Functional characterisation of UCP1 in the common carp: uncoupling activity in liver mitochondria and cold-induced expression in the brain

Multigenerational exposure to temperature influences mitochondrial oxygen fluxes in the Medaka fish (Oryzias latipes)

AIM

Thermal sensitivity of cellular metabolism is crucial for animal physiology and survival under climate change. Despite recent efforts, effects of multigenerational exposure to temperature on the metabolic functioning remain poorly understood. We aimed at determining whether multigenerational exposure to temperature modulate the mitochondrial respiratory response of Medaka fish.

METHODS

We conducted a multigenerational exposure with Medaka fish reared multiple generations at 20 and 30°C (COLD and WARM fish, respectively). We then measured the oxygen consumption of tail muscle at two assay temperatures (20 and 30°C). Mitochondrial function was determined as the respiration supporting ATP synthesis (OXPHOS) and the respiration required to offset proton leak (LEAK(Omy)) in a full factorial design (COLD-20°C; COLD-30°C; WARM-20°C; WARM-30°C).

RESULTS

We found that higher OXPHOS and LEAK fluxes at 30°C compared to 20°C assay temperature. At each assay temperature, WARM fish had lower tissue oxygen fluxes than COLD fish. Interestingly, we did not find significant differences in respiratory flux when mitochondria were assessed at the rearing temperature of the fish (i.e., COLD-20°C vs. WARM -30°C).

CONCLUSION

The lower OXPHOS and LEAK capacities in warm fish are likely the result of the multigenerational exposure to warm temperature. This is consistent with a modulatory response of mitochondrial capacity to compensate for potential detrimental effects of warming on metabolism. Finally, the absence of significant differences in respiratory fluxes between COLD-20°C and WARM-30°C fish likely reflects an optimal respiration flux when organisms adapt to their thermal conditions.

Uncoupling protein 1-driven Cre (Ucp1-Cre) is expressed in the epithelial cells of mammary glands and various non-adipose tissues

OBJECTIVE

Uncoupling protein 1 (UCP1), a mitochondrial protein responsible for nonshivering thermogenesis in adipose tissue, serves as a distinct marker for thermogenic brown and beige adipocytes. Ucp1-Cre mice are thus widely used to genetically manipulate these thermogenic adipocytes. However, evidence suggests that UCP1 may also be expressed in non-adipocyte cell types. In this study, we investigated the presence of UCP1 expression in different mouse tissues that have not been previously reported.

METHODS

We employed Ucp1-Cre mice crossed with Cre-inducible transgenic reporter Nuclear tagging and Translating Ribosome Affinity Purification (NuTRAP) mice to investigate Ucp1-Cre expression in various tissues of adult female mice and developing embryos. Tamoxifen-inducible Ucp1-CreERT2 mice crossed with NuTRAP mice were used to assess active Ucp1 expression in adult mice. Immunostaining, RNA analysis, and single-cell/nucleus RNA-seq (sc/snRNA-seq) data analysis were performed to determine the expression of endogenous UCP1 and Ucp1-Cre-driven reporter expression. We also investigated the impact of UCP1 deficiency on mammary gland development and function using Ucp1-knockout (KO) mice.

RESULTS

Ucp1-Cre expression was observed in the mammary glands within the inguinal white adipose tissue of female Ucp1-Cre; NuTRAP mice. Ucp1-Cre was activated during embryonic development in various tissues, including mammary glands, as well as in the brain, kidneys, eyes, and ears, specifically in epithelial cells in these organs. However, Ucp1-CreERT2 showed no or only partial activation in these tissues of adult mice, indicating the potential for low or transient expression of endogenous Ucp1. While sc/snRNA-seq data suggest potential expression of UCP1 in mammary epithelial cells in adult mice and humans, Ucp1-KO female mice displayed normal mammary gland development and function.

CONCLUSIONS

Our findings reveal widespread Ucp1-Cre expression in various non-adipose tissue types, starting during early development. These results highlight the importance of exercising caution when interpreting data and devising experiments involving Ucp1-Cre mice.

The zebrafish heart harbors a thermogenic beige fat depot analog of human epicardial adipose tissue

Epicardial adipose tissue (eAT) is a metabolically active fat depot that has been associated with a wide array of cardiac homeostatic functions and cardiometabolic diseases. A full understanding of its diverse physiological and pathological roles is hindered by the dearth of animal models. Here, we show, in the heart of an ectothermic teleost, the zebrafish, the existence of a fat depot localized underneath the epicardium, originating from the epicardium and exhibiting the molecular signature of beige adipocytes. Moreover, a subset of adipocytes within this cardiac fat tissue exhibits primitive thermogenic potential. Transcriptomic profiling and cross-species analysis revealed elevated glycolytic and cardiac homeostatic gene expression with downregulated obesity and inflammatory hallmarks in the teleost eAT compared to that of lean aged humans. Our findings unveil epicardium-derived beige fat in the heart of an ectotherm considered to possess solely white adipocytes for energy storage and identify pathways that may underlie age-driven remodeling of human eAT.

Uncoupling protein 1-driven Cre (Ucp1-Cre) is expressed in the epithelial cells of mammary glands and various non-adipose tissues

Objective

Uncoupling protein 1 (UCP1), a mitochondrial protein responsible for nonshivering thermogenesis in adipose tissue, serves as a distinct marker for thermogenic brown and beige adipocytes. Ucp1-Cre mice are thus widely used to genetically manipulate these thermogenic adipocytes. However, evidence suggests that UCP1 may also be expressed in non-adipocyte cell types. In this study, we investigated the presence of UCP1 expression in different mouse tissues that have not been previously reported.

Methods

We employed Ucp1-Cre mice crossed with Cre-inducible transgenic reporter Nuclear tagging and Translating Ribosome Affinity Purification (NuTRAP) mice, to investigate Ucp1-Cre expression in various tissues of adult female mice and developing embryos. Tamoxifen-inducible Ucp1-CreERT2 mice crossed with NuTRAP mice were used to assess active UCP1 expression. Immunostaining, RNA analysis, and single-cell/nucleus RNA-seq (sc/snRNA-seq) data analysis were performed to determine the expression of endogenous UCP1 and Ucp1-Cre-driven reporter expression. We also investigated the impact of UCP1 deficiency on mammary gland development and function using Ucp1-knockout (KO) mice.

Results

Ucp1-Cre expression was observed in the mammary glands within the inguinal white adipose tissue of female Ucp1-Cre; NuTRAP mice. However, endogenous Ucp1 was not actively expressed as Ucp1-CreERT2 failed to induce the reporter expression in the mammary glands. Ucp1-Cre was activated during embryonic development in various tissues, including mammary glands, as well as in the brain, kidneys, eyes, and ears, specifically in epithelial cells in these organs. While sc/snRNA-seq data suggest potential expression of UCP1 in mammary epithelial cells in adult mice and humans, Ucp1-KO female mice displayed normal mammary gland development and function.

Conclusions

Our findings reveal widespread Ucp1-Cre expression in various non-adipose tissue types, starting during early development. These results highlight the importance of exercising caution when interpreting data and devising experiments involving Ucp1-Cre mice.

Energy consumption and intestinal microbiome disorders of yellow catfish (Pelteobagrus fulvidraco) under cold stress

The yellow catfish (P. fulvidraco), as one of the economically-relevant freshwater fish found in China, cannot tolerate cold stress. Understanding the physiological and biochemical mechanisms under cold stress may provide insights for improving yellow catfish management in the cold. Therefore, we investigated the metabolic and intestinal microbiota changes in cold stress in response to induced cold stress. We found that cold stress in yellow catfish lead to a significant increase in the consumption of glucose and triglycerides, as well as increased use of cholesterol as an alternate energy source. Moreover, cold stress also activated several significant biological processes in the fish such as thermogenesis, oxidative phosphorylation, the spliceosome machinery, RNA transport, protein processing that occurs in the ER, and purine and pyrimidine metabolism pathways involved in energy production. On the other hand, many other mechanisms like insulin resistance, starch and sucrose metabolism, and the glyoxylate and dicarboxylate metabolic pathways that also served as energy production pathways were weakened. Furthermore, organic acids and their derivatives as well as the lipids and lipid-like molecules were mainly altered in cold stress; prenol lipids, steroids, and their derivatives were significantly upregulated, while fatty acyls and glycerophospholipids were significantly downregulated. Transcriptomic and metabolomic integrated analysis data revealed that carbohydrate metabolism, lipid metabolism, amino acid metabolism, and nucleotide metabolism were involved in cold stress resistance. In addition, the intestinal microbiota abundance was also reduce and the pathogenic bacteria of plesiomonas was rapidly appreciation, which suggesting that cold stress also impaired intestinal health. This research study could offer insights into winter management or the development of feed to promote cold resistance in yellow catfish.

Conditional gene targeting using UCP1-Cre mice directly targets the central nervous system beyond thermogenic adipose tissues

Objective

Uncoupling protein 1 (UCP1) is a mitochondrial protein critical for adaptive thermogenesis in adipose tissues, and it is typically believed to be restricted to thermogenic adipose tissues. UCP1-Cre transgenic mice are utilized in numerous studies to provide “brown adipose-specific” conditional gene targeting. Here, we examined the distribution of Cre and UCP1 throughout the body in UCP1-Cre reporter mice.

Methods

UCP1-Cre mice crossed to Ai14-tdTomato and Ai9-tdTomato reporter mice were used to explore the tissue distribution of Cre recombinase and Ucp1 mRNA in various tissues. UCP1-Cre mice were independently infected with either a Cre-dependent PHP.eB-tdTomato virus or a Cre-dependent AAV-tdTomato virus to determine whether and where UCP1 is actively expressed in the adult central nervous system. In situ analysis of the deposited single cell RNA sequencing data was used to evaluate Ucp1 expression in the hypothalamus.

Results

As expected, Ucp1 expression was detected in both brown and inguinal adipose tissues. Ucp1 expression was also detected in the kidney, adrenal glands, thymus, and hypothalamus. Consistent with detectable Ucp1 expression, tdTomato expression was also observed in brown adipose tissue, inguinal white adipose tissue, kidney, adrenal glands, and hypothalamus of both male and female UCP1-Cre; Ai14-tdTomato and UCP1-Cre; Ai9-tdTomato mice by fluorescent imaging and qPCR. Critically, expression of tdTomato, and thus UCP1, within the central nervous system was observed in regions of the brain critical for the regulation of energy homeostasis, including the ventromedial hypothalamus (VMH).

Conclusions

TdTomato expression in UCP1-Cre; tdTomato mice is not restricted to thermogenic adipose tissues. TdTomato was also expressed in the kidneys, adrenal glands, and throughout the brain, including brain regions and cell types that are critical for multiple aspects of central regulation of energy homeostasis. Collectively, these data have important implications for the utility of UCP1-Cre mice as genetic tools to investigate gene function specifically in brown adipose tissue.

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UCP1-Cre transgenic mice express Cre in tissues beyond thermogenic adipose tissues.

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UCP1-Cre; tdTomato reporter mice show that Cre is also expressed throughout the brain, kidney, and adrenal glands.

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Ucp1 mRNA is actively expressed in the central nervous system of adult mice.

Plate-Based Respirometry to Assess Thermal Sensitivity of Zebrafish Embryo Bioenergetics in situ

Oxygen consumption allows measuring the metabolic activity of organisms. Here, we adopted the multi-well plate-based respirometry of the extracellular flux analyzer (Seahorse XF96) to investigate the effect of temperature on the bioenergetics of zebrafish embryos (Danio rerio) in situ. We show that the removal of the embryonic chorion is beneficial for oxygen consumption rates (OCR) and penetration of various mitochondrial inhibitors, and confirm that sedation reduces the variability of OCR. At 48h post-fertilization, embryos (maintained at a routine temperature of 28°C) were exposed to different medium temperatures ranging from 18°C to 37°C for 20h prior OCR measurement. Measurement temperatures from 18°C to 45°C in the XF96 were achieved by lowering the room temperature and active in-built heating. At 18°C assay temperature, basal OCR was low due to decreased ATP-linked respiration, which was not limited by mitochondrial power, as seen in substantial spare respiratory capacity. Basal OCR of the embryos increased with assay temperature and were stable up to 37°C assay temperature, with pre-exposure of 37°C resulting in more thermo-resistant basal OCR measured at 41°C. Adverse effects of the mitochondrial inhibitor oligomycin were seen at 37°C and chemical uncouplers disrupted substrate oxidation gradually with increasing assay temperature. Proton leak respiration increased at assay temperatures above 28°C and compromised the efficiency of ATP production, calculated as coupling efficiency. Thus, temperature impacts mitochondrial respiration by reduced cellular ATP turnover at lower temperatures and by increased proton leak at higher temperatures. This conclusion is coherent with the assessment of heart rate, an independent indicator of systemic metabolic rate, which increased with exposure temperature, peaking at 28°C, and decreased at higher temperatures. Collectively, plate-based respirometry allows assessing distinct parts of mitochondrial energy transduction in zebrafish embryos and investigating the effect of temperature and temperature acclimation on mitochondrial bioenergetics in situ.

Editorial: Untangling the oxygen transport cascade: a tribute to Peter Frappell (Frapps)

This collection of research articles was put together in honour of respiratory physiologist Professor Peter Frappell's (Frapps's) academic achievements. It encompasses various topics relating to the oxygen transport cascade, which was central to Frapps' career as a comparative physiologist. This issue highlights the diversity and outreach of his influence on the field and his pioneering spirit; promoting novel perspectives, methodologies and research techniques. This issue also demonstrates how Frapps' knowledge and scientific findings answered some of the fundamental questions within the field of respiratory physiology while creating and fostering a rather unique work atmosphere in the laboratories he led. We thank Frapps for the contributions he has made and the friendships he has nurtured over his career. Cheers, Frapps - we love you mate!

Comparative functional analyses of UCP1 to unravel evolution, ecophysiology and mechanisms of mammalian thermogenesis

Brown adipose tissue (BAT), present in many placental mammals, provides adaptive nonshivering thermogenesis (NST) for body temperature regulation and has facilitated survival in diverse thermal niches on our planet. Intriguingly, several key details on the molecular mechanisms of NST and their potential ecophysiological adaptations are still unknown. Comparative studies at the whole animal level are unpragmatic, due to the diversity and complexity of thermoregulation among different species. We propose that the molecular evolution of mitochondrial uncoupling protein 1 (UCP1), a central component for BAT thermogenesis, represents a powerful opportunity to unravel key questions of mammalian thermoregulation. Comparative analysis of UCP1 may elucidate how its thermogenic function arose, how environmental selection has shaped protein function to support ecophysiological requirements, and how the enigmatic molecular mechanism of proton leak is governed. Several approaches for the assessment of UCP1 function in vitro have been introduced over the years. For comparative characterization of UCP1, we put forward the overexpression of UCP1 orthologues and mutated variants in a mammalian cell system as a primary strategy and discuss advantageous aspects in contrast to other experimental systems. In turn, we suggest how remaining experimental caveats can be solved by complimentary test systems before physiological consolidation in the animal model. Furthermore, we highlight the appropriate bioenergetic techniques to perform the functional analyses on UCP1. The comparative characterizations of diverse UCP1 variants may enable key insights into open questions surrounding the molecular basis of NST.

Molecular identification of FNDC5 and effect of irisin on the glucose metabolism in common carp (Cyprinus carpio L.)

Irisin, encoded by fibronectin type III domain-containing protein 5 (FNDC5) gene, plays a role in energy expenditure and insulin sensitivity in mice. In fish, the function of irisin related to glucose metabolism is less reported. It may increase glucose utilization in fish. The aim of the present study was to characterize the regulatory role of irisin in glucose metabolism in common carp (Cyprinus carpio L.). In this study, FNDC5a and FNDC5b were isolated from common carp. The cDNA of FNDC5a and FNDC5b were 722 bp and 714 bp, encoding 221 and 207 amino acids, respectively. FNDC5a was abundantly expressed in the brain and gonad. FNDC5b was mainly expressed in brain. Different expression pattern of FNDC5a and FNDC5b under fasting/refeeding and OGTT experiment were identified. The recombinant common carp irisinA and irisinB were prepared by prokaryotic expression system. Glucose concentration was decreased in treatment with irisinA or irisinB in the in vitro and in vivo experiments. The mRNA expression levels of gluconeogenesis-related genes were significantly down-regulated, while the mRNA expression of glycolysis-related genes were significantly up-regulated after treatment with recombinant irisinA or irisinB in liver in vivo and in primary hepatocytes in vitro. Our research shows that irisin inhibits hepatic gluconeogenesis and promotes hepatic glycolysis. Taken together, this study for the first time revealed the two subtypes of FNDC5 and explored the function and mechanisms of irisinA and irisinB in fish glucose homeostasis.

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.

Comparative assessment of purified saponins as permeabilization agents during respirometry

Saponins are a diverse group of secondary plant metabolites, some of which display hemolytic toxicity due to plasma membrane permeabilization. This feature is employed in biological applications for transferring hydrophilic molecules through cell membranes. Widely used commercial saponins include digitonin and saponins from soap tree bark, both of which constitute complex mixtures of little definition. We assessed the permeabilization power of pure saponins towards cellular membranes in an effort to detect novel properties and to improve existing applications. In a respirometric assay, we characterized half-maximal permeabilization of the plasma membrane for different metabolites, of the mitochondrial outer membrane for cytochrome C and the full solubilization of mitochondrial inner membrane protein complexes. Beyond the complete list as repository for the field, we highlight several findings with direct applicability. First, we identified and validated α-chaconine as alternative permeabilization agent in respirometric assays of cultured cells and isolated synaptosomes, superior to digitonin in its tolerability for mitochondria. Second, we identified glycyrrhizic acid to form exceptionally small pores impermeable for adenosine diphosphate. Third, in a concentration dependent manner, tomatine proved to be able to selectively permeabilize the mitochondrial outer, but not inner membrane, allowing for novel states in which to determine cytochrome C oxidase activity. In summary, we provide a list of the permeabilization properties of 18 pure saponins. The identification of two saponins, namely tomatine and chaconine, with direct usability in improved or novel cell biological applications within this small subgroup demonstrates the tremendous potential for further functional screening of pure saponins.

Is Endothermy an Evolutionary By-Product?

Endothermy alters the energetic relationships between organisms and their environment and thereby influences fundamental niches. Endothermy is closely tied to energy metabolism. Regulation of energy balance is indispensable for all life and regulatory pathways increase in complexity from bacteria to vertebrates. Increasing complexity of metabolic networks also increase the probability for endothermic phenotypes to appear. Adaptive arguments are problematic epistemologically because the regulatory mechanisms enabling endothermy have not evolved for the 'purpose' of endothermy and the utility of current traits is likely to have changed over evolutionary time. It is most parsimonious to view endothermy as the evolutionary by-product of energy balance regulation rather than as an adaptation and interpret its evolution in the context of metabolic networks.

Thermal sensitivity links to cellular cardiac decline in three spiny lobsters

Understanding mechanisms of thermal sensitivity is key to predict responses of marine organisms to changing temperatures. Sustaining heart function is critical for complex organisms to oxygenate tissues, particularly under temperature stress. Yet, specific mechanisms that define thermal sensitivity of cardiac function remain unclear. Here we investigated whole animal metabolism, cardiac performance and mitochondrial function in response to elevated temperatures for temperate, subtropical and tropical spiny lobster species. While oxygen demands increased with rising temperatures, heart function became limited or declined in all three species of lobsters. The decline in cardiac performance coincided with decreases in mitochondrial efficiency through increasing mitochondrial proton leakage, which predicts impaired compensation of ATP production. Species differences were marked by shifts in mitochondrial function, with the least thermal scope apparent for tropical lobsters. We conclude that acute temperature stress of spiny lobsters, irrespective of their climatic origin, is marked by declining cellular energetic function of the heart, contributing to an increasing loss of whole animal performance. Better understanding of physiological thermal stress cascades will help to improve forecasts of how changing environmental temperatures affect the fitness of these ecologically and commercially important species.

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'.

Quantitative analyses of adiposity dynamics in zebrafish

Adipose tissues often exhibit subtle, quantitative differences between individuals, leading to a graded series of adiposity phenotypes at the population level. Robust, quantitative analyses are vital for studying these differences. In this Commentary we highlight two articles from our lab that employ sensitive new methods in zebrafish capable of delineating complex and quantitative adiposity phenotypes. In the first article, we utilized in vivo imaging to systematically quantify zebrafish adipose tissues. We identified 34 regionally distinct zebrafish adipose tissues and developed statistical models to predict the size and variance of each adipose tissue over the course of zebrafish growth. We then employed these models to identify effects of strain and diet on adipose tissue growth. In the second article, we employed deep phenotyping to study complex disease-related adiposity traits. Using this methodology, we identified that adipose tissues have unique capacities to re-deposit lipid following food restriction and re-feeding. These distinct re-deposition potentials led to widespread fat distribution changes following re-feeding. We discuss how these novel findings may provide relevance to health conditions such as anorexia nervosa. Together, the strategies described in these two articles can be used as unbiased and quantitative methods to uncover new relationships between genotype, diet and adiposity.

Coping with exposure to hypoxia: modifications in stress parameters in gilthead seabream (Sparus aurata) fed spirulina (Arthrospira platensis) and brewer's yeast (Saccharomyces cerevisiae)

This study aimed to investigate the stress response of Sparus aurata specimens fed with nutraceutical aquafeed brewer's yeast (Saccharomyces cerevisiae) and spirulina (Arthrospira platensis). For that purpose, 96 (169.0 ± 2.8 g) animals were distributed randomly in 12 tanks (8 fish per tank, 4 replicates) and divided in 3 groups (D1, casein/gelatin, control; D2, brewer's yeast; D3, spirulina) and fed for 30 days. At the end of this period, fish from two replicates of each experimental diet were submitted to air exposure for 60 s while the fish from the other two replicates were maintained undisturbed (control). Afterwards, samples of blood, skin mucus, and head kidney were collected. The results revealed that after air exposure, cortisol, and glucose levels increased in the groups fed D1 (18.5 ± 2.6 mg/mL; 7.3 ± 0.6 mmol/L, respectively) and D2 (20.0 ± 6.2 mg/mL; 7.7 ± 0.6 mmol/L), but glucose not increased in fish fed D3 (13.7 ± 2.6 mg/mL; 5.5 ± 0.3 mmol/L). Lactate levels increased in all stressed groups, but in D1, its levels were significantly higher. After stress procedure, immunoglobulin M (IgM) levels in mucus increased only in fish fed D3 (0.1901 ± 0.0126 U/mL). Furthermore, there was a reduction in the expression of some genes involved in stress response (coxIV, prdx3, csfl-r, ucp1, and sod in fish fed D2 and D3). csf1 decreased only in stressed fish fed D2. However, cat increased in fish fed with D3. In summary, these findings points to the beneficial effects of spirulina and brewer's yeast to improve stress resistance in aquaculture practices of gilthead seabream.

Tissue-dependent DNA methylation of carp uncoupling protein 1 promoter

DNA methylation of enhancers and promoters generally inhibits gene transcription. DNA methylation occurs predominantly at the dinucleotide CpG, a methyl group that is covalently bonded to cytosine. We have previously demonstrated tissue-restricted expression of the uncoupling protein 1 (Ucp1) in common carp. Here, we characterized the methylation status of the upstream region of the transcriptional initiation site of the carp Ucp1 gene in the liver, brain, kidney, skeletal muscle, and scales. In addition, we explored the direct role of methylation on Ucp1 transcription. Ucp1 expression was higher in the liver than that in other tissues including the kidney, skeletal muscle, and scales. The extent of methylation at nt -2178 and nt -2103 was lower in the liver and kidney than that in the brain, skeletal muscle, and scales. In addition, methylation at the upstream proximal-region of the Ucp1 gene was generally less frequent in the liver compared with that in the other organs. The transcriptional activation assay using the CpG-free luciferase-based reporter suggested that the methylation of the distal and proximal regions of the carp Ucp1 gene did not affect Ucp1 transcription. Unexpectedly, mutation of cytidylic acid to guanylic acid at nt -108 decreased Ucp1 promoter activity. The present results reveal that the status of DNA methylation of the upstream region of the carp Ucp1 gene is different among different tissues, but suggest that the DNA methylation do not directly repress the transcription of Ucp1.

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.

Electron Density of Adipose Tissues Determined by Phase-Contrast Computed Tomography Provides a Measure for Mitochondrial Density and Fat Content

Phase-contrast computed tomography (PCCT) is an X-ray-based imaging method measuring differences in the refractive index during tissue passage. While conventional X-ray techniques rely on the absorption of radiation due to differing tissue-specific attenuation coefficients, PCCT enables the determination of the electron density (ED). By the analysis of respective phantoms and ex vivo specimens, we identified the components responsible for different electron densities in murine adipose tissue depots to be cellular fat and mitochondrial content, two parameters typically different between white adipose tissue (WAT) and brown adipose tissue (BAT). Brown adipocytes provide mammals with a means of non-shivering thermogenesis to defend normothermia in a cold environment. Brown adipocytes are found in dedicated BAT depots and interspersed within white fat depots, a cell type referred to as brite (brown in white) adipocyte. Localization and quantification of brown and brite adipocytes in situ allows an estimate of depot thermogenic capacity and potential contribution to maximal metabolic rate in the cold. We utilized PCCT to infer the composition of white, brite, and brown adipose tissue from ED of individual depots. As proof of principle, we imaged mice 10, 20, and 30 days of age. During this period, several WAT depots are known to undergo transient browning. Based on ED, classical WAT and BAT could be clearly distinguished. Retroperitoneal and inguinal WAT depots increased transiently in ED during the known remodeling from white to brite/brown and back to white. We systematically analyzed 18 anatomically defined adipose tissue locations and identified changes in fat content and mitochondrial density that imply an orchestrated pattern of simultaneous browning and whitening on the organismic level. Taken together, PCCT provides a three-dimensional imaging technique to visualize ED of tissues in situ. Within the adipose organ, ED provides a measure of mitochondrial density and fat content. Depending on experimental setting, these constitute surrogate markers of cellular distribution of white, brite, and brown adipocytes and thereby an estimate of thermogenic capacity.

Uncoupling protein 1 in snakehead (Channa argus): Cloning, tissue distribution, and its expression in response to fasting and refeeding

In mammals, uncoupling protein 1 (UCP1) is well known for its thermogenic role in brown adipose tissue (BAT). However, the UCP1 physiological roles are still unclear in fish, although several teleost ucp1 genes have been identified. The aim of this study is to investigate the potential roles of fish UCP1 involved in food intake regulation and energy homeostasis. We herein report on the molecular cloning, tissue distribution and the effect of fasting and refeeding on the expression of ucp1 in Channa argus. UCP1 consisted of a 921 bp open reading frame predicted to encode 306 amino acids. Sequence analysis revealed that snakehead UCP1 was highly conserved (>80%) with teleost UCP1, but shared a lower identity (60-72%) with mammals. Phylogenetic analysis supported that snakehead UCP1 was closely related to piscine UCP1. In addition, ucp1 was found to extensively expressed in all detected tissues, with the highest level in liver. Futhermore, the hepatic ucp1 was found to significantly increased after short-term and long-term food deprivation, and dramatically increased following refeeding. These findings suggested that snakehead UCP1 might play important roles in food intake regulation and fatty acid metabolism in snakehead fish, and it could be as a potential target locus to improve commercial production of this kind of fish.

Insights into the neurochemical signature of the Innervation of Beige Fat

Objective

The potential for brown adipose tissue (BAT) to be targeted as a therapeutic option to combat obesity has been heightened by the discovery of a brown–like form of inducible “beige” adipose tissue in white fat which has overlapping structural and functional properties to “classical” BAT. The likelihood that both beige and brown fat are recruited functionally by neural mechanisms, taken together with the lack of a detailed understanding of the nature of changes in the nervous system when white adipose tissue (WAT) is transformed to brown, provides the impetus for this study. Here, we aim to identify whether there is a shift in the gene expression profile in neurons directly innervating inguinal white adipose tissue (iWAT) that has undergone “beiging” to a signature that is more similar to neurons projecting to BAT.

Methods

Two groups of rats, one housed at thermoneutrality (27 °C) and the other exposed to cold (8 °C) for 7 days, were killed, and their T13/L1 ganglia, stellate ganglion (T1/T2), or superior cervical ganglion (SCG, C2/3) removed. This approach yielded ganglia containing neurons that innervate either beiged white fat (8 °C for 7 days), inguinal WAT (27 °C for 7 days), BAT (both 27 °C and 8 °C for 7 days) or non-WAT (8 °C for 7 days), the latter included to isolate changes in gene expression that were more aligned with a response to cold exposure than the transformation of white to beige adipocytes. Bioinformatics analyses of RNA sequencing data was performed followed by Ingenuity Pathway Analysis (IPA) to determine differential gene expression and recruitment of biosynthetic pathways.

Results

When iWAT is “beiged” there is a significant shift in the gene expression profile of neurons in sympathetic ganglia (T13/L1) innervating this depot toward a gene neurochemical signature that is similar to the stellate ganglion projecting to BAT. Bioinformatics analyses of “beiging” related genes revealed upregulation of genes encoding neuropeptides proopiomelanocortin (POMC) and calcitonin-gene related peptide (CGRP) within ganglionic neurons. Treatment of differentiated 3T3L1 adipocytes with αMSH, one of the products cleaved from POMC, results in an elevation in lipolysis and the beiging of these cells as indicated by changes in gene expression markers of browning (Ucp1 and Ppargc1a).

Conclusion

These data indicate that, coincident with beiging, there is a shift toward a “brown-like” neurochemical signature of postganglionic neurons projecting to inguinal white fat, an increased expression of POMC, and, consistent with a causative role for this prohormone in beiging, an αMSH-mediated increase in beige gene markers in isolated adipocytes.

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RNA Seq showed shifts in neuronal gene expression following browning of white fat.

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Gene expression in ganglia projecting to white fat became brown-like with beiging.

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Bioinformatics analyses revealed neuronal gene candidates associated with beiging.

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Prominent gene candidates associated with beiging included POMC and CGRP.

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POMC cleavage product α-MSH caused beiging of cultured fat cells.

Subspecies differences in thermal acclimation of mitochondrial function and the role of uncoupling proteins in killifish

Thermal effects on mitochondrial efficiency and ATP production can influence whole-animal thermal tolerance and performance. Thus, organisms may have the capacity to alter mitochondrial processes through acclimation or adaptation to mitigate these effects. One possible mechanism is through the action of uncoupling proteins (UCPs) which can decrease the proton motive force independent of the production of ATP. To test this hypothesis, we examined the mRNA expression patterns of UCP isoforms and characterized the effects of thermal acclimation and putative local thermal adaptation on mitochondrial capacity, proton leak, and P/O ratios in two subspecies of Atlantic killifish (Fundulus heteroclitus). Ucp1 was the dominant isoform in liver and was more highly expressed in northern killifish. We found that cold acclimation increased mitochondrial capacity (state III and maximum substrate oxidation capacity), state II membrane potential, proton leak, and P/O ratios in northern, but not southern killifish liver mitochondria. Palmitate-induced mitochondrial uncoupling was detected in northern, but not southern, killifish liver mitochondria, consistent with the differences in mRNA expression between the subspecies. Taken together, our data suggest that mitochondrial function is more plastic in response to thermal acclimation in northern killifish than southern killifish and that UCP1 may play a role in regulating the proton motive force in northern, but not southern killifish in response to thermal acclimation. These data demonstrate the potential for adaptive variation in mitochondrial plasticity in response to cold.

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.

Mitochondrial Proton Leak Compensates for Reduced Oxidative Power during Frequent Hypothermic Events in a Protoendothermic Mammal, Echinops telfairi

The lesser hedgehog tenrec (Echinops telfairi) displays reptile-like thermoregulatory behavior with markedly high variability in body temperature and metabolic rate. To understand how energy metabolism copes with this flexibility, we studied the bioenergetics of isolated liver mitochondria from cold (20°C) and warm (27°C) acclimated tenrecs. Different acclimation temperatures had no impact on mitochondrial respiration using succinate as the substrate. Mimicking the variation of body temperature by changing assay temperatures from 22 to 32°C highlighted temperature-sensitivity of respiration. The 40% reduction of respiratory control ratio (RCR) at 22°C compared to 32°C, a common estimate for mitochondrial efficiency, was caused by reduced substrate oxidation capacity. The simultaneous measurement of mitochondrial membrane potential enabled the precise assessment of efficiency with corrected respiration rates. Using this method, we show that proton leak respiration at the highest common membrane potential was not affected by acclimation temperature but was markedly decreased by assay temperature. Using membrane potential corrected respiration values, we show that the fraction of ATP-linked respiration (coupling efficiency) was maintained (70–85%) at lower temperatures. Collectively, we demonstrate that compromised substrate oxidation was temperature-compensated by the reduction of proton leak, thus maintaining the efficiency of mitochondrial energy conversion. Therefore, membrane potential data suggest that adjustments of mitochondrial proton leak contribute to energy homeostasis during thermoregulatory flexibility of tenrecs.

The evolution of metabolic regulation in animals

Energy metabolism is determined by a suite of regulatory mechanism, and their increasing complexity over evolutionary time provides the key to understanding the emergence of different metabolic phenotypes. Energy metabolism is at the core of biological processes because all organisms must maintain energy balance against thermodynamic gradients. Energy metabolism is regulated by a bewildering array of interacting molecular mechanisms, and much of what is known about metabolic regulation comes from the medical literature. However, ecology and evolutionary research would gain considerably by incorporating regulatory mechanisms more explicitly in research on topics such as the evolution of endothermy, metabolic plasticity, and energy balance. The purpose of this brief review is to summarise the main regulatory pathways of energy metabolism in animals and their evolutionary origins to make these complex interactions more accessible to researchers from a broad range of backgrounds. Some of the principal regulators of energy balance, such as the AMP-stimulated protein kinase, have an ancient prokaryotic origin. Most regulatory pathways (e.g. thyroid hormone, insulin, adipokines), however, are eukaryotic in origin and diversified substantially in metazoans and vertebrates. Diversification in vertebrates is at least partly due to genome duplications early in this lineage. The interaction between regulatory mechanisms permitted an increasingly sophisticated fine-tuning of energy balance and metabolism. Hence, regulatory complexity increased over evolutionary time, and taxa differ in their potential range of metabolic phenotypes. Choice of model organism therefore becomes important, and bacteria or even invertebrates are not good models for more derived vertebrates. Different metabolic phenotypes and their evolution, such as endothermy and metabolic plasticity, should be interpreted against this regulatory background.

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.

Phylogenetic analysis of the allometry of metabolic rate and mitochondrial basal proton leak

The mitochondrial basal proton leak (MBPL) significantly contributes to high body temperatures (T_b) and basal metabolic rates (BMR) in endotherms. In endotherms at a given body mass (M), liver MBPL is higher than in ectotherms, supporting the notion that MBPL may partly explain the evolutionary increase in metabolic rate (MR), fostering endothermy. Here, we re-addressed this assumption by performing a phylogenetic analysis comparing all available liver MBPL data for ecto- and endotherms. While MBPL within endotherms negatively scales with M and BMR as shown previously, MBPL of ectotherms does not scale allometrically with M. Phylogenetic analysis reveals that this result is confounded by a positive scaling coefficient for MBPL with M for reptiles. Strikingly, the reptilian MBPL reaches endothermic levels above a body mass of 6.6kg. Thus, phylogenetic scaling of MBPL supports previous claims of endotherm-like physiological characteristics in large reptiles. It appears that diversification of ancestral ectothermic tetrapods to a body mass of at least 6kg may have been required to reach a MBPL that is beneficial for sustained high body temperatures. Novel MBPL data for the lesser hedgehog tenrec, a protoendothermic eutherian that displays reptile-like thermoregulatory patterns, fall within the endo- and ectothermic allometric regressions. Finally, we add additional evidence that within endotherms, phylogenetic differences in MR do not correlate with MBPL. Collectively, these data suggest that MBPL does not universally scale with metabolic rate in ecto- or endotherms and that an increasing MBPL with M may have played an important physiological role in the evolutionary history of reptilian thermoregulation.

Irisin inhibition of growth hormone secretion in cultured tilapia pituitary cells

Irisin, the product of fibronectin type III domain-containing protein 5 (FNDC5) gene, is well-documented to be a regulator of energy metabolism. At present, not much is known about its biological function in non-mammalian species. In this study, a full-length tilapia FDNC5 was cloned and its tissue expression pattern has been confirmed. Based on the sequence obtained, we produced and purified recombinant irisin which could induce uncoupling protein 1 (UCP1) gene expression in tilapia hepatocytes. Further, the rabbit polyclonal irisin antiserum was produced and its specificity was confirmed by antiserum preabsorption. In tilapia pituitary cells, irisin inhibited growth hormone (GH) gene expression and secretion and triggered rapid phosphorylation of Akt, Erk1/2, and p38 MAPK. Furthermore, irisin-inhibited GH mRNA expression could be prevented by inhibiting PI3K/Akt, MEK1/2, and p38 MAPK, respectively. Apparently, fish irisin can act directly at the pituitary level to inhibit GH transcript expression via multiple signaling pathways.

Non-Mammalian Vertebrates: Distinct Models to Assess the Role of Ion Gradients in Energy Expenditure

Animals store metabolic energy as electrochemical gradients. At least 50% of mammalian energy is expended to maintain electrochemical gradients across the inner mitochondrial membrane (H⁺), the sarcoplasmic reticulum (Ca⁺⁺), and the plasma membrane (Na⁺/K⁺). The potential energy of these gradients can be used to perform work (e.g., transport molecules, stimulate contraction, and release hormones) or can be released as heat. Because ectothermic species adapt their body temperature to the environment, they are not constrained by energetic demands that are required to maintain a constant body temperature. In fact, ectothermic species expend seven to eight times less energy than similarly sized homeotherms. Accordingly, ectotherms adopt low metabolic rates to survive cold, hypoxia, and extreme bouts of fasting that would result in energy wasting, lactic acidosis and apoptosis, or starvation in homeotherms, respectively. Ectotherms have also evolved unique applications of ion gradients to allow for localized endothermy. Endothermic avian species, which lack brown adipose tissue, have been integral in assessing the role of H⁺ and Ca⁺⁺ cycling in skeletal muscle thermogenesis. Accordingly, the diversity of non-mammalian vertebrate species allows them to serve as unique models to better understand the role of ion gradients in heat production, metabolic flux, and adaptation to stressors, including obesity, starvation, cold, and hypoxia.

Thermal acclimation and subspecies-specific effects on heart and brain mitochondrial performance in a eurythermal teleost (Fundulus heteroclitus)

Mitochondrial performance may play a role in setting whole-animal thermal tolerance limits and their plasticity, but the relative roles of adjustments in mitochondrial performance across different highly aerobic tissues remain poorly understood. We compared heart and brain mitochondrial responses to acute thermal challenges and to thermal acclimation using high-resolution respirometry in two locally adapted subspecies of Atlantic killifish (Fundulus heteroclitus). We predicted that 5°C acclimation to would result in compensatory increases in mitochondrial performance, while 33°C acclimation would cause suppression of mitochondrial function to minimize the effects of high temperature on mitochondrial metabolism. In contrast, acclimation to both 33 and 5°C decreased mitochondrial performance compared to fish acclimated to 15°C. These adjustments could represent an energetic cost saving mechanism at temperature extremes. Acclimation responses were similar in both heart and brain; however, this effect was smaller in the heart which might indicate its importance in maintaining whole-animal thermal performance. Alternatively, larger acclimation effects in the brain might indicate greater thermal sensitivity compared to the heart. We detected only modest differences between subspecies that were dependent on the tissue assayed. These data demonstrate extensive plasticity in mitochondrial performance following thermal acclimation in killifish, and indicate that the extent of these responses differs between tissues, highlighting the importance and complexity of mitochondrial regulation in thermal acclimation in eurytherms.

The conserved regulation of mitochondrial uncoupling proteins: From unicellular eukaryotes to mammals

Uncoupling proteins (UCPs) belong to the mitochondrial anion carrier protein family and mediate regulated proton leak across the inner mitochondrial membrane. Free fatty acids, aldehydes such as hydroxynonenal, and retinoids activate UCPs. However, there are some controversies about the effective action of retinoids and aldehydes alone; thus, only free fatty acids are commonly accepted positive effectors of UCPs. Purine nucleotides such as GTP inhibit UCP-mediated mitochondrial proton leak. In turn, membranous coenzyme Q may play a role as a redox state-dependent metabolic sensor that modulates the complete activation/inhibition of UCPs. Such regulation has been observed for UCPs in microorganisms, plant and animal UCP1 homologues, and UCP1 in mammalian brown adipose tissue. The origin of UCPs is still under debate, but UCP homologues have been identified in all systematic groups of eukaryotes. Despite the differing levels of amino acid/DNA sequence similarities, functional studies in unicellular and multicellular organisms, from amoebae to mammals, suggest that the mechanistic regulation of UCP activity is evolutionarily well conserved. This review focuses on the regulatory feedback loops of UCPs involving free fatty acids, aldehydes, retinoids, purine nucleotides, and coenzyme Q (particularly its reduction level), which may derive from the early stages of evolution as UCP first emerged.

The distinct bioenergetic properties of the human UCP1

The uncoupling protein UCP1 from brown adipose tissue is a mitochondrial carrier which allows dissipation of metabolic energy as heat. We have characterized the human UCP1 (HsUCP1) recombinantly expressed in Saccharomyces cerevisiae and we demonstrate that HsUCP1 is activated by fatty acids and retinoids in a nucleotide sensitive manner just as its rodent orthologs. However, in the absence of regulators, rodent UCP1 presents a high ohmic proton conductance that cannot be detected in HsUCP1. Since the human protein can be activated in a nucleotide sensitive manner, we conclude that it must have lost selectively the basal proton conductance.

Evolutionary adaptation to thermosensation

Organisms continuously evolve to adapt to changing environmental conditions. Chief among these are daily and seasonal temperature fluctuations. Relatively small in terms of real physical values, temperature fluctuations of just a few degrees can profoundly affect organismal functions. In vertebrates, temperature is detected by primary afferents of somatosensory neurons, which express thermo-gated ion channels. Most of our knowledge about temperature receptors comes from seminal studies in mice and rats. Recent work uncovered thermosensory mechanisms in other vertebrates, shedding light onto the diversity of thermosensory adaptations. Here, we summarize molecular mechanisms of thermosensation in different species and discuss the need to use the standard laboratory rodents and non-standard species side-by-side in order to understand fundamental principles of somatosensation.

Longevity Is Linked to Mitochondrial Mutation Rates in Rockfish: A Test Using Poisson Regression

The mitochondrial theory of ageing proposes that the cumulative effect of biochemical damage in mitochondria causes mitochondrial mutations and plays a key role in ageing. Numerous studies have applied comparative approaches to test one of the predictions of the theory: That the rate of mitochondrial mutations is negatively correlated with longevity. Comparative studies face three challenges in detecting correlates of mutation rate: Covariation of mutation rates between species due to ancestry, covariation between life-history traits, and difficulty obtaining accurate estimates of mutation rate. We address these challenges using a novel Poisson regression method to examine the link between mutation rate and lifespan in rockfish (Sebastes). This method has better performance than traditional sister-species comparisons when sister species are too recently diverged to give reliable estimates of mutation rate. Rockfish are an ideal model system: They have long life spans with indeterminate growth and little evidence of senescence, which minimizes the confounding tradeoffs between lifespan and fecundity. We show that lifespan in rockfish is negatively correlated to rate of mitochondrial mutation, but not the rate of nuclear mutation. The life history of rockfish allows us to conclude that this relationship is unlikely to be driven by the tradeoffs between longevity and fecundity, or by the frequency of DNA replications in the germline. Instead, the relationship is compatible with the hypothesis that mutation rates are reduced by selection in long-lived taxa to reduce the chance of mitochondrial damage over its lifespan, consistent with the mitochondrial theory of ageing.

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.

Molecular cloning and tissue expression of uncoupling protein 1, 2 and 3 genes in Chinese perch (Siniperca chuatsi)

Uncoupling proteins (UCPs) are mitochondrial anion carrier proteins, which play important roles in several physiological processes, including thermogenesis, reactive oxygen species generation, growth, lipid metabolism and insulin secretion. Although the roles of UCPs are well understood in mammals, little is known in fish. To investigate the thermogenesis roles in Chinese perch (Siniperca chuatsi), we cloned the UCP1, 2 and 3. The UCP1 consisted of six exons and five introns, and the UCP2 consisted of eight exons and seven introns. The UCP1 was primarily expressed in liver, UCP2 was ubiquitously expressed, and UCP3 was primarily expressed in muscle. The mRNA levels of UCP1 and UCP2 in liver, and UCP3 in muscle were significantly increased after prolonged cold exposure, but did not change after prolonged heat exposure, suggesting that Chinese perch might have a mechanism of response to cold environment, but not to hot environment. The intestinal UCP1 mRNA level was significantly up-regulated after prolonged heat exposure, while the UCP2 mRNA level was significantly up-regulated after prolonged cold exposure, suggesting that the two paralogs might play different roles in intestine of Chinese perch. In addition, the phylogenetic analysis could shed new light on the evolutionary diversification of UCP gene family.

Neuronal UCP1 expression suggests a mechanism for local thermogenesis during hibernation

Hibernating mammals possess a unique ability to reduce their body temperature to ambient levels, which can be as low as -2.9 °C, by active down-regulation of metabolism. Despite such a depressed physiologic phenotype, hibernators still maintain activity in their nervous systems, as evidenced by their continued sensitivity to auditory, tactile, and thermal stimulation. The molecular mechanisms that underlie this adaptation remain unknown. We report, using differential transcriptomics alongside immunohistologic and biochemical analyses, that neurons from thirteen-lined ground squirrels (Ictidomys tridecemlineatus) express mitochondrial uncoupling protein 1 (UCP1). The expression changes seasonally, with higher expression during hibernation compared with the summer active state. Functional and pharmacologic analyses show that squirrel UCP1 acts as the typical thermogenic protein in vitro. Accordingly, we found that mitochondria isolated from torpid squirrel brain show a high level of palmitate-induced uncoupling. Furthermore, torpid squirrels during the hibernation season keep their brain temperature significantly elevated above ambient temperature and that of the rest of the body, including brown adipose tissue. Together, our findings suggest that UCP1 contributes to local thermogenesis in the squirrel brain, and thus supports nervous tissue function at low body temperature during hibernation.

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.

Mitochondrial acclimation capacities to ocean warming and acidification are limited in the antarctic Nototheniid Fish, Notothenia rossii and Lepidonotothen squamifrons

Antarctic notothenioid fish are characterized by their evolutionary adaptation to the cold, thermostable Southern Ocean, which is associated with unique physiological adaptations to withstand the cold and reduce energetic requirements but also entails limited compensation capacities to environmental change. This study compares the capacities of mitochondrial acclimation to ocean warming and acidification between the Antarctic nototheniid Notothenia rossii and the sub-Antarctic Lepidonotothen squamifrons, which share a similar ecology, but different habitat temperatures. After acclimation of L. squamifrons to 9°C and N. rossii to 7°C (normocapnic/hypercapnic, 0.2 kPa CO2/2000 ppm CO2) for 4-6 weeks, we compared the capacities of their mitochondrial respiratory complexes I (CI) and II (CII), their P/O ratios (phosphorylation efficiency), proton leak capacities and mitochondrial membrane fatty acid compositions. Our results reveal reduced CII respiration rates in warm-acclimated L. squamifrons and cold hypercapnia-acclimated N. rossii. Generally, L. squamifrons displayed a greater ability to increase CI contribution during acute warming and after warm-acclimation than N. rossii. Membrane unsaturation was not altered by warm or hypercapnia-acclimation in both species, but membrane fatty acids of warm-acclimated L. squamifrons were less saturated than in warm normocapnia-/hypercapnia-acclimated N. rossii. Proton leak capacities were not affected by warm or hypercapnia-acclimation of N. rossii. We conclude that an acclimatory response of mitochondrial capacities may include higher thermal plasticity of CI supported by enhanced utilization of anaplerotic substrates (via oxidative decarboxylation reactions) feeding into the citrate cycle. L. squamifrons possesses higher relative CI plasticities than N. rossii, which may facilitate the usage of energy efficient NADH-related substrates under conditions of elevated energy demand, possibly induced by ocean warming and acidification. The observed adjustments of electron transport system complexes with a higher flux through CI under warming and acidification suggest a metabolic acclimation potential of the sub-Antarctic L. squamifrons, but only limited acclimation capacities for N. rossii.

Fatty acids change the conformation of uncoupling protein 1 (UCP1)

UCP1 catalyzes proton leak across the mitochondrial inner membrane to disengage substrate oxidation from ATP production. It is well established that UCP1 is activated by fatty acids and inhibited by purine nucleotides, but precisely how this regulation occurs remains unsettled. Although fatty acids can competitively overcome nucleotide inhibition in functional assays, fatty acids have little effect on purine nucleotide binding. Here, we present the first demonstration that fatty acids induce a conformational change in UCP1. Palmitate dramatically changed the binding kinetics of 2'/3'-O-(N-methylanthraniloyl)-GDP, a fluorescently labeled nucleotide analog, for UCP1. Furthermore, palmitate accelerated the rate of enzymatic proteolysis of UCP1. The altered kinetics of both processes indicate that fatty acids change the conformation of UCP1, reconciling the apparent discrepancy between existing functional and ligand binding data. Our results provide a framework for how fatty acids and nucleotides compete to regulate the activity of UCP1.

Inter-relationship between mitochondrial function and susceptibility to oxidative stress in red- and white-blooded Antarctic notothenioid fishes

It is unknown whether Antarctic fishes can defend themselves against oxidative stress induced by elevations in temperature. We hypothesized that Antarctic icefishes, lacking the oxygen-binding protein hemoglobin, might be more vulnerable to temperature-induced oxidative stress compared with red-blooded notothenioids because of differences in their mitochondrial properties. Mitochondria from icefishes have higher densities of phospholipids per mg of mitochondrial protein compared with red-blooded species, and these phospholipids are rich in polyunsaturated fatty acids (PUFA), which can promote the formation of reactive oxygen species (ROS). Additionally, previous studies have shown that multiple tissues in icefishes have lower levels of antioxidants compared with red-blooded species. We quantified several properties of mitochondria, including proton leak, rates of ROS production, membrane composition and susceptibility to lipid peroxidation (LPO), the activity of superoxide dismutase (SOD) and total antioxidant power (TAOP) in mitochondria isolated from hearts of icefishes and red-blooded notothenioids. Mitochondria from icefishes were more tightly coupled than those of red-blooded fishes at both 2°C and 10°C, which increased the production of ROS when the electron transport chain was disrupted. The activity of SOD and TAOP per mg of mitochondrial protein was equivalent between icefishes and red-blooded species, but TAOP normalized to mitochondrial phospholipid content was significantly lower in icefishes compared with red-blooded fishes. Additionally, membrane susceptibility to peroxidation was only detectable in icefishes at 1°C and not in red-blooded species. Together, our results suggest that the high density of mitochondrial phospholipids in hearts of icefishes may make them particularly vulnerable to oxidative stress as temperatures rise.

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.

Uncoupling proteins and the control of mitochondrial reactive oxygen species production

Reactive oxygen species (ROS), natural by-products of aerobic respiration, are important cell signaling molecules, which left unchecked can severely impair cellular functions and induce cell death. Hence, cells have developed a series of systems to keep ROS in the nontoxic range. Uncoupling proteins (UCPs) 1-3 are mitochondrial anion carrier proteins that are purported to play important roles in minimizing ROS emission from the electron transport chain. The function of UCP1 in this regard is highly contentious. However, UCPs 2 and 3 are generally thought to be activated by ROS or ROS by-products to induce proton leak, thus providing a negative feedback loop for mitochondrial ROS production. In our laboratory, we have not only confirmed that ROS activate UCP2 and UCP3, but also demonstrated that UCP2 and UCP3 are controlled by covalent modification by glutathione. Furthermore, the reversible glutathionylation is required to activate/inhibit UCP2 and UCP3, but not UCP1. Hence, our findings are consistent with the notion that UCPs 2 and 3 are acutely activated by ROS, which then directly modulate the glutathionylation status of the UCP to decrease ROS emission and participate in cell signaling mechanisms.

Nonshivering thermogenesis and its adequate measurement in metabolic studies

Alterations in nonshivering thermogenesis are presently discussed as being both potentially causative of and able to counteract obesity. However, the necessity for mammals to defend their body temperature means that the ambient temperature profoundly affects the outcome and interpretation of metabolic experiments. An adequate understanding and assessment of nonshivering thermogenesis is therefore paramount for metabolic studies. Classical nonshivering thermogenesis is facultative, i.e. it is only activated when an animal acutely requires extra heat (switched on in minutes), and adaptive, i.e. it takes weeks for an increase in capacity to develop. Nonshivering thermogenesis is fully due to brown adipose tissue activity; adaptation corresponds to the recruitment of this tissue. Diet-induced thermogenesis is probably also facultative and adaptive and due to brown adipose tissue activity. Although all mammals respond to injected/infused norepinephrine (noradrenaline) with an increase in metabolism, in non-adapted mammals this increase mainly represents the response of organs not involved in nonshivering thermogenesis; only the increase after adaptation represents nonshivering thermogenesis. Thermogenesis (metabolism) should be expressed per animal, and not per body mass [not even to any power (0.75 or 0.66)]. A 'cold tolerance test' does not examine nonshivering thermogenesis capacity; rather it tests shivering capacity and endurance. For mice, normal animal house temperatures are markedly below thermoneutrality, and the mice therefore have a metabolic rate and food consumption about 1.5 times higher than their intrinsic requirements. Housing and examining mice at normal house temperatures carries a high risk of identifying false positives for intrinsic metabolic changes; in particular, mutations/treatments that affect the animal's insulation (fur, skin) may lead to such problems. Correspondingly, true alterations in intrinsic metabolic rate remain undetected when metabolism is examined at temperatures below thermoneutrality. Thus, experiments with animals kept and examined at thermoneutrality are likely to yield an improved possibility of identifying agents and genes important for human energy balance.