Analysis of Aerobic Respiration in Intact Skeletal Muscle Tissue by Microplate-Based Respirometry

Impaired regenerative capacity contributes to skeletal muscle dysfunction in chronic obstructive pulmonary disease

Locomotor skeletal muscle dysfunction is a relevant comorbidity of chronic obstructive pulmonary disease (COPD) and is strongly associated with worse clinical outcomes including higher mortality. Over the last decades, a large body of literature helped characterize the process, defining the disruptive muscle phenotype caused by COPD that involves reduction in muscle mass, force-generation capacity, fatigue-tolerance, and regenerative potential following injury. A major limitation in the field has been the scarcity of well-calibrated animal models to conduct mechanistic research based on loss- and gain-of-function studies. This article provides an overall description of the process, the tools available to mechanistically investigate it, and the potential role of mitochondrially driven metabolic signals on the regulation muscle regeneration after injury in COPD. Finally, a description of future avenues to further expand on the area is proposed based on very recent evidence involving mitochondrial metabolic cues affecting myogenesis.

Analysis of Mitochondrial Function, Structure, and Intracellular Organization In Situ in Cardiomyocytes and Skeletal Muscles

Analysis of the function, structure, and intracellular organization of mitochondria is important for elucidating energy metabolism and intracellular energy transfer. In addition, basic and clinically oriented studies that investigate organ/tissue/cell dysfunction in various human diseases, including myopathies, cardiac/brain ischemia-reperfusion injuries, neurodegenerative diseases, cancer, and aging, require precise estimation of mitochondrial function. It should be noted that the main metabolic and functional characteristics of mitochondria obtained in situ (in permeabilized cells and tissue samples) and in vitro (in isolated organelles) are quite different, thereby compromising interpretations of experimental and clinical data. These differences are explained by the existence of the mitochondrial network, which possesses multiple interactions between the cytoplasm and other subcellular organelles. Metabolic and functional crosstalk between mitochondria and extra-mitochondrial cellular environments plays a crucial role in the regulation of mitochondrial metabolism and physiology. Therefore, it is important to analyze mitochondria in vivo or in situ without their isolation from the natural cellular environment. This review summarizes previous studies and discusses existing approaches and methods for the analysis of mitochondrial function, structure, and intracellular organization in situ.

Conserved spinal cord bioenergetics in experimental autoimmune encephalomyelitis in C57BL6 mice, measured using phosphorescence oxygen analyzer

Background

We have previously reported that spinal cord respiration (cellular mitochondrial oxygen consumption) and ATP content are conserved in the studied model of experimental autoimmune encephalomyelitis (EAE), foreseeing a recovery of the diseased rats. This exemplary lesion of multiple sclerosis is used here to measure spinal cord bioenergetics in C57BL6 mice. Our hypothesis is that, despite the well-known focal axonal mitochondrial pathology, bioenergetics of the CNS is reasonably preserved in this disease.

Methods

EAE was induced with an immunodominant myelin oligodendrocyte glycoprotein epitope in complete Freund's adjuvant, appended by injections of pertussis toxin. A low- and high-dose of the encephalitogen, administered into base of tail or hind-flank, were investigated. Control mice received only the incomplete adjuvant into tail. Oxygen measurements were based on quenching the phosphorescence of Pd(II) meso-tetra (sulfophenyl) tetrabenzoporphyrin by molecular oxygen. Cellular ATP was measured using the luciferin/luciferase system.

Results

The kinetics of spinal cord oxygen consumption was zero-order (linear with time) and inhibited by cyanide, confirming oxygen was reduced by cytochrome oxidase. The rate of respiration (in μM O₂.min⁻¹.mg⁻¹; measured on Days 13–28) in control mice was (mean ± SD) 0.086 ± 0.024 (n = 8) and in immunized mice was 0.079 ± 0.020 (n = 15, P = 0.265, Mann-Whitney test). Consistently, cellular ATP (in μmol mg⁻¹ dry pellet weight; measured on Days 13–28) in control mice was 0.068 ± 0.079 (n = 11) and in immunized mice was 0.063 ± 0.061 (n = 24, P = 0.887, Mann-Whitney U test).

Conclusions

In vitro measurements of spinal cord bioenergetics show conservation of the mitochondrial function in mice with EAE. These results suggest the previously documented reduced mitochondrial electrochemical potential in this disease is alterable, and likely reflects the adverse events of neuroinflammation.

Bioisosteric Modification of To042: Synthesis and Evaluation of Promising Use-Dependent Inhibitors of Voltage-Gated Sodium Channels

Three analogues of To042, a tocainide-related lead compound recently reported for the treatment of myotonia, were synthesized and evaluated in vitro as skeletal muscle sodium channel blockers possibly endowed with enhanced use-dependent behavior. Patch-clamp experiments on hNav1.4 expressed in HEK293 cells showed that N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine, the aryloxyalkyl bioisostere of To042, exerted a higher use-dependent block than To042 thus being able to preferentially block the channels in over-excited membranes while preserving healthy tissue function. It also showed the lowest active transport across BBB according to the results of P-glycoprotein (P-gp) interacting activity evaluation and the highest cytoprotective effect on HeLa cells. Quantum mechanical calculations and dockings gave insights on the most probable conformation of the aryloxyalkyl bioisostere of To042 in solution and the target residues involved in the binding, respectively. Both approaches indicated the conformations that might be adopted in both the unbound and bound state of the ligand. Overall, N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine exhibits an interesting toxico-pharmacological profile and deserves further investigation.

Succinate Dehydrogenase (SDH)-subunit C Regulates Muscle Oxygen Consumption and Fatigability in an Animal Model of Pulmonary Emphysema

Patients with pulmonary emphysema often develop locomotor muscle dysfunction, which is independently associated with disability and higher mortality in that population. Muscle dysfunction entails reduced force-generation capacity which partially depends on fibers' oxidative potential, yet very little mechanistic research has focused on muscle respiration in pulmonary emphysema. Using a recently established animal model of pulmonary emphysema-driven skeletal muscle dysfunction, we found downregulation of succinate dehydrogenase (SDH) subunit C in association with lower oxygen consumption and fatigue-tolerance in locomotor muscles. Reduced SDH activity has been previously observed in muscles from patients with pulmonary emphysema and we found that SDHC is required to support respiration in cultured muscle cells. Moreover, in-vivo gain of SDH function in emphysema animals muscles resulted in better oxygen consumption rate (OCR) and fatigue tolerance. These changes correlated with a larger number of relatively more oxidative type 2-A and 2X fibers, and a reduced amount of 2B fibers. Our data suggests that SDHC is a key regulator of respiration and fatigability in pulmonary emphysema-driven skeletal muscles, which could be impactful to develop strategies aimed at attenuating this comorbidity.

A method for assessing tissue respiration in anatomically defined brain regions

The survival and function of brain cells requires uninterrupted ATP synthesis. Different brain structures subserve distinct neurological functions, and therefore have different energy production/consumption requirements. Typically, mitochondrial function is assessed following their isolation from relatively large amounts of starting tissue, making it difficult to ascertain energy production/failure in small anatomical locations. In order to overcome this limitation, we have developed and optimized a method to measure mitochondrial function in brain tissue biopsy punches excised from anatomically defined brain structures, including white matter tracts. We describe the procedures for maintaining tissue viability prior to performing the biopsy punches, as well as provide guidance for optimizing punch size and the drug doses needed to assess various aspects of mitochondrial respiration. We demonstrate that our method can be used to measure mitochondrial respiration in anatomically defined subfields within the rat hippocampus. Using this method, we present experimental results which show that a mild traumatic brain injury (mTBI, often referred to as concussion) causes differential mitochondrial responses within these hippocampal subfields and the corpus callosum, novel findings that would have been difficult to obtain using traditional mitochondrial isolation methods. Our method is easy to implement and will be of interest to researchers working in the field of brain bioenergetics and brain diseases.

Involvement of ammonia metabolism in the improvement of endurance performance by tea catechins in mice

Blood ammonia increases during exercise, and it has been suggested that this increase is both a central and peripheral fatigue factor. Although green tea catechins (GTCs) are known to improve exercise endurance by enhancing lipid metabolism in skeletal muscle, little is known about the relationship between ammonia metabolism and the endurance-improving effect of GTCs. Here, we examined how ammonia affects endurance capacity and how GTCs affect ammonia metabolism in vivo in mice and how GTCs affect mouse skeletal muscle and liver in vitro. In mice, blood ammonia concentration was significantly negatively correlated with exercise endurance capacity, and hyperammonaemia was found to decrease whole-body fat expenditure and fatty acid oxidation–related gene expression in skeletal muscle. Repeated ingestion of GTCs combined with regular exercise training improved endurance capacity and the expression of urea cycle–related genes in liver. In C2C12 myotubes, hyperammonaemia suppressed mitochondrial respiration; however, pre-incubation with GTCs rescued this suppression. Together, our results demonstrate that hyperammonaemia decreases both mitochondrial respiration in myotubes and whole-body aerobic metabolism. Thus, GTC-mediated increases in ammonia metabolism in liver and resistance to ammonia-induced suppression of mitochondrial respiration in skeletal muscle may underlie the endurance-improving effect of GTCs.

IL-13-driven pulmonary emphysema leads to skeletal muscle dysfunction attenuated by endurance exercise

Patients with chronic obstructive pulmonary disease (COPD) usually develop skeletal muscle dysfunction, which represents a major comorbidity in these patients and is strongly associated with mortality and other poor outcomes. Although clinical data indicates that accelerated protein degradation and metabolic disruption are common associations of muscle dysfunction in COPD, there is very limited data on the mechanisms regulating the process, in part, due to the lack of research performed on a validated animal model of pulmonary emphysema. This model deficiency complicates the translational value of data generated with highly reductionist settings. Here, we use an established transgenic animal model of COPD based on inducible IL-13-driven pulmonary emphysema (IL-13TG) to interrogate the mechanisms of skeletal muscle dysfunction. Skeletal muscles from these emphysematous mice develop most features present in COPD patients, including atrophy, decreased oxygen consumption, and reduced force-generation capacity. Analysis of muscle proteome indicates downregulation of succinate dehydrogenase C (SDH-C), which correlates with reduced enzymatic activity, also consistent with previous clinical observations. Ontology terms identified with human data, such as ATP binding/bioenergetics are also downregulated in this animal's skeletal muscles. Moreover, chronic exercise can partially restore muscle mass, metabolic and force-generation capacity, and SDH activity in COPD mice. We conclude that this animal model of COPD/emphysema is an adequate platform to further investigate mechanisms of muscle dysfunction in this setting and demonstrates multiple approaches that can be used to address specific mechanisms regulating this process.NEW & NOTEWORTHY Skeletal muscle dysfunction is a relevant comorbidity in patients with chronic obstructive pulmonary disease (COPD). Mechanistic research in the area has so far been accomplished with models based on specific exposures to otherwise healthy animals, and no investigation using an established and validated animal model of COPD has been accomplished. We present an animal model of COPD that was previously shown to recapitulate pulmonary functional and histologic features present in patients with COPD, and demonstrates most of the features present in patients with pulmonary emphysema-associated muscle dysfunction, which we proposed as an adequate tool to develop mechanistic research in the area.

Pulsed glucocorticoids enhance dystrophic muscle performance through epigenetic-metabolic reprogramming

In humans, chronic glucocorticoid use is associated with side effects like muscle wasting, obesity, and metabolic syndrome. Intermittent steroid dosing has been proposed in Duchenne Muscular Dystrophy patients to mitigate the side effects seen with daily steroid intake. We evaluated biomarkers from Duchenne Muscular Dystrophy patients, finding that, compared with chronic daily steroid use, weekend steroid use was associated with reduced serum insulin, free fatty acids, and branched chain amino acids, as well as reduction in fat mass despite having similar BMIs. We reasoned that intermittent prednisone administration in dystrophic mice would alter muscle epigenomic signatures, and we identified the coordinated action of the glucocorticoid receptor, KLF15 and MEF2C as mediators of a gene expression program driving metabolic reprogramming and enhanced nutrient utilization. Muscle lacking Klf15 failed to respond to intermittent steroids. Furthermore, coadministration of the histone acetyltransferase inhibitor anacardic acid with steroids in mdx mice eliminated steroid-specific epigenetic marks and abrogated the steroid response. Together, these findings indicate that intermittent, repeated exposure to glucocorticoids promotes performance in dystrophic muscle through an epigenetic program that enhances nutrient utilization.

Analysis of mitochondrial respiratory function in tissue biopsies and blood cells

PURPOSE OF REVIEW

The review provides an overview on latest methodological strategies to assess mitochondrial respiratory function in tissue biopsies or blood cells. In addition, it summarizes the recent literature related to this topic.

RECENT FINDINGS

Today, the study of mitochondrial function in key metabolic active tissues has been become more relevant, with increasing focus in clinical applications. In addition, assessment of mitochondrial function in blood cells by respirometry might be a sensitive biomarker of disease progression. High-Resolution Respirometry provides a modern tool to study mitochondrial respiratory physiology which allows direct measurement of cellular metabolic function during health and disease. Moreover, standard operating procedures are required regarding instrumental settings, sample collection and preparation, protocol design and respirometric data analysis of mitochondrial respiratory function in tissue biopsies (such as skeletal muscle, liver and adipose tissue), as well as isolated blood cells.

SUMMARY

Mitochondrial function is a key factor in many metabolic diseases. Although various analytical approaches are available, certain well-established protocols for isolated mitochondria are limited for the analysis of mitochondrial function in tissue biopsies or blood cells. Thus, cautious considerations in selecting appropriate protocols and analytical endpoints are crucial for the interpretation of the gained data and to draw robust conclusions.

Regulation of Insulin Receptor Pathway and Glucose Metabolism by CD36 Signaling

During reduced energy intake, skeletal muscle maintains homeostasis by rapidly suppressing insulin-stimulated glucose utilization. Loss of this adaptation is observed with deficiency of the fatty acid transporter CD36. A similar loss is also characteristic of the insulin-resistant state where CD36 is dysfunctional. To elucidate what links CD36 to muscle glucose utilization, we examined whether CD36 signaling might influence insulin action. First, we show that CD36 deletion specific to skeletal muscle reduces expression of insulin signaling and glucose metabolism genes. It decreases muscle ceramides but impairs glucose disposal during a meal. Second, depletion of CD36 suppresses insulin signaling in primary-derived human myotubes, and the mechanism is shown to involve functional CD36 interaction with the insulin receptor (IR). CD36 promotes tyrosine phosphorylation of IR by the Fyn kinase and enhances IR recruitment of P85 and downstream signaling. Third, pretreatment for 15 min with saturated fatty acids suppresses CD36-Fyn enhancement of IR phosphorylation, whereas unsaturated fatty acids are neutral or stimulatory. These findings define mechanisms important for muscle glucose metabolism and optimal insulin responsiveness. Potential human relevance is suggested by genome-wide analysis and RNA sequencing data that associate genetically determined low muscle CD36 expression to incidence of type 2 diabetes.

miR-133a function in the pathogenesis of dedifferentiated liposarcoma

Background

Sarcomas are malignant heterogeneous tumors of mesenchymal derivation. Dedifferentiated liposarcoma (DDLPS) is aggressive with recurrence in 80% and metastasis in 20% of patients. We previously found that miR-133a was significantly underexpressed in liposarcoma tissues. As this miRNA has recently been shown to be a tumor suppressor in many cancers, the objective of this study was to characterize the biological and molecular consequences of miR-133a underexpression in DDLPS.

Methods

Real-time PCR was used to evaluate expression levels of miR-133a in human DDLPS tissue, normal fat tissue, and human DDLPS cell lines. DDLPS cells were stably transduced with miR-133a vector to assess the effects in vitro on proliferation, cell cycle, cell death, migration, and metabolism. A Seahorse Bioanalyzer system was also used to assess metabolism in vivo by measuring glycolysis and oxidative phosphorylation (OXPHOS) in subcutaneous xenograft tumors from immunocompromised mice.

Results

miR-133a expression was significantly decreased in human DDLPS tissue and cell lines. Enforced expression of miR-133a decreased cell proliferation, impacted cell cycle progression kinetics, decreased glycolysis, and increased OXPHOS. There was no significant effect on cell death or migration. Using an in vivo xenograft mouse study, we showed that tumors with increased miR-133a expression had no difference in tumor growth compared to control, but did exhibit an increase in OXPHOS metabolic respiration.

Conclusions

Based on our collective findings, we propose that in DDPLS, loss of miR-133a induces a metabolic shift due to a reduction in oxidative metabolism favoring a Warburg effect in DDLPS tumors, but this regulation on metabolism was not sufficient to affect DDPLS.

Electronic supplementary material

The online version of this article (10.1186/s12935-018-0583-2) contains supplementary material, which is available to authorized users.

Classical NF-κB Metabolically Reprograms Sarcoma Cells Through Regulation of Hexokinase 2

BACKGROUND

Metabolic reprogramming has emerged as a cancer hallmark, and one of the well-known cancer-associated metabolic alterations is the increase in the rate of glycolysis. Recent reports have shown that both the classical and alternative signaling pathways of nuclear factor κB (NF-κB) play important roles in controlling the metabolic profiles of normal cells and cancer cells. However, how these signaling pathways affect the metabolism of sarcomas, specifically rhabdomyosarcoma (RMS) and osteosarcoma (OS), has not been characterized.

METHODS

Classical NF-κB activity was inhibited through overexpression of the IκBα super repressor of NF-κB in RMS and OS cells. Global gene expression analysis was performed using Affymetrix GeneChip Human Transcriptome Array 2.0, and data were interpreted using gene set enrichment analysis. Seahorse Bioscience XFe24 was used to analyze oxygen consumption rate as a measure of aerobic respiration.

RESULTS

Inhibition of classical NF-κB activity in sarcoma cell lines restored alternative signaling as well as an increased oxidative respiratory metabolic phenotype in vitro. In addition, microarray analysis indicated that inhibition of NF-κB in sarcoma cells reduced glycolysis. We showed that a glycolytic gene, hexokinase (HK) 2, is a direct NF-κB transcriptional target. Knockdown of HK2 shifted the metabolic profile in sarcoma cells away from aerobic glycolysis, and re-expression of HK2 rescued the metabolic shift induced by inhibition of NF-κB activity in OS cells.

CONCLUSION

These findings suggest that classical signaling of NF-κB plays a crucial role in the metabolic profile of pediatric sarcomas potentially through the regulation of HK2.

Gestational differences in murine placenta: Glycolytic metabolism and pregnancy parameters

The placenta is a complex and essential organ composed largely of fetal-derived cells, including several different trophoblast subtypes that work in unison to support nutrient transport to the fetus during pregnancy. Abnormal placental development can lead to pregnancy-associated disorders that often involve metabolic dysfunction. The scope of dysregulated metabolism during placental development may not be fully representative of the in vivo state in defined culture systems, such as cell lines or isolated primary cells. Thus, assessing metabolic function in intact placental tissue would provide a better assessment of placental metabolism. In this study, we describe a methodology for assaying glycolytic function in structurally-intact mouse placental tissue, ex vivo, without culturing or tissue dissociation, that more closely resembles the in vivo state. Additionally, we present data highlighting sex-dependent differences of two mouse strains (C57BL/6 and ICR) in the pre-hypertrophic (E14.5) and hypertrophic (E18.5) placenta. These data establish a foundation for investigation of metabolism throughout gestation and provides a comprehensive assessment of glycolytic function during placental development.