Featured ArticlesVolume 5 | No. 6 | June 2016
|PI3K p110β subunit is required for the acute hypophagia induced by endotoxemia
Systemic inflammation triggered by bacterial endotoxin is characterized by increased cytokines, altered energy balance via suppression of food consumption, and body weight loss. Borges and colleagues show that bacterial lipopolysaccharide (LPS) stimulates phosphoinositide 3-kinase (PI3K) and signal transducer and activator of transcription 3 (STAT3) signalling pathways in hypothalamic leptin receptor (LepR) expressing cells. Central PI3K inhibition or double deletion of PI3K p110α and p110β in LepR cells prevent LPS-induced hypophagia.
Abstract | PDF
Objective: Hypophagia and increased energy expenditure under inflammatory conditions, such as that observed after bacterial lipopolysaccharide (LPS) administration, are associated with leptin secretion. The hypophagic effect of leptin depends in part on the activation of PI3K signaling pathway. However, the role of PI3K in the endotoxemia-induced hypophagia has not been determined.
Methods: In an attempt to examine the functional contribution of the PI3K pathway in hypophagia and weight loss induced by LPS (100 ug/Kg, ip), we performed a central pharmacological PI3K inhibition (LY294002). Additionally, to gain mechanistic insights on the role of the catalytic PI3K p110α subunit in leptin responsive cells, mice expressing Cre-recombinase driven by the Lepr promoter (LepR-Cre) were crossed with mice carrying a loxP-modified p110α allele (Pi3kca gene) (LepRΔp110α). As studies have suggested that the PI3K p110β subunit has a dominant role over p110α in energy homeostasis, we further crossed LepR-Cre mice with loxP-modified p110α and p110β (Pi3kcb gene) alleles (LepRΔp110α+β). In order to verify the requirement of leptin in PI3K effects on food intake, we also used leptin-deficient ob/ob mice.
Results: We found that LPS stimulates PI3K and STAT3 signaling pathways in cells expressing the leptin receptor. Central PI3K inhibition prevented LPS-induced hypophagia and weight loss. Genetic deletion of p110α subunit selectively in LepR cells had no effect on LPS-induced hypophagia and weight loss. However, p110α and p110β double deletion in LepR cells prevented LPS-induced hypophagia and partially reversed the weight loss. Leptin deficiency blunted LPS-induced acute pAKT and pSTAT3 phosphorylation and the acute suppression of food intake.
Conclusions: Our studies show that the PI3K p110β subunit in LepR cells is required for acute endotoxemic hypophagia. The data provide promising approaches for PI3K inhibition in preventing low energy balance and cachectic states during inflammatory challenges.[Hide abstract]
|Triggering the adaptive immune system with gut bacteria protects against insulin resistanceIt has been shown before that changes in the ecology of gut bacteria characterize metabolic disease. Pomié et al. demonstrate that targeting the adaptive immune system by specific luminal ileum microbiota extracts allows the intestinal immune system to control gut microbiota most likely through the activation of CD4 T cells and the production of IgA. This mechanism is largely documented where a change in gut microbiota, by the mean of antibiotics or prebiotics and by germ free mouse colonization, is responsible for the glycemic control. The authors propose that the immunization process impacts the glycemic control by a mechanism involving the adaptive intestinal immune system.|
Abstract | PDF
Objective: To demonstrate that glycemia and insulin resistance are controlled by a mechanism involving the adaptive immune system and gut microbiota crosstalk.
Methods: We triggered the immune system with microbial extracts specifically from the intestinal ileum contents of HFD-diabetic mice by the process of immunization. 35 days later, immunized mice were fed a HFD for up to two months in order to challenge the development of metabolic features. The immune responses were quantified. Eventually, adoptive transfer of immune cells from the microbiota-immunized mice to naïve mice was performed to demonstrate the causality of the microbiota-stimulated adaptive immune system on the development of metabolic disease. The gut microbiota of the immunized HFD-fed mice was characterized in order to demonstrate whether the manipulation of the microbiota to immune system interaction reverses the causal deleterious effect of gut microbiota dysbiosis on metabolic disease.
Results: Subcutaneous injection (immunization procedure) of ileum microbial extracts prevented hyperglycemia and insulin resistance in a dose-dependent manner in response to a HFD. The immunization enhanced the proliferation of CD4 and CD8 T cells in lymphoid organs, also increased cytokine production and antibody secretion. As a mechanism explaining the metabolic improvement, the immunization procedure reversed gut microbiota dysbiosis. Finally, adoptive transfer of immune cells from immunized mice improved metabolic features in response to HFD.
Conclusions: Glycemia and insulin sensitivity can be regulated by triggering the adaptive immunity to microbiota interaction. This reduces the gut microbiota dysbiosis induced by a fat-enriched diet.[Hide abstract]
|A lipidomic screen of pancreatic β-cells defines novel features of glucose-stimulated turnover of neutral lipids, sphingolipids and plasmalogensAlthough it has been recognized for more than 40 years that β-cell lipids are acutely remodelled following glucose stimulation, these changes remain poorly defined. Pearson and colleagues apply mass spectrometry to conduct the first comprehensive and unbiased characterization of how glucose acutely regulates the turnover of phospholipids, sphingolipids, and neutral lipids in β-cells. Their results point to remodelling of all three lipid classes in response to glucose, which impacts the overall accumulation of separate diacylglycerol and monoacylglycerol species, phosphatidylcholine plasmalogens, and various sphingolipid metabolites. The findings shed new light on metabolic stimulus-secretion coupling in β-cells.
Abstract | PDF
Objective: Glucose promotes lipid remodelling in pancreatic β-cells, and this is thought to contribute to the regulation of insulin secretion, but the metabolic pathways and potential signalling intermediates have not been fully elaborated.
Methods: Using mass spectrometry (MS) we quantified changes in approximately 300 lipid metabolites in MIN6 β-cells and isolated mouse islets following 1 h stimulation with glucose. Flux through sphingolipid pathways was also assessed in 3H-sphinganine-labelled cells using TLC.
Results: Glucose specifically activates the conversion of triacylglycerol (TAG) to diacylglycerol (DAG). This leads indirectly to the formation of 18:1 monoacylglycerol (MAG), via degradation of saturated/monounsaturated DAG species, such as 16:0_18:1 DAG, which are the most abundant, immediate products of glucose-stimulated TAG hydrolysis. However, 16:0-containing, di-saturated DAG species are a better direct marker of TAG hydrolysis since, unlike the 18:1-containing DAGs, they are predominately formed via this route. Using multiple reaction monitoring, we confirmed that in islets under basal conditions, 18:1 MAG is the most abundant species. We further demonstrated a novel site of glucose to enhance the conversion of ceramide to sphingomyelin (SM) and galactosylceramide (GalCer). Flux and product:precursor analyses suggest regulation of the enzyme SM synthase, which would constitute a separate mechanism for localized generation of DAG in response to glucose. Phosphatidylcholine (PC) plasmalogen (P) species, specifically those containing 20:4, 22:5 and 22:6 side chains, were also diminished in the presence of glucose, whereas the more abundant phosphatidylethanolamine plasmalogens were unchanged.
Conclusions: Our results highlight 18:1 MAG, GalCer, PC(P) and DAG/SM as potential contributors to metabolic stimulus-secretion coupling.[Hide abstract]
|Role of mitochondrial uncoupling protein-2 (UCP2) in higher brain functions, neuronal plasticity and network oscillationMitochondria support energy demanding processes like neural transmission and synaptogenesis and are thus promising points of broadening interest in the energetics underlying the neurobiology of mental illness. Using an animal model of mitochondrial dysfunction, UCP2 KO, Hermes and colleagues explore responses to NMDA receptor blockade, synaptic density, auditory gating, and gamma power. These measures and markers relevant to the pathophysiology of mental illnesses, reveal a significant vulnerability in animals with UCP2 deficiency.|
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Background/Purpose: Major psychiatric illnesses, affecting 36% of the world's population, are profound disorders of thought, mood and behavior associated with underlying impairments in synaptic plasticity and cellular resilience. Mitochondria support energy demanding processes like neural transmission and synaptogenesis and are thus points of broadening interest in the energetics underlying the neurobiology of mental illness. These experiments interrogated the importance of mitochondrial flexibility in behavior, synaptic and cortical activity in a mouse model.
Methods: We studied mice with ablated uncoupling protein-2 expression (UCP2 KO) and analyzed cellular, circuit and behavioral attributes of higher brain regions.
Results: We found that mitochondrial impairment induced by UCP2 ablation produces an anxiety prone, cognitively impaired behavioral phenotype. Further, NMDA receptor blockade in the UCP2 KO mouse model resulted in changes in synaptic plasticity, brain oscillatory and sensory gating activities.
Conclusions: We conclude that disruptions in mitochondrial function may play a critical role in pathophysiology of mental illness. Specifically, we have shown that NMDA driven behavioral, synaptic, and brain oscillatory functions are impaired in UCP2 knockout mice.[Hide abstract]
|Raptor/mTORC1 loss in adipocytes causes lipodystrophy and fatty liver disease
White adipose tissue (WAT) functions both as the body’s major energy storage site, and as a critical endocrine tissue. Lack of adipose tissue or lipodystrophy associates with severe metabolic complications. Lee et al. investigate the in vivo role of mTORC1 in mature adipocytes by deleting Raptor with Adiponectin-Cre. They conclude that mTORC1 activity in white adipocytes is dispensable for early postnatal WAT growth but becomes essential for normal adipose tissue expansion with age. Adipocyte Raptor KO mice consuming a high fat diet are hyperphagic yet resistant to obesity. These mice appear to have a defect in adipose tissue expansion, which redistributes lipids to the liver resulting in severe hepatomegaly and hepatic steatosis and causes a dietary lipid absorption defect.
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Objective: Normal adipose tissue growth and function is critical to maintaining metabolic homeostasis and its excess (e.g. obesity) or absence (e.g. lipodystrophy) is associated with severe metabolic disease. The goal of this study was to understand the mechanisms maintaining healthy adipose tissue growth and function.
Methods: Adipose tissue senses and responds to systemic changes in growth factor and nutrient availability; in cells mTORC1 regulates metabolism in response to growth factors and nutrients. Thus, mTORC1 is poised to be a critical intracellular regulator of adipocyte metabolism. Here, we investigate the role of mTORC1 in mature adipocytes by generating and characterizing mice in which the Adiponectin-Cre driver is used to delete floxed alleles of Raptor, which encodes an essential regulatory subunit of mTORC1.
Results: RaptorAdipoq-cre mice have normal white adipose tissue (WAT) mass for the first few weeks of life, but soon thereafter develop lipodystrophy associated with hepatomegaly, hepatic steatosis, and insulin intolerance. RaptorAdipoq-cre mice are also resistant to becoming obese when consuming a high fat diet (HFD). Resistance to obesity does not appear to be due to increased energy expenditure, but rather from failed adipose tissue expansion resulting in severe hepatomegaly associated with hyperphagia and defective dietary lipid absorption. Deleting Raptor in WAT also decreases C/EBPα expression and the expression of its downstream target adiponectin, providing one possible mechanism of mTORC1 function in WAT.
Conclusions: mTORC1 activity in mature adipocytes is essential for maintaining normal adipose tissue growth and its selective loss in mature adipocytes leads to a progressive lipodystrophy disorder and systemic metabolic disease that shares many of the hallmarks of human congenital generalized lipodystrophy.[Hide abstract]