Regulation of skeletal muscle mitochondrial content during aging

Dietary Supplementation with Lauric Acid Improves Aerobic Endurance in Sedentary Mice via Enhancing Fat Mobilization and Glyconeogenesis

BACKGROUND

Lauric acid (LA), a major, natural, medium-chain fatty acid, is considered an efficient energy substrate for intense exercise and in patients with long-chain fatty acid β-oxidation disorders. However, few studies have focused on the role of LA in exercise performance and related glucolipid metabolism in vivo.

OBJECTIVES

We aimed to investigate the effect of dietary supplementation with LA on exercise performance and related metabolic mechanisms.

METHODS

Male C57BL/6N mice (14 wk old) were fed a basal diet or a diet containing 1% LA, and a series of exercise tests, including a high-speed treadmill test, aerobic endurance exercises, a 4-limb hanging test, and acute aerobic exercises, were performed.

RESULTS

Dietary supplementation with 1.0% LA accelerated the recovery from fatigue after explosive exercise (P < 0.05) and improved aerobic endurance and muscle strength in sedentary mice (P = 0.039). Lauric acid intake not only changed muscle fatty acid profiles, including increases in C12:0 and n-6/n-3 PUFAs (P < 0.001) and reductions in C18:0, C20:4n-6, C22:6n-3, and n-3 PUFAs (P < 0.05) but also enhanced fat mobilization from adipose tissue and fatty acid oxidation in the liver, at least partly via the AMP-activated protein kinase-acetyl CoA carboxylase pathway (P < 0.05). Likewise, LA supplementation promoted liver glyconeogenesis and conserved muscular glycogen during acute aerobic exercise (P < 0.05), which was accompanied by an increase in the mitochondrial DNA copy number and Krebs cycle activity in skeletal muscle (P < 0.05).

CONCLUSIONS

Dietary supplemental LA serves as an efficient energy substrate for sedentary mice to improve aerobic exercise endurance and muscle strength through regulation of glucolipid metabolism. These findings imply that LA supplementation might be a promising nutritional strategy to improve aerobic exercise performance in sedentary people.

Octanoic Acid-Enrichment Diet Improves Endurance Capacity and Reprograms Mitochondrial Biogenesis in Skeletal Muscle of Mice

Background: Medium Chain Fatty Acids (MCFAs) are a dietary supplement that exhibit interesting properties, due to their smaller molecular size. The acute consumption of MCFAs is expected to enhance exercise performance. However, the short-term effects of MCFAs on endurance performance remains poorly understood. The aim of our study is to evaluate the octanoic acid (C8)-rich diet effect on endurance capacity, and to explore their molecular and cellular effects. Methods: C57BL/6J mice were fed with a chow diet (Control group) or an octanoic acid-rich diet (C8 diet) for 6 weeks. Spontaneous activity, submaximal and maximal exercise tests were carried out to characterize the exercise capacities of the mice. Beta-oxidation and mitochondrial biogenesis pathways were explored in skeletal muscle by RT-qPCR, Western Blot (Quadriceps) and histochemical staining (Gastrocnemius). Results: Mice fed with a C8-rich diet presented a higher spontaneous activity (p < 0.05) and endurance capacities (p < 0.05) than the control, but no effect on maximal effort was observed. They also presented changes in the skeletal muscle metabolic phenotype, with a higher number of the oxidative fibers, rich in mitochondria. At the molecular level, the C8-diet induced an AMPK activation (p < 0.05), associated with a significant increase in PGC1a and CS gene expression and protein levels. Conclusion: Our study provided evidence that C8-enrichment as a food supplementation improves endurance capacities and activates mitochondrial biogenesis pathways leading to higher skeletal muscle oxidative capacities.

Lotic Environment Affects Morphological Characteristics and Energy Metabolism of Juvenile Grass Carp Ctenopharyngodon idella

This study investigated the effect of a lotic environment on morphological characteristics and energy metabolism in juvenile grass carp Ctenopharyngodon idella. The fish were stocked in the lotic environment and forced to swim for 12 h per day for 4 weeks at three water current velocities of 0.5, 2, and 4 body length s−1 (Bl s−1). The control fish were stocked in the lentic environment with water current velocities of 0 Bl s−1. The results showed that lotic environment significantly increased body weight, body length, and condition factor of grass carp. The first principal component (PC1) characterized by measured overall body size suggested that fish in a lotic environment had body stoutness and wider tail stalk. Standard metabolic rate (SMR), maximum metabolic rate (MMR), and aerobic swimming performance (Ucrit) were elevated with the increased water flow and positively correlated with PC1. The 4 Bl s−1 group showed significantly decreased contents of serum glucose and muscular glycogen, and a significantly increased level of serum lactic acid. The mRNA expression levels of AMP-activated protein kinase-phosphorylate PPAR γ coactivator 1 α-nuclear respiratory factor 1 (AMPK-PGC1α-NRF1) pathway-related genes were significantly upregulated in red muscle of grass carp in the lotic environment. Water flow environment at 4 Bl s−1 significantly increased ratios of metabolic enzymes (lactate dehydrogenase/citrate synthase) and cytochrome c oxidase/citrate synthase) in the muscle. The relationship between morphological characteristics and metabolic capacity suggested that the body size of grass carp in a lotic environment was shaped to promote energy metabolism. The study identified the evidence of the mechanism and relationship of the trade-off between energy and morphology in grass carp.

Myostatin Inhibition-Induced Increase in Muscle Mass and Strength Was Amplified by Resistance Exercise Training, and Dietary Essential Amino Acids Improved Muscle Quality in Mice

It has been frequently reported that myostatin inhibition increases muscle mass, but decreases muscle quality (i.e., strength/muscle mass). Resistance exercise training (RT) and essential amino acids (EAAs) are potent anabolic stimuli that synergistically increase muscle mass through changes in muscle protein turnover. In addition, EAAs are known to stimulate mitochondrial biogenesis. We have investigated if RT amplifies the anabolic potential of myostatin inhibition while EAAs enhance muscle quality through stimulations of mitochondrial biogenesis and/or muscle protein turnover. Mice were assigned into ACV (myostatin inhibitor), ACV+EAA, ACV+RT, ACV+EAA +RT, or control (CON) over 4 weeks. RT, but not EAA, increased muscle mass above ACV. Despite differences in muscle mass gain, myofibrillar protein synthesis was stimulated similarly in all vs. CON, suggesting a role for changes in protein breakdown in muscle mass gains. There were increases in MyoD expression but decreases in Atrogin-1/MAFbx expression in ACV+EAA, ACV+RT, and ACV+EAA+RT vs. CON. EAA increased muscle quality (e.g., grip strength and maximal carrying load) without corresponding changes in markers of mitochondrial biogenesis and neuromuscular junction stability. In conclusion, RT amplifies muscle mass and strength through changes in muscle protein turnover in conjunction with changes in implicated signaling, while EAAs enhance muscle quality through unknown mechanisms.

A ketogenic diet impacts markers of mitochondrial mass in a tissue specific manner in aged mice

Declines in mitochondrial mass are thought to be a hallmark of mammalian aging, and a ketogenic diet (KD) may prevent the age-related decreases in mitochondrial content. The objective of this study was to investigate the impact of a KD on markers of mitochondrial mass. Mice were fed an isocaloric control diet (CD) or KD from 12 months of age. Tissues were collected after 1 month and 14 months of intervention, and a panel of commonly used markers of mitochondrial mass (mitochondrial enzyme activities and levels, mitochondrial to nuclear DNA ratio, and cardiolipin content) were measured. Our results showed that a KD stimulated activities of marker mitochondrial enzymes including citrate synthase, Complex I, and Complex IV in hindlimb muscle in aged mice. KD also increased the activity of citrate synthase and prevented an age-related decrease in Complex IV activity in aged brain. No other markers were increased in these tissues. Furthermore, the impacts of a KD on liver and kidney were mixed with no pattern indicative of a change in mitochondrial mass. In conclusion, results of the present study suggest that a KD induces tissue-specific changes in mitochondrial enzyme activities, or structure, rather than global changes in mitochondrial mass across tissues.

Cancer cachexia in a mouse model of oxidative stress

BACKGROUND

Cancer is associated with muscle atrophy (cancer cachexia) that is linked to up to 40% of cancer-related deaths. Oxidative stress is a critical player in the induction and progression of age-related loss of muscle mass and weakness (sarcopenia); however, the role of oxidative stress in cancer cachexia has not been defined. The purpose of this study was to examine if elevated oxidative stress exacerbates cancer cachexia.

METHODS

Cu/Zn superoxide dismutase knockout (Sod1KO) mice were used as an established mouse model of elevated oxidative stress. Cancer cachexia was induced by injection of one million Lewis lung carcinoma (LLC) cells or phosphate-buffered saline (saline) into the hind flank of female wild-type mice or Sod1KO mice at approximately 4 months of age. The tumour developed for 3 weeks. Muscle mass, contractile function, neuromuscular junction (NMJ) fragmentation, metabolic proteins, mitochondrial function, and motor neuron function were measured in wild-type and Sod1KO saline and tumour-bearing mice. Data were analysed by two-way ANOVA with Tukey-Kramer post hoc test when significant F ratios were determined and α was set at 0.05. Unless otherwise noted, results in abstract are mean ±SEM.

RESULTS

Muscle mass and cross-sectional area were significantly reduced, in tumour-bearing mice. Metabolic enzymes were dysregulated in Sod1KO mice and cancer exacerbated this phenotype. NMJ fragmentation was exacerbated in tumour-bearing Sod1KO mice. Myofibrillar protein degradation increased in tumour-bearing wild-type mice (wild-type saline, 0.00847 ± 0.00205; wildtype LLC, 0.0211 ± 0.00184) and tumour-bearing Sod1KO mice (Sod1KO saline, 0.0180 ± 0.00118; Sod1KO LLC, 0.0490 ± 0.00132). Muscle mitochondrial oxygen consumption was reduced in tumour-bearing mice compared with saline-injected wild-type mice. Mitochondrial protein degradation increased in tumour-bearing wild-type mice (wild-type saline, 0.0204 ± 0.00159; wild-type LLC, 0.167 ± 0.00157) and tumour-bearing Sod1KO mice (Sod1KO saline, 0.0231 ± 0.00108; Sod1 KO LLC, 0.0645 ± 0.000631). Sciatic nerve conduction velocity was decreased in tumour-bearing wild-type mice (wild-type saline, 38.2 ± 0.861; wild-type LLC, 28.8 ± 0.772). Three out of eleven of the tumour-bearing Sod1KO mice did not survive the 3-week period following tumour implantation.

CONCLUSIONS

Oxidative stress does not exacerbate cancer-induced muscle loss; however, cancer cachexia may accelerate NMJ disruption.

Fine structure of the cardiac muscle cells in the orb-web spider Nephila clavata

The fine structural characteristics of cardiac muscle cells and its myofibril organization in the orb web spider N. clavata were examined by transmission electron microscopy. Although myofibril striations are not remarkable as those of skeletal muscles, muscle fibers contain multiple myofibrils, abundant mitochondria, extensive sarcoplasmic reticulum and transverse tubules (T-tubules). Myofibrils are divided into distinct sarcomeres defined by Z-lines with average length of 2.0 μm, but the distinction between the A-band and the I-bands is not clear due to uniform striations over the length of the sarcomeres. Dyadic junction which consisted of a single T-tubule paired with a terminal cisterna of the sarcoplasmic reticulum is found mainly at the A-I level of sarcomere. Each cell is arranged to form multiple connections with neighboring cells through the intercalated discs. These specialized junctions include three types of intercellular junctions: gap junctions, fascia adherens and desmosomes for heart function. Our transmission electron microscopy (TEM) observations clearly show that spider's cardiac muscle contraction is controlled by neurogenic rather than myogenic mechanism since each cardiac muscle fiber is innervated by a branch of motor neuron through neuromuscular junctions.

The Impact of Aging, Calorie Restriction and Dietary Fat on Autophagy Markers and Mitochondrial Ultrastructure and Dynamics in Mouse Skeletal Muscle

Loss of skeletal muscle mass and function is a hallmark of aging. This phenomenon has been related to a dysregulation of mitochondrial function and proteostasis. Calorie restriction (CR) has been demonstrated to delay aging and preserve function until late in life, particularly in muscle. Recently, we reported the type of dietary fat plays an important role in determining life span extension with 40% CR in male mice. In these conditions, lard fed mice showed an increased longevity compared to mice fed soybean or fish oils. In this article, we analyze the effect of 40% CR on muscle mitochondrial mass, autophagy, and mitochondrial dynamics markers in mice fed these diets. In CR fed animals, lard preserved muscle fibers structure, mitochondrial ultrastructure, and fission/fusion dynamics and autophagy, not only compared to control animals, but also compared with CR mice fed soybean and fish oils as dietary fat. We focus our discussion on dietary fatty acid saturation degree as an essential predictor of life span extension in CR mice.

Metabolic regulation by the PGC-1α and PGC-1β coactivators in larval zebrafish (Danio rerio)

The peroxisome proliferator activated receptor γ coactivator-1 (PGC-1) family is composed of three coactivators whose role in regulating mammalian bioenergetics regulation is clear, but is much less certain in other vertebrates. Current evidence suggests that in fish, PGC-1α and PGC-1β may exhibit much less redundancy in the control of fatty acid oxidation and mitochondrial biogenesis compared to mammals. To assess these roles directly, we knocked down PGC-1α and PGC-1β expression with morpholinos in zebrafish embryos, and we investigated the resulting molecular and physiological phenotypes. First, we found no effects of either morpholinos on larval hatching, heart rates and oxygen consumption over the first few days of development. Next, at 3 days post fertilization (dpf), we confirmed by real time PCR a specific knock down of both coactivators, that resulted in a significant reduction in the transcript levels of citrate synthase (CS), 3-hydroxyacyl-CoA dehydrogenase (HOAD), and medium-chain acyl-coenzyme A dehydrogenase (MCAD) in both morphant groups. However, there was no effect on transcription factors' gene expression except for a marked reduction in estrogen related receptor α (ERRα) transcripts in PGC-1α morphants. Finally, we assessed whole embryonic enzyme activity for CS, cytochrome oxidase (COX), HOAD and carnitine palmitoyltransferase I (CPT-1) at 4 dpf. The only significant effect of the knockdown was a reduced CS activity in PGC-1α morphants and a counterintuitive increase of cytochrome oxidase activity in PGC-1β morphants. Overall, our results indicate that in larval zebrafish, PGC-1α and PGC-1β both play a role in regulating expression of important mitochondrial genes potentially through ERRα.

Platelet bioenergetics correlate with muscle energetics and are altered in older adults

BACKGROUND

Physical function decreases with age, and though bioenergetic alterations contribute to this decline, the mechanisms by which mitochondrial function changes with age remains unclear. This is partially because human mitochondrial studies require highly invasive procedures, such as muscle biopsies, to obtain live tissue with functional mitochondria. However, recent studies demonstrate that circulating blood cells are potentially informative in identifying systemic bioenergetic changes. Here, we hypothesize that human platelet bioenergetics reflect bioenergetics measured in muscle biopsies.

METHODS & RESULTS

We demonstrate that maximal and ATP-linked respiratory rate measured in isolated platelets from older adults (86-93 years) correlates significantly with maximal respiration (r = 0.595; P = 0.003) measured by muscle biopsy respirometry and maximal ATP production (r = 0.643; P = 0.004) measured by 31P-MRS respectively, in the same individuals. Comparison of platelet bioenergetics in this aged cohort to platelets from younger adults (18-35 years) shows aged adults demonstrate lower basal and ATP-linked respiration. Platelets from older adults also show enhanced proton leak, which is likely due to increased protein levels of uncoupling protein 2, and correlates with increased gate speed in this cohort (r = 0.58; P = 0.0019). While no significant difference in glycolysis was observed in older adults compared to younger adults, platelet glycolytic rate correlated with fatigability (r = 0.44; P = 0.016).

CONCLUSIONS

These data advance the mechanistic understanding of age-related changes in mitochondrial function. Further, they suggest that measuring platelet bioenergetics provides a potential supplement or surrogate for muscle biopsy measurement and may be a valuable tool to study mitochondrial involvement in age-related decline of physical function.

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

Impact of Aging and Exercise on Mitochondrial Quality Control in Skeletal Muscle

Mitochondria are characterized by its pivotal roles in managing energy production, reactive oxygen species, and calcium, whose aging-related structural and functional deteriorations are observed in aging muscle. Although it is still unclear how aging alters mitochondrial quality and quantity in skeletal muscle, dysregulation of mitochondrial biogenesis and dynamic controls has been suggested as key players for that. In this paper, we summarize current understandings on how aging regulates muscle mitochondrial biogenesis, while focusing on transcriptional regulations including PGC-1α, AMPK, p53, mtDNA, and Tfam. Further, we review current findings on the muscle mitochondrial dynamic systems in aging muscle: fusion/fission, autophagy/mitophagy, and protein import. Next, we also discuss how endurance and resistance exercises impact on the mitochondrial quality controls in aging muscle, suggesting possible effective exercise strategies to improve/maintain mitochondrial health.

Sarcopenia, age, atrophy, and myopathy: Mitochondrial oxidative enzyme activities

INTRODUCTION

We studied mitochondrial impairment as a factor in the pathologic equivalent of sarcopenia, muscle fiber atrophy associated with increased age.

METHODS

Mitochondrial oxidative enzyme activities and coenzyme Q10 levels were measured in frozen human proximal limb muscles with combined age and atrophy, age alone, atrophy alone, denervation, immune myopathies, and mitochondrial disorders with ophthalmoplegia.

RESULTS

Sarcopenia (age and atrophy) had reduced mean activities of mitochondrial Complexes I, II, and II+III, with severe reduction of Complex I activity in 54% of patients. Atrophy, and specific denervation atrophy, had similar patterns of changes. Age alone had moderately reduced Complex I activity. Mitochondrial myopathies had mildly lower Complex IV activity. Immune myopathies had unchanged enzyme activities.

CONCLUSIONS

Mitochondrial oxidative enzyme activities, especially Complex I, but also Complexes II and II+III, are reduced in muscles with the pathologic equivalent of sarcopenia. Individually, atrophy and age have different patterns of oxidative enzyme changes. Muscle Nerve 56: 122-128, 2017.

The diaphragm is better protected from oxidative stress than hindlimb skeletal muscle during CLP-induced sepsis

OBJECTIVES

The aim of this study was to determine whether non-lethal sepsis induced by cecal ligation and puncture (CLP) modulates oxidative damage and enzymatic antioxidant defenses in diaphragm and hindlimb skeletal muscles (soleus and Extensor Digitorus Longus (EDL)).

METHODS

Female Wistar rats were divided into four experimental groups: (1) control animals, (2) animals sacrificed 2 hours or (3) 7 days after CLP, and (4) sham-operated animals. At the end of the experimental procedure, EDL, soleus, and diaphragm muscles were harvested and 4-hydroxynonenal (HNE)-protein adducts and protein carbonyl contents were examined in relation to superoxide dismutase and catalase expression and activities.

RESULTS

We observed that both non-respiratory oxidative (i.e. soleus) and glycolytic skeletal muscles (i.e. EDL) are more susceptible to sepsis-induced oxidative stress than diaphragm, as attested by an increase in 4-HNE protein adducts and carbonylated proteins after 2 hours of CLP only in soleus and EDL.

DISCUSSION

These differences could be explained by higher basal enzymatic antioxidant activities in diaphragm compared to hindlimb skeletal muscles. Together, these results demonstrate that diaphragm is better protected from oxidative stress than hindlimb skeletal muscles during CLP-induced sepsis.

Exercise Promotes Healthy Aging of Skeletal Muscle

Primary aging is the progressive and inevitable process of bodily deterioration during adulthood. In skeletal muscle, primary aging causes defective mitochondrial energetics and reduced muscle mass. Secondary aging refers to additional deleterious structural and functional age-related changes caused by diseases and lifestyle factors. Secondary aging can exacerbate deficits in mitochondrial function and muscle mass, concomitant with the development of skeletal muscle insulin resistance. Exercise opposes deleterious effects of secondary aging by preventing the decline in mitochondrial respiration, mitigating aging-related loss of muscle mass and enhancing insulin sensitivity. This review focuses on mechanisms by which exercise promotes "healthy aging" by inducing modifications in skeletal muscle.

Skeletal muscle mitochondrial energetic efficiency and aging

Aging is associated with a progressive loss of maximal cell functionality, and mitochondria are considered a key factor in aging process, since they determine the ATP availability in the cells. Mitochondrial performance during aging in skeletal muscle is reported to be either decreased or unchanged. This heterogeneity of results could partly be due to the method used to assess mitochondrial performance. In addition, in skeletal muscle the mitochondrial population is heterogeneous, composed of subsarcolemmal and intermyofibrillar mitochondria. Therefore, the purpose of the present review is to summarize the results obtained on the functionality of the above mitochondrial populations during aging, taking into account that the mitochondrial performance depends on organelle number, organelle activity, and energetic efficiency of the mitochondrial machinery in synthesizing ATP from the oxidation of fuels.

Mitochondrial involvement and impact in aging skeletal muscle

Atrophy is a defining feature of aging skeletal muscle that contributes to progressive weakness and an increased risk of mobility impairment, falls, and physical frailty in very advanced age. Amongst the most frequently implicated mechanisms of aging muscle atrophy is mitochondrial dysfunction. Recent studies employing methods that are well-suited to interrogating intrinsic mitochondrial function find that mitochondrial respiration and reactive oxygen species emission changes are inconsistent between aging rat muscles undergoing atrophy and appear normal in human skeletal muscle from septuagenarian physically active subjects. On the other hand, a sensitization to permeability transition seems to be a general property of atrophying muscle with aging and this effect is even seen in atrophying muscle from physically active septuagenarian subjects. In addition to this intrinsic alteration in mitochondrial function, factors extrinsic to the mitochondria may also modulate mitochondrial function in aging muscle. In particular, recent evidence implicates oxidative stress in the aging milieu as a factor that depresses respiratory function in vivo (an effect that is not present ex vivo). Furthermore, in very advanced age, not only does muscle atrophy become more severe and clinically relevant in terms of its impact, but also there is evidence that this is driven by an accumulation of severely atrophied denervated myofibers. As denervation can itself modulate mitochondrial function and recruit mitochondrial-mediated atrophy pathways, future investigations need to address the degree to which skeletal muscle mitochondrial alterations in very advanced age are a consequence of denervation, rather than a primary organelle defect, to refine our understanding of the relevance of mitochondria as a therapeutic target at this more advanced age.

Energy metabolism and cytochrome oxidase activity: linking metabolism to gene expression

Modification of mitochondrial content demands the synthesis of hundreds of proteins encoded by nuclear and mitochondrial genomes. The responsibility for coordination of this process falls to nuclear-encoded master regulators of transcription. DNA-binding proteins and coactivators integrate information from energy-sensing pathways and hormones to alter mitochondrial gene expression. In mammals, the signaling cascade for mitochondrial biogenesis can be described as follows: hormonal signals and energetic information are sensed by protein-modifying enzymes that in turn regulate the post-translational modification of transcription factors. Once activated, transcription-factor complexes form on promoter elements of many of the nuclear-encoded mitochondrial genes, recruiting proteins that remodel chromatin and initiate transcription. One master regulator in mammals, PGC-1α, is well studied because of its role in determining the metabolic phenotype of muscles, but also due to its importance in mitochondria-related metabolic diseases. However, relatively little is known about the role of this pathway in other vertebrates. These uncertainties raise broader questions about the evolutionary origins of the pathway and its role in generating the diversity in muscle metabolic phenotypes seen in nature.

Effects of Flight on Gene Expression and Aging in the Honey Bee Brain and Flight Muscle

Honey bees move through a series of in-hive tasks (e.g., “nursing”) to outside tasks (e.g., “foraging”) that are coincident with physiological changes and higher levels of metabolic activity. Social context can cause worker bees to speed up or slow down this process, and foragers may revert back to their earlier in-hive tasks accompanied by reversion to earlier physiological states. To investigate the effects of flight, behavioral state and age on gene expression, we used whole-genome microarrays and real-time PCR. Brain tissue and flight muscle exhibited different patterns of expression during behavioral transitions, with expression patterns in the brain reflecting both age and behavior, and expression patterns in flight muscle being primarily determined by age. Our data suggest that the transition from behaviors requiring little to no flight (nursing) to those requiring prolonged flight bouts (foraging), rather than the amount of previous flight per se, has a major effect on gene expression. Following behavioral reversion there was a partial reversion in gene expression but some aspects of forager expression patterns, such as those for genes involved in immune function, remained. Combined with our real-time PCR data, these data suggest an epigenetic control and energy balance role in honey bee functional senescence.

Skeletal muscle mitochondria and aging: a review

Aging is characterized by a progressive loss of muscle mass and muscle strength. Declines in skeletal muscle mitochondria are thought to play a primary role in this process. Mitochondria are the major producers of reactive oxygen species, which damage DNA, proteins, and lipids if not rapidly quenched. Animal and human studies typically show that skeletal muscle mitochondria are altered with aging, including increased mutations in mitochondrial DNA, decreased activity of some mitochondrial enzymes, altered respiration with reduced maximal capacity at least in sedentary individuals, and reduced total mitochondrial content with increased morphological changes. However, there has been much controversy over measurements of mitochondrial energy production, which may largely be explained by differences in approach and by whether physical activity is controlled for. These changes may in turn alter mitochondrial dynamics, such as fusion and fission rates, and mitochondrially induced apoptosis, which may also lead to net muscle fiber loss and age-related sarcopenia. Fortunately, strategies such as exercise and caloric restriction that reduce oxidative damage also improve mitochondrial function. While these strategies may not completely prevent the primary effects of aging, they may help to attenuate the rate of decline.

Long-term supplementation with resveratrol alleviates oxidative stress but does not attenuate sarcopenia in aged mice

This study analyzed the capacity of resveratrol, a naturally occurring polyphenol, to reduce aging-induced oxidative stress and protect against sarcopenia. Middle-aged (18 months) C57/BL6 mice were randomly assigned to receive either a control diet or a diet supplemented with 0.05% trans-resveratrol for 10 months. Young (6 months) and middle-aged (18 months) mice were used as controls. Resveratrol supplementation did not reduce the aging-associated loss of muscle mass or improve maximal isometric force production, but it appeared to preserve fast-twitch fiber contractile function. Resveratrol supplementation did not improve mitochondrial content, the subcellular localization of cytochrome c protein content, or PGC1 protein content. Resveratrol increased manganese superoxide dismutase (MnSOD), reduced hydrogen peroxide(,) and lipid peroxidation levels in muscle samples, but it was unable to significantly reduce protein carbonyl levels. The data suggest that resveratrol has a protective effect against aging-induced oxidative stress in skeletal muscle, likely through the upregulation of MnSOD activity, but sarcopenia was not attenuated by resveratrol.

Differential apoptosis-related protein expression, mitochondrial properties, proteolytic enzyme activity, and DNA fragmentation between skeletal muscles

Increased skeletal muscle apoptosis has been associated with a number of conditions including aging, disuse, and cardiovascular disease. Skeletal muscle is a complex tissue comprised of several fiber types with unique properties. To date, no report has specifically examined apoptotic differences across muscles or fiber types. Therefore, we measured several apoptotic indices in healthy rat red (RG) and white gastrocnemius (WG) muscle, as well as examined the expression of several key proteins across fiber types in a mixed muscle (mixed gastrocnemius). The protein content of apoptosis-inducing factor (AIF), apoptosis repressor with caspase recruitment domain (ARC), Bax, Bcl-2, cytochrome c, heat shock protein 70 (Hsp70), and second mitochondria-derived activator of caspases (Smac) were significantly (P < 0.05) higher in RG vs. WG muscle. Cytosolic AIF, cytochrome c, and Smac as well as nuclear AIF were also significantly (P < 0.05) higher in RG compared with WG muscle. In addition, ARC protein expression was related to muscle fiber type and found to be highest (P < 0.001) in type I fibers. Similarly, AIF protein expression was differentially expressed across fibers; however, AIF was correlated to oxidative potential (P < 0.001). Caspase-3, -8, and -9 activity, calpain activity, and DNA fragmentation (a hallmark of apoptosis) were also significantly higher (P < 0.05) in RG compared with WG muscle. Furthermore, total muscle reactive oxygen species generation, as well as Ca(2+)-induced permeability transition pore opening and loss of membrane potential in isolated mitochondria were greater in RG muscle. Collectively, these data suggest that a number of apoptosis-related indices differ between muscles and fiber types. Given these findings, muscle and fiber-type differences in apoptotic protein expression, signaling, and susceptibility should be considered when studying cell death processes in skeletal muscle.

Underdeveloped extraocular muscles in the naked mole-rat (Heterocephalus glaber)

The extraocular muscles (EOM), the effector arm of the ocular motor system, have a unique embryological origin and phenotype. The naked mole-rat (NMR) is a subterranean rodent with an underdeveloped visual system. It has not been established if their ocular motor system is also less developed. The NMR is an ideal model to examine the potential codependence of oculomotor and visual system development and evolution. Our goal was to compare the structural features of NMR EOMs to those of the mouse, a similar sized rodent with a fully developed visual system. Perfusion-fixed whole orbits and EOMs were dissected from adult NMR and C57BL mice and examined by light and electron microscopy. NMR orbital anatomy showed smaller EOMs in roughly the same distribution around the eye as in mouse and surrounded by a very small Harderian gland. The NMR EOMs did not appear to have the two-layer fiber distribution seen in mouse EOMs; fibers were also significantly smaller (112.3 +/- 46.2 vs. 550.7 +/- 226 sq microm in mouse EOMs, *P < 0.05). Myofibrillar density was less in NMR EOMs, and triad and other membranous structures were rudimentary. Finally, mitochondrial volume density was significantly less in NMR EOMs than in mouse EOM (4.5% +/- 1.9 vs. 21.2% +/- 11.6, respectively, *P < 0.05). These results demonstrate that NMR EOMs are smaller and less organized than those in the mouse. The "simpler" EOM organization and structure in NMR may be explained by the poor visual ability of these rodents, initially demonstrated by their primitive visual system.

Increased superoxide in vivo accelerates age-associated muscle atrophy through mitochondrial dysfunction and neuromuscular junction degeneration

Oxidative stress has been implicated in the etiology of age-related muscle loss (sarcopenia). However, the underlying mechanisms by which oxidative stress contributes to sarcopenia have not been thoroughly investigated. To directly examine the role of chronic oxidative stress in vivo, we used a mouse model that lacks the antioxidant enzyme CuZnSOD (Sod1). Sod1(-/-) mice are characterized by high levels of oxidative damage and an acceleration of sarcopenia. In the present study, we demonstrate that muscle atrophy in Sod1(-/-) mice is accompanied by a progressive decline in mitochondrial bioenergetic function and an elevation of mitochondrial generation of reactive oxygen species. In addition, Sod1(-/-) muscle exhibits a more rapid induction of mitochondrial-mediated apoptosis and loss of myonuclei. Furthermore, aged Sod1(-/-) mice show a striking increase in muscle mitochondrial content near the neuromuscular junctions (NMJs). Despite the increase in content, the function of mitochondria is significantly impaired, with increased denervated NMJs and fragmentation of acetylcholine receptors. As a consequence, contractile force in aged Sod1(-/-) muscles is greatly diminished. Collectively, we show that Sod1(-/-) mice display characteristics of normal aging muscle in an accelerated manner and propose that the superoxide-induced NMJ degeneration and mitochondrial dysfunction are potential mechanisms of sarcopenia.

Impact of lifelong sedentary behavior on mitochondrial function of mice skeletal muscle

This study investigated the impact of lifelong sedentariness on skeletal muscle mass and mitochondrial function. Thirty C57BL/6 strain mice (2 months) were randomly divided into three groups (young-Y; old sedentary-OS; old active-OA). Young animals were sacrificed after 1 week of quarantine, and OS and OA groups were individually placed into standard cages and in cages with running wheels, respectively, until sacrifice (25 months). Body weights and hind-limb skeletal muscle wet weights were obtained from all groups. Mitochondrial respiratory functional measures (i.e., state 3 and 4 respiration, respiratory control ratio, and ratio of nanomoles of ADP phosphorylated by nanomoles of O2 consumed [ADP/O]) and biochemical markers of oxidative damage (aconitase activity, protein carbonyl derivatives, sulfhydryl groups) were measured in isolated mitochondrial suspensions. Our results reveal that lifelong sedentary behavior has a negative impact on the age-related loss of skeletal muscle mass and on the isolated mitochondrial function of mixed skeletal muscle of mice, which is associated with an increased oxidative damage to mitochondrial biomolecules.

Aging impairs skeletal muscle mitochondrial bioenergetic function

This study investigated the influence of age on the functional status of mitochondria isolated from skeletal muscle of C57BL/6 mice aged 3 and 18 months. We hypothesized that skeletal muscle mitochondria isolated from aged animals will exhibit a decreased respiratory function. Mitochondrial respiratory functional measures (ie, State 3 and 4 respiration, respiratory control ratio and number of nanomoles of ADP phosphorylated by nanomoles of O(2) consumed per mitochondrion) and biochemical markers of oxidative damage (aconitase activity, protein carbonyl derivatives, sulfhydryl groups, and malondialdehyde) were measured in isolated mitochondrial suspensions. Along with traditional tests of mitochondrial function, an in vitro repetitive ADP-stimulation test was used to evaluate the mitochondrial capacity to reestablish the homeostatic balance between successive ADP stimulations. The number of mitochondria per mitochondrial suspension, calculated by transmission electron microscopy, was used to normalize functional and biochemical data. Our results confirm the existence of an age-associated decline in mitochondrial function of mixed skeletal muscle, which is significantly correlated with higher levels of mitochondrial oxidative damage.

Role of the PGC-1 family in the metabolic adaptation of goldfish to diet and temperature

In mammals, the peroxisome proliferator-activated receptor (PPAR) gamma coactivator-1 (PGC-1) family members and their binding partners orchestrate remodelling in response to diverse challenges such as diet, temperature and exercise. In this study, we exposed goldfish to three temperatures (4, 20 and 35 degrees C) and to three dietary regimes (food deprivation, low fat and high fat) and examined the changes in mitochondrial enzyme activities and transcript levels for metabolic enzymes and their genetic regulators in red muscle, white muscle, heart and liver. When all tissues and conditions were pooled, there were significant correlations between the mRNA for the PGC-1 coactivators (both alpha and beta) and mitochondrial transcripts (citrate synthase), metabolic gene regulators including PPARalpha, PPARbeta and nuclear respiratory factor-1 (NRF-1). PGC-1beta was the better predictor of the NRF-1 axis, whereas PGC-1alpha was the better predictor of the PPAR axis (PPARalpha, PPARbeta, medium chain acyl CoA dehydrogenase). In contrast to these intertissue/developmental patterns, the response of individual tissues to physiological stressors displayed no correlations between mRNA for PGC-1 family members and either the NRF-1 or PPAR axes. For example, in skeletal muscles, low temperature decreased PGC-1alpha transcript levels but increased mitochondrial enzyme activities (citrate synthase and cytochrome oxidase) and transcripts for COX IV and NRF-1. These results suggest that in goldfish, as in mammals, there is a regulatory relationship between (i) NRF-1 and mitochondrial gene expression and (ii) PPARs and fatty acid oxidation gene expression. In contrast to mammals, there is a divergence in the roles of the coactivators, with PGC-1alpha linked to fatty acid oxidation through PPARalpha, and PGC-1beta with a more prominent role in mediating NRF-1-dependent control of mitochondrial gene expression, as well as distinctions between their respective roles in development and physiological responsiveness.

The role of mitochondria in aging of skeletal muscle

Aging can be characterized as a time dependent decline of maximal functionality that affects tissues and organs of the whole body. Such is induced by the progressive loss of redundant components and leads to an increased susceptibility to disease and risk of death. Regarding the aging of skeletal muscle, it has been pointed out that mitochondria is a key factor behind the loss of redundancy and functionality, since this organelle has a major role in cellular homeostasis particularly at the level of the bioenergetic status. Decreased activities of the mitochondrial electron transport chain complexes and an increased release of reactive oxygen species from mitochondria are well documented with age; it is suggested that the mitochondrial loss of function results from the increased oxidative damage to proteins, lipids, and DNA of this organelle. However, it is important to be aware that the mitochondrial loss of function could also be a consequence, rather than a cause, of the cellular deterioration with age, which compromises mitochondrial biogenesis, mitochondrial protein turnover and autophagocytosis of damaged mitochondria. In this review several topics will be addressed regarding the age-related loss of skeletal muscle redundancy associated with mitochondrial dysfunction, emphasizing hypotheses for underlying mechanisms. In addition, we discuss some of the cellular mechanisms that can be pointed out as being responsible for the age-related mitochondrial dysfunction.

Mitochondrial gene expression and increased oxidative metabolism: role in increased lifespan of fat-specific insulin receptor knock-out mice

Caloric restriction, leanness and decreased activity of insulin/insulin-like growth factor 1 (IGF-1) receptor signaling are associated with increased longevity in a wide range of organisms from Caenorhabditis elegans to humans. Fat-specific insulin receptor knock-out (FIRKO) mice represent an interesting dichotomy, with leanness and increased lifespan, despite normal or increased food intake. To determine the mechanisms by which a lack of insulin signaling in adipose tissue might exert this effect, we performed physiological and gene expression studies in FIRKO and control mice as they aged. At the whole body level, FIRKO mice demonstrated an increase in basal metabolic rate and respiratory exchange ratio. Analysis of gene expression in white adipose tissue (WAT) of FIRKO mice from 6 to 36 months of age revealed persistently high expression of the nuclear-encoded mitochondrial genes involved in glycolysis, tricarboxylic acid cycle, beta-oxidation and oxidative phosphorylation as compared to expression of the same genes in WAT from controls that showed a tendency to decline in expression with age. These changes in gene expression were correlated with increased cytochrome c and cytochrome c oxidase subunit IV at the protein level, increased citrate synthase activity, increased expression of peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) and PGC-1beta, and an increase in mitochondrial DNA in WAT of FIRKO mice. Together, these data suggest that maintenance of mitochondrial activity and metabolic rates in adipose tissue may be important contributors to the increased lifespan of the FIRKO mouse.