Mitochondrial morphology is altered in atrophied skeletal muscle of aged mice

Laminin-α2 chain deficiency in skeletal muscle causes dysregulation of multiple cellular mechanisms

LAMA2, coding for the laminin-α2 chain, is a crucial ECM component, particularly abundant in skeletal muscle. Mutations in LAMA2 trigger the often-lethal LAMA2-congenital muscular dystrophy (LAMA2-CMD). Various phenotypes have been linked to LAMA2-CMD; nevertheless, the precise mechanisms that malfunction during disease onset in utero remain unknown. We generated Lama2-deficient C2C12 cells and found that Lama2-deficient myoblasts display proliferation, differentiation, and fusion defects, DNA damage, oxidative stress, and mitochondrial dysfunction. Moreover, fetal myoblasts isolated from the dy W mouse model of LAMA2-CMD display impaired differentiation and fusion in vitro. We also showed that disease onset during fetal development is characterized by a significant down-regulation of gene expression in muscle fibers, causing pronounced effects on cytoskeletal organization, muscle differentiation, and altered DNA repair and oxidative stress responses. Together, our findings provide unique insights into the critical importance of the laminin-α2 chain for muscle differentiation and muscle cell homeostasis.

Delayed and diminished postprandial lactate shuttling in healthy older men and women

Lactate, a product of glycolysis, is formed under aerobic conditions. Extensive work has shown lactate flux in young and exercising humans; however, the effect of age is not known. We tested the hypothesis that postprandial lactate shuttling (PLS) would be diminished in older adults. We used [3-13C]lactate and [6,6-2H]glucose tracers, an oral glucose tolerance test (OGTT), and arterialized blood sampling to determine postprandial lactate rates of appearance (Ra), disappearance (Rd), and oxidation (Rox) in 15 young (28.1 ± 1.4 yr) and 13 older (70.6 ± 2.4 yr) healthy men and women. In young participants, fasting blood [lactate] (≈0.5 mM) rose after the glucose challenge, peaked at 15 min, dipped to a nadir at 30 min, and rose again peaking at 60 min (≈1.0 mM). Initial responses in lactate Ra of older participants were delayed and diminished until 90 min rising by 0.83 mg·kg-1·min-1. Lactate Rox was higher throughout the entire trial in young participants by a difference of ∼0.5 mg·kg-1·min-1. Initial peaks in lactate Ra and concentration in all volunteers demonstrated the presence of an enteric PLS following an OGTT. Notably, in the systemic, but not enteric, PLS phase, lactate Ra correlated highly with glucose Rd (r2 = 0.92). Correspondence of second peaks in lactate Ra and concentration and glucose Rd shows dependence of lactate Ra on glucose Rd. Although results show both enteric and systemic PLS phases in young and older study cohorts, metabolic responses were delayed and diminished in healthy older individuals.NEW & NOTEWORTHY We used isotope tracers, an oral glucose tolerance test, and arterialized blood sampling to determine postprandial lactate flux rates in healthy young and older men and women. Lactate rates of appearance and oxidation and the lactate-pyruvate exchange were delayed and diminished in both enteric and systemic postprandial lactate shuttle phases in older participants.

Overexpression of mitochondrial fission or mitochondrial fusion genes enhances resilience and extends longevity

The dynamicity of the mitochondrial network is crucial for meeting the ever-changing metabolic and energy needs of the cell. Mitochondrial fission promotes the degradation and distribution of mitochondria, while mitochondrial fusion maintains mitochondrial function through the complementation of mitochondrial components. Previously, we have reported that mitochondrial networks are tubular, interconnected, and well-organized in young, healthy C. elegans, but become fragmented and disorganized with advancing age and in models of age-associated neurodegenerative disease. In this work, we examine the effects of increasing mitochondrial fission or mitochondrial fusion capacity by ubiquitously overexpressing the mitochondrial fission gene drp-1 or the mitochondrial fusion genes fzo-1 and eat-3, individually or in combination. We then measured mitochondrial function, mitochondrial network morphology, physiologic rates, stress resistance, and lifespan. Surprisingly, we found that overexpression of either mitochondrial fission or fusion machinery both resulted in an increase in mitochondrial fragmentation. Similarly, both mitochondrial fission and mitochondrial fusion overexpression strains have extended lifespans and increased stress resistance, which in the case of the mitochondrial fusion overexpression strains appears to be at least partially due to the upregulation of multiple pathways of cellular resilience in these strains. Overall, our work demonstrates that increasing the expression of mitochondrial fission or fusion genes extends lifespan and improves biological resilience without promoting the maintenance of a youthful mitochondrial network morphology. This work highlights the importance of the mitochondria for both resilience and longevity.

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.

Challenging Sarcopenia: Exploring AdipoRon in Aging Skeletal Muscle as a Healthspan-Extending Shield

Sarcopenia, characterized by loss of muscle mass, quality, and function, poses significant risks in aging. We previously demonstrated that long-term treatment with AdipoRon (AR), an adiponectin receptor agonist, alleviated myosteatosis and muscle degeneration in middle-aged obese mice. This study aimed to determine if a shorter AR treatment could effectively offset sarcopenia in older mice. Two groups of old mice (20-23 months) were studied, one untreated (O) and one orally-treated with AR (O-AR) at 50 mg/kg/day for three months, compared with control 3-month-old young mice (Y) or 10-month-old young-adult mice (C-10). Results showed that AR remarkably inversed the loss of muscle mass by restoring the sarcopenia index and fiber count, which were greatly diminished with age. Additionally, AR successfully saved muscle quality of O mice by halving the accumulation of tubular aggregates and aberrant mitochondria, through AMPK pathway activation and enhanced autophagy. AR also bolstered muscle function by rescuing mitochondrial activity and improving exercise endurance. Finally, AR markedly curbed muscle fibrosis and mitigated local/systemic inflammation. Thus, a late three-month AR treatment successfully opposed sarcopenia and counteracted various hallmarks of aging, suggesting AR as a promising anti-aging therapy for skeletal muscles, potentially extending healthspan.

The interplay between mitochondrial dynamics and autophagy: From a key homeostatic mechanism to a driver of pathology

The complex relationship between mitochondrial dynamics and autophagy illustrates how two cellular housekeeping processes are intimately linked, illuminating fundamental principles of cellular homeostasis and shedding light on disparate pathological conditions including several neurodegenerative disorders. Here we review the basic tenets of mitochondrial dynamics i.e., the concerted balance between fusion and fission of the organelle, and its interplay with macroautophagy and selective mitochondrial autophagy, also dubbed mitophagy, in the maintenance of mitochondrial quality control and ultimately in cell viability. We illustrate how conditions of altered mitochondrial dynamics reverberate on autophagy and vice versa. Finally, we illustrate how altered interplay between these two key cellular processes participates in the pathogenesis of human disorders affecting multiple organs and systems.

Impaired mitochondrial morphological plasticity and failure of mitophagy associated with the G11778A mutation of LHON

Mitochondrial dynamics were examined in human dermal fibroblasts biopsied from a confirmed Leber's Hereditary Optic Neuropathy (LHON) patient with a homoplasmic G11778A mutation of the mitochondrial genome. Expression of the G11778A mutation did not impart any discernible difference in mitochondrial network morphology using widefield fluorescence microscopy. However, at the ultrastructural level, cells expressing this mutation exhibited an impairment of mitochondrial morphological plasticity when forced to utilize oxidative phosphorylation (OXPHOS) by transition to glucose-free, galactose-containing media. LHON fibroblasts also displayed a transient increase in mitophagy upon transition to galactose media. These results provide new insights into the consequences of the G11778A mutation of LHON and the pathological mechanisms underlying this disease.

Functional and Structural Changes in Diaphragm Neuromuscular Junctions in Early Aging

Age-related impairment of the diaphragm causes respiratory complications. Neuromuscular junction (NMJ) dysfunction can be one of the triggering events in diaphragm weaknesses in old age. Prominent structural and functional alterations in diaphragm NMJs were described in elderly rodents, but NMJ changes in middle age remain unclear. Here, we compared diaphragm muscles from young adult (3 months) and middle-aged (12 months) BALB/c mice. Microelectrode recordings, immunofluorescent staining, electron microscopy, myography, and whole-body plethysmography were used. We revealed presynaptic (i) and postsynaptic (ii) changes. The former (i) included an increase in both action potential propagation velocity and neurotransmitter release evoked by low-, moderate-, and high-frequency activity but a decrease in immunoexpression of synapsin 1 and synaptic vesicle clustering. The latter (ii) consisted of a decrease in currents via nicotinic acetylcholine receptors and the area of their distribution. These NMJ changes correlated with increased contractile responses to moderate- to high-frequency nerve activation. Additionally, we found alterations in the pattern of respiration (an increase in peak inspiratory flow and a tendency of elevation of the tidal volume), which imply increased diaphragm activity in middle-aged mice. We conclude that enhancement of neuromuscular communication (due to presynaptic mechanism) accompanied by improved contractile responses occurs in the diaphragm in early aging.

Plasticity and structural alterations of mitochondria and sarcoplasmic organelles in muscles of mice deficient in α-dystrobrevin, a component of the dystrophin-glycoprotein complex

The dystrophin-glycoprotein complex (DGC) plays a crucial role in maintaining the structural integrity of the plasma membrane and the neuromuscular junction. In this study, we investigated the impact of the deficiency of α-dystrobrevin (αdbn), a component of the DGC, on the homeostasis of intracellular organelles, specifically mitochondria and the sarcoplasmic reticulum (SR). In αdbn deficient muscles, we observed a significant increase in the membrane-bound ATP synthase complex levels, a marker for mitochondria in oxidative muscle fiber types compared to wild-type. Furthermore, examination of muscle fibers deficient in αdbn using electron microscopy revealed profound alterations in the organization of mitochondria and the SR within certain myofibrils of muscle fibers. This included the formation of hyper-branched intermyofibrillar mitochondria with extended connections, an extensive network spanning several myofibrils, and a substantial increase in the number/density of subsarcolemmal mitochondria. Concurrently, in some cases, we observed significant structural alterations in mitochondria, such as cristae loss, fragmentation, swelling, and the formation of vacuoles and inclusions within the mitochondrial matrix cristae. Muscles deficient in αdbn also displayed notable alterations in the morphology of the SR, along with the formation of distinct anomalous concentric SR structures known as whorls. These whorls were prevalent in αdbn-deficient mice but were absent in wild-type muscles. These results suggest a crucial role of the DGC αdbn in regulating intracellular organelles, particularly mitochondria and the SR, within muscle cells. The remodeling of the SR and the formation of whorls may represent a novel mechanism of the unfolded protein response (UPR) in muscle cells.

NLRP3 inflammasome activation and altered mitophagy are key pathways in inclusion body myositis

Background

Inclusion body myositis (IBM) is the most prevalent muscle disease in adults for which no current treatment exists. The pathogenesis of IBM remains poorly defined. Inflammation and mitochondrial dysfunction are the most common histopathological findings. In this study, we aimed to explore the interplay between inflammation and mitochondrial dysfunction in IBM patients, highlighting sex differences.

Methods

We included 38 IBM patients and 22 age- and sex-matched controls without myopathy. Bulk RNA sequencing, Meso Scale Discovery ELISA, western blotting, histochemistry and immunohistochemistry were performed on frozen muscle samples from the study participants.

Results

We demonstrated activation of the NLRP3 inflammasome in IBM muscle samples, with the NLRP3 inflammasome pathway being the most upregulated. On muscle histopathology, there is increased NRLP3 immunoreactivity in both inflammatory cells and muscle fibers. Mitophagy is critical for removing damaged mitochondria and preventing the formation of a vicious cycle of mitochondrial dysfunction-NLRP3 activation. In the IBM muscle samples, we showed altered mitophagy, most significantly in males, with elevated levels of p-S65-Ubiquitin, a mitophagy marker. Furthermore, p-S65-Ubiquitin aggregates accumulated in muscle fibers that were mostly type 2 and devoid of cytochrome-c-oxidase reactivity. Type 2 muscle fibers are known to be more prone to mitochondrial dysfunction. NLRP3 RNA levels correlated with p-S65-Ubiquitin levels in both sexes but with loss of in muscle strength only in males. Finally, we identified sex-specific molecular pathways in IBM, with females having activation of pathways that could offset some of the pathomechanisms of IBM.

Conclusions

NLRP3 inflammasome is activated in IBM, along with altered mitophagy particularly in males, which is of potential therapeutic significance. These findings suggest sex-specific mechanisms in IBM that warrant further investigation.

3D Mitochondrial Structure in Aging Human Skeletal Muscle: Insights into MFN-2 Mediated Changes

Age-related atrophy of skeletal muscle, is characterized by loss of mass, strength, endurance, and oxidative capacity during aging. Notably, bioenergetics and protein turnover studies have shown that mitochondria mediate this decline in function. Although exercise has been the only therapy to mitigate sarcopenia, the mechanisms that govern how exercise serves to promote healthy muscle aging are unclear. Mitochondrial aging is associated with decreased mitochondrial capacity, so we sought to investigate how aging affects mitochondrial structure and potential age-related regulators. Specifically, the three-dimensional (3D) mitochondrial structure associated with morphological changes in skeletal muscle during aging requires further elucidation. We hypothesized that aging causes structural remodeling of mitochondrial 3D architecture representative of dysfunction, and this effect is mitigated by exercise. We used serial block-face scanning electron microscopy to image human skeletal tissue samples, followed by manual contour tracing using Amira software for 3D reconstruction and subsequent analysis of mitochondria. We then applied a rigorous in vitro and in vivo exercise regimen during aging. Across 5 human cohorts, we correlate differences in magnetic resonance imaging, mitochondria 3D structure, exercise parameters, and plasma immune markers between young (under 50 years) and old (over 50 years) individuals. We found that mitochondria we less spherical and more complex, indicating age-related declines in contact site capacity. Additionally, aged samples showed a larger volume phenotype in both female and male humans, indicating potential mitochondrial swelling. Concomitantly, muscle area, exercise capacity, and mitochondrial dynamic proteins showed age-related losses. Exercise stimulation restored mitofusin 2 (MFN2), one such of these mitochondrial dynamic proteins, which we show is required for the integrity of mitochondrial structure. Furthermore, we show that this pathway is evolutionarily conserved as Marf, the MFN2 ortholog in Drosophila, knockdown alters mitochondrial morphology and leads to the downregulation of genes regulating mitochondrial processes. Our results define age-related structural changes in mitochondria and further suggest that exercise may mitigate age-related structural decline through modulation of mitofusin 2.

Age and dietary restriction modulate mitochondrial quality in quadriceps femoris muscle of male mice

Dietary restriction (DR) is a potential intervention for ameliorating ageing-related damages. Mitochondrial quality control is the key mechanism for regulating cellular functions in skeletal muscle. This study aimed to explore the effect of age and DR on the homeostasis of mitochondrial quality control in skeletal muscle. To study the effect of age on mitochondrial homeostasis, young (3 months old) male C57BL/6J mice were fed ad libitum (AL) until 7 (Young), 14 (Middle), and 19 months (Aged) of age. For the DR intervention, 60% of AL intake was given to the mice at 3 months of age until they reached 19 months of age (16 months). The quadriceps femoris muscle was collected for further analysis. Significant changes in the skeletal muscle were noticed during the transition between middle age and the elderly stages. An accumulation of collagen was observed in the muscle after middle age. Compared with the Middle muscle, Aged muscle displayed a greater expression of VDAC, and lower expressions of mitochondrial dynamic proteins and OXPHOS proteins. The DR intervention attenuated collagen content and elongated the sarcomere length in the skeletal muscle during ageing. In addition, DR adjusted the abnormalities in mitochondrial morphology in the Aged muscle. DR downregulated VDAC expression, but upregulated OPA1 and DRP1 expressions. Taken together, greater pathological changes were noticed in the skeletal muscle during ageing, especially in the transition between middle age and the elderly, whereas early-onset DR attenuated the muscular ageing via normalising partial functions of mitochondria.

Delineating the contribution of ageing and physical activity to changes in mitochondrial characteristics across the lifespan

Ageing is associated with widespread physiological changes prominent within all tissues, including skeletal muscle and the brain, which lead to a decline in physical function. To tackle the growing health and economic burdens associated with an ageing population, the concept of healthy ageing has become a major research priority. Changes in skeletal muscle mitochondrial characteristics have been suggested to make an important contribution to the reductions in skeletal muscle function with age, and age-related changes in mitochondrial content, respiratory function, morphology, and mitochondrial DNA have previously been reported. However, not all studies report changes in mitochondrial characteristics with ageing, and there is increasing evidence to suggest that physical activity (or inactivity) throughout life is a confounding factor when interpreting age-associated changes. Given that physical activity is a potent stimulus for inducing beneficial adaptations to mitochondrial characteristics, delineating the influence of physical activity on the changes in skeletal muscle that occur with age is complicated. This review aims to summarise our current understanding and knowledge gaps regarding age-related changes to mitochondrial characteristics within skeletal muscle, as well as to provide some novel insights into brain mitochondria, and to propose avenues of future research and targeted interventions. Furthermore, where possible, we incorporate discussions of the modifying effects of physical activity, exercise, and training status, to purported age-related changes in mitochondrial characteristics.

High serum klotho levels are inversely associated with the risk of low muscle mass in middle-aged adults: results from a cross-sectional study

Objective

Muscle mass gradually declines with advancing age, and as an anti-aging protein, klotho may be associated with muscle mass. This study aims to explore the relationship between klotho levels and muscle mass in the middle-aged population.

Methods

Utilizing data from the National Health and Nutrition Examination Survey (NHANES) spanning 2011 to 2018, we conducted a cross-sectional analysis on a cohort of individuals aged 40-59. Weighted multivariable analysis was employed to assess the correlation between klotho and low muscle mass, with stratified and Restricted Cubic Spline (RCS) analyses.

Results

The cross-sectional investigation revealed a significant negative correlation between klotho levels and the risk of low muscle mass (Model 3: OR = 0.807, 95% CI: 0.712-0.915). A notable interaction between klotho and sex was observed, with a significant interaction effect (P for interaction = 0.01). The risk association was notably higher in females. The risk association was notably higher in females. Additionally, RCS analysis unveiled a significant linear relationship between klotho and low muscle mass (P for nonlinear = 0.9495, P for overall<0.0001).

Conclusion

Our observational analysis revealed a noteworthy inverse relationship between klotho and low muscle mass, particularly prominent among female participants. This discovery provides crucial insights for the development of more effective intervention strategies and offers a new direction for enhancing muscle quality in the middle-aged population.

MFN2 overexpression in skeletal muscles of young and old mice causes a mild hypertrophy without altering mitochondrial respiration and H2O2 emission

AIM

Sarcopenia, the aging-related loss of muscle mass and function, is a debilitating process negatively impacting the quality of life of affected individuals. Although the mechanisms underlying sarcopenia are incompletely understood, impairments in mitochondrial dynamics, including mitochondrial fusion, have been proposed as a contributing factor. However, the potential of upregulating mitochondrial fusion proteins to alleviate the effects of aging on skeletal muscles remains unexplored. We therefore hypothesized that overexpressing Mitofusin 2 (MFN2) in skeletal muscle in vivo would mitigate the effects of aging on muscle mass and improve mitochondrial function.

METHODS

MFN2 was overexpressed in young (7 mo) and old (24 mo) male mice for 4 months through intramuscular injections of an adeno-associated viruses. The impacts of MFN2 overexpression on muscle mass and fiber size (histology), mitochondrial respiration, and H2O2 emission (Oroboros fluororespirometry), and various signaling pathways (qPCR and western blotting) were investigated.

RESULTS

MFN2 overexpression increased muscle mass and fiber size in both young and old mice. No sign of fibrosis, necrosis, or inflammation was found upon MFN2 overexpression, indicating that the hypertrophy triggered by MFN2 overexpression was not pathological. MFN2 overexpression even reduced the proportion of fibers with central nuclei in old muscles. Importantly, MFN2 overexpression had no impact on muscle mitochondrial respiration and H2O2 emission in both young and old mice. MFN2 overexpression attenuated the increase in markers of impaired autophagy in old muscles.

CONCLUSION

MFN2 overexpression may be a viable approach to mitigate aging-related muscle atrophy and may have applications for other muscle disorders.

Dietary NMN supplementation enhances motor and NMJ function in ALS

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease that causes the degeneration of motor neurons in the motor cortex and spinal cord. Patients with ALS experience muscle weakness and atrophy in the limbs which eventually leads to paralysis and death. NAD+ is critical for energy metabolism, such as glycolysis and oxidative phosphorylation, but is also involved in non-metabolic cellular reactions. In the current study, we determined whether the supplementation of nicotinamide mononucleotide (NMN), an NAD+ precursor, in the diet had beneficial impacts on disease progression using a SOD1G93A mouse model of ALS. We found that the ALS mice fed with an NMN-supplemented diet (ALS+NMN mice) had modestly extended lifespan and exhibited delayed motor dysfunction. Using electrophysiology, we studied the effect of NMN on synaptic transmission at neuromuscular junctions (NMJs) in symptomatic of ALS mice (18 weeks old). ALS+NMN mice had larger end-plate potential (EPP) amplitudes and maintained better responses than ALS mice, and also had restored EPP facilitation. While quantal content was not affected by NMN, miniature EPP (mEPP) amplitude and frequency were elevated in ALS+NMN mice. NMN supplementation in diet also improved NMJ morphology, innervation, mitochondrial structure, and reduced reactive astrogliosis in the ventral horn of the lumbar spinal cord. Overall, our results indicate that dietary consumption of NMN can slow motor impairment, enhance NMJ function and improve healthspan of ALS mice.

The mechanisms and roles of mitochondrial dynamics in C. elegans

If mitochondria are the powerhouses of the cell, then mitochondrial dynamics are the power grid that regulates how that energy output is directed and maintained in response to unique physiological demands. Fission and fusion dynamics are highly regulated processes that fine-tune the mitochondrial networks of cells to enable appropriate responses to intrinsic and extrinsic stimuli, thereby maintaining cellular and organismal homeostasis. These dynamics shape many aspects of an organism's healthspan including development, longevity, stress resistance, immunity, and response to disease. In this review, we discuss the latest findings regarding the mechanisms and roles of mitochondrial dynamics by focussing on the nematode Caenorhabditis elegans. Whole live-animal studies in C. elegans have enabled a true organismal-level understanding of the impact that mitochondrial dynamics play in homeostasis over a lifetime.

YAP/TAZ as Molecular Targets in Skeletal Muscle Atrophy and Osteoporosis

Skeletal muscles and bones are closely connected anatomically and functionally. Age-related degeneration in these tissues is associated with physical disability in the elderly and significantly impacts their quality of life. Understanding the mechanisms of age-related musculoskeletal tissue degeneration is crucial for identifying molecular targets for therapeutic interventions for skeletal muscle atrophy and osteoporosis. The Hippo pathway is a recently identified signaling pathway that plays critical roles in development, tissue homeostasis, and regeneration. The Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are key downstream effectors of the mammalian Hippo signaling pathway. This review highlights the fundamental roles of YAP and TAZ in the homeostatic maintenance and regeneration of skeletal muscles and bones. YAP/TAZ play a significant role in stem cell function by relaying various environmental signals to stem cells. Skeletal muscle atrophy and osteoporosis are related to stem cell dysfunction or senescence triggered by YAP/TAZ dysregulation resulting from reduced mechanosensing and mitochondrial function in stem cells. In contrast, the maintenance of YAP/TAZ activation can suppress stem cell senescence and tissue dysfunction and may be used as a basis for the development of potential therapeutic strategies. Thus, targeting YAP/TAZ holds significant therapeutic potential for alleviating age-related muscle and bone dysfunction and improving the quality of life in the elderly.

Mitochondrial Dysfunction: A Key Player in Brain Aging and Diseases

Mitochondria are thought to have become incorporated within the eukaryotic cell approximately 2 billion years ago and play a role in a variety of cellular processes, such as energy production, calcium buffering and homeostasis, steroid synthesis, cell growth, and apoptosis, as well as inflammation and ROS production. Considering that mitochondria are involved in a multitude of cellular processes, mitochondrial dysfunction has been shown to play a role within several age-related diseases, including cancers, diabetes (type 2), and neurodegenerative diseases, although the underlying mechanisms are not entirely understood. The significant increase in lifespan and increased incidence of age-related diseases over recent decades has confirmed the necessity to understand the mechanisms by which mitochondrial dysfunction impacts the process of aging and age-related diseases. In this review, we will offer a brief overview of mitochondria, along with structure and function of this important organelle. We will then discuss the cause and consequence of mitochondrial dysfunction in the aging process, with a particular focus on its role in inflammation, cognitive decline, and neurodegenerative diseases, such as Huntington's disease, Parkinson's disease, and Alzheimer's disease. We will offer insight into therapies and interventions currently used to preserve or restore mitochondrial functioning during aging and neurodegeneration.

Structural Plasticity of GABAergic Pallidothalamic Terminals in MPTP-Treated Parkinsonian Monkeys: A 3D Electron Microscopic Analysis

The internal globus pallidus (GPi) is a major source of tonic GABAergic inhibition to the motor thalamus. In parkinsonism, the firing rate of GPi neurons is increased, and their pattern switches from a tonic to a burst mode, two pathophysiological changes associated with increased GABAergic pallidothalamic activity. In this study, we used high-resolution 3D electron microscopy to demonstrate that GPi terminals in the parvocellular ventral anterior nucleus (VApc) and the centromedian nucleus (CM), the two main GPi-recipient motor thalamic nuclei in monkeys, undergo significant morphometric changes in parkinsonian monkeys including (1) increased terminal volume in both nuclei; (2) increased surface area of synapses in both nuclei; (3) increased number of synapses/GPi terminals in the CM, but not VApc; and (4) increased total volume, but not number, of mitochondria/terminals in both nuclei. In contrast to GPi terminals, the ultrastructure of putative GABAergic nonpallidal terminals was not affected. Our results also revealed striking morphological differences in terminal volume, number/area of synapses, and volume/number of mitochondria between GPi terminals in VApc and CM of control monkeys. In conclusion, GABAergic pallidothalamic terminals are endowed with a high level of structural plasticity that may contribute to the development and maintenance of the abnormal increase in pallidal GABAergic outflow to the thalamus in the parkinsonian state. Furthermore, the evidence for ultrastructural differences between GPi terminals in VApc and CM suggests that morphologically distinct pallidothalamic terminals from single pallidal neurons may underlie specific physiological properties of pallidal inputs to VApc and CM in normal and diseased states.

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.

The contribution of mitochondria to age-related skeletal muscle wasting: A sex-specific perspective

As people age, their skeletal muscle (SkM) experiences a decline in mitochondrial functionality and density, which leads to decreased energy production and increased generation of reactive oxygen species. This cascade of events, in turn, might determine the loss of SkM mass, strength and quality. Even though the mitochondrial processes dysregulated by aging, such as oxidative phosphorylation, mitophagy, antioxidant defenses and mtDNA transcription, are the same in both sexes, mitochondria age differently in the SkM of men and women. Indeed, the onset and magnitude of the impairment of these processes seem to be influenced by sex-specific factors. Sexual hormones play a pivotal role in the regulation of SkM mass through both genomic and non-genomic mechanisms. However, the precise mechanisms by which these hormones regulate mitochondrial plasticity in SkM are not fully understood. Although the presence of estrogen receptors in mitochondria is recognized, it remains unclear whether androgen receptors affect mitochondrial function. This comprehensive review critically dissects the current knowledge on the interplay of sex in the aging of SkM, focusing on the role of sex hormones and the corresponding signaling pathways in shaping mitochondrial plasticity. Improved knowledge on the sex dimorphism of mitochondrial aging may lead to sex-tailored interventions that target mitochondrial health, which could be effective in slowing or preventing age-related muscle loss.

Downregulation of mitochondrial metabolism is a driver for fast skeletal muscle loss during mouse aging

Skeletal muscle aging is characterized by the loss of muscle mass, strength and function, mainly attributed to the atrophy of glycolytic fibers. Underlying mechanisms driving the skeletal muscle functional impairment are yet to be elucidated. To unbiasedly uncover its molecular mechanisms, we recurred to gene expression and metabolite profiling in a glycolytic muscle, Extensor digitorum longus (EDL), from young and aged C57BL/6JRj mice. Employing multi-omics approaches we found that the main age-related changes are connected to mitochondria, exhibiting a downregulation in mitochondrial processes. Consistent is the altered mitochondrial morphology. We further compared our mouse EDL aging signature with human data from the GTEx database, reinforcing the idea that our model may recapitulate muscle loss in humans. We are able to show that age-related mitochondrial downregulation is likely to be detrimental, as gene expression signatures from commonly used lifespan extending interventions displayed the opposite direction compared to our EDL aging signature.

Mitophagy in human health, ageing and disease

Maintaining optimal mitochondrial function is a feature of health. Mitophagy removes and recycles damaged mitochondria and regulates the biogenesis of new, fully functional ones preserving healthy mitochondrial functions and activities. Preclinical and clinical studies have shown that impaired mitophagy negatively affects cellular health and contributes to age-related chronic diseases. Strategies to boost mitophagy have been successfully tested in model organisms, and, recently, some have been translated into clinics. In this Review, we describe the basic mechanisms of mitophagy and how mitophagy can be assessed in human blood, the immune system and tissues, including muscle, brain and liver. We outline mitophagy's role in specific diseases and describe mitophagy-activating approaches successfully tested in humans, including exercise and nutritional and pharmacological interventions. We describe how mitophagy is connected to other features of ageing through general mechanisms such as inflammation and oxidative stress and forecast how strengthening research on mitophagy and mitophagy interventions may strongly support human health.

STAT3 modulates CD4+ T mitochondrial dynamics and function in aging

Aging promotes numerous intracellular changes in T cells that impact their effector function. Our data show that aging promotes an increase in the localization of STAT3 to the mitochondria (mitoSTAT3), which promotes changes in mitochondrial dynamics and function and T-cell cytokine production. Mechanistically, mitoSTAT3 increased the activity of aging T-cell mitochondria by increasing complex II. Limiting mitoSTAT3 using a mitochondria-targeted STAT3 inhibitor, Mtcur-1 lowered complex II activity, prevented age-induced changes in mitochondrial dynamics and function, and reduced Th17 inflammation. Exogenous expression of a constitutively phosphorylated form of STAT3 in T cells from young adults mimicked changes in mitochondrial dynamics and function in T cells from older adults and partially recapitulated aging-related cytokine profiles. Our data show the mechanistic link among mitoSTAT3, mitochondrial dynamics, function, and T-cell cytokine production.

Diaphragm Fatigue in SMNΔ7 Mice and Its Molecular Determinants: An Underestimated Issue

Spinal muscular atrophy (SMA) is a genetic disorder characterized by the loss of spinal motor neurons leading to muscle weakness and respiratory failure. Mitochondrial dysfunctions are found in the skeletal muscle of patients with SMA. For obvious ethical reasons, the diaphragm muscle is poorly studied, notwithstanding the very important role that respiratory involvement plays in SMA mortality. The main goal of this study was to investigate diaphragm functionality and the underlying molecular adaptations in SMNΔ7 mice, a mouse model that exhibits symptoms similar to that of patients with intermediate type II SMA. Functional, biochemical, and molecular analyses on isolated diaphragm were performed. The obtained results suggest the presence of an intrinsic energetic imbalance associated with mitochondrial dysfunction and a significant accumulation of reactive oxygen species (ROS). In turn, ROS accumulation can affect muscle fatigue, cause diaphragm wasting, and, in the long run, respiratory failure in SMNΔ7 mice. Exposure to the antioxidant molecule ergothioneine leads to the functional recovery of the diaphragm, confirming the presence of mitochondrial impairment and redox imbalance. These findings suggest the possibility of carrying out a dietary supplementation in SMNΔ7 mice to preserve their diaphragm function and increase their lifespan.

Muscle-targeted nanoparticles strengthen the effects of small-molecule inhibitors in ameliorating sarcopenia

Background: Sarcopenia is an aging-related degeneration of muscle mass and strength. Small-molecule inhibitor SW033291 has been shown to attenuate muscle atrophy. Targeted nanodrug-delivery systems can improve the efficacy of small-molecule inhibitors. Methods: The skeletal muscle cell-targeted nanoparticle was called AP@SW033291, which consisted of SW033291, modular peptide ASSLNIAGGRRRRRG and PEG-DSPE. Nanoparticles were featured with particle size, fluorescence emission spectra and targeting ability. We also investigated their effects on muscle mass and function. Results: The size of AP@SW033291 was 125.7 nm and it demonstrated targeting effects on skeletal muscle; thus, it could improve muscle mass and muscle function. Conclusion: Nanoparticle AP@SW033291 could become a potential strategy to strengthen the treatment effects of small-molecule inhibitors in sarcopenia.

Mitochondrial dysfunction and skeletal muscle atrophy: Causes, mechanisms, and treatment strategies

Skeletal muscle, which accounts for approximately 40% of total body weight, is one of the most dynamic and plastic tissues in the human body and plays a vital role in movement, posture and force production. More than just a component of the locomotor system, skeletal muscle functions as an endocrine organ capable of producing and secreting hundreds of bioactive molecules. Therefore, maintaining healthy skeletal muscles is crucial for supporting overall body health. Various pathological conditions, such as prolonged immobilization, cachexia, aging, drug-induced toxicity, and cardiovascular diseases (CVDs), can disrupt the balance between muscle protein synthesis and degradation, leading to skeletal muscle atrophy. Mitochondrial dysfunction is a major contributing mechanism to skeletal muscle atrophy, as it plays crucial roles in various biological processes, including energy production, metabolic flexibility, maintenance of redox homeostasis, and regulation of apoptosis. In this review, we critically examine recent knowledge regarding the causes of muscle atrophy (disuse, cachexia, aging, etc.) and its contribution to CVDs. Additionally, we highlight the mitochondrial signaling pathways involvement to skeletal muscle atrophy, such as the ubiquitin-proteasome system, autophagy and mitophagy, mitochondrial fission-fusion, and mitochondrial biogenesis. Furthermore, we discuss current strategies, including exercise, mitochondria-targeted antioxidants, in vivo transfection of PGC-1α, and the potential use of mitochondrial transplantation as a possible therapeutic approach.

Understanding the development of sarcopenic obesity

INTRODUCTION

Sarcopenic obesity (SarcO) is defined as the confluence of reduced muscle mass and function and excess body fat. The scientific community is increasingly recognizing this syndrome, which affects a subgroup of persons across their lifespans and places them at synergistically higher risk of significant medical comorbidity and disability than either sarcopenia or obesity alone. Joint efforts in clinical and research settings are imperative to better understand this syndrome and drive the development of urgently needed future interventions.

AREAS COVERED

Herein, we describe the ongoing challenges in defining sarcopenic obesity and the current state of the science regarding its epidemiology and relationship with adverse events. The field has demonstrated an emergence of data over the past decade which we will summarize in this article. While the etiology of sarcopenic obesity is complex, we present data on the underlying pathophysiological mechanisms that are hypothesized to promote its development, including age-related changes in body composition, hormonal changes, chronic inflammation, and genetic predisposition.

EXPERT OPINION

We describe emerging areas of future research that will likely be needed to advance this nascent field, including changes in clinical infrastructure, an enhanced understanding of the lifecourse, and potential treatments.

Mitochondrial Properties in Skeletal Muscle Fiber

Mitochondria are the primary source of energy production and are implicated in a wide range of biological processes in most eukaryotic cells. Skeletal muscle heavily relies on mitochondria for energy supplements. In addition to being a powerhouse, mitochondria evoke many functions in skeletal muscle, including regulating calcium and reactive oxygen species levels. A healthy mitochondria population is necessary for the preservation of skeletal muscle homeostasis, while mitochondria dysregulation is linked to numerous myopathies. In this review, we summarize the recent studies on mitochondria function and quality control in skeletal muscle, focusing mainly on in vivo studies of rodents and human subjects. With an emphasis on the interplay between mitochondrial functions concerning the muscle fiber type-specific phenotypes, we also discuss the effect of aging and exercise on the remodeling of skeletal muscle and mitochondria properties.

Mitochondrial heterogeneity in diseases

As key organelles involved in cellular metabolism, mitochondria frequently undergo adaptive changes in morphology, components and functions in response to various environmental stresses and cellular demands. Previous studies of mitochondria research have gradually evolved, from focusing on morphological change analysis to systematic multiomics, thereby revealing the mitochondrial variation between cells or within the mitochondrial population within a single cell. The phenomenon of mitochondrial variation features is defined as mitochondrial heterogeneity. Moreover, mitochondrial heterogeneity has been reported to influence a variety of physiological processes, including tissue homeostasis, tissue repair, immunoregulation, and tumor progression. Here, we comprehensively review the mitochondrial heterogeneity in different tissues under pathological states, involving variant features of mitochondrial DNA, RNA, protein and lipid components. Then, the mechanisms that contribute to mitochondrial heterogeneity are also summarized, such as the mutation of the mitochondrial genome and the import of mitochondrial proteins that result in the heterogeneity of mitochondrial DNA and protein components. Additionally, multiple perspectives are investigated to better comprehend the mysteries of mitochondrial heterogeneity between cells. Finally, we summarize the prospective mitochondrial heterogeneity-targeting therapies in terms of alleviating mitochondrial oxidative damage, reducing mitochondrial carbon stress and enhancing mitochondrial biogenesis to relieve various pathological conditions. The possibility of recent technological advances in targeted mitochondrial gene editing is also discussed.

Autophagy ablation in skeletal muscles worsens sepsis-induced muscle wasting, impairs whole-body metabolism, and decreases survival

Septic patients frequently develop skeletal muscle wasting and weakness, resulting in severe clinical consequences and adverse outcomes. Sepsis triggers sustained induction of autophagy, a key cellular degradative pathway, in skeletal muscles. However, the impact of enhanced autophagy on sepsis-induced muscle dysfunction remains unclear. Using an inducible and muscle-specific Atg7 knockout mouse model (Atg7iSkM-KO), we investigated the functional importance of skeletal muscle autophagy in sepsis using the cecal ligation and puncture model. Atg7iSkM-KO mice exhibited a more severe phenotype in response to sepsis, marked by severe muscle wasting, hypoglycemia, higher ketone levels, and a decreased in survival as compared to mice with intact Atg7. Sepsis and Atg7 deletion resulted in the accumulation of mitochondrial dysfunction, although sepsis did not further worsen mitochondrial dysfunction in Atg7iSkM-KO mice. Overall, our study demonstrates that autophagy inactivation in skeletal muscles triggers significant worsening of sepsis-induced muscle and metabolic dysfunctions and negatively impacts survival.

Changes in the Mitochondria in the Aging Process-Can α-Tocopherol Affect Them?

Aerobic organisms use molecular oxygen in several reactions, including those in which the oxidation of substrate molecules is coupled to oxygen reduction to produce large amounts of metabolic energy. The utilization of oxygen is associated with the production of ROS, which can damage biological macromolecules but also act as signaling molecules, regulating numerous cellular processes. Mitochondria are the cellular sites where most of the metabolic energy is produced and perform numerous physiological functions by acting as regulatory hubs of cellular metabolism. They retain the remnants of their bacterial ancestors, including an independent genome that encodes part of their protein equipment; they have an accurate quality control system; and control of cellular functions also depends on communication with the nucleus. During aging, mitochondria can undergo dysfunctions, some of which are mediated by ROS. In this review, after a description of how aging affects the mitochondrial quality and quality control system and the involvement of mitochondria in inflammation, we report information on how vitamin E, the main fat-soluble antioxidant, can protect mitochondria from age-related changes. The information in this regard is scarce and limited to some tissues and some aspects of mitochondrial alterations in aging. Improving knowledge of the effects of vitamin E on aging is essential to defining an optimal strategy for healthy aging.

Evidence for sex-specific intramuscular changes associated to physical weakness in adults older than 75 years

BACKGROUND

Physical weakness is a key component of frailty, and is highly prevalent in older adults. While females have a higher prevalence and earlier onset, sex differences in the development of frailty-related physical weakness are hardly studied. Therefore, we investigated the intramuscular changes that differentiate between fit and weak older adults for each sex separately.

METHODS

Male (n = 28) and female (n = 26) older adults (75 + years) were grouped on the basis of their ranks according to three frailty-related physical performance criteria. Muscle biopsies taken from vastus lateralis muscle were used for transcriptome and histological examination. Pairwise comparisons were made between the fittest and weakest groups for each sex separately, and potential sex-specific effects were assessed.

RESULTS

Weak females were characterized by a higher expression of inflammatory pathways and infiltration of NOX2-expressing immune cells, concomitant with a higher VCAM1 expression. Weak males were characterized by a smaller diameter of type 2 (fast) myofibers and lower expression of PRKN. In addition, weakness-associated transcriptome changes in the muscle were distinct from aging, suggesting that the pathophysiology of frailty-associated physical weakness does not necessarily depend on aging.

CONCLUSIONS

We conclude that physical weakness-associated changes in muscle are sex-specific and recommend that sex differences are taken into account in research on frailty, as these differences may have a large impact on the development of (pharmaceutical) interventions against frailty.

TRIAL REGISTRATION NUMBER

The FITAAL study was registered in the Dutch Trial Register, with registration code NTR6124 on 14-11-2016 ( https://trialsearch.who.int/Trial2.aspx?TrialID=NTR6124  ).

HIGHLIGHTS

• In female, but not male older adults, physical weakness was associated with a higher expression of intramuscular markers for inflammation. • In male, but not female older adults, physical weakness was associated with a smaller diameter of type 2 (fast) myofibers and lower PRKN expression. • Fit older adults (of both sexes) maintained expression levels comparable to young participants of weakness related genes, differing from frail participants.

Mitochondrial transplantation as a possible therapeutic option for sarcopenia

With advancing age, the skeletal muscle phenotype is characterized by a progressive loss of mass, strength, and quality. This phenomenon, known as sarcopenia, has a negative impact on quality of life and increases the risk of morbidity and mortality in older adults. Accumulating evidence suggests that damaged and dysfunctional mitochondria play a critical role in the pathogenesis of sarcopenia. Lifestyle modifications, such as physical activity, exercise, and nutrition, as well as medical interventions with therapeutic agents, are effective in the management of sarcopenia and offer solutions to maintain and improve skeletal muscle health. Although a great deal of effort has been devoted to the identification of the best treatment option, these strategies are not sufficient to overcome sarcopenia. Recently, it has been reported that mitochondrial transplantation may be a possible therapeutic approach for the treatment of mitochondria-related pathological conditions such as ischemia, liver toxicity, kidney injury, cancer, and non-alcoholic fatty liver disease. Given the role of mitochondria in the function and metabolism of skeletal muscle, mitochondrial transplantation may be a possible option for the treatment of sarcopenia. In this review, we summarize the definition and characteristics of sarcopenia and molecular mechanisms associated with mitochondria that are known to contribute to sarcopenia. We also discuss mitochondrial transplantation as a possible option. Despite the progress made in the field of mitochondrial transplantation, further studies are needed to elucidate the role of mitochondrial transplantation in sarcopenia. KEY MESSAGES: Sarcopenia is the progressive loss of skeletal muscle mass, strength, and quality. Although the specific mechanisms that lead to sarcopenia are not fully understood, mitochondria have been identified as a key factor in the development of sarcopenia. Damaged and dysfunctional mitochondria initiate various cellular mediators and signaling pathways, which largely contribute to the age-related loss of skeletal muscle mass and strength. Mitochondrial transplantation has been reported to be a possible option for the treatment/prevention of several diseases. Mitochondrial transplantation may be a possible therapeutic option for improving skeletal muscle health and treating sarcopenia. Mitochondrial transplantation as a possible treatment option for sarcopenia.

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.

Mitochondrion: A bridge linking aging and degenerative diseases

Aging is a natural process, characterized by progressive loss of physiological integrity, impaired function, and increased vulnerability to death. For centuries, people have been trying hard to understand the process of aging and find effective ways to delay it. However, limited breakthroughs have been made in anti-aging area. Since the hallmarks of aging were summarized in 2013, increasing studies focus on the role of mitochondrial dysfunction in aging and aging-related degenerative diseases, such as neurodegenerative diseases, osteoarthritis, metabolic diseases, and cardiovascular diseases. Accumulating evidence indicates that restoring mitochondrial function and biogenesis exerts beneficial effects in extending lifespan and promoting healthy aging. In this paper, we provide an overview of mitochondrial changes during aging and summarize the advanced studies in mitochondrial therapies for the treatment of degenerative diseases. Current challenges and future perspectives are proposed to provide novel and promising directions for future research.

Accumulation of polyunsaturated fatty acid-derived metabolites in the sarcopenic muscle of aging mice

AIM

Although it is known that advanced age alters skeletal muscle lipid metabolism, the role(s) of polyunsaturated fatty acid-derived metabolites (mostly eicosanoids and docosanoids) in sarcopenia are not clear. We therefore examined the changes in the metabolites of arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid in the sarcopenic muscle of aged mice.

METHODS

We used 6- and 24-month-old male C57BL/6J mice as healthy and sarcopenic muscle models, respectively. Skeletal muscles were removed from the lower limb and subjected to a liquid chromatography-tandem mass spectrometry analysis.

RESULTS

The liquid chromatography-tandem mass spectrometry analysis detected distinct changes of metabolites in the muscles of the aged mice. Of the 63 metabolites identified, nine were significantly higher in the sarcopenic muscle of aged mice compared with the healthy muscle of young mice. In particular, prostaglandin E2 , prostaglandin F2a , thromboxane B2 , 5-hydroxyeicosatetraenoic acid, and 15-oxo-eicosatetraenoic acid (arachidonic acid-derived metabolites), 12-hydroxy-eicosapentaenoic acid and 14,15-epoxy-eicosatetraenoic acid (eicosapentaenoic acid-derived metabolites) and 10-hydroxydocosa-hexaenoic acid and 14-hydroxyoctadeca-pentaenoic acid (docosahexaenoic acid-derived metabolites) were significantly higher in aged tissue compared with young tissue (all P < 0.05).

CONCLUSIONS

We observed the accumulation of metabolites in the sarcopenic muscle of aged mice. Our results may provide new insights into the pathogenesis and progression of aging- or disease-related sarcopenia. Geriatr Gerontol Int 2023; 23: 297-303.

MYTHO is a novel regulator of skeletal muscle autophagy and integrity

Autophagy is a critical process in the regulation of muscle mass, function and integrity. The molecular mechanisms regulating autophagy are complex and still partly understood. Here, we identify and characterize a novel FoxO-dependent gene, d230025d16rik which we named Mytho (Macroautophagy and YouTH Optimizer), as a regulator of autophagy and skeletal muscle integrity in vivo. Mytho is significantly up-regulated in various mouse models of skeletal muscle atrophy. Short term depletion of MYTHO in mice attenuates muscle atrophy caused by fasting, denervation, cancer cachexia and sepsis. While MYTHO overexpression is sufficient to trigger muscle atrophy, MYTHO knockdown results in a progressive increase in muscle mass associated with a sustained activation of the mTORC1 signaling pathway. Prolonged MYTHO knockdown is associated with severe myopathic features, including impaired autophagy, muscle weakness, myofiber degeneration, and extensive ultrastructural defects, such as accumulation of autophagic vacuoles and tubular aggregates. Inhibition of the mTORC1 signaling pathway in mice using rapamycin treatment attenuates the myopathic phenotype triggered by MYTHO knockdown. Skeletal muscles from human patients diagnosed with myotonic dystrophy type 1 (DM1) display reduced Mytho expression, activation of the mTORC1 signaling pathway and impaired autophagy, raising the possibility that low Mytho expression might contribute to the progression of the disease. We conclude that MYTHO is a key regulator of muscle autophagy and integrity.

The impact of high-fat feeding and parkin overexpression on skeletal muscle mass, mitochondrial respiration, and H2O2 emission

Obesity is a major risk factor for developing various health problems, including insulin resistance and type 2 diabetes. Although controversial, accumulation of mitochondrial dysfunction, and notably an increase in mitochondrial reactive oxygen species (ROS) production, was proposed as a key contributor leading to obesity-induced insulin resistance. Here, our goal was to investigate whether Parkin overexpression, a key regulator of the removal of dysfunctional mitochondria through mitophagy, could confer protection against obesity-induced mitochondrial dysfunction. To this end, intramuscular injections of adeno-associated viruses (AAVs) were performed to overexpress Parkin in limb muscle of 6-mo-old mice fed a control diet (CD) or a high-fat diet (HFD) for 12 wk. An AAV-expressing the green fluorescent protein (GFP) was used as control. HFD increased fat mass, altered glycemia, and resulted in insulin resistance. Parkin overexpression resulted in an increase in muscle mass in both CD and HFD mice. In CD mice, Parkin overexpression increased maximal mitochondrial respiration and lowered H₂O₂ emission. HFD increased mitochondrial respiration and, surprisingly, also lowered H₂O₂ emission. Parkin overexpression did not significantly impact mitochondrial function in HFD mice. Taken altogether, our results indicate that Parkin overexpression positively impacts muscle and mitochondrial health under basal conditions and challenges the notion that intrinsic mitochondrial dysfunction is involved in the development of insulin resistance caused by high-fat feeding.

Targeting Mitochondria to Control Ageing and Senescence

Ageing is accompanied by a progressive impairment of cellular function and a systemic deterioration of tissues and organs, resulting in increased vulnerability to multiple diseases. Here, we review the interplay between two hallmarks of ageing, namely, mitochondrial dysfunction and cellular senescence. The targeting of specific mitochondrial features in senescent cells has the potential of delaying or even reverting the ageing process. A deeper and more comprehensive understanding of mitochondrial biology in senescent cells is necessary to effectively face this challenge. Here, we discuss the main alterations in mitochondrial functions and structure in both ageing and cellular senescence, highlighting the differences and similarities between the two processes. Moreover, we describe the treatments available to target these pathways and speculate on possible future directions of anti-ageing and anti-senescence therapies targeting mitochondria.

Loss of epigenetic information as a cause of mammalian aging

All living things experience an increase in entropy, manifested as a loss of genetic and epigenetic information. In yeast, epigenetic information is lost over time due to the relocalization of chromatin-modifying proteins to DNA breaks, causing cells to lose their identity, a hallmark of yeast aging. Using a system called "ICE" (inducible changes to the epigenome), we find that the act of faithful DNA repair advances aging at physiological, cognitive, and molecular levels, including erosion of the epigenetic landscape, cellular exdifferentiation, senescence, and advancement of the DNA methylation clock, which can be reversed by OSK-mediated rejuvenation. These data are consistent with the information theory of aging, which states that a loss of epigenetic information is a reversible cause of aging.

Exercise preserves physical fitness during aging through AMPK and mitochondrial dynamics

Exercise is a nonpharmacological intervention that improves health during aging and a valuable tool in the diagnostics of aging-related diseases. In muscle, exercise transiently alters mitochondrial functionality and metabolism. Mitochondrial fission and fusion are critical effectors of mitochondrial plasticity, which allows a fine-tuned regulation of organelle connectiveness, size, and function. Here we have investigated the role of mitochondrial dynamics during exercise in the model organism Caenorhabditis elegans. We show that in body-wall muscle, a single exercise session induces a cycle of mitochondrial fragmentation followed by fusion after a recovery period, and that daily exercise sessions delay the mitochondrial fragmentation and physical fitness decline that occur with aging. Maintenance of proper mitochondrial dynamics is essential for physical fitness, its enhancement by exercise training, and exercise-induced remodeling of the proteome. Surprisingly, among the long-lived genotypes we analyzed (isp-1,nuo-6, daf-2, eat-2, and CA-AAK-2), constitutive activation of AMP-activated protein kinase (AMPK) uniquely preserves physical fitness during aging, a benefit that is abolished by impairment of mitochondrial fission or fusion. AMPK is also required for physical fitness to be enhanced by exercise, with our findings together suggesting that exercise may enhance muscle function through AMPK regulation of mitochondrial dynamics. Our results indicate that mitochondrial connectivity and the mitochondrial dynamics cycle are essential for maintaining physical fitness and exercise responsiveness during aging and suggest that AMPK activation may recapitulate some exercise benefits. Targeting mechanisms to optimize mitochondrial fission and fusion, as well as AMPK activation, may represent promising strategies for promoting muscle function during aging.

Caloric restriction delays age-related muscle atrophy by inhibiting 11β-HSD1 to promote the differentiation of muscle stem cells

Introduction

Calorie restriction (CR) is an important direction for the delay of sarcopenia in elderly individuals. However, the specific mechanisms of CR against aging are still unclear.

Methods

In this study, we used a CR model of elderly mice with muscle-specific 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) knockout mice and 11β-HSD1 overexpression mice to confirm that CR can delay muscle aging by inhibiting 11β-HSD1 which can transform inactive GC(cortisone) into active GC(cortisol). The ability of self-proliferation and differentiation into muscle fibers of these mouse muscle stem cells (MuSCs) was observed in vitro. Additionally, the mitochondrial function and mitochondrial ATP production capacity of MuSCs were measured by mitochondrial oxygen consumption.

Results

It was found that the 11β-HSD1 expression level was increased in age-related muscle atrophy. Overexpression of 11β-HSD1 led to muscle atrophy in young mice, and 11β-HSD1 knockout rescued age-related muscle atrophy. Moreover, CR in aged mice reduced the local effective concentration of glucocorticoid (GC) through 11β-HSD1, thereby promoting the mitochondrial function and differentiation ability of MuSCs.

Conclusions

Together, our findings highlight promising sarcopenia protection with 40% CR in older ages. Furthermore, we speculated that targeting an 11β-HSD1-dependent metabolic pathway may represent a novel strategy for developing therapeutics against age-related muscle atrophy.

Mitochondrial adaptations to calorie restriction and bariatric surgery in human skeletal muscle: a systematic review with meta-analysis

OBJECTIVE

We performed a meta-analysis to determine the changes induced by calorie restriction (CR) and bariatric surgery on human skeletal muscle mitochondria.

METHODS

A systematic search of Medline and Web of Science was conducted. Controlled trials exploring CR (≥14 days) and mitochondrial function and/or content assessment were included. Moreover, studies analyzing weight loss following gastric surgery were included for comparison purposes. Human muscle data from 28 studies assessing CR (520 muscle samples) and from 10 studies assessing bariatric surgery (155 muscle samples) were analyzed in a random effect meta-analysis with three a priori chosen covariates.

MAIN RESULTS

We report a decrease (p < 0.05) (mean (95 % CI)) in maximal mitochondrial state 3 respiration in response to CR (-0.44 (-0.85, -0.03)) but not in response to surgery (-0.33 (-1.18, 0.52)). No changes in mitochondrial content were reported after CR (-0.05 (-0.12, 0.13)) or in response to surgery (0.23 (-0.05, 0.52)). Moreover, data from CR subjects showed a reduction in complex IV (CIV) activity (-0.29 (-0.56, -0.03)) but not in CIV content (-0.21 (-0.63, 0.22)). Similar results were obtained when the length of the protocol, the initial body mass index, and the estimated energy deficit were included in the model as covariates.

CONCLUSION

The observation of reduced maximal mitochondrial state 3, uncoupled respiration, and CIV activity without altering mitochondrial content suggests that, in human skeletal muscle, CR mainly modulates intrinsic mitochondrial function.

Ageing of skeletal muscle extracellular matrix and mitochondria: finding a potential link

Aim: To discuss the progress of extracellular matrix (ECM) characteristics, mitochondrial homeostasis, and their potential crosstalk in the pathogenesis of sarcopenia, a geriatric syndrome characterized by a generalized and progressive reduction in muscle mass, strength, and physical performance.Methods: This review focuses on the anatomy and physiology of skeletal muscle, alterations of ECM and mitochondria during ageing, and the role of the interplay between ECM and mitochondria in the pathogenesis of sarcopenia.Results: Emerging evidence points to a clear interplay between mitochondria and ECM in various tissues and organs. Under the ageing process, the ECM undergoes changes in composition and physical properties that may mediate mitochondrial changes via the systematic metabolism, ROS, SPARC pathway, and AMPK/PGC-1α signalling, which in turn exacerbate muscle degeneration. However, the precise effects of such crosstalk on the pathobiology of ageing, particularly in skeletal muscle, have not yet been fully understood.Conclusion: The changes in skeletal muscle ECM and mitochondria are partially responsible for the worsened muscle function during the ageing process. A deeper understanding of their alterations and interactions in sarcopenic patients can help prevent sarcopenia and improve its prognoses.

Ageing, Metabolic Dysfunction, and the Therapeutic Role of Antioxidants

The gradual ageing of the world population has been accompanied by a dramatic increase in the prevalence of obesity and metabolic diseases, especially type 2 diabetes. The adipose tissue dysfunction associated with ageing and obesity shares many common physiological features, including increased oxidative stress and inflammation. Understanding the mechanisms responsible for adipose tissue dysfunction in obesity may help elucidate the processes that contribute to the metabolic disturbances that occur with ageing. This, in turn, may help identify therapeutic targets for the treatment of obesity and age-related metabolic disorders. Because oxidative stress plays a critical role in these pathological processes, antioxidant dietary interventions could be of therapeutic value for the prevention and/or treatment of age-related diseases and obesity and their complications. In this chapter, we review the molecular and cellular mechanisms by which obesity predisposes individuals to accelerated ageing. Additionally, we critically review the potential of antioxidant dietary interventions to counteract obesity and ageing.

From mitochondria to sarcopenia: role of 17β-estradiol and testosterone

Sarcopenia, characterized by a loss of muscle mass and strength with aging, is prevalent in older adults. Although the exact mechanisms underlying sarcopenia are not fully understood, evidence suggests that the loss of mitochondrial integrity in skeletal myocytes has emerged as a pivotal contributor to the complex etiology of sarcopenia. Mitochondria are the primary source of ATP production and are also involved in generating reactive oxygen species (ROS), regulating ion signals, and initiating apoptosis signals in muscle cells. The accumulation of damaged mitochondria due to age-related impairments in any of the mitochondrial quality control (MQC) processes, such as proteostasis, biogenesis, dynamics, and mitophagy, can contribute to the decline in muscle mass and strength associated with aging. Interestingly, a decrease in sex hormones (e.g., 17β-estradiol and testosterone), which occurs with aging, has also been linked to sarcopenia. Indeed, 17β-estradiol and testosterone targeted mitochondria and exhibited activities in regulating mitochondrial functions. Here, we overview the current literature on the key mechanisms by which mitochondrial dysfunction contribute to the development and progression of sarcopenia and the potential modulatory effects of 17β-estradiol and testosterone on mitochondrial function in this context. The advance in its understanding will facilitate the development of potential therapeutic agents to mitigate and manage sarcopenia.

Genetic Manipulation of CB1 Cannabinoid Receptors Reveals a Role in Maintaining Proper Skeletal Muscle Morphology and Function in Mice

The endocannabinoid system (ECS) refers to a widespread signaling system and its alteration is implicated in a growing number of human diseases. Cannabinoid receptors (CBRs) are highly expressed in the central nervous system and many peripheral tissues. Evidence suggests that CB1Rs are expressed in human and murine skeletal muscle mainly in the cell membrane, but a subpopulation is present also in the mitochondria. However, very little is known about the latter population. To date, the connection between the function of CB1Rs and the regulation of intracellular Ca²⁺ signaling has not been investigated yet. Tamoxifen-inducible skeletal muscle-specific conditional CB1 knock-down (skmCB1-KD, hereafter referred to as Cre^+/-) mice were used in this study for functional and morphological analysis. After confirming CB1R down-regulation on the mRNA and protein level, we performed in vitro muscle force measurements and found that peak twitch, tetanus, and fatigue were decreased significantly in Cre^+/- mice. Resting intracellular calcium concentration, voltage dependence of the calcium transients as well as the activity dependent mitochondrial calcium uptake were essentially unaltered by Cnr1 gene manipulation. Nevertheless, we found striking differences in the ultrastructural architecture of the mitochondrial network of muscle tissue from the Cre^+/- mice. Our results suggest a role of CB1Rs in maintaining physiological muscle function and morphology. Targeting ECS could be a potential tool in certain diseases, including muscular dystrophies where increased endocannabinoid levels have already been described.

Mitochondrial function and nutrient sensing pathways in ageing: enhancing longevity through dietary interventions

Ageing is accompanied by alterations in several biochemical processes, highly influenced by its environment. It is controlled by the interactions at various levels of biological hierarchy. To maintain homeostasis, a number of nutrient sensors respond to the nutritional status of the cell and control its energy metabolism. Mitochondrial physiology is influenced by the energy status of the cell. The alterations in mitochondrial physiology and the network of nutrient sensors result in mitochondrial damage leading to age related metabolic degeneration and diseases. Calorie restriction (CR) has proved to be as the most successful intervention to achieve the goal of longevity and healthspan. CR elicits a hormetic response and regulates metabolism by modulating these networks. In this review, the authors summarize the interdependent relationship between mitochondrial physiology and nutrient sensors during the ageing process and their role in regulating metabolism.