; control = 2.82 ± 0.19, LPS = 2.57 ± 0.08). However, gene expression of the pro-inflammatory cytokine IL-1β was increased (RT-PCR, control = 1.00 ± 0.30; LPS = 2.01 ± 0.67) at a more delayed time point, 48 h after LPS injection. Such changes were associated with cognitive impairments in the Barnes maze and fear chamber tests. Notably, young mice were unaffected by low-dose LPS, suggesting that mitochondrial dysfunction precedes neuroinflammation and cognitive decline in elderly patients following a low-grade systemic insult. Our findings highlight mitochondria as a potential therapeutic target for reducing delirium in elderly patients.

A semi-automatic method for extracting mitochondrial cristae characteristics from 3D focused ion beam scanning electron microscopy data

Mitochondria are the main suppliers of energy for cells and their bioenergetic function is regulated by mitochondrial dynamics: the constant changes in mitochondria size, shape, and cristae morphology to secure cell homeostasis. Although changes in mitochondrial function are implicated in a wide range of diseases, our understanding is challenged by a lack of reliable ways to extract spatial features from the cristae, the detailed visualization of which requires electron microscopy (EM). Here, we present a semi-automatic method for the segmentation, 3D reconstruction, and shape analysis of mitochondria, cristae, and intracristal spaces based on 2D EM images of the murine hippocampus. We show that our method provides a more accurate characterization of mitochondrial ultrastructure in 3D than common 2D approaches and propose an operational index of mitochondria's internal organization. With an improved consistency of 3D shape analysis and a decrease in the workload needed for large-scale analysis, we speculate that this tool will help increase our understanding of mitochondrial dynamics in health and disease.

Mitochondrial stress in GABAergic neurons non-cell autonomously regulates organismal health and aging

Mitochondrial stress within the nervous system can trigger non-cell autonomous responses in peripheral tissues. However, the specific neurons involved and their impact on organismal aging and health have remained incompletely understood. Here, we demonstrate that mitochondrial stress in γ-aminobutyric acid-producing (GABAergic) neurons in Caenorhabditis elegans ( C. elegans ) is sufficient to significantly alter organismal lifespan, stress tolerance, and reproductive capabilities. This mitochondrial stress also leads to significant changes in mitochondrial mass, energy production, and levels of reactive oxygen species (ROS). DAF-16/FoxO activity is enhanced by GABAergic neuronal mitochondrial stress and mediates the induction of these non-cell-autonomous effects. Moreover, our findings indicate that GABA signaling operates within the same pathway as mitochondrial stress in GABAergic neurons, resulting in non-cell-autonomous alterations in organismal stress tolerance and longevity. In summary, these data suggest the crucial role of GABAergic neurons in detecting mitochondrial stress and orchestrating non-cell-autonomous changes throughout the organism.

Multi-omics characterization of partial chemical reprogramming reveals evidence of cell rejuvenation

Partial reprogramming by cyclic short-term expression of Yamanaka factors holds promise for shifting cells to younger states and consequently delaying the onset of many diseases of aging. However, the delivery of transgenes and potential risk of teratoma formation present challenges for in vivo applications. Recent advances include the use of cocktails of compounds to reprogram somatic cells, but the characteristics and mechanisms of partial cellular reprogramming by chemicals remain unclear. Here, we report a multi-omics characterization of partial chemical reprogramming in fibroblasts from young and aged mice. We measured the effects of partial chemical reprogramming on the epigenome, transcriptome, proteome, phosphoproteome, and metabolome. At the transcriptome, proteome, and phosphoproteome levels, we saw widescale changes induced by this treatment, with the most notable signature being an upregulation of mitochondrial oxidative phosphorylation. Furthermore, at the metabolome level, we observed a reduction in the accumulation of aging-related metabolites. Using both transcriptomic and epigenetic clock-based analyses, we show that partial chemical reprogramming reduces the biological age of mouse fibroblasts. We demonstrate that these changes have functional impacts, as evidenced by changes in cellular respiration and mitochondrial membrane potential. Taken together, these results illuminate the potential for chemical reprogramming reagents to rejuvenate aged biological systems and warrant further investigation into adapting these approaches for in vivo age reversal.

Mitochondrial CISD1/Cisd accumulation blocks mitophagy and genetic or pharmacological inhibition rescues neurodegenerative phenotypes in Pink1/parkin models

BACKGROUND

Mitochondrial dysfunction and toxic protein aggregates have been shown to be key features in the pathogenesis of neurodegenerative diseases, such as Parkinson's disease (PD). Functional analysis of genes linked to PD have revealed that the E3 ligase Parkin and the mitochondrial kinase PINK1 are important factors for mitochondrial quality control. PINK1 phosphorylates and activates Parkin, which in turn ubiquitinates mitochondrial proteins priming them and the mitochondrion itself for degradation. However, it is unclear whether dysregulated mitochondrial degradation or the toxic build-up of certain Parkin ubiquitin substrates is the driving pathophysiological mechanism leading to PD. The iron-sulphur cluster containing proteins CISD1 and CISD2 have been identified as major targets of Parkin in various proteomic studies.

METHODS

We employed in vivo Drosophila and human cell culture models to study the role of CISD proteins in cell and tissue viability as well as aged-related neurodegeneration, specifically analysing aspects of mitophagy and autophagy using orthogonal assays.

RESULTS

We show that the Drosophila homolog Cisd accumulates in Pink1 and parkin mutant flies, as well as during ageing. We observed that build-up of Cisd is particularly toxic in neurons, resulting in mitochondrial defects and Ser65-phospho-Ubiquitin accumulation. Age-related increase of Cisd blocks mitophagy and impairs autophagy flux. Importantly, reduction of Cisd levels upregulates mitophagy in vitro and in vivo, and ameliorates pathological phenotypes in locomotion, lifespan and neurodegeneration in Pink1/parkin mutant flies. In addition, we show that pharmacological inhibition of CISD1/2 by rosiglitazone and NL-1 induces mitophagy in human cells and ameliorates the defective phenotypes of Pink1/parkin mutants.

CONCLUSION

Altogether, our studies indicate that Cisd accumulation during ageing and in Pink1/parkin mutants is a key driver of pathology by blocking mitophagy, and genetically and pharmacologically inhibiting CISD proteins may offer a potential target for therapeutic intervention.

A review of the pathophysiological mechanisms of doxorubicin-induced cardiotoxicity and aging

The population of cancer survivors is rapidly increasing due to improving healthcare. However, cancer therapies often have long-term side effects. One example is cancer therapy-related cardiac dysfunction (CTRCD) caused by doxorubicin: up to 9% of the cancer patients treated with this drug develop heart failure at a later stage. In recent years, doxorubicin-induced cardiotoxicity has been associated with an accelerated aging phenotype and cellular senescence in the heart. In this review we explain the evidence of an accelerated aging phenotype in the doxorubicin-treated heart by comparing it to healthy aged hearts, and shed light on treatment strategies that are proposed in pre-clinical settings. We will discuss the accelerated aging phenotype and the impact it could have in the clinic and future research.

Handling Difficult Cryo-ET Samples: A Study with Primary Neurons from Drosophila melanogaster

Cellular neurobiology has benefited from recent advances in the field of cryo-electron tomography (cryo-ET). Numerous structural and ultrastructural insights have been obtained from plunge-frozen primary neurons cultured on electron microscopy grids. With most primary neurons having been derived from rodent sources, we sought to expand the breadth of sample availability by using primary neurons derived from 3rd instar Drosophila melanogaster larval brains. Ultrastructural abnormalities were encountered while establishing this model system for cryo-ET, which were exemplified by excessive membrane blebbing and cellular fragmentation. To optimize neuronal samples, we integrated substrate selection, micropatterning, montage data collection, and chemical fixation. Efforts to address difficulties in establishing Drosophila neurons for future cryo-ET studies in cellular neurobiology also provided insights that future practitioners can use when attempting to establish other cell-based model systems.

Next Generation, Modifiable Cardiometabolic Biomarkers: Mitochondrial Adaptation and Metabolic Resilience: A Scientific Statement From the American Heart Association

Cardiometabolic risk is increasing in prevalence across the life span with disproportionate ramifications for youth at socioeconomic disadvantage. Established risk factors and associated disease progression are harder to reverse as they become entrenched over time; if current trends are unchecked, the consequences for individual and societal wellness will become untenable. Interrelated root causes of ectopic adiposity and insulin resistance are understood but identified late in the trajectory of systemic metabolic dysregulation when traditional cardiometabolic risk factors cross current diagnostic thresholds of disease. Thus, children at cardiometabolic risk are often exposed to suboptimal metabolism over years before they present with clinical symptoms, at which point life-long reliance on pharmacotherapy may only mitigate but not reverse the risk. Leading-edge indicators are needed to detect the earliest departure from healthy metabolism, so that targeted, primordial, and primary prevention of cardiometabolic risk is possible. Better understanding of biomarkers that reflect the earliest transitions to dysmetabolism, beginning in utero, ideally biomarkers that are also mechanistic/causal and modifiable, is critically needed. This scientific statement explores emerging biomarkers of cardiometabolic risk across rapidly evolving and interrelated "omic" fields of research (the epigenome, microbiome, metabolome, lipidome, and inflammasome). Connections in each domain to mitochondrial function are identified that may mediate the favorable responses of each of the omic biomarkers featured to a heart-healthy lifestyle, notably to nutritional interventions. Fuller implementation of evidence-based nutrition must address environmental and socioeconomic disparities that can either facilitate or impede response to therapy.

Multi-omics characterization of partial chemical reprogramming reveals evidence of cell rejuvenation

Partial reprogramming by cyclic short-term expression of Yamanaka factors holds promise for shifting cells to younger states and consequently delaying the onset of many diseases of aging. However, the delivery of transgenes and potential risk of teratoma formation present challenges for in vivo applications. Recent advances include the use of cocktails of compounds to reprogram somatic cells, but the characteristics and mechanisms of partial cellular reprogramming by chemicals remain unclear. Here, we report a multi-omics characterization of partial chemical reprogramming in fibroblasts from young and aged mice. We measured the effects of partial chemical reprogramming on the epigenome, transcriptome, proteome, phosphoproteome, and metabolome. At the transcriptome, proteome, and phosphoproteome levels, we saw widescale changes induced by this treatment, with the most notable signature being an upregulation of mitochondrial oxidative phosphorylation. Furthermore, at the metabolome level, we observed a reduction in the accumulation of aging-related metabolites. Using both transcriptomic and epigenetic clock-based analyses, we show that partial chemical reprogramming reduces the biological age of mouse fibroblasts. We demonstrate that these changes have functional impacts, as evidenced by changes in cellular respiration and mitochondrial membrane potential. Taken together, these results illuminate the potential for chemical reprogramming reagents to rejuvenate aged biological systems and warrant further investigation into adapting these approaches for in vivo age reversal.

Dual roles of myocardial mitochondrial AKT on diabetic cardiomyopathy and whole body metabolism

BACKGROUND

The PI3K/AKT pathway transduces the majority of the metabolic actions of insulin. In addition to cytosolic targets, insulin-stimulated phospho-AKT also translocates to mitochondria in the myocardium. Mouse models of diabetes exhibit impaired mitochondrial AKT signaling but the implications of this on cardiac structure and function is unknown. We hypothesized that loss of mitochondrial AKT signaling is a critical step in cardiomyopathy and reduces cardiac oxidative phosphorylation.

METHODS

To focus our investigation on the pathophysiological consequences of this mitochondrial signaling pathway, we generated transgenic mouse models of cardiac-specific, mitochondria-targeting, dominant negative AKT1 (CAMDAKT) and constitutively active AKT1 expression (CAMCAKT). Myocardial structure and function were examined using echocardiography, histology, and biochemical assays. We further investigated the underlying effects of mitochondrial AKT1 on mitochondrial structure and function, its interaction with ATP synthase, and explored in vivo metabolism beyond the heart.

RESULTS

Upon induction of dominant negative mitochondrial AKT1, CAMDAKT mice developed cardiac fibrosis accompanied by left ventricular hypertrophy and dysfunction. Cardiac mitochondrial oxidative phosphorylation efficiency and ATP content were reduced, mitochondrial cristae structure was lost, and ATP synthase structure was compromised. Conversely, CAMCAKT mice were protected against development of diabetic cardiomyopathy when challenged with a high calorie diet. Activation of mitochondrial AKT1 protected cardiac function and increased fatty acid uptake in myocardium. In addition, total energy expenditure was increased in CAMCAKT mice, accompanied by reduced adiposity and reduced development of fatty liver.

CONCLUSION

CAMDAKT mice modeled the effects of impaired mitochondrial signaling which occurs in the diabetic myocardium. Disruption of this pathway is a key step in the development of cardiomyopathy. Activation of mitochondrial AKT1 in CAMCAKT had a protective role against diabetic cardiomyopathy as well as improved metabolism beyond the heart.

Advances in mitochondria-centered mechanism behind the roles of androgens and androgen receptor in the regulation of glucose and lipid metabolism

An increasing number of studies have reported that androgens and androgen receptors (AR) play important roles in the regulation of glucose and lipid metabolism. Impaired glucose and lipid metabolism and the development of obesity-related diseases have been found in either hypogonadal men or male rodents with androgen deficiency. Exogenous androgens supplementation can effectively improve these disorders, but the mechanism by which androgens regulate glucose and lipid metabolism has not been fully elucidated. Mitochondria, as powerhouses within cells, are key organelles influencing glucose and lipid metabolism. Evidence from both pre-clinical and clinical studies has reported that the regulation of glucose and lipid metabolism by androgens/AR is strongly associated with the impact on the content and function of mitochondria, but few studies have systematically reported the regulatory effect and the molecular mechanism. In this paper, we review the effect of androgens/AR on mitochondrial content, morphology, quality control system, and function, with emphases on molecular mechanisms. Additionally, we discuss the sex-dimorphic effect of androgens on mitochondria. This paper provides a theoretical basis for shedding light on the influence and mechanism of androgens on glucose and lipid metabolism and highlights the mitochondria-based explanation for the sex-dimorphic effect of androgens on glucose and lipid metabolism.

From Physiology to Pathology: The Role of Mitochondria in Acute Kidney Injuries and Chronic Kidney Diseases

Background

Renal diseases remain an increasing public health issue affecting millions of people. The kidney is a highly energetic organ that is rich in mitochondria. Numerous studies have demonstrated the important role of mitochondria in maintaining normal kidney function and in the pathogenesis of various renal diseases, including acute kidney injuries (AKIs) and chronic kidney diseases (CKDs).

Summary

Under physiological conditions, fine-tuning mitochondrial energy balance, mitochondrial dynamics (fission and fusion processes), mitophagy, and biogenesis maintain mitochondrial fitness. While under AKI and CKD conditions, disruption of mitochondrial energy metabolism leads to increased oxidative stress. In addition, mitochondrial dynamics shift to excessive mitochondrial fission, mitochondrial autophagy is impaired, and mitochondrial biogenesis is also compromised. These mitochondrial injuries regulate renal cellular functions either directly or indirectly. Mitochondria-targeted approaches, containing genetic (microRNAs) and pharmaceutical methods (mitochondria-targeting antioxidants, mitochondrial permeability pore inhibitors, mitochondrial fission inhibitors, and biogenesis activators), are emerging as important therapeutic strategies for AKIs and CKDs.

Key Messages

Mitochondria play a critical role in the pathogenesis of AKIs and CKDs. This review provides an updated overview of mitochondrial homeostasis under physiological conditions and the involvement of mitochondrial dysfunction in renal diseases. Finally, we summarize the current status of mitochondria-targeted strategies in attenuating renal diseases.

Handling difficult cryo-ET samples: A study with primary neurons from Drosophila melanogaster

Cellular neurobiology has benefited from recent advances in the field of cryo-electron tomography (cryo-ET). Numerous structural and ultrastructural insights have been obtained from plunge-frozen primary neurons cultured on electron microscopy grids. With most primary neurons been derived from rodent sources, we sought to expand the breadth of sample availability by using primary neurons derived from 3rd instar Drosophila melanogaster larval brains. Ultrastructural abnormalities were encountered while establishing this model system for cryo-ET, which were exemplified by excessive membrane blebbing and cellular fragmentation. To optimize neuronal samples, we integrated substrate selection, micropatterning, montage data collection, and chemical fixation. Efforts to address difficulties in establishing Drosophila neurons for future cryo-ET studies in cellular neurobiology also provided insights that future practitioners can use when attempting to establish other cell-based model systems.

What do we mean by "aging"? Questions and perspectives revealed by studies in Drosophila

What is the nature of aging, and how best can we study it? Here, using a series of questions that highlight differing perspectives about the nature of aging, we ask how data from Drosophila melanogaster at the organismal, tissue, cellular, and molecular levels shed light on the complex interactions among the phenotypes associated with aging. Should aging be viewed as an individual's increasing probability of mortality over time or as a progression of physiological states? Are all age-correlated changes in physiology detrimental to vigor or are some compensatory changes that maintain vigor? Why do different age-correlated functions seem to change at different rates in a single individual as it ages? Should aging be considered as a single, integrated process across the scales of biological resolution, from organismal to molecular, or must we consider each level of biological scale as a separate, distinct entity? Viewing aging from these differing perspectives yields distinct but complementary interpretations about the properties and mechanisms of aging and may offer a path through the complexities related to understanding the nature of aging.

Using Live Cell STED Imaging to Visualize Mitochondrial Inner Membrane Ultrastructure in Neuronal Cell Models

Mitochondria play many essential roles in the cell, including energy production, regulation of Ca2+ homeostasis, lipid biosynthesis, and production of reactive oxygen species (ROS). These mitochondria-mediated processes take on specialized roles in neurons, coordinating aerobic metabolism to meet the high energy demands of these cells, modulating Ca2+ signaling, providing lipids for axon growth and regeneration, and tuning ROS production for neuronal development and function. Mitochondrial dysfunction is therefore a central driver in neurodegenerative diseases. Mitochondrial structure and function are inextricably linked. The morphologically complex inner membrane with structural infolds called cristae harbors many molecular systems that perform the signature processes of the mitochondrion. The architectural features of the inner membrane are ultrastructural and therefore, too small to be visualized by traditional diffraction-limited resolved microscopy. Thus, most insights on mitochondrial ultrastructure have come from electron microscopy on fixed samples. However, emerging technologies in super-resolution fluorescence microscopy now provide resolution down to tens of nanometers, allowing visualization of ultrastructural features in live cells. Super-resolution imaging therefore offers an unprecedented ability to directly image fine details of mitochondrial structure, nanoscale protein distributions, and cristae dynamics, providing fundamental new insights that link mitochondria to human health and disease. This protocol presents the use of stimulated emission depletion (STED) super-resolution microscopy to visualize the mitochondrial ultrastructure of live human neuroblastoma cells and primary rat neurons. This procedure is organized into five sections: (1) growth and differentiation of the SH-SY5Y cell line, (2) isolation, plating, and growth of primary rat hippocampal neurons, (3) procedures for staining cells for live STED imaging, (4) procedures for live cell STED experiments using a STED microscope for reference, and (5) guidance for segmentation and image processing using examples to measure and quantify morphological features of the inner membrane.

Leafhopper males compensate for unclear directional cues in vibration-mediated mate localization

Ambient noise and transmission properties of the substrate pose challenges in vibrational signal-mediated mating behavior of arthropods, because vibrational signal production is energetically demanding. We explored implications of these challenges in the leafhopper Aphrodes makarovi (Insecta: Hemiptera: Cicadellidae) by exposing males to various kinds of vibrational noise on a natural substrate and challenging them to find the source of the female playback. Contrary to expectations, males exposed to noise were at least as efficient as control males on account of similar searching success with less signaling effort, while playing back male-female duets allowed the males to switch to satellite behavior and locate the target without signaling, as expected. We found altered mitochondrial structure in males with high signaling effort that likely indicate early damaging processes at the cellular level in tymbal muscle, but no relation between biochemical markers of oxidative stress and signaling effort. Analysis of signal transmission revealed ambiguous amplitude gradients, which might explain relatively low searching success, but it also indicates the existence of behavioral adaptations to complex vibrational environments. We conclude that the observed searching tactic, emphasizing speed rather than thorough evaluation of directional cues, may compensate for unclear stimuli when the target is near.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

Inner mitochondrial membrane structure and fusion dynamics are altered in senescent human iPSC-derived and primary rat cardiomyocytes

Dysfunction of the aging heart is a major cause of death in the human population. Amongst other tasks, mitochondria are pivotal to supply the working heart with ATP. The mitochondrial inner membrane (IMM) ultrastructure is tailored to meet these demands and to provide nano-compartments for specific tasks. Thus, function and morphology are closely coupled. Senescent cardiomyocytes from the mouse heart display alterations of the inner mitochondrial membrane. To study the relation between inner mitochondrial membrane architecture, dynamics and function is hardly possible in living organisms. Here, we present two cardiomyocyte senescence cell models that allow in cellular studies of mitochondrial performance. We show that doxorubicin treatment transforms human iPSC-derived cardiomyocytes and rat neonatal cardiomyocytes in an aged phenotype. The treated cardiomyocytes display double-strand breaks in the nDNA, have β-galactosidase activity, possess enlarged nuclei, and show p21 upregulation. Most importantly, they also display a compromised inner mitochondrial structure. This prompted us to test whether the dynamics of the inner membrane was also altered. We found that the exchange of IMM components after organelle fusion was faster in doxorubicin-treated cells than in control cells, with no change in mitochondrial fusion dynamics at the meso-scale. Such altered IMM morphology and dynamics may have important implications for local OXPHOS protein organization, exchange of damaged components, and eventually the mitochondrial bioenergetics function of the aged cardiomyocyte.

RNA virus-mediated changes in organismal oxygen consumption rate in young and old Drosophila melanogaster males

Aging is accompanied by increased susceptibility to infections including with viral pathogens resulting in higher morbidity and mortality among the elderly. Significant changes in host metabolism can take place following virus infection. Efficient immune responses are energetically costly, and viruses divert host molecular resources to promote their own replication. Virus-induced metabolic reprogramming could impact infection outcomes, however, how this is affected by aging and impacts organismal survival remains poorly understood. RNA virus infection of Drosophila melanogaster with Flock House virus (FHV) is an effective model to study antiviral responses with age, where older flies die faster than younger flies due to impaired disease tolerance. Using this aged host-virus model, we conducted longitudinal, single-fly respirometry studies to determine if metabolism impacts infection outcomes. Analysis using linear mixed models on Oxygen Consumption Rate (OCR) following the first 72-hours post-infection showed that FHV modulates respiration, but age has no significant effect on OCR. However, the longitudinal assessment revealed that OCR in young flies progressively and significantly decreases, while OCR in aged flies remains constant throughout the three days of the experiment. Furthermore, we found that the OCR signature at 24-hours varied in response to both experimental treatment and survival status. FHV-injected flies that died prior to 48- or 72-hours measurements had a lower OCR compared to survivors at 48-hours. Our findings suggest the host's metabolic profile could influence the outcome of viral infections.

Preservation of Mitochondrial Health in Liver Ischemia/Reperfusion Injury

Liver ischemia-reperfusion injury (LIRI) is a major cause of the development of complications in different clinical settings such as liver resection and liver transplantation. Damage arising from LIRI is a major risk factor for early graft rejection and is associated with higher morbidity and mortality after surgery. Although the mechanisms leading to the injury of parenchymal and non-parenchymal liver cells are not yet fully understood, mitochondrial dysfunction is recognized as a hallmark of LIRI that exacerbates cellular injury. Mitochondria play a major role in glucose metabolism, energy production, reactive oxygen species (ROS) signaling, calcium homeostasis and cell death. The diverse roles of mitochondria make it essential to preserve mitochondrial health in order to maintain cellular activity and liver integrity during liver ischemia/reperfusion (I/R). A growing body of studies suggest that protecting mitochondria by regulating mitochondrial biogenesis, fission/fusion and mitophagy during liver I/R ameliorates LIRI. Targeting mitochondria in conditions that exacerbate mitochondrial dysfunction, such as steatosis and aging, has been successful in decreasing their susceptibility to LIRI. Studying mitochondrial dysfunction will help understand the underlying mechanisms of cellular damage during LIRI which is important for the development of new therapeutic strategies aimed at improving patient outcomes. In this review, we highlight the progress made in recent years regarding the role of mitochondria in liver I/R and discuss the impact of liver conditions on LIRI.

Mitochondrial Bioenergetics, Redox Balance, and Calcium Homeostasis Dysfunction with Defective Ultrastructure and Quality Control in the Hippocampus of Aged Female C57BL/6J Mice

Aging is a physiological process that generates progressive decline in many cellular functions. There are many theories of aging, and one of great importance in recent years is the mitochondrial theory of aging, in which mitochondrial dysfunction that occurs at advanced age could be responsible for the aged phenotype. In this context, there is diverse information about mitochondrial dysfunction in aging, in different models and different organs. Specifically, in the brain, different studies have shown mitochondrial dysfunction mainly in the cortex; however, until now, no study has shown all the defects in hippocampal mitochondria in aged female C57BL/6J mice. We performed a complete analysis of mitochondrial function in 3-month-old and 20-month-old (mo) female C57BL/6J mice, specifically in the hippocampus of these animals. We observed an impairment in bioenergetic function, indicated by a decrease in mitochondrial membrane potential, O2 consumption, and mitochondrial ATP production. Additionally, there was an increase in ROS production in the aged hippocampus, leading to the activation of antioxidant signaling, specifically the Nrf2 pathway. It was also observed that aged animals had deregulation of calcium homeostasis, with more sensitive mitochondria to calcium overload and deregulation of proteins related to mitochondrial dynamics and quality control processes. Finally, we observed a decrease in mitochondrial biogenesis with a decrease in mitochondrial mass and deregulation of mitophagy. These results show that during the aging process, damaged mitochondria accumulate, which could contribute to or be responsible for the aging phenotype and age-related disabilities.

Long-term administration of Western diet induced metabolic syndrome in mice and causes cardiac microvascular dysfunction, cardiomyocyte mitochondrial damage, and cardiac remodeling involving caveolae and caveolin-1 expression

BACKGROUND

Long-term consumption of an excessive fat and sucrose diet (Western diet, WD) has been considered a risk factor for metabolic syndrome (MS) and cardiovascular disease. Caveolae and caveolin-1 (CAV-1) proteins are involved in lipid transport and metabolism. However, studies investigating CAV-1 expression, cardiac remodeling, and dysfunction caused by MS, are limited. This study aimed to investigate the correlation between the expression of CAV-1 and abnormal lipid accumulation in the endothelium and myocardium in WD-induced MS, and the occurrence of myocardial microvascular endothelial cell dysfunction, myocardial mitochondrial remodeling, and damage effects on cardiac remodeling and cardiac function.

METHODS

We employed a long-term (7 months) WD feeding mouse model to measure the effect of MS on caveolae/vesiculo-vacuolar organelle (VVO) formation, lipid deposition, and endothelial cell dysfunction in cardiac microvascular using a transmission electron microscopy (TEM) assay. CAV-1 and endothelial nitric oxide synthase (eNOS) expression and interaction were evaluated using real-time polymerase chain reaction, Western blot, and immunostaining. Cardiac mitochondrial shape transition and damage, mitochondria-associated endoplasmic reticulum membrane (MAM) disruption, cardiac function change, caspase-mediated apoptosis pathway activation, and cardiac remodeling were examined using TEM, echocardiography, immunohistochemistry, and Western blot assay.

RESULTS

Our study demonstrated that long-term WD feeding caused obesity and MS in mice. In mice, MS increased caveolae and VVO formation in the microvascular system and enhanced CAV-1 and lipid droplet binding affinity. In addition, MS caused a significant decrease in eNOS expression, vascular endothelial cadherin, and β-catenin interactions in cardiac microvascular endothelial cells, accompanied by impaired vascular integrity. MS-induced endothelial dysfunction caused massive lipid accumulation in the cardiomyocytes, leading to MAM disruption, mitochondrial shape transition, and damage. MS promoted brain natriuretic peptide expression and activated the caspase-dependent apoptosis pathway, leading to cardiac dysfunction in mice.

CONCLUSION

MS resulted in cardiac dysfunction, remodeling by regulating caveolae and CAV-1 expression, and endothelial dysfunction. Lipid accumulation and lipotoxicity caused MAM disruption and mitochondrial remodeling in cardiomyocytes, leading to cardiomyocyte apoptosis and cardiac dysfunction and remodeling.

Mitochondrial cristae architecture protects against mtDNA release and inflammation

Mitochondrial damage causes mitochondrial DNA (mtDNA) release to activate the type I interferon (IFN-I) response via the cGAS-STING pathway. mtDNA-induced inflammation promotes autoimmune- and aging-related degenerative disorders. However, the global picture of inflammation-inducing mitochondrial damages remains obscure. Here, we have performed a mitochondria-targeted CRISPR knockout screen for regulators of the IFN-I response. Strikingly, our screen reveals dozens of hits enriched with key regulators of cristae architecture, including phospholipid cardiolipin and protein complexes such as OPA1, mitochondrial contact site and cristae organization (MICOS), sorting and assembly machinery (SAM), mitochondrial intermembrane space bridging (MIB), prohibitin (PHB), and the F₁F_o-ATP synthase. Disrupting these cristae organizers consistently induces mtDNA release and the STING-dependent IFN-I response. Furthermore, knocking out MTX2, a subunit of the SAM complex whose null mutations cause progeria in humans, induces a robust STING-dependent IFN-I response in mouse liver. Taken together, beyond revealing the central role of cristae architecture to prevent mtDNA release and inflammation, our results mechanistically link mitochondrial cristae disorganization and inflammation, two emerging hallmarks of aging and aging-related degenerative diseases.

SS-31 as a Mitochondrial Protectant in the Treatment of Tendinopathy: Evaluation in a Murine Supraspinatus Tendinopathy Model

BACKGROUND

Prior studies have demonstrated mitochondrial dysfunction in tendinopathy. The objective of this investigation was to explore the potential of SS-31 (elamipretide), a mitochondrial protectant, to improve mitochondrial function and promote tendon healing in a murine supraspinatus tendinopathy model.

METHODS

One hundred and twenty-six mice (252 limbs) were divided into 6 groups (42 limbs/group) that received (I) 4 weeks of impingement; (II) 8 weeks of impingement; (III) 8 weeks of impingement including 4 weeks of SS-31 treatment (5 mg/kg/d) starting after 4 weeks of impingement; (IV) 4 weeks of impingement ending with clip removal, followed by harvesting 4 weeks later; and (V) 4 weeks of impingement ending with clip removal, followed by 4 weeks of SS-31 treatment and harvesting; and a control group. Specimens were prepared for biomechanical testing, histological analysis, transmission electron microscopy, measurement of superoxidative dismutase (SOD) activity, and measurement of gene expression.

RESULTS

Failure force decreased after impingement, compared with the intact tendon, and the decrease was partially reversed after clip removal, SS-31 treatment, and the 2 treatments combined. A similar pattern was observed for stiffness. Histological analysis demonstrated higher modified Bonar scores in the impingement groups; however, the changes in tendon morphology were partially reversed following all treatments, especially the combined treatment. Decreased mitochondrial number and altered organization and density of cristae were observed in the impingement groups. Mitochondrial structure and number became more normal, with improvement in morphology of the cristae, after clip removal and/or SS-31 treatment. SOD activity decreased after impingement, compared with the control group, then increased significantly again after treatment, especially in the combined treatment group. Mitochondria-related gene expression decreased in the impingement groups and increased again after treatment.

CONCLUSIONS

The mitochondrial protectant SS-31 improved mitochondrial function, promoting tendon healing, especially when combined with removal of subacromial impingement.

CLINICAL RELEVANCE

Improving mitochondrial function with agents such as SS-31 may represent an effective treatment to promote healing in the setting of supraspinatus tendinopathy.

Toothed Whales Have Black Neurons in the Blue Spot

Simple Summary

Neuromelanin is a dark pigment that is present in several types of neurons of the brain. The role of human neuromelanin is a matter of controversy and, over the past few years, has been attributed to having a dual nature, either in a protective role to shield neurons from toxic compounds, or as a trigger of neuroinflammation. This pigment has been researched mainly in the human brain, but it has also been found in the neurons of monkeys, horses, giraffes, cattle, sheep, goats, dogs, rats, and even in frogs and tadpoles. Even so, neuromelanin in humans and primates presents unique features that are not shown in other animals. A study on the morphology of the locus ceruleus (a key brain structure) of the family Delphinidae highlighted the presence of a large amount of neuromelanin within this brain area. In an attempt to better define the ultrastructure of neuromelanin in toothed whales, two brain specimens of the suborder Odontoceti were investigated. The two toothed whales that were examined presented melanin granules associated with lipid droplets and membranes that bore a striking resemblance with human neuromelanin. Its accumulation takes place over the entire life span, and appears to contain the story of one’s life exposure to several endogenous and environmental metals and/or compounds.

Abstract

Neuromelanin (NM) is a dark polymer pigment that is located mostly in the human substantia nigra, and in the locus ceruleus, referred to as “the blue spot”. NM increases linearly with age, and has been described mainly in the human brain; however, it also occurs in the neurons of monkeys, horses, giraffes, cattle, sheep, goats, dogs, rats, and even in frogs. While in most of these mammals NM shows the histochemical and ultrastructural features typical of lipofuscins, human NM is confined within cytoplasmic organelles that are surrounded by a double membrane, suggesting an autophagic origin. In a study on the morphology of the locus ceruleus of the family Delphinidae, the presence of a variable quantity of NM in the interior of locus ceruleus neurons was observed for the first time; meanwhile, nothing is known about its ultrastructure and composition. Transmission electron microscopy demonstrated in two toothed whales—an Atlantic spotted dolphin (Stenella frontalis; family Delphinidae) and a Blainville’s beaked whale (Mesoplodon densirostris; family Ziphiidae)—the presence of melanin granules associated with lipid droplets and membranes that were very similar to that of human NM. The relationship between NM and neuronal vulnerability must be studied in depth, and cetaceans may offer a new natural-spontaneous comparative model for the study of NM and its implication in neurodegenerative diseases.

CG7630 is the Drosophila melanogaster homolog of the cytochrome c oxidase subunit COX7B

The mitochondrial respiratory chain (MRC) is composed of four multiheteromeric enzyme complexes. According to the endosymbiotic origin of mitochondria, eukaryotic MRC derives from ancestral proteobacterial respiratory structures consisting of a minimal set of complexes formed by a few subunits associated with redox prosthetic groups. These enzymes, which are the “core” redox centers of respiration, acquired additional subunits, and increased their complexity throughout evolution. Cytochrome c oxidase (COX), the terminal component of MRC, has a highly interspecific heterogeneous composition. Mammalian COX consists of 14 different polypeptides, of which COX7B is considered the evolutionarily youngest subunit. We applied proteomic, biochemical, and genetic approaches to investigate the COX composition in the invertebrate model Drosophila melanogaster. We identified and characterized a novel subunit which is widely different in amino acid sequence, but similar in secondary and tertiary structures to COX7B, and provided evidence that this object is in fact replacing the latter subunit in virtually all protostome invertebrates. These results demonstrate that although individual structures may differ the composition of COX is functionally conserved between vertebrate and invertebrate species.

Remission of obesity and insulin resistance is not sufficient to restore mitochondrial homeostasis in visceral adipose tissue

Metabolic plasticity is the ability of a biological system to adapt its metabolic phenotype to different environmental stressors. We used a whole-body and tissue-specific phenotypic, functional, proteomic, metabolomic and transcriptomic approach to systematically assess metabolic plasticity in diet-induced obese mice after a combined nutritional and exercise intervention. Although most obesity and overnutrition-related pathological features were successfully reverted, we observed a high degree of metabolic dysfunction in visceral white adipose tissue, characterized by abnormal mitochondrial morphology and functionality. Despite two sequential therapeutic interventions and an apparent global healthy phenotype, obesity triggered a cascade of events in visceral adipose tissue progressing from mitochondrial metabolic and proteostatic alterations to widespread cellular stress, which compromises its biosynthetic and recycling capacity. In humans, weight loss after bariatric surgery showed a transcriptional signature in visceral adipose tissue similar to our mouse model of obesity reversion. Overall, our data indicate that obesity prompts a lasting metabolic fingerprint that leads to a progressive breakdown of metabolic plasticity in visceral adipose tissue.

Neuronal induction of BNIP3-mediated mitophagy slows systemic aging in Drosophila

The effects of aging on the brain are widespread and can have dramatic implications on the overall health of an organism. Mitochondrial dysfunction is a hallmark of brain aging, but, the interplay between mitochondrial quality control, neuronal aging, and organismal health is not well understood. Here, we show that aging leads to a decline in mitochondrial autophagy (mitophagy) in the Drosophila brain with a concomitant increase in mitochondrial content. We find that induction of BCL2-interacting protein 3 (BNIP3), a mitochondrial outer membrane protein, in the adult nervous system induces mitophagy and prevents the accumulation of dysfunctional mitochondria in the aged brain. Importantly, neuronal induction of BNIP3-mediated mitophagy increases organismal longevity and healthspan. Furthermore, BNIP3-mediated mitophagy in the nervous system improves muscle and intestinal homeostasis in aged flies, indicating cell non-autonomous effects. Our findings identify BNIP3 as a therapeutic target to counteract brain aging and prolong overall organismal health with age.

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.

Macrophage mitochondrial bioenergetics and tissue invasion are boosted by an Atossa-Porthos axis in Drosophila

Cellular metabolism must adapt to changing demands to enable homeostasis. During immune responses or cancer metastasis, cells leading migration into challenging environments require an energy boost, but what controls this capacity is unclear. Here, we study a previously uncharacterized nuclear protein, Atossa (encoded by CG9005), which supports macrophage invasion into the germband of Drosophila by controlling cellular metabolism. First, nuclear Atossa increases mRNA levels of Porthos, a DEAD‐box protein, and of two metabolic enzymes, lysine‐α‐ketoglutarate reductase (LKR/SDH) and NADPH glyoxylate reductase (GR/HPR), thus enhancing mitochondrial bioenergetics. Then Porthos supports ribosome assembly and thereby raises the translational efficiency of a subset of mRNAs, including those affecting mitochondrial functions, the electron transport chain, and metabolism. Mitochondrial respiration measurements, metabolomics, and live imaging indicate that Atossa and Porthos power up OxPhos and energy production to promote the forging of a path into tissues by leading macrophages. Since many crucial physiological responses require increases in mitochondrial energy output, this previously undescribed genetic program may modulate a wide range of cellular behaviors.

Alterations in hippocampal mitochondrial dynamics are associated with neurodegeneration and recognition memory decline in old male mice

Mitochondrial dynamics is a key process that modulates the ultrastructure, quality and function of mitochondria. It is disrupted in numerous major neurodegenerative disorders including Parkinson's, Alzheimer's and Huntington's disease. Mitochondrial dysfunction has been correlated with the loss of memory. Previous studies suggest the involvement of Vdac1 and Drp1 in outer mitochondrial membrane permeabilization and promotion of mitochondrial fragmentation through Drp1 phosphorylation at S616. However, alterations in mitochondrial dynamics with respect to aging, memory loss and neurodegeneration remain unexplored. Therefore, the present study focuses on the involvement of mitochondrial dynamics in neurodegeneration and recognition memory decline during aging. The recognition memory decline was validated by the novel object recognition test and measurement of hippocampal Arc protein level during aging. The ultrastructure analysis revealed a decline in mitochondrial length and area, while an increase in the number of fragmented, round and disrupted mitochondria in the hippocampus during aging. Disruption was also evident in mitochondrial cristae and membrane with advancing age. The change in mitochondrial morphology was corroborated by an increase in the expression of phospho-Drp1 (S616) and Cyt-c proteins but decline in Mfn2, LC3B, Vdac1, Bcl-XL and Bcl-2 proteins in the hippocampus during aging. Taken together, our findings reveal that an increase in the expression of phospho-Drp1 (S616) and decrease in Mfn2 and LC3B proteins in the hippocampus bring about a reduction in mitochondrial length and area, and rise in mitochondrial fragmentation leading to reduced neuronal cell density, increased neurodegeneration and recognition memory decline in old male mice. Diagram depicts the increase in hippocampal mitochondrial fragmentation during aging of mice. Increased mitochondrial fragmentation causes distorted mitochondrial function such as decrease in ATP/ADP transportation due to decrease in Vdac1 protein level and increase in oxidative damage. These alterations result in hippocampal neurodegeneration and consequently impairment in recognition memory during aging.

MDH2 produced OAA is a metabolic switch rewiring the fuelling of respiratory chain and TCA cycle

The mitochondrial respiratory chain (RC) enables many metabolic processes by regenerating both mitochondrial and cytosolic NAD⁺ and ATP. The oxidation by the RC of the NADH metabolically produced in the cytosol involves redox shuttles as the malate-aspartate shuttle (MAS) and is of paramount importance for cell fate. However, the specific metabolic regulations allowing mitochondrial respiration to prioritize NADH oxidation in response to high NADH/NAD⁺ redox stress have not been elucidated. The recent discovery that complex I (NADH dehydrogenase), and not complex II (Succinate dehydrogenase), can assemble with other respiratory chain complexes to form functional entities called respirasomes, led to the assumption that this supramolecular organization would favour NADH oxidation. Unexpectedly, characterization of heart and liver mitochondria demonstrates that the RC systematically favours electrons provided by the 'respirasome free' complex II. Our results demonstrate that the preferential succinate driven respiration is tightly controlled by OAA levels, and that OAA feedback inhibition of complex II rewires RC fuelling increasing NADH oxidation capacity. This new regulatory mechanism synergistically increases RC's NADH oxidative capacity and rewires MDH2 driven anaplerosis of the TCA, preventing malate production from succinate to favour oxidation of cytosolic malate. This regulatory mechanism synergistically adjusts RC and TCA fuelling in response to extramitochondrial malate produced by the MAS.

Defective dimerization of FoF1-ATP synthase secondary to glycation favors mitochondrial energy deficiency in cardiomyocytes during aging

Aged cardiomyocytes develop a mismatch between energy demand and supply, the severity of which determines the onset of heart failure, and become prone to undergo cell death. The FoF1-ATP synthase is the molecular machine that provides >90% of the ATP consumed by healthy cardiomyocytes and is proposed to form the mitochondrial permeability transition pore (mPTP), an energy-dissipating channel involved in cell death. We investigated whether aging alters FoF1-ATP synthase self-assembly, a fundamental biological process involved in mitochondrial cristae morphology and energy efficiency, and the functional consequences this may have. Purified heart mitochondria and cardiomyocytes from aging mice displayed an impaired dimerization of FoF1-ATP synthase (blue native and proximity ligation assay), associated with abnormal mitochondrial cristae tip curvature (TEM). Defective dimerization did not modify the in vitro hydrolase activity of FoF1-ATP synthase but reduced the efficiency of oxidative phosphorylation in intact mitochondria (in which membrane architecture plays a fundamental role) and increased cardiomyocytes' susceptibility to undergo energy collapse by mPTP. High throughput proteomics and fluorescence immunolabeling identified glycation of 5 subunits of FoF1-ATP synthase as the causative mechanism of the altered dimerization. In vitro induction of FoF1-ATP synthase glycation in H9c2 myoblasts recapitulated the age-related defective FoF1-ATP synthase assembly, reduced the relative contribution of oxidative phosphorylation to cell energy metabolism, and increased mPTP susceptibility. These results identify altered dimerization of FoF1-ATP synthase secondary to enzyme glycation as a novel pathophysiological mechanism involved in mitochondrial cristae remodeling, energy deficiency, and increased vulnerability of cardiomyocytes to undergo mitochondrial failure during aging.

Forty years in cryoEM of membrane proteins

In a surprisingly short time, electron cryo-microscopy (cryoEM) has developed from a niche technique in structural biology to a mainstream method practiced in a rapidly growing number of laboratories around the world. From its beginnings about 40 years ago, cryoEM has had a major impact on the study of membrane proteins, in particular the energy-converting systems from bacterial, mitochondrial and chloroplast membranes. Early work on two-dimensional crystals attained resolutions ∼3.5 Å, but at present, single-particle cryoEM delivers much more detailed structures without crystals. Electron cryo-tomography of membranes and membrane-associated proteins adds valuable context, usually at lower resolution. The review ends with a brief outlook on future prospects.

Early-onset dietary restriction maintains mitochondrial health, autophagy and ER function in the left ventricle during aging

Dietary restriction (DR) exerts healthy benefits, including heart functions. However, the cardioprotective role of DR is till controversial among researchers due to the variation of DR conditions. The present study focuses on the protective effect of early-onset DR on cardiac injury using mitochondrial structure and expression of protein associated with mitochondrial homeostasis, autophagy and endoplasmic reticulum (ER) function as measures.

METHODS

Two-month-old mice were fed with a breeding diet ad libitum (AL) or DR (60% of AL) for 3 (Young) or 20 (Aged) months.

RESULTS

Body weight increased with aging, whereas DR treatment kept body weight consistent. DR mice exhibited a higher relative heart weight than AL mice. DR mice displayed lower plasma glucose levels, compared with AL groups. Furthermore, Aged-AL, but not Aged-DR mice, had increased collagen content and morphological distortions in the left ventricle (LV). Aged-DR mice had a higher ATP and lower TBARS in the LV than Aged-AL mice. Mitochondrial morphology was detected by electron microscopy; Aged-AL mice had increased abnormal morphology of mitochondria. Treatment with DR reduced abnormal mitochondrial accumulation. Aging elevated the protein expressions of mitochondrial functions and ER-induced apoptosis. Aging downregulated autophagy-related proteins and chaperones in the heart. Dietary restriction reversed those protein expressions.

CONCLUSIONS

The present study demonstrated a beneficial effect of early onset DR on cardiac aging. The age-dependent mitochondrial dysfunction and protein quality control dysregulation was significantly reversed by long-term DR, demonstrating a concordance with the beneficial effect in the heart.

Lack of age-related respiratory changes in Daphnia

Aging is a multifaceted process of accumulation of damage and waste in cells and tissues; age-related changes in mitochondria and in respiratory metabolism have the focus of aging research for decades. Studies of aging in nematodes, flies and mammals all revealed age-related decline in respiratory functions, with somewhat controversial causative role. Here we investigated age-related changes in respiration rates, lactate/pyruvate ratio, a commonly used proxy for NADH/NAD+ balance, and mitochondrial membrane potential in 4 genotypes of an emerging model organism for aging research, a cyclic parthenogen Daphnia magna. We show that total body weight-adjusted respiration rate decreased with age, although this decrease was small in magnitude and could be fully accounted for by the decrease in locomotion and feeding activity. Neither total respiration normalized by protein content, nor basal respiration rate measured in anaesthetized animals decreased with age. Lactate/pyruvate ratio and mitochondrial membrane potential (∆Ψ_mt) showed no age-related changes, with possible exceptions of ∆Ψ_mt in epipodites (excretory and gas exchange organs) in which ∆Ψ_mt decreased with age and in the optical lobe of the brain, in which ∆Ψ_mt showed a maximum at middle age. We conclude that actuarial senescence in Daphnia is not caused by a decline in respiratory metabolism and discuss possible mechanisms of maintaining mitochondrial healthspan throughout the lifespan.

Dual role of Mic10 in mitochondrial cristae organization and ATP synthase-linked metabolic adaptation and respiratory growth

Invaginations of the mitochondrial inner membrane, termed cristae, are hubs for oxidative phosphorylation. The mitochondrial contact site and cristae organizing system (MICOS) and the dimeric F₁F_o-ATP synthase play important roles in controlling cristae architecture. A fraction of the MICOS core subunit Mic10 is found in association with the ATP synthase, yet it is unknown whether this interaction is of relevance for mitochondrial or cellular functions. Here, we established conditions to selectively study the role of Mic10 at the ATP synthase. Mic10 variants impaired in MICOS functions stimulate ATP synthase oligomerization like wild-type Mic10 and promote efficient inner membrane energization, adaptation to non-fermentable carbon sources, and respiratory growth. Mic10's functions in respiratory growth largely depend on Mic10^ATPsynthase, not on Mic10^MICOS. We conclude that Mic10 plays a dual role as core subunit of MICOS and as partner of the F₁F_o-ATP synthase, serving distinct functions in cristae shaping and respiratory adaptation and growth.

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Dual role of Mic10 of mitochondrial contact site and cristae organizing system (MICOS)

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Mic10 binds to mitochondrial ATP synthase and stabilizes higher order assemblies

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Oligomerization of Mic10 is required for its function in MICOS, not at ATP synthase

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Mic10 binding to ATP synthase supports metabolic adaptation and respiratory growth

Cell Biology Meets Cell Metabolism: Energy Production Is Similar in Stem Cells and in Cancer Stem Cells in Brain and Bone Marrow

Energy production by means of ATP synthesis in cancer cells has been investigated frequently as a potential therapeutic target in this century. Both (an)aerobic glycolysis and oxidative phosphorylation (OXPHOS) have been studied. Here, we review recent literature on energy production in glioblastoma stem cells (GSCs) and leukemic stem cells (LSCs) versus their normal counterparts, neural stem cells (NSCs) and hematopoietic stem cells (HSCs), respectively. These two cancer stem cell types were compared because their niches in glioblastoma tumors and in bone marrow are similar. In this study, it became apparent that (1) ATP is produced in NSCs and HSCs by anaerobic glycolysis, whereas fatty acid oxidation (FAO) is essential for their stem cell fate and (2) ATP is produced in GSCs and LSCs by OXPHOS despite the hypoxic conditions in their niches with FAO and amino acids providing its substrate. These metabolic processes appeared to be under tight control of cellular regulation mechanisms which are discussed in depth. However, our conclusion is that systemic therapeutic targeting of ATP production via glycolysis or OXPHOS is not an attractive option because of its unwanted side effects in cancer patients.