An Adipose Tissue Atlas: An Image-Guided Identification of Human-like BAT and Beige Depots in Rodents

Non-invasive Imaging Methods for Brown Adipose Tissue Detection and Function Evaluation

Brown Adipose Tissue (BAT) has a major role in thermoregulation, producing heat by non-shivering thermogenesis. Primarily found in animals and human infants, the presence of significant brown adipose tissue was identified only recently, and its metabolic role in adults was reconsidered. BAT is believed to have an important role in many metabolic diseases, such as obesity and diabetes, and also to be associated with cancer cachexia. Therefore, it is currently a topic of great interest in the research community, and many groups are investigating the mechanisms underlying BAT metabolism in normal and pathological conditions. However, well established non-invasive methods for assessing BAT distribution and function are still lacking. The purpose of this review is to summarize the current state of the art of these methods, with a particular focus on PET, CT and MRI.

Fasting-dependent Vascular Permeability Enhancement in Brown Adipose Tissues Evidenced by Using Carbon Nanotubes as Fluorescent Probes

Brown adipose tissue (BAT), which is composed of thermogenic brown adipocytes (BA) and non-parenchymal components including vasculatures and extracellular matrix, contribute to the maintenance of body temperature. BAT distribution is detected by positron emission tomography-computed tomography (PET/CT) using ¹⁸F-fluorodeoxy glucose (¹⁸F-FDG) or single-photon-emission computed tomography-computed tomography (SPECT/CT) using [¹²³/¹²⁵I]-beta-methyl-p-iodophenyl-pentadecanoic acid. Although sympathetic nerve activity and thermogenic capacity of BA is downregulated under fasting conditions in mice, fasting-dependent structural changes and fluid kinetics of BAT remain unknown. Here we show that the fasting induces fine and reversible structural changes in the non-parenchymal region in murine BAT with widened intercellular spaces and deformed collagen bands as revealed by electron microscopy. Interestingly, a newly introduced near infrared fluorescent probe of single-walled carbon nanotubes (CNTs) coated with phospholipid polyethylene glycol (PLPEG) easily demonstrated enhanced vascular permeability in BAT by the fasting. PLPEG-CNTs extravasated and remained in intercellular spaces or further redistributed in parenchymal cells in fasted mice, which is a previously unknown phenomenon. Thus, PLPEG-CNTs provide a powerful tool to trace fluid kinetics in sub-tissue levels.

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.

Brown adipocyte glucose metabolism: a heated subject

The energy expending and glucose sink properties of brown adipose tissue (BAT) make it an attractive target for new obesity and diabetes treatments. Despite decades of research, only recently have mechanistic studies started to provide a more complete and consistent picture of how activated brown adipocytes handle glucose. Here, we discuss the importance of intracellular glycolysis, lactate production, lipogenesis, lipolysis, and beta-oxidation for BAT thermogenesis in response to natural (temperature) and artificial (pharmacological and optogenetic) forms of sympathetic nervous system stimulation. It is now clear that together, these metabolic processes in series and in parallel flexibly power ATP-dependent and independent futile cycles in brown adipocytes to impact on whole-body thermal, energy, and glucose balance.

New approaches targeting brown adipose tissue transplantation as a therapy in obesity

Brown adipose tissue (BAT) is raising high expectations as a potential target in the fight against metabolic disorders such as obesity and type 2 diabetes. BAT utilizes fuels such as fatty acids to maintain body temperature by uncoupling mitochondrial electron transport to produce heat instead of ATP. This process is called thermogenesis. BAT was considered to be exclusive to rodents and human neonates. However, in the last decade several studies have demonstrated that BAT is not only present but also active in adult humans and that its activity is reduced in several pathological conditions, such as aging, obesity, and diabetes. Thus, tremendous efforts are being made by the scientific community to enhance either BAT mass or activity. Several activators of thermogenesis have been described, such as natriuretic peptides, bone morphogenic proteins, or fibroblast growth factor 21. Furthermore, recent studies have tested a therapeutic approach to directly increase BAT mass by the implantation of either adipocytes or fat tissue. This approach might have an important future in regenerative medicine and in the fight against metabolic disorders. Here, we review the emerging field of BAT transplantation including the various sources of mesenchymal stem cell isolation in rodents and humans and the described metabolic outcomes of adipocyte cell transplantation and BAT transplantation in obesity.

Transcriptional brakes on the road to adipocyte thermogenesis

White adipocytes represent the principle site for energy storage whereas brown/beige adipocytes emerge from seemingly distinct cellular lineages and burn chemical energy to produce heat. Thermogenic adipocytes utilize cell-type selective master regulatory transcription factors to drive the expression of their adipocyte thermogenic gene program. White adipocytes harbor transcriptional mechanisms to suppress the thermogenic gene program and maintain an energy-storing function. Here, we summarize some of the key developmental and transcriptional mechanisms leading to the postnatal recruitment of thermogenic adipocytes under physiological conditions, with a particular emphasis on the transcriptional "brakes" on the thermogenic gene program. We highlight a number of recent studies, including our own work on the transcription factor, ZFP423, that illustrate the potential to engineer the subcutaneous and visceral white fat lineages to adopt a thermogenic fat cell fate by releasing the inhibition of the adipocyte thermogenic gene program. These transcriptional brakes on adipocyte thermogenesis may represent potential targets of therapeutic interventions designed to combat obesity and associated metabolic disorders.

Whole-tissue 3D imaging reveals intra-adipose sympathetic plasticity regulated by NGF-TrkA signal in cold-induced beiging

Sympathetic arborizations act as the essential efferent signals in regulating the metabolism of peripheral organs including white adipose tissues (WAT). However, whether these local neural structures would be of plastic nature, and how such plasticity might participate in specific metabolic events of WAT, remains largely uncharacterized. In this study, we exploit the new volume fluorescence-imaging technique to observe the significant, and also reversible, plasticity of intra-adipose sympathetic arborizations in mouse inguinal WAT in response to cold challenge. We demonstrate that this sympathetic plasticity depends on the cold-elicited signal of nerve growth factor (NGF) and TrkA receptor. Blockage of NGF or TrkA signaling suppresses intra-adipose sympathetic plasticity, and moreover, the cold-induced beiging process of WAT. Furthermore, we show that NGF expression in WAT depends on the catecholamine signal in cold challenge. We therefore reveal the key physiological relevance, together with the regulatory mechanism, of intra-adipose sympathetic plasticity in the WAT metabolism.

Leptin and brain-adipose crosstalks

Interactions between the brain and distinct adipose depots have a key role in maintaining energy balance, thereby promoting survival in response to metabolic challenges such as cold exposure and starvation. Recently, there has been renewed interest in the specific central neuronal circuits that regulate adipose depots. Here, we review anatomical, genetic and pharmacological studies on the neural regulation of adipose function, including lipolysis, non-shivering thermogenesis, browning and leptin secretion. In particular, we emphasize the role of leptin-sensitive neurons and the sympathetic nervous system in modulating the activity of brown, white and beige adipose tissues. We provide an overview of advances in the understanding of the heterogeneity of the brain regulation of adipose tissues and offer a perspective on the challenges and paradoxes that the community is facing regarding the actions of leptin on this system.

Lessons from Cre-Mice and Indicator Mice

The adult human adipose tissue is predominantly composed of white adipocytes. However, within certain depots, adipose tissue contains thermogenically active brown-like adipocytes, which have been evolutionarily conserved in mammals. This chapter will give a brief overview on the methods used to genetically target and trace both white and brown adipocytes using techniques such as bacterial artificial chromosome (BAC) cloning to create transgenic mouse models and the tools with which genetic recombination is mediated in vivo (e.g., Cre-loxP, CreERT, and Tet-On). The chapter furthermore critically discusses the strength and limitation of the various systems used to target mature white and brown adipocytes (ap2-Cre, Adipoq-Cre, and Ucp1-Cre). Based on these systems, it is evident that our knowledge of mature adipocyte categorization into brown, white, brite, or beige adipocytes is strongly influenced by the use of the various genetic mouse models described in this chapter. Our evaluation of different studies using the aforementioned systems focuses on key genes, which have been reported to maintain adipocyte's function (insulin receptor, Raptor, or Atgl).