Fuel Use in Mammals: Conserved Patterns and Evolved Strategies for Aerobic Locomotion and Thermogenesis

Metabolic recovery from submaximal exercise in hypoxia acclimated high altitude deer mice (Peromyscus maniculatus)

Animals living at high-altitude are faced with unremitting low oxygen availability. This can make it difficult to perform daily tasks that require increases in aerobic metabolism. An activity important for survival is aerobic locomotion, and the rapid recovery of muscle metabolism post exercise. Past work shows that hypoxia acclimated high-altitude mice (Peromyscus maniculatus) have a greater reliance on carbohydrates to power exercise than low altitude mice. However, it is unclear how quickly after aerobic exercise these mice can recovery and replenish muscle glycogen stores. The gastrocnemius muscle of high-altitude deer mice has a more aerobic phenotype and a greater capacity to oxidize lipids than low altitude deer mice. This suggests that high altitude mice may recover more rapidly from exercise than their lowland counterparts due to a greater capacity to support glycogen replenishment using intramuscular triglycerides (IMTG). To explore this possibility, we used low- and high-altitude native deer mice born and raised in common lab conditions and acclimated to chronic hypoxia. We determined changes in oxygen consumption following 15 min of aerobic exercise in 12% O2 and sampled skeletal muscles and liver at various time points during recovery to examine changes in key metabolites, including glycogen and IMTG. We found depletion in glycogen stores during exercise only in lowlanders, which returned to resting levels following 90 min of recovery. In contrast, IMTG did not change significantly with exercise or during recovery in either population. These data suggest that exercise recovery is influenced by altitude ancestry in deer mice.

What can naked mole-rats teach us about ameliorating hypoxia-related human diseases?

Ameliorating the deleterious impact of systemic or tissue-level hypoxia or ischemia is key to preventing or treating many human diseases and pathologies. Usefully, environmental hypoxia is also a common challenge in many natural habitats; animals that are native to such hypoxic niches often exhibit strategies that enable them to thrive with limited O2 availability. Studying how such species have evolved to tolerate systemic hypoxia offers a promising avenue of discovery for novel strategies to mitigate the deleterious effects of hypoxia in human diseases and pathologies. Of particular interest are naked mole-rats, which are among the most hypoxia-tolerant mammals. Naked mole-rats that tolerate severe hypoxia in a laboratory setting are also protected against clinically relevant mimics of heart attack and stroke. The mechanisms that support this tolerance are currently being elucidated but results to date suggest that metabolic rate suppression, reprogramming of metabolic pathways, and mechanisms that defend against deleterious perturbations of cellular signaling pathways all provide layers of protection. Herein, we synthesize and discuss what is known regarding adaptations to hypoxia in the naked mole-rat cardiopulmonary system and brain, as these systems comprise both the primary means of delivering O2 to tissues and the most hypoxia-sensitive organs in mammals.

CO2 scrubbing, zero gases, Keeling plots, and a mathematical approach to ameliorate the deleterious effects of ambient CO2 during 13 C breath testing in humans and animals

13 C breath testing is increasingly used in physiology and ecology research because of what it reveals about the different fuels that animals oxidize to meet their energetic demands. Here I review the practice of 13 C breath testing in humans and other animals and describe the impact that contamination by ambient/background CO2 in the air can have on the accuracy of 13 C breath measurements. I briefly discuss physical methods to avoid sample contamination as well as the Keeling plot approach that researchers have been using for the past two decades to estimate δ13 C from breath samples mixed with ambient CO2 . Unfortunately, Keeling plots are not suited for 13 C breath testing in common situations where (1) a subject's VCO2 is dynamic, (2) ambient [CO2 ] may change, (3) a subject is sensitive to hypercapnia, or (4) in any flow-through indirect calorimetry system. As such, I present a mathematical solution that addresses these issues by using information about the instantaneous [CO2 ] and the δ13 CO2 of ambient air as well as the diluted breath sample to back-calculate the δ13 CO2 in the CO2 exhaled by the animal. I validate this approach by titrating a sample of 13 C-enriched gas into an air stream and demonstrate its ability to provide accurate values across a wide range of breath and air mixtures. This approach allows researchers to instantaneously calculate the δ13 C of exhaled gas of humans or other animals in real time without having to scrub ambient CO2 or rely on estimated values.

Energy metabolic mechanisms for high altitude sickness: Downregulation of glycolysis and upregulation of the lactic acid/amino acid-pyruvate-TCA pathways and fatty acid oxidation

Hypobaric hypoxia is often associated with the plateau environment and can lead to altitude sickness or death. The underlying cause is a lack of oxygen, which limits energy metabolism and leads to a compensatory stress response. Although glycolysis is commonly accepted as the primary energy source during clinical hypoxia, our preliminary experiments suggest that hypobaric hypoxia may depress glycolysis. To provide a more comprehensive understanding of energy metabolism under short-term hypobaric hypoxia, we exposed mice to a simulated altitude of 5000 m for 6 or 12 h. After the exposure, we collected blood and liver tissues to quantify the substrates, enzymes, and metabolites involved in glycolysis, lactic acid metabolism, the tricarboxylic acid cycle (TCA), and fatty acid β-oxidation. We also performed transcriptome and enzymatic activity analyses of the liver. Our results show that 6 h of hypoxic exposure significantly increased blood glucose, decreased lactic acid and triglyceride concentrations, and altered liver enzyme activities of mice exposed to hypoxia. The key enzymes in the glycolytic, TCA, and fatty acid β-oxidation pathways were primarily affected. Specifically, the activities of key glycolytic enzymes, such as glucokinase, decreased significantly, while the activities of enzymes in the TCA cycle, such as isocitrate dehydrogenase, increased significantly. Lactate dehydrogenase, pyruvate carboxylase, and alanine aminotransferase were upregulated. These changes were partially restored when the exposure time was extended to 12 h, except for further downregulation of phosphofructokinase and glucokinase. This study demonstrates that acute high altitude hypoxia upregulated the lactic acid/amino acid-pyruvate-TCA pathways and fatty acid oxidation, but downregulated glycolysis in the liver of mice. The results obtained in this study provide a theoretical framework for understanding the mechanisms underlying the pathogenesis of high-altitude sickness in humans. Additionally, these findings have potential implications for the development of prevention and treatment strategies for altitude sickness.

Aqp5-/- mice exhibit reduced maximal body O2 consumption under cold exposure, normal pulmonary gas exchange, and impaired formation of brown adipose tissue

The fundamental body functions that determine maximal O₂ uptake (V̇o_2max) have not been studied in Aqp5^-/- mice (aquaporin 5, AQP5). We measured V̇o_2max to globally assess these functions and then investigated why it was found altered in Aqp5^-/- mice. V̇o_2max was measured by the Helox technique, which elicits maximal metabolic rate by intense cold exposure of the animals. We found V̇o_2max reduced in Aqp5^-/- mice by 20%-30% compared with wild-type (WT) mice. As AQP5 has been implicated to act as a membrane channel for respiratory gases, we studied whether this is caused by the known lack of AQP5 in the alveolar epithelial membranes of Aqp5^-/- mice. Lung function parameters as well as arterial O₂ saturation were normal and identical between Aqp5^-/- and WT mice, indicating that AQP5 does not contribute to pulmonary O₂ exchange. The cause for the decreased V̇o_2max thus might be found in decreased O₂ consumption of an intensely O₂-consuming peripheral organ such as activated brown adipose tissue (BAT). We found indeed that absence of AQP5 greatly reduces the amount of interscapular BAT formed in response to 4 wk of cold exposure, from 63% in WT to 25% in Aqp5^-/- animals. We conclude that lack of AQP5 does not affect pulmonary O₂ exchange, but greatly inhibits transformation of white to brown adipose tissue. As under cold exposure, BAT is a major source of the animals' heat production, reduction of BAT likely causes the decrease in V̇o_2max under this condition.

Thermogenesis is supported by high rates of circulatory fatty acid and triglyceride delivery in highland deer mice

Highland native deer mice (Peromyscus maniculatus) have greater rates of lipid oxidation during maximal cold challenge in hypoxia (hypoxic cold-induced V˙O2max) compared to their lowland conspecifics. Lipid oxidation is also increased in deer mice acclimated to simulated high altitude (cold hypoxia), regardless of altitude ancestry. The underlying lipid metabolic pathway traits responsible for sustaining maximal thermogenic demand in deer mice is currently unknown. The objective of this study was to characterize key steps in the lipid oxidation pathway in highland and lowland deer mice acclimated to control (23oC, 21kPa O2) or cold hypoxic (5oC, 12kPa O2) conditions. We hypothesized that capacities for lipid delivery and tissue uptake will be greater in highlanders and further increase with cold hypoxia acclimation. With the transition from rest to hypoxic cold-induced V˙O2max, both highland and lowland deer mice showed increased plasma glycerol concentrations and fatty acid availability. Interestingly, cold hypoxia acclimation led to increased plasma triglyceride concentrations at cold-induced V˙O2max, but only in highlanders. Highlanders also had significantly greater delivery rates of circulatory free fatty acids and triglycerides due to higher plasma flow rates at cold-induced V˙O2max. We found no population or acclimation differences in fatty acid translocase (FAT/CD36) abundance in the gastrocnemius or brown adipose tissue, suggesting fatty acid uptake across membranes is not limiting during thermogenesis. Our data indicate that circulatory lipid delivery plays a major role in supporting the high thermogenic rates observed in highland versus lowland deer mice.

Increased Reliance on Carbohydrates for Aerobic Exercise in Highland Andean Leaf-Eared Mice, but Not in Highland Lima Leaf-Eared Mice

Exercise is an important performance trait in mammals and variation in aerobic capacity and/or substrate allocation during submaximal exercise may be important for survival at high altitude. Comparisons between lowland and highland populations is a fruitful approach to understanding the mechanisms for altitude differences in exercise performance. However, it has only been applied in very few highland species. The leaf-eared mice (LEM, genus Phyllotis) of South America are a promising taxon to uncover the pervasiveness of hypoxia tolerance mechanisms. Here we use lowland and highland populations of Andean and Lima LEM (P. andium and P. limatus), acclimated to common laboratory conditions, to determine exercise-induced maximal oxygen consumption (V˙O₂max), and submaximal exercise metabolism. Lowland and highland populations of both species showed no difference in V˙O₂max running in either normoxia or hypoxia. When run at 75% of V˙O₂max, highland Andean LEM had a greater reliance on carbohydrate oxidation to power exercise. In contrast, highland Lima LEM showed no difference in exercise fuel use compared to their lowland counterparts. The higher carbohydrate oxidation seen in highland Andean LEM was not explained by maximal activities of glycolytic enzymes in the gastrocnemius muscle, which were equivalent to lowlanders. This result is consistent with data on highland deer mouse populations and suggests changes in metabolic regulation may explain altitude differences in exercise performance.

Positive allometry of sexually selected traits: Do metabolic maintenance costs play an important role?

Sexual selection drives the evolution of some of the most exaggerated traits in nature. Studies on sexual selection often focus on the size of these traits relative to body size, but few focus on energetic maintenance costs of the tissues that compose them, and the ways in which these costs vary with body size. The relationships between energy use and body size have consequences that may allow large individuals to invest disproportionally more in sexually selected structures, or lead to the reduced per-gram maintenance cost of enlarged structures. Although sexually selected traits can incur energetic maintenance costs, these costs are not universally high; they are dependent on the relative mass and metabolic activity of tissues associated with them. Energetic costs of maintenance may play a pervasive yet little-explored role in shaping the relative scaling of sexually selected traits across diverse taxa. Also see the video abstract here: https://youtu.be/JyuoQIeA33Q.

Lipid oxidation during thermogenesis in high-altitude deer mice (Peromyscus maniculatus)

When at their maximum thermogenic capacity (cold-induced V̇o_2max), small endotherms reach levels of aerobic metabolism as high, or even higher, than running V̇o_2max. How these high rates of thermogenesis are supported by substrate oxidation is currently unclear. The appropriate utilization of metabolic fuels that could sustain thermogenesis over extended periods may be important for survival in cold environments, like high altitude. Previous studies show that high capacities for lipid use in high-altitude deer mice may have evolved in concert with greater thermogenic capacities. The purpose of this study was to determine how lipid utilization at both moderate and maximal thermogenic intensities may differ in high- and low-altitude deer mice, and strictly low-altitude white-footed mice. We also examined the phenotypic plasticity of lipid use after acclimation to cold hypoxia (CH), conditions simulating high altitude. We found that lipids were the primary fuel supporting both moderate and maximal rates of thermogenesis in both species of mice. Lipid oxidation increased threefold in mice from 30°C to 0°C, consistent with increases in oxidation of [¹³C]palmitic acid. CH acclimation led to an increase in [¹³C]palmitic acid oxidation at 30°C but did not affect total lipid oxidation. Lipid oxidation rates at cold-induced V̇o_2max were two- to fourfold those at 0°C and increased further after CH acclimation, especially in high-altitude deer mice. These are the highest mass-specific lipid oxidation rates observed in any land mammal. Uncovering the mechanisms that allow for these high rates of oxidation will aid our understanding of the regulation of lipid metabolism.

Ancestral and developmental cold alter brown adipose tissue function and adult thermal acclimation in Peromyscus

Small, non-hibernating endotherms increase their thermogenic capacity to survive seasonal cold, through adult phenotypic flexibility. In mammals, this response is primarily driven by remodeling of brown adipose tissue (BAT), which matures postnatally in altricial species. In many regions, ambient temperatures can vary dramatically throughout the breeding season. We used second-generation lab-born Peromyscus leucopus, cold exposed during two critical developmental windows, to test the hypothesis that adult phenotypic flexibility to cold is influenced by rearing temperature. We found that cold exposure during the postnatal period (14 °C, birth to 30 days) accelerated BAT maturation and permanently remodeled this tissue. As adults, these mice had increased BAT activity and thermogenic capacity relative to controls. However, they also had a blunted acclimation response when subsequently cold exposed as adults (5 °C for 6 weeks). Mice born to cold-exposed mothers (14 °C, entire pregnancy) also showed limited capacity for flexibility as adults, demonstrating that maternal cold stress programs the offspring thermal acclimation response. In contrast, for P. maniculatus adapted to the cold high alpine, BAT maturation rate was unaffected by rearing temperature. However, both postnatal and prenatal cold exposure limited the thermal acclimation response in these cold specialists. Our results suggest a complex interaction between developmental and adult environment, influenced strongly by ancestry, drives thermogenic capacity in the wild.

High-Altitude Adaptation: Mechanistic Insights from Integrated Genomics and Physiology

Population genomic analyses of high-altitude humans and other vertebrates have identified numerous candidate genes for hypoxia adaptation, and the physiological pathways implicated by such analyses suggest testable hypotheses about underlying mechanisms. Studies of highland natives that integrate genomic data with experimental measures of physiological performance capacities and subordinate traits are revealing associations between genotypes (e.g., hypoxia-inducible factor gene variants) and hypoxia-responsive phenotypes. The subsequent search for causal mechanisms is complicated by the fact that observed genotypic associations with hypoxia-induced phenotypes may reflect second-order consequences of selection-mediated changes in other (unmeasured) traits that are coupled with the focal trait via feedback regulation. Manipulative experiments to decipher circuits of feedback control and patterns of phenotypic integration can help identify causal relationships that underlie observed genotype–phenotype associations. Such experiments are critical for correct inferences about phenotypic targets of selection and mechanisms of adaptation.

Physiological Genomics of Adaptation to High-Altitude Hypoxia

Population genomic studies of humans and other animals at high altitude have generated many hypotheses about the genes and pathways that may have contributed to hypoxia adaptation. Future advances require experimental tests of such hypotheses to identify causal mechanisms. Studies to date illustrate the challenge of moving from lists of candidate genes to the identification of phenotypic targets of selection, as it can be difficult to determine whether observed genotype-phenotype associations reflect causal effects or secondary consequences of changes in other traits that are linked via homeostatic regulation. Recent work on high-altitude models such as deer mice has revealed both plastic and evolved changes in respiratory, cardiovascular, and metabolic traits that contribute to aerobic performance capacity in hypoxia, and analyses of tissue-specific transcriptomes have identified changes in regulatory networks that mediate adaptive changes in physiological phenotype. Here we synthesize recent results and discuss lessons learned from studies of high-altitude adaptation that lie at the intersection of genomics and physiology.

Developmental and reproductive physiology of small mammals at high altitude: challenges and evolutionary innovations

High-altitude environments, characterized by low oxygen levels and low ambient temperatures, have been repeatedly colonized by small altricial mammals. These species inhabit mountainous regions year-round, enduring chronic cold and hypoxia. The adaptations that allow small mammals to thrive at altitude have been well studied in non-reproducing adults; however, our knowledge of adaptations specific to earlier life stages and reproductive females is extremely limited. In lowland natives, chronic hypoxia during gestation affects maternal physiology and placental function, ultimately limiting fetal growth. During post-natal development, hypoxia and cold further limit growth both directly by acting on neonatal physiology and indirectly via impacts on maternal milk production and care. Although lowland natives can survive brief sojourns to even extreme high altitude as adults, reproductive success in these environments is very low, and lowland young rarely survive to sexual maturity in chronic cold and hypoxia. Here, we review the limits to maternal and offspring physiology - both pre-natal and post-natal - that highland-adapted species have overcome, with a focus on recent studies on high-altitude populations of the North American deer mouse (Peromyscus maniculatus). We conclude that a combination of maternal and developmental adaptations were likely to have been critical steps in the evolutionary history of high-altitude native mammals.

Mitochondrial Nutrient Utilization Underlying the Association Between Metabolites and Insulin Resistance in Adolescents

CONTEXT

A person's intrinsic metabolism, reflected in the metabolome, may describe the relationship between nutrient intake and metabolic health.

OBJECTIVES

Untargeted metabolomics was used to identify metabolites associated with metabolic health. Path analysis classified how habitual dietary intake influences body mass index z-score (BMIz) and insulin resistance (IR) through changes in the metabolome.

DESIGN

Data on anthropometry, fasting metabolites, C-peptide, and dietary intake were collected from 108 girls and 98 boys aged 8 to 14 years. Sex-stratified linear regression identified metabolites associated with BMIz and homeostatic model assessment of IR using C-peptide (HOMA-CP), accounting for puberty, age, and muscle and fat area. Path analysis identified clusters of metabolites that underlie the relationship between energy-adjusted macronutrient intake with BMIz and HOMA-CP.

RESULTS

Metabolites associated with BMIz include positive associations with diglycerides among girls and positive associations with branched chain and aromatic amino acids in boys. Intermediates in fatty acid metabolism, including medium-chain acylcarnitines (AC), were inversely associated with HOMA-CP. Carbohydrate intake is positively associated with HOMA-CP through decreases in levels of AC, products of β-oxidation. Approaching significance, fat intake is positively associated with HOMA-CP through increases in levels of dicarboxylic fatty acids, products of omega-oxidation.

CONCLUSIONS

This cross-sectional analysis suggests that IR in children is associated with reduced fatty acid oxidation capacity. When consuming more grams of fat, there is evidence for increased extramitochondrial fatty acid metabolism, while higher carbohydrate intake appears to lead to decreases in intermediates of β-oxidation. Thus, biomarkers of IR and mitochondrial oxidative capacity may depend on macronutrient intake.

Development of homeothermic endothermy is delayed in high-altitude native deer mice (Peromyscus maniculatus)

Altricial mammals begin to independently thermoregulate during the first few weeks of postnatal development. In wild rodent populations, this is also a time of high mortality (50-95%), making the physiological systems that mature during this period potential targets for selection. High altitude (HA) is a particularly challenging environment for small endotherms owing to unremitting low O₂ and ambient temperatures. While superior thermogenic capacities have been demonstrated in adults of some HA species, it is unclear if selection has occurred to survive these unique challenges early in development. We used deer mice (Peromyscus maniculatus) native to high and low altitude (LA), and a strictly LA species (Peromyscus leucopus), raised under common garden conditions, to determine if postnatal onset of endothermy and maturation of brown adipose tissue (BAT) is affected by altitude ancestry. We found that the onset of endothermy corresponds with the maturation and activation of BAT at an equivalent age in LA natives, with 10-day-old pups able to thermoregulate in response to acute cold in both species. However, the onset of endothermy in HA pups was substantially delayed (by approx. 2 days), possibly driven by delayed sympathetic regulation of BAT. We suggest that this delay may be part of an evolved cost-saving measure to allow pups to maintain growth rates under the O₂-limited conditions at HA.

Evolution of physiological performance capacities and environmental adaptation: insights from high-elevation deer mice (Peromyscus maniculatus)

Analysis of variation in whole-animal performance can shed light on causal connections between specific traits, integrated physiological capacities, and Darwinian fitness. Here, we review and synthesize information on naturally occurring variation in physiological performance capacities and how it relates to environmental adaptation in deer mice (Peromyscus maniculatus). We discuss how evolved changes in aerobic exercise capacity and thermogenic capacity have contributed to adaptation to high elevations. Comparative work on deer mice at high and low elevations has revealed evolved differences in aerobic performance capacities in hypoxia. Highland deer mice have consistently higher aerobic performance capacities under hypoxia relative to lowland natives, consistent with the idea that it is beneficial to have a higher maximal metabolic rate (as measured by the maximal rate of O₂ consumption, VO_2max) in an environment characterized by lower air temperatures and lower O₂ availability. Observed differences in aerobic performance capacities between highland and lowland deer mice stem from changes in numerous subordinate traits that alter the flux capacity of the O₂-transport system, the oxidative capacity of tissue mitochondria, and the relationship between O₂ consumption and ATP synthesis. Many such changes in physiological phenotype are associated with hypoxia-induced changes in gene expression. Research on natural variation in whole-animal performance forms a nexus between physiological ecology and evolutionary biology that requires insight into the natural history of the study species.

Evolved Mechanisms of Aerobic Performance and Hypoxia Resistance in High-Altitude Natives

Comparative physiology studies of high-altitude species provide an exceptional opportunity to understand naturally evolved mechanisms of hypoxia resistance. Aerobic capacity (VO₂max) is a critical performance trait under positive selection in some high-altitude taxa, and several high-altitude natives have evolved to resist the depressive effects of hypoxia on VO₂max. This is associated with enhanced flux capacity through the O₂ transport cascade and attenuation of the maladaptive responses to chronic hypoxia that can impair O₂ transport. Some highlanders exhibit elevated rates of carbohydrate oxidation during exercise, taking advantage of its high ATP yield per mole of O₂. Certain highland native animals have also evolved more oxidative muscles and can sustain high rates of lipid oxidation to support thermogenesis. The underlying mechanisms include regulatory adjustments of metabolic pathways and to gene expression networks. Therefore, the evolution of hypoxia resistance in high-altitude natives involves integrated functional changes in the pathways for O₂ and substrate delivery and utilization by mitochondria.

The Ecology of Exercise: Mechanisms Underlying Individual Variation in Behavior, Activity, and Performance: An Introduction to Symposium

SYNOPSIS

Wild animals often engage in intense physical activity while performing tasks vital for their survival and reproduction associated with foraging, avoiding predators, fighting, providing parental care, and migrating. In this theme issue we consider how viewing these tasks as "exercise"-analogous to that performed by human athletes-may help provide insight into the mechanisms underlying individual variation in these types of behaviors and the importance of physical activity in an ecological context. In this article and throughout this issue, we focus on four key questions relevant to the study of behavioral ecology that may be addressed by studying wild animal behavior from the perspective of exercise physiology: (1) How hard do individual animals work in response to ecological (or evolutionary) demands?; (2) Do lab-based studies of activity provide good models for understanding activity in free-living animals and individual variation in traits?; (3) Can animals work too hard during "routine" activities?; and (4) Can paradigms of "exercise" and "training" be applied to free-living animals? Attempts to address these issues are currently being facilitated by rapid technological developments associated with physiological measurements and the remote tracking of wild animals, to provide mechanistic insights into the behavior of free-ranging animals at spatial and temporal scales that were previously impossible. We further suggest that viewing the behaviors of non-human animals in terms of the physical exercise performed will allow us to fully take advantage of these technological advances, draw from knowledge and conceptual frameworks already in use by human exercise physiologists, and identify key traits that constrain performance and generate variation in performance among individuals. It is our hope that, by highlighting mechanisms of behavior and performance, the articles in this issue will spur on further synergies between physiologists and ecologists, to take advantage of emerging cross-disciplinary perspectives and technologies.