Moderate-Intensity Exercise Enhances Mitochondrial Biogenesis Markers in the Skeletal Muscle of a Mouse Model Affected by Diet-Induced Obesity.

Skeletal muscle is composed of bundles of muscle fibers with distinctive characteristics. Oxidative muscle fiber types contain higher mitochondrial content, relying primarily on oxidative phosphorylation for ATP generation. Notably, as a result of obesity, or following prolonged exposure to a high-fat diet, skeletal muscle undergoes a shift in fiber type toward a glycolytic type. Mitochondria are highly dynamic organelles, constantly undergoing mitochondrial biogenesis and dynamic processes. Our study aims to explore the impact of obesity on skeletal muscle mitochondrial biogenesis and dynamics and also ascertain whether the skeletal muscle fiber type shift occurs from the aberrant mitochondrial machinery. Furthermore, we investigated the impact of exercise in preserving the oxidative muscle fiber types despite obesity. Mice were subjected to a normal standard chow and water or high-fat diet with sugar water (HFS) with or without exercise training. After 12 weeks of treatment, the HFS diet resulted in a noteworthy reduction in the markers of mitochondrial content, which was recovered by exercise training. Furthermore, higher mitochondrial biogenesis markers were observed in the exercised group with a subsequent increase in the mitochondrial fission marker. In conclusion, these findings imply a beneficial impact of moderate-intensity exercise on the preservation of oxidative capacity in the muscle of obese mouse models.

IFNγ causes mitochondrial dysfunction and oxidative stress in myositis.

Idiopathic inflammatory myopathies (IIMs) are severe autoimmune diseases with poorly understood pathogenesis and unmet medical needs. Here, we examine the role of interferon γ (IFNγ) using NOD female mice deficient in the inducible T cell co-stimulator (Icos), which have previously been shown to develop spontaneous IFNγ-driven myositis mimicking human disease. Using muscle proteomic and spatial transcriptomic analyses we reveal profound myofiber metabolic dysregulation in these mice. In addition, we report muscle mitochondrial abnormalities and oxidative stress in diseased mice. Supporting a pathogenic role for oxidative stress, treatment with a reactive oxygen species (ROS) buffer compound alleviated myositis, preserved muscle mitochondrial ultrastructure and respiration, and reduced inflammation. Mitochondrial anomalies and oxidative stress were diminished following anti-IFNγ treatment. Further transcriptomic analysis in IIMs patients and human myoblast in vitro studies supported the link between IFNγ and mitochondrial dysfunction observed in mice. These results suggest that mitochondrial dysfunction, ROS and inflammation are interconnected in a self-maintenance loop, opening perspectives for mitochondria therapy and/or ROS targeting drugs in myositis.

Developmental dynamics of mitochondrial fission and fusion proteins in functionally divergent skeletal muscles of goat.

During skeletal muscle development, the intricate mitochondrial network formation relies on continuous fission and fusion. This process in larger mammals differs from rodents, the most used animal models. However, the expression pattern of proteins regulating mitochondrial dynamics in developing skeletal muscle remains unexplored in larger mammals. Therefore, we characterized the cellular expression and tissue-level distribution of these proteins during development taking goat as a model. We have performed histological and immunohistochemical analyses to study metabolic features in various muscles. Neonatal muscles display uniform distribution of mitochondrial activity. In contrast, adult muscles exhibit clear distinctions based on their function, whether dedicated for posture maintenance or facilitating locomotion. Mitochondrial fission proteins like DRP-1, MFF, and fusion proteins like MFN-1 and 2 are abundantly expressed in neonatal muscles. Fission proteins exhibit drastic downregulation with limited peripheral expression, whereas fusion proteins continue to express in a fiber-specific manner during adulthood. Locomotory muscles exhibit different fibers based on mitochondrial activity and peripheralization with high SDH activity. The proximity ligation assay between MFN1 and MFN2 demonstrates that their interaction is restricted to subsarcolemmal mitochondria in adult fibers while distributed evenly in neonatal fibers. These differences between postural and locomotory muscles suggest their physiological and metabolic properties are different.

Impact of low-load resistance exercise with and without blood flow restriction on muscle strength, endurance, and oxidative capacity: A pilot study.

Low-load resistance exercise (LLRE) to failure can increase muscle mass, strength, endurance, and mitochondrial oxidative capacity (OXPHOS). However, the impact of adding blood flow restriction to low-load resistance exercise (LLBFR) when matched for volume on these outcomes is incompletely understood. This pilot study examined the impact of 6 weeks of single-legged LLBFR and volume-matched LLRE on thigh bone-free lean mass, strength, endurance, and mitochondrial OXPHOS. Twenty (12 males and 8 females) untrained young adults (mean ± SD; 21 ± 2 years, 168 ± 11 cm, 68 ± 12 kg) completed 6 weeks of either single-legged LLBFR or volume-matched LLRE. Participants performed four sets of 30, 15, 15, and 15 repetitions at 25% 1-RM of leg press and knee extension with or without BFR three times per week. LLBFR increased knee extension 1-RM, knee extension endurance, and thigh bone-free lean mass relative to control (all p < 0.05). LLRE increased leg press and knee extension 1-RM relative to control (p = 0.012 and p = 0.054, respectively). LLRE also increased mitochondrial OXPHOS (p = 0.047 (nonparametric)). Our study showed that LLBFR increased muscle strength, muscle endurance, and thigh bone-free lean mass in the absence of improvements in mitochondrial OXPHOS. LLRE improved muscle strength and mitochondrial OXPHOS in the absence of improvements in thigh bone-free lean mass or muscle endurance.

An Amino Acid Mixture to Counteract Skeletal Muscle Atrophy: Impact on Mitochondrial Bioenergetics.

Skeletal muscle atrophy (SMA) is caused by a rise in muscle breakdown and a decline in protein synthesis, with a consequent loss of mass and function. This study characterized the effect of an amino acid mixture (AA) in models of SMA, focusing on mitochondria. C57/Bl6 mice underwent immobilization of one hindlimb (I) or cardiotoxin-induced muscle injury (C) and were compared with controls (CTRL). Mice were then administered AA in drinking water for 10 days and compared to a placebo group. With respect to CTRL, I and C reduced running time and distance, along with grip strength; however, the reduction was prevented by AA. Tibialis anterior (TA) muscles were used for histology and mitochondria isolation. I and C resulted in TA atrophy, characterized by a reduction in both wet weight and TA/body weight ratio and smaller myofibers than those of CTRL. Interestingly, these alterations were lightly observed in mice treated with AA. The mitochondrial yield from the TA of I and C mice was lower than that of CTRL but not in AA-treated mice. AA also preserved mitochondrial bioenergetics in TA muscle from I and C mice. To conclude, this study demonstrates that AA prevents loss of muscle mass and function in SMA by protecting mitochondria.

Mitochondria Transplantation: Rescuing Innate Muscle Bioenergetic Impairment in a Model of Aging and Exercise Intolerance.

ABSTRACT: Arroum, T, Hish, GA, Burghardt, KJ, Ghamloush, M, Bazzi, B, Mrech, A, Morse, PT, Britton, SL, Koch, LG, McCully, JD, Hüttemann, M, and Malek, MH. Mitochondria transplantation: Rescuing innate muscle bioenergetic impairment in a model of aging and exercise intolerance. J Strength Cond Res 38(7): 1189-1199, 2024-Mitochondria, through oxidative phosphorylation, are crucial for energy production. Disease, genetic impairment, or deconditioning can harm muscle mitochondria, affecting energy production. Endurance training enhances mitochondrial function but assumes mobility. Individuals with limited mobility lack effective treatments for mitochondrial dysfunction because of disease or aging. Mitochondrial transplantation replaces native mitochondria that have been damaged with viable, respiration-competent mitochondria. Here, we used a rodent model selectively bred for low-capacity running (LCR), which exhibits innate mitochondrial dysfunction in the hind limb muscles. Hence, the purpose of this study was to use a distinct breed of rats (i.e., LCR) that display hereditary skeletal muscle mitochondrial dysfunction to evaluate the consequences of mitochondrial transplantation. We hypothesized that the transplantation of mitochondria would effectively alleviate mitochondrial dysfunction in the hind limb muscles of rats when compared with placebo injections. In addition, we hypothesized that rats receiving the mitochondrial transplantation would experience an improvement in their functional capacity, as evaluated through incremental treadmill testing. Twelve aged LCR male rats (18 months old) were randomized into 2 groups (placebo or mitochondrial transplantation). One LCR rat of the same age and sex was used as the donor to isolate mitochondria from the hindlimb muscles. Isolated mitochondria were injected into both hindlimb muscles (quadriceps femoris, tibialis anterior (TA), and gastrocnemius complex) of a subset LCR (n = 6; LCR-M) rats. The remaining LCR (n = 5; LCR-P) subset received a placebo injection containing only the vehicle without the isolated mitochondria. Four weeks after mitochondrial transplantation, rodents were euthanized and hindlimb muscles harvested. The results indicated a significant (p < 0.05) increase in mitochondrial markers for glycolytic (plantaris and TA) and mixed (quadricep femoris) muscles, but not oxidative muscle (soleus). Moreover, we found significant (p < 0.05) epigenetic changes (i.e., hypomethylation) at the global and site-specific levels for a key mitochondrial regulator (transcription factor A mitochondrial) between the placebo and mitochondrial transplantation groups. To our knowledge, this is the first study to examine the efficacy of mitochondrial transplantation in a rodent model of aging with congenital skeletal muscle dysfunction.

Loss of endogenous estrogen alters mitochondrial metabolism and muscle clock-related protein Rbm20 in female mdx mice.

Female carriers of a Duchenne muscular dystrophy (DMD) gene mutation manifest exercise intolerance and metabolic anomalies that may be exacerbated following menopause due to the loss of estrogen, a known regulator of skeletal muscle function and metabolism. Here, we studied the impact of estrogen depletion (via ovariectomy) on exercise tolerance and muscle mitochondrial metabolism in female mdx mice and the potential of estrogen replacement therapy (using estradiol) to protect against functional and metabolic perturbations. We also investigated the effect of estrogen depletion, and replacement, on the skeletal muscle proteome through an untargeted proteomic approach with TMT-labelling. Our study confirms that loss of estrogen in female mdx mice reduces exercise capacity, tricarboxylic acid cycle intermediates, and citrate synthase activity but that these deficits are offset through estrogen replacement therapy. Furthermore, ovariectomy downregulated protein expression of RNA-binding motif factor 20 (Rbm20), a critical regulator of sarcomeric and muscle homeostasis gene splicing, which impacted pathways involving ribosomal and mitochondrial translation. Estrogen replacement modulated Rbm20 protein expression and promoted metabolic processes and the upregulation of proteins involved in mitochondrial dynamics and metabolism. Our data suggest that estrogen mitigates dystrophinopathic features in female mdx mice and that estrogen replacement may be a potential therapy for post-menopausal DMD carriers.

Expression of mitochondrial oxidative stress response genes in muscle is associated with mitochondrial respiration, physical performance, and muscle mass in the Study of Muscle, Mobility, and Aging.

Gene expression in skeletal muscle of older individuals may reflect compensatory adaptations in response to oxidative damage that preserve tissue integrity and maintain function. Identifying associations between oxidative stress response gene expression patterns and mitochondrial function, physical performance, and muscle mass in older individuals would further our knowledge of mechanisms related to managing molecular damage that may be targeted to preserve physical resilience. To characterize expression patterns of genes responsible for the oxidative stress response, RNA was extracted and sequenced from skeletal muscle biopsies collected from 575 participants (≥70 years old) from the Study of Muscle, Mobility, and Aging. Expression levels of 21 protein-coding RNAs related to the oxidative stress response were analyzed in relation to six phenotypic measures, including maximal mitochondrial respiration from muscle biopsies (Max OXPHOS), physical performance (VO2 peak, 400-m walking speed, and leg strength), and muscle size (thigh muscle volume and whole-body D3Cr muscle mass). The mRNA level of the oxidative stress response genes most consistently associated across outcomes are preferentially expressed within the mitochondria. Higher expression of mRNAs that encode generally mitochondria located proteins SOD2, TRX2, PRX3, PRX5, and GRX2 were associated with higher levels of mitochondrial respiration and VO2 peak. In addition, greater SOD2, PRX3, and GRX2 expression was associated with higher physical performance and muscle size. Identifying specific mechanisms associated with high functioning across multiple performance and physical domains may lead to targeted antioxidant interventions with greater impacts on mobility and independence.

Associations between regional adipose tissue distribution and skeletal muscle bioenergetics in older men and women.

OBJECTIVE: The aim of this study was to examine associations of ectopic adipose tissue (AT) with skeletal muscle (SM) mitochondrial bioenergetics in older adults. METHODS: Cross-sectional data from 829 adults ≥70 years of age were used. Abdominal, subcutaneous, and visceral AT and thigh muscle fat infiltration (MFI) were quantified by magnetic resonance imaging. SM mitochondrial energetics were characterized in vivo (31P-magnetic resonance spectroscopy; ATPmax) and ex vivo (high-resolution respirometry maximal oxidative phosphorylation [OXPHOS]). ActivPal was used to measure physical activity ([PA]; step count). Linear regression adjusted for covariates was applied, with sequential adjustment for BMI and PA. RESULTS: Independent of BMI, total abdominal AT (standardized [Std.] ß = -0.21; R2 = 0.09) and visceral AT (Std. ß = -0.16; R2 = 0.09) were associated with ATPmax (p < 0.01; n = 770) but not following adjustment for PA (p ≥ 0.05; n = 658). Visceral AT (Std. ß = -0.16; R2 = 0.25) and thigh MFI (Std. ß = -0.11; R2 = 0.24) were associated with carbohydrate-supported maximal OXPHOS independent of BMI and PA (p < 0.05; n = 609). Total abdominal AT (Std. ß = -0.19; R2 = 0.24) and visceral AT (Std. ß = -0.17; R2 = 0.24) were associated with fatty acid-supported maximal OXPHOS independent of BMI and PA (p < 0.05; n = 447). CONCLUSIONS: Skeletal MFI and abdominal visceral, but not subcutaneous, AT are inversely associated with SM mitochondrial bioenergetics in older adults independent of BMI. Associations between ectopic AT and in vivo mitochondrial bioenergetics are attenuated by PA.

Role of the gut-muscle axis in mitochondrial function of ageing muscle under different exercise modes.

The fundamental role of the gut microbiota through the gut-muscle axis in skeletal muscle ageing is increasingly recognised. Metabolites derived from the intestinal microbiota are essential in maintaining skeletal muscle function and metabolism. The energy produced by mitochondria and moderate levels of reactive oxygen species can contribute to this process. Metabolites can effectively target the mitochondria, slowing the progression of muscle ageing and potentially representing a marker of ageing-related skeletal muscle loss. Moreover, mitochondria can contribute to the immune response, gut microbiota biodiversity, and maintenance of the intestinal barrier function. However, the causal relationship between mitochondrial function and gut microbiota crosstalk remains poorly understood. In addition to elucidating the regulatory pathways of the gut-muscle axis during the ageing process, we focused on the potential role of the "exercise-gut-muscle axis", which represents a pathway under stimulation from different exercise modes to induce mitochondrial adaptations, skeletal muscle metabolism and maintain intestinal barrier function and biodiversity stability. Meanwhile, different exercise modes can induce mitochondrial adaptations and skeletal muscle metabolism and maintain intestinal barrier function and biodiversity. Resistance exercise may promote mitochondrial adaptation, increase the cross-sectional area of skeletal muscle and muscle hypertrophy, and promote muscle fibre and motor unit recruitment. Whereas endurance exercise promotes mitochondrial biogenesis, aerobic capacity, and energy utilisation, activating oxidative metabolism-related pathways to improve skeletal muscle metabolism and function. This review describes the effects of different exercise modes through the gut-muscle axis and how they act through mitochondria in ageing to define the current state of the field and issues requiring resolution.

CISD3/MiNT is required for complex I function, mitochondrial integrity, and skeletal muscle maintenance.

Mitochondria play a central role in muscle metabolism and function. A unique family of iron-sulfur proteins, termed CDGSH Iron Sulfur Domain-containing (CISD/NEET) proteins, support mitochondrial function in skeletal muscles. The abundance of these proteins declines during aging leading to muscle degeneration. Although the function of the outer mitochondrial CISD/NEET proteins, CISD1/mitoNEET and CISD2/NAF-1, has been defined in skeletal muscle cells, the role of the inner mitochondrial CISD protein, CISD3/MiNT, is currently unknown. Here, we show that CISD3 deficiency in mice results in muscle atrophy that shares proteomic features with Duchenne muscular dystrophy. We further reveal that CISD3 deficiency impairs the function and structure of skeletal muscles, as well as their mitochondria, and that CISD3 interacts with, and donates its [2Fe-2S] clusters to, complex I respiratory chain subunit NADH Ubiquinone Oxidoreductase Core Subunit V2 (NDUFV2). Using coevolutionary and structural computational tools, we model a CISD3-NDUFV2 complex with proximal coevolving residue interactions conducive of [2Fe-2S] cluster transfer reactions, placing the clusters of the two proteins 10 to 16 Å apart. Taken together, our findings reveal that CISD3/MiNT is important for supporting the biogenesis and function of complex I, essential for muscle maintenance and function. Interventions that target CISD3 could therefore impact different muscle degeneration syndromes, aging, and related conditions.

Aging-dependent mitochondrial bioenergetic impairment in the skeletal muscle of NNT-deficient mice.

Overall health relies on features of skeletal muscle that generally decline with age, partly due to mechanisms associated with mitochondrial redox imbalance and bioenergetic dysfunction. Previously, aged mice genetically devoid of the mitochondrial NAD(P)+ transhydrogenase (NNT, encoded by the nicotinamide nucleotide transhydrogenase gene), an enzyme involved in mitochondrial NADPH supply, were shown to exhibit deficits in locomotor behavior. Here, by using young, middle-aged, and older NNT-deficient (Nnt-/-) mice and age-matched controls (Nnt+/+), we aimed to investigate how muscle bioenergetic function and motor performance are affected by NNT expression and aging. Mice were subjected to the wire-hang test to assess locomotor performance, while mitochondrial bioenergetics was evaluated in fiber bundles from the soleus, vastus lateralis and plantaris muscles. An age-related decrease in the average wire-hang score was observed in middle-aged and older Nnt-/- mice compared to age-matched controls. Although respiratory rates in the soleus, vastus lateralis and plantaris muscles did not significantly differ between the genotypes in young mice, the rates of oxygen consumption did decrease in the soleus and vastus lateralis muscles of middle-aged and older Nnt-/- mice. Notably, the soleus, which exhibited the highest NNT expression level, was the muscle most affected by aging, and NNT loss. Additionally, histology of the soleus fibers revealed increased numbers of centralized nuclei in older Nnt-/- mice, indicating abnormal morphology. In summary, our findings suggest that NNT expression deficiency causes locomotor impairments and muscle dysfunction during aging in mice.

Muscle-specific PGC-1α modulates mitochondrial oxidative stress in aged sarcopenia through regulating Nrf2.

BACKGROUND: Aged sarcopenia is characterized by loss of skeletal muscle mass and strength, and mitochondrial dysregulation in skeletal myocyte is considered as a major factor. Here, we aimed to analyze the effects of peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) on mitochondrial reactive oxygen species (ROS) and nuclear factor erythroid 2-related factor 2 (Nrf2) in aged skeletal muscles. METHODS: C2C12 cells were stimulated by 50 µM 7ß-hydroxycholesterol (7ß-OHC) to observe the changes of cellular ROS, mitochondrial ROS, and expression of PGC-1α and Nrf2. Different PGC-1α expression in cells was established by transfection with small interfering RNA (siRNA) or plasmids overexpressing PGC-1α (pEX-3-PGC-1α). The effects of different PGC-1α expression on cellular ROS, mitochondrial ROS and Nrf2 expression were measured in cells. Wild type (WT) mice and PGC-1α conditional knockout (CKO) mice were used to analyze the effects of PGC-1α on aged sarcopenia and expression of Nrf2 and CD38 in gastrocnemius muscles. Diethylmaleate, a Nrf2 activator, was used to analyze the connection between PGC-1α and Nrf2 in cells and in mice. RESULTS: In C2C12 cells, the expressions of PGC-1α and Nrf2 were declined by the 7ß-OHC treatment or PGC-1α silence. Moreover, PGC-1α silence increased the harmful ROS and decreased the Nrf2 protein expression in the 7ß-OHC-treated cells. PGC-1α overexpression decreased the harmful ROS and increased the Nrf2 protein expression in the 7ß-OHC-treated cells. Diethylmaleate treatment decreased the harmful ROS in the 7ß-OHC-treated or PGC-1α siRNA-transfected cells. At the same age, muscle-specific PGC-1α deficiency aggravated aged sarcopenia, decreased Nrf2 expression and increased CD38 expression in gastrocnemius muscles compared with the WT mice. Diethylmaleate treatment improved the muscle function and decreased the CD38 expression in the old two genotypes. CONCLUSIONS: Our study demonstrated that PGC-1α modulated mitochondrial oxidative stress in aged sarcopenia through regulating Nrf2.

Mitochondrial function in skeletal muscle contributes to reproductive endothermy in tegu lizards (Salvator merianae).

AIM: In cyclic climate variations, including seasonal changes, many animals regulate their energy demands to overcome critical transitory moments, restricting their high-demand activities to phases of resource abundance, enabling rapid growth and reproduction. Tegu lizards (Salvator merianae) are ectotherms with a robust annual cycle, being active during summer, hibernating during winter, and presenting a remarkable endothermy during reproduction in spring. Here, we evaluated whether changes in mitochondrial respiratory physiology in skeletal muscle could serve as a mechanism for the increased thermogenesis observed during the tegu's reproductive endothermy. METHODS: We performed high-resolution respirometry and calorimetry in permeabilized red and white muscle fibers, sampled during summer (activity) and spring (high activity and reproduction), in association with citrate synthase measurements. RESULTS: During spring, the muscle fibers exhibited increased oxidative phosphorylation. They also enhanced uncoupled respiration and heat production via adenine nucleotide translocase (ANT), but not via uncoupling proteins (UCP). Citrate synthase activity was higher during the spring, suggesting greater mitochondrial density compared to the summer. These findings were consistent across both sexes and muscle types (red and white). CONCLUSION: The current results highlight potential cellular thermogenic mechanisms in an ectothermic reptile that contribute to transient endothermy. Our study indicates that the unique feature of transitioning to endothermy through nonshivering thermogenesis during the reproductive phase may be facilitated by higher mitochondrial density, function, and uncoupling within the skeletal muscle. This knowledge contributes significant elements to the broader picture of models for the evolution of endothermy, particularly in relation to the enhancement of aerobic capacity.

Lack of compensatory mitophagy in skeletal muscles during sepsis.

Skeletal muscle dysfunction is a major problem in critically ill patients suffering from sepsis. This condition is associated with mitochondrial dysfunction and increased autophagy in skeletal muscles. Autophagy is a proteolytic mechanism involved in eliminating dysfunctional cellular components, including mitochondria. The latter process, referred to as mitophagy, is essential for maintaining mitochondrial quality and skeletal muscle health. Recently, a fluorescent reporter system called mito-QC (i.e. mitochondrial quality control) was developed to specifically quantify mitophagy levels. In the present study, we used mito-QC transgenic mice and confocal microscopy to morphologically monitor mitophagy levels during sepsis. To induce sepsis, Mito-QC mice received Escherichia coli lipopolysaccharide (10 mg kg-1 i.p.) or phosphate-buffered saline and skeletal muscles (hindlimb and diaphragm) were excised 48 h later. In control groups, there was a negative correlation between the basal mitophagy level and overall muscle mitochondrial content. Sepsis increased general autophagy in both limb muscles and diaphragm but had no effect on mitophagy levels. Sepsis was associated with a downregulation of certain mitophagy receptors (Fundc1, Bcl2L13, Fkbp8 and Phbb2). The present study suggests that general autophagy and mitophagy can be dissociated from one another, and that the characteristic accumulation of damaged mitochondria in skeletal muscles under the condition of sepsis may reflect a failure of adequate compensatory mitophagy. KEY POINTS: There was a negative correlation between the basal level of skeletal muscle mitophagy and the mitochondrial content of individual muscles. Mitophagy levels in limb muscles and the diaphragm were unaffected by lipopolysaccharide (LPS)-induced sepsis. With the exception of BNIP3 in sepsis, LPS administration induced either no change or a downregulation of mitophagy receptors in skeletal muscles.

Postmortem mitochondria function in longissimus lumborum of Angus and Brahman steers.

Mitochondria function and integrity may impact postmortem metabolism and meat quality development. Adaptations in heat tolerant Brahman may persist to limit cellular stress postmortem. Our objective was to evaluate glycolysis, pH decline, and mitochondria function in longissimus lumborum (LL) from Angus and Brahman steers (N = 28) early postmortem (1 to 6 h) and after rigor (24 h). We evaluated metabolites of anaerobic glycolysis, ATP, pH, and temperature, and determined mitochondria oxygen consumption rate (OCR) in permeabilized fibers. The main effects of breed (b) and time (t) and the interaction were tested. Brahman LL contained greater ATP during the first 6 h postmortem; Brahman also tended to exhibit a slower pH decline (b × t, P = 0.07) and more rapid temperature decline (b × t, P < 0.001), but metabolites of anaerobic glycolysis were not different. Mitochondria in Brahman and Angus LL were well-coupled and respired at 1 h postmortem. However, outer membrane integrity became increasingly compromised postmortem (t, P < 0.001). Brahman tended to exhibit greater electron transport system capacity (b, P < 0.1) and had greater capacity for oxidative phosphorylation (complex I and II substrates) at 6 h compared with Angus (P < 0.001). In totality, greater ATP, slower pH decline, and enhanced mitochondria capacity indicate that Brahman possess mitochondrial properties or cellular adaptations that help protect the cell during energy stress postmortem. Slower pH and more rapid temperature decline in LL from Brahman may also help preserve mitochondria function postmortem.

Chemerin regulates glucose and lipid metabolism by changing mitochondrial structure and function associated with androgen/androgen receptor.

The adipokine chemerin contributes to exercise-induced improvements in glucose and lipid metabolism; however, the underlying mechanism remains unclear. We aimed to confirm the impact of reduced chemerin expression on exercise-induced improvement in glycolipid metabolism in male diabetic (DM) mice through exogenous chemerin administration. Furthermore, the underlying mechanism of chemerin involved in changes in muscle mitochondria function mediated by androgen/androgen receptor (AR) was explored by generating adipose-specific and global chemerin knockout (adipo-chemerin-/- and chemerin-/-) mice. DM mice were categorized into the DM, exercised DM (EDM), and EDM + chemerin supplementation groups. Adipo-chemerin-/- and chemerin-/- mice were classified in the sedentary or exercised groups and fed either a normal or high-fat diet. Exercise mice underwent a 6-wk aerobic exercise regimen. The serum testosterone and chemerin levels, glycolipid metabolism indices, mitochondrial function, and protein levels involved in mitochondrial biogenesis and dynamics were measured. Notably, exogenous chemerin reversed exercise-induced improvements in glycolipid metabolism, AR protein levels, mitochondrial biogenesis, and mitochondrial fusion in DM mice. Moreover, adipose-specific chemerin knockout improved glycolipid metabolism, enhanced exercise-induced increases in testosterone and AR levels in exercised mice, and alleviated the detrimental effects of a high-fat diet on mitochondrial morphology, biogenesis, and dynamics. Finally, similar improvements in glucose metabolism (but not lipid metabolism), mitochondrial function, and mitochondrial dynamics were observed in chemerin-/- mice. In conclusion, decreased chemerin levels affect exercise-induced improvements in glycolipid metabolism in male mice by increasing mitochondrial number and function, likely through changes in androgen/AR signaling.NEW & NOTEWORTHY Decreased chemerin levels affect exercise-induced improvements in glycolipid metabolism in male mice by increasing mitochondrial number and function, which is likely mediated by androgen/androgen receptor expression. This study is the first to report the regulatory mechanism of chemerin in muscle mitochondria.

Differential impact of sex on regulation of skeletal muscle mitochondrial function and protein homeostasis by hypoxia-inducible factor-1α in normoxia.

Hypoxia-inducible factor (HIF)-1α is continuously synthesized and degraded in normoxia. During hypoxia, HIF1α stabilization restricts cellular/mitochondrial oxygen utilization. Cellular stressors can stabilize HIF1α even during normoxia. However, less is known about HIF1α function(s) and sex-specific effects during normoxia in the basal state. Since skeletal muscle is the largest protein store in mammals and protein homeostasis has high energy demands, we determined HIF1α function at baseline during normoxia in skeletal muscle. Untargeted multiomics data analyses were followed by experimental validation in differentiated murine myotubes with loss/gain of function and skeletal muscle from mice without/with post-natal muscle-specific Hif1a deletion (Hif1amsd). Mitochondrial oxygen consumption studies using substrate, uncoupler, inhibitor, titration protocols; targeted metabolite quantification by gas chromatography-mass spectrometry; and post-mitotic senescence markers using biochemical assays were performed. Multiomics analyses showed enrichment in mitochondrial and cell cycle regulatory pathways in Hif1a deleted cells/tissue. Experimentally, mitochondrial oxidative functions and ATP content were higher with less mitochondrial free radical generation with Hif1a deletion. Deletion of Hif1a also resulted in higher concentrations of TCA cycle intermediates and HIF2α proteins in myotubes. Overall responses to Hif1amsd were similar in male and female mice, but changes in complex II function, maximum respiration, Sirt3 and HIF1ß protein expression and muscle fibre diameter were sex-dependent. Adaptive responses to hypoxia are mediated by stabilization of constantly synthesized HIF1α. Despite rapid degradation, the presence of HIF1α during normoxia contributes to lower mitochondrial oxidative efficiency and greater post-mitotic senescence in skeletal muscle. In vivo responses to HIF1α in skeletal muscle were differentially impacted by sex. KEY POINTS: Hypoxia-inducible factor -1α (HIF1α), a critical transcription factor, undergoes continuous synthesis and proteolysis, enabling rapid adaptive responses to hypoxia by reducing mitochondrial oxygen consumption. In mammals, skeletal muscle is the largest protein store which is determined by a balance between protein synthesis and breakdown and is sensitive to mitochondrial oxidative function. To investigate the functional consequences of transient HIF1α expression during normoxia in the basal state, myotubes and skeletal muscle from male and female mice with HIF1α knockout were studied using complementary multiomics, biochemical and metabolite assays. HIF1α knockout altered the electron transport chain, mitochondrial oxidative function, signalling molecules for protein homeostasis, and post-mitotic senescence markers, some of which were differentially impacted by sex. The cost of rapid adaptive responses mediated by HIF1α is lower mitochondrial oxidative efficiency and post-mitotic senescence during normoxia.

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.

Muscle Mitochondrial Bioenergetic Capacities Are Associated With Multimorbidity Burden in Older Adults: The Study of Muscle, Mobility and Aging.

BACKGROUND: The geroscience hypothesis posits that aging biological processes contribute to many age-related deficits, including the accumulation of multiple chronic diseases. Though only one facet of mitochondrial function, declines in muscle mitochondrial bioenergetic capacities may contribute to this increased susceptibility to multimorbidity. METHODS: The Study of Muscle, Mobility and Aging (SOMMA) assessed ex vivo muscle mitochondrial energetics in 764 older adults (mean age = 76.4, 56.5% women, and 85.9% non-Hispanic White) by high-resolution respirometry of permeabilized muscle fibers. We estimated the proportional odds ratio (POR [95% CI]) for the likelihood of greater multimorbidity (4 levels: 0 conditions, N = 332; 1 condition, N = 299; 2 conditions, N = 98; or 3+ conditions, N = 35) from an index of 11 conditions, per SD decrement in muscle mitochondrial energetic parameters. Distribution of conditions allowed for testing the associations of maximal muscle energetics with some individual conditions. RESULTS: Lower oxidative phosphorylation supported by fatty acids and/or complex I- and II-linked carbohydrates (eg, Max OXPHOSCI+CII) was associated with a greater multimorbidity index score (POR = 1.32 [1.13, 1.54]) and separately with diabetes mellitus (OR = 1.62 [1.26, 2.09]), depressive symptoms (OR = 1.45 [1.04, 2.00]) and possibly chronic kidney disease (OR = 1.57 [0.98, 2.52]) but not significantly with other conditions (eg, cardiac arrhythmia, chronic obstructive pulmonary disease). CONCLUSIONS: Lower muscle mitochondrial bioenergetic capacities were associated with a worse composite multimorbidity index score. Our results suggest that decrements in muscle mitochondrial energetics may contribute to a greater global burden of disease and are more strongly related to some conditions than others.