Adipose tissue development during early life: novel insights into energy balance from small and large mammals

The Role of Brown Adipose Tissue and Energy Metabolism in Mammalian Thermoregulation during the Perinatal Period

Hypothermia is one of the most common causes of mortality in neonates, and it could be developed after birth because the uterus temperature is more elevated than the extrauterine temperature. Neonates use diverse mechanisms to thermoregulate, such as shivering and non-shivering thermogenesis. These strategies can be more efficient in some species, but not in others, i.e., altricials, which have the greatest difficulty with achieving thermoneutrality. In addition, there are anatomical and neurological differences in mammals, which may present different distributions and amounts of brown fat. This article aims to discuss the neuromodulation mechanisms of thermoregulation and the importance of brown fat in the thermogenesis of newborn mammals, emphasizing the analysis of the biochemical, physiological, and genetic factors that determine the distribution, amount, and efficiency of this energy resource in newborns of different species. It has been concluded that is vital to understand and minimize hypothermia causes in newborns, which is one of the main causes of mortality in neonates. This would be beneficial for both animals and producers.

Growing underground: Development of thermogenesis in pups of the fossorial rodent Ctenomys talarum

In mammals, during the pup’s development and adult life, integrated requirements of all activities of the individual must conform to a sustained rate of metabolism. Thus, partitioning the available energy according to short-term priorities at a specific moment allows animals to survive and optimize long-term reproductive success. In altricial rodents, thermal balance is a key factor for survival. When no exogenous source of heat is present, altricial pups rapidly lose heat, reaching ambient temperature (Ta). Fossorial rodents showed a strong dependence on burrows, where Ta remains relatively stable within narrow ranges. Pups of the fossorial rodent Ctenomys talarum are altricial, making them an excellent model to evaluate the development of thermogenic capacity. In this study, the ontogeny of the thermogenic capacity of pups of C. talarum was evaluated. Using respirometry techniques, non-shivering thermogenesis (NST), total thermogenic capacity (cold-induced maximum metabolic rate, MMR), and resting metabolic rate (RMR) in pups until post-weaning age (day 60) were analyzed. No NST was present in pups until day 60 despite the presence of molecular markers for NST in brown adipose tissue deposits, which became functional in adults. Although pups are altricial at birth, they maintain their thermal balance behaviorally during lactation. Total thermogenic capacity became fixed at an early age, indicating an improvement in shivering thermogenesis (ST) efficiency after day 10, which might be related to the development of musculature related to digging. Before the aboveground dispersal period (~day 60), pups gradually reached adult Tb by improving ST and thermal isolation, allowing them to confront climatic fluctuations on the surface.

Neonatal infrared thermography images in the hypothermic ruminant model: Anatomical-morphological-physiological aspects and mechanisms for thermoregulation

Hypothermia is one factor associated with mortality in newborn ruminants due to the drastic temperature change upon exposure to the extrauterine environment in the first hours after birth. Ruminants are precocial whose mechanisms for generating heat or preventing heat loss involve genetic characteristics, the degree of neurodevelopment at birth and environmental aspects. These elements combine to form a more efficient mechanism than those found in altricial species. Although the degree of neurodevelopment is an important advantage for these species, their greater mobility helps them to search for the udder and consume colostrum after birth. However, anatomical differences such as the distribution of adipose tissue or the presence of type II muscle fibers could lead to the understanding that these species use their energy resources more efficiently for heat production. The introduction of unconventional ruminant species, such as the water buffalo, has led to rethinking other characteristics like the skin thickness or the coat type that could intervene in the thermoregulation capacity of the newborn. Implementing tools to analyze species-specific characteristics that help prevent a critical decline in temperature is deemed a fundamental strategy for avoiding the adverse effects of a compromised thermoregulatory function. Although thermography is a non-invasive method to assess superficial temperature in several non-human animal species, in newborn ruminants there is limited information about its application, making it necessary to discuss the usefulness of this tool. This review aims to analyze the effects of hypothermia in newborn ruminants, their thermoregulation mechanisms that compensate for this condition, and the application of infrared thermography (IRT) to identify cases with hypothermia.

Maternal L-carnitine supplementation promotes brown adipose tissue thermogenesis of newborn goats after cold exposure

Brown adipose tissue (BAT) is an important component of energy expenditure and necessary to maintain body temperature for newborn mammals. In the previous study, we found that L-carnitine was enriched in BAT and promoted BAT adipogenesis and thermogenesis in goat brown adipocytes. However, whether dietary L-carnitine regulates BAT heat production and energy expenditure in lambs remains unclear. In this study, maternal L-carnitine supplementation elevated the rectal temperature, as well as the expression of UCP1 and mitochondrial DNA content to promote BAT thermogenesis in newborn goats. Moreover, maternal L-carnitine supplementation increased the levels of triglycerides (TG), non-esterified fatty acids (NEFA), and lactate in plasma, as well as the content of lipid droplet and glycogen in BAT of newborn goats. Lipidomic analysis showed that maternal L-carnitine supplementation remodeled the lipid composition of BAT in newborn goats. L-carnitine significantly increased the levels of TG and diglyceride (DG) and decreased the levels of glycerophospholipids and sphingolipids in BAT. Further studies showed that L-carnitine promoted TG and glycogen deposition in brown adipocytes through AMPKα. Our results indicate that maternal L-carnitine supplementation promotes BAT development and thermogenesis in newborn goats and provides new evidence for newborn goats to maintain body temperature in response to cold exposure.

Cold exposure induces lipid dynamics and thermogenesis in brown adipose tissue of goats

Background

Adaptive thermogenesis by brown adipose tissue (BAT) is important to the maintenance of temperature in newborn mammals. Cold exposure activates gene expression and lipid metabolism to provide energy for BAT thermogenesis. However, knowledge of BAT metabolism in large animals after cold exposure is still limited.

Results

In this study, we found that cold exposure induced expression of BAT thermogenesis genes and increased the protein levels of UCP1 and PGC1α. Pathway analysis showed that cold exposure activated BAT metabolism, which involved in cGMP-PKG, TCA cycle, fatty acid elongation, and degradation pathways. These were accompanied by decreased triglyceride (TG) content and increased phosphatidylcholine (PC) and phosphatidylethanolamine (PE) content in BAT.

Conclusion

These results demonstrate that cold exposure induces metabolites involved in glycerolipids and glycerophospholipids metabolism in BAT. The present study provides evidence for lipid composition associated with adaptive thermogenesis in goat BAT and metabolism pathways regulated by cold exposure.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08765-5.

Strategies for Hypothermia Compensation in Altricial and Precocial Newborn Mammals and Their Monitoring by Infrared Thermography

Thermoregulation in newborn mammals is an essential species-specific mechanism of the nervous system that contributes to their survival during the first hours and days of their life. When exposed to cold weather, which is a risk factor associated with mortality in neonates, pathways such as the hypothalamic–pituitary–adrenal axis (HPA) are activated to achieve temperature control, increasing the circulating levels of catecholamine and cortisol. Consequently, alterations in blood circulation and mechanisms to produce or to retain heat (e.g., vasoconstriction, piloerection, shivering, brown adipocyte tissue activation, and huddling) begin to prevent hypothermia. This study aimed to discuss the mechanisms of thermoregulation in newborn domestic mammals, highlighting the differences between altricial and precocial species. The processes that employ brown adipocyte tissue, shivering, thermoregulatory behaviors, and dermal vasomotor control will be analyzed to understand the physiology and the importance of implementing techniques to promote thermoregulation and survival in the critical post-birth period of mammals. Also, infrared thermography as a helpful method to perform thermal measurements without animal interactions does not affect these parameters.

Integrated application of metabolomics and RNA-seq reveals thermogenic regulation in goat brown adipose tissues

Brown adipose tissue (BAT) plays an important role on no shivering thermogenesis during cold exposure to maintain animal body temperature and energy homeostasis. However, knowledge of the cellular transition from white adipose tissue (WAT) to BAT is still limited. In this study, we provided a comprehensive metabolomics and transcriptional signatures of goat BAT and WAT. A total of 157 metabolites were significantly changed, including 81 upregulated and 76 downregulated metabolites. In addition, we identified the citric acid cycle, fatty acid elongation, and degradation pathways as coordinately activated in BAT. Interestingly, five unsaturated fatty acids (Eicosadienoic Acid, C20:2; γ-Linolenic acid, C20:3; Arachidonic Acid, C20:4; Adrenic acid, C22:4; Docosahexaenoic acid, C22:6), Succinate, L-carnitine, and L-palmitoyl-carnitine were found to be abundant in BAT. Furthermore, L-carnitine, an intermediate of fatty acid degradation, is required for goat brown adipocyte differentiation and thermogenesis through activating AMPK pathway. However, L-carnitine decreased lipid accumulation through inducing lipolysis and thermogenesis in white adipocytes. These results revealed that there are the significant alterations in transcriptomic and metabolomic profiles between goat WAT and BAT, which may contribute to better understanding the roles of metabolites in BAT thermogenesis process.

Dynamic Expression Profiles of Circular RNAs during Brown to White Adipose Tissue Transformation in Goats (Capra hircus)

Simple Summary

In our study, we launched RNA-seq in order to investigate the potential functions of circRNA during brown adipose tissue (BAT) to white adipose tissue (WAT) transformation. As a result, 6610 circRNAs and 61 differentially expressed circRNAs (DEcircRNAs) were identified. Moreover, 65 miRNAs were detected that could potentially interact with DEcircRNAs. The present study provides a detailed circRNA expression landscape and evidence for circRNA functions in the transformation from BAT to WAT.

Abstract

Adipose tissues are mainly divided into brown adipose tissue (BAT) and white adipose tissue (WAT). WAT mainly functions to buffer excess calories, whereas BAT plays a role in the non-shivering thermogenesis to maintain body temperature and energy balance. Moreover, circRNAs play important roles in various biological processes. However, knowledge of the expression profile and function of circRNAs from BAT to WAT remains largely unknown. In this study, a total of 6610 unique circRNAs were identified in the perirenal adipose tissues of 1-day, 30-days, and 1-year goats. Functional annotation revealed that host genes of circRNAs were involved in some BAT-related pathways, such as the thyroid hormone signaling pathway, MAPK signaling pathway, and VEGF signaling pathway. Furthermore, a total of 61 DEcircRNAs were detected across three stages. Additionally, five selected circRNAs were validated by RNase R assay, qPCR, and Sanger sequencing. Finally, the circRNA–miRNA network was constructed between the DEcircRNAs and their miRNA binding sites.

Both proliferation and lipogenesis of brown adipocytes contribute to postnatal brown adipose tissue growth in mice

Brown adipose tissue (BAT) is the primary non-shivering thermogenesis organ in mammals, which plays essential roles in maintaining the body temperature of infants. Although the development of BAT during embryogenesis has been well addressed in rodents, how BAT grows after birth remains unknown. Using mouse interscapular BAT (iBAT) as an example, we studied the cellular and molecular mechanisms that regulate postnatal BAT growth. By analyzing the developmental dynamics of brown adipocytes (BAs), we found that BAs size enlargement partially accounts for iBAT growth. By investigating the BAs cell cycle activities, we confirmed the presence of proliferative BAs in the neonatal mice. Two weeks after birth, most of the BAs exit cell cycle, and the further expansion of the BAT was mainly due to lipogenesis-mediated BAs volume increase. Microscopy and fluorescence-activated cell sorting analyses suggest that most BAs are mononuclear and diploid. Based on the developmental dynamics of brown adipocytes, we propose that the murine iBAT has two different growth phases between birth and weaning: increase of BAs size and number in the first two weeks, and BAs size enlargement thereafter. In summary, our data demonstrate that both lipogenesis and proliferation of BAs contribute to postnatal iBAT growth in mice.

Using RNA-Seq to Identify Reference Genes of the Transition from Brown to White Adipose Tissue in Goats

Simple Summary

Brown adipose tissue (BAT) plays important roles in unique non-shivering thermogenesis. It is necessary to select reference genes during the transition process from brown (BAT) to white adipose tissue (WAT) for Quantitative PCR (qPCR) analysis. In this study, CTNNB, PFDN5 and EIF3M, selected from RNA sequencing data, were the most suitable reference genes. The present study provides a detailed analysis of the expression stability of reference genes for the study of gene expression profiling during the transition process from BAT to WAT.

Abstract

Brown adipose tissues have unique non-shivering thermogenesis functions, can be found in newborn ruminate animals, and then are gradually replaced by white adipose tissues in adulthood. For the purpose of exploring the intrinsic mechanism underlying the conversion process from brown (BAT) to white adipose tissue (WAT), it is necessary to utilize Quantitative PCR (qPCR) to study gene expression profiling. In this study, we identified reference genes that were consistently expressed during the transformation from goat BAT to WAT using RNA-seq data. Then, twelve genes were evaluated as candidate reference genes for qPCR in goat perirenal adipose tissue using three tools (geNorm, Normfinder, and BestKeeper). In addition, the selected reference genes were used to normalize the gene expression of PGC-1α and GPAT4. It was found that traditional reference genes, such as GAPDH, RPLP0, HPRT1, and PPIA were not suitable for target gene normalization. In contrast, CTNNB, PFDN5, and EIF3M, selected from RNA sequencing data, showed the least variation and were recommended as the best reference genes during the transformation from BAT to WAT.

Turning down the heat: Down-regulation of sarcolipin in a hibernating mammal

Hibernation in mammals is a whole-body phenotype that involves profound reductions in oxygen consumption, metabolic reactions, core body temperature, neural activity and heart rate. An important aspect of mammalian hibernation is the ability to reverse this state of hypothermic torpor by rewarming and subsequent arousal. Brown adipose tissue (BAT) and skeletal muscle shivering have been characterized as the predominant driving forces for thermogenesis during arousal. Conversely, the thermogenic contribution of these organs needs to be minimized as hibernating mammals enter torpor. Because skeletal muscle accounts for approximately 40% of the dry mass of the typical mammalian body, we aim to broaden the spotlight to include the importance of down-regulating skeletal muscle non-shivering thermogenesis during hibernation to allow for whole-body cooling and long-term maintenance of a depressed core body temperature when the animal is in torpor. This minireview will briefly describe the current understanding of thermoregulation in hibernating mammals and present new preliminary data on the importance of skeletal muscle and the micro-peptide sarcolipin as a major thermogenic target.

Improving pregnancy outcomes in humans through studies in sheep

Experimental studies that are relevant to human pregnancy rely on the selection of appropriate animal models as an important element in experimental design. Consideration of the strengths and weaknesses of any animal model of human disease is fundamental to effective and meaningful translation of preclinical research. Studies in sheep have made significant contributions to our understanding of the normal and abnormal development of the fetus. As a model of human pregnancy, studies in sheep have enabled scientists and clinicians to answer questions about the etiology and treatment of poor maternal, placental, and fetal health and to provide an evidence base for translation of interventions to the clinic. The aim of this review is to highlight the advances in perinatal human medicine that have been achieved following translation of research using the pregnant sheep and fetus.

The Divergent Effect of Maternal Protein Restriction during Pregnancy and Postweaning High-Fat Diet Feeding on Blood Pressure and Adiposity in Adult Mouse Offspring

Obesity is a growing health crisis of pandemic proportions. Numerous animal and human studies have confirmed that obesity and related metabolic abnormalities, such as insulin resistance and cardiovascular disease, may be programmed during development by adverse maternal nutrition. We previously documented that offspring of female mice who were protein-restricted during pregnancy alone had no alterations to their body weights, but did display a considerable reduction in food intake, a finding which was linked to reduced expression levels of appetite regulatory genes in the hypothalamus. Whether such observations were accompanied by changes in metabolic and phenotypic parameters remained to be determined. Female pregnant MF-1 mice were fed, exclusively during the pregnancy period, a normal protein diet containing 18% casein (C) or an isocaloric protein-restricted diet containing 9% casein (PR). From birth, the lactating dams were fed a normal protein diet. At weaning, offspring were fed either the standard chow which contain 7% kcal fat (C) or high-fat diet (HF, 45% kcal fat). This yielded 4 experimental groups denoted by maternal diet/offspring diet: C/C, C/HF, PR/C, PR/HF. Our results showed that offspring adiposity was significantly increased in HF-fed offspring, and was not affected by the 50% reduction in protein content of the maternal diet fed during pregnancy. Similarly, blood glucose levels were higher in HF-fed offspring, regardless of protein content of the maternal diet. Systolic blood pressure, on the other hand, was significantly increased in both male and female offspring of dams fed the PR diet, and this was exacerbated by a postweaning HF diet. Our results show that maternal protein restriction leads to elevations in systolic blood pressure, which is exacerbated by a postweaning HF-diet. Our present findings suggest that, while changes in offspring adiposity brought about by exposure to maternal protein restriction during pregnancy may be restored by adequate maternal protein content during lactation, the same may not be true for systolic blood pressure, which was similarly impaired, regardless of the timing of maternal low-protein exposure.

Maternal n-3 PUFA supplementation promotes fetal brown adipose tissue development through epigenetic modifications in C57BL/6 mice

Brown adipose tissue (BAT) is a crucial regulator of energy expenditure. Emerging evidence suggests that n-3 PUFA potentiate brown adipogenesis in vitro. Since the pregnancy and lactation is a critical time for brown fat formation, we hypothesized that maternal supplementation of n-3 PUFA promotes BAT development in offspring. Female C57BL/6 mice were fed a diet containing n-3 PUFA (3%) derived from fish oil (FO), or an isocaloric diet devoid of n-3 PUFA (Cont) during pregnancy and lactation. Maternal n-3 PUFA intake was delivered to the BAT of neonates significantly reducing the n-6/n-3 ratio. The maternal n-3 PUFA exposure was linked with upregulated brown-specific gene and protein profiles and the functional cluster of brown-specific miRNAs. In addition, maternal n-3 PUFA induced histone modifications in the BAT evidenced by 1) increased epigenetic signature of brown adipogenesis, i.e., H3K27Ac and H3K9me2, 2) modified chromatin-remodeling enzymes, and 3) enriched the H3K27Ac in the promoter region of Ucp1. The offspring received maternal n-3 PUFA nutrition exhibited a significant increase in whole-body energy expenditure and better maintenance of core body temperature against acute cold treatment. Collectively, our results suggest that maternal n-3 PUFA supplementation potentiates fetal BAT development via the synergistic action of miRNA production and histone modifications, which may confer long-lasting metabolic benefits to offspring.

Effects of long-term sucrose overfeeding on rat brown adipose tissue: a structural and immunohistochemical study

The aim of this study was to determine the effects of long-term sucrose overfeeding on functional capacity and ultrastructural characteristics of the rat brown adipose tissue (BAT). For the study, 16 male Wistar rats, chow-fed and kept under standard laboratory conditions, were divided into 2 equal groups. The rats from a control group drank tap water, whereas those from a sucrose overfed group were allowed to drink 10% sucrose solution for 21 days. Structural changes of BAT were analysed at the level of light and electron microscopy on routinely prepared tissue sections or using immunohistochemical staining, in combination with stereological methods. Obtained results have shown that the significantly increased energy intake in sucrose overfed rats did not result in a higher gain of body mass compared with controls. The light microscopy analysis revealed that the BAT acquired the appearance of a thermogenically active tissue, with intensified vascularisation, reduced size of brown adipocytes and increased multilocularity. At the ultrastructural level, mitochondria of brown adipocytes became more abundant, enlarged and contained more cristae in comparison to control animals. The immunoexpression of uncoupling protein 1 (UCP1) and noradrenaline, as markers of BAT thermogenic status, was increased, whereas the pattern of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) was slightly modified. Taken together, the results of this investigation indicated that BAT possesses the ability to increase thermogenic capacity/activity in response to high energy intake and to prevent body mass gain. These findings are particularly relevant in view of recent reports on the existence of functional BAT in adult humans and its potential use to combat obesity.

Gene expression allelic imbalance in ovine brown adipose tissue impacts energy homeostasis

Heritable trait variation within a population of organisms is largely governed by DNA variations that impact gene transcription and protein function. Identifying genetic variants that affect complex functional traits is a primary aim of population genetics studies, especially in the context of human disease and agricultural production traits. The identification of alleles directly altering mRNA expression and thereby biological function is challenging due to difficulty in isolating direct effects of cis-acting genetic variations from indirect trans-acting genetic effects. Allele specific gene expression or allelic imbalance in gene expression (AI) occurring at heterozygous loci provides an opportunity to identify genes directly impacted by cis-acting genetic variants as indirect trans-acting effects equally impact the expression of both alleles. However, the identification of genes showing AI in the context of the expression of all genes remains a challenge due to a variety of technical and statistical issues. The current study focuses on the discovery of genes showing AI using single nucleotide polymorphisms as allelic reporters. By developing a computational and statistical process that addressed multiple analytical challenges, we ranked 5,809 genes for evidence of AI using RNA-Seq data derived from brown adipose tissue samples from a cohort of late gestation fetal lambs and then identified a conservative subgroup of 1,293 genes. Thus, AI was extensive, representing approximately 25% of the tested genes. Genes associated with AI were enriched for multiple Gene Ontology (GO) terms relating to lipid metabolism, mitochondrial function and the extracellular matrix. These functions suggest that cis-acting genetic variations causing AI in the population are preferentially impacting genes involved in energy homeostasis and tissue remodelling. These functions may contribute to production traits likely to be under genetic selection in the population.

Glucocorticoids modulate human brown adipose tissue thermogenesis in vivo

INTRODUCTION

Brown adipose tissue (BAT) is a thermogenic organ with substantial metabolic capacity and has important roles in the maintenance of body weight and metabolism. Regulation of BAT is primarily mediated through the β-adrenoceptor (β-AR) pathway. The in vivo endocrine regulation of this pathway in humans is unknown. The objective of our study was to assess the in vivo BAT temperature responses to acute glucocorticoid administration.

METHODS

We studied 8 healthy male volunteers, not pre-selected for BAT presence or activity and without prior BAT cold-activation, on two occasions, following an infusion with hydrocortisone (0.2mg.kg^-1.min^-1 for 14h) and saline, respectively. Infusions were given in a randomized double-blind order. They underwent assessment of supraclavicular BAT temperature using infrared thermography following a mixed meal, and during β-AR stimulation with isoprenaline (25ng.kg fat-free mass^-1.min^-1 for 60min) in the fasting state.

RESULTS

During hydrocortisone infusion, BAT temperature increased both under fasting basal conditions and during β-AR stimulation. We observed a BAT temperature threshold, which was not exceeded despite maximal β-AR activation. We conclude that BAT thermogenesis is present in humans under near-normal conditions. Glucocorticoids modulate BAT function, representing important physiological endocrine regulation of body temperature at times of acute stress.

Modulation of brown adipocyte activity by milk by-products: Stimulation of brown adipogenesis by buttermilk

Brown adipocytes dissipate chemical energy in the form of heat through the expression of mitochondrial uncoupling protein 1 (Ucp1); Ucp1 expression is further upregulated by the stimulation of β-adrenergic receptors in brown adipocytes. An increase in energy expenditure by activated brown adipocytes potentially contributes to the prevention of or therapeutics for obesity. The present study examined the effects of milk by-products, buttermilk and butter oil, on brown adipogenesis and the function of brown adipocytes. The treatment with buttermilk modulated brown adipogenesis, depending on the product tested; during brown adipogenesis, buttermilk 1 inhibited the differentiation of HB2 brown preadipocytes. In contrast, buttermilk 3 and 5 increased the expression of Ucp1 in the absence of isoproterenol (Iso), a β-adrenergic receptor agonist, suggesting the stimulation of brown adipogenesis. In addition, the Iso-induced expression of Ucp1 was enhanced by buttermilk 2 and 3. The treatment with buttermilk did not affect the basal or induced expression of Ucp1 by Iso in HB2 brown adipocytes, except for buttermilk 5, which increased the basal expression of Ucp1. Conversely, butter oil did not significantly affect the expression of Ucp1, irrespective of the cell phase of HB2 cells, ie, treatment during brown adipogenesis or of brown adipocytes. The results of the present study indicate that buttermilk is a regulator of brown adipogenesis and suggest its usefulness as a potential food material for antiobesity.

'Browning' the cardiac and peri-vascular adipose tissues to modulate cardiovascular risk

Excess visceral adiposity, in particular that located adjacent to the heart and coronary arteries is associated with increased cardiovascular risk. In the pathophysiological state, dysfunctional adipose tissue secretes an array of factors modulating vascular function and driving atherogenesis. Conversely, brown and beige adipose tissues utilise glucose and lipids to generate heat and are associated with improved cardiometabolic health. The cardiac and thoracic perivascular adipose tissues are now understood to be composed of brown adipose tissue in the healthy state and undergo a brown-to-white transition i.e. during obesity which may be a driving factor of cardiovascular disease. In this review we discuss the risks of excess cardiac and vascular adiposity and potential mechanisms by which restoring the brown phenotype i.e. “re-browning” could potentially be achieved in clinically relevant populations.

•

Epicardial, paracardial and thoracic perivascular adipose tissues resemble BAT at birth.

•

Despite ‘whitening’ in early life these depots remain metabolically active and potentially thermogenic into adulthood.

•

Obesity induces further ‘whitening’ and inflammation in these depots likely driving the atherogenesis.

•

Maintaining or inducing the brown phenotype in these depots could prevent atherosclerotic disease.

Cord blood irisin at the extremes of fetal growth

BACKGROUND/AIMS

Irisin, a novel myokine with antiobesity properties, drives brown-fat-like conversion of white adipose tissue, thus increasing energy expenditure and improving glucose tolerance. We aimed to investigate circulating irisin concentrations in large-for-gestational-age (LGA) and intrauterine-growth-restricted (IUGR) fetuses, both associated with metabolic dysregulation and long-term susceptibility to obesity and metabolic syndrome development.

METHODS

Plasma irisin and insulin concentrations were determined by ELISA and IRMA, respectively, in 80 mixed arteriovenous cord blood samples from LGA (n=30), IUGR (n=30) and appropriate-for-gestational-age (AGA, n=20) singleton full-term pregnancies. Fetuses were classified as LGA, IUGR or AGA, based on customized birth-weight standards adjusted for significant determinants of fetal growth.

RESULTS

Fetal irisin concentrations were lower in IUGR cases than AGA controls (p=0.031). Cord blood irisin concentrations were similar in LGA and AGA groups and positively correlated with birth-weight, as well as customized centiles (r=0.245, p=0.029 and r=0.247, p=0.027, respectively). Insulin concentrations were higher in LGA, compared to AGA fetuses (p=0.036). In the LGA group, fetal irisin concentrations positively correlated with fetal insulin concentrations (r=0.374, p=0.042).

CONCLUSIONS

Impaired skeletal muscle metabolism in IUGR fetuses may account for their irisin deficiency, which may be part of the fetal programming process, leading to increased susceptibility to later metabolic syndrome development. Furthermore, irisin down-regulation may predispose IUGR infants to hypothermia at birth, by inducing less "browning" of their adipose tissue and consequently less non-shivering thermogenesis. Irisin upregulation with increasing birth-weight may contribute to a slower fat gain during early infancy ("catch-down"), by promoting higher total energy expenditure. The positive correlation between irisin and insulin in the LGA group may reflect a counterbalance of the documented hyperinsulinemia, which is partly responsible for the excessive fat deposition in the LGA fetus.

Maternal health and eating habits: metabolic consequences and impact on child health

Apart from direct inheritance and the effects of a shared environment, maternal health, eating habits and diet can affect offspring health by developmental programming. Suboptimal maternal nutrition (i.e., either a reduction or an increase above requirement) or other insults experienced by the developing fetus can induce significant changes in adipose tissue and brain development, energy homeostasis, and the structure of vital organs. These can produce long-lasting adaptations that influence later energy balance, and increase the susceptibility of that individual to obesity and the components of the metabolic syndrome. Studies that elucidate the mechanisms behind these associations will have a positive impact on the health of the future adult population and may help to contain the obesity epidemic.

Preadipocyte and adipose tissue differentiation in meat animals: influence of species and anatomical location

Early in porcine adipose tissue development, the stromal-vascular (SV) elements control and dictate the extent of adipogenesis in a depot-dependent manner. The vasculature and collagen matrix differentiate before overt adipocyte differentiation. In the fetal pig, subcutaneous (SQ) layer development is predictive of adipocyte development, as the outer, middle, and inner layers of dorsal SQ adipose tissue develop and maintain layered morphology throughout postnatal growth of SQ adipose tissue. Bovine and ovine fetuses contain brown adipose tissue but SQ white adipose tissue is poorly developed structurally. Fetal adipose tissue differentiation is associated with the precocious expression of several genes encoding secreted factors and key transcription factors like peroxisome proliferator activated receptor (PPAR)γ and CCAAT/-enhancer-binding protein. Identification of adipocyte-associated genes differentially expressed by age, depot, and species in vivo and in vitro has been achieved using single-gene analysis, microarrays, suppressive subtraction hybridization, and next-generation sequencing applications. Gene polymorphisms in PPARγ, cathepsins, and uncoupling protein 3 have been associated with back fat accumulation. Genome scans have mapped several quantitative trait loci (QTL) predictive of adipose tissue-deposition phenotypes in cattle and pigs.

The Ontogeny of Brown Adipose Tissue

There are three different types of adipose tissue (AT)-brown, white, and beige-that differ with stage of development, species, and anatomical location. Of these, brown AT (BAT) is the least abundant but has the greatest potential impact on energy balance. BAT is capable of rapidly producing large amounts of heat through activation of the unique uncoupling protein 1 (UCP1) located within the inner mitochondrial membrane. White AT is an endocrine organ and site of lipid storage, whereas beige AT is primarily white but contains some cells that possess UCP1. BAT first appears in the fetus around mid-gestation and is then gradually lost through childhood, adolescence, and adulthood. We focus on the interrelationships between adipocyte classification, anatomical location, and impact of diet in early life together with the extent to which fat development differs between the major species examined. Ultimately, novel dietary interventions designed to reactivate BAT could be possible.

Innate hypothermia after hypoxic ischaemic delivery

The focus of this review is to collate the literature on the phenomenon of impaired thermal adaptation after hypoxic ischaemic (HI) delivery often culminating in hypothermia. This phenomenon appears different in severity and duration to a spontaneous postnatal fall in temperature observed after normal delivery. The original observation and contemporary descriptions of the temperature response to HI are described and a mechanism of action is proposed that may be utilised as a novel biomarker for HI.

Fetal programming of adipose tissue function: an evolutionary perspective

Obesity is an escalating threat of pandemic proportions and has risen to such unrivaled prominence in such a short period of time that it has come to define a whole generation in many countries around the globe. The burden of obesity, however, is not equally shared among the population, with certain ethnicities being more prone to obesity than others, while some appear to be resistant to obesity altogether. The reasons behind this ethnic basis for obesity resistance and susceptibility, however, have remained largely elusive. In recent years, much evidence has shown that the level of brown adipose tissue thermogenesis, which augments energy expenditure and is negatively associated with obesity in both rodents and humans, varies greatly between ethnicities. Interestingly, the incidence of low birth weight, which is associated with an increased propensity for obesity and cardiovascular disease in later life, has also been shown to vary by ethnic background. This review serves to reconcile ethnic variations in BAT development and function with ethnic differences in birth weight outcomes to argue that the variation in obesity susceptibility between ethnic groups may have its origins in the in utero programming of BAT development and function as a result of evolutionary adaptation to cold environments.

An evolving scientific basis for the prevention and treatment of pediatric obesity

The 2013 Pennington Biomedical Research Center's Scientific Symposium focused on the treatment and management of pediatric obesity and was designed to (i) review recent scientific advances in the prevention, clinical treatment and management of pediatric obesity, (ii) integrate the latest published and unpublished findings and (iii) explore how these advances can be integrated into clinical and public health approaches. The symposium provided an overview of important new advances in the field, which led to several recommendations for incorporating the scientific evidence into practice. The science presented covered a range of topics related to pediatric obesity, including the role of genetic differences, epigenetic events influenced by in utero development, pre-pregnancy maternal obesity status, maternal nutrition and maternal weight gain on developmental programming of adiposity in offspring. Finally, the relative merits of a range of various behavioral approaches targeted at pediatric obesity were covered, together with the specific roles of pharmacotherapy and bariatric surgery in pediatric populations. In summary, pediatric obesity is a very challenging problem that is unprecedented in evolutionary terms; one which has the capacity to negate many of the health benefits that have contributed to the increased longevity observed in the developed world.

Body mass index as a determinant of brown adipose tissue function in healthy children

OBJECTIVE

To determine whether body mass index (BMI) percentile and ethnicity influence skin temperature overlying brown adipose tissue (BAT) depots in the supraclavicular region in healthy children.

STUDY DESIGN

Infrared thermography measured supraclavicular region temperature (T(SCR)) at baseline and after exposure to a mild cool stimulus (single hand immersion in water at 20.1 °C) for 5 minutes in children aged 6-11 years (n = 55). The studies were undertaken in a normal school environment.

RESULTS

BMI percentile and ethnicity were significant predictors of baseline T(SCR), with an inverse relationship between BMI percentile persisting after adjustment for ethnicity. Twenty-four children demonstrated a significant rise in T(SCR) after exposure to the cool stimulus. BMI percentile was a significant predictor of T(SCR) response, although there was no effect of ethnicity on T(SCR) change after exposure to the cool stimulus.

CONCLUSION

We have demonstrated a negative relationship between BMI percentile and both baseline T(SCR), colocating with the primary region of BAT, and the change in T(SCR) in response to the cool stimulus. Future studies aimed at determining the primary factors regulating BAT function in healthy children should be targeted at the goal of maintaining a healthy BMI trajectory during childhood.

The developmental transition of ovine adipose tissue through early life

AIM

Hypothermia induced by cold exposure at birth is prevented in sheep by the rapid onset of non-shivering thermogenesis in brown adipose tissue (BAT). Changes in adipose tissue composition in early life are therefore essential for survival but also influence adiposity in later life and were thus examined in detail during early development.

METHODS

Changes in adipose composition were investigated by immunohistochemistry and qRT-PCR between the period from the first appearance of adipose in the mid gestation foetus, through birth and up to 1 month of age.

RESULTS

We identified four distinct phases of development, each associated with pronounced changes in tissue histology and in distribution of the BAT specific uncoupling protein (UCP)1. At mid gestation, perirenal adipose tissue exhibited a dense proliferative, structure marked by high expression of KI-67 but with no UCP1 or visible lipid droplets. By late gestation large quantities of UCP1 were present, lipid storage was evident and expression of BAT-related genes were abundant (e.g. prolactin and β3 receptors). Subsequently, within 12 h of birth, the depot was largely depleted of lipid and expression of genes such as UCP1, PGC1α, CIDEA peaked. By 30 days UCP1 was undetectable and the depot contained large lipid droplets; however, genes characteristic of BAT (e.g. PRDM16 and BMP7) and most characteristic of white adipose tissue (e.g. leptin and RIP140) were still abundant.

CONCLUSION

Adipose tissue undergoes profound compositional changes in early life, of which an increased understanding could offer potential interventions to retain BAT in later life.

Spontaneous intra-uterine growth restriction modulates the endocrine status and the developmental expression of genes in porcine fetal and neonatal adipose tissue

Low birth weight is correlated with low adiposity at birth, a phenotype that influences neonatal survival and later adiposity. A better understanding of events affecting the fetal adipose tissue development and its functionality around birth is thus needed. This study was undertaken to examine the impact of spontaneous intra-uterine growth restriction (IUGR) on circulating concentrations of hormones and nutrients together with the developmental expression patterns of various genes in subcutaneous adipose tissue of pig fetus during the last third of pregnancy and just after birth. At 71 and 112 days post-conception and 2 days postnatal, pairs of same-sex piglets were chosen within litters to have either a medium (MBW) or a low (LBW) weight (n=6 pairs at each stage). The results indicate that IUGR counteracts the temporal fall of DLK1 gene expression in developing adipose tissue across gestation. It also attenuates the time-dependent increase in expression levels of many genes promoting adipocyte differentiation (PPARG, CEBPA) and lipogenesis (LPL, SREBF1, FASN, FABP4). Opposite responses to IUGR were observed for the IGF system, so that IGF1 mRNA levels were lower (P<0.001) but IGF2 mRNA levels were greater in adipose tissue of LBW piglets compared with MBW piglets. The plasma insulin concentration and the mRNA levels of insulin receptor (INSR) and insulin-responsive glucose transporter (GLUT4) in adipose tissue were also greater in LBW piglets at day 2 postnatal. The data indicate that IUGR delays the normal ontogeny of adipose tissue across gestation and affects the insulin and IGF axes around birth.

Anatomical locations of human brown adipose tissue: functional relevance and implications in obesity and type 2 diabetes

We will review information about and present hypotheses as to the anatomy of brown adipose tissue (BAT). Why is it located where it is in humans? Its anatomical distribution is likely to confer survival value by protecting critical organs from hypothermia by adaptive thermogenesis. Ultimately, the location and function will be important when considering therapeutic strategies for preventing and treating obesity and type 2 diabetes, in which case successful interventions will need to have a significant effect on BAT function in subjects living in a thermoneutral environment. In view of the diverse locations and potential differences in responsiveness between BAT depots, it is likely that BAT will be shown to have much more subtle and thus previously overlooked functions and regulatory control mechanisms.

Brown adipose tissue growth and development

Brown adipose tissue is uniquely able to rapidly produce large amounts of heat through activation of uncoupling protein (UCP) 1. Maximally stimulated brown fat can produce 300 watts/kg of heat compared to 1 watt/kg in all other tissues. UCP1 is only present in small amounts in the fetus and in precocious mammals, such as sheep and humans; it is rapidly activated around the time of birth following the substantial rise in endocrine stimulatory factors. Brown adipose tissue is then lost and/or replaced with white adipose tissue with age but may still contain small depots of beige adipocytes that have the potential to be reactivated. In humans brown adipose tissue is retained into adulthood, retains the capacity to have a significant role in energy balance, and is currently a primary target organ in obesity prevention strategies. Thermogenesis in brown fat humans is environmentally regulated and can be stimulated by cold exposure and diet, responses that may be further modulated by photoperiod. Increased understanding of the primary factors that regulate both the appearance and the disappearance of UCP1 in early life may therefore enable sustainable strategies in order to prevent excess white adipose tissue deposition through the life cycle.