Marsupial uncoupling protein 1 sheds light on the evolution of mammalian nonshivering thermogenesis

Two-stage evolution of mammalian adipose tissue thermogenesis

Brown adipose tissue (BAT) is a heater organ that expresses thermogenic uncoupling protein 1 (UCP1) to maintain high body temperatures during cold stress. BAT thermogenesis is considered an overarching mammalian trait, but its evolutionary origin is unknown. We show that adipose tissue of marsupials, which diverged from eutherian mammals ~150 million years ago, expresses a nonthermogenic UCP1 variant governed by a partial transcriptomic BAT signature similar to that found in eutherian beige adipose tissue. We found that the reconstructed UCP1 sequence of the common eutherian ancestor displayed typical thermogenic activity, whereas therian ancestor UCP1 is nonthermogenic. Thus, mammalian adipose tissue thermogenesis may have evolved in two distinct stages, with a prethermogenic stage in the common therian ancestor linking UCP1 expression to adipose tissue and thermal stress. We propose that in a second stage, UCP1 acquired its thermogenic function specifically in eutherians, such that the onset of mammalian BAT thermogenesis occurred only after the divergence from marsupials.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

Co-Evolution of Breast Milk Lipid Signaling and Thermogenic Adipose Tissue

Breastfeeding is a unique and defining behavior of mammals and has a fundamental role in nourishing offspring by supplying a lipid-rich product that is utilized to generate heat and metabolic fuel. Heat generation from lipids is a feature of newborn mammals and is mediated by the uncoupling of mitochondrial respiration in specific fat depots. Breastfeeding and thermogenic adipose tissue have a shared evolutionary history: both have evolved in the course of homeothermy evolution; breastfeeding mammals are termed "thermolipials", meaning "animals with warm fat". Beyond its heat-producing capacity, thermogenic adipose tissue is also necessary for proper lipid metabolism and determines adiposity in offspring. Recent advances have demonstrated that lipid metabolism in infants is orchestrated by breast milk lipid signals, which establish mother-to-child signaling and control metabolic development in the infant. Breastfeeding rates are declining worldwide, and are paralleled by an alarming increase in childhood obesity, which at least in part may have its roots in the impaired metabolic control by breast milk lipid signals.

Pros and cons for the evidence of adaptive non-shivering thermogenesis in marsupials

The thermogenic mechanisms supporting endothermy are still not fully understood in all major mammalian subgroups. In placental mammals, brown adipose tissue currently represents the most accepted source of adaptive non-shivering thermogenesis. Its mitochondrial protein UCP1 (uncoupling protein 1) catalyzes heat production, but the conservation of this mechanism is unclear in non-placental mammals and lost in some placentals. Here, we review the evidence for and against adaptive non-shivering thermogenesis in marsupials, which diverged from placentals about 120-160 million years ago. We critically discuss potential mechanisms that may be involved in the heat-generating process among marsupials.

New Insights into Molecular Mechanisms Mediating Adaptation to Exercise; A Review Focusing on Mitochondrial Biogenesis, Mitochondrial Function, Mitophagy and Autophagy

Exercise itself is fundamental for good health, and when practiced regularly confers a myriad of metabolic benefits in a range of tissues. These benefits are mediated by a range of adaptive responses in a coordinated, multi-organ manner. The continued understanding of the molecular mechanisms of action which confer beneficial effects of exercise on the body will identify more specific pathways which can be manipulated by therapeutic intervention in order to prevent or treat various metabolism-associated diseases. This is particularly important as exercise is not an available option to all and so novel methods must be identified to confer the beneficial effects of exercise in a therapeutic manner. This review will focus on key emerging molecular mechanisms of mitochondrial biogenesis, autophagy and mitophagy in selected, highly metabolic tissues, describing their regulation and contribution to beneficial adaptations to exercise.

Transcriptional Landscaping Identifies a Beige Adipocyte Depot in the Newborn Mouse

The present study sought to identify gene networks that are hallmarks of the developing inguinal subcutaneous adipose tissue (iWAT) and the interscapular brown adipose tissue (BAT) in the mouse. RNA profiling revealed that the iWAT of postnatal (P) day 6 mice expressed thermogenic and lipid catabolism transcripts, along with the abundance of transcripts associated with the beige adipogenesis program. This was an unexpected finding, as thermogenic BAT was believed to be the only site of nonshivering thermogenesis in the young mouse. However, the transcriptional landscape of BAT in P6 mice suggests that it is still undergoing differentiation and maturation, and that the iWAT temporally adopts thermogenic and lipolytic potential. Moreover, P6 iWAT and adult (P56) BAT were similar in their expression of immune gene networks, but P6 iWAT was unique in the abundant expression of antimicrobial proteins and virus entry factors, including a possible receptor for SARS-CoV-2. In summary, postnatal iWAT development is associated with a metabolic shift from thermogenesis and lipolysis towards fat storage. However, transcripts of beige-inducing signal pathways including β-adrenergic receptors and interleukin-4 signaling were underrepresented in young iWAT, suggesting that the signals for thermogenic fat differentiation may be different in early postnatal life and in adulthood.

Emergent Coordination of the CHKB and CPT1B Genes in Eutherian Mammals: Implications for the Origin of Brown Adipose Tissue

Mitochondrial fatty acid oxidation (FAO) contributes to the proton motive force that drives ATP synthesis in many mammalian tissues. In eutherian (placental) mammals, brown adipose tissue (BAT) can also dissipate this proton gradient through uncoupling protein 1 (UCP1) to generate heat, but the evolutionary events underlying the emergence of BAT are unknown. An essential step in FAO is the transport of cytoplasmic long chain acyl-coenzyme A (acyl-CoA) into the mitochondrial matrix, which requires the action of carnitine palmitoyltransferase 1B (CPT1B) in striated muscle and BAT. In eutherians, the CPT1B gene is closely linked to the choline kinase beta (CHKB) gene, which is transcribed from the same DNA strand and terminates just upstream of CPT1B. CHKB is a rate-limiting enzyme in the synthesis of phosphatidylcholine (PC), a predominant mitochondrial membrane phospholipid, suggesting that the coordinated expression of CHKB and CPT1B may cooperatively enhance mitochondrial FAO. The present findings show that transcription of the eutherian CHKB and CPT1B genes is linked within a unitary epigenetic domain targeted to the CHKB gene, and that that this regulatory linkage appears to have resulted from an intergenic deletion in eutherians that significantly altered the distribution of CHKB and CPT1B expression. Informed by the timing of this event relative to the emergence of BAT, the phylogeny of CHKB-CPT1B synteny, and the insufficiency of UCP1 to account for eutherian BAT, these data support a mechanism for the emergence of BAT based on the acquisition of a novel capacity for adipocyte FAO in a background of extant UCP1.

Neofunctionalization of the UCP1 mediated the non-shivering thermogenesis in the evolution of small-sized placental mammals

The acquisition of UCP1-mediated non-shivering thermogenesis (NST) was an important event during the evolution of mammals. Here, we assessed the thermogenic neofunctionalization that occurred in the mammalian UCP1, by performing detailed comparative evolutionary genomics analyses (including phylogenetic and selection analyses) of the UCP family members across all major vertebrate classes. Heterogeneously distributed positive selection signatures were found in several UCPs, being preferably located in the mitochondrial matrix domains. Additionally, comparisons with non-mammalian orthologs showed increased evolutionary rates of the mammalian UCP1, not observable in the phylogenetically related UCP2 and UCP3 paralogs. Also, parallel signatures of episodic positive selection (ω > 1) were found in the ancestral branches of both Glires (rodents and lagomorphs) and Afroinsectivores (afrosoricids and macroscelids), underlining the importance of the UCP1 thermogenic activity in these mammalian groups. Finally, we hypothesize that the independent positive selection events that occurred in these two lineages resulted in two UCP1-mediated NST approaches, namely the cold acute response in the Glires and the reproduction success enhancement in the Afroinsectivores.

Importance of adipocyte browning in the evolution of endothermy

Endothermy changes the relationship between organisms and their environment fundamentally, and it is therefore of major ecological and evolutionary significance. Endothermy is characterized by non-shivering thermogenesis, that is metabolic heat production in the absence of muscular activity. In many eutherian mammals, brown adipose tissue (BAT) is an evolutionary innovation that facilitates non-shivering heat production in mitochondria by uncoupling food-derived substrate oxidation from chemical energy (ATP) production. Consequently, energy turnover is accelerated resulting in increased heat release. The defining characteristics of BAT are high contents of mitochondria and vascularization, and the presence of uncoupling protein 1. Recent insights, however, reveal that a range of stimuli such as exercise, diet and the immune system can cause the browning of white adipocytes, thereby increasing energy expenditure and heat production even in the absence of BAT. Here, we review the molecular mechanisms that cause browning of white adipose tissue, and their potential contribution to thermoregulation. The significance for palaeophysiology lies in the presence of adipose tissue and the mechanisms that cause its browning and uncoupling in all amniotes. Hence, adipocytes may have played a role in the evolution of endothermy beyond the more specific evolution of BAT in eutherians. This article is part of the theme issue 'Vertebrate palaeophysiology'.

The evolution of mechanisms involved in vertebrate endothermy

Endothermy, i.e. the endogenous production of metabolic heat, has evolved multiple times among vertebrates, and several strategies of heat production have been studied extensively by physiologists over the course of the twentieth century. The independent acquisition of endothermy by mammals and birds has been the subject of many hypotheses regarding their origin and associated evolutionary constraints. Many groups of vertebrates, however, are thought to possess other mechanisms of heat production, and alternative ways to regulate thermogenesis that are not always considered in the palaeontological literature. Here, we perform a review of the mechanisms involved in heat production, with a focus on cellular and molecular mechanisms, in a phylogenetic context encompassing the entire vertebrate diversity. We show that endothermy in mammals and birds is not as well defined as commonly assumed by evolutionary biologists and consists of a vast array of physiological strategies, many of which are currently unknown. We also describe strategies found in other vertebrates, which may not always be considered endothermy, but nonetheless correspond to a process of active thermogenesis. We conclude that endothermy is a highly plastic character in vertebrates and provides a guideline on terminology and occurrences of the different types of heat production in vertebrate evolution. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Molecular evolution of uncoupling proteins and implications for brain function

Uncoupling proteins (UCPs) belong to the mitochondrial anion carrier superfamily and catalyze important metabolic functions at the mitochondrial inner membrane. While the thermogenic role of UCP1 in brown fat of eutherian mammals is well established, the molecular functions of UCP1 in ectothermic vertebrates and of other UCP paralogs remain less clear. Here, we critically discuss the existence of brain UCPs and their potential roles. Applying phylogenetic classification of novel UCPs, we summarize the evidence for brain UCP1 among vertebrates, the role of UCP2 in specific brain areas, and the existence of brain-specific UCPs. The phylogenetic analyses and discussion on functional data should alert the scientific community that the molecular function of so-called UCP1 homologues is by far not clarified and possibly relates to neither thermogenesis nor mitochondrial uncoupling.

The Impact of Early Human Migration on Brown Adipose Tissue Evolution and Its Relevance to the Modern Obesity Pandemic

Genetic factors are believed to be primarily responsible for obesity; however, an understanding of how genes for obesity have become so prevalent in modern society has proved elusive. Several theories have attempted to explain the genetic basis for obesity, but none of these appear to factor in the interethnic variation in obesity. Emerging evidence is increasingly pointing to a link between reduced basal metabolism and ineffective brown adipose tissue (BAT) thermogenesis. In fact, BAT presence and function are strongly correlated with metabolic rates and directly influence obesity susceptibility. My colleagues and I recently theorized that ancestral exposure to cold necessitated the evolution of enhanced BAT thermogenesis, which, with today’s hypercaloric and sedentary lifestyle, becomes advantageous, because thermogenesis is energetically wasteful, raising basal metabolism and burning excess calories. The opposite may be true for the descendants of heat-adapted populations. This review further reconciles global evolutionary climatic exposures with obesity demographics to understand the genetic basis for the obesity pandemic, and new insights from the most recent studies are provided, including those assessing archaic human admixture. Key genetic variants influencing BAT thermogenesis are outlined that have also been linked with climatic exposure to cold and appear to support the theory that evolutionary factors relevant to climate may have shaped the modern obesity pandemic.

Insights into brown adipose tissue evolution and function from non-model organisms

Brown adipose tissue (BAT) enables adaptive thermoregulation through heat production that is catalyzed by mitochondrial uncoupling protein 1 (UCP1). BAT is frequently studied in rodent model organisms, and recently in adult humans to treat metabolic diseases. However, complementary studies of many non-model species, which have diversified to many more ecological niches, may significantly broaden our understanding of BAT regulation and its physiological roles. This Review highlights the research on non-model organisms, which was instrumental to the discovery of BAT function, and the unique evolutionary history of BAT/UCP1 in mammalian thermogenesis. The comparative biology of BAT provides a powerful integrative approach that could identify conserved and specialized functional changes in BAT and UCP1 by considering species diversity, ecology and evolution, and by fusing multiple scientific disciplines such as physiology and biochemistry. Thus, resolving the complete picture of BAT biology may fail if comparative studies of non-model organisms are neglected.

Evolution of UCP1

Brown adipose tissue (BAT), the specialized heat-producing organ found in many placental mammals including humans, may be accessible for clinical drug intervention to help combat metabolic diseases. Understanding the biology of BAT and its thermogenic uncoupling protein 1 (UCP1) will benefit from an assessment of its evolution, answering where UCP1 originated and how it has been modified and integrated into cellular energy metabolism. Here, we review topical insights regarding the molecular evolution of UCP1-also reconstructing the proximate and ultimate factors selecting for brown fat thermogenesis in placental mammals. This new thinking on "old" events will assist our understanding of how thermogenic mitochondrial uncoupling was integrated into the physiology of the brown adipocyte. Recent comparative studies examining the occurrence of UCP1 in vertebrates not only identified the ancient (pre-mammal) rise of UCP1 but also its repeated downfall during mammalian evolution as evidenced by multiple independent gene loss and/or inactivation events. Together with the comparative physiology of various species, we may be able to find conditions that favor UCP1 thermogenesis and, learning from these insights, identify molecular networks that will be useful to pharmacologically stimulate the tissue.

Muscle Non-shivering Thermogenesis and Its Role in the Evolution of Endothermy

The development of sustained, long-term endothermy was one of the major transitions in the evolution of vertebrates. Thermogenesis in endotherms does not only occur via shivering or activity, but also via non-shivering thermogenesis (NST). Mammalian NST is mediated by the uncoupling protein 1 in the brown adipose tissue (BAT) and possibly involves an additional mechanism of NST in skeletal muscle. This alternative mechanism is based on Ca²⁺-slippage by a sarcoplasmatic reticulum Ca²⁺-ATPase (SERCA) and is controlled by the protein sarcolipin. The existence of muscle based NST has been discussed for a long time and is likely present in all mammals. However, its importance for thermoregulation was demonstrated only recently in mice. Interestingly, birds, which have evolved from a different reptilian lineage than mammals and lack UCP1-mediated NST, also exhibit muscle based NST under the involvement of SERCA, though likely without the participation of sarcolipin. In this review we summarize the current knowledge on muscle NST and discuss the efficiency of muscle NST and BAT in the context of the hypothesis that muscle NST could have been the earliest mechanism of heat generation during cold exposure in vertebrates that ultimately enabled the evolution of endothermy. We suggest that the evolution of BAT in addition to muscle NST was related to heterothermy being predominant among early endothermic mammals. Furthermore, we argue that, in contrast to small mammals, muscle NST is sufficient to maintain high body temperature in birds, which have enhanced capacities to fuel muscle NST by high rates of fatty acid import.

Adiponectin is required for maintaining normal body temperature in a cold environment

BACKGROUND

Thermogenic impairment promotes obesity and insulin resistance. Adiponectin is an important regulator of energy homeostasis. While many beneficial metabolic effects of adiponectin resemble that of activated thermogenesis, the role of adiponectin in thermogenesis is not clear. In this study, we investigated the role of adiponectin in thermogenesis using adiponectin-null mice (Adipoq ^-/-).

METHODS

Body composition was measured using EchoMRI. Metabolic parameters were determined by indirect calorimetry. Insulin sensitivity was evaluated by glucose- and insulin- tolerance tests. Core body temperature was measured by a TH-8 temperature monitoring system. Gene expression was assessed by real-time PCR and protein levels were analyzed by Western blotting and immunohistochemistry. The mitochondrial density of brown adipose tissue was quantified by calculating the ratio of mtDNA:total nuclear DNA.

RESULTS

Under normal housing temperature of 24 °C and ad libitum feeding condition, the body weight, body composition, and metabolic profile of Adipoq ^-/- mice were unchanged. Under fasting condition, Adipoq ^-/- mice exhibited reduced energy expenditure. Conversely, under cold exposure, Adipoq ^-/- mice exhibited reduced body temperature, and the expression of thermogenic regulatory genes was significantly reduced in brown adipose tissue (BAT) and subcutaneous white adipose tissue (WAT). Moreover, we observed that mitochondrial content was reduced in BAT and subcutaneous WAT, and the expression of mitochondrial fusion genes was decreased in BAT of Adipoq ^-/- mice, suggesting that adiponectin ablation diminishes mitochondrial biogenesis and altered mitochondrial dynamics. Our study further revealed that adiponectin deletion suppresses adrenergic activation, and down-regulates β3-adrenergic receptor, insulin signaling, and the AMPK-SIRT1 pathway in BAT.

CONCLUSIONS

Our findings demonstrate that adiponectin is an essential regulator of thermogenesis, and adiponectin is required for maintaining body temperature under cold exposure.

Evolution of UCP1 Transcriptional Regulatory Elements Across the Mammalian Phylogeny

Uncoupling protein 1 (UCP1) permits non-shivering thermogenesis (NST) when highly expressed in brown adipose tissue (BAT) mitochondria. Exclusive to placental mammals, BAT has commonly been regarded to be advantageous for thermoregulation in hibernators, small-bodied species, and the neonates of larger species. While numerous regulatory control motifs associated with UCP1 transcription have been proposed for murid rodents, it remains unclear whether these are conserved across the eutherian mammal phylogeny and hence essential for UCP1 expression. To address this shortcoming, we conducted a broad comparative survey of putative UCP1 transcriptional regulatory elements in 139 mammals (135 eutherians). We find no evidence for presence of a UCP1 enhancer in monotremes and marsupials, supporting the hypothesis that this control region evolved in a stem eutherian ancestor. We additionally reveal that several putative promoter elements (e.g., CRE-4, CCAAT) identified in murid rodents are not conserved among BAT-expressing eutherians, and together with the putative regulatory region (PRR) and CpG island do not appear to be crucial for UCP1 expression. The specificity and importance of the upTRE, dnTRE, URE1, CRE-2, RARE-2, NBRE, BRE-1, and BRE-2 enhancer elements first described from rats and mice are moreover uncertain as these motifs differ substantially—but generally remain highly conserved—in other BAT-expressing eutherians. Other UCP1 enhancer motifs (CRE-3, PPRE, and RARE-3) as well as the TATA box are also highly conserved in nearly all eutherian lineages with an intact UCP1. While these transcriptional regulatory motifs are generally also maintained in species where this gene is pseudogenized, the loss or degeneration of key basal promoter (e.g., TATA box) and enhancer elements in other UCP1-lacking lineages make it unlikely that the enhancer region is pleiotropic (i.e., co-regulates additional genes). Importantly, differential losses of (or mutations within) putative regulatory elements among the eutherian lineages with an intact UCP1 suggests that the transcriptional control of gene expression is not highly conserved in this mammalian clade.

The hibernating South American marsupial, Dromiciops gliroides, displays torpor-sensitive microRNA expression patterns

When faced with adverse environmental conditions, the marsupial Dromiciops gliroides uses either daily or seasonal torpor to support survival and is the only known hibernating mammal in South America. As the sole living representative of the ancient Order Microbiotheria, this species can provide crucial information about the evolutionary origins and biochemical mechanisms of hibernation. Hibernation is a complex energy-saving strategy that involves changes in gene expression that are elicited in part by microRNAs. To better elucidate the role of microRNAs in orchestrating hypometabolism, a modified stem-loop technique and quantitative PCR were used to characterize the relative expression levels of 85 microRNAs in liver and skeletal muscle of control and torpid D. gliroides. Thirty-nine microRNAs were differentially regulated during torpor; of these, 35 were downregulated in liver and 11 were differentially expressed in skeletal muscle. Bioinformatic analysis predicted that the downregulated liver microRNAs were associated with activation of MAPK, PI3K-Akt and mTOR pathways, suggesting their importance in facilitating marsupial torpor. In skeletal muscle, hibernation-responsive microRNAs were predicted to regulate focal adhesion, ErbB, and mTOR pathways, indicating a promotion of muscle maintenance mechanisms. These tissue-specific responses suggest that microRNAs regulate key molecular pathways that facilitate hibernation, thermoregulation, and prevention of muscle disuse atrophy.

Brown adipose tissue: The heat is on the heart

The study of brown adipose tissue (BAT) has gained significant scientific interest since the discovery of functional BAT in adult humans. The thermogenic properties of BAT are well recognized; however, data generated in the last decade in both rodents and humans reveal therapeutic potential for BAT against metabolic disorders and obesity. Here we review the current literature in light of a potential role for BAT in beneficially mediating cardiovascular health. We focus mainly on BAT's actions in obesity, vascular tone, and glucose and lipid metabolism. Furthermore, we discuss the recently discovered endocrine factors that have a potential beneficial role in cardiovascular health. These BAT-secreted factors may have a favorable effect against cardiovascular risk either through their metabolic role or by directly affecting the heart.

Brown adipose tissue: physiological function and evolutionary significance

In modern eutherian (placental) mammals, brown adipose tissue (BAT) evolved as a specialized thermogenic organ that is responsible for adaptive non-shivering thermogenesis (NST). For NST, energy metabolism of BAT mitochondria is increased by activation of uncoupling protein 1 (UCP1), which dissipates the proton motive force as heat. Despite the presence of UCP1 orthologues prior to the divergence of teleost fish and mammalian lineages, UCP1's significance for thermogenic adipose tissue emerged at later evolutionary stages. Recent studies on the presence of BAT in metatherians (marsupials) and eutherians of the afrotherian clade provide novel insights into the evolution of adaptive NST in mammals. In particular studies on the 'protoendothermic' lesser hedgehog tenrec (Afrotheria) suggest an evolutionary scenario linking BAT to the onset of eutherian endothermy. Here, we review the physiological function and distribution of BAT in an evolutionary context by focusing on the latest research on phylogenetically distinct species.

A clinical approach to brown adipose tissue in the para-aortic area of the human thorax

BACKGROUND

Human thoracic brown adipose tissue (BAT), composed of several subdivisions, is a well-known target organ of many clinical studies; however, the functional contribution of each part of human thoracic BAT remains unknown. The present study analyzed the significance of each part of human thoracic BAT in the association between regional distribution, cellularity, and factors involved in the functional regulation of thoracic BAT.

METHODS

We analyzed 1550 healthy adults who underwent medical check-ups by positron-emission tomography and computed tomography (PET-CT) imaging, 8 cadavers, and 78 autopsy cases in an observational study. We first characterized the difference between the mediastinum and the supraclavicular areas using counts of BAT detection and conditions based on PET-CT outcomes. The measurable important area was then subjected to systematic anatomical and immunohistochemical analyses using anti-uncoupling protein 1 (UCP1) antibody to characterize the cellularity in association with age and sex.

RESULTS

In PET-CT scanning, the main site of thoracic BAT was the mediastinum rather than the supraclavicular area (P < 0.05). Systemic macroanatomy revealed that the thumb-sized BAT in the posterior mediastinal descending para-aortic area (paBAT) had feeding vessels from the posterior intercostal arteries and veins and sympathetic/parasympathetic innervation from trunks of the sympathetic and vagus nerves, respectively. Immunohistochemical analysis indicated that the paBAT exhibited immunoreactivity for tyrosine hydroxylase and vesicular acetylcholine transporter located in the pericellular nervous fibers and intracellular UCP1. The brown adipose cells of paBAT showed age-dependent decreases in UCP1 expression (P < 0.05), accompanied by a significant increase in vacuole formation, indicating fat accumulation (P < 0.05), from 10 to 37 years of age (P < 0.01).

CONCLUSIONS

paBAT may be one of the essential sites for clinical application in BAT study because of its visible anatomy with feeding vessels and sympathetic/parasympathetic innervation functionally affected by outer condition and senescence.

Brown adipose tissue and novel therapeutic approaches to treat metabolic disorders

In humans, 2 functionally different types of adipose tissue coexist: white adipose tissue (WAT) and brown adipose tissue (BAT). WAT is involved in energy storage, whereas BAT is involved in energy expenditure. Increased amounts of WAT may contribute to the development of metabolic disorders, such as obesity-associated type 2 diabetes mellitus and cardiovascular diseases. In contrast, the thermogenic function of BAT allows high consumption of fatty acids because of the activity of uncoupling protein 1 in the internal mitochondrial membrane. Interestingly, obesity reduction and insulin sensitization have been achieved by BAT activation-regeneration in animal models. This review describes the origin, function, and differentiation mechanisms of BAT to identify new therapeutic strategies for the treatment of metabolic disorders related to obesity. On the basis of the animal studies, novel approaches for BAT regeneration combining stem cells from the adipose tissue with active components, such as melatonin, may have potential for the treatment of metabolic disorders in humans.

The role of skeletal-muscle-based thermogenic mechanisms in vertebrate endothermy

Thermogenesis is one of the most important homeostatic mechanisms that evolved during vertebrate evolution. Despite its importance for the survival of the organism, the mechanistic details behind various thermogenic processes remain incompletely understood. Although heat production from muscle has long been recognized as a thermogenic mechanism, whether muscle can produce heat independently of contraction remains controversial. Studies in birds and mammals suggest that skeletal muscle can be an important site of non-shivering thermogenesis (NST) and can be recruited during cold adaptation, although unequivocal evidence is lacking. Much research on thermogenesis during the last two decades has been focused on brown adipose tissue (BAT). These studies clearly implicate BAT as an important site of NST in mammals, in particular in newborns and rodents. However, BAT is either absent, as in birds and pigs, or is only a minor component, as in adult large mammals including humans, bringing into question the BAT-centric view of thermogenesis. This review focuses on the evolution and emergence of various thermogenic mechanisms in vertebrates from fish to man. A careful analysis of the existing data reveals that muscle was the earliest facultative thermogenic organ to emerge in vertebrates, long before the appearance of BAT in eutherian mammals. Additionally, these studies suggest that muscle-based thermogenesis is the dominant mechanism of heat production in many species including birds, marsupials, and certain mammals where BAT-mediated thermogenesis is absent or limited. We discuss the relevance of our recent findings showing that uncoupling of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) by sarcolipin (SLN), resulting in futile cycling and increased heat production, could be the basis for NST in skeletal muscle. The overall goal of this review is to highlight the role of skeletal muscle as a thermogenic organ and provide a balanced view of thermogenesis in vertebrates.

White, brite, and brown adipocytes: the evolution and function of a heater organ in mammals

Brown fat is a specialized heater organ in eutherian mammals. In contrast to the energy storage function of white adipocytes, brown adipocytes dissipate nutrient energy by uncoupling of mitochondrial oxidative phosphorylation, which depends on uncoupling protein 1 (UCP1). UCP1, as well as UCP2 and UCP3, belong to the family of mitochondrial carriers inserted into the inner mitochondrial membrane for metabolite trafficking between the matrix and the intermembrane space. UCP1 transports protons into the mitochondrial matrix when activated by a rise in free fatty acid levels in the cell. This UCP1-dependant proton leak drives high oxygen consumption rates in the absence of ATP synthesis and dissipates proton motive force as heat. The enormous heating capacity of brown fat is supported by dense vascularization, high rates of tissue perfusion, and high mitochondrial density in brown adipocytes.

It has been known for more than 50 years that nonshivering thermogenesis in brown fat serves to maintain body temperature of neonates and small mammals in cold environments, and is used by hibernators for arousal from torpor. It has been speculated that the development of brown fat as a new source for nonshivering thermogenesis provided mammals with a unique advantage for survival in the cold. Indeed brown fat and UCP1 is found in ancient groups of mammals, like the afrotherians and marsupials. In the latter, however, the thermogenic function of UCP1 and brown fat has not been demonstrated as of yet. Notably, orthologs of all three mammalian UCP genes are also present in the genomes of bony fishes and in amphibians. Molecular phylogeny reveals a striking increase in the substitution rate of UCP1 between marsupial and eutherian lineages. At present, it seems that UCP1 only gained thermogenic function in brown adipocytes of eutherian mammals, whereas the function of UCP1 and that of the other UCPs in ectotherms remains to be identified.

Evolution of thermogenic function required expression of UCP1 in a brown-adipocyte-like cell equipped with high mitochondrial density embedded in a well-vascularized tissue. Brown-adipocyte-like cells in white adipose tissue, called “brite” (brown-in-white) or “beige” adipocytes, emerge during adipogenesis and in response to cold exposure in anatomically distinct adipose tissue depots of juvenile and adult rodents. These brite adipocytes may resemble the archetypical brown adipocyte in vertebrate evolution. It is therefore of interest to elucidate the molecular mechanisms of brite adipocyte differentiation, study the bioenergetic properties of these cells, and search for the presence of related brown-adipocyte-like cells in nonmammalian vertebrates.

Tissue-specific gene expression and functional regulation of uncoupling protein 2 (UCP2) by hypoxia and nutrient availability in gilthead sea bream (Sparus aurata): implications on the physiological significance of UCP1-3 variants

The aim of this study was to assess in an integrative manner the physiological regulation of uncoupling protein 2 (UCP2) in gilthead sea bream. A contig of 1,325 nucleotides in length with an open reading frame of 307 amino acids was recognized as UCP2 after searches in our transcriptome reference database ( http://www.nutrigroup-iats.org/seabreamdb ). Gene expression mapping by quantitative real-time PCR revealed a ubiquitous profile that clearly differs from that of UCP1 and UCP3 variants with the greatest abundance in liver and white skeletal muscle, respectively. The greatest abundance of UCP2 transcripts was found in the heart, with a relatively high expression level in blood cells, where UCP1 and UCP3 transcripts were practically undetectable. Functional studies revealed that UCP2 mRNA expression remains either unaltered or up-regulated upon feed restriction in glycolytic (white skeletal muscle) and highly oxidative muscle tissues (heart and red skeletal muscle), respectively. In contrast, exposure to hypoxic conditions (18-19% oxygen saturation) markedly down-regulated the UCP2 mRNA expression in blood cells in a cellular environment with increased haematocrit, blood haemoglobin content, and circulating levels of glucose and lactate, and total plasma antioxidant activity. These findings demonstrated that UCP2 expression is highly regulated at the transcriptional level, arising this UCP variant as an important piece of the complex trade-off between metabolic and redox sensors. This feature would avoid the activation of futile cycles of energy wastage if changes in tissue oxidative and antioxidant metabolic capabilities are able to maintain the production of reactive oxygen species at a low regulated level.

Life-history Constraints on the Mechanisms that Control the Rate of ROS Production

The quest to understand why and how we age has led to numerous lines of investigation that have gradually converged to consider mitochondrial metabolism as a major player. During mitochondrial respiration a small and variable amount of the consumed oxygen is converted to reactive species of oxygen (ROS). For many years, these ROS have been perceived as harmful by-products of respiration. However, evidence from recent years indicates that ROS fulfill important roles as cellular messengers. Results obtained using model organisms suggest that ROS-dependent signalling may even activate beneficial cellular stress responses, which eventually may lead to increased lifespan. Nevertheless, when an overload of ROS cannot be properly disposed of, its accumulation generates oxidative stress, which plays a major part in the ageing process. Comparative studies about the rates of ROS production and oxidative damage accumulation, have led to the idea that the lower rate of mitochondrial oxygen radical generation of long-lived animals with respect to that of their short-lived counterpart, could be a primary cause of their slow ageing rate. A hitherto largely under-appreciated alternative view is that such lower rate of ROS production, rather than a cause may be a consequence of the metabolic constraints imposed for the large body sizes that accompany high lifespans. To help understanding the logical underpinning of this rather heterodox view, herein I review the current literature regarding the mechanisms of ROS formation, with particular emphasis on evolutionary aspects.

Acid-base balance in the developing marsupial: from ectotherm to endotherm

Marsupial joeys are born ectothermic and develop endothermy within their mother's thermally stable pouch. We hypothesized that Tammar wallaby joeys would switch from α-stat to pH-stat regulation during the transition from ectothermy to endothermy. To address this, we compared ventilation (V̇e), metabolic rate (V̇o2), and variables relevant to blood gas and acid-base regulation and oxygen transport including the ventilatory requirements (V̇e/V̇o2 and V̇e/V̇co2), partial pressures of oxygen (PaO2), carbon dioxide (PaCO2), pHa, and oxygen content (CaO2) during progressive hypothermia in ecto- and endothermic Tammar wallabies. We also measured the same variables in the well-studied endotherm, the Sprague-Dawley rat. Hypothermia was induced in unrestrained, unanesthetized joeys and rats by progressively dropping the ambient temperature (Ta). Rats were additionally exposed to helox (80% helium, 20% oxygen) to facilitate heat loss. Respiratory, metabolic, and blood-gas variables were measured over a large body temperature (Tb) range (∼15–16°C in both species). Ectothermic joeys displayed limited thermogenic ability during cooling: after an initial plateau, V̇o2 decreased with the progressive drop in Tb. The Tb of endothermic joeys and rats fell despite V̇o2 nearly doubling with the initiation of cold stress. In all three groups the changes in V̇o2 were met by changes in V̇e, resulting in constant V̇e/V̇o2 and V̇e/V̇co2, blood gases, and pHa. Thus, although thermogenic capability was nearly absent in ectothermic joeys, blood acid-base regulation was similar to endothermic joeys and rats. This suggests that unlike some reptiles, unanesthetized mammals protect arterial blood pH with changing Tb, irrespective of their thermogenic ability and/or stage of development.

Molecular imaging of brown adipose tissue in health and disease

PURPOSE

Brown adipose tissue (BAT) has transformed from an interfering tissue in oncological (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to an independent imaging research field. This review takes the perspective from the imaging methodology on which human BAT research has come to rely on heavily.

METHODS

This review analyses relevant PubMed-indexed publications that discuss molecular imaging methods of BAT. In addition, reported links between BAT and human diseases such as obesity are discussed, and the possibilities for imaging in these fields are highlighted. Radiopharmaceuticals aiming at several different biological mechanisms of BAT are discussed and evaluated.

RESULTS

Prospective, dedicated studies allow visualization of BAT function in a high percentage of human subjects. BAT dysfunction has been implicated in obesity, linked with diabetes and associated with cachexia and atherosclerosis. Presently, (18)F-FDG PET/CT is the most useful tool for evaluating therapies aiming at BAT activity. In addition to (18)F-FDG, other radiopharmaceuticals such as (99m)Tc-sestamibi, (123)I-metaiodobenzylguanidine (MIBG), (18)F-fluorodopa and (18)F-14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid (FTHA) may have a potential for visualizing other aspects of BAT activity. MRI methods are under continuous development and provide the prospect of functional imaging without ionizing radiation.

CONCLUSION

Molecular imaging of BAT can be used to quantitatively assess different aspects of BAT metabolic activity.

A functional nexus between photoperiod acclimation, torpor expression and somatic fatty acid composition in a heterothermic mammal

The seasonal changes in thermal physiology and torpor expression of many heterothermic mammals are controlled by photoperiod. As function at low body temperatures during torpor requires changes of tissue lipid composition, we tested for the first time whether and how fatty acids are affected by photoperiod acclimation in hamsters, Phodopus sungorus, a strongly photoperiodic species. We also examined changes in fatty acid composition in relation to those in morphology and thermal biology. Hamsters in short photoperiod had smaller reproductive organs and most had a reduced body mass in comparison to those in long photoperiod. Pelage colour of hamsters under short photoperiod was almost white while that of long photoperiod hamsters was grey-brown and black. Short photoperiod acclimation resulted in regular (28% of days) torpor use, whereas all hamsters in long photoperiod remained normothermic. The composition of total fatty acids differed between acclimation groups for brown adipose tissue (5 of 8 fatty acids), heart muscle (4 of 7 fatty acids) and leg muscle (3 of 11 fatty acids). Importantly, 54% of all fatty acids detected were correlated (r² = 0.60 to 0.87) with the minimum surface temperature of individuals, but the responses of tissues differed. While some of the compositional changes of fatty acids were consistent with a ‘homeoviscous’ response, this was not the case for all, including the sums of saturated and unsaturated fatty acids, which did not differ between acclimation groups. Our data identify a possible nexus between photoperiod acclimation, morphology, reproductive biology, thermal biology and fatty acid composition. They suggest that some of the changes in thermal physiology are linked to the composition of tissue and organ fatty acids.

A comparative approach to understanding tissue-specific expression of uncoupling protein 1 expression in adipose tissue

The thermoregulatory function of brown adipose tissue (BAT) is due to the tissue-specific expression of uncoupling protein 1 (UCP1) which is thought to have evolved in early mammals. We report that a CpG island close to the UCP1 transcription start site is highly conserved in all 29 vertebrates examined apart from the mouse and xenopus. Using methylation sensitive restriction digest and bisulfite mapping we show that the CpG island in both the bovine and human is largely un-methylated and is not related to differences in UCP1 expression between white and BAT. Tissue-specific expression of UCP1 has been proposed to be regulated by a conserved 5′ distal enhancer which has been reported to be absent in marsupials. We demonstrate that the enhancer, is also absent in five eutherians as well as marsupials, monotremes, amphibians, and fish, is present in pigs despite UCP1 having become a pseudogene, and that absence of the enhancer element does not relate to BAT-specific UCP1 expression. We identify an additional putative 5′ regulatory unit which is conserved in 14 eutherian species but absent in other eutherians and vertebrates, but again unrelated to UCP1 expression. We conclude that despite clear evidence of conservation of regulatory elements in the UCP1 5′ untranslated region, this does not appear to be related to species or tissues-specific expression of UCP1.

Relevance of brown adipose tissue in infancy and adolescence

Brown adipose tissue (BAT) was thought to disappear after infancy. Recent findings of BAT in patients undergoing positron emission tomography/computed tomography (PET/CT) have renewed the interest in deciphering the relevance of this tissue in humans. Available data suggest that BAT is more prevalent in children than in adults and that its activation during adolescence is associated with significantly lower gains in weight and adiposity. Data also show that pediatric patients with metabolically active BAT on PET/CT examinations have significantly greater muscle volume than patients without identifiable BAT. Both the activity and the amount of BAT increase during puberty. The magnitude of the increase is higher in boys as compared with girls and is closely related to gains in muscle volume. Hence, concurrent with the gains in skeletal muscle during infancy and puberty, all infants and adolescents accumulate large amounts of BAT. These observations are consistent with in vitro investigations suggesting close interactions between brown adipocytes, white adipocytes, and myocytes. In this review, we discuss the potential role of this tissue in regulating weight and musculoskeletal development in children.

Developmental phenotypic plasticity in a marsupial

Climate change is likely to substantially affect the distribution ranges of species. However, little is known about how different mammalian taxa respond morphologically and physiologically to a rapid change of climate. Our objective was to provide the first quantitative data on the effect of continuous cold exposure during development on morphological and functional variables of a marsupial. Fat-tailed dunnarts (Sminthopsis crassicaudata, Dasyuridae) were reared at an ambient temperature (Ta) of 16°C [cold-reared (CR)] or 22°C [warm-reared (WR)] until they reached adult age (&gt;200 days). Body and head length of CR animals were significantly longer than in WR animals (mean ± s.e.m.; body: CR 80.8±6 mm, WR 76.4±5 mm; head: CR 29.4±3 mm, WR 27.5±2 mm), but other body attributes were not significantly different. Use of torpor was more frequent, torpor bout duration was longer and average daily metabolic rate and percentage of savings when using torpor were significantly higher (P&lt;0.01) in CR than in WR animals at 16°C Ta but not at 24°C. Furthermore, resting metabolic rates measured at 16°C Ta were significantly lower in CR than WR animals; at 30°C Ta values were similar. Our results do not conform to Allen’s rule, but to some extent they do conform to Bergmann’s rule. However, the data demonstrate that a relatively moderate cold exposure from birth until adulthood induces marked changes in the morphology and thermal energetics of small marsupials. Such short-term phenotypic responses without the need for long-term selection are likely important for the ability to cope with different climates over a wide range of distribution, but will also play a crucial role in enhancing the survival of species during climate change.

Expression of uncoupling proteins in a mammalian cell culture system (HEK293) and assessment of their protein function

Immortalised cultured cells are powerful tools to assess the function of ectopically expressed proteins. However, it must be ensured that the protein of interest is functional in the host system and display native behaviour. In particular, mitochondrial uncoupling proteins (UCPs) displayed (non-native) artefactual uncoupling when expressed in yeast or can possess functions upon reconstitution in proteoliposomes that cannot be reproduced in isolated mitochondria. In the light of newly discovered UCP1 orthologues and paralogues (UCP2, UCP3, plant UCP), comparative functional studies require a system with identical mitochondrial, cellular, and genetic backgrounds. In this chapter, the protocols for the ectopic expression of mouse UCP1 in the human embryonic kidney (HEK293) cell line are introduced. In isolated cell culture mitochondria, the proton leak can be measured and modulators of UCP1 activity can be tested. As mouse UCP1 in this system shows native behaviour, this may be a suitable system to directly compare the functional relationships between different UCPs.

The regulation and physiology of mitochondrial proton leak

Mitochondria couple respiration to ATP synthesis through an electrochemical proton gradient. Proton leak across the inner membrane allows adjustment of the coupling efficiency. The aim of this review is threefold: 1) introduce the unfamiliar reader to proton leak and its physiological significance, 2) review the role and regulation of uncoupling proteins, and 3) outline the prospects of proton leak as an avenue to treat obesity, diabetes, and age-related disease.

Ablations of ghrelin and ghrelin receptor exhibit differential metabolic phenotypes and thermogenic capacity during aging

Background

Obesity is a hallmark of aging in many Western societies, and is a precursor to numerous serious age-related diseases. Ghrelin (Ghrl), via its receptor (growth hormone secretagogue receptor, GHS-R), is shown to stimulate GH secretion and appetite. Surprisingly, our previous studies showed that Ghrl^-/- mice have impaired thermoregulatory responses to cold and fasting stresses, while Ghsr^-/- mice are adaptive.

Methodology/Principal Findings

To elucidate the mechanism, we analyzed the complete metabolic profiles of younger (3–4 months) and older (10–12 months) Ghrl^-/- and Ghsr^-/- mice. Food intake and locomotor activity were comparable for both null mice and their wild-type (WT) counterparts, regardless of age. There was also no difference in body composition between younger null mice and their WT counterparts. As the WT mice aged, as expected, the fat/lean ratio increased and energy expenditure (EE) decreased. Remarkably, however, older Ghsr^-/- mice exhibited reduced fat/lean ratio and increased EE when compared to older WT mice, thus retaining a youthful lean and high EE phenotype; in comparison, there was no significant difference with EE in Ghrl^-/- mice. In line with the EE data, the thermogenic regulator, uncoupling protein 1 (UCP1), was significantly up-regulated in brown adipose tissue (BAT) of Ghsr^-/- mice, but not in Ghrl^-/- mice.

Conclusions

Our data therefore suggest that GHS-R ablation activates adaptive thermogenic function(s) in BAT and increases EE, thereby enabling the retention of a lean phenotype. This is the first direct evidence that the ghrelin signaling pathway regulates fat-burning BAT to affect energy balance during aging. This regulation is likely mediated through an as-yet-unidentified new ligand of GHS-R.

Caloric restriction and longevity: effects of reduced body temperature

Caloric restriction (CR) causes a reduction in body temperature (T(b)) which is suggested to contribute to changes that increase lifespan. Moreover, low T(b) has been shown to improve health and longevity independent of CR. In this review we examine the connections between CR, T(b) and mechanisms that influence longevity and ageing. Recent findings regarding the overlapping mechanisms of CR and T(b) that benefit longevity are discussed, including changes in body composition, hormone regulation, and gene expression, as well as reductions in low-level inflammation and reactive oxygen species-induced molecular damage. This information is summarized in a model describing how CR and low T(b), both synergistically and independently, increase lifespan. Moreover, the nascent notion that the rate of ageing may be pre-programmed in response to environmental influences at critical periods of early development is also considered. Based on current evidence, it is concluded that low T(b) plays an integral role in mediating the effects of CR on health and longevity, and that low T(b) may exert independent biological changes that increase lifespan. Our understanding of the overlap between CR- and T(b)-mediated longevity remains incomplete and should be explored in future research.

Mitochondrial proton and electron leaks

Mitochondrial proton and electron leak have a major impact on mitochondrial coupling efficiency and production of reactive oxygen species. In the first part of this chapter, we address the molecular nature of the basal and inducible proton leak pathways, and their physiological importance. The basal leak is unregulated, and a major proportion can be attributed to mitochondrial anion carriers, whereas the proton leak through the lipid bilayer appears to be minor. The basal proton leak is cell-type specific and correlates with metabolic rate. The inducible leak through the ANT (adenine nucleotide translocase) and UCPs (uncoupling proteins) can be activated by fatty acids, superoxide or lipid peroxidation products. The physiological role of inducible leak through UCP1 in mammalian brown adipose tissue is heat production, whereas the roles of non-mammalian UCP1 and its paralogous proteins, in particular UCP2 and UCP3, are not yet resolved. The second part of the chapter focuses on the electron leak that occurs in the mitochondrial electron transport chain. Exit of electrons prior to the reduction of oxygen to water at cytochrome c oxidase causes superoxide production. As the mechanisms of electron leak are crucial to understanding their physiological relevance, we summarize the mechanisms and topology of electron leak from complexes I and III in studies using isolated mitochondria. We also highlight recent progress and challenges of assessing electron leak in the living cell. Finally, we emphasize the importance of proton and electron leak as therapeutic targets in body mass regulation and insulin secretion.

Adaptive thermogenesis and thermal conductance in wild-type and UCP1-KO mice

We compared maximal cold-induced heat production (HPmax) and cold limits between warm (WA; 27°C), moderate cold (MCA; 18°C), or cold acclimated (CA; 5°C) wild-type and uncoupling-protein 1 knockout (UCP1-KO) mice. In wild-type mice, HPmax was successively increased after MCA and CA, and the cold limit was lowered to -8.3°C and -18.0°C, respectively. UCP1-KO mice also increased HPmax in response to MCA and CA, although to a lesser extent. Direct comparison revealed a maximal cold-induced recruitment of heat production by +473 mW and +227 mW in wild-type and UCP1-KO mice, respectively. The increase in cold tolerance of UCP1-KO mice from -0.9°C in MCA to -10.1°C in CA could not be directly related to changes in HPmax, indicating that UCP1-KO mice used the dissipated heat more efficiently than wild-type mice. As judged from respiratory quotients, acutely cold-challenged UCP1-KO mice showed a delayed transition toward lipid oxidation, and 5-h cold exposure revealed diminished physical activity and less variability in the control of metabolic rate. We conclude that BAT is required for maximal adaptive thermogenesis but also allows metabolic flexibility and a rapid switch toward sustained lipid-fuelled thermogenesis as an acute response to cold. In both CA groups, expression of contractile proteins (myosin heavy-chain isoforms) showed minor training effects in skeletal muscles, while cardiac muscle of UCP1-KO mice had novel expression of beta cardiac isoform. Neither respiration nor basal proton conductance of skeletal muscle mitochondria were different between genotypes. In subcutaneous white adipose tissue of UCP1-KO mice, cold exposure increased cytochrome-c oxidase activity and expression of the cell death-inducing DFFA-like effector A by 3.6-fold and 15-fold, respectively, indicating the recruitment of mitochondria-rich brown adipocyte-like cells. Absence of functional BAT leads to remodeling of white adipose tissue, which may significantly contribute to adaptive thermogenesis during cold acclimation.