Properties of porcine white adipose tissue and liver mitochondrial subpopulations

The Role of Mitochondrial Adaptation and Metabolic Flexibility in the Pathophysiology of Obesity and Insulin Resistance: an Updated Overview

PURPOSE OF REVIEW

The term "metabolic flexibility" denotes the dynamic responses of the cellular oxidative machinery in order to adapt to changes in energy substrate availability. A progressive loss of this adaptive capacity has been implicated in the development of obesity-related comorbidities. Mitochondria are dynamic intracellular organelles which play a fundamental role in energy metabolism, and the mitochondrial adaptation to environmental challenges may be viewed as the functional component of metabolic flexibility. Herein, we attempt to comprehensively review the available evidence regarding the role of mitochondrial adaptation and metabolic flexibility in the pathogenesis of obesity and related morbidities, namely insulin resistance states and non-alcoholic fatty liver disease (NAFLD).

RECENT FINDINGS

Overall, there is a concrete body of evidence to support the presence of impaired mitochondrial adaptation as a principal component of systemic metabolic inflexibility in conditions related to obesity. There are still many unresolved questions regarding the relationship between the gradual loss of mitochondrial adaptability and the progression of obesity-related complications, such as causality issues, the timely appearance and reversibility of the described disturbances, and the generalizability of the findings to the mitochondrial content of every affected tissue or organ. The evidence regarding the causality between the observed associations remains inconclusive, although most of the available data points towards a bidirectional, potentially mutually amplifying relationship. The spectrum of NAFLD is of particular interest, since functional and pathological changes in the course of its development closely mirror the progression of dysmetabolism, if not constituting a dynamic component of the latter.

Systems Analyses Reveal Physiological Roles and Genetic Regulators of Liver Lipid Species

The genetics of individual lipid species and their relevance in disease is largely unresolved. We profiled a subset of storage, signaling, membrane, and mitochondrial liver lipids across 385 mice from 47 strains of the BXD mouse population fed chow or high-fat diet and integrated these data with complementary multi-omics datasets. We identified several lipid species and lipid clusters with specific phenotypic and molecular signatures and, in particular, cardiolipin species with signatures of healthy and fatty liver. Genetic analyses revealed quantitative trait loci for 68% of the lipids (lQTL). By multi-layered omics analyses, we show the reliability of lQTLs to uncover candidate genes that can regulate the levels of lipid species. Additionally, we identified lQTLs that mapped to genes associated with abnormal lipid metabolism in human GWASs. This work provides a foundation and resource for understanding the genetic regulation and physiological significance of lipid species.

Calorie restriction influences key metabolic enzyme activities and markers of oxidative damage in distinct mouse liver mitochondrial sub-populations

AIMS

The purpose of the study was to establish if enzyme activities from key metabolic pathways and levels of markers of oxidative damage to proteins and lipids differed between distinct liver mitochondrial sub-populations, and which specific sub-populations contributed to these differences.

MAIN METHODS

Male C57BL/6J mice were fed non-purified diet for one month then separated into two groups, control and calorie-restricted (CR). The two groups were fed semi-purified diet (AIN93G), with the CR group receiving 40% less calories than controls. After two months, enzyme activities and markers of oxidative damage in mitochondria were determined.

KEY FINDINGS

In all mitochondrial sub-populations, enzyme activities and markers of oxidative damage, from control and CR groups, showed a pattern of M1>M3>M10. Higher acyl-CoA dehydrogenase (β-oxidation) and β-hydroxybutyrate dehydrogenase (ketogenesis) activities and lower carbonyl and TBARS levels were observed in M1 and M3 fractions from CR mice. ETC enzyme activities did not show a consistent pattern. In the Krebs cycle, citrate synthase and aconitase activities decreased while succinate dehydrogenase and malate dehydrogenase activities increased in the M1 mitochondria from the CR versus control mice.

SIGNIFICANCE

CR does not produce uniform changes in enzyme activities or markers of oxidative damage in mitochondrial sub-populations, with changes occurring primarily in the heavy mitochondrial populations. Centrifugation at 10,000 g to isolate mitochondria likely dilutes the mitochondrial populations which show the greatest response to CR. Use of lower centrifugal force (3000 g or lower) may be beneficial for some studies.

Skeletal muscle oxidative metabolism in an animal model of pulmonary emphysema: formoterol and skeletal muscle dysfunction

Skeletal muscle dysfunction is a significant contributor to exercise limitation in pulmonary emphysema. This study investigated skeletal muscle oxidative metabolism before and after aerosol exposure to a long-acting β-agonist (LABA), such as formoterol, in the pallid mouse (B6.Cg-Pldnpa/J), which has a deficiency in serum α(1)-antitrypsin (α(1)-PI) and develops spontaneous pulmonary emphysema. C57 BL/6J and its congener pallid mice of 8-12 and 16 months of age were treated with vehicle or formoterol aerosol challenge for 120 seconds. Morphological and morphometric studies and evaluations of mitochondrial adenosine diphosphate-stimulated respiration and of cytochrome oxidase activity on skeletal muscle were performed. Moreover, the mtDNA content in skeletal muscle and the mediators linked to muscle mitochondrial function and biogenesis, as well as TNF-α and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), were also evaluated. The lungs of pallid mice at 12 and 16 months of age showed patchy areas of airspace enlargements, with the destruction of alveolar septa. No significant differences were observed in basal values of mitochondrial skeletal muscle oxidative processes between C57 BL/6J and pallid mice. Exposure to LABA significantly improved mitochondrial skeletal muscle oxidative processes in emphysematous mice, where the mtDNA content was significantly higher with respect to 8-month-old pallid mice. This effect was compared with a significant increase of PGC-1α in skeletal muscles of 16-month-old pallid mice, with no significant changes in TNF-α concentrations. In conclusion, in emphysematous mice that showed an increased mtDNA content, exposure to inhaled LABA can improve mitochondrial skeletal muscle oxidative processes. PGC-1α may serve as a possible mediator of this effect.

Caloric restriction influences hydrogen peroxide generation in mitochondrial sub-populations from mouse liver

Calorie restriction (CR) has been shown to decrease H(2)O(2) production in liver mitochondria, although it is not known if this is due to uniform changes in all mitochondria or changes in particular mitochondrial sub-populations. To address this issue, liver mitochondria from control and CR mice were fractionated using differential centrifugation at 1,000 g, 3,000 g and 10,000 g into distinct populations labeled as M1, M3 and M10, respectively. Mitochondrial protein levels, respiration and H(2)O(2) production were measured in each fraction. CR resulted in a decrease in total protein (mg) in each fraction, although this difference disappeared when adjusted for liver weight (mg protein/g liver weight). No differences in respiration (State 3 or 4) were observed between control and CR mice in any of the mitochondrial fractions. CR decreased H(2)O(2) production in all fractions when mitochondria respired on succinate (Succ), succ+antimycin A (Succ+AA) or pyruvate/malate+rotenone (P/M+ROT). Thus, CR decreased reactive oxygen species (ROS) production under conditions which stimulate mitochondrial complex I ROS production under both forward (P/M+ROT) and backward (Succ & Succ+AA) electron flow. The results indicate that CR decreases H(2)O(2) production in all liver mitochondrial fractions due to a decrease in capacity for ROS production by complex I of the electron transport chain.

Age and sex-related changes in rat brain mitochondrial function

Aging is responsible for the decline in the function of mitochondria and their increase in size and number--adaptive mechanism to restore mitochondrial function. Estrogens increase mitochondrial function, especially in female rats. The aim of this study was to determine the age-related changes in rat brain mitochondrial function focusing on sex differences. Cellular and mitochondrial protein and DNA content, mitochondrial oxidative and phosphorylative function in male and female rat brain from four different age groups (6, 12, 18 and 24 months old) were analyzed. Mitochondria protein/DNA content decreased with aging shifting toward lesser mitochondrial functional capacity and the mitochondria number increased. A sex dimorphism was determined, with female rat brain showing mitochondria with greater functional capacity than males. These sex differences gradually increased during aging.

What can mitochondrial heterogeneity tell us about mitochondrial dynamics and autophagy?

A growing body of evidence shows that mitochondria are heterogeneous in terms of structure and function. Increased heterogeneity has been demonstrated in a number of disease models including ischemia-reperfusion and nutrient-induced beta cell dysfunction and diabetes. Subcellular location and proximity to other organelles, as well as uneven distribution of respiratory components have been considered as the main contributors to the basal level of heterogeneity. Recent studies point to mitochondrial dynamics and autophagy as major regulators of mitochondrial heterogeneity. While mitochondrial fusion mixes the content of the mitochondrial network, fission dissects the mitochondrial network and generates depolarized segments. These depolarized mitochondria are segregated from the networking population, forming a pre-autophagic pool contributing to heterogeneity. The capacity of a network to yield a depolarized daughter mitochondrion by a fission event is fundamental to the generation of heterogeneity. Several studies and data presented here provide a potential explanation, suggesting that protein and membranous structures are unevenly distributed within the individual mitochondrion and that inner membrane components do not mix during a fusion event to the same extent as the matrix components do. In conclusion, mitochondrial subcellular heterogeneity is a reflection of the mitochondrial lifecycle that involves frequent fusion events in which components may be unevenly mixed and followed by fission events generating disparate daughter mitochondria, some of which may fuse again, others will remain solitary and join a pre-autophagic pool.

Sex-dependent differences in aged rat brain mitochondrial function and oxidative stress

Females show lower incidences of several neurodegenerative diseases related to oxidative stress and mitochondrial dysfunction than males. In addition, female rats show more differentiated mitochondria than males in several tissues. The aim of this work was to investigate the existence of sex-dependent differences in brain mitochondrial bioenergetics and oxidative balance in aged rats. Results showed that aged female rat brain had a lower mitochondria content than aged male brain but with a greater differentiation degree given the higher mitochondrial protein content and mitochondrial complex activities in females. Female rat brain also showed a better oxidative balance than that of males, reflected by the fact that higher mitochondrial respiratory chain function is accompanied by a similar ROS production and greater antioxidant enzyme activities, which could be responsible for the lesser oxidative damage observed in proteins and lipids in this sex. Interestingly, levels of UCP4 and UCP5--proteins related to a decrease in ROS production--were also higher in females. In conclusion, aged female rat brain had more differentiated mitochondria than male brain and showed a better control of oxidative stress balance, which could be due, in part, to the neuroprotective effect of UCPs.

Sexual dimorphism in liver mitochondrial oxidative capacity is conserved under caloric restriction conditions

Caloric restriction (CR) without malnutrition has been shown to increase maximal life span and delay the rate of aging in a wide range of species. It has been proposed that reduction in energy expenditure and oxidative damage may explain the life-extending effect of CR. Sex-related differences also have been shown to influence longevity and energy expenditure in many mammalian species. The aim of the present study was to determine the sex-related differences in rat liver mitochondrial machinery, bioenergetics, and oxidative balance in response to short-term CR. Mitochondria were isolated from 6-mo-old male and female Wistar rats fed ad libitum or subjected to 40% CR for 3 mo. Mitochondrial O2 consumption, activities of the oxidative phosphorylation system (complexes I, III, IV, and V), antioxidative activities [MnSOD, glutathione peroxidase (GPx)], mitochondrial DNA and protein content, mitochondrial H2O2 production, and markers of oxidative damage, as well as cytochrome C oxidase and mitochondrial transcription factor A levels, were measured. Female rats showed a higher oxidative capacity and GPx activity than males. This sexual dimorphism was not modified by CR. Restricted rats showed slightly increased oxygen consumption, complex III activity, and GPx antioxidant activity together with lower levels of oxidative damage. In conclusion, the sexual dimorphism in liver mitochondrial oxidative capacity was unaffected by CR, with females showing higher mitochondrial functionality and ROS protection than males.

Adipocyte- and heart-type fatty acid binding proteins are both expressed in subcutaneous and intramuscular porcine ( Sus scrofa ) adipocytes

Adipocyte- (A) and heart- (H) type fatty acid binding proteins (FABP) contribute to efficient fat storage and utilization, respectively. To understand regional-differences in lipid metabolism between tissues, A- and H-FABP transcript and protein levels were studied in adipocytes isolated from subcutaneous adipose tissue or skeletal muscle in growing pigs (Sus scrofa). Interestingly, H-FABP was expressed in adipocytes isolated from both sites. We also showed that A-FABP and H-FABP were expressed at a lower level in intramuscular adipocytes than in subcutaneous adipocytes. A discrepancy was observed between age-related changes in A-FABP content in isolated adipocytes and cell diameter or lipid content variations in tissues during growth.

Mitochondrial differentiation and oxidative phosphorylation system capacity in rat embryo during placentation period

Mitochondrial biogenesis and function are essential for proper embryo development; however, these processes have not been further studied during the placentation period, when important oxidative metabolism activation is taking place. Thus, the aim of the present study was to investigate the oxidative phosphorylation system (OXPHOS) enzymatic activities as well as the expression of genes involved in the coordinated regulation of both mitochondrial and nuclear genomes (peroxisome proliferator-activated receptor-γ coactivator-1α, nuclear respiratory factors 1 and 2, mitochondrial single-strand DNA-binding protein, mitochondrial transcription factor A), and mitochondrial function (cytochromecoxidase subunit IV, cytochromecoxidase subunit I and β-ATP phosphohydrolase) in rat embryo throughout the placentation period (gestational days 11, 12 and 13). Our results reflect that embryo mitochondria were enhancing their OXPHOS potential capacities, pointing out that embryo mitochondria become more differentiated during the placentation period. Besides, the current findings show that the mRNAs of the nuclear genes involved in mitochondrial biogenesis were downregulated, whereas their protein content together with the mitochondrial DNA expression were upregulated throughout the period studied. These data indicate that the molecular regulation of the mitochondrial differentiation process during placentation involves a post-transcriptional activation of the nuclear-encoded genes that would lead to an increase in both the nuclear- and mitochondrial-encoded proteins responsible for the mitochondrial biogenic process. As a result, embryo mitochondria would reach a more differentiated stage with a more efficient oxidative metabolism that would facilitate the important embryo growth during the second half of the pregnancy.

Sex Differences in Brown Adipose Tissue Thermogenic Features During Caloric Restriction

Caloric restriction (CR) studies have shown that females rats conserve energy more efficiently, showing a higher resistance to weight loss and higher protection of vital organs mass than male rats. Gender-dependent inactivation of thermogenesis in brown adipose tissue (BAT) has been proposed as one of these possible energy conserving mechanisms. To study the changes underlying this inactivation in rats, a three month study with 40% CR was undertaken to unravel the effects on BAT recruitment. Under ad libitum conditions female rats had greater BAT recruitment and greater oxygen consumption than their male counterparts. Total and mitochondrial protein, as well as triglyceride and DNA content were more reduced in restricted female rats than in restricted males. Similarly, the levels of key BAT functional proteins (UCP1, LPL, HSL, TFAM) were more reduced in restricted females, whereas no changes were found in mitochondrial DNA levels (mtDNA) and OXPHOS activities in males and females. Furthermore, alpha (2A)/beta (3) adrenergic receptor ratio remained constant in male rats whereas in female rats CR increased 60%. In conclusion, our results suggest that female rats, whose BAT thermogenic activity is higher in ad libitum conditions, is depressed during CR. This inactivation involves the mitochondrial differentiation process and lipolytic system and could be due, at least in part, to the unfavourable adrenergic receptor balance for thermogenic activation.

Role of cardiolipin alterations in mitochondrial dysfunction and disease

Cardiolipin (CL) is a structurally unique dimeric phospholipid localized in the inner mitochondrial membrane where it is required for optimal mitochondrial function. In addition to its role in maintaining membrane potential and architecture, CL is known to provide essential structural and functional support to several proteins involved in mitochondrial bioenergetics. A loss of CL content, alterations in its acyl chain composition, and/or CL peroxidation have been associated with mitochondrial dysfunction in multiple tissues in a variety of pathological conditions, including ischemia, hypothyroidism, aging, and heart failure. Recently, aberrations in CL metabolism have been implicated as a primary causative factor in the cardioskeletal myopathy known as Barth syndrome, underscoring an important role of CL in human health and disease. The purpose of this review is to provide an overview of evidence that has linked changes in the CL profile to mitochondrial dysfunction in various pathological conditions. In addition, a brief overview of CL function and biosynthesis, and a discussion of methods used to examine CL in biological tissues are provided.

Mitochondrial transcription factor A (TFAM) is increased in rat embryo during placentation and associated with mitochondrial differentiation

In the current study, the mitochondrial proliferationdifferentiation process was investigated in rat embryo during the placentation process, straight after organogenesis, when there is an important oxidative metabolism activation. For this purpose, on gestational days 11, 12 and 13 we studied the mitochondrial DNA (mtDNA) content and the relative gene expression of proteins involved in mtDNA replication (mitochondrial single strand DNA binding protein (mtSSB)), mtDNA transcription (mitochondrial transcription factor A (TFAM)), as well as in mitochondrial function (cytochrome c oxidase subunit I (COXI)). The results indicated that during placentation important changes in mitochondrial proliferation-differentiation process take place in rat embryo. There is a great decrease in cellular mtDNA content and a rise in the ratio between TFAM and mtDNA accompanied by an increase in COXI gene expression. Thus, we can conclude that on gestational day 13 mitochondrial differentiation predominates over mitochondrial proliferation in embryo cells. Besides, our work reveals that in a physiological condition such as embryonic development the TFAM levels change in order to regulate the transcriptional activity of mtDNA.

Brown adipose tissue mitochondrial subpopulations show different morphological and thermogenic characteristics

Rat brown adipose tissue mitochondrial subpopulations-isolated by differential centrifugation at 1000, 3000 and 8000g, giving the heavy, medium and light mitochondria-were characterized. Thus, contamination by non-mitochondrial subcellular components, morphological features, respiratory chain and antioxidant enzyme activities, both uncoupling protein 1 and mitochondrial protein content, mitochondrial DNA levels and mitochondrial integrity were measured. Results indicate that mitochondrial fractions showed important differences in the morphological, thermogenic and antioxidant properties. All the parameters studied were always higher in heavy mitochondria, which is indicative of a greater mitochondrial differentiation state.

Gender dimorphism in rat liver mitochondrial oxidative metabolism and biogenesis

In the present study, we have investigated gender differences in rat liver mitochondrial oxidative metabolism. Total mitochondrial population (M) as well as the heavy (M1), medium (M3), and light (M8) mitochondrial fractions obtained by means of differential centrifugation steps at 1,000, 3,000, and 8,000 g, respectively, were isolated. Electron microscopic analysis was performed and mitochondrial protein content and cardiolipin levels, mitochondrial O(2) flux, ATP synthase activity, mitochondrial membrane potential, and mitochondrial transcription factor A (TFAM) protein levels were measured in each sample. Our results indicate that mitochondria from females have higher protein content and higher cardiolipin levels, greater respiratory and phosphorylative capacities, and more-energized mitochondria in respiratory state 3. Moreover, protein levels of TFAM were four times greater in females than in males. Gender differences in the aforementioned parameters were more patent in the isolated heavy M1 and M3 mitochondrial fractions. The present study demonstrates that gender-related differences in liver mitochondrial function are due mainly to a higher capacity and efficiency of substrate oxidation, likely related to greater mitochondrial machinery in females than in males, which is in accord with greater mitochondrial differentiation in females.

Gender-related differences in morphology and thermogenic capacity of brown adipose tissue mitochondrial subpopulations

To investigate the possible existence of a gender dimorphism in the morphology and functionality of brown adipose tissue (BAT) mitochondrial subpopulations, we obtained three mitochondrial fractions - heavy, medium and light - by differential centrifugation. Electron microscopic analysis was carried out and mitochondrial protein content, cytochrome c oxidase and ATP synthase activities, mitochondrial DNA content and UCP1 protein levels were measured in each mitochondrial fraction. Female rats showed a greater mitochondrial size than males, with a different distribution pattern of the subpopulations. These differences were accompanied by higher oxidative and thermogenic capacities and a higher protein content in female rat BAT. This tissue also showed a greater tendency to respiratory chain uncoupling, as well as a close coordination between the oxidative, phosphorylative and thermogenic processes. These differences were found in the heavy subpopulation but not in the light one. Our results demonstrate that female rat BAT shows a highly differentiated mitochondrial pool, with the heavy mitochondrial subpopulation as the main responsible for the greater thermogenic activity of this tissue. In addition, it seems that there is a differential regulation of the mitochondrial growth cycle between genders in BAT, which leads to enhanced thermogenic capacity in female rat mitochondria.