Skeletal Muscle Oxidative Capacity Research Articles

Phenyllactic acid modulates the gut microbiota, enhances intestinal health, and alleviates physical frailty in aging mice

Phenyllactic acid (PLA) is a natural antibiotic-like compound derived from certain foods and probiotics. PLA levels have been associated with age-related sarcopenia and provide benefits to metabolic health when derived from probiotics. However, the specific regulatory effects of PLA in aging remain largely unexplored. In this study, aging mice were administered PLA via gavage, followed by fecal 16S rRNA sequencing, measurements of targeted metabolites, glucose metabolism monitoring, and physical performance assessments. Our results indicate that PLA administration significantly altered gut microbiota composition, increased the abundance of short-chain fatty acids (SCFAs) and succinate producing microbiota, and enhanced gut integrity in aging mice. Furthermore, PLA treatment raised fasting blood glucose levels and improved physical activity. Mechanistically, PLA intake elevated the levels of circulating SCFAs and succinate, promoting glycogen metabolic homeostasis and maintaining skeletal muscle oxidative capacity. This study provides evidence that PLA modulates the gut microbiota in aging mice, supports intestinal health, promotes glucose homeostasis, and enhances physical activity.

Succinate dehydrogenase-complex II regulates skeletal muscle cellular respiration and contractility but not muscle mass in genetically induced pulmonary emphysema.

Reduced skeletal muscle mass and oxidative capacity coexist in patients with pulmonary emphysema and are independently associated with higher mortality. If reduced cellular respiration contributes to muscle atrophy in that setting remains unknown. Using a mouse with genetically induced pulmonary emphysema that recapitulates muscle dysfunction, we found that reduced activity of succinate dehydrogenase (SDH) is a hallmark of its myopathic changes. We generated an inducible, muscle-specific SDH knockout mouse that demonstrates lower mitochondrial oxygen consumption, myofiber contractility, and exercise endurance. Respirometry analyses show that in vitro complex I respiration is unaffected by loss of SDH subunit C in muscle mitochondria, which is consistent with the pulmonary emphysema animal data. SDH knockout initially causes succinate accumulation associated with a down-regulated transcriptome but modest proteome effects. Muscle mass, myofiber type composition, and overall body mass constituents remain unaltered in the transgenic mice. Thus, while SDH regulates myofiber respiration in experimental pulmonary emphysema, it does not control muscle mass or other body constituents.

Daily Physical Activity Does Not Contribute to Differences in Muscle Oxidative Capacity Between Overweight and Obesity.

The interaction between physical activity, skeletal muscle health, and adiposity has been explored in normal weight and overweight/obesity grouped together; however, the overall risks associated with being overweight are less than those observed with obesity and can be confounded by disparities in both sex and race. Thus, the present study sought to investigate the intricate interplay of daily physical activity and skeletal muscle oxidative capacity (SMOC) in overweight and obesity, while exploring how sex and race impact this dynamic relationship. One hundred and forty participants were grouped by body mass index (BMI) as overweight (n = 73; BMI >25-<30 kg/m2) or obese (n = 67; BMI ≥30 kg/m2). SMOC was assessed using near-infrared spectroscopy and daily physical activity was assessed for 7 days using accelerometry. Overweight individuals exhibited a higher (p = 0.004) SMOC and engaged in more (p = 0.007) vigorous physical activity compared to obese individuals. In addition, SMOC was lower (p = 0.005) in obese non-Hispanic Black (NHB) men compared to overweight NHB men. No relationships between physical activity and SMOC were observed. Physical activity is not associated with differences in SMOC in overweight and obesity. Obese individuals engage in less vigorous physical activity and exhibit lower SMOC compared to overweight individuals and these differences are emphasised in NHB men.

Identifying the Mechanisms of a Peripherally Limited Exercise Phenotype in Patients With Heart Failure With Preserved Ejection Fraction.

We identified peripherally limited patients using cardiopulmonary exercise testing and measured skeletal muscle oxygen transport and utilization during invasive single leg exercise testing to identify the mechanisms of the peripheral limitation. Forty-five patients with heart failure with preserved ejection fraction (70±7 years, 27 females) completed seated upright cardiopulmonary exercise testing and were defined as having a (1) peripheral limitation to exercise if cardiac output/oxygen consumption (VO2) was elevated (≥6) or 5 to 6 with a stroke volume reserve >50% (n=31) or (2) a central limitation to exercise if cardiac output/VO2 slope was ≤5 or 5 to 6 with stroke volume reserve <50% (n=14). Single leg knee extension exercise was used to quantify peak leg blood flow (Doppler ultrasound), arterial-to-venous oxygen content difference (femoral venous catheter), leg VO2, and muscle oxygen diffusive conductance. In a subset of participants (n=36), phosphocreatine recovery time was measured by magnetic resonance spectroscopy to determine skeletal muscle oxidative capacity. Peak VO2 during cardiopulmonary exercise testing was not different between groups (central: 13.9±5.7 versus peripheral: 12.0±3.1 mL/min per kg; P=0.135); however, the peripheral group had a lower peak arterial-to-venous oxygen content difference (central: 13.5±2.0 versus peripheral: 11.1±1.6 mLO2/dL blood; P<0.001). During single leg knee extension, there was no difference in peak leg VO2 (P=0.306), but the peripherally limited group had greater blood flow/VO2 ratio (P=0.024), lower arterial-to-venous oxygen content difference (central: 12.3±2.5 versus peripheral: 10.3±2.2 mLO2/dL blood; P=0.013), and lower muscle oxygen diffusive conductance (P=0.021). A difference in magnetic resonance spectroscopy-derived phosphocreatine recovery time was not detected (P=0.199). Peripherally limited patients with heart failure with preserved ejection fraction identified by cardiopulmonary exercise testing have impairments in oxygen transport and utilization at the level of the skeletal muscle quantified by invasive knee extension exercise testing, which includes an increased blood flow/V̇O2 ratio and poor muscle diffusive capacity. URL: https://www.clinicaltrials.gov; Unique identifier: NCT04068844.

Near-infrared spectroscopy (NIRS) in vivo assessment of skeletal muscle oxidative capacity: a comparison of results from short versus long exercise protocols and reproducibility in non-athletic adults.

Near-infrared spectroscopy (NIRS) provides a non-invasive, cost-effective method for assessing skeletal muscle oxidative capacity when combined with a short exercise protocol and arterial occlusions. However, the impact of different exercise protocols and reproducibility of the method in non-athletic adults have not previously been assessed. Young, non-athletic adults (YA) were invited to perform a short duration, fast frequency contraction (SF) exercise protocol and a long duration slow frequency (LS) contraction protocol, combined with NIRS measurements and arterial occlusions to assess skeletal muscle oxidative capacity. YA and older non-athletic adults (OA; >65years old) were invited to perform the SF exercise protocol twice to assess the reproducibility of this oxidative capacity measurement. We included 25 participants (14 male (56%), age range: 18-86 years) in the analyses. There was a strong positive correlation and good agreement between time constants derived following the SF and LS exercise protocols (Lin's concordance correlation coefficient: 0.69, p-value < 0.001 mean bias [LoA]: -3.2 [-31.0, 24.4] seconds. There was a strong positive correlation and good agreement between time constants derived from the SF exercise protocol in the YA & OA group (Lin's concordance correlation coefficient: 0.63, p-value < 0.001; mean bias [LoA] -6.4 [-34.0, 21.3] seconds). These data provide evidence to suggest that NIRS is a reliable in vivo method for the assessment of skeletal muscle oxidative capacity irrespective of exercise protocol duration or muscle contraction frequency. NIRS-measured oxidative capacity via the SF exercise protocol was reproducible in non-athletic adults with a wide range in age.

Lactate promotes fatty acid oxidation by the tricarboxylic acid cycle and mitochondrial respiration in muscles of obese mice.

Lower oxidative capacity in skeletal muscles (SKMs) is a prevailing cause of metabolic diseases. Exercise not only enhances the fatty acid oxidation (FAO) capacity of SKMs but also increases lactate levels. Given that lactate may contribute to tricarboxylic acid cycle (TCA) flux and impact monocarboxylate transporter 1 in the SKMs, we hypothesize that lactate can influence glucose and fatty acid (FA) metabolism. To test this hypothesis, we investigated the mechanism underlying lactate-driven FAO regulation in the SKM of mice with diet-induced obesity (DIO). Lactate was administered to DIO mice immediately after exercise for over 3 wk. We found that increased lactate levels enhanced energy expenditure mediated by fat metabolism during exercise recovery and decreased triglyceride levels in DIO mice SKMs. To determine the lactate-specific effects without exercise, we administered lactate to mice on a high-fat diet (HFD) for 8 wk. Similar to our exercise conditions, lactate increased FAO, TCA cycle activity, and mitochondrial respiration in the SKMs of HFD-fed mice. In addition, under sufficient FA conditions, lactate increased uncoupling protein-3 abundance via the NADH-NAD+ shuttle. Conversely, ATP synthase abundance decreased in the SKMs of HFD mice. Taken together, our results suggest that lactate amplifies the adaptive increase in FAO capacity mediated by the TCA cycle and mitochondrial respiration in SKMs under sufficient FA abundance.NEW & NOTEWORTHY Lactate administration post-exercise promotes triglyceride content loss in skeletal muscles (SKMs) and reduced body weight. Lactate enhances fatty acid oxidation in the SKMs of high-fat diet (HFD)-fed mice due to enhanced mitochondrial oxygen consumption. In addition, lactate restores the malate-aspartate shuttle, which is reduced by a HFD, and activates the tricarboxylic acid cycle (TCA) cycle in SKMs. Interestingly, supraphysiological lactate facilitates uncoupling protein-3 expression through NADH/NAD+, which is enhanced under high-fat levels in SKMs.

Adiposity in mares induces insulin dysregulation and mitochondrial dysfunction which can be mitigated by nutritional intervention

Obesity is a complex disease associated with augmented risk of metabolic disorder development and cellular dysfunction in various species. The goal of the present study was to investigate the impacts of obesity on the metabolic health of old mares as well as test the ability of diet supplementation with either a complex blend of nutrients designed to improve equine metabolism and gastrointestinal health or L-carnitine alone to mitigate negative effects of obesity. Mares (n = 19, 17.9 ± 3.7 years) were placed into one of three group: normal-weight (NW, n = 6), obese (OB, n = 7) or obese fed a complex diet supplement for 12 weeks (OBD, n = 6). After 12 weeks and completion of sample collections, OB mares received L-carnitine alone for an additional 6 weeks. Obesity in mares was significantly associated with insulin dysregulation, reduced muscle mitochondrial function, and decreased skeletal muscle oxidative capacity with greater ROS production when compared to NW. Obese mares fed the complex diet supplement had better insulin sensivity, greater cell lipid metabolism, and higher muscle oxidative capacity with reduced ROS production than OB. L-carnitine supplementation alone did not significantly alter insulin signaling, but improved lipid metabolism and muscle oxidative capacity with reduced ROS. In conclusion, obesity is associated with insulin dysregulation and altered skeletal muscle metabolism in older mares. However, dietary interventions are an effective strategy to improve metabolic status and skeletal muscle mitochondrial function in older mares.

Short term step-reduction induces mitochondrial impairments which are not restored after reambulation, in habitually active adults

Inactivity causes physiological impairments to skeletal muscle mitochondria and disrupts metabolic homeostasis, which has predominantly been investigated through models of severe disuse. The impacts of less-restrictive inactivity through step-reduction (SR) suggests a decline in markers of mitochondrial content without a loss of function (Edwards et al. [2021]. J Appl Physiol, 131, 1653-1662). However, the impact of varying durations of SR and reambulation (RA) on skeletal muscle oxidative capacity are poorly understood. Therefore, this study aimed to investigate the effects of 14 days of SR, followed by 14 days of RA on markers of mitochondrial content and function, and also explore correlations between these markers. A cohort of 12 habitually active adults were studied from previous research (Bowden Davies et al. [2018]. Diabetologica, 61, 1282-1294), for which ethical approval was obtained from NRES Committee North West – Liverpool Central (REC:14/NW/1147). Skeletal muscle biopsy samples from the vastus lateralis were obtained at baseline (BSL), after SR and following RA, which were analysed through immunoblotting/calorimetric assays for markers of mitochondrial content (citrate synthase (CS) activity) and function (OXPHOS, PDH and CPT1A). In vivo mitochondrial function tests using 31P magnetic resonance spectroscopy (31P-MRS) was conducted in a subset of 7 participants. Markers of mitochondrial content and function were significantly reduced following SR (15-34% lower; low to medium effect size 0.3-0.5; COX I (P = 0.009), COX III (P = 0.03), COX IV (P = 0.03), 31P-MRS (P = 0.04)). COX III (P = 0.01) and 31P-MRS (P = 0.03) were lower after RA when compared to BSL. Although not significant, other markers of mitochondrial content and enzymes involved in glucose and fatty acid oxidation were numerically lower after SR (CS activity 17% lower, PDH 23% lower, CPT1A 15% lower). The changes in several complexes of OXPHOS, PDH and CPT1A were positively correlated with CS activity (r2= 0.35 to 0.53, P &lt; 0.05) and 31P-MRS (r2 = 0.46 to 0.76, P &lt; 0.05). The results suggest that a short term reduction in physical activity causes impairments to both mitochondrial content and function which are not completely restored after RA, although further studies are needed to understand the mechanisms that underpin these changes. The findings highlight the deleterious physiological consequences of just reducing physical activity, which manifests as metabolic diseases such as type 2 diabetes in the long run. This prompts for the revision of physical activity guidelines to include the importance of minimising sedentary time.

Relationship between skeletal mitochondrial function and digital markers of free-living physical activity in older adults.

This study examined the association between in vivo skeletal mitochondrial function and digital free-living physical activity patterns-a measure that summarizes biological, phenotypic, functional, and environmental effects on mobility. Among 459 participants (mean age 68years; 55% women) in the Baltimore Longitudinal Study of Aging, mitochondrial function was quantified as skeletal muscle oxidative capacity via post-exercise phosphocreatine recovery rate (τPCr) in the vastus lateralis muscle of the left thigh, using 31P magnetic resonance spectroscopy. Accelerometry was collected using a 7-day, 24-h wrist-worn protocol and summarized into activity amount, intensity, endurance, and accumulation patterning metrics. Linear regression, two-part linear and logistic (bout analyses), and linear mixed effects models (time-of-day analyses) were used to estimate associations between τPCr and each physical activity metric. Interactions by age, sex, and gait speed were tested. After covariate adjustment, higher τPCr (or poorer mitochondrial function) was associated with lower activity counts/day (β = - 6593.7, SE = 2406.0; p = 0.006) and activity intensity (- 81.5 counts, SE = 12.9; p < 0.001). For activity intensity, the magnitude of association was greater for men and those with slower gait speed (interaction p < 0.02 for both). Conversely, τPCr was not associated with daily active minutes/day (p = 0.15), activity fragmentation (p = 0.13), or endurance at any bout length (p > 0.05 for all). Time-of-day analyses show participants with high τPCr were less active from 6:00 a.m. to 12:00 a.m. than those with low τPCr. Results indicate that poorer skeletal mitochondrial function is primarily associated with lower engagement in high intensity activities. Our findings help define the connection between laboratory-measured mitochondrial function and real-world physical activity behavior.

The influence of sex, hemoglobin mass, and skeletal muscle characteristics on cycling critical power.

Critical power (CP) represents an important threshold for exercise performance and fatiguability. We sought to determine the extent to which sex, hemoglobin mass (Hbmass), and skeletal muscle characteristics influence CP. Before CP determination (i.e., 3-5 constant work rate trials to task failure), Hbmass and skeletal muscle oxidative capacity (τ) were measured and vastus lateralis (VL) muscle biopsy samples were collected from 12 females and 12 males matched for aerobic fitness relative to fat-free mass (FFM) [means (SD); V̇o2max: 59.2 (7.7) vs. 59.5 (7.1) mL·kg·FFM-1·min-1, respectively]. Males had a significantly greater CP than females in absolute units [225 (28) vs. 170 (43) W; P = 0.001] but not relative to body mass [3.0 (0.6) vs. 2.7 (0.6) W·kg·BM-1; P = 0.267] or FFM [3.6 (0.7) vs. 3.7 (0.8) W·kg·FFM-1; P = 0.622]. Males had significantly greater W' (P ≤ 0.030) and greater Hbmass (P ≤ 0.016) than females, regardless of the normalization approach; however, there were no differences in mitochondrial protein content (P = 0.375), τ (P = 0.603), or MHC I proportionality (P = 0.574) between males and females. Whether it was expressed in absolute or relative units, CP was positively correlated with Hbmass (0.444 ≤ r ≤ 0.695; P < 0.05), mitochondrial protein content (0.413 ≤ r ≤ 0.708; P < 0.05), and MHC I proportionality (0.506 ≤ r ≤ 0.585; P < 0.05), and negatively correlated with τ when expressed in relative units only (-0.588 ≤ r ≤ -0.527; P < 0.05). Overall, CP was independent of sex, but variability in CP was related to Hbmass and skeletal muscle characteristics. The extent to which manipulations in these physiological parameters influence CP warrants further investigation to better understand the factors underpinning CP.NEW & NOTEWORTHY In males and females matched for aerobic fitness [maximal oxygen uptake normalized to fat-free mass (FFM)], absolute critical power (CP) was greater in males, but relative CP (per kilogram body mass or FFM) was similar between sexes. CP correlated with hemoglobin mass, mitochondrial protein content, myosin heavy chain type I proportion, and skeletal muscle oxidative capacity. These findings demonstrate the importance of matching sexes for aerobic fitness, but further experiments are needed to determine causality.

#1553 Effects of proteinuria on muscle oxygenation and microvascular reactivity in patients with pre-dialysis CKD: a post-hoc analysis

Abstract Background and Aims Vascular dysfunction is a hallmark of CKD. Previous studies showed an impaired microvascular reactivity in the skeletal muscles, which deteriorates in advanced CKD stages. This analysis aims to examine the impact of proteinuria on skeletal muscle oxygenation and microvascular reactivity at rest, during an occlusion-reperfusion maneuver, and during exercise in patients with pre-dialysis CKD. Method 66 patients with CKD stage 2-4 were included in this post-hoc analysis; 24-h urine samples were used for evaluation of proteinuria. Continuous measurement of muscle oxygenation [tissue saturation index (TSI%)] via near-infrared-spectroscopy at rest, during occlusion-reperfusion and during a 3-min handgrip exercise (at 35% of maximal-voluntary-contraction). Results The study groups were similar in terms of age (proteinuric vs nonproteinuric: 68.4 ± 10.6 vs 67.2 ± 10.8; p = 0.676), eGFR (41.3 ± 18.9 vs 46.3 ± 14.8; p = 0.248) and BMI (28.4 ± 4.9 vs 28.1 ± 4.8; p = 0.803). Resting muscle oxygenation did not differ between study groups (proteinuric vs nonproteinuric: 63.35 ± 4.09 vs 62.34 ± 3.21; p = 0.280). During occlusion, proteinuric CKD patients had marginally lower TSI occlusion magnitude (25.77 ± 7.87 vs 29.95 ± 10.34; p = 0.074) but no difference in occlusion slope (−0.09 ± .03 vs -.10 ± .04; p = 0.134). During reperfusion, the TSI reperfusion slopes were significantly lower in proteinuric CKD (slope to max 1.03 ± .45 vs 1.39 ± 0.69; p = 0.035 and 10-sec slope: 1.34 ± .63 vs 1.92 ± .75; p = 0.002); hyperemic response was numerically lower in proteinuric CKD (7.13 ± 4.27 vs 8.89 ± 4.68; p = 0.131). Finally, during exercise no differences were detected in the average muscle oxygenation between-group (10.76 ± 6.05 vs 10.65 ± 5.39; p = 0.943). Conclusion Although no differences in resting muscle oxygenation were detected, CKD patients with proteinuria showed more impaired skeletal muscle oxidative capacity (as suggested by lower TSI magnidute during occlusion), and miscrovascular reactivity during reperfusion.

EFFECTS OF PROTEINURIA ON MUSCLE OXYGENATION AND MICROVASCULAR REACTIVITY IN PATIENTS WITH PRE-DIALYSIS CKD: A POST-HOC ANALYSIS

Objective: Vascular dysfunction is a hallmark of CKD. Previous studies showed an impaired microvascular reactivity in the skeletal muscles, which deteriorates in advanced CKD stages. This analysis aims to examine the impact of proteinuria on skeletal muscle oxygenation and microvascular reactivity at rest, during an occlusion-reperfusion maneuver, and during exercise in patients with pre-dialysis CKD. Design and method: 66 patients with CKD stage 2-4 were included in this post-hoc analysis; 24-h urine samples were used for evaluation of proteinuria. Continuous measurement of muscle oxygenation [tissue saturation index (TSI%)] via near-infrared-spectroscopy at rest, during occlusion-reperfusion and during a 3-min handgrip exercise (at 35% of maximal-voluntary-contraction). Results: The study groups were similar in terms of age (proteinuric vs nonproteinuric: 68.4±10.6 vs 67.2±10.8; p=0.676), eGFR (41.3±18.9 vs 46.3±14.8; p=0.248) and BMI (28.4±4.9 vs 28.1±4.8; p=0.803). Resting muscle oxygenation did not differ between study groups (proteinuric vs nonproteinuric: 63,35±4,09 vs 62,34±3,21; p=0.280). During occlusion, proteinuric CKD patients had marginally lower TSI occlusion magnitude (25,77±7,87 vs 29,95±10,34; p=0.074) but no difference in occlusion slope (-0,09±.,03 vs -,10±.,04; p=0.134). During reperfusion, the TSI reperfusion slopes were significantly lower in proteinuric CKD (slope to max 1,03±.,45 vs 1,39±.0,69; p=0.035 and 10-sec slope: 1,34±.,63 vs 1,92±.,75; p=0.002); hyperemic response was numerically lower in proteinuric CKD (7,13±.4,27 vs 8,89±.4,68; p=0.131). Finally, during exercise no differences were detected in the average muscle oxygenation between-group (10,76±6,05 vs 10,65±5,39; p=0.943). Conclusions: Although no differences in resting muscle oxygenation were detected, CKD patients with proteinuria showed more impaired skeletal muscle oxidative capacity (as suggested by lower TSI magnidute during occlusion), and miscrovascular reactivity during reperfusion.

Concerted regulation of skeletal muscle metabolism and contractile properties by the orphan nuclear receptor Nr2f6.

The maintenance of skeletal muscle plasticity upon changes in the environment, nutrient supply, and exercise depends on regulatory mechanisms that couple structural and metabolic adaptations. The mechanisms that interconnect both processes at the transcriptional level remain underexplored. Nr2f6, a nuclear receptor, regulates metabolism and cell differentiation in peripheral tissues. However, its role in the skeletal muscle is still elusive. Here, we aimed to investigate the effects of Nr2f6 modulation on muscle biology in vivo and in vitro. Global RNA-seq was performed in Nr2f6 knockdown C2C12 myocytes (N=4-5). Molecular and metabolic assays and proliferation experiments were performed using stable Nr2f6 knockdown and Nr2f6 overexpression C2C12 cell lines (N=3-6). Nr2f6 content was evaluated in lipid overload models in vitro and in vivo (N=3-6). In vivo experiments included Nr2f6 overexpression in mouse tibialis anterior muscle, followed by gene array transcriptomics and molecular assays (N=4), ex vivo contractility experiments (N=5), and histological analysis (N=7). The conservation of Nr2f6 depletion effects was confirmed in primary skeletal muscle cells of humans and mice. Nr2f6 knockdown upregulated genes associated with muscle differentiation, metabolism, and contraction, while cell cycle-related genes were downregulated. In human skeletal muscle cells, Nr2f6 knockdown significantly increased the expression of myosin heavy chain genes (two-fold to three-fold) and siRNA-mediated depletion of Nr2f6 increased maximal C2C12 myocyte's lipid oxidative capacity by 75% and protected against lipid-induced cell death. Nr2f6 content decreased by 40% in lipid-overloaded myotubes and by 50% in the skeletal muscle of mice fed a high-fat diet. Nr2f6 overexpression in mice resulted in an atrophic and hypoplastic state, characterized by a significant reduction in muscle mass (15%) and myofibre content (18%), followed by an impairment (50%) in force production. These functional phenotypes were accompanied by the establishment of an inflammation-like molecular signature and a decrease in the expression of genes involved in muscle contractility and oxidative metabolism, which was associated with the repression of the uncoupling protein 3 (20%) and PGC-1α (30%) promoters activity following Nr2f6 overexpression in vitro. Additionally, Nr2f6 regulated core components of the cell division machinery, effectively decoupling muscle cell proliferation from differentiation. Our findings reveal a novel role for Nr2f6 as a molecular transducer that plays a crucial role in maintaining the balance between skeletal muscle contractile function and oxidative capacity. These results have significant implications for the development of potential therapeutic strategies for metabolic diseases and myopathies.

Plasma metabolomic markers underlying skeletal muscle mitochondrial function relationships with cognition and motor function.

Lower skeletal muscle mitochondrial function is associated with future cognitive impairment and mobility decline, but the biological underpinnings for these associations are unclear. We examined metabolomic markers underlying skeletal muscle mitochondrial function, cognition and motor function. We analysed data from 560 participants from the Baltimore Longitudinal Study of Aging (mean age: 68.4years, 56% women, 28% Black) who had data on skeletal muscle oxidative capacity (post-exercise recovery rate of phosphocreatine, kPCr) via 31P magnetic resonance spectroscopy and targeted plasma metabolomics using LASSO model. We then examined which kPCr-related markers were also associated with cognition and motor function in a larger sample (n = 918, mean age: 69.4, 55% women, 27% Black). The LASSO model revealed 24 metabolites significantly predicting kPCr, with the top 5 being asymmetric dimethylarginine, lactic acid, lysophosphatidylcholine a C18:1, indoleacetic acid and triacylglyceride (17:1_34:3), also significant in multivariable linear regression. The kPCr metabolite score was associated with cognitive or motor function, with 2.5-minute usual gait speed showing the strongest association (r = 0.182). Five lipids (lysophosphatidylcholine a C18:1, phosphatidylcholine ae C42:3, cholesteryl ester 18:1, sphingomyelin C26:0, octadecenoic acid) and 2 amino acids (leucine, cystine) were associated with both cognitive and motor function measures. Our findings add evidence to the hypothesis that mitochondrial function is implicated in the pathogenesis of cognitive and physical decline with aging and suggest that targeting specific metabolites may prevent cognitive and mobility decline through their effects on mitochondria. Future omics studies are warranted to confirm these findings and explore mechanisms underlying mitochondrial dysfunction in aging phenotypes.

Mechanisms underlying exercise intolerance in long COVID: An accumulation of multisystem dysfunction.

The pathogenesis of exercise intolerance and persistent fatigue which can follow an infection with the SARS-CoV-2 virus ("long COVID") is not fully understood. Cases were recruited from a long COVID clinic (N = 32; 44 ± 12 years; 10 (31%) men), and age-/sex-matched healthy controls (HC) (N = 19; 40 ± 13 years; 6 (32%) men) from University College London staff and students. We assessed exercise performance, lung and cardiac function, vascular health, skeletal muscle oxidative capacity, and autonomic nervous system (ANS) function. Key outcome measures for each physiological system were compared between groups using potential outcome means (95% confidence intervals) adjusted for potential confounders. Long COVID participant outcomes were compared to normative values. When compared to HC, cases exhibited reduced oxygen uptake efficiency slope (1847 (1679, 2016) vs. 2176 (1978, 2373) mL/min, p = 0.002) and anaerobic threshold (13.2 (12.2, 14.3) vs. 15.6 (14.4, 17.2) mL/kg/min, p < 0.001), and lower oxidative capacity, measured using near infrared spectroscopy (τ: 38.7 (31.9, 45.6) vs. 24.6 (19.1, 30.1) s, p = 0.001). In cases, ANS measures fell below normal limits in 39%. Long COVID is associated with reduced measures of exercise performance and skeletal muscle oxidative capacity in the absence of evidence of microvascular dysfunction, suggesting mitochondrial pathology. There was evidence of attendant ANS dysregulation in a significant proportion. These multisystem factors might contribute to impaired exercise tolerance in long COVID sufferers.

Dietary Ferulic Acid Increases Endurance Capacity by Promoting Skeletal Muscle Oxidative Phenotype, Mitochondrial Function, and Antioxidant Capacity.

Endurance capacity is essential for endurance athletes' achievement and individuals' health. Nutritional supplements are a proven way to enhance endurance capacity. Previous studies have shown that ferulic acid (FA) enhances endurance capacity, but the underlying mechanism is unclear. The study is aimed to investigate the mechanism by which FA increases endurance capacity. Forty mice are divided into control and 0.5% FA-supplemented groups, and an exhaustive swimming test demonstrates increased endurance capacity with FA supplementation. This study investigates the underlying mechanism for this effect of FA. Firstly, RT-PCR and western blot analysis find that FA increases the transformation from fast to slow muscle fiber. Additionally, adenosine triphosphate concentration, metabolic enzyme activity, and mitochondrial DNA analysis find that FA increases mitochondrial biogenesis and activates nuclear factor erythroid 2-related factor (NRF)1 signaling pathway in muscle. Besides, through antioxidant capacity analysis, this study finds that FA activates NRF2 signaling pathway and improves the antioxidant capacity in muscle. Moreover, inhibiting NRF2 eliminates FA's effect on muscle fiber transformation in C2C12 cells. Our results suggest that FA increases endurance capacity by promoting skeletal muscle oxidative phenotype, mitochondrial function, and antioxidant capacity, which may be related to the NRF1 and NRF2 signaling pathways.

Association of skeletal muscle oxidative capacity with muscle function, sarcopenia-related exercise performance, and intramuscular adipose tissue in older adults.

Muscle function and exercise performance measures, such as muscle endurance capacity, maximal strength, chair stand score, gait speed, and Timed Up and Go score, are evaluated to diagnose sarcopenia and frailty in older individuals. Furthermore, intramuscular adipose tissue (IntraMAT) content increases with age. Skeletal muscle oxidative capacity determines muscle metabolism and maintains muscle performance. This study aimed to investigate the association of skeletal muscle oxidative capacity with muscle function, exercise performance, and IntraMAT content in older individuals. Thirteen older men and women participated in this study. Skeletal muscle oxidative capacity was assessed by the recoveryspeed of muscle oxygen saturation after exercise using near-infrared spectroscopy from the medial gastrocnemius. We assessed two muscle functions, peak torque and time to task failure, and four sarcopenia-related exercise performances: handgrip strength, gait speed, 30-s chair stand, and Timed Up and Go. The IntraMAT content was measured using axial magnetic resonance imaging. The results showed a relationship between skeletal muscle oxidative capacity and gait speed but not with muscle functions and other exercise performance measures. Skeletal muscle oxidative capacity was not related to IntraMAT content. Skeletal muscle oxidative capacity, which may be indicative of the capacity of muscle energy production in the mitochondria, is related to locomotive functions but not to other functional parameters or skeletal fat infiltration.

The mediation roles of intermuscular fat and inflammation in muscle mitochondrial associations with cognition and mobility.

Mitochondrial dysfunction may contribute to brain and muscle health through inflammation or fat infiltration in the muscle, both of which are associated with cognitive function and mobility. We aimed to examine the association between skeletal muscle mitochondrial function and cognitive and mobility outcomes and tested the mediation effect of inflammation or fat infiltration. We analysed data from 596 Baltimore Longitudinal Study of Aging participants who had concurrent data on skeletal muscle oxidative capacity and cognitive and mobility measures of interest (mean age: 66.1, 55% women, 24% Black). Skeletal muscle oxidative capacity was assessed as post-exercise recovery rate (kPCr) via P31 MR spectroscopy. Fat infiltration was measured as intermuscular fat (IMF) via CT scan and was available for 541 participants. Inflammation markers [IL-6, C-reactive protein (CRP), total white blood cell (WBC), neutrophil count, erythrocyte sedimentation rate (ESR), or albumin] were available in 594 participants. We examined the association of kPCr and cognitive and mobility measures using linear regression and tested the mediation effect of IMF or inflammation using the mediation package in R. Models were adjusted for demographics and PCr depletion. kPCr and IMF were both significantly associated with specific cognitive domains (DSST, TMA-A, and pegboard dominant hand performance) and mobility (usual gait speed, HABCPPB, 400m walk time) (all P<0.05). IMF significantly mediated the relationship between kPCr and these cognitive and mobility measures (all P<0.05, proportion mediated 13.1% to 27%). Total WBC, neutrophil count, and ESR, but not IL-6 or CRP, also mediated at least one of the cognitive and mobility outcomes (all P<0.05, proportion mediated 9.4% to 15.3%). Skeletal muscle mitochondrial function is associated with cognitive performance involving psychomotor speed. Muscle fat infiltration and specific inflammation markers mediate the relationship between muscle mitochondrial function and cognitive and mobility outcomes. Future studies are needed to confirm these associations longitudinally and to understand their mechanistic underpinnings.

Contraction intensity affects NIRS-derived skeletal muscle oxidative capacity but not its relationships to mitochondrial protein content or aerobic fitness.

To further refine the near-infrared spectroscopy (NIRS)-derived measure of skeletal muscle oxidative capacity in humans, we sought to determine whether the exercise stimulus intensity affected the τ value and/or influenced the magnitude of correlations with in vitro measures of mitochondrial content and in vivo indices of exercise performance. Males (n = 12) and females (n = 12), matched for maximal aerobic fitness per fat-free mass, completed NIRS-derived skeletal muscle oxidative capacity tests for the vastus lateralis following repeated contractions at 40% (τ40) and 100% (τ100) of maximum voluntary contraction, underwent a skeletal muscle biopsy of the same muscle, and performed multiple intermittent isometric knee extension tests to task failure to establish critical torque (CT). The value of τ100 (34.4 ± 7.0 s) was greater than τ40 (24.2 ± 6.9 s, P < 0.001), but the values were correlated (r = 0.688; P < 0.001). The values of τ40 (r = -0.692, P < 0.001) and τ100 (r = -0.488, P = 0.016) correlated with myosin heavy chain I percentage and several markers of mitochondrial content, including COX II protein content in whole muscle (τ40: r = -0.547, P = 0.006; τ100: r = -0.466, P = 0.022), type I pooled fibers (τ40: r = -0.547, P = 0.006; τ100: r = -0.547, P = 0.006), and type II pooled fibers (τ40: r = -0.516, P = 0.009; τ100: r = -0.635, P = 0.001). The value of τ40 (r = -0.702, P < 0.001), but not τ100 (r = -0.378, P = 0.083) correlated with critical torque (CT); however, neither value correlated with W' (τ40: r = 0.071, P = 0.753; τ100: r = 0.054, P = 0.812). Overall, the NIRS method of assessing skeletal muscle oxidative capacity is sensitive to the intensity of skeletal muscle contraction but maintains relationships to whole body fitness, isolated limb critical intensity, and mitochondrial content regardless of intensity.NEW & NOTEWORTHY Skeletal muscle oxidative capacity measured using near-infrared spectroscopy (NIRS) was lower following high-intensity compared with low-intensity isometric knee extension contractions. At both intensities, skeletal muscle oxidative capacity was correlated with protein markers of mitochondrial content (in whole muscle and pooled type I and type II muscle fibers) and critical torque. These findings highlight the importance of standardizing contraction intensity while using the NIRS method with isometric contractions and further demonstrate its validity.

GADD45A is a mediator of mitochondrial loss, atrophy, and weakness in skeletal muscle.

Aging and many illnesses and injuries impair skeletal muscle mass and function, but the molecular mechanisms are not well understood. To better understand the mechanisms, we generated and studied transgenic mice with skeletal muscle-specific expression of growth arrest and DNA damage inducible α (GADD45A), a signaling protein whose expression in skeletal muscle rises during aging and a wide range of illnesses and injuries. We found that GADD45A induced several cellular changes that are characteristic of skeletal muscle atrophy, including a reduction in skeletal muscle mitochondria and oxidative capacity, selective atrophy of glycolytic muscle fibers, and paradoxical expression of oxidative myosin heavy chains despite mitochondrial loss. These cellular changes were at least partly mediated by MAP kinase kinase kinase 4, a protein kinase that is directly activated by GADD45A. By inducing these changes, GADD45A decreased the mass of muscles that are enriched in glycolytic fibers, and it impaired strength, specific force, and endurance exercise capacity. Furthermore, as predicted by data from mouse models, we found that GADD45A expression in skeletal muscle was associated with muscle weakness in humans. Collectively, these findings identify GADD45A as a mediator of mitochondrial loss, atrophy, and weakness in mouse skeletal muscle and a potential target for muscle weakness in humans.