Neurotensin is an anti-thermogenic peptide produced by lymphatic endothelial cells

Neurotensin contributes to cholestatic liver disease potentially modulating matrix metalloprotease-7

The diagnosis and treatment of biliary atresia pose challenges due to the absence of reliable biomarkers and limited understanding of its etiology. The plasma and liver of patients with biliary atresia exhibit elevated levels of neurotensin. To investigate the specific role of neurotensin in the progression of biliary atresia, the patient's liver pathological section was employed. Biliary organoids, cultured biliary cells, and a mouse model were employed to elucidate both the potential diagnostic significance of neurotensin and its underlying mechanistic pathway. In patients' blood, the levels of neurotensin were positively correlated with matrix metalloprotease-7, interleukin-8, and liver function enzymes. Neurotensin and neurotensin receptors were mainly expressed in the intrahepatic biliary cells and were stimulated by bile acids. Neurotensin suppressed the growth and increased expression of matrix metalloprotease-7 in biliary organoids. Neurotensin inhibited mitochondrial respiration, oxidative phosphorylation, and attenuated the activation of calmodulin-dependent kinase kinase 2-adenosine monophosphate-activated protein kinase (CaMKK2-AMPK) signaling in cultured biliary cells. The stimulation of neurotensin in mice and cultured cholangiocytes resulted in the upregulation of matrix metalloprotease-7 expression through binding to its receptors, namely neurotensin receptors 1/3, thereby attenuating the activation of the CaMKK2-AMPK pathway. In conclusion, these findings revealed the changes of neurotensin in patients with cholestatic liver disease and its mechanism in the progression of the disease, providing a new understanding of the complex mechanism of hepatobiliary injury in children with biliary atresia.

β-arrestin-biased Allosteric Modulator of Neurotensin Receptor 1 Reduces Ethanol Drinking and Responses to Ethanol Administration in Rodents

Alcohol use disorders (AUDs) impose an enormous societal and financial burden, and world-wide, alcohol misuse is the 7th leading cause of premature death1. Despite this, there are currently only 3 FDA approved pharmacological treatments for the treatment of AUDs in the United States. The neurotensin (Nts) system has long been implicated in modulating behaviors associated with alcohol misuse. Recently, a novel compound, SBI-553, that biases the action of Nts receptor 1 (NTSR1) activation, has shown promise in preclinical models of psychostimulant misuse. Here we investigate the efficacy of this compound to alter ethanol-mediated behaviors in a comprehensive battery of experiments assessing ethanol consumption, behavioral responses to ethanol, sensitivity to ethanol, and ethanol metabolism. Additionally, we investigated behavior in avoidance and cognitive assays to monitor potential side effects of SBI-553. We find that SBI-553 reduces binge-like ethanol consumption in mice without altering avoidance behavior or novel object recognition. We also observe sex-dependent differences in physiological responses to sequential ethanol injections in mice. In rats, we show that SBI-553 attenuates sensitivity to the interoceptive effects of ethanol (using a Pavlovian drug discrimination task). Our data suggest that targeting NTSR1 signaling may be promising to attenuate alcohol misuse, and adds to a body of literature that suggests NTSR1 may be a common downstream target involved in the psychoactive effects of multiple reinforcing substances.

Lymphatic vessels in the age of cancer immunotherapy

Lymphatic transport maintains homeostatic health and is necessary for immune surveillance, and yet lymphatic growth is often associated with solid tumour development and dissemination. Although tumour-associated lymphatic remodelling and growth were initially presumed to simply expand a passive route for regional metastasis, emerging research puts lymphatic vessels and their active transport at the interface of metastasis, tumour-associated inflammation and systemic immune surveillance. Here, we discuss active mechanisms through which lymphatic vessels shape their transport function to influence peripheral tissue immunity and the current understanding of how tumour-associated lymphatic vessels may both augment and disrupt antitumour immune surveillance. We end by looking forward to emerging areas of interest in the field of cancer immunotherapy in which lymphatic vessels and their transport function are likely key players: the formation of tertiary lymphoid structures, immune surveillance in the central nervous system, the microbiome, obesity and ageing. The lessons learnt support a working framework that defines the lymphatic system as a key determinant of both local and systemic inflammatory networks and thereby a crucial player in the response to cancer immunotherapy.

Lymphatic malformations: mechanistic insights and evolving therapeutic frontiers

The lymphatic vascular system is gaining recognition for its multifaceted role and broad pathological significance. Once perceived as a mere conduit for interstitial fluid and immune cell transport, recent research has unveiled its active involvement in critical physiological processes and common diseases, including inflammation, autoimmune diseases, and atherosclerosis. Consequently, abnormal development or functionality of lymphatic vessels can result in serious health complications. Here, we discuss lymphatic malformations (LMs), which are localized lesions that manifest as fluid-filled cysts or extensive infiltrative lymphatic vessel overgrowth, often associated with debilitating, even life-threatening, consequences. Genetic causes of LMs have been uncovered, and several promising drug-based therapies are currently under investigation and will be discussed.

Neurotensin counteracts hair growth inhibition induced by chronic restraint stress

Stress has been considered as a potential trigger for hair loss through the neuroendocrine-hair follicle (HF) axis. Neurotensin (NTS), a neuropeptide, is known to be dysregulated in the inflammatory-associated skin diseases. However, the precise role of NTS in stress-induced hair loss is unclear. To investigate the function and potential mechanisms of NTS in stress-induced hair growth inhibition, we initially detected the expression of neurotensin receptor (Ntsr) and NTS in the skin tissues of stressed mice by RNA-sequencing and ELISA. We found chronic restraint stress (CRS) significantly decreased the expression of both NTS and Ntsr in the skin tissues of mice. Intracutaneous injection of NTS effectively counteracted CRS-induced inhibition of hair growth in mice. Furthermore, NTS regulated a total of 1093 genes expression in human dermal papilla cells (HDPC), with 591 genes being up-regulated and 502 genes being down-regulated. GO analysis showed DNA replication, cell cycle, integral component of plasma membrane and angiogenesis-associated genes were significantly regulated by NTS. KEGG enrichment demonstrated that NTS also regulated genes related to the Hippo signalling pathway, axon guidance, cytokine-cytokine receptor interaction and Wnt signalling pathway in HDPC. Our results not only uncovered the potential effects of NTS on stress-induced hair growth inhibition but also provided an understanding of the mechanisms at the gene transcriptional level.

An optimized visualization and quantitative protocol for in-depth evaluation of lymphatic vessel architecture in the liver

The liver lymphatic system is essential for maintaining tissue fluid balance and immune function. The detailed structure of lymphatic vessels (LVs) in the liver remains to be fully demonstrated. The aim of this study is to reveal LV structures in normal and diseased livers by developing a tissue-clearing and coimmunolabeling protocol optimized for the tissue size and the processing time for three-dimensional (3-D) visualization and quantification of LVs in the liver. We showed that our optimized protocol enables in-depth exploration of lymphatic networks in the liver, consisting of LVs along the portal tract (deep lymphatic system) and within the collagenous Glisson's capsule (superficial lymphatic system) in different species. With this protocol, we have shown 3-D LVs configurations in relation to blood vessels and bile ducts in cholestatic mouse livers, in which LVs were highly dilated and predominantly found around highly proliferating bile ducts and peribiliary vascular plexuses in the portal tract. We also established a quantification method using a 3-D volume-rendering approach. We observed a 1.6-fold (P < 0.05) increase in the average diameter of LVs and a 2.4-fold increase (P < 0.05) in the average branch number of LVs in cholestatic/fibrotic livers compared with control livers. Furthermore, cholestatic/fibrotic livers showed a 4.3-fold increase (P < 0.05) in total volume of LVs compared with control livers. Our optimized protocol and quantification method demonstrate an efficient and simple liver tissue-clearing procedure that allows the comprehensive analysis of liver lymphatic system.NEW & NOTEWORTHY This article showed a comprehensive 3-D-structural analysis of liver lymphatic vessel (LV) in normal and diseased livers in relation to blood vessels and bile ducts. In addition to the LVs highly localized at the portal tract, we revealed capsular LVs in mouse, rat, and human livers. In cholestatic livers, LVs are significantly increased and dilated compared with normal livers. Our optimized protocol provides detailed spatial information for LVs remodeling in normal and pathological conditions.

mitoSplitter: A mitochondrial variants-based method for efficient demultiplexing of pooled single-cell RNA-seq

Single-cell RNA-seq (scRNA-seq) analysis of multiple samples separately can be costly and lead to batch effects. Exogenous barcodes or genome-wide RNA mutations can be used to demultiplex pooled scRNA-seq data, but they are experimentally or computationally challenging and limited in scope. Mitochondrial genomes are small but diverse, providing concise genotype information. We developed "mitoSplitter," an algorithm that demultiplexes samples using mitochondrial RNA (mtRNA) variants, and demonstrated that mtRNA variants can be used to demultiplex large-scale scRNA-seq data. Using affordable computational resources, mitoSplitter can accurately analyze 10 samples and 60,000 cells in 6 h. To avoid the batch effects from separated experiments, we applied mitoSplitter to analyze the responses of five non-small cell lung cancer cell lines to BET (Bromodomain and extraterminal) chemical degradation in a multiplexed fashion. We found the synthetic lethality of TOP2A inhibition and BET chemical degradation in BET inhibitor-resistant cells. The result indicates that mitoSplitter can accelerate the application of scRNA-seq assays in biomedical research.

Mammary duct luminal epithelium controls adipocyte thermogenic programme

Sympathetic activation during cold exposure increases adipocyte thermogenesis via the expression of mitochondrial protein uncoupling protein 1 (UCP1)1. The propensity of adipocytes to express UCP1 is under a critical influence of the adipose microenvironment and varies between sexes and among various fat depots2-7. Here we report that mammary gland ductal epithelial cells in the adipose niche regulate cold-induced adipocyte UCP1 expression in female mouse subcutaneous white adipose tissue (scWAT). Single-cell RNA sequencing shows that glandular luminal epithelium subtypes express transcripts that encode secretory factors controlling adipocyte UCP1 expression under cold conditions. We term these luminal epithelium secretory factors 'mammokines'. Using 3D visualization of whole-tissue immunofluorescence, we reveal sympathetic nerve-ductal contact points. We show that mammary ducts activated by sympathetic nerves limit adipocyte UCP1 expression via the mammokine lipocalin 2. In vivo and ex vivo ablation of mammary duct epithelium enhance the cold-induced adipocyte thermogenic gene programme in scWAT. Since the mammary duct network extends throughout most of the scWAT in female mice, females show markedly less scWAT UCP1 expression, fat oxidation, energy expenditure and subcutaneous fat mass loss compared with male mice, implicating sex-specific roles of mammokines in adipose thermogenesis. These results reveal a role of sympathetic nerve-activated glandular epithelium in adipocyte UCP1 expression and suggest that mammary duct luminal epithelium has an important role in controlling glandular adiposity.

The Lymphatic Vasculature in Cardiac Development and Ischemic Heart Disease

In recent years, the lymphatic system has received increasing attention due to the fast-growing number of findings about its diverse novel functional roles in health and disease. It is well documented that the lymphatic vasculature plays major roles in the maintenance of tissue-fluid balance, the immune response, and in lipid absorption. However, recent studies have identified an additional growing number of novel and sometimes unexpected functional roles of the lymphatic vasculature in normal and pathological conditions in different organs. Among those, cardiac lymphatics have been shown to play important roles in heart development, ischemic cardiac disease, and cardiac disorders. In this review, we will discuss some of those novel functional roles of cardiac lymphatics, as well as the therapeutic potential of targeting lymphatics for the treatment of cardiovascular diseases.

Immune-Epithelial Cross Talk in Regeneration and Repair

The epithelial tissues that line our body, such as the skin and gut, have remarkable regenerative prowess and continually renew throughout our lifetimes. Owing to their barrier function, these tissues have also evolved sophisticated repair mechanisms to swiftly heal and limit the penetration of harmful agents following injury. Researchers now appreciate that epithelial regeneration and repair are not autonomous processes but rely on a dynamic cross talk with immunity. A wealth of clinical and experimental data point to the functional coupling of reparative and inflammatory responses as two sides of the same coin. Here we bring to the fore the immunological signals that underlie homeostatic epithelial regeneration and restitution following damage. We review our current understanding of how immune cells contribute to distinct phases of repair. When unchecked, immune-mediated repair programs are co-opted to fuel epithelial pathologies such as cancer, psoriasis, and inflammatory bowel diseases. Thus, understanding the reparative functions of immunity may advance therapeutic innovation in regenerative medicine and epithelial inflammatory diseases.

Exploring the heterogeneity of white adipose tissue in mouse and man

Adipose tissue is a heterogeneous organ, comprising cell types, including mature adipocytes, progenitor cells, immune cells, and vascular cells. Here, we discuss the heterogeneity of human and mouse white adipose tissue in general and white adipocytes specifically, focusing on how our understanding of adipocyte subpopulations has expanded with the advent of single nuclear RNA sequencing and spatial transcriptomics. Furthermore, we discuss critical remaining questions regarding how these distinct populations arise, how their functions differ from one another, and which potentially contribute to metabolic pathophysiology.

The physiological functions of lymphangiocrine signals

It has been demonstrated that the lymphatic system plays important roles in tissue development and the progress of diseases. Recent studies identified that lymphatic endothelial cells can secret many proteins with diverse functions. This article highlights the physiological significances of these lymphangiocrine signals in different tissues.

BCAA insufficiency leads to premature ovarian insufficiency via ceramide-induced elevation of ROS

Premature ovarian insufficiency (POI) is a disease featured by early menopause before 40 years of age, accompanied by an elevation of follicle-stimulating hormone. Though POI affects many aspects of women's health, its major causes remain unknown. Many clinical studies have shown that POI patients are generally underweight, indicating a potential correlation between POI and metabolic disorders. To understand the pathogenesis of POI, we performed metabolomics analysis on serum and identified branch-chain amino acid (BCAA) insufficiency-related metabolic disorders in two independent cohorts from two clinics. A low BCAA diet phenotypically reproduced the metabolic, endocrine, ovarian, and reproductive changes of POI in young C57BL/6J mice. A mechanism study revealed that the BCAA insufficiency-induced POI is associated with abnormal activation of the ceramide-reactive oxygen species (ROS) axis and consequent impairment of ovarian granulosa cell function. Significantly, the dietary supplement of BCAA prevented the development of ROS-induced POI in female mice. The results of this pathogenic study will lead to the development of specific therapies for POI.

Increased adipose tissue lymphatic vessel density inhibits thermogenesis through elevated neurotensin levels

During cold exposure, white adipose tissue can remodel to dissipate energy as heat under cold similar to thermogenic brown adipose tissue. This "browning" and the regulation of body temperature is under the control of neural and hormonal signaling. It was recently discovered that neurotensin, a small neuropeptide, not only acts to inhibit thermogenesis, but also that lymphatic vessels may be a surprisingly potent source of neurotensin production. We hypothesized that the induction of adipose tissue lymphangiogenesis would therefore increase tissue neurotensin levels and impair thermogenesis. Methods: We utilized AdipoVD mice that have inducible expression of vascular endothelial growth factor (VEGF)-D, a potent lymphangiogenic stimulator, specifically in adipose tissue. Overexpression of VEGF-D induced significant lymphangiogenesis in both white and brown adipose tissues of AdipoVD mice. Results: Obese Adipo-VD mice demonstrated no differences in adipose morphology or browning under room temperature conditions compared to controls but did express significantly higher levels of neurotensin in their adipose tissues. Upon acute cold exposure, AdipoVD mice were markedly cold intolerant; inhibition of neurotensin signaling ameliorated this cold intolerance as AdipoVD mice were then able to maintain body temperature on cold challenge equivalent to their littermates. Conclusion: In total, these data demonstrate that adipose tissue lymphatic vessels are a potent paracrine source of neurotensin and that lymphangiogenesis therefore impairs the tissues' thermogenic ability.

Promoting Lymphangiogenesis and Lymphatic Growth and Remodeling to Treat Cardiovascular and Metabolic Diseases

Lymphatic vessels are low-pressure, blind-ended tubular structures that play a crucial role in the maintenance of tissue fluid homeostasis, immune cell trafficking, and dietary lipid uptake and transport. Emerging research has indicated that the promotion of lymphatic vascular growth, remodeling, and function can reduce inflammation and diminish disease severity in several pathophysiologic conditions. In particular, recent groundbreaking studies have shown that lymphangiogenesis, which describes the formation of new lymphatic vessels from the existing lymphatic vasculature, can be beneficial for the alleviation and resolution of metabolic and cardiovascular diseases. Therefore, promoting lymphangiogenesis represents a promising therapeutic approach. This brief review summarizes the most recent findings related to the modulation of lymphatic function to treat metabolic and cardiovascular diseases such as obesity, myocardial infarction, atherosclerosis, and hypertension. We also discuss experimental and therapeutic approaches to enforce lymphatic growth and remodeling as well as efforts to define the molecular and cellular mechanisms underlying these processes.

Substantial impact of 3-iodothyronamine (T1AM) on the regulations of fluorescent thermoprobe-measured cellular temperature and natriuretic peptide expression in cardiomyocytes

There is growing interest in 3-iodothyronamine (T1AM), an active thyroid hormone metabolite, that induces negative inotropic and chronotropic actions in the heart and exerts systemic hypothermic action. We explored the direct impact of T1AM on cardiomyocytes with a focus on the regulation of the intracellular temperature and natriuretic peptide (NP) expression. A thermoprobe was successfully introduced into neonatal rat cardiomyocytes, and the temperature-dependent changes in the fluorescence intensity ratio were measured using a fluorescence microscope. After one-hour incubation with T1AM, the degree of change in the fluorescence intensity ratio was significantly lower in T1AM-treated cardiomyocytes than in equivalent solvent-treated controls (P < 0.01), indicating the direct hypothermic action of T1AM on cardiomyocytes. Furthermore, T1AM treatment upregulated B-type NP (BNP) gene expression comparable to treatment with endothelin-1 or phenylephrine. Of note, ERK phosphorylation was markedly increased after T1AM treatment, and inhibition of ERK phosphorylation by an MEK inhibitor completely cancelled both T1AM-induced decrease in thermoprobe-measured temperature and the increase in BNP expression. In summary, T1AM decreases fluorescent thermoprobe-measured temperatures (estimated intracellular temperatures) and increases BNP expression in cardiomyocytes by activating the MEK/ERK pathway. The present findings provide new insight into the direct myocardial cellular actions of T1AM in patients with severe heart failure.

Paracrine Regulation of Adipose Tissue Macrophages by Their Neighbors in the Microenvironment of Obese Adipose Tissue

Obesity has recently been defined as a chronic low-grade inflammatory disease. Obesity-induced inflammation of adipose tissue (AT) is an essential trigger for insulin resistance (IR) and related metabolic diseases. Although the underlying molecular basis of this inflammation has not been fully identified, there is consensus that the recruited and activated macrophages in AT are the most important culprits of AT chronic inflammation. Adipose tissue macrophages (ATMs) are highly plastic and could be polarized from an anti-inflammatory M2 to proinflammatory M1 phenotypes on stimulation by microenvironmental signals from obese AT. Many efforts have been made to elucidate the molecular signaling pathways of macrophage polarization; however, the upstream drivers governing and activating macrophage polarization have rarely been summarized, particularly regulatory messages from the AT microenvironment. In addition to adipocytes, the AT bed also contains a variety of immune cells, stem cells, as well as vascular, neural, and lymphatic tissues throughout, which together orchestrate the AT microenvironment. Here, we summarize how the aforesaid neighbors of ATMs in the AT microenvironment send messages to ATMs and thus regulate its phenotype during obesity. Deciphering the biology and polarization of ATMs in the obese environment is expected to provide a precise immunotherapy for adipose inflammation and obesity-related metabolic diseases.

The Storage Conditions of High-Fat Diet Are the Key Factors for Diet-Induced Obesity and Liver Damage

The diet-induced obesity (DIO) mouse model has been widely used for obesity studies. The effects of storage conditions on the composition of nutrients in high-fat diets (HFDs) and their impact on metabolic homeostasis have not been systemically investigated. In the current study, we tested the effects of HFDs stored under different conditions and found that mice fed a HFD stored in the fridge (HFD^fri) gained less weight than those fed HFDs stored in the freezer (HFD^fre). Further analysis revealed that changes in the relative abundance of medium-chain triglyceride (MCT) in the HFD^fri, which have much lower intestinal absorption rates, contributed to the body weight differences. In contrast, exacerbated liver damage and elevated levels of unfolded protein response (UPR) was observed in the mice fed by HFD^fri. Depletion of the UPR-regulated gene Nnmt alleviated liver damage via the inhibition of the integrated stress response (ISR). Our study, for the first time, provides evidence that HFD storage conditions can have a significant impact on both body weight changes and liver damage in the DIO model.

Leveraging GPCR signaling in thermogenic fat to counteract metabolic diseases

Background

Thermogenic brown and beige adipocytes are recognized for their unique capacity to consume extraordinary levels of metabolites and lipids from the blood to fuel heat-producing catabolic processes [[1], [2], [3], [4], [5], [6], [7]]. In humans, the functions of thermogenic adipocytes are associated with cardiometabolic protection and improved glycemic control [[8], [9], [10], [11], [12], [13]]. Consequently, engaging these macronutrient-consuming and energy-dissipating activities has gained attention as a promising therapeutic strategy for counteracting metabolic diseases, such as obesity and diabetes.

Scope of review

In this review, we highlight new advances in our understanding of the physiological role of G protein-coupled receptors (GPCRs) in controlling thermogenic adipocyte biology. We further extend our discussion to the opportunities and challenges posed by pharmacologically targeting different elements of GPCR signaling in these highly specialized fat cells.

Major conclusions

GPCRs represent appealing candidates through which to harness adipose thermogenesis. Yet safely and effectively targeting these druggable receptors on brown and beige adipocytes has thus far proven challenging. Therefore, continued interrogation across the GPCR landscape is necessary for future leaps within the field of thermogenic fat biology to unlock the therapeutic potential of adipocyte catabolism.

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Brown and beige thermogenic adipocytes robustly consume and catabolize macronutrients.

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The catabolic activity of thermogenic adipocytes promotes organismal energy balance.

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Thermogenic adipocyte functions are tightly controlled by G protein-coupled receptors (GPCRs).

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GPCRs can be potentially targeted at multiple levels to therapeutically harness thermogenic activity.

A closer look at adipose tissue lymphatics and their markers

PURPOSE OF REVIEW

Lymphatic vessels are found in most tissues, with the exception of the cornea and the central nervous system. Tissues that have high exposure to antigens, such as the skin and the intestine, have especially extensive lymphatic vascular networks. Despite being densely vascularized with blood vessels, adipose tissue is poorly permeated with lymphatic vasculature. Here, we focus on the recent advances in the research on adipose tissue lymphatics and present a lymphatic-focused analysis of published single-cell and single-nucleus RNA sequencing datasets of adipose tissues.

RECENT FINDINGS

Although lymphatic expansion in obesity may limit inflammation and promote glycerol efflux from adipose tissue, lymphatic endothelial cells (LECs) secrete factors that reduce brown adipocyte thermogenesis. Transcriptomic analyses of these cells show that they express common lymphatic markers such as Prox1, but datasets from different studies show great variation in gene expression values due to the low number of captured LECs, depot differences, and species-specific gene expression patterns.

SUMMARY

As the importance of LECs in the homeostasis of adipose tissue has become evident, investigators want to shed light on the specific interactions of lymphatics with other cell types in adipose tissues. Extracting LECs from readily available transcriptomics datasets provides a standpoint for investigators for future research. However, systematic studies are needed to reveal unique identities according to depot and species-specific LEC signatures.

New Insights in the Control of Fat Homeostasis: The Role of Neurotensin

Neurotensin (NT) is a small peptide with pleiotropic functions, exerting its primary actions by controlling food intake and energy balance. The first evidence of an involvement of NT in metabolism came from studies on the central nervous system and brain circuits, where NT acts as a neurotransmitter, producing different effects in relation to the specific region involved. Moreover, newer interesting chapters on peripheral NT and metabolism have emerged since the first studies on the NT-mediated regulation of gut lipid absorption and fat homeostasis. Intriguingly, NT enhances fat absorption from the gut lumen in the presence of food with a high fat content, and this action may explain the strong association between high circulating levels of pro-NT, the NT stable precursor, and the increased incidence of metabolic disorders, cardiovascular diseases, and cancer observed in large population studies. This review aims to provide a synthetic overview of the main regulatory effects of NT on several biological pathways, particularly those involving energy balance, and will focus on new evidence on the role of NT in controlling fat homeostasis, thus influencing the risk of unfavorable cardio–metabolic outcomes and overall mortality in humans.

Crosstalk Between Adipose and Lymphatics in Health and Disease

Adipose tissue, once thought to be an inert receptacle for energy storage, is now recognized as a complex tissue with multiple resident cell populations that actively collaborate in response to diverse local and systemic metabolic, thermal, and inflammatory signals. A key participant in adipose tissue homeostasis that has only recently captured broad scientific attention is the lymphatic vasculature. The lymphatic system's role in lipid trafficking and mediating inflammation makes it a natural partner in regulating adipose tissue, and evidence supporting a bidirectional relationship between lymphatics and adipose tissue has accumulated in recent years. Obesity is now understood to impair lymphatic function, whereas altered lymphatic function results in aberrant adipose tissue deposition, though the molecular mechanisms governing these phenomena have yet to be fully elucidated. We will review our current understanding of the relationship between adipose tissue and the lymphatic system here, focusing on known mechanisms of lymphatic-adipose crosstalk.

Latest Advancements on Combating Obesity by Targeting Human Brown/Beige Adipose Tissues

Obesity is defined as overaccumulation of white adipose tissue in the body, mainly under the skin (subcutaneous adiposity) or in the abdominal cavity (visceral adiposity). It could be the origin of various metabolic disorders including hypertension, hyperlipidemia, type 2 diabetes, cardiovascular diseases etc. Active adipose tissue was discovered in humans through ¹⁸F-fluorodeoxyglucose Positron Emission Tomography coupled with Computer Tomography (¹⁸F FDG-PET/CT), which was initially performed for tumor scanning. Since human active adipose tissue is probably composed of brown and beige adipose tissues and they burn white adipose tissue to generate heat, targeting human brown/beige adipose tissue to induce their thermogenic function is considered significant to combat obesity. In this review, we describe the latest advancements on promising therapeutic strategies to combat obesity by targeting human thermogenic adipose tissues to achieve further metabolic balance in humans.

Lymphatic endothelial cell fate specification in the mammalian embryo: An historical perspective

Development of the mammalian lymphatic vasculature is a stepwise process requiring the specification of lymphatic endothelial cell progenitors in the embryonic veins, and their subsequent budding to give rise to most of the mature lymphatic vasculature. In mice, formation of the lymphatic vascular network starts inside the cardinal vein at around E9.5 when a subpopulation of venous endothelial cells gets committed into the lymphatic lineage by their acquisition of Prox1 expression. Identification of critical genes regulating lymphatic development facilitated the detailed cellular and molecular characterization of some of the cellular and molecular mechanisms regulating the early steps leading to the formation of the mammalian lymphatic vasculature. A better understanding of basic aspects of early lymphatic development, and the availability of novel tools and animal models has been instrumental in the identification of important novel functional roles of this vasculature network.

Homeostatic maintenance of the lymphatic vasculature

The lymphatic vasculature is emerging as a multifaceted regulator of tissue homeostasis and regeneration. Lymphatic vessels drain fluid, macromolecules, and immune cells from peripheral tissues to lymph nodes (LNs) and the systemic circulation. Their recently uncovered functions extend beyond drainage and include direct modulation of adaptive immunity and paracrine regulation of organ growth. The developmental mechanisms controlling lymphatic vessel growth have been described with increasing precision. It is less clear how the essential functional features of lymphatic vessels are established and maintained. We discuss the mechanisms that maintain lymphatic vessel integrity in adult tissues and control vessel repair and regeneration. This knowledge is crucial for understanding the pathological vessel changes that contribute to disease, and provides an opportunity for therapy development.

Throttling thermogenesis with neurotensin

Lymphatic endothelial cells release neurotensin to block thermogenesis in brown adipose tissue.

Mesothelial cells are not a source of adipocytes in mice

Visceral adipose tissue (VAT) depots are associated with the adverse metabolic consequences of obesity, such as insulin resistance. The developmental origin of VAT depots and the identity and regulation of adipocyte progenitor cells have been active areas of investigation. In recent years, a paradigm of mesothelial cells as a source of VAT adipocyte progenitor cells has emerged based on lineage tracing studies using the Wilms’ tumor gene, Wt1, as a marker for cells of mesothelial origin. Here, we show that Wt1 expression in adipose tissue is not limited to the mesothelium but is also expressed by a distinct preadipocyte population in mice and humans. We identify keratin 19 (Krt19) as a highly specific marker for the adult mouse mesothelium and demonstrate that Krt19-expressing mesothelial cells do not differentiate into visceral adipocytes. These results contradict the assertion that the VAT mesothelium can serve as a source of adipocytes.

Mesothelial differentiation into adipocytes has been proposed based on fate mapping experiments using Wt1 as a marker. Westcott et al. show that Wt1 is expressed in stromal preadipocytes in addition to mesothelium and that fate mapping using a specific mesothelial marker, Krt19, does not support adipocyte differentiation.