Functional imaging of brown fat in mice with 18F-FDG micro-PET/CT

Repeatability of brown adipose tissue activation measured by [18F]FDG PET after beta3-adrenergic stimuli in a mouse model

This study aimed to evaluate the repeatability of brown adipose tissue (BAT) activation measured by [18F]FDG-PET after beta3-adrenergic stimuli with CL316243 in mice.

METHODS

Male C57BL/6 mice underwent [18F]FDG-PET at baseline without stimulation (T0-NS), on three consecutive days after intravenous administration of the selective β3-adrenergic agonist CL316243 (T1-CL, T2-CL, T3-CL), and without stimuli after 1 and 2 weeks (T7-NS and T14-NS). The standardized uptake value (SUVmax), BAT metabolic volume (BMV), and total BAT glycolysis (TBG) were measured in each scanning session, with statistical groupwise comparisons by ANOVA and post hoc Tukey test.

RESULTS

SUVmax, BMV, and TBG values showed no significant differences between the three PET scans without stimuli, but were significantly higher after CL316243 administration (p < 0.0001). The mean coefficient of variation (CoV) of PET within individuals was 49 % at baseline but only 9 % with pharmacological stimulation.

CONCLUSIONS

The study demonstrated that administration of the selective β3-adrenergic receptor agonist CL316243 (CL) in mice leads to consistent metabolic activation of brown adipose tissue (BAT), as measured by [18F]FDG-PET. We also demonstrated metabolic activation by repeated pharmacological challenge, without evidence of hysteresis. Thus, the methods used in the current work should serve for further studies on BAT metabolism in experimental animals, with translational value for clinical research.

[18F]FDG and [11C]PK11195 PET imaging in the evaluation of brown adipose tissue - effects of cold and pharmacological stimuli and their association with crotamine intake in a male mouse model

This study aimed to evaluate the role of positron emission tomography (PET) with [11C]PK11195 and [18F]FDG in the characterization of brown adipose tissue (BAT).

METHODS

Male C57BL/6 mice were studied with the glucose analogue [18F]FDG (n = 21) and the TSPO mitochondrial tracer [11C]PK11195 (n = 28), without stimulus and after cold (6-9 °C) or beta-agonist (CL316243) stimuli. PET studies were performed at baseline and after 21 days of daily treatment with crotamine, which is a peptide described to induce adipocyte tissue browning and to increase BAT metabolism. Tracer uptake (SUVmax) was measured in the interscapular BAT and translocator protein 18 kDa (TSPO) expression was evaluated by immunohistochemistry.

RESULTS

The cold stimulus increased [18F]FDG uptake compared to no-stimulus (5.21 ± 1.05 vs. 2.03 ± 0.21, p < 0.0001) and to beta-agonist stimulus (2.65 ± 0.39, p = 0.0003). After 21 days of treatment with crotamine, there was no significant difference in the [18F]FDG uptake compared to the baseline in the no-stimulus group and in the cold-stimulus group, with a significant increase in uptake after CL stimulus (baseline: 2.65 ± 0.39; 21 days crotamine: 4.77 ± 0.81, p = 0.0003). Evaluation of [11C]PK11195 at baseline shows that CL stimulus increases the BAT uptake compared to no-stimulus (4.47 ± 0.66 vs. 3.36 ± 0.68, p = 0.014). After 21 days of treatment with crotamine, there was no significant difference in the [11C]PK11195 uptake compared to the baseline in the no-stimulus group (2.94 ± 0.58, p = 0.7864) and also after CL stimulus (3.55 ± 0.79, p = 0.085). TSPO expression correlated with [11C]PK11195 uptake (r = 0.83, p = 0.018) but not with [18F]FDG uptake (r = 0.40, p = 0.516).

CONCLUSIONS

[11C]PK11195 allowed the identification of BAT under thermoneutral conditions or after beta3-adrenergic stimulation in a direct correlation with TSPO expression. The beta-adrenergic stimulus, despite presenting a lower intensity of glycolytic activation compared to cold at baseline, allowed the observation of an increase in BAT uptake of [18F]FDG after 21 days of crotamine administration. Although some limitations were observed for the metabolic changes induced by crotamine, this study reinforced the potential of using [11C]PK11195 and/or [18F]FDG-PET to monitor the activation of BAT.

Sex-dependent effects of forced exercise in the body composition of adolescent rats

Determining the body composition during adolescence can predict diseases such as obesity, diabetes, and metabolic syndromes later in life; and physical activity became an effective way to restore changes in body composition. However, current available literature assessing the body composition before, during and after adolescence in female and male rodents by in vivo techniques is scarce. Thus, by using computerized tomography, we aimed to define the baseline of the weight and body composition during the adolescence and young adulthood of female and male Sprague-Dawley rats (on P30, P60 and P90) under standard diet. Then, we determined the effect of 18 days of forced exercise on the body weight and composition during the early adolescence (P27-45). The highest percentual increments in weight, body volume and relative adipose contents occurred during the female and male adolescence. Forced running during the early adolescence decreased weight, body volume and relative adipose delta and increment values in males only. The adolescence of rats is a period of drastic body composition changes, where exercise interventions have sex-dependent effects. These results support a model that could open new research windows in the field of adolescent obesity.

Brown Adipose Tissue Heterogeneity, Energy Metabolism, and Beyond

Brown adipocyte in brown adipose tissue (BAT) specializes in expending energy through non-shivering thermogenesis, a process that produces heat either by uncoupling protein 1 (UCP1) dependent uncoupling of mitochondrial respiration or by UCP1 independent mechanisms. Apart from this, there is ample evidence suggesting that BAT has an endocrine function. Studies in rodents point toward its vital roles in glucose and lipid homeostasis, making it an important therapeutic target for treating metabolic disorders related to morbidities such as obesity and type 2 diabetes. The rediscovery of thermogenically active BAT depots in humans by several independent research groups in the last decade has revitalized interest in BAT as an even more promising therapeutic intervention. Over the last few years, there has been overwhelming interest in understanding brown adipocyte's developmental lineages and how brown adipocyte uniquely utilizes energy beyond UCP1 mediated uncoupling respiration. These new discoveries would be leveraged for designing novel therapeutic interventions for metabolic disorders.

Adaptive thermogenesis enhances the life-threatening response to heat in mice with an Ryr1 mutation

Mutations in the skeletal muscle Ca2+ release channel, the type 1 ryanodine receptor (RYR1), cause malignant hyperthermia susceptibility (MHS) and a life-threatening sensitivity to heat, which is most severe in children. Mice with an MHS-associated mutation in Ryr1 (Y524S, YS) display lethal muscle contractures in response to heat. Here we show that the heat response in the YS mice is exacerbated by brown fat adaptive thermogenesis. In addition, the YS mice have more brown adipose tissue thermogenic capacity than their littermate controls. Blood lactate levels are elevated in both heat-sensitive MHS patients with RYR1 mutations and YS mice due to Ca2+ driven increases in muscle metabolism. Lactate increases brown adipogenesis in both mouse and human brown preadipocytes. This study suggests that simple lifestyle modifications such as avoiding extreme temperatures and maintaining thermoneutrality could decrease the risk of life-threatening responses to heat and exercise in individuals with RYR1 pathogenic variants.

Diverse effects of different Akkermansia muciniphila genotypes on Brown adipose tissue inflammation and whitening in a high-fat-diet murine model

BACKGROUND

The purpose of this study was to investigate the differences in the metabolic protective effects of Akkermansia muciniphila (A.muciniphila) genotypes on high-fat diet mice and explore possible mechanisms.

METHODS

Male C57BL/6 mice were randomly divided into 6 groups, including high-fat diet (HFD)+ A. muciniphila I/II/PBS group, normal control diet (NCD)+ A. muciniphila I/II/PBS group, respectively. Dietary intervention and A. muciniphila gavage were performed simultaneously. Blood glucose and lipid metabolism, brown adipose morphology and activities, and intestinal barrier function were examined after the mice were sacrificed.

RESULTS

A.muciniphila gavage improved the impaired glucose tolerance, hyperlipidemia and liver steatosis in HFD mice, and that A. muciniphila II (Amuc_GP25) was not as effective as A. muciniphila I (Amuc_GP01). This phenomenon might be because Amuc_GP01 intervention significantly inhibited brown adipose tissue whitening and inflammation induced by HFD, by repairing the intestinal barrier and relieving endotoxemia. Amuc_GP25 did not display the same results as Amuc_GP01 in HFD mice but had stronger effects in the NCD mice.

CONCLUSIONS

This study reveals the distinct functions of different A. muciniphila genotypes on diet-induced obesity, suggesting that different A. muciniphila genotypes may affect pathological conditions differently through distinct action pathways.

Bone marrow adipose tissue is a unique adipose subtype with distinct roles in glucose homeostasis

Bone marrow adipose tissue (BMAT) comprises >10% of total adipose mass, yet unlike white or brown adipose tissues (WAT or BAT) its metabolic functions remain unclear. Herein, we address this critical gap in knowledge. Our transcriptomic analyses revealed that BMAT is distinct from WAT and BAT, with altered glucose metabolism and decreased insulin responsiveness. We therefore tested these functions in mice and humans using positron emission tomography-computed tomography (PET/CT) with ¹⁸F-fluorodeoxyglucose. This revealed that BMAT resists insulin- and cold-stimulated glucose uptake, while further in vivo studies showed that, compared to WAT, BMAT resists insulin-stimulated Akt phosphorylation. Thus, BMAT is functionally distinct from WAT and BAT. However, in humans basal glucose uptake in BMAT is greater than in axial bones or subcutaneous WAT and can be greater than that in skeletal muscle, underscoring the potential of BMAT to influence systemic glucose homeostasis. These PET/CT studies characterise BMAT function in vivo, establish new methods for BMAT analysis, and identify BMAT as a distinct, major adipose tissue subtype.

Activation of brown adipose tissue in diet-induced thermogenesis is GC-C dependent

Uroguanylin (UGN) is released from the intestine after a meal. When applied in brain ventricles, UGN increases expression of markers of thermogenesis in brown adipose tissue (BAT). Therefore, we determine the effects of its receptor, guanylate cyclase C (GC-C), on mouse interscapular BAT (iBAT) activity during diet-induced thermogenesis (DIT). The activation of iBAT after a meal is diminished in GC-C KO mice, decreased in female wild type (WT) mice, and abolished in old WT animals. The activation of iBAT after a meal is the highest in male WT animals which leads to an increase in GC-C expression in the hypothalamus, an increase in iBAT volume by aging, and induction of iBAT markers of thermogenesis. In contrast to iBAT activation after a meal, iBAT activation after a cold exposure could still exist in GC-C KO mice and it is significantly higher in female WT mice. The expression of GC-C in the proopiomelanocortin neurons of the arcuate nucleus of the hypothalamus but not in iBAT suggests central regulation of iBAT function. The iBAT activity during DIT has significantly reduced in old mice but an intranasal application of UGN leads to an increase in iBAT activity in a dose-dependent manner which is in strong negative correlation to glucose concentration in blood. This activation was not present in GC-C KO mice. Our results suggest the physiological role of GC-C on the BAT regulation and its importance in the regulation of glucose homeostasis and the development of new therapy for obesity and insulin resistance.

Noggin depletion in adipocytes promotes obesity in mice

OBJECTIVE

Obesity has increased to pandemic levels and enhanced understanding of adipose regulation is required for new treatment strategies. Although bone morphogenetic proteins (BMPs) influence adipogenesis, the effect of BMP antagonists such as Noggin is largely unknown. The aim of the study was to define the role of Noggin, an extracellular BMP inhibitor, in adipogenesis.

METHODS

We generated adipose-derived progenitor cells and a mouse model with adipocyte-specific Noggin deletion using the AdiponectinCre transgenic mouse, and determined the adipose phenotype of Noggin-deficiency.

RESULTS

Our studies showed that Noggin is expressed in progenitor cells but declines in adipocytes, possibly allowing for lipid accumulation. Correspondingly, adipocyte-specific Noggin deletion in vivo promoted age-related obesity in both genders with no change in food intake. Although the loss of Noggin caused white adipose tissue hypertrophy, and whitening and impaired function in brown adipose tissue in both genders, there were clear gender differences with the females being most affected. The females had suppressed expression of brown adipose markers and thermogenic genes including peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC1alpha) and uncoupling protein 1 (UCP1) as well as genes associated with adipogenesis and lipid metabolism. The males, on the other hand, had early changes in a few BAT markers and thermogenic genes, but the main changes were in the genes associated with adipogenesis and lipid metabolism. Further characterization revealed that both genders had reductions in VO2, VCO2, and RER, whereas females also had reduced heat production. Noggin was also reduced in diet-induced obesity in inbred mice consistent with the obesity phenotype of the Noggin-deficient mice.

CONCLUSIONS

BMP signaling regulates female and male adipogenesis through different metabolic pathways. Modulation of adipose tissue metabolism by select BMP antagonists may be a strategy for long-term regulation of age-related weight gain and obesity.

Imaging adipose tissue browning using the TSPO-18kDa tracer [18F]FEPPA

Objectives

The browning of white adipose tissue (WAT) into beige has been proposed as a strategy to enhance energy expenditure to combat the growing epidemic of obesity. Research into browning strategies are hampered by the lack of sensitive, translatable, imaging tools capable of detecting beige fat mass non-invasively. [¹⁸F]FDG is able to detect activated beige fat but provides little information on unstimulated beige fat mass. We have assessed the use of [¹⁸F]FEPPA, a tracer for the TSPO-18KDa found on the outer mitochondrial membrane, as an alternative imaging agent capable of detecting unstimulated brown fat (BAT) and beige fat.

Methods

Female Balb/c mice (n = 5) were treated for 7 days with the β3 adrenergic agonist CL-316,243 to induce the browning of inguinal WAT (beige fat). Animals were imaged longitudinally with [¹⁸F]FDG and [¹⁸F]FEPPA and uptake in interscapular BAT and inguinal WAT assessed. The browning of inguinal WAT was confirmed using H&E and immunohistochemical detection of UCP-1 and TSPO.

Results

Repeated dosing with β3-adrenergic agonist CL-316,243 caused a significant increase in [¹⁸F]FDG uptake in both interscapular BAT and inguinal WAT associated with the increased metabolic activity of brown and beige adipocytes respectively. [¹⁸F]FEPPA uptake was likewise increased in inguinal WAT but showed no increase in BAT uptake due to stimulation over the same time course. Furthermore, inguinal WAT uptake was unaffected by pharmacological blockade, indicating that [¹⁸F]FEPPA uptake is associated with the expression of mitochondria in BAT and beige adipocytes and independent of activation.

Conclusion

These data show that [¹⁸F]FEPPA can detect BAT and newly formed beige fat under non-stimulated, thermoneutral conditions and that uptake after stimulation is linked to mitochondrial expression as opposed to activation.

•

TSPO-18kDa tracers can detect BAT under non-stimulated, thermoneutral conditions.

•

TSPO-18kDa tracers can detect the formation of beige adipocytes in white adipose tissue.

•

TSPO-18kDa tracers may aid in the development of new approaches to treat obesity.

Theacrine protects against nonalcoholic fatty liver disease by regulating acylcarnitine metabolism

OBJECTIVE

Acylcarnitine metabolism disorder contributes significantly to the pathogenesis of nonalcoholic fatty liver disease (NAFLD). There are, however, few ideal medications for NAFLD, which work by targeting acylcarnitine metabolism. The aim of this study was to investigate the protective effects of theacrine, a rare purine alkaloid isolated from Camellia assamica var. kucha, against acylcarnitine metabolism disorder in NAFLD.

METHODS

The pharmacological activities of theacrine were studied using high-fat diet (HFD)-fed ApoE-/- and C57BL/6J mice models. Oleate-treated HepG2 and L-02 cells were used to investigate the molecular mechanism of theacrine on acylcarnitine metabolism. The target of theacrine was confirmed in vitro as the blockade of sirtuin 3 (SIRT3) and protein kinase A.

RESULTS

Theacrine inhibits hepatic steatosis and liver inflammation and improves energy expenditure in HFD-fed mice. Theacrine ameliorates acylcarnitine metabolism disorder in HFD-fed mice and oleate-treated hepatocytes by improving fatty acid oxidation. The underlying mechanism involves theacrine's activation of the mitochondrial deacetylase SIRT3 and consequently, the increased activity of long-chain acyl coenzyme A dehydrogenase (LCAD) through deacetylation.

CONCLUSION

Theacrine promotes acylcarnitine metabolism in NAFLD through the SIRT3/LCAD signaling pathway. The target of theacrine's activities on NAFLD is identified as SIRT3.

Assessment of the Aging of the Brown Adipose Tissue by 18F-FDG PET/CT Imaging in the Progeria Mouse Model Lmna-/

Brown adipose tissue (BAT) is an important energy metabolic organ that is highly implicated in obesity, type 2 diabetes, and atherosclerosis. Aging is one of the most important determinants of BAT activity. In this study, we used ¹⁸F-FDG PET/CT imaging to assess BAT aging in Lmna^−/− mice. The maximum standardized uptake value (SUV_Max) of the BAT was measured, and the target/nontarget (T/NT) values of BAT were calculated. The transcription and the protein expression levels of the uncoupling protein 1 (UCP1), beta3-adrenergic receptor (β3-AR), and the PR domain-containing 16 (PRDM16) were measured by quantitative real-time polymerase chain reaction (RT-PCR) and Western blotting or immunohistochemical analysis. Apoptosis and cell senescence rates in the BAT of WT and Lmna^−/− mice were determined by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and by CDKN2A/p16INK4a immunohistochemical staining, respectively. At 14 weeks of age, the BAT SUV_Max and the expression levels of UCP1, β3-AR, and PRDM16 in Lmna^−/− mice were significantly reduced relative to WT mice. At the same time, the number of p16INK4a and TUNEL positively stained cells (%) increased in Lmna^−/− mice. Collectively, our results indicate that the aging characteristics and the aging regulatory mechanism in the BAT of Lmna^−/− mice can mimic the normal BAT aging process.

Multimodal imaging approach to monitor browning of adipose tissue in vivo

The discovery that white adipocytes can undergo a browning process to become metabolically active beige cells has attracted significant interest in the fight against obesity. However, the study of adipose browning has been impeded by a lack of imaging tools that allow longitudinal and noninvasive monitoring of this process in vivo. Here, we report a preclinical imaging approach to detect development of beige adipocytes during adrenergic stimulation. In this approach, we expressed near-infrared fluorescent protein, iRFP720, driven under an uncoupling protein-1 (Ucp1) promoter in mice by viral transduction, and used multispectral optoacoustic imaging technology with ultrasound tomography (MSOT-US) to assess adipose beiging during adrenergic stimulation. We observed increased photoacoustic signal at 720 nm, coupled with attenuated lipid signals in stimulated animals. As a proof of concept, we validated our approach against hybrid positron emission tomography combined with magnetic resonance (PET/MR) imaging modality, and quantified the extent of adipose browning by MRI-guided segmentation of 2-deoxy-2-18F-fluoro-d-glucose uptake signals. The browning extent detected by MSOT-US and PET/MR are well correlated with Ucp1 induction. Taken together, these systems offer great opportunities for preclinical screening aimed at identifying compounds that promote adipose browning and translation of these discoveries into clinical studies of humans.

Gene Regulation and Targeted Therapy in Gastric Cancer Peritoneal Metastasis: Radiological Findings from Dual Energy CT and PET/CT

Gastric cancer remains fourth in cancer incidence worldwide with a five-year survival of only 20%-30%. Peritoneal metastasis is the most frequent type of metastasis that accompanies unresectable gastric cancer and is a definitive determinant of prognosis. Preventing and controlling the development of peritoneal metastasis could play a role in helping to prolong the survival of gastric cancer patients. A non-invasive and efficient imaging technique will help us to identify the invasion and metastasis process of peritoneal metastasis and to monitor the changes in tumor nodules in response to treatments. This will enable us to obtain an accurate description of the development process and molecular mechanisms of gastric cancer. We have recently described experiment using dual energy CT (DECT) and positron emission tomography/computed tomography (PET/CT) platforms for the detection and monitoring of gastric tumor metastasis in nude mice models. We have shown that weekly continuous monitoring with DECT and PET/CT can identify dynamic changes in peritoneal metastasis. The sFRP1-overexpression in gastric cancer mice models showed positive radiological performance, a higher FDG uptake and increasing enhancement, and the SUVmax (standardized uptake value) of nodules demonstrated an obvious alteration trend in response to targeted therapy of TGF-β1 inhibitor. In this article, we described the detailed non-invasive imaging procedures to conduct more complex research on gastric cancer peritoneal metastasis using animal models and provided representative imaging results. The use of non-invasive imaging techniques should enable us to better understand the mechanisms of tumorigenesis, monitor tumor growth, and evaluate the effect of therapeutic interventions for gastric cancer.

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

[¹⁸F]Fluorodeoxyglucose-PET/CT (¹⁸F-FDG-PET/CT) imaging has been invaluable for visualizing metabolically active adipose tissues in humans with potential anti-diabetic and anti-obesity effects. To explore whether mice display human-like fat depots in anatomically comparable regions, we mapped fat depots using glucose or fatty acid imaging tracers, such as ¹⁸F-FDG through PET/CT or [^123/125I]-β-methyl-p-iodophenyl-pentadecanoic acid with SPECT/CT imaging, to analogous depots in mice. Using this type of image analysis with both probes, we define a large number of additional areas of high metabolic activity corresponding to novel fat pads. Histological and gene expression analyses validate these regions as bona fide fat pads. Our findings indicate that fat depots of rodents show a high degree of topological similarity to those of humans. Studies involving both glucose and lipid tracers indicate differential preferences for these substrates in different depots and also suggest that fatty acid-based visualized approaches may reveal additional brown adipose tissue and beige depots in humans.

Loss of ADAMTS5 enhances brown adipose tissue mass and promotes browning of white adipose tissue via CREB signaling

Objective

A potential strategy to treat obesity – and the associated metabolic consequences – is to increase energy expenditure. This could be achieved by stimulating thermogenesis through activation of brown adipose tissue (BAT) and/or the induction of browning of white adipose tissue (WAT). Over the last years, it has become clear that several metalloproteinases play an important role in adipocyte biology. Here, we investigated the potential role of ADAMTS5.

Methods

Mice deficient in ADAMTS5 (Adamts5^−/−) and wild-type (Adamts5^+/+) littermates were kept on a standard of Western-type diet for 15 weeks. Energy expenditure and heat production was followed by indirect calorimetry. To activate thermogenesis, mice were treated with the β3-adrenergic receptor (β₃-AR) agonist CL-316,243 or alternatively, exposed to cold for 2 weeks.

Results

Compared to Adamts5^+/+ mice, Adamts5^−/− mice have significantly more interscapular BAT and marked browning of their subcutaneous (SC) WAT. Thermogenic pathway analysis indicated, in the absence of ADAMTS5, enhanced β₃-AR signaling via activation of the cAMP response element-binding protein (CREB). Additional β₃-AR stimulation with CL-316,243 promoted browning of WAT in Adamts5^+/+ mice but had no additive effect in Adamts5^−/− mice. However, cold exposure induced more pronounced browning of WAT in Adamts5^−/− mice.

Conclusions

These data indicate that ADAMTS5 plays a functional role in development of BAT and browning of WAT. Hence, selective targeting of ADAMTS5 could provide a novel therapeutic strategy for treatment/prevention of obesity and metabolic diseases.

•

Mice deficient in ADAMTS5 have elevated interscapular brown adipose tissue mass.

•

ADAMTS5 deficient mice show increased browning of their white adipose tissue.

•

The thermogenic profile is enhanced via adrenergic signaling and CREB activation.

•

ADAMTS5 seems an attractive therapeutic target for metabolic diseases.

Determining Glucose Metabolism Kinetics Using 18F-FDG Micro-PET/CT

This paper describes the use of ¹⁸F-FDG and micro-PET/CT imaging to determine in vivo glucose metabolism kinetics in mice (and is transferable to rats). Impaired uptake and metabolism of glucose in multiple organ systems due to insulin resistance is a hallmark of type 2 diabetes. The ability of this technique to extract an image-derived input function from the vena cava using an iterative deconvolution method eliminates the requirement of the collection of arterial blood samples. Fitting of tissue and vena cava time activity curves to a two-tissue, three compartment model permits the estimation of kinetic micro-parameters related to the ¹⁸F-FDG uptake from the plasma to the intracellular space, the rate of transport from intracellular space to plasma and the rate of ¹⁸F-FDG phosphorylation. This methodology allows for multiple measures of glucose uptake and metabolism kinetics in the context of longitudinal studies and also provides insights into the efficacy of therapeutic interventions.

Diffuse Optical Spectroscopy and Imaging to Detect and Quantify Adipose Tissue Browning

Adipose (fat) tissue is a complex metabolic organ that is highly active and essential. In contrast to white adipose tissue (WAT), brown adipose tissue (BAT) is deemed metabolically beneficial because of its ability to burn calories through heat production. The conversion of WAT-resident adipocytes to “beige” or “brown-like” adipocytes has recently attracted attention. However, it typically takes a few days to analyze and confirm this browning of WAT through conventional molecular, biochemical, or histological methods. Moreover, accurate quantification of the overall browning process is not possible by any of these methods. In this context, we report the novel application of diffuse reflectance spectroscopy (DRS) and multispectral imaging (MSI) to detect and quantify the browning process in mice. We successfully demonstrated the time-dependent increase in browning of WAT, following its induction through β-adrenergic agonist injections. The results from these optical techniques were confirmed with those of standard molecular and biochemical assays, which measure gene and protein expression levels of UCP1 and PGC-1α, as well as with histological examinations. We envision that the reported optical methods can be developed into a fast, real time, cost effective and easy to implement imaging approach for quantification of the browning process in adipose tissue.

Preclinical In vivo Imaging for Fat Tissue Identification, Quantification, and Functional Characterization

Localization, differentiation, and quantitative assessment of fat tissues have always collected the interest of researchers. Nowadays, these topics are even more relevant as obesity (the excess of fat tissue) is considered a real pathology requiring in some cases pharmacological and surgical approaches. Several weight loss medications, acting either on the metabolism or on the central nervous system, are currently under preclinical or clinical investigation. Animal models of obesity have been developed and are widely used in pharmaceutical research. The assessment of candidate drugs in animal models requires non-invasive methods for longitudinal assessment of efficacy, the main outcome being the amount of body fat. Fat tissues can be either quantified in the entire animal or localized and measured in selected organs/regions of the body. Fat tissues are characterized by peculiar contrast in several imaging modalities as for example Magnetic Resonance Imaging (MRI) that can distinguish between fat and water protons thank to their different magnetic resonance properties. Since fat tissues have higher carbon/hydrogen content than other soft tissues and bones, they can be easily assessed by Computed Tomography (CT) as well. Interestingly, MRI also discriminates between white and brown adipose tissue (BAT); the latter has long been regarded as a potential target for anti-obesity drugs because of its ability to enhance energy consumption through increased thermogenesis. Positron Emission Tomography (PET) performed with 18F-FDG as glucose analog radiotracer reflects well the metabolic rate in body tissues and consequently is the technique of choice for studies of BAT metabolism. This review will focus on the main, non-invasive imaging techniques (MRI, CT, and PET) that are fundamental for the assessment, quantification and functional characterization of fat deposits in small laboratory animals. The contribution of optical techniques, which are currently regarded with increasing interest, will be also briefly described. For each technique the physical principles of signal detection will be overviewed and some relevant studies will be summarized. Far from being exhaustive, this review has the purpose to highlight some strategies that can be adopted for the in vivo identification, quantification, and functional characterization of adipose tissues mainly from the point of view of biophysics and physiology.

Regulation of systemic energy homeostasis by serotonin in adipose tissues

Central serotonin (5-HT) is an anorexigenic neurotransmitter in the brain. However, accumulating evidence suggests peripheral 5-HT may affect organismal energy homeostasis. Here we show 5-HT regulates white and brown adipose tissue function. Pharmacological inhibition of 5-HT synthesis leads to inhibition of lipogenesis in epididymal white adipose tissue (WAT), induction of browning in inguinal WAT and activation of adaptive thermogenesis in brown adipose tissue (BAT). Mice with inducible Tph1 KO in adipose tissues exhibit a similar phenotype as mice in which 5-HT synthesis is inhibited pharmacologically, suggesting 5-HT has localized effects on adipose tissues. In addition, Htr3a KO mice exhibit increased energy expenditure and reduced weight gain when fed a high-fat diet. Treatment with an Htr2a antagonist reduces lipid accumulation in 3T3-L1 adipocytes. These data suggest important roles for adipocyte-derived 5-HT in controlling energy homeostasis.

Fluorescence Imaging of Interscapular Brown Adipose Tissue in Living Mice

Brown adipose tissue (BAT) plays a key role in energy expenditure and heat generation and is a promising target for diagnosing and treating obesity, diabetes and related metabolism disorders. While several nuclear and magnetic resonance imaging methods are established for detecting human BAT, there are no convenient protocols for high throughput imaging of BAT in small animal models. Here we disclose a simple but effective method for non-invasive optical imaging of interscapular BAT in mice using a micellar formulation of the commercially available deep-red fluorescent probe, SRFluor680. Whole-body fluorescence imaging of living mice shows extensive accumulation of the fluorescent probe in the interscapular BAT and ex vivo analysis shows 3.5-fold selectivity for interscapular BAT over interscapular WAT. Additional imaging studies indicate that SRFluor680 uptake is independent of mouse species and BAT metabolic state. The results are consistent with an unusual pharmacokinetic process that involves irreversible translocation of the lipophilic SRFluor680 from the micelle nanocarrier into the adipocytes within the BAT. Multimodal PET/CT and planar fluorescence/X-ray imaging of the same living animal shows co-localization of BAT mass signal reported by the fluorescent probe and BAT metabolism signal reported by the PET agent, ¹⁸F-FDG. The results indicate a path towards a new, dual probe molecular imaging paradigm that allows separate and independent non-invasive visualization of BAT mass and BAT metabolism in a living subject.

Responses of the insulin signaling pathways in the brown adipose tissue of rats following cold exposure

The insulin signaling pathway is critical for the control of blood glucose levels. Brown adipose tissue (BAT) has also been implicated as important in glucose homeostasis. The effect of short-term cold exposure on this pathway in BAT has not been explored. We evaluated the effect of 4 hours of cold exposure on the insulin pathway in the BAT of rats. Whole genomic microarray chips were used to examine the transcripts of the pathway in BAT of rats exposed to 4°C and 22°C for 4 hours. The 4 most significantly altered pathways following 4 hours of cold exposure were the insulin signaling pathway, protein kinase A, PI3K/AKT and ERK/MAPK signaling. The insulin signaling pathway was the most affected. In the documented 142 genes of the insulin pathway, 42 transcripts (29.6%) responded significantly to this cold exposure with the least false discovery rate (Benjamini-Hochberg Multiple Testing: −log10 (p-value)  = 7.18). Twenty-seven genes (64%) were up-regulated, including the insulin receptor (Insr), insulin substrates 1 and 2 (Irs1 and Irs2). Fifteen transcripts (36%) were down-regulated. Multiple transcripts of the primary target and secondary effector targets for the insulin signaling were also up-regulated, including those for carbohydrate metabolism. Using western blotting, we demonstrated that the cold induced higher Irs2, Irs1, and Akt-p protein levels in the BAT than in the BAT of controls maintained at room temperature, and higher Akt-p protein level in the muscle. Conclusion: this study demonstrated that 4 hours of cold exposure stimulated the insulin signaling pathway in the BAT and muscle of overnight fasted rats. This raises the possibility that acute cold stimulation may have potential to improve glucose clearance and insulin sensitivity.

A standardized infrared imaging technique that specifically detects UCP1-mediated thermogenesis in vivo

The activation and expansion of brown adipose tissue (BAT) has emerged as a promising strategy to counter obesity and the metabolic syndrome by increasing energy expenditure. The subsequent testing and validation of novel agents that augment BAT necessitates accurate pre-clinical measurements in rodents regarding the capacity for BAT-derived thermogenesis. We present a novel method to measure BAT thermogenesis using infrared imaging following β₃-adrenoreceptor stimulation in mice. We show that the increased body surface temperature observed using this method is due solely to uncoupling protein-1 (UCP1)-mediated thermogenesis and that this technique is able to discern differences in BAT activity in mice acclimated to 23 °C or thermoneutrality (30 °C). These findings represent the first standardized method utilizing infrared imaging to specifically detect UCP1 activity in vivo.

Adrenergic pathway activation enhances brown adipose tissue metabolism: a [¹⁸F]FDG PET/CT study in mice

OBJECTIVE

Pharmacologic approaches to study brown adipocyte activation in vivo with a potential of being translational to humans are desired. The aim of this study was to examine pre- and postsynaptic targeting of adrenergic system for enhancing brown adipose tissue (BAT) metabolism quantifiable by [(18)F]fluoro-2-deoxyglucose ([(18)F]FDG) positron emission tomography (PET)/computed tomography (CT) in mice.

METHODS

A β₃-adrenoreceptor selective agonist (CL 316243), an adenylyl cyclase enzyme activator (forskolin) and a potent blocker of presynaptic norepinephrine transporter (atomoxetine), were injected through the tail vein of Swiss Webster mice 30 minutes before intravenous (iv) administration of [(18)F]FDG. The mice were placed on the PET/CT bed for 30 min PET acquisition followed by 10 min CT acquisition for attenuation correction and anatomical delineation of PET images.

RESULTS

Activated interscapular (IBAT), cervical, periaortic and intercostal BAT were observed in 3-dimentional analysis of [(18)F]FDG PET images. CL 316243 increased the total [(18)F]FDG standard uptake value (SUV) of IBAT 5-fold greater compared to that in placebo-treated mice. It also increased the [(18)F]FDG SUV of white adipose tissue (2.4-fold), and muscle (2.7-fold), as compared to the control. There was no significant difference in heart, brain, spleen and liver uptakes between groups. Forskolin increased [(18)F]FDG SUV of IBAT 1.9-fold greater than that in placebo-treated mice. It also increased the [(18)F]FDG SUV of white adipose tissue (2.2-fold) and heart (5.4-fold) compared to control. There was no significant difference in muscle, brain, spleen, and liver uptakes between groups. Atomoxetine increased [(18)F]FDG SUV of IBAT 1.7-fold greater than that in placebo-treated mice. There were no significant differences in all other organs compared to placebo-treated mice except liver (1.6 fold increase). A positive correlation between SUV levels of IBAT and CT Hounsfield unit (HU) (R(2)=0.55, p<0.001) and between CT HU levels of IBAT and liver (R(2)=0.69, p<0.006) was observed.

CONCLUSIONS

The three pharmacologic approaches reported here enhanced BAT metabolism by targeting different sites in adrenergic system as measured by [(18)F]FDG PET/CT.

High-fat feeding-induced hyperinsulinemia increases cardiac glucose uptake and mitochondrial function despite peripheral insulin resistance

In obesity, reduced cardiac glucose uptake and mitochondrial abnormalities are putative causes of cardiac dysfunction. However, high-fat diet (HFD) does not consistently induce cardiac insulin resistance and mitochondrial damage, and recent studies suggest HFD may be cardioprotective. To determine cardiac responses to HFD, we investigated cardiac function, glucose uptake, and mitochondrial respiration in young (3-month-old) and middle-aged (MA) (12-month-old) male Ldlr(-/-) mice fed chow or 3 months HFD to induce obesity, systemic insulin resistance, and hyperinsulinemia. In MA Ldlr(-/-) mice, HFD induced accelerated atherosclerosis and nonalcoholic steatohepatitis, common complications of human obesity. Surprisingly, HFD-fed mice demonstrated increased cardiac glucose uptake, which was most prominent in MA mice, in the absence of cardiac contractile dysfunction or hypertrophy. Moreover, hearts of HFD-fed mice had enhanced mitochondrial oxidation of palmitoyl carnitine, glutamate, and succinate and greater basal insulin signaling compared with those of chow-fed mice, suggesting cardiac insulin sensitivity was maintained, despite systemic insulin resistance. Streptozotocin-induced ablation of insulin production markedly reduced cardiac glucose uptake and mitochondrial dysfunction in HFD-fed, but not in chow-fed, mice. Insulin injection reversed these effects, suggesting that insulin may protect cardiac mitochondria during HFD. These results have implications for cardiac metabolism and preservation of mitochondrial function in obesity.