Featured ArticlesVolume 27 | Sept. 2019
|Adipocyte Gs signaling regulates glucose homeostasisG protein coupled receptors (GPCRs) expressed in white and brown adipose tissue have pleiotropic effects on energy and glucose homeostasis. There are four main families of G proteins: Gi/Go, Gq, Gs, and G12. In rodents, most of the positive effects of the sympathetic nervous system on adipose tissue are attributed to Gs-coupled beta 3 adrenergic receptors located at the surface of adipocytes. Caron et al. used Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), chemogenetically-engineered proteins that allow spatial and temporal control of G protein signaling in vivo. They report that Gs, but not Gi signaling in adipocytes is a potent regulator of systemic glucose homeostasis.
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Objective: The sympathetic nervous system (SNS) is a key regulator of the metabolic and endocrine functions of adipose tissue. Increased SNS outflow promotes fat mobilization, stimulates non-shivering thermogenesis, promotes browning, and inhibits leptin production. Most of these effects are attributed to norepinephrine activation of the Gs-coupled beta adrenergic receptors located on the surface of the adipocytes. Evidence suggests that other adrenergic receptor subtypes, including the Gi-coupled alpha 2 adrenergic receptors might also mediate the SNS effects on adipose tissue. However, the impact of acute stimulation of adipocyte Gs and Gi has never been reported.
Methods: We harness the power of chemogenetics to develop unique mouse models allowing the specific and spatiotemporal stimulation of adipose tissue Gi and Gs signaling. We evaluated the impact of chemogenetic stimulation of these pathways on glucose homeostasis, lipolysis, leptin production, and gene expression.
Results: Stimulation of Gs signaling in adipocytes induced rapid and sustained hypoglycemia. These hypoglycemic effects were secondary to increased insulin release, likely consequent to increased lipolysis. Notably, we also observed differences in gene regulation and ex vivo lipolysis in different adipose depots. In contrast, acute stimulation of Gi signaling in adipose tissue did not affect glucose metabolism or lipolysis, but regulated leptin production.
Conclusion: Our data highlight the significance of adipose Gs signaling in regulating systemic glucose homeostasis. We also found previously unappreciated heterogeneity across adipose depots following acute stimulation. Together, these results highlight the complex interactions of GPCR signaling in adipose tissue and demonstrate the usefulness of chemogenetic technology to better understand adipocyte function.[Hide abstract]
|Impact of dietary sucrose depends on mode of ingestion: liquid or solid The causes for the current obesity epidemic are unclear. One possible risk factor is a high consumption of sucrose, which may be in the form of sugar-sweetened beverages (SSB). Togo and colleagues studied the impact of high sugar diets with different forms of delivery: solid, pelleted sucrose or sucrose dissolved in the drinking water. They found that liquid sucrose exposure contributed to higher energy consumption leading to greater body weight and body fat. Mice exposed to equivalent levels of sucrose in the solid diet were leaner and metabolically healthier than their counterparts exposed to liquid sucrose. These results strongly suggest that the consumption of SSB is an important factor that can lead to obesity and metabolic disease.|
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Objective: Although it is widely accepted that obesity results from an imbalance of energy intake and expenditure, the mechanisms underlying this process and effective strategies for prevention and treatment are unclear. Growing evidence suggests excess consumption of sugar may play an important role, yet we showed previously in mice that consuming up to 30% of calories as sucrose in the diet had no impact on weight regulation. Since in humans consumption of sugar-sweetened beverages has been widely implicated, we investigated whether the mode of ingestion (solid or liquid) had different impacts on body weight regulation and glucose homeostasis.
Methods: Dietary sucrose was delivered in solid (as part of a standard pelleted rodent chow) and liquid (in drinking water) to C57BL/6 mice for 8 weeks. Body weight, body composition, energy intake and expenditure were monitored, as well as glucose and insulin tolerance tests. Expression of sweet taste receptors on the tongue, and glycogen and fat contents of the liver were also measured.
Results: Consumption of sucrose-sweetened water, but not equivalent levels of solid sucrose, led to body fat gain in C57BL/6 mice. Glucose intolerance was positively correlated to body fatness, rather than sucrose intake.
Conclusions: Our data support the suggestion that consumption of liquid sucrose may be an important contributor to dysregulation of body weight and related metabolic syndromes.[Hide abstract]
|Phosphatidylserine decarboxylase is critical for skeletal muscle maintenance Phosphatidylethanolamine (PE) is the second most abundant phospholipid in mammals. The majority of PE is synthesized by two distinct pathways: the CDP-ethanolamine branch of the Kennedy pathway and decarboxylation of phosphatidylserine (PS) by the enzyme PS decarboxylase (PSD) located in the mitochondria. The PSD pathway is required to maintain mitochondrial structure and function and essential for life. Selathurai et al. studied the PSD pathway in muscle and found that it is important for PE synthesis in skeletal muscle and is required for maintenance of mitochondrial integrity and muscle mass. This reveals the role of mitochondria in maintaining skeletal muscle homeostasis.|
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Objective: Phosphatidylethanolamine (PtdEtn) is a major phospholipid in mammals. It is synthesized via two pathways, the CDP-ethanolamine pathway in the endoplasmic reticulum and the phosphatidylserine (PtdSer) decarboxylase (PSD) pathway in the mitochondria. While the CDP-ethanolamine pathway is considered the major route for PtdEtn synthesis in most mammalian tissues, little is known about the importance of the PSD pathway in vivo, especially in tissues enriched with mitochondria such as skeletal muscle. Therefore, we aimed to examine the role of the mitochondrial PSD pathway in regulating PtdEtn homeostasis in skeletal muscle in vivo.
Methods: To determine the functional significance of this pathway in skeletal muscle in vivo, an adeno-associated viral vector approach was employed to knockdown PSD expression in skeletal muscle of adult mice. Muscle lipid and metabolite profiling was performed using mass spectrometry.
Results: PSD knockdown disrupted muscle phospholipid homeostasis leading to an ∼25% reduction in PtdEtn and an ∼45% increase in PtdSer content. This was accompanied by the development of a severe myopathy, evident by a 40% loss in muscle mass as well as extensive myofiber damage as shown by increased DNA synthesis and central nucleation. In addition, PSD knockdown caused marked accumulation of abnormally appearing mitochondria that exhibited severely disrupted inner membrane integrity and reduced OXPHOS protein content.
Conclusions: The PSD pathway has a significant role in maintaining phospholipid homeostasis in adult skeletal muscle. Moreover, PSD is essential for maintenance of mitochondrial integrity and skeletal muscle mass.[Hide abstract]
|Evaporative cooling provides a major metabolic energy sink Thermogenesis could be harnessed to prevent obesity, which makes it important to understand sources of heat loss to maintain normothermia. Trans-epidermal water loss (TEWL) through perspiration is a cooling mechanism for over-heated mammalian bodies, but also occurs at low ambient temperatures. Kasza and colleagues calculated the energy dissipation created by evaporative cooling through mouse skin and show that increased rates of evaporative cooling account for increased energy expenditure reported in mouse strains with deficient lipid layers. They also show that obese mice show lower trans-epidermal water loss. This raises the possibility that evaporative cooling could be a significant player in energy expenditure to avoid obesity.|
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Objective: Elimination of food calories as heat could help redress the excess accumulation of metabolic energy exhibited as obesity. Prior studies have focused on the induction of thermogenesis in beige and brown adipose tissues as the application of this principle, particularly because the β-adrenergic environment associated with thermogenic activation has been shown to have positive health implications. The counterpoint to this strategy is the regulation of heat loss; we propose that mammals with inefficient heat conservation will require more thermogenesis to maintain body temperature.
Methods: Surface temperature thermography and rates of trans-epidermal water loss were integrated to profile the total heat transfer of genetically-engineered and genetically variable mice.
Results: These data were incorporated with energy expenditure data to generate a biophysical profile to test the significance of increased rates of evaporative cooling.
Conclusions: We show that mouse skins vary considerably in their heat retention properties, whether because of naturally occurring variation (SKH-1 mice), or genetic modification of the heat-retaining lipid lamellae (SCD1, DGAT1 or Agouti Ay obese mice). In particular, we turn attention to widely different rates of evaporative cooling as the result of trans-epidermal water loss; higher rates of heat loss by evaporative cooling leads to increased demand for thermogenesis. We speculate that this physiology could be harnessed to create an energy sink to assist with strategies aimed at treating metabolic diseases.[Hide abstract]
|GDF10 blocks PPARγ activation to protect against liver injury Growth differentiation factor 10 (GDF10) is an atypical member of the transforming growth factor β superfamily. It has been shown that that in vitro knockdown of GDF10 enhances adipogenesis and that transgenic mice overexpressing GDF10 are protected against diet-induced obesity and insulin resistance. Platko, Lebeau, et al. studied the impact of GDF10 on the liver. Their findings suggest that circulating GDF10 plays a critical role as a regulator of hepatic PPARγ during conditions of dietary stress and that GDF10 is capable of attenuating the progression of steatosis to steatohepatitis.|
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Objective: Growth differentiation factors (GDFs) and bone-morphogenic proteins (BMPs) are members of the transforming growth factor β (TGFβ) superfamily and are known to play a central role in the growth and differentiation of developing tissues. Accumulating evidence, however, demonstrates that many of these factors, such as BMP-2 and -4, as well as GDF15, also regulate lipid metabolism. GDF10 is a divergent member of the TGFβ superfamily with a unique structure and is abundantly expressed in brain and adipose tissue; it is also secreted by the latter into the circulation. Although previous studies have demonstrated that overexpression of GDF10 reduces adiposity in mice, the role of circulating GDF10 on other tissues known to regulate lipid, like the liver, has not yet been examined.
Methods: Accordingly, GDF10−/− mice and age-matched GDF10+/+ control mice were fed either normal control diet (NCD) or high-fat diet (HFD) for 12 weeks and examined for changes in liver lipid homeostasis. Additional studies were also carried out in primary and immortalized human hepatocytes treated with recombinant human (rh)GDF10.
Results: Here, we show that circulating GDF10 levels are increased in conditions of diet-induced hepatic steatosis and, in turn, that secreted GDF10 can prevent excessive lipid accumulation in hepatocytes. We also report that GDF10−/− mice develop an obese phenotype as well as increased liver triglyceride accumulation when fed a NCD. Furthermore, HFD-fed GDF10−/− mice develop increased steatosis, endoplasmic reticulum (ER) stress, fibrosis, and injury of the liver compared to HFD-fed GDF10+/+ mice. To explain these observations, studies in cultured hepatocytes led to the observation that GDF10 attenuates nuclear peroxisome proliferator-activated receptor γ (PPARγ) activity; a transcription factor known to induce de novo lipogenesis.
Conclusions: Our work delineates a hepatoprotective role of GDF10 as an adipokine capable of regulating hepatic lipid levels by blocking de novo lipogenesis to protect against ER stress and liver injury.[Hide abstract]
|ATGL-1 mediates longevity effects in C. elegans
Animal lifespan is increased by fasting and a reduction of insulin/IGF signaling (IIS), an effect that is conserved from nematodes to humans. Two major "signaling nodes", Forkhead box O1 (FoxO1)- and Target Of Rapamycin Complex 1 (TORC1)-centered, are responsible for the effect of nutrients and IIS on the lifespan. Zaarur et al. recently found that FoxO1 and mTORC1 control the rates of lipolysis in mammalian cells by regulating expression of adipose triglyceride lipase (ATGL). Here, they use the nematode C. elegans to show that ATGL is the common mediator of the fasting and IIS longevity effects.|
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Objective: Animal lifespan is controlled through genetic pathways that are conserved from nematodes to humans. Lifespan-promoting conditions in nematodes include fasting and a reduction of insulin/IGF signaling. Here we aimed to investigate the input of the Caenorhabditis elegans homologue of the mammalian rate-limiting lipolytic enzyme Adipose Triglyceride Lipase, ATGL-1, in longevity control.
Methods: We used a combination of genetic and biochemical approaches to determine the role of ATGL-1 in accumulation of triglycerides and regulation of longevity.
Results: We found that expression of ATGL is increased in the insulin receptor homologue mutant daf-2 in a FoxO/DAF-16-dependent manner. ATGL-1 is also up-regulated by fasting and in the eat-2 loss-of-function mutant strain. Overexpression of ATGL-1 increases basal and maximal oxygen consumption rate and extends lifespan in C. elegans. Reduction of ATGL-1 function suppresses longevity of the long-lived mutants eat-2 and daf-2.
Conclusions: Our results demonstrate that ATGL is required for extended lifespan downstream of both dietary restriction and reduced insulin/IGF signaling.[Hide abstract]
|Gs signaling in adipocytes causes striking metabolic improvements Like most other cell types, adipocytes express many G protein-coupled receptors (GPCRs). Each GPCR displays a distinct G protein coupling preference, activating either Gs- Gi-, or Gq-type G proteins which are linked to specific signaling pathways. At present, little is known about how activation of these various GPCR/G protein pathways affects glucose homeostasis. Wang et al. used Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) that can only be activated by an exogenously administered drug to elucidate the impact of Gs signaling in adipocytes. In agreement with the report by Caron et al., they found improved glucose homeostasis upon Gs activation.|
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Objective: Given the worldwide epidemics of obesity and type 2 diabetes, novel antidiabetic and appetite-suppressing drugs are urgently needed. Adipocytes play a central role in the regulation of whole-body glucose and energy homeostasis. The goal of this study was to examine the metabolic effects of acute and chronic activation of Gs signaling selectively in adipocytes (activated Gs stimulates cAMP production), both in lean and obese mice.
Methods: To address this question, we generated a novel mutant mouse strain (adipo-GsD mice) that expressed a Gs-coupled designer G protein-coupled receptor (Gs DREADD or short GsD) selectively in adipocytes. Importantly, the GsD receptor can only be activated by administration of an exogenous agent (CNO) that is otherwise pharmacologically inert. The adipo-GsD mice were maintained on either regular chow or a high-fat diet and then subjected to a comprehensive series of metabolic tests.
Results: Pharmacological (CNO) activation of the GsD receptor in adipocytes of adipo-GsD mice caused profound improvements in glucose homeostasis and protected mice against the metabolic deficits associated with the consumption of a calorie-rich diet. Moreover, chronic activation of Gs signaling in adipocytes led to a striking increase in energy expenditure and reduced food intake, resulting in a decrease in body weight and fat mass when mice consumed a calorie-rich diet.
Conclusion: Systematic studies with a newly developed mouse model enabled us to assess the metabolic consequences caused by acute or chronic activation of Gs signaling selectively in adipocytes. Most strikingly, chronic activation of this pathway led to reduced body fat mass and restored normal glucose homeostasis in obese mice. These findings are of considerable relevance for the development of novel antidiabetic and anti-obesity drugs.[Hide abstract]