Selectively breeding for high voluntary physical activity in female mice does not bestow inherent characteristics that resemble eccentric remodeling of the heart, but the mini-muscle phenotype does

Physical activity engagement results in a variety of positive health outcomes, including a reduction in cardiovascular disease risk partially due to eccentric remodeling of the heart. The purpose of this investigation was to determine if four replicate lines of High Runner mice that have been selectively bred for voluntary exercise on wheels have a cardiac phenotype that resembles the outcome of eccentric remodeling. Adult females (average age 55 days) from the 4 High Runner and 4 non-selected control lines were anaesthetized via vaporized isoflurane, then echocardiographic images were collected and analyzed for structural and functional differences. High Runner mice in general had lower ejection fractions compared to control mice lines (2-tailed p ​= ​0.023 6) and tended to have thicker walls of the anterior portion of the left ventricle (p ​= ​0.065). However, a subset of the High Runner individuals, termed mini-muscle mice, had greater ejection fraction (p ​= ​0.000 6), fractional shortening percentage (p ​< ​0.000 1), and ventricular mass at dissection (p ​< ​0.002 7 with body mass as a covariate) compared to non-mini muscle mice. Mice from replicate lines bred for high voluntary exercise did not all have inherent positive cardiac functional or structural characteristics, although a genetically unique subset of mini-muscle individuals did have greater functional cardiac characteristics, which in conjunction with their previously described peripheral aerobic enhancements (e.g., increased capillarity) would partially account for their increased V ˙ O2max.

Hippocampal, Whole Midbrain, Red Nucleus, and Ventral Tegmental Area Volumes Are Increased by Selective Breeding for High Voluntary Wheel-Running Behavior

Uncovering relationships between neuroanatomy, behavior, and evolution are important for understanding the factors that control brain function. Voluntary exercise is one key behavior that both affects, and may be affected by, neuroanatomical variation. Moreover, recent studies suggest an important role for physical activity in brain evolution. We used a unique and ongoing artificial selection model in which mice are bred for high voluntary wheel-running behavior, yielding four replicate lines of high runner (HR) mice that run ∼3-fold more revolutions per day than four replicate nonselected control (C) lines. Previous studies reported that, with body mass as a covariate, HR mice had heavier whole brains, non-cerebellar brains, and larger midbrains than C mice. We sampled mice from generation 66 and used high-resolution microscopy to test the hypothesis that HR mice have greater volumes and/or cell densities in nine key regions from either the midbrain or limbic system. In addition, half of the mice were given 10 weeks of wheel access from weaning, and we predicted that chronic exercise would increase the volumes of the examined brain regions via phenotypic plasticity. We replicated findings that both selective breeding and wheel access increased total brain mass, with no significant interaction between the two factors. In HR compared to C mice, adjusting for body mass, both the red nucleus (RN) of the midbrain and the hippocampus (HPC) were significantly larger, and the whole midbrain tended to be larger, with no effect of wheel access nor any interactions. Linetype and wheel access had an interactive effect on the volume of the periaqueductal gray (PAG), such that wheel access increased PAG volume in C mice but decreased volume in HR mice. Neither linetype nor wheel access affected volumes of the substantia nigra, ventral tegmental area, nucleus accumbens, ventral pallidum (VP), or basolateral amygdala. We found no main effect of either linetype or wheel access on neuronal densities (numbers of cells per unit area) for any of the regions examined. Taken together, our results suggest that the increased exercise phenotype of HR mice is related to increased RN and hippocampal volumes, but that chronic exercise alone does not produce such phenotypes.

Selective breeding for high voluntary exercise in mice increases maximal (V̇O2,max) but not basal metabolic rate

In general, sustained high rates of physical activity require a high maximal aerobic capacity (V̇O2,max), which may also necessitate a high basal aerobic metabolism (BMR), given that the two metabolic states are linked via shared organ systems, cellular properties and metabolic pathways. We tested the hypotheses that (a) selective breeding for high voluntary exercise in mice would elevate both V̇O2,max and BMR, and (b) these increases are accompanied by increases in the size of some internal organs (ventricle, triceps surae muscle, liver, kidney, spleen, lung, brain). We measured 72 females from generations 88 and 96 of an ongoing artificial selection experiment comprising four replicate High Runner (HR) lines bred for voluntary daily wheel-running distance and four non-selected control lines. With body mass as a covariate, HR lines as a group had significantly higher V̇O2,max (+13.6%, P<0.0001), consistent with previous studies, but BMR did not significantly differ between HR and control lines (+6.5%, P=0.181). Additionally, HR mice did not statistically differ from control mice for whole-body lean or fat mass, or for the mass of any organ collected (with body mass as a covariate). Finally, mass-independent V̇O2,max and BMR were uncorrelated (r=0.073, P=0.552) and the only statistically significant correlation with an organ mass was for V̇O2,max and ventricle mass (r=0.285, P=0.015). Overall, our results indicate that selection for a behavioral trait can yield large changes in behavior without proportional modifications to underlying morphological or physiological traits.

Weanling gut microbiota composition of a mouse model selectively bred for high voluntary wheel-running behavior

We compared the fecal microbial community composition and diversity of four replicate lines of mice selectively bred for high wheel-running activity over 81 generations (HR lines) and four non-selected control lines. We performed 16S rRNA gene sequencing on fecal samples taken 24 h after weaning, identifying a total of 2074 bacterial operational taxonomic units. HR and control mice did not significantly differ for measures of alpha diversity, but HR mice had a higher relative abundance of the family Clostridiaceae. These results differ from a study of rats, where a line bred for high forced-treadmill endurance and that also ran more on wheels had lower relative abundance of Clostridiaceae, as compared with a line bred for low endurance that ran less on wheels. Within the HR and control groups, replicate lines had unique microbiomes based on unweighted UniFrac beta diversity, indicating random genetic drift and/or multiple adaptive responses to selection.

Individual variation in heat substitution: is activity in the cold energetically cheaper for some individuals than others?

In many endotherms, a potentially important yet often overlooked mechanism to save energy is the use of the heat generated by active skeletal muscles to replace heat that would have been generated by thermogenesis (i.e., "activity-thermoregulatory heat substitution"). While substitution has been documented numerous times, the extent of individual variation in substitution has never been quantified. Here, we used a home-cage respirometry system to repeatedly measure substitution through the concomitant monitoring of metabolic rate (MR) and locomotor activity in 46 female white-footed mice (Peromyscus leucopus). A total of 117 measures of substitution were taken by quantifying the difference in the slope of the relationship between MR and locomotor activity speed at two different ambient temperatures. Consistency repeatability (±se) of substitution was 0.313±0.131 - hence, about a third of the variation in substitution occurs at the among-individual level. Body length and heart mass were positively correlated with substitution whereas surface area was negatively correlated with substitution. These two sub-organismal traits accounted for the majority of the among-individual variation (i.e., individual differences in substitution were not significant after accounting for these traits). Overall, our results imply that the energetic cost of activity below the thermoneutral zone is consistently cheaper from some individuals than others, and that the energy saved from substitution might be available to invest in fitness-enhancing activities.

Housing conditions modify seasonal changes in basal metabolism and body mass of the Siberian hamster, Phodopus sungorus

Proper housing conditions are important aspects of animal welfare. Animals housed in enriched environments show less stereotypic behaviours than animals kept in barren cages. However, different types of cage enrichment may affect the results of experimental studies and hinder comparative analyses of animal physiology and behaviour. We investigated whether access to a running wheel, availability of nesting material, and pair housing affect basal metabolic rate (BMR) of Siberian hamsters (Phodopus sungorus) under various acclimation conditions. We used 70 adult hamsters (35 males and 35 females) divided into five groups housed under different cage conditions. All individuals experienced the same acclimation procedure: first a winter (L8:D16) then a summer (L16:D8) photoperiod, at air temperatures of first 20 °C then 7 °C under both photoperiods. We found that nesting material and pair housing did not affect hamster BMR, while access to a running wheel increased BMR and body mass regardless of photoperiod and ambient temperature. Thus, we suggest that cage enrichment should be applied with caution, especially in studies on energetics or thermoregulation, particularly in seasonal animals.

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.

Rapid and longer-term effects of selective breeding for voluntary exercise behavior on skeletal morphology in house mice

Selection experiments can elucidate the varying course of adaptive changes across generations. We examined the appendicular skeleton of house mice from four replicate High Runner (HR) lines bred for physical activity on wheels and four non-selected Control (C) lines. HR mice reached apparent selection limits between generations 17 and 27, running ~3-fold more than C. Studies at generations 11, 16, and 21 found that HR mice had evolved thicker hindlimb bones, heavier feet, and larger articular surface areas of the knee and hip joint. Based on biomechanical theory, any or all of these evolved differences may be beneficial for endurance running. Here, we studied mice from generation 68, plus a limited sample from generation 58, to test whether the skeleton continued to evolve after selection limits were reached. Contrary to our expectations, we found few differences between HR and C mice for these later generations, and some of the differences in bone dimensions identified in earlier generations were no longer statistically significant. We hypothesize that the loss of apparently coadapted lower-level traits reflects (1) deterioration related to a gradual increase in inbreeding and/or (2) additional adaptive changes that replace the functional benefits of some skeletal changes.

Early-life effects of juvenile Western diet and exercise on adult gut microbiome composition in mice

Alterations to the gut microbiome caused by changes in diet, consumption of antibiotics, etc., can affect host function. Moreover, perturbation of the microbiome during critical developmental periods potentially has long-lasting impacts on hosts. Using four selectively bred high runner and four non-selected control lines of mice, we examined the effects of early-life diet and exercise manipulations on the adult microbiome by sequencing the hypervariable internal transcribed spacer region of the bacterial gut community. Mice from high runner lines run ∼3-fold more on wheels than do controls, and have several other phenotypic differences (e.g. higher food consumption and body temperature) that could alter the microbiome, either acutely or in terms of coevolution. Males from generation 76 were given wheels and/or a Western diet from weaning until sexual maturity at 6 weeks of age, then housed individually without wheels on standard diet until 14 weeks of age, when fecal samples were taken. Juvenile Western diet reduced bacterial richness and diversity after the 8-week washout period (equivalent to ∼6 human years). We also found interactive effects of genetic line type, juvenile diet and/or juvenile exercise on microbiome composition and diversity. Microbial community structure clustered significantly in relation to both line type and diet. Western diet also reduced the relative abundance of Muribaculum intestinale These results constitute one of the first reports of juvenile diet having long-lasting effects on the adult microbiome after a substantial washout period. Moreover, we found interactive effects of diet with early-life exercise exposure, and a dependence of these effects on genetic background.

Coadaptation of the chemosensory system with voluntary exercise behavior in mice

Ethologically relevant chemical senses and behavioral habits are likely to coadapt in response to selection. As olfaction is involved in intrinsically motivated behaviors in mice, we hypothesized that selective breeding for a voluntary behavior would enable us to identify novel roles of the chemosensory system. Voluntary wheel running (VWR) is an intrinsically motivated and naturally rewarding behavior, and even wild mice run on a wheel placed in nature. We have established 4 independent, artificially evolved mouse lines by selectively breeding individuals showing high VWR activity (High Runners; HRs), together with 4 non-selected Control lines, over 88 generations. We found that several sensory receptors in specific receptor clusters were differentially expressed between the vomeronasal organ (VNO) of HRs and Controls. Moreover, one of those clusters contains multiple single-nucleotide polymorphism loci for which the allele frequencies were significantly divergent between the HR and Control lines, i.e., loci that were affected by the selective breeding protocol. These results indicate that the VNO has become genetically differentiated between HR and Control lines during the selective breeding process. Although the role of the vomeronasal chemosensory receptors in VWR activity remains to be determined, the current results suggest that these vomeronasal chemosensory receptors are important quantitative trait loci for voluntary exercise in mice. We propose that olfaction may play an important role in motivation for voluntary exercise in mammals.

Genetic Basis of Aerobically Supported Voluntary Exercise: Results from a Selection Experiment with House Mice

The biological basis of exercise behavior is increasingly relevant for maintaining healthy lifestyles. Various quantitative genetic studies and selection experiments have conclusively demonstrated substantial heritability for exercise behavior in both humans and laboratory rodents. In the "High Runner" selection experiment, four replicate lines of Mus domesticus were bred for high voluntary wheel running (HR), along with four nonselected control (C) lines. After 61 generations, the genomes of 79 mice (9-10 from each line) were fully sequenced and single nucleotide polymorphisms (SNPs) were identified. We used nested ANOVA with MIVQUE estimation and other approaches to compare allele frequencies between the HR and C lines for both SNPs and haplotypes. Approximately 61 genomic regions, across all somatic chromosomes, showed evidence of differentiation; 12 of these regions were differentiated by all methods of analysis. Gene function was inferred largely using Panther gene ontology terms and KO phenotypes associated with genes of interest. Some of the differentiated genes are known to be associated with behavior/motivational systems and/or athletic ability, including Sorl1, Dach1, and Cdh10 Sorl1 is a sorting protein associated with cholinergic neuron morphology, vascular wound healing, and metabolism. Dach1 is associated with limb bud development and neural differentiation. Cdh10 is a calcium ion binding protein associated with phrenic neurons. Overall, these results indicate that selective breeding for high voluntary exercise has resulted in changes in allele frequencies for multiple genes associated with both motivation and ability for endurance exercise, providing candidate genes that may explain phenotypic changes observed in previous studies.

Variation in the response to exercise stimulation in Drosophila: marathon runner versus sprinter genotypes

Animals' behaviors vary in response to their environment, both biotic and abiotic. These behavioral responses have significant impacts on animal survival and fitness, and thus, many behavioral responses are at least partially under genetic control. In Drosophila, for example, genes impacting aggression, courtship behavior, circadian rhythms and sleep have been identified. Animal activity also is influenced strongly by genetics. My lab previously has used the Drosophila melanogaster Genetics Reference Panel (DGRP) to investigate activity levels and identified over 100 genes linked to activity. Here, I re-examined these data to determine whether Drosophila strains differ in their response to rotational exercise stimulation, not simply in the amount of activity, but in activity patterns and timing of activity. Specifically, I asked whether there are fly strains exhibiting either a 'marathoner' pattern of activity, i.e. remaining active throughout the 2 h exercise period, or a 'sprinter' pattern, i.e. carrying out most of the activity early in the exercise period. The DGRP strains examined differ significantly in how much activity is carried out at the beginning of the exercise period, and this pattern is influenced by both sex and genotype. Interestingly, there was no clear link between the activity response pattern and lifespan of the animals. Using genome-wide association studies (GWAS), I identified 10 high confidence candidate genes that control the degree to which Drosophila exercise behaviors fit a marathoner or sprinter activity pattern. This finding suggests that, similar to other aspects of locomotor behavior, the timing of activity patterns in response to exercise stimulation is under genetic control.

Electrocardiograms of mice selectively bred for high levels of voluntary exercise: Effects of short-term exercise training and the mini-muscle phenotype

Changes in cardiac function that occur with exercise training have been studied in detail, but those accompanying evolved increases in the duration or intensity of physical activity are poorly understood. To address this gap, we studied electrocardiograms (ECGs) of mice from an artificial selection experiment in which four replicate lines are bred for high voluntary wheel running (HR) while four non-selected lines are maintained as controls (C). ECGs were recorded using an ECGenie (Mouse Specifics, Inc.) both before and after six days of wheel access (as used in the standard protocol to select breeders). We hypothesized that HR mice would show innate differences in ECG characteristics and that the response to training would be greater in HR mice relative to C mice because the former run more. After wheel access, in statistical analyses controlling for variation in body mass, all mice had lower heart rates, and mice from HR lines had longer PR intervals than C lines. Also after wheel access, male mice had increased heart rate variability, whereas females had decreased heart rate variability. With body mass as a covariate, six days of wheel access significantly increased ventricle mass in both HR and C males. Within the HR lines, a subset of mice known as mini-muscle individuals have a 50% reduction in hindlimb muscle mass and generally larger internal organs, including the heart ventricles. As compared with normal-muscled individuals, mini-muscle individuals had a longer QRS complex, both before and after wheel access. Some studies in other species of mammals have shown correlations between athletic performance and QRS duration. Correlations between wheel running and either heart rate or QRS duration (before wheel running) among the eight individual lines of the HR selection experiment or among 17 inbred mouse strains taken from the literature were not statistically significant. However, total revolutions and average speed were negatively correlated with PR duration among lines of the HR selection experiment for males, and duration of running was negatively correlated with PR duration among 17 inbred strains for females. We conclude that HR mice have enhanced trainability of cardiac function as compared with C mice (as indicated by their longer PR duration after wheel access), and that the mini-muscle phenotype causes cardiac changes that have been associated with increased athletic performance in previous studies of mammals.

Enriched Environment and Effects on Neuropathic Pain: Experimental Findings and Mechanisms

Neuropathic pain inflicts tremendous biopsychosocial suffering for patients worldwide. However, safe and effective treatment of neuropathic pain is a prominent unmet clinical need. Environmental enrichment (EE) is an emerging cost-effective nonpharmacological approach to alleviate neuropathic pain and complement rehabilitation care. We present here a review of preclinical studies in ascertaining the efficacy of EE for neuropathic pain. Their proposed mechanisms, including the suppression of ascending nociceptive signaling to the brain, enhancement of the descending inhibitory system, and neuroprotection of the peripheral and central nervous systems, may collectively reduce pain perception and improve somatic and emotional functioning in neuropathic pain. The current evidence offers critical insights for future preclinical research and the translational application of EE in clinical pain management.

Reduced non-bicarbonate skeletal muscle buffering capacity in mice with the mini-muscle phenotype

Muscle pH decreases during exercise, which may impair function. Endurance training typically reduces muscle buffering capacity as a result of changes in fiber-type composition, but existing comparisons of species that vary in activity level are ambiguous. We hypothesized that high-runner (HR) lines of mice from an experiment that breeds mice for voluntary wheel running would have altered muscle buffering capacity as compared with their non-selected control counterparts. We also expected that 6 days of wheel access, as used in the selection protocol, would reduce buffering capacity, especially for HR mice. Finally, we expected a subset of HR mice with the 'mini-muscle' phenotype to have relatively low buffering capacity as a result of fewer type IIb fibers. We tested non-bicarbonate buffering capacity of thigh muscles. Only HR mice expressing the mini-muscle phenotype had significantly reduced buffering capacity, females had lower buffering capacity than males, and wheel access had no significant effect.

Mice selectively bred for high voluntary wheel-running behavior conserve more fat despite increased exercise

Physical activity is an important component of energy expenditure, and acute changes in activity can lead to energy imbalances that affect body composition, even under ad libitum food availability. One example of acute increases in physical activity is four replicate, selectively-bred High Runner (HR) lines of mice that voluntarily run ~3-fold more wheel revolutions per day over 6-day trials and are leaner, as compared with four non-selected control (C) lines. We expected that voluntary exercise would increase food consumption, build lean mass, and reduce fat mass, but that these effects would likely differ between HR and C lines or between the sexes. We compared wheel running, cage activity, food consumption, and body composition between HR and C lines for young adults of both sexes, and examined interrelationships of those traits across 6 days of wheel access. Before wheel testing, HR mice weighed less than C, primarily due to reduced lean mass, and females were lighter than males, entirely due to lower lean mass. Over 6 days of wheel access, all groups tended to gain small amounts of lean mass, but lose fat mass. HR mice lost less fat than C mice, in spite of much higher activity levels, resulting in convergence to a fat mass of ~1.7 g for all 4 groups. HR mice consumed more food than C mice (with body mass as a covariate), even accounting for their higher activity levels. No significant sex-by-linetype interactions were observed for any of the foregoing traits. Structural equation models showed that the four sex-by-linetype groups differed considerably in the complex phenotypic architecture of these traits. Interrelationships among traits differed by genetic background and sex, lending support to the idea that recommendations regarding weight management, diet, and exercise may need to be tailored to the individual level.

How low can you go? An adaptive energetic framework for interpreting basal metabolic rate variation in endotherms

Adaptive explanations for both high and low body mass-independent basal metabolic rate (BMR) in endotherms are pervasive in evolutionary physiology, but arguments implying a direct adaptive benefit of high BMR are troublesome from an energetic standpoint. Here, we argue that conclusions about the adaptive benefit of BMR need to be interpreted, first and foremost, in terms of energetics, with particular attention to physiological traits on which natural selection is directly acting. We further argue from an energetic perspective that selection should always act to reduce BMR (i.e., maintenance costs) to the lowest level possible under prevailing environmental or ecological demands, so that high BMR per se is not directly adaptive. We emphasize the argument that high BMR arises as a correlated response to direct selection on other physiological traits associated with high ecological or environmental costs, such as daily energy expenditure (DEE) or capacities for activity or thermogenesis. High BMR thus represents elevated maintenance costs required to support energetically demanding lifestyles, including living in harsh environments. BMR is generally low under conditions of relaxed selection on energy demands for high metabolic capacities (e.g., thermoregulation, activity) or conditions promoting energy conservation. Under these conditions, we argue that selection can act directly to reduce BMR. We contend that, as a general rule, BMR should always be as low as environmental or ecological conditions permit, allowing energy to be allocated for other functions. Studies addressing relative reaction norms and response times to fluctuating environmental or ecological demands for BMR, DEE, and metabolic capacities and the fitness consequences of variation in BMR and other metabolic traits are needed to better delineate organismal metabolic responses to environmental or ecological selective forces.