The spatiotemporal development of adipose tissue

From development to future prospects: The adipose tissue & adipose tissue organoids

Living organisms store their energy in different forms of fats including lipid droplets, triacylglycerols, and steryl esters. In mammals and some non-mammal species, the energy is stored in adipose tissue which is the innervated specialized connective tissue that incorporates a variety of cell types such as macrophages, fibroblasts, pericytes, endothelial cells, adipocytes, blood cells, and several kinds of immune cells. Adipose tissue is so complex that the scope of its function is not only limited to energy storage, it also encompasses to thermogenesis, mechanical support, and immune defense. Since defects and complications in adipose tissue are heavily related to certain chronic diseases such as obesity, cardiovascular diseases, type 2 diabetes, insulin resistance, and cholesterol metabolism defects, it is important to further study adipose tissue to enlighten further mechanisms behind those diseases to develop possible therapeutic approaches. Adipose organoids are accepted as very promising tools for studying fat tissue development and its underlying molecular mechanisms, due to their high recapitulation of the adipose tissue in vitro. These organoids can be either derived using stromal vascular fractions or pluripotent stem cells. Due to their great vascularization capacity and previously reported incontrovertible regulatory role in insulin sensitivity and blood glucose levels, adipose organoids hold great potential to become an excellent candidate for the source of stem cell therapy. In this review, adipose tissue types and their corresponding developmental stages and functions, the importance of adipose organoids, and the potential they hold will be discussed in detail.

Pantothenic Acid Alleviates Fat Deposition and Inflammation by Suppressing the JNK/P38 MAPK Signaling Pathway

Excessive fat deposition leads to obesity and cardiovascular diseases with abnormal metabolism. Pantothenic acid (PA) is a major B vitamin required for energy metabolism. However, the effect of PA on lipid metabolism and obesity has not been explored. We investigated the effects and molecular mechanism of PA on fat accumulation as well as the influence of adipogenic marker genes in both adult male mice and primary adipocytes. First, we demonstrated that PA attenuates weight gain in mice fed high-fat diet (HFD). Besides, PA supplementation substantially improved glucose tolerance and lipid metabolic disorder in obese mice. Furthermore, PA significantly inhibited white adipose tissue (WAT) deposition as well as fat droplets visualized by magnification in both chow and HFD group. More importantly, PA obviously suppressed the mRNA levels of CD36, IL-6, and TNF-α to alleviate inflammation and reduced the levels of PPARγ, aP2, and C/EBPα genes that are related to lipid metabolism in inguinal white adipose tissue (ing-WAT) and epididymal white adipose tissue (ei-WAT). In vitro, PA supplementation showed a lower lipid droplet aggregation as well as reduced expression levels of adipogentic genes. Finally, we identified that PA inhibits the phosphorylation levels of p38 and JNK in murine primary adipocytes. Collectively, our data demonstrated for the first time that PA attenuates lipid metabolic disorder as well as fat deposition by JNK/p38 MAPK signaling pathway.

The miRNA Contribution in Adipocyte Maturation

Mesenchymal stem cells, due to their multipotent ability, are considered one of the best candidates to be used in regenerative medicine. To date, the most used source is represented by the bone marrow, despite the limited number of cells and the painful/invasive procedure for collection. Therefore, the scientific community has investigated many alternative sources for the collection of mesenchymal stem cells, with the adipose tissue representing the best option, given the abundance of mesenchymal stem cells and the easy access. Although adipose mesenchymal stem cells have recently been investigated for their multipotency, the molecular mechanisms underlying their adipogenic potential are still unclear. In this scenario, this communication is aimed at defining the role of miRNAs in adipogenic potential of adipose-derived mesenchymal stem cells via real-time PCR. Even if preliminary, our data show that cell culture conditions affect the expression of specific miRNA involved in the adipogenic potential of mesenchymal stem cells. The in vitro/in vivo validation of these results could pave the way for novel therapeutic strategies in the field of regenerative medicine. In conclusion, our research highlights how specific cell culture conditions can modulate the adipogenic potential of adipose mesenchymal stem cells through the regulation of specific miRNAs.

The multifaceted life of macrophages in white adipose tissue: Immune shift couples with metabolic switch

White adipose tissue (WAT) is a vital endocrine organ that regulates energy balance and metabolic homeostasis. In addition to fat cells, WAT harbors macrophages with distinct phenotypes that play crucial roles in immunity and metabolism. Nutrient demands cause macrophages to accumulate in WAT niches, where they remodel the microenvironment and produce beneficial or detrimental effects on systemic metabolism. Given the abundance of macrophages in WAT, this review summarizes the heterogeneity of WAT macrophages in physiological and pathological conditions, including their alterations in quantity, phenotypes, characteristics, and functions during WAT growth and development, as well as healthy or unhealthy expansion. We will discuss the interactions of macrophages with other cell partners in WAT including adipose stem cells, adipocytes, and T cells in the context of various microenvironment niches in lean or obese condition. Finally, we highlight how adipose tissue macrophages merge immunity and metabolic changes to govern energy balance for the organism.

Adipocyte-endothelial cell interplay in adipose tissue physiology

Adipose tissue (AT) expansion through hyperplasia or hypertrophy requires vascular remodeling that involves angiogenesis. There is quite some evidence that obese white AT (WAT) displays altered vasculature. Some studies suggest that this is associated with hypoxia, which is thought to play a role in inducing inflammatory activation of the excessively expanding WAT. Increasing evidence, based on genetic manipulations or treatments with inhibitory or activator pharmaceuticals, demonstrates that AT angiogenesis is crucial for AT metabolic function, and thereby for whole body metabolism and metabolic health. Despite some contradiction between studies, disturbance of WAT angiogenesis in obesity could be an important factor driving WAT dysfunction and the comorbidities of obesity. Endothelial cells (ECs) contribute to healthy WAT metabolism via transport of fatty acids and other plasma components, secretory signaling molecules, and extracellular vesicles (EVs). This communication is crucial for adipocyte metabolism and underscores the key role that the AT endothelium plays in systemic energy homeostasis and healthy metabolism. Adipocytes communicate towards the neighboring endothelium through several mechanisms. The pro-inflammatory status of hypertrophic adipocytes in obesity is reflected in ECs activation, which promotes chronic inflammation. On the other hand, adiponectin secreted by the adipocytes is important for healthy endothelial function, and adipocytes also secrete other pro- or anti-angiogenic effector molecules and a wealth of EVs - however, their detailed roles in signaling towards the endothelium are yet poorly understood. To conclude, targeting AT angiogenesis and promoting the healthy communication between adipocytes and ECs represent potentially promising strategies to treat obesity and its comorbidities.

Altered Igf2 imprint leads to accelerated adipogenesis and early onset of metabolic syndrome in male mice following gestational arsenic exposure

Developmental exposure to environmental pollutants has been shown to promote adverse health outcomes in offspring. Exposure to heavy metals such as arsenic which also has endocrine-disrupting activity is being increasingly linked with cancers, diabetes, and lately with Metabolic Syndrome (MetS). In this work, we have assessed the effects of preconceptional plus gestational arsenic exposure on the developmental programming of MetS in offspring. In our study, only gestational arsenic exposure led to reduced birth weight, followed by catch-up growth, adiposity, elevated serum triglycerides levels, and hyperglycemia in male offspring. Significant adipocyte dysfunction was observed in offspring with increased hypertrophy, insulin resistance, and chronic inflammation in epididymal white adipose tissue. Adipose tissue regulates the metabolic health of individuals and its dysfunction resulted in elevated serum levels of metabolism-regulating adipokines (Leptin, Resistin) and pro-inflammatory cytokines (PAI-1, TNFα). The progenitor adipose-derived stem cells (AdSCs) from exposed progeny had increased proliferation and adipogenic potential with excess lipid accumulation. We also found increased activation of Akt, ERK1/2 & p38 MAPK molecules in arsenic-exposed AdSCs along with increased levels of phospho-Insulin-like growth factor-1 receptor (p-IGF1R) and its upstream activator Insulin-like growth factor-2 (IGF2). Overexpression of Igf2 was found to be due to arsenic-mediated DNA hypermethylation at the imprinting control region (ICR) located -2kb to -4.4 kb upstream of the H19 gene which caused a reduction in the conserved zinc finger protein (CTCF) occupancy. This further led to persistent activation of the MAPK signaling cascade and enhanced adipogenesis leading to the early onset of MetS in the offspring.

Two-step regulation by matrix Gla protein in brown adipose cell differentiation

OBJECTIVE

Bone morphogenetic protein (BMP) signaling is intricately involved in adipose tissue development. BMP7 together with BMP4 have been implicated in brown adipocyte differentiation but their roles during development remains poorly specified. Matrix Gla protein (MGP) inhibits BMP4 and BMP7 and is expressed in endothelial and progenitor cells. The objective was to determine the role of MGP in brown adipose tissue (BAT) development.

METHODS

The approach included global and cell-specific Mgp gene deletion in combination with RNA analysis, immunostaining, thermogenic activity, and in vitro studies.

RESULTS

The results revealed that MGP directs brown adipogenesis at two essential steps. Endothelial-derived MGP limits triggering of white adipogenic differentiation in the perivascular region, whereas MGP derived from adipose cells supports the transition of CD142-expressing progenitor cells to brown adipogenic maturity. Both steps were important to optimize the thermogenic function of BAT. Furthermore, MGP derived from both sources impacted vascular growth. Reduction of MGP in either endothelial or adipose cells expanded the endothelial cell population, suggesting that MGP is a factor in overall plasticity of adipose tissue.

CONCLUSION

MGP displays a dual and cell-specific function in BAT, essentially creating a "cellular shuttle" that coordinates brown adipogenic differentiation with vascular growth during development.

CD73-dependent generation of extracellular adenosine by vascular endothelial cells modulates de novo lipogenesis in adipose tissue

Next to white and brown adipocytes present in white and brown adipose tissue (WAT, BAT), vascular endothelial cells, tissue-resident macrophages and other immune cells have important roles in maintaining adipose tissue homeostasis but also contribute to the etiology of obesity-associated chronic inflammatory metabolic diseases. In addition to hormonal signals such as insulin and norepinephrine, extracellular adenine nucleotides modulate lipid storage, fatty acid release and thermogenic responses in adipose tissues. The complex regulation of extracellular adenine nucleotides involves a network of ectoenzymes that convert ATP via ADP and AMP to adenosine. However, in WAT and BAT the processing of extracellular adenine nucleotides and its relevance for intercellular communications are still largely unknown. Based on our observations that in adipose tissues the adenosine-generating enzyme CD73 is mainly expressed by vascular endothelial cells, we studied glucose and lipid handling, energy expenditure and adaptive thermogenesis in mice lacking endothelial CD73 housed at different ambient temperatures. Under conditions of thermogenic activation, CD73 expressed by endothelial cells is dispensable for the expression of thermogenic genes as well as energy expenditure. Notably, thermoneutral housing leading to a state of low energy expenditure and lipid accumulation in adipose tissues resulted in enhanced glucose uptake into WAT of endothelial CD73-deficient mice. This effect was associated with elevated expression levels of de novo lipogenesis genes. Mechanistic studies provide evidence that extracellular adenosine is imported into adipocytes and converted to AMP by adenosine kinase. Subsequently, activation of the AMP kinase lowers the expression of de novo lipogenesis genes, most likely via inactivation of the transcription factor carbohydrate response element binding protein (ChREBP). In conclusion, this study demonstrates that endothelial-derived extracellular adenosine generated via the ectoenzyme CD73 is a paracrine factor shaping lipid metabolism in WAT.

The Impact of Maternal Obesity on Adipose Progenitor Cells

The concept of Developmental Origin of Health and Disease (DOHaD) postulates that adult-onset metabolic disorders may originate from suboptimal conditions during critical embryonic and fetal programming windows. In particular, nutritional disturbance during key developmental stages may program the set point of adiposity and its associated metabolic diseases later in life. Numerous studies in mammals have reported that maternal obesity and the resulting accelerated growth in neonates may affect adipocyte development, resulting in persistent alterations in adipose tissue plasticity (i.e., adipocyte proliferation and storage) and adipocyte function (i.e., insulin resistance, impaired adipokine secretion, reduced thermogenesis, and higher inflammation) in a sex- and depot-specific manner. Over recent years, adipose progenitor cells (APCs) have been shown to play a crucial role in adipose tissue plasticity, essential for its development, maintenance, and expansion. In this review, we aim to provide insights into the developmental timeline of lineage commitment and differentiation of APCs and their role in predisposing individuals to obesity and metabolic diseases. We present data supporting the possible implication of dysregulated APCs and aberrant perinatal adipogenesis through epigenetic mechanisms as a primary mechanism responsible for long-lasting adipose tissue dysfunction in offspring born to obese mothers.

Partial Lipectomy of the Epididymal Fat Alters Expression of the Steroidogenic Enzymes in the Mouse Testis at Different Postnatal Ages

The epididymal fat is a type of gonadal adipose tissue, which is localized closely to the testis. Even though it has been suggested that the epididymal fat is necessary for maintenance of spermatogenesis in the testis, the influence of epididymal fat on expression of testicular steroidogenic enzymes has not been examined. In the present research, expressional changes of steroidogenic enzymes in the mouse testis after 2 weeks of the surgical partial lipectomy of epididymal fat at different postnatal ages were determined by real-time polymerase chain reaction analysis. The transcript levels of all molecules at 2 months of postnatal age were significantly increased by the lipectomy of epididymal fat. However, the lipectomy at 5 months of postnatal age resulted in decreases of expression levels of all molecules examined in the testis. Except a reduced transcript level of hydroxysteroid 17-beta dehydrogenase 3, there were no significant changes of expression levels of other steroidogenic enzymes by the lipectomy at 8 months of postnatal age. At 12 months of postnatal age, the lipectomy caused a significant increase of transcript level of steroidogenic acute regulatory protein and a significant decrease of transcript level of hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 1, without any expressional change of cytochrome P450 side chain cleavage, hydroxysteroid 17-beta dehydrogenase 3, and hydroxysteroid 17-beta dehydrogenase 3 in the testis. These findings suggest that the substances derived from epididymal fat could differentially influence on expression of steroidogenic enzymes in the testis during postnatal period.

Valvulogenesis of a living, innervated pulmonary root induced by an acellular scaffold

Heart valve disease is a major cause of mortality and morbidity worldwide with no effective medical therapy and no ideal valve substitute emulating the extremely sophisticated functions of a living heart valve. These functions influence survival and quality of life. This has stimulated extensive attempts at tissue engineering "living" heart valves. These attempts utilised combinations of allogeneic/ autologous cells and biological scaffolds with practical, regulatory, and ethical issues. In situ regeneration depends on scaffolds that attract, house and instruct cells and promote connective tissue formation. We describe a surgical, tissue-engineered, anatomically precise, novel off-the-shelf, acellular, synthetic scaffold inducing a rapid process of morphogenesis involving relevant cell types, extracellular matrix, regulatory elements including nerves and humoral components. This process relies on specific material characteristics, design and "morphodynamism".

Next-generation human adipose tissue culture methods

White adipocytes are highly specialized, lipid-storing cells. Their unique characteristics, including their large cell size and high buoyancy, have made adipocytes hard to study in vitro. Most traditional monolayered adipocyte culture models also poorly reflect the morphology and expression of their mature counterparts. The recent invent of 3D adipocyte cultures seems to circumvent many of these shortcomings, and holds promise of improved adipocyte studies in vitro. Notable advances include vascularized and immunocompetent 3D adipose tissue models and organ-on-a-chip models. This short review aims to highlight some of the most recent advances, as well as discussing what challenges still lie ahead in order to develop culture models that are easily applicable, while adequately reflecting the characteristics of human adipose tissue.

Biofabrication of vascularized adipose tissues and their biomedical applications

Recent advances in adipose tissue engineering and cell biology have led to the development of innovative therapeutic strategies in regenerative medicine for adipose tissue reconstruction. To date, the many in vitro and in vivo models developed for vascularized adipose tissue engineering cover a wide range of research areas, including studies with cells of various origins and types, polymeric scaffolds of natural and synthetic derivation, models presented using decellularized tissues, and scaffold-free approaches. In this review, studies on adipose tissue types with different functions, characteristics and body locations have been summarized with 3D in vitro fabrication approaches. The reason for the particular focus on vascularized adipose tissue models is that current liposuction and fat transplantation methods are unsuitable for adipose tissue reconstruction as the lack of blood vessels results in inadequate nutrient and oxygen delivery, leading to necrosis in situ. In the first part of this paper, current studies and applications of white and brown adipose tissues are presented according to the polymeric materials used, focusing on the studies which could show vasculature in vitro and after in vivo implantation, and then the research on adipose tissue fabrication and applications are explained.

CX3CR1hi macrophages sustain metabolic adaptation by relieving adipose-derived stem cell senescence in visceral adipose tissue

Adipose-derived stem cells (ASCs) drive healthy visceral adipose tissue (VAT) expansion via adipocyte hyperplasia. Obesity induces ASC senescence that causes VAT dysfunction and metabolic disorders. It is challenging to restrain this process by biological intervention, as mechanisms of controlling VAT ASC senescence remain unclear. We demonstrate that a population of CX3CR1^hi macrophages is maintained in mouse VAT during short-term energy surplus, which sustains ASCs by restraining their senescence, driving adaptive VAT expansion and metabolic health. Long-term overnutrition induces diminishment of CX3CR1^hi macrophages in mouse VAT accompanied by ASC senescence and exhaustion, while transferring CX3CR1^hi macrophages restores ASC reservoir and triggers VAT beiging to alleviate the metabolic maladaptation. Mechanistically, visceral ASCs attract macrophages via MCP-1 and shape their CX3CR1^hi phenotype via exosomes; these macrophages relieve ASC senescence by promoting the arginase1-eIF5A hypusination axis. These findings identify VAT CX3CR1^hi macrophages as ASC supporters and unravel their therapeutic potential for metabolic maladaptation to obesity.

Tcf21 marks visceral adipose mesenchymal progenitors and functions as a rate-limiting factor during visceral adipose tissue development

Distinct locations of different white adipose depots suggest anatomy-specific developmental regulation, a relatively understudied concept. Here, we report a population of Tcf21 lineage cells (Tcf21 LCs) present exclusively in visceral adipose tissue (VAT) that dynamically contributes to VAT development and expansion. During development, the Tcf21 lineage gives rise to adipocytes. In adult mice, Tcf21 LCs transform into a fibrotic or quiescent state. Multiomics analyses show consistent gene expression and chromatin accessibility changes in Tcf21 LC, based on which we constructed a gene-regulatory network governing Tcf21 LC activities. Furthermore, single-cell RNA sequencing (scRNA-seq) identifies the heterogeneity of Tcf21 LCs. Loss of Tcf21 promotes the adipogenesis and developmental progress of Tcf21 LCs, leading to improved metabolic health in the context of diet-induced obesity. Mechanistic studies show that the inhibitory effect of Tcf21 on adipogenesis is at least partially mediated via Dlk1 expression accentuation.

Smad4-mediated angiogenesis facilitates the beiging of white adipose tissue in mice

Beige adipocytes are thermogenic with high expression of uncoupling protein 1 in the white adipose tissue (WAT), accompanied by angiogenesis. Previous studies showed that Smad4 is important for angiogenesis. Here we studied whether endothelial Smad4-mediated angiogenesis is involved in WAT beiging. Inducible knockout of endothelial cell (EC) selective Smad4 (Smad4 ^iEC-KO) was achieved by using the Smad4 ^Floxp/floxp and Tie2 ^CreERT2 mice. Beige fat induction achieved by cold or adrenergic agonist, and angiogenesis were attenuated in WAT of Smad4 ^iEC-KO mice, with the less proliferation of ECs and adipogenic precursors. RNA sequencing of human ECs showed that Smad4 is involved in angiogenesis-related pathways. Knockdown of SMAD4 attenuated the upregulation of VEGFA, PDGFA, and angiogenesis in vitro. Treatment of human ECs with palmitic acid-induced Smad1/5 phosphorylation and the upregulation of core endothelial genes. Our study shows that endothelial Smad4 is involved in WAT beiging through angiogenesis and the expansion of adipose precursors into beige adipocytes.

Methodological advancements in organ-specific ectopic lipid quantitative characterization: Effects of high fat diet on muscle and liver intracellular lipids

OBJECTIVE

Ectopic lipid accumulation is a hallmark of metabolic diseases, linking obesity to non-alcoholic fatty liver disease, insulin resistance and diabetes. The use of zebrafish as a model of obesity and diabetes is raising due to the conserved properties of fat metabolism between humans and zebrafish, the homologous genes regulating lipid uptake and transport, the implementation of the '3R's principle and their cost-effectiveness. To date, a method allowing the conservation of lipid droplets (LDs) and organs in zebrafish larvae to image ectopic lipids is not available. Our objectives were to develop a novel methodology to quantitatively evaluate organ-specific LDs, in skeletal muscle and liver, in response to a nutritional perturbation.

METHODS

We developed a novel embedding and cryosectioning protocol allowing the conservation of LDs and organs in zebrafish larvae. To establish the quantitative measures, we used a three-arm parallel nutritional intervention design. Zebrafish larvae were fed a control diet containing 14% of nutritional fat or two high fat diets (HFDs) containing 25 and 36% of dietary fats. In muscle and liver, LDs were characterized using immunofluorescence confocal microscopy. In liver, intrahepatocellular lipids were discriminated from intrasinusoid lipids. To complete liver characteristics, fibrosis was identified with Masson's Trichrome staining. Finally, to confirm the conservation and effect of HFD, molecular players of fat metabolism were evaluated by RT-qPCR.

RESULTS

The cryosections obtained after setting up the embedding and cryopreservation method were of high quality, preserving tissue morphology and allowing the visualization of ectopic lipids. Both HFDs were obesogenic, without modifying larvae survival or development. Neutral lipid content increased with time and augmented dietary fat. Intramuscular LD volume density increased and was explained by an increase in LDs size but not in numbers. Intrahepatocellular LD volume density increased and was explained by an increased number of LDs, not by their increased size. Sinusoid area and lipid content were both increased. Hepatic fibrosis appeared with both HFDs. We observed alterations in the expression of genes associated with LD coating proteins, LD dynamics, lipogenesis, lipolysis and fatty acid oxidation.

CONCLUSIONS

In this study, we propose a reproducible and fast method to image zebrafish larvae without losing LD quality and organ morphology. We demonstrate the impact of HFD on LD characteristics in liver and skeletal muscle accompanied by alterations of key players of fat metabolism. Our observations confirm the evolutionarily conserved mechanisms in lipid metabolism and reveal organ specific adaptations. The methodological advancements proposed in this work open the doors to study organelle adaptations in obesity and diabetes related research such as lipotoxicity, organelle contacts and specific lipid depositions.

Adipose-derived stem cells exosome and its potential applications in autologous fat grafting

Recently, there has been renewed interest in autologous fat grafting both for its filler and regenerative traits. The universal application, however, has been impeded by the unstable survival rates and complications. There has been substantial research undertaken on the role of adipose-derived stem cells (ADSCs) involved in fat graft fates including angiogenesis, adipogenesis, and inflammatory regulation. As the effectors of their parental cells, ADSC-derived exosomes (ADSC-exos) encapsulating multiple bioactive cargoes mediate cell-to-cell communication in a paracrine manner. ADSC-exos have received much attention for their biocompatible and efficient therapeutic potentials as "cell-free therapy" in plastic surgery, including increasing fat grafting survival rates. In this review, we summarize the current knowledge about the biological basis of ADSC-exos, ADSC-related mechanisms of fat survival, research updates of ADSC-exos in autologous fat grafting, and discuss some challenges along with research prospects.

Modelling metabolic diseases and drug response using stem cells and organoids

Metabolic diseases, including obesity, diabetes mellitus and cardiovascular disease, are a major threat to health in the modern world, but efforts to understand the underlying mechanisms and develop rational treatments are limited by the lack of appropriate human model systems. Notably, advances in stem cell and organoid technology allow the generation of cellular models that replicate the histological, molecular and physiological properties of human organs. Combined with marked improvements in gene editing tools, human stem cells and organoids provide unprecedented systems for studying mechanisms of metabolic diseases. Here, we review progress made over the past decade in the generation and use of stem cell-derived metabolic cell types and organoids in metabolic disease research, especially obesity and liver diseases. In particular, we discuss the limitations of animal models and the advantages of stem cells and organoids, including their application to metabolic diseases. We also discuss mechanisms of drug action, understanding the efficacy and toxicity of existing therapies, screening for new treatments and pursuing personalized therapies. We highlight the potential of combining stem cell-derived organoids with gene editing and functional genomics to revolutionize the approach to finding treatments for metabolic diseases.

Towards better models for studying human adipocytes in vitro

With obesity and its comorbidities continuing to rise, we urgently need to improve our understanding of what mechanisms trigger the white adipose tissue to become dysfunctional in response to over-feeding. The recent invent of 3D culturing models has produced several noteworthy protocols for differentiating unilocular adipocytes in vitro, promising to revolutionize the obesity research field by providing more representative adipose tissue models for such mechanistic studies. In parallel, these 3D models provide important insights to how profoundly the microenvironment influences adipocyte differentiation and morphology. This commentary highlights some of the most recent 3D models, including human unilocular vascularized adipocyte spheroids (HUVASs), developed by our lab. We discuss recent developments in the field, provide further insights to the importance of the microvasculature for adipocyte maturation, and summarize what challenges remain to be solved before we can achieve a culture model that fully recapitulates all aspects of human white adipocyte biology in vitro. Taken together, the commentary highlights important recent advances regarding 3D adipocyte culturing and underlines the many advantages these models provide over traditional 2D cultures, with the aim of convincing more laboratories to switch to 3D models.

Lysosomal acid lipase promotes endothelial proliferation in cold-activated adipose tissue

Oxidative tissues such as brown adipose tissue and muscle internalize large amounts of circulating lipids and glucose as energy source. Endothelial cells (ECs) provide a platform for regulated transport and processing of blood-borne nutrients. Next to this role, it has become recognized that intercellular crosstalk between ECs and underlying parenchymal cells is indispensable for maintenance of tissue homoeostasis. Here, we comment on our recent observation that capillary ECs in thermogenic adipose tissues take up and metabolize entire triglyceride-rich lipoprotein (TRL) particles in response to cold exposure. This process is dependent on CD36, lipoprotein lipase (LPL) and lysosomal acid lipase (LAL). Remarkably, loss of LAL specifically in endothelial cells results in impaired endothelial proliferation and diminished thermogenic adaptation. Mechanistically, cell culture experiments indicate that LAL-mediated TRL processing leads to the generation of reactive oxygen species, which in turn activate hypoxia-induced factor (HIF)-mediated proliferative responses. In the current manuscript, we provide in vivo evidence that LAL-deficiency impairs proliferation of endothelial cells in thermogenic adipose tissue. In addition, we show uptake of nanoparticle-labelled TRL and LAL expression in cardiac endothelial cells, suggesting a physiological function of endothelial lipoprotein processing not only in thermogenic adipose tissue but also in cardiac muscle.

Injectable Prevascularized Mature Adipose Tissues (iPAT) to Achieve Long-Term Survival in Soft Tissue Regeneration

Soft tissue regeneration remains a challenge in reconstructive surgery. So far, both autologous fat implantations and artificial implants methods used in clinical applications lead to various disadvantages and limited lifespan. To overcome these limitations and improve the graft volume maintenance, reproducing a mature adipose tissue already including vasculature structure before implantation can be the solution. Therefore, injectable prevascularized adipose tissues (iPAT) are made from physiological collagen microfibers mixed with human mature adipocytes, adipose-derived stem cells, and human umbilical vein endothelial cells, embedded in fibrin gel. Following murine subcutaneous implantation, the iPAT show a higher cell survival (84% ± 6% viability) and volume maintenance after 3 months (up to twice heavier) when compared to non-prevascularized balls and liposuctioned fat implanted controls. This higher survival can be explained by the greater amount of blood vessels found (up to 1.6-fold increase), with balanced host anastomosis (51% ± 1% of human/mouse lumens), also involving infiltration by the lymphatic and neural vasculature networks. Furthermore, with the cryopreservation possibility enabling their later reinjection, the iPAT technology has the merit to allow noninvasive soft tissue regeneration for long-term outcomes.

Role of Adipose Tissue microRNAs in the Onset of Metabolic Diseases and Implications in the Context of the DOHaD

The worldwide epidemic of obesity is associated with numerous comorbid conditions, including metabolic diseases such as insulin resistance and diabetes, in particular. The situation is likely to worsen, as the increase in obesity rates among children will probably lead to an earlier onset and more severe course for metabolic diseases. The origin of this earlier development of obesity may lie in both behavior (changes in nutrition, physical activity, etc.) and in children's history, as it appears to be at least partly programmed by the fetal/neonatal environment. The concept of the developmental origin of health and diseases (DOHaD), involving both organogenesis and epigenetic mechanisms, encompasses such programming. Epigenetic mechanisms include the action of microRNAs, which seem to play an important role in adipocyte functions. Interestingly, microRNAs seem to play a particular role in propagating local insulin resistance to other key organs, thereby inducing global insulin resistance and type 2 diabetes. This propagation involves the active secretion of exosomes containing microRNAs by adipocytes and adipose tissue-resident macrophages, as well as long-distance communication targeting the muscles and liver, for example. Circulating microRNAs may also be useful as biomarkers for the identification of populations at risk of subsequently developing obesity and metabolic diseases.

Adipose Tissue Development Relies on Coordinated Extracellular Matrix Remodeling, Angiogenesis, and Adipogenesis

Despite developing prenatally, the adipose tissue is unique in its ability to undergo drastic growth even after reaching its mature size. This development and subsequent maintenance rely on the proper coordination between the vascular niche and the adipose compartment. In this review, the process of adipose tissue development is broken down to explain (1) the ultrastructural matrix remodeling that is undertaken during simultaneous adipogenesis and angiogenesis, (2) the paracrine crosstalk involved during adipose development, (3) the mechanical regulators involved in adipose growth, and (4) the proteolytic and paracrine oversight for matrix remodeling during adipose development. It is crucial to gain a better understanding of the complex relationships that exist between adipose tissue and the vasculature during tissue development to provide insights into the pathological tissue expansion of obesity and to develop improved soft-tissue reconstruction techniques.

Mammalian adipogenesis regulator (Areg) cells use retinoic acid signalling to be non- and anti-adipogenic in age-dependent manner

Adipose stem and precursor cells (ASPCs) give rise to adipocytes and determine the composition and plasticity of adipose tissue. Recently, several studies have demonstrated that ASPCs partition into at least three distinct cell subpopulations, including the enigmatic CD142⁺ cells. An outstanding challenge is to functionally characterise this population, as discrepant properties, from adipogenic to non- and anti-adipogenic, have been reported for these cells. To resolve these phenotypic ambiguities, we characterised mammalian subcutaneous CD142⁺ ASPCs across various experimental conditions, demonstrating that CD142⁺ ASPCs exhibit high molecular and phenotypic robustness. Specifically, we find these cells to be firmly non- and anti-adipogenic both in vitro and in vivo, with their inhibitory signals also impacting adipogenic human cells. However, these CD142⁺ ASPC-specific properties exhibit surprising temporal phenotypic alterations, and emerge only in an age-dependent manner. Finally, using multi-omic and functional assays, we show that the inhibitory nature of these adipogenesis-regulatory CD142⁺ ASPCs (Aregs) is driven by specifically expressed secretory factors that cooperate with the retinoic acid signalling pathway to transform the adipogenic state of CD142^- ASPCs into a non-adipogenic, Areg-like state.

Distinct functional properties of murine perinatal and adult adipose progenitor subpopulations

Adult white adipose tissue (WAT) harbors distinct mesenchymal stromal cell subpopulations that differentially affect WAT function and plasticity. Here we unveil the cellular landscape of the perinatal epididymal WAT primordium using single-cell transcriptomics in male mice. We reveal that adipocyte precursor cells and fibro-inflammatory progenitors (FIPs) emerge as functionally distinct PDGFRβ+ subpopulations within the epididymal WAT anlagen prior to adipocyte accrual. We further identify important molecular and functional differences between perinatal and adult FIPs, including differences in their pro-inflammatory response, adipogenic capacity and anti-adipogenic behavior. Notably, we find that transient overexpression of Pparg in PDGFRβ+ cells only during postnatal days 0.5 to 7.5 in male mice leads to hyperplastic WAT development, durable progenitor cell reprogramming, and protection against pathologic WAT remodeling and glucose intolerance in adult-onset obesity. Thus, factors that alter the adipogenic capacity of perinatal adipose progenitors can have long-lasting effects on progenitor plasticity, tissue expandability and metabolic health into adulthood.

Importance of the Microenvironment and Mechanosensing in Adipose Tissue Biology

The expansion of adipose tissue is an adaptive mechanism that increases nutrient buffering capacity in response to an overall positive energy balance. Over the course of expansion, the adipose microenvironment undergoes continual remodeling to maintain its structural and functional integrity. However, in the long run, adipose tissue remodeling, typically characterized by adipocyte hypertrophy, immune cells infiltration, fibrosis and changes in vascular architecture, generates mechanical stress on adipose cells. This mechanical stimulus is then transduced into a biochemical signal that alters adipose function through mechanotransduction. In this review, we describe the physical changes occurring during adipose tissue remodeling, and how they regulate adipose cell physiology and promote obesity-associated dysfunction in adipose tissue.

Angiogenesis in adipose tissue and obesity

While most tissues exhibit their greatest growth during development, adipose tissue is capable of additional massive expansion in adults. Adipose tissue expandability is advantageous when temporarily storing fuel for use during fasting, but becomes pathological upon continuous food intake, leading to obesity and its many comorbidities. The dense vasculature of adipose tissue provides necessary oxygen and nutrients, and supports delivery of fuel to and from adipocytes under fed or fasting conditions. Moreover, the vasculature of adipose tissue comprises a major niche for multipotent progenitor cells, which give rise to new adipocytes and are necessary for tissue repair. Given the multiple, pivotal roles of the adipose tissue vasculature, impairments in angiogenic capacity may underlie obesity-associated diseases such as diabetes and cardiometabolic disease. Exciting new studies on the single-cell and single-nuclei composition of adipose tissues in mouse and humans are providing new insights into mechanisms of adipose tissue angiogenesis. Moreover, new modes of intercellular communication involving micro vesicle and exosome transfer of proteins, nucleic acids and organelles are also being recognized to play key roles. This review focuses on new insights on the cellular and signaling mechanisms underlying adipose tissue angiogenesis, and on their impact on obesity and its pathophysiological consequences.

Role of adipose tissue macrophages in obesity-related disorders

The obesity epidemic has led researchers and clinicians to reconsider the etiology of this disease and precisely decipher its molecular mechanisms. The excessive accumulation of fat by cells, most notably adipocytes, which play a key role in this process, has many repercussions in tissue physiology. Herein, we focus on how macrophages, immune cells well known for their tissue gatekeeping functions, assume fundamental, yet ill-defined, roles in the genesis and development of obesity-related metabolic disorders. We first discuss the determinants of the biology of these cells before introducing the specifics of the adipose tissue environment, while highlighting its heterogeneity. Finally, we detail how obesity transforms both adipose tissue and local macrophage populations. Understanding macrophage diversity and their cross talk with the diverse cell types constituting the adipose tissue environment will allow us to frame the therapeutic potential of adipose tissue macrophages in obesity.

Remodeling of gene regulatory networks underlying thermogenic stimuli-induced adipose beiging

Beige adipocytes are induced by cold temperatures or β3-adrenergic receptor (Adrb3) agonists. They create heat through glucose and fatty acid (FA) oxidation, conferring metabolic benefits. The distinct and shared mechanisms by which these treatments induce beiging are unknown. Here, we perform single-nucleus assay for transposase-accessible chromatin sequencing (snATAC-seq) on adipose tissue from mice exposed to cold or an Adrb3 agonist to identify cellular and chromatin accessibility dynamics during beiging. Both stimuli induce chromatin remodeling that influence vascularization and inflammation in adipose. Beige adipocytes from cold-exposed mice have increased accessibility at genes regulating glycolytic processes, whereas Adrb3 activation increases cAMP responses. While both thermogenic stimuli increase accessibility at genes regulating thermogenesis, lipogenesis, and beige adipocyte development, the kinetics and magnitudes of the changes are distinct for the stimuli. Accessibility changes at lipogenic genes are linked to functional changes in lipid composition of adipose. Both stimuli tend to decrease the proportion of palmitic acids, a saturated FA in adipose. However, Adrb3 activation increases the proportion of monounsaturated FAs, whereas cold increases the proportion of polyunsaturated FAs. These findings reveal common and distinct mechanisms of cold and Adrb3 induced beige adipocyte biogenesis, and identify unique functional consequences of manipulating these pathways in vivo.

Prepubertal androgen signaling is required to establish male fat distribution

Fat distribution is sexually dimorphic and is associated with metabolic disease risk. It is unknown if prepubertal sex-hormone signaling influences adult fat distribution. Here, we show that karyotypically male androgen-insensitive mice exhibit pronounced subcutaneous adiposity compared with wild-type males and females. This subcutaneous adipose bias emerges prior to puberty and is not due to differences in adipocyte size or rates of adipogenesis between visceral and subcutaneous fat. Instead, we find that androgen-insensitive mice lack an adequate progenitor pool for normal visceral-fat expansion during development, thus increasing the subcutaneous-to-visceral-fat ratio. Obesogenic visceral-fat expansion is likewise inhibited in these mice, yet their metabolic health is similar to wild-type animals with comparable total fat mass. Taken together, these data show that adult fat distribution can be determined prior to the onset of puberty by the relative number of progenitors that seed nascent adipose depots.

The multifaceted roles of the adipose tissue vasculature

The prevalence of obesity and its associated pathologies continue to increase, which has led to a renewed interest in our major weight-regulating organ, the white adipose tissue. It has become clear that its development, expansion, and physiological function depend on proper crosstalk between each of its cellular constituents, with a central role for the vascular endothelium lining the blood vessels. Although first considered a mere barrier, the endothelium has emerged as a dynamic unit modulating many critical adipose tissue functions. It not only oversees the uptake of all nutrients to be stored in the adipocytes but also provides an important growth niche for adipocyte progenitors and regulates the expandability of the tissue during overfeeding and obesity. In this review, we describe the reciprocal relationship between endothelial cells, adipocytes, and obesity. We present recent studies that support an important role for endothelial cells as central mediators of many of the physiological and pathological functions of the adipose tissue and highlight several unknown aspects of adipose tissue vascular biology. This new perspective could present exciting opportunities to develop new therapeutic approaches against obesity-related pathologies and is thus of great interest in our increasingly obese society.

Diabetes and Its Cardiovascular Complications: Comprehensive Network and Systematic Analyses

Diabetes mellitus is a worldwide health problem that usually comes with severe complications. There is no cure for diabetes yet and the threat of these complications is what keeps researchers investigating mechanisms and treatments for diabetes mellitus. Due to advancements in genomics, epigenomics, proteomics, and single-cell multiomics research, considerable progress has been made toward understanding the mechanisms of diabetes mellitus. In addition, investigation of the association between diabetes and other physiological systems revealed potentially novel pathways and targets involved in the initiation and progress of diabetes. This review focuses on current advancements in studying the mechanisms of diabetes by using genomic, epigenomic, proteomic, and single-cell multiomic analysis methods. It will also focus on recent findings pertaining to the relationship between diabetes and other biological processes, and new findings on the contribution of diabetes to several pathological conditions.

Quantitative Methods to Assess Adipose Vasculature

Adipose tissue depots are invested with an extensive capillary network that is closely associated with maintenance of adipose functions and enables healthy tissue expansion. The capillary network displays a high level of plasticity, demonstrating either growth (angiogenesis) or regression (rarefaction) under various physiological/pathological conditions, which has significant consequences for cardiometabolic health. Thus, the visualization and quantification of adipose vascular networks is an important aspect of studying factors that regulate adipose tissue health. This chapter provides an overview of several methods to quantify adipose vascularization. In-depth protocols are provided for the visualization of vascular structures by staining and imaging of whole-mount adipose tissues or paraffin-embedded adipose tissue sections, together with the quantitative analysis of vascularization from these images.

Perspectives on scaling production of adipose tissue for food applications

With rising global demand for food proteins and significant environmental impact associated with conventional animal agriculture, it is important to develop sustainable alternatives to supplement existing meat production. Since fat is an important contributor to meat flavor, recapitulating this component in meat alternatives such as plant based and cell cultured meats is important. Here, we discuss the topic of cell cultured or tissue engineered fat, growing adipocytes in vitro that could imbue meat alternatives with the complex flavor and aromas of animal meat. We outline potential paths for the large scale production of in vitro cultured fat, including adipogenic precursors during cell proliferation, methods to adipogenically differentiate cells at scale, as well as strategies for converting differentiated adipocytes into 3D cultured fat tissues. We showcase the maturation of knowledge and technology behind cell sourcing and scaled proliferation, while also highlighting that adipogenic differentiation and 3D adipose tissue formation at scale need further research. We also provide some potential solutions for achieving adipose cell differentiation and tissue formation at scale based on contemporary research and the state of the field.

Global Adipose Tissue Remodeling During the First Month of Postnatal Life in Mice

During the first month of postnatal life, adipose tissue depots of mice go through a drastic, but transient, remodeling process. Between postnatal days 10 and 20, several white fat depots display a strong and sudden surge in beige adipocyte emergence that reverts until day 30. At the same time, brown fat depots appear to undergo an opposite phenomenon. We comprehensively describe these events, their depot specificity and known environmental and genetic interactions, such as maternal diet, housing temperature and mouse strain. We further discuss potential mechanisms and plausible purposes, including the tempting hypothesis that postnatal transient remodeling creates a lasting adaptive capacity still detectable in adult animals. Finally, we propose postnatal adipose tissue remodeling as a model process to investigate mechanisms of beige adipocyte recruitment advantageous to cold exposure or adrenergic stimulation in its entirely endogenous sequence of events without external manipulation.

Transcriptional and epigenetic control of adipocyte remodeling during obesity

The rising prevalence of obesity over the past decades coincides with the rising awareness that a detailed understanding of both adipose tissue biology and obesity-associated remodeling is crucial for developing therapeutic and preventive strategies. Substantial progress has been made in identifying the signaling pathways and transcriptional networks that orchestrate alterations of adipocyte gene expression linked to diverse phenotypes. Owing to recent advances in epigenomics, we also gained a better appreciation for the fact that different environmental cues can epigenetically reprogram adipocyte fate and function, mainly by altering DNA methylation and histone modification patterns. Intriguingly, it appears that transcription factors and chromatin-modifying coregulator complexes are the key regulatory components that coordinate both signaling-induced transcriptional and epigenetic alterations in adipocytes. In this review, we summarize and discuss current molecular insights into how these alterations and the involved regulatory components trigger adipogenesis and adipose tissue remodeling in response to energy surplus.

Single cell functional genomics reveals plasticity of subcutaneous white adipose tissue (WAT) during early postnatal development

OBJECTIVE

The current study addresses the cellular complexity and plasticity of subcutaneous (inguinal) white adipose tissue (iWAT) in mice during the critical periods of perinatal growth and establishment.

METHODS

We performed a large-scale single cell transcriptomic (scRNA-seq) and epigenomic (snATAC-seq) characterization of cellular subtypes (adipose stromal cells (ASC) and adipocyte nuclei) during inguinal WAT (subcutaneous; iWAT) development in mice, capturing the early postnatal period (postnatal days (PND) 06 and 18) through adulthood (PND56).

RESULTS

Perinatal and adult iWAT contain 3 major ASC subtypes that can be independently identified by RNA expression profiles and DNA transposase accessibility. Furthermore, the transcriptomes and enhancer landscapes of both ASC and adipocytes dynamically change during postnatal development. Perinatal ASC (PND06) are highly enriched for several imprinted genes (IGs; e.g., Mest, H19, Igf2) and extracellular matrix proteins whose expression then declines prior to weaning (PND18). By comparison, adult ASC (PND56) are more enriched for transcripts associated with immunoregulation, oxidative stress, and integrin signaling. Two clusters of mature adipocytes, identified through single nuclei RNA sequencing (snRNA-seq), were distinctive for proinflammatory/immune or metabolic gene expression patterns that became more transcriptionally diverse in adult animals. Single nuclei assay for transposase-accessible chromatin (snATAC-seq) revealed that differences in gene expression were associated with developmental changes in chromatin accessibility and predicted transcription factor motifs (e.g., Plagl1, Ar) in both stromal cells and adipocytes.

CONCLUSIONS

Our data provide new insights into transcriptional and epigenomic signaling networks important during iWAT establishment at a single cell resolution, with important implications for the field of metabolic programming.

Effect of vitamin A injection at birth on intramuscular fat development and meat quality in beef cattle

This study aimed to evaluate intramuscular fat and expression of genes in the muscle of Montana × Nellore treated with vitamin A at birth. We hypothesized that an injection of vitamin A after birth would increase marbling by increasing the expression of angiogenic, adipogenic, and lipogenic genes. Animals treated with vitamin A had greater marbling in the longissimus muscle (P = 0.05). The vitamin A treatment increased the expression of VEGFA gene at 40 days of age and at weaning and increased the expression of ZNF423 at weaning and at harvesting (P ≤ 0.03). The expression of WNT was higher (P = 0.01) at 40 days of age and at weaning in the animals treated with vitamin A. Vitamin A also increased the expression of SREBF1 at 40 days of age and at weaning (P ≤ 0.05). Therefore, the administration of vitamin A to cattle at birth could be a way to increase carcass marbling without affecting the performance of the animals.

Hypertrophied human adipocyte spheroids as in vitro model of weight gain and adipose tissue dysfunction

KEY POINTS

Adipocyte enlargement is a key feature of obesity and associated with insulin resistance and metabolic disease The cause and consequences of adipocyte enlargement have remained hard to study in vitro due to a lack of human cell models with representative morphology This paper provides an easily set up spheroid culture method, HUVAS (human unilocular vascularized adipocyte spheroids), for the differentiation and culturing of human adipocytes with a more unilocular morphology We show that providing adipocyte progenitors with a vascular differentiation niche is key for achieving in vitro differentiated adipocytes with large lipid droplets Lipid treatment of the HUVAS spheroids can further adipocyte enlargement and induce cellular dysfunction, mimicking the in vivo effects of weight gain The model will allow a wider research community to perform mechanistic studies of the factors impacting human adipocyte differentiation and growth, increasing our understanding of how obesity develops and why it has such detrimental consequences on whole body metabolism ABSTRACT: The rise in obesity prevalence has created an urgent need for new and improved methods to study human adipocytes and the pathogenic effects of weight gain in vitro. Despite the proven advantage of culturing adipocyte progenitors as 3D structures, the majority of studies continue using traditional 2D cultures which result in small, multilocular adipocytes with poor representability. We hypothesized that providing differentiating pre-adipocytes with a vascular growth niche would mimic in vivo adipogenesis and improve the differentiation into unilocular adipocytes. Here we present HUVAS (human unilocular vascularized adipocyte spheroids), a simple, easily applicable culture protocol that allows for the differentiation of human adipocytes with a more unilocular morphology and larger lipid droplets than previous protocols. Moreover, we offer a protocol for inducing adipocyte enlargement in vitro, resulting in larger lipid droplets and development of several key features of adipocyte dysfunction, including altered adipokine secretion, impaired lipolysis and insulin resistance. Taken together, our HUVAS model offers an improved culture system for studying the cellular and molecular mechanisms causing metabolic dysfunction and inflammation in human adipose tissue during weight gain. This article is protected by copyright. All rights reserved.

Diet-regulated production of PDGFcc by macrophages controls energy storage

The mechanisms by which macrophages regulate energy storage remain poorly understood. We identify in a genetic screen a platelet-derived growth factor (PDGF)/vascular endothelial growth factor (VEGF)-family ortholog, Pvf3, that is produced by macrophages and is required for lipid storage in fat-body cells of Drosophila larvae. Genetic and pharmacological experiments indicate that the mouse Pvf3 ortholog PDGFcc, produced by adipose tissue-resident macrophages, controls lipid storage in adipocytes in a leptin receptor- and C-C chemokine receptor type 2-independent manner. PDGFcc production is regulated by diet and acts in a paracrine manner to control lipid storage in adipose tissues of newborn and adult mice. At the organismal level upon PDGFcc blockade, excess lipids are redirected toward thermogenesis in brown fat. These data identify a macrophage-dependent mechanism, conducive to the design of pharmacological interventions, that controls energy storage in metazoans.

Bone marrow adipocytes - Good, bad, or just different?

Skeletal remodeling is essential for proper maintenance of adult bone mass, and due to its heavy energetic demands this process is closely tied to whole body metabolic. Thus, bone formation by the osteoblast, bone resorption by the osteoclast, and mechano-sensing by the osteocyte, are highly coupled processes that are essential for bone turnover. When one experiences a disruption in these processes, over time increased skeletal fragility and fracture can result. In addition to these primary cells, secondary cells within the skeletal niche are suspected to directly coordinate bone health as well. The bone marrow compartment provides a unique microenvironment in which communication occurs between white blood cells, red blood cells, platelets, and immune cells, in addition to classic bone cells (osteoblasts, osteoclasts, and osteocytes) that can both directly and indirectly impact skeletal homeostasis. One such cell population that has attracted much attention and scientific inquiry in the past decade are bone marrow adipocytes (BMAdipo) which can be found interspersed throughout the marrow compartment, and collectively are often referred to as bone marrow adipose tissue (BMAT). Although our understanding of BMAT has advanced significantly in the past decade, many questions remain relative to their lineage and function. The current collection presents recent studies and the up-to-date understanding of bone marrow adipocytes, along with future clinical perspectives related to this unique depot.

Neonatal IL-4 exposure decreases adipogenesis of male rats into adulthood

The cytokine interleukin 4 (IL-4) can increase beige adipogenesis in adult rodents. However, neonatal animals use a distinct adipocyte precursor compartment for adipogenesis as compared with adults. In this study, we address whether IL-4 can induce persistent effects on adipose tissue when administered subcutaneously in the interscapular region during the neonatal period in Sprague-Dawley rats. We injected IL-4 into neonatal male rats during postnatal days 1-6, followed by analysis of adipose tissue and adipocyte precursors at 2 wk and 10 wk of age. Adipocyte precursors were cultured and subjected to differentiation in vitro. We found that a short and transient IL-4 exposure in neonates upregulated uncoupling protein 1 (Ucp1) mRNA expression and decreased fat cell size in subcutaneous white adipose tissue (WAT). Adipocyte precursors from mature rats that had been treated with IL-4 as neonates displayed a decrease in adiponectin (Adipoq) but no change in Ucp1 expression, as compared with controls. Thus, neonatal IL-4 induces acute beige adipogenesis and decreases adipogenic differentiation capacity long term. Overall, these findings indicate that the neonatal period is critical for adipocyte development and may influence the later onset of obesity.NEW & NOTEWORTHY We used neonatal injections in rat to show that IL-4 decreases adipogenesis and increases browning of white fat. In adulthood, adipocyte precursors show persistently decreased adipogenesis but not increased browning. These studies in the neonate are the first, to our knowledge, to show that IL-4 can have long-lasting effects.

Developmental programming of offspring adipose tissue biology and obesity risk

Obesity is reaching epidemic proportions and imposes major negative health crises and an economic burden in both high and low income countries. The multifaceted nature of obesity represents a major health challenge, with obesity affecting a variety of different organs and increases the risk of many other noncommunicable diseases, such as type 2 diabetes, fatty liver disease, dementia, cardiovascular diseases, and even cancer. The defining organ of obesity is the adipose tissue, highlighting the need to more comprehensively understand the development and biology of this tissue to understand the pathogenesis of obesity. Adipose tissue is a miscellaneous and highly plastic endocrine organ. It comes in many different sizes and shades and is distributed throughout many different locations in the body. Though its development begins prenatally, quite uniquely, it has the capacity for unlimited growth throughout adulthood. Adipose tissue is also a highly sexually dimorphic tissue, patterning men and women in different ways, which means the risks associated with obesity are also sexually dimorphic. Recent studies show that environmental factors during prenatal and early stages of postnatal development have the capacity to programme the structure and function of adipose tissue, with implications for the development of obesity. This review summarizes the evidence for a role for early environmental factors, such as maternal malnutrition, hypoxia, and exposure to excess hormones and endocrine disruptors during gestation in the programming of adipose tissue and obesity in the offspring. We will also discuss the complexity of studying adipose tissue biology and the importance of appreciating nuances in adipose tissue, such as sexual dimorphism and divergent responses to metabolic and endocrine stimuli. Given the rising levels of obesity worldwide, understanding how environmental conditions in early life affects adipose tissue phenotype and the subsequent development of obesity is of absolute importance.

Thermogenic Fat: Development, Physiological Function, and Therapeutic Potential

The concerning worldwide increase of obesity and chronic metabolic diseases, such as T2D, dyslipidemia, and cardiovascular disease, motivates further investigations into preventive and alternative therapeutic approaches. Over the past decade, there has been growing evidence that the formation and activation of thermogenic adipocytes (brown and beige) may serve as therapy to treat obesity and its associated diseases owing to its capacity to increase energy expenditure and to modulate circulating lipids and glucose levels. Thus, understanding the molecular mechanism of brown and beige adipocytes formation and activation will facilitate the development of strategies to combat metabolic disorders. Here, we provide a comprehensive overview of pathways and players involved in the development of brown and beige fat, as well as the role of thermogenic adipocytes in energy homeostasis and metabolism. Furthermore, we discuss the alterations in brown and beige adipose tissue function during obesity and explore the therapeutic potential of thermogenic activation to treat metabolic syndrome.

AP2-microRNA-26a overexpression reduces visceral fat mass and blood lipids

BACKGROUND

MicroRNA-26a (miR-26a) is a key player in tumor suppression and plays important roles in glucose and lipid metabolism. However, its function in adipose tissue is not well defined.

OBJECTIVE

The study aimed to examine the effect on fat expansion and function of miR-26a in adipose tissue.

METHODS

Adipose-specific miR-26a transgenic mice (Ap2-miR-26a) were firstly generated by breeding miR-26a floxed (Mir26a^loxP/^loxP) mice with Ap2-Cre recombinase transgenic mice. The effects of miR-26a adipose-specific overexpression on body weight, body fat composition, fat pad weight, adipocyte size, blood lipid levels, glucose metabolism, and adipogenesis were investigated in mice on a chow diet and a high fat diet. White adipose tissue browning was evaluated by energy expenditure, adipocyte morphology and browning related genes expression levels both at room temperature and after cold exposure. Gene expression was determined by Real-Time quantitative PCR and western blotting.

RESULTS

MiR-26a was specifically overexpressed in adipose by ~4 folds. Ap2-miR-26a mice had a moderate decrease in body weight, body fat composition, epididymal white adipose (eWAT) weight and blood lipid levels, along with smaller adipocytes in eWAT. The favorable phenotype was not due to white adipose tissue browning (even after cold exposure) or adipogenesis or lipolysis. Ap2-miR-26a mice exhibited no significant metabolic phenotype under high-fat-diet feeding.

CONCLUSION

This study suggests that adipose-specific overexpression of miR-26a could moderately reduce visceral fat pad mass and lipid levels independent of white adipose tissue browning, adipogenesis and adipose lipolysis based on the gene expression level.

Acetyl-CoA and Metabolite Fluxes Regulate White Adipose Tissue Expansion

White adipose tissue (WAT) depends on coordinated regulation of transcriptional and metabolic pathways to respond to whole-body energy demands. We highlight metabolites that contribute to biosynthetic reactions for WAT expansion. Recent studies have precisely defined how byproducts of carbohydrate and lipid metabolism affect physiological and endocrine functions in adipocytes. We emphasize the critical emerging roles of short-chain fatty acids (SCFAs) and tricarboxylic acid (TCA) cycle metabolites that connect lipogenesis to WAT energy balance and endocrine functions. These insights address how adipocytes use small molecules generated from central carbon metabolism to measure responses to nutritional stress.

Adipose tissue plasticity and the pleiotropic roles of BMP signaling

Adipose tissues, including white, beige, and brown adipose tissue, have evolved to be highly dynamic organs. Adipose tissues undergo profound changes during development and regeneration and readily undergo remodeling to meet the demands of an everchanging metabolic landscape. The dynamics are determined by the high plasticity of adipose tissues, which contain various cell types: adipocytes, immune cells, endothelial cells, nerves, and fibroblasts. There are numerous proteins that participate in regulating the plasticity of adipose tissues. Among these, bone morphogenetic proteins (BMPs) were initially found to regulate the differentiation of adipocytes, and they are being reported to have pleiotropic functions by emerging studies. Here, in the first half of the article, we summarize the plasticity of adipocytes and macrophages, which are two groups of cells targeted by BMP signaling in adipose tissues. We then review how BMPs regulate the differentiation, death, and lipid metabolism of adipocytes. In addition, the potential role of BMPs in regulating adipose tissue macrophages is considered. Finally, the expression of BMPs in adipose tissues and their metabolic relevance are discussed.

Early Childhood Fat Tissue Changes-Adipocyte Morphometry, Collagen Deposition, and Expression of CD163+ Cells in Subcutaneous and Visceral Adipose Tissue of Male Children

Childhood obesity is a complex health problem, and not many studies have been done on adipose tissue remodeling in early childhood. The aim of this study was to examine extracellular matrix remodeling in the adipose tissue of healthy male children depending on their weight status. Subcutaneous and visceral adipose tissue was obtained from 45 otherwise healthy male children who underwent elective surgery for hernia repairs or orchidopexy. The children were divided into overweight/obese (n = 17) or normal weight groups (n = 28) depending on their body mass index (BMI) z-score. Serum was obtained for glucose, testosterone, triglyceride, total cholesterol, high-density lipoprotein (HDL), and low-density lipoprotein (LDL) measurements. Sections of adipose tissue were stained with hematoxylin and eosin to determine the adipocytes' surface area, and Masson's trichrome stain was used to detect the adipocytes' collagen content. Immunohistochemistry for CD163⁺ cells was also performed. The results showed that male children in the overweight group had higher serum triglyceride levels, greater adipocyte surface area and collagen content in their subcutaneous adipose tissue, more crown-like structures in fat tissues, and more CD163⁺ cells in their visceral adipose tissue than males in the normal weight group. In conclusion, in male children, obesity can lead to the hypertrophy of adipocytes, increased collagen deposition in subcutaneous adipose tissues, and changes in the polarization and accumulation of macrophages.

Lysosomal lipoprotein processing in endothelial cells stimulates adipose tissue thermogenic adaptation

In response to cold exposure, thermogenic adipocytes internalize large amounts of fatty acids after lipoprotein lipase-mediated hydrolysis of triglyceride-rich lipoproteins (TRL) in the capillary lumen of brown adipose tissue (BAT) and white adipose tissue (WAT). Here, we show that in cold-exposed mice, vascular endothelial cells in adipose tissues endocytose substantial amounts of entire TRL particles. These lipoproteins subsequently follow the endosomal-lysosomal pathway, where they undergo lysosomal acid lipase (LAL)-mediated processing. Endothelial cell-specific LAL deficiency results in impaired thermogenic capacity as a consequence of reduced recruitment of brown and brite/beige adipocytes. Mechanistically, TRL processing by LAL induces proliferation of endothelial cells and adipocyte precursors via beta-oxidation-dependent production of reactive oxygen species, which in turn stimulates hypoxia-inducible factor-1α-dependent proliferative responses. In conclusion, this study demonstrates a physiological role for TRL particle uptake into BAT and WAT and establishes endothelial lipoprotein processing as an important determinant of adipose tissue remodeling during thermogenic adaptation.