Endurance capacity, not body size, determines physical activity levels: role of skeletal muscle PEPCK

Reduced contextually induced muscle thermogenesis in rats with calorie restriction and lower aerobic fitness but not monogenic obesity

We have previously identified predator odor as a potent stimulus activating thermogenesis in skeletal muscle in rats. As this may prove relevant for energy balance and weight loss, the current study investigated whether skeletal muscle thermogenesis was altered with negative energy balance, obesity propensity seen in association with low intrinsic aerobic fitness, and monogenic obesity. First, weight loss subsequent to 3 wk of 50% calorie restriction suppressed the muscle thermogenic response to predator odor. Next, we compared rats bred based on artificial selection for intrinsic aerobic fitness - high- and low-capacity runners (HCR, LCR) - that display robust leanness and obesity propensity, respectively. Aerobically fit HCR showed enhanced predator odor-induced muscle thermogenesis relative to the less-fit LCR. This contrasted with the profound monogenic obesity displayed by rats homozygous for a loss of function mutation in Melanocortin 4 receptor (Mc4rK3a,4X/K314X rats), which showed no discernable deficit in thermogenesis. Taken together, these data imply that body size or obesity per se are not associated with deficient muscle thermogenesis. Rather, the physiological phenotype associated with polygenic obesity propensity may encompass pleiotropic mechanisms in the thermogenic pathway. Adaptive thermogenesis associated with weight loss also likely alters muscle thermogenic mechanisms.

Altered skeletal muscle sarco-endoplasmic reticulum Ca2+-ATPase calcium transport efficiency after a thermogenic stimulus

Exposure to predator threat induces a rapid and robust increase in skeletal muscle thermogenesis in rats. The central nervous system relays threat information to skeletal muscle through activation of the sympathetic nervous system, but muscle mechanisms mediating this thermogenesis remain unidentified. Given the relevance of sarcolipin-mediated futile calcium cycling through the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) pump to mammalian muscle nonshivering thermogenesis, we hypothesized that this plays a role in contextually induced muscle thermogenesis as well. This was assessed by measuring enzymatic activity of SERCA and sarcoplasmic reticulum Ca2+ transport, where the apparent coupling ratio (Ca2+ uptake rate divided by ATPase activity rate at a standard Ca2+ concentration) was predicted to decrease in association with muscle thermogenesis. Sprague-Dawley rats exposed to predator (ferret) odor (PO) showed a rapid decrease in the apparent coupling ratio in the soleus muscle, indicating SERCA uncoupling compared with control-odor-exposed rats. A rat model of high aerobic fitness and elevated muscle thermogenesis also demonstrated soleus muscle SERCA uncoupling relative to their obesity-prone, low-fitness counterparts. Both the high- and low-aerobic fitness rats showed soleus SERCA uncoupling with exposure to PO. Finally, no increase in sarcolipin expression in soleus muscle was detected with PO exposure. This dataset implicates muscle uncoupling of SERCA Ca2+ transport and ATP hydrolysis, likely through altered SERCA or sarcolipin function outside of translational regulation, as one contributor to the muscle thermogenesis provoked by exposure to predator threat. These data support the involvement of SERCA uncoupling in both muscle thermogenic induction and enhanced aerobic capacity.

Ins and Outs of the TCA Cycle: The Central Role of Anaplerosis

The reactions of the tricarboxylic acid (TCA) cycle allow the controlled combustion of fat and carbohydrate. In principle, TCA cycle intermediates are regenerated on every turn and can facilitate the oxidation of an infinite number of nutrient molecules. However, TCA cycle intermediates can be lost to cataplerotic pathways that provide precursors for biosynthesis, and they must be replaced by anaplerotic pathways that regenerate these intermediates. Together, anaplerosis and cataplerosis help regulate rates of biosynthesis by dictating precursor supply, and they play underappreciated roles in catabolism and cellular energy status. They facilitate recycling pathways and nitrogen trafficking necessary for catabolism, and they influence redox state and oxidative capacity by altering TCA cycle intermediate concentrations. These functions vary widely by tissue and play emerging roles in disease. This article reviews the roles of anaplerosis and cataplerosis in various tissues and discusses how they alter carbon transitions, and highlights their contribution to mechanisms of disease. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Effects of early-life exposure to Western diet and voluntary exercise on adult activity levels, exercise physiology, and associated traits in selectively bred High Runner mice

Exercise behavior is under partial genetic control, but it is also affected by numerous environmental factors, potentially including early-life experiences whose effects persist into adulthood. We studied genetic and early-life environmental effects on wheel-running behavior in a mouse model that includes four replicate high runner (HR) lines selectively bred for increased voluntary wheel running as young adults and four non-selected control (C) lines. In a full factorial design, mice from each line were granted wheel access or not and administered either standard or Western diet (WD) from weaning (3 weeks old) to 6 weeks of age (sexual maturity). In addition to acute effects, after a washout period of 8 weeks (∼6 human years) in which all mice had standard diet and no wheel access, we found both beneficial and detrimental effects of these early-life exposures. During the first week of treatments, WD increased distance run by 29% in C mice and 48% in HR mice (significant Diet × Linetype interaction), but diet effects disappeared by the third week. Across the three weeks of juvenile treatment, WD significantly increased fat mass (with lean mass as a covariate). Tested as adults, early-life exercise increased wheel running of C mice but not HR mice in the first week. Early-life exercise also reduced adult anxiety-like behavior and increased adult fasted blood glucose levels, triceps surae mass, subdermal fat pad mass, and brain mass, but decreased heart ventricle mass. Using fat mass as a covariate, early-life exercise treatment increased adult leptin concentration. In contrast, early-life WD increased adult wheel running of HR mice but not C mice. Early-life WD also increased adult lean mass and adult preference for Western diet in all groups. Surprisingly, early-life treatment had no significant effect on adult body fat or maximal aerobic capacity (VO2max). No previous study has tested for combined or interactive effects of early-life WD and exercise. Our results demonstrate that both factors can have long-lasting effects on adult voluntary exercise and related phenotypes, and that these effects are modulated by genetic background. Overall, the long-lasting effects of early-life exercise were more pervasive than those of WD, suggesting critical opportunities for health intervention in childhood habits, as well as possible threats from modern challenges. These results may be relevant for understanding potential effects of activity reductions and dietary changes associated with the obesity epidemic and COVID-19 pandemic.

Intrinsic exercise capacity induces divergent vascular plasticity via arachidonic acid-mediated inflammatory pathways in female rats

Metabolic syndrome prevalence has increased among US adults, particularly among non-hispanic white and black women. Sedentary behavior often leads to chronic inflammation, a triggering factor of metabolic syndrome. Given that intrinsic exercise capacity is genetically inherited, we questioned if low-grade chronic inflammation would be present in a female rat model of low intrinsic exercise capacity-induced metabolic syndrome, while beneficial increase of resolution of inflammation would be present in a female rat model of high intrinsic exercise capacity. In the vascular system, two primary markers for inflammation and resolution of inflammation are cyclooxygenase (COX) and lipoxygenase (LOX), respectively. Our study focused on the novel hypothesis that untrained, inherited exercise capacity induces divergent vascular plasticity via changes in the delicate balance between COX and LOX inflammatory mediators. We used divergent rat strains with low (LCR) and high (HCR) aerobic running capacity. By using animals with contrasting intrinsic exercise capacities, it is possible to determine the exact triggers that lead to inherited vascular plasticity in female rats. We observed that female LCR displayed increased periovarian fat pad and body weight, which is congruent with their obesity-presenting phenotype. Furthermore, LCR presented with vascular hypocontractility and increased COX and LOX-derived pro-inflammatory factors. On the other hand, HCR presented with a "shutdown" of COX-induced vasoconstriction and enhanced resolution of inflammation to maintain vascular tone and homeostasis. In conclusion, LCR display low-grade chronic inflammation via increased COX activity. These results provide mechanistic clues as to why lower intrinsic aerobic capacity correlates with a predisposition to risk of vascular disease. Conversely, being born with higher intrinsic aerobic capacity is a significant factor for improved vascular physiology in female rats.

Enhanced weight and fat loss from long-term intermittent fasting in obesity-prone, low-fitness rats

BACKGROUND

Intermittent fasting (IF) strategies have emerged as viable alternatives to traditional calorie-restricted diets. A key predictor of metabolic health and response to diet is cardiometabolic fitness, including intrinsic aerobic capacity. In a contrasting rat model of aerobic capacity-high- and low-capacity runners (HCR, LCR)-we found that the lean and physically active HCR were also more responsive to a standard calorie-restricted diet. Here, we assessed the ability of IF to induce weight loss on a background of high and low aerobic fitness accompanied by different levels of daily physical activity.

METHODS

Female HCR and LCR (8 per line) were subjected to IF (alternate-day fasting) for 14 weeks. Outcomes included changes in body weight, fat and lean mass, daily physical activity, and food and water intake. After initial measurements, IF was continued, and measurements were repeated after one year of IF.

RESULTS

All rats lost weight with IF, and LCR lost significantly more weight than HCR. This difference was primarily due to differential fat loss; loss of lean mass, on the other hand, was similar between HCR and LCR. Total food intake decreased with IF, and LCR showed lower intake than HCR only during the first 5 weeks of IF. Physical activity was suppressed by long-term IF. Physical activity increased on fed days compared to fasted days, and this pattern was more pronounced in HCR. The differential effects of IF in HCR and LCR persisted after one year of IF, with IF preventing the marked weight gain seen in ad libitum fed LCR during this time.

CONCLUSION

Weight and fat loss from IF was more pronounced in obesity-prone, low-aerobic capacity LCR, despite the low activity levels seen in these rats. The possibility that aerobic capacity modulates response to IF in human participants remains unexplored.

Night Migratory Songbirds Exhibit Metabolic Ability to Support High Aerobic Capacity during Migration

Aerobic metabolism in night migratory songbirds exhibit seasonal plasticity, which depends not only on annual life history stages (LHSs), viz., migratory/nonmigratory or breeding/nonbreeding, but also on the time of the day. Initially, we studied daily changes in behavior/physiology alongside aerobic metabolism intermediates using gas chromatography-mass spectrometry-based chemometric analyses of serum of migratory male redheaded buntings during low-energy wintering, that is, the nonmigrating LHS. Then, the metabolic phenotype of nonmigrating birds was compared with that of photostimulated migrating buntings, the latter representing the high-energy LHS. Diurnal changes such as daytime feeding and activity were reflected by increased fatty acid (FA, viz., palmitic, oleic, and linoleic acids) levels and protein catabolites, whereas higher night-time levels of short-chain FAs indicated lipolysis in night-fasted birds. High night-time levels of taurine, a sulfur amino acid, suggested the endogenous metabolite rendering an adaptive advantage to hyperglycaemic night migratory songbirds during the LHS with low daily energy expenditure. Conversely, migrating birds, largely night-active, exhibited higher circulatory FA, its mobilization, and increased aerobic catabolism, and the adipocyte-secreted lipid, palmitoylethanolamide (PEA), capable of activating the peroxisome proliferator-activated receptor α-PGCα axis, showed elevated levels throughout the day. PEA is known for anti-inflammatory and cannabinomimetic properties, and we show, for the first time, circadian changes in PEA levels in any migrating bird. Significantly higher levels of pyridoxal phosphate also suggested the bird's protective ability to combat metabolic stress through high aerobic capacity during migration. This study elucidates putative "serum biomarkers" with a protective role in stress accrued by enhanced aerobic capacity requirements at the organismal level.

Intrinsic Exercise Capacity and Mitochondrial DNA Lead to Opposing Vascular-Associated Risks

Exercise capacity is a strong predictor of all-cause morbidity and mortality in humans. However, the associated hemodynamic traits that link this valuable indicator to its subsequent disease risks are numerable. Additionally, exercise capacity has a substantial heritable component and genome-wide screening indicates a vast amount of nuclear and mitochondrial DNA (mtDNA) markers are significantly associated with traits of physical performance. A long-term selection experiment in rats confirms a divide for cardiovascular risks between low- and high-capacity runners (LCR and HCR, respectively), equipping us with a preclinical animal model to uncover new mechanisms. Here, we evaluated the LCR and HCR rat model system for differences in vascular function at the arterial resistance level. Consistent with the known divide between health and disease, we observed that LCR rats present with resistance artery and perivascular adipose tissue dysfunction compared to HCR rats that mimic qualities important for health, including improved vascular relaxation. Uniquely, we show by generating conplastic strains, which LCR males with mtDNA of female HCR (LCR-mt^HCR/Tol) present with improved vascular function. Conversely, HCR-mt^LCR/Tol rats displayed indices for cardiac dysfunction. The outcome of this study suggests that the interplay between the nuclear genome and the maternally inherited mitochondrial genome with high intrinsic exercise capacity is a significant factor for improved vascular physiology, and animal models developed on an interaction between nuclear and mtDNA are valuable new tools for probing vascular risk factors in the offspring.

Aerobic capacity modulates adaptive thermogenesis: Contribution of non-resting energy expenditure

Decreases in energy stores requires negative energy balance where caloric expenditure exceeds energy intake, which can induce adaptive thermogenesis-the reduction of energy expenditure (EE) beyond that accounted for by the weight lost. Adaptive thermogenesis varies between individuals. The component of total daily EE responsible for the interindividual variation in adaptive thermogenesis was investigated in this study, using a rat model that differs in obesity propensity and physical activity. Total daily EE and physical activity were examined before and after 21 days of 50% calorie restriction in male and female rats with lean and obesity-prone phenotypes-rats selectively bred for high and low intrinsic aerobic capacity (HCR and LCR, respectively). Calorie restriction significantly decreased EE more than was predicted by loss of weight and lean mass, demonstrating adaptive thermogenesis. Within sex, HCR and LCR did not significantly differ in resting EE. However, the calorie restriction-induced suppression in non-resting EE, which includes activity EE, was significantly greater in HCR than in LCR; this phenotypic difference was significant for both male and female rats. Calorie restriction also significantly suppressed physical activity levels more in HCR than LCR. When VO2max was assessed in male rats, calorie restriction significantly decreased O2 consumption without significantly affecting running performance (running time, distance), indicating increased energy efficiency. Percent weight loss did not significantly differ between groups. Altogether, these results suggest that individual differences in calorie restriction-induced adaptive thermogenesis may be accounted for by variation in aerobic capacity. Moreover, it is likely that activity EE, not resting or basal metabolism, may explain or predict the variation in individuals' adaptive thermogenesis.

Can Energetic Capacity Help Explain Why Physical Activity Reduces Cancer Risk?

Increased physical activity reduces cancer risk in humans, but why this whole-organism attribute reduces cancer remains unclear. Active individuals tend to have high capacity to generate energy on a sustained basis, which in turn can permit greater immune responses crucial for fighting emerging neoplasia. Thus, we suggest energetic capacity as a potential mechanism to explain the activity-cancer link, given that humans are intrinsically (not externally) energy limited. Human and rodent studies show that individuals with high energetic capacity mount greater immune responses and have lower cancer incidence; these trends persist after controlling for actual physical activity, supporting a direct role of energetic capacity. If true, exercise efforts might best target those that increase one's energetic capacity, which may be both individual and exercise specific.

Distribution of allele frequencies for genes associated with physical activity and/or physical capacity in a homogenous Norwegian cohort- a cross-sectional study

BACKGROUND

There are large individual differences in physical activity (PA) behavior as well as trainability of physical capacity. Heritability studies have shown that genes may have as much impact on exercise participation behavior as environmental factors. Genes that favor both trainability and participation may increase the levels of PA. The present study aimed to assess the allele frequencies in genes associated with PA and/or physical capacity, and to see if there is any association between these polymorphisms and self-reported PA levels in a cohort of middle-aged Norwegians of Scandinavian descent (n = 831; mean age mean age (± SD) 55.5 ± 3.8 years).

RESULTS

The genotype distributions of the ACTN3 R577X, ACE I/D and MAOA uVNTR polymorphisms were similar to other populations of European descent. When comparing the genotype distribution between the low/medium level PA group (LMPA) and high level PA groups (HPA), a significant difference in ACTN3 577X allele distribution was found. The X allele frequency was 10% lower in the HPA level group (P = 0.006). There were no differences in the genotype distribution of the ACE I/D or MAOA uVNTR polymorphism. Education and previous participation in sports or outdoor activities was positively associated with the self-reported PA levels (P ≤ 0.001).

CONCLUSIONS

To the best of our knowledge, this is the first study to report association between ACTN3 R577X genotype and PA level in middle-aged Scandinavians. Nevertheless, the contribution of a single polymorphism to a complex trait, like PA level, is likely small. Socioeconomic variables, as education and previous participation in sports or outdoor activities, are positively associated with the self-reported PA levels.

Physiological and Biochemical Effects of Intrinsically High and Low Exercise Capacities Through Multiomics Approaches

Regular exercise prevents lipid abnormalities and conditions such as diabetes mellitus, hypertension, and obesity; it considerably benefits sedentary individuals. However, individuals exhibit highly variable responses to exercise, probably due to genetic variations. Animal models are typically used to investigate the relationship of intrinsic exercise capacity with physiological, pathological, psychological, behavioral, and metabolic disorders. In the present study, we investigated differential physiological adaptations caused by intrinsic exercise capacity and explored the regulatory molecules or mechanisms through multiomics approaches. Outbred ICR mice (n = 100) performed an exhaustive swimming test and were ranked based on the exhaustive swimming time to distinguish intrinsically high- and low-capacity groups. Exercise performance, exercise fatigue indexes, glucose tolerance, and body compositions were assessed during the experimental processes. Furthermore, the gut microbiota, transcriptome, and proteome of soleus muscle with intrinsically high exercise capacity (HEC) and low exercise capacity (LEC) were further analyzed to reveal the most influential factors associated with differential exercise capacities. HEC mice outperformed LEC mice in physical activities (exhaustive swimming and forelimb grip strength tests) and exhibited higher glucose tolerance than LEC mice. Exercise-induced peripheral fatigue and the level of injury biomarkers (lactate, ammonia, creatine kinase, and aspartate aminotransferase) were also significantly lower in HEC mice than in LEC mice. Furthermore, the gut of the HEC mice contained significantly more Butyricicoccus than that of the LEC mice. In addition, transcriptome data of the soleus muscle revealed that the expression of microRNAs that are strongly associated with exercise performance-related physiological and metabolic functions (i.e., miR-383, miR-107, miR-30b, miR-669m, miR-191, miR-218, and miR-224) was higher in HEC mice than in LEC mice. The functional proteome data of soleus muscle indicated that the levels of key proteins related to muscle function and carbohydrate metabolism were also significantly higher in HEC mice than in LEC mice. Our study demonstrated that the mice with various intrinsic exercise capacities have different gut microbiome as well as transcriptome and proteome of soleus muscle by using multiomics approaches. The specific bacteria and regulatory factors, including miRNA and functional proteins, may be highly correlated with the adaptation of physiological functions and exercise capacity.

Housing conditions modulate spontaneous physical activity, feeding behavior, aerobic running capacity and adiposity in C57BL/6J mice

There is evidence of reduced adiposity in rodents living in a large cages (LC) as compared to animals housed in small cages (SC). Because spontaneous physical activity (SPA) provides an important portion of the total daily energy expenditure, an increase of SPA in rodents kept in LC could explain their reduced body fat accumulation. The relationship between SPA and components of physical fitness (i.e. aerobic and anaerobic fitness and body leanness) has not been previously determined. We examined the effects of eight weeks of LC exposure on SPA, body composition, feeding behavior, as well as aerobic and anaerobic running capacity in adult C57BL/6J mice. Male mice were housed in cages of two different sizes for 8 weeks: a small (SC, n = 10) and large (LC n = 10) cages with 1320 cm² and 4800 cm² floor space, respectively. SPA was measured gravimetrically, and food and water intake were recorded daily. Mice had critical velocity (CV) and anaerobic running capacity (ARC) evaluated at the beginning, middle course (4th week) and at the end of study (8th week). Despite non-significant differences in each week LC-mice were more active than SC-mice by considering all SPA values obtained in the entire period of 8 weeks. The difference in SPA over the whole day was mainly due to light phase activity, but also due to activity at dark period (from 6 pm to 9 pm and from 5 am to 6 am). LC-mice also exhibited higher food and water intake over the entire 8-wk period. LC-mice had lower content of fat mass (% of the eviscerated carcass) than SC-mice (SC: 8.4 ± 0.4 vs LC: 6.3 ± 0.3, p < 0.05). LC-mice also exhibited reduced epididymal fat pads (% of body mass) compared to SC-mice (SC: 1.3 ± 0.1 vs LC: 0.9 ± 0.1, p < 0.05) and retroperitoneal fat pads (SC: 0.4 ± 0.05 vs LC: 0.2 ± 0.02, p < 0.05). The LC-group showed significantly higher critical velocity than SC-group at the fourth week (SC: 14.9 ± 0.6 m·min^-1 vs LC: 18.0 ± 0.3 m·min^-1, p < 0.05) and eighth week (SC: 17.1 ± 0.5 m·min^-1 vs LC: 18.8 ± 0.6 m·min^-1, p < 0.05). Our findings demonstrate that eight weeks of LC housing increases SPA of C57BL/6J mice, and this may lead to reduced fat accumulation as well as higher aerobic fitness. Importantly, our study implies that SC limits SPA, possibly generating experimental artifacts in long-term rodent studies.

Metabolic Scope as a Proximate Constraint on Individual Behavioral Variation: Effects on Personality, Plasticity, and Predictability

Behavioral ecologists have hypothesized that among-individual differences in resting metabolic rate (RMR) may predict consistent individual differences in mean values for costly behaviors or for behaviors that affect energy intake rate. This hypothesis has empirical support and presently attracts considerable attention, but, notably, it does not provide predictions for individual differences in (a) behavioral plasticity or (b) unexplained variation (residual variation from mean individual behavior, here termed predictability). We outline how consideration of aerobic maximum metabolic rate (MMR) and particularly aerobic scope (= MMR - RMR) can be used to simultaneously make predictions about mean and among- and within-individual variation in behavior. We predict that while RMR should be proportional to an individual's mean level of sustained behavioral activity (one aspect of its personality), individuals with greater aerobic scope will also have greater scope to express behavioral plasticity and/or greater unpredictability in behavior (=greater residual variation). As a first step toward testing these predictions, we analyze existing activity data from selectively bred lines of mice that differ in both daily activity and aerobic scope. We find that replicate high-scope mice are more active on average and show greater among-individual variation in activity, greater among-individual variation in plasticity, and greater unpredictability. These data provide some tentative first support for our hypothesis, suggesting that further research on this topic would be valuable.

Inherently Lean Rats Have Enhanced Activity and Skeletal Muscle Response to Central Melanocortin Receptors

OBJECTIVE

Activity thermogenesis and energy expenditure (EE) are elevated in intrinsically lean rats (high-capacity runners [HCR]) and are also stimulated by melanocortin receptor activation in the ventromedial hypothalamus (VMH). This study determined whether HCR are more responsive to central modulation of activity EE compared with low-capacity runners (LCR).

METHODS

HCR and LCR rats received intra-VMH microinjections of melanotan II (MTII), a mixed melanocortin receptor agonist. Changes in EE, respiratory exchange ratio, activity EE, muscle heat, norepinephrine turnover, and muscle energetic modulators were compared.

RESULTS

HCR were significantly more responsive to intra-VMH MTII-induced changes in EE, activity EE, norepinephrine turnover to some muscle subgroups, and muscle mRNA expression of some energetic modulators. Though HCR had high muscle activity thermogenesis, limited MTII-induced modulation of muscle thermogenesis during activity was seen in LCR only.

CONCLUSIONS

An inherently lean, high-capacity rat phenotype showed elevated response to central melanocortin stimulation of activity EE and use of fat as fuel. This may be driven by sympathetic outflow to skeletal muscle, which was elevated after MTII. Central melanocortin receptor activation also altered skeletal muscle energetic modulators in a manner consistent with elevated EE and lowered respiratory exchange ratio.

Reprogramming of energy metabolism as a driver of aging

Aging is characterized by progressive loss of cellular function and integrity. It has been thought to be driven by stochastic molecular damage. However, genetic and environmental maneuvers enhancing mitochondrial function or inhibiting glycolysis extend lifespan and promote healthy aging in many species. In post-fertile Caenorhabditis elegans, a progressive decline in phosphoenolpyruvate carboxykinase with age, and a reciprocal increase in pyruvate kinase shunt energy metabolism from oxidative metabolism to anaerobic glycolysis. This reduces the efficiency and total of energy generation. As a result, energy-dependent physical activity and other cellular functions decrease due to unmatched energy demand and supply. In return, decrease in physical activity accelerates this metabolic shift, forming a vicious cycle. This metabolic event is a determinant of aging, and is retarded by caloric restriction to counteract aging. In this review, we summarize these and other evidence supporting the idea that metabolic reprogramming is a driver of aging. We also suggest strategies to test this hypothesis

Metabolic risk factors in mice divergently selected for BMR fed high fat and high carb diets

Factors affecting contribution of spontaneous physical activity (SPA; activity associated with everyday tasks) to energy balance of humans are not well understood, as it is not clear whether low activity is related to dietary habits, precedes obesity or is a result of thereof. In particular, human studies on SPA and basal metabolic rates (BMR, accounting for >50% of human energy budget) and their associations with diet composition, metabolic thrift and obesity are equivocal. To clarify these ambiguities we used a unique animal model-mice selected for divergent BMR rates (the H-BMR and L-BMR line type) presenting a 50% between-line type difference in the primary selected trait. Males of each line type were divided into three groups and fed either a high fat, high carb or a control diet. They then spent 4 months in individual cages under conditions emulating human "sedentary lifestyle", with SPA followed every month and measurements of metabolic risk indicators (body fat mass %, blood lipid profile, fasting blood glucose levels and oxidative damage in the livers, kidneys and hearts) taken at the end of study. Mice with genetically determined high BMR assimilated more energy and had higher SPA irrespective of type of diet. H-BMR individuals were characterized by lower dry body fat mass %, better lipid profile and lower fasting blood glucose levels, but higher oxidative damage in the livers and hearts. Genetically determined high BMR may be a protective factor against diet-induced obesity and most of the metabolic syndrome indicators. Elevated spontaneous activity is correlated with high BMR, and constitutes an important factor affecting individual capability to sustain energy balance even under energy dense diets.

Acclimation to hypoxia increases carbohydrate use during exercise in high-altitude deer mice

The low O₂ experienced at high altitude is a significant challenge to effective aerobic locomotion, as it requires sustained tissue O₂ delivery in addition to the appropriate allocation of metabolic substrates. Here, we tested whether high- and low-altitude deer mice (Peromyscus maniculatus) have evolved different acclimation responses to hypoxia with respect to muscle metabolism and fuel use during submaximal exercise. Using F₁ generation high- and low-altitude deer mice that were born and raised in common conditions, we assessed 1) fuel use during exercise, 2) metabolic enzyme activities, and 3) gene expression for key transporters and enzymes in the gastrocnemius. After hypoxia acclimation, highland mice showed a significant increase in carbohydrate oxidation and higher relative reliance on this fuel during exercise at 75% maximal O₂ consumption. Compared with lowland mice, highland mice had consistently higher activities of oxidative and fatty acid oxidation enzymes in the gastrocnemius. In contrast, only after hypoxia acclimation did activities of hexokinase increase significantly in the muscle of highland mice to levels greater than lowland mice. Highland mice also responded to acclimation with increases in muscle gene expression for hexokinase 1 and 2 genes, whereas both populations increased mRNA expression for glucose transporters. Changes in skeletal muscle with acclimation suggest that highland mice had an increased capacity for the uptake and oxidation of circulatory glucose. Our results demonstrate that highland mice have evolved a distinct mode of hypoxia acclimation that involves an increase in carbohydrate use during exercise.

Are Older Adults Physically Active Enough - A Matter of Assessment Method? The Generation 100 Study

Introduction

Physical activity (PA) is beneficial for general health. As a result, adults around the world are recommended to undertake regular PA of either absolute or relative intensity. Traditionally, adherence to PA recommendation is assessed by accelerometers that record absolute intensity thresholds. Since ageing often results in a decrease in cardiorespiratory fitness (CRF), older adults (aged > 65 years) might be more susceptible to not meeting the PA recommendation when measured in absolute terms. The aim of the present study was to compare the adherence to the PA recommendation using both absolute and relative thresholds. Additionally, we aimed to report the reference values for overall PA in a large sample of Norwegian older adults.

Methods

PA was assessed for 7 days using the Actigraph GT3X+ accelerometer in 1219 older adults (624 females) aged 70–77 years. Overall PA was measured as counts per minute (CPM) and steps. Absolute and relative moderate-to-vigorous PA (MVPA) thresholds were applied to quantify adherence to PA recommendation. The relative MVPA thresholds were developed specifically for the Generation 100 population sample. CRF was directly measured as peak oxygen uptake (VO_2peak).

Results

Proportions meeting PA recommendation were 29% and 71% when utilizing absolute and relative MVPA, respectively. More females met the relative PA recommendation compared to males. Overall PA was higher among the youngest age group. Older adults with medium- and high levels of CRF were more physically active, compared to those with the lowest levels of CRF.

Conclusion

This is the first study to compare adherence to PA recommendation, using absolute and relative intensity thresholds among older adults. The present study clearly illustrates the consequences of using different methodological approaches to surveillance of PA across age, gender and CRF in a population of older adults.

Ventromedial hypothalamic melanocortin receptor activation: regulation of activity energy expenditure and skeletal muscle thermogenesis

KEY POINTS

The ventromedial hypothalamus (VMH) and the central melanocortin system both play vital roles in regulating energy balance by modulating energy intake and utilization. Recent evidence suggests that activation of the VMH alters skeletal muscle metabolism. We show that intra-VMH melanocortin receptor activation increases energy expenditure and physical activity, switches fuel utilization to fats, and lowers work efficiency such that excess calories are dissipated by skeletal muscle as heat. We also show that intra-VMH melanocortin receptor activation increases sympathetic nervous system outflow to skeletal muscle. Intra-VMH melanocortin receptor activation also induced significant changes in the expression of mediators of energy expenditure in muscle. These results support the role of melanocortin receptors in the VMH in the modulation of skeletal muscle metabolism.

ABSTRACT

The ventromedial hypothalamus (VMH) and the brain melanocortin system both play vital roles in increasing energy expenditure (EE) and physical activity, decreasing appetite and modulating sympathetic nervous system (SNS) outflow. Because of recent evidence showing that VMH activation modulates skeletal muscle metabolism, we propose the existence of an axis between the VMH and skeletal muscle, modulated by brain melanocortins, modelled on the brain control of brown adipose tissue. Activation of melanocortin receptors in the VMH of rats using a non-specific agonist melanotan II (MTII), compared to vehicle, increased oxygen consumption and EE and decreased the respiratory exchange ratio. Intra-VMH MTII enhanced activity-related EE even when activity levels were held constant. MTII treatment increased gastrocnemius muscle heat dissipation during controlled activity, as well as in the home cage. Compared to vehicle-treated rats, rats with intra-VMH melanocortin receptor activation had higher skeletal muscle norepinephrine turnover, indicating an increased SNS drive to muscle. Lastly, intra-VMH MTII induced mRNA expression of muscle energetic mediators, whereas short-term changes at the protein level were primarily limited to phosphorylation events. These results support the hypothesis that melanocortin peptides act in the VMH to increase EE by lowering the economy of activity via the enhanced expression of mediators of EE in the periphery including skeletal muscle. The data are consistent with the role of melanocortins in the VMH in the modulation of skeletal muscle metabolism.

c.A2456C-substitution in Pck1 changes the enzyme kinetic and functional properties modifying fat distribution in pigs

Cytosolic phosphoenolpyruvate carboxykinase, PCK1, is one of the main regulatory enzymes of gluconeogenesis and glyceroneogenesis. The substitution of a single amino acid (Met139Leu) in PCK1 as a consequence of a single nucleotide polymorphism (SNP), c.A2456C, is associated in the pig to a negative phenotype characterized by reduced intramuscular fat content, enhanced backfat thickness and lower meat quality. The p.139L enzyme shows reduced k_cat values in the glyceroneogenic direction and enhanced ones in the anaplerotic direction. Accordingly, the expression of the p.139L isoform results in about 30% lower glucose and 9% lower lipid production in cell cultures. Moreover, the ability of this isoform to be acetylated is also compromised, what would increase its susceptibility to be degraded in vivo by the ubiquitin-proteasome system. The high frequency of the c.2456C allele in modern pig breeds implies that the benefits of including c.A2456C SNP in selection programs could be considerable.

Reciprocal Changes in Phosphoenolpyruvate Carboxykinase and Pyruvate Kinase with Age Are a Determinant of Aging in Caenorhabditis elegans

Aging involves progressive loss of cellular function and integrity, presumably caused by accumulated stochastic damage to cells. Alterations in energy metabolism contribute to aging, but how energy metabolism changes with age, how these changes affect aging, and whether they can be modified to modulate aging remain unclear. In locomotory muscle of post-fertile Caenorhabditis elegans, we identified a progressive decrease in cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), a longevity-associated metabolic enzyme, and a reciprocal increase in glycolytic pyruvate kinase (PK) that were necessary and sufficient to limit lifespan. Decline in PEPCK-C with age also led to loss of cellular function and integrity including muscle activity, and cellular senescence. Genetic and pharmacologic interventions of PEPCK-C, muscle activity, and AMPK signaling demonstrate that declines in PEPCK-C and muscle function with age interacted to limit reproductive life and lifespan via disrupted energy homeostasis. Quantifications of metabolic flux show that reciprocal changes in PEPCK-C and PK with age shunted energy metabolism toward glycolysis, reducing mitochondrial bioenergetics. Last, calorie restriction countered changes in PEPCK-C and PK with age to elicit anti-aging effects via TOR inhibition. Thus, a programmed metabolic event involving PEPCK-C and PK is a determinant of aging that can be modified to modulate aging.

Contribution of regional brain melanocortin receptor subtypes to elevated activity energy expenditure in lean, active rats

Physical activity and non-exercise activity thermogenesis (NEAT) are crucial factors accounting for individual differences in body weight, interacting with genetic predisposition. In the brain, a number of neuroendocrine intermediates regulate food intake and energy expenditure (EE); this includes the brain melanocortin (MC) system, consisting of MC peptides as well as their receptors (MCR). MC3R and MC4R have emerged as critical modulators of EE and food intake. To determine how variance in MC signaling may underlie individual differences in physical activity levels, we examined behavioral response to MC receptor agonists and antagonists in rats that show high and low levels of physical activity and NEAT, that is, high- and low-capacity runners (HCR, LCR), developed by artificial selection for differential intrinsic aerobic running capacity. Focusing on the hypothalamus, we identified brain region-specific elevations in expression of MCR 3, 4, and also MC5R, in the highly active, lean HCR relative to the less active and obesity-prone LCR. Further, the differences in activity and associated EE as a result of MCR activation or suppression using specific agonists and antagonists were similarly region-specific and directly corresponded to the differential MCR expression patterns. The agonists and antagonists investigated here did not significantly impact food intake at the doses used, suggesting that the differential pattern of receptor expression may by more meaningful to physical activity than to other aspects of energy balance regulation. Thus, MCR-mediated physical activity may be a key neural mechanism in distinguishing the lean phenotype and a target for enhancing physical activity and NEAT.

Effect of Change in VO2max on Daily Total Energy Expenditure in a Cohort of Norwegian Men: A Randomized Pilot Study

OBJECTIVE

To investigate how a change in VO2max induced through 6 weeks of high intensity aerobic interval training affects daily total energy expenditure (TEE), active energy expenditure (AEE) and mitochondrial function in people not previously exposed to structured high intensity aerobic interval training (AIT).

METHODS

Thirty healthy males (39±6 yrs) not exposed to structured exercise training were randomized to either 1x4 min AIT (1-AIT), 4x4 min AIT (4-AIT), both at 90-95% maximum heart rate (HRmax) or 47 min of MCT at 70% HRmax. TEE, AEE, number of steps, active time, sedentary time, VO2max and mitochondrial function in m. vastus lateralis were measured before and after intervention.

RESULTS

TEE increased 14% (p=0.014) and AEE increased 43% (p= 0.004) after MCT. There was no change in TEE or AEE after 1-AIT or 4-AIT, but 1-AIT had significantly lower TEE (p=0.033) and step-count (p=0.011) compared to MCT post intervention. VO2max increased 7% after 1-AIT (p= 0.004) and 9% after 4-AIT (p=0.004), with no change after MCT. No change was observed in maximal mitochondrial respiration (VMAX) or Citrate Synthase (CS) activity within or between interventions. Basal respiration (V0) increased after 1-AIT (p=0.029) and 4-AIT (p=0.022), with no significant change after MCT.

CONCLUSION

AIT interventions that increase VO2max, do not stimulate subjects to increase TEE or AEE. The intensity of exercise seems to play apart, as MCT increased TEE and AEE and AIT did not. Emphasis should be placed on the importance of maintaining everyday activities when introducing structured exercise training to untrained individuals.

Physically active rats lose more weight during calorie restriction

Daily physical activity shows substantial inter-individual variation, and low physical activity is associated with obesity and weight gain. Elevated physical activity is also associated with high intrinsic aerobic capacity, which confers considerable metabolic health benefits. Rats artificially selected for high intrinsic aerobic capacity (high-capacity runners, HCR) are more physically active than their low-capacity counterparts (low-capacity runners, LCR). To test the hypothesis that physical activity counters metabolic thriftiness, we measured physical activity and weight loss during three weeks of 50% calorie restriction (CR) in the HCR and LCR rat lines. At baseline, HCR ate more and were more active than LCR; this was seen in male rats, where LCR are considerably heavier than HCR, as well as in a set of female rats where body weight did not differ between the lines, demonstrating that this effect is consistent across sex and not secondary to body weight. We show for the first time that HCR lose more weight than LCR relative to baseline. Physical activity levels declined throughout CR, and this was more pronounced in HCR than in LCR, yet some aspects of activity remained elevated in HCR relative to LCR even during CR. This is consistent with the idea that low physical activity contributes to metabolic thriftiness during food restriction, allowing LCR to defend body mass, particularly lean mass. This has implications for physical activity during diet-induced weight loss, the genetic underpinnings of individual differences in weight loss during a diet, and the potential evolutionary opposition between metabolic thriftiness and aerobic capacity.

Potential role of branched-chain amino acid catabolism in regulating fat oxidation

Insulin-resistant or obese individuals have increased serum branched-chain amino acid (BCAA) levels. Recent findings relate increased BCAA catabolism to increased fatty acid oxidation and better metabolic health in physically active individuals. We hypothesize that, via glyceroneogenesis, BCAA catabolism mediates increased constitutive use of fatty acids for β-oxidation in subjects with increased inherent or acquired aerobic capacity both during exercise and at rest.

Leanness and heightened nonresting energy expenditure: role of skeletal muscle activity thermogenesis

A high-calorie diet accompanied by low levels of physical activity (PA) accounts for the widespread prevalence of obesity today, and yet some people remain lean even in this obesogenic environment. Here, we investigate the cause for this exception. A key trait that predicts high PA in both humans and laboratory rodents is intrinsic aerobic capacity. Rats artificially selected as high-capacity runners (HCR) are lean and consistently more physically active than their low-capacity runner (LCR) counterparts; this applies to both males and females. Here, we demonstrate that HCR show heightened total energy expenditure (TEE) and hypothesize that this is due to higher nonresting energy expenditure (NREE; includes activity EE). After matching for body weight and lean mass, female HCR consistently had heightened nonresting EE, but not resting EE, compared with female LCR. Because of the dominant role of skeletal muscle in nonresting EE, we examined muscle energy use. We found that lean female HCR had higher muscle heat dissipation during activity, explaining their low economy of activity and high activity EE. This may be due to the amplified skeletal muscle expression levels of proteins involved in EE and reduced expression levels of proteins involved in energy conservation in HCR relative to LCR. This is also associated with an increased sympathetic drive to skeletal muscle in HCR compared with LCR. We find little support for the hypothesis that resting metabolic rate is correlated with maximal aerobic capacity if body size and composition are fully considered; rather, the critical factor appears to be activity thermogenesis.

Mitochondrial function in metabolic health: a genetic and environmental tug of war

BACKGROUND

The increased prevalence of obesity and its co-morbidities and their strong association with inactivity have produced an 'exercise-deficient phenotype' in which individuals with a particular combination of disease-susceptible genes collide with environmental influences to cross a biological 'threshold' that ultimately manifests as overt clinical conditions (i.e., risk-factors for disease states). These risk-factors have been linked to impairments in skeletal muscle mitochondrial function.

SCOPE OF REVIEW

The question of whether 'inborn' mitochondrial deficiencies and/or defective mitochondrial metabolism contribute to metabolic disease, or if environmental factors are the major determinant, will be examined.

MAJOR CONCLUSIONS

We contend that impaired whole-body insulin resistance along with impaired skeletal muscle handling of carbohydrate and lipid fuels (i.e., metabolic inflexibility) is associated with a reduced skeletal muscle mitochondrial content which, in large part, is a maladaptive response to an 'inactivity cycle' which predisposes to a reduced level of habitual physical activity. While genetic components play a role in the pathogenesis of metabolic disease, exercise is a powerful environmental stimulus capable of restoring the metabolic flexibility of fuel selection and reduces risk-factors for metabolic disease in genetically-susceptible individuals.

GENERAL SIGNIFICANCE

Given the apathy towards voluntary physical activity in most Western societies, it is clear that there is an urgent need for innovative, clinically-effective exercise strategies, coupled with changes in current attitudes and methods of delivering exercise prescription and dietary advice, in order to improve metabolic health and reduce metabolic disease risk at the population level. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Region-specific differences in brain melanocortin receptors in rats of the lean phenotype

The brain melanocortin (MC) system is one of numerous overlapping systems regulating energy balance; it consists of peptides including α-melanocyte-stimulating hormone that act through melanocortin receptors (MCRs). Mutations and polymorphisms in MC3R and MC4R have been identified as one of the most common genetic contributors to obesity in human studies. Brain MC3R and MC4R are known to modulate energy expenditure (EE) and food intake, but much less is known regarding brain MC5R. To test the hypothesis that brain MC modulates physical activity (PA) and EE, we compared brain MCR profiles in rats that consistently show high versus low levels of 'spontaneous' daily PA. Compared with low-activity rats, high-activity rats show enhanced mRNA expression of MCRs in the brain, specifically of MC3R in the paraventricular nucleus (PVN), and MC4R and MC5R in the perifornical lateral hypothalamus. Next, we microinjected the MCR agonist melanotan II into the PVN region and measured PA and EE. Intra-PVN melanotan II induced a dose-dependent increase in PA and this effect was greater in high-activity rats compared with low-activity rats. These results indicate region-specific brain MCR expression in the heightened PA seen in association with high endurance capacity and identify promising targets in the brain MC system that may contribute to interindividual variability in energy balance.

Divergent skeletal muscle respiratory capacities in rats artificially selected for high and low running ability: a role for Nor1?

Inactivity-related diseases are becoming a huge burden on Western society. While there is a major environmental contribution to metabolic health, the intrinsic properties that predispose or protect against particular health traits are harder to define. We used rat models of inborn high running capacity (HCR) and low running capacity (LCR) to determine inherent differences in mitochondrial volume and function, hypothesizing that HCR rats would have greater skeletal muscle respiratory capacity due to an increase in mitochondrial number. Additionally, we sought to determine if there was a link between the expression of the orphan nuclear receptor neuron-derived orphan receptor (Nor)1, a regulator of oxidative metabolism, and inherent skeletal muscle respiratory capacity. LCR rats were 28% heavier (P < 0.0001), and fasting serum insulin concentrations were 62% greater than in HCR rats (P = 0.02). In contrast, HCR rats had better glucose tolerance and reduced adiposity. In the primarily oxidative soleus muscle, maximal respiratory capacity was 21% greater in HCR rats (P = 0.001), for which the relative contribution of fat oxidation was 20% higher than in LCR rats (P = 0.02). This was associated with increased citrate synthase (CS; 33%, P = 0.009) and β-hydroxyacyl-CoA (β-HAD; 33%, P = 0.0003) activities. In the primarily glycolytic extensor digitum longus muscle, CS activity was 29% greater (P = 0.01) and β-HAD activity was 41% (P = 0.0004) greater in HCR rats compared with LCR rats. Mitochondrial DNA copy numbers were also elevated in the extensor digitum longus muscles of HCR rats (35%, P = 0.049) and in soleus muscles (44%, P = 0.16). Additionally, HCR rats had increased protein expression of individual mitochondrial respiratory complexes, CS, and uncoupling protein 3 in both muscle types (all P < 0.05). In both muscles, Nor1 protein was greater in HCR rats compared with LCR rats (P < 0.05). We propose that the differential expression of Nor1 may contribute to the differences in metabolic regulation between LCR and HCR phenotypes.

Brain orexin promotes obesity resistance

Resistance to obesity is becoming an exception rather than the norm, and understanding mechanisms that lead some to remain lean in spite of an obesigenic environment is critical if we are to find new ways to reverse this trend. Levels of energy intake and physical activity both contribute to body weight management, but it is challenging for most to adopt major long-term changes in either factor. Physical activity outside of formal exercise, also referred to as activity of daily living, and in stricter form, spontaneous physical activity (SPA), may be an attractive modifiable variable for obesity prevention. In this review, we discuss individual variability in SPA and NEAT (nonexercise thermogenesis, or the energy expended by SPA) and its relationship to obesity resistance. The hypothalamic neuropeptide orexin (hypocretin) may play a key role in regulating SPA and NEAT. We discuss how elevated orexin signaling capacity, in the context of a brain network modulating SPA, may play a major role in defining individual variability in SPA and NEAT. Greater activation of this SPA network leads to a lower propensity for fat mass gain and therefore may be an attractive target for obesity prevention and therapy.

TRPV4 deficiency increases skeletal muscle metabolic capacity and resistance against diet-induced obesity

Transient receptor potential channel V4 (TRPV4) functions as a nonselective cation channel in various cells and plays physiological roles in osmotic and thermal sensation. However, the function of TRPV4 in energy metabolism is unknown. Here, we report that TRPV4 deficiency results in increased muscle oxidative capacity and resistance to diet-induced obesity in mice. Although no difference in body weight was observed between wild-type and Trpv4−/− mice when fed a standard chow diet, obesity phenotypes induced by a high-fat diet were significantly improved in Trpv4−/− mice, without any change in food intake. Quantitative analysis of mRNA revealed the constitutive upregulation of many genes, including those for transcription factors such as peroxisome proliferator-activated receptor α and for metabolic enzymes such as phosphoenolpyruvate carboxykinase. These upregulated genes were especially prominent in oxidative skeletal muscle, in which the activity of Ca2+-dependent phosphatase calcineurin was elevated, suggesting that other Ca2+ channels function in the skeletal muscle of Trpv4−/− mice. Indeed, gene expressions for TRPC3 and TRPC6 increased in the muscles of Trpv4−/− mice compared with those of wild-type mice. The number of oxidative type I fiber also increased in the mutant muscles following myogenin gene induction. These results strongly suggested that inactivation of Trpv4 induces compensatory increases in TRPC3 and TRPC6 production, and elevation of calcineurin activity, affecting energy metabolism through increased expression of genes involved in fuel oxidation in skeletal muscle and thereby contributing to increased energy expenditure and protection from diet-induced obesity in mice.

Suppression of skeletal muscle turnover in cancer cachexia: evidence from the transcriptome in sequential human muscle biopsies

PURPOSE

The mechanisms underlying muscle wasting in patients with cancer remain poorly understood, and consequently there remains an unmet clinical need for new biomarkers and treatment strategies.

EXPERIMENTAL DESIGN

Microarrays were used to examine the transcriptome in single biopsies from healthy controls (n = 6) and in paired biopsies [pre-resection baseline (weight-loss 7%) and 8 month post-resection follow-up (disease-free/weight-stable for previous 2 months)] from quadriceps muscle of patients with upper gastrointestinal cancer (UGIC; n = 12).

RESULTS

Before surgery, 1,868 genes were regulated compared with follow-up (false discovery rate, 6%). Ontology analysis showed that regulated genes belonged to both anabolic and catabolic biologic processes with overwhelming downregulation in baseline samples. No literature-derived genes from preclinical cancer cachexia models showed higher expression in baseline muscle. Comparison with healthy control muscle (n = 6) revealed that despite differences in the transcriptome at baseline (941 genes regulated), the muscle of patients at follow-up was similar to control muscle (2 genes regulated). Physical activity (step count per day) did not differ between the baseline and follow-up periods (P = 0.9), indicating that gene expression differences reflected the removal of the cancer rather than altered physical activity levels. Comparative gene expression analysis using exercise training signatures supported this interpretation.

CONCLUSIONS

Metabolic and protein turnover-related pathways are suppressed in weight-losing patients with UGIC whereas removal of the cancer appears to facilitate a return to a healthy state, independent of changes in the level of physical activity.

The use of a running wheel to measure activity in rodents: relationship to energy balance, general activity, and reward

Running wheels are commonly employed to measure rodent physical activity in a variety of contexts, including studies of energy balance and obesity. There is no consensus on the nature of wheel-running activity or its underlying causes, however. Here, we will begin by systematically reviewing how running wheel availability affects physical activity and other aspects of energy balance in laboratory rodents. While wheel running and physical activity in the absence of a wheel commonly correlate in a general sense, in many specific aspects the two do not correspond. In fact, the presence of running wheels alters several aspects of energy balance, including body weight and composition, food intake, and energy expenditure of activity. We contend that wheel-running activity should be considered a behavior in and of itself, reflecting several underlying behavioral processes in addition to a rodent's general, spontaneous activity. These behavioral processes include defensive behavior, predatory aggression, and depression- and anxiety-like behaviors. As it relates to energy balance, wheel running engages several brain systems-including those related to the stress response, mood, and reward, and those responsive to growth factors-that influence energy balance indirectly. We contend that wheel-running behavior represents factors in addition to rodents' tendency to be physically active, engaging additional neural and physiological mechanisms which can then independently alter energy balance and behavior. Given the impact of wheel-running behavior on numerous overlapping systems that influence behavior and physiology, this review outlines the need for careful design and interpretation of studies that utilize running wheels as a means for exercise or as a measurement of general physical activity.

Osteoblast response to ovariectomy is enhanced in intrinsically high aerobic-capacity rats

The role of exercise in promoting bone health is typically attributed to increased mechanical loading, which induces functional adaptation. Recent evidence suggests that habitual aerobic exercise has influence at the cellular level as well. The effect of aerobic capacity on osteoblast-lineage cell differentiation and function as well as skeletal phenotype is unknown. Using a rat model of high-capacity and low-capacity runners (HCRs and LCRs, respectively), in which an intrinsic functional genomic difference in aerobic capacity exists between nontrained animals, this study evaluated the effects of aerobic capacity on measures of bone mass and strength as well as osteoblast activity following ovariectomy. The ovariectomized rat emulates the clinical features of the estrogen-depleted human skeleton and represents a valuable model for studying short-term upregulation of osteoblast activity. We hypothesized that intrinsically high aerobic capacity would augment osteoblast response, which would mitigate the deleterious effects of hormone withdrawal. Femora and tibiae were assessed by micro-computed tomography, mechanical testing, and dynamic histomorphometry. HCRs had enhanced femoral tissue mineral density and estimated elastic modulus relative to LCRs. At 4 weeks postovariectomy, HCRs demonstrated a more robust osteoblast response. Markers of bone formation were upregulated to a greater extent in HCRs than LCRs, suggesting a role for aerobic capacity in governing osteoblast activity. Results from this and future studies will help to identify the influence of cellular aerobic metabolism on bone health, which may lead to new strategies for targeting diseases of the skeleton.

The biological control of voluntary exercise, spontaneous physical activity and daily energy expenditure in relation to obesity: human and rodent perspectives

Mammals expend energy in many ways, including basic cellular maintenance and repair, digestion, thermoregulation, locomotion, growth and reproduction. These processes can vary tremendously among species and individuals, potentially leading to large variation in daily energy expenditure (DEE). Locomotor energy costs can be substantial for large-bodied species and those with high-activity lifestyles. For humans in industrialized societies, locomotion necessary for daily activities is often relatively low, so it has been presumed that activity energy expenditure and DEE are lower than in our ancestors. Whether this is true and has contributed to a rise in obesity is controversial. In humans, much attention has centered on spontaneous physical activity (SPA) or non-exercise activity thermogenesis (NEAT), the latter sometimes defined so broadly as to include all energy expended due to activity, exclusive of volitional exercise. Given that most people in Western societies engage in little voluntary exercise, increasing NEAT may be an effective way to maintain DEE and combat overweight and obesity. One way to promote NEAT is to decrease the amount of time spent on sedentary behaviours (e.g. watching television). The effects of voluntary exercise on other components of physical activity are highly variable in humans, partly as a function of age, and have rarely been studied in rodents. However, most rodent studies indicate that food consumption increases in the presence of wheels; therefore, other aspects of physical activity are not reduced enough to compensate for the energetic cost of wheel running. Most rodent studies also show negative effects of wheel access on body fat, especially in males. Sedentary behaviours per se have not been studied in rodents in relation to obesity. Several lines of evidence demonstrate the important role of dopamine, in addition to other neural signaling networks (e.g. the endocannabinoid system), in the control of voluntary exercise. A largely separate literature points to a key role for orexins in SPA and NEAT. Brain reward centers are involved in both types of physical activities and eating behaviours, likely leading to complex interactions. Moreover, voluntary exercise and, possibly, eating can be addictive. A growing body of research considers the relationships between personality traits and physical activity, appetite, obesity and other aspects of physical and mental health. Future studies should explore the neurobiology, endocrinology and genetics of physical activity and sedentary behaviour by examining key brain areas, neurotransmitters and hormones involved in motivation, reward and/or the regulation of energy balance.

Disrupted sleep and delayed recovery from chronic peripheral neuropathy are distinct phenotypes in a rat model of metabolic syndrome

BACKGROUND

Sleep apnea, hypertension, atherosclerosis, and obesity are features of metabolic syndrome associated with decreased restorative sleep and increased pain. These traits are relevant for anesthesiology because they confer increased risks of a negative anesthetic outcome. This study tested the one-tailed hypothesis that rats bred for low intrinsic aerobic capacity have enhanced nociception and disordered sleep.

METHODS

Rats were developed from a breeding strategy that selected for low aerobic capacity runners (LCR) and high aerobic capacity runners (HCR). Four phenotypes were quantified. Rats underwent von Frey sensory testing (n = 12), thermal nociceptive testing (n = 12), electrographic recordings of sleep and wakefulness (n = 16), and thermal nociceptive testing (n = 14) before and for 6 weeks after a unilateral chronic neuropathy of the sciatic nerve.

RESULTS

Paw withdrawal latency to a thermal nociceptive stimulus was significantly (P < 0.01) lower in LCR than HCR rats. There were also significant differences in sleep, with LCR rats spending significantly (P < 0.01) more time awake (18%) and less time in nonrapid eye movement sleep (-19%) than HCR rats. Nonrapid eye movement sleep episodes were of shorter duration (-34%) in LCR than HCR rats. Rapid eye movement sleep of LCR rats was significantly more fragmented than rapid eye movement sleep of HCR rats. LCR rats required 2 weeks longer than HCR rats to recover from peripheral neuropathy.

CONCLUSIONS

Rodents with low aerobic capacity exhibit features homologous to human metabolic syndrome. This rodent model offers a novel tool for characterizing the mechanisms through which low aerobic function and obesity might confer increased risks for anesthesia.

Spontaneous activity, economy of activity, and resistance to diet-induced obesity in rats bred for high intrinsic aerobic capacity

Though obesity is common, some people remain resistant to weight gain even in an obesogenic environment. The propensity to remain lean may be partly associated with high endurance capacity along with high spontaneous physical activity and the energy expenditure of activity, called non-exercise activity thermogenesis (NEAT). Previous studies have shown that high-capacity running rats (HCR) are lean compared to low-capacity runners (LCR), which are susceptible to cardiovascular disease and metabolic syndrome. Here, we examine the effect of diet on spontaneous activity and NEAT, as well as potential mechanisms underlying these traits, in rats selectively bred for high or low intrinsic aerobic endurance capacity. Compared to LCR, HCR were resistant to the sizeable increases in body mass and fat mass induced by a high-fat diet; HCR also had lower levels of circulating leptin. HCR were consistently more active than LCR, and had lower fuel economy of activity, regardless of diet. Nonetheless, both HCR and LCR showed a similar decrease in daily activity levels after high-fat feeding, as well as decreases in hypothalamic orexin-A content. The HCR were more sensitive to the NEAT-activating effects of intra-paraventricular orexin-A compared to LCR, especially after high-fat feeding. Lastly, levels of cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) in the skeletal muscle of HCR were consistently higher than LCR, and the high-fat diet decreased skeletal muscle PEPCK-C in both groups of rats. Differences in muscle PEPCK were not secondary to the differing amount of activity. This suggests the possibility that intrinsic differences in physical activity levels may originate at the level of the skeletal muscle, which could alter brain responsiveness to neuropeptides and other factors that regulate spontaneous daily activity and NEAT.

Gene expression centroids that link with low intrinsic aerobic exercise capacity and complex disease risk

A strong link exists between low aerobic exercise capacity and complex metabolic diseases. To probe this linkage, we utilized rat models of low and high intrinsic aerobic endurance running capacity that differ also in the risk for metabolic syndrome. We investigated in skeletal muscle gene-phenotype relationships that connect aerobic endurance capacity with metabolic disease risk factors. The study compared 12 high capacity runners (HCRs) and 12 low capacity runners (LCRs) from generation 18 of selection that differed by 615% for maximal treadmill endurance running capacity. On average, LCRs were heavier and had increased blood glucose, insulin, and triglycerides compared with HCRs. HCRs were higher for resting metabolic rate, voluntary activity, serum high density lipoproteins, muscle capillarity, and mitochondrial area. Bioinformatic analysis of skeletal muscle gene expression data revealed that many genes up-regulated in HCRs were related to oxidative energy metabolism. Seven mean mRNA expression centroids, including oxidative phosphorylation and fatty acid metabolism, correlated significantly with several exercise capacity and disease risk phenotypes. These expression-phenotype correlations, together with diminished skeletal muscle capillarity and mitochondrial area in LCR rats, support the general hypothesis that an inherited intrinsic aerobic capacity can underlie disease risks.—Kivelä, R., Silvennoinen, M., Lehti, M., Rinnankoski-Tuikka, R., Purhonen, T., Ketola, T., Pullinen, K., Vuento, M., Mutanen, N., Sartor, M. A., Reunanen, H., Koch, L. G., Britton, S. L., Kainulainen, H. Gene expression centroids that link with low intrinsic aerobic exercise capacity and complex disease risk.

Phenotypic and evolutionary plasticity of body composition in rats selectively bred for high endurance capacity

We investigated the effects of genetic selection and prolonged wheel access (8 wk) on food consumption and body composition in lines of rats selected for high and low intrinsic (untrained) endurance running capacity (HCR and LCR, respectively) to test the generality of phenotypic correlations between physical activity levels, aerobic capacity, and body composition. HCR rats ran more minutes per day on activity wheels than LCR rats, supporting the hypothesis that voluntary activity and physiological capacity are genetically correlated (self-induced adaptive plasticity). Both treatments (selection and wheel access) significantly affected food consumption. HCR rats consumed and digested more food than LCR rats. Access to running wheels did not result in changes in overall body mass, but lean body mass increased and percent body fat decreased in both lines. Selection for high endurance capacity resulted in hypertrophy of the heart and kidneys and decreased long intestine length. We found significant phenotypic flexibility in a number of organ masses after wheel running. Specifically, access to running wheels resulted in hypertrophy of the heart, liver, kidney, stomach, and small and large intestines in LCR and HCR rats. The selected line×wheel access interaction was significantly greater in HCR rats in relative mass for the heart and lung. Compared with LCR rats, HCR rats fortify wheel running with increased food consumption along with greater hypertrophy of key organs for O2 transport.

Wheel running, skeletal muscle aerobic capacity and 1-methyl-1-nitrosourea induced mammary carcinogenesis in the rat

Emerging evidence indicates that intrinsic differences and induced changes in aerobic capacity are probably to play a critical role in the development of chronic diseases like cancer. This study was initiated: (i) to determine how citrate synthase activity, which is routinely used as a marker of aerobic capacity and mitochondrial density in skeletal muscle, was affected by voluntary running on either a motorized activity wheel or a non-motorized free wheel and (ii) to investigate the association between aerobic capacity and the carcinogenic response induced in the mammary gland by intraperitoneal injection of 1-methyl-1-nitrosurea. Overall, wheel running reduced cancer incidence (96 versus 72%, P = 0.0006) and the number of cancers per animal (2.84 versus 1.78, P < 0.0001) and induced citrate synthase activity (276 versus 353 U/mg, P < 0.0001, sedentary control versus wheel running,respectively). Both motorized and free wheel running increased citrate synthase activity (373 +/- 24, 329 +/- 11 and 276 +/- 9 U/mg protein, P < 0.0001) and reduced the average number of cancers per rat (2.84, 1.96 and 1.63, P < 0.01), sedentary control, free wheel and motorized wheel, respectively. However, regression analyses failed to provide evidence of a significant association between citrate synthase activity and either cancer incidence or cancer multiplicity. Citrate synthase activity is a single measure in a complex pathway that determines aerobic capacity. The multifaceted nature of intrinsic and inducible aerobic capacity limits the usefulness of citrate synthase activity alone in elucidating the relationship between aerobic capacity and the carcinogenic response.