Mitochondrial Oxidative Capacity Research Articles

Higher myocardial mitochondrial ketone body oxidation capacity in human heart failure

Abstract Introduction/Purpose Recent studies suggest that the ketone bodies (KB) beta-hydroxybutyrate (HBA) and acetoacetate (ACA) are relevant substrates for myocardial energy metabolism in heart failure (HF). However, the amount of KB-dependent mitochondrial respiration in HF has not yet been quantified. High-resolution respirometry (HRR) is the gold-standard method for quantifying substrate-dependent mitochondrial oxidative capacity. Therefore, we aimed to evaluate the association between myocardial KB-dependent oxidative capacity and human HF using HRR. Methods The primary endpoint of this single-center prospective cohort study was KB-dependent and overall mitochondrial oxidative capacity in permeabilized myocardium. We used two different protocols for our measurements. In the first HRR protocol, conventional respirometry substrates including fatty acids, complex I- and complex II substrates were used to quantify the maximum coupled oxidative phosphorylation capacity (OXPHOS). Leak respiration was quantified using oligomycin to calculate the respiratory control ratio (RCR, state 3/state 4o), and leak control ratio (LCR, state 4o/state u). In the second protocol which was recently established by our working group, HBA and ACA were used as respirometry substrates to quantify KB-dependent respiration down to the enzyme level. The relative contribution of the KBs was obtained by comparing HBA- and ACA-derived respiration to OXPHOS. We applied both protocols to myocardial samples from individuals with advanced HF (HF group, catheter-acquired endomyocardial biopsies from non-ischemic HF patients or samples of explanted hearts collected during orthotopic heart transplantation surgery in end-stage heart failure patients) as well as previously heart transplanted individuals without current heart failure (Control group). Results Controls and HF had similar demographic characteristics, with comparable age distribution (mean±SD, 56.3±10.3 vs. 54.2±10.7 years, p=0.36), sex (68.75% vs. 66.67% male, p=0.87), or body mass index (25.0±5.8 vs. 29.0±10.6 kg/m2, p=0.08) and differed significantly in cardiac index (3.06±0.67 vs. 1.93±0.44) and ejection fraction (55.8±10.2 vs. 26.6±9.1) (both p<0.0001). The HF group exhibited lower OXPHOS and respiration for all substrate combinations tested (Fig. 1A). In contrast, KB-dependent respiration did not differ between HF and controls (Fig. 1B). The relative HBA-dependent respiration (Fig. 1C) and the relative and absolute ACA-dependent respiration (Fig. 1C and D) were higher in the HF group compared to controls. Mitochondrial uncoupling (LCR) (0.47±0.29 vs. 0.44±0.09 [AU], p=0.79) and coupling efficiency (RCR) (2.75±1.38 vs. 2.11±0.39 [AU], p=0.15) were not different between the groups. Conclusion These findings suggest that mitochondria in the failing heart can utilize KBs for OXPHOS to a greater extent and further hint towards KB metabolism as a potential therapeutic target for HF.

Mitochondrial oxidative phosphorylation capacity in skeletal muscle measured by ultrafast Z-spectroscopy (UFZ) MRI at 3T.

To investigate the feasibility of rapid CEST MRI acquisition for evaluating oxidative phosphorylation (OXPHOS) in human skeletal muscle at 3T, utilizing ultrafast Z-spectroscopy (UFZ) combined with MRI and the Polynomial and Lorentzian line-shape Fitting (PLOF) technique. UFZ MRI on muscle was evaluated with turbo spin echo (TSE) and 3D EPI readouts. Five healthy subjects performed in-magnet plantar flexion exercise (PFE) and subsequent changes of amide, PCr, and partial PCr mixed Cr (Cr+) CEST dynamic signals post-exercise were enabled by PLOF fitting. PCr/Cr CEST signal was further refined through pH correction by using the ratios between PCr/Cr and amide signals, named PCAR/CAR, respectively. UFZ MRI with TSE readout significantly reduces acquisition time, achieving a temporal resolution of <50 s for collecting high-resolution Z-spectra. Following PFE, the recovery/decay times (τ) for both PCr and Cr in the gastrocnemius muscle of the calf were notably longer when determined using PCr/Cr CEST compared to those after pH correction with amideCEST, namely = 87.1 ± 15.8 s and = 98.1 ± 20.4 s versus = 32.9 ± 19.7 s and = 43.0 ± 13.0 s, respectively. obtained via 31P MRS ( = 50.3 ± 6.2 s) closely resemble those obtained from pH-corrected PCr/Cr CEST signals. The outcomes suggest potential of UFZ MRI as a robust tool for non-invasive assessment of mitochondrial function in skeletal muscles. pH correction is critical for the reliable OXPHOS measurement by CEST.

Loss of atrial natriuretic peptide signaling causes insulin resistance, mitochondrial dysfunction, and low endurance capacity.

Type 2 diabetes (T2D) and obesity are strongly associated with low natriuretic peptide (NP) plasma levels and a down-regulation of NP guanylyl cyclase receptor-A (GCA) in skeletal muscle and adipose tissue. However, no study has so far provided evidence for a causal link between atrial NP (ANP)/GCA deficiency and T2D pathogenesis. Here, we show that both systemic and skeletal muscle ANP/GCA deficiencies in mice promote metabolic disturbances and prediabetes. Skeletal muscle insulin resistance is further associated with altered mitochondrial function and impaired endurance running capacity. ANP/GCA-deficient mice exhibit increased proton leak and reduced content of mitochondrial oxidative phosphorylation proteins. We further show that GCA is related to several metabolic traits in T2D and positively correlates with markers of oxidative capacity in human skeletal muscle. Together, these results indicate that ANP/GCA signaling controls muscle mitochondrial integrity and oxidative capacity in vivo and plays a causal role in the development of prediabetes.

Brain Abnormalities in Young Single- and Double-Heterozygote Mice for Both Nkx2-1- and Pax8-Null Mutations.

In humans and mice, Nkx2-1 and Pax8 are crucial morphogenic transcription factors defining the early development of the thyroid and specific extrathyroidal tissues. By using 3-month-old single or double heterozygotes for Nkx2-1- and Pax8-null mutations (DHTP) mice, we studied brain abnormalities under different human-like dysthyroidisms, focusing on putative alterations of specific neurotransmitter systems, expression of markers of pre- and post-synaptic function and, given the physio-pathological role mitochondria have in controlling the bioenergetic status of neurons, of mitochondrial dynamics and oxidative balance. Compared to Wt controls, DHTP mice, bearing both systemic and brain hypothyroidism, showed altered expression of synaptic markers, generic and cholinergic (corroborated by immunohistochemistry in caudate, putamen, hippocampus, and basal forebrain) and glutamatergic ones, and reduced expression of key proteins of synaptic plasticity potency and several isoforms of glutamate receptors. The brain of DHTP mice was characterized by lower levels of H2O2 and imbalanced mitochondrial dynamics. Nkx2-1 + / - mice showed dopaminergic neuron-specific alterations, morphologically, more evident in the substantia nigra of DHTP mice. Nkx2-1 + / - mice also showed enhanced mitochondrial biogenesis and oxidative capacity likely as a global response of the brain to Nkx2-1 haploinsufficiency and/or to their elevated T3 circulating levels. Reduced transcription of both tyrosine hydroxylase and dopamine transporter was observed in Pax8 + / - euthyroid mice, suggesting a dopaminergic dysfunction, albeit likely at an early stage, but consistent with the deregulated glucose homeostasis observed in such animals. Overall, new information was obtained on the impact of haploinsufficiency of Pax8 and NKx2-1 on several brain neuroanatomical, molecular, and neurochemical aspects, thus opening the way for future targeting brain dysfunctions in the management of both overt and subclinical thyroid dysfunctions.

7086 Disrupted Mitochondrial Function in Alström Syndrome- A Monogenic Model of Insulin Resistance and Obesity

Abstract Disclosure: S. Ali: None. S. Heising: None. V. Veeranna: None. A. Vincent: None. G.S. Xavier: None. T. Geberhiwot: None. Alström syndrome (ALMS) is a rare monogenic disease typified by severe insulin resistance (IR) and obesity. IR and obesity are cardinal features of metabolic syndrome. The metabolic effects of insulin at key target tissues leads to metabolic haemostasis by promoting glucose and lipid uptake and metabolism to generate energy in cells. Although insulin is a major regulator of fuel metabolism, its effects on mitochondrial ATP production in severe insulin resistance are unclear. RNASeq data from adipose tissue (AT) from patients with ALMS show downregulation of the expression of genes associated with mitochondrial respiratory chain function, pointing towards possible altered mitochondrial function in ALMS. We therefore undertook a detailed assessment of mitochondrial oxidative phosphorylation capacity (OXPHOS) in skeletal muscle (SM) in ALMS. We performed SM biopsy in 10 ALMS and 10 healthy control volunteers that were age, gender and Body Mass Index matched. The SM was subjected to high resolution respirometry, the gold standard method to quantify mitochondrial function ex vivo, using OROBOROS Oxygraph -2k (O2k) system. Overall OXPHOS was higher in control SM compared to ALMS, with maximal respiration of 62.96±19.2 and 44.24 ± 12.53 pmol/(s*mg), respectively (p&amp;lt;0.05).Reduced oxygen influx was observed in ALMS SM in comparison to controls for fatty acid ß oxidation (6.7±5.96 vs 7.87±2.7 pmol/(s*mg);p&amp;lt;0.05, at complex I (19.4 ± 8.98 vs 29.7 ±5.3 pmol/(s*mg);p&amp;lt;0.05, and at complex II (34.98 ±11.75 vs 53.52± 13.2 pmol/(s*mg) ;p&amp;lt;0.05. Mitochondrial membrane integrity was unaffected during these experiments. Therefore, our analysis unveils a consistent reduction in mitochondrial respiration across all the electron transport pathway. This noteworthy finding suggests impairment in respiratory function and the electron transport chain in mitochondria, a crucial aspect of cellular energy metabolism. Several studies have previously linked IR with mitochondrial dysfunction, where insulin-resistant rodents and humans have shown blunted mitochondrial response to ATP generation. Our results align with this body of evidence emphasizing the significance of investigating the underlying mechanism contributing to this phenomenon. These findings could pave the way to exploring potential mitochondrial-targeted therapies to restore mitochondrial function and therefore, improve insulin sensitivity to tackle IR in ALMS. Presentation: 6/2/2024

Vgll2 as an integrative regulator of mitochondrial function and contractility specific to skeletal muscle.

During skeletal muscle adaptation to physiological or pathophysiological signals, contractile apparatus and mitochondrial function are coordinated to alter muscle fiber type. Although recent studies have identified various factors involved in modifying contractile proteins and mitochondrial function, the molecular mechanisms coordinating contractile and metabolic functions during muscle fiber transition are not fully understood. Using a gene-deficient mouse approach, our previous studies uncovered that vestigial-like family member 2 (Vgll2), a skeletal muscle-specific transcription cofactor activated by exercise, is essential for fast-to-slow adaptation of skeletal muscle. The current study provides evidence that Vgll2 plays a role in increasing muscle mitochondrial mass and oxidative capacity. Transgenic Vgll2 overexpression in mice altered muscle fiber composition toward the slow type and enhanced exercise endurance, which contradicted the outcomes observed with Vgll2 deficiency. Vgll2 expression was positively correlated with the expression of genes related to mitochondrial function in skeletal muscle, mitochondrial DNA content, and protein abundance of oxidative phosphorylation complexes. Additionally, Vgll2 overexpression significantly increased the maximal respiration of isolated muscle fibers and enhanced the suppressive effects of endurance training on weight gain. Notably, no additional alteration in expression of myosin heavy chain genes was observed after exercise, suggesting that Vgll2 plays a direct role in regulating mitochondrial function, independent of its effect on contractile components. The observed increase in exercise endurance and metabolic efficiency may be attributed to the acute upregulation of genes promoting fatty acid utilization as a direct consequence of Vgll2 activation facilitated by endurance exercise. Thus, the current study establishes that Vgll2 is an integrative regulator of mitochondrial function and contractility in skeletal muscle.

Goat Milk Supplementation Modulates the Mitochondrial Metabolic Flexibility and Orexin-A Levels Influencing the Inflammatory Pattern in Rats.

Milk and its derivatives are included in a balanced diet of humans as excellent sources of proteins, vitamins, and essential minerals that are functional nutrients. Knowledge about the nutritional benefits or harms due to milk consumption has been expanding in recent years. We previously explored, in rodent models, the metabolic effects of isoenergetic intake of milk derived from cows, donkeys, or humans, while the impact of goat's milk intake has remained unexplored. The aim of this work was to investigate, in an animal model, the effects of dietary supplementation with goat's milk on energy homeostasis and inflammatory state, focusing on the modulation of mitochondrial functions in most metabolically active organs, such as skeletal muscle and the liver. In addition, we highlighted a link between nutrient intake, substrate metabolism, and the orexinergic system. Our results indicate that goat milk improves mitochondrial oxidative capacity and reduces inflammation and oxidative stress in both organs. Notably, goat milk lowers the circulating levels of Orexin-A, a neuropeptide that plays a crucial role in regulating peripheral energy balance and central nervous system mechanisms. These data provide the first evidence that the anti-inflammatory and antioxidant effects of goat milk are mediated by the modulation of mitochondrial functions and orexinergic signaling.

Decoding the decline: unveiling drivers of sarcopenia.

There remains a critical need to define molecular pathways underlying sarcopenia to identify putative therapeutic targets. Research in the mechanisms of aging and sarcopenia relies heavily on preclinical rodent models. In this issue of the JCI, Kerr et al. implemented a clinically-relevant sarcopenia classification system of aged C57BL/6J mice, capturing sarcopenia prevalence across both sexes. The authors performed detailed physiological, molecular, and energetic analyses and demonstrated that mitochondrial biogenesis, oxidative capacity, and AMPK-autophagy signaling decreased as sarcopenia progressed in male mice. Sarcopenia was less prevalent in female mice with fewer alterations compared with the male-affected processes. The findings highlight factors beyond age as necessary for classifying the sarcopenic phenotype in rodent models, reveal sexual dimorphism across the trajectory of age-related declines in muscle mass and function in a commonly used rodent model, and provide insight into sex-dependent molecular alterations associated with sarcopenia progression.

Culture of Bovine Aortic Endothelial Cells in Galactose Media Enhances Mitochondrial Plasticity and Changes Redox Sensing, Altering Nrf2 and FOXO3 Levels.

Understanding the complex biological processes of cells in culture, particularly those related to metabolism, can be biased by culture conditions, since the choice of energy substrate impacts all of the main metabolic pathways. When glucose is replaced by galactose, cells decrease their glycolytic flux, working as an in vitro model of limited nutrient availability. However, the effect of these changes on related physiological processes such as redox control is not well documented, particularly in endothelial cells, where mitochondrial oxidation is considered to be low. We evaluated the differences in mitochondrial dynamics and function in endothelial cells exposed to galactose or glucose culture medium. We observed that cells maintained in galactose-containing medium show a higher mitochondrial oxidative capacity, a more fused mitochondrial network, and higher intercellular coupling. These factors are documented to impact the cellular response to oxidative stress. Therefore, we analyzed the levels of two main redox regulators and found that bovine aortic endothelial cells (BAEC) in galactose media had higher levels of FOXO3 and lower levels of Nrf2 than those in glucose-containing media. Thus, cultures of endothelial cells in a galactose-containing medium may provide a more suitable target for the study of in vitro mitochondrial-related processes than those in glucose-containing media; the medium deeply influences redox signaling in these cells.

Exercise Hemodynamics and Mitochondrial Oxidative Capacity in Disease Stages of Wild-Type Transthyretin Amyloid Cardiomyopathy.

Wild-type transthyretin amyloid (ATTRwt) cardiomyopathy is increasingly recognized in the development of heart failure. The link between cardiac performance, hemodynamics, and mitochondrial function in disease stages of ATTRwt has not previously been studied but may provide new insights into the pathophysiology and clinical performance of the patients. The study investigated 47 patients diagnosed with ATTRwt at Aarhus University Hospital, Denmark. Patients were stratified according to the disease stages of the National Amyloidosis Centre (NAC) as NAC I with low levels of NT-proBNP (N-terminal pro-B-type natriuretic peptide) (NAC I-L, n=14), NAC I with high levels NT-proBNP (NAC I-H, n=20), and NAC II-III (n=13). Exercise testing with simultaneous right heart catheterization was performed in all patients. Endomyocardial biopsies were collected from the patients and the mitochondrial oxidative phosphorylation capacity was assessed. All NAC disease groups, even in the NAC I-L group, a significant abnormal increase in biventricular filling pressures were noted during exercise while the filling pressures was normal or near normal at rest. The inotropic response to exercise was reduced with diminished increase in cardiac output which was significantly more pronounced in the NAC I-H (Diff. -2.4, 95% CI (-4.2: -0.7), P=0.00) and the NAC II-III group (Diff: -3.1 L/min, 95% CI (-5.2: -1.1), P=0.00) compared with the NAC I-L group. The pulmonary artery wedge pressure to cardiac output ratio at peak exercise was significantly different between NAC I-L and NAC II-III (Diff: 1.6 mm Hg*min/L, 95% CI (0.01:3.3, P=0.04)). Patients with ATTRwt had a reduced oxidative phosphorylation capacity which correlated to left ventricular mass but not to cardiac output capacity. An abnormal restrictive left ventricle and right ventricle response to exercise was demonstrated, even present in patients with early-stage ATTRwt. In more advanced disease stages a progressive impairment of the pressure-flow relationship was noted. The myocyte energetics is deranged but not associated to the contractile reserve or restrictive filling characteristics in ATTRwt.

Reduced adipocyte glutaminase activity promotes energy expenditure and metabolic health

Glutamine and glutamate are interconverted by several enzymes and alterations in this metabolic cycle are linked to cardiometabolic traits. Herein, we show that obesity-associated insulin resistance is characterized by decreased plasma and white adipose tissue glutamine-to-glutamate ratios. We couple these stoichiometric changes to perturbed fat cell glutaminase and glutamine synthase messenger RNA and protein abundance, which together promote glutaminolysis. In human white adipocytes, reductions in glutaminase activity promote aerobic glycolysis and mitochondrial oxidative capacity via increases in hypoxia-inducible factor 1α abundance, lactate levels and p38 mitogen-activated protein kinase signalling. Systemic glutaminase inhibition in male and female mice, or genetically in adipocytes of male mice, triggers the activation of thermogenic gene programs in inguinal adipocytes. Consequently, the knockout mice display higher energy expenditure and improved glucose tolerance compared to control littermates, even under high-fat diet conditions. Altogether, our findings highlight white adipocyte glutamine turnover as an important determinant of energy expenditure and metabolic health.

Flies on the rise: acclimation effect on mitochondrial oxidation capacity at normal and high temperatures in Drosophila melanogaster.

Increased average temperatures and extreme thermal events (such as heatwaves) brought forth by climate change impose important constraints on aerobic metabolism. Notably, mitochondrial metabolism, which is affected by both long- and short-term temperature changes, has been put forward as an important determinant for thermal tolerance of organisms. This study examined the influence of phenotypic plasticity on metabolic and physiological parameters in Drosophila melanogaster and the link between mitochondrial function and their upper thermal limits. We showed that D. melanogaster acclimated to 15°C have a 0.65°C lower critical thermal maximum (CTmax) compared with those acclimated to 24°C. Drosophila melanogaster acclimated to 15°C exhibited a higher proportion of shorter saturated and monounsaturated fatty acids, concomitant with lower proportions of polyunsaturated fatty acids. No mitochondrial quantitative changes (fractional area and number) were detected between acclimation groups, but changes of mitochondrial oxidation capacities were observed. Specifically, in both 15°C- and 24°C-acclimated flies, complex I-induced respiration was increased when measured between 15 and 24°C, but drastically declined when measured at 40°C. When succinate and glycerol-3-phosphate were added, this decrease was however compensated for in flies acclimated to 24°C, suggesting an important impact of acclimation on mitochondrial function related to thermal tolerance. Our study reveals that the use of oxidative substrates at high temperatures is influenced by acclimation temperature and strongly related to upper thermal tolerance as a difference of 0.65°C in CTmax translates into significant mitochondrial changes.

Muscle mitochondrial function is impaired in adults with type 1 diabetes

AimsType 1 diabetes has been associated with mitochondrial dysfunction. However, the mechanism of this dysfunction in adults remains unclear. MethodsA secondary analysis was conducted using data from several clinical trials measuring in-vivo and ex-vivo mitochondrial function in adults with type 1 diabetes (n = 34, age 38.8 ± 14.6 years) and similarly aged controls (n = 59, age 44.6 ± 13.9 years). In-vivo mitochondrial function was assessed before, during, and after isometric exercise with 31phosphorous magnetic resonance spectroscopy. High resolution respirometry of vastus lateralis muscle tissue was used to assess ex-vivo measures. ResultsIn-vivo data showed higher rates of anaerobic glycolysis (p = 0.013), and a lower maximal mitochondrial oxidative capacity (p = 0.012) and mitochondrial efficiency (p = 0.024) in adults with type 1 diabetes. After adjustment for age and percent body fat maximal mitochondrial capacity (p = 0.014) continued to be lower and anaerobic glycolysis higher (p = 0.040) in adults with type 1 diabetes. Ex-vivo data did not demonstrate significant differences between the two groups. ConclusionsThe in-vivo analysis demonstrates that adults with type 1 diabetes have mitochondrial dysfunction. This builds on previous research showing in-vivo mitochondrial dysfunction in youths with type 1 diabetes and suggests that defects in substrate or oxygen delivery may play a role in in-vivo dysfunction.

Mouse sarcopenia model reveals sex- and age-specific differences in phenotypic and molecular characteristics.

Our study was to characterize sarcopenia in C57BL/6J mice using a clinically relevant definition to investigate the underlying molecular mechanisms. Aged male (23-32 months old) and female (27-28 months old) C57BL/6J mice were classified as non-, probable-, or sarcopenic based on assessments of grip strength, muscle mass, and treadmill running time, using 2 SDs below the mean of their young counterparts as cutoff points. A 9%-22% prevalence of sarcopenia was identified in 23-26 month-old male mice, with more severe age-related declines in muscle function than mass. Females aged 27-28 months showed fewer sarcopenic but more probable cases compared with the males. As sarcopenia progressed, a decrease in muscle contractility and a trend toward lower type IIB fiber size were observed in males. Mitochondrial biogenesis, oxidative capacity, and AMPK-autophagy signaling decreased as sarcopenia progressed in males, with pathways linked to mitochondrial metabolism positively correlated with muscle mass. No age- or sarcopenia-related changes were observed in mitochondrial biogenesis, OXPHOS complexes, AMPK signaling, mitophagy, or atrogenes in females. Our results highlight the different trajectories of age-related declines in muscle mass and function, providing insights into sex-dependent molecular changes associated with sarcopenia progression, which may inform the future development of novel therapeutic interventions.

In Vitro Hypoxia/Reoxygenation Induces Mitochondrial Cardiolipin Remodeling in Human Kidney Cells.

Renal ischemia/reperfusion is a serious condition that not only causes acute kidney injury, a severe clinical syndrome with high mortality, but is also an inevitable part of kidney transplantation or other kidney surgeries. Alterations of oxygen levels during ischemia/reperfusion, namely hypoxia/reoxygenation, disrupt mitochondrial metabolism and induce structural changes that lead to cell death. A signature mitochondrial phospholipid, cardiolipin, with many vital roles in mitochondrial homeostasis, is one of the key players in hypoxia/reoxygenation-induced mitochondrial damage. In this study, we analyze the effect of hypoxia/reoxygenation on human renal proximal tubule epithelial cell (RPTEC) cardiolipins, as well as their metabolism and mitochondrial functions. RPTEC cells were placed in a hypoxic chamber with a 2% oxygen atmosphere for 24 h to induce hypoxia; then, they were replaced back into regular growth conditions for 24 h of reoxygenation. Surprisingly, after 24 h, hypoxia cardiolipin levels substantially increased and remained higher than control levels after 24 h of reoxygenation. This was explained by significantly elevated levels of cardiolipin synthase and lysocardiolipin acyltransferase 1 (LCLAT1) gene expression and protein levels. Meanwhile, hypoxia/reoxygenation decreased ADP-dependent mitochondrial respiration rates and oxidative phosphorylation capacity and increased reactive oxygen species generation. Our findings suggest that hypoxia/reoxygenation induces cardiolipin remodeling in response to reduced mitochondrial oxidative phosphorylation in a way that protects mitochondrial function.

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.

Investigating the thermal sensitivity of key enzymes involved in the energetic metabolism of three insect species.

The metabolic responses of insects to high temperatures have been linked to their mitochondrial substrate oxidation capacity. However, the mechanism behind this mitochondrial flexibility is not well understood. Here, we used three insect species with different thermal tolerances (the honey bee, Apis mellifera; the fruit fly, Drosophila melanogaster; and the potato beetle, Leptinotarsa decemlineata) to characterize the thermal sensitivity of different metabolic enzymes. Specifically, we measured activity of enzymes involved in glycolysis (hexokinase, HK; pyruvate kinase, PK; and lactate dehydrogenase, LDH), pyruvate oxidation and the tricarboxylic acid cycle (pyruvate dehydrogenase, PDH; citrate synthase, CS; malate dehydrogenase, MDH; and aspartate aminotransferase, AAT), and the electron transport system (Complex I, CI; Complex II, CII; mitochondrial glycerol-3-phosphate dehydrogenase, mG3PDH; proline dehydrogenase, ProDH; and Complex IV, CIV), as well as that of ATP synthase (CV) at 18, 24, 30, 36, 42 and 45°C. Our results show that at high temperature, all three species have significantly increased activity of enzymes linked to FADH2 oxidation, specifically CII and mG3PDH. In fruit flies and honey bees, this coincides with a significant decrease of PDH and CS activity, respectively, that would limit NADH production. This is in line with the switch from NADH-linked substrates to FADH2-linked substrates previously observed with mitochondrial oxygen consumption. Thus, we demonstrate that even though the three insect species have a different metabolic regulation, a similar response to high temperature involving CII and mG3PDH is observed, denoting the importance of these proteins for thermal tolerance in insects.

Obesity-induced tissue alterations resist weight loss: A mechanistic review.

Interventions aimed at weight control often have limited effectiveness in combating obesity. This review explores how obesity-induced dysfunction in white (WAT) and brown adipose tissue (BAT), skeletal muscle, and the brain blunt weight loss, leading to retention of stored fat. In obesity, increased adrenergic stimulation and inflammation downregulate β-adrenoreceptors and impair catecholaminergic signalling in adipocytes. This disrupts adrenergic-mediated lipolysis, diminishing lipid oxidation in both white and brown adipocytes, lowering thermogenesis and blunting fat loss. Emerging evidence suggests that WAT fibrosis is associated with worse weight loss outcomes; indeed, limiting collagen and laminin-α4 deposition mitigates WAT accumulation, enhances browning, and protects against high-fat-diet-induced obesity. Obesity compromises mitochondrial oxidative capacity and lipid oxidation in skeletal muscle, impairing its ability to switch between glucose and lipid metabolism in response to varying nutrient levels and exercise. This dysfunctional phenotype in muscle is exacerbated in the presence of obesity-associated sarcopenia. Additionally, obesity suppresses sarcolipin-induced sarcoplasmic reticulum calcium ATPase (SERCA) activation, resulting in reduced oxidative capacity, diminished energy expenditure, and increased adiposity. In the hypothalamus, obesity and overnutrition impair insulin and leptin signalling. This blunts central satiety signals, favouring a shift in energy balance toward energy conservation and body fat retention. Moreover, both obese animals and humans demonstrate impaired dopaminergic signalling and diminished responses to nutrient intake in the striatum, which tend to persist after weight loss. This may result in enduring inclinations toward overeating and a sedentary lifestyle. Collectively, the tissue adaptations described pose significant challenges to effectively achieving and sustaining weight loss in obesity.

Exercise Training Mitigates Inhibition of Skeletal Muscle Mitochondrial Respiration with Empagliflozin Treatment in Male and Female Mice

Sodium-glucose cotransporter-2 inhibitors (SGLT2i) are antihyperglycemic drugs designed to lower renal glucose reabsorption, yet recent evidence indicates that SGLT2i have off-target effects on skeletal muscle to inhibit mitochondrial respiration. Such inhibition raises important questions regarding the interaction of SGLT2i and exercise on regulation of skeletal muscle mitochondrial metabolism. The purpose of the current study was to test the hypothesis that SGLT2i treatment (empagliflozin, EMPA) would blunt improvements to skeletal muscle mitochondrial oxidation capacity following aerobic training. Male and female C57BL/6J mice consumed a western diet (40% kcal from fat) for 8 weeks, then performed an additional 8 weeks of either treadmill training (5 times per week progressing up to 23 m/min) or remained sedentary. During the intervention period, all mice consumed either western diet or western diet enriched with EMPA (10mg/kg diet) in a 2x2 design (n=10/group for each sex). We isolated mitochondria from quadriceps muscles collected 72 hours after the final training session (or rest) and measured ex vivo mitochondrial substrate oxidation capacity using high-resolution respirometry. There was an interaction between exercise and EMPA (p=0.037) on complex I-II respiration whereby sedentary mice had lower respiration following EMPA versus untreated (-17% p=0.01) while exercise-trained mice had similar respiration between EMPA and untreated. The interaction was less prominent in complex I-supported respiration (p=0.06) or complex-II alone (p=0.09). Our results show that, contrary to expectations, EMPA treatment did not impair mitochondrial adaptations to exercise training. Instead, exercise training prevented the suppression of mitochondrial respiration during EMPA treatment. These findings indicate that aerobic exercise has positive effects on the off-target inhibition of mitochondrial respiration by SGLT2i. Supported by funding from the Collins Medical Trust (Wilsonville, Oregon) awarded to Matthew Robinson. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A New Genetic Mouse Model of Lean NAFLD Uncovers a Sexual Dimorphism for Hepatic Damages and Cardiometabolic Impairments

Background: The prevalence of non alcoholic fatty liver disease (NAFLD) is ~30% higher in the obese population. However, among patients living with NAFLD, 40% are not obese and half of them are lean. Although being mostly studied in obese individuals, the progression of NAFLD in lean patients may involve cardiac manifestations as well. Our hypothesis states that lean NAFLD patients also develop cardiac complications, independently of pre-existent metabolic disruptions, mechanisms of which remain to be fully understood, especially when considering the sexual dimorphism of the disease. Method: To address the underlying mechanisms of the progression of NAFLD independently of obesity, we used a murine model of hepatic mitochondrial deficiency, achieved through a hepato-specific knockout (KO) of Lrpprc. KO mice thus develop microvesicular steatosis without obesity in both sexes which was extensively characterized at the molecular and metabolic level in liver, plasma and heart. Our results unveiled a sex-dependent hepatic and cardiac phenotypic remodeling. KO male mice, to a greater extent than females, showed sustained fasting hypoglycemia and increased insulin sensitivity. Males also showed a more inflammatory hepatic profile characterized by a significant increase of F4/80 staining in immunohistological analyses (3-fold). Furthermore, only males exhibited an increase in the circulating hepatokine FGF21 (1.74-fold) and an increase in VLDLs lipoproteins while HDLs lipoproteins were decreased. In contrast, KO females showed a modest and global decrease of the lipoprotein profile. As such, sex differences occurred also in the heart since only males showed diastolic dysfunction while females’ cardiac function remained unaffected despite mitochondrial defects such as impairments in mitochondrial biogenesis, dynamics and oxidative capacities. Noteworthy, we also observed a significant and inverse correlation between glycemia and an index for cardiac relaxation only in males. Conclusion: Overall, this study underscores the value of this model for NAFLD without obesity where sex-dependent damage in the liver, the plasma and the heart are highlighted. Canadian Institutes of Health Research (CIHR), Montreal Heart Institute Foundation, Heart and Stroke Foundation of Canada, Cardiometabolic Health, Diabetes and Obesity Research Network. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.